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Investigation of the tribological properties of silicon containing zinc–aluminum based journal bearings
Tribology International 37 (2004) 433–440 www.elsevier.com/locate/triboint
Investigation of the tribological properties of silicon containing zinc–aluminum based journal bearings Hamdullah C ¸ uvalcı , Hasan Bas¸ Mechanical Engineering Department, Karadeniz Technical University, 61080 Trabzon, Turkey
Abstract In this study, frictional behavior of thin-walled journal bearings produced from Zn–Al–Cu–Si alloys was investigated using a purpose-built journal bearing test rig. The alloys were produced by permanent mould casting. Mechanical properties such as ultimate tensile strength, elongation, hardness and microstructure of these alloys were determined. The friction properties of the bearings produced from these alloys were also investigated. In this investigation, the effects of surface roughness and bearing pressure on the frictional properties of the journal bearings were taken into account. The results showed that friction factor decreased with increasing bearing pressure especially in the mixed and full-film lubrication zones. It was found that high surface roughness led to high friction factor. The ZnAl27Cu2Si1 and ZnAl40Cu2Si1 bearings showed full Stribeck curve tendency while ZnAl27Cu2Si2 bearing did not exhibit the typical diagram having no full-film lubrication zone at the pressure of 0.7 and 1.1 MPa. # 2003 Published by Elsevier Ltd. Keywords: Journal bearing; Surface roughness; Friction factor; Zn–Al–Cu–Si alloys; Sommerfeld number; Lubricated wear
1. Introduction Many studies have been going on to replace widely used conventional journal bearing materials such as white metal (babbitt), cast iron and bronze with zinc– aluminum-based new journal bearing materials having superior properties [1]. Over the last decade, many investigations have been carried out to improve the properties of the zinc–aluminum based alloys and to increase their applications in field [2,3]. Zinc–aluminum based alloys have superior mechanical and tribological properties and are more economic than those of conventional journal bearing materials [4,5]. The alloys have been substituted for conventional journal bearing materials in a wide range of industrial applications [6]. Nowadays the journal bearings produced from these alloys have been used particularly in high load and low speed applications such as earthmoving equipments, mining and milling machines, cable winches, etc., [7]. In the earlier investigations, copper has been added to zinc–aluminum-based alloys and its effects on these alloys studied in detail [8–10]. These studies have
Corresponding author. Fax: +90-462-3255526. E-mail address: [email protected] (H. C ¸ uvalcı).
0301-679X/$ - see front matter # 2003 Published by Elsevier Ltd. doi:10.1016/j.triboint.2003.10.006
shown that copper addition improves mechanical properties of these alloys but it causes dimensional instability especially at high working temperatures [11]. In order to reduce this problem and to improve wear resistance, silicon was added to zinc–aluminum based alloys [12]. In the literature there is more information about friction properties of silicon containing quaternary Zn–Al–Cu–Si alloys [13] but there is not enough information about friction properties of the journal bearings. In journal bearing tribology, three lubrication regimes are often defined: boundary, mixed and fullfilm lubrication. These lubrication regimes can be explained by the Stribeck diagram obtained first by the McKee brothers [14]. As shown in Fig. 1, this diagram presents the change in the coefficient of friction, against the bearing parameter, gn=p, where g is the dynamic viscosity of the lubricant (N s/m2), n is the rotational speed (rev./s), and p is the nominal bearing pressure (N/m2). Since it defines the stability of lubrication and helps one to understand the lubrication regimes, it may be used as a rough design parameter or as an approximate means for quickly deciding if the bearing is operating near a danger zone. However, it does not include the clearance ratio, W ¼ c=r, where r is the journal
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Fig. 1.
alloy was prepared in the laboratory. Aluminum addition was made in the form of AlSi12, Cu–Al alloy and pure aluminum metal. The purity of metals used to prepare the master and Zn–Al–Cu–Si alloys was 99.99% for zinc and copper, 99.97% for aluminum. The melting process was carried out in the temperature controlled electrical crucible furnace. The melted alloys were then cast into steel moulds preheated at a v temperature of 300 C. The mould was designed in the form of a conical-cylinder for proper feeding and directional solidification. The mould had a casting cavity having dimensions of 45 mm internal diameter at the bottom and 60 mm internal diameter at the top and 200 mm length, respectively. The chemical compositions of the alloys were determined by the atomic absorption method. The results of chemical analysis of the alloys were tabulated in Table 1.
A schematic Stribeck diagram.
radius and c is the radial clearance. In view of the above, a more comprehensive parameter, the Sommerfeld number ðS ¼ gn=PW2 Þ was used in this study to take into account the variables generally specified by a designer. Tribological properties of the journal bearings were then obtained via the graphics plotted using the friction factor, (Ff ¼ l=W), versus the Sommerfeld number. This paper presents the results of an experimental study conducted to determine tribological properties of zinc–aluminum based journal bearings having silicon addition using a purpose-built journal bearing test rig. Mechanical properties such as ultimate tensile strength (UTS), elongation, hardness and microstructure of the alloys were determined. Friction properties of the bearings produced from the alloys were also studied. Surface roughness of the journal bearings was investigated to determine its effects on their frictional properties. 1.1. Alloy preparation Three zinc–aluminum based (Zn–Al–Cu–Si) alloys containing silicon were prepared by permanent mould casting process. In order to prepare Zn–Al–Cu–Si alloys, silicon and copper additions were made as AlSi12 and intermetallic copper–aluminum (46.5% Al) master alloys, respectively. AlSi12 alloy was procured from a commercial source, while intermetallic Cu–Al
1.2. Microstructure of alloys The samples taken from the alloy castings for microstructural studies were prepared using standard metv allographic techniques and etched at 70 C with 25% Nital etchant (25% nitric acid, 75% ethanol). Their micrographs were taken using an optical microscope. The size of silicon particles in the microstructure was measured according to the intercept method mentioned elsewhere [15]. The working surfaces of the thin-walled journal bearings were investigated using a Jeol 6400 scanning electron microscope (SEM). 1.3. Determination of mechanical properties and surface roughness Hardness measurements were carried out using a Brinell hardness tester at 31.25 kg load with a 2.5 mm diameter ball indenter. Standard specimens having 50.8 mm gauge length and 12.8 mm gauge diameter prepared from the alloys were tested at a 2:56 103 s1 strain rate in order to determine their UTS and percent elongation. The density of the alloys was calculated from the mass to volume ratio. The initial (after running-in process) and the final (after working at 0.1, 0.3, 0.7 and 1.1 MPa pressures) surface roughness of the journal bearings and the sleeve was measured by using a stylus surface rough-
Table 1 Chemical compositions of the alloys and measured silicon particles size range Alloy
Chemical composition (Weight%)
No.
