i The Development of Whisker and Cubic Boron Nitride Reinforced Titanium Carbide Ceramic Matrix Composites A Thesis submitted in fulfilment of the requirements of the degree of Master of Science in Metallurgy and Materials Engineering Prepared by Shaheeda Petersen (693270) Submitted to School of Chemical and Metallurgical Engineering, Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, South Africa Supervisor: Professor I. Sigalas March, 2023 ii DECLARATION 20 March 2023 III ABSTRACT TiC as the matrix in ceramic matrix composites (CMC’s) is limited due to its poor fracture toughness. In order to increase their use in interrupted cutting applications, it is necessary to improve their fracture toughness and hardness. The purpose of this study was to improve the fracture toughness of TiC without causing a decrease in its other mechanical properties by adding SiC whiskers to the starting TiC powder before Spark Plasma Sintering (SPS). To improve the hardness cBN was also added as a secondary hard phase prior to sintering. The reinforced powders where then sintered at temperatures between 1550°C and 1650°C, under pressures of 50-70 MPa and hold times between 5-20 minutes. The resulting materials were then characterized by density, hardness, fracture toughness, biaxial strength, sliding wear testing and scanning electron microscopy. Analysis of the hardness and fracture toughness of the sintered TiC matrix compacts with/without SiO2 concluded that the sintered sample with the highest hardness was found to be 90TiC-8Al2O3-2Y2O3 SPS’d at 1625°C, 70MPa and the sintered sample with the highest fracture toughness was determined to be 90TiC-8Al2O3-2Y2O3 SPS’d at 1625°C, 50MPa. From the XRD results we observed that the SPS material with the highest fracture toughness had formed YAP instead of YAG during sintering. The 77.8TiC-6.9Al2O3-1.7Y2O3-14 (20 vol.%) SiCw composition had the greatest ultimate fracture strength of 152.67 GPa and a Weibull modulus of 26.973 which is higher than unreinforced engineering ceramics but similar to other CMC’s reinforced with ceramic fibres. Out of all the compositions tested the 78.6TiC-3.5Al2O3-5.6Y2O3- 4.9SiO2-7.5 (10 vol.%) cBN removed the most material and had the lowest frictional coefficient making it suitable for use as a cutting blade. IV ACKNOWLEDGMENTS This research was made possible by the financial support of Element Six Production (Pty) Limited, South Africa and the Department of Science and Technology/ National Research Foundation: Centre of Excellence in Strong Materials, South Africa. I would like to express my sincere gratitude to my academic supervisor Prof Iakovos Sigalas, School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa and my research advisor Dr Mathias Herrmann, IKTS Fraunhofer Institute for Ceramic Technology and Systems, Dresden, Germany for their continuous support, motivation and immense knowledge. I would also like to thank the academic staff, technical staff and my fellow postgraduate students at the School of Chemical and Metallurgical Engineering, University of the Witwatersrand for their assistance, guidance and encouragement during the duration of my research and beyond. I am eternally grateful for the emotional support and encouragement I have received from my colleagues and management at the Department of Mechanical Engineering, Cape Peninsula University of Technology and also the Mellon Mays Undergraduate Fellowship program for their guidance throughout the early stages of my academic undertakings until now. And last, but not least I would like to thank my family for their unparalleled emotional, financial and spiritual support during my academic journey. I am immeasurably grateful to you all. V TABLE OF CONTENTS Declaration ....................................................................................................................................................... ii Abstract ........................................................................................................................................................... iii Acknowledgments .......................................................................................................................................... iv List of figures ................................................................................................................................................. xii List of tables ................................................................................................................................................. xix 1. INTRODUCTION ........................................................................................................................................... 1 1.1 Subject of the thesis .............................................................................................................................. 1 1.2 Background to the thesis ...................................................................................................................... 1 1.3 Problem Statement ................................................................................................................................ 2 1.4 Objectives of the thesis ......................................................................................................................... 3 1.5 Plan of development .............................................................................................................................. 4 2. LITERATURE REVIEW ................................................................................................................................. 5 2.1 Titanium Carbide .................................................................................................................................... 5 2.1.1 Background ...................................................................................................................................... 5 2.1.2 Structure of TiC ............................................................................................................................... 5 2.1.3 Properties of TiC .............................................................................................................................. 6 2.1.4 Sintering TiC .................................................................................................................................... 6 2.1.5 Liquid phase sintering aids ............................................................................................................ 7 2.1.6 Sintering TiC with liquid phase sintering aids ............................................................................... 8 2.1.7 Interaction between TiC and SiC ............................................................................................... 9 2.1.8 Interaction of SiO2 and liquid phase sintering aids.................................................................. 9 2.1.9 Interaction between SiC and liquid phase sintering aids ........................................................ 9 VI 2.1.10 Interaction between TiC and Cubic Boron Nitride (cBN) .......................................................... 10 2.1.11 Interaction between cBN and liquid phase sintering aids (SiO2) .............................................. 10 2.2 Whisker reinforcement of ceramics .................................................................................................... 11 2.2.1 Processing of whisker reinforced ceramics ................................................................................ 11 2.2.2 Whisker/powder mixing ................................................................................................................. 11 2.2.3 Whisker toughening in ceramic materials ................................................................................... 11 2.2.4 Proposed toughening mechanisms for whisker reinforced CMC’s ........................................... 12 2.3 Sintering and densification of ceramics ............................................................................................. 15 2.3.1 Spark Plasma Sintering (SPS) ...................................................................................................... 15 2.4 Reinforcement materials ..................................................................................................................... 15 2.4.1 Silicon Carbide Whiskers (SiCw) ................................................................................................... 15 2.4.2 Boron Nitride .................................................................................................................................. 17 2.5 Fracture in ceramics ............................................................................................................................ 19 2.5.1 Contact failure .......................................................................................................................... 19 2.5.2 Ball on three balls (B3B) biaxial strength testing .................................................................. 20 2.6 Wear by hard particles .................................................................................................................... 21 2.6.1 Models for abrasive wear ......................................................................................................... 22 2.6.2 Types of physical interactions that occur during abrasive wear .......................................... 25 2.6.3 Factors that affect abrasive wear ............................................................................................ 26 2.6.4 Testing methods for abrasive wear ......................................................................................... 28 2.6.5 Effect of secondary phases on abrasive wear ............................................................................. 29 3. METHODOLOGY ........................................................................................................................................ 30 3.1 Experimental Approach ....................................................................................................................... 30 3.2 Materials used and their characterization .......................................................................................... 31 3.2.1 Scanning Electron Microscopy of Reinforcement Materials ...................................................... 31 VII 3.3 Experimental Apparatus ...................................................................................................................... 32 3.3.1 Planetary ball mill .......................................................................................................................... 33 3.3.2 Attritor mill ..................................................................................................................................... 33 3.3.3 Turbula mixer ................................................................................................................................. 34 3.3.4 Spark plasma sintering (SPS) machine ........................................................................................ 34 3.3.5 Materialography (Sample preparation) equipment ...................................................................... 35 3.3.6 X-ray Diffractometry (XRD) ........................................................................................................... 35 3.3.7 Scanning Electron miscroscopy (SEM) and Energy Dispersive Spectroscopy (EDX) .............. 35 3.4 Powder processing .............................................................................................................................. 36 3.4.1 Preparation of matrix powders ..................................................................................................... 36 3.4.2 Preparation of reinforced matrix powders ................................................................................... 36 3.5 Analysis of sintered samples .............................................................................................................. 