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Friction and wear properties of a surface-modified TiO2 nanoparticle as an additive in liquid paraffin
WEAR ELSEVIER
Wc:tr -nl'~. ! I t)t)7 ~ .'~t)-.-'~'*
Friction and wear properties of a surface-modified TiO2 nanoparticle as an additive in liquid paraffin Qunji xue
....
~:. Weimin Liu ', Zhijun Zhang"
" l.(d~or¢/tt;t~v o/'S, di, I l.td, rictttitm. L t m z h , m In,slittll," ,d ('heroical I'hv.si,'.s. ('him'~c ..~c,,l,'mv .I Sci,,nc,'. h m z h o t ¢ 7 3 t ~ H I . P e o p h " x R,'lml,li," , . f Chim~ *' t "h,,mi.~trv I)~'l,.rtm,'n/, t l c n t m I 'ni~ 'c r~ilv, Kaili 'og 4 7.¢r7o I. I'~,~l,h," ~ h',,l,tddit. ~l" China Rcc¢ixctl 14 .lul.'. 19~)7: accepted 27 ,,'~tl~?|l',,t It)q7
Abstract A 2-ethyl hexoic acid (EHA) surthce-mt~dilied "I'i(), ilanoparliclc with an average diameter of 5 nm was chemically synthesized. The friction-reduction and antiwear behaviors of the prepared EHA-Ti(), as an oil additive in liquid paraffin were evaluated with a lore-ball wear tester. The rubbed surfaces after friction tests were invesligated with X-ray photoelectrtm .,pectroscopy ( XPS 1. Friction tests indicate that EHA-TiO_~ exhibits good performance in friction and wear reduction as well an in Ioad-carryine capacity. Results of XPS analysis show that a boundary film mainly comprised of TIC), and titanatcs was formed (.111lhe rubbed sttrfaccs. ~ 1997 Elsevier Science S.A. Kevu'm'ds: Oil additi~,e: 2-ethyl hexoic acid ( EHAI r.urface-modilicd
Tit,).,nanol~arliclc: Friction and ¢,ear behavion.~: XPS analy~,i~
I. Introduction
2. Experimental details
Nanometer size particles have received considerable attention in recent years because of their special physical and chemical properties. A variety of methods of preparing nanoparticles and in evaluating their properties have been developed [I-31. In previous work. nanometer size MoS: and Mo-S compounds were prepared chemically, and their tribological behaviour as an oil additive in liquid paraffin was investigated 14.51. The synthesized nanometer size MoS.., and Mo-S compounds could be well dissolved in a paraffin oil, and have shown excellent antiwear performance and high load-carrying capacity. Investigation of the coatings with TiO_~ and TiS.~ have shown that they provide good friction reduction and excellent antiwear properties 161. Titanium dialkyldithiophosphate (TiDDP) was proved to be effective its an extreme-pressure (EP) oil additive, and friction tests have found that its performance was even better than that of the zinc dialkyldithiophosphate (ZDDP i 17 I. In the present work, a 2-ethyl hcxoic acid. CHaCHC.,H.~CH,CH_,CH~COOH, ! EHA) surface-modilied TiO, nanoparticle was synthesized using a surlace-modilied method in organic solvent, and its tribological hehaviours were investigated using a tbur-ball wear tester. It is expected Ihat this effort will be helpful in understanding the antiwear mechanism of the EHA-TiO, nanoparticles as an oil additive.
