Synthesis of TiB2 by electric discharge assisted mechanical milling

Article abstract of DOI:10.1016/j.jallcom.2006.09.073

In this study, synthesis of titanium diboride from elemental powders of Ti and B by electric discharge assisted mechanical milling technique was investigated. This recently developed technique has the following advantages: rapid reaction rate, controlled

Full text of DOI:10.1016/j.jallcom.2006.09.073

Journal of Alloys and Compounds 440 (2007) 346–348
Synthesis of TiB by electric discharge assisted mechanical milling
2
A. Calka , D. Oleszakb
Faculty of Engineering, University of Wollongong, Northfields Ave., Wollongong, NSW 2522, Australia
a,∗
a
b
Faculty of Materials Science and Engineering, Warsaw University of Technology, Poland
Received 28 July 2006; accepted 10 September 2006
Available online 23 October 2006
Abstract
In this study, synthesis of titanium diboride from elemental powders of Ti and B by electric discharge assisted mechanical milling technique
was investigated. This recently developed technique has the following advantages: rapid reaction rate, controlled reaction, direct reaction between
Ti and B without adding another element into the system and cost effectiveness. TiB
with an ac high voltage transformer, generating impulses within kV/mA range. The structures of reaction products were characterized by X-ray
diffractometry, and powder morphologies by SEM. X-ray diffraction studies showed that the milling product was TiB with small fraction of TiB.
2006 Elsevier B.V. All rights reserved.
2
samples were prepared using an in-house built reactor
2
©
Keywords: Mechanochemical processing; Mechanical alloying
1
. Introduction
2. Experimental procedure
Titanium powder of particle size <250 ␮m and minimum purity of 99.9%
was mixed with amorphous boron powder of particle size <50 ␮m (purity 99.8%)
to give composition of TiB2. Before electric discharge milling experiments Ti
and B powders were pre-mixed for 1 h, under high purity argon in conventional
laboratory ball mill. Electric discharge milling was performed in a modified
vibrational laboratory rod mill. The mill was designed to produce a milling
mode combining a repeated impact of a hardened curved rod end on powder
particles, placed on a vibrating hemispherical container under electrical condi-
tions of pulsed arc discharges (Fig. 1). The electric discharges were generated
during milling in the gaps between the vibrating mill base, the powder parti-
cles and a loosely suspended conducting plunger. The power supply used in
this study was connected through an ac high voltage transformer, generating
impulses within the kV/mA range. During vibration, small gaps between the
stainless steel rod and the chamber wall resulted in an arc discharge (Fig. 1).
The localized temperature of reacting powder increased by Joule heating gen-
erated simultaneously by the electric arc discharge and mixing by the vibrating
Titanium diboride (TiB2) is an advanced ceramic material
with relatively high strength and durability as characterized
by high melting point, hardness, strength to density ratio, and
wear resistance. It can withstand an oxidizing atmosphere up
◦
to 1000 C [1–3]. TiB2 is tough enough to be used as military
armor and improves the fracture toughness of ceramic cutting
tools and other components. It is also use as a cathodes material
in the electro-chemical reduction of alumina to aluminum metal.
TiB2 maybe hot-pressed (HP) or hot isostatic pressed (HIP)
into desired shapes or electro-discharged machined due to its
electrical conductivity.
The TiB2 powders are generally prepared by a borothermic
reduction of titania, fused-salt electrolysis, solution phase pro-
cessing, gas combustion synthesis technique using the gas phase
flame reaction and self-propagating high temperature synthesis
3
electrode. Milling was carried in a high purity Ar flowing with the rate 3 m /min.
In all experiments we used milling times 2, 5 and 10 min. XRD analysis of
the as-milled powders was performed using a Phillips PW1730 diffractometer
with a graphite monochromator and Cu K␣ radiation. Phase identification was
carried out using the International Centre for Diffraction Data (JCPDS-ICDD
(
SHS) technique [4–8].
In the present work we describe rapid synthesis of TiB2 using
novel electric discharge assisted mechanical milling method
2000) powder diffraction files (PDF).
(
EDAMM) [9].
The crystallite size of the powders was estimated using the method of inte-
gral breadths as described by Varin et al. [10]. The full width at half maximum
FWHM) values were corrected for instrument broadening using Warren’s cor-
rection [11–13]:
(
∗
Corresponding author. Tel.: +61 2 4221 4945; fax: +61 2 4221 3112.
E-mail addresses: Andrzej Calka@uow.edu.au, acalka@uow.edu.au
A. Calka).
2
2
2
(
B = Bt − Bi
(1)
0
925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.09.073

