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Sliding wear of cast zinc-based alloy bearings under static and dynamic loading conditions
Wear 252 (2002) 693–703
Sliding wear of cast zinc-based alloy bearings under static and dynamic loading conditions T. Sava¸skan a,∗ , G. Pürçek a , S. Murphy b a
Mechanical Engineering Department, Karadeniz Technical University, 61080-Trabzon, Turkey b School of Engineering and Applied Science, Aston University, Birmingham B4 7ET, UK
Received 3 May 2001; received in revised form 25 September 2001; accepted 17 October 2001
Abstract The lubricated wear behaviour of cast journal bearings, produced from a series of zinc-based alloys and SAE 660 bronze as a reference material, was investigated under both static and dynamic loading conditions using a bearing test rig. All of the zinc-based alloys had higher wear resistance than the SAE 660 bronze. Among the zinc-based alloys, the wear resistance of the monotectoid-based alloys was superior to those based on near-eutectoid composition, and the best wear performance under both static and dynamic loading conditions was obtained with ZnAl40Cu2Si1 alloy. Copper content affected the wear resistance of monotectoid zinc-based alloys. Under dynamic loading conditions, it increased with increasing copper content up to 2%, but declined thereafter. Tensile properties and hardness of the monotectoid alloys were also affected by their copper content. Loading conditions had a strong influence on the wear rate. Under static loading conditions, as-cast zinc-based alloys showed higher wear resistance than the equivalent heat-treated alloys, but this behaviour was reversed for dynamic loading. Possible reasons for this are briefly discussed. © 2002 Published by Elsevier Science B.V. Keywords: Zinc-based alloys; Sliding wear; Static and dynamic loading
1. Introduction Zinc-based alloys have been used for many years for the production of small components and to a certain extent for plain bearings [1,2]. They have good physical, mechanical and tribological properties [3–5]. Their main advantages are low cost, high resistance to wear and abrasion, ability to withstand high bearing loads and good emergency or dry-running characteristics [5]. However, their major disadvantages are low mechanical and creep strength at elevated temperatures and long-term dimensional instability at ambient or slightly elevated temperatures [6,7]. In applications requiring close tolerances like journal bearings, dimensional instability may cause a serious problem. It has been shown that an irreversible expansion on ageing of the copper-containing zinc–aluminium alloys is due to the conversion of a copper-rich metastable phase formed during casting to the equilibrium phase [8–10]. However, a stabilising heat treatment may be used to complete these changes before machining [2,7,8,11].
∗ Corresponding author. Tel.: +90-462-377-2919; fax: +90-462-325-7405. E-mail address: [email protected] (T. Sava¸skan).
0043-1648/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 8 7 6 - 6
Bearings produced from the zinc-based alloys are known to have good tribological properties under static loading [1,7], but no work appears to have been published on their tribological behaviour under dynamic loading conditions. Therefore, the purpose of this work is to study the wear properties of cast bearings produced from a range of zinc-based alloys under both static and dynamic loading conditions, compare the results, and assess their wear performance vis-a-vis a conventional bronze bearing.
2. Experimental procedure 2.1. Production and testing of alloys A series of zinc–aluminium, zinc–aluminium–copper and zinc–aluminium–copper–silicon alloys were produced for this work. These were in two groups: one based on the near-eutectoid (27% Al) and the other on the monotectoid (40% Al) compositions as per the Zn–Al phase diagram. The alloys were melted in a crucible using an electrical furnace and poured at temperatures between 600 and 700 ◦ C into a steel mould pre-heated to 100 ◦ C. Ingots had a conical shape with a length of 180 mm, a bottom diameter of 57 mm and a top diameter of 70 mm. Some of the ingots
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Table 1 Chemical composition of the alloys
2.2. Wear testing
Alloy
Half-sleeves were produced from the alloys by fine machining and used as the experimental bearings for the wear tests. This type of bearings have been used in some engineering applications including automotive, aviation and agricultural industries [2]. The shape and dimensions of the bearing are shown in Fig. 1. The test bearing had different surface areas including load-bearing and unloaded areas, an oil hole and an oil groove. The load-bearing areas were selected to obtain a certain level of pressure on the bearing surface. This selection was necessary because of the limitation of hydraulic loading system of the rig. The wear tests were carried out using a bearing test machine. A sectional view of the test machine is shown in Fig. 2. The test rig consisted of a 3 kW drive motor, a test bearing and bearing housing, two shafts, two rolling bearings to support the main shaft, a hydraulic loading unit, a speed control unit and a lubricating system. The main shaft having a diameter of 44.480 ± 0.010 mm was made of St 37 steel (Fe–0.18% C–0.019% P–0.027% S) and its surface was hardened to 50 HRC by carburising. The part of the shaft acting as the rotating element of the bearing was ground and polished with a 0.05 m alumina suspension. The hydraulic loading system of the machine consists of a motor with 1.5 kW capacity, a pump, a pressure relief valve, a proportional valve, an electronic regulator, a pressure transducer, a manometer, a hydraulic cylinder, an oscilloscope and a computer with a control card to control the loading. Each bearing was ultrasonically cleaned and weighed accurately before each test. The test bearing was removed after an interval of 20 h corresponding to a sliding distance of 93.6 km, cleaned in solvents and weighed to determine the mass loss. Since the bearing life is determined by the
ZnAl27Cu2 ZnAl27Cu2Si1 ZnAl40 ZnAl40Cu0.5 ZnAl40Cu1 ZnAl40Cu2 ZnAl40Cu3 ZnAl40Cu4 ZnAl40Cu2Si1 SAE 660 bronze
Chemical composition (wt.%) Zn
Al
Cu
Si
71.2 69.4 61.5 60.2 61.2 57.6 59.7 58.8 57.1 Cu–7 wt.% Pb
26.6 27.3 38.5 39.3 37.8 40.3 37.4 37.1 39.9 Cu–7 wt.% Sn
2.2 2.2 – 0.5 1.0 2.1 2.9 4.1 2.1 Cu–3 wt.% Zn
– 1.1 – – – – – 0.9
were subsequently heat treated by ageing at 150 ◦ C for 240 h, conditions which are known to effectively complete the equilibrium phase transformations at the expense of some structural coarsening [7]. The chemical compositions of the alloys were determined using atomic absorption analysis (Table 1). Samples for electron microscopy were prepared using standard metallographic techniques and etched in 4% nital, followed by examination with a scanning electron microscope (SEM) using atomic number (BSE) contrast. Tensile tests were carried out on round specimens with a gauge diameter of 8 mm and gauge length of 40 mm at a strain rate of 0.0075 s−1 . From the test results, tensile strength and percentage elongation were determined (Table 2). Brinell hardness of the alloys was measured using a 2.5 mm diameter ball indentor under a load of 62.5 kgf. The hardness of each alloy was determined by taking the average of five readings (Table 2). Table 2 Hardness, tensile strength, percentage elongation and density of the alloysa Alloy
Condition
Hardness (BSD)
Tensile strength (MPa)
Elongation (%)
Density (kg m−3 )
ZnAl27Cu2
A B
110 81
306 273
2.0 5.0
4910 4890
ZnAl27Cu2Si1
A B
120 87
337 250
1.8 3.0
4850 4830
ZnAl40
A
103
260
4.5
3980
ZnAl40Cu0.5
A
107
270
3.9
4210
ZnAl40Cu1
A
110
294
1.9
4230
ZnAl40Cu2
A B
117 85
316 285
1.8 4.2
4280 4250
ZnAl40Cu3
A
120
314
2.0
4295
ZnAl40Cu4
A
125
303
1.2
4355
ZnAl40Cu2Si1
A B
122 89
339 255
1.5 3.1
4270 4255
SAE 660 bronze
A
85
205
7.2
8780
a
A: as-cast condition; B: heat-treated at
150 ◦ C
for 10 days.
