Oxidation behaviour of an aluminium nitride-hafnium diboride ceramic composite

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