Editing Lubricant (section)

== Properties ==

A good lubricant generally possesses the following characteristics:
* A high boiling point and low freezing point (in order to stay [[liquid]] within a wide range of temperature)
* A high [[viscosity index]]
* Thermal stability
* Hydraulic stability
* Demulsibility
* Corrosion prevention
* A high resistance to [[oxidation]]
* Pour Point (the minimum temperature at which oil will flow under prescribed test conditions)

=== Formulation ===

Typically lubricants contain 90% base oil (most often [[petroleum]] fractions, called [[mineral oil]]s) and less than 10% [[Oil additive|additives]]. [[Vegetable oil]]s or synthetic liquids such as hydrogenated [[polyolefin]]s, [[ester]]s, [[silicone]]s, [[fluorocarbon]]s and many others are sometimes used as base oils. Additives deliver reduced friction and wear, increased [[viscosity]], improved viscosity index, resistance to [[corrosion]] and [[oxidation]], aging or contamination, etc.

Non-liquid lubricants include powders (dry [[graphite]], [[polytetrafluoroethylene|PTFE]], [[molybdenum disulphide]], [[tungsten disulphide]], etc.), PTFE tape used in plumbing, air cushion and others. [[Dry lubricant]]s such as graphite, molybdenum disulphide and tungsten disulphide also offer lubrication at temperatures (up to 350&nbsp;°C) higher than liquid and oil-based lubricants are able to operate. Limited interest has been shown in low friction properties of [[compacted oxide glaze layer]]s formed at several hundred degrees Celsius in metallic sliding systems; however, practical use is still many years away due to their physically unstable nature.

=== Additives ===

{{main|Oil additive}}

A large number of additives are used to impart performance characteristics to the lubricants. Modern automotive lubricants contain as many as ten additives, comprising up to 20% of the lubricant, the main families of additives are:<ref name=Fundamentals/>
* [[Pour point]] depressants are compounds that prevent crystallization of waxes. Long chain [[alkylbenzene]]s adhere to small crystallites of wax, preventing crystal growth.
* [[Anti-foaming agent]]s are typically [[silicone]] compounds which increase [[surface tension]] in order to discourage foam formation.
* [[Viscosity index improver]]s (VIIs) are compounds that allow lubricants to remain viscous at higher temperatures. Typical VIIs are [[polyacrylate]]s and [[butadiene]].
* [[Antioxidant]]s suppress the rate of oxidative degradation of the hydrocarbon molecules within the lubricant. At low temperatures, free radical inhibitors such as hindered phenols are used, e.g. [[butylated hydroxytoluene]]. At temperatures >90&nbsp;°C, where the metals [[Catalysis|catalyze]] the oxidation process, dithiophosphates are more useful. In the latter application the additives are called [[metal deactivator]]s.
* [[Detergent]]s ensure the cleanliness of engine components by preventing the formation of deposits on contact surfaces at high temperatures.
* [[Corrosion inhibitor]]s (rust inhibitors) are usually alkaline materials, such as alkylsulfonate salts, that absorb acids that would corrode metal parts.
* [[AW additive|Anti-wear]] additives form protective 'tribofilms' on metal parts, suppressing [[wear]]. They come in two classes depending on the strength with which they bind to the surface. Popular examples include [[tricresylphosphate|phosphate ester]]s and [[zinc dithiophosphate]]s.<ref>{{Cite journal|last=Spikes|first=H.|date=2004-10-01|title=The History and Mechanisms of ZDDP|journal=Tribology Letters|language=en|volume=17|issue=3|pages=469–489|doi=10.1023/B:TRIL.0000044495.26882.b5|s2cid=7163944|issn=1023-8883}}</ref> 
* [[EP additive|Extreme pressure]] (anti-scuffing) additives form protective films on sliding metal parts. These agents are often sulfur compounds, such as dithiophosphates.
* [[Friction modifier]]s reduce friction and wear, particularly in the boundary lubrication regime where surfaces come into direct contact.<ref>{{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|url=http://spiral.imperial.ac.uk/bitstream/10044/1/25879/2/post%20review%20Friction%20modifier%20additives.pdf|hdl-access=free|access-date=23 September 2019|archive-date=22 September 2017|archive-url=https://web.archive.org/web/20170922050548/http://spiral.imperial.ac.uk/bitstream/10044/1/25879/2/post%20review%20Friction%20modifier%20additives.pdf|url-status=live}}</ref> 
<!--* [[Friction modifier]]s
* Stickiness improver
* Complexing agent (in case of greases)
Note that many of the basic chemical compounds used as detergents (example: calcium sulfonate) serve the purpose of the first seven items in the list as well. Usually it is not economically or technically feasible to use a single do-it-all additive compound. Oils for [[hypoid]] gear lubrication will contain high content of EP additives. Grease lubricants may contain large amount of solid particle friction modifiers, such as graphite, molybdenum sulfide.-->

