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Dependence of adhesion and friction on porosity in porous anodic alumina films
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Scripta Materialia 58 (2008) 870–873 www.elsevier.com/locate/scriptamat
Dependence of adhesion and friction on porosity in porous anodic alumina films Dukhyun Choi,a Sangmin Lee,a Seonghan Kim,a Pyungsoo Lee,b Kunhong Lee,b Hyunchul Parka and Woonbong Hwanga,* a
Department of Mechanical Engineering, Pohang University of Science and Technology, San 31, Hyoja, Namgu, Pohang, Gyungbuk 790-784, South Korea b Department of Chemical Engineering, Pohang University of Science and Technology, San 31, Hyoja, Namgu, Pohang, Gyungbuk 790-784, South Korea Received 28 September 2007; revised 12 November 2007; accepted 2 January 2008 Available online 11 January 2008
We explore, using atomic force microscopy, adhesion and friction characteristics of porous anodic alumina (PAA) films as the porosity varies. Pore formation greatly reduces the adhesive force, by as much as 30 times, but the porosity of PAA films has little influence on its adhesion properties, showing good agreement with the relative contact area estimated by the Hertzian model and indentation theory. Frictional coefficients vary nonlinearly with the porosity of the PAA films due to intrinsic characteristics of the surface morphology of PAA films. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Porous material; Adhesion; Friction; Atomic force microscopy
Uniformly arrayed nanoporous materials have recently been proposed in the medical, biological, electrical, optical and micromechanical fields as a template for the growth of low-dimensional nanomaterials and as a three-dimensional micro/nanostructure itself. These include nanodots, nanowires and nanotubes, as well as photonic-crystal waveguide devices, nanocomposite structures, membranes for bioseparation, high-density magnetic memories, single-electron devices and biotechnological structures such as scaffold for tissue growth [1–6]. There is a wide variety of fabrication techniques for creating nanometer-sized structures, including UV photolithography, electro- and ion-beam lithography, X-ray lithography, and scanning probe microscopybased lithography. Not all of these approaches are economically viable, and some are unsuitable for the fabrication of large areas of nanostructured surfaces. Alternative techniques have been proposed to fabricate large arrays of highly ordered massively parallel nanostructures at low cost, including nanochannel glassbased lithography [7], nanosphere lithography [8] and * Corresponding author. Tel.: +82 54 279 2174; fax: +82 54 279 5899; e-mail: [email protected]
techniques based on nanoporous templates [9], such as porous anodic alumina (PAA). PAA as a nanoporous template produces a hole configuration with long-range ordering and a high aspect ratio, and the combined process of pre-texturing aluminum and appropriate anodization generates porous alumina with an ideally ordered hole array of various configuration shapes [10]. The structural properties of PAA have been well investigated, mainly for its mechanical characteristics and optical properties [11,12]. Such PAA films are rigid, well defined, self-supporting and self-stable. Properly annealed, they possess acceptable optical transparency in the visible region of the spectrum. PAA, because of its highly robust and versatile characteristics, is an excellent candidate for use in micro/nanoelectromechanical systems (MEMS/NEMS) and integrated biosensing/ analyzing functional devices [13]. As the surface-to-volume ratio increases in ever-smaller components in MEMS/NEMS devices, a variety of material properties, including magnetic, electronic, optical, mechanical and tribological properties, become strongly dependent on the size, shape and regularity of the material structures [14–16]. Furthermore, since in small-scale devices a high proportion of atoms are located at surface and interface sites (rather than in
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.01.001
D. Choi et al. / Scripta Materialia 58 (2008) 870–873