Description
Zn
Al
Cu
Si
1 2 3
ZnAl27Cu2Si1 ZnAl27Cu2Si2 ZnAl40Cu2Si1
69.4 68.3 56.3
27.6 27.8 40.8
1.97 1.96 1.95
1.03 1.94 0.95
Size range of silicon particles (1 lm) 7–15 12–20 7–15
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ness apparatus. Surface roughness parameters such as centre line average (Ra), average peak to valley height (Rz) and maximum peak to valley height (Rmax) were measured by using a driving unit at a tracing speed of 0.5 mm/s with traversing and cut-off lengths, 15 and 2.5 mm, respectively. The unit was connected to a measuring indicator in order to read measured roughness values. The average values of five roughness measurements were taken. 1.4. Determination of friction properties 1.4.1. Specimen preparation and test procedure Thin-walled journal bearings produced from each alloy (total nine bearings for three alloys) were tested to investigate their friction properties using a journal bearing test rig modified specifically for this research. There was a circular oil groove at the centre of the bearing wall to enable sufficient supply of oil on the contacting surfaces as illustrated in Fig. 2. The effective bearing length was taken as 10 mm to operate at a pressure of 1.1 MPa because the maximum allowable load of the test rig was 542 N. The total bearing length and wall thicknesses were 30 and 2.95 mm, respectively. The bearings had a bearing clearance of 40 lm and an inner diameter of 50 mm. A schematic representation of the rig is shown in Fig. 2. The rig consists of a rigid tubular steel frame equipped with a pneumatically operated pressurized oil supply, a direct current solid state variable speed control unit, split journal bearing assembly and instrumentation for indicating speed and armature current. The apparatus operates a 240 V single phase electrical supply and an air supply at a pressure of not less than 0.6 MPa. The pressurized oil system consists of a double acting pneumatic cylinder driving an oil cylinder in a reciprocating manner. Incorporated in the system are two over pressure relief valves which limit the supply pressure to the bearing and hydrostatic pad to approximately 0.15 and 1.6 MPa, respectively. In this test rig, the hydrostatic pad is made of mild steel and accurately lapped to fit the journal bearing housing. Eleven jets provided the oil for the hydrostatic lift enabling the load to increase up to 542 N to be transmitted to the bearing without affecting the sensitivity of friction torque measurements. Oil leakage from the bearing and hydrostatic pad is collected in the drip tray and returned to the reservoir. The journal bearing housing is made of cast iron and has an accurately honed bore to accommodate thin-walled bearings as used in motor car engines. The sleeve was made of cold worked tool steel hardened to 722 HB. The rotational speed could be adjusted in the range of 0–1100 rpm (0–2.88 m/s) using a direct current speed control unit. The detailed information can be obtained from Ref. [16].
Fig. 2. Journal bearing test rig and test specimen.
The friction torque was measured by using a Wheatstone bridge circuit with strain gauges on the torque plate. The torque signal obtained from the bridge circuit was transmitted to the recorder and this enabled monitoring and recording the signals. The recorded signal values were then converted into friction torque and friction factor using the calibration line obtained from strain gauges. All experiments were carried out using SAE 15 lubricating oil at the flow rate of 50 cm3/min. Oil temperatures at the outlet of the bearing was measured using a
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thermometer and viscosity values were taken from the calibration chart of viscosity–temperature given by the manufacturer for this type of oil. In all experiments, v the oil temperature remained in the range 25–30 C with corresponding dynamic viscosity 0.0168–0.0138 N s/m2. Before conducting tests, the journal bearing surfaces were subjected to running-in process under a pressure of 0.3 MPa at a constant rotational speed (500 rpm, 1.3 m/s) for 10 min. After this process, the journal bearings were tested at rotational speeds ranging from 25 rpm (0.065 m/s) to 1100 rpm (2.88 m/s) for 5 min at each speed. The hydrostatic pad and lubricating oil pressures selected were 1.2 and 0.1 MPa, respectively. Only one sleeve was used for experiments due to its high hardness (722 HB) compared to the bearings (97– 125 HB). In order to determine the effect of the bearing pressure on the frictional properties, the bearings were tested at different pressures of 0.1, 0.3, 0.7 and 1.1 MPa at the constant clearance ratio of W ¼ 0:0008. After testing a bearing at a pressure such as 0.1 MPa, it was subjected to a test at 0.3, 0.7 and 1.1 MPa. The process repeated for three bearings having same chemical composition. After that, the mean frictional torque was taken into account while plotting the friction factor versus the Sommerfeld number. The lubricant film thickness was calculated by using the formula: h0 ¼ W:r½1e . Here, r and E are the bearing radius and eccentricity ratio, respectively. Eccentricity ratio (e) was taken from charts according to the bearing properties such as pressure, viscosity, rotational speed, bearing length, etc., [17]. The lubricant film thickness was calculated only for ZnAl27Cu2Si2 bearings working at the 900 rpm and different pressures because chemical composition has no effect on the frictional behavior in full-film lubrication regime [17].
2. Results 2.1. Microstructure of alloys Microstructures of the alloys are shown in Fig. 3a–c. In all alloys, the matrix structure consisted of aluminum rich a dendrites (light areas), zinc rich g phase (dark areas) in its surrounding, and silicon particles (eutectic and primary). Furthermore, at the rims of the a dendrites there was a mixture of a and g phases from the reaction product of b phase. The size of the silicon particles in the alloys is listed in Table 1. The table shows that ZnAl27Cu2Si2 alloy revealed coarser and more silicon particles than the other alloys. SEM images of the wear surfaces of the journal bearings are shown in Fig. 4a–c. It can be seen from the figures that there are slight wearing tracks and friction layer smeared on the surface.
Fig. 3. The microstructure of (a) ZnAl27Cu2Si1, (b) ZnAl27Cu2Si2, and (c) ZnAl40Cu2Si1 alloys.
