ff3 fs6 fc0 sc0 ls0 ws11">roughness of the counter face was varied. It was postu-
lated that this parameter directly relates to the develop-
ment of the transfer film on the counter face; the rougher
surface facilitates transfer film growth. Similar to the pre-
vious studies, maximum wear resistance was found at
weight pe r centages below 10.
As the percentage of filler increased above this opti-
mum, the composite mater ial experienced more wear than
the unfilled counterpart. Unlike the previous work with
PEEK, the coefficient of friction increased monotonically
with increasing filler concentrations. Li et al. [24] filled
PTFE with nano particles of ZnO. Wear resistance was
improved by nearly two orders of magnitude with a maxi-
mum wear resistance at ZnO concentrations of roughly 15%
by volume. In that study, the friction coefficient of the
nano-composite was higher than the unfilled PTFE. Pet-
rovicova et al. [38] filled Nylon 11 with silica. Wear
resistance increased with increasing concentrations of
nanoscale silica up to 15% by volume. They found that
the friction coefficient of the nano-composite was lower
than that of the unfilled Nylon. Avella et al. [39] filled
polymethylmethacrylate (PMMA) with nano-scale CaCO3.
The abrasion resistance increased as the filler content
was increased, improving by a factor of 2% with 3%
CaCO3 by weight. Yu et al. [40] filled polyoxymethylene
(POM) with micrometer and submicron copper particles.
The nano-composite had less wear and a lower coeffi-
cient of friction than the composites filled with mi-
crometer sized particles of copper and the unfilled POM.
It was hypothesized that the increased surface area of the
submicron copper filler particles improved the bonding
strength at the filler/matrix interface. In Figure 1, the
reported wear-rates of the nano-composites are each nor-
malized by the reported wear-rate of the unfilled matrix
and plotted versus the volume fraction of filler particles.
In Table 1. the wear-rate for the most wear-resistant
formulation of each nano-composite is give n [25].
This compilation of data showed that the optimum
concentrations of nanometer sized filler particles is 2% -
5% by volume. A more typical optimum in polymer
composites made with micrometer sized filler particles is
on the order of 30%. It is also shown that consistent ob-
servation that improvements in wear can be realized with
polymer nano-composites. But the origin of wear resis-
tance improvements in polymer nano-composites is an
open question.
Copyright © 2012 SciRes. WJNSE
A. A. ALY ET AL.
Copyright © 2012 SciRes. WJNSE
34
Figure 1. A plot of normalized wear improvements for polymer nano-composites as a function of filler volume fraction [25].
3. Chemical Surface Modification
Table 1. A comparison of the lowest reported wear-rates for
various polymer nano-composites [25]. Chemical surface modification is widely used to obtain a
high wettability for a solid surface, a good dispersion of
particles, and adhesion of fillers in composite materials
[43]. Chemical surface modification can be categorized
as follows:
Matrix/nano-filler Lowest wear-rate, k
(×10 - 6 mm3/Nm)
PTEE/ZnO
PPS/Al2O3
PEEK/SiC
PEEK/Si3N4
PEEK/ZrO2
PEEK/SiO2
13
10.4
3.4
1.3
3.9
1.4
Surface modificat ion by chemical reacti on
This type of surface modification promotes a chemical
reaction between the polymer matrix and an inorganic
filler to strengthen the adhesion. Two reagents are mainly
used in chemical surface modification to obtain a hydro-
phobic surface. One example of such a reagent is an alkyl
silane coupling agent and another is an alcohol [44-46].
Surface modifi cat ion by non-reactive modifier
As a PTFE matrix, it has been successfully filled with
nano-particles of alumina, zinca, and carbon nano-tubes.
