Where thick film lubrication fails, thin film lubrication is done. Thin film or boundary Lubrication is done for those cases in which the continuous film of lubrication cannot persist and direct metal to metal contact is possible.
- A Shaft starts moving from rest
- The speed is very low
- Load is very high and iv) viscosity of lubricating oil is very low.
A thin lubricant should have high viscosity index, good resistance to heat and oxidation, good oiliness and low pour point. Since the thickness of the oil film between the friction surfaces is smaller than their surface roughness in boundary lubrication.
In boundary lubrication, the oily lubricant is filled in between the sliding or moving metallic surfaces, which is adsorbed on both the metallic surfaces by any physical or chemical means and cannot be removed easily. The adsorbed lubricant from two thin layers between the sliding metallic surfaces and avoid the direct metal to metal contact. As a result of this, the frictional resistance between the sliding metallic surfaces is reduced.
Vegetable and animal oils and their soaps possess great property of adsorption and are used for thin film lubrication. Graphite and molybdenum disulphide in oil are also used for boundary lubrication. These lubricants form a thin film on the metallic surfaces, which reduces the friction and can bear compression as well as high temperature.
Wear in boundary lubrication
The main types of wear in boundary lubrication are as follows.
- Adhesive wears: Caused by adhesion of micro asperities to friction surfaces.
- Abrasive wears: Caused by cutting by hard, foreign particles or abrasive particles in oil.
- Surface fatigue wears: Caused by surface fatigue resulting from repeated stress and chemical interactions. Chemical wear: Dominated by chemical reactions between friction surfaces and lubricant molecules.
For the boundary lubrication, the lubricant molecules should possess these properties. They should have long hydrocarbon chains and the lateral attraction between the chains.