Process for preparing ceramic materials

A method of preparing ceramic materials or articles by the pyrolysis of preceramic silazane polymers, wherein the preceramic silazane polymers are rendered infusible prior to pyrolysis by treatment with steam or a steam and oxygen mixture, is disclosed. This method is especially suited for the preparation of ceramic fibers.

BACKGROUND OF THE INVENTION 
This invention relates to the preparation of ceramic materials or articles 
by the pyrolysis of preceramic silazane polymers wherein the preceramic 
silazane polymers are rendered infusible prior to pyrolysis by treatment 
with steam or a steam and oxygen mixture. This method is especially suited 
for the preparation of ceramic fibers. 
The prior art discloses that ceramic materials have been prepared by the 
pyrolysis of preceramic silazane polymers. Gaul in U.S. Pat. No. 4,312,970 
(issued Jan. 26, 1982) obtained ceramic materials by the pyrolysis of 
preceramic silazane polymers, which polymers were prepared by reacting 
organochlorosilanes and disilazanes. The preceramic silazane polymers were 
pyrolyzed in an inert atmosphere without any separate treatment to render 
the silazane preceramic polymer infusible. 
Gaul in U.S. Pat. No. 4,340,619 (issued July 20, 1982) obtained ceramic 
materials by the pyrolysis of preceramic silazane polymers, which polymers 
were prepared by reacting chlorine-containing disilanes and disilazanes. 
Fibers prepared from such preceramic silazane polymers were given a "mild 
heat treatment" in air before pyrolysis but there is no teaching that such 
a treatment rendered the fibers infusible. 
Cannady in U.S. patent application Ser. No. 555,755, filed Nov. 28, 1983 
obtained ceramic materials by the pyrolysis of preceramic silazane 
polymers, which polymers were prepared by reacting trichlorosilane and 
disilazane. The preceramic silazane polymers were not rendered infusible 
prior to pyrolysis, in order to form ceramic materials. 
What has been discovered is a method of rendering preceramic silazane 
polymers infusible prior to pyrolysis. This method represents a 
significant advance in the art of preparing ceramic materials or articles, 
especially in the art of preparing ceramic fibers. 
THE INVENTION 
This invention relates to a method of preparing a ceramic material, which 
method comprises (1) treating a preceramic silazane polymer with steam for 
a time sufficient to render the preceramic silazane polymer infusible 
wherein the treatment temperature is sufficiently low so that the 
preceramic silazane polymer remains unfused during the treatment step and 
(2) heating the infusible preceramic silazane polymer of step (1) in an 
inert atmosphere, vacuum or ammonia-containing atmosphere to a temperature 
of at least 750.degree. C. until said infusible preceramic silazane 
polymer is converted to a ceramic material. 
This invention also relates to a method of preparing a ceramic material, 
which method comprises (1) treating a preceramic silazane polymer with 
steam at a temperature below the penetration temperature of the preceramic 
silazane polymer for a time sufficient to render the preceramic silazane 
polymer infusible and (2) heating the infusible preceramic silazane 
polymer of step (1) in an inert atmosphere, vacuum or ammonia-containing 
atmosphere to a temperature of at least 750.degree. C. until said 
infusible preceramic silazane polymer is converted to a ceramic material. 
This invention also relates to a method of preparing a ceramic material, 
which method comprises (1) treating a preceramic silazane polymer with a 
steam and oxygen mixture for a time sufficient to render the preceramic 
silazane polymer infusible wherein the treatment temperature is 
sufficiently low so that the preceramic silazane polymer remains unfused 
during the treatment step and (2) heating the infusible preceramic 
silazane polymer of step (1) in an inert atmosphere, vacuum or 
ammonia-containing atmosphere to a temperature of at least 750.degree. C. 
until said infusible preceramic silazane polymer is converted to a ceramic 
material. 
This invention also relates to a method of preparing a ceramic material, 
which method comprises (1) treating a preceramic silazane polymer with a 
steam and oxygen mixture at a temperature below the penetration 
temperature of the preceramic silazane polymer for a time sufficient to 
render the preceramic silazane polymer infusible and (2) heating the 
infusible preceramic silazane polymer of step (1) in an inert atmosphere, 
vacuum or ammonia-containing atmosphere to a temperature of at least 
750.degree. C. until said infusible preceramic silazane polymer is 
converted to a ceramic material. 
