Page 175 - Handbook of Surface Improvement and Modification

170 The Coefficient of Friction

faces (solid-solid adhesion, liquid-
mediated adhesion, dry friction,
wet friction, and friction models −
all illustrated with examples and
1
mathematical equations.
Macro-scale surface patterns
are often used to improve gripping
3
and sliding. Polyacetal samples
with micron-scale surface struc-
tures were examined to determine
their effects on the sliding friction
and compared with similarly pat-
Figure 11.5. Correlation between elastic modulus, crystallinity, and terned polypropylene. Dynamic
3
coefficient of friction in copolymers 6/12. [Adapted, by permis-
sion, from Álvarez-Láinez, ML; Palacio, JA, Wear, 372-373, 76- friction levels depended on surface
80, 2017.] pressure. The micro-pattern influ-
3
enced friction at low loads (a fric-
tion minimum at the surface
pressure range of 0.5-1 MPa for
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both PP and POM). Figure 11.4
shows that the main wear mecha-
nism for both patterned materials
is a plastic deformation of the
micro-bump tops producing
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shaved debris. It can be deduced
that flake-like wear debris do not
3
stick to wearing surface. Lower
contact area causes a decrease in
friction as long as the high load
Figure 11.6. Set-up of the tribology cell. [Adapted, by permission,
from Álvarez-Láinez, ML; Palacio, JA, Wear, 372-373, 76-80, does not flatten the entire struc-
3
2017.] ture.
PA-6 crystallinity and elas-
4
ticity modulus are higher than the values for PA-12. At the same time, the friction coeffi-
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cient in PA-6 is lower than that of PA-12. The combination of both in 6/12 copolymer
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shows that this relation of values also holds for copolymer (Figure 11.5). The lower the
value of either the crystallinity or the elastic modulus the higher the coefficient of friction
as measured by with rotational rheometer with tribology cell arrangement given in Figure
4
11.6.
Coefficient of friction and transfer film formation were investigated by varying sur-
face texture and roughness features of ultra high molecular weight polyethylene pins
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against steel plates using an inclined pin-on-plate sliding tester. The transfer film forma-
tion of UHMWPE on steel material surface depends on the coefficient of friction (depen-
5
dent on the texture of harder material surface). The surface texture was influenced by the
coefficient of friction because of the variation of the hysteresis of friction components of
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different surfaces.