Adhesion and Friction Properties of Molecularly Thin

Feb 1, 2012 - The friction forces on the Z-03 films having different thicknesses were ... For a more comprehensive list of citations to this article, ...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/Langmuir

Adhesion and Friction Properties of Molecularly Thin Perfluoropolyether Liquid Films on Solid Surface Hiroshi Tani* and Norio Tagawa Department of Mechanical Engineering, Kansai University, 3-3-35, Yamate-cho, Suita-shi, Osaka 564-8680, Japan ABSTRACT: The adhesion and friction properties of molecularly thin perfluoropolyether (PFPE) lubricant films dip-coated on a diamond-like carbon (DLC) overcoat of magnetic disks were studied using a pin-on-disk-type micro-tribotester that we developed. The load and friction forces were simultaneously measured on a rotating disk surface under an increasing/decreasing load cycle and slow sliding conditions. Experiments were performed using two types of PFPE lubricants: Fomblin Z-tetraol2000S with functional end-groups and Fomblin Z-03 without any end-group. The curves of the friction force as a function of the applied load agree with the curves estimated using the Johnson−Kendall−Roberts (JKR) model. The friction forces on the Z-03 films having different thicknesses were not found to decrease drastically; however, the friction forces on the Z-tetraol film were found to decrease drastically when the film thickness is more than ∼1.2 nm. This drastic change in the case of the Z-tetraol film is estimated to be affected by the coverage of the lubricant film. (0−2000 μm/s) and at small applied loads (1−100 nN) using a sharp probe with a curvature radius of ∼20 nm. The contact pressure of the AFM probes is very high (in the order of gigapascals) because of the extremely small contact area of the sharp probe. Because of its measurement capabilities and the range of the magnitude of the testing parameters, neither the SFA nor the AFM is directly applicable to a practical tribological measurement of a thin lubricant film on magnetic disks because the size of the slider pads ranges from tens to hundreds of micrometers. Thomas et al. measured a novel region of tribological interaction by inducing near contact between the magnetic recording slider and the disk.11 They observed that there is a sudden transition to high friction and vibration (bridged state) at some low touchdown pressure when the ambient pressure is decreased with the magnetic recording slider initially flying over the lubricated disk surface; they also discussed the HDI phenomena at the slider contact on the disk surface. On the other hand, to measure the tribological properties, He et al. developed a pin-on-disk-type micro-tribotester that can measure adhesion and friction forces.12,13 Their tester mainly consists of a spindle motor, a friction sensor, a slider load/unload switching mechanism, an incremental loading actuator mechanism, and an optical displacement sensor. The friction force is measured on the rotating disk surface, and the pull-off force is measured on the stationary disk surface. The sensors of this device have high sensitivities; however, the device has a few drawbacks that are expected to generate certain data measurement errors. The first drawback is that the clamping on the spindle hub causes deformation of the disk surface, which leads to a runout of the

1. INTRODUCTION The adhesion and friction mechanisms in nanoscale systems such as microelectrical mechanical systems (MEMSs) and the head−disk interfaces (HDIs) in hard disk drives (HDDs) should be better understood to improve these technologies.1−3 In the case of HDDs, the control of perfluoropolyether (PFPE) liquid films is necessary to increase the recording density because high-density HDDs require a very low flying height, in the order of a few nanometers. To achieve this low flying height, the thickness of the lubricant film is designed to be molecularly thin (i.e., film thickness ∼ 1 nm). Therefore, in recent times, PFPE lubricants having a hydroxyl functional endgroup strongly adsorbent on the carbon surface of magnetic disks have been used widely.4−6 However, the fundamental tribological characteristics of PFPE lubricants having functional end-groups subjected to a stratified molecularly thin constitution are not precisely known. In the near future, the head slider is expected to be operated in intermittent contact or sustained contact modes, which will require the use of better lubricants to coat over magnetic disks to improve the tribological properties. Therefore, we must better understand and accurately measure the friction and adhesion properties of the lubricant films at small scales.7−9 The friction properties at a small scale have thus far been usually discussed on the basis of measurements obtained using a surface force apparatus (SFA) or an atomic force microscope (AFM). Brunner et al. discussed the relationship between the adhesive force and the surface energy on the magnetic disk surfaces having a PFPE lubricant film by AFM force curve measurements.10 They reached an interesting conclusion that the adhesive force was proportional to the surface energy. The SFA operates at an ultraslow velocity (0.5−5 μm/s) and at a relatively high applied load (1−200 mN) between two surfaces with a curvature radius of 1 cm. On the other hand, the AFM operates over a wide range of velocities © 2012 American Chemical Society

