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-on-disc 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.