Types of Lubricants (Liquid, Solid, Gas, and Semi-Solid)

What are lubricants?

Lubricants are substances which reduce friction and wear between two surfaces. It is applied at the interface of the two surfaces. Lubricants also help reduce the heat that is generated at the interface of the two surfaces that are in contact with each other and moving.

Some of the main functions of lubricants are:

• Keep parts moving smoothly
• Transfer heat to prevent overheating
• Reduce friction and increase efficiency
• Anti-stick coating on surfaces that tend to stick to one another
• Carry away debris to prevent clogging
• Transporting foreign particles
• Transmit power from one equipment to another
• Protect against wear and lengthen the life of the equipment
• Prevent corrosion when a piece of equipment is being used in a corrosive or wet environment
• Seal for gas to prevent it from escaping or losing pressure

Besides industrial applications, lubricants have some interesting domestic applications as well. For example, lubricants are used as oils in cooking and baking. They are also used in medical applications like ultrasound therapy.

Properties of lubricants

Lubricants that are considered good or high-quality will normally possess the following properties:

• High boiling point
• Low freezing point
• High viscosity index
• Thermal stability
• Hydraulic stability
• Demulsibility
• Prevention of corrosion
• High resistance to oxidation

Types of lubricants

Based on the molecular structure of the lubricant material as well as its shear strength, lubricants are classified as follows:

Solid Lubricants

A solid lubricant is a solid material which is applied or inserted between two moving surfaces or bearing surfaces. This material will shear a lot more easily than the bearing or moving surfaces. The three main requirements for a material to be a solid lubricant are the ability to support the applied load without major distortion, a low coefficient of friction, and a low rate of wear. Solid lubricants are generally used when the conditions are extreme. Solid lubricants are used in a powder form, as lubricating grease, in suspensions, in metallic films, or in bonded lubricants. Anti-friction linings use solid lubricants.

The durability of solid lubricants is increased by coating the binders along with lubricating pigments. These bonded coatings provide greater film thickness and increase the wear life of the lubricant and the surface on which the lubricant is applied. The popular applications of bonded coating of solid lubricants are cylindrical brushes, separator cage of rolling bearing, and electrical brushes.

Solid lubricants can be further classified into four sub-types, namely polymer, metal-solid, carbon and graphite, and ceramic and cermet.

Polymers

Polymers are one of the largest groups of solid lubricants. They are suitable for use with light loads. They have a lower thermal conductivity which the amount of heat that they are able to dissipate. There are three main polymer solid lubricants, polytetrafluoroethylene (PTFE), Nylon, and synthetic polymers.

Polytetrafluoroethylene is a polymer which is derived from ethylene. All the hydrogen atoms in an ethylene molecule are replaced by fluorine atoms to give polytetrafluoroethylene. It is more popularly known as Teflon, a trade name given to PTFE by the famous company Du Pont. It is commonly used as a solid lubricant because of its low friction, its chemical stability, its low surface energy, and its greater chemical inertness. PTFE also happens to be non-toxic and hence is suitable for use in industries such as food and pharmaceuticals.

For all the plus points of polytetrafluoroethylene, there are some downsides as well. Firstly, it has a relatively high rate of wear. Secondly, it has high thermal expansion and low thermal conductivity which makes it less desirable for use in high-temperature environments. Lastly, it has low load capacity.

However, many of these disadvantages can be addressed through the use of synthetic polymers. Synthetic polymer lubricant can be prepared by mixing glass and carbon-based fillers with PTFE. Impregnating PTFE with metal structures such as bronze or lead is also an option. This modification also allows the synthetic PTFE to withstand higher loads and improve the wear rate.

Metal-solid

These solid lubricants contain lamellar solids and achieve low friction through a process known as film transfer. Molybdenum disulfide is the most commonly used metal-solid lubricant. Some of the advantages of a metal-solid lubricant like molybdenum disulfide are a high load carrying capacity, good high-temperature performance, and low friction. It is also stable in vacuum up to 1000 degrees Celsius. Hence, molybdenum disulfide also finds use in space applications. Its disadvantages are sub-optimal performance in the presence of moisture and high film thickness. A thicker film does not last as long because it is more prone to wear and tear.

Carbon and graphite

Carbon graphite seals are used as solid lubricants. They have desirable properties such as high-temperature stability, high oxidation stability, and sustainable performance in high sliding speed applications. Graphite as a material has low friction and can withstand moderate loads. Graphite, however, is prone to corrosion and does not work very well in vacuum. The lubrication performance of graphite actually increases with an increase in temperature. However, beyond 500 degrees Celsius, the incidence of corrosion increases.

Ceramic and cermet

Ceramic and cermet coatings are used as lubricants in situations where a lower wear rate is more important than low friction. Ceramic/cermet coatings can be used at high-temperature ranges of around 1000 degrees Celsius. A 0.5mm thick coating of ceramic/cermet material offers a low-cost way of utilizing its wear resistance. The coating can be sprayed using a detonation gun, plasma spraying, or electrolytic deposition using an electrolyte that contains ceramic particles.

