Oxidation behavior and mechanisms of TiAIN/VN coatings

Hard wear-resistant coatings require excellent oxidation resistance for high-speed machining operations. Moreover, the oxide formed is integral to the frictional behavior and therefore the success of the coating. The oxidation behavior of TiAlN/VN nanoscale multilayer coatings was investigated using high-resolution techniques and was compared with TiN and TiAlN coatings. Static oxidation of TiAlN/VN films was studied in the range 550 degrees C to 700 degrees C, and characterized by high-temperature in-situ X-ray diffraction (XRD) and scanning transmission electron microscopy/energy-dispersive X-ray/electron energy loss spectroscopy (STEM/EDX/EELS) of selected surface cross sections. The oxidation resistance of TiAlN/VN was found to be controlled by the VN layers, and consequently, oxidation was initiated at a lower temperature than TiN and TiAlN coatings. The onset of oxidation of the TiAlN/VN coating was found to be >= 550 degrees C with the VN being the first component to oxidize. At temperatures > 600 degrees C, a duplex oxide structure was formed; the inner layer comprised a porous region of Ti-rich and V-rich nanocrystallites, while several phases were observed in the outer region, including V2O5, TiO2, and AlVO4. V2O5 was the dominant oxide at the outer layer at >= 638 degrees C. The outward diffusion of V depended on the species present; in the inner layer, V was present as V3+, V4+, whereas a significant V5+ was dominant in the outer layer of oxide at >= 638 degrees C. An Au marker study suggested roughly equal diffusivity of cations outward, and oxygen inward diffusion occurred during oxidation.