Trans. Nonferrous Met. Soc. China 28(2018) 946-956

Tribological behavior of self lubricating Cu/MoS₂ composites fabricated by powder metallurgy

Mohammad MOAZAMI-GOUDARZI, Aram NEMATI

Department of Materials Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran

Received 25 March 2017; accepted 30 September 2017

Abstract: The effects of MoS₂ content on microstructure, density, hardness and wear resistance of pure copper were studied. Copper-based composites containing 0-10% (mass fraction) MoS₂ particles were fabricated by mechanical milling and hot pressing from pure copper and MoS₂ powders. Wear resistance was evaluated in dry sliding condition using a pin on disk configuration at a constant sliding speed of 0.2 m/s. Hardness measurements showed a critical MoS₂ content of 2.5% at which a hardness peak was attained. Regardless of the applied normal load, the lowest coefficient of friction and wear loss were attained for Cu/2.5MoS₂ composite. While coefficient of friction decreased when the applied normal load was raised from 1 to 4 N at any reinforcement content, the wear volume increased with increasing normal load. SEM micrographs from the worn surfaces and debris revealed that the wear mechanism was changed from mainly adhesion in pure copper to a combination of abrasion and delamination in Cu/MoS₂ composites.

Key words: copper matrix composite; microstructure; coefficient of friction; wear mechanism

1 Introduction

Lubricants are widely applied between contacting surfaces to minimize friction and wear. While liquid lubricants are typically used in most of industrial and engineering applications, solid lubricants such as graphite and molybdenum disulfide (MoS₂) are employed in tribosystems when liquid lubrication is limited by operational or environmental considerations. In this respect, solid lubricants may be used in space technology, ultra-high vacuum or automotive transport [1,2]. However, the effective performance of solid lubricants requires a continuous supply of them between contacting surfaces. This is usually achieved by incorporation of solid lubricant material within a metallic matrix and creating a composite material. In this regard, metal matrix composites (MMCs) reinforced with solid lubricant particles are being used as self-lubricating materials for various engineering applications [2].

While both graphite and MoS₂ have low shear strength due to lamellar structure and reduce friction in tribo-interfaces, most of the investigations in this field are focused on MMCs reinforced with graphite particles. This might be due to higher cost and complexities in processing of MoS₂ reinforced MMCs. Both liquid and solid metallurgy processing routes are widely used to synthesize self lubricating Cu/graphite composite [3]. However, liquid metallurgy processing method such as stir casting could not be utilized to fabricate Cu/MoS₂ composites owing to high reactivity of MoS₂ particles with molten copper. Conventional powder metallurgy (PM), as the most widely used solid processing method, has been successfully applied to a large number of metal/non-metal combinations. However, agglomeration of MoS₂ particles during process of mixing and their dissociation during solid state sintering are among the problems encountered in fabrication of Cu/MoS₂ composites using PM method [3,4].

MoS₂ offers better lubrication than graphite under extreme condition of load in dry and vacuum environments [2,5]. It is reported that impregnating MoS₂ into sintered bronze provides even a greater improvement in friction and wear life than impregnation with graphite [6]. Therefore, it would be influential to study the tribological behavior of copper-based composites containing various amounts of embedded MoS₂ particles.

BAO et al [7] reported the reduced coefficient of friction of Cu/MoS₂ composite against a bronze counterface under electrical sliding by increasing of MoS₂ content. SENTHIL KUMAR et al [8,9] studied the effect of MoS₂ addition on the wear behavior of Cu-Sn matrix composites and reported MoS₂ as a potential solid lubricant for the sintered copper-based composites. Contradictory results especially at higher reinforcement contents were reported by others [10] expressing that MoS₂ is an ineffective solid lubricant in sintered Cu-Sn composites due to its reactivity with the matrix at the sintering temperature. KOVALCHENKO et al [11] described the influence of incorporated MoS₂ particles within a copper matrix on prevention of scuffing damage against a sliding copper counterface.

