Molybdenum disulfide is a solid lubricant comprising molybdenum and sulfur.The chemical formula for molybdenum disulfide is MoS2. This is a transition metal and is silvery and black whereas its nature is solid. It originally became popular due to military and aerospace applications and is now being massively used in lubrication applications. 

The most common use of molybdenum disulfide in today's world is its use in greases and products that contain greases such as pastes, fluid lubricants including automotive and industrial gear oils, sputtered coatings, resin bonded coatings, and impingement coatings.

Introduction

Molybdenum disulfide is a very beneficial transition metal that has a lot to offer in today's world. It is dark greyish when present in its powder form which depends upon the size of the particle. It has excellent properties and characteristics which enable it to grow in larger means and parameters. A lot of work goes on in the preparation and working of these metals so that a better and very positive outcome can be obtained through them.

Molybdenum Disulfide Properties

The following are the properties that are exhibited by molybdenum disulfide

• It has a density of 4.9.
• Its color is blue-gray to black.
• It has a molecular weight of 160.08.
• Its crystal form a hexagonal and rhombohedral.
• It has an electrical conductivity that is low but still variable.
• It has excellent vacuum lubrication.
• It has good radiation stability.
• It has diamagnetic magnetic properties.
• Their dissociation temperature is 1370oC in any non-oxidizing environment.
• It has a melting point of 1700oC under pressure.
• It has a sublimation temperature of 1050oC in a high vacuum.

Forms of Molybdenum Disulfide

Molybdenum disulfide is capable of being presented in two crystalline forms which are hexagonal and rhombohedral. However, out of both these forms, the hexagonal form is the one that is commonly and mostly used in the areas of commercial ores whereas the rhombohedral form is said to be found in natural sources and was initially found in a synthetic material.

The hexagonal form is the most used and has a characterization based on the MoS2 layers and the Mo atoms show coordination with six of the sulfur atoms having two molecules in each unit cell. This crystalline shows a sandwich-like appearance because the hexagonal layer consisting of molybdenum atoms is distributed between the two layers of sulfur atoms that are present.

Classification of Molybdenum Disulfide

Molybdenum disulfide has various classifications depending upon the characteristic property of its being a transition metal dichalcogenide (TMD) which has disulfides, diselenides, and cof various metals including Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W. The compounds that are mentioned above exist in different types of crystalline forms but these compounds exist only in the hexagonal type of crystal structure. Therefore, it has been proved that the excellent lubricant properties of MoS2 are due to the presence of large spacing between S-Mo-S and the weak Van der Waals that exists between them. However, the differences in the lubrication behaviors are owing to the electron distribution in the case of constituent atoms.

Use of Molybdenum Disulfide (MoS2)

MoS2 is used for various purposes in different areas and industries however its most common use is in the boundary lubrication regime in the case in which the metal-to-metal contact exists in comparison to the thick fluid films that are present for the prevention of asperity and wears do not take place there.

Several factors play a role in contributing to the existence of boundary lubrication which are high operating temperatures, low sliding speeds, oscillatory motion, and shock loading. However, molybdenum disulfide naturally occurs in its hexagonal form and it is unable to dissolve in oil and water as its unreactive while interacting with most acids whereas it shows dissolving properties in the strong oxidizing agents for example aqua regia.

Conversion of Molybdenum Disulfide MoS2

The entire process of the conversion of molybdenum metal is quite long and demanding as a lot of tiny steps go into this entire process. The process of conversion takes place through the heating of molybdenum metal in hydrogen gas with the help of intermediate compounds Mo2S3 and MoO3 through an exothermic oxidation reaction which takes place in the air at around 500 to 600oC.

The next step includes the generation of molybdenum pentachloride through the reaction of natural MoS2 chlorine gas while at high temperatures and with no air around. The oxidation characteristics of naturally present MoS2 are studied by various researchers and it has been shown that MoS2 has exhibited tremendous properties they are the reason these have embarked on paving their way in the industries at a much larger scale as the progress has been reported to be very smooth and positive.

Smaller Particles and Their Characterization

It has been observed that the particles of MoS2 which are smaller in size have a rather much higher rate of oxidation in comparison to the larger particles. The reason is that the particles having edgy sites carry rather much higher oxidation rates in comparison to the particles having basal planar areas. Following are the important reasons why the behavior of oxidation of molybdenum disulfide holds practical importance:

• The oxidation of MoS2 has the tendency to cause corrosion in a few areas.
• The oxidation of MoS2 has the tendency of determining the life of MoS2 films that are either bonded or burnished.
• The products of MoS2 oxidation have the tendency of causing increased friction.
• The oxidation of MoS2 has the tendency of causing problems that deal with gelling capabilities in a few greases.
• The oxidation of MoS2 has the tendency of lowering the maximum temperature that is used for MoS2.
• The oxidation process has the tendency of determining the shelf life of MoS2.

