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The formation of an ultra-low friction system was observed for ta-C coated surfaces lubricated with biodiesel containing petrodiesel. The tribosystem was compared to pure petrodiesel and engine oil lubrication and to uncoated steel surfaces. Friction was found to be lower for more than 50% over the petrodiesel lubricated or the uncoated state. Different tribological oscillating test setups, simulating high and low contact pressure, were used to characterise friction and wear. Unusual high wear was found for some ultra-low friction experiments. Also, in one test setup the beginning of the ultra-low friction state was observed as a sudden drop of the friction coefficient after several minutes of running in. An assessment of biodiesel components and related lubricants showed that all investigated fatty-acid based lubricants contribute to the formation of the ultra-low friction system. Obviously not their chemical head function is important but rather their fatty-acid origin

Coatings used in tribological applications often exhibit high hardness and stiffness to achieve high wear resistance. One coating characterization method frequently used is nanoindentation which allows the determination of indentation hardness and indentation modulus among other material properties. The indentation modulus describes the elastic surface behavior during indentation and is, among hardness, a direct indicator for wear resistance. To obtain the true indentation modulus of a coating, it must be measured with varying loads and then extrapolated to zero load. Current recommendation of the standard ISO 14577-4:2016 is a linear extrapolation which fits poorly for nonlinear curves. Such nonlinear curves are commonly found for high hardness mismatches between coating and substrate, for example, superhard tetrahedral amorphous carbon coatings (ta-C) on a steel substrate. In this study, we present a new empirical fit model, henceforth named sigmoid. This fit model is compared to several existing fit models described in the literature using a large number of nanoindentation measurements on ta-C coatings with wide ranges of indentation modulus and coating thickness. This is done by employing a user-independent and model agnostic fitting methodology. It is shown that the sigmoid model outperforms all other models in the combination of goodness of fit and stability of fit. Furthermore, we demonstrate that the sigmoid model’s fit parameter directly correlates with coating thickness and thus allows for a new approach of determining ta-C coating thickness from nanoindentation.

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