Journal of Materials Science & Technology  2020 , 40 (0): 158-167 https://doi.org/10.1016/j.jmst.2019.09.025

Novel insight into dry sliding behavior of Cu-Pb-Sn in-situ composite with secondary phase in different morphology

B.W. Dong, S.H. Wang, Z.Z. Dong, J.C. Jie*, T.M. Wang, T.J. Li

School of Material Science and Engineering, Dalian University of Technology, Dalian 116024, China

Corresponding authors:   *Corresponding author.E-mail address: jiejc@dlut.edu.cn (J.C. Jie).*Corresponding author.E-mail address: jiejc@dlut.edu.cn (J.C. Jie).

Received: 2019-07-23

Revised:  2019-09-2

Accepted:  2019-09-3

Online:  2020-03-01

Copyright:  2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

The Cu-24Pb-xSn (wt%) (x = 0, 2, 4, 6) alloys with Pb-rich second-phase particles (SPPs) in different shapes show obviously differently mechanical and self-lubricating properties. The influence of the SPPs’ shape difference on the alloys’ mechanical and self-lubricating properties was revealed. Cu-24Pb alloy with continuously netty SPPs shows much more intensive stick-slip phenomenon during dry sliding than the other three alloys with independently rodlike SPPs. That is mainly due to insufficient lubrication resulted by the netty SPPs’ splitting matrix. With the SPPs transforming from netty to rodlike shape under the addition of Sn, the stick-slip phenomenon was notably weakened, which was proven to be related to the higher self-lubricating property of alloys with rodlike SPPs. Simultaneously, the simultaneous increase of ductility and tensile strength was observed in the Cu-24Pb-xSn alloys with increasing Sn content, which is because the netty SPPs’ splitting behavior will be weakened with them replaced by the rodlike SPPs.

Keywords: Second-phase particles ; Netty shape ; Rodlike shape ; Stick-slip ; Mechanical property ; Self-lubricating property

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B.W. Dong, S.H. Wang, Z.Z. Dong, J.C. Jie, T.M. Wang, T.J. Li. Novel insight into dry sliding behavior of Cu-Pb-Sn in-situ composite with secondary phase in different morphology[J]. Journal of Materials Science & Technology, 2020, 40(0): 158-167 https://doi.org/10.1016/j.jmst.2019.09.025

1. Introduction

In the recent years, hypomonotectic Cu-Pb-Sn alloys have become the dominant bearing alloy used in high load and high speed engine due to its excellent self-lubricating property and high strength [[1], [2], [3]]. It has been pointed out that, during the friction process, the low-melting-point SPPs distributed in the α-Cu matrix will melt and provide a Pb-rich lubricating film on the friction surface, which can serve as a lubricant and effectively decrease the friction coefficient μ [[4], [5], [6]]. However, Cu-Pb alloy is a typical immiscible alloy. With temperature declining, the solubility of Pb in Cu decreases fastly, which usually results in the formation of SPPs with extremely different shape and size under different solidification condition in immiscible alloys [[7], [8], [9], [10]]. It was pointed out by Dong that SPPs in Cu-20Pb (wt%) alloys prepared under a low undercooling usually exist in a form of filling among the α-Cu grains (called here netty SPPs). But when the undercooling increases to a high enough value (exceeding 200 K) [2], the SPPs will transform to fine and independently rodlike one which is thought to be the optimal microstructure of this alloy with relatively high performance [11]. The different microstructures of the alloys are sure to affect this material’s property, such as mechanical and tribological property [5,[12], [13], [14], [15]]. However, such high undercooling needed for rodlike SPPs can be hardly reached in industrial production. Sn addition is often used to optimize the Cu-Pb alloys’ microstructure, which can promote the SPPs’ transformation from continuously netty to independently rodlike shape. The function mechanism has been detailedly discussed in the previous work [11].

