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Tribological properties of alcohols as lubricating additives for aluminum-on-steel contact
WEAR ELSEVIER
Wear 218 11998) 244-249
Tribological properties of alcohols as lubricating additives for aluminum-on-steel contact Yanhong Hu *, Weimin Liu Laboratoryof Solid Lubrication.LanzhouInstitute of ChemicalPhysics. ChineseAcademyof Sciences. Lanzhon 730000, China
Received 22 July 1997:accepted30 January 1998
Abstract The tribological properties of a series of alcohols as lubricating additives in liquid paraffin were investigated using a Timken tester with a SAE52100 steel ring sliding against an A12024 block. The boundary film formed on the aluminum block was studied with X-ray photoelectron spectrometry (XPS) and .scanning electron microscopy (SEM). It was found that these compounds possess high load-currying capacities, good friction reduction and aatiwear properties. In particular, the Ioad-carryin8 capacity and antiwear ability increased with the increase of hydroxy group number in the alcohols. XPS analysis suggests that the lubricating mechanism of the alcohols compounds as additives is the formation of complex of aluminum and alcohols. SEM analysis reveals that adhesion, scoring and plastic deformation are the dominative wear. © 1998 Elsevier Science S.A. All rights reserved. Ke~'ords: Alcohols:Aluminum-on-steel:Frictionand wear behaviors.TribopolymerXPS and SEM analyses
1. I n t r o d u c t i o n Owing to the excellent resistance to corrosion, high strength/density rate, aluminum alloys have been widely used in automotive industry and aviation. Commonly. lubrication is required in operations. Alcohols are well-known effective lubricants, especially in the process of aluminum foil, sheet and extrusions [ I - 3 ] . Hotten [4] and Wan et al. [ 5 ] reported that linear et- and ~-diols are effective boundary lubricants for aluminum. Hironaka and Sakurai [2] pointed out that pentaerythritol partial ester which contains both hydroxyl and ester group is more effective as compared to the full ester. The practical and theoretical sradies of lubrication with lubricants containing fatty alcohols have been conducted for decades [6-11 ]. However, not all is known about the action mechanism of tbese compounds, especially in the aluminumon-steel system. Kajdas [ 12] put forward a viewpoint of anionic-radical concept which explains the lubrication mechanism of alcohols considerably. in this paper, the wear behavior of 2024AI against steel under the lubrication of liquid paraffin containing alcohols was evaluated The tribological properties of the system were * Correspondingauthor. Lanzhoa Institute of Chemical Physics. Lab. of Solid Lubrication,AcademiaSinica. 730000 Lanzhou,China. Tel.: +86931-8417088.
correlated to the molecular structures especially the numbers of hydroxy group in the alcohols. It is expected that this investigation will be helpful to understand the action mechanism of alcohols for lubrication of aluminum.
2. E x p e r i m e n t a l 2. I. Specimen
Liquid paraffin ( L P ) . which has a boiling point of above 300°C, density of 0.86 g cm -3, viscosity at 50 and 100 of 10.28 mm 2 s - i and 3.36 mm 2 s - i respectively, was used as base oil in the tests because of its simple structure and uniform friction and wear behaviors. The used additives, n-butanol, ethandiol, 1,3-butanediol, 1,4-butanediol and propanetriol, are all AR grade reagents used without further purification. The formulas and molecular weights of these compounds are listed in Table I. 2.2. Apparatus
The friction and wear tests were conducted on a block-onring wear tester (MRH-3 Block-on-Ring Wear Tester, Ji'nan Tester Factory, China), which is similar to the Timken Tester. The rings used in the tests are made of A1SI52100 steel with
0043-1648/98/$ - .see front matter © 1998 El~vier Science S.A, All rights reserved. PII S0043-1648 (98)00162-8
Y. Hu. W, Liu / Wear 218 t 1998) 244-249
Table I Formulaand molecularweightof the selectedcompound
2
Compound
Formula
Molecular weight
n-butanol ethandiol 1.3-butanediol ] .4-butanediol propanetriol
CH~CH,CH,CH~OH HOCH:CH:OH OHCH:CH:CH (OH )CH,
76 62 92 92 92
I .
