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Investigation of the structure and tribological characteristics of monolayers deposited by the Langmuir–Blodgett technique
Applied Surface Science 167 Ž2000. 152–159 www.elsevier.nlrlocaterapsusc
Investigation of the structure and tribological characteristics of monolayers deposited by the Langmuir–Blodgett technique Peihong Cong, Hidetaka Nanao, Takashi Igari, Shigeyuki Mori ) Faculty of Engineering, Iwate UniÕersity, 4-3-5 Ueda, Morioka 020-8551, Japan Received 14 February 2000; accepted 12 May 2000
Abstract A series of partially fluorinated ethyl esters with the same hydrophobic group length were deposited by the Langmuir– Blodgett ŽLB. technique on an Al plate coated with a NiP film. Film structures were inferred through surface pressure– molecular area isotherms Žp –A isotherms.. The results indicate that the film structures are determined by van der Waals forces between hydrophobic chains. Stronger mutual forces lead to stable solid films. Frictional properties and load-carrying capacities of the monolayers were evaluated using a ball-on-plate-type sliding apparatus. The friction coefficient and load-carrying capacity are dependent on the monolayer film structures. The solid film shows a lower friction coefficient and a higher load-carrying capacity. Friction tracks of the monolayers were examined by time-of-flight secondary ion mass spectroscopy ŽTOF-SIMS.. We found that the liquid film may be a fluid monolayer, and that the structure of the solid film changes under frictional force. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Microtribology; LB film; Surface free energy; Frictional properties; Load-carrying capacity
1. Introduction With the development of microminiaturized, high-performance technological devices such as magnetic storage devices, lubricant films that are thinner, more durable, and with lower friction are required w1,2x. Therefore, it is necessary to elucidate the tribological mechanisms of ultra-thin organic lubricants, which are also termed boundary lubricants. Recently, atomic force microscopy ŽAFM., friction force microscopy ŽFFM. and scanning force ) Corresponding author. Tel.: q81-19-621-6335; fax: q81-19621-6335. E-mail address: [email protected] ŽS. Mori..
microscope ŽSFM. w3–7x, coupled with computer simulation w8,9x, have led to significant progress in understanding the microtribological mechanisms of organic thin films. However, the relations between tribological properties, molecular structure, and organic-film structures at the mesoscopic scale are not well understood. Organic films deposited by the Langmuir–Blodgett ŽLB. technique are one of the best boundarylubricating model systems for studying the relationships between structures and tribological properties, and organic films of fluorinated surfactants are useful materials because of their high chemical stability. This system is also one of the best choices for solving lubricating problems in micromachines.
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 4 4 1 - 4
P. Cong et al.r Applied Surface Science 167 (2000) 152–159
In this study, the LB technique was used to deposit monolayers of a series of partially fluorinated molecules on an Al plate coated with a NiP film. Tribological properties of the monolayers were investigated under milli-Norton loads. The objectives of this research are to clarify the influence of molecular structures and monolayer film structures on mesoscopic tribological characteristics, and to obtain useful information about structures and wear characterization of ultra-thin films by time-of-flight secondary ion mass spectroscopy ŽTOF-SIMS..
2. Experimental details Three kinds of partially fluorinated long-chain ethyl esters, which have the same chain length of hydrophobic group, were selected as samples. The chemical structure of the samples, the simplified names used in this paper and theoretical lengths of the hydrophobic chains are shown in Table 1. All simplified names of the samples are written as m y n, where m is the fluorocarbon atom number and n is the hydrocarbon atom number. The total lengths of the hydrophobic chains are equal, and, therefore, its effect on film structures and tribological properties may be neglected. Solutions of the samples were made at a concentration of about 1 mM in chloroform. Substrates were Al plates coated with NiP films Žabbreviated as NiP. with surface roughnesses of 1–2 nm. The substrate was rinsed in benzenerethanol solution and ultra-pure water in succession using an ultrasonic dispersed instrument, after which the substrate was treated in O 3 for 20 min to remove surface organic contaminants. Surface pressure–molecular area isotherms Žp –A isotherms. and area variation with increasing time
153
Ž A–t curves. of the samples were obtained using a computer-controlled Langmuir trough. The solutions were spread uniformly over a subphase of ultra-pure water at 20.0 " 0.18C. At least 30 min was allowed for evaporation of the solvent prior to compression. Films were compressed at a rate of 0.1 mmrs. According to the p –A isotherms, the preset pressure of the A–t curves and deposition process of the monolayers was 20 mNrm. The deposition speed was 1 mmrmin. Properties of each monolayers were described by molecular area at the preset pressure of 20 mNrm on the trough. A ball-on-plate sliding friction tester was used in the study. Fig. 1Ža. shows the schematic diagram of the friction tester. The slider was an optical glass lens composed mainly of SiO 2 Ž68.9%., B 2 O 3 Ž10.1%., Na 2 O Ž8.8%. and K 2 O Ž8.4%. with a diameter of 10 mm and a curvature radius of 7.19 mm. The treatment process of the slider before the friction experiments was the same as the substrate. The slider was fixed, and the specimen was moved by an automicrometer. Sliding of the specimen was started after load having been applied for about 30 s. Friction tests were carried out three times on each slider using unworn films in each experiment. The sliding speed was 0.02 mm sy1 , the sliding distance was 1 mm, and the load ranged from 10 to 80 mN. All friction tests were performed at a room temperature of 24 " 18C and a relative humidity of 48–62%. During the sliding process, the applied load could be adjusted. The frictional force was measured with a spring attached to the slider arm. The applied load and frictional force were recorded and calculated automatically by an attached computer. For monolayers sliding with a clean slider, obvious stick–slip phenomena were observed at the beginning of the friction tests. Fig. 1Žb. shows a typical frictional force variation with an increase in sliding
Table 1 Chemical structure, simplified name and theoretical lengths of the hydrophobic chains of the samples Chemical structure
Simplified name
CF3 ŽCF2 . 6 CH 2 CHIŽCH 2 . 8 COOC 2 H 5 CF3 ŽCF2 . 7 CH 2 CHIŽCH 2 . 7 COOC 2 H 5 CF3 ŽCF2 .11CH 2 CHIŽCH 2 . 3 COOC 2 H 5
7–10 8–9 12–5
Theoretical lengths of the hydrophobic chains Žnm. Fluorocarbon chain
Hydrocarbon chain
Total
1.0 1.3 1.6
1.3 1.0 0.7
2.3 2.3 2.3
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P. Cong et al.r Applied Surface Science 167 (2000) 152–159
3. Results and discussion 3.1. Characterization of the monolayers
Fig. 1. Schematic diagram of the friction tester and typical variation of the frictional force with an increase in sliding time Žab: baseline of applied load; bc: applied load; AB: baseline of frictional force; B: friction start; DE: steady-state frictional force; E: friction stop..
