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Friction and wear properties of bronze–graphite composite under water lubrication
Tribology International 37 (2004) 423–429 www.elsevier.com/locate/triboint
Friction and wear properties of bronze–graphite composite under water lubrication Jun-hong Jia , Jian-min Chen, Hui-di Zhou, Jing-bo Wang, Hua Zhou State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 73000, China Received 23 June 2003; received in revised form 2 December 2003; accepted 30 December 2003
Abstract Bronze–graphite composite was prepared using powder metallurgy. The friction and wear behaviors of the resulting composites in dry- and water-lubricated sliding against a stainless steel were comparatively investigated on an MM-200 friction and wear tester in a ring-on-block contact configuration. The wear mechanisms of the bronze–graphite composite were discussed based on examination of the worn surface morphologies of both the composite block and the stainless steel ring by means of scanning electron microscopy equipped with an energy dispersion spectrometry and on determination of some typical elements on the worn surfaces by means of X-ray photoelectron spectroscopy. It was found that the friction coefficient was higher under water lubrication than that under dry sliding and it showed margined change with increasing load under the both sliding conditions. A considerably decreased wear rate of the bronze–graphite composite was registered under water-lubricated sliding than under dry sliding, though it rose significantly at a relatively higher load. This was attributed to the hindered transfer of the composite onto the counterpart steel surface under water-lubricated sliding and the cooling effect of the water as a lubricant, while its stronger transfer onto the steel surface accounted for its higher wear rate under dry sliding. Thus, the bronze–graphite composite with much better wear-resistance under water-lubricated sliding than under dry sliding against the stainless steel could be a potential candidate as the tribo-material in aqueous environment. # 2003 Elsevier Ltd. All rights reserved. Keywords: Bronze–graphite; Powder metallurgy; Water lubrication; Wear mechanism
1. Introduction In many cases, bearings are required to operate in aqueous medium in which water is either deliberately introduced as a coolant, e.g., in rolling mill bearing, or present as a working fluid, e.g., in submerged pumps and journal bearings of rolling-mills. In the latter case, a water environment can pose special problems because of abrasive contamination and the corrosiveness of water towards many metals [1,2]. Therefore, most of the power hand tools used by navy divers are operated by oil hydraulic motors. However, the use of oil hydraulic system has many disadvantages, e.g., the leakage of working fluid may lead to contamination to the environment [3]. Moreover, food and pharmaceu Corresponding author. Tel.: +86-931-8272179; fax: +86-9318277088. E-mail address: [email protected] (J.-h. Jia).
0301-679X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2003.12.013
tical industries are demanding for the substitution of traditional oils with water-based lubricants because of the pollution of their products caused by their machinery. These disadvantages would be eliminated by use of specifically designed hydraulic motor to operate in water medium. Nevertheless, the selection of materials having the desired corrosion and friction and wear properties in this respect becomes a major challenge to the development of fluid power transmission, since water of corrosiveness may cause cavitation and has low viscosity and poor lubricity [4]. Accordingly, many efforts have been made to develop materials suitable to aqueous environment. The behavior of waterlubricated ceramic journal bearings has been studied [5]. The obtained results suggest to extending water lubrication to systems working in water like submerged pumps. Encouraging results of water lubrication have been achieved with couplings of polymeric materials against steels that have been proposed for drills and
424
J.-h. Jia et al. / Tribology International 37 (2004) 423–429
stainless steel ring and the bronze–graphite composite block were abraded with No. 900 water-abrasive paper and ultrasonically cleaned with acetone. The average surface roughness (Ra) of the rings and blocks is about 0.1–0.2 lm. The water lubrication between the sliding surfaces was realized by continuously dropping of distilled water onto the sliding surface at a rate of 65–70 drops per minute. The friction force between the tested block and the counterpart steel ring was measured with a torque shaft equipped with strain gauges. The width of the wear tracks of the blocks was measured with a naked-eye microscope to an accuracy of 0.01 mm. Then the wear volume loss V of the blocks was calculated as below: 2 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 2 pr b b b2 r2 5 arcsin ð1Þ V ¼ B4 2r 2 180 4
drifters in mines [6]. In this work, the authors investigate the possibility of a rational use of metal-based materials to solve tribological problems in water environment. Sliding bearings produced by power metallurgical processes are employed preferentially in mechanical engineering, in the car industry, and the production of electric motors, optical instruments and precision mechanics [7]. Advance in powder metallurgy technology offers various types of self-lubricating bearings, such as solid lubricant embedded bearings, oil-impregnated bronze, iron, iron–graphite bearing and so on [8]. Such materials were been mostly focused on the application in ambient dry sliding and high temperature. However, few are reported on their friction and wear properties and wear mechanisms in water environment. Therefore, in an effort to investigating the feasibility to use bronze-based composites in aqueous environment. The commercial bronze–graphite powder, which is widely used in different bearing applications, was chosen for the present work. It was prepared by powder metallurgy and tested in water medium. Thus, the friction and wear behaviors of bronze–graphite composite sliding against stainless steel under dry- and water-lubricated condition were comparatively investigated. The friction and wear mechanisms of the bronze–graphite composite were discussed as well. It was expected that the present work would be helpful to provide references to the selection of appropriate metal-based materials for the components subjected to motion and sealing in aqueous environment.
