Modify the friction between steel ball and PDMS disk under water lubrication by surface texturing

Transcription

1 Meccanica (2011) 46: DOI /s ASPERITY CONTACTS & LUBRICATION ASPECTS Modify the friction between steel ball and PDMS disk under water lubrication by surface texturing Jinfeng Li Fei Zhou Xiaolei Wang Received: 22 September 2009 / Accepted: 25 March 2010 / Published online: 11 June 2010 Springer Science+Business Media B.V Abstract Surface texture has been recognized as an effective means of surface engineering to improve the tribological properties of sliding surfaces. Various lubrication mechanisms of surface texture have been studied for the surfaces of metals and ceramics. In order to investigate the effects of surface texture for soft material, the patterns of micro-dimple with different dimple diameters and area ratios were fabricated on a polymer material poly(dimethylsiloxane) (PDMS) by lithography & replica techniques. A preliminary study on the friction of the polymer disk sliding against a bearing steel ball under water lubrication was carried out. The effects of surface texture on the friction of polymer material were evaluated. It is found that surface texture is capable of reducing friction at low sliding speed, as well as increasing friction at high sliding speed condition. Keywords Surface texture Friction Soft material PDMS Nomenclature a Contact radius d Dimple diameter E Reduced elastic modulus. E = 1.38 MPa for steel on PDMS J. Li F. Zhou X. Wang ( ) College of Mechanical & Electrical Engineering, Nanjing University of Aeronautics & Astronautics, Nanjing , China wxl@nuaa.edu.cn h Dimple depth h c Central film thickness h min Minimum film thickness l Pitch between dimples r = πd 2 /(4l 2 ), area ratio of dimples, % R Radius of the ball U Sliding speed W Normal load δ Depressed displacement of the ball η Viscosity. For water, η = Pa s 1 Introduction The basic idea of controlling surface roughness to improve tribological performance has been recognized since the early 1900s. Now, it is proven that surface texture is an effective way of surface engineering to decrease friction or increase the load carrying capacity of sliding surfaces. Hence, surface texture has been successfully applied to sliding bearings, mechanical seals, and piston/cylinder pair etc. [1]. Advanced techniques, such as laser, micro-abrasive jet [2], micro-lithography [3] etc., make it possible to fabricate various desirable texture patterns accurately on the surface of metal [4] and ceramics [5]. The friction reduction mechanisms of surface textures are reasonably understood for low load and high speed conformal contacts, such as sliding bearings and mechanical seals. The micro-dimples on the surface can provide additional hydrodynamic pressure