In 1999, an estimated 37,300,000 tons of lubricants were consumed worldwide.<ref>{{Cite book |doi=10.1002/14356007.a15_423|chapter=Lubricants and Lubrication|title=Ullmann's Encyclopedia of Industrial Chemistry|year=2003|last1=Bartels|first1=Thorsten|last2=Bock|first2=Wolfgang|last3=Braun|first3=Jürgen|last4=Busch|first4=Christian|last5=Buss|first5=Wolfgang|last6=Dresel|first6=Wilfried|last7=Freiler|first7=Carmen|last8=Harperscheid|first8=Manfred|last9=Heckler|first9=Rolf-Peter|last10=Hörner|first10=Dietrich|last11=Kubicki|first11=Franz|last12=Lingg|first12=Georg|last13=Losch|first13=Achim|last14=Luther|first14=Rolf|last15=Mang|first15=Theo|last16=Noll|first16=Siegfried|last17=Omeis|first17=Jürgen|isbn=978-3527306732}}</ref> Automotive applications dominate, including electric vehicles<ref>{{Cite book|last1=Beyer|first1=Monica|last2=Brown|first2=Gareth|last3=Gahagan|first3=Michael|last4=Higuchi|first4=Tomoya|last5=Hunt|first5=Gregory|last6=Huston|first6=Michael|last7=Jayne|first7=Doug|last8=McFadden|first8=Chris|last9=Newcomb|first9=Timothy|last10=Patterson|first10=Suzanne|last11=Prengaman|first11=Christopher|last12=Shamszad|first12=Mariam|date=2019-12-12|chapter=Lubricant Concepts for Electrified Vehicle Transmissions and Axles|chapter-url=https://www.jstage.jst.go.jp/article/trol/14/5/14_428/_article/-char/ja/|pages=428–437|doi=10.2474/trol.14.428|title=Tribology Online|volume=14|issue=5|s2cid=210160024|access-date=17 February 2020|archive-date=17 February 2020|archive-url=https://web.archive.org/web/20200217161625/https://www.jstage.jst.go.jp/article/trol/14/5/14_428/_article/-char/ja/|url-status=live}}</ref> but other industrial, marine, and metal working applications are also big consumers of lubricants. Although air and other gas-based lubricants are known (e.g., in [[fluid bearing]]s), liquid lubricants dominate the market, followed by solid lubricants.

Lubricants are generally composed of a majority of base [[oil]] plus a variety of additives to impart desirable characteristics. Although generally lubricants are based on one type of base oil, mixtures of the base oils also are used to meet performance requirements.

=== Mineral oil ===

The term "[[mineral oil]]" is used to refer to lubricating base oils derived from [[crude oil]]. The [[American Petroleum Institute]] (API) designates several types of lubricant base oil:<ref>{{Cite web |url=http://www.api.org/certifications/engineoil/pubs/index.cfm |title=Engine Oil Publications |access-date=30 August 2007 |archive-date=1 July 2010 |archive-url=https://web.archive.org/web/20100701174323/http://www.api.org/certifications/engineoil/pubs/index.cfm |url-status=live }}</ref>

* Group I – Saturates < 90% and/or [[sulfur]] > 0.03%, and [[Society of Automotive Engineers]] (SAE) [[viscosity index]] (VI) of 80 to 120
: Manufactured by solvent extraction, solvent or catalytic dewaxing, and hydro-finishing processes. Common Group I base oil are 150SN (solvent neutral), 500SN, and 150BS (brightstock)
* Group II – Saturates > 90% and sulfur < 0.03%, and SAE viscosity index of 80 to 120
: Manufactured by hydrocracking and solvent or catalytic dewaxing processes. Group II base oil has superior anti-oxidation properties since virtually all hydrocarbon molecules are saturated. It has water-white color.
* Group III – Saturates > 90%, sulfur < 0.03%, and SAE viscosity index over 120
: Manufactured by special processes such as isohydromerization. Can be manufactured from base oil or slax wax from dewaxing process.
* Group IV – [[Polyalphaolefins]] (PAO)
* Group V – All others not included above, such as naphthenics, polyalkylene glycols (PAG), and [[polyester]]s.