Up to now, scientists have carried out a great deal of researches on the properties of Cu-Pb alloy. Bowden et al. investigated the correlation between the metallic lubricating film’ thickness and μ in hypomonotectic Cu-Pb alloy, indicating that significant decrease of μ can be achieved when the film’s thickness reaches a critical value [4]. Buchanan et al. investigated the friction behavior of non-equilibrium Cu-Pb alloys during dry sliding under different contact pressure. A transition from high fiction to low friction was observed with the contact pressure up to a specific value, which was attributed to the “extrusion” of Pb under high contact pressure [5]. Molian et al. investigated the friction behavior of Cu-Pb alloys with different Pb content, indicating that the wear rate increases with Pb content up to 40 wt%, and a drop in wear rate was observed when Pb content surpass 40 wt% [16]. Although a great deal of investigations have been conducted on Cu-Pb alloys’ mechanical and tribological properties, the frequently-used variants in these researches were the content of Pb and the contact pressure during friction test. There exist few researches with respect to the correlation between the SPPs’ shape and the properties of Cu-Pb or other monotectic alloys, which is sure to limit the further development of the alloys with high performance.

In this work, the Cu-24Pb-xSn (wt%) alloys with SPPs in different shapes are found to show obviously different mechanical and self-lubricating properties. Cu-24Pb alloy with netty SPPs shows abnormally intensive stick-slip phenomenon and obviously lower tensile performance, compared with the other Cu-24Pb-xSn alloys with rodlike SPPs. The corresponding function mechanism of the SPPs with different shapes is discussed in detail. This work can serve as a microstructure standard in industrial production, which can help identify whether the products with the corresponding microstructure are up to standard. Simultaneously, it will give direction to the design and development of new bearing alloys with higher performance.

2. Experimental

The details of the Cu-24Pb-xSn alloys’ preparation and microstructure characterization have been listed in the previous work [11].

The hardness of each sample was measured by a Brinell tester under 62.5 kgf loads at a dwelling of 15 s. At least eight tests were conducted on each sample to ensure the repeatability of the experimental results. Tensile tests were conducted on a universal tensiletesting machine (DNS100) with a strain rate of 1 mm/min at room temperature. Four tensile bars machined from the same position of the ingot were tested for each alloy in order to ensure the data’s accuracy. Each tensile bar is 3.5 mm in diameter and 80 mm in length, as shown in Fig. 1(a). The fracture surface of each fractured tensile bar was observed on a Zeiss Supra55 scanning electron microscopy (SEM) linked with an energy-dispersive spectrometer (EDS).

Fig. 1.   Schematic diagram of (a) tensile bars, (b) pin-on-ring device and (c) stick-slip phenomenon.

Dry sliding wear tests were conducted on a pin-on-ring friction wear testing machine (MMW-1A) in order to investigate friction behavior of the samples. The schematic diagram of the device is shown in Fig. 1(b). The Cu-24Pb-xSn pins were rotating against a fixed 0.45 wt% C steel ring along the vertical axis at a constant speed during the tests. All the pin specimens with a cylindrical shape were cut from the central region of the corresponding ingots, of which the diameter and height are respectively 4.8 mm and 12.7 mm. The steel ring used as the counterface material is 31.7 mm in diameter and 10 mm in thickness. The wear tests were conducted at room temperature and with a rotate speed of 320 r/min. The experimental time of each test is 20 min. Five tests were conducted on each sample in order to ensure the experimental data’s validity.

The element distribution on the alloys’ friction surface was characterized by field emission electron probe (JXA-8530 F PLUS). The friction surface topographies of the Cu-24Pb-xSn alloys were examined by a laser scanning confocal microscope (OLS4000) at a magnification of ×20. The friction surface outline of the alloys in y-z plane were measured, and each measurement was repeated for five times.

The friction coefficient μ(t) as a function of sliding time t is obtained, of which the average value ($\bar{μ}$) and fluctuation intensity (ω) are defined respectively by Eqs. (1) and (2):

$\bar{μ}=\sum^{t/\Delta t}_{i=0}\frac{μ_{i*\Delta t}}{t/\Delta t}$ (1)

$\omega =\sum^{\frac{t}{\Delta t}}_{i=0}(\mu_{i*\Delta t}-\bar{\mu})^{2} $ (2)

where Δt = 10 s; t = 1200s.

