Oxidation of ZrB2 powder in the temperature range of 650-800 °C

Article abstract of DOI:10.1016/j.jallcom.2008.04.006

The isothermal oxidation of ZrB₂ powder was carried out in the range of 650-800 °C in a flowing air using thermogravimetric analysis (TGA). The evolution of the phase characterization and morphology of ZrB₂ powder oxidized at 700 °C

Full text of DOI:10.1016/j.jallcom.2008.04.006

Journal of Alloys and Compounds 471 (2009) 502–506
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
◦
Oxidation of ZrB powder in the temperature range of 650–800 C
2
∗
Wei-Ming Guo, Guo-Jun Zhang , Yan-Mei Kan, Pei-Ling Wang
State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Shanghai 200050, China
a r t i c l e i n f o
a b s t r a c t
◦
Article history:
The isothermal oxidation of ZrB2 powder was carried out in the range of 650–800 C in a flowing air using
thermogravimetric analysis (TGA). The evolution of the phase characterization and morphology of ZrB2
Received 23 February 2008
Received in revised form 1 April 2008
Accepted 3 April 2008
◦
powder oxidized at 700 C for varying durations was studied by X-ray diffraction (XRD) and scanning
electron microscopy (SEM), respectively. The excellent fit of TG curves by multiple-law model suggests
that the oxidation of ZrB2 powder in air follows para-linear kinetics, and based on the fitted results, oxi-
dation mechanisms can also be obtained. The reaction product of ZrB2 powder with oxygen is metastable
Available online 19 May 2008
Keywords:
Kinetics
Microstructure
Oxidation
◦
tetragonal ZrO2 at 700 C, and the tetragonal phase transforms to the monoclinic phase with oxidation.
The oxidation of ZrB2 powder is associated with surface microcrack formation, which is attributed to vol-
ume expansion resulting from oxidation of ZrB2 to tetragonal ZrO2 and tetragonal-to-monoclinic phase
transformation of ZrO2. In the last stage of oxidation, each ZrB2 particle breaks into fragments.
©
2008 Elsevier B.V. All rights reserved.
1. Introduction
an external glassy B O3 layer and an underlying porous ZrO2 layer
2
filled with B O . The glassy B O film acts as an effective barrier
2
3
2
3
Many transition-metal diborides have attracted considerable
attention owing to the excellent combination of physicochemical
properties such as high melting temperature, high hardness, and
high thermal conductivity. ZrB2 is one of these diborides and a
candidate for use in the thermal protection systems and scramjet
engine components for hypersonic flight vehicles, as well as high
temperature electrodes, molten metal containment systems, and
incinerators, because of its good resistance to ablation at high tem-
peratures and thermal-shock resistance in addition to the attractive
aforementioned properties [1,2].
against oxygen diffusion, and the porous ZrO2 layer does not offer
effective protection. At temperatures below 1100 C, a parabolic
◦
behavior was reported for the oxidation of ZrB2 due to the pres-
ence of a continuous layer of B O [3,8–10]. B O rapidly vaporizes
2
3
2
3
◦
at temperatures above 1100 C, thus reducing the effectiveness of
the diffusion barrier. Para-linear kinetics have been observed in the
temperature range of 1100–1400 C [4,8–10]. Because the removal
◦
◦
of the B O3 layer above 1400 C leaves behind a porous ZrO2 scale,
2
ZrB2 exhibits rapid linear oxidation kinetics at these temperatures
[9,10].
When ZrB2 is exposed to oxidizing environments at high tem-
peratures, it oxidizes, which will degrade its properties. Thus, a
large number of studies on oxidation of ZrB2 and its composites
in air have been carried out in order to better understand the oxi-
dation mechanisms and enhance oxidation resistance [3–12]. The
reaction of ZrB2 with O2 is as follows:
It has been almost 50 years since researchers started to explore
the oxidation of ZrB . However, previous investigations have mainly
2
focused on the oxidation of bulk materials, oxidation mechanisms
of ZrB2 powder as well as phase composition and microstructures
that evolve during oxidation have not been studied in detail. In this
◦
paper, the oxidation behavior of ZrB2 powder from 650 to 800 C
in air was investigated. Firstly, the isothermal oxidation kinetics
and mechanisms were studied by multiple-law model. Then the
evolution of phase characterization and morphology of the samples
before and after oxidation was described.
5
ZrB + 2 O (g) → ZrO + B O (l)
(1)
2
2
2
2
3
◦
B O is known to have a low melting point of 450 C and high vapor
pressure, so it readily vaporizes at high temperatures.
2
3
B O (l) → B O (g)
(2)
2
3
2
3
2. Experimental
Researchers have studied the products resulting from the oxi-
The ZrB2 powder employed in this study was a commercial product (Kojundo
dation of ZrB , and discovered that the oxide scale is composed of
Chemical Laboratory Co. Ltd, Saitama, Japan). The surface area of the sample was
2
2
0.3042 m /g, as measured by the BET method. Oxidation tests were carried out by a
Netzsch STA449C simultaneous thermal analyzer under isothermal conditions. The
◦
samples were heated to the desired temperature (650, 700, 750, and 800 C) for
∗ Corresponding author. Tel.: +86 21 52411080; fax: +86 21 52413122.
E-mail address: gjzhang@mail.sic.ac.cn (G.-J. Zhang).
the isothermal oxidation in flowing nitrogen (99.99%, 50 ml/min). The fast heating
◦
stage with a heating rate of 50 C/min prior to the isothermal period was applied
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.04.006

