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Oil retaining capability and sliding friction behaviour of porous copper with elongated cylindrical pores
Journal of Materials Processing Technology 212 (2012) 1796–1801
Contents lists available at SciVerse ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Oil retaining capability and sliding friction behaviour of porous copper with elongated cylindrical pores Hao Du ∗ , Jianzhong Qi, Yuanxia Lao, Tianying Xiong Division of Surface Engineering of Materials, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China
a r t i c l e
i n f o
Article history: Received 15 January 2012 Received in revised form 19 March 2012 Accepted 26 March 2012 Available online 3 April 2012 Keywords: Porous copper Solidification Oil content Sliding friction behaviour
a b s t r a c t Porous copper with elongated cylindrical pores aligned either axially or radially was fabricated under a high pressure of mixture gas of hydrogen and argon. Structure characterization indicated that pore size increased, pore density decreased, pore size distribution became wider with an increase in porosity for the porous copper. The dependence of oil retaining capability and sliding friction coefficient on porosity and pore size of the porous copper were investigated. It was found that the oil content of the porous copper depended mainly on the porosity, and reached 27.6% on the specimen with a porosity of 47.1%. On the other hand, the oil efficiency was not satisfactory, and became worse when the porosity increased, which was attributed to the increase in pore size and the wider pore size distribution for the porous copper. It was proven that the impregnated oil in the pores played an important role in improving the sliding friction behaviour of the porous copper. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Porous metal has been expected to be used in construction, automotive and aerospace applications from the viewpoint of both energy conservation and environmental preservation (Banhart, 2001). However, pores formed in the metal by typical methods such as foaming or powder sintering are round or irregular, acting as stress promoters, as reported by Hyun et al. (2001). As a result, the traditional porous metals break up easily under stress, which restricts their application, especially for construction purposes. A new method for producing porous metals with elongated cylindrical pores ordered regularly was reported and called as gasar by Shapovalov (1993). In this method, the gas solubility difference between in liquid and in solid is utilized for the gas pore formation during solidification of the porous metals. Compared with the traditional fabrication techniques, this method allows an effective control of porosity, pore morphology and pore aligning direction for porous metals, reported by Nakajima (2007). Generally, the elongated cylindrical pores align axially or radially in the porous metal, which depends on the solidification direction. Nakajima (2007) named it as ‘lotus-type porous metal’ when pores aligned axially in the porous metal for its appearance as a lotus root on the cross section. It has been reported by Simone and Gibson (1997), Hyun and Nakajima (2003), and Hyun et al.
∗ Corresponding author. E-mail address: [email protected] (H. Du). 0924-0136/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2012.03.024
(2004) that gasar metals or lotus-type porous metals even with considerable porosity exhibited much superior strengths (mechanical properties) than those formed by other techniques, such as foaming or powder sintering, which is promising to extend their application especially when higher mechanical properties are necessary. It is possible to fill the pores in the porous metals with materials of different compositions for various applications. Loutfy et al. (2002) investigated the technical feasibility of using several porous matrices with elongated cylindrical pores and abrasives as filler for the development of high performance brake components. They reported that porous cast iron and steel exhibited high friction coefficient, perfect stability and low wear, and it was also possible to control the braking abilities of the porous matrix filled with abrasives by altering the matrix porosity and the abrasive composition. Du et al. (2010a) reported that porous copper with elongated cylindrical pores aligned radially possessed a good oil content, which was uniform even if there was a gradient in the pore structure. However, further investigation about the oil content and the effect of a lubricating oil as the filler on friction behaviour of the porous copper is not available. The purpose in the present work is to investigate the relationship among pore structure, oil retaining capability and sliding behaviour for the porous copper with elongated cylindrical pores, considering its potential application on self-lubricating components. To achieve this goal, porous copper with elongated pores aligned either axially or radially was fabricated under different partial pressures of hydrogen and argon for altering pore structure, by our previous work (Du et al., 2010b). The oil retaining
H. Du et al. / Journal of Materials Processing Technology 212 (2012) 1796–1801
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Fig. 1. Optical images of the porous copper rings: (a) with radial pores, (b) with axial pores.
capability was evaluated by an impregnation method. The sliding friction behaviour of the corresponding porous copper rings with axial pores impregnated with oil was evaluated and compared to that of as-fabricated rings, as well as that of bored copper rings from nonporous copper. The effect of pore structure on oil retaining capability and sliding friction behaviour is discussed for the porous copper. 2. Experimental procedure The porous copper with elongated cylindrical pores aligned axially or radially, and a nonporous copper were fabricated by a vacuum-assisted and pressurized casting apparatus, which was previously described in detail (Du et al., 2010a). Two cooling units were employed with either bottom or lateral side of the mold being cooled down. In the case of porous copper with axial pores, the mold with a thin ceramic wall adjacent to the lateral side was directionally cooled from the bottom. In the case of porous copper with radial pores, the mold was cooled from the lateral wall, and there was a thick alumina plate under the bottom for a minimal heat loss via the bottom. High purity copper (99.99 wt.%) was melted in the crucible by middle frequency heating after the chamber was evacuated to 5.0 Pa. When the temperature reached 1523 K, monitored by a W-5Re/W-26Re thermocouple, high purity hydrogen and argon were introduced into the chamber, respectively. The partial pressures of hydrogen and argon for each specimen are listed in Table 1. For each condition, two porous copper specimens with either axial or radial pores were fabricated. In order to make hydrogen dissolve uniformly in liquid copper, each pressurized condition was maintained for 1.8 ks at the selected temperature. In addition, nonporous copper was fabricated in a 0.5 MPa argon atmosphere by bottom cooling as a reference, labeled as specimen 7. The obtained ingots with either axial or radial pores were 135 mm in diameter and 200–300 mm in length, which were cut on transverse sections using an electric discharge machine (DK7763, Longhao Digital-Controlled Machine Corp., China) between 45 mm Table 1 Fabrication conditions for the porous copper specimens. Specimen
