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polyelectrolyte multilayer films by AFM and micro-tribometer
Tribology International 44 (2011) 906–915
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Tribology International journal homepage: www.elsevier.com/locate/triboint
Adhesion and friction of nanoparticles/polyelectrolyte multilayer films by AFM and micro-tribometer Yan-Bao Guo n, De-Guo Wang n, Si-Wei Zhang College of Mechanical and Transportation Engineering, China University of Petroleum, 18 Fuxue Road, Changping, Beijing 102249, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2010 Received in revised form 6 March 2011 Accepted 9 March 2011 Available online 17 March 2011
The in situ CuS nanoparticle/polyelectrolyte multilayer (NPs/PEM) films have been prepared using the layer-by-layer method and the in situ synthesize method. The NPs/PEM films were characterized by atomic force microscope (AFM), ultraviolet–visible (UV–vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), and transmission electron microscope (TEM). It was found that the adhesive forces between probe and sample are decreased by compositing in situ CuS nanoparticles. From the investigation of the nano- and micro-tribological behaviors, the NPs/PEM film has a lower friction force and a better anti-wear property than the pure polyelectrolyte multilayer film. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Polyelectrolyte multilayer film Nanoparticles Adhesion Friction
1. Introduction Over the previous decades, there are some methods for deposition of ultrathin films on a substrate such as layer-by-layer (LBL), Langmuir–Blodgett (LB), spin coating, sputtering, and so on [1–3]. Layer-by-layer (LBL) method developed by Decher et al. [4–6] consists of alternate dipping of the substrate in oppositely charged polycation and polyanion electrolyte solutions. The LBL method is simple and cheaper, easy to automate, and friendly to the environment. Especially, the film can be deposited on a wide variety of substrates, and devices with complex architectures can be coated uniformly over large areas. The LBL film is a novel material studied extensively in recent years due to their numerous potential applications, versatility, and simplicity [1]. The LBL technique is ideally suited to combat the tribological challenges in microelectromechanical systems (MEMS). Up to now, the tribological properties of polyelectrolyte multilayer (PEM) film were studied popularly. Pavoor et al. [7,8] has reported that the deposition of a poly(acrylic acid) and poly(allylamine hydrochloride) polyelectrolyte multilayer film using the LBL technique, and the capacity for polyelectrolyte multilayer film induced wear reduction at large scales under the dry state. The PEM film is able to decrease the adhesive force on a surface, and to modify the friction surface and reduce the friction force [9–11]. And, nanoparticles were applied in tribology of ultrathin film. Graphite oxide, TiO2, SiO2, Ag nanoparticles, and so on composite polyelectrolyte multilayer films
n
Corresponding authors. E-mail addresses: [email protected] (Y.-B. Guo), [email protected] (D.-G. Wang).
0301-679X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2011.03.007
have been investigated to enhance the wear life of PEM film [12–15]. It was found that these films can improve the tribological performance greatly. It has been reported that CuS were used as oil additive and filling material to improve the tribological property [16–18]. In this paper, the adhesion and friction of the in situ CuS nanoparticles/polyelectrolyte multilayer (NPs/PEM) film have been investigated by AFM and micro-tribometer.
2. Experiment 2.1. Materials All materials were used without further purification. The polyelectrolytes used in this studies were poly(diallyl dimethylammonium chloride) and poly(acrylic acid), hereafter referred to as PDDA and PAA, supplied by Aldrich Chemical Co., American. The averaged molecular weight (Mw) of the PDDA and PAA used in the experiments were 100,000–200,000 and 30,000, respectively. Mili-Q ultrapure water was used for preparation of all aqueous solutions, and during rinsing procedures ( 418 MO cm, Milipore Mili-Q). Copper chloride (CuCl2) and Sodium sulfide (Na2S) were supplied by Sinopharm Chemical Reagent Co., Ltd. All the other reagents are analytical reagents. Prior of deposition, the glass and quartz substrates were cleaned according to the following procedures: (1) The substrates were ultrasonic treated in acetone, chloroform, and anhydrous ethanol (volume ratio¼3:3:1) bath for 30 min
Y.-B. Guo et al. / Tribology International 44 (2011) 906–915
first and rinsed to eliminate the residues on substrates using de-ionized water flushing. (2) The substrates were treated in an 80 1C piranha solution (98%H2SO4/30%H2O2, volume ratio¼7:3) bath for 1 h (caution: piranha solution reacts violently with organic solvents and is a skin irritant. Extreme caution should be exercised when handling piranha solution). The piranha treatment allows removal of residues of organic and inorganic impurities from the substrates and makes the slides completely hydrophilic at the same time, through making the substrates to be hydroxylated. In the end, the substrates were rinsed with plenty of de-ionized water and dried with nitrogen to prepare for the deposition of polyelectrolytes.
2.2. Instruments Ultraviolet–visible (UV–vis) absorption spectrums of films were recorded on a Unico 2102 UV–vis spectrophotometer, which was used to monitor the influence of absorbance on the in situ nanoparticle/polyelectrolyte multilayer film. The X-ray photoelectron spectroscopy (XPS) measurements were performed with the PHI-5300 spectroscope using Al Ka (hn ¼1486.6 eV) X-ray radiation. The vacuum inside the analysis chamber was 1.333 10 8 Pa during the analysis. The binding energy (BE) of C1s (284.8 eV) was used as the reference. Contact angle measurements were conducted using the sessile drop technique performed on a contact angle goniometer (JC2000A, China) at 25 1C. A NanoMan VS (VEECO) atomic force microscopy was used to evaluate the morphology of the MD films by tapping mode, and it was operated under air-ambient temperatures (2571 1C) and a relative humidity of 20% using commercial silicon nitride probes (spring constant 0.2022 N/m, which was checked by the AFM instrument). AFM has been used extensively to measure adhesive force between surfaces at nano-scale [19–21]. The adhesive force between the AFM tip and the film surface by force-curves mode under ambient condition is shown in Fig. 1. a-b is the probe approaching the sample surface gradually, and the micro-cantilever of the probe does not bend with the force between the sample surface and probe zero. b-c is probe contacting with the sample surface suddenly due to the surface tension when both close infinitely, and then the micro-cantilever is curved downward slightly. c-d is loading process where the downward micro-cantilever transforms into without any bending, then curved upward, and the degree of bending gradually increases.
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The probe retraces to unload when achieving the Point ‘d’, d-f is the unloading process, where the bending degree of microcantilever upward decreases to zero (Point ‘e’), then the microcantilever bends downward gradually due to the adhesive force between the probe and the sample surface. The probe will pull-off the sample surface suddenly when the adhesive force equals to the bending force of the micro-cantilever (Point ‘f ’). The Point ‘a’ indicates no bending occurs after the probe pulling off the sample surface. The adhesive force (pull-off force) was calculated by multiplying the cantilever spring constant by the horizontal distance between Points ‘e’ and ‘a’ [19] Fa ¼ K lae
ð1Þ
The friction forces of PEM and NPs/PEM films were measured using the friction force mode of AFM (FFM) [22]. Fig. 2 shows the working principles of FFM, and the main components are a thin cantilever with a probing tip (L is the lever length of cantilever; w is the lever width of cantilever, h is the lever thickness, i is the height of the tip mounted at the end of the cantilever). The size of the AFM probe is listed in Table 1. The laser beam deflection caused by the cantilever is determined by a quadrant photodiode of the optical system. The normal load and the frictional force are proportional to the normal and torsional deflections of the cantilever, which are recorded simultaneously via the output of the photodiode detector. The signal VA þ B VC þ D is the measurement for normal bending and VA þ C VB þ D for the torsional deflection of the cantilever. When the lateral force Fy is applied at the tip, as in the case of the AFM cantilever, lateral deflection is accompanied by twisting or torsion along the cantilever axis [23]. Fy ¼ kyT Dy
ð2Þ
kyT is the bending stiffness. Dy is the deflection in the y direction. For a wide, thin cantilever (b bh) experiencing a rotation f. The torsional stiffness of the beam kf, is given as M ¼ kf f
ð3Þ
Fig. 2. The working principles of FFM, and the main components a thin cantilever with a probing tip.
