Super low friction of DLC applied to engine cam follower lubricated with ester-containing oil

This paper presents a material combination that reduces the friction coefficient markedly to a superlow friction regime (below 0.01) under boundary lubrication. A state approaching superlubricity was obtained by sliding hardened steel pins on a hydrogen-free diamond-like carbon (DLC) film (ta-C) lubricated with a poly-alpha-olefin (PAO) oil containing 1 mass% of an ester additive. This ta-C/steel material combination showed a superlow friction coefficient of 0.006 at a sliding speed of 0.1 m/s. A hydrogencontaining DLC coating/steel combination also showed a lower friction coefficient in air than a steel/steel combination, 0.1 vs. 0.8, but no large reduction was observed when the sliding surfaces were lubricated with ordinary 5W-30 engine oil and the PAO oil containing an ester additive. The friction coefficient of the hydrogen containing DLC/steel combination lubricated with the PAO containing an ester additive was above 0.05. On the other hand, the superlow friction performance demonstrates that the rolling contact friction level of needle roller bearings can be obtained in sliding contact under a boundary lubrication condition. It is planned to apply this advanced DLC coating technology to valve lifters lubricated with a newly formulated engine oil in actual mass-produced gasoline engines. A larger friction reduction of more than 45% is expected to be obtained at an engine speed of 2000 rpm.