HOCH:CH:CH-CH-OH
OHCH:CH(OH CH,OH
21H)
41111
~X)
Illln
iiiiii
I~)
141wt
^l~he,I tn'~JlNI
a HRc between 58 and 61, whereas the blocks are made of 2024AI with a Vicker hardness of 194.6. and their compositions arc listed in Table 2. Before each test, both the block and ring were polished with a No. 800 abrasive paper and cleaned with petroleum ether. Tests were carried out at a sliding velocity of 2.05 m s - ~( 800 rpm ) at room temperature with the dropping oil ( 0.7 ml rain - z) lubrication. The test time was selected to be 20 min because such a test duration will enable it to give a stable friction curve and an-easy-tomeasure wear scar. During friction test, the block was linear connected with the ring, and each length of the wear scars was almost equal to the width of the block as well. Different wear scar widths of the aluminum blocks after friction tests were observed under different loads with the lubrication of base oil containing different additives, which was used to assess the wear of the aluminum block. The wear scar widths were measured using an optical microscope. 2.3. Analytical technique
Since X-ray photoelectron spectroscopy I XPS) is very sensitive to investigate the chemical composition and chemical environment of the elements in a material, the chemical composition of the boundary film formed on the surface of 2024AI during the sliding process was evaluated using XPR, XPS analysis was conducted on a PHI-5702 electron spectrometer using a pass energy of 29.35 eV and the Mg K a line excitation source using the reference of C~s at 284.6 eV. Furthermore, the rubbed surfaces were studied by scanning electron microscopy (SEM). SEM was conducted on a EPMA-SIOQ electron probe microanalyzer, 3. Results and discussion
3. I. A n t i w e a r behaviors
Fig. l shows the wear scar width of aluminum block as a function of applied load with the lubrication of base oil con-
n-bulamll 1.3-bulancdud
+
@ +
inlFUp;l~lrllll
clhancdml 1.4-haam:du,I
Fig. 1. ~,'ear scar width of aluminumblock as a function of applied load v,ith the luhncationof base oil con~ning 3 wt.% alcohols.
•
o° oO
~ o3 f~'lO
II00
l(t~O
1200
14N10 I6(I0
inl~
Apldied Io~JqN) --I.--,3v4%
- - I 1 ~ twO.
--,dk--o.swt% --)4,--0.lwl% Fig. 2. Wear ~ar width as a funclion of ptopanetriol concentrationunder differemloads. taining 3 wt.% alcohols. Results in Fig. I clearly indicate that different alcohol exhibits quite different antiwear ability ( see also Table 3 ). With the lubrication of base oil containing nbutanol, the 2024AI-on-steel system failed at a load higher than 600 N. whereas with the lubrication of base oil c o n t ~ n ing ethandiol, the failure load was higher than 1000 N. However, base oil containing propanetriol is very effective to lubricate the 2024Al-on-stecl system even at a load of 1600 N. Results in Fig. I also show that propanetriol is the best antiwear additive, and the antiwear ability of 1,3-hotanediol is better than that of 1.4-butanediol. which can he explained by Hotton's and Wan's six-number ring bidentate bonding hypothesis [ 4,5 ]. Among ethandiol. 1.3-butanedioL and 1,4butanediol which have the same numbers of hydroxy groups, the 1,3-butanediol and 1+4-butanediol possess better antiwear and load-carrying capacities, which can he attributed to the alkyl chain length [ 13,14]. Fig. 2 gives the influence of load and propanetriol concentration on the wear of the aluminum block. Under normal loads lower than 1000 N, the effect of the concentration is
Table 2 Chemicalcompositionsof the SAE52I00 steel and A12024
SAE52100 steel A12024
C
Cr
0.95-1.05
1.30-1.65 0.15~0.35 0.204).40 0.50 0.34).9
Si
Mn
Cu
Mg
Zn
Fe
AI
Balance 3.8-4.9
1.2-1.8
0.25
Balance
246
Y. Hu. W. Liu / Wear 211¢(19981 244-249
Table 3 Mean ~'alue and dispersion of wear scar width of AI 2024 under different loads with the lubrication of base oil containing 3 wt.Cbdifferent alcohols
200 300 4O0 500 60O 800 I000 1200 1400
n-Butanol
Ethandiol
1.3-Butanediol
1.4-Butanediol
0.49±0.010 0.48 ± 0.026 0.64+_0.12 0.95 ± 11.058 1.90+_0.17
0.91+_0.026
0.56±0.044
0.66±0.052
0.99+-0.~1 1.36+_0.093 1.73±0.082
O.77+-0.012 0.83+_0.14 1.18±0.018 1.80±0.~7
0.92+-0.~8 1.00±0.024 1.38±0.037 1.88+_0.~0
Table 4 Mean value and dispersion of wear sear width of AL 2024 under different loads with the lubrication of base oil containing propanetriol at different concentrations
600 800 1000 1200 1400 16{10 1800
3wt.~
Iwt.~
0.5wt.~
0.1wt.~
0.59 + 0.046 0.97+-0.O60 0.91 :[::0.020 0.90 4-0.049 0.91 +-0.063 failure
0.79 +_0.013 0.6O+_0.042 0.86+0.055 1.63 -I-0.40
0.69_+O.II 0.71 +_0.0,41 0.72+0.045 0.82 + 0.018 0.86+_0.029 0.975:0.12 failure
0.43+0.0094 0.72+_0.042 1.66 +0.096 1.98 + 0.095
:
:
:!
