Preparation and Characterization of Carbon Nanofiber ... · Preparation and Characterization of...

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 12, Number 10 (2017) pp. 2253-2261

© Research India Publications. http://www.ripublication.com

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Preparation and Characterization of Carbon Nanofiber Reinforced Copper

Composite Electrodes via Powder Metallurgy Process for Electrical

Discharge Machining Applications

Pay Jun Liew¹*, Nurlishafiqa Zainal¹ and Qumrul Ahsan²

¹Department of Manufacturing Process, Faculty of Manufacturing Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100, Durian Tunggal, Melaka, Malaysia.

²Department of Material Engineering, Faculty of Manufacturing Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100, Durian Tunggal, Melaka, Malaysia.

* Corresponding author *ORCID: 0000-0001-6729-1576

Abstarct

In this paper, new carbon nanofiber (CNF) reinforced copper

(Cu) composite electrodes were fabricated via powder

metallurgy (PM) process for electrical discharge machining

(EDM) applications. The concentrations of CNF in the

composite electrode were varied and the properties of Cu-

CNF composite electrodes such as relative density, porosity,

hardness and electrical conductivity were determined and

compared with that of pure Cu electrode. The Cu and CNF

powders were mixed using high-energy planetary ball mill,

followed by cold compaction process and sintering operation.

The results indicated that high concentration of CNF in

composite electrodes decreased the relative density and

hardness of the electrode. However, the porosity and electrical

conductivity of Cu-CNF composite electrodes increased with

the increasing of CNF concentration. These findings provide

possibility of using Cu-CNF for high EDM machining

efficiency particularly on low conductivity ceramic materials.

Keywords: Cu-CNF composite electrode, Electrical discharge

machining (EDM), Density, Porosity, Hardness, and Electrical

conductivity.

INTRODUCTION

Electrical discharge machining (EDM) is the most versatile

machining tool in today’s manufacturing world. The

performance of EDM process is largely depends on the

electrode and the main role of electrode is to convey electrical

current and to allow erosion of workpiece (Patel, 2015).

Previous studies have proven that the comparison of physical,

mechanical, and electrical properties of electrodes have

considerable influence on the machining performance in terms

of material removal rate (MRR), electrode wear ratio (EWR),

surface roughness (SR), spark gap, surface topography and

etc. (Sultan et al., 2014; Klocke et al., 2013). In EDM,

although most researchers tried to optimize process

parameters to achieve optimum machining performance but

several researchers tried innovative ways to improve MRR

and EWR. The research work in the area of tooling design can

be classified into two categories: (i) by investigating suitable

electrode material for a particular workpiece material; and (ii)

by trying different kind of electrode geometries and designs

(Garg et al., 2010).

The selection of appropriate reinforcement material for Cu-

based composite electrodes for EDM applications is a critical

problem in enhancing the physical and mechanical properties

of electrodes. Previously, many researchers were investigated

the characterizations of carbon nanotube (CNT) reinforced Cu

composite. For example, Daoush et al. (2009) have

investigated the electrical and mechanical properties of CNT

reinforced Cu nanocomposites fabricated by electroless

deposited process. These authors reported that the use of CNT

as reinforcement in Cu matrix has resulted in decreased

electrical conductivity by increasing CNTs volume fraction,

but the hardness was increased when increasing volume

fraction of CNT. Besides that, Shukla et al. (2013) have

studied the processing of Cu-CNT composites by vacuum hot

pressing technique and they revealed that the significant

improvement in hardness of Cu-based single wall carbon



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nanotube (Cu-SWCNT) composite was observed with

increased in CNTs content by using vacuum hot pressing

technique. Uddin et al. (2010) fabricated CNT reinforced Cu

and Cu alloy (bronze) composites by well-established hot-

press sintering method of powder metallurgy (PM). It is found

that CNT has improved the mechanical properties of highly-

conductive low-strength Cu metals, whereas in low-

conductivity high-strength Cu alloys the electrical

conductivity was improved. However, when nanoparticles

reinforced Cu matrix composites with high concentration were

used, it tends to agglomerate during preparation due to the

strong van der Waals force among nanomaterial, resulting in

poor properties (Gao et al., 2016).

