Preparation of Cast Mo2B5 by Self-Propagating High-Temperature Synthesis Metallurgy Methods

Article abstract of DOI:10.1134/S0020168518120051

Abstract—: This paper reports the preparation of cast dimolybdenum pentaboride by a self-propagating high-temperature synthesis (SHS) metallurgy method. Experiments were carried out in SHS reactors at an initial excess gas (Ar) pressure p₀ = 5

Full text of DOI:10.1134/S0020168518120051

ISSN 0020-1685, Inorganic Materials, 2018, Vol. 54, No. 12, pp. 1216–1222. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © V.A. Gorshkov, N.V. Sachkova, N.Yu. Khomenko, 2018, published in Neorganicheskie Materialy, 2018, Vol. 54, No. 11, pp. 1256–1262.
Preparation of Cast Mo2B5 by Self-Propagating High-Temperature
Synthesis Metallurgy Methods
V. A. Gorshkova, *, N. V. Sachkovaa, and N. Yu. Khomenkoa
aInstitute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences,
ul. Akademika Osip’yana 8, Chernogolovka, Moscow oblast, 142432 Russia
*e-mail: gorsh@ism.ac.ru
Received February 12, 2018; in final form, May 24, 2018
Abstract⎯This paper reports the preparation of cast dimolybdenum pentaboride by a self-propagating high-
temperature synthesis (SHS) metallurgy method. Experiments were carried out in SHS reactors at an initial
excess gas (Ar) pressure p0 = 5 MPa. In the experiments, we used thermite-type starting mixtures consisting
of molybdenum oxide, aluminum, boron oxide, and elemental boron powders. The combustion of such mix-
tures yields two types of final product: Mo2B5 and Al2O3. The combustion temperatures of the starting mix-
tures used in our experiments have been shown to exceed the melting points of the final products, which are
thus formed in a liquid (cast) state. Under the effect of gravity, the heavier phase Mo2B5 settles down to form
a lower ingot, whereas the lighter phase Al2O3 forms an upper ingot. The synthesis products have been char-
acterized by X-ray diffraction and local microstructural analysis. The results demonstrate that the composi-
tion and amount of the starting mixture have a significant effect on the synthesis parameters and the compo-
sition and microstructure of the Mo2B5. We have optimized conditions for the preparation of single-phase
cast dimolybdenum pentaboride.
Keywords: boride ceramics, cast dimolybdenum pentaboride, self-propagating high-temperature synthesis
metallurgy, combustion, crystallization, phase composition, microstructure
DOI: 10.1134/S0020168518120051
INTRODUCTION
Metal borides constitute an important and large
class of inorganic compounds possessing low fusibility
exceed 19.5–20.8 wt %.) This phase is an imperfect
structure with the ideal composition Mo2B5. At a tem-
perature of 1600°C, it transforms into a phase with the
and high chemical stability in various aggressive media MoB2 structure, which melts at ~2100°C [5]. Dimo-
and exhibiting metal-like behavior, which shows up as
high electrical and thermal conductivity, magnetic
properties, and an unusual electronic structure. Their
high hardness in combination with plastic properties
and chemical inertness open up great possibilities for
the use of borides as abrasive tools capable of ensuring
high-quality surface finish and materials for cutting
tools and tool steel heavily doped with borides. They
are also used as alloys with some transition metals for
the fabrication of critical parts; for boronizing articles
made from steel and other metals in order to improve
their hardness, wear resistance, and corrosion resis-
tance; and as catalysts and semiconductors [1–4].
lybdenum pentaboride has a wide homogeneity range:
66.7–70 at % boron. It is insoluble in water and has
tm = 1600°C and ρ = 7.2 g/cm3 [6]. The best known
process for the preparation of Mo2B5 is reaction
between MoO3, boron carbide, and carbon black. In a
vacuum from 10–2 to 10–1 mm Hg at temperatures
from 1200 to 1300°C, this reaction reaches completion
in 0.5°C 1 h. Dimolybdenum pentaboride can also be
prepared by heating a mixture of molybdenum and
boron powders in a hydrogen atmosphere at a tem-
perature from 1500 to 1600°C (2Mo + 5B = Mo2B5).
