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FRACTURE TOUGHNESS OF YTTRIUM STABILIZED ZIRCONIA SINTERED IN
CONVENTIONAL AND MICROWAVE OVENS
by
Aristotelis Marinis
A thesis submitted in partial fulfillment
of the requirements for the
Master of Science degree in Oral Science
in the Graduate College of
The University of Iowa
May 2011
Thesis Supervisor: Professor Steven A. Aquilino
UMI Number: 1495718
All rights reserved
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The quality of this reproduction is dependent on the quality of the copy submitted.
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and there are missing pages, these will be noted. Also, if material had to be removed,
a note will indicate the deletion.
UMI 1495718
Copyright 2011 by ProQuest LLC.
All rights reserved. This edition of the work is protected against
unauthorized copying under Title 17, United States Code.
ProQuest LLC.
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Copyright by
ARISTOTELIS MARINIS
2011
All Rights Reserved
Graduate College
The University of Iowa
Iowa City, Iowa
CERTIFICATE OF APPROVAL
_______________________
MASTER’S THESIS
_______________
This is to certify that the Master’s thesis of
Aristotelis Marinis
has been approved by the Examining Committee
for the thesis requirement for the Master of Science Degree
in Oral Science at the May 2011 graduation.
Thesis Committee:
Steven A. Aquilino, Thesis Supervisor
Peter S. Lund
David G. Gratton
Ana M. Diaz-Arnold
Clark M. Stanford
To my sister
ii
When you set out on your journey to Ithaca,
pray that the road is long,
full of adventure, full of knowledge.
C.P.Cavafy, Ithaca.
iii
ACKNOWLEDGMENTS
First and foremost I would like to express my sincere gratitude to my thesis
supervisor Dr. Steven A. Aquilino for his invaluable guidance, inspiration and
willingness to motivate me in this research project. I also would like to thank my
graduate committee for their encouragement and constant support. I acknowledge the
contribution of Dr. Fang Qian in assisting the statistical analysis.
Special thanks to my University of Athens educators who encouraged me to
pursue this degree.
I am forever grateful to my parents and my sister who supported my graduate
studies fully and unconditionally. Without their love, useful advice, endless patience and
moral support I would not have been able to achieve my goals.
I would like to thank the University of Iowa for providing me the facilities to
complete this project.
Finally, I want to acknowledge the Greater New York Academy of Prosthodontics
for the financial support on this research study. I also want to express my gratitude to the
KaVo Dental GmbH (Bismarckring, Biberach/Riß, Germany), 3M ESPE (St. Paul, MN,
USA) and DLMS (Scottsdale, AZ, USA) for providing me the required material for this
research.
iv
TABLE OF CONTENTS
LIST OF TABLES ......................................................................................................
viiii
LIST OF FIGURES ....................................................................................................
viii
CHAPTER
I
II
INTRODUCTION ..............................................................................
1
Significance of Problem - Purpose of the Study..........................
Experimental Hypotheses ............................................................
Long-Term Objective ..................................................................
2
3
3
REVIEW OF THE LITERATURE ....................................................
4
Historical Overview of All-Ceramic Systems - Review of
Current Systems ................................................................
Strengthening Mechanisms for Dental Ceramic Systems ...........
Zirconium Biomaterial.................................................................
Zirconium Dioxide - Transformation Toughening ......................
Fabrication of Yttrium Stabilized Zirconia for Dental
Applications .......................................................................
Zirconia Compared to Other All-Ceramic Biomaterials .............
Veneering of All-Ceramic Restorations ......................................
Tests for Predicting Clinical Performance of Dental Ceramic
Materials .............................................................................
Fracture Toughness Methods .......................................................
Comparison of Fracture Toughness in All-Ceramic Materials....
The Importance of Oral Environment in Zirconia Physical
Properties ...........................................................................
Clinical Studies of All-Ceramic Restorations..............................
Utilization of Microwave Technology in Dental Ceramics .........
Comparison of Zirconia Sintered in Conventional and
Microwave Ovens ..............................................................
III
IV
4
7
8
8
10
12
15
18
20
21
22
25
28
29
MATERIALS AND METHODS ........................................................
35
Pilot Study ...................................................................................
Main study - Sample Fabrication, Number, Size and Design......
Sintering Technique .....................................................................
Specimens Storage .......................................................................
Testing Method ............................................................................
Statistical Analysis.......................................................................
35
37
43
44
44
47
RESULTS ...........................................................................................
49
Interaction Between Sintering Type and Manufacturer...............
Differences Between Sintering Type Within Each
Manufacturer ......................................................................
50
v
52
Differences Among Three Manufacturers Within Each
Sintering Type ....................................................................
53
DISCUSSION .....................................................................................
55
Limitations of the Study ..............................................................
Clinical Relevance .......................................................................
Avenues for Future Research.......................................................
61
62
64
CONCLUSIONS.................................................................................
65
.............................................................................................................
66
Raw Data ...........................................................................................................
66
REFERENCES ...........................................................................................................
69
V
VI
APPENDIX
vi
LIST OF TABLES
Table
1.
Flexural Strength and Fracture Toughness of All-Ceramic Materials .................
15
2.
Descriptive Statistics of the Fracture Toughness (MPa√m) of ZrO2 Specimens
36
3.
Sintering Protocol for ZrO2 Specimens ...............................................................
42
4.
Descriptive Statistics of Fracture Toughness (MPa√m) by Sintering Types
and Manufacturer .................................................................................................
49
5.
Result of Two-Way ANOVA for the Fracture Toughness ..................................
50
6.
Mean Fracture Toughness (MPa√m) by Manufacturer .......................................
51
7.
Mean Fracture Toughness (MPa√m) by Type of Sintering .................................
51
8.
Mean Fracture Toughness (MPa√m) of ZrO2 Specimens ...................................
52
vii
LIST OF FIGURES
Figure
1.
Molecular configuration of ZrO2: Tetragonal crystal structure (top),
Monoclinic crystal structure (bottom) .................................................................
9
2.
Partially sintered ZrO2 block placed in the positioning aid .................................
37
3.
Specimens' expected dimensions after sintering..................................................
38
4.
Pre-sintered ZrO2 milled specimens ....................................................................
40
5.
Straight notch created by diamond disc ...............................................................
40
6.
Fabrication of V-shaped notch in the microparallelometer milling machine ......
41
7.
Heavily distorted and fractured specimens ..........................................................
43
8.
Four-Point bending test (Zwick 1445 Universal Testing Machine) ....................
45
9.
Locations of three measurements used to calculate notch depth in SEM image .
46
10.
Fracture toughness equations, ISO standard 6872 for ceramic materials ............
47
viii
1
CHAPTER I
INTRODUCTION
Esthetics is a personal preference and is influenced by one’s individual
personality and cultural factors (Lombardi, 1973). Looking back at history, the perception
of what constitutes an ideal smile has changed. Currently a bright, white smile with teeth
in perfect alignment is considered an integral part of a beautiful appearance. Nowadays a
patient’s dental treatment requires materials that will fulfill these expectations.
Porcelain fused to metal restorations (PFMs) have been widely used in the last 50
years but with increasing patients’ esthetic perceptions, material costs and a shortage in
skilled technical support there has been a push towards all-ceramic restorations. Initially
the all-ceramic restorations used were predominantly glass materials which could provide
high esthetic results. However, due to inferior mechanical properties, these materials
remain in use primarily for single anterior restorations (Kelly, 2004). Fillers were
dispersed in the glassy material to broaden the clinical use of the all-ceramic restorations.
The particle filled glasses provided increased strength. Most recently polycrystalline
ceramics have been introduced to provide improved physical properties. Polycrystalline
ceramics such as zirconium dioxide (ZrO2) are tougher and stronger than predominantly
glassy materials or particle filled glasses. The utilization of these materials in dentistry
provided the ability to construct high strength single unit and multi unit all-ceramic
prostheses (Kelly, 2004).
The promising development in all-ceramic materials in combination with the high
cost of noble metal alloys increased the demand for ZrO2 ceramics. These are produced
through complicated procedures. The fabrication of fully sintered ZrO2 requires an
extended sintering process (8-10 hours) under high temperature in a conventional oven.
Microwave sintering of ZrO2 ceramics is an alternative technique and requires shorter
sintering time (2 hours) compared to conventional sintering. In the past few years,
2
microwave ovens for sintering dental ceramics have been developed. The microwave
oven manufacturer DLMS (Scottsdale, AZ, USA) claims a superiority of their microwave
oven compared to conventional ovens. They report an 80% reduction in sintering time,
better physical properties and up to 90% reduction in energy consumption
(www.sinteringovens.com). However, the physical properties of microwave sintered
ZrO2 have not been extensively studied. The purpose of this study was to compare the
fracture toughness of 3 mol% yttrium stabilized tetragonal zirconia polycrystals (3YTZP) sintered in a conventional oven relative to sintering in a microwave oven.
Significance of Problem - Purpose of the Study
Microwave technology is a well established procedure in domestic and industrial
heating of products. Food, wood, rubber, chemicals, polymers, semiconductors and
ceramics are among the materials that are now commonly processed with microwave
heating equipment (Clark, Folz and West, 2000). The utilization of microwave energy for
sintering dental ceramics has been introduced. However, there are very few studies
comparing microwave and conventionally sintered ZrO2 for dental prostheses and these
studies provide conflicting results. Wilson and Kunz (1988) showed that ultra rapid
microwave heating could lead to poorer mechanical properties. Nightingale and Dunne
(1996) suggested that microwave sintering of ZrO2 would shorten sintering time, increase
densification, and improve production rates and lower energy requirements. Chen et al.
(2006) concluded that the microwave technique provided ceramics with superfine grain
size, favorable microstructure and substantially decreased sintering time. Vaderhobli and
Saha (2007) suggested comparable mechanical properties for specimens sintered in
microwave and conventional ovens. The small number and conflicting results of these
studies indicate the need to validate whether microwave sintering will produce ZrO2 with
similar or improved physical properties compared to conventional sintering.
3
Experimental Hypotheses
Null Hypothesis (Ho) {1}: There is no difference between the fracture toughness
of 3Y-TZP sintered in a microwave oven and in a conventional oven.
Alternative Hypothesis (Ha) {1}: There is a difference between the fracture
toughness of 3Y-TZP sintered in a microwave oven and in a conventional oven.
Null Hypothesis (Ho) {2}: There is no difference among the fracture toughness of
3Y-TZP made by the three different manufacturers.
Alternative Hypothesis (Ha) {2}: There is a difference among the fracture
toughness of 3Y-TZP made by the three different manufacturers.
Null Hypothesis (Ho) {3}: There is no significant interaction between the
sintering technique and the ZrO2 manufacturer.
Alternative Hypothesis (Ha) {3}: There is a significant interaction between the
sintering technique and the ZrO2 manufacturer.
Long-Term Objective
The overall goal of this research is to evaluate the feasibility and reliability of
sintering 3Y-TZP in a microwave oven for fabrication of substructures of all-ceramic
dental prostheses.
4
CHAPTER II
REVIEW OF THE LITERATURE
Historical Overview of All-Ceramic Systems - Review of
Current Systems
The all-ceramic materials used for dental prostheses have been categorized in
three groups: predominantly glassy materials, particle filled glasses and polycrystalline
ceramics (Kelly, 2004). In general, highly esthetic dental materials are predominantly
glassy and higher strength ceramic substructures are mainly crystalline. Predominantly
glassy ceramics are derived from feldspar and they are based on alumina and silica. Their
structure is amorphous; their atoms having minimal ordered crystalline properties. These
materials tend to have good optical properties. Manufacturers use small amount of fillers
to control the optical effects of the porcelain (i.e. opacity, opalescence, color) (Kelly,
2008). The other two categories have crystalline content from 55% to 100%. Particle
filled glasses are glass ceramics with fillers to enhance mechanical properties. The first
filler incorporated in feldspathic glass was alumina oxide (Al2O3). Leucite has been more
recently used through a uniform dispersion in the glass phase (dispersion strengthening).
The fillers improve the mechanical properties and affect the optical properties of the
ceramic material such as the translucency and opalescence (Kelly, 2008). Depending on
the shade and the opacity of the existing teeth this characteristic can have favorable
influence. For instance, a moderately discolored tooth can be masked with these
restorations. However, a predominantly glassy ceramic could provide better esthetic
results when trying to match a very translucent tooth. Polycrystalline ceramics have
fillers packed into regular arrays and do not have a glassy component. The matrix is
Al2O3 or ZrO2 (Kelly, 2008). The filler powder can be packed up to 70% of the
theoretical density and it achieves full density through a sintering process. Polycrystalline
ceramics are much tougher and stronger ceramic materials than the particle filled glasses
5
or the predominantly glassy ceramics. A crack is more difficult to propagate in a
polycrystalline ceramic with packed atoms compared to a ceramic material with less
dense fillers and irregular network (Kelly, 2008). Polycrystallines are considerably
opaque and difficult to process in complex shapes. They are mainly used as a
substructure material in dentistry with a glassy ceramic material veneered on the
substructure for esthetics (Kelly, 2004). The development of the polycrystalline ceramics
has been enhanced with computer aided manufacturing technology.