Received: November 4, 2011 Revised: February 1, 2012 Published: February 1, 2012 3814

dx.doi.org/10.1021/la204327h | Langmuir 2012, 28, 3814−3820

Langmuir

Article

Figure 1. Schematic representation of micro-tribotester for measurement of adhesion and friction forces.

disk surface; this causes errors in the friction force measurement because a runout of a few micrometers to 10 μm changes the applied load force from 37 to 184 μN. The second drawback is that the friction sensor continuously experiences the friction force during the measurement because of which the zero load point drifts easily; this causes a fluctuation in the measurement on the rotating disk using the friction sensor. In this study, we developed a new pin-on-disk-type microtribotester that does not suffer from the above-mentioned drawbacks to study the fundamental tribological properties of molecularly thin lubricant films coated on magnetic disks. This tester has a special feature in that the load force increase/ decrease mechanism is very simple. This mechanism is used by the up/down motion of the disk surface caused by the runout of the disk rotation. Using this mechanism, we can simultaneously measure the adhesion force when the pin is sliding on the disk surface.

Figure 2. Schematic representation of friction and adhesion measurement on the basis of up/down motion of disk surface due to runout. and the disk surface are separated at the trailing slope of the runout peak, as shown in Figure 2. When the pin comes into contact with the disk surface from the leading slope to the peak, the applied load increases. On the other hand, this load decreases when the pin separates from the top to the trailing slope. We were able to measure the friction forces as a function of the applied load by using this method. All tests were conducted at 28 ± 2 °C and at a relative humidity (RH) of 60 ± 20%. B. Measurement Procedure. The method for measuring the adhesion and friction forces is very simple. The surface of the pin (semispherical optical lens) mounted on the beam was washed by ethanol before each measurement. A magnetic disk was mounted on the spindle, and the pin was positioned near the disk surface using the micrometer to roughly control the stage height while monitoring the Newton’s ring through the pin. The pin was incrementally moved downward to the disk surface by the step control of the piezo stage when the spindle was rotating. The load and friction forces were measured from an index signal to the two sliding revolutions. The pin was loaded at a radius of 25 mm, and the rotational speed of the disks was fixed at 6 rpm, which corresponded to a relative velocity of 15.7 mm/s. We confirmed the dependence of the rotational speed for the friction force. The friction force did not depend on the rotational speed from 2 to 7 rpm. This result implied that the friction force was not affected by the hydrodynamic effects. The wear of the glass pin was confirmed after each test by the interference light intensity of Newton’s ring, as shown in Figure 3. The Newton’s ring was observed at the contact region, and the contact area was roughly estimated by using this image. Figure 3 shows that the pin surface did not wear after a friction measurement because the profile of the interference light intensity before the measurement approximately agreed with that after the measurement. We confirmed the profiles of the interference light intensity after each friction measurement. Meniscus formation could not be observed in this image because the meniscus area and the normal contact area could not be distinguished in the image. In the case of a sliding velocity higher than a few tens of rotations per minute, the pin easily wore after the measurement.