where V refers to the wear volume loss (mm3), B to the width of the block specimen (mm), r to the radius of the stainless steel ring (mm), and b to the width of the wear track (mm). The specific wear rate (K0) was calculated from the volume loss using the following equation: K0 ¼V =ðL dÞ ½mm3 ðN mÞ1
ð2Þ
where d is the sliding distance in meter and L is the load in Newton. Three replicate friction and wear tests were carried out for each specimen with a relative error of 10%, and the average of the three replicate test results is reported in this article. At the end of the friction and wear tests, the blocks were cleaned and dried, followed by the analyses of the worn surface and elemental distributions thereon with a JEM-5600LV scanning electron microscope (SEM) equipped with a KEVEX energy dispersive X-ray spectrometer (EDS) attachment. The chemical states of the elements on the worn surface were determined using an X-ray photoelectron spectroscope (XPS). The XPS analysis was conducted at 400 W and pass energy of 29.35 eV, using Mg-Ka radiation as the excitation source and the binding energy of C1s (284.6) as a reference. The worn surface was sputtered with Ar+ ions for 10 s before analysis.
2. Experimental procedures The bronze–graphite composite samples were prepared by using power metallurgy method. The resulting rectangular cubic blocks had a size of 30 mm 7 mm 6 mm. The friction and wear behavior of the bronze–graphite composite sliding against stainless steel was evaluated on an MM-200 model ring-on-block test rig. The rings of 1 50 mm 10 mm were made of stainless steel (1Cr18Ni9Ti, HB ¼ 1:48 GPa). The chemical composition of the frictional pair is given in Table 1. The tests were carried out at a linear velocity of 1.06 and 0.53 m/s, loads from 100 to 400 N, and duration of 120 min. Before each test, the Table 1 Chemical composition of the frictional pair C
Si
Mn
Cr
Ni
Ti
Fe
0.12
1.00
2.00
17.00 19.00
8.00 11.00
0.02
Balance
Sn 7
Cu 85
Graphite 8
Steel ring
Block
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Fig. 1. Variations in friction coefficient and wear rate of bronze– graphite composite with load at a sliding speed of 0.53 m/s.
3. Results and discussion 3.1. Friction and wear properties The variations in friction coefficient and wear rate of bronze–graphite composites with load at a sliding speed of 0.53 m/s are shown in Fig. 1. The friction coefficient under the dry sliding at various loads is around 0.23, which is small in terms of the dry sliding condition. This could be attributed to the self-lubricity of the composite at a relatively high temperature under the dry sliding. Surprisingly, the friction coefficient is higher under water lubrication than under dry sliding, though the friction coefficients assume small fluctuation with increasing load under both conditions. We suppose that the water as a lubricant might function to significantly hinder the friction-induced thermal effect and hence to hinder the self-lubricity of the composite as well, while the boundary lubricating action of the water could not compensate for such a hindrance in the self-lubricity of the composite under dry sliding if the applied normal is high enough. Further investi-
425
gation might be meaningful in this respect. Besides, the wear rate of the composite under dry sliding decreases with increasing load. This could also be related to the increased thermal effect at increased load, which helps to strengthen the self-lubricity of the composite under dry sliding. The wear rate of the composite under water-lubricated sliding decreases with increasing load up to 200 N and then it rises gradually with the further increase of the load, but it is always smaller than that under dry sliding. This could be related to the greatly decreased thermal heat under water lubrication, which was beneficial for the composite to retain its mechanical strength and resist wear. In other words, although the increased friction heat under dry sliding is beneficial for the composite to play the solid lubricity [9], it is harmful to the mechanical strength of the composite and leads to increased softening and plastic deformation and hence increased wear of the composite. Therefore, the composite records a larger wear rate under dry sliding than under water lubrication. Accordingly, it might be imperative to pay attention to the balance between the self-lubricity and wear resistance of the composite subject to dry sliding condition, while this would not be a problem with respect to the application of the composite under water-lubricated condition. The variations in the friction coefficient and wear rate of bronze–graphite composites with sliding speed at 200 N are shown in Table 2. The friction coefficients are smaller at high sliding speed under both dry- and water-lubricated conditions. The wear rate at high sliding speed under dry sliding is about one-third of that at low speed. However, the difference in the wear rates of the composites at various sliding speeds becomes margin under water lubrication. The lowest wear rate of the composite is recorded as 1:01 106 mm3 ðN mÞ1 at a normal load of 200 N and a sliding speed of 0.53 m/s. Similarly, the relatively smaller friction coefficient at higher sliding speed might also be attributed to the increased friction heat [10]. In other words, a larger sliding speed corresponded to increased friction heat, which helped to lowering the shear strength by enhancing the softening and melting of the composite surface material under dry sliding. Subsequently, a lowered friction coefficient is registered at higher sliding speed under dry sliding. the decrease of the friction coefficient with sliding speed under water lubrication might
