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Process development and characterisation of (Ta,Hf)C ultra-high temperature ceramics

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Title: Process development and characterisation of (Ta,Hf)C ultra-high temperature ceramics
Authors: Cedillos Barraza, Omar
Item Type: Thesis or dissertation
Abstract: Tantalum carbide (TaC), hafnium carbide (HfC) and compounds in the TaC-HfC system have extremely high melting points (>3700ºC) 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 2050-2450ºC. 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 ≥2350ºC for 20 min and 30 MPa. In addition, TaC, HfC, 1TaC-1HfC and 1TaC-4HfC ceramics were sintered by SPS at 2350ºC. 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 ºC, and the highest melting temperature for any known compound. Tm for TaC was measured at 3768 ± 40 ºC and the solid solutions fall in between the single member carbide values with 4TaC-1HfC at 3905 ± 40 ºC. Microstructural characterisation using SEM and TEM on samples after the laser testing experiments is reported.
Content Version: Open Access
Issue Date: May-2015
Date Awarded: Nov-2015
URI: http://hdl.handle.net/10044/1/32138
DOI: https://doi.org/10.25560/32138
Supervisor: Lee, William
Sponsor/Funder: Consejo Nacional de Ciencia y Tecnología (Mexico)
Funder's Grant Number: 214654
Department: Materials
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Materials PhD theses

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