Dry lubricant

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Dry lubricants or solid lubricants are materials that, despite being in the solid phase, are able to reduce friction between two surfaces sliding against each other without the need for a liquid oil medium. [1]

Contents

The two main dry lubricants are graphite and molybdenum disulfide. They offer lubrication at temperatures higher than liquid and oil-based lubricants operate. Dry lubricants are often used in applications such as locks or dry lubricated bearings. Such materials can operate up to 350 °C (662 °F) in oxidizing environments and even higher in reducing / non-oxidizing environments (molybdenum disulfide up to 1100 °C, 2012 °F). The low-friction characteristics of most dry lubricants are attributed to a layered structure on the molecular level with weak bonding between layers. Such layers are able to slide relative to each other with minimal applied force, thus giving them their low friction properties.

However, a layered crystal structure alone is not necessarily sufficient for lubrication. In fact, there are some solids with non-lamellar structures that function well as dry lubricants in some applications. These include certain soft metals (indium, lead, silver, tin), polytetrafluroethylene, some solid oxides, rare-earth fluorides, and even diamond. [2]

Limited interest has been shown in low friction properties of compacted oxide glaze layers formed at several hundred degrees Celsius in metallic sliding systems. However, practical use is still many years away due to their physically unstable nature.

The four most commonly used solid lubricants are:

  1. Graphite. Used in air compressors, food industry, railway track joints, brass instrument valves, piano actions, open gear, ball bearings, machine-shop works, etc. It is also very common for lubricating locks, since a liquid lubricant allows particles to get stuck in the lock worsening the problem. It is often used to lubricate the internal moving parts of firearms in sandy environments.
  2. Molybdenum disulfide (MoS2). Used in CV joints and space vehicles. [3] Does lubricate in vacuum.
  3. Hexagonal boron nitride. Used in space vehicles. Also called "white graphite."
  4. Tungsten disulfide. Similar usage as molybdenum disulfide, but due to the high cost only found in some dry lubricated bearings.

Graphite and molybdenum disulfide are the predominant materials used as dry lubricants.

Structure-function relationship

The lubricity of many solids is attributable to a lamellar structure. The lamellae orient parallel to the surface in the direction of motion and slide easily over each other resulting in low friction and preventing contact between sliding components even under high loads. Large particles perform best on rough surfaces at low speed, finer particles on smoother surfaces and at higher speeds. These materials may be added in the form of dry powder to liquid lubricants to modify or enhance their properties.

Other components that are useful solid lubricants include boron nitride, polytetrafluorethylene (PTFE), talc, calcium fluoride, cerium fluoride, and tungsten disulfide.

Applications

Solid lubricants are useful for conditions when conventional lubricants are inadequate, such as:

Graphite

Graphite is structurally composed of planes of polycyclic carbon atoms that are hexagonal in orientation. The distance of carbon atoms between planes is longer and, therefore, the bonding is weaker.

Graphite is best suited for lubrication in air. Water vapor is a necessary component for graphite lubrication. The adsorption of water reduces the bonding energy between the hexagonal planes of the graphite to a lower level than the adhesion energy between a substrate and the graphite. Because water vapor is a requirement for lubrication, graphite is not effective in vacuum. [4] Because it is electrically conductive, graphite can promote galvanic corrosion. In an oxidative atmosphere, graphite is effective at high temperatures up to 450 °C continuously and can withstand much higher temperature peaks.

Graphite is characterized by two main groups: natural and synthetic.

For applications where only a minor lubricity is needed and a more thermally insulating coating is required, then amorphous graphite would be chosen (80% carbon).

Molybdenum disulfide

MoS2 is mined from some sulfide-rich deposits and refined to achieve a purity suitable for lubricants. Like graphite, MoS2 has a hexagonal crystal structure with the intrinsic property of easy shear. MoS2 lubrication performance often exceeds that of graphite and is effective in vacuum as well, whereas graphite is not. The temperature limitation of MoS2 at 400 °C is restricted by oxidation. Particle size and film thickness are important parameters that should be matched to the surface roughness of the substrate. Large particles may result in excessive wear by abrasion caused by impurities in the MoS2, and small particles may result in accelerated oxidation.

