Hot wear properties of ceramic and basalt fiber reinforced hybrid friction materials

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doi:10.1016/j.tr

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Tribology International 40 (2007) 37–48

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Hot wear properties of ceramic and basalt fiberreinforced hybrid friction materials

Bulent Ozturka,�, Fazlı Arslana, Sultan Ozturkb

aDepartment of Mechanical Engineering, Karadeniz Technical University, Trabzon, TurkeybBes-ikduzu Technical College, Karadeniz Technical University, Trabzon, Turkey

Received 22 April 2005; received in revised form 17 January 2006; accepted 26 January 2006

Available online 14 March 2006

Abstract

In the present study, hybrid friction materials were manufactured using ceramic and basalt fibers. Ceramic fiber content was kept

constant at 10 vol% and basalt fiber content was changed between 0 to 40 vol%. Mechanical properties and friction and wear

characteristics of friction materials were determined using a pin-on-disc type apparatus against a cast iron counterface in the sliding

speeds of 3.2–12.8m/s, disc temperature of 100–350 1C and applied loads of 312.5–625N. The worn surfaces of the specimens were

examined by SEM. Experiments show that fiber content has a significant influence on the mechanical and tribological properties of the

composites. The friction coefficient of the hybrid friction materials was increased with increasing additional basalt fiber content. But the

specific wear rates of the composites decreased up to 30 vol% fiber content and then increased again above this value. The wear tests

showed that the coefficient of friction decreases with increasing load and speed but increases with increasing disc temperature up to

300 1C. The most important factor effecting wear rate was the disc temperature followed by sliding speed. The materials showing higher

specific wear rates gave relatively coarser wear particles. XRD studies showed that Fe and Fe2O3 were present in wear debris at severe

wear conditions which is indicating the disc wear.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Hybrid friction material; Basalt; Ceramic

1. Introduction

Automotive friction materials are traction devices usedin clutches and brake systems. Convention dictates that,although all of the various friction components containedwithin clutch and brake assemblies could be referred to asfriction materials, the label is traditionally reserved for theexpendable or consumable triboelement. The opposingtriboelement to the friction material, which is generally acast and turned metallic, i.e. drum or disc rotors in brakesor the pressure plate and flywheel in clutches, areparochially referred to as the countermember or counter-face [1].

The formulation and production of friction materials forbraking systems of automobiles, trucks, airplanes racing

ee front matter r 2006 Elsevier Ltd. All rights reserved.

iboint.2006.01.027

ing author. Fax: +90462 3255526.

ess: [email protected] (B. Ozturk).

cars, and other vehicles have undergone major changes inthe last two decades. Until recently, asbestos fibers wereused to friction materials due to its outstanding properties,such as well-suited with matrix, low cost and goodmechanical and heat resistance. Because asbestos can causehealth problems, brake lining designers have been scram-bling to find a replacement for it, using, ceramic fiber,mineral fiber, metallic fiber, glass fiber and others [2].Friction materials for an automotive brake and clutch

system are one of the most complicated compositematerials and usually formed by hot compaction of coarsepowders including many different ingredients. However,details about ingredients contained in the commercialfriction material are difficult to find in the literature due toproprietary reasons [3–5]. The combination of severalingredients and their synergism in a commercial frictionmaterial makes it rather difficult to analyze its frictionand wear characteristics completely. However, the fibrous

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Table 1

The composition of the friction materials studied in this work

Material and content (vol%)

Specimen

code

Fibers Binder Fillers

Ceramic Basalt Phen

form.

Barite Bronze, plaster,

graphite

FF10 10 0 30 40 20

FF10BA10 10 10 30 30 20

FF10BA20 10 20 30 20 20

FF10BA30 10 30 30 10 20

FF10BA40 10 40 30 0 20

Fig. 1. SEM micrographs of fibers: (a) basalt and (b) ceramic.

B. Ozturk et al. / Tribology International 40 (2007) 37–4838

reinforcement can have a prominent effect on thetribological properties and this may form the basis forformulating the material [6].

