University of IowaIowa Research Online

Theses and Dissertations

2011

Fracture toughness of yttrium stabilized zirconiasintered in conventional and microwave ovensAristotelis MarinisUniversity of Iowa

Copyright 2011 Aristotelis N/A Marinis

This dissertation is available at Iowa Research Online: http://ir.uiowa.edu/etd/1017

Follow this and additional works at: http://ir.uiowa.edu/etd

Part of the Oral Biology and Oral Pathology Commons

Recommended CitationMarinis, Aristotelis. "Fracture toughness of yttrium stabilized zirconia sintered in conventional and microwave ovens." master's thesis,University of Iowa, 2011.http://ir.uiowa.edu/etd/1017.

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

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

ii

To my sister

iii

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.

iv

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.

v

TABLE OF CONTENTS

LIST OF TABLES ...................................................................................................... viiii

LIST OF FIGURES .................................................................................................... viii

CHAPTER

I INTRODUCTION .............................................................................. 1

Significance of Problem - Purpose of the Study .......................... 2 Experimental Hypotheses ............................................................ 3 Long-Term Objective .................................................................. 3

II REVIEW OF THE LITERATURE .................................................... 4

Historical Overview of All-Ceramic Systems - Review of Current Systems ................................................................ 4

Strengthening Mechanisms for Dental Ceramic Systems ........... 7 Zirconium Biomaterial ................................................................. 8 Zirconium Dioxide - Transformation Toughening ...................... 8 Fabrication of Yttrium Stabilized Zirconia for Dental

Applications ....................................................................... 10 Zirconia Compared to Other All-Ceramic Biomaterials ............. 12 Veneering of All-Ceramic Restorations ...................................... 15 Tests for Predicting Clinical Performance of Dental Ceramic

Materials ............................................................................. 18 Fracture Toughness Methods ....................................................... 20 Comparison of Fracture Toughness in All-Ceramic Materials .... 21 The Importance of Oral Environment in Zirconia Physical

Properties ........................................................................... 22 Clinical Studies of All-Ceramic Restorations .............................. 25 Utilization of Microwave Technology in Dental Ceramics ......... 28 Comparison of Zirconia Sintered in Conventional and

Microwave Ovens .............................................................. 29

III MATERIALS AND METHODS ........................................................ 35

Pilot Study ................................................................................... 35 Main study - Sample Fabrication, Number, Size and Design ...... 37 Sintering Technique ..................................................................... 43 Specimens Storage ....................................................................... 44 Testing Method ............................................................................ 44 Statistical Analysis ....................................................................... 47

IV RESULTS ........................................................................................... 49

Interaction Between Sintering Type and Manufacturer ............... 50 Differences Between Sintering Type Within Each

Manufacturer ...................................................................... 52

vi

Differences Among Three Manufacturers Within Each Sintering Type .................................................................... 53

V DISCUSSION ..................................................................................... 55 Limitations of the Study .............................................................. 61 Clinical Relevance ....................................................................... 62 Avenues for Future Research ....................................................... 64

VI CONCLUSIONS................................................................................. 65

APPENDIX ............................................................................................................. 66

Raw Data ........................................................................................................... 66

REFERENCES ........................................................................................................... 69

vii

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

viii

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

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 (3Y-

TZP) 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

Al 2O3 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 leucite-

reinforced 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

Al 2O3 dry press (IA dry pressed), In-Ceram Al2O3 slip (IA slip), In-Ceram ZrO2 (IZ), In-

Ceram 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

Al 2O3, 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 all-

ceramic 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

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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).

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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).

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Comparison of Fracture Toughness in All-Ceramic

Materials

Fracture toughness is the first step in predicting the clinical performance of all-

ceramic 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 all-

ceramic 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 Y-

TZP 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 air-

steamed 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 Al 2O3

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

Variable N Mean Std Dev Minimum Maximum

Conventional Sintering 5 4.48 0.21 4.26 4.78

Microwave Sintering 5 4.63 0.29 4.21 4.89

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.

