The Effect of MoS2 on Friction & Wear Behavior of PTFE Composites
©2016
Academic Paper
61 Pages
Summary
The wear resistance of PTFE can be significantly improved by addition of suitable filler materials. Besides the type, the shape and size of the materials added also influence the tribological properties. In the past, research in this area has been confined to the PTFE filled with conventional filler materials like glass fibers, graphite, carbon fibers, etc. However, with the growing demand for utilizing PTFE in a variety of applications, significant effort is needed towards developing the novel composite materials by adding one or more non-conventional filler materials possessing the potential of increasing the wear resistance. It is established that PTFE exhibits significantly low coefficient of friction when sliding against steels. The low coefficient of friction results from the ability of its extended chain linear molecules, – (CF2–CF2) n–, to form low shear strength films upon its surface and mating counter-faces during sliding. PTFE is extensively used for a wide variety of structural applications as in aerospace, automotive, earth moving, medical, electrical, electronics, computer and chemical industries. On account of its good combination of properties, these are used for producing a number of mechanical components such as gears, cams, wheels, brakes, clutches, bearings, gaskets, seals as well as wires, cables, textile fibers, electronic components, medical implants, surgical instruments etc.
Excerpt
Table Of Contents
2
PTFE is extensively used for a wide variety of structural applications as in aerospace,
automotive, earth moving, medical, electrical, electronics, computer and chemical
industries. On account of its good combination of properties, these are used for producing
a number of mechanical components such as gears, cams, wheels, brakes, clutches,
bearings, gaskets, seals as well as wires, cables, textile fibers, electronic components,
medical implants, surgical instruments etc.
The needs and advantages of accelerated wear testing are discussed, with particular
reference to aerospace applications. Examples are given from recent work on plastics
based dry-bearing liners to illustrate how accelerated wear tests can provide information
relevant to materials selection, identification of the main parameters influencing wear and
definition of the relationships between wear, material composition and structure.
Prediction of the service life of dry-bearing liners, however, presents problems and full-
scale component tests are usually required.
[4]
Surface texturing is the combination of the imperfection on the surface of a part,
roughness, waviness, lay and flaws make a surface texture. Surface texturing takes place
on the disc which consists of the different dimple size in µ and different dimple density.
So, compare the readings of same dimple sizes in µ and dimple density and discuss on the
result on wear and coefficient of friction. We use EN304 stainless steel disc. On that disc,
three dimple tracks are takes place having different diameters and µ sizes (100 µ, 200 µ,
300 µ). On plane surface also takes the reading. Compare the reading on plane surfaces
and texturing surfaces and discuss the results on wear and coefficient of friction.
In this work attention is given to investigate the tribological properties of
composite materials considering various conditions so as to observe the comparative
friction and wear behavior of PTFE composite interfaced with graded fillers under
constant loads and sliding velocities by using a pin-on-disc type wear tester at NTP.
Experimental work will be carried out considering velocity i.e.0.12 m/skeepingrest of the
parameters constant. The test will be carried out for two materials, 85 % PTFE + 15 %
Glass Fiber and 80 % PTFE + 15 % GF + 5 % MoS
2
by weight, in dry condition. In this
work EN304 stainless steel disc will be used as counterpart surface and tests will be
carried out at ambient conditions using a pin-on-disc Tribometer (TR-20LE).The
influence of the fillers, on the wear of the PTFE composite under dry sliding conditions
against the EN304 stainless steel disc will try to investigate and their sliding wear
performance will compared with PTFE + GF material.
3
2
.
LITERATURE
REVIEW
JaydeepKhedkar, IoanNegulescu, Efstathios I. Meletis(2002) "Sliding wear behavior of
PTFE composites" They experimented that the tribological behavior of
polytetrafluroethylene (PTFE) and PTFE composites with filler materials such as carbon,
graphite, E glass fibers, MoS2 and poly-p-phenyleneterephthalamide (PPDT) fibers, was
studied. The present filler additions found to increase hardness and wear resistance in all
composites studied. The highest wear resistance was found for composites containing (i)
18% carbon + 7% graphite, (ii) 20% glass fibers + 5% MoS
2
and (iii) 10% PPDT fibers.
