tribology sudarshan rao

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Tribological behavior of Fiber Reinforced Polymer Composites

Mr. Sudarshan Rao K a , Dr. Y S Varadarajan b & Dr. N Rajendra c

a. Research scholor, Department of Industrial & Production Engineering, NIE, Mysore.Mobile: 9448252890. Email: [email protected].

b. Professor & Head, Department of Industrial & Production Engineering, NIE, Mysore.

c. Professor, Department of Industrial & Production Engineering, NIE, Mysore.

Abstract:A Composite is a combination of two

or more materials that has properties which thecomponent materials do not have by themselves and the properties of the resultant material is superior to the

properties of the individual material which has made upthe composite. Fiber Reinforced Polymer compositesare the result of embedding high strength, high stiffnessfibers of one material in a surrounding matrix of thermoset or thermo plastic polymers. Fibers are the principalload carrying members where in the matrix keep them

in position. The pioneering works on the friction and

wear properties of polymer composites have identifiedseveral tribological factors affecting the tribology of polymer composites have attempted to clarify thegoverning mechanism of friction and wear. The studyof wear has two major purposes: to predict wear and to

reduce wear.Keywords: Composites, Tribology, friction, wear.

Introduction:Fiber Reinforced Polymer composites are

increasingly being used for wear sensitive applications

owing to their excellent specific strength, specificstiffness, light weight, corrosion resistance & self

lubricity, extreme flexibility in material propertiesthrough the control of fibers, matrix composition andfiber lay-up.

Because of the wear, mechanical properties of

the polymers like low stiffness, the application have been limited to low load carrying parts. There is a

strong driving force for using composites as low frictionand low wear material. The use of Fiber ReinforcedPolymer Composites in sliding applications, such as bearings & seals, has grown significantly due to the

composites self lubricating characteristics & enhancedmechanical and tribological properties provided by the

unique combination of properties of the fibers and the

matrices. In order to design or develop a good bearingmaterial it is necessary to characterize & understand thefriction & wear behavior of composites. An

understanding of the friction & wear mechanisms of polymers would aid the development of composites for

the solution of technological problems.Coefficient of friction of such material is of

importance as in braking materials (higher values) orgears (limiting values). Friction and wear of compositeshas been the subject of many studies and

experimentations over many years. In spite of thenumerous investigations, Fiber Reinforced Polymercomposites have unique capability of operating under

un lubricated conditions. In addition, many differentmaterial combination and designs can be tailored for a particular application.

This paper is a review paper concentrates onthe study of tribological behavior i.e. friction and wear behavior of Fiber Reinforced Polymer composites basedon the fiber orientation, fiber volume fraction, fiber

size, & fiber type etc. Initially the mechanisms of wear:film transfer, adhesion and delamination are discussed.Then the affect of the internal factors (fiber orientation,fiber volume fraction, types of fiber & matrix, andmodulus of elasticity), and external factors (normalload, sliding velocity, surface roughness of the sliding

mate, sliding distance & temperature) on the friction

and wear behavior of fiber reinforced polymercomposite are studied. Finally friction and wear properties of few composites like Kevlar 49 reinforcedin epoxy matrix, glass fiber in PEEK matrix, and carbonfiber in PEEK matrix are studied.

Mechanisms of friction and wear:Three mechanisms have been postulated to

explain sliding friction & wear of polymers: filmtransfer (abrasion), adhesion & delamination (fatigue).

Film transfer [1] is the most significant

macroscopic phenomenon which differentiates a polymer from a metal in friction & wear. Thus an

understanding of polymer wear must take into accountthe formation & subsequent removal of any transferredfilm & its chemical changes in addition to the acceptedmechanisms that have been proposed for metals. This

film can smooth out the metal counterface and reduce both friction & wear rate as the sliding condition

changes from that of the polymer sliding steel to one ofthe polymer sliding on polymer. The friction & wear behavior is also affected by the film thickness which isrelated to the temperature and thus to the sliding speed

