Tribological behavior of microwave processed copper–nanographite composites

Tribological behavior of microwave processed copper–nanographite composites

Tribology International 57 (2013) 282–296 Contents lists available at SciVerse ScienceDirect Tribology International journal homepage: www.elsevier...

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Tribology International 57 (2013) 282–296

Contents lists available at SciVerse ScienceDirect

Tribology International journal homepage: www.elsevier.com/locate/triboint

Tribological behavior of microwave processed copper–nanographite composites K. Rajkumar, S. Aravindan n Department of Mechanical Engineering, Indian Institute of Technology—Delhi, New Delhi 110016, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 January 2012 Received in revised form 15 June 2012 Accepted 27 June 2012 Available online 4 July 2012

Excellent properties offered by nanographite particles are exploited as a reinforcement to the copper matrix. The effects of graphite particle size, spatial distribution, normal load and sliding speed on the friction and wear performance of microwave sintered copper metal matrix composites were studied using a pin-on-disc tribometer. Copper–nanographite composites show higher wear resistance and low coefficient of friction compared to copper–graphite composites. High surface area of nanographite particles embedded in copper matrix exhibited high adherent graphite tribo-layer at the contact surface. Formed graphite layer reduces the sub-surface deformation of the composite by way of reduced frictional force. & 2012 Elsevier Ltd. All rights reserved.

Keywords: Sliding wear Metal–matrix composite Solid lubricants Electrical contact

1. Introduction Copper–graphite composite is widely used as electrical sliding contacts in motor/generator applications and it is also used as bush and bearing for industrial applications [1,2]. The inherent properties of copper provide good electrical conductivity, and the graphite provides self-lubrication ability to this composite. Graphite prevents the sticking or welding during sliding by way of forming a conductive tribo-layer between the tribo-couple. Moreover this graphite layer does not increase the contact resistance between the tribo-couple [3]. Many researchers investigated the tribological properties of copper–graphite composites which were fabricated using a micron size graphite powder (5–400 mm) for the realization of self-lubrication ability in copper matrix [1,4–6]. A few researchers also reported contradictory views on using the micron size graphite particles (50 mm) which deteriorates the tribological properties of self-lubricating composites [7]. Gibson et al. [8] have reported on the improvement in wear resistance of aluminum alloys with low addition levels (2 wt%) of graphite. However, higher addition of (8 wt%) graphite weakens the alloy, causing significant yielding and increased wear rate. The graphite particles may also accelerate the rate of damage accumulation and hence deteriorate the wear resistance of aluminum alloy matrix [9]. Further, micron size graphite reinforced particles can largely affect the electrical conductivity of copper–graphite composite by hindering the continuous copper matrix network, though it has moderate electrical

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Corresponding author. Tel.: þ91 11 26596350; fax: þ91 11 26582053. E-mail address: [email protected] (S. Aravindan).

0301-679X/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.triboint.2012.06.023

conductivity. Electrical conductivity of copper–graphite composite is not only affected by the presence of larger size graphite particles at the grain boundary but also by the poor interface characteristics which leads to more electron scattering [3]. Moreover, the mechanical properties of self-lubricating composites and the formation of self-lubricating layer at the contact surface are strongly influenced by the size and distribution of the solid lubricants [3,10]. The formed graphite layer is reducing/alleviating the plastic deformation of the sub-surface of the self-lubricating composites [11]. Only a very few researchers attempted to study the effect on tribological properties of self-lubricating composites using different size of graphite particles. Kovacik et al. [10] showed better tribological properties of copper–graphite composites using 16 mm graphite particle size when compared to Moustfa et al. [1] findings who fabricated the copper–graphite composites using 40 mm with same volume percentage. The good spatial distribution of fine graphite particles in copper matrix provides a continuous self-lubricating layer at the contact surface. It is evident from the above research findings that the size reduction of graphite particle has major influence on the tribological properties of copper–graphite composites. In recent years, nanosized materials have emerged as a new alternative reinforcement owing to their exotic properties [12,13]. The application of nanoparticles in tribology has received considerable attention [14]. Handful of researchers reported that the composites with fine-size solid lubricants exhibit better tribological properties than those composites made of micron particles [15,16]. In this work, commercially available graphite nanoparticles were used as solid lubricant in copper metal matrix composites in order to obtain composite microstructure with fine lubricants.

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Nanographite is characterized by stacking of finite flat graphene sheets having open edges, whereas fullerenes and carbon nanotubes have closed surfaces. Therefore, the presence of open edges around the peripheral region adds specific features to nanographite systems which are different from their closed surface counterparts, such as fullerenes and carbon nanotubes [17]. The difference in nanographite from bulk graphite is the large contribution of edge carbon atoms in the periphery of nanographite, however the edge state governs the electronic structure [18]. The edges of the nanographite are acting as potential sites for absorbing oxygen and moisture which in turn can enhance the self-lubricating properties of graphite. Some researchers have used graphite nanoparticles as an additive in commercial oil lubricant to reduce the coefficient of friction and wear of a tribo-couple. Lee et al. [19] have reported that the nanographite (55 nm) lubricant is effective in decreasing the wear and friction of gray cast iron tribological pair compared to dry sliding. The presence of nanoparticles between the friction surfaces has reduced the contact between the tribo-couple by acting as ball-bearing spacers. The worn surface analysis indicated that the addition of nanoparticles decreased the wear resulting in a smooth surface with fewer scars. This finding indicates that there is a significant reduction in direct metal contact with the presence of nanographite particles. Generally copper–graphite self-lubricating composites are fabricated through the powder metallurgy route due to poor wetting between the constituting elements. Conventionally sintered components generally possess coarser microstructure with inherent porosity due to longer processing time [20,21]. To surpass the negative aspects of conventional sintering the development of a novel processing route like microwave sintering is necessitated. Microwave processing has gained a lot of significance in recent times for materials synthesis and sintering, mainly because of its intrinsic advantages such as rapid heating rates, reduced processing time with improved properties, finer microstructures, and environmentally more benign nature [22,23]. Moreover, microwave imparts the graphitization of nano-carbon materials and preserves the nanostructures of

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nano-sized graphite and carbon nanotube during microwave heating [24]. Thus the damage to graphite nanoparticles can be avoided during the fabrication stage of composite material. This work emphasizes on microwave sintering of copper– nanographite composites and their tribological properties under dry sliding condition. In order to study the effect of size of graphite particles on tribological properties, the obtained results were compared with the microwave sintered copper–coarse size graphite composites.

2. Experimental procedure 2.1. Materials Commercially available electrolytic copper powder (99.98% purity, average particle size 12 mm) was used as the matrix material. Graphite is one of the softer materials, hence graphite nanoparticles have been chosen as a solid lubricant material. Commercially supplied nanographite (purity: 99.9%, surface area (BET): 40–60 m2/g, density: 2.25 g/cm3) with an average particle size of 35 nm was used as a reinforcement. Graphite powder (99.8% purity level) with an average grain size of 50 mm was used as reinforcement to fabricate copper–graphite composites for comparative evaluation. Fig. 1 shows the powder morphology of copper, graphite and nanographite. Copper has a dendritic structure; graphite has flake like whereas nanographite exhibits spherical morphology. 2.2. Electroless copper coating Prone to cluster formation, poor wettability with metal matrix and non-uniform distribution are the major impediments in using nanographite (NG) as a reinforcement in metal matrix composites. The attainment of good mechanical properties of composite depends on the interfacial strength between matrix and reinforcement. In order to improve the wettability of NG reinforcement to metal matrix, as well as to reduce the inter-particle force, the

Fig. 1. Powder morphology of used materials.

