Recent Advances in Ultra-High-Temperature Ceramic Coatings for Various Applications

In the past, the research on high-temperature applications of materials has focussed mainly on SiC and Si3N4 only. The recent advances in propulsion and hypersonic concepts and the related applications have resulted in the search for new categories of materials capable of withstanding very high temperature. The borides, nitrides and carbides of various transition metals can be employed for synthesising ultra-high-temperature ceramic (UHTC) coatings. These materials possess excessively huge melting point along with substantial mechanical properties at extreme temperature, making them suitable for several high-temperature structural and other environmental applications such as in rockets, hypersonic vehicles and engine components. The inherent brittleness as well as extremely feeble shock resistance of ceramic materials can be overcome to a greater extent with the application of ultra-high-temperature ceramic coatings and fibre-reinforced ultra-high-temperature ceramic materials. UHTC coatings are extremely useful as thermal shock absorbers, surface seals and leak minimisers. The carbothermal reduction method is the oldest method employed for the synthesis of UHTCs. However, many other solid-state powders-based and solution-based methods have been employed recently to enhance the unique properties of them. The synthesis methods face a few challenges and require highly refined approaches to synthesis high purity UHTC powders, suitable chemical synthesis reactions and proper selection of precursors delivering excellent chemical yield and less degradation. The microstructural aspects of the synthesised UHTCs can be identified through suitable characterisation techniques including in situ characterisation. Apart from the excellent mechanical properties like excellent elasticity, flexural strength, and fracture toughness, UHTCs must be assessed for its machinability when they are employed for hypersonic and space applications. The thermodynamic properties such as the coefficient of thermal expansion, thermal conductivity and total hemispherical conductivity play a significant role in determining the high-temperature applications of UHTCs. Oxidation resistance at high-temperature environment is the most wanted property of materials especially when they are employed for various space applications. Recent advances show that the use of multilayers of the high-temperature ceramic protective coating reduces oxidation of carbon–carbon (C–C) composite materials and protect them from aerothermal heating at temperatures above 3273 K. Hence, the mechanisms, kinetics and the end products of oxidation mechanisms help the researchers to identify the strengths and weaknesses of the UHTCs when employed for specific applications. This chapter focusses mainly on the recent advances in the synthesis and characterisation techniques of UHTCs giving due importance to their various properties and oxidation mechanisms. The atomistic computational modelling and simulation studies on the impacts of defects on the thermal and mechanical properties of UHTCs are found to be extremely useful to identify their drawbacks prior to their employment in various device configurations. In a similar way, the computational studies on UHTCs by employing thermal shock modelling tools provide useful information about their thermal shock resistance. This chapter also provides a brief overview of the applications of UHTCs along with a description of the properties and applications of the emerging high entropy UHTCs.