2.2. Mechanical properties The mechanical and physical properties of the test specimens are shown in Table 2. The maximum tensile strength (325 MPa) and hardness (125 HB) were obtained from ZnAl27Cu2Si2 alloy containing 2% Si. All alloys exhibited low elongation and brittle fracture behavior. It is apparently seen that increasing silicon content from 1% to 2% increases the UTS and hardness of alloys. The ZnAl27Cu2Si1 alloy attained UTS
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as 287 MPa whereas the ZnAl27Cu2Si2 alloy demonstrated as 325 MPa UTS. Further, increasing aluminum content from 27% to 40% decreases mechanical properties of the alloys. The monotectoid-based ZnAl40Cu2Si1 alloy shows lower UTS and hardness than the eutectoid based ZnAl27CuSi1 alloy. The ZnAl40Cu2Si1 alloy had the highest aluminum content and exhibited the lowest density among the alloys. 2.3. Surface roughness Table 3 shows the surface roughness values of the journal bearings and sleeves. Final roughness represents the roughness obtained at the end of the experiments, that is, after the testing of the journal bearings at sequentially 0.1, 0.3, 0.7 and 1.1 MPa pressures. The initial roughness was measured from bearings after the running-in period. The maximum initial roughness value (1.45 lm) was obtained for the ZnAl27Cu2Si2 alloy journal bearing and it also showed the maximum final roughness (1.37 lm) after experiments. Minimum initial (1.05 lm) and final roughness values (1.0 lm) were obtained from ZnAl40Cu2Si1 journal bearing. In addition to journal bearings the hard sleeve shows minimum initial (0.52 lm) and final (0.48 lm) surface roughness. 2.4. Friction behavior The friction factor versus Sommerfeld number plots for the bearings are plotted in Fig. 5a–d at different pressures. The curves obtained at 0.1 and 0.3 MPa pressures were similar to typical Stribeck diagram revealing boundary, mixed and full-film lubrication regimes as illustrated in Fig. 5a,b. As clearly seen from the figures, the friction factor decreased rapidly with increasing Sommerfeld number in the boundary lubrication regime. After exhibiting a minimum in the mixed lubrication zone, the friction factor increased with increasing Sommerfeld number in the full-film lubrication zone. On the contrary, as shown in Fig. 5c,d, the friction factor of the journal bearings containing 2% Si continuously decreased with increasing Sommerfeld number whereas the plots for remaining alloy bearings showed a slight increase at the pressures of 0.7 and 1.1 MPa in the full-film lubrication zone.
Fig. 4. SEM images of working surfaces of the journal bearings of (a) ZnAl27Cu2Si1, (b) ZnAl27Cu2Si2, and (c) ZnAl40Cu2Si1 after friction tests.
Table 2 Some mechanical and physical properties of the alloys No.
Material
Ultimate tensile strength (MPa)
Elongation (%)
Brinell hardness (HB)
Density (kg/m3)
1 2 3
ZnAl27Cu2Si1 ZnAl27Cu2Si2 ZnAl40Cu2Si1
287 325 247
2.0 0.8 1.2
108 125 97
4810 4630 4110
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Table 3 Average surface roughness of bearings and sleeve (0.02 lm) Condition
Material
Ra (lm)
Rz (lm)
Rmax (lm)
Initial roughness (After running-in, 0.3 MPa, 500 rpm)
ZnAl27Cu2Si1 ZnAl27Cu2Si2 ZnAl40Cu2Si1 Sleeve ZnAl27Cu2Si1 ZnAl27Cu2Si2 ZnAl40Cu2Si1 Sleeve
1.15 1.45 1.05 0.52 1.06 1.37 1.00 0.48
4.9 6.6 4.8 2.2 4.2 5.0 4.4 1.8
6.3 7.3 5.9 2.3 4.4 6.2 5.2 1.8
Final roughness (After experiments, testing sequentially at 0.1, 0.3, 0.7 and 1.1. MPa pressures)
Ra, centerline average; Rz, average peak-to-valley height; Rmax, maximum peak-to-valley height.
The lubricant film thickness calculated by using the method described in Ref. [17] as a function of bearing pressure is shown in Table 4. The lubricant film thickness decreased with increasing bearing pressures. At the pressure of 0.1 MPa, the lubricant film thickness was 14 lm while it reduced to 5 lm at 1.1 MPa. 3. Discussion Generally monotectoid Zn–Al alloys (40–60% Al) show lower mechanical properties than eutectoid (25–28% Al) alloys [18]. The agreeable results achieved in our study show that the monotectoid-based ZnAl40Cu2Si1 alloy containing high aluminum (40%) showed lower UTS and lower hardness than the eutectoid alloys (Table 2). This may be due to their different microstructures forming at the end of the different phase transformations (monotectoid and eutectoid). In alloys (27% Al), the eutectoid material (b) decomposes into the equilibrium constituents (a þ g) in less than 2 min after quenching from above the eutectoid temperature. Similarly the supersaturated a solid solution transforms to the equilibrium constituents (a þ g) by precipitation of the excess zinc (g-phase) in a relatively short period of time. The decomposition reactions become more complicated at medium aluminum content. Most alloys near to 50% Al decompose on slow cooling via a monotectoid reaction, two successive eutectoid reactions and a solubility change [19]. Increasing silicon content from 1% to 2% in the Zn– Al based alloys improved their mechanical properties (Table 2). Silicon is used as a good alloying and dispersion strengthening element for aluminum alloys [20]. It plays a role as an obstacle to dislocation movement for slip in microstructure and hence mechanical properties increase [21]. Therefore, the addition of 2% silicon to the ZnAl27Cu2Si2 alloy led to the highest UTS and hardness among the alloys tested. More and coarse silicon means a more and huge dislocation barrier, which leads to higher mechanical properties of the alloys. The addition of silicon to the bearing alloys such as 2% coarsens its particles and it may cause increased wear
of the journal bearing and the sleeve. In order to prevent this likely wear process, the silicon size must be controlled with other additions such as strontium. In mixed and full-film lubrication zones, silicon particles (15–20 lm) are almost in the same size range as the lubricant film thickness (Table 4). The silicon particles detached from the journal bearing surface during the working period would lead to scoring and wear of the bearings. Smearing and adhering of worn debris on the surface with limited wear tracks demonstrated adhesive type wear mechanism for the Zn–Al based bearings as shown in Fig. 4. After the running-in process, the journal bearings showed different surface roughness. This may be attributed to different chemical composition and varying mechanical properties of the bearings. For materials having high mechanical properties, the asperity flattening and smoothing of the surface becomes more difficult than the materials having low mechanical properties [22]. Smoothening of the asperity of the harder sleeve (722 HB) was less than that of the journal bearings (97–108 HB) after the tests although it worked longer than the bearings. There was 0.04 lm decrease in surface roughness on the sleeve whereas the decrease was 0.05–0.09 lm for the journal bearings. As shown in Fig. 5, friction factor decreases with increasing bearing pressure especially in the mixed and full-film lubrication zones for all bearings. It can be seen in Fig. 5a,d that the minimum friction factor was 60.4 at pressure of 0.1 MPa, while it decreases to 14.6 at 1.1 MPa for ZnAl40Cu2Si1 bearing. Friction depends on oil film thickness partially in the mixed lubrication zone but completely in full-film lubrication regime [23]. In full-film lubrication, the surfaces are completely separated by a fluid lubricant. Friction solely arises due to shearing of the lubricant film layers by the rotating sleeve. In this study, as clearly seen from Table 4, the film thickness between the surfaces decreased with the increasing bearing pressure. This explains why friction factor decreased with the increasing bearing pressure.
