Effect of Zn on Microstructure and Wear Resistance of Sn-Based Babbitt Alloy

Abstract

Tin-based Babbitt alloys are a widely used bearing bushing material which have good comprehensive properties. However, problems such as high-temperature softening and insufficient bearing capacity occur during their use, so the optimization of tin-based Babbitt alloys has become a research hotspot. In this paper, ZChSnSb11-6 alloy was mainly prepared by the gravity casting method, and different amounts of Zn were added to the alloy (the mass fraction values were 0 wt.%, 0.05 wt.%, 0.1 wt.%, 0.15 wt.%, and 0.2 wt.%, respectively). Through the hardness test, the tensile test, the friction and wear test, and the microstructure observation of the prepared alloy, the influence of Zn on the organization and properties of the ZChSnSb11-6 alloy was analyzed. The results show that the size of the SnSb hard phase changed with the increasing content of Zn. The size of the hard phase of the SnSb tended to increase first and then decrease, and the number of phase particles increased first and then decreased, resulting in changes in performance. Through comparison, it was learned that the addition of Zn can effectively improve the hardness, tensile strength, yield strength, and wear resistance of the alloy, but the elongation rate was reduced. When the Zn content was 0.1 wt.%, the hardness value of the alloy reached the maximum value, 25.82 HB, which increased by 7.3% when compared with the sample without Zn. The hardness of the Zn, 0.15 wt.%, is close to that of the Zn, 0.1 wt.%. Compared to the sample without Zn, the tensile strength and elongation of the alloy were maximized at a Zn content of 0.15 wt.%. Compared to the sample without the Zn, the tensile strength was increased by 21.29%, and the elongation rate was increased by 46%. An analysis showed that the alloy has good comprehensive mechanical properties when the Zn content is 0.15 wt.%.

1. Introduction

The rolling bearing is a precision part in mechanical equipment, which plays the role of supporting, positioning, bearing, and reducing the wear of rotating parts, such as shafts. Therefore, it is an indispensable and important part of high-end equipment. Slide bearing is one of the key components of large steam turbine and gas turbine units, which play a role in supporting the weight of the rotor and ensuring the high speed and stable operation of the rotor [1]. During the operation of rolling bearing, there is relative movement between the rolling body, holder, and sleeve ring, which leads to heavy friction wear under the conditions of a heavy load, high speed, and skidding; the performance of the rolling bearings is closely related to the material [2]. Babbitt alloys are widely used in the sliding support bearings of steam turbine generator sets due to their superior friction reduction, adaptability, and pressure inability, ensuring that bearings have better performance and protect the expensive rotor [3].

Babbitt alloys were invented by Babbitt in the 19th century. Due to the differences in the main components, they are mainly tin, lead, cadmium, and other similar metals [4]. Currently, tin-based and lead-based alloys are widely used in industry, but with the enhancement of environmental awareness, toxic lead-based pasteurized alloys will inevitably be replaced, so high-performance tin-based Babbitt alloys have become a new research hotspot [5]. The main components of tin-based Babbitt alloy are tin, lead, antimony, and copper, among which antimony and copper, as alloying elements, can significantly improve the hardness of the alloy. The structure of a tin-based Babbitt alloy is composed of a soft matrix and a hard point. The soft-phase matrix ensures that the alloy has very good retention, compliance, and occlusion resistance, which can play a role in reducing friction. Hard particles like SbSn and Cu₆Sn₅ are distributed on the soft matrix. When rubbed, the soft base becomes concave, the hard point becomes convex, and the lubricating oil can be stored in the middle area, which can play role in lubrication. The harder part has a strong carrying capacity and plays a supporting role [6]. Therefore, the harsh working conditions of bearing alloys require a specific organization, which requires both soft and hard features. Compared to other bearing materials, the tin-based Babbitt alloy’s friction coefficient is small, and it has moderate hardness, as well as good toughness, run-in, corrosion resistance, thermal conductivity, compliance, and latent and lubrication ability, so it is widely used in electric power, heavy industry, and shipbuilding, being found in steam turbines, gas engines, internal combustion engines, and other large mechanical bearings, shafts, and shaft sleeves [7,8,9,10,11].