The mounting material was left to set for a minimum of 30 minutes. The

positioning aid was secured with

Dental GmbH) manufacturer provided the

specimens from each ZrO

depending on the apparatus of the fracture toughness test

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

strategy provided by KaVo

specimens, considering that the final sintering would create 20% shrinkage of the

partially sintered specimens. The expected dimensions after the final sintering for ea

beam specimen were w= 4mm, b=

Figure 3. Specimens’ expected dimensions after sintering.

material was left to set for a minimum of 30 minutes. The

aid was secured with the two screws in the Everest yoke. KaVo

manufacturer provided the CAM strategy for milling three rectangular

ZrO2 block. According to ISO standards for dental ceramics,

apparatus of the fracture toughness test, the specimens

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

provided by KaVo (KaVo Dental GmbH) calculated the dimensions of the

specimens, considering that the final sintering would create 20% shrinkage of the

sintered specimens. The expected dimensions after the final sintering for ea

beam specimen were w= 4mm, b=3mm, c= 0.12mm and the length was 32 mm (Fig 3).

Figure 3. Specimens’ expected dimensions after sintering.

38

material was left to set for a minimum of 30 minutes. The

oke. KaVo (KaVo

for milling three rectangular

block. According to ISO standards for dental ceramics,

the specimens should have the

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

The software

calculated the dimensions of the

specimens, considering that the final sintering would create 20% shrinkage of the

sintered specimens. The expected dimensions after the final sintering for each

3mm, c= 0.12mm and the length was 32 mm (Fig 3).

Figure 3. continued.

The milling process consisted of two stages. In the first stage, the milling

milled half the thickness of the specimens. The recommended

Milling Pin ZS1 and ZS3; KaVo

were used. One set of tools

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 ou

the yoke (Fig 4). The Inwax was softened using an industrial heat gun until the specimens

became loose in the wax. The specimens

same process was continued with the rest of the

The specimens were 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

machine (MP 3000; Metalo

straight notch was created with a diamond disc (Brasseler,

depth of the notch was 1mm.

process consisted of two stages. In the first stage, the milling

milled half the thickness of the specimens. The recommended milling tool

Milling Pin ZS1 and ZS3; KaVo (KaVo Dental GmbH) for milling partial

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 ou

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.

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; Metalor Technologies SA, Neuchatel, Switzerland). An initial

straight notch was created with a diamond disc (Brasseler, Savannah, GA), (Fig 5).

depth of the notch was 1mm.

39

process consisted of two stages. In the first stage, the milling engine

milling tools (Everest

for milling partial sintered 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 of

the yoke (Fig 4). The Inwax was softened using an industrial heat gun until the specimens

removed from the positioning aid and the

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

Microparallelometer milling

r Technologies SA, Neuchatel, Switzerland). An initial

Savannah, GA), (Fig 5). The

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

KaVo

Sintering Oven

Total Sintering Time

Maximum Temperature

Group 1:

KaVo Everest Therm 4180

10 hours

1450°C for 2 hours

Group 4:

MicroSinterWave A1614

2 hours

1520°C for 35 minutes

Lava 3M

Sintering Oven

Total Sintering Time

Maximum Temperature

Group 2:

3M ESPE's Lava Furnace 200

8 hours 30 minutes

1500°C for 2 hours

Group 5:

MicroSinterWave A1614

2 hours

1520°C for 35 minutes

Crystal HS

Sintering Oven

Total Sintering Time

Maximum Temperature

Group 3:

Zircar Hot Spot 110

10 hours

1510°C for 2 hours

Group 6:

MicroSinterWave A1614

2 hours

1520°C for 35 minutes

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 post-

hoc 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

Interaction between

type of sintering

and manufacturer

2 0.14 0.07 0.09 0.9174

Within Groups 90 71.90 0.80

Total 95 86.39

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 two-

way 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

Manufacturer Conventional Sintering Mean

Fracture Toughness (SD)