2. I, Svntlwsis and .S'II'IQCIIII't'chara('leri:.alion of TiO..
* Corresponding author. 0043-1648/97/$17.00 ¢-~ 1997 Elsevier Science S.A. All right.,, reserved PIIS00431 6 4 8 ( 97 ) 0 0 2 n 0 - 7
nanoptlrli~'h,s The surface-modilied T i O . nanoparticlcs was synthesized with a method similar to that o f Severin el al, 181. and its
structure was characterized by infrared spectroscopy (IR). elemental :m:dysis (EA), X-ray photoelectron spectroscopy ( XPS ) and transmission electron microscopy ( TEM ). in the synthesis process, titanium alkoxide (TAOI was used as a precursor and the molar ratio of TAO:EHA:H:O was 1:2:l in the reaction process. TEM examination has shown that the diameter of the prepared particles was at~mt 5 nm. FTIR spectra, :is indicated in Fig. ! where the samples were prepared by coating nano-TiO_, solution on KBr cells, indicated that a very small amount of --OH groups or HaO exist on the surface hut no free EHA was detected in the nanoparticles. The vibration absorption at low frequencies, such as at wavenumbers of 797 c m l showed the existence of a TiO - T i backbone, illustrating the existence of the nano-TiO: core. The TiO: nanoparticles capped by a organic carbon hydrogen chain disperse well in non-polar and weak polar organic solvenls such as toluene. THF, acetone, chlorolbnn and liquid parafiin. XPS analyses to the prepared EHA-TiO: nanoparticles have Iound the elements C, O and Ti, and the results were shown in Fig. 2. The C., spectra measured consist of peaks at 284.6 eV (reference peak). 288.3 eV and
30
Q. Xm' ~'taL / Wear 213 t/997) 29-32 F
120
R
o/"%o
100 ,0
|
/O\c
/.o\_
e eo
E i
s~oo
so'oo
doo
i ~
2~
[500
i i,O00
o/ \ o ~.c. .;/
¢
500
W-ven,~m bet(e..m"~)
I
Fi~. I. I"TIRspeclra of lilt?prepared I-HA-TiO_~nanoparticlcs.
R ~ C;H~s
R
Fig. 3. The suggested slruclurc of ~he prepared EHA-TiO: nanopanicles. 7,80
00t l cls
?.S$
• 284.6
2.2. Trih,logical properties
2D6 I
~7N 7.05 S.It0 470
465 460 Binding
455
Energy(¢V)
O1s
?~oo
450
295
2gO 285 Binding Em'~y. (eV)
280
5298
531
Liquid paraffin, with boiling point 3(X)°C and viscosity at 50°C of 10.28 mm -~s ~, was used as the base oil because of its simple structure and unique tdbological behaviour. The friction and wear tests were carded out with a tour-ball wear tester in which a ball rotated against three stationary balls. The load-carrying capacity was also evaluated with a fourball tester according to ASTM D2783. The balls used in all tests were made of SAE521 ( ~ steel with a hardness of HRc from 59 to 6 I. Friction and wear tests were conducted at 1450 rpm under room temperature for 30 min. After testing, the mean wear scar diameter was measured using an optical microscope.
5.00
2.3. Analysis a f the rubbed s.C.'ace 3.00
~
1.00 54O
535 530 Blndiltg I~ngt'~, (¢1,,)
.~2~
Fig...2. XP."; spectra of the prepared I:HA-TiO~ nanop;mide~.
286.1 eV. corresponding to alkyl, carboxylale carbon, and alkoxide carbon, respectively. The binding energy of Ti,p~z, was 458.6 eV. which was in good agreement with the binding energy of Ti,p,j~ of TiO:. The O~, spectra measured consist peaks at 529.8 eV, 5 3 1 . 8 eV and 533.4 eV. corrcsponding to TiO,. carhoxylate and alkoxide oxygen, respectively. These results illustrate that [l'e prepared sample consists of a TiO: core and a modilied laye; of organic acid on the surface. In another words, the prepared sample is a kind of organic group-capped TiO2 nanoparticles or nanoparucles of organic-inorganic composite. From the above analyses the structure of the prepared EHA-TiO_, nanoparliclcs was suggested, as indicated in Fig. 3.
Alter wear tests, the steel balls were rinsed with ethanol and acetone, and the chemical composition and chemical state of the elements on the worn surfaces were analyzed by XPS. XPS analyses were carried out on an ESCALAB 210 Electron Spectrometer using a pass energy of 30 eV and the Mg K a line excitation source, and the given binding energies used the reference binding energy of C,, at 284.6 eV. Tahle I The maximum non-seizure load of EIfA-TiO: as an additive Addilivc
Bust oil 2-Ethyl h,:xoic acid F.HA-TiO: F.flA-TiO: EHA-TiO,
('oncen|ralion
P. value
( vol/~ )
(N )
I IX) 0.5 O. I 0.5
4(X) 6(X) 800 > I I(g)
1.0
> I I(X)
Four-ball, 1450 rpm. 3(X) N, !(} s. 2Y'C.
