Relationship between contact geometry and average plastic strain during scratch tests on amorphous polymers

The scratch behavior of two amorphous polymers is investigated to understand how the materials characteristics affect the scratch resistance. A thermosetting resin and a thermoplastic polymer are studied using both an experimental set-up allowing in situ observations of the contact area during indentation and scratching with spherical tips, and a finite element modeling (FEM). The rheological properties of polymer surfaces are modeled assuming a linear elastic behavior and a plastic law with high strain hardening, described by G'Sell-Jonas equation. The local friction coefficient μloc at the interface between the indenter and the material was modeled with a Coulomb's friction coefficient, for each computed ratio a/R, where a being the contact radius, and R the tip radius. A description of the plastic strain field, beneath the indenter during scratch is proposed as a function of the ration a/R set between 0.1 and 0.4 and the friction coefficient varying between 0 and 0.4. The 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 local friction coefficient and also the rheological parameters of tested material. An equivalent average plastic strain is calculated with FEM over a representative volume deformed plastically. The average plastic strain increases with the ratio a/R, as predicted by the empiric Tabor's rule, but also with the local friction coefficient, for a given ratio a/R, while the strain hardening ability tends to decrease plastic strain imposed during the contact. Clear correlations are demonstrated between the average plastic strain and geometrical parameters, classically used to describe the geometry of the contact area.