Effect of carbon and titanium carbide on sintering behaviour of zirconium diboride

Article abstract of DOI:10.1016/j.jallcom.2007.11.004

Systematic sintering studies on the ZrB₂ powder were carried out with the addition of carbon (C) in the range of 0 < x < 10 (x = wt.%) and (titanium carbide) TiC in the range of 0 < y < 30 (y = wt.%). Zirconium diboride powder made by different

Full text of DOI:10.1016/j.jallcom.2007.11.004

Journal of Alloys and Compounds 465 (2008) 547–555
Effect of carbon and titanium carbide on sintering behaviour
of zirconium diboride
S.K. Mishra∗, L.C. Pathak
National Metallurgical Laboratory, Jamshedpur 831007, India
Received 13 June 2007; received in revised form 31 October 2007; accepted 1 November 2007
Available online 20 February 2008
Abstract
Systematic sintering studies on the ZrB2 powder were carried out with the addition of carbon (C) in the range of 0 < x < 10 (x = wt.%) and (titanium
carbide) TiC in the range of 0 < y < 30 (y = wt.%). Zirconium diboride powder made by different routes showed similar sintering behaviour and
amongst all the powders, the SHS made powder exhibited most homogeneous morphology. The addition of C facilitate densification in the range
of 0 < x < 4 and hinders densification for 4 < x < 10, whereas, the addition of TiC above 5 wt.% was found to be detrimental for the sintering of
ZrB2. The addition of C also found to inhibit the grain size and fine grained ZrB2–C composite could be obtained with high densification. The
increased densification with the C addition was due to removal of oxygen impurities on the ZrB2 powder surfaces that prevented the commonly
observed exaggerated grain growth through evaporation condensation of oxide phases during sintering of boride ceramics. At higher percentages
of C addition, the densification rates decreased due to increased volume fraction of second phase that acted as diffusion barrier.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Borides; Composites; SHS; Sintering; Microstructure
1. Introduction
exaggerated grain growth during sintering [6,7], which hinders
the densification process. Following the initial stage of rapid
shrinkage (presumably by grain-boundary diffusion process),
excessive coarsening is induced by the partial predominance of
evaporation–condensation mechanism at the densification tem-
peratures and results in a rapid diminution of the driving force for
sintering. The presence of thin oxygen rich layer on the surface
of boride powders is reported to be detrimental for densification
of borides [8,9].
Improvement of physical and chemical behaviour of the ZrB2
ceramics is possible by incorporating other high-temperature
ceramic materials into the ZrB2 matrix. So far several addi-
tives have been used to make the diboride-based composites
and it has also been aimed to counter the grain growth for
enhancing the sintering process and yield good sintered bod-
ies. Most of the studies on sintering additives such as NiB, TiC,
TaC, CrB2 Ni, Fe, Co, Mo, B4C, etc. are being reported on
the TiB2 system [10–14] and a few are reported on the ZrB2
system, which is expected to behave similar to the TiB2 sys-
tem. The addition of NbB2 and CrB2 was found to prevent
the grain growth during hot pressing of TiB2 and the addition
of Ni and TaC increased the pressureless shrinkages of TiB2
via liquid phase and chemical gradient effects [15,16]. A very
The diborides, particularly the ZrB2 and TiB2 are important
high temperature ceramics. They have similar crystal structures
and possess excellent high temperature properties such as high
hardness, high elastic modulus, excellent oxidation resistance
and chemically stability in many corrosive environments [1,2].
Zirconium diboride does not have many intermediate phases
compared to TiB2, which makes ZrB2 a favourable candidate
over TiB2. It is a potential material for use at elevated tem-
peratures with good abrasion, wear and corrosion resistances
both in terms of bulk as well as protective coatings. For bulk
applications, loosely packed compacts need to be sintered and
fabrication of monolithic zirconium diboride by sintering tech-
nique is difficult due to its high melting temperature and covalent
bonding. Most often ZrB2 is sintered at 2000–2200 ◦C [3–5]
and the pressureless sintering studies are very limited. One of
the crucial problems faced during sintering of borides is the
∗
Corresponding author. Tel.: +91 657 2271709; fax: +91 657 2270527.
E-mail addresses: skm smp@yahoo.co.in,
suman@nmilindia.org (S.K. Mishra).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.11.004

