Tribol Lett (2013) 49:431–437 DOI 10.1007/s11249-012-0086-6
ORIGINAL PAPER
Tribological Behaviors of Lubricants Modified Nanoporous Anodic Alumina Film Ying Wang • Li Xia • Jianning Ding Ningyi Yuan • Yuanyuan Zhu
•
Received: 6 August 2012 / Accepted: 10 December 2012 / Published online: 18 December 2012 Ó Springer Science+Business Media New York 2012
Abstract A uniform closely packed hexagonal array of porous anodic alumina (PAA) film with an average pore diameter of *100 nm and a pore height of *330 nm was obtained by anodizing in oxalic acid solution. Low surface energy molecules of perfluoropolyether (PFPE) and octadecyltrichlorosilane (OTS) were used to enhance the hydrophobicity of the PAA and aluminum (Al) surface. The tribological performances were investigated in detail. The results showed that the static friction coefficient of PAA was much lower than that of smooth Al, which could be due to the higher hardness of the PAA than that of Al. PAA modified with PFPE was characterized by lower friction coefficient and longer wear life as compared with bare Al, PAA, and PAA modified with OTS. This is caused by the combined effects of flexibility, mobility, and inherent lubricity of PFPE molecules and the nanoporous structure of PAA as a reservoir for lubricants and wear particles. Keywords
Alumina film PAA PFPE OTS Friction
1 Introduction Owing to high electrical and thermal conductivity, outstanding resistance against corrosion and lightweight [1], aluminum-based materials have gained various applications Y. Wang L. Xia J. Ding N. Yuan Y. Zhu Center for Low-dimensional Materials, Micro-nano Devices and System, Changzhou University, Changzhou 213164, China Y. Wang L. Xia J. Ding (&) N. Yuan Y. Zhu Jiangsu Key Laboratory for Solar Cell Materials and Technology, Changzhou 213164, China e-mail:
[email protected];
[email protected]
in fabrication of micro/nanoelectromechanical systems (MEMS/NEMS) [2]. However, some shortcomings of aluminum materials including low hardness, high friction coefficient, and difficulty to lubricate limited their extended application [3, 4]. Porous anodic alumina (PAA) fabricated by anodization of aluminum makes favorable nanohoneycomb structure which mainly consist of amorphous A12O3 [5] and greatly increases hardness [6]. Resulting from its highly robust and versatile characteristics, PAA is an excellent candidate for use in MEMS/NEMS as well as integrated biosensing/analyzing functional devices [7]. However, the friction coefficient of PAA is still high and the abrasion of counterpart is found to be increased [6]. To overcome these disadvantages of PAA, a self-lubricating surface is required. Many researchers improved the tribological properties of PAA by surface modification. For example, Zhang et al. [3] successfully incorporated polytetrafluoroethylene (PTFE) within the PAA to reduce the friction coefficient. Skeldon and Xu et al. [6, 8] filled MoS2 into the pores of PAA to decrease wear. Other researchers have padded materials including carbon nanofibers [1], nickel [9], iodine compound [10], etc., into the pores of PAA to improve the wear-resistance properties. Tribological properties of PAA have been investigated. However, the experimental parameters, especially the applied loads, are several newton to hundreds of newton [1, 3] or lower than 1 mN [11]. Although deliver forces of micromotors are usually well below 1 mN, a reasonable level of contact load involved in emerging MEMS is reported in the range of 1–1,000 mN [12]. The tests were performed under the loads of 100, 200, and 300 mN, which is a reasonable level of contact load as described above. Perfluoropolyether (PFPE) and octadecyltrichlorosilane (OTS) were selected as lubricants based on the following considerations. PFPE has superior properties, such as chemical and
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Fig. 1 Surface morphology of the as-obtained PAA, which was prepared by a two-step anodization process. a AFM image; b SEM image, showing the cross-sectional morphology of the PAA; the pore diameter and height are about 100 and 330 nm, respectively
thermal stability, low vapor pressure, low hydrophilic character, low surface tension, and good lubricity, which make it a potential candidate for tribological application [13–15]. Moreover, PFPE is safe, nontoxic, and environmental friendly [16]. The OTS, which is easy to form selfassembled monolayer (SAM), is known for its good bonding strength, hydrophobic property, and low surface energy, making them attractive candidates for use in several applications [17]. The corresponding tribological mechanisms and analysis of the reasons were further investigated.
2 Experimental Details 2.1 Materials High-purity aluminum foils (99.999 % with the thickness about 150 lm); octadecyltrichlorosilane (OTS, 95 %) were purchased from Acros Organics; PFPE (formula HOCH2 CF2O–(CF2–CF2O)m–(CF2O)n–CF2CH2OH, m and n are integers, MW3800, with the commercial name Zdol 3800) was provided by Aldrich Chem. Co. Ltd; other chemicals were analytical grade and used as received. Ultra pure water was used throughout the experiment.