Contribution to Lubrication Capabilities

Molybdenum disulfide has a huge amount of contribution towards the lubrication capabilities while it goes through the process of oxidation as the product that is obtained through the oxidation of molybdenum disulfide is MoO3 which is non-abrasive. It is due to this reason that molybdenum disulfide is capable of delivering good lubrication performances despite the high degrees of oxidation. A lot of conductivity measurement processing has also been carried out which shows that the large single crystals are capable of possessing more conductive behaviors and therefore, all these measurements are carried out on them, particularly most of the time.

Lansdown's Observation

In the light of Lansdown's observation, there is no clear evidence present that supports the agreement referring to the electrical conductivity of MoS2 but there is a general view that was presented in this observation that molybdenum disulfide can therefore be classified as a 'p-type semiconductor. MoS2 exhibits a very strong relationship between its conductivity and temperature. They both are directly proportional to each other because if the temperature is increased, the conductivity of MoS2 is also increased and if the temperature is decreased, the conductivity is also decreased.

To find out more, you can read A review case on dry lubricants PTFE and graphite.

Electrical Conductivity for Molybdenum Disulfide

The electrical conductivity for MoS2 at room temperature in the case of single crystals ranges from 0.16 to 5.12 Ω -1 cm -1 that is along the cleavage plane whereas it ranges from 1.02 x 10 -4 to 5.89 x 10 -4 Ω -1 cm -1 parallel to it 10. Other than that it was observed that in the case of 19 oC the resistance of MoS2 remains the same despite the current that is flowing in the crystalline structure. A lot of other parameters are there as well that share a correlation with the resistivity of MoS2. However, temperature and level of impurities remain the dominant parameters.

Greases

Molybdenum disulfide presents greases that have a wide range of applications in almost all the markets and industries where the greases have massive uses and advantages like bentonite clay, lithium, lithium 12-hydroxy stearate, lithium complex, aluminum complex, and polyuria. It is known as a very useful additive as well which is why the range of applications that it carries, maximizes to greater levels. Generally, it is known that grease contains only 1 to 2 percent of molybdenum disulfide with very critical parameters including speed, load, and the roughness of the surface. The metal surfaces certainly found rough are desirable to have larger particle sizes because they are capable of providing a better finish and better results can be obtained through this application.

Smoother Finish

To obtain a smoother finish, medium-sized and small-sized particles work for providing load-carrying abilities in the specified concentration of MoS2. In this case, corrosion issues can be caused when smaller particles are present owing to the higher acid numbers. This is why the most common size to be used of molybdenum disulfide in greases is ideally the medium-sized particle that has a size equivalent to ~ 6 µm. The market of molybdenum disulfide greases has a lot of things that are a part of it including the transportation sector, construction sector, mining sector, agricultural sector, and military or aerospace sector. All these things have progressed a lot in their relevant ways with the progress and implementation of molybdenum disulfide which is enhancing the quality and quantity.

Uses of Grease

Greases have many uses in constant velocity joints, most commonly known as CVJ, which is particularly used in front-wheel drive cars and a few rear-view drive cars as they contain CVJ as a part of their drivetrain. In the case of CVJ and all the other applications that are processed by grease, it moves around as a whole in the contact zone and does not only become a part of the oil component of the grease. It is said that the role played by grease in CVJ has the maximum amount of molybdenum disulfide.

Molybdenum Disulfide for the Construction Industry

In the construction industry, mining industry, and agricultural markets, greases are capable to contain MoS2 as being used as an OEM lubricant as they are also supplied to the aftermarket for obtaining quality results that can be proved beneficial for future circumstances and applications. All these applications include kingpins, ball joints, pivot pins, and spherical frame pivot joints as well. These greases are capable of enhancing the quality of the new components that are in progress so that protective measures can be taken to continue operating the systems in arduous conditions as well. Grease-like products contain higher percentages of molybdenum disulfide and that is why their applications have multiplied over time owing to the excellent features that the molybdenum disulfide exhibits.

Automotive and Military Applications of Molybdenum Disulfide

There are a lot of military applications as well for molybdenum disulfide which consists of greases in comparison to the markets of nonmilitary. For example, Mil G-23549 is a clay used for various purposes and contains grease that has 5 percent of molybdenum disulfide in it which is used for various lubrication purposes in the automotive vehicles bringing ease and enhancement in the quality of the product that it becomes a part of.

Fluid Lubricants

Molybdenum disulfide comprising fluid lubricants is very effectively used for reducing the friction and wear in fluid lubricants in terms of a few conditions holding boundaries so that an increase in the carrying of load can be provided and all the catastrophic events can be minimized at maximum levels. However, the concentrations of molybdenum disulfide in the case of dispersions lie between 0.1 to 60 percent whereas the range of dispersion in the case of oils lies between 3 to 8 percent. A few of the gears are also famously named when we discuss molybdenum disulfide products as they become super responsive through the presence of molybdenum disulfide in them including worm gears, helical gears, and hypoid gears.