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small percentage of Fe (0.5 wt.%) initially enhanced the densi-
fication rates but at higher percentages poor densification was
resultedwithoccurrenceoflargegraingrowth. TheB4Caddition
along with 0.5 wt.% of Fe had shown remarkable suppression
of grain growth but when added with higher percentages (more
than 0.5 wt.%) no significant effect was observed and B4C was
trapped inside of large titanium diboride grains [17]. The addi-
tion of TiN was also shown to improve the properties [18]. The
addition of silicon carbide to TiB2 showed remarkable improve-
ment in densification behaviour of TiB2 by eliminating the oxide
layer that existed on the surface of the powder [19]. The addition
of Si3N4 to titanium diboride resulted in better densification but
the glassy phases of Si–Ti–O–N at grain-boundaries degraded
the mechanical properties [20].
So far, a few studies reported the addition of NiB, Cu, Mo,
etc. with ZrB2 [21]. The addition of 15 wt.% Mo to ZrB2 found
to be the optimum composition for obtaining the highest density.
It was also observed that the addition of Mo formed the MoB and
ZrC phases during processing. The addition of Cu resulted liquid
phase sintering and the addition of metal up-to a few percentage
only help in sintering. It also limited the strength, hardness and
other properties of the composites. Besides the sintering aids,
the powder preparation techniques also have significant effect on
the final properties of the product. Compared to the conventional
carbothermic process other non-equilibrium processes such as
plasma arc technique had shown better properties due to high
purity of the product and high surface area of the synthesised
powder [5]. The SHS processing is an attractive non-equilibrium
processing technique, which is fast & energy efficient [22,23].
The SHS produced powder also had high defect concentration
in the powders, which could be helpful in enhanced sintering
[24]. In the present paper, a comparative study on the effect of
C and TiC on the sintering behaviour of ZrB2 powder prepared
by different techniques has been reported.
2.2. Preparation of ZrB2 powder by carbothermic process
In the carbothermic process, zirconium diboride powder was prepared using
zirconium dioxide, boron oxide and carbon by the following reaction:
ZrO2 + B2O3 + 5C = ZrB2 + 5CO
The starting powders of zirconia, boron oxide and carbon were taken in
stoichiometric quantities and then mixed in a planetary ball mill (Retsch,
PM400 Germany) for 30 min at 250 rpm rotation. The mixture was subse-
quently heated at 1800 ◦C for 30 min in a resistive heating graphite furnace
(Thermal Technology Inc., USA) and then furnace cooled down to room
temperature.
2.3. Powder compaction and sintering study
Zirconium diboride powder synthesized by SHS and carbothermic routes
and the commercial ZrB2 powder were mixed with 2, 3, 4, 5 and 10 wt.% of C in
five different batches. Similarly, 5, 10, 20 and 30 wt.% of TiC (made by the SHS
technique) were also added with the ZrB2 powders. The C and TiC added ZrB2
powder was milled in a planetary ball mill to increase homogeneity of the powder
mixture. The mixed composite powder was pressed into the form of 3 mm thick
pellets having a diameter of 15 mm. All these pellets were sintered at 1800 ◦C
for 30 min in a graphite furnace under flowing argon gas followed by slow
coolingdownto1300 ◦Candthenfurnacecooleddowntoroomtemperature. The
microstructure and composition analyses were carried out using SEM (JEOL-
JSM 800, Japan). The phase analyses were carried out by X-ray diffraction
pattern using Simens powder diffractometer (Germany). The densities of the
samples were measured by liquid immersion technique in water medium using
Archmedies principle with an accuracy of 1%.
3. Results and discussion
3.1. Characterisation of the starting powders
The X-ray diffraction (XRD) analyses of the synthesised
ZrB2 powders (by SHS and carbothermic routes) showed the
peaks of ZrB2 as the main phase in the patterns. Minor peaks
due to ZrO2 were also observed in the patterns. The SHS made
ZrB2 and TiC powder was found to be sub-micrometer in size,
whereas, the carbothermically prepared powder were found to
be micrometer in size. The commercial powder was found to be
single-phase materials (without any zirconia peaks) and its par-
ticle sizes were also in the micrometer range. The SHS produced
TiC powder was found to be a single-phase material and they
were agglomerated with sizes >10 ␮m. The scanning or trans-
mission electron micrographs of the ZrB2 powder prepared by
different processes and the SHS made TiC powder are shown in
Fig. 1.
2. Experimental
Zirconium diboride powders used in the present investigation were made by
SHS and carbothermic routes. The details of the synthesis procedures are briefly
described in the following sections. The sintering behaviour of the synthesised
powders was also compared with the commercially available zirconium diboride
powder (Aldrich, USA).
2.1. Preparation of ZrB2 and TiC powder by SHS process
Zirconium diboride (ZrB2) and TiC powders were synthesised by SHS pro-
cess (arc ignition technique) using zirconia, boric acid and magnesium as the
starting powder. The details of the process are described elsewhere [25]. Tita-
niumcarbidewasalsopreparedusingthemixtureofTiandamorphousCpowder.
The details of the process for the TiC powder preparation using SHS technique
is also been reported [26]. The SHS reactions for the synthesis of zirconium
diboride and TiC are given below.
3.2. Effect of C addition to zirconium diboride
The XRD patterns of sintered ZrB2 pellets (made from the
SHS prepared powder with various C contents, 2–10 wt.%)
showed the presence of ZrB2 phase with very feeble peaks of
ZrC0.7 phase (Fig. 2). The densification behaviour of the ZrB2–C
pellets sintered at various temperatures is shown in Fig. 3a,
which showed that the increase of measured densities from 87 to
94% with the increase of C content from 0 to 4 wt.% and further
increase of C to 10 wt.% led to the decrease of densification.
The observed increase in the density for smaller percentages
of the carbon addition was due to the fact of that the borides
ZrO2 + 5Mg + 2H3BO3 = ZrB2 + 3H2O + 5MgO
Ti + C = TiC
The by-product of SHS reaction MgO was leached by dissolving in HCl and
the product (ZrB2) was filtered and washed with distilled water and alcohol. The
powder was then dried in an oven at 150 ◦C.

