Abrasive wear of some commercial polymers

Abrasive wear of some commercial polymers J.M. Thorp* Cylindrical test pins of some commercial polymer-based bearing materials (comprising two nylons 6, a filled nylon 6/6, a filled ultra-high molecular weight polyethene (uhmwpe) and three polyurethanes) were rotated, in dry conditions and at constant load and sliding speed, on circular tracks on stationary discs of steel gauze and abrasive paper. Wear against run-in steel gauze was proportional to the sliding time (distance), with the specific wear rate, Vsp, (wear volume per unit area per unit sliding distance) varying with the nominal pressure, p, according to Vsp = Kp~. Values of K and c~ are presented enabling comparison of the fatigue wear of the materials at various loads against steel (or a counterface with rounded asperities) in non-transfer film conditions. Nylon 6 showed the least wear and the polyurethanes showed the greatest wear, up to pressures of 3.43 MN m-" (500 Ibf in -2 ). With abrasive paper, the circular path became progressively clogged with transfer films and wear debris, and the wear volume, A W, diminished with time, t, throughout the test duration, following the relationship A W = Dt c, where both c and D are functions of the wear path diameter, c appears to be related to the film transfer capability of the polymer. The best overall abrasive wear resistance (in transfer film conditions) was exhibited by the filled uhmwpe, followed by two polyurethanes. Nylon 6 showed relatively poor abrasion resistance under these conditions. The mechanical properties indicate, with one exception, a similar ranking order for non-transfer film conditions Keywords: wear resistance, friction, polymers, pick-up, fatigue

Plastics are increasingly being used in industry as bearings, chute liners and wear guides due to their general resistance to corrosion, abrasion, galling and seizure; because of their tolerance to small smounts of misalignment and shock loading; because, provided frictional heat can be dissipated, plastics can perform satisfactorily with sparse or no lubrication; and for noise and weight reduction. In certain applications the coefficient of friction ~ is of importance (as in braking materials (high/~) or gravity feed wear strips (low ~)), but largely it is the mechanical properties and the wear life of the component that determine its acceptability in industrial applications. In this work an investigation is carried out of two types of wear of a number of commercial polymer-based bearing materials against rough surfaces. Polymers wear abrasively by two extreme mechanisms: by plastic deformation and microcutting of the surface by sharp projections on the abrading surface, particularly of rigid polymers; and elastic deformation followed by fatigue wear, by rounded asperities, particularly of the more elastic polymers 1-4 . In the first type of wear (abrasive) longitudinal furrows, ie in the direction of sliding, are formed on the abraded surface. In the second type (frictional), transverse furrows are observed. A combination of these two types of surface degradation and their interaction *Physics and Engineering Laboratory, Private Bag, Lower Hurt, New Zealand

determines the wear mechanism of the material, the ratio of frictional to abrasive wear depending on the elasticity of the polymer and the sharpness of the abrading asperities. It is deemed necessary, therefore, to test the abrasion resistance of a polymeric material by the use of both abrasive paper (sharp projections) and metal gauze (rounded wire mesh intersection asperities) 1-3 . During repeated sliding on a counter(ace and, particularly in dry conditions, modifications to the counter(ace by transfer fdms, trapped debris or surface damage further influence the wear process. Commercial polymer-based bearing materials generally incorporate fillers (asbestos, carbon black, bronze powder, silica, talc etc), fibres (glass, carbon, textile) and solid lubricants (graphite, molybdenum disulphide (MoS2), polytetrafluoroethene (ptfe)) to improve the mechanical, thermal and frictional properties of the base polymer. Recent studies indicate that effective Idlers increase the adhesion of the transferred layer to the counter(ace and hence reduce the rate of polymer wear s . Some fillers (eg glass fibre) may increase the wear rate of the counter(ace, however. In abrasive wear tests performed on pin-on-disc type rigs, the specimen pins are designed to traverse a spiral path on abrasive paper since a circular path tends to become clogged with wear debris and transfer films. Circular tracks are generally used on metal gauze, however, since its abrasive capacity remains unchanged for some considerable time

TRIBOLOGY international April 1982 0301-679X/82/020059-10 $03.00 © 1982 Butterworth & Co (Publishers) Ltd

59

T,5orp -- Abrasive wear o f some commerc/a/ po/Fmers

iv. use. Such tests do not determine the benefits endowed when a polymer N m or incorporated solid lubricant becomes tra~sferred to a dry counterface during repeated traversais. The results are thus only applicable to single traversal or non-transfer film conditions, such as met by wear strips, liners used in the transportation of abrasive materials, and sleeve bearings and bushes in liquid environments which prevent transfer flare formation+

proportional to the load, in contrast to resu] ~s obt~.med wi~k gauze 4 (see Eq (1)). Ln this work, a tri-pin-on-disc rig has been used t o determi+~e the friction, fatigue and abrasive wear propert_~es 05 a number of commercial polymer-based bearing mates'~s against steel gauze and abrasive paper. The aim was to obtain comparative data under fixec~ conditioes, in o-dot to assist in material selection for appropriate ir~dust:iai applications. In the case of commercial plastic~ and ei.asto+ mers there is tittte data ava8able other than that p~avided by manufacturers, generally using a variety of test rigs ~t;> different loads, speeds, counterfaces and cor~tact geometo ties, often poorly specified.

single traversat (non-transfer N m ) conditions the wear volume of a polymer is proportions1 to the sliding distance, with the specific wear rate vsp (the wear volume per unit area per unit sliding distance) varying with the nominat pressure p according to:

v~p =Kp+

Ln these tests loaded polymer test pins were rotated a: a fixed sliding speed (0.1 m/s) in dry conditions on the same circular tracks on horizontal stationary discs o# s~ain2eas steel gauze and abrasive paper.

