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Preparation and characterization of arachidic acid self-assembled monolayers on glass substrate coated with sol–gel Al2O3 thin film
Surface and Coatings Technology 176 (2004) 229–235
Preparation and characterization of arachidic acid self-assembled monolayers on glass substrate coated with sol–gel Al2O3 thin film Jinqing Wang, Shengrong Yang*, Miao Chen, Qunji Xue State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Received 6 January 2003; accepted in revised form 21 March 2003
Abstract Ultra-thin film of Al2O3 was prepared on glass substrates via a sol–gel method, followed by sintering at 500 8C. The selfassembled arachidic acid monolayers (AHDA-SAM) were then prepared on the glass substrate coated with sol–gel Al2 O3 thin film by means of a molecular self-assembly process. The wetting behavior, structure, and morphology of the AHDA-SAM were characterized by means of contact angle measurement, X-ray photoelectron spectroscopic analysis, Fourier transformation infrared spectroscopy, and atomic force microscopy. The friction behavior of the AHDA-SAM on the glass substrate coated with Al2O3 thin film sliding against a Si3 N4 ball was examined on a unidirectional friction and wear tester. The worn surfaces of the AHDASAM were observed with a scanning electron microscope. Distilled water had a contact angle of approximately 1058 on the AHDA-SAM. AHDA-SAM showed excellent antiwear and friction-reducing behavior under a low load (0.5 N) and a low sliding speed (90 mm miny1). The AHDA-SAM was dominated by abrasive wear and plastic deformation in dry sliding against the Si3N4 ball. The glass substrate was endowed with good friction-reducing and antiwear behavior after duplex surface-modification with sol–gel Al2O3 and top layer of AHDA-SAM. This treatment might be of relevance to the surface-modification of single crystal Si and SiC in microelectromechanical systems applications. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Sol–gel process; Self-assembled monolayers; Al2O3 film; Arachidic acid; Friction and wear behavior
1. Introduction Self-assembled monolayers (SAMs) have been the topic of extensive research lately for both their fundamental importance in understanding interfacial properties as well as their potential applications in protective coatings, catalysts, biological sensors, surface and modified electrodes, optoelectronic devices, information storage, adhesion systems, protein films etc. w1–5x. They are suitable substitutes for Langmiur–Blodgett monolayers, because of their orientation and ordering, and chemical bonding to the substrate w6,7x. It is feasible to make use of SAMs to design functional surfaces by attaching different reactive groups to the substrate of interest at the molecular scale w8x. The systems claimed to form highly organized films include alkylsiloxanes on natively oxidized SiO2, n-alkanoic acids on natively *Corresponding author. Tel.: q86-931-8277851; fax: q86-9318277088. E-mail address: [email protected] (S. Yang).
oxidized Al and Ag, dialkyl disulfides and dialkyl sulfides on Au, and alkanethiol on Au, Ag, Cu, and GaAs w3,6,9,10x. To date, the formation of SAMs on metal and metal oxide surfaces has been widely employed for the fabrication of model surfaces with highly controlled chemical properties w11x. For example, the monolayer of alkylsianes, thiols, and methosphonates deposited on metal oxide particles of a larger diameter has been suggested as a closer model for 2D SAM, since intercalation is impossible in this case and the socalled planar metal oxide surfaces are rough on a microscopic level. The adsorption of carboxylic acid on a metal surface has been the subject of many studies, mainly due to the interest in the areas of lubrication, corrosion, and catalysis, of which the formation of SAM of n-alkanoic acid on aluminum w12x, the SAM of arachidic acid and n-perfluorocarboxylic acid on Ag w7x, and the formation of a thin Al2O3 overlayer of 40–60 ˚ thick to protect the underlying metal by the passivaA tion of Al in atmosphere w13x, have been specifically
0257-8972/04/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972Ž03.00632-7
