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Comparison of Macro- and Nanotribological Behavior with Surface Plastic Deformation of Polystyrene Taku Aoike,* Hiroki Uehara, Takeshi Yamanobe, and Tadashi Komoto Department of Chemistry, Gunma University, Kiryu, Gunma 376-8515, Japan Received September 15, 2000. In Final Form: January 23, 2001 The macrotribological behavior observed when a Si3N4 ball of a friction tester slid on polystyrene (PS) surface was compared with the nanotribological behavior observed when a Si3N4 tip of a scanning force microscope (SFM) slid on the same surface. The macro- and nanofriction forces were measured by a conventional ball-on-disk type friction tester and a SFM, respectively, as a function of the applied load. The morphological changes of the rubbed PS surfaces were also characterized by optical microscopy or SFM observations. Surface plastic deformation of PS always occurred under both tribological conditions examined in this study. The morphology formations of the macrocrack and the nanoperiodic pattern that might be attributed to the crack opening were observed at the rubbed PS surfaces under the macro- and nanotribological conditions. It should be noted that these formations occurred at the same level of apparent average contact pressure estimated based on the Johnson-Kendall-Roberts contact theory. Furthermore, under a comparable contact pressure, these surface plastic deformations yielded similar values of the macro- and nanofriction coefficients. The friction coefficient was considerably influenced by surface plastic deformation regardless of measurement scales. These results demonstrated that the macro- and nanotribological mechanisms of PS were similar.
Introduction Macro-, micro-, and nanotribological properties are important for many modern technologies, including medical devices,1 magnetic storage devices, and microelectromechanical systems.2 The essentials of tribology have long been studied but are still matters of conjecture because of the complex of friction, lubrication, and wear. In contrast, the origin of tribological phenomena between two surfaces has gradually been revealed in recent years.2-4 On the basis of the recent evolution of surface analysis, e.g., surface force apparatus,5 quartz-crystal microbalance,6 and the family of scanning force microscopes7 (SFMs), micro- and nanotribological studies have been extensively developed. In particular, SFM is a powerful tool for understanding tribological behavior of very small spatial fields. The friction and wear behavior on atomic, nanometer, and micrometer scales for various materials, such as metallic,8,9 inorganic,9-12 organic13,14 (Langmuir* To whom correspondence should be addressed. E-mail: aoike@ polymer.chem.gunma-u.ac.jp. (1) Wang, A.; Essner, A.; Polineni, V. K.; Stark, C.; Dumbleton, J. H. Tribol. Int. 1998, 31, 17. (2) Bhushan, B. Handbook of Micro/Nanotribology, 2; CRC Press: Boca Roton, FL, 1999. (3) Bhushan, B.; Israelachivili, J. N.; Landman, U. Nature 1995, 374, 607. (4) Langmuir (Special Issue: Proceedings of a Workshop on the Physical and Chemical Mechanisms of Tribology) 1996, 12, 4481. (5) Israelachivili, J. N. Surf. Sci. Rep. 1992, 14, 109. (6) Krim, J.; Solina, D. H.; Chiarello, R. Phys. Rev. Lett. 1991, 66, 181. (7) (a) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986, 56, 930. (b) Mate, C. M.; McClelland, G. M.; Erlandsson, R.; Chinag, S. Phys. Rev. Lett. 1987, 59, 1942. (8) Jiang, Z.; Lu, C.-J.; Bogy, D. B.; Miyamoto, T. ASME J. Tribol. 1995, 117, 328. (9) Koinker, V. N.; Bhushan, B. J. Vac. Sci. Technol. A 1996, 14, 2378. (10) Miyake, S. Appl. Phys. Lett. 1995, 67, 2925. (11) Liu, E.; Blanpain, B.; Celis, J.-P.; Roos, J. R. J. Appl. Phys. 1998, 84, 4859. (12) Hu, J.; Xiao, X.-d.; Ogletree, D. F.; Salmeron, M. Surf. Sci. 1995, 327, 358. (13) Bhushan, B.; Kulkarni, A. V.; Koinkar, V. N.; Boehm, M.; Odoni, L.; Martelet, C.; Belin, M. Langmuir 1995, 11, 3189.
