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Resistance to particle abrasion of selected plastics
WEAR BLSEVIER
Wear 203-204 ( 1997) 302-309
Resistance to particle abrasion of selected plastics Kenaeth G. Budinski EastmanKodakCornpony,3177Latto Road,Suite146, Rochrsrer.NY 14612-3092. USA
Ahstnwt Accelerated abrasive wear of plastic parts in a piece of productionmachineryprompteda laboratorystudy to find a materialwith better abrasionresistance.The abrasionoccurredin a machinethatcompacted‘sand-like’pardclesof an inorganic compound.Theabrasionresistance of a -wide variety of plastics and different durometer polyurethanes (21 materials) was tested with a modification of the ASTM dry-sand rubber wheel three-budy abrasion test. Only one material, a polyurethane, bad better abrasion r&stance tban tbe material tbat was currently in use. Hardness, friction and scratch tests were conducted on the test materials to try to understand the mle of material properties in this type of abrasion. None of these correlatedwith the wear data.Rcvious investigatorsof plasticabrasionrelatedabrasionresistanceto the fracture energy and friction.The wear data developed in this study did not cormlate with the specific modal pmposed by Ratner. However, it was possible to obtain a reasonablecorrelationwith a deformationfactor that included the friction of the abrasive on tbe plastic and a term that related to the energy required to deform the material plastically. A test similar to a Brine11hardness test was used to arrive at tbe &format&n energy of the 21 test materials.The more easily the materialdeforms in contactwitha particularabrasive,the betterthe abrasionresistance. 0 1997 Elsevier Science S.A. All rightsreserved. Keywordr: Abrasion resistaaee; Ptastia
1. IntYoduction
This study was prompted by an equipment problem that was occurring in the production of crystnilites of an inorganic compound used in the preparation
of photographic emulsions. The material was similar to ordinary table salt in size, appearance and compressive strength. The problem to be addressed by this study was the accelerated wear of plastic guides that directed the powder into a briqueting press. The powder was directed into compacting rollers with two parallel plastic plates of ultrahigh molecular weight polyethylene (UHMWPE) that were about 6 mm thick. The plates were only lasting about two weeks before they had to be replaced because of excessive abrasive wear from the powder rubbing on the plate as it entered a roller nip. The plates wem not particularly expensive, but replacing these plates was a very expensive operation. They were ‘buried’ well into the machine and their replacement required the loss of up to two days of production. The plates were made from plastics because metals may introduce contamination (wear debris) and because ceramics were too brittle to withstand that occurs in the plates. Our assignment was to conduct laboratory tests to determine whether another material would provide improved service life over the UHMWF’E.
theflexing
OW3-1648/97/$t7SlCI0 t997EtsevkrSeteaeeS.A.All ii&Is -ed PIlSOO43-1648(96)07346-4
Many studies have been conducted on the wear of plastics mated to metals, but the studies on the abrasion resistance of plastics are fewer and less conclusive. Most reports seem to recommend additional studies [ l-101. There are some standard tests for the abrasion resistance of plastics; one of the most widely used tests is the Taber Abraser test, ASTM D 4060 [ 111. This test involves abrasion of a flat sample of plastic with rotating rubber/abrasive wheels or with sandpaperwheels.Anothertest,ASTMD1242ProcedureA[12], uses loose abrasive distributed on a rotating platen. lhe loose abrasive is pressed into the rotating sample. ASTM D 1242 Procedure B [ 131 uses what is essentially a belt sander to abrade specimens on a conveyor that cycles in and out of contact with the sandpaper. ASTM G 56 [ 141 has been used to measure the abrasion resistance of plastic to paper. A bail rider is rubbed on a large paper-covered drum. ASTM G 132 [ 151 is an abrasion test in which the ends of vertical pins rub on a large sandpaper-covered drum. Although this test was developed for metals, the concept has been used by others to test plastics 161. All of these tests were considered as caudidates for a laboratory test to screen materials to address the above production problem. The test selected to rank materials was yet another ASTM test, ASTM G 65, the dry-sand rubber wheel abrasion test [ 171. This test, which is illustrated in Fig. 1, was developed
[
K.G. Budhki/
Wear 203-204 (1997) 302-309 Tabk
to rank the abrasion resistanceof both hard and soft metals. The material to be tested is line-contact loaded against a rubberwheel and silica sand is meteredinto the nip. Wear is assessed by measuring rye volume of material (by mass change) removed from the specimen in a fixed period of rubbing. This particulartest is one of the most used abrasion
I
Base polymer
wucr w%)/wiafollmnYit
Polyphmylene sume (PPS) PdYrtyrar m) epOxy (EP) PolyphmylenesuRide (PPS) Phedk (Pm PolytanauomeIhylaK (Pm?) Polyoxymhykne (PDM) Acrylwiuikbtadiadstyme (ABS) EPOXY@PI Phenolic (PF) Poly&rrtherlraonc (PEEK) PolylarpRuom(hylcae (Prm) Polyimide (PI) Polyethyknc (HDPE) Polyamtde.(PA) Polyurahane (PtJtt) 55A Polyamhane (PUR) 9OA Polyumhaae (PUR) 75D Polo (Putt) 85D P01yethykm (UHMwpe) Polycthykae wHMwm) cotltml
4o%cwbmtiber(cP) Now 4o%vmwa* PlpelcboPPcd LLU KG) wovmmnGd6berwDM) Er+=(cG) None wova dotb (cot) wovendotb (cot) None NODI? NOSE Nolv Me None NOD? NOtIC NaU oil Note
Referraametlls:AlSItype316ruinkn~zl(~92HRB).Stdlite 6B(hudnss43HRc).typeA2~l~(~6oHRc).