the bulk of the material), surface forces such as friction and adhesion dominate over the available actuation and restoring forces. In particular, moving components in integrated multifunctional systems at micro/nanometer scales cause serious interface problems due to adhesion, friction and wear. For example, the adhesive force prevents high-precision positioning in microgripping systems, and also causes critical system failure for moving elements in microsystems [17,18]. Thus, a comprehensive micro/nanotribological characterization of the scaledependent nature of material properties would have great importance, both in the design of reliable industrial applications and to provide a bridge between science and engineering on micro/nanoscales. Conventional tribological and lubrication techniques used for large objects can be ineffective at the nanometer scale, which requires new methods for control [19]. Much research has therefore aimed to optimize tribological properties between the components in microsystems by using surface coating or chemical deposition methods. These methods work only for short times, and are expensive and laborious. Below, tunable adhesion and friction behavior is investigated on PAA films according to the porosity, using atomic force microscopy (AFM). To fabricate PAA films having different initial pore size and interpore distance but similar porosities, we applied oxalic acid and phosphoric acid, respectively, during the anodization process. Since tribological properties are influenced by several factors, including contact area, surface roughness and mechanical properties such as hardness and elasticity [20–22], we examined the surface morphology, the indentation modulus and the relative contact area of PAA films as the porosity varies. Typical PAA films were fabricated by two-step anodization of aluminum, as described elsewhere [7–12]. In summary, a pure Al sheet (99.999%) of 1 mm thickness was electropolished in a mixture of perchloric acid and ethanol (HClO4:C2H5OH = 1:4 volumetric ratio) to remove surface irregularities, at a constant voltage of 20 V and temperature of 7 °C. In the anodization process, 0.3 M oxalic acid and 0.1 M phosphoric acid were used to fabricate PAA films with different interpore distances and different initial pore diameters. In the oxalic acid, a PAA film with an interpore distance of 100 nm and an initial pore diameter of 31 nm was obtained by anodizing at 40 V and 15 °C for 12 h. In the phosphoric acid, a PAA film with interpore distance 500 nm and initial pore diameter 100 nm was fabricated at 195 V and 0 °C for 16 h. When phosphoric acid was used, ethylene glycol was added to prevent the circulating water used for temperature control from freezing. After first anodization, the PAA layer was removed in a mixture of 1.8 wt.% chromic acid and 6 wt.% phosphoric acid at 65 °C for 6 h. Highly ordered PAA films were then obtained by a second anodization, under the same conditions as the first. The pore size of PAA films was readily adjusted by controlling the etching time, in 0.1 M phosphoric acid at 30 °C. An atomic force microscope (SPA 400, Seiko) was used to investigate the dependence of adhesion and friction on the porosity of PAA films. An alumina thin film without pores was prepared for use as a control sample. We used AFM tips with SiO2 beads with radii of 380
871
and 2280 nm (Navascan company); the beads were mounted on an aluminum-coated rectangular Si3N4 cantilever with a normal spring constant of 0.6 N m1. All adhesion and friction data were measured under the same normal humidity condition (45% RH) since the adhesion and friction behaviors are strongly sensitive to the humidity. The adhesion force was determined by a single point measurement with a force calibration plot. The frictional force was determined from the friction loop curves determined by AFM. We calibrated the frictional forces measured by AFM using the method of Choi et al. [23]. The frictional coefficients were determined from the gradient of the graph of friction force vs. normal force. For the friction test, the scanning distance was 5 lm, and the normal forces were in the range 0–140 nN. Figure 1 shows typical scanning electron microscopy (SEM; XL30SFEG, Philips) images of PAA films fabricated in phosphoric acid. The surface morphology becomes sharper as the pore size (i.e. the porosity) increases. The inset shows a top view of the PAA films; the scale bar is 500 nm. Table 1 sets out the pore diameters and the porosities of the PAA films fabricated in this work. The porosity of PAA films can be determined by subtracting the pore area from the unit cell area for a PAA film (see Fig. 3b inset, where q is the interpore distance and r is the pore radius in the triangle unit cell). Figure 2 shows the dependence of the adhesive forces on the porosity of the PAA samples for each tip radius. The initial pore formation from the flat surfaces without pores reduced the adhesive force by as much as 30 times, depending on the AFM tip size. As the porosity increases, the adhesive forces hardly change, however, merely showing small variations in the PAA samples.
Figure 1. Typical SEM images of PAA films fabricated in phosphoric acid. The pore diameters were 159 nm (left) and 372 nm (right), with an interpore distance of 500 nm. The surface morphology becomes sharper as the porosity (i.e. pore diameter) increases. The inset shows the top view of the corresponding samples; the scale bar is 500 nm.
