Self Lubricating Behavior of Copper Graphite Composites

Copper metal matrix composites reinforced with particulate graphite are the potential candidate materials for electrical sliding contact application such as brushes in the electric motor and generator [28-35]. In the case of low voltage and high current densities, typically for sliding parts of welding machines, it is required to employ materials with a very high specific electrical conductivity, good thermal conductivity and low friction coefficient. Such conditions are fulfilled only by copper-graphite composite material. Copper-graphite particulate composites retain combined properties of copper, i.e., excellent thermal and electrical conductivities, and properties of graphite, i.e., solid lubricating and low thermal expansion coefficient.

Fig. 17.10 Variations of wear loss of the composites with loads as a function of Ce content in the magnesiumgraphite composite [27]

Fig. 17.10 Variations of wear loss of the composites with loads as a function of Ce content in the magnesiumgraphite composite [27]

120 160 200

Load/N

Considerable efforts have been made to study the tribological performance of copper-graphite composites. Tu et al. [36] studied the tribological behavior of zinc-coated graphite-copper matrix self-lubricating composite contact strip (ZGCCS) and common graphite-copper matrix self-lubricating composite (GCCS). Figure 17.11 shows the variation of wear rate with sliding velocity and electrical current. It was observed that the wear rate of both the ZGCCS and the GCCS increased with the increment of the current density and the sliding velocity. However, the ZGCCS exhibited wear resistance superior to the GCCS, indicating the electroplating Zn on graphite played an important role in improving the wear resistance and the wear rate of the former was only one-third of the latter at the sliding velocity of 41.7 m/s and with the current density of 285 A/cm2. Arc erosion wear, oxidative wear and adhesive wear were the dominant mechanisms during the electrical sliding process. The arc erosion of the composite contact strip was remarkably reduced by the electroplating of Zn on graphite since the Zn-coated graphite particles were homogeneously distributed in the Cu matrix. Kovacik et al. [37] investigated the effect of composition on the friction coefficient of copper-graphite composites in the range of 0-50 vol% of graphite at constant load. The study was specifically done to determine the critical graphite content above which the coefficient of friction of the composite remains almost composition independent and constant. Hence, uncoated copper-graphite composite in the composition range of 0-50 vol% of graphite and coated composites with 30 and 50 vol% of graphite were prepared and their tribological properties were measured. Figure 17.12 shows the composition dependence of friction coefficient of (a) uncoated and (b) coated copper-graphite composites. It was found that with increasing concentration of graphite the coefficient of friction and wear rate of coated and uncoated composites at first decreases until certain critical concentration threshold of graphite is reached. Then the coefficient of friction of composites becomes independent on the composition and corresponds to the dynamic

Fig. 17.11 The variation of wear rate with a sliding velocity, and b electrical current [36]

Fig. 17.11 The variation of wear rate with a sliding velocity, and b electrical current [36]

coefficient of friction of used graphite material (0.15-0.16 for used graphite) while the wear rate decreases further. The results were also compared with work done by Moustafa et al. [31] on copper-graphite composites.

Efforts have also been made to study the friction and wear performance of copper-graphite composites against different counterpart materials. Ma et al. [38] investigated sliding tribological behaviors of copper-graphite composite against different counterparts, specified as 2024 aluminum alloy, AZ91D magnesium alloy and Ti-6Al-4V titanium alloy. Figure 17.13 shows the variation of friction coefficient and wear rate with sliding speed. It was found that the tribological performance of copper-graphite composite strongly depended on its counterpart materials. Copper-graphite composite could provide friction reduction in sliding against 2024 and Ti-6Al-4V. Copper-graphite composite was a good self-lubricating material in sliding against AZ91D at low speeds. The authors described that the transfer layer of copper-graphite composite on counterface is the key to friction reduction. Wear mechanism of copper-graphite composite was related to the transfer (from copper-

Fig. 17.12 Composition dependence of friction coefficient of a uncoated, and b coated copper-graphite composites [37]

Fig. 17.12 Composition dependence of friction coefficient of a uncoated, and b coated copper-graphite composites [37]

Graphite Copper Composite

graphite composite to light weight alloys) and counter-transfer (from light weight alloys to copper-graphite composite).

