Tribological behaviour of DLC coatings in combination with biodegradable lubricants

The tribological behaviour of coated components lubricated with a biodegradable saturated ester and unsaturated ester has been studied. Different diamond-like carbon (DLC)-based coatings were selected as low friction coatings: pure DLC, Si-doped, Ti-doped and W-doped DLC. The performance of the studied ester oils has been compared with sunflower oil and mineral oil lubrication as reference. The oils were all formulated in the same way with conventional anti-wear (AW) and extreme-pressure (EP) additives. Ball-on-disc tests were performed for friction and wear analyses. It was found that DLC/steel contacts lubricated with biodegradable lubricants show similar friction as steel/steel contacts. AW additive activity which was clear in steel/steel contacts was not found in DLC/steel contacts. The wear was low in the DLC/steel contact both for the coating as for the steel surface even when pure base oil was used as lubricant.