Table 1. Pore diameter and the porosity of PAA films PAAoxalic (q = 100 nm)
I II III IV V
PAAphosphoric (q = 500 nm)
Pore diameter
Porosity
Pore diameter
Porosity
31 ± 4.6 41 ± 5.5 47 ± 3.9 52 ± 5.5 60 ± 7.2
0.09 0.16 0.20 0.25 0.32
159 ± 6.6 208 ± 11.5 271 ± 11.1 320 ± 8.8 372 ± 13.6
0.09 0.16 0.27 0.37 0.50
872
D. Choi et al. / Scripta Materialia 58 (2008) 870–873
of the indenter; bc = 1 for flat, cylindrical indenters, bc = 1.012 for Vickers indenters and bc = 1.034 for Berkovich indenters. From the measured indentation modulus, the elastic properties of the sample can be determined by 1 1 v2 1 v2i ¼ þ M E Ei
Figure 2. Porosity dependence of the adhesive force for PAA samples fabricated in oxalic acid (PAAoxalic) and in phosphoric acid (PAAphosphoric); m is the mean value of the adhesive force for each sample and r is the standard deviation. Pore formation reduced the adhesive forces by as much as 30-fold, but the porosity had little influence on the adhesion.
This behavior can be quantitatively estimated through the relative contact area, which can be defined by the ratio of the contact radii of PAA samples, based on a Hertzian contact [24] and indentation theory [25]. In a Herzian model, when a sphere in contact with a flat surface is under the action of an applied load P, the contact radius a is given by the following expression: 3PR ð1Þ 4K where R is the radius of contacting sphere and K is the reduced stiffness defined by
a3 ¼
1 1 m21 1 m22 ¼ þ K E1 E2
ð2Þ
where E1 and E2 are Young’s moduli, and m1 and m2 are Poisson’s ratios for the contacting two bodies. During loading and unloading indentation, the indentation modulus M can be specified by pffiffiffi 2 ð3Þ S ¼ bc pffiffiffi M A p where S is initial unloading stiffness, A is contact area and bc is a correction factor that depends on the shape
ð4Þ
where E and m are Young’s modulus and Poisson’s ratio for the sample and Ei and mi are the same parameters for the indenter. Based on Eqs. (1), (2) and (4), the relative contact radius, aˆ, between PAA samples can be then estimated as follows: 3 aA MB ^a3 ¼ ¼ ð5Þ aB MA where MA and MB respectively denote the indentation moduli of material A and B. In the case of PAA films, the relative contact area, Ar, should be determined by subtracting the pore area to give the apparent contact area (see Fig. 3b, inset), as follows: " # p^a2 1 2 2 Ar ¼ p^a pffiffiffi pr 2 3=4q2 " 2 # 2p r ð6Þ ¼ p^a2 1 pffiffiffi ¼ p^a2 ð1 P Þ 3 q where P is the porosity of the PAA samples. Eq. (6) shows that the contact area on PAA films is strongly influenced by the porosity. Figure 3 shows the indentation modulus and the estimated relative contact area. For the indentation tests, a TriboScopeÒ nano-indenter (Hysitron Inc.) was used, with a Berkovich tip. As shown in Figure 3a, the indentation moduli of PAA films decreases dramatically with the porosity. The relative contact area determined by Eqs. (5) and (6) displays small variations as a function of the porosity, in good agreement with the variations of the adhesive forces according to the porosity. Figure 4 shows the coefficients of friction as a function of the porosity. The frictional coefficients increased nonlinearly with the porosity due to surface roughness and the contacting asperities. As the porosity increases,
Figure 3. (a) Indentation modulus on PAA samples. The modulus decreased strongly with the porosity. (b) Relative contact area for PAA films according to the porosity estimated by a Hertzian model and indentation theory. The inset shows the pore configuration, the unit cell geometry and the apparent contact area in a PAA sample.
D. Choi et al. / Scripta Materialia 58 (2008) 870–873
873
This research was supported by Grant No. R012006-000-10585-0 (2006) from the Korea Science and Engineering Foundation (KOSEF) funded by the Korea government (MOST).
