Processing and properties of TiB2 with MoSi2 sinter-additive: A first report

Article abstract of DOI:10.1111/j.1551-2916.2005.00652.x

The densification of non-oxide ceramics like titanium boride (TiB ₂) has always been a major challenge. The use of metallic binders to obtain a high density in liquid phase-sintered borides is investigated and reported. However, a non-metallic sintering additive needs to be used to obtain dense borides for high-temperature applications. This contribution, for the first time, reports the sintering, microstructure, and properties of TiB ₂ materials densified using a MoSi₂ sinter-additive. The densification experiments were carried out using a hot-pressing and pressureless sintering route. The binderless densification of monolithic TiB₂ to 98% theoretical density with 2-5 μm grain size was achieved by hot pressing at 1800°C for 1 h in vacuum. The addition of 10-20 wt% MoSi₂ enables us to achieve 97%-99%ρ_th in the composites at 1700°C under similar hot-pressing conditions. The densification mechanism is dominated by liquid-phase sintering in the presence of TiSi₂. In the pressureless sintering route, a maximum of 90%ρ_th is achieved after sintering at 1900°C for 2 h in an (Ar+H₂) atmosphere. The hot-pressed TiB₂-10 wt% MoSi₂ composites exhibit high Vickers hardness (～26-27 GPa) and modest indentation toughness (～4-5 MPa·m¹²).

Full text of DOI:10.1111/j.1551-2916.2005.00652.x

J. Am. Ceram. Soc., 89 [1] 131–138 (2006)
DOI: 10.1111/j.1551-2916.2005.00652.x
r 2006 The American Ceramic Society
ournal
J
Processing and Properties of TiB2 with MoSi2 Sinter-additive:
A First Report
T. S. R. Ch. Murthy, B. Basu,w and R. Balasubramaniam
Department of Materials and Metallurgical Engineering, Indian Institute of Technology, IIT-Kanpur, India
A. K. Suri, C. Subramanian, and R. K. Fotedar
Material Processing Division, Bhabha Atomic Research Center, Mumbai, India
The densification of non-oxide ceramics like titanium boride
(TiB2) has always been a major challenge. The use of metallic
binders to obtain a high density in liquid phase-sintered borides
is investigated and reported. However, a non-metallic sintering
additive needs to be used to obtain dense borides for high-tem-
perature applications. This contribution, for the first time, re-
ports the sintering, microstructure, and properties of TiB2
materials densified using a MoSi2 sinter-additive. The densifi-
cation experiments were carried out using a hot-pressing and
pressureless sintering route. The binderless densification of mon-
olithic TiB2 to 98% theoretical density with 2–5 lm grain size
was achieved by hot pressing at 18001C for 1 h in vacuum. The
addition of 10–20 wt% MoSi2 enables us to achieve 97%–99%
of addition of B4C along with 0.5 wt% Fe, abnormal grain
growth was suppressed considerably, and an increase in sintered
density up to 95% rth was measured.4 However, the presence of
a metallic binder is not desirable for high-temperature structural
applications. Therefore, non-metallic sinter-additives need to be
used, with an aim to improve sinterability and also to retain
high-temperature properties with good oxidation resistance.
Different non-metallic additives such as AlN,5 ZrO2,6–12
SiC,8,10,12,13 Si3N4,14 CrB2,15 B4C,9,16 TaC,17 TiC,17,18 WC,17
17
TiN,17 ZrN,17 and ZrB2 have been used for attaining densifi-
cation of TiB2 with good mechanical properties. Torizuka and
Kishi8 and Torizuka et al.12 observed the formation of grain-
boundary liquid phase (amorphous SiO2) when SiC was used as
an additive. Moreover, they identified the presence of a partial
liquid phase, (Ti,Zr)5Si3, at triple points in a TiB2–19.5 wt%
ZrO2–2.5/5.0 wt% SiC composite processed via the hot isostatic
pressing (HIP) route. Watanabe and Shoubu19 reported the for-
mation of a (Ti,Zr)B2 solid solution in hot-pressed TiB2–30 wt%
ZrO2 composites. Telle et al.6 also observed the formation of a
(Zr,Ti)O2 solid solution in addition to a (Ti,Zr)B2 solid solution,
when 25 vol% ZrO2 was added to TiB2. In another interesting
investigation, Torizuka et al.10 did not report any solid solution
formation when 20 wt% ZrO2 binder was used to densify TiB2
via the HIP route. According to Murata et al.,17 TaC and TaN
were quite effective in densifying TiB2. In their work, (Ti,Ta)B2
and (Ta,Ti)(C,N) solid solutions were observed to form after hot
pressing at 20001C, when TaC was added to TiB2. Li et al.5
identified the formation of BN, TiN, and Al2O3 phases in a
TiB2–20 wt% AlN composite, fabricated by hot pressing at
18001C. In another work, Ho Park et al.14 reported the forma-
tion of different reaction products like TiN and BN in TiB2–2.5–
10 wt% Si3N4 composites (sintered at 18001C), and observed
that Si3N4 acts as a grain growth inhibitor.