38 3.5.1 Density Measurements .................................................................................................................. 38 3.5.2 Vickers Hardness Testing ............................................................................................................. 39 3.5.3 B3B Biaxial strength testing ........................................................... Error! Bookmark not defined. 3.5.4 Sliding wear testing ....................................................................................................................... 41 4. Analysis of unreinforced TiC with Al2O3, Y2O3 AND SiO2 sINTERING AIDS ........................................... 43 4.1 Particle size analysis of TiC, liquid phase sintering aids and reinforcement material ................ 43 4.2 Analysis of the TiC Matrix powders .................................................................................................... 44 4.2.1 XRD results of Milled TiC matrix powders with/without SiO2 ..................................................... 44 4.2.2 SEM analysis of the TiC Matrix powders...................................................................................... 45 4.3 SPS Sintering of TiC matrix powders ................................................................................................. 46 4.3.1 Theoretical densities of TiC powder mixes without SiO2 ............................................................ 46 4.3.2 Theoretical densities of TiC powder mixes with the addition of Al2O3, Y2O3 and SiO2 ............. 46 4.3.3 SPS sintering parameters for TiC mixes without SiO2 ................................................................ 46 VIII 4.3.4 SPS sintering parameters for TiC mixes with the addition of SiO2 ............................................ 48 4.3.5 Percentage of theoretical density of TiC mix sintered samples with/without SiO2 ................... 48 4.4 Phases identified by XRD measurements of SPS sintered samples ................................................ 52 4.5 Microstructural Analysis ..................................................................................................................... 54 4.5.1 Scanning Electron Microscopy TiC mix sintered samples with/without SiO2 ........................... 54 4.5.2 Energy-dispersive X-ray spectroscopy on TiC mix sintered samples with/without SiO2 ......... 54 4.6 Hardness and fracture toughness of sintered samples .................................................................... 57 5. Analysis of TiC based CMC’s with SiC whisker or cBN particle reinforcement .................................... 59 5.1 Analysis of the TiC Matrix powders with/without SiO2 and the addition of SiC whiskers and cBN respectively ................................................................................................................................................ 59 5.1.1 Examples of typical XRD patterns XRD of milled TiC matrix powders with/without SiO2 and the addition of SiC whiskers and cBN respectively ................................................................................... 59 5.2 SPS Sintering of TiC matrix powders with/without SiO2 and the addition of SiC whiskers and cBN .................................................................................................................................................................... 63 5.2.1 Theoretical density of TiC matrices with/without SiO2 and the addition of SiC whiskers and cBN respectively ............................................................................................................................................. 63 5.2.2 SEM analysis of TiC matrix powder samples with the addition of reinforcing materials ......... 64 5.2.3 SPS sintering parameters for TiC mixes using TiC powder without SiO2 and with the addition of cBN ...................................................................................................................................................... 65 5.2.4 SPS sintering parameters for TiC mixes using TiC powder with SiO2 and with the addition of cBN .......................................................................................................................................................... 65 5.2.5 SPS sintering parameters for TiC mixes using 2µm TiC powder without SiO2 and with the addition of SiC whiskers ........................................................................................................................ 65 5.2.6 SPS sintering parameters for TiC mixes using TiC powder with SiO2 and with the addition of SiC whiskers ........................................................................................................................................... 65 5.3 Density of SPS sintered samples ........................................................................................................ 66 5.3.1 Densities of TiC powder mixes without SiO2 and with the addition of SiC whiskers ................ 66 5.3.2 Densities of TiC powder mixes with SiO2 and with the addition of SiC whiskers...................... 69 IX 5.3.3 Densities of TiC powder mixes with/without SiO2 and with the addition of cBN ....................... 71 5.4 Phases identified by XRD measurements of SPS sintered samples ................................................ 75 5.4.1 Phases identified by XRD analysis of SPS’d samples of TiC + SiCw with the highest percentage theoretical density .................................................................................................................................. 75 5.4.2 Phases identified by XRD analysis of SPS’d samples of TiC + cBN with the highest percentage theoretical density .................................................................................................................................. 79 5.5 Microstructural Analysis by SEM ........................................................................................................ 82 5.5.1 SEM analysis of SPS sintered samples with 85 wt. % TiC, liquid phase sintering aids (Al2O3- Y2O3) and SiC whiskers of various compositions and sintering parameters ..................................... 82 5.5.2 SEM analysis of SPS sintered samples with 85 wt. % TiC, liquid phase sintering aids (Al2O3- Y2O3-SiO2) and SiC whiskers of various compositions and sintering parameters ............................. 83 5.5.3 SEM analysis of SPS sintered samples with 90 wt. % TiC, liquid phase sintering aids (Al2O3- Y2O3) and SiC whiskers of various compositions and sintering parameters ..................................... 84 5.5.4 SEM analysis of SPS sintered samples with 90 wt. % TiC, liquid phase sintering aids (Al2O3- Y2O3-SiO2) and SiC whiskers of various compositions and sintering ................................................. 84 5.5.5 SEM analysis of SPS sintered samples with 85 wt. % TiC, liquid phase sintering aids (Al2O3- Y2O3-SiO2) and cBN of various compositions and sintering parameters............................................ 85 5.5.6 SEM analysis of SPS sintered samples with 90 wt. % TiC, liquid phase sintering aids (Al2O3- Y2O3-SiO2) and cBN of various compositions and sintering................................................................ 86 5.6 Hardness and fracture toughness ...................................................................................................... 88 5.6.1 Hardness and fracture toughness of SPS sintered samples with 85 wt.% TiC, liquid phase sintering aids and SiC whiskers with the highest density ................................................................... 88 5.6.2 Hardness and fracture toughness of SPS sintered samples with 85 wt.% TiC, liquid phase sintering aids and cBN with the highest density .................................................................................. 90 5.6.3 Hardness and fracture toughness of SPS sintered samples with 90 wt.% TiC, liquid phase sintering aids and SiC whiskers with the highest density ................................................................... 91 5.6.4 Hardness and fracture toughness of SPS sintered samples with 90 wt.% TiC, liquid phase sintering aids and cBN with the highest density .................................................................................. 92 6. Biaxial strength testing using Ball on 3 Ball testing ............................................................................... 94 X 6.1 Analysis of the flexural strength tests of sintered TiC samples with the best combination of density and fracture toughness. ............................................................................................................................ 94 6.1.1 85TiC-12Al2O3-3Y2O3 SPS’d at 1610°C, 70MPa and 10 min hold ................................................. 96 6.1.2 79TiC-11Al2O3-3Y2O3-7(10 vol.%) SiCw SPS’d at 1600°C, 70MPa and 10 min hold .................... 98 6.1.3 73TiC-10Al2O3-3Y2O3-14(20 vol.%) SiCw SPS’d at 1625°C, 70MPa and 10 min hold ................ 100 6.1.4 79TiC-4.5Al2O3-5.6Y2O3-4.9SiO2-7.1(10 vol.%) SiCw SPS’d at 1600°C, 70MPa and 10 min hold ............................................................................................................................................................... 102 6.1.5 78.6TiC-3.5Al2O3-5.6Y2O3-4.9SiO2-7.5 (10 vol.%) cBN SPS’d at 1575°C, 70MPa and 5 min hold ............................................................................................................................................................... 105 6.1.6 77.8TiC-6.9Al2O3-1.7Y2O3-14(20 vol.%) SiCw SPS’d at 1625°C, 70MPa and 20 min hold ......... 107 6.1.7 Comparison of the Weibull moduli of sintered TiC with various liquid phase sintering aids and with/without the addition of reinforcement materials ........................................................................ 110 7. Ball on Disc sliding wear testing ............................................................................................................ 112 7.1 85TiC-12Al2O3-3Y2O3 SPS’d at 1610°C, 70MPa and 10 min hold ...................................................... 113 7.2 79TiC-11Al2O3-3Y2O3-7(10 vol.%) SiCw SPS’d at 1600°C, 70MPa and 10 min hold ........................ 116 7.3 73TiC-10Al2O3-3Y2O3-14(20 vol.%) SiCw SPS’d at 1625°C, 70MPa and 10 min hold ...................... 119 7.4 79TiC-4.5Al2O3-5.6Y2O3-4.9SiO2-7.1(10 vol.%) SiCw SPS’d at 1600°C, 70MPa and 10 min hold .... 122 7.5 78.6TiC-3.5Al2O3-5.6Y2O3-4.9SiO2-7.5 (10 vol.%) cBN SPS’d at 1575°C, 70MPa and 5 min hold.... 125 7.6 77.8TiC-6.9Al2O3-1.7Y2O3-14 (20 vol.%) SiCw SPS’d at 1625°C, 70MPa and 20 min hold .............. 128 7.7 Mass lost on ball bearing during wear testing ................................................................................. 131 7.8 Friction and sliding wear results from CSM Tribometer ................................................................. 132 8 Conclusions and Recommendations ...................................................................................................... 134 8.1 Conclusions ........................................................................................................................................... 134 8.1.1 Results of the analysis of the TiC matrices with/without SiO2 ..................................................... 134 8.1.2 Results of the analysis of the TiC Matrices with/without SiO2 and the addition of SiC whiskers and cBN respectively ............................................................................................................................... 135 XI 8.1.3 Results of the analysis of the biaxial strength tests of sintered TiC samples with the best combination of density and fracture toughness. ................................................................................... 137 8.1.4 Results of the analysis of the friction and sliding wear tests of sintered TiC samples with the best combination of density and fracture toughness. ................................................................................... 