IMPROVEMENT IN THE HIGH-TEMPERATURE OXIDATION RESISTANCE OF TIAL BY SOL-DERIVED SIO2 COATING

TiAl coupon specimens were coated with sol-derived SiO2 films of about 0.2 mum thickness and subjected to oxidation tests at 1100, 1200 and 1300 K in a flow of purified oxygen under atmospheric pressure. At 1100 and 1200 K the oxidation was suppressed very much by applying the coating. This is attributable to the formation of alumina layers beneath the coating. The formation of the protective alumina layer can be explained in terms of the slow diffusion of oxygen through the coating which can favor the preferential oxidation of aluminum. However, at 1300 K the coating suppresses the oxidation a little during the initial oxidation period, after which its protectiveness decreases as the oxidation proceeds. This can be explained by the diffusion of oxygen which is enhanced by the temperature rise and the development of mechanical failure in the coating which may be caused by the oxide growth.