Blodgett and self-assembled monolayer films), and polymeric15-19 materials, has been studied by using SFM. Sliding between two surfaces causes various surface deformations in polymeric materials regardless of observation scales. Under conventional macrotribological conditions, abrasive wear and wear debris formation are observed for glassy polymeric materials. A “Schallmach wave” pattern is formed at the surface during sliding for rubbery material.20 In contrast, under nanotribological conditions created by using SFM, sliding a SFM tip on a glassy polymeric solid such as polystyrene (PS) produces a periodic pattern21 at low applied loads and a scratch mark19 at high loads. Macro- and nanofriction forces have been measured in order to investigate the tribological properties and viscoelasticity of a polymer surface.15-17,22-26 It has been reported that the sliding velocity dependence of the macrofriction coefficient for rubbery material, which has a lower glass transition temperature (Tg) than the measurement temperature, is closely related to the bulk viscoelastic properties.22 On the nanometer scale, Glad(14) (a) Lio, A.; Charych, D. H.; Salmeron, M. J. Phys. Chem. B 1997, 101, 3800. (b) Mowery, M. D.; Kopta, S.; Ogletree, D. F.; Salmeron, M.; Evans, C. E. Langmuir 1999, 15, 5118. (15) Michel, D.; Kopp-Marsaudon, S.; Aime´, J. P. Tribol. Lett. 1998, 4, 75. (16) Matzelle, T. R.; Herkt-Bruns, Ch.; Heinrich, L. A.; Kruse, N. Surf. Sci. 2000, 454-456, 1010. (17) Haugstad, G.; Gladfelter, W. L.; Jones, R. R. J. Vac. Sci. Technol. A 1996, 14, 1869. (18) Schmidt, R. H.; Haugstad, G.; Gladfelter, W. L. Langmuir 1999, 15, 317. (19) Kaneko, R.; Hamada, E. Wear 1993, 162-164, 370. (20) Schallamach, A. Wear 1971, 17, 301. (21) Leung, O. M.; Goh, M. C. Science 1992, 255, 64. (22) Grosch, K. A. Nature 1963, 197, 858. (23) Kajiyama, T.; Tanaka, K.; Takahara, A. Macromolecules 1997, 30, 280. (24) Kajiyama, T.; Tanaka, K.; Satomi, N.; Takahara, A. Macromolecules 1998, 31, 5150. (25) Hammerschmidt, J. A.; Gladfelter, W. L.; Haugstad, G. Macromolecules 1999, 32, 3360. (26) Ge, S.; Pu, Y.; Zhang, W.; Rafailovich, M.; Sokolov, J.; Buenviaje, C.; Buckmaster, R.; Overney, R. M. Phys. Rev. Lett. 2000, 85, 2340.
