​Dry Lubricants and Their Use in Machinery

Dry lubricants are basically solid lubricants that are present in the solid state but even though when they are in the solid state they are capable of minimizing the friction between different surfaces. This in itself is one of the best properties of dry lubricants as it does not require any liquid medium to carry out this desired process.

There are four types of dry lubricants among which graphite and molybdenum disulfide are the most commonly used ones in the industry of machinery. Their applications are vast and are easily adaptable with the environment as they are the ultimate source of bringing ease to the industries throughout the world that is why a major number of applications are to be found in regard to graphite and molybdenum disulfide as the dry lubricants.

Introduction

Despite being in the solid phase, solid lubricants or dry lubricants are the materials that have the ability to lessen the friction between the two surfaces which are sliding against one another. Dry lubricants don’t even need the liquid oil medium to lessen that friction. Molybdenum disulfide and graphite are the two main dry lubricants. The oil and liquid-based lubricants operate at high temperatures but dry lubricants provide lubrication at even higher temperatures. In applications like dry lubricated bearings or locks, dry lubricants are usually utilized. In non-oxidizing/reducing environments, such materials operate at higher temperatures than 350 C (662 F), whereas in oxidizing environments, they operate at 350 °C (662 °F) (molybdenum disulfide at 1100 °C, 2012 °F).

Most of the dry lubricants have low-friction properties which are because of the layered structure on the molecular level. The bonding between the layers is weak that's why with minimal force applied, such layers slide relative to each other, therefore, providing the layers their low friction characteristics. Although, when it comes to lubrication, only a layered crystal structure is not enough. Also, just like dry lubricants, some of the solids which have non-lamellar structures function the same. Diamond, rare-earth fluorides, some solid oxides, polytetrafluoroethylene, and some particular soft metals like tin, silver, lead, and indium, are included in them. Compacted oxide glaze layers are made in metallic sliding systems at several hundred degrees Celsius and their low friction characteristics didn’t get much attention. Although, because of their physically unstable nature, their practical usage is very far away.

Solid lubricants are of different types but four of the most usually utilized solid lubricants are:

Tungsten disulfide: They are utilized in space vehicles and CV joints but they can only be seen in some of the dry lubricated bearings because of their high cost.

Hexagonal boron nitride: It is also known as white graphite and can be utilized in space vehicles.

Molybdenum disulfide (MoS2): In a vacuum, it lubricates. Its usages and functions are the same as tungsten disulfide but they are affordable, unlike tungsten disulfide.

Graphite: When it comes to lubricating the rocks, graphite is very common as the liquid lubricant aids the particles in getting stuck in the lock, therefore making the problem even more complicated. Graphite is utilized in machine-stop works, ball bearings, open gear, piano actions, brass instrument valves, railway track joints, food industry, and air compressors. In sandy environments, graphite is utilized for lubricating the firearms’ internal moving parts.

The predominant materials that are most commonly utilized as dry lubricants are molybdenum disulfide and graphite.

Graphite

The structure of graphite is made up of polycyclic carbon atoms’ planes that are hexagonal in orientation. The bonding between the planes is weaker as the distance of the carbon atoms is longer between the planes. When it comes to lubrication in the air, the most suitable one is graphite. The important component for graphite lubrication is water vapor. As compared to the adhesion energy between the graphite and a substrate, the bonding energy between graphite's hexagonal planes is at a lower level because of the water's absorption as it lessens the bonding energy. In a vacuum, graphite is not an efficient use as water vapor is needed for lubrication. Galvanic corrosion can be promoted by graphite as graphite is electrically conductive. In an oxidative atmosphere, graphite can withstand very high-temperature peaks but works effectively at 450 °C continuously.

Synthetic and Natural are the two main groups in which graphite has been characterized

• Mining provides Natural graphite so the quality of the ore and the post-mining processes determine the natural graphite's quality as the quality varies. Graphite with some content of ash, SiO2, sulfur, and carbon (high-grade graphite 96−98% carbon), is the end product. The resistance to oxidation and lubricity depends on the graphitization's degree and carbon content. If the carbon content and graphitization's degree are higher, the lubricity and resistance to oxidation will be better.
• Carbon’s high purity (99.5-99.9%) in synthetic graphite characterizes the synthetic graphite. It is a high-temperature sintered product. The quality natural graphite has very good lubricity which can be approached by the primary grade synthetic graphite.

