Coating of preceramic articles with silicon and/or carbon during pyrolysis to ceramic

A process for the preparation of carbon and/or silicon coated ceramic fibers is disclosed involving contact of a preceramic fiber with a hydrocarbon or a silane which decomposes at a temperature no higher than 500.degree. C. at 1 atmosphere.

FIELD OF THE INVENTION 
This invention relates to the production of coated ceramic-shaped articles 
from organosilicon polymers. 
More particularly, the invention relates to imparting a coating of carbon 
and/or silicon to preceramic-shaped articles such as fibers while 
pyrolyzing the same to ceramics. 
BACKGROUND OF THE INVENTION 
Ceramic materials are of critical importance for a number of high 
temperature, high performance applications such as gas turbines. These 
applications require a unique combination of properties such as high 
specific strength, high temperature mechanical property retention, low 
thermal and electrical conductivity, hardness and wear resistance, and 
chemical inertness. Design reliability and the need for economical 
fabrication of complex shapes, however, have prevented ceramic materials 
from fulfilling their potential in these critical high temperature, high 
performance applications. 
The design reliability problems with ceramics, and the resultant failure 
under stress, are due largely to the relatively brittle nature of 
ceramics. This, in combination with the high cost of fabricating complex 
shapes, has limited the usage of ceramics. 
Ceramics made from organosilicon polymers have the potential to overcome 
these problems. To this end, polymers based on silicon, carbon and/or 
nitrogen and oxygen have been developed. See, e.g., "Siloxanes, Silanes 
and Silazanes in the Preparation of Ceramics and Glasses" by Wills, et 
al., and "Special Heat-Resisting Materials from Organometallic Polymers" 
by Yajima, in Ceramic Bulletin, Vol. 62, No. 8, pp. 893-915 (1983), and 
the references cited therein. 
The major and most critical application for ceramics based on polymer 
processing is high strength, high modulus, shaped articles such as fibers. 
Such fibers are spun from organosilicon preceramic polymers, and then 
cured and pyrolyzed to their ceramic form. The low molecular weight and 
highly branched structure of typical preceramic polymers, however, alters 
the spinning and subsequent fiber handling behavior of these polymers from 
that of conventional polymers. In particular, gelation and foaming 
tendencies in the melted polymers used for melt spinning may lead to the 
presence of undesirable flaws in the resulting fiber. Such flaws are 
undesirable in fine diameter fibers since they are believed to be the 
source of cracking and lowered tensile strength. Furthermore, because of 
the low molecular weight of the preceramic polymers used, the fibers spun 
therefrom have relatively low tensile strength and are difficult to handle 
in spinning, curing, and subsequent pyrolysis operations. 
It is known that ceramic fibers can be improved if compounded or admixed 
with certain materials which enhance their physical properties such as 
carbon or silicon. Materials of this type which are physical mixtures of 
preceramic fibers with various fillers are disclosed in U.S. Pat. Nos. 
4,340,619, 4,404,153, 4,482,689 and 4,460,640, the disclosure of said 
patents being incorporated by reference. 
The process of the instant invention differs from said composite materials 
and prior art processes for their preparation in that a carbon and/or 
silicon compound is introduced during the pyrolysis step thereby 
converting the polymer to a ceramic which has a carbon and/or silicon 
coating. 
SUMMARY OF THE INVENTION 
Accordingly, a primary object of the present invention is to provide an 
improved process for the production of coated ceramic fibers from 
organosilicon preceramic polymers involving the use of carbon and/or 
silicon compounds during pyrolysis. 
Another object of the present invention is to provide an improved process 
for the production of organosilicon ceramic fibers having high tensile 
strength. 
Another object of the present invention is to provide an improved process 
for the production of ceramic fibers based upon organosilicon preceramic 
polymers, which fibers have improved handleability, e.g., increased 
toughness and protection of the organosilicon preceramic material from 
abrasion and the atmosphere. 
These and other objects, aspects and advantages, as well as the scope, 
nature and utility of the present invention, will be apparent from the 
following description and appended claims. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The Organosilicon Preceramic Polymers 
Organosilicon preceramic polymers are well known in the art. Such polymers 
contain silicon, carbon and/or nitrogen, and are fiber-forming, and can be 
cured and pyrolyzed to ceramic form. See, e.g., U.S. Pat. Nos. 4,310,651; 
4,312,970; 4,342,712; 4,482,689; and 4,340,619; which are incorporated 
herein by reference. 
