Simultaneous synthesis and densification of titanium nitride/titanium titanium diboride composites by high nitrogen pressure combustion

Article abstract of DOI:10.1111/j.1151-2916.2002.tb00564.x

Composites of TiN/TiB₂ were synthesized by a combustion process of BN, Ti in a nitrogen atmosphere. The effect of the BN/Ti ratio and the nitrogen gas pressure on the synthesis of these composites was investigated. Dense TiN/TiB₂ composites with relatively high hardness and toughness were fabricated by combustion synthesis from Ti and BN under a nitrogen pressure of 4.0 MPa. The Vickers microhardness of the products obtained from reactants with a BN/Ti mole ratio of 0.11 increased with an increase in nitrogen pressure and had a maximum value of ～25 GPa. Fracture toughness, K_IC, of the products increased from 3.1 to 5.9 MPa·m^1/2 as the BN/Ti ratio increased from 0.11 to 0.20. However, products formed under nitrogen pressures higher than 6.0 MPa exhibited circumferential macrocracks due to thermal shock.

Full text of DOI:10.1111/j.1151-2916.2002.tb00564.x

J. Am. Ceram. Soc., 85 [12] 2965–70 (2002)
journal
Simultaneous Synthesis and Densification of Titanium Nitride/
Titanium Diboride Composites by High Nitrogen Pressure Combustion
Masachika Shibuya and Manshi Ohyanagi*
Department of Materials Chemistry and High-Tech Research Center, Ryukoku University, Ohtsu 520-2134, Japan
Zuhair A. Munir*
Facility for Advanced Combustion Synthesis, Department of Chemical Engineering and Materials Science,
University of California, Davis, California 95616
Composites of TiN/TiB2 were synthesized by a combustion
process of BN, Ti in a nitrogen atmosphere. The effect of the
BN/Ti ratio and the nitrogen gas pressure on the synthesis of
these composites was investigated. Dense TiN/TiB2 composites
with relatively high hardness and toughness were fabricated
by combustion synthesis from Ti and BN under a nitrogen
pressure of 4.0 MPa. The Vickers microhardness of the
products obtained from reactants with a BN/Ti mole ratio of
0.11 increased with an increase in nitrogen pressure and had a
maximum value of ϳ25 GPa. Fracture toughness, KIC, of the
products increased from 3.1 to 5.9 MPa⅐m1/2 as the BN/Ti ratio
increased from 0.11 to 0.20. However, products formed under
nitrogen pressures higher than 6.0 MPa exhibited circumfer-
ential macrocracks due to thermal shock.
were free of the circumferential macrocracks. The fracture tough-
ness tended to increase with the content of nickel but the Vickers
microhardness decreased correspondingly.
Another high-temperature material which has been prepared by
combustion synthesis is TiB2. It also possesses attractive proper-
ties of hardness, electrical conductivity, and chemical stability.12
Thus the addition of TiB2 to form TiN/TiB2 composites has the
potential of improving the hardness and toughness of TiN.13–15
TiN/TiB2 composites are useful in applications such as cutting tool
and friction resistance materials. In the present work, we have
investigated the synthesis of dense TiN/TiB2 composites by a
high-nitrogen-pressure combustion reaction between Ti and BN
powders. In a recent work we have investigated the synthesis of dense
TiN/TiB2 nanocomposites using mechanical and field activation.16
II. Experimental Procedure
I. Introduction
Powders of titanium, with an average particle size of about 22
␮m and powders of BN with an average particle size of about 10
␮m were used in this study. The titanium, obtained from Sumi-
tomo SITIX, Inc. (Osaka, Japan), was reported to be 99.5% pure
and the boron nitride powders were obtained from Kojundo
Chemical Laboratory Co., Ltd. (Tokyo, Japan) and reported to be
99% pure. Powders of Ti and BN were weighed out in several
mole ratios and were dry-mixed in an automated agate mortar for
0.5 h. From these mixed powders, cylindrical compacts, ϳ16 mm
in diameter and 25 mm long, were formed in a stainless steel die
with double-acting rams by uniaxial pressing at 3.0 MPa. Each
compact was then placed in a porous crucible inside the combus-
tion chamber. Details of the apparatus and the experimental
methods have been provided in previous a publication.11 An
ignition device made of a slender carbon ribbon was placed on the
top end of the compact. The chamber, which had a volume of
5.0 ϫ 10Ϫ3 m3, was evacuated and back-filled with nitrogen three
times. Then it was filled with nitrogen to a pressure in the range of
0.1–10 MPa. The compact was then ignited by passing a current
through the carbon ribbon.
