Influence of B source materials on the synthesis of TiB2-Al2O3 nanocomposite powders by mechanical...

International Journal of Minerals, Metallurgy and Materials

V olume 20 , Number 12 , December 2013 , 214

DOI: 10.1007/s12613-013-0857-6

Influence of B source materials on the synthesis of TiB2-Al2O3

nanocomposite powders by mechanical alloying

Majid Abdellahi1), Javad Heidari2), and Rahman Sabouhi2)

1) Materials Engineering Department, Islamic Azad University, Najafabad Branch, Najafabad, Iran

2) Materials Engineering Department, Islamic Azad University, Saveh Branch, Saveh, Iran

(Received: 14 May 2013; revised: 15 June 2013; accepted: 23 June 2013)

Abstract: An Al2O3-TiB2 nanocomposite was successfully synthesized by ball milling of Al, TiO2 and two B source

materials of B2O3 (system (1)) and H3BO3 (system (2)). Phase identification of the milled samples was examined by X-

ray diffraction. The morphology and microstructure of the milled powders were monitored by scanning electron microscopy

and transmission electron microscopy. It was found that the formation of this composite was completed after 15 and 30 h

of milling time in systems (1) and (2), respectively. More milling energy was required for the formation of this composite

in system (2) due to the lubricant properties of H3BO3 and also its decomposition to HBO2 and B2O3 during milling. On

the basis of X-ray diffraction patterns and thermodynamic calculations, this composite was formed by highly exothermic

mechanically induced self-sustaining reactions (MSR) in both systems. The MSR mode took place around 9 h and 25 h of

milling in systems (1) and (2), respectively. At the end of milling (15 h for system (1) and 30 h for system (2)) the grain

size of about 35-50 nm was obtained in both systems.

Keywords: nanocomposites; powders; alumina; titanium diboride; mechanical alloying

1. IntroductionTitanium diboride (TiB2) has an attractive combina-

tion of good electrical and thermal conductivity, high hard-

ness, and inertness with the melting of nonferrous metals.

However, its applications are presently limited due to low

sinter ability and poor mechanical properties, such as flex-

ural strength and fracture toughness [1-4].

Addition of secondary phase, such as Al2O3 to

TiB2matrix, widely improves its sinter ability and mechan-

ical properties [2]. Accordingly, TiB2-Al2O3 nanocom-

posite is synthesized by several methods, such as self-

propagating high-temperature synthesis [5], pressureless

metal infiltration [6], exothermic dispersion [7], and me-

chanical alloying [8-9]. Mechanical alloying (MA) is a pow-

der technique that allows production of homogeneous ma-

terials from blended elemental powder mixtures. MA in-

volves repeated cold welding, fracturing, and rewelding of

powder particles in a high-energy ball mill. The transfer

of mechanical energy to the powder particles leads to the

introduction of strain into the powders through the gener-

ation of dislocations and other defects that act as fast dif-

fusion paths. In addition, refinement of particle and grain

sizes occurs, and consequently, the diffusion distances are

reduced. Furthermore, a slight rise in powder temperature

occurs during milling. This method is well known for fab-

rication of novel materials, such as nanocomposite [10-12].

Recently, TiB2-Al2O3 nanocomposite has been pro-

duced from a mixture of Al, B2O3, and TiO2 as raw mate-

rials via mechanical alloying [8]. In this research, the prod-

ucts were synthesized after 60 h of milling, while the ball to

powder weight ratio (BPR) and the rotational speed (RS)

of vials were 10:1 and 500 r/min, respectively. In another

research [9], this nanocomposite was synthesized from Al,

H3BO3, and TiO2 mixture in which the formation of prod-

ucts occurred after 1.5 h due to the high energy of milling

media (BPR = 20:1 and RS = 600 r/min). In the above-

mentioned researches, there is no report about the effect of

B source materials on the mechanism of synthesis reaction

of this composite. Furthermore, the events during milling

of the raw materials powder mixture were not completely

Corresponding author: Majid Abdellahi E-mail: [email protected]

c© University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2013

M. Abdellahi etal., Influence of B source materials on the synthesis of TiB2-Al2O3 nanocomposite ... 1215

studied in mentioned researches.