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Fig. 1. Sectional view and dimensions of the test bearing.
clearance between the journal and the bearing, the volume loss is a more significant parameter than the mass loss, so the measured mass losses were converted into volume losses using the measured density values of the alloys. After the test, the wear surfaces of the bearings were examined using the SEM in SE mode. Wear tests were performed under both static and dynamic loading conditions. In the first case, the tests were carried out under a constant pressure of 20 MPa (20 kN) and a sliding speed of 1.3 m s−1 . In the second case, the tests were performed under a dynamic loading of 15–20 MPa (15–20 kN) with a loading frequency of 10 Hz and at a sliding speed of 1.3 m s−1 (560 rpm). In all the cases, the test bearing was continuously lubricated with a SAE 20W/50 oil at a flow rate
of 240 ml h−1 and the test procedure was repeated for each bearing until a total sliding distance of 850 km was completed. Normally, single wear test was conducted for each data point, but some of them were checked several times using different samples of the same alloys and no considerable amount of change was observed.
3. Results 3.1. Chemical composition and microstructure The chemical compositions of the alloys tested are given in Table 1. The microstructure of the binary ZnAl40Cu
Fig. 2. Sectional view of the bearing test rig.
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Fig. 3. Microstructure of the as-cast ZnAl40 alloy.
alloy in the as-cast condition consisted of cored aluminiumrich ␣-dendrites with zinc-rich -phases in the inter-dendritic regions, Fig. 3 The addition of copper produced some copper-rich inter-metallic particles in the inter-dendritic regions, but it had no effect on the shape and size of the dendrites of the alloys as shown in Fig. 4. The zinc– aluminium–copper–silicon alloys in the as-cast condition consisted of aluminium-rich ␣-dendrites, zinc- and copperrich inter-dendritic phases and silicon particles. Fig. 5 shows the general microstructure of the ZnAl40Cu2Si1 alloy in the as-cast condition. A detailed microstructure of this alloy is also shown in Fig. 6. The ageing heat-treatment had no discernible effect on the dendritic structure of the zinc-based alloys, but coarsened the decomposition products of the  phase only, as seen in the microstructures of the heat-treated ZnAl40Cu2 and ZnAl40Cu2Si1 alloys, Figs. 7 and 8. The microstructure of the SAE 660 bronze was observed to be consisted of copper-rich ␣-dendrites and eutectoid ␣- and ␦-phases as seen in Fig. 9.
Fig. 4. Microstructure of the as-cast ZnAl40Cu2 alloy.
Fig. 5. Microstructure of the as-cast ZnAl40Cu2Si1 alloy.
Fig. 6. Micrograph showing details of the microstructure of the as-cast ZnAl40Cu2Si1 alloy.
Fig. 7. Microstructure of the heat-treated ZnAl40Cu2 alloy.
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3.2. Mechanical testing
Fig. 8. Microstructure of the heat-treated ZnAl40Cu2Si1 alloy.
The hardness, tensile strength, percentage elongation and density of the alloys in the as-cast and heat-treated conditions are given in Table 2. This table shows that amongst the as-cast alloys, the highest tensile strength was obtained from the ZnAl40Cu2Si1 alloy while the ZnAl40 alloy had the lowest. The highest elongation was obtained from the binary ZnAl40 alloy and the lowest from the ZnAl40Cu4 alloy. The highest hardness was obtained from the ZnAl40Cu4 alloy and the lowest hardness from the ZnAl40 alloy. The addition of 1% Si improved the hardness and tensile strength of the ternary ZnAl27Cu2 and ZnAl40Cu2 alloys but adversely affected their elongation. Heat treatment reduced the hardness and tensile strength of all the zinc-based alloys, but improved their ductility. The hardness and tensile strengths of the monotectoid zinc–aluminium–copper alloys increased with increasing the copper content up to 2%, above which the tensile strength decreased slightly as their copper content increased. 3.3. Wear testing
Fig. 9. Microstructure of the SAE 660 bronze.
3.3.1. Static loading The plots of volume loss versus sliding distance for the bearings tested under static loading condition are shown in Fig. 10. This figure shows that the wear resistance of all the zinc-based alloys was considerably higher than the SAE 660 bronze. The heat treatment decreased the wear resistance of the zinc-based alloys. The highest wear resistance was obtained from as-cast ZnAl40Cu2Si1 alloy while heat-treated ZnAl27Cu2 alloy showed the lowest wear resistance. The ZnAl27Cu2Si1 alloy showed the lowest wear resistance amongst the as-cast zinc-based alloys.
Fig. 10. The cumulative volume loss vs. sliding distance for the bearings produced from zinc-based alloys and SAE 660 bronze tested under static loading.
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Fig. 11. The cumulative volume loss vs. sliding distance for the bearings produced from zinc–aluminium-based alloys and SAE 660 bronze tested under dynamic loading.
3.3.2. Dynamic loading The plots of volume loss versus sliding distance for the bearings tested under dynamic loading condition are given in Fig. 11. It can be seen from this figure that all the zinc– aluminium-based alloys had a higher wear resistance than the SAE 660 bronze. The heat-treated zinc-based alloys showed lower volume loss than the corresponding as-cast version. The lowest volume loss was obtained from the heat-treated ZnAl40Cu2Si1 alloy with the as-cast version of the same alloy showing the lowest volume loss in the as-cast group. The ZnAl27Cu2Si1 alloy exhibited the highest volume loss of the as-cast alloys. To compare the wear behaviour of the bearings tested under static and dynamic loading conditions more clearly, the
volume loss versus sliding distance curves are presented together in Figs. 12–16. It can be seen from the figures that the volume loss of the as-cast alloys in general under static loading condition was higher than that in the dynamic loading condition at the shorter sliding distances. However, the trend reversed at longer sliding distances and the volume loss of these alloys under static loading condition was lower than that in the dynamic loading condition. In contrast, the volume loss of the heat-treated zinc-based alloys under dynamic loading condition was lower than that in the static loading condition. To demonstrate the effect of copper content on the tensile strength and wear behaviour of the monotectoid zinc–aluminium–copper alloys, additional wear tests were
Fig. 12. Volume loss vs. sliding distance for the bearings produced from ZnAl27Cu2 alloy in the as-cast and heat-treated conditions tested under both static and dynamic loadings.