Scanning electron microscopy (SEM) was utilized to examine composite microstructures
and study modes of failure. Wear testing and SEM analysis showed that three-body
abrasion was probably the dominant mode of failure for PTFE + 18% carbon + 7%
graphite composite, while fiber pull out and fragmentation caused failure of PTFE + 20%
glass fiber + 5% MoS
2
composite. The composite with 10% PPDT fibers caused wear
reduction due to the ability of the fibers to remain embedded in the matrix and
preferentially support the load. Differential scanning calorimetry (DSC) analysis was also
performed to study the relative heat absorbing capacity and thermal stability of the
various composites in an effort to correlate these properties to the tribological
performance. The results indicated that composites with higher heat absorption capacity
exhibited improved wear resistance. The dominant interactive wear mechanisms during
sliding of PTFE and its composites are discussed in view of the present findings. From
that they concluded that Addition of filler materials such as carbon, graphite, glass fibers
and PPDT to PTFE causes an increase in hardness and wear resistance, while the
coefficient of friction is slightly affected and remains low. Filler materials in general are
effective in impeding large-scale fragmentation of PTFE, thereby reducing the wear rate.
The wear process in the composites depends mainly on three factors: thermal stability,
thermal conductivity, and the characteristics of the filler materials. IN the presence of a
strengthening phase such as glass fibers, MoS
2
particles were found to be very effective in
improving the wear properties of the composite.
[5]
Dr. G.J.Vikhe, K.B.Kale&J.L.Shindein(2008) "An Investigation of tribological
behavior of MoS
2
on PTFE composites" they experimented that effect of addition of
MoS
2
in PTFE composites (Bronze & Glass Fiber) on tribological properties are studied.
In this paper, experimental evaluation of tribological properties is done with help of Pin-
on-Disc (TR-20) Tribometer to find the wear rate and coefficient of friction. The tests
were carried out in different environments like dry & humid. Also a case study is being
4
done on sealing materials in compressors and they concluded that it is observed from the
graphs of wear that in dry condition addition of 5% of MoS
2
toPTFE+15%GF matrix
increases wear against both counter surface Grey CI & ChromiumPlate. This is according
to Liu et al. that MoS
2
was not very effective for reducing friction andcaused an increase
in wear. Also Bijwe et al. proposed that the addition of MoS
2
alone didnot impart a good
wear resistance to PTFE, especially during severe conditions of slidingGrey C.I. &
Chromium Plate.While addition of 5% MoS
2
in PTFE+60% Bronze wear decreases
against both countersurface Grey CI & Chromium Plate. This is due to formation of PTFE
bronze composite filmon the counter surface (disc). It is the result of the fact that
adhesion force between the firstcomposite wear products (pin) and counter surface (disc)
is greater than the cohesion forcebetween first composite wear products and counter
surface causing good compositetransfer film reducing wear. The filler particles in the
current composite are expected toprovide a blocking action on the slip of the PTFE
lamellae and thereby to reduce the wearrate.Frictional force decreases with addition of
MoS
2
in PTFE + 15% GF as well as for PTFE +60% Bronze. Also reduction of frictional
force is seen more in PTFE + 60% Bronze comparedto PTFE + 15% GF matrix. This may
be due to addition of lubricating MoS
2
in the presence ofa strengthening phase has the
potential to reduce the friction by the latter by maintaining allow friction film. Wear
increases with addition of MoS
2
in PTFE +15% GF in humid environment. Highhumidity
enables the rate at which MoS
2
oxidizes to increase. The oxidation productmolybdenum
trioxides (MoO
3
) possess abrasive properties, and the sulphur acid which forms when this
occurs may cause severe corrosive wear. While it remains all the most same forthe PTFE
+ 60% Bronze matrix. Frictional force increases with addition of MoS
2
in PTFE + 15%
GF in humid environment. An increase in the temperature in the friction zone causes
partial oxidation of the MoS
2
in the areas of the actual contact surface forming H
2
S
immediately in large quantities, and thefriction coefficient increases until it reached a
maximum value, after which it graduallydropped again to a lower value. Frictional force
decreases with addition of PTFE+60%Bronzematrix in humid environment against both
counter surfaces.Wear resistance of PTFE +15% GF + MoS
2
shows slightly better results
as compared toPTFE+60%Bronze + MoS
2
but, properties of PTFE + 60%Bronze + MoS
2
are more suitable for gas compressors like thermal conductivity, coefficient of linear
expansion, compressivestrength & is thus recommended for the piston rings of gas
compressors. High humidity enables the rate at which MoS
2
oxidizes to increase, causing
corrosive wear.Glass fibers with the presence of a small amount of MoS
2
particulates
5
(5%) in PTFE cause asignificant decrease in the wear resistance.PTFE + 60 % Bronze
composite with the presence of a small amount of MoS
2
particulates (5%) cause a
significant increase in the wear resistance and frictional force in both humid &dry
condition & is thus recommended for gas compressors.
