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Effects of surface texturing on the frictional behavior of cast iron surfaces
Tribology International 70 (2014) 128–135
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Tribology International journal homepage: www.elsevier.com/locate/triboint
Effects of surface texturing on the frictional behavior of cast iron surfaces Beomkeun Kim a,n, Young Hun Chae b, Heung Soap Choi c a
High Safety Vehicle Core Technology Research Center, Inje University, 607 Obang-Dong, Gimhae, Gyeongnam 621-749, Republic of Korea Department of Mechanical Engineering, Kyungpook National University, 1370 Sankyuk-Dong, Pook-Gu, Daegu 702-701, Republic of Korea c Department of Mechanical and Design Engineering, Hongik University, 2639 Sejong-Ro, Jochiwon-Eup, Sejong 339-701, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 19 June 2013 Received in revised form 1 October 2013 Accepted 8 October 2013 Available online 15 October 2013
The purpose of this study is to investigate the effects of the geometry and distribution of microdimples on the frictional behavior of surfaces for applications in automotive engines. A square array of microscale circular dimples was selected as the texture pattern. A laser beam was used to create microdimples with various dimensions on cast iron surfaces. Frictional tests were performed with selected loads and speeds to simulate the operation conditions of automotive engine parts. The effects of dimple distribution were also investigated. The aspect ratio of the dimples was found to be the most significant factor, while the effect of the surface density of the dimples on the coefficient of friction was found to be only marginal. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Surface texture Laser machining Coefficient of friction Depth to diameter ratio
1. Introduction The increasing demand for environmentally friendly automobiles has led to much research interest in improving the fuel efficiency of automotive engines. Because a significant proportion of the energy in an automotive engine is consumed by frictional losses, the tribology of the mechanical parts is an important factor in determining the engine efficiency. Automotive engine parts experience more than one regime of lubrication during their operation. For example, the lubrication regimes associated with a cam and follower are boundary, mixed and elastohydrodynamic [1]. The lubrication regimes for typical automotive engine parts are shown in Fig. 1 [1]. Modifying the lubrication conditions for a given automotive engine part may potentially improve the efficiency of the system. Vehicle performance could also be improved by utilizing reduced-friction engines. The effects of dimples on a surface were first reported by Etsion et al. [2–4], who investigated the effect of surface texturing and the optimal geometric arrangement of the surface features, both theoretically and experimentally, for water-lubricated sealing ring applications. They found that surface texturing substantially increased the load- carrying capacity of bearing surfaces. It is also well known that surface texturing of solid surfaces modifies the frictional and wear properties. Dumitru et al. [5] investigated the
n
Corresponding author. Tel.: þ 82 11 296 3816; fax: þ 82 55 320 3439. E-mail address: [email protected] (B. Kim).
0301-679X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.triboint.2013.10.006
effect of surface textures using tribological tests on laserstructured substrates. They reported that the lifetimes of laserprocessed samples increased significantly in comparison with unstructured samples. Menezes et al. [6] studied the effects of surface roughness and textures on the coefficient of friction; however, they only examined the effect of the existence of dimples, and did not investigate their geometry. Wang et al. [7,8] investigated the effects of microdimples on the loadcarrying capacity of SiC thrust bearings. The contact surface was textured by reactive ion etching, and water lubrication was used in the tests. They showed that the textured surface was able to maintain hydrodynamic lubrication for a longer time than the non-textured surface. Andersson et al. [9] demonstrated the benefits of laser-textured tool surfaces for friction reduction. They found that laser-textured patterns significantly reduced friction and wear under lubricated sliding conditions. They performed experiments using three different types of oil, but did not include sufficient variation in the dimensions of the dimples to investigate the effect of their geometry. Wakuda et al. [10] studied the frictional properties of dimpled silicon nitride ceramic surfaces against steel. They evaluated samples with a variety of microdimple dimensions, under boundary or mixed lubrication conditions, and observed that there was a critical dimple size, below which the texturing was ineffective. Petterson and Jacobson [11] performed friction tests on steel surfaces textured with a diamond tool. They reported that the coefficients of friction of the textured surfaces indicated improved lubrication at high pressure and low sliding speed. Nakano et al. [12] investigated the relationship between texture geometry and
B. Kim et al. / Tribology International 70 (2014) 128–135
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All of these studies indicate that textured surfaces (whether laser, chemically or mechanically structured) offer the benefit of reduced coefficients of friction, not only in the hydrodynamic regime, but also in the mixed or boundary lubrication regime. This leads to decreased wear on solid surfaces. However, to date there has been insufficient investigation of the effect of the dimple geometry on the tribological behavior for applications in automobile engines. The purpose of this study was to investigate the effects of the geometry and distribution of microdimples on the tribological behavior of surfaces for applications in automobile engines. Surface texturing has proved to be an effective way of improving the tribological behavior of contact surfaces. As found in previous studies, surface texturing increased load-carrying capacity [2–4]. While this aspect of surface texturing is relevant to automotive engine parts, the scope of this study was limited to friction reduction. In this study, the friction-modifying effects of texturing were examined, and relationships between the geometry and the distribution of microdimples and the coefficients of friction were investigated for the different sliding velocity, viscosity of lubricant and contact pressure, with a focus on piston rings, cam followers, and engine bearings. Microtextured cast iron surfaces were prepared by laser processing, and experiments with various loads and speeds were conducted under a thin layer of oil (by removing excess oil from the surface) to simulate the operating conditions of automotive engine parts.
Fig. 1. A modified Stribeck curve associated with automotive engine parts [1].
