Page 35 - Tribology in Machine Design

22 Tribology in machine design

models of fatigue wear usually include the concept of fatigue failure and also
of simple plastic deformation failure, which could be regarded as low-cycle
fatigue or fatigue in one loading cycle. Theories for the fatigue-life
prediction of rolling metallic contacts are of long standing. In their classical
form, they attribute fatigue failure to subsurface imperfections in the
material and they predict life as a function of the Hertz stress field,
disregarding traction. In order to interpret the effects of metal variables in
contact and to include surface topography and appreciable sliding effects,
the classical rolling contact fatigue models have been expanded and
modified. For sliding contacts, the amount of material removed due to
fatigue can be estimated from the expression

where 77 is the distribution of asperity heights, y is the particle size constant,
Si is the strain to failure in one loading cycle and H is the hardness.
It should be mentioned that, taking into account the plastic-elastic stress
fields in the subsurface regions of the sliding asperity contacts and the
possibility of dislocation interactions, wear by delamination could be
envisaged.

2.8.4. Wear due to chemical reactions
It is now accepted that the friction process itself can initiate a chemical
reaction within the contact zone. Unlike surface fatigue and abrasion,
which are mainly controlled by stress interactions and deformation
properties, wear resulting from chemical reactions induced by friction is
influenced mainly by the environment and its active interaction with the
materials in contact. There is a well-defined sequence of events leading to
the creation of wear particles (Fig. 2.10). At the beginning, the surfaces in
contact react with the environment, creating reaction products which are
deposited on the surfaces. The second step involves the removal of the
reaction products due to crack formation and abrasion. In this way, a
parent material is again exposed to environmental attack. The friction
process itself can lead to thermal and mechanical activation of the surface
layers inducing the following changes:
(i) increased reactivity due to increased temperature. As a result of that the
formation of the reaction product is substantially accelerated;
(ii) increased brittleness resulting from heavy work-hardening.

Figure 2.10 contact between asperities