Page 34 - Tribology in Machine Design

Basic principles of tribology 21

where E is the elastic modulus, H is the hardness of the softer material, K ]c is
the fracture toughness, n is the work-hardening factor and P y is the yield
strength.
The simplified model takes only hardness into account as a material
property. Its more advanced version includes toughness as recognition of
the fact that fracture mechanics principles play an important role in the
abrasion process. The rationale behind the refined model is to compare the
strain that occurs during the asperity interaction with the critical strain at
which crack propagation begins.
In the case of abrasive wear there is a close relationship between the
material properties and the wear resistance, and in particular:
(i) there is a direct proportionality between the relative wear resistance
and the Vickers hardness, in the case of technically pure metals in an
annealed state;
(ii) the relative wear resistance of metallic materials does not depend on
the hardness they acquire from cold work-hardening by plastic
deformation;
(iii) heat treatment of steels usually improves their resistance to abrasive
wear;
(iv) there is a linear relationship between wear resistance and hardness for
non-metallic hard materials.
The ability of the material to resist abrasive wear is influenced by the extent
of work-hardening it can undergo, its ductility, strain distribution, crystal
anisotropy and mechanical stability.

2.8.3 Wear due to surface fatigue
Load carrying nonconforming contacts, known as Hertzian contacts, are
sites of relative motion in numerous machine elements such as rolling
bearings, gears, friction drives, cams and tappets. The relative motion of the
surfaces in contact is composed of varying degrees of pure rolling and
sliding. When the loads are not negligible, continued load cycling
eventually leads to failure of the material at the contacting surfaces. The
failure is attributed to multiple reversals of the contact stress field, and is
therefore classified as a fatigue failure. Fatigue wear is especially associated
with rolling contacts because of the cycling nature of the load. In sliding
contacts, however, the asperities are also subjected to cyclic stressing, which
leads to stress concentration effects and the generation and propagation of
cracks. This is schematically shown in Fig. 2.9. A number of steps leading to
the generation of wear particles can be identified. They are:
(i) transmission of stresses at contact points;
(ii) growth of plastic deformation per cycle;
(iii) subsurface void and crack nucleation;
(iv) crack formation and propagation;
(v) creation of wear particles.
A number of possible mechanisms describing crack initiation and propag-
Figure 2.9 ation can be proposed using postulates of the dislocation theory. Analytical