138 8.2 Recommendations ................................................................................................................................. 139 Bibliography ................................................................................................................................................ 140 Appendices .................................................................................................................................................. 147 Appendix A – Theoretical Density calculations ..................................................................................... 147 Appendix B – SPS sintering parameters for TiC mixes without SiO2 ................................................... 148 Appendix C – Preliminary Densification results of Sintered TiC mixes with Al₂O₃ and Y₂O₃ ............. 151 Appendix D – SPS sintering parameters for TiC mixes with the addition of SiO2 ............................... 152 Appendix E - SPS sintering parameters for TiC mixes using TiC powder without SiO2 and with the addition of cBN ........................................................................................................................................ 154 Appendix F - SPS sintering parameters for TiC mixes using TiC powder with SiO2 and with the addition of cBN ....................................................................................................................................................... 156 Appendix G -SPS sintering parameters for TiC mixes using 2µm TiC powder without SiO2 and with the addition of SiC whiskers ......................................................................................................................... 157 Appendix H -SPS sintering parameters for TiC mixes using TiC powder with SiO2 and with the addition of SiC whiskers ........................................................................................................................................ 158 XII LIST OF FIGURES Figure 2.1 (a) Crystal structure of TiC and (b) Projection of Ti–C atoms along [111] direction ...................... 5 Figure 2.2 Displacement rate Vs temperature for powder processed with the initial pressure and densification mechanisms. ................................................................................................................................................ 7 Figure 2.3 Alumina-Yttria phase diagram ...................................................................................................... 7 Figure 2.4 Alumina-Yttria phase diagram of the Alumina rich portion of the Al2O3-Y2O3 system. The metastable phase diagram is superimposed with dashed lines and the phase fields are labelled ................................... 8 Figure 2.5 Phase relation in the ternary system SiC–Al2O3–Y2O3 at 1400°C ............................................... 10 Figure 2.6 Typical processes occurring during crack propagation in whisker reinforced ceramics .............. 13 Figure 2.7 Schematic representation of the spark plasma sintering apparatus ........................................... 15 Figure 2.8 SEM images of Silicon Carbide whiskers ................................................................................... 16 Figure 2.9 Crystal structures of Boron Nitride ............................................................................................. 17 Figure 2.10 SEM morphologies of cBN powders......................................................................................... 18 Figure 2.11 Contact damage/cracks on the surface of ceramics: a) Hertzian ring crack in silicon nitride and b) crack caused in operation by the rolling of high alloyed steel wires. c) Radial cracks in the corners of a Vickers indent in silicon nitride and d) breakout due to lateral crackers under a Vickers indent in silicon carbide. ... 19 Figure 2.12-1 a) and b) Experimental setup of the B3B test: a) shows the set-up with applied pre-load b) the set up (with the guide lowered) just before the start of the fracture. .................................................................. 20 Figure 2.13 Graphs of wear loss by hard particles as a function of material properties and operating parameters a) hardness of the abrasive particle, b) ratio of hardness of the abrasive particle and hardness of the wearing material, c) abrasive particle size, normal load and impact velocity and d) impact angle without considering lubrication. .................................................................................................................................................. 21 Figure 2.14 Geometry of contact between an idealised conical abrasive particle and a surface: (a) in elevation; (b) in the plane view ................................................................................................................................... 22 Figure 2.15 Diagram showing crack formation in a brittle material due to point indentation. The normal load increases from (a) to (c), and is then progressively reduced from (d) to (f) ................................................ 23 Figure 2.16 Schematic representation of different interactions between sliding abrasive particles and the surface of materials, scanning electron micrograph of a wear groove on an austenitic steel and a draft of a taper section through the wear groove. ............................................................................................................................ 24 Figure 2.17 Ratio of microcutting to microploughing as a function of the ratio of the attack angle to the critical attack angle. ............................................................................................................................................... 25 Figure 2.18 Schematic illustrations of four common methods used to measure abrasive wear rates of materials: (a) Pin on abrasive disc; (b) pin on abrasive plate; (c) pin on abrasive drum; (d) rubber wheel abrasion test.27 Figure 2.19 Schematic representation of abrasive wear resistance of two-phase structures as a function of volume fraction of a reinforcing phase. ....................................................................................................... 28 Figure 3.1 A brief outline of the experimental approach .............................................................................. 29 XIII Figure 3.2 SEM image of SiC whiskers ...................................................................................................... 31 Figure 3.3 SEM images of SiC whiskers (higher magnification) .................................................................. 31 Figure 3.4 A SEM image of cBN crush grade 6 .......................................................................................... 31 Figure 3.5 SEM image of cBN crush grade 6 (higher magnification) ........................................................... 31 Figure 3.6 Fritsch Pulverisette 6 planetary ball mill ..................................................................................... 32 Figure 3.7 FCT group Systeme GmbH Spark Plasma Sintering Machine ................................................... 33 Figure 3.8 Sketch of the B3B experimental setup ....................................................................................... 39 Figure 3.9 Ball on disc sliding wear testing rig ............................................................................................ 41 Figure 3.10 Diagram of the positioning of the sample disc and wear ball in a ball on disc sliding wear testing rig ................................................................................................................................................................... 41 Figure 3.11 Diagram of a spherical cap region of a sphere ......................................................................... 41 Figure 4.2a) 85 wt. % TiC +12 wt. % Al2O3 + 3 wt. % Y2O3 powder ............................................................ 43 Figure 4.2b)85 wt. % TiC-3.75 wt. % Al2O3-6wt. % Y2O3-5.25 wt. % SiO2 powder ...................................... 43 Figure 4.3A Micrograph of 95% TiC mix ..................................................................................................... 44 Figure 4.3B Micrograph of 90% TiC mix ..................................................................................................... 44 Figure 4.3C Micrograph of 85% TiC mix ..................................................................................................... 44 Figure 4.3D Micrograph of 80% TiC mix ..................................................................................................... 44 Figure 4.4 Graph of the trends in the density of SPS sintered samples with 85 wt. % TiC and liquid phase sintering aids .............................................................................................................................................. 48 Figure 4.5 Graph of the rends in the density of SPS sintered samples with 90 wt. % TiC and liquid phase sintering aids .............................................................................................................................................. 50 Figure 4.6 Y2O3-Al2O3 phase diagram)........................................................................................................ 52 Figure 4.7 Isothermal section of the system Y2O3-Al2O3-SiO2 at 1600°C in air (in mol. %) .......................... 52 Figure 4.8A: SEM image of the microstructure of 85TiC-12Al2O3-3Y2O3 SPS’d at 1550 °C, 50MPa and 10 min hold. ........................................................................................................................................................... 53 Figure 4.8B: SEM image of 85TiC-3.75Al2O3-6Y2O3-5.25SiO2 SPS’d at 1610°C, 70MPa and 10 min hold. 53 Figure 4.8C: SEM image of 90TiC-8Al2O3-2Y2O3 SPS’d at 1550°C, 50MPa and 10 min hold ..................... 53 Figure 4.8D: SEM image of 90TiC-2.5Al2O3-4Y2O3-3.5SiO2 SPS’d at 1625°C, 70MPa and 10 min hold ..... 53 Figure 4.9 A: Image of the area of sintered 85TiC-12Al2O3-3Y2O3 SPS’d at 1550 °C, 50MPa and 10 min hold scanned by EDS for composition ................................................................................................................ 54 Figure 4.9 B: Results of EDS qualitative analysis on the area highlighted in Fig. 4.9 A ............................... 54 Figure 4.10 A: Image of the area of sintered 85TiC-12Al2O3-3Y2O3 SPS’d at 1550 °C, 50MPa and 10 min hold scanned by EDS for composition ................................................................................................................ 54 XIV Figure 4.10 B: Results of EDS qualitative analysis on the area highlighted in Fig. 4.110 A ......................... 54 Figure 4.11 A: Image of the area of sintered 85TiC-12Al2O3-3Y2O3 SPS’d at 1550 °C, 50MPa and 10 min hold scanned by EDS for composition ............................................................................................................... 54 Figure 4.11 B: Results of EDS qualitative analysis on the area highlighted in Fig. 4.11 A ........................... 54 Figure 4.12 A: Image of the area of sintered 90TiC-2.5Al2O3-4Y2O3-3.5SiO2 SPS’d at 1625°C, 70MPa and 10 min hold scanned by EDS for composition.................................................................................................. 55 Figure 4.12 B: Results of EDS qualitative analysis on the area highlighted in Fig. 4.12 A ........................... 55 Figure 4.13 A: Image of the area of sintered 90TiC-2.5Al2O3-4Y2O3-3.5SiO2 SPS’d at 1625°C, 70MPa and 10 min hold scanned by EDS for composition.................................................................................................. 55 Figure 4.13: Results of EDS qualitative analysis on the area highlighted in Fig. 4.13 A .............................. 55 Figure 4.14 A: Image of the area of sintered 90TiC-2.5Al2O3-4Y2O3-3.5SiO2 SPS’d at 1625°C, 70MPa and 10 min hold scanned by EDS for composition.................................................................................................. 