S.K. Mishra, L.C. Pathak / Journal of Alloys and Compounds 465 (2008) 547–555
549
Fig. 1. Scanning electron micrographs of the starting powder (a) ZrB2 powder by SHS route, (b) ZrB2 powder by carbothermic, (c) commercial ZrB2 powder and
(d) TiC powder by SHS route.
always have oxygen at the surface, which was reduced by added
carbon while sintering at high temperature. Hence the surface
became more active and helped in sintering. From the densi-
fication curve (Fig. 3a) it was apparent that beyond a certain
concentration, the excess free carbon was not taking part in the
sintering process, rather it hinderedthemasstransportprocesses.
Hence with higher percentages of C contents, the sintered pel-
lets were less dense and remained porous. Scanning electron
microscopy on the fractured surfaces of the sintered ZrB2 pellets
with 0–5 wt.% of C addition showed insignificant grain growth
during sintering (Fig. 4). From the microstructures compared
to that of the samples sintered without C addition, the grain
growth could be inhibited to a good extent. The microstructure of
Fig. 2. XRD patterns of ZrB2 pellets (SHS) sintered at 1800 ◦C for 30 min with
Fig. 3. (a–c) Densification behaviour of the SHS produced ZrB2 powder with
different C contents: (a) 0, (b) 2, (c) 5 and (d) 10 wt.%.
different C contents.

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S.K. Mishra, L.C. Pathak / Journal of Alloys and Compounds 465 (2008) 547–555
Fig. 4. SEM micrographs of the fractured surfaces of ZrB2 pellets (SHS) sintered at 1800 ◦C for 30 min with different C concentrations: (a) 0, (b) 2, (c) 3, (d) 4, (e)
5, and (f) 10 wt.%.
the sintered pellet was also found to be homogeneous through-
out the samples and the porosity was much less compared to
the samples where no C was added. The SEM/EDX analyses
showed the presence of Zr only in all the sintered grains (the
present detector cannot detect B & C peaks). From the XRD
analyses, the sintered samples were found to be a single-phase
material and therefore the grains showing Zr peaks in the EDX
spectra were from the ZrB2 grains only. The sintered sample
surface added with carbon was found to be electrically con-
ducting with a conductivity in the range of 300–400 ␮ꢀ cm),
whereas the top upper layer surface was non-conducting when
carbon was not added and the inside bulk material was conduct-
ing. The formation of zirconia layer on the surface of the pellets
sintered without C addition is shown in Fig. 5. This oxide scal-
ing on the surface of sintered pellets indicated the movement
of oxygen towards the surface of the pellets during sintering
when no carbon was added. In the presence of C, the oxy-
gen reacted with C and no oxide formation on the surface was
observed. Therefore, the C addition was found be beneficial in
purifying the material, which needed optimisation of the carbon
content.
Fig. 5. SEM micrographs of ZrB2 pellets (SHS) sintered without C addition at
1800 ◦C for 30 min showing the oxide scale formation.