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felter et al.17,25 have studied the sliding velocity dependence of nanofriction for several polymeric materials by using SFM and clearly explained these results on the basis of surface viscoelasticity. Also, Kajiyama et al.23 have investigated the sliding velocity dependence of lateral (friction) force by varying the molecular weight of PS having a bulk Tg exceeding room temperature (RT). They found that as molecular weight decreases, transition from glassy to rubberlike behavior of the surface region is demonstrated at RT. This phenomenon was explained by the excess free volumes at the surface induced by surface segregation of the chain end group. Furthermore, both groups23-25 have revealed that Tg and the activation energy for molecular motion at the surface are lower than those of bulk. In contrast, Ge et al.26 measured the temperaturedependent shear force on a SFM tip modulated parallel to the PS surface. This result demonstrated that the measured surface Tg agrees well with the bulk Tg over a large range of molecular weights. Adhesion measurements of poly(tert-butyl acrylate) by Tsui et al.27 using SFM have also revealed that no enhanced molecular relaxation is found at the free polymer surface. As shown in these conflicting results, the nature of molecular mobility at a polymer surface is still imperfectly understood. As studies on the micro- and nanotribological phenomena have progressed, the influence of the measurement scale on tribological behavior has become of increasing interest from scientific and technical viewpoints. It was reported that, for inorganic materials such as muscovite mica and highly oriented polygraphite, the nanofriction coefficient measured by SFM was quite lower than that measured by the conventional macrotribological tester.9,11 This difference was interpreted as ploughing contribution occurring under the macroscopic measurement. No wear was observed under the nanotribological conditions investigated in these reported works. On the other hand, Carpick et al.28 have stated that a sudden or anomalous increase in friction is observed when extensive wear starts to occur, indicating that shear processes between the SFM tip and surface cause the wear. Regardless of measurement scales, the friction force might be closely related to surface deformation caused by opposite substrate sliding. The friction coefficient for glassy polymeric materials, which are softer and brittler than metallic and inorganic solids, is expected to be significantly influenced by surface deformation; this influence may be observed in all tribological systems. In this paper, the macrotribological behavior observed when a Si3N4 ball slides on a PS surface is compared with the nanotribological behavior observed when a Si3N4 tip of SFM slides on the same surface. The obtained results are discussed from the viewpoints of a relation between the friction coefficient and surface deformation caused by a ball or tip sliding and of a comparison of the macro- and nanotribological phenomena that are standardized based on the Johnson-KendallRoberts (JKR) contact theory.29 Experimental Section Materials and Sample Preparation. PS used in this study, which was purchased from Scientific Polymer Products, Inc., was reprecipitated from tetrahydrofran into n-hexane. The obtained purified powder was vacuum-dried. The weight- and number-average molecular weights determined via gel permeation chromatography were 183 000 and 74 900, respectively. (27) Tsui, O. K. C.; Wang, X. P.; Ho, J. Y. L.; Ng, T. K.; Xiao, X. Macromolecules 2000, 33, 4198. (28) Carpick, R. W.; Salmeron, M. Chem. Rev. 1997, 97, 1163. (29) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London A 1971, 324, 301.
Aoike et al. Freshly cleaved muscovite mica was also used as a nanotribological reference. PS films for the macro- and nanotribological tests were prepared by casting onto a stainless steel plate (12 mm × 12 mm, 0.3 mm thickness) and into a stainless steel dish disk (φ 70 mm, depth ca. 2 mm) from 10 wt % toluene solution. Both samples were dried at RT in the atmosphere for 24 h, followed by holding at RT and 120 °C in a vacuum for more than 24 h. The films were ∼200 µm thick for macrotribological tests and ∼50 µm thick for nanotribological tests. The physical properties of the film for the macrotribological test were evaluated by using tensile tests and differential scanning calorimetry. The