Diamond-Like Carbon Coating for Reducing Valvetrain Friction

Improving the fuel efficiency of automobiles has become more important in recent years. Reducing mechanical losses in the engine directly contributes to resolving this issue. Friction between cam lobes and direct-acting valve lifters in particular accounts for about 30% of total engine friction at low speed. Accordingly, many friction- reducing techniques have been adopted for automotive engines today, such as surface finishing to obtain a smoother cam lobe surface, adding a thin hard TiN or Cr₂N coating to direct-acting valve lifters, and adding a friction modifier like molybdenum dithiocarbamate (MoDTC) to the engine oil.

This paper presents a study concerning the application of a diamond- like carbon (DLC) coating to valve lifters. The amorphous carbon film (a-C) obtained by an ion-plating physical vapor deposition (PVD) process was selected for examination because of its lower friction property. It has been shown that friction decreases with decreasing hydrogen content in the film under an engine oil-lubricated condition.

The effect of the a-C coating on reducing friction was evaluated using a cylinder head in which the camshaft was driven by a motor. Other coatings such as a hydrogen-containing a-C:H coating obtained by chemical vapor deposition (CVD), a TiN coating obtained by PVD, a Mo coating (MoS₂ with resin) and a Mn-phosphate coating were also evaluated and compared. The test results showed that the average friction torque at an engine speed of 2000 rpm was the lowest when the camshaft was mated with a-C film-coated valve lifters. Even if the other coatings were lubricated with oil containing MoDTC, their friction torque exceeded that of the a-C coating at the same engine speed. That result is attributed to the following two factors. Firstly, due to the higher hardness of the a-C coating, the mating cam lobe surface was smoothly polished during the test. Secondly, the low hydrogen content (below 0.5 at%) of the a-C coating also presumably contributed to a more wettable film that reacted with the oiliness agent in the engine oil. This supposition is supported by the fact that the friction torque of the a-C coating lubricated with a poly-alpha-olefin (PAO) oil containing 1 mass% glycerol monooleate (GMO) was by far the lowest of the coatings tested.

The parameters of the a-C coating were examined in order to meet the specifications needed for application to valve lifters. Finally, an engine motoring test was conducted to verify the durability of the a-C coating on the valve lifters with respect to its adhesion to the base metal. Throughout the test, no flaking of the a-C coating was observed, indicating that good film adhesion was obtained, which contributed to a continuous reduction of friction.