In addition, the improved wear resistances of Ni [12], Mo [13], Al [14-16], Mg [17,18] and Fe [19] based composites by incorporation of MoS₂ particles in the matrix have also been reported. However, because of difficulties in embedding and uniformly distributing MoS₂ particles within a metallic matrix such as copper, this type of solid lubricant is often utilized on metallic substrates by different coating methods. For example, the improved sliding wear performance of Cu/MoS₂ composite coatings fabricated by cold spray method was recently reported by ZHANG et al [20]. Nevertheless, the working period of a PM self lubricating material is much longer than a composite coating because solid lubricants are uniformly distributed within the whole bulk material [11].

Addition of high amount of MoS₂ (20%-40%, volume fraction) to copper alloys was reported to lead to very high wear rates [10]. Therefore, in the present work, the effect of lower contents of MoS₂ (i.e., 0-10%, mass fraction) on friction and wear behavior of self lubricated copper matrix composites is evaluated. More specifically, the relation between microstructure and hardness of Cu/MoS₂ composites with their wear resistance is discussed. In order to alleviate the aforementioned problems in processing of Cu/MoS₂ composites through the PM method, mechanical milling and hot pressing were respectively used for mixing and consolidation of copper and MoS₂ powders.

2 Experimental

In the present work, copper and MoS₂ powders were used as the matrix and reinforcement, respectively. The SEM images of as-received copper (99% in purity and 8 μm in size) and MoS₂ (98% in purity and 5 μm in size) particles are shown in Fig. 1. As observed, copper powders were spherical in shape and MoS₂ particles possessed sheet-like structure. The pure copper and copper matrix composites containing 2.5%, 5% and 10% (mass fraction) of MoS₂ (equivalent to 4%, 8% and 16%, volume fraction, respectively) were fabricated by a powder metallurgy route including ball milling (mixing) and hot pressing. The copper and MoS₂ powders were mixed together in an attrition ball mill using a plastic container and steel balls at 20 r/min for 20 min. The balls were 6 mm in diameter and ball to charge ratio was 5:1. These milling setup and parameters were chosen to minimize any work hardening effect during the mixing stage. The powder mixtures were consolidated in a hot work tool steel die at a constant temperature of 370 °C under 500 MPa for 20 min. Since the oxidation rate of MoS₂ in air is very low at temperatures below 370 °C [21,22], the powder compaction was carried out at this constant temperature. The hot-pressed compacts were cylindrical in shape with 25 mm in diameter and about 10 mm in height.

Fig. 1 SEM micrographs of as-received copper powders (a) and MoS₂ particles (b)

In order to investigate the possibility of formation of new deleterious products during the fabrication process, X-ray diffraction (XRD) studies were performed on both ball milled Cu/MoS₂ powders and hot consolidated samples using a Philips PW-1730 diffractometer. The target was Cu K_α (λ=0.154 nm) and the tests were carried out with a scanning speed of 2 (°)/min.

Microstructural studies were performed on polished samples using an Olympus BX51M optical microscope. Density of consolidated compacts was measured using Archimedes’ method. Vickers hardness of hot-pressed samples was obtained by applying 300 N load using the ESE WAY hardness tester device.

Coefficient of friction and wear loss of prepared samples were measured using a pin on disk tribometer. The wear tests were carried out on rotating disk samples under normal loads of 1, 2 and 4 N at a constant sliding speed of 0.2 m/s for a sliding distance of 1000 m at room temperature (24 °C). The disks were polished to the surface roughness (R_a) of about 0.2 μm. The counterface was round ended AISI 52100 steel pin, 3 mm in diameter and 15 mm in height with a hardness of HRC 62. The wear tests were carried out in the atmosphere with relative humidity of (40±2)%. Prior to each test, the samples and disks were cleaned in acetone using an ultrasonic device. The wear of the specimens and the counterface was determined by measuring their mass loss using an electric balance with the precision of 0.1 mg. The wear volume was calculated using the mass loss and the known density of each sample. The worn surfaces and generated debris were observed and analyzed with a Tescan VEGA-LMU scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS).

