Analysing the Tribological Behaviour of DLC-Coated Dry-Running Deep Groove Ball Bearings with Regard to the Ball Material

In many applications conventional lubricants which are based on mineral oils cannot be used because it is physically impossible or forbidden by regulations. Operating machine elements, such as rolling element bearings, under dry running conditions is highly demanding in regard to the materials that are being used. Applying thin films to steel substrates was identified as an approach to achieve wear resistant and low friction surfaces at reasonable cost. Furthermore, a substitution of mineral oil based lubricants by coatings is an achievement in terms of sustainability, environmental friendliness and conserving resources. In this paper wear and friction behaviour of deep groove ball bearings with two types of diamond-like-carbon (DLC) coating systems on the inner and the outer ring are investigated. The coating systems are modified hydrogenated amorphous carbon films, one with a metallic doping element (a‑C:H:Me) functional layer and one with a non-metallic (a‑C:H:X). As ball materials hardened steel (100Cr6) and ceramics (Si3N4) are considered. The tests have been conducted on a four-bearing-test-rig under radial load and a constant rotational speed. The combination of a‑C:H:Me with Si3N4 balls shows the best results in this sample by reaching the predefined time limit without exhibiting an increase in the friction torque trend.