On the Pressure Dependence of Shear Strengths in Sliding, Boundary-Layer Friction

Abstract

Density functional theory, as implemented in a full-potential linearized augmented plane wave method, flair, is used to calculated the pressure-dependent shear strength S of KCl on a Fe(100) substrate and the results are compared to the experimental dependence given by \( S = S_{0} + \alpha P \), where P is the contact pressure and S0 = 65 ± 5 MPa and α = 0.14 ± 0.02. Calculations were performed for a KCl bilayer enclosed between two Fe(100) slabs, where the energy was found to vary harmonically as a function of the separation between the outermost layers. Thus, a simple analytical model was developed for the pressure-dependent shear strength of the film, which includes both linear and quadratic pressure dependence. However, the coefficient of the quadratic term was found to be much smaller than the linear term, leading to the linear shear-strength pressure dependence found experimentally. The calculated values of S0 〈10〉 = 64 ± 9 and S0 〈11〉 = 69 ± 8 MPa are in excellent agreement with experiment, while α 〈10〉 and α 〈11〉 equal 0.05 ± 0.01, somewhat lower than, but within the same range as the experimental values.