Preceramic compositions and ceramic products

Preceramic polymer dispersions which have particular utility in providing protective ceramic coatings having low moisture sensitivity on carbon/carbon composites, graphite, carbon fibers, and other normally oxidizable materials are prepared by dispersing about 0-3 parts by weight of aluminum-silicon eutectic, about 0-4 parts by weight of silicon carbide, about 1.5-5 parts by weight of silicon boride, and about 0.4-5 parts by weight of silicon metal in a solution or dispesion obtained by dispersing about 0.1-1.0 parts by weight of a Group IIA metal salt in an organoborosilazane polymer obtained by reacting about 0.25-20 parts by weight of a trialkoxy-, triaryloxy-, or tri(arylalkoxy)boroxine with one part by weight of a polysilazane in an organic solvent and, if desired, heating the dispersion to convert it to a solution.

FIELD OF INVENTION 
This invention relates to ceramic materials derived from polysilazanes and 
more particularly to such materials which are useful in protecting 
substrates that are normally susceptible to oxidative deterioration. 
BACKGROUND 
It is known that many materials, such as carbon/carbon composites, carbon 
fibers, graphite, and certain metals, have properties which make them 
attractive for use in aerospace and other applications in which their 
susceptibility to oxidative deterioration at elevated temperatures is a 
serious disadvantage. It would be desirable to find a means of protecting 
those materials from oxidation at high temperatures, and it has been 
proposed to provide such protection with ceramic coatings. However, known 
ceramic coatings have proved to be inadequate. 
Copending applications Ser. No. 242,493 (Niebylski-I), filed Sept. 9, 1988, 
and Ser. No. 272,481 (Niebylski-II), filed Nov. 17, 1988, and now U.S. 
Pat. No. 4,921,925 teach organoborosilazane polymers which can be coated 
onto substrates and pyrolyzed to ceramics to provide improved protection 
from oxidative deterioration at elevated temperatures; and copending 
application Ser. No. 261,104 (Niebylski-III), filed Oct. 24, 1988, teaches 
that coating compositions comprising the organoborosilazane polymers can 
be improved by the incorporation therein of a mixture of aluminum-silicon 
eutectic, silicon carbide, silicon boride, and silicon metal. However, the 
utility of these compositions is limited by the moisture sensitivity of 
the ceramics obtained from them. 
THE INVENTION 
It has now been found that oxidizable substrates can be protected from 
oxidative deterioration at elevated temperatures with ceramics having low 
moisture sensitivity and derived from compositions obtained by dispersing 
(A) a homogenized mixture of about 0-3 parts by weight of aluminum- 
silicon eutectic, about 0-4 parts by weight of silicon carbide, about 
1.5-5 parts by weight of silicon boride, and about 0.4-5 parts by weight 
of silicon metal in (B) a solution or dispersion obtained by dispersing 
about 0.1-1.0 part by weight of a Group IIA metal salt in an 
organoborosilazane polymer solution obtained by reacting about 0.25-20 
parts by weight of a trailkoxy-, triaryloxy-, or tri(arylalkoxy)boroxine 
with one part by weight of a polysilazane in an organic solvent and, if 
desired, heating the dispersion to convert it to a solution. 
The organoborosilazane polymers employed in preparing the 
organometallosilazane polymers are those disclosed in Niebylski-I and 
Niebylski-II, i.e., polymers prepared by reacting about 0.25-20 parts by 
weight of a trialkoxy-, triaryloxy-, or tri(arylalkoxy)boroxine with one 
part by weight of polysilazane. 