Hybrid composites are materials made by combiningtwo or more different types of fibers in a common matrix.They offer a range of properties that cannot be obtainedwith a single kind of reinforcement. These are related to(i) the cost savings which can be achieved by replacingexpensive one fibers by less expensive the other fibers(ii) the wider range of physical and mechanical propertieswhich can be obtained by optimizing the choice of thefibers used and their volume fractions, and (iii) thepossibility of obtaining unique properties, singly or incombination which are not readily obtained when usingone type of fiber alone. Therefore, brake pad manufac-turers are studying on the combining of different types offibers [7,8].

Ceramic fibers are a family of high temperature fibersdesigned to be used in a variety of industrial andcommercial applications. Although ceramic fibers are offersuperior properties (high temperature stability, low thermalconductivity, low heat storage etc.), these materials aremore expensive. Basalt fiber can have high strength,excellent fiber/resin adhesion and ability to be easilyprocessed using conventional process and equipment. Atthe same time the basalt will be selling at a price well belowthat of ceramics. But it has a melting point below that ofceramic fiber.

In this work, ceramic and basalt fiber containing hybridphenolic composites were manufactured and tested with apin-on-disc type friction tester. In hybrid composites,ceramic fiber content was kept constant at 10 vol% andbasalt fiber content was changed from 0 to 40 vol%. Theeffects of fiber content, the sliding speed, the applied loadand the disc temperature on the coefficient of friction andthe specific wear rates of the composites were investigated.The morphological features of worn surfaces and weardebris were analyzed in order to understand the frictionand wear mechanism of this tribosystem.

2. Experimental procedure

2.1. Fabrication and characterization of the specimen

Friction material specimens for this experiment weremanufactured based on a typical non-asbestos organic(NAO) type formulation containing a binder, reinforce-ments, filler, etc. The compositions of the manufacturedfriction materials (in vol%) were listed in Table 1. Thenumbers used in specimen codes represent fiber volumecontents. Phenolic resin was used as the matrix material.Curing of resin was done by adding 10% hexamethylene-tetramine to the base resin. Basalt (Deutsche Basaltstein-wolle GmbH with the trade name Basarits, average fiberdiameter of 13 mm and length of up to 3mm) and ceramic(Carborundum Resistant Materials GmbH with the tradename Fiberfraxs, average fiber diameter of 5 mm and

length of up to 100mm) were used as the inorganic fibers[9,10]. Bronze, plaster, graphite, and barite were added tothe friction materials as the filler. The SEM micrographs ofbasalt and ceramic fibers are shown in Fig. 1.The components of friction materials were weighed with

sensitivity of 1mg, and mixed for 2min in a finned typemixer. The mixture was loaded into a mold of size25mm� 25mm and hot pressed at about 150 1C and15MPa for 10min to cure. After the molding process, thespecimens were post-cured in an oven at 200 1C for about

ARTICLE IN PRESSB. Ozturk et al. / Tribology International 40 (2007) 37–48 39

2 h and were machined to a size of 25� 25� 10mm. Thefrictional surface of the specimen was abraded with 400grade abrasive paper prior to tests.

Table 2

Chemical compositions of the cast iron disc

Element C Si Mn P S

Wt% 2.54 2.31 0.65 0.015 0.08

2.2. Testing machine

Friction and wear tests were carried out using a purposebuilt pin-on-disc type apparatus under dry conditions.Schematic diagram of the apparatus used is shown inFig. 2. A commercial cast iron brake disc was used as thecounterface friction material. The disc material waslamellar graphite cast iron and the chemical compositionis given in Table 2. The disc had a size of 272mm indiameter, 13mm in thickness and the hardness of 210 HB.

The disc surface temperature was measured, via an iron-constantan thermocouple positioned in the back of the discat a depth of 2mm from the sliding interface and connectedto a slip ring arrangement. The temperature of the discwas controlled by using a heater and an air blower. Thetemperatures mentioned in this paper refer to the disctemperatures and not the sliding interface temperatures.The friction force, the normal load, the temperature andthe rotational speeds were transferred to a PC by a specialdata program and were displayed on the monitor real timebasis. The disc was connected to a 4 kW motor through avariable speed clutch capable of imparting speeds up to1400 rpm to the disc.

