Tribological properties of polytetrafluoroethylene-based composite in different lubricant media

WEAR ELSEVIER

Wear 196(1996)164--170

Tribological properties of polytetrafluoroethylene-based composite in different lubricant media Zhao-ZhuZhang,Wei-ChangShen,Wei-MinLiu,Qun-JiXue,Tong-ShengLi L~oratoryofSolid.~brication,Lan~a in~tituteofChemicalPhysics,ChineseAcademyof Sciences,Lanzhou730000,PR China Received24 July1995;accepted27 November1995

Abslract A kind of pelytetrafluomethylene(PTFE) based multilayerself-lubricatingcompositematerial,madeof a steel backing,a alnteredporous bronze middlelayer and a surfacelayer consistingof a mixtureof PTFE and Pb powders,was prepared.The frictionand wear propertiesas well as the limitingPV (pressuretimesvelocity) valuesof the PTFEcompositeslidingagainststainlesssteel with lubricationof 10* gasoline engine lubricantoil, glyceroland triethanolaminewere studied.An MPV-1500 frictiontester with a steel axis rotatingon a sliding bearing was used.The frictioncoefficientwas determinedby measuringthe frictiontorqueduringtesting,whilethe wear was detectedby the weight loss of the P'I"FEcompositebearingafter each test. The worn surfacesof ~ compositeand the steel counterfacewere then examinedby usingelectronprobe microanalysis(EPMA). Experimentalresultsshowthat the frictionaM wear propertiesas wellas the limitingPV value of PTFE composite can be greatly improvedusing the above lubricant media. The limiting PV value of this PTFE composite is above 120 MPa m s", under lubricationof these kinds of lubricantmedia, whichis about I00 times that of pure FIFE in dry frictionconditions. EPMA investigationsshow that the wear and the transferof FIFE compositeonto the steel cotmterfacewere significantlyreduced,but still took place. geywords: PTFE-basedmultilayerself-lubdcalmgcomposite;Oillubrication;Frictionandwear;Ffi~ionalsurfaces;EPMA

I. Introduction

Polytetrafluorocthylene ( P T ~ ) based self-lubricating composites have been widely used in dry friction conditions, but up to now, veryfe'~resultson the tribologicalcharacteristicsof FIFE-based self-lubricatingcomposites under oil lubricationhave been reported,though the frictionand wear propertiesof FIFE-based composites in aqueous (water) environments have been studiedby many workers [I-5]. It is known that~ composites wear much more in water than in air [6--8].So investigationof the tribologiealprop. ertiesof ~ compositesunder oillubricationisbecoming more and more importantand urgent. In thispaper,thefrictionand wear prol~rtiesas wellas the limitingPV (pressuretimesvelocity) valuesof a ~ compositounder differentlubricantmedia arereported.The worn surfacesof the FrFE composite and the steelcounterface were also examined by using EPMA. These studieswere conductedwith theaim of providingsome practicalguidance for the use of thisPTFE compositeunder differentlubricant media. 0043-1~8/%1515.00© 1996Elsevie~,%'ienceS,A.All dgMsreserved SSD10043-1648( 95 ) 06892-9

2. Experimental details In this experiment, an MPVo1500 friction tester with a steel axis rotating on a sliding bearing was used as shown in Fig. 1. The load was added gradually at various constant sliding speeds: the maximum load applied was 14 700 N (1500 kg) and the maximum speed used was 4 m s- ~. The material of the steel axis is a 45* carbon steel, and the chemical composition is given in Table 1. The diameter of the steel axis was 35.95 ram, and its length was 42.00 mm. Before each test, the surface of the steel axis was polished with number 900 grade SiC abrasive paper, washed in acetone and dried

A

O

P

Fig. 1. Contactingmodeof the friction pair: A, axis of ¢ad~n steel;B, bering bush(testedsample);P, appliedload; V, speed.

