Magnéli phase formation of PVD Mo–N and W–N coatings

The binary systems of Mo–N and W–N offer a high potential to enhance the tribological properties of common hard coatings, owing to their ability of forming lubricious oxides (often also referred to as Magnéli phases) at elevated temperatures. The aim of this work is to characterize Mo–N and W–N hard coatings prepared by reactive unbalanced magnetron sputtering, and to verify the concept of solid/liquid oxide lubrication. Oxidation of the coatings and possible melting of the oxides are investigated by dynamical thermo-gravimetric analysis and differential scanning calorimetry (DSC) up to 900 °C in air. During heating the samples, the exothermal reactions, detected during the DSC measurements, indicate oxidation in the temperature range between 200 and 750 °C. These exothermic reactions are superimposed by endothermic melting reactions between 650 and 850 °C. The formed oxide phases are investigated by X-ray diffraction after DSC and tribometer testing. To evaluate the effect of formation and melting of the oxides on the coefficient of friction, dry sliding experiments in the temperature range between 25 and 700 °C against alumina and austenitic stainless steel balls are performed using a ball-on-disc tribometer. All coatings investigated show lubricious oxide formation, resulting in a decreasing coefficient of friction at temperatures above 500 °C. For Mo–N, these beneficial properties suffer from the formation of the volatile oxide MoO3 at temperatures higher than 500 °C. heless, the lubricious oxide concept within both systems strongly contributes to the reduction of friction, which is important for several industrial applications. We therefore infer that the addition of Mo–N and/or W–N phases to hard coatings can effectively improve their tribological properties, especially in the temperature range between 200 and 700 °C.