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Table 4 Calculated lubricant film thickness for ZnAl27Cu2Si2 bearing at 900 rpm
Fig. 5. Friction factor–Sommerfeld number plots for the bearings at the bearing pressure of (a) 0.1 MPa, (b) 0.3 MPa, (c) 0.7 MPa, and (d) 1.1 MPa.
Bearing pressure (MPa)
Lubricant film thickness (lm, 0.5)
0.1 0.3 0.7 1.1
14 9 7 5
The ZnAl27Cu2Si2 bearings showed higher friction factor than the other bearings in all lubrication zones at all pressures. This may be due to the high surface roughness of the journal bearings. The coefficient of friction increases with increasing surface roughness leading to high energy dissipation. This is due to the resistance offered by the surface roughness against the lubricant flow, which increases with increasing roughness [24,25]. As clearly seen in Fig. 5c,d, ZnAl27Cu2Si2 bearing showed different frictional behavior at 0.7 and 1.1 MPa bearing pressures than at 0.1 and 0.3 MPa, Fig. 5a,b. Friction factor in this case continuously decreased with increasing rotational speed at 0.7 and 1.1 MPa (Fig. 5c,d) and full-film lubrication zone did not occur at all. As pointed out previously, the bearings had the highest surface roughness that becomes more important at high pressures for determining lubrication zone. In our study, the working conditions for the bearings were still in mixed lubrication zone and metal-to-metal contact occurred although the other bearings experienced full-film lubrication condition. It can be said that the aluminum content affects the frictional properties of the bearings, especially in boundary and mixed lubrication zones where metal-tometal contact occurs. Comparing the friction data for ZnAl27Cu2Si1 and ZnAl40Cu2Si1 bearings (Fig. 5), it can be noted that the friction factor data for ZnAl40Cu2Si1 are lower than those of ZnAl27Cu2Si1 at all pressures. For example, at a pressure of 0.1 MPa and at 75 rpm ZnAl40Cu2Si1 bearing shows minimum friction factor as 60.4, whereas this factor was found to be 74.6 for ZnAl27Cu2Si1 bearing. ZnAl40Cu2Si1 alloy has lower UTS (247 MPa) than the ZnAl27Cu2Si1 alloy (287 MPa). In boundary and mixed lubrication zones, mechanical properties as well as surface roughness also affect the frictional force. The surface asperities having low shear strength can be easily deformed by hard asperities of the sleeve resulting in a low friction force as compared to high shear strength materials [26]. Running-in is the process of removing high spots and mating the bearing surfaces [26]. The bearings were subjected to work at low speed and pressure before actual working conditions. During the process roughness of surface decreases and then low friction and wear rates can be obtained [27]. Surface roughness also
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determines the full-film lubrication initiation point at the bearings. Low friction and wear in boundary and mixed lubrication zones can be obtained by decreasing surface roughness with the extent of running-in process or with other methods in Zn–Al based bearings containing 2% Si. 4. Conclusions The results of current study indicate that all the investigated factors like Al and Si content of the alloys and test pressure/speed play an important role in controlling the frictional behavior of the thin-walled journal bearings. It is shown that high silicon content improves mechanical properties of the Zn–Al based alloys but the size of the silicon particles for bearing having especially 2% Si must be taken into account to prevent excessive wear of sleeve and bearing. It is further shown that high surface roughness leads to high friction in all lubrication zones. Moreover, ZnAl27Cu2Si2 bearings having high roughness do not show full-film lubrication zone at the bearing pressures of 0.7 and 1.1 MPa for all the test speeds. In addition, surface roughness has effects on frictional properties of Zn–Al based journal bearings containing Si.
References [1] Barhurst RJ. Guidelines for designing zinc alloy bearings—a technical manual. Society of Automotive Engineers, Paper no. 880289, 1989. [2] Durman M, Murphy S. Precipitation of metastable e-phase in a hyper-eutectic zinc–aluminum alloy containing copper. Acta Metall Mater 1991;39(10):2235–42. [3] Dahotre NB, McCay TD, McCay MH. Laser surface modification of zinc-base composites. JOM 1990:44–7. [4] Seah KHW, Sharma SC, Girish BM. Mechanical properties of as-cast and heat-treated ZA-27/graphite particulate composites. Compos, Part A: Appl Sci Manuf 1997;28(3):251–6. [5] Gervais E. ZA alloys—a challenge to the metals industry. CIM Bull 1987;80:67–92.
