Production of UHTC Complex Shapes and Architectures

State Of The Art On The Production Of Dense Uhtcs

The performance of a structural ceramic is closely linked with the parameters involved in the production process: starting powder, compositions, forming and densification process, microstructure, intergranular secondary phases of the sintered materials and so on. SiC, Si₃N₄, oxide ceramics, and composites of these materials are typical structural materials for use in high-temperature oxidizing environments. The limit temperature of using the silicon-based ceramics in an oxidizing atmosphere is 1600 °C. Above this temperature, the protective SiO₂ surface film, responsible for the thermal stability, begins to soften dramatically and develops substantial vapour pressure (Berton et al., 1992; Guo, 2009; Upadhya et al., 1997). Among the refractory oxides that are stable in an oxidizing environment at T ≥ 2000 °C, zirconia (ZrO₂) and hafnia (HfO₂) typically have the highest melting points, ~2700 °C and ~2800 °C, respectively (Scott, 1975; Upadhya et al., 1997). However, they are susceptible to thermal shock, and exhibit high creep rates and phase transition at higher temperatures. The need of structural materials for using in oxidizing environments at temperature over 1600 °C has moved the research interest toward the development of the so called ultra-high temperature ceramics (UHTCs). As for common structural ceramics, the understanding of the relation of performance with all the parameters involved in the production process is fundamental for exploiting the potentialities of these ceramics.