These organosilicon precursor polymers may be made in a variety of ways as 
is known in the art. For example, they may be made by first dechlorinating 
an alkylchlorosilane, e.g., dimethyldichlorosilane, and polymerizing the 
product to form a polysilane, e.g., polydimethylsilane. This material is 
then heated to convert its backbone of silicon atoms to a backbone of 
alternating silicon and carbon atoms by forming a polycarbosilane. 
Preferably, the organosilicon preceramic polymers utilized in the present 
invention consist essentially of silicon, carbon, nitrogen and oxygen. 
Such polymers are typically prepared by reacting a disilazane and a 
dichlorodisilane or methylchlorodisilane. 
Most preferably, the organosilicon preceramic polymers of the present 
invention are characterized as polysilazanes prepared from 
methylchlorodisilanes and hexamethyldisilazane. Particularly preferred are 
the polysilazanes, containing N--Si--Si--N linkages. Optionally, the 
addition of difunctional monosilanes as co-reactants may be used to 
enhance spinning and/or subsequent fiber handling properties. Such 
difunctional monosilanes include preferably R.sub.1 R.sub.2 SiCl.sub.2, 
where R.sub.1 and R.sub.2 may independently be a methyl, ethyl, phenyl or 
vinyl group. 
Such organosilicon preceramic polymers may be further modified, for 
example, by incorporating vinyl functionality by reacting with the polymer 
itself. This may be achieved, for example, by co-reacting the polymer with 
a vinyl (Vi) halosilane such ViR.sub.1 R.sub.2 SiCl, where R.sub.1 and 
R.sub.2 may each independently be methyl or phenyl. 
Another preferred type of organosilicon polymer which is thermally 
sensitive and which may be especially suitable in the present invention 
comprises a plurality of cyclic and/or linear precursor residues of the 
repeating units of formula I: 
##STR1## 
linked together by Si.sub.2 W.sub.2 bridges of formula II, 
##STR2## 
wherein R is hydrogen, a lower alkyl group having from 1 to about 6 carbon 
atoms, a substituted or unsubstituted vinyl group, a substituted or 
unsubstituted alkyl group, a substituted or unsubstituted benzyl group, a 
substituted or unsubstituted lower aryl group having from 6 to about 10 
carbon atoms, a tri(lower)alkyl or di(lower)alkylsilyl group, a 
di(lower)alkylamino group, a lower alkoxy group having from 1 to about 6 
carbon atoms and n is an integer greater than one. The substituted groups 
are substituted with lower alkyl and lower aryl groups. 
These polymers form novel ladder-like or planar array structures that are 
soluble in common organic solvents, stable at room temperature and 
thermally stable up to about 200.degree. C. The ladder-like or planar 
array polymers of the present invention are formed in high yield by 
reaction of the cyclic and/or linear residues in the presence of a basic 
catalyst capable of deprotonating an NH function adjacent to silicon. 
These polymers and their preparation are described more fully in U.S. Pat. 
No. 4,482,669, to Seyferth et al., assigned on its face to Massachusetts 
Institute of Technology, which patent is herein incorporated by reference. 
Molecular weight (M.sub.n) for the above organosilicon preceramic polymers 
may vary from about 500 to 20,000, but typically 2,000 (g/mole-GPC 
method); higher molecular weight polymers are preferred. These polymers 
may also have softening temperatures (T.sub.s) of about 50.degree. C. to 
308.degree. C., preferably about 60.degree. C. to 200.degree. C., and most 
preferably 70.degree. C. to 150.degree. C. 
Spinning of the Fibers 
As indicated earlier, the above-described organosilicon preceramic polymers 
are dry spun, melt spun or extruded as fibers or filaments. 
To melt spin, the solid organosilicon polymer is melted at a temperature 
and rate sufficient to avoid gelation and foaming, and substantially 
immediately thereafter the melted polymer is preferably spun or extruded 
to form optically flaw-free, fine diameter organosilicon preceramic fiber. 
Prior to spinning, any gel detected in the polymer blend should be removed 
such as by filtration. In addition, the polymer should be essentially free 
of other contaminants such as small insoluble particulates or bubbles. 
The relatively short residence time of the polymer in the melt is critical 
to achieving optically flaw-free, fine diameter fibers. If an 
organosilicon polymer is brought up to a melt-processable or extrudable 
temperature and held at such temperatures for too long a period time, 
gelation will occur, which in turn will lead to the presence of noticeable 
flaws in the fiber and a concomitant loss of tensile properties. In 
addition, the melt temperature of the organosilicon polymer should be less 
than that needed to cause foaming of the polymer, which foaming will also 
lead to the presence of voids or flaws in the fiber. The typical 
organosilicon polymer is significantly more melt sensitive as compared to 
other conventional fiber-forming polymers, e.g., polyethylene 
terephthalate. 
