Zeitschrift Kunststofftechnik Journal of Plastics Technology

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Wissenschaftlicher
Gyurova, Schlarb
Arbeitskreis der
Improving sliding friction and wear performanceUniversitäts-
Zeitschrift Kunststofftechnik
Journal of Plastics Technology
Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern.
Professoren der
Kunststofftechnik
© 2008 Carl Hanser Verlag, München
www.kunststofftech.com
archival, peer-reviewed online Journal of the Scientific Alliance of Polymer Technology
archivierte, peer-rezensierte Internetzeitschrift des Wissenschaftlichen Arbeitskreises Kunststofftechnik (WAK)
www.plasticseng.com, www.kunststofftech.com
handed in/eingereicht:
accepted/angenommen:
24.10.2007
18.06.2008
Lada Antonova Gyurova, M.Sc., Prof. Dr.-Ing. Alois K. Schlarb,
Institut für Verbundwerkstoffe GmbH, University of Kaiserslautern
State-of-the-Art: On the Action of Various
Reinforcing Fillers and Additives for Improving
the Sliding Friction and Wear Performance of
Polymer Composites. Part 1: Short Fibers,
Internal Lubricants, Particulate Fillers
The present study deals with some of the fundamental knowledge discovered over the last ten years
about the role of various reinforcing fillers and additives dispersed in polymer matrices for improving
their friction and wear behavior. At the beginning our focus will be placed on traditional fillers such as
short fibers, internal lubricants and their combinative action. In the next section we will concentrate on
the emerging field of polymer nanocomposites and demonstrate what is possible with nanocomposites
for friction and wear applications and provide comparison over various length scales from micro to
nano-level.
Zum Einfluss von Zusatzstoffen auf die
Leistungsfähigkeit von Tribocompounds.
Teil 1: Kurzfasern, interne Schmierstoffe,
partikelförmige Füllstoffe
Die aktuelle Studie befasst sich mit den über die letzten zehn Jahre gewonnenen grundlegenden
Erkenntnissen zum Einfluss von verschiedenen Verstärkungsstoffen und Additiven in polymeren
Matrizes zur Verbesserung des Reibungs- und Verschleißverhaltens. Zu Beginn betrachten wir
Standardfüllstoffe wie Kurzfasern, interne Schmierstoffe und deren Interaktion. Im nächsten Abschnitt
konzentrieren wir uns auf den jungen Bereich des Zusatzes von Nanofüllstoffen. Wir werden die
Möglichkeiten der Nanokomposite in diesem Bereich aufzeigen und einen Vergleich der
verschiedenen Größenordnungen von Mikro bis Nano liefern.
© Carl Hanser Verlag
Zeitschrift Kunststofftechnik / Journal of Plastics Technology 4 (2008) 6
Gyurova, Schlarb
Improving sliding friction and wear performance
State-of-the-Art: On the Action of Various
Reinforcing Fillers and Additives for Improving
the Sliding Friction and Wear Performance of
Polymer Composites. Part 1: Short Fibers,
Internal Lubricants, Particulate Fillers
L. A.. Gyurova, A. K. Schlarb
1
INTRODUCTION
Over the past decades polymer composites are likely to play an important role
in more mundane technologies, where self-lubricity combined with light weight
and non-toxic nature, hence avoidance of contamination problems, is of special
advantage. A number of these applications involve the service as high-tech,
tribo-materials for gears, cams, clutches and brakes as well as sliding bearings
and seals in the automotive, aerospace, electronic and chemical sectors. In
most cases virgin polymers are not appropriate for such function due to their
inherent weaknesses such as low load carrying capacity, high friction and wear
rates (higher when compared to liquid lubricated metals [1]), poor thermal
conductivity, and high coefficient of thermal expansion. However, the key
advantage of polymers is their affinity for various fillers, reinforcing agents and
additives, which allows tailoring desired performance profile that cannot be
achieved with other materials. Short fibers (glass, aramid or carbon) are
regarded as typical tribo-reinforcement used to enhance properties such as
stiffness, strength, impact resistance, and thermal conductivity yielding
improved load carrying capacity, accordingly [2, 3]. Due to their layered
structure, solid lubricants, (e.g. graphite, PTFE, and molybdenum disulfide),
assist in the formation of a thin, coherent transfer film on the counterface; this is
one of the essential prerequisites for lessening friction and facilitating sliding [4].
More recently nano-scale inorganic particles proved to be effective in improving
both the wear resistance and mechanical performance of the neat matrix
material [5-9].
Friction and wear in sliding contact is generally of great interest because of its
common occurrence in many machine elements. In practice, numerous
geometrical test configurations for sliding wear evaluation have been developed
(Figure 1).
Journal of Polymer Technology 4 (2008) 6
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Gyurova, Schlarb
Figure 1: Schematic illustrations of common sliding wear test configurations: (a)
block-on-ring, (b) pin-on-disc
The extent of realism in the data as well as the possibility of making reliable
conclusions about performance or usability in particular service conditions,
normally decreases when we go from the field test to the simple model test.
Therefore, the basic criterion in evaluating materials for specific applications is
to choose a model test with the highest possible level of realism. Model tests
based on block-on-ring (ASTM G77) or pin-on-disc (ASTM G99) are examples
of American standards used most frequently [10]. These tests allow material
ranking and evaluation of key material properties such as the specific wear rate
ws in accordance with
ws =
∆m
ρ ⋅ v ⋅ t ⋅ FN
[mm3/Nm]
(1)
Where ∆m is the mass loss, ρ is the density of the material being tested, v is the
sliding speed, t is the testing time, FN is the normal force applied on the
specimen during sliding.
