Effect of lead and molybdenum disulfide additives on wear resistance and physical properties of copper–graphite composite

In the transportation industry, copper (Cu) matrix composites have been used as electro-sliding contact materials in bearings. Although they have excellent qualities like high thermal conductivity and heat transfer, Cu-based composites have a low wear resistance, which prevents them from being widely used. During the friction process, it is crucial to have a composite with a high wear resistance and higher lubricant properties. Recent research has mostly focused on carbon-based materials such as carbon black, carbon nanotubes, and carbon fibers [1]. Carbon-based reinforcement could increase the Cu matrix’s durability. Adding more carbon will improve low-carbon Cu matrix composites’ mechanical and wear resistance because of their lower density and higher surface energy. Adding more carbon increases the cost of composite production and the aggregation of carbon in the composite matrix [2,3].

Using Gr might significantly reduce the friction coefficient and lower the cost of the carbon/Cu composites due to its low price, lower friction coefficient, and self-lubricating performance. Gr–Cu composites can have a wide range of applications due to their excellent performance and wear resistance [4]. The friction and wear properties of Gr–Cu composites with different Gr concentrations were examined by Kováčik et al. [5]. When the concentration of Gr dropped below a certain critical level, the friction coefficient decreased [6]. Combining two or more lubricants would synergistically improve the lubricating capacities of composites, as compared to using Gr lubricants alone. Huang et al. [7] developed Cu-based self-lubricating materials comprising Gr and MoS₂ solid lubricants by hot pressing powder metallurgy. It was shown that the combination of Gr and MoS₂ may effectively reduce the friction and wear coefficients of Cu-based composites [4]. Solid lubricants are oil additives that regulate the performance of sliding pairs, significantly impacting the lubricant system’s overall response. Self-lubricating particles, such as polytetrafluoroethylene (PTFE), Gr, and MoS₂, are also used to reduce the wear [8,9]. They provide a self-lubricating composite coating, lowering the friction [10]. MoS₂ is an essential layered chemical for solid lubrication. The laminated structure of MoS₂ is composed of strong S–Mo–S covalent bonds within the layers and a weak van der Waals force between the layers. The extraordinary lubricity of MoS₂ is often attributed to its easy layer-to-layer sliding. MoS₂ at the nanoscale often exhibits more excellent tribological properties than MoS₂ at the microscale, especially regarding wear resistance and friction reduction [11,12]. According to He et al. [13], 7.9 wt% MoS₂ decreased the Ni–P friction coefficient from 0.4 to 0.05, while MoS₂ decreased the Ni–W friction coefficient from 0.27 to 0.14 [12].

Also, lead (Pb) is a well-known solid lubricant that can be found in a variety of materials. Dry sliding wear studies on Pb-dispersed Cu-based composites revealed that low-Pb content materials perform poorly in composites. The softness of Pb particles has been proposed to decrease the wear rate [9]. The melting point of Pb is 326°C. In contrast to MoS₂ and Cu, a substance with such a low melting point generates liquid phase sintering (LPS) [14]. LPS is used at a temperature suitable as a densification agent. The eutectic melting point of Pb–Cu is related to the liquid phase [15]. No stoichiometric binary solid phases of the Cu–Pb system are currently known. At 326°C and 99.94 wt% Pb, the eutectic point can be found. At 700 and 900°C, the solubility of Cu in Pb increases to about 2 and 7.5 wt%, respectively [15,16]. In addition, based on the friction test and the observation of the wear surface, the result showed the influence of varying the Gr concentrations on the wear mechanism. However, the impact of self-lubricating characteristic of MoS₂ and liquid-phase synthesis of Pb on the friction performance and wear resistance has rarely been examined [10,17].

In recent years, the effect of Pb and solid lubricants such as MoS₂ on the wear resistance of Cu-based composites has not been sufficiently investigated; thus, the current study investigated the effects of Pb and MoS₂ additives in a Cu–Gr composite. This research aims to develop a composite with high conductivity, wear resistance, and lubrication. Using scanning electron microscopy (SEM), differential thermal analysis (DTA), wear testing, and hardness measurements, the effects of different amounts of Pb and MoS₂ on Cu–Gr composites were investigated.