(1)

where K is a proportionality constant (representing the specific wear rate at unit pressure) and ~ is a parameter dependent on the properties of the polymer and the abrading surface ~-a . For abrasion on a metal gauze ~ > I, whereas for abrasion on fresh abrasive paper oz= 1.

With the steel gauze the relationship given by Eq ( ! ) w a s confirmed and values of K and ~ were determined for the range of materials tested, comprising an extruded and cas~ nylon 6. a fNed nylon 6/6, polyurethanes, ptfe and a ~ass-filled ultra-high molecular weight polyethene (uhmwpe).

A correlation has been established by Ratner et al and others ~-+ between the wear of poiymers on metal gauze and single traversal wear on a (continuous) steel surface. Accordingly, useful data can be obtained by gauze wear tests, provided application is limited to non4ransfer N m conditions. With repeated traverssls against a steel counterface, as m dry sleeve bearings and bushings, it has been shown that the equilibrium wear rate fkrla~y attained is

The (gauze wear) results should thus ~ve an indication of the comparative fatigue wear of these materials against steel (in where cutting wear is not predo~Jnan0 in singt¢ traversal or non-transfer N m conditions, as met by guide and wear strips or beaffmgs is_ !iquid enviro~_ments+

Tabme I Typica[ physical properties at room temperature {from manufacturers" data) for the poh/mer-based bearing materials t ~ e d Code Base No, polymer

Filler

Colour' Specific Uitimate tensime gravity strength, MN/m 2 Ibf in -2

Break e~ongation, %

Hardness Rockwe!i

Shore D

119

--

Uzod !mpact strength, m N m-: (ft lbf/~n)

Max. recommended ?V qmil {L~n;ubr;cated)+ -~N m m -2 s-~ ~Jb[ ft/in~ min}

L~m[~ic# speed. m/~ {ftf~Er}

53 (1.0)

87,5 ,[2500) 70 i2000)

0.05 1i O~ 0.5

525 ( 15 000)

2,5 :,590}

105 (3000)

i ,5 {300!

ThermoNast c 100" Nylon 6 niH (A/B) (extruded)

t01

Nyton 6/6

white/ 1.14 opaque

81.4

11 880

20

ptfe, pe

greyblack

t.18

72.4

10 500

10

116

86

102" Nylon 8 (A/B) {cast)

nucleated with MoS 2 (+5% dye)

dark grey

1o16

82.7

11 800

30

--

85

103

uhmwpe

silica sand

grayblue

0.96

38.8

5600

334

70

104

ptfe

nil

white

2.1

6.9

1000

I00

58

53 f .1.0,

(10o;,

1500 (28.0) 52

! 55 {2,9)

42 (1200) 63 ~1800)

0.05 (i0} 0.5 ~I00~

--

Thermosetting 200

Polyurethane

about 2% (unknown)

b~eck

1.24

34.5

5000

219

--

68

254 (4.8)

595 17 000

201

Pomy urethane

insignificant

green

1.17

43.4

5400

86

--

78

64 (1 °2)

857.5 (24 500}

202

Polyurethane

insignificant

cream

1.t0

32.1

5100

240

--

88

280 {5.3)

717.5 {20 500)

~{A/B) samples from different batches

60

TR IBOLOGY

internationa~ Apri! t9S2

Thorp - Abrasive wear o f some c o m m e r c i a l p o l y m e r s

In contrast to metal gauze, the circular wear tracks on abrasive paper became clogged with debris and transfer films which progressively reduced its abrasive capacity. Accordingly, the wear was found to diminish with time and an empirical exponential relationship was obtained of similar form to that proposed by Rhee for the braking of a metal drum by asbestos-filled polymer friction materials: _~ w =

..t~ v h t ~

Pros we=ghed and replaced

where AW is the wear, p is the nominal pressure, V is the sliding speed, t is the sliding time, A is the wear constant, and a, b and c are a set of parameters specific to the friction pair.

The validity of an extrapolation to one-cycle abrasive wear is uncertain but the criterion based on the mechanical properties of the material is then used to ascertain an approximaUon of the relative wear of the materials in single traversal conditions.

6mm 3mm

F-

r-' Pins weighed and replaced

The coefficient of friction is calculated from the recorder deflection, the calibration constant, the wear track diameter and the load. Wear is measured by removing and weighing the test pins which are of softer material than the disc. Polymer-based materials The commercial polymer-based bearing materials selected,

their code numbers, and typical (room temperature) physical properties (from manufacturers' data) are listed in Table 1. Fifteen test pins (with a contact diameter of 3.0 ram) were machined from each of these materials. Prior to each test, three test pins were numbered, cleaned in an ultrasonic cleaner containing Shell X55 solvent, dried, weighed and placed in one of the five available locations in the pinholder so that the pins rotated on the same circular track on the disc.

Pros weighed and replaced 2mm

¢

,,

~,=o86

d lo zo Material 102B/SL C paper Apphed load 20 N d 69rnm

The tri-pin-on-disc tribometer 7'8 has, basically, three loaded equispaced cylindrical test pins which rotate on one of five available circular tracks (comprising concentric circles of 69, 84, 9q, 114 and 129 mm diameter) on a stationary homzontal disc supported on a hydrostatic oil bearing. The sliding speed is infinitely variable between 0.0002 and 18 m/s and the applied force can be varied in 10 N increments up to 310 N. The minimum force on the three pins (ie with a zero applied force) is 37 N, comprising the weight of the pin holder and loading components. The disc is restrained by a torque arm/flexure spring, movement (0.5 mm maximum) of the latter, due to the torque generated by the sliding system, being determined by a transducer/recorder system. Calibration torques are provided to calibrate the torque measuring system.

Pros weNhed and replaced

6mm

In general, a combinaUon of both microcutting and frictional weal occurs so that the wear performance may he somewhere between that extubited on metal gauze and abrasive paper.

