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Two-body and three-body abrasive wear behaviour ofpolyaryletherketone composites

A.P. Harsha, U.S. Tewari *Industrial Tribology, Machine Dynamics and Maintenance Engineering Centre, Indian Institute of Technology, Hauz-Khas, New

Delhi 110 016, India

Received 25 July 2002; accepted 5 September 2002

Abstract

Single and multipass two-body and three-body abrasive wear behaviour of various polyaryletherketone compositeshave been carried out by using a pin-on-disc machine and a rubber wheel abrasion test (RWAT) rig. Abrasive wearstudies were conducted under different loads and sliding distances. Comparative wear performance of all the compositesat different loads shows that specific wear rate (Ko) of two-body abrasive wear (single pass condition) is 30-50 timesgreater than three-body abrasive wear. Also, specific wear rate (Ko) of two-body abrasive wear (multipass condition)is 5-12 times greater than three-body abrasive wear. Efforts were made to correlate the abrasive wear performancewith the appropriate mechanical properties. The operating wear mechanisms under different experimental conditionhave also been studied by using scanning electron microscopy.

Keywords: Two and three-body abrasive wear; Polyaryletherketone; Composites; Wear mechanism; Scanning electron microscopy

1. Introduction

Wear is defined [1] as damage to a solid surface, gen-erally involving progressive loss of material, due to rela-tive motion between that surface and contacting subst-ance or substances. The five main types of wear areabrasive, adhesive, fretting, erosion and fatigue wear,which are commonly observed in practical situations.Abrasive wear is the most important among all the formsof wear because it contributes almost 63% of the totalcost of wear [2]. Abrasive wear is caused due to hardparticles or hard protuberances that are forced againstand move along a solid surface [3]. The abrasive wearprocess is traditionally divided into two groups: two-body and three-body abrasive wear. A distinction ismade between two-body abrasion, in which wear is

caused by hard protuberances on one surface which canonly slide over the other and three-body abrasion, inwhich particles are trapped between two solid surfacesbut are free to roll as well as slide. The distinctionbetween these and also typical practical examples aregiven in Table 1. The rate of material removal in three-body abrasion is one order of magnitude lower than thatfor two-body abrasion [4], because the loose abrasiveparticles abrade the solid surfaces between which theyare situated only about 10% of the time, while they spendabout 90% of time in rolling. Most of the abrasive wearproblems, which arise in agricultural and industrialequipment, are three-body, while two-body abrasion isencountered primarily in material removal operations.Despite the importance of three-body abrasion the vastmajority of abrasive wear studies have dealt with two-body abrasion. Also in the literature, two-body and three-body abrasion is usually discussed separately.

Engineering polymers and polymer based compositesare widely used in design. Polymers and their compositesare often required to move in contact with hard abrasive

404 A.P. Harsha, U.S. Tewari/Polymer Testing 22 (2003) 403-418

Table 1Classifications of abrasive wear, and practical examples and operating conditions

Wear mode Wear mechanism Typical practical examples Operating conditions [2]

Two-bodyabrasion

Three-bodyabrasion

Pipe and chute liners used to transport Abrasive grit size 50—500 |im andabrasive materials, rotors of powder mixers, sliding speeds up to 3 m/s; moderatepivot pins in construction machinery, blades load conditions.and components in agricultural and earthmoving machinery, impellers in pumpshandling sewage and abrasive fluid wastes,wear strips and guides.Sleeve bearing and bushes operating inabrasive environment, lower sleeve bearingin vertical sewage pumps, vehicle springbushes, marine stern tube bearings, chainwear strips, rope sleeve bearings etc.

Abrasive grit size 100-300 |im,sliding speeds up to 2.4 m/s;Moderate load conditions.

particles as counterfaces. Some practical situations arelisted in Table 1. The increasing use of polymers andtheir composites in such practical situations, makesabrasive wear studies imperative from a technologicaland commercial standpoint. In general, it is the mechan-ical load carrying capacity and specific wear rate of thematerials that determine their acceptability in practicalapplications. Different types and amounts of fibrereinforcement and/or solid lubricants are known toimprove significantly many physical and/or mechanicalproperties of polymers, and their potential in improvingthe abrasive wear resistance is worth exploring. Wear isnot a simple material property, but depends strongly onthe system in which two surfaces function (Fig. 1).

