THE EFFECTS OF SINTERING ADDITIVES ON THE MECHANICAL PROPERTIES AND MICROSTRUCTURE

EVOLUTION OF 3 MOL% Y-TZP

NUR NADIA BINTI AHMAD HASAN

FACULTY OF ENGINEERING

UNIVERSITY OF MALAYA KUALA LUMPUR

2017

THE EFFECTS OF SINTERING ADDITIVES ON THE

MECHANICAL PROPERTIES AND MICROSTRUCTURE

EVOLUTION OF 3 MOL% Y-TZP

NUR NADIA BINTI AHMAD HASAN

DISSERTATION SUBMITTED IN FULFILMENT OF

THE REQUIREMENTS FOR THE DEGREE OF MASTER

OF ENGINEERING SCIENCE

FACULTY OF ENGINEERING

UNIVERSITY OF MALAYA

KUALA LUMPUR

2017

ii

UNIVERSITY OF MALAYA

ORIGINAL LITERARY WORK DECLARATION

Name of Candidate:Nur Nadia Binti Ahmad Hasan

Matric No: KGA160010

Name of Degree: Master of Engineering Science

Title of Dissertation (“this Work”):

The effects of sintering additives on the mechanical properties and microstructure

evolution of 3 mol% Y-TZP Field of Study: Engineering Material

I do solemnly and sincerely declare that:

(1) I am the sole author/writer of this Work;

(2) This Work is original;

(3) Any use of any work in which copyright exists was done by way of fair dealing

and for permitted purposes and any excerpt or extract from, or reference to or

reproduction of any copyright work has been disclosed expressly and

sufficiently and the title of the Work and its authorship have been

acknowledged in this Work;

(4) I do not have any actual knowledge nor do I ought reasonably to know that the

making of this work constitutes an infringement of any copyright work;

(5) I hereby assign all and every rights in the copyright to this Work to the

University of Malaya (“UM”), who henceforth shall be owner of the copyright

in this Work and that any reproduction or use in any form or by any means

whatsoever is prohibited without the written consent of UM having been first

had and obtained;

(6) I am fully aware that if in the course of making this Work I have infringed any

copyright whether intentionally or otherwise, I may be subject to legal action

or any other action as may be determined by UM.

Candidate’s Signature Date:

Subscribed and solemnly declared before,

Witness’s Signature Date:

Name:

Designation:

iii

ABSTRACT

3mol% Yttria stabilized Tetragonal Zirconia Polycrystalline (3Y-TZP) ceramic is one of

the most demanding material in biomedical application because it possesses the best

combination of mechanical properties, biocompatibility and aesthetic properties. These

extraordinary properties were attributed to its transformation toughening mechanism as it

provides resistance to crack propagation by externally applied stress as a consequence of

phase transformation from tetragonal(t) to monoclinic(m) at room temperature. One

major drawback with use of the zirconia is the significant loss in mechanical properties

attributed by undesired transformation phase tetragonal to monoclinic(tm) when

undergo ageing or low temperature degradation(LTD). The current work investigates the

effects of sintering additives, particularly focused on 0.5 wt% manganese oxide (MnO2)

and 0.1 wt% alumina (Al2O3), and co-doping both additives together (i.e. MnO2 + Al2O3),

on the microstructure and mechanical properties of 3 mol% Y-TZP. The doped powders

were prepared by attrition milling and subsequently green samples were sintered in air at

temperatures ranging from 1250℃ to 1550°C with 2 hours holding time. The results

showed that the dopants have not disrupted the tetragonal phase stability. All the dopants

aided sintering at 1250°C but with MnO2 being most effective in promoting densification

with samples, recording about 97.6 % relative density. Similar improvement in the

mechanical properties was also observed for the doped zirconia. Young’s modulus,

Vickers hardness and fracture toughness as high as 192 GPa, 13.6 GPa and 4.6 MPam1/2,

respectively were obtained for both, the MnO2-doped and co-doped Y-TZPs, when

sintered at low temperature of 1250°C. Microstructural examinations however revealed

that the MnO2 dopant promoted exaggerated grain growth when sintered at higher

temperatures. In addition, the study also found that LTD phenomena in superheated steam

condition was suppressed when average grain size not exceed 0.32 µm, particularly for

samples sintered at lower temperature.

iv

ABSTRAK

3mol% Yttria menstabilkan Tetragonal Zirconia Polycrystalline (3Y-TZP) seramik

adalah salah satu bahan yang mendapat permintaan paling tinggi dalam aplikasi

bioperubatan kerana ia mempunyai kombinasi sifat mekanik, sifat biokompatibiliti dan

estetik yang terbaik. Ciri-ciri luar biasa ini disebabkan oleh mekanisme transformasi

penguatnya kerana ia memberikan rintangan kepada retakan dengan tekanan luaran yang

digunakan sebagai akibat daripada transformasi fasa dari tetragonal (t) kepada

monoklinik (m) pada suhu bilik. Satu kelemahan utama dengan penggunaan zirkonia

adalah kerugian yang ketara dalam sifat mekanik yang disebabkan oleh fasa transformasi

yang tidak diinginkan, tetragonal kepada monoklinik (t m) apabila mengalami penuaan

atau degradasi suhu yang rendah (LTD). Kerja semasa adalah menyiasat kesan tambahan

pensinteran, terutamanya tertumpu pada 0.5 wt% manganese oksida (MnO2) dan 0.1 wt%

alumina (Al2O3), dan co-doping kedua-dua bahan tambahan bersama-sama (iaitu MnO2

+ Al2O3) pada struktur mikro dan sifat mekanik 3 mol% Y-TZP. Serbuk didopkan telah

disediakan oleh pergeseran pengilangan dan kemudian sampel hijau disinter di udara pada

suhu antara 1250℃ hingga 1550℃ dengan masa pegangan 2 jam. Keputusan

menunjukkan bahawa dopan tidak mengganggu kestabilan fasa tetragonal. Semua dopan

membantu pensinteran pada 1250℃ tetapi MnO2 yang paling berkesan dalam

mempromosikan kepadatan dengan sampel, mencatatkan kepadatan relatif 97.6%.

Peningkatan yang sama dalam sifat-sifat mekanik juga diperhatikan untuk zirconia

didopkan. Young’s Modulus, kekerasan Vickers dan ketumpatan fraktur setinggi 192

GPa, 13.6 GPa dan 4.6 MPam1/2, masing-masing diperolehi untuk kedua-dua, yang

MnO2-didopkan dan bersama-didopkan Y-TZP, apabila disinter pada suhu rendah

1250℃. Pemeriksaan mikrostruktural, bagaimanapun mendedahkan bahawa dopan

MnO2 mempromosikan pertumbuhan bijirin yang besar apabila sintered pada suhu yang

lebih tinggi. Di samping itu, kajian ini juga menemui bahawa fenomena LTD dalam

v

keadaan stim haba yang panas telah ditindas apabila purata saiz biji tidak melebihi 0.32

μm, terutamanya untuk sampel yang disinter pada suhu yang lebih rendah.

vi

ACKNOWLEDGEMENTS

Firstly, I would like to thank God, for giving me an opportunity to further study in

master degree level in this top 100th university ranking in the world, providing me a path

to explore His knowledge with passion especially in advance material engineering field.

I would like to express my sincere appreciation and gratitude to my supervisor, Prof.

Ir Dr. Ramesh Singh and co-supervisor Assoc. Prof. Dr. Tan Chou Yong, for their

guidance and wise words of encouragement throughout the duration of this study. Their

valuable time, advice and extensive technical knowledge very much appreciated.

Furthermore, they help me to build up confidence and shaped me into a better person.

In addition, I would like to convey my appreciation towards:

1. University Malaya (UM) for the research grant (FP056-2015A) and funding my

fees by GRAS

2. SIRIM Berhad Malaysia for the usage of Young’s Modulus measurement

equipment to completed this research.

3. My collegues in UM, especially Mohaymen, Dr Ali Niakan, Nurhusna Zulkifli for

their guidance and support.

Last but not least, special thanks to my father Ahmad Hasan bin Saidin, my mother

Naazian binti Abd Rahman, my husband Hariz Izaan bin Kamarudzaman, beloved family

members and friends for their never ending support, prayers and belief in me.

vii

TABLE OF CONTENTS

Abstract ............................................................................................................................ iii

Abstrak ............................................................................................................................. iv

Acknowledgements .......................................................................................................... vi

Table of Contents ............................................................................................................ vii

List of Figures ................................................................................................................... x

List of Tables................................................................................................................... xii

List of Symbols and Abbreviations ................................................................................ xiii

List of Appendices .......................................................................................................... xv

CHAPTER 1: INTRODUCTION .................................................................................. 1

1.1 Bioceramic ............................................................................................................... 1

1.2 Problem statement ................................................................................................... 5

1.3 Objective .................................................................................................................. 5

1.4 Scope of the study .................................................................................................... 6

1.5 Thesis Structure ....................................................................................................... 6

CHAPTER 2: LITERATURE REVIEW ...................................................................... 8

2.1 Introduction to Zirconia .............................................................................................. 8

2.1.1 Zirconia structure ....................................................................................... 9

2.1.2 Transformation toughening mechanism ................................................... 10

2.1.3 Yttria-stabilized Tetragonal Zirconia Polycrystals (Y-TZP) .................... 11

2.1.4 Mechanical properties of Y-TZP .............................................................. 13

2.1.5 Application of zirconia ............................................................................. 14

2.1.5.1 Clinical application ................................................................... 14

2.1.5.2 Cutting tool application ............................................................. 14

viii

2.1.5.3 Refractory application ............................................................... 15

2.1.5.4 Heat Engine application ............................................................ 16

2.2 Low Temperature Degradation (LTD) phenomena on Y-TZP .............................. 17

2.2.1 Mechanism of degradation ....................................................................... 18

2.2.1.1 Corrosion mechanism ................................................................ 18

2.2.1.2 Destabilization mechanism ....................................................... 20

2.2.1.3 Stress-induced transformation ................................................... 22

2.2.2 Prevention of degradation ........................................................................ 24

2.2.2.1 Reduction of grain size .............................................................. 24

2.2.2.2 Increase of stabilizer concentration ........................................... 26

2.2.2.3 Inhomogeneous stabilizer distribution ........................................ 28

2.2.2.4 Introduction of dopant ................................................................. 29

2.3 Effect of sintering additives on mechanical properties and microstructure evolution

on Y-TZP ............................................................................................................... 31

2.3.1 Effect of manganese (IV) oxide doped Y-TZP ........................................ 31

2.3.2 Effect of alumina doped Y-TZP ............................................................... 32

2.3.3 Effect of manganese (IV) oxide and alumina co-doped Y-TZP............... 33

CHAPTER 3: METHODOLOGY ............................................................................... 35

3.1 Preparation samples ............................................................................................... 35

3.1.1 Powder ...................................................................................................... 35

3.1.2 Mixing powder ......................................................................................... 35

3.1.3 Green body preparation ............................................................................ 36

3.1.4 Sintering ................................................................................................... 36

3.2 Mechanical properties measurement ..................................................................... 37

3.2.1 Relative density ........................................................................................ 37

3.2.2 Young Modulus ........................................................................................ 38

ix

3.2.3 Vickers hardness ....................................................................................... 39

3.2.4 Fracture toughness .................................................................................... 41

3.3 Microstructure and Phase Composition ................................................................. 42

3.3.1 FESEM Analysis and Average Grain Size ............................................... 42

3.3.2 XRD Analysis ........................................................................................... 43

3.3.3 Ageing ...................................................................................................... 45

CHAPTER 4: RESULTS AND DISCUSSION .......................................................... 47

4.1 Phase analysis ........................................................................................................ 47

4.2 Mechanical Properties Analysis ............................................................................ 48

4.2.1 Relative Density ....................................................................................... 48

4.2.2 Young’s Modulus ..................................................................................... 49

4.2.3 Vickers Hardenss ...................................................................................... 51

4.2.4 Fracture Toughness .................................................................................. 52

4.3 Microstructural Development and Grain Size ....................................................... 54

4.4 Low Temperature Degradation Behavior .............................................................. 57

CHAPTER 5: CONCLUSION AND FUTURE WORK ........................................... 61

5.1 Conclusion ............................................................................................................. 61

5.2 Future Work ........................................................................................................... 64

References ....................................................................................................................... 65

List of Publications ......................................................................................................... 71

APPENDIX A: EXPERIMENT APPARATUS ......................................................... 72

Appendix B: Density Table for Distilled Water ............................................................. 79

Appendix C: XRDML Reference File ............................................................................ 80

x

LIST OF FIGURES

Figure 1.1 Clinical uses of bioceramics (Ravaglioli et al., 1992) ..................................... 3

Figure 2.1 Baddeleyite from Phalaborwa, South Africa in nature (courtesy from

Wikipedia: Baddeleyite).................................................................................................... 8

Figure 2.2 Schematic representation of the polymorphs phase of zirconia and

corresponding space group a) monoclinic b) tetragonal c) cubic (Hannink et al., 2000) . 9

Figure 2.3 Transformation toughening process (Piconi & Maccauro,1999) .................. 10

Figure 2.4 Phase diagram for the zirconia-yttria system (Hannink et al.,2000) ............. 12

Figure 2.5 a) A one-piece zirconia implant b) Clinical aspect of the abutment part of

zirconia implant before cementation of crown (courtesy of Prof. Andrea Enrico

Borgonovo, University of Milan). ................................................................................... 14

Figure 2.6 Zirconia blades (Courtesy of American Cutting Edge) ................................. 15

Figure 2.7 Turbine blade with thermal barrier coating based on zirconia (Courtesy of

Institute of Material Research) ........................................................................................ 16

Figure 2.8 The proposed degradation mechanism of Yoshimura et al. (1989) ............... 19

Figure 2.9 TEM sample before and after ageing in humid air (Schmauder and Schubert,

1986) ............................................................................................................................... 22

Figure 2.10 Mechanism proposed for microcracking and monoclinic phase propagation

(Munoz-Tabares et al., 2011) .......................................................................................... 23

Figure 2.11 XRD analysis of 3Y-TZP sintered sample after hydrothermal degradation in

humid environment at 130℃ and 140 hours (Elshazly et al., 2011) .............................. 26

Figure 2.12 The effect of monoclinic content and ageing temperature with different yttria

content in Y-TZP sintered at 1400℃ to 1600℃ and aged for 50 hours (Sato et al., 1985b)

......................................................................................................................................... 28

Figure 2.13 Monoclinic content measured by XRD (Nogiwa-Valdez et al., 2013)........ 33

Figure 3.1 Schematic diagram of an indentation for measurement ................................ 40

Figure 3.2 Typical indentation formed (Flavio T. da Silva et al.,2007) ......................... 40

Figure 3.3 Cross section of autoclaves vessel ................................................................. 45

Figure 4.1 XRD pattern of sintered undoped and doped 3Y-TZP samples .................... 47

xi

Figure 4.2 Effect of dopants and sintering temperatures on the relative density of Y-TZP.

