Page 33 - Tribology in Machine Design

20 Tribology in machine design

estimated from the expression proposed by Archard

where k is the wear coefficient, L is the sliding distance and H is the hardness
of the softer material in contact.
The wear coefficient is a function of various properties of the materials in
contact. Its numerical value can be found in textbooks devoted entirely to
tribology fundamentals. Equation (2.14) is valid for dry contacts only. In
the case of lubricated contacts, where wear is a real possibility, certain
modifications to Archard's equation are necessary. The wear of lubricated
contacts is discussed elsewhere in this chapter.
While the formation of the adhesive junction is the result of interfacial
adhesion taking place at the points of intimate contact between surface
asperities, the failure mechanism of these junctions is not well defined.
There are reasons for thinking that fracture mechanics plays an important
role in the adhesive junction failure mechanism. It is known that both
adhesion and fracture are very sensitive to surface contamination and the
environment, therefore, it is extremely difficult to find a relationship
between the adhesive wear and bulk properties of a material. It is known,
however, that the adhesive wear is influenced by the following parameters
characterizing the bodies in contact:
(i) electronic structure;
(ii) crystal structure;
(iii) crystal orientation;
(iv) cohesive strength.
For example, hexagonal metals, in general, are more resistant to adhesive
wear than either body-centred cubic or face-centred cubic metals.

2.8.2. Abrasive wear
Abrasive wear is a very common and, at the same time, very serious type of
wear. It arises when two interacting surfaces are in direct physical contact,
and one of them is significantly harder than the other. Under the action of a
normal load, the asperities on the harder surface penetrate the softer surface
thus producing plastic deformations. When a tangential motion is intro-
duced, the material is removed from the softer surface by the combined
action of micro-ploughing and micro-cutting. Figure 2.8 shows the essence
of the abrasive-wear model. In the situation depicted in Fig. 2.8, a hard
conical asperity with slope, 0, under the action of a normal load, W, is
traversing a softer surface. The amount of material removed in this process
can be estimated from the expression

Figure 2.8