The frictional surface of the disc was abraded with a 320grade abrasive paper and was cleaned by rubbing with adry cloth before each test in order to start with samesurface roughness.

1

2 3 4

5

9

10 12

13

14

15 6

A

Fig. 2. Schematic diagram of the testing machine: 1. Motor, 2. Gear box, 3.

Normal load cell and specimen holder, 7. Specimen, 8. Friction disc, 9. Loading

counter, 14. Heater, 15. Horizontal load cell.

2.3. Testing procedure

Prior to friction and wear test, a run-in process wasapplied to all specimens to obtain complete mating withthe counterface. This was achieved using a normal loadof 437.5N (0.7MPa) and the sliding speed of 4.5m/s,while the limiting temperature was maintained constant at10075 1C.The frictional tests consisted of continuous application

of the specimen on the disc beginning at a temperature of10075 1C and ending at a temperature of 35075 1C.During this period, the speed was maintained at 6.7m/swhile the applied load was 625N (1MPa). The frictionforce was measured at a intervals of 2 s using a horizontalload cell. The continuous frictional tests were lasted about15min (sliding distanceE6030m). The specimen center was118.5mm off the disc center during the tests, sliding speedand distance was calculated at this point.Investigation of the effects of sliding speeds on the

friction coefficient and the specific wear rates, the load andthe temperature were maintained at about 625N and20075 1C, respectively, and the sliding speeds were selectedas 3.2, 6.7, 9.7, and 12.8m/s. The effects of applied loadson the friction and wear behavior were determined at thesliding speed of 6.7m/s and the temperature of 20075 1C.

11

7

8

Gear lever, 4. Thermocuple and slip-ring arrangements, 5. Air blower, 6.

weights, 10. Main body, 11. PC, 12. Electrical panel, 13. Magnetic rotation

ARTICLE IN PRESSB. Ozturk et al. / Tribology International 40 (2007) 37–4840

Here, the loads were taken as 312.5, 406.25, 500 and 625N.The effects of the temperature on the coefficient offriction and the specific wear rates were determined at thesliding speed of 6.7m/s and the load of 625N, while thetemperature was selected as 100, 200, 300, 35075 1C. Theweight loss was obtained by weighing the specimens withsensitivity of 1 milligramme before and after each test runlasted 10min. The specific wear rate was calculated usingthe following equation [11]:

W s ¼Dm

rFn V t,

where Ws is the specific wear rate (mm3/Nm), Dm the massloss, r the density of the specimen, Fn the normal load, V

the sliding speed, and t the time of operation.After each test, the friction material was dismantled

and the rubbing surface was cleaned using soft brushover a clean sheet of aluminum foil to collect the weardebris that was released from the friction material. Mostof the wear debris was collected from the rivet holeswhich was drilled on the specimen prior to test. The wornsurfaces of some specimens and the loose wear debris wereexamined by a scanning electron microscopy (JSM—6400)to provide information about the wear mechanisms. X-raydiffraction (XRD) measurements were conducted with aRigaku D/Max-3C diffractometer using monochromaticCuKa (l ¼ 1:5418 A) radiation. X-ray diffraction anddiffraction line fitting were applied to identify the differentphases.

3. Results and discussion

3.1. Physical and mechanical properties

The composites were characterized for their variousphysical and mechanical properties. The properties of themanufactured composites are given in Table 3. As isevident from Table 3, the incorporation of fibers hasalways contributed to the decrease in density but increasein hardness. Flexural strength and shear strength increasedup to 30 vol% total fiber content. Above this value, shearstrength was decreased while flexural strength was remain-ing almost constant. Compression strength, in general,increased with increasing fiber content.