Boron nitride

Hexagonal boron nitride is a ceramic powder lubricant. The most interesting lubricant feature is its high temperature resistance of 1200 °C service temperature in an oxidizing atmosphere. Furthermore, boron nitride has a high thermal conductivity. (Cubic boron nitride is very hard and used as an abrasive and cutting tool component.)

Polytetrafluorethylene

Polytetrafluorethylene (PTFE) is widely used as an additive in lubricating oils and greases. Due to the low surface energy of PTFE, stable unflocculated dispersions of PTFE in oil or water can be produced. Contrary to the other solid lubricants discussed, PTFE does not have a layered structure. The macro molecules of PTFE slip easily along each other, similar to lamellar structures. PTFE shows one of the smallest coefficients of static and dynamic friction, down to 0.04. Operating temperatures are limited to about 260 °C.

Application methods

Spraying/dipping/brushing

Dispersion of solid lubricant as an additive in oil, water, or grease is most commonly used. For parts that are inaccessible for lubrication after assembly, a dry film lubricant can be sprayed. After the solvent evaporates, the coating cures at room temperature to form a solid lubricant. Pastes are grease-like lubricants containing a high percentage of solid lubricants used for assembly and lubrication of highly loaded, slow-moving parts. Black pastes generally contain MoS2. For high temperatures above 500 °C, pastes are composed on the basis of metal powders to protect metal parts from oxidation necessary to facilitate disassembly of threaded connections and other assemblies.[ citation needed ]

Free powders

Dry-powder tumbling is an effective application method. The bonding can be improved by prior phosphating of the substrate. Use of free powders has its limitations, since adhesion of the solid particles to the substrate is usually insufficient to provide any service life in continuous applications. However, to improve running-in conditions or in metal-forming processes, a short duration of the improved slide conditions may suffice.[ citation needed ]

Anti-friction coatings

Anti-friction (AF) coatings are "lubricating paints" consisting of fine particles of lubricating pigments, such as molydisulfide, PTFE or graphite, blended with a binder. After application and proper curing, these "slippery" or dry lubricants bond to the metal surface and form a dark gray solid film. Many dry film lubricants contain special rust inhibitors which offer exceptional corrosion protection. Most long-wearing films are of the bonded type but are still restricted to applications where sliding distances are not too long. AF coatings are applied where fretting and galling is a problem (such as splines, universal joints and keyed bearings), where operating pressures exceed the load-bearing capacities of ordinary oils and greases, where smooth running in is desired (piston, camshaft), where clean operation is desired (AF coatings will not collect dirt and debris like greases and oils), and where parts may be stored for long periods. [5]

Composites

Self-lubricating composites: Solid lubricants such as PTFE, graphite, MoS2 and some other anti-friction and anti-wear additives are often compounded in polymers and all kind of sintered materials. MoS2, for example, is compounded in materials for sleeve bearings, elastomer O-rings, carbon brushes, etc. Solid lubricants are compounded in plastics to form a "self-lubricating" or "internally lubricated" thermoplastic composite. For example, PTFE particles compounded in the plastic form a PTFE film over the mating surface, resulting in a reduction of friction and wear. MoS2 compounded in nylon reduces wear, friction and stick-slip. Furthermore, it acts as a nucleating agent effecting in a very fine crystalline structure. The primary use of graphite lubricated thermoplastics is in applications operating in aqueous environments.[ citation needed ]

Related Research Articles

<span class="mw-page-title-main">Boron nitride</span> Refractory compound of boron and nitrogen with formula BN

Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexagonal form corresponding to graphite is the most stable and soft among BN polymorphs, and is therefore used as a lubricant and an additive to cosmetic products. The cubic variety analogous to diamond is called c-BN; it is softer than diamond, but its thermal and chemical stability is superior. The rare wurtzite BN modification is similar to lonsdaleite but slightly softer than the cubic form.

