Influence of a hard surface layer on the limit of elastic contact. II. Analysis using a modified GW model

The Greenwood and Williamson microcontact model of rough surfaces is modified to include the presence of a surface layer which is suffer and harder than the substrate. The axisymmetric contact between a rigid spherical asperity and an elastic layered halfspace is analyzed numerically and correction factors for the contact area, load and the maximum von Mixes stress are approximated to a closed form by using curve fits of the numerical results. The correction factors for the contact area and load are applied to the GW model to reflect the effect of the finite layer thickness and the substrate material. The correction factor for the maximum von Mises stress is used to calculate the plasticity index for layered surfaces. Parametric calculation of the ratio of plastic contact area to real contact area is carried out for a TiN-coated steel surface. The modified GW model is compared with a more rigorous real surface model and the validity of the present model is discussed. When the layer thickness is sufficiently large, the influence of the soft substrate can be neglected. A simple criterion for realizing the contact free of the effect of the substrate is proposed.