Low density silicon nitride ceramics having improved bending and specific strengths over known silicon nitride ceramics derived from silicon nitride powder and polysilazane precursors are prepared by (A) intimately mixing about 50-85% by weight of silicon nitride powder with about 15-50% by weight of a preceramic polysilazane binder, (B) pulverizing the mixture to form particles having a particle size smaller than 105 micrometers, (C) separating from those particles any particles having a particle size larger than about 105 micrometers, (D) molding the resultant composition having a particle size not larger than about 105 micrometers, and (E) pyrolyzing the molded composition in an inert atmosphere to a temperature of about 1200.degree.-1450.degree. C.; said preceramic binder being at least one polysilazane prepared by reacting an organodihalosilane 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.

FIELD OF INVENTION 
This invention relates to silicon nitride ceramics, i.e., ceramics composed 
predominantly of silicon nitride, and more particularly to preceramic 
compositions and processes for preparing them. 
BACKGROUND 
It is known that silicon nitride ceramics having acceptably high strength 
can be obtained by a reaction bonding process but that such processes have 
the disadvantage of being very time-consuming, requiring, e.g., 72 hours 
or more for completion. As disclosed in U.S. Pat. Nos. 4,543,344 (Cannady) 
and 4,612,383 (Laine et al.), it is also known that silicon nitride 
ceramics can be derived from polysilazane precursors. However, the 
strengths of silicon nitride ceramics derived from mixtures of silicon 
nitride powders and polysilazane precursors have hitherto been quite poor. 
To overcome the disadvantages of these known methods of preparing silicon 
nitride ceramics, it would be desirable to find a way of deriving silicon 
nitride ceramics from mixtures of silicon nitride powders and polysilazane 
precursors that would result in the ceramics' having high strength, i.e., 
a strength higher than that of the known silicon nitride ceramics derived 
from silicon nitride powders and polysilazane precursors, e.g., a bending 
strength of at least about 23 kg/mm.sup.2 and a specific strength of at 
least about 10 (kg/mm.sup.2)/(g/cc). 
U.S. Pat. Nos. 4,482,669 (Seyferth et al.-I), 4,645,807 (Seyferth et 
al.-II), 4,650,837 (Seyferth et al.-III), and 4,659,850 (Arai et al.) 
disclose the utility of polysilazanes as preceramic materials, and 
Seyferth et al.-I teach that their polysilazanes are especially useful as 
binders for ceramic powders such as silicon nitride. Silicon nitride 
ceramics prepared from these silicon nitride powder/polysilazane binder 
compositions are composed predominantly of silicon nitride, e.g., about 
92% silicon nitride or more, with the balance being mostly silicon 
carbide. Seyferth et al.-I do not disclose the typical densities and 
bending strengths of ceramics made from their preceramic compositions. 
However, Wiseman, "The Development and Application of Polysilazane 
Precursors to Ceramics," a Massachusetts Institute of Technology thesis, 
1984, shows that these densities were generally about 2.1-2.2 g/cc and the 
bending strength were poor. Wiseman showsa recognition, though, of its 
being desirable to minimize alkali metal contamination and to use 
preceramic polymers having a sufficiently high molecular weight, or 
mixtures (such as 80/20 mixtures) of such polymers with lower molecular 
weight polymers, to maximize strength. 
SUMMARY OF INVENTION 
An object of this invention is to provide novel preceramic polysilazane 
compositions capable of forming high strength silicon nitride ceramics 
having low density. 
Another object is to provide processes for preparing such compositions and 
converting them into high strength silicon nitride ceramics having low 
density. 
These and other objects are attained by (A) intimately mixing about 50-85% 
by weight of silicon nitride powder with about 15-50% by weight of a 
preceramic polysilazane binder, (B) pulverizing the mixture to form 
particles having a particle size smaller than 105 micrometers, i.e., 
particles which pass through a 105 micrometer sieve, (C) separating from 
those particles any particles having a particle size larger than about 105 
micrometers, i.e., particles which are retained on a 105 micrometer sieve, 
(D) molding the resultant composition having a particle size not larger 
than about 105 micrometers, and (E) pyrolyzing the molded composition in 
an inert atmosphere to a temperature of about 1200-450.degree. C.; said 
binder consisting essentially of at least one polysilazane prepared by 
reacting an organodihalosilane 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. 