time recorded by the attached computer. At the mesoscopic scale, it is believed that the effect of adhesive force on the friction coefficient could be ignored if the relative humidity is less than 70% w10x. Therefore, the friction coefficient of the monolayers was obtained by dividing the steady-state frictional force by the load Žindicated as Fdynamic and Load in Fig. 1Žb., respectively.. The load-carrying capacity of the monolayer was evaluated by monitoring the change of the frictional force with an increase in the applied load. Film failure was defined as the point where the amplitude of the frictional force vibration exceeded 10 mN, and the corresponding load was defined as the load-carrying capacity of the monolayer. Friction tracks were observed by means of TOF-SIMS following friction tests. Operational pressure in the ultra-high vacuum system during analysis was lower than 10y8 Torr and analysis area was 120 = 120 mm. Static contact angles of the monolayers and the substrate were measured at ambient conditions with distilled water and CH 2 I 2 . Surface free energies were calculated from the measurement of the contact angles.
p –A isotherms of the samples are shown in Fig. 2. The molecular area of the monolayers at the surface pressure of 20 mNrm on the trough, and the theoretical area of the hydrophilic group are shown in Table 2. The areas occupied by each molecule on the trough at 20 mNrm range from 0.32 to 0.49 nm2 , which are greater than the theoretical area Ž0.25 nm2 . of ethyl ester hydrophilic group. This may be due to the methods of computer calculation about the theoretical area of the ethyl ester hydrophilic group. It was assumed that the ethyl group attached to the hydrophilic group and the hydrophobic chain were in a straight line, in other words, the steric effect of the ethyl group was neglected in the computer calculation. Moreover, the large iodine atom attached to the hydrocarbon chain may also prevent a more condensed monolayer being formed. In addition, molecular cross-section area of the fluorocarbon chain is 0.28 nm2 w11x, which is also larger than the theoretical area of the ethyl ester. It is also observed that the shapes of the p –A isotherms and collapse pressures of 7–10 and 8–9, which have shorter fluorinated chain, are different from 12–5. While the compression process is proceeding, surface pressures of the 7–10 and 8–9 monolayers increase gradually, and the collapse pressures are lower than that of the 12–5 monolayer. Area variations of the monolayers on the trough with an increase in time Ž A–t curves. at a surface
Fig. 2. Surface pressure–molecular area isotherms Žp – A isotherms. of the samples at 20.0"0.18C.
P. Cong et al.r Applied Surface Science 167 (2000) 152–159 Table 2 Molecular area of the monolayers at the surface pressure of 20 mNrm on the trough and the theoretical area of the hydrophilic group Sample
Molecular area Žnm2 rmolecule.
Theoretical area of the hydrophilic group Žnm2 .
7–10 8–9 12–5
0.49"0.01 0.43"0.02 0.32"0.01
0.25 0.25 0.25
pressure of 20 mNrm are shown in Fig. 3. Under the preset pressure, with time increasing the occupied areas of the 7–10 and 8–9 monolayers decrease gradually, however, the area of the 12–5 monolayer is nearly a constant. From Figs. 2 and 3, it could be concluded that 7–10 and 8–9 formed mechanically unstable monolayers, whereas 12–5 formed a stable monolayer on the trough. The molecular area at a preset pressure on the trough is one useful parameter to characterize the monolayer film structure. When the area occupied by each molecule on the trough at the preset pressure is near the extreme area Žtheoretical area of hydrophilic group., the molecules are nearly perpendicularly oriented, and are closely packed in a two-dimensional solid-like film. If the area occupied by each molecule on the trough at the preset pressure is smaller or larger than the extreme area, then an unstable film or liquid film was obtained w12x. Based on the abovementioned viewpoint and the experimental results shown in Figs. 2 and 3 and Table 2, it could be inferred that 7–10 and 8–9 formed liquid films Žsee Fig. 4Ža.., 12–5 formed solid film Žsee Fig. 4Žb.. on the trough even under the same experimental conditions. It is also found that the molecular area of the 12–5 monolayer at 20 mNrm Žsee Table 2. is larger than the molecular cross-section area of fluorocarbon chain Ž0.28 nm2 . and the theoretical area of ethyl ester Ž0.25 nm2 .. This indicates that even the 12–5 molecule cannot form the most closely packed monolayer, which may be due to the steric effects of the ethyl ester hydrophilic group and the iodine atom attached to the hydrocarbon chain. It is also interested to note that an increase in fluorination ratio correlates to a change in the monolayer film structure from liquid to solid film.
