3.1 Effect of Speed on the Tribological Characteristics
In this section, the test object is the ‘Experiment 1’ in Table 2. The contact pressure was adjusted to 3 MPa, and the speed was 500 r/min and 1800 r/min, respectively. Due to the difference in surface hardness between the upper and lower samples, wear mainly occurs on the upper samples made of CuPb15Sn5. Therefore, only the wear rate of the upper samples is calculated as the measured mass loss divided by the sliding distance, the unit is mg/m.
The friction coefficients at 500 r/min and 1800 r/min are respectively shown in Figure 8. At the speed of 500 r/min, it gradually increases, then reaches a stable wear stage; the stable value is 0.105. In regard to 1800 r/min, the friction coefficient fluctuates between 0.045 and 0.08, and there was a huge noise during the test. After the test, it was found that a large amount of loose copper particles or wear debris were accumulated between the friction pairs. All of these phenomena indicate that the friction at 1800 r/min is unstable. Their corresponding wear rates at 500 r/min and 1800 r/min are shown in Figure 9, i.e., 2.25 mg/m and 5.81 mg/m, respectively.
As shown in Figure 10(a), the surface of the upper samples at 500 r/min does not show severe adhesive wear; however, there is severe adhesive wear on the surface under the condition of 1800 r/min (Figure 10(b)). After the test, a lot of CuPb15Sn5 debris were found on the friction surface under the condition of 1800 r/min. The reasons may be as follows: on the one hand, it is impossible to keep a complete oil film between the contact surfaces under the combined action of the centrifugal force and the contact pressure of 3 MPa; on the other hand, friction heat might soften the upper sample. In this test, both frictional heat and centrifugal force increase significantly with speed. Therefore, adhesive wear is aggravated as the rotational speed increases.
In addition, it can be inferred from Figure 10 that the fluctuation in the friction coefficient at 1800 r/min might result from three-body abrasive wear. The process is generally as follows: adhesion wear occurs because of high contact pressure and high rotational speed, then adhered copper material is knocked off and is crushed into a large amount of loose wear abrasives, these loose abrasive particles will slide across the wearing surface, just acting as rolling bearings between the friction surfaces, which will convert the sample from sliding friction to rolling friction, thereby reducing the friction coefficient. However, due to the differences in the shape and size of these loose abrasive copper particles, and they are not evenly distributed between the friction surfaces, which will make the friction pair rotate unevenly, and then the friction coefficient curve fluctuates.
3.2 Effect of Contact Pressure on Tribological Characteristics
In this section, the samples are from ‘experiment 1’ in Table 2. The operating speed was 1800 r/min. During the test, the contact pressure was set as 1 MPa, 1.5 MPa, 2 MPa, 2.5 MPa and 3 MPa, respectively. As shown in Figure 11, under the condition of 1 MPa, 1.5 MPa, 2 MPa and 2.5 MPa, the friction pair goes through a short running-in wear stage in which the friction coefficient gradually decreases, then it reaches a stable wear stage, their average friction coefficients at stable stage are 0.075, 0.07, 0.065 and 0.08, respectively.
Within the contact pressure range of 1–2 MPa, the friction coefficient decreases with the increase of contact pressure; as the contact pressure increases to 2.5 MPa, the friction coefficient is about 0.08. This trend matches the relation between friction and Sommerfield Number [25]. The Sommerfeld Number is a dimensionless quantity used extensively in hydrodynamic lubrication analysis. From a macroscopic point of view, when the rotational speed and fluid viscosity are constant, the Sommerfield Number decreases as the contact pressure increases. Under full fluid film lubrication condition, the friction coefficient decreases as Sommerfield Number decreases. Once the conditions like the contact pressure decreasing the Sommerfield Number enough, the interface transitions to mixed lubrication or boundary lubrication. Under the condition of mixed lubrication or boundary lubrication, the contact of the surfaces begins to occur between the asperities, so the friction level increases sharply. In addition, it should pointed out that a large amount of debris were found in the oil box under the condition of 2.5 MPa after the experiment, and the maximum temperature have reached 50 ℃ (as shown in Figure 12), which was higher than that under 2 MPa condition as for 1–2 MPa, there is little debris in the oil box after experiment.
