Study on Friction and Wear Properties of New Self-Lubricating Bearing Materials

Abstract

In this paper, friction and wear tests were carried out: 45# steel (carbon steel) was rubbed with aluminum bronze, aluminum bronze-based inlaid solid self-lubricating bearing (ISSLB) material, tin bronze, and tin bronze-based ISSLB material under different loads. The friction and wear properties (friction coefficient, wear amount and friction temperature) of the above four materials were studied. The friction and wear properties of the new self-lubricating bearing material and the traditional copper alloy under the same load were compared. The friction mechanism of each material in the friction process was discussed. The effect and mechanism of C-MoS₂ composite solid lubricant on friction and wear properties were analyzed. Under the experimental loads of 100 to 500 N, the average friction coefficients of aluminum bronze-based ISSLB material were maintained in the range of 0.18~0.14, while the average friction coefficients of tin bronze-based ISSLB material were maintained between 0.26~0.20, which is much lower than those of copper alloy. The wear amounts of tin bronze-based ISSLB material were always in the range of 14.7~34.4 mg, which were much less than those of aluminum bronze-based ISSLB materials and copper alloy. The results show that the copper-based ISSLB materials possess excellent wear resistance, and the friction and wear properties of tin bronze-based ISSLB material are better than that of aluminum bronze-based ISSLB material. Therefore, if the copper-based ISSLB materials are applied to self-lubricating bearings, the friction and wear resistance of bearings will be greatly improved and the service life of bearings prolonged.

1. Introduction

The self-lubricating bearing completely breaks through the limitation of relying on grease lubrication. The new inlaid solid self-lubricating bearing (ISSLB) is composed of a metal substrate and solid lubricant that is embedded in it. In service, the metal substrate bears most of the load, and the solid lubricant in holes or grooves is transferred or reverse transferred to the surface, forming a solid transfer film with good lubrication and uniform coverage on the surface, greatly reducing wear. Copper alloy bearings are a very important sliding bearing material, and they are widely used because of their high hardness and strength, good wear resistance, and many other excellent properties [1]. The working environment of bearings is very tough. Their friction and wear process is a complex coupling process of mechanical action, thermodynamics, and other factors. The friction coefficient, friction temperature, and wear amount are important parameters for studying friction behavior [2].

At the beginning of the last century, inlaid self-lubricating composites began trial production. In recent years, Wei et al. studied the tribological properties of composite self-lubricating materials such as lead powder, graphite, and glass fiber in polytetrafluoroethylene-based materials, and compared the tribological properties of copper-based samples by adjusting different ratios of self-lubricating materials [3]. Shen and others prepared a new type of Cu-Pb self-lubricating material by liquid metal infiltration [4]. Through dry friction and wear experiments, it was concluded that the friction coefficient decreased further with increased load and sliding. Chen et al. prepared nano graphite-reinforced copper-based friction materials and nano zinc oxide [5]. The effect of nano zinc oxide on the friction properties of copper-based materials was studied by inertial wear testing. The results showed that the friction coefficients of friction materials added to nano zinc oxide and nano graphite were high and stable. Wang and others further studied the effects of two different graphites (S1 and S2) and coverages (32% and 43%) on the wear resistance of copper alloy Cu15Ni8Sn base [6]. The results showed that the friction coefficient and wear amount changed differently with increasing coverages of S1 and S2 graphite. In addition to graphite, other researchers have also carried out relevant research on solid lubricating materials for polymer- and molybdenum-disulfide-inlaid sliding bearings [7]. Ma et al. used the discrete element software to establish a numerical model for simulating the sliding friction process of copper matrix composite and 45# steel friction pair, and to explore the influence of graphite content on the compressive strength and tribological behavior of copper matrix bearing materials [8]. The results showed that the compressive strength of the material continues to decrease and the wear amount first decreases and then increases with increasing graphite content. The wear amount of copper matrix composites with 10% graphite volume fraction is the lowest. Jabinth and Selvakumar studied the friction properties of pure copper before and after the addition of different proportions of vanadium and graphene reinforcement, and concluded that the wear resistance was further enhanced by increasingly mixed compositions [9]. This provides a reliable means to improve the friction resistance of copper matrix metal matrix composites. Koranteng et al. prepared a reinforced copper-based friction material through powder metallurgy, and studied the effects of sliding speed and temperature on friction [10]. To date, further research has been performed on the composition and composition ratio of ISSLB solid lubricant [11,12].