Figure 4. Dependence of frictional coefficients on the porosity on PAAoxalic and PAAphosphoric samples. Nonlinear behavior of the frictional coefficients was observed with respect to porosity, as a result of the surface morphology of the PAA samples.
so does the surface roughness of the PAA films (see Fig. 1); contact with adjacent bodies also increases as a result of the reduction in stiffness of the porous material [13]. Frictional behavior depends strongly on the interface conditions of the contacting materials, and the overall effect gives rise to nonlinear behavior. In particular, the frictional mode in AFM was of pin-on-disk type, so that the frictional behavior was affected mainly by the surface morphology due to the stick–slip mechanism between the AFM tip and pores on the PAA films. For different AFM tip radii the same trend in friction was found, but the magnitude of the frictional coefficient varied due to the changing contact area. In conclusion, we have investigated the dependence on porosity of friction and adhesion in PAA films by AFM. Adhesion was scarcely influenced by the porosity, and was in good agreement with the relative contact area, whereas pore formation from a flat surface dramatically reduces the adhesive forces by as much as 30 times. The friction shows nonlinear behavior with respect to the porosity as a result of the intrinsic characteristics of the surface morphology. The tribological behavior of PAA films is robust, stable and permanent because it stems from their structural properties; there has been increasing deployment of sophisticated tipbased tools in NEMS/MEMS systems. Knowledge of the tribological characteristics of the PAA films will be vital in the design of various types of contact devices in multifunctional micro/nanostructure systems.
[1] C.R. Martin, Science 266 (1994) 1962. [2] H. Dai, J.H. Hafner, A.G. Rinzler, D.T. Colbert, R.E. Smalley, Nature 384 (1996) 147. [3] H. Masuda, K. Yasui, K. Nishio, Adv. Mater. 12 (2000) 1031. [4] S.B. Lee, D.T. Mitchell, L.N. Trofin, T.K. Nevanen, H. Soderlund, C.R. Martin, Science 296 (2002) 2198. [5] X.Y. Zhang, G.H. Wen, Y.F. Chan, R.K. Zheng, X.X. Zhang, N. Wang, Appl. Phys. Lett. 83 (2003) 3341. [6] C.L. Xu, H. Li, T. Xue, H.L. Li, Scripta Mater. 54 (2006) 1605. [7] D.H. Pearson, R.J. Tonucci, Science 270 (1995) 68. [8] O. Lyandres, N.C. Shah, C.R. Yonzon, J.T. Walsh, M.R. Clucksberg, R.P. Van Duyne, Anal. Chem. 77 (2005) 6134. [9] T.D. Clark, M.R. Ghadiri, J. Am. Chem. Soc. 117 (1995) 12364. [10] H. Masuda, K. Fukuda, Science 268 (1995) 1466. [11] A.P. Li, F. Mu¨ller, A. briner, K. Nielsch, U. Go¨sele, Adv. Mater. 11 (1999) 483. [12] D. Choi, S.M. Lee, C.W. Lee, P.S. Lee, J.H. Lee, K.H. Lee, H.C. Park, W. Hwang, J. Micromech. Microeng. 17 (2007) 501. [13] F. Matsumoto, M. Harada, K. Nishio, H. Masuda, Adv. Mater. 17 (2005) 1609. [14] B. Bhushan, S. Sundararajan, Acta Mater. 46 (1998) 3793. [15] R.W. Carpick, Science 313 (2006) 184. [16] M. Palacio, B. Bhushan, Scripta Mater. 57 (2007) 821. [17] B. Bhushan, M. Nosonovsky, Acta Mater. 52 (2004) 2461. [18] F.W. DelRio, M.L. Dunn, L.M. Phinney, C.J. Bourdon, M.P. de Boer, Appl. Phys. Lett. 90 (2007) 163104. [19] M. Urbakh, J. Klafter, D. Gourdon, J. Israelachvili, Nature 430 (2004) 525. [20] C.K. Bora, E.E. Flater, M.D. Street, J.M. Redmond, M.J. Starr, M.E. Plesha, R.W. Carpick, Trib. Lett. 19 (2005) 37. [21] L. Sirghi, F. Rossi, Appl. Phys. Lett. 89 (2006) 243118. [22] M.J. Brukman, G.O. Marco, T.D. Dunbar, L.D. Boardman, R.W. Carpick, Langmuir 22 (2006) 3988. [23] D. Choi, W. Hwang, E.S. Yoon, J. Microsc.-Oxford 228 (2007) 190. [24] H. Hertz, J. Reine Angew. Math. 92 (1881) 156. [25] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564.