This invention further relates to ceramic fibers prepared by the method 
comprising the steps of (1) preparing a preceramic silazane polymer, (2) 
preparing preceramic fibers from said preceramic silazane polymer, (3) 
treating the preceramic fibers prepared in step (2) with steam or a steam 
and oxygen mixture for a time sufficient to render the preceramic fibers 
infusible wherein the treatment temperature is sufficiently low so that 
the preceramic silazane fibers remain unfused during the treatment step 
and (4) heating the infusible preceramic fibers of step (3) in an inert 
atmosphere, vacuum or ammonia-containing atmosphere to a temperature of at 
least 750.degree. C. until said infusible preceramic fibers are converted 
to ceramic fibers. 
This invention also relates to ceramic fibers prepared by the method 
comprising the steps of (1) preparing a preceramic silazane polymer, (2) 
preparing preceramic fibers from said preceramic silazane polymer, (3) 
treating the preceramic fibers prepared in step (2) with steam or a steam 
and oxygen mixture at a temperature below the penetration temperature of 
the preceramic fibers for a time sufficient to render the preceramic 
fibers infusible and (4) heating the infusible preceramic fibers of step 
(3) in an inert atmosphere, vacuum or ammonia-containing atmosphere to a 
temperature of at least 750.degree. C. until said infusible preceramic 
fibers are converted to ceramic fibers. 
Passage of steam or a steam and oxygen mixture over a preceramic silazane 
polymer will result in an infusible preceramic silazane polymer suitable 
for pyrolysis to form a ceramic material. Preceramic silazane polymers in 
the form of pellets, powders, flakes, foams, fibers, and the like are 
especially suitable for treatment with steam or a steam and oxygen mixture 
by the method of this invention. The pressure of the steam or the steam 
and oxygen mixture is not critical. High pressure steam may be used. It is 
preferred, however, that pressures at or near atmospheric pressure be used 
for ease of operation. The steam or steam and oxygen mixture may, if 
desired, contain inert gas dilutents such as argon, nitrogen and the like. 
The oxygen in the steam and oxygen mixture may be pure or relatively pure 
oxygen or may be in the form of air. By "steam" we mean both essentially 
100 weight percent water vapor and water vapor in an inert carrier gas 
such as nitrogen, argon, helium, carbon dioxide and the like. 
The temperature of the treatment of the preceramic silazane with the steam 
or steam and oxygen mixture must be sufficiently low so that the 
preceramic polymer does not melt or fuse during the treatment step. 
Preferably the treatment temperature is below the penetration temperature 
or softening temperature of the preceramic silazane polymer. The treatment 
temperature must be above the temperature at which the steam will condense 
as water droplets on the preceramic silazane polymer being treated. As one 
skilled in the art would realize, the penetration temperature of 
individual preceramic silazane polymers will depend in large part upon the 
reactants and reaction conditions employed to prepare each preceramic 
silazane polymers. Therefore the penetration temperature of a given 
preceramic silazane polymer should be determined to establish the 
preferred upper temperature limit of treatment with steam or a steam and 
oxygen mixture. More preferably the temperature of the treatment with 
steam or a steam and oxygen mixture should be between about 35.degree. C. 
and about 10.degree. C. below the penetration temperature of the 
preceramic silazane polymer. The treatment temperature must be, however, 
above the temperature at which steam will condense as water droplets on 
the preceramic silazane polymer being treated. The pressure of the 
treatment gas can be varied to help control condensation. Shorter 
treatment times to render the preceramic silazane polymer infusible can be 
expected if the treatment temperature is kept as high as possible. Within 
these constraints, it generally has been found that temperatures between 
about 35.degree. and 200.degree. C. are suitable. One way to avoid the 
possibility of water condensing on the ceramic article during treatment is 
to use air or oxygen saturated with water vapor at room temperature and a 
treatment temperature greater than about 10.degree. C. above room 
temperature. 
The preceramic silazane polymers are treated with steam or a steam and 
oxygen mixture for a time sufficient to render the preceramic silazane 
polymer infusible. What is meant by "infusible" in this specification is 
that the treated preceramic silazane polymer when heated rapidly up to the 
pyrolysis temperature will not fuse together. A crude screen for 
infusibility is provided by the solubility of the preceramic silazane 
polymer in toluene. Prior to treatment with steam or a steam and oxygen 
mixture the preceramic silazane polymers are almost completely soluble in 
toluene. The infusible preceramic silazane polymers obtained by treatment 
by the method of this invention are either insoluble in toluene or have 
only limited solubility in toluene. The time required to render the 
preceramic silazane polymer infusible by the method of this invention will 
depend, in part, on the size of the preceramic silazane polymer object, 
the temperature of the treatment, the amount of water vapor or water vapor 
and oxygen present, and the specific preceramic silazane polymer employed. 