Copper-alloy-inlaid graphite technology has both the excellent mechanical properties of copper alloy and the low friction and self-lubricating properties of graphite. The novelty of the new self-lubricating bearings is that it is not necessary to add lubricant either before or during use. To ensure that the entire friction surface of the sliding bearing is covered with lubricant, several circular holes of the same size were arranged in an orderly fashion on the metal matrix. The solid inlay material is especially bonded and subjected to aging treatment in order to integrate the matrix with the solid lubricant. To ensure that the lubricant is able to cover the whole moving direction, the inlay holes are overlapped tangentially and arranged alternately in the moving direction. The new self-lubricating bearings break through the limitations of lubricating oil (grease), and are especially suitable for narrow working environments with heavy loads and conditions limit filling with lubricating oil. At the same time, they are generally applicable in all rotary mechanisms, having a relatively simple structure and less pollution. At present, this technology is widely used for bearings under special working conditions such as in nuclear power, aviation, and aerospace. To achieve optimal mechanical and tribological properties, factors such as graphite content, solid lubricant ratio, and copper alloy type should be considered, which are also the main research directions for scholars at home and abroad. At present, most research is focused on using powder metallurgy (PM) techniques for preparing complete solid self-lubricating materials [13]. The disadvantage is that high strength in the friction process cannot be guaranteed. By means of filling solid lubricants into the holes of the matrix, not only can the strength of the materials be ensured, but the lubrication requirements can also be met. This paper starts with a study of copper alloy materials with the aim of improving the friction and wear performance of self-lubricating bearings. The effects of friction time and normal load on the friction coefficient, friction temperature, and wear amount are studied. Based on the parameters and wear morphology of wear experiment, the wear properties of aluminum bronze, tin bronze, and ISSLB materials based on these two materials as bearing materials in friction operation are compared.

2. Experimental Details

The experimental materials are aluminum bronze 9-4, tin bronze 6-6-3, aluminum bronze-based ISSLB material, and tin bronze-based ISSLB material. The opposite grinding part is 45# steel [Fe-(0.42~0.50)C-(0.50~0.80)Mn-(0.17~0.37)Si-(≤0.25)Ni-(≤0.25)Cr, wt.%], which is equal to S45C in Japan, 1045 in the US, and C45 in Germany. 45# steel is one of the most common high-quality carbon steels, possessing outstanding wear performance, and is often used as the opposite grinding part in friction and wear experiments on metal materials. Additionally, 45# steel has excellent comprehensive mechanical properties such as strength and plasticity. The hardness of 45# steel (240 ± 10 HV) is higher than those of tin bronze 6-6-3 (177 ± 8 HV) and aluminum bronze 9-4 (140 ± 7 HV). Therefore, 45# steel is suitable as the opposite grinding part for studying the friction performance of copper alloys. The copper-based ISSLB used in this test was provided by Beijing Jin’an Co. The solid lubricant raw materials were composed of graphite and MoS₂, the mass ratio of which was 1:1. After batching, they were mixed and dispersed evenly in a high-speed mechanical mixer. Six open holes with a diameter of 8 mm and a depth of 4 mm were made on the copper alloy surface according to a certain arrangement, and the configured solid lubricant was compressed into the holes on the copper base surface at a pressure of 30 MPa. The weight of the lubricant used for each sample was 1.82 g. It was sintered at 370 °C for 2 h. Finally, the experimental materials were ground and machined in a manner suitable for the MM200 wear tester. The outer and inner diameters of the tested sample were 40 mm and 16 mm, respectively.

In this paper, the ring-block wear test was carried out on a MM200 wear tester. The samples for testing were polished with 1200-mesh silicon carbide water sandpaper, soaked in acetone solution for 30 min. Then, the samples were washed with absolute ethanol solution, blow dried, and transferred into a drying oven, where they were held at 60 °C for 2 h. The samples were weighed using an electronic balance, the accuracy of which was 0.1 mg.

Under room temperature conditions and dry grinding, 100 N, 200 N, 300 N, 400 N and 500 N were selected as normal loads, respectively. The rotating speed was 200 r/min. The sliding friction was conducted for 120 min for each pair of test pieces. The temperature was measured using an industrial infrared thermometer and the friction torque value in the scale was read every 2000 revolutions (10 min). After each group of tests, the test piece was weighed again, the wear amount was calculated, and the surface wear morphology of the test piece was observed. Figure 1 shows the samples after friction and wear test under different loads. The materials from left to right are aluminum bronze, aluminum bronze ISSLB, tin bronze, and tin bronze ISSLB.