DTA was performed to determine the impact of the number of additives on the sintering temperature. Before sintering, three powder samples, M4P5, M4.5P4.5, and M5P4, were analyzed using DTA. Figure 1 depicts the obtained DTA results for powder samples. The absence of endothermic peaks indicates that there were no phase transitions or melting points detected during the analysis. Each curve of M4P5 and M5P4 consisted of four exothermic peaks, and the curves of M4.5P4.5 comprised three peaks. DTA revealed the effect of additives on the sintering temperature. Below 450°C, the temperature changes for all three powder samples were comparable. According to the DTA of the samples, the sintering process was conducted at 700°C. Figure 1 demonstrates that increasing the amount of MoS₂ can influence the thermal stability and reactions during sintering, potentially affecting the sintering temperature due to its higher thermal stability compared to Pb. The sintering temperature of sample M5P4 decreased as the amount of Pb additives was increased.

Figure 1

DTA of powder samples before sintering.

The porosity percentage of the sample was determined using the Archimedes fluid handling method. As can be observed in the diagram in Figure 2a, the highest porosity was observed for the sample M4P4, 14.1%, and the lowest porosity for sample M5P5, 10.5%, which indicate that the porosity reduced with the increase of the addition of powders. The effect of variation of porosity with the amount of each powder is illustrated in Figure 2b–d for Pb, MoS₂, and Gr, respectively. Figure 2b shows that increasing the amount of Pb powder significantly reduced porosity, which can be attributed to the lower melting point of Pb at 327°C, resulting in LPS at 700°C [18] and provided more liquid phase to fill the porosities and hindered their formation. The same trend, decreasing porosity, can be observed for MoS₂ powder addition, as can be detected in Figure 2c. It is shown that the lowest porosity was reached in the samples with 5 wt% of Pb, where enough transient liquid phase was available to fill the porosities by using a capillary force and surrounding the MoS₂, Cu, and Gr powders. Pb has a lower melting point than MoS₂; therefore, Pb has a higher flow rate and diffusion rate than MoS₂ during pressing and sintering. The results indicate that adding 1% MoS₂ resulted in a 1% reduction in porosity, while adding 1% Pb resulted in a 2.5% reduction in porosity. In Figure 2d, the variation of the porosities versus the variation of the Gr amount is presented. Since the sum of Pb and MoS₂ powders varied from 8 to 10% and a constant amount of Cu powders (63%) was used, the amount of Gr powders must be varied from 27 to 28%, and the effect of this variation was also investigated. As shown in Figure 2d, the porosity increased by increasing the Gr powder. In other words, by expanding the Gr powder, there was a lesser amount of Pb powder as a source of the transient liquid phase within the LPS, and lesser porosities could be filled, thus the porosities increased. Furlan et al. [19] studied the effect of adding 1% MoS₂ to solid iron-based lubricant matrices. The mechanical and tribological properties of composites were affected by the type and amount of solid lubricant (Gr and MoS₂), the combination of solid lubricants, the initial particle size, and the sintering temperature. Additionally, the samples’ porosity increased. On the other hand, according to the results [20], the density decreased with increasing Gr volume. The porosity of flaky Gr increased, and the percentage of porosity in each volume remained the same regardless of its size. Previous research [21] showed that closed porosity decreased with an increase in the fine-grained Gr content. However, it remained almost constant for the other two cases. A composite containing 15% fine-grained Gr had a 50% greater open porosity than medium and coarse-grained Gr composites. This difference can be attributed to the size of the flakes in the same volume fraction [21]. Furthermore, because of its higher surface, fine-grained Gr interacts more with the composite matrix than medium-grained Gr or coarse-grained Gr. Thus, fine-grained Gr contains more open porosity than medium- or large-grained Gr [22]. In addition to increasing pressure, the density also increased. The force applied to the samples reduced their porosity by increasing their density [23].

Figure 2

(a) Variation of the measured porosities, (b) Pb wt%, (c) MoS₂ (wt%), and (d) Graphite (wt%).

The density of the obtained samples was measured by regarding the measured porosities through immersion in water in the Archimedes method, and the results are presented in Figure 3. It is obvious that the porosity has a reverse relation to the density. Figure 3a compares the calculated theoretical density with the actual density of different samples. The results show that the highest actual density, 4.6 g/cm³, was found in sample M5P5, which also had the lowest porosity. In contrast, the lowest actual density, 4 g/cm³, was observed in sample M4P4, which had the highest porosity. There is a linear relationship between the theoretical and actual densities, although the actual density was lower than the calculated theoretical values. Figure 3b–d illustrates how the density changes with varying amounts of Pb, MoS₂, and Gr added to the samples. It can be observed from Figure 3b that with increasing amount of Pb, the density increased too, which can be attributed to the decrease in porosity due to the presence of the transient liquid phase during the sintering (LPS). The microstructural results revealed that elements with a lower boiling point, such as Pb, evaporated from the structure during sintering, increasing the density, as proposed by Su et al. [24].