The present research was focused on the synthesis of

TiB2-Al2O3 nanocomposite via mechanical alloying of Al,

B2O3 and TiO2 (system (1)) and Al, H3BO3 and TiO2

(system (2)). Effect of B source materials and milling time

were also investigated on the mechanism of synthesis reac-

tion of this composite.

2. ExperimentalTitanium dioxide (TiO2, Merck, 99%, 1-3 µm), alu-

minum (Al, Merck, 99%, 10-50 µm), boron oxide (B2O3,

Merck, 99.99% , 5-100 µm), and boric acid (H3BO3, Merck,

99.9%, 20-30 µm) were used as raw materials in two sys-

tems (1) and ( 2). The raw materials were mixed according

to reaction (1) and (2) for systems (1) and (2), respectively:

10Al + 3TiO2 + 3B2O3 = 3TiB2+ 5Al2O3, ΔH�298 =

–2725 kJ/mol (1)

10Al + 3TiO2 + 6H3BO3 = 3TiB2+ 5Al2O3 + 9H2O,

ΔH�298 = –2519 kJ/mol (2)

A high-energy planetary ball mill with two hardened

stainless steel vials was used for MA experiments. The

BPR of 10:1 and the main disk speed of 250 r/min were

used for both systems. Sampling was performed in various

intervals. The surface temperature of the vial was mea-

sured by a digital thermometer at different milling time

The morphology of milled powders was monitored us-

ing a Hitachi S4160 SEM operated at 15 kV and ZEISS EM

10C TEM operated at 100 kV. Phase identification of the

milled samples was examined by X-ray diffraction (XRD)

with Cu Kα radiation at 30 kV and 25 mA. The mean

grain size and micro-strain were calculated on the basis of

Rietveld refinement method [13] by using of X’Pert high

score plus software (developed by PANalytical BV Com-

pany, Almelo, the Netherlands, and version 2.2b). In this

method, peak profile fitting, size broadening, and strain

broadening were calculated based on the following equa-

tions:

Gik= ΓC 0.50 /Hkπ[1 + C0X

2ik]−1 + (1 − γ)C0.5

1 /

H0.5kπ exp[–C1X

2ik],

Hk = (Utan2θ + V tanθ + W )0.5,

Di = (180/π) λ/(Wi − Wstd)0.5,

ηi=[(Ui – Ustd) – (Wi – Wstd)]0.5/

0.04[180/π](2ln2)0.5 ,

where Gik is the Pseudo-Voigt function, C0 = 2, C1 =

4×ln2, Hk is the full width at half maximum of the

Kth brag reflection, Γ is the shape parameter, Xik = (2θi –

2θk)/Hk, Di, ηi, λ, U and W are the grain size function,

strain function, wavelength, strain parameter, and size pa-

rameter of the peak profile, respectively. In the size and

strain functions, i and std are referred to analyzed and

standard samples, correspondingly. In this project, pure

TiB2 and Al2O3 that annealed at 1500◦C for 15 h were

used as standard materials for deconvolution of instrumen-

tal broadening.

3. Results and discussion

3.1. Thermodynamic calculations and routeof reactions

Thermodynamic calculation, based on thermody-

namic databases, reveals that reactions (1) and (2) take

place in two and three steps, respectively. For reaction

(1), in the first step, the aluminothermy reduction reac-

tions (3) and (4) take place to form Al2O3, elemental Ti

and B:

4Al + 3TiO2 = 2Al2O3 + 3Ti, ΔG�298 = –500 kJ/mol,

ΔH�298 = –520 kJ/mol (3)

2Al + B2O3 = Al2O3 + 2B, ΔG�298 = –417 kJ/mol,

ΔH�298= –403 kJ/mol (4)

In the second step, released heat from the above re-

actions provides the activation energy for the formation of

TiB2:

Ti + 2B = TiB2, ΔG�298 = –319 kJ/mol, ΔH�

298= –342

kJ/mol (5)

The negative Gibbs free energies of the above reac-

tions confirm that these reactions are favorable at room

temperature during milling [14].

