Tools to Probe Nanoscale Surface Phenomena in Cellulose Thin Films: Applications in the Area of Adsorption and Friction - The Nanoscience and Technology of Renewable Biomaterials - page 98

4.5 Lubrication

Lubrication phenomena are involved when a finish or lubricant is applied to (moving)
objects as means to reduce friction between them. Amonton's law was proposed in the
17th century in order to analytically describe sliding friction at the macroscopic scale
(Dowson 1998):

µ = F f /N (4.1)

where µ is the coefficient of friction, a dimensionless scalar value that describes the
ratio of the force of friction between two bodies, F f , the force pressing them together
and the normal force applied, N . From a macroscopic perspective, µ is a constant
related to the nature of both contacting objects. The frictional force ( F f ) is independent
of the apparent contact surface. The Amonton equation can be applied in many cases
at the macroscopic scale and for sliding objects directly in contact. However, simple
experimental observation has shown that frictional forces do depend on the contact area ,
the surface roughness as well as the chemical nature of the sliding substances.
When dealing with fluid lubricants the situation becomes more complicated since the
gap between the two moving objects may vary. The friction coefficient may depend
on the gap between the sliding surfaces as well as the sliding speeds or shear rates.
According to Hamrock (Hamrock, Schmid et al . 2004), four different regimes of fluid
film lubrication can be defined, i.e. boundary, mixed, elasto-hydrodynamic and hydrody-
namic regimes. These regimes depend on a liquid film parameter, . A plot of friction
coefficient as a function of is illustrated by the Stribeck curve (Figure 4.5). The film
parameter, , represents the minimum film thickness separating the two surfaces and
can be quantified by using Equation (4.2):

= V × η b /P (4.2)

Boundary lubrication

a

b

Elastohydrodynamic
lubrication (EHL)

Hydrodynamic
lubrication

Film parameter, Λ

Mixed lubrication

Figure 4.5 Stribeck curve displaying the different regimes of lubrication. Figure redrawn
fromHamrock,Schmidetal. (2004).