1
2
3
4
5
6
7
PH2 (MPa) PAr (MPa) Total pressure PH2 + PAr (MPa)
0.7 0.1 0.8
0.9 0.1 1.0
0.2 0.3 0.5
0.7 0.3 1.0
0.2 0.5 0.7
0.2 0.1 0.3
0 0.5 0.5
and 110 mm from the bottom. Then, they were further cut into six rings at the points 39.5 and 47.5 mm from outer surface and a height of 8 mm, as shown in Fig. 1. All porous copper rings were examined using an optical technique (Power Shot SX200 IS Camera, Canon, Japan). In the case of the rings with radial pores, regions with a central angle no more than 7◦ on the ring surface were chosen to avoid the distortion on pore boundary. The optical images were analyzed using a SISC Image Analyzing software (KYKY Technology Development Ltd., China) for pore size and pore density. The pore size and the pore density are reported as the average value ± standard deviation from the six rings for each specimen, and the porosity calculated by a relative density method. Then, the six rings were divided into two groups, three for evaluation of oil retaining capability, the others as reference. The porous copper rings were impregnated with a synthetic lubricating oil (SJ 10W-40, Great wall lubricant Co. Ltd., China) for 7.2 ks under 80 ◦ C by oil bath heating, following another 7.2 ks under room temperature. Then, the impregnated rings were weighed for oil content immediately, as well as after being exposed in air for 1.8, 3.6, 7.2, 10.8, 21.6, 32.4, 43.2, 86.4, 172.8, 259.2 and 345.6 ks. The oil efficiency was calculated as the percentage of total pores in a ring that was accessible and could be filled with oil. At the same time, nonporous copper rings obtained by the same cutting process were bored using a 2 mm drill to obtain some axial penetrable holes on the surface uniformly to the porosity in range of the porous copper rings. The same impregnation process was carried out on the bored copper rings for the oil content. A vertical universal friction–abrasion testing machine (MMW1A, Jinan Yihua Tribology Testing Technology Company Ltd., China) was used to evaluate sliding properties of the copper rings with axial pores at room temperature in air with a relative humidity of 40–45%. The disc was a copper ring, which surface was polished using #1000 SiC abrasive papers for 1.2 ks before the evaluation. The sliding part was a WC-Co ball with 6.35 mm in diameter and HV1750 in hardness. The copper rings were fixed on the ball-ondisc testing configuration, and the ball was revolved on the rings for 1.8 ks with a sliding speed of 1 r/s under a normal load of 50 N. The turn radius for the ball was 24 mm. The porous copper rings both as-fabricated and impregnated with the lubricating oil, as well as the drilled copper rings were evaluated. The friction coefficient was recorded continuously by a computer connected to the machine during sliding. After evaluation, the copper rings were observed on worn surface by both optical and SEM micrography (JSM 6401, JEOL, Japan).
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H. Du et al. / Journal of Materials Processing Technology 212 (2012) 1796–1801 70
35
50 40 30
30
25
Oil content, %
60
Distribution frequency, %
(a)
Specimen 1 2 3 4 5 6
20 10
20
15
10
0 0
100
200
300
400
500
600
700
800
900
5
Pore size, μm
0
Fig. 2. Size distribution of pores in the porous copper.
0
5
10
15
20
25
30
35
40
45
50
55
Porosity, %
3. Results and discussion 3.1. Pore structure
3.2. Oil retaining capacity The oil content of the porous copper exhibits significant isotropy, which is not consistent with the mechanical properties reported by Nakajima (2007). The oil content after being exposed in air for 7.2 ks increases with the increase in porosity for the porous copper, as shown in Fig. 3(a). The highest value (27.6%) corresponds to the highest porosity (47.1%), which is higher than the value reported for commercial self-lubricating copper-based bearings-25% (Durak, 2003). The relationship between oil content and porosity of the porous copper almost agrees with a linear rule in Fig. 3(a). However, the oil content is a little higher than the value in the simulated line when the porosity is lower, while much smaller when the porosity is higher. This relationship indicates that the oil content to porosity ratio cannot be constant for the porous copper. The oil efficiency, defined as the oil content to porosity ratio for the porous copper, or the percentage of pores that can be filled with oil is shown in Fig. 3(b). The oil efficiency decreases with the increase in porosity, and there is a gap on specimens 2 and 4 (also on oil content) although they have almost the same porosity 30.2%
Specimen 5 (201.1μm)
(b) Specimen 4 (229.4μm)
75
Specimen 2 (261.3μm) 70
Oil efficiency, %
The average and standard deviation values of porosity, pore size and pore density of the porous copper specimens are listed in Table 2. It is clear that the pore structure depends on partial pressures of hydrogen and argon regardless of pore direction: the pore size decreases with increasing partial pressures of both hydrogen and argon; while the pore density increases with increasing hydrogen partial pressure. The specimen with higher porosity reveals the smaller number of pores and the bigger pore size. The relatively small standard deviation on porosity, pore size and pore density indicates that the porous copper possesses uniform macroscopic structure along the ingot axis. In addition, the corresponding porosity of the drilled copper rings is 27.7%, obtained by mathematical calculation. For the specimens fabricated under higher pressure, the pore size was distributed in a narrower range. In the case of specimens 1, 2, 4 and 5, about half of the pores or even more are around their average pore sizes, and distributions approximately obey the Gaussian distribution, as shown in Fig. 2. However, in the case of specimen 6, fabricated under 0.2 MPa in hydrogen pressure and 0.1 MPa in argon pressure, the pore size distributes in a wide range, and the content of pores bigger than its average size is higher than the other specimens. The reason for the wide distribution in pore size is not clear at present.
