Lubrication - Solid Lubricants

       

Nonfluid Lubrication

Solid Lubrication

 Definition of solid lubricant. A solid lubricant is a material used as powder or thin film to provide protection from damage during relative movement and to reduce friction and wear. Fluids are frequently used as a medium or as a lubricant with solid additives. Most commonly used solid lubricants are the inorganic compounds graphite and molybdenum disulfide (MoS2) and the polymer material polytetrafluoroethylene (PTFE).

 Types of solid lubricants.

(1) Lamellar solids. The most common materials are graphite and molybdenum disulfide.

(a) Graphite. Graphite has a low friction coefficient and very high thermal stability (2000 °C and above). However, practical application is limited to a range of 500 to 600 °C due to oxidation. Furthermore, because graphite relies on adsorbed moisture or vapors to achieve low friction, use may be further limited. Graphite has a very noble potential of + 0.25V, which can lead to severe galvanic corrosion of copper alloys and stainless steels in saline waters.

(b) Molybdenum disulfide (MoS2). Like graphite, MoS2 has a low friction coefficient, but, unlike graphite, it does not rely on adsorbed vapors or moisture. In fact, adsorbed vapors may actually result in a slight, but insignificant, increase in friction. MoS2 also has greater load-carrying capacity and its manufacturing quality is better controlled. Thermal stability in non-oxidizing environments is acceptable to1100˚C, but in air it may be reduced to a range of 350 to 400 ˚C .

 (2) Soft metal films. Many soft metals such as lead, gold, silver, copper, and zinc, possess low shear strengths and can be used as lubricants by depositing them as thin films on hard substrates. Deposition methods include electroplating, evaporating, sputtering, and ion plating. These films are most useful for high temperature applications up to 1000 °C and roller bearing applications where sliding is minimal.

(3) Surface treatments. Surface treatments commonly used as alternatives to surface film depositions include thermal diffusion, ion implantation, and chemical conversion coatings.

(a) Thermal diffusion. This is a process that introduces foreign atoms into a surface for various purposes such as increasing wear-resistance by increasing surface hardness; producing low shear strength to inhibit scuffing or seizure; and in combination with these to enhance corrosion-resistance.

(b) Ion implantation. This is a recently developed method that bombards a surface with ions to increase hardness, which improves wear- and fatigue-resistance.

(c) Chemical conversion coatings. Frequently, solid lubricants will not adhere to the protected metal surface. A conversion coating is a porous non lubricating film applied to the base metal to enable adherence of the solid lubricant. The conversion coating by itself is not a suitable lubricant.

(4) Polymers. Polymers are used as thin films, as self-lubricating materials, and as binders for lamellar solids. Films are produced by a process combining spraying and sintering. Alternatively, a coating can be produced by bonding the polymer with a resin. Sputtering can also be used to produce films. The most common polymer used for solid lubrication is PTFE The main advantages of PTFE are low friction coefficient, wide application range of -200 to 250 ˚C , and lack of chemical reactivity.

Disadvantages include lower load-carrying capacity and endurance limits than other alternatives.

 Low thermal conductivity limits use to low speed sliding applications where MoS2 is not satisfactory. Common applications include anti-stick coatings and self-lubricating composites.

 Characteristics.

The properties important in determining the suitability of a material for use as a solid lubricant are discussed below.

(1) Crystal structure. Solid lubricants such as graphite and MoS2 possess a lamellar crystal structure with inherently low shear strength. Although the lamellar structure is very favorable for materials such as lubricants, non-lamellar materials also provide satisfactory lubrication.

(2) Thermal stability. Thermal stability is very important since one of the most significant uses for solid lubricants is in high temperature applications not tolerated by other lubricants. Good thermal stability ensures that the solid lubricant will not undergo undesirable phase or structural changes at high or low temperature extremes.

(3) Oxidation stability. The lubricant should not undergo undesirable oxidative changes when used within the applicable temperature range.

(4) Volatility. The lubricant should have a low vapor pressure for the expected application at extreme temperatures and in low-pressure conditions.

(5) Chemical reactivity. The lubricant should form a strong, adherent film on the base material.

(6) Mobility. The life of solid films can only be maintained if the film remains intact. Mobility of adsorbates on the surfaces promotes self-healing and prolongs the endurance of films.

