Based on its physical states, you can classify lubricants as:
Liquid lubricants, which include:
Vegetable oil and animal oil
Mineral oil from petroleum
Blended oil, doped oil, or compound oil
Semisolid lubricants
Solid lubricants
Synthetic Lubricants
Liquid lubricants reduce friction and wear between two moving or sliding metallic surfaces by providing a continuous fluid film in between them. They act as a cooling medium, a sealing agent, and a corrosion preventor. Liquid lubricants are classified into many types, depending on the type of base oil used.
These oils are the most commonly used lubricants. They are quite oily and are absorbed by all the metallic surfaces. However, they decompose at high temperature, undergo oxidation easily, forming gummy and acidic hydrolyzed products, and thicken on coming in contact with air, restricting the smooth movement of moving surfaces. Consequently, these oils are blended with mineral oils to overcome these restrictions.
Mineral oils that are used as lubricants are obtained by fractional distillation of petroleum. Petroleum oils have long-chain hydrocarbons, ranging between 12-50 Carbon atoms. The shorter chain oils have lower viscosity than the longer chain hydrocarbons do. Shorter chain hydrocarbons are widely used as lubricants because they possess good stability. At very high pressure and speed, petroleum oil possesses poor oiliness but the oiliness of these oils can be improved by adding oils, such as Oleic and Stearic acid.
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