An atomistic approach to the initiation mechanism of galling

Sliding metallic contacts are accompanied by a severe wear phenomenon named galling. This is manifested in adhesion followed by a substantial increase of friction and subsequent deterioration of the contact surfaces. Experiments on titanium- and vanadium-nitride coated surfaces slid against steels indicate that VN exhibits low friction and improved galling characteristics, clearly outperforming the widely used TiN coating. Using first principles simulations, based on the exact muffin-tin orbital and pseudopotential methods, we present an insight to the atomic level processes initiating galling at Ti(V)N–Fe interfaces. It is found that the equilibrium atomic-scale contacts between nitrides and Fe are adhesive. We demonstrate that the close-packed (0 0 1) VN–Fe interface exhibits a lower atomic-level roughness and a reduced atomic-friction coefficient than that in the case of TiN. These theoretical results indicate that VN is less prone to galling compared to TiN.