Ultra-high-temperature ceramics (UHTCs) are a group of materials that can withstand ultra high temperatures (1600-3000 oC) which will be encountered by future hypersonic re-entry vehicles. Future re-entry vehicles will have sharp edges to improve flight performance. The sharp leading edges result in higher surface temperature than that of the actual blunt edged vehicles that could not be withstood by the conventional thermal protection system materials. To withstand the intense heat generated when these vehicles dip in and out of the upper atmosphere, UHTC materials are needed. UHTC materials are composed of borides of early transition metals. From the larger list of borides, ZrB2 and HfB2 have received the most attention as potential candidates for leading edge materials because their oxidation resistance is superior to that of other borides due to the stability of the ZrO2 and HfO2 scales that form on these materials at elevated temperatures in oxidizing environments. Processing of these materials is very difficult as these materials are very refractory in nature. In this chapter, processes available for powder synthesis, fabrication of dense bodies, and coating processes is discussed.
Top1. Introduction
Ultra-high-temperature ceramics (UHTC) are a group of ceramic, which can withstand ultra high temperature (>2000oC) in oxidizing conditions. Such a high temperature will be encountered by future hypersonic re-entry vehicles (Fahrenholtz et al., 2007 & Opeka et al., 2004). Future re-entry vehicles will have sharp edges to improve aerodynamic performance. Sharp leading edges can improve the cross range (the ability of a vehicle to deviate from the most direct or planned entry path, thereby accessing a greater range of landing opportunities) and can contribute to accuracy and improved safety, especially for crewed vehicles (Johnson, 2012). The sharp leading edges will cause higher surface temperature than that of the actual blunt edged vehicles. The sharp edges have less surface area to dissipate heat and thus the temperature gets increased. To withstand the intense heat generated when these vehicles re- enters to the earth’s upper atmosphere, UHTC materials are needed.
Ability of withstanding such conditions depends on melting point, oxidation resistance and high temperature mechanical and physical properties of the material. Carbon-carbon composites have very high melting point but are not oxidation-resistant. Coatings are being developed for providing oxidation-resistance, but thermal expansion coefficient (CTE) mismatch between the C-C composite and the coating systems results in thermal stresses. The SiC-based composites exhibit oxidation resistance up to 1600◦C in hypersonic environments, but CTE- mismatch-induced matrix cracking allows direct oxidation of the carbon fiber reinforcement (Opeka et al., 2004).
UHTC materials are suitable for these application due to attractive combination of properties such as high melting point (>3000oC), high thermal conductivity, low thermal expansion coefficient, strength at high temperature, good thermal shock resistance, oxidation resistance and erosion resistance. UHTC materials are composed of borides, nitrides and carbides of early transition metals. Borides are considered better material compared to nitrides and carbides due to superior mechanical and thermal properties as well as higher oxidation resistance (Opeka et al., 1999). For the sharp wing leading edge applications, in addition to oxidation resistance, high thermal conductivity is desirable. As compared to carbide and nitrides, the borides typically have higher thermal conductivity. High thermal conductivity improves a material’s thermal shock resistance by reducing temperature gradients and thermal stresses in the material (Gasch et al., 2004). ZrB2 and HfB2 based UHTCs also have high electrical conductivity which is good for manufacturing complex shapes by using electric discharge machining .
Among all refractory metal borides, ZrB2 and HfB2 are considered as potential candidates for leading edge materials because of their superior oxidation resistance which is attributed to the stability of the ZrO2 and HfO2 scales. The performance of material in ultra high temperature environment is determined not only by the properties of borides but also by the high temperature properties of their respective oxides. ZrO2 and HfO2 have high melting point (>2700 oC) and thus ZrB2 and HfB2 are superior to other metal borides for the high temperature oxidizing conditions. Moreover suitable additives such as SiC can be added to enhance the properties of these materials (Fahrenholtz et al., 2007 & Sonber et al., 2011). SiC improves oxidation resistance of UHTC materials by formation of borosilicate glass which prevents further oxidation. Compared to liquid B2O3 in monolithic borides, the borosilicate glass in the SiC containing ZrB2 and HfB2 ceramics has higher viscosity, higher melting temperature, lower oxygen diffusivity and lower vapor pressure thus providing much more effective oxidation protection (Opeka et al., 1999). Silicon carbide also prevents the grain growth and thus improves mechanical properties. Silicon carbide is also available in the form of fibers, which is used for enhancement of strength and fracture toughness.