All-ceramic prostheses can also be divided into four different groups according to
the fabrication procedure: powder condensation (layering), slip casting, hot pressing, and
computer aided designed – computer aided manufactured (CAD/CAM) (Griggs, 2007).
Powder condensation is the traditional method of fabricating PFMs and all-ceramic
dental prostheses. In this technique, moist porcelain powder is built up with a brush. The
porcelain is condensed with removal of the excess fluid and during firing under vacuum.
This porcelain can be porous and the crystalline particles do not form a network. The
crystalline particles are dispersed in the glassy material. The nature of this “porcelain”
material (which is primarily a feldspathic amorphous glass) and the variability in the
porosity create a low strength material. The glassy component in combination with the
lack of extensive fillers provides better esthetic translucent properties. This technique is
used for veneering substructures or frameworks made of stronger but less esthetic
materials.
Slip casting involves the fabrication of a framework which is stronger than the
applied porcelain. This substructure later is veneered using the powder condensation
technique. Initially a mold is fabricated with a material, such as stone. This can support
the slip casting mix and at the same time absorbs water. The “slip” is a viscous mixture of
ceramic powder particles suspended in water. This forms a thin layer on the mold that can
be removed after partial sintering or infiltration with molten glass. This porcelain product
has a high concentration of crystalline particles forming a network that improves the
6
mechanical properties of the all-ceramic prosthesis. Slip casting has lost popularity
because of the complicated procedure, the possibility of internal defects and the challenge
to achieve a good fit (Griggs, 2007).
Hot pressed ceramics are based on the lost wax technique. A wax up of the
prosthesis is made and invested. The investment is heated and the wax is eliminated from
the mold. Prefabricated ingots made of crystalline particles dispersed in a glassy matrix
are used. The material is heated until a highly viscous liquid is created, and slowly
pressed in the lost wax mold. The pressable ceramics have more crystalline particles and
less porosity than the porcelain applied with the condensation technique creating a
controlled and more homogenous material. This technique can produce a monolithic
restoration or a substructure which will be veneered in order to maximize the optical
properties (Griggs, 2007).
The CAD/CAM fabricated ceramics are based on three procedures. Initially the
prepared tooth/teeth are scanned either intra-orally or from the definitive cast. The data is
collected and processed by computer software that will aid in the design of the final
prosthesis. The digital information is sent to a milling engine that will mill the product
from a prefabricated ingot. The prefabricated material may be in a partially sintered or
fully sintered stage. A partially sintered stage makes the milling process easier (material
is softer) but requires a final sintering stage to provide maximum density and strength.
The milling procedure creates an enlarged partially sintered structure compensating for
the shrinkage of the ceramic during the final sintering. Different types of materials can be
used with the CAD/CAM systems. Glass infiltrated and hot pressed ceramics are milled
in their final stage compared to stronger materials (i.e. ZrO2) that are preferably milled in
the partially sintered stage.
7
Strengthening Mechanisms for Dental Ceramic Systems
Dental porcelain is a highly esthetic material that can successfully imitate tooth
structure, however like other ceramic materials it is susceptible to fracture. Different
methods have been introduced to enhance the mechanical properties of the dental
ceramics. Current methods include: framework support, dispersion strengthening,
transformation toughening, residual surface stressing and surface treatment. The
utilization of a framework substructure provides support to the ceramic material and
allows the porcelain superstructure to withstand higher tensile forces, which could be
detrimental to the ceramic material. In order for the whole system to work effectively, the
main requirements are the bonding and adjusted compatibility in coefficient of thermal
expansion (CTE) between the framework and the applied layering veneer porcelain.
Dispersion strengthening consists of a fine crystalline material incorporated in the glassy
matrix, preventing the propagation of cracks within the ceramic material. This is achieved
via the compressive stress that is created in the dispersion phase, caused by the difference
in CTE between the glassy matrix and the fine particles. The compressive stress diverts
the cracks around the particle. Transformation toughening is a characteristic property of
ZrO2. Under certain conditions, such as the initiation of a crack, the molecular
configuration of the ceramic material undergoes phase transformation. This results in
local volumetric changes that create compressive stress, which counteract the crack
propagation. The overall result is an increase in the strength of the ceramic material.
Residual surface stressing is based on ion exchange on the surface of the material. The
porcelain is coated with potassium salt and heated at low temperature. Smaller ions (i.e.
sodium) are replaced by larger ions (i.e. potassium) which create a layer of compressive
stress in the surface of the dental ceramic. Surface treatment consists of polishing and
glazing and results in a reduction of the surface flaws. In conclusion, the main
prerequisite of the strengthening mechanisms is to create compressive stress which is
8
more favorable than tensile stress and reduces flaws in the ceramic material (O’Brien,
2000).
Zirconium Biomaterial
Zirconium (Zr) is a chemical element and its name originates from the Persian
“Zar-Gun” meaning golden in color. Zr belongs to the transitional metals and its atomic
number is 40 and its atomic mass is 91.224g·mol-1. The melting temperature of Zr is
1855°C and the boiling temperature is 4371°C. Zr was originally discovered by the
chemist Martin Heinrich Klaproth in Germany in 1789 and was isolated by the Swedish
chemist Jöns Jacob Berzelius in 1824. The first reported biomedical application of Zr was
in 1969 by Helmer and Driskell; however Christel (1989) first utilized Zr to fabricate the
ball head for total hip replacement (Piconi and Maccauro, 1999). Zr is never found as a
native metal in nature. It is part of igneous rocks mixed with other elements such us iron,
titanium and silicon oxide. The main source of Zr is Zircon (ZrSiO4) which is found
primarily in Australia, South Africa, Brazil, India, Russia, and the United States. Zr also
occurs in many other mineral species including baddeleyite (Hisbergues, Vendeville and
Vendeville, 2009).
Zirconium Dioxide - Transformation Toughening
The most popular of the polycrystalline ceramic materials in dentistry is currently
ZrO2. It is a white crystalline oxide of Zr. ZrO2 has a melting temperature of 2715°C and
a boiling temperature of 4300°C. It is produced though a series of steps that separate the
ZrO2 and the impurities from the ore (i.e. ZrSiO4). ZrO2 ceramics have three different
crystallographic forms depending on temperature. At room temperature and up to 1170
°C the material is in the monoclinic phase (M) (Fig 1). Over this temperature and up to
2370 °C the material transforms to the tetragonal phase (T) and then to the cubic phase
(C) at yet higher temperatures (Piconi and Maccauro, 1999). The transformation of the
ZrO2 material from tetragonal to monoclinic phase is combined with 3-4% volumetric
9
expansion. This transformation takes place on cooling at about 950 °C (Denry and Kelly,
2008).
Figure 1. Molecular configuration of ZrO2: Tetragonal crystal structure (top), Monoclinic
crystal structure (bottom). (Source: Dambreville A, Phillipe M, Ray A. 1999. “Zirconia
ceramics or by night, all cats are grey.” Maîtrise Orthop 78:1-11.)
It is this transformation of ZrO2 from the monoclinic to the tetragonal phase that
differentiates it from other ceramic materials. The addition of oxides such as CaO, MgO,
CeO2, Y2O3 to pure ZrO2 can create partially stabilized zirconia (PSZ) which is stable at
room temperature. PSZ usually consists of all three phases (Piconi and Maccauro, 1999).
The cubic phase is not a favorable phase because it accumulates the yttrium and the
remaining tetragonal phase is not stable (Chevalier et al, 2004). It is also possible to
create PSZ at room temperature consisting of primarily the tetragonal phase (Tetragonal
Zirconia Polycrystalline-TZP) by the addition of 2-3% mol yttrium oxide (Y2O3) (Piconi
and Maccauro, 1999). The ability to retain the tetragonal phase at room temperature
10
provides very favorable mechanical properties. Under stress, i.e., at the tip of a crack, the
3Y-TZP undergoes a phase transformation from tetragonal to monoclinic phase. This
phase transformation results in a 3-4% volumetric expansion inducing a compressive
stress in the area of the crack and theoretically prevents crack propagation (Piconi and
Maccauro, 1999). This strengthening mechanism is known as transformation toughening
and makes ZrO2 much stronger compared to all other ceramic materials.
Fabrication of Yttrium Stabilized Zirconia for Dental
Applications
3Y-TZP has been used in dentistry as a substructure material for fabrication of
crowns and fixed partial dentures. Implants, implant abutments and posts can be
fabricated by this material as well. It is also popular in other fields of dentistry such as
orthodontics where it is used for fabrication of brackets. 3Y-TZP can be used in dentistry
through two different procedures. The first procedure employs soft machining of partially
sintered ZrO2 blanks that are finally sintered in high temperature in sintering ovens.
According to ISO specifications for ceramic materials (ISO 6872, 2008), sintering is “the
process whereby the heat and potentially other parameters (e.g. pressure and atmosphere)
are applied to a ceramic powder or powder compact, in order to densify the ceramic into
its required form”. Initially, the ZrO2 blanks are manufactured from ZrO2 powder which
contains yttrium and a binder that makes it suitable for pressing. The binder is eliminated
in the pre-sintering stage. The powder is compacted through cold isostatic pressing and
becomes partially sintered ZrO2. These blanks have approximately 40% of the expected
density. The die or wax pattern is scanned in CAD machines and a ZrO2 partially sintered
blank is milled by CAM engine in an enlarged dimension, based on the calculated
shrinkage during the sintering process (approximately 25% shrinkage). The final
sintering procedure depends on the manufacturer and usually requires 8 to 10 hours at a
sintering temperature between 1350°C and 1550 °C. The sintering temperature and the
11
sintering time should be well controlled because they affect the grain size which
subsequently dictates the mechanical properties of the material. After fabrication of the
ZrO2 coping, compatible feldspathic porcelain is applied to create the final esthetics and
morphology of the restoration (Denry and Kelly, 2008).
The second method to fabricate crowns or Fixed Partial Dentures (FPDs), with
3Y-TZP is through hard machining of fully sintered ZrO2 blocks. The same powder is
used to fabricate these blocks. The block is sintered at a temperature below 1500 °C and
it reaches a density at least 95% of the expected density. The block is pressed at high
temperature (between 1400°C and 1500 °C) (Hot Isostatic Pressure or HIP) (Denry and
Kelly, 2008). This produces a fully sintered block of ZrO2. The density is more than 99%
of the theoretical density and the block is milled according to the design processed by the
software. In this approach, the milling program creates a framework in the exact
dimensions of the restoration. This framework is veneered with compatible feldspathic
porcelain.
The fabrication of prostheses, through soft machining of partially sintered ZrO2,
provides the advantage of easier milling than the fully sintered ZrO2. It requires less
milling time and causes less wear of the cutters (Raigrodski, 2005; Beuer, Schweiger and
Edelhoff, 2008). In hard machining of fully sintered ZrO2, no sintering shrinkage is
expected and there is no need for a sintering oven; however, micro cracks maybe
introduced (Raigrodski, 2005). The HIP process creates highly densified ceramics, with
limited grain growth (Li, Liao, and Hermansson, 1996). The grinding of the fully sintered
ZrO2 causes a certain degree of transformation (from tetragonal to monoclinic phase) in
the surface of this material (Rekow et al., 2011). When comparing the final surface of the
soft machined ZrO2 to the hard machined ZrO2, it is expected that the former will have a
more consistent final state; given that is left intact (no sandblasting or grinding) after the
final sintering (Denry and Kelly, 2008).
12
Zirconia Compared to Other All-Ceramic Biomaterials
Since the introduction of all-ceramic restorations, many different systems have
come on the market. Initially, all-ceramics were predominantly used for anterior single
crowns and were made of feldspathic porcelain. The optical properties of these materials
were exceptional but they were considerably brittle restorations with low fracture
toughness (Cesar et al., 2007). Subsequent systems had enhanced mechanical properties
and minimized the possibilities of fracture. In-Ceram Al2O3 followed by In-Ceram
Spinell (MgAl2O4) and In-Ceram ZrO2 were introduced for single crowns and anterior
short span FPDs (Guazzato et al., 2002). These systems were comprised of crystal
particles infiltrated with low fusing glass through the slip cast process. Heat pressed
ceramics became another very popular all-ceramic system. IPS Empress, a leucitereinforced glass ceramic, and IPS Empress 2, a lithium disilicate ceramic, provided better
marginal fit, decreased porosity and good mechanical properties compared to traditional
particle filled glasses and feldspathic all-ceramic restorations. The polycrystalline
ceramics were introduced to further enhance the mechanical properties of the all-ceramic
restorations.
