# Boundary Lubrication

The type of lubrication, between two surfaces in relative motion, in which the primary factor affecting the lubricating performance is the lubricant′s frictional properties, rather than the lubricant′s viscometric properties.

Sometimes referred to as thin film lubrication, connoting that, under high loads, the lubricant film is not thick enough to prevent surface asperities from contacting each other.

On a microscopic level, boundary lubrication is caused by asperity contact. Boundary lubrication arises from contact between many asperities of various shapes, it is normally treated stochastically because the distribution of asperity heights is essentially random.

The Greenwood and Tripp model treats the asperities as a statistical distribution and gives the contact pressure as:

Where

P_{c} = the nominal contact pressure between the surfaces

d = the mean separation of the two surfaces

η = is the asperity density per unit area

β = asperity peak radius of curvature

φ(z) = the probability distribution of asperity heights

z = offset between asperity height mean and surface height mean

Young′s modulus (E) and standard deviation of asperity heights (σ) are taken to be composite values, where E_{1}, E_{2} and σ_{1}, σ_{2} represent the respective values for Young′s modulus and asperity standard deviation for each surface:

### Reference

Luke Moughon, "Effects of Piston Design and Lubricant Selection on Reciprocating Engine Friction", Mechanical Engineering Georgia Institute of Technology, 2004

**See also: **Hydrodynamic Lubrication, Lubricating Oil, Lubrication, Mixed Lubrication, Oil.

**Subjects: ** Mechanical Engineering