There is an increased demand for high temperature structural materials suitable for use at >1400°C, including heat exchangers, Generation IV nuclear reactor components (e.g. control rod sheath & fuel constituents), fusion energy (first wall materials), the receiver materials of concentrated solar power (CSP) systems, high temperature thermoelectric devices and static components in gas turbines and aero engines. Ultra-high temperature ceramics (UHTCs) exhibit especially high melting temperatures and include the diborides and carbides of zirconium and hafnium (ZrC, TaC, ZrB2, HfB2 & HfC). Unfortunately, UHTCs also suffer from poor fracture toughness, relatively poor oxidation resistance and are very difficult to process since the densification temperature required is also very high. EREMOZ addressed these challenges by using a non-conventional process route along with a combination of continuous fibre reinforcement and rare earth oxide (RE) coating / dopants. Specifically, processing was based on the energy efficient process of chemical vapour infiltration (CVI), slurry impregnation and pyrolysis (SIP) rather than more conventional sintering methods such as hot pressing / spark plasma sintering; the former allows nano sized particle composites to be produced at much lower temperatures. The poor fracture toughness is improved by the introduction of continuous carbon fibres (Cf) or pre SiC / RE coated carbon fibres, which enhance resistance to crack propagation and allows the fibre pull out mechanism. Finally, oxidation & ablation resistance can also be improved by the introduction of second phases, such as those based on silicon and/or rare earths metal oxides (yttria & ceria), which facilitate the formation of oxide scales that inhibit oxygen diffusion.
The Specific Research Objectives (SRO) of EREMOZ were: 1) processing of ultra-high temperature ceramic matrix composites (UHTCMCs), these are typically a continuous fibre reinforced with matrix of ultra-high temperature ceramic (UHTC), 2) detailed microstructural characterisation and 3) evaluation of the thermal properties via oxidation and ablation studies.
