Mechanical and Tribological Behavior of Polyester Reinforced With Short Glass Fibers

8/9/2019 Mechanical and Tribological Behavior of Polyester Reinforced With Short Glass Fibers

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Wear, 137 (1990) 251- 266

251

MECHANICAL AND TRIBOLOGICAL BEHAVIOR OF POLYESTER

REINFORCED WITH SHORT GLASS FIBERS

S. BAHADUR and Y. ZHENG

~ec~~i c~l E~gineeri ~ paFtment, iowa State University, Ames, fowa 50011 (U.S.A.)

(Received April 4,1989; revised August 8,1989; accepted October 11, 1989)

Summary

The effect of the reinforcement of thermosett~g polyester with short

glass fibers has been investigated in terms of the resulting mechanical prop-

erties and tribological behavior. The optimum catalyst proportion for max-

imum strengthening was determined and the effect of coupling agent on

both the mechanical and the tribological properties was investigated. The

proportion by weight of fiber-glass used for the re~forcement of the poly-

mer was varied from 0 to 50 . The friction and wear-behavior was studied

as a function of sliding speed and fiber-glass proportion. The possibility of

oxidation of the unsaturated cross-linked polyester during sliding was exam-

ined by infrared spectroscopy. Scanning electron microscopy was used to

study the wear surfaces. The variation in wear rate was finally examined in

terms of observations related to the variation in coefficient of friction, tem-

perature rise, wear particle size and electron microscopy studies.

1. Introduction

There are a wide range of applications for polymers in wear-related

situations such as bearings, gears, sprockets, sleeves, valve guides, seals,

brakes etc. The attraction of polymers for these applications lies in their high

specific strength, low coefficient of friction, favorable wear characteristics,

ease of processing and economic feasibility [ I]. A number of material-pro-

cessing strategies have been used to improve the wear performance of poly-

mers. These include the incorporation of organic and inorganic fillers as well

as the addition of glass, carbon or aramid fibers. The addition of these

second-phase materials results not only in improved tribological properties

but also in superior physical and mechanical properties.

Because of the technolo~c~ importance of composite materials, the

friction and wear behavior of reinforced polymers has been actively investig-

* Some results from this paper were presented at the 60th Colloid and Surface

Science Symposium, American Chemical Society, Atlanta, GA, June 1986.


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252

ated [2 - 61. The wear process for these materials is fairly complex. The

interaction of the matrix and fiber materials, and in some cases the coupling

agent as well, complicates the fracture process which results in the formation

of wear particles. It has been reported [7 - 91 that the coefficient of friction

and the wear rate of reinforced polymers depend on the fiber orientation,

with superior wear resistance when the axis of the fibers is normal to the

plane of sliding. This is presumably so because the fibers in this orientation

are in the best position to support load and resist detachment from the

matrix. The magnitude of the effect of orientation on wear decreases as the

fiber content increases. Tsukizoe and Ohmae ]7] accounted for the good wear

resistance of fiber-reinforced plastics in terms of the self-lubricating ability,

high modulus of elasticity and high interlaminar shear strength of the com-

posite material. They proposed a model suggesting that wear proceeded by

the wear-thinning of fiber reinforcements, the subsequent breakdown of

fibers and by the peeling action of the fibers from the matrix. According to

Tanaka [lo], a fiber-rich transfer film is formed during the sliding of fiber-

filled polymers and it is this film which plays a vital role in reducing wear.

