Mechanochemical synthesis and pressureless sintering of TiB2-AlN composites

Article abstract of DOI:10.1111/j.1151-2916.2002.tb00216.x

TiB₂-AlN composites have been fabricated by the pressureless sintering of a mechanochemically processed Ti, Al, and BN powder mixture. TiB₂-AlN powder was obtained from the mixture of Ti, Al, and BN, which had a composition corresponding to 45.7 wt% TiB₂-54.3 wt% AlN, after mechanochemical processing for longer than 24 h. X-ray diffraction and transmission electron microscopy analysis showed that the powder subjected to mechanochemical processing for 60 h consisted of crystallites less than 300 nm in size with a disordered crystal structure. TiB₂-AlN composites with 95% relative density, a flexural strength of 172 MPa, a fracture toughness of 4.6 MPa·m^1/2, a hardness of 12.0 GPa, and an electrical resistivity of 1488 μΩ·cm were obtained by pressureless sintering at 1700°C for 2 h of the powder subjected to mechanochemical processing for 60 h.

Full text of DOI:10.1111/j.1151-2916.2002.tb00216.x

J. Am. Ceram. Soc., 85 [4] 1022–24 (2002)
journal
Mechanochemical Synthesis and Pressureless Sintering of
TiB2–AlN Composites
Hyung-Jong Kim, Heon-Jin Choi,* and June-Gunn Lee*
Multifunctional Ceramics Research Center, Korea Institute of Science and Technology, Seoul 130-650, Korea
TiB2–AlN composites have been fabricated by the pressureless
sintering of a mechanochemically processed Ti, Al, and BN
powder mixture. TiB2–AlN powder was obtained from the
mixture of Ti, Al, and BN, which had a composition corre-
sponding to 45.7 wt% TiB2–54.3 wt% AlN, after mechano-
chemical processing for longer than 24 h. X-ray diffraction and
transmission electron microscopy analysis showed that the
powder subjected to mechanochemical processing for 60 h
consisted of crystallites less than 300 nm in size with a
disordered crystal structure. TiB2–AlN composites with 95%
relative density, a flexural strength of 172 MPa, a fracture
toughness of 4.6 MPa⅐m1/2, a hardness of 12.0 GPa, and an
electrical resistivity of 1488 ␮⍀⅐cm were obtained by pressure-
less sintering at 1700°C for 2 h of the powder subjected to
mechanochemical processing for 60 h.
II. Experimental Procedure
Ti (99.8%, Ϫ200 mesh, Cerac., USA), Al (99.9%, Ϫ325 mesh,
High Purity Chemicals, Japan), and BN (grade A01, H. C. Stark,
Germany) powder were used as starting materials. A powder
mixture was prepared with a mole ratio of 1:2:2 of Ti, Al, and BN.
This is the composition corresponding to 45.7 wt% TiB2–54.3
wt% AlN. Ten grams of the powder mixture was loaded into a
cylindrical WC vial with three WC balls. Mechanochemical
processing was conducted in a shaker mill (Model 8000 Mixer/
Mill, Supreme Technology, Inc., USA) for 60 h at room temper-
ature. The processed powder was compacted uniaxially and
isostatically pressed at 25 000 psi for 5 min. Sintering was
performed in the range of 1500–2000°C for 2 h under flowing Ar.
X-ray diffractometry (XRD, MXP, Mac-Science, Japan) was
used to determine the crystalline phases in the powder. From the
XRD patterns, full width at half-maximum (FWMH) of diffraction
peaks was calculated after peak deconvolution assuming a Gauss-
ian function, and size and strain variance of TiB2 crystallites in the
powder were determined using the Williamson–Hall method.13
Commercial TiB2 powder (99.9%, Ϫ325 mesh, ART Inc., Buffalo,
NY) was used as a standard powder for the correction of
instrumental peak broadening. Scanning electron microscopy
(SEM, Jeol, JSM 6000F, Jeol, Japan) was used to characterize the
size and morphology of powder and fracture surfaces of sintered
composites. Transmission electron microscopy (TEM, CM 30,
Philips, The Netherlands) was also used to characterize the
powder. The density of specimens was measured using the
Archimedes method. The theoretical density of the specimen was
calculated according to the rule of mixtures based on the theoret-
ical densities of the target materials. To test mechanical properties,
the pressureless sintered specimens were cut and polished to
rectangular bars with dimension of 1.5 mm ϫ 2 mm ϫ 25 mm.
Strength was measured in a four-point bending configuration with
a crosshead speed of 0.5 mm/min, and inner and outer spans of 10
and 20 mm, respectively. The fracture toughness of the sintered
composite sample was characterized by the single-edge notched
beam (SENB) method using rectangular bars with dimensions of 3
mm ϫ 4 mm ϫ 30 mm. The Vickers indentation method was used
to measure the hardness of sintered samples. Electrical resistivity
was characterized by using the four-point terminal method.
I. Introduction
ECAUSE of its excellent mechanical, chemical, and electrical
B
properties, TiB2-based composites are suitable materials for
vacuum metallization applications.1–3 Densification of TiB2-based
composites is difficult because of its covalent bonding nature. Hot
pressing and large amounts of sintering aids have been used to
prepare dense TiB2-based composites.4–8 However, hot pressing
has limited shape-forming capability. Furthermore, sintering aids
can leave deleterious impurities in the final product, such as
metallic phases.
Recently, the mechanochemical process has been investigated for
obtaining ceramic powders such as LaAlO3, Pb(Mg1/3Nb2/3)O3,
and Ti–Al2O3 composites.9–11 The process utilizes mechanical
energy instead of thermal energy to provide the activation energy
for solid-state reaction and has several advantages over both
conventional solid-state reaction and wet-chemical processes in-
cluding the use of low-cost raw materials, simplicity of the
process, and the ability to obtain fine particles. It is interesting that
the powders obtained by this process usually have a crystallite size
in the nanometer regime and a highly disordered crystal structure12
that can act as a strong driving force for densification.
In this study, TiB2–AlN composites have been obtained by the
pressureless sintering of TiB2–AlN powder that has been prepared
by using the mechanochemical processing of a mixture of Ti, Al,
and BN. The properties of the pressureless sintered TiB2–AlN
composites indicate that the mechanochemical process can be
considered as a candidate for synthesizing covalently bonded
materials that are more easily densified.
III. Results and Discussion
Figure 1 shows the XRD patterns of the mixed Ti, Al, and BN
powder that was subjected to mechnochemical processing. Sharp
peaks for Al and Ti were observed for the powder subjected to the
mechanochemical processing for 12 h. However, these peaks
vanished and peaks corresponding to TiB2 and AlN appeared for
the powder processed for 24 h. This result indicates that solid-state
reactions involving Ti, Al, and BN occurred during mechanochem-
ical processing. Peaks corresponding to TiN were also observed
for the powder mechanochemically processed for 24 and 36 h. The
peaks corresponding to minor phases (i.e., AlN and TiN) vanished
after mechanochemical processing for longer than 36 h, which may
O. C. Wilson Jr.—contributing editor
Manuscript No. 187788. Received April 5, 2001; approved January 23, 2002.
*Member, American Ceramic Society.
1022

