Friction modifier

{{short description|Lubricant additive to reduce friction and wear}}

Friction modifiers are added to lubricants in order to reduce friction and wear in machine components. They are particularly important in the boundary lubrication regime, where they can prevent solid surfaces from coming into direct contact, substantially reducing friction and wear.

Several classes of friction modifier additives exist, the main examples being organic friction modifiers (OFMs), oil-soluble organo-molybdenum additives, functionalized polymers, and dispersed nanoparticles.{{Cite journal|last=Spikes|first=Hugh|date=2015-10-01|title=Friction Modifier Additives|journal=Tribology Letters|language=en|volume=60|issue=1|pages=5|doi=10.1007/s11249-015-0589-z|issn=1023-8883|hdl=10044/1/25879|s2cid=137884697|hdl-access=free}}

• OFMs are amphiphilic surfactants, such as fatty acids, often derived from fats and vegetable oils. OFMs are important additives in modern engine oils and are also employed in fuels. They adsorb on metal surfaces and self-assemble to form incompressible monolayers which prevent asperity contact and reduce friction and wear.{{Cite journal|last1=Hardy|first1=W. B.|last2=Doubleday|first2=Ida|date=1922-03-01|title=Boundary Lubrication. The Paraffin Series|journal=Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences|language=en|volume=100|issue=707|pages=550–574|doi=10.1098/rspa.1922.0017|bibcode=1922RSPSA.100..550H|issn=1364-5021|doi-access=free}}
• Organo-molybdenum compounds, were initially developed as antiwear additives but were later recognized to be very effective in reducing boundary friction.{{Cite journal|last1=Braithwaite|first1=E. R.|last2=Greene|first2=A. B.|date=1978-02-01|title=A critical analysis of the performance of molybdenum compounds in motor vehicles|journal=Wear|volume=46|issue=2|pages=405–432|doi=10.1016/0043-1648(78)90044-3}} 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.{{Cite journal|last1=Grossiord|first1=C|last2=Varlot|first2=K|last3=Martin|first3=J. -M|last4=Le Mogne|first4=Th|last5=Esnouf|first5=C|last6=Inoue|first6=K|date=1998-12-01|title=MoS2 single sheet lubrication by molybdenum dithiocarbamate|url=http://www.sciencedirect.com/science/article/pii/S0301679X98000942|journal=Tribology International|language=en|volume=31|issue=12|pages=737–743|doi=10.1016/S0301-679X(98)00094-2|issn=0301-679X}}
• Functionalized polymers, which can be tailored to adsorb specifically on polar surfaces, have been shown to markedly reduce friction and wear.{{Cite journal|last1=Guangteng|first1=G|last2=Smeeth|first2=M|last3=Cann|first3=P M|last4=Spikes|first4=H A|date=1996-03-01|title=Measurement and Modelling of Boundary Film Properties of Polymeric Lubricant Additives|journal=Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology|language=en|volume=210|issue=1|pages=1–15|doi=10.1243/PIME_PROC_1996_210_473_02|s2cid=136658009|issn=1350-6501}}
• Dispersed nanoparticles have been shown to reduce boundary friction,{{Cite journal|last1=Dai|first1=Wei|last2=Kheireddin|first2=Bassem|last3=Gao|first3=Hong|last4=Liang|first4=Hong|author4-link=Hong Liang|date=2016-10-01|title=Roles of nanoparticles in oil lubrication|url=http://www.sciencedirect.com/science/article/pii/S0301679X16301116|journal=Tribology International|language=en|volume=102|pages=88–98|doi=10.1016/j.triboint.2016.05.020|issn=0301-679X}} 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 emissions.{{Cite journal|last1=Holmberg|first1=Kenneth|last2=Andersson|first2=Peter|last3=Erdemir|first3=Ali|date=2012-03-01|title=Global energy consumption due to friction in passenger cars|journal=Tribology International|volume=47|issue=Supplement C|pages=221–234|doi=10.1016/j.triboint.2011.11.022}} One approach has been to progressively reduce lubricant viscosity 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 experiments{{Cite journal|last1=Campen|first1=Sophie|last2=Green|first2=Jonathan|last3=Lamb|first3=Gordon|last4=Atkinson|first4=David|last5=Spikes|first5=Hugh|date=2012-11-01|title=On the Increase in Boundary Friction with Sliding Speed|journal=Tribology Letters|language=en|volume=48|issue=2|pages=237–248|doi=10.1007/s11249-012-0019-4|s2cid=135749402|issn=1023-8883}} and molecular dynamics simulations{{Cite journal|last1=Ewen|first1=James P.|last2=Gattinoni|first2=Chiara|last3=Morgan|first3=Neal|last4=Spikes|first4=Hugh A.|last5=Dini|first5=Daniele|date=2016-05-10|title=Nonequilibrium Molecular Dynamics Simulations of Organic Friction Modifiers Adsorbed on Iron Oxide Surfaces|journal=Langmuir|volume=32|issue=18|pages=4450–4463|doi=10.1021/acs.langmuir.6b00586|pmid=27064962|issn=0743-7463|doi-access=free|hdl=10044/1/30875|hdl-access=free}} have given new insights into their behaviour under boundary lubrication conditions.