Calorimetric evidence for frictional self-adaptation of TiAlN/VN superlattice coatings

TiAlN/VN superlattices are potential candidates for dry machining due to their high hardness and excellent tribological properties. It has been reported that VN easily oxidizes at relatively low temperatures and forms V2O5 having lubricious properties. The aim of this work is to investigate the effects of vanadium oxides on tribological properties of TiAlN/VN superlattices at high temperatures. During differential scanning calorimetry (DSC) in an argon/oxygen atmosphere it was found that the coatings oxidize at approximately 450 °C. Melting and boiling point of the formed oxides could be determined by DSC to approximately 635 °C and 1400 °C, respectively. It was observed by DSC and X-ray diffraction that the melting phase, a V2O5 containing oxide, transforms into a VO2 containing oxide causing the loss of the liquid phase. Dry sliding tests showed that up to 500 °C the friction coefficient increases from 0.55 to 0.95. It drops to approximately 0.18 at 700 °C and remains there as long as a liquid surface oxide is present. If most of V2O5 is converted into lower-oxidized vanadium the friction coefficient increases to a steady state value of approximately 0.55 at 700 °C. The results obtained show that addition of VN to hard coatings has high potential to achieve a low friction effect due to the formation and melting of a V2O5 containing oxide, in addition to its conversion into easily shearable lower-oxidized vanadium phases.