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

Abstract

Freestanding CrN x and Cr 1 − x Al x N films with two different Al atomic percentages with respect to the metal sublattice ( x = 0.23 and x = 0.60) were produced by pulsed closed field unbalanced magnetron sputtering (P-CFUBMS). The dynamic oxidation behavior of the films has been characterized by thermal analysis using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The structure of the films at different thermal-annealing temperatures were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) in an effort to understand different phase transitions and oxidation reactions observed on the DSC curves. The peak temperatures of the main exothermic/endothermic oxidation reactions in the DSC signals at different heating rates were applied to the Kissinger model for determination of activation energies. The mechanical properties of the films at different heat-annealing states were measured by nano-indentation. It was found that the CrN x films oxidized in air after 600 °C by the dissociation of fcc (face center cubic)-CrN to h(hexagonal)-Cr 2N and nitrogen and, after 900 °C by the dissociation of h-Cr 2N 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-Cr 2N 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) 2O 3 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 Cr 0.77Al 0.23N and Cr 0.40Al 0.60N films were found at 900 and 1000 °C respectively, accompanied with crystalline Al 2O 3 formation. After that, both Cr-Al-N films oxidized rapidly after the dissociation of h-Cr 2N to Cr and nitrogen. In addition, Cr 0.40Al 0.60N films exhibit higher oxidation resistance than Cr 0.77Al 0.23N films. The fcc-CrAlN was retained up to 900 °C and the precipitation of h-AlN phase took place after 1000 °C in Cr 0.40Al 0.60N films. Cr 0.40Al 0.60N 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 CrN x , Cr 0.77Al 0.23N and Cr 0.40Al 0.60N films in the current study were found to be 2.2, 3.2 and 3.9 eV atom − 1 respectively.