The development of silicon carbide-silicon nitride fibers (SiC-Si3N4) by the pyrolysis of polycarbosilazane precursors is reviewed. Precursor resin, which was prepared by heating tris(N-methylamino)methylsilane or tris(N-methylamino)phenylsilane to about 520 C, was drawn into fibers from the melt and then made unmeltable by humidity conditioning at 100 C and 95 percent relative humidity. The humidity treated precursor fibers were pyrolyzed to ceramic fibers with good mechanical properties and electrical resistivity. For example, SiC-Si3N4 fibers derived from tris(N-methylamino)methylsilane had a tensile rupture modulus of 29 million psi and electrical resistivity of 6.9 x ten to the 8th power omega-cm, which is ten to the twelfth power times greater than that obtained for graphite fibers

Clemons, J. M.

Daniels, J. G.

Ledbetter, F. E., III

Penn, B. G.

English

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J
INASA -Td - 66505) FhEPIRATIGII O F SILICON
CARBIDE-SILICCN IIITbICE FlEFbS EY THE
PYROLYSIS OF POLYCIRBUSILAZAYE PRECURSORS
NASA	 Final Report JNAS l) 12 p HC s0218F A0 1
Technical	 CSCL 11G G3/27
Memorandum
N85-28107
Unclas
21420
NASA TM-86505
PREPARATION OF SILICON CARBIDE-SILICON
NITRIDE FIBERS BY THE PYROLYSIS OF
POLY CARBOSILAZANE PRECURSORS
(Center Director's Discretionary Fund Final Report)
By B. G. Penn, J. G. Daniels, F. E. Ledbetter, III,
and J. M. Clemons	 r
Space Science Laboratory and
Materials and Processes Laboratory
Science and Engineering Directorate
March 1985
X
^c	 JUN 1^95
NASA
Naticnai Aeronautics and
S pace Adrnl n strahon
George C. Marshall Space Fl;ght Center
MSFC - Form 3190 (Rev. May 1963)
Wlli
71rP'4larlr`A1 OtOAOT lT&Mr%AMr% TITt i Oaf]C
1.	 REPORT NO. 2,	 GOVERNMENT ACCESSION NO.. M f.	 RECIPIENT'S CATALOO.NO.
NASA TM-86505
4.	 TITLE AND SUBTITLE S.	 REPORT SATE
Preparation of Silicon Carbide-Silicon Nitride Fibers by March 1985
the Pyrolysis of Polycarbosilazane Precursors 6.	 PERFORMING ORGANIZATION CEVE
Center Director's Discretionary Fund Final Report)
7. AUTHOR(S)	 B. G. Penn ,* 	an @ s, 6, PERFORMING ORGAnIZAT ION REPAR f N
Ledbetter	 III** and J. M. Clemons**
9.	 PERFORMING ORGANIZATION NAME AND ADDRESS 10.	 WORK UNIT NO.
George C. Marshall Space Flight Center 11.	 CONTRACT OR GRANT NO.
Marshall Space Flight Center, Alabama	 35812
12. TYPE OF REPOR'i IN PERIOD COVERED
12.	 SPONSORING AGENCY NAME AND ADDRESS
National Aeronautics and Space Administration Technical Memorandum
Washington, D.C. 	 20546 14.	 SPONSORING AGENCY CODE
15.	 SUPPLEMENTARY NOTES
*Space Science Laboratory, Science and Engineering.
**Materials and Processes Laboratory, Science and Engineering.
14.	 ABSTRACT
The development of silicon carbide-silicon nitride fibers (SiC-Si 3N 4) by the
pyrolysis of polycarbosilazane precursors that was carried out in this laboratory is
reviewed.	 Precursor resin, which was prepared by heating tris(N-methylamino)-
methylsilane or tris(N-methylamino)phenylsilane to about 520°C, was drawn into
fibers from the melt and then made unmeltable by humidity conditioning at 100°C
and 95 percent relative humidity.
	 The humidity treated precursor fibers were
pyrolyzed to ceramic fibers with good mechanical properties and electrical resistivity.
For example, SiC-Si 3N 4
 fibers derived from tris(N-methylamino)methylsilane had a
tensile rupture modulus of 29 x 10 6 psi and electrical resistivity of 6.9 x 10 8 0-cm
which is 10 12 times greater than that obtained for graphite fibers.
This research was sponsored by the MSFC Center Director's Discretionary Fund
Project [No. 82-13, "Preparation of New Continuous Silicon Carbide-Silicon Nitride
(SiC-Si 3N 4 ) Fibers by the Controlled Pyrolysis of Organosilane Polymeric Precursors"] .
17.	 KEY WORDS 16.	 DISTRIBUTION STATEMENT
Silicon Carbide-Silicon Nitride I1,^„,,.,.	 d - 39_ro^
Ceramic Fiber, Silicon Carbide,
Silicon Nitride, Polycarbosilazane Unlimited---Unclassified
19,
	