ferent normal forces and test durations. llte test procedure used in this study employeda 45 N force and a test duration of only 200 wheel revolutions (60 s). This procedurewas used successfullyby the ASTM G02.3 Abrasive Wear Subcommitteein 1990to conduct in&l&xatq tests on polymeric coatings. llte test mamriais included the plastics and elastomerslisted in Table 1. These materials were selected because of their successful performance in other plant operations. All test samplesweremadefrom bulk materials.The samples with wovenreinforcementsweretestedon the& flat face. Where possible, the abrasion tests were conducted on asmolded surfaces.Wear volumes were calculatedfrom mass changesduring the test. 3. T&results 2. Prncedure
The dry-sandrubber wheel testeruses a 228 mm diameter chlorobutyl rubber wheel (6OShoreA) as an abrader.‘l’hc ,wheel is 12.7 mm wide and runs at a single speed, 20.9 rad s- ‘. ‘Ibetest samplesare from 4 to 12.7mm in thickness, 25 mm wide, and 76 mm long. Tbe wear surfaces are the 25 X 76 mm*faces.The loading forceof the specimenagain-t the wheel can be up to 140N. The abrasive is 215 to 300 km silica. All test users purchase test sand from the same source. The send flow is in the range 30&4OCtg mitt-‘. There are threeproceduresin the ASTA4test standardthat employ dif-
The averagevolume lossesare comparedin Fig. 2. Typical wearseerson the plastic specimensare shownin Fig. 3. Only onematerial,a90ShonAdurometerpolyurrthaae.hadbetter wear resistancethan the UHMWPEplastic currently in use. An oil-lubricatedUHMWPEwas comparablein wear msistanceto the controlmaterialas were severalotherhardnesses of polyurethaneelastomer. Theglassandcarbonfi~r-ninforcedplssticshadtheworst abrasion resistanceof the reinforcedmaterials. Cotton and linen-reinforcedmaterialshad better abrasionresistancethan the glass-reinforcedmaterials. llte worst wear on a nonreinforcedplastic was on polystymne.lltere appeamdto be a preferredhardnessfor polyurethane-the top of the Shore
KG. Budi&i/
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was more abrasionresistantthanallof thetestmaterials,but itwasnotsignificantly moreabrasionresistantthan a9OShore polyurethane. These.testssuggestthatundercertainconditions,someplasticsandelastomerscanhaveparticleabraA
sion resistance comparable
with that of hard steels.
4. Dlscuselon
/ I I 0
i
100 200 200 400 so2 coo 700 a00 w8w -
h-2)
Fig. 2. Wear voIumesfor plastic caadidatlesin a modified ASTM G 65
abrssioa test.
4.1. Role of hardness What controls the abrasion resistance of plastics?Theclassic relationship for abrasive wear of metals is that wear is inversely proportional to the hardness of the metal [ 181. The harder the metal, the lower the abrasion rate. Unfortunately there is no single plastic hardness scale that is suitable for the wide range of plasticsieiastomers inch&d in this study. Nonetheless, Shore D and recoil hardness tests were conducted on the test plastics to explore the plausibility of a hardness-abrasion relationship. As shown in Fig. 4, the hardest plastic, glass-reinforced epoxy, was harder than U?MWF’E by a factor of about 1.4. but their abrasion rates varied by a factor of about 60. It was thought that the resilience cf the plastics may play some role in resisting indentation and scratching by hard Fig. 3. Typial appmma
ofwaarscmoaplnticrpedmenr.
A scale. Harder (Shore D) and softer materials did not wear as well. All of dte plastics tested, except UHMWPE and polyurethane, had lower abrasion resistance than the scft stainless steel reference material. A2 tool steel at 60 HRC
particles.Thereis a commercially availablehardnesstester thatuses thereboundvelocityof a spherical-ended sabotto measure the hardness of metals. The harder the metal, the greater the rebound velocity. This device was used on the test materials. As shown in Fig. 5, dtese rabound hardnesses did not show an apparent correlation with the abrasion volume losses.
K.G. Budin&/
War 203-204 (1997) 302-309
305
of plastics. The scratch hardness is measured by the wid*b of the furrow produced by the scratching stylus:
PB PPSG
EPvlu~ PPS*PTFE
H- Plb’
PF’NDU ABB POM.PTFE
where His the scratch hardness,
EP*OtitlMl PEEK P7FE-x
P is the stylus load, and b is
the furrow width.
The tes: o!astics were scratched for a distance of about 50
PI HDPE
stylus. Thefo~rerjuiredtoproduccthcscratch was&~~ded continuously and this force was converted into static and kinetic f&ion coefficients by dividing by the normal load. Typical scratches arc showo in Fig. 6.
PAMCB2 P7FE PUR-MD
-ks shown in Fig. 7, the mat&&
0
10 20
20
40
BD BO 70
80
96 100
Hv&r~ Bbom D Fig. 4 Shorehsrdsessof testmaterials
with ‘he highest scratch
hardness. such as the eooxies and ohenolics. had the ooorest abrasion resistance. T&o of the eiastomers; PUR-55’ A and PUR-90 A, did not scratch at all. The various plastics appeared ;o scratch by different mechanisms (Fig. 8). The. reinforced plastics displayed ragged edges on the scratch scratch furrows; some produced
furrows that varied in width suggesling what appeared to be a stick-slip type motion during surface deformation. Four metals and a cemented carbide were scratched with the plastics/elastomers to see whether they responded ‘properiy’ to hardness differences. The
Ps PPSGF EP*gluS PPti*PTFE PF*NDM ABB POMrP7FE EP’co1tott PEEK PTFE-CO PI HDPE PA’YOSZ PTFE PUR-SSD 31B BB PUR-76D PUS-SSA SIetIIU SB UHktWPE*o” U+iMWPE PUMOA A2 tool ?? wl 1 0
100200220400SO0BCO700
(ubnwy unn. 1 to loo01 Fie. 5. Reboundhardnessof testmaterials(basedon the recoilvelocitvof a endedsabot).
s&h.4
4.2. The role of scratch resistance
For malAy years the Taber Abraser has been used to rank the abrasion &stance of plastics [ I ,6.16,19]. As mentioned previously, this device can be used for two-body or threebody abrasion. This type of test is not unlike a scratch test where the abrader is & gbrasive-filled rubber or a sandpapercovered wheel. Fixed sharp particles are imposed on the test surface. In an attempt to simulate this type of material removal, scratch tests were conducted on the test plastics using a 60’ included angle diamond cone stylus with a tip radius of 200 Frn. Yamaguchi [ 161 and Briscoe et al. [20] suggested that scratch hardness is a factor in abrasive wear
..
__
Fig.dSentchaintcst~m~~~~S~~~ NC
ma@ifiatiw:
molybdenum
(a)
disttlfidc.
mthm-&forced
phmolic.
(b)
polyamide+
KG.