Z-Z 72umgeta!. / We¢.r196(1996) 164-170

165

T~le I Chemical compositionof 45" carbon steel (Wt.%) C

Mn

Si

Cr

P

S

Ft,

0.420-0.500

0.500-o.g00

0.170-0.370

< 0.250

<0.040

~g0.040

Remaiader

Table 2 Main propertiesof 10" gasofineengine lubricantoil Kinematicviscosity ( × 10-4 m~"$- t ) ( I00 °C)

Pour point (°C)

Flash point (°C)

10-12

< - 15

>209

Table3 The main physical and mechanicalpropertiesof the PTFE composite Compressives~ength (MPa)

Linear expansioncoefficient (°C-')

Thermalconductivitycoeflk'ient (W m-~ K-a)

Temlx-n~,e scope of th~pltcatioa {'C)

>280

15.6× 10-6

2.2

-2C0 to +280

in air. The slidingbearings made of the FIVE composite were prepared with a 36.05 mm inside diameter, 39.00 mm outside diameter, 14.00 mm height and 1.45-1.48 mm thickness of the bearing wall. The surfaces of the sliding bearings were washed in acetone and dried in air before each test. The lubricant media used in this experiment were glycerol, triethanolamine and 10" gasoline engine lubricant oil. The 10' gasoline engine lubricant oil was a mineral oil containing a small amount of antioxidant additive and a small amount of rug inhibitor. Glycerol and triethanolamine were analytically pure. The main properties of 10" gasoline engine lubricant oil [9] are listed in Table 2. R e FIFE composite prepared in this experiment is a metal-plastic multilayer self-lubricating composite, which is composed of a steel backing, a sintered porous bronze middle layer and a surface layer consisting of a mixture of PTFE and Pb powders. This FIFE composite possesses the beneficial properties of the original metal and plastic, such as high mechanical strength, low thermal expansion coefficient and friction coefficient, good thermal conductivity,excellent antiwear properties etc. The main preparation process of this PTFE composite is as follows: selecting steel plate -~ plating copper ~ laying bronze powder -> high temperature sintering --, laying FIFE powder filled by Pb powder ~ roller compacting ---, sintering --, sample processing. Its structure is similar to that of DU material (the commercial name of a PTFE-based multilayer composite invented by the Glacier Company of the United Kingdom) [ 10-12], and Fig. 2 gives a micrograph of the profile structure of the prepared PTFE composite. The main physical and mechanical properties of this FI'FE composite have ~eady been described and discussed elsewhere [ 13,14]. Table 3 lists the main physical and mechanical properties of this FIFE composite. Friction and wear tests were performed at room temperature with sliding speeds ranging from I to 4 m s- i and with various loads ranging from 1960 to 14 700 N. The lubricant

media were addedto the frictional surface through an oil hole during testing at arate of 20 drops per minute. The wear was detected by the weight loss of the FI'FE composite beating after each test. The fri~on coefficient was determined by measuring the friction torque. The friction tO,ClUe was recorded by atoalee reconkr, sothe frictioncoefficientcoeld calculated by the formula ~t=M/PR,where/t is the friction coefficient, M is the friction torque, P is the radial load applied, and R is the radius of the steel axis. The limiting PV value was evaluated according to GB7948-ST, the national standard of PR O,.i~. According to this national standard, when the friction torque increases sharply, or the temperature of the frictional surface is above 120'C under oil lubdcatioa, the PV value at this moment is considered as the limiting FV value of a hearing. The temperature of the frictional surface was measured by using a thermecoeple. Bec~me the real temperature

~g.2. MiaeSal~eftk lae~ ~ plastic layer;,b, layer of ~ ~ ; baking.

ofk vrfe ~ :

a, m'ra~

c, la~f of 10~tedO0¢la~ d, steel

166

Z.Z Zhang el al. / Wear196 (1996) 164-170

of the frictional interface was very difficult to measure, in this experiment the thermoeouple was set to contact the outer surface (steel backing) of the bearing bush. The measured temperature was not the real temperature of the frictional surface, but the bulk ten'lperature of the friction contacting zone. Because the bearing wall of this PTFE composite is thin and it has good thermal conductivity, so the measured temperature can be approximately considered as the temperature of the frictional surface.

