Page 193 - Handbook of Surface Improvement and Modification

188 The Coefficient of Friction

cate that 0.5 wt% of MoS hollow microspheres in liquid paraffin can sufficiently reduce
2
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the friction coefficient. Figure 11.31 illustrates lubrication wear mechanism in the pres-
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ence of MoS hollow microspheres. The MoS hollow microspheres are present between
2
2
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the surfaces of the friction pair together with the liquid paraffin. They perform according
to the common lubricating mechanisms resulting from their spherical structures rolling
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and deformation, which reduce the friction and wear. When the concentration of additive
is small (0.02 wt%), the amount of hollow microspheres entering the friction pair is insuf-
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ficient to bear the pressure, therefore, some of them are crushed (Figure 11.31a). When
MoS concentration is higher (0.30 wt%), there is a sufficient amount of additive between
2
the surfaces of friction pair, and the damage of hollow microspheres can be avoided, as
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showed in Figure 11.31b. The exfoliation becomes the dominant form of lubrication
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mechanism.
Effect of 25 keV proton irra-
diation with fluence of 2.25x10 17
2
ion/cm on the friction and wear
properties of polyimide has been
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studied. The proton irradiation
induced bond breaking to form the
carbon-enriched structure on poly-
imide surface and increased the
surface hardness and the surface
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energy (Figure 11.32). The irra-
Figure 11.32. The wear mechanism. (a) before, and (b)
after irradiation. [Adapted, by permission, from Lv, M; Zheng, F; diation depth was restricted to 514
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Wang, Q; Wang, T; Liang, Y, Wear, 316, 30-6, 2014.] nm. Proton irradiation increased
the initial friction coefficient (the
friction coefficient was closely related to the surface hardness of the material) and
decreased the steady friction coefficient of polyimide (the main wear mechanism includes
three-body abrasion wear, three-body abrasion, and the low surface energy which reduced
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the wear rate and the friction coefficient).
Gamma irradiation (up to 3 MGy) effects on thermal, physical and tribological prop-
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erties of polyetheretherketone under water lubricated conditions have been studied. The
glass transition and melting temperatures, hardness, and density were increased after
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exposure to gamma irradiation. The highest glass transition and melting temperatures
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were observed at 0.5 MGy. Surface stresses generated due to crosslinking increased
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coefficient of friction to its highest value at 0.5 MGy dose.
Environmental exposures affected the sliding behavior of polyetheretherketone com-
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posites. Polyetheretherketone compounds were filled with graphite flakes, carbon nano-
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tubes, or titanium dioxide particles. Pure polyetheretherketone is environmentally
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sensitive, leading to a lower friction but a high wear under vacuum conditions. A very
low friction was obtained with composites containing graphite and carbon nanotubes in
hydrogen, while the lowest wear rate was achieved with the addition of titanium dioxide
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particles both in vacuum and hydrogen environments.