Nanoparticle-doped Lubricants: Potential of Inorganic Fullerene-like (IF-) Molybdenum Disulfide for Automotive Applications
The growing environmental concerns, along with the continuous increase in the price of fossil fuels, have highly motivated car manufacturers worldwide to improve the efficiency of their vehicles. The tribological properties of engine and gearbox lubricants have a significant impact on the global efficiency of vehicles, as they contribute to reducing friction in many contacts and allow the downsizing of various components by providing their surfaces with anti-wear protection. The recent breakthroughs in nanoparticle synthesis have opened new prospects in terms of lubricant additivation, with the discovery of the excellent friction and wear reducing properties of nanoparticles such as Inorganic Fullerene-like (IF-) molybdenum or tungsten disulfides. The tribological potential of IF-MoS2 for automobile applications was investigated in this work. The respective influences of nanoparticle size and structure were first of all studied, revealing that poorly crystalline nanoparticles were more efficient in maintaining low-friction tribofilms on steel substrates in severe boundary lubrication regimes regardless of size (for the range studied). All the nanoparticles tested however showed similar performances when proper oil recirculation was ensured, providing a continuous feeding of the contact in nanoparticles. The IF-MoS2 nanoparticles lost their lubricating abilities when added to fully-formulated lubricants. This behavior was attributed to the presence of dispersants in the oil, which dispersed the nanoparticles effectively but prevented them from forming tribofilms on the rubbing surfaces. The well-dispersed IF-MoS2 were shown to enter the contact and exfoliate, but an excessive adsorption of the dispersants on the released MoS2 platelets and/or the steel surfaces is thought to prevent tribofilm adhesion. A balance between nanoparticle dispersion and tribological performance was then found, by using very low concentrations of dispersants. The behavior of nanoparticle-doped oils in various scenarios related to automobile applications was finally explored. The IF-MoS2 provided significant friction and wear reduction at ambient temperature and in milder rolling/sliding test conditions, for smooth and rough surfaces. The risks related to the presence of nanoparticles in the oil in full-film lubrication regimes were partially lifted, with no significant influence on friction witnessed for all the test conditions considered. The ability of IF-MoS2 nanoparticles to protect steel surfaces from surface-initiated Rolling Contact Fatigue was finally shown.