& the normal load. Knowledge of the chemistry of thefilm and of the nature of the degradation products

produced by the very high local temperatures on the

polymeric sliding surfaces and the presence of themetallic counterface should assist in understanding itsrole in friction and wear. If the temperature due to the

high normal load and sliding speed is high, fibertransfer may reduce the coefficient of friction and thus

affect its correlation with wear rate.The adhesion theory of friction & wear [1] for

unlubricated metals has been widely accepted in thefield of tribology. This theory postulates that friction &wear are controlled by the adhesion of contacting

asperities. Friction is due to the shearing of the junctions formed at the regions of contact. Wear is dueto separation within the bulk of the weaker material by

a fracture process at the junction. Mathematical modelshave been derived to predict friction from the bulk properties of the weaker material and wear from the

hardness of the wearing material, the normal load & the

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sliding distance. Such models do not take into accountthe adhesion energy or the surface properties of thesliding material.

In the delamination theory of wear [2]

proposed by suh, repeated normal & tangential loads,which are transmitted through the contact points by

adhesive and ploughing actions, deform & sometimesfracture the asperities on the softer surface, forming

small wear particles and generating a relatively smoothsurface. The contact thus becomes an asperity- plane

contact rather than an asperity – asperity contact; theasperities of the harder surface plough the softer surface

which then experiences cyclic loading. The surfacetraction exerted by the harder asperity on the softersurface initially induces plastic shear deformationwhich accumulates with repeated loading leading tocrack nucleation: cracks which are nucleated below thesurface propagate parallel to the surface. Ultimately

they shear to the surface, generating delamination ofwear sheets of thickness equal to the crack propagationdepth which is controlled by the normal and tangentialloads at the surface.

Suh suggested that thermoplastics withinclusions wear by the delamination mechanism; where

as pure thermoplastics wear by surface melting orcontinuous deformation.

Factors affecting friction and wear of Fiber

Reinforced Polymer composites: The abrasive wear of polymer composites has

been investigated by various researchers who studiedthe effect of numerous parameters on the wear rate. Thewear rate increased with the increasing surfaceroughness of a steel disk in dry & wet environments.The abrasive wear rate of polymer compositescorrelated with the mechanical properties (hardness andfracture energy) and micro structural parameters (fiber

fraction, fiber aspect ratio, fiber diameter, roughnessand frictional coefficients). The abrasive wear rate isalso depends on the normal load, sliding distance,temperature, moisture content.

Fiber orientation:

The effect of fiber orientation on compositewear is still controversial [3]. Lancaster say that theeffect of fiber orientation on composite wear depends

on the fiber fraction in the composites: for compositeswith low volume fraction (20%) the longitudinaldirection has higher wear, where as for composites with

high volume fraction (>40%) there is no fiberorientation effect. The effect of the fiber orientation onthe wear rate also depends on the type of compositematerial under consideration as well as on the type oftribological system under which it operates.

In general the composites exhibit the greatest

wear resistance when fiber alignment is normal to the plane of contact (normal orientation), & lowest wear

resistance when fiber alignment is in the plane ofcontact and perpendicular to the sliding direction

(transverse orientation). Between these two extremes inwear resistance is the longitudinal orientation, where

fiber alignment is in the contact plane and parallel to thesliding direction.

The highest wear & friction coefficients areobtained for fiber oriented in the transverse direction,while intermediate for longitudinal direction. As thefiber orientation is varied from normal to transverse,

both the friction coefficient & the Wear rate increasegradually.

The orientation of fiber may vary between parallel and perpendicular to the wearing surface [4].

Parallel and transverse sliding direction on compositescontaining fiber lying parallel to the wearing surface

must be distinguished. Intermediate orientation canoccur between these extremes. The affect of fibers in

reducing wear loss depends on the ratio of indentationdepth of abrasive particles to the embedded length ofthe fibers. Fibers parallel to the surface may dug outmore easily than that perpendicular to it. The abrasivewear loss increased by changing the fiber orientationfrom perpendicular to parallel to the wearing surface.

Reinforcing constituents of a size smaller than theindentation depth of abrasive particles are easily dugout. Hence, the abrasive wear loss may increase withdecreasing size of the reinforcing constituents.

Fibers of predominantly normal orientationtended to produce lower wear intensity than those

parallel or anti parallel. Fiber lying normal to thewearing surface tend to result in lower wear intensity but sometimes in a greater coefficient of friction thanthose lying parallel or anti parallel.

