ZrN, ZrxAlyN and ZrxGayN thin films – novel materials for hard coatings grown using pulsed laser deposition

Abstract.

High-quality thin films of ZrN, Zr_xAl_yN and Zr_xGa_yN have been grown by pulsed reactive crossed-beam laser ablation using Zr, Zr-Al and Zr-Ga ablation targets, respectively, and a N₂ gas pulse. The films were characterized for their chemical, crystallographic and tribological properties. All the films had very low impurity levels and a cubic rock salt crystal structure over the entire investigated temperature range between room temperature and 600 °C. High-quality epitaxial films could be grown on Si (001) at 400 °C, though the crystallinity was disrupted at 525 °C by Si diffusion into the film bulk and the formation of ZrSi₂ crystallites. Films grown on stainless steel were polycrystalline. The ratios of the metals in the alloy targets were in general not equal to those in the films: the Al content in the Zr_xAl_yN films was lower than the target value, which we attribute to differential scattering in the ablation plume. The Ga content in the Zr_xGa_yN films fell with increasing substrate temperature, indicative of re-evaporation of Ga from the substrate surface. Those Zr_xGa_yN films with the highest Ga content, grown at the lowest temperatures, were particularly nitrogen-deficient, which we attribute to the low reactivity of Ga with N₂. The Zr_xAl_yN films had an exceptionally low coefficient of friction (0.20) versus steel and the greatest nanohardness of 28 GPa.