Strengthening Effect of Soft Thin Coatings

A finite element analysis was used to study the onset of plasticity of a coated sphere compressed by a rigid flat. This was done for soft coatings on a harder substrate. Generally, the results agree very well with the findings in the literature for the opposite case of an indentation of a rigid sphere into a coated fiat, with a soft coating. In this case, a weakening prevails over the entire range of coating thicknesses, resulting in critical loads (at yield inception) lower than the critical load of the uncoated contact. However, a surprising strengthening effect was discovered, in this study, for very thin coating thicknesses resulting in a higher resistance to plasticity, compared to an uncoated sphere. This phenomenan resembles a mirror image of the opposite weakening effect which was reported for very thin hard coatings. These results may suggest that when very thin coatings are to be applied in a spherical contact, a softer, rather than harder, coating should be considered in order to increase the contact's resistance to plasticity inception.