Y2O3-sealed Ni-Al protective coatings for Inconel 625

The surfaces of flame-sprayed Ni-Al coatings deposited on to Inconel 625 substrates were sealed by Y2O3 film (0.5-1.5 mu m) derived pyrolytically from an yttrium acetate tetrahydrate-based precursor solution. Y2O3-sealed Ni-Al coating specimens were placed into an automated cyclic 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 usefulness of this coating system in preventing oxidation of the underlying Inconel. After 75 cycles, the weight gains of sealed Ni-Al coatings were similar to 50% lower than those of unsealed coatings. Such effectiveness of the Y2O3 sealer in improving the efficacy of the Ni-Al protective coatings was due mainly to its reactivity with Al2O3 formed by oxidation of Al within the coatings. This selective reaction led to the formation of a crystalline YAlO3 phase. Extended oxidation resulted in 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 significantly to filling and eliminating inherent open spaces in the Ni-Al coatings, thereby suppressing the diffusion of oxygen through the pores. Such oxygen-impervious layers endow the Ni-Al layers with a long useful life-span as oxidation-resistant coatings for Inconel. (C) 1998 Elsevier Science S.A.