During the relative movement of two surfaces, solid lubricants are used to reduce damage due to friction and wear. The solid lubricant provides low and controlled friction between the two surfaces. For an application, the lubricant should be chosen based on the working conditions.
Solid lubricants chosen should be chemically stable over the operating temperature range and not attack or damage the surfaces. It should preferably adhere to the surfaces so that it is not lost rapidly from the surfaces. It should have sufficient resistance to wear to provide a useful life to the components. It should be non-toxic and economical to use.
Solid lubricants are used in aerospace applications where low temperatures could cause the liquid lubricants to freeze or become too viscous to provide lubricity. They are used in dry machining operations when the cost of using large amounts of liquid detergent is not affordable. Solid lubricants can simplify lubrication and reduce weight.
They can be used in high vacuum and ultra-high vacuums where the liquor lubricants may evaporate and may not be able to provide lubrication. At high temperatures, the liquid lubricants may decompose or oxidise therefore solid lubricants are used.
Some of the high-pressure applications may exceed the load-carrying capacity of liquid lubricants. Liquid lubricants may not be preferred as they tend to pick up dirt, dust and other contaminants from the environment. Solid lubricants help to overcome these problems.
One of the solid lubricants is Molybdenum Disulfide (MoS2) which can be used in a wide range of applications requiring dry conditions. MoS2 can be used in applications where the use of oil and grease is unsuitable. For example, in food and textile applications the use of oil and greases can contaminate the products therefore solid lubricants like MoS2 are used.
Molybdenum disulfide (MoS2)
Molybdenum disulfide is classified as a 2D Transition Metal Dichalcogenide (TMD). It is an inorganic compound made up of Molybdenum (Mo) and Sulphur (S). It occurs naturally as a mineral ore Molybdenite and Jordisite. The ore is processed by floatation to get pure MoS2. It can also be obtained by treating Molybdenum compounds with Hydrogen Sulfide or elemental Sulphur.
MoS2 has a layered lattice structure with a layer of Molybdenum atoms sandwiched between two layers of Sulfide ions. The Mo-S bonds are covalent and the S layers are bonded by weak Van der Waal forces. This allows it to shear more easily parallel to the layers than across them. The layers can easily slide against each other thus leading to a low coefficient of friction. The low friction occurs only parallel to the layers therefore MoS2 is effective when their layers are parallel to the direction of sliding.
MoS2 is chemically inert and therefore it can be used in reactive chemical environments. It has very low volatility therefore it can be used in high vacuum and high temperature applications. Molybdenum disulfide (MoS2) powder is non-toxic and has good load-carrying capacity. It is stable to radioactivity and can be used in nuclear power plants.
MoS2 begins to oxide at a temperature of 350° C in air but it can be used for short periods up to a temperature of 450° C. The oxidation of MoS2 produces Molybdic oxide which is a lubricant at higher temperatures but wears quickly. MoS2 is attacked by strong oxidising acids and alkalis.
As the shear strength of MoS2 increases the coefficient of friction increases. This property is called superlubricity. The coefficient of friction of MoS2 is 0.150 at ambient conditions.
Applications of MoS2
- MoS2 can be used as a coating or as an additive in compounds to improve the lubricating properties. It is used as a coating for bullets to ensure that it travels through the barrel effectively. MoS2 is added to aviation engine oil to improve the tribological properties.
- It is used in space applications where liquid lubricant cannot be used. Moving systems in space like satellites, telescopes, antennas and others operate for long periods of time under a wide range of extreme environmental conditions. The space application works under vacuum environments solid lubricant like MoS2 is used. MoS2 performs best in vacuum environments.
- MoS2 can be added to plastics to form a composite which has improved strength and reduced friction.
- MoS2 can be used as a lubricant in operating conditions such as vacuum, humidity, high-temperature and radiation. It can also be used for components exposed to dusty or abrasive environments.
- It is used as a metal pretreatment lubricant to reduce friction and wear and prevent metal to metal contact and seizure.