306
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Wear 203-204
(1997)
302-309
PF-NOM
cinct. These results suggest a friction effect. Stellite 6B did not scratcheven though it is much softer than the tool steel and cemented carbide. Overall, the scratch tests did not lead to a relationship that clearly related the scratch hardness to the observed volume losses in the abrasion tests. A second and thiid series of scratch tests were conducted with carbide styli with larger radii (2 and IO mm diameterballs). Only a
HWE PAwoSz PTPE Pun-am 3mss
none scratched with the IO mm stylus. The scratch tests were concluded and the scratch force data were evaluated for possible correlation with abrasion rates.
?Ps+eF EPyllvr PPS*TPE
few of the test materials scratchedwith the 2 nun stylus and
_._ __
PUIWW
4.3. Friction
Pun-s3A awlI* .a UHSSWPM UHMWPE Pua-wA
A2tooIti.ut
0
!
10
,
I
20
30
40
Fig. 7. Scratchhaninw of testmaterhIs.
scratch hardnesses of all of the plastics were lower than the hardnesses of the metals (Table 2). Tbe metals disuiayed scratch hardnesses that geaerallycorrelated with their hardness, but the relationship was not suc-
Fig.
considerations
The force measurements obtained in the scratch tests suggest that when a scratch stylus produces plastic deformation/ fracture of the surface, the force is probably a reflection of the energy required to produce deformation and removal of material. The friction ‘coefficients’ p measured in scratch tests on the test plastics are given in Table 3. These results suggest that me friction coefficient depends to a significant degree on the degree.of plastic deformation produced in the scratching operation. The wear test resuha did not correlate with any of these scratch test results. In the
8. Appearance of 600diamondscratches in varioustestmaterials.
K.G. Budieski/ Wear203-204 (1997) 302-309
307
Table 2 Scratchhe&err of four metatsaedcemeeted carbide Mated
Scratchhardness tkgmm-‘)
Vickersluh~ss
6061-T6 aluminum 3 I6 stainlesssteel Stellie 6B A2 tool steel Cementedcarbide(C2)
25 loo No scratch 400 10000
47 143 435 625 1854
Table 3 Ceeffic~ent of friction fi measuredin scratchtestson plasticswith various scratchstyli Styluslloedingmass
Avdnge p
60°diameedlKJOOg 2 mm diametercarbide/1000 g 10 mmdiametercubidc/1OOOg
0.58 G.21 0.14
I
0.14 0.2 0.13
rmge
0.2-0.9 0.2-I 0.05-0.7 Fig. 9. Fricuoecc&icienu (kinetic) of siticeseadstidiag011testms&iats.
’Standarddeviatiw.
4.4. Abrasion models the scratching material is 50-70 mesh silica sand. In an effort to determine whether scratching with silica makes a difference in scratch results. a scratch stylus with silica grams on the rubbing surface was fabricated. Friction coefficients were calculated from the force measurements made with a silica sand stylus. The steal stylus was 6 mm in diameterand a monolayerof sand was adheredto the hemisphericalend of the stylus with a cellulosic lacquer. As shown in Fig. 9, the friction coefficient of the plastic/sand couple did not correlate with the wear test results. The average friction coafficient for the sand/plastic couples was 0.32 (u=O.22. r=O.l to 1.1). Most test samples were permanently scratched by the sand stylus with a normal force produced by a 500 g mass on the stylus. The scratches and the friction coefficients were smaller than for the diamond stylus. These data suggest that the friction coefficient of this tribosystem includes a measure of deformation as proposed many years ago by Bikennan [ 211. abrasion test, however,
surface
Up to this point, the traditional tribological proper& of tiictionand hardnesshavefailedtocomlatewiththeabrasion results. Some additional plastic abrasion models from the litemtttre were reviewed (Table 4) for dition. One model that seemed to be quite reasonable was that proposed by Ranter et al. [22], where tlte rate of material removal was said to he inversely propcmtonal to the product of ‘stress and strain at rupture’. It was not clear how stress and strain at rupture could he ohtaincd from the G 65 abrasion blocks that were available as test materials. It was decided that the load/ deflection curve for plastic deformation of the surface by a hemispherical indenter may be a predictor of at least the ability of a material to deform plastically as in scratclSng. With this reasoning, the abrasion test coupons were ittdcnted to a fixed depth of I.25 mm with a 6 mm diameter htdenter in a universal tension/compression tester. The area under the load/deflection curve was integrated and it was considered
Table 4 Some of the models pmposedfor the abrasiveWW ‘Of plastics
a, yield strength;L load;E elasticmcdulus;Kc home
roughness: H hardness; W wear
w-@VPd p ebrasivewearfactor;N scmtchingefficiencyfactor;P normal load; d slidingdistance
Yamsgacni[I61
L Rmlcrctal.
w-p:;,,
[U]
p friction coefficient;L load;H hardness; mestressandstrainat mpnrre w-tans tee 6 dampingparameterobtainedfromdynamicmechanicalanalysis(DMA) Iv-- puta: Y p centactforce;u slidingvelocity; f time; a mughes
Blw I241
visweluJuthendBellow [zs] of surface; y surface energy
K.G. &din&i/
Wear203-204 (1997) 302-309
2. The hard, reinforced and filled engineering plastics had relatively poor abrasion resistance to silica sand in the three-body test used in this study. 3. ‘llte plastics tbat defcrm easily when acted on by loose abrasive particles (e.g. silica) are less likely to produce material removal by scratching/fracture.
6. Sumruury
0
Twenty-one plastics/elastomers were subjected to a threebody abrasion test to find an improved material to solve a production problem. The tests did not identifya material with 2
4
6
e
10
Raformmtion Factor(r=--aruB
i2
wu-1u(D~v.s,
Corralatlon&CtfiClMt
-0.7273 (deformation factor)
as a measure of the energy required to deform plastically the test material. Wear data were plotted vs. the reciprocal of this energy term as proposed by Ratner. l’bere was poor correlation. After manipulating the energy data in a variety of ways it was determined that the best correlation existed with a deformation factor that included the friction coefficirbii -f the sand on the test material: w-/&Q) where w is the abrasion rate, /.c is the friction coefficient of sand on the plastic surface, and Se is the area under the load/ deflection curve from tbe bag indent test. The lower the product of friction and deformation energy, thelower theabrasion.As shownin Fig. IO, the correlation is much less than perfect (correlation of 0.73), but this correlation is certainly better than the correlation obtained with the hardness and scratch parameters. This relationship also seems reasonable. The deformation factor for elastomers is low and they have good abrasion resistance. The same can he said aboutthe UHMWPRs. The contribution of friction
coefficientis thoughtto be thatlow frictioncanreducewear becausethe abrasive(in thme-bodyabrasion)is less likely to dig in and form a plow mark or scratch. However,a highfriction material such as PUR has good abrasion resistance because. the abrasive grains tend to roll through the wear interface rather than hecotue fixed on one member and plow a furrow. The correlation seemed plausible and further work with the other models was decided against.
