In situ synthesis of ceramic composite materials in the Ti-B-C-N system by a mechanically induced self-sustaining reaction

Article abstract of DOI:10.1111/j.1551-2916.2012.05174.x

The synthesis of multicomponent ceramic materials in the titanium-diboride-carbide-nitride-carbonitride system by the mechanochemical process known as the mechanically induced self-sustaining reaction (MSR) was investigated. Ceramic composite powders containing TiB₂and TiC, TiN or TiC_xN_1-xwere prepared from a blended mixture of the elements by exploiting the highly exothermic nature of the formation reactions. The synthesis of the composite materials was made possible by the ability of the MSR to simultaneously induce independent self-sustaining reactions, generating a mixture of ceramic phases. The composition of the ceramic composites was designed using the initial atomic ratio of the reactants, and the achieved microstructure was characterized by TiB₂particles in the micrometric range, surrounded by submicrometric and nanometric TiC, TiN, or TiC _xN_1-xcrystals.

Full text of DOI:10.1111/j.1551-2916.2012.05174.x

J. Am. Ceram. Soc., 1–7 (2012)
DOI: 10.1111/j.1551-2916.2012.05174.x
©
2012 The American Ceramic Society
ournal
J
In Situ synthesis of Ceramic Composite Materials in the Ti–B–C–N System by
a Mechanically induced Self-Sustaining Reaction
Miguel A. Aviles, Ernesto Chicardi, Jose M. Cordoba, Marıa J. Sayagues, and Francisco J. Gotor†
´ ´ ´ ´ ´
Instituto de Ciencia de Materiales de Sevilla (CSIC-US), Ame
´
rico Vespucio 49, 41092 Sevilla, Spain
1
3–15
The synthesis of multicomponent ceramic materials in the
titanium-diboride-carbide-nitride-carbonitride system by the
mechanochemical process known as the mechanically induced
self-sustaining reaction (MSR) was investigated. Ceramic com-
reduction) featuring titania in a controlled atmosphere.
However, the appearance of secondary phases, such as TiO
2
,
Ti O C, and TiBO , in the final product lowers the purity of
x
y
3
the powders produced from the reduction reaction. In recent
years, self-propagating high-temperature synthesis (SHS),
which has the advantages of time and energy efficiency, has
been a promising route for the production of advanced mate-
rials, including borides, carbides, nitrides, and carbonit-
The mechanochemical process referred to as the
mechanically induced self-sustaining reaction (MSR) is simi-
lar to thermally ignited SHS and requires highly exothermic
posite powders containing TiB
2
and TiC, TiN or TiC
x
N
1ꢀx
were prepared from a blended mixture of the elements by
exploiting the highly exothermic nature of the formation reac-
tions. The synthesis of the composite materials was made possi-
ble by the ability of the MSR to simultaneously induce
independent self-sustaining reactions, generating a mixture of
ceramic phases. The composition of the ceramic composites
was designed using the initial atomic ratio of the reactants,
1
6–19
rides.
2
0
chemical reactions. The MSR has proved to be adequate in
producing TiB , TiC, and Ti(C,N) from mixtures of elemen-
tal titanium with boron or carbon in an inert or nitrogen
and the achieved microstructure was characterized by TiB
particles in the micrometric range, surrounded by submicromet-
2
2
2
1–23
ric and nanometric TiC, TiN, or TiC N
x
crystals.
atmosphere.
In addition, MSR has allowed for the pro-
1ꢀx
duction of nanocrystalline carbonitride powders with homo-
geneous and controlled chemical compositions by adequately
adjusting the milling parameters and the metal-to-carbon
I. Introduction
2
3
ERAMIC materials in the Ti–B–C–N system are interest-
ing because single phases, such as TiB , TiC, TiN, and
1ꢀx solid solutions, have excellent mechanical, thermal,
and electrical properties, which make them very useful for
atomic ratio in the starting mixture.
TiB –TiN, TiB –TiC, and TiB –TiC
C
2
2
2
2
x
N
1ꢀx ceramic com-
TiC
x
N
posites are traditionally prepared from the premixed powders
of single-phase components. Recently, research attention has
turned to the reactive in situ synthesis of these ceramic com-
posites, which has usually been performed by SHS or other
1
–4
structural applications.