Studies have been performed to compare the properties of pressable ceramics, slip
cast ceramics and polycrystalline ceramics. These studies have used polycrystalline
ceramics comprised of Al2O3 or ZrO2 substructures. Guazzato el al. (2004a and b) studied
the strength, fracture toughness and microstructure of various all-ceramic systems: IPS
Empress (E1), IPS Empress 2 (E2), an experimental pressable ceramic (EC), In-Ceram
Al2O3 dry press (IA dry pressed), In-Ceram Al2O3 slip (IA slip), In-Ceram ZrO2 (IZ), InCeram ZrO2 slip (IZ slip), an experimental Y-TZP (YZ), and DC-Zirkon (DZ). They
13
concluded that ZrO2 based ceramics had more favorable properties compared to glassy
ceramics and they suggested that these materials may have better clinical performance.
DZ had the highest fracture toughness followed by the YZ. The IZ, IA slip and IZ dry
pressed exhibited lower fracture toughness without statistically significant differences
among materials. IA dry pressed, E2 and EC had statistically significant less fracture
toughness and E1 was the least tough material. The investigators examined the crystal
structure of the broken samples with X-ray diffraction and concluded that the better
physical properties of the ZrO2 material were due to its metastability from tetragonal to
monoclinic phase. A system is in a metastable state when it is not changing by time, but it
is susceptible to fall into lower-energy states with only slight interaction. They also
emphasized the importance of grain size, shape and porosity in the physical properties of
all-ceramic materials and that the increase in crystalline phase corresponded to better
physical properties. The differences in the strength and toughness of all-ceramic materials
with equal crystalline content, was related to the porosity (Guazzato et al., 2004a;
Guazzato et al., 2004b).
Tinschert et al. (2001) compared the fracture resistance of three unit all-ceramic
FPDs fabricated by five different materials: IPS Empress, IPS Empress 2, In-Ceram
Al2O3, In-Ceram ZrO2, and DC Zirkon. The thicknesses of the core material and
connector size were standardized (0.8mm thickness and 4.0mm occluso-gingival
connector height). The frameworks were cemented on a metal master model with zinc
phosphate cement. They calculated the load to fracture for five core substructures and for
five specimens veneered with the recommended feldspathic porcelain. These specimens
were not tested under fatigue loading or stress corrosion caused by the oral environment.
14
The authors concluded that fracture load was significantly higher for the DC Zirkon
group followed by In-Ceram ZrO2 and IPS Empress 2. The lowest values were obtained
for IPS Empress followed by In-Ceram Al2O3. They suggested higher strength for
restorations made of core ceramics and recommended ZrO2 based material for highly
loaded all-ceramic restorations.
Raigrodski (2005) presented an overview of current all-ceramic systems. He
suggested using leucite reinforced glass ceramics (LRG) on single anterior crowns. He
suggested lithium disilicate glass ceramics (LDG) (i.e., Empress 2) for single anterior or
posterior restorations and anterior FPDs and recommended etching the intaglio surface
for adhesive cementation. He emphasized that the strength of these materials relies on the
adhesive bond to the tooth. Glass infiltrated ceramics {In-Ceram Al2O3 (GIA), In-Ceram
ZrO2 (GIAZ)} were recommended for anterior crowns and three unit FPDs or posterior
crowns. Densely sintered high purity Al2O3 (DSHPA) (i.e., All Ceram) requires the use
of CAD/CAM and was recommended for anterior and posterior crowns but the utilization
of this material for FPDs was reported to be questionable. The Y-TZP (i.e., Cercon, Lava,
Cerec in Lab, Procera All Zircon) was suggested for anterior or posterior crowns and
FPDs, when a stronger material is desired. The reported flexural strength and fracture
toughness of these materials are summarized in Table 1.
The above in vitro studies evaluated the mechanical properties of the current allceramic systems. It is well documented that the increase in the crystalline content will
provide ceramic materials which can better withstand applied forces than traditional
ceramic materials. In addition, factors other than the mechanical properties (i.e. opacity,
15
translucency, marginal adaptation, compatibility with the veneering porcelain) should be
considered for selecting the appropriate all-ceramic system.
Table 1. Flexural Strength and Fracture Toughness of All-Ceramic Materials.
Material
Flexural Strength
(MPa)
Fracture Toughness
(MPa√m)
LRG
105-120
1.5- 1.7
LDG
300-400
2.8-3.5
GIA
236-600
3.1-4.61
GIAZ
421-800
6 -8
DSHPA
500-650
4.48-6
Y-TZP
900-1200
9-10
Source: Raigrodski AJ. 2005. “All-ceramic full-coverage restorations: Concepts and
guidelines for material selection.” Pract Proced Aesthet Dent 17:249-256.
Veneering of All-Ceramic Restorations
The fabrication of all-ceramic restorations supported by a core substructure
requires veneering of the core material with compatible feldspathic porcelain. The effect
of the veneering procedure, the bonding of the glass ceramic to the core material and the
performance of the restoration under loading has been studied. Sundh, Molin and Sjögren
(2005) investigated the effect of veneering, heat treatment and fatigue loading on the
fracture resistance of Y-TZP frameworks. Three unit FPDs were produced using
16
prefabricated blanks of sintered (HIP) Y-TZP (Denzir, Cad Esthetics). The thickness of
the ZrO2 substructure was 0.5mm and the connectors were 3x3 mm. They determined the
resistance to fracture 1) after machining the fully sintered material, 2) after heat treatment
to increase particle density and 3) after the frameworks were veneered with glass
ceramic. The FPDs were cemented with zinc-phosphate cement and fatigue loaded
(100,000 cycles, 90 loads per min 0-50 N) in water at 37 °C. None of the specimens
fractured during fatigue loading. Testing for fracture resistance followed the fatigue
loading. They observed significant (P<0.05) reduction in fracture resistance of the
specimens that were heat treated and/or veneered with glass ceramic. They concluded
that veneering affected the mechanical properties of the Y-TZP frameworks (Sundh,
Molin and Sjögren, 2005).
Tinschert et al. (2001) compared the fracture resistance of three unit all-ceramic
FPDs fabricated by different all-ceramic materials before and after veneering with the
recommended feldspathic porcelain. They calculated the load to fracture of the veneered
restorations and the core substructure alone. They concluded that veneering of the core
substructure increased the fracture resistance of the FPDs.
Al-Dohan et al. (2004) studied the strength of the substructure and veneered
porcelain interface in all-ceramic systems. They fabricated specimens from 5 all-ceramic
systems layered with the suggested feldspathic porcelain: IPS-Empress 2 with Eris (IE),
Procera AllCeram with Degussa-Ney All (PA), Procera AllZircon with Cerabien CZR
(PZ), and DC-Zircon with Vita D (DC). Metal ceramic specimens were used for the
control group. The manufacturers’ suggested firing cycles were followed. A shear
strength test was performed. The broken specimens were evaluated to determine the
17
mode of failure, cohesive or adhesive. The highest value was found for the IE group
followed by the PZ and the DC. The PA showed the weakest bond. Most of the failures
(55% -60%) were cohesive in the veneering porcelain. The rest of the failures were near
the interface with residual veneering material remaining on the core. IE was the only
material where they found a combination of cohesive failure in both the core and
veneering porcelain. They also concluded that there was no significant difference in mean
shear strength of IE, PZ, DC and the PFM control group.
Guazzato et al. (2004c) investigated the strength, reliability and mode of fracture
of bilayered porcelain ZrO2 core ceramics. Disk specimens were fabricated using four
different configurations: a) monolithic porcelain, b) monolithic specimens of Y-TZP core
material, c) bilayered specimens with the porcelain on top (facing the loading piston
during testing) and d) bilayered specimens with core material on top. They compared
flexural strength of the four groups and concluded that monolithic core specimens and
bilayered samples with core material on the bottom were statistically significantly
stronger than monolithic porcelain disks and bilayered samples with porcelain on the
bottom. They emphasized the importance of the framework design and the actual
distribution of tensile stress. They reported that advantages of the stronger core materials
may be offset by the weaker veneering porcelain if the prosthesis design does not take
into account stress distribution. They suggested that the core material can improve crown
strength and that in FPDs the weak porcelain underlining the connectors’ site will be
under tension and adversely affect the prosthesis prognosis.
The majority of studies performed on specimens veneered with feldspathic
porcelain, indicated an important interaction with the supporting material. The design of
18
the substructure, the type of forces applied in the bilayered system, the processing cycle
and testing environment all affected material performance. It is widely accepted that
every factor that relates to veneering of the substructure should be controlled, in order to
achieve predicable properties.
Tests for Predicting Clinical Performance of Dental
Ceramic Materials
A prerequisite for utilizing techniques or materials in patients is the in vitro
evaluation of their performance. Many different tests have been suggested in order to
compare and evaluate ceramic systems and processing procedures. Anusavice, Kakar and
Ferree (2007) evaluated different mechanical and physical tests in order to suggest those
that could predict the clinical performance of ceramic materials. They concluded there
was no single test predicting the performance of these materials but rather there should be
a combination of tests. They also suggested that there was too much uncertainty when
trying to correlate in vitro test data to clinical performance. According to them, the main
focus should be placed on promoting standardized clinical trials and rigorous standards
should be established for mechanical and physical property tests designed to stimulate the
oral environment.
There are many parameters that affect the performance of dental materials.
According to Kelly (2004) three main properties affect the clinical performance of
ceramic materials: strength, fracture toughness and susceptibility towards chemically
assisted crack propagation. Physical properties such as flexural strength and fracture
toughness are the first parameters to investigate when studying the clinical potential of
dental ceramics (Yilmaz, Aydin and Gul, 2007).
19
Strength is the ultimate stress that is required to cause plastic deformation or
fracture and is affected by the flaw size and defects present on the surface of the tested
material. The most common test to determine the strength of dental ceramics is the three
point bend test (Wagner and Chu, 1996). Two other methods to determine the flexural
strength are the four-point bend test and the biaxial flexural test. The latter two tests are
closer to pure bending (Mecholsky, 1995). In three and four point bending tests, the load
is applied to the edges of the tested materials. The main problems with those testing
methods are that the results are affected by material flaws at the specimen edges. It is
very difficult to eliminate all flaws and as a consequence there is great variability in the
results. The biaxial flexural strength eliminates edge effect because the load is not applied
directly to the surface imperfections. Biaxial flexural strength will provide less variation
in the determination of ceramic strength (Wagner and Chu, 1996; Yilmaz, Aydin and
Gul, 2007). Wagner and Chu (1996) suggested that biaxial flexural strength was the
method of choice, when comparing flaw free materials and that porcelain strength
depends on the defects produced during production, processing and handling.
Considering that dental ceramic restorations are not flat flawless surfaces, they suggested
that the best method to evaluate the performance of the ceramic materials was the fracture
toughness test.
Toughness is defined as the amount of energy needed to be applied to a material
to propagate a critical flaw or defect and take the system to rupture failure. Fracture
toughness is the resistance of a material to rapid crack propagation. It is not generally
affected by surface flaws and is independent of the initial crack size. As a result, the
evaluation of fracture toughness is more valuable than the comparison of the strength of
20
ceramic materials (Mecholsky, 1995). According to Kelly (2004), strength depends on
the materials’ properties and the condition of the experiment. Fracture toughness is a
more inherent property of ceramics. Based on ISO specifications for dental ceramics,
“fracture toughness is an important property since it is inherent to the material and can be
used to predict other properties, such as strength (which is sensitive to flaw size and flaw
population” (ISO 6872, 2008). Mecholsky (1995) mentioned the importance of fracture
toughness in comparison with strength. Fracture toughness tests are independent of initial
crack size and can provide more accurate data concerning the failure mode of the tested
material. Different methods for determining the fracture toughness have been used in the
literature (Antis et al., 1981; Chantikul et al., 1981; ISO 6872, 2008; Taskonak et al.,
2008).
Fracture Toughness Methods
Many different methods have been described for calculating the fracture
toughness of a ceramic material. The methods are single edge precracked beam, surface
crack in flexure and chevron notched beam (ISO 6872, 2008). Chantikul et al. (1981)
proposed a method for determining fracture toughness using the indentation strength
method. The accuracy of their equation was comparable to the previously used direct
crack technique. Guazzato et al. (2004a) used the equation proposed by Chantikul et al.
(1981) to determine the fracture toughness of all-ceramic specimens. Yilmaz, Aydin and
Gul (2007) determined the fracture toughness of their specimens, by indentating in three
equidistant locations as proposed by Anstis et al. (1981). International ISO standards
suggest the use of the single edge V-shaped notch beam model, to calculate the fracture
toughness of ceramic materials (ISO 6872, 2008).
21
Comparison of Fracture Toughness in All-Ceramic
Materials
Fracture toughness is the first step in predicting the clinical performance of allceramic materials and several studies have been performed to evaluate the difference in
fracture toughness of all-ceramic restorations. Yilmaz, Aydin and Gul (2007) measured
the fracture toughness of six all-ceramic materials. The results of their study showed the
indentation fractured toughness for Finesse was 1.88 MPa√m, Cergo 1.73 MPa√m, IPS
Empress 2.40 MPa√m, In-Ceram Al2O3 4.78 MPa√m, In Ceram ZrO2 5.56 MPa√m and
Cercon ZrO2 6.27 MPa√m. They concluded that ZrO2 based ceramic core materials had
significantly higher fracture toughness values.