The combustion synthesis reactions were videotaped at 30
frames per second (Sony CCD-V800 with a Kenko ND400 filter).
The reaction temperature was monitored through a fused silica
window with an optical pyrometer (Model IR-AQ, range 1000–
3100°C, Chino, Inc.). The combustion wave velocity was mea-
sured with a video-measuring gauge (IV-560, For.A, Inc.). Fol-
lowing the synthesis process, products were polished with
diamond powders and examined by scanning electron microscopy
(SEM) (JSW-5200, JEOL) and analyzed by X-ray diffraction
(RAD-C system, Rigaku, Inc.) using CuK␣ radiation. For lattice
parameter determinations, X-ray scans were made stepwise with
⌬2␪ ϭ 0.02° and a collection time of 2 s using five peaks of the
sample as well as the Si standard.
VARIETY of high-temperature materials have been successfully
synthesized by self-propagating combustion using solid–solid
A
or solid–gas reactions.1,2 An example of the latter is TiN, a
material that has the attractive properties of relatively high
hardness, high chemical stability and corrosion resistance, and
high electrical conductivity.3 However, products of combustion
synthesis are generally porous and require further processing to
obtain dense bodies.4 Densification can be accomplished by such
processes as hot pressing,5 hot isostatic pressing,6 explosive
consolidation,7 or hot-forging.8
TiN has been synthesized in porous form by reacting Ti with
nitrogen gas under high pressure.9,10 More recently, Shibuya et
al.11 achieved simultaneous combustion synthesis and densifica-
tion of TiN under a nitrogen pressure of up to 10 MPa. The TiN
products fabricated under a nitrogen pressure of 2–10 MPa were
dense bodies, which maintained the shape of the initial specimen.
It was observed that the nitrogen pressure had an effect on
hardness and fracture toughness of the products. However, circum-
ferential macrocracks were generated in the product due to thermal
shock. To prevent cracking, the addition of controlled amounts of
elemental nickel was found necessary. Products with a Ni/Ti mole
ratio in the range of 0.1–0.6 were synthesized in dense form and
J. J. Petrovic—contributing editor
Manuscript No. 187352. Received November 6, 2001; approved September 23,
2002.
Support for M. Ohyanagi was provided by the Japanese Ministry of Education and
the High-Tech Research Center. Support for A. A. Munir was provided by the U.S.
Army Research Office (ARO).
Hardness and fracture toughness of the products were deter-
mined by Vickers microhardness (HMV-2000, Shimazu, Inc.;
*Member, American Ceramic Society.
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Journal of the American Ceramic Society—Shibuya et al.
Vol. 85, No. 12
load: 200 g; dwell time: 10 s) with indentation crack measure-
ments.17 Densities of the products were determined by the
Archimedes method, except in cases of porous products, where
the calculations were made based on weight and volume
measurements.
III. Results and Discussion
(1) Effect of BN Addition under Constant Nitrogen Pressure
The use of hexagonal BN as a reactant was made to provide a
solid source of nitrogen and thus minimize the problem of
permeation-limited reactions with gaseous nitrogen.10 The reaction
conditions were changed by changing the amount of BN under a
constant nitrogen pressure of 4.0 MPa. Figure 1(a) shows the effect
of BN addition on the maximum combustion temperature and the
combustion wave velocity. The maximum combustion temperature
decreased from ϳ3100° to 2500°C as the molar ratio BN/Ti
increased from 0 to 1.0. As will be seen later, the product of
combustion contained the two phases TiB2 and TiN only and thus
the role of BN in the combustion reaction can be depicted as
Fig. 2. XRD patterns of products formed from reactants with: (a)
BN/Ti ϭ 1.0, (b) BN/Ti ϭ 0.33, (c) BN/Ti ϭ 0.11 (nitrogen pressure: 4.0
MPa).
