a new approach to fabricating tib2-tic composite using

International Journal of Engineering & Technology Sciences (IJETS) 2 (2): 193-203, 2014 ISSN 2289-4152 © Academic Research Online Publisher

Research paper

A new approach to fabricating TiB2-TiC composite using self-propagation

high temperature synthesis via pressureless sintering

Mitra Akhtari Zavareh

a,*, Ahmed Aly Diaa Mohammed Sarhan

b, Malihah Amiri Roudan

c,

Parisa Akhtari Zavareh d

a,b,c,d

Department of Mechanical Engineering, Faculty of Engineering, University Malaya, Kuala Lumpur,

Malaysia

* Corresponding author. Tel.: 0060176443409;

E-mail address:[email protected]

A b s t r a c t

Keywords:

Self- propagation,

Synthesis,

Titanium diboride,

Titanium carbide,

Ceramics composite,

Microstructure.

Titanium diboride/titanium carbide (TiB2- TiC) composites are favorable materials

with appeal in different industrial components, such as wear parts and high-

temperature parts. Conventional sintering techniques have many limitations with

respect to material density. Thus, a simultaneous synthesis technique was used in

this study with different amounts of carbon content and constant processing time

and temperature. The chemical reaction between titanium metal, boron and carbon

particles completed at 1200oC after 1 hour. Pure TiB2- TiC ceramic composite was

ultimately produced. The hardness values of the TiB2- TiC compacts obtained

decreased with increasing carbon per titanium molar ratio.

Accepted:12 April2014 © Academic Research Online Publisher. All rights reserved.

1. Introduction

Titanium diboride/titanium carbide (TiB2- TiC) composites are a favorable material with appeal in

different industries such as aircraft wear and high temperature components. Significant activity exists

in the development and fabrication of TiB2- TiC composites. This composite has many characteristics,

among which high melting point and hardness, good thermal shock resistance and high temperature

stability, electrical conductivity, and elastic modulus. It is also an exceptional option in applications

with high performance cutting tools and abrasives [1-5].

Furthermore, this material has low density, which makes it applicable for aircraft propulsion systems

and space vehicle thermal protection [6, 7]. However, the high, strong covalent bonding and low self-

diffusion coefficients of the component cause difficult densification of this composition by traditional

Mitra Akhtari Zavareh et al. / International Journal of Engineering & Technology Sciences (IJETS) 2 (2): 193-203, 2014

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methods like hot pressing or hot-isostatic pressing (HIPing) since high temperatures around 3300⁰C

are required. These properties make TiB2 and TiC the subject of numerous, extensive studies [8-10].

Reaction sintering and hot pressing, with or without sintering aids, are common fabrication methods

for producing dense, solid bodies of these ceramics [11]. However, both methods require liquid-

forming additives or high sintering temperatures. During liquid phase sintering, a low melting phase is

created at the grain boundaries. For instance, a small percentage of nickel in the chemical composition

of titanium, boron and carbide causes a direct reaction with raw materials, and a dense TiB2- TiC

composite is fabricated [12]. Meanwhile, at higher temperatures grain growth becomes predominant

[13]. Another process that can be synthesized is self-propagation high-temperature synthesis (SHS)

[14]. These processing methods have many advantages; however, a high degree of covalent bonding

and low self-diffusion create some limitations in achieving fully-dense composites [15].

Nonetheless, such limitations can be overcome by applying an external force during synthesis. The

external load causes TiB2-TiC powder to consolidate into dense solid bodies [16, 17]. The process is

high-pressure self-combustion synthesis (HPCS) and a maximum density of ∼96.5% is obtained. Self-

propagation synthesis and dynamic compaction of elemental powders can also be used to fabricate

dense TiB2 [18, 19].

Producing and fabricating TiB2-TiC in one step via SHS is an economically effective method that can

be applied in industry. The SHS process consumes extreme internal thermal energy generated from

the chemical reaction, making SHS an energy efficient means of producing advanced, high

temperature materials such as TiB2-TiC. Besides, the high temperature reaction of initial element

powders including Ti, B, and C provides far-from-equilibrium conditions that may facilitate self-

organized processes, self-purification and homogenization processes. Very unique structures can be

achieved at far from equilibrium SHS conditions another advantage of the process [20-23].