The pv-factor or load-carrying capacity can be considered as basic performance
criterion for characterizing bearing materials. In practice it may be represented
in two different ways: (i) the pv-factor for permanent function at a given specific
wear rate; (ii) the “limiting pv” above which a rapid increase in wear appears
[11]. In terms of the aforesaid the major goals in designing tribo-materials is
enhancement of the “limiting pv” factor and reduction of the basic wear factor.
These goals can be successfully accomplished with the appropriate combination of diverse fiber reinforcements, fillers and lubricants in a polymer matrix.
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2
SHORT FIBERS AND INTERNAL LUBRICANT
2.1
Short Fibers
The bulk properties of pristine polymer can be changed remarkably by the
addition of short fiber reinforcement. The classic benefits of such reinforcement
in polymer composites may be an improvement in (i) mechanical properties
such as compressive strength, impact strength, creep resistance, load-carrying
capacity; (ii) coefficient of thermal expansion along with thermal conductivity;
(iii) coefficient of friction, wear rate as well as life endurance. Furthermore,
these fillers are uncomplicated to handle with respect to standard manufacturing
techniques. Table 1 summarizes information on the physical properties of fibers
commonly used for polymer reinforcement.
Tensile
Modulus
[GPa]
60-180
Density
[g/cm3]
Diameter
[µm]
Aramid
Tensile
Strength
[GPa]
3.6-3.8
1.44-1.47
12
Carbon
2.1-7.1
230-830
1.7-2.18
5-10
Glass
3.5*
72
2.54
10-20
Fiber
Type
*Virgin strength values. Actual strength values prior to
incorporation into composite are about 2.1 GPa.
Table 1: Selected properties of common fibers used in polymer composites [12]
2.1.1
Short Glass Fibers
Short glass fibers (SGFs) are preferred common reinforcing agents owing to
their low cost. Still, these are abrasive in nature, which results in increase in the
coefficient of friction of polymer composites. As a rule, the reinforcement with
glass fibers yields approximately 10 to 100 times higher abrasiveness when
compared to short carbon fibers. By the incorporation of mineral and lubricating
graphite the wear properties of glass fiber reinforced material can be enhanced
to an intermediate level between those of neat glass and neat carbon fiber
reinforcement [4, 13].
In general, the improved wear resistance of short glass fiber reinforced polymer
systems is coupled to the increased rigidity and high modulus of fibers
embedded in the polymer matrix [14-16]. Under sliding wear conditions (Figure
2) the glass fibers are depleted layer by layer and small abrasive fiber particles
are only detaching from the fiber ends. The wear process is a combination of
adhesive and abrasive components. Depending on the corresponding matrix
material as well as the test conditions the proportions of these two wear effects
might alter [17, 18].
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Gyurova, Schlarb
10 µm
50 µm
Figure 2: A schematic illustration of the wear process of short glass fiberreinforced PA46 composite [17]
2.1.2
Short Carbon Fibers
Of all three types of fiber reinforcement, short carbon fibers (SCFs) are
regarded as fundamental filler in improving wear resistance of polymers.
Compared to glass fibers, they perform better, owing to their excellent mechaniccal properties and their less abrasive nature [1, 2, 15]. Moreover, in the
process of sliding SCFs form a smooth carbon film on the counterface and
increase thermal conductivity, as well as resistance to heat distortion of the
polymer matrix [19]. Carbon fibers are derived either from polyacrilonitrile (PAN)
or a special petroleum pitch. The PAN-derived fibers are generally referred to
as high strength fibers, while pitch-based fibers are identified as high modulus
fibers, which make them appealing for stiffeness-decisive applications [4, 20].
Because of their low price, the pitch-based carbon fibers could be an alternative
to PAN-carbon fibers for tribological applications [21].
Physical Property
PAN
Pitch
Shape and Size
Round,
7µm dia.
Round,
10µm dia.
Density [kg/m³]
1770
1990
Tensile Strength [MPa]
3.65-4.28
1.38-3.10
Modulus of Elasticity [GPa]
230-241
159-931
1.4
0.5
-0.54 (long.)
-1.30 (long.)
Elongation at Failure at 25 oC [%]
Coefficient of Thermal Expansion [*10-6 oC-1]
Table 2:
Selected properties of commercial carbon fibers used in polymer
composites [4, 20]
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Gyurova, Schlarb
Typical wear stages with carbon fibers embedded into polyphenylenesulfide
(PPS) matrix are depicted in Figure 3 and include fiber thinning, break-down
and removal (pull-out). This wear scenario has also been observed in various
other thermoplastic and thermosetting short carbon-fiber-reinforced composites
[9, 11, 19, 22].
(a) fiber thinning
(c) fiber pull-out
(b) fiber breakage and fiber/matrix
interfacial debonding
70 µm
(a)
70 µm
(b)
Figure 3: Micrograph of the wear process of short carbon fiber-reinforced PPS
composite: (a) fiber thinning, (b) fiber breakage and fiber/matrix
interfacial debonding, (c) fiber pull-out
2.1.3
Short Aramid Fibers
Aramid fibers (AFs) are industrial fibers made of organic material. In contrast to
glass and carbon fibers, aramid fibers are rather soft and flexible. This kind of
fibers possesses almost 100 % paracrystalline structure and a very high degree
of orientation of fibrils along the fiber axis [23, 24]. Besides high strength AFs
possess a very good damage tolerance, impact resistance and vibrational
damping characteristics, as well as lack of notch sensitivity. In the last few years
short forms of aramid fibers have found important applications in wear parts
such as clutches, brakes, thermosetting bulk molding compounds [25]. The
advantage of aramid fibers over the other fiber types is that they cause minimal
wear to counter materials [26]. Still, low stability at high temperatures combined
with moisture absorption behavior can sometimes be a problem with AFs in the
field of tribology [21]. Moreover, due to its high crystallinity the surface of AF is
chemically inert and smooth which leads to poor fiber-matrix adhesion.