Q. Xu ; ez al. / Wear 21.1 t IVt)7J 2v- 32
3. Results and discussions
31
'i[I
1. 3. I, The tribulogical properties ~?I the mud(fied TiO,_ nam~particles
Table I gives the maximum non-seizure load referred as the P~ value of EHA modilied nano-TiO, as an additive in liquid paraffin at different concentrations. The results in Table I show that the maximum non-seizure load of the base oil is only 400 N, and with the addition of 0.5c/~ 2-ethyl hexoic acid, the Pa value reaches 600 N. However, with the addition of only O. I% EHA-TiO, nanoparticles, the ~'1~value increases to 800 N. and when the concentration of EHA-TiO, nanoparticles is 0.5% the maximum non-seizure loads is even higher than I I 0 0 N. Since 2-ethyl hexoic acid is a polar compound, and it easily adsorbs on the steel surface or even forms soap on the sliding surfaces to protect the occurrence of wear, so a relatively higher Pn value of liquid paraflin containing hexoic acid was observed. As to the liquid paraflin containing EHA-TiO_~ nanoparticles, the extreme pressure mechatfism might be different, because the maximum nonseizure load is much higher as compared to that of liquid paraffin containing 2-ethyl hexoic acid. The antiwear property of the EHA-TiO2 nanoparlicle as function of additive concentration in liquid paraflin is given in Fig. 4, Results indicates that as little as 0. I ?~ EHA-TiO, nanoparticle in base oil possesses excellent antiwear ability, and it can signilicantly reduce the wear scar diameter of a steel ball. The more interesting results in Fig. 4 are that a larger amount of EHA-TiO: nanoparticle in liquid paraffin could not further decrease wear. Fig. 5 gives the friction c o e f licient as a function of additive concentration. Results also
E 1 E o.g
I
EHA-TiO2 I
0.0 0.7
I
0.6
~ 0,S 0.4
~ 0.30.2
~ 0A00
0.$
1 1.S
2 2.5
m _.o**
I
0.8 I m 0A ,~ O.2 ~:
O0 100 200 300 400 500 600 7P9 Applied Load ( N ) I:i~. ~. Wear near diamcler a.,,a functi~mof applied load wittl the lubrication ~,l"the ha.~¢t)il or I).5',~ EHA-TiO2 ( four-hall. 1450 rpm. 311(IN. 3t1 rain. 25'('~. illustrate that. with the addition of EHA-TiO, nanoparticle to the base oil, the friction coefficient reduced remarkably, However, unlike the results in Fig. 4, the friction coefficient decreases gradually as the additive concentration increa~s. Fig. 6 show.~ the wear scar diameter as a function of applied load, with the lubrication of the base oil containing 0.5% EHA-TiO, nanoparticle. As a comparison, the results of the base oil arc al,,o given in Fig. 6. With the lubrication of liquid paraflin only, a relative bigger wear scar diameter was observed, and the friction system sculling at a load of 400 N. However. with the lubrication of liquid paraffin containing 0.5')~ EHA-TiO_~ .mnoparticles, a smaller wear .~ardiameter was generated, and the friction system could he lubricated effectively even at a load of 6IX) N. 3.2. 7he re.vult.v o./XPS unalysex on the worn .vu~aces Since XPS is very sensitive at investigaling the chemical composition and chemical environment of the elements in a material, it was used to evaluate the chemical coml~}sition of tile boundary lilm formed on the worn surfaces. Figs. 7-9 show the binding energies of C,~. O~p and Tizr,. respectively. all on the worn surfaces alter a four-ball test at 300 N for 30 rain wilh the lubrication of a base oil conlaining 0.5% EHATiO~ nanoparticles. Results in Fig. 7 show that before sputtering the binding energy of C~, is 288.0 eV and 284.6 eV:
3 3.S
Concentration ( % } Fig. 4, Wear scar diaineter a.~ a function of additive concci)tration ( flinthall. 1451)rpm. 3181N. 3111nin.25"(?I. 0.2 0.18 ,t 0,16 u 0,14 0.12
EHA-Ti02__ ]
0.1 0.08 ,~ 0,00
0,04 0.02 0(~ 0:5
1
1,5
2
2.5
;
3.5
Con~ntmUon ( % ) Fig. 5. Friction coefficient as a functionof addlliVeconcentration ( limr-ball, 1450 rpm, 300 N, 30 rain. 25"C).