yTi ϩ xBN ϩ 0.5͑yϪ1.5x͒N2 ϭ 0.5xTiB2 ϩ ͑yϪ0.5x͒TiN
(1)
The role of nitrogen in the above reaction depends on the ratio of
BN/Ti (ϭx/y). Up to a ratio of x/y ϭ 0.67, nitrogen gas is a
reactant, but for higher ratios, it is a product. In the range of the
ratio BN/Ti investigated in this work (0.11–1.0), the composites
had a volume percent of TiB2 ranging from 7.3% to 57.5%. Based
on the reaction (1), the dependence of the adiabatic temperature on
the BN/Ti ratio was calculated and the results are shown in Fig.
1(b). The calculations were made using the HSC software with its
imbedded thermodynamic and thermophysical data.18 The short
plateau at 2920°C represents the melting point of TiB2. Although
one does not anticipate quantitative agreement between adiabatic
and experimental temperatures, a qualitative trend is usually
anticipated. However, this was not the case in this study. The
calculated results showed a marked decrease in temperature with
an increase in the BN/Ti ratio, while the experimental values
showed only a modest decrease. Moreover, the measured temper-
ature at BN/Ti ϭ 1.0 was nearly 1500°C higher than the calculated
value, 1093°C. This implies that the measured surface temperature
was not a true representation of the combustion temperature but
was dominated by a gas–solid reaction between Ti and nitrogen.
The adiabatic temperature for this reaction was high, 4630°C.2
The combustion wave velocity decreased markedly with an
increase in the BN/Ti ratio, as seen in Fig. 1(a). As the ratio
increased from 0 to 1.0, the velocity decreased from about 85 to
3.0 mm⅐sϪ1. These results are more consistent with the trend in the
calculated temperature, suggesting that the observed steady-state
wave propagation was representative of the bulk reaction under a
constant nitrogen pressure of 4.0 MPa and not the limited surface
reaction influencing the measured temperature.
Fig. 1. (a) Effect of BN addition on the maximum combustion temper-
ature and combustion wave velocity (nitrogen pressure: 4.0 MPa). (b)
Effect of the BN/Ti ratio on the calculated adiabatic temperature.
Fig. 3. Effect of BN/Ti ratio on the lattice constant and x of TiNx in the
products.

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2967
Fig. 4. Effect of BN/Ti ratio on the bulk relative densities of the products
(nitrogen pressure: 4.0 MPa).
Fig. 6. Effect of BN/Ti ratio on the Vickers microhardness and fracture
toughness KIC of the products (nitrogen pressure: 4.0 MPa).
As indicated above, the products of combustion were found to
contain TiN and TiB2 only with no evidence for unreacted Ti.
Figure 2 shows X-ray diffraction patterns of the products for
reaction with BN/Ti ratios of (a) 1.0, (b) 0.33, and (c) 0.11. The
relative peak intensity of TiB2 decreased, expectedly, with a
decrease in this ratio. As a phase, TiNx is stable over a wide range
of nitrogen stoichiometry and its lattice parameter depends corre-
spondingly on composition.19 From the published relationship and
our XRD results, we calculated the effect of the BN/Ti mole ratio
on the lattice constant and x in TiNx of the products as shown in
Fig. 3. The ratio of Ti/N increased toward the stoichiometric ratio
as the amount of BN in the reactants increased. This observation
supports the concept that BN is acted as a (solid) source of
nitrogen. The phase in the product with a BN/Ti ratio of 0.11 was
TiN0.79
.
