Triboactive CrAlN+X hybrid dcMS/HPPMS PVD nitride hard coatings for friction and wear reduction on components

Increasing environmental awareness and energy costs are major driving forces behind the development of energy efficient machines. Simultaneously, increased energy efficiency often leads to higher power densities. Besides diamond-like carbon (DLC) coatings, nitride hard coatings deposited by physical vapor deposition (PVD) show a high potential for wear reduction on machine components. However, regarding friction reduction in lubricated tribological contacts, nitride hard coatings still exhibit a high demand for research since state-of-the art lubricants are tailored to interact with steel surfaces in order to form friction reducing tribolayers. Therefore, the addition of triboactive elements (X) into nitride hard coatings is a promising approach to enhance tribological interactions with lubricants and to reduce friction. The aim of the paper is the analysis of the tribological interaction between the nitride hard coating (Cr,Al)N+X and lubricants. Therefore, a mineral oil based reference and a mineral oil doped with a sulphur additive were investigated regarding interactions with the coatings under tribological conditions. The coatings were deposited in a low temperature T ≤ 200 °C hybrid PVD coating process on case hardened steel AISI5115 (16MnCr5E). The coating and compound properties were investigated. The tribological behavior of the coatings was tested in a pin on disc (PoD) tribometer against inert ceramic Si3N4 counter bodies. The tests were conducted at temperatures T = 90 °C and T = 130 °C at Hertzian contact pressures p0 ≈ 1,600 MPa and p0 ≈ 1,900 MPa. Under the given tribological conditions a friction reduction was achieved by adding the triboactive element Mo into the coatings. Raman spectroscopy revealed that MoS2 was formed in-situ during the tribological tests. The investigated (Cr,Al)N+Mo coatings are therefore a promising approach for friction reduction in highly loaded tribological systems.