80
65
Specimen 1 (324.3μm) Specimen 3 (339.5μm)
60
Specimen 6 (428.7μm) 55
50 20
25
30
35
40
45
50
55
Porosity, % Fig. 3. Relationship between porosity and oil content (a), oil efficiency(b) of the porous copper rings.
and 29.8%, which is attributed to the variation of pore size and its distribution in the specimens. In addition, the oil efficiency of the drilled copper is very low due to its low oil content, which is about 3.2% and may be a consequence of the remaining oil on the pore walls. It is supposed that the oil content is proportional to porosity of the porous copper as open pores supply the space for oil entering. On the other hand, oil in big-sized pores will escape by the net force of gravity and surface tension exposed in air (Fig. 4), which is simplified as: F = 2r cos ˛ − gr 2 h
(1)
where is the surface tension coefficient of the oil, r is the radius of the pore, ˛ the angle of contact between the oil and the pore, the density of the oil, g the acceleration of gravity, and h the height of the oil standing in the pore. The biggest r appears when the gravity and the surface tension of oil balance, or F = 0. In this work, and are chosen as 0.02 N/m and 0.85 g/cm3 by manufacture’s production information, h is 8 mm, so the largest pore radius is about 250 m for ˛ = 60◦ . Thus, the oil content of the porous copper depends on the balance of the two effects-porosity and pore size. However, the effect of pore size on oil content was always covered by porosity as pore size also increased with the increase in porosity for the porous copper. Upon the results of this work,
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Table 2 The average and standard deviation values of porosity, average pore size and pore density of the porous copper specimens. Specimen
1
2
3
4
5
6
I
Porosity(%) Pore size(m) Pore density(cm−2 )
32.6 ± 0.9 324.3 ± 9.0 128 ± 12
30.2 ± 0.7 261.3 ± 7.1 185 ± 8
37.1 ± 1.0 339.5 ± 8.6 139 ± 11
29.8 ± 0.8 229.4 ± 5.7 211 ± 8
26.2 ± 0.6 201.1 ± 4.8 183 ± 7
47.1 ± 1.6 428.7 ± 12.5 106 ± 9
II
Porosity(%) Pore size(m) Pore density(cm−2 )
33.1 ± 0.8 315.3 ± 7.9 116 ± 9
30.1 ± 0.8 270.4 ± 7.4 180 ± 7
36.0 ± 0.9 330.9 ± 8.9 142 ± 12
30.6 ± 0.7 232.1 ± 5.9 203 ± 12
25.6 ± 0.8 198.3 ± 2.9 181 ± 9
47.8 ± 1.3 431.2 ± 10.9 105 ± 6
I and II means specimen with pores aligned axially and radially, respectively.
pores escapes into the atmosphere due to expansion caused by heat. Regardless of pore direction, big-sized pores cannot hold oil under the atmosphere as the capillary tension is lower than the oil weight, so part of the oil in the pores loses until a new balance appears. For the rings with smaller size pores, the oil content changes no more than 5% even after 345.6 ks exposing in air. The results indicate that the oil retaining capability is also correlated with pore size for the porous copper although porosity plays a more important role. At the same time, the oil efficiency of the porous copper is much lower than that obtained by sintering, which is about 95% or even more, as reported by Durak (2003). It is predicted that the oil efficiency can be improved if the pore size can be decreased to some tens of micrometers as that of sintered porous copper by adjusting fabrication parameters or secondary treatment on the porous copper. Fig. 4. Sketch map of lubricating oil in pores of the porous copper.
3.3. Friction behaviour it is considered that the increase in oil content with the increase in porosity is mainly caused by the combination of pore volume, and the decrease in oil efficiency due to the increase in pore size. In the case of specimen 6, the pore size distributes in a wide range, and about 30% of pores are larger than the 500 m, resulting in the lowest oil efficiency. The variation of the oil content with the exposed time in air for each specimen proves the assumption on balance between force of gravity and surface tension of oil in pores, as shown in Fig. 5. Initially, there is a decrease in the oil content for all specimens, which may be attributed to the oil loss from the big-sized pores. The decrease is significant on specimen 6, which has the highest porosity and many big-sized pores; its value changes from 31.6% to 27.6% after the initial 7.2 ks, to 26.6% after 86.4 ks and 26.2% after 345.6 ks. It is supposed that the oil pours into the open pores when the porous copper is dropped into the hot oil and the air in the
40
Spicemen 1 2 3 4 5 6
Oil content, %
35
30
25
20
0 2 4 6 8
50
100
150
200
250
300
350
Exposed time, ks Fig. 5. Variation of oil content of the porous copper rings with the exposed time.
The friction coefficients of the porous copper and the drilled copper rings sliding against a WC-Co ball are shown in Table 3, which are the average values of three measurements. The wear rates of all copper rings were not calculated for the very small copper loss on wear surface, especially under lubrication. It is clear that the friction coefficient decreases markedly with the oil impregnation, indicating an oil layer may form on the sliding surface, which can prevent the direct contact and decrease the friction force between the ring and the ball. However, the gap on the drilled ring is much smaller between lubricated condition and unlubricated condition compared with those on the porous rings, which is attributed to its poor oil content. In addition, the drilled copper ring shows a higher friction coefficient, which may stem from both its poor oil content and its comparatively less plastic deformation for not only denser structure but also work hardening during the drilling. The porous copper rings impregnated with oil show a decrease in friction coefficient with the increase in porosity, which may stem from both an increase in contact area between the ring and the ball and the increase in oil content. According to Zhang et al. (2003), the friction coefficient was determined by the real contact area, contact state and lubricant role of debris. As the strength of the porous copper decreases with increasing porosity (Nakajima, 2007), it is predicted that the depth the WC-Co ball pressed into the porous copper ring increases, resulting in an increase in the contact area between the ring and the ball. There is an abrupt drop in friction coefficient for porosities of 30.2% and 29.8% for the porous copper ring. The exceptional variation may indicate that pore size or oil efficiency also plays a significant role in friction coefficient. The variation of the friction coefficient with sliding time for the copper rings impregnated with oil is shown in Fig. 6. Smooth friction coefficient curves are not achieved on both the porous copper ring with higher porosity and the drilled copper ring. The friction coefficient of the drilled copper ring fluctuates acutely during the whole sliding time, while for the porous copper rings with lower porosity, the friction coefficient keeps relatively stable during the sliding period.