5. Conclusions
1. Polyurethane with a durometer of 90Shore A has more abrasionresistanceto AFS 50-70 silica in a three-body abrasion test than WE.
the desiredIO-foldincreasein abrasionlife. ‘Ihe improvementwas only of the order of two times, much less tban anticipated. However, this study reconfirmed that UHMWPE and high Shore A polyurethanes have better abrasion resistance than most other plastics and elastomers. The explanation for their excellent abrasion resistance appears to be theirabilityto deformeasilyandtheir favorable
frictioncharacteristics againstmost other materials.The modelsuggestedby thisstudyneedsmoredevelopment, but it is feltthatit mayhavecorrelated betterif this study did not systems (glass-nininclude suchdiverse plastic/elastomer forced composites, injection moldable commodity plastics, carbon fiber-reinforcc.$t engineering plastics, and elrstomers) . They were not from similar groups or families. Future studies need a less diversegroup of test materials. Also, it appears that the abrasionmodel shouldinclude a fracture toughness term since the more brittle materials lost material by brittle fracture in the scratch tests. The elastomers may need a term relating to tbeii rcsiliencc (branching) and the neat materials may need a more well defined friction term. In
1981, BartenevandLavrcntcv[ 191 concludedthe chapter on abrasive wear in their book on the friction and wear of polymers with the statement: “To the present there is no theory of abrasive wear of polymers.” This situation appears to prevail, and it may not be possible IO improve on tbe UHhIWPEs and polyurctbanes for abrasionresistanceuntil a bettermodelis deduced.
References
[II J.M. Tborp. Abrasive wear ofsomecommeminl polymers. Tritd. hr.. I5 ( 1982)59-68. 1211. Larsen-Basx and T. Abmad. Slurry abrasion of polymers under simuktcd submarine conditions. Wear, 122 (2) (1988) 131-149. [31 D.H. Buckky and R.L. Johnson. Friction wear and decomposition mechanisms for variouspolymercompcsitesin vacuumto IO+ mm Kg.NASATech.NorcD-2373, Dccmber 1963 (NASA, Washington, DC). f41 G.F. Cok and R. Travksco. Wear by paper on nylon matrix composites. In L.H. Lee (cd.), Adwnces in Polymer Friction and Wear. Vol. SB. PIcnum New York, 1974, pp. 689-702. IS1 I.M. Hu~cbings. Trih&~.CRCPress.London. 1992. pp. 156-162. t6108.PowerandJ.H.Dumbleton,Animprovedmethodfordemrmining thewearof polymeric coadngs, Wear.25 ( 1973) 373-380.
KG. &din&i/
Wear203-204 (1997) 302-309
171 B.J. Briscoeand P.D. Evans.Tk influence of asperity defomwion conditions on the abmive wear of imdii PIPE. lo K.C. Ludma (cd.). Wear of Maferialr 1989, Vol. 2. Amclican sociay of Mcchaniul Enghan. New York, 1989. pp. 449457. [El R.KamkoaadE.~Microwearpoceuerofpolymcrrurf~. wear, 162-164 ( 1993) 370-377. I91 D.G.BellowandN.S. Virw~alll.An~ysiroflhcwearofpolymm, Wear, 162-164 (1993) 1048-1053. [IO] S. Bahadur md D. Don& tbmdation of the model for opumal pmpmtiw of Clkr in polymer for weat n&tatce. In K.C. L&ma (cd.), Wear of Mamialr 1991. Anhan Sociay of Mechanical Engineers. New York. 1991. pp. 177-187. [lIlASTMD4060.T~me~forabrasion~sisulneeoforganic~ngs by Taivr Abrascr, Amacan Society for Testing and Materials. W. Conshohocken. PA. [ 121 ASH D 1242. Test method for resistance of plastic mawi& to abrasion. prowdun A. American Society for Testingand Materials. W. Conshohockcn, PA. 1131 ASTM D 1242, Test method for raistance of plastic materials to abrasion, pucedue B. AmericanSocietyfor Testing and Mat&d.s. W. Conhhocken, PA. ( 14) ASTM G 56. Test maid for the abrasiveness of ink-impregnated fabric prinlerribbons. Amriran Sac&y forTestinS and Mauials. W. Conshohocken. PA. 1151 AsrMGI32.Sundsrdttstmahodforp~abrasiontffting.Amrican Saciity for Testing and Materials. W. Conshohocken, PA.
309
( I61 Y. Yanquchi. Tribo&y OfPJasticMamiaJs, E&via, 1990.9.125.
hmadq
[l7lASTMG65,PmcticeforconduchSdry-smdrubbu~~ W.Is. Americ=m Soday for Testing and M*cr*lr. W. coarhohodrto. PA. Rabinwin. Friction and Wear ~Mat&7Jr. Wii. Nnv Y& 1966. p. 168. f I91 G.M. Bartcnev and V.V. Lawewcv. in KC. Luknu sld LR. h (edr.). Fricdun and Wearo/PoJymers.Ekvier. Amsmbm, 1981. p. 239. [ZOI B.J. Briscoe.S.K.BerinamdS.S.Paoesw.ll~scrachhudacrrlod hictionofaMArigidm*Q*1.~~C.LUdCm;LUSdR.G.B8~(~.), wearof MOI.zriaLv 1991. Amaiaa .9ociuy oft&&&alE, NewYork. 1991.pp.451~56. (211 J.J. Bikerman.The nahnt of polyma friction. In L.H. Lee (cd.). PolymerScienceand Techwlo8y. Vol. 5%Pkaum, New Yak. 1974. p. 168.
t181 E.
t221 S.B. hmer. 1.1. Pattwmva, O.V. RaJyakaidt PIllsI... ( M4) 37.
and E.G. Lw. Sov.
(231~. Hombogen.Tltcmkoffmctuetoughcssinthe~ofnwb, wear. 33 ( 1975) 251-259. [Xl PJ. Blau. Friction Schce awJ Techmb~. k4me1 Jkkka. Ncv Yolk, 1996,p. 194. WI N. Viswanathand D.G. BcUow.Developmentof 60 apai~~ fa the wearof polymers.Wear. 181-183 ( 1995) 42-49.