[6] Calayag T, Ferres D. High-performance, high-aluminum zinc alloys for low-speed bearings and bushings. Society of Automotive Engineers, Paper no. 820643, 1983. [7] Altorfer KJ. Zinc Alloys Compete with Bronze in Bearings and Bushings. Metal Prog 1982;122(6):29–31. [8] Pratt GC. Int Met Rev 1973;18:1–27. [9] Gervais E, Levert H, Bess M. Development of a family of zinc base foundry alloys. Trans AFS 1980;88:183–94. [10] Kubel EJ. Expanding horizons for ZA alloys. Adv Mater Process 1987;132:51–7. [11] Prasad BK. Wear 2000;240:100–12. [12] Savas¸kan T, Purcek G, Murphy S. Wear 2002;252:693–703. [13] Purcek G, Savas¸kan T, Kucukomeroglu T, Murphy S. Wear 2002;252:894–901. [14] McKee SA, McKee TR. Journal bearing friction in the region of thin film lubrication. SAE J 1932;31:371–7. [15] Shubat GJ. Metallography, structures and phase diagrams. Metals Handbook, vol. 8. Metals Park, Ohio: ASM; 1973 [ASM Handbook Committee Chairman]. [16] Cuvalci H. An investigation of tribological properties of Zn–Al based alloys and journal bearing produced from these alloys. Ph.D. thesis, Karadeniz Technical University, Trabzon, Turkey, 1996. [17] Fuller DD. Theory and practice of lubrication for engineers. USA: John Wiley and Sons Inc; 1956. [18] Skenazi AF, Pelerin J, Coutsouradis D, Magnus B, Meeus M. Some recent developments in the improvements of the mechanical properties of zinc foundry alloys. Metallurgy 1983;37(9): 898–902. [19] Mykura N, Zhu YH, Murphy S. Solid-state reactions in Zn–Al based alloys. Can Metall Q 1986;25(2):151–9. [20] Askeland DR. The science and engineering of materials, SI ed. Hong Kong: Van Nostrand Reinhold (International); 1988. [21] Gahr KHZ. Microstructure and wear of materials. Amsterdam: Elseiver; 1987. [22] Anderson P, Juhanko J, Nikkila AP, Lintula P. Influence of topography on the running-in of water-lubricated silicon carbide journal bearings. Wear 1996;201:1–9. [23] Hutchings IM. Tribology: friction and wear of engineering materials. UK: Edward Arnold Publishers Ltd; 1992. [24] Kumar P, Jain SC, Ray S. Study of surface roughness effects in elastohydrodynamic lubrication of rolling line contacts using a deterministic model. Tribol Int 2001;34:713–22. [25] Hata H, Nakahara T, Aoki H. Surface roughness effects in hydrodynamic and mixed lubrication. The Winter Annual Meeting of The American Society of Mechanical Engineers Chicago, Illinois, November 16–21, 1980. [26] Freeman P. Lubrication and friction. London: Sir Isaac Pitman and Sons Ltd; 1962. [27] ASM Handbook, Friction, lubrication and wear technology, 18, ASM International; 1992.
Investigation of the tribological properties of silicon containing zinc–aluminum based journal bearings Hamdullah C ¸ uvalcı , Hasan Bas¸ Mechanical Engineering Department, Karadeniz Technical University, 61080 Trabzon, Turkey
Abstract In this study, frictional behavior of thin-walled journal bearings produced from Zn–Al–Cu–Si alloys was investigated using a purpose-built journal bearing test rig. The alloys were produced by permanent mould casting. Mechanical properties such as ultimate tensile strength, elongation, hardness and microstructure of these alloys were determined. The friction properties of the bearings produced from these alloys were also investigated. In this investigation, the effects of surface roughness and bearing pressure on the frictional properties of the journal bearings were taken into account. The results showed that friction factor decreased with increasing bearing pressure especially in the mixed and full-film lubrication zones. It was found that high surface roughness led to high friction factor. The ZnAl27Cu2Si1 and ZnAl40Cu2Si1 bearings showed full Stribeck curve tendency while ZnAl27Cu2Si2 bearing did not exhibit the typical diagram having no full-film lubrication zone at the pressure of 0.7 and 1.1 MPa. # 2003 Published by Elsevier Ltd. Keywords: Journal bearing; Surface roughness; Friction factor; Zn–Al–Cu–Si alloys; Sommerfeld number; Lubricated wear
1. Introduction Many studies have been going on to replace widely used conventional journal bearing materials such as white metal (babbitt), cast iron and bronze with zinc– aluminum-based new journal bearing materials having superior properties [1]. Over the last decade, many investigations have been carried out to improve the properties of the zinc–aluminum based alloys and to increase their applications in field [2,3]. Zinc–aluminum based alloys have superior mechanical and tribological properties and are more economic than those of conventional journal bearing materials [4,5]. The alloys have been substituted for conventional journal bearing materials in a wide range of industrial applications [6]. Nowadays the journal bearings produced from these alloys have been used particularly in high load and low speed applications such as earthmoving equipments, mining and milling machines, cable winches, etc., [7]. In the earlier investigations, copper has been added to zinc–aluminum-based alloys and its effects on these alloys studied in detail [8–10]. These studies have
Corresponding author. Fax: +90-462-3255526. E-mail address: [email protected] (H. C ¸ uvalcı).
0301-679X/$ - see front matter # 2003 Published by Elsevier Ltd. doi:10.1016/j.triboint.2003.10.006
shown that copper addition improves mechanical properties of these alloys but it causes dimensional instability especially at high working temperatures [11]. In order to reduce this problem and to improve wear resistance, silicon was added to zinc–aluminum based alloys [12]. In the literature there is more information about friction properties of silicon containing quaternary Zn–Al–Cu–Si alloys [13] but there is not enough information about friction properties of the journal bearings. In journal bearing tribology, three lubrication regimes are often defined: boundary, mixed and fullfilm lubrication. These lubrication regimes can be explained by the Stribeck diagram obtained first by the McKee brothers [14]. As shown in Fig. 1, this diagram presents the change in the coefficient of friction, against the bearing parameter, gn=p, where g is the dynamic viscosity of the lubricant (N s/m2), n is the rotational speed (rev./s), and p is the nominal bearing pressure (N/m2). Since it defines the stability of lubrication and helps one to understand the lubrication regimes, it may be used as a rough design parameter or as an approximate means for quickly deciding if the bearing is operating near a danger zone. However, it does not include the clearance ratio, W ¼ c=r, where r is the journal
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Fig. 1.
alloy was prepared in the laboratory. Aluminum addition was made in the form of AlSi12, Cu–Al alloy and pure aluminum metal. The purity of metals used to prepare the master and Zn–Al–Cu–Si alloys was 99.99% for zinc and copper, 99.97% for aluminum. The melting process was carried out in the temperature controlled electrical crucible furnace. The melted alloys were then cast into steel moulds preheated at a v temperature of 300 C. The mould was designed in the form of a conical-cylinder for proper feeding and directional solidification. The mould had a casting cavity having dimensions of 45 mm internal diameter at the bottom and 60 mm internal diameter at the top and 200 mm length, respectively. The chemical compositions of the alloys were determined by the atomic absorption method. The results of chemical analysis of the alloys were tabulated in Table 1.