Table 2 Variations in friction coefficient and wear rate of bronze–graphite composite with speed under dry- and water-lubricated sliding at 200 N Sliding speed
Dry sliding 0.53 m/s 1.06 m/s
Wear rate (106 mm3 (N m)1)
Friction coefficient
0.26 0.24
Water lubrication
Dry sliding
Water lubrication
0.31 0.25
14.24 5.01
1.01 1.10
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Fig. 2. SEM morphologies of worn surfaces of the composite under dry sliding (a) and water lubrication (b), distributions of Cu (c) and Fe (e) under dry sliding, and distributions of Cu (d) and Fe (f) under water lubrication at 1.06 m/s and 200 N.
depend on the strengthened boundary lubricating ability of the water film at increased sliding speed. At the same time, the cooling effect of water was also significant, which helped to decrease the surface temperature and then restrain the adhesion transference and plastic deformation of the composite effectively, though the corrosion of the counterpart steel by water might also
play a role in affecting the friction and wear behavior. The action of water could be well understood if one noticed that the sliding surface of the porous composite was able to adsorb water and entrap the adsorbed water therein. Thus, in terms of the comprehensive friction-reducing and antiwear ability, bronze–graphite composite
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Fig. 3. SEM morphologies of worn surface of the steel rings under dry sliding (a) and water lubrication (b), distributions of Cu (c) and Fe (e) under dry sliding, and distributions of Cu (d) and Fe (f) under water lubrication against the composite at 1.06 m/s and 200 N.
could be potential candidates as tribo-material in sliding against stainless steel under water-lubricated condition. 3.2. The examinations of the worn surfaces The SEM morphologies of and the corresponding elemental distributions on the worn surfaces of the
bronze–graphite composites under dry- and waterlubricated sliding against the stainless steel at a normal load of 200 N and a sliding speed of 1.06 m/s are given in Fig. 2. The worn surface of the bronze–graphite composite under dry sliding shows signs of plastic deformation, microcracking, and scuffing (Fig. 2(a)), which is attributed to the adhesion and abrasive wear of the bronze–graphite composite. The worn surface of
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Fig. 4.
XPS spectra of Cu and Fe on the worn steel surfaces under dry- and water-lubricated sliding against the composite.
the composite under water lubrication is also characterized by scuffing, with the microcracking and plastic deformation to be significantly hindered (Fig. 2(b)). Meanwhile, more holes are seen on the worn composite surface sliding against the stainless steel under water lubrication, as compared with Fig. 2(a). This could be attributed to the corrosiveness of the water and might partly account for the increased friction coefficient under water-lubricated sliding at a relatively smaller speed. In addition, the worn composite surface under dry sliding records smaller concentration of Fe (Fig. 2(e)) than that under water-lubricated sliding (Fig. 2(f)). This implies that Fe is more liable to transfer onto the worn surface of the composite under water-lubricated sliding, while such a kind of Fe transfer onto the composite surface is hindered under dry sliding. Thus, it might be rational to infer that the powder metallurgical composite of bronze–graphite could be suitable to water-lubricated applications. The SEM morphologies of the worn surfaces of the stainless steel rings under dry- and water-lubricated sliding at a load of 200 N and a speed of 1.06 m/s are given in Fig. 3. The worn steel surface under dry sliding shows obvious signs of adhesion which is attributed to the transfer of the bronze–graphite composite onto the steel surface (Fig. 3(a)). Such a kind of transfer of the composite is also confirmed by the elemental distri-