[6]
ArashGolchin, Gregory F Simmons and Sergei B Glavatskih " Break-away
friction of PTFE materials in lubricated conditions" experimented that This study
investigates the tribological characteristics at the initiation of sliding (break-away
friction) of several polytetrafluoroethylene (PTFE) based materials including virgin PTFE
(PP), PTFE filled with 25% black glass (PG), PTFE filled with 40% Bronze (PB), PTFE
filled with 25% Carbon (PC), andPTFE filled with 20% glass fiberand 5% Molybdenum
disulphide (PM), as well as standard white-metal Babbitt (BA) in lubricated sliding
contact with a steel counter-face. Experiments were carried out using a reciprocating
tribo-meter in the block on plate configuration with the specific goal of determining the
friction characteristics at break-away under varying conditions. Apparent contact
pressures of 1 to 8 MPa were applied with oil temperature levels of 25° to 85°C. Bronze-
and carbon-filled PTFE and virgin PTFE were found to provide generally lower break-
away friction and less variation in break-away friction over the course of testing than the
other tested materials. Break-away friction tests after an extended stop under loading
found bronze- and carbon-filled PTFE and virgin PTFE to be minimally affected by the
extended stop whereas Babbitt produced a significant increase in break-away friction in
the first cycle after stopping. Break-away friction for the four tested materials after an
extended stop returned to pre-stop values after 1 stroke.
[7]
H. Unal, A. Mimaroglu, U. Kadioglu, H. Ekiz (2006) "Sliding friction and wear
behavior of polytetrafluoroethylene and its composites under dry conditions"they
experimented that and they studied and explored the influence of test speed and load
values on the friction and wear behavior of purepolytetrafluoroethylene (PTFE), glass
fiber reinforced (GFR) and bronze and carbon (C) filled PTFE polymers. Friction and
wearexperiments were run under ambient conditions in a pin-on-disc arrangement. Tests
were carried out at sliding speed of 0.32, 0.64,0.96 and 1.28 m/s and under a nominal load
of 5, 10, 20 and 30 N. The results showed that, for pure PTFE and its compositesused in
this investigated, the friction coefficient decrease with the increase in load. The maximum
reductions in wear rate and frictioncoefficient were obtained by reinforced PTFE +17%
glass fibers. The wear rate for pure PTFE was in the order of 10 7 mm
2
/N, whilethe wear
rate values for PTFE composites were in the order of 10 8 and 10 9 mm
2
/N. Adding glass
6
fiber, bronze and carbon fillers toPTFE were found effective in reducing the wear rate of
the PTFE composite. In addition, for the range of load and speeds used inthis
investigation, the wear rate showed very little sensitivity to test speed and large sensitivity
to the applied load, particularly athigh load values. And they concluded that Wear studies
against AISI 440C stainless steel disccounter face under various loads and sliding
speeds,materials used in this study were ranked as followsfor their wear performance.
PTFE+ 17% GFR>PTFE+ 25% bronze>PTFE + 35% C>pure PTFE.PTFE + 17% GFR
exhibited best wear performance (K0 in the order of 10_8 mm
2
/N) and can be
consideredas a very good tribo-material between materials used in this study. The friction
coefficient of pure PTFE and its compositesdecreases when applied load increases. Pure
PTFE is characterized by high wear because ofits small mechanical properties. Therefore,
the reinforcementPTFE with glass fibersimproves the loadcarrying capability that lowers
the wear rate of thePTFE.For the specific range of load and speed explored inthis study,
the load has stronger effect on the wear behavior of PTFE and its composites than the
sliding velocity.