2. Experiments 2.1. Test equipment
coefficients of friction by testing microtextured surfaces prepared by shot blasting. They found that a dimpled pattern reduced the coefficients of friction, whereas a grooved pattern increased them. Kovalchenko et al. [13] studied the impact of laser surface texturing on lubricating regime transitions. They observed that laser surface texturing could reduce the friction on components operating in a boundary-lubricated regime. This study included different sized dimples, but focused on the tribological behavior rather than the effects of the dimple dimensions. Voevodin [14] explored the applications of laser-texturing the surfaces of hard TiCN coatings with adaptive solid lubrication. Tests revealed increased wear life for specimens employing the dimple reservoir concept. Borghi et al. [15] investigated the effects of surface texturing on nitride steel. They showed that texturing the surface reduced the coefficient of friction, and avoided the transition from the hydrodynamic regime to the mixed or boundary lubrication regime, resulting in an increase in the wear lifetime. That study was limited to specimens where the dimple dimensions were not varied. Etsion et al. [16–19] conducted experimental and theoretical studies to investigate the friction-reducing effects of surface texturing on automotive components. They suggested using textured surfaces for piston rings to achieve a reduction in the coefficient of friction, and they conducted experimental investigations to support their theoretical estimates of the effect of surface texturing using a novel test rig for the reciprocating automotive components. Ren et al. [20] conducted numerical analyses to determine the effect of the distribution of shallow dimples on the tribological properties of a surface. Golloch et al. [21] investigated the benefits of laser-structured cylinder liners by carrying out a series of tests on a single-cylinder diesel engine. They found that a laser-structured liner not only reduced frictional losses and oil consumption, but also decreased cylinder wear. Park et al. performed computational fluid dynamics (CFD) analyses to investigate the lubrication characteristics of an infinitely long slider bearing with dimples, and found that the pressure distribution was strongly related to the dimple dimensions [22].
Friction tests were carried out using a pin-on-disk tribometer [23]. The upper pin was loaded against the surface of a flat disk specimen. The loading direction was parallel to the axis of rotation, and the magnitude of the normal force was controlled by adding dead weights on the upper pin. The frictional force was recorded using strain gauges attached to the holder. The coefficient of friction was determined using these two force values. A schematic diagram of the tribometer and the test setup is shown in Fig. 2. The speed of the test was controlled by changing the rotational speed of the disk or adjusting the length of the arm holding the pin. 2.2. Test specimens The pin and disks were prepared from cast iron, which is suitable material for automotive engine parts such as liners, piston rings, or bearings. Table 1 shows an elemental analysis of the cast iron material used in this study. One end of the pin was machined as a sphere with a diameter of 8 mm and a flat area (Fig. 3). The other end was machined so that it could be attached to the arm of the tribometer. The final flat contact area created on the pin was 3.3 mm in diameter. The surface roughness of the contact area of the pin was 0.8 μm Rz. The disk specimens were also made from cast iron, each with a diameter of 60 mm and a thickness of 5 mm. Surface textures, consisting of microdimples of various sizes and depths, were fabricated on the disks by laser-beam machining. Table 2 lists the surface texturing specifications of the various specimens used in this study. The surface area to density ratio (total dimple area/total surface area) ranged from 10% to 30%. The diameters and depths of the microdimples were controlled by adjusting the pulse energy of the laser. A YAG laser (AHTSE010T model, λ¼1064 nm, tpulse E15 ns) was used to create the microdimples on the disks. By trial and error under various laserprocessing conditions, appropriate laser parameters were established to generate the desired pattern sizes. Table 3 lists the laser-processing parameters and the corresponding dimple sizes. For the cast iron
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Fig. 2. Equipment setup for the tribology test. Table 1 Elemental analysis of the FC 25 cast iron used in this study. Elemental composition (wt%)
C 3.16
Si 2.35
Mn 0.66
P 0.21
S 0.083
Cr 0.19
Cu 0.25
B 0.065
Fe Bal.
Fig. 3. Pin and disk specimens. (a) Pin, (b) Disk.
material, a pulse energy of 24.4 W was used to create dimples with a diameter of 106 μm and a depth of 14.84 μm. Each disk specimen was moved laterally via a high-precision XY-translation stage. A diagram of the laser-beam machining setup is shown in Fig. 4. After laser machining, the craters that formed around the microdimples were removed by manual grinding with abrasive emery paper. Polishing with alpha alumina powder followed the grinding process, resulting in a surface roughness of 0.8 μm Rz. The roughness of the textured surfaces was measured on the upper area between the dimples. Fig. 5 shows threedimensional (3D) scanning laser microscopy images of the microdimples created on the cast iron surfaces. Table 4 lists measurement results for five randomly selected laser-machined dimples after polishing. The diameters of the dimples were close to the target dimensions, but the deviations from the target dimensions were greater for the depths of the dimples than for the diameters. Fig. 6 shows scanning electron microscopy (SEM) images of the textured surfaces.
2.3. Experimental procedures Tribological tests of 145 specimens were conducted using the pin-on-disk tribometer. Prior to each test, the pins and disks were sonicated in alcohol to remove metal fragments and oil from the surface. Oil was applied to the top surface of each disk, and excess oil was removed using a rubber blade under constant pressure. The properties of the oil used in this study, which did not contain any extreme pressure additives, are listed in Table 5. The measurements were started immediately after the oil was applied, and continued until the coefficient of friction converged. Non-textured specimens were used, as well as specimens with circular dimples. The duration of the measurements were determined by considering the type of specimen and the lubrication parameters (i.e., sliding velocity viscosity/contact pressure). Here, the contact pressure was the total normal force between the mating surfaces and can be considered as the load-carrying capacity. The coefficients of friction converged over a relatively
B. Kim et al. / Tribology International 70 (2014) 128–135
short distance (from 130 to 250 m) for the textured specimens. For the non-textured specimens, however, the coefficients of friction only converged over a short distance at the high and mid-range of the lubrication parameters (0.462 to 2.310); at the low range of the lubrication parameters (0.0369 to 0.0972), they converged over a longer distance (from 300 to 700 m). Tests were carried out with the aim of determining the coefficient of friction of the various laser-textured surfaces. Table 6 lists the conditions for each test.
