 Myself Dr. Nitin Gramopadhyay working as assistant professor in Department of Humanities and Sciences. Today we are going to learn the topic lubricant and one of the beat from that topic that is types of lubricants. This is the learning outcome for today's session. At the end of this session students will be able to describe different types of lubricants. This is the content for today's session. We will learn the different types of lubricants such as solid lubricant, liquid lubricant and semi-solid lubricant. Now first we will start with the solid lubricant. Solid lubricants are used where it is not possible to form and maintain a film of liquid lubricant due to the operating conditions of the machine. If there is a possibility of contamination of a material with lubricant, if the operating temperature or load is too high. So these are the probable conditions of the machine where the solid lubricants are best such as when it is not possible to form a film of liquid lubricant due to operating conditions of the machine. When there is a question of contamination of a material with lubricant and the operating conditions are too extreme. The two most usual solid lubricants used are graphite and molybdenum disulphite. Graphite consists of layers of flat plates which are held together by only weak bonds. Consequently the parallel layers slide over one another easily. Usually some organic substances are mixed with solid lubricants so that they may stick firmly to the metal surface. Graphite is most widely used of all solid lubricants. It is very soapy in nature. It is non-inflammable and not oxidized in air below 375 degree centigrade. So graphite is a best solid lubricant. When graphite is dispersed in oil, it is called as oil dag and when it is dispersed in water it is called as the aqua dag. So it is the one method of using graphite as a lubricant. Sometimes graphite is directly used as a solid material but somewhere if it is not possible to use graphite as a solid material it is a practice to convert graphite into slurry either by adding little quantity of oil or the little quantity of water. When a graphite is mixed with little quantity of oil the mixture is called as the oil dag and when it is mixed with little quantity of water it is known as the aqua dag. So oil dag is found particularly useful in internal combustion engines because it forms a film between the piston rings and the cylinder and gives a tight fit contact thereby increasing compression. On the other hand oil dag is useful where a lubricant free from oil is needed. Example the foodstuff industry also graphite is mixed with greases to form graphite based greases which are used at still higher temperatures. This is the second example of the solid lubricant that is molybdenum disulfide. Molybdenum disulfide has a sandwich like structure in which a layer of molybdenum atoms lies between two layers of sulfur atoms. The poor inter laminar attraction is responsible for low shear strain in direction parallel to the layers and is stable in air up to 400 degree centigrade. It is also used along with solvents and in greases. Besides graphite and molybdenum disulfide other substances like soapstone, talc, mica, etcetera are also used as the solid lubricants. So this is the structure of graphite and this is the structure of the molybdenum disulfide where graphite exists in a multilayered structure with a weak bonds and during the operation these slides over each other and it gives flexibility to use such a solid material as lubricant. So here in case of molybdenum disulfide the atom of molybdenum is linked with the sulfur the alternative layers are there. So these are the two structures of one is graphite and one is molybdenum disulfide. So this is one more variety of lubricant that is called as the synthetic lubricant. Petroleum based lubricants can be used under abnormal conditions like extremely high temperature, chemically reactive atmosphere, etcetera by employing certain specific additives. However, synthetic lubricants have been developed which alone can meet the most drastic and severe conditions such as those existing in aircraft engines in which the same lubricant may have to use in temperature range of minus 50 to plus 250 degree centigrade. So synthetic lubricants are used in a machine which operates at the extreme conditions of temperature, speed, load, etcetera. So this is the one example of aircraft engine where the engine shows the temperature range lowest minus 50 and the plus 250 degree centigrade. So synthetic lubricants serves even in such conditions to the machine. Synthetic lubricants possesses the following distinguishing characteristics such as non-inflammable, high flash points, high thermal stability, high viscosity index, chemical stability. The important synthetic lubricants are polymerized hydrocarbons, polyglycols, etcetera. Now this is one more very important type of lubricant known as liquid lubricant and one of the example of liquid lubricant is animal and vegetable oils. The vegetable and animal oils were the most commonly used liquid lubricants before the modern industrialization. Animal and vegetable oil possesses good oiliness and viscosity property. However, animal and vegetable oils are costly, undergo oxidation and get thickened on coming in contact with moist air or aqueous medium. So they are rarely used as such. They are used with blending agent with other lubricating oils. This is the second example of liquid lubricant which is known as mineral or petroleum oil. These oils are obtained by distillation of crude oil. The shorter chain oils have lower viscosity than the longer chain hydrocarbons. These are most widely used lubricants because they possess the properties like low in cost available in large scale and quite stable under service conditions. So mineral oils, petroleum oils are quite stable over the wide range of the operating conditions whereas if you compare mineral oils with the animal oil and vegetable oil, they are good in viscosity, oiliness, but they possess very poor resistance to the extreme operating conditions. However, they possess poor oiliness as compared to that of animal and vegetable oils. The oiliness of petroleum oils can be increased by addition of high molecular weight compounds like olec acid, steric acid etcetera. Now, here I would like to ask you one question. Animal and vegetable oils are option A, good in viscosity, option B, good in thermal stability, option C, good in corrosion resistance and option D, all of these. Think for a moment and the correct answer is option A that is animal and vegetable oils are good in viscosity. Now, this is also one of the example of liquid lubricant which is known as blended oils. Till now, we have learnt animal oil, petroleum oil, these are the natural we are taking from the nature, but they possess certain drawbacks and therefore, not a single oil serves as the most satisfactory lubricant for many of the modern machineries. Typical properties of petroleum oils are improved by incorporating specific additives. These so called blended oils give desired lubricating properties required for a particular machine. The following additives are employed to improve quality of the liquid lubricant such as oiliness improvers, extreme pressure additives, power point depressing additives, viscosity index improvers, polymeric thickeners, antioxidants or inhibitors, corrosion preventers and anti foaming agents. So, generally animal oil, vegetable oil or petroleum oils are best blended with the help of these agents to give the best mixture which is known as the blended oils. Then, one more type of the lubricant is semi-solid lubricants, greases are prepared by saponification of fat with alkali like lime, caustic soda, etc. followed by adding hot lubricating oil under agitation. The total amount of mineral oil added determines the consistency of the finished greases. Greases have higher shear or fractional resistance than oils and therefore, can support much heavier loads at lower speeds. They also require as much attention unlike the lubricating liquids, but greases have a tendency to separate in to oil science soaps. Greases that is known as semi-solid lubricants are used in situation where oil cannot remain in place due to high load, low speed, sudden jerks, etc. For example, rail access in bearing gears and gears that work at high temperatures. In situation where bearing needs to be sealed against dust, dirt or moisture because greases are less liable to contamination, greases if used do not flash or drip over article being prepared by the machine. So, these are the conditions where greases that is semi-solid lubricants are best. So, all these material I have prepared by using a textbook of engineering chemistry written by Jane and Jane. Thank you.