W.-M. Guo et al. / Journal of Alloys and Compounds 471 (2009) 502–506
503
Fig. 1. Thermogravimetric curves of ZrB2 powder oxidized in air at different tem-
peratures: (a) 650 C; (b) 700 C; (c) 750 C; and (d) 800 C.
Fig. 2. Square of mass gain versus time for ZrB2 powder oxidized in air at different
temperatures: (a) 650 C; (b) 700 C; (c) 750 C; and (d) 800 C.
◦
◦
◦
◦
◦
◦
◦
◦
to minimize oxidation effects before reaching the target temperatures. When the
desired temperature was reached, the sample was held at that temperature for
about 5 min before the gas was switched from nitrogen to air. Roughly 60 mg of ZrB2
powder was placed into an alumina crucible and oxidized in flowing air (50 ml/min).
Due to the low oxidation temperatures, no obvious interaction between the sample
and the crucible was observed. Phase composition and morphology of the samples
before and after oxidation were studied by X-ray diffraction (XRD) with Cu K␣
radiation and scanning electron microscopy (SEM), respectively.
3. Results and discussion
3.1. Oxidation kinetics and mechanisms
◦
TG curves of ZrB2 powder oxidized in air at 650–800 C are
shown in Fig. 1, where w is the mass gain per unit surface area and
◦
◦
◦
◦
Fig. 3. Multiple-law modeling of TG curves of ZrB2 powder oxidized in air at different temperatures: (a) 650 C; (b) 700 C; (c) 750 C; and (d) 800 C.