Fig. 1. Effect of wear rate on fiber orientation.

Composites containing normally orientedfibers lead to lower wear intensity but may be greatercoefficient of friction than those with parallel orantiparallel fibers. Continuous unidirectional fibers

result in the lowest wear intensity. Particulatereinforced composites result in relatively poorresistance, since they are easily detached from awearing surface.

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Modulus of elasticity:A low modulus of elasticity of the matrix or the fibersfavors debonding at interfaces [5]. Hard but brittle

fibers may be insufficiently supported by a matrix oflow modulus of elasticity, and are thus fractured or

pulled out during abrasion. Hence, abrasive wear lossmay increase with decreasing modulus of elasticity. The

ratio of hardness of abrasive particles to the reinforcingconstituents influences the abrasive wear. Constituents

harder than the abrasive particles act as strong barriersagainst grooving and reduce wear loss effectively.

Abrasive wear loss increases when the hardness of theabrasive particles increases relatively to the hardness ofthe constituents. Brittle constituents or matrix favorcracking and flaking due to abrasion. This result inincreasing abrasive wear loss.

High modulus fibers result in lower wear

intensity than high strength fibers. Wear intensity tendsto decrease with increasing elastic modulus of the fiberand the composite. Improved fiber matrix bondinglowers the wear intensity.

Some of the external variables affect thetribological properties of fiber reinforced polymer

composites like normal load, sliding velocity, surfaceroughness of the sliding mate, & the temperature.

Influence of normal load, sliding velocity on the

friction & wear:With the increase in normal load the

coefficient of friction decreases in the case of fiberreinforced polymer composites [6]. Depending upon thenormal load, the hard abrasive particles could merely plow, deforming the matrix under small loads, whichthey could crack and cut the material. It is known thatdebris plowing would bend the fibers along eith thematrix, resisting the cracking, while with higher normal

load; the fiber could be cut without being bend ordeforming. This is the case for increased plowingfriction under small loads.Wear rate generally decreases as the sliding velocityincreases under a low normal load & the wear rate tendsto remain constant under a high load when sliding

velocity increases..

Fig.2. specific wear rate of FRP as a function of fibervolume fraction against sliding velocity for twodifferent loads

Fig.3. Specific wear rate of FRP against SlidingVelocity

Fig. 4. Wear rate v/s sliding speed for different fiber

volume fraction of Glass fiber reinforced composite.

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Surface roughness: Both friction coefficient and wear loss increase

if a critical surface roughness is exceeded [5]. The valueof surface roughness resulting in minimum friction and

wear loss depends on the test condition used. Theoccurrence of the minimum can be explained by

decreasing adhesion with increasing roughness on theleft side & increasing abrasion with further increasing

roughness on the right side of the minimum. Atoptimum roughness, a firmly attached transferred film

may also be involved.

Fig.6. Specific wear rate against sliding speed for Glassand carbon fiber reinforced PEEK composite.

Fig.5. coefficient of friction & Wear loss against thesurface roughness.

A particularly attractive composite system isthat of carbon fiber reinforced polymers resulting from

the high stiffness and self lubricity of the fibers [7]. Asa polymer matrix, polyetheretherketone (PEEK) resin isalso of interest owing to its excellent resistance tochemical attack while maintaining an adequate stiffnessat high temperature. Thus a carbon fiber reinforcedPEEK composite is likely to be very attractivetribological material for demanding applications.

Addition of glass, carbon, Kevlar fibers to polymers will improve their mechanical and tribological properties. The wear rate decreases up to a certainlimiting value with increasing the fiber content. Carbonfibers are more effective than glass fibers in increasingthe wear resistance of composites. Increased thermal

conductivity and resistance to heat distortion, a lowwear rate has been reported in carbon fiber reinforcedcomposites.

The combination of superior thermomechanical properties & good friction & wearcharacteristics renders the potential of polyetheretherketone composites as performance tribo

materials for use at high temperatures a very promisingone [8].

Fig.7. Specific wear rate against sliding speed for Glassfiber reinforced PEEK composite for two orientations.