Sung and Suh [9] contradicted Tanaka’s explanation, saying that this did

not account for the difference in wear rates in the longitudinal and trans-

verse sliding directions. They instead relied on their delamination theory

to model the sequence of events during wear. Cleric0 and Patierno [ 111 pos-

tulated that subsurface deformation led to crack nucleation at the matrix-m

fiber interface. Any crack would be propagated parallel to the surface at a

depth dependent on the friction coefficient, leading finally to surface shear-

ing and the formation of a wear sheet. In his studies on hybrid carbonglass-

fiber-epoxy composites, Hawthorne [12] could not find any evidence of

subsurface plastic deformation or cracking which could lead to delamination

failure. Wear was instead influenced by the relative abrasiveness, fatigue

resistance, transfer film and relative load-bearing ability of the two constitu-

ent phases. Working with short fibers of glass, carbon and hybrid materials,

Voss and Friedrich

[

131 observed that the addition of fibers improved the

wear resistance of both polyamide and polyethersulfone and the improve-

ment was much more significant in polyethersulfone which itself wore

orders of magnitude faster than polyamide. The minimum wear rate occur-

red with a fiber volume proportion of 15

-

20 which seemed to depend

upon the pressure-velocity product.

In a follow-up work by the same

authors [ 141, similar behavior has been reported for short-fiber-reinforced

PEEK as well. Hanmin et al. [ 151 studied the friction and wear behavior of

poly(phenylene sulphide) reinforced with carbon fibers and noted a similar

dependence of wear rate on fiber content, with the exception that the wear

rate decreased again with fiber proportions in excess of 50 when higher

loads were used. This decrease was attributed to the steel counterface polish-

ing action at high fiber proportions and loads, accompanied by a temper-

ature rise leading to the formation of a dense film of Fe0 which was hard

and adhesion proof. They also found that the coefficient of friction was at

a minimum for a fiber content of 20

-

30 wt. . Lhymn and Light [16] stu-


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died the effect of sliding velocity on the wear rate of PBT reinforced with

short glass and carbon fibers and ~lyethylene terephth~a~ reinforced with

glass fibers. Their results indicated that the curve of wear rate us. velocity

could be divided into three regions. For low velocities the specific wear rate

is inversely proportional to velocity. The plateau region in the medium speed

range is due to the lubricating action of molten polymer at the surface. The

third region in which the wear rate rises with velocity corresponds to a severe

rise in tem~rat~e at the interface.

Arkles et ad. [17] studied the wear behavior of fluoropolymers rein-

forced with randomly dispersed glass and carbon fibers. They found that the

wear rate decreased as the fiber content increased up to a limiting value.

Tanaka [lo] noted that carbon fibers were more effective than glass fibers

in increasing the wear resistance of polyacetal. The improvement in perform-

ance brought about by carbon fibers has been attribute to the transfer of a

smoother film to the counterface and an increase in polymer thermal con-

ductivity and resistance to heat distortion [ 181. Nevertheless, Briscoe and

Steward [19] and Arkles et al. [17] reported that glass fibers were more

effective in reducing wear. This was so because glass fibers were harder and

less friable than carbon fibers.

As indicated by Lancaster 120 J, reinforced compositions based on

polyester resins incorporating fibers of cotton, cellulose and asbestos have

been widely used over the years as bearing materials. Eleiche and Amin [21]

studied the friction and wear behavior of polyester reinforced with uni-

directional continuous cotton fibers as a function of the sliding speed, fiber

volume-fraction and fiber orientation within the matrix. These natural

fibers have now been replaced with glass, graphite and aramid fibers. The

present work is, therefore, related to tribological studies on polyester re-

inforced with discontinuous glass fibers. Short fibers were used because

the composite material in this case may be fabricated by inexpensive com-

mercial processes such as extrusion and injection molding. The scope of the

work involved determining the optimum molding conditions, the investiga-

tion of the effect of fiber-glass proportion and sliding speed on friction, wear

and temperature rise, the relationship between wear rate and wear particle

size, and the wear mechanisms.

2. Experimental details

A commercial thermosetting polyester, Resinal no. WOW2 from the

Industrial Arts Supply Co., containing styrene as cross-linking agent and

cobalt as promoter, was used in this study. The catalyst for this resin con-

tamed 60 methyl ethyl ketone and cyclohexanone peroxides in dimethyl

phthalate. The polyester was reinforced with chopped fiber-glass strands

17 pm in diameter and 6 mm long. The coupling agent used was Dow

Corning Z-6030 silane. The glass fibers were immersed in an aqueous solu-

tion of 0.2 wt. of the coupling agent and dried in an oven at 120 “C for 2 h.