A lubricant 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, transporting foreign particles, or heating or cooling the surfaces. The property of reducing friction is known as lubricity.

<span class="mw-page-title-main">Polytetrafluoroethylene</span> Synthetic polymer

Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene and is a PFAS that has numerous applications. The commonly known brand name of PTFE-based composition is Teflon by Chemours, a spin-off from DuPont, which originally discovered the compound in 1938.

<span class="mw-page-title-main">Motor oil</span> Lubricant used for lubrication of internal combustion engines

Motor oil, engine oil, or engine lubricant is any one of various substances used for the lubrication of internal combustion engines. They typically consist of base oils enhanced with various additives, particularly antiwear additives, detergents, dispersants, and, for multi-grade oils, viscosity index improvers. The main function of motor oil is to reduce friction and wear on moving parts and to clean the engine from sludge and varnish (detergents). It also neutralizes acids that originate from fuel and from oxidation of the lubricant (detergents), improves the sealing of piston rings, and cools the engine by carrying heat away from moving parts.

<span class="mw-page-title-main">Molybdenum disulfide</span> Chemical compound

Molybdenum disulfide is an inorganic compound composed of molybdenum and sulfur. Its chemical formula is MoS
2
.

<span class="mw-page-title-main">Plain bearing</span> Simplest type of bearing, comprising just a bearing surface and no rolling elements

A plain bearing, or more commonly sliding contact bearing and slide bearing, is the simplest type of bearing, comprising just a bearing surface and no rolling elements. Therefore, the journal slides over the bearing surface. The simplest example of a plain bearing is a shaft rotating in a hole. A simple linear bearing can be a pair of flat surfaces designed to allow motion; e.g., a drawer and the slides it rests on or the ways on the bed of a lathe.

<span class="mw-page-title-main">Galling</span> Form of wear caused by adhesion between sliding surfaces

Galling is a form of wear caused by adhesion between sliding surfaces. When a material galls, some of it is pulled with the contacting surface, especially if there is a large amount of force compressing the surfaces together. Galling is caused by a combination of friction and adhesion between the surfaces, followed by slipping and tearing of crystal structure beneath the surface. This will generally leave some material stuck or even friction welded to the adjacent surface, whereas the galled material may appear gouged with balled-up or torn lumps of material stuck to its surface.

Grease is a solid or semisolid lubricant formed as a dispersion of thickening agents in a liquid lubricant. Grease generally consists of a soap emulsified with mineral or vegetable oil.

Fretting refers to wear and sometimes corrosion damage of loaded surfaces in contact while they encounter small oscillatory movements tangential to the surface. Fretting is caused by adhesion of contact surface asperities, which are subsequently broken again by the small movement. This breaking causes wear debris to be formed.

<span class="mw-page-title-main">Extreme pressure additive</span>

Extreme pressure additives, or EP additives, are additives for lubricants with a role to decrease wear of the parts of the gears exposed to very high pressures. They are also added to cutting fluids for machining of metals.

<span class="mw-page-title-main">Vespel</span>

Vespel is the trademark of a range of durable high-performance polyimide-based plastics made by DuPont.

Solid film lubricants are paint-like coatings of very fine particles of lubricating pigment blended with a binder and other additives. The lubricant is applied to a substrate by spray, dip or brush methods and, once cured, creates a solid film which repels water, reduces friction and increases the wear life of the substrate to which it has been applied. Certain film lubricants also offer additional properties such as corrosion inhibition. Solid film lubricants are used in the automotive, transportation and aerospace industries. 2 commonly used ones are graphite and molybdenum disulfide.

