Friction modifiers are added to

lubricants A lubricant (sometimes shortened to lube) is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, t ...

in order to reduce

friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...

and

wear Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical (e.g., erosion) or chemical (e.g., corrosion). The study of wear and related processes is referred to as tribology. Wear in m ...

in machine components. They are particularly important in the boundary

lubrication Lubrication is the process or technique of using a lubricant to reduce friction and wear and tear in a contact between two surfaces. The study of lubrication is a discipline in the field of tribology. Lubrication mechanisms such as fluid-lubric ...

regime, where they can prevent solid surfaces from coming into direct contact, substantially reducing

and

. Several classes of friction modifier additives exist, the main examples being organic friction modifiers (OFMs), oil-soluble organo-

molybdenum Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...

additives, functionalized

polymers A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic an ...

, and dispersed

nanoparticles A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 1 ...

. * OFMs are

amphiphilic An amphiphile (from the Greek αμφις amphis, both, and φιλíα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compoun ...

surfactants Surfactants are chemical compounds that decrease the surface tension between two liquids, between a gas and a liquid, or interfacial tension between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsion#Emulsifiers , ...

, such as

fatty acids In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, f ...

, often derived from fats and

vegetable oils Vegetable oils, or vegetable fats, are oils extracted from seeds or from other parts of fruits. Like animal fats, vegetable fats are ''mixtures'' of triglycerides. Soybean oil, grape seed oil, and cocoa butter are examples of seed oils, or fat ...

. OFMs are important additives in modern engine oils and are also employed in

fuels A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy but ...

. They

adsorb Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a fl ...

on metal surfaces and self-assemble to form incompressible monolayers which prevent asperity contact and reduce

and

. * Organo-

compounds, were initially developed as antiwear additives but were later recognized to be very effective in reducing boundary friction. They are currently used in many engine oils and, more recently, in gear oils. They reduce friction by forming two-dimensional molybdenum disulphide layers on rubbing surfaces. * Functionalized

, which can be tailored to adsorb specifically on polar surfaces, have been shown to markedly reduce

and

. * Dispersed

have been shown to reduce boundary

, but they have not yet found widespread employment in industrial applications. Reduction of frictional losses and through more efficient lubrication is a key target in order to reduce

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

emissions. One approach has been to progressively reduce

lubricant A lubricant (sometimes shortened to lube) is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, t ...

viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

to minimize hydrodynamic shear, churning and pumping losses. However, this means that an increased number of components operate under boundary lubrication conditions. This has led to a resurgence in interest in friction modifier additives, particularly OFMs. For example, recent

tribology Tribology is the science and engineering of interacting surfaces in relative Motion (physics), motion. It includes the study and application of the principles of friction, lubrication and wear. Tribology is highly interdisciplinary, drawing on m ...

experiments and

molecular dynamics Molecular dynamics (MD) is a computer simulation method for analyzing the physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a fixed period of time, giving a view of the dynamic "evolution" of the ...

simulations have given new insights into their behaviour under boundary lubrication conditions.

References

Lubricants Oil additives

See also

References