The structure and properties of CrAlN films deposited by mid-frequency unbalanced magnetron sputtering at different substrate bias duty cycles

The CrAlN films were deposited on silicon and stainless steel substrates by mid-frequency unbalanced magnetron sputtering system. The influence of duty cycle on composition, structure, morphology and properties of the CrAlN films were investigated. The results showed that the CrAlN films presented NaCl-type crystal structure, indicating that the crystalline structures of the CrAlN films were predominantly cubic as that of CrN. Meanwhile, the Al and N concentration, grain size and surface roughness declined with the increase of the duty cycle. The cross section of the films changed from random porous to fine continuous column structure with duty cycle varied from 50% to 80%. The internal stress was changed to the compressive stress and declined with the increase of the duty cycle. The behavior of the incremental hardness as the duty cycle increased was also investigated. In addition, the oxidation resistance of the films deposited at different duty cycle was evaluated after annealed in air at 500–900 °C. The extent of oxidation and the oxidizing state of the element were analyzed by X-ray diffraction, X-ray photoelectron spectroscope and energy dispersive X-ray spectroscopy techniques. The results showed that the thermal stability of the film was influenced by the composition and the microstructure of the film. It was found that the film deposited at 70% had the best oxidation resistance compared with other films, where the ratio of O/N kept still about the 35.22% after annealed at 700 °C. However, the hardness of the film was significantly decreased as the heat–treatment temperature increases, which was attributed to the formation of amorphous oxides and θ-Al2O3 phase as well as the relaxation of the internal stress.

► The microstructure of the CrAlN is analyzed in detail as a function of the duty cycle.
► The relationship between structure and mechanical properties of CrAlN film is investigated.
► The mechanism of the thermal stability is expounded.
► The composition and the microstructure have a co-action on the thermal stability of the films.