The polysilazane which is reacted with the boroxine may be any polysilazane 
that is soluble in common organic solvents, such as aliphatic or aromatic 
hydrocarbons, dialkyl or alicyclic ethers, etc.; and it may be, e.g., a 
polysilazane of any of U.S. Pat. Nos. 4,397,828 (Seyferth et al.-I), 
4,482,669 (Seyferth et al.-II), 4,645,807 (Seyferth et al.-III), 4,650,837 
(Seyferth et al.-IV), and 4,659,850 (Arai et al.), the teachings of all of 
which are incorporated herein in toto by reference. However, it is 
preferably a polysilazane of the type tauqht by Seyferth et al.-II, i.e., 
a polysilazane prepared by reactinq an orqanodihalosilane with ammonia, 
treating the ammonolysis product with a basic catalyst which is capable of 
deprotonating an NH group that is adjacent to an SiH group, and quenching 
the resultant product with an electrophilic quenching reagent, a mixture 
of such polysilazanes, or, alternatively, an oligomeric ammonolysis 
product formed as an intermediate in the process of Seyferth et al.-II and 
isolated as in Seyferth et al.-I. For example, it may be one or more 
polysilazanes prepared by reacting methyldichlorosilane with ammonia, 
treating the ammonolysis product with potassium hydride, and quenching the 
resultant product with methyl iodide or dimethylchlorosilane; or it may be 
one or more polysilazanes prepared by reacting methyldichlorosilane with 
ammonia and isolating the ammonolysis product. 
The boroxine reactant used in preparing the organoborosilazane polymer is a 
compound corresponding to the formula: 
##STR1## 
wherein R is an alkoxy, aryloxy, or arylalkoxy group, preferably an 
alkoxy, phenoxy, alkylphenoxy, phenalkoxy, or alkylphenalkoxy group in 
which any alkyl or alkoxy group contains 1-6 carbons, such as the 
trimethoxy-, triethoxy-, tripropoxy-, tributoxy-, tripentoxy-, trihexoxy-, 
triphenoxy-, tritolyloxy-, tri(2-ethylphenoxy)-, tribenzyloxy-, 
triphenethoxy-, tri(3-phenylpropoxy)-, tri(4-phenylbutoxy)-, 
tri(5-phenylpentoxy)-, and tri(6-phenylhexoxy)boroxines, the corresponding 
triphenalkoxyboroxines having non-linear alkyl chains, 
tritolylethoxyboroxine, etc. It is preferably trimethoxyboroxine or 
triphenoxyboroxine. 
Regardless of the particular boroxine used, the amount employed is about 
0.25-20 parts per part by weight of the polysilazane. However, when the 
boroxine is a trialkoxyboroxine, it is generally preferred to use about 
1-6, most preferably about 3-4 parts per part by weight of polysilazane; 
and, when the boroxine is a triaryloxyboroxine, it is generally preferred 
to employ about 1-10, most preferably about 6-8 parts per part by weight 
of polysilazane. 
To prepare the organoborosilazane polymers, the neat boroxine reactant (if 
sufficiently low melting) or a solution thereof in an organic solvent is 
added to a solution of a polysilazane in an organic solvent to initiate an 
exothermic reaction which is preferably controlled to a temperature below 
50.C. for a period of time sufficient to allow the formation of an 
organoborosilazane polymer. In a preferred embodiment of the invention, 
the polysilazane is used as a clear solution having a solids content of 
about 10-40%, preferably about 30% by weight; and the total amount of 
solvent employed is such as to provide an organoborosilazane polymer 
solids content of about 5-75%, preferably about 30-60% by weight. 
The solvent employed for the polysilazane and optionally also the boroxine 
may be any suitable organic solvent, such as hexane, heptane, and other 
aliphatic hydrocarbons; benzene, toluene, xylene, and other aromatic 
hydrocarbons; cyclohexanone, 1-methyl-2-pyrrolidone, and other ketones; 
etc.; and mixtures thereof. 
The Group IIA metal salt which is mixed with the organoborosilazane polymer 
is a salt, such as a fluoride, tetrafluoroborate, oxide, oxyfluoride, 
oxynitride, acetate, benzoate, etc., of beryllium, magnesium, calcium, 
strontium, or barium. The calcium and barium salts are preferred, with the 
fluorides and tetrafluoroborates thereof being particularly preferred. 