2. EXPERIMENTAL APPARATUS AND PROCEDURES A. Developed Micro-tribotester. The adhesion and friction forces were measured using a pin-on-disk-type micro-tribotester custommade by us. A schematic representation of the micro-tribotester is shown in Figure 1. The tester mainly consists of a friction sensor, a load sensor, a precise height control stage, a direct drive spindle motor, and an optical microscope system to observe the contact area between a semispherical lens and the disk surface. The friction and load force sensors include force transducers consisting of parallel leave springs and capacitive displacement sensors (MTI Instruments Inc.: Accumasure 9000) to measure the strain of these springs. The sensitivities of these sensors were less than 1 μN. The measurement pin is a glass semispherical optical lens (BK7) with a surface roughness of Ra ≈ 0.6 nm and a curvature radius of 1.82 mm. The pin is firmly attached to a rigid beam of stainless steel, which is hardly deformed by the applied load. The load sensors and the pin were mounted during the height control stage with the piezoactuator, and the clearance between the disk surface and the pin was controlled by the stage. The natural frequency of the load force sensor including a glass pin and a rigid beam was 350 Hz, and that of the friction sensor was 300 Hz. Therefore, these sensors could dynamically measure phenomena slower than ∼3 ms. A schematic representation of the measurement method is presented in Figure 2. The disk surface is moved up/down by the runout, which is caused by the deformation due to the clamping on the spindle hub.14 Therefore, the disk surface and the pin surface are separated by the runout at the pin position when the disk is rotating. If the height of the pin from the disk surface is controlled precisely by the height control stage, the pin comes into contact with the disk surface at the leading slope of the runout peak, and the pin 3815

dx.doi.org/10.1021/la204327h | Langmuir 2012, 28, 3814−3820

Langmuir

Article

Figure 3. Interference light intensity of the Newton’s ring at the contact region before and after the friction measurement. The disk lubricant was 1.2 nm thick Z-tetraol film, and the load force was ∼1 mN. C. Disks and Lubricants. Magnetic disks made of a 2.5 in. diameter glass substrate were used; the surface of the disks was an ∼4 nm thick nitrogenated DLC overcoat, and the average surface roughness Ra was ∼0.2 nm. PFPE lubricants (Solvay Solexis: Z-03 and Z-tetraol), shown in Figure 3, were used in this study. They have the same mainchain structure, but Z-tetraol has two hydroxyl functions on each chain end, whereas Z-03 has no functional end-group. The lubricant films were coated on the magnetic disks by dip-coating. The lubricant film thickness was controlled by adjusting the lubricant concentration in the solution. The solvent used for all lubricants was Vertrel-XF (DuPont), and the lubricant film thickness was measured using a scanning ellipsometer (FiveLab: MARY-102).

3. RESULTS AND DISCUSSION First, we confirmed the contact detection between the pin and the disk surface by using both the load and friction forces. Figure 4 shows both the forces on a disk with an ∼1.2 nm thick coating of Z-tetraol at several height steps δ ranging from 0 to −2000 nm. Both forces were not observed at a stage height step δ of 0 nm. At a stage height step δ of −1000 nm, a small peak corresponding to the load and friction forces was observed along with a negative peak corresponding to the load force signal. Moreover, two peaks of both the forces were observed when the height step was decreased to less than −1500 nm. Therefore, the contact points between the pin and the disk surface with the runout increased when the pin was moved downward to the disk surface. The maximum load force in a revolution increased with decreasing height step, and the maximum friction force exhibited the same trend. The magnified views of the graphs shown in Figure 4 corresponding to a height step δ of −1000 and −1500 nm are shown in Figures 5 and 6, respectively. As shown in Figure 5, the friction force increased sharply to ∼21 μN when the pin and the disk surface came into contact with each other; subsequently, the friction force increased gradually with an increase in the applied load. However, the applied load gradually decreased to a negative value of −1.23 mN after the maximum peak and increased to zero immediately after the minimum value. The friction force gradually decreased to zero when the negative applied load changed to zero. As shown in Figure 6, the friction force increased to 25 μN in almost the same manner as that shown in Figure 4. The applied load was negative at the initial contact and gradually decreased to −1.27 mN, which was approximately in agreement with the value of −1.23 mN shown in Figure 5. These results generally suggested that the friction force F was related to the apparent

Figure 4. Measured load and friction forces on a disk with an ∼1.2 nm thick coating of Z-tetraol at several height steps δ ranging from 0 to −2000 nm. Both forces were measured over two revolutions.