In recent years research has been devoted to exploringthe potential advantage of a thermoplastic matrix forcomposite materials. One such matrix is polyaryletherke-tone (PAEK), which shows exceptional properties due toits semicrystalline character and molecular rigidity of itsrepeating units. Its high strength and modulus, bettertoughness, thermal stability, easy processing, chemicalinertness, wear and radiation resistance, make it an idealmaterial for advanced composites. PAEKs such as polye-theretherketone (PEEK), polyetherketone (PEK), polye-therketoneketone (PEKK) have similar crystal structuresbut differ in ketone content (Table 2). A large amountof data on abrasive wear behaviour of PEEK and theircomposites reinforced with fibres and fillers have beenreported [6-12]. Cirino et al. [6,7] reported the sliding aswell as the abrasive wear behaviour of continuous glass,carbon and aramid fibre reinforced PEEK. Lhymn et al.[8] investigated the abrasive wear of short carbon fibre(CF) reinforced PEEK. Voss and Friedrich [9] studiedthe sliding and abrasive wear behaviour of short glassand CF reinforced PEEK composites at room tempera-ture. Briscoe et al. [10] reported abrasive wear behaviour

of PEEK filled PTFE and PTFE filled PEEK. Incorpor-ation of PEEK in PTFE reduced wear rate of PTFE whilewear rate increased in the latter case. Harsha and Tewari[11,12] investigated two-body abrasive wear behaviourof various short fibres reinforced PAEK composites. Theabove investigations were concentrated on two-bodyabrasion studies of PAEKs and their composites againstabrasive papers. According to the author's knowledge,no study is available in the literature on three-bodyabrasive wear behaviour of PAEK composites. Also,comparison between two-body and three-body abrasivewear of polymer composites is not reported in the litera-ture. In view of the above, the objective of the presentinvestigation is to study the two-body and three-bodyabrasive wear performance of PAEKs and their com-posites under different experimental conditions and tostudy the comparative wear performance. Another aimof the present study is to study the effect of chemicalstructure and ketone/ether linkage ratios, the amount andtypes of short fibres and filler reinforcement on abrasivewear resistance of PAEKs having different mechanicalproperties.

2. Experimental details

2.1. Materials

Three types of neat resins of PAEKs, having differentchemical structure and ketone/ether linkage ratios, inves-tigated in the present study are given in Table 2. Detailsof mechanical properties of various PAEKs and theircomposites are given in Table 3. Initial surface rough-ness (arithmetic mean average, Ra) of all the specimenswere also measured and given in Table 3. Test specimens

A.P. Harsha, U.S. Tewari/Polymer Testing 22 (2003) 403-418 405

Composite materials

Polymer matrixType and structure ofreinforcementFiller/Fibre/ Matrix interfaceLubricantsCompositionFibre orientation/ Length

Relative movements

Single pass conditionMultipass conditionsSliding

Wear mechanism

Micro ploughingMicro cuttingMicro crackingFractureFatigue

Counter body asperties/abradingsurface

ShapeSizeSharpnessToughnessHardnessParticle density per unit

Test parameters

LoadVelocityContact areaContact pressureRelative humidityTemperature

Fig. 1. Influence of various parameters on abrasive wear performance of composite materials [5].

Table 2Chemical structures and ketone/ether linkage ratios of PAEKs investigated in the present study

Structure Name

PEEK

PEK

PEKK

Ketone (%)

33

50

67

Table 3Mechanical properties of PEEK, PEK and PEKK composites

Material Designation F i l l e r a n d i t s D e n s i t y T e n spercentage(W/W)

DensityASTMD792(gem"3)

Tensilestrength

[SIASTMD638(MPa)

TensilemodulusASTMD 638(MPa)

Tensileelongation atbreak [e]ASTMD 638 (%)

FlexuralstrengthASTMD 790(MPa)

FlexuralmodulusASTMD 790(GPa)

IzodimpactstrengthASTMD256(KJm-2)

HardnessH] ASTMD785

SurfaceRoughness

M

PEEKa

PEKa

PEKKb

ABCDE

FGHIJKL

—GF (20%)GF (30%)CF (30%)CF (10%)PTFE (10%)Graphite (10%)-GF (10%)GF (20%)GF (30%)-

CF (30%)CF (10%)PTFE (10%)Graphite (10%)

1.321.421.491.441.48

1.301.381.441.531.311.361.45

101120144224141

1120140155165111249159

36006800970013000-

40007000105001250044162760012420

>405.814.242.02.5

206.034.143.88121.22

170199220350210

183265290350194387249

4.06.78.2208.1

5.127.99.913.54.5524.1512.42

6.47.5109.06.3

7.07.07.07.0-—

122125125125127

124126126126126128127

979710510495

105107109110110115100

115115115115113

112111115115115117114

0.1700.3040.3041.240.450

0.2400.1980.2610.2680.3423.831.74

1 Victrex Plc., USA, supplied PEEK, PEK and their composites.' Infinite Polymer System II, USA, supplied PEKK and its composite.