The actual melt temperatures may vary, but will generally be above the 
softening temperature (T.sub.s) of the organosilicon polymer, but below 
that at which foaming, gelation or other degradation occurs within the 
total melt residence time. Typically, such melt temperature will range 
from about 30.degree. C. to 130.degree. C., and most preferably 60.degree. 
C. to 80.degree. C. above the T.sub.s of the polymer blend. 
As the preceramic fibers are melt spun or extruded, fiber handling is 
preferably minimized to avoid abrasion of the fibers sufficient to cause 
fiber breakage during fiber take-up and/or sufficient to induce latent 
stressing sufficient to cause fiber breakage during subsequent curing and 
pyrolysis to ceramics. Thus, those type of conventional fiber take-up 
apparatus which induce high levels of stress in fiber are preferably not 
used. The preceramic fibers as spun are relatively brittle due to their 
relatively low molecular weight as compared to conventional fiber-forming 
polymers. 
The preceramic fibers as spun may be taken up in any appropriate take-up 
speed. Take-up speed of up to about 1400 meters/minute, typically 100 to 
1000, and preferably 300 to 800, may be used. 
To dry spin, the solid organosilicon polymer is dissolved in a solvent at a 
relatively high polymer solids concentration, and thereafter the polymer 
is spun or extruded to form flaw-free organosilicon preceramic fiber. 
Any solvent in which the organosilicon polymer may be dissolved at the 
relatively high solids concentration may be used. Suitable aliphatic 
hydrocarbon solvents may include those having from 1 to 8 carbon atoms and 
having boiling points ranging from about 0.degree. C. to about 190.degree. 
C. Typical aliphatic hydrocarbon solvents include n-hexane, cyclo-hexane, 
cyclo-hexene, n-pentane, cyclopentadiene, iso-octane, acetonitrile, 
dichloroethane, trichloroethane, hexachloroethane, chloroform, 
methylchloroform, methylene chloride, methyl acetate, ethyl acetate, 
carbon tetrachloride, and tetrahydrofuran. Suitable aromatic hydrocarbon 
solvents may include those having from 6 to 10 carbon atoms and have 
boiling points ranging from about 70.degree. C. to 180.degree. C. Typical 
aromatic hydrocarbon solvents include toluene, xylene, styrene, benzene, 
chlorobenzene, dichlorobenzene, ethylbenzene, and isopropylbenzene. 
Toluene and xylene are particularly preferred. 
Prior to spinning, any gel detected in the polymer should be removed such 
as by filtration. In addition, the polymer should be essentially free of 
other contaminants such as small insoluble particulates. 
As indicated above, the relatively high solids concentration of the polymer 
in the spinning solution is critical to achieving a self-supporting 
threadline with these low-molecular weight polymers. If insufficiently 
high and organosilicon polymer solids concentrations are used, threadline 
breakdown will frequently occur. Preferably, polymer solids concentrations 
of at least about 70 percent, and most preferably at least about eighty 
percent are used. 
After dissolution and prior to spinning, the polymer should be maintained 
in solution in an essentially gel-free state, i.e., kept under conditions 
insufficient to cause gel formation of polymer in the solvent. 
The actual solution temperatures at spinning may vary, but will generally 
be near the boiling point of the solvent (to improve solvent evaporation) 
but below that at which foaming, gelation or other degradation occurs 
within the total dry spinning residence time. Typically, at spinning such 
solution temperatures will be between about 70.degree. C. and 250.degree. 
C., preferably 70.degree. C. to 200.degree. C. and most preferably 
90.degree. C. to 160.degree. C. 
As the preceramic fibers are dry spun and solvent-extracted, fiber handling 
is preferably minimized to avoid abrasion of the fibers sufficient to 
cause fiber breakage during fiber take-up and/or sufficient to induce 
latent stressing sufficient to cause fiber breakage during subsequent 
curing and pyrolysis. Thus, those types of conventional fiber take-up 
apparatus which induce high levels of stress in fiber are preferably not 
used. The preceramic fibers during and after spinning and solvent 
extraction are relatively brittle due to their relatively low molecular 
weight as compared to conventional fiber-forming polymers. 
Curing of the Spun Fibers 
The novel process of this invention includes curing the preceramic fibers 
as spun by contacting the same with gaseous hydrogen halide which is 
preferably admixed with an inert gas such as nitrogen and argon, etc. The 
curing of the preceramic polymer can take place during either of two 
stages. It can be cured in a batch manner after it has been formed into a 
desired shape, e.g. fibers can be made and cured after they have been 
placed on a take-up reel. The most preferred method of cure, however, is 
to treat the fiber immediately after it is spun or before it is placed on 
the conventional take-up reel. 
