The wear resistance of ZSnSb11-6 babbitt alloy and ZQSn10-1 tin bronze under different loads was compared by using MFT-5000 friction and wear tester. The results show that when the load is 20 N, the wear rate of tin bronze is 78.3% lower than that of babbitt alloy, and when the load is 140 N, the wear rate of tin bronze is still 36.3% lower than that of babbitt alloy; ZSnSb11-6 babbitt alloy has deeper wear marks and furrows due to its lower hardness, and it is easy to peel off at the hard phase β phase (SnSb) with the increase of load; ZQSn10-1 tin bronze has shallower wear marks and furrows due to its higher hardness, and the degree of peeling is light due to the smaller size of (α + δ) hard phase and no sharp edges. Therefore, the wear resistance of ZQSn10-1 tin bronze is better than that of ZSnSb11-6 babbitt alloy under loads of 20 to 140 N.

ZSnSb11-6 babbitt alloy is widely used in ship engine bearings due to its good wear resistance, compliance and friction reduction. Fu Yanchao et al. studied the effect of load on the friction and wear properties of 11-6 tin-based babbitt alloy under dry friction conditions and found that as the load increased from 10 N to 50 N, the wear rate also increased. When the load was 50 N, fatigue spalling was the most serious and the wear resistance decreased significantly. Wu et al. studied the effect of load on the friction and wear properties of 16-16-2 babbitt alloy in seawater environment and found that as the load increased from 10 N to 120 N, the wear rate also increased. When the load reached 120 N, the plastic deformation was serious and the wear resistance decreased significantly. Therefore, the wear resistance of Babbitt alloy is related to the load. As the load increases, the wear resistance decreases.

As the engine bearing is developing towards high speed and heavy load, the traditional ZSnSb11-6 Babbitt alloy has low strength and poor load-bearing capacity. As the working temperature increases, the wear resistance decreases and even fails early. Unlü et al. compared CuSn10 tin bronze and CuZn30 brass under a load of 20 N and a speed of 0. The tribological properties under the condition of 78 m/s were compared, and it was found that CuSn10 tin bronze had a lower wear rate than CuZn30 brass and had better wear resistance; Yang Qing et al. compared the friction and wear properties of ZA27 alloy, ZCuAl10Fe3 aluminum bronze and ZCuSn6Zn6Pb3 tin bronze, and found that the wear volume of the three alloys gradually increased with the increase of load; Zhao et al. compared the friction and wear properties of QAl9-4 aluminum bronze and ZQSn6-6-3 tin bronze, and found that tin bronze had better wear resistance; ZQSn10-1 tin bronze has the characteristics of high hardness, excellent corrosion resistance and wear resistance, and is suitable for wear-resistant parts working under heavy load and high speed, and is expected to become a substitute material for babbitt alloy ship engine bearings. In summary, tin bronze has good wear resistance, but there are few reports on the comparison of the wear resistance of tin bronze and babbitt alloy for different bearings under different loads, especially heavy loads, and whether tin bronze can replace babbitt alloy as a material for ship engine bearings.

This paper compares the friction and wear properties of ZSnSb11-6 babbitt alloy and ZQSn10-1 tin bronze under different loads, observes and analyzes the surface morphology after wear, and explores its wear mechanism under heavy load conditions, in order to provide reference for the selection of ship engine bearings.

1 Test materials and methods

The chemical compositions of ZSnSb11-6 babbitt alloy and ZQSn10-1 tin bronze used in the study are shown in Tables 1 and 2, respectively.

The friction and wear test was carried out on the MFT-5000 multifunctional friction and wear tester. The pin samples are ZSnSb11-6 babbitt alloy and ZQSn10-1 tin bronze, with a size of 5 mm × 10 mm. The friction pair material is 35CrMoA steel, with a size of 50.80 mm × 6.35 mm, and a surface roughness of 0.1 mm. Oil lubrication is adopted, and the lubricant is Great Wall No. 68 anti-wear extreme pressure oil. The test load is set to 20, 50, 80, 110, and 140 N, the speed is 300 r/min, the rotation radius is 16 mm, the friction and wear time is 60 min, and the test is repeated 3 times. The friction coefficient during the test is collected and output by the testing machine. The calculation formula of the wear rate K is shown in the figure formula (1). Where: K is the wear rate, cm3 /(N·m); Δm is the wear mass loss before and after the test, g; FN is the normal load, N; ρ is the density of the pin material, g/cm3; S is the total sliding distance, m. The mass of the sample before and after wear was weighed using an electronic analytical balance with an accuracy of 0.1 mg. Ultrasonic cleaning was used for 15 min before weighing to remove adsorbent impurities on the surface of the sample. The surface morphology of the sample before and after wear was observed using a SIGMA-300 scanning electron microscope, and the wear scar depth was measured using a Bruker Contour GT-K optical profiler. The hardness of the sample before and after wear was measured using an MH-3 micro-Vickers hardness tester. The test force was 2 N and the holding time was 10 s.