Therefore, surface modification is crucial to improve the reinforcing effect with
this filler [27, 28].
A comparison of the reinforcing effect of all three kinds of fibers has been done
by Song and Ehrenstein [13]. Short carbon and aramid fibers showed very
similar effects in lessening wear of polyamide 66 (PA66). However, the wear
reduction was stronger with the carbon fibers. In contrast to carbon and aramid
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Gyurova, Schlarb
fibers, the coefficient of friction increased by the addition of glass fibers. Yet, the
wear rate dropped down [13].
2.1.4
Important Aspects in Short Fiber Reinforcement
2.1.4.1
Fiber Loading Fraction
A key factor in designing composites is the fiber loading fraction as it governs
the mechanical and thermo-mechanical responses of these materials. In the
process of optimizing the wear performance of polymer composites several
authors recognized the role of fiber volume content. The results of some of
these studies are summarized in Figure 4.
Figure 4: Influence of short fiber loading fraction on the sliding wear behavior in
various polymer matrices
Voss and Friedrich [15] showed that fiber loading fraction of about 20 vol.% SCF
in thermotropic liquid crystal polymer (LCP) yields minimum in the specific wear
rate. The same has principally been observed for polyethernitrile (PEN)- and
polyetherketone (PEEK)- based systems [15]. Xian et al. [29] reported a
noticeable improvement in the wear behavior of short carbon fiber-reinforced
polyetherimide (PEI), especially in the high temperature and pv-range (80 times
lower specific wear rate at 150°C). Moreover, the specific wear rate of the PEI
composites exhibited almost no change when the carbon fiber volume fraction
was varied in the range of 5-20 vol.% [29]. Based on these works, as well as
the studies of Friedrich and coworkers [9, 11, 30], it can be concluded that a
favorable friction and wear behavior can be achieved with carbon fiber loading
in the range of 10-20 vol.% (Figure 4). A content higher than 20 vol.%,
especially in the upper pv-range, can lead to stick-slip occurrence.
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Gyurova, Schlarb
2.1.4.2
Fiber/Matrix Interface. Fiber Orientation
Another important factor concerning reinforcement with short fibers involves the
quality of the fiber/matrix interface along with fiber orientation. Strong
fiber/matrix interfacial bonding helps keep the broken fiber pieces within the
composite surface, thus preventing early formation of third body abrasives and
enhanced wear. In this respect different techniques can be applied to improve
the adhesion between fiber and matrix. Zhang et al. [31] employed air oxidation
and cryogenic treatment and showed that enhanced fiber/matrix interfacial
bonding resulted in much better wear resistance. Voss and Friedrich [32]
demonstrated for short fiber reinforced PEEK composites a marked variation of
the wear rate with the fiber orientation. The analysis of the corresponding wear
mechanisms disclosed reduced fiber breakage and pulverization for fibers in
normal direction leading to lower wear rates compared to parallel or anti-parallel
fiber orientation. Consequently, scientists and engineers working with fiberreinforced composites should consider all these issues when designing a
material for specific service.
2.2
Internal Lubricants
In general it has been recognized that in metal-polymer sliding systems the
formation of thin, uniform, coherent transfer film is a must for reducing the
specific wear rate. This transfer film prevents direct contact between the soft
polymer surface and the much harder metal counterface leading to reduced
abrasive action and improved wear resistance [33-39]. One of the mechanisms
for facilitating transfer film development is the incorporation of internal lubricants
such as polytetrafluoroethylene (PTFE), molybdenum disulfide (MoS2) and
graphite (GR).
2.2.1
Polytetrafluoroethylene (PTFE)
Among polymers, PTFE (Figure 5a) forms transfer film most readily [4]. PTFE is
a unique polymer in the development of composite materials since it may either
be the matrix forming polymer or a solid phase lubricant. It is chemically inert
and does not absorb water leading to excellent dimensional stability [41, 42]. In
the process of sliding the molecules of PTFE are stretching out parallel to the
sliding direction. The symmetrical, non-polar configuration of the PTFE chains is
responsible for weak intermolecular bonding. Once an oriented layer is built-up,
slippage in this layer becomes very easy (reduced shear strength). The end
result is a low coefficient of friction [4, 36]. This effect is used to improve the
frictional properties of other materials by lubricating them internally with PTFE.
The filler is adherent, cannot be worn out easily and prevents stick-slip motion
instabilities [36, 43]. The positive contribution of PTFE in diminishing wear has
already been established in different polymers e.g. poly(m-phenylene
isophalamide) (PMIA) [44], polyphenylenesulfide (PPS), polyvinylchloride
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Gyurova, Schlarb
(PVC), polyarylate (PAr), polyoxymethylene (POM), polyimide (PI) and diallylphthalate [45].
(a)
(b)
Figure 5: Crystal structure of (a) PTFE reproduced after [40], (b) graphite
reproduced after [46]
2.2.2
Graphite
The key to graphite (Figure 5b) as a lubricant is its layered-lattice structure and
its ability to form strong chemical bonds with gases from the environment. The
latter weakens the interlayer bonding forces, accordingly yields easy shear and
transfer of crystallite platelets to the mating surface. For that reason graphite is
very often used as filler with polymers run under lubrication or wet environment
[47].