As indicated above, the dispersions of the invention may be heated to 
convert them to solutions if desired. It is believed that thermal 
treatment of the dispersions causes the salt to react with the 
organoborosilazane polymer, although it is possible that heating merely 
solubilizes the salt. When solution formation is desired, it is generally 
accomplished by heatinq the dispersion at a temperature in the range of 
about 120-150.degree. C. for a suitable time, e.g., about 12-24 hours. 
The solids which are intimately mixed with the aforedescribed dispersions 
and solutions to form the dispersions of the invention are constituted by 
about 0-3 parts by weight of aluminum-silicon eutectic, about 0-4 parts by 
weight of silicon carbide, about 1.5-5 parts by weight of silicon boride, 
and about 0.4-5 parts by weight of silicon metal per part by weight of 
polysilazane employed in making the organoborosilazane polymer. The 
silicon carbide is preferably .alpha.-silicon carbide, the silicon boride 
may be silicon tetraboride and/or silicon hexaboride, and the silicon 
metal is preferably amorphous. 
In the preparation of the dispersions, it is preferred to premix the 
silicon boride, silicon metal, and any aluminum-silicon eutectic and/or 
silicon carbide, homogenize and dry them, and then intimately mix them 
with the Group IIA metal-organoborosilazane polymer solutions or 
dispersions. Generally, the Group IIA metal-organoborosilazane polymer 
solutions or dispersions are added to the homogenized solids, whether 
predispersed or not, and the resultant dispersions are agitated until they 
are uniform. 
When the homogenized solids are predispersed in an organic medium, the 
amount of organic medium used is generally such that the ultimate 
dispersion has a total solids content of about 5-75% by weight, preferably 
about 30-60% by weight, if the dispersions are to be used as coating 
and/or infiltration materials. 
The dispersions of the invention are preceramic materials which are useful 
for making ceramics, such as coatings, structural composites, etc.; and, 
like other preceramic materials, they may be used in combination with 
other ingredients, such as ceramic powders or whiskers, etc., when 
appropriate. 
An application in which they find particular utility is as coating 
compositions for normally oxidizable materials, especially those which 
need protection from oxidative deterioration at elevated temperatures. 
(Such materials include, e.g., fibers, tows, hanks, mats, and composites 
of carbon; carbon or graphite slabs, rods, and structures; and oxidizable 
metals, such as magnesium, aluminum, silicon, niobium, molybdenum, 
lanthanum, hafnium, tantalum, tungsten, titanium, and the metals of the 
lanthanide and actinide series.) When used in such an application in which 
the substrate is porous, the compositions also serve as infiltrants. 
In addition to providing protection from oxidative deterioration, the 
coating compositions can also serve to improve the physical properties and 
thermal stability of substrates, such as those mentioned above, silica 
foams, ceramic cloths (e.g., clothes formed from alumina, silica, and/or 
lithia), etc. 
The coating compositions are also useful for patching ceramic coatings 
formed from the same or different formulations. 
When the dispersions are to be used to provide protective ceramic coatings 
on substrates, the surfaces to be coated are usually cleaned prior to the 
application of the coating composition in order to improve the bonding of 
the ceramic coating to the substrate. The bonding can sometimes be further 
improved by pre-etching the surfaces to be coated. 
The coating compositions may be applied to the substrates in any suitable 
manner, such as by spraying, swabbing, or brushing, to form coatings 
having the desired thickness, generally a thickness of up to about 1000 
micrometers, frequently a thickness of about 10-250 micrometers. A coating 
of a desired thickness can be achieved by applying a single coating of 
that thickness or by applying the precursor coating composition in 
multiple thinner layers, e.g., by applying the coating composition in 
layers of about 25-100 micrometers, each layer being dried by driving off 
the solvent before the next layer is applied. 