2 500 Meccanica (2011) 46: resulted from the asymmetric pressure distribution caused by the micro diverging-converging geometry of the dimples [6]. Therefore, the hydrodynamic lubrication regime is expanded, and the transition point from mixed to hydrodynamic lubrication of Stribeck curve is shifted to a left and low position [7]. It is well accepted that surface texturing is most efficient for parallel sliding. Analytical models based on the Reynolds equation explain most of the experimental data very well [3, 8]. Some principles for surface texture design have been proposed and approved. For example, the dimple depth over diameter ratio is the most important parameter and it has a preferable range in order to obtain high load carrying capacity [3, 9]. However, the surface texture effects are still complicated at the condition with low speed and high load, particularly, in the case associated with non-conformal contacts [10 12], where full film lubrication could not be established, and boundary lubrication influences the friction very much. People still doubt about the effects of surface texturing because a part of experimental results show that surface texture is beneficial for friction reduction while others show negative effects. Recently, there are numbers of analytical and numerical results based on hydrodynamic effect, which suggests that the optimal surface texture for minimizing friction of a convergent thrust bearing is no texture at all [13, 14]; surface texture increases the load carrying capacity only in low convergence bearings [15, 16]; and the size of dimple is critical for improving load carrying capacity in the case of line contact [11]. On the other hand, the experimental results show that the small features with shallow depth were effective for friction reduction at low speed and high load conditions [17, 18]. Plastic and elastic deformation was supposed to influence the effect of surface texture in these cases [19]. Recently, the reason has been well explained by EHL experiments [20], which showed that a significant increase in lubricant film thickness is induced by a shallow micro-cavity in the elastohydrodynamic lubrication regime. On the other hand, there are many so called soft tribological contact phenomena in our life. For example, the contact of animal skin with the environment, rain wipers with windscreen, lip seals with shaft, vehicle tyres with ground etc. The friction control of these soft contacts is always an important issue of research. Lee and Spencer observed a dramatic reduction of friction forces in aqueous lubrication upon the hydrophilization of poly(dimethylsiloxane) (PDMS) surface by oxygen-plasma treatment [21]. Bongaerts et al. found that neither surface roughness nor hydrophobicity influence the friction coefficient of PDMS within the EHL regime [22]. Surface texture has already been considered to be applied to the contact with soft materials. Hadinata and Stephens [23] conducted a numerical analysis to investigate the elastohydrodynamic effect of deterministic microasperities on the shaft of a lip seal. The microasperities on the shaft surface show significant reduction of friction and increase of the reverse pumping rate. Shinkarenko et al. [24] developed a theoretical model to analyze the lubrication between a textured rigid surface and a smooth elastomeric. It is found that the optimum aspect ratio depends exclusively on the stiffness index. On the aspect of experiments, He et al. [25] carried out dry friction tests on the textured PDMS surface using a nanoindentation-scratching system with the loads in micro-newton scale, and found the friction coefficient of pillar-textured surface much lower than the smooth surface. The tribological problem of elastomer under lubrication is complicated, not only because the relative larger scale of elastic deformation during contact, but also the special surface chemistry, such as hydrophobicity etc. Therefore, in order to understand the effect of surface texture in soft tribological contact under lubrication, an elastomer, PDMS was used as the specimen. The patterns of micro-dimple with different dimple diameters and area ratios were fabricated on the surface of PDMS by lithography & replica techniques. The frictional properties of this PDMS disks sliding against a bearing steel ball were investigated and compared to an untextured surface. 2 Experimental setting 2.1 Surface texture fabrication Poly (dimethylsiloxane, PDMS) named as Sylgard 184 made by Dow Corning Corp., USA was selected as the material for disk. The pattern of evenly distributed dimples was fabricated on the surface of PDMS. The procedure of surface texture fabrication is shown in Fig. 1 as following steps.

3 Meccanica (2011) 46: Fig. 1 Procedure of surface texture fabrication Fig. 2 PDMS disk. (a) Picture of the disk. (b) Optical microscope image of a dimple pattern. (c) 3Dprofileofadimple.(d) Cross section of a dimple Table 1 Parameters of the dimple patterns Dimple diameter d, µm Pitch l,µm Depthh, µm Area ratio r,% 1 30, 50, 100, , 200, 400, , 112.5, , 225, (1) Fabricate the opposite mold of the photoresist on a polished brass disk by lithography process. (2) Mix the prepolymer and its curing agent by the ratio of 10:1; pour carefully to the previous mold; after complete degassing by a vacuum pump, place in a convection oven with temperature of 60 C for 16 hours for curing. (3) Peel PDMS off the mold after cooling down for 24 hours. Several patterns with different dimple diameters and different dimple area ratios were fabricated in this research. The dimple depth is kept constantly as the thickness of photoresist around 5 µm. In order to avoid shape effect of dimples related to the sliding direction, circular dimples on the top surface were used. Due to the fabrication method, the dimple has the side wall nearly perpendicular to the bottom surface. Table 1 lists three groups of the patterns, which are used in this research to investigate the surface texture effects with different dimple diameters and different dimple area ratios. Figure 2 shows the picture of the PDMS disk, optical microscope image of the dimple pattern which