Additionally, graphite is an excellent thermal conductor [4]. This property is
particularly important in tribo-contacts, where frictional heat generated at the
interface must be effectively dissipated. A comparison of the lubricating function
of PTFE and graphite in epoxy nanocomposites was provided by Li [9]. For the
composite with PTFE the lowest peak value of the coefficient of friction and the
shortest duration in the running-in stage was observed. However, graphite
contributed to a lower stable coefficient of friction and an average wear rate,
predominantly under high pressures [9].
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Gyurova, Schlarb
2.2.3
Molybdenum disulfide (MoS2)
The layer-lattice structure of MoS2 (Figure 6) is similar to that of graphite (Figure
5b).
Figure 6: Crystal structure of MoS2 reproduced after [48]
The very low sliding friction of MoS2 is due to very low shear strength parallel to
the basal plane of the crystal lamellae, compared to the high strength or
hardness perpendicular to the basal plane [41]. Apart from its low friction
properties, the other beneficial feature of MoS2, important in lubrication, is its
high load carrying capacity. Studies on the effect of MoS2 incorporation into
different polymers are summarized in [41]. MoS2 is reported to decrease wear
rate in PTFE, PA 66, polyamide-imide (PAI), polyimide (PI). It is recommended
that the volume fraction of MoS2 in polymer matrices does not exceed 30 vol.%,
unless the material is designated to supply transfer lubrication. Another
important aspect concerning wear with this lubricant is the particle size. This
effect is clearly shown in a study with PA 66 [41]. In this case a lower wear rate
was derived for a particle size of 4 µm when compared to that of 0.3 µm. A
negative outcome with MoS2 added to PPS and PA 1010 has been reported by
WU et al. [49] and Wang et al. [50]. It has been demonstrated that while graphite
and PTFE contribute to an increase in wear resistance of PPS, MoS2
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Gyurova, Schlarb
deteriorates the wear resistance of the composite. The latter effect is related to
poor mechanical properties and a tendency of the filler to segregate and
extrude out of the matrix material in the process of sliding. However, it should
be taken into account that for both studies the lubricant concentration was equal
or greater than 10 vol.%. It has already been stated in the literature that with
some polymers at concentrations of 10 vol.% or more MoS2 can significantly
reduce the structural strength of a composite [41].
2.3
Combining Short Fibers and Internal Lubricants
The effects on the tribological performance of polymer composites when
combining short fibers and internal lubricants were studied in numerous
publications. Bolvari et al. [45] observed a considerable reduction in both,
friction and wear of PA 66 reinforced with aramid fiber and PTFE. A synergistic
effect was also reported for PA 1010 reinforced with SCF and MoS2 [50]. As
already mentioned before, the addition of MoS2 as a single phase to PA 1010,
failed to bring a positive outcome. At the same time it was found that carbon
fiber reinforcement combined with MoS2 could effectively reduce the wear rate
of PA, particularly under high loads. The beneficial interaction between the fiber
reinforcement and internal lubricant led to the formation of thin, uniform and
continuous transfer film. We previously pointed out, it is well established in the
community that the transfer film is crucial in controlling wear behavior of such
tribo-materials. The transfer film of the composition with MoS2 only was poor
and weak. Moreover, in the case of MoS2 acting alone, the polymer and MoS2
debris cannot support higher loads in contrast to the fiber reinforcement. Zhang
and coworkers [51, 52] found a beneficial interaction when PTFE powders and
graphite flakes were added to SCF-reinforced PEEK and PEI. This effect
became more explicit for PEI under severe sliding conditions (higher sliding
temperatures and pressures). Reinicke et al. [18] and Bijwe et al. [53, 54]
investigated the influence of solid lubricants and fiber reinforcement in various
polymer matrices such as polyphthalamide (PPA), polyamide (PA) 46,
polyethersulfone (PES) as well as PEI, and established wear rate reduction as a
result of combined positive interaction of the different phases. This interaction
can be summarized in terms of load-supporting effect of the fibers and transfer
film formation via the lubricants. In addition, Lu [55] gave a broad summary on
the present subject for materials being tested at different temperatures. The
results demonstrated that high load bearing capacities can be realized with
such systems at elevated temperatures (220°C), which opens new opportunities
in industrial sectors like the chemical process industry and the transportation
sector. Klein [56] investigated different tribo-systems based on PTFE. PTFE is
widely used as lubricant; still it undergoes marked cold flow under stress and
exhibits the lowest wear resistance among semi-crystalline polymers [42]. Due
to its low strength, PTFE is seldom used as matrix material for machine parts.
The effect of various reinforcements on friction and wear behavior in PTFE
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Gyurova, Schlarb
composites is summarized in Figure 7. The picture highlights the beneficial
action of carbon fibers (CF) as key tribo-reinforcement for PTFE.
Figure 7: Friction and wear data of PTFE composites under standard test
conditions: pv = 1 MPa*m/s; Rz = 2.1 µm (modified after [56])
Until recently the low temperature technology has been applied to space applications, superconductivity or medical diagnostics. However, new applications
such as hydrogen technologies for environmentally friendly energy supply and
transportation are on the way to win interest [57]. Therefore, it is important to
search for adequate system design that can meet the requirements of these
new applications. Klein [56] summarized the research done on different PTFEbased systems in cryogenic environment (-196°C). It has been established that
at room temperature, factors like filler volume fraction dominate the wear
behavior of the composites, whereas in the low temperature range properties of
the matrix and not those of the filler play the key role.
