When two surfaces move in relative motion, they must be lubricated to operate smoothly and reduce friction and wear. Friction causes moving surfaces to wear down and lubrication helps to reduce wear. There are various types of dry lubricants like Graphite, Tungsten Disulfide (WS2), Molybdenum Disulfide (MoS2), Hexagonal Boron Nitride (hBN) and PTFE (polytetrafluoroethylene).
In machines when the moving surfaces roll, slide or rub against each other it leads to mutual motion and a resistance is generated called friction. Friction causes wear and tear on the surfaces of moving parts. Therefore a substance is introduced between the two surfaces to reduce friction known as lubricants. The main purpose of the lubricants is to keep the moving surfaces apart so the friction reduces and thus the wear. The process of adding a substance to reduce friction is called lubrication.
The amount and the type of lubricant to be used depends on factors like operating speed, temperature, surrounding conditions and more. At high temperatures, the liquid lubricants fail and decompose and even vaporise. In such situations, dry lubricants are used. Dry lubricants are preferred in applications that require high temperatures and high pressure where liquid lubricants cannot be used. Liquid lubricants can be difficult to dispose of and recycle. Dry lubricants can be used instead.
Dry lubricants are preferred in applications that require high temperatures and high pressure where liquid lubricants cannot be used. Dry lubricants are used in situations where the lubrication cannot be secured by the lubricating oils or grease. They are used where the contamination of the lubricating oils is unacceptable. When the operating temperature or load is too high for grease to stay in position, dry lubricants can be used. They are also used in situations where combustible lubricants cannot be used.
Dry lubricants can be used in powder form or with binders to make them stick to the metal surfaces during the operation. Dry lubricant powders can be used in continuously operating conditions. They can also be added to mineral oils and grease to improve their load-carrying capacity.
Types of dry lubricants
Graphite
It is widely used because of its good lubricating performance. This is due to its structure in which the atoms within the layer have strong covalent bonds and the layers are bonded by weak Van der Waals forces. However, Graphite is inferior to oxidation resistance at elevated temperatures. Graphite gets oxidised to Carbon monoxide at temperatures above 400° C and Carbon dioxide above 500° C. It is used as a lubricant for applications requiring medium temperatures.
Tungsten Disulphide
It is the most lubricious substance known and occurs naturally as a mineral ore ‘Tungstenite’. It has a layered structure with W (Tungsten) atoms located between the layers of S (Sulphur) atoms. It is ideal for applications requiring high load and extreme pressure (EP) conditions. It has a long life in dry environments and can last for longer cycles. However, it is highly reactive in humid environments.
Molybdenum Disulfide
It is an inorganic compound and is found naturally as mineral ores Molybdenite and Jordisite. It has a layered structure with a plane of Molybdenum atoms sandwiched between the planes of Sulphur ions. It is relatively inactive and does not get affected by oxygen and dilute acids. When used as a dry lubricant it provides a protective layer between the two moving surfaces. However, it does not adhere easily to the moving surfaces.
Hexagonal Boron Nitride
It possesses superior anti-friction and anti-wear properties. This is due to its laminated structure where strong covalent bonds exist between Boron and Nitrogen and the layers are held together by weak Van der Waals forces. It is a cheap lubricant and has better biodegradability. It can be used as a lubricant for low and high-temperature applications. It can withstand temperatures up to 900° C even in an oxidising atmosphere. It is used as a lubricant in applications where the electrical conductivity and the chemical reactivity of Graphite are not acceptable.
PTFE
It is a fluorocarbon solid consisting of Carbon and Fluorine. It is hydrophobic and neither water nor water-containing substances can wet it. It is known by the brand name Teflon and is used as a non-stick coating for cookware. It is non-reactive because of its strong Carbon and Fluorine bonds. It is used as a lubricant and it can help to reduce friction, wear and energy consumption in machinery. Its coefficient of friction is the third lowest of any known solid. The only chemicals that can affect it are highly reactive metals like alkali metals.