3. Results and discussion The frictional behavior of textured and non-textured plates was compared. There were significant differences in the frictional behavior of the test samples at low values of the lubrication parameter. At a lubrication parameter of 0.0369, the lubrication regime changed, and the coefficient of friction increased as the pin began to slide on the non-textured plate. The coefficient of friction converged to a value of 0.343 after sliding a distance of 650 m. A similar trend was observed at a lubrication parameter of 0.0972. The post-test oil residue on the non-textured plates revealed that the oil film became mixed with wear particles during the test, and no longer lubricated the sliding movement of the pin against the plate. This shows that the system was in the boundary lubrication regime. The pin slid smoothly on the surface of textured plates for a lubrication parameter of 0.0369, and the coefficient of friction remained at a relatively low value of 0.171. The coefficient of friction converged relatively quickly compared with the case of a Table 2 Specimen specifications. Specimen no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Properties of the dimples Diameter (μm)
Depth (μm)
Depth to diameter ratio
Surface area Distance between consecutive dimples density ratio (%) (μm)
80 80 80 80 80 80 106 106 106 106 106 106 130 130 130 130 130 130
11.2 11.2 11.2 24 24 24 14.84 14.84 14.84 31.8 31.8 31.8 18.2 18.2 18.2 39 39 39
0.14 0.14 0.14 0.30 0.30 0.30 0.14 0.14 0.14 0.30 0.30 0.30 0.14 0.14 0.14 0.30 0.30 0.30
225 158 130 225 158 130 303 212 174 303 212 174 365 256 208 365 256 208
10 20 30 10 20 30 10 20 30 10 20 30 10 20 30 10 20 30
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non-textured plate. The oil films on the textured plates also remained relatively clean, since the dimples were able to retain not only the lubricants, but also any wear particles that were formed during the tests [12,15]. Textured plates with different dimple dimensions followed similar trends for low lubrication parameters. Fig. 7 compares the coefficients of friction between the textured and non-textured plates measured for a lubrication parameter of 0.0369 over a long sliding distance. The textured plates had regular arrays of circular dimples with diameters of 106 μm, depths of 14.84 μm, and surface density ratios of 10%. Tribological measurements of the textured and non-textured plates with different normal loads (from 1.47 to 14.70 N) and different speeds (from 0.063 to 0.340 m/s) were performed to investigate the various ranges of the lubrication parameter. Each test continued until the coefficients of friction converged; the converged coefficient of friction was measured at the end of each test. The results showed that the coefficients of friction converged between 130 m and 250 m for most samples. Fig. 8 shows the differences in frictional behavior between the textured and nontextured plates with respect to the lubrication parameter. These results indicate that all samples followed the trends of previous studies [13,15]. Typical Stribeck curves with a transition from the boundary to mixed lubrication regime were obtained for the nontextured plates as the lubrication parameter decreased, whereas no transition was detected for the textured plates. At low values of the lubrication parameter (0.037 and 0.097), the coefficients of friction of the non-textured plates were high, while the coefficients of friction of the textured plates remained low. As the lubrication parameter increased, the coefficients of friction of the non-textured plates dropped to the level of the textured plates. When the lubrication parameter increased above 1.5, the coefficients of friction of both plates tended to increase slightly. All textured plates followed the same trends. Fig. 9 shows the frictional behavior of the textured plates with different dimple diameters in terms of the lubrication parameter. The tests were carried out with respect to two different parameters: surface area density ratio and depth to diameter ratio. Samples with three different surface area density ratios (10%, 20%, and 30%) and two different depth to diameter ratios (0.14 and 0.30) were investigated. Three different test samples were machined for each combination of surface area density ratio and depth to diameter ratio. The friction tests were repeated with three different samples for each test condition, and the measured coefficients of friction were averaged. Fig. 10 shows the coefficients of friction as functions of the surface area density ratio, summarized from the results of the basic friction tests. At low values of the lubrication parameter (0.037 and 0.097), the difference between the textured and non-textured plates was significant. Similar trends were found for the textured plates with dimple diameters of 106 μm and 130 μm, as shown in Figs. 11 and 12. For moderate values of the lubrication parameter (0.462, 0.924, and 1.382), no significant differences between the non-textured and textured plates were observed. At high values of the lubrication parameter (1.848 and 2.310), no
Table 3 Laser-processing parameters for each specimen. Laser processing no.
1 2 3 4 5 6
Properties of the dimples
Processing properties
Diameter (μm)
Depth (μm)
Aperture (μm)
Beam Expander
Frequency (kHz)
Current (A)
Pulse per dot
80 80 106 106 130 130
11.2 24 14.84 31.8 18.2 39
2.0 2.0 2.3 2.0 2.6 2.6
4 4 4 4 3 3
10 10 10 10 10 10
24.5 24.5 23.5 28 23 23
14 26 17 26 22 42
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Fig. 4. Diagram of the laser-beam machining setup.
Fig. 5. Scanning laser microscopy 3D images of microdimples created on the disks. (a) Scanned image of a dimple and (b) contours of the scanned image.
Table 4 Dimple dimensions. Dimple type
Type #1
Type #2
Type #3
Target dimension (μm)
Diameter Depth Depth Diameter Depth Depth Diameter Depth Depth
80 11.2 24 106 14.84 31.8 130 18.2 39
Random measurements results (μm) Average
Std. deviation
81.25 10.64 24.08 105.6 15.43 31.67 132.8 19.34 38.58
2.20 1.63 2.15 2.35 1.93 2.76 2.77 3.29 1.63
differences between the test samples were identified. When the lubrication parameter was high and the depth to diameter ratio was 0.3, the textured plates exhibited slightly higher coefficients of friction than the non-textured plates, whereas the textured plates with a depth to diameter ratio of 0.14 maintained similar coefficients of friction. This phenomenon may result from the fluid dynamics of the oil near the dimples [16,20,22]. The results for the textured plates with dimple diameters of 106 μm and 130 μm followed trends similar to those observed for a dimple diameter of 80 μm at moderate and high lubrication parameter values. Samples with a surface area density ratio of 10% exhibited the lowest coefficients of friction
among the textured plates under many test conditions. However, the differences were insignificant. The curves in each graph were obtained at seven different lubrication parameters: 0.037, 0.097, 0.462, 0.924, 1.382, 1.848, and 2.310. The test results at low values of the lubrication parameter (0.037 and 0.097) indicated little dependency on the depth to diameter ratio, unlike the test results at moderate and high lubrication parameter values (0.462–2.310). In this regime of hydrodynamic lubrication parameters (where elastohydrodynamic or hydrodynamic lubrication applies), the textured plates with a depth to diameter ratio of 0.14 had lower coefficients of friction than those with a depth to diameter ratio of 0.30. Similar results were obtained for the textured plates with dimple diameters of 106 μm and 130 μm. Fig. 13 compares the measured coefficients of friction at a high lubrication parameter value (2.310) sorted by the depth to diameter ratio, while Fig. 14 compares those sorted by the surface area density ratio. In the high lubrication parameter regime, the variation due to the depth-to-diameter ratio was greater than that due to the surface area density. This is consistent with the results of Ronen et al. [16], where the simulated optimum depth-to-diameter ratio of textured plates was found to be near 0.14 in the high-lubrication parameter regime. The surface area density was also found to have little effect on the coefficient of friction, which is also consistent with the simulation analysis reported in Ref. [16]. The coefficient of friction of was more strongly related to the dimensions of the dimples than to the
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Fig. 6. SEM images of the textured surfaces. (a) 1 magnification and (b) 100 magnification.