......................................................................................................................................... 49

Figure 4.3 Young’s modulus variation with dopant addition and sintering temperatures.

......................................................................................................................................... 50

Figure 4.4 Effect of dopants and sintering temperature on the Vickers hardness of Y-TZP.

......................................................................................................................................... 51

Figure 4.5 Variation in fracture toughness as a function of dopant addition and sintering

temperatures. ................................................................................................................... 53

Figure 4.6 Microstructural development for Y-TZP sintered at 1250 °C [(a) undoped, (b)

0.5 wt% MnO2-doped, (c) 0.1 wt% Al2O3-doped and (d) co-doped Y-TZP] and 1550 °C

[(e) undoped, (f) 0.5wt% MgO2-doped, (g) 0.1 wt% Al2O3-doped and (h) co-doped Y-

TZPs]. .............................................................................................................................. 55

Figure 4.7 Effect of grain size on ageing-induced monoclinic content for Y-TZPs sintered

after 1h exposure in superheated steam. ......................................................................... 59

Figure A-1 Powder for making green sample ................................................................. 72

Figure A-2 Mixing powder with attrition milling ........................................................... 72

Figure A-3 Ultrasonification of mixture powder ............................................................ 73

Figure A-4 Dried powder ................................................................................................ 73

Figure A-5 Crush and sieved to form fine powder ......................................................... 74

Figure A-6 Bench Press used for powder compaction .................................................... 74

Figure A-7 Sinter the sample by conventional sintering ................................................. 75

Figure A-8 Grinding the sample by using Silicon Carbide paper ................................... 75

Figure A-9 Polishing sample by using diamond paste .................................................... 76

Figure A-10 Electronic Balance to measure relative density .......................................... 76

Figure A-11 Field Emission Scanning Electron Microscope (FESEM) ......................... 77

Figure A-12 Young’s Modulus determination via impulse excitation technique

(GrindoSonic: MK5 “Industrial”, Belgium) ................................................................... 77

Figure A-13 Aging test in autoclave ............................................................................... 78

xii

LIST OF TABLES

Table 1.1 Type of bioceramic and functions ..................................................................... 2

Table 2.1 Comparison of properties between commercial Y-TZP with steel ................. 13

Table 4.1 Average grain size (µm) of Y-TZPS sintered at different temperatures. ........ 57

Table 4.2 Monoclinic content (%) of 3Y-TZPs sintered after exposure in superheated

steam ............................................................................................................................... 57

xiii

LIST OF SYMBOLS AND ABBREVIATIONS

Al2O3 : Alumina or Aluminium Oxide

AFM : Atomic Force Microscopy

𝜌 : Bulk Density

CIP : Cold Isostatic Pressing

CeO : Ceria

CSZ : Ceria Stabilized Zirconia

FESEM : Field Emission Scanning Electron Microscope

FIB : Focused Ion Beam

KIC : Fracture Toughness

CaO : Lime

LTD : Low Temperature Degradation

MnO2 : Manganese (IV) Oxide

MgO : Magnesia

MSZ : Magnesia Stabilized Zirconia

m : Monoclinic phase

PSZ : Partially Stabilized Zirconia

µ : Poisson’s Ratio

t : Tetragonal phase

tm : Tetragonal to monoclinic

TZP : Tetragonal Zirconia Polycrystal

TTZ : Transformation Toughened Zirconia

Hv : Vickers Hardness

XRD : X-Ray Diffraction

E : Young’s modulus

xiv

Y-TZP : Yttria-stabilized Tetragonal Zirconia Polycrystals

ZrO2 : Zirconia or Zirconium Dioxide

ZrSiO4 : Zircon or Zirconium Silicate

xv

LIST OF APPENDICES

Appendix A: Experiment Apparatus 7

Appendix B: Density Table for Distilled Water

Appendix C: XRDML Reference File

Appendix D: Sample calculation to determine phase content

1

CHAPTER 1: INTRODUCTION

1.1 Background

Humans discovered that burnt clay could be transformed into ceramic pottery over

thousands of years ago. Ceramic pots are practically used as usable vessels for food and

stored grains for long periods of time with minimal deterioration. A method of cooking

was also invented because of invulnerable ceramic to fire and ability to keep water. This

development of ceramic radically transforms human civilization in the quality and length

of life.

Ceramic can be classified into two categories, traditional and advanced ceramic.

Generally, traditional ceramics were manufactured from natural based and have been used

for many years which include consumer products like dinnerware and construction

product like tiles or window. Whereas, advanced ceramics were manufactured from

modified and refined raw material by controlling their chemical composition for specific

application which have been discovered over last several decades (Brook,2012). They

offer unique and powerful physical and mechanical properties that have opened up

opportunities to contribute in society.

Advanced ceramic gives a boost in demand by providing an extensive range of

advancements in catalyst system, bioceramic, solid oxide fuel cells and

telecommunications. Bioceramics are important in the biomedical field due to their

chemical similarity to bone; and are ideal for surgical implants due to their thermal and

chemical inertness, and have high strength, wear resistance, and durability. Ceramic

biomaterials also stimulate bone growth and have low friction coefficients. They do not

create strong biologically relevant interfaces with bones, but they do promote strong

adhesions to bones. Bioceramic can repair and reconstruct diseased, damaged or “worn

2

out” parts of the human body. The use of ceramics for biological use have increased

greatly and can be divided into the following categories:

Table 1.1 Type of bioceramic and functions

Bioceramic Bioinert Absorbable Bioactive

Function retain structure

after implantation

and do not provoke

or undergo reactions

filling materials which make

it possible during and after their

dissolution,

reformation of the tissues

increased

phenomenon

of adhesion

through

stimulation of

bone regrowth

Examples Alumina, zirconia Hydroxyapatite Bioglass

(Salinas et al., 2013)

Bioceramic are produced in a variety of forms and phases and serve many

different functions in the repair of the body, which are summarized in Figure 1.1. The

main applications of ceramic biomaterials include joint/tissue replacement, metal coating

to improve biocompatibility and absorbable lattice to provide a temporary structure that

is eventually replaced by the body’s tissues.

Bioceramics are made in many different phases. They can be single crystals

(sapphire), polycrystalline (alumina or hydroxyapatite), glass (bioglass), glass ceramics

(A/W glass-ceramic) or composites (polyethylene-hydroxyapatite). The phase or phases

used depends on the properties and functions required.

3

Figure 1.1 Clinical uses of bioceramics (Ravaglioli et al., 1992)

For examples, Yttria-stabilized Tetragonal Zirconia Polycrystalline(Y-TZP)

ceramic is one of the most demanding ceramic especially in orthopedic because it

possesses the best combination of strength above 800 MPa, high fracture toughness (>10

MPam1/2), biocompatibility and aesthetic properties. The basic color of Y-TZP is opaque

to white allowing more natural appearance especially in dental restoration.

4

These extraordinary mechanical properties of Y-TZP are attributed to its

transformation toughening mechanism as it provides resistance to crack propagation by

externally applied stress as a consequence of phase transformation from tetragonal to

monoclinic (tm) at room temperature. This transformation causes 3-5 % increase in

volume, this results in generation of compressive stress at the crack tip and prevent the

crack from further propagating (Feder and Anglada,2005; Kim et al. 2009).

However, Kobayashi et al. (1981) reported when the ceramic is exposed to hot

humid conditions, it makes Y-TZP ceramics prone to low temperature degradation (LTD)

or called ageing process, where spontaneous tetragonal to monoclinic (tm)

transformation is unintentionally initiated resulting in its failure. This problem contributes

an extensive microcracking within the transformed surface layer due to compressive

stress produced by transformed grains and eventually leads to degradation of mechanical

properties (Ramesh et al. 2008; Keuper et al.,2014). Clarke et al. (2003) have reported

that LTD was partially responsible regarding the failure episode when a high number of

3Y-TZP femoral heads fracture shortly after implementation that occurred in 2001.

Therefore, Chevalier (2006) suggested that it is important to develop reliable materials

that might have interesting behavior in LTD over longer period of time (>15 – 20 years)

without compromising its attractive fracture toughness especially for orthopedic

applications. This major drawback of Y-TZP has gotten the attention of numerous

researchers and has opened a research gap for further investigation to develop excellent

mechanical properties and ageing behavior in Y-TZP ceramic.

5

1.2 Problem statement

Zirconia is known by its incredible mechanical properties especially fracture

toughness. This fracture toughness is associated by its unique trait; transformation

toughening mechanism. This phase transformation can then put the crack into

compression, retarding its growth, and enhance the fracture toughness. However,

undesirable phase transformation was unfavorable to zirconia when exposed to humid

environment, where it leads to volume expansion produced by transformed grains and

eventually deteriorate the mechanical properties when undergo ageing or low temperature

degradation (LTD). This major drawback of 3Y-TZP has gotten the attention of numerous

researchers where they discovered that grain boundary modification to be the most

economical method. By adding proper selection of sintering additive or dopant for

tetragonal lattice of zirconia, it can extend the reliability and lifetime of product made

with stabilized zirconia under hydrothermal conditions. Thus, the ultimate goal of this

research is to study the effect of sintering additives to zirconia to enhance LTD properties

without sacrificing other properties. The suitable amount of sintering additives to be used

in this present work was studied based on latest findings.

1.3 Objectives

This research work is aimed to study the sintering additives to 3Y-TZP to enhance its

LTD properties without sacrificing mechanical properties. With this main objectives, the

following aspects are to be achieved:

1. To investigate the effect of alumina and manganese (IV) oxide on mechanical

properties, microstructure of 3Y-TZP and retain in tetragonal phase at room

temperature.

2. To identify the optimum condition to prepare 3Y-TZP sintered body that exhibit

improved mechanical properties.

3. To evaluate the Low Temperature Degradation (LTD) or aging behavior in 3Y-

TZP when exposed in superheated steam at 185℃/10 bar.

6

1.4 Scope of the study

The research focuses on the enhancement of the alumina and manganese (IV)

oxide co-doped Y-TZP ceramic in mechanical properties and evaluation of microstructure

when sintered at temperature ranging from 1250℃ to 1550℃ with an interval of 100℃.

In the present study, the behavior in 3 mol% Y-TZP doped with 0.1 wt% of Al2O3 and

0.5 wt% of MnO2 was examined. The mixing was accomplished by attrition milling, using

zirconia balls as the milling media and ethanol as the mixing medium. The phase present

in the sintered body was characterized by using X-ray diffraction (XRD). Field Emission

Scanning Electron Microscopy (FESEM) was used to observe the particle agglomeration

and microstructures development. The sintered samples were evaluated in terms of

mechanical properties which include relative density, fracture toughness, Vicker

Hardness and Young’s Modulus. The experiment for Low Temperature Degradation

(LTD) phenomena was carried out in an autoclaves containing superheated steam

185℃/10 bar. The phase transformation was evaluated by XRD analysis.

1.5 Thesis Structure

In general, the thesis is divided into five chapters which will cover the introduction

of research, extensive literature review, experimental technique, results and discussion

and finally conclusion and future work suggestions. List of the collected experimental

data and other additional information will also be included in the form of appendix.

Chapter 1 presents the introduction to bioceramics, in particular zirconia as the

main subject for this research. Furthermore, the major concern faced with the used of

zirconia is presented in the problem statement followed by the objective of the research.

Chapter 2 reviews the introduction of zirconia, its crystalline structure with

specific applications. The present of transformation toughening mechanism, ageing

phenomena in zirconia properties are also discussed. In addition, the effect of sintering

7

additive on the mechanical properties and microstructure are also deliberated. The chapter

end with the summary of the literature review in order to correlate with main objective

during the research.

Chapter 3 specifies the experimental technique and method that were used in the

current research which include the apparatus and equipment.

Chapter 4 reports on the results and discussion from the finding of this research.

The comparison of the undoped, single doped and co-doped Y-TZP in terms of

mechanical properties and microstructure behavior are discussed in this chapter.

Chapter 5 presents the conclusion of the whole research and list of several

suggestions for future work.

The appendices section contains the additional experimental data and relevant

figures along with other necessary documentation to support the research.

8

CHAPTER 2: LITERATURE REVIEW

2.1 Introduction to Zirconia

The capability of ceramics as biomaterials are compatible because they are

composed of ions showing limited toxicity to body tissue (zirconium and titanium) and

generally found in the physiological environment (calcium, potassium, magnesium,

sodium). This chapter deals with one of the inert ceramics that is most used in surgical

implants: zirconia.

Figure 2.1 Baddeleyite from Phalaborwa, South Africa in nature (courtesy from

Wikipedia: Baddeleyite)

Native zirconia or zirconium dioxide (ZrO2) is a rare crystalline oxide mineral that

naturally exist in shades of brown, brownish black or greenish brown as shown in Fig 2.1

named as baddeleyite. It is named after its founder, Joseph Baddeley who had brought

this specimen from Rakwana, Ceylon (now known as Sri Lanka) and it is found richly in

South Africa. Along with cessation of mining activities, the availability of baddeleyite

gradually declined because its high purity of zirconia (96.5-98.5%). Then, zircon became

the majority of world’s supply of zirconia. Zircon or zirconium silicate(ZrSiO4) sand that

9

is found in India and Australia (Lughi & Sergo,2010) requires significant processing and

fabricating to obtain zirconia since it is not as pure as baddeleyite.

2.1.1 Zirconia structure

Pure zirconia, ZrO2, undergoes a process of unit cell expansion during its manufacturing

process (sintering/heating). Pure zirconia is a polymorphic material that has the same

chemical composition but is able to exist in three well-defined of crystalline structure;

monoclinic (m), tetragonal(t) and cubic (c) phase. Zirconia-based materials in each

specific phase is used for specific application such as a simulant of gems, diamond and

electroceramic from cubic zirconia and dental implants from tetragonal zirconia. These

phase changes take place with change in temperature as shown in Figure 2.2. Zirconia

exist as monoclinic phase from ambient temperature to 1170℃, tetragonal phase exists

between 1170℃ to 2370℃ and cubic phase can exist up to 2370℃.

