Friction-Induced Tribochemical Processes: From Experimental Analysis to Simulation in Minimal Mixtures

Abstract

Friction-induced tribochemical reactions play a crucial role in the frictional behaviour, wear, and performance of high-load tribological systems. This study investigates the formation of third-body layer in epoxy-based minimal mixtures during sliding friction. Experiments were conducted using a pin-on-disc tribometer with different minimal mixture compositions. The results showed that coarse alumina samples containing around 15 wt% copper significantly enhanced the formation of tribochemically generated 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) confirmed that these patches consist mainly of magnetite (Fe₃O₄) and hematite (Fe₂O₃), which are mechanically bonded rather than chemically diffused. Nanohardness tests showed that the patches were harder than the matrix and the abrasives, while thermogravimetric analysis (TGA) showed that mixtures containing coarse abrasives degraded thermally at lower temperatures. To investigate the role of temperature in patch formation, a three-dimensional cellular automata thermal simulation was developed to model heat distribution in the heterogeneous friction material. Preliminary simulations for the silica mixture predicted transient surface temperatures above 800 °C under continuous braking, in agreement with literature values, and around 70 °C at 2.7 mm depth, in agreement with experimental observations. However, extended experimental results indicated that patch formation depends not only on temperature, but also on abrasive particle size and material properties. This research provides new insights into tribochemically driven third-body formation in frictional contacts and supports the development of predictive models for tribochemical reactions, aiding the optimisation of friction materials for improved wear resistance.

Keywords: Tribochemistry, Third-body layer, Friction-induced oxidation, Minimal mixtures, Cellular automata simulation, Epoxy-based composites