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Influence of particle-abrasion and aging on biaxial flexural-strength of three Zirconia materials
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Influence of particle-abrasion and aging on biaxial flexural-strength of three Zirconia materials
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Content
Influence of Particle-Abrasion and Aging on Biaxial Flexural-Strength of Three
Zirconia Materials
by
Nazanin Forghani, DDS
A Thesis Presented to the
FACULTY OF THE USC HERMAN OSTROW SCHOOL OF DENTISTRY
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirement for the Degree
MASTER OF SCIENCE
(BIOMATERIALS AND DIGITAL DENTISTRY)
December 2022
Copyright 2022 Nazanin Forghani
ii
Dedication
I want to dedicate my work to my parents, sisters, and brothers-in-law. Without whom,
none of my success would be possible.
iii
Acknowledgments
First, I would like to thank the committee members for their efforts and constant
encouragement throughout my journey at USC.
A special appreciation is to my supervisor and the committee chairman Dr. Jin-
Ho Phark, for his unlimited time, prompt response, and continuous support he offered
to me.
Dr. Phark kept pushing and supporting me to be a better researcher and be more
organized. I've learned so much from him; I really appreciate his support and help.
Dear Dr. Phark, I had the honor of working with you, and it was a beautiful
scientific experience. Thank you can never fully express my gratitude to you.
Also, I would like to give Dr. Duarte, my mentor and co-advisor, extraordinary
gratitude for teaching me.
Dr. Duarte, thank you for your tremendous support, professionalism, motivation,
and immense knowledge. I am thankful for believing in and supporting me throughout
these 26 months of residency. You saw that I have potential; since then, you have
supported and inspired me to go beyond my ability, always do my best, and try to be
the best!
Dr. Duarte, words will not express my gratitude toward you. Thank you for your
faith, constant encouragement, and for helping me become a better clinician.
Dear Dr. Duarte, I was honored to be one of your students. I'll always be Dr. Duarte's
student. Learning from you not only contributed to my academic performance but also
influenced my personal life. Thank you, Dr. Duarte, from the bottom of my heart.
iv
Additionally, I should thank Dr. Alena Knezevic for her generous heart and
support.
Dear Dr.Knezevic, I learned from you the dedication to work with enjoying the
process. You are an example of the successful, robust, responsible woman I'm looking
forward to. Your achievements inspired me personally. Thank you for all your
instructions, for sharing information, and for your love for spreading science.
I also thank my parents for their support, love, and constant prayers all my life.
They have never hesitated to offer everything I need to succeed and fulfill my
ambitions.
My beloved mom "Fatima Emami," you are my shield and my strength in this
challenging life, and you flooded me with love. So, I dedicate this work to you.
My lovely dad, "Dr. Reza Forghani," I will always be your spoiled girl, and you'll
be my first love. I gained much experience working with you in your clinic. You were
with me on my pathway and shared your experience with me. You motivated me to
reach my goals and helped me with difficulties, and I hope I can satisfy you. So now, I
am here to fulfill the dream that you saw!
I should express my gratitude with special appreciation to my amazing man, Dr.
Eddie Sheh, for standing by my side, hand in hand, to reach where I am now. He
encourages me to cope with my obstacles in the right way. I have learned a lot from
you. Your humble dedication to work was a great inspiration for me
I want to thank Dr. Jenny Son. Thank you for teaching me and encouraging me
to be a better dentist. Thank you for all your advice and guidance.
v
To Mrs. Karen Guillen, thank you for all your help and kindness. Thank you for
being patient and taking care of everything.
I am lucky to have this fantastic opportunity to meet those supportive people
beside me. This program was a remarkable learning experience and a memorable
training stage that helped me become a better dentist. I am grateful to all those who
helped make it happen.
vi
Table of Contents
Dedication .............................................................................................................................. ii
Acknowledgments .................................................................................................................. iii
List of Tables ........................................................................................................................ ix
List of Figures ........................................................................................................................ x
Abstract................................................................................................................................ xiii
Chapter 1: Introduction ........................................................................................................... 1
1) History of Using Ceramics in Dentistry ........................................................................ 1
2) Dental Ceramic Classification ...................................................................................... 3
1.3) Classification of CAD/CAM Ceramic Restorative Materials ...................................... 5
1.4) Zirconia .................................................................................................................. 11
1.5) Properties of Zirconia ............................................................................................. 14
1.5.1) Physical Properties.............................................................................................. 14
1.5.2) Mechanical Properties ......................................................................................... 14
1.5.3) Chemical Properties ............................................................................................ 17
1.5.4) Biocompatibility ................................................................................................... 18
1.5.5) Radioactivity ........................................................................................................ 18
1.6) Types of Zirconia in Dentistry ................................................................................. 19
1.7) Manufacturing and Fabrication of Zirconia Restorations ........................................ 24
1.7.1) Synthesis of Zirconia Nanopowder for Dental Restorations ................................. 25
1.7.2) Types of Zirconia Blanks ..................................................................................... 27
1.7.3) Manufacturing Procedure to Fabricate Blocks ..................................................... 27
1.7.4) CAD/CAM Milling ................................................................................................ 28
1.7.5) Sintering Techniques for Non-Sintered Blocks .................................................... 29
1.7.6) Glazing or Polishing ............................................................................................ 31
1.7.7) Luting of a Zirconia Restoration ........................................................................... 32
1.7.7.1) Conventional Luting Agents .............................................................................. 33
1.7.7.2) Resin Luting Agents ......................................................................................... 33
1.7.7.2.1) Chemical Bonding technique ......................................................................... 35
1.7.7.2.2) Micromechanical Bonding Techniques .......................................................... 37
vii
1.8) Test Methods and Flexural Strength Tests ............................................................. 41
Objective and Specific Aims ................................................................................................. 46
Chapter 2: Material and Methods ......................................................................................... 46
2.1) Preparation of Specimens ......................................................................................... 48
2.1.1) Cutting the Samples ............................................................................................ 48
2.1.2) Polishing ............................................................................................................. 53
2.1.3) Sintering .............................................................................................................. 54
2.2) Artificial Aging ........................................................................................................ 57
2.3) Surface treatment ................................................................................................... 58
2.4) Biaxial Flexure Test (Piston-on-Three-Ball Test) .................................................... 59
2.5) Calculation of Flexural Strength, σ: ........................................................................ 61
2.6) Statistical Analysis ................................................................................................. 62
Chapter 3: Results ............................................................................................................... 64
3.1) Descriptive Findings ............................................................................................... 64
3.1.1) All Groups Based on Yttria Content, Aging, and Surface Treatment ....................... 64
3.1.2) All Samples Based on Material ............................................................................... 67
3.1.3) All Samples Based on Aging .................................................................................. 68
3.1.4) All Samples Based on Surface Treatment .............................................................. 69
3.2) Inferential Findings ................................................................................................. 70
3.3) Weibull Reliability Analysis ..................................................................................... 77
3.3.1) Weibull Plots for 3Y-TZP (DD Bio ZX
2
) ................................................................... 81
3.3.2) Weibull Plots for 4Y-TZP (DD cube One) ................................................................ 85
3.3.3) Weibull Plots for 5Y-TZP (DD cubeX
2
) .................................................................... 89
3.4) Comparison of Weibull Results Using R and SPSS results using ANOVA .............. 94
Chapter 4: Discussion .......................................................................................................... 97
4.1) Different Methods to Examine the Flexural Strength Values ...................................... 97
4.2) Different Techniques of Surface Treatment ............................................................. 100
4.3) Aging ....................................................................................................................... 103
viii
4.4) Different Compositions of Materials ......................................................................... 105
Chapter 5: Conclusion ........................................................................................................ 109
References ........................................................................................................................ 110
ix
List of Tables
Table 1: Generations of representative dental zirconia materials ............................................................... 23
Table 2: Classification of samples based on the content of yttria, aging, and surface treatment... 47
Table 3: Sintering parameters .................................................................................................................................... 54
Table 4: Characteristics parameters for calculation of biaxial flexural strength based on ISO
6872:2015 (137) ............................................................................................................................................................... 62
Table 5: Mean, maximum, minimum, and std. deviation of the biaxial flexural strength of samples
based on materials, aging and sandblasting ........................................................................................................ 65
Table 6: Mean, maximum, and minimum of biaxial flexural strength of samples based on yttria
content .................................................................................................................................................................................. 67
Table 7: Mean, maximum, and minimum of biaxial flexural strength grouped by aging .................... 68
Table 8: Mean, maximum, and minimum of biaxial flexural strength grouped by surface treatment
................................................................................................................................................................................................. 70
Table 9: Evaluating normality by the indicators of Kolmogorov-Smirnov and Shapiro-Wilk ............. 71
Table 10: Evaluating normality by the indicators of Skewness and Kurtosis .......................................... 72
Table 11: Evaluating normality by the indicators of Skewness and Kurtosis for different groups of
samples ................................................................................................................................................................................ 73
Table 12: Levene test results for homogeneity of variances ......................................................................... 74
Table 13:Tests of between-subjects effects ......................................................................................................... 75
Table 14: The results of the Bonferroni test for comparing the biaxial flexural strength of three
materials .............................................................................................................................................................................. 76
Table 15: Summary of Weibull parameter results for the three materials based on surface
treatment ............................................................................................................................................................................. 93
Table 16: Summary of Weibull parameter results for the three types based on aging condition ... 94
Table 17: Summary of impacts by material, aging, and treatment using ANOVA test........................ 95
Table 18: Summary of impacts by material, aging, and treatment by using Weibull reliability
analysis ................................................................................................................................................................................ 96
x
List of Figures
Figure 1: Classification of dental ceramics in 2010 ............................................................................................. 4
Figure 2: Classification of dental ceramics in 2014. ............................................................................................ 5
Figure 3: CAD/CAM ceramic restorative materials ............................................................................................ 10
Figure 4: Phases of Zirconia ....................................................................................................................................... 12
Figure 5: Transformation toughening ...................................................................................................................... 16
Figure 6: Transformation t to m .................................................................................................................................. 16
Figure 7: Zirconia having undergone low-temperature degradation ........................................................... 18
Figure 8: Manufacturing and fabrication of zirconia restorations ................................................................. 25
Figure 9: Zirconia surface treatment ........................................................................................................................ 34
Figure 10: 3-point flexural test .................................................................................................................................... 42
Figure 11: 4-point flexure test ..................................................................................................................................... 43
Figure 12: Three loading schemes of biaxial flexural strength ...................................................................... 44
Figure 13: Three types of zirconia based on yttria content ............................................................................ 48
Figure 14: STL file design by Meshmixer............................................................................................................... 49
Figure 15: Zirkonzahn M5 CAD/CAM Milling Machine ..................................................................................... 50
Figure 16: 1) Milling zirconia blocks 2) Milled zirconia block ......................................................................... 50
Figure 17: 1) Removing cylindrical samples from milled block 2) Prepared cylindrical samples 3)
Polished cylindrical samples and metal sprue 4) Cylindrical samples glued into metal sprue ........ 51
Figure 18: Cutting cylindrical samples to disc shape samples using IsoMet 1000 ............................... 52
Figure 19: The thickness and diameter of disc-shaped samples before sintering ................................ 52
Figure 20: Sintering furnace( Dekema Austromat 674i Sintering Furnace, Freilassing, Germany 55
Figure 21: The measurement of samples' diameter and thickness before and after sintering ........ 56
Figure 22: Ultrasonic Cleaning Systems, Quantrex, Kearny, NJ, USA ..................................................... 56
Figure 23: Different translucency based on yttria content............................................................................... 57
xi
Figure 24: 1) Midmark M11 UltraClave Dental Steam Autoclave Sterilizer, USA 2) 3.5" x 5.25"
Peelvue sterilization pouch 3) After the steam autoclave ............................................................................... 58
Figure 25: 1) Air-borne particle abrasion 2) 50 µm Al2O3 3) Wet particle abrasion .............................. 59
Figure 26: 1) Universal testing machine 2) Piston-on-three balls 3) Piston-on-three balls
technique inside a universal testing machine ....................................................................................................... 59
Figure 27: Three hardened steel balls and placing the sample concentrically ....................................... 60
Figure 28: Fixing the disc-shape sample on three hardened steel balls concentrically ..................... 60
Figure 29: A typical plot of the linear form of the Weibull modulus ............................................................. 62
Figure 30: Summary of sample preparation ......................................................................................................... 64
Figure 31: Box plot of the biaxial flexural strength of the different test groups based on materials,
aging, and surface treatment ...................................................................................................................................... 66
Figure 32: Box plot of biaxial flexural strength of the different test groups based on materials ...... 68
Figure 33: Box plot of biaxial flexural strength of the different test groups based on aging ............. 69
Figure 34: Box plot of biaxial flexural strength of the different test groups based on materials and
surface treatment regardless of aging. ................................................................................................................... 70
Figure 35: A typical plot of the linear form of the Weibull modulus ............................................................. 78
Figure 36: A typical Weibull plot with confidence bounds ............................................................................... 79
Figure 37: (A): Weibull CDF and Confidence Intervals (CI) of biaxial flexural strength for dry and
wet abrasion for 3Y material. (B) Contour plot of beta values vs. eta in the Weibull CDF analysis
for 3Y zirconia under none, dry and wet abrasion. ............................................................................................ 82
Figure 38: (A) Weibull CDF and Confidence Intervals (CI) of biaxial flexural strength (σ) for aged
and non-aged 3Y material. (B) Contour plot of biaxial flexural strength for the 3Y material based
on aging condition ............................................................................................................................................................ 84
Figure 39: (A) Weibull CDF and Confidence Intervals (CI) of biaxial flexural strength (σ) for none,
dry and wet abrasion for 4Y material. (B): Contour plot of beta values vs. eta in the Weibull CDF
analysis for 4Y zirconia under none, dry and wet abrasion ............................................................................ 86
xii
Figure 40: (A) Weibull CDF and Confidence Intervals (CI) of biaxial flexural strength (σ) for aged
and non-aged 4Y material. (B) Contour plot of the biaxial flexural strength for the 4Y material
based on aging condition .............................................................................................................................................. 88
Figure 41: (A)Weibull CDF and Confidence Intervals (CI) of biaxial flexural strength for dry, wet,
and none abrasion for 5Y material. (B) Contour plot of beta values vs. eta in the Weibull CDF
analysis for 5Y zirconia under none, dry and wet abrasion ............................................................................ 90
Figure 42: (A) Weibull CDF and Confidence Intervals (CI) of biaxial flexural strength for aged and
non-aged 5Y material. (B) Contour plot of the biaxial flexural strength for the 5Y material based
on aging condition. .......................................................................................................................................................... 92
xiii
Abstract
Title: Impact of Particle-Abrasion and Aging on Biaxial Flexural-Strength of Three
Zirconia Materials
Objective: This study examines the zirconia materials’ biaxial flexural strength (BFS)
concerning yttria content, surface treatment, and artificial aging.
Material and methods: Three zirconia materials [Bio ZX² (3Y-TZP), DD cube One(4Y-
TZP), and DD cubeX² (5Y-TZP)] were selected, and a total of 270 zirconia disks were
divided into two groups: aged and non-aged. Based on surface treatment, these
groups were further subclassified into three groups: none, wet particle abrasion, and
airborne abrasion (n=15). For disk fabrication, cylinders (ø 15 mm, length 14 mm) were
milled from round zirconia blocks using Zirkonzahn M5 (Zirkonzahn GmbH, South
Tyrol, Italy ).The cylinders were attached to CAD/CAM block metal sprues and then
sliced into 1.4 mm thick disks using a low-speed precision saw (IsoMet 1000; Buehler,
Lake Buff, IL, USA). All disks were polished to a thickness of 1.5 mm with polishing
paper up to grit 1200 and sintered according to the manufacturer’s recommendations.
Aging was performed via the accelerated aging test, exposing samples to steam at
134±2°C under a pressure of 0.2 MPa for 5 h. Both particle abrasions were conducted
by 50 µm Al2O3 particles at 0.24 MPa (35 psi) for 15 s. Lastly, a universal testing
machine (Model 6596; Instron, Norwood, MA, USA) was used to evaluate BFS using
the piston-on-three-balls technique. A three-way ANOVA (IBM SPSS Statistics 22;
α=0.05) and Weibull analysis (R software) were conducted for statistical data analysis.
xiv
Results: Yttria content significantly impacted BFS, with the highest for 3Y and the
lowest for 5Y. Particle abrasion increased the BFS of all materials. Wet abrasion
resulted in a lower Weibull modulus and showed higher dispersion of the measured
data than dry abrasion. Aging had an insignificant effect on BFS of 3Y and 4Y, but it
decreased BFS of 5Y.