Based on the previous studies, carbon nanofiber (CNF) also

has received considerable attention as nano-sized

reinforcement for composite materials (Vincent et al., 2011;

Al-Saleh & Sundararaj, 2011; Al-Saleh & Sundararaj, 2009;

Tibbetts et al., 2007; Hammel et al., 2004). Owing to the

excellent properties of CNF such as extremely high aspect

ratios, low density, high tensile strength, high electrical and

thermal conductivity (Guadagno et al., 2015; Larionova et al.,

2014), this reinforcement can provide enhanced material

properties as compared with conventional micro-sized

spherical reinforcement due to their excellent mechanical and

physical properties (Gao et al., 2016).

Therefore, the present work aims to fabricate the CNF

reinforced Cu composite electrode for EDM application via

PM process. The produced composite electrodes were

underwent mechanical properties measurements to evaluate

the different composition of Cu-CNF composite electrodes.

Introduction of CNF in Cu electrode may lead to a new

venture in strengthening the Cu and reducing the EWR in

EDM process. Most importantly, the excellent electrical

conductivity of CNF may help in initiating the discharge and

improving the stability of the arc during the EDM.

EXPERIMENTAL PROCEDURE

Materials:

Copper fine powder with 99.999% purity (average particle

size > 15 μm) acquired from Merck Schuchardt, Germany was

used as the matrix material and CNF with average diameter of

< 20 nm and average length < 100 μm was used as the

reinforcement. Table 1 and Table 2 show the specifications of

carbon nanofiber (CNF) and Cu particle, respectively. Figure

1(a) and Figure 1(b) show a Field Emission Scanning Electron

Microscope (FESEM) micrograph of the CNFs and Cu,

respectively.

Table 1: Specification of carbon nanofiber

Material Average diameter (nm) Range of length (μm) Producer/Supplier

CNF < 20 < 100 Magna Value Sdn. Bhd.

Table 2: Specification of metal particle

Metal powder Particle size (μm) Particle shape Producer/Suppliers

Cu > 15 Dendritic Merck Sdn. Bhd.



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(a)

(b)

Figure 1: FESEM micrograph of (a) carbon nanofibers and (b) copper.

Mixing Powders and Composite Fabrication:

Different CNF concentrations ranging from 0.0 to 4.0 wt. %

were used in this study. The required amount of Cu and CNF

were measured separately before being mixed together. In

order to achieve uniform dispersion and distribution of CNF

throughout the matrix material, both Cu powder and CNF

were mixed together using Insmart Mini high-energy

planetary ball mill at room temperature with 150 rpm of

milling speed for 2 hours and 10:1 of ball-to-powder ratio

(BPR). 1 wt. % of stearic acid was added into the mixture as a

process control agent (PCA). The stearic acid was absorbed

on the surface of the powder particles and helped to minimize

cold welding between powder particles and thereby inhibits

agglomeration. No functionalization of the CNF was carried

out prior to mixing process. After that, the mixed powder was

uniaxially compacted in a manual hydraulic press in order to

obtain green samples with dimension of 6 mm diameter and

25 mm length with load of 7 kN. The green samples were

sintered in horizontal tube furnace with the presence of argon

gas (purity level ≥ 99.999%) in a flow rate of 1 ƪ/min as the

protective atmosphere. The sintering operation was carried out



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by heating the furnace at a rate of 10˚C/min until it reached to

a temperature of 850˚C ± 5˚C where the sintered samples

were soaked for 30 minutes. During sintering operation, the

samples were covered by graphite powder placed inside the

crucible boat. This is to ensure that the temperature of the

sample was uniformly distributed during heating and cooling

process. Figure 2 illustrates the schematic diagram of

fabrication Cu-CNF composite electrode via PM process.

Figure 2: Schematic diagram of fabrication Cu-CNF composite electrode via powder metallurgy (PM) process (a) raw materials;

(b) mixing; (c) cold-compaction; and (d) sintering operation.

Characterizations of the composite Electrode :

The theoretical density of sintered Cu-CNF composite

electrodes was determined using the rule-of mixture in

accordance to ASTM B962 – 15 using archimedes’ principle

because the electrodes have surface-connected porosity. In

this study, Equation (1) and Equation (2) were used to

calculate the experimental and theoretical densities of Cu-

CNF composite electrode, respectively.