A.P. Harsha, U.S. Tewari /Polymer Testing 22 (2003) 403-418 407

were cut from the same injection moulded plaque inorder to avoid variation of surface roughness.

2.2. Abrasive wear studies

Two types of abrasive wear tests were carried out.Two-body abrasive wear studies under single and multi-pass conditions were carried out by using a pin-on-discmachine and three-body abrasive wear studies were car-ried out by using a dry sand/RWAT rig.

2.2.1. Two-body abrasive wear studiesTwo-body abrasive wear studies were carried out on

the pin-on-disc machine described in detail elsewhere[11]. Abrasive paper was fixed on a rotating disc and thepolymer pin was fixed in a holder. The polymer pin 10x 10 x 6 mm3 was initially abraded against waterproof1200 grade (grit size~5 Mm) silicon carbide (SiC) abras-ive paper for uniform contact. During single pass abras-ive wear studies, after each traversal of the polymer pinnew abrasive paper was fixed until the total abrading dis-tance was reached. However, under multipass conditionsthe polymer pin traversed the same wear track until thetotal abrading distance was reached. A scanning electronmicrograph (SEM) of SiC abrasive paper of 120 (gritsize—175 Mm) grade before the wear test is shown inFig. 2.

2.2.2. Three-body abrasive wear studiesThree-body abrasive wear studies were carried out on

a dry sand/ RWAT rig as shown in Fig. 3. The abrasivesare introduced between the test specimen and the rotatingwheel with a chlorobutyl rubber tire. The test specimenis pressed against the rotating wheel at a specified forceby means of lever arm while a controlled flow of gritabrades the test surface. The rotation of wheel is suchthat its contact face moves in the direction of grit flow.

— HOPPER

SILICA SAND

WEIGHTS

SPECIMEN

RUBBER LINED WHEEL

Fig. 3. Schematic diagram of dry sand/rubber wheel abrasivewear test rig.

The pivot axis of the lever arm lies within a plane, whichis approximately tangential to the rubber wheel surfaceand normal to the horizontal diameter along which theload is applied. The tests were carried out for differentloads and sliding distances. The rubber wheel was rotat-ing at a speed of 2.4 m/s. Abrasive particles used wassilica sand (r = 2600 kg/m3; Knoop hardness 880 [13])of size 150-300 Mm. A SEM of the silica sand is shownin Fig. 4. Sand flow rate between rubber wheel andspecimen was 250 ± 5 g/min The size of the specimenwas 76 x 25 x 5-12 mm3 Weight loss measurementswere made at regular test intervals of 60 s using an ana-lytical balance reading to 1 x 10~5g. The specimenholder was designed to ensure that samples are removedand replaced during each test such that the wear scar wasalways at the same location. In all the above tests, wear

Fig. 2. SEM of SiC abrasive paper of 120 grade (gritsize~175 |im) before wear test. Fig. 4. SEM of silica sand of size 150-300 |im.


was measured by loss in weight, which was then con-verted into wear volume using density data. The specificwear rate was calculated for two-body and three-bodyabrasive wear from the following equation:

Ld[Nm (1)

where, AV is volume loss in cubic millimetres or cubicmeters; L, the load in Newton's; and d, the sliding dis-tance in meters.

2.3. Scanning electron micrograph studies

Worn polymer pins and abrasive paper surfaces wereexamined by using a SEM (S 360 Cambridge and Phil-ip's 515). Before taking micrographs, the samples werecoated with a thin layer of gold by sputtering.

3. Results and discussion

Wear volume as a function of sliding distance and loadis shown in Figs. 5(a-d) and 6(a-c). Specific wear rate

« A A B * C » D « E o F + G - H - l n J i K i L

2 4

Sliding distance [m]

450

0 50 100 150 200 250 300 350 400 450


140

120 •

-z 100 •

= 805*

> 60 -i

40

20

0 •

- A D B I C

•

•

, , , 1 L.

• D «E

•

•

4

• it aI *T

—'—'—h

O F

•

t

a8T

+ G AH

•

•

•« A

• ^

9. $

—•—•—h

A l

•

4

A

•A

O?

