A new low friction concept for high temperatures: lubricious oxide formation on sputtered VN coatings

The binary system of vanadium and nitrogen offers promising phases in order to enhance the tribological properties of common hard coatings, owing to their ability to form lubricious oxides, often also referred to as Magneli phases, at elevated temperatures. The aim of this work is to characterize VN coatings prepared by reactive unbalanced magnetron sputtering, and to verify the new concept of solid/liquid oxide lubrication. Oxidation of the coatings and possible melting of the oxides were investigated by dynamic Thermo-Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) up to 900degreesC in air. On heating the coating samples, exothermal reactions appeared during the DSC measurements indicating oxidation in the temperature range of 500-650degreesC. Subsequently, endothermal melting reactions were observed between 650 and 850degreesC. To investigate the effect of oxide formation and melting, dry sliding experiments against alumina and austenitic stainless steel balls were performed using a ball-on-disc tribometer in the temperature range between 25degreesC and 700degreesC. For all coatings evaluated, a significant decrease of the friction coefficient at temperatures above 500degreesC was observed compared to room temperature. Additionally, oxide phases were identified by X-ray Diffraction (XRD) after DSC and tribometer testing. Finally, it can be concluded that the lubricious oxide concept should strongly contribute to the reduction of friction for future machining applications.