(7) Melting point. If the melting point is exceeded, the atomic bonds that maintain the molecular structure are destroyed, rendering the lubricant ineffective.

(8) Hardness. Some materials with suitable characteristics, such as those already noted, have failed as solid lubricants because of excessive hardness. A maximum hardness of 5 on the Mohs’ scale appears to be the practical limit for solid lubricants.

(9) Electrical conductivity. Certain applications, such as sliding electric contacts, require high electrical conductivity while other applications, such as insulators making rubbing contact, require low conductivity.

Applications.

Generally, solid lubricants are used in applications not tolerated by more conventional lubricants. The most common conditions requiring use of solid lubricants are discussed below.

(1) Extreme temperature and pressure conditions. These are defined as high-temperature applications up to 1926 °C , where other lubricants are prone to degradation or decomposition; extremely low temperatures, down to -212 °C , where lubricants may solidify or congeal, applications, such as space, where lubricants may volatilize.

(2) As additives. Graphite, MoS2, and zinc oxide are frequently added to fluids and greases. Surface conversion coatings are often used to supplement other lubricants.

(3) Intermittent loading conditions. When equipment is stored or is idle for prolonged periods, solids provide permanent, noncorrosive lubrication.

(4) Inaccessible locations. Where access for servicing is especially difficult, solid lubricants offer a distinct advantage, provided the lubricant is satisfactory for the intended loads and speeds.

(5) High dust and lint areas. Solids are also useful in areas where fluids may tend to pick up dust and lint with liquid lubricants; these contaminants more readily form a grinding paste, causing damage to equipment.

(6) Contamination. Because of their solid consistency, solids may be used in applications where the lubricant must not migrate to other locations and cause contamination of other equipment, parts, or products.

(7) Environmental. Solid lubricants are effective in applications where the lubricated equipment is immersed in water that may be polluted by other lubricants, such as oils and greases.

 Advantages of solid lubricants.

(1) More effective than fluid lubricants at high loads and speeds.

(2) High resistance to deterioration in storage.

(3) Highly stable in extreme temperature, pressure, radiation, and reactive environments.

(4) Permit equipment to be lighter and simpler because lubrication distribution systems and seals are not required.

Disadvantages of solid lubricants.

(1) Poor self-healing properties. A broken solid film tends to shorten the useful life of the lubricant.

(2) Poor heat dissipation. This condition is especially true with polymers due to their low thermal conductivities.

(3) Higher coefficient of friction and wear than hydro dynamically lubricated bearings.

 (4) Color associated with solids may be undesirable.

Methods of applying solid Lubricants.

(a)        Burnishing. Burnishing is a rubbing process used to apply a thin film of dry powdered solid lubricant such as graphite, MoS2, etc., to a metal surface. This process produces a highly polished surface that is effective where lubrication requirements and wear-life are not stringent. Surface roughness of the metal substrate and particle size of the powder are critical to ensure good application

(b)       Spraying: Spray application of bonded lubricants either by an aerosol method or by a spray nozzle.

(c)        Hand rubbing/Brushing. Hand rubbing is a procedure for loosely applying a thin coating of solid lubricant

(d)       Dusting. Powder is applied without any attempt to evenly spread the lubricant. This method results in a loose and uneven application that is unsatisfactory.

(e)        Tumbling. Parts to be lubricated are tumbled in a powdered lubricant. Although adhesion is not very good, the method is satisfactory for noncritical parts such as small threaded fasteners and rivets.

(f)         Dipping : This involves dipping of parts in bonded solid lubricants , usually applied under heated conditions- adhesion is limited to the resin used. Electro- deposition with dipping gives lasting results.

         

                                    Types of Solid Lubricants

(1)       Powdered solids.

The application of powdered solids is limited by surface preparation of the substrate over which it has to be applied. Solids in powdered form when applied by burnishing, tumbling, hand rubbing and dusting tend to adhere less to the substrate and arrangement is required to retain them within the lubricating zone.

They are applied as dispersions , which are mixtures of solid lubricants with grease or oils as substrate.

The most common solids used for dispersion are graphite, MoS2 , PTFE, and Teflon.  The grease or fluid provides normal lubrication while the solid lubricant increases lubricity and provides extreme pressure protection.