Experimental

7mln

N

(2)

These results thus give the comparati,~e abrasive wear of the matermls tested in conditions where repeated traversals are made on a rough surface, where microcutting conditions predominate but may be reduced by transfer films.

l

i lOm,n

lOmm ~,f=054

r ~

~',=0 83

o ,o zb Material 102B/SLC paper Apphed load 20N d 84mrn

Fig I Friction traces given by test phls o f material 102B (a cast tLvlon 6) sliding on 69 m m and 84 mm diameter wear tracks on a (silicon carbide-coated) abrasive paper disc/'or a total time a l l 0 min. Load 19 N (2. 68 M N m -2 ) per pin, sliding speed O.1 m/s (i = initial, f = final) size 400) paper. Each disc was glued to the upper face of a new chemotextile diamond polishing cloth. The latter possessed a self-adhesive backing which facilitated attachmerit (and removal) of the assembly to a steel sub-plate fixed to the base of the disc container. Procedure Dry conditions and a constant sliding speed of 0.l m/s (about 20 ft/min) were maintained throughout the tests. The wear was measured by removing, cleaning and weighing the test pins after timed intervals. (The test pins were replaced in the pin-carrier in exactly the same position by ahgning marks on the pin shoulder and pin-holder.) Loose wear debris was blown off the wear track before the pincarrier was replaced and the test continued. A running-in period was required for abrasion on metal gauze before the plot of the wear volume against time reached linearity, With abraswe paper, however, the wear versus time curves remained of exponential form throughout the entire specimen wear. Friction traces were recorded at the beginning and end of each time interval, as shown in Fig 1. Results Steel gauze ,Coefficient o f f r i c t i o n

Disc materials Discs were cut from stainless steel plain weave gauze (aperature size 300/~m) and silicon carbide-coated (grain

TRIBOLQGY

i n t e r n a t i o n a l A p r i l 1982

Corresponding values of the coefficient of friction, ~, generated by the polymers sliding on stainless steel gauze at the start of each test on a fresh track,/ai, and at equilibrium at

61

Thorp - A b r a s i v e w e a r o f s o m e c o m m e r c i M p o l y m e r s

the end of each test (up to 1-2 h), #f, were found to be remarkably reproducible and independent of the wear track diameter. Average values are compared with those exhibited on abrasive paper in Table 2. The decrease in friction A# =/~i - #f during a test started on a fresh track gives a measure of the effectiveness of the transfer film deposited on the wear path. Marked differences show up on abrasive paper which is considerably clogged by some of the materials. The effect is much less marked with gauze, how~ ever, which is not clogged (at least to the levei of the raised abrading wire intersections) by polymer wear debris for a considerable time.

150,

]40~

!OOA?

4

/

tP° i

i

~oo ~o ~70

/

~, 6o

5C

4°i

/

~o~

/

Final (equiLibrium) friction coefficients given by gauze vary from 0°57 for a titled polyurethane (200) to 0.28 for the filled ul~nwpe (103). o

Wear

Typical plots of the average weight loss per pin, AW, agains~ sliding time on gauze (under constant load and speed condb rions) are shown in Fig 2 for the nylons (100A, 101,102B) and the glass-filled uhmwpe (103). In all cases after a running-in period a linear region was reached, the slope a (calculated using the method of least squares) giving the mass wear rate. tn those cases where the system had already been pan in (ie using worn pins on a used track), the straight tine passed through the origin (eg 100A and 10! in Fig 2). The reproducibility of &e wear rate was within i%. Typical plots of the wear, AN, as a function of load is illustrated m Fig 3 for the gauze abrasion of the glassfilled uhmwpe (t03). Table 2 Comparison of the average initial (#i) and final (~,f) values of the coefficient of friction given by the polymers sliding on fresh tracks o~ steel gauze and abrasive paper a

Mater]a~

Code No.

Stee~gauze #i Nf

#[%

Abrasive paper Deposit #i #f A#/ thick#]% heSS

A#/

ptfe

104

0.48

0.33

3t

--

--

Polyurethane

202

0.48

0.33

31

0.70

0.21~ 70

heavy

Nylon 6/5 (filled}

I01

0.42

0.29

31

0.63

0.28

59

heavy

Polyurethane

201

0.39

0.31

21

0.59

0.32

46

medium heaw (+ ~oose debris)

Nylon 6 (fiHed)

102B

uhmwpe (filled)

103

0.39

0.28

28

0.64

0.48

25

medium

Nylon 8

1008

--

-

--

0.83

0.84

23

medium small (+ loose debris)

0.54

0.55

2

0.83

0.55

33

medium

2

5

4

5

6

Average weight loss per p~p,AW,mg

8

9

Fig 2 Average toss in weight per pin against '" " *" a steelgauze disc for materials IOOA, i02B, 13i and i03 Load 32.3 N (4o5 7 MAr m -2 ) per pin; sfiding speed &i m/s

8o[

7o! 60

t

Loo0M.m24

552 0

•

504

50

_~4o x:J

= 30 O9 20 Lo

0

t

2

5 4 5 6 7 8 Average weight loss per pin, A W, mg

9

200

0.75

0.67

11

0.85

0.66

22

medium

Nylon ~ (filled)

I02A

--

--

--

0.88

0.53

22

medium sma]}

Nyton 8

100A

0.47

0.36

23

0,72

0.67

7

small (+ loose debris

aThe percentage reduction in #i (= 100 (#i the effectiveness o f the transfer film

- $~)/I~i)

gives a measure o f

~0

Fig 3 Average loss in weight per pin against sliding ti;~e o~,. a steel gauze disc for (filled) uhmwpe {103) for various loads

The specific wear rate vsp (ram a m-2 m-1 ) was caiculated from a

VsP=ApV

Polyurethane (filled)

62

~

(3)

where A is the nominal contact area, p the material density, V the sliding speed and a the linear mass wear rate deter° mined from the plot of AW against t for a specific load. Linear plots of v_ against the nominal contact vressure , , • -~Y • { using logarithmic scales, are shown m F.g 4 s"~or ~all the materials tested. Vatues of the exponent a and parameter K listed in Table 3 were obtained respectively from the sieve and intercept of each plot, according to: lgvsp = igK + ce lgp