p ,
,
:
:
:
•
:
:
:
:
:
Propanetriol
1).59+_0.~6 0.97±0.000 0.91±0.020 0.90+_0.~9 0.91+_0.003
not obvious, the w e a r has a tendency to increase slightly. With load increasing, it can be found that there are o p t i m u m concentrations. At the concentration o f 0.5 wt.%. propanetriol exhibits g o o d load-carrying capacity, it w a s considered that at the concentration lower than 0.5 wt.%, no effective and continuous adsorption film formed as well as at rather higher concentrations than 0.5 wt.%, propanetriol w a s readily to corrode the substrate. It w a s a little difficult to understand w h y the limiting load under the lubrication o f base oil conraining I wt.% propanetriol w a s lower than that o f 3 wt.%. it w a s probably subjected to the experimental errors which are presented in Table 4. It w a s also found that under the limiting loads, certain w e a r o f the a l u m i n u m block lubricated by base oil containing additives w a s so serious that even it could not endure a test duration, which led to the tests cut o f f and no definite w e a r scar widths obtained. W e l d w e a r took place under such conditions. T h e dotted vertical lines were introduced to demonstrate the weld loads. 3.2. X P S analyses o f worn su .rfaces
• ~7 " 76
~5 " q4
~3
i~
71
Binging Enexgy (eV) Fig. 3. XPS spectra of AI,r on the rubbed surface of aluminum block lubricated with ethandiol.
Figs. 3 - 7 g i v e the X P S spectra o f AI2o. Ci • and Ot~ o f w o r o scars o f the a l u m i n u m blocks lubricated with ethandiol and propanediol at the load o f 800 N and concentration o f 3 wt.%. respectively. Results in Figs. 3 and 4 s h o w that AI m i g h t be at least in two forms, one is AI203 with the binding e n e r g y o f AI2e at about 74.6 eV. another is probable the c o m p l e x e s
!......... i
.
.
.
.
.
. . . . . . . . . . . . . . . . .
74 73 72 71 Binding Energy (eV) Fig. 4. XPS spectra of Al.,r on the rubbed surface of aluminum block lubricated with propanetriol. 76
75
2~
2~
2U
2~
216
2B5
284
.
2~
2S2
Binding Energy (:V) Fig. 5. XPS spectra of C i, on the rubbed surface of aluminum block lubricated with ethandiol.
2~
2S9
2n
2S?
2~
2S5
2O4
2S~
~.~
Binding Energy(eV) Fig.6. XPSspectraof C,, on the rubbedsurfaceof aluminumblocklubricated with propanetriol.
3.4. Discus.~ion
S31.~TS
S34
S~
S~O
lubricate the surface well, aluminum transfer is serious under such a load as 600 N. adhesion wear is the dominative wear. Similar phenomena can be found under the lubrication of base oil containing 3 wt.% n-butanol or 1,3-butanediol. It is noticeable under the lubrication of base oil containing 0.5 wt.% propanetriol. There are some ridges beside the shallow grooves, plastic deformation took place as well as ploughing fainted which lead to mild wear under the load of 800 N ( Fig. 8h). Deep and narrow grooves accompanied some higher ridges are found under the high load of 1800 N (Fig. 8i). Scoring and plastic deformation contributed to the severe wear. It can be considered that ptopanetriol inhibited the transfer and adhesion of aluminum effectively and exhibited high load-carrying property.
~
S~
Binding Energy(eV) Fig. 7. XPS spectraof O,, on the rubbed surfaceof aluminumblock lubricated withethandioland ptopanetriol. of aluminum and ethandiol or propanetriol, with the corresponding binding energy of Al_~pfrom about 73.6 eV to 73.8 eV. An interesting phenomenon is the binding energy of Al_,p at 71.46 eV or 71.66 cV, which could not he attributed to any known forms of aluminum, one probable explanation is the formation of AIC compound. The binding energies (BE) of Ct~ in Figs. 5 and 6 fluctuated around 287.68, 285.98 and 284.46 eV suggest the formation of polyalkylene glycol during friction which was also proved in Kajdas and Mori's tests.
In boundary lubrication of an aluminum-on-steel system, the chemical inter,tction especially the chemical reaction of a lubricant with the rubbing surface could be very important. Hotten has investigated the lubricity of diol and ketol for aluminum, and has suggested the antiwear mechanism is due to the formation of aluminum diol complex or aluminum ketol complex [4l. Investigations in Laboratory of Solid Lubrication, Chine~ Academy of Sciences have proved the formation of aluminum diol complex, and have found that the complex of aluminum with 1.3-diol is more chemically stable than corresponding 1.4-diol or 2.3-diol [5]. The t~sults in this work illustrate that alcohols are effective additives in the lubrication of aluminum-on-steel system. This can be attributed to the chem-adsoq~on of alcohols and the formation of complex between aluminum and alcohols or the generation of friction polymer. The effect of the hydroxy numbers may include two aspects. With the inct~a~ of the hydroxy group numbers, the molecular polarity improves, consequently, a more stable chem-adsorption film was formed. On the other hand. it may make the cross-link of the friction polymer easier. Fig. 9 gives the suggested schematic diagram of the action mechanism of alcohols [ 12,15].
