Tribological behavior of lubricating oil additives in lubricated aluminum-on-steel contact

UliiilUlU|ill i

WEAR ELSEVIER

Wear 196 (1996) ~7 91

Tribological behavior of lubricating oil additives in lubricated aluminum-on-steel contact YongWan *, QunjiXue,WeiminL;.u Laboratoryof Solid Lubrication,Lua,zb.ouitutimte of ChemicalPhysics,ChineseAcademyof Scienees,Lanz~ 73000~,People'sRepublicof CIw Recc~.ved30 June 1995;accepted 9 November 1995

Abstract Effectsof somecommerciallubricatingoil additiveson the tribologicalprope~es of alun~numalloyin slidingcontactwith beating steel were investigatedunder boundarylubricationusinga reciprocatingwear tester. It was shownthat base stock comainingI wt.% additi.~eis not effectiveat reducingthe frictionand wearof aluminumalloy.Furthermore,additives,exceptsulfuri~..edolefin,at 5 wt.~, in the base s~ock have pro-wearproperties.The beneficialeffectsof base stockcontaining5 wt.~ solfurizedolelin werea~.',uted m the formationof A1203 filmat the contactarea, whichwas identifiedby X-rayphotoelectronspectroscopy(XPS) and electronprobemicroanalysis(EPMA). feywords: Additives;Lubricated wear, Aluminumalloys; Tdbological properties: XPS; EPMA

1. lntroduedon There has been an increased use of aluminum alloy in tribological systems in recent years due to its excellent resistance to corrosion, superior fatigue resistance, good thermal conductivity and moderate costs. However, the major draw. back of aluminum alloy is its poor resistance to seizure and galling. The aluminum-against-steel metal couple is an extremely difficult system to !ub.n_'c.ateeven at m~le,xt loads [ 1,2]. The situation is more difficult under conditions where aluminum is subjected to high pressure, as for the example in various aluminum-workingprocesses. Lubricants are usually needed to lower the friction and wear between the mating parts, In the past, the tribological behavior of aluminum alloy in a lubricated aluminum-on-steel system has been studied in order to provide the effective lubricants or additives for aluminum. Montgomery [3-5] and St. Pierre et al. [6] reported d~at~lar longchaincompounds, such as fattyalcohol,ester and acid,~d unsaturatedhydrocm'oons were effectivein reducingthe wear of aluminum alloyunderboundary lubrication.However, less systematic research has been reported about the effects of conventional lubricating oil additives on the tribological peffo~ance of aluminum alloy. In this research, the friction and wear behavior of aluminum alloy in a sliding steel contact was investigated to study * CotTespondingauthor,Cunemlyat: Depamnentof CIzmis~.Tsinghua University,lkijing 100084,People's Republic of China. 0043-1~8/901515,00 © 1996 Elsevier S,~ace S.A. All figl~ n~'ved SSDi 0043-1648 (95) 06868-6

the potemial application of conunercialadditives on the ale* minum/steel contact. The triboehemical reaction of additives was studied using X-ray photoelectron spectroscopy (XPS) and electron probe microanalysis (EPMA).

2. Experimemi details A schematic drawing of the recil~ocating we~ tester used in this study is shown in Fig. 1. This tester has a ball-on-plate type of contact geomeuy. The tribological specimens consisted of GCrl5 bearing steel (AISI 52100 steel) and AA2024 aluminum alloy. The balls were 10 mm in diameter and plates were 19 nun long, 12.5 nun wide and 8 mm thick. All samples were degreased and cleaned using acetone in an ultrasoniccleaner,thenair-driedbeforethe wear tests. The wear tests were carried out at the followingconditions: 50 N load, 30rain testing time, 20 Hz f~uency, I mm amplitude and room temlgraturc (about 20 ~C). The wear volumes of aluminum plates were measured by surface prosr~d~om

l.oed

I

el

g Steelbel

88

Y. Wan a al. / Wear I96 (1996) 87-91

Table I Typicalpropertiesof liquidparaffin Density(g cm-3)

Viscosity(ram2s -j)

0,8465

40°C

100°C

21.49

4.42

Viscosityindex

S content(ppm)

Boilingpoint(°C)

117

1

> 300

Table2 Summaryof the additivesusedforstudy Additives

Function

Sulfonate(a) Sulfonate(b) Sulfonate(c) Sulfonate(d)

Detergent

Suifudzedolefin

AW/EP additives

S

Ca

Zn

N

(%)

(%)

[%)

(%)

1.73 12.11 2.93 3.25 I 1.17 3.30

TBN (mg KOHg-1)

M.p. (°C)

Density (gem-~)

Viscosity (mm2s -i)

Rash Source' point (°c)

293.68 41.76 282.04 39.14

C.P. C.P. C.P. C.P.

40

C.P.

Dibwylphosphite Tdcresylphosphate

C.R.

Zinc

15.1

8.2

C.R. C.P.

8,2

dialkyldithiophosphate La~! acid

C,R.

Frictionmodifying agent

Laurylalcohol Ethyllaurylate

C,R,

CR.