April 2002
Communications of the American Ceramic Society
1023
Fig. 1. XRD patterns of Al, BN, and Ti mixture powder subjected to
mechanochemical processing for 60 h.
be due to peak broadening of the TiB2 phase or alloying of AlN
and TiN with the TiB2 phase during the extensive milling. The
broad peaks of the TiB2 phase in the powder processed for 60 h
imply that the powder had a very small crystallite size and/or some
degree of amorphous state, which are frequently observed in
mechanochemically processed powders.14,15
Figure 2 shows the size and the strain variances of the TiB2
crystallites in the powder calculated using the Williamson–Hall
equations. The crystallite size was about 1.5 ␮m for the powder
processed for 24 h and decreased to ϳ300 nm for 48 h. The strain
variances increased with processing time until a maximum of
0.53% was achieved, after which the strain variance decreased
exponentially to a saturation value of 0.38–0.39%. The increase in
strain variance indicates the disordered state of the crystal struc-
ture. Therefore, the crystallite size and strain variance calculations
indicate that the powder had a disordered structure as well as small
crystallite size. During mechanochemical processing, the starting
powders are heavily impacted under impact pressures in the
gigapascal range, fractured to nanometer scale, and highly activat-
ed.16 Then the TiB2 (and AlN and TiN although it was not
analyzed in this study) crystallites may be formed via solid-state
reaction at many contact points between starting materials. As
compared with the thermal process, it proceeds intermittently and
locally and, therefore, sufficient time for ordering of atoms by
diffusion and rearrangement for a well-ordered crystal structure
may not be available and a disordered crystal structure results.
Fig. 3. SEM (a) and TEM (b) micrographs of the powder subjected to
mechanochemical processing for 60 h.
Figure 3 shows SEM and TEM micrographs of the powder
subjected to mechanochemical processing for 60 h. The powder
consisted of spherical grains in the range of 1–10 ␮m with larger
agglomerates (Fig. 3(a)). However, the TEM investigation of the
powder indicated that it consisted of crystallites with sizes below
300 nm, although particle agglomerates in the micrometer size
range were visible (Fig. 3(b)).
Figure 4 shows the relative densities of the TiB2–AlN compos-
ites prepared by pressureless sintering of the powder that was
subjected to mechanochemical processing for 60 h. The density
increased with the sintering temperature from 67% to 98% for
Fig. 2. Crystallite sizes and strain variance of TiB2 in the powder
Fig. 4. Relative density of TiB2–AlN composites as a function of
subjected to mechanochemical processing for 60 h.
sintering temperature.

1024
Communications of the American Ceramic Society
Vol. 85, No. 4
Table I. Compositions, Relative Densities, Sintering Conditions, and Mechanical and Electrical Properties of
TiB2-Based Composites
Mechanical properties
Relative
density
(%)
Flexural
strength
(MPa)
Fracture
Electrical
resistivity
(␮⍀⅐cm)
Composition
(wt%)
toughness
Hardness
(GPa)
Sintering conditions
(MPa⅐m1/2
5.21
5.05
3.7
)
Ref.
33.3TiB2⅐66.7AlN
Hot pressing, 200°C,
1 h, 25 MPa
Hot pressing, 1740°C,
0.5 h, 25 MPa
Gas pressure sintering, 1750°C,
1 h, 0.2 MPa
Pressureless sintering,
1700°C, 2 h
96.8
98.2
96.5
96.1
352
539
220
172
6
33.0TiB2⅐64.0AlN
17.8
9.5
12
7
8
10.0TiB2⅐88.0AlN(⅐2.0Y2O3)
45.7TiB2⅐54.3AlN
4.6
1488
This study
IV. Summary
By using a mechanochemical process, TiB2–AlN powder can be
obtained from a mixture of Ti, Al, and BN, having a composition
corresponding to 45.7 wt% TiB2–54.3 wt% AlN. Dense TiB2–AlN
composites can also be prepared by pressureless sintering of the
powder at low temperature. This study demonstrates the feasibility
of using the mechanochemical process to synthesize covalently
bonded materials that are easier to densify.
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