Table 3

Physical and mechanical properties of the composites

Properties FF10 FF10B

Density (g/cm3) 2.70 2.58

Hardness (Rockwell-M) 9476 1027Flexural strength (MPa), ASTM-D790 49 59

Shear strength (MPa), ASTM-D732 21.1 24.6

Compression strength (MPa), ASTMD695 114 174

3.2. Effect of temperature and sliding time on friction

The effects of the temperature and the sliding time on thecoefficients of friction of the specimens were shown inFig. 3(a)–(e).The coefficient of friction increased with increasing

basalt fiber content in hybrid composites as can be seenin Fig. 3(a)–(e). The increase in the friction coefficient withthe addition of the basalt fiber can be understood if oneremembers that the friction coefficient in these systems ismostly governed by the ploughing component of friction,and the addition of this hard fiber enhances the ploughingaction of the specimen surface on the metallic disc surface[2]. Also, the strength of the manufactured specimensincreases with increasing basalt fiber content as can be seenin Table 3. The coefficient of friction of the FF10,FF10BA10, FF10BA20, FF10BA30 and FF10BA40 wasabout 0.36, 0.43, 0.45, 0.46 and 0.49 at the beginning of thetest and about 0.39, 0.45, 0.51, 0.52 and 0.57 at the end ofthe test, respectively.As in all sliding contact situations, the friction forces

are transferred by the area of real contact. Due to thetopography of all the specimens, the area of real contact isconfined within the contact plateaus (contact patches). Inany instant, the real contact area is however very smallcompared to the total area of the plateaus. The normalload may be carried by direct, spot-wise contact againstthe counterface, but also via very minute particles ormore continuous films, thin enough to pass between theplateau and the counterface. The size and composition ofthe plateaus obviously has a crucial influence on thefriction behavior of all the specimens. The friction in-crease with increasing the time is primarily correlatedto the formation of primary plateaus which are formed byfibers of the composites. When the rough surface of aspecimen is worn, the primary plateaus form, therebyincreasing the possible area of real contact between thespecimen and the disc. Further, as the possible area of realcontact increases and the surfaces are worn smooth, theelement of elastic contact increases, and accordingly, thearea of real contact. An increased area of real contact for agiven normal load is believed to result in an increasedcoefficient of friction [12].Due to the friction, the temperature increased with

increasing the sliding time in all experiments. Since noabrupt changes in friction coefficient of all specimens

A10 FF10BA20 FF10BA30 FF10BA40

2.43 2.23 2.02

10 10378 102710 10677

71 69 70

29.0 28.2 24.2

203 222 217

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(a) (b)

(c) (d)

0 100 200 300 400 500 600 700 800 9000.1

0.2

0.3

0.4

0.5

0.6

0.7

FF10

T

Coe

ffic

ient

of f

rict

ion

(µ)

Sliding time (s)

µ

0

100

200

300

400

500

600

700

Tem

pera

ture

(°C

)

0 1000 2000 3000 4000 5000 6000

Accumulated sliding distance (m)

0 100 200 300 400 500 600 700 800 9000.1

0.2

0.3

0.4

0.5

0.6

0.7

FF10BA10

T

Coe

ffic

ient

of

fric

tion

(µ)

Sliding time (s)

µ

0

100

200

300

400

500

600

700

Tem

pera

ture

(°C

)

0 1000 2000 3000 4000 5000 6000


0 100 200 300 400 500 600 700 800 9000.1

0.2

0.3

0.4

0.5

0.6

0.7

Coe

ffic

ient

of

fric

tion

(µ)

Sliding time (s)

FF10BA20

0

100

200

300

400

500

600

700

T

µ

Tem

pera

ture

(°C

)

Tem

pera

ture

(°C

)

0 1000 2000 3000 4000 5000 6000


0 100 200 300 400 500 600 700 800 9000.1

0.2

0.3

0.4

0.5

0.6

0.7

Coe

ffic

ient

of

fric

tion

(µ)

Sliding time (s)

FF10BA30

0

100

200

300

400

500

600

700

Tem

pera

ture

(°C

)

0 1000 2000 3000 4000 5000 6000


T

µ

(e) 0 100 200 300 400 500 600 700 800 900

0.1

0.2

0.3

0.4

0.5

0.6

0.7

T

µ

Coe

ffic

ient

of f

rict

ion

(µ)

Sliding time (s)

FF10BA40

0

100

200

300

400

500

600

700

0 1000 2000 3000 4000 5000 6000


Fig. 3. Variations in friction coefficients of the specimens with sliding time (or accumulated sliding distance) and temperature (P ¼ 625N, V ¼ 6:7m=s):(a) FF10; (b) FF10BA10; (c) FF10BA20; (d) FF10BA30; and (e) FF10BA40.