Conclusion: Particle abrasion and decreasing yttria content can enhance the BFS of
zirconia.
Keywords: Zirconia, Particle Abrasion, Aging, Yttria Content, Biaxial Flexural
Strength, Weibull
Chapter 1: Introduction
Ceramic materials have been the most popular material in restorative dentistry in
terms of esthetics for more than a century. John McLean introduced aluminous porcelain
in 1965 (1).
The improvement of ceramics in terms of esthetic, strength, and fabrication
methods has continued due to the quick progression in computer technology and dental
restorative technologies, such as 3D printing and computer-aided design and
manufacturing systems (CAD/CAM) (2, 3).
Subsequently, various products are available for dentists to choose from
CAD/CAM blocks (4). Nevertheless, because of the fast-growing number of new products,
clinicians often go through complicated decision-making processes to select a ceramic
material for a specific indication (5). Their decision would be according to specific criteria
such as strength, translucency, manufacturing method, recognized laboratory
technicians, or advertising demands (6).
1) History of Using Ceramics in Dentistry
Ceramics originated from the Greek Keramos, meaning pottery or burnt stuff. Historically,
three basic ceramic materials have been developed: earthenware, stoneware, and
porcelain (7). Earthenware is fired at low temperatures and has relatively porosity.
Stoneware, which emerged in China around 100 B.C., is fired at a greater temperature
than earthenware; in both materials, firing enhances strength and presents it more
2
impervious to water. The third material is porcelain, achieved by fluxing white China clay
with China stone to create white translucent stoneware (7).
The initial porcelain tooth material was introduced in 1789 by a French dentist, Nicholas
Dubois de Chemant (8). The production was developed from the last version made in
1774.
In 1808, an Italian dentist, Fonzi designed a terrometallic porcelain tooth kept in place by
a platinum pin or frame. Terrometal is composed of several clays possessing specific
hardness when baked. Planteau, a French dentist, presented porcelain teeth to the United
States in 1817, and Peale, an artist in Philadelphia, improved the baking process for them
in 1822 (8).
Dr. Charles Land introduced the initial successful fused feldspathic porcelain crown and
inlay to dentistry in 1886 (9). Land explained a method for fabricating ceramic crowns with
platinum foil as a substructure with control of a gas furnace.
Due to the reliable chemical bond of feldspathic porcelains, it has been used in metal-
ceramic prostheses for over 35 years. On the other hand, feldspathic porcelains have
been weak to operate reliably in constructing all-ceramic crowns without a cast-metal or
metal foil coping. Furthermore, their firing shrinkage leads to considerable discrepancies
in fit and adaptation of margins. McLean and Hughes introduced a noticeable
improvement in porcelain crowns' fracture resistance and improved a high-alumina
reinforced porcelain restoration in 1965 (10). They made alumina of 95% purity as an
aluminous core ceramic containing a glass matrix and 45-50% Al2O3 (11). Because of the
insufficient translucency of the aluminous porcelain core material, a veneer of feldspathic
3
porcelain was needed to gain appropriate aesthetics. In addition, aluminous porcelain
crowns can provide low flexural strength (around 131 MPa); as a result, this type of
porcelain crown can be used for anterior restorations (11). There has been development
in dental ceramics in terms of their properties and fabrication methods. Another progress
in the 1950s was the fabrication of the castable Dicor® crown system , resulting in new
restorations. Then, in the late 1980s, the first pressable ceramic material (Empress® 1)
was introduced. During this time, Vita using a slip-casting process, produced the In-
Ceram® system (12).
In 1971, Dr. Duret introduced a dental CAD/CAM device in the dental field, and in 1983,
the first dental CAD/CAM restoration was produced. In 1985, Dr. Mörmann and Dr.
Brandestini fabricated the first chairside inlay using CEREC 1. Then in 1994, CEREC 2
was able to produce copings, partial and full crowns, veneers, onlays, and inlays.
CEREC 1 and 2 had two-dimensional design programs. Finally, in 1998 Ivoclar
produced IPS Empress II was lithium disilicate fabricating as a single- and multiple-unit
framework indicated for the anterior area (12).
In 2005, the first 3D design program, CEREC 3, was introduced with virtual automatic
occlusal adjustments (13).
2) Dental Ceramic Classification
Several classification methods have been presented according to clinical application,
components, the probability of surface treatments, boding methods, translucency, and
mechanical properties (14).
4
In 2010, Edward A. McLaren and Russell Giordano classified dental ceramics into four
groups based on microstructural level: 1) glass-based systems (predominantly silica), 2)
glass-based systems (primarily silica) with crystalline fillers, 3) crystalline-based with
glass fillers (mostly alumina), and 4) polycrystalline solids (alumina and zirconia) (Figure
1: Figure 1) (2, 6).
Figure 1: Classification of dental ceramics in 2010
The most frequent classification is based on the Kelly and Benetti explained (15) ceramic
materials based on the glass contents and classified dental ceramic as (1) primarily
glassy materials, (2) particle-filled glasses, and (3) polycrystalline ceramics (no glass). In
this categorizing, novel materials (resin-matrix materials) were not included.
Subsequently, Stefano Gracis et al. (2014) classified dental ceramic materials into three
families: (1) glass-matrix ceramics (2) polycrystalline ceramics, and (3) resin-matrix
ceramics (Figure 2) (2, 6) .
5
Figure 2: Classification of dental ceramics in 2014.
1.3) Classification of CAD/CAM Ceramic Restorative
Materials
CAD/CAM technology (Error! Reference source not found.) is utilized in indirect d
ental restorations (crowns, fixed partial dentures, veneers, inlays, or dental implants) (15).
This technology permits providers to scan the preparations, design, mill, and deliver the
restorations on the same day. Some chairside CAD CAM methods do not need further
processing stages after milling, and lab-side systems need extra lab work, such as
crystallization through sintering (3). Recently, lab technicians and dentists could select
from a wide variety of materials to match the needed properties of each patient (like high
esthetics, durability, biocompatibility, and functionality) (16, 17).
Dental ceramics
Glass-matrix ceramics
Feldspathic Synthetic
Leucite-based
Lithium disilicate
and derivatives
Fluorapatite-
based
Glass-infiltrated
Alumina
Alumina and
magnesium
Alumina and
zirconia
Polycrystalline ceramics
Alumina
Stabilized
zirconia
Zirconia-
toughened
alumina
Alumina-
toughened
zirconia
Resin-matrix ceramics
Resin
nanoceramic
Polymer Infiltrated
Ceramic
Network(PICN)
Zirconia-silica
ceramic in resin
matrix
6
1.3.1) Glass-Matrix Ceramics:
1.3.1.1) Feldspathic ceramic: The initial CAD/CAM fine-structure feldspar ceramics
(VITAMark II (VITA Zahnfabrik, Bad Säckingen, Germany)) developed from traditional
feldspathic ceramics and have been used in clinics. The feldspathic CAD/CAM ceramic
contains sodium potassium aluminum silicate (18). Due to their low flexural strength
(154MPa) are indicated for single-tooth restorations such as veneers, onlays, inlays,
anterior and posterior crowns, and partial crowns (18).
1.3.1.2) Synthetic glass-ceramics
A) Leucite-reinforced ceramic: IPS Empress CAD (Ivoclar Vivadent, Schaan
Liechtenstein) is a proper ceramic that originated from initial generations of
CAD/CAM blocks consisting of leucite crystals up to 40% implanted in a
feldspathic glass-ceramic (19).
B) Lithium disilicate ceramic: A glass-ceramic by precipitating (controlled
nucleation and growth) lithium disilicate crystal leads to noticeably higher
strength. With two successful pioneer systems (Empress 2 and IPS e.max
Press; Ivoclar Vivadent, Schaan, Liechtenstein), a lithium disilicate glass-
ceramic IPS e.max CAD (Ivoclar Vivadent, Schaan, Liechtenstein) has been
designed (20).
C) Zirconia-reinforced lithium silicate: In 2013, a modern lithium silicate/phosphate
(LSP) CAD/CAM material, also termed zirconia-reinforced lithium silicate glass-
ceramics ((VITA SUPRINITY, VITA Zahnfabrik, Bad Säckingen, Germany), Celtra,
Celtra Duo, (Dentsply Sirona, Hanau-Wolfgang, Germany)), was presented (21).
7
D) Fluorapatite glass-ceramics: This type of ceramic consists of
fluorapatite crystals Ca5(PO4)3F in different sizes embedded into the glassy
matrix (IPS e.max Ceram and IPS e.max ZirPress, Ivoclar Vivadent,
Schaan, Liechtenstein) (21).
1.3.1.3) Glass-Infiltrated ceramics
Glass-infiltrated ceramic is utilized as VITA In-Ceram systems (Vita Zahnfabrik; Bad
Säckingen, Germany), including In-Ceram Spinell, In-Ceram Alumina, and In-Ceram
Zirconia. The ceramic's optical features and mechanical behaviors are associated with
the chemical composition. VITA In-Ceram
TM
Spinell, composed of alumina and magnesia
(MgAl2O4), with high translucency and low strength (400 MPa); therefore, single crowns
in the anterior region are an appropriate indication for Spinell. The amount of alumina in
VITA In-Ceram
TM
Alumina is 80% to gain optimal strength (500 MPa) and high
translucency; therefore, its indication includes single crowns in the anterior and posterior
areas and 3-unit bridges in the anterior region. VITA In-Ceram
TM
Zirconia consists of
alumina and zirconia, with the highest strength (600 MPa) compared to VITA In-Ceram
TM
Spinell and VITA In-Ceram
TM
Alumina. VITA In-Ceram
TM
Zirconia can be indicated for
crowns in the posterior region and 3-unit bridges (21).Generally, In-Ceram has a porous
matrix that the pores are then filled with lanthanum-aluminosilicate glass (22).
1.3.2) Polycrystalline Ceramics:
1.3.2.1) Alumina ceramics:They are made of more than 99.9% alumina and has a
flexural strength of about 600 MPa. Procera® AllCeram (Nobel Biocare AB, Göteborg,
Sweden ) is the initial fully dense polycrystalline ceramic (20).
8
1.3.2.2) Zirconia Ceramics: In the early 1990s, CAD/CAM technology made zirconia
widely available in dentistry. It will be explained in the next section [(Zirconia] (21).
1.3.2.3) Alumina-toughened zirconia (ATZ) and zirconia-toughened alumina (ZTA):
ATZ consisted of > 50% by weight of zirconia while ZTA made of > 50% by weight of
alumina. ZTA is mainly indicated for the fabrication of medical prostheses (21).
In 2000, pink-colored alumina-zirconia hip joints (BIOLOX®delta, CeramTec GmbH,
Plochingen, Germany) were utilized for orthopedics, and ZTA has not yet been used in
dentistry. The primary cause is derived from its opacity and is improper for esthetic
applications (22).
Alumina-reinforced zirconia (ATZ) such as Ziraldent® (Metoxit, Thayngen, Switzerland)
and NANOZR® (Panasonic Electric Works, Osaka, Japan) can be used for dental
implants (22).
1.3.3) Resin-Matrix-Ceramics:
1.3.3.1) Resin Nano Ceramic: in 2011, Lava Ultimate (3M EPSE, Seefeld, Germany)
was introduced based on polymerizable resin composite Filtek Supreme Ultra. It contains
dispersed nanometric colloidal silica and ZrO2 spherical particles in agglomerated and
non-agglomerated form (80% wt, 65% vol) embedded in a dimethacrylate resin (21).
1.3.3.2) Polymer Infiltrated Ceramic Network (PICN): VITA Enamic (Vita Zahnfabrik,
Bad Sackingen, Germany) consists of a PICN containing urethane dimethacrylate and
triethylene glycol dimethacrylate cross-linked polymers and an exemplary open porous
feldspathic ceramic structure network (86% wt, 75%) (2).
9
1.3.3.3) Zirconia-silica ceramic in resin matrix:
It is the combination of organic and inorganic materials. For example, Shofu Block HC
(Shofo, Kyoto, Japan) consists of silica powder, zirconium silicate, TEGDMA, UDMA,
micro-fumed silica, and pigments. The content of inorganic is more than 60% by weight.
Another example is Paradigm MZ-100 Block (3M EPSE, Seefeld, Germany), consisting
of 85% ultrafine zirconia-silica ceramic particles and a polymer matrix (BISGMA and
TEGDM) (21).
10
Figure 3: CAD/CAM ceramic restorative materials
11
1.4) Zirconia
The discovery of zirconia was related to the German chemist Martin Heinrich Klaproth
in 1789. Zirconium is the metal, while zirconia ceramic ("zirconia") is zirconia-dioxide-
ceramic (ZrO2) (3). The name zirconium derives from the Arabic "Zargun" (golden in
color), originating from the two Persian words "Zar" (Gold) and "Gun" (Color) (3).
Zircon or zirconium silicate, ZrSiO4 (67.2% of ZrO2 and 32.8% of SiO2), is the most
critical zirconium mineral (23).
The impure zirconium was initially isolated by Jöns Jakob Berzelius, a Swedish
chemist, in 1824 by heating a mixture of potassium and potassium zirconium fluoride
in a small iron tube. However, gaining pure zirconium was impossible until the early
19th century. Pure zirconium oxide was initially prepared in 1914 by Herzfel (24).
He invented the process of crystallizing zirconium oxychloride octahydrate from a
concentrated hydrochloric acid solution to eliminate extensive contents of silica and
oxychloride, then crystallized under cooling in1916 (24).
Very pure zirconium was initially made in 1925 by van Arkel and de Boer (25) through
an iodide decomposition process. Commercial-grade zirconium contains from 1 to 3%
hafnium (25).
Zirconium oxide was initially utilized for medical goals in 1969 for orthopedic
applications. It was recommended as a novel material for hip head replacement
instead of titanium or alumina prostheses (4). Its application in the dental field began
in the 1990s for root canal posts, then for prosthetic abutments, but the development
of its implementation in dental prosthetics began with the chance to manufacture
ceramic posterior fixed partial prostheses (26) .
12
Many studies compare the survival rate of all-ceramic and metal-ceramic restorations.
Recent studies were performed to compare the survival rate of implant-supported
single zirconia and metal-ceramic crown restorations. In 2008, a study showed that
the survival rate of zirconia-based restorations for five years was 97.6%, while the
survival rate of metal-ceramic crowns for three years was 98.3% (27).
Different studies showed the survival rate of zirconia materials related to different
factors, such as the content of yttria and surface treatment methods (27).
1.4.1) Phases of Zirconia:
In nature, zirconia is available in the polymorphic shape and demonstrates various
crystal structures at various temperatures without alternation in chemistry. It is
available in three crystal shapes monoclinic (M), tetragonal (T), and cubic (C). At room
temperature and heating to1170°C, the structure is monoclinic. The structure is
tetragonal between 1170 and 2370°C and cubic above 2370°C and up to the melting
point (Figure 4) (28).
Figure 4: Phases of Zirconia
The transformation from the tetragonal (T) to the monoclinic (M) phase upon cooling
is along with a substantial enhancement in volume ( ∼4.5%), resulting in catastrophic
failure (29). The ceramic material displays a hysteretic martensitic t → m
Monoclinic Tetragonal
Cubic
1170 ℃
2370 ℃
13
transformation during heating. This transformation is reversible and begins at ∼950 °C
on cooling (30).
Passerini and Ruff et al (31). found that alloying pure zirconia with stabilizing oxides
such as CaO, MgO, Y2O3, or CeO2 permits the retention of the tetragonal structure at
room temperature (31).
When the amount of stabilizing oxides is less than needed for complete stabilization,
zirconia is partially stabilized in a multi-phase shape called partially stabilized zirconia
(PSZ). It includes cubic zirconia as the chief phase and monoclinic and tetragonal
zirconia as the minor phase. The entire material consists of transformable t-zirconia
grains known as tetragonal zirconia polycrystals (TZP). The best form for dental
applications is zirconia stabilized with Y2O3 (32, 33) . Yttrium-oxide partially stabilized
zirconia (Y-PSZ) is an entirely tetragonal small-grained zirconia ceramic material,
including 100% metastable tetragonal grains (Y-TZP) followed by adding around 2-3%
mol% yttrium oxide (Y2O3) as a stabilizing element (33).
The percentage of yttria could be various in the zirconia, in which the content
considerably impacts both mechanical and optical properties (34). As zirconia is
originally dull-white, similar to a household ceramic tile's opacity, its opacity conceals
the underneath structure (like a tetracycline-stained tooth structure or metal). The
desired goal is to increase translucency, which can be demonstrated by the isotropic
quality of the cubic form that inhibits the birefringence of the residual tetragonal phase,
decreasing the visional scattering coefficient (35).
