Oxidation behavior, microstructure evolution and thermal stability in nanostructured CrN/AlN multilayer hard coatings

CrN/AlN multilayer coatings with modulation period of 4 and 12.3 nm were manufactured by RF magnetron sputtering. The films were annealed at temperatures 800–950 °C for 1 h in air environment. The microstructure evolution and chemical composition of the formed oxides after heat treatment were identified by transmission electron microscope (TEM) and energy dispersive spectrometer (EDS). After heat treatment at 800 °C for 1 h, three regions which formed on the surface of CrN/AlN coatings with 12.3 nm modulation was observed, including the Al-rich layer covered at the topmost surface, the mixed nano-crystalline Al2O3 and Cr2O3 film and the spherical nano-voids embedded in between. On the contrary, for CrN/AlN coating with a modulation period of 4 nm, the dense oxide layer around 37 nm formed on the top of the un-reacted film was much thinner than that of CrN/AlN coating with 12.3 nm. Besides, no nano-voids was detected which implied that the outward diffusion of atom species was suppressed as compared to that in the film with 12.3 nm. The presence of the topmost Al-rich layer protected the multilayer coating from the outward diffusion of Cr and Al, as well as the inward diffusion of oxygen. After 950 °C for 1 h and 800 °C for 16 h, the grain growth of surface oxides occurred and non-uniform interface between oxide and coating was also determined by TEM in the CrN/AlN multilayer coating with 12.3 nm. However, no substantial oxidations were detected in the coating with modulation period of 4 nm. It was evident that the CrN/AlN multilayer with smaller modulation period exhibited better oxidation resistance.