2. Test results and analysis

2.1 Wear rate

Figure 1 compares the wear rates of ZSnSb11-6 Babbitt alloy and ZQSn10-1 tin bronze under different loads. It can be seen that with the increase of load, the wear rate of ZSnSb11-6 Babbitt alloy and ZQSn10-1 tin bronze both increase, among which the wear resistance of ZQSn10-1 tin bronze is better than that of ZSnSb11-6 Babbitt alloy. When the load is 20 N, the wear rate of tin bronze is 78.3% lower than that of Babbitt alloy; when the load is 140 N, although the wear rate of Babbitt alloy and tin bronze both increase, the wear rate of tin bronze is still 36.3% lower than that of Babbitt alloy.

2.2 Microstructure

Figure 2 shows the microstructure of ZSnSb11-6 Babbitt alloy and ZQSn10-1 tin bronze. As can be seen from Figure 2 (a), the matrix of ZSnSb11-6 babbitt alloy is an α solid solution with Sn as the main component, the square and sharp-edged intermetallic compound is the β phase (SnSb), and the short rod-shaped intermetallic compound is the ε phase (Cu6 Sn5). As can be seen from Figure 2 (b), the cast ZQSn10-1 tin bronze is mainly composed of three phases: α-Cu, δ, and Cu3P. The matrix α phase is a solid solution of Sn in Cu, and the δ phase is a solid solution with Cu31 Sn8 as the matrix. According to the literature, the grayish-white island block in Figure 2 (b) is an (α + δ) eutectoid, which is small in size and has no sharp edges.

2. 3 Wear morphology and mechanism

Figure 3 shows the wear surface morphology of ZSnSb11-6 Babbitt alloy and ZQSn10-1 tin bronze under different loads. As shown in Figure 3 (a, c, e, g, i), with the increase of load, the number of wear marks and furrows on the surface of ZSnSb11-6 Babbitt alloy increases, and the peeling area increases. According to the surface morphology and energy spectrum analysis results at position 1 in Figure 3 (c), this position contains elements such as Fe and O, among which Fe and other elements come from the grinding wheel material 35CrMo steel, which indicates that adhesive wear occurs during the friction and wear process, and the presence of O element indicates that slight oxidation wear occurs. Therefore, the wear mechanism of ZSnSb11-6 Babbitt alloy is mainly abrasive wear, contact fatigue wear, and slight oxidation wear and adhesive wear. As the load increases, the abrasives generated by adhesive wear and abrasive wear pass through the wear marks and furrows during the friction and wear process, and cover them to fill part of the furrows. Since the β phase (SnSb) in the babbitt alloy has sharp edges and corners, it is easy to cause stress concentration at the interface with the matrix. As the load increases, its contact stress continues to increase, resulting in the early formation of fatigue cracks. When the crack expands to a critical size, it causes spalling under the action of shear stress. Observing Figure 3 (b, d, f, h, j), it can be seen that the wear mechanism of ZQSn10-1 tin bronze is mainly abrasive wear. As the load increases, the number of wear marks and furrows on the surface of ZQSn10-1 tin bronze continues to increase, and the furrow width increases slightly. Compared with ZSnSb11-6 babbitt alloy, the size of the (α + δ) hard phase in ZQSn10-1 tin bronze is smaller and the degree of spalling is lighter. The wear scar depths of ZSnSb11-6 Babbitt alloy and ZQSn10-1 tin bronze at a load of 140 N were measured by an optical profilometer to be 7.962 and 1.531 μm, respectively.

The hardness of ZSnSb11-6 Babbitt alloy and ZQSn10-1 tin bronze after friction and wear at a load of 140 N was 21.6 and 166.1 HV0.2, respectively. Due to the lower hardness of ZSnSb11-6 Babbitt alloy, its wear scars and furrows are deeper.

In summary, the wear resistance of ZQSn10-1 tin bronze is better than that of ZSnSb11-6 babbitt alloy under loads of 20 to 140 N. Although the wear rate of ZQSn10-1 tin bronze increases faster with increasing load, the hardness of ZSnSb11-6 babbitt alloy is lower, the wear marks and furrows are deeper, and it is easy to peel off at the hard phase β phase (SnSb); in addition, ZQSn10-1 tin bronze has higher hardness, shallower wear marks and furrows, and its (α + δ) hard phase is smaller in size, has no sharp edges, and has a lighter degree of peeling. The maximum specific pressure of this test is 7MPa. Considering that the specific pressure of babbitt alloy engine bearings usually does not exceed 5MPa, the wear resistance of ZQSn10-1 tin bronze is better than that of ZSnSb11-6 babbitt alloy in actual service.

3 Conclusions
(1) Compared with ZSnSb11-6 babbitt alloy, ZQSn10-1 tin bronze has better wear resistance. When the load is 20 N, the wear rate of ZQSn10-1 tin bronze is 78.3% lower than that of ZSnSb11-6 babbitt alloy; when the load is 140 N, the wear rate of ZQSn10-1 tin bronze is still 36.3% lower than that of ZSnSb11-6 babbitt alloy.
(2) The wear resistance of ZQSn10-1 tin bronze is better than that of ZSnSb11-6 babbitt alloy under different loads. The hardness of ZSnSb11-6 babbitt alloy is relatively low, and the wear marks and furrows are relatively deep. Moreover, as the load increases, it is easy for the hard phase β phase (SnSb) to peel off. However, the hardness of ZQSn10-1 tin bronze is relatively high, and the wear marks and furrows are relatively shallow. Moreover, the size of its (α + δ) hard phase is relatively small, without sharp edges, and the degree of peeling is relatively light.