Table 1 The size of the AFM probe.
Fig. 1. A typical force–distance plot and schematic illustration for adhesion force calculation.
L (mm)
w (mm)
h (mm)
i (mm)
130
25
1
8
908
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where M is the torque or torsion moment. From beam theory, the torsional stiffness is defined as follows [23,24]: 3
Kf ¼
Gbh 3L
ð4Þ
where G is the modulus of rigidity or shear modulus (G ¼(E/2(1þ n))), E and n are Young’s modulus and Poisson’s ratio for cantilever, respectively. The lateral stiffness of the cantilever-tip assembly with the lateral load applied at the end of the tip (torque) is defined as [25,26] KyT ¼
kf
ð5Þ
i2
The signal of the torsional deflection of cantilever can be related to the degree of twisting, hence to the magnitude of friction force. While friction force contributes to the FFM signal, it may not be the only contributing factor in commercial FFM instruments. There may be some unrelated signal to friction force or to the actual twisting of the cantilever, but rather to cross-talk effects. This part of the FFM signal is called ‘‘FFMF’’. This would be a detrimental effect. It has to be understood and eliminated from the data acquisition before any quantitative measurement of friction force becomes possible. The part which is truly related to friction force is called ‘‘FFMT’’. By the following method, the FFMF signal can be eliminated. Firstly, the sample is scanned in both y and y directions, and the FFM signal for scans in each direction is recorded. Since friction force reverses its direction when the scanning direction is reversed from y to y direction, the FFMT signal will have opposite signs as the scanning direction of the sample is reversed (i.e. FFMT(y) ¼ FFMT( y)). Hence the FFMT signal can be canceled out if we take the sum of the FFM signals for the two scans. The average value of the two scans will be related to FFMF FFMðyÞ þ FFMðyÞ ¼ 2FFMF
ð6Þ
This value can be subtracted from the original FFM signals of each two scans to obtain the FFMT. Another way, by taking the difference of two FFM signals, the FFMT can be directly obtained [23] FFMðyÞ FFMðyÞ ¼ FFMTðyÞ FFMTðyÞ ¼ 2FFMT
ð7Þ
From Eq. (7), we can obtain the friction force signal (V). Then the friction force can be get from Ff ¼ kv FFMT
ð8Þ
where kn is the transfer coefficient (N/V). It can be expressed as kn ¼ kyT a
ð9Þ
a is the deflection sensitivity which can be obtained by the AFM instrument. From Eqs. (4), (5), and (9), the transfer coefficient (N/V) kn is kv ¼ a
Gbh3 3Li2
ð10Þ
In this experiment, Young’s modulus and Poisson’s ratio for Si3N4 are 238 GPa and 0.29, respectively, and the deflection sensitivity a is 51.84 10 9 m/V. The transfer coefficient kn is 4.79 10 6N/V. As above mentioned, the friction force can be calculated by Eq. (8). The calculation is approximative, therefore the calculated friction force is called ‘‘relative friction force’’. The micro-tribological properties of the PEM films were studied by a ball-on-disk reciprocating type micro-tribometer [14]. A sample platform slides against a ball reciprocating at a selected speed and travel distance. A steel ball of 3 mm in diameter was fixed on the tester. Before each test, the steel ball were ultrasonically cleaned in acetone and then thoroughly dried. The sample on the platform moved horizontally contact with the steel ball on one-way reciprocating friction mode (the ball was raised from the surface on the reverse cycle) under a frequency of 1 Hz (10 mm/s, unless otherwise noted) and a traveling distance of 5 mm. The initiation of wear on the sample surface led to increase in the friction coefficient, and a sharp increase of the friction coefficient indicated the failure of the film. The change in the friction coefficient was monitored versus sliding cycles (passes). All the tests were conducted at room temperature (2571 1C) and a relative humidity of 20%. 2.3. Assembly of the nanoparticle/polyelectrolyte multilayer (NPs/PEM) film The polycation solution of PDDA and polyanion solution of PAA were prepared into 1 10 4 mol/L by directly dissolving in deionized water, respectively (in which the concentration was based on the molecular weight per repeat unit). The CuCl2 and Na2S aqueous solutions used for synthesize of CuS nanoparticles were prepared into 1 10 4 mol/L freshly. All the solutions were prepared at room temperature. The glass and quartz substrates which have been processed by hydroxylation were dipped into PDDA and PAA polyelectrolyte solutions for 20 min alternately. Ultrapure water and nitrogen were used for rinsing and drying after each immersion, respectively. One
Fig. 3. Schematic diagram of the preparation of CuS nanoparticles/polyelectrolyte multilayer (NPs/PEM) films.
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monolayer of PDDA and one of PAA constitute one bilayer polyelectrolyte film, which can be expressed as PDDA/PAA. Thus the PDDA/ PAA PEM films of different layer numbers were prepared by repeating the above process. Deposition always ended with the first half of a layer pair in order to terminate the film with a PDDA layer. The schematic diagram of layer-by-layer deposition process was shown in Fig. 3(a). The in situ synthesize process of CuS nanoparticles was shown in Fig. 3(b). Firstly, the substrates coating with PDDA/PAA which have been prepared by the method that has been mentioned above were dipped into CuCl2 solution for 2 min, and the following with ultrapure water washing. The Cu2 þ ions have been adsorbed to the carboxyl group of PAA [27,28]. Then the nucleation of CuS nanoparticles within the PEM film occurred by dipping the PEM film with Cu2 þ ions into the fresh Na2S aqueous solutions for 2 min. The above synthesize process can be repeated (Fig. 3(b)). The in situ synthesize CuS nanoparticles composite PEM film (NPs/PEM) can be expressed as (PDDA/PAA)m/(CuS)n, where m is the number of bilayers and n is the number of synthesis cycle.
3. Results and discussion 3.1. Characterize of NPs/PEM film In situ synthesize process of CuS nanoparticles in the (PDDA/ PAA)4.5 PEM film was monitored using UV–vis spectrophotometer. Fig. 4 shows the UV–vis absorption spectrum of the (PDDA/PAA)4.5/(CuS)n, in which n ¼1, y 7. As can be seen from the chart, the absorbance increased with the synthesis cycle numbers, and the absorption peak has a red shift. It is due to this the increasing of nanoparticles in amount with the increasing of synthesis cycle number [29]. Plotting the absorbance at 242 nm versus the cycle n resulted in nearly straight line (inset in Fig. 4). This indicates that the CuS nanoparticles are involved in the PEM films and the forming process is reproducible. To identify the elemental composition of the sample, XPS of the NPs/PEM film on quartz substrate were measured by a PHI5300 spectroscope. Although the XPS gives only semi-quantitative elemental composition, the presence of S2p, Cu2p elements in NPs/PEM film can be confirmed. The XPS spectrums of Cu2p, S2p are shown in Fig. 5. A typically high resolution XPS spectrum of Cu2p is shown in Fig. 5(a). The Cu2p spectrum can be split by the
Fig. 4. UV–vis spectrum of the (PDDA/PAA)4.5/(CuS)n films with n¼ 1, y, 7 on quartz substrates. The plot of the absorbance values at 242 nm versus the synthesis cycle n (1–7), and the straight line fits to the data points.
Fig. 5. A typical XPS spectrums of Cu2p (a) and S2p (b) in (PDDA/PAA)4.5/(CuS)2 on quartz substrate.
