Hi
-
Nicalon
TM
-
S SiC fiber
strengths
and Weibull moduli
were measured after oxidation
for up to 100 hours
between 700° and 1
400°C
in wet
and dry
air
.
SiO
2
s
cale
thickness
and crystallization extent were
measured by
T
EM.
Th
e effect of furnace e
nvironment
and bake
-
out on trace element levels in SiO
2
scales was characterized
by secondary ion mass spectroscopy (
SIMS
).
C
rystallization kinetics
and Deal
-
Grove
oxidation kinetics for
glass
and crystalline scale
, and the transition between them,
were determined
and modeled
.
Crystallization retards
oxidation kinetics, and scale that forms in the crystalline state is heavily deformed
by the growth stress
accompanying SiC oxidation volume expansion
.
Glass scales formed in dry air sli
ghtly increase
fi
ber strength.
Glass sc
ales formed in wet air do
not increase strength
,
and in some cases
significantly
decrease strength
. Scales
more than 200 nm thick were usually
partially
or completely
crystallized
,
and
these
f
ibers
had
low
strength
s
.
Contamination of s
cales by trace impurities such as
A
l
and Ca during heat
-
treatment significantly inhibited
crystallization.
The oxidation
kinetics
and the strengths of oxidized
Hi
-
Nicalon
TM
-
S fibers are compared with
results from previous stu
dies.
Empirical r
elationships between oxidation
temperature, time, scale thickness,
and
strength are
determined and
discussed.