J. Am. Ceram. Soc., 94 [9] 2796–2799 (2011)
DOI: 10.1111/j.1551-2916.2011.04726.x
©
2011 The American Ceramic Society
ournal
J
Oxidation of ZrB Ceramics Containing SiC as Particles,
2
Whiskers, or Short Fibers
†
Laura Silvestroni and Diletta Sciti
CNR-ISTEC, Institute of Science and Technology for Ceramics, Via Granarolo 64, I-48018, Faenza, Italy
The thermal stability of ZrB
chopped fibers was tested in a bottom-up furnace at 1200°C,
500°C, and 1700°C for 30 min. The oxidation behavior was
2
-based composites containing SiC-
In this prospect, the oxidation behavior of SiC short fiber-
reinforced ZrB ceramics was tested in a bottom-up furnace
up to 1700°C for 30 min and was compared to a typical
2
1
studied by X-ray diffraction, scanning electron microscopy, and
weight gain. The degradation induced by oxidation was also
evaluated considering the strength decrease of oxidized bars.
ZrB material containing either SiC particles or SiC whisker.
2
To assess the materials degradation, the pristine strength was
compared to the strength of the oxidized specimens.
Is there a difference among the oxidation behavior of a
Baseline ZrB -composites containing the same amount of SiC
2
particles or SiC whiskers were tested in the same conditions for
comparison.
ZrB matrix containing SiC particles, fibers or whiskers? Can
2
we obtain a tougher material than the conventional ZrB –
2
SiC without losing the oxidation resistance properties at high
temperature? These issues will be the target of this study.
I. Introduction
IRCONIUM diboride (ZrB
2
) belongs to the class of materi-
als called ultra-high temperature ceramics, attributable
II. Experimental Procedure
Z
to its high melting point above 3000°C. It possesses interest-
ing engineering properties, such as high hardness, strength,
thermal and electrical conductivity, and finds application in
The materials investigated in the present work were prepared
by hot-pressing ZrB with 5 vol% of Si N and 20 vol% of
2 3 4
SiC-chopped fibers (20f), or 20 vol% of SiC particles (20p),
or 10–20 vol% of whiskers (10w and 20w, respectively).
Commercial powders were used to prepare the ceramic
1
refractory and energy production industries. Increasing
interest raised by ZrB is related to its potential use for high
2
temperature aerospace applications, such as hypersonic vehi-
cles or rocket propulsion systems. The main obstacle to the
use of ZrB -based ceramics in a wider span of applications is
2
composites: ZrB Grade B (H.C. Starck, Goslar, Germany),
specific surface area 1.0 m /g, maximum impurity content:
0.25 wt% C, 2 wt% O, 0.25 wt% N, 0.1 wt% Fe, 0.2 wt%
2
2
the discrete oxidation resistance, associated to low thermal
shock resistance and low fracture toughness.
3 4
Hf, particle size range 0.1–8 mm; a-Si N Baysinid (Bayer,
2
Leverkusen, Germany), specific surface area 12.2 m /g,
Concerning the first issue, the addition of SiC has been
proved to improve the oxidation resistance of ZrB2 and
HfB , owing to the formation of a protective SiO -glassy
maximum impurity content: 1.5 wt% O; b-SiC BF12 (H.C.
Starck), 3% a-SiC, s.s.a. 11.6 m /g, silica ∼1.65 wt%; SiC HI
2
2
2
Nicalon-chopped fibers, Si:C:O = 62:37:0.5, 15 lm diameter,
1 mm length; for SiC whiskers, no data sheet was available,
by image analysis: average diameter 0.6 lm and average
length 30 lm. Details on the materials preparation, densifica-
tion, and mechanical properties have been reported in a
2
layer which hinders the oxygen penetration in the bulk. The
oxidation mechanisms and the microstructural features of
2
–7
ZrB –SiC composites have been extensively studied.
The
2
morphology of the oxidized scale is constituted by an outer-
most borosilicate-glassy layer, an underlying coarse or
columnar ZrO layer with SiO filling the voids, and a ZrB
8
previous work.
The oxidation tests were carried out in a bottom-up
loading furnace at 1200°C, 1500°C, and 1700°C for 30 min
on rectangular 13 mm9 2.5 mm9 2 mm bars in static air.
Specimens were located in the furnace when the maximum
temperature was achieved and then removed and air-
quenched after the exposure time. Mass and bars’ dimensions
were measured before and after the oxidation. The as-
sintered material and oxidized specimens were examined
on the surface using X-ray diffraction (Siemens D500,
Karlsruhe, Germany) to identify the crystalline phases.
Microstructural modifications induced by oxidation were
analyzed by scanning electron microscopy (SEM; Cambridge
S360, Cambridge, U.K.) and energy dispersive spectroscopy
(EDS; INCA Energy 300, Oxford Instruments, High
Wycombe, U.K.) on the surface and on the polished cross-
section of the specimens. For comparison, composites
containing 20 vol% of SiC particles, or 10–20 vol% SiC
whiskers were also oxidized under the same conditions. For
all the composites, four-point flexural strength tests were
conducted on as-sintered materials at room temperature and
at 1200°C on a universal screw-type machine, Instron 1195
(Instron 6025, High Wycombe, U.K.) with a crosshead speed
2
2
2
SiC-depleted region above the bulk. On the other hand, the
addition of SiC fibers or whiskers proved to be effective in
1
/2
increasing the fracture toughness from 3.8 to 5.7 MPa·m
which is an increase of more than 50%.
,
8
9
A previous study on HfB
2
–SiC revealed that the oxida-
tion mechanisms occurring at ambient pressure at 2000°C for
h developed microstructural features very similar to those
formed upon an arc jet test at lower temperature for shorter
1
9
time. Arc jet tests are commonly used to study the material
response in simulated re-entry conditions and the study
pointed out that the oxidation in static air for 30–60 min
could give indications on the microstructure evolution of a
material during atmospheric re-entry.
M. Cinibulk—contributing editor
Manuscript No. 29654. Received April 27, 2011; approved June 08, 2011.
Author to whom correspondence should be addressed. e-mail: laura.silvestroni@
†
istec.cnr.it
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