(1)

(2)

where is experimental density (g/cm³), is the mass of

electrode in air (g), is the density of water which is 1

g/cm³, is the mass of the oil-impregnated electrode (g),

is the mass of the oil-impregnated electrode in water (g),

is the theoretical density (g/cm³), is the volume

fraction of reinforcement, is the density of reinforcement

(g/cm³) which is 2g/cm³, and is the density of matrix

material (g/cm³) which is 8.96 g/cm³. Later, the porosity of

Cu-CNF composite electrode was calculated using Equation

(3),

(3)

where is the density of air which is 0.001225 g/cm³.

Hardness tester (Model Mitutoyo Hardness Testing Machine –

MicroWizard) was used for measuring micro-vicker’s

hardness for the pure Cu and Cu-CNF composite electrodes.

The hardness testing was repeated for 5 times at different

points in each electrode. The electrical conductivity for the

prepared electrodes was measured using JANDEL

Multiheight Microposition Probe (model RM3-AR) system.

This equipment consisted of four probes arranged linearly in a

straight line at equal distance from each other. The set up

current in this equipment was 1A and the reading was

measured in resistivity. This testing was repeated for 10 times

at different points in each electrode to get the average value of

resistivity. The average value of conductivity was calculated

from the average value of resistivity by using Equation (4)

and Equation (5).

(4)



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(5)

Where is the spacing of the probe, is the measured

voltage, and is the test current, and are the symbol for

resistivity and conductivity, respectively.

RESULTS AND DISCUSSION

Density and Porosity:

Table 4 shows the theoretical and experimental densities of

sintered Cu-CNF composite electrodes. Relative density of

Cu-CNF composite electrodes was calculated by the ratio of

the measured Archimedes’ density relative to the calculated

density of composite electrodes by the rule-of-mixture. From

this table, it is observed that the theoretical and experimental

densities of electrodes are reduced with the increases of CNF

concentration in Cu matrix. The result also shows that the

relative density of Cu-CNF composite electrodes decreases

when concentration of CNF increases.

Table 4: Density of sintered Cu electrode and Cu-CNF

composite electrode

Composition Theoretical density (g/cm³)

Experimental density (g/cm³)

Relative density

(%)

Cu 8.96 8.35 93.19

Cu – 0.5 wt.

% CNF

8.85 7.73 87.34

Cu – 1.0 wt.

% CNF

8.75 7.33 83.77

Cu – 2.0 wt.

% CNF

8.54 7.10 83.14

Cu – 4.0 wt.

% CNF

8.14 6.65 81.69

These finding are consistent with those reported by Daoush et

al. (2009) and Rajkumar & Aravindan (2011). This might be

due to the lower density of the reinforcement fiber in the Cu

matrix. The density of CNFs used in this study was 2 g/cm³

which is 4.48 times lighter than matrix material. The

differences densities of Cu matrix and CNF reinforcement

give large effect to the density of the fabricated composite

electrodes. Besides that, sintering at 850˚C which is below the

melting point of Cu did not lead to high densifications. In this

condition, the homogeneity and Cu-CNF distribution play an

important role because atomistic Cu solid state diffusion

across favorable path is the prevailing sintering mechanism

and increase the densification of composite electrode

(Abbaszadeh et al., 2012). In addition, when using CNF with

high concentration, they have large specific surface area and

high value of surface energy. This high surface energy results

in strong interfacial interaction between CNF particles and

cannot disperse well inside the Cu matrix therefore

agglomeration of CNF’s may occur (Uddin et al., 2010). This

possibly prevents the effective interfacial bonding between the

Cu matrix and the CNF. The distribution of CNF in the Cu

matrix depends on the mixing of Cu and CNF powders (Kim

et al., 2006). If CNFs are well distributed within Cu powder

during the ball milling process, more homogenerous

distribution of CNF may be obtained. The effect of CNF

concentrations on the porosity of composite electrodes is

presented in Figure 3. The porosity of fabricated composite

electrodes is increases when the concentrations of CNF are

increased. This behaviour is also confirmed by Trinh et al.

(2015), where increasing the nano-filler contents, the

homogeneous distribution of CNFs in Cu matrix decreased

will lead to a decrease in density and increase the porosity of

composite.

Figure 3: Effect of the CNF concentration to the porosity of

electrodes.

Hardness:

The hardness of Cu-CNF composite electodes as well as the

pure Cu were measured using micro-vicker’s hardness testing.