Here, the term ω is calculated by sum of squares of deviation from mean, and is used to characterize the fluctuation of μ which is related to the so-called stick-slip phenomenon during sliding. The stick-slip phenomenon was firstly proposed by Bowden and Leben. As shown in Fig. 1(c), the friction force firstly increases from Fk (kinetic friction force) to a specific value during the stick phase. Then slip will occur at the interface when a force high enough is applied to overcome the static friction force Fs [17,18]. In this case, a higher ω means that the stick-slip is more intensive than that with a lower one. The stick-slip phenomenon is usually companied with squeal (about 0.6-2 kHz) and chatter (< 0.6 kHz) during sliding, which has been recognized as a major cause of frictional dissipation [18,19]. Therefore, it is necessary to search an effective method to weaken the stick-slip phenomenon during friction.

3. Results

Fig. 2 shows the SEM images of the Cu-24Pb-xSn alloys deeply etched. SPPs with obviously different shapes are observed. An obvious variation tendency of the SPPs from continuously netty to independently rodlike shape can be observed with increasing Sn content in Fig. 2(a)-(d). According to the EDS results in Fig. 2(e), Sn mainly dissolves in the α-Cu matrix, which may simultaneously increase alloys’ strength through solution strengthening.

Fig. 2.   3D shapes of the SPPs in the Cu-24Pb-xSn alloys: (a) Cu-24Pb alloy; (b) Cu-24Pb-2Sn alloy; (c) Cu-24Pb-4Sn alloy; (d) Cu-24Pb-6Sn alloy; (e) EDS results of Cu-24Pb-6Sn alloy.

Fig. 3(a)-(d) shows the friction surface topology of the Cu-24Pb-xSn alloys. In the Cartesian coordinate system, regions with a color close to blue means that z-value of this region is lower than that of a region with a color close to red. The friction surface outline’s fluctuation intensity can be thus reflected by the color difference among the regions. One can easily find that the fluctuation intensity in Cu-24Pb alloy is much more intensive than that in the other three alloys, which can be characterized by the much higher proportion of the blue region on its friction surface, and usually corresponds to a higher frictional dissipation. Term λ is used to characterize the fluctuation intensity of the friction surface outline (Fig. 3(f)) in y-z plane (x =640 μm), and is defined by Eqs. (3) and (4) in this case:

$\bar{z}=\sum_{i=0}^{y_{max}/\Delta y}\frac{z_{y=i*\Delta y}}{y_{max}/\Delta y}$ (3)

$\lambda =\sum_{i=0}^{y_{max}/\Delta y}(z_{y=i_\Delta y}-\bar{z})^{2}$ (4)

where $\bar{z}$ is the average value of the points’ z-value; ymax is 1280 μm; Δy is 1.25 μm; zy=i*Δy is z-value of the point corresponding to a specific y-value.

Fig. 3.   Surface topographies of (a) Cu-24Pb, (b) Cu-24Pb-2Sn, (c) Cu-24Pb-4Sn and (d) Cu-24Pb-6Sn alloys by confocal laser microscopy; (e) μ curves of the Cu-24Pb-xSn alloys; (f) surface profiles of the Cu-24Pb-xSn alloys.

A higher value of λ corresponds to a more rough friction surface which usually reflects a higher frictional dissipation. λ in the Cu-24Pb-xSn alloys has been presented in Table 1. The minimum of λ appears in Cu-24Pb-6Sn alloy (53858.770). λ in Cu-24Pb-2Sn (59091.730) and Cu-24Pb-4Sn alloys (61955.014) present no significant difference, and are respectively 9.71% and 15.03% higher than that in Cu-24Pb-6Sn alloy. However, λ in Cu-24Pb alloy is 43.81% significantly higher than that in Cu-24Pb-6Sn alloy, indicating that obviously higher frictional dissipation existed in Cu-24Pb alloy during sliding.

Table 1   Values of λ, $\bar{μ}$, ω and H in the Cu-24Pb-xSn alloys during dry sliding wear tests.