3. Experimental Results and Analysis

According to the experimental results, the friction and wear resistance of aluminum bronze, aluminum bronze-based ISSLB material, tin bronze and tin bronze-based ISSLB material were compared with respect to three aspects: friction coefficient, friction temperature, and wear amount.

3.1. Friction Coefficient

During the experiment, the friction coefficient was calculated based on the friction torque. The calculation formula is as follows [14]:

f = μN,

(1)

T = fR,

(2)

μ = T/NR,

(3)

where μ is the friction coefficient; N is the normal load (N); R is the outer diameter of the tested sample (m); and T is the friction torque (N·m).

The calculated friction coefficients were sorted, and the experimental results are shown in Figure 2.

It is obvious that the friction coefficient of the aluminum bronze-based ISSLB material is lower than that of aluminum bronze under the same load in Figure 2a,b. At the initial stage of the experiment, the friction coefficient of aluminum bronze was the highest. With the progression of the running-in stage (the first 2000 revolutions), the friction coefficient gradually decreased, and then fluctuated around a lower fixed value. This is because the friction coefficient of the bearing in the running-in stage can be many times higher than that of the bearing after already having been run in [15]. The contact surface of the friction pair is not smooth enough, resulting in a large friction force in the initial stage. With the progress of friction, the contact surface becomes more and more smooth, and the friction coefficient gradually decreased. After that, the friction coefficient fluctuated up and down, but changed little due to the continuous generation and elimination of wear particles. The friction coefficient of the aluminum bronze-based ISSLB material under various loads was more stable than that of aluminum bronze. This is because a stable solid lubrication film was produced in the friction process, and the lubrication friction entered a steady state.

Figure 2c,d show the general trends of the friction coefficients of tin bronze and tin bronze-based ISSLB materials with the number of revolutions under each load. Tin bronze was in the running-in stage before 2000 revolutions. Because the hardness of tin bronze is low, it was sheared to produce abrasive particles during friction, which caused the contact surface to become rough. Therefore, abrasive wear was enhanced and the friction coefficient increased gradually. After successful running in, with the generation and discharge of abrasive particles reaching equilibrium, a relatively stable friction coefficient was obtained. The friction coefficient of tin bronze-based ISSLB material is low and stable under 100 N and 200 N. At the beginning, the solid lubricant was slowly squeezed out of the holes, and solid lubricant film was gradually formed under the action of load with the rotation of the sample. Thus, the friction coefficient decreased first. After the whole solid lubrication film was stably formed, direct contact between tin bronze-based ISSLB material and the opposite grinding part was completely avoided, so that the friction coefficient would not change and remained stable. Under large loads of 300 N, 400 N and 500 N, the friction heat increased, and the lubricating film softened obviously. Thus, it became easy for the lubricating film to bet stuck to the opposite wear parts, which led to part of the lubricating film being taken away by the rotating opposite wear parts during the friction process. Therefore, the lubricating film formed between the friction pairs was destroyed and the lubricating area was reduced, which resulted in a continuous increase in the friction coefficient. In the case of high speed and high load, a lot of heat was generated, plastic deformation was aggravated, and the adhesion phenomenon was generated. The friction coefficient increases when the lubricating film is peeled off [16]. Figure 2c,d show that the friction coefficient of the tin bronze-based ISSLB material was lower than that of tin bronze under each load.

In general, the friction coefficients of copper alloy and copper-based ISSLB materials decrease with increasing load. The friction coefficients of the two bearing materials Thordon ThorPlas Blue (ThorPlas) and Orkot TXMMarine (Orkot), used in the hydropower industry and studied in reference [17], also follow this law. Modern friction theory points out that the real contact area will increase with increasing normal load, which means a decrease in contact stress. This explains why the increase in normal load leads to a decrease in the friction coefficient. However, the case of tin bronze ISSLB material does not conform to the above law under loads of 300 N, 400 N and 500 N. The reason for this is that the lubricating film will be destroyed with a continuous increase in load, the friction conditions will become harsh, and the friction coefficient will increase.

