On the oxidation behaviour of monolithic TiB2 and Al2O3-TiB2 and Si3N4-TiB2 composites

Abstract

In view of the susceptibility of TiB2 to oxidation, the thermal stability of monolithic TiB2 and of Al2O3-30 vol% TiB2 and Si3N4-20 vol% TiB2 composites was investigated. The temperature at which TiB2 ceramic starts to oxidize is about 400°C, oxidation kinetics being controlled by diffusion up toT≈900°C and in the first stage of the oxidation at 1000°C and 1100°C (up to 800 min and 500 min respectively), and by a linear law at higher temperatures and for longer periods. Weight gains in the Al2O3-TiB2 composite can be detected only at temperatures above ≈700°C and the rate governing step of the oxidation reaction is characterized by a one-dimensional diffusion mechanism atT=700°C andT=800°C and by two-dimensional diffusion at higher temperatures. Concerning the Si3N4-TiB2 composite, three different oxidation behaviours related to the temperature were observed, i.e. up to ≈1000°C the reaction detected regards only the second phase; at ≈1000<T<≈1200°C, the diffusion of O2 or N2 through an oxide layer is proposed as the rate-governing step; atT〉=1200°C, a linear kinetic indicates the formation of a non protective scale.