Lubricants help to reduce the friction and wear between the rubbing parts by preventing direct contact between them. Lubrication is required for the proper functioning of the rubbing parts and to improve their life. As it reduces the damage caused by friction it helps to reduce the downtimes.
Different types of lubricants exist: Solid, Semi-Solid, Liquid and Gas. The type of lubricant to use depends on the application and operating conditions. Liquid lubricant cannot be used in extreme conditions like high temperature and high vacuum. The lubricant can deteriorate due to thermal instability and may degrade. In such conditions, dry lubricants are used for lubrication.
In liquid lubricants, high temperatures can lead to the breakage of the chains in the molecules. This can lead to polymerisation and sludge formation which affects the lubricating capability. If it is used in bearings operating at high temperatures then it will reduce load-carrying capacity of the bearing. This can lead to failure of equipment. In such cases dry lubricants are used for lubrication. Dry lubricants can work efficiently in bearings operating at high temperatures.
In aerospace/space applications low temperatures are encountered under such conditions the liquid lubricants can become too viscous or solidify. Under such extreme conditions, dry lubricants can be used.
There are different types of dry lubricants like Molybdenum Disulfide (MoS2), Tungsten Disulfide (WS2), Hexagonal Boron Nitride (hBN), PTFE (Polytetrafluoroethylene) and Graphite (C). The dry lubricant powders help to reduce the friction and wear between the rubbing parts. These powders can make a separating film between the rubbing parts.
Dry lubricants
Dry lubricants are used to reduce the wear and friction between two moving surfaces. The solid material is interposed as a film between the rubbing parts. An adequate amount of dry lubricant has to be used for lubrication under extreme operating conditions.
Dry lubricants can function well under a wide temperature range of -200° C- 850° C and corrosive atmospheres. Dry lubricants have a lamellar crystalline structure which gives low shear strength and minimises friction. The bonds between the crystalline layers is weak and sets up a low friction mechanism as the layers slide against each other under low shearing forces.
Dry lubricants are used in powder form or as bonded solid films. The main purpose of the lubricant is to keep the rubbing parts apart so that the frictional resistance and the consequent destruction of the materials is minimised.
Use in bonded coatings
In bonded coatings, the dry lubricant in powder form is mixed with a binder and an inorganic or organic solvent to form a suspension. This composition can be applied to the mating surfaces as a thin solid film. The type of binder used decides the operating temperature, bonding ability, abrasion resistance, chemical stability and application procedure to be used.
Bonded coatings can prevent direct contact between the two rubbing parts. The use of dry lubricant helps to reduce the friction and wear. You can get dry and long-lasting lubrication without using grease or oil. The bonded coating is not sensitive to dust, soiling, moisture and chemicals.
The bonded coating has a high-temperature resistance and can function well under a broad temperature range. It can provide long-term corrosion protection and has extreme pressure resistance. The bonded coating can be applied by spraying, burnishing or mechanical impingement, dip application and sputtering vacuum.
Use as a release agent
Dry lubricants are widely used as release agents in moulding. The dry lubricant does not get affected by the mould material or the material being moulded. The transfer of the lubricant on the moulded parts is negligible and does not interfere with post moulding operations.
It is widely used while moulding rubber and plastic parts. The lubricant can be sprayed from a distance and dried for 5 -10 minutes at room temperature.
Use in boundary lubrication
Dry lubricants are used in powder form or as stable emulsions in oil. It helps to reduce the friction between the two rubbing parts by forming films on the surfaces of the parts. They can bear compression and high-temperature.
When are dry lubricants used?
- Dry lubricants are preferred where the semi-solid lubricating film cannot be maintained or their presence is undesirable.
- They are used when the operating temperature is too high for a semi-solid lubricant to remain in place. Or where combustible lubricants have to be avoided.
- When parts to be lubricated are not easily accessible then dry lubricants are used.
- When the machine has to be operated at high speed and moderate load or high load and slow speed.