With the continuous emergence of large, industrial, high-grade, and heavy-load mechanical equipment, traditional tin-based pasteurized alloy materials and their preparation process cannot meet the new development needs. In order to improve the mechanical properties of tin-based bar alloy materials and meet the demand of industrial equipment, the modification of tin-based Babbitt alloy material optimization has become a research hotspot. Researchers at home and abroad are committed to conducting more deep, comprehensive, and systematic research on the development of improved bar alloy materials and preparation technology, in order to meet the increases in industrial demand.

Many scholars have improved the alloy’s performance through composition optimization and preparation process optimization. The preparation process has been updated, and many new methods have been derived. Many scholars have used arc spraying [12], laser cladding [13], the melting of extremely inert gas protection welding (MIG) [14,15], the TIG arc brazing method [16,17], and the high-pressure cold-spraying method [18] in processes for the preparation of tin-based bar alloys. Through organization and performance analysis, the bar alloy and steel matrix layer microstructure are made fine and uniform, with no obvious segregation phenomena, and the strength, combined reliability, hardness, and tribology properties are also improved. In addition to controlling the process, many researchers mainly use the alloying method to improve the performance and wear resistance of the alloy. For example, Pan Junyi [19] and others prepared composite powder by evenly mixing tin-based pasteurized alloy powder, metal Co powder, and WC powder in the mixer, and prepared the tin-based pasteurized alloy layer by the plasma spraying process. The friction and wear test results show that this method significantly improved the hardness and wear resistance. Some scholars [20,21,22] studied the influence of alloying on pasteurized alloys and found that the nature of the tin-based alloys with trace cadmium, nickel, Cd, Cu, and Pb changed, and the mechanical properties of the pasteurized alloys were improved through fine crystal strengthening and solid solution reinforcement.

Alcover Junior [23], through the pin-on-disk wear testing of a lower wear of coatings obtained by thermal spraying processes, compared the conventional casting processes in dry and oil-lubricated conditions. The results obtained indicate that in the oil-lubricated condition, the C.O.F. stabilized with values much lower than those obtained under dry conditions, between 0.05 and 0.07. In the oil-lubricated condition, the ASP and FS thermally sprayed coatings showed similar behavior, with a lower C.O.F. compared to the conventionally casted sample.

Jingyu Zhao [24] studied the effect of Zn additions on the microstructure and mechanical properties of Sn-Babbitt alloys fabricated by arc deposition. The experimental results revealed that SnSb phases and Cu₆Sn₅ phases became more uniformly dispersed in the Sn-matrix as Zn composition increased, which resulted in the improvement of the hardness and tensile strength. The specimen with the addition of 2 wt.% Zn exhibited the highest hardness increase of 16.3%, from 34.3 HB to 39.9 HB. In addition, this specimen had the highest performance, with an enhancement of 22.9% in tensile strength over the Babbitt alloys.

Fatemeh Ghasemi [25] studied the microstructures and the hardening and tribological behaviors of tin matrix composites reinforced with SiC and Zn particles. He compared the microstructure, hardness, and the pin-on-disk wear resistance of pure tin and the fabricated composites of a Sn–7.5 Sb–3.5 Cu Babbitt alloy. He found that the hardening effect of Zn was much higher than that of SiC. No hardening effect was observed following the addition of SiC particles within the tin matrix. The addition of SiC particles into pure tin or the presence of reinforcing Cu₆Sn₅ precipitates within the cast Babbitt alloy increased the resistance to the surface fatigue damage. However, subsurface crack propagation through the delamination mechanism, together with the intensification of the abrasive wear, increased the wear rates of the Sn/SiC and Babbitt pins worn at high normal loads.