1800
H. Du et al. / Journal of Materials Processing Technology 212 (2012) 1796–1801
Table 3 Friction coefficients of porous copper rings and drilled copper rings sliding against a WC-Co ball. Specimen
1
2
3
4
5
6
7
FC1 FC2
0.258 ± 0.014 0.172 ± 0.011
0.234 ± 0.011 0.161 ± 0.006
0.238 ± 0.012 0.166 ± 0.005
0.229 ± 0.009 0.152 ± 0.004
0.262 ± 0.015 0.187 ± 0.009
0.237 ± 0.010 0.165 ± 0.004
0.292 ± 0.017 0.261 ± 0.015
FC1 means friction coefficient of the as-fabricated rings with the WC-Co ball, FC2 means friction coefficient of the impregnated rings with the WC-Co ball.
0.6
Specimen 7 6 4 1 3
Friction coefficient
0.5
0.4
0.3
0.2
0.1
0.0 0
200
400
600
800
1000
1200
1400
1600
Sliding time, second Fig. 6. Variation of the friction coefficient with the sliding time of the porous copper rings.
In order to understand the underlying friction coefficient of the porous copper, observations on the worn surface were conducted, as shown in Fig. 7. On the drilled copper ring, the presence of shallow plough tracks and occasionally some abrasive particles generated from loose debris can be seen, as shown in Fig. 7(a)-3. As a little lubricating oil participates in the sliding, and the lubricating film from copper debris and/or copper oxide debris cannot form easily between the sliding couple since the debris are easily pushed into the large drilled pores, the friction coefficient is higher and fluctuates severely. In the case of porous copper ring 4 with 29.8% porosity, the worn surface is relatively smooth, and exhibits rather irregular sliding marks with micro-cutting and micro-ploughing, as shown in Fig. 7(b)-3. Both the debris of copper and/or its oxide and the extruded oil contribute to the decrease in friction coefficient. Furthermore, a good lubrication from the debris and the oil film plays an important role in the relatively stable friction coefficient. In the case of the porous copper ring 6 with 47.1% porosity shown in Fig. 7(c)-3, the worn surface reveals much deeper grooves due to higher level of structure disruption and plastic deformation under load, which agrees with its unstable friction coefficient.
Fig. 7. Observations on the worn surface of specimen 7 (a), 4(b) and 6(c) with optical Image 1, SEM image with lower magnification 2, and with bigger magnification 3.
H. Du et al. / Journal of Materials Processing Technology 212 (2012) 1796–1801
Porous copper with elongated cylindrical pores and a proper structure has shown good oil retaining capability and good sliding friction behaviour. It is predicted that copper-based porous alloys with the same pore structure will have better sliding friction behaviour or wear resistance if the porous alloys have higher strength and hardness. 4. Conclusions Porous copper with elongated cylindrical pores aligned either axially or radially was fabricated under different partial pressures of hydrogen and argon. Structural parameters including porosity, pore size and distribution, and pore density were characterized, and properties including oil content, oil efficiency and friction coefficient were evaluated for the porous copper. It is indicated that: 1. The pore size increases from 198.3 m to 431.2 m, the pore size distribution becomes wider and the pore density decreases from 211 cm−2 to 105 cm−2 , when the porosity increases from 25.6% to 47.8% for the porous copper. 2. The oil content increases with the increase in porosity for the porous copper, and reached 27.6% on the specimen with the highest porosity of 47.1%. On the other hand, the oil efficiency decreases with the increase in porosity for the porous copper. The highest oil efficiency is 75.8%, which is much lower than that of sintered porous metal. 3. The friction coefficient of the porous copper rings impregnated with oil sliding against a WC-Co ball is in range of 0.15–0.19, which depends on the porosity and the pore size. The impregnated oil in the pores played an important role in improving the sliding friction behaviour of the porous copper.
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Acknowledgments The financial supports of Natural Science of Foundation from Liaoning Province (no. 201104137), and Science and Technology project (no. 2010AZ2010) from Jiaxing City are acknowledged. The first author would like to express his appreciation to Professor Hideo Nakajima in the Institute of Scientific and Industrial Research, Osaka University for his valuable comment. References Banhart, J., 2001. Manufacture, characterisation and application of cellular metals and metal foams. Progress in Materials Science 46, 559–632. Durak, E., 2003. Experimental investigation of porous bearing under different lubricant and lubricating conditions. KSME International Journal 17 (9), 1276–1286. Du, H., Qi, J.Z., Du, S.Q., Xiong, T.Y., Li, T.F., Lee, S.W., 2010a. Structure and oil retaining capacity of gasar copper fabricated by radial solidification with a combined crystallizer. Journal of Materials Processing Technology 210 (11), 1523–1528. Du, H., Qi, J.Z., Wu, J.X., Du, S.Q., Xiong, T.Y., 2010b. Structure of porous copper fabricated by unidirectional solidification under pressurized hydrogen. Materials Science Forum 654–656, 1030–1033. Hyun, S.K., Murakami, K., Nakajima, H., 2001. Anisotropic mechanical properties of porous copper fabricated by unidirectional solidification. Materials Science and Engineering A 299, 241–248. Hyun, S.K., Nakajima, H., Boyko, L.V., Shapovalov, V.I., 2004. Bending properties of porous copper fabricated by unidirectional solidification. Materials Letters 58, 1082–1086. Hyun, S.K., Nakajima, H., 2003. Anisotropic compressive properties of porous copper produced by unidirectional solidification. Materials Science and Engineering A340, 258–264. Loutfy, R.L., Boyko, L.V., Shapovalov, V.I., 2002. Is Gasar Brakes Having Future., http://www.metalfoam.net/conference02.html. Nakajima, H., 2007. Fabrication, properties and application of porous metals with directional pores. Progress in Materials Science 52, 1091–1173. Shapovalov, V.I., 1993. Method for Manufacturing Porous Articles, U.S. Patent, 5,181,549. Simone, A.E., Gibson, L.J., 1997. The compressive behaviour of porous copper made by the gasar process. Journal of Materials Science 32, 451–457. Zhang, D.K., Ge, S.R., Qiang, Y.H., 2003. Research on the fatigue and fracture behavior due to the fretting wear of steel wire in hoisting rope. Wear 255, 1233–1237.