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Fig. 13. Volume loss vs. sliding distance for the bearings produced from ZnAl27Cu2Si1 alloy in the as-cast and heat-treated conditions tested under both static and dynamic loadings.
Fig. 14. Volume loss vs. sliding distance for the bearings produced from ZnAl40Cu2 alloy in the as-cast and heat-treated conditions tested under both static and dynamic loadings.
Fig. 15. Volume loss vs. sliding distance for the bearings produced from ZnAl40Cu2Si1 alloy in the as-cast and heat-treated conditions tested under both static and dynamic loadings.
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Fig. 16. Volume loss vs. sliding distance for the bearings produced from SAE 660 bronze tested under both static and dynamic loadings.
Fig. 17. The effect of copper content on the tensile strength of as-cast ZnAl40 alloy and the volume loss of the bearing produced from this alloy under dynamic loading where a sliding distance of 750 km at a sliding speed of 1.3 m s−1 and pressure of 15–20 MPa (±2.5 MPa).
carried out on as-cast monotectoid alloys under dynamic loading condition. Volume losses after 750 km and measured tensile strengths are plotted as a function of copper content in Fig. 17. The figure shows that both the tensile strength and wear resistance increased with increasing copper content up to 2%, above which both decreased slightly. The SEM images obtained from bearings produced using the ZnAl27Cu2Si1 alloy and tested under both static and dynamic loading conditions are shown in Figs. 18–21 to illustrate the general features. The worn surfaces of the bearings produced from all of the zinc-based alloys were found to be quite similar to each other, but heat-treated versions of the alloys showed a greater degree of smearing than as-cast versions. No smearing was observed on the worn surfaces of the bearings produced using the SAE 660 bronze, but instead they had deep scratches and extensive gouging as seen in Figs. 22 and 23.
Fig. 18. Worn surface of the bearing produced from as-cast ZnAl27Cu2Si1 alloy tested under static loading condition.
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Fig. 19. Worn surface of the bearing produced from heat-treated ZnAl27Cu2Si1 alloy tested under static loading condition. Fig. 22. Worn surface of the bearing produced from SAE 660 bronze tested under static loading condition.
Fig. 20. Worn surface of the bearing produced from as-cast ZnAl27Cu2Si1 alloy tested under dynamic loading condition. Fig. 23. Worn surface of the bearing produced from SAE 660 bronze tested under dynamic loading condition.
4. Discussion
Fig. 21. Worn surface of the bearing produced from heat-treated ZnAl27Cu2Si1 alloy tested under dynamic loading condition.
The excellent sliding wear properties of the bearings produced from zinc-based alloys have been attributed to their multiphase structure and the formation of zinc and aluminium oxide films on the bearing surface [12–14]. The aluminium-rich ␣-phase, copper-rich inter-metallic phases (ε and T ) and silicon particles improve the load-bearing capacity of the alloys [8]. The zinc-rich -phase facilitates sliding and provides solid lubrication characteristics by smearing on the mating surfaces under extreme running conditions, particularly in the presence of thermally stable phases like silicon [15]. Oxidative wear occurs in these alloys, and the surface oxide films contribute to wear resistance. The aluminium oxide is a hard compound and therefore also acts as a load-bearing phase, while the zinc oxide is a much softer compound which allows it to act as a lubricant [15,16].
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The SEM images of the worn surfaces showed that different wear mechanisms operated for bearings produced from the zinc-based alloys and SAE 660 bronze. Material transfer and smearing were observed to be more influential in the zinc-based alloy bearings as shown in Figs. 18–21. Although the bronze is an excellent bearing alloy having a strong but ductile matrix reinforced by hard particles of ␦-phase, it did not have the advantage of the compliant matrix and tribologically advantageous oxide films unlike the zinc-based alloys. Figs. 22 and 23 show deep scratches and extensive gouging in the bronze bearings. These bearings are much more susceptible to loss of surface material as detached particles which were less easily re-imbedded in the surface. These observations suggest that adhesion and smearing is the main mechanism for the zinc-based alloys, while adhesive wear dominates for the bronze. The running-in behaviour of the zinc-based alloys and the bronze differed according to test conditions. For the zinc-based alloys, the volume loss of the bearings reduced sharply after running for distances in excess of about 500 km under static loading (Fig. 10), but under dynamic loading, the volume loss showed a small but continuous increase up to the maximum distance of 850 km. This may result from the volumetric change which takes place in the as-cast alloys after a certain period of test time, and surface fatigue due to dynamic loading. The bronze exhibited the same behaviour of a small but steady increase under both conditions. Under static loading condition, Fig. 10 shows a great amount of increase in volumetric loss of material at 850 km for heat-treated versions of the four zinc-based alloys compared with their as-cast versions. Any improvement in conformability, embeddability and damping capacity, or added support by formation of hard T -phase, resulting from the prolonged heat treatment [8,17] was evidently more than that offset by lower matrix strength, and thus this poor behaviour of the heat-treated bearings under static loading is readily understandable. In the dynamic tests, the load cycle used was effectively a fixed load of 17.5 kN with a superposed cyclic load of ±2.5 kN, giving an average load of 12.5% less than for static loading at 20 kN. The reduced mean load would, thus, be expected to improve the wear resistance. A comparison of Figs. 10 and 11 shows that for the heat-treated versions of the alloys this expected behaviour was obtained. The volume loss for the heat-treated versions of all four zinc-based alloys in the dynamic test was lower than that of the alloys in the static test. However, the effect was quite the opposite for the as-cast versions of the same alloys. The volume loss for these alloys tested under dynamic loading condition was much higher than that of the same alloys tested under static loading condition. There is no ready explanation for this poor behaviour of the as-cast alloys under dynamic loading, but one possibility is that the brittleness of the as-cast alloys had played a part. All cast alloys are porous. And in these zinc-based alloys, it is predominately shrinkage porosity wherein the irregular