155
It is well known that van der Waals forces between hydrocarbon chains are stronger than that between fluorocarbon chains w8,9x. However, the greater fluorinated molecule of 12–5 could form solid-like monolayer Žsee Fig. 2. at the air–water interface. This may be due to the fact that the steric effects of the hydrophilic group and the iodine atom cause hydrocarbon chains to separate greatly from each other, and that mutual intermolecular forces between hydrocarbon chains become weaker than that between fluorocarbon chains w13,14x in partially fluorinated ordered ultra-thin films. It is generally assumed that if the deposition process is well controlled, films transferred to substrates are very similar to those on the trough w15x. Static contact angle measurements were carried out to ensure that the transferred films maintained their original structures, and the contact angles were converted into their surface free energies. Fig. 5 shows surface free energies of the monolayers on the NiP substrate. For comparison, the surface free energy of the NiP substrate is also shown in Fig. 5. It can be seen that the surface free energies of the monolayers are lower than that of the substrate, and surface free energy of the 12–5 monolayer is lower than that of the 7–10 and 8–9 monolayers, despite the fact that they have the same end group of `CF3 . These results further confirm that the 12–5 solid film is more well ordered than the 7–10 and 8–9 liquid films. 3.2. Tribological behaÕior of the monolayers Fig. 6 shows friction coefficients of the monolayers under a load of 10 mN. To compare the friction
Fig. 3. Area variations of the monolayers on the trough with an increase in time at a surface pressure of 20 mNrm Žtemperature: 20.0"0.18C..
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Fig. 4. Schematic diagram of the monolayer film structures on the trough.
coefficients of the monolayers with the substrate, we also measured the friction coefficient of the clean NiP substrate. As can be seen from Fig. 6, all of the monolayers caused a significant reduction in the friction coefficient of the substrate. The friction coefficient of the 12–5 monolayer is lower than that of the 7–10 and 8–9 monolayers. Computer simulation w8,9x of the friction of LB films showed that the frictional force of perfluorocarboxylic acids and semifluorocarboxylic acids were larger than that of hydrocarboxylic acids. In our research work on partially fluorinated ethyl esters, it is found that with an increase in fluorination ratio correlates to a decrease in friction coefficient. This result indicates that film structure is a major influence on frictional properties of the monolayers, and the film structure is determined by van der Waals forces between hydrophobic chain.
Relations between the surface free energies and the friction coefficients of the monolayers are shown in Fig. 7. It can be seen that a lower surface free energy correlates to a lower friction coefficient, which indicates that well-ordered solid monolayer can offer better frictional properties. In order to measure the load-carrying capacity of the monolayers, the applied load was increased in a stepwise way during friction tests. Besides the monolayers sliding with a clean slider, stick–slip frictional phenomena were also observed for liquid films at loads higher than 10 mN. Fig. 8 shows the typical frictional force variations of the monolayers with an increase in sliding time under different loads. The results of molecular dynamic ŽMD. simulation has clarified that the stick–slip frictional phenomena of ordered organic monolayers depend on such factors as load, terminal groups, chain packing
Fig. 5. Surface free energies of the monolayers and the NiP substrate.
Fig. 6. Friction coefficients of the monolayers and the NiP substrate Žload: 10 mN; speed: 0.02 mmrs..
P. Cong et al.r Applied Surface Science 167 (2000) 152–159
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shown in Fig. 9. The amplitude of the frictional force vibration of the 12–5 monolayer is 0 mN, even when load was increased to 80 mN, stick–slip phenomena were not observed. When load is increased to 40–50 mN, the amplitudes of the frictional forces vibration of the 7–10 and 8–9 monolayers are larger than 10 mN. When load was increased to 80 mN, the amplitudes of the frictional force vibration exceeded maximum which the friction tester could be obtained. These indicate that under sliding at higher load, the Fig. 7. Relations between the surface free energy and the friction coefficient of the monolayers Žload: 10 mN; speed: 0.02 mmrs..