The time required to render the preceramic silazane polymer infusible will 
normally be in the range of a few minutes to several hours or longer. It 
is best to determine the treatment time by routine experimentation. 
The amount of steam or steam and oxygen mixture that the preceramic 
silazane polymer should be exposed to is the amount sufficient to render 
the preceramic silazane polymer infusible. This required amount will vary 
from case to case depending, in part, upon the temperature, pressure, the 
time of exposure and the actual preceramic silazane polymer used as well 
as other variables. When the preceramic silazane polymer is in the shape 
of a formed object such as a fiber it is not necessary to render the 
entire shaped article infusible. Rather only the outer surfaces, and 
sufficient interior portions directly adjacent to the outer surfaces, need 
be rendered infusible. The interior portion of the shaped article may be 
cured during the pyrolysis of the shaped article to elevated temperature. 
Simply rendering the exterior infusible will prevent the shaped articles 
from fusing together during the pyrolysis unless a break in the exterior 
surface occurs which allows the nonfused interior to leak out. 
Preceramic silazane polymers suitable for use in this present invention are 
well known in the art. The preceramic silazane polymers suitable for use 
in this invention must be capable of being converted to a ceramic material 
at elevated temperatures. It is generally preferred that the preceramic 
silazane polymers used in this invention be capable of being converted to 
a ceramic material in at least 40 weight percent yield. Mixtures of 
preceramic silazane polymers may also be used in this invention. Examples 
of preceramic silazane polymers or polysilazanes suitable for use in this 
invention include polysilazanes as described by Gaul in U.S. Pat. Nos. 
4,312,970 (issued Jan. 26, 1982), 4,340,619 (issued July 20, 1982), 
4,395,460 (issued July 26, 1983), and 4,404,153 (issued Sept. 13, 1983), 
all of which are hereby incorporated by reference. Suitable polysilazanes 
also include those described by Haluska in U.S. Pat. No. 4,482,689 (issued 
Nov. 13, 1984) and by Seyferth et al. in U.S. Pat. No. 4,397,828 (issued 
Aug. 9, 1983), both of which are hereby incorporated by reference. Other 
polysilazanes suitable for use in this invention are disclosed by Cannady 
in U.S. patent applications Ser. No. 555,755 (filed Nov. 28, 1983), Ser. 
No. 627,260 (filed July 2, 1984), and Ser. No. 689,258 (filed Jan. 7, 
1985), by Bujalski in U.S. patent application Ser. No. 653,003 (filed 
Sept. 21, 1984), and by Baney et al. in U.S. patent applications Ser. No. 
652,938 (filed Sept. 21, 1984) and Ser. No. 653,939 (filed Sept. 21, 
1984), all of which are hereby incorporated by reference. Still other 
polysilazanes may be suitable for use in this invention. 
Preceramic silazane polymers especially useful in this invention are 
described in U.S. Pat. Nos. 4,312,970 and 4,340,619 and U.S. patent 
application Ser. No. 555,755 filed Nov. 28, 1983, all of which have been 
incorporated by reference. 
The preceramic silazane polymers described in U.S. Pat. No. 4,312,970 are 
prepared by contacting and reacting in an inert, essentially anhydrous, 
atmosphere, an organochlorosilane or a mixture of organochlorosilanes of 
the general formula 
EQU R'.sub.c SiCl.sub.(4-c) 
with a disilazane having the general formula 
EQU (R.sub.3 Si).sub.2 NH 
at a temperature in the range of 25.degree. C. to 300.degree. C. while 
distilling byproduced volatile products, wherein R' is selected from the 
group consisting of vinyl, phenyl, and alkyl radicals containing 1 to 3 
carbon atoms; R is selected from the group consisting of vinyl, hydrogen, 
phenyl, and alkyl radicals containing 1 to 3 carbon atoms; and c has a 
value of 1 or 2. 
The organochloromonosilanes of U.S. Pat. No. 4,312,970 are those having the 
general formula 
EQU R'.sub.c SiCl.sub.(4-c) 
where R' is vinyl or an alkyl radical containing 1-3 carbon atoms or the 
phenyl group. Thus, those groups which are contemplated as being useful in 
this invention are methyl, ethyl, propyl, vinyl, and phenyl. The R' groups 
can all be the same or they can be different. The organochloromonosilanes 
are common commodity chemicals and are commercially available and, 
therefore, an explanation as to their preparation does not appear to be 
necessary. The value of c is 1 or 2. Thus, single organic group 
substituted silanes such as CH.sub.3 SiCl.sub.3, C.sub.6 H.sub.5 
SiCl.sub.3, CH.sub.2 .dbd.CHSiCl.sub.3, CH.sub.3 CH.sub.2 SiCl.sub.3 or 
CH.sub.3 (CH.sub.2).sub.2 SiCl.sub.3 and double organic substituted 
silanes such as (CH.sub.3).sub.2 SiCl.sub.2, (C.sub.2 H.sub.5).sub.2 
SiCl.sub.2 and (CH.sub.2 .dbd.CH)(CH.sub.3)SiCl.sub.2 and mixtures of such 
silanes, for example CH.sub.3 SiCl.sub.3 and (CH.sub.3).sub.2 SiCl.sub.2, 
can be used. It is preferred that when organochlorosilane mixtures are 
used, the number of units of diorganosubstituted silicon atoms should not 
exceed the number of units of monoorgano-substituted silicon atoms. 
