In the present day scenario, the development of metal matrix composites (MMCs) has been receiving global recognition because of its superior properties in terms of strength, stiffness, wear resistance, and corrosion resistance compared to monolithic metals. MMCs are generally formed by the blend of a hard reinforcement phase in the metallic matrix. The different matrix materials used are copper, magnesium, aluminum, and their alloys [1–4]. Alumina, silicon carbide, titanium carbide, tungsten carbide, carbon nanotubes, boron nitrate, graphite, and molybdenum disulfide are the reinforcement materials used widely along with the metal matrix to form MMCs [5–11]. In the ductile matrix the stress concentrations and sudden propagation of cracks are reduced because of plastic deformation behavior, which enables improvement in fracture toughness. Applications such as aerospace, automobile, marine, and turbine‐compressor engineering use components made of MMCs in the recent years, because of their low density, greater strength and stiffness, and resistance to wear and high temperature [12–17]. In the automobile industry MMCs are used to manufacture components such as brake disk and drums, cylinder blocks, and liners drive shaft [18, 19]. The aircraft industry uses MMCs to manufacture components such as rotor vanes, rotor blades, and drive shafts.
Copper and its alloys are widely used in various industries due to the significant properties such as good thermal and electrical conductivity, corrosive resistance, nonmagnetic, and stability at elevated temperatures [20, 21]. Copper has higher thermal conductivity, which is an important property for making components requiring fast heat dissipation; however, properties such as high deformability and low strength make copper inappropriate for many industrial applications . To increase the strength, wear, and frictional resistance, an additional reinforcement needs to be added to the copper matrix. Moustafa et al.  reported in their study that high electrical conductivity and excellent lubrication properties render the copper–graphite composites useful in the manufacture of sliding contacts. Kalinin et al.  reported that copper based composites have applications in making components such as connectors, electrodes for spot welding processes, lead wires, and other electronic components because of the exclusive combination of strength and conductivity at high temperatures. Kato et al.  elaborated that copper based composites made by powder metallurgy are commonly used for making components such as bushes and bearings. Copper–tin composites have been developed to make self‐lubricating bearings working in extreme load and temperature conditions. Hussain and Kit  investigated the copper–alumina composites prepared through mechanical alloying and ball milling for studying the spot welding behavior and mechanical properties; their results revealed that a mechanically alloyed composite shows good performance on spot welding behavior, since the electrical current is constrained within a small weld region due to higher hardness achieved through mechanical alloying. Hong et al.  manufactured a composite with copper alloy as matrix and tungsten carbide as reinforcement with or without indium for the application of drill bit in oil or gas industry. Ram Prabhu et al.  reported the potential use of copper based composites for applications such as clutches and brakes and recommended copper hybrid composites prepared with SiC and graphite as reinforcements for brake friction applications.
Fly ash is a waste by‐product obtained from thermal power plants that use coal as the primary fuel for power production. Fly ash is available at low cost and has low density, which makes it suitable for use as reinforcement in MMCs. Fly ash is one of the cheapest and low‐density reinforcements obtained in large quantities as a waste by‐product during combustion of coal in thermal power plants, which can be utilized to develop metal matrix composites  . Some researchers recommended fly ash cenosphere particles for foam based applications [30, 31]. Owing to its low cost and low density, fly ash can be used as reinforcement in metal matrix [32, 33]. Since it is an underutilized by‐product from burning of coal from the thermal power , reduction in environmental pollution created due to land filling can be achieved by utilizing it. Mahendra and Radhakrishna  studied the characterization of stir cast Al–Cu–(fly ash + SiC) hybrid metal matrix composites. They conveyed that the addition of fly ash decreases the density and increases the tensile strength, compression strength, impact strength, and wear resistance. Kumar et al.  performed the dry sliding wear behavior study at elevated temperatures on AA6061 to find out the effect of addition of particulate fly ash on the aluminum alloy. The composite was manufactured through powder metallurgy route. They proved that addition of fly ash on AA6061 delivered better wear behavior at both room and elevated temperatures. They stated that incorporation of fly ash improved the wear resistance up to 300 °C due to hardening effect on the surface by the fly ash particles. David Raja Selvam et al.  developed the AA6061 aluminum alloy composites by the compo‐casting technique; they reported the microstructure and some mechanical properties of fly ash particulate reinforcement in the aforesaid alloy. Their results revealed that the addition of fly ash particles enhanced the micro hardness and tensile strength of the AMCs. AA6061/12 wt% fly ash composite showed 132.21% higher micro hardness and 56.95% higher ultimate tensile strength compared to unreinforced AA6061 alloy. Sreekanth et al.  explained the effect of addition of fly ash on polymer based composites with varying particle size and reinforcement percentage. They reported that addition of fly ash improved the strength and thermal stability; on the contrary, elongation at break decreases drastically. They claimed significant improvement in mechanical properties that was found in the composites prepared with smaller particle size of fly ash. Sai et al.  reported that with the addition of fly ash up to 8 wt%, the wear resistance of the composite increases. Vogiatzis et al.  prepared syntactic foams comprising of aluminum and ceramic cenospheres with different densities through powder metallurgy technique by varying the compaction pressure, ranging from 200 to 300 MPa. From the compression study, they claimed that fracture propagation is from the bottom to top, detachment of the external wall is from the main body, and the top surface of the specimen is not damaged; shear planes are formed in the core of the specimens. Fan et al.  explained the reaction effect between the fly ash and Al–Mg melt for different holding times during the stir casting process. The considered holding time varies from 10 to 40 h. Their results show that for longer reaction time the fly ash decomposed and generated θ‐alumina and MgAl2O4. SiO2 is converted to Si, and the magnesium alloy reacts with the Si to form Mg2Si. When the reaction time is more than 30 h nearly all the fly ash in the material decomposes.
