Influence of contact conditions on tribological behaviour of DLC coatings

By providing excellent frictional properties and greatly improving wear resistance of sliding contact surfaces, diamond-like carbon coatings (DLC) offer a great opportunity for improving performance and durability, and reducing frictional losses in many mechanical systems. However, for the successful application of coated machine components, coatings have to perform reliably under dry and oil-lubricated conditions, with the majority of machine components still being oil-lubricated and operating under diverse contact conditions.The aim of the present investigation was to determine the influence of contact conditions on the tribological behaviour of boundary lubricated DLC coatings, when using oils with extreme-pressure (EP) and anti-wear (AW) additives. Tests with ball-bearing steel ball being loaded against DLC coated discs were performed in the contact pressure range of 1.0 to 3.0 GPa, average sliding speeds of 0.01 to 0.15 m/s and at oil temperatures ranging from 20 to 200 °C.While phosphorous-based AW additives had only minor effects, sulphur-based EP additives were found to reduce friction and wear of W-containing DLC coatings. Furthermore, increase in contact pressure or temperature accelerates the process of friction reduction and results in lower friction while increase in sliding speed has the opposite effect.