Figure 3

Variation of the obtained density. (a) Theoretical, (b) Pb (wt%), (c) MoS₂ (wt%), and (d) Graphite (wt%).

The same trend can be observed in Figure 3c, which shows that the density increases with the amount of MoS₂. The thickness of the composites can be calculated through equation (3), where each component’s density and volume fraction were determined. The densities of Cu, Pb, MoS₂, and Gr were equal to 8.96, 11.34, 5.06, and 2.09 g/cm³, respectively. (3) ρ total = ρ Cu ⋅ X Cu + ρ Pb ⋅ X Pb + ρ MoS 2 ⋅ X MoS 2 + ρ G ⋅ X G . {\rho }_{\text{total }}={\rho }_{\text{Cu}}\cdot {X}_{\text{Cu}}+{\rho }_{\text{Pb}}\cdot {X}_{\text{Pb}}+{\rho }_{{\text{MoS}}_{2}}\cdot {X}_{{\text{MoS}}_{2}}+{\rho }_{\text{G}}\cdot {X}_{\text{G}}. Regarding the variation of the volume fraction of the added elements of Pb, MoS₂, and Gr, it is evident that increasing the sum of Pb and MoS₂ caused the decrease in the amount of Gr by considering the mentioned density of each component and the constant value of the term ρ _Cu·X _Cu; therefore, it can be concluded that with the increase of the amount of MoS₂ and Pb and decrease of the amount of Gr, the density of composites increased, which was confirmed by experimental data results presented in Figure 3b–d. Furthermore, it can be concluded that the effect of Pb addition is higher than that of MoS₂, as shown in Figure 3b and c, due to the presence of transient liquid and higher density of Pb compared to MoS₂. MoS₂ increased the density by 0.15%, and Pb increased it by 0.25%. Gr contributes to an increase in porosity. The density varied by 0.48%, whereas the Gr density varied by 2%. In addition, Gr density reduced by around 0.22% for each percentage increase in weight. Pb has the most significant effect on increasing density among Pb, MoS₂, and Gr weight percentages. Higher pressure increased the density and lowered the porosity, whereas the porosity decreased when the pressure was increased. As the Gr content increased, the density decreased [20].

The surface hardness of the synthesized samples was measured by the Vickers hardness method, and the results are illustrated in Figure 4. The maximum hardness was achieved in sample M4.5P5, 279 HV_0.3, and the minimum hardness was achieved in sample M4P4, 248 HV_0.3. Considering the hardness of the sample M5P5, which has the highest density and lowest porosity, 277 HV_0.3, it can be concluded that increasing the density increases the surface hardness. In other words, hardness has a direct relationship with density and a reverse relationship with porosity. In addition, as the density increased, the defects and cavities decreased, which resulted in their locking and thus improved the hardness of the material. According to previous studies, poor bonding between Cu and Gr in composites of these two materials resulted in a weak joint with a large microstructure and cavities [25]. Increasing the pressure during sintering due to the presence of the transient liquid phase increased the hardness and decreased the number of pores. Large amounts of Gr were utilized to reduce friction. Increasing the Gr content in the material generally results in a decrease in mechanical properties, such as hardness. While Pb exhibits a relatively low hardness of 5 HV, MoS₂ demonstrates a significantly higher hardness of 654 HV. As illustrated in Figure 4b and c, incorporating Pb and MoS₂ nanoparticles into the material led to a gradual increase in hardness as their weight percentage increased. This trend generally aligned with the corresponding changes in porosity and density. As a result, Pb and MoS₂ reduced the porosity and increased the density; they can also increase hardness. In contrast, increasing the percentage of Gr reduced the hardness, as shown in Figure 4d. The increased porosity and Gr’s low hardness may contribute to this phenomenon. As shown in Figure 4c, MoS₂ caused a more significant increase in hardness than Pb. For every percent increase in MoS₂, samples became approximately 20 units harder. Furthermore, about 15 units of hardness increase were obtained for every percent increase in Pb, which might be because the MoS₂ particles are denser than Pb particles. Figure 4d depicts composites’ hardness as a Gr percentage function. The composite’s hardness reduced as the Gr content increased. Furthermore, Gr is more porous than other components due to its softness. Each percent of Gr reduced hardness by roughly 15 units. Gr is a soft material; its hardness decreased with increasing percentage [26]. The grain size of Gr is the primary reinforcing subject, which has increased the hardness of Cu–Gr composites. Furthermore, the Cu powder particles are partially coated with Gr powder at high pressures, which improves the bond between Cu and Gr.