. . . . . . . Binding Energy ( eV ) Fig. 7. The hindmg ¢l|ergy o1"Ci, in the i~)unda~ lilnt at difler~n! :puttering tinge.,,,.
32
Q. Xue et al. / Wear 213 (19~7) 29-32
Binding Energy ( eV ) Fig. 8. The binding energy of O~, in the boundary tihn at different sputtering
times.
9, it is concluded that the boundary film is mainly composed of TiO_,. Nanometer materials possess many special physical and chemical properties, such as quantum size effect, small size effect. ~trface and interface effects, it is anticipated that the nanometer materials as oil or grease additives will provide a well bonded boundary film to the steel surfaces which will enable it to work at high temperature and extremely high load. Furtl'termore, an organic acid modified nanoparticle may be effective in lubricating a sliding system both at low or high toad. At mild load, the organic acid might react with the metal surface to form a chemical adsorbed or reacted boundary film, while at higher load the inorganic core of TiO., nanoparticles might react with the steel surface or be deposited on the steel surfaces to form a ceramic-like film, In this investigation, the four-ball wear tests and XPS analyses have proved that the organic acid capped TiO,, owns excellent loadcarrying capacity due to the formation of a TiO_, boundary lilm on the steel surface.
4a Conclusions
470
4~'~
460
45~
4~0
Binding Energy ( eV ) Fig. 9. The binding energy ol'Ti:r in the boundary film at different sputtering times.
the binding energy at 288.0 eV corresponds to carboxylate, while at 284.6 eV corresponds to hydrogen chain or absorbed carbon. This result illustrates that the fatty acid is strongly bonded to the friction surface. Results in Fig. 7 also indicate that after 23 min sputtering the peak at 288.00 eV disappeared, and the peak at 284.6 eV became weaker and a new peak at 282.6 eV appeared. This new peak may correspond to FeC or correspond to the reduction of carbon by argon ions. Results in Fig. 8 of the binding energy of O~, show that on the original surfaces the binding energy of O ~ is about 530 eV and 531.5 eV: the binding energy at 531.5 eV corresponds to carboxylate, while 530 eV corresponds to TiO,. Results in Fig. 8 also show that after sputtering, lbr 23 min no carboxylate could be detected and only the binding energy of O ~ at 530 eV could be observed. From Fig. 9, it is clear that the binding energy of Ti_,~,~z is 458.3 eV and has almost no apparent change with sputtering time increase. This result reveals that the TiO, exists in the rubbed surface. After 23 min sputtering, the peak of Ti,~,~/_, is still strong, while at this time the peak o f C ~, is quite weak. From the results in Figs. 7-
I. Under boundary lubrication conditions, 2-ethyl hexoic acid ( E H A ) surface-modified TiO: nanoparticles possess excellent load-carrying capacity and good antiwear and friction-reduction properties. . 2-ethyl hexoic acid modified TiO., nanoparticles as an oil additive could form a boundary film, mainly composed of TiO-,, to provide an antiwear function and load-carrying capacity.
Acknowledgements The authors wish to acknowledge the generous support of Chinese Academy of Sciences and National Natural Science Foundation of China.