A. Calka, D. Oleszak / Journal of Alloys and Compounds 440 (2007) 346–348
347
tional Centre for Diffraction Data powder diffraction file (PDF)
card 05-0682 which corresponds to ␣-Ti. Surprisingly this
XRD pattern do not show any boron XRD peaks. The absence
of any boron XRD peaks could be due to amorphous nature
of the boron powder. The absence of clearly define crystalline
structure and much lower mass absorption coefficient of boron
in comparison with Ti makes boron very difficult to detect by
XRD analysis in the presence of titanium.
Fig. 2b shows that after 2 min of milling the XRD pattern
contains strong TiB2 peaks and weak peaks corresponding
to TiB phase and unreacted ␣-Ti. After milling for 5 and
Fig. 1. Schematics of electrical discharge milling device used in this work.
1
0 min (Fig. 2c and d) there are only strong TiB2 and weak
where B is the corrected peak broadening, Bt the total broadening and
Bi is the instrumental broadening. The instrumental broadening was deter-
mined using the strongest peak from the XRD pattern of a silicon reference
sample.
Powder morphologies were characterized using scanning electron micro-
scope (SEM).
TiB peaks. Interestingly TiB peaks become less intense and
broader after 10 min of milling indicating nanostructural
character of TiB phase. The peak broadening analysis indicates
that the TiB crystallite size estimated to be approximately
8
0 nm.
Fig. 3 shows SEM micrographs of starting powder (Fig. 3a)
3
. Results and discussion
and milled after 2, 5 and 10 min (Fig. 3b–d). Fig. 3a illustrates
microstructureofmixtureofirregularlargeTiparticlesandsmall
boron particles. After 2 min of milling the size of Ti particles
becomes smaller—below 100 ␮m. Fig. 3c and d shows pow-
der morphology after 5 and 10 min of milling. One can notice
that between 5 and 10 min of milling powder fracturing is much
slower than within the first 2 min of milling. Prolonged milling
time up to 10 min causes formation of large number of parti-
cles about 2 ␮m in size (Fig. 3d) predominantly in the form of
agglomerates.
Fig. 2 shows the X-ray diffraction patterns of ␣-Ti and B
powders milled for 2, 5, and 10 min (Fig. 2b–d). Diffraction
pattern of pre-mixed, starting powder is shown in Fig. 2a. The
peaks in this pattern give an excellent match with the Interna-
The phase diagram of the TiB system shows presence of TiB,
Ti3B4 and TiB2 phases. Our work shows that TiB2 phase can be
rapidly formed as a result of direct reaction between Ti and B
powders.
When the high intensity electric discharge acts on the mate-
rial, it creates a plasma channels with the localized high temper-
ature areas as a result of Joule heating. The high temperatures in
plasma channels accelerate diffusion and in consequence lead
to fast reaction between Ti and B within short times. Subse-
quent rapid quenching in these areas induces supersaturaion of
reactants which in turn, leads to formation of compounds with
desired chemistry and formation of fine powders as a combined
effect of fracturing caused by moving plunger and vapor con-
densation.
The XRD pattern revealed very low intensity and broad XRD
peaks corresponding to TiB phase. Small intensity of these peaks
indicate of presence of small volume of this phase.
Broad shape of these peaks indicate nanoscale grain size
and/or effect of high lattice strain concentration. We believe that
TiB2 particles are formed as a result of direct diffusion between
Ti and B particles. As the process progresses more TiB2 parti-
cles surround Ti particles and towards the end of reaction there
are small number of particles that are not in direct contact with
Ti surfaces which may lead to formation of TiB areas as a result
of much slower diffusion of Ti through TiB2 phase. If TiB areas
are formed within TiB2 particles with different coefficient of
thermal expansion value this may lead to additional generation
of internal lattice strain which may contribute to the XRD peak
broadening.
Fig. 2. X-ray diffraction patterns of (a) pre-mixed starting powder, and electric
discharge milled for (b) 2 min, (c) 5 min, and (d) 10 min.

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48
A. Calka, D. Oleszak / Journal of Alloys and Compounds 440 (2007) 346–348
Fig. 3. Scanning electron micrographs of (a) pre-mixed starting powder, and electric discharge milled for (b) 2 min, (c) 5 min, and (d) 10 min.
4
. Conclusions
It was found that milling of Ti and B elemental powders,
References
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milling dramatically enhance reaction rate presumable to high
local temperatures generated by electric discharges which in
consequence speeds up diffusion. XRD peaks intensities indi-
cate presence of large volume of TiB2 and small volume of
TiB phases in nanocrystalline form. Prolonged milling time up
to 10 min causes formation of large number of particles about
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