Steady-state frictional sliding contact on surfaces of plastically graded materials

Tailored gradation in elastic–plastic properties is known to offer avenues for suppressing surface damage during normal indentation and sliding contact. In tribological applications, sliding contact analysis provides a more representative mechanism for fundamental understanding and design as it offers a tool to test materials under conditions of controlled abrasive wear. However, no such study exists for plastically graded materials, although the sliding behavior for elastically graded materials has been reasonably well understood. This study has established a systematic methodology to quantify the mechanics of steady-state frictional sliding response for a plastically graded material. Specifically, the effect of linear gradient in yield stress on the frictional sliding response is examined through parametric finite-element (FEM) computation of the instrumented scratch test. Gradients in yield strength affect both the load carrying capacity of the surface and its pile-up around the sliding indenter. An increase in yield strength with distance beneath the surface shifts the peak values of von Mises stress below the surface, thus improving the resistance of the surface to onset of plasticity and damage. For a given elastic–plastic property, an increasing yield strength gradient causes a reduction in total apparent friction through a reduction in the ploughing coefficient. The contact-load-bearing capacity of plastically graded surfaces follows a similar trend during indentation and scratch. However, significant differences between the pile-up and the friction response are observed between normal indentation and steady-state frictional sliding. In particular, an increase in interfacial friction is found to cause an increase in pile-up during scratch, while it causes a decrease in pile-up during indentation. The implications of the present results to the design of graded surfaces are discussed. 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.