Page 29 - Tribology in Machine Design

16 Tribology in machine design

For most engineering materials this ratio is of the order of 0.2 and means
that the friction coefficient may be of the same order of magnitude. In the
case of clean metals, where the junction growth is most likely to take place,
the adhesion component of friction may increase to about 10-100. The
presence of any type of lubricant disrupting the formation of the adhesive
junction can dramatically reduce the magnitude of the adhesion com-
Figure 2.2
ponent of friction. This simple model can be supplemented by the surface
energy of the contacting bodies. Then, the friction coefficient is given by (see
Fig. 2.2)

/
where W 12 =yi+y 2 —Tia is the surface energy.
Recent progress in fracture mechanics allows us to consider the fracture
of an adhesive junction as a mode of failure due to crack propagation

where <7 12 is the interfacial tensile strength, 6 C is the critical crack opening
displacement, n is the work-hardening factor and H is the hardness.
It is important to remember that such parameters as the interfacial shear
strength or the surface energy characterize a given pair of materials in
contact rather than the single components involved.

2.4. Friction due to Ploughing occurs when two bodies in contact have different hardness. The
ploughing asperities on the harder surface may penetrate into the softer surface and
produce grooves on it, if there is relative motion. Because of ploughing a
certain force is required to maintain motion. In certain circumstances this
force may constitute a major component of the overall frictional force
observed. There are two basic reasons for ploughing, namely, ploughing by
surface asperities and ploughing by hard wear particles present in the
contact zone (Fig. 2.3). The case of ploughing by the hard conical asperity is
shown in Fig. 2.3(a), and the formula for estimating the coefficient of
friction is as follows:

Asperities on engineering surfaces seldom have an effective slope, given by
0, exceeding 5 to 6; it follows, therefore, that the friction coefficient,
according to eqn (2.9), should be of the order of 0.04. This is, of course, too
low a value, mainly because the piling up of the material ahead of the
moving asperity is neglected. Ploughing of a brittle material is inevitably
associated with micro-cracking and, therefore, a model of the ploughing
process based on fracture mechanics is in place. Material properties such as
Figure 2.3 fracture toughness, elastic modulus and hardness are used to estimate the