L

oJ |

M

A

•AO

t

- S N

- A nB . C «D . E oF +G AH i l »J

400 800 1200


400 800 1200


1600

Fig. 5. Wear volume as a function of sliding distance. (a) Two-body abrasive wear in single pass condition (n = 5 cm/s, L = 10N, SiC abrasive paper of 120 grade (grit size~175 |im); (b) Two-body abrasive wear in multipass condition (n = 0.36 m/s, L = 6N, SiC abrasive paper of 120 grade (grit size~175 |im) (c); and (d) Three-body abrasive wear (n = 2.4 m/s, L = 5, 12 N, abrasiveparticle size 150-300 |im).


• A A B O C X D X E » F + G - H - I O J D K A L -A-H

25-

20 -

i s -

le-

s '

0-

A

-

s\ •

—'—I—•-

A

8

A

i•

A

sS

}»•

— i — • -

A

X

D

X

o™

•

—I—'

[a]

A

X

D

X

o

%

i

.6 8 10

Load [N]

12 14

-A-I

-B-J

-C-E

1000 T

: 100

1000

12 15

Load [N]

Fig. 6. Wear volume as a function of load. (a) Two-body abrasive wear in ingle pass condition (n = 5 cm/s, d = 4 m, SiC abrasivepaper of 120 grade (grit size~175 |im); (b) Two-body abrasive wear in multipass condition (n = 0.36 m/s, d= 250 m, SiC abrasivepaper of 120 grade (grit size=175 \im) (c) Three-body abrasive wear (n = 2.4 m/s, d= 1433 m, abrasive particle size 150-300 \im).

as a function of load is shown in Fig. 7. Wear volumeas a function of various mechanical properties is shownin Fig. 8. Comparative abrasive wear performance ofPAEKs and their composites under two-body and three-body abrasive wear conditions is shown in Fig. 9. Salientfeatures of the present study are as follows:

• With increase in sliding distance and load, wear volumeincreases linearly for two-body (single pass condition)and three-body abrasive wear. The case of two-bodyabrasive wears under multipass conditions showed an

initial rapid increase for all the PAEKs and their com-posites. Subsequently, wear volume stabilised to analmost constant value for some materials although others(D, E, K and L) continued to show a linear increase withabrading distance and load.The specific wear rate (Ko) decreases with increase inload for two-body abrasive wear under single passconditions, whereas in cases of two-body abrasivewear under multipass condition and three-body abras-ive wear specific wear rate increases with increasein load.


Fig. 7. Specific wear rate (Ko) as a function of load. (a) Two-body abrasive wear in single pass condition (n = 5 cm/s, d = 4 m,SiC abrasive paper of 120 grade (grit size~175 |im); (b) Two-body abrasive wear in multipass condition (n = 0.36 m/s, d= 250 m,SiC abrasive paper of 120 grade (grit size=175 |im) (c) Three-body abrasive wear (n = 2.4 m/s, d= 1433 m, abrasive particle size150-300 nm).

The specific wear rate (Ko) of the materials for two-body abrasive wear under single pass condition wereobserved to be of the order of 1CT10 m3/Nm, whereasin cases of two-body abrasive wear under multipassconditions and three-body abrasive wear it was in theranges of 10"11 to 1(T10 and 1(T12 to 10"11

m3/Nm, respectively.The fillers such as PTFE and graphite were observedto be detrimental to abrasive wear performance (Eand L).The CF reinforcement to PAEK matrix (composite Dand K) had worse abrasion resistance as compared toglass fibre reinforced PAEK composites.Correlations between wear volume and selectedmechanical properties for two-body (single passcondition) and three-body abrasive wear wereobtained. However, in cases of two-body abrasivewear under multipass condition no linear trend wasobserved.

• The comparative wear performance of all the com-posites abraded under two-body and three-bodyabrasive wear situation at different loads can be seenfrom the histogram in Fig. 9. It was observed thattwo-body abrasion (single pass condition) is moreeffective in removing material as compared to theother two abrasive wear conditions.

Archard's equation [14] is generally used to describethe sliding wear of metal caused by adhesion, but it hasproven very useful in abrasive wear as well. The equ-ation states that

(2)

where V is the wear volume; L, the applied load; d, thesliding distance; H, the hardness of material; and K, isa constant referred to as wear coefficient. Eq. (2) clearlyindicates wear volume is directly proportional to both


• 2-bodySPC A2-bodyMPC

X 3-body (5 N) o 3-body (12 N)S 2-body SPC B 2-body MPC B 3-body]

1000

0.001 0.002 0.003 0.004

[Se]"1

o 2-body SPC o 2-body MPC

A 3-body (5 N) x 3-body (12 N)