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The reinforced samples were prepared both with and without the coupling

agent.

The fiber-glass was mixed with polyester resin containing catalyst and

cured in a mold at room temperature (about 23 “C) overnight. The fiber-

glass-reinforced polyester was molded in the form of disks, 12 mm thick and

140 mm in diameter, and the specimens for flexure and wear tests were

machined out of it. The specimens for flexure were 6.4 mm thick, 12.7 mm

wide and 127 mm long. Friction and wear tests were performed using cylin-

drical specimens 6.4 mm in diameter and 15 mm long.

The flexure tests were conducted according

t

the ASTM Standard

D790 using a cross-head speed of 2.8 mm mini. The friction and wear

experiments were performed in a pin-on-disk machine with the circular end

of the polymer composite pin riding against the periphery of a steel disk

(R, 55), 6.9 mm thick and 92 mm in diameter. The polymer specimen was

secured to a cylindrical loading arm which had two strain gauges mounted

on its surface on opposite sides to provide a measure of friction force. The

tests were conducted under ambient conditions at sliding speeds of 1.0, 1.5,

2.0 and 2.5 m s-l, and a normal load of 9.8 N was applied over the pin.

Before starting the test, the pin was abraded against a disk of the same

diameter as was to be used in later experiments so as to make sure that the

curved surface of the pin was in contact with the disk periphery. In addition,

the sliding surface of the steel disk in the wear machine was polished with

600 grade emery paper and cleaned with acetone. This provided a base sur-

face roughness of 0.09

- 0.11

pm (AA) for the steel disk. The wear data were

obtained by weighing the polymer composite pin in a precision balance

every half-hour during the first hour and every hour thereafter up to nine

hours. The friction force was recorded continuously using a strip chart

recorder.

The temperature rise during sliding was monitored by focusing a

Raytek SL 280 SF Infrared Thermal System with close focusing arrangement

at about 2 - 3 mm below the disk sliding surface. In addition to this, infrared

spectra were obtained from a Perkin-Elmer Fourier Transform Infrared

Spectrometer 1800 using a photoacoustic cell accessory (MTEC --. Model

100). For this purpose, a section 3 mm thick was cut from the worn side

of the pin and inserted directly into the photoacoustic cell. The spectra were

normalized with the CH, band at 2920 cm-” as an internal standard.

3. Results and discussion

3.1. Mechanical properti es

Initially, the optimum catalyst proportion which would provide the

greatest strength was investigated. This was done by molding pure polyester

specimens with varying catalyst proportions from 1.0 to 1.35 by weight

in increments of 0.1 . Higher proportions of the catalyst were not used

because catalyst added to the impurity content of the material. The hard-


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255

1.00

1.10 1.20 1.30

1.40

PROPORTI ON OF CATALYST, UT%

Fig. 1. Variation of mechanical properties of polyester with varying catalyst proportion.

ness, flexure strength and flexure modulus data with varying catalyst propor-

tions are

plotted in Fig. 1. Whereas the flexure modulus keeps increasing

with increasing catalyst proportion, the flexure strength and hardness show

the highest values for 1.23 catalyst. So all the other specimens were pre-

pared with this catalyst proportion of 1.23 .

Next, the effect of coupling agent on the mechanical properties of re-

inforced polyester was investigated. Here the specimens for hardness and

flexure strength were prepared in five weight proportions of fiber-glass both

with and without the coupling agent. The hardness and flexure data corre-

sponding to these conditions are plotted in Fig. 2. Addition of the coupling

agent results in improved flexure strength and flexure modulus for any

proportion of fiber-glass, as would be expected because of better polyester-

glass interface bonding in the presence of a coupling agent. Whereas the

hardness and flexure modulus keep increasing with increasing fiber-glass

proportion, this is not so with the flexure strength because when the pro-

portion of fiber-glass is small, the result is a lower flexure strength than

that of the unreinforced polyester. The reason for this lack of strength

is an earlier failure caused by the separation between the polyester and fiber-

glass which is prompted by flexure when deformation becomes considerable.