B. Ozturk et al. / Tribology International 40 (2007) 37–48 41

occurred up to 350 1C, it can be concluded that hightemperature fade resistances were good.

3.3. Effect of speed, load and temperature on friction and

wear properties

Friction and wear characteristics are primarily deter-mined by the nature of interactions between the two

friction surfaces in contact. The nature and degree ofinteractions between the two surfaces are dependent on theproperties of the surfaces as well as those of the bulkmaterials comprising the couple. The relevant surfaceproperties include surface geometry, surface energy,chemical reactivity, and physical and mechanical propertiesof the surface under given conditions of temperature,speed, load and environment [2].

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2 4 6 8 10 12 140.24

0.30

0.36

0.42

0.48

0.54

0.60

0.66

0.72

Coe

ffic

ient

of

fric

tion

(µ)

Sliding speed (m/s)

FF10 FF10BA10 FF10BA20 FF10BA30 FF10BA40

Fig. 4. Effect of sliding speed on the coefficient of friction (T ¼ 200 1C,

P ¼ 625N).

Fig. 5. SEM micrographs of the worn surfaces of FF10BA20 specimen at

different sliding speeds (T ¼ 200 1C, P ¼ 625N): (a) 3.2m/s; and (b)

9.7m/s (the arrow indicates disc sliding direction).


The variation in coefficient of friction with sliding speedfor all specimens is shown in Fig. 4. As can be seen inFig. 4, in general, the coefficient of friction decreased withincreasing sliding speed except for the relatively low fibercontaining FF10 and FF10BA10 specimens. The decreasein friction coefficient was attributed to the increase insliding interface temperature. Softening and charring ofmatrix resin with increasing interface temperature causefiber–matrix debonding, detachment of the polymer matrixand cracking of the deteriorated surface layers [13,14]. Lowfiber containing specimens such as FF10 and FF10BA10are more resistant to fiber–matrix debonding due tokeeping matrix integrity at high temperatures. Thecoefficient of friction of the FF10 and FF10BA10 speci-mens increased up to 6.7m/s then decreased. In theFF10BA20 specimen, the coefficient of friction is foundto be steadier in comparison to the others. The highestfluctuation of the coefficient of friction was seen inFF10BA40 specimen.

Scanning electron micrographs (SEM) of the wornsurfaces of FF10BA20 and FF10BA40 specimens tested atsliding speeds of 3.2 and 9.7m/s are shown in Figs. 5(a, b)and 6(a, b) respectively. At relatively low sliding speed of3.2m/s, the rubbed surfaces of the composites exhibitedlarger secondary plateaus in addition to the primaryplateaus as shown in Figs. 5(a) and 6(a). But with increasingsliding speed, fiber–matrix debonding and fiber cracking wasobserved due to softening and charring of matrix resin ascan be seen in Figs. 5(b) and 6(b). This resulted in destroyingof secondary plateaus. The abrasive effect of the brokenfiber particles further increases wear rate (Figs. 5(b), 6(b)and 9) [15].