1000

0.00E+00 1.00E-05 2.00E-05 3.00E-05 4.00E-05

[HSe]"1

Fig. 8. Wear volume as a function of: (a) [Se]"1; (b) \HSe\~1.Two-body abrasive wear in single pass condition (n = 5 cm/s,L = 10 N, SiC abrasive paper of 120 grade (grit size~175 |im);two-body abrasive wear in multipass condition (n = 0.36 m/s,I = 6N, SiC abrasive paper of 120 grade (grit size=175 |im);Three-body abrasive wear (n = 2.4 m/s, L = 5, 12 N, abrasiveparticle size 150-300 |im).

the load and sliding distance and it is also inversely pro-portional to hardness. However, the above equation doesnot hold good for polymers and their composites, thougha linear trend has been reported [15] for somepolymers/composites. This is because a combination ofseveral properties and experimental parameters influ-

1.00E-08

1.00E-12

A B C D E F G H I J K L

Materials

E 2-body SPC B 2-body MPC m 3-body]

1.00E-08

s2=• 1.00E-09-:

1.00E-10

u£a 1.00E-11Q.

1.00E-12

A B C D E F G H I

Materials

J K L

Fig. 9. Comparative wear performance of PAEKs and theircomposites for different load at two-body and three-body wearconditions. Two-body abrasive wear in single pass condition:(a) L = 6 N; (b) 12 N, n = 5 cm/s, d = 4 m, SiC abrasive paperof 120 grade (grit size~175 |im); two-body abrasive wear inmultipass condition: (a) L = 6 N; (b) 12 N, n = 0.36 m/s, d =250 m, SiC abrasive paper of 120 grade (grit size=175 |im);three-body abrasive wear: (a)L = 5 N ; (b) L = 12 N, n = 2.4m/s, d = 1433m, abrasive particle size 150—300 |im.

ences the abrasive wear performance ofpolymers/composites. In the present study, wear volumeincreases linearly with increase in sliding distance andload in two-body abrasive wear under single pass con-dition and three-body abrasive wear. However, in thecase of two-body abrasive wear under multipass con-ditions non-linear increase in wear volume has beenobserved (Figs. 5 and 6). This is because of the effectsof transfer of a polymer film to the abrading surface andtrapped debris in multipass conditions. Such transferfilms and wear debris collect in the crevices ordepressions on the abrasive paper leading to a clogging


effect. Thus, after a certain number of traverses, abrasionof the materials eventually stops and the system reachesan equilibrium condition. In the case of singlepass andthree-body abrasive wear, the polymer specimen alwaysfaces fresh abrasive particles, hence wear volumeincreases linearly with sliding distance and load.

Fig. 7 shows specific wear rate as a function of normalload for PAEKs and their composites. In two-body abras-ive wear under single pass conditions, specific wear ratefor all the materials decreased with increase in load.Lhymn [16] developed a mathematical model based onthe concepts of crack propagation and classical mech-anics. According to that mathematical model, the follow-ing equation is applicable when thermal activation isinsignificant, as in the case of low abrading speed andloads.

Ws =K1

EHEJI

where, Ws is the specific wear rate, Vs, the sliding speed;E, is the elastic modulus; e, the critical strain; m, thefriction coefficient; Vc, the crack growth velocity; andFN, the normal load. If FN or Vs is large, thermal effectsbecome significant and this relationship is not applicable.In the present study both FN and Vs are small in the caseof single pass conditions, and Eq. (3) is applicable and,hence, wear rate decreases linearly with increase in load.Lhymn [16] also confirmed the above relationship in thecase of glass fibre (GF) reinforced PA 66, polyoxyme-thylene (POM), and polybutyleneterephthalate (PBT). Intwo-body abrasive wear under multipass conditions andthree-body abrasive wear, specific wear rate increases forall the materials with increase in load. This is becausethe sliding speed was high, the material softens some-what and causes crack propagation. This results inincrease of specific wear rate. The equation (3) does nothold well because of high sliding speed.