This paper is mainly based on the gearbox bearings of large ships; the research objects were based on the gearbox bearings in order to meet the high bearing capacity and the required needs of high wear resistance. Using a single metal cannot meet the comprehensive performance requirements. The gearbox bearings are of Babbitt and 20 steel bimetallic. At present, the binding strength of the tin-based bearing alloy and steel substrate cannot meet the performance requirements. After flaw detection, it is found that there are many alloy surface defects, so the alloy layer easily falls off during use and its lifespan becomes shorter. New technologies will need to be improved urgently to improve the binding performance and surface quality of the Babbitt and 20 steel members. Furthermore, the service performance of the bimetallic parts should be enhanced, on the basis of the addition of different contents of Zn, to improve the organization and performance of the friction wear material.

Furthermore, this paper studies the change in friction coefficient with time under a low-oil friction state. Through the 3D images of the wear, we can observe the wear depth and the wear width and analyze the influence of the elements on the friction wear performance of the alloy.

2. Experimental Procedure

2.1. Preparation of As-Casted ZSnSb11Cu6 Alloy

The material used in this experiment are the Sn-Babbitt alloys ZSnSb11Cu6. The effect of Zn on the microstructure and mechanical properties of the ZSnSb11Cu6 alloy was studied by adding different contents of trace element Zn with mass fraction 0 wt.%, 0.05 wt.%, 0.1 wt.%, 0.15 wt.%, and 0.2 wt.% on the basis of this conventional alloy. The chemical composition of the tin-based Babbitt alloy is shown in

Table 1. The material ratio of the experimental scheme (wt.%).

No.	Sn	Sb	Cu	Pb	Zn
1	Bal.	11	6	0.35	0
2	Bal.	11	6	0.35	0.05
3	Bal.	11	6	0.35	0.1
4	Bal.	11	6	0.35	0.15
5	Bal.	11	6	0.35	0.2

.

2.2. Experimental

The experiment was carried out by metal casting. Before casting, it is necessary to pretreat the prepared raw materials, the metal molds (as shown in Figure 1a), and the related tools according to the requirements to prevent the occurrence of casting defects and the damage to the mold by excessive temperature differences.

The roadmap is shown in Figure 2. The SXW-18-13 smelting furnace parameters are set before the smelting alloy so that the temperature of the smelting furnace slowly rises to 700 °C; the Cu powder, Sn, and Sb are fitted into the preheated crucible, melted in the melting furnace, and carbon powder is added to prevent oxidation. After all the pre-added metals are melted, Zn and Pb are added to the alloy solution according to the melting point of the alloy elements; trace Zn is added at 0 wt.%, 0.05 wt.%, 0.1 wt.%, 0.15 wt.%, 0.2 wt.%. After the last metal is completely melted, at the intermediate interval of 5 min, after all the metal is melted with a preheated 300 °C graphite rod, the surface impurities are removed, the temperature is set to 430 °C into the preheated metal mold, and, after air cooling for 20 min, the Y-type sample is obtained. According to the national standard GB/T 1176-2013 [26], the cast Y-type sample is shown in Figure 1b.

2.3. Test Method

The cast alloy was sampled by a wire cutter; the sampling diagram is shown in Figure 3. The tensile test samples are cut, as in the red part of Figure 3, machined according to GB/T1348-2009 [27], and processed into tensile standard samples. The friction wear sample is cut as in the yellow part of the image; the standard sample size is a 12.32 mm × 12.32 mm × 19.05 mm cuboid, and the upper surface of the yellow part is used as the friction wear surface. The lower surface of the blue part of the gate is polished for metallographic analysis and the side is used for hardness determination.

The mechanical tensile test was carried out on a WDW-20/30 universal electronic mechanical testing machine; the loading tensile rate was 2.4 mm/min and the gauge length of the tensile specimen was 40 mm. The Brinell hardness was measured on a HB-300B Brinell hardness tester; each sample was measured 5 times and the average value was taken at the end.

