Most engineering applications concerned with exposure to extremely high temperatures, (>2000°C), and harsh environmental conditions require the use of ceramic materials possessing melting points in excess of ~ 3000°C. Such ceramics, more commonly referred to as ultra high temperature ceramics (UHTCs), are required to possess a desired combination of mechanical and physical properties, which are retained despite the extremely high temperatures as demanded by their applications. However, there are some drawbacks of such materials, with respect to both their processing as well as their properties, which limit their applications to a considerable extent and demand careful engineering of their composition and microstructure to circumvent those limitations. Continuing research efforts have been focused on addressing such issues. Against this backdrop, the present review summarizes the various properties possessed by the UHTCs and critically analyzes the issues concerned with such materials. Through such analysis, an overview of the more recent research efforts that have been conducted to solve the various issues related to this material class is presented. This also highlights the difficulties associated with experimental assessments of the various properties of such materials. Lastly, the various existing applications and potential future applications for such materials are mentioned, with an outlook towards the issues that need to be addressed in the near future.
TopIntroduction
Ceramics with melting points in excess of 3000oC are usually classified as ultra high temperature ceramics (UHTCs) (Cutler, 1991; Telle, 1994; Cotton, 2010; Basu, 2006; Clougherty, 1968; Wuchina, 2007; Bellosi, 2006). The applications of UHTCs are extensive, which range from more conventional applications, like in process metallurgy or chemical plants, to more advanced applications such as in hypersonic vehicles. A very important feature of the UHTCs is that not only do they possess excellent mechanical properties (such as hardness, strength) and thermophysical properties at room temperature, but also such properties can be maintained at elevated temperatures. Due to their refractoriness, these materials also possess appreciable creep resistance. Hence UHTCs are amongst the few material classes that are suitable for various structural applications demanding constant exposure to extremely high temperatures, sometimes in excess of ~ 2500oC. Most of the ultra high temperature structural applications are also associated with corrosive environment and even highly aggressive environments. Hence, resistances against oxidation, corrosion and thermal shock are also some of the pre-requisites for use at such high temperatures, which are satisfied by the UHTCs. In addition to such beneficial thermomechanical and thermochemical properties of the ultra-high temperature ceramics, their attractive physical properties such as lower densities and low coefficient of thermal expansion are advantageous when compared to the refractory metals (such as W, Mo, Ir). This combination of properties makes UHTCs an ideal class of materials for applications in air/space-borne structures that are amongst the most important ultra-high-temperature applications, demanding a higher property-to-weight ratio.
In most cases, the materials characterized by such unique combination of properties are covalently bonded ceramics (Cutler, 1991; Telle, 1994; Cotton, 2010). More precisely, mainly the borides (Cutler, 1991; Telle, 1994; Basu, 2006; Fahrenholtz, 2007; Murthy, 2006; Gasch, 2004; Tang, 2007; Mukhopadhyay, 2008; Mukhopadhyay, 2009; Raju, 2008; Raju, 2009; Levinea, 2002; Chamberlain, 2004; Ferber, 1983; Kang, 2001; Kang, 1989; Einarsrud, 1997; Biswas, 2006; Torizuka, 1996; Torizuka, 1992; Murata, 1967; Bellosi, 2006; Monteverde, 2005; Sayir, 2004; Blum, 2004; Savino, 2008; Upadhya, 1997; Marschall, 2004; Gasch, 2004; Pierson, 1996) of transition metals classify themselves as ultra high temperature ceramics, although some carbides as well as nitrides (Monteverde, 2005; Sayir, 2004; Blum, 2004; Savino, 2008) are also used for such applications, either solely or as components of boride-based composites. The strength and directional nature of the primary covalent bond are the basic factors responsible for such promising properties. It is ironical that the covalent bonding is also a basic cause for the shortcomings of such materials, which are some of the focuses of this review.