Publications

Oxidation of Ni-Al-base electrodeposited composite coatings. II: Oxidation kinetics and morphology at 1000°C

Susan, D.F.; Marder, A.R.

The oxidation behavior of nickel-matrix/aluminum-particle composite coatings was studied using thermogravimetric (TG) analysis and long-term furnace exposure in air at 1000°C. The coatings were applied by the composite-electrodeposition technique and vacuum heat treated for 3 hr at 825°C prior to oxidation testing. The heat-treated coatings consisted of a two-phase mixture of γ (Ni) + γ′(Ni3Al). During short-term exposure at 1000°C, a thin α-Al2O3 layer developed below a matrix of spinel NiAl2O4, with θ-Al2O3 needles at the outer oxide surface. After 100 hr of oxidation, remnants of θ-Al2O3 are present with spinel at the surface and an inner layer of θ-Al2O3. After 1000-2000 hr, a relatively thick layer of α-Al2O3 is found below a thin, outer spinel layer. Oxidation kinetics are controlled by the slow growth of the inner Al2O3 layer at short-term and intermediate exposures. At long times, an increase in mass gain is found due to oxidation at the coating-substrate interface and enhanced scale formation possibly in areas of reduced Al content. Ternary Si additions to Ni-Al composite coatings were found to have little effect on oxidation performance. Comparison of coatings with bulk Ni-Al alloys showed that low Al γ-alloys exhibit a healing Al2O3 layer after transient Ni-rich oxide growth. Higher Al alloys display Al2O3-controlled kinetics with low mass gain during TG analysis.