Yet another process for its synthesis is heating a mix-
ture of molybdenum, boron oxide, and boron carbide
in a hydrogen atmosphere at a temperature of 2000°C
(28Mo + 5B2O3 + 15B4C = 14Mo2B5 + 15CO). It is
The Mo–B system is known to contain six boride
phases: Mo2B, Mo3B2, α- and β-MoB, MoB2, and
Mo2B5. According to its phase diagram (Fig. 1), the
higher molybdenum boride is Mo2B5 (calculated worth noting that all of the existing industrial dimo-
lybdenum pentaboride preparation processes require
deficiency: boron content of this boride does not much energy and have low efficiency.
boron content, 22 wt %). (Actually, there is boron
1216

PREPARATION OF CAST Mo2B5
Temperature, °C
1217
2600
2200
1800
Mo2B5
MoB
2180
Mo3B2
Mo2B
MoB2
2100
2070
β
2060
MoB
2000
2000
1600
Mo3B2
1850
MoB2
MoB
Mo2B
Mo2B5
1400
200
0
4
8
12
wt % B
16
20
24
Fig. 1. Phase diagram of the Mo–B system.
In this respect, self-propagating high-temperature mixtures containing an excess of the boron-containing
synthesis (SHS) appears very attractive [7–10]. One component and deficient in aluminum relative to the
promising direction in this approach is SHS metal- stoichiometric composition. The ratio of the reagents
lurgy. This method utilizes mixtures of metal oxides, a was calculated for the following chemical reactions:
metallic reducing agent (aluminum), and a nonmetal
1. 4MoO3 + 18Al + 5B2O3 → 2Mo2B5 + 9Al2O3 (X),
(carbon, boron, or silicon). Combustion temperatures
2. 2MoO3 + 4Al + 5B → Mo2B5 + 2Al2O3 (Y),
3. Х/Y = 0.5/0.5,
of such mixtures typically exceed the melting points of
the starting reagents and the final products, which are
formed in a liquid (cast) state in a combustion wave.
This approach was used to obtain a large number of
cast borides, silicides, and carbides of refractory met-
als and related composite materials [11–13].
4. 0.9 (Х/Y) + 0.1 (Al/B2O3).
In our experiments, we visually observed the syn-
thesis process through the inspection holes of the SHS
reactor. The initial and final (maximum) pressures in
the reactor were monitored with a standard pressure
gage. Separation into the desired boride phase and an
oxide phase was quantified by the yield of the desired
product to the ingot (ηy), which was calculated as ηy =
(MMB/Mmix) × 100%, where MMB is the mass of the
ingot of the desired product and Mmix is the mass of the
starting mixture. The scattering process (ejection of
material from the reaction vessel) was quantified by
the degree of scatter (ηs), which was calculated as ηs =
Δm/Mmix) × 100%, where Δm = Mi – Mf is the mass of
the material ejected from the reaction vessel, Mf is the
final mass of the vessel with the synthesis products,
and Mi is the initial mass of the vessel with the starting
mixture.
The purpose of this work is to study fundamental
aspects of the preparation of cast Mo2B5 by the SHS
metallurgy method.
EXPERIMENTAL
In our experiments, we used mixtures of analytical-
grade molybdenum(VI) oxide and pure-grade
boron(III) oxide with ASD-1 aluminum and boron.
Starting mixtures were burned in quartz and graphite
beakers 20 and 80 mm in diameter and 50 and 100 mm
in height, respectively. The weight of the mixtures was
20 or 2000 g. It is worth noting that the synthesis under
atmospheric conditions was accompanied by severe
scatter of products from the reaction vessel because of
the formation of a large amount of final and interme-
diate gaseous combustion products and the high
MoO3 volatility, which can be suppressed by creating
an excess gas (Ar) pressure in the reactor. In view of
this, all of the experiments were carried out in 3- and
20-L SHS reactors at an initial gas pressure p0 =
The phase composition of the desired products was
determined by X-ray diffraction on a DRON-3M dif-
fractometer with Cu Kα radiation. The phases present
were identified using Powder Diffraction File (PDF)
data. For microstructural characterization and ele-
mental analysis of the structural components of the
5 MPa. We used both stoichiometric mixtures and desires products, we used a Zeiss Ultra Plus ultrahigh-
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GORSHKOV et al.