5
04
W.-M. Guo et al. / Journal of Alloys and Compounds 471 (2009) 502–506
t is oxidation time. With increasing temperature, it is obvious that
the weight gain increases at identical time. It can be seen that the
◦
TG curve approaches linear behavior for 650 C, whereas TG curves
clearly show deviation from linear behavior with the increase of
temperature.
Fig. 2 presents the square of mass gain per unit surface area as
a function of oxidation time for ZrB2 powder. It is found that the
square of mass gain per unit surface area is a nonlinear function
of oxidation time, which indicates that oxidation of ZrB2 powder
deviates from parabolic kinetics. Therefore, a simple parabolic law
is unable to describe the kinetic behavior. In order to give a com-
plete analytical description, the TG curves were fitted according to
a multiple-law model including a linear and parabolic term, devel-
oped by Nickel [13]. The mass change per unit area w is expressed
as a function of time, according to the following equation:
√
w = A + Klint + Kpar
t
(3)
where A is a constant, Klin is linear rate constant and Kpar is
parabolic rate constant. The fit allows for the quantification of the
individual contributions of the multiple-law model and accounts
for para-linear kinetics. Using multiple linear-regression analysis,
multiple-law modeling of TG curves of ZrB2 powder oxidized in air
at different temperatures was performed, with the results illus-
◦
Fig. 4. X-ray diffraction patterns of ZrB2 powder oxidized at 700 C for varying
durations: (a) no oxidation; (b) 0.5 h; (c) 4 h; and (d) 15 h.
2
trated in Fig. 3. The K parameters and R values corresponding
monoclinic ZrO2 peaks also appear, while intensities of ZrB2 peaks
simultaneously decrease, as shown in Fig. 4c. When oxidation time
is increased to 15 h at 700 C, the XRD patterns show monoclinic
to each fitting curve are reported in Table 1. The excellent fit of
the solutions confirms that oxidation of ZrB2 powder follows para-
◦
2
linear kinetics in such a temperature range (R > 0.99).
ZrO2 as a major phase, a minor phase of tetragonal ZrO , and the
2
As seen in Fig. 3, the dominating term is the parabolic one in the
◦
◦
disappearance of the ZrB2 phase (Fig. 4d).
range of 650–800 C, accounting for oxygen diffusion in oxide scale.
Generally, the stable polymorph of zirconia at room tempera-
ture and atmospheric pressure is monoclinic, which transforms at
From 650 to 700 C, the linear term is positive, which is related to
mass gain due to chemical reaction between ZrB2 and O at the
2
◦
◦
◦
1170 C to tetragonal and then at 2370 C to cubic structure. Previous
researchers [15] have investigated the effect of the crystallite size
on the phase transformation and pointed out that the phase trans-
formation was closely related to the growth of the crystallite size.
When the particle size reaches nano-scale, a metastable tetragonal
ZrO2 can also exist at room temperature, which is so-called nano-
size effect of particle phase [16]. So, the reaction of ZrB2 powder
with oxygen can generate metastable tetragonal ZrO2 instead of
diboride–oxide interfaces. In the range of 750–800 C, the linear
term is negative, which is associated with mass loss due to vapor-
ization of B O . It can therefore be inferred that oxidation of ZrB
2
3
2
powder in air is governed by oxygen diffusion and chemical reaction
◦
at 650–700 C, but controlled by oxygen diffusion and the evapora-
◦
tion of B O at 750–800 C.
2
3
Even though the vapor pressure of B O may be low in the tem-
2
3
◦
perature range of 750–800 C, oxidation kinetics revealed that the
evaporation of B O should not be neglected during the oxidation of
◦
stable monoclinic phase at 700 C. Broad ZrO2 peaks also validate
2
3
this deduction in Fig. 4b. However, the tetragonal crystallites grow
during annealing. Upon reaching a critical diameter, the tetragonal
phase transforms immediately to the monoclinic phase. Therefore,
monoclinic ZrO peaks appear with oxidation. The increase of hold-
ing time will induce more phase transform of ZrO , and as a result,
ZrB powder. This deduction can be also supported by the study on
2
oxidation of AlN–SiC–ZrB composites reported by Brach et al. [14].
2
◦
In the range 700–900 C, Brach et al. [14] indicated the oxidation
2
of AlN–SiC–ZrB2 composites followed mixed para-linear kinetics,
where the linear term was negative, accounting for mass loss due
2
the oxidized samples after 15 h contains major monoclinic ZrO2
phase with minor tetragonal ZrO2 phase.
to vaporization of B O .
2
3
Morphology changes occurring to the ZrB2 powder as a result
of oxidation were examined using SEM. Fig. 5 is a montage of SEM
images illustrating the progression of oxidation at 700 C. The as-
3
.2. Phase characterization and morphology of the oxidized
samples
◦
received ZrB2 powder consists of irregularly sized particles with
mainly columnar shape, as shown in Fig. 5a. The first visible evi-
dence of appreciable oxidation of the samples is the appearance of
circumferential microcracks at the edge of the ends of a number of
cylinders, for example, on the arrow sites in Fig. 5b. At the begin-
ning of oxidation, the particles keep their original shape, while the
surfaces become rough. With further oxidation, microcracks at the
surface of the oxide layers running parallel to the central axes of the
cylinders are also observed (indicated by arrows in Fig. 5c). At the
same time, the oxide scale at the ends of certain columnar parti-
cles flake off. After 15 h, every ZrB2 particle has broken into several
fragments (Fig. 5d).
◦
Fig. 4 shows the XRD patterns of ZrB powder oxidized at 700 C
2
for varying durations. Before the exposure, only ZrB2 peaks are
detected as shown in Fig. 4a. In the initial stage of oxidation, tetrag-
onal ZrO2 peaks appear (Fig. 4b). With increasing oxidation time,
Table 1
Kinetic parameters of the oxidation fitting curve, relatively to parabolic and linear
contributions, and R2 (fit goodness) values.
Oxidation temperature
Kpar
(mg cm min
Klin
R2
◦
−2
−0.5
−2
−1
(
C)
)
(mg cm min )
6
50
0.00192
0.00536
0.02068
0.03405
0.00013
0.00007
0.99852
0.99947
0.99744
0.99886
7
7
8
00
50
00
It is assumed that oxidation products have theoretical density.
Therefore, the oxidation from ZrB2 to tetragonal ZrO2 is accompa-
nied by 9% volume expansion, based on the densities of 6.09 and
−0.00040
−0.00122