qth in the composites at 17001C under similar hot-pressing con-
ditions. The densification mechanism is dominated by liquid-
phase sintering in the presence of TiSi2. In the pressureless sin-
tering route, a maximum of 90% qth is achieved after sintering
at 19001C for 2 h in an (Ar1H2) atmosphere. The hot-pressed
TiB2–10 wt% MoSi2 composites exhibit high Vickers hardness
(B26–27 GPa) and modest indentation toughness (B4–5
MPa m1/2).
.
I. Introduction
ITANIUM BORIDE (TiB2) is a potential candidate material for
high-temperature structural applications because of an ex-
T
cellent combination of properties such as high melting point
(B32001C), high thermal conductivity (60–120 W ꢀ (mꢀ K)ꢁ1),
high elastic modulus (B500 GPa), and high hardness (25–32
GPa).1 Despite having the above-cited useful combination of
properties, the applications of monolithic TiB2 are rather limited
because of poor sinterability and exaggerated grain growth at
high temperature. To overcome this problem, various metallic
and non-metallic binders are used to obtain dense borides.
Extensive research has been reported in the open literature on
the role of metallic binders in the densification of TiB2. Ferber
et al.2 have used up to 10 wt% Ni to achieve more than 99%
theoretical density by the hot-pressing route (14251C). The
strength and toughness of TiB2 ceramics are significantly en-
hanced with a finer grain size (4 mm) and low Ni content
(o2 wt%).2 Kang et al.3 observed that simultaneous addition of
0.5 wt% Fe and 0.5 wt% Cr enhances the densification of TiB2
in the temperature range of 18001–19001C. They have shown
that when a small amount of Fe (0.5 wt%) is added, abnormal
grain growth occurs and the sintered density is low. In the case
Among various high-temperature intermetallics, MoSi2 has a
high thermal conductivity (B53 W ꢀ (mꢀ K)ꢁ1) and good oxida-
tion resistance (up to 14001C).20 Additionally, MoSi2 is reported
to have good thermo-mechanical compatibility with TiB2,20 as
can also be predicted from the small mismatch in the thermal
expansion co-efficient (8.1ꢂ 10ꢁ6
K
ꢁ1 for TiB21 and 8.5ꢂ 10ꢁ6
Kꢁ1 for MoSi2).20 While MoSi2 can be a potential reinforce-
ment/sinter-additive for developing TiB2 materials for high-tem-
perature applications, no attempt, according to the best of the
authors’ knowledge, has been made to assess such a feasibility.
In this perspective, the present work reports the processing,
microstructure, and mechanical properties of TiB2–MoSi2
composites.
R. Koc—contributing editor
II. Experimental Procedure
(1) Processing
Manuscript No. 20324. Received October 31, 2004; approved June 8, 2005.
Supported by Board of Research in Nuclear Science (BRNS) programme of Depart-
ment of Atomic Energy (DAE), Government of India.
TiB2 and MoSi2 powders, prepared in the laboratory, were se-
lected as starting powders for composite production. The mean
particle diameter and particle size distribution of both the TiB2
wAuthor to whom correspondence to addressed. e-mail: bikram@iitk.ac.in
131

132
Journal of the American Ceramic Society—Murthy et al.
Vol. 89, No. 1
and MoSi2 powders were measured using a laser particle size
analyzer (Analsette 22, Fritsch, Germany). The specific surface
area was obtained using the BET (Coulter, Miami, FL, SA300)
method. In the production of TiB2 powders, B4C powders, syn-
thesized in our laboratory are used. B4C powders were processed
using carbothermic reduction of commercial boric acid (H3BO3,
laboratory reagent grade):
4H3BO3 þ 7C ¼ B4C þ 6CO þ 6H2O
(1)
Carbon, used in the above reaction, was commercially available
high-purity (499% purity) petroleum coke (M/S Assam car-
bon, Guwahati, India). The size of the coke was ꢁ325 mesh size,
and loss on ignition (LOI) of the coke used was found to be o
0.5%. During the initial stage of reaction (1), H3BO3 is reduced
to form B2O3, which, above 12001C, is further reduced by car-
bon to form B4C lumps. These lumps are mechanically crushed
and ball milled to produce fine powers, which are purified by
acid leaching followed by washing with water to remove impu-
rities. The details of B4C powder production can be found
elsewhere.21 As part of the present work, TiB2 powder was pro-
duced by a borothermic reaction, as follows:
2TiO2 þ B4C þ 3C ! 2TiB2 þ 4CO
(2)
Oxygen and carbon, the major impurities in as-synthesized TiB2,
were determined by the vacuum fusion analysis technique (Leco
Industries). The details of the starting powders are summarized
in Table I.