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focused on. It is worth placing special emphasis on the thin Al2O3 overlayer, because it allows the chemisorption of carboxylic acids as monocarboxylate anions thereon, which makes it possible to prepare self-assembled ultra-thin organic films with significantly improved friction-reducing and antiwear behavior by using the ceramic-based film as the buffering layer. Such a technique would find great potential in the lubrication and protection of many devices in microelectromechanical systems (MEMS). The thin film of Al2O3, especially, Al2O3 thin films prepared by sol–gel method, is worth special stress, because of the excellent properties such as chemical inertness, good mechanical strength, high hardness, excellent antiwear ability, transparency, high abrasive and corrosion resistance, as well as insulating and optical properties, which is imperative for their mechanical, optical, and microelectronic applications w14,15x. In the present work, arachidic acid is used as a reactive group to form a SAM on a glass substrate coated with sol–gel Al2O3 thin film as a covering layer, as a tentative to develop novel tribological surfacemodification routes for the devices and elements in MEMS. 2. Experimental 2.1. SAM preparation Ultra-thin Al2O3 films were deposited onto glass substrates and single-crystal silicon wafers via a sol–gel process. The alumina precursor sol at a concentration of 0.2 mol ly1 was prepared in a similar manner as previously reported w16,17x. Thus, 4.08 g aluminum isopropoxide ((C3H7O)3Al) was mixed with 100 ml anhydrous ethanol, followed by adding of 4 ml concentrated nitric acid and 2.5 ml acetylacetone, and kept in a constant-temperature oil bath of 70–80 8C with stirring for 1 h. The Al2O3 sols were transparent at room temperature and no precipitate was formed even after storing for half a year. The glass sheets were pretreated with piranha solution (a mixed solution of 30% H2O2 with 98% H2SO4, at a volume fraction of 3:7) at 80 8C for 30 min, then rinsed with distilled water and acetone. Nano-Al2O3 thin films were deposited by dip-coating method in air at a relatively humidity of 45–55% and withdrawal speed of 42.4 cm miny1. The gel films were dried at 50 8C, for 15 min, then sintered in an oven at a heating rate of 10 8C miny1 up to 500 8C, for 20 min, finally cooled to room temperature in the oven. A commercial reagent, arachidic acid (for short AHDA, L. Light and Co. Ltd, Poyle, Colnbrook, Bucks, England), was used as the raw material for preparation of self-assembled arachidic acid monolayers (for short AHDA-SAM). This was dissolved in hexane solution at a concentration of 0.5 mmol ly1. After that the bare
substrates and Al2O3 coatings deposited on the glass substrate were, respectively, placed in the AHDA solution and kept at ambient temperature for approximately 24 h, to allow the self-assembly. Then the substrates were lifted from the solution, washed with hexane, acetone, and distilled water, and finally dried in a flow of nitrogen. 2.2. Characterization of AHDA-SAM X-ray photoelectron spectroscopic (XPS) analysis was performed on a PHI-5702 multi-functional X-ray photoelectron spectroscope (Physical Electronics, USA) using Mg Ka irradiation (hns1253.6 eV) at a pass energy of 29.35 eV, with the binding energy of contaminated carbon (C1s: 284.6 eV) as the reference. The resolution for the measurement of the binding energy is approximately 0.3 eV. The static contact angles of distilled water on the film surface were measured in ambient air (relative humidity 40%) using a CA–A contact angle measurement device (Kyowa Scientific Co., Ltd). The values reported are the averaged values of at least 5 measurements of three identical samples. The error for the measurement of the contact angles was below 28. Fourier transformation infrared spectra (FTIR) of the AHDA-SAM were recorded on a Bruker IFS66V Fourier transformation infrared spectrometer. Using transmission mode, the spectrum was collected for 500 scans at a resolution of 4 cmy1. A freshly cleaned single-crystal silicon wafer was used as the reference (in order to eliminate the H2O and CO2, the pressure in the sample chamber and optical chamber was kept below 3.0=10y4 Pa). The thickness of the films was measured on a Gaertner L116-E ellipsometer which was equipped with a He–Ne laser (632.8 nm) set at an incident angle of 708 and a spot size of 30=500 mm2. A single-layer model was employed to calculate the thickness from ellipsometric measurement. Namely, a single measurement gives the film thickness of a single-layer transparent lubricant film. A silicon substrate wafer reference sample with a single-layer oxide film thickness of approximately 2 nm supplied with the ellipsometer was used to calibrate the instrument. Thickness data were obtained by averaging five to six thickness values measured at different spots of each sample surface. The index of refraction for the AHDA was taken to be 1.47. The thickness was recorded to an accuracy of "0.3 nm. An atomic force microscope (AFM) of model SPM9500 (Shimadzu Corp., Kyoto, Japan) with a Si3N4 probe was used to observe the morphology of the Al2O3 thin film and AHDA-SAM, using ‘constant force’ mode to obtain the morphology image.
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Fig. 1. XPS spectra of typical elements in Al2O3 thin film and AHDA-SAMyAl2 O3 film. (a) Al2p of Al2 O3 film; (b) O1s of Al2O3 film; (c) C1s of AHDA-SAMyAl2 O3 film; (d) O1s of AHDA-SAMyAl2 O3 film.