shape of the voids contribute to low fracture toughness. Heat treatment ameliorates this low toughness. In sliding wear, the stress system at a point near the bearing surface comprises a normal stress due to the applied load, combined with a shear stress parallel to the sliding surface which is determined by the coefficient of friction at the bearing interface and the normal load. A simple Mohr’s Circle analysis shows that this combination produces a tensile principal stress acting at a low angle to the sliding surface, the exact angle determined by the relative magnitudes of the normal and shear stresses. Macroscopically the shear and normal stresses are directly related by the coefficient of friction, but on a small scale different parts of the sliding surface will bear different proportions of the applied load, and the coefficient of friction will be much higher where local welding takes place. Thus, from point to point on the surface and from time to time there will be a constant change in the angle and magnitude of the induced tensile stress even under static loading. When dynamic loading is applied there will be an additional cyclic change during which the local components of stress become even more de-coupled from the macroscopic values, thus, widening the angular range of tensile stress variation. In brittle as-cast alloys, this could activate cracking from additional groups of defects such as shrinkage pores leading to more rapid loss of surface material. Another possibility is that the thermal cycling associated with the load cycling causes near-surface cracking, which would again be more important in the more brittle matrix material of as-cast alloys. Either of these possibilities might also provide an explanation of the observation that with dynamic loading wear continues slightly after the running-in period, but further research is needed in this area. The effect of silicon on wear resistance was found to vary considerably amongst these zinc-based alloys. For the monotectoid alloys, the addition of 1% Si increased the wear resistance of both as-cast and heat-treated versions under both static and dynamic loadings. The silicon-containing alloy has a hard silicon phase which provides load carrying capability in addition to that provided by the copper-rich inter-metallic compounds [11]. For the near-eutectoid-based alloys the effect was less clear. Figs. 10 and 11 show that the addition of 1% Si decreased the wear resistance of the as-cast near-eutectoid alloy under static loading condition but increased that of the heat-treated version under both static and dynamic loading conditions. More detailed examination is needed to provide a scientific explanation for this effect. Aluminium content had a powerful effect on wear resistance. Figs. 10 and 11 show that the monotectoid alloys (ZnAl40Cu2 and ZnAl40Cu2Si1) had superior wear properties to the near-eutectoid-based alloys (ZnAl27Cu2 and ZnAl27Cu2Si1). This was probably due to the aluminium-rich matrix in these alloys, as it is well known that the hardness and tensile strength of zinc-based alloys increases with increasing aluminium content [8],
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which is supported by the mechanical properties listed in Table 2. Both, the tensile strength and wear resistance of the monotectoid-based zinc–aluminium–copper alloys increased with increasing copper content up to 2%, above which both decreased slightly (Fig. 17). This may be explained in terms of the effect of added copper on the structure and properties of zinc–aluminium alloys. It has been shown previously [8], and confirmed by our own results (Table 2 and Fig. 17), that the hardness and tensile strength of zinc–aluminium alloys increase with increasing copper content up to 2%. However, when the copper content of the zinc–aluminium–copper alloys exceeds this level (2%) copper-rich T and ε inter-metallic phases are formed in the inter-dendritic regions [18,19]. These are hard but brittle phases and this is why the tensile strength of the zinc-based alloys containing more than 2% Cu decreases with the increasing copper content while the hardness increases continuously (Table 2). It can be concluded that the wear resistance of the bearings produced from zinc-based alloys depends on the tensile strength, rather than the hardness of these alloys under dynamic loading.
5. Conclusions 1. Adhesion and smearing is the main mechanism for wear in zinc-based alloys, while abrasive wear dominates for the commercial SAE 660 bronze. 2. The bearings produced from the zinc-based alloys exhibit higher wear resistance than the bearings produced from bronze. These bearings appear to be good alternatives to the bronze bearings during operation under both static and dynamic loadings. 3. Under static loading conditions, the as-cast zinc-based alloys showed higher wear resistance than the same alloys in the heat-treated condition. 4. Under dynamic loading conditions, the volume loss of as-cast zinc-based alloys increased compared with static loading tests, while the overall trend was reversed for those of heat-treated alloys. 5. Monotectoid zinc–aluminium-based alloys had superior wear resistance to the near-eutectoid-based alloys in both as-cast and heat-treated conditions, and under both static and dynamic loading conditions. 6. The highest relative wear resistance was obtained from the ZnAl40Cu2Si1 alloy in both as-cast and heat-treated conditions and under both static and dynamic loading. The best performance was obtained for the as-cast version under static loading. 7. Under dynamic loading conditions, the wear resistance of the monotectoid-based zinc–aluminium–copper alloys increased with increasing the copper content up to 2%
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but declined rapidly at higher levels. Wear resistance correlated strongly with the tensile strength. Acknowledgements This work was supported by the Research fund of Karadeniz Technical University. The authors would like to thank Dr. Tevfik Küçükömero g lu and all the technicians in the Materials Division of the Mechanical Engineering Department of this university for their help. References [1] S. Murphy, T. Sava¸skan, Comparative wear behaviour of Zn–Al-based alloys in an automotive engine application, Wear 98 (1984) 151–161. [2] T. Calayag, D. Ferres, High performance, high aluminum zinc alloys for low speed bearings and bushings, in: Proceedings of the SAE Annual Conference, Paper No. 820643, 1983, pp. 2241–2251. [3] T. Calayag, Zinc alloys replace bronze in mining equipment bushings and bearings, Mining Eng. 1983, 727–728. [4] E. Gervais, H. Levert, M. Bess, The development of a family of zinc-based foundry alloys, Trans. Am. Foundrym. Soc. 88 (1980) 183–194. [5] H. Geng, J. Ma, Friction and wear of Al–Zn–Pb bearings alloy, Wear 169 (1993) 201–207. [6] Y. Zhu, B. Yan, W. Huan, Bearing wear resistance of monotectoid Zn–Al based alloy (ZA-35), Mater. Sci. Technol. 11 (1995) 109–113. [7] T. Sava¸skan, S. Murphy, Mechanical properties and lubricated wear of Zn–Al25-based alloys, Wear 116 (1987) 211–224. [8] T. Sava¸skan, The structure and properties of zinc–aluminium-based bearing alloys, Ph.D. Thesis, University of Aston, Birmingham, 1980. [9] T. Ma, Q.D. Chen, S.C. Li, H.M. Wang, Effect of Mn on lubricated friction and wear properties of Zn–Al alloys, Wear of Materials, Louyang Institute of Technology, Louyang, PR China,1989. [10] M. Durman, S. Murphy, Precipitation of metastable ε-phase in a hypereutectic zinc–aluminum alloy containing copper, Acta Metal. Matter. 39 (1991) 2235–2242. [11] B.K. Prasad, Microstructure, mechanical properties and sliding wear characteristics of Zn-based alloys: effects of partially substituting Cu by Si, Z. Metallkde. 88 (1997) 929–933. [12] R.J. Marczak, R. Ciach, Tribological properties of concentrated Al–Zn alloys, in: Proceedings of the 1st Europe Tribology Congress, London, 1973, pp. 223–227. [13] B.K. Prasad, Effect of microstructure on the sliding wear performance of a Zn–Al–Ni alloy, Wear 240 (2000) 100–112. [14] B. Bhushman, B.K. Gupta, Handbook of Tribology, Materials, Coatings and Surface Treatments, McGraw-Hill, New York, 1991. [15] B.K. Prasad, Effects of silicon addition and test parameters on sliding wear characteristics of zinc-based alloys containing 37.5% aluminium, Mater. Trans., JIM 38 (1997) 701–706. [16] H. Tarabian, J.P. Pathak, S.N. Tiwari, Wear characteristics of Al–Si alloys, Wear 172 (1994) 49–58. [17] B.K. Prasad, Influence of heat treatment on the physical, mechanical and tribological properties of a zinc-based alloy, Z. Metallkde. 87 (1996) 226–232. [18] S. Murphy, Solid-phase reactions in the low-copper part of the Al–Cu–Zn system, Z. Metallkde. 71 (1980) 96–102. [19] T. Sava¸skan, S. Murphy, Decomposition of Zn–Al alloys on quench-aging, Mater. Sci. Technol. 6 (1990) 695–700.
