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1. INTRODUCTION

A metal matrix composite (MMC) is a type of composite material with at least two

constituent parts, one being a metal. The other material may be a different metal or another

material, such as a ceramic or organic compound [1].

Copper is widely used because of a high thermal conductivity, ease of fabrication and

a good erosion-resistance. On the other hand, copper has low hardness and yield strength.

Therefore, particles like oxides, carbides and borides are dispersed to improve mechanical

properties such as strength and hardness [2, 6].

Those dispersing particles must have high strength, high melting point and low

solubility in the metal matrix. Especially TiB2

particles are excellent candidates for

dispersoids of Cu alloys because of their high melting point, high hardness, low resistance

and excellent thermal conductivity [4].

The combination of high electrical and hardness is particularly attractive for welding

electrodes and sliding contact. Copper and copper-based alloys are widely used as electricand electrode materials due to their good conductivity. However, in the case of precipitationhardened copper alloys (such as Cu-Zr and Cu-Cr) with high strength, there is a problem that

the mechanical property decreases rapidly due to the presence of a coarse precipitate phase at

high temperature. This problem limits the application of copper alloy in electrical and

resistance welding applications. There have been several efforts to develop copper alloys

which exhibit good mechanical properties even at elevated temperatures. Copper-base metal

matrix composites (MMC) with reinforcing ceramic particles such as oxides, borides and

carbides were developed as electrode materials because the ceramic particles are stable at

high temperatures. TiB2 was also found to be a potential candidate for reinforcement of the

copper alloy because of its high melting point, hardness, thermal conductivity as well as

electrical conductivity [2, 4-5].

Powder metallurgy is a method of consolidation of metal powder (non-metallic

powder or metal powder mixture) into finished product. The advantages of powder

metallurgy technology have made it the key to solve the problem of new materials, whose

development plays an important role in todays world. It can be used to prepare materials

having excellent electrical, magnetic, optical, mechanical properties and having very high


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Melting point without segregation; can easily achieve a variety of types of complex, give full

play to the characteristics of each group element, is a low-cost production of high

performance metal matrix and ceramic composite materials; Can achieve near net shape

forming and automated mass production, and thus, can effectively reduce the production of

resources and energy consumption.

Titanium carbide and TiB2 have high melting points, very high hardness and strength,

and high chemical stability. These compounds also have large absolute values of heat of

formation and are normally produced by high-temperature processes such as the self-

propagating high-temperature synthesis (SHS).


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2. Literature Survey

One of the current problems facing Material Science is the production of bulk materials with

fine nano-level microstructure. Methods of mechanical alloying and mechanical treatment are

widespread and well known for making it possible producing a great variety of composite

powders with nano-level grains or inclusions. But it is understood that when nanocomposite

is in powder form it becomes very important to find an appropriate method of consolidation

to obtain high-density material and not to sacrifice its fine microstructure [15].

Copper matrix composites are promising candidates for applications in electrical

sliding contacts such as those in homopolar machine and railway overhead current collection

system [13], where good wear resistant properties and high thermal and electrical

conductivity are needed. Addition of non-metallic second phase particles such as oxides,

carbides and borides can dramatically improve mechanical properties and wear resistance of

metal matrix composites [16].

A COPPER-BASED composite formed by dispersion strengthening with TiB2 is a

leading candidate for applications where an excellent combination of high thermal and

electrical conductivity and high-temperature mechanical strength are required [3].Unlike the

precipitation strengthens copper alloys like Cu-Zr and Cu-Cr, Cu-TiB2

maintains its strength

upto very high temperatures because TiB2 particles have excellent thermal stability [2 ,15].

Titanium diboride is a hard high-melting ceramics with high stiffness and wear resistance.

Thermal and electrical conductivities of titanium diboride are higher compared to other

ceramics. The behaviour of TiB2 particles during of heat treatment of mechanically alloyed

TiB2-Cu mixtures has been studied in a number of works [3, 4-15]. Due to the TiB2

nanoparticles, tensile strength and hardness of the Cu-TiB2 composite improved. But the

electric conductivity of the nanocomposites decreases with increasing the TiB2 content [4].

Various methods have been employed to manufacture Copper Titanium diboride

composites namely SHS, SPS, Coated filler method etc. each having its own advantages.

Powder metallurgical fabrication of a metal-matrix composite conventionally involves

mixing the discontinuous reinforcement and matrix metal powder and subsequent sintering.