DETAILED DESCRIPTION 
Silicon nitride powders that can be employed in the practice of the 
invention are commercially-available materials that vary from very fine to 
coarse powders. However, the preferred silicon nitride powders are those 
which have a particle size of about five micrometers or less, preferably 
one micrometer or less; and particularly good results have been obtained 
with silicon nitride powders having mean particle sizes of about 0.1-1.0 
micrometer. 
The binder that is mixed with the silicon nitride powder is a polysilazane 
of Seyferth et al.-I (the teachings of which are incorporated herein in 
toto by reference), i.e., a polysilazane prepared by reacting an 
organodihalosilane 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, or a mixture of such polysilazanes. 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. The utilizable polysilazanes are solids which are 
soluble in common organic solvents, such as aliphatic or aromatic 
hydrocarbons, dialkyl or alicyclic ethers, etc., including solid mixtures 
of normally solid polysilazanes and normally liquid polysilazanes. The 
solid, soluble polysilazanes having the higher molecular weights are 
preferred to permit the use of faster pyrolysis rates. 
The amount of polysilazane used is such that the preceramic composition 
comprises about 50-85% by weight of silicon nitride powder and about 
15-50% by weight of binder, preferably about 70-80% by weight of silicon 
nitride powder and about 20-30% by weight of binder. 
If desired, the compositions may be modified by the inclusion of optional 
ingredients, such as polyisobutenyl succinimides, other dispersing agents, 
and other additives that have been used in known ceramic molding 
compositions. For example, one or more lubricants such as higher fatty 
acids and the esters and amides thereof, higher alcohols, paraffin wax, 
and low molecular weight polyolefins can be used. When employed, such 
additives are used in minor amounts, e.g., up to about 5% by weight of 
dispersing agent or up to about 15% by weight of a lubricant, based on the 
weight of the remainder of the composition. 
The preceramic compositions of the invention are prepared by intimately 
mixing the silicon nitride powder and binder, pulverizing the mixture to 
form particles having a particle size smaller than 105 micrometers, as 
hereinabove defined, and separating from those particles any particles 
having a particle size larger than about 105 micrometers. Neither the 
particular manner in which the silicon nitride powder and binder are mixed 
nor the particular manner in which the particle size is reduced appears to 
be critical. For example, mills in general are useful for the reduction in 
particle size. However, it is particularly convenient to conduct the 
process by dispersing the silicon nitride powder in an organic solvent 
solution of the binder (e.g., a solution in an aliphatic or aromatic 
hydrocarbon, such as hexane, toluene, etc., or a dialkyl or alicyclic 
ether, such as diethyl ether, tetrahydrofuran, etc.) preferably at room 
temperature, removing the solvent (e.g., by rotary evaporation followed by 
vacuum distillation), ball milling the resultant chunks of powder/binder, 
and then sieving to remove any particles having a particle size larger 
then about 105 micrometers. 
Ceramics may be prepared from the preceramic compositions by molding them 
at a temperature and pressure suitable for the parts being made, usually 
at a temperature of about 60.degree.-225.degree. C. and a pressure of 
about 6.8-343 MPa, using any suitable shaping process, such as 
compression, injection, or transfer molding, or extrusion, and then 
pyrolyzing the molded composition in an inert atmosphere, such as 
nitrogen, argon, etc., to a temperature of about 1200.degree.-1450.degree. 