[8]
Seong Su Kim, HakGu Lee, &Dai Gil Lee (2007)"Thetribological behavior of
polymer coated carbon compositesunder dry and water lubricating conditions
"
were
performed wear experiments on pin-on-disk on carbon epoxy composite specimens with
many small surface grooves with respect to epoxy (EP) and polyethylene (PE) surface
coating materials with and without self-lubricating powders (MoS
2
and PTFE). From the
experiments they have showed that, the friction coefficient of the grooved specimen for
the case of PE-based coating decreased 5% compared to that of uncoated specimen
because of the low hardness and friction coefficient of the PE-based coating. The friction
coefficients of the PE + MoS
2
and EP + MoS
2
decreased 9% compared to those with PE +
PTFE and EP + PTFE. The PE + MoS
2
coated specimen showed an excellent wear
resistance under water lubricating condition.In this work pin on disk wear experiments
were performedon carbon epoxy composite specimens with many small surface grooves
of 100 µm with respect to epoxy (EP) and polyethylene (PE) surface coating materials
with and without self-lubricating powders (MoS
2
and PTFE). From the experiments, the
following results were obtained The friction coefficient of the grooved specimen for the
case of PE-based coating decreased 5% compared to that of uncoated specimen because
of the low hardness and friction coefficient of the PE-based coating. The friction
coefficients of the PE + MoS
2
and EP + MoS
2
decreased 9% compared to those with PE +
PTFE and EP + PTFE. The PE coated and PE + MoS
2
coated specimen reduced the wear
7
on the sliding surface due to the embedment of abrasive particles in the coating layer,
which prevented fibers from being deboned out of the ridges. The PE + MoS
2
coated
specimen showed an excellent wear resistance under water lubricating condition, because
the mixture of harder MoS
2
lamellaand soft PE lamellae was less prone to blistering
compared to the specimen coated with soft film alone.
[9]
Yunxia Wang and Fengyuan Yan (2006) "Tribological properties of transfer films
of PTFE-based composites" have carried out the wear test on PTFE-based composites
containing 15 vol. % MoS
2
, graphite, aluminum and bronze powder. Transfer films of
pure PTFE and these composites were prepared on the surface of AISI-1045 steel bar
using a pin on disc wear tester.Tribological properties of these transfer films were
investigated using another tribometer by sliding against GCr15 steel ball in a point-
contacting configuration. Morphology of the transfer films and worn surface of the steel
ball were observed and analyzed using SEM and optical microscopy. From this study they
have showed that, the wear life of the transfer film of PTFE is short because PTFE cannot
form durable transfer film on the steel counter face. PTFE is apt to form big flakes and
left the contacting region during the friction process. They concluded that The wear life of
the transfer film of PTFE is short because PTFE cannot form durable transfer film on the
steel counterface. PTFE is apt to form big flakes and left the contacting region during the
friction process. The transfer films containing different fillers have different
morphologies and structures and hold different load bearing capabilities. Transfer film of
the composite could carry out obvious back-transfer to the composite, which effectively
reduced wear of the composite. Some inorganic materials, for example, MoS
2
, graphite,
aluminum and bronze power as fillers could effectively prolong the wear life of transfer
film of PTFE-based composites. This was mainly achieved by strongly adhering transfer
film and smaller wear debris particles or fine fillers stably stay in the roughness valley.
Tribological properties of these transfer films are sensitive to load change. Generally,
increased load shortened wear life of transfer film.
[10]
H. Unal a, U. Sen a, A. Mimarogluhas (2006) "An approach to friction and wear
properties of polytetrafluoroethylene composite" and explored the influence of test speed
and load values on the friction and wear behavior of pure Polytetrafluoroethylene (PTFE),
glass fiber reinforced (GFR) and bronze and carbon (C) filled PTFE polymers. Friction
and wear experiments were run under ambient conditions in a pin-on-disc arrangement.
Tests were carried out at sliding speed of 0.32 m/s, 0.64 m/s, 0.96 m/s and 1.28 m/s and
under a nominal load of 5 N, 10 N, 20 N and 30 N. From this study the have observed
8
that, PTFE + 17% GFR exhibited best wear performance and is a very good tribo-material
between materials used in this study. The friction coefficient of pure PTFE and its
composites decreases when applied load increases. Pure PTFE is characterized by high
wear because of its small mechanical properties. Therefore, the reinforcement PTFE with
glass fibers improves the load carrying capability that lowers the wear rate of the PTFE.
For the specific range of load and speed explored in this study, the load has stronger
effect on the wear behavior of PTFE and its composites than the sliding velocity.