Guazzato et al. (2004a and b) studied the fracture toughness of nine all-ceramic
materials using the equation proposed by Chantikul et al. (1981). They used bar
specimens with dimensions 20x3x4mm and four of the studied materials were ZrO2
based. They found that dry-pressed In-Ceram ZrO2 had a fracture toughness of 4.9
MPa√m, slip-cast In-Ceram ZrO2 4.8 MPa√m, an experimental Y-TZP 5.5 MPa√m and
DC Zirkon 7.4 MPa√m. They concluded that ZrO2 based ceramics were stronger and
tougher materials. They also suggested that the increased crystalline content in fully
sintered ZrO2 materials corresponded to better physical properties which in turn may
have a positive influence on the materials’ clinical performance.
Wagner and Chu (1996) investigated the fracture toughness value of three allceramic systems: Empress, In-Ceram and Procera All-Ceram core materials. The
respective fracture toughness values were 1.74 MPa√m, 4.49 MPa√m, and 4,48 MPa√m.
Lazar et al. (2008) compared three commercially available reinforced dental ceramic
22
materials with an experimental ZrO2 based ceramic core material. They calculated
Vickers Hardness and fracture toughness of these materials. They created blocks
6x5x5mm and the mean fracture toughness was determined for Procera All-Ceram 4.2
MPa√m, In-Ceram ZrO2 block 5.2 MPa√m, In-Ceram ZrO2 5.5 MPa√m and for an
experimental material 3Y-TZP was 6.0 MPa√m.
In summary, those studies compared the 3Y-TZP with other ZrO2 based ceramic
materials and other existing all-ceramic systems. It is evident that the 3Y-TZP exhibited
considerably higher and more favorable initial fracture toughness. The mean fracture
toughness value of 3Y-TZP varied among the different studies depending on the
manufacturer and testing methodology. It should be noted that these in vitro studies
compared the substructure of all-ceramic materials without including bilayered
specimens in their experiments.
The Importance of Oral Environment in Zirconia Physical
Properties
Physical properties such as flexural strength and fracture toughness are the first
parameters used to investigate the clinical potential of dental ceramics (Yilmaz, Aydin
Gul, 2007). In addition, the importance of other parameters such as hydrothermal
degradation, the oral environment and fatigue loading, have been emphasized
(Anusavice, Kakar and Ferree, 2007; Rekow and Thompson, 2007; Wheeler and Peralta,
2010). Rekow and Thompson (2007) suggested that factors such as prosthesis design,
fatigue (especially in the oral environment), and fabrication techniques, could be
detrimental to the success of advanced ceramics despite the significant improvement in
material properties and toughening mechanisms.
23
Different experiments have examined whether the properties of the ceramic
materials are affected by an aqueous environment, ageing, fatigue loading, thermal
cycling and finishing treatment. The role of water in crack propagation has been
documented since 1985 (Kelly, 2004). Water is entrapped in the cone cracks produced
due to the loading force and under fatigue loading the hydraulic pressure of the enclosed
liquid propagates the crack and reduces the lifetime of the ceramic material (Zhang, Song
and Lawn 2005). Kohorst et al. (2008) suggested crack growth in polycrystalline
materials was due to water-assisted breakage of ceramic bonds and degradation of the
ceramic toughening mechanism. According to Bermejo et al. (2008) the crack
propagation results from a stress assisted reaction of water with the metal oxide bonds on
the crack tip. They also pointed out that crack propagation increased further under cyclic
loading. The influence of water and temperature on crack rates of Y-TZP was studied by
Chevalier, Olagnon and Fantozzi, 1999. According to their study, the increase in crack
rates was caused by reduction of transformation toughening. They also suggested that the
main mechanism for crack growth in the polycrystalline ceramics was stress corrosion
caused by water. According to Studart et al. (2007a) the susceptibility of crack growth in
Y-TZP was due to degradation of the transformation toughening mechanism.
Sundh, Molin, and Sjögren (2005) studied the effect on fracture resistance of YTZP specimens after veneering and fatigue loading in water. They reported that heat
treatment and/or veneering reduced the fracture resistance of Y-TZP frameworks;
however, there was no significant effect of fatigue loading on fracture resistance of the
material. Mante et al. (1993) measured fracture toughness of high Al2O3 core dental
ceramics after storing the specimens for a short period of time (one week) in a dry
24
environment, water, and artificial saliva. They did not find any deterioration in the
properties of the core material. Studart et al. (2007b) proved that cyclic loading in water,
enhanced propagation of subcritical cracks. Even so they inferred that the ZrO2 FPDs
could still be functional for longer than 20 years.
The degradation of ZrO2 has been described as “hydrolytic ageing” or “low
temperature aging” and characterized by increase in the monoclinic phase and reduction
in density and toughness (Piconi and Maccauro, 1999). The ageing is affected by
temperature and the presence of water, and depends on the concentration of yttrium, the
distribution of yttrium, the grain size and the presence of flaws in the ZrO2 material. The
life time performance of the ZrO2 ceramic will be impacted by the stability of the ZrO2
structure. Papanagiotou et al. (2006) examined the effect of aging, finishing and low
thermal degradation (LTD) treatment in the flexural strength of Y-TZP. They mentioned
that there was a decrease in concentration of yttrium after LTD treatment, which could
compromise the stability of the tetragonal phase. However, they found that those
procedures did not significantly affect the flexural strength of the tested material.
The foundation of an abutment tooth and the cement as well as the ceramic
substructure can be affected by the wet environment. These changes in the properties of
the core material can induce changes in the fracture resistance of the ceramic material.
Rekow and Thompson (2007) showed that water diffusion in a composite core and in the
cement underneath a crown increased the stress in the porcelain substructure. This
mechanism induced fracture on the ceramic restorations.
All these studies reveal the importance of the environment in which the ceramic
material functions, for the long term performance of the material. It is important that
25
studies which evaluate the properties of ceramics be conducted in an environment that
simulates oral conditions, so results can be clinically relevant.
Clinical Studies of All-Ceramic Restorations
In vitro studies provide useful information concerning the mechanical and
physical properties of all-ceramic materials. However, the performance of all-ceramic
restorations in patients is not always predictable due to multiple factors that cannot be
controlled in clinical conditions. Della Bona and Kelly (2008) reported a literature review
for clinical success of all-ceramic systems. They included studies for single-tooth
restorations made of In-Ceram Al2O3, In-Ceram MgAl2O4, Procera, IPS Empress, IPS
Empress 2, and Dicor crowns. For FPDs, the studies included the following systems: In
Ceram, In Ceram Zr, IPS Empress 2, and Cercon Zr. Based on this literature review,
veneer restorations exhibited more than 90% survival rate over a 10-13 year time frame.
Partial coverage restorations (inlays and onlays) showed a 90% survival rate for 10 years
of service (except those that had been cemented with dual cured cement, which resulted
in a significantly lower success rate of 77%). All clinical trials reported survival rates for
single full coverage restorations over 90%, apart from the Dicor system (no longer on the
market), which survival rate was 87%. The survival rates of all-ceramic, anterior tooth
full coverage restorations were comparable to conventional PFM restorations. Pressed
ceramics and polycrystalline ceramics also provided predictable results in single molar
restorations. Three unit FPDs with two conventional retainers suggested relatively
inconsistent success rates, with generally higher survival rates anteriorly. According to
the authors Y-TZP was the most successful system but chipping of the veneering
porcelain continued to be one of the major problems (Della Bona and Kelly, 2008).
26
Only a few clinical studies evaluated the reliability of ZrO2 core all-ceramic
restorations. Furthermore, these clinical trials were conducted over a limited time frame
(3-5 years). Örtorp, Kihl and Carlsson (2009) reported on a 3 year retrospective study of
ZrO2 single crowns, in private practice patients. In this study, 204 Nobel Procera crowns
were cemented in 161 patients. The majority of the crowns (78%) were placed in
premolars and molars. After 3 years of service, none of the cores had fractured, but 16%
of the crowns demonstrated some type of complication. The most remarkable
complications were: extraction of abutment tooth (2.5%), remake of crown due to loss of
retention (2%), veneer fracture (1%) and persistent pain (0.5%). Patients’ satisfaction was
recorded high. The cumulative survival rate was 92.7% for 3 years.
Encke et al. (2009) organized a prospective study of ZrO2 based full coverage
crowns on posterior teeth when compared with conventional gold crowns. The aim of this
randomized controlled clinical trial was to evaluate survival rate over a 5-year period.
This study reported results of the first 24 months. In the study, 123 patients were restored
with all-ceramic crowns (KaVo Everest) and 101 with gold crowns. The prospective
survival rates (Kaplan-Meier) for an observation period of 6, 12 and 24 months, were
97.9%, 95.1% and 89.8% for the KaVo Everest crowns and 100%, 94.8% and 92.7%, for
the gold crowns respectively. There were no significant differences between the two
groups (P=0.2). Marginal discrepancies that could be detected with the explorer were
more common in the ceramic crowns (49.5%) compared with gold crowns (26.1%). They
suggested that KaVo Everest crowns could be used for posterior restorations, but the
marginal fit showed potential for improvement.
27
Schmitt et al. (2010) in a clinical trial of severely compromised anterior teeth
evaluated the survival rate of all-ceramic single crowns with ZrO2 substructure. The
substructure was 0.3mm thick and the marginal preparation was feather edged. Ten
patients received 19 restorations and no failure was recorded for an observation period of
39.2 months.
Roediger et al. (2010) studied the survival rate of three and four-unit FPDs with
frameworks fabricated of Y-TZP. Ninety-nine posterior FPDs were fabricated and
cemented with zinc-phosphate cement. The overall survival rate after 48 months was
94% (Kaplan-Meier analysis). Seven restorations were lost with four due to technical
complications and three due to biologic complications. There were 23 events that
required clinical intervention for restoration maintenance out of which 13 were ceramic
veneer chippings (polishing), six were losses of retention (recementation), three were
caries lesions (direct restoration) and one was loss of vitality (endodontic treatment).
They concluded that sufficient survival rate was recorded for an observation period of
four years.
Sailer et al. (2007) reported a five year clinical study of posterior three to five unit
all-ceramic FPDs with ZrO2 substructure. Fifty-seven FPDs frameworks (Cercon) were
fabricated, layered with feldspathic porcelain and cemented with resin cement. Seven
patients with 17 FPDs were lost to follow up and seven FPDs in seven patients were
replaced due to biologic or technical complications. During five years of observation, 12
FPDs in 12 patients had to be replaced. One five-unit FPD fractured as a result of trauma.
The success rate of the ZrO2 frameworks was 97.8%; however prosthesis survival rate
was 63.9% due to other complications. Secondary caries was found in 21.7% of the
28
FPDs, and chipping of the veneering ceramic in 15.2%. The authors suggested that there
are needs for improvement in the fit and veneering of the FPDs with ZrO2 substructure.
Many clinical studies have reported the performance of all-ceramic restorations.
However, it is evident that most studies had a short term evaluation period (up to 2-5
years) and there was no consensus among them, in the criteria used to determine clinical
performance. The majority of those studies suggested a more predictable clinical
performance for single all-ceramic restorations in anterior teeth over posterior teeth or
FPDs. They also showed improvement in the survival rate of newer systems and
materials compared to the ones initially introduced. There is a tendency to utilize core
substructure ceramic systems layered with veneering porcelain, when maximum strength
is required. However, many complications of the veneering porcelain have been reported.
These studies suggested that all-ceramic restorations had comparable survival rates to
conventional PFM restorations, for the short period of time they were examined.
Utilization of Microwave Technology in Dental Ceramics
Percy Spencer in 1945 was the first to notice the heating effect of microwaves
(Murray, 1958). Microwave technology is utilized in industrial fabrication of products
including the sintering of ceramic materials. In a conventional furnace, the heat is
transmitted to the material surface and reaches the core of the material by thermal
conduction. A microwave oven works by creating non-ionizing microwave radiation,
usually at a frequency of 2.45 GHz. This radiation creates movement of the polarized
molecules within the object that is in the microwave. The microwave oven alters the
electric field causing constant movement of the dipole molecules. This movement
produces heat that is dispersed in the material through a process called dielectric heating.
29
Upadhyaya et al. (2001) suggested that sintering ZrO2 in a conventional oven produced
high thermal gradients and stress, while the temperature in a microwave oven was
distributed uniformly through the ceramic material.
Microwave energy does not have the same efficiency on all types of materials.