4 502 Meccanica (2011) 46: has the diameter of 100 µm, dimple depth of 5 µm, and area ratio of 15.5%, 3D profile of a dimple obtained by an optical profiler, and the cross section of the dimple. 2.2 Mechanical properties of PDMS Based on CTM 0099 (Dow Corning Corp., USA) standard, the hardness of PDMS was measured by Shore A type sclerometer. In accordance with ASTM D standard, the compressive elastic modulus was calculated from the stress and strain data obtained with the universal mechanical tester UMT-CETR. The measurements show that the hardness of PDMS (prepolymer/curing agent mass ratio of 10:1) was 50 ± 2 Shore A, and compression elastic modulus E was ± MPa. The surface roughness of PDMS disk is dependent on the quality of the brass disk. The optical profiler was utilized to measure the surface roughness of the area between dimples. The results show that the PDMS disks have an average surface roughness Ra around 140 nm. The contact angle of the PDMS surface without surface texture was 112 ± 2 degree. 2.3 Frictional testing procedure Friction tests were carried out using a pin-on-disk tester. The lower specimen was the disk of PDMS with Fig. 3 Schematic figure of the contact between PDMS disk and bearing ball diameter of 36 mm and thickness of 5 mm. The upper specimen is a standard GCr15 bearing steel ball (Shanghai bearing ball corp.), which has diameter of 8 mm, hardness of HRC, and surface roughness Ra of µm. Figure 3 is the schematic diagram of the contact condition of ball and disk. The bearing ball was stationary, hold by the arm. The load was applied on the ball by dead weight. The disk was driven by a motor rotating with a constant speed. The friction coefficient was obtained by measuring the strain of the arm caused by friction force. Before the test, GCr15 steel bearing balls were ultrasonic cleaned with acetone and alcohol to remove the oil remained on the surface. The PDMS disk were ultrasonic cleaned by isopropyl alcohol and deionized water. Then the friction test was conducted under the lubrication of deionized water at room temperature with the normal loads of 0.95 N and 1.88 N within the sliding speed range from 20 to 200 mm/s. All the specimens have a run-in distance of 2 m, and then the friction coefficient was obtained by averaging the data within the sliding distance of next 1 m. Based on Hertz contact theory, the contact radius and the contact pressure were calculated as listed in Table 2. 3 Experimental results 3.1 Frictional properties of untextured surface under water lubrication Figure 4 shows the friction coefficients of untextured PDMS disks sliding against the GCr15 bearing ball under water lubrication at the loads of 0.95 N and 1.88 N. The test at each condition was repeated several times with different specimens. So there is an error bar for each data showing the deviation of friction coefficient. At the sliding speed 20 mm/s and 50 mm/s, the friction coefficients were all above 1, and as high as 1.5 in some cases, showing a state of high friction. When the Table 2 Contact parameters Normal load W (N) Contact radius a (mm) Average contact pressure (MPa) Max contact pressure (MPa)

5 Meccanica (2011) 46: sliding speed was increased to over 60 mm/s, the friction coefficient decreased rapidly, and stayed in a low friction coefficient range from 0.04 to This phenomenon indicates that there was a transition of lubrication condition around the sliding speed of 60 mm/s. The load seems to have no obvious influence on the position of transition. This trend is similar to Bongaerts s results [22], which supposed the lubrication transited from boundary to mixed regime. 3.2 Frictional properties of textured surface under water lubrication Figure 5 shows the friction coefficients of the PDMS disks textured with dimple patterns which have the Fig. 4 Friction coefficients of untextured PDMS sliding against bearing ball same depth of 5 µm, same area ratio of 4.9%, and different dimple diameters from 30 µm to 500 µm. Compared to the untextured PDMS, the friction coefficients still decreased while the sliding speed increased, but the transition is not as clear as that of untextured surface. In the high speed range (U 60 mm/s), most of the friction coefficients of textured surface are higher than that of untextured surface, which is usually between 0.04 and The pattern with dimple diameter of 500 µm has less increase of friction coefficient within these four kinds of patterns. And the patterns with dimple diameters of 30 µm or 100 µm could increase friction coefficient to the value as high as 0.3, which is near ten times higher than that of untextured surface. However, in the low speed range (U <60 mm/s), most of the surface textures have the effect of friction reduction, and it becomes obvious when sliding speed was increased from 20 mm/s to 50 mm/s. Figure 6 compares the friction coefficients of the specimens with the same area ratio and different dimple diameters at sliding speed of 50 mm/s. The pattern with dimple diameter of 30 µm did not decrease friction very much. The patterns with dimple diameter from 50 µm to 500 µm expressed an obvious friction reduction effect. At the load of 0.95 N and sliding speed of 50 mm/s, the texture with diameter of 50 µm reduced the friction coefficient about 89% of that of untextured surface. And the reduction rate of friction coefficient is almost the same at the load of 1.88 N. Figure 7 shows the friction coefficients of the PDMS disk textured with dimple patterns which have Fig. 5 Friction coefficients of textured PDMS with different dimple diameters

