Ultra-low friction of biodiesel lubricated ta-C coatings

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.