Sawyer et al. [41] used 38 nm Al2O3 filler to improve the
wear performance of PTFE, and the wear resistance of
this nanocomposite increased monotonically with filler
wt%, eventually being 600 times more wear resistant
than unfilled PTFE at a loading of 20 wt%. Burris and
Sawyer [42] and Li et al. [24] have found similar im-
provements for metal-oxide nano-composites of PTFE at
high weight percents of filler. The promise of nano-com-
posites, however, was that low weight percents could
provide such improvements. This study is the first in
which low weight p ercentag e of filler p articles are shown
to provide over 1000× improvements in wear rate. The
difference between these composites and previous nano-
composites of PTFE is that the nano-particles are irregu-
lar in shape, as opposed to the spherical shape of previ-
ous composites.
In most cases the surface of the nano-particles used for
nano-particle filled polymer composites prepared by dis-
persive mixing for tribological applications have not been
pre-treated. If no specific surface treatment is applied be-
forehand, the unique nano-effect of nano-particles cannot
be fully brought into play. Therefore, pre-treatment of
nano-particles before every experiment is necessary.
A nonreactive modifier reduces the interaction between
the filler particles within agglomerates by reducing the
physical attraction rather than by any chemical reaction
[47,48].
Stearic acid has been widely used as a non-interacting
surface modifier. Modified nano-filler can easily be in-
corporated into a polymer matrix, resulting in a decrease
in the melt viscosity and, in most cases, an improved dis-
persion of the nano-filler in the composite [49,50]. The
wettability of silica nano-particles and the adhesion be-
A. A. ALY ET AL. 35
tween the filler and the polymer matrix were improved
by modifying the silica nano-particles with stearic acid
[51]. The non-reactive stearic acid reduces the interaction
between the filler particles within agglomerates by re-
ducing the physical attraction, rather than by any chemi-
cal reaction. However there has been little research re-
lated to the mechanical properties of silica nano-particle
reinforced PEN composites with the modification of stea-
ric acid.
Surface modified silica nano-particle reinforced PEN
composites were melt blended to investigate the effect of
stearic acid modification on the mechanical properties,
crystallization behavior, and the processibility of the sil-
ica nano-particle reinforced PEN composites. The inter-
facial properties of the composites with stearic acid modi-
fication were quantitatively analyzed from tensile test
results with various theoretical models [52].
The attempts to improve the tribological properties of
pure poly-electrolyte multilayers (PEMs) have been largely
focused on transforming the structure of PEMs. It was
found that the load carrying capacity of the polyelectro-
lyte multilayer film increased and better anti-wear prop-
erties can be obtained with compositing nanoparticles
[53].
Zhang et al. [54-57] have systematically studied the
tribological behavi or of composi t e Langmuir-Blodgett (LB)
films consisted of organic molecule and inorganic nano-
particles, they found that the nano-particles play a key
role in increasing antiwear life of LB films, enhancing
the load-carrying capacity of the films. In the present
years, the organic-inorganic hybrid thin films have also
attracted widespread attention because of their low fric-
tion coefficient and relatively long antiwear life [58].
Nucleation and growth of ZnS nano-particles were
achieved in a poly (diallyl dimethylammonium chloride)
(PDDA)-poly(acrylic acid) (PAA) film prepared by the
layer-by-layer deposition technique. It was found that ZnS
nano-particles within PEMs possess load-carrying capac-
ity and enhance antiwear life. Moreover, the PEMs with
three reaction cycles show considerably lower friction
coefficient and higher antiwear life than the PEMs with
six reaction cycles [59].
Attapulgite (or palygorskite) is a clay mineral that to-
gether with sepiolite forms the group of fibrous clay
minerals. The structure of attapulgite was first proposed
by Bradley [60]. It finds important uses as an animal
waste adsorbent, pesticide carrier, decolorizing agent, in
the oil refining and pharmaceutical industries and catalyst
and catalyst support [61-63]. Due to its especial layer-
chain structure and low price, nowadays attapulgite is
receiving a great deal of attention. However, until now,
not much information has been available on the friction
and wear behaviors of the PTFE composite filled with
nano-attapulgite.