The synthesized TiB2 powder was further ground to break
down the agglomerates to form finer particles having a D50
around 1.1 mm. MoSi2 was synthesized from elemental powders
of Mo (499% purity, Leco Industries) and Si (499% purity,
Merck, Germany). Mechanical grinding further reduced the size
of the synthesized MoSi2 powders, leading to a finer particle size
with a D50 around 1.4 mm. SEM images of the starting powders
are shown in Fig. 1. While finer equiaxed TiB2 particles can be
observed in Fig. 1(a), some agglomerated MoSi2 particles can be
noted in Fig. 1(b). To this end, it can be noted that an alternative
route to obtain submicrometer TiB2 is reported elsewhere.22
Approximate amounts of boride and silicide powders of var-
ious compositions (0, 10, 15, 20, and 25 wt% MoSi2) were mixed
using WC grinder. The densification was performed by hot press-
ing for 1 h in vacuum (10ꢁ5 Pa), using a 10 mm diameter graphite
die at 32 MPa pressure. The samples were hot pressed at 17001C
at a heating rate of 151C/min. Selected experiments were also
carried out at 16501 and 18001C. After completion of sintering,
vacuum and pressure were maintained till the end of cooling. The
thickness of the hot-pressed pellets was around 6–8 mm.
To study the feasibility of densification via the pressureless
sintering route, the green pellets of various TiB2–MoSi2 com-
posites (without organic binder) were used. The green density of
the cold-pressed specimens was 65% of the theoretical density.
The green samples were sintered for 2 h at 19001C at a heating
rate of 151C/min in a reducing atmosphere (Ar1H2). Prior to
the sintering experiments, the vacuum chamber was evacuated
and backfilled with Ar to ensure that no residual oxygen was
present.
Fig. 1. SEM images showing the morphology of starting powders: (a)
titanium boride (TiB2) and (b) MoSi2.
(2) Microstructure and Mechanical Properties
Characterization
The cleaned dense pellets were weighed, and the density was
measured in water using the Archimedes principle. The crystal-
line phases in the starting powders and hot-pressed sample were
analyzed using XRD (Rich-Seifert, Germany, 2000D). Micro-
structural investigation of the phase assemblage was performed
by means of SEM on polished and fractured surfaces. Detailed
microstructural analysis was carried out using EPMA (WDS
analysis) and 200 kV TEM (JEM 2000 FX, JEOL, Tokyo,
Japan). A thin foil for TEM was prepared following standard
ion beam thinning procedures.
The mechanical properties were measured only for hot-
pressed samples. The Vickers hardness on the smoothly pol-
ished hot-pressed samples was obtained with a load of 10 kg
(dwell time 15 s). The indentation fracture toughness (KIc) data
were evaluated by crack length measurement of the crack pat-
tern formed around Vickers indents, adopting the model for-
mulation proposed by Anstis et al.23: KIc 5 0.016(E/H)1/2P/c3/2
,
where E is the Young’s modulus, H the Vickers’s hardness, P the
applied indentation load, and c the half crack length. The re-
ported values are the average of five indentation tests.
Table I. Details of the Starting Powders Used in the Present Work
Metallic Impurities
Median particle
Surface area
(m2/g)
Powder
Supplier
Carbon (wt%) Oxygen (wt%) Nitrogen (wt%)
(wt%)
diameter, D50 (mm)
TiB2
MoSi2
TiO2
In-house
In-house
Merck (Germany)
0.6
0.37
—
0.5
0.3
—
0.6
—
—
—
—
1.1
1.4
0.8
1.360
0.389
—
Pbr0.005
Asr0.0005
Fer0.005
B4C
In-house
19.5
o1.0
—
Fe B2000 ppm Si B2000 ppm
6.7
—
TiB2, titanium boride.