Studies were also made to investigate the influence of various self-lubricating particles on tribological behavior of copper-graphite composites. Chen et al. [39] studied the tribological properties of solid lubricants (graphite, h-BN) for copper-based friction composites. Copper-based friction composites containing graphite at weight fractions in the range of 0, 2, 5, 8 and 10%, corresponding to the hexagonal boron nitride (h-BN) at weight fractions in the range of 10, 8, 5, 2 and 0%, were fabricated by a powder metallurgy hot press method, respectively. Figure 17.14 shows the variation of coefficient of friction and wear rates for different normal loads. Results indicate that lubrication effects of graphite are superior to those of h-BN. With the increase of graphite content wear rates and friction coefficient decreased significantly.

Fig. 17.13 a Friction coefficients. b Wear rates of copper-graphite composite sliding against 2024, AZ91D and Ti-6Al-4V [38]

Fig. 17.13 a Friction coefficients. b Wear rates of copper-graphite composite sliding against 2024, AZ91D and Ti-6Al-4V [38]

Different heat treatments were used to improve the tribological properties of copper-graphite composites. Rajkumar and Aravindan [40] studied friction and wear properties of microwave-heat-treated copper-5 wt% graphite composites and untreated copper-graphite composites. Figure 17.15 shows the variation of friction coefficient and wear rates of untreated and microwave-treated copper-graphite composites. It was found that the microwave-heat treated composites exhibited reduced coefficient of friction and specific wear rate when compared to the untreated ones. The untreated copper-graphite composites exhibited the highest coefficient of friction and specific wear rate due to weaker interface contact. Microwave heat treating is helpful for the formation of steady graphite layer at the contact region of tribo-surface, thereby reducing coefficient of friction and specific wear rate. Further, Rajkumar and Aravindan [41] studied the tribological performance of microwave sintered copper-TiC-graphite hybrid composites. Figure 17.16 shows the variation of friction coefficient and wear rate of hybrid composite with normal load. Coefficient

Fig. 17.14 Variation of a friction coefficient, and b wear rates with loads for various copper-based composites [39]

Fig. 17.14 Variation of a friction coefficient, and b wear rates with loads for various copper-based composites [39]

of friction and wear rate of hybrid composites and unreinforced copper increases with increase in normal load. Coefficient of friction and wear rates of hybrid composites are lower than those of unreinforced copper. Wear rate of hybrid composites is reduced with increasing %TiC and %graphite, due to the cooperative effect offered by both the reinforcements. Increased content of TiC reinforcement for a given volume fraction of graphite, leads to higher coefficient of friction. Coefficient of friction of hybrid composites is decreased with increase in % graphite reinforcement. The presence of mixed layer particularly in higher content graphite hybrid composites was found to have influence on the tribological properties. Increasing sliding velocity increased the coefficient of friction and wear rate of hybrid composites. Wear mechanism involved in Cu-TiC-graphite hybrid composites for given volume fraction of graphite and 5% TiC was oxidative wear with plastic deformation. The wear mechanism operating in hybrid composite with 15% TiC and 5% graphite hybrid composites was oxidative wear with high strained delamination and it was oxidative wear with delamination wear for 15% TiC and 10% graphite. Ramesh et al. [42] analyzed tribological performance of novel cast copper-SiC-Gr hybrid composites. Coefficient of friction of hybrid composites was higher than that of copper

Fig. 17.15 Variation of a friction coefficient, and b wear rates of untreated and microwave treated copper-graphite composites [40]

and wear rates of hybrid composites are lower when compared to copper. Increased content of hard reinforcement for a given volume fraction of soft reinforcement leads to higher coefficient of friction and lower wear rates of hybrid composites.

As regards to the influence of sliding speed on the tribological performance of copper-graphite composite, Ma and Lu [43] studied the effect of sliding speed on surface modification and tribological behavior of copper-graphite composite. Figure 17.17 shows the variation of coefficient of friction and wear rates for different sliding speeds. The results showed that the friction coefficient and wear rate of copper-graphite composites are largely dependent on sliding speed. When the speed exceeds the critical value, a transition of the friction and wear regime occurs. The formation of a lubricant layer on the contact surface is regarded as an

Fig. 17.16 Variation of friction coefficient and wear rate of hybrid composite with normal load [41]