According to Rao et al.  stir casting method is preferred because of its ease, flexibility, and suitability to large scale production. They also stated that the only problem associated with the stir casting process is the distribution of the particulate in a non‐uniform manner, due to segregation of particles by gravity and low wettability. Sajjadi et al.  reported that because of the better wettability of particles in compo‐casting technique than in the stir casting, higher densification and lower grain size are achieved in compo‐casting. Sapate et al.  reported that for making copper matrix composite, powder metallurgy technique is appropriate, since copper has a high melting point, and also possesses poor wettability with the reinforcement particles. Narayanasamy et al.  reported that powder metallurgy route is preferred for making materials such as cutting tools made of tungsten carbide, and nickel based super alloys blended through mechanical alloying. Powder metallurgy is popular globally due to its high productivity, ability to make intricate shapes with precision, and cost‐effectiveness on mass production. Ayyappadas et al.  fabricated the graphene reinforced copper matrix composites with varying proportions through powder metallurgy technique. They suggested graphene as the solid lubricant for MMCs in dry sliding conditions, since the friction coefficient becomes low with increase in graphene content. They also reported that microwave sintered composites show better performance than the conventional sintered composites due to higher densification.
Rajkumar and Aravindan  prepared a hybrid copper based composite with titanium carbide and graphite as reinforcements through microwave sintering and studied its performance on tribological characteristics. They stated that wear rate and coefficient of friction (CoF) of hybrid composites and unreinforced copper increase with increase in normal load. Unreinforced copper has higher wear rates and CoF compared to hybrid composites. Formation of oxide layer in the contact surface and deformation of the microstructure occur, which signifies higher wear rate on unreinforced copper. Jia et al.  studied the wear behavior and friction characteristics of the bronze graphite composites. The studies were carried out in dry sliding condition and water lubricated condition. Stainless steel was used as the counter surface material for the wear studies. They reported that during dry sliding condition plastic deformation, micro‐cracking, and scuffing were attributed to adhesion and abrasive wear behavior of the composite. They also reported the transfer of Fe from the counter face to the specimen under dry sliding condition, which is not noticed in the specimen tested under lubricated condition.
In the present chapter, dry sliding wear behavior of copper fly ash graphite hybrid composites with varying graphite percentage is studied under different sliding velocities and loads.
10.2 Materials and Methods
Copper metal powder with 99.7% purity with an average particle size of 30 μm was purchased from the Oxford Chemicals, which was used as the matrix. Copper is a chemical element with symbol Cu (from Latin: cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. The melting point of copper is 1085 °C and the density is 8940 kg m−3. A freshly exposed surface of pure copper has a reddish‐orange color. The purchased copper powder is shown in Figure 10.1.
Fly ash was used as the reinforcement material. Fly ash is one of the residues generated in combustion and the fine particles rise with the flue gases as flue particles. Fly ash is usually referred to the ash produced during combustion of coal. The melting point of fly ash is 1400 °C and its density is 890 kg m−3. The properties of fly ash are low density, high wear, and abrasion resistance. Fly ash obtained from Tuticorin Thermal Power Station, India, with an average particle size of 5 μm was used as reinforcement. The composition of the fly ash obtained is as follows: SiO2 – 61.75%; Al2O3 – 25.24%; Fe2O3 + Fe3O4 – 4.77%; CaO – 1.3%; MgO – 0.87%. The morphology of copper and fly ash was obtained through scanning electron microscope (SEM), which is shown in the Figure 10.2. From the SEM images, it is found that copper powder is in the form of dendrite structure and fly ash is in the form of hollow spheres.
10.2.2 Preparation of the Composite by Powder Metallurgy Process
Three proportions of composites with copper as matrix and fly ash with 10 wt% along with 1, 2, and 3 wt% graphite were prepared by maintaining copper +10 wt% fly ash as the common proportion for all composites. The copper, fly ash, and graphite powders of required weight proportions were taken by weighing the powders using a digital weighing balance having 0.0001 g accuracy. The proportions taken were blended manually and heated to 100 °C under argon atmosphere to remove the moisture content before compacting. After heating, compaction was done using single action compaction die in the universal testing machine (UTM) at the compaction pressure of 450 MPa. Before compaction process, the inner surfaces of the die were cleaned with acetone, and after drying the surfaces were lubricated with wax to avoid the adhesion of the powder with the die and to enhance the removal of specimen from the die easily. The die along with the UTM during the compaction process is shown in the Figure 10.3.