bution of Cu on the steel surface (Fig. 3(c)). Under water lubrication, the worn steel surface becomes smoother than that under dry sliding and the adhesion signs are considerably abated (Fig. 3(b)). The abated adhesion on the steel surface under water lubrication, i.e. the abated transfer of the composite onto the steel surface in this case is also confirmed by the elemental distribution of Cu (Fig. 3(d)), where less Cu is present as compared with that under dry sliding. Besides, it is equally interesting to notice that the Fe distributions on the worn steel surfaces under dry- and water-lubricated sliding against the bronze–graphite composite conform to the different friction and wear behaviors of the composites in the both sliding conditions as well. In other words, since a smaller amount of the composite has been transferred onto the worn steel surface, more Fe is hence detected thereon by EDAX (Fig. 3(f)). Vise versa, a smaller amount of Fe is observed on the worn steel surface under dry sliding against the bronze– graphite composite, because more composite matrix has been transferred thereon (Fig. 3(e)). Though the transference of the composite matrix on the counterpart steel surface is relatively lighter under water-lubricated sliding than that under dry sliding, it together with the adsorbed water boundary layer contributed to the better wear resistance of the composite in the former case. Therefore, it can be concluded that
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the transfer of the bronze–graphite onto the steel surface is effectively hindered under water lubrication, which accounts for decreased wear of the composite and sets up the prerequisite for it to be used in aqueous environment. The abated transfer of the composite onto the steel surface under water lubrication could be well understood if one took the cooling and lubricating effect of the adsorbed water layer between the sliding surfaces into account. Under dry sliding, the increased frictional heat would lead to strong softening or even local melting of the composite, which makes it easier for the molten material to flow and transfer onto the counterpart surface to form a transfer layer under repetitive sliding. The water as a lubricant and a cooling medium as well, however, acts to hinder the friction heat and prevent or weaken the softening of the composite in the sliding process. In this way, the large-scale surface melting of the composite would be eliminated. Therefore, the transference of the composite matrix onto the counterpart steel surface is considerably hindered under water-lubricated sliding than under dry sliding. Naturally, the transfer of a small amount of the worn composite material onto the counterpart surface is unavoidable even under the water-lubricated sliding, thus, a small amount of Cu was detected on the counterpart surface under water lubrication by EDAX (Fig. 3(d)). The chemical states of the Cu and Fe elements on the worn surfaces of the steel rings under dry- and water-lubricated sliding at a normal load of 200 N and a speed of 1.06 m/s are given in Fig. 4. The peak of Cu2p around 933.6 eV is assigned to Cu or CuO (Fig. 4(a) and (c)), which indicates that the Cu in the composite does transfer onto the stainless steel surface under both dry- and water-lubricated sliding and is possibly accompanied by oxidation. Almost no Fe is observed on the worn surface of the stainless steel, which illustrates that a relatively thicker transfer film of the composite is formed on the surface of the steel under dry sliding (Fig. 4(b)). Contrary to the above, the transfer film of the composite on the stainless steel under water-lubricated sliding is relatively thinner, because the peak of Fe2p assigned to Fe2O3 as an oxidation product of the steel is observed around 710.9 eV (Fig. 4(d)). This observation agrees well with the corresponding SEM and EDAX analytical results. Thus, the powder metallurgical composite of bronze– graphite could be potential candidates as tribo-material in sliding against stainless steel under water lubrication.
4. Conclusions 1. The powder metallurgical composite of bronze– graphite gave friction coefficients of small fluctu-
429
ation with increasing load under both dry- and water-lubricated sliding against stainless steel. In the tested load range, a smaller wear rate of the bronze– graphite composite was always observed under water lubrication than under dry sliding. This was largely attributed to the hindered transfer of the composite onto the steel surface under water lubrication and partly attributed to the cooling effect and boundary lubrication action of the water as a lubricating medium. 2. The bronze–graphite composite was dominated by adhesion wear in dry sliding against the stainless steel, which was significantly abated and accompanied by corrosion to the steel under waterlubricated sliding. Since the bronze–graphite composite showed much better wear-resistance under water-lubricated sliding than under dry sliding against the stainless steel, it could be a potential candidate as the tribo-material in aqueous environment.
Acknowledgements The work was financially supported by National Natural Science Foundation of China (Grant No. 59925513) and National Defence Innovation Foundation of Chinese Academy of Sciences. We also gratefully acknowledge the financial support by State Key Laboratory of Fluid Power Transmission and Control of Zhejiang University (Grant No. 9905).
