Oxidation behaviour of sputter-deposited Ni-Cr-Al micro-crystalline coatings

Unbalanced magnetron sputter deposition was used to produce Ni-20Cr-xAl (s = 2, 3, 5 wt%) coatings on the substrates of the same materials. The effects of coating grain size and Al content on oxidation behaviour at 1000 degrees C have been studied. It a as found that the oxidation products were determined by both grain size and Al content. alpha-Al2O3 scales formed on the coatings containing as little as 2.4 wt% Al when the coating grain size was <100 nm. The oxidation behaviour was described using Wagner's theory to include short-circuit diffusion. With the formation of alpha-Al2O3 scale on coatings, the oxidation kinetics did not follow a parabolic law, but could be described with a new model that considered diffusion through both the lattice and grain boundaries. Based on the experimental results, a map showing the relationship between Al content, alloy grain size, and oxidation products has been established. (C) 1998 Acta Metallurgica Inc.