3.3. SEM analyses t~f worn surface.~
4. Conclusion
Fig. 8 shows SEM micrographs of the worn surfaces of AI blocks under their mild loads and failing loads, respectively. Several fine or flake-like wear debris were found on the worn tracks. Results clearly indicate that with the lubrication of base oil or base oil containing additives, the original polished surfaces were rubbed. There are deep and narrow grooves on the worn block surface under the lubrication of base oil under 300 N (Fig. 8b). The Al.,O.~ particles indented into the 2024AI surface were the abrasives that may cause ploughing wear. The surface is relatively flat and the grooves are not so clear under 600 N (Fig. 8c). it is suggested that LP can not
I. An aluminum-on-steel system can be lubricated effectively by alcohols. Their antiwear prapeaies increase as the alkyl chain length and hydroxy number increase. 2. The antiwear mechanism of the alcohols can be attributed to the tribochemical reaction on the robbed surfaces which produce aluminum alkoxide, aluminum complex and friction polymer. 3. The dominative wear is scuffing, plastic deformation and adhesion. Propauetriol is an excellent additive as an inhibitor from transfer.
....
248
........
~
....
E Hu. W. Liu/Wear218 (1998) 2 4 4 - . 2 4 9
) LP. wt.%
Y. Hu. W. Liu I Wear 218 ¢19981 244-249
Fig. 9. The schematic diagram of alcohols chem-adsorption and interaction with aluminum during friction process. Acknowledgements T h e authors gramfully a c k n o w l e d g e the financial support o f Natural Science Foundation o f Lanzhou Branch o f Chinese A c a d e m y o f Sciences, and National Natural Science Foundation o f China.
249
19] K.V. Shooter. The lubrication of metals by long-chain organic compounds, nr. J. Appl. Phys. Suppl. ( 1951 ) 49.-5 I. [10] W.A. Zisman. Friction, durability and wetability properties of monomolecular films on solids. Prec. Syrup. on Friction and Wear, Detroit, MI. 1957. Elsevier, Almlcrdam, 1959. I I 1I Cz. Kajdas. L Luczkiewicz, D. Ozimina, E. Wawak. Estimation of tribological properties of long-chain acids and alcohols, Prec. 2rid conL on Tribology, Budapest. October 24-26,1979, Vol. 2. pp. 619625. 112l K.Czeslaw.Aboutan~alonic-redicalconce~ofthelubricationmechanism of alcohols. Wear 116 (1987) 167-180. [ 13] L.E. St.Pierre, R.S.Owans, R,V.Kliar.Chemicaleffectsintheboondary lubrication of aluminum. Wear 9 (1966) 160-168. [ 141 P.C. Nauiyal, J.A. Schey. Transfer of aluminum to steel in sliding contact: effects of lubricanL J, Tribol. I 12 (1990) 282-287. ] 15] S. Mori. M. Sugmoya. Y. TamaL ~ of ocgani¢ compounds on a clean AI surface prepared by cutting under high vacuum. ASLE Trans. 25 (2) (1982) 261-266.
Biographi~ References [ I [ R.S. Montgomery, The effect of 'alcohols and ethers on the wear behavior of'aluminum, Wear 8 (1965) 466--473. [ 21 S. Hironaka, T. Sakural, The effect of pcntacrythfitol partial ester on the wear of aluminum, Wear 50 (1978) 105-114. [31 R.S. Montgomery. H.L. Garret, An electron-microscopic study of aluminum wear particles formed during sliding in the presence of polyglycols, Wear IO (1967) 310-318. [4l B.W. Hottcn. Bidentate organic compounds as boundary lubricants for alumianm, Lubr. Eng. 30 ( 19741 398-403. [ 51 Y- Wan. W. Liu. Q. Xue, Effects of diol compounds on the friction and wear of aluminum alloy in a lubricated aluminum-on-steel contact. Wear 193 (1996) 99-104. 161 W.G. neare. F.P. Bowdan, Physical properties of surfaces: I. Kinetic friction. Phil. Trans. R. Soc. London. Ser. A 234 (1935) 329--354. [7] W.B. Hardy. Collected Scientific Papers. Cambridge Univ. Pregs, London, 1936. [81 3. Sameshima, M. Kidokoro. H. Akamatsu, Oiliness of liquids: I. Measurements of the static friction coefficients. Bull Chem. Soc. Jpn. I I (1936) 659-666.
Y a n h o n g Hu received her B S f r o m the D c p a r l m c n t o f C h e m istry, H e ' n a n University in 1994. Currently, she is a studant enrolled in doctorate plrogran~ at the ~ o r y o f Solid Lubrication, Lanzhou Institute o f Clmmical Physics, Chinese A c a d e m y o f Sciences. W e i m i n Liu received his B S in C h e m i s t r y at S h a n d o n g Normal University in 1984 and P h D in Tribology at Lanzhou I n s u m t e o f Chemical Physics in 1990. H e j o i n e d the [,almratory o f Solid Lubrication, Lanzlmu Institute o f C h e m i c a l Physics, Chinese A c a d e m y o f Sciences in 1990, and f r o m 1993 to 1994 he spent I y e a r as a visiting scientist at I ~ m e n t o f Chemical Engineering, T h e Pennsylvania State University. His research interests include lubricant additive interactions, lubrication o f ceramic, solid lubrication, tribochemistry and fuel additives. Cuffcntly, he is a professor and Deputy Director o f the Laboratory o f Solid Lubrication.