2.3. Tribological behavior The friction and wear behaviors of the films were evaluated on a dynamicystatic friction precision measurement apparatus (DF-PM unidirectional sliding tester) by sliding the glass substrate sheet coated with Al2O3 thin film or AHDA-SAM against a Si3N4 ceramic ball (diameter 4 mm, HV 1400–1700) at a sliding velocity of 90 mm miny1, a sliding distance of 7 mm, and a load of 0.5 N. The friction test rig provides a unidirectional sliding configuration. All the tests were conducted at room temperature and a relative humidity of 40–45%. The friction coefficient and sliding cycles were recorded automatically. It was assumed that lubrication failure of the film occurred as the friction coefficient rose sharply to a higher and stable value similar to that of a cleaned bare glass substrate against the same counterface. The number of sliding cycles at this point was recorded as the antiwear life of the film. The morphologies of the wear track of the films were observed on a JSM-5600LV scanning electron microscope (SEM). 3. Results and discussion 3.1. Characterization of AHDA-SAM XPS is significantly useful for the characterization of thin layers and surface products, because information
on the chemical states of the surface products can be obtained according to the chemical shifts of XPS binding energies w18x. XPS was used in the present work to detect the chemical states of some typical elements in the as-prepared Al2O3 thin film and AHDA-SAM on the surface of the Al2O3 thin film, the results are shown in Fig. 1. The resulting binding energies of Al2p at 74.30 eV and O1s at 531.0 eV (Fig. 1a and b) are consistent with that of Al2O3 in sapphire Al2O3 w14x. For AHDASAM, the binding energies of C1s and O1s are worth emphasizing. The C1s spectrum consists of two peaks (Fig. 1c), in which the peak at 284.6 eV can be assigned to the contaminated carbon, while the one at 288.8 eV can be assigned to C in the –COOH group of AHDA molecule. The O1s spectrum consists of a peaks and a shoulder, the peak at 530.8 eV corresponds to the O element in Al2O3, while the shoulder at 531.6 eV corresponds to the O element in AHDA molecule. The FTIR spectrum of AHDA-SAM on the Al2O3 thin film in 3100–2700 cmy1 is shown in Fig. 2. For comparison, the spectrum of a bulk sample of AHDA is also presented. It is seen that asymmetric and symmetric methylene vibrations, nas (CH2) and ns (CH2), show a trend to shift to higher frequencies in the ultrathin film of AHDA. Namely, nas (CH2) and ns (CH2) of the bulk sample of AHDA shift from 2913 and 2846 cmy1 to 2923 and 2852 cmy1, respectively, in AHDA-
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Fig. 2. FTIR spectrum of AHDA-SAM on Al2 O3 thin film in the high-frequency region. The spectrum of a bulk sample of AHDA is also presented on the top left corner for a comparison.
SAM. Such a shift suggests that AHDA-SAM is in a crystallized environment w6x. Contact angle measurement is used as a simple, useful and sensitive tool to assess the surface energy of solids w19x. Table 1 gives the static contact angles of distilled water on the glass substrate sheet and the ones coated with Al2O3 thin film and AHDA-SAMyAl2 O3 film. The contact angle of distilled water on the surface of glass sheet is approximately 58, indicating that the surface of glass substrate sheet is very clean and hydrophilic. The contact angle on the glass substrate sheet coated with an Al2O3 thin film was unchanged. However, it rises sharply to 1058 on the AHDA-SAM topped on the surface of Al2O3 thin film, which is in agreement with that reported elsewhere w3x. The marked variation of the contact angle implies that the composition and features of the surface have changed considerably, which suggests that the densely packed and highly oriented AHDA-SAM has been successfully prepared on the glass substrate sheet coated with sol–gel Al2O3 thin film. Interestingly, the contact angle of water on the bare glass sheet immersed with AHDA for 24 h is approximately 258, which indicates that the AHDA molecules can only be physico-chemically absorbed on the surface of the bare glass substrate and hence no self-assembly monolayer is formed in this case. The thickness of the Al2O3 thin film and AHDASAM was measured using an ellipsometer. The averaged thicknesses of mono-Al2O3 thin film and AHDA-SAM
were determined to be approximately 35.5 and 2.75 nm, respectively, based on the ellipsometric measurement. Comparison of the thickness of the AHDA-SAM to that of a fully extended AHDA chain (2.80 nm) w6x, indicates that the self-assembled film has an average tilt of approximately 118, if neglecting the changes in bonding due to the reactive chemistry between the carboxylic acid head-group and the surface of Al2O3 thin film. We suppose that, a spontaneous absorption of AHDA molecules, based on an acid-base reaction, with the formation of a surface salt from the carboxylate anion and a surface metal cation, occurs as illustrated in Fig. 3. AFM surface morphologies of the glass sheet, Al2O3 thin film coated-glass substrate sheet and SAM of AHDAyAl2O3 are shown in Fig. 4. It can be seen that the surface of glass sheet is rough (Fig. 4a), with a relative roughness (RMS) approximately 3.0 nm. The Al2O3 thin film coated on the glass substrate sheet is compact, uniform, and crack-free and is composed of nanoscale crystallites (Fig. 4b), with a roughness of RMS less than 2.0 nm. The AHDA-SAM is more smooth than the Al2O3 thin film, with a roughness of RMS less than 0.5 nm (Fig. 4c).
Table 1 Water contact angles on the surfaces of glass substrate and modified glass substrate Surface
Contact angle ("28)
Glass Glass Glass Glass
5 5 25 105
substrate sheet coated with Al2O3 immersed with AHDA coated with AHDA-SAMyAl2 O3
Fig. 3. Schematic diagram of the adsorption of AHDA to a smooth surface of Al2O3 thin film.
