Polymeric precursors to Si-C-O, SI-B-N and Si-C were evaluated for use as ceramic interfaces in ceramic matrix composites. Use of the preceramic polymers allows for easy dip coating of fibers from dilute solutions of a polymer, which are then pyrolyzed to obtain the ceramic. SCS-0 fibers (Textron Specialty Materials, Lowell, MA) were coated with polymers from three systems: polysilsesquioxanes, polyborosilazanes and polycarbosilanes. The polysilsesquioxane systems were shown to produce either silicon oxycarbide or silicon oxynitride, depending on the pyrolysis conditions, and demonstrated some promise in an RBSN (reaction-bonded silicon nitride) matrix model system. Polyborosilazanes were shown, in studies of bulk polymers, to give rise to oxidation resistant Si-B-N ceramics which remain amorphous to temperatures of 1600 C, and should therefore provide a low modulus interface. Polycarbosilanes produce amorphous carbon-rich Si-C materials which have demonstrated oxidation resistance

Baranwal, R.

Dickerson, R.

Eldridge, J.

Heimann, P. J.

Hurwitz, F. I.

Wheeler, D.

English

NASA Technical Reports Server

NASA-TM-llI755 /
PRECERAMIC POLYMERS FOR USE AS FIBER COATINGS
P. J. Heimann, Cleveland State University, Cleveland, OH; F. I. Hurwitz, D.
Wheeler and J. Eldridge, NASA Lewis Research Center, Cleveland, OH; R.
Baranwal, University of Michigan, Ann Arbor, MI; R. Dickerson, Case Western
Reserve University, Cleveland, OH.
ABSTRACT
Polymeric precursors to Si-C-O, Si-B-N and Si-C were evaluated for use
as ceramic interfaces in ceramic matrix composites. Use of the preceramic
polymers allows for easy dip coating of fibers from dilute solutions of a polymer,
which are then pyrolyzed to obtain the ceramic. SCS-0 fibers (Textron Specialty
Materials, Lowell, MA.) were coated with polymers from three systems:
polysilsesquioxanes, polyborosilazanes and polycarbosilanes. The
polysilsesquioxane systems were shown to produce either silicon oxycarbide or
silicon oxynitride, depending on the pyrolysis conditions, and demonstrated some
promise in an RBSN matrix model system. Polyborosilazanes were shown, in
studies of bulk polymers, to give rise to oxidation resistant Si-B-N ceramics
which remain amorphous to temperatures of 1600°C, and should therefore provide
a low modulus interface. Polycarbosilanes produce amorphous carbon-rich Si-C
materials which have demonstrated oxidation resistance.
INTRODUCTION
Using polymeric precursors for interface coatings offers a process with
potential for reduced cost compared with conventional chemical vapor deposition
techniques. The coating could be applied to the fiber by dip coating from a
polymer solution, followed by pyrolysis to convert the coating to a ceramic. This
could be done in an on-line process. In the polymeric state, the coating might
serve as a sizing to protect the fiber during further handling and/or weaving,
taking the place of a sizing that would require removal. This process could be
https://ntrs.nasa.gov/search.jsp?R=19970003316 2020-06-16T03:38:39+00:00Z
appliedto coatingtowsor, alternatively,mightbeusedto coatcloth. Thechoice
of apolymericprecursor,rheologyandconcentrationof polymersolutionswould
beusedto controlcoatingthickness.
Thepresentwork focuseson surveyingseveralpotentialprecursorsystems
to establishinitial parametersfor coatings,usinglargediameter(143p.m)SCS-0
fibers. Theuseof largediameterfiberspermitshanddippingof thefibers into the
polymersolutions.
Thepolysilsesquioxanesystemhasseveralspecificadvantages.First, it is
easyto synthesizethepolymer. It involvesasimplelabbenchprocedurethat can
beeasilydonewithoutanyspecialequipment.Thepolymeris handledin air,
making it readilyadaptableto coatingfiberson-linewith few alterationsto the