Wear 218 11998) 244-249
Tribological properties of alcohols as lubricating additives for aluminum-on-steel contact Yanhong Hu *, Weimin Liu Laboratoryof Solid Lubrication.LanzhouInstitute of ChemicalPhysics. ChineseAcademyof Sciences. Lanzhon 730000, China
Received 22 July 1997:accepted30 January 1998
Abstract The tribological properties of a series of alcohols as lubricating additives in liquid paraffin were investigated using a Timken tester with a SAE52100 steel ring sliding against an A12024 block. The boundary film formed on the aluminum block was studied with X-ray photoelectron spectrometry (XPS) and .scanning electron microscopy (SEM). It was found that these compounds possess high load-currying capacities, good friction reduction and aatiwear properties. In particular, the Ioad-carryin8 capacity and antiwear ability increased with the increase of hydroxy group number in the alcohols. XPS analysis suggests that the lubricating mechanism of the alcohols compounds as additives is the formation of complex of aluminum and alcohols. SEM analysis reveals that adhesion, scoring and plastic deformation are the dominative wear. © 1998 Elsevier Science S.A. All rights reserved. Ke~'ords: Alcohols:Aluminum-on-steel:Frictionand wear behaviors.TribopolymerXPS and SEM analyses
1. I n t r o d u c t i o n Owing to the excellent resistance to corrosion, high strength/density rate, aluminum alloys have been widely used in automotive industry and aviation. Commonly. lubrication is required in operations. Alcohols are well-known effective lubricants, especially in the process of aluminum foil, sheet and extrusions [ I - 3 ] . Hotten [4] and Wan et al. [ 5 ] reported that linear et- and ~-diols are effective boundary lubricants for aluminum. Hironaka and Sakurai [2] pointed out that pentaerythritol partial ester which contains both hydroxyl and ester group is more effective as compared to the full ester. The practical and theoretical sradies of lubrication with lubricants containing fatty alcohols have been conducted for decades [6-11 ]. However, not all is known about the action mechanism of tbese compounds, especially in the aluminumon-steel system. Kajdas [ 12] put forward a viewpoint of anionic-radical concept which explains the lubrication mechanism of alcohols considerably. in this paper, the wear behavior of 2024AI against steel under the lubrication of liquid paraffin containing alcohols was evaluated The tribological properties of the system were * Correspondingauthor. Lanzhoa Institute of Chemical Physics. Lab. of Solid Lubrication,AcademiaSinica. 730000 Lanzhou,China. Tel.: +86931-8417088.
correlated to the molecular structures especially the numbers of hydroxy group in the alcohols. It is expected that this investigation will be helpful to understand the action mechanism of alcohols for lubrication of aluminum.
2. E x p e r i m e n t a l 2. I. Specimen
Liquid paraffin ( L P ) . which has a boiling point of above 300°C, density of 0.86 g cm -3, viscosity at 50 and 100 of 10.28 mm 2 s - i and 3.36 mm 2 s - i respectively, was used as base oil in the tests because of its simple structure and uniform friction and wear behaviors. The used additives, n-butanol, ethandiol, 1,3-butanediol, 1,4-butanediol and propanetriol, are all AR grade reagents used without further purification. The formulas and molecular weights of these compounds are listed in Table I. 2.2. Apparatus
The friction and wear tests were conducted on a block-onring wear tester (MRH-3 Block-on-Ring Wear Tester, Ji'nan Tester Factory, China), which is similar to the Timken Tester. The rings used in the tests are made of A1SI52100 steel with
0043-1648/98/$ - .see front matter © 1998 El~vier Science S.A, All rights reserved. PII S0043-1648 (98)00162-8
Y. Hu. W, Liu / Wear 218 t 1998) 244-249
Table I Formulaand molecularweightof the selectedcompound
2
Compound
Formula
Molecular weight
n-butanol ethandiol 1.3-butanediol ] .4-butanediol propanetriol
CH~CH,CH,CH~OH HOCH:CH:OH OHCH:CH:CH (OH )CH,
76 62 92 92 92
I .