2p spin–orbit interactions into the 2p1/2 and 2p3/2 regions. Two peaks with binding energies of 932 and 952 eV can be observed, which indicated that the valence of copper element is divalent. The characteristic absorption peak of S2p is at 162 eV, which indicated that the valence of sulfur element is negative divalent. The above analysis indicated that the CuS nanoparticles have been fabricated in the PEM film in situ. This was consistent with the results of the related literatures [30,31]. The surface morphologies of the PEM films were investigated using AFM with tapping mode. Fig. 6 shows the AFM surface morphology of the (PDDA/PAA)4.5 PEM film and the (PDDA/ PAA)4.5/(CuS)n NPs/PEM films with different synthesis cycles on glass substrate in accordance with the aforementioned experimental conditions. The three-dimensional morphology of polyelectrolyte (PDDA/PAA)4.5 PEM and 1, 2 ,and 5 synthesis cycle NPs/PEM films were shown in Fig. 6(a)–(d), respectively. The roughness (Ra) of corresponding PEM and NPs/PEM films were listed in Table 2. It can be seen from Table 2 that a small increase in surface roughness of NPs/PEM film. Fig. 7 shows the TEM picture of in situ CuS nanoparticle/ polyelectrolyte multilayer film. TEM micrographs revealed that the nanoparticles have been synthesized in PEM film in situ. The ultrapure water contact angle on hydroxylated substrate surface was below 51. When the PEM and NPs/PEM film were deposited onto the substrate surface, the contact angle increased,
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Fig. 6. AFM surface topographic images of the (PDDA/PAA)4.5 film (a), the (PDDA/PAA)4.5/(CuS)1 film (b), the (PDDA/PAA)4.5/(CuS)2 film (c), and the (PDDA/PAA)4.5/(CuS)5 film (d).
Table 2 The surface roughness (Ra) of PEM and NPs/PEM films.
Ra (nm)
(PDDA/ PAA)4.5
(PDDA/ PAA)4.5/ (CuS)1
(PDDA/ PAA)4.5/ (CuS)2
(PDDA/ PAA)4.5/ (CuS)5
1.8
1.82
1.83
1.87
Fig. 8. The adhesive force of the (PDDA/PAA)4.5 PEM and (PDDA/PAA)4.5/(CuS)2 NPs/PEM films, the inset chart shows the force–distance curves of the (PDDA/ PAA)4.5 (a) and (PDDA/PAA)4.5/(CuS)2 films (b).
Fig. 7. TEM image of CuS nanoparticles/polyelectrolyte multilayer film.
with the contact angles were 20 72.31 and 2572.71, respectively. This indicated that the films were formed on the substrates. The surface energy of the solid can be estimated using the semiempirical equation developed by Owens and Wendt theory, applicable for a wide range of surface energies and materials [32–34]. The surface energy of the solid and contact angle are inversely proportional relationship. From the ultrapure water
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contact angles of hydroxide substrates, PEM and NPs/PEM films, the hydroxide substrates have a higher surface energy, and the NPs/PEM film has a lower surface energy than the PEM film. It indicates that the compositing CuS nanoparticles led to the reduction of the surface energy [35].
Fig. 9. The friction force versus load of different synthesis cycle numbers (PDDA/ PAA)4.5/(CuS)n films at a sweep speed of 5 mm/s and a scanning area of 300 300 nm2. The solid lines are non-linear fits with the JKR model.
Table 3 The energy per unit contact area g and shear strength t of (PDDA/PAA)4.5/(CuS)n films calculated by JKR model. n 2
g (mJ/m ) t (MPa)
0
1
2
3
4
100 171
57 160
21 146
22 148
42 152
911
3.2. Adhesive forces measurements under ambient conditions Ten points have been selected on PEM film surface of size of 1 1 mm2 randomly. Firstly the scan area and the scan size were selected. At each point, the AFM took the height data and then does a force–distance curve, as described in instruments, followed by the adhesion force analysis of each force–distance curve. The adhesive force of PEM and NPs/PEM films measured by AFM were summarized in Fig. 8. And the inset shows the force– distance curves of the (PDDA/PAA)4.5 PEM film (a) and (PDDA/ PAA)4.5/(CuS)2 NPs/PEM film (b). It can be seen that the adhesive forces between the probe and sample surface decreased, indicating that the surface interactions between probe and sample are reduced by compositing in situ CuS nanoparticles. The average adhesive forces of the (PDDA/PAA)4.5 PEM and (PDDA/PAA)4.5/ (CuS)2 NPs/PEM films are 14.16 76.24 and 2.9370.69 nN, respectively. The discrete degree of the adhesion force decreased after compositing in situ CuS nanoparticles. The adhesive force is mainly dependent on the surface interaction between probe and sample surfaces. It is well known that, when the surface is hydrophilic, they would easily form meniscus by adsorbed water molecules in air, thus they had higher adhesive force. From the ultrapure water contact angle measurement, the NPs/PEM film was more hydrophobic than PEM film, and shown a lower adhesion [36]. 3.3. Nano-tribological properties Nano-friction forces of PEM and NPs/PEM film were characterized with the AFM (friction force mode). The relative friction force could be obtained by monitoring the lateral torsion of the cantilever. Fig. 9 shows the relative friction forces versus load of different synthesis cycle number NPs/PEM films under a sweep speed of 5 mm/s and a scanning pitch of 5 nm. It was demonstrated by Carpick et al. [37,38] that, for an AFM elastic singleasperity contact on bare mica, the friction force F is proportional to the
Fig. 10. The AFM wear tracks of (PDDA/PAA)4.5 and (PDDA/PAA)4.5/(CuS)2 films. (a)–(c) For the (PDDA/PAA)4.5 wear tracks after 20, 100, and 200 scanning cycles. (d)–(f) For the (PDDA/PAA)4.5/(CuS)2 wear tracks after 20, 100, and 200 scanning cycles.
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tip-sample contact area A through a constant shear strength t. While the contact area is greater than the Hertz contact area due to the adhesive elastic contact. The JKR model was used to explain the results of the friction data. In the JKR model, the contact radius a can be given as [39–41] qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R a3 ¼ P þ 3gpR þ 6gpRP þ ð3gpRÞ2 ð11Þ K 1 , where R¼R1R2/(R1 þR2), K ¼ ð4=3Þ 1u21 =E1 þ 1u22 =E2 E1, E2 are Young’s module and v1, v2 are the Poisson ratios for each material, respectively. g is the energy per unit contact area (i.e. the two surface). P is the applied normal load. The pull-off force is [42] 3 PJKR ¼ gpR 2
ð12Þ
Following this approach, we apply the JKR model to our friction versus load plots. The JKR model predicts the dependence of the tip-sample contact area A upon the applied load P. Thus, assuming friction is proportional to the contact area, we have the relation given in Eq. (13) [43,44] F ¼ tA ¼ tpa2 ¼ tp
R ðP þ3gpR þ K
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2=3 6gpRP þð3gpRÞ2 Þ
ð13Þ
If R and K are known, and the g can be estimated work of adhesion measurements by Eq. (12). In this experiment, R is 30 nm, K is 80.8 Gpa, the g is shown in Table 3. The JKR fitted friction versus normal load plots were shown in Fig. 9. The shear strength t was also listed in Table 3.