1170℃ 2370℃

Monoclinic (m) Tetragonal (t) Cubic (c)

Figure 2.2 Schematic representation of the polymorphs phase of zirconia and

corresponding space group a) monoclinic b) tetragonal c) cubic (Hannink et al.,

2000)

10

2.1.2 Transformation toughening mechanism

Particularly, the tetragonal to monoclinic transformation is of great significance

that leads to the interesting mechanical properties of high strength and toughness

displayed by zirconia as it is similar to the hardening mechanism in steels. Garvie et al.

(1975) published in their paper “Ceramic Steel?” about the presence of transformation

toughening mechanism in zirconia, which leads to increasing demand of zirconia

especially in engineering application as well as biomedical application. This mechanism

is beneficial in limiting the growth of cracks and hindering the propagating crack by

transforming the metastable tetragonal to monoclinic to overcome the matrix constraint

which leads to a favorable compressive stress as shown in Figure 2.3.

Figure 2.3 Transformation toughening process (Piconi & Maccauro,1999)

However, such a phenomenal mechanism of action against crack propagation has

been questioned because of the so-called low-temperature degradation process, a sort of

aging of zirconia. This phase transformation results in large volume changes range

between 3-4% which leads to the formation of macro- and micro-cracking as claimed by

Saridag et al. (2013) and Wirwicki & Topolinski (2012). This is cause by the large volume

and shape deformations that set up large strains in the structure. These strains cannot be

11

relieved by diffusion, instead they are accommodated by elastic or plastic deformation of

the surrounding matrix. Due to the martensitic nature of zirconia, manufacturer find it

impossible to fabricate different items from pure zirconia as martensitic reactions are

usually athermal, occuring only when the temperature is changing.

Alternatively, incorporation of oxides soluble like yttria (Y2O3), ceria (CeO), lime

(CaO) and magnesia (MgO) are commonly added to pure zirconia to allow the retention

of tetragonal or cubic phase at room temperature to remain stable (Saridag et al., 2013;

Viazzi et al., 2006). These materials are referred to as stabilizers.

2.1.3 Yttria-stabilized Tetragonal Zirconia Polycrystals (Y-TZP)

Material selection will always depend on not just one consideration (such as the

avoidance of low temperature degradation) but the full application

specification/structure-property relationship. However, based on previous findings, for

high stress, high temperature applications, Magnesia Stabilized Zirconia (MSZ) can be

useful. To protect against low temperature degradation, Ceria-Stabilized Zirconia (CSZ)

is often chosen instead. Alternatively, YTZP can be manipulated to increase resistance to

low temperature degradation, either through doping or the use of surface treatments.

The addition of various stabilizers results in two types of transformation-

toughened zirconia (TTZ) which are partially stabilized zirconia (PSZ) and tetragonal

zirconia polycrystalline (TZP) materials, even though both are partially stabilized, their

microstructure and stabilizers are different. By adding some oxides to zirconia, it is

possible to stabilize the tetragonal and/or cubic phases. The so-called partially stabilized

zirconia (PSZ) consists mainly of a cubic phase, with monoclinic and tetragonal zirconia

as minor phases. By adding 2–3% of yttria (yttrium oxide, Y2O3), it is possible to obtain

a completely tetragonal zirconia, the so-called yttria-stabilized tetragonal zirconia

polycrystal (Y-TZP).

12

The solubility of yttria (Y2O3) enables greater stability of t-ZrO2 grains against

the t- m transformation, allowing the production of a 100% t-ZrO2 material and allows

the retention of metastable t phase when the zirconia is cooled after sintering to room

temperature. The amount of alloying oxide required to produce stabilization is determined

from the relevant phase diagram. The fraction of the T-phase retained at room temperature

is dependent on the processing temperature and yttria content as shown in Figure 2.4.

Figure 2.4 Phase diagram for the zirconia-yttria system (Hannink et al.,2000)

Besides, the size of grain also plays a role in retained tetragonal phase, where the

zirconia particle must be below critical grain size for retention to occur (Hirvonen et al.,

2006). The ceramic was not prone to LTD with small grain sizes (below a certain value)

below 0.5 mm (Khan et al., 2014) to obtain a metastable tetragonal structure at room

13

temperature (3 mol% Y2O3-stabilized tetragonal ZrO2). A spontaneous t-m

transformation of grains takes place˙ if the grain size is higher than critical value as shown

in Figure 2.4. The t-m transition in TZP materials depends not only on the Y2O3 content,

but also on its distribution. The stabilizing oxide is introduced in ZrO2 during the early

stages of the ceramic powder manufacturing process.

2.1.4 Mechanical properties of Y-TZP

Y-TZP is the most performing zirconia from a mechanical point of view and the

most used in high temperature chemistry and ceramic biomaterial applications (Piconi &

Macauro, 1991; Basu et al, 2004). It is interesting and in some cases unique mechanical

properties are the reasons why zirconia is often called “ceramic steel”: a high corrosion

and wear resistance, high Young’s modulus (200 GPa), a very high flexural strength (up

to 1200 MPa), a high fracture toughness as compare with steel as shown in Table 2.1.

Table 2.1 Comparison of properties between commercial Y-TZP with steel

3Y-

TZP1

3Y-TZP2 Steel3

Density (g/cm3) 6.09 6.05 7.8

Bending strength (MPa) 1200 999.05 1550

Fracture toughness (MPam1/2) 5-6 10 50

Elastic Modulus (GPa) 210 206.7 200

Thermal Expansion Coefficient (x10-6/℃) 10.5 10 14

1’KZ-3YF-E Type P’ powder by Tosoh Corporation, Japan

2’Technox 2000’ powder by Dynamic-Ceramic Ltd,UK

3’Typical steel properties by Ashby and Jones (2012)

14

2.1.5 Application of zirconia

Most recently, the attractiveness of advanced ceramic has various engineering

application due to its mechanical properties and chemical inertness, which offer stiff

competition to steel. Detailed applications of zirconia are discussed as below:

2.1.5.1 Clinical application

Zirconia implant is mostly applied in dentistry because it has excellent aesthetic

as it is metal free, has a natural white color and biocompatible. It is also the first choice

in patients with titanium allergy. Currently, commercial implant zirconia system is

available and the example implantation is shown in Figure 2.5.

(a) (b)

Figure 2.5 a) A one-piece zirconia implant b) Clinical aspect of the abutment

part of zirconia implant before cementation of crown (courtesy of Prof. Andrea

Enrico Borgonovo, University of Milan).

2.1.5.2 Cutting tool application

Zirconia material can be used as kitchen appliances such as knives, scissors and

blades. The zirconia ceramic blades can only be ground using diamond because of their

very high hardness and the cutting edges that can achieve 2 to 4 RMS finish. These

ceramic blades had specific applications which are used for cutting capsules, packaging,

rubber, fiber, plastics, film, foil, mat board as well as other products.

15

Figure 2.6 Zirconia blades (Courtesy of American Cutting Edge)

2.1.5.3 Refractory application

Due to its high melting temperature and low thermal conductivity, zirconia

material can be used as a refractory component and brick for metallurgical processing and

refractory fibers. It provides thermal insulation to separator in aerospace batteries, hot gas

filter and electrolysis diaphragm. Besides, it has good chemical stability in environments

such as inside a gas turbine and good thermal shock resistant. Zirconia can be used as

thermal barrier coating on turbine blades (Figure 2.7) which allows the operating

temperature of the engine to be increased without incurring any rise of temperature of the

metal blade, thus increasing the efficiency of the engine.

16

Figure 2.7 Turbine blade with thermal barrier coating based on zirconia

(Courtesy of Institute of Material Research)

2.1.5.4 Heat Engine application

The desirable properties of zirconia such as low thermal conductivity can lower heat

loss from the combustion chamber and increase the efficiency of the engine. Various parts

in the adiabatic diesel engines are made from zirconia material such as piston crown, head

face plates and piston liners. Low coefficient of friction and good tribological properties

enable parts made of zirconia have a long lifespan and requires less maintenance.

17

2.2 Low Temperature Degradation (LTD) phenomena on Y-TZP

This sub-chapter review the process and causes of degradation of Y-TZP, and

measure that need to be taken to prevent them from occurring based on previous

experimental observation. Despite the superior mechanical properties of Y-TZP, this

ceramic is prone to age in the wet environment, showing degradation of their mechanical

properties (Lughi & Sergo,2010; Ramesh et al., 2012). To one degree or another, most

materials experience deterioration of its mechanical properties, other physical properties,

or appearance. Sometimes, to the frustration of design engineer, the degradation behavior

of a material for some application is ignored, with adverse consequences.

Degradation of zirconia happens in low temperatures between 65℃ to 500℃ in

the presence of water or water vapor (Castkova et al, 2004). This condition, so-called low

temperature degradation, a sort of aging of zirconia, is a transformation of undesired

tetragonal phase to monoclinic phase with microcracking as first reported by Kobayashi

in 1981. The main steps of TZP degradation were summarized by researchers (Kobayashi

et al., 1981; Swab, 1991; Lawson, 1995; Ramesh,1998; Feder & Anglada,2005; Matsui

et al., 2014) in the following way:

(1) The most critical temperature range is 150℃ to 300℃, degradation process occurs

depending on residual stress, ageing time and environment of deterioration.

(2) The effects of ageing are the reduction in strength, toughness and density, and an

increase in monoclinic phase content.

(3) Degradation of mechanical properties is due to the t-m transition, resulting in

volume expansion and takes place with micro and macrocracking of the material.

(4) t-m transition starts on the surface and progresses into material bulk with ageing

time.

18

(5) t-m transformation is enhanced in water or in vapor.

(6) Degradation can be suppressed by reduction in grain size and/or increase in

concentration of stabilizing oxide.

2.2.1 Mechanism of degradation

Thirty years after the initial discovery of the problem, research into the exact

nature of LTD is still ongoing, and for long this phenomenon remains only partially

understood. Although a single model which successfully describes all experimental

observation has yet to be agreed, mechanical and chemical mechanisms that are widely

assumed to phenomena LTD in Y-TZP can be classified into mechanisms of corrosion,

destabilization and stress-induces transformation as summarized by Ramesh (1999).

2.2.1.1 Corrosion mechanism

Primarily, this mechanism involves water reaction with the constitutes of Y-TZP.

Yoshimura et al. (1989) suggested that the residual stress was created once Zr react with

water to for Zr-OH on the surface as shown in Figure 2.8 (Stage 1 and 2). Further

accumulation of these stresses happen by the migration of OH- ions in the lattice which

causes the development of monoclinic nuclei as shown in Figure 2.8 (Stage 3 and 4). As

a result, nucleation of monoclinic domains in tetragonal matrix and lowering the barrier

for the phase transformation to occur.

19

Figure 2.8 The proposed degradation mechanism of Yoshimura et al. (1989)

Sato and Shimada (1985a) also proposed a similar mechanism involving

chemisorption of water to form Zr-OH at the surface but they predicted that the OH

formation will result in the release of the strain which stabilizes the tetragonal grain

attribute the accumulation of strain energy to the phase transformation. Kim (1997)

also reported that the presence of water does not necessary indicate the beginning of the

phase transformation but the amount of residual stress on t grains of zirconia triggers the

degradation process.

Str

ess

accu

mula

tion

Stage 4 Stage 3

Stage 2 Stage 1

vacancy

20

Guo (2001) further improved the mechanism deduced by Yoshimura (1987) by

proposing a new degradation mechanism involving annihilation of oxygen (anion)

vacancies by OH- ions as summarized below:

1) Chemical absorption of water on zirconia surface

2) Reaction of water with O2- on the surface to form OH-

3) Penetration of OH- in the lattice by grain boundary diffusion

4) Annihilation of oxygen vacancies by OH-

5) t-m transformation occurs when oxygen vacancy concentration not stable, further

degradation expands along grain boundaries and crack.

This model mechanism is able to explain the influence of grain size and stabilizer

content in the zirconia ceramic. The author explained that diffusion of OH- ions and

oxygen vacancies remain sufficiently high at the ageing temperature while increasing the

stabilizer content as well as decreasing grain size can account for lower phase

transformation due to large number of oxygen vacancies at the surface and grain

boundaries.

2.2.1.2 Destabilization mechanism

Unlike corrosion mechanism, destabilization mechanism focuses on the reaction

of water with the yttrium in the Y-TZP matrix. It seems that in the presence of water, the

yttrium ions react to form cluster rich of Y(OH)3 which were frequently found adjacent

to transform monoclinic and cubic grains (Lange et al,1986). The author observed clusters

of small crystallites which bridged gaps in the foils and which upon examination by

EDAX, it was confirmed as yttrium rich cluster. Lange proposed that when water react

with yttria, shortage amount of yttria on the surface of the grain causes destabilization

and provoking monoclinic nucleation. The monoclinic nucleus grew by further depletion

of yttrium until critical size was exceeded. Lange also emphasized that nucleation, growth

21

and transformation of subsurface tetragonal grains will be limited by the long-range

diffusion of yttrium to the surface if the transformed grain size is smaller than critical

size.

Wang and Steven (1989) have also supported the idea in the degradation model

proposed by Lange et al. (1986). However, the author further suggested a two-step

mechanism to explain the structural instability of Y-TZP ceramics:

1) Nucleation process: numerous transformation of (t) domain where yttria content

is lower than an average value in regions of lack in yttria or defect area, such as

cracks, etc.

2) Growth process: the autocatalytic propagation of the small transformed zones to

complete the phase transformation.

Hernandez et al. (1991) support the mechanism of Lange et al. where the author found

compelling evidence when carrying out an XPS study on hydrothermally degraded TZP

surface. The degradation begins on the surface of t grains along the establishment of

YO(OH) and the pure yttria free t grain within the zirconia lattice. Chevalier et al. (1999)

also have made observation to associate the underlying LTD mechanism of Y-TZP to the

nucleation and growth of monoclinic phase, where increasing of monoclinic phase

content after increase the duration from 0 hours to 7 hours exposure of steam at 134℃.

However, Kimel and Adair (2002) established a diffusion control leaching model which

did not show any significant diffusion of hydroxide into the 3Y-TZP from the aqueous

solution at room temperature. Besides, they also found that yttrium not has no effect on

the amount of monoclinic and tetragonal phase in room temperature as confirmed by

XRD.

22

2.2.1.3 Stress-induced transformation

Besides both corrosion and destabilization mechanism, stress-induced

transformation has also been reported by Schmauder and Schubert (1986) who inferred

that the stress situation of the grains was the prime factor governing the phase stability.

This conclusion was made after they studied ageing behavior of thin foil sintered sample

when exposed in humid air at 250℃ for up to for 168 hours. A schematic representation

of the samples before and after ageing is shown in Figure 2.9.

