Effects of mechanical properties and surface friction on elasto-plastic sliding contact

Indentation hardness has been used extensively for material characterization and many recent computational studies have established quantitative relationships between elasto-plastic mechanical properties and the response in instrumented indentation. In contrast, very few studies have systematically quantified the effect of the plastic deformation characteristics on the frictional sliding response of metals and alloys. Building upon dimensional analysis and finite element computations, a parametric study was carried out to extend our previous work to different contact friction conditions. For a wide range of elasto-plastic and contact friction parameters, we established closed form universal functions, for various contact conditions, that relate elasto-plastic properties (Young’s modulus, yield strength, and power law hardening exponent) to steady state frictional sliding response (scratch hardness, pile-up height and overall sliding frictional coefficient). Distribution of the plastic strain beneath the indenter was studied to rationalize the deformation modes versus elasto-plastic properties and pile-up. In parallel, experiments were conducted for the effect of plastic flow characteristics on the frictional sliding (or scratch) response under different surface friction conditions. Pure copper and a brass alloy were heat-treated to vary yield strength and strain hardening exponent and the contact friction coefficient was varied by applying a liquid lubricant on the surface. Frictional sliding experiments were conducted using a nanoindentation testing system, where grain size and alloy composition were found to influence the response. Although variations in the frictional sliding response versus yield strength, strain hardening and friction were invariably coupled, the combined computational and experimental approach enabled us to isolate the relative contributions of each parameter. The results clearly demonstrated that an increase in the strain hardening exponent can significantly decrease the pile-up height, with known and further potential implications for the evaluation of tribological damage.