High-temperature hardness of ReB2 single crystals

Article abstract of DOI:10.1016/j.jallcom.2008.10.094

Vickers micro-hardness was measured on the (0 0 0 1) and (1 0 - 1 0) planes of ReB₂ single crystals. By increasing the temperature from 20 to 1000 °C, the hardness decreased from 30.8 to 19.8 GPa and from 35.8 to 14.3 GPa, respectively. ReB

Full text of DOI:10.1016/j.jallcom.2008.10.094

Journal of Alloys and Compounds 477 (2009) L28–L29
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Letter to the Editor
High-temperature hardness of ReB single crystals
2
S. Otani , M.M. Korsukova , T. Aizawaa
a,∗
b
a
National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
A. F. Ioffe Physico-Technical Institute, Russian Academy of Science, 26 Politekhnicheskaya, St. Petersburg 194021, Russian Federation
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 September 2008
Available online 10 December 2008
Vickers micro-hardness was measured on the (0 0 0 1) and (1 0 - 1 0) planes of ReB single crystals. By
increasing the temperature from 20 to 1000 C, the hardness decreased from 30.8 to 19.8 GPa and from
2
◦
35.8 to 14.3 GPa, respectively. ReB2 was found to have the highest hardness among refractory borides, but
exhibited deliquescence.
Keywords:
Hardness
©
2008 Elsevier B.V. All rights reserved.
ReB2
Single crystals
Deliquescence
1
. Introduction
ambient argon gas by the floating zone method [7]. The crystal orientation was
examined by X-ray using the Laue back-reflection method. The crystals were cut
into rectangular blocks, 5 mm × 5 mm × 10 mm, so that the large planes would be
the (0 0 0 1) and (1 0 - 1 0)planes of the hexagonal lattice. The planes were mirror-like
polished using 2 ␮m diamond paste, after coarsely grinding the crystal planes with
B4C abrasive.
Hardness measurement is a simple method of examining
mechanical properties. We have used hardness, especially high-
temperature hardness, to estimate the quality of refractory crystals
prepared by the floating zone method [1–3]. ReB , one of refractory
◦
Hardness measurement was carried out from 20 C, room temperature, to
2
◦
−3
1
000 C in a vacuum <10 Pa, using a micro-hardness tester (QM-12, Nikon Ltd.)
and a diamond Vickers indenter. The load was 0.98 N (100 g) and 1.96 N (200 g) for
0 s. Hardness was obtained from the average value of more than ten measurements
because of experimental errors due to the small indentation.
borides, was recently reported to have high hardness, 48 GPa under
a load of 0.49 N, and to leave scratch marks on a diamond surface, as
well as to have possible applications in cutting because it was syn-
thesized under ambient pressure without using a high-pressure
technique [4]. However, Dubrovinskaia et al. [5] commented that
the hardness should be 30 GPa in a range of no load dependence
and that it was not high enough to scratch a diamond surface. In
response to the comment, AFM (Atomic Force Microscopy) data
were presented to show that ReB2 scratched diamond, making a
groove 2.4 ␮m wide and 0.23 ␮m deep [6]. It is not yet clear how the
1
3. Results and discussion
◦
Rhenium diboride, ReB , which congruently melts at 2400 C
2
[
8], was easily synthesized as a single phase by controlling the
mixing ratio of Re and B, as speculated from the ease of syn-
thesis of borides, such as LaB6 and YB , which congruently melt.
4
diamond was scratched. The ReB samples were polycrystalline and
2
Single crystals 1 cm in diameter were prepared from feed rods
with a little excess boron, 67–68 at% of B, using the floating zone
method [7].
the orientation was not clearly identified. In this report, using sin-
gle crystals, the hardness was examined and compared with other
refractory crystals.
The hardness of the (0 0 0 1) plane of ReB crystal was 29.8(± 2.4)
2
and 30.8(± 1.5) GPa under a load of 0.98 and 1.96 N, respectively,
2.
Experimental procedure
at room temperature. The values are within experimental error,
ReB2 was synthesized from rhenium and amorphous boron powders by means
±
6%. Accordingly, the load was determined to be 1.96 N. The mea-
of self-propagation high-temperature reaction (SHS reaction) [7]. The product was
ground into powder using a silicon nitride mortar and pestle. The powder was
mixed with a small amount of polyvinylbutyral ethanol solution as binder, and
then isostatically pressed in a rubber bag at 200 MPa. The pressed rod was sin-
sured hardness on the (0 0 0 1) and (1 0 - 1 0) planes was 30.8(± 1.5)
and 35.8(± 1.0) GPa at room temperature, respectively, as shown
in Fig. 1. The reported hardness at 48 GPa under a load of 0.49 N
decreased to 30–34 GPa under a load higher than 1.96 N [4], which is
◦
tered in vacuum at 1800 C. Single crystals of ReB2 were prepared in 0.5 MPa of
consistent with our data reported above. Therefore, ReB was found
2
to have high hardness among the borides, as shown in Table 1, but
∗ Corresponding author. Tel.: +81 29 860 4320; fax: +81 29 860 4680.
E-mail address: OTANI.shigeki@nims.go.jp (S. Otani).
not as high as superhard materials such as c-BN at ∼72 GPa and B O
6
at ∼59 GPa.
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.10.094

