The aim of this study was to gain knowledge of tribological layers on a steel disc. This steel disc has been used in a transmission system in the field. ATF-oil (automatic transmission fluid), which contains different surface-adhering additives, serves as a lubricant and as a cooling fluid in the tribological system. Over time, the tribological characteristics of the system changed. Special interest in this study lies in the modifications of the steel surface. The field-used disc was therefore compared with model tested discs. In a disc-on-disc tribometer, all samples are tested for a short period of time to evaluate the actual condition of the system regarding friction behaviour. Analysis and characterisation of surfaces and layer formation were carried out with, among other techniques, 3D topography, SEM / EDX, AFM, XPS, and ToF-SIMS. Results indicate that in some regions a complex composed tribological layer is formed. Differences were detected between the steel discs used in the field and the model discs. This indicates the problem of evaluation of long-term behaviour exclusively by use of short or accelerated model experiments. A combination of carefully selected and sophisticated analytical methods is necessary to trace small changes of the system.

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.