January 2006
Densification of Non-Oxide Ceramics
133
Table III. Details of the Densification Data for Ceramic
Samples, Pressureless Sintered at 19001C for 2 h in an (Ar1H2)
Atmosphere
100
98
96
94
92
90
88
86
0
Theoretical
Sample
density
(g/mL)
Density
(g/mL)
Relative
% weight
loss
Ã
designation
density (%)
PS10
PS15
PS20
PS25
PS0
4.65
4.71
4.78
4.85
4.52
3.83
4.00
4.23
4.43
Not
densified
82.36
84.92
88.49
91.34
Not
densified
4.4
5.0
5.6
6.2
Not
densified
1650°C
1700°C
1800°C
Ã
PS, pressureless sintered sample. Digit represents composition: 10—10%
MoSi21TiB2; 15—15% MoSi21TiB2; 20—20% MoSi21TiB2; 25—25% Mo-
Si21TiB2; 0—0% MoSi21TiB2. TiB2, titanium boride.
5
10
15
20
25
Wt. % MoSi2
rth was reported to be obtained for monolithic TiB2 after hot
pressing at 18001C for 1 h in Argon.14 The attainment of B98%
Fig. 2. Relative density of hot-pressed samples as a function of MoSi2
content. The lines joining the data points are connecting the densificat-
ion data obtained at varying hot-pressing temperatures.
r
th at 18001C, in the present case, for binderless TiB2 by the hot-
pressing route is a new result.
Critical observation of the data presented in Fig. 2 reveals that
the sintered density of TiB2 increases with the amount of MoSi2
sinter-additive, and a maximum of 99% rth can be obtained with
10% MoSi2 addition after hot pressing at 17001C. For 20%
MoSi2-reinforced composites, a similar high density of around
98% rth is obtained at 17001C. Additional hot-pressing exper-
iments at 17001C revealed that 98%–99% rth can be obtained
with 15% and 25% MoSi2 additions. From the above observa-
tions, it should be clear that MoSi2 improves the sinterability of
TiB2 and more than 95% rth can be obtained by the hot-pressing
route at a lower temperature of 17001C. Considerable weight
loss (during hot pressing) of around 4%–9% is measured de-
pending on the sintering temperature and MoSi2 content.
Because of the excellent densification achieved with a higher
amount of MoSi2 additions and in order to study the feasibility
of densifying them at lower temperature, limited experiments
were carried out by hot pressing TiB2–15% MoSi2 and TiB2–
20% MoSi2 composites at lower temperature of 16501C for 1 h.
The densification results reveal that 87%–89% rth can be ob-
tained for these composites with a lower density for higher sili-
cide additions.
It should be mentioned here that the measured toughness is
dependent on the test technique, which are widely classified into
long- and short-crack methods. Short-crack techniques involve
measurement of the crack lengths (radial/median) around hard-
ness indentations, from which the toughness is computed using
established formulas.23,24 Although the loading conditions are less
well characterized and crack growth is ‘‘slow’’ relative to conven-
tional long crack fracture toughness tests, careful use can yield
reproducible results for indentation toughness measurements.
In view of the above as well as to validate the indentation
toughness data, another established model proposed by Palmq-
vist24 was also used: KIc 5 0.0089(E/H)2/5P/(al1/2), where a is the
half diagonal length, and l 5 cꢁa. It can be noted that Palmq-
vist’s formula is valid for 0.25ol/ao2.5.