56 Figure 4.14 B: Results of EDS qualitative analysis on the area highlighted in Fig. 4.14 A ........................... 56 Figure 4.15 Comparison of % of full density of 85 wt.% TiC vs. 85 wt.% TiC(SiO2) at various sintering parameters ................................................................................................................................................. 57 Figure 4.16 Comparison of % of full density of 90 wt.% TiC vs. 90 wt.% TiC(SiO2) at various sintering parameters ................................................................................................................................................. 57 Figure 5.1 a) 90 wt.% TiC (2µm) + 10 vol.% SiCw powder ......................................................................... 59 Figure 5.1 b) 90 wt.% TiC (2µm) + 7 vol.% cBN powder ............................................................................. 59 Figure 5.1 c) 90 wt.% TiC (+SiO2) (5:8:7) + 20 vol.% SiCw powder............................................................ 60 Figure 5.1 d) 90 wt.% TiC (+SiO2) (5:8:7) + 10 vol.% cBN powder ............................................................. 60 Figure 5.2 SEM images of TiC matrix powder mixes after the addition of SiC whiskers .............................. 63 Figure 5.3 Densification rate of 85 wt.% TiC matrices with/without SiO2 and 10 vol.% cBN SPS’d at 1575°C72 Figure 5.4 Y2O3-Al2O3 phase diagram ......................................................................................................... 75 Figure 5.5 Isothermal section of the system Y2O3-Al2O3-SiO2 at 1600°C in air (in mol. %) ♦ - Represents liquid phase sintering aid composition used. ........................................................................................................ 75 Figure 5.6 Phase relation in the ternary system SiC- Al₂O₃-Y₂O₃ at 1400°C .............................................. 75 Figure 5.7 SEM micrographs of A - crack path from Vickers Hardness indentation in a sample with the composition 79TiC-11Al2O3-3Y2O3-7 (10 vol.%) SiCw SPS’d at 1600°C, 70MPa and 10 min hold time and B - crack path from Vickers Hardness indentation in a sample with the composition 73TiC-10Al2O3-3Y2O3-14 (20 vol.%) SiCw SPS’d at 1625°C, 70MPa and 10 min hold. ............................................................................ 81 Figure 5.8 SEM micrographs of A - 85TiC-3.75Al2O3-6Y2O3-5.25SiO2 SPS’d at 1610°C, 70MPa and 10 min hold time and B - crack path from Vickers Hardness indentation in 73TiC-3Al2O3-5Y2O3-4SiO2-15 (20 vol.%) SiCw SPS’d at 1625°C, 70MPa and 10 min ................................................................................................ 82 XV Figure 5.9 SEM micrographs of A – 77.2TiC-6.9Al2O3-1.7Y2O3-14.2 (20 vol.%) SiCw SPS’d at 1625°C, 70MPa and 20 min hold time and B - crack path from Vickers Hardness indentation in 77.2TiC-6.9Al2O3-1.7Y2O3-14.2 (20 vol.%) SiCw SPS’d at 1625°C, 70MPa and 20 min hold time magnified by 15 K X. .............................. 83 Figure 5.10 SEM micrographs of A – 77TiC-2.1Al2O3-3.4Y2O3-3SiO2-14.5 (20 vol.%) SiCw SPS’d at 1640°C, 70MPa and 10 min hold time and B - crack path from Vickers Hardness indentation in 77TiC-2.1Al2O3-3.4Y2O3- 3SiO2-14.5 (20 vol.%) SiCw SPS’d at 1640°C, 70MPa and 10 min hold time magnified by 10 K X ............. 83 Figure 5.11 SEM micrographs of A – 78.6TiC-3.5Al2O3-5.6Y2O3-4.9SiO2-7.5 (10 vol.%) cBN SPS’d at 1625°C, 70MPa and 10 min hold time and B - crack path from Vickers Hardness indentation in 78.6TiC-3.5Al2O3- 5.6Y2O3-4.9SiO2-7.5 (10 vol.%) cBN SPS’d at 1625°C, 70MPa and 10 min hold time magnified by 10 K X 84 Figure 5.12 SEM micrographs of A – 78.6TiC-3.5Al2O3-5.6Y2O3-4.9SiO2-7.5 (10 vol.%) cBN SPS’d at 1575°C, 70MPa and 10 min hold time viewed with the Secondary detector magnified by 15 K X and B - 78.6TiC- 3.5Al2O3-5.6Y2O3-4.9SiO2-7.5 (10 vol.%) cBN SPS’d at 1575°C, 70MPa and 10 min hold time viewed via Backscatter detector magnified by 15 K X .................................................................................................. 84 Figure 5.13 SEM micrographs of A – 83.3TiC-2.3Al2O3-3.7Y2O3-3.2SiO2-7.5 (10 vol.%) cBN SPS’d at 1575°C, 70MPa and 10 min hold time and B - crack path from Vickers Hardness indentation in 83.3TiC-2.3Al2O3- 3.7Y2O3-3.2SiO2-7.5 (10 vol.%) cBN SPS’d at 1575°C, 70MPa and 10 min hold time magnified by 10.00 KX 85 Figure 5.14 SEM micrographs of A – 83.3TiC-2.3Al2O3-3.7Y2O3-3.2SiO2-7.5 (10 vol.%) cBN SPS’d at 1575°C, 70MPa and 10 min hold time viewed with the Secondary detector magnified at 15.1 K X and B - 83.3TiC- 2.3Al2O3-3.7Y2O3-3.2SiO2-7.5 (10 vol.%) cBN SPS’d at 1575°C, 70MPa and 10 min hold time viewed via Backscatter detector magnified by 15.1 K X ............................................................................................... 86 Figure 6.1 Photograph of fractured sample of 85TiC-12Al2O3-3Y2O3 .......................................................... 95 Figure 6.2 A- Outer limit of fractured edge (outside circumference of sample furthest from the site of loading), B- mid-length of fractured edge and C-inner edge of fracture of edge (center of sample directly under the site of loading) ...................................................................................................................................................... 96 Figure 6.3 A - Fracture surface analysed under high magnification (10.0 K X) using Backscatter Detector and B- same surfaced analysed under high magnification using Secondary Electron Detector ......................... 96 Figure 6.4 Graph of the regression line used to determine the Weibull modulus for 85TiC-12Al2O3-3Y2O3 . 97 Figure 6.5 Photograph of fractured sample of 79TiC-11Al2O3-3Y2O3-7 (10.0 vol.%) SiCw .......................... 97 Figure 6.6 A- Outer limit of fractured edge (outside circumference of sample furthest from the site of loading), B- mid-length of fractured edge and C-inner edge of fracture of edge (center of sample directly under the site of loading) ...................................................................................................................................................... 98 Figure 6.7 A - Fracture surface analysed under high magnification (10.0 K X) using Backscatter Detector and B- same surfaced analysed under high magnification using Secondary Electron Detector ......................... 98 Figure 6.8 Graph of the regression line used to determine the Weibull modulus for 79TiC-11Al2O3-3Y2O3- 7(10.0 vol.%) SiCw ..................................................................................................................................... 99 Figure 6.9 Photograph of fractured sample of 73TiC-10Al2O3-3Y2O3-14 (20.0 vol.%) SiCw ...................... 100 Figure 6.10 A- Outer limit of fractured edge (outside circumference of sample furthest from the site of loading), B- mid-length of fractured edge and C-inner edge of fracture of edge (center of sample directly under the site of loading) .................................................................................................................................................... 100 XVI Figure 6.11 A - Fracture surface analysed under high magnification (10.0 K X) using Backscatter Detector and B- same surfaced analysed under high magnification using Secondary Electron Detector ....................... 100 Figure 6.12 Graph of the regression line used to determine the Weibull modulus for 73TiC-10Al2O3-3Y2O3-14 (20.0 vol.%) SiCw ..................................................................................................................................... 101 Figure 6.13 Photograph of fractured sample of 79TiC-4.5Al2O3-5.6Y2O3-4.9SiO2-7.1 (10.0 vol.%) SiCw .. 101 Figure 6.14 A- Outer limit of fractured edge (outside circumference of sample furthest from the site of loading), B- mid-length of fractured edge and C-inner edge of fracture of edge (center of sample directly under the site of loading) .................................................................................................................................................... 102 Figure 6.15 A - Fracture surface analysed under high magnification (10.0 K X) using Backscatter Detector and B- same surfaced analysed under high magnification using Secondary Electron Detector ....................... 102 Figure 6.16 Graph of the regression line used to determine the Weibull modulus for 79TiC-4.5Al2O3-5.6Y2O3- 4.9 SiO2-7.1 (10.0 vol.%) SiCw ................................................................................................................. 103 Figure 6.17 Photograph of fractured sample of 78.6TiC-3.5Al2O3-5.6Y2O3-4.9SiO2-7.5 (10.0 vol.%) cBN . 104 Figure 6.18 A- Outer limit of fractured edge (outside circumference of sample furthest from the site of loading), B- mid-length of fractured edge and C-inner edge of fracture of edge (center of sample directly under the site of loading) .................................................................................................................................................... 104 Figure 6.19 A - Fracture surface analysed under high magnification (10.0 K X) using Backscatter Detector and B- same surfaced analysed under high magnification using Secondary Electron Detector ....................... 105 Figure 6.20 Graph of the regression line used to determine the Weibull modulus for 78.6TiC-3.5Al2O3-5.6Y2O3- 4.9SiO2-7.5 (10.0 vol.%) cBN ................................................................................................................... 106 Figure 6.21 Photograph of fractured sample of 77.8TiC-6.9Al2O3-7Y2O3-14 (20.0 vol.%) SiCw ................ 106 Figure 6.22 A- mid-length of fractured edge and B- inner edge of fracture of edge (center of sample directly under the site of loading) .......................................................................................................................... 107 Figure 6.23 A - Fracture surface analysed under high magnification (10.0 K X) using Backscatter Detector and B- same surfaced analysed under high magnification using Secondary Electron Detector ....................... 107 Figure 6.24 Graph of the regression line used to determine the Weibull modulus for 77.8TiC-6.9Al2O3-1.7Y2O3- 14 (20.0 vol.%) SiCw ................................................................................................................................ 108 Figure 7.1 A: wear track on the surface of the 85TiC-12Al2O3-3Y2O3 matrix only sample disc. B: High magnification image of the wear track on the 82.6 vol.%TiC-14.6 vol.%Al2O3-2.9 vol.%Y2O3 matrix only sample disc ........................................................................................................................................................... 112 Figure 7.2 A: Spectrum 2 Area alongside the wear track scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 2. .......................................................................................................... 112 Figure 7.3 A: Spectrum 1 Area on the wear track scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 1 ........................................................................................................... 113 Figure 7.4 A: Wear scar on the surface of the worn AISI 52100 steel ball bearing. B: Diameter and area of the wear scar on the worn AISI 52100 steel ball bearing. (Diameter: 1.041mm and Area: 0.850mm2) ........... 113 Figure 7.5 A: High magnification image of the edge of the wear scar on the worn AISI 52100 steel ball bearing. B: Higher magnification image of the center of the wear scar on the worn AISI 52100 steel ball bearing .. 114 XVII Figure 7.6 A: Spectrum 2 Area inside the wear scar on the worn AISI 52100 steel ball bearing scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 2 ..................................................... 114 Figure 7.7 A: wear track on the surface of the 79TiC-11Al2O3-3Y2O3-7 (10.0 vol.%) SiCw sample disc. B: High magnification image of the wear track on the 79TiC-11Al2O3-3Y2O3-7 (10.0 vol.