The primary reasons for adding fillers or reinforcingfibres to polymers is to improve their mechanical proper-ties, but the effects on wear rate are not invariably ben-

eficial. According to Lancaster [17] the product of S ande (where S is the ultimate tensile strength and e is theultimate elongation) is very important factor which con-trols the abrasive wear behaviour of composites.Reinforcement with fibres/fillers increases tensilestrength (S) of neat polymer, they usually greatlydecrease ultimate elongation to break (e) and hence theproduct Se may become smaller than that of neat poly-mer. Hence, reinforcement usually leads to deteriorationin abrasive wear resistance. Various theories have beenproposed to interpret the abrasive wear performance ofcomposites. Various researchers have also found a var-iety of factors such as S, e, H (hardness), cohesiveenergy, ploughing component of friction, fracture energyof composites and probability factor for microcrackingas abrasive wear controlling factors. Table 4 providessome of the models proposed by various researchers [18—23] to correlate abrasive wear performance ofpolymers/composites. One model that seemed to be quitereasonable was that proposed by Ratner et al. [18], wherethe rate of material removal was said to be inversely pro-portional to the product of stress and strain at rupture.In the present studies, Fig. 8 indicates the relationshipbetween wear volume and [Se]^1 and [HSe]'1. Reason-ably good linearity was observed between wear volumeand relevant mechanical properties for two-body abras-ive wear (single pass condition) and three-body abrasivewear. However, in the case of two-body abrasive wearunder multipass conditions the data points were scatteredand no linear trend were observed. This is because theextent of counterface modification or damage plays anequally important role. According to Lancaster [24], dur-ing abrasion the condition of the abrasive paper is modi-fied to a different extent by different types of compositesdue to the different nature of the fillers or fibres. Someof the fillers (PTFE or graphite) transfer to the abrasivepaper clogging up the depressions whereas other fillerssuch as fibres (CF and GF) might cause fracture or pullout of abrasive grains. Eventually the condition of thepaper will reach a steady state after a large number of

Table 4Some of the models for abrasive wear of polymer/composites

1 W~/iL/H<7£ , friction coefficient; L, load; H, hardness; ere, stress and strainrupture

2 W~f5NPd b, abrasive wear factor; N, scratching efficiency factor; P, normalload; d, sliding distance

3 wV = {FNS(1 + 4f2)0.5/sy} FN, normal load; S, sliding distance;f, frictioncoefficient; sy, rupture stress of polymer; Wv, wear volume

4 W—Clff5/ GIC fi, probability factor for microcracking; H, hardness ofcomposites; GIC, fracture energy

5 W^lIC0-5 C, cohesive energy6 W~K\ivIHea mp, is the ploughing contribution to the coefficient of friction; H, Vaziri et al. [23]

hardness; e, percentage elongation to failure; s, tensile strength and K is theconstant

Ratner et al. [18]

Yamaguchi [19]

Czichos [20]

Friedrich and Cyffka [21]

Giltrow [22]

Table 5Wear volumes, the relative ranking order of the material under different experimental condition

Material Two-body abrasive wear, single passconditionL = 10N, v = 5cm/s, d= 6.6m

Two-body abrasive wear, multipassconditionL = 6N, v = 0.36m/s, d = 400m

Three-body abrasive wearL = 5N, v = 2.4m/s, d = 430m

Three-body abrasive wear ^L = 12N, v = 2.4m/s, d = 430m ^

AV (mm3) AV/AVA Rankingorder

AV (mm3) AV/AVA Rankingorder

AF(mm 3 ) AV/AVO Rankingorder

AV (mm3) AF/AF Rankingorder

ABCDEFGHIJKL

11.69217.87325.10027.62536.14814.83315.52518.26319.12816.72329.62340.246

1.01.275622.142.363.091.261.321.561.631.432.533.44

158911236741012

45.57558.443161.879184.121365.81053.84661.24474.263149.54056.671210.640403.190

1.01.283.554.058.021.181.3418.23.281.244.628.84

148911256731012

15.46217.92924.28129.97050.14816.04614.070680.23025.39017.022

1.091.271.722.133.561.141.01.291.801.20

1/257910316

42.72762.86699.476117.52234.2565.75069.97080.23082.2 9474.816

1.01.472.322.755.481.531.631.871.921.75

12891034675


passes. Thus, correlation between wear and mechanicalproperties is not possible under multipass conditionsbecause of the combination of material properties, whichinfluence the counterface. In two-body abrasive wearunder single pass conditions and three-body abrasivewear, the wear volume Vs \Se\~l, [HSe]^1 showed bettercorrelation indicating all the three factors i.e. S, e, Hplayed a role in controlling wear behaviour of selectedcomposites rather than just one. In the present study, theproduct of Se was a maximum for neat PEEK, whichshowed the highest wear resistance under differentexperimental conditions. This product decreased with theinclusion of fibre and/or fillers which results in deterio-ration of abrasive wear performance.

