UHTC composites sintering

Figure 7. SEM back-scattered micrographs of spark plasma sintered UHTC composite materials (a) ZS, (b) HS, (c) TS, (d) ZZS, (e) HHS, and (f) TTS (adapted from Licheri et al., 2007 Licheri etai, 2008 Licheri et al., 2009 Licheri etal., 2010b)...

The composition or the presence of second phases also affects the hot hardness properties of the UHTCs. For instance, the effect of sintering additive on the hot hardness properties of TiB is presentedin Figure 7 (Raju, 2009 Raju, 2009 Jungling, 1993). It can also be noticed that the hardness of TiB samples decreases with temperature. It is well known that hardness has an exponential dependency on temperature and can be given by the following relation... 

These are just some basic features of Mo and Ta siiicides, however, in combination with UHTC matrices and in the sintering environment, the chemistry of these compounds can locally change, inclnding the incorporation of carbon, oxygen, boron in the crystal lattice, which in tnrn indnces the formation of metastable structures with different properties. As an example, creep and oxidation properties of MoSi can be improved with small addition of boron, which facilitate the development of mnltiphase composite material in the Mo-Si-B system (Meyer, 1996). 

Emerging potential applications for UHTCs require complex shapes and/or functionalized surfaces. In this chapter, recent achievements in the development of monolithic and composites based on Hf and Zr borides and carbides are presented and discussed, especially with reference to the effects of shaping and sintering technologies. Novel UHTC structures and architectures, dense and porous, open new potential fields of applications for UHTCs not only in aerospace applications, but also in energy sectors (such as solar energy), hot gases and molten metal filtration and catalysis. 

In the following sections, practical examples of application of the possibilities offered by wet forming techniques to produce UHTC components are presented. A systematic study of ZrB2-4 vol% SijN and ZrB2-20 vol% SiC-4 vol% SijN compositions, conducted by the authors to produce complex shape devices by various wet-forming techniques and PLS, is reported. Beside, ZrB with no sintering aid is also proposed. 

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