Combustion, Explosion, and Shock Waves, Vol. 38, No. 1, pp. 37 40, 2002
Nonthermal Nature of Unsteady Combustion of Chromium in Nitrogen
B. Sh. Braverman,1 M. Kh. Ziatdinov,1 and Yu. M. Maksimov1 UDC 536.46
Translated from Fizika Goreniya i Vzryva, Vol. 38, No. 1, pp. 43 46, January February, 2002.
Original article submitted January 17, 2001; revision submitted April 10, 2001.
When specimens pressed from a chromium powder react with nitrogen at pressures
of 1 8 MPa and relative densities of 0.47 0.55, unsteady combustion of a nonthermal
nature, which was not known previously, is observed. It is shown that the unsteadiness
is due to cracks formed ahead of the combustion front. The specimens fail as a result
of an increase in the condensed-phase volume upon formation of nitrides.
INTRODUCTION 49% when it transforms into Cr2N and CrN, respec-
tively. Compared to pure metals, the change in the
Stimulated by an exothermic reaction, macrodis- volume of chromium nitrides is greater than that of ni-
placement of a condensed substance can influence the
trides of other metals. Chromium combustion occurs in
combustion process and formation of the product. At
the solid phase [5], and, hence, stresses caused by ex-
present, these processes are better understood for the
pansion cannot be compensated by melting and liquid
case of gasless combustion.
spreading.
Merzhanov [1] and Naiborodenko et al. [2] showed
In the experiments, we used a PKh1S chromium
that motion of the specimen material under the action
powder and gaseous nitrogen. The combustion was
of absorbed and diluted gases released in combustion
studied in a constant-pressure bomb. Cylindrical spec-
affects the velocity of the reaction zone and the shape
imens of diameter 25 mm and height 50 70 mm were
of the final products. The steady regime of combus- pressed from the powders. The relative density of the
tion can be converted to the self-oscillatory regime [3]
specimens was varied from 0.47 to 0.55, and the nitrogen
owing to the displacement of the condensed substance
pressure was varied from 1 to 8 MPa. The specimens
behind the combustion front of gasless compositions and
were ignited at the upper end. Some specimens were
associated delamination of the products. Using the ex- burnt in cylindrical tubes made of a metal mesh with a
perimental method of pulsed radiography, Proskudin et
0.1-mm cell to ensure access of nitrogen to the lateral
al. [4] showed that, behind the combustion front of the
surface of the specimens. Nitrogen consumption was de-
Ti C system, the specimens deform, which may result in
termined by a change in the specimen mass. The com-
unsteady combustion. However, the effect of deforma- bustion rate was measured by a stopwatch or an FR-14
tions and cracking of specimens on filtration combustion
photorecorder. To study the character of combustion-
of metal powders has not been studied experimentally.
front propagation, it was arrested by an abrupt drop of
the nitrogen pressure and filling the bomb by an inert
gas (argon).
EXPERIMENTAL PROCEDURE
In this work, we study the combustion of chromium
DISCUSSION OF
powders in nitrogen. The volume of the material in-
EXPERIMENTAL RESULTS
creases upon chromium nitration. Given the densities
of chromium nitrides, simple calculations show that the
The combustion of powders in a metal-mesh shell
volume of a chromium particle increases by 27% and
was steady in the entire range of pressures and densi-
ties. The combustion front was seen as a plane luminous
1
Department of Structural Macrokinetics,
band moving with a constant velocity toward the axis
Tomsk Science Center, Russian Academy of Sciences,
Tomsk 634021; [e-mail]. of the cylindrical specimen.
0010-5082/02/3801-0037 $27.00 © 2002 Plenum Publishing Corporation 37
38 Braverman, Ziatdinov, and Maksimov
Fig. 1. Shape of the combustion front of a pressed Fig. 3. Specimen with an arrested combustion front.
chromium powder in nitrogen (nitrogen pressure is
4 MPa).
a
b
Fig. 4. Diagram of intersection of the combustion
front with a crack (the arrows show the direction of
combustion-front propagation from the crack).
