Ceramic articles containing silicon carbide

A shaped article is described comprising a ceramic matrix and dispersed therein in the range of 5 to 30 weight percent mechanically added silicon carbide, said article having a modulus of elasticity (E) value of at least 10 percent greater than the inherent elastic modulus value of the fully dense host matrix. The articles are useful as high temperature stable reinforcement materials in composites requiring a high modulus of elasticity.

FIELD OF THE INVENTION 
This invention relates to shaped articles having a high modulus of 
elasticity comprising a ceramic matrix with silicon carbide dispersed 
therein and to a method of their production. The articles are useful as 
high temperature stable reinforcement materials in composites requiring 
high modulus of elasticity. 
BACKGROUND OF THE INVENTION 
Non-vitreous inorganic articles are becoming increasingly important in 
commerce as high performance materials. For example, non-vitreous ceramic 
fibers are finding utility not only as high temperature insulating 
materials, but also as reinforcing materials in composite structures, for 
example, in metals, glasses and ceramics. The reinforcement application 
requires fibers to have a high tensile strength and a high modulus of 
elasticity. 
It is known that an oxide ceramic must be fully dense and have a 
polycrystalline structure if it is to achieve optimum tensile strength and 
modulus of elasticity (E). Whenever porosity is present, reduced or lower 
tensile strengths and modulus of elasticity can be expected. To reduce 
porosity in inorganic materials, the process of sintering is used which is 
normally accompanied by growth of the crystallites. Unfortunately, large 
crystallites or grains have the effect of reducing the tensile strength of 
polycrystalline fibers. Thus, the improvement in tensile strength 
attributed to the reduction in porosity by sintering is partially offset 
by the larger crystallites which have grown during sintering. Therefore, 
to produce inorganic fibers with a high tensile strength and a high 
modulus of elasticity (E), a dense ceramic (minimum porosity) with the 
smallest crystallites possible is preferred. 
It is known to use organic precursors to produce a second SiC phase in 
oxide ceramics. U.S. Pat. No. 4,010,233 discloses inorganic fibers wherein 
a metal oxide phase contains a second dispersed phase. In all cases, the 
dispersed pahse is an in situ precipitation or chemical reaction product; 
for the examples utilizing SiC, it is obtained via chemical reaction of an 
organic precursor. The particle size is dependent upon the firing 
conditions used; for example, time, temperature and atmosphere. E values 
up to 269 GPa (39.times.10.sup.6 psi) are reported. 
U.S. Pat. Nos. 4,298,558 and 4,314,956 disclose alkoxylated and 
phenoxylated methyl polysilane which are useful for the preparation of 
fine grained silicon carbide-containing ceramics. Pyrolysis and reaction 
of the ceramic precursor polymers provide the silicon carbide-containing 
ceramics. 
SUMMARY OF THE INVENTION 
Briefly, the present invention provides a shaped article comprising a 
ceramic matrix and having therein 5 to 30 weight percent 
mechanically-added silicon carbide, the article having a modulus of 
elasticity (E) value of at least ten percent higher, preferably 25 
percent, more preferably 50 percent higher than that inherent in the fully 
dense host ceramic matrix. The silicon carbide is added to the ceramic 
matrix prior to densification as crystalline particles ahving an average 
diameter 0.1 micrometers or less. 
Preferably, the surface of the shaped article is smooth, i.e. the average 
peak to valley surface roughness is less than 0.2 micrometer. 
Although the concept of raising the modulus of elasticity by incorporation 
of a second higher modulus phase is know, see Kingery et al. "Introduction 
to Ceramics", John Wiley & Sons, New Yokr, N.Y. 1976, pages 723-777 
(1976), it has not been proven practical for application to fibers or 
other sol-gel derived products having small dimensions. Commercially 
available, high modulus powders such as SiC, can be incorporated into 
these articles but the relatively large particle size (typically greater 
than 0.1 micrometer diameter and more typically greater than 1.0 
micrometer diameter) leads to difficulties in spinning fibers, and more 
importantly leads to the formation of large flaws (voids, cracks, surface 
roughness) which negate any advantage which might be derived from the high 
modulus phase. 
This invention provides ceramic articles having incorporated therein 
sufficient quantities of SiC such that the additive effect of the second 
phase can be achieved leading to a modulus of elasticity much higher than 
that inherent in the fully dense oxide ceramic. 
U.S. Pat. No. 4,010,233 demonstrated improvements in the modulus of 
elasticity of alumina up to values of 269 GPa (39.times.10.sup.6 psi) 
using different dispersed phases to limit grain growth and minimize 
porosity. However, the improvements obtained are still well below the 
inherent modulus of elasticity of fully dense alumina [414 GPa 
(60.times.10.sup.6 psi)]. 