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metal coating on reinforcement particles is necessitated. This coating increases the metal to metal contact between the reinforcement and metal matrix, and thereby increases the densification of composites during sintering. NG particles were purified and oxidized using nitric acid treatment by sonication for 10 min at 60 1C for further processing. NG particles were coated with a layer of copper by electroless plating using the established twostep sensitization–activation method. Prior to electroless plating, the purified NG particles were pre-treated in a sensitization solution (0.1 M SnCl2–0.1 M HCl) and activation solution (0.0014 M PdCl2–0.25 M HCl) for 30 min each. The activated NG particles were introduced into the electroless copper bath having a composition of 4.75 g/l CuSO4  5H2O, 12.5 g/l KNa(C4H4O6)  4H2O, 1% formaldehyde and 2 g/l NaOH. The solution was stirred for 30 min using ultrasonicator. The pH was maintained at 12 during the electroless coating process. Transmission Electron Microscope (TEM) was used to observe the copper coating on NG particles. TEM image of copper coated NG is shown in Fig. 2. The surface of NG was completely covered and encapsulated by the coated copper. The coating thickness was observed to be around 50 nm and well adherence of coating to the NG surface could also be observed. Copper coating can reduce the interparticle force which results in the reduced possibility of agglomeration of nanographite particles.

2.3. Fabrication of composites The powder metallurgy technique was used to fabricate the copper coated NG–copper composites with varying NG volume fraction (5%, 10%, 15% and 20%). Coated NG particles were dispersed in ethanol with vigorous sonication for 20 min. Then copper powder was introduced in ethanol solution where NG particles were suspended. Solution was stirred and it was dried at 120 1C allowing ethanol to evaporate. Dried copper powder with NG was well mixed in electric agate pestle mortar for more than 2 h. The nano-size graphite particles tend to fill in the interstices between the copper powders during mixing. The mixed powder was uniaxialy compacted in a hydraulic press at a pressure of 450 MPa, in order to obtain green disk like pellets. The green compacts were introduced into the hybrid microwave sintering setup. This hybrid setup was designed in such a way that outer envelope was transparent to microwave (alumina wool) and acting as a thermal insulator to preserve the heat during sintering. The inner envelope was an absorber (SiC susceptor) of microwaves. SiC susceptor provides hybrid heating facility, reduces thermal gradient and promotes crack free components. The specimens were sintered in an industrial microwave furnace at the temperature of 750 1C. Then sintered specimens were allowed

to cool in the furnace itself. In all the cases, the power of microwave was controlled at 640 W and the heating rate was also set within 12 1C/min. Copper–15 vol% graphite composites were also fabricated through the microwave sintering technique for the comparative study. 2.4. Characterization of microwave sintered composites The sinterability of the composite was evaluated through relative density and the procedure for determination of relative density was explained elsewhere [23]. Sintered density of the composite samples was determined by Archimedes principle. The samples were weighed using an electronic balance having an accuracy of 0.001 g. Theoretical density of sample was calculated by rule of mixture. The hardness of sintered composites was measured on a Vickers hardness testing and used load was 10 kg. Electrical conductivity of developed composites was measured using four-point electrical conductivity meter and reported in the International Annealed Copper Standard (IACS). The percentage of IACS is the standard conductivity (resistivity) used to judge the material’s property of conduction, and is based on the International Annealed Copper Standard (IACS) adopted by IEC in 1913. For each type of composition, five compacts were tested for evaluating the physical and mechanical properties and the average values were reported. Distribution of graphite particles and chemical composition of microwave sintered copper–NG samples were studied through Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Analysis (EDAX) respectively. 2.5. Tribological test Tribological properties of composites were characterized using a pin-on-disc tribo-meter under dry sliding condition. The samples have the form of cylindrical pins of 15 mm diameter and 8 mm height. Pin surfaces were prepared by grinding against 1000-grit silicon carbide paper and cleaning with acetone. The stationary pin was vertically positioned on the rotating EN 30 steel counter surface material having a hardness of 63 HRC. All experiments were conducted in ambient temperature condition. The normal loads were varied from 12 to 60 N in the step of 12 N and the sliding speeds were varied from 0.77 to 2.77 m/s in the step of 1 m/s. For each sliding conditions, three testing runs at a constant sliding distance 12,330 m were carried out. Prior to each wear test, the disk was ground by surface grinding machine to Ra 5.4 mm and followed by cleaning with acetone. At the end of wear test, the pins were weighed using a high sensitive electronic balance having an accuracy of 0.0001 g to determine the mass loss. All mass loss data were converted to volume loss using their

Fig. 2. TEM images of copper coated NG.

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corresponding sintered densities. Wear rate was calculated from the ratio of volume loss to sliding distance. Volumetric wear rates were reported from the volume losses as the basis of averaging the three wear tests. The worn surfaces and wear debris collected were examined and analyzed through scanning electron microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDAX). The temperature rise of pin during wear testing was measured using ‘Rt’ type thermocouple (accuracy 1 1C) which had been embedded at the pin’s periphery. 2.6. Tribo-layer characterization Tribo-layer formation and sub-surface deformation of worn surface were studied using a new technique of Focused Ion Beam (FIB) milling (FIB Quanta 3D machine). This technique was utilized for measuring thickness of tribo-layer and sub-surface deformation of the worn surfaces of composites. Dedicated FIB milling procedure has been followed to cut the tribo-layer in the direction perpendicular to sliding direction. Rectangular patterns of size 5 mm  2 mm with a depth of 2 mm were cut in the worn out surface using an ion beam with a current of 3 nA. The walls of cut section is cleaned with low ion beam current (3 pA) for more clear view of tribo-layer and sub-surface of wear tested composites. The walls of the cut section were viewed under Field Emission-Scanning Electron Microscopy (FE-SEM) in a tilted position at 521 to clearly view the cross-section of the tribo-layer; the sub-surface and substrate of wear tested composites.

3. Results and discussion 3.1. Microstructure of sintered composites Typical SEM images of copper–graphite (Cu–Gr) and copper– nanographite (NG) composites are presented in Fig. 3. The black areas in copper matrix are graphite particles. These graphite particles are uniformly distributed throughout the copper matrix, as seen from the copper–graphite SEM image. Near equiaxed grain morphology with finer microstructure is observed. No cracks or fissures are seen in SEM micrographs which confirm the advantages of microwave sintering. Pores are rarely observed in the micrographs. Typical SEM images of copper–NG composites with 5 and 15% show homogeneous distribution of NG in copper matrix and it also reveal lower order of porosity. As observed from micrographs, the nanographite particles are very intact with the matrix material. A good interfacial integration can be

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observed due to metallic bonding of copper particle to copper coated NG. EDAX profile of copper–15% nanographite composite shows the peaks of carbon, copper and oxygen elements. The presence of oxygen peak cannot be attributed to microwave heating since microwaves accelerate the deoxidation process due to arcing between the copper powder and edges of compacts [25,26]. It can be attributed to the acid pretreatment during copper coating of nanographite. The increase in volume fraction of nanographite beyond 15 vol% in copper matrix leads to agglomeration of particles. SEM image of copper–20% NG composites reveals the agglomerated nanographite particles in copper matrix due to high order inter-particle force. 3.2. Properties of sintered composites The properties such as relative density, sintered density, hardness and electrical conductivity of microwave sintered copper–graphite composite and copper–nanographite composites are presented with standard deviation in Table 1. It is observed that the increase in percentage of nanographite in copper matrix influences the physical and mechanical properties. There is an increase in relative density with the increase in vol% of nanographite. This can be attributed to selective coupling of microwave interaction with the nanographite particles. When the nanographite percentage is increased beyond 15% volume, the decrease in relative density is observed. This is due to the formation of larger agglomerates of nanographite particles in copper matrix. Graphite particles are having the capacity to absorb microwaves. Owing to microwave absorption of graphite particles, it is heated to a higher temperature within a short duration [27]. The nano-sized particles can absorb more microwave power when compared to micron sized particles and immediately heated to higher temperature [28]. Heat generation

Table 1 Properties of sintered composites. Composites

Relative density (%)

Sintered density (g/cm3)

Hardness (HV)

Electrical conductivity (%IACS)

Cu–15% Gr Cu–5% NG Cu–10% NG Cu–15% NG Cu–20% NG

92.3 7 0.13 95.82 7 0.14 96.41 7 0.13 96.92 7 0.12 88.42 7 0.15

7.327 0.02 87 0.02 7.787 0.04 7.437 0.03 6.377 0.06

72 71.6 94 71.9 90 71.0 81.5 71.6 56 72

65 71.5 79.8 71.9 72.4 71.0 70 71.3 38.7 72.5

Fig. 3. Typical SEM images of copper–graphite (Cu–Gr) and copper–nanographite (Cu–NG) composites.