Figure 2.9 TEM sample before and after ageing in humid air (Schmauder and

Schubert, 1986)

The author found that unconstrained grain (4) was stable and did not undergo the

phase transformation compared to constrained grains (2 and 3) which transformed to

monoclinic phase. The grains (2 and 3) and grain (4) have a similar free surface area along

with homogeneous surface energy status. But these grains behaved differently during

exposure which showed that the surface energy term is not the major factor affecting the

tetragonal phase stability. Thus, the stresses that destabilized and assist the transformation

in the zirconia matrix are shear and tensile stress while compressive stresses stabilize the

grains as suggested by Schubert and Frey (2005). The shear stress is normally largest and

most likely to nucleate or control the transformation.

23

Munoz-Tabares et al. (2011) also proposed the stress induced transformation by

microcracking; i.e, interaction between a martensite plate and a grain boundary. In Figure

2.10a shows that once the single martensite plate formed in grain 1 reach its grain

boundary, a local tensile stress will appear at the grain boundary if grain 2 is unable to

accommodate shear component of the transformation (shape change). First plate will be

formed from the lattice correspondence able to aid the deformation corresponding with

the shape change by surface uplift as shown in Figure 2.10b.

Figure 2.10 Mechanism proposed for microcracking and monoclinic phase

propagation (Munoz-Tabares et al., 2011)

24

Subsequently, microcraking is the result of shear strain accommodation produced

by formation of the first martensite plate in partially transformed grains. The mechanism

proceeds when the associated microcracking releases the constraint, thereby promoting

transformation within the grain by autocatalytic transformation, spreading into the matrix

across several grains away from the nucleation grain. This is a result of martensite plate

providing local stress in the surrounding grains which undergoes induced transformation.

The rate of monoclinic phase propagation increases drastically until saturation.

2.2.2 Prevention of degradation

The observation and evidence of hydrothermal degradation experiment is well

documented by numerous researcher. There are conditions under which the LTD

phenomena can be retarded by proper control of specific parameter such as composition

and microstructure through sintering additives or by manipulating sintering conditions.

However, in some improvement in ageing resistance may cause reduction of other

mechanical properties such as fracture toughness. Hence, the degradation can be

minimized by applying one of the following strategies: (1) reduction of grain size; (2)

increase of stabilizer concentration; (3) inhomogeneous stabilizer distribution (4)

introduction of inert materials as dopants.

2.2.2.1 Reduction of grain size

Chevalier et al. (2004) observed that sintering duration and time influence 3Y-

TZP grain size. Increasing sintering temperature from 1450℃ to 1550℃ at the holding

time of 2 hours resulted in significant increase in grain size from 0.4 µm to 0.6 µm.

Similarly, increasing holding time of the samples from 2 to 5 hours at a constant sintering

temperature of 1450℃ resulted an increase in grain size.

25

Feder and Anglada (2005) also studied the grain size effect during ageing process.

They sintered the Y-TZP at different holding time so they can obtain grain size higher

than critical grain size. The resulted grain size increases from 0.3 µm to 1.05 µm and 1.37

µm with 1 and 2 hours holding time, respectively. The sample with longer holding time

of 2 hours transformed the most monoclinic phase compare to the samples having holding

time of 1 hour which showed lower monoclinic phase content. The author suggested that

tetragonal grain size controls the microcracks formation on the surface of the transformed

grains with increase in grain size resulted into the increase in shear strain which is

associated to the transformation and thus causes microcrack in the matrix.

Gaillard et al. (2008) also conducted similar experiments by varying the t grain

size during the heat treatment of the Y-TZP samples in air. By using the combination of

AFM analysis and Focused Ion Beam cross section (FIB), they obtained much thicker

degradation layer in the sintered samples as compared to the starting material. Thus, they

concluded that Y-TZP with larger t grain size easy to degrade than smaller grain

tetragonal zirconia.

Similarly, to the recent finding by Elshazly et al. (2011) aged three different 3Y-

TZP samples sintered at temperature 1450℃, 1550℃ and 1650℃. The XRD analysis

revealed the highest monoclinic content for the sample sintered at the higher temperature

as shown in Figure 2.11.

26

Figure 2.11 XRD analysis of 3Y-TZP sintered sample after hydrothermal

degradation in humid environment at 130℃ and 140 hours (Elshazly et al., 2011)

Hence, it can be agreed upon that the sintering condition plays an important role

in grain maturity and size of the 3Y-TZP ceramic. Increase in temperature and holding

time will increase the grain size and show greater susceptibility to degrade compared to

3Y-TZP with smaller grain size.

2.2.2.2 Increase of stabilizer concentration

One of the major controlling variable to slow down the undesirable t to m phase

transformation of LTD is yttria content of zirconia. To generate the significant mechanical

properties, it is mandatory to have zero content of monoclinic phase in microstructure

which would act as a flaw, and the optimum level of stabiliszer added. Xu et al. (2004)

and Huang et al. (2005) has done extensive research work on TZP ceramics. This work

has dispute the amount of yttria required to achieve optimum properties, including

fracture toughness.

Sato et al (1985b) proposed that higher amount of yttria in Y-TZP led to better

resistance to LTD and the degradation is marked more in the case lower dopant content

as depicted in Figure 2.12 which showed that the monoclinic zirconia content in 4 mol %

27

Y-TZP is lower than 2 mol % Y-TZP. Furthermore, there is no monoclinic formed in

samples of 3 and 4 mol % Y-TZP when sintered below 1400℃ and 1450℃, respectively.

The study also revealed that an increase in yttria content also helps in lowering the critical

ageing temperature at which monoclinic phase transformation occurs.

Sato and Shimada (1985a) also conducted experiment on the amount of

percentage of yttria concentration in zirconia matrix and its subsequent effect on the

hydrothermal degradation. They found that 4 mol% Y-TZP sample yielded ~48% of

monoclinic content compared to 3 mol% of Y-TZP sample which resulted into ~98% of

monoclinic phase transformation when sintered at 1600℃. Similar study by Wang and

Stevens (1989) revealed that 2 mol % of yttria had lower stabilizing effect on the zirconia

matrix compared to 2.5 and 3 mol % of yttrium oxide. Ageing of 2Y-TZP yielded higher

nucleation boundary region which led in a decrease in surface toughness and higher

monoclinic phase transformation. Furthermore, sever cracking on the sample was

observed due to the large volume expansion and shear strain. On the other hand, 2.5 and

3 mol % Y-TZP showed lower phase transformation making the zirconia matrix more

stable.

However, increasing the stabilizer content may over-stabilize the tetragonal

structure. Lawson et al. (1993) who studied the ageing behavior of Y-TZPs with yttria

content ranging from 2 to 4 mol% found that formation of monoclinic decreased when

yttria content was raised. Nevertheless, the highest yttria content suffered a sudden failure

after 100 hours of exposure despite exhibiting the least phase transformation. This poor

ageing resistant of the Y-TZP is due to yttria depletion from tetragonal grain into cubic

grains which led to phase instability of the tetragonal structure.

28

Figure 2.12 The effect of monoclinic content and ageing temperature with

different yttria content in Y-TZP sintered at 1400℃ to 1600℃ and aged for 50

hours (Sato et al., 1985b)

2.2.2.3 Inhomogeneous stabilizer distribution

This study focuses on yttria as stabilizer, where having an inhomogeneous yttria

distribution in the zirconia enable it to protect the grain boundary region from hydroxyl

reaction. For example, having higher yttria concentration near grain boundary region.

Ruhle et al. (1983) upon examining several commercially available Y-TZP samples found

that the samples having inhomogeneous yttria distribution were more degradation

resistant compared to the sample with homogeneous yttria distribution.

While in 1998, Piconi et al. who performed hydrothermal treatment at 120℃ for

both coated and co-precipitated Y-TZPs, concluded that Y-TZP made from coated

29

technique had enhanced ageing resistance when compared to co-precipitate technique.

Co-precipitation and coating Y-TZP has homogeneous and inhomogeneous distribution

yttria in zirconia grain, respectively. The authors observed that co-precipitated Y-TZP

suffer bending strength reduction and transformed a thick layer after 48 hours of exposure

while coated Y-TZP remained intact even after 120 hours without any property

degradation. Ramesh et al. (1999) also attributed the enrichment of yttria at the grain

boundaries coated Y-TZP to slower ageing kinetic when compared to co-precipitated Y-

TZP. Impulse excitation technique (IET) test by Roebben et al. (2003) supported the

better ageing resistance of Y-TZP with heterogeneous yttria distribution when compared

to Y-TZP with homogeneous distribution yttria. Basu et al. (2004) also reported that Y-

TZP ceramic with inhomogeneous yttria distribution were less prone to degradation when

compared to those of homogeneous yttria distribution. The author further postulated that

the difference in LTD behavior to be from the microstructural constraint imposed by the

higher yttria containing grains to prevent the transformation initiated in the low yttria

containing grains, which is possible in coated Y-TZP since a gradient exist for yttria

distribution in the zirconia matrix.

2.2.2.4 Introduction of dopant

Another effective way of preventing Y-TZP to degrade is to dope Y-TZP with

certain amount of sintering aid. This is because doping with additives is the simplest and

economical solution to problem encountered during the sintering of undoped Y-TZP. The

effect of dopant on the tetragonal phase stability of Y-TZP can be beneficial or

detrimental, depending on the type and amount of additive, optimum condition such as

sintering parameter. The amount of dopant and controlling the sintering parameter are

necessary to induce degradation of Y-TZP ceramics.

30

Kondo et al. (2000) reported that a dense YTZP body could be obtained with the

addition of 0.3mol% NiO. However, the authors also found that higher amounts of NiO

were detrimental as this resulted in formation of monoclinic phase and microcracks in the

sintered Y-TZP bodies. It is clear that careful selection of sintering additives and the

amount used are crucial so as not to disrupt the sinterability of Y-TZP. Gill et al. (1996)

and Ramesh et al. (1999) reports on copper oxide, CuO addition to Y-TZP seemed to

suggest 0.2 wt% CuO to be the apparent limit for the retention of high tetragonal phase,

beyond which transformation of tetragonal to monoclinic zirconia proceed. Ran et al.

(2006) observed the formation of monoclinic phase during heating stage from 1100℃ to

1200℃ and small cracks on the surface after sintering when 0.5 wt% CuO doped Y-TZP.

Lemaire et al. (1999) proposed that poor tetragonal phase retention can be attributed to

segregation of yttria to the grain boundary followed by formation of Y2Cu2O5 product as

a result of its reaction with CuO and thus, leaving zirconia grain with insufficient yttria

to stabilize the tetragonal structure.

Shi and Yen (1997) suggested another possibility of destabilization is the

overwhelming growth of the tetragonal grains as a result of CuO addition which could

have caused the grain size to exceed the critical limit, thus triggering the spontaneous

transformation to monoclinic symmetry upon cooling from sintering. Further experiment

by Kanellopous and Gill (2002) revealed that CuO-doped Y-TZP sintered at 1315℃ with

12 minutes holding time showed better ageing resistance compared to 2 hours holding

time, and this is attributed to the grain boundary modification caused by CuO which

protected the region against hydroxyl reaction.

Zhi-kai Wun et al. (2013) also highlighted that the improvement of tetragonal

phase stability of Y-TZP using alumina as sintering additives. Introduction of 0.5–5.0

wt% Al2O3 addition into 3Y-TZP effectively delayed the tetragonal-monoclinic phase

31

transformation and the degradation of mechanical property during aging, when sintered

at 1400–1600℃. However, it is interesting to note that a secondary phase, α-Al2O3 was

detected by XRD when 0.5-5 wt% Al2O3-doped Y-TZP sintered at 1350℃ even though

they did not show any monoclinic phase content as reported by Yang et al. (2004). The

author ascribed this observation to the amount of alumina addition used that exceeded the

solubility limit of alumina in 3Y-TZP, which is about 0.2 wt% at 1350℃.

2.3 Effect of sintering additives on mechanical properties and microstructure

evolution on Y-TZP

The major drawback of 3Y-TZP that prone to LTD has gotten the attention of

numerous researcher where Li et al. (1996) and Deville (2003) discovered that grain

boundary modification to be the most economical method. By adding proper selection of

sintering additive or dopant for tetragonal lattice of zirconia, it can increase life-time

under hydrothermal conditions. The interest in this research is an addition of dopant

particularly focus on MnO2 and Al2O3 which have been examined by researcher and they

found that different dopant provides different role in improving the zirconia ceramic.

2.3.1 Effect of manganese (IV) oxide doped Y-TZP

Meenaloshini et al. (2008) studied the effect of up to 1 wt% doped MnO2 addition on

sinterability of tetragonal zirconia over the temperature range of 1250-1500℃. It was

found that for all ≥0.3 wt% doped MnO2 the relative density of 98% can be obtained at

lower temperature 1300 ℃ however fracture toughness only increase in 1 wt% of MnO2

doped sample sintered above 1400℃.

In another study, Ramesh et al. (2013) studied the effect of MnO2 dopant up to 1wt%

on the sintering behavior of 3Y-TZP at sintering temperature range of 1250℃ to 1500℃.

They found that mechanical properties of 3Y-TZP were dependent on the content of

dopant and sintering temperature. Doped sample with 0.5 wt% MnO2 achieve higher

32

relative density 97.5% as compared to undoped 91.8% when sintered at low temperature

range of 1250℃ to 1300℃. It was revealed that 1 wt% of MnO2 were not enhanced in

the densification of 3Y-TZP when sintered beyond sintering temperature of 1300℃. This

result is supported by XRD analysis which showed that 1 wt% MnO2 doped 3Y-TZP

exhibited monoclinic phase in zirconia matrix 3% and 55% at sintering temperature of

1500℃ and 1600℃ respectively. The presence of monoclinic phase in the zirconia matrix

also affected the fracture toughness of the ceramic where 1 wt% MnO2 doped 3Y-TZP

shows increasing of fracture toughness drastically when sintered above 1400℃, from 4.8

MPam1/2 to 7 MPam1/2. This was due to dissolution of yttria in zirconia matrix, where the

potential of stabilizer to retain tetragonal phase was reduced, which results in monoclinic

phase transformation upon cooling.

2.3.2 Effect of alumina doped Y-TZP

Vasylkiv et al. (2003) used 0.2-0.7 wt% of fine grained alumina as sintering additives

for 3Y-TZP. The sintering studies were carried out at temperature of 1150℃ with 2 to 30

hours of holding time. The authors noticed that as the sintering holding time was increased

from 2 to 12 hours then to 30 hours, the relative density of undoped samples also increase

from 95 to 97.5% and 99.1% respectively. The small amount of dopant alumina helped

in enhancing the density of the material by reducing sintering time, with 12 hours of soak

temperature yielded 99.2% of relative density. The 0.2-0.35 wt% addition of alumina was

beneficial in restricting microstructure to nano-scale region and producing relative

density of 99.5% with highest hardness value of 16.8 GPa.

