Atomistic Modeling of Polymeric Nanotribology

Abstract

Molecular dynamics simulations, supported by experimental characterizations, show that the tribology of nano-structured polymer interfaces are largely influenced by the inter-locking of diffusion-induced polymer chains. The degree of interfacial locking was found to determine the mechanisms of tribological behavior. The instantaneous friction coefficient shows regular stick–slip behavior at low sliding speed due to the concurrency of molecular deformation. Surface melting was found at the commencement of slipping. With increasing sliding speed, the regularity of the stick–slip cycles is less obvious and the magnitude of the fluctuations in friction coefficient decreased due to increasing atomic collisions and less durable slip rebound. Stick–slip phenomenon is reduced gradually before finally converging to complete dynamic frictional sliding at high sliding speed. Based on interfacial diffusion conditions, three mechanisms of interface deformation as ‘brushing’, ‘combing’ and ‘scissoring’ were proposed, among which reversible ‘brush’ was considered as the main source of interfacial deformation and dominant mechanism of the tribological behavior. Comparatively, ‘combing’ and ‘scissoring’ only took place in case of significant interfacial diffusion, and were not reversible. The interfacial structure also imposes effects on the influences of other factors to the frictional characteristics. In models with non-diffusive interface, higher pressure from the slider flattens the substrate surface, and thereby reduces the friction coefficient. In conclusion, once the interfacial structure was known, the tribological behaviors become predictable and qualitatively consistent with experimental observations.