Evolution of NiO Island Size Distributions during the Oxidation of a Ni-5Cr Alloy: Experiment and Modeling

The classic models of metal oxidation developed by Wagner and Cabrera and Mott presuppose the existence of a planar oxide film and develop expressions for the rate at which the film thickens. Missing from those models is a description of how that initially planar film forms. Using scanning tunneling microscopy, we study the growth of NiO islands on the (100) surface of a Ni-5Cr alloy during the oxidation regime where the initial planar film is formed as oxide islands. The island height and area distributions as a function of the oxygen exposure in Langmuir (1 L = 10^-6 Torr s) are measured. Lateral island growth and thickening occur as seemingly separate processes, and after a critical thickness of ≈0.4 nm is achieved, growth is purely in the lateral direction. We develop a surface diffusion model for the evolution of the island size distribution that accounts for the lateral growth and coalescence of the NiO islands. Our results indicate that the oxygen surface diffusion screening length ξ=Dτ controls the island evolution. The screening length is found to be 0.3-0.4 nm, which suggests that the processes leading to island growth are highly localized to the island edge.