Oxidation of CrAlN thin films at 800 °C and 900 °C in air

CrAlN films with a composition of 31.3Cr-12.9Al-55.3 N (at.%) were deposited on steels by closed field unbalanced magnetron sputtering to a thickness of 1.2 mu m. They consisted of dense, polycrystalline CrN and AlN fine columns. Their oxidation characteristics were studied at 800 degrees C and 900 degrees C for up to 20 h in air. The formed oxides consisted primarily of crystalline Cr2O3 incorporated with Al2O3. The amount of Al in the film was not large enough to precipitate out Al2O3 from Cr2O3. The oxide layers were thin and compact so as to make CrAlN films more protective than CrN films. During oxidation, Cr, Al and nitrogen diffused outward, whereas oxygen diffused inward. At the same time, Fe diffused outward from the substrate toward the outermost oxide surface owing to the concentration gradient.