Lubrication is the process of reduce wear and tear of mating materials by interposing a lubricant between two surfaces moving relative to each another. The principle is that lubricants form a physical barrier of thin layer to separate the moving parts.
Lamellar materials of metal disulphide type such as MoS2, WS2, NbS2 etc. are widely used either as solid lubricant for space applications or as additives dispersed in a lubricating base as they have excellent lubricating properties. The most efficient anti-friction additives currently used in the field of the automotive lubrication is the molybdenum dithiocarbamate (MoDtc), in solution in oil, but has problems of toxicity and pollution such as sulphur gas emissions produced by MoDtc and are extremely harmful for the environment. But the tribological performance of common lubricants can be altered by adding small amounts of nanoparticles which provide reduced wear and low friction.
Nano lubricants
Nanostructures such as nanotubes, onions, fullerenes are used as additive of lubrication in a lubricating base due to their spectacular tribological performance and hence serve as a substitute to conventional additives. The nanomaterial such as carbon nanotubes, carbon onions, inorganic fullerenes of MoS2, WS2 offer a strong reduction of the friction coefficient from 50 to 70%, according to the type of nanomaterial, the concentration in particles and the contact pressure compared to those without addition of particles. The reduction of friction of the MS2 type fullerenes are due to progressive delamination, release of MS2 layers in the contact zone and layers forming a tribofilm on the rubbing surfaces.
Examples of nano lubricant additives
Grease containing 10% Cu nanoparticles produced the highest surface quality and the lowest tool wear. NiMoO2S2 nanoparticles and also copper nanopowder as an additive in SAE 30 motor oil effectively reduces friction at high loads and high sliding speeds.
Surface-modified Pd nanoparticles offer stable dispersions in lubricant oils with excellent tribological properties.
Nanoparticle impregnated polymer or metal coatings provide antifriction properties in porous metal parts and used to make self-lubricating components.
C60 and C70 fullerenes have good tribological properties. Spherical fullerenes function as nanoscale ball bearing with the addition of small amounts of quasi-spherical WS2 or MoS2 nanoparticles.
Carbon onion layer compounded with gold dispersion exhibit a low-friction property.
Spherical and cylindrical CeF3 and TiO2 nanoparticles as lubricant additives dispersed in organic solvent in liquid paraffin have excellent load-carrying capacity, good extreme pressure and friction reducing properties.
Graphite nanosheets and WS2 nanorods added to liquid paraffin provide excellent friction reducing properties.