Page 190 - Handbook of Surface Improvement and Modification

11.1 Methods and mechanisms of improvement of the coefficient of friction 185

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wear. Smaller particles (10-15
nm) were more effective in reduc-
tion of wear but had much less
influence on the friction coeffi-
31
cient. Nanoparticles may act as a
rolling element, which can
decrease friction and increase
wear resistance of polymer com-
31
posites.
Rare earth oxide (La O )
2
3
Figure 11.28. a) Light optical microscopy of micro-constituent microparticles were added to poly-
(short carbon fibers), b) scanning transmission electron micros-
copy of nano constituent (silica nanoparticles). [Adapted, by per- imide composites aiming at
mission, from Österle, W; Dmitriev, AI; Wetzel, B; Zhang, G; improvement of its tribological
Häusler, I; Jim, BC, Mater. Design, 93, 474-84, 2016.] 32
properties. With La O micropar-
2
3
ticles, composite had a larger surface roughness, a lower surface energy, and a higher
32
hydrophobicity than the neat polyimide. It had beneficial layered structure differing
32
from the compact structure of neat polyimide (Figure 11.27). The introduction of La O 3
2
microparticles resulted in altered surface composition of polyimide which reduced polar
32
contribution of surface energy. The 70% reduction in the friction force and the coeffi-
32
cient of friction and a 30% reduction in wear rate were achieved. With the increase in the
content of La O microparticles, the friction force decreased and then increased after the
3
2
32
optimal loading of 1.5 wt%.
The effect of carbon fibers and silica nanoparticles on friction and wear reduction of
33
an advanced polymer matrix composite has been studied. Figure 11.28 shows compo-
33
nents of micro- and nano-filling system. The composite had lasting antiwear and anti-
friction properties because flash temperatures at micron-sized carbon fibers lead to
33
polymer degradation and subsequent release of nanoparticles. The released particles
were mixed with other wear products and they formed stable films preventing further
33
severe oxidational wear. The released wear product embedded carbon fibers preventing
33
fiber fragmentation and participation in the third body abrasion.
The hierarchical porous polyetheretherketone composites with mesoporous titanium
34
oxide whisker have self-lubricating properties. The nano-microporous polyetheretherke-
tone composites with 30 wt% micro-porogen (NaCl added to composition which after
o
extrusion molding was leached out to water at temperature of 80 C) and 5 wt% mesopo-
rous titanium oxide whiskers (synthesized from the sintered K Ti O , which involves a
2
5
2
hydrolytic step for the mesoscopic microphase separation) reached the lowest friction
16
3
coefficient and specific wear rate, which were recorded as 0.0194 and 2.13x10 m /Nm
under the load of 200 N (15 wt% carbon fiber-reinforced polyetheretherketone composite,
34
widely used in industry, has the wear resistance 41 times higher).
Fluoropolymers have low friction coefficients but their use as solid lubricants is
3
4
35
inhibited by high wear rates (1-5x10 mm /Nm). The addition of certain types of α-alu-
mina reduced the wear rate of polytetrafluoroethylene by over three orders of magni-
35
tude. Perfluorinated copolymer of tetrafluoroethylene and perfluorinated alkylvinyl
35
ether were injection molded with various weight fractions of α-alumina. The composites