tensile modulus and strength were 2.4 GPa and 34 MPa, and Tg was 100 °C. The mechanical properties of this film were comparable to the previous reported values of PS films.30 Macrotribological Test. Data of the macrofriction force were collected by a conventional ball-on-disk type friction tester in the atmosphere (in air, 18-22 °C, relative humidity of ∼40%). A 2.5 mm-radius ball made of Si3N4 slid on a disk rotating at a speed of 1 mm/s under applied loads of 1.96-29.4 N. The rotator diameter was 50 mm. The data were collected in the first round. The macrofriction force was determined by distortion of a strain gauge attached to the arm of the tester. The morphology of the wear track made by the sliding ball was characterized by polarized optical microscopy. Nanotribological Test. The nanotribological test was done with SFM in the same atmosphere as the macrotribological test. The SFM used in this study was a SPA 400 with a SPI 3800N controller (Seiko Instruments Industry Co., Ltd.). A commercial rectangular 100 µm cantilever (Olympus Optical Co., Ltd.) having a spring constant of 0.37 N/m with a Si3N4 integrated tip (tip radius, ∼20 nm) was used. The spring constant for the cantilever used in this study was confirmed by using a continuum elasticity model after measuring the cantilever size by scanning electron microscopy (SEM).31,32 The tip radius was also confirmed by SEM. The same tip was always used for all the nanotribological tests. Before each nanofriction force measurement, the force curve measurement, which is the vertical cantilever deflection as a function of vertical sample displacement toward and away from a tip, was carried out to evaluate the applied load and the pulloff force. The frictional data were obtained in the tip reciprocatingscan at 0.3-30 µm/s for 1 µm along the direction perpendicular to the cantilever long axis. A sliding velocity of 1 µm/s was mainly used to obtain the data. An applied load from 5 to 50 nN that was controlled by changing the vertical cantilever deflection was maintained during the perpendicular scanning. The nanofriction force of mica was measured using a method in which the tip reciprocates on the surface along both perpendicular and parallel directions of the cantilever axis to determine the absolute value of the nanofriction force.2,33 To investigate the surface deformation caused by the sliding tip, a scratch test was done under the same sliding conditions as in the nanofriction force measurement. In the scratch test, a 1 µm × 1 µm area was scanned at 1 µm/s under an applied load of 5-50 nN. Each test includes 256 line scratches with the perpendicular axis of the scan direction. The topographical images after the scratch test were obtained by scanning over a wider area under a weakly repulsive force of 200 GPa) is much higher than E1 (2.4 GPa). The Poisson ratio of PS, ν1, has
(39) Bowden, F. P.; Tabor, D. The Friction and Lubrication of Solids part II; Clarendon Press: Oxford, England, 1964. (40) Buenviaje, C.; Ge, S.; Rafailovich, M.; Sokolov, J.; Drake, J. M.; Overney, R. M. Langmuir 1999, 15, 6446.
(41) Elkaakour, Z.; Aime´, J. P.; Bouhacina, T.; Odin, C.; Masuda, T. Phys. Rev. Lett. 1994, 73, 3231. (42) Meyers, G. F.; DeKoven, B. M.; Seitz, J. T. Langmuir 1992, 8, 2330.
P)
Fa
(2)
πa2
Fa is the applied load, and a is the radius of the contact area defined as
3 a3 ) kR[Fa + 3πRW + x6πRWFa + (3πRW)2] (3a) 4 k)
(
) ( )
1 - ν1 1 - ν2 1 - ν1 + ≈ E1 E2 E1
(3b)
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Figure 7. Topographical images of PS (A-G) and mica (H) surfaces after scratch test under applied loads of (A) 5, (B) 10, (C) 15, (D) 20, (E) 30, (F) 40, and (G, H) 50 nN. These images were obtained under a repulsive force of e1 nN.
been reported as 0.33.43 The work of adhesion, W, was estimated by
3 Fp ) - πRW 2
(4)
force curve measurement using SFM. This estimated value of work for adhesion between PS and Si3N4 under atmospheric conditions was similar to the value calculated using Hamker constants of PS,44 Si3N4,45 and H2O,44 considering that the two substrates across the atmosphere acted as a water thin film.45 Additionally, the surfaces of
where Fp represents the pull-off force evaluated by the (43) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3; Wiley: New York, 1989; p V-81.
(44) Israelachvili, J. N. Intermolecular and Surface Forces, 2; Academic Press: London, 1992. (45) Eastaman, T.; Zhu, D.-M. Langmuir 1996, 12, 2859.