When the fiber-glass proportion exceeds 40 wt. , in the presence of the

coupling agent, the flexure strength of the composite exceeds that of the

matrix material because the strengthening from the presence of the stronger

fiber material exceeds the weakening effect of debonding.

On the basis of the above results, it would ordinarily have been ex-

pected that the coupling agent would help in improving the wear resistance.


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PROPORTION OF FI BERGLAS, W'

Fig. 2. Effect of coupling agent on the mechanical properties of fiber-glass-reinforced

polyester.

0. 4 -

f

F

LJ 0. 3

4c

pl

4 0. 2-

O.l -

I I

I

I

0 10

20 30 40

PROPORTI ON OF FI BERGLASS, WT"

Fig. 3. Effect of coupling agent on the wear rate and coefficient of friction for polyester

filled with fiberglass: sliding speed, 2.5 m s-l; normal load, 9.8 N.

This is not necessarily the case as may be seen from Fig. 3. This figure shows

the variation of steady state wear rate with varying fiber-glass proportion for

a sliding speed of 2.5 m s-l, both with and without the coupling agent. Con-

sidering the scatter in data, one would conclude that the coupling agent af-

fects neither friction nor wear rate in any significant manner. This would


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tend to imply that the bonding between the constituents of the multiphase

structures should not affect the wear rate. This point will be elaborated later.

In view of these observations, the reinforced polyester specimens used in

later work were made without the coupling agent.

3.2. Friction and wear results

The variation of the coefficient of friction with sliding time for a sliding

speed of 2.0 m s-l

is shown in Fig. 4. Whereas the cyclic ~uctuatious in the

coefficient of friction were large, with +25 spread about the mean, only

the mean values are plotted here. It is noted that the coefficient of friction

stabilizes after about 2 h of sliding. During this transient period, a number of

phenomena occur, such as changes in counterface surface topography and

polymer rubbing surface, the development of polymer transfer film on the

steel disk surface, and a rise in interface temperature owing to the dissipation

of energy in rubbing. When the fiber content is increased from 10 to 50

wt. , the steady state coefficient of friction increases from 0.35 to 0.67.

The friction coefficient is at its lowest for 10 wt. fiber, because it also in-

creases when the proportion of fiber decreases below this, as shown in Fig. 5.

This behavior is similar to that reported by Hanmin et al. [151. Since the

variation of friction with varying fiber-glass proportion is identical to that of

flexural strength (Figs. 2 and 5), the friction of the composite material seems

to be governed by its shear strength which influences the rupture of adhesive

bonds at the interface. The likelihood of fiber-glass disrupting the transfer

film, as exhibited during the test by large cyclic fluctuations in friction

force, could also account for increased friction with the higher proportions

of fiber-glass. Little can be deduced from the variation in the coefficient of

friction with sliding velocity because of the overlapping of the data points

for different velocities (Fig. 5).

The variation of wear with sliding time for a sliding speed of 2 m s-l

and a load of 9.8 N is shown in Fig. 6 for varying fiber-glass weight percent-

ages. As with the coefficient of friction, the wear rate also reaches a steady

state in 2 h of sliding which is therefore assumed to be the time needed for

the build-up of transfer film on the mating steel surface, and for the tem-

SLI DI NG TI NE, h

Fig. 4. Variation of coefficient of friction with sliding time for varying fiberglass propor-

tions (by weight) in polyester: sliding speed, 2 m s-l; load, 9.8 N.


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PROPORTI ON OF FI BERGLASS

Fi g. 5. Variation of coefficient of friction with fiber-glass proportion (by weight) in poly-

ester for varying sliding speeds.

