Experimental and finite-element analysis of scratches on amorphous polymeric surfaces

The scratch behaviour of two amorphous polymers is investigated to understand how the materials characteristics affect the scratch resistance. A thermosetting resin (CR39) and a thermoplastic polymer polymethylmethacrylate (PMMA) are studied using both an experimental set-up allowing in situ observations of the contact area during indentation or scratching with spherical tips, and a finite-element modelling (FEM). The rheological properties of CR39 and PMMA surfaces are modelled assuming a linear elastic behaviour and a plastic law with high strain hardening, described by G'sell Jonas equation. A description of the plastic strain field, beneath the indenter during scratch is proposed. The main parameter of the description is the level of the deformation imposed on the samples, related in first approximation to the ratio a/R set between 0.1 and 0.4, a being the contact radius, and R the tip radius. The true friction μ_loc at the interface between the indenter and the material was modelled with a Coulomb's friction coefficient varying between 0 and 0.4, for each computed ratio a/R. The two polymers display the same scratching behaviour for low levels of deformation, whereas at high strain, PMMA shows a more pronounced plastic behaviour, with high level of average plastic strain, especially for higher friction coefficient. This results obtained by FEM are in good correlation with experimental observations and show that the plastic strain gradient beneath the indenter depends clearly on the ratio a/R, on the true friction coefficient and also the rheological parameters of tested material.