(a)
(b)
2500
2000
1500
1000
500
Mo2B5
Al2O3
AlBMo
AlB10
Fig. 2. Appearance of the synthesis products. The upper
and lower ingots consist of Al O and Mo B , respectively.
2
3
2 5
0
10
20
30
40
50
60
70
80
2θ, deg
resolution field emission scanning electron micro-
scope (based on the Ultra 55).
Fig. 3. X-ray diffraction pattern of the molybdenum boride
prepared from mixture 1 (M = 20 g, stoichiometric
mix
composition:
).
= 0%
ΔB O
Al
3
2
EXPERIMENTAL RESULTS
Visual observations and video recording of the
combustion process, as well as X-ray diffraction char-
acterization and local microstructural analysis of the
synthesis products, provided a qualitative idea of the
synthesis process. The combustion temperature of
mixtures 1–4 exceeded the melting point of the syn-
thesis products, which were thus obtained in a liquid
state. In general, the synthesis process consists of three
sequential steps. The first step is combustion and
chemical transformation of the components of the
starting mixtures (MoO3, B2O3, B, and Al) into a two-
phase melt (high-temperature synthesis). The second
step is the gravity separation of the combustion prod-
ucts (MoxBy boride and Al2O3 oxide phases), which
produces two layers: upper, consisting of the “light”
oxide (Al2O3), and lower, consisting of the “heavy”
MoxBy boride. The third step is cooling and the forma-
tion of the phase composition and crystal structure of
the metallic and oxide layers.
After cooling and crystallization, there was no
bonding between the ingots, which were easy to sepa-
rate mechanically (Fig. 2). The present experimental
data are summarized in Table 1. It is seen that, during
combustion of the stoichiometric mixture 1, the final
pressure in the reactor is pf = 7 MPa, the degree of
scatter is ηs = 5%, and the yield of the desired product
(molybdenum boride) is ηy = 32%.
According to the X-ray diffraction data, the major
phase in the sample was Mo2B5. We also observed lines
of the AlBMo phase and Al2O3. The presence of AlB10
also cannot be ruled out: its strongest lines were seen,
whereas its other lines could not be separated from the
Table 1. Synthesis parameters at different starting mixture compositions (p0 = 5 MPa, Mmix = 20 g, Vreactor = 3 L)
рf, MPa ηy = (MMB/Мmix) × 100% ηs = (Δm/Мmix) × 100%
Mixture
System
1
2
3
4
Х = 4MoO3/18Al/5B2O3
Y = 2MoO3/4Al/5B
7
32
17
28
29
5
18
10
8
18
Х/Y = 0.5/0.5 = 5MoO3/11Al/2.5B2O3/2.5В 10
0.9 (Х/Y) + 0.1 (2Al/B2O3)
9
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PREPARATION OF CAST Mo2B5
1219
Phase*
Point
1
B
O
Al
Mo
0.2
2
composition
3
Al2O3
46.8
53.0
1
7
Al2O3
52.4 47.5
2
3
4
5
0.1
76.1
77.4
Mo2B5
Mo2B5
0.7
0.4
0.2
22.8
22.1
6
0.3
0.2
0.4
5
MoAlB
MoAlB
AlB10
72.5
71.6
0.2
18.9
19.8
8.4
8.2
4
8
6
7
8
19.1
0.5
80.2
81.1
100 μm
AlB10
0.3 18.2
0.4
* Calculated from the elemental
analysis and X-ray diffraction data.
Fig. 4. Microstructure and elemental composition (wt %) of the molybdenum boride prepared from mixture 1 (M =20g, stoichiometric
mix
composition:
).
= 0%
ΔB O
Al
3
2
general background in the X-ray diffraction pattern of the stoichiometric mixture 2, we had pf = 18 MPa,
(Fig. 3).