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in the electrically conducting particles is characterized by penetration depth. Heat generation occurred when the penetration depth is equal to or less than the constituent powders to be processed in microwave sintering. Penetration depth of graphite particles is in the order of 30 mm [23]. However, the nanographite particle size used in this work is 35 nm which is less than penetration depth of graphite. Hence the nanographite particles are volumetrically heated to higher temperature. Carbon nanoparticles and carbon nanotubes are the efficient absorbers of microwaves [29]. Due to uniform distribution of nano-sized graphite particles in copper matrix, the absorbed heat by the nanographite is transferred uniformly to surrounding matrix. Simultaneously dendritic structure of electrolytic copper powder can couple well with the microwaves [23]. Subsequently the matrix material can be heated volumetrically within a shorter duration. This phenomenon prevails upto 15% nanographite particles reinforced composite. Owing to rapid heating, neck growth of copper particles and their coalescence leads to faster densification. The heating of copper particles and reinforcements takes place through Joule’s effect caused by electromagnetically induced electrical current loss in the constituent particles due to the electrical conductive nature [22]. The high surface area of graphite nanoparticles has the possibility of more number of active atoms in their surface which results in higher order of Joule heating than the micron size graphite particles. When the volume fraction of nanographite goes beyond 15%, more availability of nanoparticles in the matrix has resulted in very low inter-particles distance which leads to formation of larger agglomerated particles. The size of the agglomerated particles is approximately equal to the micron size graphite particles and comparatively the agglomerated particles are weakly bonded. It could affect the microwave absorption during sintering and thereby reduction in microwave coupling. Ultimately it decreased the relative density of 20% nanographite composite. This is supported by the findings of Janowska et al. [30] that the agglomerates of the carbon particles reduce the absorption capacity of microwaves which produced the low order graphitization of carbon particles. From the Table 1, it is observed that density of the composites decreases with the increase in volume percentage of reinforcement due to lower density of the reinforcement. Copper–15% NG composites exhibited higher density than the copper–15% graphite composites. Due to relatively strong microwave absorbing capacity nanographite enhances the heating capability of composites during sintering. Also nanographite particles tend to occupy the very small size pores occurred during the sintering stage. It has resulted in the reduced porosity for the copper–nanographite composites. Copper–NG composites exhibited higher hardness value when compared to the copper–graphite composites for the same volume percentage, as seen from Table 1. This could be attributed to faster microwave heating or higher rate of volumetric heating which leads to finer refined microstructure of copper–nanographite composites. The increase in volume percentage of NG in copper matrix leads to reduction in the hardness of composites. Graphite is a well known soft material. Geim and Novoselov [31] inferred that the mechanical strength of graphite nanocrystallites reinforced composites could not match with carbon nanotube reinforced composites due to lower order reinforcement efficiency. Accordingly reinforcement of nanographite in copper matrix leads to reduction in the hardness. Similar reduction in hardness of the aluminum–nanographite was reported with increasing amount of nanographite in aluminum matrix [32]. Due to agglomeration effect at 20% NG composites, the hardness is relatively lowered and this is attributed to its higher porosity. The electrical conductivity of copper matrix decreases with increase in nanographite volume percentage. Generally, a hindrance in the continuous copper network strongly influences the electrical

conductivity of a particulate-reinforced copper matrix composite. Copper–NG composites exhibited higher electrical conductivity when compared to copper–graphite composites due to relatively higher electrical conductivity of nanographite and strong interface between copper matrix and nanographite reinforcements as a result of copper coating of reinforcements. Graphite particles occupy the grain boundaries, however the small pores are not filled by micron sized particles. In the case of nanographite particles due to size effect the very small pores are filled with these nanosized particles and thereby results in reduced electron scattering. Moreover uniform distribution of nanographite particles acts as an isolated particle in the high conductivity copper phase. It does not affect the continuity of copper phase which leads to higher electrical conductivity. Deprez and McLachlan [33] demonstrated that graphite compact made of small grain size (1.6–32 mm) exhibited the higher electrical conductivity than coarser one (20–160 mm). Due to exotic electrical properties of nanographite, the electrical conductivity of composites is not adversely affected. Umar et al. [34] claimed that few staked layer of graphite exhibited extraordinary electrical properties. When volume percentage is increased the possibility of hindrance to conductivity copper phase is also increased. Finally it results in the reduction in electrical conductivity. This is supported by Kovacik and Bielek [35] findings that the electrical conductivity of copper matrix is decreased with the increased amount of graphite. Electrical conductivity of copper– nanographite composites is deteriorated rapidly beyond the 15% of reinforcements due to agglomeration of nanographite particles which largely affect the continuous network of copper phase. It is understood that electrical conductivity of copper–nanographite composite is much higher than those of copper–graphite composites. It shows that nanographite is one of the promising reinforcements to copper for the electrical sliding applications. 3.3. Friction behavior Variations of coefficient of friction with normal load for the copper–graphite and copper–nanographite composites are shown in Fig. 4. It is observed that coefficient of friction is high in the initial unsteady state and relatively lower at the later steady zone. Initial coefficient of friction and steady state coefficient of friction are influenced by changes in the nature of contact between tribocouple resulting from initial metal–metal contact to tribo-contact separated by graphite layer. It is observed that copper–nanographite composites exhibited lower coefficient of friction when compared to 0.34

Cu-15% Gr Cu-5% NG Cu-10% NG Cu-15% NG Cu-20% NG

0.32 0.30 Coefficient of friction

286

0.28 0.26 0.24 0.22 0.20 0.18 0.16 0

12

24 36 Normal Load (N)

48

60

Fig. 4. Coefficient of friction with normal load at sliding speed 0.77 m/s.

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copper–graphite composite for the same volume percentage. The nanographite in copper matrix significantly decreased the friction coefficient even below 0.1. It is understood that degree of selflubrication is influenced by the size of the particle size and nanographite offered this great self-lubrication level to copper matrix under all sliding conditions. Owing to formation of highly adherent and continuous graphite layer at the sliding surfaces, the coefficient of friction is reduced to a greater extent. The formation of adherent graphite layer is related to many factors such as the distance between graphite particles (inter-particle distance), surface area of the particles, moisture adsorption capacity, penetration into contact surface, and cohesion between the graphite layer and adhesion to the counter surface material. Nanographite is transferred in the form of layer by layer along the sliding direction and completely smears at the contact zone with normal load which eventually forms the graphite layer. Despite lamellar crystalline structure of graphite, the lubricating property of graphite is not intrinsic. The requirement for the self-lubrication of graphite particles is to

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absorb the moisture or water vapor at an adequate level. Water vapor reduces the shear strength of graphite along the basal plane by easy preferential cleavage [36]. Due to more open edges and large surface area of nanographite having more activated sites, almost all nanographite particles can absorb the available water vapor from the environment. This absorbed water vapor to nanographite imparts a good self-lubricating capacity. In the case of nanographite, rich presence of moisture can help shearing along the basal plane without breaking the covalent bond. In contrast, low surface area of graphite particles can absorb relatively lower amount of moisture. It results in higher order of transverse breakage of basal plane or fragmentation of graphite particles which produces more dangling bonds. These are possibly increasing the adhesive energy with sliding surfaces and this leads to increased coefficient of friction for the copper–graphite composites. In the case of copper–nanographite composites, low friction may also result from the complete deactivation of all the dangling bonds which are created during sliding, because of the more absorbed moisture.

