High-temperature oxidation resistance of Cr1−xAlxN thin films deposited by reactive magnetron sputtering

The thermal stability against oxidation of Cr1−xAlxN films with 0≤x≤0.63 has been investigated by isochronal (15 min) heating in air at various temperatures up to 1173 K. Cr1−xAlxN thin films were deposited by reactive magnetron sputtering from Cr and Al targets in a mixed Ar/N2 atmosphere at a substrate temperature of 573 K. All the films crystallize in the pseudo binary, rocksalt-type cubic structure, showing a (111) preferential orientation. Oxidation proceeds by de-nitridation and the formation of a pseudo binary, mixed, Cr/Al oxide with the corundum structure. The degree of film oxidation was evaluated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Rutherford backscattering spectroscopy (RBS). The substitution of Cr atoms by Al atoms leads to two oxidation behaviors. Cr1−xAlxN films with low Al content (x<0.2) exhibit poor resistance against oxidation and aluminum alloying is actually detrimental. These films show signs of oxidation at 773 K already. In contrast Cr1−xAlxN films with high Al content (x>0.2) are more resistant to high temperatures compared to pure CrN. Films with the highest Al content (x=0.63) are stable up to 1173 K due to the formation of an amorphous, aluminum-rich oxide which blocks oxygen diffusion and prevents further film oxidation.