After the friction test, the microstructure and morphology of the worn surface were observed by a metallographic microscope (AXIO Scope.A1, ZEISS, Germany) and the etching solution was 4% nitric acid alcohol solution. The morphology of the Babbitt alloy was observed on a thermal field emission scanning electron microscope (SU5000, Tecnai G2 20, USA) and was combined with its built-in energy spectrometer function to perform energy spectrum analysis on the composition of the alloy structure and the composition of the precipitated phase. And, using the grain evaluation software (Image J 1.53) that comes with the device, the size and morphology of the SnSb particles in the Babbitt alloy structure were evaluated. The sizes and morphological distribution of the SnSb particles were analyzed in the same field of view to determine the size and distribution of the SnSb particles. The Setaram Labsys synchronous thermal analyzer was used to detect and analyze the DSC experimental data. The temperature measurement range was 20~600 °C, the heating rate was 5 °C/min, and the cooling rate was 10 °C/min.

The tribological properties of the Babbitt alloy are also particularly important; in order to study the effect of Zn addition on the tribological properties of the alloy, the friction test was carried out on the MRH-3 high-speed “ring-on-block” wear tester with a ZSnSb11Cu6 sample as the lower sample and 45 steel rings as the upper sample in Figure 4. The research is mainly based on the existing ship project, and the grinding material used in this project is 45 steel, so, in order to ensure the authenticity of the experimental data, 45 steel is selected as the grinding surface. The standard used for tribological testing is GB/T 12444-2006 [28].

The dual ring part is placed at the front of the spindle to rotate with the spindle and the friction block is fixed above the ring piece after applying the load. The test temperature of the friction and wear test was room temperature, and the lubrication state was oil lubrication. Tests were conducted with the 250 N load and the 500 r/min rotational speed; the time of test was 30 min, and the lubricating oil used was 15 W-40#. And the experiment was carried out in the oil tank. The changes in friction coefficient, wear rate, and wear morphology were observed by changing the Zn content. The same experiment was repeated three times and averaged, where the value of the coefficient of friction was an average of the 30 min values. Before the test, each sample was cleaned by rough grinding, fine grinding, and polishing to ensure the surface roughness Ra ≤ 0.8 μm, and this is defined as its quality. After wear, each sample was washed with acetone and dried, and then their mass was weighed with a 1/10,000 electronic analytical balance. Their wear rates were calculated through the weight of the wear loss (the average of multiple weighings). The morphology after wear was observed by a friction topography observer (HT-1000).

3. Results and Discussion

3.1. Observation and Analysis of Alloy Microstructure

3.1.1. Microstructure

The microstructure diagram of the alloy is shown in Figure 5. It can be seen that no new phase is formed in the microstructure. The black matrix in Figure 6 is the β solid solution of Sb dissolved in the Sn soft phase; the large, massive polygonal crystal is the intermetallic compound SnSb phase, which is the hard phase; the needle or star hard phase is in the copper tin compound Cu₆Sn₅ phase.

Through observing the metallographic organization under different Zn contents, we can see that the phase composition of the alloy did not change after the addition of Zn. However, it is obvious that with the addition of Zn, the SnSb hard phase particle size first increased and then decreased. Compared to Figure 5a,b, the number of SnSb phases decreased, mainly due to the addition of the Zn which changed the Cu₆Sn₅ phase from a small, short dendrite to a long needle, which hinders the growth of the SnSb crystal nucleus and turn the large, massive SnSb phase into smaller SnSb phase particles. Comparing with Figure 5a–c, the phase particle size is larger, and even the number of particles decreases. Comparing with Figure 6a,e, it is obvious that the hard phase SnSb size decreases, the number increases, and the microstructure distribution is more uniform. When the amount of Zn is 0.15 wt.%, a large needle appears, and the Cu₆Sn₅ phase changes its morphology to become a dendrite. When the addition of Zn increased to 0.2 wt.%, the microstructure was small and uniform, and the SnSb particle phase was minimal and evenly distributed in the microstructure.

