Abrasive and abrasive–adhesive behaviour and tribologically transformed zones in gross-slip fretting: Rationalisation and quantification via a physically-based oxygen transport and consumption model

In non-noble metals, tribologically transformed zones (TTZs) form in gross-slip fretting contacts when oxygen cannot access the entire interface to sustain oxide debris formation, leading to direct first–body metal–metal contact. Existing models either rely on unsupported assumptions or do not accord with experimental observations. This work presents the first physically consistent oxygen transport–consumption model for circular, square and rectangular fretting contacts. Predicted oxygen concentration distributions identify a critical specific consumption rate separating fully oxide-covered (abrasive) contacts from those containing adhesive regions without a protective debris layer. The model quantifies the adhesive zone area fraction (with its accompanying TTZ) and explains why expelled debris remains oxide-based: metallic particles formed in adhesive regions oxidise at the adhesive–abrasive boundary, acting as a local oxygen sink that sustains contact oxygen ingress. Predictions agree with published observations and provide a unified framework for interpreting fretting scar characteristics and advancing predictive wear-rate models.