Fig. 17.16 Variation of friction coefficient and wear rate of hybrid composite with normal load [41]

important characteristic for the good tribological performance of copper-graphite composites. Due to a large strain gradient in subsurface deformation zone, the graphite-rich lubricant layer can easily form on the sliding surface when the speed is lower than the critical value. At speeds exceeding the critical value, however, formation of the lubricant layer is difficult due to the delamination wear by the high strain rate. The wear mechanism is found to be mild wear caused by ratch-etting at speeds less than the critical value and severe wear induced by delami-nation at speeds exceeding the critical value. Ma and Lu [44] also studied the effect of surface texture on transfer layer formation and tribological behavior of copper-graphite composite. Three kinds of texture, namely Parallel Groves (PG), Random Groves (RG) and Polished Surfaces (PS), were prepared on the surface of steel discs. The influences of surface textures on the friction and wear behaviour of copper-graphite composite were investigated under both low and high load condition at a fixed speed. They found that the textures had different ratcheting effects on the contact surface of copper-graphite composite and thus influence friction and wear behaviors as shown in the Fig. 17.18.

Efforts have also been made to study the tribological performance of copper-graphite composite at high temperature. Zhan and Zhang [45] studied the role of graphite particles in the high-temperature wear of copper hybrid composites against steel. Dry sliding wear properties of copper matrix composite reinforced with SiC and graphite (Gr) particles were tested in the temperature range 373-723 K. Figure 17.19 shows the variation of friction coefficient and wear rate for the two composites. It was found that the addition of graphite particles simultaneously decreased the wear rates of the composite and effectively avoided the occurrence of severe wear up to 723 K. The friction coefficient of copper hybrid composite was more stable and lower than that of SiC/Cu. It was concluded that the graphite particle was an effective addition agent for copper matrix composite applied in high-temperature sliding wear condition.

Fig. 17.17 Variation of a average friction coefficient, and b wear rate of copper-graphite composites at different sliding speeds [43]

Fig. 17.17 Variation of a average friction coefficient, and b wear rate of copper-graphite composites at different sliding speeds [43]

Various novel composites were developed to enhance self-lubricating properties for specific application. Chen et al. [46] developed new composites with 1 and 2 wt% graphite for the application of frictional bearing under the high speed low-load sliding frictional conditions. More specifically, new Cu-based self-lubricating composites with 1 and 2 wt% graphite that were prepared using atomized Cu-10Ni-3Sn-3Pb (wt%) powder as the matrix alloy with 0.5 wt% Y2O3 as the matrix. Figure 17.20 shows the variation of friction and wear rate. They found that the composite with 1 wt% graphite possesses better mechanical and frictional properties, while the composite with 2% graphite possesses better self-lubricating properties.

For use in maglev transportation system, Ma et al. [47] studied the sliding wear behavior of copper-graphite composite material using a specially designed sliding

Fig. 17.18 Variation of average friction coefficient and wear rates with sliding distance for the pin sliding against various surface textures [44]

Fig. 17.19 a Variation of specific wear rate with temperature. b Variation of friction coefficient with sliding distance for various temperatures [45]

Fig. 17.19 a Variation of specific wear rate with temperature. b Variation of friction coefficient with sliding distance for various temperatures [45]

wear apparatus, which simulates the tribological conditions of sliding current collectors in a maglev system. The material was slid against a stainless steel band under unlubricated conditions. Within the studied range of normal pressure and electrical current, the wear loss increased with the increasing normal pressure and electrical current. More specifically, at a sliding speed of 50 km/h after sliding of cumulative distance 100 km without electrical current, the wear loss under a stress of 40 N is about five times to that with 20 N [Fig. 17.21a]. The wear loss almost doubled under electrical current, compared to the case without electrical current at an applied stress of 20 N at a sliding speed of 25 km/h [Fig. 17.21b]. Adhesive wear, abrasive wear and electrical erosion wear are the dominant wear mechanisms during the electrical sliding wear processes. The authors concluded that the present results provides a better understanding principle of design suitable sliding counter parts for the current collection device in maglev systems.

Fig. 17.20 a Friction coefficient. b Wear loss of the composites with different graphite contents [46]

0.40

0.40

0.12

0 500 1000 1500 2000 2500

0 500 1000 1500 2000 2500

W (graphite) (%)

Copper-graphite composites are used in plane bearings for trucks, cranes, bulldozers and automotives; drive shafts, bearings; stoker chain, screw conveyor, roller conveyor and roller bearings; spherical bushings for automotive transmission; slider for electric cars, overhead railways, dams and flood gates; slide bushing for discharge valves in hydraulic turbines; electrical contacts and brushes.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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