Solid lubricants - Unique Lubrication
The major challenge was faced by the Lubrication Industry when it came to the lubrication of military jet fighters and space applications. The requirement of better lubrication led to the development of a unique form of lubrication using solid particles. These are now known as Solid Lubricants.
Industries usually have a myth that extreme conditions are uncommon in their plant. However, usually, every manufacturing plant has at least one application which can be termed as a critical and extreme condition on the basis of lubrication viewpoint. Typical examples of extreme conditions are high or low shaft speeds, high pressures, high or low temperatures, atmospheric and process contaminants, and hard-to-reach areas.
Lubricants provide effective lubrication when the surface area (metal to metal or metal to plastic or plastic to plastic etc.) comes in contact and prevalent speed at that moment allows for effective formation of oil film and temperature range falls within the limit.
The limit for lubricants is regardless of base oil type and usually, it’s the condition that causes a change in the state of the fluid that prohibits the formation of lubrication film.
Solid Lubricants are a unique way of lubrication that can protect interacting surfaces even after the wet lubricating film is lost. Materials usually used as solid lubricants are either discovered or created. These solid lubricants can be applied on the surface in the form of additive to fluid lubricants or in a pure form. The resultant film on the surface is characterized as dry film.
The Most Common Type Of Solid Lubricants are:
- Molybdenum disulfide (MoS2) – known as moly
- Polytetrafluoroethylene (PTFE) – known as teflon
- Boron nitride
- Calcium fluoride
- Cerium fluoride
- Tungsten disulfide
Moly, Graphite, and PTFE are most commonly used in the lubrication industry. Moly and graphite are extracted from mined ore whereas PTFE was created by Dupont chemical company and is manufactured by the company for many purposes.
Solid lubricants have a lamellar structure preventing direct contact between the two surfaces coming in contact even at extreme conditions. Graphite and moly are commonly used lubricants. Solid lubricants are used as additives to oils and greases. They can also be added or alloyed into the surface when the component is been manufactured.
Properties of Solid lubricants
Different Solid lubricants have different properties e.g. Lamella materials have a good load-carrying capacity in rolling and sliding mode, Graphite has high-temperature capability and functions well in radiation atmospheres, Moly performs satisfactorily in hard vacuum and can tolerate higher loads better than graphite.
Let’s understand the commonly used solid lubricants that are Moly, Graphite, and PTFE
Mos2 occurs naturally in the form of thin solid veins within the granite. It is a purified form of the mineral molybdenite. It has a hexagonal crystalline structure. The total surface resistance is reduced among interacting surfaces thus reducing surface friction and resistance. Molybdenum disulfide is thermally stable in vacuum or inert environments, but in air or oxygen, it begins to oxidize to MoO3 at approximately 400oC.
What are the benefits of molybdenum di-sulfide?
- Adhesion is excellent
- Wide temperature range
- It protects against fretting corroison
- Friction is decreased with increasing load
- Load carrying capacity is high
- Stick – slip prevention
Limitations of molybdenum disulfide include :
- It is hydroscopic in nature, so it cannot be used in wet conditions as it increases friction
- Oxidation can cause corrosion
- Oxidation lower maximum temperature
Major applications with Moly:
Automotive sector, rails, mining, construction, agriculture, military and aerospace.
Graphite occurs naturally in rocks such as marble, schist, etc. It has properties of a metal and a non-metal, which makes it opt for much industrial application. It is a layer lattice lamellar hexagonal structure. Graphite comprises carbon and water vapor. It has excellent lubricating properties till the time moisture is available and will work satisfactorily up to the temperature limit of 788°C.
What are the benefits of graphite?
- Excellent lubrication in humidity
- Protects against fretting corrosion
- Good temperature stability
- Low co-efficient of friction under high temperature
Limitations of graphite include:
- It can not be used in a hard vacuum
- It can not provide lubricity in absence of moisture
- The oxidation product is Carbon dioxide which is hazardous to health
Major applications with Graphite:
Hot and cold forming, wire drawing and billet coatings, mold release for die cast, plastic and rubber mold, automotive engine and many common industrial applications.
PTFE Coating constitutes carbon and fluorine atoms and is one of the most slippery materials since it has very low surface tension. Unlike other solid lubricants, PTFE does not have a layered structure. The molecules of PTFE slip along with each other very easily similar to other lamellar structures. PTFE Coating can be used up to a temperature limit of 260 deg C.
What are the benefits of PTFE?
- Good sliding – friction reduction
- Excellent chemical resistance
- Low load carrying capacity
- Low Co-effecient of friction at low loads
- Colorless lubricating film
- Chemically inert
Limitations of PTFE include:
- Low melting point
- Low thermal conductivity
- Load-carrying properties is poor
Major applications with PTFE:
Acts as a coating in cookware, stain repellent for fabrics and textile industry, Chemical industry.
As we have understood the properties, benefits, and limitations of solid lubricants, It is important to understand where to use solid film lubricants. Since we have a variety of solid materials with excellent lubricating properties. The most commonly used solid lubricants are mentioned above i.e Moly, graphite, and PTFE.
Major properties to be considered while selecting solid film lubricants are
- Coefficient of friction
- Load – carrying capacity
- Corrosion resistivity
- Electrical conductivity
- Surrounding in which solid film lubricant has to perform
- Environmental factors such as temperature, pressure, humidity, oxygen content, radiation, etc.
- Pertaining to the solid lubricants benefits and limitation, we can use above mentioned solid lubricants For eg:
- Moly has the highest load-carrying capacity with a low coefficient of friction. But it starts oxidation at a temperature of 400 deg c. It can be used in high load-carrying capacity applications but not recommended for use at high temperatures.
- PTFE is commonly used as anti-wear additives and performs well in presence of acids, bases, and solvents. It can be used in applications where acid resistivity is required and it can’t be used at very high temperatures and high loads.
MOSIL has a varied range of greases, oils, and anti-size in a combination with the solid film lubricants as mentioned above and can be used depending upon the application in which it has to perform.