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. The density functional theory calculated pressure-dependent shear strength S of a four-layer slab of KCl on a Fe(100) substrate is compared to previous calculations for a bilayer slab to gauge the effect of film thickness on the shear properties of the film. It is found that the shear strength varies with pressure as S=S0+αP, where P is the contact pressure. The resulting calculated values for the four-layer slab are S0〈10〉=62±15 and S0〈11〉=65±11MPa while α〈10〉 and α〈11〉 are 0.06±0.01. The values are very close to those calculated for the bilayer slab of S0〈10〉=64±9 and S0〈11〉=69±8MPa and α〈10〉 and α〈11〉 of 0.05±0.01, and in reasonable agreement with the experiment values. These results suggest that the thickness of the film does not have a profound effect on the shear properties.