3.1. Comparisons of wear test

It is well known that abrasion resistance of polymerswill deteriorate with short fibre reinforcement. However,in many practical applications, e.g. gears, chute liners,bearings, and pipe, fibre reinforced plastic will be used.Therefore, it is necessary to examine the performance ofthe material with different types and amount of fibreunder different experimental condition before it is usedin a particular application. Fig. 9 shows comparativeabrasive wear performance of PAEKs and their com-posites at different loads under different experimentalconditions. It was observed that specific wear rate oftwo-body abrasive wear (single pass condition) is 30-50times greater than three body abrasive wear. Also, spe-cific wear rate of two-body abrasive wear under multi-pass condition is 5-12 times greater than three-bodyabrasive wear. In the two-body abrasion process particlesor asperities are rigidly attached to the second body,whereas in three-body abrasion the abrasive particles areloose and free to roll. Hence, two-body abrasion con-ditions are expected to produce higher wear rates thanthree-body conditions because the contact between theabrasive particle and the wearing surfaces is slidingrather than rolling. It was observed that CF reinforce-ment to PAEK matrix (composite D and K) had worseabrasion resistance as compared to glass fibre reinforcedPAEK composites. Also, fillers such as PTFE and graph-ite reinforcement to PAEK matrix showed higher spe-cific wear rate. If fibre reinforced plastic has to be usedin any abrasive wear situation, it is always preferable tohave a low amount of glass fibre reinforcement ratherthan a combination of CF and filler reinforcement.

The relative performance of the different PAEKs andtheir composites under different experimental conditionsis given in Table 5. The ranking order of the materialsunder different experimental conditions is slightlyvaried. This type of ranking is useful for the selectionof material for particular situation i.e. non-film transfer(single pass condition), film transfer (multipasscondition) and three-body wear. From Table 5, two-body

Fig. 10. Surface topography of SiC abrasive paper of 120 (gritsize~175 |im) grade after wear against: (a) neat PEK (singlepass condition, n = 5 cm/s, L = 4 N, d = 4 m); (b) CF reinforcedPEEK (composite D) (multipass condition, n = 0.36 m/s, L =6 N , d = 250 m).

abrasive wear (multipass condition) can be comparedwith three-body abrasive wear, which has almost similarexperimental conditions except sliding velocity. It isquite clear that two-body abrasion is more effective inremoving material compared to three-body abrasive

3.2. SEM observation of worn surfaces

SEM micrographs of SiC abrasive paper and pin sur-faces are shown in Figs. 10,11 and 13. Several mech-anisms have been proposed to explain how material isremoved from the surface during abrasion. These mech-anisms include fracture, fatigue and melting. Because of


(f)

Fig. 11. SEM of surfaces of neat PEEK: (a) L = 4 N; (c) L = 12 N (two-body abrasive wear (single pass condition), n = 5 cm/s,d = 4 m, counterface SiC abrasive paper of 120 grade (grit size~175 |im)); (b) L = 4 N ; (d) L = 10 N (two-body abrasive wear(multipass condition) ν = 0.36 m/s, d = 250 m, counterface SiC abrasive paper of grade 120 (grit size~175 |im) (e); and (f) L = 12N (three-body abrasive wear), n = 2.4 m/s, d = 1433 m.

Direction of particle motion

Entrance zone

central zone exit zone

Fig. 12. Schematic representation of different zone on thewear scar in three-body wear situation.

the complexity of abrasion, no one mechanism com-pletely accounts for all the loss. In general, the abrasivewear process involves four different mechanismsmicroploughing, microcutting, microfatigue andmicrocracking [4]. Using SEM images it is possible toidentify qualitatively the dominant wear mechanisms thatoperate under two-body and three-body abrasive pro-cesses. In Fig. 10a and b micrographs of abrasive papers(120 grade, grit size—175 um) are shown after wearagainst neat PEK and CF reinforced PEEK at differentload conditions (4 and 6 N) under single pass conditionsand multipass conditions. The noticeable differencebetween the two types of abrasion condition is that lesswear debris transferred to the abrasive paper in singlepass conditions (Fig. 10a). SEM studies seem to supportthe difference in wear behaviour observed under differ-ent experimental conditions.


Fig. 13. SEM of surfaces of neat PEK: (a) L = 4 N; (d) L = 12 N, two-body abrasive wear (single pass condition), n = 5 cm/s, d= 4 m, counterface SiC abrasive paper of grade 120 (grit size~175 |im); (b)L = 4 N; (e) L = 10 N two-body abrasive wear (multipasscondition), n = 0.36 m/s d = 250 m, counterface SiC abrasive paper of grade 120 (grit size~175 |im); (c) L = 5 N and (f) L = 12N, three-body abrasive wear, n = 2.4 m/s, d= 1433 m, abrasive particle size 150-300 |im.