6 504 Meccanica (2011) 46: the same diameter of 50 µm and different dimple area ratios. The overall tendency of friction coefficient of textured surfaces is similar to that in Fig. 5, presenting that surface texture decreases friction coefficients in low speed range (U <60 mm/s) and increases friction coefficient in high speed range (U 60 mm/s). The surface textures with high area ratio do not decrease friction at low speed as much as that by the texture with area ratio of 4.9%. Figure 8 shows the comparison of friction coefficients of specimens with different area ratios at sliding speed of 50 mm/s. The surface texture with the dimple area ratio of 15.5% and 22.9% decrease friction in the range from 8% to 27% under the loads of 0.95 N and 1.88 N. Figure 9 shows the friction coefficients of the PDMS disks textured with dimple patterns which have the same diameter of 100 µm and different dimple area ratios. The phenomenon that the friction coefficient increased along with the increase of area ratio becomes more obvious. For the dimple pattern with diameter of 100 µm and area ratio of 22.9%, the fiction coefficients at low speed range were slightly reduced at the load of 0.95 N, and almost the same as that of untextured specimen at the load of 1.88 N. Even the sliding speed was increased to over 60 mm/s, there was no obvious decrease in friction coefficient. In most cases, the friction coefficients were above 1, which shows an effective way to increase friction of PDMS surface at high speed. Figure 10 shows the comparison of friction coefficients at sliding speed of 200 mm/s. 3.3 Discussion Fig. 6 Comparison of friction coefficients of the specimens with the same area ratio and different dimple diameters The friction properties of untextured PDMS surface presented in this research are similar to Bongaerts s results. Because the contact angle of PDMS surface is around 110 degree, showing the hydrophobic property, the water is excluded easily from the contact area. Hence, the friction coefficients are above 1 in water lubrication at low sliding speed (20 mm/s, and 50 mm/s), which were very similar to the condition of dry friction. While the sliding speed was increased to over 60 mm/s, the friction coefficient decreased to Fig. 7 Friction coefficients of textured PDMS with diameter of 50 µm and different dimple area ratios

7 Meccanica (2011) 46: the range of , a typical value of mixed lubrication regime. As suggested by Bongaerts, more fluid is entrained in the contact as sliding speed increases, resulting in less asperity contact, EHL becoming an important role and hence a reduction in friction. According to the soft-ehl theory, and assuming that h c = 4/3h min, the central film thickness in this regime isgivenby[26]: h c 4.37R 0.76 (ηu) 0.66 W 0.21 E At the loads of 0.95 N, the central film thickness is 53 nm at sliding speed of 20 mm/s, 154 nm at Fig. 8 Comparison of friction coefficients of the specimens with the same dimple diameter and different dimple area ratio 100 mm/s, and 244 nm at 200 mm/s. At the loads of 1.88 N, the central film thickness is 46 nm at sliding speed of 20 mm/s, 134 nm at 100 mm/s, and 211 nm at 200 mm/s. The change of load does not result in the obvious change in the central film thickness. That would be the reason why the friction curve at the load of 0.95 N is similar to that at load of 1.88 N. Because the contact in this research is between a steel ball and a PDMS disk, the elastic deformation mainly happens on the surface of PDMS disk. The depressed displacement is given by: δ = a2 R. It is 405 µm at the load of 0.95 N, and 639 µm at the load of 1.88 N, showing that the steel ball was pressed into the PDMS disk deeply, with the value much greater than the surface roughness. During the sliding of ball on disk, the ball needs to do work to deform the PDMS disk continually so that the friction coefficient was in the range of , and didn t decrease further while sliding speed was increased. From this viewpoint, the full lubrication film of EHL probably exists even the friction coefficients are a little bit higher than usual. While micro-dimples were introduced on the surface of PDMS, the textured area acted as a resistive factor to the water escaping from the contact, so that the water could be kept within contact area even at low speed, as result, friction coefficients of textured surface are lower than that of untextured surfaces in the Fig. 9 Friction coefficients of textured PDMS with diameter of 100 µm and different dimple area ratios