This conventional method of powder metallurgy is called the admixture method. A less

conventional method of powder metallurgy is the coated filler method, which involves

coating the discontinuous reinforcement with the matrix particle and then sintering. The


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coated filler method is more effective than the admixture method when reinforcement volume

fraction is high [3, 14].

Cu-TiB2 nanocomposites may be fabricated through combination of mechanical alloy,

SHS and SPS [4, 15]. Recently, an in situ technique has been developed to fabricate ceramic

particle-reinforced metal matrix composites an improved reinforcementmatrix interface

bond could be achieved because the reinforcement surfaces generated in situ tend to remain

free of contamination, such as gas absorption, oxidation and other detrimental surface

reactions [16]. Often mechanical alloying has been combined with other techniques to create

dispersion-strengthened copper alloys [17].

Cu-TiB2 composites with high volume content of titanium diboride introduce a

potential for a new kind of composite structures with both matrix and reinforcement forming

3D interpenetrating networks [8]. Particle connectivity and spatial distribution of networks

define the properties of these composites [15]. The strengthening effect has been analyzed by

various hardening models such as shear-lag model [7], Orowan-looping mechanism [8,9], and

the empirical Hall petch relationship [10]. Nardone and Prewo suggested a modified shear-

lag model [11] which considered the load transfer by normal stress at the reinforcement ends.

The entire strengthening, effect was described to the load-bearing feature of the hard

reinforcements. This model, however, is irrelevant in explaining the strengthening of the TiB2

particulate reinforced Cu composite since the continuum mechanics model does not take into

account the obvious influence of size, distribution, and properties of the reinforcement. The

Orowan mechanism is not suitable either for explaining the strengthening of the alloys

containing coarse hard particles. Dislocation generation at the reinforcement and matrix

interface, due to a local stress concentration, is expected instead of Orowan looping. This

assumption is in agreement with the observation of the many dislocation tangles in the

vicinity of the coarse particles reported previously [12].

In all the study so far parameters like sintering temperature ,sintering time and

porosity and the size and shape of particles have been found to affect the properties of the

composite drastically. Porosity is the dominant factor which affects almost all properties of

the compact, though the critical porosity levels may be different for different compositions

and samples prepared by different mechanisms. The sintering time and temperature are also

important as they determine the extent of interaction between the matrix and reinforcement

which will affect the harness and other mechanical properties.


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3. Experimental Procedure

To fabricate Cu-TiB2 composite we have followed a procedure similar to that of the

admixture method. First the TiB2 powder was prepared by Self propagating High

Temperature synthesis followed by leaching and pure copper powder was taken and they

were mechanically alloyed and pressed to obtain the pellets. The pellets were then sintered

and were characterized. The details of the various processes have been discussed in detail

below.

3.1 Self Propagating High Temperature Synthesis

Of all the intermetallics of titanium and boron system, TiB2 is the most stable

compound. It has properties such as high melting point (2970C), high hardness (1800

Knoop), wear resistance in acid environment and chemical inertness. These excellent

properties of TiB2 make it desirable for many applications such as ballistic armor, reinforced

magnesium matrix composites and cutting tools [22].

Recently, the process for synthesis of TiB2 have attracted much attention and the

successful process to synthesized TiB2 were preferred such as the mechanical alloying of

titanium (Ti) and boron (B) powders, carbothermal reduction of the titanium dioxide (TiO2)

and Boric acid (H3BO3), solid-state reaction of the TiCl4, Mg and MgB2. The self-

propagating high-temperature synthesis method has been developed to produce ceramics,

intermetallics, catalysts and magnetic materials at low cost. This method exploits self-

sustaining solid-flame combustion which develops very high temperature inside materials

over a short period. It therefore offers many advantages over traditional methods, such as a

much lower energy lost, a lower environmental impact, a convenient many fracturing process

and unique properties of the product. In the present study, TiB2 powders were synthesized by

SHS method [22].

In the SHS process, once the combustion reaction is ignited, extremely high

temperatures can be achieved within a very short time. The outstanding features of SHS such

as the internal heat release and the high-burning velocity make it a highly productive and

resource-saving manufacturing technique for various materials. New advanced materials such

as, nano-composite powders, ceramic foams, ceramic containing number of activators for

sintering, anisotropy ceramics and oxygen free single crystal are developed using SHS


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techniques. Special attention is also paid to the development of macro-inhomogeneous

structure, multilayer and functionally graded materials, etc. [20].

The SHS has several advantages that include less processing times, low processing

temperatures, simple operation and generation of fine microstructures due to high cooling

rates. In the SHS process, volatilization of impurities in the reactant powder takes place; this

improves the purity of the product. The SHS technique has the capability to synthesize

ultrafine microstructures; the particle size grows due to high adiabatic temperatures and

enhanced mass transfer during combustion reaction propagation [20].