It has been reported that ceramic
composites obtained within these phases possess superior
mechanical properties (hardness, wear resistance, and frac-
ture toughness) compared with their constituent ceramic
2
4–27
related combustion methods. The reactants commonly
used in these studies are elemental Ti and B C and BN pow-
ders as the sole source of B, C, and N, which unfortunately
fix the final composition of the ceramic composite. To extend
the range of compositions available through these methods,
the addition of elemental B and C to the reactant mixture
4
5
components. Moreover, due to an excellent combination of
mechanical, wear, and electrical properties as well as good
corrosion resistance, some of these composites, such as TiB2–
TiN and TiB –TiC, are candidate materials for high-technology
2
applications, in which the operating conditions can be
defined as severe. In addition, their use in other nonstructural
and the use of a nitrogen atmosphere instead of an inert one
6
28–31
have been proposed. Moreover, only few works have
reported the synthesis of these composites by milling pro-
cesses in which relatively long milling periods were always
applications, such as molten-metal crucibles, cathodes in
aluminum electro-melting, and vaporizing elements in vac-
7
,8
32–34
uum-metal deposition installations, have also been studied.
Furthermore, compared with conventional cermets based on
WC and TiC, cermets based on TiC N –TiB2 composites
necessary.
and B
tants, and a subsequent thermal treatment was often compul-
Longer milling times were required when BN
4
C rather than elemental mixtures were used as reac-
x
1ꢀx
3
5,36
exhibit a higher hardness and chemical stability at high
temperatures and are good alternatives for wear-resistant
sory to achieve full conversion.
The MSR is a simple, fast, and direct method and, in con-
trast to SHS, requires only a single step to mix the reactants,
produce the product, and promote its subsequent homogeni-
zation. Thus, in this study, we intended to prepare different
ceramic composites in the Ti–B–C–N system by performing
simultaneous MSR processes from mixtures of elements in a
controlled atmosphere. We report that the MSR is suitable
for obtaining composite materials because excellent powder
homogeneity and a fine microstructure can be simultaneously
achieved.
9
materials. In this framework, ceramic composites in the
Ti–B–C–N system represent promising materials for applica-
tions as wear-resistant parts and in high-performance cutting
tools and also exhibit good behavior as high-temperature
1
0–12
structural components in heat exchangers and engines.
Usually, TiN, TiC, TiC powders are pro-
duced by reduction processes (carbothermal or borothermal
x
N
1ꢀx, and TiB
2
R. Cutler—contributing editor
II. Experimental Procedure
Titanium powder (99% purity, <325 mesh; Strem Chemicals,
Newburyport, MA), boron powder (95%–97% purity, amor-
phous powder; Fluka, St. Louis, MO), and graphite powder
Manuscript No. 30754. Received December 02, 2011; approved February 22, 2012.
Supported by the Spanish government under grant No. MAT2010-17046, which
was financed in part by the European Regional Development Fund 2007-2013. E. Chi-
2
(11 m /g, Fe ꢁ 0.4%; Merck, Whitehouse Station, NJ) were
cardi and J. M. Cordoba were supported by CSIC through JAE-Pre and JAE-Doc
´
grants, respectively, financed in part by the European Social Fund (ESF).
Author to whom correspondence should be addressed. e-mail: fgotor@cica.es
used in this work. The different powder mixtures were ball
†
2
milled under 6 bars of high-purity helium gas (H O < 3 ppm,
1

2
Journal of the American Ceramic Society—Avile´s et al.
O
2
< 2 ppm and C
n
H
m
< 0.5 ppm; Air Liquide Espan
O and O < 3 ppm;
a) using a modified planetary ball mill
model Micro Mill Pulverisette 7; Fritsch, Idar-Oberstein,
˜
a,
spectra were deconvoluted for plural scattering with the Fou-
rier-ratio method, and semi-quantitative analyses were per-
formed. All of these treatments were performed within the
EL/P program (Gatan).