density Žtilt angle. and friction anisotropy w16–18x. In our study, we selected the samples with the same terminal group of `CF3 , then it could be deduced that under the same load the difference of the packing density of the solid and liquid monolayers is the reason to cause stick–slip frictional phenomena for liquid film. In fact, we have found that the liquid film of carboxylic acid showed a larger title angle than its solid film by measurements of the monolayer thickness. According to the molecular areas shown in Table 2, we know that the packing densities of the 7–10 and 8–9 liquid films are 2.0 and 2.3 moleculesrnm2 , respectively, which are lower than that of the 12–5 solid film Ž3.1 moleculesrnm2 .. Higher frictional force in stick–slip frictional phenomena may be due to that lower packing density and weaker mutual molecular interactions in the liquid film are not enough to carry the applied load, the slider contact to the substrate during sliding process. On the other hand, the lower frictional force in stick–slip frictional phenomena could be due to the transferred molecule on the slider w19x. It can also be seen from Fig. 8 that the amplitude of the frictional force vibration is greater under a load of 80 mN than that under a load of 40 mN, indicating that the amplitude of the frictional force vibration may be used to characterize load-carrying capacity of the monolayers. During sliding under a constant load, if the stick–slip phenomena could be observed and the amplitude of frictional force vibration is larger than 10 mN, the monolayer is deduced to have collapsed in this paper. The amplitudes of the frictional force vibration of the monolayers as a function of applied load are
Fig. 8. Typical frictional force variations of the monolayers with an increase in sliding time under different loads Žspeed: 0.02 mmrs..
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P. Cong et al.r Applied Surface Science 167 (2000) 152–159
Fig. 9. Amplitudes of the frictional force vibration of the monolayers as a function of applied load Žspeed: 0.02 mmrs..
liquid film collapses more easily, on the other words, the load-carrying capacity of the liquid film is lower than that of the solid film. Fig. 10 shows variations in the frictional forces of the monolayers vs. applied load. When the amplitude of the frictional force vibration exceeds 10 mN, the monolayer is defined as having failed, so the frictional forces of the 7–10 and 8–9 monolayers under loads higher than 40 mN are not shown in Fig. 10. It can be seen that the frictional force of the 12–5 solid film is lower than that of the 7–10 and 8–9 liquid films under all of the loads. In addition, we tried to extend the applied loads to 0 mN, and found that the corresponding frictional forces are not 0 mN for all the monolayers. This indicates that some factors, such as the adhesive forces between the slider and the monolayers, perhaps have some influences on the friction coefficients of the monolayers at the mesoscopic scale. According to the intercepts of the approximate lines, which are also shown in Fig. 10, we found that these influences are very little. In order to get more friction and wear information of the monolayers with different film structures,
Fig. 10. Variations in the frictional forces of the monolayers vs. applied load Žspeed: 0.02 mmrs..
Fig. 11. Total secondary-ion mapping of friction track of the 12–5 monolayer Žload: 20 mN; speed: 0.02 mmrs..
friction tracks of the 8–9 liquid film and 12–5 solid film were observed by means of TOF-SIMS. For the 8–9 liquid film, the corresponding friction tracks could not be found despite extensive searching. Before TOF-SIMS observation, it was necessary to put the specimen into the vacuum chamber for more than 1 h. It is possible that the liquid film is a kind of fluid monolayer and has self-repairing properties, the friction tracks have been repaired before the TOFSIMS observation. Fig. 11 shows total secondary-ions mapping of the friction track of the 12–5 solid film after sliding under a load of 20 mN. An obvious friction track of about 20 mm is observed, which proves that solid film structure have a change during rubbing process. It has also been found that this film structure change affected the frictional properties of the LB film w13x. Some white points are also observed on the mapping, these are inferred to be defects in the 12–5 solid film.
4. Conclusions The relations between monolayer film structures and tribological properties of a series of partially fluorinated ethyl esters, which share the same length of the hydrophobic chain, were investigated. It is concluded that the monolayer film structure is determined by mutual intermolecular force between adjacent hydrophobic groups. Stronger van der Waals forces lead to the formation of well-ordered solid film on the trough. It is found that the frictional properties and load-carrying capacity of the monolayers are dependent on the monolayer film struc-
P. Cong et al.r Applied Surface Science 167 (2000) 152–159
tures. Solid film can offer a lower friction coefficient and a higher load-carrying capacity. The results of TOF-SIMS analysis indicate that the liquid film is a kind of fluid monolayer film, and the structure of the solid film changes under frictional force.
References w1x M.L. Gee, P.M. McGuiggan, J.N. Israelachvili, A.M. Homola, J. Chem. Phys. 93 Ž1990. 1895. w2x B. Bhushan, J.N. Israelachvili, U. Landman, Nature 374 Ž1995. 607. w3x R.M. Overney, E. Meyer, J. Frommer, D. Brodbeck, R. Luthi, M. Fujihira, H. Takano, ¨ L. Howald, H.-J. Guntherodt, ¨ Y. Gotoh, Nature 359 Ž1992. 133. w4x R.M. Overney, E. Meyer, J. Frommer, H.-J. Guntherodt, M. ¨ Fujihira, H. Takano, Y. Gotoh, Langmuir 10 Ž1994. 1281. w5x R.M. Overney, H. Takano, M. Fujihira, E. Meyer, H.-J. Guntherodt, Thin Solid Films 240 Ž1994. 105. ¨ w6x R.M. Overney, E. Meyer, J. Frommer, H.-J. Guntherodt, G. ¨ Decher, J. Reibel, U. Sohling, Langmuir 9 Ž1993. 341.