Amorphous graphite is best for applications in which there is a requirement of minor lubricity and a more thermally insulating coating. Amorphous graphite is 80% carbon.

Molybdenum disulfide

Some sulfide-rich deposits provide MoS2 when they are mined, and they are refined for obtaining a purity that is appropriate for lubricants. MoS2 has a hexagonal crystal structure like graphite and contains the intrinsic characteristic of easy shear. The performance of MoS2 lubrication is more than graphite. Graphite is also not effective in vacuum whereas MoS2 lubrication is effective in a vacuum. Oxidation restricts MoS2’s temperature limitation at 400 °C. The significant parameters that should match with the substrate’s surface roughness are film thickness and particle size. The impurities in the MoS2 cause excessive wear of the large particles by abrasion and cause accelerated oxidation in smaller particles.

Polytetrafluorethylene

In lubricating greases and oils, polytetrafluorethylene (PTFE) is utilized broadly as an additive. PTFE’s stable unflocculated dispersions can be formed in water or oil because of the PTFE’s low surface energy. PTFE’s structure is not layered as compared to the other solid lubricants. Like lamellar structures, PTFE’s macro-molecules slip along each other easily. One of the smallest coefficients of dynamic and static friction is showed by PTFE. The temperature at which they operate is restricted to about 260 °C.

Boron nitride

The hexagonal boron nitride is a ceramic powder lubricant. In the oxidizing atmosphere, boron nitride's high-temperature resistance of 1200 C service temperature is its most interesting feature. High thermal conductivity is possessed by boron nitride. Being extremely hard, cubic boron nitride is utilized as a cutting tool component and an abrasive.

Properties

Generally, a good lubricant has the following properties, for instance, it has thermal stability, high viscosity index, demulsibility, thermal stability, hydraulic stability, high resistance to oxidation, corrosion prevention, and a low freezing but a high boiling point (so they can stay liquid within a broad temperature range).

Formulation

Lubricants usually have 10 % additives or less and 90% or more of the base oil (petroleum fractions, known as mineral oils). Sometimes, base oils are synthetic liquids or vegetable oils like fluorocarbons, silicones, esters, hydrogenated polyolefins, and many others.Following changes occur due to the additives; contamination or aging, resistance to oxidation and corrosion, enhanced viscosity index, increased viscosity, and reduced wear and friction. Powders (for instance, tungsten disulfide, molybdenum disulfide, PTFE, dry graphite, etc.) are included in the non-liquid lubricants, so the non-liquid lubricants and PTFE tape is utilized in air cushion, plumbing, and others. At a temperature higher than 350 °C, lubrication is offered by dry lubricants.

Additives

Almost 10 additives are contained by modern automotive lubricants. 20% of the lubricant is made up of additives. Following are the additives’ main families:

1. Friction modifiers are the additives that lessen the wear and friction, specifically in the regime of boundary lubrication where there is a direct contact between the surfaces.

2. On sliding metal parts, anti-scuffing (extreme pressure) additives produce protective films.

3. Anti-wear additives suppress wear by producing protective 'tribofilms' on metal parts.

4. Anti-foaming agents discourage the formation of foam as they increase the surface tension. They are silicone compounds.

5. Rust inhibitors (corrosion inhibitors) are typically alkaline materials.

6. Pour point depressants prevents waxes from being crystallized.

7. Detergents make sure that the components of the engine stay clean.

8. Viscosity index improvers (VIIs) enable the lubricants to stay viscous at higher temperatures.

9. Antioxidants suppress hydrocarbon molecule's rate of oxidative degradation within the lubricant. The additives are known as metal deactivators.

Benefits in machinery

Molybdenum disulfide and graphite are the two main solid lubricants. There are special operating conditions of solid lubricants:

1. Solid lubricants can be used if, under high temperature or heavy load, the lubricating film cannot be maintained by using either grease or oil.

2. In some machines (commutator brushes, motors, electric generators, etc), the contamination of solid particles or dust through oil or grease is not acceptable so the solid lubricants which are in pure form can be used here.