3 Results and discussion

3.1 Microstructure, density and hardness

Figure 2 shows the optical microstructures of hot pressed copper and Cu/MoS₂ composites. The microstructure of pure copper sample shown in Fig. 2(a) includes some microporosities which are typical in powder metallurgy processed parts. The microstructure of Cu/2.5MoS₂ composite exhibits dark particles of MoS₂ dispersed in the light matrix of copper, as shown in Fig. 2(b). Some voids are also observed which are probably generated by removing of loose MoS₂ particles in the polishing stage. Such a microstructure in which solid lubricant reinforcements are reasonably distributed within the matrix phase is anticipated to provide self lubricating property. When the mass fraction of MoS₂ was 5% or higher, the continuous matrix phase in the microstructure changed from metallic copper to MoS₂, as shown in Figs. 2(c) and (d). These microstructures resemble to those containing much higher content of solid lubricant particles. This is mainly due to smearing of copper powders with finely divided MoS₂ particles during mechanical milling in powder mixing process. In fact, while Cu/2.5MoS₂ is a copper matrix composite reinforced with soft MoS₂ particles, Cu/5MoS₂ and Cu/10MoS₂ are a kind of MoS₂ matrix composite reinforced with hard copper particles. Similar microstructures were reported for Cu/WS₂ composites by other authors [23].

Fig. 2 Optical micrographs of hot-pressed pure copper (a) and Cu/MoS₂ composites containing 2.5% (b), 5% (c) and 10% (d) of MoS₂

The results of X-ray diffraction studies on Cu/5MoS₂ powder mixture and the hot-pressed composite are shown in Fig. 3. The analyses show the presence of Cu and MoS₂ diffraction peaks for both the powder mixture and the consolidated compact. The absence of copper oxide, molybdenum oxide, mixed copper and molybdenum sulfides (Cu-Mo-S compounds) or other related peaks indicates that no oxidation or dissociation reaction has taken place during hot pressing.

Fig. 3 XRD patterns for Cu/5MoS₂ powder mixture and hot-pressed composite

Figure 4 shows the variation of relative density and hardness of hot pressed samples with MoS₂ content. It is found that relative density of the compacts increases with MoS₂ content. Generally, compaction process of powders consists of rearrangement and movement of particles followed by their plastic deformation [24]. Since all the samples were compacted at a constant applied pressure, the contribution of plastic deformation in the achieved densification would be nearly the same for all the tested materials. However, MoS₂ reduces the internal friction among copper particles sliding against each other under the applied pressure. Therefore, particle sliding of compacting powders increases with increasing MoS₂ content. As a result, the improved densification of powder mixtures with MoS₂ content could be mainly attributed to the enhanced particle rearrangement arisen from lubricating effect of MoS₂ particles.

The Vickers hardness of various specimens is also presented in Fig. 4. It is found that the hardness of Cu/2.5MoS₂ composite is 11% higher than that of pure copper compact produced by hot pressing of the milled powders. This is due to enhanced densification which compensates any possible reduction of hardness due to the addition of soft MoS₂ particles to the powder compact. However, when the MoS₂ content exceeds this threshold value, the hardness is decreased by further MoS₂ addition. This reversed trend in hardness variation results from microstructural changes in the composites, as previously shown in Fig. 2. The MoS₂ phase covers a larger area than copper phase in the microstructure of the composites containing 5% and 10% of soft MoS₂ particles. The softer matrix imposes lower constraint against indentation and thereby results in reduction of hardness.