The effect of CNF’s concentration on the hardness of the

prepared Cu-CNF composite electrodes is illustrated in Figure

4. It was plotted against increasing proportions of CNFs. It is



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clearly seen that the hardness of electrodes is decreases almost

linearly by the increase of CNF concentration. It is accepted

that there is a direct relation between the relative density and

mechanical behaviour which is the higher the density, the

greater the hardness. The relative density decreased is due to

the increased porosity of sintered composites which in turn

resulting lower hardness. Pure Cu electrode presented the

highest hardness of electrode which is 33.5 HV, however, the

hardness for 4.0 wt.% CNF reinforced Cu matrix was reduced

56.4% if compare with pure Cu. This is due to the increased

number of porosity in composite electrodes, which is

supported by the Figure 5. Figure 5 shows the relationship

between hardness and porosity of Cu-CNF composite

electrodes with different concentration of CNF. Based on this

figure, the reduced hardness of Cu-CNF composite electrodes

is attributed by the increasing porosity of sintered composite

electrode. It is proven by referring to Figure 6(a), pure Cu

electrode exhibites highest value of hardness because it was

observed that less dark surface in the optical micrograph after

grinding and polishing compare with electrode containing 4

wt. % CNF in Figure 6(b) and this dark surface represents the

porosity inside the electrode. Similar results were obtained by

Samal et al. (2013) when using conventional sintering

method. This might be due to the less bonding between Cu

matrix and reinforcement in the composite. According to these

authors, the dispersion strengthening and grain size

refinement are the main strengthening mechanism which is

responsible for increase in hardness of composite materials.

Uddin et al. (2010) supported that if high interface can be

achieved due to the reduced particle size of Cu and more

homogeneous distribution of reinforcement surrounding the

Cu particle, it will provide higher interfacial strength with

higher relative density, which in fact help to increase the

hardness of composite. Therefore the decrease in hardness at

higher CNF content may be explained by thickening of the

grain boundary CNFs clusters while retaining the same Cu

matrix grain size.

Figure 4: Effect of CNF concentration on the hardness of Cu-

CNF composite electrodes.

Figure 5: The relationship between hardness and porosity of

Cu-CNF composite electrodes with different concentration of

CNF.

Figure 6: Optical images of sintered (a) 0 wt. % CNF

electrode and (b) 4 wt. % CNF composite electrode revealing

their microstructures.

a

20µm

20µm

b

b



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Electrical Conductivity :

The electrical conductivity was measured by using a four-

point probe in-line technique. The electrical conductivity of

Cu-CNF composite electrodes with varying concentration of

CNF is shown in Figure 7. The results obtained show that the

electrical conductivity increases with the increasing in CNF

content. This finding is consistent with those reported by

Mendoza et al. (2012) and Yang et al. (2008). From the

results, the value of electrical conductivity for Cu matrix with

4.0 wt.% of CNF added in the electrode is nearly 1.6 times

higher than the fabricated pure Cu electrode. In addition, the

electrical conductivity of sintered 4.0 wt. % CNF composite

electrode also higher than the wrought Cu electrode which is

59.59 MS/m [Uddin et al. (2010)]. This result shows the

significant improvement of developing new Cu-based

composite electrode with addition of CNF via PM route. The

presence of CNF in Cu matrix improved the electrical

conductivity of Cu-CNF composite electrode might be due to

the smallest size of CNFs fill the interstitial space between the

Cu grains and help to reduce electrical resistivity. Therefore,

the improved electrical conductivity of Cu-CNF composite

electrodes enhanced the potential usage of CNF as a

reinforcement in metal matrix, especially for EDM

applications. Ji et al. (2014) stated that when electrode has

high electrical conductivity, the discharge channel is formed

relatively easily, the discharge delay time is shorter and the

discharge energy is high, which can enhances the workpiece

material removal during EDM process.

Figure 7: Electrical conductivity of Cu-CNF composite electrode with different concentration of CNF.

CONCLUSION

CNF reinforced Cu matrix composite electrodes for EDM

applications were successfully fabricated using PM process.

The characterization of pure Cu and Cu-CNF composite

electrodes were investigated and the results obtained were

analyzed. From this study, it is found that the density and

hardness of composite electrodes were decreased when

increased the concentration of CNF. However, the porosity

and electrical conductivity of composite electrodes were

increased with the increased of CNF concentrations.

ACKNOWLEDGEMENT

The authors are grateful for the financial support provided by

Universiti Teknikal Malaysia Melaka, under the grant number

RACE/F3/TK4/FKP/F00300 for the accomplishment of this

work.

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