References [1] Lancaster JK. A review of the influence of environment humidity and water of friction lubrication and wear. Tribol Int 1990;23:371–89. [2] Al-Hashem A, Riad W. The role of microstructure of nickel-aluminium-bronze alloy on its cavitation corrosion behavior in natural seawater. Mater Charact 2002;48:37–41. [3] Bhushan B, Gray S. Investigation of material combinations under high load and speed in synthetic seawater. Lubr Eng 1978;35(11):628–39. [4] Stolarski TA. Wear of water lubricated composite materials. Wear 1980;58:103–8. [5] Andersson P, Lintula P. Load-carrying capability of water lubricated ceramic journal bearings. Tribol Int 1994;27:315–21. [6] Clarke CG, Allen C. The water lubricated, sliding wear behavior of polymeric materials against steel. Tribol Int 1991;10:109–18. [7] Junghans R, Neukirchner J, Schumann D, Lippman KH. The use of sintered metal bearing for high sliding velocities. Tribol Int 1996;9(3):181–92. [8] Rapoport L, Lvovsky M, Lapsker IL, Leshchinsky W. Friction and wear of bronze powder composites including fullerene-like WS2 nanoparticles. Wear 2002;35:47–53. [9] Bowden FP, Tabor D. The friction and lubrication of solids. Oxford: Clarendon Press; 1986. p. 285–90 [Chapter 14]. [10] La PQ, Xue QJ, Liu WM. Tribological properties of Ni3AlCr7C3 composite coating under water lubrication. Tribol Int 2000;33:469–75.
Friction and wear properties of bronze–graphite composite under water lubrication Jun-hong Jia , Jian-min Chen, Hui-di Zhou, Jing-bo Wang, Hua Zhou State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 73000, China Received 23 June 2003; received in revised form 2 December 2003; accepted 30 December 2003
Abstract Bronze–graphite composite was prepared using powder metallurgy. The friction and wear behaviors of the resulting composites in dry- and water-lubricated sliding against a stainless steel were comparatively investigated on an MM-200 friction and wear tester in a ring-on-block contact configuration. The wear mechanisms of the bronze–graphite composite were discussed based on examination of the worn surface morphologies of both the composite block and the stainless steel ring by means of scanning electron microscopy equipped with an energy dispersion spectrometry and on determination of some typical elements on the worn surfaces by means of X-ray photoelectron spectroscopy. It was found that the friction coefficient was higher under water lubrication than that under dry sliding and it showed margined change with increasing load under the both sliding conditions. A considerably decreased wear rate of the bronze–graphite composite was registered under water-lubricated sliding than under dry sliding, though it rose significantly at a relatively higher load. This was attributed to the hindered transfer of the composite onto the counterpart steel surface under water-lubricated sliding and the cooling effect of the water as a lubricant, while its stronger transfer onto the steel surface accounted for its higher wear rate under dry sliding. Thus, the bronze–graphite composite with much better wear-resistance under water-lubricated sliding than under dry sliding against the stainless steel could be a potential candidate as the tribo-material in aqueous environment. # 2003 Elsevier Ltd. All rights reserved. Keywords: Bronze–graphite; Powder metallurgy; Water lubrication; Wear mechanism
1. Introduction In many cases, bearings are required to operate in aqueous medium in which water is either deliberately introduced as a coolant, e.g., in rolling mill bearing, or present as a working fluid, e.g., in submerged pumps and journal bearings of rolling-mills. In the latter case, a water environment can pose special problems because of abrasive contamination and the corrosiveness of water towards many metals [1,2]. Therefore, most of the power hand tools used by navy divers are operated by oil hydraulic motors. However, the use of oil hydraulic system has many disadvantages, e.g., the leakage of working fluid may lead to contamination to the environment [3]. Moreover, food and pharmaceu Corresponding author. Tel.: +86-931-8272179; fax: +86-9318277088. E-mail address: [email protected] (J.-h. Jia).
0301-679X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2003.12.013
tical industries are demanding for the substitution of traditional oils with water-based lubricants because of the pollution of their products caused by their machinery. These disadvantages would be eliminated by use of specifically designed hydraulic motor to operate in water medium. Nevertheless, the selection of materials having the desired corrosion and friction and wear properties in this respect becomes a major challenge to the development of fluid power transmission, since water of corrosiveness may cause cavitation and has low viscosity and poor lubricity [4]. Accordingly, many efforts have been made to develop materials suitable to aqueous environment. The behavior of waterlubricated ceramic journal bearings has been studied [5]. The obtained results suggest to extending water lubrication to systems working in water like submerged pumps. Encouraging results of water lubrication have been achieved with couplings of polymeric materials against steels that have been proposed for drills and
424
J.-h. Jia et al. / Tribology International 37 (2004) 423–429
stainless steel ring and the bronze–graphite composite block were abraded with No. 900 water-abrasive paper and ultrasonically cleaned with acetone. The average surface roughness (Ra) of the rings and blocks is about 0.1–0.2 lm. The water lubrication between the sliding surfaces was realized by continuously dropping of distilled water onto the sliding surface at a rate of 65–70 drops per minute. The friction force between the tested block and the counterpart steel ring was measured with a torque shaft equipped with strain gauges. The width of the wear tracks of the blocks was measured with a naked-eye microscope to an accuracy of 0.01 mm. Then the wear volume loss V of the blocks was calculated as below: 2 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 2 pr b b b2 r2 5 arcsin ð1Þ V ¼ B4 2r 2 180 4
drifters in mines [6]. In this work, the authors investigate the possibility of a rational use of metal-based materials to solve tribological problems in water environment. Sliding bearings produced by power metallurgical processes are employed preferentially in mechanical engineering, in the car industry, and the production of electric motors, optical instruments and precision mechanics [7]. Advance in powder metallurgy technology offers various types of self-lubricating bearings, such as solid lubricant embedded bearings, oil-impregnated bronze, iron, iron–graphite bearing and so on [8]. Such materials were been mostly focused on the application in ambient dry sliding and high temperature. However, few are reported on their friction and wear properties and wear mechanisms in water environment. Therefore, in an effort to investigating the feasibility to use bronze-based composites in aqueous environment. The commercial bronze–graphite powder, which is widely used in different bearing applications, was chosen for the present work. It was prepared by powder metallurgy and tested in water medium. Thus, the friction and wear behaviors of bronze–graphite composite sliding against stainless steel under dry- and water-lubricated condition were comparatively investigated. The friction and wear mechanisms of the bronze–graphite composite were discussed as well. It was expected that the present work would be helpful to provide references to the selection of appropriate metal-based materials for the components subjected to motion and sealing in aqueous environment.