Fig. 4 Variation of relative density and hardness with MoS₂ content

3.2 Tribological behavior

3.2.1 Friction

Figure 5 shows the average coefficient of friction as a function of MoS₂ content under different applied normal loads. It is found that at all the applied loads the composite containing 2.5% of MoS₂ particles exhibits the least coefficient of friction. For the pure copper, the mean coefficient of friction varies between 0.34 and 0.53, while it fluctuates between 0.20 and 0.40 in 2.5% MoS₂ composite at various normal loads. Friction rises when the content of solid lubricant particles increases from 2.5% to 5% and then remains nearly constant. However, most of reports in this field indicate that coefficient of friction decreases initially with percentage of solid lubricating particles and then levels off without any subsequent increase [25-28]. The decrease of coefficient of friction with addition of 2.5% of MoS₂ to the base copper is related to lubricating characteristic of MoS₂ particles. The MoS₂ particles incorporated within the copper matrix shear on the mating surfaces during sliding and decrease metal to metal contact area, which results in reduction of coefficient of friction. However, it is reported that overall effectiveness of solid lubricating particles depends on the ability of the sheared material layers to adhere to the sliding surfaces [29]. In fact, properties of the substrate material affect the lubricating characteristic of the sheared layer. When a thin film is deposited on a harder substrate, the coefficient of friction (f) could be expressed as [30]

                                  (1)

where S_f is the shear strength of the film material and H_s is the hardness of the substrate. This equation suggests that coefficient of friction is inversely related to the hardness of substrate material. Therefore, the increased coefficient of friction by increasing MoS₂ content from 2.5% to 5% (Fig. 5) could be attributed to the reduced hardness value (Fig. 4) which in turn is affected by microstructure of the material.

Fig. 5 Variation of coefficient of friction with MoS₂ content at different normal loads

It is also clear from Fig. 5 that the coefficients of friction of both base copper and Cu/MoS₂ composites decrease with increase in the applied normal load from 1 to 4 N. Similar trends for reduced coefficient of friction by increasing of normal load have been reported by other researchers [31-33]. For pure copper, the increased applied load is accompanied with increased frictional heating which in turn softens the surface asperities. In this condition, local bonds formed due to welding in contacting surface asperities are easily broken. Therefore, lower tangential force is needed to shear these adhesive bonds leading to a lower friction. In addition, higher normal load introduces higher tendency of oxidation, which prevents metal-metal contact and therefore reduces friction coefficient. Moreover, higher normal load leads to heavier work hardening, resulting in a harder tribolayer and thus reducing the friction according to Eq. (1). In the case of Cu/MoS₂ composites, reduction of coefficient of friction could be attributed to the enhanced squeezing-out of MoS₂ particles from the matrix at higher normal loads. As a result, a wider part of the contact area is covered with the lubricating film and metal to metal contact is effectively reduced leading to a lower coefficient of friction. Moreover, the effect of applied normal pressure (P) on coefficient of friction for thin film lubrication is given by [3]

                                 (2)

where α is a constant of film material. As the normal pressure increases, the contribution of the first term in Eq. (2) is reduced and the coefficient of friction approaches to the constant value of α, a characteristic of MoS₂ lubricating film.

Generally, the coefficient of friction is to be minimized for system efficiency [5]. However, the fluctuation level of friction is also of special importance since it affects frictional behavior in tribosystems. Figure 6 shows the variation of coefficient of friction with sliding distance for the base copper and Cu/MoS₂ composites. For pure copper, it is found that the coefficient of friction exhibits a fluctuant trend at all the normal loads (Fig. 6(a)). It is evident from Fig. 6(b) that Cu/2.5MoS₂ composite has reasonably stable frictional traces. This is typical of lubricated sliding ensuring the formation of a stable lubricating film between the mating surfaces. The friction traces of Cu/10MoS₂ composite (Fig. 6(c)) show unsteady data particularly at higher normal loads. This could be attributed to the instability of lubricating film due to the reduced hardness of substrate material at high MoS₂ content. A higher fluctuation of the coefficient of friction for copper matrix composites containing higher solid lubricant contents was previously reported by other authors [11].