Semi-Solid Lubricants/Grease

Grease is considered to be one of the most versatile forms of lubricants. It can be used in a wide range of environments across various temperatures, load conditions, or speeds. Be it a dry or wet environment, dusty or clean environment, or even corrosive environment, grease finds use in all sorts of applications.

Grease is a type of pseudo-plastic fluid. One of the most significant properties of grease is consistency. Consistency is nothing but the relative hardness or softness of any material. Grease consists of lubricating oils which have low viscosity and are thickened by finely dispersed solids known as thickeners. Grease is made up of the following:

Base Oil

Petroleum and synthetic base oils are used in the manufacturing of grease. The properties of the base oil are very important, as they affect the properties of the grease that gets produced from the oil. A low viscosity and light base oil is used to produce grease that works at low temperatures. A heavier and high viscosity base oil is used to produce high-temperature grease.

Additives

Certain chemical additives are added to the grease in order to improve its properties. The choice of additives depends completely on the end use or application of the grease. Factors like performance parameters, environmental impact, sustainability parameters, compatibility, cost, and color all play a role in the choice of additives.

Thickeners

Thickeners are added to the base oil in order to thicken the material and produce grease. There are two types of thickeners, organic thickeners and inorganic thickeners. Inorganic thickeners are non-soap based while organic thickeners can be soap-based or non-soap based.

Some of the advantages of using grease as a lubricating material are its water resistance, its binding strength to the surface where applied, lower frequency of application, its ability to reduce noise and vibration, its ability to seal against contaminants, and its usability with vertical/inclined shafts.

Some of the disadvantages of using grease as a lubricant are its poor heat dissipation, its vulnerability to being contaminated by dust, and the inability to filter out contaminants from the grease.

Lubricants may also be used in a paste form in heavy load applications, in sliding applications, and with slow running bearings. The paste form of lubricant may also be used as assembly paste or as a high-temperature paste.

Liquid Lubricants

Liquid lubricants are used extensively in applications that are high in terms of speed and load size. Liquid lubricants are the most dominant type of lubricant in the market. Liquid lubricants are comprised of base oil and some additives. The various types of liquid lubricants are as follows:

Mineral Oil

Mineral-based lubricants are extracted from crude oil. Mineral oil lubricants are of four types. The first type is paraffinic oil. It has good resistance to oxidation. It exhibits good thermal stability, is less volatile, and has a high flash point.

The second type of mineral oil lubricant is naphthenic oil. This type of lubricant is good for low-temperature applications. It has a lower flash point than paraffinic oil lubricant. When naphthenic oil lubricant is burnt, soft deposits are formed which in turn lowers the abrasive wear.

The third type of mineral oil lubricant is multigrade oil. It is made by adding polymers in mineral oils, thus enhancing the viscosity index of the lubricant. These lubricants have different grade levels whereby a specific grade of lubricant oil can offer optimal performance in low temperatures or in high temperatures.

Lastly, synthetic oil is another type of mineral oil lubricant. This type of lubricant was created to withstand harsh operating conditions. Jet engines use synthetic lubricants. These lubricants are expensive but they can withstand high levels of heat and stress. Some commonly used synthetic oils are esters, silicon, polyglycols, perfluoropolyalkylether, and perfluoropolyethers.

Vegetable Oil

Oil-based lubricant made from rapeseed and castor is known as vegetable oil lubricant. Vegetable oil contains more natural boundary lubricant than what is observed in mineral oil. However, vegetable oil lubricant is less stable than mineral oil lubricant at high-temperature ranges.

Animal Oil

Fats extracted from fish and animals are the sources of animal oil. Animal oil is sometimes also known as fixed oil. It is added to mineral oil in order to improve the film forming ability of the mineral oil. Animal oil does not volatilize. The main drawback with animal oil is its availability.

Gas Lubricants

Gases like nitrogen and helium are used as lubricants in applications where film thickness between tribo-pair is ultra small. The advantages of using gas lubricants are large temperature range, no sealing required for lubrication, very low friction due to low viscosity, no vaporization, no solidification, and no decomposition.

The downsides of using gas lubricants are low load capacity, lower tolerance for any errors in load estimation, and the need for a specialist designer in creating smooth surfaces with low clearance.

Selection of the right lubricant

You may know the different types of lubricants as well as the pros and cons of each type. However, you ultimately have to be able to pick the right kind of lubricant for the job at hand. The major factors that one needs to look at when selecting lubricants are load level, speed, sealing requirements, and environmental parameters.

One also needs to look at the coefficient of friction, the operating temperature, the area of contact, and the thermal conductivity. Once you have studied these metrics and have a good idea of the environment in which the lubricant is to be used, you will be able to select the right kind of lubricant.

Generally speaking, for lower loads and speeds, solid lubricants are a good bet. Then as you move towards higher loads and higher speeds, grease becomes a better bet. Grease is then followed by high viscosity oils, low viscosity oils, and finally gas lubricants. For the highest speed applications, gas lubricants are used.