S. A., Madrid, Spain) or nitrogen gas (H
Air Liquide Espan
(
2
2
˜
Germany) operated at a constant gas pressure; self-propagating
reactions were detected during milling. These reactions were
detected by connecting the vial to a gas cylinder via a rotat-
ing union (model 1005-163-038; Deublin, Waukegan, IL) and
to a flexible polyamide tube and continuously monitoring the
gas pressure with an SMC solenoid valve (model EVT307-
III. Results and Discussion
Table I shows different elemental mixtures submitted to mill-
ing to obtain ceramic composite materials belonging to the
Ti–B–C–N system by the MSR process. To provide insight
into the MSR process leading to these ceramic composites,
5
DO-01F-Q; SMC Co., Tokyo, Japan) connected to an
ADAM-4000 series data acquisition system (Esis Pty Ltd.,
Pennant Hills, Australia). When the self-sustaining reaction
occurs, the increase in temperature due to the exothermic
reaction produces an instantaneous increase in the total pres-
sure. The ignition time (tig) for each mixture, e.g., the milling
time required to ignite the reactant mixture, was obtained
from the spike observed in the time-pressure record. After
ignition, the milling was prolonged for 30 min to ensure full
conversion and to obtain a homogeneous product.
TiB
2
, TiC, and TiC
x
N
1ꢀx monophasic products were also
obtained by the same procedure. For all mixtures, the time-
pressure record showed a single MSR effect, and the ignition
times (t ) are shown in Table I. The products obtained after
the MSR processes were identified by XRD, and the
observed phases are also depicted in Table I. The XRD dia-
grams corresponding to the three ceramic composite products
(samples TBN, TBC, and TBCN) are shown in Fig. 1. Sam-
ples TBC and TBCN were constituted by a biphasic product
ig
All milling experiments were performed under the same
experimental conditions. Five grams of powder, together
with seven tempered steel balls, were placed in a 45-mL tem-
pered steel vial (67 Rc). The diameter and weight of balls
were 15 mm and 12.39 g, respectively. The powder-to-ball
mass ratio (PBR) was 1/17.35. The vial was purged several
times, and the desired gas pressure was selected before mill-
ing. A spinning rate of 600 rpm for both the rotation of the
supporting disk and the superimposed rotation in the direc-
tion opposite to that of the vial rotation was used.
X-ray diffraction (XRD) analysis of the resultant ceramic
composite powders was performed on a X′Pert Pro MPD dif-
fractometer (Panalytical, Almelo, the Netherlands) equipped
with a graphite diffracted beam monochromator and a solid-
state detector (X′Cellerator), with an angular aperture of
featuring TiB
2
/TiC and TiB
2
/TiC
x
N
1ꢀx, respectively, whereas
sample TBN was composed of three distinct phases (TiB2
and TiN as the main phases and TiB as a minority phase).
In the case of samples TB, TC, and TBC, the syntheses of
which were performed in a helium environment, only solid-
state reactions were induced during milling; however, in the
production of samples TCN, TBN, and TBCN, nitrogen
acted as a reagent gas and was incorporated into the product,
leading to more complex reactions.
In Table I, the ratio of the reaction heat of formation and
0
the room temperature heat capacity of the products (DH 298
/
is also shown for each sample. This is a useful
3
8
Cp298
)
parameter that correlates closely with the adiabatic tempera-
ture and provides information regarding the ignition capacity
of a reactant mixture. In all cases, this parameter was well
above the 2000K value that is regarded as a necessary crite-
rion to conduct the MSR process. For samples TB, TC, and
TBC, for which solid-state reactions were performed, an
2
0
2
.12° (2Θ), using CuKa radiation (45 kV, 40 mA) over a 2Θ-
range of 20–150° and a step size of 0.017°, with a counting
time of 300 s/step. Using the freely distributed program
37
0
FULLPROF,
the Rietveld refinement method was
expected inverse correlation between DH 298/Cp298 and tig
employed for the quantitative phase analysis of composite
products.
was observed with a decrease in ignition time as the exother-
mic character of the process increased. Although the presence
of nitrogen increased the exothermic character of the mix-
tures, the ignition times for samples TCN, TBN, and TBCN
The morphology of the samples was characterized by scan-
ning electron microscopy (SEM) using a FEG S-4800 micro-
scope (Hitachi High-Tech, Tokyo, Japan). Transmission
electron microscopy (TEM), electron diffraction (ED), and
electron energy loss spectroscopy (EELS) were performed
using a CM-200 microscope (FEI, Hillsboro, OR) operating
were not proportionally reduced.