Sliding wear of cast zinc-based alloy bearings under static and dynamic loading conditions T. Sava¸skan a,∗ , G. Pürçek a , S. Murphy b a
Mechanical Engineering Department, Karadeniz Technical University, 61080-Trabzon, Turkey b School of Engineering and Applied Science, Aston University, Birmingham B4 7ET, UK
Received 3 May 2001; received in revised form 25 September 2001; accepted 17 October 2001
Abstract The lubricated wear behaviour of cast journal bearings, produced from a series of zinc-based alloys and SAE 660 bronze as a reference material, was investigated under both static and dynamic loading conditions using a bearing test rig. All of the zinc-based alloys had higher wear resistance than the SAE 660 bronze. Among the zinc-based alloys, the wear resistance of the monotectoid-based alloys was superior to those based on near-eutectoid composition, and the best wear performance under both static and dynamic loading conditions was obtained with ZnAl40Cu2Si1 alloy. Copper content affected the wear resistance of monotectoid zinc-based alloys. Under dynamic loading conditions, it increased with increasing copper content up to 2%, but declined thereafter. Tensile properties and hardness of the monotectoid alloys were also affected by their copper content. Loading conditions had a strong influence on the wear rate. Under static loading conditions, as-cast zinc-based alloys showed higher wear resistance than the equivalent heat-treated alloys, but this behaviour was reversed for dynamic loading. Possible reasons for this are briefly discussed. © 2002 Published by Elsevier Science B.V. Keywords: Zinc-based alloys; Sliding wear; Static and dynamic loading
1. Introduction Zinc-based alloys have been used for many years for the production of small components and to a certain extent for plain bearings [1,2]. They have good physical, mechanical and tribological properties [3–5]. Their main advantages are low cost, high resistance to wear and abrasion, ability to withstand high bearing loads and good emergency or dry-running characteristics [5]. However, their major disadvantages are low mechanical and creep strength at elevated temperatures and long-term dimensional instability at ambient or slightly elevated temperatures [6,7]. In applications requiring close tolerances like journal bearings, dimensional instability may cause a serious problem. It has been shown that an irreversible expansion on ageing of the copper-containing zinc–aluminium alloys is due to the conversion of a copper-rich metastable phase formed during casting to the equilibrium phase [8–10]. However, a stabilising heat treatment may be used to complete these changes before machining [2,7,8,11].
∗ Corresponding author. Tel.: +90-462-377-2919; fax: +90-462-325-7405. E-mail address: [email protected] (T. Sava¸skan).
0043-1648/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 8 7 6 - 6
Bearings produced from the zinc-based alloys are known to have good tribological properties under static loading [1,7], but no work appears to have been published on their tribological behaviour under dynamic loading conditions. Therefore, the purpose of this work is to study the wear properties of cast bearings produced from a range of zinc-based alloys under both static and dynamic loading conditions, compare the results, and assess their wear performance vis-a-vis a conventional bronze bearing.
2. Experimental procedure 2.1. Production and testing of alloys A series of zinc–aluminium, zinc–aluminium–copper and zinc–aluminium–copper–silicon alloys were produced for this work. These were in two groups: one based on the near-eutectoid (27% Al) and the other on the monotectoid (40% Al) compositions as per the Zn–Al phase diagram. The alloys were melted in a crucible using an electrical furnace and poured at temperatures between 600 and 700 ◦ C into a steel mould pre-heated to 100 ◦ C. Ingots had a conical shape with a length of 180 mm, a bottom diameter of 57 mm and a top diameter of 70 mm. Some of the ingots
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Table 1 Chemical composition of the alloys
2.2. Wear testing
Alloy
Half-sleeves were produced from the alloys by fine machining and used as the experimental bearings for the wear tests. This type of bearings have been used in some engineering applications including automotive, aviation and agricultural industries [2]. The shape and dimensions of the bearing are shown in Fig. 1. The test bearing had different surface areas including load-bearing and unloaded areas, an oil hole and an oil groove. The load-bearing areas were selected to obtain a certain level of pressure on the bearing surface. This selection was necessary because of the limitation of hydraulic loading system of the rig. The wear tests were carried out using a bearing test machine. A sectional view of the test machine is shown in Fig. 2. The test rig consisted of a 3 kW drive motor, a test bearing and bearing housing, two shafts, two rolling bearings to support the main shaft, a hydraulic loading unit, a speed control unit and a lubricating system. The main shaft having a diameter of 44.480 ± 0.010 mm was made of St 37 steel (Fe–0.18% C–0.019% P–0.027% S) and its surface was hardened to 50 HRC by carburising. The part of the shaft acting as the rotating element of the bearing was ground and polished with a 0.05 m alumina suspension. The hydraulic loading system of the machine consists of a motor with 1.5 kW capacity, a pump, a pressure relief valve, a proportional valve, an electronic regulator, a pressure transducer, a manometer, a hydraulic cylinder, an oscilloscope and a computer with a control card to control the loading. Each bearing was ultrasonically cleaned and weighed accurately before each test. The test bearing was removed after an interval of 20 h corresponding to a sliding distance of 93.6 km, cleaned in solvents and weighed to determine the mass loss. Since the bearing life is determined by the
ZnAl27Cu2 ZnAl27Cu2Si1 ZnAl40 ZnAl40Cu0.5 ZnAl40Cu1 ZnAl40Cu2 ZnAl40Cu3 ZnAl40Cu4 ZnAl40Cu2Si1 SAE 660 bronze
Chemical composition (wt.%) Zn
Al
Cu
Si
71.2 69.4 61.5 60.2 61.2 57.6 59.7 58.8 57.1 Cu–7 wt.% Pb
26.6 27.3 38.5 39.3 37.8 40.3 37.4 37.1 39.9 Cu–7 wt.% Sn
2.2 2.2 – 0.5 1.0 2.1 2.9 4.1 2.1 Cu–3 wt.% Zn
– 1.1 – – – – – 0.9
were subsequently heat treated by ageing at 150 ◦ C for 240 h, conditions which are known to effectively complete the equilibrium phase transformations at the expense of some structural coarsening [7]. The chemical compositions of the alloys were determined using atomic absorption analysis (Table 1). Samples for electron microscopy were prepared using standard metallographic techniques and etched in 4% nital, followed by examination with a scanning electron microscope (SEM) using atomic number (BSE) contrast. Tensile tests were carried out on round specimens with a gauge diameter of 8 mm and gauge length of 40 mm at a strain rate of 0.0075 s−1 . From the test results, tensile strength and percentage elongation were determined (Table 2). Brinell hardness of the alloys was measured using a 2.5 mm diameter ball indentor under a load of 62.5 kgf. The hardness of each alloy was determined by taking the average of five readings (Table 2). Table 2 Hardness, tensile strength, percentage elongation and density of the alloysa Alloy
Condition
Hardness (BSD)
Tensile strength (MPa)
Elongation (%)
Density (kg m−3 )
ZnAl27Cu2
A B
110 81
306 273
2.0 5.0
4910 4890
ZnAl27Cu2Si1
A B
120 87
337 250
1.8 3.0
4850 4830
ZnAl40
A
103
260
4.5
3980
ZnAl40Cu0.5
A
107
270
3.9
4210
ZnAl40Cu1
A
110
294
1.9
4230
ZnAl40Cu2
A B
117 85
316 285
1.8 4.2
4280 4250
ZnAl40Cu3
A
120
314
2.0
4295
ZnAl40Cu4
A
125
303
1.2
4355
ZnAl40Cu2Si1
A B
122 89
339 255
1.5 3.1
4270 4255
SAE 660 bronze
A
85
205
7.2
8780
a
A: as-cast condition; B: heat-treated at
150 ◦ C
for 10 days.
