Influence of the friction coefficient on the contact geometry during scratch onto amorphous polymers

The scratch behaviour of an amorphous polymer was investigated to determine how the characteristics of the material affect the scratch resistance. A thermoplastic polymer (polymethylmethacrylate) was studied using both an experimental device allowing in situ observation of the contact area during scratching with spherical tips and a numerical approach using finite element modelling (FEM). The rheological properties of the PMMA surface were approximated by a simplified bilinear law, while the friction at the interface between the indenter and the material was modelled with Coulomb's local friction coefficient μad varying between 0 and 1, for each computed ratio a/R from 0.1 to 0.6 (where a is the contact radius and R the radius of the tip). FEM results for elastic–plastic contacts indicate that the contact geometry is directly related to the plastic strain field in the deformed volume beneath the indenter during scratching. The dimensions of the plastically deformed volume and the gradient of plastic strain both are shown to depend on the ratio a/R and also on the local friction coefficient μad. An equivalent average plastic strain is calculated by FEM over a representative plastically deformed volume. The average plastic strain increases with a/R, as predicted by Tabor's empirical rule, and with the local friction coefficient for a given ratio a/R. Clear correlations are demonstrated between the average plastic strain and the geometrical parameters classically used to describe the shape of the contact area.