SiC derived from organic precursors may help control grain growth and 
porosity in oxide fibers and generally contains C and SiO.sub.2 which 
lower its effective modulus of elasticity to 207 GPa (-30.times.10.sup.6 
psi). Thus SiC derived from such materials would not be expected to 
produce a significant increase in the moduli of oxides already having 
moduli of elasticity in this range. In contrast, higher purity forms of 
SiC have moduli of elasticity greater than 690 GPa (100.times.10.sup.6 
psi) making such materials much more effective as an additive to produce a 
modulus increase above that which would be expected from the oxide itself. 
In the present invention, the modulus of elasticity of fibers such as 
aluminum-borosilicates and zirconium silicates can be raised to volues 
over 100 percent greater than that which could be obtained from the fully 
dense oxide fibers. 
In this application: 
"ceramic" means inorganic nonmetallic material consolidated by the action 
of heat, such as metal and nonmetal oxides; 
"fully dense" means essentially free of pores or voids; 
"sol" means a fluid solution or a colloidal suspension; 
"non-vitreous" means not formed from a melt of the final oxide composition; 
"green" refers to the ceramic articles which are unfired, that is, not in 
their ceramic form; 
"amorphous" means a material having a diffuse X-ray diffraction pattern 
without definite lines to indicate the presence of a crystalline 
component; 
"crystalline" means having a characteristic x-ray or electron diffraction 
pattern; 
"dehydration gelling" or "evaporative gelling" means that sufficient water 
and volatile material are removed from the shaped green fibers so that the 
form or shape of the fiber is sufficiently rigid to permit handling or 
processing without significant loss or distortion of the desired fibrous 
form or shape; all the water in the shaped fiber need not be removed. 
Thus, in a sense, this step can be called partial dehydrative gelling; and 
"continuous fiber" means a fiber (or multi-fiber article such as a strand) 
which has a length which is infinite for practical purpose as compared to 
its diameter. 
DETAILED DESCRIPTION 
This invention provides an inorganic non-vitreous ceramic article 
comprising a fully dense ceramic matrix and having therein 5 to 30 weight 
percent silicon carbide, which is dispersed throughout the ceramic matrix. 
The crystalline ultrafine (i.e., diameter less than 0.1 micrometer) 
silicon carbide particles are dispersed into the ceramic article precursor 
before shaping and converting to the ceramic form for the improvement of 
high temperature mechanical properties, e.g. modulus of elasticity. 
In the present invention, the particles of SiC have a primary particle size 
of less than 0.1 micrometer, preferably less than 0.03 micrometer. It has 
been found that if particles, aggregates, and agglomerates greater than 
0.1 micrometer are eliminated from the system, then the concept of 
utilizing a high modulus second phase to enhance the modulus of elasticity 
can be successfully applied to fibers. 
In another aspect, the present invention provides a process for preparing 
the ceramic, high modulus of elasticity, articles of the present 
invention. The articles can be flakes, microspheres, bubbles, or random 
shaped particles, but preferably they are fibers. 
In the process of the present invention, in preparing fibers the matrix 
phase is provided by a non-melt process comprising shaping a viscous 
concentrate of a mixture of precursor liquid and dispersed silicon carbide 
filler into a fiber form and then dehydratively or evaporatively gelling 
or hydrolyzing the drawn or spun fibers. These fibers can subsequently be 
dried to result in "green" or non-refractory fibers. Heating and firing 
the shaped green fibers removes water, decomposes and volatilizes 
undesired fugitive constituents and converts them into the refractory 
fibers of the invention. 
Shaped and fired refractory fibers of this invention can be made by 
extruding in air the viscous fiberizable concentrate and then heating and 
firing the resulting green fibers to form continuous uniformly round, or 
oval, rod-like (elongated ovoid) or ribbon-like, strong, flexible, smooth, 
glossy refractory fibers. The fibers are useful in making textile fabrics, 
but are particularly useful as fillers and reinforcement for plastic, 
ceramic and metal matrix composites. 
In one embodiment, the starting material or fiber precursor composition 
from which the refractory alumina-silica fibers of this invention can be 
made comprises a liquid mixture of a silicon compound, e.g., an aqueous 
dispersion of colloidal-silica and a compatible aqueous solution or 
dispersion of a water-soluble or dispersible aluminum compound, and, where 
used, compatible compounds, e.g., boron, zirconium, titanium, thorium, or 
phosphorus compounds. The compounds used are those which can be calcined 
to their respective oxides. 
