Argon ion sputtering bridging plasma nitriding and GLC film deposition: Effects on the mechanical and tribological properties

A duplex coating consisting of a nitrided layer and a GLC (Graphite-like carbon) film was deposited on the surface of the M50NiL steel via a one-step process in a plasma nitriding system. The influence of argon ion sputtering, a critical process bridging plasma nitriding and GLC film deposition, was studied. Effects of argon ion sputtering on the growth of the GLC film were investigated by means of Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. The mechanical and tribological properties of the duplex coating were evaluated by a Vickers hardness tester, a scratching tester and a rotating ball-on-disc tribometer. Results reveal that pre-treatment by ion sputtering improves the surface condition of the nitrided layers and increases its surface energy, thus facilitating nucleation growth and GLC film adhesion. The combined effect of better film-substrate adhesion, strong interfacial chemical interaction and the self-lubricating features of the GLC films are the main reasons for the excellent tribological properties of the coating. This study may provide a new idea for optimizing the duplex coating (nitrided layer + GLC film) with excellent tribological properties and high durability.