Fig. 6 Variation of coefficient of friction versus sliding distance for pure copper (a), Cu/2.5MoS₂ (b) and Cu/10MoS₂ (c) composites under different normal loads

3.2.2 Wear loss

Figure 7 shows the wear loss of pure copper and Cu/MoS₂ composite disks as a function of MoS₂ mass fraction under different applied normal loads. As observed, the volume loss of disk samples at a constant applied load follows almost the same tendency of the coefficient of friction (Fig. 5). A critical MoS₂ content of 2.5% (mass fraction) is found in which the lowest wear loss occurred at a constant normal load. Further increase in amount of solid lubricating reinforcement resulted in a higher wear loss. The reduced wear loss of 2.5% MoS₂ composite as compared to that of the pure copper is attributed to the antifriction properties of solid lubricating film of MoS₂ as discussed before. The subsequent increase in volume loss of composite disks with further addition of MoS₂ particles is associated with the deteriorated mechanical properties of the substrate material (Fig. 4). The results presented in Fig. 7 also indicate that the wear loss increases with increasing normal load regardless of the MoS₂ content of substrate material. These results might be interesting considering that the coefficient of friction decreased with normal load as shown in Fig. 5. The effect of normal load (N) on volume loss per unit sliding distance (V) is given by Archard wear equation [34]:

                                  (3)

where K is wear coefficient indicating the severity of the wear process. The variations of wear coefficient with applied normal load for pure copper and self lubricating Cu/MoS₂ composites are presented in Fig. 8. It is found that the wear coefficient for pure copper significantly increased with normal load, indicating deterioration of tribological performance of the copper at high normal loads. However, the wear coefficients of Cu/MoS₂ composites show little changes with normal load. The wear coefficient of Cu/2.5MoS₂ composite, in particular, is almost constant through the entire range of the applied load. This suggests that properly selected and processed self lubricating Cu/MoS₂ composites have high potentials to be utilized in applications requiring high load carrying capacity.

Fig. 7 Wear loss of pure copper and Cu/MoS₂ composites at different applied loads

Fig. 8 Variation of wear coefficient with normal load for pure copper and Cu/MoS₂ composites

The mass loss of the counterfaces running against pure copper and composite disks as a function of MoS₂ content is shown in Fig. 9. It is evident that the addition of MoS₂ particles into the copper matrix decreases the mass loss of counterfaces at all the applied normal loads. The decrease of mass loss of the counterfaces reaches the minimum at 5% MoS₂ content and remains almost constant with further increase of MoS₂ content. The smearing of MoS₂ particles on the composite surface and formation of a separating film between the mating surfaces, i.e. transfer layer, govern the wear loss of both composites and the counter material. This layer is continuously replenished by MoS₂ dispersoids within the composite compacts. The formed transfer layer protects the surface of the counter material from further wear damage through avoiding metal to metal contact of the rubbing surfaces. In addition, the reduced hardness of composite disks with addition of MoS₂ particles affects the wear of counterface pins. That is why the minimum mass loss for the counterface material is found to occur at 5% or 10% mass fraction of MoS₂ particles.

Fig. 9 Mass loss of counterface pins as function of MoS₂ content worn at different applied loads

From above results, it may be concluded that MoS₂ particles enclosed in a copper matrix provide effective lubrication of the material surfaces in contact. At an optimum content of MoS₂, i.e. 2.5%, friction surfaces are always covered with a lubricating MoS₂ film which separates the mating surfaces effectively. This results in a low and stable coefficient of friction together with improved wear resistance of the tribosystems. However, in a similar work by KOVALCHENKO et al [11], it was reported that a minimum content of 5% (mass fraction) MoS₂ in the copper matrix is needed to obtain the best lubricating effect. These different results may arise from using of different synthesizing processes. In the present work, ball milling was utilized to mix pure copper with MoS₂ particles. The soft MoS₂ powders are easily sheared and fractured to very fine particles during mechanical milling. It would be easier for these well dispersed and finely powdered particles to emerge from the copper matrix. As a result, the milled MoS₂ particles could be efficiently smeared on the surface of composite compacts during the sliding wear. In addition, no decomposition of MoS₂ was found due to utilizing of hot pressing as the consolidating process in synthesizing of the Cu/MoS₂ composites. Consequently, all the added MoS₂ particles (i.e. 2.5%) were effectively used for the self lubricating purpose. Therefore, it is evident that even an addition of as low content as 2.5% MoS₂ results in a high level of lubrication.