S.K. Mishra, L.C. Pathak / Journal of Alloys and Compounds 465 (2008) 547–555
551
Similar densification and phase formation behaviour was
observed in the case of carbon added ZrB2 (carbothermic)
powder when sintered at 1800 ◦C for 30 min. The pellets sin-
tered with 2 wt.% of C showed the appearance of ZrB2, ZrO2
along with very feeble peak of ZrC and further increase of
C showed only the presence of ZrB2 and ZrC phases in
the XRD patterns. The density was found to increase ini-
tially (∼89% for C = 4 wt.%) and later decreased for further
increase of carbon addition (Fig. 3b). Scanning electron micro-
graphs of the fractured surfaces of the ZrB2 samples sintered
at 1800 ◦C with 2, 5, 10 wt.% of carbon additions showed
controlled grain growth and homogeneous morphology through-
out the sintered samples (Fig. 6). The samples, which had
10 wt.% of C showed large micro-pores with exaggerated grain
growth in some localised places, which was the reason for
observed decrease of density. The observed increase in grain
size at higher percentages of the carbon was due to inho-
mogenieties in the mixture, which on sintering facilitated the
grain growth where no carbon was present. In that case, the
boride–boride grains grow as it happens in normal sintering
process. The increased porosity in the higher carbon content
in the sample was resulted in abnormal grain growth at localised
places.
The commercial ZrB2 powder having only ZrB2 phase also
showed the presence of feeble ZrC peaks in the XRD patterns.
Fig. 6. SEM microstructure of the fracture surfaces of ZrB2 pellets (carbother-
mic powder) sintered at 1800 ◦C for 30 min with different weight percentages
of carbon: (a) 2, (b) 5 and (c) 10 wt.%.
Fig. 7. SEM microstructure of the fracture surface of ZrB2 (commercial powder)
sintered at 1800 ◦C for 30 min with different percentages of carbon: (a) 2, (b) 5
and (c) 10 wt.%.

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S.K. Mishra, L.C. Pathak / Journal of Alloys and Compounds 465 (2008) 547–555
Fig. 8. TEM micrographs of ZrB2 powder showing agglomeration.
Similar trend of densification was also observed with the addi-
tion of C, where densification increased to 85% for 3 wt.% of
C addition and further increase of C content led to decrease in
densification (Fig. 3c). The commercial powder revealed two
types of particles during SEM observation, one was very fine
and other was large in size. Whereas, the microstructure of
the sintered samples showed a contrasting features of homo-
geneous morphology with fine grains through out the samples
(Fig. 7). With the addition of C up to only 2 wt.%, uniform
microstructure is resulted and further addition of C to 5 wt.%
did not reveal any significant changes in the microstructure. The
samples added with 10 wt.% of C expectedly showed significant
grain growth during sintering. The porosity of the samples also
increased with the grain growth. The abnormal grain growth and
decreased densification is possibly due to same reason as in the
case of sintering of carbothermic or SHS made ZrB2 powder
(Fig. 7c).
The apparent reduction in grain size is due to deagglomera-
tion of the powder during high temperature sintering. It is well
known that agglomerated particles undergo coarsening before
the first stage of sintering and one of the concerns in ceramic
processing technique is the control of agglomeration, since an
agglomerated powder is of little benefit during densification as
coarsening prevails over sintering. From the transmission elec-
tron microscopy studies it was apparent that the starting powders
were agglomerated and it contained large numbers of crystallites
(Fig. 8). It is also known that Vander walls or hydrogen type of
bonding are the cause of weak-agglomeration in ceramics. In
this case, these bonds are due to presence of oxide layer on the
powder surface and it is irrespective of the powder preparation
techniques the oxide layer is present on the powder surface. Dur-
ing sintering, C reacted with the oxide layer and removed the
agglomeration. Therefore, the apparent decrease in grain size is
due to deagglomeration.
Fig. 9. (a) TEM micrograph of nano-crystalline ZrB2 powder. (b) SEM
microstructure of the ZrB2 pellet sintered without carbon at 1800 ◦C for 30 min
and (c) SEM microstructures of the ZrB2 pellet sintered with 5 wt.% carbon at
1800 ◦C for 30 min.
using WC grinding media), which was sintered at 1800 ◦C for
30 min. TheTEMimageofthemilledpowderisshownin Fig. 9a.
The sintered pellets without any carbon addition achieved a
densification of 90% of the theoretical density, whereas the
5 wt.% carbon added samples achieved densification >94%. The
samples sintered without any carbon addition showed large
The effect of carbon is quite clear for the fine SHS made
ZrB2 powder (<100 nm prepared by milling in vibratory cup mill