Our analysis assumed that the interfacial shear strength is constant and have assumed no load dependency as is sometimes assumed by researchers [45]. From the calculate result and JKR fitting, it can be seen that the interfacial shear strength t decreased with the naoparticles doped into the polymer layer. Though, the friction force was mainly depended on the surface adhesive energy, from the results of the adhesion, it is found that the adhesive forces between probe and sample are decreased by compositing in situ CuS nanoparticles. Therefore the friction force of the PEM film is reduced. In order to investigate the effect of nanoparticles to the antiwear behavior of the PEM film, the wear properties of the (PDDA/ PAA)4.5 PEM and (PDDA/PAA)4.5/(CuS)2 NPs/PEM film were tested contrastively by Si3N4 probe scanning on the surface repeatedly under the conditions of setting load 8.5 nN, sweeping speed 5 mm/ s, scanning area 300 300 nm2, and a relative humidity of 20%. To investigate the wear tracks after repeated scanning, we obtained the surface AFM morphology by enlarging the scan area (1000 1000 nm2) in situ. The average height of the surface of AFM image can be obtained using ‘‘area analysis’’. The average height of AFM image of unworn range was h1. The average height of worn range was h2. Then the mean surface wear depth h0 can be expressed as h0 ¼h1 h2. Fig. 10 shows the AFM images of (PDDA/PAA)4.5 PEM and (PDDA/ PAA)4.5/(CuS)2 NPs/PEM films after different scanning cycles. Fig. 10(a)–(c) was the AFM images of (PDDA/PAA)4.5 PEM film after 20, 100, and 200 scanning cycles, respectively. Fig. 10(d)–(f) was the AFM images of (PDDA/PAA)4.5/(CuS)2 NPs/PEM films after commensurate scanning cycles with (PDDA/PAA)4.5 PEM film. From
Fig. 13. The fluctuations in the friction coefficient with the one-way reciprocating sliding cycles.
Fig. 11. The surface wear of the PEM and NPs/PEM films using AFM.
Z Data (nm)
30 nm
15
0
0
200
400 600 Distance (nm)
800
1000 nm
Fig. 12. Micrograph and fluctuation of NPs/PEM film after 300 times repeated friction movement by AFM.
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these images, it can be seen that the (PDDA/PAA)4.5/(CuS)2 NPs/PEM film has a better wear resistant than the (PDDA/PAA)4.5 PEM film. The average surface wear of different scanning number is shown in Fig. 11. Compared to the two curves, it can be found that the average surface wear of NPs/PEM film is lower than that of the PEM film. The average surface wear of PEM film has no change basically when scanning number from 160 to 200. This may be due to that the PEM film has been worn out. But the average surface wear of the NPs/PEM film continued to increase with the scanning number increasing, the NPs/PEM film still play a protection to the substrate.
913
Fig. 12 shows the micrograph and surface morphology variation curve of the NPs/PEM film after 300 scanning cycles.
3.4. Micro-tribological properties In order to further reveal the tribological properties of the NPs/ PEM film and the effect of the nanoparticles on the anti-wear life of PEM films, the friction tests were conducted using a microtribometer.
Fig. 14. The AFM micrographs of worn track of in situ nanoparticle/polyelectrolyte multilayer film: after 120 sliding cycles (a), after 300 sliding cycles (b).
Fig. 15. SEM micrograph of wear track of (PDDA/PAA)4.5 polyelectrolyte MD film after 90 sliding cycles (magnification: 10,000) (a). SEM micrographs of wear tracks of (PDDA/PAA)4.5/(CuS)2 composite MD film: after 120 sliding cycles (magnification: 20,000) (b) and after 450 sliding cycles (magnification: 20,000) (c).
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Fig. 16. SEM images of wear scars of steel balls. (a) The ball paring with PDDA/PAA PEM film after 90 sliding cycles and (b) the ball paring with (PDDA/PAA)/CuS NPs/PEM film after 450 sliding cycles (magnification: 20,000).
Fig. 13 shows the fluctuation of the friction coefficient with the single-reciprocating sliding cycles. The friction coefficient of the (PDDA/PAA)/CuS NPs/PEM film is lower than that of PDDA/PAA PEM film. It is qualitatively in agreement with that obtained with the AFM as discussed above except for the magnitude. When the sliding cycles was around 70, the friction coefficient of the PDDA/ PAA PEM film increased sharply, suggesting that the failure of the PEM film. In contrast, the friction coefficient of the (PDDA/PAA)/ CuS NPs/PEM film increased sharply when the sliding cycles reaches 450. The friction coefficient of (PDDA/PAA)/CuS NPs/ PEM film was around 0.1 before 160 sliding cycles. After 160 sliding cycles the friction coefficient increased to 0.15, it was due to the surface of polymer matrix has been destroyed with the reciprocating sliding cycles and the nanoparticles pull-off from the polymer matrix. It can be seen from the AFM images of (PDDA/PAA)4.5/(CuS)2 NPs/PEM films after friction (Fig. 14). The nanoparticles gradually appeared in the wear track with the increasing of wear, which was shown in Fig. 14 from 120 sliding cycles (a) to 300 sliding cycles (b). The SEM results are consistent with the tribological behavior of the PEM films. The binding force between the polyelectrolyte and the substrate was weak, and the polyelectrolyte was peeled from the substrate after 90 times repeated friction (Fig. 15(a)). Fig. 15(b) and (c) was the SEM micrographs after 120 sliding cycles and 450 sliding cycles of NPs/PEM film, respectively. It can be seen that the NPs/PEM film has a better anti-wear behavior than the PEM film. To contrast the degree of ball wear for the two specimens, the wear scars of the steel balls were also investigated by SEM. Fig. 16 shows the wear scars of steel balls against PEM film (a) and NPs/PEM film (b) when the films failed. Comparing the two SEM pictures, it can be seen that the steel ball wear scar is smaller for NPs/PEM film than that for PEM film. The steel ball against the NPs/PEM film has a lower wear rate than the PEM film till the film was out of service. In Fig. 16(b), some scratches appeared on the wear track after 450 sliding cycles, it is maybe due to the nanoparticles falling off and accumulated after the repeated sliding. The nanoparticles/polyelectrolyte multilayer film has a lower friction force and better anti-wear property than the pure polyelectrolyte molecular deposition film. It is due to that the nanoparticles make the load carrying capacity of the PEM film increase [15]. Furthermore, the surface energy of the NPs/PEM film was lower than that of PEM film, the high friction coefficients are found for sliding materials with high surface energy [35]. The polymer multilayer film matrix with nanoparticles would decrease the mobility of the polymer chain segments to make the film can undergo reversible shear deformation.
4. Conclusions CuS nanoparticle/polyelectrolyte multilayer films were prepared using the layer-by-layer deposition and the in situ synthesize method. Adhesion and friction of the film were investigated by AFM and micro-tribometer. It was found that the adhesive forces between probe and sample are decreased by compositing in situ CuS nanoparticles. From the nano and micro-tribological tests, the NPs/ PEM film has a lower friction force and a better anti-wear property than the pure polyelectrolyte multilayer film. It is considered that the load carrying capacity of the polyelectrolyte multilayer film increased with compositing nanoparticles.