This shows that the Zn can effectively refine the grains in a certain range. The finer the grain, the greater the force that can be dispersed into the adjacent grains by an external force, the plastic deformation is more uniform, and the stress concentration is reduced. And increasing the grain boundaries can prevent dislocation movement and prevent cracks from spreading.

3.1.2. SnSb Particles Size Evaluation

The size of the SnSb particle phase was analyzed using the image analysis software (Image J 1.53). First, the image file of the 100 × microstructure to be analyzed is imported in Image J 1.53; then, the scale bar is set, the area is selected using the rectangular selection tool box, and the particles are screened by adjusting the color range. To ensure the accuracy of the analysis, error and noise can be excluded by setting the upper and lower limits of the particles. After all the particle statistics are completed, the data are exported and the corresponding tables and charts are generated, which can be used to draw the particle size distribution map of the SnSb particles. The rating results are shown in

Table 2. Summary of SnSb particle evaluation.

Serial Number	Zn Content	The Average Particle of SnSb	A Total Number of Grains of SnSb	0~450 μm2 SnSb Particle Number	Particle Proportion of SnSb in 0~450 μm2
1	0.00 wt.%	48.19 μm2	389	384	98.7%
2	0.05 wt.%	18.39 μm2	1012	1009	99.70%
3	0.1 wt.%	62.09 μm2	209	199	95.22%
4	0.15 wt.%	14.50 μm2	472	470	99.58%
5	0.2 wt.%	13.10 μm2	1768	1768	100%

below.

From the data in the Table 2, it is demonstrated that the SnSb phase size decreases with increasing Zn content. The number of large size particles decreased and the number of small size increased. This indicates that the increase of Zn refines the grains.

Based on the data results in Table 2, the size and distribution of the SnSb phase are drawn, as shown in Figure 6.

As can be seen from Figure 6, since the number of added SnSb particles increases, small particles increase, and large particles decrease. The number of SnSb particles in a range of less than or equal to 450 μm² is the highest, with the content of the Zn 0.2 wt.% in the alloy about 4.6 times the number of the particles in the original sample, and the smallest average particle size is 72.8% lower than the original sample; the number of SnSb particles with Zn 0.1 wt.% decreases and the number of large particles increases; when the Zn content is 0.15%, the number of SnSb particles in the range of 0 to 450 μm² is about 1.2 times that of the original sample, and the average particle size is small.

3.1.3. DTA Analysis

A DTA analysis was made of the alloy material to determine the phase structure. Figure 7 shows the change plot of the alloy difference heat data. It is obvious that there are three exothermic peaks in two curves. Combining the formation point of the phase in the Cu-Sn and Sn-Sb phase diagram and the temperature transition point in the DTA, it can be found that the solid-phase β-Sn phase is first precipitated during solidification. The addition of Zn decreases the phase transition point, hindering the formation of the β-Sn phase. During solidification, when the temperature drops to the range of 400 °C to 350 °C, the β -Sn phase begins to precipitate, and the amount of precipitation is small, so the peak is low. The addition of Zn reduced the β-Sn phase precipitation point temperature and decreased the temperature by 6 °C. The precipitation of the β-Sn phase is hindered by the decreasing temperature. The reduced number of β-Sn phases resulted in a decrease in the exothermic peak, which was reduced by 39.4%.

When the temperature decreases to the range of 300 °C~200 °C, another peak occurs, and the exothermic peak is the precipitation point of the SnSb phase according to the Sn-Sb phase diagram analysis. The main microstructure of the alloy was the β-Sn phase + SnSb phase. The precipitation temperature of the SnSb phase is 259 °C without Zn. After the addition of 0.2% Zn, the precipitation point temperature of 252 °C decreased by 7 °C, and the heat flow decreased from −0.0829 to −1.1424 and decreased by 1.0595 μv.

