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Investigation of Sintering Mechanism and Phase Evolution in Ternary System of TaC-HfC-ZrC

Ghaffari, Amir | 2013

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 43922 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Faghihi Sani, Mohammad Ali; Golestani-fard, Farhad
  7. Abstract:
  8. Ultra-high temperature ceramics (UHTCs) are a class of materials that have melting points above 3000°C and include borides, carbides and nitrides of some transition metals such as ZrB2, ZrC, HfB2, HfC, HfN, TaC. These compounds are of particular interest because of the unique property combination such as high electrical and thermal conductivity, chemical inertness against molten metals or non basic slags, and good thermal shock resistance. Sintering of these compounds is very difficult due to their high melting point and utilization of sintering aids or new consolidation routes is necessary to achieve high density sintered bodies. In this research, phase evolution and kinetics of diffusion were investigated in primary phase. New silicide sintering aid was used for pressureless sintering of transition metal carbides. In addition, spark plasma sintering route (SPS), as a new sintering method, was utilized to reduce the sintering temperature and the amount of sintering aid. In the first part of the research, phase evolution and diffusion behavior of TaC-HfC, TaC-ZrC and HfC-ZrC, binary carbides were investigated. It was demonstrated that TaC act as favorable host to form TaC-based solid solution and solubility of Zr inside TaC structure is more than that of Hf. The inter-diffusion coefficient of Ta, Hf and Zr was calculated based on Boltzmann-matano method. Pressureless sintering was carried out to achieve 95%RD of Ta0.8Hf0.2C in the presence of 24 vol. MoSi2. Increasing the HfC content in presence of fixed value of MoSi2 led to decrease the final relative density of TaC-HfC samples. Spark plasma sintering was successfully done using 12 vol. sintering aid (MoSi2 or TaSi2) to reach ~100%RD at 1650°C for 5 min. Sintering curve revealed that sintering mechanism in the presence of MoSi2 (reactive liquid phase sintering) is quite different from TaSi2 (conventional liquid phase sintering). MoSi2 had a significant effect on formation of SiC and increasing the fracture toughness of samples. In contrast, TaSi2 increased the hardness of sintered samples. Substitution of Hf by Zr in Ta0.8Hf0.2C or Ta0.8Hf0.1Zr0.1C samples did not sensibly change the mechanical properties of sintered smaples, except in TZ20T12 which formation of SiC was activated by TaSi2-ZrC reaction
  9. Keywords:
  10. Zirconium Carbide ; Sintering ; Tantalum Carbide (TaC) ; Hafnium Carbide (HfC) ; Ultra High Temperature Ceramics (UHTC)

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