3.2.3 Wear mechanism

The worn surfaces of pure copper disks and their corresponding wear debris are shown in Figs. 10 and 11, respectively. The results of EDS analysis performed on those surfaces are also inserted. Figure 10(a) shows minor plastic deformation for pure copper loaded with 1 N, indicating the domination of adhesive wear. The EDS analysis of the surface shows only copper element. A characteristic of adhesive wear is large unstable coefficient of friction [35], as presented in Fig. 6(a). Figure 11(a) shows differently shaped fine wear particles produced in wear test of pure copper sample under normal load of 1 N.

Fig. 10 SEM images of wear surface of pure copper under normal load of 1 N (a) and 4 N (b) together with results of their corresponding EDS analyses

Figure 10(b) shows massive plastic deformation in surface of the pure copper disk worn under normal load of 4 N. The region marked with dashed lines shows a wearing out particle developing on the disk surface due to the adhesive action of the counterface. As a result of adhesive wear, irregular large wear fragments are generated, as shown in Fig. 11(b). The presence of iron and oxygen on the wear surface as confirmed by EDS analysis might indicate the material transfer from the counterface and formation of oxides, respectively. Increasing the normal load in sliding wear from 1 to 4 N leads to a temperature rise of contacting asperities especially in steel counterface with lower heat conduction than the copper disc. Therefore, the thermal softening of asperities in steel counterface and consequently the material transfer occur. In addition, the formation of metallic oxides on the mating surfaces decreases the coefficient of friction between sliding surfaces [36,37]. This results in the reduction of coefficient of friction with increasing of normal load for pure copper samples, as previously shown in Fig. 5.

Fig. 11 SEM images from wear debris of pure copper disks worn under normal load of 1 N (a) and 4 N (b)

Fig. 12 SEM images of wear surface of Cu/2.5MoS₂ (a, b) and Cu/10MoS₂ (c, d) composites under normal load of 1 N (a, c) and 4 N (b, d) together with results of their corresponding EDS analyses

Figure 12 shows the wear surface and results of EDS analysis for Cu/MoS₂ composites. It is observed that the surface of Cu/2.5MoS₂ composite worn under normal load of 1 N (Fig. 12(a)) is relatively smooth and covered with a dark and continuous film of MoS₂ confirmed by EDS analysis. This solid lubricating film is created by transferring of MoS₂ particles from the bulk of composite to the surface under the applied normal load. Since no signs of adhesion are observed, it can be concluded that the main role of a solid lubricant film is to suppress or decrease the adhesive wear between sliding surfaces. Instead, some shallow scratches are evident indicating the dominating abrasion marks. This wear mechanism results in formation of fine powder like debris and some approximately equiaxed particles smaller than 5 μm in size as presented in Fig. 13(a). However, the wear surface of the same composite under the applied load of 4 N (Fig. 12(b)) reveals brittle fracture of the lubricant film which results in formation of both fine powdery and coarse irregularly shaped wear debris (Fig. 13(b)). Similar wear debris morphology was observed for Al/9Gr composite disks sliding against a steel counterface by other researchers [28]. The formation of pits by brittle fracture on the worn surface might suggest the activation of fatigue wear mechanism in the absence of the adhesive wear due to the well lubricated contacts.