The preceramic silazane polymers of U.S. Pat. No. 4,340,619 are prepared by 
contacting and reacting in an inert, essentially anhydrous, atmosphere, a 
chlorine-containing disilane or a mixture of chlorine-containing 
disilanes, of the general formula 
EQU (Cl.sub.d R'.sub.e Si).sub.2 
with a disilazane having the general formula 
EQU (R.sub.3 Si).sub.2 NH 
at a temperature in the range of 25.degree. C. to 300.degree. C. while 
distilling byproduced volatile products, wherein R' is selected from the 
group consisting of vinyl, phenyl, and alkyl radicals containing 1 to 3 
carbon atoms; R is selected from the group consisting of vinyl, hydrogen, 
phenyl, and alkyl radicals containing 1 to 3 carbon atoms; d has a value 
of 0.5-3; e has a value of 0-2.5 and the sum of (d+e) is equal to three. 
The chlorine-containing disilanes of U.S. Pat. No. 4,340,619 are those 
disilanes having the general formula 
EQU (Cl.sub.d R'.sub.e Si).sub.2 
where R' is vinyl, an alkyl radical containing 1-3 carbon atoms or the 
phenyl group. Thus, the R' groups are methyl, ethyl, propyl, vinyl and 
phenyl. The R' groups can all be the same or they can be different. The 
chlorine-containing disilanes can be those found in the residue from the 
Direct Process for producing halosilanes (Eaborn, C., "Organosilicon 
Compounds", Butterworth Scientific Publications, London, 1960, pg. 1). The 
Direct Process is the reaction between silicon metal and aliphatic 
halides, generally methyl chloride, at elevated temperature in the 
presence of catalyst, generally copper, to produce chlorosilanes. For the 
chlorine-containing disilanes described above, the value of d and e is 
from 0.5-3 and 0-2.5 respectively, and the sum of (d+e) is equal to three. 
Examples of chlorine-containing disilanes are (Cl.sub.2 
(CH.sub.3)Si).sub.2, (Cl(CH.sub.3).sub.2 Si).sub.2, (Cl.sub.2 C.sub.2 
H.sub.5 Si).sub.2, (Cl(C.sub.6 H.sub.5).sub.2 Si).sub.2 and (Cl.sub.2 
(CH.sub.2 .dbd.CH)Si).sub.2. Monosilanes can also be used in admixtures 
with the above described chlorine-containing disilanes. Examples include 
CH.sub.3 SiCl.sub.3, (CH.sub.3).sub.2 SiCl.sub.2, H(CH.sub.3).sub.2 SiCl, 
(CH.sub.3).sub.3 SiCl, (CH.sub.2 .dbd.CH)(CH.sub.3).sub.2 SiCl, (C.sub.2 
H.sub.5).sub.2 SiCl.sub.2, C.sub.6 H.sub.5 SiCl.sub.3, as well as (C.sub.6 
H.sub.5).sub.2 SiCl.sub.2, and (C.sub.6 H.sub.5).sub.3 SiCl. When 
polysilazane polymers are prepared in accordance with U.S. Pat. No. 
4,340,619 for use in this invention it is preferred that mixtures of 
chlorine-containing disilanes be employed where the number of units of 
diorgano-substituted silicon atoms does not exceed the number of units of 
monoorgano-substituted silicon atoms. 
The preceramic silazane polymers of application Ser. No. 555,755 are 
prepared by contacting and reacting in an inert, essentially anhydrous 
atmosphere, trichlorosilane with a disilazane at a temperature in the 
range of 25.degree. to 300.degree. C. while removing byproduced volatile 
products, wherein said disilazane has the general formula 
EQU (R.sub.3 Si).sub.2 NH 
where R is selected from the group consisting of vinyl, hydrogen, phenyl, 
and alkyl radicals containing 1 to 3 carbon atoms. It appears that some 
component, possibly a hydrolysis product, in aged trichlorosilane is 
detrimental in the preparation of this preceramic silazane polymer. Such 
contaminated trichlorosilanes can be suitably purified by distillation. 
