THE INFLUENCE OF CRYSTALLOGRAPHIC ORIENTATION OF NICKEL SURFACE ON OXIDATION-INHIBITION BY CERIA COATINGS

The growth of NiO on (100) and (111) crystal faces of Ni, both pure an d coated with nano-sized CeO2 particles, has been studied at temperatu res in the range of 873-1073 K. It is shown that the crystallographic orientation of the substrate exerted an essential influence on the gro wth rate and microstructure of pure and CeO2-modified NiO. The 14 nm t hick coatings of CeO2 deposited on (100)Ni markedly inhibited the oxid ation process; however, oxide growth rate on this face was still highe r than that measured for uncoated (111)Ni. The same coatings applied t o (111)Ni, when incorporated into the oxide without the coarsening of CeO2 particles, decreased the oxidation rate to values as low as that predicted from the lattice diffusion of Ni in NiO. The differences in the oxidation rate of both Ni crystal faces are interpreted by the cha nges in oxide microstructure and texture, in terms of the type and den sity of grain boundaries. It is suggested that, at high temperatures, the NiO grain boundaries act as easy-diffusion paths for Ce4+ ions and primarily affect the distribution of relative element in oxide. Subse quently, the Ce4+ segregation affects the relative suppression of Ni2 and O-2(-) diffusion, and the dominant oxidation mechanism. (C) 1998 Acta Metallurgica Inc.