The BN/Ti ratio influenced the final and relative densities of the
products based on Eq. (1), as shown in Fig. 4. The product with no
BN additive had circumferential macrocracks. However, the prod-
ucts with BN/Ti ratios in the approximate range 0.10–0.25 were
Fig. 5. SEM micrographs of polished product: (a) BN/Ti ϭ 0.11, (b) BN/Ti ϭ 0.25, (c) BN/Ti ϭ 0.5, (d) BN/Ti ϭ 1.0 (nitrogen pressure: 4.0 MPa).

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Journal of the American Ceramic Society—Shibuya et al.
Vol. 85, No. 12
Fig. 7. Effect of nitrogen pressure on the maximum combustion temper-
ature and combustion wave velocity (BN/Ti ϭ 0.11).
Fig. 9. XRD patterns of products synthesized at different nitrogen gas
pressure: (a) 0.1, (b) 4.0, (c) 10 MPa (BN/Ti ϭ 0.11).
relatively dense with no circumferential macrocracks. In the case
of BN/Ti ratios of 0.33–1.0, the products were porous but had no
cracks. The relative density of the product was calculated from a
combination of the reported density for TiB2, 4.50 g⅐cmϪ3, and the
density calculated with the lattice parameter for TiNx. The calcu-
lated density of TiNx was 5.20 g⅐cmϪ3 at x ϭ 0.79. The maximum
bulk density of the product was 4.80 g⅐cmϪ3 for a BN/Ti ratio of
0.11. This value corresponds to a relative density of 92.6%. As
indicated above, the products of samples with BN/Ti ratios higher
than 0.33 were porous, with the degree of porosity increasing as
this ratio was increased. The difference between the low and high
BN content samples was related to the presence of a liquid phase.
For samples with BN/Ti ratios in the range 0–0.25, the tempera-
ture was at or higher than the melting point of TiB2. The presence
of a liquid phase led to the formation of higher density product, as
has been observed before in many investigations. In contrast,
samples with BN/Ti ratios higher than about 0.3 had combustion
temperatures lower than the melting point of TiB2. Moreover, as
indicated above, in samples with BN/Ti Ն 0.67 nitrogen was
produced rather than consumed during combustion. The generation
of nitrogen led to further porosity generation, in a manner similar
to the effect of impurity evolution.20
indicated above. On the other hand, the products with BN/Ti ratios
of 0.50 and 1.0 were porous, as shown in Figs. 5(c) and (d).
Figure 6 shows the effect of BN addition on the Vickers
microhardness and on the fracture toughness, KIC, of the relatively
dense products (0 Յ BN/Ti Յ 0.3). The hardness showed a
dependence on the BN/Ti ratio, exhibiting a maximum value of
about 25 GPa at a ratio of 0.11. At this ratio the composite was
dense and its TiB2 content represented a value of 7.3 vol%. The
fracture toughness of the products showed a weak dependence on
BN addition, increasing from 3.1 to 5.9 MPa⅐m1/2 as the BN/Ti
ratio increased from 0.10 to 0.20. The maximum microhardness
value obtained for the dense products, 25 MPa (at a BN/Ti ratio of
0.11), was between the values reported for the pure phases: 33.0
and 20.1 GPa for TiB2 and TiN, respectively.21,22 Similarly, the
maximum value for the fracture toughness obtained in this work,
5.9 MPa⅐m1/2 (at a BN/Ti ratio of 0.20) was in the range between
the values of the pure phases. The fracture toughness for TiB2 is
reported as 6.4 MPa⅐m1/2 (Ref. 21) while that for TiN is reported
to be in the range of 3.4–4.3 MPa⅐m1/2 (Refs. 22 and 23).
(2) Effect of Nitrogen Pressure at Constant BN Addition
Powder compacts with a BN/Ti ratio of 0.11 were reacted under
nitrogen pressures ranging from 0.1 to 10 MPa. Figure 7 shows the
effect of nitrogen pressure on the maximum combustion temper-
ature and on the combustion wave velocity. Both parameters
increased with increasing nitrogen pressure. It should be recalled
that at this BN/Ti ratio, nitrogen is a reactant and the observed
Evidence of melting can be seen from the SEM micrographs of
the polished cross sections of products with BN/Ti ratios of 0.11
and 0.25 (Figs. 5(a) and (b)). The rounded particles of TiN (light
gray) are surrounded by the TiB2 phase (dark gray). The black
regions are pores. These products were relatively dense, as
Fig. 8. Effect of nitrogen pressure on sample cooling rate (BN/Ti ϭ
0.11).
