Electroless plating method is used as it can deposit many metals on almost any substrates regardless of size and shape, also the carbon nanotubes can be ultrasonically dispersed in plating solution, so each Nanotube may have probability to be coated [8]. Electroless plating is a two-step process: 1) Sensitization (0.1 M

SnCl₂/0.1 M HCl) 30 minutes; 2) Activation (0.0014 M PdCl₂/0.25 M HCl) 30 minutes. Preactivation of carbon nanotube was accomplished by dispersing the carbon nanotubes in a solution of 0.1 M SnCl₂/0.1 M HCl for 30 minutes. The sensitized carbon nanotubes were further activated in an aqueous solution of 0.0014 M PdCl₂/0.25 M HCl for another 30 minutes. The activated carbon tubes were then introduced into an electroless plating bath. The composition of plating solution and the reaction conditions are given in Table 2. The whole procedure was carried out at 25˚C and the carbon nanotubes were washed with distilled water after each step. The washed CNT’s were heated in an oven at 150˚C to remove the moisture present in the coated CNT. The dried CNTs were used for further ball milling, compaction and sintering processes.

Al-CNT powders were stirred thoroughly to mix well before adding it into the ball milling urn to perform mechanical alloying on a Retch PM4000 planetary ball milling machine. The severity of the cold rolling is avoided by the addition of stearic acid (1% wt) to the urn containing the powders during the ball milling, ball to powder ratio of 5:1 was maintained and a speed of 120 RPM used throughout the process [14]. The Ball milling duration was 4 hours, with an interim duration of 10 minutes for each hour that was set to avoid overheating. After this stage, the Al-CNT blended powder was filled between the bottom and upper punch of the die, wherein the bottom punch acts as a support as well as an ejector, at the time and at the end of the compaction respectively. The sintering of the green compacted Al-CNT composite was carried out at 600˚C [15] for 2 hours in a muffle furnace (Figure 1).

Scanning electron microscope (SEM) is a tool for the characterization of materials from nanometer to micrometer scale. It is one of the most versatile instruments that are available for the examination and analysis of the microstructure morphology. Before observing the microstructure, the surface is prepared. First

the specimen surface was cleaned using a cotton swab and ethyl alcohol. After this, polish the section on 800 grit emery paper in single direction and note that no excess pressure is applied during the process as it may lead to permanent scarring on the surface. The specimen is rotated by 90˚ and the same procedure is followed on a 1000 grit sand paper [16]. The surface is cleaned at each stage with cotton swab and ethyl alcohol before taking a finer Emery paper (1200, 1500, 1800, and 2000 grit size). The cleaning with ethyl alcohol is necessary to make sure that abraded particles or the soot do not get embedded on the specimen surface. After achieving a near smooth surface, further polishing is done using a velvet cloth and diamond paste. The diamond paste is applied to the surface to be polished; velvet cloth is fixed to a disc spinning at a fixed rpm. The specimen with the applied diamond paste is pressed against the rotating velvet cloth. Due to the friction, the diamond paste polishes the surface. This process is carried out till the required finish is obtained.

The differential thermal analysis was performed on the Al-CNT MMCs which was subjected to continuous increase in temperature at a constant heating rate and linear with time. 10 mg of Al-CNTs were placed in a pan inside the setup and DTA was performed at a heating rate of 10˚C/min to keep a continuous record till it reaches the melting point as shown in Figure 2 [17].

Transmission Electron microscopy (TEM) was performed on the coated CNTs. Figure 3 clearly shows that Ni is coated on the carbon nanotube evenly.

Transmission electron microscope (TEM) micrographs of multiwall carbon nanotubes (MWCNT) and coated MWCNT are shown in Figures 3(a)-(f). Based on the TEM micrograph Figure 3(e) the inner diameter of MWCNT is in the range of 12.91 to 20 nm and the outer diameter is in the range of 17.93 to 30 nm. It can also be observed from the micrograph that the ends of MWCNT are

closed in the form of end caps. Figures 3(b)-(d) show a well decoration of MWCNT with nickel (Ni) and also the Ni nano particles are closely attached to the MWCNT surface. The coating thickness or the average size of Ni nano particles is in the range of 5 - 20 nm. In addition to this, the Ni particles are spread homogeneously on the outer surface of MWCNT.

It can also be observed from Figures 3(b)-(d) that Ni appears as aggregates on the outer surface of the nanotubes indicating the activated sites were formed. Ni on coating appears as balls on the outer surface of carbon nanotubes. Also the Ni coating layer on the nanotube is discontinuous, this is probably due to absence of active sites or due to the formation of nonuniform active sites on the surface of the nanotube. This is attributed to the rate of reaction which is dependent on the pH value of the solution which can be adjusted by varying the concentration of NH₃∙H₂O [9].

The microstructures of the specimens are taken for 0%, 1%, 1.5% and 2% composition of the reinforcements are studied. From Figure 4 and Figure 5 microstructure study shows good bonding between matrix and reinforcement with fairly distinguishable grain boundaries. The observations showed that CNT

reinforcement was dispersed uniformly into the aluminum matrix. This leads to strengthening of the composite at the microscopic level and enhancing the mechanical properties of the composite as compared to its unmixed counterpart.

The fabricated specimens are named as pure aluminum S1, 1 wt% of CNT as S2, 1.5 wt% of CNT as S3 and 2 wt% of CNT as S4. Since the specimens are smaller in dimension, the usual hardness test cannot be conducted on it as the ball indenter might break the specimen. Hence Micro Vickers hardness tests were conducted on the specimens to compare their hardness, which is appropriate for the chosen dimension of the specimen. The tests were conducted by WILSON-VH1102, BUEHLER, make hardness tester and test results recorded using diamet software. The machine has Magnification of 10× to 50×, load range 10 gm 2 kg with a diamond indenter and square base pyramidal shape. A total of five indentations were recorded with a distance of 1 mm. with a load of 0.5 kg and average value was noted to maintain consistency.

From Table 3, it is observed that there is an increase in the hardness of the specimens reinforced with varying wt% of CNT as compared to pure Al6061 alloy (0 wt% CNT). From Figure 6, it can be observed that the Al alloy with 2 wt% CNT as reinforcement shows higher hardness when compared to other specimens. It is also observed that the specimen S4 with 2 wt% CNT as reinforcement shows 32% increase in the strength when compared to unreinforced Al6061 alloy.

The compression test is carried out on the specimen to find out the compression strength of the composite synthesized. Compression test were carried out in a Computerized Universal Testing Machine with a capacity of 1000 KN (Figure 7). Specimens are of standard cylindrical shape having l/d ratio less than 2 (short specimen) were prepared as per ASTM standard EN09. From Figure 8, it can be observed that compressive strength increases with increase in the percentage of CNT up to 2 wt%. Higher value is observed in the sample S4 having 2 wt% of CNT as reinforcement. This clearly indicates that the increase in the strength is attributed to the presence of CNT in the matrix which acts as an obstacle for crack propagation in turn increasing the strength of the composites. It is also clear that there is adequate bond between the CNT reinforcement and the Al-6061 matrix material. It is also observed that the specimen S4 with 2 wt% CNT as reinforcement shows 12% increase in the strength when compared to unreinforced Al-6061 alloy.