Study of thickness dependence of sputtered-carbon coating for low friction valve lifters

This paper presents test and stress calculation results of two thicknesses (0.7 and 1.7 μm) of amorphous a-C:Cr-coated standard rollers for a cam roller follower valve train mechanism in a diesel truck engine. The coated rollers were tested for 100 h on equipment simulating near-normal engine running conditions. For the thicker coating, the results show mainly polishing wear and low wear on the cam surface. The thinner coating delaminates and the cam shows higher wear. The delamination may be the product of high tensional stresses in the thinner coating, as determined by finite element calculations. These tensions allow surface cracks to propagate down to the chromium interlayer and vice versa.