References I I I D, Chahavorty. A.K. Girt, Chemistry for tile 21st Century, in C.N.R. Ran 4Ed.). Chemistry of Advanced Materials. Blackwell Scientilic Publication. London. 1993, pp. 217. 121 H. Gleiter. Prong.Mater. Sci. 33 ( 1991 ) 223-227. 131 D. Papoutsi, P. Llanos, P. Yianoulis, P. Koutsoukos, Langmuir I0 ( 1994 } 1684-1687. 141 Z. Zhang, J. Zhang. Q. Xue, J. Physical Chemistry t.lq ( 1994 ) 1297312977. 151 Q. Xue. Z. Zhang, J. Zhang, Prtv.:eedingsof the International Conference ~n Surface Science and Engineering. in R. Zhu lEd.), International Academic Publishers. Beijing. P.R. China. 1995, pp. 553-556. 161 Y. Wang. Wear 128 11988) 277-290. 171 D. Wang, H. Li. B. Li, R. Wang, Lubrication Engineering 48 (1992) 497-499, 181 K.G. Severin. J.S.I.edfiwd. B.A. Torgerson. K.A. Berglund. Chem. Mater. 6 i 1994} 89{~898.
Wc:tr -nl'~. ! I t)t)7 ~ .'~t)-.-'~'*
Friction and wear properties of a surface-modified TiO2 nanoparticle as an additive in liquid paraffin Qunji xue
....
~:. Weimin Liu ', Zhijun Zhang"
" l.(d~or¢/tt;t~v o/'S, di, I l.td, rictttitm. L t m z h , m In,slittll," ,d ('heroical I'hv.si,'.s. ('him'~c ..~c,,l,'mv .I Sci,,nc,'. h m z h o t ¢ 7 3 t ~ H I . P e o p h " x R,'lml,li," , . f Chim~ *' t "h,,mi.~trv I)~'l,.rtm,'n/, t l c n t m I 'ni~ 'c r~ilv, Kaili 'og 4 7.¢r7o I. I'~,~l,h," ~ h',,l,tddit. ~l" China Rcc¢ixctl 14 .lul.'. 19~)7: accepted 27 ,,'~tl~?|l',,t It)q7
Abstract A 2-ethyl hexoic acid (EHA) surthce-mt~dilied "I'i(), ilanoparliclc with an average diameter of 5 nm was chemically synthesized. The friction-reduction and antiwear behaviors of the prepared EHA-Ti(), as an oil additive in liquid paraffin were evaluated with a lore-ball wear tester. The rubbed surfaces after friction tests were invesligated with X-ray photoelectrtm .,pectroscopy ( XPS 1. Friction tests indicate that EHA-TiO_~ exhibits good performance in friction and wear reduction as well an in Ioad-carryine capacity. Results of XPS analysis show that a boundary film mainly comprised of TIC), and titanatcs was formed (.111lhe rubbed sttrfaccs. ~ 1997 Elsevier Science S.A. Kevu'm'ds: Oil additi~,e: 2-ethyl hexoic acid ( EHAI r.urface-modilicd
Tit,).,nanol~arliclc: Friction and ¢,ear behavion.~: XPS analy~,i~
I. Introduction
2. Experimental details
Nanometer size particles have received considerable attention in recent years because of their special physical and chemical properties. A variety of methods of preparing nanoparticles and in evaluating their properties have been developed [I-31. In previous work. nanometer size MoS: and Mo-S compounds were prepared chemically, and their tribological behaviour as an oil additive in liquid paraffin was investigated 14.51. The synthesized nanometer size MoS.., and Mo-S compounds could be well dissolved in a paraffin oil, and have shown excellent antiwear performance and high load-carrying capacity. Investigation of the coatings with TiO_~ and TiS.~ have shown that they provide good friction reduction and excellent antiwear properties 161. Titanium dialkyldithiophosphate (TiDDP) was proved to be effective its an extreme-pressure (EP) oil additive, and friction tests have found that its performance was even better than that of the zinc dialkyldithiophosphate (ZDDP i 17 I. In the present work, a 2-ethyl hcxoic acid. CHaCHC.,H.~CH,CH_,CH~COOH, ! EHA) surface-modilied TiO, nanoparticle was synthesized using a surlace-modilied method in organic solvent, and its tribological hehaviours were investigated using a tbur-ball wear tester. It is expected Ihat this effort will be helpful in understanding the antiwear mechanism of the EHA-TiO, nanoparticles as an oil additive.