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Fig. 1. Sectional view and dimensions of the test bearing.
clearance between the journal and the bearing, the volume loss is a more significant parameter than the mass loss, so the measured mass losses were converted into volume losses using the measured density values of the alloys. After the test, the wear surfaces of the bearings were examined using the SEM in SE mode. Wear tests were performed under both static and dynamic loading conditions. In the first case, the tests were carried out under a constant pressure of 20 MPa (20 kN) and a sliding speed of 1.3 m s−1 . In the second case, the tests were performed under a dynamic loading of 15–20 MPa (15–20 kN) with a loading frequency of 10 Hz and at a sliding speed of 1.3 m s−1 (560 rpm). In all the cases, the test bearing was continuously lubricated with a SAE 20W/50 oil at a flow rate
of 240 ml h−1 and the test procedure was repeated for each bearing until a total sliding distance of 850 km was completed. Normally, single wear test was conducted for each data point, but some of them were checked several times using different samples of the same alloys and no considerable amount of change was observed.
3. Results 3.1. Chemical composition and microstructure The chemical compositions of the alloys tested are given in Table 1. The microstructure of the binary ZnAl40Cu
Fig. 2. Sectional view of the bearing test rig.
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Fig. 3. Microstructure of the as-cast ZnAl40 alloy.
alloy in the as-cast condition consisted of cored aluminiumrich ␣-dendrites with zinc-rich -phases in the inter-dendritic regions, Fig. 3 The addition of copper produced some copper-rich inter-metallic particles in the inter-dendritic regions, but it had no effect on the shape and size of the dendrites of the alloys as shown in Fig. 4. The zinc– aluminium–copper–silicon alloys in the as-cast condition consisted of aluminium-rich ␣-dendrites, zinc- and copperrich inter-dendritic phases and silicon particles. Fig. 5 shows the general microstructure of the ZnAl40Cu2Si1 alloy in the as-cast condition. A detailed microstructure of this alloy is also shown in Fig. 6. The ageing heat-treatment had no discernible effect on the dendritic structure of the zinc-based alloys, but coarsened the decomposition products of the  phase only, as seen in the microstructures of the heat-treated ZnAl40Cu2 and ZnAl40Cu2Si1 alloys, Figs. 7 and 8. The microstructure of the SAE 660 bronze was observed to be consisted of copper-rich ␣-dendrites and eutectoid ␣- and ␦-phases as seen in Fig. 9.
Fig. 4. Microstructure of the as-cast ZnAl40Cu2 alloy.
Fig. 5. Microstructure of the as-cast ZnAl40Cu2Si1 alloy.
Fig. 6. Micrograph showing details of the microstructure of the as-cast ZnAl40Cu2Si1 alloy.
Fig. 7. Microstructure of the heat-treated ZnAl40Cu2 alloy.
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3.2. Mechanical testing
Fig. 8. Microstructure of the heat-treated ZnAl40Cu2Si1 alloy.
The hardness, tensile strength, percentage elongation and density of the alloys in the as-cast and heat-treated conditions are given in Table 2. This table shows that amongst the as-cast alloys, the highest tensile strength was obtained from the ZnAl40Cu2Si1 alloy while the ZnAl40 alloy had the lowest. The highest elongation was obtained from the binary ZnAl40 alloy and the lowest from the ZnAl40Cu4 alloy. The highest hardness was obtained from the ZnAl40Cu4 alloy and the lowest hardness from the ZnAl40 alloy. The addition of 1% Si improved the hardness and tensile strength of the ternary ZnAl27Cu2 and ZnAl40Cu2 alloys but adversely affected their elongation. Heat treatment reduced the hardness and tensile strength of all the zinc-based alloys, but improved their ductility. The hardness and tensile strengths of the monotectoid zinc–aluminium–copper alloys increased with increasing the copper content up to 2%, above which the tensile strength decreased slightly as their copper content increased. 3.3. Wear testing
Fig. 9. Microstructure of the SAE 660 bronze.
3.3.1. Static loading The plots of volume loss versus sliding distance for the bearings tested under static loading condition are shown in Fig. 10. This figure shows that the wear resistance of all the zinc-based alloys was considerably higher than the SAE 660 bronze. The heat treatment decreased the wear resistance of the zinc-based alloys. The highest wear resistance was obtained from as-cast ZnAl40Cu2Si1 alloy while heat-treated ZnAl27Cu2 alloy showed the lowest wear resistance. The ZnAl27Cu2Si1 alloy showed the lowest wear resistance amongst the as-cast zinc-based alloys.
Fig. 10. The cumulative volume loss vs. sliding distance for the bearings produced from zinc-based alloys and SAE 660 bronze tested under static loading.
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Fig. 11. The cumulative volume loss vs. sliding distance for the bearings produced from zinc–aluminium-based alloys and SAE 660 bronze tested under dynamic loading.
3.3.2. Dynamic loading The plots of volume loss versus sliding distance for the bearings tested under dynamic loading condition are given in Fig. 11. It can be seen from this figure that all the zinc– aluminium-based alloys had a higher wear resistance than the SAE 660 bronze. The heat-treated zinc-based alloys showed lower volume loss than the corresponding as-cast version. The lowest volume loss was obtained from the heat-treated ZnAl40Cu2Si1 alloy with the as-cast version of the same alloy showing the lowest volume loss in the as-cast group. The ZnAl27Cu2Si1 alloy exhibited the highest volume loss of the as-cast alloys. To compare the wear behaviour of the bearings tested under static and dynamic loading conditions more clearly, the
volume loss versus sliding distance curves are presented together in Figs. 12–16. It can be seen from the figures that the volume loss of the as-cast alloys in general under static loading condition was higher than that in the dynamic loading condition at the shorter sliding distances. However, the trend reversed at longer sliding distances and the volume loss of these alloys under static loading condition was lower than that in the dynamic loading condition. In contrast, the volume loss of the heat-treated zinc-based alloys under dynamic loading condition was lower than that in the static loading condition. To demonstrate the effect of copper content on the tensile strength and wear behaviour of the monotectoid zinc–aluminium–copper alloys, additional wear tests were
Fig. 12. Volume loss vs. sliding distance for the bearings produced from ZnAl27Cu2 alloy in the as-cast and heat-treated conditions tested under both static and dynamic loadings.