The microstructure also impacts translucency and displays an association between
grain size and the number of grain boundaries (36). The increasing sintering
temperature and the content of yttria (which means a higher cubic form) lead to a
14
higher fraction of cubic crystals inside the zirconia (34). Therefore, zirconia with a
lower yttria content (or less cubic phase) shows more opacity (less translucency).
1.5) Properties of Zirconia
1.5.1) Physical Properties
1.5.1.1) Thermal Properties:
Thermal properties are associated with the conductivity of heat. Zirconia has high
stability of temperature and melting point (2680°C), high thermal expansion (>10 x 10-
6 1/K), low thermal conductivity (<1 W/mK), and an excellent thermo-shock resistance
(ΔT=400-500°C) (37).
1.5.1.2) Optical Characteristic:
The refractive index is one of the optical features, meaning how much the light pattern
is refracted or bent when entering a material. For example, YSZ displays a high
refractive index (around 2.2) (38).
Another feature is the absorption coefficient, meaning how far into a material light of a
particular wavelength is able to penetrate before absorption. Zirconia has a low
absorption coefficient and high opacity. Its high radiopacity (opacity to X-rays) is
comparable to metal alloys leading to an easy evaluation on the radiograph when
assessing marginal integrity and excess cement (38).
1.5.2) Mechanical Properties
Zirconia is particular among oxide ceramics due to its appropriate mechanical
properties.
1.5.2.1) Hardness, Flexural Strength, and Fracture Toughness:
15
In vitro studies, zirconia samples display a flexural strength of 900 -1200 MPa and a
fracture toughness of 9 -10 MPa/m
2
(27), which is approximately twice as robust as
aluminum oxide ceramics and Young's modulus (210 GPa), similar to that of stainless-
steel alloy (193 GPa) (29). Also, zirconia has a high hardness (1200-1350 HVN) (2,
33).
The high primary fracture toughness and strength of zirconia are derived from a
physical feature of partially stabilized zirconia known as transformation toughening
(t → m transformation leads to more resistance to crack propagation) (4).
An average load-bearing capacity of 755 N was reported for zirconia restorations.
Fracture loads ranging between 706 N, 2,000 N, and 4,100 N were recorded; all
studies show that zirconia demonstrates higher fracture loads than alumina or lithium
disilicate in dental restorations (38). In addition, the transformation toughening
capabilities of this material lead to excellent mechanical properties.
1.5.2.2) Transformation Toughening
It is the process leading to resistance to crack propagation. It is associated with
transforming the tetragonal to monoclinic form due to some stress-generating, along
with enhancing the volume and creating compressive stresses on the surface by
modifying the phase integrity and increasing flexural strength. (33) The enhancement
of volume results in a localized halt of the crack propagation. This process is known
as "transformation toughening," with resistance to the propagation of the crack (Figure
5, Figure 6). (39).
Several factors, inducing stress (like aging) and surface treatments (such as grinding
and airborne-particle abrasion), can enhance the susceptibility of t→m transformation.
(39)
16
Air abrasion propels a thin stream of abrasive particles ( silica or aluminum oxide) via
compressed air. This method could be used to remove caries, stains, and old
restorations and prepare teeth for bonding of new restoration. There are different
names for particle abrasion, such as sandblasting and micro-etching. A dental air
abrasion unit, micro etcher, or sandblaster can be utilized without or with water (dry
and wet abrasion) (40, 41).
Transformation t → m
Mastication
load>100N
Figure 5: Transformation toughening
Figure 6: Transformation t to m
17
1.5.3) Chemical Properties
The chemical properties of zirconia are dissolution resistance, chemical etching, and
discoloration. In addition, low-temperature degradation, an essential feature of
zirconia, is often considered a degradation due to the phase transformation in the
moist condition (22).
1.5.3.1) Low -temperature degradation (LTD):
This phenomenon is spontaneous and refers to the slow transformation of metastable-
t to stable-m phase without mechanical stress, happening gradually at low
temperatures. The existence of fluid, steam, or water exacerbates this phenomenon.
The starting point can be at the surface and later progress toward the material's bulk.
The transformation t-m, along with the volume increase of one grain, stimulates the
stresses on the surrounding grains and microcracks. Microcracks, the cause of water
penetration, lead to the progress of LTD from neighbor to neighbor. The slow crack
growth can lead to premature failure (22).
The significant elements impacting LTD are (1) the content and type stabilizer, (2) the
residual stress, and (3) the grain size.
The reaction of water with the Zr-O-Zr at the crack tip, leading to zirconium hydroxides
(Zr-OH), promotes crack growth and t-m phase transition (42, 43). The effective
temperature range for the development of aging is 200 to 300°C. LTD reduces
strength, density, and toughness and increases monoclinic phase content, resulting in
micro and macro cracking of the material (38). It starts on the surface and moves
toward the material's mass. t-m transformation is increasing in water or vapor ( Figure
7) .
18
Figure 7: Zirconia having undergone low-temperature degradation
Hydrothermal aging or exposing water vapor can cause and accelerate reducing the
mechanical properties of Y-TZP ceramics (44).
1.5.4) Biocompatibility
Biocompatibility means that the material is not harmful to living tissue. The higher
biocompatibility of high-purity Y-TZP powders has been proven in vivo and in vitro
studies.
Similar cytotoxicity compared with alumina was found in Y-TZP powders because of
the low content of TiO2. Any mutagenic impacts on blood cells or fibroblasts were not
seen. Osseo-integration for zirconia in implants is similar to titanium. However,
particles derived from the low-temperature degradation of zirconia (LTD) or the
process of manufacturing could be released, leading to localized inflammation (38).
1.5.5) Radioactivity
Radioactivity is a feature of the atomic nuclei of definite isotopes, leading to a
spontaneous alternation in the nucleus structure and the emission of energetic
radiation. Zirconia powder consists of low content of radionuclides derived from the
H
2
O H
2
O
H
2
O H
2
O
19
uranium-radium (226Ra) and thorium (228Th) actinide series. Its radioactivity is
comparable to Co-Cr alloys and alumina ceramics (38).
1.6) Types of Zirconia in Dentistry
1) Tetragonal zirconia polycrystals (3Y-TZP) (1
st
generation)
It comprises 3 mol% Y2O3 and 0.25 wt% Al2O3 (alumina) and has a grain size of 0.3-
0.5 μm. It shows a significant fracture toughness (9-10 MPa/m2) and flexural strength
(900-1200 MPa). Due to its poor optical properties, the first-generation zirconia was
initially utilized as the framework for single and multi-unit restorations. Feldspathic
porcelain was used for veneering the framework (5).
2) Tetragonal zirconia polycrystals(3Y-TZP) with small alumina
content (2
nd
generation)
It is made up of 3 mol% of Y2O3 and 0.1 or 0.05 wt.%Al2O3. Compared to the first, this
generation showed improvement in optical features because of the reduction in the
number and size of the alumina grains, leading to increasing translucency.
Subsequently, its translucency enhanced to T.P. = 24- 31, and the biaxial strength
declined to 900 -1150 MPa. The usage of this material is limited to frameworks for
single or multi-unit restorations and partially veneered restorations (45).
When yttria is between 3-8 mol%, there is a mix of tetragonal and cubic phases at
room temperature called partially stabilized zirconia (PSZ). But tetragonal zirconia
polycrystal (TZP) is made of 3 mol% of yttria and 100% tetragonal phases at room
temperature, called toughened zirconia (46).
20
3) 5Y- PSZ (3
rd
generation)
5-mol% Yttria Partially Stabilized Zirconia (5Y-PSZ) was introduced in 2015 to
enhance the optical feature through increasing yttria (Y2O3) content to 5-mol% to
stabilize the zirconia with up to 53% cubic phase, and thus, the material has a
combination of cubic/tetragonal structure. The cubic phase has a bigger crystallite size
with fewer boundaries leading to higher translucency. There is a direct correlation
between the amount of the cubic phase and translucency. On the other hand, cubic
zirconia is more brittle and weaker compared to its tetragonal counterpart, which
imperils the strength of the zirconia. Its translucency is enhanced to T.P. = 30- 43, and
the biaxial flextural strength declined to 450-740 MPa. (47) This material can be an
alternative to high-strength glass ceramics because of its high flexural strength.
In a study by X-ray fluorescence (XRF), Kolakarnprasert et al. (48) claimed Katana
UTML zirconia (Kuraray Noritake, Dental, Tokyo, Japan) had the most considerable
amount of yttria content (5.4 mol%). However, Katana STML zirconia had the yttria
content of 4.8 mol%. The different amount of yttria mol% leads to a variety of zirconia-
phase composition. First, the enhancement in the cubic phase leads to a noticeable
decrease in the strength of zirconia since it cannot tolerate stress-stimulated
transformation (48, 49)Subsequently, the decreasing amount of yttria mol% increases
the strength of zirconia and fracture resistance.
4) 4-Y-PSZ (4
th
generation)
4-mol% Yttria Partially Stabilized Zirconia (4Y-PSZ) was introduced in 2017. It consists
of more cubic Zirconia (50-80%) due to 4mol% of Y2O3 and has a grain size of 1-4 μm.
The duration and temperature of sintering are enhanced compared to the second
generation. It is recommended for single and short-span FDPs (43).
21
5) Zirconia-Toughened-Alumina and Alumina-Toughened Zirconia
Adding alumina to partially stabilized zirconia can increase fracture toughness, and
there are two kinds of material, including zirconia-alumina (alumina-toughened
zirconia [ATZ]) and alumina-zirconia (zirconia-toughened alumina [ZTA]) (50).
The amount of alumina or zirconia can be changed based on manufacturers'
manipulation. Overall, ZTA should have more than 50% by weight of Al, while ATZ
should have more than 50% by weight of Zr, and compared with Y-TZP, it has better
mechanical properties (51).
ZTA BIOLOX®delta (Courtesy Ceram Tec GmbH, Plochingen, Germany) is in the
market consisting of 81.6vol % Alumina matrix and 17 vol % zirconia. (22) ZTA is
mainly used in orthopedics but is never used in dentistry due to its high opacity. (22)
ATZ, Ceramys(Ceramys, Mathys, Bettlach, Switzerland), is in the market with 80 w.%
of zirconia and 20 w.% of alumina. Moreover, it is formed by 1% monoclinic
zirconia,17% cubic zirconia, and. 61 % tetragonal zirconia. (52)
6) Glass Infiltrated into Zirconia:
Another type of zirconia is glass infiltrated into zirconia. The glass leads to impeding
water sorption and limiting hydrothermal degradation. Also, this structure displays
promoted fracture resistance and aesthetics compared with homogeneous Y-TZP(52).
In-Ceram Zirconia (Vita Zahnfabrik; Bad Säckingen, Germany) consists of AI2O3
(62%), ZnO (20%), La2O3 (12%), SiO2 (4.5%), CaO (0.8%), and can be indicated for
masking discolored teeth and three-unit posterior bridges due to lower translucency
and high strength (22).
22
7) Nanostructured Zirconia
There is a limitation in increasing the cubic phase to improve translucency. An
alternative and experimental method is the reduction in the grain size to improve the
light transmittance of zirconia material. In order to have good transmittance in Y-TZP
ceramic, the size of grains should be less than 100 nm (47).
Moreover, Green et al. showed that small grain sizes led to good light transmittance
and can prevent t → m transformations in the zirconia material (53).
23
Table 1: Generations of representative dental zirconia materials
Group Name Manufacturers
3Y
(Conventional)
(With 0.25–0.5 wt%
alumina)
Cercon base
inCoris Zi
Vita YZ T
Aadva ST
Ceramill zi
DD Bio Z
Z-CAD HD
Copran Zri
Dentsply Sirona, Bensheim , Germany
Dentsply Sirona ,Bensheim, Germany
Vita Zahnfabrik , Bad Sackingen , Germany
GC Tech, Leuven, Belgium
Amann Girrbach , Koblach,Austria
Dental Direkt GmbH , Spenge, Germany
Metoxit, Thayngen, Switzerland
WhitepeaksDental Solutions, Wesel, Germany
3Y
(High translucent)
(> 0.05 wt% alumina)
Katana HT/ML
Cercon ht
Vita YZ HT
Aadva EI
ceramill zolid
inCoris TZI
DD Bio ZX
2
Z-CAD HTL
IPS e.max ZirCAD MO/LT
Zpex
Lava Plus
Kuraray Noritake, Dental, Tokyo, Japan
Dentsply Sirona, Bensheim , Germany
Vita Zahnfabrik , Bad Sackingen , Germany
GC Tech, Leuven, Belgium
Amann Girrbach, Koblach, Austria
Dentsply Sirona, Bensheim , Germany
Dental Direkt , Spenge, Germany
Metoxit , Thayngen, (Switzerland)
Ivoclar Vivadent, Schaan Liechtenstein
Tosoh, Tokyo, Japan
3M ESPE, Maplewood, USA
4Y-PSZ
Katana ST/STML
ceramill zolid HT+
DD cube ONE®
Z-CAD One4
All opraSupreme
IPS e.max ZirCAD MT
Vita YZ ST
Zpex 4
3M™ Chairside Zirconia
Kuraray Noritake, Dental, Tokyo, Japan
Amann Girrbach , Koblach, Austria
Dental Direkt , Spenge, Germany
Metoxit, Thayngen, Switzerland
Whitepeaks Dental Solutions Gmb, Wesel, Germany
Ivoclar Vivadent, Schaan Liechtenstein
Vita Zahnfabrik , Bad Sackingen , Germany
Tosoh, Tokyo, Japan
3M ESPE, Maplewood, USA
5Y-PSZ
Katana UT/UTML
Aadva NT
Prettau Anterior
DD cube X
2
Vita YZ XT
Z-CAD Smile
CopraSmile
Ceramill zolid fx
Cercon xt
Zpex Smile
Lava Esthetic
Kuraray Noritake Dental, Tokyo,Japan
GC Tech, Leuven, Belgium
Zirkonzahn, Taufers, Italy
Dental Direkt , Spenge, Germany
Vita Zahnfabrik , Bad Sackingen , Germany
Metoxit , Thayngen, Switzerland
Whitepeaks Dental Solutions, Wesel, Germany
Amann Girrbach AG, Koblach, Austria
Dentsply Sirona, Bensheim, Germany
Tosoh, Tokyo, Japan
3M ESPE, Maplewood, USA
24
1.7) Manufacturing and Fabrication of Zirconia
Restorations
Gradually improving CAD/CAM systems and zirconia materials led to using zirconia
wildly for single crowns and fixed partial dentures (FPDs) in prosthetic dentistry.
Zirconia has been adopted in many functions, involving fixed partial dentures,
complete- and partial-coverage restorations, implant fixtures, all-ceramic frameworks,
and post and core systems (54, 55).
There are two types of zirconia-based restorations involving:
1) Layered Zirconia Restorations, in which a robust zirconia core is veneered
with porcelain, are usually mentioned as "porcelain fused to zirconia" or "PFZ."
They have esthetics and strength. The different coefficients of thermal
expansion between zirconia and porcelain can lead to residual stresses and the
propagation of cracks. The major problem of this group is the chipping of the
veneering porcelain (56).
2) Solid Zirconia Restorations or Monolithic Zirconia: because of the major
failure of layered zirconia that was chipping and fracturing, monolithic zirconia
was introduced. This group is sintered for over six hours at 1,500 °C. Compared
to layered ones, monolithic crowns are more resistant and cost-effective. (26)
Monolithic Zirconia: has had outstanding mechanical properties. As a result, it
can be used for different applications, from long-span fixed partial dentures to
complicated all-ceramic restorations (26).
There are several steps to fabricate zirconia restorations that should be performed
from the synthesis of zirconia nanopowder by the final step, which is delivery of the
restoration in dental clinics (Figure 8).
25
1.7.1) Synthesis of Zirconia Nanopowder for Dental
Restorations
The initial phase of fabricating the monolithic zirconia restorations is the synthesis of
the proper yttrium stabilized zirconia nanopowder that would be high purity and with
fine particle sizes (57).
Different ways to synthesize nanoscale zirconia include co-precipitation, hydrothermal
treatment, sol-gel, solution combustion synthesis, and mechanochemical processing.
The Co-Precipitation Method is derived from dissolving the favorite cations in an
aqueous solution along with adding a precipitant agent, leading to the precipitation of
metal hydroxides. Growth mechanisms and nucleation can be controlled by modifying
the temperature and pH. Although it is a low-cost method, broad particle size
distribution and grain agglomeration are unavoidable (58-60).
Hydrothermal Treatment includes co-precipitation and heat treatment at high
temperatures to gain an anhydrous crystalline powder. It can have the microwave-
hydrothermal route to provide further heat for the reactions (53, 61).