ApNano Materials is a nanotechnology company, wholly owned and operated by Nanotech Industrial Solutions (NIS) with R&D lab, manufacturing, blending and packaging facilities in Avenel, New Jersey, United States, and Yavne, Israel. NIS is the only company in the world with an exclusive license to manufacture inorganic fullerene-like tungsten disulfide (IF-WS2) submicron (nanosized) spherical particles on a commercial scale with the patent from the Weizmann Institute. These inorganic fullerene-like tungsten disulfide-based nanomaterials opened up new possibilities for developing extreme performance industrial lubricants, coatings, and polymer composites.

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.

Oil additives are chemical compounds that improve the lubricant performance of base oil. The manufacturer of many different oils can utilize the same base stock for each formulation and can choose different additives for each specific application. Additives comprise up to 5% by weight of some oils.

Chameleon coating, also known as nano composite tribological coating, is an adaptive adhesive that uses nanotechnology to adjust to environmental fluctuations to make living conditions more suitable to the object that the coat has been applied to.

Space tribology is a discipline in the field of tribology which deals with tribological systems for spacecraft applications. Research in the field aims to design reliable tribological systems that can withstand the harsh environment of space.

<span class="mw-page-title-main">Materials for use in vacuum</span>

Materials for use in vacuum are materials that show very low rates of outgassing in vacuum and, where applicable, are tolerant to bake-out temperatures. The requirements grow increasingly stringent with the desired degree of vacuum to be achieved in the vacuum chamber. The materials can produce gas by several mechanisms. Molecules of gases and water can be adsorbed on the material surface. Materials may sublimate in vacuum. Or the gases can be released from porous materials or from cracks and crevices. Traces of lubricants, residues from machining, can be present on the surfaces. A specific risk is outgassing of solvents absorbed in plastics after cleaning.

A composite bearing is a bearing made from a combination of materials such as a resin reinforced with fibre and this may also include friction reducing lubricants and ingredients.

Extreme tribology refers to tribological situations under extreme operating conditions which can be related to high loads and/or temperatures, or severe environments. Also, they may be related to high transitory contact conditions, or to situations with near-impossible monitoring and maintenance opportunities. In general, extreme conditions can typically be categorized as involving abnormally high or excessive exposure to e.g. cold, heat, pressure, vacuum, voltage, corrosive chemicals, vibration, or dust. The extreme conditions should include any device or system requiring a lubricant operating under any of the following conditions:

References

  1. Thorsten Bartels et al. "Lubricants and Lubrication" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Weinheim. doi : 10.1002/14356007.a15_423
  2. Scharf, T. W.; Prasad, S. V. (2013-01-01). "Solid lubricants: a review". Journal of Materials Science. 48 (2): 511–531. Bibcode:2013JMatS..48..511S. doi:10.1007/s10853-012-7038-2. ISSN   1573-4803. S2CID   135902634.
  3. harshvs (2016-10-28). "Space tribology – An overview of application of tribology in space..." tribonet. Archived from the original on 2016-12-03. Retrieved 2016-12-02.
  4. "SKF DryLube bearings". SKF dry lubricated bearings. SKF. Retrieved 2 December 2011.[ permanent dead link ]
  5. "Coating Terminology". DECC.


Further reading

[1] [2]

  1. Singh, H.; Mutyala, K. C.; Mohseni, H.; Scharf, T. W.; Evans, R. D.; Doll, G. L. (July 2015). "Tribological Performance and Coating Characteristics of Sputter-Deposited Ti-Doped MoS2 in Rolling and Sliding Contact". Tribology Transactions. 58 (5): 767–777. doi:10.1080/10402004.2015.1015758. S2CID   136631684.
  2. Singh, H.; Mutyala, K.C.; Evans, R.D.; Doll, G.L. (December 2015). "An investigation of material and tribological properties of Sb2O3/Au-doped MoS2 solid lubricant films under sliding and rolling contact in different environments". Surface and Coatings Technology. 284: 281–289. doi: 10.1016/j.surfcoat.2015.05.049 .