III. Results
(1) Densification
(A) Hot Pressing: The densification data of the hot-
pressed samples are presented in Fig. 2. For monolithic TiB2,
a poor densification of B88% theoretical density (rth) was ob-
tained after hot pressing at 17001C, while an excellent densifi-
cation of B98% rth was achieved at 18001C. The density and
the material property data (literature data and present work) of
monolithic TiB2 as well as TiB2 sintered with various sinter-ad-
ditives are summarized in Table II. Observing the data in Table
II, it is apparent that there have been few attempts to densify
TiB2 without any sinter-additive. Earlier, a maximum of 90%
(B) Pressureless Sintering: Table III presents the dens-
ification results of pressureless sintering experiments, carried out
at 19001C for 1 h in an inert (Ar1H2) atmosphere. The sintered
density of the composites increases with increasing MoSi2 addi-
tion. A maximum sintered density of 91% rth is obtained with
25 wt% MoSi2 addition. The weight loss during sintering varies
between 4.4 and 6.2 wt% for different MoSi2 additions and the
measured weight loss is observed to increase with an increase in
MoSi2 content. From the obtained results, it is evident that
pressureless sintering cannot be adopted to densify TiB2 even at
Table II. Density, Hardness, and Fracture Toughness of the Newly Developed TiB2-Based Materials
Relative
density
Vickers
hardness
(Hv), GPa
Indentation
toughness
(KIc), MPa ꢀ m1/2
Material
Processing details
(% rth
)
References
Monolithic TiB2
TiB2–2.5% Si3N4
TiB2–10% Si3N4
TiB2–5% AlN
TiB2–10% AlN
TiB2–2.5% SiC
Monolithic TiB2
TiB2–10% MoSi2
TiB2–20% MoSi2
HP 18001C, 1 h, Argon
HP 18001C, 1 h, Argon
HP 18001C, 1 h, Argon
HP 18001C, 1 h, Argon
90
99
96
98
88.5
96
97.5
99.3
98.7
23
27
20
22
14
—
26
27
25
5.8
5.1
4.4
6.8
5.2
4.3
5.1
4.0
5.0
Ho Park et al.14
Ho Park et al.14
Ho Park et al.14
Li et al.5
HP 18001C, 1 h, Argon
Li et al.5
PS 17001C, vaccum and HIP, 16001C, Argon
HP 18001C, 1 h Vaccum
HP 17001C, 1 h Vaccum
HP 17001C, 1 h Vaccum
Torizuka et al.13
Present work
Present work
Present work
For comparison, the literature data are also mentioned. Different processing routes are also indicated. TiB2, titanium boride; HP, hot pressing; PS, pressureless sintering;
HIP, hot isostatic pressing.

134
Journal of the American Ceramic Society—Murthy et al.
Vol. 89, No. 1
a high temperature of 19001C, and large addition of MoSi2 does
not aid in obtaining TiB2–MoSi2 with closed porosity i.e., with
more than 95% rth sintered density.
A summary of the experimental results clearly indicates that
the densification of TiB2 is enhanced with the addition of MoSi2
content at a lower hot-pressing temperature of 17001C. With
pressureless sintering, a maximum of 91% rth could be obtained
at 19001C for TiB2–25% MoSi2 composites.
As the major purpose of the material development work is to
obtain dense borides with minimal sinter-additive content, de-
tailed microstructural and mechanical properties were investi-
gated for selected hot-pressed composites with 10% and 20%
silicide additives. A comparison has also been made with the
monolithic borides.
grain size of the monolithic TiB2 particles in hot-pressed samples
is around 2–5 mm (Fig. 4(a)). Considering the starting particle
size of TiB2 (D50B1.1 mm), this observation indicates that hot
pressing at 18001C does not promote any significant grain
growth. The topography of the fracture surface (Fig. 4(a)) indi-
cates that the transgranular fracture is the predominant mode of
fracture. TEM investigation of the TiB2–20% MoSi2 composite
was carried out, and representative bright field TEM images of
the TiB2–20% MoSi2 composite is presented in Figs. 4(b)–(d).
The presence of finer TiB2 grains with sizes of 1–2 mm or less can
be noticed along with the dispersion of coarser MoSi2 particles
(Figs. 4(b) and (c)). The typical aspect ratio of elongated silicides
particles is around 4–5. Some equiaxed-shaped MoSi2 particles,
with sizes around 1 mm and having sharp interfaces with TiB2
grain, can be observed in Fig. 4(c). The presence of a crystalline
phase at the triple pocket is observed at grain-boundary triple
pockets (Fig. 4(d)). EDS analysis (not shown) from the pocket
phase reveals a strong Ti peak along with a weaker Si signal. The
presence of Mo or B was not detected. The weaker Si signal may
be because of the smaller size of the pocket phase and/or the use
of a Si detector. More detailed TEM analysis of the investigated
composite using high-resolution microscopy is presented else-
where.25 The above observation along with the XRD results
suggests that TiSi2, a reaction product, forms at grain-boundary
triple pockets during sintering.
(2) Microstructure and Properties
XRD analyses of starting powders and hot-pressed specimens
were carried out, and the results are shown in Fig. 3. These re-
sults indicate that the microstructure of the developed compos-
ites predominantly contain TiB2 and MoSi2. However, addition
of 10 and 20 wt% MoSi2 resulted in the formation of a small
amount of TiSi2. No presence of TiO2 or SiO2 or other Mo-
silicide is observed within the detection limit of XRD.
The SEM image of fracture surfaces of monolithic TiB2, hot
pressed at 18001C for 1 h, is shown in Fig. 4(a). The average
The mechanical properties, in particular hardness and frac-
ture toughness, were measured for the selected hot-pressed ma-
terials using the Vickers indentation technique. A comparison
of the mechanical properties of the newly developed materials
with the TiB2 sintered using various non-metallic additives is
made in Table II. The Vickers hardness, recorded using 100 N
indent load, is plotted against the MoSi2 content in Fig. 5.