SLI DI NG TI ME, h

Fig. 6. Variation of wear with sliding time for varying fiber-glass proportions (by weight)

in polyester: sliding speed, 2 m s-‘; load, 9.8 N.

05

0.4

f

E"

- 0. 3

z

20. 2

Y

0.1

0.0

I I I 1

I

0

10 20 30 40

50

PROPORTI ON OF FI BERGLASS, WT%

Fig. 7. Variation of wear rate with fiber-glass proportion (by weight) in polyester for

varying sliding speeds.

perature to rise to an equilibrium state along with other effects. The steady

state wear rates were determined by fitting straight lines, using the method

of least squares, to the steady state wear data. The slopes of these lines, in

Fig. 6, provide steady state wear rates which are plotted in Fig. 7 as a func-

tion of fiber-glass proportion. The wear rate of unreinforced polyester had


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259

an excessively high value of 1.7 mg h-’ at a sliding speed of 2.0 m s-l. With

the addition of a mere 5 wt. of fiber-glass to the polyester, the wear rate

was reduced to 0.23 mg h

-l. This result is similar to that observed by Voss

and Friedrich [ 131 for polyethersulfone reinforced with glass fibers. The

wear rate increases with increasing fiber-glass content with the values in

excess of lo , for all sliding speeds, and the lowest wear rate is found for

about 10 fiber-glass. Such a minimum in wear rate with fiber proportion

has been reported for other composites as well [5,6,13,14]. The variation of

wear rate with fiber-glass proportion is similar to that of the coefficient of

friction (Fig. 5) discussed earlier. For fiber proportions higher than the

optimum for minimum wear rate, the wear rate increases with increasing

sliding velocity as has been reported by others [ 14, 161.

3.3.

Temperat ure ri se and oxi dat ion

The variation in the temperature rise, which was measured 2 - 3 mm

below the contact surface during sliding, as a function of the sliding speed

and fiber proportion, is shown in Fig. 8. It is noted that for fiber-glass pro-

portions of 10 - 50 , the disk temperature rises both when the fiber pro-

portion is increased, for any sliding speed, and when the sliding speed is

increased, for any fiber proportion. The first part of this statement is sup-

ported by the variation of the coefficient of friction with fiber-glass propor-

tion shown in Fig. 5. The increased temperature rise with increasing sliding

speed here is mainly due to the reduced cooling time per revolution of the

disk because the variation in the coefficient of friction with sliding speed was

fairly small (Fig. 5). The data on temperature rise presented in Fig. 8 indi-

cate only the trend in variation. The actual temperatures closer to or at the

interface are considerably higher as revealed by the different measurements

by Bark [22]. Jian

et al. [23]

successfully studied the photo-oxidation of

60 -

0"

1.0 1.5 2.0

2.5

SLI DI NG SPEED, m s

Fig. 8. Variation of disk temperature with sliding speed for varying fiber-glass propor-

tions.


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260

unsaturated cross-linked polyester using infrared spectra. They found that

the spectra showed distinctive changes following ~hoto~oxidat~on, specific-

ally the broadening of a peak at 3200 - 3500 cm- ’ attributed to the forma-

tion of a hydroxy/hydroperoxide group. The possibility of oxidation during

the wear process was studied in view of the above result. The infrared spectra

of specimens with varying fiber-glass proportions corresponding to the two

conditions - before sliding and after 6 hours of sliding - were obtained and

are shown in Fig. 9. There is a marked broadening seen in the CW stretching

mode peak at 3200 - 3500 cm ml caused by the wear process. As mentioned

earlier, this is because CH has been changed to COW or COOH, whicth indi-

cates that the considerable temperature rise on the mating surfaces has

caused some oxidation of polyester. It is also noted that the peak broadening

in wear specimens increases with increasing fiber proportion in a way which

2692

1961 5

a)

bj

Fig. 9. Infrared spectra of fiber-glass-reinforced polyester (opd = 0.25) : percentages reiate

to fiber-glass content; sliding speed, 2 m s-l.

