Adhesion and sliding tribological properties of polyolefins treated by diffuse coplanar surface barrier discharges

A study is presented on the plasma treatment of two different ultra-high molecular weight polyethylene (UHMWPE) and polypropylene (PP) surfaces by using diffuse coplanar surface barrier discharges under atmospheric air conditions. The plasma-treated polymer surfaces are characterized in terms of wettability, surface chemistry, topography, adhesion and tribology. It is found that plasma treatment significantly improves wettability through formation of various oxygen containing functionalities at the surface. The plasma treatment consequently improves adhesion for either polymer/polymer or polymer/steel joints. Under dry sliding conditions the coefficient of friction slightly increases after plasma treatment in parallel with the higher adhesion. In contrast, the coefficient of friction significantly decreases under oil lubrication conditions and it remains low even after cleaning the oil likely due to the improved oil retention capability of the plasma-treated surface. The observed tendencies in adhesion and friction are further analyzed in terms of surface chemistry after plasma treatment and after sliding by use of spectroscopic methods and chemical imaging.