Ultra-high temperature ceramics reinforced with SiC short fiber or whisker

The IV and V group transition metals borides, carbides and nitrides are widely known as ultra-high temperature ceramics (UHTCs), thanks to their high melting point above 2500°C. These ceramics possess outstanding physical and engineeristic properties, such as high hardness and strength, low electrical resistivity and good chemical inertness. The materials more deeply investigated are the ZrB2-based ones, in view of the lower density, compared to Hf and Ta composites, and the better oxidation resistance of the borides compared to the carbides and nitrides. So far, the research activity has mainly focused on the achievement of high strength UHTCs usually based on ZrB2 and HfB2 composites and SiC particles in amounts of 20-30vol%. These ceramics can in fact reach strength up to 1 GPa and even higher, through careful tailoring of processing parameters. However, these strong ceramics still have very low fracture toughness, with values ranging from 2.5 to 4 MPam1/2 for ZrB2-based materials. The demand to design and realize reinforced UHTC materials comes from the need to overcome this brittleness, which makes the scale-up of big components a technological issue. The basic concept is the improvement of the fracture toughness through the introduction of elongated reinforcing phases, like SiC fibers (macro-reinforcement), or whiskers (micro-reinforcement) into UHTC matrices. A number of critical issues rise when elongated secondary phases are added to the matrix: it is difficult to obtain a homogeneous dispersion, fracture of the reinforcing phase occurs during milling and it is problematic to achieve dense ceramics without deteriorate the reinforcing phase. For example, whiskers tend to degenerate into particles if the sintering temperature is too high and then lose their reinforcing action. For carbon or silicon carbide fibers there is a strong interface reaction with the ZrB2 matrix, which should be instead avoided to promote significant pull out. This chapter presents the latest results on ZrB2-based materials reinforced with SiC fibers or whiskers. Several sintering additives are experienced in order to preserve the integrity of the reinforcing element, thus to exploit at best their toughening action. The microstructure of the dense materials is studied by SEM and TEM techniques to investigate the effect of the sintering additive and the interface between matrix and reinforcing phase. The mechanical properties are compared to those of the reference material to assess the real variation obtained by the introduction of these reinforcing agents. Following the inspection of the crack/microstructure interaction, the experimental fracture toughness is quantitatively compared to theoretical models. The addition of such reinforcements can improve the fracture toughness, from 3.7 to 5.5-6.3 MPam1/2 upon a proper choice of sintering additive and right amount of reinforcing phase. The main mechanisms invoked for explaining such improvements are crack deflection and crack bridging. The values of high temperature strength and fracture toughness up to 1500 °C are also presented and discussed. Mention is also made to the effect of the introduction of SiC short fibers to other UHTCs, namely to HfB2 and TaC.