Figure 5. Load and friction forces measured at a height step δ of −1000 nm. 3816

dx.doi.org/10.1021/la204327h | Langmuir 2012, 28, 3814−3820

Langmuir

Article

the releasing load force. Further, the friction force suddenly changed to zero when the pin was released from the disk surface. The difference in the normal force, which is zero minus the force at the release point, is defined as the pull-in force. We compared the relationship between the normal load and the friction force from the friction hysteresis curves. For smooth and elastic surfaces between which adhesion occurs, the real contact area was described by the Johnson−Kendall−Roberts (JKR) model15−17 as follows: a3 =

3R W0 + 3γπR + 4E*

{

6γπRW0 + (3γπR )2

Figure 6. Load and friction forces measured at a height step δ of −1500 nm.

W = W0 + Wa = W0 + 3γπR +

applied load W0 and the adhesion force Wa between the pin and the disk. Therefore, the measured force profiles were considered as a schematic representation of the contact between the pin and the disk surface, as shown in Figure 7.

F = m(W0 + Wa)

}

(1)

6γπRW0 + (3γπR )2 (2) (3)

where E* is the reduced elastic modulus of the sphere and the plane, R is the reduced curvature radius of the sphere and the plane, γ is the adhesive energy of the sphere and the plane, and μ is the friction coefficient. The hysteresis curve of friction along the release is fitted using eqs 2 and 3 with μ and γ considered to be the fitting parameters. The fitted line, for which γ = 105 mJ/m2 and μ = 0.0125, is plotted as a solid curve in Figure 8. This fitting line revealed that a good approximation was obtained. The contact radius could be estimated from eq 1, and the contact radius a was 6.0 μm when W0 = 1.0 mN, R = 1.82 mm, E* = 5.53 × 1010 Pa (Edisk = 1.4 × 1011 Pa, Epin = 8.5 × 1010 Pa, Poisson ratio νdisk = 0.21, and νpin = 0.208), and γ = 105 mJ/m2. This estimated contact radius roughly agreed with the measured contact radius shown in Figure 3. The fitted value of μ = 0.0125 appeared remarkably low for the friction coefficient on the magnetic disk with the supersmooth surface. The reason for this low coefficient was not clear, although we repeated the experiments on the disks with Z-tetraol. We need a further study to clarify the unexpected causes (e.g., adsorbed water or contamination on the disk surface) affecting the measurement results. On the other hand, to verify the parameter of the adhesive energy γ, we obtained the surface energies from the contact angles on the glass surface as it was the same material as that of the pin and the disk surface with a 1.2 nm thick Z-tetraol coating. The dispersive and polar surface energies of the liquid films on the disk were obtained by measuring the contact angles. Here, the polar surface energy was the nondispersive surface energy, which included the hydrogen and dipole−dipole interactions. In this study, n-hexadecane and water were used as the apolar and polar test liquids, respectively. The contact angles were measured 60 s after the test liquids were dropped onto the sample surfaces to reduce the influence of the growth of the film roughness. The dispersive and polar surface energies of the liquid films on the disk were obtained by measuring the contact angles.18−21 The dispersive surface energy γLd was obtained from the contact angle θd by using the Girifalco− Good−Fowkes−Young equation.

Figure 7. Schematic representation of contact between pin and disk surface during rotation.

No load and friction forces were measured when there was no contact between the pin and the disk surface, as shown in Figure 7a. Positive applied load and friction forces were measured at contact, as shown in Figure 7b, and the adhesion force as the maximum negative load force and the fiction force were measured when the pin was about to be released from the disk surface, as shown in Figure 7c. At this moment, as shown in Figure 7c, the pin was estimated to be in contact with the disk surface by only the adhesion force. This contact cycle is depicted as a hysteresis curve of the friction force and a function of the apparent load force on the 1.2 nm thick Z-tetraol surface in Figure 8.

cos θd = −1 + 2

γ1d γT

(4)

where γ1, γT, and γLT are the surface energies of the air/sample surface, air/test liquid, and sample surface/test liquid interface, respectively, and subscript d denotes the dispersive components. Note that γTd = γT for n-hexadecane because it does not have any polar surface energy. Because the surface energy γT for

Figure 8. Typical hysteresis curve of friction force as a function of normal load force. The lubricant thickness for Z-tetraol was 1.2 nm.