Sn content (wt%)λ$\bar{μ}$ωH (HBS)
077454.7840.3341.36930.68
259091.7300.2110.14844.72
461955.0140.1950.13551.04
653858.7700.1820.11659.34

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μ curves as a function of t in the Cu-24Pb-xSn alloys are presented in Fig. 3(e). Compared with the other three samples, abnormally high value and intensive fluctuation of μ can be observed in Cu-24Pb alloy. The values of $\bar{μ}$ and ω in the alloys are also presented in Table 1, both of which are found to decrease monotonously with increasing Sn content. However, the value of ω in Cu-24Pb alloy is approximately 10 times more than that in the other samples, which reflects the abnormally intensive stick-slip phenomenon in Cu-24Pb alloy during sliding. It is noted that the variation tendency of ω is, to some extent, similar to that of λ in the Cu-24Pb-xSn alloys.

Fig. 4(a)-(d) shows the backscatter photographs of the Cu-24Pb-xSn alloys’ friction surface. According to the map scanning results by EPMA in Fig. 4(e) and (f), the white areas in Fig. 4(a)-(d) are the Pb-rich lubricating films covering the surface. Similar Pb-rich films were also observed in the work of H. Cui et al [1]. Simultaneously, as shown in Fig. 4(a), it is noted that the film’s area in Cu-24Pb alloy is not as obvious as that in the other three alloys.

Fig. 4.   Backscatter photographs of the Cu-24Pb-xSn alloys’ friction surface: (a) Cu-24Pb alloy; (b) Cu-24Pb-2Sn alloy; (c) Cu-24Pb-4Sn alloy; (d) Cu-24Pb-6Sn alloy; (e, f) map scanning results of the friction surface in Cu-24Pb-6Sn alloy by EPMA.

The Brinell hardness H (HBS) of the Cu-24Pb-xSn alloys are also listed in Table 1, which is found to increase with increasing Sn content mainly due to solution strengthening. The microstructures of Cu-24Pb and Cu-24Pb-6Sn alloys around the indentation are shown in Fig. 5. As shown in Fig. 5(a) and (b), obvious cracks with a high continuity are observed on the SPPs at the indentation boundary’s both side in Cu-24Pb alloy, which may occur due to the deformation under load. It is noted that all the cracks are mainly along the netty SPPs’ contours, indicating that the SPPs’ high continuity may promote the formation of cracks under enough load. In contrast, no cracks are observed in Cu-24Pb-6Sn alloy as shown in Fig. 5(c) and (d). That may be benefitted from the SPPs’ lower continuity and more regular shape. According to the previous work, it was proven that SPPs with irregular shape are more likely to lead to stress concentration than that with regular shape during deformation [20,21], and cracks are more likely to occur in the former [22].

Fig. 5.   SEM images of the longitudinal section in Cu-24Pb and Cu-24Pb-6Sn alloys: (a, b) Cu-24Pb alloy; (c, d) Cu-24Pb-6Sn alloy.

Fig. 6(a)-(d) shows the SEM micrographs of the tensile samples’ fracture surfaces. According to the EDS results in Fig. 6(e), it’s found that all the samples’ fracture surfaces are covered with Pb-rich phase. Fig. 7(a)-(d) shows the map scanning results of Cu-24Pb-6Sn alloy’s fracture surface, it can be further verified that there exist a Pb-rich layer covering the fracture surface. Ductile fracture is found to be the main fracture mode in all the tensile tests, which may be attributed to the Pb-rich phase high ductility [23,24]. According to the Cu-Pb-xSn pseudobinary phase diagrams [11], the Pb-rich layer covering the fracture surface may form and segregate among the α-Cu grains below monotectic temperature due to the rejection of Pb by the solidifying α-Cu. Simultaneously, referring to the DSC curves in Fig. 7(e), the residual liquid L1 surrounding the solidified α-Cu grains will break up into a α-Cu (S1) and a Pb (S2) solid at the corresponding eutectic reaction temperature [2,25,26]. There is no doubt that S1 will nucleate and grow on the grain boundary of the existed α-Cu, and Pb will thus be rejected to the grain boundary clearance and finally solidify as the filling among the α-Cu grains through eutectic reaction [[27], [28], [29]]. Therefore, during the fracture, cracks will firstly occur and extend along the Pb-rich phase among the α-Cu grains due to its relatively low strength [30,31], which is, to some extent, similar to the fracture mode in Fig. 5(a) and (b).