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Gyurova, Schlarb
3
PARTICULATE FILLERS
3.1
From Micro- To Nano- Level
Composites reinforced with micron-sized particles of different materials belong
to the most widely used composites in everyday materials. Such materials offer
low cost and ease of fabrication. Usually, they are added to increase the matrix
elastic modulus and yield strength [58, 59]. The tribological behavior of
polymers filled with inorganic micro-fillers (e.g. CuO, CuS, PbO3, CuF2, PbS,
Ag2S) was extensively studied in the past. In a number of publications Bahadur
and coworkers [60-65] showed that certain fillers like CuO, CuS, CuF2, Pb3O4,
CaS, CaO and Ag2S were beneficial for the wear resistance of neat PEEK
and/or PA. At the same time other fillers like sulfides and fluorides of Zn and Pb
brought adverse effects in PA 11 [64]. The wear resistance of PPS was
improved by microparticles such as Ag2S, NiS, CuS and CuO and deteriorated
with PbTe, PbSe, ZnF2, CaF2 and SnS [33, 34, 37, 66]. The mechanism of wear
reduction with such micro-fillers were explained in terms of chemical reaction
between filler and metal counterface, leading to enhanced adhesion of the
transfer film along with mechanical interlocking of the transfer film into the
crevices of the counterface asperities [34, 36, 66, 67]. Further, the deformability
of the particles might also contribute to better wear resistance [37].
When working with micro-scale particles it must be taken into account that the
mechanical behavior of the fillers tends to follow the one of the bulk filler
material. The particle angularity plays a decisive role for such compounds. In
this respect, rigid fillers are not suitable due to unwanted abrasive effects [35].
Therefore, it is expected that the smaller the particle size, the more efficient
their reinforcing action should be. This statement was confirmed by the results
for the sliding wear behavior of epoxy filled with silica (particle size from 120 to
510 nm) [68]. Yet, opposite size effects were also found in epoxy reinforced
materials with ceramic particles (5-100 µm) [59]. Particles having an average
size smaller than 20 µm were removed together with matrix debris, next these
remained entrapped between the two sliding bodies thus acting as abrasive
grits. However, in this case not a metal, but an alumina counterbody was used.
Moreover, the improvement in the composite wear herein was at the expense of
the counterbody, since the reinforced polymer became more aggressive to it
and increased its wear.
By scaling the particle size down to a nanometer range the influence from the
particle´s angularity has greatly shrunk (Figure 8). Nanoparticles possess a
large surface-to-volume ratio and are referred to as being “interface-dominated”
[69] materials. The latter increases the possibility of enhanced bonding between
the filler and the surrounding, and would produce a stronger transfer film [38].
Additionally, material removal would be restricted due to the smaller particle
size, being of the same order as the segments of the surrounding polymer
chains (severe abrasion would be replaced by mild abrasive wear) [70].
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Gyurova, Schlarb
Figure 8: Correlations among nanoparticle diameter, inter-particle distance and
volume percentage based on assumptions of spherical particle, cubic
distribution (modified after [69])
In different studies the potential of this method for designing wear resistant
materials has already been investigated and proved. Xue et al. [71] compared
the effect of nano, micron and whisker SiC particles in a PEEK matrix at relative
low filler loading (about 4 vol.%). The nanoparticles yielded the most effective
reduction, which was related to their ability to form thin, uniform, transfer film.
Shi et al. [72] reported that both, friction and wear were reduced for epoxy filled
with nanometer Si3N4. Wang et al. [73] varied the particle size of ZrO2 in the
range 10-100 nm and confirmed the tendency the smaller the particle size, the
lower the wear. Zhang et al. [74] showed that nano-silica can simultaneously
secure friction and wear reduction of the epoxy composites at low filler content
(2.17 vol.%). This could have not been achieved with micrometer silica [74]. In
the subsequent section the relationship between the nano-filler volume content
and wear resistance of nanocomposites will be more comprehensively
analyzed.
3.2
Nanoparticle Volume Fraction
Because of their small sizes, it is extremely difficult to disperse nanoparticles
uniformly. Nanoparticle agglomeration arises normally as a result of van der
Waals bonding [58]. This problem is even more complicated for high filler
loadings. Several observations have been made so far concerning the influence
of nanoparticle volume fraction on the tribo-performance of such materials.
Bahadur et al. [35, 39] studied the tribological behavior of PPS filled with
different inorganic nanoparticles such as alumina, TiO2, ZnO, CuO and SiC.
The maximum filler loading in these studies was 10 vol.%. A decrease in the
wear rate was observed with 2 vol.% alumina, TiO2 and CuO. Increasing the
filler content to 3 vol.% or more worsened the wear resistance of the
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Gyurova, Schlarb
composites when compared to that of unfilled PPS. The wear rate was
increased with the higher filler loading due to: (i) abrasion of the transfer film by
the hard filler, (ii) weakening of the transfer film bonding aroused from the
polishing effect by the nanoparticles on the counterface [35]. Wang et al. [71,
73, 75-78] recognized similar tendency for PEEK reinforced with SiC, ZrO2,
Si3N4, and SiO2. The corresponding wear mechanisms changed from adhesive
wear and scuffing for pure PEEK to slight fatigue and transfer wear for the
nanocomposites. A volume fraction higher than 4 vol.% led to severe abrasive
wear and deterioration of the composite’s cohesion. Wetzel and coworkers [5,
6, 72], Sreekala and Eger [79] investigated the effect of different size nanoreinforcement in epoxy. A gradual increase in the wear resistance as well as
stiffness and impact strength of epoxy filled with TiO2 (300 nm) was observed
with rising filler content until an optimum value of 4 vol.% was reached. Higher
filler loadings led to properties´ deterioration [5]. The addition of small amounts
of alumina (13 nm) seemed effective also only at low filler volume (1-2 vol.%)
[6]. It should be underlined that in both cases homogeneous particle distribution
was crucial for achieving such properties´ enhancement. Similar to alumina in
[6], nano-Si3N4 (20 nm in size) proved effective at very low volume fraction (less
than 1 vol.%) [72]. The incorporation of nano-SiO2 (very narrow size range
between 6 and 20 nm) into reactive epoxy resin via sol-gel technique led to
noticeable wear rate reduction at a filler loading fraction on the order of 3 to 6
vol.% [79]. The addition of approximately 2 wt% (~ 1.3 vol.%) silica (20 nm) in
PA 6 matrix resulted in a three fold reduction coefficient of friction and 140-fold
lower specific wear rate. The use of higher silica loadings was less successful
[80]. Zhang et al. [70, 74, 81] employed grafting polymerization technique with
polyacrylamide (PAAM) as grafting polymer in conjunction with SiO2 nanoreinforcement (9 nm) and SiC nano-reinforcement (61 nm). This resulted in
better interfacial adhesion between the nano-filler and the epoxy matrix via
chemical bonding. Consequently, the material removal was reduced, the
coefficient of friction was lowered and the load bearing capacity amplified. With
filler loading fraction in the range from 2 to 6 vol.%, the wear resistance of
epoxy could have been increased roughly by a factor of 20 by the inclusion of
SiO2-g-PAAM [70]. In Figure 9 are displayed reported wear rates for selected
nanocomposites as a function of nanoparticle volume fraction.