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Fig. 4. (a) AMF morphologies of the glass sheet; (b) Al2O3 thin film, and (c) ADHA-SAMyAl2 O3 .
3.2. Friction and wear behavior Because the SAMs form densely packed and often ordered structures on solid surfaces, they are ideal to model lubricant films for fundamental studies of friction. Fig. 5 gives the macroscale friction coefficients of Al2O3 thin film coated-glass sheet and AHDA-SAM thereon sliding against Si3N4 ball. The glass substrate shows a friction coefficient as high as 0.7–0.8. The thin film of Al2O3 on the glass substrate registers a relatively low friction coefficient (0.22) and considerably increased antiwear life as compared with the bare glass substrate. Under the same testing condition, the AHDASAM records further decreased friction coefficients by 0.10 relative to the Al2O3 covering layer on the glass substrate and the small friction coefficient remained almost unchanged even at an extended sliding cycles of 200. This value is smaller than the macroscale friction coefficient of silane-based SAM sliding against an alumina ball (0.17 at 0.1 N), reported by Bhushan et al. w20x, and than that of carboxyl acid SAM sliding against AISI 52100 steel ball (0.12 under 0.5 N), reported by Ren et al. w21x. Thus it can be concluded that the AHDA-SAM coating on the Al2O3 covering layer pro-
vides good friction-reduction and wear-resistance behavior at a small load (0.5 N) and sliding speed (90 mm miny1). This is further proved from the measured antiwear life of the AHDA adsorption layer on the glass
Fig. 5. Friction coefficient as a function of sliding cycles against Si3N4 ball.
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Fig. 6. (a) SEM micrographs of worn surfaces of glass sheet; (b) Al2O3 thin film, and (c) AHDA-SAMyAl2 O3 composite film sliding against Si3N4 ceramic ball at 0.5 N for 10 cycles, 45 cycles, and 200 cycles.
substrate (Fig. 5). Namely, the adsorption layer of the AHDA on the glass substrate is maintained only for approximately 10 cycles. Though it registers the same initial friction coefficient as the AHDA-SAM, it increases rapidly thereafter. This can be better understood by taking into account the correlation between the friction behavior of a thin film and its structure and surface energy w20,22x. In particular, the poor packing and low orientation in the AHDA layer deposited directly on the glass substrate give rise to more energy dissipating modes (chain bending and tilting, rotations, and formation of gauche defects, etc.) and the higher surface energy gives higher adhesive force (Fig. 4), which in turn determine an increased friction force (Fig. 5). Consequently, the AHDA-SAM displays superior friction-reducing and antiwear performance compared to both the AHDA on glass and also the Al2O3 thin film itself. In order to gain more insights into the friction and wear mechanisms of the films, the worn surfaces of the
bare glass substrate, the Al2O3 thin film coated-glass substrate sheet, and the AHDA-SAMyAl2 O3 sliding against Si3N4 ceramic ball were observed by SEM. As shown in Fig. 6, the worn surface of the bare glass substrate sliding against Si3N4 ceramic ball after 10 sliding cycles at 0.5 N is very rough and characterized by severe adhesion wear and scuffing (Fig. 6a). The worn surface of the Al2O3 thin film coated-glass substrate after 45 sliding cycles at the same testing conditions is relatively rough and characterized by fatigue wear, slight abrasion, and scuffing (Fig. 6b), with some large wear particles appearing on the wear track. The worn surface of the AHDA-SAMyAl2 O3 after 200 sliding cycles under the same testing conditions is much more smooth and characterized by slight plastic deformation and scuffing near the wear track edge (Fig. 6c). This indicates that the glass substrate can be successfully surface-modified to obtain better friction-reducing and antiwear behavior, by using dual surface treatment of deposition of sol–gel Al2O3 thin film followed by the
J. Wang et al. / Surface and Coatings Technology 176 (2004) 229–235
topping with the AHDA-SAM. This finding might be helpful to the surface-modification of single crystal Si and SiC in MEMS applications. 4. Conclusions AHDA-SAM on glass substrate coated with sol–gel Al2O3 thin film was prepared successfully by means of molecular self-assembly process. The wetting behavior, structure and morphology of the AHDA-SAM were characterized by means of XPS analysis, contact angle measurement, Fourier transformation infrared spectroscopy, and atomic force microscopy. The friction behavior of the glass substrate coated with Al2O3 thin film and the AHDA-SAM sliding against a Si3N4 ball was examined. Results showed that the AHDA-SAM on the glass substrate coated with sol–gel Al2O3 thin film was dominated by slight scuffing and plastic deformation. The glass substrate was endowed with good frictionreducing and antiwear behavior after duplex surfacemodified with sol–gel Al2O3 and topping of AHDA-SAM. The relevant results might be helpful to guiding the surface-modification of single crystal Si and SiC in MEMS applications. Acknowledgments The authors are grateful to the National Natural Science Foundation of China (Grant No. 50023001) and ‘Top Hundred Talents Program’ of Chinese Academy of Sciences for financial support.