Scripta Materialia 58 (2008) 870–873 www.elsevier.com/locate/scriptamat
Dependence of adhesion and friction on porosity in porous anodic alumina films Dukhyun Choi,a Sangmin Lee,a Seonghan Kim,a Pyungsoo Lee,b Kunhong Lee,b Hyunchul Parka and Woonbong Hwanga,* a
Department of Mechanical Engineering, Pohang University of Science and Technology, San 31, Hyoja, Namgu, Pohang, Gyungbuk 790-784, South Korea b Department of Chemical Engineering, Pohang University of Science and Technology, San 31, Hyoja, Namgu, Pohang, Gyungbuk 790-784, South Korea Received 28 September 2007; revised 12 November 2007; accepted 2 January 2008 Available online 11 January 2008
We explore, using atomic force microscopy, adhesion and friction characteristics of porous anodic alumina (PAA) films as the porosity varies. Pore formation greatly reduces the adhesive force, by as much as 30 times, but the porosity of PAA films has little influence on its adhesion properties, showing good agreement with the relative contact area estimated by the Hertzian model and indentation theory. Frictional coefficients vary nonlinearly with the porosity of the PAA films due to intrinsic characteristics of the surface morphology of PAA films. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Porous material; Adhesion; Friction; Atomic force microscopy
Uniformly arrayed nanoporous materials have recently been proposed in the medical, biological, electrical, optical and micromechanical fields as a template for the growth of low-dimensional nanomaterials and as a three-dimensional micro/nanostructure itself. These include nanodots, nanowires and nanotubes, as well as photonic-crystal waveguide devices, nanocomposite structures, membranes for bioseparation, high-density magnetic memories, single-electron devices and biotechnological structures such as scaffold for tissue growth [1–6]. There is a wide variety of fabrication techniques for creating nanometer-sized structures, including UV photolithography, electro- and ion-beam lithography, X-ray lithography, and scanning probe microscopybased lithography. Not all of these approaches are economically viable, and some are unsuitable for the fabrication of large areas of nanostructured surfaces. Alternative techniques have been proposed to fabricate large arrays of highly ordered massively parallel nanostructures at low cost, including nanochannel glassbased lithography [7], nanosphere lithography [8] and * Corresponding author. Tel.: +82 54 279 2174; fax: +82 54 279 5899; e-mail: [email protected]
techniques based on nanoporous templates [9], such as porous anodic alumina (PAA). PAA as a nanoporous template produces a hole configuration with long-range ordering and a high aspect ratio, and the combined process of pre-texturing aluminum and appropriate anodization generates porous alumina with an ideally ordered hole array of various configuration shapes [10]. The structural properties of PAA have been well investigated, mainly for its mechanical characteristics and optical properties [11,12]. Such PAA films are rigid, well defined, self-supporting and self-stable. Properly annealed, they possess acceptable optical transparency in the visible region of the spectrum. PAA, because of its highly robust and versatile characteristics, is an excellent candidate for use in micro/nanoelectromechanical systems (MEMS/NEMS) and integrated biosensing/ analyzing functional devices [13]. As the surface-to-volume ratio increases in ever-smaller components in MEMS/NEMS devices, a variety of material properties, including magnetic, electronic, optical, mechanical and tribological properties, become strongly dependent on the size, shape and regularity of the material structures [14–16]. Furthermore, since in small-scale devices a high proportion of atoms are located at surface and interface sites (rather than in
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.01.001
D. Choi et al. / Scripta Materialia 58 (2008) 870–873
the bulk of the material), surface forces such as friction and adhesion dominate over the available actuation and restoring forces. In particular, moving components in integrated multifunctional systems at micro/nanometer scales cause serious interface problems due to adhesion, friction and wear. For example, the adhesive force prevents high-precision positioning in microgripping systems, and also causes critical system failure for moving elements in microsystems [17,18]. Thus, a comprehensive micro/nanotribological characterization of the scaledependent nature of material properties would have great importance, both in the design of reliable industrial applications and to provide a bridge between science and engineering on micro/nanoscales. Conventional