process.Theability to changetheratiosof monomers,which in turn changesthe
carboncontentof theresultingpolymer,is alsobeneficial. Thisallowstailoring
interfacepropertiesto aspecificfiber/matrix system.
PolyborosilazanederivedSi-B-N ceramicsoffer theadvantageof being
oxidatively stable.This is necessaryfor thecoatingsto survivetheanticipated
useconditions.Theceramicremainsamorphousto veryhigh temperatures.
Crystallizationof thecoatingwould resultin ashrinkage,whichwouldmost
likely leadto cracking,exposingthe fiber to oxygen.
Polycarbosilanepyrolysedto carbon-richsiliconcarbide. Crosslinking
agents,suchas1,2-disilylethane,polybutadiene,trivinylmethylsilaneand
ammonia,canbeeasilyincorporatedinto thepolyvinylsilane. Thisallows for
tailoring thepolymerrheologyandcontrollingcuring.
EXPERIMENTAL
Polysilsesquioxanesweresynthesizedby copolymerizinga phenyl-and
methyl-trimethoxysilaneusingamethoddescribedpreviouslyI. Theratioof the
phenyl(P)to methyl(M) presentin thepolymerdirectlyeffectedthecarbon
contentof thefinal ceramicand alsoinfluencedtherheology. A greater
percentageof phenylin thepolymerloweredthepolymerviscosityandresultedin
increasedcarbonin theceramic. This studyexaminespolymerswith phenylto
methyl ratiosof 5/95,10/90,30/70and50/50to determinetheeffectsof
copolymercompositiononthecoatingproperties.After thepolymerswere
synthesized,theyweredilutedin methanol to concentrationsrangingbetween5-
15percent,andSCS-0fibersweredippedinto this solution. Thesecoatingswere
thenpyrolyzedin bothargonandammoniaatmospheresat 1200°Cfor 30
minutes. Someof thefiberswerethenincorporatedinto areactionbondedsilicon
nitride (RBSN)composite.Thefiberswerealsocharacterizedby scanning
electronmicroscopy(SEM),Auger,andpushout tests.
Two polyborosilazanes, synthesized by SRI International, were examined
for use as coatings. The first polymer examined was synthesized by reacting
borane with cyclomethylsilazane (CMS) in three different ratios of boron to
silicon (1:8, 1:4 and 1:2). The polymerization of dichloromethylsilane and
borontrichloride, at a boron to silicon ratio of 1:4, produced the second polymer
examined. The fibers were coated from a 20 percent tetrahydrofuran solution.
Coated fibers were characterized by SEM and Auger. Bulk powders were also
prepared and characterized by thermogravimetric analysis (TGA), transmission
electron microscopy (TEM) and x-ray diffraction (XRD).
Polyvinylsilane polymerization and pyrolysis is discussed in detail
elsewhere 2'3'4. Four different crosslinking agents have been examined for the
vinylsilane polymer. They are 1,2-disilylethane (BSE), polybutadiene (PBD),
trivinylmethylsilane (TVM) and ammonia ('NH3). The crosslinking agents help
to tailor properties of the polymer and influence the observed viscosity of the
material. The various compositions of the crosslinking agent would also be
expected to somewhat alter the ceramic composition. Fibers were coated from a
10 percent cyclohexene solution.
RESULTS AND DISCUSSION
All of the polymers examined showed good fiber coverage by SEM. The
coatings were smooth and virtually free of defects in all cases. Auger done on the
fibers indicated that the coating thicknesses were usually in the range of 40 - 200
am.
Polysilsesquioxanes
The polysilsesquioxane system has been characterized in both the polymer
and ceramic state in more detail previously 1'5'6. The fiber coatings were examined
to determine the effect of the carbon content and pyrolysis atmosphere on the final
ceramic composition. Auger depth profiles showed that both carbon content and
pyrolysis conditions had an influence on final polymer composition. After
pyrolysis in argon the 50P/50M composition showed a large amount of carbon