A direct relationship
0
between DH 298/Cp298 and tig was observed, and the ignition
time increased with the relative amount of nitrogen with
respect to the solid reactants in the associated reaction. This
was a direct consequence of the fact that MSR processes are
more difficult to induce when solid-gas reactions are involved
than when only solid phases are used.
˚
at 200 kV, with a LaB filament (point resolution 2.3 A), a
6
double-tilt goniometer, and a Peels spectrometer. EELS spec-
tra were acquired using a Gatan model 766-2K parallel
detection electron spectrometer (Gatan, Inc., Pleasanton,
CA) and were recorded in the diffraction mode at a collec-
tion angle of 1.45 mrad. The measured energy resolution at
the zero-loss peak of the coupled microscope/spectrometer
system was ~2 eV. A low-loss spectrum was also recorded
with each edge in the same illuminated area. After back-
ground subtraction with a standard power-law function, the
2
6,31
In the literature,
it has been reported that the initia-
tion of SHS reactions in the Ti–B–C–N system is controlled
by the rate of the surface reaction among the reagents, and is
essential for the propagation of the combustion wave and to
achieve full conversion to reach the formation temperature
of a Ti-containing eutectic melt; this facilitates the contact
between the reactants. It is for this reason that, to assist the
Table I. Elemental Mixtures Submitted to Milling in the Ti–B–C–N System, Indicating the Initial Atomic Ratio of the Reactants,
0
the Atmosphere Used, the Ignition Time (tig) of the MSR Process, the DH 298/Cp298 Ratio as a Measurement of the Self-Sustaining
Capacity of the Mixture, and the Obtained Products, as Identified by XRD.
0
39
Sample
Solid Reactants (atomic ratio)
Atmosphere
t
ig (min)
DH 298/Cp298 (K)
Products
TB
TC
Ti + B (1/2)
Ti + C (1/1)
Ti + C (1/0.5)
Ti + B (1/1)
Ti + B + C (2/2/1)
Ti + B + C (2/2/0.5)
He
He
N2
32
62
72
57
55
44
6214
5427
7357
6911
5820
6785
TiB
TiC
TiC N
2
TCN
TBN
TBC
TBCN
↓
x
1ꢀx
↓
N
2
TiB
TiB
2
2
+ TiN + TiB
+ TiC
He
N2
TiB + TiC N
2
x
1ꢀx
minor phase.

Ceramic Composite Materials in the Ti–B–C–N System
3
(
a)
(
b)
Fig. 1. X-ray powder diffraction diagrams of products obtained
from the MSR process for samples TBN, TBC, and TBCN. (●)
TiB
2
x
; (■) TiC N1ꢀx; (□) TiC; (▲) TiN; (○) TiB.
self-sustaining reaction, the addition of Ni to the reactant
mixture has been proposed to promote the formation of a
Ti–Ni eutectic melt at a lower temperature at which the reac-
3
1,39
tants are dissolved and the final product is precipitated.
The MSR process affords the advantage of extremely inti-
mate mixing of reagents; moreover, when the reaction is
ignited, it spreads quickly throughout the entire sample, com-
pletely transforming the reactants into products.
It is worth noting that in samples TBN, TBC, and TBCN,
the different phases constituting the composite products were
actually formed through independent self-sustaining reac-
tions. The fact that only a single combustion process was
detected in the time-pressure record suggests that these inde-
pendent reactions occurred simultaneously. It can be
observed in Table I that sample TB (Ti + B) reacted after
(c)
32 min of milling treatment in a He atmosphere, while sam-
ple TC (Ti + C) ignited at 62 min, requiring a greater extent
of activation. However, in sample TBC (Ti + B + C), which
can be seen as the TB and TC collection, carbon first pre-
vented the reaction between titanium and boron, delaying
the ignition; however, when it occurred after 55 min of mill-
ing, the heat released by this reaction triggered the reaction
between titanium and carbon, creating a unique propagation
front that caused the consumption of all the reactants, which
were instantly transformed into TiB and TiC. It is for this
2
reason that two different MSR events were not detected;
instead, only one event corresponding to these two simulta-
neous formation reactions was observed. This same explana-
tion can be extended to sample TBCN, which is a collection
of samples TB and TCN.