Acknowledgments This research is sponsored by the National Natural Science Foundation of China (Grant no. 50975288), the National Basic Research Program of China (973, Grant no. 2007CB607604), and the State Key Laboratory of Tribology Opening Fund (SKLT05–02). References [1] Decher G, Schlenoff JB, editors. Multilayer thin films. New York/Weinheim: Wiley-VCH; 2002. [2] Zhang SW, Lan HQ. Developments in tribological research on ultrathin films. Tribology International 2002;35:321–7. ¨ ¨ H, Yılmazo˘glu M, et al. A novel approach for highly [3] Yılmazturk S, Deligoz proton conductive electrolyte membrances with improved methanol barrier properties: layer-by-layer assembly of salt containing polyelectrolytes. Journal of Membrane Science 2009;343:137–46. [4] Decher G, Hong JD, Schmitt J. Buildup of ultrathin multilayer films by a selfassembly process: I. Consecutively alternating adsorption of anionic and cationic polylectrolytes on charged surface. Thin Solid Films 1992;210–211: 831–5. [5] Decher G. Layered nanoarchitectures via directed assembly of anionic and cationic molecules. Comprehensive Supramolecular Chemistry 1996;9: 507–28. [6] Decher G. Fuzzy nanoassemblies: toward layered polymeric multicomposites. Science 1997;277:1232–7. [7] Pavoor PV, Gearing BP, Bellare A, et al. Tribological characteristics of polyelectrolyte multilayers. Wear 2004;256:1196–207. [8] Pavoor PV, Gearing BP, Muratoglu O, et al. Wear reduction of orthopaedic bearing surfaces using polyelectrolyte multilayer nanocoatings. Biomaterials 2006;27:1527–33. [9] Wang QB, Gao ML, Zhang SW. Tribological application of molecular deposition (MD) films. Material Science Engineering C 1999;10:127–9. [10] Wang QB, Gao ML, Zhang SW. Nanofriction properties of molecular deposition films. Science in China Series B 2000;43(2):137–42. [11] Ma HX, Yang GB, Yu LG, et al. Effect of counterparts on the friction and wear behavior of polyelectrolyte multilayers. Tribology International 2009;42: 869–74. [12] Zhang SW, Feng DP, Wang DG. Tribological behaviors of composite molecular deposition films. Tribology International 2005;38:959–65.
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Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Adhesion and friction of nanoparticles/polyelectrolyte multilayer films by AFM and micro-tribometer Yan-Bao Guo n, De-Guo Wang n, Si-Wei Zhang College of Mechanical and Transportation Engineering, China University of Petroleum, 18 Fuxue Road, Changping, Beijing 102249, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 September 2010 Received in revised form 6 March 2011 Accepted 9 March 2011 Available online 17 March 2011
The in situ CuS nanoparticle/polyelectrolyte multilayer (NPs/PEM) films have been prepared using the layer-by-layer method and the in situ synthesize method. The NPs/PEM films were characterized by atomic force microscope (AFM), ultraviolet–visible (UV–vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), and transmission electron microscope (TEM). It was found that the adhesive forces between probe and sample are decreased by compositing in situ CuS nanoparticles. From the investigation of the nano- and micro-tribological behaviors, the NPs/PEM film has a lower friction force and a better anti-wear property than the pure polyelectrolyte multilayer film. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Polyelectrolyte multilayer film Nanoparticles Adhesion Friction
1. Introduction Over the previous decades, there are some methods for deposition of ultrathin films on a substrate such as layer-by-layer (LBL), Langmuir–Blodgett (LB), spin coating, sputtering, and so on [1–3]. Layer-by-layer (LBL) method developed by Decher et al. [4–6] consists of alternate dipping of the substrate in oppositely charged polycation and polyanion electrolyte solutions. The LBL method is simple and cheaper, easy to automate, and friendly to the environment. Especially, the film can be deposited on a wide variety of substrates, and devices with complex architectures can be coated uniformly over large areas. The LBL film is a novel material studied extensively in recent years due to their numerous potential applications, versatility, and simplicity [1]. The LBL technique is ideally suited to combat the tribological challenges in microelectromechanical systems (MEMS). Up to now, the tribological properties of polyelectrolyte multilayer (PEM) film were studied popularly. Pavoor et al. [7,8] has reported that the deposition of a poly(acrylic acid) and poly(allylamine hydrochloride) polyelectrolyte multilayer film using the LBL technique, and the capacity for polyelectrolyte multilayer film induced wear reduction at large scales under the dry state. The PEM film is able to decrease the adhesive force on a surface, and to modify the friction surface and reduce the friction force [9–11]. And, nanoparticles were applied in tribology of ultrathin film. Graphite oxide, TiO2, SiO2, Ag nanoparticles, and so on composite polyelectrolyte multilayer films
n
Corresponding authors. E-mail addresses: [email protected] (Y.-B. Guo), [email protected] (D.-G. Wang).
0301-679X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2011.03.007
have been investigated to enhance the wear life of PEM film [12–15]. It was found that these films can improve the tribological performance greatly. It has been reported that CuS were used as oil additive and filling material to improve the tribological property [16–18]. In this paper, the adhesion and friction of the in situ CuS nanoparticles/polyelectrolyte multilayer (NPs/PEM) film have been investigated by AFM and micro-tribometer.
2. Experiment 2.1. Materials All materials were used without further purification. The polyelectrolytes used in this studies were poly(diallyl dimethylammonium chloride) and poly(acrylic acid), hereafter referred to as PDDA and PAA, supplied by Aldrich Chemical Co., American. The averaged molecular weight (Mw) of the PDDA and PAA used in the experiments were 100,000–200,000 and 30,000, respectively. Mili-Q ultrapure water was used for preparation of all aqueous solutions, and during rinsing procedures ( 418 MO cm, Milipore Mili-Q). Copper chloride (CuCl2) and Sodium sulfide (Na2S) were supplied by Sinopharm Chemical Reagent Co., Ltd. All the other reagents are analytical reagents. Prior of deposition, the glass and quartz substrates were cleaned according to the following procedures: (1) The substrates were ultrasonic treated in acetone, chloroform, and anhydrous ethanol (volume ratio¼3:3:1) bath for 30 min
Y.-B. Guo et al. / Tribology International 44 (2011) 906–915
first and rinsed to eliminate the residues on substrates using de-ionized water flushing. (2) The substrates were treated in an 80 1C piranha solution (98%H2SO4/30%H2O2, volume ratio¼7:3) bath for 1 h (caution: piranha solution reacts violently with organic solvents and is a skin irritant. Extreme caution should be exercised when handling piranha solution). The piranha treatment allows removal of residues of organic and inorganic impurities from the substrates and makes the slides completely hydrophilic at the same time, through making the substrates to be hydroxylated. In the end, the substrates were rinsed with plenty of de-ionized water and dried with nitrogen to prepare for the deposition of polyelectrolytes.
2.2. Instruments Ultraviolet–visible (UV–vis) absorption spectrums of films were recorded on a Unico 2102 UV–vis spectrophotometer, which was used to monitor the influence of absorbance on the in situ nanoparticle/polyelectrolyte multilayer film. The X-ray photoelectron spectroscopy (XPS) measurements were performed with the PHI-5300 spectroscope using Al Ka (hn ¼1486.6 eV) X-ray radiation. The vacuum inside the analysis chamber was 1.333 10 8 Pa during the analysis. The binding energy (BE) of C1s (284.8 eV) was used as the reference. Contact angle measurements were conducted using the sessile drop technique performed on a contact angle goniometer (JC2000A, China) at 25 1C. A NanoMan VS (VEECO) atomic force microscopy was used to evaluate the morphology of the MD films by tapping mode, and it was operated under air-ambient temperatures (2571 1C) and a relative humidity of 20% using commercial silicon nitride probes (spring constant 0.2022 N/m, which was checked by the AFM instrument). AFM has been used extensively to measure adhesive force between surfaces at nano-scale [19–21]. The adhesive force between the AFM tip and the film surface by force-curves mode under ambient condition is shown in Fig. 1. a-b is the probe approaching the sample surface gradually, and the micro-cantilever of the probe does not bend with the force between the sample surface and probe zero. b-c is probe contacting with the sample surface suddenly due to the surface tension when both close infinitely, and then the micro-cantilever is curved downward slightly. c-d is loading process where the downward micro-cantilever transforms into without any bending, then curved upward, and the degree of bending gradually increases.