Contents lists available at SciVerse ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Oil retaining capability and sliding friction behaviour of porous copper with elongated cylindrical pores Hao Du ∗ , Jianzhong Qi, Yuanxia Lao, Tianying Xiong Division of Surface Engineering of Materials, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China
a r t i c l e
i n f o
Article history: Received 15 January 2012 Received in revised form 19 March 2012 Accepted 26 March 2012 Available online 3 April 2012 Keywords: Porous copper Solidification Oil content Sliding friction behaviour
a b s t r a c t Porous copper with elongated cylindrical pores aligned either axially or radially was fabricated under a high pressure of mixture gas of hydrogen and argon. Structure characterization indicated that pore size increased, pore density decreased, pore size distribution became wider with an increase in porosity for the porous copper. The dependence of oil retaining capability and sliding friction coefficient on porosity and pore size of the porous copper were investigated. It was found that the oil content of the porous copper depended mainly on the porosity, and reached 27.6% on the specimen with a porosity of 47.1%. On the other hand, the oil efficiency was not satisfactory, and became worse when the porosity increased, which was attributed to the increase in pore size and the wider pore size distribution for the porous copper. It was proven that the impregnated oil in the pores played an important role in improving the sliding friction behaviour of the porous copper. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Porous metal has been expected to be used in construction, automotive and aerospace applications from the viewpoint of both energy conservation and environmental preservation (Banhart, 2001). However, pores formed in the metal by typical methods such as foaming or powder sintering are round or irregular, acting as stress promoters, as reported by Hyun et al. (2001). As a result, the traditional porous metals break up easily under stress, which restricts their application, especially for construction purposes. A new method for producing porous metals with elongated cylindrical pores ordered regularly was reported and called as gasar by Shapovalov (1993). In this method, the gas solubility difference between in liquid and in solid is utilized for the gas pore formation during solidification of the porous metals. Compared with the traditional fabrication techniques, this method allows an effective control of porosity, pore morphology and pore aligning direction for porous metals, reported by Nakajima (2007). Generally, the elongated cylindrical pores align axially or radially in the porous metal, which depends on the solidification direction. Nakajima (2007) named it as ‘lotus-type porous metal’ when pores aligned axially in the porous metal for its appearance as a lotus root on the cross section. It has been reported by Simone and Gibson (1997), Hyun and Nakajima (2003), and Hyun et al.
∗ Corresponding author. E-mail address: [email protected] (H. Du). 0924-0136/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2012.03.024
(2004) that gasar metals or lotus-type porous metals even with considerable porosity exhibited much superior strengths (mechanical properties) than those formed by other techniques, such as foaming or powder sintering, which is promising to extend their application especially when higher mechanical properties are necessary. It is possible to fill the pores in the porous metals with materials of different compositions for various applications. Loutfy et al. (2002) investigated the technical feasibility of using several porous matrices with elongated cylindrical pores and abrasives as filler for the development of high performance brake components. They reported that porous cast iron and steel exhibited high friction coefficient, perfect stability and low wear, and it was also possible to control the braking abilities of the porous matrix filled with abrasives by altering the matrix porosity and the abrasive composition. Du et al. (2010a) reported that porous copper with elongated cylindrical pores aligned radially possessed a good oil content, which was uniform even if there was a gradient in the pore structure. However, further investigation about the oil content and the effect of a lubricating oil as the filler on friction behaviour of the porous copper is not available. The purpose in the present work is to investigate the relationship among pore structure, oil retaining capability and sliding behaviour for the porous copper with elongated cylindrical pores, considering its potential application on self-lubricating components. To achieve this goal, porous copper with elongated pores aligned either axially or radially was fabricated under different partial pressures of hydrogen and argon for altering pore structure, by our previous work (Du et al., 2010b). The oil retaining
H. Du et al. / Journal of Materials Processing Technology 212 (2012) 1796–1801
1797
Fig. 1. Optical images of the porous copper rings: (a) with radial pores, (b) with axial pores.
capability was evaluated by an impregnation method. The sliding friction behaviour of the corresponding porous copper rings with axial pores impregnated with oil was evaluated and compared to that of as-fabricated rings, as well as that of bored copper rings from nonporous copper. The effect of pore structure on oil retaining capability and sliding friction behaviour is discussed for the porous copper. 2. Experimental procedure The porous copper with elongated cylindrical pores aligned axially or radially, and a nonporous copper were fabricated by a vacuum-assisted and pressurized casting apparatus, which was previously described in detail (Du et al., 2010a). Two cooling units were employed with either bottom or lateral side of the mold being cooled down. In the case of porous copper with axial pores, the mold with a thin ceramic wall adjacent to the lateral side was directionally cooled from the bottom. In the case of porous copper with radial pores, the mold was cooled from the lateral wall, and there was a thick alumina plate under the bottom for a minimal heat loss via the bottom. High purity copper (99.99 wt.%) was melted in the crucible by middle frequency heating after the chamber was evacuated to 5.0 Pa. When the temperature reached 1523 K, monitored by a W-5Re/W-26Re thermocouple, high purity hydrogen and argon were introduced into the chamber, respectively. The partial pressures of hydrogen and argon for each specimen are listed in Table 1. For each condition, two porous copper specimens with either axial or radial pores were fabricated. In order to make hydrogen dissolve uniformly in liquid copper, each pressurized condition was maintained for 1.8 ks at the selected temperature. In addition, nonporous copper was fabricated in a 0.5 MPa argon atmosphere by bottom cooling as a reference, labeled as specimen 7. The obtained ingots with either axial or radial pores were 135 mm in diameter and 200–300 mm in length, which were cut on transverse sections using an electric discharge machine (DK7763, Longhao Digital-Controlled Machine Corp., China) between 45 mm Table 1 Fabrication conditions for the porous copper specimens. Specimen
