3.1. Microstructure and Properties of Composites
Relative densities of about 98% or higher were measured in all sintered specimens (Table 2
). These high values are in agreement with those typically achieved by field-assisted hot pressing [13
shows typical micrographs of composites with 1–4 wt % graphite. There is no sign of porosity in the specimens. The dark gray contrast in the micrographs corresponds to graphite. Agglomerations of graphite are observed; the average size and surface density of the agglomerates were measured for each composite to study the graphite distribution in the copper matrix.
The Cu–1Gr composite has very few graphite agglomerates. However, as more graphite is incorporated into the composites, agglomerates become clearly visible and have an average size of about 10 µm. The Cu–2Gr composites exhibit a surface density of graphite agglomerates of about 1.4% (0.28 wt %). Since the total amount of graphite for this composite is 2 wt %, we can conclude that about 14% of the total graphite is poorly dispersed around the Cu grains while forming agglomerates. The surface densities of the graphite agglomerates in the Cu–3Gr and Cu–4Gr composites are very similar and slightly increased to 2.1 and 2.3%, respectively. Therefore, for these composites, ~12–14% of the graphite forms agglomerates, but the rest is well dispersed in the copper matrix. Thus, the powder processing routine followed in this work leads to a moderately homogeneous dispersion of graphite in the copper matrix.
Since graphite is a soft material, a significant reduction in hardness is realized by adding 1 wt % graphite. The hardness of the composites slightly decreases with further increases in the amount of graphite, as was expected on the basis of previous studies [3
] (Table 2
): for example, Nayak and Debata [7
] observed a decrease in hardness from 51 HV to 36 HV when the graphite content increased from 5 vol % to 15 vol %. The composites in the present study exhibit higher hardness values (ca. 70 HV) compared with the composites fabricated by cold pressing followed by 1 hour of sintering, but their values are similar to those of composites fabricated using microwave sintering, considering similar levels of reinforcement.
Samal et al. [9
] studied the hardness of Cu–graphite (1 and 10 vol %, i.e., about 0.2 and 2 wt %) fabricated by field-assisted hot pressing and conventional techniques. Our hardness measurement results for pure copper are very close to what they obtained. However, our results for the Cu–Gr composites are slightly higher than those obtained by conventional sintering. The reason for these differences in samples with the same composition likely lies in the samples’ grain size, which strongly depends on the sintering technique. Thus, the high heating rate with the short sintering time results in a fine grain size, which is responsible for the improvement in the hardness value [9
3.2. Friction and Wear Behavior
shows the SEM images of the wear tracks of the pure copper sample. In our tests, the applied normal load was 8 N using a stainless-steel ball as the counterbody. Because of the geometrical shape of the ball, the initial contact area was very limited, but as the test proceeded and the wear occurred, this contact area increased, and consequently, the stress applied to the material decreased. Therefore, a considerable plastic deformation with some adhesive wear is observed in the center of the wear track (Figure 2
a). The main wear mechanism observed in the material is plastic deformation, as clearly seen in both the SEM and 3D optical profiler micrographs of the worn track (Figure 2
a,c), with the lateral zone of the track clearly showing a pile-up effect. Also, the EDX analysis shows a clear oxygen peak in the worn track in the EDX zone marked B (Figure 2
b), confirming the existence of an oxidative wear mechanism as well, as was also found by Moustafa et al. [4
]. They studied the wear mechanism of pure copper and copper–graphite using a wide load range (50–500 N). With the low wear regime, as in the present work, they found that oxidation-delamination was the operating wear mechanism in pure copper. They also observed fine particles consisting of copper and Cu2
O from the debris of pure copper samples.
In the present work, the load used was even lower than that in the above study, promoting the formation of a mechanically mixed layer (MML) as a result of deformed oxidized copper protecting the material to be worn. Usually, when these oxides are produced between the ball and the substrate, a ‘three bodies’ wear mechanism is initiated because of the fragile behavior of the oxide, with significant increases in the wear rate and clear plowing grooves in all directions in the worn track. The plastic deformation of the Cu debris produces an adhesive phenomenon on the wear track that prevents the progression of the ‘three bodies’ wear mechanism, which is usually more aggressive than any other wear mechanism, as observed in Figure 2
In copper–graphite composites, the situation is different as a result of the presence of graphite. The role of graphite in tribological behavior can be summarized as follows: due to their softness and lamellar structure, graphite particles in the subsurface deform with the subsurface matrix and are squeezed out to the wearing surface during the sliding friction process. Then, the graphite smears the wearing surface layer by layer and mixes with the other debris detached from the contacting face. This mechanical mixing process results in the formation of an MML rich in graphite [15
]. Graphite is a natural lubricating material used in solid bearings. In the composite system reported in this work, graphite acts in the same way by introducing a soft lubricant phase between the counterpart and the worn surface. Graphite also reduces the wear effect; however, some hard particles, such as copper oxide particles, are present in the tribological system.