W.-M. Guo et al. / Journal of Alloys and Compounds 471 (2009) 502–506
505
◦
Fig. 5. SEM photographs of ZrB2 powder oxidized at 700 C for varying durations: (a) No oxidation; (b) 0.5 h, arrows indicated the microcracks of the ends of columnar
particles; (c) 4 h, arrows indicated microcracks of the oxide layers running parallel to the central axes of columnar particles; and (d) 15 h.
.10 g/cm3 for ZrB2 and tetragonal ZrO , respectively. In addition,
◦
6
at 700 C, and the tetragonal phase transforms to the monoclinic
2
the tetragonal-to-monoclinic phase transformation is accompanied
by 5% volume expansion [17]. It is thus understandable that oxi-
dation of ZrB2 powder is associated with the generation of surface
microcracks. Compared with a flat surface, greater stresses induced
by volume expansion exist in the edges of columnar ZrB2 parti-
cles. Therefore, microcracks firstly emerge at these edges. With
the progress of oxidation, the volume expansion which develops
in the oxide layer increases and eventually breaks the particles into
fragments.
phase with oxidation. The oxidation of ZrB2 powder is associ-
ated with surface microcrack formation, which can be attributed
to volume expansion resulting from oxidation of ZrB2 to tetrag-
onal ZrO2 and tetragonal-to-monoclinic phase transformation of
ZrO2. Finally, in the last stage of oxidation, each particle breaks into
fragments.
Acknowledgement
Financial support from the Chinese Academy of Sciences under
the Program for Recruiting Outstanding Overseas Chinese (Hun-
dred Talents Program), the National Natural Science Foundation of
China (No. 50632070 and No. 50602048) is gratefully appreciated.
4
. Conclusions
The oxidation of ZrB2 powder follows para-linear kinetics in air
◦
at 650–800 C, where the dominating term is the parabolic one,
accounting for oxygen diffusion in the oxide scale. From 650 to
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