Tribological performance of TiN, TiAlN and CrN hard coatings lubricated by MoS2 nanotubes in Polyalphaolefin oil

• TiN, CrN and TiAlN PVD coatings were lubricated with MoS2 nanotubes (NTs) in PAO.
• MoS2 NTs were shown to be an excellent friction modifier for PVD coatings.
• MoS2 NTs reduced both friction and wear more than conventional MoS2 platelets.
• The friction reduction for CrN and TiN was up to 65%, while for TiAlN it was up to 25%.

Hard coatings deposited by PVD (physical vapor deposition) tend to have a relatively high coefficient of friction, which means that in applications they are lubricated with conventional lubricants. Molybdenum disulfide (MoS2) is a lubricant additive and a friction modifier, and when using the recently developed MoS2 multi-wall nanotubes it is possible to decrease this coefficient of friction even further. For this reason the tribological behavior of MoS2 nanotubes added to polyalphaolefin (PAO) synthetic oil was investigated for the lubrication of cold-work tool steel AISI D2 coated with TiN, TiAlN and CrN hard coatings. The experiments were performed with a ball-on-flat reciprocating machine under a contact pressure of 1.0–2.0 GPa (Hertz, max) and a sliding velocity of 0.5–1.0 cm/s, with a 100Cr6 steel counterpart body. The results were compared to pure PAO oil and to a mixture of commercially available MoS2 platelets with PAO. It was found that the addition of the MoS2 nanotubes leads to a significant reduction in the friction and an improvement in the wear behavior.