In view of the aforesaid, it can be concluded that the optimum filler content for
acquiring adequately good enhancement in friction and wear behavior with
nanoparticles lies within the limits 1-5 vol.%, except for nano-reinforced PTFE
[82], which case will be discussed subsequently. Comparable tendency has
already been established formerly by Zhang and coworkers [7-9] as well as by
Sawyer et al. [83]. With the grafting polymerization technique even lower filler
loading fractions (about 0.2 vol.%) are possible, which is explained in terms of
improving the filler/matrix adhesion as well as chunking the nanoparticle
agglomerates [81]. It should be borne in mind that the optimum filler loading for
polymer composites reinforced with microparticles is on the order of 30 vol.%
[83].
15
Gyurova, Schlarb
Figure 9: Specific wear rate for selected polymer nanocomposites as a function
of nanoparticle volume fraction
3.3
Nanoparticle Volume Fraction in PTFE
Contrary to other polymer matrices, the wear of PTFE reinforced by 20 vol.%
TiO2 (agglomeration of fine particles less than 300 nm in size) was much higher
than that of PTFE incorporating ZrO2 (ranging from several microns to about 50
µm) particles. This was explained in terms of the inability of small scale fillers
(e.g. nanoparticles) to prevent large scale destruction of the PTFE banded
structure [42]. Nevertheless, with the booming of nanotechnology in the recent
years few attempts have also been made to improve the wear behavior of PTFE
by incorporating nano-ZnO (average particle size 50 nm) [82] or nano-alumina
(average particle size 40 nm) [83] as fillers. The reported optimum nano-filler
content was on the order of 15 vol.% or more for both sliding wear [82] and
fretting wear [83] conditions. As already mentioned, this content was much
higher when compared to other polymer matrices. LI et al. [82] suggested that
the nanoparticles being distributed uniformly on the subsurface of PTFE
composites prevent the destruction of the PTFE banded structure. Also, it has
been suggested by Sawyer et al. [83] that in this case the nanoparticles provide
casing for the material. This casing interrupts surface crack propagation and,
therefore, keeps the virgin PTFE islands isolated so that damage in one region
cannot easily spread into the other. However, it is our hypothesis that with the
higher nanoparticle volume fraction agglomerates built-up, which agglomerates
function similarly to microparticles and have load-supporting function for the soft
PTFE matrix. The reported increase in the coefficient of friction might be
explained by the degree of hardness of the nano-filler agglomerates, which
16
intensify abrasive wear. Recently, Burris et al. [84] incorporated irregularly
shaped alumina (80 nm) and reported a 3000-fold improvement over unfilled
PTFE at low filler content of 1 wt% (~ 2.2 vol.%). The very much improved wear
resistance was related to the irregular shape of the nano-filler. The latter allows
sufficient mechanical entanglement with the matrix to take place, consequently
facilitates filler accumulation at the sliding interface. Another interesting
investigation was that of Lai et al. [85] who evaluated the friction and wear
properties of PTFE filled with ultrafine diamond (10 nm in diameter). Ultrafine
diamond (UFD) possesses unique properties of nano-scale material combined
with those of a diamond (high hardness, good thermal conductivity, good wear
and chemical resistance). The experimental results showed no significant
change on the coefficient of friction, but orders of magnitude lower wear rate
with increasing filler concentration when compared to neat PTFE. This effect
was analyzed in terms of improved heat absorption capacity of the composite
as well as the assistance in transfer film formation, and enhancement of the
bonding between the transfer film and the counterpart. Finally, it is suggested
that the UFD particles function as roll bearing in the frictional interface [85].
3.4
Counterface Surface Roughness
One additional aspect, relevant to the effect of nanoparticle incorporation in
polymer matrices, involves the counterpart surface roughness. In general, the
surface roughness of the steel counterpart correlates directly to the real
average distance of two sliding surfaces [86] and plays an important role for the
formation of thin, uniform, transfer film as well as for wear reduction. An
example of this effect was provided by Zhang et al. [22] and Bahadur et al. [35]
(Figure 10).