Table 5 Oil properties. Property
Mineral oil (CAS 8042-47-5)
Density at 15 1C (kg/m3) Viscosity at 40 1C (mm2/s) Vapor pressure at 20 1C (mmHg)
877 65–75 0.001
Table 6 Measurement conditions. Test no.
1 2 3 4 5 6 7
Parameters Speed (m/s)
Load (N)
Lubrication parameter
0.0628 0.0628 0.262 0.262 0.392 0.272 0.340
14.710 5.590 4.903 2.452 2.452 1.275 1.275
0.0369 0.0972 0.462 0.924 1.382 1.848 2.310
Fig. 8. Coefficients of friction of the textured and non-textured plates vs. the lubrication parameter.
Fig. 9. Coefficients of friction of the textured plates with various texturing conditions vs. the lubrication parameter.
Fig. 7. Coefficients of friction of the textured and non-textured plates during the long range sliding.
surface area density of the dimples. This may result from the fluid dynamics of the oil near the dimples [16,20,22].
4. Conclusion Tribological measurements with various geometric parameters were performed on textured plates to investigate the effect of
texturing on friction reduction. A comparison between textured and non-textured plates showed that textured plates maintained elastohydrodynamic or mixed lubrication regimes longer than non-textured plates under a given lubrication condition. Friction test revealed that the measured coefficients of friction for the non-textured plates, plotted as functions of the lubrication parameter, took the form of typical Stribeck curves with a transition from the boundary to mixed lubrication regime, whereas for the textured plates, low coefficients of friction were maintained throughout the entire range of lubrication parameter values. The friction tests showed that the textured plates had markedly reduced coefficients of friction at low values of the lubrication parameter, regardless of dimple diameter. However, texturing had little effect on the coefficients of friction at moderate and high values of the lubrication parameter.
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Fig. 10. Coefficients of friction of the textured plates (dimple diameter¼80 μm) vs. surface area density for various lubrication parameters at depth to diameter ratio of (a) 0.14, (b) 0.30.
Fig. 12. Coefficients of friction of the textured plates (dimple diameter¼130 μm) vs. surface area density for various lubrication parameters at depth to diameter ratio of (a) 0.14, (b) 0.30.
Fig. 13. Comparison of coefficients of friction from two different depth to diameter ratios at a high lubrication parameter value for various surface area densities.
Fig. 11. Coefficients of friction of the textured plates (dimple diameter¼ 106 μm) vs. surface area density for various lubrication parameters at depth to diameter ratio of (a) 0.14, (b) 0.30.
Friction tests were performed to investigate the effect of the depth to diameter ratio, based on two different values of the ratio. At a high lubrication parameter value, the textured plates with a depth to diameter ratio of 0.14 had lower coefficients of friction than those with a depth to diameter ratio of 0.30, which was predicted in the previous study of Ronen et al. [16]. At low values of the lubrication parameter, the depth to diameter ratio did not significantly affect the frictional behavior. For surface area density ratio, only a marginal effect on frictional behavior was observed. Under some lubrication parameter conditions, the surface area density ratio had no apparent effect on the coefficients of friction. Under these conditions, the effect of the depth to diameter ratio was greater than that of the surface area density ratio. This also was in agreement with the numerical predictions of Ronen et al. [16].
B. Kim et al. / Tribology International 70 (2014) 128–135
Fig. 14. Comparison of coefficients of friction from three different surface area densities at a high lubrication parameter value for two different depth to diameter ratios.
In this study, a basic tribological study of the effect of surface texturing was provided for applications in automobile engines. Advanced experiments considering operating conditions, as described in Refs. [17–19,21], may provide more detailed data for application purposes in automobile engines. Acknowledgement This work was supported by the 2011 Inje University research grant. References [1] Priest M, Taylor CM. Automobile engine tribology-approaching the surface. Wear 2000;241:193–203. [2] Etsion I, Kligerman Y, Halperin G. Analytical and experimental investigation of lase-textured mechanical seal faces. Tribol Trans 1999;42:511–6. [3] Etsion I, Halperin G. A laser surface textured hydrostatic mechanical seal. Tribol Trans 2002;45:430–4. [4] Etsion I, Halperin G, Brizmer V, Kligerman Y. Experimental investigation of laser surface textured parallel thrust bearings. Tribol Lett 2004;17:295–300.