Additionally, the SHS process is an exothermic reaction. During the formation of some refractory

material, extreme heat is generated between either two or more solid reactants and gas. This reaction

is propagated spontaneously and helps the reaction to convert raw materials into pure refractory

product. The self-sustaining merit from the exothermic reaction and low time and energy

requirements is an advantage of this process material [20-23]. Conventional methods have some

limitations regarding method complexities and high cost. The SHS process has overcome such

limitations with its simplicity of facilities and reaction heat [24, 25]. Therefore, SHS is a good choice

for fabricating different refractory materials. This research work applies a novel experimental design

to produce and fabricate a pure composite in one step, with the characterization and development of

TiB2-TiC composite through self-propagating high-temperature synthesis via pressureless sintering at

a low temperature of about 1200°C.


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2. Experimental procedures

The fundamental aspect behind Ti-B-C design is to investigate the porosity reduction at different

amounts of carbon content. The initial powders were Ti (150 μm; Sigma-Aldrich, St Louis, MO,

USA), B (amorphous; Sigma-Aldrich) and C (50 μm; Merck) and these were dried and blended before

mixing. The Ti+2B+xC mixture, according to the predetermined compositions for each specimen (x=

1, 1.1, 1.2 and 1.3), was prepared in a planetary ball mill, separately, at 300 rpm for 5 h. Then, each

mixture in a cylindrical die of 12.7 mm diameter was subsequently pressed in a uniaxial hydraulic

press under 3 tons, producing a cylindrical specimen approximately 5-8 mm high.

The specimens were sintered in a tube furnace filled with Argon gas. The sintering cycle profile from

30 to 1200ºC is shown in Figure 1. A holding time of 1 hour for a sintering temperature of 1200°C

was applied in order to ensure that expansion throughout the specimen occurred coherently during

heating.

Fig. 1: Thermal cycle used to produce TiB2-TiC

3. Results and discussion

3.1. Combustion characteristics

Figure 2 shows an optical image of combusted [(1Ti+2B) + (1Ti+xC)] mixture when x=1.1 and 1.2,

while Figure 3 illustrates an optical image of combusted [(1Ti+2B) + (1Ti+xC)] mixture when x= 1

and 1.3. The results of this study indicate that the amount of carbon content has direct influence on the

combustion behavior of the compacted matrix. Pores and crevices may be generated by unbalanced


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diffusion between the reagent particles or by vaporization and expulsion of volatile impurities due to

high temperatures.

Fig. 2: Optical image of combusted [(1Ti+2B) + (1Ti+xC)] mixture when x=1.1 and 1.2

Fig. 3: Optical image of combusted [(1Ti+2B) + (1Ti+xC)] mixture when x=1 and 1.3

As shown in Figure 2, when the carbon content x=1.1 and 1.2 the results indicate the effect of powder

composition on synthesis and consolidation. The two end sides are swollen, while the interface

between the two pieces is concaved, meaning that a violent combustion reaction occurred. The major

cracks observed in the samples mean that considerable out-gassing occurred. In addition, the color of


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the samples completely changed from black to light gray. But when the amount of carbon was x=1

and 1.3, as seen in Figure 3, the color did not change significantly. Furthermore, the specimens’

dimensions did not change considerably, so no shrinkage or expansion took place.

3.2 The morphology and phase analysis of the surface

To investigate the surface phase analysis, XRD analyses corresponding to the as-synthesized

composites with different amounts of carbon in the initial mixture are shown in Figures 4(a), (b), (c)

and (d).

(a) x=1

(b) x=1.1


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(c) x=1.2

(d) x=1.3

Fig. 4: XRD analysis of synthesized [TiB2-TiC] mixture with different carbon content (a) x=1, (b) x=1.1, (c)

x=1.2 and (d) x=1.3

Figure 4(a) signifies that where carbon content x=1, TiB2, TiC and TiO0.325 are observed in the

product. It means some amount of C was oxidized and left the system, therefore the insufficient C

caused the reaction of Ti with the oxygen available in the chamber. The source of oxygen is either

from air trapped inside the sample during compaction or the oxygen content mixed with argon gas. In

Figures 4(b) and (c), where x=1.1 and x=1.2, the products are composed of TiC or TiB2 with no

intermediate phases, suggesting complete phase conversion of the reactants with no remaining pure


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materials and oxidation. Apparently, an increase in carbon provides Ti with sufficient carbon for

reaction, and it prevents oxidation from occurring in the sample (Figure 4(d), where x=1.3 indicates

TiB2, TiC and C in the product). This means the percentage of carbon in the mixture was high;

therefore, the amount of carbon in this sample was more than sufficient. Although some C was

oxidized and left the system, this amount of carbon is not acceptable.