Suitable aluminum compounds which can be used as alumina precursors include 
water-dispersible alumina sols and water soluble aluminum salts such as 
aluminum formoacetate, aluminum nitrate, aluminum isopropylates, basic 
aluminum acetate, and mixtures thereof. The aluminum formoacetate Al(OH) 
(OOCH) (OCOCH.sub.3) is a preferred source. 
Where the refractory fibers of this invention are to contain boria, a 
suitable precursor is boric acid. Basic aluminum acetate, Al(OH).sub.2 
(OCOCH.sub.3) 1/3 H.sub.3 BO.sub.3, e.g., boric acid stabilized aluminum 
acetate, can be used as a boria precursor, alone or in combination with 
boric acid. 
The precursor silica sol can be used with SiO.sub.2 concentrations of 1 to 
50 weight percent, preferably 15 to 35 weight percent; silica sols of 
varying concentrations are commercially available. The silica sol is 
preferably used as an aqueous dispersion or aquasol, but can also be used 
in the form of an organosol where the silica is colloidally dispersed in 
such water-miscible polar organic solvents as tehylene glycol or dimethyl 
formamide. 
In a zirconia-silica system, the precursor zirconia sol can be used in the 
form of an aqueous solution of a suitable organic or inorganic acid 
water-soluble salt, the zirconia salts of aliphatic or acyclic mono or 
dicarboxylic acids having dissociation constants of at least 
1.5.times.10.sup.-5, such as formic, acetic, oxalic, malic, citric, 
tartaric and lactic acids and their halogenated derivatives such as 
chloroacetic acid. Zirconium diacetate is preferred because of its 
compatibility with colloidal silica and commercial availability and 
relatively low cost of its aqueous solution. Typical inorganic zirconium 
salts which can be used are zirconyl nitrate, zirconium carbonate and the 
like. 
Preparation of different aqueous liquid mixtures, sols, or dispersible 
colloids or mixtures thereof which can be used for individual components 
of the matrix fibers of the invention are disclosed, for example, as 
follows: 
______________________________________ 
Fiber matrices U.S. Pat. Nos. 
______________________________________ 
titania 4,166,147 
alumina-chromia-metal (IV) oxide 
4,125,406 
alumina-silica 4,047,965 
thoria-silica-metal (III) oxide 
3,909,278 
alumina-boria and alumina-boria-silica 
3,795,524 
zirconia-silica 3,793,041 
zirconia-silica 3,709,706 
alumina-phosphorus oxide 
______________________________________ 
The starting material or ceramic precursor compositions form the matrix 
phase to which the silicon carbide filler is added. The silicon carbide 
preferred for addition to alumina:boria:silica fibers is produced by radio 
frequency plasma synthesis from silane and methane starting materials as 
is known in the art. The SiC has an average size of 2.times.10.sup.-2 
micrometer, with an estimated size range of 5.times.10.sup.-3 to 
3.times.10.sup.-2 micrometers (50 to 300 A), as measured by gas adsorption 
surface area measurement procedures in combination with X-ray diffraction 
and electron microscopy. However, in the ceramic matrix, the mechanically 
dispersed SiC filler may be present as a discrete phase or it may be 
dissolved in the ceramic matrix. 
The specific surface area of the plasma synthesized SiC was measured to be 
82 to 104 m.sup.2 /g. X-ray diffraction of the samples showed beta-SiC. 
Emission spectrographic analysis shows 30 ppm Al, 5 ppm Mg and 10 ppm Ni. 
A silicon carbide preferred for the zirconiasilica fibers was produced by a 
carbothermal process according to the reaction: 
EQU SiO.sub.2 +3C.fwdarw.SiC+2CO 
where the carbon black was dispersed into a silica sol, the mixture was 
then dried, crushed, and fired in a vacuum furance at 1400.degree. C. The 
resultant SiC material was ball milled in a solvent, e.g., acetone and 
filtered to the desired particle size. Silicon carbide in powder form 
(20nm diameter) can be dispersed into the zirconia-silica precursors by 
sonicating a mechanical mixture. A preferred method is to partially 
oxidize the SiC by heating at 600.degree. C. in air for about three hours. 
The oxidized SiC is then mixed into the zirconia-silica precursor sol and 
fully dispersed by sonication. 
When a fiber with high emissivity is desired, as is described in assignee's 
copending patent application Ser. No. 912,829, now U.S. Pat. No. 
4,732,070, it is desirable to incorporate carbon into the structure of the 
ceramic fibers. 