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Preparation and characterization of arachidic acid self-assembled monolayers on glass substrate coated with sol–gel Al2O3 thin film Jinqing Wang, Shengrong Yang*, Miao Chen, Qunji Xue State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Received 6 January 2003; accepted in revised form 21 March 2003
Abstract Ultra-thin film of Al2O3 was prepared on glass substrates via a sol–gel method, followed by sintering at 500 8C. The selfassembled arachidic acid monolayers (AHDA-SAM) were then prepared on the glass substrate coated with sol–gel Al2 O3 thin film by means of a molecular self-assembly process. The wetting behavior, structure, and morphology of the AHDA-SAM were characterized by means of contact angle measurement, X-ray photoelectron spectroscopic analysis, Fourier transformation infrared spectroscopy, and atomic force microscopy. The friction behavior of the AHDA-SAM on the glass substrate coated with Al2O3 thin film sliding against a Si3 N4 ball was examined on a unidirectional friction and wear tester. The worn surfaces of the AHDASAM were observed with a scanning electron microscope. Distilled water had a contact angle of approximately 1058 on the AHDA-SAM. AHDA-SAM showed excellent antiwear and friction-reducing behavior under a low load (0.5 N) and a low sliding speed (90 mm miny1). The AHDA-SAM was dominated by abrasive wear and plastic deformation in dry sliding against the Si3N4 ball. The glass substrate was endowed with good friction-reducing and antiwear behavior after duplex surface-modification with sol–gel Al2O3 and top layer of AHDA-SAM. This treatment might be of relevance to the surface-modification of single crystal Si and SiC in microelectromechanical systems applications. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Sol–gel process; Self-assembled monolayers; Al2O3 film; Arachidic acid; Friction and wear behavior
1. Introduction Self-assembled monolayers (SAMs) have been the topic of extensive research lately for both their fundamental importance in understanding interfacial properties as well as their potential applications in protective coatings, catalysts, biological sensors, surface and modified electrodes, optoelectronic devices, information storage, adhesion systems, protein films etc. w1–5x. They are suitable substitutes for Langmiur–Blodgett monolayers, because of their orientation and ordering, and chemical bonding to the substrate w6,7x. It is feasible to make use of SAMs to design functional surfaces by attaching different reactive groups to the substrate of interest at the molecular scale w8x. The systems claimed to form highly organized films include alkylsiloxanes on natively oxidized SiO2, n-alkanoic acids on natively *Corresponding author. Tel.: q86-931-8277851; fax: q86-9318277088. E-mail address: [email protected] (S. Yang).
oxidized Al and Ag, dialkyl disulfides and dialkyl sulfides on Au, and alkanethiol on Au, Ag, Cu, and GaAs w3,6,9,10x. To date, the formation of SAMs on metal and metal oxide surfaces has been widely employed for the fabrication of model surfaces with highly controlled chemical properties w11x. For example, the monolayer of alkylsianes, thiols, and methosphonates deposited on metal oxide particles of a larger diameter has been suggested as a closer model for 2D SAM, since intercalation is impossible in this case and the socalled planar metal oxide surfaces are rough on a microscopic level. The adsorption of carboxylic acid on a metal surface has been the subject of many studies, mainly due to the interest in the areas of lubrication, corrosion, and catalysis, of which the formation of SAM of n-alkanoic acid on aluminum w12x, the SAM of arachidic acid and n-perfluorocarboxylic acid on Ag w7x, and the formation of a thin Al2O3 overlayer of 40–60 ˚ thick to protect the underlying metal by the passivaA tion of Al in atmosphere w13x, have been specifically
0257-8972/04/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972Ž03.00632-7
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focused on. It is worth placing special emphasis on the thin Al2O3 overlayer, because it allows the chemisorption of carboxylic acids as monocarboxylate anions thereon, which makes it possible to prepare self-assembled ultra-thin organic films with significantly improved friction-reducing and antiwear behavior by using the ceramic-based film as the buffering layer. Such a technique would find great potential in the lubrication and protection of many devices in microelectromechanical systems (MEMS). The thin film of Al2O3, especially, Al2O3 thin films prepared by sol–gel method, is worth special stress, because of the excellent properties such as chemical inertness, good mechanical strength, high