The error bar in the hardness and toughness data indicates
the standard deviation in the measurement with five indenta-
tions. While monolithic TiB2 exhibits an average hardness of
26 GPa, a slightly lower hardness of 25 GPa is recorded for
TiB2–20% MoSi2 composites. A high hardness of 26 GPa in
monolithic binderless TiB2 is a promising result, and this is at-
tributed to the use of extremely finer TiB2 particles. Also, the
absence of hardness enhancement in the composites is because
of the lower hardness of the MoSi2 phase ( ꢃ 10 GPa for pure
MoSi2) as well as the presence of another brittle intermetallic
phase, TiSi2. It can be noted here that high hardness of 19, 22–
23, and 28 GPa were also reported for TiB2–10 vol% B4C,9
15
TiB2–15 wt% TiC,17 and TiB2–3 wt% CrB2 composites,
respectively.
Based on the indentation crack length measurements and as-
suming theoretical E-modulus values for TiB2 and TiB2–MoSi2
composites, the indentation toughness was evaluated. Table IV
presents all the indentation data, as measured from SEM images
of the indented surfaces. In order to study the reliability of the
indentation toughness, the models proposed by Anstis et al.23 as
well as Palmqvist24 were used. As the normalized crack length
parameter, ‘‘l/a,’’ lies between 0.25 and 2.5, the model proposed
by Palmqvist was also used to compute indentation toughness in
the present work. In fact, the crack length measurement reveals
that the ratio (l/a) is usually more than 1.5. A critical look at
Table IV indicates that the ranking of the investigated materials
in terms of indentation toughness remains the same, independ-
ent of the model used to compute toughness. However, the
toughness measured using Anstis et al.’s model underestimates
the toughness and hence these values are considered while com-
paring the material properties with earlier developed TiB2, sin-
tered using various non-metallic additives (see Table II). The
toughness of the investigated materials, measured using the
model of Anstis et al., varies in the range of 4–6 MPa ꢀ m1/2
.
While the average toughness of monolithic TiB2 and TiB2–20%
MoSi2 lies around 5.1 MPa ꢀ m1/2, a little lower toughness of 4.3
MPa ꢀ m1/2 was recorded for TiB2–10% MoSi2 composites. The
toughness improvement in composites is not achieved primarily
because of the presence of brittle ceramic phases like MoSi2 and
small amount of TiSi2.
Fig. 3. Phase analysis of starting powder and hot-pressed samples (a)
titanium boride (TiB2)–10 wt% MoSi2 and (b) TiB2–20 wt% MoSi2. The
Ã
different crystalline phases are identified: TiB2 ( ); MoSi2 (m); TiSi2 ( &).

January 2006
Densification of Non-Oxide Ceramics
135
Fig. 4. SEM image of the fracture surface of monolithic titanium boride (TiB2), hot pressed at 18001C for 1 h (a). Bright field (BF) TEM image of TiB2–
20 wt% MoSi2 composite (b) and (c), hot pressed at 17001C for 1 h. High-magnification BF TEM image revealing the presence of a crystalline phase at
grain-boundary triple pocket (indicated by an arrow) (d).
IV. Discussion
In this section, an explanation for the high density obtained with
monolithic TiB2 as well as enhancement in densification of TiB2–
MoSi2 composites will be presented. Additionally, the mechanical
properties, obtained with the newly developed materials, will be
discussed with particular reference to the earlier work.
10
9
8
7
6
5
4
3
2
1
0
30
28
26
24
22
20
18
16
14
12
10
(1) Densification Mechanism
The better densification of monolithic TiB2 can be explained on
the basis of the particle size effect on sintering. As no binder/
sinter-additive is used, the densification, in the present case, is
governed by a solid-state sintering mechanism. The sintering ki-
netics strongly depends on the neck growth rate during the
densification process. Following the classical two-particle sinte-
ring model,26 the neck growth in case of solid-state sintering of
monolithic TiB2 can be explained following the classical rela-
tionships:
Fracture Toughness
10 15 20
Hardness
5
ꢂ
ꢂ
ꢃ
ꢀ ꢁ
6
x
192dgDbO
KT
t
r4
0
¼
¼
(3)
(4)
r
% MoSi2 Content (Wt %)
ꢃ
ꢀ ꢁ
Fig. 5. Vickers hardness and indentation toughness as a function of
MoSi2 content. Monolithic titanium boride (TiB2) is sintered at 18001C,
whereas the composites containing MoSi2 are hot pressed at 17001C.