Furthermore, Nogiwa-Valdez et al. (2013) has reported that 0.1 wt% of alumina give

significant deceleration of ageing without modifying the mechanical properties of 3Y-

TZP especially fracture toughness when sintered for 3 hours at 1450℃. The monoclinic

33

content for 0.1 wt% of alumina was less than 10% as compared to undoped near to 20%

when undergo ageing for 10 hours as shown in Figure 2.13.

Figure 2.13 Monoclinic content measured by XRD (Nogiwa-Valdez et al., 2013)

2.3.3 Effect of manganese (IV) oxide and alumina co-doped Y-TZP

Alternatively, the addition of ternary dopant proved to be an effective method by

segregation of ternary dopant to the grain boundary to retard the LTD rate as

recommended by Ross et al. (2001). The latest finding by Ragurajan et al. (2016)

discovered that the addition of alumina and manganese (IV) oxide was effective in

enhancing mechanical properties particularly when addition of alumina is more than

manganese (IV) oxide, but this finding requires confirmation. Therefore, the purpose of

this work was to investigate the combine effect of 0.1 wt% of alumina and 0.5 wt% of

manganese (IV) oxide co-doping on the mechanical properties and microstructure of 3Y-

TZP ceramics sintered at different temperature at range of 1250-1550℃.

34

In the summary, there are promising results in term of mechanical properties and

ageing resistance reported by various researcher who used sintering additives in Y-TZP.

However, it is observed that the effect of co-doping two or more additives on the sintering

of Y-TZP of Y-TZP was not fully exploited in the literature especially in co-doping

(MnO2 and Al2O3). Based on the literature review, there are potential benefits that could

be attained from the use these dopants.

35

CHAPTER 3: METHODOLOGY

3.1 Preparation samples

3.1.1 Powder

In this present study, the starting powder used was 3 mol% Yttria-Stabilized

Tetragonal Zirconia Polycrystalline (Y-TZP) powder (Tosoh ,Japan) having major

impurities of Al2O3, SiO2, Fe2O3 and Na2O, particle diameter of 0.4 µm and specific

surface of 16±3 m2/g. In this work, the Y-TZP powder would be known as undoped

Y-TZP.

Two dopant powder were used, 0.5 wt% of manganese(IV) oxide (MnO2)

provided by R&M Chemicals, UK and 0.1 wt% of alumina (Al2O3). The weight

percentage of dopants was chosen based on the latest studies in 3Y-TZP (Ramesh et

al., 2013; Nogiwa-Valdez et al., 2013).

3.1.2 Mixing powder

There are numerous methods used to mix the as-received Y-TZP powder with

sintering additives (MnO2 and Al2O3) and in the present work an attrition mill (Union

Process Inc. Akron,Ohio) was utilized. Before milling process took place, the

weighted powder and 150 ml of ethanol was poured in the beaker as mixing medium,

and subjected to ultrasonic vibrator for 20 min to obtain homogeneous mixture.

Besides, a stir was used to avoid deposition of powder at the bottom of beaker.

Then, ultrasonified powder mixture was placed in a stationary jacketed grinding

tank together with the grinding media (5 mm zirconia balls). The media was stirred at

550 rpm for 30 min with the help of rotating shaft attached with cross arm. After the

milling process, the zirconia balls were taken out through a sieve and the resulting

mixture was poured into a bowl and dried in an oven at 60℃ for one night. The dried

cake was then crushed and sieved using 212 𝜇m mesh stainless steel sieve and the

36

resulting powder referred as doped Y-TZP. In this mixing procedure, the additional

ethanol was used while transferring the mixing powder from one medium to another

to avoid any loss of powder mixture.

3.1.3 Green body preparation

An uniaxial press was used to compact the undoped and doped Y-TZP powder

into disc and rectangular bar sample at 3 MPa pressure using hydraulic bench press

(SPX Hydraulic Technologies, Rockford, Illinios, USA). Disc samples (diameter 20

mm, thickness 4 mm, each weighing 2.5 g) and rectangular bar samples (4x13x32

mm, each weighing 3 g) were compacted in steel die to ensure the uniform volume in

the samples.

Since the pressing process was a dry process, an oil based liquid such as WD-40

was used to clean, remove the remain powder and avoid contamination powder in

samples. After uniaxial compaction, the samples undergo cold isostatically pressed

(CIP) (Riken Seiki,Japan) at 200 MPa for 1 min to improve the compaction and

densification of the green body when the samples are subjected to high pressure from

every direction (Koizumi & Nishihar,1991). Prior to CIP, the samples are placed and

secured in latex glove before undergoing the high pressure.

3.1.4 Sintering

The aim of sintering process is to produce denser matrix with much finer grain

size. The conventional pressureless sintering was conducted in a box furnace (Box

Furnace, Malaysia) hold for 2 hours under sintering temperature ranging from 1250℃

to 1550℃ at a ramp rate of 10 ℃/min. These parameters were selected based on

previous works (Ramesh et al.,2013; Ramesh & Gill,2001). Grinding and polishing

37

The sintered samples were grounded and polished to obtain optical reflective

surface prior to further experiments. Grinding and polishing processes were

conducted manually on a machine. Only one surface for each sample were

grounded at a speed ranging below 70 rpm by using silicon carbide (SiC) papers

of 120, 240, 600, 800 and 1200 grades. After grinding, the samples were polished

using 6 𝜇m and 1 𝜇m diamond paste.

3.2 Mechanical properties measurement

3.2.1 Relative density

The bulk density of the undoped and doped 3Y-TZP samples was determined

by water immersion technique based on Archimedes principle while distilled

water was used as an immersion medium. An electronic balance (Shimadzu

AY220, Japan) retrofitted with a density determination kit was used for the

measurement of bulk density. The weight of samples in air and water were

measured to determine its bulk density. The relative density of the sample was

obtained by using equation (3.1):

𝜌 =

𝑊𝑎𝑊𝑎−𝑊𝑤

𝜌𝑤

6.09 x 100 (3.1)

Where,

𝜌 = bulk density

𝑊𝑎 = Weight of samples in air

𝑊𝑤 =Weight of samples in water

𝜌𝑤 =density of distilled water

38

The density of distilled water (𝜌𝑤) at room temperature 25℃ was obtained from

the table shown in Appendix A. The theoretical density of Y-TZP, 6.09 Mgm-3

was taken.

3.2.2 Young Modulus

The Young’s Modulus by sonic resonance technique was determined for

rectangular bar samples using a commercial testing instrument (GrindoSonic:

MK5 “Industrial”,Belgium). The instrument allows determination of the

resonance frequency by placing the transducer sensor near to samples, where

vibration is physically induced in the sample by tapping. The modulus of elasticity

is calculated using the experimentally determined resonance frequencies,

according to standard test method ASTM E1876-97. The Young’s Modulus (E) is

calculated using equation (3.2):

𝐸 = 0.9465(𝑚𝐹𝑓

2

𝑏)(

𝐿

𝑡)3𝑇1 (3.2)

Where,

E = Young’s Modulus

𝑚= Mass of bar

𝑏= Width of bar

𝐿= Length of bar

𝑡= Thickness of bar

𝐹𝑓 = Fundamental resonant frequency of bar in flexural (Hz)

39

𝑇1= Correction factor for fundamental flexural mode, calculated using equation

(3.3):

𝑇1 = 1 + 6.585(1 + 0.0752𝜇 + 0.8109𝜇2)(𝑡

𝐿)2 − 0.868(

𝑡

𝐿)4

−8.340( 1 + 0.2023𝜇 + 2.173𝜇2)(

𝑡𝐿)4

1 + 6.338(1 + 0.1408𝜇 + 1.536𝜇2)(𝑡𝐿)2

Where,

𝜇= Poisson ratio of Y-TZP taken as 0.23

3.2.3 Vickers hardness

Vickers hardness of the polished samples was determined by using Vickers

indentation method. Indentation was performed using the Vickers micro-hardness

tester from Mitutoyo AVK-C2, USA. The indentation was applied by pyramidal

diamond indenter and impression formed on the indented surface took the form

of an inverted pyramid as shown in Figure 3.1 and 3.2. A load of 10 kgf was

applied slowly and smoothly and was held for 10 second.

40

Figure 3.1 Schematic diagram of an indentation for measurement

Figure 3.2 Typical indentation formed (Flavio T. da Silva et al.,2007)

Generally, after the load is removed, the indentation appears to be square and two

diagonals, whereby D1 and D2 have similar lengths. The crack lengths of L1, L2, L3 and

L4 were measured. Three indentations were made and average diagonal length was taken

to improve the accuracy of the Vickers Hardness value. The hardness value was

calculated using equation (3.4):

𝐻𝑉 =1.854 𝑃

𝐷2 (3.4)

41

Where,

Hv= Vickers hardness value, Pa

P= Applied load, N

D= Mean diagonal length, m

3.2.4 Fracture toughness

After each Vickers indentation, the length of four cracks L1, L2, L3 and L4 were

taken to calculate the mean length. The fracture toughness, KIC of material is

determined using derived equation by Niihara et al. (1982), as shown by equation (3.5):

𝐾𝐼𝐶 = 0.035(𝐿

𝑎)

12⁄ (

𝐸Φ

𝐻𝑣)

25⁄ (

𝐻𝑉

Φ)𝑎

12⁄ (3.5)

Where,

𝐾𝐼𝐶= Fracture toughness value, MPam1/2

L= Mean crack length, m

a= half of average diagonal length, m

E= Young’s Modulus, MPa

Φ= Constraint factor for tough ceramic taken as 3 (Evans & Charles,1976)

42

3.3 Microstructure and Phase Composition

3.3.1 FESEM Analysis and Average Grain Size

Prior to conducting FESEM analysis, the samples were subjected to thermal

etching to outline the grain boundaries. The samples were thermal etched at

temperature 50℃ below their respective sintering temperature (For examples, sample

sintered at 1550℃ were thermal etched at 1500℃) at ramp rate of 10 ℃/min for 30

minutes.

The thermal etched samples were placed on small pins with the help of carbon

tape and were mounted in the microscope chamber. An electron beam was fired on

the samples and secondary electron emitted by the samples was used to capture

images at various position at a voltage of 1-1.5 kV and a magnification of 3K X,5K

X, 10K X and 20K X.

The average grain size of the samples was determined by line intercept method

introduced by Mendelson (1969), after obtaining high quality images for both

undoped and doped 3Y-TZP from FESEM. A test line was drawn across the FESEM

micrograph printed on A4 paper and the number of intercept between the line drawn

and grain boundaries were counted. Then, the average grain size was calculated by

using proposed equation by Mendelson (1969), as shown equation (3.6) :

�̅� = 1.56�̅� (3.6)

Where,

�̅�= Average grain size

�̅�= Average interception length over large number of grains, calculated using equation

(3.7):

43

�̅� =𝐶

𝑀𝑁 (3.7)

Where,

C= Total length of the test line

M= Magnification of FESEM micrograph

N= Number of intercept

3.3.2 XRD Analysis

X-Ray Diffraction (XRD) was used to analyze the present phase in the sintered

samples for both undoped and doped 3Y-TZP. XRD is a rapid analytical method

which is used to identify the phase present in material due to dual wave or particle

nature X-rays portray the information of the structure materials. The incident beam

of X-rays interacts with the sample and results in scattering of X-rays from atoms

within the samples to produce a diffraction pattern that provide necessary information

about atomic arrangement in the crystal structure of the materials.

The XRD machine’s generator works at 40 mA and 40 kV, with anode of Cu-K𝛼

having a step scan of 0.5 °/min and a step size of 0.02° (2𝜃). The range of 2𝜃 graph

of 3Y-TZP was chosen on the tetragonal and monoclinic detection range that has one

peak for tetragonal (t) phase at ~30.2° for (1 1 1)t and two peaks for monoclinic (m)

phase at ~28.2° for (1 1 1̅)m and ~31.5° for (1 1 1)m. The 2𝜃 range for this research

was chosen from 26° and 34°.

44

The monoclinic phase present in both undoped and doped 3Y-TZP samples can

be calculated by using derived equation by Toraya et al. (1984) as shown in equation

(3.8):

𝑉𝑚 =1.311 𝑋𝑚

1+0.311 𝑋𝑚 (3.8)

Where,

𝑉𝑚= Volume fraction of monoclinic zirconia

𝑋𝑚= Integrated instensity ration, can be calculated using equation (3.9):

𝑋𝑚 =𝐼(1 1 1)𝑚+𝐼(1 1 1̅)𝑚

𝐼(1 1 1)𝑚+𝐼(1 1 1̅)𝑚+𝐼(1 1 1)𝑡

(3.9)

Where,

𝐼(1 1 1)𝑚= Intensity peak at monoclinic phase at (1 1 1) plane

𝐼(1 1 1̅)𝑚= Intensity peak at monoclinic phase at (1 1 1) plane

𝐼(1 1 1)𝑡= Intensity peak at tetragonal phase at (1 1 1) plane

The volume fraction of tetragonal zirconia (Vt) in term of percentage can be

calculate by using equation (3.10):

𝑉𝑡% = 100% − 𝑉𝑚% (3.10)

45

3.3.3 Ageing

The hydrothermal degradation or ageing experiment was performed for both

undoped and doped 3Y-TZP under hostile environment to study the effect of

ageing on the mechanical properties and grain size of the samples. Both undoped

and doped 3Y-TZP sample were placed in an autoclaves vessel (digestion

chamber) obtained from Parr Instrument, USA as shown in Figure 3.3. The vessel

was filled with 10 ml of distilled water at room temperature. The ageing vessel

was placed inside an oven which was held at 185℃ to convert water into

superheated steam. The ageing was performed in the superheated steam for 1 hour

and 3 hours, then followed by conducting XRD analysis to determine phase

transformation.

Figure 3.3 Cross section of autoclaves vessel

46

3.4 Research work flow chart

Mixing powder

(attrition milling)

Dry, crush, sieved powder

Weighing powder and

compress

Sintered sample

Polishing and grinding

Mechanical properties test Thermal etched sample

FESEM analysis

Aging the sample

XRD analysis

XRD analysis

47

CHAPTER 4: RESULTS AND DISCUSSION

This chapter covers the findings of the current research. The starting powders used in

the present study were commercially available high purity 3 mol% yttria-stabilized

tetragonal zirconia polycrystals (TZ-3YB, Tosoh Corporation, Japan). The co-doping was

accomplished by mixing 0.5 wt% of MnO2 and 0.1 wt% of alumina simultaneously with

the Y-TZP powder. For comparison purpose, the dopant was also added separately with

the Y-TZP powder.