Table 1 WCAs of different samples studied in the research
Sample
WCA (°)
Al
54.3
PAA
73.5
Al–OTS
82.8
PAA–OTS Al–PFPE PAA–PFPE
101.3 89.6 106.4
1 h. Subsequently, the obtained PAA layer was removed in a mixture of chromic acid and phosphoric acid at 60 °C for 1 h. Finally, PAA with hexagonal unidirectional pore structure were obtained by secondary anodization under the same conditions as the first anodization for 3 min. In order to improve the tribological properties of PAA, PFPE, and OTS were used to modify the as-prepared PAA. At first, the as-prepared PAA were immersed into dilute solutions of PFPE and OTS (1 mM) for 24 h, respectively. Then, the samples were taken out and ultra-sonicated in HFE 7100 and toluene to remove the physical-adsorbed molecules, respectively. Finally, the samples were dried under a flow of N2. 2.3 Films Characterization
2.2 Sample Preparation PAA with highly ordered pore structures were fabricated by a two-step anodization method [18, 19]. First, cleaned highpurity aluminum foils were electropolished under a constant voltage of 12 V for 3 min in a mixture of perchloric acid (HClO4:C2H5OH = 1:4 in volumetric ratio) to remove surface irregularities. Then, the electropolished aluminum foils were first anodized in 0.3 M oxalic acid solution under a constant voltage of 40 V and temperature 3 * 5 °C for
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The surface morphology of as-prepared PAA was observed on a Nanoscope IIIa multimode atomic force microscope (AFM, Veeco) in tapping mode. Water contact angles (WCAs) were determined using a Contact Angle System (HARKE-SPCA). A 5-lL droplet was used for the WCA measurements, and average values of at least five repeat measurements for each sample were recorded. The surface chemical compositions were examined with a PHI-5702 multifunctional X-ray photoelectron spectroscope (XPS,
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Fig. 2 XPS spectra of PAA surface modified with PFPE (a) and OTS (b), respectively
Perkin-Elmer, America), using Mg Ka radiation as the excitation source. And, the binding energy of contaminated carbon (C1 s: 284.8 eV) was used as reference. Tribological properties were investigated on a UMT2MT tribometer (CETR, USA) in ambient condition (20–25 °C, RH = 40–50 %), using a reciprocating-sliding mode. The upper counterparts used here were commercially available AISI52100 bearing steel balls (A = 3 mm). Applied loads of 100–300 mN and a sliding rate of 10 mm/s were applied for all measurements. The friction coefficient versus time plots were recorded automatically, and at least three repeated measurements were performed. And, the images of worn surfaces were observed on a ContourGT non-contact 3D profile (Bruker, Germany).
3 Results and Discussion 3.1 Surface Morphology of the PAA Figure 1 shows the typical topographic morphology of the fabricated PAA. It can be clearly seen that a uniform closely packed hexagonal pore structure is observed on the surface of the PAA (Fig. 1a). And, the average pore diameter and pore height of the PAA are around 100 and 330 nm (Fig. 1a, b), respectively. 3.2 WCA Measurement The low wettability/surface energy is one of the important properties that are required for MEMS components as surfaces with high energy lead to stiction and early failure [20]. WCAs were measured to characterize the surface wetting/ dewetting properties [21]. The results are shown in Table 1. Under the same conditions, WCAs of PAA surfaces are larger than those of Al surfaces. It is well known that the
Fig. 3 Friction coefficients as function of time for OTS (a) and PFPE (b) sliding against AISI52100 steel ball at applied load 100–300 mN and a sliding velocity of 10 mm/s
contact angle is mainly determined by two factors, such as the surface topography and the surface chemistry [22]. PAA with nanohoneycomb structures could decrease the contact
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enhance the hydrophobicity of Al or PAA. The contact angles of PFPE-coated surfaces are higher than that of OTScoated surfaces, which can be explained based on chemical factor. The hydrophobic CF2 branches in the perfluoropolyether would result in higher contact angles, compared with that of OTS with CH3 chain [15, 24]. 3.3 X-ray Photoelectron Spectroscopy Results To obtain insights in chemical composition of the PAA surfaces modified with PFPE and OTS, XPS investigation was performed on the surfaces and the results are presented in Fig. 2. The scan survey spectrum of PFPE shows three elements: fluorine (F1s), carbon (C1s) and oxygen (O1s), which indicates that the PFPE is adsorbed successfully on the surface of PAA (Fig. 2a). The adsorption of OTS is confirmed by Fig. 2b, which displays the characteristic elements of OTS in particular: carbon (C1s), oxygen (O1s), and silicon (Si2s and Si2p). 3.4 Microtribological Behaviors 3.4.1 Tribological Performance Under Different Loads
Fig. 4 Friction coefficients as function of sliding time for Al and PAA under different surface modifications at an applied load of 100 mN. a Pure Al and PAA under dry conditions at static friction stage, b pure Al, c PAA