Materials with high purity in Al2O3 or ZrO2 are low microwave absorbers at temperatures
below 300-500 °C, they have low thermal conductivity at ambient temperatures, and they
are prone to cracking (Sutton, 1992). According to Nigthingale and Dunne (1996), the
ZrO2 material is not a good absorber of microwave temperature until it is heated to 400
°C. Hybrid (indirect) heating has been introduced to heat materials that do not absorb
microwave energy in ambient temperatures. Microwave energy initially increases the
temperature in Silicon Carbide (SiC) susceptors which are incorporated in crucibles. This
elevates the temperature of the low microwave absorbing ZrO2 through thermal
conduction. The initial slow rate heating process eliminates the development of hot spots
and localized thermal runaway is avoided (Sutton, 1992). When the ZrO2 reaches a
temperature of 300-500 °C it becomes susceptible to further heating by microwave
energy thereby enhancing the sintering process. This sintering process enables controlled
power increase without creating cracks in the ZrO2 material (Sutton, 1992). Thus
microwave hybrid heating increases the heating rate compared to conventional heating
process.
30
Comparison of Zirconia Sintered in Conventional and
Microwave Ovens
Non-dental studies have compared ZrO2 sintered using conventional and
microwave techniques. There are also a few studies in the dental literature that compared
microwave and conventionally sintered ZrO2 and provided conflicting results.
Wilson and Kunz (1988) evaluated microwave sintering of partially stabilized
ZrO2. Initially, they tried to rapidly heat the ZrO2 specimens but realized that the material
did not absorb microwave energy at room temperature and concluded that rapid heating
of the ZrO2 was impossible. They placed ZrO2 material in SiC susceptors which absorbed
microwave energy and transferred heat to the specimens. After initial sintering, the ZrO2
material could then absorb microwave energy and be heated further. Bar specimens
(25x6x3mm) in the green stage were placed in the susceptors, inside a beryllia crucible
which served as a heat shield for different heating rates and sintering times. They
concluded that ultra rapid heating (less than 600s) caused cracking of the ZrO2 material
and suggested similar physical properties for conventional and microwave sintered ZrO2.
The fracture toughness for their specimens (considering that for each specimen they used
different sintering parameters) was from 4.55 to 4.99 MPa√m.
Upadhyaya et al. (2001) studied sintering and grain growth of 3Y-TZP as well as
3Y-TZP with titanium dioxide (TiO2) and 3Y-TZP with magnesium dioxide (MnO2). The
rational for adding TiO2 or MnO2 was to improve the microwave coupling at a given
temperature. They fabricated cylindrical samples that were placed in a casket and SiC
powder was used for susceptors. The sintering temperature was 1350 °C for the
microwave oven and 1400 °C for the conventional oven. Vickers indentation was used to
31
determine hardness and toughness of the materials. The fracture toughness was calculated
from the radical crack using the procedure described by Anstis et al. (1981). The fracture
toughness was 8.7 MPa√m for the 3Y-TZP sintered in conventional oven, 8.6 MPa√m for
the same material sintered in microwave oven, 4.3 MPa√m for 3Y-TZP with MnO2
sintered in microwave oven and 4.8 MPa√m for the 3Y-TZP with TiO2 sintered in
microwave oven. They suggested that the general morphology of the microwave sintered
samples was identical to the conventional ones. The addition of either TiO2 or MnO2
considerably reduced the properties of the 3Y-TZP and the crystals of ZrO2 sintered in
microwaves appeared relatively fine and uniform in size. They also suggested that the
grain boundaries were significantly thinner. These observations could be beneficial for
the mechanical properties and the microstructural design of the ZrO2, considering that the
grain size and the boundaries affect the toughening mechanism.
Nightingale and Dunne (1996) studied the density and grain growth of 3Y-TZP
sintered in conventional and microwave ovens. Trunec (2008) studied the effect of grain
size on the mechanical properties of 3Y-TZP ceramics. He suggested the larger the grain
size the greater the toughness of the ceramic material. Maximum toughness lies near a
critical grain size which varies from 1-6µm depending on the material. Further increase
of the grain size may lead to a spontaneous tetragonal to monoclinic transformation. In
contrast to the fracture toughness, the strength of 3Y-TZP usually reaches its maximum at
smaller grain size. Nightingale and Dunne (1996) mentioned a number of potential
advantages in sintering 3Y-TZP in microwave ovens. The authors suggested that, in
ceramics, it is desirable to achieve maximum density while maintaining minimum grain
growth. The sintering temperature and sintering time partially affect grain growth and
32
density of the final product. Microwave sintering is expected to achieve shortened
sintering time, higher densification, improved production rates and controlled
microstructure. They found that microwave sintered specimens were denser than
conventionally sintered specimens. This difference was eliminated when 96% densities
were achieved. They also noticed that grain growth in microwave sintered specimens was
accelerated once densification was near completion and after aging of the specimens in
1500°C. They concluded that microwave sintering produces high density ZrO2 material
but suggested that sintering should be controlled to avoid extensive grain growth. Trunec
(2008) found that the fracture toughness of ZrO2 was almost constant at 5.1 MPa√m for a
grain size of up to 0.4µm. The maximum fracture toughness of 7.8 MPa√m was achieved
for a grain size of 1.8µm.
Wheeler and Peralta (2010) compared the flexural strength of ZrO2 based material
sintered in microwave and conventional ovens. They suggested ZrO2 material is prone to
aging effect which can progressively degrade its strength. Due to this they studied the
flexural strength of ZrO2 samples exposed to hydrothermal ageing (75 hours in airsteamed environment at 125 °C and 200 Kpa pressure). Specimens were cut, with a
diamond wafering saw, from three types of partially sintered ZrO2 blocks {Crystal HS
(HS= High Strength ZrO2), Crystal HT (HT= High Translucency- different composition
that enhance the translucency of the ZrO2, according to the manufacturer) and Lava 3M}.
The specimens’ cross sectional dimensions were 3x3 mm, to simulate the connector size
of a fixed partial denture and three point bend test was performed according to
ANSI/ADA specification No 69 for dental ceramics. The flexural strength of the Crystal
HS ZrO2 specimens sintered in a microwave oven was 8% higher than the flexural
33
strength of the same material sintered in a conventional oven. Considering the small
standard deviation of the results this was a statistically significant difference. After 75
hours of hydrothermal degradation, the flexural strength of the ZrO2 material sintered in a
conventional oven was reduced 43% compared to a 14% reduction in microwave sintered
specimens. The flexural strength of the Crystal HT ZrO2 material was lower than the
flexural strength of Crystal HS ZrO2 material. The microwave sintered Crystal HT ZrO2
had 17% higher flexural strength than the conventionally sintered Crystal HT ZrO2.
When they compared the flexural strength of the Crystal HS ZrO2 material to the Lava
3M, results were identical. They observed a smaller grain size in the microwave sintered
samples and suggested that this could be the reason why the microwave samples were
less affected by hydrothermal degradation than conventionally sintered specimens.
Chen et al. (2006) investigated the feasibility and reliability of sintering Al2O3
and ZrO2 powder in a microwave oven. They used 5 disc specimens for each group and
maintained the microwave temperature at 1600°C for 10 minutes. They examined, using
X-ray diffraction and scanning electron microscopy, the phase composition, grain size
and microstructure of their samples. They observed that the monoclinic ZrO2 had been
transformed to tetragonal and there was no change in the crystal phase of the Al2O3
samples. They concluded that microwave sintering could reduce the sintering time, and
could also provide ceramics with superfine grain size and favorable microstructure.
Vaderhobli and Saha (2007) evaluated the feasibility of sintering dental ceramic
crowns and compared the properties of ZrO2 crowns sintered using the two different
techniques. Fourteen ZrO2 substructures were layered with glass porcelain and sintered in
microwave and conventional ovens at a temperature of 800°C. Their density, indentation
34
hardness, toughness, microstructure and modulus of rupture were compared. They
noticed that the microwave samples had uniform grains and less voids. Based on the
calculations of the fracture toughness and the hardness of these materials, they suggested
that the microwave technique could provide ceramics with improved mechanical
properties as well as energy savings.
The increasing demand for all-ceramic restorations with ZrO2 substructure, the
introduction of microwave ovens for sintering of ZrO2 and the lack of controlled studies
that can predict the performance of microwave sintered restorations suggests the
requirement for assiduous investigation. There is a need to validate that microwave
sintering will produce ZrO2 with similar or improved physical properties when compared
to conventional sintering.
35
CHAPTER III
MATERIALS AND METHODS
The fabrication of a ZrO2 substructure for all-ceramic restorations requires a
sintering process for the ZrO2 framework, which takes place in conventional ovens.
Microwave sintering is an alternative technique to oven sintered ZrO2. Specific benefits
can be derived from this technique in terms of shortened sintering time, improved
mechanical properties, higher densification, improved production rates and lower energy
requirements (Upadhyaya et al, 2001; Chen et al, 2006). This study compared the fracture
toughness of 3Y-TZP sintered in a conventional furnace and the same material sintered in
a microwave sintering oven. The fracture toughness of the specimens made by three
different manufacturers, sintered in conventional and microwave ovens and stored in
artificial saliva, were compared.
Pilot Study
A pilot study was performed in order to evaluate the feasibility of the experiment,
to identify possible difficulties in creating and testing the samples and to determine
sample size. Ten 3Y-TZP KaVo specimens (KaVo Dental GmbH) were milled in a KaVo
Everest engine (KaVo Dental GmbH). The specimen dimensions followed ISO standards
for dental ceramics (ISO 6872, 2008). A uniform 1mm deep notch was made in the
specimens before final sintering. Five specimens were sintered in the KaVo Everest
Therm 4180 (KaVo Dental GmbH) conventional oven, and five specimens in the
MicroSinterWave A1614 (DLMS, Scottsdale, AZ) microwave oven. The sintering time
followed the recommendations of each manufacturer (10 hours sintering in the
conventional oven with a maximum temperature of 1450°C for 2 hours, and 2 hours
36
sintering in the microwave oven with a maximum temperature of 1520°C for 35
minutes). Specimens were stored in ambient temperature before the facture toughness
test. The thickness and the width of each specimen were measured with a digital caliper
(Mitutoyo 500-196-20, Hauppauge, NY). Specimens were tested for fracture toughness
using a four-point bend test and the load to fracture was recorded using a Zwick 1445
Universal Testing Machine (Zwick DmbH & Co.KG, Ulm, Germany). The depths of the
V-shaped notches were measured using SEM (Amray 1820D, Beldford, MA) at 50x. The
fracture toughness values were calculated according to the equation described by the ISO
standard for dental ceramics (ISO 6872, 2008). A two-sample t-test was used to compare
the fracture toughness between the two types of sintering (Alpha=0.05).
According to the results the mean fracture toughness of microwave sintering
(4.63±0.29 MPa√m) was not significantly greater than conventional sintering (4.48± 0.21
MPa√m) (p=0.3732), (Table 2). The power analysis revealed that a sample size of 85-110
per group would have 90% or higher power to detect any differences between the two
groups assuming that the common standard deviation was 0.30 and effect size was 0.50
using a two sample t-test with a 0.05 two-sided significance level.
Table 2. Descriptive Statistics of the Fracture Toughness (MPa√m) of ZrO2 Specimens
N
Mean
Std Dev
Minimum
Conventional Sintering
5
4.48
0.21
4.26
4.78
Microwave Sintering
5
4.63
0.29
4.21
4.89
Variable
Maximum
37
Main study - Sample Fabrication, Number, Size and Design
Partially sintered ZrO2 blocks (42x20x16 mm) were obtained from three
manufacturers, KaVo (KaVo Dental GmbH), Lava 3M (3M ESPE) and Crystal HS
(DLMS). Twenty-four ZrO2 blocks from each company, measuring 42x20x16 mm, were
placed in the positioning aid of a KaVo Milling Engine (KaVo Dental GmbH) (Fig 2).
The Everest Universal Inplast components A and B (KaVo Dental GmbH) were mixed
according to manufacturer’s directions and poured in the positioning aid around the ZrO2
block.
Figure 2. Partially sintered ZrO2 block placed in the positioning aid.
38
The mounting material was left to set for a minimum of 30 minutes. The
positioning aid was secured with the two screws in the Everest yoke.
oke. KaVo (KaVo
Dental GmbH) manufacturer provided the CAM strategy for milling three rectangular
specimens from each ZrO2 block. According to ISO standards for dental ceramics,
depending on the apparatus of the fracture toughness test
test, the specimens should have the
following dimensions: width w= 4.0mm ± 0.2mm, thickness b=1.2mm to 3.0mm ±
0.2mm (with 3mm recommended), chamfer c=0.09mm to 0.15mm and length at least
2mm longer than the support span used for the test (ISO 6872, 2008). The software
strategy provided by KaVo (KaVo Dental GmbH) calculated the dimensions of the
specimens, considering that the final sintering would create 20% shrinkage of the
partially sintered specimens. The expected dimensions after the final sintering for each
ea
beam specimen were w= 4mm, b=
b=3mm,
3mm, c= 0.12mm and the length was 32 mm (Fig 3).
