OXIDATION RESISTANCE OF TIAL IMPROVED BY COCRAL AND COCRALY COATING

Oxidation resistance of TiAl coupons coated with Co-30Cr-4Al and Co-30Cr-4Al-0.2Y (mass%) coatings has been assessed by isothermal oxidation test in a temperature range 1100 to 1400 K and by cyclic oxidation test at 1200 K. The coating, consisting of very fine grains, was prepared by magnetron sputtering with resulting thicknesses of 30-40 mum. Both the coatings are very effective to suppress the oxidation below 1300 K. This effect is attributable to the formation of alpha-alumina scales. However, after long oxidation periods, the oxidation manner changes from parabolic to accelerated oxidation. This acceleration is explained in terms of local growth of rutile on the scale. Relatively large voids are formed at the scale/coating interface, within the coating, and the coating/substrate interface. The addition of Y to the coating results in the flat scales with no voids at the scale/coating interface, though the oxidation rate slightly increases and the recrystallization of the coating is delayed.