Microwave Sintering Mean

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

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 Shapiro-

Wilk test was used to verify normality of residuals. Table 8 presents the results of post-

hoc 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 post-

hoc 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 (2-

5 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 W B A1 A2 A3 Fract. Load Kic

1kc 4.06 3.08 1.17 1.19 1.22 188.48 4.578 2kc 4.07 3.08 0.83 0.865 0.895 186.24 3.643 3kc 4.06 3.05 1.13 1.12 1.12 336.32 7.897 4kc 4.09 3.07 1.01 1.06 1.07 204.64 4.477 5kc 4.06 3.07 0.97 0.985 0.985 194.56 4.144 6kc 4.06 3.07 1.09 1.1 1.1 227.84 6.227 7kc 4.06 3.06 0.885 0.915 0.945 305.92 6.267 8kc 4.07 3.06 1.08 1.07 1.04 249.6 5.595 9kc 4.08 3.06 1.23 1.21 1.22 193.76 4.763 10kc 4.07 3.05 1.22 1.23 1.25 248.32 6.21 11kc 4.04 3.05 1.16 1.14 1.1 230.24 5.5 12kc 4.09 3.08 0.94 0.945 0.93 368.96 7.508 13kc 4.07 3.05 1.47 1.46 1.29 281.12 7.832 14kc 4.07 3.06 1.1 1.1 1.12 335.68 7.733 15kc 4.09 3.07 1.08 1.06 1.07 206.4 4.582 16kc 4.08 3.07 1.15 1.13 1.12 236.32 5.51 1km 4.07 3.06 1.1 1.1 1.3 316.32 7.565 2km 4.08 3.07 1 0.995 1 311.84 6.651 3km 4.08 3.07 0.99 0.98 0.935 205.76 4.305 4km 4.05 3.05 1.12 1.16 1.19 165.44 3.988 5km 4.09 3.07 1.12 1.16 1.19 283.84 6.651 6km 4.08 3.07 1.11 1.07 1.05 288.48 6.466 7km 4.08 3.05 1.11 1.07 1.05 215.04 4.851 8km 4.06 3.05 0.92 0.875 0.835 262.88 5.268 9km 4.1 3.08 1.13 1.13 1.15 206.24 5.085 10km 4.07 3.06 0.98 0.99 0.95 306.4 6.535 11km 4.06 3.05 1.05 1.06 1.07 244.32 5.513 12km 4.07 3.05 0.97 0.975 0.97 388.8 8.249 13km 4.09 306 1.22 1.09 1.01 293.44 6.687 14km 4.07 3.06 1.14 1 1.16 242.24 5.674 15km 4.07 3.07 1.01 1.04 1.05 315.36 6.914 16km 4.08 3.07 1.21 1.25 1.28 223.68 4.428 1mc 4.09 3.05 0.935 0.945 0.93 299.25 6.143 2mc 4.09 3.02 1.15 1.14 1.21 183.36 4.395 3mc 4.09 3.08 1.15 1.14 1.21 258.08 6.065 4mc 4.1 3.09 1.32 1.36 1.37 194.56 5.076 5mc 4.08 3.07 1.05 1.09 1.12 235.68 4.284