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[5] Dumitru G, Romano V, Weber HP, Haefke H, Gerbig Y, Pflueger E. Laser microstructuring of steel surfaces for tribological applications. Appl Phys A 2000;70:485–7. [6] Menezes PL, Kishore SV, Kailas. Influence of surface texture on coefficient of friction and transfer layer formation during sliding of pure magnesium pin on 080 M40 (EN8) steel plate. Wear 2006;261:578–91. [7] Wang X, Kato K, Adachi K, Aizawa K. The effect of laser texturing of SiC surface on the critical load for the transition of water lubrication mode from hydrodynamic to mixied. Tribol Int 2001;34:703–11. [8] Wang X, Kato K, Adachi K, Aizawa K. Loads carrying capacity map for the surface texture design of SiC thrust bearing sliding in water. Tribol Int 2003;36:189–97. [9] Andersson P, Koskinen J, Varjus S, Gerbig Y, Haefke H, Georgiou S, et al. Microlubrication effect by laser-textured steel surfaces. Wear 2007;262:369–79. [10] Wakuda M, Yamauchi Y, Kanzaki S, Yasuda Y. Effect of surface texturing on friction reduction between ceramic and steel materials under lubricated sliding contact. Wear 2003;254 (369–363). [11] Pettersson U, Jacobson S. Textured surfaces for improved lubrication at high pressure and low sliding speed of roller/piston in hydraulic motors. Tribol Int 2007;40:355–9. [12] Nakano M, Korenaga A, Korenaga A, Miyake K, Murakami T, Ando Y, et al. Applying micro-texture to cast iron surfaces to reduce the friction coefficient under lubricated conditions. Tribol Lett 2007;28:131–7. [13] Kovalchenko A, Ajayi O, Erdemir A, Fenske G, Etsion I. The effect of laser surface texturing on transitions in lubrication regimes during unidirectional sliding contact. Tribol Int 2005;38:219–25. [14] Voevodin AA, Zabinski JS. Laser surface texturing for adaptive solid lubrication. Wear 2006;261:1285–92. [15] Borghi A, Gualtieri E, Marchetto D, Moretti L, Valeri S. Tribological effects of surface texturing on nitriding steel for high-performance engine applications. Wear 2008;265:1046–51. [16] Ronen A, Kligerman Y, Etsion I. Friction-reducing surface-texturing in reciprocating automotive components. Tribol Trans 2001;44:359–66. [17] Ryk G, Etsion I. Testing piston rings with partial laser surface texturing for friction reduction. Wear 2006;261:792–6. [18] Ryk G, Kligerman Y, Etsion I. Experimental investigation of laser surface texturing for reciprocating automotive components. Tribol. Trans. 2002;45:444–9. [19] Ryk G, Kligerman Y, Etsion I, Shinkarenko A. Experimental investigation of partial laser surface texturing for piston-ring friction reduction. Tribol Trans 2005;48:583–8. [20] Ren N, Nanbu T, Yasuda Y, Zhu D, Wang Q. Micro textures in concentratedconformal-contact lubrication: effect of distribution patterns. Tribol Lett 2007;28:275–85. [21] Golloch R, Merker GP, Kessen U, Brinkmann S. Functional properties of microstructured cylinder liner surfaces for internal combustion engines. Tribotest 2005;11:295–388. [22] Park TJ, Hwang YG, Sohn JD, Chung HG. CFD analysis of an infinitely long slider bearing with two-dimensional micro-pockets. J KSTLE 2009;25:43–8. [23] ASTM Ed., Standard test method for wear testing with a pin-on-disk apparatus, Annual Book of ASTM Standards G99 (2006).
Contents lists available at ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
Effects of surface texturing on the frictional behavior of cast iron surfaces Beomkeun Kim a,n, Young Hun Chae b, Heung Soap Choi c a
High Safety Vehicle Core Technology Research Center, Inje University, 607 Obang-Dong, Gimhae, Gyeongnam 621-749, Republic of Korea Department of Mechanical Engineering, Kyungpook National University, 1370 Sankyuk-Dong, Pook-Gu, Daegu 702-701, Republic of Korea c Department of Mechanical and Design Engineering, Hongik University, 2639 Sejong-Ro, Jochiwon-Eup, Sejong 339-701, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 19 June 2013 Received in revised form 1 October 2013 Accepted 8 October 2013 Available online 15 October 2013
The purpose of this study is to investigate the effects of the geometry and distribution of microdimples on the frictional behavior of surfaces for applications in automotive engines. A square array of microscale circular dimples was selected as the texture pattern. A laser beam was used to create microdimples with various dimensions on cast iron surfaces. Frictional tests were performed with selected loads and speeds to simulate the operation conditions of automotive engine parts. The effects of dimple distribution were also investigated. The aspect ratio of the dimples was found to be the most significant factor, while the effect of the surface density of the dimples on the coefficient of friction was found to be only marginal. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Surface texture Laser machining Coefficient of friction Depth to diameter ratio
1. Introduction The increasing demand for environmentally friendly automobiles has led to much research interest in improving the fuel efficiency of automotive engines. Because a significant proportion of the energy in an automotive engine is consumed by frictional losses, the tribology of the mechanical parts is an important factor in determining the engine efficiency. Automotive engine parts experience more than one regime of lubrication during their operation. For example, the lubrication regimes associated with a cam and follower are boundary, mixed and elastohydrodynamic [1]. The lubrication regimes for typical automotive engine parts are shown in Fig. 1 [1]. Modifying the lubrication conditions for a given automotive engine part may potentially improve the efficiency of the system. Vehicle performance could also be improved by utilizing reduced-friction engines. The effects of dimples on a surface were first reported by Etsion et al. [2–4], who investigated the effect of surface texturing and the optimal geometric arrangement of the surface features, both theoretically and experimentally, for water-lubricated sealing ring applications. They found that surface texturing substantially increased the load- carrying capacity of bearing surfaces. It is also well known that surface texturing of solid surfaces modifies the frictional and wear properties. Dumitru et al. [5] investigated the
n
Corresponding author. Tel.: þ 82 11 296 3816; fax: þ 82 55 320 3439. E-mail address: [email protected] (B. Kim).
0301-679X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.triboint.2013.10.006
effect of surface textures using tribological tests on laserstructured substrates. They reported that the lifetimes of laserprocessed samples increased significantly in comparison with unstructured samples. Menezes et al. [6] studied the effects of surface roughness and textures on the coefficient of friction; however, they only examined the effect of the existence of dimples, and did not investigate their geometry. Wang et al. [7,8] investigated the effects of microdimples on the loadcarrying capacity of SiC thrust bearings. The contact surface was textured by reactive ion etching, and water lubrication was used in the tests. They showed that the textured surface was able to maintain hydrodynamic lubrication for a longer time than the non-textured surface. Andersson et al. [9] demonstrated the benefits of laser-textured tool surfaces for friction reduction. They found that laser-textured patterns significantly reduced friction and wear under lubricated sliding conditions. They performed experiments using three different types of oil, but did not include sufficient variation in the dimensions of the dimples to investigate the effect of their geometry. Wakuda et al. [10] studied the frictional properties of dimpled silicon nitride ceramic surfaces against steel. They evaluated samples with a variety of microdimple dimensions, under boundary or mixed lubrication conditions, and observed that there was a critical dimple size, below which the texturing was ineffective. Petterson and Jacobson [11] performed friction tests on steel surfaces textured with a diamond tool. They reported that the coefficients of friction of the textured surfaces indicated improved lubrication at high pressure and low sliding speed. Nakano et al. [12] investigated the relationship between texture geometry and
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All of these studies indicate that textured surfaces (whether laser, chemically or mechanically structured) offer the benefit of reduced coefficients of friction, not only in the hydrodynamic regime, but also in the mixed or boundary lubrication regime. This leads to decreased wear on solid surfaces. However, to date there has been insufficient investigation of the effect of the dimple geometry on the tribological behavior for applications in automobile engines. The purpose of this study was to investigate the effects of the geometry and distribution of microdimples on the tribological behavior of surfaces for applications in automobile engines. Surface texturing has proved to be an effective way of improving the tribological behavior of contact surfaces. As found in previous studies, surface texturing increased load-carrying capacity [2–4]. While this aspect of surface texturing is relevant to automotive engine parts, the scope of this study was limited to friction reduction. In this study, the friction-modifying effects of texturing were examined, and relationships between the geometry and the distribution of microdimples and the coefficients of friction were investigated for the different sliding velocity, viscosity of lubricant and contact pressure, with a focus on piston rings, cam followers, and engine bearings. Microtextured cast iron surfaces were prepared by laser processing, and experiments with various loads and speeds were conducted under a thin layer of oil (by removing excess oil from the surface) to simulate the operating conditions of automotive engine parts.