When the contact pressure is further increased to 3 MPa, the experimental friction coefficient becomes extremely unstable. As can be seen from Figure 11(e), the friction coefficient fluctuates between 0.045 and 0.08. The reason is that the loose abrasive particles generated by the adhesive wear are present on the friction surface, which act as rolling bearings between the surfaces to reduce the friction coefficient.
The wear rate at the contact pressure of 1 MPa, 1.5 MPa, 2 MPa, 2.5 MPa and 3 MPa are shown in Figure 13. As shown, the wear rate increases nonlinearly as the contact pressure increases. It increases slowly when the contact pressure is less than 2.5 MPa; once the contact pressure exceeds 2.5 MPa, it is sharply increased.
Figures 14, 15 illustrate the surface morphology of the sample after tests. Since the rate of wear is low at the contact pressure of 1 MPa and 1.5 MPa, the original processing marks on the surface of upper samples are still clearly visible, as shown in Figure 14(a), (b). At the contact pressure of 2 MPa, the original morphology of the upper sample disappears, and its surface is smoother than that before the experiment, as shown in Figure 14(c). There are no scratches or adhesive wear on the surface of the t upper samples shown in Figure 14(a)–(c).
As described in Section 2.2, the surface roughness of the upper and lower samples are 0.2 μm and 0.4 μm, respectively. In other words, there are some asperities on every surface. These asperities may be preserved under light load and full film lubrication conditions, such as 1 MPa and 1.5 MPa. As the load is greater than the critical value, the tips of these asperities contact each other because of the breakdown of the lubricant film, and the “real” contact area is much smaller than the dimensions of friction pair, then the asperities are quickly loaded beyond their yield stress. As a result, the plastic deformation makes the surface of the friction pair look smoother. This phenomenon is consistent with the test results under 2 MPa operating conditions (Figure 14(c), (f)).
As the load is further increased, the subsequent relative movement of the friction pair would make the joined asperities shear each other, and then new junctions form. If the force of adhesion at the junction between the two surfaces is higher than cohesion, the shearing might occur at the base of one of the asperities. As a result, the material from one surface may be transferred to the other one, which is known as adhesive wear. This description is consistent with the test results under 2.5 MPa condition. As shown, a small amount of adhesive wear appears on the sample surface shown in Figure 15(a), and the surface of the lower sample is slight oxidized due to friction heat (Figure 15(c)).
As the contact pressure is increased to 3 MPa, severe wear occurs on the upper sample (Figure 15(b)), and several pieces of bronze are torn away from its surface. For the lower sample shown in Figure 15(d), some bronze particles adhere to its surface, and its surface is severely oxidized by the friction heating. The results indicate that the valve plate/cylinder block friction pairs cannot reach the specified life at the contact pressure of 3 MPa.
Obviously, it can be concluded from Figures 11(a)–(c), 13 and 14 that the interface transitions to the mixed lubrication condition in the range of 1.5–2 MPa. The wear rate is sharply increased when the contact pressure is higher than 2.5 MPa. Based on the above results, 2.5 MPa and 1800 r/min are selected as subsequent experimental parameters.
3.3 Effect of Material and Processing Treatment on the Tribological Performance
3.3.1 Friction Coefficient and the Wear Rate
In this section, the contact pressure is 2.5 MPa and the operating speed is 1800 r/min. Figure 16(a) shows the friction coefficients of 38CrMoAl samples with different treatment. As shown, the friction coefficient of the ‘control group 1’ gradually increases from 0.06 to 0.1 in the initial stage, then it rapidly increases to a certain value and starts to fluctuate, the average value is about 0.12. The friction coefficients of the ‘control group 2’ and the ‘experiment 1’ are between 0.07 and 0.08, it seems that the curve of ‘experiment 1’ is more stable than that of ‘control group 2’.