%) SiCw sample disc. ..... 115 Figure 7.8 A: Spectrum 2 Area alongside the wear track scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 2 ........................................................................................................... 115 Figure 7.9 A: Spectrum 1 Area on the wear track scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 1. .......................................................................................................... 116 Figure 7.10 A: Wear scar on the surface of the worn AISI 52100 steel ball bearing. B: Diameter and area of the wear scar on the worn AISI 52100 steel ball bearing. (Diameter: 1.032mm and Area: 0.837mm2) ...... 116 Figure 7.11 A: High magnification image of the edge of the wear scar on the worn AISI 52100 steel ball bearing. B: Higher magnification image of the center of the wear scar on the worn AISI 52100 steel ball bearing ................................................................................................................................................................. 117 Figure 7.12 A: Spectrum 2 Area inside the wear scar on the worn AISI 52100 steel ball bearing scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 2 ............................................. 117 Figure 7.13 A: wear track on the surface of the 73TiC-10Al2O3-3Y2O3-14 (20 vol.%) SiCw sample disc. B: High magnification image of the wear track on the 73TiC-10Al2O3-3Y2O3- 14 (20.0 vol.%) SiCw sample disc ... 118 Figure 7.14 A: Spectrum 2 Area alongside the wear track scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 2 ........................................................................................................... 118 Figure 7.15 A: Spectrum 1 Area on the wear track scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 1 ........................................................................................................... 119 Figure 7.16 A: Wear scar on the surface of the worn AISI 52100 steel ball bearing. B: Diameter and area of the wear scar on the worn AISI 52100 steel ball bearing. (Diameter: 0.929mm and Area: 0.678mm2) ...... 119 Figure 7.17 A: High magnification image of the edge of the wear scar on the worn AISI 52100 steel ball bearing. B: Higher magnification image of the center of the wear scar on the worn AISI 52100 steel ball bearing ................................................................................................................................................................. 120 Figure 7.18 A: Spectrum 3 Area inside the wear scar on the worn AISI 52100 steel ball bearing scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 3 ............................................. 120 Figure 7.19 A: wear track on the surface of the 79TiC-4.5Al2O3-5.6Y2O3-4.9SiO2-7.1(10.0 vol.%) SiCw sample disc. B: High magnification image of the wear track on the 79TiC-4.5Al2O3-5.6Y2O3-4.9 SiO2-7.1(10.0 vol.%) SiCw sample disc ..................................................................................................................................... 121 Figure 7.20 A: Spectrum 2 Area alongside the wear track scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 2 ........................................................................................................... 121 Figure 7.21 A: Spectrum 1 Area on the wear track scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 1 ........................................................................................................... 122 Figure 7.22 A: Wear scar on the surface of the worn AISI 52100 steel ball bearing. B: Diameter and area of the wear scar on the worn AISI 52100 steel ball bearing. (Diameter: 0.798mm and Area: 0.500mm2) ...... 122 XVIII Figure 7.23 A: High magnification image of the edge of the wear scar on the worn AISI 52100 steel ball bearing. B: Higher magnification image of the center of the wear scar on the worn AISI 52100 steel ball bearing ................................................................................................................................................................. 123 Figure 7.24 A: Spectrum 2 Area inside the wear scar on the worn AISI 52100 steel ball bearing scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 2 ............................................. 123 Figure 7.25 A: wear track on the surface of the 78.6TiC-3.5Al2O3-5.6Y2O3-4.9 SiO2-7.5(10.0 vol.%) cBN sample disc. B: High magnification image of the wear track on the 78.6TiC-3.5Al2O3-5.6Y2O3-4.9SiO2-7.5(10.0 vol.%) cBN sample disc ............................................................................................................................ 124 Figure 7.26 A: Spectrum 1 Area alongside the wear track scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 1 ........................................................................................................... 124 Figure 7.27 A: Spectrum 2 Area on the wear track scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 2. .......................................................................................................... 125 Figure 7.28 A: Wear scar on the surface of the worn AISI 52100 steel ball bearing. B: Diameter and area of the wear scar on the worn AISI 52100 steel ball bearing. (Diameter: 1.376mm and Area: 1.486mm2) ...... 125 Figure 7.29 A: High magnification image of the edge of the wear scar on the worn AISI 52100 steel ball bearing. B: Higher magnification image of the center of the wear scar on the worn AISI 52100 steel ball bearing ................................................................................................................................................................. 126 Figure 7.30 A: Spectrum 3 Area inside the wear scar on the worn AISI 52100 steel ball bearing scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 3 ............................................. 126 Figure 7.31 A: wear track on the surface of the 77.8TiC-6.9Al2O3-1.7Y2O3-14(20.0 vol.%) SiCw sample disc. B: High magnification image of the wear track on the 77.8TiC-6.9Al2O3-1.7Y2O3-14(20.0 vol.%) SiCw sample disc ................................................................................................................................................................. 127 Figure 7.32 A: Spectrum 2 Area alongside the wear track scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 2. .......................................................................................................... 127 Figure 7.33 A: Spectrum 1 Area on the wear track scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 1 ........................................................................................................... 128 Figure 7.34 A: Wear scar on the surface of the worn AISI 52100 steel ball bearing. B: Diameter and area of the wear scar on the worn AISI 52100 steel ball bearing. (Diameter: 0.876mm and Area: 0.603mm2) ...... 128 Figure 7.35 A: High magnification image of the edge of the wear scar on the worn AISI 52100 steel ball bearing. B: Higher magnification image of the center of the wear scar on the worn AISI 52100 steel ball bearing ................................................................................................................................................................. 129 Figure 7.36 A: Spectrum 2 Area inside the wear scar on the worn AISI 52100 steel ball bearing scanned by EDS for composition B: Results of EDS qualitative analysis on Spectrum 2 ............................................. 129 Figure 7.37 Graphs for results given by the CSM Tribometer for the sliding wear tests of the sintered TiC discs with/without reinforcements ...................................................................................................................... 131 XIX LIST OF TABLES Table 2.1 Properties of TiC ........................................................................................................................... 6 Table 2.2 Typical properties for hexagonal and cubic boron nitride ............................................................ 18 Table 3.1: Materials used for processing .................................................................................................... 31 Table 3.2: Preparation method for grinding sintered TiC matrices samples ................................................ 35 Table 3.3: Preparation method for polishing sintered TiC matrices samples ............................................... 35 Table 3.4: Components TiC matrices .......................................................................................................... 36 Table 3.5: Components TiC matrices with SiC whisker (SiCw) reinforcements ........................................... 37 Table 3.6: Components TiC matrices with Cubic boron nitride (cBN) reinforcements ................................. 37 Table 4.1 Results of the Particle size analysis of the starting powders ....................................................... 43 Table 4.2: Results for XRD patterns for milled TiC matrix powders ............................................................. 45 Table 4.3: Theoretical densities of TiC mixes with Al₂O₃ and Y₂O₃ ............................................................. 46 Table 4.4: Theoretical densities of TiC mixes with Al₂O₃, Y₂O₃ the addition of SiO2 ................................... 46 Table 4.5: Average density measurements for samples of TiC mixes with Al₂O₃ and Y₂O₃ sintered at 1450°C………………………………………………………………………………………………………………….47 Table 4.6: Average density measurements for samples of TiC mixes with Al₂O₃ and Y₂O₃ sintered at 1550°C ……………………………………………….…………………………………………………………………………47 Table 4.7: Average density measurements for samples of TiC mixes with Al₂O₃ and Y₂O₃ sintered at 1650°C……………………………………………………………………………………………………………….…47 Table 4.8: Density of SPS sintered samples with 85 wt. % TiC and liquid phase sintering aids .................. 48 Table 4.9: Density of SPS sintered samples with 90 wt. % TiC and liquid phase sintering aids .................. 50 Table 4.10: Phases identified by XRD analysis of SPS’s samples with the highest percentage theoretical density. (YAG-Y3Al5O12, YAP- YAlO3 and Y2Si2O7) ..................................................................................... 52 Table 4.11 Hardness and Fracture Toughness of SPS samples with highest percentage of Theoretical Density ................................................................................................................................................................... 58 Table 5.1 of Results for XRD patterns for milled 90 wt.% TiC matrix powders with/without SiO2 and the addition of SiC whiskers and cBN respectively ......................................................................................................... 62 Table 5.2 Results for XRD patterns for milled 85 wt.% TiC matrix powders with/without SiO2 and the addition of SiC whiskers and cBN respectively ............................................................................................................ 62 Table 5.3 Composition of TiC matrix samples without SiO2 and with the addition of SiC whiskers and cBN respectively ................................................................................................................................................ 63 Table 5.4 Composition of 85 wt.% TiC matrix samples with SiO2 and with the addition of SiC whiskers and cBN respectively ................................................................................................................................................ 63 Table 5.5 Composition of 90 wt.% TiC matrix samples with SiO2 and with the addition of SiC whiskers and cBN respectively ................................................................................................................................................ 64 XX Table 5.6: Density of SPS sintered samples with 85 wt. % TiC, liquid phase sintering aids (Al2O3-Y2O3), SiC whiskers and cBN respectively of various compositions and sintering parameters ..................................... 