2. I, Svntlwsis and .S'II'IQCIIII't'chara('leri:.alion of TiO..
* Corresponding author. 0043-1648/97/$17.00 ¢-~ 1997 Elsevier Science S.A. All right.,, reserved PIIS00431 6 4 8 ( 97 ) 0 0 2 n 0 - 7
nanoptlrli~'h,s The surface-modilied T i O . nanoparticlcs was synthesized with a method similar to that o f Severin el al, 181. and its
structure was characterized by infrared spectroscopy (IR). elemental :m:dysis (EA), X-ray photoelectron spectroscopy ( XPS ) and transmission electron microscopy ( TEM ). in the synthesis process, titanium alkoxide (TAOI was used as a precursor and the molar ratio of TAO:EHA:H:O was 1:2:l in the reaction process. TEM examination has shown that the diameter of the prepared particles was at~mt 5 nm. FTIR spectra, :is indicated in Fig. ! where the samples were prepared by coating nano-TiO_, solution on KBr cells, indicated that a very small amount of --OH groups or HaO exist on the surface hut no free EHA was detected in the nanoparticles. The vibration absorption at low frequencies, such as at wavenumbers of 797 c m l showed the existence of a TiO - T i backbone, illustrating the existence of the nano-TiO: core. The TiO: nanoparticles capped by a organic carbon hydrogen chain disperse well in non-polar and weak polar organic solvenls such as toluene. THF, acetone, chlorolbnn and liquid parafiin. XPS analyses to the prepared EHA-TiO: nanoparticles have Iound the elements C, O and Ti, and the results were shown in Fig. 2. The C., spectra measured consist of peaks at 284.6 eV (reference peak). 288.3 eV and
30
Q. Xm' ~'taL / Wear 213 t/997) 29-32 F
120
R
o/"%o
100 ,0
|
/O\c
/.o\_
e eo
E i
s~oo
so'oo
doo
i ~
2~
[500
i i,O00
o/ \ o ~.c. .;/
¢
500
W-ven,~m bet(e..m"~)
I
Fi~. I. I"TIRspeclra of lilt?prepared I-HA-TiO_~nanoparticlcs.
R ~ C;H~s
R
Fig. 3. The suggested slruclurc of ~he prepared EHA-TiO: nanopanicles. 7,80
00t l cls
?.S$
• 284.6
2.2. Trih,logical properties
2D6 I
~7N 7.05 S.It0 470
465 460 Binding
455
Energy(¢V)
O1s
?~oo
450
295
2gO 285 Binding Em'~y. (eV)
280
5298
531
Liquid paraffin, with boiling point 3(X)°C and viscosity at 50°C of 10.28 mm -~s ~, was used as the base oil because of its simple structure and unique tdbological behaviour. The friction and wear tests were carded out with a tour-ball wear tester in which a ball rotated against three stationary balls. The load-carrying capacity was also evaluated with a fourball tester according to ASTM D2783. The balls used in all tests were made of SAE521 ( ~ steel with a hardness of HRc from 59 to 6 I. Friction and wear tests were conducted at 1450 rpm under room temperature for 30 min. After testing, the mean wear scar diameter was measured using an optical microscope.
5.00
2.3. Analysis a f the rubbed s.C.'ace 3.00
~
1.00 54O
535 530 Blndiltg I~ngt'~, (¢1,,)
.~2~
Fig...2. XP."; spectra of the prepared I:HA-TiO~ nanop;mide~.
286.1 eV. corresponding to alkyl, carboxylale carbon, and alkoxide carbon, respectively. The binding energy of Ti,p~z, was 458.6 eV. which was in good agreement with the binding energy of Ti,p,j~ of TiO:. The O~, spectra measured consist peaks at 529.8 eV, 5 3 1 . 8 eV and 533.4 eV. corrcsponding to TiO,. carhoxylate and alkoxide oxygen, respectively. These results illustrate that [l'e prepared sample consists of a TiO: core and a modilied laye; of organic acid on the surface. In another words, the prepared sample is a kind of organic group-capped TiO2 nanoparticles or nanoparucles of organic-inorganic composite. From the above analyses the structure of the prepared EHA-TiO_, nanoparliclcs was suggested, as indicated in Fig. 3.