[11]
J.K. Lancaster (1982) "Accelerated wear testing as an aid to failure diagnosis and
materials selection" wasperformedthe needs and advantages of accelerated wear testing
are discussed, with particular reference to aerospace applications. Examples are given
from recent work on plastics-based dry-bearing liners to illustrate how accelerated wear
tests can provide information relevant to materials selection, identification of the main
parameters influencing wear and definition of the relationships between wear, material
composition and structure. Prediction of the service life of dry-bearing liners, however,
presents problems and full-scale component tests are usually required.
[12]
9
3. BASIC
FUNDAMENTALS
3.1 Engineering
Materials
The knowledge of materials and their properties is a great significance for a design
engineers. Materials are divided into following four types.
1.
Metals
2. Ceramics
3. Polymers
4. Composites
3.1.1 Metals
Metals are elements that generally have good electrical and thermal conductivity. Many
metals have high strength, high stiffness, and have good ductility. Some metals, such as
iron, cobalt and nickel are magnetic. The most important properties of metals include
density, fracture toughness, strength and plastic deformation. The atomic bonding of
metals also affects their properties.
[13]
i) Pure Metals
Pure metals are elements which come from a particular area of the periodic table.
Examples of pure metals include copper in electrical wires and aluminum in cooking foil
and beverage cans.
ii) Metal Alloys
Metal Alloys contain more than one metallic element. Their properties can be changed by
changing the elements present in the alloy. Examples of metal alloys include stainless
steel which is an alloy of iron, nickel, and chromium; and gold jewelry which usually
contains an alloy of gold and nickel. Many metals and alloys have high densities and are
used in applications which require a high mass-to-volume ratio. Some metal alloys, such
as those based on Aluminum, have low densities and are used in aerospace applications
for fuel economy. Many metal alloys also have high fracture toughness, which means
they can withstand impact and are durable.
[13]
3.1.2 Ceramics
A ceramic is often broadly defined as any inorganic nonmetallic material. Examples of
such materials can be anything from NaCl (table salt) to clay (a complex silicate). Some
of the useful properties of ceramics and glasses include high melting temperature, low
density, high strength, stiffness, hardness, wear resistance, and corrosion resistance. Many
ceramics are good electrical and thermal insulators. Some ceramics have special
10
properties: some ceramics are magnetic materials; some are piezoelectric materials; and a
few special ceramics are superconductors at very low temperatures. Ceramics and glasses
have one major drawback that they are brittle in nature.
[13]
3.1.3 Polymers
A polymer has a repeating structure, usually based on a carbon backbone. The repeating
structure results in large chainlike molecules having chemical bonding and synthesized by
the polymerization process. Polymers are useful because they are lightweight and
corrosion resistant, are easy to process at low temperatures, have good strength to weight
ratio and are generally inexpensive. Also they are poor conductors of electricity and heat,
which makes them good insulators. Some important characteristics of polymers include
their low molecular weight, softening and melting points, crystallinity, and structure. The
mechanical properties of polymers generally include low strength and high toughness.
Their strength can be improved by reinforced composite structures.
[14]
Classification of polymers
According to their mechanical and thermal behavior, polymers are classified as
thermoplastics, thermosetting polymers and elastomers.
i) Thermoplastics
Thermoplastics are composed of long chains produced by joining together monomers,
typically behave in a plastic ductile manner and may or may not have branches. On
heating they become soft and melt to mold into shapes and are easily recycled.
Polyamide, polyethylene, PVC, PTFE, acrylic, nylon, PEEK etc. are the examples of
thermoplastics.
ii) Thermosetting Polymers
Thermosetting Polymers are composed of long chains (linear or branched) of molecules
that strongly cross linked to one another to form three dimensional network structures.
They are stronger but brittle than thermoplastics. On heating they do not melt but
decompose and hence recycling is difficult. Phenolic, urethanes, amines, polyesters
epoxies etc are the examples of thermosetting polymers.
iii) Elastomers
Elastomers are nothing but rubbers and have high elastic deformation (>200%). The
polymer chain consists of coil like molecules that can reversibly stretch by applying a
force. Natural rubbers, polyisoprene, polybutadiene, butadiene styrene, silicones etc. are
the examples of elastomers.
11
3.1.4 Composites
Composite materials (composites) are engineered materials made from two or more
constituent materials with significantly different physical or chemical properties and
which remain separate and distinct on a macroscopic level within the finished structure.
