Dry lubricants are used to reduce friction and wear between the two moving surfaces. They are mostly used in applications that require extreme conditions like high temperature, pressure, vacuum, and load.
Some dry lubricants are Graphite (C), Tungsten Disulfide (WS2), Molybdenum Disulfide (MoS2), and Hexagonal Boron Nitride (hBN). Dry lubricant powders can be placed between the two moving surfaces to reduce friction and wear.
Most dry lubricants have a layered structure where the atoms within the layer are bonded by strong covalent bonds and the layers are connected by weak Van der Waals forces. The layers have low shear strength and can easily slide against each other when a shear force is applied. This results in a low coefficient of friction.
When placed between two moving surfaces the layers align themselves in the moving direction and slide over one another to form a lubricating film. This helps to reduce friction between the moving surfaces. There is a strong bond between the atoms within the layers that have a good load-carrying capacity. This helps to prevent severe wear on the surfaces.
For dry lubricants to work efficiently the lubricating film should adhere to the moving surface or tribo-surface. The method used to apply the lubricant can affect the adhesion between the lubricating film and the surface. The dry lubricants can be adsorbed, deposited or bonded on the surfaces.
Methods of applying dry lubricants
The main reason for using dry lubricants is to form a continuous lubricating film on the moving surfaces. The different methods used to create a lubricating film on the metal surfaces are spluttering, burnishing, and plasma spraying. These methods can be classified into traditional methods and modern methods.
Traditional methods
One of the widely used methods is spraying or painting a mixture of dry lubricants on hard surfaces. Usually, a binder is used like acrylic or alkyd. Another method used is friction transfer of material.
Modern methods
The modern methods used are physical vapour deposition and chemical vapour deposition.
In physical vapour deposition, hard films are deposited in vacuum conditions. The solid lubricant is vapourised and deposited on the surface as a thin film. Some of the techniques used are spluttering, plasma spraying, cathodic arc and electron beam.
In chemical vapour deposition, a high-quality coating can be produced under a vacuum. In this, the chemical reaction between the precursors and other gases is used to deposit a film on the surfaces. Different techniques used are ultra-high vacuum, low pressure, atmospheric pressure, atomic layer deposition and more.
Advantages of using dry lubricants
- Dry lubricants are used in situations where liquid lubricants or greases cannot be used. They are used in space technology, high vacuum applications, aggressive environments, automotive transport, extreme conditions and high loads.
- Dry lubricants can be used in boundary and mixed frictional regimes for low-speed and high-temperature applications.
- Dry lubricants can be used in different forms like dispersions, suspensions, pastes, powders, and as additives to composites and greases.
- Dry lubricants are available as powders embedded in resin or wax film or as dispersion in oil or water. The dispersion and suspension made using dry lubricants and water can be used to coat mass elements.
- In lubricating varnishes dry lubricants and additives are added to an inorganic or organic binding agent. Hydrocarbons or water are used as a solvent. Some of the binding agents are cellulose, organic resins, inorganic silicates and phosphates. The mixture is sprayed on the surface and the solvent is evaporated to form a film. After hardening the varnishes form a highly adhesive lubricant film.
- Dry lubricants are used where the loading at the contact point is high and in high-speed conditions.
- Dry lubricants are used in metalworking for cold and hot forging, pressure die casting, non-cutting forming and more.
Types of dry lubricants
Graphite (C)
It is a crystalline allotrope form of Carbon and is used as a lubricant in extreme conditions such as aerospace applications. The presence of water and oxygen helps to improve the lubricating properties of Graphite.
Tungsten disulfide
It has the lowest coefficient of friction when compared to other lubricants. It has the property of a state of zero friction or superlubricity. It can be used for lubrication in high-temperature and high-pressure applications.
Molybdenum disulfide
It is an inorganic compound and is found naturally as mineral ore Molybdenite and Jordisite. It is used as a lubricant in space applications.
Hexagonal boron nitride
It is also known as ‘White Graphite’ and can be used as a lubricant at both high and low temperatures.