Page 32 - Tribology in Machine Design

Basic principles of tribology 19

is not only a function of the usual variables, such as load, contact area
diameter and sliding velocity, but also of the angular velocity. Furthermore,
there is an additional force orthogonal to the direction of linear motion. In
Fig. 2.6, a spherically ended pin rotates about an axis normal to the plate
with angular velocity co and the plate translates with linear velocity V.
Assuming that the slip at the point within the circular area of contact is
opposed by simple Coulomb friction, the plate will exert a force T dA in the
direction of the velocity of the plate relative to the pin at the point under
Figure 2.6 consideration. To find the components of the total frictional force in the x
and y directions it is necessary to sum the frictional force vectors, x dA, over
the entire contact area A. Here, i denotes the interfacial shear strength. The
integrals for the components of the total frictional force are elliptical and
must be evaluated numerically or converted into tabulated form.

2.8. Types of wear and Friction and wear share one common feature, that is, complexity. It is
their mechanisms customary to divide wear occurring in engineering practice into four broad
general classes, namely: adhesive wear, surface fatigue wear, abrasive wear
and chemical wear. Wear is usually associated with the loss of material from
contracting bodies in relative motion. It is controlled by the properties of
the material, the environmental and operating conditions and the geometry
of the contacting bodies. As an additional factor influencing the wear of
some materials, especially certain organic polymers, the kinematic of
relative motion within the contact zone should also be mentioned. Two
groups of wear mechanism can be identified; the first comprising those
dominated by the mechanical behaviour of materials, and the second
comprising those defined by the chemical nature of the materials. In almost
every situation it is possible to identify the leading wear mechanism, which
is usually determined by the mechanical properties and chemical stability of
the material, temperature within the contact zone, and operating
conditions.

2.8.1. Adhesive wear
Adhesive wear is invariably associated with the formation of adhesive
junctions at the interface. For an adhesive junction to be formed, the
interacting surfaces must be in intimate contact. The strength of these
junctions depends to a great extent on the physico-chemical nature of the
contacting surfaces. A number of well-defined steps leading to the
formation of adhesive-wear particles can be identified:
(i) deformation of the contacting asperities;
(ii) removal of the surface films;
(iii) formation of the adhesive junction (Fig. 2.7);
(iv) failure of the junctions and transfer of material;
(v) modification of transferred fragments;
(vi) removal of transferred fragments and creation of loose wear particles.
Figure 2.7 The volume of material removed by the adhesive-wear process can be