HOCH:CH:CH-CH-OH
OHCH:CH(OH CH,OH
21H)
41111
~X)
Illln
iiiiii
I~)
141wt
^l~he,I tn'~JlNI
a HRc between 58 and 61, whereas the blocks are made of 2024AI with a Vicker hardness of 194.6. and their compositions arc listed in Table 2. Before each test, both the block and ring were polished with a No. 800 abrasive paper and cleaned with petroleum ether. Tests were carried out at a sliding velocity of 2.05 m s - ~( 800 rpm ) at room temperature with the dropping oil ( 0.7 ml rain - z) lubrication. The test time was selected to be 20 min because such a test duration will enable it to give a stable friction curve and an-easy-tomeasure wear scar. During friction test, the block was linear connected with the ring, and each length of the wear scars was almost equal to the width of the block as well. Different wear scar widths of the aluminum blocks after friction tests were observed under different loads with the lubrication of base oil containing different additives, which was used to assess the wear of the aluminum block. The wear scar widths were measured using an optical microscope. 2.3. Analytical technique
Since X-ray photoelectron spectroscopy I XPS) is very sensitive to investigate the chemical composition and chemical environment of the elements in a material, the chemical composition of the boundary film formed on the surface of 2024AI during the sliding process was evaluated using XPR, XPS analysis was conducted on a PHI-5702 electron spectrometer using a pass energy of 29.35 eV and the Mg K a line excitation source using the reference of C~s at 284.6 eV. Furthermore, the rubbed surfaces were studied by scanning electron microscopy (SEM). SEM was conducted on a EPMA-SIOQ electron probe microanalyzer, 3. Results and discussion
3. I. A n t i w e a r behaviors
Fig. l shows the wear scar width of aluminum block as a function of applied load with the lubrication of base oil con-
n-bulamll 1.3-bulancdud
+
@ +
inlFUp;l~lrllll
clhancdml 1.4-haam:du,I
Fig. 1. ~,'ear scar width of aluminumblock as a function of applied load v,ith the luhncationof base oil con~ning 3 wt.% alcohols.
•
o° oO
~ o3 f~'lO
II00
l(t~O
1200
14N10 I6(I0
inl~
Apldied Io~JqN) --I.--,3v4%
- - I 1 ~ twO.
--,dk--o.swt% --)4,--0.lwl% Fig. 2. Wear ~ar width as a funclion of ptopanetriol concentrationunder differemloads. taining 3 wt.% alcohols. Results in Fig. I clearly indicate that different alcohol exhibits quite different antiwear ability ( see also Table 3 ). With the lubrication of base oil containing nbutanol, the 2024AI-on-steel system failed at a load higher than 600 N. whereas with the lubrication of base oil c o n t ~ n ing ethandiol, the failure load was higher than 1000 N. However, base oil containing propanetriol is very effective to lubricate the 2024Al-on-stecl system even at a load of 1600 N. Results in Fig. I also show that propanetriol is the best antiwear additive, and the antiwear ability of 1,3-hotanediol is better than that of 1.4-butanediol. which can he explained by Hotton's and Wan's six-number ring bidentate bonding hypothesis [ 4,5 ]. Among ethandiol. 1.3-butanedioL and 1,4butanediol which have the same numbers of hydroxy groups, the 1,3-butanediol and 1+4-butanediol possess better antiwear and load-carrying capacities, which can he attributed to the alkyl chain length [ 13,14]. Fig. 2 gives the influence of load and propanetriol concentration on the wear of the aluminum block. Under normal loads lower than 1000 N, the effect of the concentration is
Table 2 Chemicalcompositionsof the SAE52I00 steel and A12024
SAE52100 steel A12024
C
Cr
0.95-1.05
1.30-1.65 0.15~0.35 0.204).40 0.50 0.34).9
Si
Mn
Cu
Mg
Zn
Fe
AI
Balance 3.8-4.9
1.2-1.8
0.25
Balance
246
Y. Hu. W. Liu / Wear 211¢(19981 244-249
Table 3 Mean ~'alue and dispersion of wear scar width of AI 2024 under different loads with the lubrication of base oil containing 3 wt.Cbdifferent alcohols
200 300 4O0 500 60O 800 I000 1200 1400
n-Butanol
Ethandiol
1.3-Butanediol
1.4-Butanediol
0.49±0.010 0.48 ± 0.026 0.64+_0.12 0.95 ± 11.058 1.90+_0.17
0.91+_0.026
0.56±0.044
0.66±0.052
0.99+-0.~1 1.36+_0.093 1.73±0.082
O.77+-0.012 0.83+_0.14 1.18±0.018 1.80±0.~7
0.92+-0.~8 1.00±0.024 1.38±0.037 1.88+_0.~0
Table 4 Mean value and dispersion of wear sear width of AL 2024 under different loads with the lubrication of base oil containing propanetriol at different concentrations
600 800 1000 1200 1400 16{10 1800
3wt.~
Iwt.~
0.5wt.~
0.1wt.~
0.59 + 0.046 0.97+-0.O60 0.91 :[::0.020 0.90 4-0.049 0.91 +-0.063 failure
0.79 +_0.013 0.6O+_0.042 0.86+0.055 1.63 -I-0.40
0.69_+O.II 0.71 +_0.0,41 0.72+0.045 0.82 + 0.018 0.86+_0.029 0.975:0.12 failure
0.43+0.0094 0.72+_0.042 1.66 +0.096 1.98 + 0.095
:
:
:!