SECURITY CLASSIF. (of this repo”) 20.	 SECURITY CL ASSIF. (of this r^go) 21,	 NO. OF PAGES 22.	 PRICE
Unclassified Unclassified 11 NTIS
rTc - Fors 7297 (r1 1969)
For No by Nstloaal Tooltoloal latonmal4oa Ssrvlos. W6 WK Vlt¢ola 22161
sk,
TABLE OF CONTENTS
3
Page
INTRODUCTION ..............................................................
	 1
MONOMER PREPARATION .....................................................
	
1
POLYMERIZATION OF THE TRI(N-SUBSTITUTED)SILANE
T RIAMIN E MONOMERS ........................................................ 	 2
PREPARATION AND CHARACTERIZATION OF POLY CARBOSILAZANE
FIBERS ...................................................................... 	 3
PREPARATION AND CHARACTERIZATIONS OF SILICON
CARBIDE-SILICON NITRIDE FIBERS ..........................................	 3
CONCLUSIONS............................................................... 	 4
REFERENCES ................................................................. 	 8
l'RI:CLI 1;:G lJ A(-,E' I31.ANK NOT I'II,WD
iii	 I -1s l
TECHNICAL MEMORANDUM
PREPARATION OF SILICON CARBIDE-SILICON NITRIDE FIBERS
BY THE PYROLYSIS OF POLYCARBOS ILA ZANE PRECURSORS
(Center Director's Discretionary Fund Final Report)
INTRODUCTION
Carbon fibers are widely used in fabricating fiber reinforced composite materials;
however, undesirable characteristics such as severe oxidation at temperatures greater
than 400°C [421,  high electrical conductivity which may present a safety hazard
during manufacture [ 3] , and poor fiber-matrix adhesion between carbon fibers and
thermoplastic or metal matrices [ 1, 2] make the development of alternate reinforcement
fibers desirable. Another problem that significantly limits the use of carbon fibers is
their reactivity with various metals that are used as matrices in composite materials [ 2] .
Silicon carbide-silicon nitride (SiC-Si 3N 4 ) fibers first developed by Verbeek i4]
are an excellent candidate for use in applications that are unsuitable for carbon fibers.
These silicon-uased ceramic fibers, which are prepared by the pyrolysis of poly-
carbosilazane precursors, are reported to have good physical and mechanical lroper-
ties. For example, SiC-Si 3N 4
 fibers derived from tris(N-methylamino)methyl-,..aiie are
reported to have a tensile strength of 18.8 x 10 4 psi and elastic modulus of 29 x 106
psi. Moreover, these fibers are stab:e in air up to 1200°C [4].
This report reviews the work carried out in this laboratory to further develop
and characterize SiC-Si 3N 4 fibers.
MONOMER PREPARATION
The preparation of silicon carbide-silicon nitride fibers consisted of the follow-
ing steps: preparation of monomeric tris(N-methylamino)alkylsilane, polymerization of
the monomer to form a polycarbosilazane precursor, treatment of the precursor to
make it unmeltable and pyrolysis of the precursor in an inert environment.
The tri(N- substituted)silanetriamine monomer was prepared by the reaction of
a trihalosilane and monomethlyamine as shown below:
RSiC1 3
 + 6 CH3NH2 -► RSi(NHCY 3 ) 3 + 3[CH 3NH 3+ ] Cl-	(1)
in this work met hyltrichlorosilane or phenyltrichlorosilane was reacted with methyl-
amine to yield tris(N-methylamino)methylsilane or tris(N-methylamino)phenylsilane
[5-7]. In the first step of the reaction, the trihalosilane was added dropwise to
excess monomethylamine dissolved in petroleum ether or toluene at -40°C. The
mixture was then refluxed for 1 hr and the product recovered by distillation. Excess
monomethylamine was added for reaction with the HCl generated during the reaction
as shown by equation (1) . This is necessary since HCl reacts with SiN bonds
[equation (2) ] thereby decreasing the monomer yield [ 8] .
R 3SiNH 2 + HC1 + R 3SiC1 + NH 4 C1	 (2)
Another precaution was to rigorously exclude water during the reaction since it will
cleave the silicon-nitrogen bonds of the monomer.
The monomer, tris(N-methylamino)methylsilane has been prepared by Verbeek
[ 4] and Tansjo [ 9] . Verbeek prepared the silaneamine by reacting methylamine with
met hyltrichlorosilane dissolved in petroleum ether at 40 0 C. Tansjo synthesized the
monomer in high yield by the dropwise addition of met hyltrichlorosilane to an ice-
cooled mixture of excess methylamine dissolved in ether followed by refluxing of the
mixture for 1 hr. This procedure resulted in a yield of 71 percent.
POLYMERIZATION OF THE TRI(N- SUBSTITUTED)SILANE
TRIAMINE MONOMLRS
Tris(N-methylamino)phenylsilane and tris(N-rr.ethylamino)methylsilane were used
as monomers in the preparation of the polycarbosilazane precursors. This was
accomplished by heating the monomers for 1.5 to 4.5 hr in a system that is basically
a total reflux distillation system (Fig. 1) . The structure shown in equation (1) has
been proposed for the polycarbosilazane resulting from the polymerization of
tris(N-methylamino)methylsilane [ 9] .
CH3
I
NH CH3
CH3 S1	 CH3
	