When the temperature continues to 229 °C, Cu₆Sn₅ is formed, the peak heat flow is large, and the precipitation temperature point of the two curves is the same, but due to the inconsistent precipitation morphology and quantity, the exothermic peak is inconsistent. Zn added by an amount of 0.2 wt.% increased heat flow by 8% compared to no addition.

In conclusion, it can be seen from the DTA data that the addition of Zn can hinder the precipitation quantity of β-Sn phase, change the precipitation morphology of SnSb phase, and change the precipitation temperature and precipitation morphology of Cu₆Sn₅.

3.2. Mechanical Performance Analysis

3.2.1. Hardness Analysis

The results of the Brinell hardness test are shown in Figure 8; when the Zn content is 0.1%, the hardness value increases by about 7.3% compared to the sample without the Zn; when the Zn content is 0.15%, the hardness value is 25.36 HB, which is about 5.4% compared to the sample without the Zn.

The hardness performance of the pasteurized alloy mainly depends on the microstructure of the alloy, and the size, quantity, and distribution in the structure of the alloy. Based on the metallographic photos and the summary data of the phase particles, the hard phase at the Zn content of 0.1 wt.% is thick with the largest hardness value, which fits the experimental data. According to the hardness curves of the different Zn contents in Figure 8, with the increase in the Zn content, the hardness value shows a trend of increasing first and then decreasing, so the content of the Zn should be strictly controlled for different needs.

3.2.2. Tensile Test Results and Analysis

The results of the parameters measured in the tensile test are shown in Figure 9.

It can be clearly seen from the comparison data that after the addition of the Zn, the tensile strength of the alloy is improved, and the tensile strength value is not much different. When the Zn content is 0.05 wt.%, the tensile strength value is 110.1 Mpa, which increased by 10.5% when compared to the original sample. When the Zn content is 0.2 wt.%, the tensile strength value reached the peak and the intensity value was 120.8 Mpa, which increased by 21.29% compared to the original sample. The tensile test data show that the addition of Zn can effectively improve the tensile strength of the alloy, which has a lot to do with the microorganization of the alloy. Combined with the metallographic organization and the particle distribution, the alloy reaches the maximum quantity and the minimum size with the Zn content of 0.2 wt.%. The fine particles hinder the dislocation movement, thus improving the strength.

Figure 9a shows the change plot of the tensile strength and yield strength of the tin-based Babbitt alloy after the addition of different amounts of Zn; Figure 9c shows the stress and strain curve plot. It can be seen that the yield strength of the four samples supplemented with Zn was increased compared to the original sample. With a Zn content of 0.05 wt.%, the yield strength of the alloy is 90 MPa, 57% more than the original sample. With a Zn content of 0.1 wt.%, the yield strength of the alloy reaches 96.2 MPa, 68% more than the original sample; with 0.15 wt.%, the yield strength increases to 74.8 MPa, which is 31% higher compared to the original sample. The yield strength data map line increased first and then decreased, and the extension first decreased and then increased.

Figure 9b shows the plot of the tin-based Babbitt alloy elongation after the addition of different amounts of Zn. It can be seen that the extension of the four samples with Zn first decreases and then increases. The elongation of the alloy with a Zn content of 0.05 wt.% was 2.125%, which is 6.3% lower compared to the original sample. With the Zn of 0.1 wt.%, the elongation of the alloy was 1.925%, which was 34.5% lower compared to the original sample. Increasing the Zn content to 0.15 wt.% extended the alloy by 3.55%, 56.4% higher than the original sample. When increasing to 0.2 wt.%, the alloy extension rate was 4.25%, 87.2% higher than the original sample. This indicates that the addition of Zn will increase the plasticity of the alloy.

