The role of surface oxidation in friction processes on molybdenum nitride thin films

Molybdenum nitride Mo2N films were deposited on 440C steel substrates by reactive magnetron sputtering. The film composition was analyzed with EDX/WDX. X-ray diffraction was used to determine their crystallographic structure. Friction tests, with an alumina ball as a counterpart, were carried out in the temperature range of 20-400 °C. Film surface inside and outside wear track was studied with Raman spectroscopy. We discuss the crucial role of surface oxide layer, created due to thermal and tribo-oxidation, in friction and wear processes. Room temperature friction leads to residual tribo-oxidation within the wear track. In normal humidity conditions, it results in low friction values μ≈0.4 and very low wear. At higher temperatures, brittle oxide layer is easily worn out and friction increases. Above 250 °C, oxide softening leads to pronounced decrease of friction.