Coalescence and breakup of contact areas: Effects on surface temperatures

In a study of the mechanisms by which thin polymeric films can prevent or delay the onset of fretting corrosion, experimental observations were made of the apparent real area of contact and temperatures generated by friction in a dry-sliding system consisting of stationary polymer-coated steel balls loaded against a vibrating sapphire disk. Five different polymers were used in the original study at vibrating frequencies ranging from 100 to 200 Hz and amplitudes from 20 to 100 μm; but this paper focuses on only one of these—polystyrene coated steel balls in contact with sapphire. Surface temperatures generated by friction were measured using an infrared microscope system. A photomacro/video technique was developed to view the fretting contact interface and to measure the size and distribution of the real areas of contact. The experiments revealed several complex patterns and unusual phenomena. In one example of behavior, a number of small contact patches would suddenly coalesce into one larger contact patch and then break up again into a similar collection of separate patches. This coalescence and breakup occurred at a regular frequency which was much lower than the oscillating frequency. In addition, significant surface temperature spikes corresponding to the occurrence of coalesced areas were observed.A general thermal model previously developed was used to theoretically predict the temperatures corresponding to the experimental conditions [Furey MJ, Vick B, Foo SJ, Weick BL. A theoretical and experimental study of surface temperatures generated during fretting. In: Proceedings of Japan international tribology conference, Nagoya, Japan, 29 October 1990–1 November 1990, vol. II, pp. 809–14; Vick B, Furey MJ. A basic theoretical study of the temperature rise in sliding contact with multiple contacts. Tribology International 2001;34:823–29]. The thermal model consists of a sliding pair of any material combination with three-dimensional and transient conditions. The key feature is the contact area, which is modeled as a collection of rectangular patches in which each patch can have any specified size, shape, position, and time duration. In this way, each contact has a unique start and finish time and the entire collection of contacts can evolve with time in any specified manner. This provides the flexibility to model everything from hard, brittle surfaces such as ceramics to softer, deformable surfaces such as polymers.Using the changes in the apparent real area of contact as observed in the experiments, the theoretical model predicted surface temperatures in close agreement with experimental values. The results of this study show not only that the area of contact is complex and dynamically changing, but that the surface temperatures produced are extremely sensitive to the real area of contact. Although the fundamental mechanisms for the observed phenomena of breakup, coalescence and motion of contact areas are unknown, the study is important since it illustrates the connection between areas of contact and surface temperature—a key unknown which influences physical and chemical behavior in tribological processes.