Improvement of oxidation resistance of TiAl by ion-beam-enhanced deposition coatings

Silicon nitride films of different thicknesses were synthesized on the intermetallic compound TiAl by ion-beam-enhanced deposition. Auger electron spectroscopy shows that the silicon nitride film is composed of a thin silicon-enriched top layer, a stoichiometric Si3N4 layer and a mixing layer at the film-substrate interface. It was found that a small amount of aluminum and titanium diffused into coatings from X-ray photoemission spectroscopy measurements. Cyclic oxidation tests were carried out on the TiAl coupons coated with nitride films of thicknesses of 0.5, 1 and 2 μm. The nitride film of 0.5 μm thickness demonstrated an excellent oxidation resistance at 1300 K for at least 30 cycles (600 h). However, this effect decreases as the thickness of the coating increases. Scanning electron microscopy observation and energy-dispersive X-ray spectroscopy revealed that the excellent oxidation resistance came from the formation of a thin layer rich in Al2O3 beneath the outer TiO2 layer during the oxidation period. The less effectiveness for oxidation resistance of the thicker film resulted from the local fracture of the coating and the spalling off of the oxide scale.