,.4)

(For wear against metal gauze ~ > i. Hence d~e specific wear is dependent on both the load and test specimen area~ A. Accordingly, the latter is included in the definition of the specific wear rate given by Eq (3).) The rapid rise in the wear ram observed (see Fig 4) for @ o n 6 (100A) above a critical pressure of about

T R JB O L O G Y internationa~ Apri~ ! £82

Thorp - Abrasive wear o f some c o m m e r c i a l p o l y m e r s

5.7 MN m -2 (823 lbf in -2 ) is attributed to thermal softening of the polymer~. The values of ~ and K listed for nylon 6 in Table 3 are accordingly deduced from the linear region below the critical load.

Table 3 Values of the pressure exponent e and the wear factor K (from Vsp = Kp c~) for the polymer-based materials abraded on metal gauze at a sliding speed of 0.1 m/s. (The linear correlation coefficient R is for the plot of Ig Vsp against Igp)

Pressure e x p o n e n t

It has been shown that the pressure exponent a depends on the properties of both the abrading surface and the polymer type and is specific to fatigue wear 2'3 . It has also been established that c~increases with the number of cycles required for degradation of the polymer surface during abrasion. Thus a increases with increase in 'the degree of smoothness' of the gauze (defined by n2/r, where n is the number of grid meshes per cm 2 and r is the radius of the grid wire) and with increase in the molecular interaction forces of the material 2 . Values reported by Rather and Faberova 3 range from 1 to 3 for rigid thermoplastics, as shown in Table 4, which compares their values (a(R & F)) for nylon 6, high density pe and pfte with values reported here (a(T)) for similar materials (nylon 6, glass-filled uhmwpe and ptfe). On average, values of a(T) are about 1.5 times greater than the corresponding values of c~(R & F) but otherwise are in the same relative order, indicating that 201

100

Material code No.

cx

K, m m 3 N 4 m 4

R

100A 101 102B 103 104 200 201 202

2.7 2.4 3.9 2.7 1.6 1.2 4.1 2.2

9.1 8.3 3.6 4.9 1.1 1.7 1.7 3.1

0.999 0.995 0.981 0.999 0.971 0.936 0.993 0.999

x 10 -s x 10 -6 X 10 -9

x x x x x

10 -7 10 4 10 -3 1 0 -6 10 -5

Table 4 Comparison of values of a (T) + with e (R & F)# for similar polymers abraded on a metal gauze Material

cz (T)

a (R & F)

Ratio (e (T)/ (R & F))

Nylon 6/6 (filled) Nylon 6 (unfilled) High density pe u h m w p e (filled) ptfe Babbitt metal

2.4

-

-

2.7

1.5

1.8

2.7 1.6 -

2.1 1.2 1.0

) 1.3 ) 1.3 -

Aluminium

-

2.8

-

+ T = Thorp

2(3

#R & F = R a t n e r and Farberova 3

~-~ 15

200

, J/y0

"'E E

LO

either a rougher gauze was used by the Soviet workers or that the difference is due to the increased frictional heat generated by their faster sliding speed (0.3 m/s).

I

%

~, o5 ,ooA Q_ O3 025

The low values of cx reported for ptfe (a(T) = 1.6; c~(R & F) = 1.2) confirm low surface inter-molecular forces and are within the range of values reported by Uchiyama and Tanaka ~° (1.26 and 1.63) for 3 mm diameter pins of ptfe rotating on a chromium-plated brass disc maintained at 29 °C and 80 °C respectively. Relative wear performance

0O5

0 I 0025

3=0 L5

[ 4LO

2~0 Contoct pressure, p, MN/m

z

Fig 4 Logarithmic plots of the specific wear rate (Vsp) against the nominal pressure p for the polymeric materials abraded on steel gauze at a sliding speed of O.1 m/s

T R I B O L O G Y international Aprd 1982

0

The relative wear resistance of the materials compared to nylon 6 (100A), which exhibits least wear on abrasion by gauze, changes with pressure, as shown in Table 5( with pressures of 0.10, 0.69 and 3.43 MN m -2 (about 14,100 and 500 lbf in -2)). Overall, unfilled nylon 6 (from source A, coded IOOA) and the (dyed) cast nylon 6 tfrom source 6, coded 102B) show superior (frictional) wear resistance. Filled uhmwpe (103) is consistently third best but with five times the wear of nylon 6. The polyurethanes show the worst overall performance and exhibit the most drastic changes with increase in load, both adverse (eg 201 in Table 5) and to advantage (eg 200 and 202). Correlation w i t h wear o f p o l y m e r s on steel

A good correlation has been established between polymer wear on a metal gauze and a rough steel surface under single traversal conditions 1-3. The fatigue wear processes

63

Thorp -- Abrasive wear of some commercial polymers

are thus considered to be essentialy the same, provided the rounded abrading asperities do not cause p!asdc deformation and mAcrocutting.

7F

The reiative performance shown in Table 5 can be expected to apply in practice, therefore, to the fatigue wear of these materi~s against steel in conditions which prevent polymer transfer film build up on the counter~ace. Polymer wear strips and guides fall in this category°

I

In agreement w~th these findings, it is knoww that materiais 100A, 102B and t03 (ie ~ylon 6 and uhmwpe) are used very successfully in tb_is type of applicationo Thus ny!on sheaves can replace steel and i~crease wire rope life by up to 4°5 limes ~ . uhmwpe is used successf,ally as guides and star wheels in bottling plant and against steel in hip prostheses where transfer film buildmp is prevented by the presence of synovial fluid.