Fig. 5. (a) Distribution of nanographite in matrix, (b) distribution of graphite in matrix, (c) contact profile nanographite composite, (d) contact profile of graphite composite, (e) and (f) conceptual wear generation model for nanographite and graphite reinforced composite respectively, (g) and (h) typical wear debris at 48 N and 0.77 m/s for copper–nanographite and copper–graphite respectively.

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The adherent graphite layer formation is attributed to continuous smearing of very fine particles at the contact zone. Nanographite from composite is squeezed out to the contact zone under the action of normal load. By virtue of its smaller size, particles could have penetrated deep inside the asperities of composite and counter surface. Further, mechanical comminution of nanographite particles also could have occurred between the sliding surfaces during the wearing process. During sliding, the comminuted nanographite and nanographite particles could have filled most of the asperities of the pin surface with subsequent formation of graphite layer at the pin–disc interface. This process is continued, upto the formation of thick adherent graphite layer, and then mechanical comminution of nanographite particles at the contact zone is slowed down. Conversely, micron size graphite particles also underwent similar mechanical comminution process; however it could not penetrate into the very narrow grooves which formed during the wearing process or gaps between the asperities of sliding contact easily [37]. Hence its effect on improving anti-wear properties was not remarkable. In fact, formed graphite layer is well adhered to the steel counter surface due to higher order absorbed moisture to reduce the surface energy of graphite layer to steel counter surface [2]. The adsorbed moisture can reduce the cohesion between the graphite layers and aids the easy smearing of graphite at the contact zone. Combined effects of these things lead to formation of a highly adherent graphite layer at the contact zone. This graphite layer is capable of sustaining the higher loads also. It is observed that the coefficient of friction decreases with increase in the amount of nanographite in copper matrix. Presence of graphite layer and coverage of graphite layer at the contact zone are directly proportional to the amount of graphite particles at the sliding surface and distribution of graphite particles in copper matrix. It is also seen from Fig. 4 that the coefficient of friction for lower amount of nanographite composites is steadily increasing with the normal load. Steady increase in coefficient of friction is attributed to low availability of selflubricating layer at the contact zone. It leads to more number of metal to metal contacts between the copper matrix and steel counter surface. It results in the higher coefficient of friction for the copper–5% nanographite composites. It can be noted that the addition of sufficient amount of nanographite particles in the copper matrix decreases the steeper rise of the friction coefficient with normal load. As the volume fraction of nanographite increased, the coverage of graphite layer at the contact zone is also increased. It provides significant self-lubricating ability to copper–10 and 15% nanographite composites which reduces remarkable level of frictional force. Also it is observed that there is a gradual increase in the coefficient of friction with normal load. This is attributed to the increased amount of copper wear debris at the contact zone. Further, abundantly available graphite nanoparticles in the copper matrix having lower mean free path (distance between the two particles) between the graphite nanoparticles (Fig. 5a) when compared to same volume fraction of micron size graphite particles (Fig. 5b). It could produce relatively smaller size asperities and also less space between the asperities which can be completely filled by nanographite particles during the wearing process as shown in Fig. 5c, whereas graphite reinforced composite produce the larger size asperities (Fig. 5d). This completely filled nanographite particles apparently produce the more continuous graphite layer that reduces the direct contact between the composite pin surface and steel surface. Thus formed high adherent graphite layer reduces the frictional coefficient. Finally it reduces the wear debris size, as shown in Fig. 5e when compared to graphite reinforced composite (Fig. 5f). These findings can be confirmed by wear debris analysis through SEM. Fig. 5g shows the

SEM image of wear debris morphology of 15% nanographite at 48 N and 0.77 m/s. The wear debris particles are smaller in size and has flake shaped particles morphology due to higher amount of graphite self-lubrication. The wear debris observed for copper– graphite composite is larger and equiaxed, as shown in Fig. 5h. This indicates that the 15% graphite composite has undergone significant plastic deformation. Owing to pull out micron size graphite particles and subsequent fracture of copper grains which can lead to the formation of large wear fragments from the composite pin surface. In the case of 20% nanographite, due to large amount of agglomeration, the graphite cluster forms in an isolated region of the copper matrix which leads to incomplete spreading of graphite at the contact zone. Larger amount fracture of copper grains at the sliding surface is the reason for the very high coefficient of friction. The severity of fracture of copper grains increases with the increase of normal load which in turn results in increasing trend of coefficient of friction. 3.4. Wear behavior Fig. 6 shows that the variation of wear rate of copper–graphite and copper–nanographite composites with the normal load. Copper–10% and 15% nanographite composites exhibited the lowest wear rate for all the range of normal loads. It is understood from the figure that copper–15% graphite composites is showing two times higher wear rate than copper–15% nanographite composite. This increased wear rate is attributed not only to the lower order interfacial strength between the copper and uncoated graphite but also to the relatively lower hardness, as evident from the Table 1. Higher hardness, lower porosity and finer microstructure are attributed to improved wear resistance of nanographite reinforced composites. It is observed that the wear rate of copper–nanographite composites decreases with increase in the nanographite content. Similar results have also been reported by investigators using graphite reinforced copper composites processed by the conventional sintering route [1,10]. The wear behavior of composites is mainly influenced by the volume content of nanographite, as observed from the Fig. 6. The wear rate of 5% and 20% nanographite composites shows the linear trend with normal load. In the 10% and 15% nanographite composites, wear rate with normal load is not following the similar trend. This is due to the nature of self-lubrication at the 14 Cu-15% Gr Cu-5% NG Cu-10% NG Cu-15% NG Cu-20% NG

12 Wear rate X10-4 (mm3/m)

288

10

8

6

4 0

12

24

36

48

60

Normal Load (N) Fig. 6. Wear rate of composites with normal load at sliding speed 0.77 m/s.