66 Table 5.7: Density of SPS sintered samples with 90 wt. % TiC, liquid phase sintering aids (Al2O3-Y2O3), SiC whiskers and cBN respectively of various compositions and sintering parameters ..................................... 67 Table 5.8: Density of SPS sintered samples with 85 wt. % TiC, liquid phase sintering aids (Al2O3-Y2O3-SiO2) and SiC whiskers of various compositions and sintering parameters .......................................................... 69 Table 5.9: Density of SPS sintered samples with 90 wt. % TiC, liquid phase sintering aids (Al2O3-Y2O3-SiO2) and SiC whiskers of various compositions and sintering parameters .......................................................... 70 Table 5.10: Density of SPS sintered samples with 85 wt. % TiC, liquid phase sintering aids (Al2O3-Y2O3-SiO2) and cBN of various compositions and sintering parameters ........................................................................ 71 Table 5.11: Density of SPS sintered samples with 90 wt. % TiC, liquid phase sintering aids (Al2O3-Y2O3-SiO2) and cBN of various compositions and sintering parameters ........................................................................ 73 Table 5.12 Phases identified by XRD analysis of SPS’d samples of 85 wt. % TiC + SiCw with the highest percentage theoretical density. (YAG-Y3Al5O12, YAP- YAlO3) ..................................................................... 75 Table 5.13 Phases identified by XRD analysis of SPS’d samples of 90 wt. % TiC + SiCw with the highest percentage theoretical density. (YAG-Y3Al5O12, YAP- YAlO3) ..................................................................... 77 Table 5.14 Phases identified by XRD analysis of SPS’d samples of 85 wt. % TiC + cBN with the highest percentage theoretical density. (YAG-Y3Al5O12, YAP- YAlO3) ..................................................................... 79 Table 5.15 Phases identified by XRD analysis of SPS’d samples of 90 wt. % TiC + cBN with the highest percentage theoretical density. (YAG-Y3Al5O12, YAP- YAlO3) ..................................................................... 81 Table 5.16: Hardness and fracture toughness of SPS sintered samples with 85 wt. % TiC, liquid phase sintering aids (Al2O3-Y2O3) and SiC whiskers with the highest density ........................................................ 88 Table 5.17: Hardness and fracture toughness of SPS sintered samples with 85 wt. % TiC, liquid phase sintering aids (Al2O3-Y2O3-SiO2) and SiC whiskers with the highest density. ............................................... 89 Table 5.18: Hardness and fracture toughness of SPS sintered samples with 85 wt.% TiC, liquid phase sintering aids (Al2O3-Y2O3-SiO2) and cBN with the highest density. ........................................................................... 90 Table 5.19: Hardness and fracture toughness of SPS sintered samples with 90 wt. % TiC, liquid phase sintering aids (Al2O3-Y2O3) and SiC whiskers with the highest density ........................................................ 91 Table 5.20: Hardness and fracture toughness of SPS sintered samples with 90 wt. % TiC, liquid phase sintering aids (Al2O3-Y2O3-SiO2) and SiC whiskers with the highest density. ............................................... 92 Table 5.21: Hardness and fracture toughness of SPS sintered samples with 90 wt.% TiC, liquid phase sintering aids (Al2O3-Y2O3-SiO2) and cBN with the highest density. ........................................................................... 92 Table 6.1. Table of sintering parameters, sintering density and fracture toughness for selected sample compositions .............................................................................................................................................. 94 Table 6.2. Table of results for the force at the moment of fracture for samples of various compositions ..... 94 Table 6.3 Table of Poisson’s Ratios and Young’s moduli for the constituents of the composite materials .. 95 XXI Table 6.4 Table of results for theoretical Poisson’s Ratios and Young’s moduli of various compositions based on Rule of Mixtures ..................................................................................................................................... 95 Table 6.5 Table of results for the calculation of Poisson’s Ratio (υ), Young’s Modulus (E) and maximum tensile stress (σmax) for samples of various composition and sintering parameters ................................................. 96 Table 6.6 Table of results for the calculation of the probability of fracture and Weibull modulus of 85TiC- 12Al2O3-3Y2O3 ............................................................................................................................................ 98 Table 6.7 Table of results for the calculation of the probability of fracture and Weibull modulus of 79TiC- 11Al2O3-3Y2O3-7(10.0 vol.%) SiCw ........................................................................................................... 100 Table 6.8 Table of results for the calculation of the probability of fracture and Weibull modulus of 73TiC- 10Al2O3-3Y2O3-14(20.0 vol.%) SiCw ......................................................................................................... 102 Table 6.9 Table of results for the calculation of the probability of fracture and Weibull modulus of 79TiC- 4.5Al2O3-5.6Y2O3-4.9SiO2-7.1(10.0 vol.%) SiCw ……………………………………………………………………………………………………………….…104 Table 6.10 Table of results for the calculation of the probability of fracture and Weibull modulus of 78.6TiC- 3.5Al2O3-5.6Y2O3-4.9 SiO2-7.5 (10.0 vol.%) cBN…………………………………………………………………………………………………………………….106 Table 6.11 Table of results for the calculation of the probability of fracture and Weibull modulus of 77.8TiC- 6.9Al2O3-1.7Y2O3-14(20.0 vol.%) SiCw ..................................................................................................... 109 Table 6.12: Typical values of the Weibull modulus m for some materials ................................................. 110 Table 6.13: Weibull moduli of sintered TiC with various liquid phase sintering aids and with/without the addition of reinforcement materials ........................................................................................................................ 110 Table 7.1 Mass lost on ball bearing during wear testing............................................................................ 131 Table 7.2 Coefficient of friction (µ) for the sliding wear testing of sintered TiC discs with/without reinforcement materials ................................................................................................................................................... 133 1 1. INTRODUCTION 1.1 Subject of the thesis Titanium carbide (TiC) is successfully used as a component of heat resistant materials intended for parts operating at high temperature. In addition to high hardness and wear resistance, TiC is readily available at a comparatively low cost. As a result of its low comparative cost it is commonly used as a matrix of sintered hard alloys for wear resistant parts and cutting tools (Samsonov & Sergeev, 1971). However, the widespread use of TiC in cutting tools and other applications has been limited because the fracture strength of the material has resulted in poor fracture toughness. (Sorrell, 2001) The insufficient fracture toughness of conventionally produced TiC is a characteristic that could possibly be overcome by the use of TiC as a matrix in ceramic matrix composites (CMC’s). The use of TiC matrices in ceramic matrix composites (CMC’s) is limited because of the methods used to produce CMC’s. Complex microstructures, changeability of the distribution of reinforcement materials, sintering defects and unusual material behaviour can be created during the production processes. This results in a large obstacle to the use of CMC’s as there is limited knowledge with regards to the relationship between sintering parameters, microstructure and physical properties (Gowayed et al., 2018). This project intended to determine the suitability of TiC based CMC’s as a material for cutting tools. The research was aimed at improving the fracture toughness of TiC without causing a decrease in its other mechanical properties by introducing and comparing two types of reinforcing phases in the TiC matrix i.e. SiC whiskers and cubic boron nitride (cBN) particles. The project will span the processing of the TiC matrix powders and Spark Plasma Sintering (SPS) of such materials, followed by characterization and mechanical property evaluation; particularly in terms of density, hardness, fracture toughness, biaxial flexural strength and sliding wear resistance. 1.2 Background to the thesis Various toughening methods have been considered in order to increase the toughness of ceramics to levels where they can be used as engineering components.The addition of whiskers to many ceramics have a significant effect on their toughening (Wei & Becher, 1984) (Claussen, Petow 1986) (Buljan, Pasto & Kim 1989) (Lio et al. 1989, Gadkaree, Chyung 1986). The ease of fabrication and higher degree of isotropy in material properties of whisker reinforced ceramics are directly related to the smaller aspect ratios involved. Due to chemical incompatibility issues between matrices and fibers, only a few fibers have the potential to be used as reinforcements, especially under the high temperatures experienced during the sintering process. One of the possible candidates as a reinforcement material for ceramic matrix composites is SiC whiskers (Lundberg et al. 1987). 2 SiC whiskers have been shown to be an exceptional reinforcement material in glass and ceramic composites, giving improved high temperature properties and preparing the materials for use as high temperature engineering composites (Becher et al. 1988, Chokshi, Porter 1985, Homeny, Vaughn & Ferber 1987, Tiegs, Becher 1986). SiC fiber composites possessed a high potential for significantly improving the fracture toughness as well as the tendency towards catastrophic failure in brittle load bearing systems (Petrovic et al., 1985). Cubic boron nitride was commonly used as 2nd hard phase in the TiC composites. Cubic boron nitride (cBN) is the second hardest material next to diamond. It has many desirable physical and chemical properties (thermal stability up to 1200°C) and is highly resistant to chemical damage. Cubic boron nitride (cBN); characterized both by high compressive strength, hardness and oxidative resistance, is extensively used in applications in the manufacturing industry. Tools fabricated for cutting hardened steel, cast iron or other materials can be difficult to process. cBN compacts are commonly bonded with Ti-based alloys or compounds (Benko et al., 2003). Composites with metals of the elements in groups IV-VI of the period table or their compounds are most usually used as a binding phase. TiC and TiN display the highest chemical activity towards Boron Nitride. Previously experimental cBN-TiN/TiC composites were prepared by high pressure hot (HPHT) pressing and then followed by heat treatments. Those samples were found to have a dense polycrystalline microstructure and a thin layer of TiB2 which was visible at the interface between the BN and the binder. It was also found that after the heat treatment there was a significant decrease in the hardness (Benko et al., 1999). 