Scratching polycarbonate: A quantitative model

Generally it is understood that friction is additively decomposed into an adhesion- and a deformation-related component, suggesting independence. Experimentally these components cannot be separated and only by combining experiments with simulations, a decoupled analysis is possible. We apply this hybrid experimental–numerical approach in the single-asperity scratch test, simplifying the friction geometry. Simulations without adhesive interaction between tip and surface result in friction forces that are only half of the experimental ones, and are almost not influenced by the sliding velocity. In case of an additive decomposition, this would imply a large contribution of the adhesive component which, moreover, should take care of all rate dependency. This sounds unrealistic. By inclusion of constant friction between tip and polymer, we find that the adhesive component strongly influences the contribution of the deformation component by the formation of a bow wave in front of the sliding tip. Experimental friction forces are quantitatively predicted, including the rate dependency. This entails that the suggested additive decomposition is not applicable and the large macroscopic deformation response proves to be the result of small changes in local processes. Using the model, for the first time, quantitative relations between the polymer's intrinsic mechanical properties and its frictional properties are established.