Fig. 6.   SEM images of the Cu-24Pb-xSn alloys’ fracture surface: (a) Cu-24Pb; (b) Cu-24Pb-2Sn; (c) Cu-24Pb-4Sn; (d) Cu-24Pb-6Sn. (e) EDS results of the Cu-24Pb-xSn alloys’ fracture surfaces.

Fig. 7.   (a-d) Map scanning results of Cu-24Pb-6Sn alloy’s fracture surface, (e) DSC curves of Cu-24Pb and Cu-24Pb-6Sn alloys and (f) UTS and δ of the Cu-24Pb-xSn alloys.

4. Discussion

4.1. Effect of Sn content on the tensile performance of the Cu-24Pb-xSn alloys

The bearing material should be deformed to possess a half round outline through stamping process before it can be put into use in industry, which makes it necessary for the bearing material to possess definite strength and plasticity. It is thus essential to investigate the influence of SPPs in different shapes on the tensile performance of the Cu-24Pb-xSn alloys.

The Cu-24Pb-xSn alloys’ ultimate tensile strength (UTS) and elongation to fracture (δ) are shown in Fig. 7(f). An increase of tensile strength can be observed in the alloys with increasing Sn content, and a simultaneous increase of UTS and δ is observed in Cu-24Pb-6Sn alloy. δ of Cu-24Pb-2Sn and Cu-24Pb-4Sn alloys are also obviously higher than that of Cu-24Pb alloy. This is an unusual phenomenon and cannot be explained by solution strengthening introduced with Sn addition, because UTS and δ of Cu-Sn alloys will respectively increase and decline with Sn addition due to solution strengthening according to the Ref. [32] and the industrial standard (GB/T 4423-2007).

With consideration of the Pb-rich phase covering the fracture surface (see Fig. 6, Fig. 7), the formation and extension of cracks in the four alloys may firstly occur in the Pb-rich phase among the α-Cu grains due to the Pb-rich phase’s relatively low strength. The above phenomenon points out that the Pb-rich phase filling the grain space can deteriorate the material’s mechanical property, which is mainly attributed to the filling’s splitting the matrix. Therefore, it can be proven that the less amount of netty SPPs is the main cause to lead to the simultaneous increase of UTS and δ in the Cu-24Pb-xSn alloy.

4.2. Friction behavior of the Cu-xSn alloys

Fig. 8(a) shows the Brinell hardness of both the Cu-24Pb-xSn and Cu-xSn alloys prepared under the same experimental condition. One can easily observe that the hardness monotonically increases with increasing Sn content in the both systems, which is mainly due to solution strengthening as mentioned above.

Fig. 8.   (a) Hardness of the Cu-24Pb-xSn and Cu-xSn alloys and (b) μ curves of the Cu-xSn alloys.

It was pointed out that the adhesion in softer materials is usually more intensive than that in the harder materials during sliding, because of the former’s relatively high real area of contact (Arp) under the same load [33]. Arp was proven to be inversely proportional to the hardness of the material’s the sliding surface layer, which can be expressed by Eq. (5) [19]:

$A_{rp}=(p_{a}\cdot A_{a})/H$ (5)

where pa is apparent pressure; Aa is apparent area.

The increase of Arp in softer material usually leads to the increase of adhesion which can result in the increase of the static friction coefficient μs [19,34,35]. Simultaneously, it has been pointed out that the stick-slip phenomenon usually occurs when μs is markedly higher than the kinetic friction coefficient μk [12,35,36] as shown in Fig. 1(c). Therefore, the abnormally high ω in Cu-24Pb alloy may be related to its relatively low hardness corresponding to a higher adhesion. In order to accurately reveal the causing factors of the obvious ω difference between Cu-24Pb alloys and the other three samples, it is necessary to firstly investigate the influence of H on the Cu-24Pb-xSn alloys’ μ curves.