Fig. 1. A modified Stribeck curve associated with automotive engine parts [1].
2. Experiments 2.1. Test equipment
coefficients of friction by testing microtextured surfaces prepared by shot blasting. They found that a dimpled pattern reduced the coefficients of friction, whereas a grooved pattern increased them. Kovalchenko et al. [13] studied the impact of laser surface texturing on lubricating regime transitions. They observed that laser surface texturing could reduce the friction on components operating in a boundary-lubricated regime. This study included different sized dimples, but focused on the tribological behavior rather than the effects of the dimple dimensions. Voevodin [14] explored the applications of laser-texturing the surfaces of hard TiCN coatings with adaptive solid lubrication. Tests revealed increased wear life for specimens employing the dimple reservoir concept. Borghi et al. [15] investigated the effects of surface texturing on nitride steel. They showed that texturing the surface reduced the coefficient of friction, and avoided the transition from the hydrodynamic regime to the mixed or boundary lubrication regime, resulting in an increase in the wear lifetime. That study was limited to specimens where the dimple dimensions were not varied. Etsion et al. [16–19] conducted experimental and theoretical studies to investigate the friction-reducing effects of surface texturing on automotive components. They suggested using textured surfaces for piston rings to achieve a reduction in the coefficient of friction, and they conducted experimental investigations to support their theoretical estimates of the effect of surface texturing using a novel test rig for the reciprocating automotive components. Ren et al. [20] conducted numerical analyses to determine the effect of the distribution of shallow dimples on the tribological properties of a surface. Golloch et al. [21] investigated the benefits of laser-structured cylinder liners by carrying out a series of tests on a single-cylinder diesel engine. They found that a laser-structured liner not only reduced frictional losses and oil consumption, but also decreased cylinder wear. Park et al. performed computational fluid dynamics (CFD) analyses to investigate the lubrication characteristics of an infinitely long slider bearing with dimples, and found that the pressure distribution was strongly related to the dimple dimensions [22].
Friction tests were carried out using a pin-on-disk tribometer [23]. The upper pin was loaded against the surface of a flat disk specimen. The loading direction was parallel to the axis of rotation, and the magnitude of the normal force was controlled by adding dead weights on the upper pin. The frictional force was recorded using strain gauges attached to the holder. The coefficient of friction was determined using these two force values. A schematic diagram of the tribometer and the test setup is shown in Fig. 2. The speed of the test was controlled by changing the rotational speed of the disk or adjusting the length of the arm holding the pin. 2.2. Test specimens The pin and disks were prepared from cast iron, which is suitable material for automotive engine parts such as liners, piston rings, or bearings. Table 1 shows an elemental analysis of the cast iron material used in this study. One end of the pin was machined as a sphere with a diameter of 8 mm and a flat area (Fig. 3). The other end was machined so that it could be attached to the arm of the tribometer. The final flat contact area created on the pin was 3.3 mm in diameter. The surface roughness of the contact area of the pin was 0.8 μm Rz. The disk specimens were also made from cast iron, each with a diameter of 60 mm and a thickness of 5 mm. Surface textures, consisting of microdimples of various sizes and depths, were fabricated on the disks by laser-beam machining. Table 2 lists the surface texturing specifications of the various specimens used in this study. The surface area to density ratio (total dimple area/total surface area) ranged from 10% to 30%. The diameters and depths of the microdimples were controlled by adjusting the pulse energy of the laser. A YAG laser (AHTSE010T model, λ¼1064 nm, tpulse E15 ns) was used to create the microdimples on the disks. By trial and error under various laserprocessing conditions, appropriate laser parameters were established to generate the desired pattern sizes. Table 3 lists the laser-processing parameters and the corresponding dimple sizes. For the cast iron
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Fig. 2. Equipment setup for the tribology test. Table 1 Elemental analysis of the FC 25 cast iron used in this study. Elemental composition (wt%)
C 3.16
Si 2.35
Mn 0.66
P 0.21
S 0.083
Cr 0.19
Cu 0.25
B 0.065
Fe Bal.
Fig. 3. Pin and disk specimens. (a) Pin, (b) Disk.
material, a pulse energy of 24.4 W was used to create dimples with a diameter of 106 μm and a depth of 14.84 μm. Each disk specimen was moved laterally via a high-precision XY-translation stage. A diagram of the laser-beam machining setup is shown in Fig. 4. After laser machining, the craters that formed around the microdimples were removed by manual grinding with abrasive emery paper. Polishing with alpha alumina powder followed the grinding process, resulting in a surface roughness of 0.8 μm Rz. The roughness of the textured surfaces was measured on the upper area between the dimples. Fig. 5 shows threedimensional (3D) scanning laser microscopy images of the microdimples created on the cast iron surfaces. Table 4 lists measurement results for five randomly selected laser-machined dimples after polishing. The diameters of the dimples were close to the target dimensions, but the deviations from the target dimensions were greater for the depths of the dimples than for the diameters. Fig. 6 shows scanning electron microscopy (SEM) images of the textured surfaces.