N	 N CH3
	
IN Si	 Si -- N
\ N \CH3
I
CH3
CH3Si(NHCH3)3 520_C
n
Weight average molecular weights that range from 1533 to 4222 have been obtained by
the use of this polymerization technique.
The preparation of a polycarbosilazane by the pyrolysis of tris(N-methylamino)-
methylsilane has been described by Verbeek [4] . In this method, the liquid monomer
which boils at 150°C is conducted for 3 hr through a glass tube that is heated to
520°C, and which is filled with Raschig rings. This mixture is then separated into
2
the desired carbosilazane resin and redistillable products, which are recycled into
the pyrolysis, by heating at 4500C in a cylinder. The molecular weights of the
polymers prepared by this method were not reported.
PREPARATION AND CHARACTERIZATION OF
POLY CARBOSILAZANE FIBERS
Fibers were drawn from the polycarbosilazane derived from trio (N--methylamirio)-
methylsilane by touching the surface of the polymer melt with a glass rod and drawing
fibers [ 5] . Lengths up to 6 ft with diameters of 0.5 to 4 mils were obtained using
this crude technique. These fibers were made unmeltable by humidity conditioning
at 100°C and 95 percent relative humidity for 20 hr. These fibers were found to have
tensile strengths of 13,800, 4000 and 1800 psi for diameters of 0.52, 1.74, and 4.97
mil, respectively. This is comparable to the tensile strength value of 1440 psi found
for polycarbosilane which is a precursor for SiC fibers [101.
Using the crude fiber drawing technique previously described, fibers were
drawn from the polycarbosilazane melt derived from tris(N-methylamino)phenylsilane
[ 6] . A fiber with a diameter of 2.7 mil had a tensile strength of 23,500 psi which is
comparable to that reported for the silicon carbide precursor (polycarbosilane) and
the polycarbosilazane derived from tris(N-methylamino)phenylsilane (Table 1).
Verbeek has demonstrated that polycarbosilazane resins can be spun into fibers
using conventional spinning methods [4]. For example, staple fibers with a cross-
section of about 10 to 20 W m were produced by a nozzle blowing process or centrifugal
spinning process. In addition, polycarbosilazane resin is reported to be suitable for
spinning by conventional dry or wet spinning processes because of its solubility in a
large number of organic solvents [4].  The spinning characteristics of the solutions
are considerably improved by the addition of auxiliary polymeric material having a
molecular weight in excess of 10,000.
The suitability of polycarbosilazane resin for fiber spinning was recently demon-
strated by Dunn and Pomplum [111.  These investigators produced fibers by a con-
ventional melt spinning technique using a precursor resin derived from tris(N-
methylamino)methylsilane. Polycarbosilazane fibers with diameters of 25 to 30 microns
were produced.
PREPARATION AND CHARACTERIZATIONS OF SILICON
CARBIDE-SILICON NITRIDE FIBERS
Before pyrolysis to ceramic fibers, the polycarbosilazane precursor fibers were
made unmeltable by humidity conditioning at 100°C and 95 percent relative humidity
for 20 hr. In addition to this method, Verbeek reported that other pretreatments