contact surface and relative densification of nanographite composites. Self-lubrication is mainly affected by volume content, spatial distribution and size of graphite particles [10]. When the nanographite is at the lowest percentage (5 vol%), the ability of forming graphite layer is inadequate at the contact zone which increased the metal to metal contact. The possibility of more metal to metal contact is also increased with increase in normal load. Consequently, hard asperities of counter surface material tend to plow in the surface of the composite pin. This action is severed with the increase in normal load. It results in the steady increase of wear rate of composites with normal load. Hence the wear rate is in the highest order for 5 vol% nanographite composites when compared to 10 and 15% nanographite reinforced composites. The formation of graphite layer is evident at the contact zone upto 15% of nanographite which results in the improvement of wear resistance of composites. Owing to size effect of the nanographite particles the availability of more graphite individual particles at the contact zone is increased. It results in low interparticle distance between the nanographite particles. It provides the complete coverage of graphite layer due to the smearing of nanographite at the sliding surfaces. However the steady formation of graphite layer is influenced by normal load and sliding conditions. In the case of 10% and 15% nanographite, the wear rate is marginally increasing from 12 N to 48 N, as observed from Fig. 6. The wear rate of composites is moderately increased beyond the normal load of 48 N. The applied load is increased from 12 N to 48 N, the extrusion of graphite particles from the sub-surface of copper matrix to sliding surface is increased. These extruded graphite particles smear on the sliding surface along the sliding direction which fairly provides the complete graphite layer at the sliding zone. It could reduce the plastic deformation of composite pin further by reducing the rough mating between the tribo-couple. Accordingly, the wear rate of copper–nanographite composite is marginally increased with the increase in normal load from 12 N to 48 N. The formed graphite layer can be affected by increased normal load beyond 48 N. Graphite layer may, however, loose its lubricating properties due to rupturing of formed graphite layer by the hard asperities of counter surface material during sliding. Further it could expose the virgin composite material to sliding zone which in turn increases metal to metal contact. In this sliding condition, the wear rate is increased moderately due to combined action of smearing of graphite particles and fracturing of copper grains under higher load. The increased local pressure exerted on the copper asperities by the counter surface material leads to fracture of copper grains resulting in copper wear debris. The fracturing of copper grains at higher load is at higher order compared to smearing of the graphite particles in the sliding surface. The mixing of smeared graphite particles and copper debris would occur during the subsequent sliding. However, rich presence of small sized copper debris in graphite layer with hard asperities of counter surface abrades the copper matrix surface which certainly leads to increased wear rate of composites. In the case of 20% nanographite composites, due to severe agglomeration of nanographite particles and presence of higher level porosity in composites result in deterioration of mechanical properties. These are attributed to the increase in wear rate with increase in normal load for the 20% nanographite composites. 3.5. Effect of sliding speed Variation of friction coefficient of copper–nanographite composites with sliding speed is shown in Fig. 7. In the case of 5% nanographite, the coefficient of friction is decreased with sliding speed up to 1.77 m/s, and then starts to increase slightly with

Coefficient of friction

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0.38 0.36 0.34 0.32 0.30 0.28 0.26 0.24 0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06

289

5% NG 10% NG 15% NG 20% NG

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Sliding speed (m/s) Fig. 7. Variation of coefficient of friction with sliding speed at 36 N.

sliding speed, as observed from Fig. 7. When the sliding speed is increased beyond 1.77 m/s, the formed thin and less adherent graphite layer is likely to peel off from the surface of the pin. It results in the depletion of self-lubricating layer at higher speed. As a consequence, the coefficient of friction is increased slightly at higher sliding speed. There is a difficulty in forming the lubricant layer at high sliding speeds due to non-availability of sufficient nanographite particles at the contact surface. In the case of 10–15% nanographite, the coefficient of friction decreases with the increase in sliding speed. Coefficient of friction of composites attains the steady state value beyond 1.77 m/s. The coefficient of friction of copper–nanographite is not affected by the sliding speed especially for the composites having more nanographite content, since the contact surface is covered with the highly adherent graphite layer. Hence the coefficient of friction of composite shows very small variation with change in sliding speed. However this reason is only valid for the uniformly distributed nanographite in copper matrix. Similar observation is made on copper–graphite composites when wear tested under different sliding velocities i.e coefficient of friction was independent to sliding velocity [38]. With a view to the friction coefficient, at lower sliding speed, the formation of graphite layer is not continuous as seen from SEM image of worn surface, Fig. 8a. The formation of steady and continuous graphite layer is increasing with increase of sliding speed as evident from SEM image of worn surfaces, Fig. 8b and c. However the copper–20% nanographite reinforced composite is showing the continuous increase in coefficient of friction with sliding speed. The poor mechanical property leads to higher order of copper grain fracture. This action is intensified with increase in the sliding speed, finally large scale presence of copper wear debris at the contact zone is attributed to the continuous increase of coefficient of friction. 3.6. Wear mechanism SEM micrograph of worn surface of copper–graphite composite shows a distinct characteristic of wear scar, as shown in Fig. 9. SEM image of copper–graphite worn surface at lower load shows many slim and deep wear grooves along the sliding direction due to mild plastic deformation occurred during sliding, as shown in Fig. 9a. These grooved lines on the worn surface indicate the occurrence of plastic deformation. During sliding, considerable amount of plastic deformation occurred in the matrix which results in the exposure of the graphite particles to the contact

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Fig. 8. SEM image of worn surface of copper–15% nanographite composites at different sliding speeds at 36 N (a), 0.77 m/s (b) 1.77 m/s and (c) 2.77 m/s.

Fig. 9. SEM images of worn surface of copper–15% graphite composite: (a) 12 N and 0.77 m/s, (b) 60 N and 0.77 m/s, (c) and (d) high magnification SEM image of wear track, (e) EDAX of worn surface of copper–graphite 12 N and 0.77 m/s and (f) EDAX of worn surface of copper–graphite at 60 N and 0.77 m/s.

surface. These graphite particles smeared at the sliding surface under the normal load along the sliding direction form a thin graphite layer. This thin graphite layer covered at the worn surface prevents further plastic deformation of the matrix. Plastic deformation is observed to be increased with the increase in normal load. It leads to the formation of many wider wear grooves on the worn surface at higher load, as seen from Fig. 9b. The grooves observed in Fig. 9b is magnified and observed to have clear understanding. The wear track and ploughing of materials are observed from Fig. 9c and d. Due to increased localized pressure, the formed thin graphite layer ruptured during sliding and again fresh exposure of graphite particles at the contact surface is observed. Formation and rupture of graphite layer are repeated during the entire wear test. Corresponding EDAX results of the worn surface of copper– graphite composites at 15 N and 60 N are shown adjacent to SEM micrographs. The compositional analysis of the worn surface shows that the worn surface has a mixture of ingredients from composite pin, counter surface material and tribo-oxidized products. Peaks of copper and carbon and the residual peak of O and Fe are observed. Copper–graphite composite tested at 12 N shows a relatively low intensity Cu peak, high intensity peak of C, low intensity peaks of Fe and O when compared to 60 N normal load. Higher intensity peak of C indicates the presence of relatively higher amount of smeared graphite particles at the worn surface. It reduces the plastic deformation of copper matrix which is evident from low intensity peak of Cu in EDAX profile, as seen in Fig. 9e. These are indicating that the occurrence of lower order of plastic deformation of copper matrix at lower load (12 N). Owing to plastic deformation of matrix, the copper debris was formed at

the contact zone. This entrapped copper debris is abrading the matrix and counter surface materials. The appearance of O and Fe peaks in EDAX is attributed to the oxidation of the worn surface and occurrence of mild plastic deformation of counter surface material. Subsequently these products are transferred to the tribo layer of worn surface. The ploughed mark on the worn surface at 60 N can be seen from Fig. 9b. It leads to considerable plastic deformation of copper matrix that results the high intensity Cu peak as shown in Fig. 9f. The presence of Fe implies the transfer of counter surface materials to the contact surface while the oxygen peak indicates the oxidation reaction in the layers. SEM micrograph of worn surface of copper–5% nanographite composite and corresponding EDAX profile is shown in Fig. 10. At lower load, worn surface is having many slim wear grooves as seen from Fig. 10a. The lack of self-lubricating ability is shown by this composite due to insufficient nanographite particles which is ineffective in fully preventing the direct contact. This effect deteriorates the friction and wear performance. It is also observed that there is a patch like covered graphite layer at the worn surface. During sliding, the mixed mode of contact between the tribo-surface has occurred like metal to metal where no graphite layer exists and contact of tribo-surface separated by graphite layer. Under this condition, the softer copper matrix is subjected to mild plastic deformation. The availability of graphitic layer at the contact surface is scarce at increased normal load which results in reduced lubrication level and thereby increased metal to metal contact. Hence the localized stress on the matrix material increases which further results in plastic deformation of matrix during sliding. It eventually produces many deeper and wider wear grooves at the worn surface, as shown in Fig. 10b. Due