Figure 8: Manufacturing and fabrication of zirconia restorations
26
The Sol-Gel Method can combine metal-organic compounds and inorganic salts
through hydrolysis and condensation, forming a sol that can be converted into a gel.
Then, a homogenous nanopowder can be obtained by drying the gel and treating
thermally. A modified, nonaqueous sol-gel process is a modern green technique to
prepare dispersive three mol% yttria-stabilized zirconia nano-powder with particle
sizes of 15–25 nm (62).
The solution, Combustion Synthesis, is derived from dissolving various kinds of
organic fuels (like urea, sucrose, glycine, and glucose) in a solvent (like water or
hydrocarbons) with metal nitrate hydrates (63) For the mixture, there are two modes
of thermal treatment available. The first method (thermal explosion or volume
combustion) includes sequential thermal therapy in different phases. The second
method (self-propagating combustion) is derived from local heating to start an
exothermic reaction in the shape of a combustion wave (64).
Mechanochemical processing is a mechanical mixture of significant pure oxide
powders milled before the calcination of yttrium towards the grain surface (59, 61-65).
The synthesis can be chemically or mechanically. Naturally, the mechanical way is
cheaper than the chemical method. However, the chemical process indicates a strict
control of the powder features.
27
1.7.2) Types of Zirconia Blanks
Three types of Zirconia blanks: (62)
A) Non-sintered or green state
B) Fully sintered zirconia
C) Partially sintered zirconia
1.7.3) Manufacturing Procedure to Fabricate Blocks
After synthesizing the powder of zirconia nanopowder, compacting ceramic powders
are needed to fabricate the ceramic blocks, and then the blocks are ready for milling.
The production of CAD/CAM zirconia blocks can be based on “soft machining” of non-
sintered blocks or “hard machining” of fully sintered blocks (66).
A) Non-sintered/green stage” zirconia: The fabrication is based on milling
non-sintered or non-sintered/green stage blanks (67).
Compacting zirconia powders with a binder through a cold isostatic pressing
(CIP) can produce non-sintered blocks. During CIP, pressure is performed at
environment temperature, gradually enhancing from 50 to 400–1000 MPa and
conveyed evenly on zirconia powder until the green pacts attain 40% to 60% of
their theoretical density before sintering. The green stage (non-sintered) `is
further stabilized and condensed up to approximately 95% of the theoretical
density using sintering without pressure (68). Partially sintered zirconia for
fabrication belongs to this method (67).
28
B) Fully sintered or HIP type of zirconia: Fully sintered blocks are prepared
by pre-sintering at temperatures < 1500ºC to gain a density of 95% of the
theoretical density. Next, hot isostatic pressing (HIP) is conducted to enhance
densification by performing high pressure (50 to 200 MPa) and high
temperature (1400-1500ºC) through an isostatic gas pressure (like argon or
nitrogen). This step produced very hard, dense (99% of the theoretical density),
and homogeneous blocks of fully sintered zirconia. Performing HIP on Y-TZP
leads to a gray-black material that frequently needs consequent heat treatment
to oxidize and restore whiteness (69).
C) Partially sintered zirconia: After compacting the powder mixture into the
green state, partial sintering under moderate firing conditions (for example,
1250 °C for 30 min) can be conducted (67-69).
1.7.4) CAD/CAM Milling
Zirconia can be mostly used in dentistry by milling the zirconia block utilizing a
computer-aided design/computer-aided manufacturing (CAD/CAM).
There are two methods for milling the blocks based on manufacturing, mentioned in
the previous section.
1) Soft machining: Soft milling is used for non-sintered blocks (green state).
A non-sintered block is soft after compacting powder to fabricate blocks like
chalk. In this stage, restorations are 20-25% larger than the desired final size
due to sintering shrinkage in the next step (3).
29
2) Hard machining: The fully sintered zirconia is milled at a ratio of 1:1 in hard
machining. Because of the hardness of blocks, the milling of fully sintered
blocks is very difficult and time-consuming, but the final dimension can be
achieved. Mostly, the fully sintered zirconia milling blocks are pre-shaded and
multi-layer to fabricate fabrication of natural-tooth colored. This procedure leads
to the elimination of staining during sintering (66).
After machining, there are separate ways for non-sintered and full-sintered zirconia
until the bonding procedure.
a) For non-sintered zirconia blocks, polishing and staining should be considered after
milling. After milling, the creation of surface textures through finishing and polishing
can be performed. Zirconia has a porous structure after milling, leading to low
strength and deficiencies like cracks. Therefore, polishing after milling can be
helpful (51). Also, staining of zirconia should be performed after finishing and
polishing. In the following step, non-sintered zirconia materials must be sintered in
a furnace to convert to strong material. After sintering, there is a shrinkage of
around 25% to produce the desired dimensions (25).
b) Fully sintered blocks are mostly pre-shaded and multi-layer to eliminate the
staining. Before the bonding procedure, polishing or glazing and staining (if
needed) should be considered (66).
1.7.5) Sintering Techniques for Non-Sintered Blocks
There are two sintering techniques for non-sintered blocks, including conventional
and two-step.
30
a) Conventional Sintering
The most frequent sintering technique for zirconia utilized conventional furnaces
between 1,300-1,700°C and retaining times between 2-4 h (70).
Hot isostatic pressure and hot press are used in conventional sintering furnaces,
including resistively heated atmospheric furnaces. In 2013, a study utilized
conventional sintering to investigate the impact of sintering temperature on grain size
and flexural strength (71). In that study, authors used Ceramill ZI Y-TZP blanks, the
highest sintering temperatures ranging from 1,300 -1,700 °C, a heating rate of
8°C/min, and a duration of 120 min. The authors concluded that conventional sintering
was ideal in terms of flexural strength (1,100–1,250 MPa) for sintering temperatures
between 1,400°C and 1,550°C (71).
In 2008, one study examined conventional sintering (1,100 -1500°C) for Zpex 3Y-TZP
for 1 min at a heat rate of 5°C/min. The conclusion showed that the highest density
was 98.8% (72).
Different studies showed conventional sintering could obtain grain sizes that create
proper mechanical and optical behaviors; however, the high temperatures cause fast
grain growth, damaging quality consistency (72).
A short "speed" sintering protocol in conventional furnaces is an alternative method.
According to the manufacturer's instructions, the "speed" sintering protocol includes a
temperature between 500-1,600°C and a holding time of 30 minutes.
Until now, any manufacturer has a specific protocol for its products. Also, various
methods were introduced, such as vacuum furnaces, microwaves, and plasma
sintering(70, 71, 73).
31
b) Two-Step Sintering (TSS)
TSS can be used to obtain full density without grain growth in the final sintering stage.
It includes high-temperature heating to T1°C (a peak temperature) to gain an
intermediate density, and then the temperature should be declined to a dwell
temperature (T2°C). Then, the temperature is held at T2 to reach the full density. (73)
The proper density that should be obtained at T1 is 70% or greater. This method
showed a slower grain growth rate than conventional sintering under similar conditions
(74, 75).
It also created higher densification at a smaller grain size (110 nm compared to 275
nm), with the ideal situation being the 1300–1050°C, 30 h cycle (75).
A study conducted in 2004 al investigated the TSS-CW method on 3Y-TZP.
It used a heat rate of 10°C/min to reach a T1 of 1500 C, was held for 5 min, then cooled
to a T2 of 1300 C, and held for ten h. It resulted in flexural strength of 1078 MPa
compared to the 295 MPa achieved using conventional sintering. In addition, grain
size at optimal density was also smaller using TSS-CW (560 nm compared to
1050 nm) (76).
Changes in sintering parameters showed alteration of zirconia's microstructural,
mechanical, and optical properties. For example, enhancing the sintering temperature
and reducing the sintering time can develop the translucency of zirconia; on the other
hand, it impacts mechanical behavior negatively (77).
1.7.6) Glazing or Polishing
Glazing and polishing are required to smoothen the surface of monolithic translucent
zirconia restorations leading to esthetic and biological integration. This part is after
32
milling and staining for fully sintered zirconia and after sintering and staining for non-
sintered zirconia.
Glazing is made by firing a slight transparent glass coating onto the surface or heating
the framework to glazing temperatures for 60 to 120 seconds to provide shiny surfaces
like glass (26).
Glazing is a laboratory process to create original gloss and color stability and decrease
antagonist tooth wear and plaque retention. After glazing, the ceramic surface can
become more robust, glossy, smoother, and stable in color and translucency (78, 79).
Due to functional stress in the oral cavity, the glazing of the occlusal surface can be
removed over time, leading to an enhancement in the wear of the antagonist.
Therefore, polishing can be preferable (80). It is essential to be considered that
polishing after sintering remained a critical step to reduce the surface roughness and
enhance the flexural strength of the zirconia restorations (80).
The cementation procedure plays a vital role in the clinical prosperity of ceramic
restorations, and adhesive cementation to ZrO2 ceramics is desirable (81, 82). Since
it improves retention, (83) marginal adaptation, and fracture resistance (84), it reduces
the possibility of recurrent decay and enables more conservative cavity preparations.
Surface treatment of zirconia can promote adequate adhesion between the resin
cement and zirconia (85).
1.7.7) Luting of a Zirconia Restoration
Zirconia ceramic restorations might be luted with conventional luting agents or resin-
based luting agents (87).
33
1.7.7.1) Conventional Luting Agents
The luting process is critical for the clinical success of all-ceramic restorations. Since
zirconia is a strong material, conventional cement can be used. Therefore, traditional
preparations, providing mechanical retention and resistance form, along with
conventional luting agents, such as zinc-phosphate, polycarboxylate, and glass-
ionomer cements, can be considered to deliver zirconia restorations. Conventional
cements are less technique-sensitive than the bonding procedure but cannot provide
enough bond strength for some clinical applications. Also, these types of cement are
opaque; as a result, they are not the first option because they affect the restoration's
appearance. Consequently, surface treatments and resin-ceramic bonding can show
higher bond strength than conventional luting agents (86).
1.7.7.2) Resin Luting Agents
Resin luting agents are classified into three groups dual cure, self-cure, and light cure.
Ceramic/resin cement bonds can be more effective and durable with the treatment of
ceramic intaglio surfaces (87). Before using resin-luting agents, zirconia surface
treatments are required (Figure 9 ). The zirconia surface treatments are classified into
two major groups: micromechanical bonding and chemical bonding.
34
Piwowarczyk et al. evaluated zinc phosphate; conventional and modified glass-
ionomer cement cannot create a lasting bond with zirconia. Only self-adhesive
universal resin cement (Rely X Unicem ) and dual-cure resin cement containing MDP
monomer (Panavia F2.0) show promising outcomes even after aging (86).
Zirconia surface
treatment
Micromechanical
Bonding
Mechanical
Treatment
Particle Abrasion
Diamond & Disk
Grinding
Selective Infiltration
Etching (SIE)
Zirconia Particle
Suspension
Chemical
Various
Acids(HF,HCL)
Laser
Chemical Bonding
Silicon coating
Cojet
Rocatec
Pyrosil Pen
Coupling agent
Silanes
Zirconia &metal
primers
10-MDP & Others
Figure 9: Zirconia surface treatment
35
Luthy et al. showed that the bond strength of glass-ionomer cement and conventional
Bis-GMA-based composites is noticeably lower, followed by thermocycling aging.
Only Rely X Unicem and Panavia F2.1 showed high bond strength (87).
The best method for cementing zirconia restorations is combining airborne-particle
abrasion with aluminum oxide (Al2O3) at 50μm and Panavia F2.0 containing
phosphate monomer 10 methacryloyloxydecyl dihydrogen phosphate (MDP) (88).
1.7.7.2.1) Chemical Bonding technique
Chemical bonding means the adhesion between dissimilar molecules of atoms of
substances bonding together. Adhesion is different from cohesion. In cohesion, there
is the bonding between atoms and molecules of similar or like materials.
I. Functional Monomer:
Specific functional monomers can be utilized to promote the adhesion to ZrO2,
providing a chemical affiliation to metal oxides, and involved in resin cement and
adhesives or conducted directly on the ceramic surface (52). Phosphate ester
monomers, such as 10-methacryloyloxyidecyl-dihydrogen phosphate (MDP), has a
chemical reaction with ZrO2, enhancing a water-resistant bond to densely sintered
zirconia ceramics. In addition, a phosphonic acid monomer, 6-methacryloyloxyhexyl
phosphonoacetate (6-MHPA), indicated some shape of chemical bonding to the
zirconia surface (5). Another monomer frequently utilized in ceramic primer materials,
like MEPS (thiophosphoric methacrylate), showed without clear benefits (89).
Some nonphosphate metal primers were examined like 6-(4-Vinylbenzyl-n-propyl)amino-
1,3,5-triazine-2,4-dithione (VBATDT), 4-methacryloxyethyl trimellitic anhydride (4-
36
META), ,6-methacryloyloxyhexyl-2-thiouracil-5-carboxylate (MTU-6), and phosphoric
acid acrylate monomers, permitting extra chemical bond with zirconium/metal oxides
(85).
The surface treatment with primers containing functional monomers such as MDP (e.g.,
Alloy Primer and Clearfil Ceramic Primer, Kuraray Medical Inc., Japan) or other
phosphoric acid acrylate monomer (e.g., Metal/Zirconia Primer, Ivoclar-Vivadent) are
frequently suggested to promote the bonding to ZrO2. However, since results are not
always noticeable, primers and air-abrasion techniques can create a stronger bond and
more longevity. In addition, using novel zirconia primers (a mixture of organophosphate
and carboxylic acid monomers) or a phosphonic acid monomer (6-MHPA) showed good
instant consequences (90-92).
II. Silica Coating:
Another technique to enhance the bond strength is tribochemical silica coating (TSC),
generating a fresh silica-rich ceramic surface. Tribochemistry can make a strong
chemical bond derived from the kinetic energy from sandblasting without any
application of light or heat. This procedure, before the luting, can conduct the chemical
bonding between the outer surface of ceramic enriched with silica and silane.
It began in 1984 with silicate technology (93), and in 1989 the Rocatec system
(Rocatec, 3M ESPE, Seefeld, Germany), a device used in the laboratory, was
improved. Later the CoJet system (CoJet Sand, 3M ESPE, Seefeld, Germany), a
chairside device, was presented to the market (94). These methods are related to
applying 30 𝜇 m (CoJet) or 110 𝜇 m (Rocatec) Si-coated alumina particles that are
blown onto the ceramic surface. Before cementation, this method can provide micro
retentive roughness (32, 95).
37
Another method that can enhance the bond strength is tribochemical Si-coating on
ceramic surfaces (96-98).
The air-abrasion pressure used is mostly 2.5–2.8-bar (99, 100); on the other hand,
CoJet with higher pressure leads to higher bond strength (101). In addition, some
researchers found that shear bond strength without and with Si-coating by air-abrasion
techniques is similar (102, 103). Si-deposition by air abrasion could create a more
reactive surface with silane (104). It is prone to making a surface with minor roughness
and subsequently less probability of mechanical engagement with resin cement (105).
Plasma spraying Hexamethyldi-siloxane, creating a thin (<1 mm) siloxane coating, and
PyrosilPen technology, depositing silicon and forming a SiOx-C layer with a thickness
of 0.1 µm are other techniques in this group (105).
1.7.7.2.2) Micromechanical Bonding Techniques
Micromechanical bonding means the mechanical interlocking related to bonding of an
adhesive to a roughened adherend surface.
A. Mechanical Treatment
The mechanical treatment can be subclassified into additive and subtractive methods.
In the subtractive method, particle abrasion and grinding can be mentioned. Selective
infiltration etching and zirconia particle suspension are categorized into additive
methods.
I. Particle Abrasion:
Dentistry's most common abrasive particles include boron carbide (BC), aluminum
oxide, and silicon carbide (SiC). Air abrasion is a method in which abrasive particles
are utilized to alter tooth or material structure. Air-abrasion units can be used in
38
different customization, including as attachments to a dental unit, water flow rate,
particle flow rate, and modifications to air pressure. Intraoral air abrasion is usually
worked by spraying water (106).
Particle abrasion with air-borne alumina particles (Al2O3), which is the most common
abrasive particle, was initially demonstrated in 1945 by Black (107).
Intraoral air abrasion can be primarily used to prepare a cavity with a minimally
invasive method (100, 108, 109). Alumina particles are produced from compressed air
and aerosolized water in water- airborne-particle abrasion.
Airborne-particle abrasion can roughen the surfaces of restorative materials and
enhance bond strength between dental restorative and resin-luting materials (110).
Airborne-particle abrasion (APA) could be conducted on ceramics and metals (108)
and hard dental tissues (dentin, enamel) (111) and has also been suggested to provide
the rough surface for zirconia as a pathway of enhancing mechanical interlock and the
entire contact area (112-114).