2,6-tert-Butyl-p-cresol

Andoxidant

Thiadizolederivative

Antirust agent

69-70 28

C.P.

5.3

Benzotrizolederivative

C.P. 1.01 (20°C)

12.5(50°C)

150

C.P.

' C.P.= commercialproduct;C.R.= chemicalpurityreagent. filometry. Friction ceefficient was recorded automatically. Two tests were run ~ each test condition and if the results differed by more tha 10%, additional tests were run. Chemical puri_'ty-g!ade liquid p~m~..n was used as base stock, Table I lists physical and chemical properties of liquid paraffin. The additives used in the study are summarized in Table 2. All were obtained commercially. The additive concentration in base stock was 1 wt.% and 5 wt.%. In the study, XPS and EPMA were used to characterize the composition and microstrncture of the worn specimens. XPS spectra were obtained on a PHi 550 model XPS spectrometer using a monochromatic Mg Ket X.ray source with the pass energy of 50 eV. Electron binding energies were calibrated against the contaminated C Is peak assumed to be 284.6 eV. EPMA analysis was conducted with an EPMA 810Q model spectrometer. 3. Reml(s and discussion 3.1. Tribological results

The tribological properties of base stock containing addifives were evaluated at a load of 50 N and testing time of

30 min. Table 3 details the wear volume obtained from wear tests conducted with base stockcontaining I wt.% and 5 wt.% additives. It should be noted that base stock without any additives is a good lubricant for aluminum alloy. The addition of additives to base stock at the concentration of 1 wl.% is not effective at reducing the wear of aluminum. However, 5 wt.% additives, except sulfurized olefin, produce pro-wear effects when compared with base stock. The variation of friction coefficient as the test progressed was also recorded. A summary of data on commercial lubricating oil additives is given in Table 3. Three types of friction traces were observed. The first one (type A), as shown in Fig. 2(a), is exhibited by base stock and 1 wt.% additives. The friction coefficient remained at 0.10 as the test progressed. This friction behavior can be set as the standard of performance measurement. The base stockcontaining 5 wt.% additives, except sulfudzed olefin, showed a different kind of friction curve (type B). initially, friction was low, bet increased as the wear progressed. Final coefficient of friction was generally bigger than 0.10, a value that the base stock provided. This behavior indicates relatively poor lubrication, e.g, bigger wear volume and rougher sttfface of aluminum

E Wan et aL I Wear 196 (1996) 87-91 Table 3

Effect of additives on the tribological Im~e~ie~ of aluminum alloy in lebrica~ aluminum-on-steelcontact at a load of 50 N mat tmiu$ time of 30 rain Additives

Friction coefficient Base stock

Basestock

Wmt volume ( X l0 -~ mm3)

Friction type *

I wt,%

5 wt,%

0,10

Bate stock

I wl,%

5 wt.%

A

namestock

I mr.%

5 wl+%

3.27

Sulfonate (a) Sulfonate (b) Sulfonate (c) Sulfonate (d)

0.I0 0.I0 0.I0 0.10

0.I0 0.I0 0.I0 0.I0

A A A A

A A A A

6.64 5.98 7.57 6.46

10.89 11.28 10.89 14.67

Sulfurizedolefin Dibutyl phosphite Tricresyl phosphate Zinc dialkyldithiophosphale

0.10 0.10 0.10 0.10

0.07 0.07-0.12 0.09 0.10

A A A A

C

3.14

B

4.56

A A

3.30 3.12

2.02 50.22 7.31 8.48

Lauryl acid Lauryl ~hohol Ethyl laurylate

0.10 0,I0 0.10

0.06--0.095 0.065-0.095 0.06-0.I05

B B B

3.02 4.16 3.05

22.80 14,49 27.60

2,f-left-Butyl-p-cresol

0.10

A A A A

Thiadizole derivative Benzotrizolederivative

0.10

0.06-0,12 0.105

A A

B

0.10

4.99 A

7,46 5.17

34.20

• As describedin Fig. 2.

{b) ~

.I(]

In short, of the commercial lubricating oils studied, only sulfurized olefin at the concert[rationof 5 wt.% is effective at reducingthe frictionand wearofaluminum alloy in lubricated aluminum-on-steelcontact.

0.0(

3.2. Surfacefilm analysis by XPS and EPMA

"~ 0.06

10

2O

3O

Tu~, (rain) Fig. 2. Friction traces for lubricated aluminum-on-steelcontact when lubdcaring with (a) base stock (type A), (b) 5% dibutyl phosphite (type B) and (c) 5% sulfudzed olefin (type C).

It can be inferred from the tribological results that the performanceof additives is different.Thus, Ihe emphasis will be put on the analysis of the chemical characteristicsof surface 61m of steel bah and aluminum plate using XPS EPMA in order to determine the acting mechanism of addifives in the aluminum-sloel frictional pair. Sulfurized oh:fin

plate than that in the presence of base stock. The third one (type C) is only attributed to base stock containing 5 wt.% sulfurized olefin. The friction coefficient remained at 0.07 as the test progressed. This friction behavior indicates good lubrication.