Figure 3 displays the measured friction coefficients as a function of the time at the same sliding speeds of 10 mm/s at different normal forces. It can be clearly seen that the friction coefficients tend to decrease when the normal forces increase from 100 to 300 mN, which is in that the lubricant membranes on PAA become compact with the applied load increasing. When the lubricants in the subsurface pores are squeezed out, it would form interfacial layer with low shear strength, which makes the friction coefficient decrease. The higher applied loads result in the larger area of interfacial layer with low shear strength and the lowering of friction coefficient. When the applied load increases to 300 mN, the anti-wear life of the OTS-modified PAA is shortened to *950 s (Fig. 3a). Since the lubricant membranes are worn out, the wear scar of the composite film specimen will reveal a white aluminum matrix and lubrication protection was completely ineffective. In contrast, the PFPEs on the PAA surface remain as an effective lubricant layer for more than 1,600 s (Fig. 3b). It is in that the flexibility, mobility, and inherent lubricity of PFPE molecules offer lower resistance to sliding, which contribute to improving the wear life and lowering the friction coefficient [20, 25]. 3.4.2 Synergies of Nanotextured and Lubricants
spacing and air may be trapped in the contact surface, resulting in a composite solid–air–liquid interface, which results in higher contact angles [23]. Compared with bare Al or PAA, the higher WCAs for OTS- or PFPE-modified Al or PAA indicate that the low surface energy materials could
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Figure 4 presents the curves of friction coefficient-versussliding time for surfaces under different modification at an applied load of 100 mN. Figure 4a shows that the static friction coefficient of PAA is lower than that of pure Al. It is
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Fig. 5 3D images of different worn surfaces: Pure Al (a), PAA (b), Al–OTS (c), PAA–OTS (d), Al–PFPE (e), and PAA–PFPE (f). All the tests were run against steel ball at load of 100 mN and a sliding velocity of 10 mm/s for 5 min
because of the fact that the hardness of the PAA is higher than that of the pure Al, and hence, the contact area between the asperity on the PAA surface and the ball specimen is smaller than that of pure Al. However, there is no significant discrepancy in the dynamic friction coefficient (*1.1), which is similar to the previous research [26]. Figure 4b, c reveal a significant increase of wear life and a decrease of friction coefficient of PAA modified by OTS or PFPE, compared with that of barely PAA or pure Al modified with different lubricants. Especially, PFPE-modified PAA presents the best tribological behavior, the friction coefficient of
which is only 0.16 and is particularly stable. It may be due to the combined effect of the effective lubrication of PFPE and the excellent wear resistance of PAA. It has potential advantageous for tribological application as the nanoporous structure can be utilized as a reservoir for lubricants and wear particles, which could result in longer wear life [27, 28]. 3.4.3 Wear Scar A 3D non-contact profilometer was used to observe the worn surfaces of the different samples at an applied load of
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100 mN and a constant sliding velocity of 10 mm/s for 5 min (Fig. 5). It could be found that wear scars of left figures (Al) are more serious than that of the corresponding right figures (PAAs). There are a lot of abrasive grains and severe scratch in the left figures. While for the right figures, there are nearly no abrasive grains and the scratch was inhibited to some degree. The introduction of specific textures on the sliding surface contributes to improve its tribological properties [29]. The texture can trap wear particles from the interface, reducing the plowing and deformation components of friction [30]. The scratch of the OTS-modified PAA is inhibited to only a certain degree due to its low molecular flexibility and mobility [31], Fig. 5d. During the six samples in this research, it can be clearly seen that the PFPE-modified PAA presents the best anti-wear property. The wear on the surface is significantly reduced, and the scratch is nearly indistinguishable (Fig. 5f). The reason is that PFPE molecules have good flexibility, mobility, and inherent lubricity, and the texture is acting as a reservoir which feeds the lubricant directly between the two contacting surfaces [32].
4 Conclusions PAAs with an average diameter of *100 nm and a depth of *330 nm were achieved by anodizing in oxalic acid solution. PFPE and OTS were used to improve the wettability and tribological performance of the as-obtained PAA and Al films. The WCAs of PAA surfaces are larger than that of Al surfaces. Combining with the chemical modification of low surface energy materials, the hydrophobicity is further enhanced. The PFPE-modified PAA presents the best tribological properties, which is due to good flexibility, mobility, and inherent lubricity of PFPE molecules and the ordered nanopores as a reservoir feeding the lubricant directly between the two contacting surfaces. It is expected that tribological performance could be largely improved by designing suitable surface topography combined with lubricant films. In our future researches, PAA with different pore diameter and pore depth will be designed, and the tribological behaviors will be investigated systematically. Acknowledgments The authors are grateful to the National Natural Science Foundation of China (Grant no. 51102028) and the Priority Academic Program Development of Jiangsu Higher Education Institutions for financial support.
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