C., preferably about 1300.degree. C. The time required for the pyrolysis 
varies with the ultimate pyrolysis temperature, being at least one hour at 
the preferred pyrolysis temperature of about 1300.degree. C., a shorter 
time at higher temperatures, and a longer time at lower temperatures. It 
is particularly useful to pyrolyze the molded composition by (1) heating 
it to 1300.degree. C. at rates of 60.degree. C./hour from room temperature 
to 60.degree. C., 30.degree. C./hour from 60.degree. C. to 260.degree. C., 
120.degree. C./hour from 260 .degree. C. to 1260.degree. C., and 
60.degree. C./hour from 1260.degree. C. to 1300.degree. C., maintaining 
the temperature at 1300.degree. C. for one hour, cooling to 900.degree. C. 
at a rate of 120.degree. C./hour, and conducting the remainder of the 
cooling at an ambient rate or (2) heating it to 1400.degree. C. at rates 
of 60.degree. C./hour from room temperature to 60.degree. C., 15.degree. 
C./hour from 60.degree. C. to 260.degree. C., 120.degree. C./hour from 
260.degree. C. to 1260.degree. C., and 60.degree. C./hour from 
1260.degree. C. to 1400.degree. C. maintaining the temperature at 
1400.degree. C. for 45 minutes, and cooling to room temperature. 
Ceramics prepared from the preceramic silicon nitride/polysilazane 
compositions of the invention have comparable densities but better bending 
strengths than ceramics prepared from comparable preceramic compositions 
having a larger particle size. In fact, the use of the novel preceramic 
compositions can lead to the formation of silicon nitride ceramics having 
a specific strength (i.e., a bending strength/density ratio) of at least 
10 (kg/mm.sup.2)/(g/cc) --a strength particularly desirable for aerospace 
and other demanding and critical structural applications. The fact that 
this combination of properties in the ceramic can be achieved by the 
reduction in particle size of the preceramic composition is surprising, 
and the reason for the beneficial effect of the particle size reduction is 
not understood. 
The following examples are given to illustrate the invention and are not 
intended as a limitation thereof. In the processes described in these 
examples, thoroughly-dried equipment, purified raw materials, and an inert 
atmosphere were used to protect the polysilazanes from attack by water and 
other substances having active hydrogens during synthesis and storage of 
the polysilazanes and during processing and storage of the 
polysilazane-containing materials used to make the silicon nitride 
ceramics.

EXAMPLE I 
Synthesis of Polysilazane A 
Part A 
A suitable reaction vessel was charged with 14L of anhydrous 
tetrahydrofuran and cooled to about 0 C., after which 1497 g (13.01 mols) 
of methyldichlorosilane was added to the vessel, and stirring at about 60 
rpm was begun. A slow steady stream of 745 g (43.7 mols) of anhydrous 
ammonia gas was introduced into the vessel at a flow rate such that the 
reaction pressure was maintained at or below 206.8 kPa, and the reaction 
temperature stayed in the range of 0.degree.-10.degree. C. Then the 
reaction mixture was stirred at 0.degree. 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 mass was poured into flasks and filtered in a dry box with a 
sintered glass filter having pore diameters of 4.0-5.5 micrometers. 
Part B 
The clear filtrate from Part A was discharged into a polymerization vessel 
which had previously been charged with a suspension of 2.5g (0.063 mol) of 
KH powder in about 100 mL of anhydrous tetrahydrofuran and chilled to 0 C. 
to begin the polymerization reaction. The reaction mixture was maintained 
at 0 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.degree.-22.degree. C., the reaction was quenched by adding about 11.9 g 
(0.063mol) of dimethylchlorosilane to the polymerization solution. 
The polymer product was isolated to a dry powder by vacuum distillation, 
after which the dry residue was redissolved in anhydrous cyclohexane. The 
cyclohexane solution was filtered and the filtrate was vacuum dried to 
provide a white solid which was designated as Polysilazane A. Proton NMR 
spectra of the polymer in deuterated chloroform solvent had resonances 
consistent with those reported in Seyferth et al.-I for polysilazane and 
with a small amount, i.e., 2.2% by weight, of residual tetrahydrofuran, as 
well as about 7% by weight of cyclohexane. Elemental oxygen by neutron 
activation was about 0.2%, corrected for residual solvent; and the 
potassium content was determined by inductively coupled plasma emission 
spectroscopy to be less than 5 ppm, based on the weight of polymer. 