The friction coefficient of the copper alloy ISSLB material is lower than that of the copper alloy, showing that the interlayer adhesion between graphite and MoS₂ is very weak. MoS₂ is a very soft solid and has good stability. The metal surface cannot be easily damaged by it. MoS₂ crystal is a layered structure formed by multiple stacked layers. Each layer consists of two sulfur atom planes and a molybdenum atom plane embedded between them. The strength of the S-Mo-S valence bond in one layer is very high, while the bonding strength between two adjacent layers caused by van der Waals force is relatively low [18]. Therefore, it is difficult to destroy the bonding between S-Mo-S layers; however, it is easy to slide between layers. The lubrication mechanism of graphite is very similar to MoS₂, and both of them are easily sheared and interlaminar between layers [19,20]. The direct contact between copper alloy and wear parts was isolated by the formed solid lubrication film, resulting in a reduced friction coefficient and good lubrication effect. The solid lubricant was squeezed and sheared by the interaction between friction pairs. The lubricating materials were evenly distributed on the surface of the substrate, and finally a solid lubricating film was formed. Because the shear strength of the lubricating film is very low, the solid lubricating film on the substrate surface will be transferred to the surface of the opposite grinding material to form a transfer film during the friction process. Thus, friction occurred between the transfer film and the lubrication film; that is, the friction occurred inside the solid lubricant. Therefore, the anti-wear effect was achieved, the friction and wear were reduced, and the service life was extended. Considering the mechanism of solid lubrication film from a microscopic point of view, it can be considered that the effect on the shear force between grinding specimens was actually a result of the interaction between lubricant molecules. Shear occurred between the layers of lubricating film. The low molecular force between layers results in the phenomenon of an extremely low friction coefficient [21]. Friction is a complex and changing process. The value of the friction coefficient depends not only on the characteristics of the material itself, but also on the external conditions. It is the result of the comprehensive action of many aspects.

3.2. Friction Temperature

The friction surface temperatures were measured every 2000 revolutions (10 min) using an industrial infrared thermometer. The changes in the surface temperature of aluminum bronze and tin bronze during friction are shown in Figure 3a,c, respectively. The surface temperature of the two materials had the same trend with the number of revolutions under different loads during friction. The temperature rose sharply in the running-in stage of the first 2000 revolutions, and then fluctuated stably. Both materials produced serious abrasive wear due to the existence of micro-convex parts on the surface of the wear parts. At this stage, a large amount of heat was generated rapidly and could not be effectively transmitted by the friction surface and lubricated fluid. Therefore, the surface temperature of the aluminum bronze and tin bronze materials increased sharply. After that, the micro-convex part gradually became smooth, and part of the heat was taken away by conduction, convection, and wear debris in the subsequent friction process, resulting in the macro dynamic balance of the friction system. Hence, the surface temperature basically remained stable with increasing numbers of revolutions.

In Figure 3b,d, the change in the surface friction temperature of aluminum bronze-based ISSLB and tin bronze-based ISSLB materials with number of revolutions under each load can be seen, respectively. The variation trends of the surface temperature for the two materials with number of revolutions during friction were consistent with those of aluminum bronze and tin bronze. In the running-in stage, the temperature rose rapidly and then reached a stable state. The main reason for this is that when the friction pairs are in contact at the beginning, the solid lubricant cannot form a stable solid lubrication film in time, resulting in more heat being generated by the direct contact of the friction pairs. In addition, less wear debris was produced at the beginning, resulting in less heat being taken away. After the running-in stage, a solid lubricating film was gradually formed. The heat present in the lubricating film can be taken up by the environment for heat dissipation. The friction temperature remained stable with increasing number of revolutions. The reason the tin bronze-based ISSLB material did not conform to the above law under the load of 500 N is that the load was too high. The stable solid lubricating film cannot be formed under high load (500 N) in the friction process. The constantly damaged lubricating film is difficult to discharge after being mixed with wear debris, resulting in a decline in heat conduction and heat dissipation performance. Thus, the friction temperature rises, and is higher than that of tin bronze. The change trend of surface temperature of the tin bronze-based ISSLB material under 500N load was consistent with that of the friction coefficient under the same condition. Additionally, Figure 3c,d show that the surface temperature of tin bronze-based ISSLB material was lower than that of the tin bronze material during friction.

The friction temperature of these four materials increased with increasing load, and remained stable after heating. This is because the greater the load, the greater the shear force between the films, resulting in an increase in temperature.

