Boundary Lubrication or Thin-Film Lubrication

This is done, while a continuous film of lubricant cannot persist and direct metal to metal contact is possible due to certain reasons. This happens while

1.    a shaft starts moving from rest, or
2.    the speed is very low, or
3.    the load is very high, and
4.    viscous force of the oil is too low under such conditions, the clearance space among the sliding surfaces is lubricated with an oil lubricant, a thin layer of that is absorbed (that is surface associated) by physical or chemical forces or both by the metallic surfaces. This absorbed layer ignores direct metal to metal contact. The load is carried through the layers of the absorbed lubricant on both of the metal surfaces. The coefficient of friction in such circumstances is usually 0.05 to 0.15.

Vegetable & animal oils & their soaps possess property of adsorption (or surface attachment) either physically or react chemically with along metal surface forming a thin film of metallic soap, which behaves as lubricant. The load is carried by the two layers of adsorbed lubricant.

For boundary lubrication lubricant molecules should have following

1.    long hydrocarbon chain,
2.    polar groups to promote spreading over the metallic surfaces at high pressure,
3.    lateral attraction among the chains, and
4.    active groups or atoms which might form chemical linkages along with the metal or other surfaces.
High viscosity, resistance to heat up & oxidation, good oiliness, and low pour point are some good characteristics of boundary lubricants.