(a) Before sliding and (b) after sliding for

6 h.


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261

parallels the temperature rise variation

presented in Fig. 8. The information

presented here related to the temperature rise and oxidation will be used

later in explaining the wear behavior.

3.4.

Wear part i cl e si ze

The

size of polymer wear particles generated in sliding was measured

with an optical microscope for three different fiber-glass proportions. The

surface of a particle was approximated as an ellipse and the area was estim-

ated from the measurements of major and minor axes. A total of 50 particles

were used in each case in plotting the histograms shown in Fig. 10. It is

noted that about 75 of the wear particles rubbed off the 10 fiber com-

posite had surface areas less than 1 X

10m5

cm2 and 20 were in the range

(1

-

10) X lop5 cm2. The number of particles with areas less than 1

X

lo-’

cm2 decreased to 62 for 30 fiber composite and to 50 for 50 fiber

composite. It was not possible to make such measurements on 5 fiber com-

posite because the material was worn in the form of filaments (Fig. 11, see

Section 3.5). This observation, related to the generation of larger sized

polymer wear particles with higher proportions of fiber, may be looked at

in the context of our temperature rise data presented in Figs. 8 and 9. As

the temperature increases in discrete contact zones, localized softening

occurs, thereby increasing the area of asperity junctions which are later

sheared giving rise to wear particles.

3.5.

Scanning electron microscopy studies

The wear mechanisms were investigated by scanning electron micro-

scopy (SEM) of the worn surfaces of polymer composite pins. Figure 11

shows that the filaments of polyester material have been torn apart because

a fiber-glass proportion of 5 is too small to provide real support to the

polymeric material during sliding. When the proportion of fiber-glass is

increased from 5 to 10 . the filaments disappear and wear occurs through

60

*

w

Y

240

4

G

20

0

I

0

AREA OF PARTI CLE x 10' cm

(a)

AREA OF PARTI CLE x l o5 cm

(b)

AREA OF PARTI CLE x l o5 cm2

cc)

Fig. 10. Histograms showing wear particle distributions for polyester, reinforced with (a)

10 ; (b) 30 and (c) 50 fiber-glass; sliding velocity, 2 m s-l; load, 9.8 N.


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262

Fig. 11. Worn surface of polyester pin reinforced with 5 fiber-glass: sliding velocity, 2 m

s-r; load, 9.8 N.

Fig. 12. Worn surface of polyester pin reinforced with 10 fiber-glass: sliding velocity,

2 m s-l; load, 9.8 N.

the generation of wear particles (Fig. 12). The fibers are aligned at an angle

to the plane of sliding. In some locations, polyester has not been able

t

fill

the space between the fibers thereby producing a void. One such void is

shown at a higher magnification in Fig. 13. This also shows the degradation

of intervening polyester material because of oxidation and the development

of cracks following deformation. The features on the worn surfaces of 30

and 50 fiber composites are similar, as shown in Figs. 14 and 15. The

surface has been severely deformed and is covered with a large number of


2 m s-l; load, 9.8 N.


2 m SC’; load, 9.8 N.


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263

Fig. 15. Worn surface of polyester pin reinforced with 50 fiber-gims: sliding velocity,

2 m s-l; load, 9.8 N.


2 m s-‘; load, 9.6 N.

wear particles. The oval shape of the fiber ends clearly indicates that they

were involved in a wearing process. This is better seen in Fig. 16 which

was obtained by tilting the specimen stage so as to be able to look along

the axis of the fibers. Like this, the fiber ends appear to be circular in

shape. The angle of tilt needed was 32” which means that the fibers were

aligned at an angle of 32” to the perpendicular direction. The alignment

of the fibers in the vicinity of the surface seems to be influenced by the

resultant action of the normal load and frictional force because tan 32”

equals 0.62 which is approximately the coefficient of friction for 50

fiber composite.

