1.1 UHTCs’ Background
In the last 10 years, the attention towards ultra-refractory compounds, also known as Ultra-high Temperature Ceramics (UHTCs), has been raised again after some forty years of apparent inactivity (Kaufman, 1966; Fahrenholtz, 2007; Guo, 2009). This class of materials, constituted by borides and carbides of group IV-VI transition metals, is gaining particular attention owing to the unique combination of physico-chemical features coupled with promising engineering properties. The main peculiar feature of these compounds is the high melting point exceeding 3000°C, which places them among the most refractory compounds known, especially the carbides. They also possess interesting engineering properties, such as high hardness, high stiffness and potential high strength, which can be further increased by addition of secondary phases (Cutler, 1991; Opeka, 1999; Schaffer, 1999). Other distinguishing characteristics are strong covalent bonds and low self-diffusion coefficient, rendering these ceramics very difficult to sinter to full density. One direct consequence of these aspects is the difficult process associated to the densification of UHCTs: temperature above 2000°C and pressure in the order of 30-100 MPa are required to achieve high density, but the resulting microstructure is characterized by coarse grains and trapped porosity, which thwarts all the engineering potentialities (Kalish, 1966). The introduction of sintering additives and secondary phases can notably reduce the processing conditions and ending up in promising microstructural features, in turn influencing the final thermo-mechanical properties (Monteverde, 2002; Monteverde, 2003; Sciti, 2006; Zou, 2009).
High temperature stability is also a key requirement. If exposed to oxidizing environment at high temperature, carbides and borides oxidize to the corresponding oxide that generally is constituted by a porous and cracked microstructure (Levine, 2003). An improvement of the oxidation resistance can be obtained upon addition of suitable secondary phases, such as SiC or Si-based phases, which could help producing a refractory glassy layer at intermediate temperature up to 1600°C (Tripp, 1973; Karlsdottir, 2009; Rezaie, 2007), or through addition of oxide melting temperature depressants, such as W-based compounds, which favor the melting and densification of the oxide at ultra-high temperature forming a compact scale able to inhibit further oxygen penetration in the bulk (Zhang, 2008).
The fields needing further investigations to fill the gap in the understanding of UHTCs relationships among processing-microstructure-properties are still numerous. However, notable steps forward the obtainment of dense composites, possessing superior strength up to 1 GPa, Young’s modulus above 500 GPa, hardness in the range 21-25 GPa and improved oxidation resistance have been done (Chamberlain, 2004; Fahrenholtz, 2007). Other unsolved issues, or still needing more efforts, involve the improvement of fracture toughness, the enhancement of the oxidation resistance in middle-high temperature range both in oxidizing and reducing atmosphere and the design of the models to be employed in new hypersonic space vehicles or propulsion components. While this last topic is a matter mainly for engineers, materials scientists will be busy in unraveling such other problems still for some time.
In this chapter we will focus on the sintering problems affecting this family of ceramics. Despite most of the activity performed on UHTC in the last years with regards the addition of SiC to boride matrices, in our lab we developed a series of composites containing transition metal disilicides, mainly MoSi2 and TaSi2 (Sciti, 2006a; Sciti, 2006b; Sciti, 2008; Sciti 2009), which offer a wide range of tunable properties, processing advantages, but of course some unavoidable drawbacks. In the following pages, the main properties and the motivation which lead to the choice of these two silicides are presented.