907
The probe retraces to unload when achieving the Point ‘d’, d-f is the unloading process, where the bending degree of microcantilever upward decreases to zero (Point ‘e’), then the microcantilever bends downward gradually due to the adhesive force between the probe and the sample surface. The probe will pull-off the sample surface suddenly when the adhesive force equals to the bending force of the micro-cantilever (Point ‘f ’). The Point ‘a’ indicates no bending occurs after the probe pulling off the sample surface. The adhesive force (pull-off force) was calculated by multiplying the cantilever spring constant by the horizontal distance between Points ‘e’ and ‘a’ [19] Fa ¼ K lae
ð1Þ
The friction forces of PEM and NPs/PEM films were measured using the friction force mode of AFM (FFM) [22]. Fig. 2 shows the working principles of FFM, and the main components are a thin cantilever with a probing tip (L is the lever length of cantilever; w is the lever width of cantilever, h is the lever thickness, i is the height of the tip mounted at the end of the cantilever). The size of the AFM probe is listed in Table 1. The laser beam deflection caused by the cantilever is determined by a quadrant photodiode of the optical system. The normal load and the frictional force are proportional to the normal and torsional deflections of the cantilever, which are recorded simultaneously via the output of the photodiode detector. The signal VA þ B VC þ D is the measurement for normal bending and VA þ C VB þ D for the torsional deflection of the cantilever. When the lateral force Fy is applied at the tip, as in the case of the AFM cantilever, lateral deflection is accompanied by twisting or torsion along the cantilever axis [23]. Fy ¼ kyT Dy
ð2Þ
kyT is the bending stiffness. Dy is the deflection in the y direction. For a wide, thin cantilever (b bh) experiencing a rotation f. The torsional stiffness of the beam kf, is given as M ¼ kf f
ð3Þ
Fig. 2. The working principles of FFM, and the main components a thin cantilever with a probing tip.
Table 1 The size of the AFM probe.
Fig. 1. A typical force–distance plot and schematic illustration for adhesion force calculation.
L (mm)
w (mm)
h (mm)
i (mm)
130
25
1
8
908
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where M is the torque or torsion moment. From beam theory, the torsional stiffness is defined as follows [23,24]: 3
Kf ¼
Gbh 3L
ð4Þ
where G is the modulus of rigidity or shear modulus (G ¼(E/2(1þ n))), E and n are Young’s modulus and Poisson’s ratio for cantilever, respectively. The lateral stiffness of the cantilever-tip assembly with the lateral load applied at the end of the tip (torque) is defined as [25,26] KyT ¼
kf
ð5Þ
i2
The signal of the torsional deflection of cantilever can be related to the degree of twisting, hence to the magnitude of friction force. While friction force contributes to the FFM signal, it may not be the only contributing factor in commercial FFM instruments. There may be some unrelated signal to friction force or to the actual twisting of the cantilever, but rather to cross-talk effects. This part of the FFM signal is called ‘‘FFMF’’. This would be a detrimental effect. It has to be understood and eliminated from the data acquisition before any quantitative measurement of friction force becomes possible. The part which is truly related to friction force is called ‘‘FFMT’’. By the following method, the FFMF signal can be eliminated. Firstly, the sample is scanned in both y and y directions, and the FFM signal for scans in each direction is recorded. Since friction force reverses its direction when the scanning direction is reversed from y to y direction, the FFMT signal will have opposite signs as the scanning direction of the sample is reversed (i.e. FFMT(y) ¼ FFMT( y)). Hence the FFMT signal can be canceled out if we take the sum of the FFM signals for the two scans. The average value of the two scans will be related to FFMF FFMðyÞ þ FFMðyÞ ¼ 2FFMF
ð6Þ
This value can be subtracted from the original FFM signals of each two scans to obtain the FFMT. Another way, by taking the difference of two FFM signals, the FFMT can be directly obtained [23] FFMðyÞ FFMðyÞ ¼ FFMTðyÞ FFMTðyÞ ¼ 2FFMT
ð7Þ
From Eq. (7), we can obtain the friction force signal (V). Then the friction force can be get from Ff ¼ kv FFMT
ð8Þ
where kn is the transfer coefficient (N/V). It can be expressed as kn ¼ kyT a
ð9Þ
a is the deflection sensitivity which can be obtained by the AFM instrument. From Eqs. (4), (5), and (9), the transfer coefficient (N/V) kn is kv ¼ a
Gbh3 3Li2
ð10Þ
In this experiment, Young’s modulus and Poisson’s ratio for Si3N4 are 238 GPa and 0.29, respectively, and the deflection sensitivity a is 51.84 10 9 m/V. The transfer coefficient kn is 4.79 10 6N/V. As above mentioned, the friction force can be calculated by Eq. (8). The calculation is approximative, therefore the calculated friction force is called ‘‘relative friction force’’. The micro-tribological properties of the PEM films were studied by a ball-on-disk reciprocating type micro-tribometer [14]. A sample platform slides against a ball reciprocating at a selected speed and travel distance. A steel ball of 3 mm in diameter was fixed on the tester. Before each test, the steel ball were ultrasonically cleaned in acetone and then thoroughly dried. The sample on the platform moved horizontally contact with the steel ball on one-way reciprocating friction mode (the ball was raised from the surface on the reverse cycle) under a frequency of 1 Hz (10 mm/s, unless otherwise noted) and a traveling distance of 5 mm. The initiation of wear on the sample surface led to increase in the friction coefficient, and a sharp increase of the friction coefficient indicated the failure of the film. The change in the friction coefficient was monitored versus sliding cycles (passes). All the tests were conducted at room temperature (2571 1C) and a relative humidity of 20%. 2.3. Assembly of the nanoparticle/polyelectrolyte multilayer (NPs/PEM) film The polycation solution of PDDA and polyanion solution of PAA were prepared into 1 10 4 mol/L by directly dissolving in deionized water, respectively (in which the concentration was based on the molecular weight per repeat unit). The CuCl2 and Na2S aqueous solutions used for synthesize of CuS nanoparticles were prepared into 1 10 4 mol/L freshly. All the solutions were prepared at room temperature. The glass and quartz substrates which have been processed by hydroxylation were dipped into PDDA and PAA polyelectrolyte solutions for 20 min alternately. Ultrapure water and nitrogen were used for rinsing and drying after each immersion, respectively. One
Fig. 3. Schematic diagram of the preparation of CuS nanoparticles/polyelectrolyte multilayer (NPs/PEM) films.
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monolayer of PDDA and one of PAA constitute one bilayer polyelectrolyte film, which can be expressed as PDDA/PAA. Thus the PDDA/ PAA PEM films of different layer numbers were prepared by repeating the above process. Deposition always ended with the first half of a layer pair in order to terminate the film with a PDDA layer. The schematic diagram of layer-by-layer deposition process was shown in Fig. 3(a). The in situ synthesize process of CuS nanoparticles was shown in Fig. 3(b). Firstly, the substrates coating with PDDA/PAA which have been prepared by the method that has been mentioned above were dipped into CuCl2 solution for 2 min, and the following with ultrapure water washing. The Cu2 þ ions have been adsorbed to the carboxyl group of PAA [27,28]. Then the nucleation of CuS nanoparticles within the PEM film occurred by dipping the PEM film with Cu2 þ ions into the fresh Na2S aqueous solutions for 2 min. The above synthesize process can be repeated (Fig. 3(b)). The in situ synthesize CuS nanoparticles composite PEM film (NPs/PEM) can be expressed as (PDDA/PAA)m/(CuS)n, where m is the number of bilayers and n is the number of synthesis cycle.
