Research on Diamond-Like Carbon Coatings for Low-Friction Valve Lifters

One important development area for obtaining better fuel economy is to reduce mechanical friction losses in engine components. The valvetrain is a significant source of mechanical friction loss in an automobile engine, especially at low speeds where fuel economy is most important.

This paper describes the potential use of diamond-like carbon (DLC) coatings at the cam/follower interface in a bucket-type valvetrain. Using a pin-on-disk tester, a motored valvetrain friction apparatus and a bench test rig, the frictional performance of DLC coatings was tested. Experimental data indicate that under a lubricated condition, DLC coatings produced by a plasma CVD (chemical vapor deposition) technique did not show a sufficient effect on reducing friction (only a 20-25% reduction) contrary to our expectations. DLC coatings prepared by arc-ion plating and containing less hydrogen showed superior frictional performance compared with CVD-DLC coatings under a lubricated condition. Results obtained with a motored valvetrain friction tester indicate that an arc ion-plated DLC coating on a valve lifter can reduce friction loss by almost 45% compared with a phosphated steel valve lifter in a conventional valvetrain. Surface hardness, film thickness and surface roughness can be designed to match the intended valvetrain application.