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w7x V.V. Tsukruk, V.N. Bliznyuk, J. Haze, D. Visser, Langmuir 12 Ž1996. 4840. w8x A. Koike, M. Yoneya, Langmuir 13 Ž1997. 1718. w9x A. Koike, M. Yoneya, J. Chem. Phys. 105 Ž1996. 6060. w10x Y. Inoue, Tribologist ŽJ. Jpn. Soc. Tribol.. 39 Ž1994. 201. w11x H. Le Breton, B. Bennetan, J. Dunogues, J.-F. Letard, R. Lapouyde, L. Vignau, J.-P. Morand, Langmuir 11 Ž1995. 1353. w12x A.W. Adamson, Physical Chemistry of Surfaces, 3rd edn., John Wiley and Sons, Inc., New York, 1976. w13x T. Igari, N. Oyamada, H. Nanao, S. Mori, Tribologist ŽJ. Jpn. Soc. Tribol.. 45 Ž2000. 414. w14x P. Cong, T. Igari, H. Nanao, S. Mori, J. Jpn. Pet. Inst. 43 Ž2000. 241. w15x R.A. Hann, in: G. Roberts ŽEd.., Molecular Structure and Monolayer Properties in Langmuir–Blodgett Films, Plenum, New York, 1990, pp. 17–92. w16x T. Ohzono, J.N. Glosli, M. Fujihira, Jpn. J. Appl. Phys. 38 Ž1999. L675. w17x M. Fujihira, T. Ohzono, Jpn. J. Appl. Phys. 38 Ž1999. 3918. w18x T. Ohzono, J.N. Glosli, M. Fujihira, Jpn. J. Appl. Phys. 37 Ž1998. 6535. w19x H. Nanao, P.H. Cong, S. Mori, Tribologist ŽJ. Jpn. Soc. Tribol.. 44 Ž1999. 642.
Investigation of the structure and tribological characteristics of monolayers deposited by the Langmuir–Blodgett technique Peihong Cong, Hidetaka Nanao, Takashi Igari, Shigeyuki Mori ) Faculty of Engineering, Iwate UniÕersity, 4-3-5 Ueda, Morioka 020-8551, Japan Received 14 February 2000; accepted 12 May 2000
Abstract A series of partially fluorinated ethyl esters with the same hydrophobic group length were deposited by the Langmuir– Blodgett ŽLB. technique on an Al plate coated with a NiP film. Film structures were inferred through surface pressure– molecular area isotherms Žp –A isotherms.. The results indicate that the film structures are determined by van der Waals forces between hydrophobic chains. Stronger mutual forces lead to stable solid films. Frictional properties and load-carrying capacities of the monolayers were evaluated using a ball-on-plate-type sliding apparatus. The friction coefficient and load-carrying capacity are dependent on the monolayer film structures. The solid film shows a lower friction coefficient and a higher load-carrying capacity. Friction tracks of the monolayers were examined by time-of-flight secondary ion mass spectroscopy ŽTOF-SIMS.. We found that the liquid film may be a fluid monolayer, and that the structure of the solid film changes under frictional force. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Microtribology; LB film; Surface free energy; Frictional properties; Load-carrying capacity
1. Introduction With the development of microminiaturized, high-performance technological devices such as magnetic storage devices, lubricant films that are thinner, more durable, and with lower friction are required w1,2x. Therefore, it is necessary to elucidate the tribological mechanisms of ultra-thin organic lubricants, which are also termed boundary lubricants. Recently, atomic force microscopy ŽAFM., friction force microscopy ŽFFM. and scanning force ) Corresponding author. Tel.: q81-19-621-6335; fax: q81-19621-6335. E-mail address: [email protected] ŽS. Mori..
microscope ŽSFM. w3–7x, coupled with computer simulation w8,9x, have led to significant progress in understanding the microtribological mechanisms of organic thin films. However, the relations between tribological properties, molecular structure, and organic-film structures at the mesoscopic scale are not well understood. Organic films deposited by the Langmuir–Blodgett ŽLB. technique are one of the best boundarylubricating model systems for studying the relationships between structures and tribological properties, and organic films of fluorinated surfactants are useful materials because of their high chemical stability. This system is also one of the best choices for solving lubricating problems in micromachines.
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 4 4 1 - 4
P. Cong et al.r Applied Surface Science 167 (2000) 152–159
In this study, the LB technique was used to deposit monolayers of a series of partially fluorinated molecules on an Al plate coated with a NiP film. Tribological properties of the monolayers were investigated under milli-Norton loads. The objectives of this research are to clarify the influence of molecular structures and monolayer film structures on mesoscopic tribological characteristics, and to obtain useful information about structures and wear characterization of ultra-thin films by time-of-flight secondary ion mass spectroscopy ŽTOF-SIMS..
