Reduced Friction Losses and Wear by DLC Coating of Piston Pins

A Diamond Like Carbon (DLC) coating is well known to offer superior wear and friction behaviour. This combination of properties makes DLC suitable for many different areas of tribology. This paper concerns itself with usage in the power cylinder environment of automotive diesel engines. To estimate the potential of DLC coatings applied to piston pins in internal combustion engines, linearly reciprocating sliding wear examinations have been performed on uncoated and DLC coated component segments versus different counterpart materials as present in the power cylinder environment, including: piston (aluminium alloy), bushing (brass), piston pin (steel) and connecting rod (steel). Evaluation criteria for the tests include friction and wear performance in dry and lubricated conditions. Test results show how the DLC coatings offer impressive wear reductions for each of the different counterpart materials used. Furthermore, special emphasis is given to the analysis of the friction behaviour. As expected, the coefficient of friction (COF) decreased for aluminium and steel counterparts when the piston pin segments were DLC coated. However, for the combination of DLC with brass the COF increased in the dry condition. This surprising outcome is explained with SEM and EDX investigations of the wear traces. The tests at elevated temperature with lubrication show an inverse relationship with respect to friction criteria when compared with the dry room temperature tests for the DLC with brass combination. Examined engine tests confirm the results of the non-engine wear test rig, showing that DLC coatings applied on piston pins also exhibit properties and good potential to decrease frictional losses and fuel consumption in modern engines.