Third body effects in the wear of polyamide: Micro-mechanisms and wear particles analysis

The wear micro-mechanisms at a frictional interface between a polyamide 4,6 (PA 46) substrate and a spherical sapphire counterface have been investigated using in situ contact observation. Experiments were carried out under reciprocating sliding conditions with varying values of the overlap ratio, Δ, defined as the ratio of the displacement amplitude to the contact diameter. Using this procedure, it was found possible to vary the extent of wear particles accumulation within coherent third body agglomerates. For low overlap ratios (Δ < 0.2), particles detached from the PA 46 substrate were extensively agglomerated within distinct third body corrugations. In this regime, the rate limiting process regarding the wear degradation was found to be the extrusion of fibril-like wear particles from the extremities of sheared third body agglomerates. On the other hand, no substantial debris accumulation was found to occur when the overlap ratio was greater than 0.2. Additional infrared thermal contact imaging showed that these wear mechanisms were induced under essentially isothermal conditions. From differential scanning calorimetry (DSC) and size exclusion chromatography (SEC) analysis, micro-structural changes within wear particles were found to be associated with a strong reduction in the molecular weight of the PA 46 molecules by virtue of stress-induced chain scission mechanisms.