Abstract
Powder materials are widely used in journal bearings since they provide a good tribological performance with journal bearings. These bearings are self-lubricated and can be used in places where no lubricating is possible. In this study, tribological and mechanical properties of copper based CuSn10, ferrous based Fe–graphite and copper+ferrous based CuSnFe–graphite bearings manufactured by powder metallurgy method have been determined and compared. Wear tests were carried out at 20 N load and 1500 rev min−1 every 30 min for 2·5 h using radial journal bearing wear test rig. Hardness, tensile, compressive, bending and radial fracture mechanical tests of these bearing materials were carried out. As a result, tribological and mechanical properties improved in CuSnFe–C bearings.
Introduction
In the past few years, wood, iron and skin have been used as journal bearing materials. Later, brass, bronze and white metal have also found some applications. Currently, in addition to these bearing materials, aluminium and zinc based materials are used as journal bearing materials. With technological improvements, self-lubricated sintered bearings and plastic materials are used where continuous lubricating is impossible. Therefore, it is essential that the bearing material be chosen depending on application area.
Wear resistance is one of the most important properties that journal bearings should possess. There are several studies and investigations dealing with wear resistance improvements of these materials.1–4 Self-lubricating ferrous and copper based powder metal bearings are used. To improve their mechanical properties, powder metal bearings are produced by alloying. Alloying technique, powder characteristics and pressing methods affect lubrication, wear and microstructure of bearings material.5,6 Pore size and distribution are the most important parameters affecting mechanical properties.7,8 Amount of pore distribution and powder form are controlled in order to improve self-lubrication conditions.9,10 Oil impregnated sintered bearings are manufactured by powder metallurgy (PM) technique. Cost of these bearings is low. These bearings are used in computers, electrical motors, automotives, sewing machines, typewriters, agriculture and packing machines where lubrication is impossible. When pores of these bearings are filled up with oil, they can work under hydrodynamic lubrication conditions.11
Manufacturing parameters such as pressing pressure and sintering temperature affect mechanical properties of PM bearings. Porosity decreases with increasing pressure. Mechanical properties especially radial fracture strength also increase.12 Therefore, porosity affects mechanical properties of these materials, especially fatigue strength. To improve these properties, material density must be increased. Porosities are more dangerous at surface. Therefore, surface hardening processes are important.13 Porosity type, amount, scattering and powder form are controlled for better self-lubrication of bearings and for arrangement of the amount of impregnated oil. In addition, oil impregnation process is carried out by waiting in oil under vacuum or atmosphere pressure at 80°C for 6–12 h. Process temperature, applied load, tolerance and environment conditions must be paid attention at oil selection.14 Self-lubrication bearings have different densities and microstructures at different sintering temperatures and conditions. With increasing pressure density, porosity decreases in these bearings, resulting in less oil impregnation. Graphite addition increases density and decreases wear.15,16 Since pressure and velocity affect adhesive wear, bearings wear must have suitable PV values.5
In this study, tribological and mechanical properties of copper based CuSn10, ferrous based Fe–graphite and copper+ferrous based CuSnFe–graphite bearings manufactured by PM method have been determined and compared.
Experimental
Preparation of experimental materials
In this study, CuSn10, Fe–C (1% graphite) and 50%CuSn10+50%Fe–C CuSnFe–C (1% graphite) materials were used as journal bearing and the SAE 1050 was used as shaft. The chemical compositions of the materials which are used in the experiments are given in Table 1. Dimensions of bearing specimens are as follows: inner diameter (d = 12+0·05 mm), width (B = 12 mm) and outer diameter (D = 16 mm).
Chemical composition of journal material, wt-%
The specimens have been worn in radial journal bearing wear test at lubricated condition according to the procedure by Atik et al.17 The wear losses have been measured at lubricated conditions of 20 N load and 1500 rev min−1 (velocity v = 0·785 m s−1) every 30 min for 2·5 h (7065 m sliding distance). Lubricating has been carried out by SAE 90 gear oil. The microstructures of wear surfaces were photographed using optical (Hund Wetzlar CCD-290) and scanning electron microscope (Jeol JSM-6060). Surface roughness values have been performed on Mitutoyo-CE surface roughness test rig.
Tensile strength, compressive strength, three-point bending strength and radial fracture strength were measured using ALSA tensile test rig depending on TS-138 and TS-269 (Turkish Standard) for mechanical properties. Moreover, the hardness was measured using a digital metal hardness tester (SADT HARTIP-3000).
Radial journal bearing wear test rig
Bearing materials in journal bearings are generally selected from materials which have lower wear strength than the shaft material, thereby lowering the wear of the shaft significantly. Therefore, journal bearing wear test apparatus is designed to examine the wearing of bearing materials. In this study, a special bearing wear test apparatus has been designed to examine the wearing behaviour of bearing material and the shaft together. Therefore, it is possible to investigate different bearing and shaft materials and the effects of heat treatments on these materials. Such a mechanism provides wear of bearings rather than using standard methods as this is more appropriate.17
The system is formed by a weight applied by a rigid bar, a steel bar connected to the bearing from a distance and a comparator. Friction coefficient is determined from the friction force formed along the rotating direction of the bearing and from the movement of the steel bar connected to the bearing.18 Radial wear test rig is illustrated in Fig. 1.
Radial journal bearing wear test rig
In the experiments under lubricated conditions, very little movement took place due to high comparator's spring coefficient and low friction. Therefore, a tensile spring of k = 0·004 N mm−1 has been connected on the opposite side of the comparator. The movements formed by the effect of the friction force have been measured by this method.