4.1 Phase analysis

The phase analysis was performed through X-Ray diffraction (XRD) at two stages

of experimentation: as-sintered samples and aged samples exposed to autoclave

conditions at different aging time. This section will discuss the phase analysis of as-

sintered samples while results of the aged samples will be presented in section 4.4. The

entire data of phase analysis can be found in Appendix D.

Figure 4.1 XRD pattern of sintered undoped and doped 3Y-TZP samples

25 27 29 31 33 35

°2 Theta, 2θ°

t

d) undoped

c) 0.5 wt% MnO2

b) 0.1 wt% Al2O3

a) Co-doped

48

After sintering at temperature of 1250℃ to 1550℃, all samples showed

effectiveness in controlling tetragonal phase stability with no traces of monoclinic phase

contents as all the sintered samples showed fully 100% tetragonal phase during XRD

analysis. Figure 4.1 revealed that the dopant additions as well as the sintering

temperatures have not disrupted the tetragonal phase stability and all samples regardless

of the type of dopant used revealed the presence of a fully tetragonal structure after

sintering.

4.2 Mechanical Properties Analysis

4.2.1 Relative Density

The effects of sintering temperature on relative density of undoped and doped Y-

TZP samples are as shown in Figure 4.3. The results revealed that addition of dopants

was beneficial in enhancing the densification of Y-TZP at low temperature. In particular,

sintering at 1250°C resulted in highest relative density of 97.6% as compared to 92.9 %

for the undoped sample. While the low relative density of undoped samples at low

sintering temperature can be related to the presence of residual pores in the samples as

shown in Figure 4.2a.

However, as the sintering temperature increased to 1350°C, the co-doped and 0.5

wt% MnO2 doped Y-TZP did not show any improvement in the relative density. This is

in agreement with Ramesh et al. (2005) that found addition of manganese oxide to be

most beneficial at low sintering temperature of 1250℃ to 1300℃. The addition of 0.1

wt% Al2O3 was also effective in aiding densification and the samples achieved relative

density of 98.8% as compared to 98.4% for undoped ceramic when sintered at 1350°C.

49

Figure 4.2 Effect of dopants and sintering temperatures on the relative density

of Y-TZP.

In contrast, as sintering proceeded beyond 1350°C, all the dopants did not exhibit

much effect in promoting densification as shown in Figure 4.3. On the contrary, the

relative density of the undoped Y-TZP was found to increase gradually with increasing

sintering temperature and attained a maximum of 99% when sintered at 1550°C.

4.2.2 Young’s Modulus

The variation in Young’s Modulus (E) for the Y-TZP with sintering temperature

and dopants are shown in Figure 4.4. In agreement with the relative density trend, the

addition of dopants was effective in enhancing the stiffness of the tetragonal zirconia

matrix when sintered below 1450°C. The results indicated that co-doped samples had

lower E value when compared to single dopants regardless of sintering temperature

employed. The addition of MnO2 was beneficial in enhancing the elastic modulus of Y-

TZP at low sintering temperatures of 1250℃. It was found that, the addition of 0.5 wt%

MnO2 achieved the highest E value of 194 2 GPa at 1250°C while the addition of Al2O3

gave the highest E value of 200 1.5 GPa at 1350°C. In agreement with the bulk density

90.5

91.5

92.5

93.5

94.5

95.5

96.5

97.5

98.5

99.5

1250 1350 1450 1550

Rel

ati

ve

Den

sity

(%

)

Sintering Temperature (℃)

Undoped

0.5wt% MnO2

0.1 wt% Al2O3

Co-doped

50

trend, the E value of the undoped Y-TZP increased gradually with increasing temperature

and reached a maximum of 203 2 GPa at 1450°C before declining slightly to 197 1.5

GPa at 1550°C.

Figure 4.3 Young’s modulus variation with dopant addition and sintering

temperatures.

The relative lower E value of the co-doped samples at higher temperatures of

1450℃ to 1550℃ could be attributed to the dissolution of yttria into dopant phase. This

is believed to have been caused by depletion of yttria content at grain boundary which

results into reduced matrix stiffness of 3Y-TZP (Khan et al, 2014). Different studies

which adopted varying amounts of MnO2 and Al2O3 separately at various temperature

reported similar trends in E of Y-TZP ceramics (Ramesh et al.,2011; Nogiwa -Valdez et

al.,2013).

90

110

130

150

170

190

210

1250 1350 1450 1550

You

ng's

Mod

ulu

s (G

Pa)

Sintering Temperature (℃)

Undoped

0.5wt% MnO2

0.1wt% Al2O3

Co-doped

51

4.2.3 Vickers Hardness

The influence of sintering temperature and dopants addition on the Vickers

hardness of the Y-TZP are shown in Figure 4.5. It was found that the hardness of co-

doped Y-TZP was relatively high when sintered at 1250°C and 1350°C. The highest

hardness achieved by this sample was ~14.6 GPa at 1350°C when compared to 12.7 GPa

for undoped sample. The 0.1 wt% Al2O3-doped Y-TZP samples exhibited similar trend

and have almost similar hardness values as that of the co-doped Y-TZP. In all cases, the

trend of hardness for the doped sample declined with further sintering beyond 1350°C

while undoped samples declined beyond 1450°C. This observation is consistent with

other researchers where they found that the hardness gradually increased to maximum at

certain sintering temperature followed by a decreased with further sintering (Bowen et

al., 1998; Hodgson et al., 1999; Ramesh et al., 2011; Khan et al.,2014).

Figure 4.4 Effect of dopants and sintering temperature on the Vickers hardness

of Y-TZP.

8.5

9.5

10.5

11.5

12.5

13.5

14.5

1250 1350 1450 1550

Vic

ker

s H

ard

nes

s (G

Pa)

Sintering Temperature (℃)

Undoped

0.5wt% MnO2

0.1wt% Al2O3

Co-doped

52

Generally, there are several factors influencing the hardness of a material. These

includes grain size, porosity, crystal structure and composition. The hardness increase can

be attributed to the reduction of porosity and higher tetragonal retention. These results

agree well with the results obtained by Ramesh et al. (2013). The Hall-Petch equation

states that smaller grain size leads to higher hardness value (Yan et al., 2013). This can

explain the hardness of co-doped and alumina doped 3Y-TZP samples sintered at 1350℃

are higher compared to sample sintered at 1450℃. Similarly, the reduction of hardness

of the samples sintered at 1550℃ is also attributed to the grain size effects.

4.2.4 Fracture Toughness

The variation in the fracture toughness of undoped and doped samples with

sintering temperatures is shown in Figure 4.6. The addition of 0.1 wt% of alumina has

negligible effect on the fracture toughness of Y-TZP for entire sintering temperature

investigated. The trend of fracture toughness of this sample was fluctuated in the range

of 4.5 MPam1/2 and 4.71 MPam1/2. The fact that KIC value did not change significantly

indicates that the addition of 0.1 wt% of alumina did not affect the tetragonal phase

stability of Y–TZP.

The fluctuate trend of fracture toughness for both additions of 0.5 wt% MnO2 and

co-doped Y-TZP was observed for sintering temperature below 1450℃. However, the

KIc value of both additions of 0.5 wt% MnO2 and co-doped Y-TZP started to increase

significantly after sintering above 1450℃ as shown in Figure 4.6. The optimum KIc value

of 6 MPam1/2 was achieved with 0.5 wt% MnO2-doped Y-TZP and followed up by co-

doped samples which recorded a value of 5.3 MPam1/2 when sintered at 1550°C.

This remarkable rise in fracture toughness could be associated with the

transformation toughening effect which is a unique attribute of tetragonal zirconia.

Lawson et al. (1995) have emphasized that the KIc value could be used to demonstrate the

53

stability of the tetragonal grains in the zirconia matrix. According to these authors, a high

KIc indicates that the transformation toughening mechanism was in operative since the

tetragonal grains were retained in the metastable state and hence responded instantly to

the stress field of a propagating crack during the Vickers indentation test. It also can be

hypothesized that during sintering, influx of yttria to certain tetragonal grain will over-

stabilized the tetragonal grains, leading to exaggerated grain growth and the formation of

yttria-rich cubic phase as shown in Figure 4.2f and 4.2h.

Figure 4.5 Variation in fracture toughness as a function of dopant addition and

sintering temperatures.

3

3.5

4

4.5

5

5.5

6

6.5

7

1250 1350 1450 1550

Fra

ctu

re T

ou

gh

nes

s (M

Pam

1/2

)

Sintering Temperature (℃)

Undoped

0.5wt% MnO2

0.1wt% Al2O3

codoped

54

4.3 Microstructural Development and Grain Size

The microstructures of undoped and doped Y-TZPs sintered at 1250°C and

1550°C are shown in Figure 4.2. At low sintering temperature of 1250°C, residual pores

are generally visible as shown in Figures 4.2 (a) and (c) for the undoped and 0.1 wt%

Al2O3-doped Y-TZP, respectively. This observation is consistent with the low bulk

density (below 97%) measured at this temperature for these samples as shown in Figure

4.3. In comparison, the 0.5 wt% MnO2-doped (Figure 4.2b) and the co-doped Y-TZPs

(Figure 4.2d) revealed dense microstructure when sintered at 1250°C.

In general, it was difficult to distinguish the tetragonal and cubic grains due

mainly to the very uniform microstructure and distribution of equiaxed fine grains

particularly for the undoped and the 0.1 wt% Al2O3-doped Y-TZPs. In contrast, a more

bimodal microstructure comprising of fine tetragonal grains and relatively large abnormal

grains believed to be of the cubic type was clearly observed for the 0.5 wt% MnO2-doped

Y-TZP (Figure 4.2f) and the co-doped samples (Figure 4.2h) when sintered at 1550°C.

Such microstructural feature was not seen for the undoped and Al2O3-doped Y-TZPs.

It is envisaged that during sintering, the MnO2 could have aided densification

through forming a transient liquid phase that facilitated particle rearrangement and could

help in accelerating densification. As sintering proceeded, coalescence of tetragonal

grains could have taken place resulting in the formation of abnormally large grains as that

observed in the present work. This process, could possibly cause the adjacent smaller

grains to suffer yttria depletion thus making the grains metastable and hence promoting

transformation toughening effect as evident from the high fracture toughness measured

for the 0.5 wt% MnO2-doped samples sintered at 1550°C as shown in Figure 4.6.

55

Figure 4.6 Microstructural development for Y-TZP sintered at 1250 °C [(a)

undoped, (b) 0.5 wt% MnO2-doped, (c) 0.1 wt% Al2O3-doped and (d) co-doped Y-

TZP] and 1550 °C [(e) undoped, (f) 0.5wt% MgO2-doped, (g) 0.1 wt% Al2O3-doped

and (h) co-doped Y-TZPs].

56

This results were also correlated with Ramesh et al. (1996) and Ross et al. (2001)

findings where the presence of the large-size cubic grains in Y-TZPs, particularly when

sintered at high temperature of above 1450℃. According to these authors, the large-size

cubic grains normally have a higher yttria content than the tetragonal grains and thus

considered overstabilized.

The average tetragonal grain size for the Y-TZPs determined from FESEM

micrographs for polished and thermally etched sintered at 1250℃, 1350℃, 1450℃ and

1550℃ are presented in Table 4.1. In general, it was found that the average grains size

varied from 0.20 µm to 1.17 µm with increasing temperature from 1250°C to 1550°C.

Similar trend of grain size in which higher sintering temperature leads to grain coarsening

has been proven by many studies. For example, Kawai et al. (2001) observed higher grain

growth in their Y-TZPs samples when sintered at temperature ranging from 1350℃ to

1450℃. Further investigation into LTD phenomena also helped them to relate grain size

to ageing behavior when they observed that the sample having coarse grains showed

higher susceptibility to aged induced phase transformation and material properties

deterioration.

57

Table 4.1 Average grain size (µm) of Y-TZPS sintered at different

temperatures.

Dopants Sintering temperature (℃)

1250 1350 1450 1550

Undoped 0.20 0.32 0.54 0.78

0.5 wt% MnO2 0.20 0.27 0.44 1.08

0.1 wt% Al2O3 0.21 0.25 0.42 0.80

Co-doped 0.21 0.30 0.48 1.17

4.4 Low Temperature Degradation Behavior

In order to evaluate the effectiveness of the dopant in suppressing the low

temperature degradation (LTD) phenomena in Y-TZP, sintered samples were exposed to

superheated steam at 185℃/10 bar in an autoclave. It is widely reported that zirconia

ceramics are highly susceptible to the ageing in steam environment and leads to

catastrophic failure in early stages of exposure (Ramesh et al., 1996). The monoclinic (m)

development with ageing time for Y-TZPs sintered at 1350℃, 1450℃ and 1550℃ as

shown in Table 4.2.

Table 4.2 Monoclinic content (%) of 3Y-TZPs sintered after exposure in

superheated steam

Sintering temperature 1350℃ 1450℃ 1550℃

Aging time 1h 3h 1 h 3 h 1h 3h

Undoped 0 0 3.74 41.3 46.27 53.12

0.5 wt% MnO2 0.10 0.10 21.71 23.29 57.49 81.43

0.1 wt% Al2O3 0 0 4.33 44.38 52.54 55.43

Co-doped 0 0.13 23.66 26.21 69.25 82.04

58

The XRD analysis showed that the 0.1 wt% of alumina doped and undoped 3Y-

TZP samples sintered at temperature below 1450℃ showed 100 % tetragonal grains after

both 1 hour and 3 hours of ageing. It confirms that sintering at low temperature range of

1250 ℃ to 1350 ℃ generally led to zero monoclinic content upon ageing process as

compared to higher sintering temperatures. However, 0.5 wt% of MnO2 and co-doped

3Y-TZP samples encountered slight monoclinic content about 0.1% after 1 hour and

0.13% after 3 hours, respectively.

At 1450℃, all samples were susceptible to LTD, reaching above 23% of

monoclinic phase content after 3 hours. The most severe situation happened in co-doped

3Y-TZP samples sintered at 1550℃ when the monoclinic content reached to 82% after 3

hours of ageing. In general, the ageing resistance of undoped and doped Y-TZP was found

to be significantly affected by the sintering temperature. As the sintering temperature

increased, the LTD resistance of Y-TZP regardless of the dopant additions decreased

substantially.