S.K. Mishra, L.C. Pathak / Journal of Alloys and Compounds 465 (2008) 547–555
553
Fig. 10. XRD patterns of ZrB2 pellets made from SHS powder and sintered at
1800 ◦C for 30 min with: (a) 0 and (b) 20 wt.% of TiC.
coarsening of grains (Fig. 9b) and the 5 wt.% carbon added
samples had fine grain structures throughout the pellets after
sintering (Fig. 9c). This clearly indicated the role of C on grain
growth inhibition of zirconium diboride ceramics.
3.3. Effect of TiC addition to zirconium diboride
The XRD pattern for the samples added with 5 wt.% of TiC
(SHS made) did not show any additional peaks, but the 10 wt.%
of TiC added ZrB2 sample showed the low intensity additional
peaks of TiB2. No TiC peak could be observed in the XRD
pattern. This is due to reaction between ZrB2 and TiC during
sintering at 1800 ◦C by the following reaction:
ZrB2 + TiC → TiB2 + ZrC
The ZrC peaks could not clearly be observed in the XRD
patterns due to low percentage of it in the sample. However,
careful observation of the pattern could reveal the small peak
due to ZrC phase. The XRD patterns of sintered samples with
different percentages of TiC are shown in Fig. 10. During sin-
Fig. 12. SEM microstructures of ZrB2 pellets sintered sample with 10 wt.% TiC:
(a) showing two types of regions, (b) Ti rich area showing elongation of grains
and (c) Zr rich area.
Fig. 11. Schematic of the EDX point analyses: (a) ZrB2 grains and (b) Ti-containing grains.

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S.K. Mishra, L.C. Pathak / Journal of Alloys and Compounds 465 (2008) 547–555
Table 1
Table 3
Density ZrB2–TiC (By SHS) composites sintered at 1800 ◦C
Bulk EDX analyses of Zr and Ti rich area in ZrB2 sample having 10% TiC
Sample
Theoretical
density
Measured density
%Densification
Element
Zr rich area
Tr rich area
Ti (wt.%)
Zr (wt.%)
4.31
95.69
69.03
30.17
ZrB2 (initial powder)
ZrB2 (5% TiC)
ZrB2 (10% TiC)
6.0
5.656
5.349
5.22
5.33
3.57 (4.6)
87
94.2
86
elemental analyses are presented in Table 2. From which it is
clear that Ti/TiC had diffused in the ZrB2 grains making it as
complex boride and had graded diffusion from grain-boundary.
The bulk EDX analyses of the Ti and Zr rich areas are given
in Table 3. The Ti rich areas showed some elongation of grains
whereas Zr rich areas showed controlled grain growth (Fig. 11).
The densification of 10 wt.% of TiC added samples decreased
to 85%. This observed decrease in densification was due to the
observed segregation and also due to formation of TiB2, which
was not considered during density calculation. Besides these,
the other causes may be the presence of second phases, which
hinders the sintering of the composite.
Similar densification and phase formation behaviour was
observed for the carbothermic and commercial powder. The
scanning electron microscopy studies reveal that the samples
became more porous and fragile as the TiC content increased
above 5 wt.% and it was difficult to measure densities of the
tering some of the TiC might be going into solution with ZrB2
forming a complex boride and the rest is reacting and formed
the TiB2 phase. The density was found to increase to 94% of
the theoretical density compared to 87% for the sample sintered
without TiC (Table 1). The microstructures of the sintered sam-
ples are shown in Fig. 11, which showed less grain growth in the
TiC added samples. Low magnification studies revealed that the
sample added with 10 wt.% of TiC had two types of areas in the
samples, one having larger percentages of Ti and other with Zr.
The EDX analysis at the grain boundaries and inside the grain
for 5 wt.% TiC added sample showed the diffusion of TiC/Ti into
ZrB2 grains, but no diffusion of Zr was observed in Ti-containing
grains. For understanding the interdiffusion of Zr and Ti, system-
atic EDX point analyses, with probe size of the order of 20 nm,
at different points of the grain from grain boundary to middle
of the grain and beyond schematically shown in Fig. 12 and the
Table 2
EDX analyses of ZrB2 grain and grain boundary for ZrB2 sintered sample having 5% TiC
Element
X1 (grain boundary)
X2
X3
X4 (middle)
X5
4.04
X6
4.99
Ti (wt.%)
Zr
5.39
94.61
5.01
94.99
4.18
95.82
4.01
95.99
95.96
95.01
Fig. 13. SEM microstructures of the pellets sintered with different percentages of TiC: (a) and (b) carbothermic ZrB2 powder, (c) & commercial ZrB2 powder. (a)
5, (b) 30, (c) 5 and (d) 30 wt.% of TiC.