Based on the above studies, the tensile strength and the yield strength of Zn both increase, but the extension decreases first and then increases. With increasing Zn content, the tensile strength increases from 100.4 MPa to 120.8 MPa, and the yield strength increases from 42.4 MPa to a maximum of 96.2 MPa (Zn content is 0.1 wt.%), where the elongation is minimal. As the Zn content continues to increase, the elongation rate begins to increase again, from 1.725% to 4.25%, and the yield strength begins to decrease. It can be seen that the alloy tensile strength and yield strength can be improved, and the elongation rate can also increase.

3.3. Tribological Performances

3.3.1. Friction Coefficient Analysis

Figure 10 shows the time curve of the friction coefficient for alloys with different Zn contents and the time curve of the average friction coefficient.

It can be clearly seen in Figure 10 that the friction coefficient of the four samples shows a decreasing trend with increasing time. Before 10 min, it belongs to the run-in period and fluctuates greatly. After 10 min, it reaches a steady state and fluctuates around a fixed value. At the Zn content of 0.15 wt.% and 0.2 wt.%, the friction coefficient fluctuates around 0.005 in the range of 10 to 15 min, and after 15 min, the friction coefficient stabilizes and fluctuates below 0.005, with a Zn content of 0.15%.

Figure 10b shows the change in the average friction coefficient before and after the addition of different Zn contents under the test force of 250 N, the speed of 500 r/min, and the test time of 30 min. The average friction coefficient is larger without adding Zn. After adding the Zn, the average friction coefficient increases and then decreases with the Zn content.

The hard points of the ZChSnSbll-6 alloy in the friction process begin to wear as the friction coefficient, in the friction process, heats, caused by high temperature; this deforms the soft matrix of the ZChSnSbll-6 alloy and increases the friction area, leading to the increase of the friction coefficient, which affects the formation of the ZChSnSbll-6 alloy in the process of friction wear. The variation in the friction coefficient is greatly associated with its microorganization. An alloy with a Zn content of 0.05 wt.% has more small and large hard particles, and the Zn content of 0.15 wt.% has more small particles and small, large particles, so the friction coefficient between the two schemes is not different. The hard phase particles in the alloy microstructure with a Zn content of 0.15 wt.% are thicker than the original sample, so the friction coefficient is larger than that of the original sample. The alloy with a Zn content of 0.2 wt.% has the smallest size and no large particles, and the grains are fine and uniform, so its friction coefficient is the smallest.

3.3.2. Outline of 3D Wear Marks Analysis

Through the three-dimensional grinding marks analysis of the alloy, the three parameters of grinding depth, grinding width, and grinding area were compared. Figure 11 shows the 3D grinding pattern of the tin-based Babbitt alloy with different Zn contents after the friction wear test. The upper surface in the figure is the opposite grinding surface, and the blue area is the wear site. It can be seen that the the wear area gradually decreases with the increase in the Zn content, indicating that the addition of Zn can increase the wear resistance of the alloy. This is mainly because the addition of the Zn-refined organization makes the hard SnSb phase size smaller and more evenly distributed in the matrix organization; so, the soft matrix uniform becomes inlaid with fine hard SnSb particles, its soft base body becomes concave, its hard point convex, and the middle area is formed, allowing the storage of lubricating oil. This makes the alloy in the friction process have a good lubrication effect; at the same time, the hard part has a strong carrying capacity, a good supporting effect, and when both are together, the effect of wear can be realized. The specific wear depth and wear width can be analyzed in Figure 12.

The grinding depth and grinding width analysis from the x-axis and y-axis directions are shown in Figure 12. The grinding data showed that the different Zn content samples differed in grinding morphology and depth after the grinding test. It can be found that the grinding depth gradually becomes shallow as the Zn content increases. The wear depth is 25 microns when Zn is not added; 20 microns when Zn consists of 0.05 wt.% and 0.1 wt.%. There were 15 microns when Zn at 0.15 wt.% was added, and 10 microns when the Zn content was increased to 0.2 wt.%. After observing the change pattern of wear width and wear area, it is found that the change trend of the wear width and wear area is consistent with the Zn content; that is, Zn:0 wt.% > Zn:0.15 wt.% > Zn:0.1 wt.% > Zn:0.15 wt.% > Zn:0.2 wt.%. The comprehensive data show that the addition of Zn can increase the wear resistance of tin-based Babbitt alloys. Without Zn, the largest wear trace is produced after the friction experiment under the same conditions, and the smallest wear trace is that of Zn 0.2 wt.%.