present in the coating ( Figure 1). In comparison, the 5P/95M showed oxygen to
be the predominant species. When the same compositions are examined after
being pyrolyzed in ammonia, the 50P/50M shows very little incorporation of
nitrogen into the system, with the coating being primarily SiO2. The 5P/95M on
the other hand shows significant incorporation of nitrogen into the system, with
significantly less oxygen present. Surface chemistry performed on the two
coatingswhichhadbeenpyrolyzedin ammoniaalsosupportedthefinding that
therewasaproportionallylargeramountof nitrogenpresentin the5P/95M
derivedcoating. This difference in the amount of nitrogen incorporation can be
explained by the way in which carbon is present in the two systems. The carbon
present in the 5P/95M has been shown to be predominately incorporated into the
network. This is in contrast to the 50P/50M polymer, in which a higher
percentage of the carbon is in the form of free carbon. It was determined based on
this information, that both silicon oxycarbides and silicon oxynitrides can be
formed from the polysilsesquioxane polymer, depending on the pyrolysis
environment.
o
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O
100
8o
6O
40
20
0
lO0
0
0
80
so
0
40
20
Argon
C
20 40 60 80 100 120 140 160 180
Depth (nMeters)
Ammonia
100
8o
60
40
20:
0
100
I,o
Argon
0
. c_ /..... '__ ....-__
20 40 60 80 100 120 140 160 180
Depth (nMeters)
Ammonia
............ 40
.... sJ
10 20 30 40 50 0 50 1O0 150 200 250 300
Depth (nMeters) Depth (nMeters)
(A) (B)
Figure 1. Auger depth profile of (A) 50P/50M and (B) 5P/95M after
pyrolysis in indicated atmosphere.
Fiber push out tests on fibers pyrolyzed in argon, then incorporated into a
RBSN matrix, show that the coated fibers provide a weaker interface than the
uncoated SCS-0 fibers, although it is still a stronger interface than desired. The
coatings that were tested are felt to be too thin. It is expected that a weaker
interface will result as the coating thicknesses are increased. Auger analysis of
the coating that remained on the pushed out fiber showed evidence of nitrogen
incorporation and no carbon. It was determined that the nitrogen must have been
incorporatedduringthenitridingof the RBSN composite. For this reason, it is
felt that the RBSN matrix is not an ideal model system to study these coatings.
Future work will focus on studying the coatings in mierocomposites fabricated by
chemical vapor deposition (CVD) of a SiC matrix.
Polyborosilazane
The bulk ceramics formed by the two polyborosilazanes were examined to
determine their potential effectiveness as a ceramic interface. Both polymers
were found to yield Si-B-N ceramics. These ceramics were oxidatively stable
after an initial pyrolysis in Nil 3 to 900°C, followed by further heating in argon to
temperatures of 1200°C or greater, with the weight loss at 1400°C in air being
less than 1 percent (Figure 2). The ceramic that was only heated to 900°C, was
phase separated, so both the gold and black phases were tested for oxidative
stability.
w_a_
wt% --_._._._.
0.o
°C wt%
1600 99.74
I I
250.0 500.0
900, Black
96.57
""_._ 900, Gold
_'_._. _.48
I I I J
750.0 1000.0 1250.0 1500.0
Figure 2. TGA profile of MeHSiC12/BCI3 in air, after pyrolysis in argon
to the indicated temperatures.
The effect of boron on the ceramic was also examined.
Polycyclomethylsilazane (polymerization of CMS without boron) that had been
heated to 1600°C was crystallized, primarily ot-SiaN4 with some I3. After boron
incorporation, at a ratio of I boron for every 8 silicon atoms present, TEM results
indicated that there was still areas that were crystalline 13-Si3N4 with some ct.
There were also turbostratic areas in which the boron had inhibited