As previous studies have pointed out, the size, volume fraction, and homogeneity of the distribution of graphite have significant effects on the formation of the graphite-rich layer [4
]. Firstly, a finer size of graphite particles leads to a lower wear rate and friction coefficient for the same graphite concentration. For example, copper–graphite composites with 16 μm graphite particles [5
] had a lower friction coefficient than composites with 40 μm graphite particles [4
]. Moreover, Rajumar et al. [6
] reported that copper composites with 35 nm nanographite had a higher wear resistance and lower friction coefficient compared with composites containing 50 μm graphite particles. In our work, the graphite powder comprises particles with sizes lower than 40 μm, and consistent with the above, our friction coefficient values are lower than those in [4
] but higher than the values reported in [5
]. Secondly, the amount of graphite added to the matrix could directly affect the thickness and extent of the graphite-rich layer, which plays an important role in the wear behavior of composites. As the amount of graphite addition increases, more graphite is released to the wear surface smeared on the layer. Therefore, the graphite-rich layer on the contact surface becomes thicker and denser [16
]. With increased graphite content, this layer can effectively decrease the friction coefficient and the wear. Many similar results have been observed not only in the copper matrix but also in other metal matrices [17
]. However, it is confirmed that with an increasing graphite fraction, the friction coefficient and wear rate decrease until a critical threshold of graphite concentration is reached [5
]. This threshold concentration is 23 vol % (5.7 wt %) for graphite sizes between 40 and 25 μm, 12 vol % (2.7 wt %) for a graphite size of about 16 μm [5
], and approximately 15 vol % (3.4 wt %) for graphite with a nanosize of 35 nm [6
]. Beyond this threshold concentration, the friction coefficient becomes almost independent of the composition; in fact, copper–nanographite composites can even show a higher friction coefficient and wear rate due to the agglomeration of nanographite particles.
shows the wear tracks of composites with graphite content ranging from 1 to 4 wt %. In the case of Cu–1Gr (Figure 3
a), an adhesive wear mechanism in conjunction with an abrasive phenomenon on the surface is observed. Particles detached from the copper adhered to the counterpart are then transferred to the wear track, and because of the limited lubricating effect in this sample due to the small quantity of graphite, several zones of attached particles are observed in the wear track, together with some abrasion grooves (Figure 3
a). The main wear mechanism of this Cu–1Gr specimen is adhesion. In Cu–2Gr (Figure 3
b), many wide grooves along the sliding direction can be observed, revealing that abrasion is the main wear mechanism in this sample. The number and size of adhered particles are decreased in comparison with the Cu–1Gr sample. The increase in graphite content lubricates the wear track, and some debris particles are not transferred to the final track. When the graphite content increases to 3 wt % in the Cu–3Gr composite, the abrasion grooves become slimmer, as shown in the worn surface (Figure 3
c), which indicates that there is a sufficient amount of graphite to increase the self-lubricating effect at the contact surface. The adhesive wear mechanism can hardly be observed. The Cu–4Gr composite shows a smooth worn surface with small wear scars: these characteristics indicate that a thick graphite-rich layer is formed (Figure 3
d). EDX analysis was used to determine the carbon content on the worn surface. Table 3
shows the results of EDX analysis of the copper–graphite composites. The increased weight and atomic percent of carbon detected on the surface indicate that the graphite-rich layer is enhanced by increasing the graphite content.
The variation in the friction coefficient as a result of increasing the graphite content is shown in Table 2
. A sharp decrease in the friction coefficient is observed: from 0.92 in pure copper to 0.32 in Cu–1Gr. Further increasing the graphite content does not significantly change the friction coefficient, whose value is in the range of 0.29–0.3. This reduction in the friction coefficient of copper–graphite composites could be attributed to the presence of the graphite layer on the sliding surface of the wear sample since the graphite layer decreases the metal–metal contact points.
shows the changes in the friction coefficient as a function of the sliding distance for different tested specimens. The substantial noise on the curve of pure copper reflects that the contact lacks stability. Meanwhile, the Cu–3Gr and Cu–4Gr curves present little noise, which indicates a much more stable contact.
Pure copper exhibits a very low wear rate value compared with the samples containing graphite (Table 2
), where the wear rate is calculated from the weight loss of the samples before and after the wear test. Then, several cross-sectional profiles of the sample worn track were examined to investigate the reason for such wear behavior. Figure 5
shows the cross-sectional profile for pure copper; the profiles related to the composites are included for the sake of comparison. From this figure, it is observed that when the pure copper sample slides against the counterpart in a low wear regime, because of its ductility, the copper debris formed is plastically deformed on the contacting surface, then repeatedly pressed on the wear track by the counterpart, and finally transferred to the wear track. This, together with the oxidation of the copper, is the reason that the weight loss and thus the wear rate value is so small in this sample. In any case, wear damage appears on pure copper, and the wear resistance of this material is low in the tested conditions, although very little penetration of the copper by the counterbody is observed in the cross-sectional profile (Figure 5
), and because the weight loss is also small, this leads to a very low specific wear rate (Table 2
However, the presence of graphite improves the mobility of the debris as a result of the self-lubrication effect of graphite, and it is difficult for the debris to adhere to the wear surface of the composite compared with that of the pure copper sample. It is interesting that copper with 1 wt % graphite has the highest weight loss (see Table 2
). The reason for this may be that by adding 1 wt % graphite, the hardness of the composite decreases and the mobility of the debris improves, but the amount of graphite is insufficient to form a uniform graphite-rich layer. With the increase in graphite content, as mentioned previously, the wear surface is gradually covered with a graphite-rich layer, and the wear loss of the composites decreases, as seen in the cross-sectional profiles in Figure 5
. Further increasing the graphite content (2–4 wt %) has a slight effect on the wear loss of the composites. Similar results have also been reported by other researchers using the conventional sintering route [4
In Cu–3Gr, the lowest wear rate of 12.7 × 10−4
/Nm is obtained, which seems to indicate a threshold graphite concentration of about 3 wt % under our experimental conditions. The value of this threshold is in agreement with Kovacik et al.’s study [5
]. However, the existence of this threshold concentration of 3 wt % graphite could be more clearly demonstrated in our work by investigating the tribological behavior of composites containing higher graphite amounts.