Oxidation behaviour of an aluminium nitride-hafnium diboride ceramic composite

An aluminium nitride-hafnium diboride electroconductive particulate composite was produced via hot isostatic pressing without sintering aids. Oxidation kinetics studies were performed up to 1350 °C under a flow of pure oxygen using a microbalance. The reaction products were analysed using SEM and XRD techniques. This composite had a high oxidation resistance up to 1200 °C. The kinetic curves had an asymptotic or paralinear shape. The formation of a protective oxide scale containing hafnia (HfO2) and aluminium borate (Al18B4O33) phases embedded in a glassy phase was observed. The evaporation of B2O3 was limited by the formation of refractory aluminium borate. Above 1200 °C, morphological observations showed the formation of a Maltese cross structure associated with the cracking of the oxide scale along the edges resulting in sigmoidal oxidation kinetics and in a high oxidation rate of the ceramic composite.