Alter wear tests, the steel balls were rinsed with ethanol and acetone, and the chemical composition and chemical state of the elements on the worn surfaces were analyzed by XPS. XPS analyses were carried out on an ESCALAB 210 Electron Spectrometer using a pass energy of 30 eV and the Mg K a line excitation source, and the given binding energies used the reference binding energy of C,, at 284.6 eV. Tahle I The maximum non-seizure load of EIfA-TiO: as an additive Addilivc
Bust oil 2-Ethyl h,:xoic acid F.HA-TiO: F.flA-TiO: EHA-TiO,
('oncen|ralion
P. value
( vol/~ )
(N )
I IX) 0.5 O. I 0.5
4(X) 6(X) 800 > I I(g)
1.0
> I I(X)
Four-ball, 1450 rpm. 3(X) N, !(} s. 2Y'C.
Q. Xu ; ez al. / Wear 21.1 t IVt)7J 2v- 32
3. Results and discussions
31
'i[I
1. 3. I, The tribulogical properties ~?I the mud(fied TiO,_ nam~particles
Table I gives the maximum non-seizure load referred as the P~ value of EHA modilied nano-TiO, as an additive in liquid paraffin at different concentrations. The results in Table I show that the maximum non-seizure load of the base oil is only 400 N, and with the addition of 0.5c/~ 2-ethyl hexoic acid, the Pa value reaches 600 N. However, with the addition of only O. I% EHA-TiO, nanoparticles, the ~'1~value increases to 800 N. and when the concentration of EHA-TiO, nanoparticles is 0.5% the maximum non-seizure loads is even higher than I I 0 0 N. Since 2-ethyl hexoic acid is a polar compound, and it easily adsorbs on the steel surface or even forms soap on the sliding surfaces to protect the occurrence of wear, so a relatively higher Pn value of liquid paraflin containing hexoic acid was observed. As to the liquid paraflin containing EHA-TiO_~ nanoparticles, the extreme pressure mechatfism might be different, because the maximum nonseizure load is much higher as compared to that of liquid paraffin containing 2-ethyl hexoic acid. The antiwear property of the EHA-TiO2 nanoparlicle as function of additive concentration in liquid paraflin is given in Fig. 4, Results indicates that as little as 0. I ?~ EHA-TiO, nanoparticle in base oil possesses excellent antiwear ability, and it can signilicantly reduce the wear scar diameter of a steel ball. The more interesting results in Fig. 4 are that a larger amount of EHA-TiO: nanoparticle in liquid paraffin could not further decrease wear. Fig. 5 gives the friction c o e f licient as a function of additive concentration. Results also
E 1 E o.g
I
EHA-TiO2 I
0.0 0.7
I
0.6
~ 0,S 0.4
~ 0.30.2
~ 0A00
0.$
1 1.S
2 2.5
m _.o**
I
0.8 I m 0A ,~ O.2 ~:
O0 100 200 300 400 500 600 7P9 Applied Load ( N ) I:i~. ~. Wear near diamcler a.,,a functi~mof applied load wittl the lubrication ~,l"the ha.~¢t)il or I).5',~ EHA-TiO2 ( four-hall. 1450 rpm. 311(IN. 3t1 rain. 25'('~. illustrate that. with the addition of EHA-TiO, nanoparticle to the base oil, the friction coefficient reduced remarkably, However, unlike the results in Fig. 4, the friction coefficient decreases gradually as the additive concentration increa~s. Fig. 6 show.~ the wear scar diameter as a function of applied load, with the lubrication of the base oil containing 0.5% EHA-TiO, nanoparticle. As a comparison, the results of the base oil arc al,,o given in Fig. 6. With the lubrication of liquid paraflin only, a relative bigger wear scar diameter was observed, and the friction system sculling at a load of 400 N. However. with the lubrication of liquid paraffin containing 0.5')~ EHA-TiO_~ .mnoparticles, a smaller wear .~ardiameter was generated, and the friction system could he lubricated effectively even at a load of 6IX) N. 3.2. 7he re.vult.v o./XPS unalysex on the worn .vu~aces Since XPS is very sensitive at investigaling the chemical composition and chemical environment of the elements in a material, it was used to evaluate the chemical coml~}sition of tile boundary lilm formed on the worn surfaces. Figs. 7-9 show the binding energies of C,~. O~p and Tizr,. respectively. all on the worn surfaces alter a four-ball test at 300 N for 30 rain wilh the lubrication of a base oil conlaining 0.5% EHATiO~ nanoparticles. Results in Fig. 7 show that before sputtering the binding energy of C~, is 288.0 eV and 284.6 eV:
3 3.S
Concentration ( % } Fig. 4, Wear scar diaineter a.~ a function of additive concci)tration ( flinthall. 1451)rpm. 3181N. 3111nin.25"(?I. 0.2 0.18 ,t 0,16 u 0,14 0.12
EHA-Ti02__ ]
0.1 0.08 ,~ 0,00
0,04 0.02 0(~ 0:5
1
1,5
2
2.5
;
3.5
Con~ntmUon ( % ) Fig. 5. Friction coefficient as a functionof addlliVeconcentration ( limr-ball, 1450 rpm, 300 N, 30 rain. 25"C).