3. Resells and discmsion

3.1. Thefriction property of PTFE composi~e The friction and wear properties of this FIFE composite in dry friction conditions have already been studied and discussed elsewhere [ 14]. Table 4 summarizes the tribological results of this ~ composite in dry friction conditions. The variations of the friction coefficients with load for this P'fFE composite in different lubricant med!a are shown in Fig. 3.

A comparison of the dry friction properties of this PTFE composite shows that the frictional property of this PTFE composite can be geatly improved with the above lubricant media and the friction coefficient can be decreased by two to three orders of magnitude. Meanwhile the results in Fig. 3 indicate that the friction coefficients of the ~ composite in different lubricant media decrease with an increase in load and sliding speed, and the friction properties of the composite in glycerol and triethanolamine are better than those of 10" gasoline engine lubricant oil, When the sliding speed is constant, the variation of friction coefficient with load for the PTFE composite in different lubricant media can be described by the formula/~x rlNIP, where ~ is the friction coefficient, ~/is the viscosity of l,bricant medium, N is the rotation speed of the steel axis, and P is the load applied on the bearing bush [ 15,16]. At a censtant sliding speed, the temperature of the frictional surface increases with increase in load, and the viscosity of lubricant medium decreases with increase in temperature but increases with the increase in load. This kind of variation of viscosity suggests that the effect of viscosity on the friction coefficient

"table 4

Tribologiealresult.cof the PTFEcompositein dry frictionconditions Speed (ms -I )

1

Maximum load (N)

Limiting PV value (MPam s-t)

4410

Friction coefficient (F)

8.82

•.. 6

t.96

Vt

!,

0.07

0.13

224.4

246

t4.70

i'" t

1.96

(b)

(°C)

V2 S " V3 2- w

V4

o

~

S.M 9,W Lo~(xIO~)

4 2

~

Temperatut~ of friction surface

~

(;I)

i

Wear (ms)

•

I

e.R

"

!

LO0

Line Ix~O'~l

'

i

I

14.70

I

1.96

(c)

i

I

5.88

,,

!

"w

9.80

•

14.70

Load(xtO,~)

Fig, 3. Variations of friction coe~cicnts with load for the PTFB composile in different lubricant media: (a) 10" gasoline engine lubricant oil; (b) glycerol; (c) triethanolaminc,Vl, V2,V3 and V4 are I ms-~, 2 m s-l,3 ms-~ and4 ms -s respectively.

Z,-Z7_,hang e! a[I Wear 196 (1996) 164-I70

is much smaller than that of load, thereforethe formulation ~x~lP can be approximated by p~N/P, so the friction coefficientdecreaseswith load increase. When the load is constant, the friction coefficientof the PTFE composite in different lubricantmedia decreaseswith increaseof sliding speed. It is believed that, with increaseof sliding speed, a layer of oil film can be more easily formed on the frictionalsurface: the lubricationconditionof the frictional surface can be greatly improved,thereforethe friction coefficientdecreaseswith increaseof stidingspeed.This variationin the behaviorof frictioncoefficientwith slidingspeed may be explainedby the Stribeckdiagram [ i7].

3.2. The wear property of PTFE composite Variations of wear with sliding speed for the FIFE composite in different lubricant media are shown in Fig. 4. Compared with the wear of this FIVE composite in dry friction (see Table 4), it is clear that the wear of the FIVE composite can be greatly reduced with the above lubricant media and the wear can be decreasedby one to two ordersof magnitude, though it increases with the increase of sliding speed. It can also be seen that the wear of the FIFE composite in 10" gasolineengine lubricant,oil is lower than that in glyceroland triethanolamine.10* gasolineengine lubricantoil can be seen to be the most effective lubricant medium in reducing the wear of the ~ composite, and triethanolaminethe worst one. However,more work should be done so as to determine the anti-wear mechanismsof the above lubricantmedia. The thickness of the surface plastic layer of the FIFE compositeis about 0.05 to 0.10 mm. The bronzepowdermay be exposed as soon as the surface plastic layer is worn off. Then contact of bronze powder with stainlesssteel may take place. It has been discoveredthat, with lubricationof glycerol and triethanolamine,a selective transfer effect occurs in the friction pair system of bronze-steel [ lg]. When the selective transfer effect takes place, the friction pair systemexhibits a very low friction coefficient and zero-wear properties.