5
x
80gDlO
KT
t
r3
r

136
Journal of the American Ceramic Society—Murthy et al.
Vol. 89, No. 1
Table IV. Summary of the Indentation Data i.e., Average Indent Diagonal Length and Total Crack Length as well as the Fracture
Toughness Values, Computed using Models Proposed by Anstis et al.23 and Palmqvist,24 for the Investigated Materials
Material
Indent diagonal
Crack
Indentation toughness,
MPa ꢀ m1/2 (Anstis et al.)
Indentation toughness,
MPa ꢀ m1/2 (Palmqvist)
designation
(2a), mm
length (2c), m
l/a
TiB2
TiB2–10MoSi2
TiB2–20MoSi2
84.4
82.3
86.1
245.2
281.4
231.8
1.9
2.4
1.7
5.1
4.0
5.0
7.5
6.7
7.1
Another crack length parameter (l) is defined as l 5 cꢁa. TiB2, titanium boride.
where Dl is the lattice self-diffusion coefficient for the rate-lim-
iting diffusing species, Db is the boundary self-diffusion coeffi-
cient for the rate-limiting diffusing species, d is the boundary
thickness, K is the Boltzman constant, O is the vacancy volume,
x is the neck dimension, t is the time of sintering, T is the tem-
perature during sintering, g is the surface energy, and r is the
particle size of the powders.
results clearly indicate the formation of TiSi2. Additionally, the
weight loss measured after sintering can be correlated with the
formation of volatile compounds like B2O3(g) and MoO3(g),
which evaporate at a hot-pressing temperature of 17001C. The
vaporization of MoO3 is reported for oxidation of MoSi2.29
From the above discussion, it is quite likely that the densifi-
cation of TiB2 with a MoSi2 sinter-additive occurs via liquid-
phase sintering, involving the rearrangement of grains in the
presence of a wetting liquid phase (TiSi2).
While Eq. (3) is derived for the intermediate stage of sintering in
which the neck growth is controlled by grain-boundary diffusion,
Eq. (4) is related to the lattice diffusion-controlled neck growth
process occurring at the final stage of sintering.26 From Eqs. (3)
and (4), it is evident that the neck growth rate increases signifi-
cantly with little reduction in particle size. Thus, one can intui-
tively expect a large increase in the densification rate, in the present
case, with the use of finer TiB2 particles (average size B1 mm).
Comparing the data presented in Fig. 2 and Table II, it should
be clear that MoSi2, like other ceramic additives (Si3N4, AlN,
SiC), can be effectively used as a sinter-additive to densify TiB2-
based materials. In fact, the use of MoSi2 enables to achieve high
densification even at a lower hot press temperature of 17001C,
when compared with earlier literature results (see Table II). To
this end, it can be noted that 98% rth was obtained with TiB2–5
wt% AlN, hot pressed at 18001C by Li et al.5 Ho Park et al.14
achieved 99% rth using 2.5 wt% Si3N4 additives for TiB2 hot
pressed at 18001C for 1 h. Thus, comparing our experimental
results with literature reports, it can be said that high density
obtained in TiB2–10 wt% MoSi2 composites at 17001C, in the
present case, is a new and promising result.
The existence of the grain triple pocket phase (TiSi2) provides
a useful insight into the densification mechanism. As the melting
point of TiSi2 is 15001C, liquid TiSi2 presumably forms during
hot pressing (sintering temperature of 17001C). It is reported in
the literature that a surface layer of TiO2 and B2O3 exists on the
surface of TiB2 particles, and B2O3 vaporizes rapidly above
11271C.27 In our hot-pressing experiments, 4%–6% weight loss-
es are measured and this can be attributed to the evaporation of
volatile oxides like B2O3, MoO3.
(2) Mechanical Properties
A comparison of the mechanical properties of newly developed
TiB2 materials with the earlier developed materials is presented
in Table II. In earlier experiments, various researchers have opt-
imized the amount of sinter-additives like AlN, Si3N4 during
sintering experiments and the results obtained with optimized
sinter-additives, for example 2.5% Si3N4, 5% AlN, are also
mentioned in Table II. For comparison, the material property
data obtained with 10% ceramic additive are also indicated in
Table II. Observing the data in Table II, it should be clear that
TiB2 materials with a combination of high hardness (25 GPa or
more) and modest fracture toughness (B5 MPa ꢀ m1/2) could be
achieved with the use of various non-metallic sinter-additive.