may be used such as oxidation and/or sulfidation [4]. Suitable reagents for these
processes are summarized in Table 2.
After humidity treatment, the hydrolyzed polycarbosilazane fibers were pyro-
lyzed to SiC-Si 3N 4 fibers at ambient to 1500°C [5]. The pyrolysis process has been
_^A_W
:i
examined using thermogravimetric analysis (12).  Humidity conditioned polycarbosila-
zane fibers derived from tris(N-methylamino)methylsilane were heated at cmhi: nt to
1000°C in a nitrogen atmosphere. As shown in Figure 2, most of the 32.27 percent
weight loss occurred at 7580 to 8080 C. This implies that the precursor fibers s:!.:)uld
be heated slowly and carefully in this temperature range to decrease flaw formation
due to the rapid escape of volatiles.
The stability of silicon carbide-silicon nitride fiber in air at ambient to 1000°C
was examined [131.  As shown in Figure 3, the weight loss was insignificant. This
was the case after the sample was heated for fifteen minutes under these conditions.
Verbeek claims that these fibers have thermal oxidative stability at 12000C.
The mechanical properties and electrical resistivity of silicon carbide-silicon
nitride fibers -were founa to be good. The tensile strength for SiC-Si 3N 4 fiber
derived from tris(N-methylamino)methylsilane was 105 x 10 3 psi which is comparable
to that reported by Verbeek (188 x 10 3
 psi) for the same material [ 5] . The tensile
rupture modulus was found to be 29 x 10 6
 psi and the electrical resistivity was 6.9 x
10 8 0-cm for a 0.6 mil diameter which is 10 12
 times greater than the value reported
for graphite fibers.
CONCLUSIONS
The full potential applications of SiC-Si 3N 4
 fibers prepared by the pyrolysis of
polycarbosilazane precursors cannot be determined without further work; however,
data presented here and that of Verbeek [ 4] indicate that these fibers are an
excellent candidate for use as a reinforcement in ceramic, polymer, and metal matrices.
For example, the tensile modulus (29 x 10 6
 psi) of silicon carbide-silicon nitride fibers
derived from tris(N-methylamino)methylsilane is comparable to other reinforcement
fibers and the electrical resistivity value of 6.9 x 10 8 n-cm is 10 12 times greater than
that of graphite. Moreover, these fibers have excellent thermal-oxidative stability as
shown by thermogravimetric analysis.
4
TABLE 1. TENSILE STRENGTHS OF SILICON-BASED
CERAMIC FIBER PRECURSORS
Pyrolyzed
Fiber Precursor [Monomer]
SiC-Si 3N 4 Polycarbosilazane
[Tris(N-methylamino)methylsilane]
Sic-Si 3N 4 Polycarbosilazane
[ Tris(N-methylamino)phenylsilane]
Sic Polycarbosilane
Fiber Tensile
Diameter Strength
(mil) (psi) Reference
0.52 13,800 5
2.7 23,500 6
— 1,440 10
TABLE 2.	 PRETREATMENT REAGENTS THAT MAY BE USED TO
MAKE POLYCARBOSILAZANE F 3ERS UNMELTABLE [ 4]
Reagents Suitable for Pretreatment
A.	 Oxidizing, Dehydrizing, or Dehydrating Agents
1. Air
2. Oxygen
3. Ozone
4. Halogens
5. Nitrogen Oxides
6. Sulfur Vapors
7. Hydre,`en Peroxide
8. Organic Peroxides
9. Potassium Permanganate
10. Hydrogen Sulfide
11. Sulfur Dioxide
B.	 Lewis Acids
1. Aluminum Trichloride
2. Boron Trihalide
5
_ FRIEDRICHS
CONDENSER
T/S 24/40 JOINTS