tribological and lubrication techniques used for large objects can be ineffective at the nanometer scale, which requires new methods for control [19]. Much research has therefore aimed to optimize tribological properties between the components in microsystems by using surface coating or chemical deposition methods. These methods work only for short times, and are expensive and laborious. Below, tunable adhesion and friction behavior is investigated on PAA films according to the porosity, using atomic force microscopy (AFM). To fabricate PAA films having different initial pore size and interpore distance but similar porosities, we applied oxalic acid and phosphoric acid, respectively, during the anodization process. Since tribological properties are influenced by several factors, including contact area, surface roughness and mechanical properties such as hardness and elasticity [20–22], we examined the surface morphology, the indentation modulus and the relative contact area of PAA films as the porosity varies. Typical PAA films were fabricated by two-step anodization of aluminum, as described elsewhere [7–12]. In summary, a pure Al sheet (99.999%) of 1 mm thickness was electropolished in a mixture of perchloric acid and ethanol (HClO4:C2H5OH = 1:4 volumetric ratio) to remove surface irregularities, at a constant voltage of 20 V and temperature of 7 °C. In the anodization process, 0.3 M oxalic acid and 0.1 M phosphoric acid were used to fabricate PAA films with different interpore distances and different initial pore diameters. In the oxalic acid, a PAA film with an interpore distance of 100 nm and an initial pore diameter of 31 nm was obtained by anodizing at 40 V and 15 °C for 12 h. In the phosphoric acid, a PAA film with interpore distance 500 nm and initial pore diameter 100 nm was fabricated at 195 V and 0 °C for 16 h. When phosphoric acid was used, ethylene glycol was added to prevent the circulating water used for temperature control from freezing. After first anodization, the PAA layer was removed in a mixture of 1.8 wt.% chromic acid and 6 wt.% phosphoric acid at 65 °C for 6 h. Highly ordered PAA films were then obtained by a second anodization, under the same conditions as the first. The pore size of PAA films was readily adjusted by controlling the etching time, in 0.1 M phosphoric acid at 30 °C. An atomic force microscope (SPA 400, Seiko) was used to investigate the dependence of adhesion and friction on the porosity of PAA films. An alumina thin film without pores was prepared for use as a control sample. We used AFM tips with SiO2 beads with radii of 380
871
and 2280 nm (Navascan company); the beads were mounted on an aluminum-coated rectangular Si3N4 cantilever with a normal spring constant of 0.6 N m1. All adhesion and friction data were measured under the same normal humidity condition (45% RH) since the adhesion and friction behaviors are strongly sensitive to the humidity. The adhesion force was determined by a single point measurement with a force calibration plot. The frictional force was determined from the friction loop curves determined by AFM. We calibrated the frictional forces measured by AFM using the method of Choi et al. [23]. The frictional coefficients were determined from the gradient of the graph of friction force vs. normal force. For the friction test, the scanning distance was 5 lm, and the normal forces were in the range 0–140 nN. Figure 1 shows typical scanning electron microscopy (SEM; XL30SFEG, Philips) images of PAA films fabricated in phosphoric acid. The surface morphology becomes sharper as the pore size (i.e. the porosity) increases. The inset shows a top view of the PAA films; the scale bar is 500 nm. Table 1 sets out the pore diameters and the porosities of the PAA films fabricated in this work. The porosity of PAA films can be determined by subtracting the pore area from the unit cell area for a PAA film (see Fig. 3b inset, where q is the interpore distance and r is the pore radius in the triangle unit cell). Figure 2 shows the dependence of the adhesive forces on the porosity of the PAA samples for each tip radius. The initial pore formation from the flat surfaces without pores reduced the adhesive force by as much as 30 times, depending on the AFM tip size. As the porosity increases, the adhesive forces hardly change, however, merely showing small variations in the PAA samples.
Figure 1. Typical SEM images of PAA films fabricated in phosphoric acid. The pore diameters were 159 nm (left) and 372 nm (right), with an interpore distance of 500 nm. The surface morphology becomes sharper as the porosity (i.e. pore diameter) increases. The inset shows the top view of the corresponding samples; the scale bar is 500 nm.