Fig. 7(a, b) and 8(a, b) show the wear debris ofFF10BA20 and FF10BA40 specimens with sliding speedof 3.2 and 9.7m/s, respectively. The micrographs revealthat the higher the sliding speed and fiber content, thecoarser are the particles of the resulting wear debris

indicating severe wear (Figs. 7(b) and 8(b)). The natureof the typical wear debris consists of sheared-deformedpolymer matrix containing small broken fiber elements,wear powder of the metallic counterpart. The particles caneither be lost from the contact zone immediately after beingbroken from the composite surface, or remain there for awhile as transferred and back-transferred layers.Fig. 9 shows the variation in specific wear rates with

sliding speed for all specimens investigated. The figureindicates that the specific wear rate increased withincreasing sliding speed as expected.An increase in sliding speed increases the rate of impact-

type repeated loading caused by the hard asperities presenton the counterface. This increases the frictional thrust,which in turn causes localized vibration and chattering atthe sliding interface, thereby increasing the debonding andfracture of the reinforcing fibers [11,16]. These fibers cancause an increase in wear rate of the specimen by damagingthe counterface film. The highest specific wear rate isobserved in the FF10BA40 specimen, while the lowestspecific wear rate is observed FF10BA20 specimen. Thismay be attributed to increasing abrasive wear rate withincreasing fiber content (Fig. 6(b)).The variation in coefficient of friction with load for all

specimens is shown in Fig. 10. As can be seen in Fig. 10, the

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Fig. 6. SEM micrographs of the worn surfaces of FF10BA40 specimen at


9.7m/s.

Fig. 7. SEM micrographs of the wear debris of FF10BA20 specimen at


9.7m/s.


coefficient of friction decreased with increasing appliedload. This can be explained that the increased loadreducing the space between the specimen and disc. Whenthe height between specimen and disc is reduced, moredebris will become jammed. As a result, wear debris will bemore prone to sinter, forming agglomerates or evencontinuous films. A continuous film or agglomerates onthe specimen caused lubrication [12]. Therefore, thecoefficient of friction decreased with the increasing load.

SEM micrographs of the worn surfaces of FF10BA20and FF10BA40 specimen tested at applied load of 312.5and 625N are shown in Figs. 11(a, b) and 12(a, b)respectively. Because of the high friction coefficient of thecomposites at lower load of 312.5N, heat generation wasrelatively higher. It seems that patches of the softenedpolymer matrix was peeled of while sliding against themetal surface as shown in Figs. 11(a) and 12(a). Also, ascan be seen in Fig. 12(a), melt flow of the polymer tookplace and fiber-resin adhesion reduced and pulled out fromthe matrix. At higher loads, the surface damage mechan-isms, i.e. fiber and fiber-matrix interfacial fracture, areobserved but show protection offered by debris spreadingon the surfaces, Fig. 11(b). The formation of such tribofilmat high load is probably due to the enhancement of plasticdeformation process that favors debris agglomeration and

adhesion to the substrate. This coherent debris layer isparticularly effective in decreasing the specific wear rateand the coefficient of friction of friction materials at higherloads [17].Figs. 13(a, b) and 14(a, b) show the wear debris of

FF10BA20 and FF10BA40 specimens with appliedload of 312.5 and 625N, respectively. The figures indi-cate that the lower the applied load, the coarser are theparticles of the resulting wear debris indicating relativelysevere wear.The variation in specific wear rate with applied load for

all specimens investigated was shown in Fig. 15.The figure indicates that the specific wear rate, in

general, decreases with increase in applied load. This trendis in agreement with the previously published data ofGopal et al. [6]. As can be seen in Fig. 15, the highestspecific wear rate is observed in the high fiber containingFF10BA30 and FF10BA40 specimens, while the FF10,FF10BA10 and FF10BA20 specimens show nearly thesame wear rate.The variation in coefficient of friction with disc

temperature for all specimens is shown in Fig. 16. Thefigure indicates that the coefficient of friction increased upto 300 1C, then decreased up to 350 1C. This increase ismore pronounced in relatively high fiber containing

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300 400 500 6000.30

0.36

0.42

0.48

0.54

0.60

0.66

0.72

Coe

ffic

ient

of

fric

tion

(µ)

Applied load (N)


Fig. 10. Effect of load on the coefficient of friction (T ¼ 200 1C,

V ¼ 6:7m=s).



9.7m/s.