67

6mc 4.1 3.06 0.865 0.875 0.87 210.4 4.099 7mc 4.09 3.06 0.985 1 1.01 215.04 4.578 8mc 4.06 3.06 0.945 0.97 0.965 296.16 6.248 9mc 4.06 3.07 1.09 1.1 1.09 235.04 5.369 10mc 4.07 3.06 1.35 1.3 1.33 181.06 4.797 11mc 4.1 3.06 1.28 1.23 1.23 193.44 4.781 12mc 4.07 3.01 1.16 1.21 1.15 175.2 4.277 13mc 4.09 3.06 1.25 1.23 1.24 190.24 4.709 14mc 4.09 3.02 1.19 1.21 1.15 172.96 4.189 15mc 4.09 3.01 1.12 1.17 1.2 187.68 4.504 16mc 4.06 3.08 1.21 1.21 1.22 205.24 5.037 1mm 4.09 3.07 1.04 1.02 1.03 237.92 5.151 2mm 4.08 3.05 1.08 1.05 1.02 238.56 5.292 3mm 4.09 3.08 1.14 1.17 1.2 189.92 4.473 4mm 4.09 3.06 1.15 1.13 1.16 250.24 5.847 5mm 4.1 3.06 1 1.09 1.11 253.44 5.603 6mm 4.06 3.05 1.06 1.05 1.07 248.32 5.603 7mm 4.1 3.07 1.17 1.22 1.17 224 5.318 8mm 4.09 3.06 1.37 1.41 1.44 202.24 5.493 9mm 4.07 3.03 0.986 1.01 1.07 183.36 4.044 10mm 4.06 3.07 1.09 1.11 1.09 225.76 5.179 11mm 4.1 3.08 0.925 0.935 0.96 184 3.729 12mm 4.06 3.05 1.09 1.09 1.09 195.2 4.489 13mm 4.1 3.07 1.25 1.22 1.2 222.4 5.401 14mm 4.08 3.05 1.29 1.27 1.24 175.2 4.448 15mm 4.1 3.08 0.99 1.03 0.99 201.44 4.252 16mm 4.09 3.07 1.31 1.26 1.22 205.44 5.142 1lm 4.01 2.99 1.01 1.08 1.05 231.84 5.435 2lm 4.03 3.03 1.15 1.19 1.21 217.76 5.433 3lm 4.02 3.01 1.03 1.04 1.04 232.48 5.351 4lm 4.04 3.01 1.17 1.16 1.17 215.36 5.323 5lm 4.02 2.99 1.29 1.32 1.35 169.44 4.695 6lm 4.03 3.02 1.18 1.19 1.21 200.16 5.042 7lm 4.03 3 1.39 1.37 1.37 187.68 5.34 8lm 4.01 3.04 0.995 1 0.995 256.96 5.738 9lm 4.02 2.98 0.996 1.01 1.02 201.12 4.592 10lm 4.01 3.02 1.31 1.35 1.41 179.68 5.074 11lm 4.04 3.02 0.996 1.02 1 236.64 5.264 12lm 4.02 3.01 1.05 1.04 1.06 235.68 5.47 13lm 4.02 2.98 1.13 1.17 1.21 231.2 5.848 14lm 4.01 2.96 1.27 1.29 1.31 160.48 4.433 15lm 4.02 2.97 0.93 1.04 1.14 193.76 4.52 16lm 4.02 2.97 1.23 1.22 1.2 199.52 5.215

68

1lc 4.01 2.98 1.1 1.09 1.1 227.68 5.529 2lc 4 2.99 1.17 1.16 1.11 228.32 5.736 3lc 4 2.99 1.39 1.36 1.3 212.32 6.066 4lc 4 2.97 1.25 1.21 1.21 219.2 5.819 5lc 4 3.02 0.876 0.916 0.981 223.2 4.811 6lc 4 3.03 1.04 1.03 0.995 221.28 5.065 7lc 4 2.98 1.24 1.27 1.3 164.16 4.473 8lc 4.01 3.02 1.06 1.1 1.1 257.44 6.13 9lc 4.01 2.99 1.41 1.36 1.33 176.16 5.057 10lc 4.01 3.01 1.14 1.17 1.18 186.08 4.666 11lc 4.01 3.02 1.08 1.02 1.01 207.2 4.779 12lc 3.99 2.98 0.925 0.905 0.875 234.72 5.077 13lc 4 3.01 1.06 1.06 1.04 235.04 5.527 14lc 4.02 2.98 0.972 0.907 0.832 225.92 4.816 15lc 4.02 2.99 0.83 0.87 0.89 246.88 5.105 16lc 4.01 3.03 1.21 1.24 1.25 181.44 4.724

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.

69

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