Other purification methods may also be employed. It is also preferred that 
the reactants be added in such a manner that the initial reaction exotherm 
is kept to a minimum. One reactant may be added slowly to the other 
reactant, or the added reactant may be cooled, or the reaction vessel may 
be cooled to keep the reaction exotherm low. Other methods or combination 
of methods may also be used. In general, it is preferred that the reaction 
be controlled such that the initial reaction temperature due to the 
exotherm is less than about 50.degree. C., and most preferably, less than 
35.degree. C. In general, more reproducible results are obtained when 
purified trichlorosilane is used and when the initial reaction exotherm is 
controlled carefully. 
The second reactant in U.S. Pat. Nos. 4,312,970, 4,340,619, and application 
Ser. No. 555,755 is a disilazane of the general formula (R.sub.3 Si).sub.2 
NH. R in this formula is vinyl, hydrogen, an alkyl radical of 1-3 carbon 
atoms or the phenyl group. Therefore, R, for purposes of this formula, is 
represented by hydrogen, methyl, ethyl, propyl, vinyl and phenyl. Each R 
group in this formula can be the same or they can be different. Examples 
of the disilazanes include ((CH.sub.3).sub.3 Si).sub.2 NH, (C.sub.6 
H.sub.5 (CH.sub.3).sub.2 Si).sub.2 NH, ((C.sub.6 H.sub.5).sub.2 CH.sub.3 
Si).sub.2 NH, (CH.sub.2 .dbd.CH(CH.sub.3).sub.2 Si).sub.2 NH, (CH.sub.2 
.dbd.CH(CH.sub.3)C.sub.6 H.sub.5 Si).sub.2 NH, (CH.sub.2 .dbd.CH(C.sub.6 
H.sub.5).sub.2 Si.sub.2 NH, (CH.sub.2 .dbd.CH(C.sub.2 H.sub.5).sub.2 
Si).sub.2 NH, (H(CH.sub.3).sub.2 Si).sub.2 NH and (CH.sub.2 
.dbd.CH(C.sub.6 H.sub.5)C.sub.2 H.sub.5 Si).sub.2 NH. 
The reactants in U.S. Pat. Nos. 4,312,970, 4,340,619, and application Ser. 
No. 555,755 are brought together in an inert, essentially anhydrous 
atmosphere. By "inert" we mean that the reaction is carried out under a 
blanket of inert gas, such as argon, nitrogen, or helium. What we mean by 
"essentially anhydrous" is that the reaction is preferably carried out in 
an absolutely anhydrous atmosphere but minute amounts of moisture can be 
tolerated. 
When the reactants are contacted with each other, as described in U.S. Pat. 
Nos. 4,312,970 and 4,340,619 and application Ser. No. 555,755 the reaction 
begins which forms an intermediate amino compound. Upon heating, 
additional amino compound is formed and upon continued heating, R.sub.3 
SiCl is distilled from the reaction mixture and a silazane polymer is 
formed. The order of addition of the materials does not appear to be 
critical. As the temperature is raised higher, more condensation takes 
place and crosslinking occurs with residual R.sub.3 Si-- that is not 
distilled from the mixture acting as a chain stopper. This control allows 
one to stop the reaction at any point to obtain almost any desired 
viscosity. The desirable temperature range for this reaction is 25.degree. 
to 300.degree. C. A preferred temperature range for this reaction is 
125.degree. to 300.degree. C. The length of time that the reaction 
requires depends on the temperature employed and the viscosity one wishes 
to achieve. What is meant by "volatile products" are the distillable 
byproduced products that are formed by the reactions set forth above. 
These materials can be represented by (CH.sub.3).sub.3 SiCl, (CH.sub.2 
.dbd.CH)(C.sub.6 H.sub.5).sub.2 SiCl, CH.sub.3 (C.sub.6 H.sub.5).sub.2 
SiCl, (CH.sub.3).sub.2 C.sub.6 H.sub.5 SiCl and (CH.sub.2 
.dbd.CH)(CH.sub.3).sub.2 SiCl. Sometimes, the process requires the use of 
a vacuum along with the heat in order to remove these materials from the 
reaction mixture. 
After the preceramic silazane polymer has been rendered infusible by 
treatment with steam or a steam and oxygen mixture, the infusible 
preceramic silazane polymer is fired to an elevated temperature of at 
least 750.degree. C. in an inert atmosphere, vacuum or ammonia-containing 
atmosphere until the mixture is converted to a ceramic material. 