1
2
3
4
5
6
7
PH2 (MPa) PAr (MPa) Total pressure PH2 + PAr (MPa)
0.7 0.1 0.8
0.9 0.1 1.0
0.2 0.3 0.5
0.7 0.3 1.0
0.2 0.5 0.7
0.2 0.1 0.3
0 0.5 0.5
and 110 mm from the bottom. Then, they were further cut into six rings at the points 39.5 and 47.5 mm from outer surface and a height of 8 mm, as shown in Fig. 1. All porous copper rings were examined using an optical technique (Power Shot SX200 IS Camera, Canon, Japan). In the case of the rings with radial pores, regions with a central angle no more than 7◦ on the ring surface were chosen to avoid the distortion on pore boundary. The optical images were analyzed using a SISC Image Analyzing software (KYKY Technology Development Ltd., China) for pore size and pore density. The pore size and the pore density are reported as the average value ± standard deviation from the six rings for each specimen, and the porosity calculated by a relative density method. Then, the six rings were divided into two groups, three for evaluation of oil retaining capability, the others as reference. The porous copper rings were impregnated with a synthetic lubricating oil (SJ 10W-40, Great wall lubricant Co. Ltd., China) for 7.2 ks under 80 ◦ C by oil bath heating, following another 7.2 ks under room temperature. Then, the impregnated rings were weighed for oil content immediately, as well as after being exposed in air for 1.8, 3.6, 7.2, 10.8, 21.6, 32.4, 43.2, 86.4, 172.8, 259.2 and 345.6 ks. The oil efficiency was calculated as the percentage of total pores in a ring that was accessible and could be filled with oil. At the same time, nonporous copper rings obtained by the same cutting process were bored using a 2 mm drill to obtain some axial penetrable holes on the surface uniformly to the porosity in range of the porous copper rings. The same impregnation process was carried out on the bored copper rings for the oil content. A vertical universal friction–abrasion testing machine (MMW1A, Jinan Yihua Tribology Testing Technology Company Ltd., China) was used to evaluate sliding properties of the copper rings with axial pores at room temperature in air with a relative humidity of 40–45%. The disc was a copper ring, which surface was polished using #1000 SiC abrasive papers for 1.2 ks before the evaluation. The sliding part was a WC-Co ball with 6.35 mm in diameter and HV1750 in hardness. The copper rings were fixed on the ball-ondisc testing configuration, and the ball was revolved on the rings for 1.8 ks with a sliding speed of 1 r/s under a normal load of 50 N. The turn radius for the ball was 24 mm. The porous copper rings both as-fabricated and impregnated with the lubricating oil, as well as the drilled copper rings were evaluated. The friction coefficient was recorded continuously by a computer connected to the machine during sliding. After evaluation, the copper rings were observed on worn surface by both optical and SEM micrography (JSM 6401, JEOL, Japan).
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35
50 40 30
30
25
Oil content, %
60
Distribution frequency, %
(a)
Specimen 1 2 3 4 5 6
20 10
20
15
10
0 0
100
200
300
400
500
600
700
800
900
5
Pore size, μm
0
Fig. 2. Size distribution of pores in the porous copper.
0
5
10
15
20
25
30
35
40
45
50
55
Porosity, %
3. Results and discussion 3.1. Pore structure
3.2. Oil retaining capacity The oil content of the porous copper exhibits significant isotropy, which is not consistent with the mechanical properties reported by Nakajima (2007). The oil content after being exposed in air for 7.2 ks increases with the increase in porosity for the porous copper, as shown in Fig. 3(a). The highest value (27.6%) corresponds to the highest porosity (47.1%), which is higher than the value reported for commercial self-lubricating copper-based bearings-25% (Durak, 2003). The relationship between oil content and porosity of the porous copper almost agrees with a linear rule in Fig. 3(a). However, the oil content is a little higher than the value in the simulated line when the porosity is lower, while much smaller when the porosity is higher. This relationship indicates that the oil content to porosity ratio cannot be constant for the porous copper. The oil efficiency, defined as the oil content to porosity ratio for the porous copper, or the percentage of pores that can be filled with oil is shown in Fig. 3(b). The oil efficiency decreases with the increase in porosity, and there is a gap on specimens 2 and 4 (also on oil content) although they have almost the same porosity 30.2%
Specimen 5 (201.1μm)
(b) Specimen 4 (229.4μm)
75
Specimen 2 (261.3μm) 70
Oil efficiency, %
The average and standard deviation values of porosity, pore size and pore density of the porous copper specimens are listed in Table 2. It is clear that the pore structure depends on partial pressures of hydrogen and argon regardless of pore direction: the pore size decreases with increasing partial pressures of both hydrogen and argon; while the pore density increases with increasing hydrogen partial pressure. The specimen with higher porosity reveals the smaller number of pores and the bigger pore size. The relatively small standard deviation on porosity, pore size and pore density indicates that the porous copper possesses uniform macroscopic structure along the ingot axis. In addition, the corresponding porosity of the drilled copper rings is 27.7%, obtained by mathematical calculation. For the specimens fabricated under higher pressure, the pore size was distributed in a narrower range. In the case of specimens 1, 2, 4 and 5, about half of the pores or even more are around their average pore sizes, and distributions approximately obey the Gaussian distribution, as shown in Fig. 2. However, in the case of specimen 6, fabricated under 0.2 MPa in hydrogen pressure and 0.1 MPa in argon pressure, the pore size distributes in a wide range, and the content of pores bigger than its average size is higher than the other specimens. The reason for the wide distribution in pore size is not clear at present.