hardness, excellent antiwear ability, transparency, high abrasive and corrosion resistance, as well as insulating and optical properties, which is imperative for their mechanical, optical, and microelectronic applications w14,15x. In the present work, arachidic acid is used as a reactive group to form a SAM on a glass substrate coated with sol–gel Al2O3 thin film as a covering layer, as a tentative to develop novel tribological surfacemodification routes for the devices and elements in MEMS. 2. Experimental 2.1. SAM preparation Ultra-thin Al2O3 films were deposited onto glass substrates and single-crystal silicon wafers via a sol–gel process. The alumina precursor sol at a concentration of 0.2 mol ly1 was prepared in a similar manner as previously reported w16,17x. Thus, 4.08 g aluminum isopropoxide ((C3H7O)3Al) was mixed with 100 ml anhydrous ethanol, followed by adding of 4 ml concentrated nitric acid and 2.5 ml acetylacetone, and kept in a constant-temperature oil bath of 70–80 8C with stirring for 1 h. The Al2O3 sols were transparent at room temperature and no precipitate was formed even after storing for half a year. The glass sheets were pretreated with piranha solution (a mixed solution of 30% H2O2 with 98% H2SO4, at a volume fraction of 3:7) at 80 8C for 30 min, then rinsed with distilled water and acetone. Nano-Al2O3 thin films were deposited by dip-coating method in air at a relatively humidity of 45–55% and withdrawal speed of 42.4 cm miny1. The gel films were dried at 50 8C, for 15 min, then sintered in an oven at a heating rate of 10 8C miny1 up to 500 8C, for 20 min, finally cooled to room temperature in the oven. A commercial reagent, arachidic acid (for short AHDA, L. Light and Co. Ltd, Poyle, Colnbrook, Bucks, England), was used as the raw material for preparation of self-assembled arachidic acid monolayers (for short AHDA-SAM). This was dissolved in hexane solution at a concentration of 0.5 mmol ly1. After that the bare
substrates and Al2O3 coatings deposited on the glass substrate were, respectively, placed in the AHDA solution and kept at ambient temperature for approximately 24 h, to allow the self-assembly. Then the substrates were lifted from the solution, washed with hexane, acetone, and distilled water, and finally dried in a flow of nitrogen. 2.2. Characterization of AHDA-SAM X-ray photoelectron spectroscopic (XPS) analysis was performed on a PHI-5702 multi-functional X-ray photoelectron spectroscope (Physical Electronics, USA) using Mg Ka irradiation (hns1253.6 eV) at a pass energy of 29.35 eV, with the binding energy of contaminated carbon (C1s: 284.6 eV) as the reference. The resolution for the measurement of the binding energy is approximately 0.3 eV. The static contact angles of distilled water on the film surface were measured in ambient air (relative humidity 40%) using a CA–A contact angle measurement device (Kyowa Scientific Co., Ltd). The values reported are the averaged values of at least 5 measurements of three identical samples. The error for the measurement of the contact angles was below 28. Fourier transformation infrared spectra (FTIR) of the AHDA-SAM were recorded on a Bruker IFS66V Fourier transformation infrared spectrometer. Using transmission mode, the spectrum was collected for 500 scans at a resolution of 4 cmy1. A freshly cleaned single-crystal silicon wafer was used as the reference (in order to eliminate the H2O and CO2, the pressure in the sample chamber and optical chamber was kept below 3.0=10y4 Pa). The thickness of the films was measured on a Gaertner L116-E ellipsometer which was equipped with a He–Ne laser (632.8 nm) set at an incident angle of 708 and a spot size of 30=500 mm2. A single-layer model was employed to calculate the thickness from ellipsometric measurement. Namely, a single measurement gives the film thickness of a single-layer transparent lubricant film. A silicon substrate wafer reference sample with a single-layer oxide film thickness of approximately 2 nm supplied with the ellipsometer was used to calibrate the instrument. Thickness data were obtained by averaging five to six thickness values measured at different spots of each sample surface. The index of refraction for the AHDA was taken to be 1.47. The thickness was recorded to an accuracy of "0.3 nm. An atomic force microscope (AFM) of model SPM9500 (Shimadzu Corp., Kyoto, Japan) with a Si3N4 probe was used to observe the morphology of the Al2O3 thin film and AHDA-SAM, using ‘constant force’ mode to obtain the morphology image.
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Fig. 1. XPS spectra of typical elements in Al2O3 thin film and AHDA-SAMyAl2 O3 film. (a) Al2p of Al2 O3 film; (b) O1s of Al2O3 film; (c) C1s of AHDA-SAMyAl2 O3 film; (d) O1s of AHDA-SAMyAl2 O3 film.