ηy = 17%, and ηs = 18%. It is worth noting that, in this
case, the final pressure in the reactor and the degree of
scatter were maximal, whereas the yield of the desired
product was minimal. This suggests that the mixture in
question is technologically unsuitable, because the
combustion of large masses in a reactor may lead to a
hazardous situation (because of the high final pres-
sure), with a very low yield of the desired product. For
this reason, this mixture was not used in subsequent
experiments. In the case of the combustion of mix-
tures 3 and 4, the processes were similar in synthesis
parameters and had low final (maximal) pressure, a
low degree of scatter, and a high yield of the desired
product. Thus, these compositions are the most
attractive for the preparation of cast dimolybdenum
pentaboride.
Figure 4 illustrates the microstructure and the ele-
mental and phase compositions of the ingot. These
data correlate well with the above X-ray diffraction
results. The major phase is Mo2B5 (points 3 and 4).
There are also the Al2O3 phase (points 1 and 2), the
MoAlB eutectic (points 5 and 6), and a small amount
of AlB10 (points 7 and 8). In the case of the combustion
1800
Mo2B5
AlBMo
1600
1400
1200
1000
800
600
400
200
0
Figures 5 and 6 present the phase composition,
microstructure, and elemental composition of the
ingot prepared from the stoichiometric mixture 3
(Mmix = 20 g) in the 3-L SHS reactor. It is seen that, in
addition to the major phase Mo2B5, there is an apprecia-
ble amount of AlBMo. In the case of the combustion of
20 g of mixture 4, containing an excess of boron-contain-
10
20
30
40
50
60
70
80
ing reagents (
), no AlBMo was detected
= 10%
ΔB O
Al
3
2θ, deg
2
in the reaction products, which consisted largely of the
molybdenum boride Mo2B5 and aluminum boride
AlB10 (Figs. 7, 8). The X-ray diffraction pattern in
Fig. 7 contains reflections from not only Mo2B5 but
Fig. 5. X-ray diffraction pattern of the molybdenum boride
prepared from mixture 3 (M = 20 g, stoichiometric
mix
composition:
).
= 0%
ΔB O
Al
3
2
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GORSHKOV et al.
Phase*
Point
1
B
Al
Mo
78.6
78.0
composition
Mo2B5
0.3
21.01
1
21.6
0.4
19.7
19.9
0.2
Mo2B5
MoAlB
MoAlB
2
3
4
71.8
71.9
2
8.5
8.2
21.2
21.9
78.6
77.5
Mo2B5
Mo2B5
3
5
6
4
0.6
* Calculated from the elemental analysis
and X-ray diffraction data.
5
6
30 μm
Fig. 6. Microstructure and elemental composition (wt %) of the molybdenum boride prepared from mixture 3 (Mmix = 20 g, stoichiometric
composition: ).
ΔB O
= 0%
Al
3
2
also AlB10. It is worth noting that, despite the appre- The X-ray diffraction pattern in Fig. 9 demonstrates
that the major phase in the synthesis product is
Mo2B5. In addition, there are weak lines (at the level of
the background) characteristic of AlB10 and AlBMo.
Figure 10 illustrates the microstructure and elemental
and phase compositions of the synthesis product
obtained in the last experiment. It is seen that the
desired product consists almost entirely of dimolybde-
num pentaboride. Small amounts (under 3%) of AlB10
and AlBMo can be present as intergranular phases.
ciable intensity of the reflections from AlB10, the per-
centage of this phase is relatively small: it has a higher
reflecting power than does Mo2B5. This follows as well
from analysis of the data presented in Fig. 8.
As shown earlier [14, 15], increasing the mass of the
starting mixture (scale factor) leads to an increase in the
percentage of the desired phase in the final product. For
this purpose, we synthesized mixture 4, with an excess of
boron-containing reagents (
Mmix = 2 kg).
ΔB O
= 10%,
Al
2
3
DISCUSSION
Analysis of the combustion process and final prod-
ucts shows that, in the case of the combustion of mix-
ture 1, the reaction product contains a large amount of
oxide inclusions. This is due to the low combustion
temperature, as a result of which the “lifetime” of the
melt is short, so that the Al2O3 and Mo2B5 phases can-
not fully separate. This is also evidenced by the low
final (maximum) pressure in the reactor (pf = 7 MPa)
and the low degree of scatter (ηs = 5%). Mixture 2
seems to have a very high combustion temperature,
which leads to the formation of a large amount of gas-
eous products. As a result, the final pressure in the
reactor reaches a very high level (pf = 18 MPa), as does
the degree of scatter of the material from the reaction
vessel (ηs = 18%). In this case, the final pressure in the
reactor and the degree of scatter have maximum val-
ues, and the yield of the desired product has the min-
Mo2B5
AlB10
2500
2000
1500
1000
500
0
10
20
30
40
50
60
70
80
2θ, deg
Fig. 7. X-ray diffraction pattern of the molybdenum boride pre-
pared from mixture 4 (M = 20 g, excess ).