 Detergent additives in some oils can also inhibit the wear-reducing ability of MoS2 and graphite, and some anti-wear additives may actually increase wear.

 Solid lubricants can also affect the oxidation stability of oils and greases. Consequently, the concentration of oxidation inhibitors required must be carefully examined and controlled.

Aerosol sprays are frequently used to apply solid lubricant in a volatile carrier or in an air-drying organic resin. However, this method should be limited to short-term uses or to light- or moderate-duty applications where thick films are not necessary.

          (2) Bonded coatings.

 Bonded coatings provide greater film thickness and increased wear life and are the most reliable and durable method for applying solid lubricants.

Under carefully controlled conditions, coatings consisting of a solid lubricant and binding resin agent are applied to the material to be protected by spraying, dipping, or brushing.

The most commonly used lubricants are graphite, MoS2, and PTFE. Binders include organic resins, ceramics, and metal salts. Thermosetting resin binders requiring heat-cure generally provide longer wear-life The choice of binder is also influenced by mechanical properties, environmental compatibility, and facility of processing, apart from operating temperatures.

Air-cured coatings are generally limited to operating temperatures below 260 ºC  

Aerosol applied are used for moderate-duty applications

Heat -cured coatings are generally used to 370 ºC

Organic resins are stable below 300ºC.

Inorganic binders such as metal salts or ceramics permit bonded films to be used in temperatures above 650 ºC 

Surface preparation is very important to remove contaminants and to provide good surface topography for lubricant adhesion.

Other pretreatments used as alternatives or in conjunction with roughness include phosphating for steels and analogous chemical conversion treatments for other metals.

(3) Self-lubricating composites.

 The primary applications for self-lubricating composites include dry bearings, gears, seals, sliding electrical contacts, and retainers in roller bearings.

Composites may be polymer, metal-solid, carbon and graphite, and ceramic and cermets.

(a) Polymer. The low thermal conductivity of polymers inhibits heat dissipation, which causes premature failure due to melting. This condition is accelerated if the counter face material has the same or similar thermal conductivity.

Two polymers in sliding contact will normally operate at significantly reduced speeds than a polymer against a metal surface. The wear rate of polymer composites is highly dependent upon the surface roughness of the metal counter faces.

In the initial operating stages, wear is significant but can be reduced by providing smooth counter faces. As the run-in period is completed, the wear rate is reduced due to polymer film transfer or by polishing action between the sliding surfaces.

Environmental factors also influence wear rate. Increased relative humidity inhibits transfer film formation

in polymer composites such as PTFE, which rely on transfer film formation on counter faces. The presence of hydrocarbon lubricants may also produce similar effects. Composites such as nylons and acetyl s, which do not rely on transfer film formation, experience reduced wear in the presence of small amounts of hydrocarbon lubricants.

(b) Metal-solid. Composites containing lamellar solids rely on film transfer to achieve low friction.

The significant amount of solids required to improve film transfer produces a weak composite with reduced wear life. Addition of non-lamellar solids to these composites can increase strength and reduce wear.

Metal-solid composites are manufactured by powder metallurgy, infiltration of porous metals, plasma spraying, and electrochemical co deposition.

Applications:

-Fabrication technique of machine parts  requiring drilling holes in machine parts and packing the holes with solid lubricants.

 -Self-lubricating roller bearing retainers used in vacuum or high temperatures up to 400°C

-Fail-safe operations, where the bearing must continue to operate for a limited time

  following failure of the normal lubrication system.

(c) Carbon and graphite. The primary limitations of bulk carbon are low tensile strength and lack of ductility. However, their high thermal and oxidation stability at temperatures of 500 to 600 ºC (higher with additives) enable use at high temperatures and high sliding speeds.

For graphitic carbons in dry conditions, the wear rate increases with temperature. This condition is accelerated when adsorbed moisture inhibits transfer film formation.

Dusting also causes failure at high temperatures and sliding speeds, which is overcome by additives.

(d) Ceramics and cermet. Ceramics and cermets can be used in applications where low wear rate is more critical than low friction. These composites can be used at temperatures up to 1000 ºC .

Cermets have a distinct advantage over ceramics in terms of toughness and ductility. However, the metal content tends to reduce the maximum temperature limit. Solid lubricant use with bulk ceramics is limited to insertion in machined holes or recesses.