Figure 3. Specimens’ expected dimensions after sintering.
39
Figure 3. continued.
The milling process consisted of two stages. In the first stage, the milling engine
milled half the thickness of the specimens. The recommended milling tools
tool (Everest
Milling Pin ZS1 and ZS3; KaVo (KaVo Dental GmbH) for milling partial sintered ZrO2
were used. One set of tools was used for every 12 ZrO2 blocks. Everest ZS Inwax (KaVo
Dental GmbH) was melted in a small pot and poured over the half milled specimens
inside the positioning aid without removing it from the yoke. The material was left to set
for 30 minutes and the milling was completed. The positioning aid was then taken out
ou of
the yoke (Fig 4). The Inwax was softened using an industrial heat gun until the specimens
became loose in the wax. The specimens were removed from the positioning aid and the
same process was continued with the rest of the ZrO2 blocks.
The specimens were
ere placed in a square base, made of type IV stone (Whip Mix
Corporation, Louisville, KY). The specimens were secured on one of the flat surfaces
with sticky wax and tape. The stone base was mounted in a Microparallelometer milling
machine (MP 3000; Metalo
Metalorr Technologies SA, Neuchatel, Switzerland). An initial
straight notch was created with a diamond disc (Brasseler, Savannah, GA), (Fig 5). The
depth of the notch was 1mm.
40
Figure 4. Pre-sintered ZrO2 milled specimens.
Figure 5. Straight notch created by diamond disc.
41
The positioning table was tilted upward and downward as much as the table could
be tilted and the diamond disc was used to open the single notch and create a V-shaped
notch (Fig 6). Attention was given not to change the initial depth of the single notch. A
razor blade was introduced into the starter notch. Light force was applied, with gently
back and forward motion as straight as possible, in order to polish and create a uniform
notch. Following the ISO standards for dental ceramics, the overall goal was to create a
predictable depth V-shaped notch in the range of 0.8mm to 1.2mm deep, after final
sintering.
Figure 6. Fabrication of V-shaped notch in the microparallelometer milling machine.
42
The specimens from each manufacturer were randomly assigned into two groups
representing the two different sintering methods. Six groups of specimens were created (3
manufacturers, 2 sintering methods), (Table 3).
Table 3. Sintering Protocol for ZrO2 Specimens
Manufacturers
Conventional sintering
Microwave sintering
Group 1:
Group 4:
KaVo Everest Therm 4180
MicroSinterWave A1614
10 hours
2 hours
1450°C for 2 hours
1520°C for 35 minutes
Group 2:
Group 5:
3M ESPE's Lava Furnace 200
MicroSinterWave A1614
Total Sintering Time
8 hours 30 minutes
2 hours
Maximum Temperature
1500°C for 2 hours
1520°C for 35 minutes
Group 3:
Group 6:
Zircar Hot Spot 110
MicroSinterWave A1614
10 hours
2 hours
1510°C for 2 hours
1520°C for 35 minutes
KaVo
Sintering Oven
Total Sintering Time
Maximum Temperature
Lava 3M
Sintering Oven
Crystal HS
Sintering Oven
Total Sintering Time
Maximum Temperature
43
Sintering Technique
The specimens that were sintered in conventional ovens followed the
specifications of their manufacturers, including using sintering ovens that were suggested
by the manufacturers. Group 1 was sintered in a KaVo Everest Therm 4180 (KaVo
Dental GmbH), group 2 was sintered in a Lava Furnace 200 (3M ESPE) and group 3 was
sintered in a Hot Spot 110 (Zircar Zirconia, Florida, NY). Groups 4-6 were sintered in a
MicroSinterWave A1614 (DLMS). Sintering cycles are described in (Table 3). During
the sintering process five specimens of the Crystal HS ZrO2 were heavily distorted and
fractured in areas other than the V-shaped notch (2 from Group 3; conventional oven and
3 from group 6; microwave oven), (Fig 7). All broken specimens were discarded and an
equal number of specimens were randomly discarded from the remaining groups in order
to maintain equal numbers per test group.
Figure 7. Heavily distorted and fractured specimens.
44
Specimens Storage
The samples were stored for 10 days in containers with artificial saliva solution
before final testing was conducted. The artificial saliva was fabricated as described by
Birkeland in 1973. For fabrication of 1 liter artificial saliva, 1.6802g of NaHCO3,
0.41397g of NaH2PO4.H2O and 0.11099g CaCl2 were mixed with 1 liter of double
distilled water. The pH of the final solution was 6.97.
Testing Method
The thickness, b, and width, w, of each specimen were measured with a
micrometer (Mitutoyo 500-196-20, Hauppauge, NY) capable of measuring to two
decimal places. The specimens were tested for fracture toughness using four–point
bending test. The four-point bending apparatus followed recommendations of the ISO
standard for dental ceramics (ISO 6872, 2008). It consisted of four 5 mm diameter
stainless steel rollers. The supporting rollers were placed 24 mm apart and the loading
rollers were positioned on the top of the specimen, 12 mm apart. The centers between the
two supporting and the two loading rollers were the same (Fig 8). This ensured that equal
forces were applied to the loading rollers and that torsional loading was minimized. The 3
mm width face, with the V-notch, was placed down and the middle of the specimen was
positioned in the center. The specimens were loaded to failure with a crosshead speed of
0.5 mm/min at room temperature in air. The fracture load was recorded using a Zwick
1445 Universal Testing Machine (Zwick DmbH & Co.KG). The specimens were
evaluated under 10x magnification to assure that the fracture started at the bottom of the
V-shaped notch and continued over its entire length (if this was not the case the specimen
45
had to be discarded), (ISO 6872 2008). All the specimens had fractured in the notch and
no specimen was discarded at this point.
Figure 8. Four-Point bending Test (Zwick 1445 Universal Testing Machine).
46
The right part of each sample was chosen arbitrarily, considering the written side
of each specimen as the frontal side and the side of the notch of each sample as the
inferior side. These samples were mounted in stubs using Zapit acrylic glue (Dental
Ventures of America, Corona, CA). They were positioned in a Balzers SCD 040 sputter
coater machine (Balzers Union, Balzers, Liechenstein). The specimens were coated with
gold palladium (AuPd) at a working distance of 35mm. The sputtering time was 2
minutes and the expected coat had 25nm thickness. The ZrO2 specimens were examined
in an 1820D Amray Scanning Electron Microscope (Amray, Beldford, MA). Photos were
captured at 50x magnification and the depth of the V-notch of each sample was
calculated using Image Pro Software (Media Cybernetics, Bethesda, MD). Three
measurements were taken for each sample (Fig 9).
Figure 9. Locations of three measurements used to calculate notch depth in SEM image.
47
The fracture toughness was calculated according to the equation described by the
ISO standards (ISO 6872, 2008), (Fig 10).
Kic
is the fracture toughness in megapascals
S1, S2 are the support spands (S1>S2) in meters
by square foot meter
Y
is the stress intensity shape factor
σ
is the fracture strength in megapascals
a
is the average notch depth in meters
P
is the fracture load in meganewtons
α
is the relative V-notch depth
b
is the specimen’s thickness in meters
w
is the specimen’s width in meters.
Figure 10. Fracture toughness equations, ISO standard 6872 for ceramic materials.
Statistical Analysis
Mean fracture toughness was calculated for each subgroup as described above. A
two-way ANOVA was performed to detect significant main effects on the type of
sintering, type of manufacturer, and their interaction on fracture toughness. A main effect
48
determines whether or not that variable displays a significant main effect, if a given
predictor variable is not involved in any significant interaction. This means that there is a
difference between at least two of the levels of that predictor with respect to the criterion
variable (in this study the fracture toughness). An interaction effect is a change in the
simple effect of one variable over levels of the second factor. Simple effects are the
effects of one factor at the level of the other factor. When an interaction is significant, the
simple effects rather than the main effects are normally tested and interpreted.
A two-sample t-test was used to test differences in fracture toughness between
two types of sintering for each manufacturer. Additionally, one-way ANOVA with posthoc Tukey’s HSD (Honestly Significant Difference) was used to test differences among
different manufacturers within each type of sintering. In order to use the ANOVA, an
underlying assumption, is the normality of the residuals. The normality of residuals was
checked using a non significant Shapiro-Wilk test and normal probability plots.
All tests employed a 0.05 level of statistical significance. Statistical analyses were
carried out with the statistical package SAS® System version 9.1(SAS Institute Inc, Cary,
NC, USA).
49
CHAPTER IV
RESULTS
Ninety six specimens were randomly and equally divided into six treatment
groups (n=16 per group). Raw data is presented in Appendix 1. Table 4 displays the
descriptive statistics of fracture toughness by type of sintering and manufacturer.
Table 4. Descriptive Statistics of Fracture Toughness (MPa√m) by Sintering Types and
Manufacturer
Manufacturer
N
Mean
Std Dev
Minimum
Maximum
Conventional Sintering
KaVo
16
5.78
1.39
3.64
7.90
Lava 3M
16
5.21
0.52
4.47
6.13
Crystal HS
16
4.91
0.71
4.10
6.25
Microwave Sintering
KaVo
16
5.93
1.22
3.99
8.25
Lava 3M
16
5.17
0.42
4.43
5.85
Crystal HS
16
4.97
0.63
3.73
5.85
50
Interaction Between Sintering Type and Manufacturer
The normality of residuals was checked with a non significant Shapiro-Wilk test
and normal probability plots. Since the assumption of normality was valid, a two-way
ANOVA was performed to assess the effects of type of sintering and type of
manufacturer on the fracture toughness, including their interaction. This analysis revealed
a significant main effect on the type of manufacturer (p=0.0003), (Table 5).
Table 5. Result of Two-Way ANOVA for the Fracture Toughness
Source
d.f.
SS
MS
F
P value
Type of Sintering
1
0.07
0.07
0.09
0.7612
Manufacturer
2
14.28
7.14
8.94
0.0003
2
0.14
0.07
0.09
0.9174
Within Groups
90
71.90
0.80
Total
95
86.39
Interaction between
type of sintering
and manufacturer
d.f.: degrees of freedom; SS: sum of squares; MS: mean square; N=96
The post-hoc Tukey’s HSD indicated that the mean fracture toughness observed
in the KaVo group was significantly greater than those observed for the other two
51
manufacturers, and no significant difference was found between Lava 3M and Crystal HS
(Table 6). The main effect of the type of sintering proved to be non significant
(p=0.7612) (Table 7), and the interaction between the type of sintering and the type of
manufacturer proved to be non significant (p=0.9174) (Table 5). The results of the twoway ANOVA are summarized in tables 5-7.
Table 6. Mean Fracture Toughness (MPa√m) by Manufacturer
Manufacturer
N
Mean Fracture Toughness (SD)
KaVo
32
5.85 (1.29)1
Lava 3M
32
5.19 (0.47)2
Crystal HS
32
4.94 (0.66)2
Means with the same number are not significantly different using post-hoc Tukey’s HSD
test (p>.05).
Table 7. Mean Fracture Toughness (MPa√m) by Type of Sintering
Type of Sintering
N
Mean Fracture Toughness (SD)
Microwave Sintering
48
5.36 (0.92)1
Conventional Sintering
48
5.30 (1.00)1
Means with the same number are not significantly different using a two-sample t-test
(p>.05).
52
Differences Between Sintering Type Within Each
Manufacturer
The normality of residuals was checked with a non significant Shapiro-Wilk test
and normal probability plots. Since the assumption of normality was valid under each
condition, a two sample t-test was used to test the differences in fracture toughness
between the two types of sintering for each manufacturer (Table 8).
Table 8. Mean Fracture Toughness (MPa√m) of ZrO2 Specimens
Conventional Sintering Mean
Microwave Sintering Mean
Fracture Toughness (SD)
Fracture Toughness (SD)
KaVo
Group 1:
5.78(1.39)1
Group 4:
5.93(1.22)1
Lava 3M
Group 2:
5.21(0.52)1,2
Group 5:
5.17(0.42)2
Crystal HS
Group 3:
4.91(0.71)2
Group 6:
4.97(0.63)2
Manufacturer
Row means are not significantly different using two-sample t-test (P>.05).
Column means with the same number are not significantly different using post-hoc
Tukey’s HSD test (P>.05).
The results of the two sample t-test revealed there was no significant statistical
effect of the type of sintering on the fracture toughness for each manufacturer; KaVo
(p=0.7524), Lava 3M (p=0.8224) and Crystal HS (p=0.8115). No significant difference
was observed between conventional and microwave sintering within each manufacturer.
53
Differences Among Three Manufacturers Within Each
Sintering Type
One-way ANOVA followed by the post-hoc Tukey’s HSD was used to test
differences between the types of manufacturers within each sintering type. The ShapiroWilk test was used to verify normality of residuals. Table 8 presents the results of posthoc Tukey’s HSD tests.
