Molybdenum Disulfide (MoS2) is a two-dimensional Transition Metal Dichalcogenide (TMC) inorganic material. It occurs naturally as a mineral ore Molybdenite and Jordisite. It is silvery black sold and relatively unreactive.
MoS2 has a layered structure with two hexagonal planes of S atoms separated by a plane of Mo atoms. It is oriented like S-Mo-S. The layers are tacked together by weak Van der Waal forces. MoS2 has exceptional properties like low coefficient of friction, wear resistance and excellent thermal stability. It is used as a common solid/dry lubricant.
MoS2 can function effectively as a lubricant up to a temperature of 350° C in oxidising environments. At higher temperatures it forms Molybdenum trioxide. It does not get affected by dilute acids and oxygen. The shear strength of MoS2 increases if the coefficient of friction increases. This property is called superlubricity.
MoS2 can be obtained by purifying the mineral ore, Molybdenite. At ambient temperatures the coefficient of friction of MoS2 is 0.150. It is thermally stable in vacuum and inert environments.
MoS2 can be used in applications where low speed, high load, high temperature and start-stop conditions exist. It is widely used as a lubricant additive in the transportation, military, space, mining and construction industries.
Use as a solid lubricant
Solid lubricants are mostly used to lubricate the mechanical components operating under low pressure and high temperatures. The lubricant can be used in harsh environments like high vacuum, high temperatures, fretting wear and corrosive gasses.
They are used in various applications like deployment devices, hold-down and release mechanisms, pointing mechanisms and solar ray drive mechanisms. Solid lubricants are used in applications where challenging conditions exist.
Liquid lubricants cannot be used in such an environment because they cannot function without oxidising and disappearing due to their volatility. At cryogenic temperatures, the liquid lubricants may solidify and not be able to function effectively as a lubricant. The liquid lubricants may decompose in radioactive, ultraviolet and corrosive environments.
Solid lubricants have emerged as an alternative for providing effective lubrication in challenging environmental conditions. It also does not require frequent replenishment.
Solid lubricant can be used as a single solid or composite material that helps reduce friction and wear between two moving surfaces. Solid lubricants can offer advantages like better thermal stability and high melting point and work effectively under humid and high load conditions. Another advantage is that solid lubricants are lighter than liquid lubricants and help to reduce weight.
To work effectively solid lubricants should have properties like low-shear strength, excellent adherence to substrate material and thermodynamic stability within the operating conditions. The lubricant selected should be softer than the surfaces they lubricate which means they should have low abrasivity.
Selecting the most effective lubricant depends on the application requirements. MoS2 stands out due to its exceptional properties and low coefficient of friction. For lubrication, it can be used in cryogenic and high temperatures.
Applications of MoS2
- MoS2 is used in boundary lubrication regimes where metal-to-metal contact exists. One of the properties of MoS2 is that it can be physically rubbed on most metal substrates to establish a lubricating film. The burnished coating can be produced by using molybdenum disulfide (MoS2) powder, dispersions, wire-bushing or impingement onto the metal substrate to form a thin adherent film. The thin MoS2 coating helps to reduce friction between the moving surfaces.
- Spluttered coating of MoS2 can be achieved by bombarding the target materials with compacted MoS2 with a charged gas. This process takes place in a magnetron vacuum chamber under low pressure. It produces an extremely adherent coating of MoS2. The coating is much thinner, more adherent and shows greater endurance. MoS2 sputtering can be performed on a wide range of substrates. This method is primarily used in vacuum environments.
- Resin-bonded lubricant coatings are one of the most common ways of achieving dry self-lubricating films on metal surfaces. In this method, the solid lubricant is dispersed in a binder and attached to the surface. Different types of binders and additives can be used. MoS2 can be used with other solid lubricants, a binder and a solvent. The coating can be applied using different methods used like spraying, roll-coating and dip-coating. A range of MoS2 particle sizes can be used depending on the application.
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