For reaction (2), before aluminothermy reduction and

in the first step, it is required for H3BO3 to decompose to

B2O3 and H2O as follows:

H3BO3= HBO2 + H2O, ΔH�298 = 50 kJ/mol, ΔG�

298 =

–45 kJ/mol (6)

2HBO2 = H2O + B2O3, ΔH�298 = 95 kJ/mol, ΔG�

298 =

–91 kJ/mol (7)

It was reported that mechanically induced self-

sustaining reactions (MSR) take place in highly exothermic

powder mixtures [15]. The MSR is ignited when the pow-

der reaches a well-defined critical state [15]. Once started,

the reaction propagates through the powder charge as a

combustion process. In combustion type reactions, there

is an induction milling time after which the reaction initi-

ates and proceeds at a high transformation rate, producing

a rapid rise in temperature, which can be detected by the

increased temperature at the wall of the mill vial [15-16]

or by XRD patterns with sampling in very small time in-

tervals. A reaction can perform in MSR mode, if ΔH/C,

the magnitude of reaction heat to the room temperature

heat capacity of the products, is higher than about 2000

K [15]. This parameter for reactions (1) and (2) is 5000 K

and 4620 K, respectively. Therefore, these reactions will

take place in MSR mode.

1216 Int. J. Miner. Metall. Mater., V ol. 20 , No. 12 , Dec. 2013

Reactions (6) and (7) show that in system (2), about

150 kJ/mol energy must be provided for decomposition of

boric acid to boron oxide. This energy is obtained from two

sources: released heat of reactions (3) and milling media.

It was reported that 2 GPa pressure is required for decom-

position of H3BO3 to B2O3 [17], whereas during milling, 6

GPa pressure is produced [18]. It means that some of the

milling energy in this system is consumed for this event,

and therefore, this system needs more milling time for the

formation of products in comparison with system (2).

3.2. Phase analysis and structural featuresFig. 1 depicts the XRD patterns of the milled powders

in system (1). There is no change in the reflections of start-

ing materials up to 9 h of milling except peak broadening,

which is attributed to crystallite size refinement. By in-

creasing the milling time to 15 h, all peaks of the initial

powder mixture (Al, B2O3, and TiO2) disappeared. On

the other hand, Al2O3 and TiB2 were formed in the range

of 9-15 h of milling.

Fig. 1. XRD patterns of Al, B2O3 and TiO2 milled

powders in system (1).

Fig. 2 shows the XRD patterns of the milled powders

related to system (2). The formation of Al2O3 and TiB2

was delayed to 30 h of milling. As discussed, some of the

milling energy in system (2) was consumed for boric acid

transformation to boron oxide. By increasing the milling

time, the required energy was provided and ignition took

Fig. 2. XRD patterns of Al, H3BO3 and TiO2 milled

powders in system (2).

place in the range of 25-30 h of milling caused the forma-

tion of products.

It is well known that boric acid is a layered mate-

rial with a triclinic crystal structure [19], as shown in

Fig. 3. The unique structure of boric acid is similar to

that of graphite that acts as a lubricant [19]. This prop-

erty leads to the delay in the fracture of raw materials

in first hours of milling in system (2). By increasing the

milling time and the formation of metaboric acid (HBO2),

the lubricant property is deactivated in system (2). Fur-

ther milling time leads to the decomposition of metaboric

acid to boron oxide (B2O3). This boron oxide is known as

the glass-like B2O3 [20] and has larger hardness in com-

parison with boron oxide that was spent as raw material

in system (1)[20]. It means that the lubricant property of

Fig. 3. Schematic illustration of the layered-triclinic

structure of boric acid [19].


boric acid and subsequently formation of glass-like B2O3

in system (2) increase the required energy in system (2)

for the formation of products in comparison with system

(1).