A schematic Stribeck diagram.
radius and c is the radial clearance. In view of the above, a more comprehensive parameter, the Sommerfeld number ðS ¼ gn=PW2 Þ was used in this study to take into account the variables generally specified by a designer. Tribological properties of the journal bearings were then obtained via the graphics plotted using the friction factor, (Ff ¼ l=W), versus the Sommerfeld number. This paper presents the results of an experimental study conducted to determine tribological properties of zinc–aluminum based journal bearings having silicon addition using a purpose-built journal bearing test rig. Mechanical properties such as ultimate tensile strength (UTS), elongation, hardness and microstructure of the alloys were determined. Friction properties of the bearings produced from the alloys were also studied. Surface roughness of the journal bearings was investigated to determine its effects on their frictional properties. 1.1. Alloy preparation Three zinc–aluminum based (Zn–Al–Cu–Si) alloys containing silicon were prepared by permanent mould casting process. In order to prepare Zn–Al–Cu–Si alloys, silicon and copper additions were made as AlSi12 and intermetallic copper–aluminum (46.5% Al) master alloys, respectively. AlSi12 alloy was procured from a commercial source, while intermetallic Cu–Al
1.2. Microstructure of alloys The samples taken from the alloy castings for microstructural studies were prepared using standard metv allographic techniques and etched at 70 C with 25% Nital etchant (25% nitric acid, 75% ethanol). Their micrographs were taken using an optical microscope. The size of silicon particles in the microstructure was measured according to the intercept method mentioned elsewhere [15]. The working surfaces of the thin-walled journal bearings were investigated using a Jeol 6400 scanning electron microscope (SEM). 1.3. Determination of mechanical properties and surface roughness Hardness measurements were carried out using a Brinell hardness tester at 31.25 kg load with a 2.5 mm diameter ball indenter. Standard specimens having 50.8 mm gauge length and 12.8 mm gauge diameter prepared from the alloys were tested at a 2:56 103 s1 strain rate in order to determine their UTS and percent elongation. The density of the alloys was calculated from the mass to volume ratio. The initial (after running-in process) and the final (after working at 0.1, 0.3, 0.7 and 1.1 MPa pressures) surface roughness of the journal bearings and the sleeve was measured by using a stylus surface rough-
Table 1 Chemical compositions of the alloys and measured silicon particles size range Alloy
Chemical composition (Weight%)
No.
Description
Zn
Al
Cu
Si
1 2 3
ZnAl27Cu2Si1 ZnAl27Cu2Si2 ZnAl40Cu2Si1
69.4 68.3 56.3
27.6 27.8 40.8
1.97 1.96 1.95
1.03 1.94 0.95
Size range of silicon particles (1 lm) 7–15 12–20 7–15
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ness apparatus. Surface roughness parameters such as centre line average (Ra), average peak to valley height (Rz) and maximum peak to valley height (Rmax) were measured by using a driving unit at a tracing speed of 0.5 mm/s with traversing and cut-off lengths, 15 and 2.5 mm, respectively. The unit was connected to a measuring indicator in order to read measured roughness values. The average values of five roughness measurements were taken. 1.4. Determination of friction properties 1.4.1. Specimen preparation and test procedure Thin-walled journal bearings produced from each alloy (total nine bearings for three alloys) were tested to investigate their friction properties using a journal bearing test rig modified specifically for this research. There was a circular oil groove at the centre of the bearing wall to enable sufficient supply of oil on the contacting surfaces as illustrated in Fig. 2. The effective bearing length was taken as 10 mm to operate at a pressure of 1.1 MPa because the maximum allowable load of the test rig was 542 N. The total bearing length and wall thicknesses were 30 and 2.95 mm, respectively. The bearings had a bearing clearance of 40 lm and an inner diameter of 50 mm. A schematic representation of the rig is shown in Fig. 2. The rig consists of a rigid tubular steel frame equipped with a pneumatically operated pressurized oil supply, a direct current solid state variable speed control unit, split journal bearing assembly and instrumentation for indicating speed and armature current. The apparatus operates a 240 V single phase electrical supply and an air supply at a pressure of not less than 0.6 MPa. The pressurized oil system consists of a double acting pneumatic cylinder driving an oil cylinder in a reciprocating manner. Incorporated in the system are two over pressure relief valves which limit the supply pressure to the bearing and hydrostatic pad to approximately 0.15 and 1.6 MPa, respectively. In this test rig, the hydrostatic pad is made of mild steel and accurately lapped to fit the journal bearing housing. Eleven jets provided the oil for the hydrostatic lift enabling the load to increase up to 542 N to be transmitted to the bearing without affecting the sensitivity of friction torque measurements. Oil leakage from the bearing and hydrostatic pad is collected in the drip tray and returned to the reservoir. The journal bearing housing is made of cast iron and has an accurately honed bore to accommodate thin-walled bearings as used in motor car engines. The sleeve was made of cold worked tool steel hardened to 722 HB. The rotational speed could be adjusted in the range of 0–1100 rpm (0–2.88 m/s) using a direct current speed control unit. The detailed information can be obtained from Ref. [16].
Fig. 2. Journal bearing test rig and test specimen.
The friction torque was measured by using a Wheatstone bridge circuit with strain gauges on the torque plate. The torque signal obtained from the bridge circuit was transmitted to the recorder and this enabled monitoring and recording the signals. The recorded signal values were then converted into friction torque and friction factor using the calibration line obtained from strain gauges. All experiments were carried out using SAE 15 lubricating oil at the flow rate of 50 cm3/min. Oil temperatures at the outlet of the bearing was measured using a
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thermometer and viscosity values were taken from the calibration chart of viscosity–temperature given by the manufacturer for this type of oil. In all experiments, v the oil temperature remained in the range 25–30 C with corresponding dynamic viscosity 0.0168–0.0138 N s/m2. Before conducting tests, the journal bearing surfaces were subjected to running-in process under a pressure of 0.3 MPa at a constant rotational speed (500 rpm, 1.3 m/s) for 10 min. After this process, the journal bearings were tested at rotational speeds ranging from 25 rpm (0.065 m/s) to 1100 rpm (2.88 m/s) for 5 min at each speed. The hydrostatic pad and lubricating oil pressures selected were 1.2 and 0.1 MPa, respectively. Only one sleeve was used for experiments due to its high hardness (722 HB) compared to the bearings (97– 125 HB). In order to determine the effect of the bearing pressure on the frictional properties, the bearings were tested at different pressures of 0.1, 0.3, 0.7 and 1.1 MPa at the constant clearance ratio of W ¼ 0:0008. After testing a bearing at a pressure such as 0.1 MPa, it was subjected to a test at 0.3, 0.7 and 1.1 MPa. The process repeated for three bearings having same chemical composition. After that, the mean frictional torque was taken into account while plotting the friction factor versus the Sommerfeld number. The lubricant film thickness was calculated by using the formula: h0 ¼ W:r½1e . Here, r and E are the bearing radius and eccentricity ratio, respectively. Eccentricity ratio (e) was taken from charts according to the bearing properties such as pressure, viscosity, rotational speed, bearing length, etc., [17]. The lubricant film thickness was calculated only for ZnAl27Cu2Si2 bearings working at the 900 rpm and different pressures because chemical composition has no effect on the frictional behavior in full-film lubrication regime [17].