. . . . . . . Binding Energy ( eV ) Fig. 7. The hindmg ¢l|ergy o1"Ci, in the i~)unda~ lilnt at difler~n! :puttering tinge.,,,.
32
Q. Xue et al. / Wear 213 (19~7) 29-32
Binding Energy ( eV ) Fig. 8. The binding energy of O~, in the boundary tihn at different sputtering
times.
9, it is concluded that the boundary film is mainly composed of TiO_,. Nanometer materials possess many special physical and chemical properties, such as quantum size effect, small size effect. ~trface and interface effects, it is anticipated that the nanometer materials as oil or grease additives will provide a well bonded boundary film to the steel surfaces which will enable it to work at high temperature and extremely high load. Furtl'termore, an organic acid modified nanoparticle may be effective in lubricating a sliding system both at low or high toad. At mild load, the organic acid might react with the metal surface to form a chemical adsorbed or reacted boundary film, while at higher load the inorganic core of TiO., nanoparticles might react with the steel surface or be deposited on the steel surfaces to form a ceramic-like film, In this investigation, the four-ball wear tests and XPS analyses have proved that the organic acid capped TiO,, owns excellent loadcarrying capacity due to the formation of a TiO_, boundary lilm on the steel surface.
4a Conclusions
470
4~'~
460
45~
4~0
Binding Energy ( eV ) Fig. 9. The binding energy ol'Ti:r in the boundary film at different sputtering times.
the binding energy at 288.0 eV corresponds to carboxylate, while at 284.6 eV corresponds to hydrogen chain or absorbed carbon. This result illustrates that the fatty acid is strongly bonded to the friction surface. Results in Fig. 7 also indicate that after 23 min sputtering the peak at 288.00 eV disappeared, and the peak at 284.6 eV became weaker and a new peak at 282.6 eV appeared. This new peak may correspond to FeC or correspond to the reduction of carbon by argon ions. Results in Fig. 8 of the binding energy of O~, show that on the original surfaces the binding energy of O ~ is about 530 eV and 531.5 eV: the binding energy at 531.5 eV corresponds to carboxylate, while 530 eV corresponds to TiO,. Results in Fig. 8 also show that after sputtering, lbr 23 min no carboxylate could be detected and only the binding energy of O ~ at 530 eV could be observed. From Fig. 9, it is clear that the binding energy of Ti_,~,~z is 458.3 eV and has almost no apparent change with sputtering time increase. This result reveals that the TiO, exists in the rubbed surface. After 23 min sputtering, the peak of Ti,~,~/_, is still strong, while at this time the peak o f C ~, is quite weak. From the results in Figs. 7-
I. Under boundary lubrication conditions, 2-ethyl hexoic acid ( E H A ) surface-modified TiO: nanoparticles possess excellent load-carrying capacity and good antiwear and friction-reduction properties. . 2-ethyl hexoic acid modified TiO., nanoparticles as an oil additive could form a boundary film, mainly composed of TiO-,, to provide an antiwear function and load-carrying capacity.
Acknowledgements The authors wish to acknowledge the generous support of Chinese Academy of Sciences and National Natural Science Foundation of China.
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