ROLLING-CONTACT FATIGUE AND WEAR-RESISTANCE OF HARD COATINGS ON BEARING-STEEL SUBSTRATES

The ever-increasing need for high-performance ball- and roller-bearing components that endure extreme applications has led to a growing interest in hard coatings for improved fatigue life and wear resistance. In particular, hard TiN and TiC coatings and, quite recently, diamond-like carbon films have attracted much attention from manufacturers that produce bearing systems for both rolling and sliding contact applications. This paper presents an overview that highlights recent incremental progress in achieving improved fatigue and wear resistance in bearing-steels through the use of hard coatings. In general, previous research has shown that hard coatings can effectively prolong the fatigue lives of bearing-steel substrates. In particular, thinner hard coatings, 0.2 to 1 mum thick, were found to provide the best overall performance in rolling-contact fatigue (RCF) tests, whereas, thicker hard coatings, > 2 mum, underwent severe delamination and/or fracture and, in most cases, reduced the RCF life of the bearing steels. It was concluded that for these coatings to have any beneficial effect on the RCF life of a bearing steel substrate, it is important that they remain intact on the rolling contact surfaces. Specific references are made to a few mechanistic models that correlate coating thickness and adhesion to improved fatigue life and wear resistance.