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Surface Hardening of Titanium Alloys By SURFACE HARDENING OF TITANIUM ALLOYS BY GAS PHASE NITRIDATION UNDER KINETIC CONTROL by LIZHI LIU Submitted in partial fulfillment of the requirements For the degree of Doctor of Philosophy Dissertation Advisor: Prof. Arthur H. Heuer Co-Advisor: Prof. Frank Ernst Department of Materials Science and Engineering CASE WESTERN RESERVE UNIVERSITY January, 2005 CASE WESTERN RESERVE UNIVERSITY SCHOOL OF GRADUATE STUDIES We hereby approve the dissertation of Lizhi Liu candidate for the Ph.D. degree *. (signed) Arthur Heuer (chair of the committee) Frank Ernst Gary Michal Roberto Ballarini John Lewandowski Harold Kahn (date) 6 August, 2004 *We also certify that written approval has been obtained for any proprietary material contained therein. Copyright 2004 by Lizhi Liu All rights reserved I grant to Case Western Reserve University the right to use this work, irrespective of any copyright, for the University’s own purpose without cost to the University or to its students, agents and employees. I further agree that the University may reproduce and provide single copies of the work, in any format other than in or from microforms, to the public for the cost of reproduction. Lizhi Liu (sign) Dedicated to my wife Yin Tang Table of Contents Table of Contents.................................................................................................... 1 List of Tables .......................................................................................................... 5 List of Figures......................................................................................................... 6 Acknowledgments................................................................................................. 13 Abstract................................................................................................................. 14 Chapter 1 INTRODUCTION ............................................................................ 16 1.1 History and Current Applications of Titanium ..................................... 16 1.2 Surface Hardening Methods ................................................................. 17 1.3 The Goal................................................................................................18 Works Cited ...................................................................................................... 20 Chapter 2 LITERATURE REVIEW ................................................................. 22 2.1 Titanium and Titanium Alloys.............................................................. 22 2.2 Basic Properties of Titanium Alloys..................................................... 31 2.3 Current Surface Hardening Methods .................................................... 34 2.3.1 Physical vapor deposition and chemical vapor deposition ............... 36 2.3.2 Ion implantation techniques.............................................................. 37 2.3.3 Laser nitriding................................................................................... 39 2.3.4 Plasma nitriding................................................................................ 41 2.3.5 Gas nitriding......................................................................................42 2.4 Gas Adsorption Kinetics....................................................................... 45 2.4.1 Physisorption and chemisorption...................................................... 45 1 2.4.2 Nitrogen adsorption on metal surface............................................... 46 2.4.3 The interaction of nitrogen with titanium ......................................... 47 2.5 Diffusion Model of Nitridation............................................................. 51 2.5.1 Solution of the diffusion equation..................................................... 51 2.5.2 Diffusion in titanium......................................................................... 53 2.5.3 Diffusivity of nitrogen in titanium.................................................... 54 2.6 Kinetic Control of Gas Nitriding .......................................................... 59 Work Cited........................................................................................................ 62 Chapter 3 THERMODYNAMIC AND KINETIC CALCULATIONS ............ 69 3.1 Equilibrium Nitrogen Pressure............................................................. 69 3.2 Gas Concentration................................................................................. 79 3.3 Kinetic Equilibrium Nitrogen Partial Pressure ..................................... 80 3.4 Nitrogen Pressure for Si3N4/Si Powder Pack........................................ 87 3.5 Sticking Coefficient.............................................................................. 91 3.6 Nitrogen pressure for CrN/Cr powder packs ........................................ 93 3.7 Nitrogen pressure for Cr2N/Cr powder packs....................................... 96 3.8 Summary............................................................................................... 99 Work Cited........................................................................................................ 99 Chapter 4 EXPERIMENTAL PROCEDURES............................................... 101 4.1 Selection of Materials ......................................................................... 101 4.2 Heat Treatment....................................................................................103 4.3 Characterization Techniques............................................................... 107 4.3.1 Light optical microscopy ................................................................ 107 2 4.3.2 Microhardness measurement .......................................................... 107 4.3.3 XRD Analysis................................................................................. 111 4.3.4 XPS and AES.................................................................................. 116 4.3.5 SEM ................................................................................................ 117 Work Cited...................................................................................................... 119 Chapter 5 CrN/Cr2N/Cr POWDER PACK TREATMENT ............................ 120 5.1 CrN/Cr Powder Pack Treatment ......................................................... 120 5.2 Cr2N/Cr Treatment.............................................................................. 133 5.3 CrN/Cr and Cr2N/Cr Powder Packs.................................................... 141 5.4 Treatment at Other Conditions............................................................ 144 5.5 Goal-Oriented Nitriding Practice........................................................ 149 5.6 Discussion........................................................................................... 157 5.6.1 Nitrogen pressure for CrN/Cr and Cr2N/Cr powder pack............... 157 5.6.2 Surface adsorption kinetics............................................................. 159 5.6.3 Nitridation kinetics..........................................................................162 5.6.4 Microstructural analysis.................................................................. 164 Work Cited...................................................................................................... 170 Chapter 6 MECHANICAL PROPERTIES ..................................................... 172 6.1 Tensile Testing.................................................................................... 172 6.2 Microhardness Profile......................................................................... 182 6.3 Discussion........................................................................................... 184 Chapter 7 CONCLUSION AND FUTURE WORK ....................................... 185 APPENDIX A. TiN/Ti POWDER PACK TREATMENT.............................. 188 3 A.1 Surface Properties of CP Titanium after Heat Treatment................... 188 A.1.1 Heat Treatment at 860 °C .............................................................. 190 A.1.2 Heat Treatment at 1150 °C ............................................................ 195 A.2 Surface Properties of Ti-6Al-4V after Heat Treatment ...................... 199 A.3 Microhardness Measurement.............................................................. 204 A.4 Discussion........................................................................................... 207 A.5 Summary............................................................................................. 209 Work cited....................................................................................................... 210 APPENDIX B. Si3N4/Si POWDER PACK TREATMENT............................ 211 B.1 Si3N4/Si Powder Pack Treatment without getter ................................ 211 B.2 Si3N4/Si Powder Pack Treatment with Getter..................................... 217 B.3 Discussion........................................................................................... 220 B.4 Summary............................................................................................. 222 APPENDIX C. Non-linear Viscoelastic Properties of Cucumaria frondosa Dermis........................................................................................... 223 Bibliography ....................................................................................................... 261 4 List of Tables Table 2.1 Some of the most important
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