Table 1. Pore diameter and the porosity of PAA films PAAoxalic (q = 100 nm)
I II III IV V
PAAphosphoric (q = 500 nm)
Pore diameter
Porosity
Pore diameter
Porosity
31 ± 4.6 41 ± 5.5 47 ± 3.9 52 ± 5.5 60 ± 7.2
0.09 0.16 0.20 0.25 0.32
159 ± 6.6 208 ± 11.5 271 ± 11.1 320 ± 8.8 372 ± 13.6
0.09 0.16 0.27 0.37 0.50
872
D. Choi et al. / Scripta Materialia 58 (2008) 870–873
of the indenter; bc = 1 for flat, cylindrical indenters, bc = 1.012 for Vickers indenters and bc = 1.034 for Berkovich indenters. From the measured indentation modulus, the elastic properties of the sample can be determined by 1 1 v2 1 v2i ¼ þ M E Ei
Figure 2. Porosity dependence of the adhesive force for PAA samples fabricated in oxalic acid (PAAoxalic) and in phosphoric acid (PAAphosphoric); m is the mean value of the adhesive force for each sample and r is the standard deviation. Pore formation reduced the adhesive forces by as much as 30-fold, but the porosity had little influence on the adhesion.
This behavior can be quantitatively estimated through the relative contact area, which can be defined by the ratio of the contact radii of PAA samples, based on a Hertzian contact [24] and indentation theory [25]. In a Herzian model, when a sphere in contact with a flat surface is under the action of an applied load P, the contact radius a is given by the following expression: 3PR ð1Þ 4K where R is the radius of contacting sphere and K is the reduced stiffness defined by
a3 ¼
1 1 m21 1 m22 ¼ þ K E1 E2
ð2Þ
where E1 and E2 are Young’s moduli, and m1 and m2 are Poisson’s ratios for the contacting two bodies. During loading and unloading indentation, the indentation modulus M can be specified by pffiffiffi 2 ð3Þ S ¼ bc pffiffiffi M A p where S is initial unloading stiffness, A is contact area and bc is a correction factor that depends on the shape
ð4Þ
where E and m are Young’s modulus and Poisson’s ratio for the sample and Ei and mi are the same parameters for the indenter. Based on Eqs. (1), (2) and (4), the relative contact radius, aˆ, between PAA samples can be then estimated as follows: 3 aA MB ^a3 ¼ ¼ ð5Þ aB MA where MA and MB respectively denote the indentation moduli of material A and B. In the case of PAA films, the relative contact area, Ar, should be determined by subtracting the pore area to give the apparent contact area (see Fig. 3b, inset), as follows: " # p^a2 1 2 2 Ar ¼ p^a pffiffiffi pr 2 3=4q2 " 2 # 2p r ð6Þ ¼ p^a2 1 pffiffiffi ¼ p^a2 ð1 P Þ 3 q where P is the porosity of the PAA samples. Eq. (6) shows that the contact area on PAA films is strongly influenced by the porosity. Figure 3 shows the indentation modulus and the estimated relative contact area. For the indentation tests, a TriboScopeÒ nano-indenter (Hysitron Inc.) was used, with a Berkovich tip. As shown in Figure 3a, the indentation moduli of PAA films decreases dramatically with the porosity. The relative contact area determined by Eqs. (5) and (6) displays small variations as a function of the porosity, in good agreement with the variations of the adhesive forces according to the porosity. Figure 4 shows the coefficients of friction as a function of the porosity. The frictional coefficients increased nonlinearly with the porosity due to surface roughness and the contacting asperities. As the porosity increases,
Figure 3. (a) Indentation modulus on PAA samples. The modulus decreased strongly with the porosity. (b) Relative contact area for PAA films according to the porosity estimated by a Hertzian model and indentation theory. The inset shows the pore configuration, the unit cell geometry and the apparent contact area in a PAA sample.
D. Choi et al. / Scripta Materialia 58 (2008) 870–873
873
This research was supported by Grant No. R012006-000-10585-0 (2006) from the Korea Science and Engineering Foundation (KOSEF) funded by the Korea government (MOST).