Figure 4

(a) Variation of the achieved hardness of synthesized samples, (b) Pb (wt%), (c) MoS₂ (wt%), and (d) Graphite (wt%).

According to the investigation of Su et al. [24], after the addition of an element with a low melting point, such as Sn, to a Gr–MoS₂–Cu composite, it is challenging for the slower-moving Sn atoms to diffuse. Gr and MoS₂ must enter liquid phases and dissolve into a thicker Cu coating. When the sintering temperature was above 760°C, the movement of Sn atoms accelerated dramatically. Therefore, increasing the temperature caused more Pb to diffuse and increase the density of samples. According to the Sn–Cu and Pb–Cu binary phase diagrams, the dissolvability of Pb to Cu is higher than that of Sn to Cu. Su et al. [24] reported that the addition of Sn increased the maximum hardness of the composite consisting of 85 wt% Cu-15% Sn alloy powder, 10 wt% Cu-coated Gr powder, and 5 wt% Cu-coated MoS₂ powder, reaching 88 HV. In this study, 15 wt% Sn powder with an average particle size of approximately 5.1 µm was used. Still, in this research, by adding Pb to the Cu–Gr composite, the hardness increased to 273 HV. Increasing the pressure and decreasing the Gr content in the samples caused an increase in the hardness of the samples. Thus, among the three parameters, Pb weight percentage, MoS₂ weight percentage, and Gr weight percentage, increasing the MoS₂ weight percentage significantly influenced the density growth.

SEM was used to observe the samples’ morphology, size, and porosity, and the results are illustrated in Figures 5–7. SEM was employed to examine the morphology, size, and porosity of the samples, with the results presented in Figures 5–7. Following the sintering process, these figures demonstrate how the morphology of the composites changes with varying amounts of Pb and MoS₂ particles. Increasing the MoS₂ content while reducing Pb enhances the flake-like structure of the material. Furthermore, a comparison of the SEM images in Figures 5 and 7 shows that decreasing the Pb content also reduces the transient liquid phase. Cu has soft powder particles, flattened during milling, after cold welding, and rewilding. When two metal balls collide, some powder becomes trapped between them. Following the plastic deformation of the particles under the influence of force, hard work and eventual failure are the results. As a result, the new surfaces of the particles can fuse, increasing the grain size due to the softness of the particles in the early stages of milling and their tendency to connect and form larger particles. The particle size multiplies by several factors. This process results in composite particles with a layered structure and a different composition than the original particles. With more surface contact and better penetration, Cu alloying with graphene becomes easier.

Figure 5

SEM image of sample M4P5 after sintering.

Figure 6

SEM image of sample M4.5P4.5 after sintering.

Figure 7

SEM image of sample M5P4 after sintering.

The wear behavior of the samples was evaluated using a pin-on-disk test. The samples’ weight loss and the average coefficient of friction (COF) were measured, and the results are reported in Table 3. The maximum weight loss (43 mg) was observed for sample M5P5, indicating lower wear resistance. It can be attributed to the higher amount of transient liquid phase in this sample, and due to quick solidification, the brittle phases were achieved, which have a lower resistance to wear; Thus, it can be concluded that with the increase of the amount of Pb, the wear resistance of the composites decreased due to the increase of the transient liquid phase and subsequent brittle phase formation because of the fast solidification. Furthermore, the brittle intrinsic causes the more effortless movement of the wear debris from the contact surfaces and acts as abrasive particles between the contact surfaces, yielding more significant damage. Also, the minimum weight loss (36 mg) was observed for M4P4.5 and M4P5 samples, implying their better wear resistance than other samples. MoS₂ is one of the most commonly used solid lubricants for Cu–Fe-based friction materials, according to the research of Peng et al. [27]. It can stabilize the friction and increase the wear resistance of composites. Even though numerous studies have demonstrated that MoS₂ reacts with Cu or Fe matrices when sintered at temperatures above 950°C, a recent study revealed that lubricity is preserved.