Many studies evaluated the impact of airborne-particle abrasion on zirconia's
mechanical properties and bonding credibility. Airborne-particle abrasion can cause
the creation of microcracks, leading to a compressive layer on the zirconia surface
and increasing the flexural strength, accompanied by t→m phase transformation of
the surface grains (115).
There are various factors in APA, including grain size of alumina (25 to 250 µm),
propulsion pressure (0.05- 0.45 MPa), distance (5-20 mm) from the nozzle to the
samples, and the duration of APA (5 to 30 seconds) (90, 116).
Until now, studies have not compared wet and dry abrasion, impacting zirconia's
mechanical properties and bonding credibility.
39
II. Grinding with Disks and Diamond Rotary Instruments:
The most common method to roughen the surface of zirconia is the grinding technique,
improving the mechanical bonding. In this technique, abrasive paper or disks, or
diamond burs can be used. The major downside of grinding techniques is the probable
initiation of microcracks on the surface (117).
Grinding with coarse diamond rotary instruments (120 -200 mm grain size) only can
be effective because of the significant hardness of zirconia (118). Previously, coarse-
grained diamond grinding had been examined, presenting a more uneven surface than
other methods and enhancing bond strength. However, it was unacceptable since it is
an aggressive way that could initiate microcracks and lead to the damaging surface of
zirconia (119).
In addition, grinding Y-TZP by tungsten carbide burs was not recommended in
previous studies due to a considerable decrease in the biaxial flexural strength of
zirconia (120, 121).
III. Selective Infiltration Etching (SIE):
In this method, the zirconia surface should be coated with a thin layer of glass along
with a conditioning agent. Then, this layer should be heated, and molten glass can
split grains at the grain boundary. In the next step, the glass should be rinsed in an
acid bath, then cooled to room temperature. This method makes a novel retentive
surface. Selective infiltration etching and silane coupling agents can significantly
enhance the bond strength between zirconia and resin cement (122-124).
Some studies claimed airborne particle abrasion had less roughness of the surface
than SIE (125).
40
IV. Zirconia Particle Suspension:
Nobelbond (Nobel Biocare AB, Gothenburg, Sweden) is a porous ceramic and is
produced by coating a non-sintered or a fully sintered and milled zirconia framework
with a slurry containing zirconia ceramic powder and a pore former. Then the slurry-
coated ceramic is sintered while the pore former burns off, leaving a porous surface.
This technique is a novel way to rough the zirconia surface (125).
B. Chemical Treatment by Acid Etching
Different agents like nitric acid, hydrofluoric acid, or sulfuric acid can conduct. Heat
treatment after acid etching can smoothen the sharp edges resulting from the etching
process. For example, Hydrofluoric (H.F.) can remove the glassy matrix of glass
ceramics, enhancing surface energy and resulting in micromechanical interlocking. On
the other hand, H.F. etching cannot alter the roughness (Ra) of ZrO2 (122). The negligible
influence of the H.F. on the ZrO2 surface happens because of the lack of a glassy matrix,
leading to low bond strength values (126).
Another study in 2014 showed that etching zirconia with H.F. could only create nano-
irregular patterns, leading to weak bonding. Because of this, etching zirconia cannot be
recommended to enhance bond strength (127).
C. Laser Treatment
Because of the developing usage of lasers in the dental field, laser irradiation can be an
alternative way to roughen the ceramic surface and enhance the bond strength.
Several kinds of lasers can cut hard dental substances, improving the capacity of zirconia
bonding. Different types of lasers (125) include:
a) Neodymium-Doped Yttrium Aluminum Garnet (Nd:YAG) Laser
41
b) Carbon Dioxide (CO2) Laser
c) Erbium-Doped Yttrium Aluminum Garnet Laser (Er: YAG) Laser
d) High-Speed Pulse Lasers (Femtosecond Lasers)
Laser treatment can enhance the temperature of the zirconia surface and absorption of
the temperature by zirconia, leading to the creation of porosity on the surface. Based on
some previous studies, the roughness made from the laser treatment is not considerable
compared to airborne particle abrasion (128). Also, similar to airborne particle abrasion
and any mechanical abrasion can make micro-cracks in zirconia for retention (129, 130).
1.8) Test Methods and Flexural Strength Tests
There are many test ways to investigate the mechanical behaviors of dental ceramics,
such as the compressive test, tensile test, hardness test, fracture toughness test,
diametral tensile test, and flexural test. However, among all tests, flexural strength is the
most common in evaluating the mechanical properties of brittle materials like dental
ceramics. Flexural strength is essential in assessing dental materials' stability- such as
zirconium oxide. It shows the ability of a material to resist fracture. There are three tests
to evaluate flexural strength: the four-point, three-point (transverse strength), and biaxial
flexural tests (131).
A) Three-Point Flexural Test (Transverse Strength):
The highest tensile strength is required to break the rectangular model that is supported
from two ends. The load should be applied to the model from a point in the middle (Figure
10).
42
Strength is known as compressive strength, which is related to the type of dominant
tension(131).
Figure 10: 3-point flexural test
B) Four-Point Flexural Test:
The specimen is rectangular and supported from two ends. Also, the load should
be applied to the model from two points in the middle (Figure 11). The values
gained with four-point flexural tests are 30–40% less than the achieved value from
43
the three-point flexural test (132).
Figure 11: 4-point flexure test
C) Biaxial Flexural Test:
Samples should be in a disc shape, and force should be applied from the outer sides of
the model and its center (132).
Uniaxial flexural tests (4-point and 3-point flexural tests) have a noticeable downside
due to unpleasant edge failures,. The biaxial flexural test can overcome this problem
because of disc-shaped samples and can also assess ceramic materials' fracture
behavior (131, 132).
Three loading schemes can produce biaxial tension in disc-shaped samples, including
piston-on-three-ball, ball-on-ring, and ring-on-ring (Figure 12).
In the ball-on-ring test, a specimen is supported on a ring and centrally forced with a
ball, compared to the ring-on ring that is forced with a ring. In piston-on-three-ball, the
44
model is supported o on three hardened steel balls positioned 120° apart, and force
should be applied centrally. The biaxial flexural test via piston-on-three-ball is
recommended by ISO protocol (133).
Figure 12: Three loading schemes of biaxial flexural strength
The standard technique for studying the differences in the mechanical behavior of
ceramics is Weibull statistical path (133). Weibull analysis can expect a flexural
strength gained by various tests (133, 134).
Weibull Analysis or Weibull Distribution
Weibull analysis is a statistical technique to evaluate strength, reliability, and variability.
This statistical method was presented by Walloddi Weibull (Swedish engineer, scientist,
and mathematician) in 1939. The Weibull Distribution is a distribution of continuous
probability performed to analyze model failure times, life data, and product reliability. It is
able to fit a huge range of data from different fields like engineering sciences, economics,
and biology. For example, it can be used for brittle materials like ceramics to show the
amount of force and time until failure. It is defined with the main factors included are the
45
characteristic strength and Weibull modulus. Weibull modulus can determine the possible
variabilities and flaw distributions in brittle materials such as ceramics (135, 136).
There are two forms of Weibull Distribution:
A. Two-parameter Weibull: it consists of Weibull Modulus (m) and Weibull
Characteristic Strength (σ0)
B. Three-parameter Weibull: in this form, there is an additional parameter, the
waiting time parameter.
1)The Weibull Modulus (m) or Beta or Shape Parameter:
The shape parameter is called the Weibull slope or the threshold parameter. As a
failure probability function, it is a parameter explaining the data dispersion by the
strength distribution's shape (including width). A large Weibull modulus means less
dispersion in failure data, which is advantageous and shows higher test reliability
(135).
2) The Weibull Characteristic Strength (σ0) or Eta or Scale Parameter:
It is defined as the strength occurring at a probability of failure of 63.2 % for a given
sample and tension setting. It conveys an idea of the expected value of the biaxial
flexural strength among all samples (137).
3) Location Parameter(μ) or Shift Parameter: The threshold parameter is the
lowest probable value in a Weibull distribution. This parameter can be called the
location parameter or the waiting time (137).
46
Objective and Specific Aims
The objective of the present study is to investigate the impact of the content of yttria, wet
and dry abrasion, and artificial aging on biaxial flexural strength of three zirconia
materials: 3Y-TZP (DD Bio ZX²), 4Y-TZP (DD cube One), 5Y-TZP (DD cubeX²).
The null hypotheses tested are:
(1) Zirconia yttria content: There is no significant difference between biaxial flexural
strength in three zirconia materials based on the content of yttria (3Y-TZP (DD Bio ZX²),
4Y-TZP (DD cube One), 5Y-TZP (DD cubeX²)).
(2) Different surface treatments: Including none, wet, and dry abrasion, do not affect
biaxial flexural strength.
(3) Artificial aging: There is no significant difference in biaxial flexural strength of three
zirconia (3Y-TZP (DD Bio ZX²), 4Y-TZP (DD cube One), 5Y-TZP (DD cubeX²))
Chapter 2: Material and Methods
A total of 270 zirconia specimens based on the content of zirconia 3Y-TZP (DD Bio ZX²),
4Y-TZP ( DDcube One),5Y-TZP (DD cubeX²) (Figure 8) were selected and divided into
two groups: aging and non-aging. Based on the hypothesis, three different surface
treatments were used to modify the zirconia surface, including none( control group), wet
and dry abrasion. In each experiment, 15 samples were collected (Table 2). Also, the
biaxial flexural test (piston-on-three-ball test) was used.
47
Material
Yttria content
(wt.%)
Non-aged Aged
None
Wet
abrasion
Dry
abrasion
None
Wet
abrasion
Dry
abrasion
DD Bio ZX²
(3Y-TZP)
<6
N=15 N=15 N=15 N=15 N=15 N=15
DD cube ONE
(4Y-TZP)
<8
N=15 N=15 N=15 N=15 N=15 N=15
DD cubeX²
(5Y-TZP)
<10
N=15 N=15 N=15 N=15 N=15 N=15
Table 2: Classification of samples based on the content of yttria, aging, and surface treatment
48
2.1) Preparation of Specimens
According to ISO 6872:2015 (137), disc-shaped samples are needed for the biaxial
flexural test. Therefore, a total of 270 disk-shaped (12±1 mm diameter, 1.2 ± 0.2
mm thickness) zirconia specimens ((DD Bio Z X² (3Y-TZP), 4Y-TZP (DD cube
One), 5Y-TZP (DD cubeX²)) were be fabricated for the biaxial flexural test based
on the below explanation (Figure 13).
Figure 13: Three types of zirconia based on yttria content
2.1.1) Cutting the Samples
First, an STL file was designed and provided for cutting the samples via Meshmixer
(Meshmixer Version 3.5.474, Autodesk, San Rafael, California, USA). The object was
created as a cylindrical shape (15 mm diameter, 14 mm height) that is 20% larger than
the dimension of the final measurement after sintering (Figure 14).
49
Figure 14: STL file design by Meshmixer
In the next step, the STL file was sent to a Zirkonzahn M5 CAD/CAM Milling Machine
(Zirkonzahn GmbH, South Tyrol, Italy) (Figure 15). Zirconia blocks were milled based on
the provided design (Figure 16) Then, the cylindrical samples were attached with glue
(JB Weld Steel Reinforced Epoxy Putty Stick - 2 oz, J-B Weld, USA) to the remaining
metal sprue of CAD-CAM blocks that were milled previously (Figure 17)
50
Figure 15: Zirkonzahn M5 CAD/CAM Milling Machine
Figure 16: 1) Milling zirconia blocks 2) Milled zirconia block
1
2
51
Figure 17: 1) Removing cylindrical samples from milled block 2) Prepared cylindrical
samples 3) Polished cylindrical samples and metal sprue 4) Cylindrical samples glued
into metal sprue.
In the following stage, cylindrical samples were cut to disc shape samples using a
precision sectioning saw (IsoMet 1000; Buehler, Lake Buff, IL, USA) with a diamond blade
(102 mm dimension, 0.17mm thickness; IsoMet Blade 15LC, Buehler, Lake Buff, IL, USA)
at a speed of approximately 350 RPM under continuous water cooling (Figure 18)
2
3 4
1
52
In this stage, disc-shaped samples were cut at the initial desired size to account for an
anticipated 20% sintering shrinkage, 0.13 mm thickness of the blade, and anticipated
polishing will be considered and cut in diameter of 15 mm and thickness of 1.5 mm (Figure
19).
Figure 18: Cutting cylindrical samples to disc shape samples using IsoMet 1000
Figure 19: The thickness and diameter of disc-shaped samples before sintering
1
2
3
1 2
53
2.1.2) Polishing
Before sintering, both flat disc surfaces were manually polished with 600 and 1200-grit
silicon carbide paper (Buehler, Lake Buff, IL, USA) with water coolant. The anticipated
20% sintering shrinkage was considered in this stage, and the disc-shaped samples (15
mm diameter, 1.5 mm thickness) were provided. The thickness for all samples before
sintering was adjusted through polishing and verified by digital calipers (Mitutoyo digital
caliper: Mitutoyo Crop, Kawasaki, Japan) with a 0.001 mm accuracy.
54
2.1.3) Sintering
In the next step, sintering was performed according to the manufacturer's
recommendation (Table 3) using the Dekema Austromat 674i sintering furnace
(Dekema Austromat 674i Sintering Furnace, Freilassing, Germany) (Figure 20)
Table 3: Sintering parameters
Temp.1
[°C]
Temp.2
[°C]
Heating
rate
[°C/h]
Heating
rate
[°C/min]
Dwell
time
[min]
Time
[min]
Heating 20 900 480 8 - 110
Dwell 900 900 - - 30 30
Heating 900 1,450 200 3 165
Dwell 1,450 1,450 - - 120 120
Cooling 1,450 200 600 10 - 12
Total
time
550
min.9.2 h
Sintering was done based on the recommended sintering program by Dental Direkt
GmbH company. (138)
55
Figure 20: Sintering furnace( Dekema Austromat 674i Sintering Furnace, Freilassing,
Germany
2 1
56
During sintering, 20% sintering shrinkage was seen, and the measurements before and
after sintering shrinkage showed a diameter of 12 mm and thickness of 1.2 mm for the
samples after sintering(.Figure 21 ).
Figure 21: The measurement of samples' diameter and thickness before and after
sintering
Then ultrasonically(Ultrasonic Cleaning Systems, Quantrex, Kearny, NJ, USA) cleaned
in 99% isopropyl alcohol (Mountain Falls Products, TN, USA) for 10 min and thoroughly
air-dried using an oil-free compressed air stream for 5 s (Figure 22)
Figure 22: Ultrasonic Cleaning Systems, Quantrex, Kearny, NJ, USA
57
After cleaning, the different translucency for three zirconia based on yttria contents can
be seen in Figure 23.
Figure 23: Different translucency based on yttria content
2.2) Artificial Aging
After sintering, half of the specimens were stored in distilled water for 24 h before testing
(non-aged). The rest of the specimens were aged using a steam autoclave (Midmark M11
UltraClave Dental Steam Autoclave Sterilizer, USA) according to ISO standard 13356
(139), at 134±2 ℃, under 0.2 MPa (under 20 psi), for 5 h. and placed samples inside 3.5"
x 5.25" Peelvue sterilization pouches (KaVo Kerr, Brea, California, USA). After using the
steam autoclave, all pouches were cooled down for 20 minutes. ( Figure 24)
58
2.3) Surface treatment
The surface treatment of zirconia specimens was classified into two groups. In the first
group (airborne-particle abrasion or dry abrasion), using 50 µm Al2O3 particles
(Deldent ,Petach Tikva, Israel) at 0.24 MPa (35 psi) for 15 s at a distance of 10 mm, with
nozzle motion in both lateral directions. Airborne abrasion was performed using a spot
blasting unit (Basic Professional Model Sandblaster; Renfert, Germany) (Figure 25).
In the second group, wet abrasion was performed using a spot blasting unit (Rondoflex
Plus 360) (KaVo, Biberach an der Riss, Germany) and 50 µm Al2O3 at 0.24 MPa (35 psi)
for 15 s at a distance of 10 mm, with nozzle motion in both lateral directions. Keeping the
distance of 10 mm was achieved using the piece of a metal paper clip (length:10 mm)
that was attached to the nozzle (Figure 25).
Figure 24: 1) Midmark M11 UltraClave Dental Steam Autoclave Sterilizer, USA 2) 3.5" x
5.25" Peelvue sterilization pouch 3) After the steam autoclave
1 2 3
59
2.4) Biaxial Flexure Test (Piston-on-Three-Ball Test)
According to ISO 6872 (137), samples were subjected to a biaxial flexural test using the
piston-on-three balls technique (Biaxial Flexion-ISO, Odeme Dental Research, Luzerna
SC, Brazil) in a universal testing machine (Model 6596; Instron, Norwood, MA, USA) at a
crosshead speed of 0.5 mm/min and applying loads of between 10 N and around 1,450
N (±1 %) (Figure 26).