Figure 4. Dependence of frictional coefficients on the porosity on PAAoxalic and PAAphosphoric samples. Nonlinear behavior of the frictional coefficients was observed with respect to porosity, as a result of the surface morphology of the PAA samples.
so does the surface roughness of the PAA films (see Fig. 1); contact with adjacent bodies also increases as a result of the reduction in stiffness of the porous material [13]. Frictional behavior depends strongly on the interface conditions of the contacting materials, and the overall effect gives rise to nonlinear behavior. In particular, the frictional mode in AFM was of pin-on-disk type, so that the frictional behavior was affected mainly by the surface morphology due to the stick–slip mechanism between the AFM tip and pores on the PAA films. For different AFM tip radii the same trend in friction was found, but the magnitude of the frictional coefficient varied due to the changing contact area. In conclusion, we have investigated the dependence on porosity of friction and adhesion in PAA films by AFM. Adhesion was scarcely influenced by the porosity, and was in good agreement with the relative contact area, whereas pore formation from a flat surface dramatically reduces the adhesive forces by as much as 30 times. The friction shows nonlinear behavior with respect to the porosity as a result of the intrinsic characteristics of the surface morphology. The tribological behavior of PAA films is robust, stable and permanent because it stems from their structural properties; there has been increasing deployment of sophisticated tipbased tools in NEMS/MEMS systems. Knowledge of the tribological characteristics of the PAA films will be vital in the design of various types of contact devices in multifunctional micro/nanostructure systems.
[1] C.R. Martin, Science 266 (1994) 1962. [2] H. Dai, J.H. Hafner, A.G. Rinzler, D.T. Colbert, R.E. Smalley, Nature 384 (1996) 147. [3] H. Masuda, K. Yasui, K. Nishio, Adv. Mater. 12 (2000) 1031. [4] S.B. Lee, D.T. Mitchell, L.N. Trofin, T.K. Nevanen, H. Soderlund, C.R. Martin, Science 296 (2002) 2198. [5] X.Y. Zhang, G.H. Wen, Y.F. Chan, R.K. Zheng, X.X. Zhang, N. Wang, Appl. Phys. Lett. 83 (2003) 3341. [6] C.L. Xu, H. Li, T. Xue, H.L. Li, Scripta Mater. 54 (2006) 1605. [7] D.H. Pearson, R.J. Tonucci, Science 270 (1995) 68. [8] O. Lyandres, N.C. Shah, C.R. Yonzon, J.T. Walsh, M.R. Clucksberg, R.P. Van Duyne, Anal. Chem. 77 (2005) 6134. [9] T.D. Clark, M.R. Ghadiri, J. Am. Chem. Soc. 117 (1995) 12364. [10] H. Masuda, K. Fukuda, Science 268 (1995) 1466. [11] A.P. Li, F. Mu¨ller, A. briner, K. Nielsch, U. Go¨sele, Adv. Mater. 11 (1999) 483. [12] D. Choi, S.M. Lee, C.W. Lee, P.S. Lee, J.H. Lee, K.H. Lee, H.C. Park, W. Hwang, J. Micromech. Microeng. 17 (2007) 501. [13] F. Matsumoto, M. Harada, K. Nishio, H. Masuda, Adv. Mater. 17 (2005) 1609. [14] B. Bhushan, S. Sundararajan, Acta Mater. 46 (1998) 3793. [15] R.W. Carpick, Science 313 (2006) 184. [16] M. Palacio, B. Bhushan, Scripta Mater. 57 (2007) 821. [17] B. Bhushan, M. Nosonovsky, Acta Mater. 52 (2004) 2461. [18] F.W. DelRio, M.L. Dunn, L.M. Phinney, C.J. Bourdon, M.P. de Boer, Appl. Phys. Lett. 90 (2007) 163104. [19] M. Urbakh, J. Klafter, D. Gourdon, J. Israelachvili, Nature 430 (2004) 525. [20] C.K. Bora, E.E. Flater, M.D. Street, J.M. Redmond, M.J. Starr, M.E. Plesha, R.W. Carpick, Trib. Lett. 19 (2005) 37. [21] L. Sirghi, F. Rossi, Appl. Phys. Lett. 89 (2006) 243118. [22] M.J. Brukman, G.O. Marco, T.D. Dunbar, L.D. Boardman, R.W. Carpick, Langmuir 22 (2006) 3988. [23] D. Choi, W. Hwang, E.S. Yoon, J. Microsc.-Oxford 228 (2007) 190. [24] H. Hertz, J. Reine Angew. Math. 92 (1881) 156. [25] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564.