Structural and mechanical evolution of reactively and non-reactively sputtered Zr-Al-N thin films during annealing

The influence of reactive and non-reactive sputtering on structure, mechanical properties, and thermal stability of Zr_1 - xAl_xN thin films during annealing to 1500 °C is investigated in detail. Reactive sputtering of a Zr_0.6Al_0.4 target leads to the formation of Zr_0.66Al_0.34N thin films, mainly composed of supersaturated cubic (c) Zr_1 - xAl_xN with small fractions of (semi-)coherent wurtzite (w) AlN domains. Upon annealing, the formation of cubic Zr-rich domains and growth of the (semi-)coherent w-AlN domains indicate spinodal-like decomposition. Loss of coherency can only be observed for annealing temperatures above 1150 °C. Following these decomposition processes, the hardness remains at the as-deposited value of ~ 29 GPa with annealing up to 1100 °C. Using a ceramic (ZrN)_0.6(AlN)_0.4 target and sputtering in Ar atmosphere allows preparing c-Zr_0.68Al_0.32N coatings with a well-defined crystalline single-phase cubic structure combined with higher hardnesses of ~ 31 GPa. Due to the absence of (semi-)coherent w-AlN domains in the as-deposited state, which could act as nucleation sites, the decomposition process of c-Zr_1 - xAl_xN is retarded. Only after annealing at 1270 °C, the formation of incoherent w-AlN can be detected. Hence, their hardness remains very high with ~ 33 GPa even after annealing at 1200 °C. The study highlights the importance of controlling the deposition process to prepare well-defined coatings with high mechanical properties and thermal stability.