¢

8O

167

It therefore possesses excellent anti-friction and anti-wear prop~es [ 19-21]. In this experiment, the surface plastic layer of the FIFE composite is not worn away, and the friction coefficientof the PTFE compositein glyceroland lriethanolamineis lower than that of 10" gasoline engine lutgicant oil, though wear is higher than that of the latter. However,as soon as the surface plastic layer of the PTFE composite is worn away, the selective transfer effect may occur for the friction pair system in glycerol and triethanohmine, but not in 10' gasoline engine lubricant oil. Under these conditions, the antifrictionand anti-wearpropertiesof the frictionpairsystem in glyceroland trkthanolaminemay be much betterthan thosein 10" gasolineenginelubricantoil.

3.3. The limiting PVpropertyof PTFEcomposite The limitingIN valuesof the FrFE compositein different lubricantmedia ate shown in Fig. 5. Comparedwith the limiting PV value of the FIFE composite in dry friction conditions (see Table 4), it can be seen that the limitingIN value of the PTFE compositecan be increasedby at least one order of magnitudeusing the above lubricant media. The limiting PV value of the FIFE compositein differentlubricantmedia is higher than 120 MPa m s-'. Such a limiting IN value is about IO0 times that of pure PTFE in dry friction conditions [22]. According to the GB7948-87 national standard of PR China, the temperatureof frictionalsurface (see Table 5) is consideredas a key parameterto decide whetherthe limiting PV value of FIFE composite is reached or not. Combining the results of Fig. 3, Fig. 5 and Table 5, it is evident that the limiting PV value of the FIFE composite in glycerol and

I

HII b

I

4O .-.

r

L~x~M

¢ ~

30

Fig.5. LimitingIN valuesof the FIFEcompesitein diffep,.r,tleveret medixa. 10' gasolmesginelebriuatoil;b,glycml;c. tr~mdam~

10

Table 5 Temperatereof the ~ sat'faceia d i ~ 14 70ON;velocity,4 in s- e)

o

i

i

i

I

t

2

~

4

sad~ spud v(vattl Fig. 4. Variationsof wear with sliding speedsfor the PTFEcompmitein differentlubricantmedia:a. 10" gasolineenginelubricantoil, b. glycerol; c. Ldetlmolamine.

lubt'k~ media (load,

Lubrkaat modem

T ~

clyce~ Trktha~lam~ |0' gasolinee~ne lulmcmtoil

102 Ill

(~c)

168

Z..Z Zhanget al. /Wear 196H996) 164-170

triethanolamine is higher than that in 10' gasoline engine lubricant oil. Moreover, when the frictional surface of the FIFE composite is not properJy lubricated by the lubricant medium, the surface plastic layer of this P T ~ composite may act as a solid lubricant, and the worst result is that the surface plastic layer may be worn away. However, for metal bearings under such conditions, the frictional surfaces or even the whole bearing may be damaged especially at high sliding speed and pressure. Therefore, with lubrication using the above lubricant media, this PTFE composite may be used in mechanical equipment as a kind of sliding bearing that has a high limiting PV value.

3.4. EPMA investigations offrictional surfaces

The electron micrograph and X-ray images of Pb and F elements of the steel counterface in lubrication of 10' gasoline engine lubricant oil are shown in Fig. 6. Compared with those obtained in dry friction conditions [ 14,23], it is found that the transfer of the FIFE composite onto the steel counterrace can ~ greatly reduced with lubricant media, but transfer of the ~ composite onto the steel counterface still takes place. Also the amount of Pb in the steel counterface is much higher than that ofF. So it is deduced that the frictional sure'aceof the PTFE compositeremains in partial contact with

Fig. 6. X-rayimagesof F and Pb elementsand the electronmibm~: of ~ii~terface in I0" gasolineenginelubricantoil: (a) electronmicmgraph; (b) X-r~)"ir~,~.geofF; (c) X-rayimageof Pb.

ms-'; (d) u=4 ms-'.