Gyurova, Schlarb
Figure 10: Steady state wear rate for PPS-filled with nano-scale alumina (33 nm
average diameter) at various counterpart surface roughnesses Ra
(modified after [35])
17
Gyurova, Schlarb
It has been shown that both, the wear rate and coefficient of friction of the
nanocomposites increased strongly when sliding against the smoother
counterface, having lower arithmetic average surface roughness (30 nm) than
the nanoparticles´ average size (300 nm) [22]. For PPS filled with nano-scale
alumina (33 nm) [35], the lowest wear rate was reached again when sliding
against counterfaces having surface roughness higher than the nanoparticles
average size (Figure 10). In the opposite case (sliding against a counterface of
smaller surface roughness than the particle size) the transfer film was patchy
and could not cover the asperities completely. Therefore, nanoparticles could
not position themselves in the counterface asperities and provide the required
effective anchoring for the transfer film [35]. In tailoring nanomaterials, it is
indispensable to consider the issue of selecting optimum surface roughness,
which will compensate for these unfavorable effects.
4
SUMMARY
In recent years the use of polymer composites as structural components, where
friction and wear are critical issues, has undergone rapid growth. Typical
examples are manifold, from gears and bearings to seals in automotive,
aerospace and chemical industries. Polymers are favored in tribo-applications
because of the prospect to tailor desired material features by proper
incorporation of different phases as well as self-lubricity of the matrix itself. In
this paper a review of the action of diverse reinforcing agents, fillers and
additives on the sliding friction and wear properties of polymer composites has
been given. After an initial treatment of traditional reinforcing phases, such as
short fibers and internal lubricants and their mechanisms of wear, a discussion
concerning the potential of particulate fillers (micro- and nanoparticles) to lower
friction and wear was presented. The incorporation of nano-scale filler resulted
in significant improvement of the wear resistance of polymers at very low filler
loading (1-4 vol.%), much lower when compared to micro-scale particulate
fillers. In the following part of our review efforts will be concentrated on the
combinative action of traditional fillers and nanoparticles. Possible mechanisms
concerning their synergistic effect will be discussed. Finally, stimulating insights
on the prospective of carbon nanotubes for improving the friction and wear
performance of polymer composites will be given.
18
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Appendix: Selected Friction and Wear Test Data
27
Filler
[vol.%/wt.%]
-
15 vol.% PTFE,
30 vol.% GF
-
15 vol.% PTFE,
30 vol.% GF
30 vol.% CF
30 vol.% CF
-
-
-
25 wt.% CF
25 wt.% CF
25 wt.% CF
-
PA66 [14]
PA66 [14]
PPS [14]
PPS [14]
PPS [14]
PEEK [14]
Acetal [19]
Acetal [19]
Acetal [19]
Acetal [19]
Acetal [19]
Acetal [19]
PEEK [21]
pin-on-disc
pin-on-disk
pin-on-disk
pin-on-disk
pin-on-disk
pin-on-disc
pin-on-disc
-
-
-
-
-
-
Testing
mode
23
23
23
23
23
23
23
23
23
23
23
23
23
T [°C]
Matrix
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.50
0.50
0.50
0.50
0.50
0.50
v [m/s]
1 MPa /16 N
75 N
55 N
45 N
75 N
55 N
45 N
(pv)lim = 6.0
[MPa*m/s]
(pv)lim = 6.6
[MPa*m/s]
(pv)lim = 9.9
[MPa*m/s]
(pv)lim = 1.8
[MPa*m/s]
(pv)lim = 6.6
[MPa*m/s]
(pv)lim = 0.9
[MPa*m/s]
p [MPa]/FN [N]/
pv [MPa*m/s]
0.40
0.20
0.18
0.23
0.35
0.37
0.32
0.40
0.20
0.17
0.24
0.26
0.28
µ [1]
3
3
3
3
3
3
3
3
-6
-6
-6
-6
-6
-3
-3
3
3
3
3
-3
-3
-5
1.00*10
6.00*10
3.00*10
[mm /min]
[mm /min]
[mm /min]
[mm /min]
-3
2.50*10
[mm /min]
3
13.00*10
-3
6.00*10 [mm /min]
5.00*10 [mm /min]
4.00*10 [mm /Nm]
3.20*10 [mm /Nm]
2.10*10 [mm /Nm]
10.70*10 [mm /Nm]
0.31*10 [mm /Nm]
4.00*10 [mm /Nm]
-6
Wear volume [mm ]
3
3
Linear wear rate [mm /min]
3
Specific wear rate [mm /Nm]
Gyurova, Schlarb
28
Filler
[vol.%/wt.%]
10 vol.% PAN-CF