Experimental

The raw materials used in this paper were TiO2, H3BO3 and Mg powders whose

properties are listed in Table

Reactant Purity (%)

TiO2 99.5

H3BO3 99.5

Mg 99.8

TiO2, H3BO3 and Mg powder were weighted as stoichiometric ratio and mixed manually for

long enough to ensure homogenisation of the mixture. The mixture was pyrolysed in a

tubular (resistance heating) furnace with continuous flow of argon gas. After the combustion

synthesis, the by-product MgO was leached out from the synthesized powder by boiling in

dilute HCl. The residue is filtered using the filter paper and allowed to dry in the oven for 2

hour at 80C Here after, the final powder prepared.

In the combustion synthesis technique, the self-sustainability of the exothermic

reaction and the propagation wave front mostly depends upon the enthalpy change associated

with the reaction. Since there is very little time for the heat dissipation to the surrounding, it

can be assumed that a thermally isolated exothermic system exists. Therefore, the maximum


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temperature that can be attained during the combustion is assumed to be the adiabatic

temperature. The overall chemical reactions can be expressed as:

TiO2(s) + H3BO3(s) +5Mg(s) = TiB2(s) + 5MgO(s) + 3H2O (g)

Using the thermodynamic data from a recent compilation, the heat of formation was

calculated to be 220 kcal/mol [20] and the adiabatic temperature was found to be 2831.8C

[22].

During the process of SHS, TiO2, H3BO3, and Mg may be interacted to form some

possible compounds as following intermediate chemical reactions below:

4Mg(s) + 2TiO2(s) = 2Ti(s) + 4MgO(s)

2H3BO3 = B2O3(s) + 3H2O (g)

3Mg(s) + B2O3(s) = 2B(s) + 3MgO(s)

Ti(s) + 2B(s) = TiB2(s)

Equilibrium composition of TiO2- H3BO3-Mg system in Ar gas atmosphere

It can be seen from Figure 3 that it is thermodynamically feasible to synthesis composites by

heat up the system of reaction. As accepted that the reaction can be self-sustained combustion

when adiabatic temperature of the reaction higher than 1800C [22].

(1)

(2)

(3)

(4)

(5)

Fig 3


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3.2 Mechanical alloying and Compaction

After preparing the TiB2 powder the next step is making the pellets. In order to make

the pellets pure copper (99.99%) and the TiB2 powder were weighed and were taken in a

ceramic bowl where they were grinded and mixed long enough to ensure that the TiB2 has

been evenly distributed. This is very important as this is not done properly it may lead to non-

uniform distribution of the reinforcement in the matrix and hence will not improve the

mechanical properties. Once the alloying is completed the mixture was the compacted into

cylindrical pellets, using a 10 mm die set and a pelletizer, by applying load of 2 ton for 30

sec. It should be noted that the die surface should be clean and smooth else the pellet surface

will be rough and also Zinc Stearate was also applied for lubrication. About 4 set of pellets,

each set having 5 compositions of 0%, 5%, 10%, 20% and 30% TiB2 respectively, were

prepared.

Table 3.1.1 The table shows the amount of TiB2 added to produce different samples.

Composite Copper

(%weight)

TiB2

(%weight)

TiB2

(weight in gm)

Sample I 100 0 0

Sample II 95 5 0.15

Sample III 90 10 0.30

Sample IV 80 20 0.60

Sample V 70 30 0.90

3.3 Sintering

After the pellets were prepared the next step was sintering.Stability and growth ofnanoparticulate reinforcements in metal matrix composites during heating are widely studied,

which contain several volume percent of reinforcing phase. When high volume content of

nanoparticles distributed within a matrix is concerned results of particles aggregation and

growth as well as crystallization mechanisms are evident [13]. In this experiment we have

used pressureless sintering to sinter the pellets. A set of pellets was taken in a alumina boat


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Fig 3.1 Microstructure of 5% TiB2 composite

(100x)Fig 3.2 Microstructure of 10% TiB2 composite(100x)

and kept inside a tube furnace, an inert atmosphere was also maintained to prevent the sample

from being oxidized. The heat cycle consisted of the various sintering temperatures with a

heating rate of 12.5oC/min and a soaking time of 30 min was also given so as to allow the

sample to homogenize and then cooled in the furnace itself. The sintering was carried out at

temperatures of 700oC, 750oC, 800oC, 900oC each for a set of pellets and the pellets were

carefully stored after cooling. The soaking or holding time for the 750oC set was 1 hour.