2.3. Tribological behavior The friction and wear behaviors of the films were evaluated on a dynamicystatic friction precision measurement apparatus (DF-PM unidirectional sliding tester) by sliding the glass substrate sheet coated with Al2O3 thin film or AHDA-SAM against a Si3N4 ceramic ball (diameter 4 mm, HV 1400–1700) at a sliding velocity of 90 mm miny1, a sliding distance of 7 mm, and a load of 0.5 N. The friction test rig provides a unidirectional sliding configuration. All the tests were conducted at room temperature and a relative humidity of 40–45%. The friction coefficient and sliding cycles were recorded automatically. It was assumed that lubrication failure of the film occurred as the friction coefficient rose sharply to a higher and stable value similar to that of a cleaned bare glass substrate against the same counterface. The number of sliding cycles at this point was recorded as the antiwear life of the film. The morphologies of the wear track of the films were observed on a JSM-5600LV scanning electron microscope (SEM). 3. Results and discussion 3.1. Characterization of AHDA-SAM XPS is significantly useful for the characterization of thin layers and surface products, because information
on the chemical states of the surface products can be obtained according to the chemical shifts of XPS binding energies w18x. XPS was used in the present work to detect the chemical states of some typical elements in the as-prepared Al2O3 thin film and AHDA-SAM on the surface of the Al2O3 thin film, the results are shown in Fig. 1. The resulting binding energies of Al2p at 74.30 eV and O1s at 531.0 eV (Fig. 1a and b) are consistent with that of Al2O3 in sapphire Al2O3 w14x. For AHDASAM, the binding energies of C1s and O1s are worth emphasizing. The C1s spectrum consists of two peaks (Fig. 1c), in which the peak at 284.6 eV can be assigned to the contaminated carbon, while the one at 288.8 eV can be assigned to C in the –COOH group of AHDA molecule. The O1s spectrum consists of a peaks and a shoulder, the peak at 530.8 eV corresponds to the O element in Al2O3, while the shoulder at 531.6 eV corresponds to the O element in AHDA molecule. The FTIR spectrum of AHDA-SAM on the Al2O3 thin film in 3100–2700 cmy1 is shown in Fig. 2. For comparison, the spectrum of a bulk sample of AHDA is also presented. It is seen that asymmetric and symmetric methylene vibrations, nas (CH2) and ns (CH2), show a trend to shift to higher frequencies in the ultrathin film of AHDA. Namely, nas (CH2) and ns (CH2) of the bulk sample of AHDA shift from 2913 and 2846 cmy1 to 2923 and 2852 cmy1, respectively, in AHDA-
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Fig. 2. FTIR spectrum of AHDA-SAM on Al2 O3 thin film in the high-frequency region. The spectrum of a bulk sample of AHDA is also presented on the top left corner for a comparison.
SAM. Such a shift suggests that AHDA-SAM is in a crystallized environment w6x. Contact angle measurement is used as a simple, useful and sensitive tool to assess the surface energy of solids w19x. Table 1 gives the static contact angles of distilled water on the glass substrate sheet and the ones coated with Al2O3 thin film and AHDA-SAMyAl2 O3 film. The contact angle of distilled water on the surface of glass sheet is approximately 58, indicating that the surface of glass substrate sheet is very clean and hydrophilic. The contact angle on the glass substrate sheet coated with an Al2O3 thin film was unchanged. However, it rises sharply to 1058 on the AHDA-SAM topped on the surface of Al2O3 thin film, which is in agreement with that reported elsewhere w3x. The marked variation of the contact angle implies that the composition and features of the surface have changed considerably, which suggests that the densely packed and highly oriented AHDA-SAM has been successfully prepared on the glass substrate sheet coated with sol–gel Al2O3 thin film. Interestingly, the contact angle of water on the bare glass sheet immersed with AHDA for 24 h is approximately 258, which indicates that the AHDA molecules can only be physico-chemically absorbed on the surface of the bare glass substrate and hence no self-assembly monolayer is formed in this case. The thickness of the Al2O3 thin film and AHDASAM was measured using an ellipsometer. The averaged thicknesses of mono-Al2O3 thin film and AHDA-SAM
were determined to be approximately 35.5 and 2.75 nm, respectively, based on the ellipsometric measurement. Comparison of the thickness of the AHDA-SAM to that of a fully extended AHDA chain (2.80 nm) w6x, indicates that the self-assembled film has an average tilt of approximately 118, if neglecting the changes in bonding due to the reactive chemistry between the carboxylic acid head-group and the surface of Al2O3 thin film. We suppose that, a spontaneous absorption of AHDA molecules, based on an acid-base reaction, with the formation of a surface salt from the carboxylate anion and a surface metal cation, occurs as illustrated in Fig. 3. AFM surface morphologies of the glass sheet, Al2O3 thin film coated-glass substrate sheet and SAM of AHDAyAl2O3 are shown in Fig. 4. It can be seen that the surface of glass sheet is rough (Fig. 4a), with a relative roughness (RMS) approximately 3.0 nm. The Al2O3 thin film coated on the glass substrate sheet is compact, uniform, and crack-free and is composed of nanoscale crystallites (Fig. 4b), with a roughness of RMS less than 2.0 nm. The AHDA-SAM is more smooth than the Al2O3 thin film, with a roughness of RMS less than 0.5 nm (Fig. 4c).
Table 1 Water contact angles on the surfaces of glass substrate and modified glass substrate Surface
Contact angle ("28)
Glass Glass Glass Glass
5 5 25 105
substrate sheet coated with Al2O3 immersed with AHDA coated with AHDA-SAMyAl2 O3
Fig. 3. Schematic diagram of the adsorption of AHDA to a smooth surface of Al2O3 thin film.