Fig. 11a and c shows SEMs of neat PEEK surfacesabraded at different load under single pass conditions.The wear debris appears as fine fibrils cut from the poly-mer surface and the nature of the surface damage con-firms the cutting type of wear mechanism. It is alsoobserved that at low load the abrasive tips have cut thematerial much like a machine tool does. The curled chipsare reflected on the surface of polymer (Fig. 11a). Thisresults in removal of material, but very little displacedmaterial relative to the size of the groove. At higher loadthe surface of neat PEEK is highly plastically deformedwith much fine wear debris, which is adhered to the sur-face (Fig. 11c). Fig. 11b and d shows SEMs of neatPEEK surface abraded at different load under multipassconditions. The surface damage to the neat PEEK aftersliding under multipass conditions appears to have beencaused by a severe ploughing type of wear mechanism.

At low load (Fig. 1 1b), a number of parallel ploughingmark appeared on the surface and also the sharp ridgeformation is reflected in the micrograph. At higher load(Fig. 11d), neat surface is plastically deformed and sev-ere plough marks are also reflected in the micrograph.Fig. 11e and f shows damage morphologies of neatPEEK matrix at 12 N under three-body abrasive wearconditions. Fig. 12 shows schematic representation ofdifferent zones on the wear scar under three-body wearconditions. At the entrance and exit zone, where thepressure applied to the abrasive is lowest, the damagemorphologies were consistent with particle rolling. Fig.11e shows a micrograph of the entrance zone, smallergrooves were formed on the surface because most of thetime the particles were rolled between the surfaces,rather than sliding. In the centre of the wear scar (Fig.1 1f), parallel grooves were formed, typical of particle

A.P. Harsha, U.S. Tewari/Polymer Testing 22 (2003) 403-418 All

sliding, a result of the higher pressure forcing abrasiveinto the rubber wheel. Displacement of material hadoccurred within the grooves, forming shallow ridgessimilar to the ploughing process. A long microcrack maybe observed, predominantly in the transverse direction.The transverse microcracks observed on the surface areconsistent with a repeated ploughing mechanism causingsurface fatigue. The grooves observed in the wear direc-tion on the surface are formed by microcutting by theabrasive particles.

Fig. 13a-f shows SEM of neat PEK worn under differ-ent experimental conditions at different load conditions.The different abrasive wear mechanisms can be clearlyseen from the micrographs. Fig. 13a and d shows micro-graphs of neat PEK abraded under single pass conditions,the furrows in the abrasion direction are clearly visible.Also, wear debris appears as fine fibrils cut from thepolymer and which are adhered to the surface. The dam-age of polymer confirms cutting type of wear mech-anism. Fig. 13b and e shows micrographs of neat PEKabraded under multipass conditions. The abraded surfacehas many parallel ploughing marks in the abrasion direc-tion. Fig. 13c and f shows neat PEK surface abradedunder three-body abrasive wear situation. The micro-graphs show the matrix damage and subsequent matrixremoval due to the formation and propagation of micro-as well as macro-cracks at the surface. Comparison ofthese SEM micrographs confirms different wear mech-anisms. In single pass condition cutting, multipass con-dition ploughing and three-body abrasive wear conditionmicrocracking mechanisms were observed to be domin-ating.

4. Conclusions

An experimental study of two-body (single pass andmultipass conditions) and three-body abrasive behaviourof PAEK and their composites at different loads and slid-ing distances reveals the following characteristics.

1. Abrasive wear of PAEK and their compositesstrongly depends on the experimental test parameterssuch as load and sliding distance.

2. Comparative wear performance of all the compositesat different loads shows that specific wear rate (Ko)of two-body abrasive wear (single pass condition) is30-50 times greater than three-body abrasive wear.Also specific wear rate (Ko) of two-body abrasivewear (multipass condition) is 5-12 times greater thanthree-body abrasive wear.

3. The fillers such as PTFE and graphite were observedto be detrimental to abrasive wear performance (Eand L). Glass fibre (GF) reinforcement to the PAEKmatrix provides better abrasion resistance as com-pared to the CF reinforced PAEK composites.

4. Better correlations between wear volume and selectedmechanical properties for two-body (single passcondition) and three-body abrasive wear wereobtained. However, in case of two-body abrasivewear under multipass condition no linear trend isobserved.

5. SEM studies of worn surfaces indicate cutting,ploughing, cracking wear mechanisms under differentexperimental conditions.

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