80
65
Specimen 1 (324.3μm) Specimen 3 (339.5μm)
60
Specimen 6 (428.7μm) 55
50 20
25
30
35
40
45
50
55
Porosity, % Fig. 3. Relationship between porosity and oil content (a), oil efficiency(b) of the porous copper rings.
and 29.8%, which is attributed to the variation of pore size and its distribution in the specimens. In addition, the oil efficiency of the drilled copper is very low due to its low oil content, which is about 3.2% and may be a consequence of the remaining oil on the pore walls. It is supposed that the oil content is proportional to porosity of the porous copper as open pores supply the space for oil entering. On the other hand, oil in big-sized pores will escape by the net force of gravity and surface tension exposed in air (Fig. 4), which is simplified as: F = 2r cos ˛ − gr 2 h
(1)
where is the surface tension coefficient of the oil, r is the radius of the pore, ˛ the angle of contact between the oil and the pore, the density of the oil, g the acceleration of gravity, and h the height of the oil standing in the pore. The biggest r appears when the gravity and the surface tension of oil balance, or F = 0. In this work, and are chosen as 0.02 N/m and 0.85 g/cm3 by manufacture’s production information, h is 8 mm, so the largest pore radius is about 250 m for ˛ = 60◦ . Thus, the oil content of the porous copper depends on the balance of the two effects-porosity and pore size. However, the effect of pore size on oil content was always covered by porosity as pore size also increased with the increase in porosity for the porous copper. Upon the results of this work,
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Table 2 The average and standard deviation values of porosity, average pore size and pore density of the porous copper specimens. Specimen
1
2
3
4
5
6
I
Porosity(%) Pore size(m) Pore density(cm−2 )
32.6 ± 0.9 324.3 ± 9.0 128 ± 12
30.2 ± 0.7 261.3 ± 7.1 185 ± 8
37.1 ± 1.0 339.5 ± 8.6 139 ± 11
29.8 ± 0.8 229.4 ± 5.7 211 ± 8
26.2 ± 0.6 201.1 ± 4.8 183 ± 7
47.1 ± 1.6 428.7 ± 12.5 106 ± 9
II
Porosity(%) Pore size(m) Pore density(cm−2 )
33.1 ± 0.8 315.3 ± 7.9 116 ± 9
30.1 ± 0.8 270.4 ± 7.4 180 ± 7
36.0 ± 0.9 330.9 ± 8.9 142 ± 12
30.6 ± 0.7 232.1 ± 5.9 203 ± 12
25.6 ± 0.8 198.3 ± 2.9 181 ± 9
47.8 ± 1.3 431.2 ± 10.9 105 ± 6
I and II means specimen with pores aligned axially and radially, respectively.
pores escapes into the atmosphere due to expansion caused by heat. Regardless of pore direction, big-sized pores cannot hold oil under the atmosphere as the capillary tension is lower than the oil weight, so part of the oil in the pores loses until a new balance appears. For the rings with smaller size pores, the oil content changes no more than 5% even after 345.6 ks exposing in air. The results indicate that the oil retaining capability is also correlated with pore size for the porous copper although porosity plays a more important role. At the same time, the oil efficiency of the porous copper is much lower than that obtained by sintering, which is about 95% or even more, as reported by Durak (2003). It is predicted that the oil efficiency can be improved if the pore size can be decreased to some tens of micrometers as that of sintered porous copper by adjusting fabrication parameters or secondary treatment on the porous copper. Fig. 4. Sketch map of lubricating oil in pores of the porous copper.
3.3. Friction behaviour it is considered that the increase in oil content with the increase in porosity is mainly caused by the combination of pore volume, and the decrease in oil efficiency due to the increase in pore size. In the case of specimen 6, the pore size distributes in a wide range, and about 30% of pores are larger than the 500 m, resulting in the lowest oil efficiency. The variation of the oil content with the exposed time in air for each specimen proves the assumption on balance between force of gravity and surface tension of oil in pores, as shown in Fig. 5. Initially, there is a decrease in the oil content for all specimens, which may be attributed to the oil loss from the big-sized pores. The decrease is significant on specimen 6, which has the highest porosity and many big-sized pores; its value changes from 31.6% to 27.6% after the initial 7.2 ks, to 26.6% after 86.4 ks and 26.2% after 345.6 ks. It is supposed that the oil pours into the open pores when the porous copper is dropped into the hot oil and the air in the
40
Spicemen 1 2 3 4 5 6
Oil content, %
35
30
25
20
0 2 4 6 8
50
100
150
200
250
300
350
Exposed time, ks Fig. 5. Variation of oil content of the porous copper rings with the exposed time.
The friction coefficients of the porous copper and the drilled copper rings sliding against a WC-Co ball are shown in Table 3, which are the average values of three measurements. The wear rates of all copper rings were not calculated for the very small copper loss on wear surface, especially under lubrication. It is clear that the friction coefficient decreases markedly with the oil impregnation, indicating an oil layer may form on the sliding surface, which can prevent the direct contact and decrease the friction force between the ring and the ball. However, the gap on the drilled ring is much smaller between lubricated condition and unlubricated condition compared with those on the porous rings, which is attributed to its poor oil content. In addition, the drilled copper ring shows a higher friction coefficient, which may stem from both its poor oil content and its comparatively less plastic deformation for not only denser structure but also work hardening during the drilling. The porous copper rings impregnated with oil show a decrease in friction coefficient with the increase in porosity, which may stem from both an increase in contact area between the ring and the ball and the increase in oil content. According to Zhang et al. (2003), the friction coefficient was determined by the real contact area, contact state and lubricant role of debris. As the strength of the porous copper decreases with increasing porosity (Nakajima, 2007), it is predicted that the depth the WC-Co ball pressed into the porous copper ring increases, resulting in an increase in the contact area between the ring and the ball. There is an abrupt drop in friction coefficient for porosities of 30.2% and 29.8% for the porous copper ring. The exceptional variation may indicate that pore size or oil efficiency also plays a significant role in friction coefficient. The variation of the friction coefficient with sliding time for the copper rings impregnated with oil is shown in Fig. 6. Smooth friction coefficient curves are not achieved on both the porous copper ring with higher porosity and the drilled copper ring. The friction coefficient of the drilled copper ring fluctuates acutely during the whole sliding time, while for the porous copper rings with lower porosity, the friction coefficient keeps relatively stable during the sliding period.