Preferably the pyrolysis temperature is from about 1000.degree. C. to 
about 1600.degree. C. Since the preceramic silazane polymers of this 
invention have been rendered infusible prior to pyrolysis, the pyrolysis 
step may be carried out by quickly raising the temperature to the desired 
level. If the preceramic silazane polymer is of sufficient viscosity or if 
it possesses a sufficiently low melt temperature, it can be shaped first, 
then rendered infusible, and then finally pyrolyzed to give a ceramic 
shaped article such as a fiber. Preferably the preceramic silazane 
polymers used in the practice of this invention have a penetration 
temperature of about 50.degree. to 300.degree. C. and most preferably in 
the range of 70.degree. to 200.degree. C. Such a penetration temperature 
allows for the formation of preceramic silazane fibers by known spinning 
techniques. 
So that those skilled in the art can better appreciate and understand the 
invention, the following examples are given. Unless otherwise indicated, 
all percentages are by weight. The preceramic polymers were fired to 
elevated temperature using either an Astro Industries Furnace 1000A water 
cooled graphite resistance heated model 1000.3060-FP-12, a Lindberg tube 
furnace (Heavy Duty SB Type S4877A) or a Thermolyne F-21100 tube furnace. 
Oxygen was determined using LECO analysis. 
The penetration temperature of the preceramic silazane polymer was measured 
with a DuPont Instruments Thermoanalyzer Model 1090 equipped with a Model 
1091 DuPont Disk Memory and a DuPont Model 943 Thermomechanical Analyzer. 
The penetration temperature is related to the softening point temperature. 
The tensile strength and elastic modulus were determined on a single fiber 
employing a computer controlled Instron tester Model 1122 equipped with 
pneumatic jaws and a 500 g load cell. Values reported are an average of 
ten individual tests. The procedure used was similar to ASTM 3379-75.

EXAMPLE 1 
A preceramic silazane polymer, labeled polymer A, was prepared by reacting 
a mixture of disilanes obtained from the direct process and 
phenylvinyldichlorosilane with hexamethyldisilazane. The mixture of 
disilanes and phenylvinyldiclorosilane contained 49.1 weight percent 
Cl.sub.2 CH.sub.3 SiSiCH.sub.3 Cl.sub.2, 28.2 weight percent Cl.sub.2 
CH.sub.3 SiSi(CH.sub.3).sub.2 Cl, 6.4 weight percent Cl(CH.sub.3).sub.2 
SiSi(CH.sub.3).sub.2 Cl, 0.7 weight percent low boiling impurities, and 
15.4 weight percent (C.sub.6 H.sub.5)(CH.sub.2 .dbd.CH)SiCl.sub.2. The 
hexamethyldisilazane was added to the disilane mixture at a level 
equivalent to 0.75 moles of N--H present in the hexamethyldisilazane per 
mole of Si--Cl present in the disilane and silane mixture. The resulting 
mixture was heated at a rate of 1.1.degree. C./min. to 230.degree. C. All 
reaction steps were carried out under a nitrogen atmosphere. Volatiles 
were removed by distillation throughout the heating process. The resulting 
silazane polymer A had a penetration temperature of about 70.degree. C. 
Preceramic fibers were prepared by standard spinning techniques at 
144.degree. C. by extrusion through a spinneret with 0.01 inch diameter 
holes. The preceramic silazane fibers prepared from polymer A were 
approximately 30 microns in diameter. 
Another preceramic silazane polymer, labeled polymer B, was prepared in a 
similar manner except as noted. The mixture of disilanes and 
phenylvinyldiclorosilane contained 49.8 weight percent Cl.sub.2 CH.sub.3 
SiSiCH.sub.3 Cl.sub.2, 27.7 weight percent Cl.sub.2 CH.sub.3 
SiSi(CH.sub.3).sub.2 Cl, 6.7 weight percent Cl(CH.sub.3).sub.2 
SiSi(CH.sub.3).sub.2 Cl, 1.0 weight percent low boiling impurities, and 
14.7 weight percent (C.sub.6 H.sub.5)(CH.sub.2 .dbd.CH)SiCl.sub.2. The 
resulting mixture was heated at a rate of 1.1.degree. C./min. to 
230.degree. C. and held at 230.degree. C. for 30 minutes. The resulting 
polymer B had a penetration temperature of about 136.degree. C. Preceramic 
fibers were prepared by standard spinning techniques at 219.degree. C. The 
preceramic silazane fibers prepared from polymer B were approximately 15 
microns in diameter. 