The lubricant industry commonly extends this group terminology to include:
* Group I+ with a [[viscosity index]] of 103–108
* Group II+ with a viscosity index of 113–119
* Group III+ with a viscosity index of at least 140

Can also be classified into three categories depending on the prevailing compositions:
* Paraffinic
* Naphthenic
* Aromatic

=== Synthetic oils ===
Petroleum-derived lubricant can also be produced using synthetic hydrocarbons (derived ultimately from petroleum), "[[synthetic oil]]s".

These include:
* [[Polyalpha-olefin]] (PAO)
* Synthetic [[esters]]
* [[Polyalkylene glycols]] (PAG)
* [[Phosphate ester]]s
* [[Perfluoropolyether]] (PFPE)
* [[Alkylated naphthalenes]] (AN)
* [[Silicate esters]]
* [[Ionic fluids]]
* [[Multiply alkylated cyclopentanes]] (MAC)

=== Solid lubricants ===
{{main|Dry lubricant}}

'''PTFE:''' [[polytetrafluoroethylene]] (PTFE) is typically used as a coating layer on, for example, cooking utensils to provide a non-stick surface. Its usable temperature range up to 350&nbsp;°C and chemical inertness make it a useful additive in special [[Grease (lubricant)|greases]], where it can function both as a thickener and a lubricant. Under extreme pressures, PTFE powder or solids is of little value as it is soft and flows away from the area of contact. Ceramic or metal or alloy lubricants must be used then.<ref>{{cite journal|title=Historical developments and new trends in tribological and solid lubricant coatings
|author1=Donnet, C.|author2=Erdemir, A.|journal=Surface and Coatings Technology|year=2004|volume=180-181|pages=76–84|doi=10.1016/j.surfcoat.2003.10.022}}</ref>

'''Inorganic solids:''' [[Graphite]], hexagonal [[boron nitride]], [[molybdenum disulfide]] and [[tungsten disulfide]] are examples of [[solid lubricant]]s. Some retain their lubricity to very high temperatures. The use of some such materials is sometimes restricted by their poor resistance to oxidation (e.g., molybdenum disulfide degrades above 350&nbsp;°C in air, but 1100&nbsp;°C in reducing environments.

'''Metal/alloy:''' Metal alloys, composites and pure metals can be used as grease additives or the sole constituents of sliding surfaces and bearings. [[Cadmium]] and [[gold]] are used for plating surfaces which gives them good corrosion resistance and sliding properties, [[Lead]], [[tin]], [[zinc]] alloys and various [[bronze]] alloys are used as sliding bearings, or their powder can be used to lubricate sliding surfaces alone.

=== Aqueous lubrication ===
Aqueous lubrication is of interest in a number of technological applications. Strongly hydrated [[Polymer brush|brush polymer]]s such as PEG can serve as lubricants at liquid solid interfaces.<ref>{{Cite journal| doi=10.1007/s11249-009-9549-9| title=Macrotribological Studies of Poly(L-lysine)-graft-Poly(ethylene glycol) in Aqueous Glycerol Mixtures| journal=Tribology Letters| volume=37| issue=3| pages=541–552| year=2010| last1=Nalam| first1=Prathima C.| last2=Clasohm| first2=Jarred N.| last3=Mashaghi| first3=Alireza| last4=Spencer| first4=Nicholas D.| hdl=20.500.11850/17055| s2cid=109928127| url=http://doc.rero.ch/record/314245/files/11249_2009_Article_9549.pdf| hdl-access=free| access-date=13 July 2019| archive-date=27 April 2019| archive-url=https://web.archive.org/web/20190427091427/http://doc.rero.ch/record/314245/files/11249_2009_Article_9549.pdf| url-status=live}}</ref> By continuous rapid exchange of bound water with other free water molecules, these polymer films keep the surfaces separated while maintaining a high fluidity at the brush–brush interface at high compressions, thus leading to a very low coefficient of friction.