Fig. 2. Appearance of the combustion product of a
pressed specimen for nitrogen pressures of 1 (a) and
8 MPa (b).
The combustion of specimens with a free surface parts of the specimen that did not react (light fragments
was steady when the nitrogen pressure was 1 8 MPa in Fig. 2b). A comparison of Fig. 2a with Fig. 2b shows
and the relative density of pressing was 0.47 0.55. The that the failure of specimens becomes more pronounced
reaction front propagated over the specimen as sepa- as the nitrogen pressure increases. The possible reason
rate, randomly moving luminous sites. This fact dis- is that, as the pressure increases from 1 to 8 MPa, the
tinguishes this combustion regime from all the known transformation depth increases substantially [5]. The
modes of unsteady combustion (spin, self-oscillations, average combustion rates of the specimens with a free
etc.). A distinctive feature of the combustion examined surface are 1.2 times lower than that of specimens of
was that additional sites occurred at a distance from the same density enclosed in a metal-mesh shell, the
the luminous reaction front. Behind the site, the ini- nitrogen pressure and relative density being identical.
tial substance remained (Fig. 1). The specimen failed The most probable reason for this occurrence of the
partly during combustion. There were many cracks on process is that cracks are formed ahead of the combus-
the specimens (Fig. 2). One can see failure marks and tion front, which is supported by the following facts.
Nonthermal Nature of Unsteady Combustion of Chromium in Nitrogen 39
First, one can see from Fig. 1 that the luminous com-
bustion sites arrive at the specimen surface along the
cracks ahead of the continuous front. Between the com-
bustion front and leading sites, there is a dark region
of the nonreacted substance. Second, there are cracks
in the unburnt part of the specimens with an arrested
combustion front. Figure 3 shows a  quenched speci-
men. One can see a large crack ahead of the arrested
combustion front. Moreover, there are small cracks that
cannot be seen in the photograph. Third, the char-
acter of failure indicates that it is precisely ahead of
the front where the cracks are formed. During com-
bustion, pieces of various size are separated from the
specimens. As a result of this failure, cylindrical speci-
mens with a smooth surface acquire a  corroded form
similar to that shown in Fig. 2b. Inspection of the sepa-
rated material shows that it consists mainly of the initial
chromium. Hence, cracking, failure of the specimens, Fig. 5. Specimen burnt in a shell.
and separation of fragments from the specimens occur
in the nonreacted part ahead of the combustion front.
front. When chromium powder burns in a metal-mesh
Figure 4 shows schematically what happens when the
shell, radial expansion of the specimen is constrained,
combustion front is intersected by a crack. The dark
strains are small, cracks are not formed, and the com-
region denotes the burnt part of the specimen. The
bustion regime is steady. There are no cracks on the
crack is a channel of accelerated (easier) filtration com-
burnt specimens (Fig. 5).
pared to the bulk of the pressed specimen; therefore,
The increase in the specimen diameter cannot be
the velocity of combustion-front propagation along the
caused by absorbed gases released. To verify this, we
crack is much higher than the average combustion rate.
pressed specimens from a chromium powder annealed
Propagating along the crack, the reaction site arrives
in vacuum (10-3 Pa) within an hour. In this case, the
at the lateral surface of the specimen ahead of the com-
character of combustion remained unchanged: the spec-
mon combustion front. In the following, similar new
imen failed and combustion was unsteady. This shows
combustion sites propagate over the specimen surface
that absorbed gases are not responsible for the forma-
along a complex trajectory, which is sometimes close
tion of cracks. Hence, the most probable reason for
to a horizontal line. In this case, the local velocity of
expansion and failure of the specimens is the volume
the sites can exceed the average velocity of combustion-
change due to the formation of chromium nitrides.
front propagation toward the specimen axis. This differ-
ence between the local and average velocities is observed
for spin combustion [3, 6]. Since the cracks ahead of the
CONCLUSIONS
combustion front are distributed irregularly, the prop-
agation of the sites is not strictly periodical, which is
the case of self-oscillating or spin regimes of combus-
Unsteady filtration combustion associated with
tion [3, 6]. The cracks are produced by stresses acting
cracking of the unburnt part of the specimen has been
during deformation of a burning specimen. One can
revealed. The cracking is caused by an increase in the
see from Fig. 3 that the diameter of the burnt part is
specific volume upon the formation of chromium ni-
greater than the diameter of the initial specimen.
trides. It is of interest to study the above-described
In a pressed specimen, particles are bonded by au-
phenomenon by, for example, pulsed radiography [4] (or
tohesion and internal-friction forces that ensure the con-
a similar method), which would allow one to observe
nectivity of the specimen and its strain resistance [7].
cracking during combustion.
An increase in the diameter during combustion causes
This work was supported by the Russian Foun-
the stress waves to propagate. Since the stress-wave ve-
dation for Fundamental Research (Grant No. 01 03
locity is usually higher than the thermal-wave velocity,
32055).
stresses occur ahead of the combustion front. If these
stresses exceed the ultimate strength of the pressed
specimen, cracks are formed ahead of the combustion
40 Braverman, Ziatdinov, and Maksimov
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