Dry sliding wear tests were thus conducted on the Cu-xSn alloys in order to eliminate the disturb of Pb which may provide lubrication and hinder analyzing the influence of H on the μ curves. The Cu-xSn alloys’ μ curves are presented in Fig. 8(b) in which an intensive fluctuation of μ is observed in the all samples during sliding. The values of ω in the Cu-xSn alloys are also presented in Table 2. The maximum and minimum of ω appear respectively in Cu-4Sn (4.816) and Cu-2Sn (1.742) alloys, both of which are higher than that in Cu-24Pb alloy. No obvious correlation between H and ω are found among these μ curves. That means the difference of H among the samples respectively in the Cu-xSn and Cu-24Pb-xSn alloys is not the main factor leading to the abnormally high ω. Simultaneously, with consideration of the fact that the abnormally high ω can be observed in all the Cu-xSn alloys with the lack of Pb, it is indicated that the lubrication provided by Pb should act as an important role to weaken the stick-slip phenomenon in this case.

Table 2   Values of $\bar{μ}$ and ω in the Cu-xSn alloys.

Sn content
(wt%)
H (HBS)$\bar{μ}$ω
037.640.7214.216
254.410.6091.742
467.550.6784.816
677.720.5822.660

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4.3. Lubricating behavior of the Cu-24Pb-xSn alloys

According to the previous work, Pb in Cu-Pb alloys will be extruded during sliding and form a thin lubricating film on the surface, which can effectively decrease the friction coefficient and weaken the stick-slip phenomenon [5,12,37]. Such Pb-rich lubricating film has also been observed in this case (see Fig. 4). However, it was point out by Bowden that effective lubrication can be obtained in Cu-Pb alloys only if the lubricating film’s thickness reaches or surpass a critical value. For example, when the substrate is pure Cu, the critical thickness of the lubricating film for the best lubrication in Cu-Pb alloy is 1×10-3 cm. The critical thickness becomes higher if the hardness of the substrate increases. In contrast, little reduction in the friction coefficient will occur if the lubricating film isn’t thick enough [4,18].

Therefore, in order to investigate the causing factor of the abnormally high ω, it is necessary to investigate the correlation between the lubricating behavior and the μ curve in Cu-24Pb alloy. Another sliding wear test was conducted on Cu-24Pb alloy. A lithium base grease was used as additional lubrication to cover the ring before test. The μ curves of the Cu-24Pb alloys with and without additional lubrication are both presented in Fig. 9(a). Compared to the red line (without additional lubrication), an obvious decrease of μ is observed on the black line (with additional lubrication). Simultaneously, ω of the black line is calculated to be 0.316 which is much lower than the red line (1.369). The additional lubrication is found to significantly decrease the value of ω. Simultaneously, the friction surface outline of Cu-24Pb alloy with additional lubrication is shown in Fig. 9(b), and λ in it is calculated to be 61768.807 which is very close to that in Cu-24Pb-2Sn and Cu-24Pb-4Sn alloys. Referring to the relatively low lubricating area on the friction surface of Cu-24Pb alloy (see Fig. 4(a)), it is indicated that the abnormally high ω in Cu-24Pb alloy without additional lubrication is very likely to be resulted from insufficient lubrication, and lubrication provided by Pb acts as an important role to weaken the stick-slip phenomenon in this case.

Fig. 9.   (a) Friction coefficients and (b) surface profiles of Cu-24Pb alloys with and without additional lubrication.

In addition, three parameters (λ, μ and ω) are presented to characterize the self-lubricating property of Cu-24Pb-xSn alloys in this section. It may be helpful for understanding to clarify the internal relationship among these three parameters. Firstly, ω is proposed to characterize the fluctuation of μ which cannot be well characterized by the average value of μ. For example, when μ(1) = 1.0 and μ(2) = 0.0, the average value of μ is (1.0 + 0.0)/2 = 0.5. However, when μ(1) = 0.5 and μ(2) = 0.5, the average value of μ is also 0.5. Obviously, the fluctuation in the former is more intensive, which cannot be reflected by the average value of μ. Similarly, λ is proposed to characterize the fluctuation of the friction surface’s outline. It’s guessed that the abnormally intensive fluctuation of μ or the highest value of ω in Cu-24Pb alloy is very likely to correspond to serious frictional dissipation as mentioned in section 2. The abnormally high value of λ can just reflect the serious frictional dissipation in Cu-24Pb alloy. Simultaneously, the variation tendency of λ in the Cu-24Pb-xSn alloys is very similar to that of ω (see Table 1), which can further prove that: (1) the increasing lubricating property of the Cu-24Pb-xSn alloys effectively weaken the stick-slip phenomenon characterized by the variation of ω; (2) the weaken stick-slip phenomenon in the Cu-24Pb-xSn alloys closely corresponds to the decreasing frictional dissipation characterized by the vibration of λ.