3.4 Analysis of Microstructure

Microstructure of as obtained composites were studied in an optical microscope and

the corresponding images were obtained using an image analyzer. To study the sample they

were etched using FeCl3-H2O-ethyl alcohol and dried and then viewed under microscope. Allthe sample were viewed at 100x in order to study the connectivity of the TiB 2 skeleton and

the changes that occur during the sintering in the skeleton because the connectivity affects the

final properties of the sample.

Microstructure of sample sintered at various sintering temperatures

1. Microstructure of sample sintered at 700oC


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Fig 3.3 Microstructure of 20% TiB2 composite(100x)


(100x)


Fig 3.6 Microstructure of 10% TiB2 composite(100x).


(100x).

Fig 3.7 Microstructure of 20% TiB2 composite.

(100x)



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(100x).



5. Microstructure of non-sintered Sample


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4. Results and observations

Various tests were carried out to characterize the composite and to evaluate the effect

of sintering on the properties of the compact.

4.1 Porosity:

Since the powder is consolidated to form a specimen by compaction and sintering, it

is expected to have a good amount of pores in it. The specimen is not subjected to any further

thermo mechanical treatment and hence found to have large amount of porosity. The micro

structural images obtained from image analyzer were observed for pore distribution on the

surface of specimen. Size of the pores was not consistent throughout the surface. Pore

distribution on the surface was quiet uniform. As the amount of TiB2 in the specimens

increased, the porosity also increased. It can also be noted that the porosity has been found to

decrease as the sintering temperature increases. Similarly the porosity increases with the

amount of TiB2. The porosity can be calculated by considering the following equations:

Consider a composite of mass Mc and volume Vc. The total mass of the composite is

the sum total of the masses of reinforcement and matrix, that is,

The volume of the composite, however, must include the volume of voids Vv. Thus,

Dividing the equation 1 byMc and equation 2 by Vc and denoting the mass and volume

fractions byMR, MMand VF, VM, VV, respectively, we can write

MR + MM =1

VF +VM +VV = 1


(6)

(7)

(8)

(9)


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The composite densityDc

is given by

Dc = DrVR + DmVM

Representing the Previous equations in terms of mass fractions we get

Hence using this formula we can calculate the volume fraction of voids and find the porosity.

Table 1 Porosity for different pellets obtained after sintering to 700oC

S No PercentageComposition

TheoreticalDensity(g/cm3)

ObservedDensity (g/cm3)

Porosity

1 0 8.96 6.11 0.31

2 5 8.54 6.12 0.28

3 10 8.15 5.71 0.29

4 20 7.49 4.60 0.38

5 30 6.92 4.34 0.39


S No Percentage

Composition

Theoretical

Density(g/cm

3

)

Observed

Density (g/cm

3

)

Porosity

(10)

(11)


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1 0 8.96 6.37 0.29

2 5 8.54 6.50 0.24

3 10 8.15 6.04 0.25

4 20 7.49 4.9 0.33

5 30 6.92 4.37 0.37


S No Percentage

Composition

Theoretical

Density(g/cm3)Observed

Density (g/cm3)

Porosity

1 0 8.96 6.20 0.30

2 5 8.54 6.31 0.25

3 10 8.15 5.63 0.26

4 20 7.49 4.67 0.373

5 30 6.92 4.43 0.38


S No Percentage

Composition

Theoretical

Density(g/cm3)Observed

Density (g/cm3)

Porosity

1 0 8.96 6.00 0.33


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2 5 8.54 6.42 0.24

3 10 8.15 5.74 0.25

4 20 7.49 4.90 0.34

5 30 6.92 4.44 0.36

Graph 1 The graph shows the variation of porosity with percentage of TiB2 at different

sintering temperatures

This graph depicts the effect of holding time on the porosity. The sample which was

sintered to 750 oC had a holding time of 1 hour while that of 800 oC had a holding time of 30

min. So it can be concluded that as the holding time at mximum temperature increases the

porosity decreases. This may be because more time is available for homogenization and

hence the entrapped air leaves the composite thereby creating pores at the surface as was seen

in the microstucture. The other point woth noting is that even though the second sample was

help at a higher temperature the porosity was not reduced by much extent, this shows the

importance of holding time. Moreover indirectly affects the mechanical properties too as they

depend on the extent of porosity.Hence optimum holding time is very important.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0% 5% 10% 15% 20% 25% 30% 35%

Porosity

Percentage of TiB2

Porosity 750 degrees Porosity 800 degrees


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Graph 2 The graph shows the variation of porosity with percentage of TiB2 at different

sintering temperatures

This graph depicts the importance of sintering temperature on porosity. It can be clearly

concluded from the graph that as the temperature of sintering is increased the porosity

decrease. It is because at higher temperature it becomes easier for the entrapped gases tocome out resulting in lesser porosity of the composite.