3. Solid lubricants can also be used in machines where combustible lubricants are avoided.

Mixing with fatty oils and fats:

(a) Dispersed in suspension or oils.

(b) Added to grease.

Dry coating:

a. Solid lubricants that are dispersed in vehicles are sprayed or coated on solid surfaces and dried later.

b. Sputtering: Solid lubricants, like molybdenum disulfide, are placed in a vacuum at the cathode, and the inert gas bombards it for accumulating ejected particles onto the solid surfaces to coat them.

c. Ion plating: The plating material (like silver or gold) is accelerated and ionized by an electric field for depositing a metal coating in a vacuum.

Composite materials:

They are utilized for filling the irregularities of ceramics and resins’ surfaces.

Solid lubricants are directly applied to the sliding surfaces:

In all of these methods, a coating is produced by the solid lubricants on friction surfaces for solid lubrication even when there is no oil. Graphite and molybdenum disulfide are utilized usually as thermally stable extreme-pressure agents. In the air, molybdenum disulfide is stable at 350 C whereas molybdenum sulfide is stable in vacuum at 1200 C, however, graphite is stable in vacuum at 3600 C and in the air at 500 C. Solid films are physically produced by the lubricity that is provided by these lubricants. Chemically reactive extreme-pressure agents corrode the substrate metals whereas dry lubricants don’t but the molybdenum disulfide can decompose, therefore releasing sulfur, which can result in corrosive wear.

The reaction-generated coating is one of the other applications of solid lubricants. On metal surfaces, the coating is formed by chemical reactions or during lubrication by friction heat and other factors. One of the former pretreatment method's examples is iron sulfide films' formation when the metal surfaces are treated with hydrogen sulfide. Oiliness improvers like fatty acids are absorbed well by the resulting surfaces, and the wear resistance is enhanced by the adsorption. Under friction heat, this compound decomposes and a molybdenum disulfide coating is formed by this compound on friction surfaces, displaying extreme pressure characteristics.

Applications of Molybdenum disulfide

Lubricant

A low coefficient of friction is possessed by MoS2 because of the weak van der Waals interactions between the sulfide atoms’ sheets. MoS2 of 1–100 µm size is a common dry lubricant. In oxidizing environments, few alternatives confer stability and high lubricity at up to 350 °C. MoS2 gives less than 0.1 friction coefficient values. Low friction is a need of MoS2 as it is a component of composites and blends. For instance, MoS2 is added to graphite for improving sticking. Different greases and oils are utilized as they get their lubricity back. The lubricity is retained even in almost total oil loss cases, therefore finding usage in critical applications, for instance, aircraft engines. MoS2 produces a composite with lessened friction and enhanced strength when MoS2 is added to plastics.

Vespel, Teflon, and Nylon (Nylatron is its trade name) are the kind of MoS2 that fills the polymers. For the applications of high temperature, self-lubricating composite coatings are made up of titanium nitride and molybdenum disulfide by utilizing chemical vapor deposition.There are many MoS2-based lubricants' applications, which include bullets, ski waxes, universal joints, automotive CV, bicycle coaster brakes, and two-stroke engines (motorcycle engines). Graphite is included in the other layered inorganic materials which display lubricating characteristics (also called dry lubricants or solid lubricants) and needs hexagonal boron nitride and volatile additives.

In catalysis process

In petrochemistry, MoS2 is used for desulfurization as a co-catalyst. Catalysis does not take place at crystallites’ regular sheet-like regions but occurs at these planes’ edge. MoS2 is utilized as a hydrogenationcatalyst for organic synthesis.When the major concern is resistance to sulfur poisoning or the price of the catalyst, MoS2 is good to be used. For forming secondary amines through reductive alkylation and nitro compounds’ hydrogenation to amines, MoS2 is effective and can be used.

Applications of Graphite

Batteries

Since the 1970s, graphite usage has been increased in batteries. In major battery technologies, electrodes are constructed by synthetic and natural graphite as they are utilized as an anode material. In the early 1990s and late 1980s,the demand for graphite rise because of the rise in the demand for batteries, for instance, lithium-ion and primarily nickel–metal hydride batteries.This demand growth was driven by portable electronics like power tools and portable CD players. The demand for batteries has also increased in smartphone products, tablets, mobile phones, and laptops.