2. Experimental details Three kinds of partially fluorinated long-chain ethyl esters, which have the same chain length of hydrophobic group, were selected as samples. The chemical structure of the samples, the simplified names used in this paper and theoretical lengths of the hydrophobic chains are shown in Table 1. All simplified names of the samples are written as m y n, where m is the fluorocarbon atom number and n is the hydrocarbon atom number. The total lengths of the hydrophobic chains are equal, and, therefore, its effect on film structures and tribological properties may be neglected. Solutions of the samples were made at a concentration of about 1 mM in chloroform. Substrates were Al plates coated with NiP films Žabbreviated as NiP. with surface roughnesses of 1–2 nm. The substrate was rinsed in benzenerethanol solution and ultra-pure water in succession using an ultrasonic dispersed instrument, after which the substrate was treated in O 3 for 20 min to remove surface organic contaminants. Surface pressure–molecular area isotherms Žp –A isotherms. and area variation with increasing time
153
Ž A–t curves. of the samples were obtained using a computer-controlled Langmuir trough. The solutions were spread uniformly over a subphase of ultra-pure water at 20.0 " 0.18C. At least 30 min was allowed for evaporation of the solvent prior to compression. Films were compressed at a rate of 0.1 mmrs. According to the p –A isotherms, the preset pressure of the A–t curves and deposition process of the monolayers was 20 mNrm. The deposition speed was 1 mmrmin. Properties of each monolayers were described by molecular area at the preset pressure of 20 mNrm on the trough. A ball-on-plate sliding friction tester was used in the study. Fig. 1Ža. shows the schematic diagram of the friction tester. The slider was an optical glass lens composed mainly of SiO 2 Ž68.9%., B 2 O 3 Ž10.1%., Na 2 O Ž8.8%. and K 2 O Ž8.4%. with a diameter of 10 mm and a curvature radius of 7.19 mm. The treatment process of the slider before the friction experiments was the same as the substrate. The slider was fixed, and the specimen was moved by an automicrometer. Sliding of the specimen was started after load having been applied for about 30 s. Friction tests were carried out three times on each slider using unworn films in each experiment. The sliding speed was 0.02 mm sy1 , the sliding distance was 1 mm, and the load ranged from 10 to 80 mN. All friction tests were performed at a room temperature of 24 " 18C and a relative humidity of 48–62%. During the sliding process, the applied load could be adjusted. The frictional force was measured with a spring attached to the slider arm. The applied load and frictional force were recorded and calculated automatically by an attached computer. For monolayers sliding with a clean slider, obvious stick–slip phenomena were observed at the beginning of the friction tests. Fig. 1Žb. shows a typical frictional force variation with an increase in sliding
Table 1 Chemical structure, simplified name and theoretical lengths of the hydrophobic chains of the samples Chemical structure
Simplified name
CF3 ŽCF2 . 6 CH 2 CHIŽCH 2 . 8 COOC 2 H 5 CF3 ŽCF2 . 7 CH 2 CHIŽCH 2 . 7 COOC 2 H 5 CF3 ŽCF2 .11CH 2 CHIŽCH 2 . 3 COOC 2 H 5
7–10 8–9 12–5
Theoretical lengths of the hydrophobic chains Žnm. Fluorocarbon chain
Hydrocarbon chain
Total
1.0 1.3 1.6
1.3 1.0 0.7
2.3 2.3 2.3
154
P. Cong et al.r Applied Surface Science 167 (2000) 152–159
3. Results and discussion 3.1. Characterization of the monolayers
Fig. 1. Schematic diagram of the friction tester and typical variation of the frictional force with an increase in sliding time Žab: baseline of applied load; bc: applied load; AB: baseline of frictional force; B: friction start; DE: steady-state frictional force; E: friction stop..
time recorded by the attached computer. At the mesoscopic scale, it is believed that the effect of adhesive force on the friction coefficient could be ignored if the relative humidity is less than 70% w10x. Therefore, the friction coefficient of the monolayers was obtained by dividing the steady-state frictional force by the load Žindicated as Fdynamic and Load in Fig. 1Žb., respectively.. The load-carrying capacity of the monolayer was evaluated by monitoring the change of the frictional force with an increase in the applied load. Film failure was defined as the point where the amplitude of the frictional force vibration exceeded 10 mN, and the corresponding load was defined as the load-carrying capacity of the monolayer. Friction tracks were observed by means of TOF-SIMS following friction tests. Operational pressure in the ultra-high vacuum system during analysis was lower than 10y8 Torr and analysis area was 120 = 120 mm. Static contact angles of the monolayers and the substrate were measured at ambient conditions with distilled water and CH 2 I 2 . Surface free energies were calculated from the measurement of the contact angles.
p –A isotherms of the samples are shown in Fig. 2. The molecular area of the monolayers at the surface pressure of 20 mNrm on the trough, and the theoretical area of the hydrophilic group are shown in Table 2. The areas occupied by each molecule on the trough at 20 mNrm range from 0.32 to 0.49 nm2 , which are greater than the theoretical area Ž0.25 nm2 . of ethyl ester hydrophilic group. This may be due to the methods of computer calculation about the theoretical area of the ethyl ester hydrophilic group. It was assumed that the ethyl group attached to the hydrophilic group and the hydrophobic chain were in a straight line, in other words, the steric effect of the ethyl group was neglected in the computer calculation. Moreover, the large iodine atom attached to the hydrocarbon chain may also prevent a more condensed monolayer being formed. In addition, molecular cross-section area of the fluorocarbon chain is 0.28 nm2 w11x, which is also larger than the theoretical area of the ethyl ester. It is also observed that the shapes of the p –A isotherms and collapse pressures of 7–10 and 8–9, which have shorter fluorinated chain, are different from 12–5. While the compression process is proceeding, surface pressures of the 7–10 and 8–9 monolayers increase gradually, and the collapse pressures are lower than that of the 12–5 monolayer. Area variations of the monolayers on the trough with an increase in time Ž A–t curves. at a surface
Fig. 2. Surface pressure–molecular area isotherms Žp – A isotherms. of the samples at 20.0"0.18C.