A small sample (0.1-0.5 g) of fibers was placed in a glass tube inserted in 
a tube furnace. The fibers were cured or rendered infusible by exposure to 
humidified air at various temperatures for varying lengths of time. The 
humidified air was prepared by bubbling air through liquid water at room 
temperature before passage over the fibers. The humidified air was at 
about 100 percent relative humidity at room temperature. After treatment 
with humidified air the fibers were pyrolyzed in an argon atmosphere by 
heating the fibers at a rate of 3.degree. C./min. to 1200.degree. C. The 
results are presented in Table I. For runs where only one temperature is 
given, the fibers were exposed to the humidified air at a constant 
temperature for the listed amount of time. Where a temperature range is 
given, the fibers were exposed to the humidified air by heating from the 
lower to higher temperature at a rate of 3.75.degree. C./min and then 
holding at the higher temperature for the remainder of the specified time. 
None of the fibers treated with humidified air under the conditions 
indicated in Table I melted or fused together upon pyrolysis to 
1200.degree. C. Preceramic fibers which were not cured or rendered 
infusible did fuse together upon pyrolysis to 1200.degree. C. 
TABLE I 
______________________________________ 
CERAMIC FIBERS 
POLY- CURE CONDITIONS OXYGEN CERAMIC 
MER TEMP (.degree.C.) 
TIME (hrs) (%) YIELD (%) 
______________________________________ 
A 65 17.0 10.8 74.3 
B 65-150 22.5 24.0 74.0 
B 65-175 31.0 27.0 75.0 
B 65-200 36.0 30.0 77.0 
______________________________________ 
EXAMPLE 2 
Another preceramic silazane polymer was prepared using the procedures 
outlined in U.S. patent application Ser. No. 555,755 (filed Nov. 28, 
1983). The preceramic polymer was prepared by mixing one equivalent of 
trichlorosilane with 2.25 equivalents of hexamethyldisilazane at 
0.degree.-15.degree. C. The mixture was held at room temperature overnight 
and then heated at a rate of 1.degree. C./min. to 250.degree. C. The 
reaction mixture was held at 250.degree. C. for one hour. All reaction 
steps were carried out under an argon atmosphere. During the heating 
period, volatiles were removed by distillation. The preceramic polymer was 
cooled, dissolved in toluene, filtered through a 0.45 micron membrane, and 
then strip distilled at 250.degree. C. under vacuum in a molecular still. 
The resulting preceramic polymer had a penetration temperature of 
76.degree. C. The preceramic polymer was spun into fibers using a melt 
rheometer with a single 0.02 inch orifice at a temperature of 154.degree. 
C. The preceramic fibers had diameters of 40-50 microns. 
One sample of these preceramic fibers was fired in a 100 volume percent 
ammonia atmosphere. The fibers were not cured prior to the ammonia 
pyrolysis. After the pyrolysis to 1200.degree. C. the fiber had melted and 
fused together. 
Another sample of these preceramic fibers was cured by exposure to 
humidified air. The fiber were exposed to humidified air (about 8 cfh flow 
and 100 percent relative humidity at room temperature) where the 
temperature was raised from 35.degree. to 165.degree. C. at a rate of 
2.7.degree. C./hr. 
Both cured and uncured fiber samples were pyrolyzed to 1200.degree. C. in a 
100 volume percent ammonia atmosphere. Samples with no cure melted and 
fused during pyrolysis to 1200.degree. C. Cured fibers did not melt or 
fuse when pyrolyzed to 1200.degree. C. The results are given in Table II. 
Uncured preceramic fibers gave a white, fused, ceramic mass with a ceramic 
yield of 50.7 weight percent. Cured preceramic fibers gave white ceramic 
fibers with a ceramic yield of 63.6 weight percent which contained 31.4 
weight percent nitrogen and about 0.1 weight percent carbon and which had 
a tensile strength of 26 MPa and an elastic modulus of 6.9 GPa. 
EXAMPLE 3 
Preceramic silazane fibers similar to those used in Example 2 were used in 
this example. The fibers were treated with humidified air containing 
approximately 100 percent relative humidity at room temperature using the 
procedure of Example 1. The cured fibers were then pyrolyzed in an argon 
atmosphere to 1200.degree. C. as in Example 1. The results are presented 
in Table II. None of the fibers treated with humidified air under the 
conditions indicated in Table II melted or fused together upon pyrolysis 
to 1200.degree. C. Preceramic fibers which were not cured or rendered 
infusible did fuse together upon pyrolysis to 1200.degree. C. 
TABLE II 
______________________________________ 
CERAMIC FIBERS 
CURE CONDITIONS CERAMIC 
TEMP (.degree.C.) 