Abrasive paper Coefficients of friction As with gauze, corresponding values o f # at the start of ~ test on a fresh abrasive paper track,/zi, and at the end of each test (after 10 min, in general), ~f were found to be reprcdudble and independent of the wear path diameter. Average values are compared with those given by abrasion on gauze in Table 2, where the materials are ranked in order of their transfer film capacity on abrasive paper as indicate£ by the percentage reduction in #i tisted in column 8o Clearly, in terms of reducing friction, polyureti~ane 202, followed by the pe/ptfe fflbd nylon 6/6 material (101), have the most effective and the nylons 6 the bast effective transfer properties of the materials tested. in the case of those materials observed to deposit medium (or medium/heavy) transfer films on to abrasive paper, the final value of I~f on the dogged paper ~s, in generA, in close agreement with that given by gauze (eg with materials 201, 102B and 9_002. For a heavy low-friction deposit the value of #f on the clogged track is even less than that on gauze (eg with materials 202 and 101). For materials with poor transfer film properties, however, #f on abrasive paper is greater than that on gauze (eg 100A). Wear as a function o f time

the tests on abrasive paper the polymer test pins were rotated, under constant toad (2.68 MN m -~ ) and sliding speed (0.t m/s) conditions, for up to a maximum of 280 cycles on each of the five circular concentric tracks. Owing to the progressive contamination of the abrasive path the

Wear path diameter,ram

99/

69 84

/

%

/

/ / ,'

E |

/

~41

/ /

/

/

/

,/

/

////

J

/ 29

.P

/ t ,

/

I

I i

J. . . . .

4

5

6

7 8 9 ;C Average weight l o s s per pro,

~2

II

A

~5

,2

W, ,"rig

Fig 5 Average loss #~ weight per pin against s#d#~g time fc, r polyuretha~'oe-based polymer (202) sliding o;~ circuIo~r rracks o f different diameter on a SiC - coated ebmsive pape,, disc. Load 19 N (2. 68 M N m -2) per pin; sEdh~g speed oA m/s Polymer base

uhmwpe

Nylo~ 6

IO3

~0

~OOB

Po!yure~l~Qse

08

E 04 _,]O2

v~

Logarithm

of QverQge wear(m@ ;oss per ~Jq [Igz~ A/)

Fig 6 Linear logarithmic plots o f the average loss 9; weight per pin against sliding time for materials 100B, 103, 20] and 202 sliding on wear tracks o f d%~eren~ diameter 6~ SiC-eoa ted paper disc. Load 19 N {2. 68 M N ~ -~ ) per pm; sliding speed O.J m/s wear versus time curves fotowed ar~ exponenti£ re~at!o~sbip~ shown in Fig 5~ given by: AW =Dr c

.<~;

(for constar~t p, V, d) where AW is the average weight loss per pin, t is the ~!dmg

Tabme 5 Specific gauze wear relative to nylon 6 {100A} at pressures of 0 . I 0 , 0.69 and 3.43 MN m -2 (ie about 14,100 and 500 Ibf in -2 }. For each pressure, the ranking order of the materials is given, with 1 being most wear resistant and 8 being ~east wear resistant Material 0.10 MN m -2 Ranking Code No, (14 Ibf in -2) order

0.69 MN rn -2 Ranking (100 Ibf b, -2 ) order

3.43 MN m -2 (500 Ibf in -~)

Ranking order

100A t02B 103 t01 202 104 201 200

1.0 0.4 5.4 50.8 127.4 145.3 291.2 1031

1,0 2.8 5.4 31.3 57.0 24,7 2772 92.2

t 2 3 5 6 4 8 7

64

1.0 0.04 5.4 91 o0 337 1236 19,1 19 101

2 1 3 5 6 7 4 8

2 1 3 4 5 6 7 8

T R I B O L O G Y internadonaJ Apr;l !982

Thorp - Abrasive wear o f some commercial polymers

[

time and parameters D and c are specific to the material and dependent on the wear track diameter.

g

Eq (5) is of similar form to the empirical wear relationship (see Eq (2)t proposed by Rhee 6 for asbestos-filled polymer friction materials. Linear plots of lg AW against lg t for the materials sliding on different wear tracks (see Fig 6) enabled the values of c and D, listed in Tables 6 and 7 respectively, to be determined as a function of the wear track diameter from the slopes and intercepts respectively, according to: lg A14' =c lg t + lgD (for constant p, V, d).

(6)

2 I0 ~ IIq

202

, I

"\,/

/•/

o

69

84

99

114

129

I 82 L

•

/

69

0.727 0.738 0.580 0.535 0.690 0.770

0.751 0.569 0.564 0.716 0.752

103 200 201 202

26 20 20 20

0.337 0.379 0.210 0.267 0.323 0.528 0.368 0.300 0.287

0.365 0.306 0.613 0.262

0.351 0.358 0.654 0.239

0.756 0.662 0.548 0.536 0.638 0.685

Table 7 Values of D for each material as a function of the abrasive paper wear track diameter d

20 25 26 20 23 24 26 20 20 20

/'

• 0 50

,q 70

0 40 ~50 LogOrllhm of expQnent c (Iq c',

0 80

Fig 7 Logarithmic plots o f the exponcnt c against the wear track diameter for the wear oj poiymer-hased materials against SiC-coated abrasive paper ,

Jhm~pe

, rib

%vice C, rJ~l, n 6,, 10Lh C'C'&l L'~ I" (to /

i q

P,,I~ J,enr,,,~e

2I /'

t

/

/~/

/

q4

0.753 0.712 0.539 0.586 0.673 0.692

100A 100B 101 102A 102B 102B (repeat) 103 200 201 202

,'

/

/

0.722 0.637 0.470 0.687 0.719

Mean ambient temperature, °C

/ /

/ / ,/

,/

/

,

/ /

20 25 26 20 23 24

Material Code No.

/

/ \

~'25

r

100A 100B 101 102A 102B 102B (repeat)

Wear path diameter, mm

,

',,

/

~' ,l~r i~r to

Mean ambient temperature, °C

'~

\

/

d ,n 1

Material Code No.