crystallization. At a boron to silicon ratio of 1:4, the ceramic powder is shown by
X-ray diffractionto beamorphousfor bothof thepolymersystemsstudied.The
two polyborosilazanesexaminedresultin aceramicthatoffersthepotentialto
provideastable,amorphousfiber coating.
Polycarbosilane
Thepolyvinylsilanepolymerandits resultingceramichavebeen
characterizedin detailpreviously2'3'4.Thepolymerwasshownto form a
polycarbosilanebackbonewhichresultedin anamorphous,carbonrich Si-C upon
pyrolysisin argon.Thematerial,originallypyrolyzedin argonto 1400°Cfor 30
minutes,showsnoweightchangeonreheatingin air to the sametemperaturefor
30minutes.(Figure3). This indicates the potential to provide a stable fiber
coating. Work on this polymer system has also determined that smooth
continuous coatings could be formed with the polymers when any of four
crosslinking agents were added to the polyvinylsilane.
100
90,
._ 80
70
60,
J
_._._..___ Weight %
.... 1st Deflvative
V Reheated In air
!,
,! i ii
500 750 1000 1250
Temperature, =C
Figure 3. TGA ofpolyvinlysilane showing stability on reheating in air,
after an initial pyrolysis in argon.
CONCLUSIONS
All of the polymer systems examined have resulted in smooth, thin fiber
coatings on large diameter fibers. Pushout tests that were done on the
polysilsesquioxane system show that the resulting ceramic does result in a weaker
interface than uncoated SCS-0 fibers. These results should improve as both the
composition and thickness of the coating is optimized. Both the polycarbosilane
and polyborosilazane were found to be oxidatively stable to 1400°C. This will
provide for a stable coating under anticipated use conditions. The ability of the
polyborosilazane to remain amorphous to high temperatures, means that it is
unlikely thatshrinkagewill occur. Shrinkage of the coating would cause it to
crack exposing the fiber to oxygen which would result in loss of fiber properties.
The final ceramic composition of each of the three polymers are different.
The polysilsesquioxane polymer results in a Si-C-O ceramic, the polyborosilazane
forms a Si-B-N ceramic and the polysilsesquioxane pyrolyzes to a carbon-rich Si-
C. These differences in composition, and their related properties, could allow for
better matching the requirements of a fiber/matrix system with an interface
coating.
REFERENCES
1. P.J. Heimann, F. I. Hurwitz, A. L. River, "Synthesis of
Polysilsesquioxanes for Use as Ceramic Precursors," Cream. Trans. 19 (Adv.
Comps. Mater.), 27-33 (1991)
2. F.I. Hurwitz, T. A. Quack, X. Y. Bu, J. Mason, P. J. Heimann and K.
Beyond, "Pyrolytic Conversion of Methyl- and Vinylsilane Polymers to Si-C
Ceramics," submitted to J Mater. Sci.
3. F.I. Hurwitz, T. A. Kacik, X. Y. Bu, J. Masnovi, P. J. Heimann and K.
Beyene, "Conversion of Polymers of Methyl- and Vinylsilane to Si-C Ceramics,"
Mater. Res. Soc. Proc., to be published. Also NASA TM 106544 (1994).
4. J. Masnovi, X. Y. Bu, K. Beyene, P. J. Heimann, T. A. Kacik, A. H.
Andrist and F. I. Hurwitz, "Syntheses, Structures and Properties of
Polycarbosilanes Formed Directly by Polymerization of Alkenylsilanes," Mater.
Res. Soc. Proc. 271, (Better Ceramics Through Chemistry V) 771-776 (1992)
5. F.I. Hurwitz, P. J. Heimann, J. Z. Gyekenyesi, J. Masnovi and X. Y. Bu,
"Polymeric Routes to Silicon Carbide and Silicon Oxycarbide CMC," Ceram.
Eng. Sci. Proc. 12, 1292-1303 (1991), also NASA TM 104372.
6. F.I. Hurwitz, P. J. Heimann, S. Farmer and D. M. Hembree,
"Characterization of the Pyrolytic Conversion of Polysilsesquioxanes to Silicon
Oxycarbides," J.. Mater. Sci. 28, 6622-6630 (1993).
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Preceramic Polymers for Use as Fiber Coatings

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