=== Biolubricant ===
Biolubricants<ref>{{Cite journal |last1=Duan |first1=Zhenjing |last2=Li |first2=Changhe |last3=Zhang |first3=Yanbin |last4=Yang |first4=Min |last5=Gao |first5=Teng |last6=Liu |first6=Xin |last7=Li |first7=Runze |last8=Said |first8=Zafar |last9=Debnath |first9=Sujan |last10=Sharma |first10=Shubham |date=2023-02-20 |title=Mechanical behavior and semiempirical force model of aerospace aluminum alloy milling using nano biological lubricant |url=https://doi.org/10.1007/s11465-022-0720-4 |journal=Frontiers of Mechanical Engineering |language=en |volume=18 |issue=1 |pages=4 |doi=10.1007/s11465-022-0720-4 |bibcode=2023FrME...18....4D |issn=2095-0241}}</ref> are derived from vegetable oils and other renewable sources. They usually are [[triglyceride]] esters (fats obtained from plants and animals). For lubricant base oil use, the vegetable derived materials are preferred. Common ones include high oleic [[canola oil]], [[castor oil]], [[palm oil]], [[sunflower seed oil]] and [[rapeseed oil]] from vegetable, and [[tall oil]] from tree sources. Many vegetable oils are often hydrolyzed to yield the acids which are subsequently combined selectively to form specialist synthetic esters. Other naturally derived lubricants include [[lanolin]] (wool grease, a natural water repellent).<ref>{{cite journal|title=Biolubricants: raw materials, chemical modifications and environmental benefits|author1=Salimon, Jumat|author2=Salih, Nadia|author3=Yousif, Emad
|journal=European Journal of Lipid Science and Technology|year=2010|volume=112|issue=5|pages=519–530|doi=10.1002/ejlt.200900205}}</ref>

[[Whale oil]] was a historically important lubricant, with some uses up to the latter part of the 20th century as a friction modifier [[oil additive|additive]] for [[automatic transmission fluid]].<ref>{{Cite book |url=https://books.google.com/books?id=buwQ8a2RCUcC&q=whale+oil+dexron&pg=PA20 |page=20|title=The Turbo Hydra-Matic 350 Handbook|isbn=9780895860514|last1=Sessions|first1=Ron|year=1985|publisher=Penguin }}</ref>

In 2008, the biolubricant market was around 1% of UK lubricant sales in a total lubricant market of 840,000 tonnes/year.<ref>[[National Non-Food Crops Centre]]. [http://www.nnfcc.co.uk/publications/nnfcc-conference-poster-improved-winter-rape-varieties-for-biolubricants NNFCC Conference Poster. Improved winter rape varieties for biolubricants] {{Webarchive|url=https://web.archive.org/web/20150204193558/http://www.nnfcc.co.uk/publications/nnfcc-conference-poster-improved-winter-rape-varieties-for-biolubricants |date=4 February 2015 }}</ref>

{{as of|2020}}, researchers at Australia's [[CSIRO]] have been studying [[safflower]] oil as an engine lubricant, finding superior performance and lower emissions than [[petroleum]]-based lubricants in applications such as [[engine]]-driven [[lawn mower]]s, [[chainsaw]]s and other agricultural equipment. [[Grain]]-growers trialling the product have welcomed the innovation, with one describing it as needing very little refining, [[biodegradable]], a [[bioenergy]] and [[biofuel]]. The scientists have reengineered the plant using [[gene silencing]], creating a variety that produces up to 93% of oil, the highest currently available from any plant. Researchers at [[Montana State University]]’s Advanced Fuel Centre in the US studying the oil’s performance in a large [[diesel engine]], comparing it with conventional oil, have described the results as a "game-changer".<ref>{{cite web | title=Safflower oil hailed by scientists as possible recyclable, biodegradable replacement for petroleum | website=ABC News | series=Landline | publisher=Australian Broadcasting Corporation | first=Tim | last=Lee | date=7 June 2020 | url=https://www.abc.net.au/news/2020-06-07/safflower-oil-new-biofuel-to-replace-petroleum/12321028 | access-date=7 June 2020 | archive-date=7 June 2020 | archive-url=https://web.archive.org/web/20200607012058/https://www.abc.net.au/news/2020-06-07/safflower-oil-new-biofuel-to-replace-petroleum/12321028 | url-status=live }}</ref>

=== Greases ===
Greases are a solid or semi-solid lubricant produced by blending thickening agents within a liquid lubricant. Greases are typically composed of about 80% lubricating oil, around 5% to 10% thickener, and approximately 10% to 15% additives. In most common greases, the thickener is a light or alkali metal soap, forming a sponge-like structure that encapsulates the oil droplets. Beyond lubrication, greases are generally expected to provide corrosion protection, typically achieved through additives. To prevent drying out at higher temperatures, dry lubricants are also added. By selecting appropriate oils, thickeners, and additives, the properties of greases can be optimized for a wide range of applications. There are greases suited for high or extremely low temperatures, vacuum applications, water-resistant and weatherproof greases, highly pressure-resistant or creeping types, food-grade, or exceptionally adhesive greases.<ref>{{Cite web |last=CarArco |date=2024-10-18 |title=Graisses lubrifiantes : Normes internationales |url=https://wodoil.ch/graisses-lubrifiantes-normes-internationales/ |access-date=2024-10-30 |website=Wodoil |language=de-DE}}</ref>