Wear 203-204 ( 1997) 302-309
Resistance to particle abrasion of selected plastics Kenaeth G. Budinski EastmanKodakCornpony,3177Latto Road,Suite146, Rochrsrer.NY 14612-3092. USA
Ahstnwt Accelerated abrasive wear of plastic parts in a piece of productionmachineryprompteda laboratorystudy to find a materialwith better abrasionresistance.The abrasionoccurredin a machinethatcompacted‘sand-like’pardclesof an inorganic compound.Theabrasionresistance of a -wide variety of plastics and different durometer polyurethanes (21 materials) was tested with a modification of the ASTM dry-sand rubber wheel three-budy abrasion test. Only one material, a polyurethane, bad better abrasion r&stance tban tbe material tbat was currently in use. Hardness, friction and scratch tests were conducted on the test materials to try to understand the mle of material properties in this type of abrasion. None of these correlatedwith the wear data.Rcvious investigatorsof plasticabrasionrelatedabrasionresistanceto the fracture energy and friction.The wear data developed in this study did not cormlate with the specific modal pmposed by Ratner. However, it was possible to obtain a reasonablecorrelationwith a deformationfactor that included the friction of the abrasive on tbe plastic and a term that related to the energy required to deform the material plastically. A test similar to a Brine11hardness test was used to arrive at tbe &format&n energy of the 21 test materials.The more easily the materialdeforms in contactwitha particularabrasive,the betterthe abrasionresistance. 0 1997 Elsevier Science S.A. All rightsreserved. Keywordr: Abrasion resistaaee; Ptastia
1. IntYoduction
This study was prompted by an equipment problem that was occurring in the production of crystnilites of an inorganic compound used in the preparation
of photographic emulsions. The material was similar to ordinary table salt in size, appearance and compressive strength. The problem to be addressed by this study was the accelerated wear of plastic guides that directed the powder into a briqueting press. The powder was directed into compacting rollers with two parallel plastic plates of ultrahigh molecular weight polyethylene (UHMWPE) that were about 6 mm thick. The plates were only lasting about two weeks before they had to be replaced because of excessive abrasive wear from the powder rubbing on the plate as it entered a roller nip. The plates wem not particularly expensive, but replacing these plates was a very expensive operation. They were ‘buried’ well into the machine and their replacement required the loss of up to two days of production. The plates were made from plastics because metals may introduce contamination (wear debris) and because ceramics were too brittle to withstand that occurs in the plates. Our assignment was to conduct laboratory tests to determine whether another material would provide improved service life over the UHMWF’E.
theflexing
OW3-1648/97/$t7SlCI0 t997EtsevkrSeteaeeS.A.All ii&Is -ed PIlSOO43-1648(96)07346-4
Many studies have been conducted on the wear of plastics mated to metals, but the studies on the abrasion resistance of plastics are fewer and less conclusive. Most reports seem to recommend additional studies [ l-101. There are some standard tests for the abrasion resistance of plastics; one of the most widely used tests is the Taber Abraser test, ASTM D 4060 [ 111. This test involves abrasion of a flat sample of plastic with rotating rubber/abrasive wheels or with sandpaperwheels.Anothertest,ASTMD1242ProcedureA[12], uses loose abrasive distributed on a rotating platen. lhe loose abrasive is pressed into the rotating sample. ASTM D 1242 Procedure B [ 131 uses what is essentially a belt sander to abrade specimens on a conveyor that cycles in and out of contact with the sandpaper. ASTM G 56 [ 141 has been used to measure the abrasion resistance of plastic to paper. A bail rider is rubbed on a large paper-covered drum. ASTM G 132 [ 151 is an abrasion test in which the ends of vertical pins rub on a large sandpaper-covered drum. Although this test was developed for metals, the concept has been used by others to test plastics 161. All of these tests were considered as caudidates for a laboratory test to screen materials to address the above production problem. The test selected to rank materials was yet another ASTM test, ASTM G 65, the dry-sand rubber wheel abrasion test [ 171. This test, which is illustrated in Fig. 1, was developed
[
K.G. Budhki/
Wear 203-204 (1997) 302-309 Tabk
to rank the abrasion resistanceof both hard and soft metals. The material to be tested is line-contact loaded against a rubberwheel and silica sand is meteredinto the nip. Wear is assessed by measuring rye volume of material (by mass change) removed from the specimen in a fixed period of rubbing. This particulartest is one of the most used abrasion
I
Base polymer
wucr w%)/wiafollmnYit
Polyphmylene sume (PPS) PdYrtyrar m) epOxy (EP) PolyphmylenesuRide (PPS) Phedk (Pm PolytanauomeIhylaK (Pm?) Polyoxymhykne (PDM) Acrylwiuikbtadiadstyme (ABS) EPOXY@PI Phenolic (PF) Poly&rrtherlraonc (PEEK) PolylarpRuom(hylcae (Prm) Polyimide (PI) Polyethyknc (HDPE) Polyamtde.(PA) Polyurahane (PtJtt) 55A Polyamhane (PUR) 9OA Polyumhaae (PUR) 75D Polo (Putt) 85D P01yethykm (UHMwpe) Polycthykae wHMwm) cotltml
4o%cwbmtiber(cP) Now 4o%vmwa* PlpelcboPPcd LLU KG) wovmmnGd6berwDM) Er+=(cG) None wova dotb (cot) wovendotb (cot) None NODI? NOSE Nolv Me None NOD? NOtIC NaU oil Note
Referraametlls:AlSItype316ruinkn~zl(~92HRB).Stdlite 6B(hudnss43HRc).typeA2~l~(~6oHRc).
