Question

Discuss any one in detail: (i) solid lubricants and (ii) determination of cloud-point and pour-point of oil.

Answer 1

As mentioned in the question I am discussing about only solid lubricant

Definition

Solid or dry lubricant are materials which are able to reduce friction between two surfaces sliding against each other without the need for a liquid oil medium.

Application

Solid lubricants are useful for conditions when conventional lubricants are inadequate, such as:

• Reciprocating motion. A typical application is a sliding or reciprocating motion that requires lubrication to minimize wear, as, for example, in gear and chain lubrication. Liquid lubricants will squeeze out while solid lubricants do not escape, preventing fretting, corrosion, and galling.
• Ceramics. Another application is for cases where chemically active lubricant additives have not been found for a particular surface, such as polymers and ceramics.
• High temperature. Graphite and MoS₂ act as lubricants at high temperature and in oxidizing atmosphere environments, where liquid lubricants typically will not survive. A typical application include fasteners that are easily tightened and unscrewed after a long stay at high temperatures.
• Extreme contact pressures. The lamellar structure orients parallel to the sliding surface, resulting in high bearing-load combined with a low shear stress. Most applications in metal forming that involve plastic deformation use solid lubricants.

Examples of solid lubricant

The two main dry lubricants are graphiteand molybdenum disulfide. They offer lubrication at temperatures higher than liquid and oil-based lubricants operate. Dry lubricants are often used in applications such as locks or dry lubricated bearings. Such materials can operate up to 350 °C (662 °F) in oxidizing environments and even higher in reducing / non-oxidizing environments (molybdenum disulfide up to 1100 °C, 2012 °F). The low-friction characteristics of most dry lubricants are attributed to a layered structure on the molecular level with weak bonding between layers. Only layered structure is not sufficient such layers are able to slide relative to each other with minimal applied force, thus giving them their low friction properties.