20 vol.% PAN-CF
30 vol.% PAN-CF
10 vol.% pitch-CF
20 vol.% pitch-CF
30 vol.% pitch-CF
10 vol.% mod.PANCF
-
10 vol.% PAN-CF
10 vol.% pitch-CF
10 vol.% mod.PANCF
30 vol.% PAN-CF
-
PEEK [21]
PEEK [21]
PEEK [21]
PEEK [21]
PEEK [21]
PEEK [21]
PEEK [21]
PEEK [21]
PEEK [21]
PEEK [21]
PEEK [21]
PEEK [21]
PEEK [31]
block-onring
pin-on-disc
pin-on-disc
pin-on-disc
pin-on-disc
pin-on-disc
pin-on-disc
pin-on-disc
pin-on-disc
pin-on-disc
pin-on-disc
pin-on-disc
pin-on-disc
Testing
mode
-
150
150
150
150
150
23
23
23
23
23
23
23
T [°C]
Matrix
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
v [m/s]
1 MPa
3 MPa /48 N
3 MPa /48 N
3 MPa /48 N
3 MPa /48 N
3 MPa /48 N
3 MPa /48 N
3 MPa /48 N
3 MPa /48 N
1 MPa /16 N
1 MPa /16 N
1 MPa /16 N
1 MPa /16 N
p [MPa]/FN [N]/
pv [MPa*m/s]
-
-
-
-
-
-
0.26
0.25
0.35
0.28
0.30
0.25
0.27
µ [1]
3
3
3
3
3
3
3
3
3
3
3
3
-7
-6
-7
-6
-6
-7
-5
-6
-6
-6
-6
6.92*10
-6
3
[mm /Nm]
1.80*10 [mm /Nm]
1.40*10 [mm /Nm]
2.50*10 [mm /Nm]
1.00*10 [mm /Nm]
2.90*10 [mm /Nm]
7.50*10 [mm /Nm]
1.70*10 [mm /Nm]
1.80*10 [mm /Nm]
7.50*10 [mm /Nm]
1.00*10 [mm /Nm]
7.00*10 [mm /Nm]
8.00*10 [mm /Nm]
-7
Wear volume [mm ]
3
3
Linear wear rate [mm /min
3
Gyurova, Schlarb
29
-
30 vol.% Ag2S
15 vol.% CF
15 vol.% PTFE
15 vol.% CF
15 vol.% PTFE
15 vol.% CF;
9 vol.% PEEK
15 vol.% CF;
9 vol.% PEEK
-
30 vol.% NiS
30 vol.% PbSe
-
2.5 wt.% SiC
5 wt.% SiC
10 wt.% SiC
PPS [34]
PPS [34]
PEEK [57]
PEEK [57]
PTFE [57]
PPS [66]
PPS [66]
PPS [66]
PEEK [77]
PEEK [77]
PEEK [77]
PEEK [77]
PTFE [57]
Filler
[vol.%/wt.%]
block-on-ring
block-on-ring
block-on-ring
block-on-ring
pin-on-disk
pin-on-disk
pin-on-disk
pin-on-disc
pin-on-disc
pin-on-disc
pin-on-disc
pin-on-disk
pin-on-disk
Testing
mode
20
20
20
20
23
23
23
-196
23
-196
23
23
23
0.45
0.45
0.45
0.45
1.00
1.00
1.00
0.20
0.20
0.20
0.20
1.00
1.00
v [m/s]
0.93 MPa /196 N
0.93 MPa /196 N
0.93 MPa /196 N
0.93 MPa /196 N
0.65 MPa /19.5 N
0.65 MPa /19.5 N
0.65 MPa /19.5 N
16 N
16 N
16 N
16 N
0.65 MPa /19.6 N
0.65 MPa /19.6 N
p [MPa]/FN [N]/
pv [MPa*m/s]
0.22
0.28
0.32
0.38
0.30
0.32
0.43
-
-
-
-
0.36
0.43
µ [1]
3
3
-6
3
3
3
3
3
3
3
-6
-5
-6
-6
-6
-6
3.65*10 [mm /Nm]
3.60*10 [mm /Nm]
3.40*10 [mm /Nm]
7.40*10 [mm /Nm]
8.10 *10 [mm /Nm]
3.68*10 [mm /Nm]
9.68*10 [mm /Nm]
-6
0.025 [mm ]
3
0.075 [mm ]
3
0.001 [mm ]
3
0.020 [mm ]
3
2.31*10 [mm /Nm]
9.68*10 [mm /Nm]
-6
Wear volume [mm ]
3
3
Linear wear rate [mm /min
3
T [°C]
Matrix
Gyurova, Schlarb
30
Gyurova, Schlarb
Keywords:
sliding friction, sliding wear, polymer–matrix composites, fiber reinforcement,
internal lubricant, microparticle, nanoparticle
Schlüsselwörter:
Reibungskoeffizient, Gleitverschleiß, Polymer-Matrix Verbundwerkstoffe, Faser
Verstärkung, Interne Schmierstoffe, Mikropartikel, Nanopartikel
Author/Autor:
Lada Antonova Gyurova, M.Sc.
Prof. Dr.-Ing. Alois K. Schlarb
Institut für Verbundwerkstoffe GmbH
Erwin-Schroedinger-Straße
67663 Kaiserslautern, Germany
Editor/Herausgeber:
Europe/Europa
Prof. Dr.-Ing. Dr. h.c. G. W. Ehrenstein, verantwortlich
Lehrstuhl für Kunststofftechnik
Universität Erlangen-Nürnberg
Am Weichselgarten 9
91058 Erlangen
Deutschland
Phone: +49/(0)9131/85 - 29703
Fax.:
+49/(0)9131/85 - 29709
E-Mail: [email protected]
Publisher/Verlag:
Carl-Hanser-Verlag
Jürgen Harth
Ltg. Online-Services & E-Commerce,
Fachbuchanzeigen und Elektronische Lizenzen
Kolbergerstrasse 22
81679 Muenchen
Phone.: 089/99 830 - 300
Fax: 089/99 830 - 156
E-mail: [email protected]
Website: www.ivw.uni-kl.de
Phone.: +49(0)631 / 2017-102
Fax: +49(0)631/2017-199
The Americas/Amerikas
Prof. Dr. Tim A. Osswald,
responsible
Polymer Engineering Center,
Director
University of Wisconsin-Madison
1513 University Avenue
Madison, WI 53706
USA
Phone: +1/608 263 9538
Fax.:
+1/608 265 2316
Editorial Board/Beirat:
Professoren des Wissenschaftlichen
Arbeitskreises Kunststofftechnik/
Professors of the Scientific Alliance
of Polymer Technology
31

Zeitschrift Kunststofftechnik Journal of Plastics Technology

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Zeitschrift Kunststofftechnik Journal of Plastics