It is also possible to use curing agents other than hydrogen halide and 
typical prior art curing agents are disclosed in U.S. Pat. Nos. 3,853,567, 
4,535,007 and 4,399,232, the disclosures of which are incorporated by 
reference. The temperature employed for curing can range from room 
temperature up to the glass transition temperature of the particular 
polymer. It is preferred to operate at temperature at least 20.degree. C. 
below said glass transition temperature. 
Following the above-described curing, the preceramic polymers are ready to 
be coated with carbon and/or silicon in accordance with the novel process 
of this invention. 
This is accomplished by contacting the preceramic polymers with a 
carbon-containing compound or a silicon-containing compound during 
pyrolysis. 
The expression "carbon-containing compound" is intended to mean a 
hydrocarbon which decomposes at no higher than 500.degree. C. at 1 
atmosphere of pressure. The most preferred hydrocarbons are acetylene, 
propylene and isoprene, although any aromatic, aliphatic cycloaliphatic or 
olefinic hydrocarbon meeting the above criteria can be used. 
The expression "silicon-containing compound" is intended to mean any 
compound of the formula R.sub.4 Si, wherein each R is hydrogen, C.sub.1 
-C.sub.6 alkyl, phenyl or vinyl providing that said compound decomposes at 
a temperature of at least 500.degree. C. and 1 atmosphere. The most 
preferred component is silane. 
The treatment is usually carried out in an inert atmosphere of nitrogen, 
argon or the like at atmospheric pressure. Pyrolysis temperatures may be 
from 500.degree.-2000.degree. C., preferably from 600.degree. C. to 
1600.degree. C., and most preferably from 1100.degree. C. to 1400.degree. 
C. 
The amount of silicon of carbon-containing compound is not critical and 
enough is used to get the desired coating on the ceramic fiber.

EXAMPLE 1 
An organosilicon preceramic polymer is prepared according to the general 
procedure of Example 1 of U.S. Pat. No. 4,340,619 and is introduced into a 
melt extruder after filtration. The polymer is passed through a spinneret 
at a temperature of 180.degree. C. and immediately passed through a curing 
chamber into which a gas mixture of hydrogen chloride and nitrogen is 
continuously flowed at about 150 ml/min. and is then collected on a 
take-up reel. 
Fibers were removed from said take-up reel and placed in an alumina boat. 
A control experiment was carried out wherein said fibers were subjected to 
the following heat treatment in argon flowing at 100 cc/min. 
______________________________________ 
Temperature Time 
______________________________________ 
RT-500.degree. C. 1 hr. 
500-1200.degree. C. 2 hrs. 
Cool to RT (room temperature) 
______________________________________ 
A second experiment was conducted wherein acetylene at flow rates of 15 
mL/min. and 5 mL/min. was introduced at the RT-500.degree. C. stage, the 
rest of the heat treatment being identical. 
The physical properties of the resulting ceramic fiber were as follows: 
______________________________________ 
Tensile Strength 
Elastic Modulus 
Fiber Diameter 
Sample Ksi.sup.1 Msi.sup.2 (micrometers) 
______________________________________ 
Control 
37 11 19 
Acetylene 
96 12 21.5 
______________________________________ 
.sup.1 1,000 lbs/sq. in. 
.sup.2 1,000,000 lbs/sq. in. 
The physical properties of the fibers coated at the two different flow 
rates of acetylene were identical although the thickness of the carbon 
coating was different at 0.7 micrometers for the 15 mL/min. and 0.3 
micrometers for the 5 mL/min. 
EXAMPLE 2 
The control of Example 1 (after 1200.degree. C. heat treatment) was coated 
with acetylene to produce a carbon coating of 0.7 micrometers. Its 
physical properties were as follows: 
______________________________________ 
Tensile Strength 
Elastic Modulus 
Fiber Diameter 
Ksi.sup.1 Msi.sup.2 (micrometers) 
______________________________________ 
68 17 20 
______________________________________ 
.sup.1 1,000 lbs/sq. in. 
.sup.2 1,000,000 lbs/sq. in. 
Thus it can be seen that there is a dramatic difference in strength when 
ceramic fibers are coated after pyrolysis (Example 2) than coating 
preceramic polymers during initial pyrolysis. 
EXAMPLE 3 
The procedure of Example 1 was repeated with the exception that 10% silane 
in argon was used instead of acetylene. A silicon metal coated ceramic was 
obtained.