In Situ Film-Forming and Friction-Reduction Mechanisms for Carbon-Nanotube Dispersions in Lubrication

Current requirements in automotive lubrication impose extremely complex formulation. For environmental reasons, it is important to reduce or eliminate the presence of sulphur and phosphorus contained in tribological additives. For that purpose, multi-walled carbon nanotubes have been dispersed in oil in various concentrations. The lubrication mechanisms of such dispersions in mixed and EHL regimes have been investigated by means of the IRIS tribometer that allows us simultaneous contact visualization, film thickness and friction measurement under controlled contact kinematics. The lubricant film-forming capability has been determined as a function of the entrainment velocity and the nanotube content: the presence of carbon nanotubes within the contact results in a local increase in the film thickness and it can be shown that the contact acts as a filter of carbon-nanotube aggregates. Introduction of sliding results in a diminution of the number of aggregates passing through the contact. Moreover, a reduction in friction and a drift in the wear onset have been observed under controlled contact kinematics: this behaviour originates from the transient propagation of carbon-nanotube aggregates through the contact and a friction law is proposed taking into account the contact heterogeneity.