The mean intensity of raw materials reflection (2θ =

45◦) versus milling time is shown in Fig. 4. One can ob-

serve there is no reaction in systems (1) and (2) up to 9

and 25 h milling, respectively. In other words, almost no

chemical interaction between raw materials occurred dur-

ing milling until the self-sustaining reaction. Trapp and

Kieback [21] mentioned that the volume fraction of phases

formed before ignition is very small for higher enthalpy

formation and rapidly approaches 100% for ΔH298 < 5

kJ·cm−3, i.e., the reaction will then be gradual (Fig. 5).

Calculations on systems (1) and (2) based on reactions (1)

and (2) showed that the value of ΔH298 is higher than 100

kJ·cm−3. Therefore, it can be concluded that in the first

stage of milling, the volume fraction of phases formed is

near zero, and the intensity decreasing of raw materials is

attributed to peak broadening and grain size decreasing.

Therefore, it can be argued that the grain size of starting

materials decreases gradually and has approximately same

size up to 9 h and 25 h of milling in system (1) and (2), re-

spectively. With further milling (9-15 h for system (1) and

25-30 h for system (2)), the intensity suddenly decreases

due to the consumption of starting materials in reactions

(1) and (2).

Fig. 4. Mean intensities of raw material peaks in sys-

tems (1) and (2) versus milling time.

For system (1), it was estimated that ignition occurred

between 9-15 h because raw materials peaks were com-

pletely disappeared in this stage of milling. More milling

experiments were performed in this range of milling (Fig.

6). The products were formed after 10 h of milling, while

the small peak of TiO2 was still observable. It means that

reaction (1) took place in MSR mode, but it was not com-

pletely performed after ignition. Incomplete propagation

of reaction (1) by MSR mode can be explained as this:

heating loss of the powders to the milling media and gas at-

mosphere, insufficient mixing of the reactants before com-

bustion, formation of the oxide layer on the Al powders,

and even a little amount of impurity in the reactants.

Fig. 5. Volume fraction of phases formed until ignition

versus the enthalpy of phase formation.

Fig. 6. XRD patterns of the milled powders in the

range of 9-12 h.

As can be seen in Fig. 6, the Al peak was disappeared

after 10 h, and it was appeared again after 12 h of milling.

It seems that incomplete reaction and heat releasing led to

the diffusion of unreacted Al to the lattice of alumina and

titanium diboride. Since reaction (1) is not completely per-

formed, the milled powders are not stable thermodynam-

ically. By increasing the milling time (12 h), the system

tends to reduce its internal energy and reach to a stable


condition. Therefore, the unreacted Al diffuses out to re-

act with TiO2 and B2O3 to form more thermodynamically

stable phases. The entire remained Al was consumed after

15 h of milling.

On the other hand, in system (2), ignition occurred

between 25-30 h of milling. Here, it is estimated that the

previous events have occurred. Fig. 7 shows the product

peaks at 10 h and 30 h of milling for systems (1) and (2),

respectively. One can observe that the intensity peaks in

system (1) are more than those in system (2) because more

heat is released from MSR mode in system (1). In other

words, heat released from reaction (1) is more than that

in reaction (2), leading to the formation of larger grains in

Fig. 7. Comparison of heat released from MSR mode

in both systems.

Fig. 8. Changes of vial temperature during milling of

the powder mixture in both systems.

system (1) and, hence, more intensity peaks. To confirm

this, the temperature of vials was measured during milling

of the powder mixture, which is shown in Fig. 8. Clearly,

the vial temperature shows a sudden increase, suggesting

that exothermic combustion has occurred after 9 and 25

h of milling in systems (1) and (2), respectively. As we

expected, the vial temperature in system (2) is lower than

that in system (1) because of the more milling energy con-

sumption in system (2).