When compared with TiB2 sintered with 5% or 10% AlN sinter-
additives, TiB2–MoSi2 exhibits higher hardness. The indentation
toughness, measured with our materials, however remains mod-
est. Importantly, one can notice that the maximum hardness
achievable in TiB2-based materials densified using a non-metal-
lic sinter-additive is limited to around 7 MPa ꢀ m1/2 (TiB2–5%
AlN). Also, the combination of hardness and toughness ob-
tained with the newly developed materials with a MoSi2 sinter-
additive is comparable with the earlier developed TiB2–2.5%
Si3N4 materials. However, when compared with TiB2–2.5%
Si3N4 ceramic, our materials are densified at a lower hot press
temperature at 17001C.
The indentation-induced radial crack patterns emanating
from the Vickers indentations are shown in Fig. 6. In both
monolith as well as TiB2–10% MoSi2 composites, sharp and
perfect indentations are visible, and the observation of smaller
Vickers indents conforms well to the measured high hardness
(Fig. 6(a)). The wider residual crack opening of the Vickers in-
dentation induced cracks in case of the TiB2–10% MoSi2 com-
posite implicates a lower fracture toughness (Fig. 6(b)). Figure
6(c) provides the evidence of the deflection of indentation-in-
duced crack by ceramic particulates. No indication of crack
bridging is observed in our materials. This confirms that the
crack deflection is the only toughening mechanism.
The sintering mechanism involving the formation of TiSi2 can
be explained by the following reactions:
TiO2 þ MoSi2 ! TiSi2 þ MoO3ðgÞ
(5)
(6)
TiB2 þ 3O2ðgÞ þ MoSi2
! TiSi2 þ B2O3ðgÞ þ MoO3ðgÞ
Based on the data available for the free energy of formation of
different compounds,28 it was found that the overall free energy
change for reaction (5) was DG540 at high temperature (41773
K). Hence, the first reaction involving chemical interaction of
TiO2 and MoSi2 resulting in the formation of TiSi2 is not fea-
sible thermodynamically. However, the thermodynamic cal-
culations reveal the overall free energy change for the other
possible reaction, i.e., DG6o0 at and above 1773 K (15001C).
For example, the thermodynamic calculations indicate that DG6
at 1800 K is ꢁ183.018 Kcal.28 At T41800 K, DG6 becomes
more negative. Hence, reaction (6), leading to the formation of
TiSi2, is thermodynamically feasible. It can be noted here that
this reaction can take place even at a low oxygen partial pres-
sure, as relevant in our hot-pressing experiments. Also, XRD
As a concluding remark, it can be said that the present study
clearly indicates that MoSi2 can be potentially used as a ceramic
binder in densifying borides. While high hardness of borides can
be retained in the composites, the fracture toughness remains
moderate. Further research should be directed toward opt-
imizing the silicide content in the narrow window of 0–10
wt%, while obtaining high densification and hardness. Prelim-
inary electrical conductivity data reveal that the 20% silicide-
containing boride composite has better electrical conductivity
than monolithic boride. Recent thermal property measurements
using the laser flash technique revealed that the TiB2–20%
MoSi2 composite has a higher thermal conductivity of 64.8

January 2006
Densification of Non-Oxide Ceramics
137
ture of 18001C results in obtaining B98% rth dense binderless
TiB2 with a finer microstructure (B2–5 mm), high hardness (25–
26 GPa), and modest indentation fracture toughness (5.1
MPa ꢀ m1/2).
(b) Our experimental results clearly reveal that MoSi2 can
be used as a sinter-additive for densification of TiB2. A high
sintered density of 97% rth is obtained in a TiB2–10 wt% MoSi2
composite, hot pressed at 17001C for 1 h. In case of pressureless
sintered materials, a maximum of 91% rth is obtained with 25%
MoSi2 composites, when sintering is carried out at 19001C for
2 h in an inert (Ar1H2) atmosphere.
(c) XRD and TEM results indicate the formation of TiSi2, a
reaction product formed during hot pressing at 17001C. TiSi2,
being a liquid at hot-pressing temperature, promotes liquid-
phase sintering and hence enhances densification.
(d) The optimized composite (TiB2–10 wt% MoSi2) exhibits
a significantly higher hardness (Hv B26.5 GPa) and modest in-
dentation toughness (KIc B4.3 MPa ꢀ m1/2). The attainment of
high hardness is presumably because of a finer microstructure
containing TiB2 (2–3 mm) and MoSi2 (3–4 mm). The crack de-
flection is observed as the only toughening mechanism.
Acknowledgment
The authors also thank Krishanu Biswas for help with TEM investigation.
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&