On the film thickness dependence of shear strengths in sliding, boundary-layer friction

► Uses first-principle quantum calculations in the form of density functional theory (DFT) to compute the pressure-dependent shear strength of a model KCl tribofilm. ► Shows that first principles quantum theory can reproduce the experimentally measure value of the zero-pressure limit of shear strength. ► Analyses the origin of pressure-dependent shear strengths in boundary lubrication. ► Demonstrates that the difference between the experimental and theoretical values of the proportionality constant between shear strength and pressure cannot be accounted for by differences in film thickness.