1.3 Problem Statement TiC compacts can be sintered by hot pressing TiC powder with a ceramic binder (Al2O3 and/or Y2O3) at temperatures in the range of 1850-2100°C. During sintering, the plastic deformation of the powder particles occurs simultaneously with the chemical reaction with the binder in a complicated process. The microhardness of TiC prepared by spark plasma sintering (SPS) at a temperature range of 1450-1600°C was between 24-30 GPa, and its fracture toughness KIC was from 3.7-5 MPa.m1/2 (Cheng et al., 2012). Previous research has shown that addition of whiskers to CMC’s at a volume fraction exceeding 30 vol. % results in a significant decrease in the relative density of the composites (Dusza et al., 1992); nevertheless there is a tendency of higher volume percentages of whiskers in CMC’s resulting in higher fracture toughness (Evans & Faber, 1984). Therefore, determining the correct ratio of whiskers to matrix material is essential. Consideration will also need to be given to the fact that the different whiskers will have different mechanical and chemical properties which could also affect the ratio of whiskers to matrix material needed to significantly improve the fracture toughness of the TiC composite (Cheng et al., 2012). Shahedi Asl et. al produced SiCw reinforced TiC via SPS sintering at a temperature of 1900°C. The specimens with 20 volume % SiC whiskers showed the highest flexural strength. They also found that the SiC whiskers could act as a grain growth inhibitor (Shahedi Asl et al., 2019). Grain growth was a concern during the study due to the high sintering temperature required in order to achieve full density. 3 In studies that analysed the behaviour of cBN reinforced α-Sialon ceramic produced by SPS sintering it was found that higher densities were achieved at a sintering temperature 1550°C as it was below the temperature at which hexagonalization occurs. The addition of cBN to the α-Sialon increased the hardness and fracture toughness at sintering temperatures of 1550°C (Garrett et al., 2015) To improve the hardness and potential use as a cutting tool material cBN would be added as a secondary hard phase. CBN is commonly bonded with Ti based compounds. The more reactive the binding phase is with the cBN crysrallites the better the properties of the composite. (Benko et al., 2003) The purpose of this study is to improve the fracture toughness of TiC without causing a decrease in its other mechanical properties by adding SiC whiskers and cBN powder allong with liquid phase sintering aids to the starting TiC powder before sintering. In order to produce a TiC composite material with the best possible mechanical properties it will be essential to achieve the highest possible density. At a sintering temperature of 1450°C, and pressure of 50MPa, a relative density of ±96% can be achieved for TiC only material (no whiskers) (Cheng et al., 2012). With the addition of whiskers, the density will most likely decrease and therefore higher temperatures (> 1450°C) and pressures (> 50MPa) will be needed to produce samples of equal or similar density. To further increase the fracture toughness and facilitate the densification of the whisker-reinforced TiC liquid phase sintering aids will be added that will bind with the TiC to form new phases which will increase densification. cBN would be added to the TiC powder before sintering to increase the hardness and fracture toughness . 1.4 Objectives of the thesis The objectives of this project are:  To analyse the TiC-based ( with liquid phase sintering aids) matrix powders with/without reinforcement materials after milling prior to sintering using SEM, EDS and XRD phase analysis  To compare the SPS sintering parameters of TiC-based ( with liquid phase sintering aids) matrix powders with/without reinforcement materials  To analyse TiC-based (with liquid phase sintering aids) matrix with/without reinforcement materials after SPS sintering using SEM, EDS and XRD microstructure and phase analysis  To determine the Density, Vickers Hardness and Fracture Toughness of the sintered TiC-based (with liquid phase sintering aids) matrix with/without reinforcement materials  To select the TiC-based CMC compositions with the best combination of Density, Vickers Hardness and Fracture toughness for additional testing  To determine the Biaxial Flexural Strength, Weibull Modulii and analyse the fracture surfaces of the selected TiC-based CMC’s 4  To perform Sliding Wear Testing and determine the wear resistance and co-efficent of friction of the selected TiC-based CMC’s 1.5 Plan of development An overview of the TiC, SiC whiskers and cBN materials are presented in Chapter 2. The preparation and properties of TiC-based materials are also reviewed in this chapter. At the end of chapter 2 some relevant phase diagrams (binary Al2O3-Y2O3, BN-TiC and ternary Al2O3-Y2O3-SiO2 and Al2O3-Y2O3-SiC) are discussed. Chapter 2 also summarizes the findings and develops the strategy for the research on which this thesis is based. Chapter 3 describes the powders, the equipment and the experimental techniques used. The results of the investigation into the density, microstructure, hardness and fracture toughness of the sintered TiC matrices are given in Chapter 4, which includes the results of the investigation into the reactions between TiC, Al2O3, Y2O3 and SiO2. Chapter 5 focuses on the sintering of TiC matrices with the addition of SiC whiskers and cBN respectively. The hardness and fracture toughness properties of the resultant TiC-based CMC’s and their microstructures are analysed and discussed at the end of this chapter. Chapter 6 summarizes the preliminary studies of the Ball-on- 3 ball biaxial strength testing allong with the calculations for the Weibull Moduli of the TiC-based CMC’s. Chapter 7 focuses on ball-on-disc sliding wear testing on the TiC-based CMC’s for the determination of their co-efficients of friction. The results are discussed at the end of each chapter. The conclusions and recommendations for necessary future work are presented in Chapter 8. Appendices with additional tables and graphs can be found at the back of the document. 5 2. LITERATURE REVIEW 2.1 Titanium Carbide 2.1.1 Background Titanium carbide (TiC) is an ideal structural material due to its stability at high temperatures, low density and high hardness. The extensive use of TiC in cutting tools and similar applications has been limited because the high production cost of TiC and poor fracture toughness of the material (Sorrell, 2001). 2.1.2 Structure of TiC TiC is a standard faceted crystal with a NaCl type crystal structure. It has been hypothesised that the growth unit of TiC is a TiC6 octahedron. An ideal morphology of TiC particles would be an octahedral shape if the TiC6 grew freely in liquid under equilibrium during solidification. Different growth kinetics, conditions and mechanisms can however affect the growth morphology. TiC particles with dendritic, whisker/fibre, rod and hollow spherical shapes, etc. have reportedly been found (Li et al., 2008). Figure 2.1 (a) Crystal structure of TiC and (b) Projection of TiC atoms along [111] direction. The blue atoms are Ti and the black atoms are C atoms (Li et al., 2008). 6 2.1.3 Properties of TiC Table 2.1 Properties of TiC Mechanical Properties Property Value Unit Young's modulus 430 - 451 GPa Compressive strength 2500 - 2500 MPa Bending strength 420 - 420 MPa Physical Properties Property Value Unit Thermal expansion 4.1 - 7.7 e-6/K Thermal conductivity 110 - 110 W/m.K Melting temperature 3100 - 3100 °C Service temperature 20 - 1600 °C Density 4940 - 4940 kg/m3 Resistivity 0.2 - 100 Ohm.mm2/m Friction coefficient 0.07 - 0.07 - Reference: (Matweb - Material Property Database, “Titanium Carbide / TiC Chemical, mechanical, physical and environmental properties of materials,” n.d.) 2.1.4 Sintering TiC TiC has strong covalent bonds which makes it difficult to obtain fully dense ceramics using traditional sintering methodologies without substantial amounts of sintering aids and sintering temperatures upwards of 2000°C. High temperatures and lengthy heat treatments are unsuitable for use as they limit the types of reinforcements which can be added to the matrix. A rapid densification method is therefore more desirable. Spark plasma sintering (SPS) allows for rapid densification of metal and/or ceramic powders at considerably lower temperatures. The advantage of SPS is that highly dense samples can be produced within a few minutes under a high mechanical pressure although it does require a large electric current. SPS technology is broadly used in the sintering of carbides, nitrides and nano-ceramics (Cheng et al., 2013). The graph below shows examples of the densification curves produced during SPS sintering. The peaks seen represent localized deformation, bulk deformation and mass transport phenomena (Diouf & Molinari, 2012). 7 Figure 2.2 Displacement rate Vs temperature for powder processed with the initial pressure and densification mechanisms (Diouf & Molinari, 2012). 2.1.5 Liquid phase sintering aids Alumina and Yttria are used as additives for liquid phase sintering of Si3N4, SiC and SiAlON ceramics. Phase interactions in the Y2O3-Al2O3 system are of importance due to even small changes in the physico-chemical conditions during sintering could result in the generation of new phase equilibria and the development of alternative microstructures which would thus influence the properties of the resulting sintered materials. It was discovered that the Y3Al5O12 (YAG) phase and the YAG-Al2O3 eutectic are potential materials for the reinforcement of ceramics and intermetallic matrix composites for use in structural applications (Fabrichnaya & Seifert, 2001). Figure 2.3 Alumina-Yttria phase diagram (Fabrichnaya & Seifert, 2001) Metastable eutectics of Yttrium-Aluminium Perovskite (YAP, YAlO3) and alumina (Al2O3) were initially determined by Caslavsky and Viechnicki (Caslavsky & Viechnicki, 1980). The method of formation of metastable eutectics is not well defined but it has been suggested that it is carried out via the solidification of immiscible eutectic liquids heated at a minimum 40°C above the liquidus point (Caslavsky & Viechnicki, 1980). 8 A resurgence in interest in the YAP-Alumina eutectic was driven by its potential use as a high temperature stable structural ceramic. It was noted that after extended periods of time at temperatures above 1200°C, deformation accompanying a reaction to YAG prevented the consideration of this eutectic ceramicfor high temperature stable structural applications (Hay, 1994). 𝐴𝑙 𝑂 + 3𝑌𝐴𝑙𝑂 = 𝑌 𝐴𝑙 𝑂 𝑎𝑙𝑢𝑚𝑖𝑛𝑎 + 𝑌𝐴𝑃 = 𝑌𝐴𝐺 The reactants and reaction products have equivalent stiffness and creep resistance and hence the strain energy effects should be of greater importance (Hay, 1994). Figure 2.4 Alumina-Yttria phase diagram of the Alumina rich portion of the Al2O3-Y2O3 system. The metastable phase diagram is superimposed with dashed lines and the phase fields are labelled (Caslavsky & Viechnicki, 1980). Porosity can develop during sintering adjacent to the alumina-YAG interface and can be deduced to be Kirkendall porosity. This porosity results from the Aluminium and Oxygen diffusion through the YAG phase or is related to stresses developed during the reaction. Yttrium is the slowest diffusing of the species which supports the Kirkendall porosity hypothesis (Hay, 1994). 2.1.6 Sintering TiC with liquid phase sintering aids In their investigations, Chae et al. have shown that the addition of Y2O3 in small amounts is very effective in the densification of TiC-Al2O3 composite during sintering at 1700°C. The added Y2O3 had been shown to form a solid solution film at the surface of the alumina particles and to suppress the gas generating reaction that occurs during sintering (Chae & Kim, 1993). Since the eutectic reaction in the Al2O3-Y2O3 has been reported to occur at 1760°C, the sintering of Al2O3- TiC composite with Y2O3 additive at higher temperatures is expected to occur in the presence of liquid and result in enhanced densification (Chae et al., 1995). 9 In the sintering of Al2O3- TiC composites, it has been suggested that vacuum atmosphere is detrimental for densification because of the gas generating reactions. Among the various possible reactions, the following is known to be the most severe: 𝐴𝑙 𝑂 + 𝑇𝑖𝐶 = 𝐴𝑙 𝑂 + 𝑇𝑖𝑂 + 𝐶𝑂 The equilibrium partial pressure for CO for the above reaction is reported to be around 24 Pa at 2000K and thus the vacuum atmosphere which enhances the reaction will have an adverse effect on densification. However, under the conditions of rapid heating and where the gas generating reaction of Al2O3 with TiC is sufficiently suppressed by the addition of Y2O3, vacuum sintering is expected to provide better densification than sintering in an inert gas atmosphere. There is also a possibility of formation of YAG (Y3Al5O12) in the cooled liquid phase after sintering (Chae et al., 1995). 