Friction, Wear, and Surface Film Formation Characteristics of Diamond-Like Carbon Thin Coating in Valvetrain Application

Abstract

The friction and wear characteristics of thin diamond-like carbon (DLC) coatings have been investigated extensively in recent years mostly in laboratory bench tests. These coatings are known to provide significant friction reduction in the absence of lubricants. In the presence of lubricants, the friction benefits of these coatings are not clearly demonstrated. The current investigation is focused on exploring the friction reduction potential of a DLC coating obtained from a supplier in laboratory bench tests and in a motored valve train test. The DLC coating was deposited on the bucket tappet. In laboratory bench tests, results showed significant friction reduction in the absence of any lubricant but not in the presence of engine oil. In motored valve train tests a significant reduction in friction torque was observed when compared against a slightly rougher uncoated bucket, but no reduction was observed when compared against uncoated bucket tappet with comparable surface finish. Under boundary lubrication conditions, no lubricant-derived surface films were present on the DLC-coated surface. However, under mixed lubrication conditions, evidence of patchy antiwear surface films could be observed on DLC-coated buckets. The antiwear film appears to be primarily composed of calcium phosphate.

KEY WORDS:

• DLC Coating
• Friction
• Wear
• Valvetrain
• Boundary Lubrication
• Mixed Lubrication

ACKNOWLEDGEMENT

The authors acknowledge many helpful discussions held with Steve Matera, Bob Furby, and Kirk Sheffer, V-Engine Engineering, Ford Motor Company, during this investigation.

Review led by Victor Wong