Fig. 2. SEM micrographs obtained for the (a) TBN, (b) TBC and
c) TBCN ceramic composites.
(
For sample TBN, the aim of the reactant mixture was to
obtain an equimolar TiB /TiN product through the multiple
of TiB in sample TBN as a minor phase was the result of a
limited absorption of nitrogen by the sample (incomplete
2
reaction 2Ti + 2B + 1/2N = TiB + TiN such that the heat
2
2
released during the formation of TiB
2
would promote the
reaction between Ti and N
the intended TiB /TiN mixture but allowed for the consump-
tion of all of the elemental Ti, which was not detected in the
final product.
2
); this prevented the formation of
self-sustaining reaction between the exceeding Ti and nitro-
gen gas. However, despite the fact that the reaction between
titanium and nitrogen (a reactant gas) is extremely exother-
2
0
mic (DH 298/Cp298 = 9129 K), its induction by a milling pro-
The microstructural characterization of the three ceramic
composite powders (TBN, TBC, and TBCN) was performed
using scanning and transmission electron microscopy tech-
niques (SEM, TEM, ED and EELS), and the representative
results are presented in Figs 2–4. SEM micrographs (Fig. 2)
show the morphology and particle size distribution of the
three samples. As can be observed, all the materials were
quite similar and were formed by large faceted particles (0.5–
2 lm) surrounded by smaller rounded ones (0.1–0.3 lm).
The EDX analysis did not provide valuable information
because B, C, and N have very low atomic masses.
cess is more difficult to achieve than the reaction between
titanium and boron or carbon (solid reactants). This was evi-
denced by the fact that the attempted self-sustaining reaction
of titanium with gaseous nitrogen, under the same milling
conditions used in the present work and without any other
element, was not detected after several hours of treatment.
The combustion synthesis of TiN induced by the high-energy
ball milling of Ti under a nitrogen atmosphere has already
been reported, although only after prolonged treatment in an
4
0
extremely intensive milling regime. Therefore, the existence

4
Journal of the American Ceramic Society—Avile´s et al.
(
a)
(
b)
(
c)
Fig. 3. TEM and ED representative results obtained for (a) TBN,
b) TBC, and (c) TBCN composites. TiB , TiB, TiN, TiC, and
TiC
1ꢀx phases are marked in the images.
(
2
x
N
A representative TEM micrograph corresponding to the
TBN sample, which is depicted in Fig. 3(a), confirmed the
heterogeneous particle size distribution observed by SEM.
An ED study established the notion that the larger particles
in the micrometric range corresponded to the TiB
(
2
phase
marked in the image) with a hexagonal symmetry and P6/
mmm space group (191). The corresponding ED pattern was
oriented along the [101] zone axis and is presented in
Fig 3(a). Some of the medium crystalline diffraction domains
(
(
in the submicrometric range) corresponded to the TiB phase
[031] zone axis depicted) with an orthorhombic symmetry
and Pnma space group (62). Other medium-sized crystals
belonged to the TiN phase ([100] zone axis is presented) with
a cubic symmetry and Fm3m space group (225). The smallest
particles in the nanometric range that surrounded the TiB
2
particles also corresponded to the TiN phase. When an elec-
tron beam was diffracted along an area, such as the area that
is circled in the image that contained these small diffraction
domains, ED ring patterns were observed.
The EELS experiments performed using the TBN compos-
ite confirmed the presence of the three different phases; the
characteristic spectra for TiB , TiB, and TiN are presented in
Fig. 4. EELS representative spectra obtained for TiB
TiC, and TiC
composites.
