In-process structuring of CrN coatings, and its influence on friction in dry and lubricated sliding

The growing trend to improve component lifetimes coupled with the need to conserve resources is driving new technologies in fields such as tool and forming industries. The use of in-process structuring applied to hard coatings on surfaces is one way of creating lubricating surfaces on the microscale, with superior tribological properties and improved lifetimes. In this study, CrN coatings deposited by plasma-activated physical vapor deposition (PAPVD) on hard metal substrates were structured by variation of the deposition parameters. The parameter combinations favorable for surface structuring were identified. Of the various deposition parameters that were varied in this study, the bias voltage was determined to have a dominating influence on the surface structure of the coatings. A wide variety of structures were fabricated, ranging from flat to highly creviced, with grain sizes ranging from 5 to 500 nm, as determined using scanning electron microscopy (SEM). Profilometer measurements show that the surface roughness, R a, could be varied from 0.04 to 0.12 μm. The highly creviced surfaces however exhibit a somewhat reduced hardness as well as lower adhesion to the substrates, relative to flat CrN surfaces. Even so, ball-on-disk (BoD) experiments, performed under conditions of minimum lubrication at high loads exhibited a longer wear life on the highly structured coatings compared to the relatively flat, unstructured surfaces. This is attributed to lubricant accumulation in microfissures present in the structured coatings. These microreservoirs not only provide critical lubrication at the contacting surfaces but also act as traps for wear-generated debris. Furthermore, the advantages of surface structuring are even more evident under low load conditions; this effect is the result of the reduced contact area and directed lubrication provided by the surface structuring.