Composites are widely used because overall properties of the composites are superior to
those of the individual components.
There are two constituent materials of composite; matrix and reinforcement. The
matrix material surrounds and supports the reinforcement materials by maintaining their
relative positions. The reinforcements impart their special mechanical and physical
properties to enhance the matrix properties. A synergism produces material properties
unavailable from the individual constituent materials, while the wide variety of matrix
and strengthening materials allows the designer of the product or structure to choose an
optimum combination.
A variety of molding methods can be used according to the end-item design
requirements and natures of the chosen matrix and reinforcement materials. Most
commercially produced composites use a polymer matrix material often called as a resin
solution. There are many different polymers like polyester, vinyl ester, epoxy, phenolic,
polyimide, polyamide, polypropylene, PEEK, etc. are available. The reinforcement
materials are often in the form of fibers but also commonly ground minerals. The strength
of the product is greatly dependent on the percentage of fiber content.
A number of material-processing strategies have been used to improve the wear
performance of polymers. This has prompted many researchers to cast the polymers with
fibers/fillers. Considerable efforts are being made to extend the range of applications.
Various researchers have studied the tribological behavior of FRPCs. Studies have been
conducted with various shapes, sizes, types and compositions of fibers in a number of
matrices. In general these materials exhibit lower wear and friction when compared to
pure polymers. An understanding of the friction and wear mechanisms of FRPC's would
promote the development of a new class of materials.
[14]
Use of inorganic fillers dispersed in polymeric composites is increasing. Fillers
not only reduce the cost of the composites, but also meet performance requirements,
which could not have been achieved by using reinforcement and resin ingredients alone.
In order to obtain perfect friction and wear properties many researchers modified
polymers using different fillers like Al
2
O
3
, ZnO, CuO, Pb
3
O
4
, ZrO
2
, TiO
2
, CuS, MoS
2
,
12
Bronze, Brass, Glass fiber, Carbon, Rubber, Graphite, Oxide Particles, Carbide particles,
etc. and even other polymer materials also.
[13]
Fiber reinforced polymers or FRPs include wood (comprising cellulose fibers in a
lignin and hemicelluloses matrix), carbon-fiber reinforced plastic or CFRP, and glass
reinforced plastic or GRP. If classified by matrix then there are thermoplastic composites,
short fiber thermoplastics, long fiber thermoplastics or long fiber reinforced
thermoplastics. There are numerous thermoset composites, but advanced systems usually
incorporate aramid fiber and carbon fiber in an epoxy resin matrix.
Composites can also use metal fibers reinforcing other metals, called as metal
matrix composites (MMC). Magnesium is often used in MMCs because it has similar
mechanical properties as epoxy. Ceramic matrix composites include bone (hydroxyapatite
reinforced with collagen fibers), Cermets (ceramic and metal) and concrete. Ceramic
matrix composites are built primarily for toughness, not for strength. Chobham armor is a
special composite used in military applications.
The thermoplastic composite materials can also be formulated with specific metal
powders resulting in materials with a density range from 2 g/cc to 11 g/cc. These
materials can be used in place of traditional materials such as aluminum, stainless steel,
brass, bronze, copper, lead, and even tungsten in weighing, balancing, vibration damping,
and radiation shielding applications.
Composite materials are popular in high-performance products that need to be
lightweight, yet strong enough to take harsh loading conditions such as aerospace
components (tails, wings, fuselages, propellers), boat, bicycle frames and racing car
bodies. Other uses include fishing rods and storage tanks. The new Boeing787 structure
including the wings is composed of over 50 percent composites. Carbon composite is a
key material in today's launch vehicles and space crafts. It is widely used in solar panel
substrates, antenna reflectors and yokes of space crafts.
PTFE, phenolic, nylon, acetalpolyimide, polysulfone, polyphenylenesulfide,
Ultrahigh molecular weight polyethylene, lubronetc and their composites can be used as
bearing materials.