8 506 Meccanica (2011) 46: coefficient was increased over the untextured surface. For the pattern with diameter of 500 µm and area ratio of 4.9%, it has the maximum pitch 2 mm of dimples among all patterns in this research. With the relative small contact radius of 1.27 mm between steel ball and PDMS disk at load of 0.95 N, the ball has enough chance to slide against untextured area. Therefore, the friction coefficient of the pattern with diameter of 500 µm is close to that of untextured specimen at high speed range as shown in Fig Concluding remarks Fig. 10 Comparison of friction coefficients of the specimens with the same dimple diameter and different dimple area ratios at sliding speed of 200 mm/s low speed range. A relative high speed would enhance the EHL lubrication effect. Hence, the friction coefficient has an obvious reduction at 50 mm/s. As reported in the previous research, small and shallow dimples are better for friction reduction at high load and low speed condition, particularly for the case of point contact geometry. That is probably the reason why the pattern with dimple diameter of 50 µm obtained the best result of friction reduction compared to the untextured surfaces in this research. However, it is not correct for the pattern with dimple diameter of 30 µm. It is wondered if the limited friction reduction effect is due to its high depth over diameter ratio or other unknown reasons. When the area ratio was increased to 15.5% and 22.9%, the friction reduction effect was not as obvious as that with 4.9%, it might because the increase of dimples equals to the increase of surface roughness. As mentioned above, the surface roughness Ra of the steel ball and PDMS disk are 25 nm and 140 nm respectively. At the load of 0.95 N and sliding speed of 60 mm/s, the calculated central film thickness h c is 110 nm, hence a full EHL film probably has been formed in the high sliding speed range although the friction coefficient was a little high. While a dimple pattern was introduced on the surface, on the other hand, the dimples would increase the surface roughness, and interrupt the continuance of lubrication film to generate a contact-localized transient condition of non-steady film thickness [27]. As a result, friction In order to investigate the effects of surface texture on friction for soft material, the lithography and replica process were successfully used for the surface texture fabrication on the surface of PDMS. The surface texture of dimple pattern with dimple depth of 5 µm, diameters from 30 to 100 µm and area ratio from 4.9% to 22.5% were fabricated. Friction tests between the PDMS disk against a steel bearing ball were carried out under water lubrication preliminarily. The conclusions are as following: (1) The surface texture has an obvious effect of friction reduction at low speed range (<60 mm/s). The friction coefficient could be reduced up to 89% of that of untextured specimen by the surface texture, which has the dimple diameter of 50 µm, depth of 5 µm and area ratio of 4.9%. (2) At high speed range (>60 mm/s), the surface texture with dimple diameter of 100 µm, depth of 5 µm and area ratio of 22.5% can induce high friction coefficients, which were above 1 in most cases. Above results shows that surface texture provides an effective way to modify the friction of the PDMS surface. Further experiments and theoretical analyses need to be done to understand the phenomena involved deeply. Acknowledgements We would like to thank the National Nature Science Foundation of China (NSFC) (No ), and Science Foundation of Jiangsu province (No. BK ) for their financial supports. References 1. Etsion I (2005) State of the art in laser surface texturing. J Tribol Trans ASME 127:248

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