A model for the weakening effect of very thin hard coatings

It was observed in several studies involving coated spherical contact, that very thin hard coatings may have a weakening effect of reducing the resistance to yield inception compared to an uncoated case. A numerical approach is utilized in this paper to study the effect of various mechanical properties of the substrate and the coating on the range of thicknesses where this weakening effect is encountered. A coating thickness associated with a maximum weakening is found and a universal dimensionless solution for the range of the detrimental coating thicknesses as well as for the maximum weakening thickness is presented. Good qualitative agreement is found between the model and a few published experimental results.