S. Otani et al. / Journal of Alloys and Compounds 477 (2009) L28–L29
L29
values, corresponding well with the above hardness data. The finish
of the mirror-like planes was achieved by polishing with a cloth
using 2 ␮m diamond paste.
The mirror-like (0 0 0 1) and (1 0 - 1 0) planes of the ReB2 crys-
tals were easily scratched with the tip of the diamond indenter
by hand, which was easily determined since the hardness was
measured with a diamond indenter. However, the diamond plane,
one of four side-planes of the tip of the diamond indenter, could
not be scratched by the edge of the (0 0 0 1) and (1 0 - 1 0) planes
of the ReB2 crystal. The ReB2 crystal merely slipped on the dia-
mond surface while being pushed. Although ReB2 was reported
to scratch the diamond plane [4], this was not reproduced in our
experiments.
Fig. 1 also shows the dependence of hardness on the tem-
perature. Relatively high hardness was maintained during the
process of raising the temperature: 19.8(± 1.4) and 14.3(± 0.6) GPa
◦
on the (0 0 0 1) and (1 0 - 1 0) planes, respectively, at 1000 C.
High-quality refractory crystals without subgrain boundaries are
generally grown by the floating zone method, in cases where the
hardness is higher than 5 GPa at 40% of the melting point [2],
◦
∼
800 C in the case of ReB . This empirical rule was found to apply
2
Fig. 1. Vickers micro-hardness of ReB2 crystal.
to the crystal growth of ReB , too [7]. Table 1 shows a comparison of
hardness with other refractory materials at 1000 C [2,9]. ReB was
2
◦
2
Table 1
◦
found to have the highest hardness even at 1000 C, although the
Vickers micro-hardness of some refractory borides and carbides as measured on
single crystals [2,9].
◦
2
400 C melting point of ReB is not high among refractory borides.
2
In summary, ReB was found to be one of the hardest borides, but
2
◦
m.p. ( C)
Hardness (GPa)
not as hard as superhard materials such as c-BN and B O. A severe
6
◦
◦
C
20
C
1000
problem exhibited by ReB is deliquescence, that is, it absorbs water
2
(
0 0 0 1)
(1 0 - 1 0)
(0 0 0 1)
(1 0 - 1 0)
in the air and becomes covered with a viscous solution within a few
months. In addition, rhenium is an expensive element. Therefore,
it is not easy to use ReB2 as a material for cutting tools.
ReB2
WB2
TaB2
HfB2
ZrB2
YB4
(2400)
(2365)
(3037)
(3380)
(3220)
(2800)
(2715)
(3067)
30.8
20.4
29.2
17.8
22.2
13.2
35.8
19.4
22.2
24.5
19.5
19.8
9.3
14.3
13.3
11.9
8.8
14.2
16.4
7.8
References
5.5
11.9
+
++
+
++
17.1
9.6
[
[
[
1] S. Otani, T. Tanaka, Y. Ishizawa, J. Alloys Compd. 202 (1993) L25–L28.
2] S. Otani, T. Ohsawa, J. Crystal Growth 200 (1999) 472–475.
3] S. Otani, H. Nakagawa, Y. Nishi, N. Kieda, J. Solid State Chem. 154 (2000)
238–241.
LaB6
TiC
18.9*
25.7*
–
–
8.4*
2.5*
+, ++
*
: On the (0 0 1) and (1 0 0) planes of the tetragonal lattice, respectively.
: On the (1 0 0) plane of the cubic lattice.
[
4] H.-Y. Chung, M.B. Weinberger, J.B. Levine, A. Kavner, J.-M. Yang, S.H. Tolbert, R.B.
Kaner, Science 316 (2007) 436–439.
[
[
5] N. Dubrovinskaia, L. Dubrovinsky, V.L. Solozhenko, Science 318 (2007) 1550c.
6] H.-Y. Chung, M.B. Weinberger, J.B. Levine, R.W. Cumberland, A. Kavner, J.-M. Yang,
S.H. Tolbert, R.B. Kaner, Science 318 (2007) 1550d.
In the process of finishing mirror-like planes for the hardness
measurement, the (0 0 0 1) plane was not easily ground by the green
[
7] S. Otani, T. Aizawa, Y. Ishizawa, J. Alloys Compd. 252 (1997) L19–L21.
silicon carbide, ∼28 GPa, but was easily ground by the B C abrasive,
[8] T.B. Massalski (Ed.), Binary Alloy Phase Diagram, second edition, ASM, OH, 1990.
9] S. Otani, M.M. Korsukova, T. Mitsuhashi, J. Crystal Growth 194 (1998) 430–433.
4
[
∼
33 GPa. Accordingly, the hardness of ReB was between these two
2