When temperatures as high as about 200-250.degree. C. are used to drive off 
high boiling solvents, some pyrolysis of the preceramic material is 
initiated during the drying of the coating composition. However, higher 
temperatures, i.e., about 675-900.degree. C., preferably about 
825-875.degree. C., are required to convert the preceramic coating to a 
ceramic coating. This pyrolysis may be delayed until the final desired 
thickness of preceramic coating has been deposited. However, it is 
generally preferred to pyrolyze each one or two layers of dried preceramic 
coating before applying the next layer of coating composition. The time 
required for the pyrolysis is generally about 1-60 minutes, depending on 
the particular pyrolysis temperature selected. In the preferred embodiment 
of the invention where the coating is applied in multiple layers, each one 
or two of which is pyrolyzed before the application of the next layer, and 
the pyrolysis temperature is about 825-875.degree. C., it is generally 
preferred to pyrolyze the first coat for only about five minutes and then 
to pyrolyze subsequent coats for longer times up to about 15 minutes. 
When the coating is intended to protect a substrate from oxidative 
deterioration at very high temperatures, e.g., temperatures higher than 
800.degree. C., the final pyrolysis is followed by thermal treatment of 
the coated substrate at about 1075-1250.degree. C., preferably about 
1100-1175.degree. C., most preferably about 1125.degree. C., in an 
atmosphere containing not more than a minor amount of oxygen, e.g., in a 
nitrogen, argon, or helium atmosphere, to convert the ceramic coating into 
a homogeneous film. This treatment may be accomplished by raising the 
temperature in the vessel used for the pyrolysis or by transferring the 
coated substrate to a vessel maintained at the higher temperature; and it 
is preferably continued for about five minutes for the first coat and 
longer periods, e.g., about 15-20 minutes, for subsequent coats. 
After the pyrolysis or pyrolysis/heat treatment, the coated substrate is 
cooled. Optimum results are attained when this cooling is accomplished at 
a rate not greater than about 50.C/minute, preferably about 20-30.degree. 
C./minute, until the substrate temperature is below 500.degree. C., at 
which time further cooling may be accomplished at ambient air temperature. 
Although not essential, it is preferred to keep the starting polysilazane 
and the organoborosilazane polymers and compositions formed from it in a 
dry atmosphere until a layer of ceramic has been formed because of the 
susceptibility of the preceramic materials to attack by water and other 
compounds having active hydrogens. 
As already indicated, the dispersions of the invention are useful in 
preparing a variety of ceramic objects, but the major advantage of the 
invention is its provision of compositions capable of resisting hydrolytic 
attack and protecting normally oxidizable materials from oxidative 
deterioration at elevated temperatures. This advantage is of particular 
importance in the protection of carbon/carbon composites, graphite, and 
metals used in aerospace applications, such as engine components, advanced 
nozzle system components, and high-temperature vehicle structures.

The following examples are given to illustrate the invention and are not 
intended as a limitation thereof. 
EXAMPLE I 
Synthesis of Polysilazane 
Part A 
A suitable reaction vessel was charged with 14L of anhydrous 
tetrahydrofuran and cooled to about 0.C, after which 1545g (13.43 mols) of 
methyldichlorosilane was added to the vessel, and stirring at about 60 rpm 
was begun. A slow steady stream of 1058g (62.12 mols) of anhydrous ammonia 
gas was introduced into the vessel at a flow rate such that the reaction 
pressure was maintained at or below 400 kPa, and the reaction temperature 
stayed in the range of 0-10.degree. C. Then the reaction mixture was 
stirred at 0.C for about three hours, after which the coolant flow on the 
vessel was shut off, and the system was put under gentle nitrogen purge to 
allow the reaction mass to warm to room temperature and the majority of 
the excess ammonia to vent off. Then the reaction vessel was pressurized 
with sufficient nitrogen gas to pump the product mass through a bag filter 
assembly into a holding tank, where it was verified that the filtrate 
solution was free of particulates. 
Part B 
The clear filtrate from Part A was discharged into a polymerization vessel 
and chilled to about 0.degree. C., and a suspension of 3.6g (0.089 mol) of 
potassium hydride powder in about 100 mL of anhydrous tetrahydrofuran was 
added to begin the polymerization reaction. The reaction mixture was 
maintained at 0.degree. C. for about 8 hours and then allowed to warm 
gradually to about 22.degree. C. After a total of about 26 hours of 
polymerization at 0-22.C, the reaction was quenched by adding about 12.6g 
(0.13 mol) of dimethylchlorosilane to the polymerization solution. 