Δ
= 10%
B2O3 Al
mix
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PREPARATION OF CAST Mo2B5
1221
Phase*
O
Point
B
Al
0.3
Mo
76.5
77.2
0.2
composition
Mo2B5
1
2
3
22.7
22.5
80.4
0.5
0.2
0.4
0.3
0.1
Mo2B5
AlB10
19.0
2
4
80.6
18.7
0.4
AlB10
3
* Calculated from the elemental
analysis and X-ray diffraction data.
1
100 μm
Fig. 8. Microstructure and elemental composition (wt %) of the molybdenum boride prepared from mixture 4 (Mmix = 20 g,
excess ).
ΔB O
= 10%
Al
3
2
imum value among all of the experiments carried out desired product consists almost entirely of dimolybde-
(Table 1). Mixture 3 is a 1 : 1 combination of the stoi-
chiometric mixtures 1 and 2. In the case of the com-
bustion of 20 g of this mixture, the reaction product
consists of Mo2B5, AlB10, and eutectic MoAlB.
Increasing the percentage of boron in the starting mix-
ture causes eutectic MoAlB to disappear in the com-
position of the desired product. Combustion of a large
mass of a mixture (Mmix = 2 kg) in the reactor increases
the “lifetime” of the melt, thereby leading to an
increase in the extent of the reaction. As a result, the
num pentaboride.
It is seen from the X-ray diffraction and local
microstructural analysis results that all of the synthe-
ses yielded the same phases, but their relative amounts
differed significantly. As the synthesis conditions are
varied, the synthesis product approaches the desired
phase (Mo2B5), which prevails in the last sample
(Figs. 9, 10). It is worth noting that, in most cases, the
reflections from the Mo2B5 phase in X-ray diffraction
patterns are rather narrow, which points to a well-
formed crystal structure.
CONCLUSIONS
2000
Mo2B5
We have found conditions for the preparation of
cast dimolybdenum pentaboride, Mo2B5, by the SHS
metallurgy method. The microstructure and phase com-
position of reaction products obtained under various syn-
thesis conditions have been determined by X-ray diffrac-
tion and local microstructural analysis. Taking into
account results of previous research on the preparation
of cast materials by SHS metallurgy methods, we have
shown that the key parameters influencing the funda-
mental aspects of synthesis and the phase composition
and microstructure of the final products are the initial
pressure in the reactor and the composition and
amount of the starting mixture. Varying these param-
eters, one can effectively control the fundamental
aspects of the synthesis of cast dimolybdenum pent-
aboride. We have optimized conditions for the prepa-
ration of a single-phase product.
1500
1000
500
0
10
20
30
40
50
60
70
80
2θ, deg
Fig. 9. X-ray diffraction pattern of the molybdenum boride pre-
pared from mixture 4 (M = 2 kg, excess ).
ΔB O
= 10%
Al
3
mix
2
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1222
GORSHKOV et al.
Point
B
Al
0.4
0.1
0.1
Mo Phase composition*
1
2
3
21.0
22.1
20.8
78.6
77.8
79.1
Mo2B5
Mo2B5
Mo2B5
3
* Calculated from the elemental analysis and X-ray
diffraction data
2
1
70 μm
Fig. 10. Microstructure and elemental composition (wt %) of the molybdenum boride prepared from mixture 4 (M = 2 kg,
mix
excess
).
= 10%
ΔB O
Al
3
2
10. Levashov, E.A., Mukasyan, A.S., Rogachev, A.S., and
Shtansky, D.V., Self-propagating high-temperature
synthesis of advanced materials and coatings, Int.
Mater. Rev., 2017, vol. 62, no. 4, pp. 203–239. doi
10.1080/09506608.2016.1243291
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Translated by O. Tsarev
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