Friction and wear - Orthopaedic Biomaterials in Research and Practice - page 264

small. Because of their extreme hardness, ceramic components can be
polished to a fine surface finish that will facilitate their potential for
fluid film lubrication during walking.
Boundary . The other extreme occurs at low values of ϕ that permit
surface-surface contact. Lubricant films are very thin, on the order
of the asperity height, and lubrication occurs not by lubricant shear
but by modification of surface properties. For a material, either
liquid or solid, to be a good boundary lubricant, it is necessary for
it to interact with the surface. Long-chain molecules with a chemi-
cally active region, such as fatty acids, are highly effective in this
respect, reacting with surfaces to form soaps. However, both fric-
tional coefficients and wear rates are relatively high, compared
with those in other lubrication regimens. Because of the relatively
high roughness of polyethylene, traditional MOP arthroplasty con-
tact articulations fall into the boundary lubrication regime.
Elastohydrodynamic . The transition from hydrodynamic to boundary
lubrication is not abrupt but passes through two other regimes. In the
first of these regimes, the elastrohydrodynamic, the lubricant prevents
the surfaces from interacting directly, but pressure waves conducted
through it can produce elastic deformations in one surface opposite
asperities in the other. This permits maintenance of a thicker lubricant
film than would be otherwise expected. The result is excellent lubrica-
tion, with slightly higher μ D than in pure hydrodynamic lubrication,
and with relatively low wear rates.
Mixed . At still lower values of ϕ, the lubricant film becomes discon-
tinuous and a mixed regimen of elastohydrodynamic and bound-
ary lubrication occurs. This regimen is not terribly efficient and
is usually accompanied by higher wear rates than the other three.
Traditional metal-on-metal bearings typically fall into the mixed
lubrication category.
In addition to these four classical lubrication regimes, there are addi-
tional important lubrication processes that may occur in special cases
( Fig u re 11.6).
Hydrostatic . In some engineering applications, an external pressure
source is provided to maintain h above some critical limit, thus
producing lower values of μ D .
Weeping. If the bearing surfaces are porous and deformable, relative
motion may squeeze additional lubricant out of the surfaces into
the separating film. This is a highly efficient mechanism and is
believed to be a contributing factor to the low coefficients of fric-
tion observed in articular cartilage-lined joints.*
* The mechanisms of lubrication in natural joints are still a matter of some debate. The
present consensus is that it is a combination of boundary, elastohydrodynamic, and
weeping lubrication. Lubrication in replaced joints is primarily elastohydrodynamic,
with the lubricant, produced by regenerating synovial tissues, resembling normal joint
fluid.