ferent normal forces and test durations. llte test procedure used in this study employeda 45 N force and a test duration of only 200 wheel revolutions (60 s). This procedurewas used successfullyby the ASTM G02.3 Abrasive Wear Subcommitteein 1990to conduct in&l&xatq tests on polymeric coatings. llte test mamriais included the plastics and elastomerslisted in Table 1. These materials were selected because of their successful performance in other plant operations. All test samplesweremadefrom bulk materials.The samples with wovenreinforcementsweretestedon the& flat face. Where possible, the abrasion tests were conducted on asmolded surfaces.Wear volumes were calculatedfrom mass changesduring the test. 3. T&results 2. Prncedure
The dry-sandrubber wheel testeruses a 228 mm diameter chlorobutyl rubber wheel (6OShoreA) as an abrader.‘l’hc ,wheel is 12.7 mm wide and runs at a single speed, 20.9 rad s- ‘. ‘Ibetest samplesare from 4 to 12.7mm in thickness, 25 mm wide, and 76 mm long. Tbe wear surfaces are the 25 X 76 mm*faces.The loading forceof the specimenagain-t the wheel can be up to 140N. The abrasive is 215 to 300 km silica. All test users purchase test sand from the same source. The send flow is in the range 30&4OCtg mitt-‘. There are threeproceduresin the ASTA4test standardthat employ dif-
The averagevolume lossesare comparedin Fig. 2. Typical wearseerson the plastic specimensare shownin Fig. 3. Only onematerial,a90ShonAdurometerpolyurrthaae.hadbetter wear resistancethan the UHMWPEplastic currently in use. An oil-lubricatedUHMWPEwas comparablein wear msistanceto the controlmaterialas were severalotherhardnesses of polyurethaneelastomer. Theglassandcarbonfi~r-ninforcedplssticshadtheworst abrasion resistanceof the reinforcedmaterials. Cotton and linen-reinforcedmaterialshad better abrasionresistancethan the glass-reinforcedmaterials. llte worst wear on a nonreinforcedplastic was on polystymne.lltere appeamdto be a preferredhardnessfor polyurethane-the top of the Shore
KG. Budi&i/
c :
Wear 203-204
3
(1997i 302409
was more abrasionresistantthanallof thetestmaterials,but itwasnotsignificantly moreabrasionresistantthan a9OShore polyurethane. These.testssuggestthatundercertainconditions,someplasticsandelastomerscanhaveparticleabraA
sion resistance comparable
with that of hard steels.
4. Dlscuselon
/ I I 0
i
100 200 200 400 so2 coo 700 a00 w8w -
h-2)
Fig. 2. Wear voIumesfor plastic caadidatlesin a modified ASTM G 65
abrssioa test.
4.1. Role of hardness What controls the abrasion resistance of plastics?Theclassic relationship for abrasive wear of metals is that wear is inversely proportional to the hardness of the metal [ 181. The harder the metal, the lower the abrasion rate. Unfortunately there is no single plastic hardness scale that is suitable for the wide range of plasticsieiastomers inch&d in this study. Nonetheless, Shore D and recoil hardness tests were conducted on the test plastics to explore the plausibility of a hardness-abrasion relationship. As shown in Fig. 4, the hardest plastic, glass-reinforced epoxy, was harder than U?MWF’E by a factor of about 1.4. but their abrasion rates varied by a factor of about 60. It was thought that the resilience cf the plastics may play some role in resisting indentation and scratching by hard Fig. 3. Typial appmma
ofwaarscmoaplnticrpedmenr.
A scale. Harder (Shore D) and softer materials did not wear as well. All of dte plastics tested, except UHMWPE and polyurethane, had lower abrasion resistance than the scft stainless steel reference material. A2 tool steel at 60 HRC
particles.Thereis a commercially availablehardnesstester thatuses thereboundvelocityof a spherical-ended sabotto measure the hardness of metals. The harder the metal, the greater the rebound velocity. This device was used on the test materials. As shown in Fig. 5, dtese rabound hardnesses did not show an apparent correlation with the abrasion volume losses.
K.G. Budin&/
War 203-204 (1997) 302-309
305
of plastics. The scratch hardness is measured by the wid*b of the furrow produced by the scratching stylus:
PB PPSG
EPvlu~ PPS*PTFE
H- Plb’
PF’NDU ABB POM.PTFE
where His the scratch hardness,
EP*OtitlMl PEEK P7FE-x
P is the stylus load, and b is
the furrow width.
The tes: o!astics were scratched for a distance of about 50
PI HDPE
stylus. Thefo~rerjuiredtoproduccthcscratch was&~~ded continuously and this force was converted into static and kinetic f&ion coefficients by dividing by the normal load. Typical scratches arc showo in Fig. 6.
PAMCB2 P7FE PUR-MD
-ks shown in Fig. 7, the mat&&
0
10 20
20
40
BD BO 70
80
96 100
Hv&r~ Bbom D Fig. 4 Shorehsrdsessof testmaterials
with ‘he highest scratch
hardness. such as the eooxies and ohenolics. had the ooorest abrasion resistance. T&o of the eiastomers; PUR-55’ A and PUR-90 A, did not scratch at all. The various plastics appeared ;o scratch by different mechanisms (Fig. 8). The. reinforced plastics displayed ragged edges on the scratch scratch furrows; some produced
furrows that varied in width suggesling what appeared to be a stick-slip type motion during surface deformation. Four metals and a cemented carbide were scratched with the plastics/elastomers to see whether they responded ‘properiy’ to hardness differences. The
Ps PPSGF EP*gluS PPti*PTFE PF*NDM ABB POMrP7FE EP’co1tott PEEK PTFE-CO PI HDPE PA’YOSZ PTFE PUR-SSD 31B BB PUR-76D PUS-SSA SIetIIU SB UHktWPE*o” U+iMWPE PUMOA A2 tool ?? wl 1 0
100200220400SO0BCO700
(ubnwy unn. 1 to loo01 Fie. 5. Reboundhardnessof testmaterials(basedon the recoilvelocitvof a endedsabot).
s&h.4
4.2. The role of scratch resistance
For malAy years the Taber Abraser has been used to rank the abrasion &stance of plastics [ I ,6.16,19]. As mentioned previously, this device can be used for two-body or threebody abrasion. This type of test is not unlike a scratch test where the abrader is & gbrasive-filled rubber or a sandpapercovered wheel. Fixed sharp particles are imposed on the test surface. In an attempt to simulate this type of material removal, scratch tests were conducted on the test plastics using a 60’ included angle diamond cone stylus with a tip radius of 200 Frn. Yamaguchi [ 161 and Briscoe et al. [20] suggested that scratch hardness is a factor in abrasive wear
..
__
Fig.dSentchaintcst~m~~~~S~~~ NC
ma@ifiatiw:
molybdenum
(a)
disttlfidc.
mthm-&forced
phmolic.
(b)
polyamide+
KG.