In a study conducted through a laboratory practice for the worm gear lubricants in the case of worm gear speed reducer, a mineral oil-based compound was discovered and when molybdenum disulfide was added to it, an increased efficiency was seen in the working mechanics of those worm gear lubricants.

Automotive Hypoid Axle Tests

To check for authenticity and efficiency, a few automotive hypoid axle tests were also conducted in this regard in a few laboratories in certain conditions and it was observed that an increased amount of efficiency was seen when molybdenum disulfide became a part of the automotive vehicles. Therefore, under laboratory experiments, it was proved that the addition of molybdenum disulfide is very good for the products to flourish and gain positive growth in the market. This not only enhances product growth but also increases the economic value of the markets in which these products are a part.

Solid Film Coatings

Solid film coatings which are known as solid film lubricants as well are materials that have lubricating properties as they have a firm grip over the surface of the substrate to which they are attached.

Burnished Films

One of the basic properties of molybdenum disulfides is that they have the tendency to be rubbed over the metal substrates and then a lubricant film is formed having a thickness of 1 to 5µmand this value of thickness remains for the longest periods. This burnished coating remains the longest and most frequently used lubricant applications including metal forming dies, threaded parts, switches, liquid oxygen valves, and a few electronic contacts.

Characteristics of The Applications

In the case of the above-mentioned applications, molybdenum disulfide burnished coating can be obtained through various means including loose powder, dispersions, or sandblasting on the metal surfaces so that a thin film can be obtained. However, the process of sandblasting is quite hard and requires a few hard steps to complete the entire process. Owing to the excellent characteristics that the molybdenum disulfide is capable of showing, it becomes easier for the products to flourish and make a good mark on the market. The reduction in negative outcomes and promotion of positive outcomes is all due to the presence of molybdenum disulfide for the smooth running of automotive vehicles.

Sputtered Coatings

To obtain the sputtered coatings of MoS2, targeted materials of MoS2 get bombarded with gas for example argon that becomes responsible for releasing the atoms and affects any nearby substrate, and this way a sputtered-coated molybdenum disulfide is obtained. A magnetron chamber is needed to carry out this entire process and certain temperature conditions are said to be kept under consideration so that this entire process can run smoothly and the desired outcomes can be obtained effectively and efficiently. These sputtered coatings of molybdenum disulfide become much more enhanced versions of the molybdenum disulfide thus making it easier for automotive vehicles to use them to obtain the desired results.

Sputtered Films

Just like sputtered coatings of molybdenum disulfide, sputtered films of molybdenum disulfide also exist and are casually used in vacuum environments mainly in deep spaces so that low friction and wear can be achieved through these sputtered films. However, to continue enjoying the advantages of sputtered coatings and films, it is necessary to keep the desired conditions and circumstances for them to run in full consideration otherwise their disadvantages start overcoming their advantages. Critical aerospace is the most common area in which the implementation of molybdenum disulfide applications takes place making it the most used and wide range of areas for automotive applications of MoS2. All these areas become more authentic with the addition of MoS2 and become a reason for the flourishment of industries overall.

Resin-Bonded Coatings

All the products that are achieved through the addition of molybdenum disulfide have different areas of applications and different kinds of products are obtained through them. The resin bonded lubricants are the most common product to be obtained through the addition of molybdenum disulfide and with this product a dry and self-lubricating film is obtained on the metal surfaces which has a wide range of advantages in its area. However, commercially a lot of different compositions are present which can be used as per the need of attaining the desired outcome but all these compositions must have molybdenum disulfide as their major component to flourish effectively.

Organic Resins

The organic resins are also formulated by the addition of molybdenum disulfide and the newer organic resins include polybenzimidazole and poly benzothiazole. Different products require different compositions but the most integral part has to be molybdenum disulfide to ensure the active running of the product. The processes to carry out the formulation methods are hard and require a few steps that are hard to attain but once attained they can work wonders for the growth of the markets and in return benefit the entire economy.

Other Resin-bonded Coatings

A few other resin-bonded coatings are also present which are basically oil compatible and they help provide a reduction in friction and wear while considering certain boundaries that are not harmful to the products that are involved.

For example in automotive piston skirt coating, MoS2 is used for improving the fuel economy and thus the extension of the durability of the engines. The other application for these bonded coatings is the fasteners. This allows the bonded coatings to work in their parameters so that undesirable outcomes can be avoided completely by all means.

Conclusion

Molybdenum disulfide is a transition metal that has rich properties and characteristics that differentiate it from other transition metals. Owing to its great properties and characteristics, molybdenum disulfide has massively progressed in the field of automotive applications. These applications are rapidly increasing and are providing a strong base to the industries that rely on them and this has been repeatedly proved by the multiple types of research that have been carried out in this regard.

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References

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