Y2O3-SEALED NI-AL PROTECTIVE COATINGS FOR INCONEL-625

The surfaces of flame-sprayed Ni-Al coatings deposited on to Inconel 6 25 substrates were sealed by Y2O3 film (0.5-1.5 mu m) derived pyrolyti cally from an yttrium acetate tetrahydrate-based precursor solution. Y 2O3-sealed Ni-Al coating specimens were placed into an automated cycli c furnace and then subjected to heating-cooling cycle tests (one cycle = 12 h at 900 degrees C + 12 h at 25 degrees C) to evaluate the usefu lness of this coating system in preventing oxidation of the underlying Inconel. After 75 cycles, the weight gains of sealed Ni-Al coatings w ere similar to 50% lower than those of unsealed coatings. Such effecti veness of the Y2O3 sealer in improving the efficacy of the Ni-Al prote ctive coatings was due mainly to its reactivity with Al2O3 formed by o xidation of Al within the coatings. This selective reaction led to the formation of a crystalline YAlO3 phase. Extended oxidation resulted i n the transformation of YAlO3 into the Y3Al5O12 phase as a result of a secondary reaction between YAlO3 and Al2O3. Consequently, the sealing layers consisting of Y2O3, YAlO3, and Y3Al5O12 contributed significan tly to filling and eliminating inherent open spaces in the Ni-Al coati ngs, thereby suppressing the diffusion of oxygen through the pores. Su ch oxygen-impervious layers endow the Ni-Al layers with a long useful life-span as oxidation-resistant coatings for Inconel. (C) 1998 Elsevi er Science S.A.