3.2
PTFE and PTFE Composites
Polytetrafluoroethylene (PTFE) resin is a paraffinic thermoplastic polymer that has some
or all of the hydrogen replaced by fluoride. It is discovered in 1938 by a DuPont chemist,
Mr. Roy J. Plunkett at DuPont's Jackson Laboratory in New Jersey. Upon examination,
13
he learned that PTFE provided a combination of friction, temperature, chemical,
mechanical and electrical resisting properties. PTFE is recorded the lowest coefficient of
static and dynamic friction as 0.02 - equivalent to wet ice on wet ice. PTFE revolutionized
the plastics industry and, in turn, gave birth to limitless applications of benefit to
mankind. PTFE is used extensively for a wide variety of structural applications as in
aerospace, automotive, earth moving, medical, electrical, electronics, computer and
chemical industries.
[15]
PTFE has extended chain of linear repeating molecules of CF
2
CF
2
. PTFE is a
crystalline polymer with a melting point of about 327ºC. PTFE has useful mechanical
properties from cryogenic temperature of -260ºC to higher temperature of 280ºC. Pure
PTFE has virtually universal chemical resistance, light and weather resistant, resistant
against hot water vapor, excellent sliding properties, anti-adhesive behavior, non-
combustible, good electric and dielectric properties, no absorption of water,
physiologically harmless so as to use in food industry applications. But it has some
adverse properties like cold flow behavior, relatively low wear resistance, low resistance
to high-energy radiation, poor adhesive behavior and PTFE cannot be injected.
[13, 15]
PTFE is a high performance engineering plastics which is widely used in industry
due to its properties of self-lubrication, low friction coefficient, high temperature stability
and chemically resistant. In fact, PTFE exhibits poor wear and abrasion resistance. To
improve the wear resistance suitable fillers are added to PTFE. The most commonly used
are glass fiber, carbon, bronze and graphite, in the form of powder intimately mixed with
the PTFE, other fillers are molybdenum disulfide, metal powders, ceramics, metal oxides
and mixtures of two or more additives.
[11]
Molecular structure of PTFE
PTFE is a completely fluorinated polymer manufactured by free radical polymerization of
tetrafluoroethylene with a linear molecular structure of repeating
CF
2
CF
2
units.
Molecular structure Polytetrafluoroethylene( PTFE) is a crystalline polymer with a
melting point of about 621ºF (327ºC). PTFE has useful mechanical properties from
cryogenic temperatures (-260ºC) to 500ºF (280ºC).
Its coefficient of friction is lower than almost any other material. The chain
structure of PTFE has two interesting peculiarities.
[15]
14
3.3 Filler
Material
Typical Fillers
a) Glass fiber
PTFE is reinforced with glass fibers, the percentage varying between 5% and 40%. The
added glass fiber improves the wear properties to a minor degree, also the deformation
strength under load while leaving substantially unchanged the electrical and chemical
characteristics.
b) Carbon
Carbon is added to the PTFE in a percentage by weight between 10 and 35%, along with
small percentage of graphite. Also, the carbon tends to improve wear & deformation
strength .e, while leaving practically unchanged the chemical resistance, but substantially
modifying the electrical properties.
c) Bronze
Bronze, when used as filler, is added in percentages of weight between 40% and 60%.
Bronze filled PTFE has the best wear properties, remarkable deformation strengths and
good thermal conductivity, but poor electrical characteristics and chemical resistance.
d) Graphite
The percentage of graphite varies between 5% and 15%. Graphite lowers the coefficient
of friction & is, therefore, often added to other types of filled PTFE for improving this
property. It improves the deformation under load, strength &in a minor degree, wears
properties.
e) Molybdenum Disulphide
Molybdenum disulfide, minimize friction, furnishes high load performance, reduce power
input, prevent metal to metal contact, provide superior protection against wear, adhere
well to all types of surfaces, work themselves into surfaces for longer life.
[15]
3.4
Manufacturing of PTFE Component
Processing of PTFE is more difficult than that of standard thermoplastics. At high
temperatures (340-380°C) PTFE will merely become highly viscous, which means that
injection-molding or regular extrusion is impossible. For this reason, semi-finished
products are manufactured by means of compression sintering or ram-extrusion. PTFE
can be turned, milled, drilled, pierced, broached, ground and polished.
Details
- Pages
- Type of Edition
- Erstausgabe
- Year
- 2016
- ISBN (PDF)
- 9783960675259
- ISBN (Softcover)
- 9783960670254
- File size
- 1.2 MB
- Language
- English
- Publication date
- 2016 (March)
- Keywords
- PTFE Friction MoS2 Molybdenite Tribology Engineering material PTFE Composite Wear Behavior Polytetrafluoroethylene Polymer