Poor IO£N qOOB

20i

/ \

The parameters c and D, which are specific to the polymer, were found to vary with the wear track diameter d accordmg to"

Wear path diameter, mm

IO?A 101

" ~\

E t~O-

Table 6 Values of the exponent c for each material as a function of the abrasive paper wear track diameter d

Tronsfer ftlm pro0erlles

Good 202

129 L

~206[

g

69

84

99

114

129

,'

/ /'

/

/

/

,,' /

/

/

/ / / •

q, c,4 t

, 4,lr,'Ym

,.

, t ~tlF

I 14

i , ir,

I I L3

Fig 8 Logarithmic plots o f tlle parameter D against the wear track diameter for the wear o f polymer-based materials against SiC-coated abrasive paper c = Cd ~

(7)

and D = Gd v

(8)

as illustrated (using logarithmic scales) in Figs 7 and 8, respectively. Accordingly, substituting for D in Eq (5) and dividing by the material density to obtain the wear volume: Av = G' d 7 tc (9) where G' = G/O

2.400 2.636 3.025 1.934 2.041 2.135 3.774 4.428 2.778 3.194 3.478 2.077 2.363 2.388 2.331 2.453 2.457

3.301 2.248 4.673 3.903 3.078 2.942

2.456 5.318 3.708 3.007 3.031

1.609 1.852 2.057 4.891 5.724 6.034 5.800 13.357 20.767 5.179 6.128 7.311 7.608

2.367 5.977 23.367 8.637

T R I B O L O G Y international April 1982

Values of the parameters C and/3 from Eq (7) and G and 7 from Eq (8) are listed in Tables 8 and 9 respectively, thereby enabling the wear volume to be estimated from Eq (5) for any of the materials after cycling for time t on a circular wear track of diameter d on abrasive (SiC-coated) paper. Wear and number o f cycles traversed

At constant shding speed the time t is proportional to the product dN, where N is the number of cycles traversed by the test pins on a circular path of diameter d. It is more meaningful therefore to substitute for the sliding time in Eq (9) in terms of d and N: AV = G* dT+C N c (10) where for a sliding speed of 0.1 m/s: G* = G' (n/6000) c

(11)

65

Thorp - Abrasive wear o f some commercial polymers

iPnus a iogarithmic plot of the wear volume (tg Av) againsc the number of cycles (ig N) is linear with the slope giving the same parameter c as that abtained from the slope of the plot of lg Av (or lg A W) against Ig t. The magnitude of the exponent c (shown as a function of the abrasive paper path diameter d in Table 5) appears to be related to the film transfer capability of the po!ymer. Thus materials with relatively low values of c (eg 202, 200, 103) were observed to buitd up stabte transfer films on abrasive paper wi~ch proved difficult to remove. Converseiy, materials with Ngh values a r c (eg I00A, 100B, 201) produced 1ease non-clogging debris which was easily swept off the wear path. Estimated one-cycle=wear on a fresh path

The wear volume for a one cycle (N = 1) traversal is given from Eq (10) by: (12)

AvN = 1 = G* d v+c

Running-in phenomena can be expected to affect the onecycle-wear behaviour, however, at values of d within the experimental range owing to the very short sliding times involved. Thus the largest diameter (129 ram) is traversed in 4 s. Accordingly, wear volumes have been estimated (see Table 10) for a hypothetical large diameter (1910 ram) requiring a traversal time of i min, in an attempt to eliminate initial idiosyncrasies relating to the wear of machine Tabme 8 Values of C and/~ (from c = Cd~, ie from a |inear plot of ~gc against Ig d} for each material cycling on tracks of diameter d on abrasive paper MateriN Code No.

C, mm -~-

100A 100B 10I 102A 102B 103 200 201 202

0.437 0.191 0.310 0,142 0.201 1.476 0.008 0.130 6.737

~

Materiai code

Av, mm 3 (A; = I, d = !910 ram)

0,120 0.284 0.127 0.283 0.261 -0o296 0.790 0.330 -0,687

0.851 0.987 0.701 0.996 0.994 ~0.9997 0.945 0,994 0.9998

Materiaf Code No.

G, mg min -1 mm -~

?

Linear correlatio,~ coefficient

t00A 100B + 101 102A 102B* 103 200 201 202

0.150 0.552 + 0.132 0,170 0.149 0.133 1.232 0.298 0.177

0.651 0,296 + 0,759 0.661 0.522 0.584 0.326 0.898 0.801

0,994 0.998 0.987 0.997 0,933 0.967 0.999 0.999 0.991

~Combined results of two test runs +Results atypical of the nylon 6/filled nylon 6 specimens

&v/~v~o.~

No.

100B t 03 200 102B 100A t02A t 01 202 201

Orde~ e ~ wear

res;stapcG ;siqle t:aversa;} 4.5 t 1.4 11.7 15.2 18.0 21.6 34.6 68o3 225.!

0.4 i o0 1.0(3) 1.3 1.5 1.9 3.0 S,0 19.7

! 2/3 2/2 4 '5, 3 "7 ~., 9

TaNe 11 The reciprocal of the breaking work of the poiymeric materials and values relative to that for filled uhmwpe {103}, compared with the relative wear {to {103)) calculated for 104 cycles on a 99 mm diameter abrasive paper wear path aed for 1 cycle on a 1910 mm path Materiai Code No.