Figure 26: 1) Universal testing machine 2) Piston-on-three balls 3) Piston-on-three balls
technique inside a universal testing machine
Figure 25: 1) Air-borne particle abrasion 2) 50 µm Al2O3 3) Wet particle abrasion
1
2
3
1 2
3
60
Each disc was placed centrally on three hardened steel balls with a diameter of 4,5±2
mm positioned 120° apart on a support circle with a diameter of 11±1 mm shall be
provided. The samples were placed concentrically on these supports, and the load was
applied with a flat punch with a diameter of 1,4± 0,2 mm at the center of the specimen
(Figure 27).
Since the load should be applied centrally, fixing the disc-shaped samples is needed. For
this purpose, it was decided to provide an object, helping samples to be set centrally. The
mentioned object has a hole measured in a diameter of 1.5 that keeps samples. The
object was printed using a 3D printer (Photon Mono X, Anycubic, Shenzhen, China)
similar to the below picture, and samples were placed inside its hole (Figure 28).
Figure 28: Fixing the disc-shape sample on three hardened steel balls concentrically
Figure 27: Three hardened steel balls and placing the sample concentrically
61
2.5) Calculation of Flexural Strength, σ:
The flexural strength of ceramic material can be calculated by Error! Reference source n
ot found. [ISO 6872-2015] (137):
Eq. 1: σ = −0,2387 P(X-Y)/𝑏 2
Where:
σ is the maximum center tensile stress in megapascals (MPa)
P is the total load causing fracture; in Newton (N)
X and Y are parameters calculated based on the following equations [ISO 6872:2015]
(137):
Eq. 1:
Eq. 2:
where ν is Poisson's ratio (if the value for the ceramic concerned is not known, use
Poisson's ratio = 0.25)
r1 is the radius of the support circle in millimeters.
r2 is the radius of the loaded area in millimeters.
r3 is the radius of the specimen in millimeters.
b is the specimen thickness at fracture origin in millimeters.
The following table (Table 4) shows the values associated with these parameters:
62
Table 4: Characteristics parameters for calculation of biaxial flexural strength based on
ISO 6872:2015 (137)
Poisson’s ratio v 0.25
the radius of the support circle (mm) r1 5
the radius of the loaded area (mm) r2 0.7
the radius of the specimen (mm) r3
measured by a caliper for each
sample
specimen thickness at fracture origin
(mm)
b
measured by a caliper for each
sample
***The data, including r3 and b for each sample, must be measured separately using
Mitutoyo digital caliper (Mitutoyo Corp, Kawasaki, Japan)
2.6) Statistical Analysis
Lastly, the values of Weibull modulus (beta) vs. characteristics strength (eta) for the 90%
CI of the data are plotted against each other in the form of contour plots using the plot
contour function. The contour plot helps in understanding the statistical difference
between the data distribution for different groups. If there is no statistical similarity in the
data between two groups, the contours will appear separate from each other, and if they
do have similarities, they will appear overlapping. As an example, for two fully identical
datasets, the contours will overlap. Further, the shape of the contour will reflect the
dispersity of Weibull modulus (beta) and Eta with respect to each other. The more the
shape of the contour deviated from a full circle, the less similarity between the dispersity
63
of Weibull modulus (beta) vs. characteristics strength (eta) variable. Full details and
instructions on how the Weibull analysis can be done using WeibullR are found in the
WeibullR user’s manual (140, 141).
64
Chapter 3: Results
3.1) Descriptive Findings
This research examined the biaxial flexural strength characteristics of the Zirconia
materials concerning three factors: yttria content, surface treatment, and artificial aging.
Three zirconia materials, including 3Y-TZP (DD Bio ZX²), 4Y-TZP (DD cube One), and
DD cubeX² (5Y-TZP), were selected, and a total of 270 zirconia specimens were divided
into two groups: aging and non-aging. Based on the surface treatments, samples are
subclassified into three groups: none, wet abrasion, and dry abrasion (n=15) (Figure 30).
Figure 30: Summary of sample preparation
3.1.1) All Groups Based on Yttria Content, Aging, and Surface Treatment
Table 5 shows the average, maximum, and minimum biaxial flexural strength for three
materials based on aging and surface treatment. The highest biaxial mean of biaxial
flexural strength was related to 3Y, particularly non-aged and none abrasion.
Conversely, the minimum observed biaxial flexural strength was associated with 5Y,
particularly non-aged and none abrasion.
65
Table 5: Mean, maximum, minimum, and std. deviation of the biaxial flexural strength of
samples based on materials, aging and sandblasting
Material Aging
Surface
Treatment
N Minimum Maximum Mean
Std.
Deviation
3Y
Non-
aged
None 15 1085.05 1379.97 1267.68 89.64
Wet 15 1002.43 1414.12 1212.50 159.15
Dry 15 1021.79 1435.91 1246.94 149.80
Aged
None 15 950.23 1383.08 1232.04 155.53
Wet 15 1050.97 1435.92 1259.51 143.66
Dry 15 949.71 1424.68 1237.74 152.92
4Y
Non-
aged
None 15 866.22 1226.55 1012.48 114.11
Wet 15 781.62 1375.10 1052.49 188.28
Dry 15 915.86 1191.42 1048.91 82.41
Aged
None 15 775.99 1152.94 990.22 114.43
Wet 15 968.36 1262.92 1104.90 96.51
Dry 15 905.27 1234.94 1079.95 112.27
5Y
Non-
aged
None 15 612.10 824.98 703.53 71.15
Wet 15 588.97 1095.38 790.00 135.76
Dry 15 618.36 1077.04 787.25 134.34
Aged
None 15 645.39 915.97 791.05 81.22
Wet 15 561.65 922.43 705.73 120.31
Dry 15 636.20 863.30 727.96 80.94
66
The box plot of biaxial flexural strength for different groups (combinations of aging,
material, and treatment method) is depicted in Figure 31. Each box shows the
corresponding group's minimum, first quartile, median, third quartile, and maximum
samples.
Figure 31: Box plot of the biaxial flexural strength of the different test groups based on
materials, aging, and surface treatment
The box plot in Figure 31 is used to visualize further the results presented in Table 5. The
highest flexural strength of the zirconia pertains to the 3Y type, whereas the 4Y and 5Y
types show lower strength, with the 5Y material having the lowest strength among the
three types. In terms of the variance of the data samples at each study group, represented
67
by a separate box, the maximum variance is usually observed in wet particle abrasion
(yellow boxes).
3.1.2) All Samples Based on Material
The mean, maximum, and minimum of biaxial flexural strength of samples based on the
yttria content is shown in Table 6.
According to Table 6. and Figure 32, the highest biaxial flexural strength belongs to 3Y,
and the lowest was for 5Y.
Table 6: Mean, maximum, and minimum of biaxial flexural strength of samples based on
yttria content
Material N Minimum Maximum Mean Std. Deviation
3Y 90 949.71 1435.92 1242.74 140.86
4Y 90 775.99 1375.10 1048.16 125.29
5Y 90 561.65 1095.38 750.92 111.54
68
Figure 32: Box plot of biaxial flexural strength of the different test groups based on
materials
3.1.3) All Samples Based on Aging
Table 7 shows the average, maximum, and minimum of biaxial flexural strength based on
aging. Based on the findings, aged and non-aged showed similar data. This data is further
illustrated in Figure 33.
Table 7: Mean, maximum, and minimum of biaxial flexural strength grouped by aging
Aging N Minimum Maximum Mean Std. Deviation
Non-aged 135 588.97 1435.91 1013.53 236.84
Aged 135 561.65 1435.92 1014.35 241.20
69
Figure 33: Box plot of biaxial flexural strength of the different test groups based on aging
3.1.4) All Samples Based on Surface Treatment
Table 8 shows the average, maximum, and minimum biaxial flexural strength based on
surface treatment. Figure 34 indicates a box plot of three materials' biaxial flexural
strength for different surface treatments. Based on the results, wet particle abrasion and
airborne abrasion enhance biaxial flexural strength slightly compared to none abrasion.
In addition, Figure 32 shows that wet abrasion generally shows higher strength variability
and standard deviation in the samples compared to dry ones.
70
Table 8: Mean, maximum, and minimum of biaxial flexural strength grouped by surface
treatment
Surface Treatment N Minimum Maximum Mean Std. Deviation
None 90 612.10 1383.08 999.50 233.18
Wet 90 561.65 1435.92 1020.86 249.63
Dry 90 618.36 1435.91 1021.46 234.62
Figure 34: Box plot of biaxial flexural strength of the different test groups based on
materials and surface treatment regardless of aging.
71
3.2) Inferential Findings
A) Normality
Checking the normality assumption is mandatory to decide whether a parametric or
non-parametric test requires to be used to analyze the difference between the means
of the sample groups. Different ways are suggested in the literature to use for checking
normality. Table 9 lists the normality test results based on Kolmogorov-Smirnov and
Shapiro-Wilk variables (p-value >0.05) (128).
Table 9: Evaluating normality by the indicators of Kolmogorov-Smirnov and Shapiro-
Wilk
Material Aging Surface
Treatment
Kolmogorov-Smirnov
a
Shapiro-Wilk
Statistic df Sig. Statistic df Sig.
3Y Non-
aged
None .126 15 .200
*
.909 15 .132
Wet .157 15 .200
*
.879 15 .046
Dry .159 15 .200
*
.905 15 .113
Aged None .255 15 .009 .817 15 .006
Wet .200 15 .107 .892 15 .071
Dry .133 15 .200
*
.913 15 .148
4Y Non-
aged
None .158 15 .200
*
.919 15 .186
Wet .136 15 .200
*
.940 15 .387
Dry .229 15 .033 .921 15 .197
Aged None .108 15 .200
*
.960 15 .701
Wet .115 15 .200
*
.953 15 .571
Dry .129 15 .200
*
.939 15 .367
5Y Non-
aged
None .127 15 .200
*
.933 15 .298
Wet .222 15 .045 .890 15 .066
Dry .253 15 .010 .843 15 .014
Aged None .248 15 .014 .925 15 .232
Wet .119 15 .200
*
.920 15 .190
Dry .152 15 .200
*
.897 15 .086
72
Skewness and Kurtosis values were computed as an alternative method to evaluate the
normality further. Table 10 shows the values in the range of −1.96 and +1.96,
demonstrating the normal data distribution. Thus, normal distribution in the data was
observed, and the parametric test (ANOVA) can be used (142, 143).
Table 10: Evaluating normality by the indicators of Skewness and Kurtosis
Skewness and Kurtosis values for the distribution of the dependent variable in each group
(18 groups) were shown in Table 11,Table 10 where the values are in the range of −1.96
and +1.96, indicating the data distribution is normal.
N Skewness Kurtosis
Statistic Statistic Std. Error Statistic Std. Error
270 -.023 .148 -1.063 .295
73
Table 11: Evaluating normality by the indicators of Skewness and Kurtosis for different
groups of samples
Material Aging Surface
Treatment
N Skewness Kurtosis
Statistic Statistic Std.
Error
Statistic Std.
Error
3Y Non-
aged
None 15 -.912 .580 .214 1.121
Wet 15 -.077 .580 -1.749 1.121
Dry 15 -.174 .580 -1.597 1.121
Aged None 15 -.856 .580 -.831 1.121
Wet 15 -.266 .580 -1.558 1.121
Dry 15 -.634 .580 -.262 1.121
4Y Non-
aged
None 15 .725 .580 -.481 1.121
Wet 15 .071 .580 -.660 1.121
Dry 15 .469 .580 -.838 1.121
Aged None 15 -.429 .580 -.639 1.121
Wet 15 .165 .580 -1.054 1.121
Dry 15 -.028 .580 -1.357 1.121
5Y Non-
aged
None 15 .329 .580 -1.141 1.121
Wet 15 .831 .580 1.320 1.121
Dry 15 1.246 .580 1.577 1.121
Aged None 15 -.474 .580 -.795 1.121
Wet 15 .545 .580 -.634 1.121
Dry 15 .473 .580 -1.144 1.121
74
B) Homogeneity of Variances
Levene test results (Table 12) for homogeneity of variances are non-significant (p>0.05).
Accordingly, the assumption of homogeneity of variances has been correctly observed.
Table 12: Levene test results for homogeneity of variances
Test of Homogeneity of Variances
Factor Levene Statistic df1 df2 Sig.
Material 1.433 2 267 .240
Aging .064 1 268 .801
Surface Treatment .542 2 267 .582
75
C) The Three-way ANOVA
The three-way ANOVA (using IBM SPSS Statistics 22) is used to determine if there is an
interaction effect between three independent variables (material, surface treatments,
aging) on the dependent variable (biaxial flexural strength) (Table 13).
Table 13:Tests of between-subjects effects
Source Type III Sum
of Squares
df Mean Square F Sig. Partial Eta
Squared
Corrected
Model
11344722.21
a
17 667336.60 42.37 .000 .741
Intercept 277581544.29 1 277581544.29 17625.75 .000 .986
material 11042807.37 2 5521403.68 350.59 .000 .736
aging 44.91 1 44.91 .00 .957 .000
treatment 28162.80 2 14081.40 .89 .410 .007
material * aging 17182.25 2 8591.12 .54 .580 .004
material *
treatment
78386.44 4 19596.61 1.24 .293 .019
aging *
treatment
6228.46 2 3114.23 .19 .821 .002
material * aging
* treatment
171909.96 4 42977.49 2.72 .030 .042
Error 3968656.03 252 15748.63
Total 292894922.55 270
Corrected Total 15313378.24 269
76
Based on the results in Table 13, only the effect of the material and interactive impact of
aging * treatment * material on biaxial flexural strength were significant (p<0.05).
The interactive effect of aging and treatment, aging and material, treatment and material
on Tests of Between-Subjects Effects was insignificant (p>0.05).
Table 14: The results of the Bonferroni test for comparing the biaxial flexural strength of
three materials
(I) Material (J)
Material
Mean
Difference
(I-J)
Std.
Error
Sig. 95% Confidence Interval
Lower
Bound
Upper
Bound
3Y 4Y 194.57
*
18.70 .000 149.49 239.66
5Y 491.81
*
18.70 .000 446.72 536.90
4Y 3Y -194.57
*
18.70 .000 -239.66 -149.49
5Y 297.23
*
18.70 .000 252.15 342.32
5Y 3Y -491.81
*
18.70 .000 -536.90 -446.72
4Y -297.23
*
18.70 .000 -342.32 -252.15
Table 14 confirms that biaxial flexural strength was different based on the content of yttria
in three materials, and the mean values were significantly different in all three groups
(p<0.05). The highest mean was 3Y, and the lowest belonged to 5Y. This test confirms
that yttria's content significantly influences biaxial flexural strength, and there is a
statistically significant difference between materials. In summary, the data show no
statistically significant differences between aged and non-aged samples (p>0.05). In
addition, different techniques of surface treatments, including none, wet, and dry
abrasion, showed an insignificant effect on biaxial flexural strength (p>0.05).
77
3.3) Weibull Reliability Analysis
Weibull Analysis:
Weibull distribution is a statistical distribution introduced in 1939 to characterize the
fracture strength of different materials. (144) Today, Weibull analysis is extensively used
in reliability, failure, and lifecycle analysis. Besides, it is a key tool for studying and
improving measurements' precision in which a wide deviation in the data may occur. In
this case, the two-parameter Weibull distribution can describe the probability (P) of a
measured sample occurring at or smaller than a given value (145). The Weibull
distribution has two parameters; one is the scale parameter or characteristic value σ0,
represented by ɳ (Eta) in the plots, and the other one is the shape or modulus parameter
(m), represented by β (Beta) in the plots (146).
The Weibull modulus is a parameter similar to the inverse of standard deviation, which
represents the data disparity. The characteristic strength resembles the normal
distribution's mean parameter, which shows the average value of the measurements.
The Cumulative Distribution Function (CDF)
2
of the Weibull distribution is expressed as:
Eq. 6: 𝑃 𝑓 =1- 𝑒 𝑥𝑝 [ − (
𝜎 𝜎 0
)
𝑚 ]
By applying the ln(x) function to this equation, the Weibull distribution can be represented
in a linear form as follows:
Eq. 5: ln 𝑙𝑛 [
1
1 − 𝑃 𝑓 ] = 𝑚 ( ln ( 𝜎 ) − ln ( 𝜎 0
) )
2
CDF is defined as a function whose value is the probability of occurrence of the values at or
smaller than the given input of the function.