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Fig. 13. Volume loss vs. sliding distance for the bearings produced from ZnAl27Cu2Si1 alloy in the as-cast and heat-treated conditions tested under both static and dynamic loadings.
Fig. 14. Volume loss vs. sliding distance for the bearings produced from ZnAl40Cu2 alloy in the as-cast and heat-treated conditions tested under both static and dynamic loadings.
Fig. 15. Volume loss vs. sliding distance for the bearings produced from ZnAl40Cu2Si1 alloy in the as-cast and heat-treated conditions tested under both static and dynamic loadings.
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Fig. 16. Volume loss vs. sliding distance for the bearings produced from SAE 660 bronze tested under both static and dynamic loadings.
Fig. 17. The effect of copper content on the tensile strength of as-cast ZnAl40 alloy and the volume loss of the bearing produced from this alloy under dynamic loading where a sliding distance of 750 km at a sliding speed of 1.3 m s−1 and pressure of 15–20 MPa (±2.5 MPa).
carried out on as-cast monotectoid alloys under dynamic loading condition. Volume losses after 750 km and measured tensile strengths are plotted as a function of copper content in Fig. 17. The figure shows that both the tensile strength and wear resistance increased with increasing copper content up to 2%, above which both decreased slightly. The SEM images obtained from bearings produced using the ZnAl27Cu2Si1 alloy and tested under both static and dynamic loading conditions are shown in Figs. 18–21 to illustrate the general features. The worn surfaces of the bearings produced from all of the zinc-based alloys were found to be quite similar to each other, but heat-treated versions of the alloys showed a greater degree of smearing than as-cast versions. No smearing was observed on the worn surfaces of the bearings produced using the SAE 660 bronze, but instead they had deep scratches and extensive gouging as seen in Figs. 22 and 23.
Fig. 18. Worn surface of the bearing produced from as-cast ZnAl27Cu2Si1 alloy tested under static loading condition.
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Fig. 19. Worn surface of the bearing produced from heat-treated ZnAl27Cu2Si1 alloy tested under static loading condition. Fig. 22. Worn surface of the bearing produced from SAE 660 bronze tested under static loading condition.
Fig. 20. Worn surface of the bearing produced from as-cast ZnAl27Cu2Si1 alloy tested under dynamic loading condition. Fig. 23. Worn surface of the bearing produced from SAE 660 bronze tested under dynamic loading condition.
4. Discussion
Fig. 21. Worn surface of the bearing produced from heat-treated ZnAl27Cu2Si1 alloy tested under dynamic loading condition.
The excellent sliding wear properties of the bearings produced from zinc-based alloys have been attributed to their multiphase structure and the formation of zinc and aluminium oxide films on the bearing surface [12–14]. The aluminium-rich ␣-phase, copper-rich inter-metallic phases (ε and T ) and silicon particles improve the load-bearing capacity of the alloys [8]. The zinc-rich -phase facilitates sliding and provides solid lubrication characteristics by smearing on the mating surfaces under extreme running conditions, particularly in the presence of thermally stable phases like silicon [15]. Oxidative wear occurs in these alloys, and the surface oxide films contribute to wear resistance. The aluminium oxide is a hard compound and therefore also acts as a load-bearing phase, while the zinc oxide is a much softer compound which allows it to act as a lubricant [15,16].
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The SEM images of the worn surfaces showed that different wear mechanisms operated for bearings produced from the zinc-based alloys and SAE 660 bronze. Material transfer and smearing were observed to be more influential in the zinc-based alloy bearings as shown in Figs. 18–21. Although the bronze is an excellent bearing alloy having a strong but ductile matrix reinforced by hard particles of ␦-phase, it did not have the advantage of the compliant matrix and tribologically advantageous oxide films unlike the zinc-based alloys. Figs. 22 and 23 show deep scratches and extensive gouging in the bronze bearings. These bearings are much more susceptible to loss of surface material as detached particles which were less easily re-imbedded in the surface. These observations suggest that adhesion and smearing is the main mechanism for the zinc-based alloys, while adhesive wear dominates for the bronze. The running-in behaviour of the zinc-based alloys and the bronze differed according to test conditions. For the zinc-based alloys, the volume loss of the bearings reduced sharply after running for distances in excess of about 500 km under static loading (Fig. 10), but under dynamic loading, the volume loss showed a small but continuous increase up to the maximum distance of 850 km. This may result from the volumetric change which takes place in the as-cast alloys after a certain period of test time, and surface fatigue due to dynamic loading. The bronze exhibited the same behaviour of a small but steady increase under both conditions. Under static loading condition, Fig. 10 shows a great amount of increase in volumetric loss of material at 850 km for heat-treated versions of the four zinc-based alloys compared with their as-cast versions. Any improvement in conformability, embeddability and damping capacity, or added support by formation of hard T -phase, resulting from the prolonged heat treatment [8,17] was evidently more than that offset by lower matrix strength, and thus this poor behaviour of the heat-treated bearings under static loading is readily understandable. In the dynamic tests, the load cycle used was effectively a fixed load of 17.5 kN with a superposed cyclic load of ±2.5 kN, giving an average load of 12.5% less than for static loading at 20 kN. The reduced mean load would, thus, be expected to improve the wear resistance. A comparison of Figs. 10 and 11 shows that for the heat-treated versions of the alloys this expected behaviour was obtained. The volume loss for the heat-treated versions of all four zinc-based alloys in the dynamic test was lower than that of the alloys in the static test. However, the effect was quite the opposite for the as-cast versions of the same alloys. The volume loss for these alloys tested under dynamic loading condition was much higher than that of the same alloys tested under static loading condition. There is no ready explanation for this poor behaviour of the as-cast alloys under dynamic loading, but one possibility is that the brittleness of the as-cast alloys had played a part. All cast alloys are porous. And in these zinc-based alloys, it is predominately shrinkage porosity wherein the irregular