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Fig. 10. SEM image of worn surface and corresponding EDAX of copper–5% nanographite: (a)12 N and 0.77 m/s and (b) 60 N and 0.77 m/s.

to insufficient self-lubrication at 5% nanographite composites showed plastic deformation as the wear mechanism at all ranges of normal load. It can be confirmed by EDAX analysis of worn surface. Corresponding EDAX of worn surface of copper–5% nanographite at 12 N and 60 N at 0.77 m/s are shown adjacent to worn surface. At lower load (12 N), the EDAX of the worn surfaces showed a very low intensity peak of C (0.82 wt%), high intensity peak of copper (90.14 wt%) and O (8.01 wt%) and Fe (1.03 wt%) intensity peaks. The low intensity peak of carbon indicates the lack of formation of graphite film at the worn surface. Hence this composite cannot provide self-lubrication. The presence of high intensity copper peak indicates the plastic deformation of copper matrix during sliding. At higher load (60 N), increased intensity of copper peak, Fe and O peaks are observed. It indicates that a significant amount of plastic deformation is occurred in composite microstructure and counter surface material. Similarly counter surface material is transferred to the formed tribo-layer. It confirms that plastic deformation is the operating wear mechanism for copper–5% nanographite composite. It is noted that 10 vol% nanographite additions are sufficient to produce the self-lubricating effect at the contact surface. This could reduce the plastic deformation level of composite matrix. Fig. 11 a and b shows typical SEM micrographs of the worn surfaces of copper–10 vol% nanographite composites tested at the sliding speed of 0.77 m/s. The examination of the worn surfaces at lower load (12 N) showed that the worn surface of the composite is very smooth and covered with graphite layer than at the higher load, as observed from Fig. 11a. It is observed that nanographite particles are comminuted between the tribo-surface under the load, gradually fill up the gaps between the asperities or wear grooves of composite pin and counter surface. The filled graphite nanoparticles in the grooves are loose particles. As a consequence of continuous sliding with normal load, these loose particles are tightly packed with themselves. It forms an adherent film over the contact surfaces which results in smoother surface than that graphite reinforced copper composites. Owing to formation of continuous thick graphite layer at the contact surface, it prevents the metal to metal contact. Finally it leads to reduction in plastic deformation of pin surface. The observed mild plastic deformation is found to be increased with increasing normal load. Especially at higher loads, tear out of formed graphite adherent layer on worn surface is observed, as seen from Fig. 11b. In some regions in the

worn surface the graphite layer spalling pit or delaminated scar is observed. Delaminated feature size is relatively smaller at lower load. Delaminated scar size is observed to be increased with increase in normal load. The mechanism of spalling or delaminating of formed graphite layer is peeling of thin sheet or detachment of flake like wear debris from the worn surface of composite pin. Formation of flake like debris has occurred due to wear process proceeded by the formation of sub-surface cracks initiated from the graphite particles or at the weaker sites of interface of graphite and matrix. These cracks are propagated to the contact surface, eventually joined together to form a thin sheet of wear debris. Corresponding EDAX of worn surface of copper–10% nanographite composites at 12 N and 60 N at 0.77 m/s is shown in Fig. 11. Both the EDAX profiles show a high intensity peak of carbon which indicates the formation of an adherent graphite self-lubricating layer. The formed graphite layer reduced the plastic deformation and results in reduced copper peak. At higher load (60 N), the intensity of carbon peak is reduced due to tear out of formed layer. This turns out in rise of copper peak due to fresh exposure of virgin matrix material at the contact surface. Fig. 12a and b shows the worn surface of copper–15% nanographite composites at the sliding condition 12 and 60 N for 0.77 m/s respectively. As compared to copper–15% graphite composites, copper–15% nanographite shows very smooth worn surface with completely covered graphite layer and also wear feature size is smaller. Similar results have also been reported by JolyPottuz etal. [39] that wear scar of the steel tribo-couple was 20–30% lower for the carbon nano-onions (mean diameter 10 nm) additive lubrication condition when compared to graphite (60 mm) additive for all ranges of pressures. Delaminating scar in the worn surface was observed at 12 N from the higher magnification of SEM images. The delaminating scar size is increased with increase in normal load. Corresponding EDAX profile of 12 and 60 N is also shown adjacent to Fig. 12. EDAX profiles show a very high intensity peak of carbon and considerable low level intensity peak of copper. This result confirms the formation of a very thick adherent graphite layer for 15% nanographite composite. This thick adherent graphite layer is compacted between the tribo surfaces which renders the reduced plastic deformation of composite pin even for increased normal load, as observed from EDAX profile. There is no increase in copper peak as compared to lower load. Thus nanographite reinforced copper composites shows a higher load carrying

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Fig. 11. SEM images of worn surface and EDAX of copper–10% nanographite composites: (a)12 N and 0.77 m/s and (b) 60 N and 0.77 m/s.

Fig. 12. SEM images of worn surface and EDAX of copper–15% nanographite composites. (a)12 N and 0.77 m/s and (b) 60 N and 0.77 m/s.

capacity. The macro appearance of the worn surface of 15% nanographite composites is similar to copper–10% nanographite composites at low load and high load. Hence the operating wear mechanism for copper–15% nanographite composites is delamination of graphite layer. Typical SEM micrographs of the worn surfaces of copper–20% nanographite composites are shown in Fig. 13. These worn surfaces show the irregular and wider grooves along the sliding direction. This composite shows poor mechanical and physical properties due to higher order of agglomeration. Large amount of fragmented copper debris from composite are entrapped between the sliding surfaces and subsequently work hardened. This resembles the third body abrasion to composite pin surface. The hard asperities of counter surface material is also abraded the composite surface. This combined action results the severe damage to worn surface. The severity of formation of irregular grooves is also increased with increased normal load.

By comparing the EDAX analysis of the worn surfaces of copper–graphite and copper–5% to 15% nanographite composites, the amount of Fe observed also is increased substantially with increasing normal load. During the sliding process, fracture of copper grains from the composite’s matrix is occurred and the wear debris is formed. Copper wear debris are entrapped between the tribo-surface. As a consequence of continuous sliding, these wear debris abrade the counter surface material. Subsequently the very low level of plastic deformation is occurred in counter steel surface due to higher hardness. This action is more severe with increase of normal load and it leads to transfer of Fe into tribo-layer of the worn surface. The amount of Fe transfer is decreased with increase of nanographite percentage at a given normal load. Increased nanographite volume fraction results in more coverage of graphite layer at the contact surface. It reduces the tendency of plastic deformation of counter surface material. Hence, the Fe transfer to tribo-layer of the worn surface is

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Fig. 13. SEM images of worn surface of copper–20% nanographite composites. (a)12 N and 0.77 m/s and (b) 60 N and 0.77 m/s.