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Table 3 Friction coefficients of porous copper rings and drilled copper rings sliding against a WC-Co ball. Specimen
1
2
3
4
5
6
7
FC1 FC2
0.258 ± 0.014 0.172 ± 0.011
0.234 ± 0.011 0.161 ± 0.006
0.238 ± 0.012 0.166 ± 0.005
0.229 ± 0.009 0.152 ± 0.004
0.262 ± 0.015 0.187 ± 0.009
0.237 ± 0.010 0.165 ± 0.004
0.292 ± 0.017 0.261 ± 0.015
FC1 means friction coefficient of the as-fabricated rings with the WC-Co ball, FC2 means friction coefficient of the impregnated rings with the WC-Co ball.
0.6
Specimen 7 6 4 1 3
Friction coefficient
0.5
0.4
0.3
0.2
0.1
0.0 0
200
400
600
800
1000
1200
1400
1600
Sliding time, second Fig. 6. Variation of the friction coefficient with the sliding time of the porous copper rings.
In order to understand the underlying friction coefficient of the porous copper, observations on the worn surface were conducted, as shown in Fig. 7. On the drilled copper ring, the presence of shallow plough tracks and occasionally some abrasive particles generated from loose debris can be seen, as shown in Fig. 7(a)-3. As a little lubricating oil participates in the sliding, and the lubricating film from copper debris and/or copper oxide debris cannot form easily between the sliding couple since the debris are easily pushed into the large drilled pores, the friction coefficient is higher and fluctuates severely. In the case of porous copper ring 4 with 29.8% porosity, the worn surface is relatively smooth, and exhibits rather irregular sliding marks with micro-cutting and micro-ploughing, as shown in Fig. 7(b)-3. Both the debris of copper and/or its oxide and the extruded oil contribute to the decrease in friction coefficient. Furthermore, a good lubrication from the debris and the oil film plays an important role in the relatively stable friction coefficient. In the case of the porous copper ring 6 with 47.1% porosity shown in Fig. 7(c)-3, the worn surface reveals much deeper grooves due to higher level of structure disruption and plastic deformation under load, which agrees with its unstable friction coefficient.
Fig. 7. Observations on the worn surface of specimen 7 (a), 4(b) and 6(c) with optical Image 1, SEM image with lower magnification 2, and with bigger magnification 3.
H. Du et al. / Journal of Materials Processing Technology 212 (2012) 1796–1801
Porous copper with elongated cylindrical pores and a proper structure has shown good oil retaining capability and good sliding friction behaviour. It is predicted that copper-based porous alloys with the same pore structure will have better sliding friction behaviour or wear resistance if the porous alloys have higher strength and hardness. 4. Conclusions Porous copper with elongated cylindrical pores aligned either axially or radially was fabricated under different partial pressures of hydrogen and argon. Structural parameters including porosity, pore size and distribution, and pore density were characterized, and properties including oil content, oil efficiency and friction coefficient were evaluated for the porous copper. It is indicated that: 1. The pore size increases from 198.3 m to 431.2 m, the pore size distribution becomes wider and the pore density decreases from 211 cm−2 to 105 cm−2 , when the porosity increases from 25.6% to 47.8% for the porous copper. 2. The oil content increases with the increase in porosity for the porous copper, and reached 27.6% on the specimen with the highest porosity of 47.1%. On the other hand, the oil efficiency decreases with the increase in porosity for the porous copper. The highest oil efficiency is 75.8%, which is much lower than that of sintered porous metal. 3. The friction coefficient of the porous copper rings impregnated with oil sliding against a WC-Co ball is in range of 0.15–0.19, which depends on the porosity and the pore size. The impregnated oil in the pores played an important role in improving the sliding friction behaviour of the porous copper.
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Acknowledgments The financial supports of Natural Science of Foundation from Liaoning Province (no. 201104137), and Science and Technology project (no. 2010AZ2010) from Jiaxing City are acknowledged. The first author would like to express his appreciation to Professor Hideo Nakajima in the Institute of Scientific and Industrial Research, Osaka University for his valuable comment. References Banhart, J., 2001. Manufacture, characterisation and application of cellular metals and metal foams. Progress in Materials Science 46, 559–632. Durak, E., 2003. Experimental investigation of porous bearing under different lubricant and lubricating conditions. KSME International Journal 17 (9), 1276–1286. Du, H., Qi, J.Z., Du, S.Q., Xiong, T.Y., Li, T.F., Lee, S.W., 2010a. Structure and oil retaining capacity of gasar copper fabricated by radial solidification with a combined crystallizer. Journal of Materials Processing Technology 210 (11), 1523–1528. Du, H., Qi, J.Z., Wu, J.X., Du, S.Q., Xiong, T.Y., 2010b. Structure of porous copper fabricated by unidirectional solidification under pressurized hydrogen. Materials Science Forum 654–656, 1030–1033. Hyun, S.K., Murakami, K., Nakajima, H., 2001. Anisotropic mechanical properties of porous copper fabricated by unidirectional solidification. Materials Science and Engineering A 299, 241–248. Hyun, S.K., Nakajima, H., Boyko, L.V., Shapovalov, V.I., 2004. Bending properties of porous copper fabricated by unidirectional solidification. Materials Letters 58, 1082–1086. Hyun, S.K., Nakajima, H., 2003. Anisotropic compressive properties of porous copper produced by unidirectional solidification. Materials Science and Engineering A340, 258–264. Loutfy, R.L., Boyko, L.V., Shapovalov, V.I., 2002. Is Gasar Brakes Having Future., http://www.metalfoam.net/conference02.html. Nakajima, H., 2007. Fabrication, properties and application of porous metals with directional pores. Progress in Materials Science 52, 1091–1173. Shapovalov, V.I., 1993. Method for Manufacturing Porous Articles, U.S. Patent, 5,181,549. Simone, A.E., Gibson, L.J., 1997. The compressive behaviour of porous copper made by the gasar process. Journal of Materials Science 32, 451–457. Zhang, D.K., Ge, S.R., Qiang, Y.H., 2003. Research on the fatigue and fracture behavior due to the fretting wear of steel wire in hoisting rope. Wear 255, 1233–1237.