STEADY-STAGE GROWTH OF NIO SCALES ON CERIA-COATED POLYCRYSTALLINE NICKEL

The effect of CeO2 ceramic coatings with thicknesses in the range of 1 4 to 42 nm on the oxidation of high purity polycrystalline nickel at 9 73 K in 1 atm O-2 has been studied. The ceria coatings decrease the Ni oxidation rate after 125 h bg a factor up to 45, which is significant ly higher than the reduction achieved during early stages. Growth feat ures, including oxide thickness, surface morphology, and the texture f or both the pure and CeO2 modified NiO, demonstrate a definite depende nce on the crystallographic orientation of the Ni substrate. Oxides wi th thicknesses of up to 1 mu m developed on CeO2-coated Ni consisted o f three sublayers. The Ce-rich part, composed of small NiO grains and CeO2 particles, was located inside the scale and shifted deeper into t he scale with increased oxidation time. The growth kinetics and micros tructural evolutions of modified NiO are discussed in terms of the mec hanism of inhibition of grain boundary diffusion in NiO by Ce+4 ions.