Reciprocal of breaking work, m2/MN

Breaking work (reciprocal} relative to { 103}

Relative wear, 104 cycles/ 99 mm (transfer ~.i!m conditions)

Relative wear, I cycie/ 1910 mm {s~ngle traverse conditions}

103 200 202 201 * 102(A/B) 100 (B/A) 101 +

0.8 1.3 1.3 2.7 4.0 6.! "~3.8

1.0 1.6 1.6 3.4 5.0 7.5 i7.3

i .0 i o5 ! .8 29.0 5.5, 7.4 7.4, I3.4 4.9

t .0 ! .0 &0 '~.~ ~ -" I~9 i 3 0.4, !.5 3.0

Linear correlation coefficient

Table 9 Values of G and 3' (from D = Gd% ie from a linear plot of Ig D against ig d) for each material cycling on tracks of diameter d on abrasive paper

66

TaMe 10 Estimated wear volumes and retative wear ratios (to fNed uhmwpe 1103}) for one cycle on a h'/pothetieam fresh abrasive paper track of 1910 mm diameter requiring a traverse] time of 1 mira {There is some doubt as to the validiW of the e×trapolation, however}

*Faulty material, with dispersed cavities +Contains ptfe and pe

grooves on the test specimens. However, there is some doubt as to the vMidity of Table i0 (based on Eq (12)) since the calculated specific wear rate for ?7 = ! Oe Av~7 = 1/~d) is not independent of the wear park Hameter, as expected for a fresh abrasive path, uNess 7 + c-1 = O. The latter is approximately true for only two of the materials, 103 and 202. Abrasive wear in non-transfer film conditions is suffered in practice by pipe and chute liners used to transport abrasive materials, by blades and components in agricultural and earthmoving machinery, by impeders in pumps handling sewage and abrasive fluid wastes and by wear strips and guides. Materials recommended by manufacturers for these applications inc!ude glass-fiUed uhmwpe (103) a~.d poty° urethanes (200, 201,202) which are Naced respectively in positions 2, 3, 9 and 8 kr~Table 10. [t has been shown that there is reasonable correlation between the single pass wear rate of a polymer sliding over a rough counterface under a fixed toad and the

TRIBOLOGY i,qternat~onM Aprii t982

Thorp - Abrasive wear o f some c o m m e r c i a l p o l y m e r s

reciprocal of the work required to rupture the bulk polymer 442 . The rupture (breaking) work is estimated from the product of the stress at break and the associated strain (or percentage elongation) as determined in a conventional tensile test. Table 11 lists the reciprocal of (typical breaking work for each polymer (with values of the ultimate tensile stress (uts) and break elongation from Table 1) and also (in column 3) the value relative to that for materiaI 103, in increasing order. The estimated wear relative to that given by (103) for a one cycle traversal on a hypothetical 1910 mm diameter track is compared in column 5. According to the mechanical properties, the ranking order of the materials in terms of decreasing abrasion resistance, is given by 103,200, 202,201,102, 100 and 101. In agreement, materials 103 and 200 exhibit low relative wear (ie high abrasion resistance) when N = 1 (see column 5, Table 11). The faulty (cavity-filled) nature of material 201 is evident from the exceptionally high relative wear. However, the nylon 6 based materials appear to wear to a lesser extent and material 202 to a greater extent than predicted by their bulk properties. Better correlation is obtained in fact, but probably fortuitously, for the calculated relative wear on traversing a 99 mm diameter track for 10 000 cycles (see Table 11, column 4). (This is discussed in the following section.) The extrapolation to one-cycle wear is thus uncertain, being affected by running in phenomena at values of d within the experimental range. However, materials 103 and 200 appear to have the best abrasion resistance of the materials tested in terms of both mechanical properties of the bulk polymer and extrapolated one-cycle wear volumes. Abrasive wear u n d e r transfer f i l m c o n d i t i o n s

The experimental results for polymer test pins cycling on the same circular abrasive paper path are given by Eq (10). Plots of the wear volume (calculated with d = 99 ram) against the number of cycles N, shown in Fig 9 with N up to 25 and extended to 1000 cycles respectively, are valid at least up to 300 cycles. Extrapolation to higher values of N (Fig 9(b)) was carried out in order to extend and clarify the trends observed within the experimental range (N = 15 to 280). Further extrapolation for 103 , 104 and 106 cycles is illustrated in Table 12 which shows the calculated wear volumes, the relative wear to the most wear resistant material (103) and the ranking order of the materials for each of these values of N. The best overall abrasive wear resistance under continued cycling (ie transfer film) conditions is clearly exhibited by material 103, followed by polyurethanes 200 and 202, as predicted (for single traversals) by their mechanical properties. Table 12 illustrates that with increase in the number of cycles the relative wear of polyurethanes 200 and 202 is reduced whereas that of the other materials is increased. This is in agreement with evidence found in the literature which suggests that transfer films from polymers with high elongations (eg 103,200, 202, with respective break elongations of 334,219 and 240%) are beneficial and reduce the wear rate, whereas those from less ductile polymers (eg 100, 101,102, with respective elongations of 20, 10 and 30%) are detrimental and increase wear 4 . Material 101, a nylon 6/6 containing ptfe and pe tillers, is in fourth place (see Table 12), exhibiting wear under transfer f'flm conditions which is considerably less than that pre-

T R I B O L O G Y international April 1982

7C~:

/

60[

202

/

•- '

PoI,urelh....

/

_-2°° loll

)40 ~5 O[

~ -

o

....

__. . . . .

5

~

g

~s

,ooA

ii

I01 ~jIOOB ---~C'I02A

.-:p~"

/"

,i

/f

.

I,

•

"-I Nylons

/I

,g

I1~ I

,l' I t

M

~

Numt~r of c~:les ~/

soF aol / //Polyurethane 32~ I /

LO2A IOOAINyI°n

"

i-

•

.7" ~~

....

8 I

/"

o

,6o 2bo sbo 4bo ~6o do

I I

,o2B

;

...... _~ ~ .....