From Table 4.2, it is clearly show that the ageing induced phase transformation

was more severe in Y-TZP sintered at the higher sintering temperatures. The declined of

ageing resistance observed could be attributed to the increase in the average grain size

with the higher sintering temperatures, which was confirmed by FESEM results in Table

4.1. This is consistent with the findings of Feder et al. (2005) who reported that the

monoclinic phase development in Y-TZP increased significantly with larger grain sizes.

The reason the 0.1 wt% of alumina doped and undoped 3Y-TZP samples sintered

at 1350 ℃ possessed excellent ageing resistance could be due to higher relative densities

as compared to 0.5 wt% of MnO2 and co-doped 3Y-TZP samples. Besides, grain size also

played a crucial role in determining the ageing resistance of the 3Y-TZP ceramics. It can

be discussed that at sintering temperatures of 1250℃ to 1350℃, smaller grain sizes

59

inhibit phase transformation i.e. in the range of 0.2 µm to 0.32 µm as shown in Figure

4.7. Having larger grain sizes above 0.32 µm were observed for the samples sintered at

higher temperature (>1450℃), hence leading to rapid phase transformation upon ageing.

The present results concur with the findings of many researchers who reported that

samples with small grain size (below a certain value) were not to prone to LTD as

compared to the one with bigger grain sizes (Castkova et al., 2004; Khan et al., 2014).

Figure 4.7 Effect of grain size on ageing-induced monoclinic content for Y-TZPs

sintered after 1h exposure in superheated steam.

The results obtained in this study correlated well with a recent study by F. Zhang

et al. (2014) who revealed that the LTD susceptibility of 3Y-TZPs doped with 0.25 wt%

alumina reduces as sintering temperature was reduced. However, the authors were not

able to obtain a LTD resistant 3Y-TZP with optimized mechanical properties. The results

of undoped and 0.25 wt% of alumina doped 3Y-TZP showed that high sintering

temperature of 1550℃ led to highest monoclinic phase content after 40 hours of ageing,

in comparison with the lower monoclinic content at lower sintering temperature of

1350℃, 1400℃ and 1450℃.

0

10

20

30

40

50

60

70

80

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Mon

ocl

inic

con

ten

t (%

)

Average grain size (µm)

Undoped

0.5 wt% MnO2

0.1 wt% Al2O3

Co-doped

60

In summary, the results obtained in the present work demonstrated that the

addition of MnO2-Al2O3 as the co-dopants did not play any substantial role in suppressing

the degradation phenomena of the 3Y-TZP ceramics. However, it should be mentioned

that the addition of MnO2 and Al2O3 as sintering additives for 3Y-TZP was beneficial in

aiding densification, particularly at lower sintering temperature which helped to achieve

higher mechanical properties. Moreover, based on the current work and that reported in

literatures, it can be inferred that the Y-TZP in the present work did not undergo the

ageing-induced tetragonal to monoclinic phase transformation if the grain size was not

exceed the limit of 0.32 µm during sintering.

61

CHAPTER 5: CONCLUSION AND FUTURE WORK

5.1 Conclusion

In this present work, the influence of dopants on microstructure and mechanical

properties in 3mol% Y-TZP doped were evaluated. The following conclusions were

obtained.

1) The tetragonal phase stability of the zirconia matrix was not disrupted in the

presence of dopant throughout the sintering regime employed as confirmed by

XRD analysis.

2) The microstructure of Y-TZPs contain very uniform microstructure and

distribution of equiaxed fine grains particularly for the undoped and the 0.1 wt%

Al2O3-doped Y-TZPs when compared to 0.5 wt% MnO2 and co-doped Y-TZP

ceramic when sintered at 1550℃. Grain coarsening was only evident when the

sintering temperature was raised.

3) The addition of 0.5 wt% MnO2 and co-doped revealed a bimodal microstructure

consisting of abnormal large grains of the cubic phase, in the vicinity of a finer

tetragonal matrix when sintered at1550℃.

4) 0.5 wt% of MnO2 addition enhanced ZrO2 grain growth in co-doped sample

compared to 0.1 wt% of alumina and thermal etched process triggered in self

transformation phase when co-doped sample sintered at 1550℃ as confirmed by

XRD analysis.

62

5) The addition of manganese(IV) oxide was beneficial in enhancing the

densification of Y-TZP. In particular, sintering at 1250°C resulted in highest

relative density of 97.6% for co-doped 3Y-TZP as compared to 92.9% for the

undoped sample. the study revealed that 1350℃ was the optimum sintering

temperature for all Y-TZPs to achieve >97.5% of relative density.

6) The variation in Young’s Modulus (E) for the Y-TZP with sintering temperature

of all composition studied were in good agreement with the relative density trend.

The co-doped samples had lower E value when compared to single dopants

regardless of sintering temperature employed. It was found that the addition of 0.5

wt% MnO2 achieved the highest E value of 194 2 GPa at 1250°C while the

addition of Al2O3 gave the highest E value of 200 1.5 GPa at 1350°C.

7) The beneficial effect of MnO2-Al2O3 as co-dopants in improving the mechanical

properties has been revealed. The co-doped 3Y-TZP sample exhibited higher

hardness 14.6 GPa at 1350°C when compared to 12.7 GPa for the undoped

sample. however, as sintering temperature increased above 1450℃, the hardness

co-doped samples was found to decreased.

8) The addition of MnO2-Al2O3 in 3Y-TZP, particularly at lower sintering

temperature was discovered to be beneficial in improving fracture toughness of

the sintered 3Y-TZP matrix. The highest value of ~4.7 MPam1/2 was recorded for

co-doped samples sintered at 1350℃ compared to ~4.5 MPam1/2 for undoped

samples.

63

9) The KIc value of both additions of 0.5 wt% MnO2 and co-doped Y-TZP started

increased significantly after sintering above 1450℃. The optimum KIc value of 6

MPam1/2 was achieved with 0.5 wt% MnO2-doped Y-TZP and followed up by co-

doped samples which recorded a value of 5.3 MPam1/2 when sintered at 1550°C.

the enhancement of toughness was attributed to the enhanced transformation

toughening effect of the metastable tetragonal grains.

10) Superior ageing resistance was exhibited by all sample Y-TZPs sintered below

1450℃ where samples did not undergo any ageing-induced phase transformation

after 3 hour of exposure autoclave condition. Moreover, the Y-TZP in the present

work did not undergo the ageing-induced tetragonal to monoclinic phase

transformation if grain size limit of 0.32 µm was not exceed during sintering.

11) LTD experiment revealed that the higher sintering temperature range of 1450℃

to 1550℃ was detrimental for all sample of Y-TZPs. Larger grain size at higher

temperature led to ageing induced phase transformation for 0.5 wt% MnO2 and

co-doped Y-TZP that showed greater than 81% monoclinic phase content after

just 3 hours of hydrothermal ageing.

12) Overall, the optimum sintering temperature for co-doped 3Y-TZP is at 1350℃,

by attained the highest Hv value at 1350℃ with 14.6 GPa and highest KIC values

at 1350℃ with 4.8MPam1/2 if compared to undoped ceramic. Despite with the

improvement in hardness and toughness, co-doped enhance the densification by

obtained 97.6% of relative density at 1250℃. All Y-TZPs sample did not prone

to degradation when the average grain size is below grain size limit (0.32 µm)

after ageing in autoclaves at 185℃/10 bar.

64

5.2 Future Work

Following are some suggestion for future works include:

1) Many other advanced techniques of sintering of Y-TZP ceramics have been

reported such as microwave sintering, hot pressing, spark plasma. These

techniques are reported to produce fully dense Y-TZPs with minimal grain

growth. Therefore, it would be interesting to compare the mechanical properties

as well as LTD behavior of Y-TZP ceramics using different techniques.

2) In order to enhance the mechanical properties of Y-TZPs, altering the amount of

wt% MnO2 smaller than Al2O3 as it have been reported and seemed to show very

promising results. As these dopant amount have been reported to achieve higher

mechanical properties especially in hardness value than current studies.

3) Performing Transmission Electron Microscopy (TEM) on Y-TZPs in the quest to

seek the effect of co-dopants on the grain boundary modification. This may

provide further useful information to explain superior ageing resistance in LTD.

65

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Zhi-kai Wun, N. L., Chao Jian, Wan-qian Zhao & Jia-zhen Yan. (2013). Low temperature

degradation of Al2O3-doped 3Y-TZP sintered at various temperatures. Ceramic

International, 39, 7199–7204.

71

LIST OF PUBLICATIONS

Nur Nadia A.H., Ramesh, S., Tan, C. Y., Wong, Y.H., Zainal Abidin, N.I., Teng, W.D.,

Sutharsini,U. & Ahmed, A.D., Sarhan,.(2017). The effect of sintering additives

on the mechanical properties and microstructure evolution of 3 mol% Y-TZP.

Journal of Ceramic Proccessing Research. 18(7),483-487.

72

APPENDIX A: EXPERIMENT APPARATUS

Figure A-1 Powder for making green sample

Figure A-2 Mixing powder with attrition milling

73

Figure A-3 Ultrasonification of mixture powder

Figure A-4 Dried powder

74

Figure A-5 Crush and sieved to form fine powder

Figure A-6 Bench Press used for powder compaction

75

Figure A-7 Sinter the sample by conventional sintering

Figure A-8 Grinding the sample by using Silicon Carbide paper

76

Figure A-9 Polishing sample by using diamond paste

Figure A-10 Electronic Balance to measure relative density

77

Figure A-11 Field Emission Scanning Electron Microscope (FESEM)

Figure A-12 Young’s Modulus determination via impulse excitation technique

(GrindoSonic: MK5 “Industrial”, Belgium)

78

Figure A-13 Aging test in autoclave

79

APPENDIX B: DENSITY TABLE FOR DISTILLED WATER

Sources: Operating Induction, Density Determination Kit, Mettler Toledo, Switzerland,

Manual No: P706039, pp. 14)

80

APPENDIX C: XRDML REFERENCE FILE

C1- XRDML File for Monoclinic Zirconia

Name and formula Reference code: 98-006-2993

Mineral name: Baddeleyite

Compound name: Baddeleyite

Common name: Baddeleyite

Chemical formula: O2Zr1

Crystallographic parameters Crystal system: Monoclinic

Space group: P 1 21/c 1

Space group number: 14

a (Å): 5.1510

b (Å): 5.2120

c (Å): 5.3170

Alpha (°): 90.0000

Beta (°): 99.2300

Gamma (°): 90.0000

Calculated density (g/cm^3): 5.81

Volume of cell (10^6 pm^3): 140.90

Z: 4.00

RIR: 4.94

Subfiles and quality Subfiles: User Inorganic

User Mineral

Quality: User From Structure (=)

Comments

Creation Date: 6/19/1989

Modification Date: 8/1/2007 Original ICSD space group: P121/C1. Cell from 3rd ref

(Edmonds et al., round robin): 5.14844-5.14897, 5.20952-5.20973, 5.31820-5.31863, 99.232-99.245

Stable up to 1273 K (2nd ref., Tomaszewski), 1273-2573 K: P42/nmc. Rwp is .0466, Rb is

.0195. Temperature factors for all atoms as B eq.. No R value given in the paper.

Structure type: ZrO2(mP12). Temperature factors available The structure has been assigned a PDF number (experimental powder diffraction data):

37-1484. Neutron diffraction (powder) Compound with mineral name: Baddeleyite

81

Recording date: 6/19/1989

Modification date: 8/1/2007 Mineral origin: synthetic

ANX formula: AX2 Z: 4

Calculated density: 5.81

Pearson code: mP12 Wyckoff code: e3

PDF code: 00-037-1484 Publication title: Structures of the Zr O2 polymorphs at room temperature by high resolution

neutron powder diffraction. ICSD collection code: 62993

Structure: ZrO2(mP12)

Chemical Name: Zirconium Oxide Second Chemical Formula: Zr O2

References

Structure: Reichert, B.E.;Hill, R.J.;Howard, C.J., Powder Diffraction, 1, 66 - 76, (1986)

Peak list No. h k l d [Å] 2Theta[deg] I [%]

