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.