A. Conventional Sintering
The results of the one-way ANOVA revealed a significant effect of the type of
manufacturers on the fracture toughness with conventional sintering (p=0.0404). The
post-hoc Tukey’s HSD test indicated that the mean fracture toughness observed in KaVo
specimens was significantly greater than what was observed in Crystal HS specimens.
Moreover, no significant differences were observed between KaVo and Lava 3M, and
between Lava 3M and Crystal HS (Table 8).
B. Microwave Sintering
Results of one-way ANOVA revealed a significant effect for type of
manufacturers on the fracture toughness with microwave sintering (p=0.0053). The posthoc Tukey’s HSD test indicated that the mean fracture toughness observed in KaVo
specimens was significantly greater than observed in Lava 3M and Crystal HS
specimens, and no significant difference was observed between Lava 3M and Crystal HS
(Table 8).
In summary, based on the results and the statistical analysis, the mean fracture
toughness of the ZrO2 specimens sintered in conventional oven was 5.30 MPa√m and
those sintered in microwave oven was 5.36 MPa√m. There was no statistical significant
54
difference based on the type of sintering (p>0.5). The mean fracture toughness of KaVo
specimens was 5.85 MPa√m, of Lava 3M specimens was 5.19 MPa√m and of Crystal HS
specimens was 4.94 MPa√m. There was a statistical significant difference between KaVo
specimens and the other two manufacturer’s specimens (p=0.0003). However, no
significant difference was found between Lava 3M and Crystal HS (p>0.5). The
interaction between the type of sintering and the type of manufacturer proved to be non
significant (p=0.9174).
55
CHAPTER V
DISCUSSION
The physical properties of 3Y-TZP materials can be affected by numerous factors
such as the composition of the ZrO2 product, sintering cycle, microstructure and
morphology, and the fabrication procedures (Nightingale and Dunne 1996; Nightingale,
Worner and Dunne 1997; Piconi and Maccauro, 1999; Laberty et al., 2003). In this study,
pre-sintered materials from three different manufacturers were evaluated and the
respective manufacturers’ recommendations followed for conventional post-milling
sintering. The microwave sintering was performed according to the protocol that the
microwave sintering manufacturer provided. All specimens were prepared under the
same conditions and milled in one KaVo milling engine with the same software build.
This created identical specimens for each manufacturer. No significant difference was
observed in fracture toughness of the ZrO2 specimens sintered in conventional (5.30
±1.00 Mpa√m) vs. microwave ovens (5.36 ±0.92 Mpa√m, p=0.7612) and there was no
interaction between sintering and ZrO2 manufacturer (p=0.9174).
Upadhyaya et al. (2001) studied sintering and grain growth of 3Y-TZP as well as
3Y-TZP with TiO2 and 3Y-TZP with MnO2. The rationale for adding TiO2 or MnO2 was
to improve the microwave coupling at a given temperature. Fracture toughness was
calculated from the radial crack using the procedure described by Anstis et al. (1981).
The fracture toughness was 8.7 MPa√m for the 3Y-TZP sintered in conventional oven,
8.6 MPa√m for the same material sintered in microwave oven, 4.3 MPa√m for 3Y-TZP
with MnO2 sintered in microwave oven and 4.8 MPa√m for the 3Y-TZP with TiO2
sintered in microwave oven. It was concluded that microwave sintering had a number of
56
benefits in mechanical properties and microstructural design, but the results were similar
for both techniques. However, the addition of TiO2 or MnO2 considerably reduced the
properties of the ZrO2 material. Upadhyaya et al. (2001) measured considerably higher
fracture toughness for the 3Y-TZP than those calculated in this study and this could be
due to different ZrO2 composition, sintering cycles or technique for determining the
fracture toughness.
Vaderhobli and Saha (2007) compared ZrO2 sintered in conventional and
microwave ovens and calculated fracture toughness. It was concluded that the microwave
technique provided ceramics with improved mechanical properties. Unfortunately, the
materials and methods of this study were only summarized in a 2007 IADR abstract.
Sixty ZrO2 cylinders and 14 dental copings layered with glass ceramic were tested. The
ZrO2 cylinders were sintered at different temperatures ranging from 1100 °C to 1450 °C.
The dental copings were layered with glass ceramics and sintered in the microwave and
conventional furnace at 800°C. Their fracture toughness was 2.26±0.8 MPa√m. The
interesting part of the study is that they utilized ZrO2 substructure layered with glass
ceramic which makes the study more clinically relevant. However, the detailed procedure
of the study was not described.
The ZrO2 materials used for fabrication of all-ceramic restorations and for
fracture toughness tests have different compositions. Many of the studies included
experimental products in addition to products from established manufacturers. This could
be due to the fact that manufacturers are still developing the ZrO2 material in order to
achieve maximum physical properties and minimize complications. Therefore, an ideal
ZrO2 product may not yet exist. Yilmaz, Aydin and Gul (2007) determined the mean
57
fracture toughness of Cercon ZrO2 as 6.27 MPa√m. Lazar et al. (2008) determined that
the fracture toughness of 3Y-TZP (experimental material) was 6.0 MPa√m. Guazzato et
al. (2004b) calculated the fracture toughness of an experimental Y-TZP as 5.5 MPa√m
and for DC Zirkon as 7.4 MPa√m. Studies performed by different authors may not be
directly comparable. However, based on the studies that evaluated materials prepared and
tested under the same conditions, it is evident that differences in the composition of ZrO2
affected the physical properties of the final product.
Three different manufacturers of ZrO2 were evaluated in the present study. KaVo
and Lava 3M were chosen because they are two established suppliers of ZrO2 based
dental ceramics. Crystal HS is partially sintered ZrO2 produced by DLMS which
manufactures the microwave sintering oven. When comparing the mean fracture
toughness of these ZrO2 materials with conventional sintering, there was no significant
difference between KaVo and Lava 3M, and between Lava 3M and Crystal HS. There
was statistically greater mean fracture toughness for the KaVo ZrO2 compared to the
Crystal HS. For microwave sintering, the mean fracture toughness of KaVo ZrO2 was
significantly greater than for Lava 3M and Crystal HS ZrO2, which were not statistically
different from each other. Even though the KaVo ZrO2 had a higher mean fracture
toughness, it is important to emphasize that the ZrO2 material from KaVo and Lava 3M
were not formulated specifically for microwave sintering, as was the Crystal HS. The
reasons for the difference in mean fracture toughness of the materials tested is beyond the
scope of this investigation.
Wilson and Kunz (1988) evaluated microwave sintering of ZrO2 specimens. The
authors followed different heating rates and sintering times for each specimen and
58
concluded that ultra rapid heating (less than 600 seconds) caused cracking of the ZrO2
material. Similar properties were suggested for conventionally and microwave sintered
material. The fracture toughness of their microwave sintered specimens ranged from 4.55
to 4.99 MPa√m (Wilson and Kunz, 1988). In the present study, five specimens of the
Crystal HS ZrO2 were heavily distorted and fractured during the sintering process (2 from
Group 3; conventional oven and 3 from group 6; microwave oven), (Fig 7). The reason
for the catastrophic failure in sintering of these specimens is unknown. It could be
attributed to the sintering cycle, the sintering oven that was utilized for those groups,
internal voids, thermal shock related to the doping within the ZrO2 structure and/or
localized regions of internal transformation (t-m) leading to internal critical size defect,
the composition of the material or interaction among these factors. Sutton (1992) in his
overview study for microwave sintering of ceramics suggested that ceramic materials
with high purity in ZrO2 are difficult to initially heat with microwaves and are prone to
cracking. Therefore, it appears that either the formulation of the Crystal HS ZrO2 or the
microwave sintering process may need further refinement in order to provide a reliable
method to fabricate dental prostheses.
This study focused on the fracture toughness of three types of ZrO2 material. The
literature suggests several methods for measuring the fracture toughness of ceramic
materials including the single edge precracked beam, a surface crack in flexure or a
chevron notched beam tests (ISO 6872, 2008). Chantikul et al. (1981) suggested a
method for determining the fracture toughness of ceramics using an indentation/strength
technique. Specimen dimension and shape differ in different studies depending on the
protocol and testing specifications of each study. Wilson and Kunz (1988) utilized
59
25x6x3mm specimens. Guazzato et al (2004c) used bar specimens with dimensions
20x3x4mm. Lazar et al. (2008) created 6x5x5mm blocks in order to calculate the mean
fracture toughness. Taskonak et al. (2008) used the 1995 ISO standard to fabricate
specimens and measure strength and fracture toughness. The crack initiation flaws were
measured from SEM images in order to determine the fracture toughness of the material
(Taskonak et al., 2008). The 2008 ISO standards for dental ceramics recommended the
single edge V-notch beam method to calculate the fracture toughness of the ceramic
materials (ISO 6872, 2008). This study was done according to the ISO standards because
it was the most recent and systematic method for testing the properties of dental
ceramics.
The effect of sintering technique and manufacturer on the physical properties of
the material could be further studied with an analysis of the specimens’ microstructure.
Nightingale and Dunne (1996) used the Archimedes’ method to evaluate the density of
their specimens and electron microscopy to measure the grain growth of their samples.
The Archimedes’ method determines the density of an object by measuring its mass and
its volume. They concluded that for densities lower than 96%, the density of microwave
sintered specimens was significantly higher than the conventional sintered specimens.
The densification difference disappeared at higher densities due to continued grain
growth. They suggested that microwave sintering should be controlled to restrict grain
growth and ensure the desired microstructure. Other studies used X-ray diffraction to
examine the phase composition, the grain size and the density of the material
(Nightingale, Worner and Dunne 1997; Laberty et al., 2003; Chen et al., 2006). Chen et
al. (2006) examined the phase composition, grain size and microstructure of microwave
60
sintered ZrO2 using X-ray diffraction and scanning electron microscopy. They concluded
that microwave sintering reduced the sintering time and provided ceramics with superfine
grain size and favorable microstructure. Vaderhobli and Saha (2007) suggested that
microwave samples had uniform grains and less voids. Wheeler and Peralta (2010)
observed smaller grain size in the microwave sintered samples and suggested this as one
of the reasons that the microwave samples were less affected by hydrothermal
degradation than the conventional sintered specimens. The present investigation did not
evaluate the sintered ZrO2 microstructure.
Rekow and Thompson (2007) emphasized that factors other than physical
properties, such as prosthesis design, fatigue, especially in the oral environment, and the
fabrication techniques, could be detrimental to the success of advanced ceramics. These
factors are critical and can cause catastrophic failures despite the significant improvement
in material properties and toughening mechanisms. Guazzato et al (2004c) investigated
strength, reliability and mode of fracture of bilayered porcelain ZrO2 core ceramics. The
authors emphasized restoration design and the actual distribution of tensile stress. They
suggested that the advantages of the stronger core materials may be offset by the weaker
veneering porcelain (though radial crack growth) if the design of the restoration does not
take into account the stress distribution. There is some controversy with this hypothesis
with an alternative theory of subsurface microscopic voids as a nidus for the early bilayer
ceramic failure mode (R. Kelly, personal communication, 3/2011). Wheeler and Peralta
(2010) compared the flexural strength of three ZrO2 based materials sintered in
microwave and conventional ovens, before and after hydrothermal aging. After 75 hours
of hydrothermal degradation, the flexural strength of the ZrO2 material sintered in a
61
conventional oven was reduced 43% compared to a 14% reduction in the strength of the
microwave sintered specimens. They concluded that with both types of sintering the
hydrothermal degradation would reduce the flexural strength of the ZrO2 material, but the
reduction would be considerably less in microwave sintered specimens. When they
compared the flexural strength of the Crystal HS ZrO2 material to the Lava 3M, the
results were identical. The present study also found no significant difference in the
fracture toughness of Crystal HS and Lava 3M sintered with either conventional or
microwave techniques. Samples were stored in artificial saliva for 10 days, however no
other aging process was incorporated and the design of the specimens did not reflect the
exact clinical conditions since they did not simulate the design of a restoration and were
not layered with feldspathic porcelain.
All the available studies that compared microwave sintering of dental ceramics to
conventional sintering were in vitro studies. The lack of consensus in the literature
regarding ZrO2 materials, sintering process and testing methodology, makes direct
comparisons of the results of this study with other published studies difficult. Even so,
the existing studies suggest that microwave sintering may be a feasible technique and it
can produce ZrO2 with comparable properties to conventionally sintered ZrO2. Critically,
the role of fatigue testing in an aqueous environment with follow-up x-ray diffraction
analysis is needed to assess if the transformation toughened state (t-m) is a symmetric or
asymmetric distribution on the surface of the final ceramic surface.
Limitations of the Study
This study evaluated the fracture toughness of 3Y-TZP sintered in conventional
and microwave ovens based on the recently published ISO specifications for dental
62
ceramics (ISO 6872, 2008). Although this testing methodology was the latest available
for evaluating the performance of ceramic materials, there was no evidence that this
methodology was the most appropriate to test the fracture toughness of polycrystalline
ceramics. While fracture toughness is one of the mechanical properties of the ceramic
materials, there are other important properties (i.e. flexural strength) that were not
examined in this study. The fabrication of the specimens was very consistent, due to the
high accuracy of the CAD/CAM engine, but the depth of the V- shaped notch was not as
consistent as the rest of the dimensions because it was fabricated manually.