~-

Number of cycles

-

I--- ~

Polyurethanes

200

760 86o 960 ,obo ,,do ,2'o0

Fig 9 The wear volume o f the materials against the number o f cycles (N) on a 99 mm diameter wear path on SiC-coated abrasive paper for N up to (a) 25 and (b) extended to 1000 cycles

dicted by its mechanical properties, as shown by comparison of columns 3 and 4 in Table 11. The ptfe/pe transfer fdms deposited by this material clearly reduce both friction (see Table 2) and wear under conditions of repeated sliding on the same counterface. The beneficial effects decrease with increase in N, however, as observed with the other nylon based materials. The relative abrasion resistance of the nylon 6 (particularly the unfdled materials) is poor, decreasing with increase in N, the loose wear debris proving detrimental. Finally, the presence of cavities within a material (201)is clearly disastrous, the relative wear of this material being highest of all. The rupture work of the bulk base polymer indicates the material should have moderate abrasion resistance had it been of good quality. The manufacturers of the uhmwpe based material (103) recommend its use particularly in abrasive conditions as guide and wear strips, eg bar strips under saw-mill chains, chain channel guides, hopper and chute liners, conveyor star wheels and rope guides and pulleys. The polyurethanes (200, 201 and 202) are recommended mainly as sleeve bearings and bushes in dirty conditions. Applications include use as the lower sleeve bearings in vertical sewage pumps, vehicle spring bushes (200), marine stern tube bearings (200), chain wear strips (202), rope sheave bushings (201) and railroad brake gear pivot points (200). The results presented in Fig 9(b) and Table 12 confirm the basic good abrasive wear resistance and transfer f'dm properties of materials 103,200 and 202 under conditions in

67

Thorp - A brasive wear o f some commercial polymers Table 12 Wear volumes {cNcuiated from Eq {10)), the re~at[ve wear compared "to that for filled uhmwpe {No. ~0.~0: ~ arid the ranking order for {a hypothetical} 10 ? , 104 and 106 cycles on a 99 mm diameter abrasive paper wear path No. of cyc!es, N

Material

Code No.

! O*

103 Ranking 2w, mm a ZXv/ZXVlos order

! O~ Ranking

~',;mm ~

&v/£svze3 order

Ranki:~ ~u,mm ~ ,~v/~v-_o3 steer

uhmwpe (fil!ed)

103

9.0

1.0

I

21.7

1.0

*

I24

1 oC

"

Polyurethane {filled}

200

15,9

1.8

2

33.5

1.5

2

!48

!.2

2/3

Polyurethane

202

19.8

2.2

3

38°4

1.8

3

i44

1.2

2/i:

Nylon 6/6 (filled}

101

30.7

3.4

4/5

106.3

4.9

4

1272

10.3

z

Nylon 6 (filled}

102A

30.9

3°4

4/5

! 19.1

5~5

5

!770

I4.3

5

Nylon 6 {filled}

102B

33.2

3.7

7

160.0

7°4

6/7

37!6

30°0

5

Nylon 6

100B

31,4

3.5

6

161.6

7.4

6/7

4290

34~5

Nylon 6

100A

51.2

5.7

8

290.0

t3.4

8

9300

75.0

8

Polyurethane

20t

162.1

t8.0

9

630.5

29.0

9

9544

77.0

9

which repeated cycling atlows surface films to be transferred to a rough counterface. In single pass abrasive conditions, however, the transfer N m properties of the materials are o f no significance and abrasive wear is dependent sole!y on the mechanical properties of the polymer. Compa~son of the reciprocals o f the breaking work o f the bulk polymers may then be used as a rough guide as to their relative wear resistances. Materials 1 0 3 , 2 0 0 and 202 retain their effectiveness in single traverse] conditions owing to the basic cohesive strengths of these materials. Material 101, however, would lose the benefits endowed by its good transfer f'rim properties.

References t.

2. 3.

68

Rather S.B. Comparison of the Abrasion of Rubbers and Plastics. Abrasion o f Rubber. Edited by D.L James, Maclaren and Son Ltd, London, UK; Palmerston Pub. Co. inc., NY. English translation, 1967, 2 3 - 3 5 Klitenik G.S. and Rather S.B. Features of the Abrasion of Rubber on a Metal Gauze. Ibid, 64- 73 Rather S.B. and Faxberova LL Mechanical Testing of PlasLics Wear. Ibid, 2 9 7 - 3 i 2

4.

Lancas~e~JoKeAbrasive Wear of Pob'mers. Wear. 1959, ~_4,223

5.

Steward M.P. Friction and Wear of PTFE Composites. ~h.D Thesis, University of Ci~mbridge, 1978

6.

Rhee S.KoWear Mechanisms for Asbestos-Reinforced Automotive Friction Materials. Weal5 1974, 29(3). 3971--392~

7.

Thorp J.M° A Novel Tri-Pin-On-Disc Tribomete~. Lubrieal~o~, Frietiol,~ and Wear in Engineering ] 980 Conference, Melhoun~oe, 1 - 5 December ]980, Inst. Engo Australia. National Confi P'Jbo No. 80/12. d8

8.

Thorp J.M, A Novel Tri-Pin-On-Disc Tribom%e~ Designed to Retain Lubricants. Tribology International April 1982, 14f2;~ 12]-t25

9.

Evans D.C. and Lancaster J.K. The Wear of Polymers. Tzea~.ise on Materials Science a~Td Technology, VoL !3, Wear, Edized b), D. ScotL Academic Press, 1979, 8 6 - I 3 9 (~ee 109)

10. Uchiyama Y° and Tanaka Ko Wear Laws for PolytetraCuoroethylene. Wear, 1981,58(2/. 223-235 1i. Chert J.H. and Ursetl C.R. Comparison of Wire Rope Life; Using Nylon and Steel Sheaves, Part I: Test Methodology and Compm~son of Wire Rope Endurance Life. SAE paper 790~04, i97P 12. Briseoe Be Wear of Polymers: An Essay on Funda~en~.al Aspects. Tribotogy International, Aug. 1981, t4(4), 2 3 t - 2 4 3

TRIBOLOGY internationa! Ap,d 1982