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(Ta,Hf)C Ultra-High Temperature Ceramics

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(Ta,Hf)C Ultra-High Temperature Ceramics Process development and characterisation of (Ta,Hf)C ultra-high temperature ceramics Omar Cedillos Barraza A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy Department of Materials Imperial College London London, UK, SW7 2AZ May 2015 1 Declaration I declare that all the work contained herein is my own and any work of others included in this work is appropriately referenced and acknowledged. Omar Cedillos Barraza ‘The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work’ 2 Abstract Tantalum carbide (TaC), hafnium carbide (HfC) and compounds in the TaC-HfC system have extremely high melting points (>3700 °C) making them potential candidates for thermal protection structures in hypersonic space vehicles. Information regarding mechanical and thermal properties of these compounds and their solid solutions is scarce. Synthesis and sintering of 4TaC-1HfC compounds was conducted using efficient reactive routes using self-propagating high temperature synthesis (SHS) and a spark plasma sintering (SPS) furnace. Reactive one-step reactive spark plasma sintering (RSPS) and a combination of SHS+SPS were used as the processing routes to produce TaC-HfC ceramics. Relative density >98% was achieved by the SHS+SPS method without sintering aids at 2100 °C for 20 min and 60 MPa. Product conversion of the reactants after SHS and after sintering was characterised by XRD. Analysis of microstructures was conducted by SEM and EDS. TaC, HfC and different TaC-HfC compounds were sintered using SPS at temperatures up to 2450 °C using commercial powders of TaC and HfC. Microstructural evolution and solid solution formation was analysed in 4TaC-1HfC ceramics fabricated using SPS from 2050-2450 °C. XRD, SEM and EDS were used to analyse the formation of (Ta,Hf)C solid solutions. TEM was conducted and the diffusion mechanisms during sintering were analysed. Single-phase solid solutions were formed at sintering temperatures ≥2350 °C for 20 min and 30 MPa. In addition, TaC, HfC, 1TaC-1HfC and 1TaC-4HfC ceramics were sintered by SPS at 2350 °C. Measurements of mechanical properties (hardness, elastic modulus and fracture toughness) and thermal properties (thermal diffusivity, thermal conductivity and coefficient of thermal expansion) are reported. Melting temperatures (Tm) were reassessed using a laser melting technique with a 4.5 kW, 1064 nm Nd:YAG CW laser programmed to deliver pulses with time ranging from 100 to 1000 ms and power up to 3980 W. HfC showed the highest melting temperature at 3959 ± 50 °C, and the highest melting temperature for any known compound. Tm for TaC was measured at 3768 ± 40 °C and the solid solutions fall in between the single member carbide values with 4TaC-1HfC at 3905 ± 40 °C. Microstructural characterisation using SEM and TEM on samples after the laser testing experiments is reported. 3 This work is dedicated to Leticia, Thanks for all your motivation and support through these years and for sharing this adventure with me. 4 Acknowledgements First of all, I would like to thank my parents Dagoberto and Esperanza for their unconditional support. Thanks for being my guidance and my inspiration. To all my family, for always being there, without you this degree would not have been possible. Thanks to my supervisor Prof. Bill Lee for all your support and guidance throughout this PhD and to Dr. Luc Vandeperre for his valuable discussions and help. Thanks to all the technical staff at Imperial College, Dr. Mahmoud Ardakani for all the training and technical support with SEM and TEM, Mrs. Ecaterina Ware for helping me with the preparation of FIB samples, Mr. Richard Sweeney for all the help with XRD, Mr. Gary Stakalls for all the countless times he helped me by cutting and preparing samples and with equipment training and Dr. Richard Chater. Thanks to Dr. Doni Daniel Jayaseelan for his ideas and input on this project and his help with the TEM work, to Dr. Salvatore Grasso at Queen Mary University London for helping me in the fabrication of samples by SPS and all his valuable recommendations, discussions and feedback and Dr. Dario Manara at the Institute for Transuranium Elements (ITU) for all his attentions during my visit to Germany, his invaluable help with the laser experiments and useful feedback. Thanks to all my colleagues at CASC for always helping me, all their constructive discussions and having those words of support in the good and the bad days. Finally, I would like to thank CONACyT (Consejo Nacional de Ciencia y Tecnología, México) for the financial support of this PhD. 5 Publications “Enhanced oxidation resistance of ZrB2/SiC composite through in situ reaction of gadolinium oxide in patterned surface cavities”, Jesus Gonzalez-Julian, Omar Cedillos- Barraza, Sven Doering, Stefan Nolte, Olivier Guillon, and William E. Lee, Journal of the European Ceramic Society. 34, 4157-4166 (2014) “Flash Spark Plasma Sintering (FSPS) of Pure ZrB2”, Salvatore Grasso, Theo Saunders, Harshit Porwal, Omar Cedillos-Barraza, Daniel Doni Jayaseelan, William E. Lee, and Mike John Reece, Journal of the American Ceramic Society, 97 [8] 2405–2408 (2014). Presentations Microstructure and mechanical properties of (Ta,Hf)C ultra-high temperature ceramics, 38th International Conference and Exposition on Advanced Ceramics and Composites, Daytona Beach, FL, USA. (2014) Process development and microstructural characterisation of (Ta,Hf)C ultra-high temperature ceramics, 13th Conference of the European Ceramics Society, Limoges, France. (2013) Synthesis and sintering of TaC-HfC ceramics by SHS and RSPS techniques, 37th International Conference and Exposition on Advanced Ceramics and Composites, Daytona Beach, FL, USA. (2013) Fabrication of TaC-HfC for ultra-high temperature applications, Poster presentation at Ultra-High Temperature Ceramics: Materials for Extreme Environment Applications II. Schloss Hernstein, Austria. (2012). Best poster prize winner. 6 Table of Contents 1 Introduction ................................................................................................................................... 23 2 Literature review ........................................................................................................................... 25 2.1 Background ........................................................................................................................... 25 2.2 Historic research ................................................................................................................... 27 2.3 Bonding and crystal structure of tantalum and hafnium carbide phases ............................... 28 2.4 Phase equilibria of TaC and HfC .......................................................................................... 31 2.5 The TaC-HfC system ............................................................................................................ 33 2.6 Powder preparation techniques ............................................................................................. 38 2.6.1 Self-propagating high-temperature synthesis (SHS) ..................................................... 40 2.7 Densification ......................................................................................................................... 43 2.7.1 Hot pressing (HP).......................................................................................................... 45 2.7.2 Pressureless sintering (PS) ............................................................................................ 48 2.7.3 Spark plasma sintering .................................................................................................. 49 2.7.4 Reactive sintering .......................................................................................................... 53 2.8 Mechanical properties ........................................................................................................... 54 2.9 Thermal properties ................................................................................................................ 57 2.9.1 Thermal conductivity .................................................................................................... 57 2.9.2 Thermal expansion ........................................................................................................ 59 2.10 Oxidation ............................................................................................................................... 60 2.11 Melting temperature measurements ...................................................................................... 63 2.12 Aims and objectives .............................................................................................................. 66 3 Experimental methods .................................................................................................................. 67 7 3.1 Starting materials and powder processing............................................................................. 67 3.2 Elemental analysis ...............................................................................................................
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