4.4. Influence of the SPPs’ shape on the self-lubricating property

It has been proven that cracks in Cu-24Pb alloy may firstly occur on the netty SPPs under enough load (see Fig. 5), which can split the matrix and finally lead to fracture. With consideration of the fact that contact and load also exist on the interface between the pin and disk during dry sliding tests, it is necessary to investigate the influence of load on the interfacial microstructure of the samples. Fig. 10 shows the SEM images of the friction surface in Cu-24Pb-6Sn (Fig. 10(a) and (b)) and Cu-24Pb (Fig. 10(c) and (d)) alloys. Cracks are observed on the friction surface of both Cu-24Pb and Cu-24Pb-6Sn alloys, which has been recognized to be a common phenomenon during friction dissipation. However, it is noted that complete fracture and a number of fragments with a size of about 5 μm can be observed on the surface of Cu-24Pb alloy, and many cracks without extension are also observed as shown in Fig. 10(c) and (d). In contrast, as shown in Fig. 10(a) and (b), similar fragments are not observed, and there only exists partly fracture in Cu-24Pb-6Sn alloy. Fig. 10(e) and (f) shows the map scanning results (EPMA) on Cu-24Pb alloy’s friction surface. A continuous Pb-rich region is observed along the crack’s boundary (red broken line in Fig. 10(d)) and surrounds an obvious Cu-rich region. That means the fractured SPPs still connect with the separated α-Cu fragments.

Fig. 10.   SEM images of (a, b) Cu-24Pb-6Sn alloy’s and (c, d) Cu-24Pb alloy’s friction surface; (e, f) Map scanning results of the friction surface in Cu-24Pb alloy by EPMA.

It is concluded from the above discussion that crack and fracture are more likely to occur in Cu-24Pb alloy with netty SPPs during dry sliding due to the SPPs’ high continuity and irregular shape. The continuously netty SPPs surrounding the α-Cu crystalline grains will thus lead to the “breaking up” of the matrix. Simultaneously, it is found that the SPPs still connect with the α-Cu fragments after fracturing, which is sure to result in the wasting of Pb and weaken the self-lubricating property of this alloy. With the poor lubrication, Cu-24Pb alloy thus shows an abnormally high ω during dry sliding.

5. Conclusions

The present study has investigated the effect of the SPPs’ shape on the Cu-24Pb-xSn alloys’ mechanical and self-lubricating property. It is found that, compared with the other three alloys with rodlike SPPs, Cu-24Pb alloys with netty SPPs shows abnormally intensive stick-slip phenomenon during dry sliding wear test and obviously lower tensile performance. The relevant causing factors were revealed as follow:

(1)The self-lubricating and mechanical properties of the Cu-24Pb-xSn alloys increase with increasing Sn content.

(2)No obvious correlation is found between H and ω, indicating that it is not the relatively low strength of Cu-24Pb alloy that lead to the abnormally high μ and ω.

(3)Additional lubrication effectively weakens the stick-slip phenomenon in Cu-24Pb alloy, indicating that the abnormally high ω in Cu-24Pb alloy can be resulted from insufficient lubrication.

(4)The vibration tendency of λ is similar to that of ω in the Cu-24Pb-xSn alloys, indicating that the increasing lubricating property leads to the weakening of both stick-slip phenomenon and frictional dissipation.

(5)The netty SPPs splitting the matrix directly result in poor lubrication and lower mechanical properties of Cu-24Pb alloy.

Acknowledgements

This work was supported financially by the National Key Research and Development Program of China (Nos. 2016YFB0301303 and 2017YFB0306105), the National Natural Science Foundation of China (Nos.51871041, 51771040and51690163) and the Fundamental Research Funds for the Central Universities of China (No. DUT17JC44).


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