Fig° 3, M i c r o ~

was chosen as the example. Micrographs of the worn scar of alumintiin alloy plate when lubricated with base stockand sulfurized olefm systems are shown in Fig. 3. A larger patch of deposit mm be seen in the presence of 5 wt.% sulfurized olefin than with base stock

o f ~ worn sew of fl~ alumimnn block whm l ~ a ~ d i ~ wida (a) lure mdr~ (b) I ~ ~ ( c )

5~ s,~fs~ed o k ~ .

90

¥. Wan et aL /Wear 196 (1996)87-91

Fig. 4. M i c r o ~ of ~ ~ ~

Of~

~ ,

(b) I% and (c) S°&sulfurizedolefin.

Fig. 5. The distributionof aluminumon the worn scar of the steel ball WheniuMicalingwith (a) base stock, (b) 1~ and (c) 5% sulfurizedolefin.

and 1% sulfurizedolefin.The results give support to optical observationof the worn scar. For the specimens tested with base stock and 1% sulfurizedolefin respectively,the surface was bright a~d lustrous. However, the surface was dark in color when lubricatedwith 5% suifurizedolefin. XPS was used to detect the chemical compositionof the correspondingworn scar. In the presence of base stock, the bindingenergy peak locatedat 72.6 eV, correspondingto the metallic aluminum, was detected. For the specimen tested with 5 wt.% suifurized olefin, the binding energy located at 75.3 eV, correspondingto the A1203,in addition to 72.6 eV, was also observed.Moreover,the concentrationof sulfur was too !or; to investigatethe clear chemical :,alence which existedon the surface.The presentresultsclearlydemonstrate that sulfurizedolefin in base stock is effective at providing the formationof AI20 3 on the surface. Micrographs of the typical worn scar of steel balls after wear tests, when lubricated with base stock and base stock containing 1 wt.% and 5 wt.% sulfurizedolefinrespectively, are shownin Fig. 4, The distributionof aluminumis included in Fig. 5. Fewergroovescan be seen in the presenceof5 wt.% suifurized olefin than with base stock or I wt.% additive. Furthermore, the transfer or smearing of aluminum in large patches on the steel counterface is clearly evident from Fig. 5(a) when base stock has been used as the lubricant,In the presenceof 1 wt.%sulfurizedolefin,transferof aluminum to the steel counterface,although to a lesser extent, was also

evident (Fig. 5(b)). This proves that the I wt.% sulfurized olefin is not effectiveat preventingthe transfer of aluminum to steel. However, much less transfer of aluminum can be found for the oil containing 5 wt.% sulfurized olefin (Fig. 5(c)). That is say that the wear.induced transfer of aluminum to the steel counterface could be effectively controlled by 5 wt.% sulfurizedolefin. Takingthe XPS and EPMAresults into account,the effectivenessof 5 wt.% sulfurizedolefinin base stock can be attributed to the formation of an AI203 film which prevented the adhesion and transfer of aluminum to the steel counted'ace, then reduced the wear.

4. Conclusion The tribological performance of commercial lubricating oil additives in lubricated aluminum-on-steelcontact were evaluatedby an OptimolSRV tester in which a steel ball was held againsta reciprocatingaluminumalloy plate at a load of 50 N and testing time of 30 min. It was found that the base stock withoutany additivesis a good lubricantfor aluminum. The additionof 1 wt.% additivesto base stock is not effective at improving ttibological performance. Moreover, the high concentration(5 wt.%) of additives,exceptsulfurizedolefin, producedpro-weareffects.The improvementof the tribolog. ical performanceof aluminumalloy by using 5 wt.% sulfur-

E Wan etaL/Wear 196 (1996) 87-91

ized olefin is attributed to the formation of A1203 film on the contact area which prevented the transfer of aluminum to the counterface during sliding of the ah:minurt:-on-steel contact.

Acknowledgements The research reported here was supposed by the National Natural Science Foundation of China. The authors wish to thank Mr, Shangkui Qi, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,forconducting the surface analysisexperiments.

91

Rderenees [i] ].A. Sd~-yandPC. Nauzi~, Elfectsof sun'acerou~L-ss m andmetalu'~m~ ia l~fi~zd s l i ~ of a l t o alloys~pist ~ t surfaces,Wear, 146 (1991)37-5l. [2] P.C.Naudyaland J.A.Schey,Tnmfer of aluminumto steelin s l ' ~ co~t,~'t:Effectso f l ~ T ~ . ASldE J. TnboL, 112 (1990)28228/. [3] R,S. Montgomery,The effe~ of alcoholswtd ethers on the we~ behaviorof~ Wear.8 (1965)466--473. [4] R.S.gontgomry, The~ ofaht~ctmtby plahalic~ d e~'s, Wear, 9 (1966)297-299. [5] R.S. Montgomay, ~ effectson we~¢ in the luI~'ic~ionof aluminum,W~', 8 (1965)2~)-302. [6] L.E. St. i~ene, R.S. Oweasam1R.V. Klim, Chemicaleffec~in the boundarylubr~ ofal~ Wear,9 (1966) !f~-t68.