Z-Z Zhang et aLI Wear 196 (1996) 164-I70

fhe steel countefface, and the Ph filler preferentially transfers ~nto the steel countefface. Electron micrographs of the worn surfaces of the FIFE :omposite in 10" gasoline engine lubricant oil are shown in Fig. 7. Compared with those in dry friction conditions [ 14], it is evident that the width and depth of the worn traces of the FIFE composite in a lubricant medium are much smaller than those in dry friction conditions. It can also be seen from Fig. 7 that the severity of the wear tra,-.es increases with increase of sliding speed. So it can be concluded that the wear of the PTFE composite in lubricant media is much lower than that in dry friction and the wear increases with the increase of sliding speed. These results are consistent with the wear data. Combining the results of Fig. 6 and Fig. 7, it is shown that the wear and transfer of the FTFE composite still take place f v f n In " ~mefen~ A:¢~ . lubricant media. Also the Pb filler not only increases the load canting capacity of the PTFE composite, but preferentially transfers onto the steel counterface. Therefore, the improvement of tribological properties of the FIFE composite in lubricant media can be largely attributed to the action of the Pb filler.

4. Conclusions 1. Using the above lubricant media, the ~ction coefficient of the FIFE composite can be decreased by two to three orders of magnitude, and the friction coefficient decreases with the increase of load and sliding speed. 2. Using certain lubricant media, the wear of ~be PTFE compcsite can he decreased by one to two orders of magnitude, and the wear increases with an increase of sliding speed. 3. The limiting PV value of the FIFE composite can be increased at least by one order of magnitude with the above lubricant media. The limiting PV value is above 120 MPa m s- i about 100 times that of pure FIFE in dry friction conditions. 4. The friction and limiting PV properties of the FIFE composite in glycerol and triethanolamine are better than those of 10' gasoline engine lubricant oil, but the anti-wear properties are worse than those of 10" gasoline en-ir lubricant oil. 5. The wear and transfer of the FIFE, composite can be greatly reduced with the above lubricant media, but the wear and transfer of the FIFE composite still take place. The Pb filler not only increases the load carrying capacity of the FIFE composite, but also preferentially transfers onto the steel counterface. 6. When the surface plastic layer of the FIFE composite is worn away, a selective transfer effect may occur with the friction pair in glycerol and triethanolamine, but not in 10" gasoline engine lubricant oil. As soon as the selective transfer effect is achieved, the friction and wear reducing properties of the friction pair in glycerol and triethanolamine may be much better than those in 10" gasoline engine lubricant oil.

169

References [ i ] WD. C~g, l,~brication Engineering,20 (1964) 456. 12l S. Ho~her-Lushington,Tribal. Int.. 9 (1976) 257. [3] T.A. Stolarski, Wear,58 (1980) 103. [4 ] Y. Y~ctda and K. Tanaka, in K. Friedfich (ed. ). Frictionand Wear of PolymerCom#osites,Elsevier, Amsterdam, 1986, p. 137. [5] J.WM. Mens, and A.WJ. de Gee, Wear. 149 ( 1991) 255. [6] J.K. L a n ~ , Wear,20 (1972) 315. [7] Y. Zonggian, L. Ma~guing and K. Hailing, in Wear o[Materids, ASME, New YeA, 1981, p, 153. [8l M. Watanabe, Wear, 158 (I992) 79. 19] Editorial Baaed of Mechanical Engineering Handbook and Elecltic E~gineering Handbook, MechanicalEngineeringHandbook. Part 22, Friction. Wearand L~brication, Mech~ical Indestry Press, Beijing, 1978, p. 61. [ 10] British Patents657080, 657085, 756950 (Glacier, UK).

[I i] G.C Pratt, ?'r~o/. Int., 6 (1973) 135. [ 12] J.K.Lancaster,Tribal. Int., 12 (1979) 65.

