Spark Plasma Sintering of Ultrahigh Temperature Ceramics
The ultrahigh-temperature ceramics (UHTCs) have been considered as the emerging class of materials, suitable for the high-temperature (exceeding 2000 °C) structural applications in oxidation environment, including hypersonic aviation, rocket propulsion, high-temperature electrodes for furnaces, etc. As UHTCs have higher melting point (>3000 °C), it is worthwhile to adopt novel sintering techniques to fabricate the structural frames from UHTCs powder for different niche applications. Effective utilization of UHTCs requires full densification of the materials from powder. UHTCs, especially, transition metal borides and carbides, have lower sinterability due to their high melting point (>3000 °C), strong covalent bonding, low bulk diffusion coefficient, and higher vapor pressure of the constituent elements present in the UHTCs. Among the available sintering techniques, spark plasma sintering (SPS) has been widely utilized to achieve fully dense powder compact along with uniform fine-grain microstructure, suitable for high-temperature applications. SPS is found to be advantageous over the conventional sintering techniques such as hot pressing (HP), hot isostatic pressing (HIP), pressureless sintering (PS), etc. because uniform fine-grain microstructure could be obtained via SPS due to considerably lower sintering temperature, high heating rate, lower holding time, and application of pulsed DC current along with uniaxial mechanical pressure during sintering schedule. In this chapter, the efficacy of SPS technique to sinter different UHTCs, especially monolithic transition metal borides, carbides, and mixed borides and carbides with/without sinter-aid or other secondary phase is elaborated. Microstructural evolution and enhancement of different properties, particularly the mechanical properties, by adopting suitable sintering scheme and sintering parameters are discussed in detail. Also, entropy-stabilized multicomponent UHTC borides (contains at least five transition metal borides in equimolar ratio) have been discussed to highlight the effectiveness of SPS technique to consolidate these types of materials. Finally, the application-oriented development of UHTCs for sharp leading edges of hypersonic space vehicle is briefed.