Tribological characterization of diamond-like carbon (DLC) coatings sliding against DLC coatings

Two different qualities (single-layer and multi-layer) of thick diamond-like carbon (DLC) coatings were prepared on both extremely polished (R_a∼0.01 μm) stainless steel disks and 6 mm diameter stainless steel balls. Morphological and micro-structural evaluations reveal that multi-layer DLC coatings are superior in nature compared to the single-layer coatings. Tribological characterisations using a pin (ball)-on-disk tribometer shows that, even though the multi-layer DLC coating exhibits superior qualities, when it slides against the same displays relatively higher friction and wear compared to that of single-layer DLC sliding against the same. Multi-layer DLC coated steel ball sliding against the single-layer DLC coated disk displays the best tribological performance. However, stress cracking was observed very rarely at the sliding edge of the multi-layer film prepared on the ball and is believed to be due to the relatively higher compressive stress in the film combined with the applied stress. The tribological performance of the single-layer DLC sliding against the same is also found to be excellent, and is comparable to that of the multi-layer DLC coated ball against the single-layer DLC coated disk. No cracks were observed on the low stress single-layer DLC coated ball after the friction test. The study reveals that the single-layer DLC sliding against the same would provide the best tribological DLC pair for relatively higher load applications such as in orthopaedic implants.