306
Eudinski/
Wear 203-204
(1997)
302-309
PF-NOM
cinct. These results suggest a friction effect. Stellite 6B did not scratcheven though it is much softer than the tool steel and cemented carbide. Overall, the scratch tests did not lead to a relationship that clearly related the scratch hardness to the observed volume losses in the abrasion tests. A second and thiid series of scratch tests were conducted with carbide styli with larger radii (2 and IO mm diameterballs). Only a
HWE PAwoSz PTPE Pun-am 3mss
none scratched with the IO mm stylus. The scratch tests were concluded and the scratch force data were evaluated for possible correlation with abrasion rates.
?Ps+eF EPyllvr PPS*TPE
few of the test materials scratchedwith the 2 nun stylus and
_._ __
PUIWW
4.3. Friction
Pun-s3A awlI* .a UHSSWPM UHMWPE Pua-wA
A2tooIti.ut
0
!
10
,
I
20
30
40
Fig. 7. Scratchhaninw of testmaterhIs.
scratch hardnesses of all of the plastics were lower than the hardnesses of the metals (Table 2). Tbe metals disuiayed scratch hardnesses that geaerallycorrelated with their hardness, but the relationship was not suc-
Fig.
considerations
The force measurements obtained in the scratch tests suggest that when a scratch stylus produces plastic deformation/ fracture of the surface, the force is probably a reflection of the energy required to produce deformation and removal of material. The friction ‘coefficients’ p measured in scratch tests on the test plastics are given in Table 3. These results suggest that me friction coefficient depends to a significant degree on the degree.of plastic deformation produced in the scratching operation. The wear test resuha did not correlate with any of these scratch test results. In the
8. Appearance of 600diamondscratches in varioustestmaterials.
K.G. Budieski/ Wear203-204 (1997) 302-309
307
Table 2 Scratchhe&err of four metatsaedcemeeted carbide Mated
Scratchhardness tkgmm-‘)
Vickersluh~ss
6061-T6 aluminum 3 I6 stainlesssteel Stellie 6B A2 tool steel Cementedcarbide(C2)
25 loo No scratch 400 10000
47 143 435 625 1854
Table 3 Ceeffic~ent of friction fi measuredin scratchtestson plasticswith various scratchstyli Styluslloedingmass
Avdnge p
60°diameedlKJOOg 2 mm diametercarbide/1000 g 10 mmdiametercubidc/1OOOg
0.58 G.21 0.14
I
0.14 0.2 0.13
rmge
0.2-0.9 0.2-I 0.05-0.7 Fig. 9. Fricuoecc&icienu (kinetic) of siticeseadstidiag011testms&iats.
’Standarddeviatiw.
4.4. Abrasion models the scratching material is 50-70 mesh silica sand. In an effort to determine whether scratching with silica makes a difference in scratch results. a scratch stylus with silica grams on the rubbing surface was fabricated. Friction coefficients were calculated from the force measurements made with a silica sand stylus. The steal stylus was 6 mm in diameterand a monolayerof sand was adheredto the hemisphericalend of the stylus with a cellulosic lacquer. As shown in Fig. 9, the friction coefficient of the plastic/sand couple did not correlate with the wear test results. The average friction coafficient for the sand/plastic couples was 0.32 (u=O.22. r=O.l to 1.1). Most test samples were permanently scratched by the sand stylus with a normal force produced by a 500 g mass on the stylus. The scratches and the friction coefficients were smaller than for the diamond stylus. These data suggest that the friction coefficient of this tribosystem includes a measure of deformation as proposed many years ago by Bikennan [ 211. abrasion test, however,
surface
Up to this point, the traditional tribological proper& of tiictionand hardnesshavefailedtocomlatewiththeabrasion results. Some additional plastic abrasion models from the litemtttre were reviewed (Table 4) for dition. One model that seemed to be quite reasonable was that proposed by Ranter et al. [22], where tlte rate of material removal was said to he inversely propcmtonal to the product of ‘stress and strain at rupture’. It was not clear how stress and strain at rupture could he ohtaincd from the G 65 abrasion blocks that were available as test materials. It was decided that the load/ deflection curve for plastic deformation of the surface by a hemispherical indenter may be a predictor of at least the ability of a material to deform plastically as in scratclSng. With this reasoning, the abrasion test coupons were ittdcnted to a fixed depth of I.25 mm with a 6 mm diameter htdenter in a universal tension/compression tester. The area under the load/deflection curve was integrated and it was considered
Table 4 Some of the models pmposedfor the abrasiveWW ‘Of plastics
a, yield strength;L load;E elasticmcdulus;Kc home
roughness: H hardness; W wear
w-@VPd p ebrasivewearfactor;N scmtchingefficiencyfactor;P normal load; d slidingdistance
Yamsgacni[I61
L Rmlcrctal.
w-p:;,,
[U]
p friction coefficient;L load;H hardness; mestressandstrainat mpnrre w-tans tee 6 dampingparameterobtainedfromdynamicmechanicalanalysis(DMA) Iv-- puta: Y p centactforce;u slidingvelocity; f time; a mughes
Blw I241
visweluJuthendBellow [zs] of surface; y surface energy
K.G. &din&i/
Wear203-204 (1997) 302-309
2. The hard, reinforced and filled engineering plastics had relatively poor abrasion resistance to silica sand in the three-body test used in this study. 3. ‘llte plastics tbat defcrm easily when acted on by loose abrasive particles (e.g. silica) are less likely to produce material removal by scratching/fracture.
6. Sumruury
0
Twenty-one plastics/elastomers were subjected to a threebody abrasion test to find an improved material to solve a production problem. The tests did not identifya material with 2
4
6
e
10
Raformmtion Factor(r=--aruB
i2
wu-1u(D~v.s,
Corralatlon&CtfiClMt
-0.7273 (deformation factor)
as a measure of the energy required to deform plastically the test material. Wear data were plotted vs. the reciprocal of this energy term as proposed by Ratner. l’bere was poor correlation. After manipulating the energy data in a variety of ways it was determined that the best correlation existed with a deformation factor that included the friction coefficirbii -f the sand on the test material: w-/&Q) where w is the abrasion rate, /.c is the friction coefficient of sand on the plastic surface, and Se is the area under the load/ deflection curve from tbe bag indent test. The lower the product of friction and deformation energy, thelower theabrasion.As shownin Fig. IO, the correlation is much less than perfect (correlation of 0.73), but this correlation is certainly better than the correlation obtained with the hardness and scratch parameters. This relationship also seems reasonable. The deformation factor for elastomers is low and they have good abrasion resistance. The same can he said aboutthe UHMWPRs. The contribution of friction
coefficientis thoughtto be thatlow frictioncanreducewear becausethe abrasive(in thme-bodyabrasion)is less likely to dig in and form a plow mark or scratch. However,a highfriction material such as PUR has good abrasion resistance because. the abrasive grains tend to roll through the wear interface rather than hecotue fixed on one member and plow a furrow. The correlation seemed plausible and further work with the other models was decided against.