p ,
,
:
:
:
•
:
:
:
:
:
Propanetriol
1).59+_0.~6 0.97±0.000 0.91±0.020 0.90+_0.~9 0.91+_0.003
not obvious, the w e a r has a tendency to increase slightly. With load increasing, it can be found that there are o p t i m u m concentrations. At the concentration o f 0.5 wt.%. propanetriol exhibits g o o d load-carrying capacity, it w a s considered that at the concentration lower than 0.5 wt.%, no effective and continuous adsorption film formed as well as at rather higher concentrations than 0.5 wt.%, propanetriol w a s readily to corrode the substrate. It w a s a little difficult to understand w h y the limiting load under the lubrication o f base oil conraining I wt.% propanetriol w a s lower than that o f 3 wt.%. it w a s probably subjected to the experimental errors which are presented in Table 4. It w a s also found that under the limiting loads, certain w e a r o f the a l u m i n u m block lubricated by base oil containing additives w a s so serious that even it could not endure a test duration, which led to the tests cut o f f and no definite w e a r scar widths obtained. W e l d w e a r took place under such conditions. T h e dotted vertical lines were introduced to demonstrate the weld loads. 3.2. X P S analyses o f worn su .rfaces
• ~7 " 76
~5 " q4
~3
i~
71
Binging Enexgy (eV) Fig. 3. XPS spectra of AI,r on the rubbed surface of aluminum block lubricated with ethandiol.
Figs. 3 - 7 g i v e the X P S spectra o f AI2o. Ci • and Ot~ o f w o r o scars o f the a l u m i n u m blocks lubricated with ethandiol and propanediol at the load o f 800 N and concentration o f 3 wt.%. respectively. Results in Figs. 3 and 4 s h o w that AI m i g h t be at least in two forms, one is AI203 with the binding e n e r g y o f AI2e at about 74.6 eV. another is probable the c o m p l e x e s
!......... i
.
.
.
.
.
. . . . . . . . . . . . . . . . .
74 73 72 71 Binding Energy (eV) Fig. 4. XPS spectra of Al.,r on the rubbed surface of aluminum block lubricated with propanetriol. 76
75
2~
2~
2U
2~
216
2B5
284
.
2~
2S2
Binding Energy (:V) Fig. 5. XPS spectra of C i, on the rubbed surface of aluminum block lubricated with ethandiol.
2~
2S9
2n
2S?
2~
2S5
2O4
2S~
~.~
Binding Energy(eV) Fig.6. XPSspectraof C,, on the rubbedsurfaceof aluminumblocklubricated with propanetriol.
3.4. Discus.~ion
S31.~TS
S34
S~
S~O
lubricate the surface well, aluminum transfer is serious under such a load as 600 N. adhesion wear is the dominative wear. Similar phenomena can be found under the lubrication of base oil containing 3 wt.% n-butanol or 1,3-butanediol. It is noticeable under the lubrication of base oil containing 0.5 wt.% propanetriol. There are some ridges beside the shallow grooves, plastic deformation took place as well as ploughing fainted which lead to mild wear under the load of 800 N ( Fig. 8h). Deep and narrow grooves accompanied some higher ridges are found under the high load of 1800 N (Fig. 8i). Scoring and plastic deformation contributed to the severe wear. It can be considered that ptopanetriol inhibited the transfer and adhesion of aluminum effectively and exhibited high load-carrying property.
~
S~
Binding Energy(eV) Fig. 7. XPS spectraof O,, on the rubbed surfaceof aluminumblock lubricated withethandioland ptopanetriol. of aluminum and ethandiol or propanetriol, with the corresponding binding energy of Al_~pfrom about 73.6 eV to 73.8 eV. An interesting phenomenon is the binding energy of Al_,p at 71.46 eV or 71.66 cV, which could not he attributed to any known forms of aluminum, one probable explanation is the formation of AIC compound. The binding energies (BE) of Ct~ in Figs. 5 and 6 fluctuated around 287.68, 285.98 and 284.46 eV suggest the formation of polyalkylene glycol during friction which was also proved in Kajdas and Mori's tests.
In boundary lubrication of an aluminum-on-steel system, the chemical inter,tction especially the chemical reaction of a lubricant with the rubbing surface could be very important. Hotten has investigated the lubricity of diol and ketol for aluminum, and has suggested the antiwear mechanism is due to the formation of aluminum diol complex or aluminum ketol complex [4l. Investigations in Laboratory of Solid Lubrication, Chine~ Academy of Sciences have proved the formation of aluminum diol complex, and have found that the complex of aluminum with 1.3-diol is more chemically stable than corresponding 1.4-diol or 2.3-diol [5]. The t~sults in this work illustrate that alcohols are effective additives in the lubrication of aluminum-on-steel system. This can be attributed to the chem-adsoq~on of alcohols and the formation of complex between aluminum and alcohols or the generation of friction polymer. The effect of the hydroxy numbers may include two aspects. With the inct~a~ of the hydroxy group numbers, the molecular polarity improves, consequently, a more stable chem-adsorption film was formed. On the other hand. it may make the cross-link of the friction polymer easier. Fig. 9 gives the suggested schematic diagram of the action mechanism of alcohols [ 12,15].
