Investigation on structure and properties of arc-evaporated HfAlN hard coatings

HfAlN hard coatings were synthesized by cathodic arc evaporation from Hf0.75Al0.25 targets at different bias voltages and N2 pressures. Structural investigations by x-ray diffraction (XRD) revealed the formation of a face-centered cubic (fcc) structure with a change in the preferred orientation depending on the energetic growth conditions. This change in the crystal structure did not significantly influence the mechanical properties, e.g., hardness, where values ranging from 24 to 26 GPa were measured. The obtained fcc phase is stable up to 1000 deg. C as no significant changes in the crystal structure could be observed after annealing in vacuum. Similar experiments in ambient air and subsequent analysis of the coatings by XRD and Raman spectroscopy revealed an onset temperature for oxidation in the range of 700-800 deg. C and the formation of monoclinic HfO2 as the dominating oxide phase. Ball-on-disk tests at different temperatures were performed in order to evaluate the tribological properties. The coefficient of friction ranged from 0.8 to 1.2 depending on the testing temperature while the best wear performance was obtained in the temperature range of 300-500 deg. C.