5. Conclusions
1. Polyurethane with a durometer of 90Shore A has more abrasionresistanceto AFS 50-70 silica in a three-body abrasion test than WE.
the desiredIO-foldincreasein abrasionlife. ‘Ihe improvementwas only of the order of two times, much less tban anticipated. However, this study reconfirmed that UHMWPE and high Shore A polyurethanes have better abrasion resistance than most other plastics and elastomers. The explanation for their excellent abrasion resistance appears to be theirabilityto deformeasilyandtheir favorable
frictioncharacteristics againstmost other materials.The modelsuggestedby thisstudyneedsmoredevelopment, but it is feltthatit mayhavecorrelated betterif this study did not systems (glass-nininclude suchdiverse plastic/elastomer forced composites, injection moldable commodity plastics, carbon fiber-reinforcc.$t engineering plastics, and elrstomers) . They were not from similar groups or families. Future studies need a less diversegroup of test materials. Also, it appears that the abrasionmodel shouldinclude a fracture toughness term since the more brittle materials lost material by brittle fracture in the scratch tests. The elastomers may need a term relating to tbeii rcsiliencc (branching) and the neat materials may need a more well defined friction term. In
1981, BartenevandLavrcntcv[ 191 concludedthe chapter on abrasive wear in their book on the friction and wear of polymers with the statement: “To the present there is no theory of abrasive wear of polymers.” This situation appears to prevail, and it may not be possible IO improve on tbe UHhIWPEs and polyurctbanes for abrasionresistanceuntil a bettermodelis deduced.
References
[II J.M. Tborp. Abrasive wear ofsomecommeminl polymers. Tritd. hr.. I5 ( 1982)59-68. 1211. Larsen-Basx and T. Abmad. Slurry abrasion of polymers under simuktcd submarine conditions. Wear, 122 (2) (1988) 131-149. [31 D.H. Buckky and R.L. Johnson. Friction wear and decomposition mechanisms for variouspolymercompcsitesin vacuumto IO+ mm Kg.NASATech.NorcD-2373, Dccmber 1963 (NASA, Washington, DC). f41 G.F. Cok and R. Travksco. Wear by paper on nylon matrix composites. In L.H. Lee (cd.), Adwnces in Polymer Friction and Wear. Vol. SB. PIcnum New York, 1974, pp. 689-702. IS1 I.M. Hu~cbings. Trih&~.CRCPress.London. 1992. pp. 156-162. t6108.PowerandJ.H.Dumbleton,Animprovedmethodfordemrmining thewearof polymeric coadngs, Wear.25 ( 1973) 373-380.
KG. &din&i/
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171 B.J. Briscoeand P.D. Evans.Tk influence of asperity defomwion conditions on the abmive wear of imdii PIPE. lo K.C. Ludma (cd.). Wear of Maferialr 1989, Vol. 2. Amclican sociay of Mcchaniul Enghan. New York, 1989. pp. 449457. [El R.KamkoaadE.~Microwearpoceuerofpolymcrrurf~. wear, 162-164 ( 1993) 370-377. I91 D.G.BellowandN.S. Virw~alll.An~ysiroflhcwearofpolymm, Wear, 162-164 (1993) 1048-1053. [IO] S. Bahadur md D. Don& tbmdation of the model for opumal pmpmtiw of Clkr in polymer for weat n&tatce. In K.C. L&ma (cd.), Wear of Mamialr 1991. Anhan Sociay of Mechanical Engineers. New York. 1991. pp. 177-187. [lIlASTMD4060.T~me~forabrasion~sisulneeoforganic~ngs by Taivr Abrascr, Amacan Society for Testing and Materials. W. Conshohocken. PA. [ 121 ASH D 1242. Test method for resistance of plastic mawi& to abrasion. prowdun A. American Society for Testingand Materials. W. Conshohockcn, PA. 1131 ASTM D 1242, Test method for raistance of plastic materials to abrasion, pucedue B. AmericanSocietyfor Testing and Mat&d.s. W. Conhhocken, PA. ( 14) ASTM G 56. Test maid for the abrasiveness of ink-impregnated fabric prinlerribbons. Amriran Sac&y forTestinS and Mauials. W. Conshohocken. PA. 1151 AsrMGI32.Sundsrdttstmahodforp~abrasiontffting.Amrican Saciity for Testing and Materials. W. Conshohocken, PA.
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hmadq
[l7lASTMG65,PmcticeforconduchSdry-smdrubbu~~ W.Is. Americ=m Soday for Testing and M*cr*lr. W. coarhohodrto. PA. Rabinwin. Friction and Wear ~Mat&7Jr. Wii. Nnv Y& 1966. p. 168. f I91 G.M. Bartcnev and V.V. Lawewcv. in KC. Luknu sld LR. h (edr.). Fricdun and Wearo/PoJymers.Ekvier. Amsmbm, 1981. p. 239. [ZOI B.J. Briscoe.S.K.BerinamdS.S.Paoesw.ll~scrachhudacrrlod hictionofaMArigidm*Q*1.~~C.LUdCm;LUSdR.G.B8~(~.), wearof MOI.zriaLv 1991. Amaiaa .9ociuy oft&&&alE, NewYork. 1991.pp.451~56. (211 J.J. Bikerman.The nahnt of polyma friction. In L.H. Lee (cd.). PolymerScienceand Techwlo8y. Vol. 5%Pkaum, New Yak. 1974. p. 168.
t181 E.
t221 S.B. hmer. 1.1. Pattwmva, O.V. RaJyakaidt PIllsI... ( M4) 37.
and E.G. Lw. Sov.
(231~. Hombogen.Tltcmkoffmctuetoughcssinthe~ofnwb, wear. 33 ( 1975) 251-259. [Xl PJ. Blau. Friction Schce awJ Techmb~. k4me1 Jkkka. Ncv Yolk, 1996,p. 194. WI N. Viswanathand D.G. BcUow.Developmentof 60 apai~~ fa the wearof polymers.Wear. 181-183 ( 1995) 42-49.