4.2 Density:

The dimensions of specimens of different compositions were measured with the help of

vernier callipers. After calculating the average values of diameter and length of the specimen,

volume of specimen is found. The Specimen being cylinder, following formula is used

After finding out the volume, weight of each sample is measured using a micro balance.

Density of the specimen is calculated using the obtained values of weight and volume.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0% 5% 10% 15% 20% 25% 30% 35%

Porosity

Percentage of TiB2

Porosity 700 degrees Porosity 800 degrees Porosity 900 degrees

(13)

(14)


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All the values are noted in different tables. Each table has density values of specimens with a

common sintering temperature. The density values obtained are as follows.

Table 1: density values of 0%, 5%, 10%, 20%, and 30% by weight Cu-TiB2

S. No. Type of

sample

Diameter Length Weight Volume Density

% TiB2

Added

mm mm g mm g/cm

1 0% 14.98 7.23 8 1273.59 6.00

2 5% 9.99 4.46 2.24 349.41 6.05

3 10% 9.99 6.68 3.05 523.33 5.52

4 20% 10.02 7.37 2.94 580.86 4.56

5 30% 10.18 8.9 2.98 724.02 4.11

Table 2: density values of 0%, 5%, 10%, 20%, and 30% by weight Cu-TiB2 composites at 700C

sintering temperature.

S. No Type of

sample


% TiB2

Added

mm mm g mm g/cm

1 0% 14.97 7.37 7.926 1296.52 6.11

2 5% 10.1 6.01 2.95 481.26 6.12

3 10% 10.08 6.76 3.08 539.18 5.71

4 20% 10.16 8.39 3.13 679.85 4.60

5 30% 10.04 8.66 2.98 685.25 4.34


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Table 3: density values of 0%, 5%, 10%, 20%, and 30% by weight Cu-TiB2 composites at 750C


S. No Type of

sample


% TiB2

Added

mm mm g mm g/cm

1 0% 14.98 7.06 7.926 1243.64 6.37

2 5% 9.95 5.89 2.978 457.75 6.50

3 10% 10.01 6.62 3.15 520.70 6.04

4 20% 10.1 7.52 3.01 602.18 4.99

5 30% 10 8.76 3.01 687.66 4.47

Table 4: density values of 0%, 5%, 10%, 20% and 30% by weight Cu-TiB2 composites at 800C


S. No Type of

sample


% TiB2

Added

mm mm g mm g/cm

1 0% 14.95 7.3 7.95 1280.78 6.20

2 5% 10.1 5.75 2.91 460.44 6.31

3 10% 10.08 6.85 3.08 546.36 5.73

4 20% 10.3 7.97 3.1 663.74 4.67

5 30% 10.03 8.65 3.03 683.10 4.40


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Table 5: density values of 0%, 5%, 10%, 20% and 30% by weight Cu-TiB2 composites at 900C


S. No Type of

sample


% TiB2

Added

Mm mm g mm g/cm

1 0% 15.09 7.51 8.0557 1342.41 6.28

2 5% 10.03 5.83 2.96 460.40 6.42

3 10% 10.1 6.98 3.21 558.94 5.84

4 20% 10.16 7.72 3.07 625.56 4.90

5 30% 9.98 8.97 3.05 701.33 4.44

Graphs were plotted between the sintering temperature and the density of Specimen of a

given composition.

Fig 1: Graph plotted between sintering temperature and density for 0% TiB2 sample

6

6.11

6.37

6.2

6.28

5.95

6

6.05

6.1

6.15

6.2

6.25

6.3

6.35

6.4

0 100 200 300 400 500 600 700 800 900 1000

Density[g/cm3]

Temperature [ oC ]


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6.05

6.12

6.5

6.31

6.42

6

6.05

6.1

6.15

6.2

6.25

6.3

6.35

6.4

6.45

6.5

6.55

0 100 200 300 400 500 600 700 800 900 1000

Density[g/cm3]

Temperature [ oC ]

5.52

5.71

6.04

5.73

5.84

5.4

5.5

5.6

5.7

5.8

5.9

6

6.1

0 100 200 300 400 500 600 700 800 900 1000

Density[g/cm3]

Temperature [ oC ]


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Fig 4: Graph plotted between sintering temperature and density for 20% TiB2 sample.