Silica/polymer composites were found to posses unique
physical, chemical and electromechanical properties, which
have extensive applicatio n potential [64]. The addition of
various nano-sized fillers into polymer which may cause
an improvement in the tribological feature at low filler
contents due to a change of the wear mechanisms were
reported by Wang and Xue [65]. Su and co-workers [66]
revealed the friction and wear behaviors of the resulting
carbon fabric composites sliding against AISI-1045 steel
in a pin-on-disk apparatus. Nanofillers, such as TiO2,
ZnO, SiO2 and Si3N4 were reported to be effective in
improving the friction and wear p roperties of some po ly-
mer and fabric composites [67,68].
4. Use of Multi Functional Nano-Fillers
Integrating various functional fillers is a principal route
to develop high performance composite materials which
cannot be achieved by using the single filler alone [69].
Considerable attempts have been made to incorporate
different additional fillers in to SFRPs in order to further
improve the tribological performance. In particular,
lubricating particulates such as polytetrafluoroethylene
(PTFE), graphite and molybdenum disulfide (MoS2) have
been successfully used to reduce the friction coefficient
and the wear rate of SFRPs [70-73]. These solid lubri-
cants are generally helpful in developing a uniform trans-
fer layer on the surface of metallic counterparts, which
protects fib ers from severe abrasive wear.
More recently, nano-sized inorganic particles have also
come under consideration. For instance, Cho and Ba-
hadur [74] reported that the addition of 2 vol% nano-
CuO could generally enhance the wear resistance of short
fiber-reinforced polyphenylene sulfide. The beneficial
effect of nano-particles was attributed to the dev elop ment
of a thin and unifor m transfer film. Zhang et al. [75] and
Chang et al. [76] systematically studied the effect of
nano-TiO2 on short fiber-reinforced epoxy under differ-
ent loading conditions. They found that the addition of 5
vol% nano-TiO2 could significantly reduce the friction
coefficient and the wear rate of epoxy composites than
filling only with traditional fillers. The reduction was
more pronoun ced at high pv (the product of pressure and
velocity) conditions. The rolling effect of nano-particles
was proposed to explain the low friction and wear loss of
the nano-composites.
Guo et al. [77] used surface modified nano-SiO2 filler
to enhance the tribo-properties of epoxy composites filled
with short carbon fibers. In order to improv e th e interface
bonding between nanop articles and the po lymeric matrix,
the nanoparticles were pretreated by graft polymerization.
The additional nanoparticles (4 wt%) proved to be useful
in enhancing the wear resistance and reducing the fric-
tion coefficient of the SFRPs. This improvement mecha-
Copyright © 2012 SciRes. WJNSE
A. A. ALY ET AL.
36
nisms of nano-particles could be caused by the increased
strength of the matrix and better proper- ties o f the trans-
fer film. These results have clearly shown that the addi-
tion of nano-particles is potentially useful to improv e the
tribological performance of SFRPs, even at a relatively
low content. Nevertheless, the improvement mechanisms
using nano-particles have not been deeply understood,
although the pioneer researchers have addressed a number
of significant factors affecting the wear behavior of these
materials.
5. Conclusion
A survey study has been conducted and showed that the
interest in polymer-based composites for technical ap-
plications, in w hich lo w frictio n and low wear, is increa se-
ing. The survey showed that there is a significant im-
provement in mechanical properties of the composite due
to the addition of the n ano-p articles. Many typ es of nano-
filling martials, including SiC, Si3N4, SiO2, ZrO2, ZnO,
CaCO3, Al2O3, TiO2, and nano-CuO, have been used to
different types of polymers such as PEEK; PMMA; PTFE
and epoxy. The mechanical properties which have been
improved include fatigue resistance, fracture toughness,
tensile strength, wear resistance, and friction coefficient.
From the results of the conducted studies there is a con-
siderable increase in wear resistance and decrease in the
coefficient of friction. The change of wear resistance and
friction coefficient depends on the size and volume frac-
tion of the nano-filling materials. It is also possible to use
multi functional fillers to develop high performance com-
posite materials which cannot be achieved by using a
single filler.
6. Acknowledgements
This study is supported by Taif University under a con-
tract No. 1-432-1171. The University is highly acknowl-
edged for its fi nanci al s u pp ort.
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