The Wear of Polymers during Sliding under Fluid-Contaminated Conditions

Abstract

Polymer-based dry bearings are widely used in the aircraft industry. In many applications, these materials may fail to achieve their required performances as a result of contamination by fluids, such as, water, fuel, oils and greases. The purpose of this project is to examine in detail some of the fundamental mechanisms underlying the effects produced by contaminants. Meaningful measurements of wear are only possible when elastohydrodynamic effects are negligible and the surfaces remain in solid contact within the boundary lubrication regime. A rotating pin-on-ring apparatus has been built to measure the friction and wear of polymers under these conditions. The polymer used was Polyphenylene Oxide (PPO) and the lubricants were polydimethy! siloxane silicone fluids of different viscosities. The worn surfaces of the polymer were examined for the presence of silicon by Electron Probe Microanalysis and Rutherford backscattering. A depth of silicon to approximately 8um was found in samples worn under boundary-lubricated sliding conditions. When the external lubricant was removed, one might expect the coefficient of friction to rise from its value under boundary-lubricated sliding to its value under dry sliding conditions after about 8um of wear. In fact, the rise in friction did not occur until several hundred microns of wear of the polymer. Scanning electron microscopy was used to identify any change in the surface and an indentation experiment was devised to detect changes in mechanical properties of the surface layers. The general conclusion from the experimental results is that a plasticised layer is formed on the surface of the polymer, which migrates further into the polymer during wear under starved lubrication conditions, still maintaining a low coefficient of friction. Finally, an attempt is made to quantify the changes in some of the mechanical properties of the polymer which have occurred as a result of sliding under boundary-lubricated conditions, using a theory of boundary lubrication.