Lubrication can be defined as the application of a material between two surfaces moving in relative motion to each other. This material is called lubricant and there are different types of lubricants such as solid/dry, semi-solid, liquid and gaseous.
Dry lubricants are used to reduce friction and wear between two moving surfaces by avoiding direct contact between them. It also helps to reduce the expansion of metal due to frictional heat and destruction of material. It helps to reduce the maintenance costs of parts.
Different types of dry lubricants are Tungsten Disulfide (WS2), Molybdenum disulfide (MoS2) and Hexagonal Boron Nitride (hBN). Different dry lubricants have different properties like load-carrying capacity, corrosion resistance and coefficient of friction.
These lubricants are also available as aerosol sprays. Such sprays can be used for interior locks, internal hinges, slides, drawers and more. Dry lubricant sprays can also be used where dust and dirt-free lubrication is required. They can be used in many applications to increase the ease of use. They can be used to protect the tools from rust and corrosion. Spraying lubricants on sliding tracks helps to keep them moving easily and smoothly.
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Lubrication
For lubrication under high vacuum, high temperature or under high radiation liquid lubricants cannot be used. In such cases, solid lubricants are used to reduce friction between two moving surfaces. It is used as a film between the two sliding or rolling surfaces. Dry lubricants are used for lubrication where extreme operating conditions exist like high or low temperatures like -200 C to 850 C and corrosive atmospheres.
The dry lubricants have a layered crystalline structure which leads to low shear strength. As they have low shear strength a low friction mechanism is set up due to the slippage of the crystalline layers under shear forces. Dry lubricants are used in the form of powders or bonded solid coatings.
The solid lubricant forms an adhesive film over the surface providing full coverage and malleability. The lubricant should be chosen based on the operating conditions and the environment.
In high vacuum conditions, liquid lubricants cannot be used due to their volatility of organic compounds. In such cases, inorganic solid film lubrication can be used with the lubricant dispersed in an inorganic binder. This mixture can be applied and bonded to the sliding surfaces as a thin solid film. The binder helps to hold the lubricant particles. An effective binder can be used to get favorable characteristics.
Advantages of using dry lubricants
- Dry lubricants are stable at high temperatures, cryogenic temperatures, high pressure and vacuum.
- High thermally conductive dry lubricants can be used for high heat dissipation.
- They have high resistance to high radiation, high dust and reactive environments.
- They are more effective than liquid lubricants for high loads, high speed and intermittent loading conditions.
- As lubrication distribution systems and seals are not required dry lubricants are lighter and using them is simpler.
- They are preferred in locations where the servicing is difficult.
- They can provide translucent or transparent coating as desired.
Sometimes under high vacuum, high temperature, cryogenic temperatures, radiation, dust, corrosive and clean environments solid lubricants may be the only feasible system. Dry lubricants can offer the required coefficient of friction and durability under environments.
Dry lubricants
Tungsten Disulfide (WS2)
It is an inorganic compound consisting of Tungsten and Sulphur. It has a hexagonal layered structure in which a layer of Tungsten atoms is sandwiched between two layers of Sulphur atoms. The bonds within the atoms are strong and layers are held together by weak Van der Waal forces. This allows the layers to easily slide past each other and leads to a low friction coefficient. It can function effectively in a temperature range of -273° C to 650° C.
Molybdenum Disulfide (MoS2)
It is an inorganic compound consisting of Molybdenum and Sulphur. It occurs naturally as a mineral ores Molybdenite and Josdisite. It has a layered structure with a plane of Molybdenum atoms sandwiched between two planes of Sulphur ions. It can work effectively up to a temperature of 350° C in oxidising environments.
Hexagonal Boron Nitride (hBN)
It is a compound made up of Boron and Nitrogen and is the most stable form of Boron Nitride. It has a hexagonal crystalline structure where a layer of Boron atoms is sandwiched between two layers of Nitrogen atoms. Strong covalent bonds exist within the layer whereas the layers are held together by weak Van der Waal forces. It can be used as a lubricant for low and high temperatures up to 900° C.