2. Results 2.1. Microstructure of alloys Microstructures of the alloys are shown in Fig. 3a–c. In all alloys, the matrix structure consisted of aluminum rich a dendrites (light areas), zinc rich g phase (dark areas) in its surrounding, and silicon particles (eutectic and primary). Furthermore, at the rims of the a dendrites there was a mixture of a and g phases from the reaction product of b phase. The size of the silicon particles in the alloys is listed in Table 1. The table shows that ZnAl27Cu2Si2 alloy revealed coarser and more silicon particles than the other alloys. SEM images of the wear surfaces of the journal bearings are shown in Fig. 4a–c. It can be seen from the figures that there are slight wearing tracks and friction layer smeared on the surface.
Fig. 3. The microstructure of (a) ZnAl27Cu2Si1, (b) ZnAl27Cu2Si2, and (c) ZnAl40Cu2Si1 alloys.
2.2. Mechanical properties The mechanical and physical properties of the test specimens are shown in Table 2. The maximum tensile strength (325 MPa) and hardness (125 HB) were obtained from ZnAl27Cu2Si2 alloy containing 2% Si. All alloys exhibited low elongation and brittle fracture behavior. It is apparently seen that increasing silicon content from 1% to 2% increases the UTS and hardness of alloys. The ZnAl27Cu2Si1 alloy attained UTS
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as 287 MPa whereas the ZnAl27Cu2Si2 alloy demonstrated as 325 MPa UTS. Further, increasing aluminum content from 27% to 40% decreases mechanical properties of the alloys. The monotectoid-based ZnAl40Cu2Si1 alloy shows lower UTS and hardness than the eutectoid based ZnAl27CuSi1 alloy. The ZnAl40Cu2Si1 alloy had the highest aluminum content and exhibited the lowest density among the alloys. 2.3. Surface roughness Table 3 shows the surface roughness values of the journal bearings and sleeves. Final roughness represents the roughness obtained at the end of the experiments, that is, after the testing of the journal bearings at sequentially 0.1, 0.3, 0.7 and 1.1 MPa pressures. The initial roughness was measured from bearings after the running-in period. The maximum initial roughness value (1.45 lm) was obtained for the ZnAl27Cu2Si2 alloy journal bearing and it also showed the maximum final roughness (1.37 lm) after experiments. Minimum initial (1.05 lm) and final roughness values (1.0 lm) were obtained from ZnAl40Cu2Si1 journal bearing. In addition to journal bearings the hard sleeve shows minimum initial (0.52 lm) and final (0.48 lm) surface roughness. 2.4. Friction behavior The friction factor versus Sommerfeld number plots for the bearings are plotted in Fig. 5a–d at different pressures. The curves obtained at 0.1 and 0.3 MPa pressures were similar to typical Stribeck diagram revealing boundary, mixed and full-film lubrication regimes as illustrated in Fig. 5a,b. As clearly seen from the figures, the friction factor decreased rapidly with increasing Sommerfeld number in the boundary lubrication regime. After exhibiting a minimum in the mixed lubrication zone, the friction factor increased with increasing Sommerfeld number in the full-film lubrication zone. On the contrary, as shown in Fig. 5c,d, the friction factor of the journal bearings containing 2% Si continuously decreased with increasing Sommerfeld number whereas the plots for remaining alloy bearings showed a slight increase at the pressures of 0.7 and 1.1 MPa in the full-film lubrication zone.
Fig. 4. SEM images of working surfaces of the journal bearings of (a) ZnAl27Cu2Si1, (b) ZnAl27Cu2Si2, and (c) ZnAl40Cu2Si1 after friction tests.
Table 2 Some mechanical and physical properties of the alloys No.
Material
Ultimate tensile strength (MPa)
Elongation (%)
Brinell hardness (HB)
Density (kg/m3)
1 2 3
ZnAl27Cu2Si1 ZnAl27Cu2Si2 ZnAl40Cu2Si1
287 325 247
2.0 0.8 1.2
108 125 97
4810 4630 4110
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Table 3 Average surface roughness of bearings and sleeve (0.02 lm) Condition
Material
Ra (lm)
Rz (lm)
Rmax (lm)
Initial roughness (After running-in, 0.3 MPa, 500 rpm)
ZnAl27Cu2Si1 ZnAl27Cu2Si2 ZnAl40Cu2Si1 Sleeve ZnAl27Cu2Si1 ZnAl27Cu2Si2 ZnAl40Cu2Si1 Sleeve
1.15 1.45 1.05 0.52 1.06 1.37 1.00 0.48
4.9 6.6 4.8 2.2 4.2 5.0 4.4 1.8
6.3 7.3 5.9 2.3 4.4 6.2 5.2 1.8
Final roughness (After experiments, testing sequentially at 0.1, 0.3, 0.7 and 1.1. MPa pressures)
Ra, centerline average; Rz, average peak-to-valley height; Rmax, maximum peak-to-valley height.