Sung & suh, studied three types of fiberreinforced polymer composites [8]. They are

unidirectionally oriented composite with graphite fibersreinforced in epoxy matrix, unidirectionally orientedKevlar 49 fibers reinforced in epoxy matrix, & glassfibers in polytetraflouoroethelene (PTFE) with fibers

oriented in three orthogonal directions with fiber ratios2:1:0.

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For graphite epoxy composite the friction coefficientand wear volume of each specimen were measured as afunction of sliding distance for three different fiberorientations, Normal (N), Transverse (T), &

Longitudinal (L). Significant differences are found in both friction coefficient and wear volume for three

orientations. The lowest wear & friction coefficients areobtained for fiber oriented normal to the sliding surface.

Fig.8. Friction coefficient and wear volume as afunction of sliding distance in graphite fiber epoxycomposite with fiber orientation Normal, Longitudinal,& transverse to the sliding direction.

For Kevlar 49 epoxy composite, more wear

rate observed in normal direction, with highest frictioncoefficient. The fiber orientation in increasing order ofwear- normal, transverse & longitudinal, were exactlyreversed in friction coefficient values. The maximum

wear and the minimum friction coefficient wereobserved with longitudinal fiber orientation.

Fig.9. Friction coefficient and wear volume as afunction of sliding distance in Kevlar 49 fiber epoxy

composite with fiber orientation Normal, Longitudinal,& transverse to the sliding direction.

For Glass fiber PTFE composite the lowest

wear was obtained when the largest fraction of fiberswas oriented normal to the sliding plane, where as the

highest wear can exhibited when no fibers wereoriented normal to the sliding plane. The friction

coefficient were virtually identical being 0.34 for allthree orthogonal planes.

Fig.10. Friction coefficient and wear volume as afunction of sliding distance in glass fiber PTFE

composite with fiber orientation Normal, Longitudinal,

& transverse to the sliding direction.

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Conclusion:

The following observations and conclusions can be

drawn from this study:i. The effect of fiber orientation on composite

wear depends on the fiber volume fraction inthe composites. If the composites with high

volume fraction, there is no fiber orientationeffect.

ii. The friction and wear of composites dependson the types of fiber and matrix, for carbon

fiber wear rate is less, where as for Kevlar 49or glass fibers wear rate is high.

iii. The friction and wear depends on the modulusof elasticity of matrix and fiber, low modulusof elasticity favors debonding at interfaces andare then fractured or pulled out during

abrasion.iv. Both friction and wear increases if the critical

surface roughness exceeded.wear rategenerally decreases as the sliding velocityincreases under a low normal load and thewear rate tends to remain constant under a high

load when sliding velocity increases.

References:

[1]. M Clerico, V Patierno “Sliding wear of polymeric

composites”, Wear 53 (1979) 279-301.[2]. N P Suh, “The delamination theory of wear”,

Wear 25 (1973) 11.[3]. B Vishwanath, AP Verma, CVS Kameswara Rao

“Effect of reinforcement on friction & wear offabric reinforced polymer composites”, Wear 167

(1993) 93-99.[4]. T C Ovaert “Wear of unidirectional polymer

matrix Composites with fiber orientation in the plane of contact”, Tribology Transaction Vol. 40(1997), 2, 227-234.

[5]. Karl-Heinz-ZumGahr “Microstructure and wear ofmaterials”, Elsevier (1987), 292-294, 323-328,461-477.

[6]. M Cirino, K Friedrich, R B Pipes “The effect offiber orientation on the abrasive wear behavior of polymer composite materials”, Wear 121 (1998)127-141.654

[7]. H.Voss & K. Friedrich “On the wear behavior ofshort fiber reinforced PEEK composites”, Wear

116 (1987) 1-18.[8]. N H Sung & N P Suh, “ Effect of fiber orientation

on friction & wear of fiber reinforced polymericcomposites”, Wear 53 (1979) 129-141.

[9]. C.Lhymn “Analysis of wear statistics for polymercomposites”, Wear 114 (1987) 223-239.

[10]. T C Ovaert “On the wear behavior oflongitudinally oriented unidirectional FiberReinforced polymer Composites”, TribologyTransaction Vol. 38 (1995), 1, 27-34.

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