The polymer product was isolated by (1) concentrating the product solution 
to about 4L of volume by vacuum distillation, (2) centrifuging the 
concentrated solution to obtain a clear supernatant solution and a white 
precipitate, (3) decanting off the supernatant solution from the 
precipitate, and (4) flashing off the volatiles from the supernatant 
solution by vacuum distillation to provide a white solid. Proton NMR 
spectra of the polymer in deuterated chloroform solvent had resonances 
consistent with those reported in Seyferth et al.-II for polysilazane and 
with the presence of a small amount, i.e., 2.4% by weight, of residual 
tetrahydrofuran. 
EXAMPLE II 
Synthesis of Organoborosilazane Polymer 
A clear solution of four parts by weight of trimethoxyboroxine in a mixture 
of 0.5 part by weight of xylene and 0.5 part by weight of 
1-methyl-2-pyrrolidone was slowly added to a clear solution of one part by 
weight of the polysilazane of Example I in a mixture of 1.5 parts by 
weight of xylene and 1.5 parts by weight of 1-methyl-2-pyrrolidone. An 
exothermic reaction occurred to form a solution of an organoborosilazane 
polymer. 
EXAMPLE III 
Synthesis of Group IIA Metal-Organoborosilazane 
Polymer Solution 
Composition A, a solution, was prepared by dispersing 5g of a fine 
anhydrous barium fluoride powder in 100g of the organoborosilazane polymer 
solution of Example II and stirring the composition continuously while 
heating it overnight at 130.C to form a solution from which undissolved 
salt (about 0.8g) was removed. 
EXAMPLE IV 
Preparation of Dispersion 
A mixture of 0.3 part by weight of aluminum-silicon eutectic, 1.7 parts by 
weight of o-silicon carbide, two parts by weight of silicon hexaboride, 
and 1.8 parts by weight of amorphous silicon metal was homogenized and 
vacuum-dried for at least two hours, after which the Composition A 
solution of Example III was added and intimately mixed with it to form a 
dispersion designated as Composition B. 
EXAMPLE IV 
Graphite coupons having nominal dimensions of about 3.8 cm x 2.5 cm x 0.3 
cm were abraded to provided a smooth finish, cleaned, vacuum dried, 
thoroughly swab-coated in an inert atmosphere with Composition B, dried, 
heated at 100.degree. C. for five minutes, heated to 150.C at a rate of 
about 10.C/minute, held at 150.degree. C. for 15-30 minutes, allowed to 
cool to room temperature, recoated and held at 150 C for 30 minutes, 
heated to about 200-225.degree. C., maintained at that temperature for at 
least 15 minutes, and cooled to provide coupons having a coating thickness 
of about 0 08-0.1 mm. 
The polymer coatings were then pyrolyzed to ceramic coats by heating the 
coated coupons to 800-825.degree. C., holding at that temperature for 30 
minutes, subsequently heatinq them at about 1125.degree. C. for at least 
five minutes, and cooling to room temperature at a rate of 10-20.degree. 
C./minute. 
The effectiveness of the ceramic coats thus obtained in protecting the 
graphite substrate from oxidation was determined by an oxidation test. The 
coated specimen was mounted horizontally in a half section of a silicon 
carbide tube which was used as a holder and which allowed over 99% of the 
coupon surface to be directly exposed to hot ambient convecting air. The 
holder and specimen were placed in a box furnace which had been preheated 
to 1100.degree. C. Periodically the holder and specimen were removed from 
the furnace and quenched in ambient air, the cooled specimen was weighed 
and remounted in its holder, and the holder and specimen were replaced in 
the heated furnace for additional heating in air. The weight loss on 
oxidation was determined to be only 13% after four hours, compared with a 
weight loss of 67% after four hours when uncoated graphite coupons were 
subjected to the same test and a weight loss of 56% when the graphite 
coupons subjected to the test were coated with Composition A instead of 
Composition B. 
It is obvious that many variations may be made in the products and 
processes set forth above without departing from the spirit and scope of 
this invention.