EXAMPLE II 
Synthesis of Polysilazane B 
Example I was essentially repeated except that the reaction mixture for the 
polymerization was prepared by adding the KH suspension to the ammonolysis 
product, the amount of KH employed was 0.6 mol %, based on the amount of 
methyldichlorosilane charged in the ammonolysis reaction, and the 
polymerization was conducted entirely at 0.degree. C. for a total of 0.7 
hour. The Polysilazane B formed by the process was a viscous liquid having 
a tetrahydrofuran content of 0.5% by weight, an elemental oxygen content 
of 0.4% (corrected for residual solvent), and a potassium content of less 
than 5 ppm. 
EXAMPLE III 
Synthesis of Polysilazane C 
Example II was repeated to form another viscous liquid which was designated 
as Polysilazane C. It was not analyzed for impurities. 
EXAMPLE IV 
Synthesis of Polysilazane D 
Example II was essentially repeated except that the polymerization time at 
0.degree. C. was 10 hours. The Polysilazane D formed by the process was a 
solid having a potassium content of about 10 ppm. It was not analyzed for 
other impurities. 
The following examples describe molding formulations prepared from the 
polysilazanes of the preceding examples and commercial silicon nitride 
powders. Each of the silicon nitride powders is predominantly 
alpha-silicon nitride. The silicon nitride powder designated as Si.sub.3 
N.sub.4 -1 has an average particle size of 0.8 micrometer and a specific 
surface area of 7-10 m.sup.2 /g, the silicon nitride powder designated as 
Si.sub.3 N.sub.4 -2 has an average particle size of 0.1-0.3 micrometer and 
a specific surface area of 10 m.sup.2 /g, the silicon nitride powder 
designated as Si.sub.3 N.sub.4 -3 has an average particle size of 0.7 
micrometer and a specific surface area of 19-22 m.sup.2 /g, and the 
silicon nitride powder designated as Si.sub.3 N.sub.4 -4 has an average 
particle size of 0.6 micrometer and a specific surface area of 12 m.sup.2 
/g. 
EXAMPLE V 
Preparation of Formulation I 
A mixture of 14.4g of Polysilazane A, 3.8g of Polysilazane B, and 0.3g of a 
commercial polyisobutenyl succinimide dispersant in 200 g of anhydrous 
toluene was stirred magnetically for about 15 minutes to obtain a 
homogeneous solution, after which 42 g of Si.sub.3 N.sub.4 -1 powder was 
added to the solution. The mixture was stirred magnetically for 60 minutes 
and then ultrasonicated for about one hour to disperse the silicon nitride 
powder, and the majority of the toluene was then flashed off to provide a 
non-flowing residue. The residue was dried under high vacuum for several 
days and then pulverized lightly with a mortar/pestle to obtain a 
free-flowing formulation powder which was then ball milled for about one 
hour with about 200 cc of silicon carbide milling balls having a diameter 
of about 0.25 inch in a 2.5-pint mill jar, after which the milling balls 
were removed. The milled powder was then dry-sieved through a screen 
having size openings of 106 micrometers. The powder that did not pass 
through the sieve was ball-milled again until all of the formulation 
passed through. 
EXAMPLE VI 
Preparation of Additional Formulations 
The general procedures of Example V, i.e., slurry-blending, 
ultrasonicating, drying, coarse-grinding, ball-milling, and sieving, were 
used to prepare additional formulations having the compositions shown in 
Table I. The dispersant included in the formulations was a commercial 
polyisobutenyl succinimide dispersant. 