2.3. Experimental procedures Tribological tests of 145 specimens were conducted using the pin-on-disk tribometer. Prior to each test, the pins and disks were sonicated in alcohol to remove metal fragments and oil from the surface. Oil was applied to the top surface of each disk, and excess oil was removed using a rubber blade under constant pressure. The properties of the oil used in this study, which did not contain any extreme pressure additives, are listed in Table 5. The measurements were started immediately after the oil was applied, and continued until the coefficient of friction converged. Non-textured specimens were used, as well as specimens with circular dimples. The duration of the measurements were determined by considering the type of specimen and the lubrication parameters (i.e., sliding velocity viscosity/contact pressure). Here, the contact pressure was the total normal force between the mating surfaces and can be considered as the load-carrying capacity. The coefficients of friction converged over a relatively
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short distance (from 130 to 250 m) for the textured specimens. For the non-textured specimens, however, the coefficients of friction only converged over a short distance at the high and mid-range of the lubrication parameters (0.462 to 2.310); at the low range of the lubrication parameters (0.0369 to 0.0972), they converged over a longer distance (from 300 to 700 m). Tests were carried out with the aim of determining the coefficient of friction of the various laser-textured surfaces. Table 6 lists the conditions for each test.
3. Results and discussion The frictional behavior of textured and non-textured plates was compared. There were significant differences in the frictional behavior of the test samples at low values of the lubrication parameter. At a lubrication parameter of 0.0369, the lubrication regime changed, and the coefficient of friction increased as the pin began to slide on the non-textured plate. The coefficient of friction converged to a value of 0.343 after sliding a distance of 650 m. A similar trend was observed at a lubrication parameter of 0.0972. The post-test oil residue on the non-textured plates revealed that the oil film became mixed with wear particles during the test, and no longer lubricated the sliding movement of the pin against the plate. This shows that the system was in the boundary lubrication regime. The pin slid smoothly on the surface of textured plates for a lubrication parameter of 0.0369, and the coefficient of friction remained at a relatively low value of 0.171. The coefficient of friction converged relatively quickly compared with the case of a Table 2 Specimen specifications. Specimen no.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Properties of the dimples Diameter (μm)
Depth (μm)
Depth to diameter ratio
Surface area Distance between consecutive dimples density ratio (%) (μm)
80 80 80 80 80 80 106 106 106 106 106 106 130 130 130 130 130 130
11.2 11.2 11.2 24 24 24 14.84 14.84 14.84 31.8 31.8 31.8 18.2 18.2 18.2 39 39 39
0.14 0.14 0.14 0.30 0.30 0.30 0.14 0.14 0.14 0.30 0.30 0.30 0.14 0.14 0.14 0.30 0.30 0.30
225 158 130 225 158 130 303 212 174 303 212 174 365 256 208 365 256 208
10 20 30 10 20 30 10 20 30 10 20 30 10 20 30 10 20 30
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non-textured plate. The oil films on the textured plates also remained relatively clean, since the dimples were able to retain not only the lubricants, but also any wear particles that were formed during the tests [12,15]. Textured plates with different dimple dimensions followed similar trends for low lubrication parameters. Fig. 7 compares the coefficients of friction between the textured and non-textured plates measured for a lubrication parameter of 0.0369 over a long sliding distance. The textured plates had regular arrays of circular dimples with diameters of 106 μm, depths of 14.84 μm, and surface density ratios of 10%. Tribological measurements of the textured and non-textured plates with different normal loads (from 1.47 to 14.70 N) and different speeds (from 0.063 to 0.340 m/s) were performed to investigate the various ranges of the lubrication parameter. Each test continued until the coefficients of friction converged; the converged coefficient of friction was measured at the end of each test. The results showed that the coefficients of friction converged between 130 m and 250 m for most samples. Fig. 8 shows the differences in frictional behavior between the textured and nontextured plates with respect to the lubrication parameter. These results indicate that all samples followed the trends of previous studies [13,15]. Typical Stribeck curves with a transition from the boundary to mixed lubrication regime were obtained for the nontextured plates as the lubrication parameter decreased, whereas no transition was detected for the textured plates. At low values of the lubrication parameter (0.037 and 0.097), the coefficients of friction of the non-textured plates were high, while the coefficients of friction of the textured plates remained low. As the lubrication parameter increased, the coefficients of friction of the non-textured plates dropped to the level of the textured plates. When the lubrication parameter increased above 1.5, the coefficients of friction of both plates tended to increase slightly. All textured plates followed the same trends. Fig. 9 shows the frictional behavior of the textured plates with different dimple diameters in terms of the lubrication parameter. The tests were carried out with respect to two different parameters: surface area density ratio and depth to diameter ratio. Samples with three different surface area density ratios (10%, 20%, and 30%) and two different depth to diameter ratios (0.14 and 0.30) were investigated. Three different test samples were machined for each combination of surface area density ratio and depth to diameter ratio. The friction tests were repeated with three different samples for each test condition, and the measured coefficients of friction were averaged. Fig. 10 shows the coefficients of friction as functions of the surface area density ratio, summarized from the results of the basic friction tests. At low values of the lubrication parameter (0.037 and 0.097), the difference between the textured and non-textured plates was significant. Similar trends were found for the textured plates with dimple diameters of 106 μm and 130 μm, as shown in Figs. 11 and 12. For moderate values of the lubrication parameter (0.462, 0.924, and 1.382), no significant differences between the non-textured and textured plates were observed. At high values of the lubrication parameter (1.848 and 2.310), no
Table 3 Laser-processing parameters for each specimen. Laser processing no.
1 2 3 4 5 6
Properties of the dimples
Processing properties
Diameter (μm)
Depth (μm)
Aperture (μm)
Beam Expander
Frequency (kHz)
Current (A)
Pulse per dot
80 80 106 106 130 130
11.2 24 14.84 31.8 18.2 39
2.0 2.0 2.3 2.0 2.6 2.6
4 4 4 4 3 3
10 10 10 10 10 10
24.5 24.5 23.5 28 23 23
14 26 17 26 22 42
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Fig. 4. Diagram of the laser-beam machining setup.
Fig. 5. Scanning laser microscopy 3D images of microdimples created on the disks. (a) Scanned image of a dimple and (b) contours of the scanned image.
