Progress in the fabrication of ultra-high-temperature ceramics: “in situ” synthesis, microstructure and properties of a reactive hot-pressed HfB2-SiC composite

An ultra-high-temperature HfB2-SiC composite was successfully fabricated by reactive hot-pressing. Solid reagents like Hf/Si/B4C, mechanically mixed in molar ratio 2.2/0.8/1, were “in situ” converted into the basic ingredients (i.e., HfB2, SiC), and then directly hot-pressed until full density was achieved (1900 °C final temperature). The microstructure consisted of faceted diboride grains (mean size 3 μm), with HfC (6 vol%) and SiC (22 vol% and mean size 1 μm) evenly distributed intergranularly. The combination of some mechanical properties was of considerable significance: about 19 GPa of micro-hardness, 520 GPa of Young's modulus, 770 ± 35 and 315 ± 10 MPa of flexural strength at 25 and 1500 °C, respectively. A relevant merit characterized the resistance to oxidation: repeated exposures at 1700 °C, or at 1450 °C for 20 h, involved limited mass gains and small changes of the original microstructure. The marked refractoriness of HfB2 and SiC, which constitute the framework of the composite, dominates beneficially its thermostructural stability.