decreased. In all EDAX results of composites worn surface of copper–graphite and copper–5% to 15% nanographite, the presence of oxygen peak is observed. This indicates that there has been some oxide formation of the formed tribo-layer. The direct contact of copper matrix with the counter surface is prevented due to the formation graphite layer at lower load. When the normal load is increased, considerable deformation of copper has occurred and transfer of copper to counter surface is also observed at 5% nanographite composites. Deformation of copper matrix increases the temperature rise of the pin which increased local oxidation of copper. During sliding of composite with steel counter surface, the combined action of temperature rise and environment reaction accelerated the oxidation of the formed tribo-layer. In the case of 10–15% nanographite composites, more nanographite particles exposed at the contact surface lead to form the self-lubricating tribo-layer. This prevents the direct oxidization of copper matrix instead nanographite itself readily reacts with the environmental oxygen due to larger surface area of nanographite. In fact, the fragmented copper and fine wear debris from composite and counter surface are oxidized. The operating wear mechanism of copper–5% nanographite composites is observed to be oxidative with plastic deformation at all ranges of normal load. The operating wear mechanism of copper–10% and 15% nanographite composites is observed to be oxidative with delamination at lower and higher load. 3.7. Tribo-layer analysis FIB milled cross-sectional images of worn surface of copper– nanographite are shown in Fig. 14. Fig. 14a shows the typical FIB milled trench in the section of the wear track of copper– nanographite composites. Fig. 14b shows the enlarged view of a section of wear track of copper–5% nanographite composites. It shows a more clear view of tribo-layer and sub-surface deformation of the section. FE-SEM image shows the presence of graphite layer at the worn surface. The plastic deformation of the material near the worn surface could be observed at all normal loads. It is observed that a large gradient of plastic deformation occurred in 5% nanographite composite due to the limited self-lubrication capacity. This indicates that the surface material has undergone relatively larger strain. The high order of sub-surface deformation has occurred with increased normal load, as observed from Fig. 14c. Fig. 14d and e shows the enlarged view of a section of wear track of copper–15%nanographite composites at 12 N and 60 N respectively. Fig. 14d shows the fully covered and considerable thickness of graphite layer on the worn surface and also low level sub-surface deformation. The thickness and the area covered by the graphite layer is increased by increasing nanographite

graphite content. During sliding the graphite layer formation on the worn surface is attributed to deformation of bulk matrix along with transferred graphite to contact surface. The increase in nanographite percentage in copper matrix can form the adherent graphite layer and it reduces the friction force thereby, the strain gradient which ultimately leads to a reduction in sub-surface deformation. Ma and Lu [38] reported that the formed graphite layer could reduce the sub-surface deformation of copper– graphite composite. Also this self-lubricating graphite layer is a main contributor to the reduction in coefficient of friction and wear rate since it prevents the direct contact with counter surface. It reduced the shear stress transmitted from the contact surface to the subsurface during sliding. Since the transmitted shear stress from contact surface to the subsurface is reduced, the wear debris detaches as an ultra thin surface layer. Graphite layer thickness is decreased, and consequently the sub-surface deformation is increased with increase in normal load, as observed from Fig. 14e. This can be attributed to the large stress concentration in the copper matrix when tearing off the existing graphite layer in the contact zone and subsequently underwent considerable plastic deformation which results in increased sub-surface deformation. 3.8. Effect of temperature rise Transformation of frictional energy to heat can raise the temperature of tribo-couple and that may affect the performance. Also it might have influenced on the self-lubricating action at the contact zone. The temperature rise (1C) of the composite pin is defined as:

DT ¼ T composites 2T

ð1Þ

where DT is the temperature rise in 1C, Tcomposites is the composite pin temperature in 1C at the end of wear test and T is the wear test starting temperature in 1C. The composite pin temperature is not necessarily the hottest spot or flash point temperature during sliding. Friction temperature rise of tribo-couple is a complex process and many factors such as contact area, normal load, sliding speed, thermal conduction and properties of intermediate tribo-layer can affect it. Sliding contact area (real contact area) is increasing with increase in normal load which increases the temperature rise of sliding body. Generally increasing in sliding speed will increase the temperature rise of sliding body. Among these, temperature rise is largely affected by the formed graphite layer. This intermediate graphite layer at contact zone lowered the coefficient of friction; thus reduced metal–metal contact, which resulted in less generated heat. Fig. 15 shows the temperature rise of the composites pin at sliding speed of 0.77 m/s due to frictional

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Fig. 14. Typical FE-SEM images of FIB milled copper–nanographite composites. (a) Typical FIB milled trench, (b) 5% nanographite at 12 N and 0.77 m/s, (c) 5% nanographite at 60 N and 0.77 m/s, (d) 15% nanographite at 12 N and 0.77 m/s and (e) 15% nanographite at 60 N and 0.77 m/s.

heating between the tribological pair. Temperature rise observed to be increasing with the normal load for all the composites. The temperature rise is observed to be decreased with the increase of nanographite. The reduction in temperature rise due to increase in graphite layer thickness at the contact zone, as evident from Fig. 14c and e. Similar findings by Kestursatya et al. [5] for centrifugally cast copper–micron sized graphite composites were reported. The formed graphite layer reduced the temperature rise of the composite. It is also observed that copper–nanographite composites exhibited smaller temperature rise when compared to copper–graphite composites, as same volume percentage. Due to relatively higher thickness of the graphite layer, quick transfer and dissipation of frictional heat to the environment from the contact surfaces, bulk material is attributed to the high thermal conductivity of nanographite and copper. It suggests that the finer size of graphite solid lubricant particles not only improves the self-lubrication level of composites but also composites pin runs

at a cool condition. Better tribological properties showed by nanographite particles at 15%, however to reach the same effect, higher percentage of the micron sized graphite particles is required. Adding higher amount of micron sized graphite particles to metal matrix negatively impacts the electrical conductivity and mechanical properties. It is observed from this research work, copper–nanographite composites are the best suited ones for the high technology application such as self-lubricating bearing and electrical sliding contact in terms of high endurance to normal load, better electrical and mechanical and better tribological performance.

4. Conclusion Copper–NG composites exhibited higher physical and mechanical properties when compared to the copper–graphite composites

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28 26

Cu-15% Gr Cu-5% NG Cu-10% NG Cu-15% NG Cu-20% NG

ΔT Temperature rise °C

24 22 20 18 16 14 12 10 8 6 12

24

36 Load (N)

48

60

Fig. 15. Typical temperature rise of copper–graphite and copper–nanographite composite at sliding speed 0.77 m/s.

for the same volume percentage. The inherent exotic properties and high order microwave absorption capacity are attributed to improvement of properties. Nanographite (20 vol%) composites exhibited poor physical and mechanical properties due to agglomeration effect. It is observed that maximum volume fraction of nano-sized reinforcement to the metal matrix composites is limited to 15 % of volume fraction. Nanographite reinforced copper composites exhibited higher load withstanding capacity and lower coefficient of friction compared to copper–graphite composites by way of forming a thick graphite layer at the contact surface. Copper–nanographite composite exhibited a lower coefficient of friction even less than 0.1 compared to copper–graphite composites. Nanographite reinforced composites produce a relatively smaller size asperities and also less space between the asperities which can be completely filled by nanographite particles during the wearing process. This completely filled nanographite particles apparently produce a more continuous graphite layer. It reduces the wear debris size as confirmed by SEM studies. The formed continuous and high adherent graphite layer largely reduces the sub-surface deformation of composites as confirmed through Focused Ion beam (FIB) characterization. Operating wear mechanism for 5 vol% nanographite composite is oxidative with plastic deformation whereas 10–15 vol% nanographite composite is oxidative with delamination.

Acknowledgment The authors kindly acknowledge FIST-RP01933, DST, India, for FIB related research study. References [1] Moustfa SF, Badry EI, Sannad AM. Friction and wear of copper–graphite composite made with Cu-coated and uncoated graphite powders. Wear 2002;253:699–710. [2] He Da Hai, Manory Rafel R. A novel electrical contact material with improved self-lubrication for railway current collectors. Wear 2001;249:626–36. [3] Bravunovic M, Konchits VV, Myshkin NK. Electrical contacts: fundamentals, applications and technology. 1st ed. New York: CRC Press; 2007. [4] Yasar I, Canakci A, Arslan F. The effect of brush spring pressure on the wear behaviour of copper–graphite brushes with electrical current. Tribology International 2007;40:1381–6.