2 4 6 8 10 12 140

20

40

60

80

100

Spec

ific

wea

r ra

te (

10-6m

m3 /N

.m)

Sliding speed (m/s)


Fig. 9. Effect of sliding speed on the specific wear rate (T ¼ 200 1C,

P ¼ 625N).

Fig. 11. SEMmicrographs of the worn surfaces of FF10BA20 specimen at

different applied loads (T ¼ 200 1C, V ¼ 6:7m=s): (a) 312.5N; and (b)

625N.


composites. At higher temperatures the friction behavior isstrongly influenced by thermal degradation and softeningof the phenolic resin. Herring [18] proposed that at highdisc temperatures, the organic material releases gas as a

result of thermal degradation. Some of this gas maybecome trapped at the sliding interface. This gas may exertan opposing force to the applied load and reduce thefrictional force.

ARTICLE IN PRESS

Fig. 12. SEMmicrographs of the worn surfaces of FF10BA40 specimen at


625N.



625N.


SEM micrographs of the worn surfaces of FF10BA20and FF10BA40 composites tested at different temperaturesof 200 and 350 1C are shown in Figs. 11(b), 17(a) and 12(b),17(b), respectively. As can be seen in Fig. 11(b), theFF10BA20 specimen showed a smooth worn surface incomparison with Fig. 17(a). FF10BA20 specimen showedmatrix degradation, fiber–matrix debonding, fiber pull-outand fiber fracture. These effects more apparent in theFF10BA40 specimen which has relatively higher fibercontent (Fig. 17(b)).

Figs. 13(b), 18(a) and 14(b), 18(b) show the wear debrisof FF10BA20 and FF10BA40 composites with tempera-ture of 200 and 350 1C, respectively. The figures indicatethat the higher the temperature and fiber content, thecoarse and plate shape are the particles of the resultingworn debris which are the signs of increasing wear rate.

Fig. 19 shows the variation in specific wear rate with disctemperature for all composites investigated. The figureindicates that the specific wear rate increases with increas-ing disc temperature. The highest specific wear rate isobserved in the FF10BA40 composite, while theFF10BA10 and FF10BA20 composites show nearly thesame wear rate.

The increase in temperature causes thermal penetration,which results in softening of the matrix resin. This causes

weakening of the bonds at the fiber–matrix interface;the reinforcing fibers thus become loose in the matrixand break due to the frictional thrust. The frictionalheat, however, chars the matrix resin at the contact zone.The combined effect thermal softening and charringof the matrix resin result in debonding of reinforc-ing fibers, and sometimes in loss of structural inte-grity, thus enhancing fiber pull-out (Figs. 17(a) and (b)).Both the thermal degradation and the pull-out of fiberscontribute to higher specific wear rate at high tempe-ratures [16]. Also, thermal softening of polymers canlead to a drop in surface hardness, which can lead toincreases in the real contact areas. This can lead to rapidincreases both in the coefficient of friction and thewear rate.The XRD patterns of the FF10BA20 and FF10BA40

specimens after a wear test at 200 and 350 1C is shown inFigs. 20–23. As can be seen from Fig. 20, the wear debris ofthe FF10BA20 specimen consist of various complex oxidessuch as Al2SiO5 (silimanite), SiO2, BaSO4 (barite) andAl2O3 phases under 200 1C/625N/6.7m/s. The phases ofSiO2 and Al2O3 come from fibers of ceramic and basalt.But the wear debris of the same specimen compose of thesame phases as well as Fe and Fe2O3, which are theelements that result from disc wear under 350 1C/625N/

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625N.

300 400 500 60020

30

40

50

60

Spec

ific

wea

r ra

te (

10-6 m

m3 /N

.m)

Applied load (N)


Fig. 15. Effect of applied load on the specific wear rate (T ¼ 200 1C,

V ¼ 6:7m=s).

100 150 200 250 300 3500.30

0.36

0.42

0.48

0.54

0.60

0.66

Coe

ffic

ient

of

fric

tion

(µ)

Temperature (°C)


Fig. 16. Effect of temperature on the coefficient of friction (P ¼ 625N,

V ¼ 6:7m=s).