3.3. Wear Amount

The samples before and after wear testing were weighed by an electronic balance with an accuracy of 0.1 mg. The wear amount was calculated from the difference in quality. Figure 4a clearly shows the change trend of the wear amount of the aluminum bronze and aluminum bronze-based ISSLB materials with increasing load. With the increase in load, the wear amount of both materials also increased. The wear amount of aluminum bronze-based ISSLB material was lower than that of aluminum bronze. Under the loads of 100 N, 200 N and 300 N, due to the existence of complete solid lubrication film, the wear amount of aluminum bronze-based ISSLB material was far lower than that of aluminum bronze, and remained basically stable. The specific values of the wear amount of aluminum bronze-based ISSLB material were 107.3 mg, 73.7 mg and 98.8 mg, respectively, and did not increase linearly with increasing load. Under 100~300 N, the wear amount of aluminum bronze-based ISSLB material had no specifically corresponding relationship with the load, showing excellent wear resistance. The plastic deformation of the matrix material was serious under the action of repeated extrusion and cutting force when the loads were increased to 400~500 N, while the solid lubrication film was considerably peeled off and the wear amount increased.

A comparison of the wear amount of tin bronze and tin bronze-based ISSLB materials with load is shown in Figure 4b. It is clear that the wear amount of the tin bronze-based ISSLB material was much lower than that of tin bronze. The wear amount of tin bronze under 200 N was the lowest, at 180.4 mg, and the wear amount under 300 N was the highest, at 299.6 mg. The wear amount of tin bronze-based ISSLB material under 200 N was the lowest, at 14.7 mg, and the wear amount of tin bronze-based ISSLB material under 300 N was the highest, at 34.4 mg. Under loads of 100~500 N, the maximum wear amount of the tin bronze-based ISSLB material was far lower than the minimum wear amount of tin bronze. This is due to a lubricating film composed of graphite and MoS₂ being formed on the surface of tin bronze-based ISSLB material during friction. The direct wear of the material was largely avoided.

Wear amount is an important index for evaluating the wear resistance of materials [22]. In general, copper-based ISSLB has better friction and wear properties than copper alloy.

3.4. Surface Morphology Analysis

Figure 5a–c suggest that there were wide furrows and peeling pits on the worn surface of aluminum bronze materials, and the furrows were connected with the peeling pits. The particles peeled from the matrix destroy the wear surface under load and then form furrows. This is typical abrasive wear. Because the hardness of the grinding pair is higher than that of aluminum bronze, the micro-convex parts on the surface of grinding pair become embedded in the surface of aluminum bronze, which is another reason for the obvious furrow on the surface of aluminum bronze. In Figure 5d–f, the furrows on the friction surface of aluminum bronze-based ISSLB material are fine, and there are few peeling pits, indicating that few particles fell off during the friction process. Under the load of 500 N, the stable state of the lubricating film was destroyed during the friction process. Thus, the aluminum bronze-based ISSLB material was in direct contact with the grinding parts, resulting in a rough friction surface and more serious wear. At the same time, adhesive wear appeared. By comparing the three-dimensional morphology of aluminum bronze and the aluminum bronze-based ISSLB material in Figure 6, it can be seen that the friction surface of the aluminum bronze-based ISSLB material was smoother than that of aluminum bronze, which was due to the presence of the lubricating film.

Figure 7a–c show that there were obvious adhesion traces on the friction surface of tin bronze. The adhesion occurs when the local compressive stress at some contact points on the surface exceeds the yield strength during the friction process of tin bronze. Then, the surface wear occurs when the friction pairs move relative to each other. Figure 8a–c also show that the friction surface of tin bronze was uneven. Figure 7d–f suggest that there were clear and flat marks parallel to the rotation direction on the surface of the tin bronze-based ISSLB material. During the initial stage, the lubrication effect was poor, due to there being less lubricant on the surface. During the running-in stage, the particles on the material surface caused slight abrasive wear. On the basis of macroscopic observation, it was observed that there was no adhesive wear on the surface of the tin bronze-based ISSLB material. Figure 8d–f show that the surface of the tin bronze-based ISSLB material was smooth and basically free of wear marks, and this was mainly because of the lubricating film hindering direct wear between the friction pairs.

