World Journal of Nano Science and Engineering, 2012, 2, 32-39
http://dx.doi.org/10.4236/wjnse.2012.21006 Published Online March 2012 (http://www.SciRP.org/journal/wjnse)
Friction and Wear of Polymer Composites Filled by
Nano-Particles: A Review
Ayman A. Aly*, El-Shafei B. Zeidan, AbdAllah A. Alshennawy, Aly A. El-Masry, Wahid A. Wasel
Mechatronics Section, Department of Mechanical Engineering, Faculty of Engineering,
Taif University, Taif, Saudi Arabia
Received November 14, 2011; revised December 19, 2011; accepted January 10, 2012
ABSTRACT
Composites formed by adding nano-scale particles to a polymer matrix results in improving electrical, mechanical, and
thermal properties of the composite. Good tribological properties can be obtained for polymers filled with nano-scale
fillers compared to that filled with micro-scale particles. The friction and wear resistance of these composites is found
to increase with increasing filler concentration. It is also possible to use multi-functional fillers to develop high per-
formance composites which cannot be achieved by using a single filler.
Keywords: Friction; Wear; Polymer Composites; Nano-Particles
1. Introduction
In the past few decades, researchers and engineers inter-
est has been shifting from monolithic materials to rein-
forced polymeric materials. These composite materials
now dominate the pharmaceutical, aerospace, leisure, auto-
motive, construction, and sporting industries [1-3].
A polymer nano-composite is defined as a composite
material with a polymer matrix and filler particles that
have at least one dimension less than 100 nm. These en-
gineering composites are desired due to their low density,
high corrosion resistance, ease of fabrication and low
cost [4-7]. Glass fibers are the most widely used to rein-
force plastics due to their good mechanical properties
and low cost. Various kinds of polymers and polymer
matrix composites reinforced with metal particles have a
wide range of industrial applications such as heaters,
electrodes, actuators etc. [8]. When silica nano-particles
are added to a polymer matrix to form a composite, they
play an important role in improving electrical, mechanic-
cal, and thermal properties of composites [3,9]. The in-
clusion of inorganic fillers into polymers for commercial
applications is primarily aimed at the cost reduction and
stiffness improvement [10].
Polymer composites can be fabricated by the incorpo-
ration of inorganic reinforcements into the polymer ma-
trix. The properties of the resulting polymer composites
depend on the characteristics, the dimensions, and the
shapes of the inorganic fillers, and also on the interfacial
bonding strength. It is proposed that with decreasing
filler dimensions or increasing filler content a significan t
improve in the contact area between the filler and matrix,
and in turn it would greatly and effectively improve the
transfer of the load between the fillers and the polymer
matrix [11]. The inorganic nan o-fillers, ranging fro m 1 to
50 nm, were successfully incorporated into the polymeric
matrix to strengthen and improve the ductile polymer to
be more stiff and resistant for abrasion [12,13].
The inclusion of the ceramic nano-filler into the more
flexible and lower thermal resistance polymer can sub-
stantially improve its stiffness and thermal stability [14-
17]. The nano-sized silica or alumina particles without
any chemical modification were incorporated into the
PEEK polymer. It appears that there are occasional clus-
tering occurrences for two to five nano-particles to clus-
ter or align together, the majority of the nano-particles
were seen to disperse semi-homogeneously in the poly-
etheretherketone (P EEK) m a t rix [18].
With the booming of nano-phased materials in the re-
cent years, attempts are being made to develop nano-
particle filled-polymer composites with improved tribo logi-
cal performance of the materials. It is expected that good
tribological properties can be obtained for the polymers
filled with nano-scale fillers compared to those filled
with micro-scale particles [19,20]. Due to their lower
strength and stiffness compared with synthetic fibers,
natural fibers use in polymer composites has been limited
to non-tribological applications. Very little information
concerning the tribological performance of natural fiber
reinforced composite materials has been reported [21].
*Corresponding a uthor.
C
opyright © 2012 SciRes. WJNSE

A. A. ALY ET AL. 33
Polytetrafluoroethylene (PTFE) exhibits many desir-
able tribological characteristics, including high melting
temperature, low friction, and chemical inertness. PTFE
is an excellent solid lubricant and used commonly in
bearing and seals applications [22] Unfortunately, PTFE
exhibits high wear rate under normal friction conditions,
which limits its application fields. Therefore, many kinds
of PTFE-based composites have been produced to im-
prove the wear resistance of PTFE [23,24]. It was found
that some micro-scale inorganic fillers showed distinct
effect on the friction and wear behaviors of PTFE com-
posites [ 2 5].
The present paper presents a survey on nano-filler
polymer-based composites with improved mechanical
properties for low friction and low wear applications.
The effect of filler particle size is summarized. Then the
effect of chemical surface modification and multi func-
tional fillers is reviewed. Finally some conclusions are
drawn.
2. Effect of Nano-Filler Particle Size and
Ratio
The shape, size, volume fraction, and specific surface area
of added particles have been found to affect mechanical
properties of the composites greatly. Currently, many
studies have focused on how single-particle size affects
mechanical properties of the composites [26-29]. Ho-
shino et al. [26] discussed the effects of size and shape of
silica particle on the strength and fracture toughness
based on particle-matrix adhesion and they found an in-
crease of the flexural and tensile strength as specific sur-
face area of particles increased. Yamamoto et al. [27]
reported that the structure and shape of silica particle have
significant effects on the mechanical properties such as
fatigue resistance, tensile and fracture properties.
The effects of inorganic nanometer particles, such as
SiC, SiO2, Si3N4 and ZrO2 on the tribological properties
of some polymers have been studied. Li et al. [24] re-
ported that filling nanometer ZnO to PTFE could greatly
reduce the wear of this polymer and the best anti-wear
property was obtained with the composite containing 15
vol% nanometer ZnO, but the friction coefficient of
nano-composite was higher than that of the unfilled PTFE.
Sawyer et al. [25] investigated the tribological properties
of PTFE composites filled with 40 nm Al2O3, and they
found that the friction coefficient of the composite in-
creased slightly compared to the unfilled sample and the
wear resistance increased monotonically with increasing
filler concentration.
Wang et al. [30-36] have filled PEEK with various
weight fractions of SiC, Si3N4, SiO2, and ZrO2. The addi-
tion of the filler in fractions less than 10% by weight
improved the wear resistance and reduced the friction
coefficient. The improved wear and friction is attributed
to two factors: the smoothing of the steel counterface,
and the development of a transfer film. Schwartz and
Bahadur [37] filled polyphenylene sulfide (PPS) with
alumina nano-particles. Examination of these samples
with scanning electron microscopy showed good disper-
sion of filler particles in the PPS matrix. Wear tests were
performed on a four stations pin-on-disk tribometer. The
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.

A. A. ALY ET AL.
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-

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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