Fig 5: Graph plotted between sintering temperature and density for 30% TiB2 sample.

Density of the specimens is found to decrease with an increase in percentage of TiB 2.

This is because TiB2 has very less density when compared to that of copper. Sintering

4.56

4.6

4.99

4.67

4.9

4.5

4.55

4.6

4.65

4.7

4.75

4.8

4.85

4.9

4.95

5

5.05

0 100 200 300 400 500 600 700 800 900 1000

Density[g/cm3]

Temperature [ oC ]

4.11

4.34

4.47

4.4

4.44

4.05

4.1

4.15

4.2

4.25

4.3

4.35

4.4

4.45

4.5

0 100 200 300 400 500 600 700 800 900 1000

Density[g/cm3]

Temperature [ oC ]


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temperature variation was not found to have an orderly effect on the density. For four

samples out of five, leaving out 30% TiB2 sample, are found to have maximum density at

750C sintering temperature (because of greater holding time). Whereas 30% TiB2 sample

has its maximum density value at 800C.

4.3 Microhardness:

The five Cu-TiB2 specimens with compositions ranging from 0% to 30% TiB2 by

weight, sintered at different temperatures, have been checked for their micro hardness. The

specimens were first viewed through the eye piece to select an area of interest to carry out

indentation. Load 300 gms have been used for different specimen. A Vickers harness test

indenter is used. Once a dent is made in the specimen, the indenter is flipped around to bring

lens on to area of indentation of specimen. Specimen is viewed for indentation and the lines

in eye piece are adjusted to coincide with the horizontal diagonal ends and then button

provided on the viewing equipment is clicked to allow the equipment feed in the value of

diagonal length. The eye piece is turned by 90 and the same procedure is repeated in

vertical direction. After repeating this on both the sides, length is read by equipment and a

micro hardness value is given. This test has been carried out at four different areas of

specimen at different radial distances to ensure an accurate average micro hardness value.

Once the values are obtained average hardness values at different sintering

temperatures are calculated for specimen of different compositions.

Table 6: Microhardness values 0% TiB2 sample at different sintering temperatures.

S.No Temperature

Micro hardness values

HvavgHv 1 Hv 2 Hv 3 Hv 4

1 Green

sample 18.6 18.4 18.7 18.8 18.625

2 700C 21.6 21.9 22 22.1 21.9

3 750C 23.9 24.3 24.2 24 24.1

4 800C 28.7 29.1 28.9 29.3 29

5 900C 35.4 35.7 35.9 36 35.75


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S.No Temperature


Hvavg

Hv 1 Hv 2 Hv 3 Hv 4

1 Green sample 20 20.4 20.6 20.2 20.3

2 700C 24.6 24.2 24.3 24.3 24.35

3 750C 32 31.3 31.4 31.6 31.575

4 800C 41.6 41.7 42 41.9 41.8

5 900C 56 55.6 55.2 55.9 55.675


S.No Temperature


HvavgHv 1 Hv 2 Hv 3 Hv 4

1 Green sample 21.4 21.7 21.9 21.6 21.65

2 700C 25.6 25.1 25.5 25.3 25.375

3 750C 36.8 37.1 37.3 36.9 37.025

4 800C 43 43.5 43.1 43.4 43.25

5 900C61.9 62.3 62.4 62.7 62.325


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Graphs were plotted between the Hardness values obtained and the sintering temperatures for

different specimen compositions.

Fig 6: Graph plotted between hardness value and sintering temperature for 0% TiB2 composite

Fig 7: Graph plotted between hardness value and sintering temperature for 5% TiB2 composite

18.625

21.9

24.1

29

35.75

0

5

10

15

20

25

30

35

40

0 100 200 300 400 500 600 700 800 900 1000

Hardness(Hv)

Temperature (C)

20.3

24.35

31.575

41.8

55.675

0

10

20

30

40

50

60

0 100 200 300 400 500 600 700 800 900 1000

H

ardness(Hv)

Temperature (C)


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Fig 8: Graph plotted between hardness value and sintering temperature for 10% TiB2 composite.


21.65

25.375

37.025

43.25

62.325

0

10

20

30

40

50

60

70

0 100 200 300 400 500 600 700 800 900 1000

Hardness(Hv)

Temperature (C)

23.175

26.8

42.475

46.45

66.3

23.175

26.8

42.475

46.45

66.3

0

10

20

30

40

50

60

70

0 100 200 300 400 500 600 700 800 900 1000

Hardness(Hv)

Temperature (C)


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From the above collected data it can be concluded that the hardness is directly proportional tothe sintering temperature. At higher temperatures various interactions occur between the

matrix and the reinforcing material resulting in the formation of various intermediate phases

which are formed near the interface and also the reduction in porosity occurs as a result of

which the composite becomes more and more harder.

