Offset Linear Relationship between Friction Force and Real Contact Area in Polyacetal Ball Sliding against Glass Disk

The frictional behavior of polymers is strongly influenced by the evolution of the real contact area and the interfacial structure during sliding; however, their quantitative relationship remains unclear. In this study, we investigated the friction interface of a polyacetal (polyoxymethylene, POM) ball sliding against a flat transparent glass disk using in situ optical observations. The friction force and real contact area were measured simultaneously at various sliding speeds, loads, and ball diameters under dry conditions at room temperature. The friction force exhibited a linear dependence on the real contact area under all the tested conditions. However, the linear relationship did not pass through the origin. All linear fits converged to common intersection points for a given load and ball diameter. While the intercept remained nearly constant, the slope varied significantly with sliding speed. This behavior cannot be explained by classical adhesion-based friction models, which assume a uniform interfacial shear strength. Based on detailed observations of wear debris generation, accumulation, and migration, we propose that the friction interface consists of mechanically distinct regions: a load-bearing contact region that supports the applied load and an additional weakly load-bearing region associated with wear debris. This mechanically heterogeneous interface explains the observed offset linear relationship, with load-bearing asperity contacts contributing a nearly constant friction component and debris-mediated regions providing a speed-dependent shear resistance. These findings offer valuable insights into polymer friction beyond classical friction laws.