where V refers to the wear volume loss (mm3), B to the width of the block specimen (mm), r to the radius of the stainless steel ring (mm), and b to the width of the wear track (mm). The specific wear rate (K0) was calculated from the volume loss using the following equation: K0 ¼V =ðL dÞ ½mm3 ðN mÞ1
ð2Þ
where d is the sliding distance in meter and L is the load in Newton. Three replicate friction and wear tests were carried out for each specimen with a relative error of 10%, and the average of the three replicate test results is reported in this article. At the end of the friction and wear tests, the blocks were cleaned and dried, followed by the analyses of the worn surface and elemental distributions thereon with a JEM-5600LV scanning electron microscope (SEM) equipped with a KEVEX energy dispersive X-ray spectrometer (EDS) attachment. The chemical states of the elements on the worn surface were determined using an X-ray photoelectron spectroscope (XPS). The XPS analysis was conducted at 400 W and pass energy of 29.35 eV, using Mg-Ka radiation as the excitation source and the binding energy of C1s (284.6) as a reference. The worn surface was sputtered with Ar+ ions for 10 s before analysis.
2. Experimental procedures The bronze–graphite composite samples were prepared by using power metallurgy method. The resulting rectangular cubic blocks had a size of 30 mm 7 mm 6 mm. The friction and wear behavior of the bronze–graphite composite sliding against stainless steel was evaluated on an MM-200 model ring-on-block test rig. The rings of 1 50 mm 10 mm were made of stainless steel (1Cr18Ni9Ti, HB ¼ 1:48 GPa). The chemical composition of the frictional pair is given in Table 1. The tests were carried out at a linear velocity of 1.06 and 0.53 m/s, loads from 100 to 400 N, and duration of 120 min. Before each test, the Table 1 Chemical composition of the frictional pair C
Si
Mn
Cr
Ni
Ti
Fe
0.12
1.00
2.00
17.00 19.00
8.00 11.00
0.02
Balance
Sn 7
Cu 85
Graphite 8
Steel ring
Block
J.-h. Jia et al. / Tribology International 37 (2004) 423–429
Fig. 1. Variations in friction coefficient and wear rate of bronze– graphite composite with load at a sliding speed of 0.53 m/s.