The lubricant film thickness calculated by using the method described in Ref. [17] as a function of bearing pressure is shown in Table 4. The lubricant film thickness decreased with increasing bearing pressures. At the pressure of 0.1 MPa, the lubricant film thickness was 14 lm while it reduced to 5 lm at 1.1 MPa. 3. Discussion Generally monotectoid Zn–Al alloys (40–60% Al) show lower mechanical properties than eutectoid (25–28% Al) alloys [18]. The agreeable results achieved in our study show that the monotectoid-based ZnAl40Cu2Si1 alloy containing high aluminum (40%) showed lower UTS and lower hardness than the eutectoid alloys (Table 2). This may be due to their different microstructures forming at the end of the different phase transformations (monotectoid and eutectoid). In alloys (27% Al), the eutectoid material (b) decomposes into the equilibrium constituents (a þ g) in less than 2 min after quenching from above the eutectoid temperature. Similarly the supersaturated a solid solution transforms to the equilibrium constituents (a þ g) by precipitation of the excess zinc (g-phase) in a relatively short period of time. The decomposition reactions become more complicated at medium aluminum content. Most alloys near to 50% Al decompose on slow cooling via a monotectoid reaction, two successive eutectoid reactions and a solubility change [19]. Increasing silicon content from 1% to 2% in the Zn– Al based alloys improved their mechanical properties (Table 2). Silicon is used as a good alloying and dispersion strengthening element for aluminum alloys [20]. It plays a role as an obstacle to dislocation movement for slip in microstructure and hence mechanical properties increase [21]. Therefore, the addition of 2% silicon to the ZnAl27Cu2Si2 alloy led to the highest UTS and hardness among the alloys tested. More and coarse silicon means a more and huge dislocation barrier, which leads to higher mechanical properties of the alloys. The addition of silicon to the bearing alloys such as 2% coarsens its particles and it may cause increased wear
of the journal bearing and the sleeve. In order to prevent this likely wear process, the silicon size must be controlled with other additions such as strontium. In mixed and full-film lubrication zones, silicon particles (15–20 lm) are almost in the same size range as the lubricant film thickness (Table 4). The silicon particles detached from the journal bearing surface during the working period would lead to scoring and wear of the bearings. Smearing and adhering of worn debris on the surface with limited wear tracks demonstrated adhesive type wear mechanism for the Zn–Al based bearings as shown in Fig. 4. After the running-in process, the journal bearings showed different surface roughness. This may be attributed to different chemical composition and varying mechanical properties of the bearings. For materials having high mechanical properties, the asperity flattening and smoothing of the surface becomes more difficult than the materials having low mechanical properties [22]. Smoothening of the asperity of the harder sleeve (722 HB) was less than that of the journal bearings (97–108 HB) after the tests although it worked longer than the bearings. There was 0.04 lm decrease in surface roughness on the sleeve whereas the decrease was 0.05–0.09 lm for the journal bearings. As shown in Fig. 5, friction factor decreases with increasing bearing pressure especially in the mixed and full-film lubrication zones for all bearings. It can be seen in Fig. 5a,d that the minimum friction factor was 60.4 at pressure of 0.1 MPa, while it decreases to 14.6 at 1.1 MPa for ZnAl40Cu2Si1 bearing. Friction depends on oil film thickness partially in the mixed lubrication zone but completely in full-film lubrication regime [23]. In full-film lubrication, the surfaces are completely separated by a fluid lubricant. Friction solely arises due to shearing of the lubricant film layers by the rotating sleeve. In this study, as clearly seen from Table 4, the film thickness between the surfaces decreased with the increasing bearing pressure. This explains why friction factor decreased with the increasing bearing pressure.
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Table 4 Calculated lubricant film thickness for ZnAl27Cu2Si2 bearing at 900 rpm
Fig. 5. Friction factor–Sommerfeld number plots for the bearings at the bearing pressure of (a) 0.1 MPa, (b) 0.3 MPa, (c) 0.7 MPa, and (d) 1.1 MPa.
Bearing pressure (MPa)
Lubricant film thickness (lm, 0.5)
0.1 0.3 0.7 1.1
14 9 7 5
The ZnAl27Cu2Si2 bearings showed higher friction factor than the other bearings in all lubrication zones at all pressures. This may be due to the high surface roughness of the journal bearings. The coefficient of friction increases with increasing surface roughness leading to high energy dissipation. This is due to the resistance offered by the surface roughness against the lubricant flow, which increases with increasing roughness [24,25]. As clearly seen in Fig. 5c,d, ZnAl27Cu2Si2 bearing showed different frictional behavior at 0.7 and 1.1 MPa bearing pressures than at 0.1 and 0.3 MPa, Fig. 5a,b. Friction factor in this case continuously decreased with increasing rotational speed at 0.7 and 1.1 MPa (Fig. 5c,d) and full-film lubrication zone did not occur at all. As pointed out previously, the bearings had the highest surface roughness that becomes more important at high pressures for determining lubrication zone. In our study, the working conditions for the bearings were still in mixed lubrication zone and metal-to-metal contact occurred although the other bearings experienced full-film lubrication condition. It can be said that the aluminum content affects the frictional properties of the bearings, especially in boundary and mixed lubrication zones where metal-tometal contact occurs. Comparing the friction data for ZnAl27Cu2Si1 and ZnAl40Cu2Si1 bearings (Fig. 5), it can be noted that the friction factor data for ZnAl40Cu2Si1 are lower than those of ZnAl27Cu2Si1 at all pressures. For example, at a pressure of 0.1 MPa and at 75 rpm ZnAl40Cu2Si1 bearing shows minimum friction factor as 60.4, whereas this factor was found to be 74.6 for ZnAl27Cu2Si1 bearing. ZnAl40Cu2Si1 alloy has lower UTS (247 MPa) than the ZnAl27Cu2Si1 alloy (287 MPa). In boundary and mixed lubrication zones, mechanical properties as well as surface roughness also affect the frictional force. The surface asperities having low shear strength can be easily deformed by hard asperities of the sleeve resulting in a low friction force as compared to high shear strength materials [26]. Running-in is the process of removing high spots and mating the bearing surfaces [26]. The bearings were subjected to work at low speed and pressure before actual working conditions. During the process roughness of surface decreases and then low friction and wear rates can be obtained [27]. Surface roughness also
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determines the full-film lubrication initiation point at the bearings. Low friction and wear in boundary and mixed lubrication zones can be obtained by decreasing surface roughness with the extent of running-in process or with other methods in Zn–Al based bearings containing 2% Si. 4. Conclusions The results of current study indicate that all the investigated factors like Al and Si content of the alloys and test pressure/speed play an important role in controlling the frictional behavior of the thin-walled journal bearings. It is shown that high silicon content improves mechanical properties of the Zn–Al based alloys but the size of the silicon particles for bearing having especially 2% Si must be taken into account to prevent excessive wear of sleeve and bearing. It is further shown that high surface roughness leads to high friction in all lubrication zones. Moreover, ZnAl27Cu2Si2 bearings having high roughness do not show full-film lubrication zone at the bearing pressures of 0.7 and 1.1 MPa for all the test speeds. In addition, surface roughness has effects on frictional properties of Zn–Al based journal bearings containing Si.
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