Figure 16(b) shows the friction coefficients of 42CrMo samples with different heat treatment. The friction coefficient of the ‘control group 3’ slowly decreases from 0.08 to 0.07. In the ‘control group 4’, it remains stable after a brief increase, but exceeds 0.11 and fluctuates greatly in the second half of the test. After the quenching and tempering processes, the ‘experiment 2’ shows a significant improvement compared with the ‘control group 4’, but there still exists fluctuations in the later stage.
Figure 17 illustrates the wear rate of upper sample in each experiment. It can be found that the wear rate is consistent with the friction coefficient. For 38CrMoAl with different treatment, the ‘experiment 1’ experiences the lowest wear rate, namely, 2.7 mg/m; and those of ‘control group 1’ and ‘control group 2’ are 8.95 mg/m and 6.35 mg/m, which are 3.3 times and 2.4 times as high as that of the ‘experiment 1’, respectively. As for the 42CrMo with different treatment, the ‘control group 3’ has the lowest wear rate, namely, 3.65 mg/m; and those of ‘control group 4’ and ‘experiment 2’ are 8.15 mg/m and 4.4 mg/m.
Comprehensive comparison of the above tests, it can be found that the wear characteristics of 38CrMoAl and 42CrMo are quite different. The wear rate of 38CrMoAl without any heat treatment is 145% higher than that of 42CrMo under the same condition; the wear rate of 38CrMoAl treated by nitriding process is 22% lower than that of 42CrMo; the wear rate of 38CrMoAl treated by tempering, quenching and nitriding process is 38% lower than that of 42CrMo.
3.3.2 Surface Topography
Figure 18 shows surface topography of the friction pairs with 38CrMoAl lower sample. For ‘control group 1’, the upper sample is severely worn (Figure 18(a)), the marks of adhesive wear exceeds 198 μm in diameter and scratches produced by abrasive also exceeds 70 μm; there also exists severe wear on the lower sample shown in Figure 18(d), it can be found that scratches with a width greater than 100 μm appear on it, and bronze particles adhere to the surface.
Compared with the test results of the ‘control group 1’, the surface wear of the ‘control group 2’ is slight. As illustrated in Figure 18(b), both scratches and adhesive wears are found on the upper sample surface, and the plastic deformation occurs on its surface, too; Figure 18(e) is the lower sample paired with it, it can be found that its surface roughness has disappeared, some bronze particles accumulate and have been transferred to its surface. Bronze particles transferred to the lower sample would come into contact with the upper sample, and eventually form adhesive wear or indentation (plastic deformation) on the upper sample.
Figure 18(c), (f) depicts the surface topography from ‘experiment 1’, its wear seems to be the slightest. For the upper sample shown in Figure 18(c), some adhesive wear and scratches exist on its surface as well, but these are negligible by comparison with Figure 18(a), (b); the lower sample paired with it is shown in Figure 18(f), it can be found that its surface is smooth and bronze particles are transferred to its surface as well.
Figure 19 shows the surface topography of the friction pairs with 42CrMo lower sample. For ‘control group 3’, there is a small amount of adhesive wear and slight indentations on its upper sample surface (Figure 19(a)); and some bronze particles are evenly spread between the lower-sample roughness peaks (Figure 19(d)), thereby smoothing its surface.
With respect to the ‘control group 4’, the upper sample seems to experience severe adhesive wear, and there are also serious scratches on it (Figure 19(b)). It can be found from Figure 19(e) that the roughness peaks on the lower sample have disappeared, and a large amount of bronze has accumulated on its surface; in addition, its entire friction area is covered with oxidized marks.
As depicted in Figure 19(c), adhesive wear and scratches also occur on the upper sample from the ‘experimental 2’, but the wear rate is slighter than that of the ‘control group 4’. Apart from that, the lower sample also shows an improvement in wear resistance, as illustrated in Figure 19(f). It could be found that only a small amount of bronze adheres to its surface, and its surface scratches and oxidation phenomena are also significantly reduced.