High-Temperature Oxidation Kinetics of TiB2 Powders in Air

The oxidation kinetics of TiB2 powders in air at temperatures up to 1,673 K was investigated using thermal analysis [thermogravimetry and differential scanning calorimetry (DSC)]. DSC-curves had three well distinguishable exo-effects in the temperature ranges of about 760-780, 960-980 and 1,630-1,650 K and a slightly lower intensity in the temperature range 1,000-1,600 K. With the temperature (T) and conversion fraction (alpha) increasing, the thickness of a scale, its composition and microstructure continuously changed. This led to a change in the limiting-rate step mechanism of the oxygen diffusion transport to the diboride-scale boundary. The powders oxidation macro-mechanism is slightly dependent on their dispersion and morphology, but these factors substantially influence the kinetics. It has been determined that the reaction is of the first formal order. The activation energy is 300-390 kJ/mol at low temperatures and alpha a parts per thousand currency sign5-7 % in the beginning of the reaction. In the end of the reaction the activation energy is similar to 20 kJ/mol, when T and alpha are high.