Tribological behavior of thick DLC coatings under lubricated conditions

Abstract DLC coatings, with properties like high wear resistance, corrosion protection and low friction coefficient under unlubricated conditions, are becoming more and more popular in different fields every year, and offer a wide application spectrum in many industrial areas. A new approach is the deposition of thick DLC films with thickness up to 55 μm. With this level of thickness, it is possible to create interesting tribological systems with high mechanical load carrying capacity even on softer steels, which are used in general engineering. However, under lubricated conditions, DLC films can show very different behavior compared to dry conditions. In this work three different DLC coatings were investigated. A “Hard” and a “Soft” coating, as well as a Multilayer DLC film. As substrate material, samples of steel grade 4140 were used. In order to determine the influence of the surface condition, each coating was deposited on a rough (sandblasted) and a polished substrate. The friction coefficient (COF) was determined by a pin-on-disc tribometer with a 100Cr6 (AISI 52100) ball counterpart and a sliding distance of 2000 m. In addition, the wear track and the surface defects were investigated by confocal microscopy. The used lubricants were water and engine oil. The experiments showed that under same tribological parameters all lubricated DLC coatings on polished samples suffered under strong fatigue wear on the surface. By using thick coatings, it could be realised that this is not caused by an adhesion problem, because the resulting defects do not reach the steel interface. Fatigue wear was found for lubrication with oil as well as for lubrication with water. In contrast DLC coated rough surfaces showed no fatigue wear, also under lubricated condition. By using rough DLC coatings the wear of the counterpart is obviously remarkable increased. If the tribological experiments were done under excessive use of oil (not on a thin oil film), no fatigue wear on the surface could be found as well. On smooth surfaces the adhesion forces caused by the thin oil or water film seem to be responsible for the observed fatigue wear defects. It is also shown that with a special topography of the DLC coating fatigue wear under slightly lubricated condition can be eliminated and simultaneously the wear of the counterpart can be kept very low.