Fig. 17. SEM micrographs of the worn surfaces after testing at 350 1C

(V ¼ 6:7m=s, P ¼ 1MPa): (a) FF10BA20; and (b) FF10BA40.


6.7m/s. The wear debris of the FF10BA40 specimenconsist of Al2SiO5, SiO2, Al2O3, Fe and Al2SiO5, SiO2,Al2O3, Fe, Fe2O3 under 200 1C/625N/6.7m/s and 350 1C/625N/6.7m/s, respectively.

4. Conclusions

Fiber content, sliding speed, normal load and tempera-ture have a strong influence on the wear behavior of hybridphenol formaldehyde based composites reinforced with

ARTICLE IN PRESS

Fig. 18. SEM micrographs of the wear debris after testing at 350 1C

(V ¼ 6:7m=s, P ¼ 1MPa): (a) FF10BA20; and (b) FF10BA40.

100 150 200 250 300 35010

20

30

40

50

60

70

80

90

Spec

ific

wea

r ra

te (

10-6m

m3 /N

.m)

Temperature (°C)


Fig. 19. Effect of temperature on the specific wear rate (V ¼ 6:7m=s,P ¼ 625N).

10 20 30 40 50 60

1. Al2SiO5

2. SiO2

3. BaSO4

4. Al2O3

4 32

1

2

1(200 °C, 625 N, 6.7 m/s)

Inte

nsity

(A

rbitr

ary

unit)

2θ (degree)

FF10BA20

Fig. 20. XRD patterns of the wear debris of FF10BA20 specimen:

(200 1C/625N/6.7m/s).

10 20 30 40 50 60

6

1. Al2SiO5

2. Al2O3

3. Fe4. Fe2O3

5. BaSO4

6. SiO2

54 32

2

1

1(350 °C, 625 N, 6.7 m/s)

Inte

nsity

(A

rbitr

ary

unit)

2θ (degree)

FF10BA20


(350 1C/625N/6.7m/s).


ceramic and basalt fibers. From the friction and wear tests,following conclusions are obtained.

(1)
The coefficient of friction increased with increasingtotal fiber content.
(2)
The coefficient of friction was found to decrease withincreasing speed and load but increased with increasingdisc temperature up to 300 1C.
(3)
The specific wear rate was found to decrease withincreasing total fiber content up to 30 vol%. Butincreases again above this value due to abrasive natureof wear debris. Thus the best wear behavior wasobtained in FF10BA20 composite.
(4)
Increasing in sliding speed and disc temperatureresulted in increase of wear rate while increasing loadcaused to decrease in wear rate.
(5)
The size of wear debris was found to increase withincreasing wear rate. Thus the size of wear debris wasfound to increase with increasing speed and disctemperature but decrease with increasing load.

ARTICLE IN PRESS

10 20 30 40 50 60

1. Al2SiO5

2. Al2O

3

3. SiO2

4. Fe2O

3

5. Fe

(350 °C, 625 N, 6.7 m/s)

Inte

nsity

(A

rbitr

ary

unit)

2θ (degree)

FF10BA40

543 2

1

1


(350 1C/625N/6.7m/s).

10 20 30 40 50 60

1. Al2SiO5

2. SiO2

3. Al2O3

4. Fe

4

3

21

1(200 °C, 625 N, 6.7 m/s)

Inte

nsity

(A

rbitr

ary

unit)

2θ (degree)

FF10BA40


(200 1C/625N/6.7m/s).


(6)
The wear debris was flake shaped containing variouscomplex oxides such as Al2SiO5 (silimanite), SiO2,BaSO4 (barite) and Al2O3 phases under mild wearconditions. But Fe and Fe2O3 was found within thewear debris at severe wear conditions indicating thedisc wear.
Acknowledgments

The authors would like to thank Karadeniz TechnicalUniversity Research Fund for the financial support of thisresearch work.

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Documents

Hot wear properties of ceramic and basalt fiber reinforced hybrid friction materials