2.1.7 Interaction between TiC and SiC TiC-SiC composites are hard to densify owing to the low self-diffusion rate of SiC into TiC. TiC-SiC composites are usually produced by hot pressing techniques, including SPS. The properties of these composites largely depend on their microstructure. The intragranular nature of the microstructure is effective in improving the mechanical properties of the composite material. Intragranular composites are challenging to fabricate by the previously mentioned sintering methods (Chen et al., 2009). 2.1.8 Interaction of SiO2 and liquid phase sintering aids The system Y2O3-Al2O3-SiO2 is important as a system as it involves the most widely used sintering additives for sintering Si3N4, SiAlON and SiC ceramics. Y2O3-Al2O3-SiO2 based glasses have become more important for technical use due to their astonishing physico-chemical properties (Kolitsch et al., 1999). The system Y2O3-Al2O3 features the monoclinic compound Y4Al2O9 (YAM), the Perovskite-type YAlO3 (YAP), and the garnet-type Y3Al5O12 (YAG), with mole ratio Y2O3:Al2O3 of 2:1, 1:1 and 3:5 respectively. Because of the very complex (and partly metastable) behaviour of this system, many different interpretations of the phase diagram had been published. Even though an extensive amount of experimental work performed on this system, there still are some inconsistencies concerning the melting and crystallization behaviour of YAP and YAG, the composition and the temperature of specific eutectics, the liquidus data on the Y2O3-rich side, and the thermal stability of YAP at lower temperature (Kolitsch et al., 1999). 2.1.9 Interaction between SiC and liquid phase sintering aids The traditional method of solid-state sintering SiC is by sintering at temperatures up to 2200°C where additions of small amounts of boron, aluminium and carbon are added to improve densification. Instead, liquid phase sintering with the use of rare earth metals together with alumina and AlN, allow for sintering at lower temperatures and improve fracture toughness and mechanical strength (Neher et al., 2011). The observed phase relations in the SiC-Al2O3-Y2O3 system indicates that at least some dissolution of SiC in the liquid phase is required. The 10 observed integration of minute amounts of alumina into the SiC grains and the quick phase transformation from β- to α-SiC during liquid phase sintering are evidence of the existence of this solution. The ratios and the melting temperature of the various components are shown to have an effect on the low solubility of SiC in the oxide solution (Neher et al., 2011). Figure 2.5 Phase relation in the ternary system SiC–Al2O3–Y2O3 at 1400°C (Neher et al., 2011) 2.1.10 Interaction between TiC and Cubic Boron Nitride (cBN) One of the most essential criteria for selecting the matrix phase for sintering cBN-containing composites is the nature of chemical reaction occurring at the BN-matrix interface. Formation of new phases on the-BN-matrix interface can give rise to composite materials with advantageous mechanical properties. Exploring the reactions taking place at the BN-matrix interface is also of interest from a technological perspective. The TiC matrix material reacts with BN to form two new phases, TiB2 and TiN (Benko et al., 2001). BN-TiC with a molar ratio equating to 1:1 and at a temperature of 1400°C, co-exists as four phases in the system at constant pressure (102 Pa). These are as follows: cBN(solid), TiC0.8N0.2(solid); TiB2(solid), C(solid). Under low pressure three solid phases co-exist in the system: TiB2, C and TiC0.8N0.2 in molar quantities of 0.50 mol, 0.60 mol and 0.50 mol respectively, as well as Nitrogen (gas) at the amount 0.45 mol. At pressures higher than 1X10- 1 Pa, four solid phases exist in the system. These are TiB2, C, TiC0.8N0.2 and BN in molar quantities of 0.09 mol, 0.27 mol, 0.90 mol, and 0.82 mol respectively (Benko et al., 2001). 2.1.11 Interaction between cBN and liquid phase sintering aids (SiO2) CBN and SiO2 have low coefficients of thermal expansion and electrical conductivity. But cBN has high hardness values and thermal conductivity unlike SiO2. cBN does not readily react with SiO2. Spark plasma sintering would restrict the cBN to hBN phase transformation due to the short sintering time involved. The phase transformation temperature was higher than usual (1973 K) in the presence of SiO2 than with other secondary phases (Zhang et al., 2014). 11 2.2 Whisker reinforcement of ceramics 2.2.1 Processing of whisker reinforced ceramics Reinforcement using fibre has been successfully used to increase the fracture toughness of ceramics. Only a few types of fibres can be considered possible candidates for the reinforcement of high performance ceramics due to the effects of chemical incompatibility between fibres and the matrix and/or the high temperatures required during sintering and/or use. Silicon carbide whiskers are accepted as the most promising fibres thus far since they retain their high strength at high temperatures up to at least 1600⁰C. Scanning electron microscopy (SEM) of multiple whiskers showed that the whiskers contained both particulate material in the form of smooth crystals and in the form of bundles of fibres. These particulates could possibly act as flaws in a ceramic matrix composite structure (Lundberg et al., 1987). 2.2.2 Whisker/powder mixing A wet processing method has been used to ensure a homogeneous distribution of whiskers in the whisker/powder mixture. Mixing can be carried out in either a dispersed or flocced state depending on which technique is used to form the composite. In slip casting a properly stabilized and dispersed slip mixture with a low viscosity and high solids content must be used to reach high green densities after casting. This can be obtained by choosing a pH where both the matrix and the whiskers have a high zeta potential with an identical charge. Surfactants can be used to increase the stability and change the zeta potential curve for one or both components so that a wider range of pH can be used. For the remaining forming methods such as pressing, flocced mixing must be considered. Systems consisting of multiple components can be effectively mixed in a few minutes in a flocced state. Flocculation occurs when whiskers and matrix powders are differently charged or have zeta potentials near zero. The aggregates of the flocs formed contain both components but they are not very homogeneously mixed. The flocs may be broken up with a mixer with relatively high shear rate but only if the shearing continues. Once the flocs leave the high shear-rate field they reform and “freeze” the in the homogeneous state the mixture achieved. The best possible mixers are high speed homogenizers and ball mills with plastic or even ceramic balls. Once a mixed floc has formed it remains stable for several weeks and will not segregate (Lundberg et al., 1987). 2.2.3 Whisker toughening in ceramic materials The inclusion of whiskers in many ceramics has resulted in significant benefits being achieved. Whisker toughening is just as effective as ZrO2 toughening at an ambient temperature. In comparison to ZrO2 toughening, the amount of toughening remains relatively consistent at high (>1000⁰C) temperatures. Fracture toughness values can further be increased by the incorporation of both ZrO2 and whiskers and in some cases both types of toughening has also been observed. 12 Fracture toughness levels in fibre reinforced ceramic matrix composites (CMCs) are the highest among all ceramic composites. The shorter whisker reinforcement offers many advantages i.e. easier fabrication and a higher degree of isotropy in material properties due to the smaller aspect ratios involved (Bengisu et al., 1991). To achieve maximum toughening the following parameters need to be considered: • τi (interface shear stress): partial resistance to slip is needed but it should not be too high otherwise pull-out will not occur. Lower shear stresses are preferable in order to increase pull-out and debonding lengths. • lpo (pull-out length) pull-out lengths should be increased for maximum toughening. This can be accomplished by minimizing the compressive stresses acting on the whiskers, lowering the mechanical clamping force by using smooth surfaced whiskers and coating the whiskers with lubricants to prevent chemical reactions at the interface and decrease interfacial friction. • r (whisker radius) decreasing whisker radii provide higher toughening. • Vf (whisker volume fraction): the quantity of whiskers should be as high as processing method allows. • σw (whisker strength): high whisker strengths are most favourable since higher strengths provide more significant improvements in toughening. • lDB (debond length): this factor should be maximized as much as possible by decreasing whisker/matrix interaction. • Φ (deflection angle): deflection angles should also be maximized by monitoring whisker/matrix interface characteristics (Bengisu et al., 1991). 2.2.4 Proposed toughening mechanisms for whisker reinforced CMC’s Observations in whisker toughening in whisker reinforced CMC’s allude to the possibility of five toughening mechanisms to be taking place. These mechanisms are crack deflection, micro-cracking, whisker pull-out, crack bowing and crack bridging (Bengisu et al., 1991). 13 Figure 2.6 Typical processes occurring during crack propagation in whisker reinforced ceramics (Bengisu et al., 1991). 2.2.4.1 Microcracking Microcracking is not uncommon in ceramics reinforced with whiskers. Recent experimental and theoretical analyses show that toughening due to microcracking in non-transformational ceramics is negligible at best. Evans and Farber’s Model of microcracking induced toughening is based on the concept that microcracking leads to dilatations in the material which produces compressive stresses on a macrocrack. Their model predicted that toughness only increased by 10% (Evans & Faber, 1984). Another model by Dolgopolsky et al. considers the possibility of two opposing effects due to microcracking. One effect is the diminishing of crack growth resistance due to linking of microcracks and the other effect is crack shielding by nucleated microcracks (Dolgopolsky et al., 1989). 2.2.4.2 Crack bowing Crack bowing initiates from resistant second phase species in the path of a propagating crack. Faber and Evans’ calculated that the highest toughening by this mechanism is achieved in the case of discs with high aspect ratios whereas the lowest toughening is achieved in the case of rods (Evans & Faber, 1984). The crack bowing theory postulates that toughening is predominantly a function of obstacle penetrability (related to an obstacles toughness, strength and the coherency of said obstacle/matrix interface), the volume fraction of second phase and the size obstacle itself. Following that, whiskers are therefore theoretically less effective in bowing a crack front than e.g. metal discs/inclusions as the whiskers have a rod-like shapes and hence easier to penetrate by the crack due to their lower comparative fracture toughness. Furthermore, it can be debated that crack bowing is not a major contributor as a toughening mechanism in whisker reinforcement, although it should not be disregarded completely. Crack pinning observations suggest that a minor contribution is possible although, though crack bowing theory is not fully established it can be used to calculate this contribution with any confidence. (Dolgopolsky et al., 1989) 14 2.2.4.3 Whisker pull-out Becher et al. determined whisker pull-out toughening can be considered as the work done by the sliding of whiskers within the matrix. Whisker pull-out is often seen in whisker reinforced CMC’s even though in many cases pull-out length is limited to 1-2µm. Becher et al. calculated the average pull-out lengths as 1.2-2.0 µm in Al2O3 or SiCw, 0.4-0.8µm in mullite or SiCw, and 0.2-0.4µm in glass or SiCw composi