The friction force increased rapidly with an increase in the applied load force and gradually decreased with a decrease in 3817

dx.doi.org/10.1021/la204327h | Langmuir 2012, 28, 3814−3820

Langmuir

Article

n-hexadecane was known, the dispersive surface energy γ1d could be obtained from the measured contact angle θd. Next, the polar surface energy of the sample surface θ1p was obtained from the measured contact angle θp, the dispersive surface energy γ1d, and the Owens and Wendt equation and the Young−Dupre equation: γ1T = γ1 + γT − 2 γ1dγTd − 2 γ1pγTp

(5)

γ1T = γ1 − γT cos θp

(6)

of the friction forces are shown in Figure 10. The left figure shows the hysteresis curve of the friction force on the nonlubricated disk surface, the middle and right figures in the top column show the curves of Z-03, and the bottom figures shows the curves of Z-tetraol. From a rough comparison of these friction forces, we can deduce that the disk with no lubrication showed the highest friction force, and the disks with Z-03 showed higher friction forces than the disks with Z-tetraol. A drastic change in the friction force was observed with the variation in the lubricant thickness of Z-tetraol; however, no such change was observed in the case of Z-03. The friction forces as a function of the lubricant thickness are compared in Figures 11 and 12. The vertical axis in Figure 11 represents the friction force with only the adhesion force and without the normal load, and that in Figure 12 represents the pull-in force, explained in Figure 8. The pull-in force datum on the nonlubricated disk is not shown in Figure 12 because the large friction force on this disk causes it to exhibit a stick−slip motion in the negative load region. The plot in Figure 11 shows the exact difference between the friction force of both lubricants. Z-03 showed a gradual decrease with a decrease in the lubricant thickness, but Z-tetraol showed a drastic decrease at 12.1 Å. There was a substantial difference between the pull-in forces for Z-03 and Z-tetraol. Further, the force in the case of Z-tetraol was clearly less than that in the case of Z-03, irrespective of the lubricant thickness. Two differences between both the lubricant films were considered to mainly cause the large difference in the friction hysteresis. First, the film coverage increased with an increase in the lubricant thickness and the lubricant film completely covered more than 1.0 nm of the disk surface in the case of Z-tetraol, as illustrated in Figure 13a−c. In fact, we previously confirmed the dependence of the coverage of the lubricant film on the lubricant thickness from the direct AFM observations.24 In the case of Figure 13a, the load W was the sum of the apparent applied load W0 and the adhesion force Wc between the pin and the disk surface without the lubricant film. The adhesion force Wc was more than the contact force in the case of Figure 13b or 13c because the surface free energy of the disk with no lubrication was generally higher than that of the lubricated disk. Consequently, the friction force on the disk surface without the lubricant film was very large because of the large adhesive force Wc. In the case of Figures 13b and 13c, the load W could be described as follows:

The total surface energy γtotal was obtained as the sum of the dispersive and polar surface energies (γtotal = γ1d + γ1p). This method was used for the characterization of the PFPE lubricant surface on the magnetic disks, and the results obtained by using this method were discussed from the perspective of the molecular conformation of the PFPE lubricant film. The adhesion energy γ was derived from the extended Fowkes theory22,23 as follows: γ = 2 γ1dγ2d + 2 γ1pγ2p

(7)

where the indexes 1 and 2 denote the surface of the glass and that of the disk. The surface energies of the glass were γ1d = 22 mJ/m2 and γ1p = 45 mJ/m2, and those of the disk surface were γ2d = 14 mJ/m2 and γ2p = 26 mJ/m2. The adhesive energy γ obtained from these values was γ = 104 mJ/m2. This value agreed well with the value obtained from the fitting curve, as shown in Figure 8. This indicated that the friction on the disk with the Z-tetraol lubricant was described by the JKR model. On the other hand, the hysteresis curves of the friction forces on a nonlubricated disk and the 1.2 nm thick Z-03 disk are shown in Figure 9.

W = W0 + (1 − α)Wc + αWl Figure 9. Hysteresis curves of friction force on nonlubricated disk and 1.2 nm thick Z-03 disk.