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555
samples. This desintering effect that increased the porosity of
composites with higher amount of TiC content can be due to
combination of several reasons such as solubility of dispersoid
phase to matrix or the vice versa, molar volume change dur-
ing sintering, differential diffusivity of the phases [27]. It has
been found that though the solubility of ZrB2 is less into TiB2
but TiB2 has higher solubility in ZrB2. It has been observed
that when additive has more solubility into matrix phase the
swelling can occur in the product. Also the change in molar vol-
ume during sintering due to some reaction product or the phase
also leads to crack or pores due to stresses. The difference in
the diffusivity of the two phases, segregation of phases can also
lead to fragility and more pores during sintering. Considering all
these, it was concluded that during high temperature sintering
with higher percentages of TiC, formation of TiB2 was observed.
Some TiB2/Ti must have gone into solution with ZrB2, which
has been observed in the SHS process by EDX analysis but no
diffusion of Zr was observed into Ti containing grain. Also the
molar volume of TiC (81.6) when gets converted to TiB2 (25.6)
during sintering at high temperature will lead to pore formation
and swelling. Hence porous mass was obtained for the higher
percentages of TiC addition.
The SEM microstructures of the fractured surfaces of sin-
tered samples made from carbothermic and commercial powder
is shown in Fig. 13. From the microstructures, it was apparent
that very small amount of TiC was helping in sintering and the
increase of TiC content led to segregation, porosity and less den-
sification. The larger grain growth in pockets was also observed
during sintering when segregation are there and normal grain
growthtookplacesimilartothenormalsinteringofZrB2 without
any additive.
geneous material in all the cases of ZrB2 prepared by different
routes. Formation of TiB2 and TiB phase was observed for 10,
20 and 30 wt.% of TiC added samples. The porous structure
was observed due to complex sintering and segregation during
sintering.
Hence on the basis of the above studies, it is concluded that
a very fine grained structure of the ZrB2–C composite could be
obtained with higher density by addition of 2–4% of C. Higher
percentages of C are not beneficial to enhance the property in
terms of sintering. TiC is not a good addition for sintering of
ZrB2, but the C additions in very small percentage (3–4 wt.%)
can enhance the density drastically along with the control of
grain growth. Composites prepared from SHS made ZrB2 pow-
der proved to give the most homogeneous product.
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The effect of different amounts of TiC and C presence in the
ZrB2 matrix prepared by three different methods namely SHS,
carbothermic and commercial powder was studied. The density
was found to increase with the addition of C only upto 5 wt.%.
Further increase in C content led to decrease in densification due
to slow mass transport between the ZrB2 grains and C does not
take part in the sintering and stays at the grain-boundary. Out
of all the three methods, the SHS process was best in terms of
homogeneity of the sample the density could be increased from
87 to 94% of the theoretical density. It is believed that the smaller
amount of carbon acted to reduce the surface oxygen of borides
and led to enhancement of densification. In the case of TiC addi-
tion, it was observed that though there is increase in density with
5 wt.% TiC addition but the microstructure is not homogeneous.
Further increase of TiC content leads to porous and inhomo-
[27] R.M. German, Sintering Theory & Practice, vol. 5, JWilly & Sons Publ
ch., 1996, p. 179.