Friction-induced tribochemical processes: From experimental analysis to simulation in minimal mixtures

Friction-induced tribochemical reactions play a crucial role in the frictional behaviour, wear, and performance of high-load tribological systems. This study examines the formation of third-body layers in epoxy-based minimal mixtures as a model system for fundamental tribochemical investigations. Pin-on-disc experiments were conducted on minimal mixtures containing abrasives and copper in varying compositions. Coarse alumina samples with around 15 wt% copper exhibited significant formation of surface patches. Imaging and chemical analysis using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy, focused ion beam (FIB), and transmission electron microscopy (TEM) revealed that these patches consist mainly of magnetite (Fe₃O₄) and hematite (Fe₂O₃), and are mechanically bonded to the sample surface. Nanohardness tests showed that the patches were harder than the matrix and the abrasives. Thermogravimetric analysis (TGA) indicated that mixtures containing coarse abrasives degraded thermally at lower temperatures. To evaluate the role of temperature in patch formation, a three-dimensional cellular automata thermal model was developed to simulate heat distribution in the heterogeneous friction materials. Preliminary simulations for the silica mixture predicted transient surface temperatures above 800 °C under continuous sliding contact and subsurface temperature around 70 °C at 2.7 mm depth, consistent with literature and experimental observations. However, extended tests showed that patch formation depends not only on temperature, but also on abrasive particle size and material properties. This study provides fundamental insights into tribochemically induced third-body formation in sliding frictional contacts and supports the development of predictive models for tribochemical reactions, aiding the optimisation of friction materials for improved wear resistance.