Temperature-dependent structural transformation and friction behavior of nanocomposite VCN-(Ag) coatings

Abstract

Nanocomposite VCN-Ag coatings with 10–11 at.% of Ag were obtained by pulsed DC magnetron co-sputtering of V and C (graphite) targets and simultaneous sputtering of Ag target and characterized as promising materials for wide temperature range tribological applications. The friction behavior of the Ag-doped coatings was studied during dynamic temperature ramp tribological tests in the range of 100–700 °C in comparison with their Ag-free counterparts. The sequence of phase transformations upon heating was determined using a combination of high-temperature in situ grazing incidence XRD and DSC/TGA analysis. In situ TEM heating to 450 °C was carried out to evaluate the thermal stability of VCN-Ag coatings. Raman spectroscopy, SEM, TEM, and EDS were employed to compare the coating structure inside and outside of wear tracks after high-temperature tribological tests. The temperature dependence of Ag shear moduli and the shear moduli of two experimentally observed vanadium oxides, V3O7 and V6O13, were obtained using DFT calculations to support and verify experimental observations. The obtained results indicated that the Ag-doped coatings exhibited a lower oxidation threshold. An observed significant drop in the friction coefficient of the VCN-Ag coatings in comparison with their Ag-free counterparts can be explained by (i) the surface self-oxidation initiated by Ag and the tribo-activated formation of AgVO3 phase in the temperature range of 250–350 °C and (ii) the formation of Ag0.4V2O5 phase and its tribo-activated melting above 650 °C. Keywords: Nanocomposite coatings, Lubrication, High-temperature tribology, DFT simulation