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

Abstract

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 V 2O 5 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 V 2O 5 containing oxide, transforms into a VO 2 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 V 2O 5 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 V 2O 5 containing oxide, in addition to its conversion into easily shearable lower-oxidized vanadium phases.