Development of Hydrogen-Free Diamond-Like Carbon Coating for Piston Rings

The first piston ring application of a hydrogen-free diamond-like carbon (H-free DLC) coating to a mass-production gasoline engine reduces friction. It is found that the friction reduction effect between the cylinder bore and piston is 18%. When H-free DLC is applied to top ring, friction is over 10% reduced. The effect is larger than H-free DLC applied to oil rings. Because DLC as low friction coating is effective in a thin oil film and there are few oil at a large area near piston Top Dead Center (TDC). The friction reduction effect is enhanced when lower viscosity engine oil is used. In this study, the adhesion strength of H-free DLC coating on piston rings is also improved. It is shown that the relation between the adhesion strength and 2 coating parameters; i.e. providing substrate roughness and coating thickness. Adhesion strength is verified by test results using actual parts considered to piston ring environment by gasoline engine. Piston rings with low friction and high reliability are developed.