From the observation of wear morphology, it can be concluded that the antifriction performance of the solid lubricating film is excellent. As can be seen from Figure 4 and Figure 5, the wear marks and the corresponding profile under the three-dimensional surface view on the aluminum bronze friction surface are more obvious compared to those of the aluminum bronze ISSLB material. From Figure 7 and Figure 8, it can also be concluded that the friction and wear properties of the tin bronze ISSLB material are better than those of tin bronze. By comparing the two-dimensional morphologies of the four materials, it can be concluded that the wear resistance of the bronze-based ISSLB material is better than that of the corresponding copper alloy, and the wear resistance of the tin bronze ISSLB material is better than that of aluminum bronze ISSLB material.

Determining the damage behavior of bearings is of great significance in improving their long-term reliability and efficiency [23]. Scholars generally believe that the wear forms of bearings are mainly adhesive wear and abrasive wear, accompanied by a certain degree of fatigue wear [24]. By observing and analyzing the wear morphology after dry friction testing, the wear forms of copper alloy and the copper alloy-based ISSLB materials are mainly abrasive wear and a slight amount of adhesive wear. The wear surface of copper alloy is worse than that of the copper alloy-based ISSLB material.

Yang et al. [25] studied the friction properties of 3D needled C/SiC-MoS₂ self-lubricating composites under loads of 2 N, 5 N and 10 N. This paper studies the friction properties under loads of 100~500 N, and the results indicate that the new self-lubricating bearing materials with good friction properties can be expected to be useable under heavy load conditions. Mushtaq and Wani [26] studied the tribological properties of iron–copper–tin alloy with graphite as solid lubricant at a rotation speed of 0.157~0.314 m/s. However, the friction experiment in this paper was carried out at a speed of 0.837 m/s with a higher load. Both abrasive and adhesive wear occurred in both of these studies. The wear resistance of the ISSLB material performs well even under conditions of relatively high rotation speed and heavy load.

According to the experimental data analysis presented in the above sub-sections, the friction coefficient of the aluminum bronze ISSLB material is lower than that of aluminum bronze, and the friction coefficient of the tin bronze ISSLB material is lower than that of tin bronze. A comparison of the friction temperature between the copper alloy ISSLB material and copper alloy is not obvious. The wear amounts of the copper alloy ISSLB materials were much lower than those of the copper alloys, and the wear amount of the aluminum bronze base was about 10 times that of the tin bronze base. These results show that the wear resistance of the copper alloy ISSLB material is better than that of copper alloy, and that the wear resistance of the tin bronze ISSLB material is better than that of the aluminum bronze ISSLB material.

4. Conclusions

In this study, on the basis of the wear tests of aluminum bronze, aluminum bronze-based ISSLB materials, tin bronze, and tin bronze-based ISSLB materials under different loads, their friction and wear characteristics were analyzed on the basis of the test results, and the following main conclusions were obtained:

Due to the presence of the solid lubricating film, the friction coefficient of the copper-based ISSLB material was much lower than that of the copper alloy. Under the loads of 100~500 N, the average friction coefficient of the aluminum bronze-based ISSLB material was maintained in the range of 0.18~0.14, and that of aluminum bronze was in the range of 0.29~0.20. The average friction coefficient of the tin bronze-based ISSLB material was maintained between 0.26 and 0.20, and that of tin bronze was in the range of 0.52~0.41.

The wear amount of the copper-based ISSLB material was much lower than that of the copper alloy. The wear amount of the aluminum bronze-based ISSLB material under the loads of 100~300 N was maintained at about 100 mg. Under the loads of 100~500 N, the wear amount of the tin bronze-based ISSLB material was always in the range of 14.7~34.4 mg, showing excellent wear resistance.

On the basis of a comprehensive analysis of the friction coefficient, friction temperature, wear amount, and friction surface morphology, it is not hard to find that the copper-based ISSLB material has superior wear resistance than traditional copper alloy bearing materials. The friction and wear performance of the tin bronze ISSLB material was more outstanding than that of the aluminum bronze ISSLB material.

Considering the mechanism of solid lubrication film from a microscopic point of view, it can be considered that the effect on the shear force between grinding pairs is actually the interaction between lubricant molecules. The solid lubrication film composed of graphite and MoS₂ slides easily between layers due to the weak interlayer adhesion of graphite and MoS₂. Thus, the copper alloy ISSLB material shows the lowest friction coefficient and the least wear from a macroscopic point of view.