1 1 0 0 5.08431 17.428 5.7

2 0 1 1 3.69816 24.045 8.0

3 1 1 0 3.63945 24.438 11.3

4 1 1 -1 3.16575 28.165 100.0

5 1 1 1 2.84181 31.455 69.9

6 0 0 2 2.62408 34.141 20.7

7 0 2 0 2.60600 34.385 12.9

8 2 0 0 2.54215 35.277 15.6

9 1 0 -2 2.50105 35.876 3.2

10 0 1 2 2.34379 38.374 0.0

11 0 2 1 2.33408 38.540 2.9

12 1 2 0 2.31911 38.799 0.2

13 2 1 0 2.28486 39.405 1.0

14 1 1 -2 2.25487 39.951 0.4

15 2 1 -1 2.21506 40.700 13.9

16 1 0 2 2.19288 41.130 5.1

17 1 2 -1 2.18105 41.364 4.9

18 1 2 1 2.06609 43.780 0.0

19 1 1 2 2.02126 44.803 7.2

20 2 0 -2 1.99254 45.485 6.9

21 2 1 -2 1.86117 48.897 2.0

22 0 2 -2 1.84908 49.238 9.7

23 2 2 0 1.81973 50.087 24.2

24 1 2 -2 1.80447 50.540 13.6

25 2 2 -1 1.78387 51.165 5.9

26 2 0 2 1.69503 54.059 12.2

27 3 0 0 1.69477 54.068 1.5

28 1 2 2 1.67788 54.657 0.7

29 2 2 1 1.66128 55.249 4.3

30 0 1 3 1.65846 55.351 5.0

31 1 1 -3 1.65252 55.567 6.8

82

32 0 3 1 1.64931 55.685 0.1

33 1 3 0 1.64400 55.881 8.5

34 2 1 2 1.61193 57.093 0.0

35 3 1 0 1.61170 57.102 0.7

36 3 1 -1 1.61120 57.121 8.9

37 1 3 -1 1.59257 57.852 5.9

38 2 2 -2 1.58287 58.241 3.9

39 1 3 1 1.54612 59.764 10.2

40 3 0 -2 1.54074 59.994 8.0

41 1 1 3 1.51044 61.326 6.7

42 2 1 -3 1.49709 61.932 7.8

43 3 1 1 1.47869 62.790 11.8

44 3 1 -2 1.47753 62.845 0.0

45 0 2 -3 1.45247 64.057 0.7

46 0 3 -2 1.44861 64.247 0.5

47 1 2 -3 1.44848 64.254 1.8

48 2 3 0 1.43437 64.963 0.9

49 1 3 -2 1.42686 65.348 2.7

50 2 2 2 1.42090 65.656 5.4

51 3 2 0 1.42075 65.664 2.2

52 3 2 -1 1.42041 65.682 0.5

53 2 3 -1 1.41661 65.880 1.2

54 1 3 2 1.36175 68.897 2.6

55 2 3 1 1.35284 69.416 0.1

56 1 2 3 1.34992 69.588 1.0

57 2 2 -3 1.34037 70.156 0.7

58 3 0 2 1.32977 70.799 0.0

59 3 2 1 1.32712 70.962 0.2

60 3 2 -2 1.32628 71.013 2.5

61 1 0 -4 1.32279 71.229 4.8

62 0 0 4 1.31204 71.903 1.0

63 2 3 -2 1.30947 72.066 1.6

64 0 4 0 1.30300 72.481 1.4

65 2 1 3 1.30140 72.584 1.2

66 3 1 2 1.28849 73.429 0.0

67 3 1 -3 1.28721 73.514 0.5

68 1 1 -4 1.28214 73.853 0.0

69 0 1 -4 1.27234 74.518 0.0

70 4 0 0 1.27108 74.605 2.2

71 0 4 1 1.26461 75.052 2.7

72 1 4 0 1.26221 75.219 0.4

73 2 0 -4 1.25052 76.047 0.0

74 4 1 -1 1.24613 76.363 3.1

75 1 4 -1 1.23848 76.921 0.3

76 4 1 0 1.23488 77.186 0.9

77 0 3 3 1.23272 77.346 0.4

78 1 3 -3 1.23028 77.529 0.6

79 1 0 4 1.22381 78.015 1.0

80 4 0 -2 1.22352 78.037 0.6

81 1 4 1 1.21625 78.594 0.1

82 2 1 -4 1.21601 78.612 0.6

83 2 3 2 1.21325 78.826 0.3

84 3 3 0 1.21315 78.834 1.5

85 3 3 -1 1.21294 78.850 1.3

86 2 2 3 1.19448 80.314 0.6

87 1 1 4 1.19141 80.563 0.1

88 4 1 -2 1.19114 80.585 0.2

83

89 3 2 2 1.18447 81.133 0.0

90 3 2 -3 1.18348 81.215 0.7

91 1 2 -4 1.17954 81.544 2.4

92 0 2 -4 1.17189 82.190 0.4

93 1 3 3 1.16816 82.510 2.4

94 0 4 -2 1.16704 82.606 0.3

95 4 1 1 1.16235 83.013 0.5

96 2 3 -3 1.16195 83.049 3.0

97 2 4 0 1.15956 83.258 1.0

98 1 4 -2 1.15558 83.609 0.4

99 3 3 1 1.15328 83.813 1.8

100 3 3 -2 1.15273 83.862 1.0

101 4 2 -1 1.15132 83.989 0.3

102 2 4 -1 1.15011 84.097 1.8

103 4 2 0 1.14243 84.794 2.2

104 3 0 -4 1.12881 86.063 2.1

105 2 2 -4 1.12744 86.193 0.4

106 1 4 2 1.12017 86.891 0.4

107 2 4 1 1.11519 87.376 3.0

108 1 2 4 1.10775 88.114 2.9

109 4 2 -2 1.10753 88.136 0.6

110 3 1 3 1.10436 88.455 3.7

111 3 1 -4 1.10323 88.569 0.1

112 2 0 4 1.09644 89.263 1.4

113 4 1 -3 1.09222 89.701 4.0

114 2 4 -2 1.09053 89.878 1.2

Structure

No. Name Elem. X Y Z Biso sof Wyck.

1 O1 O 0.55040 0.25690 0.02080 0.4600 1.0000 4e

2 O2 O 0.07000 0.33170 0.34470 0.5500 1.0000 4e

3 ZR1 Zr 0.27540 0.03950 0.20830 0.3300 1.0000 4e

Stick Pattern

84

C2- XRDML File for Tetragonal Zirconia

Name and formula Reference code: 98-006-2994

Compound name: Zirconium Yttrium Oxide (0.94/0.07/1.97)

Common name: Zirconium Yttrium Oxide (0.94/0.07/1.97)

Chemical formula: O1.968Y0.065Zr0.935

Crystallographic parameters

Crystal system: Tetragonal Space group: P 42/n m c

Space group number: 137

a (Å): 3.6060

b (Å): 3.6060

c (Å): 5.1800

Alpha (°): 90.0000

Beta (°): 90.0000

Gamma (°): 90.0000

Calculated density (g/cm^3): 6.04

Volume of cell (10^6 pm^3): 67.36

Z: 2.00

RIR: 10.31

Subfiles and quality

Subfiles: User Inorganic Quality: User From Structure (=)

Comments

Creation Date: 6/19/1989 Modification Date: 4/1/2007

Original ICSD space group: P42/NMCZ. Stable from 1273 to 2573 K (2nd ref.,

Tomaszewski), below P21/c, m.p. 2950 K. R(Bragg)=.0350 Structure type: ZrO2(HT). Temperature factors available

The structure has been assigned a PDF number (experimental powder diffraction data): 42-1164

The structure has been assigned a PDF number (calculated powder diffraction data):

01-078-1808. Neutron diffraction (powder) Structure type: ZrO2(HT)

Recording date: 6/19/1989 Modification date: 4/1/2007

ANX formula: AX2 Z: 2

85

R value: 0.0898

Pearson code: tP6 Wyckoff code: d a

PDF code: 00-042-1164 Structure TIDY: TRANS Origin 1/2 1/2 0

Publication title: Structures of the Zr O2 polymorphs at room temperature by high resolution

neutron powder diffraction. ICSD collection code: 62994

Structure: ZrO2(HT) Chemical Name: Zirconium Yttrium Oxide (0.94/0.07/1.97)

Second Chemical Formula: Zr0.935 Y0.065 O1.968

References Structure: Reichert, B.E.;Hill, R.J.;Howard, C.J., Golden Book of Phase

Transitions, Wroclaw, 1, 1 - 123, (2002)

Peak list No. h k l d [Å] 2Theta[deg] I [%]

1 0 1 1 2.95951 30.173 100.0

2 0 0 2 2.59000 34.605 8.2

3 1 1 0 2.54983 35.167 13.6

4 0 1 2 2.10362 42.960 1.0

5 1 1 2 1.81704 50.166 36.0

6 0 2 0 1.80300 50.584 18.6

7 0 2 1 1.70280 53.792 0.0

8 0 1 3 1.55734 59.290 12.9

9 1 2 1 1.53976 60.036 24.8

10 0 2 2 1.47975 62.739 6.2

11 1 2 2 1.36897 68.483 0.3

12 0 0 4 1.29500 73.000 2.1

13 2 2 0 1.27491 74.342 4.8

14 0 2 3 1.24705 76.296 0.0

15 0 1 4 1.21879 78.399 0.2

16 1 2 3 1.17856 81.626 9.4

17 0 3 1 1.17089 82.276 4.6

18 1 1 4 1.15462 83.694 3.4

19 2 2 2 1.14384 84.665 2.7

20 1 3 0 1.14032 84.988 2.5

21 1 3 1 1.11365 87.528 0.0

22 0 3 2 1.09031 89.901 0.0

Structure

No. Name Elem. X Y Z Biso sof Wyck.

1 O1 O 0.25000 0.25000 0.04130 0.9800 0.9840 4d

2 Y1 Y 0.75000 0.25000 0.25000 0.6500 0.0650 2b

3 ZR1 Zr 0.75000 0.25000 0.25000 0.6500 0.9350 2b

Stick Pattern

86

87

C3-XRDML File for Manganese (IV) Oxide MnO2

Name and formula Reference code: 98-015-0462

Compound name: Manganese(IV) Oxide - Gamma

Common name: Manganese(IV) Oxide - Gamma

Chemical formula: Mn1O2

Crystallographic parameters

Crystal system: Monoclinic Space group: C 1 2/m 1

Space group number: 12

a (Å): 13.7000

b (Å): 2.8670

c (Å): 4.4600

Alpha (°): 90.0000

Beta (°): 90.5000

Gamma (°): 90.0000

Calculated density (g/cm^3): 4.94

Volume of cell (10^6 pm^3): 175.17

Z: 6.00

RIR: 1.36

Subfiles and quality

Subfiles: User Inorganic Quality: User From Structure (=)

Comments

Creation Date: 4/1/2006 Modification Date: 12/30/1899

Original ICSD space group: C12/M1. The model is an intergrowth of ramsdellite and

pyrolusite slabs in equal proportions. At least one temperature factor missing in the paper.. No R value given in the paper..

Structure calculated theoretically. Standard deviation missing in cell constants

The structure has been assigned a PDF number (calculated powder diffraction data):

01-073-2509 Recording date: 4/1/2006

ANX formula: AX2 Z: 6

Calculated density: 4.94 Pearson code: mS18

88

ICSD collection code: 150462

Chemical Name: Manganese(IV) Oxide - Gamma Second Chemical Formula: Mn O2

References

Structure: Verbaere, A.;Hill, L.I., Journal of Solid State Chemistry, 177, 4706 - 4723, (2004)

Peak list No. h k l d [Å] 2Theta[deg] I [%]

1 2 0 0 6.84974 12.914 2.9

2 0 0 1 4.45983 19.892 42.6

3 2 0 -1 3.75245 23.692 93.7

4 2 0 1 3.72262 23.884 100.0

5 4 0 0 3.42487 25.996 1.2

6 1 1 0 2.80621 31.864 0.0

7 4 0 -1 2.72785 32.805 48.3

8 4 0 1 2.70495 33.091 49.4

9 3 1 0 2.42811 36.992 58.8

10 1 1 -1 2.37706 37.817 53.6

11 1 1 1 2.37323 37.880 46.4

12 6 0 0 2.28325 39.433 27.8

13 0 0 2 2.22992 40.417 9.3

14 3 1 -1 2.13671 42.263 54.1

15 3 1 1 2.12839 42.436 2.7

16 2 0 -2 2.12585 42.489 0.4

17 2 0 2 2.11496 42.719 0.1

18 6 0 -1 2.03962 44.379 1.6

19 6 0 1 2.02523 44.711 23.7

20 5 1 0 1.98081 45.770 0.8

21 4 0 -2 1.87622 48.480 0.0

22 4 0 2 1.86131 48.894 0.4

23 5 1 -1 1.81454 50.240 11.5

24 5 1 1 1.80607 50.492 17.2

25 1 1 -2 1.74735 52.315 0.6

26 1 1 2 1.74431 52.413 0.2

27 8 0 0 1.71243 53.465 0.3

28 3 1 -2 1.64620 55.799 55.0

29 3 1 2 1.63861 56.081 50.2

30 7 1 0 1.61638 56.922 0.8

31 8 0 -1 1.60333 57.428 3.3

32 6 0 -2 1.60231 57.468 20.1

33 8 0 1 1.59399 57.796 3.2

34 6 0 2 1.58840 58.019 22.4

35 7 1 -1 1.52317 60.758 19.1

36 7 1 1 1.51615 61.070 15.0

37 0 0 3 1.48661 62.418 0.9

38 5 1 -2 1.48558 62.466 0.2

39 5 1 2 1.47630 62.903 0.4

40 2 0 -3 1.45542 63.911 2.1

41 2 0 3 1.45017 64.170 3.4

42 0 2 0 1.43350 65.008 21.6

43 2 2 0 1.40310 66.596 0.0

44 10 0 0 1.36995 68.428 0.1

89

45 4 0 -3 1.36805 68.536 5.5

46 0 2 -1 1.36473 68.726 1.0

47 8 0 -2 1.36393 68.772 0.8

48 4 0 3 1.35936 69.036 3.5

49 8 0 2 1.35248 69.437 0.1

50 9 1 0 1.34443 69.913 30.9

51 2 2 -1 1.33911 70.232 6.0

52 2 2 1 1.33774 70.314 6.6

53 4 2 0 1.32234 71.257 0.1

54 1 1 -3 1.31463 71.739 9.7

55 7 1 -2 1.31323 71.828 0.4

56 10 0 -1 1.31278 71.856 4.8

57 1 1 3 1.31269 71.862 6.6

58 10 0 1 1.30636 72.265 4.0

59 7 1 2 1.30427 72.399 0.0

60 9 1 -1 1.28996 73.332 1.6

61 9 1 1 1.28448 73.696 0.9

62 3 1 -3 1.27048 74.645 0.1

63 4 2 -1 1.26895 74.751 6.4

64 4 2 1 1.26663 74.912 6.5

65 3 1 3 1.26524 75.008 6.4

66 6 0 -3 1.25082 76.026 2.8

67 6 0 3 1.24087 76.745 0.0

68 6 2 0 1.21406 78.763 6.1

69 0 2 -2 1.20583 79.406 1.5

70 5 1 -3 1.19262 80.465 2.7

71 2 2 -2 1.18853 80.799 0.1

72 2 2 2 1.18662 80.956 0.0

73 5 1 3 1.18541 81.055 3.5

74 6 2 -1 1.17281 82.112 0.2

75 10 0 -2 1.17184 82.195 0.4

76 6 2 1 1.17005 82.348 4.6

77 10 0 2 1.16275 82.979 0.1

78 9 1 -2 1.15530 83.634 2.3

79 9 1 2 1.14745 84.337 2.4

80 11 1 0 1.14229 84.807 0.7

81 12 0 0 1.14162 84.868 1.8

82 4 2 -2 1.13908 85.102 0.0

83 4 2 2 1.13572 85.414 0.1

84 8 0 -3 1.12748 86.189 0.7

85 8 0 3 1.11779 87.123 1.6

86 0 0 4 1.11496 87.399 7.1

87 11 1 -1 1.10870 88.019 1.5

88 12 0 -1 1.10829 88.060 1.3

89 11 1 1 1.10445 88.446 1.4

90 12 0 1 1.10365 88.526 0.0

91 2 0 -4 1.10200 88.694 0.0

92 8 2 0 1.09920 88.980 0.1

93 2 0 4 1.09895 89.005 0.0

94 7 1 -3 1.09815 89.087 3.6

95 7 1 3 1.09029 89.903 3.1

90

Structure

No. Name Elem. X Y Z Biso sof Wyck.

1 O1 O 0.21700 0.00000 0.27400 0.5000 1.0000 4i

2 O2 O 0.10950 0.00000 0.74000 0.5000 1.0000 4i

3 O3 O 0.55350 0.00000 0.23200 0.5000 1.0000 4i

4 MN1 Mn 0.33900 0.00000 0.49400 0.5000 1.0000 4i

5 MN2 Mn 0.00000 0.00000 0.00000 0.5000 1.0000 2a

Stick Pattern