shape of the voids contribute to low fracture toughness. Heat treatment ameliorates this low toughness. In sliding wear, the stress system at a point near the bearing surface comprises a normal stress due to the applied load, combined with a shear stress parallel to the sliding surface which is determined by the coefficient of friction at the bearing interface and the normal load. A simple Mohr’s Circle analysis shows that this combination produces a tensile principal stress acting at a low angle to the sliding surface, the exact angle determined by the relative magnitudes of the normal and shear stresses. Macroscopically the shear and normal stresses are directly related by the coefficient of friction, but on a small scale different parts of the sliding surface will bear different proportions of the applied load, and the coefficient of friction will be much higher where local welding takes place. Thus, from point to point on the surface and from time to time there will be a constant change in the angle and magnitude of the induced tensile stress even under static loading. When dynamic loading is applied there will be an additional cyclic change during which the local components of stress become even more de-coupled from the macroscopic values, thus, widening the angular range of tensile stress variation. In brittle as-cast alloys, this could activate cracking from additional groups of defects such as shrinkage pores leading to more rapid loss of surface material. Another possibility is that the thermal cycling associated with the load cycling causes near-surface cracking, which would again be more important in the more brittle matrix material of as-cast alloys. Either of these possibilities might also provide an explanation of the observation that with dynamic loading wear continues slightly after the running-in period, but further research is needed in this area. The effect of silicon on wear resistance was found to vary considerably amongst these zinc-based alloys. For the monotectoid alloys, the addition of 1% Si increased the wear resistance of both as-cast and heat-treated versions under both static and dynamic loadings. The silicon-containing alloy has a hard silicon phase which provides load carrying capability in addition to that provided by the copper-rich inter-metallic compounds [11]. For the near-eutectoid-based alloys the effect was less clear. Figs. 10 and 11 show that the addition of 1% Si decreased the wear resistance of the as-cast near-eutectoid alloy under static loading condition but increased that of the heat-treated version under both static and dynamic loading conditions. More detailed examination is needed to provide a scientific explanation for this effect. Aluminium content had a powerful effect on wear resistance. Figs. 10 and 11 show that the monotectoid alloys (ZnAl40Cu2 and ZnAl40Cu2Si1) had superior wear properties to the near-eutectoid-based alloys (ZnAl27Cu2 and ZnAl27Cu2Si1). This was probably due to the aluminium-rich matrix in these alloys, as it is well known that the hardness and tensile strength of zinc-based alloys increases with increasing aluminium content [8],
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which is supported by the mechanical properties listed in Table 2. Both, the tensile strength and wear resistance of the monotectoid-based zinc–aluminium–copper alloys increased with increasing copper content up to 2%, above which both decreased slightly (Fig. 17). This may be explained in terms of the effect of added copper on the structure and properties of zinc–aluminium alloys. It has been shown previously [8], and confirmed by our own results (Table 2 and Fig. 17), that the hardness and tensile strength of zinc–aluminium alloys increase with increasing copper content up to 2%. However, when the copper content of the zinc–aluminium–copper alloys exceeds this level (2%) copper-rich T and ε inter-metallic phases are formed in the inter-dendritic regions [18,19]. These are hard but brittle phases and this is why the tensile strength of the zinc-based alloys containing more than 2% Cu decreases with the increasing copper content while the hardness increases continuously (Table 2). It can be concluded that the wear resistance of the bearings produced from zinc-based alloys depends on the tensile strength, rather than the hardness of these alloys under dynamic loading.
5. Conclusions 1. Adhesion and smearing is the main mechanism for wear in zinc-based alloys, while abrasive wear dominates for the commercial SAE 660 bronze. 2. The bearings produced from the zinc-based alloys exhibit higher wear resistance than the bearings produced from bronze. These bearings appear to be good alternatives to the bronze bearings during operation under both static and dynamic loadings. 3. Under static loading conditions, the as-cast zinc-based alloys showed higher wear resistance than the same alloys in the heat-treated condition. 4. Under dynamic loading conditions, the volume loss of as-cast zinc-based alloys increased compared with static loading tests, while the overall trend was reversed for those of heat-treated alloys. 5. Monotectoid zinc–aluminium-based alloys had superior wear resistance to the near-eutectoid-based alloys in both as-cast and heat-treated conditions, and under both static and dynamic loading conditions. 6. The highest relative wear resistance was obtained from the ZnAl40Cu2Si1 alloy in both as-cast and heat-treated conditions and under both static and dynamic loading. The best performance was obtained for the as-cast version under static loading. 7. Under dynamic loading conditions, the wear resistance of the monotectoid-based zinc–aluminium–copper alloys increased with increasing the copper content up to 2%
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but declined rapidly at higher levels. Wear resistance correlated strongly with the tensile strength. Acknowledgements This work was supported by the Research fund of Karadeniz Technical University. The authors would like to thank Dr. Tevfik Küçükömero g lu and all the technicians in the Materials Division of the Mechanical Engineering Department of this university for their help. References [1] S. Murphy, T. Sava¸skan, Comparative wear behaviour of Zn–Al-based alloys in an automotive engine application, Wear 98 (1984) 151–161. [2] T. Calayag, D. Ferres, High performance, high aluminum zinc alloys for low speed bearings and bushings, in: Proceedings of the SAE Annual Conference, Paper No. 820643, 1983, pp. 2241–2251. [3] T. Calayag, Zinc alloys replace bronze in mining equipment bushings and bearings, Mining Eng. 1983, 727–728. [4] E. Gervais, H. Levert, M. Bess, The development of a family of zinc-based foundry alloys, Trans. Am. Foundrym. Soc. 88 (1980) 183–194. [5] H. Geng, J. Ma, Friction and wear of Al–Zn–Pb bearings alloy, Wear 169 (1993) 201–207. [6] Y. Zhu, B. Yan, W. Huan, Bearing wear resistance of monotectoid Zn–Al based alloy (ZA-35), Mater. Sci. Technol. 11 (1995) 109–113. [7] T. Sava¸skan, S. Murphy, Mechanical properties and lubricated wear of Zn–Al25-based alloys, Wear 116 (1987) 211–224. [8] T. Sava¸skan, The structure and properties of zinc–aluminium-based bearing alloys, Ph.D. Thesis, University of Aston, Birmingham, 1980. [9] T. Ma, Q.D. Chen, S.C. Li, H.M. Wang, Effect of Mn on lubricated friction and wear properties of Zn–Al alloys, Wear of Materials, Louyang Institute of Technology, Louyang, PR China,1989. [10] M. Durman, S. Murphy, Precipitation of metastable ε-phase in a hypereutectic zinc–aluminum alloy containing copper, Acta Metal. Matter. 39 (1991) 2235–2242. [11] B.K. Prasad, Microstructure, mechanical properties and sliding wear characteristics of Zn-based alloys: effects of partially substituting Cu by Si, Z. Metallkde. 88 (1997) 929–933. [12] R.J. Marczak, R. Ciach, Tribological properties of concentrated Al–Zn alloys, in: Proceedings of the 1st Europe Tribology Congress, London, 1973, pp. 223–227. [13] B.K. Prasad, Effect of microstructure on the sliding wear performance of a Zn–Al–Ni alloy, Wear 240 (2000) 100–112. [14] B. Bhushman, B.K. Gupta, Handbook of Tribology, Materials, Coatings and Surface Treatments, McGraw-Hill, New York, 1991. [15] B.K. Prasad, Effects of silicon addition and test parameters on sliding wear characteristics of zinc-based alloys containing 37.5% aluminium, Mater. Trans., JIM 38 (1997) 701–706. [16] H. Tarabian, J.P. Pathak, S.N. Tiwari, Wear characteristics of Al–Si alloys, Wear 172 (1994) 49–58. [17] B.K. Prasad, Influence of heat treatment on the physical, mechanical and tribological properties of a zinc-based alloy, Z. Metallkde. 87 (1996) 226–232. [18] S. Murphy, Solid-phase reactions in the low-copper part of the Al–Cu–Zn system, Z. Metallkde. 71 (1980) 96–102. [19] T. Sava¸skan, S. Murphy, Decomposition of Zn–Al alloys on quench-aging, Mater. Sci. Technol. 6 (1990) 695–700.