3. Results and discussion 3.1. Friction and wear properties The variations in friction coefficient and wear rate of bronze–graphite composites with load at a sliding speed of 0.53 m/s are shown in Fig. 1. The friction coefficient under the dry sliding at various loads is around 0.23, which is small in terms of the dry sliding condition. This could be attributed to the self-lubricity of the composite at a relatively high temperature under the dry sliding. Surprisingly, the friction coefficient is higher under water lubrication than under dry sliding, though the friction coefficients assume small fluctuation with increasing load under both conditions. We suppose that the water as a lubricant might function to significantly hinder the friction-induced thermal effect and hence to hinder the self-lubricity of the composite as well, while the boundary lubricating action of the water could not compensate for such a hindrance in the self-lubricity of the composite under dry sliding if the applied normal is high enough. Further investi-
425
gation might be meaningful in this respect. Besides, the wear rate of the composite under dry sliding decreases with increasing load. This could also be related to the increased thermal effect at increased load, which helps to strengthen the self-lubricity of the composite under dry sliding. The wear rate of the composite under water-lubricated sliding decreases with increasing load up to 200 N and then it rises gradually with the further increase of the load, but it is always smaller than that under dry sliding. This could be related to the greatly decreased thermal heat under water lubrication, which was beneficial for the composite to retain its mechanical strength and resist wear. In other words, although the increased friction heat under dry sliding is beneficial for the composite to play the solid lubricity [9], it is harmful to the mechanical strength of the composite and leads to increased softening and plastic deformation and hence increased wear of the composite. Therefore, the composite records a larger wear rate under dry sliding than under water lubrication. Accordingly, it might be imperative to pay attention to the balance between the self-lubricity and wear resistance of the composite subject to dry sliding condition, while this would not be a problem with respect to the application of the composite under water-lubricated condition. The variations in the friction coefficient and wear rate of bronze–graphite composites with sliding speed at 200 N are shown in Table 2. The friction coefficients are smaller at high sliding speed under both dry- and water-lubricated conditions. The wear rate at high sliding speed under dry sliding is about one-third of that at low speed. However, the difference in the wear rates of the composites at various sliding speeds becomes margin under water lubrication. The lowest wear rate of the composite is recorded as 1:01 106 mm3 ðN mÞ1 at a normal load of 200 N and a sliding speed of 0.53 m/s. Similarly, the relatively smaller friction coefficient at higher sliding speed might also be attributed to the increased friction heat [10]. In other words, a larger sliding speed corresponded to increased friction heat, which helped to lowering the shear strength by enhancing the softening and melting of the composite surface material under dry sliding. Subsequently, a lowered friction coefficient is registered at higher sliding speed under dry sliding. the decrease of the friction coefficient with sliding speed under water lubrication might
Table 2 Variations in friction coefficient and wear rate of bronze–graphite composite with speed under dry- and water-lubricated sliding at 200 N Sliding speed
Dry sliding 0.53 m/s 1.06 m/s
Wear rate (106 mm3 (N m)1)
Friction coefficient
0.26 0.24
Water lubrication
Dry sliding
Water lubrication
0.31 0.25
14.24 5.01
1.01 1.10
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Fig. 2. SEM morphologies of worn surfaces of the composite under dry sliding (a) and water lubrication (b), distributions of Cu (c) and Fe (e) under dry sliding, and distributions of Cu (d) and Fe (f) under water lubrication at 1.06 m/s and 200 N.
depend on the strengthened boundary lubricating ability of the water film at increased sliding speed. At the same time, the cooling effect of water was also significant, which helped to decrease the surface temperature and then restrain the adhesion transference and plastic deformation of the composite effectively, though the corrosion of the counterpart steel by water might also
play a role in affecting the friction and wear behavior. The action of water could be well understood if one noticed that the sliding surface of the porous composite was able to adsorb water and entrap the adsorbed water therein. Thus, in terms of the comprehensive friction-reducing and antiwear ability, bronze–graphite composite
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Fig. 3. SEM morphologies of worn surface of the steel rings under dry sliding (a) and water lubrication (b), distributions of Cu (c) and Fe (e) under dry sliding, and distributions of Cu (d) and Fe (f) under water lubrication against the composite at 1.06 m/s and 200 N.
could be potential candidates as tribo-material in sliding against stainless steel under water-lubricated condition. 3.2. The examinations of the worn surfaces The SEM morphologies of and the corresponding elemental distributions on the worn surfaces of the
bronze–graphite composites under dry- and waterlubricated sliding against the stainless steel at a normal load of 200 N and a sliding speed of 1.06 m/s are given in Fig. 2. The worn surface of the bronze–graphite composite under dry sliding shows signs of plastic deformation, microcracking, and scuffing (Fig. 2(a)), which is attributed to the adhesion and abrasive wear of the bronze–graphite composite. The worn surface of
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Fig. 4.
XPS spectra of Cu and Fe on the worn steel surfaces under dry- and water-lubricated sliding against the composite.
