Dry lubricant

Dry lubricants or solid lubricants are materials which despite being in the solid phase, are able to reduce friction between two surfaces sliding against each other without the need for a liquid media.

Such lubricants, including materials such as graphite, hexagonal boron nitride, molybdenum disulfide and tungsten disulfide are also able to offer lubrication at temperatures higher than liquid and oil-based lubricants are able to operate. These lubricants are often to be found in applications such as locks or dry lubricated bearings. Such materials can operate up to 350°C in oxidizing environments and even higher in reducing / non-oxidizing environments (molybdenum disulfide up to 1100°C). 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 also 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.

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, foodstuff industry, railway track joints, 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.
2. Molybdenum disulfide. Used in CV joints and space vehicles. Does also 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 (MoS₂) are the predominant materials used as solid lubricant. In the form of dry powder these materials are effective lubricant additives due to their lamellar structure. The lamellas orient parallel to the surface in the direction of motion.

Even between highly loaded stationary surfaces the lamellar structure is able to prevent contact. In the direction of motion the lamellas easily shear over each other resulting in a low friction. Large particles best perform on relative rough surfaces at low speed, finer particle on relative smooth surface and higher speeds.

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:

• Reciprocating motion. A typical application is a sliding or reciprocating motion that requires lubrication to minimize wear as for example in gear and chain lubrication. Liquid lubricants will squeezed out while solid lubricants do not escape and prevent for fretting corrosion and galling.
• Ceramics. Another application is for cases where chemically active lubricant additives have not been found for a particular surface, such as polymers and ceramics.
• High temperature. Graphite and MoS₂ exhibit high temperature and oxidizing atmosphere environments, whereas liquid lubricants typically will not survive. A typical application include fasteners which are easily tightened and unscrewed after a long stay at high temperatures.
• Extreme contact pressures. The lamellar structure orient parallel to the sliding surface resulting in high bearing-load combined with a low shear stress. Most applications in metal forming that involve plastic deformation will utilize solid lubricants.

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 a regular atmosphere. 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. In an oxidative atmosphere graphite is effective at high temperatures up to 450°C continuously and can withstand much higher temperature peaks. The thermal conductivity of graphite is generally low: approximately 1.3 W/mK at 40°C.

Graphite is characterized by two main groups: natural and synthetic. Synthetic graphite is a high temperature sintered product and is characterized by its high purity of carbon (99.5-99.9%). The primary grade synthetic graphite can approach the good lubricity of quality natural graphite.

Natural graphite is derived from mining. The quality of natural graphite varies as a result of the ore quality and post mining processing of the ore. The end product is graphite with a content of carbon (high grade graphite 96-98% carbon), sulfur, SiO₂ and ash. The higher the carbon content and the degree of graphitization (high crystalline) the better the lubricity and resistance to oxidation.

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 a mined material found in the thin veins within granite and highly refined in order to achieve a purity suitable for lubricants. Just like graphite has MoS₂ 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 does not. The temperature limitation of MoS₂ at 400°C is restricted by oxidation. The 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, small particles may result in accelerated oxidation.

Boron Nitride

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 has a high thermal conductivity. Boron is available in two chemical structures, i.e. cubic and hexagonal where the last is the lubricating version. The cubic structure 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 common 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.

Free powders

Dry-powder tumbling is an effective application method. The bonding can be improved by priory phosphating 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.

AF-coatings

Anti-friction 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 lubricants bond to the metal surface and form a dark gray solid film. Many dry film lubricants also 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), where parts may be stored for long periods of time.

Composites

Self lubricating composites: Solid lubricants 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, elastomere O-rings, carbon brushes etc. Solid lubricants are compounded in plastics to form a "self-lubricating" or "internally lubricated" thermoplastic composite. PTFE particles for example 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.

References

www.tribology-abc.com/abc/solidlub.htm