Soon, there is an anticipation of an increase in the demand for graphite from electric-vehicle batteries. In a fully electric Nissan Leaf, graphite of almost 40 kg is possessed by its lithium-ion battery. Old nuclear reactors provided radioactive graphite which is being searched right now as the fuel. When it comes to supplying energy to electronics and things for a longer duration, the potential for that is seen in the nuclear diamond battery.

Refractories

Before 1900, graphite was started being utilized as a refractory material. Molten metal was used to being held by graphite crucibles. Now, this all is a small part of refractories. Since 2017, the importance is in this order: alumina-graphite shapes, carbon-magnesite brick, Monolithics, crucibles. Extremely large flake graphite started to get used by Crucibles. Not so large flake graphite is needed by the carbon-magnesite bricks. Now, the flake’s size is flexible, more than it was back then. Also, now there is no more restriction for amorphous graphite to the low-end refractories. Some parts of the blast furnace linings use graphite blocks as graphite’s high thermal conductivity is important to assure the furnace’s hearth and the bottom’s sufficient cooling. Instead of carbon-magnesite bricks, high-purity Monolithics are usually utilized as a continuous furnace lining.

In 2000-2003, a crisis was fallen upon the European and the US refractories industry. The crisis was due to many plant closures, the decline in refractory consumption per tonne of steel underlying firm buyouts, and an unmoved steel market. Due to Harbison-Walker Refractories’ acquisition by RHI, many plants were closed and some of them had their equipment auctioned off. Graphite’s consumption was more in manganese bricks but after the capacity was lost from bricks, graphite consumption also decreased in the bricks and the capacity moved to Monolithics, along with the consumption of graphite. China is the major source of carbon-magnesite brick.

Steelmaking

The content of carbon can be raised in molten steel by the natural graphite in steelmaking. This can aid in lubricating the dyes that are utilized for extruding hot steel. Alternatives like Petroleum coke, synthetic graphite powder, and other carbon forms, give competition to the carbon additives in pricing. In order to improve the content of carbon to a particular level in the steel, a carbon raiser is added. In 2005, the US steelmakers utilized 10,500 tonnes according to the USGS's graphite consumption statistics.

Brake linings

Fine flake graphite and natural amorphous graphite are utilized in brake shoes or brake linings for heavier vehicles. When the need of substituting asbestos increases, they became important. The market share of graphite is being reduced by the non-asbestos organic (NAO) compositions as they have been significant for some time now because of their use as a substitute for asbestos. There have been no benefits or any kind of indifferent automotive market by a brake-lining industry shake-out with some plant closures. In 2005, the consumption of graphite in brake linings was 6,510 tonnes according to the USGS.

Foundry facings:

Fine flake or amorphous graphite’s water-based paint is a foundry-facing mold wash. If we used it to paint the mold’s inside and let it dry, it will leave a fine graphite coating which can be used in separating the object cast after the hot metal cools down.At extremely low or high temperatures, graphite lubricants can be used for lubricating rocks, as a gear lubricant for mining machinery, as an anti-seize agent, and as a forging die lubricant.There is a high desire of having no-grit graphite (as it is of ultra-high purity) and low-grit graphite. It can be utilized as and in various forms, like, colloidal graphite (a permanent suspension in a liquid), in oil or water, and as a dry powder.

In 2005, 2200 tonnes of graphite was consumed in this fashion according to the estimate provided by USGS on USGS graphite consumption statistics. A self-lubricating alloy can be created when graphite is impregnated with the metal and it has applications in extreme conditions, for instance, bearings for machines exposed to low or high temperatures.

Conclusion:

Dry lubricants, specifically known as solid lubricants are categorized into 4 types out of which two are the most commonly known ones which include graphite and molybdenum disulfide. Graphite is rich in mechanical properties as well as applications and so is the case with molybdenum disulfide that is why these two are rather used more excessively than the rest two dry lubricants. Their excellent properties make them one of a kind and enhance their productivity and usage.

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