4.4 Electrical Resistivity

To calculate the electrical resistivity two terminals were connected to either side of the

sample and then the resistance offered was measured. Then the electrical resistivity was

found by using the formula:

24.6

36.25

48.35

52.775

76.325

0

10

20

30

40

50

60

70

80

90

0 100 200 300 400 500 600 700 800 900 1000

Hardness(Hv)

Temperature (C)

(15)


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29

Once the resistance was found the resistivity values were calculated and the graph was

plotted for the sample sintered at 900oC.

Fig 10: Graph plotted between Electrical Resistivity and different composite.

The above graph shows the variation of electrical resistivity(x 108 /m) with

percentage composition .This is due to the formation of the nanocomposites. These

nanocomposites increase the scattering surfaces for the conduction electrons in the copper

matrix. So the electrical conductivity of the copper matrix composite was reduced. When the

content of TiB2particles is more, the electric conductivity decreases sharply [4]. Moreover

the porosity also increases within increase in amount of reinforcement and this also reduces

the conductivity. Further studies have also shown that after reaching a critical porosity level

the resistivity increases sharply [14].

4.5 Microstructure and SEM

The pellets were observed under microscope and also under SEM the images obtained

by the image analyzer have already been produced.The temperature of sintering is rather low

and equal to 0.7 times melting point of copper and 0.3 titanium diboride melting point. At

this temperature diffusion processes in copper matrix are accelerated. It appears that titanium

diboride nanoparticles move in solid copper matrix. We assume this moving as an anomalous

1.79

2.3882.5

2.86

3.2

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30 35

Electrical

Resistivity(x108o

hm/m)

Percentage of TiB2


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30

mass transfer in non-uniform field of stresses. Non-uniform field of stresses in the composite

powders is due to high-energy mechanical treatment used in the synthesis processing.

Annealing of vacancies in metal matrix may lead to situations when a plane of matrix at one

side of particulate inclusion disappears and appears at the other side [18]. These processes

result in moving of the inclusion as a whole. Crystallization by formation of aggregates from

nanoparticles is known for systems with liquid as a matrix [19]. It is proposed that so called

heterogeneous events in crystallization are favoured at high concentrations of solids and low

solubility in the matrix. Drawing the analogy to our system, these conditions are satisfied in

our case by very low solubility of titanium diboride in copper and by high content of

nanoparticles in copper matrix. Association of nanoparticles in aggregates becomes clear

when high diffusion activity in dispersed systems and increased sinterability of nanoparticles

are taken into account [18]. The Img.1shows an interface between the TiB2 and copper matrix

and the other images show the distribution of TiB2 in the matrix after sintering one can also

see some micro crack appearing which may be due to mechanical loads during the storage

period.

Fig 11 SEM image of Cu-30%TiB2 showing the presence of

minute cracks and the surface porosity


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31

Fig 12 SEM image of Cu-20% TiB2 sintered at 900oC

showing an interface between Cu matrix and the TiB2

phase. Porosity and the TiB2 network can be observed.


showing distribution of TiB2 phase in Cu matrix and the.

TiB2 network can be observed.5. Conclusions

The following conclusions can be drawn from the present study:

1. Pressureless Sintering leads to densification of samples but the extent of porosity is high.

2. The porosity increases with increase in the amount of TiB2 and decreases with the increase

in sintering temperature. Porosity also decreases with increase in holding time at maximum

temperature.

3. The TiB2 phase is well distributed in the matrix resulting in formation of a TiB2 network

with small amount of agglomeration at places. The TiB2 phase may be considered to be

moving as a whole in the bulk matrix during sintering and diffuse through appearing on the

other side of pellet, as is evident from the samples at 700oC and 800oC (5% TiB2 ).

4. The presence of finely dispersed TiB2 particles is the main reason for the high micro

hardness values of the Cu-TiB2 composite. The hardness has been found to increase as the

amount of reinforcement and sintering temperature increase. But overall the sample have


showing distribution of TiB2phase in Cu matrix and the

high level of porosity can be seen.


showing distribution of TiB2 phase in Cu matrix and the

agglomeration can be seen in the image.


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32

high porosity hence the hardness can be further increased is pellets with lesser porosity can

be prepared by SPS.

