Substrate-affected contact deformation and hardness of an elastoplastic film coated on purely elastic substrates

The elastoplastic contact deformation of coating/substrate bilayer composites is examined through finite element analyses. The author focuses on the plastic flow within an elastoplastic film that is enhanced by the geometrical constraint of elastic substrate. To gain a deep physical insight into the effect of the substrate on the plastic flow of film, a specific elastoplastic film is supposed to be coated on purely elastic substrates, ranging from very compliant to very stiff. The elastic mismatch of coating/substrate plays an essential role in controlling the elastoplastic deformation and flow of the film. The film significantly changes its characteristic elastic/plastic and contact behaviors, depending on the substrate compliance; a stiffer substrate enhances the plastic flow within the coated film, while a more compliant substrate, in contrast, depresses the plastic flow. In other words, at a given depth of penetration, the film accommodates the penetration in a more elastic manner, when the substrate is compliant, whereas the film becomes more plastic and ductile, when the substrate is stiff. Accordingly, the concept of “substrate-independent–film-only properties” has none of engineering significances from a practical point of view, if the information on the mechanical characteristics of the substrate is limited.