P1: PSG/BBS P2: SDI/MIG P3: RJU+SNH/ATR QC: SNH 12-5086-98 January 2, 1999 11:28
JOURNAL OF MATERIALS SCIENCE 34 (1999)515–533
Processing, physical and thermal properties of
Blackglas
TM
matrix composites reinforced with
Nextel
TM
fabric
SRIRAM RANGARAJAN
∗
, RONALD BELARDINELLI, PRANESH B. ASWATH
†
Materials Science and Engineering Program, University of Texas @ Arlington, P.O. Box 19031,
Arlington, TX 76019
The cure and pyrolysis behavior of a Blackglas
TM
resin and Nextel
TM
440 impregnated with
Blackglas
TM
resin were studied. Cure of the Blackglas
TM
resin is an exothermic process and
DSC studies indicate that with an increase of catalyst content from 0.1 to 1.0%, the onset
and peak temperature of cure are decreased coupled with an increase in the enthalpy of
cure indicating a greater extent of cross linking. However, pyrolysis char yield of the
pyrolyzate is relatively insensitive to cure conditions.
Cure pressure and pyrolysis environment are variables in the processing of Blackglas
TM
matrix composite reinforced with Nextel
TM
440 plain weave fabric. Variations in cure
pressure from 30 to 80 psi had no discernible effect on the chemistry of the pyrolyzate.
However, the higher cure pressure resulted in top and bottom ply damage. Pyrolysis in an
Ar environment resulted in incorporation of up to 12 wt%Cofwhich 8 wt % was graphitic
in nature in the ceramic matrix. Pyrolysis in NH
3
resulted in 3.9 wt % nitrogen and 1.5 wt %
carbon in the matrix, with all the nitrogen and carbon bonded to Si. The cured panels have
to be pyrolyzed/densified between 6–7 times to achieve required density and porosity
content. Oxidation behavior of the composites at 1000
◦
C indicate that the argon pyrolyzed
CMC’s lose more weight due to decomposition of the pyrolytic carbon, whereas, NH
3
pyrolyzed CMC’s are stable as both the N and C are bonded to Si in the matrix. Dielectric
constants K’ and K” measured at 1 GHz in the as-processed condition are high in the argon
pyrolyzed CMC, (K’= 11–28) due to the presence of pyrolytic carbon. On the other hand NH
3
pyrolyzed CMC exhibit low dielectric constant (K’= 4). On oxidation, the dielectric constant
in both the Ar and NH
3
pyrolyzed panels is approximately 4.0.
C
1999
Kluwer Academic
Publishers
1. Introduction
The polymer pyrolysis route to processing of ceramics
and ceramic matrix composites (CMCs) is being ac-
tively developed. Many different preceramic polymers
have been discovered since their original development
byYajimaetal. [1,2].Thesepolymers onthermaltreat-
ment transform into a variety of ceramics, such as, SiC
from polysilanes, Si
3
N
4
from polysilazanes, Si–O–C
silicon oxycarbides from polysiloxanes, silicon oxyni-
tride, silicon carbonitride [3–7]. The interest in these
materials has been spurred by their potential for low
cost, ease and lower temperature of processing.
Recent work with polysiloxanes has attracted at-
tention because they can be transformed into rela-
tively high yielding (75–80% ceramic yield) Si–O–C
ceramics when pyrolyzed in the 800–1000
◦
C range
[6–9]. Pyrolysis beyond 1100
◦
C results in a carbother-
mal reduction into SiC, SiO
2
, SiO(g) and CO(g). The
∗
Present address: Post Graduate Research Associate, Center for Ceramic Research, Rutgers University, Piscataway, NJ.
†
Author to whom all correspondence should be addressed.
simultaneous incorporation of carbon and oxygen in
the matrix results in a metastable glassy phase with
an increased resistance to devitrification [10], which is
useful as a matrix in CMC’s [11] in the intermediate
temperature range (800–1100
◦
C).
In general, preceramic polymers contain a backbone
chain of silicon or silicon and O/N with alkyl (e.g.
–CH
3
,–C
2
H
5
etc.) or aryl (e.g. –C
6
H
5
) substituents
attached to the silicon. In siloxane based precursors,
the backbone is made up of Si–O units. These poly-
mers are typically converted to ceramics in two stages.
In stage one, the polymer is cured and crosslinked
into a solid. In stage 2, the cured solid is pyrolyzed
in an inert/reactive environment, where it is trans-
formed into a ceramic with the simultaneous emission
of gases such as hydrogen and hydrocarbons. This is
schematically illustrated in Fig. 1a. Typically, silicon
oxycarbide glasses (SiO
x
C
y
), have silicon in multiple
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1999 Kluwer Academic Publishers
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