[ 13} ResearchGroupof Plastic-BasedSelf-LubricatingMateri~ls,J. SOl/d Lubrication, 2 (1982) 24. [14] ~an-Zhu ~ang, Woi-Cl~mgShen, and Wei.MinLie eta]., The Iriboiogicalchata~etislicsof JS n ~ a l under lubrkationof oil, Wear, in press. [ 15] F.P.Bowdenand D. Tabor,TheFriction and Lubrication of Solids, Clarendon, Oxford,1954, p. 250. [16] P.M. Dickens,J.L Sullivanand J.K. L,lw,astet, Wear, 112 (1986) 273. 117] V. Stepinaand V. Vesely,l,ubncants and Special Fluids, Elsevier Science.Netherlands,1992,p. 4. [ 18] L V. lcd'a~cI~kyandV?¢. Allsin,Friction Wear Lubrication. Triboloffy Handbook,Vol.2.Mir,Moscow,I981,p. 128. [ 19] N.J.Fury, Wear, 26 (1973) 369. [20] V.A.Belyand A.I, Svhid~okezal.. Friction and Wear in Potyrezr. Based Marcels, P e ~ ,

Oxford, 1982,p. 227.

[21] J. Vihcrsalo,The selectiveIntasfereffectin mecKudcalengineering, presentedat 8th Int. Colloq. 'Tribology 2000" G:rn~y, Jan. 14-16, 1992, [22] ~ao-Zhu ~ang, Wei-ChangShen~d Jia-23"~ngZhao, J. TriboL, 13 ~1993) 228. [23] C.M.Pooleyand D.Tabor,Prec. I~ Sac. Lee,don, A 329 (197"2)251.

Biographies Zhao-Zhu Zhang, born in 1965, graduated with a B.S. degree in Solid Physics f~m the Physics Department of Lanzhou University in 1988. He received his M.S. degree in Physical Chemistry. from L~q~ou Institute of Chemical Physics, Chinese Academy of Sciences in 1991. Now he is a Ph.D. student. His current research interests include studies of tribological properties and their mechanisms, as well as the tribochemistry and ~bophysics of frictional surfaces and interfaces of polymer-based self.lubricating composites under dry friction and oil lubrication conditions. Wei-Chang Shen, born in 1940, graduated from the Chemical Engineering Department of Zhejiang University in 1964. Since then he has worked in Lanzhou Institute of Chemical Physics, Chinese .Academy of Sciences. Now he is an associate professor, and his current interests include studies of new polymeric self-lubricating composites and their tribological lxoperties.

170

7.-7. Zlu~;get at. I Wear 196 (1996) 164-170

Wei.-MinLiu graduatedwitha B+S.degreein Chemistryfrom ShandongNormalUniversityin 1984,and receivedhis M.S. degree and Ph.D in Tribolo~ at LanzhouInstituteof Chemical Physics in 1987 and 1990 respectively.He joined the Laboratoryof SolidLubrication,LanzhouInstituteof Chem. icalPhysics,ChineseAcademyof Sciencein 1990,and from 1993to 1994he silentoneyearatthe Dep~LrlmentofChemical Engineering, PennsylvaniaState University. His research interests include lubricantadditiveinteractiens,lubrication of ceramics,solid lubricationand tribochemistry. Qun-Ji Xue graduated from the Departmentof Chemistry, ShandongUniversityof Chinaand receivedhis M.Sc. degree at Lanzhouinstituteof ChemicalPhysicsin 1967.From 1980 to 1982he workedat the UniversityofMichigan.Since1965,

he has workedin the fieldof [ribologywithspecialempht,~is on lubricatedmaterials.He is now a professorand head of the Laboratoryof Solid Lubrication,Lanzhou Institute of Chemical Physics, Chinese Academy of Science, and also head of the Lubricationand TribochemistryBranch of the ChineseSocietyof MechanicalEngineers. Tong-ShengLi, born in 1953,graduatedfromthe Chemistry Departmentof the ChineseUniversityof Scienceand Technologyin 1977.Now he is an associateprofessorin Lanzhou InstituteofChemicalPhysics,ChineseAcademyof Sciences+ He has researchedself-lubricatingcoatingsby resin binders, and his currentresearch interestsinclude studies of the tfibologicalpropertiesof ceramics,polymersand their blends and composites.