W = W0 + Wl

for α ≥ 1

for α < 1

(8) (9)

where α is the coverage of the lubricant film and Wl is the adhesive force of the contact of the Z-tetraol lubricant film with full coverage. Equation 8 implies a gradual decrease in the load W with an increase in the coverage α because the adhesion energy of the contact with Z-tetraol lubricant is less than that of the disk with no lubrication. For lubricant films composed of bonded molecules, such as Z-tetraol, the dependence of friction on the loading force could be described by the JKR model, as shown in Figure 8. The contact area between the pin and the disk surface may be filled with bonded molecules, as shown in Figure 13c. Second, lubricant molecules composed only of mobile molecules, as is the case for Z-03, are easily deformed or expelled under load because they are not bonded to the disk surface. Thus, the load force could be described as follows:

The fitted lines obtained by using the JKR model agreed with the measured friction curves. However, the friction force in the negative load region could not be fully measured because the stick−slip vibration due to the large friction force disturbed the adhesion between the pin and disk surface. The obtained adhesive energy and the friction force for the nonlubricated disk were γ = 950 mJ/m2 and μ = 0.356, respectively, and those for the Z-03 disk were γ = 840 mJ/m2 and μ = 0.149, respectively. The obtained adhesion energies were considerably higher than that of Z-tetraol. This could be attributed to the fact that the carbon surface is tribologically activated by the pin sliding on the carbon surface, which increases the adhesion energy. We then measured the friction and normal load forces on the disks with Z-03 and Z-tetraol. The lubricant thicknesses were varied from 0 to 1.6 nm in several steps. Some hysteresis curves

W = W0 + (1 − β)Wc + βWl 3818

(10)

dx.doi.org/10.1021/la204327h | Langmuir 2012, 28, 3814−3820

Langmuir

Article

Figure 10. Hysteresis curves of friction forces measured on disks having various lubricant thicknesses of Z-03 and Z-tetraol. (a)-1: nonlubricated disk; (b)-1 and (b)-2: lubricated disks with 7.9 and 15.2 Å thick layers of Z-03, respectively; and (c)-1 and (c)-2: lubricated disks with 8.5 and 15.8 Å thick layers of Z-tetraol.

Figure 13. Models for PFPE-lubricant-mediated contact: (a) direct contact between pin and disk surface, (b) contact between pin and Z-tetraol lubricant film with insufficient coverage, (c) contact between pin and Z-tetraol lubricant film with full coverage, and (d) contact between pin and Z-03 lubricant film. Figure 11. Friction force without normal load as a function of lubricant thickness. The friction force on the vertical axis is the force at a normal load of 0 mN.

These results of our measurements indicated that the adhesive force at the contact of the magnetic head and the disk surface must be of the same order as the head load force and that the friction and adhesion forces depended considerably on the lubricant thickness. Thus, this methodology should support the design of a lubricant film to reduce the slider vibration and slider wear at the contacts on the disk surface and also support the design of HDIs for surfing recording in which the slider surfs in the lubricant film of the disk.25

4. CONCLUSIONS A microtribotester was developed to study the adhesion and friction properties of a molecularly thin PFPE lubricant film dip-coated on magnetic disks. The tester measured both the adhesion and the friction forces as a function of the applied load force under slow sliding conditions. The adhesion and friction properties of molecularly thin PFPE lubricant (Z-tetraol and Z-03) films on magnetic disks as a function of the lubricant thickness were investigated for two lubricants. The results of this study can be summarized as follows: 1. The friction and adhesion forces of Z-tetraol decreased with an increase in the lubricant thickness, and the friction force drastically decreased when the lubricant thickness was more than 1.2 nm. This could be attributed to the fact that the

Figure 12. Pull-in force as a function of lubricant thickness except in the case of the disk with no lubrication.

where β is the contact ratio of the lubricant with nonfunctional end-groups, such as Z-03. The pin and the disk surface might be in contact through intervening molecules having a flat conformation or might be in contact locally, through solid-to-solid contact, as shown in Figure 13d. 3819

dx.doi.org/10.1021/la204327h | Langmuir 2012, 28, 3814−3820

Langmuir

Article

(20) Tyndall, G. W.; Waltman, R. J.; Pocker, D. J. Langmuir 1998, 14, 7527−7536. (21) Fukuzawa, K.; Shimuta, T.; Yoshida, T.; Mitsuya, Y.; Zhang, H. Langmuir 2006, 22, 6951−6955. (22) Fowkes, F. M. J. Phys. Chem. 1963, 67, 2538−2541. (23) Fowkes, F. M. Ind. Eng. Chem. 1964, 56, 40−52. (24) Tani, H.; Tagawa, N. IEEE Trans. Magn. 2009, 45, 862−866. (25) Liu, B.; Zhang, M. S.; Yu, S. K.; Hua, W.; Ma, Y. S.; Zhou, W. D.; Gonzaga, L.; Man, Y. J. IEEE Trans. Magn. 2009, 45, 899−904.