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Fig. 4. (a) AMF morphologies of the glass sheet; (b) Al2O3 thin film, and (c) ADHA-SAMyAl2 O3 .
3.2. Friction and wear behavior Because the SAMs form densely packed and often ordered structures on solid surfaces, they are ideal to model lubricant films for fundamental studies of friction. Fig. 5 gives the macroscale friction coefficients of Al2O3 thin film coated-glass sheet and AHDA-SAM thereon sliding against Si3N4 ball. The glass substrate shows a friction coefficient as high as 0.7–0.8. The thin film of Al2O3 on the glass substrate registers a relatively low friction coefficient (0.22) and considerably increased antiwear life as compared with the bare glass substrate. Under the same testing condition, the AHDASAM records further decreased friction coefficients by 0.10 relative to the Al2O3 covering layer on the glass substrate and the small friction coefficient remained almost unchanged even at an extended sliding cycles of 200. This value is smaller than the macroscale friction coefficient of silane-based SAM sliding against an alumina ball (0.17 at 0.1 N), reported by Bhushan et al. w20x, and than that of carboxyl acid SAM sliding against AISI 52100 steel ball (0.12 under 0.5 N), reported by Ren et al. w21x. Thus it can be concluded that the AHDA-SAM coating on the Al2O3 covering layer pro-
vides good friction-reduction and wear-resistance behavior at a small load (0.5 N) and sliding speed (90 mm miny1). This is further proved from the measured antiwear life of the AHDA adsorption layer on the glass
Fig. 5. Friction coefficient as a function of sliding cycles against Si3N4 ball.
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Fig. 6. (a) SEM micrographs of worn surfaces of glass sheet; (b) Al2O3 thin film, and (c) AHDA-SAMyAl2 O3 composite film sliding against Si3N4 ceramic ball at 0.5 N for 10 cycles, 45 cycles, and 200 cycles.
substrate (Fig. 5). Namely, the adsorption layer of the AHDA on the glass substrate is maintained only for approximately 10 cycles. Though it registers the same initial friction coefficient as the AHDA-SAM, it increases rapidly thereafter. This can be better understood by taking into account the correlation between the friction behavior of a thin film and its structure and surface energy w20,22x. In particular, the poor packing and low orientation in the AHDA layer deposited directly on the glass substrate give rise to more energy dissipating modes (chain bending and tilting, rotations, and formation of gauche defects, etc.) and the higher surface energy gives higher adhesive force (Fig. 4), which in turn determine an increased friction force (Fig. 5). Consequently, the AHDA-SAM displays superior friction-reducing and antiwear performance compared to both the AHDA on glass and also the Al2O3 thin film itself. In order to gain more insights into the friction and wear mechanisms of the films, the worn surfaces of the
bare glass substrate, the Al2O3 thin film coated-glass substrate sheet, and the AHDA-SAMyAl2 O3 sliding against Si3N4 ceramic ball were observed by SEM. As shown in Fig. 6, the worn surface of the bare glass substrate sliding against Si3N4 ceramic ball after 10 sliding cycles at 0.5 N is very rough and characterized by severe adhesion wear and scuffing (Fig. 6a). The worn surface of the Al2O3 thin film coated-glass substrate after 45 sliding cycles at the same testing conditions is relatively rough and characterized by fatigue wear, slight abrasion, and scuffing (Fig. 6b), with some large wear particles appearing on the wear track. The worn surface of the AHDA-SAMyAl2 O3 after 200 sliding cycles under the same testing conditions is much more smooth and characterized by slight plastic deformation and scuffing near the wear track edge (Fig. 6c). This indicates that the glass substrate can be successfully surface-modified to obtain better friction-reducing and antiwear behavior, by using dual surface treatment of deposition of sol–gel Al2O3 thin film followed by the
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topping with the AHDA-SAM. This finding might be helpful to the surface-modification of single crystal Si and SiC in MEMS applications. 4. Conclusions AHDA-SAM on glass substrate coated with sol–gel Al2O3 thin film was prepared successfully by means of molecular self-assembly process. The wetting behavior, structure and morphology of the AHDA-SAM were characterized by means of XPS analysis, contact angle measurement, Fourier transformation infrared spectroscopy, and atomic force microscopy. The friction behavior of the glass substrate coated with Al2O3 thin film and the AHDA-SAM sliding against a Si3N4 ball was examined. Results showed that the AHDA-SAM on the glass substrate coated with sol–gel Al2O3 thin film was dominated by slight scuffing and plastic deformation. The glass substrate was endowed with good frictionreducing and antiwear behavior after duplex surfacemodified with sol–gel Al2O3 and topping of AHDA-SAM. The relevant results might be helpful to guiding the surface-modification of single crystal Si and SiC in MEMS applications. Acknowledgments The authors are grateful to the National Natural Science Foundation of China (Grant No. 50023001) and ‘Top Hundred Talents Program’ of Chinese Academy of Sciences for financial support.
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