Macro- and Nanotribological Behavior
Figure 8. Macro- and nanofriction coefficient as a function of apparent average contact pressure estimated based on JKR theory. The pressure regions of both macrocrack and nanoperiodic pattern formations, which are evaluated from Figures 4 and 7, are also denoted.
the ball and SFM tip were assumed to be perfectly smooth. However, the calculated average contact pressure might not correspond to the actual average contact pressure, since the contact between the ball or SFM tip and PS surface is not elastic but plastic as shown in Figures 4 and 7. Both macro- and nanofriction coefficients are plotted in Figure 8 as a function of the apparent average contact pressure estimated on the basis of JKR contact theory. This standardization enables us to compare macro- and nanotribological behavior as a function of a common quantitative parameter. The pressure regions of both macrocrack and nanoperiodic pattern formations, which are evaluated from Figures 4 and 7, are also denoted in Figure 8. It should be noted that both formations of the macrocrack and the nanoperiodic pattern occurring under quite different conditions, including the contact scale and the applied load, are observed at the same level of apparent average contact pressure. The origin of both formations might be the same. For the nanotribological conditions, further applied pressure erased the nanoperiodic pattern and yielded at scratch mark and wear debris. We believe that the scratch mark with wear debris would be formed if the macrotribological experiment were done under very high pressures, excluding those of the present study. Finally, we estimated the absolute value of the nanofriction force for PS using the relationship between the nanofriction force derived voltage and the absolute value of the nanofriction force converted from the apparent height for mica. The nanofriction force of mica was almost proportional to the normal force. The nanofriction coefficient was 0.02, which was much lower than the value (0.3) measured by the conventional macroscopic test.9 For PS specimens, the nanofriction coefficient increased from 0.2 to 0.3 with increasing applied pressure, but was still lower than the macrofriction coefficient of 0.4. However, the difference between the macro- and nanofriction coefficients for PS was much smaller than that for metallic, inorganic, and organic materials.9,11 Surface plastic deformations occurring under comparable contact pressures
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for both systems yield similar values for the macro- and nanofriction coefficients. By comparing both tribological results, we found that the macro- and nanotribological mechanisms with surface plastic deformation of PS were similar as compared with those of metallic and inorganic materials.9,11 The observed behavior might be ascribed to low surface free energy and the higher brittleness of glassy polymeric material. Since the plastic deformation of polymeric solid easily occurs at the contact face between this solid and a hard slider, e.g., ceramic and metallic materials, we consider that the roughness of the contact faces, which is much smaller than the size of the deformation, hardly influences tribological behavior. However, the mechanical properties of metallic and inorganic materials are much higher than those of a polymeric material. Thus, according to the surface roughnesses of the hard sample and hard slider, the applied load might be distributed unevenly at the contact face between the surfaces, and the surface deformation might not occur evenly regardless of measurement scales. Since the surface free energies of metallic and inorganic materials are high, the effect of the surface force becomes large. These two factors, i.e., low surface free energy and high brittleness of polymeric material, seem to decrease both surface force effects and contact problems concerning whether the contact between two surfaces is single asperity or multiple asperity. Conclusions The macrotribological behavior observed when a Si3N4 ball slid on a PS surface was compared with the nanotribological behavior observed when a Si3N4 tip slid on the same surface. Under the macrotribological conditions, abrasive wear occurred as ploughing, and a macrocrack was observed almost perpendicular to the ball sliding direction under high applied loads. The macrofriction coefficient was constant at 0.4. This macrotribological phenomenon obeyed the Amontons-Coulomb law known as the classical theory. In contrast, under the nanotribological conditions created by using SFM, the tip produced a persistently periodic pattern perpendicular to the sliding direction on the PS surface at low applied loads. Increasing the applied load erased this pattern and yielded scratch mark and wear debris. The nanofriction coefficient of PS changed from 0.2 to 0.3 with increasing applied load. The SFM observations of the PS surfaces after the scratch test revealed that this change in the nanofriction coefficient was attributed to the change in the mode of surface plastic deformation of PS. Comparing both tribological phenomena, the macrocrack and the nanoperiodic pattern, which were observed in different tribological systems, were formed at the same level of apparent average pressure estimated on the basis of JKR contact theory. These surface deformations under different conditions yielded similar values of the macro- and nanofriction coefficients. We found that the friction coefficient is considerably influenced by surface plastic deformation regardless of measurement scales. Consequently, these results showed that macroand nanotribological mechanisms of PS were similar. LA001326O
