Biomedical Engineering Reference
In-Depth Information
change, from the boundary lubrication value of 0.1 to the best value achieved in
hydrodynamic lubrication which approximates 0.001.
The region of full fluid-film lubrication is shown on the right-hand side of
Figure 5.2. It is characterized by a rising coefficient of friction as the speed
increases or the load decreases. If the coefficient of friction is recorded in a bearing
as the speed and load are changed, the results are similar to a Stribeck curve
(Figure 5.2).
Generation of bearing wear debris or surface damage is studied using the
boundary and mixed lubrication regimes, where direct contact occurs between
two bearing surfaces at asperity levels [10]. Wear mechanisms of an ultrahigh
molecular weight polyethylene (UHMWPE), commonly used in bearing material
of joint prostheses, have been well established qualitatively, by observation of
the polyethylene surface and debris formation under high magnification. Three
classical mechanisms of wear have been established for the sliding of a hard
counterface on polyethylene. These are (i) adhesive wear; (ii) counterface or
third-body abrasive wear (Figure 5.3); and (iii) fatigue wear derived from plowing
friction (Figure 5.4). Particles such as bone and bone cement have been found
embedded beneath and on the surface of the polyethylene components after clinical
retrieval and these particles can cause third-body debris (abrasive grains). Some
researchers have concluded that third-body debris is the main cause of increased
polyethylene wear and premature failure
in vivo
and suggest that the use of bone
cement in joint replacement can increase the probability of increased counterface
roughness.
As can be seen from the examples of testing protocol of the American Society
for Testing and Materials (ASTM) F732-82 [11], the pin-on-plate wear test is one of
the most widespread and useful configuration tests. Despite the development and
extensive use of simple pin-on-plate tests, there have been relatively few studies
concerning the effect of the pin geometry on wear rate (Figure 5.5) [12].
Abrasive wear
Abrasive wear
(third-body wear)
Adhesive wear
Figure 5.3
Wear mechanisms.