2
, TiB, TiN,
x
N1ꢀx phases in the TBN, TBC, and TBCN ceramic
2
2
Fig. 4. B-K and Ti-L2-3 edges were recorded in the TiB and
TiB phases, and N-K and Ti-L2-3 edges were recorded in the
TiN phase. It is worth mentioning that the Ti-L2-3 edge dis-
played the same shape in the TiB and TiB phases; however,
[001] zone axis, and its ED pattern is depicted in Fig. 3(b).
The area marked with a circle, in which submicrometric and
nanometric crystallites were observed, corresponded to the
TiC phase, as derived from the ED ring pattern [Fig. 3(b)].
Representative EELS spectra are presented in Fig. 4. B-K
2
it showed a slightly different shape (marked with arrows)
when it was recorded in the TiN phase. This was due to the
Ti bonds that are similar in TiB and TiB, which differ from
2
those found in TiN. A semi-quantitative analysis was con-
ducted in the TiB and TiB phases, and the calculated Ti : B
ratio was verified to conform to the 2 : 1 and 1 : 1 stoichi-
ometries, respectively.
The micrograph presented in Fig. 3(b) corresponds to the
TBC composite; a heterogeneous particle size distribution
was observed. The largest crystalline domains belonged to
the TiB2 phase. The marked crystal was oriented along the
and Ti-L2-3 edges were recorded in the TiB phase, and C-K
2
2
and Ti-L2-3 edges were recorded in the TiC phase. The
shapes of the Ti-L2-3 edges appearing in both phases were
different (marked with arrows); this difference was greater
than the Ti-L2-3 edge from the TiN phase.
Microscopic characterizations of the TBCN composite are
presented in Fig. 3(c), confirming the presence of two phases,
TiB and TiC N . As in the two previous composites, the
2
x
1ꢀx

Ceramic Composite Materials in the Ti–B–C–N System
5
TiB
2
phase showed quite large diffraction domains, which
phases, which was confirmed by quantification using the
Rietveld refinement (Table II).
can be seen in the micrograph [Fig. 3(c)], with the corre-
sponding ED pattern along the [001] zone axis. The smaller
crystallites corresponded to the TiC N
x
dispersion in the diffraction domain size was observed, as
confirmed by the fact that dot-ED [the upper left side of
Fig. 3(c)] and ring-ED [the lower right side of Fig. 3(c)] pat-
terns were observed for this phase. The EELS spectra
recorded for different crystals showed that the presence of a
C-K edge was always associated with N-K and Ti-L2-3 edges
Finally, Fig. 7 and Table II show the compositional varia-
tions performed in the TBCN series. The ignition time values
in Table II show two trends. On one hand, ignition occurred
at shorter times as the relative amount of boron in the start-
ing mixture was increased, in line with previous results dem-
onstrating the importance of the presence of boron to induce
the ignition of the mixtures. On the other hand, for the same
Ti/B ratio, the ignition time was further reduced when the
relative amount of graphite was decreased. In a previous
study on the synthesis of transition metal carbonitrides by
the MSR process, the ignition of the reactant mixture was
solid solution, and
1ꢀx
(
Fig. 4), confirming the formation of a TiC N
x
solid solu-
1ꢀx
tion phase. In summary, the microstructural characterization
of the three ceramic composite powders revealed that the
TiB
than the sizes of the other constituents of the composites,
i.e., TiN, TiC, or TiC
4
1
2
phase was always obtained with a particle size larger
determined by the synergistic effect of nitrogen and carbon.
Although the presence of carbon was important to self-
sustain the reaction, and a minimal amount of carbon was
always necessary, the ignition of the mixture was favored by
the presence of nitrogen.
x
N
1ꢀx, which were usually on the nano-
meter scale. These results are consistent with the XRD dia-
grams of Fig. 1, which show broader reflections for the TiN,
TiC, and TiC N
x
ing to the TiB phase.