TIME (hrs) OXYGEN (%) YIELD (%) 
______________________________________ 
35-100 48 3.0 58.6 
35-120 22 1.2 -- 
35-155 32 2.1 -- 
35-165 48 17.8 64.0 
35-211 47 10.9 -- 
______________________________________ 
EXAMPLE 4 
A preceramic silazane polymer was prepared by reacting a mixture of 
disilanes obtained from the direct process, methylvinyldichlorosilane, and 
methylhydrogendichlorosilane with hexamethyldisilazane. The mixture of 
disilanes, methylvinyldichlorosilane, and methylhydrogendichlorsilane 
contained 50.0 weight percent Cl.sub.2 CH.sub.3 SiSiCH.sub.3 Cl.sub.2, 
36.6 weight percent Cl.sub.2 CH.sub.3 SiSi(CH.sub.3).sub.2 Cl, 2.9 weight 
percent Cl(CH.sub.3).sub.2 SiSi(CH.sub.3).sub.2 Cl, 5.8 weight percent 
(CH.sub.3)(CH.sub.2 .dbd.CH)SiCl.sub.2, and 4.7 weight percent CH.sub.3 
(H)SiCl.sub.2. The hexamethyldisilazane was added to the disilane and 
silane mixture at a level equivalent to 0.75 moles of N--H present in the 
hexamethyldisilazane per mole of Si--Cl present in the disilane and silane 
mixture. The resulting mixture was heated at a rate of 2.5.degree. C./min. 
to 80.degree. C. and held at 80.degree. C. for 20 minutes. The temperature 
was then raised to 220.degree. C. at a rate of 1.0.degree. C./min. and 
held at 220.degree. C. for 10 minutes. All reaction steps were carried out 
under an argon atmosphere. Volatiles were removed throughout the heating 
process. This preceramic silazane polymer had a penetration temperature of 
112.degree. C. Fibers were prepared from this material with an average 
diameter of about 40 microns. 
The preceramic silazane fibers were exposed to steam or steam and oxygen 
atmosphere in the same tube furnace used for the later pyrolysis step. The 
preceramic fibers were placed in a boat in the center of the tube furnace. 
The system was first flushed with argon. Then the desired gas was passed 
over the sample boat at the desired temperature. Steam was generated by 
passage of the desired gas over liquid water prior to entering the tube 
furnace. The amount of steam in the gas phase was controlled by varying 
the temperature of the liquid water. The temperature of the liquid water 
and the preceramic material being treated were controlled independently. 
After completion of the steam or steam and oxygen treatment, the system 
was purged with argon and the temperature of the tube furnace increased to 
the desired pyrolysis temperature under an argon atmosphere. 
Two techniques were employed to gauge the effectiveness of the steam or 
steam and oxygen mixture treatment step in rendering the preceramic 
silazane fibers infusible. A simple solubility test was used to estimate 
the effectiveness of the treatment step of this invention. A small amount 
of the fibers was placed in toluene at room temperature and the degree of 
solubility was observed. Uncured material was essentially completely 
soluble in toluene whereas material which had been rendered infusible was 
essentially insoluble in toluene. The second method to evaluate the degree 
of cure or degree to which the preceramic silazane polymer had been 
rendered infusible was actual pyrolysis. Pyrolysis is a direct test to 
measure the effectiveness of the treatment. If the fibers do not fuse 
together, the treatment was sufficient to render the material or fibers 
infusible. Two basic pyrolysis schedules were employed. With "slow 
pyrolysis", preceramic fibers were heated at 5.degree. to 13.degree. 
C./min. to 1200.degree. C. in argon. With "fast pyrolysis", preceramic 
fibers were heated at 100.degree. C./min. to 600.degree. C. in argon. 
Fibers which did not fuse during the pyrolysis treatment were considered 
to have been rendered infusible by the steam or steam and oxygen mixture 
treatment and were rated "pass". Fibers which were uncured failed both the 
slow and fast pyrolysis tests. The results are presented in Table III. 
TABLE III 
______________________________________ 
SOLU- 
CURE CONDITIONS BIL- PYROLYSIS 
GAS TEMP (.degree.C.) 
TIME (min.) 
ITY SLOW FAST 
______________________________________ 
O.sub.2 /H.sub.2 O 
152 1 slightly 
pass pass 
soluble 
O.sub.2 /H.sub.2 O 
155 2 very pass pass 
slightly 
soluble 
O.sub.2 /H.sub.2 O 
160 3 in- pass pass 
soluble 
argon/ 158 1 mostly 
pass pass 
H.sub.2 O in- 
soluble 
______________________________________