the composite under water lubrication is also characterized by scuffing, with the microcracking and plastic deformation to be significantly hindered (Fig. 2(b)). Meanwhile, more holes are seen on the worn composite surface sliding against the stainless steel under water lubrication, as compared with Fig. 2(a). This could be attributed to the corrosiveness of the water and might partly account for the increased friction coefficient under water-lubricated sliding at a relatively smaller speed. In addition, the worn composite surface under dry sliding records smaller concentration of Fe (Fig. 2(e)) than that under water-lubricated sliding (Fig. 2(f)). This implies that Fe is more liable to transfer onto the worn surface of the composite under water-lubricated sliding, while such a kind of Fe transfer onto the composite surface is hindered under dry sliding. Thus, it might be rational to infer that the powder metallurgical composite of bronze–graphite could be suitable to water-lubricated applications. The SEM morphologies of the worn surfaces of the stainless steel rings under dry- and water-lubricated sliding at a load of 200 N and a speed of 1.06 m/s are given in Fig. 3. The worn steel surface under dry sliding shows obvious signs of adhesion which is attributed to the transfer of the bronze–graphite composite onto the steel surface (Fig. 3(a)). Such a kind of transfer of the composite is also confirmed by the elemental distri-
bution of Cu on the steel surface (Fig. 3(c)). Under water lubrication, the worn steel surface becomes smoother than that under dry sliding and the adhesion signs are considerably abated (Fig. 3(b)). The abated adhesion on the steel surface under water lubrication, i.e. the abated transfer of the composite onto the steel surface in this case is also confirmed by the elemental distribution of Cu (Fig. 3(d)), where less Cu is present as compared with that under dry sliding. Besides, it is equally interesting to notice that the Fe distributions on the worn steel surfaces under dry- and water-lubricated sliding against the bronze–graphite composite conform to the different friction and wear behaviors of the composites in the both sliding conditions as well. In other words, since a smaller amount of the composite has been transferred onto the worn steel surface, more Fe is hence detected thereon by EDAX (Fig. 3(f)). Vise versa, a smaller amount of Fe is observed on the worn steel surface under dry sliding against the bronze– graphite composite, because more composite matrix has been transferred thereon (Fig. 3(e)). Though the transference of the composite matrix on the counterpart steel surface is relatively lighter under water-lubricated sliding than that under dry sliding, it together with the adsorbed water boundary layer contributed to the better wear resistance of the composite in the former case. Therefore, it can be concluded that
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the transfer of the bronze–graphite onto the steel surface is effectively hindered under water lubrication, which accounts for decreased wear of the composite and sets up the prerequisite for it to be used in aqueous environment. The abated transfer of the composite onto the steel surface under water lubrication could be well understood if one took the cooling and lubricating effect of the adsorbed water layer between the sliding surfaces into account. Under dry sliding, the increased frictional heat would lead to strong softening or even local melting of the composite, which makes it easier for the molten material to flow and transfer onto the counterpart surface to form a transfer layer under repetitive sliding. The water as a lubricant and a cooling medium as well, however, acts to hinder the friction heat and prevent or weaken the softening of the composite in the sliding process. In this way, the large-scale surface melting of the composite would be eliminated. Therefore, the transference of the composite matrix onto the counterpart steel surface is considerably hindered under water-lubricated sliding than under dry sliding. Naturally, the transfer of a small amount of the worn composite material onto the counterpart surface is unavoidable even under the water-lubricated sliding, thus, a small amount of Cu was detected on the counterpart surface under water lubrication by EDAX (Fig. 3(d)). The chemical states of the Cu and Fe elements on the worn surfaces of the steel rings under dry- and water-lubricated sliding at a normal load of 200 N and a speed of 1.06 m/s are given in Fig. 4. The peak of Cu2p around 933.6 eV is assigned to Cu or CuO (Fig. 4(a) and (c)), which indicates that the Cu in the composite does transfer onto the stainless steel surface under both dry- and water-lubricated sliding and is possibly accompanied by oxidation. Almost no Fe is observed on the worn surface of the stainless steel, which illustrates that a relatively thicker transfer film of the composite is formed on the surface of the steel under dry sliding (Fig. 4(b)). Contrary to the above, the transfer film of the composite on the stainless steel under water-lubricated sliding is relatively thinner, because the peak of Fe2p assigned to Fe2O3 as an oxidation product of the steel is observed around 710.9 eV (Fig. 4(d)). This observation agrees well with the corresponding SEM and EDAX analytical results. Thus, the powder metallurgical composite of bronze– graphite could be potential candidates as tribo-material in sliding against stainless steel under water lubrication.
4. Conclusions 1. The powder metallurgical composite of bronze– graphite gave friction coefficients of small fluctu-
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ation with increasing load under both dry- and water-lubricated sliding against stainless steel. In the tested load range, a smaller wear rate of the bronze– graphite composite was always observed under water lubrication than under dry sliding. This was largely attributed to the hindered transfer of the composite onto the steel surface under water lubrication and partly attributed to the cooling effect and boundary lubrication action of the water as a lubricating medium. 2. The bronze–graphite composite was dominated by adhesion wear in dry sliding against the stainless steel, which was significantly abated and accompanied by corrosion to the steel under waterlubricated sliding. Since the bronze–graphite composite showed much better wear-resistance under water-lubricated sliding than under dry sliding against the stainless steel, it could be a potential candidate as the tribo-material in aqueous environment.
Acknowledgements The work was financially supported by National Natural Science Foundation of China (Grant No. 59925513) and National Defence Innovation Foundation of Chinese Academy of Sciences. We also gratefully acknowledge the financial support by State Key Laboratory of Fluid Power Transmission and Control of Zhejiang University (Grant No. 9905).
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