Results and discussion
Mechanical properties
Values of mechanical test are shown in Table 2. Hardness values were found to be 39–67 HB. Tensile strength of these bearing materials was found to be around 94–148 MPa. Mechanical properties of ferrous based Fe–C bearing materials appeared higher than those of PM copper based CuSn10 bronze bearing materials. The highest mechanical properties appeared in ferrous based Fe–C bearing materials because of high mechanical properties of Fe. The lowest mechanical properties appeared in copper based CuSn10 bearing materials because of lower mechanical properties of bronze depending on Fe. The values for mechanical properties of 50%CuSn10+50%Fe–C filled CuSnFe–C bearing materials were between those of CuSn10 and Fe–C bearing materials.
Mechanical properties of PM bearing materials
Demir and Sarıtaş7 investigated mechanical properties of powder metal steels. Kurt14 investigated mechanical properties of PM bronze bearing materials. They obtained standard mechanical properties in these materials. They reported that porosity, temperature and environment conditions affect mechanical properties. Kato et al.19 investigated mechanical properties of sintered copper–tin composites. They reported that mechanical properties of the composites decreased with increasing amount of added graphite. In this study, similar mechanical properties were obtained.
Tribological properties
Values of surface roughness before wear and after wear are given in Table 3. These values decreased due to few adhesive wear after wear tests.
Roughness of bearing PM materials
Values of friction coefficient, bearing temperature, bearing and journal wear loss are shown in Figs. 2–5. The highest friction coefficient, bearing temperature and journal wear loss appeared in Fe–graphite bearing because of hard Fe phase, while the lowest friction coefficient appeared in CuSn10 bearing because of good wear properties. The lowest bearing wear loss appeared in Fe–graphite bearing, while the lowest journal wear loss appeared in CuSnFe–graphite bearing. From these bearings, CuSn10 wear loss of 9·6 mg, Fe–graphite wear loss of 8·4 mg and CuSnFe–graphite wear loss of 9·8 mg appeared at 2·5 h. Journal wear loss for CuSn10 was 6·4, that for Fe–graphite was 41·1 mg and that for CuSnFe–graphite was 3·4 mg. Bearing wear rate of CuSn10 was 8·3×10−6 mm3 Nm−1 (Fig. 6), that of Fe–graphite was 8·08×10−6 mm3 Nm−1 and that of CuSnFe–graphite was 8·94×10−6 mm3 Nm−1. So, the highest bearing wear loss and the lowest journal wear loss appeared in CuSnFe–graphite bearing. The lowest bearing wear loss and the highest journal wear loss appeared in Fe–graphite bearing.
Friction coefficient–time variation in PM bearings
Temperature–time variation in PM bearings
Wear loss of bearing–time variation in PM bearings
Wear losses of journal–time variation in PM bearings
Comparison of bearing materials at different wear rates
Varol5 investigated wear properties of Cu and Fe based PM bearing materials at 25 and 40 N loads, 0·48 and 0·68 m s−1 speeds and 4000 m sliding distance. He obtained 8 mg wear loss for Cu bearing and 9 mg wear loss for Fe bearing at 0·48 m s−1. However, he obtained different wear losses at different loads and speeds. He reported that pressure and velocity (PV) affect adhesive wear. Kurt14 investigated wear properties of PM bronze bearing materials. He reported that porosity, pressing methods and environment conditions affect wear and self-lubrication properties. Kato et al.19 investigated wear properties of sintered copper–tin composites. They reported that wear properties of the composites increased with increasing amount of added graphite. Jia et al.20 determined friction coefficient 0·3 and wear rate 1×10−6 mm3 Nm−1 for bronze–graphite PM composites at 200 N load and 0·53 m s−1 sliding velocity for 2 h sliding duration under pin on disc wear water lubricated test conditions. They reported that the wear resistance in water lubricated test specimens was ∼14 times that in dry test specimens.
The differences in the author's results and those of other previous studies may be attributed to the fact that their materials were different from the author's materials. In addition, the author's results showed that radial journal bearing test rig gave more accurate measurements.
Wear surface properties
When microstructures of samples were examined using optical and scanning electron microscopes (Figs. 7 and 8), the highest adhesive wear tracks and torn particles observed in CuSn10 bearings because of soft Sn phase structure (Figs. 7a and 8a). The lowest adhesive wear tracks and torn particles were observed in Fe–graphite and CuSnFe–graphite bearings because of hard Fe phase (Figs. 7b and c and 8b and c). Figure 6
Microstructure of wear surface ×100
Microstructure (SEM) of wear surface ×1000
Few wear tracks were observed in graphite added bronze composites depending on non-graphite bronze composites.19 Jia et al.20 observed plastic deformation, microcracking, scuffing, abrasive and adhesive wear tracks at dry conditions in bronze–graphite PM composites. They reported that these wear tracks decreased at water lubricated test conditions. In this study, similar wear tracks were obtained at lubricated conditions.
Conclusion
The highest friction coefficient, bearing temperature and journal wear loss appeared in Fe–graphite bearings, while the lowest friction coefficient appeared in CuSn10 bearings. The lowest bearing wear loss appeared in Fe–graphite bearing, while the lowest journal wear loss appeared in CuSnFe–graphite bearing. The highest adhesive wear tracks and torn particles were observed in CuSn10 bearings, while the lowest adhesive wear tracks and torn particles were observed in Fe–graphite and CuSnFe–graphite bearings. Tribological and mechanical properties improved in 50%CuSn10+50%Fe–C filled CuSnFe–C bearings.