The resistance to oxidation of an HfB2-SiC composite

The oxidation resistance of an hot-pressed HfB2-–SiC composite was studied through non-isothermal and isothermal treatments at temperatures up to 1600°C in air. The most severe oxidation conditions consisted of repeated heating-cooling cycles at 1600°C for up to 80 min of exposure. A thermogravimetric test for over 20 h at 1450°C provided evidence that, at this temperature, the oxidation kinetics fits a paralinear law until 10 h, when a partial rupture of external oxide scale occurs (i.e. a break-away reaction). Afterwards, the weight gain data fit a linear law. The main secondary phases formed in the composite during hot-pressing, namely BN, Hf(C,N) and a Si-based compound, although in limited amounts, influenced the oxidation resistance at temperatures below 1350°C. At temperatures higher than about 1400°C, the presence of SiC particles markedly improved the oxidation resistance due to the formation of a protective borosilicate glassy coating on the exposed surfaces.