Experimental study and numerical modelling of the nickel oxide coating on the Ni(111) surface

The evolution of the nickel oxide coating developed on the mono-crystalline Ni(111) surface was studied both experimentally and by means of numerical modelling. The NiO/Ni system was submitted to a high-temperature creep in the oxygen atmosphere. Using the O18 diffusion method we were able to observe several deformation modes (diffusional creep, gliding at the interfaces, etc.) and periodic crack generation perpendicular to the stress direction. The periodicity of the cracks array depends strongly on the external stress applied to the oxide scale. Two-step numerical modelling of the process was performed using first a simplified 1D model and then the 3D finite elements approach. The modelling procedure takes into account epitaxial deformations due to the scale ordering and to the mismatch of the lattice parameters, the growth process and the periodic crack generation. Identification of the visco-elastic material law permits then to obtain the strain (and the stress) distribution in the scale and in the substrate. The comparison of the experimental data with the numerical results will allow the formulation of several criteria for the oxide scale adherence.