DLC coatings (DLC = Diamond-like carbon) are applied to surfaces in sliding contacts for friction reduction. Due to the covalent bond, they have a low potential for adhesion, which results in a low friction coefficient. Additional, the high hardness provides a very good wear protection of the coated surface.

The properties of DLC coatings are determined by the chemical composition, the micro structure and the macroscopic appearance, like e.g. porosity. In addition to the coating properties, the mechanical properties of the underlying basic material are also important. Especially the hardness is crucial, because a too soft basic material can be plastically deformed and the support of the DLC coating is not given anymore. In case of a heavy deformation, this would lead to cracks in the coating and finally to a chipping off of the coating.

With a focused ion beam (FIB), coated engine parts were prepared and analyzed in a SEM (scanning electron microscope). The results showed that quite even a small deformation of the base material might locally influence the durability of the DLC coatings. Due to the deformation, raised material occurs, which propagates through coating to the surface. Therefore, locally the hertzian pressure increases and an unusual high polishing wear of the DLC coating occurs at this site.

For monitoring the DLC coating properties during mass production and to ensure a constant coating quality, currently, parameters like coating hardness, adhesion force and coating thickness are checked.

However, our investigations have shown that the scope of inspection is not sufficient enough or is too inexact for an identification of small to medium changes of the coating quality. In wear tests, engine parts from two different production batches showed a clearly different wear resistance. The parts from the first batch showed distinct polishing wear in large surface areas, while on the surface of the parts from the second investigated batch nearly no wear occurred. The coating hardness of both batches was comparable and was also well within the new part specification. To clarify this problem, scientific methods were used for a detailed characterisation of the DLC coatings. SNMS sputter profiles (Secondary Neutral Mass Spectrometry) revealed slightly higher hydrogen content in the batch with the worse wear resistance. As the difference was only 2 wt%, the different wear behaviour could not be explained by the different hydrogen content alone. Unusual chemical impurities were not observed in any of the investigated coatings. Our evaluations of EELS (Electron Energy Loss Spectroscopy) spectra showed a 15 wt% - 20 wt% higher sp²-hybridized carbon in the batch with the worse wear behaviour. Furthermore, micrographs taken with a high resolution SEM (HR-SEM), showed for this batch a worse coverage of the surface. Especially, on some grinding grooves, the worse coverage was obvious.

From our own observations and also from literature it is known that certain engine oils or additives of these oils can take influence on the wear resistance of DLC coatings. In case of MoDTC (molybdenum-dialkyl-dithiophosphate) containing oils, an increased wear of DLC coatings was observed. From literature, it can be seen that especially a-C:H coatings are affected. That means coatings with higher hydrogen content and a higher contingent of sp²-hybridized carbon.

Although the above mentioned higher wear of the first investigated production batch was observed in an engine oil without MoDTC, the different coating composition might cause a different interaction and reactivity to other components of the engine oil. Further investigations will target on adsorption and absorption of engine oil components on or in the coating, to specify a possible interaction between DLC coatings and engine oils.