78
Based on this equation, the linear form of Weibull function is characterized by the
parameter m, which is the slope of the line and is known as the shape parameter.
As a result, this formula 𝑙𝑛 𝑙𝑛 [
1
1 − 𝑃 𝑓 ] = 𝑚 𝑙𝑛 𝜎 − 𝑚 𝑙𝑛 𝜎 0
can be converted to Y=m X -
𝑚 𝑙𝑛 𝜎 0
.
Figure 35: A typical plot of the linear form of the Weibull modulus
a) Weibull plots with confidence bounds:
Similar to Figure 35 ,another form of Weibull plot is a Weibull probability plot with
additional confidence bounds (as explained in section
2.6) Statistical Analysis), illustrated in Figure 36. Probability plots allow an understanding
of the actual samples of the data and the approximate lines fitted to the data., i.e., failure
data and failure modes. Weibull probability plot consists of the following:
0
0.5
1
1.5
2
2.5
3
3.5
4
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Y = ln[ln1/1-pf]
x = lnσ
79
• A double-logarithmic y-axis (unreliability),
• A logarithmic x-axis (force to failure),
• A Weibull line and median ranks of the given data,
• Confidence bounds (two-sided),
• Sample size n (shown in legend),
• Confidence interval (CI) bounds type (shown in legend),
• The estimated slope of the Weibull line (which is the shape parameter β or
Weibull modulus)
• The estimated scale parameter η is defined as the x-axis value for the
unreliability of 63.2 %.
Figure 36: A typical Weibull plot with confidence bounds
80
If the data tends to fit a straight line, it shows that the data follows the Weibull distribution.
Another characteristic is the rank of the sample, where the weakest sample is ranked 1,
and the strongest is ranked n. Using the Weibull package in R, the median rank of the
samples and the 90% confidence interval (CI) is shown in the Weibull plots. Each plot has
a median rank, which is the value that the actual probability of strength should have at
the j
th
sample out of sample size N units at a 50% confidence level. In other words, each
fit has three lines corresponding to the 5%, 50%, and 95% confidence levels. So, the 90%
CI is the data spanning between the 5% and 95% levels. The Weibull plots for the three
types of materials are populated in the following sections.
b) Contour plots:
In addition to the Weibull plots, contour plots are created to evaluate the cross behavior
between the modulus (beta) and characteristics strength (eta) of the three zirconia across
all measured samples. The contour plot is a popular graphical method for comparing the
data distribution, which involves plotting the data at one or multiple confidence bounds
and seeing whether the bounds overlap or separate at the point of interest (147). The
overlap between contours indicates the absence of statistical difference.
The contour plot is also a measure of the precision of the flexural strength estimates. The
smaller the area of contour, the higher precision obtained in the group measurements
(148).
In what follows, the contour plots of 90% confidence interval of the data for three types of
zirconia materials are illustrated.
81
3.3.1) Weibull Plots for 3Y-TZP (DD Bio ZX
2
)
Figure 37 (A) shows that the modulus parameter in the Weibull CDF for the wet, dry, and
none abrasion is 10.1, 9.8, and 9.6, respectively. This indicates that the data dispersion
in all groups is similar, thus, similar predictability. Other than that, the wet abrasion shows
the lowest characteristic strength ( 𝜎 0
) of 1296. The characteristic strength ( 𝜎 0
) of the
Weibull distribution is the value under which 63.2% of the data holds. As observed, the
characteristics strength is slightly higher with dry abrasion. Based on the characteristic
strength values, there are no major differences in the biaxial flexural strength of the 3Y
zirconia according to the analysis of the Weibull distribution concerning the different
abrasion techniques.
Figure 37 (B) shows the contour plot for 3Y material with the 90% CI bounds (the area
inside the curve shows where 90% of the values fall in). As seen, the circular curves have
a high intersection, thus, showing a high overlap in the data. The characteristic strength
and the modulus are not affected by the method of abrasion in the 3Y material. The lowest
Weibull modulus (m=6) among all the groups was presented by wet abrasion, while the
non-abrasion group showed the highest Weibull modulus (m=15.4) among all the groups.
82
Figure 37: (A): Weibull CDF and Confidence Intervals (CI) of biaxial flexural strength for dry and wet abrasion for 3Y
material. (B) Contour plot of beta values vs. eta in the Weibull CDF analysis for 3Y zirconia under none, dry and wet
abrasion.
83
Figure 38 (A) depicts the Weibull CDF and CI of biaxial flexural strength of 3Y material
grouped by aging condition, regardless of the abrasion method. The Weibull modulus is
11.27 for non-aged and 9.03 for aged material, indicating that the data is slightly more
dispersed in the aged group. The characteristic strength ( 𝜎 0
) is similar between the non-
aged and aged samples. Further, the contour plot,( Figure 38 (B)) shows a high overlap
in the data. This result confirms that the aging condition does not impact the biaxial
flexural strength in the 3Y zirconia.
84
Figure 38: (A) Weibull CDF and Confidence Intervals (CI) of biaxial flexural strength (σ) for aged and non-aged 3Y
material. (B) Contour plot of biaxial flexural strength for the 3Y material based on aging condition
85
3.3.2) Weibull Plots for 4Y-TZP (DD cube One)
Figure 39 (A) shows the Weibull CDF and CI for biaxial flexural strength of 4Y material
per different particle abrasion, regardless of the aging condition. Weibull modulus (beta)
parameter for the CDF is lower for the wet abrasion, meaning that the data for this group
had higher dispersion. However, the characteristic strength of the wet abrasion is higher
than dry and none. It can be observed that the wet abrasion resulted in a higher strength
in the 4Y material group.
Figure 39 (B) shows the contour plot for 4Y material with the 90% CI bounds (the area
inside the curve shows where 90% of the values fall in). As seen, the circular curves do
not have a significant intersection. Hence, the characteristic strength and the modulus
are affected by the air abrasion method in the 4Y material. Weibull modulus (beta) value
is lower in the wet abrasion and has a gap with dry and none abrasion. The
characteristic strength ( 𝜎 0
) spans to larger values of 1,150 and above is only obtained
through wet abrasion.
86
Figure 39: (A) Weibull CDF and Confidence Intervals (CI) of biaxial flexural strength (σ) for none, dry and wet
abrasion for 4Y material. (B): Contour plot of beta values vs. eta in the Weibull CDF analysis for 4Y zirconia under
none, dry and wet abrasion
87
Figure 40 (A) depicts the Weibull CDF and CI of the biaxial flexural strength of 4Y material
grouped by aging condition, regardless of the surface treatment method. The Weibull
modulus is 9.7 for non-aged and 11.4 for aged material, indicating that the data is more
dispersed in the non-aged group. The characteristic strength ( 𝜎 0
) shows higher at 1,111
for the aged material group.Figure 40 (B) is contour plot of the biaxial flexural strength for
aged and non-aged 4Y material and shows the contour plot for the same data. The values
of Weibull modulus in the two contours are different, showing different dispersion in the
data. However, the range of eta or characteristic strength ( σ0) values is similar, andthere
is an overlap in the characteristic strength ( σ0), showing similar results concerning the
aging condition.
88
Figure 40: (A) Weibull CDF and Confidence Intervals (CI) of biaxial flexural strength (σ) for aged and non-aged 4Y
material. (B) Contour plot of the biaxial flexural strength for the 4Y material based on aging condition
89
3.3.3) Weibull Plots for 5Y-TZP (DD cubeX
2
)
Figure 41 (A) shows the Weibull CDF and CI for flexural strength of 5Y material per
different abrasion, regardless of the aging condition. The CI for the wet and dry abrasion
is slightly larger than none. Correspondingly, the modulus (beta) parameter for the CDF
is lower for the wet abrasion, meaning that the data for this group had higher dispersion.
This is consistent with a similar observation in 4Y material. The characteristic strength for
wet and dry abrasions are the same ( 𝜎 0
=795) and higher than the none ( 𝜎 0
=782).
Figure 41 (B) shows the contour plot for 5Y material for the 90% CI bounds (the area
inside the curve shows where 90% of the values fall in). The curves corresponding to dry
and wet abrasion intersect, but the curve for the non-abrasion is separate. Further, the
characteristic strength ( 𝜎 0
) of 820-850 can only be obtained through dry and wet abrasion,
where Weibull modulus (m) is smaller than none abrasion. Thus, the characteristic
strength and the modulus are affected particularly by dry and wet abrasion compared to
the none abrasion for the 5Y material, and the particle abrasion method tends to show
higher strength values compared to the none-abrasion group. The modulus for the none
abrasion is higher than that of dry and wet abrasion for the 5Y material. This indicates
that applying the particle abrasion has increased the dispersion of the data compared to
none abrasion.
90
Figure 41: (A)Weibull CDF and Confidence Intervals (CI) of biaxial flexural strength for dry, wet, and none abrasion
for 5Y material. (B) Contour plot of beta values vs. eta in the Weibull CDF analysis for 5Y zirconia under none, dry
and wet abrasion
91
Figure 42 (A) and (B) show the Weibull CDF and CI of the biaxial flexural strength of 5Y
material grouped by aging condition regardless of the treatment method. Weibull
modulus is similar for the non-aged and aged samples; thus, the data is similarly
dispersed. Based on contour plot ,the characteristic strength ( 𝝈 𝟎 ) is slightly higher for
non-aged in this material group, and there is no overlap in the contour plot. Thus, it is
inferred that the aging condition impacts the strength of the 5Y material.
92
Figure 42: (A) Weibull CDF and Confidence Intervals (CI) of biaxial flexural strength for aged and non-aged 5Y
material. (B) Contour plot of the biaxial flexural strength for the 5Y material based on aging condition.
B
93
Table 15 summarizes the Weibull parameters for each type of material based on surface
treatment in the Weibull plots. Based on Table 15 and the contour plots, in 3Y and 5Y,
the wet and dry abrasion results in similar improvements in biaxial flexural strength.
However, in the 4Y material, wet abrasion demonstrates the highest value of
characteristic strength (σo), meaning the largest enhancement in biaxial flexural strength.
Table 15: Summary of Weibull parameter results for the three materials based on surface
treatment
Material Surface treatment n
Weibull Modulus
(m)
Characteristic
strength (σ0)
3Y-TZP (DD Bio ZX²) None 15 9.61 1299
3Y-TZP (DD Bio ZX²) Wet abrasion 15 10.13 1296
3Y-TZP (DD Bio ZX²) Dry abrasion 15 9.86 1305
4Y-TZP (DD cube One) None 15 11.23 1053
4Y-TZP (DD cube One) Wet abrasion 15 8.50 1140
4Y-TZP (DD cube One) Dry abrasion 15 13.73 1104
5Y-TZP (DD cubeX²) None 15 10.63 782.1
5Y-TZP (DD cubeX²) Wet abrasion 15 7.28 795.1
5Y-TZP (DD cubeX²) Dry abrasion 15 9.50 795.2
94
Table 16 summarizes the Weibull parameters for three materials based on aging
conditions in the Weibull plots. Based on Table 16 and the contour plots, 3Y and 4Y did
not show significant differences between the aged and non-aged groups. But, in 5Y, non-
aged showed higher Characteristic strength (σ0)
compared to the aged group, and aging
reduced biaxial flexural strength. Also, the aged group has a higher Weibull modulus
meaning less dispersion in data than the non-aged group.
Table 16: Summary of Weibull parameter results for the three types based on aging
condition
3.4) Comparison of Weibull Results Using R and SPSS results
using ANOVA
In the previous sections, analysis of the strength data based on ANOVA and Weibull
tests was demonstrated. It is also interesting to compare the results between the two
methods and see how additional information can be inferred from each of the tests.
Group Variable Weibull Characteristics Strength Weibull Modulus
Aging
3Y-Aged: 1301
3Y-non-aged: 1298
3Y-Aged: 9.03
3Y-non-aged: 11.27
4Y-Aged: 1111
4Y-non-aged: 1091
4Y-Aged: 11.41
4Y-non-aged:9.70
5Y-Aged: 781
5Y-non-aged: 800
5Y-Aged: 9.10
5Y-non-aged:8.837
95
Table 18 reveals that the surface treatment enhances the biaxial flexural strength in
contrast with the ANOVA test inTable 17,showing insignificant influence (p>0.05). The
surface treatment showed different impacts on the biaxial flexural strength in the ANOVA
test. As detailed in Table 17, ANOVA results showed no significant differences between
age and non-aged groups. While Weibull showed differences between 5Y-nonaged and
5y aged groups. (Table 16)
As detailed in Table 17, the ANOVA test confirms that the impact of materials is significant
(p< 0.05). This result agrees with Table 18 (summary of the Weibull parameters based
on the analysis of three factors separately), which showed the highest strength in the 3Y
material group.
Table 17: Summary of impacts by material, aging, and treatment using ANOVA test
Source Sig. Partial Eta Squared
Material .000 .736
Aging .957 .000
Treatment .410 .007
Material * Aging .580 .004
Material * Treatment .293 .019
Aging * Treatment .821 .002
Material * Aging * Treatment .030 .042
96
Table 18: Summary of impacts by material, aging, and treatment by using Weibull
reliability analysis
Group Variable Weibull Characteristics Strength Weibull Modulus
Material
3Y: 1300
4Y: 1099
5Y: 790.8
3Y: 9.86
4Y: 11.15
5Y: 9.13
Aging
Aged: 1064.5
Non-aged: 1063.2
Aged: 9.84
Non-aged: 9.76
Treatment
None: 1044.7
Wet: 1077.03
Dry: 1068
None: 10.49
Wet: 8.63
Dry: 11.03
97
Chapter 4: Discussion
The present study assessed the influence of three factors on the biaxial flexural strength
of three zirconia materials such as DD Bio ZX² (3Y-TZP), 4Y-TZP (DD cube One), and
DD cube X²(5Y-TZP). These factors include the content of yttria, abrasion, and artificial
aging.
Our results demonstrated the influence of particle abrasion and yttria content on the
biaxial flexural strength. Also, artificial aging showed an insignificant impact on the
strength of 3Y and 4Y materials, while artificial aging was an influential factor for 5Y.
4.1) Different Methods to Examine the Flexural Strength Values
The three-point bend test is the most frequent method to define the strength of dental
ceramics (Wagner and Chu, 1996) (1). The biaxial flexural and the four-point bend test
are other techniques to examine flexural strength. The last two tests are nearly pure
bending (Mecholsky, 1995) (149). In three and four-point bending tests, the application of
the load is at the edges of the examined materials. The major issues with these testing
ways are that the results are impacted by flaws in the material at the sample edges. It is
hard to remove all faults, and as a subsequent, there is enormous variability in the
findings. The biaxial flexural test can eliminate the edge impact due to the lack of applying
the load directly to the surface deficiencies. Biaxial flexural strength can cater to the fewer
diversities in the definition of ceramic strength (Wagner and Chu, 1996; Yilmaz, Aydin,
and Gul, 2007) (148, 150, 151).
98
Wagner and Chu (1996) recommended that biaxial flexural strength was the proper
technique for comparing defect-free materials and that porcelain strength with flaws was
created during production, processing, and handling (148, 152).
A comparative study of flexural strength test methods on CAD/CAM Y-TZP dental
ceramics in 2015 was conducted (153). This study showed that the sample's sizes and
process of preparation based on recommendations in ISO 6872 (137) had flexural
strength values as follows: Biaxial flexural strength> three-point flexural test (3PF) > four-
point flexural test (4PF) (153). The biaxial flexural test has several benefits compared to
the uni-axial flexural tests (three-point flexural test and four-point flexural). Due to disc-
shaped samples and applying load at the center of the surface, edge failures are
eliminated. Moreover, the biaxial flexural test showed less coefficient of variation (C.V)
compared to uni-axial flexural tests (148). Subsequently, the biaxial flexural test (BFT)
has high value than uni-axial flexural tests.
On the other hand, another study evaluated the reliability of different bending test
methods for dental press ceramics. This study ranked the three flexural strengths in the
following order: Three-point flexural strength ≥ biaxial flexural strength > four-point flexural
strength (132).
Also, Weibull's statistical fracture theory can analyze the correlation between various uni-
axial test methods (3PF& 4PF), the same as the relation between the variety of sample
sizes (153). The discrepancy between uni-axial test methods and biaxial flexural strength
was primarily derived from the impact of edge flaws. Uni-axial test methods were less
99
reliable than biaxial flexural strength tests (153). Materials with low flexural strength
showed more pores and flaws than those with higher flexural strength.
The ring-on-ring test gives less failure stress values than piston-on-three-ball and three-
point bending tests due to the relatively large area tested in the ring-on-ring test (154).