coverage of the lubricant film in the contact area was maintained by the bonding of the lubricant molecules, and the bonded molecules reduce the adhesion force. 2. Z-03 without a functional end-group exhibited relatively large friction and adhesion forces. The probable reason for this was because the pin and the disk surface are in contact through intervening molecules having a flat conformation or in contact locally, through solid-to-solid contact. 3. The friction force curves measured on the disk surface with the PFPE lubricant using the developed microtribotester were described by the JKR model. The friction force at an applied load force of 0 mN and the pull-in force were the effective parameters to understand the tribological properties of the molecularly thin lubricant.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +81-6-6368-0771; Fax: +81-6-6368-0771; e-mail: hrstani@ kansai-u.ac.jp. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by a “Strategic Project to Support the Formation of Research Bases at Private Universities”: Matching Fund Subsidy from MEXT (Ministry of Education, Culture, Sports, Science and Technology).

■

REFERENCES

(1) Bhushan, B.; Israelachvili, J. N.; Landman, U. Nature 1995, 374, 607−616. (2) Bhushan, B. Tribology Issues and Opportunities in MEMS; Kluwer Academic Publishers: Dordrecht, 1997. (3) Mate, C. M. Tribology on the Small Scale; Oxford University Press: New York, 2008. (4) Mate, C. M.; Toney, M. F.; Leach, K. A. IEEE Trans. Magn. 2001, 37, 1821−1823. (5) Sinha, S. K.; Kawaguchi, M.; Kato, T. J. Phys. D: Appl. Phys. 2004, 37, 2247−2255. (6) Tyndall, G. W.; Karis, T. E. Tribol. Trans. 1999, 42, 463−470. (7) McGuiggan, P. M.; Zhang, J.; Hsu, S. M. Tribol. Lett. 2001, 10, 217−223. (8) Sinha, S. K.; Kawaguchi, M.; Kato, T.; Kennedy, F. E. Tribol. Int. 2003, 36, 217−225. (9) Gao, J.; Luedtke, W. D.; Gourdon, D.; Ruths, M.; Israelachvili, J. N.; Landman, U. J. Phys. Chem. B 2004, 108, 3410−3425. (10) Karis, T. E.; Guo, X.-C.; Juang, J.-Y. Tribol. Lett. 2008, 30, 123− 140. (11) Brunner, R.; Tyndall, G. W.; Waltman, R. J.; Talke, F. E. Tribol. Lett. 2010, 40, 41−48. (12) He, Y.; Fujikawa, Y.; Zhang, H.; Fukuzawa, K.; Mitsuya, Y. Tribol. Lett. 2007, 27, 1−11. (13) Zhang, H.; Mitsuya, Y.; Fujikawa, Y.; Fuwa, A.; He, Y.; Fukuzawa, K. Tribol. Lett. 2007, 28, 163−170. (14) Chang, J. Y. IEEE Trans. Magn. 2011, 47, 1855−1861. (15) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London 1971, A324, 301−313. (16) Zappone, B.; Rosenberg, K. J.; Israelachivili, J. Tribol. Lett. 2007, 26, 191−201. (17) Hilda, W.; Ahmeda, S. I.-U.; Hungenbacha, G.; Scherge, M.; Schaefer, J. A. Tribol. Lett. 2007, 25, 1−7. (18) Israelachivili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 1992; p 203. (19) Adamson, W.; Gast., A. Physical Chemistry of Surfaces; Wiley: New York, 1997; pp 352−364. 3820

dx.doi.org/10.1021/la204327h | Langmuir 2012, 28, 3814−3820