Lubrication is the process of interposing a substance called lubricant between two moving surfaces relative to each another in order to reduce wear of one or both in close proximity in order to allow the surfaces to carry the imposed load and the resulting pressure generated between the opposing surfaces. The interposed lubricant film can be a solid such as graphite, MoS2 or solid/liquid dispersion, a liquid or liquid-liquid dispersion or exceptionally a gas.

Lubricants

Typically lubricants contain 90% base oil such as petroleum fractions called mineral oils and less than 10% additives. Vegetable oils or synthetic liquids such as hydrogenated polyolefins, esters, silicones, fluorocarbons and many others are sometimes used as base oils. Additives act to reduced friction and wear. The most common lubricants form a physical barrier of thin layer of lubricant to separate the moving parts. The tribological performance of common lubricants can be altered by adding small amounts of nanoparticles which provide reduced wear and low friction. Modification of conventional lubricants by adding various additives for different situations is discussed below. But one of the main difficulties of using nanoparticles as additives is their dispersion or dissolution in lubricant oils, typically of hydrocarbon nature. With the surface modification of nanoparticles through long chain high molecular weight hydrocarbons, stable dispersions in lubricant oils become feasible. Nano-lubes may reduce the frequency of oil changes and increase the value proposition of condition-based maintenance practices. Numerous studies have shown that different nanoparticles, impregnated into metal, polymer, ceramic and other coatings, can provide these materials with enhanced tribological performance.
Cu nanoparticles

Lubrication is a key issue in diamond turning of reaction-bonded SiC hard materials. The type and concentration of dispersed nanoparticles significantly affected lubricating performance. Grease containing 10% Cu nanoparticles produced the highest surface quality and the lowest tool wear. Lubrication is because nanoparticle-induced solid lubricating film is formed at the tool-workpiece interface. Also copper nanopowder as an additive in SAE 30 motor oil effectively reduces friction at high loads and high sliding speeds.

Pd nanoparticles

Using surface-modified Pd nanoparticles with tetraalkylammonium chains, stable dispersions in lubricant oils become feasible with excellent tribological properties. The use of these nanoparticles decreases the electrical resistance of the contact up to 99% in comparison with the base oil alone. This outstanding performance is attributed to a combination of factors as metallic character of palladium, nanometric size, and replenishment of Pd nanoparticles onto the contact forming a transfer layer.

Function of nano-lubes

In severe conditions, ordinary lubes can be squeezed out from between contact areas. Adding nanoparticles to the oil can reduce friction and wear rates, and increase load-bearing capacity. Nanoparticles also can be impregnated into polymer or metal coatings to provide antifriction properties, and into porous metal parts to make self-lubricating components. Impregnating does not sacrifice the mechanical properties of the base part. Maintenance-free solid lubricants are particularly suited to ultra-clean environments.

Nanotechnology-based extreme-pressure and anti-wear additives are found to have high chemical and physical stability, even under extreme conditions leading to longer equipment operation life, increased machine performance efficiency, extended maintenance intervals, reduced noise, heat and vibration, reduced energy consumption and decreased air pollution. This is due to improved coverage even on rough surfaces as the nanoscale particles infiltrate even the tiniest spaces between contacting surfaces.

Fullerenes

C60 and C70 fullerenes have a good tribological properties. the nearly spherical fullerenes might behave as nanoscale ball bearings by entering into crevices or valleys and separate the asperities of the mating metal surfaces. The addition of small amounts of quasi-spherical WS2 or MoS2 nanoparticles with fullerene-like structure to various kinds of lubricant greatly improves their tribol-ogical characteristics. Nanoparticles are capable of withstanding a severe hydrostatic pressure, caused by compression. The inner layers of these nanoparticles seem to remain intact. The broken outer layers are expected to be transferred to friction surfaces, providing superior tribological properties of rubbed surfaces.

Carbon onion

Japanese researchers formed carbon onion layer compounded with gold by dispersing carbon onions on a silicon wafer coated with gold. The carbon onion layer compounded with gold has kept lower-friction coefficient for a longer time than gold layer in a certain range of gold film thicknesses and normal forces. in addition, carbon onion layer on a self-assembled monolayer exhibited the low-friction property under a wide range of normal forces.

Molybdenum-Sulfur compounds

Molybdenum disulfide is used as a dry lubricant in, e.g. greases, dispersions, friction materials and bonded coatings. Molybdenum-sulfur complexes may be used in suspension but more commonly dissolved in lubricating oils at concentrations of a few percent. Molybdenum disulfide, MoS2, the most common natural form of molybdenum, is extracted from the ore and then purified for direct use in lubrication. Since molybdenum disulfide is of geothermal origin, it has the durability to withstand heat and pressure. This is particularly so if small amounts of sulfur are available to react with iron and provide a sulfide layer which is compatible with MoS2 in maintaining the lubricating film.

A combination of molybdate and water soluble sulfides can provide both lubrication and corrosion inhibition in cutting fluids and metal forming materials. Oil soluble molybdenum-sulfur compounds, such as thiophosphates and thiocarbamates, provide protection against wear, oxidation and corrosion.
Other nanoparticles

TiO2 nanoparticles as lubricant additives can be dispersed in organic solvent in liquid paraffin. TiO2 nanoparticles have excellent load-carrying capacity, good extreme pressure and friction reducing properties.

Graphite nanosheets and WS2 nanorods, which form a physical deposition film on the rubbing surface whenadded to liquid paraffin provide excellent AW and frictionreducing properties.

CeF3 nanoparticles are spherical and cylindrical have excellent extreme-pressure and friction-reducing properties whenadded to liquid paraffin.

NiMoO2S2 nanoparticles have very favorable anti-frictional properties both as an additive in lubricating oils at room temperature and as a solid lubricant at high.

Commercial products

ApNano Materials manufactures a nanoparticle-based solid lubricant and additive that’s depicted as the rolling of billions of miniature ball bearings. The company has already launched NanoLub, the world's first commercial solid lubricant based on spherical inorganic nanoparticles. ApNano claims that NanoLub reduces friction and wear significantly better than conventional lubricants especially at high loads, prolongs device service life, and lowers maintenance costs and downtimes.