5. The Electrical Resistivity was found to be good which was the main requirement but even

the resistivity can be decreased further owing to the fact that the sample had porosity. The

electrical resistivity was found to increase with increase in the composition of TiB2 as the

porosity level increases.


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6. Reference

1. A.K. Khanra,L.C. Pathak, S.K. Mishra, M.M. Godkhindi Materials Letters 58(2004) 733738.

2. Mechanical, electrical and wear properties of Cu-TiB2 nanocompositesfabricated by MA-SHS and SPSDae-Hwan Kwon, Thuy Dang Nguyen, Khoa Xuan

Huynh, Pyuck-Pa Choi, Myung-Gyu Chang, Young-Jin Yuma, Ji-Soon Kim* and

Young-Soon Kwon. Journal of Ceramic Processing Research. Vol. 7, No. 3, pp.

275~279 (2006)

3. Formation of a TiB2-Reinforced Copper-Based Composite by MechanicalAlloying and Hot PressingS.J. DONG, Y. ZHOU, Y.W. SHI, and B.H. CHANG.

4. Mechanical Properties of Cu-TiB2Nanocomposite by MA/SPSYoung-Jin Yum,Kyung-Sik Lee, Ji-Soon Kim et al

5. Investigation of In situ Cu-TiB2 Composite on the Copper Using Laser MeltingSynthesisL.Y. Gu, G.Y. Liang, and Z.B. Zheng.

6. Strengthening Mechanism of a Spray-Formed Cu-TiB2 CompositeJongsangLee*, J. Y. Jung, Eon-Sik Lee, Nack J. Kim* and S. Ahn

7. Load Bearing Fibre Composites,M. R. Piggott, p. 62,Pergamon Press (1980).8.

Symposium on Internal Stresses in Metals and Alloys,

E. Orowan, in p. 451,Institute of Metals, London, England (lq48).

9. Micromechanisms in Particle Hardened Alloys,J. W. Martin, p. 62, CambridgeUniversity Press, Cambridge, England (1980).

10. R. M. Aikin, Jr. and L. Christodoulou, Scripta metall .mater. 25, 9 (1991).11. V. C. Nardone and K. M. Prewo, Scripta metall . 20, 43 (1986).12. Strengthening Methods in CrystalsM. F. Ashby (eds.,A. Kelly and R. B.

Nicholson), p. 137, Applied Science Pub., London, England (1971).

13. Microstructure changes in TiB2-Cu nanocomposite under sintering D. V.DUDINA, M. A. KORCHAGIN, O. I. LOMOVSKY JOURNAL OF MATERIALS

SCIENCE39(2004) 5325 5331

14.A comparative study of the coated filler method and the admixture method ofpowder metallurgy for making metal-matrix compositesPay Yih, D. D. L. Chung

Journal of Materials Science 32 (1997) 2873-2882


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15. Microstucture of Spark-Plasma Sintered TiB2-Cu NanocompositesD.V.Dudina, MA Korchagin, 0.1. Lomovsky, Y.-S. Kwon" Institute of Solid State Chemistry

and Mechanochemistry, SB RAS Kutateladze 18, Novosibirsk, 630128, Russia

16. Dry sliding wear behavior of in situ CuTiB2 nanocomposites against mediumcarbon steelJ.P. Tu a,, W. Ronga, S.Y. Guob, Y.Z. Yang aWear 255 (2003) 832

835

17.Heat-Resistant Dispersion-Strengthened Copper AlloysJoanna GrozaJMEPEG91992- 1:113-121 ASM International.

18.Dispersion Strengthened MaterK. I . PORTNOY and B. N. BABICH (Metallurgiya,Moscow, 1974, in Russian)p. 53

19.C. SURYANARAYANA and C. C. KOCH, Hyperf. Interact 130 (2000) 5.20.Effect of NaCl on the Synthesis of TiB2 powder by self-propagation high-

temperature synthesis technique, A.K. Khanra, Mishra, L.C.P., and Godkhindi,

M.M. Mater. Lett., 58, 2004, pp. 733-738.

21.Fabrication of TiB2 and TiB2-TiC particulates reinforced magnesium matrixcompositesWang, H.Y., Jiang, Q.C., Zhao, Y.Q., Ma, B.X. and Wang, Y. Mater.

Sci. Eng. A, 372, 2004, pp. 109-114.

22.Synthesis of Titanium Diboride Powder by Self-propagating High TemperatureSynthesisN. Chaichana, N. Memongkol , J. Wannasin, S. Niyomwas.

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