J. Am. Ceram. Soc., 86 [5] 838–45 (2003)
journal
Suppression of Active Oxidation of Polycarbosilane-Derived
Silicon Carbide Fibers by Preoxidation at High Oxygen Pressure
Toshio Shimoo, Yoshiaki Morisada,† and Kiyohito Okamura*
Department of Metallurgy and Materials Science, Graduate School of Engineering, Osaka Prefecture University,
Sakai, Osaka 599-8531, Japan
Three types of polycarbosilane-derived SiC fibers (Nicalon,
Hi-Nicalon, and Hi-Nicalon S) with different SiO2 film thick-
nesses (b) were subjected to exposure tests at 1773 K in an
argon-oxygen gas mixture with an oxygen partial pressure of 1
Pa. The suppression effect of a SiO2 coating on active oxidation
was examined through TG, XRD analysis, SEM observation,
and tensile tests. All the as-received fibers were oxidized in the
active-oxidation regime. The mass gain and the SiO2 film
development showed a suppression of active oxidation at b
values of м0.070 m for Nicalon, м0.013 m for Hi-Nicalon,
and м0.010 m for Hi-Nicalon S fibers. Considerable strength
was retained in the SiO2-coated fibers. For Hi-Nicalon fibers,
the retained strength was 71%–90% of the strength in the
as-received state (2.14–2.69 GPa).
an SiO2 film suppressed the escape of decomposition gases (SiO
and CO), resulting in the retardation of thermal decomposition of
the fibers.13,15,19 In addition, the SiO2 film is an effective
protective coating against oxidizing environments. Therefore,
active oxidation, as well as further passive oxidation of the fibers
is thought to be retarded by the SiO2 film.
In the present work, prevention of active oxidation was inves-
tigated for three types of fibers: Nicalon, Hi-Nicalon, and Hi-
Nicalon S. The fibers were coated with SiO2 films of different
thicknesses and were exposed, at 1773 K, to an Ar-O2 gas mixture
with an oxygen partial pressure of 1 Pa. The thermal stability of the
fibers in the active oxidation region was related to the changes in
crystal phase, morphology, and strength that resulted from the
environmental exposure.
I. Introduction
II. Experimental Methods
OLYCARBOSILANE-DERIVED silicon carbide (SiC) fibers, such as
Nicalon, Hi-Nicalon, and Hi-Nicalon S (Nippon Carbon Co.,
The samples used in the present work included three types of
polycarbosilane-derived silicon carbide fibers: Nicalon, Hi-
Nicalon, and Hi-Nicalon S fibers (Nippon Carbon Co., Tokyo,
Japan). The compositions and properties of the fibers are shown in
Table I.20 The SiO2 coating was produced by oxidation at different
times, at 1773 K, in air. Thicknesses of thin SiO2 films could not
be determined by SEM observation. In addition, SEM observation
of thick films introduced relatively large errors, because of the
formation of spalling, cracks, and blisters. Therefore, the apparent
film thickness, b, was determined from the mass gain of the fibers
P
Tokyo, Japan) are of great importance as reinforcing materials for
ceramic-matrix composites. The composites are exposed to
oxygen-containing environments at high temperature in fabrication
and service. The oxygen partial pressure plays an important part in
the oxidation of the fibers; passive oxidation is characterized by
the mass gain and the formation of SiO2 film at high oxygen partial
pressures and active oxidation by mass loss and vaporization of
SiO at low oxygen partial pressures. Numerous reports have been
made about passive oxidation of silicon carbide fibers.1–15 How-
ever, little has been reported on the active oxidation of the fibers.16
Polycarbosilane-derived silicon carbide fibers are composed of
SiC crystallites, free carbon, and a silicon oxycarbide (SiCXOY)
phase. High-temperature exposure causes thermal decomposition
of the SiCXOY phase that is accompanied by crystallization into
-SiC and generation of SiO and CO gases. Thermal decomposi-
tion of the SiCXOY phase produces a significant growth of SiC
grains and a very large decrease in fiber strength. The present
authors investigated the oxidation behavior of silicon carbide
1/ 2
b ϭ r0͓1 Ϫ ͑1 Ϫ X͒
͔
(1)
where r0 and X are the initial radius of the fibers and the oxidized
fraction, respectively. The value X is estimated from the stoichi-
ometry of oxidation reactions
for Nicalon:
SiC1.20O0.41͑s͒ ϩ 1.395O2͑g͒ ϭ SiO2͑s͒ ϩ 1.20CO͑g͒
X ϭ 4.435⌬W/W0
(2)
(3)
fibers at 1773 K in Ar-O2 gas mixtures with a wide range of
17,18
oxygen partial pressures, pO
.
Active-to-passive oxidation
2
transition at 1773 K occurred at pO ϭ 102 Pa for Nicalon, pO
ϭ
for Hi-Nicalon:
SiC1.387O0.014͑s͒ ϩ 1.687O2͑g͒ ϭ SiO2͑s͒ ϩ 1.387CO͑g͒ (4)
2
2
10 Pa for Hi-Nicalon, and pO ϭ 1 Pa for Hi-Nicalon S, causing a
2
significant increase in porosity and a serious degradation in
strength of the fibers. In the extreme case, the fibers disappeared as
a result of active oxidation. Prevention of active oxidation is of
great importance for technical requirements. It has been shown that
X ϭ 3.167⌬W/W0
for Hi-Nicalon S:
SiC1.046O0.007͑s͒ ϩ 1.520O2͑g͒ ϭ SiO2͑s͒ ϩ 1.046CO͑g͒ (6)
(5)
X ϭ 2.103⌬W/W0
(7)
D. Butt—contributing editor
where ⌬W is the mass gain of the oxidized fibers and W0 is the
initial mass of the fibers. Because slight thermal decomposition of
the SiCXOY phase occurs at the earliest stage of oxidation,
unavoidable errors are contained in the calculated values.11–15 The
apparent SiO2 film thicknesses of the as-oxidized fibers subjected
to exposure tests are shown in Table II.
Manuscript No. 187428. Received October 1, 2001; approved December 2, 2002.
Supported in part by a grant from the Japanese Ministry of Education, Science,
Sports and Culture, under Grant No. 11450255.
*Member, American Ceramic Society.
†Graduate Student at Osaka Prefecture University.
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