1" O.D. TUBE
JACKING
1/4" RASCHIG RING
i
POWER
LEADS
WATER
CONNECTIONS
III
k I
HIGH TEMPERATURE /
CYLINDRICAL FURNACE
Figure 1. High temperature polymerization system.
140
120
100	
7".0°C
i^ 80	 32.27 % DECOMPOSITION
r	 789.70C (7.22 nv)
807.80C
40
20
0
100 200 300 400 500 600 700 800 900 1000 1100 1200
TEMPERATURE (°C)
Figure 2. TGA for moisture conditioned poly carbosilazane.
6
i in
1
140
120
100 a
X
80
a
W
60
s
40
0	 100 200 300 400 500 600 700 800 900 1000 1100
TEMPERATURE (°C)	 ,
Figure 3. TGA analysis of silicon carbide-silicon nitride fibers
( 5.63 MG).  Heating rate of 10° /min in air.
20
n
f'
Syl'
\i
nREFERENCES
1. Harris, B.: Carbon Fibers in Engineering. Marcus Langleygi	 g.	 g  (ed.) , McGraw-Hill,
	 i
New York, 1973, pp. 1-45.
2. Jenkins, G. M. and Kawamura, K.: Polymeric Carbons-Carbon Fibers, Glass,
and Char. Cambridge University Press, New York, Chapters 7 and 8, 1976.
3. Cagliostro, D. E.: Textile Research Journal, October 1980, p. 632.
1. Verbeek, W. 	 Seer. Offen. 2, 218, 960, 1973.
5. Penn, B. k. , Ledbetter, F. E. III, Clemons, J. N1. , and Daniels, J. G.: J.
Appl. Polym . Sci. , Vol. 27, 1982, p. 3751.
6. Crouse, D. J . , Penn, B. G. , Ledbetter, F. E. III , Daniels, J. G. , and
Clemons, J. M.: J. Appl. Polym. Sci. (paper submitted).
7. Penn, B. G . , Ledbetter, F. E. III, and Clemons, J. M.: I • EC Process
Design and Development, Vol. 23, 1984, p. 217.
8. Fessenden, R. and Fessenden, J.: Chemical Reviews, Vol. 61, 1961, p. 361.
9. Markle, R. A., Sekercioglu, I., Hill, D. L., Wills, R. R., and Sinclair, R. G.:
Preparation of SixNy Cz Fibers by the Controlled Pyrolysis of Novel Organosilicon
Polymeric Precursors. Battelle Columbus Laboratories Technical Report, Contract
No. NAS8-33223, 1980.
10. Andersson, C-H. and Warren, R.: Composites, Vol. 15, No. 1, 1984, p. 16.
1 ] . Pomplum , A. R. and Dunn, S. A.: Bjorksten Research Laboratories, Final Report
in Preparation:	 Development of a Continuous Process for Producing Silicon
Carbide-Silicon Nitride Precursor Fibers. Contract No. NAS8-34648, NASA,
MSFC, 1985.
12. Ledbetter, F. E. III, Daniels, J. G., Clemons, J. M., Hundley, N. H., and	 i
Penn, B. G.: J. Material Science Letters, Vol. 3, 1984, p. 802.
13. Daniels, J. G., Ledbetter, F. E. III, Clemons, J. M., and Penn, B. G.:
SAMPE Quarterly, Vol. 15, No. 4, 1984, p. 39.
8
APPROVAL
PREPARATION OF SILICON CARBIDE-SILICON NITRIDE FIBERS BY THE
PYROLYSIS OF POLYCARBOSILAZANE PRECURSORS
(Center Director's Discretionary Fund Final Report)
i;y B. G. Penn, J. G. Daniels, F. E. Ledbetter, M, and J. M. Clemons
The information in this report has been reviewed for technical content. Review of any
information concerning Department of Defense or nuclear energy activities o- programs has
been made by the MSFC Security Classification Officer. This report, in its entirety, has been
determined to be unclassified.
T.77-0 H-- W--I R  HA MrR
Director, ;Materials and Processes Laboratory
*US. CJVERNMENT FAINTING OFFICE 1"5-9"-04W101{2 	 9


Preparation of silicon carbide-silicon nitride fibers by the pyrolysis of polycarbosilazane precursors

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