A study of the oxidation behavior of CrN and CrAlN thin films in air using DSC and TGA analyses

It was found that the CrNx films oxidized in air after 600 °C by the dissociation of fcc (face center cubic)-CrN to h(hexagonal)-Cr2N and nitrogen and, after 900 °C by the dissociation of h-Cr2N to Cr and nitrogen in the film. The addition of Al to CrN film can further improve the oxidation resistance, especially for the high temperature above 800 °C. The oxidation degradation in two Cr-Al-N films started with dissociation of fcc-CrAlN to h-Cr2N and nitrogen in the film. The presence of thermally stable Al-N bonding in the fcc-CrAlN structure can suppress the reduction of nitrogen in the film. A dense (Cr,Al)2O3 layer (either amorphous or crystalline) formed at early oxidation stage (< 700 °C) can act as an effective diffusion barrier slowing down the inward diffusion of the oxygen at high temperatures. Precipitation of h-AlN phase in Cr0.77Al0.23N and Cr0.40Al0.60N films were found at 900 and 1000 °C respectively, accompanied with crystalline Al2O3 formation. After that, both Cr-Al-N films oxidized rapidly after the dissociation of h-Cr2N to Cr and nitrogen. In addition, Cr0.40Al0.60N films exhibit higher oxidation resistance than Cr0.77Al0.23N films. The fcc-CrAlN was retained up to 900 °C and the precipitation of h-AlN phase took place after 1000 °C in Cr0.40Al0.60N films. Cr0.40Al0.60N films also retained a hardness of 25 GPa after annealing at 800 °C in ambient air for 1 h. The activation energies of the final oxidation exothermic peaks in CrNx, Cr0.77Al0.23N and Cr0.40Al0.60N films in the current study were found to be 2.2, 3.2 and 3.9 eV atom− 1 respectively.