3. Results and discussion 3.1. Characterize of NPs/PEM film In situ synthesize process of CuS nanoparticles in the (PDDA/ PAA)4.5 PEM film was monitored using UV–vis spectrophotometer. Fig. 4 shows the UV–vis absorption spectrum of the (PDDA/PAA)4.5/(CuS)n, in which n ¼1, y 7. As can be seen from the chart, the absorbance increased with the synthesis cycle numbers, and the absorption peak has a red shift. It is due to this the increasing of nanoparticles in amount with the increasing of synthesis cycle number [29]. Plotting the absorbance at 242 nm versus the cycle n resulted in nearly straight line (inset in Fig. 4). This indicates that the CuS nanoparticles are involved in the PEM films and the forming process is reproducible. To identify the elemental composition of the sample, XPS of the NPs/PEM film on quartz substrate were measured by a PHI5300 spectroscope. Although the XPS gives only semi-quantitative elemental composition, the presence of S2p, Cu2p elements in NPs/PEM film can be confirmed. The XPS spectrums of Cu2p, S2p are shown in Fig. 5. A typically high resolution XPS spectrum of Cu2p is shown in Fig. 5(a). The Cu2p spectrum can be split by the
Fig. 4. UV–vis spectrum of the (PDDA/PAA)4.5/(CuS)n films with n¼ 1, y, 7 on quartz substrates. The plot of the absorbance values at 242 nm versus the synthesis cycle n (1–7), and the straight line fits to the data points.
Fig. 5. A typical XPS spectrums of Cu2p (a) and S2p (b) in (PDDA/PAA)4.5/(CuS)2 on quartz substrate.
2p spin–orbit interactions into the 2p1/2 and 2p3/2 regions. Two peaks with binding energies of 932 and 952 eV can be observed, which indicated that the valence of copper element is divalent. The characteristic absorption peak of S2p is at 162 eV, which indicated that the valence of sulfur element is negative divalent. The above analysis indicated that the CuS nanoparticles have been fabricated in the PEM film in situ. This was consistent with the results of the related literatures [30,31]. The surface morphologies of the PEM films were investigated using AFM with tapping mode. Fig. 6 shows the AFM surface morphology of the (PDDA/PAA)4.5 PEM film and the (PDDA/ PAA)4.5/(CuS)n NPs/PEM films with different synthesis cycles on glass substrate in accordance with the aforementioned experimental conditions. The three-dimensional morphology of polyelectrolyte (PDDA/PAA)4.5 PEM and 1, 2 ,and 5 synthesis cycle NPs/PEM films were shown in Fig. 6(a)–(d), respectively. The roughness (Ra) of corresponding PEM and NPs/PEM films were listed in Table 2. It can be seen from Table 2 that a small increase in surface roughness of NPs/PEM film. Fig. 7 shows the TEM picture of in situ CuS nanoparticle/ polyelectrolyte multilayer film. TEM micrographs revealed that the nanoparticles have been synthesized in PEM film in situ. The ultrapure water contact angle on hydroxylated substrate surface was below 51. When the PEM and NPs/PEM film were deposited onto the substrate surface, the contact angle increased,
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Fig. 6. AFM surface topographic images of the (PDDA/PAA)4.5 film (a), the (PDDA/PAA)4.5/(CuS)1 film (b), the (PDDA/PAA)4.5/(CuS)2 film (c), and the (PDDA/PAA)4.5/(CuS)5 film (d).
Table 2 The surface roughness (Ra) of PEM and NPs/PEM films.
Ra (nm)
(PDDA/ PAA)4.5
(PDDA/ PAA)4.5/ (CuS)1
(PDDA/ PAA)4.5/ (CuS)2
(PDDA/ PAA)4.5/ (CuS)5
1.8
1.82
1.83
1.87
Fig. 8. The adhesive force of the (PDDA/PAA)4.5 PEM and (PDDA/PAA)4.5/(CuS)2 NPs/PEM films, the inset chart shows the force–distance curves of the (PDDA/ PAA)4.5 (a) and (PDDA/PAA)4.5/(CuS)2 films (b).
Fig. 7. TEM image of CuS nanoparticles/polyelectrolyte multilayer film.
with the contact angles were 20 72.31 and 2572.71, respectively. This indicated that the films were formed on the substrates. The surface energy of the solid can be estimated using the semiempirical equation developed by Owens and Wendt theory, applicable for a wide range of surface energies and materials [32–34]. The surface energy of the solid and contact angle are inversely proportional relationship. From the ultrapure water
Y.-B. Guo et al. / Tribology International 44 (2011) 906–915
contact angles of hydroxide substrates, PEM and NPs/PEM films, the hydroxide substrates have a higher surface energy, and the NPs/PEM film has a lower surface energy than the PEM film. It indicates that the compositing CuS nanoparticles led to the reduction of the surface energy [35].
Fig. 9. The friction force versus load of different synthesis cycle numbers (PDDA/ PAA)4.5/(CuS)n films at a sweep speed of 5 mm/s and a scanning area of 300 300 nm2. The solid lines are non-linear fits with the JKR model.
Table 3 The energy per unit contact area g and shear strength t of (PDDA/PAA)4.5/(CuS)n films calculated by JKR model. n 2
g (mJ/m ) t (MPa)
0
1
2
3
4
100 171
57 160
21 146
22 148
42 152
911
3.2. Adhesive forces measurements under ambient conditions Ten points have been selected on PEM film surface of size of 1 1 mm2 randomly. Firstly the scan area and the scan size were selected. At each point, the AFM took the height data and then does a force–distance curve, as described in instruments, followed by the adhesion force analysis of each force–distance curve. The adhesive force of PEM and NPs/PEM films measured by AFM were summarized in Fig. 8. And the inset shows the force– distance curves of the (PDDA/PAA)4.5 PEM film (a) and (PDDA/ PAA)4.5/(CuS)2 NPs/PEM film (b). It can be seen that the adhesive forces between the probe and sample surface decreased, indicating that the surface interactions between probe and sample are reduced by compositing in situ CuS nanoparticles. The average adhesive forces of the (PDDA/PAA)4.5 PEM and (PDDA/PAA)4.5/ (CuS)2 NPs/PEM films are 14.16 76.24 and 2.9370.69 nN, respectively. The discrete degree of the adhesion force decreased after compositing in situ CuS nanoparticles. The adhesive force is mainly dependent on the surface interaction between probe and sample surfaces. It is well known that, when the surface is hydrophilic, they would easily form meniscus by adsorbed water molecules in air, thus they had higher adhesive force. From the ultrapure water contact angle measurement, the NPs/PEM film was more hydrophobic than PEM film, and shown a lower adhesion [36]. 3.3. Nano-tribological properties Nano-friction forces of PEM and NPs/PEM film were characterized with the AFM (friction force mode). The relative friction force could be obtained by monitoring the lateral torsion of the cantilever. Fig. 9 shows the relative friction forces versus load of different synthesis cycle number NPs/PEM films under a sweep speed of 5 mm/s and a scanning pitch of 5 nm. It was demonstrated by Carpick et al. [37,38] that, for an AFM elastic singleasperity contact on bare mica, the friction force F is proportional to the
Fig. 10. The AFM wear tracks of (PDDA/PAA)4.5 and (PDDA/PAA)4.5/(CuS)2 films. (a)–(c) For the (PDDA/PAA)4.5 wear tracks after 20, 100, and 200 scanning cycles. (d)–(f) For the (PDDA/PAA)4.5/(CuS)2 wear tracks after 20, 100, and 200 scanning cycles.
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tip-sample contact area A through a constant shear strength t. While the contact area is greater than the Hertz contact area due to the adhesive elastic contact. The JKR model was used to explain the results of the friction data. In the JKR model, the contact radius a can be given as [39–41] qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R a3 ¼ P þ 3gpR þ 6gpRP þ ð3gpRÞ2 ð11Þ K 1 , where R¼R1R2/(R1 þR2), K ¼ ð4=3Þ 1u21 =E1 þ 1u22 =E2 E1, E2 are Young’s module and v1, v2 are the Poisson ratios for each material, respectively. g is the energy per unit contact area (i.e. the two surface). P is the applied normal load. The pull-off force is [42] 3 PJKR ¼ gpR 2
ð12Þ
Following this approach, we apply the JKR model to our friction versus load plots. The JKR model predicts the dependence of the tip-sample contact area A upon the applied load P. Thus, assuming friction is proportional to the contact area, we have the relation given in Eq. (13) [43,44] F ¼ tA ¼ tpa2 ¼ tp
R ðP þ3gpR þ K
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2=3 6gpRP þð3gpRÞ2 Þ
ð13Þ
If R and K are known, and the g can be estimated work of adhesion measurements by Eq. (12). In this experiment, R is 30 nm, K is 80.8 Gpa, the g is shown in Table 3. The JKR fitted friction versus normal load plots were shown in Fig. 9. The shear strength t was also listed in Table 3.