3.3. Morphology and MicrostructureFigs. 9 and 10 show that the morphology of powder

Fig. 9. SEM micrographs of the milled powders in sys-

tem (1): (a) 3 h, (b) 9 h, and (c) 30 h.


particles after different milling time is related to systems

(1) and (2), respectively. The agglomeration can be seen

in both systems at 3 h of milling (Figs. 9(a) and 10(a)),

and it is likely because of the ductility of aluminum. This

property gives way to the diffusion of TiO2 and B2O3 pow-

der particles through the ductile aluminum and hence the

agglomerates form. In addition, due to the increase of

temperature during the first few hours of milling, the ther-

mally controlled dynamic recovery is dominant. This leads

Fig. 10. SEM micrographs of the milled powders in

system (2): (a) 3 h, (b) 9 h, and (c) 10 h.

to the fact that fracturing is prevented because of the an-

nihilation of generated dislocation during the deformation.

On the other hand, since the milling energy in system (2) is

less than one in system (1), this system has larger agglom-

erates compared to system (1) at 3 h of milling (Figs. 9(a)

and 10(a)). In fact, a higher energy leads to a more work

hardening and fracturing of agglomerates. By increasing

the milling time to 9 h, work hardening of the milled pow-

ders leads to the decrease in particles size (Figs. 9(b) and

10(b)). As can be seen in Figs. 9(c) and 10(c), after the

products formed by MSR mode, the released heat led to

the reagglomeration of the powder particles.

Table 1 shows the mean grain size of the milled pow-

ders for both systems. In the same condition of milling,

the mean grain size of the milled powders in system (2) is

larger than that in system (1). This difference is due to the

more milling energy consumption for the transformation of

acid boric to boron oxide. On the other hand, in system

(1), the whole of milling energy in the first stage of milling

(0-9 h) was consumed for microstructural refinement, such

as grain size decrease. Based on the values in Table 1, it

is concluded that immediately after the formation of prod-

ucts, the crystallite size is increased in both systems. This

is due to the heat released by the ignition of reactions (1)

and (2), which leads to grain growth during milling. In

general, in both systems, the longer the milling time is,

the smaller the mean grain size is. It means that in both

systems, nanostructure powders with the mean grain size

less than 50 nm were obtained at the end of milling.

A bright-field TEM image for the 15 h milled prod-

uct in system (1) is shown in Fig. 11. There are very fine

grains less than 50 nm in this figure, which is consistent

with Reitveld analysis results. In the image, TiB2 appears

as the dark phase and Al2O3 as the bright phase.

4. ConclusionsAn Al2O3-TiB2 nanocomposite was successfully syn-

thesized with two mixtures of starting materials. This com-

Table 1. Mean grain size of both systems in various

milling time nm

System Milling time / h TiO2 B2O3 Al Al2O3 TiB2

System (1)

3 46 41 62 — —

6 40 35 51 — —

9 34 28 37 — —

10 — — — 54 49

15 — — — 46 40

25 — — — 38 37

30 — — — 23 35

System (2)

3 53 51 69 — —

6 47 41 54 — —

9 39 34 39 — —

10 35 29 32 — —

15 — — — — —

25 30 27 29 — —

30 — — — 44 40


Fig. 11. TEM bright field image of the 15 h milled

product in system (1).

posite was formed after 15 and 30 h in system (1) (B2O3

source) and system (2) (H3BO3 source), respectively. In

system (2), a more milling energy was required for the

formation of this composite because of transformation of

boric acid to boron oxide. SEM results confirmed that at

the beginning of milling, the size of agglomerates in sys-

tem (2) is larger than that in system (1). Furthermore,

particle adhesion was seen when the products were formed

due to MSR mode. The vial temperature shows a sud-

den increase, suggesting that exothermic combustion has

occurred after 9 and 25 h of milling in systems (1) and

(2), respectively. As we expected, the vial temperature

for system (2) is less than that for system (1) because of

a more energy consumption in system (2). In both sys-

tems, nanostructure powders with the mean grain size less

than 50 nm were obtained at the end of milling, which is

consistent with the TEM image.

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