As MoS₂ content in the composite disk loaded with 1 N increased from 2.5% to 10%, the morphology of the wear surface changed from fine scratches (Fig. 12(a)) to deep grooves (Fig. 12(c)) indicating sever abrasive wear mechanism. This could be explained by the reduced hardness of the composite disks with increasing MoS₂ content leading to lowered abrasion resistance of the materials. In addition, delamination is observed to be operative during sliding wear of 10% MoS₂ composite loaded with 1 N, as shown in Fig. 12(c). This could be attributed to the fact that microcracks propagate more easily in high lubricant content composites than in those containing less lubricant [38].

Fig. 13 SEM images from wear debris of Cu/2.5MoS₂ (a, b) and Cu/10MoS₂ (c, d) composite disks worn under normal load of 1 N (a, c) and 4 N (b, d)

Figure 12(d) shows the morphology of the wear surface of Cu/10MoS₂ composite disk worn under normal load of 4 N. It is observed that solid lubricant film is smashed into particles up to 100 μm in diameter at this normal load. As a result, in some regions, the fresh uncoated composite surface is exposed to the steel counterface leading to a highly fluctuant friction, as illustrated in Fig. 6(c). The load-carrying capacity of coated systems generally decreases with decreasing hardness of substrate material [39]. Therefore, the smashing of solid lubricant film under normal load of 4 N could be attributed to the low hardness of substrate composite containing 10% MoS₂ particles, as presented in Fig. 4. This wear mechanism generates irregular large wear debris, as shown in Fig. 13(d).

4 Conclusions

The effects of MoS₂ content (0-10%, mass fraction) on microstructure, density, hardness and wear behavior of the pure copper synthesized by mechanical milling and hot pressing of powder particles were studied. Microstructural studies showed that the continuous matrix phase changed from copper to MoS₂ when MoS₂ content increased from 2.5% to 5% in the composites. While the relative density of the fabricated samples increased continuously with MoS₂ content, their hardness values showed a peak at 2.5% MoS₂. Results of the wear tests demonstrated that with increasing MoS₂ content to the critical content of 2.5%, both the coefficient of friction and the wear loss decreased at all the applied normal loads. This was attributed to the formation of an intact lubricant film of MoS₂ on the composite surface. Higher contents of MoS₂, however, contributed to the reduction of wear resistance due to the reduced hardness of the materials. In the case of Cu/10MoS₂ composite, a sever fragmentation of solid lubricant layer was observed on the surface worn at normal load of 4 N causing instability and increasing of coefficient of friction. It was also confirmed that increasing the normal load in wear tests led to reduction of coefficient of friction regardless of MoS₂ mass fraction. Nevertheless, wear volume loss increased with increasing applied loads. The wear mechanism was found to be mostly adhesive in pure copper and a mixture of abrasive and delamination wear in Cu/MoS₂ composites.

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粉末冶金自润滑Cu/MoS₂复合材料的摩擦行为

Mohammad MOAZAMI-GOUDARZI, Aram NEMATI

Department of Materials Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran

摘  要：研究MoS₂ 含量对纯铜的显微组织、密度、硬度和耐磨性能的影响。采用纯铜粉和MoS₂粉末，通过机械球磨和热压法，制备含0~10% (质量分数) MoS₂颗粒的铜基复合材料。在干滑动摩擦条件下，采用销-盘式磨损实验装置，测试材料的耐磨性能，固定滑动速率为0.2 m/s。硬度测试结果显示，MoS₂含量为2.5% 的复合材料的硬度达到峰值。当载荷一定时，Cu/2.5MoS₂复合材料具有最低的摩擦因数和磨损量。当载荷从1 N增加到4 N，不同含量增强相复合材料的摩擦因数均减小，同时，磨损量增大。磨损表面和磨屑的SEM照片显示，Cu/MoS₂复合材料的磨损机理由纯铜的粘着磨损为主转变为磨粒磨损和剥层磨损相结合的机制。

关键词：铜基复合材料；显微组织；摩擦因数；磨损机制

(Edited by Bing YANG)

Corresponding author: Mohammad MOAZAMI-GOUDARZI; E-mail: moazami@srbiau.ac.ir

DOI: 10.1016/S1003-6326(18)64729-6