It is expected that due to the lubricating features of MoS₂ and Gr particles, their influences showed a synergic effect. They increased the wear resistance of the samples, but due to the chemical variation of the composites, the balance between MoS₂ and Gr particles was changed, which impressed the wear resistance. Regarding weight loss, it can be concluded that Gr particles have a more positive effect on improving the wear resistance than MoS₂. Also, the wear traces’ width and depth can be considered as wear behavior criteria. As shown in Figure 8, the wear trace of sample M4P5 is higher than that of other samples. However, the depth of the wear trace of sample M5P4 is more than that of different samples, which confirms the formation of the brittle phase in samples with a higher amount of Pb. In other words, increasing the amount of Gr particles improved the wear resistance, and increasing the amount of MoS₂ particles decreased the wear resistance, which indicates the better lubricating feature of Gr and its higher amount in the chemical composition. On the other hand, the variations of the average COF of three different samples with high, low, and medium weight loss are illustrated in Figure 8. The lowest and highest COF of samples M4P5 and M4.5P4.5, respectively, were 0.25–0.54. Compared to Ni–P and Ni–W composites, a Cu–Gr-based composite’s COF was reduced from 0.54 to 0.25 by adding 0.05 wt% of MoS₂ [12]. Figures 8–10 show EDS maps of different samples. The oxygen content of the sintered samples remained relatively consistent, ranging from 5.4 to 8.1 at%. This stability indicates that the sintering and composite formation processes did not significantly alter the oxygen levels, suggesting minimal oxidation or reduction reactions. The presence of oxygen likely arises from residual impurities in raw materials or surface oxidation during processing. Despite this stability, the wear resistance and COF values varied significantly among the samples, highlighting the dominant influence of Pb, MoS₂, and Gr on the composite’s tribological behavior rather than the oxygen content. Additionally, SEM/EDS mapping revealed localized oxygen distribution, possibly associated with oxide layers or specific interfacial phases, which warrants further investigation into its role in phase formation and mechanical properties (Figure 11).

Figure 8

Variation of the COF and wear trace of samples: (a) M4P5, (b) M4.5P4.5, and (c) M5P4.

Figure 9

EDS map of sample M4.5P5.5.

Figure 10

EDS map of sample M5P4.

Figure 11

EDS map of sample M4P5.

Figure 8 demonstrates that decreasing MoS₂ and increasing Pb led to a decrease in friction coefficient, which can be related to a higher density of MoS₂ than Pb. Pb particles have a lower melting point and a softer surface than MoS₂; hence, a temperature increase resulted in more Pb penetration than MoS₂ into the composite. The hardness of MoS₂ was 8.9 GPa. It can affect the friction coefficient of samples; therefore, increasing the quantity of MoS₂ can significantly influence the weight loss compared to that caused by increasing the amount of Pb.

This study investigated the effects of different Pb and MoS₂ content percentages on Cu–Gr composites’ physical, mechanical, and tribological properties. 700°C was chosen as the proper sintering temperature for the obtained powder mixture to form a composite based on the DTA results. Exploiting the Pb particles with lower melting points than other components caused an LPS phenomenon. Subsequently, due to the presence of transient liquid phase through LPS, the porosity was decreased, and the density and hardness of the achieved samples increased by increasing the Pb content. High hardness was reached, 279 HV_0.3. In contrast to the surface hardness, wear results illustrated that the weight loss was increased by increasing the Pb content, which can be attributed to the brittle feature of formed phases due to the fast solidification of the transient liquid phase. The samples with the lowest and highest COF values were M4P5 and M4.5P4.5, with values ranging from 0.25 to 0.54. Therefore, it can be concluded that MoS₂ and Gr have a better effect on the wear behavior than Pb. Additionally, the lubrication behavior of Gr has a better influence on the wear behavior and friction coefficient than MoS₂ particles.

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Maryam Hosseini: Investigation, Methodology, Formal Analysis; Amirreza Mashtizadeh: Investigation, Methodology, Formal Analysis, Writing-Original Draft, review & editing; Arvin Taghizadeh Tabrizi: Writing-Original Draft, review & editing, Formal Analysis; Hossein Aghajani: Conceptualization, Supervision.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.

Data will be available on reasonable request.

Not applicable.

Not applicable.