2
phases compared with those correspond-
It is worth noting that in this TBCN series, the MSR pro-
cedure allowed us to modify both the molar proportions of
1ꢀx
Table II shows a new set of reactant mixtures, which cor-
responds to some variations of the TBN, TBC, and TBCN
composite samples from Table I. These milling experiments
were designed to verify the ability of the MSR process to tai-
lor the chemical composition of the final product. The XRD
diagrams of the TBN family samples are shown in Fig. 5,
the TiB
as the chemical composition of the TiC N
2
and TiC
x
N
1ꢀx constituent phases (Table II) as well
solid solution
x
1ꢀx
by an appropriate adjustment of the reactants’ ratio. The rel-
ative amounts of the TiB and TiC
2
x
N
1ꢀx phases in the final
product were fixed by both the Ti/(B+C) and B/C atomic
ratios. All boron were consumed to form TiB , and the
remaining Ti reacted simultaneously with graphite and the
2
which clearly shows that the relative amounts of TiB
2
and
TiN can be modulated simply by modifying the initial Ti/B
atomic ratio. These findings were confirmed by quantification
using the Rietveld refinement (Table II). Moreover, the tig
values in Table II show how the ignition time increased with
decreasing relative amounts of boron in the starting mixture
and illustrate the importance of the presence of boron to
induce ignition. This trend is a direct consequence of the
increasing difficulty of igniting elemental mixtures, in which
the extent of the self-sustaining reaction between titanium
and nitrogen becomes increasingly important. At the same
time, the amount of TiB in the final product, as a conse-
quence of the incomplete reaction between titanium and
nitrogen, increased with the decrease in boron content. TiB
was not detected in the mixture with a Ti/B atomic ratio of
nitrogen atmosphere to produce TiC
x
N
1ꢀx. The stoichiome-
try of the solid solution was controlled by the ratio (Ti-½B)/
C. This is illustrated in the inset of Fig. 7 for samples
TBCN-c, TBCN, and TBCN-d, which shows a shift in the
(111) and (200) reflections of the TiC
was attributed to the different C/N ratios in the solid solu-
tion. The lattice parameter of TiC
x
N
1ꢀx phase; this shift
x
N
1ꢀx was calculated from
˚
the XRD diagrams, and values of 4.304, 4.292, and 4.279 A
were obtained for samples TBCN-c, TBCN, and TBCN-d,
respectively. These values are in accordance with the final
TiC
x
N
1ꢀx stoichiometry and are close to the value expected
23
from the atomic ratio of the starting mixture. The presence
of TiB in some products slightly enriched the carbon content
of the carbonitride phase. The presence of TiB in this TBCN
series in samples with higher Ti/(B+C) starting ratios was
due, as in the previous TBN series, to an incomplete reaction
of Ti with nitrogen to form, in this case, the TiC N
2/3. In any case, TiB always appeared as a secondary phase,
the maximum amount of which was 15 wt% in the less
favorable mixture, with a Ti/B atomic ratio of 2/1.
Figure 6, which shows the variations in the TBC series,
clearly demonstrates how modifying the boron and carbon
ratio in the starting mixture gives rise to composite materials
x
1ꢀx
phase.
The overall results shown in Table II indicate that TiB
tended to form when the Ti/(B+C) atomic ratio was ~1 or
higher. This was due to the difficulty of completing the
with different relative amounts of the TiB and TiC constituent
2
Table II. Elemental Mixtures Corresponding to Variations of the Composite Materials in Table I, Modifying the Initial Atomic
Ratio of the Reactants and Indicating the Used Atmosphere, the Ignition Time (t ) of the MSR Process, and the Phase
ig
2
Quantification in the Products, as Calculated by the Rietveld Refinement with Goodness of Fit (v ).