P. Cong et al.r Applied Surface Science 167 (2000) 152–159 Table 2 Molecular area of the monolayers at the surface pressure of 20 mNrm on the trough and the theoretical area of the hydrophilic group Sample
Molecular area Žnm2 rmolecule.
Theoretical area of the hydrophilic group Žnm2 .
7–10 8–9 12–5
0.49"0.01 0.43"0.02 0.32"0.01
0.25 0.25 0.25
pressure of 20 mNrm are shown in Fig. 3. Under the preset pressure, with time increasing the occupied areas of the 7–10 and 8–9 monolayers decrease gradually, however, the area of the 12–5 monolayer is nearly a constant. From Figs. 2 and 3, it could be concluded that 7–10 and 8–9 formed mechanically unstable monolayers, whereas 12–5 formed a stable monolayer on the trough. The molecular area at a preset pressure on the trough is one useful parameter to characterize the monolayer film structure. When the area occupied by each molecule on the trough at the preset pressure is near the extreme area Žtheoretical area of hydrophilic group., the molecules are nearly perpendicularly oriented, and are closely packed in a two-dimensional solid-like film. If the area occupied by each molecule on the trough at the preset pressure is smaller or larger than the extreme area, then an unstable film or liquid film was obtained w12x. Based on the abovementioned viewpoint and the experimental results shown in Figs. 2 and 3 and Table 2, it could be inferred that 7–10 and 8–9 formed liquid films Žsee Fig. 4Ža.., 12–5 formed solid film Žsee Fig. 4Žb.. on the trough even under the same experimental conditions. It is also found that the molecular area of the 12–5 monolayer at 20 mNrm Žsee Table 2. is larger than the molecular cross-section area of fluorocarbon chain Ž0.28 nm2 . and the theoretical area of ethyl ester Ž0.25 nm2 .. This indicates that even the 12–5 molecule cannot form the most closely packed monolayer, which may be due to the steric effects of the ethyl ester hydrophilic group and the iodine atom attached to the hydrocarbon chain. It is also interested to note that an increase in fluorination ratio correlates to a change in the monolayer film structure from liquid to solid film.
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It is well known that van der Waals forces between hydrocarbon chains are stronger than that between fluorocarbon chains w8,9x. However, the greater fluorinated molecule of 12–5 could form solid-like monolayer Žsee Fig. 2. at the air–water interface. This may be due to the fact that the steric effects of the hydrophilic group and the iodine atom cause hydrocarbon chains to separate greatly from each other, and that mutual intermolecular forces between hydrocarbon chains become weaker than that between fluorocarbon chains w13,14x in partially fluorinated ordered ultra-thin films. It is generally assumed that if the deposition process is well controlled, films transferred to substrates are very similar to those on the trough w15x. Static contact angle measurements were carried out to ensure that the transferred films maintained their original structures, and the contact angles were converted into their surface free energies. Fig. 5 shows surface free energies of the monolayers on the NiP substrate. For comparison, the surface free energy of the NiP substrate is also shown in Fig. 5. It can be seen that the surface free energies of the monolayers are lower than that of the substrate, and surface free energy of the 12–5 monolayer is lower than that of the 7–10 and 8–9 monolayers, despite the fact that they have the same end group of `CF3 . These results further confirm that the 12–5 solid film is more well ordered than the 7–10 and 8–9 liquid films. 3.2. Tribological behaÕior of the monolayers Fig. 6 shows friction coefficients of the monolayers under a load of 10 mN. To compare the friction
Fig. 3. Area variations of the monolayers on the trough with an increase in time at a surface pressure of 20 mNrm Žtemperature: 20.0"0.18C..
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Fig. 4. Schematic diagram of the monolayer film structures on the trough.
coefficients of the monolayers with the substrate, we also measured the friction coefficient of the clean NiP substrate. As can be seen from Fig. 6, all of the monolayers caused a significant reduction in the friction coefficient of the substrate. The friction coefficient of the 12–5 monolayer is lower than that of the 7–10 and 8–9 monolayers. Computer simulation w8,9x of the friction of LB films showed that the frictional force of perfluorocarboxylic acids and semifluorocarboxylic acids were larger than that of hydrocarboxylic acids. In our research work on partially fluorinated ethyl esters, it is found that with an increase in fluorination ratio correlates to a decrease in friction coefficient. This result indicates that film structure is a major influence on frictional properties of the monolayers, and the film structure is determined by van der Waals forces between hydrophobic chain.
Relations between the surface free energies and the friction coefficients of the monolayers are shown in Fig. 7. It can be seen that a lower surface free energy correlates to a lower friction coefficient, which indicates that well-ordered solid monolayer can offer better frictional properties. In order to measure the load-carrying capacity of the monolayers, the applied load was increased in a stepwise way during friction tests. Besides the monolayers sliding with a clean slider, stick–slip frictional phenomena were also observed for liquid films at loads higher than 10 mN. Fig. 8 shows the typical frictional force variations of the monolayers with an increase in sliding time under different loads. The results of molecular dynamic ŽMD. simulation has clarified that the stick–slip frictional phenomena of ordered organic monolayers depend on such factors as load, terminal groups, chain packing
Fig. 5. Surface free energies of the monolayers and the NiP substrate.