3.3. SEM analyses t~f worn surface.~
4. Conclusion
Fig. 8 shows SEM micrographs of the worn surfaces of AI blocks under their mild loads and failing loads, respectively. Several fine or flake-like wear debris were found on the worn tracks. Results clearly indicate that with the lubrication of base oil or base oil containing additives, the original polished surfaces were rubbed. There are deep and narrow grooves on the worn block surface under the lubrication of base oil under 300 N (Fig. 8b). The Al.,O.~ particles indented into the 2024AI surface were the abrasives that may cause ploughing wear. The surface is relatively flat and the grooves are not so clear under 600 N (Fig. 8c). it is suggested that LP can not
I. An aluminum-on-steel system can be lubricated effectively by alcohols. Their antiwear prapeaies increase as the alkyl chain length and hydroxy number increase. 2. The antiwear mechanism of the alcohols can be attributed to the tribochemical reaction on the robbed surfaces which produce aluminum alkoxide, aluminum complex and friction polymer. 3. The dominative wear is scuffing, plastic deformation and adhesion. Propauetriol is an excellent additive as an inhibitor from transfer.
....
248
........
~
....
E Hu. W. Liu/Wear218 (1998) 2 4 4 - . 2 4 9
) LP. wt.%
Y. Hu. W. Liu I Wear 218 ¢19981 244-249
Fig. 9. The schematic diagram of alcohols chem-adsorption and interaction with aluminum during friction process. Acknowledgements T h e authors gramfully a c k n o w l e d g e the financial support o f Natural Science Foundation o f Lanzhou Branch o f Chinese A c a d e m y o f Sciences, and National Natural Science Foundation o f China.
249
19] K.V. Shooter. The lubrication of metals by long-chain organic compounds, nr. J. Appl. Phys. Suppl. ( 1951 ) 49.-5 I. [10] W.A. Zisman. Friction, durability and wetability properties of monomolecular films on solids. Prec. Syrup. on Friction and Wear, Detroit, MI. 1957. Elsevier, Almlcrdam, 1959. I I 1I Cz. Kajdas. L Luczkiewicz, D. Ozimina, E. Wawak. Estimation of tribological properties of long-chain acids and alcohols, Prec. 2rid conL on Tribology, Budapest. October 24-26,1979, Vol. 2. pp. 619625. 112l K.Czeslaw.Aboutan~alonic-redicalconce~ofthelubricationmechanism of alcohols. Wear 116 (1987) 167-180. [ 13] L.E. St.Pierre, R.S.Owans, R,V.Kliar.Chemicaleffectsintheboondary lubrication of aluminum. Wear 9 (1966) 160-168. [ 141 P.C. Nauiyal, J.A. Schey. Transfer of aluminum to steel in sliding contact: effects of lubricanL J, Tribol. I 12 (1990) 282-287. ] 15] S. Mori. M. Sugmoya. Y. TamaL ~ of ocgani¢ compounds on a clean AI surface prepared by cutting under high vacuum. ASLE Trans. 25 (2) (1982) 261-266.
Biographi~ References [ I [ R.S. Montgomery, The effect of 'alcohols and ethers on the wear behavior of'aluminum, Wear 8 (1965) 466--473. [ 21 S. Hironaka, T. Sakural, The effect of pcntacrythfitol partial ester on the wear of aluminum, Wear 50 (1978) 105-114. [31 R.S. Montgomery. H.L. Garret, An electron-microscopic study of aluminum wear particles formed during sliding in the presence of polyglycols, Wear IO (1967) 310-318. [4l B.W. Hottcn. Bidentate organic compounds as boundary lubricants for alumianm, Lubr. Eng. 30 ( 19741 398-403. [ 51 Y- Wan. W. Liu. Q. Xue, Effects of diol compounds on the friction and wear of aluminum alloy in a lubricated aluminum-on-steel contact. Wear 193 (1996) 99-104. 161 W.G. neare. F.P. Bowdan, Physical properties of surfaces: I. Kinetic friction. Phil. Trans. R. Soc. London. Ser. A 234 (1935) 329--354. [7] W.B. Hardy. Collected Scientific Papers. Cambridge Univ. Pregs, London, 1936. [81 3. Sameshima, M. Kidokoro. H. Akamatsu, Oiliness of liquids: I. Measurements of the static friction coefficients. Bull Chem. Soc. Jpn. I I (1936) 659-666.
Y a n h o n g Hu received her B S f r o m the D c p a r l m c n t o f C h e m istry, H e ' n a n University in 1994. Currently, she is a studant enrolled in doctorate plrogran~ at the ~ o r y o f Solid Lubrication, Lanzhou Institute o f Clmmical Physics, Chinese A c a d e m y o f Sciences. W e i m i n Liu received his B S in C h e m i s t r y at S h a n d o n g Normal University in 1984 and P h D in Tribology at Lanzhou I n s u m t e o f Chemical Physics in 1990. H e j o i n e d the [,almratory o f Solid Lubrication, Lanzlmu Institute o f C h e m i c a l Physics, Chinese A c a d e m y o f Sciences in 1990, and f r o m 1993 to 1994 he spent I y e a r as a visiting scientist at I ~ m e n t o f Chemical Engineering, T h e Pennsylvania State University. His research interests include lubricant additive interactions, lubrication o f ceramic, solid lubrication, tribochemistry and fuel additives. Cuffcntly, he is a professor and Deputy Director o f the Laboratory o f Solid Lubrication.