Oxidation of thin chromium nitride films: Kinetics and morphology

Magnetron-sputtered, 150-500 nm thick chromium nitride films of CrN and Cr2N stoichiometry were exposed to air at 500-800 degrees C. Oxide scale growth and morphology and the phase transformations during oxidation were investigated by means of Rutherford backscattering spectrometry, resonant nuclear reaction analysis, scanning force microscopy (SFM) and x-ray diffraction. A combined logarithmic and parabolic Cr2O3 scale growth was found and interpreted as due to inwards diffusion of oxygen via grain boundaries (logarithmic) and outwards lattice diffusion of Cr (parabolic). Pt-marker experiments confirmed this interpretation. The activation energies of the two processes were determined to be Q = 60(20) and 196(17) kJ mol(-1). During oxidation of Cr2N a transformation into CrN was observed. A linear increase in the surface roughness and surface grain size with the oxide layer thickness was found by SFM.