2
Sample
Solid Reactants (atomic ratio)
Atmosphere
t
ig (min)
Phase Content (wt%)
v
TBN-a
TBN-b
TBN
TBN-c
TBN-d
TBC-a
TBC
TBC-b
TBCN-a
TBCN-b
TBCN-c
TBCN
TBCN-d
TBCN-e
TBCN-f
Ti + B (2/3)
Ti + B (3/4)
Ti + B (1/1)
Ti + B (3/2)
N
2
35
40
57
102
147
44
55
55
36
22
43
44
31
45
40
TiB
TiB
2
/TiN/TiB 73/27/0
/TiN/TiB 66/30/4
2.5
4.6
2.0
4.1
4.6
4.2
4.7
4.2
3.1
5.3
3.5
3.2
4.0
2.6
3.9
2
TiB /TiN/TiB 47/39/14
2
TiB /TiN/TiB 35/58/7
2
TiB
Ti + B (2/1)
2
/TiN/TiB 19/66/15
TiB /TiC 66/34
TiB /TiC 51/49
Ti + B + C (3/4/1)
Ti + B + C (2/2/1)
Ti + B + C (3/2/2)
Ti + B + C (3/4/0.7)
Ti + B + C (3/4/0.3)
Ti + B + C (2/2/0.7)
Ti + B + C (2/2/0.5)
Ti + B + C (2/2/0.3)
Ti + B + C (3/2/1.4)
Ti + B + C (3/2/0.6)
He
2
2
2
TiB /TiC 36/64
N
2
TiB
TiB
2
/TiC
/TiC
x
N
1ꢀx/TiB 63/37/0
2
x
N1ꢀx/TiB 64/36/0
TiB /TiC N /TiB 51/49/0
2
2
2
x
x
x
1ꢀx
TiB
TiB
/TiC
/TiC
N
1ꢀx/TiB 50/50/0
N1ꢀx/TiB 49/48/3
TiB /TiC N /TiB 36/64/0
2 x
TiB /TiC N /TiB 27/63/10
2 x
1ꢀx
1ꢀx

6
Journal of the American Ceramic Society—Avile´s et al.
self-sustaining solid-gas reactions induced by milling, which
resulted in a lower incorporation of nitrogen into the final
product than was required to obtain stoichiometric TiB
TiN and TiB –TiC N mixtures. In any case, TiB was the
2
–
2
x
1ꢀx
only secondary phase detected, and it should be noted that
this phase is also an interesting refractory compound with
4
2–44
properties similar to those of TiB2.
Moreover, the forma-
tion of TiB prevented the presence of unreacted Ti in the
ceramic composite, which would have been detrimental to
the final properties of the material.
In summary, the MSR procedure represents an alternative
procedure for obtaining composite materials in the Ti–B–C–
N system with great control over the composition of the final
product. According to the results shown in Table II, the
overall chemical reactions associated with the MSR processes
studied in this work were derived and are as follows:
y
ðx þ yÞTi þ 2xB þ N2 ! xTiB2 þ yTiN
(1)
(2)
2
Fig. 5. X-ray powder diffraction diagrams of the products obtained
ðx þ yÞTi þ 2xB þ yC ! xTiB2 þ yTiC
from the MSR process for the TBN series. (●) TiB ; (▲) TiN; (○)
2
TiB.
ꢀ
ꢁ
1
ꢀ z
ðx þ yÞTi þ 2xB þ ðyzÞC þ
N2 ! xTiB2
2
(3)
þ yTiCzN1ꢀz
;
0:3\z\1
IV. Conclusions
The experimental results in this work show that the MSR
process can be very useful for the synthesis of multiphase
materials. Powdered ceramic composites composed of TiB
and TiC, TiN, or TiC
2
,
x
N
1ꢀx were prepared in a single step
from the constituent elements in a controlled atmosphere.
The ability of the MSR to tailor the chemical composition of
composite materials belonging to the Ti–B–C–N system was
demonstrated, and a broad range of compositions was
obtained. The composition of the ceramic composite powders
was designed by carefully adjusting the atomic ratio of the
solid reactants (titanium, boron, and carbon) and appropri-
ately choosing the type of surrounding atmosphere, inert or
reactive, employed during milling. The microstructural char-
acterization of composite powders showed an excellent distri-
bution of different components, which was mainly composed
of TiB2 micrometric faceted particles surrounded by TiC,
Fig. 6. X-ray powder diffraction diagrams of the products obtained
from the MSR process for the TBC series. (●) TiB ; (□) TiC.
2
TiN, or TiC
and nanometric ranges.
x
N1ꢀx rounded particles in the submicrometric
Acknowledgments
The authors wish to thank Ms. C. Gallardo for her assistance in the high-
energy ball-milling experiments.
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