Fundamental studies of the tribological behavior of thin polymeric coatings in fretting contact using infrared and photo/video techniques

Abstract

Direct measurements of surface temperatures produced during fretting contact are an unknown area in the discipline of tribology; in addition, the possible effects of such temperatures on the behavior of protective anti-fretting coatings (e.g., polymeric) have never been investigated. An oscillating contact device was designed and built to study fretting contact behavior in tribological processes. The contact geometry consisted of a stationary spherical test specimen loaded against a vibrating sapphire disk driven by an electromagnetic shaker. Surface temperatures generated by frictional heating were measured during fretting contact using an infrared microscope. A photo/video technique was developed to view the fretting contact interface during an experiment and to measure the size and distribution of real area(s) of contact. The effects of size and distribution of the areas on the experimental surface temperatures for polymer-coated steel spheres-on- sapphire were investigated. Archard's theoretical model was also modified to account for multiple contact areas, and the calculated surface temperatures were compared to the experimental results.

Polymeric coatings - including polystyrene (PS), polymethylmethacrylate (PMMA), polysulfone (PSO), polyvinylchloride (PVC), and polyvinylidenechloride (PVDC) were studied at a given load (20 N), frequency (150 Hz), amplitude (100 JLm), and film thickness (55 p.m). The surface temperatures generated were generally low and below the glass transition temperatures of the rigid polymers studied. The magnitude of the surface temperatures was found to be particularly dependent on the size and distribution of real area(s) of contact. The most extensive studies were performed using polystyrene coatings. Effects of load, frequency, amplitude, and film thickness on surface temperature rise and the size and distributions of real area of contact were examined. In addition, uncoated steel specimens were studied under various loads and fretting amplitudes. The observed formation of iron oxide at low surface temperature (60°C) tribologica1 experiments was explained in terms of exoelectron emission.

There were considerable differences observed in the behavior of polymeric coatings under various fretting conditions. The fretting behavior of the coatings was explained in terms of mechanical and thermo-elastic effects. Thermo-elastic predictions of size distributions of real contact areas (patches) showed good agreement with the observed photo/video studies. A mechanism was proposed for tribological behavior and fretting protection of polystyrene coatings.