Yet, the standard deviations obtained (an indirect measure of fabrication variation
specimen to specimen) were relatively small suggesting the notch formation was not a
major source for the observed variation. The number of the specimens used was 48 for
each sintering method. This was half the number of the specimens that was indicated
from the pilot study and the power analysis (90-110 samples). Fewer specimens were
utilized due to discarded samples and the limitations in acquiring more material for the
study. The shape of the examined specimens did not represent the shape of a dental
prosthesis. In addition, the performance of the ZrO2 material could be affected by
veneering and cementation procedures that were not included in this test. The main
limitation is the fact that this study examined the performance of a material that functions
in a multi-dynamic oral environment using a static test.
Clinical Relevance
The first step to evaluate a dental material or technique is to use in vitro studies to
determine the mechanical properties of the material. Further prediction of the clinical
performance of a restorative material or technique requires fatigue loading, aging and
63
conditions that simulate the oral environment. Anusavice, Kakar and Ferree (2007)
concluded that a combination of tests was necessary to predict the performance of
ceramic based materials. Rekow and Thompson (2007) also stated that there was too
much uncertainty when trying to correlate in vitro test data to clinical performance.
Therefore, the results of this investigation should be carefully applied to clinical
decisions.
Clinical studies follow in vitro studies in the evaluation of the performance of a
material or a technique. In the literature, there were no clinical studies comparing
conventional and microwave sintering techniques. In fact, there were very few studies
evaluating survival rates of all-ceramic restorations and most of them were short-term (25 years), (Sailer et al., 2007; Örtorp, Kihl and Carlsson, 2009; Roediger et al., 2010). A
possible reason for this may be that the all-ceramic materials have been recently
introduced to clinical practice. Manufacturers are also inducing practitioners to utilize
their new products and techniques before long term data of clinical studies are available.
During this process, manufacturers are modifying their products in order to improve their
performance. This creates a reality that many of the available products or techniques have
been modified and the studies that were initiated a few years earlier do not represent
currently available products. Yet, critical evaluation of existing studies is important for
stating conclusive results. For instance, the survival rate of a ZrO2 framework will not
represent a high clinical success rate if this is accompanied by complications of the
overlaying feldspathic porcelain that can adversely affect function and esthetics of the
restoration. As a result, the extrapolation of data that will determine use of these materials
64
or techniques is a complicated procedure that requires long-term randomized controlled
prospective clinical studies.
Avenues for Future Research
Microwave technology for sintering ZrO2 has been introduced in the dental
laboratory procedures. However, many aspects of this process have not been studied
thoroughly. It is critical to examine the bonding of the veneering porcelain to the
microwave sintered ZrO2 substructure. The fitting and the marginal discrepancy of ZrO2
frameworks sintered in microwave oven have not been evaluated. The performance of the
ZrO2 material under different sintering cycles needs to be investigated, as well. The
evaluation of the microstructure of the microwave sintered ZrO2 will provide evidence of
potential advantages or differences from the conventionally sintered ZrO2. It is
imperative to confirm the presence of a stable tetragonal phase following microwave
sintering. Further investigation of elements that could be incorporated in the material and
would increase the ability of the ZrO2 to absorb microwave energy in ambient
temperature may simplify the sintering process. The establishment of microwave
sintering technique, for fabricating ZrO2 prostheses, requires in vitro studies combined
with long term prospective clinical investigations. This will ascertain the viability and the
predictability of microwave sintering.
65
CHAPTER VI
CONCLUSIONS
Considering the limitations of this study the following conclusions were drawn:
1. There was no significant statistical difference in the mean fracture toughness of
3Y-TZP sintered in microwave or conventional ovens. The Null Hypothesis Ho {1} was
accepted.
2. For conventional sintering, there was no significant statistical difference in the
mean fracture toughness between KaVo and Lava 3M, and between Lava 3M and Crystal
HS ZrO2 materials. There was statistically significant greater mean fracture toughness for
the KaVo compared to Crystal HS ZrO2. The Null Hypothesis Ho {2} was rejected and
the Alternative Hypothesis Ha {2} was accepted.
3. For microwave sintering, the mean fracture toughness of KaVo ZrO2 material
was significantly greater than for Lava 3M and Crystal HS ZrO2. There was no statistical
significant difference between Lava 3M and Crystal HS ZrO2. The Null Hypothesis Ho
{2} was rejected and the Alternative Hypothesis Ha {2} was accepted.
4. The interaction between type of sintering and type of manufacturer proved to
be non significant. The Null Hypothesis (Ho) {3} was accepted.
66
APPENDIX
Raw Data
ID
1kc
2kc
3kc
4kc
5kc
6kc
7kc
8kc
9kc
10kc
11kc
12kc
13kc
14kc
15kc
16kc
1km
2km
3km
4km
5km
6km
7km
8km
9km
10km
11km
12km
13km
14km
15km
16km
1mc
2mc
3mc
4mc
5mc
W
4.06
4.07
4.06
4.09
4.06
4.06
4.06
4.07
4.08
4.07
4.04
4.09
4.07
4.07
4.09
4.08
4.07
4.08
4.08
4.05
4.09
4.08
4.08
4.06
4.1
4.07
4.06
4.07
4.09
4.07
4.07
4.08
4.09
4.09
4.09
4.1
4.08
B
3.08
3.08
3.05
3.07
3.07
3.07
3.06
3.06
3.06
3.05
3.05
3.08
3.05
3.06
3.07
3.07
3.06
3.07
3.07
3.05
3.07
3.07
3.05
3.05
3.08
3.06
3.05
3.05
306
3.06
3.07
3.07
3.05
3.02
3.08
3.09
3.07
A1
1.17
0.83
1.13
1.01
0.97
1.09
0.885
1.08
1.23
1.22
1.16
0.94
1.47
1.1
1.08
1.15
1.1
1
0.99
1.12
1.12
1.11
1.11
0.92
1.13
0.98
1.05
0.97
1.22
1.14
1.01
1.21
0.935
1.15
1.15
1.32
1.05
A2
1.19
0.865
1.12
1.06
0.985
1.1
0.915
1.07
1.21
1.23
1.14
0.945
1.46
1.1
1.06
1.13
1.1
0.995
0.98
1.16
1.16
1.07
1.07
0.875
1.13
0.99
1.06
0.975
1.09
1
1.04
1.25
0.945
1.14
1.14
1.36
1.09
A3
1.22
0.895
1.12
1.07
0.985
1.1
0.945
1.04
1.22
1.25
1.1
0.93
1.29
1.12
1.07
1.12
1.3
1
0.935
1.19
1.19
1.05
1.05
0.835
1.15
0.95
1.07
0.97
1.01
1.16
1.05
1.28
0.93
1.21
1.21
1.37
1.12
Fract. Load
188.48
186.24
336.32
204.64
194.56
227.84
305.92
249.6
193.76
248.32
230.24
368.96
281.12
335.68
206.4
236.32
316.32
311.84
205.76
165.44
283.84
288.48
215.04
262.88
206.24
306.4
244.32
388.8
293.44
242.24
315.36
223.68
299.25
183.36
258.08
194.56
235.68
Kic
4.578
3.643
7.897
4.477
4.144
6.227
6.267
5.595
4.763
6.21
5.5
7.508
7.832
7.733
4.582
5.51
7.565
6.651
4.305
3.988
6.651
6.466
4.851
5.268
5.085
6.535
5.513
8.249
6.687
5.674
6.914
4.428
6.143
4.395
6.065
5.076
4.284
67
6mc
7mc
8mc
9mc
10mc
11mc
12mc
13mc
14mc
15mc
16mc
1mm
2mm
3mm
4mm
5mm
6mm
7mm
8mm
9mm
10mm
11mm
12mm
13mm
14mm
15mm
16mm
1lm
2lm
3lm
4lm
5lm
6lm
7lm
8lm
9lm
10lm
11lm
12lm
13lm
14lm
15lm
16lm
4.1
4.09
4.06
4.06
4.07
4.1
4.07
4.09
4.09
4.09
4.06
4.09
4.08
4.09
4.09
4.1
4.06
4.1
4.09
4.07
4.06
4.1
4.06
4.1
4.08
4.1
4.09
4.01
4.03
4.02
4.04
4.02
4.03
4.03
4.01
4.02
4.01
4.04
4.02
4.02
4.01
4.02
4.02
3.06
3.06
3.06
3.07
3.06
3.06
3.01
3.06
3.02
3.01
3.08
3.07
3.05
3.08
3.06
3.06
3.05
3.07
3.06
3.03
3.07
3.08
3.05
3.07
3.05
3.08
3.07
2.99
3.03
3.01
3.01
2.99
3.02
3
3.04
2.98
3.02
3.02
3.01
2.98
2.96
2.97
2.97
0.865
0.985
0.945
1.09
1.35
1.28
1.16
1.25
1.19
1.12
1.21
1.04
1.08
1.14
1.15
1
1.06
1.17
1.37
0.986
1.09
0.925
1.09
1.25
1.29
0.99
1.31
1.01
1.15
1.03
1.17
1.29
1.18
1.39
0.995
0.996
1.31
0.996
1.05
1.13
1.27
0.93
1.23
0.875
1
0.97
1.1
1.3
1.23
1.21
1.23
1.21
1.17
1.21
1.02
1.05
1.17
1.13
1.09
1.05
1.22
1.41
1.01
1.11
0.935
1.09
1.22
1.27
1.03
1.26
1.08
1.19
1.04
1.16
1.32
1.19
1.37
1
1.01
1.35
1.02
1.04
1.17
1.29
1.04
1.22
0.87
1.01
0.965
1.09
1.33
1.23
1.15
1.24
1.15
1.2
1.22
1.03
1.02
1.2
1.16
1.11
1.07
1.17
1.44
1.07
1.09
0.96
1.09
1.2
1.24
0.99
1.22
1.05
1.21
1.04
1.17
1.35
1.21
1.37
0.995
1.02
1.41
1
1.06
1.21
1.31
1.14
1.2
210.4
215.04
296.16
235.04
181.06
193.44
175.2
190.24
172.96
187.68
205.24
237.92
238.56
189.92
250.24
253.44
248.32
224
202.24
183.36
225.76
184
195.2
222.4
175.2
201.44
205.44
231.84
217.76
232.48
215.36
169.44
200.16
187.68
256.96
201.12
179.68
236.64
235.68
231.2
160.48
193.76
199.52
4.099
4.578
6.248
5.369
4.797
4.781
4.277
4.709
4.189
4.504
5.037
5.151
5.292
4.473
5.847
5.603
5.603
5.318
5.493
4.044
5.179
3.729
4.489
5.401
4.448
4.252
5.142
5.435
5.433
5.351
5.323
4.695
5.042
5.34
5.738
4.592
5.074
5.264
5.47
5.848
4.433
4.52
5.215
68
1lc
2lc
3lc
4lc
5lc
6lc
7lc
8lc
9lc
10lc
11lc
12lc
13lc
14lc
15lc
16lc
4.01
4
4
4
4
4
4
4.01
4.01
4.01
4.01
3.99
4
4.02
4.02
4.01
2.98
2.99
2.99
2.97
3.02
3.03
2.98
3.02
2.99
3.01
3.02
2.98
3.01
2.98
2.99
3.03
1.1
1.17
1.39
1.25
0.876
1.04
1.24
1.06
1.41
1.14
1.08
0.925
1.06
0.972
0.83
1.21
1.09
1.16
1.36
1.21
0.916
1.03
1.27
1.1
1.36
1.17
1.02
0.905
1.06
0.907
0.87
1.24
ID: Specimen’s code
A1: Depth of V-shaped notch in the left side
A2: Depth of V-shaped notch in the middle
A3: Depth of V-shaped notch in the right side
Fract. Load: Fracture load in Newtons
Kic: Fracture Toughness in MPa√m
kc: KaVo specimen sintered in conventional oven
km: KaVo specimen sintered in microwave oven
mc: Crystal HS specimen sintered in conventional oven
mm: Crystal HS specimen sintered in microwave oven
lm: Lava 3M specimen sintered in microwave oven
lc: Lava 3M specimen sintered in conventional oven.
1.1
1.11
1.3
1.21
0.981
0.995
1.3
1.1
1.33
1.18
1.01
0.875
1.04
0.832
0.89
1.25
227.68
228.32
212.32
219.2
223.2
221.28
164.16
257.44
176.16
186.08
207.2
234.72
235.04
225.92
246.88
181.44
5.529
5.736
6.066
5.819
4.811
5.065
4.473
6.13
5.057
4.666
4.779
5.077
5.527
4.816
5.105
4.724
69
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