To investigate the surface morphology, SEM micrographs of samples with various amounts of carbon

are shown in Figure 5.

(a) x=1

(b) x=1.1

Magnification 24000x Magnification 110x



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(c) x=1.2

(d) x=1.3

Fig. 5: SEM patterns of synthesized [(1Ti+2B) + (1Ti+xC)] mixture with (a) x=1, (b) x=1.1, (c) x=1.2 and (d)

x=1.3

In Figure 5 (a) where x=1 the SEM micrograph indicates less pores in the microstructure compared

with other samples where x is bigger than 1 (x>1). In addition some smaller and bright, white faces

are also observed, which indicate titanium oxide. Clearly, the oxide phase appeared on selected

regions, where a Carbon deficit occurred near Ti. When the carbon content increased up to x=1.1

(Figure 5(b)) a solidified liquid formation in cloudy shapes with the role of matrix for the small grains

is observed. The small grains developed in the matrix and are distributed uniformly throughout. The

microstructure demonstrates a porous product with shapeless grains joining to each other in some

parts, while small grains are discrete crystals distributed to them. Further increasing the carbon

content to x=1.2, as shown in Figure 5(c), sponge-shaped sample features are observed. The cavities

are likely to be the result of entrapped gases related to the rapid growth of TiB2 grains as well as

volume changes during phase transformation. Grain size in this specimen is approximately big. It




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means that the specimen temperature during combustion synthesis increased, causing all the grains to

melt and compete to grow and integrate. Sample cracking and failure may have occurred during the

reaction of titanium and boron, because this chemical reaction liberates a great amount of heat [25].

Eventually, at carbon content of x=1.3 (Figure 5(d)) the sample features include a black matrix with

bright grains growing on it. Light cracks are observed between large grains in the matrix and

microcracks are evident in the grains. Pores of various sizes are distributed in the grains.

3.3 Hardness analysis

Vickers hardness is presented in Figure 6. Grain and porosity size has obvious influence on the

results.

Fig. 6: Microhardness of the combusted mixtures

When the amount of carbon is x=1.1 and 1.2, hardness sharply increases. According to the XRD of

these groups, the amount of TiB2-TiC is greater than the other groups and besides, they have less

amorphous content. On the other hand, when the carbon content is x=1.3 the difference in

microhardness is not as much as when x=1. The XRD of these groups signifies that the amount of

TiB2-TiC decreases compared with x=1.1 and 1.2. This is related to the role of pore size, shape, and

distribution throughout the sample. Although the samples changed microscopically, their

microhardness remained in the same range without significant change. It is suggested that even if any

changes occurred in the microstructure, their influence was not significant on microhardness.

However, microscopically significant changes were observed. This can strongly be related to the pore

distribution dominating the microhardness measurement results.

It is clear that increasing the amount of carbon has a direct effect on product hardness owing to the

fact that carbon has higher porosity. Carbon also has low hardness and is softer compared with other

composite materials. This explains why increasing the amount of carbon to x=1.3 in the composite

reduces composite material hardness.


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4. Conclusion

The system’s chemical reaction depends on solid state diffusion between B, C and Ti. Moreover, the

diffusion of carbon in the titanium matrix during sintering is faster than boron, which is why the

emergence of TiC occurs earlier than TiB2. A pure TiB2-TiC composite was produced and fabricated

from Ti, B and C powders using SHS in one step by optimizing the carbon content. Furthermore,

oxidation was prevented by varying the C:Ti ratio. The best yield was achieved when C:Ti=1:1 and

1:2. Maximum hardness was achieved when the amount of carbon was 1.2 and hardness was HV=

563.

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a new approach to fabricating tib2-tic composite using

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