Our analysis assumed that the interfacial shear strength is constant and have assumed no load dependency as is sometimes assumed by researchers [45]. From the calculate result and JKR fitting, it can be seen that the interfacial shear strength t decreased with the naoparticles doped into the polymer layer. Though, the friction force was mainly depended on the surface adhesive energy, from the results of the adhesion, it is found that the adhesive forces between probe and sample are decreased by compositing in situ CuS nanoparticles. Therefore the friction force of the PEM film is reduced. In order to investigate the effect of nanoparticles to the antiwear behavior of the PEM film, the wear properties of the (PDDA/ PAA)4.5 PEM and (PDDA/PAA)4.5/(CuS)2 NPs/PEM film were tested contrastively by Si3N4 probe scanning on the surface repeatedly under the conditions of setting load 8.5 nN, sweeping speed 5 mm/ s, scanning area 300 300 nm2, and a relative humidity of 20%. To investigate the wear tracks after repeated scanning, we obtained the surface AFM morphology by enlarging the scan area (1000 1000 nm2) in situ. The average height of the surface of AFM image can be obtained using ‘‘area analysis’’. The average height of AFM image of unworn range was h1. The average height of worn range was h2. Then the mean surface wear depth h0 can be expressed as h0 ¼h1 h2. Fig. 10 shows the AFM images of (PDDA/PAA)4.5 PEM and (PDDA/ PAA)4.5/(CuS)2 NPs/PEM films after different scanning cycles. Fig. 10(a)–(c) was the AFM images of (PDDA/PAA)4.5 PEM film after 20, 100, and 200 scanning cycles, respectively. Fig. 10(d)–(f) was the AFM images of (PDDA/PAA)4.5/(CuS)2 NPs/PEM films after commensurate scanning cycles with (PDDA/PAA)4.5 PEM film. From
Fig. 13. The fluctuations in the friction coefficient with the one-way reciprocating sliding cycles.
Fig. 11. The surface wear of the PEM and NPs/PEM films using AFM.
Z Data (nm)
30 nm
15
0
0
200
400 600 Distance (nm)
800
1000 nm
Fig. 12. Micrograph and fluctuation of NPs/PEM film after 300 times repeated friction movement by AFM.
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these images, it can be seen that the (PDDA/PAA)4.5/(CuS)2 NPs/PEM film has a better wear resistant than the (PDDA/PAA)4.5 PEM film. The average surface wear of different scanning number is shown in Fig. 11. Compared to the two curves, it can be found that the average surface wear of NPs/PEM film is lower than that of the PEM film. The average surface wear of PEM film has no change basically when scanning number from 160 to 200. This may be due to that the PEM film has been worn out. But the average surface wear of the NPs/PEM film continued to increase with the scanning number increasing, the NPs/PEM film still play a protection to the substrate.
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Fig. 12 shows the micrograph and surface morphology variation curve of the NPs/PEM film after 300 scanning cycles.
3.4. Micro-tribological properties In order to further reveal the tribological properties of the NPs/ PEM film and the effect of the nanoparticles on the anti-wear life of PEM films, the friction tests were conducted using a microtribometer.
Fig. 14. The AFM micrographs of worn track of in situ nanoparticle/polyelectrolyte multilayer film: after 120 sliding cycles (a), after 300 sliding cycles (b).
Fig. 15. SEM micrograph of wear track of (PDDA/PAA)4.5 polyelectrolyte MD film after 90 sliding cycles (magnification: 10,000) (a). SEM micrographs of wear tracks of (PDDA/PAA)4.5/(CuS)2 composite MD film: after 120 sliding cycles (magnification: 20,000) (b) and after 450 sliding cycles (magnification: 20,000) (c).
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Fig. 16. SEM images of wear scars of steel balls. (a) The ball paring with PDDA/PAA PEM film after 90 sliding cycles and (b) the ball paring with (PDDA/PAA)/CuS NPs/PEM film after 450 sliding cycles (magnification: 20,000).
Fig. 13 shows the fluctuation of the friction coefficient with the single-reciprocating sliding cycles. The friction coefficient of the (PDDA/PAA)/CuS NPs/PEM film is lower than that of PDDA/PAA PEM film. It is qualitatively in agreement with that obtained with the AFM as discussed above except for the magnitude. When the sliding cycles was around 70, the friction coefficient of the PDDA/ PAA PEM film increased sharply, suggesting that the failure of the PEM film. In contrast, the friction coefficient of the (PDDA/PAA)/ CuS NPs/PEM film increased sharply when the sliding cycles reaches 450. The friction coefficient of (PDDA/PAA)/CuS NPs/ PEM film was around 0.1 before 160 sliding cycles. After 160 sliding cycles the friction coefficient increased to 0.15, it was due to the surface of polymer matrix has been destroyed with the reciprocating sliding cycles and the nanoparticles pull-off from the polymer matrix. It can be seen from the AFM images of (PDDA/PAA)4.5/(CuS)2 NPs/PEM films after friction (Fig. 14). The nanoparticles gradually appeared in the wear track with the increasing of wear, which was shown in Fig. 14 from 120 sliding cycles (a) to 300 sliding cycles (b). The SEM results are consistent with the tribological behavior of the PEM films. The binding force between the polyelectrolyte and the substrate was weak, and the polyelectrolyte was peeled from the substrate after 90 times repeated friction (Fig. 15(a)). Fig. 15(b) and (c) was the SEM micrographs after 120 sliding cycles and 450 sliding cycles of NPs/PEM film, respectively. It can be seen that the NPs/PEM film has a better anti-wear behavior than the PEM film. To contrast the degree of ball wear for the two specimens, the wear scars of the steel balls were also investigated by SEM. Fig. 16 shows the wear scars of steel balls against PEM film (a) and NPs/PEM film (b) when the films failed. Comparing the two SEM pictures, it can be seen that the steel ball wear scar is smaller for NPs/PEM film than that for PEM film. The steel ball against the NPs/PEM film has a lower wear rate than the PEM film till the film was out of service. In Fig. 16(b), some scratches appeared on the wear track after 450 sliding cycles, it is maybe due to the nanoparticles falling off and accumulated after the repeated sliding. The nanoparticles/polyelectrolyte multilayer film has a lower friction force and better anti-wear property than the pure polyelectrolyte molecular deposition film. It is due to that the nanoparticles make the load carrying capacity of the PEM film increase [15]. Furthermore, the surface energy of the NPs/PEM film was lower than that of PEM film, the high friction coefficients are found for sliding materials with high surface energy [35]. The polymer multilayer film matrix with nanoparticles would decrease the mobility of the polymer chain segments to make the film can undergo reversible shear deformation.
4. Conclusions CuS nanoparticle/polyelectrolyte multilayer films were prepared using the layer-by-layer deposition and the in situ synthesize method. Adhesion and friction of the film were investigated by AFM and micro-tribometer. It was found that the adhesive forces between probe and sample are decreased by compositing in situ CuS nanoparticles. From the nano and micro-tribological tests, the NPs/ PEM film has a lower friction force and a better anti-wear property than the pure polyelectrolyte multilayer film. It is considered that the load carrying capacity of the polyelectrolyte multilayer film increased with compositing nanoparticles.
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