The hardness, adhesion and wear resistance of coatings developed for cobalt-base alloys

One potential approach for reducing the level of nuclear plant radiation exposure that results from activated cobalt wear debris is the use of a wear resistant coating. However, large differences in stiffness between a coating and substrate can result in high interfacial stresses and subsequent coating de-adhesion when a coated substrate is subjected to high stress sliding contact. Scratch adhesion and indentation tests have been used to identify four promising coating processes that result in coatings that are adherent and have stiffness values close to the substrate [1,2]: (1) the use of a thin, Cr–nitride coating with a hard and less-stiff interlayer; (2) the use of a thick, multilayered Cr–nitride coating with graded layers; (3) use of the duplex approach, or nitriding to harden the material subsurface followed by application of a multilayered Cr–nitride coating and (4) application of nitriding alone. The processing, characterization and adhesion of these coating systems are discussed. The wear-resistance and performance has been evaluated using laboratory pin-on-disc, 4-ball and high-stress rolling contact tests. Based on the results of these wear tests, the best coating candidate from the high-stress rolling contact wear test was the thin, duplex coating, which consists of ion-nitriding followed by deposition of a thin Cr–nitride coating, while the thin Cr–nitride coating exhibited the best results in the 4-ball wear test.