Fig. 10. Effect of nitrogen pressure on the lattice constant and x of TiNx
(BN/Ti ϭ 0.11).

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Simultaneous Synthesis and Densification of Titanium Nitride/Titanium Diboride Composites
2969
enhancement of heat conduction by the presence of a molten
phase. At pressures of 4.0 MPa or higher, the combustion
temperature exceeds the melting point of TiB2.
Circumferential macrocracks formed inside the fabricated prod-
ucts under a nitrogen pressure higher than 6.0 MPa. Figure 8
shows the effect of the nitrogen pressure on the cooling rate from
the maximum temperature down to 1000°C. The cooling rate
increased with an increase in nitrogen pressure, consistent with the
role of a gas in convective heat loss, as stated above. For cases
with pressures higher than 6.0 MPa, the cooling rate was higher
than 60°C⅐sϪ1. The presence of the cracks is thus likely to be the
result of thermal shock for products synthesized under nitrogen
pressures higher than 6.0 MPa.
Figure 9 shows X-ray diffraction patterns of the products under
nitrogen pressure of (a) 0.1, (b) 4.0, and (c) 10 MPa. The products
were identified as TiNx and TiB2. The peak intensity of TiNx
increased with increasing nitrogen pressure. Unreacted titanium
was observed in the product under 1 atm (0.1 MPa) nitrogen
pressure only, consistent with previous observations.10 Figure 10
shows the effect of nitrogen pressure on the lattice constant and on
x in TiNx, as calculated from the XRD results. The x value of TiNx
increased with an increase in nitrogen pressure. For a pressure of
10 MPa, x was equal to 0.87. The effect of nitrogen pressure on the
bulk and relative density of the products is shown in Fig. 11. In the
pressure range between 0.1 and 4.0 MPa the density increased
significantly, but with a further increase in the pressure, the density
showed only a slight increase. At the highest pressure, 10 MPa, the
density was 4.90 g⅐cmϪ3, which correspond to a relative density of
94.0%, based on a nitride phase of TiN0.87 as indicated by the
XRD results. At a pressure of 4.0 MPa and higher, the temperature
exceeds the melting point of TiB2. Figure 12 shows SEM micro-
graphs of cross-sectioned TiN/TiB2 products obtained under nitro-
gen pressures of (a) 0.1, (b) 2.0, (c) 4.0, and (d) 10 MPa. The
Fig. 11. Effect of nitrogen pressure on bulk relative densities of the
products (BN/Ti ϭ 0.11).
dependence on its pressure is consistent with general expectations.
The maximum combustion temperature reached 3000°C under a
nitrogen pressure of about 4.0 MPa and increased further as the
pressure increased. The combustion wave velocity increased mark-
edly from ϳ3.0 to 78 mm⅐sϪ1 as the nitrogen pressure increased
from 0.1 to 10 MPa. The combustion wave velocity usually
decreases with an increase in pressure due to convective heat loss,
but in this case, the effect is dominated by the role of nitrogen as
a reactant, as indicated above. A pressure increase signifies an
increase in the local availability of nitrogen at the gas/solid
interface. The increase in wave velocity is probably related to the
Fig. 12. SEM micrographs of products synthesized at different nitrogen gas pressure: (a) 0.1, (b) 2.0, (c) 4.0, (d) 10 MPa (BN/Ti ϭ 0.11).

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Journal of the American Ceramic Society—Shibuya et al.
Vol. 85, No. 12
macrocracks. However, samples synthesized under nitrogen pres-
sures higher than 6.0 MPa contained macrocracks due to thermal
shock.
Acknowledgments
This work is part of a collaborative agreement between Ryukoku University in
Japan and the University of California at Davis.
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Ⅺ