When the ring-on-ring test gives the highest Weibull modulus, the stress distribution is
more uniform in this test (154).
In 2008, Fischer et al. evaluated the flexural strength of veneering ceramics for zirconia.
(154) They found that the three-point flexural strength values of veneering ceramics for
zirconia were similar to those of veneering ceramics for the metal-ceramic method. The
four-point flexure test among all three tests displayed the highest discrimination between
the various ceramic materials (154).
A greater Weibull modulus value displays a lower scattering in the test results. Therefore,
the small scattering test technique is noticeably desirable for comparing flexural
strengths.
Moreover, another explanation for the Weibull modulus (m) can explain the asymmetrical
distribution or variation of strength, meaning the microcracks and flaws developing in the
microstructure (155). The lower value of the Weibull modulus shows more defects and
flaws in the materials, leading to a decline in reliability.
On the other hand, the higher values of the Weibull modulus show less range of error
and, therefore, more structural reliability (155).
100
Mostly, "m" values in ceramics are recorded between 5–15, while, in metals, the values
are between 30–100 (156). The Weibull modulus of opaque porcelain from the 4-point
bending test was less than that of the biaxial flexural test. However, the Weibull modulus
of body porcelain from the biaxial flexural test was less than that from the 4-point bending
test (157).
In this study, the same observation is made about the modulus, and the modulus value
for all the material types (3Y, 4Y, and 5Y) lie in the range of 5-15. Also, based on
Table18, concerning the yttria content, the Weibull modulus is higher in 4Y material and
based on the aging condition, the modulus values are similar. Also, the lowest modulus
is observed in wet treatment based on treatment. Therefore, a lower modulus value
indicates higher dispersion in the data.
One of the aspects that can affect the biaxial flexural strength of zirconia is the shade of
the zirconia. In this study, unshaded zirconia was used. Several studies showed that the
unshaded zirconia had a higher biaxial flexural strength than the shaded zirconia.
However, they did not have a statistically significant difference. This slight increase can
be related to presenting foreign metal oxides in zirconia's structure (158-160). On the
other hand, Behrens et al. in 2004 and Farsi et al. in 2006 showed no significant difference
in flexural strength between colored and uncolored zirconia samples (160, 161).
4.2) Different Techniques of Surface Treatment
Another factor that can affect zirconia's biaxial flexural strength is the different surface
treatment techniques. Therefore, this study evaluated the differences between wet and
dry abrasion for surface treatment.
101
After sintering, Ishgi et al. (162) showed that sandblasted or polished zirconium oxide
ceramic samples had higher flexural strength than control sintered zirconium oxide
samples (163).
Kosmac et al. in 1999 and Kosmac et al. in 2000 found that airborne-particle abrasion
significantly enhanced biaxial flexural strength. Moreover, fine grinding cannot alter the
strength (164, 165). However, coarse grinding also decreased the strength of zirconia
and is the more anticipated result. Coarse grinding might induce surface flaws that cause
strength degradation (165, 166).
The impact of dry and wet grinding and airborne-particle abrasion with alumina (110µm)
on the microstructure, biaxial flexural strength, and reliability of two yttria-stabilized
tetragonal Zirconia (Y-TZP) ceramics were investigated by Kosmac in 1999. He found
that airborne-particle abrasion with alumina (110µm) gives more surface roughness than
dry and wet grinding surface treatments (166).
Airborne-particle abrasion is a frequently utilized surface treatment method to enhance
bond strength in crown restoration. After airborne-particle abrasion, an enhancement in
flexural bond strength is associated with transforming the tetragonal to a monoclinic
phase, which reduces the remaining compressive stress. The stress concentration at the
cracks' tip (before or after airborne-particle abrasion) avoids crack propagation and
enhances the substrate's strength (167, 168).
In 2013, Ravi Kiran Chintapalli et al. studied the impact of various airborne-particle
abrasion conditions on the mechanical features of yttria-stabilized tetragonal zrconia (Y-
TZP). Two different particle sizes (110, 250μm) of airborne-particle abrasion, two
102
pressures (2 and 4bar), and two impact angles (30° and 90°) were used. Results of the
study showed that airborne-particle abrasion with particle sizes less than or equal to 110
μm and pressures less than 4bar enhances the bi-axial strength of the zirconia ceramics.
Also, strength showed a slight increase when airborne-particle abrasion with a 30° impact
angle regardless of the particle size (168, 169).
In 2009, Arakoca and Yilmazj showed Biaxial flexural Strength of the Y-TZP declined after
grinding and considerably enhanced after airborne-particle abrasion (170).
Similarly, Curtis et al. (2005) claim that airborne-particle abrasion (alumina abrasion) and
fine grinding showed no difference in the biaxial flexural Strength of Lava™ samples;
however, the biaxial flexural strength was considerably reduced when samples were
coarse ground (120-150 µm) (170).
Regarding surface treatment methods, zirconia materials have been studied in the
literature from different aspects of airborne-particle abrasion effects. For example, some
studies showed that changing applied pressure during airborne particle abrasion cannot
significantly affect the biaxial flexural strength of zirconia. Inokoshi et al. (2017) claimed
that 0.2 MPa airborne-particle abrasion could enhance the biaxial flexural strength of
monolithic zirconia (3Y-TZP) (171).
Zeighami et al. (2017) showed that changes in the distance and angle of airborne-particle
abrasion could not lead to noticeable alternation in the flexural strength of zirconia (172).
103
In 2016, Aurélio et al. concluded that airborne-particle abrasion enhanced the flexural
strength of Y-TZP ceramics regardless of the pressure, particle size, and duration of
airborne-particle abrasion (173).
The enhancement in air pressure is in line with a currently published study in which air
pressure of 0.2, 0.4, and 0.6 MPa was utilized. On the other hand, the flexural strength of
Y-TZP declined with enhancing pressure (174).
However, the impact of wet abrasion compared to dry one on the biaxial flexural strength
of zirconia materials has not been addressed yet. Therefore, this study aimed to show
their differences.
In this study, based on Weibull analysis using R software, particle abrasion enhances the
biaxial flexural strength of three materials compared to none abrasion group. However,
based on ANOVA results, particle abrasion does not significantly affect biaxial flexural
strength.
4.3) Aging
There are different conditions for aging, such as mechanical cycling and autoclave aging.
Therefore, various studies evaluated the impact of aging on the mechanical features of
zirconia. Autoclave aging did not significantly affect the flexural strength of zirconia. This
is a frequent finding in many studies (175, 176) and is associated with the superficial
transformation compared to the samples' dimensions, which are not able to change their
mass features (175).
104
Regarding thermal cycling, there is no standardized protocol for making findings.
Therefore, it is challenging to compare. (177) The ISO/TS 11405 suggests using two
baths with temperatures of 5°C–55°C and a dwell time ⩾of 20 min. However, in the oral
cavity, regulatory mechanisms are prone to restore the temperature to 37°C, while
patients could not withstand such excessive stimuli for prolonged periods. Subsequently,
setting an intermediate temperature of 37°C to simulate better temperature transitions in-
vivo. Cotes et al. (177) evaluated different aging protocols such as (AUT)12h in autoclave
at134◦C/2bars; (T) thermal cycling (6,000cycles/5–55◦C/30s); (TM) thermo-mechanical
cycling (1,200,000cycles/3.8Hz/200N with a temperature range from 5-55°C for 60s
each); (M)mechanical cycling (15,000,000cycles/3.8Hz/200N); and (STO) storage in
distilled water(37°C /400days). They found that autoclave aging and water storage for
400 days could not affect the flexural strength of Y-TZP ceramic. However, mechanical
and thermomechanical cycling could decline the strength of examined zirconia material.
Similarly, another study with TC up to 5000 cycles showed a considerable reduction in
flexural strength (178).
Kelesi et al.(2020) used autoclave aging for one hour and thermal cycling for 22,500
cycles, corresponding to 3–5 years of clinical aging. The authors demonstrated a similar
and slight influence on the flexural strength of two monolithic zirconia ceramics (BruxZir,
Zenostar). Differences in composition and processing might cause considerable
differences between the two ceramics (179).
Pereira et al. (2016) reported that three m-phase contents were between 0- 14% prior to
aging and enhanced to 11-40% after aging for 10 hours. When m-phase content was
more than 50%, flexural strength was reduced (151).
105
Based on ISO Standard 13356:2008 (139), aging yttria-stabilized tetragonal zirconia (Y-
TZP) by steam autoclave for five hours at 0.2 MPa at 134 °C can stimulate t-m
transformation. However, this might not be sufficient to reduce the ceramics' mechanical
behaviors. Similarly, an accelerated aging test was used in this study, and samples were
exposed to steam at (134±2) °C under a pressure of 0,2 MPa for five hours.
According to this study's results using ANOVA, accelerated aging had an insignificant
effect on the biaxial flexural strength of three zirconia materials. There were no significant
differences between aged and non-aged samples. On the other hand, Weibull analysis
showed that aging is an influential factor in 5Y material. Weibull results demonstrated that
the 5Y-aged group had lower biaxial flexural strength compared to the non-aged group.
4.4) Different Compositions of Materials
Regarding the composition of zirconia, several studies compare the mechanical
properties of different Zirconia materials in terms of composition and company.
This study evaluated the biaxial flexural strength of different zirconia materials based on
the content of yttria from the same manufacturer, including 3Y-TZP (DD Bio ZX²), 4Y-TZP
(DD cube One), 5Y-TZP (DD cubeX²). The experiment results analyzed by ANOVA and
Weibull demonstrate that yttria content had a statistically significant impact on biaxial
flexural strength; the highest strength belonged to 3Y, and the lowest strength was 5Y.
The content of yttria has been shown to impact static flexural strength significantly.
Pereira et al. (2018) found that increasing the content of yttria ( ∼5-mol%) compared to
106
less content of yttria (3-mol%) leads to considerably less dynamic fatigue and static
flexural strength (180).
Kim HK et al. (2021) showed that conventional 3Y-TZP showed slower abrasion than high
translucent zirconia such as 4Y- and 5Y-PSZ conventional 3Y-TZP was similar to the
result of Zhao et al.'s study (180, 181). This finding will further entail the necessity of
analyzing the yttria content impacts.
Mao et al. (2018) inspected the relationship between the flexural Strength of ultra-
translucent zirconia (5Y-PSZ) and the surface treatments compared to the standard high
cubic, including 3Y-TZP. The strength reduced considerably in 5Y-PSZ, followed by high
polishing. Also, they found that by enhancing translucency, the high content of the cubic
phase decreased the strength of zirconia because of a decline in the transformation from
tetragonal to monolithic. (182)
The flexural strength in different materials is different and decreases in zirconia with the
increase in the cubic phase. In 2020, a study evaluated newer zirconia materials' flexural
strength (FS) degradation compared to traditional tetragonal zirconia materials using a 3-
point bend test device on a universal testing machine. The 3Y-TZP zirconia materials had
the highest flexural strength. The lithium-disilicate material had the lowest strength value
compared to zirconia materials. The degree of strength degradation in zirconia materials
was material dependent, with the 4Y-PSZ or 5Y-PSZ cubic containing zirconia materials
showing more significant or similar strength degradation than the primarily tetragonal 3Y-
TZP materials (183).
107
Like previous literature reviews, this study showed that yttria's content significantly
affected the biaxial flexural strength of zirconia material. 3Y-TZP (DD Bio ZX²) had the
highest value of biaxial flexural strength regardless of surface treatment and aging, and
the lowest value was associated with 5Y-TZP (DD cubeX²).
On the impact of artificial aging, Sulaiman et al. (2017) reported that airborne particle
abrasion decreased the flexural strength of fully stabilized Zirconia and enhanced the
flexural strength of partially stabilized monolithic Zirconia (184). Furthermore, they
claimed that artificial aging could not impact flexural strength, similar to the reported
results by Stawarczyk et al. (185).
Particle abrasion stimulates the transformation of the t-ZrO2 to the m-ZrO2 phase,
enhancing residual compressive stress on the surface. The material containing the lowest
yttria content has the highest transformability of the t-ZrO2 phase. Therefore, 3Y,
compared to 4Y and 5Y, can show higher compressive stress on the surface, increasing
mechanical behaviors such as biaxial flexural strength. Subsequently, increasing yttria
content showed a lower t-m transformation that causes the reduction in compressive
stress and efficacy of particle abrasion compared to 3Y (186).
In this study, LTD, due to aging, transformed smaller tetragonal grains into larger
monoclinic grains. The tested 5Y-TZP (DD cubeX2) contains less than 1% monoclinic
zirconia, and more than 50% cubic phase. Therefore, the less t-m transformation in 5Y
reduced its mechanical behavior, such as fracture toughness and resistance to crack
propagation. As a result, aging reduced the biaxial flexural strength of 5Y compared to
3Y and 4Y (166, 186).
108
Moreover, the BFS of zirconia is directly related to increasing m-ZrO2 content since the
m-ZrO2 content is crucial in transformation toughening (induced by mechanical stress like
particle abrasion) and LTD (induced by aging). Therefore, decreasing m-ZrO2 content led
to t-m transformation and decreasing mechanical behavior of zirconia (166). In summary,
the BFS of 5Y material showed the lowest values in this study, and aging reduced its BFS
compared to 3Yand 5Y.
109
Chapter 5: Conclusion
The conclusions that can be gained from this research are as follows:
1. The content of yttria had a significant influence on the biaxial flexural strength of
zirconia material. 3Y-TZP (DD Bio ZX²) had the highest value of biaxial flexural
strength regardless of surface treatment and aging, and the lowest value was
associated with 5Y-TZP (DD cubeX²).
2. The accelerated aging test (exposing to steam at 134°C under a pressure of 0,2 MPa
for five hours) showed an insignificant impact on the biaxial flexural strength of 3Y-
TZP (DD Bio ZX²) and 4Y-TZP (DD cube One). In contrast, aging reduced the strength
of 5Y-TZP (DD cubeX²).
3. Particle abrasion can improve the biaxial flexural strength of the three materials
compared to none abrasion. While dry and wet abrasion led to similar changes in 3Y-
TZP (DD Bio ZX²) and with 5Y-TZP (DD cubeX²), wet abrasion in 4Y-TZP (DD cube
One) resulted in larger improvement compared to dry and none abrasion.
110
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Abstract (if available)
Abstract
Objective: This study examines the zirconia materials’ biaxial flexural strength (BFS) concerning yttria content, surface treatment, and artificial aging.
Material and methods: Three zirconia materials [Bio ZX² (3Y-TZP), DD cube One(4Y-TZP), and DD cubeX² (5Y-TZP)] were selected, and a total of 270 zirconia disks were divided into two groups: aged and non-aged. Based on surface treatment, these groups were further subclassified into three groups: none, wet particle abrasion, and airborne abrasion (n=15). For disk fabrication, cylinders (ø 15 mm, length 14 mm) were milled from round zirconia blocks using Zirkonzahn M5 (Zirkonzahn GmbH, South Tyrol, Italy ).The cylinders were attached to CAD/CAM block metal sprues and then sliced into 1.4 mm thick disks using a low-speed precision saw (IsoMet 1000; Buehler, Lake Buff, IL, USA). All disks were polished to a thickness of 1.5 mm with polishing paper up to grit 1200 and sintered according to the manufacturer’s recommendations.
Aging was performed via the accelerated aging test, exposing samples to steam at 134±2°C under a pressure of 0.2 MPa for 5 h. Both particle abrasions were conducted by 50 µm Al2O3 particles at 0.24 MPa (35 psi) for 15 s. Lastly, a universal testing machine (Model 6596; Instron, Norwood, MA, USA) was used to evaluate BFS using the piston-on-three-balls technique. A three-way ANOVA (IBM SPSS Statistics 22; α=0.05) and Weibull analysis (R software) were conducted for statistical data analysis.
Results: Yttria content significantly impacted BFS, with the highest for 3Y and the lowest for 5Y. Particle abrasion increased the BFS of all materials. Wet abrasion resulted in a lower Weibull modulus and showed higher dispersion of the measured data than dry abrasion. Aging had an insignificant effect on BFS of 3Y and 4Y, but it decreased BFS of 5Y.
Conclusion: Particle abrasion and decreasing yttria content can enhance the BFS of zirconia.
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Creator
Forghani, Nazanin
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Core Title
Influence of particle-abrasion and aging on biaxial flexural-strength of three Zirconia materials
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School of Dentistry
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Master of Science
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Biomaterials and Digital Dentistry
Degree Conferral Date
2022-12
Publication Date
12/19/2022
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10/27/2022
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aging,biaxial flexural strength,OAI-PMH Harvest,particle abrasion,Weibull,yttria content,Zirconia
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Tags
aging
biaxial flexural strength
particle abrasion
Weibull
yttria content
Zirconia