Table 4 Dimple dimensions. Dimple type
Type #1
Type #2
Type #3
Target dimension (μm)
Diameter Depth Depth Diameter Depth Depth Diameter Depth Depth
80 11.2 24 106 14.84 31.8 130 18.2 39
Random measurements results (μm) Average
Std. deviation
81.25 10.64 24.08 105.6 15.43 31.67 132.8 19.34 38.58
2.20 1.63 2.15 2.35 1.93 2.76 2.77 3.29 1.63
differences between the test samples were identified. When the lubrication parameter was high and the depth to diameter ratio was 0.3, the textured plates exhibited slightly higher coefficients of friction than the non-textured plates, whereas the textured plates with a depth to diameter ratio of 0.14 maintained similar coefficients of friction. This phenomenon may result from the fluid dynamics of the oil near the dimples [16,20,22]. The results for the textured plates with dimple diameters of 106 μm and 130 μm followed trends similar to those observed for a dimple diameter of 80 μm at moderate and high lubrication parameter values. Samples with a surface area density ratio of 10% exhibited the lowest coefficients of friction
among the textured plates under many test conditions. However, the differences were insignificant. The curves in each graph were obtained at seven different lubrication parameters: 0.037, 0.097, 0.462, 0.924, 1.382, 1.848, and 2.310. The test results at low values of the lubrication parameter (0.037 and 0.097) indicated little dependency on the depth to diameter ratio, unlike the test results at moderate and high lubrication parameter values (0.462–2.310). In this regime of hydrodynamic lubrication parameters (where elastohydrodynamic or hydrodynamic lubrication applies), the textured plates with a depth to diameter ratio of 0.14 had lower coefficients of friction than those with a depth to diameter ratio of 0.30. Similar results were obtained for the textured plates with dimple diameters of 106 μm and 130 μm. Fig. 13 compares the measured coefficients of friction at a high lubrication parameter value (2.310) sorted by the depth to diameter ratio, while Fig. 14 compares those sorted by the surface area density ratio. In the high lubrication parameter regime, the variation due to the depth-to-diameter ratio was greater than that due to the surface area density. This is consistent with the results of Ronen et al. [16], where the simulated optimum depth-to-diameter ratio of textured plates was found to be near 0.14 in the high-lubrication parameter regime. The surface area density was also found to have little effect on the coefficient of friction, which is also consistent with the simulation analysis reported in Ref. [16]. The coefficient of friction of was more strongly related to the dimensions of the dimples than to the
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Fig. 6. SEM images of the textured surfaces. (a) 1 magnification and (b) 100 magnification.
Table 5 Oil properties. Property
Mineral oil (CAS 8042-47-5)
Density at 15 1C (kg/m3) Viscosity at 40 1C (mm2/s) Vapor pressure at 20 1C (mmHg)
877 65–75 0.001
Table 6 Measurement conditions. Test no.
1 2 3 4 5 6 7
Parameters Speed (m/s)
Load (N)
Lubrication parameter
0.0628 0.0628 0.262 0.262 0.392 0.272 0.340
14.710 5.590 4.903 2.452 2.452 1.275 1.275
0.0369 0.0972 0.462 0.924 1.382 1.848 2.310
Fig. 8. Coefficients of friction of the textured and non-textured plates vs. the lubrication parameter.
Fig. 9. Coefficients of friction of the textured plates with various texturing conditions vs. the lubrication parameter.
Fig. 7. Coefficients of friction of the textured and non-textured plates during the long range sliding.
surface area density of the dimples. This may result from the fluid dynamics of the oil near the dimples [16,20,22].
4. Conclusion Tribological measurements with various geometric parameters were performed on textured plates to investigate the effect of
texturing on friction reduction. A comparison between textured and non-textured plates showed that textured plates maintained elastohydrodynamic or mixed lubrication regimes longer than non-textured plates under a given lubrication condition. Friction test revealed that the measured coefficients of friction for the non-textured plates, plotted as functions of the lubrication parameter, took the form of typical Stribeck curves with a transition from the boundary to mixed lubrication regime, whereas for the textured plates, low coefficients of friction were maintained throughout the entire range of lubrication parameter values. The friction tests showed that the textured plates had markedly reduced coefficients of friction at low values of the lubrication parameter, regardless of dimple diameter. However, texturing had little effect on the coefficients of friction at moderate and high values of the lubrication parameter.
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Fig. 10. Coefficients of friction of the textured plates (dimple diameter¼80 μm) vs. surface area density for various lubrication parameters at depth to diameter ratio of (a) 0.14, (b) 0.30.
Fig. 12. Coefficients of friction of the textured plates (dimple diameter¼130 μm) vs. surface area density for various lubrication parameters at depth to diameter ratio of (a) 0.14, (b) 0.30.
Fig. 13. Comparison of coefficients of friction from two different depth to diameter ratios at a high lubrication parameter value for various surface area densities.
Fig. 11. Coefficients of friction of the textured plates (dimple diameter¼ 106 μm) vs. surface area density for various lubrication parameters at depth to diameter ratio of (a) 0.14, (b) 0.30.
Friction tests were performed to investigate the effect of the depth to diameter ratio, based on two different values of the ratio. At a high lubrication parameter value, the textured plates with a depth to diameter ratio of 0.14 had lower coefficients of friction than those with a depth to diameter ratio of 0.30, which was predicted in the previous study of Ronen et al. [16]. At low values of the lubrication parameter, the depth to diameter ratio did not significantly affect the frictional behavior. For surface area density ratio, only a marginal effect on frictional behavior was observed. Under some lubrication parameter conditions, the surface area density ratio had no apparent effect on the coefficients of friction. Under these conditions, the effect of the depth to diameter ratio was greater than that of the surface area density ratio. This also was in agreement with the numerical predictions of Ronen et al. [16].
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Fig. 14. Comparison of coefficients of friction from three different surface area densities at a high lubrication parameter value for two different depth to diameter ratios.
In this study, a basic tribological study of the effect of surface texturing was provided for applications in automobile engines. Advanced experiments considering operating conditions, as described in Refs. [17–19,21], may provide more detailed data for application purposes in automobile engines. Acknowledgement This work was supported by the 2011 Inje University research grant. References [1] Priest M, Taylor CM. Automobile engine tribology-approaching the surface. Wear 2000;241:193–203. [2] Etsion I, Kligerman Y, Halperin G. Analytical and experimental investigation of lase-textured mechanical seal faces. Tribol Trans 1999;42:511–6. [3] Etsion I, Halperin G. A laser surface textured hydrostatic mechanical seal. Tribol Trans 2002;45:430–4. [4] Etsion I, Halperin G, Brizmer V, Kligerman Y. Experimental investigation of laser surface textured parallel thrust bearings. Tribol Lett 2004;17:295–300.
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