Effects of phosphorus on friction behaviour in wet clutch systems: A comparison of various coatings

Abstract

This paper presents the investigation of effects of phosphorus-containing coatings on the friction behaviour. Three types of technologically fabricated coatings are investigated: zinc phosphate, iron phosphate, and a third the composition of which is still confidential as it is being patented. For the description of the composition and the properties of these coatings, scanning electron microscopy, energy dispersive X-ray analysis, and electron backscattered diffraction are used among other methods. Friction behaviour is characterized by a disc-on-disc tribometer, in which coated steel discs rotate against so-called paper friction discs under lubricated conditions. Tests are carried out with two different types of lubricants and at two temperatures. The disc-ondisc tests show the influence of coatings on the level and stability of the coefficient of friction (the occurrence of stick-slip or vibrations). Results show that phosphorus-containing coatings reduce or -in certain cases -even prevent the occurrence of stick-slip. Therefore, one focus of the discussion is the investigation of layers formed during tribological tests. For detailed chemical information of the surface, selected systems are also characterized by X-ray photoelectron spectroscopy. Differences in the chemical state of otherwise equal zinc phosphate coatings can explain differences in the experimental results.