From the y-axis direction, the wear moves up and down. This is because the ordinary casting of the alloy microstructure’s hard phase has a thick diameter; in the process of friction, the wear will fall off and cause the friction surface to have a deep furrow or the metal compounds to be broken into fine particles, which are then stuck on the friction surface of the micro-convex body, causing fluctuations. According to the sectional profile shown in the y-axis, the fluctuations without Zn are stable but have the deepest depth, and the grinding outline is shallow when the content of Zn is 0.2 wt.%.

Overall, when the content of the Zn is 0.2 wt.%, the 3D wear depth, width, and wear amount are all small, which shows a good tribological performance.

3.3.3. Friction Surface Analysis

The wear track morphology of the Sn-Babbitt alloys with 0 wt.% and 0.2 wt.% Zn content are shown in Figure 13. It can be seen that the sample with 0 wt.% Zn content is mainly subjected to abrasive wear, and a clear debris extrusion layer is observed. The wear debris consists mainly of the oxides of Cu and Sn, as shown in Figure 13c. The sample with 0.2 wt.% Zn content is also mainly subjected to abrasive wear, as shown in Figure 13b. However, the addition of Zn significantly decreased the depth of groove as well as the amount of wear debris. This is mainly due to the improvement of the material properties caused by the addition of Zn. In addition, since the thermal conductivity of the debris is lower than the bulk matrix, the debris extrusion layer will experience a higher temperature. Therefore, the alloy with 0.2 wt.% Zn content also shows a higher oxidative wear resistance.

4. Conclusions

In this study, the microscopic structure, performance, and tribological properties of a ZSnSb11Cu6 alloy with different trace contents of Zn were investigated. And a series of significant results were obtained.

• The addition of Zn makes the size and number of the SnSb phase in the microstructure generally increase first and then decrease. Therefore, the size and distribution of the alloy microstructure are better when the Zn content is 0.2%.
• After the addition of Zn, the overall tensile strength tends to increase, and the yield strength shows a trend of first increasing and then decreasing. When the Zn content is 0.1 wt.%, the yield strength reaches the maximum value, and then begins to decrease with the Zn content continuing to increase. Conversely, the elongation rate showed a tendency to first decrease and then increase, reaching a minimum Zn content of 0.1 wt.% and subsequently increasing as the Zn content continues to increase. When the Zn content is 0.1 wt.%, the hardness value of the alloy reaches the maximum value 25.82 HB, which increases by 7.3% compared to the sample without Zn. The hardness of Zn 0.15 wt.% is close to that of Zn 0.1 wt.%. Compared to the sample without Zn, the tensile strength and elongation of the alloy were maximized at a Zn content of 0.15 wt.%. Compared to the sample without Zn, the tensile strength was increased by 21.29%, and the elongation rate was increased by 46%. The changes in performance are mainly related to the organization. The increase in hardness is mainly caused by the hard Cu₆Sn₅ phase changing from independent short needles to slender branches coexisting with the matrix.
• The average friction coefficient of the alloy fluctuates in the range of 0.007 to 0.013. With the increase of Zn content, the friction coefficient tends to first decrease and then increase. When the Zn content is 0.1%, the friction coefficient is the largest, and then begins to decline with the further increase in Zn content. When the Zn content is 0.2%, the friction coefficient is the smallest, and the mean value is 0.007.
• With the increasing amount of Zn, the depth of the marks gradually become shallow. The addition of Zn can increase the wear resistance of the tin-based Babbitt alloy, with the largest friction trace after the same conditions without Zn and the addition of Zn to 0.2 wt.% as the minimum grinding mark.