295

[5] Kestursatya M, Kim J, Rohatgi P. Friction and wear behavior of a centrifugally cast lead-free copper alloy containing graphite particles. Metallurgical and Materials Transactions A 2001;32:2115–25. [6] Cho Keun Hyung, Hong Ui Seok, Lee Kang Sun, Jang Ho. Tribological properties and electrical signal transmission of copper–graphite composites. Tribology Letters 2007;27:301–6. [7] Jha AK, Prasad SV, Upadhyaya GS. Dry sliding wear of sintered 6061 aluminum alloy–graphite particle composites. Tribology International 1989;22: 321–7. [8] Gibson PR, Clegg AJ, Davis AA. Wear of cast Al–Si alloys containing graphite. Wear 1984;95:193–8. [9] Ames W, Alpas AT. Wear mechanisms in hybrid composites of graphite-20 pct SiC in A356 aluminum alloy (Al–7 pct Si–0.3 pct Mg). Metallurgical and Materials Transactions A 1995;26:85–98. [10] Kovacik J, Emmer S, Bielek J, Kelesi L. Effect of composition on friction coefficient of Cu–graphite composites. Wear 2008;256:417–21. [11] Basvarajappa S, Chandramohan G, Mahadevan A, Thangavelu M, Subramanian R, Gopalakrishnan P. Influence of sliding speed on the dry sliding wear behavior and the subsurface deformation on hybrid metal matrix. Wear 2007;262:1007–12. [12] Street Jr Kenneth W, Miyoshi Kazuhisa, Vander Wal Randy L. Application of carbon based nano-materials to aeronautics and space lubrication. In: Erdemir JM, Martin JM, editors. Superlubricity. Kidlington-UK: Elsevier; 2007. p. 311–40. [13] Hirata Atsushi, Igarashi Masaki, Kaito Takahiro. Study on solid lubricant properties of carbon onions produced by heat treatment of diamond clusters or particles. Tribology International 2004;37:899–905. [14] Rapoport L, Fleischer N, Tenne R. Applications of WS2 (MoS2) inorganic nanotubes and fullerene-like nanoparticles for solid lubrication and for structural nanocomposites. Journal of Materials Chemistry 2005;15:1782–8. [15] Manish Chhowalla, Amaratunga Gehan AJ. Thin films of fullerene-like MoS2 nanoparticles with ultra-low friction and wear. Nature 2000;407:164–7. [16] Cizaire L, Vacher B, Le Mogne T, Martin JM, Rapoport L, Margolin A, Tenne R. Mechanisms of ultra-low friction by hollow inorganic fullerene-like MoS2 nanoparticles. Surface and Coatings Technology 2002;160:282–7. [17] Andersson OE, Prasad BLV, Sato H, Enoki T. Structure and electronic properties of graphite nanoparticles. Physical Review B 1998;58:16387–95. [18] Takai K, Sato H, Enoki T, Yoshida N, Okino F, Touhara H, et al. Novel electronic properties of a nano-graphite disordered network and their iodine doping effects. Journal of the Physical Society of Japan 2000;69:754–67. [19] Lee CH, Hwang YJ, Choi YM, Lee JK, Choi C, Oh JM. A study on the tribological characteristics of graphite nano lubricants. International Journal of Precision Engineering and Manufacturing 2009;10:85–90. [20] Queipo P, Granda M, Santamaria R, Menendez R. Preparation of pitch based carbon–copper composites for electrical applications. Fuel 2004;83:25–34. [21] Savitskii AP, Zhdanova VN, Girya EN. The effect of sintering temperature on the strength of powder copper subjected to repeated pressing. Journal of Powder Metallurgy and Metal Ceramics 1964;3:311–5. [22] Roy R, Agrawal D, Cheng J, Gedevanishvili S. Full sintering of powdered-metal bodies in a microwave field. Nature 1999;399:668–70. [23] Rajkumar K, Aravindan S. Microwave sintering of copper–graphite composites. Journal of Materials Processing Technology 2009;209:5601–5. [24] Lin Wei, Moon Kyoung-Sik, Zhang Shanju, Ding Young, Shang Jitang, Chen Mingxiang, et al. Microwave makes carbon nanotubes less defective. ACS Nano 2010;4:1716–22. [25] Reddy GC, Rajkumar K, Aravindan S. Fabrication of copper–TiC–graphite hybrid metal matrix composites through microwave processing. International Journal of Advanced Manufacturing Technology 2010;48:645–53. [26] Sethi G, Upadhyaya A, Agrawal D. Microwave and conventional sintering of premixed and prealloyed Cu–12Sn bronze. Science of Sintering 2003;35:49–65. [27] Ubaldo OM, Oxana VK, Manuel R. Synthesis and morphology of nanostructures via microwave heating. Reviews on Advanced Materials Science 2003;5:398–402. [28] Chen Kai, Wang Chunlei, Ma Ding, Huang Weixin, Bao Xinhe. Graphitic carbon nanostructures via facile microwave-induced solid-state process. Chemical Communications 2008:2765–7. [29] Imholt TJ, Dyke CA, Hasslacher B, Perez JM, Price DW, Roberts JA, et al. Nanotubes in microwave fields: light emission, intense heat, outgassing, and reconstruction. Chemistry of Materials 2003;15:3969–70. [30] Izabela Janowska, Ovidiu Ersen, Timo Jacob, Philippe Vennegues, Dominique Begin, Marc-Jacques Ledoux, et al. Catalytic unzipping of carbon nanotubes to few-layer graphene sheets under microwaves irradiation. Applied Catalysis A: General 2009;371:22–30. [31] Geim AK, Novoselov KS. The rise of graphene. Nature Materials 2007;6: 183–91. [32] Estrada-Guel I, Carreno-Gallardo C, Mendoza-Ruiz DC, Miki-Yoshida M, Rocha-Rangel E, Martı´nez-Sa´nchez R. Graphite nanoparticle dispersion in 7075 aluminum alloy by means of mechanical alloying. Journal of Alloys and Compounds 2009;483:173–7. [33] Deprez N, McLachlan DS. The analysis of the electrical conductivity of graphite conductivity of graphite powders during compaction. Journal of Physics D: Applied Physics 1988;21:101–7. [34] Khan Umar, O’Connor Ian, Gun’ko Yurii K, Coleman Jonathan N. The preparation of hybrid films of carbon nanotubes and nano-graphite/graphene with excellent mechanical and electrical properties. Carbon 2010;48: 2825–30.

296

K. Rajkumar, S. Aravindan / Tribology International 57 (2013) 282–296

[35] Kovacik J, Bielek J. Electrical conductivity of Cu/graphite composite material as function of structural characteristics. Scripta Materialia 1996;35:151–6. [36] Hu ZL, Chen ZH, Xia JT. Study on surface film in the wear of electrographite brushes against copper commutators for variable current and humidity. Wear 2008;264:11–7. [37] Huang HD, Tu JP, Gan LP, Li CZ. An investigation on tribological properties of graphite nanosheets as oil additive. Wear 2006;261:140–4.

[38] Ma Wenlin, Lu Jinjun. Effect of sliding speed on surface modification and tribological behavior of copper–graphite composite. Tribology Letters 2011;41:363–70. [39] Joly-Pottuz L, Vacher B, Ohmae N, Martin JM, Epicier T. Anti-wear and friction reducing mechanisms of carbon nano-onions as lubricant additives. Tribology Letters 2008;30:69–80.