Thermal\nand Mechanical Mechanisms of Polymer Wear\nat the Nanoscale

Wear\nis a ubiquitous phenomenon that limits the life of many engineered\ncomponents with sliding interfaces through the gradual removal of\nmaterial. The wear of polymers is crucial in many applications, ranging\nfrom bearings to orthopedic implants to nanolithography processes.\nThe wear rate of polymers is strongly affected by the stress and temperature\nat the interface. The effects of temperature and stress are often\ndescribed empirically since the wear process involves complex interactions\nbetween multiple asperities on rough surfaces over a range of length\nscales. Nanoscale tribology experiments at the single-asperity level\nhave provided new insights into the underlying mechanisms of wear.\nExperiments on hard covalently bonded materials, including silicon\nand diamond, have demonstrated that wear is an atomic attrition wear\nprocess that can be modeled using stress-assisted transition state\ntheory. Here, we examine the wear of a common polymer, polymethylmethacrylate\n(PMMA), at the nanoscale as a function of stress and temperature and\nshow that the polymer wear is controlled by a combination of atomic\nattrition and viscoelastic relaxation. While the wear experiments\nare conducted at the nanoscale via atomic force microscopy, the results\nshow that accounting for the local stress distribution at the contact\ninterface is critical to understanding the wear behavior, an effect\nthat was not considered in earlier studies on hard materials. Using\na model that accounts for the stress distribution, we demonstrate\nthe ability to predict the wear volume within 8%.