Dynamic changes of mechanical properties induced by friction in the Archard wear model

Fretting is small-amplitude, oscillatory motion between two bodies leading to surface damage. During the fretting process, a tribologically transformed structure (TTS) which has different mechanical properties and microstructure than the starting material is formed on the surface. The commonly-used Archard wear equation relating wear volume to the hardness of the worn material does not account for changes in mechanical properties induced by friction in fretting. To investigate that effect, ball-on-plane fretting tests were conducted on three engineering materials (type 316 stainless steel, pure copper, and titanium alloy Ti-6Al-4V) against an alumina ball to generate TTS layers. The evolution of mechanical properties and microstructures of TTS layers were investigated using nanoindentation and focused ion beam-scanning electron microscope (FIB-SEM). Wear volumes after different fretting cycles were measured with a white light interference microscope. Results show that the mechanical properties of TTS layers evolve differently on different materials during the fretting process. Microstructures of TTS layers also vary from one material to the other. A modified wear model that accounts for friction-induced dynamic changes in mechanical properties is proposed. In these tests the modified model was able to predict the wear volume of 316 steel and pure copper more accurately than the classical Archard model, but it was less successful in predicting wear on Ti6Al4V where there is added complexity from changing microstructure, oxidation, porosity and cracking.