Nano-rheological properties of polymeric third bodies generated within fretting contacts

The accumulation and compaction of polymer wear particles into coherent third body compacts has been investigated in a contact between steel and poly(methylmethacrylate) under small amplitude cyclic micro-motions, i.e. fretting. Under such contact conditions, the preferential accumulation of the PMMA wear particles at the centre of the contact was found to result in the formation of a distinct third body agglomerate. Hardness and modulus measurements carried out at the nanoscale revealed that the polymer wear particles within this third body formation were progressively compacted to form an agglomerate, for which the mechanical properties were ultimately close to those of the initial PMMA substrate. The efficiency of the third body compaction process was interpreted by considering the evolving load-carrying capacity of the contact, which was characterised by progressive redistribution of the contact pressure within the third body layer. In addition, an assessment of the dissipation of frictional energy within the worn contact area demonstrated that particle compaction was enhanced by the highly energy-intensive nature of the fretting process.