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, V 3 O 7 and V 6 O 13 , 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 AgVO 3 phase in the temperature range of 250–350 °C and (ii) the formation of Ag 0.4 V 2 O 5 phase and its tribo-activated melting above 650 °C.