3.6. Discussion

On the basis of the above obse~ations, it is possible to explain the wear

behavior presented earlier in Fig. 7. Voss and Friedrich [13, 141 have re-

ported similar variation of wear rate with fiber proportion for other polymer

composites and developed a semiquantitative rule-of-mixtures equation.

They consider the total wear to comprise the following four components:

wear of matrix, wear from fiber sliding, wear from fiber cracking and finally

the wear by fiber/matrix separation at the interface. It is impossible to mea-

sure these components individually in any sliding situation. We will thus

examine the variation in wear rate (Fig. 7) in terms of our observations

related to the variation in coefficient of friction, temperature rise and

wear particle size, and with reference to our electron microscopy studies.

We account for the high wear rate (1.7 mg h-’ at a speed of 2.0 m s-i) of

unreinforced polyester in terms of considerable heating at the sliding inter-

face owing to its high coefficient of friction of 0.70 - 0.75 (Fig. 5). As a

result of this, thick blobs of material were observed to have been removed

during sliding. This process was consistent with the fact that the unrein-


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264

forced material was weak. The addition of 5 fiber-glass lowers the coeffi-

cient of friction and increases the stiffness of the material. Consequently,

the heating at the interface is less and the worn material is no longer in the

form of blobs. The wear rate is thus much lower. The addition of 10

fiber-glass results in the lowest attainable value of the coefficient of friction

and the material removed during sliding is in the form of small particles. The

wear rate is at its lowest for this fiber proportion. With smaller fiber propor-

tions the wear can be considered as wear of the matrix material, but with

larger proportions the processes related to the amount of fiber and its

interaction with the matrix material greatly affect the wear rate. These

processes involve fiber sliding, fiber pull-out, voids produced through fiber--

matrix debonding, along with those already present, sources of stress con-

centration such as where crack initiation is caused by non-uniform stress

distribution arising from the segregation of fibers, stress at the crack tips

and the like. All of these are important factors in a dynamic material failure

situation such as that of wear, but their individual contributions cannot be

measured. Since all of the above processes lead to failure at the micro-level,

they can all be detrimental, as causes of wear. In addition to the above

phenomena, it is noted that with increasing fiber-glass proportion, the coef-

ficient of friction increases (Fig. 5). This leads to a higher temperature rise

as evidenced in Fig. 8 and more oxidation as shown in Fig. 9. These pro-

cesses cause more wear damage because they produce localized weakening

and degradation and lead to the formation of larger sized wear particles

(Fig, 10). The wear also increases with increasing sliding velocity because

then there is a greater rise in temperature at the sliding interface.

4. Conclusions

The results obtained from this study of the mechanical properties and

of the friction and wear behavior of glass-fiber-reinforced polyester sliding

against steel may by summarized as follows:

(1) The use of a coupling agent results in improved strength properties

but its effect on the coefficient of friction and on the wear rate is insignif-

icant.

2) With an increasing proportion of fiber-glass in polyester, the hard-

ness and flexural modulus of the composites increase continuously whereas

the flexural strength initially decreases and then shows a continuous in-

crease.

(3) The wear rate of polyester composites is much lower than that of

the unreinforced polyester.

(4) The wear rate and the coefficient of friction are both at a minimum

with a fiber-glass proportion of 10 wt. , and they both increase when this

proportion is made either lower or higher.

(5) With increasing sliding speeds, the wear rate increases but there is no

significant effect on the coefficient of friction,


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265

(6) The disk temperature rises with an increase in sliding speed and also

with an increase in fiber-glass proportion. There is oxidation of polyester,

detectable by infrared spectroscopy, because of this rise in temperature.

(7) There is no direct relationship between mechanical strengthening

and wear or friction.

The authors gratefully acknowledge the help of Dr. John McClelland

of the Ames Laboratory at Iowa State University, Ames, IA, with infrared

studies.

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Documents

Mechanical and Tribological Behavior of Polyester Reinforced With Short Glass Fibers