TABLE I 
______________________________________ 
Formulation Ingredient Parts 
______________________________________ 
II Polysilazane A 
16 
Polysilazane C 
4 
Si.sub.3 N.sub.4 --1 
80 
Dispersant 0.5 
III Polysilazane A 
24 
Polysilazane C 
6 
Si.sub.3 N.sub.4 --1 
70 
Dispersant 0.5 
IV Polysilazane D 
24 
Polysilazane C 
6 
Si.sub.3 N.sub.4 --2 
70 
Dispersant 0.5 
V Polysilazane D 
24 
Polysilazane C 
6 
Si.sub.3 N.sub.4 --3 
70 
Dispersant 0.5 
VI Polysilazane D 
24 
Polysilazane C 
6 
Si.sub.3 N.sub.4 --4 
70 
Dispersant 0.5 
______________________________________ 
EXAMPLE VII 
Molding of Formulation I 
Each of six green discs having a nominal diameter of 12.7 mm and a nominal 
thickness of 2.54 mm was molded from Formulation I. In the preparation of 
each of these discs, about 0.8 g of the formulation was loaded into a 
suitable mold in a nitrogen glovebox; and the mold was evacuated to less 
than about 133 pascals, sealed under vacuum, transported to a hydraulic 
press, reconnected to a vacuum line, and evacuated to a pressure of not 
more than about 67 pascals--a vacuum level that was maintained throughout 
the remainder of the molding process. The evacuated mold was placed snugly 
between the press platens, which were preheated to about 182.degree. C. 
and allowed to preheat for 10 minutes, after which a force of 2268-2722 kg 
(175-210 MPa pressure) was applied to the mold and maintained for about 
five minutes. After compression, the mold was sealed under vacuum and 
transported back into the glovebox, where it was allowed to cool for about 
five minutes. After cooling, the molded green disc was removed from the 
mold and stored in the glovebox. The density was determined to be about 
2.05 g/cc. 
EXAMPLE VIII 
Pyrolysis of Formulation I 
The green discs prepared in Example VII were pyrolyzed in a nitrogen 
atmosphere by heating them to 1300.degree. C. at rates of 60 /hour from 
room temperature to 60.degree. C., 30.degree. C./hour from 60.degree. C. 
to 260.degree. C., 120.degree. C./hour from 260.degree. C. to 1260.degree. 
C., and 60.degree. C./ hour from 260.degree. C. to 1300.degree. C., 
maintaining the temperature at 1300.degree. C. for one hour, cooling to 
900.degree. C. at a rate of 120.degree. C./hour, and allowing ambient-rate 
cool down from 900.degree. C. to about room temperature, i.e., shutting 
down the furnace heaters when the temperature reached 900.degree. C. and 
allowing the resultant furnace conditions to determine the rate of the 
remainder of the cool down. After the pyrolyzed specimens had cooled to 
below 100.degree. C., they were removed from the furnace and stored 
immediately in a dry nitrogen atmosphere. Their densities were calculated 
from weight and dimension data to be 2.23 g/cc. All of the disc specimens 
underwent uniform, linear shrinkage of about 6% as a result of the 
pyrolysis. 
The pyrolyzed specimens were subsequently stored in air at ambient 
temperature and humidity for several weeks, during which time their weight 
increased by 2-3% --the maximum weight gain having been reached in about 
seven days. After the specimens had equilibrated to constant weight, their 
bending strengths were determined by the biaxial-loading-stress method 
described in Godfrey, Materials Science & Technology, Vol. 1, No. 7 
(1985), pp. 510-515. The discs formed from Formulation I were determined 
to have an average bending strength of 27.4 kg/mm.sup.2 and an average 
specific strength of 12.3 (kg/mm.sup.2)/(g/cc). 
EXAMPLE IX 
Molding and Pyrolysis of Additional Formulations 
About six green discs were molded from each of Formulations II-VI 
essentially as in Example VII, and the green discs were pyrolyzed 
essentially as in Example VIII. The pyrolyzed disc specimens were 
equilibrated in ordinary air, after which their bending strengths were 
determined as in Example VIII. The densities, average bending strengths, 
and average specific strengths of the discs formed from the various 
formulations are shown in Table II. 
TABLE II 
______________________________________ 
Density Bending Strength 
Specific Strength 
Formulation 
(g/cc) (kg/mm.sup.2) 
(kg/mm.sup.2)/(g/cc) 
______________________________________ 
II 2.27 23.2 10.2 
III 2.22 30.1 13.6 
IV 2.08 25.2 12.1 
V 2.22 27.4 12.3 
VI 2.11 27.0 12.8 
______________________________________ 
It is obvious that many variations can be made in the products and 
processes set forth above without departing from the spirit and scope of 
this invention.