The product obtained by the compaction process is called green compact. The prepared green compact specimen was of 20 mm diameter. In green compact the particles were only mechanically bonded. To make the particles metallurgically bonded, the prepared green compact was sintered using the tubular furnace under argon atmosphere at 900 °C for a period of 60 min. The sintered specimen was machined to the required dimension for further testing. The specimen after compacting is shown in the Figure 10.4.
10.2.3 Wear Studies
The sintered specimen was machined to the dimension of 30 mm height and 10 mm diameter. Wear studies were carried out using pin‐on‐disk setup as per ASTM G99‐2010. The prepared composite specimens were tested for the loads of 10, 30, and 50 N under the sliding velocities of 1, 2, and 3 m s−1. Before conducting the experiment, the specimen was polished with 800, 1000, and 1200 grit emery paper followed by polishing with alumina particles on a double disk polishing machine. The counter disk for wear testing was made of EN31 Steel having 65HRC and dimensions of 165 mm diameter and 8 mm thickness with the arithmetic average roughness (Ra) of 0.84 μm. Analysis of wear mechanism was carried out using SEM/EDX. The pin‐on‐disk setup is shown in Figure 10.5. The typical wear specimen is shown in Figure 10.6.
10.3 Results and Discussion
Figure 10.7 shows the microstructure of the composite with 1 wt% graphite. Figures 10.8 and 10.9 show the microstructures of the composite with 2 and 3 wt% graphite respectively. From the microstructure results it is inferred that reinforcements have a fairly uniform distribution in the matrix. The white background portions represent the copper matrix. The black rounded dot‐like structure represents the fly ash and the black patches in non‐uniform shape represent the graphite.
Figure 10.10a–c shows the wear test results of the different composites prepared under sliding velocities of 1, 2, and 3 m s−1 in the loading condition of 10, 30, and 50 N. Figure 10.10a shows the effect of load and sliding velocity on wear rate of the composite with 10% fly ash and 1% graphite. The results indicate that as the load increases the wear rate increases under all sliding velocities, or vice versa – for the same loading condition as the velocity increases the wear rate decreases; this is due to the decrease in CoF. As the sliding velocity increases CoF decreases due to the formation of a mechanically mixed layer between the sliding and the courter parts. Similar results are observed for the composites with higher graphite contents, which are shown in Figure 10.10b,c.
It is evident from Figure 10.11a–c that as the sliding velocity increases the CoF decreases for all composites prepared. The results also show that as the reinforcement percentage of graphite increases from 1% to 3% there is a reasonable decrease in CoF, which is achieved through increased lubrication layer formation with the increase in graphite content.
To find the influence of each input parameter (load, sliding velocity, and percentage reinforcement) on the output responses (wear rate and CoF) gray relational analysis was done. The gray relational analysis was done based on L27 orthogonal array. The input and output responses, signal‐to‐noise ratio, normalization gray relational coefficient, and gray relational grade are shown in Table 10.1. The signal‐to‐noise ratio for the output response is calculated based on Equation (10.1):
Table 10.1 Grey relational analysis.
|S. No||Load||Sliding velocity||% graphite||Error||Wear rate||CoF||Signal‐to‐noise ratio (wear rate)||Signal‐to‐noise ratio (CoF)||Normalization (wear rate)||Normalization (CoF)||GRC (wear rate)||GRC (CoF)||GRG||Order|
where X is the output response.
The normalized value is calculated using Equation (10.2):
where Yij is the output response for the combination based on L27 orhtogonal array.
The preliminary values are calculated by finding the absolute difference between Yo(k) and Yij where Yo(k) = 1.
The gray relational coefficient is calculated using Equation (10.3):
ξ is assumed as 0.5.
Gray relational grade is calculated based on Equation (10.4):
where is the gray relational grade for the jth experiment, and k is the number of output responses.
Gray relational order is found by ranking the gray relational coefficient. The order indicates the best optimal combinations of input parameter to get better output response.
Table 10.2 shows the response table for the input parameters, which indicates that optimal combination of input parameters are load at 10 N, sliding velocity at 3 m s−1, and reinforcement of graphite with 3%. Table 10.3 shows the ANOVA results; it is obvious that sliding velocity (SV) has the major contribution of 44.99% followed by percentage reinforcement and load with contribution of 43.99% and 9.91% in influencing the output responses wear rate and CoF for the studied composite. The contribution due to error is very minimal compared to other responses, which clearly shows that the achieved result is appropriate.
Table 10.2 Response table.
|Average mean: 0.6122|
Table 10.3 ANOVA for various input parameters.
|Parameters||Degrees of freedom||Sum of square||Mean square||Contribution of input parameters (%)|
Composite with fly ash and graphite as reinforcement was successfully fabricated through powder metallurgy technique and the following conclusions have been drawn:
- With increase in load, the wear rate and CoF increase.
- The CoF decreases with increase in sliding velocity and graphite content.
- Sliding velocity has a high contribution in affecting the output responses with 44.99%.
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