Fig. 6. Friction coefficients of the monolayers and the NiP substrate Žload: 10 mN; speed: 0.02 mmrs..
P. Cong et al.r Applied Surface Science 167 (2000) 152–159
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shown in Fig. 9. The amplitude of the frictional force vibration of the 12–5 monolayer is 0 mN, even when load was increased to 80 mN, stick–slip phenomena were not observed. When load is increased to 40–50 mN, the amplitudes of the frictional forces vibration of the 7–10 and 8–9 monolayers are larger than 10 mN. When load was increased to 80 mN, the amplitudes of the frictional force vibration exceeded maximum which the friction tester could be obtained. These indicate that under sliding at higher load, the Fig. 7. Relations between the surface free energy and the friction coefficient of the monolayers Žload: 10 mN; speed: 0.02 mmrs..
density Žtilt angle. and friction anisotropy w16–18x. In our study, we selected the samples with the same terminal group of `CF3 , then it could be deduced that under the same load the difference of the packing density of the solid and liquid monolayers is the reason to cause stick–slip frictional phenomena for liquid film. In fact, we have found that the liquid film of carboxylic acid showed a larger title angle than its solid film by measurements of the monolayer thickness. According to the molecular areas shown in Table 2, we know that the packing densities of the 7–10 and 8–9 liquid films are 2.0 and 2.3 moleculesrnm2 , respectively, which are lower than that of the 12–5 solid film Ž3.1 moleculesrnm2 .. Higher frictional force in stick–slip frictional phenomena may be due to that lower packing density and weaker mutual molecular interactions in the liquid film are not enough to carry the applied load, the slider contact to the substrate during sliding process. On the other hand, the lower frictional force in stick–slip frictional phenomena could be due to the transferred molecule on the slider w19x. It can also be seen from Fig. 8 that the amplitude of the frictional force vibration is greater under a load of 80 mN than that under a load of 40 mN, indicating that the amplitude of the frictional force vibration may be used to characterize load-carrying capacity of the monolayers. During sliding under a constant load, if the stick–slip phenomena could be observed and the amplitude of frictional force vibration is larger than 10 mN, the monolayer is deduced to have collapsed in this paper. The amplitudes of the frictional force vibration of the monolayers as a function of applied load are
Fig. 8. Typical frictional force variations of the monolayers with an increase in sliding time under different loads Žspeed: 0.02 mmrs..
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Fig. 9. Amplitudes of the frictional force vibration of the monolayers as a function of applied load Žspeed: 0.02 mmrs..
liquid film collapses more easily, on the other words, the load-carrying capacity of the liquid film is lower than that of the solid film. Fig. 10 shows variations in the frictional forces of the monolayers vs. applied load. When the amplitude of the frictional force vibration exceeds 10 mN, the monolayer is defined as having failed, so the frictional forces of the 7–10 and 8–9 monolayers under loads higher than 40 mN are not shown in Fig. 10. It can be seen that the frictional force of the 12–5 solid film is lower than that of the 7–10 and 8–9 liquid films under all of the loads. In addition, we tried to extend the applied loads to 0 mN, and found that the corresponding frictional forces are not 0 mN for all the monolayers. This indicates that some factors, such as the adhesive forces between the slider and the monolayers, perhaps have some influences on the friction coefficients of the monolayers at the mesoscopic scale. According to the intercepts of the approximate lines, which are also shown in Fig. 10, we found that these influences are very little. In order to get more friction and wear information of the monolayers with different film structures,
Fig. 10. Variations in the frictional forces of the monolayers vs. applied load Žspeed: 0.02 mmrs..
Fig. 11. Total secondary-ion mapping of friction track of the 12–5 monolayer Žload: 20 mN; speed: 0.02 mmrs..
friction tracks of the 8–9 liquid film and 12–5 solid film were observed by means of TOF-SIMS. For the 8–9 liquid film, the corresponding friction tracks could not be found despite extensive searching. Before TOF-SIMS observation, it was necessary to put the specimen into the vacuum chamber for more than 1 h. It is possible that the liquid film is a kind of fluid monolayer and has self-repairing properties, the friction tracks have been repaired before the TOFSIMS observation. Fig. 11 shows total secondary-ions mapping of the friction track of the 12–5 solid film after sliding under a load of 20 mN. An obvious friction track of about 20 mm is observed, which proves that solid film structure have a change during rubbing process. It has also been found that this film structure change affected the frictional properties of the LB film w13x. Some white points are also observed on the mapping, these are inferred to be defects in the 12–5 solid film.
4. Conclusions The relations between monolayer film structures and tribological properties of a series of partially fluorinated ethyl esters, which share the same length of the hydrophobic chain, were investigated. It is concluded that the monolayer film structure is determined by mutual intermolecular force between adjacent hydrophobic groups. Stronger van der Waals forces lead to the formation of well-ordered solid film on the trough. It is found that the frictional properties and load-carrying capacity of the monolayers are dependent on the monolayer film struc-
P. Cong et al.r Applied Surface Science 167 (2000) 152–159
tures. Solid film can offer a lower friction coefficient and a higher load-carrying capacity. The results of TOF-SIMS analysis indicate that the liquid film is a kind of fluid monolayer film, and the structure of the solid film changes under frictional force.
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