Each of the fiber precursor materials, initially will be a relatively 
dilute liquid, generally containing about 10-30 weight percent equivalent 
oxide, which can be calculated from a knowledge of the equivalent solids 
in the original materials and the amount used, or determined by calcining 
samples of the component starting materials. For the preparation of 
fibers, it is necessary to concentrate or viscosify the dilute liquid in 
order to convert it to a viscous or syrupy fluid concentrate which will 
readily gel when the concentrate is fiberized and dehydrated, for example 
when the concentrate is extruded and drawn in air to form the fibers. The 
mixture can be concentrated with a rotary evaporation flask under vacuum. 
The concentration procedures are well known in the prior art; see U.S. 
Pat. No. 3,795,524. Sufficient concentration will be obtained when the 
equivalent solids content is generally in the range of 25 to 55 weight 
percent (as determined by calcining a sample of the concentrate), and 
viscosities (Brookfield at ambient room temperature) are in the range of 
10,000 to 100,000 mPa sec., preferably 40,000 to 100,000 mPa secd., 
depending on the type of fiberizing or dehydrative gelling technique and 
apparatus used and the desired shape of gelled fiber. High viscosities 
tend to result in fibers which are more circular in cross-section whereas 
low viscosities (e.g., less than 50,000 mPa sec.) may have a greater 
tendency to result in fibers which are more oval or rod-like (elongated 
ovoid) in cross-section. 
In making continuous fiber, the viscous concentrates can be extruded 
through a plurality of orifices (e.g., a total of 10 to 400) from a 
stationary head and the resulting green fibers allowed to fall in air by 
the force of gravity or drawn mechanically in air by means of drawing 
rolls or a drum or winding device at a speed faster than the rate of 
extrusion. The concentrate can also be extruded through orifices from a 
stationary or rotating head and at the orifice exit blown by a parallel, 
oblique or tangential stream of high pressure air. The resulting blown 
green fibers are in essentially staple or short from with lengths generaly 
25 cm or less (rather than the continuous filament form) and collected on 
a screen or the like in the form of a mat. Any of these forces exerted on 
the extruded, green fibers cause attenuation or stretching of the fibers, 
and can reduce their diameter by about 50 to 90 percent or more and 
increase their length by about 300 to 1,000 percent or more and serve to 
hasten or aid the drying of the green fibers. 
The dehydrative gelling of the green fibers can be carried out in abmient 
air, or heated air if desired for faster drying. The drying rate can 
affect the shape of the fiber. The relative humidity of the drying air 
should be controlled since excess humidity will cause the gelled green 
fibers to stick together and excessively dry air tends to result in fiber 
breakage. Generally, air with relative humidity in the range of 20 to 60 
percent at an operative temperature of 15.degree.-30.degree. C. is most 
useful, although drying air temperatures of 70.degree. C. or more can be 
used. Where continuous green fibers are made and gathered together in 
parallel alignment or juxtaposition in the form of a multi-fiber strand, 
the fibers or strand should be treated with a size to prevent the fibers 
from sticking together. The fibers in the green or unfired gel form are 
dry in the sense that they do not adhere or stick to one another or other 
substrates and feed dry to the touch. However, they still may contain 
water and organics, and it is necessary to heat and fire the green fibers 
in order to remove these remaining fugitive materials and convert the 
green fibers into refractory fibers. These green fibers in their 
continuous form are preferably gathered or collected in the form of a 
strand. The strand then accumulates in a relaxes, loose, unrestrained 
configuration of offset or superimposed loops as in a "FIG. 8". 
In firing the green fibers, care should be exercised to avoid ignition of 
combustible material (such as organics within or size upon the fiber) in 
or eveolved from the fibers. Such combustion may tend to cause overheating 
of the fibers resulting in imporper rate of temperature rise of the firing 
cycle and cause degradation of fiber properties. 
The refractory products of this invention are useful as reinforcement in 
composites where in particular a high modulus is required. Of special 
importance are ceramic reinforcement materials capable of performing in a 
high temperature (upt to 1300.degree. C.) oxidative atmosphere. 
Representative samples of the fired fibers were characterized for tensile 
strength and modulus of elasticity. The procedure for testing tensile 
strength used a metal chain attached to a single fiber. The load applied 
to the fiber was measured by increasing the chain length 
electromechanically until a break occurred and then weighing the minimum 
length of chain necessary for break. The tensile strenth (TS) is 
calculated as 
##EQU1## 
W=weight of chain length at break, and A=cross-section area of the fiber. 
The modulus of elasticity was determined from flexural vibration as 
described by E. Schreiber et al., "Elastic Constants and Their 
Measurement", McGraw-Hill Publishing Co., NY (1973) pages 88 to 90. The 
general equation which relates modulus of elasticity (Young's modulus) and 
the flexural resonant frequency (f.sub.E) is: 
##EQU2## 
where K=radius of gyration of the cross-section about the axis 
perpendicular to the plane of vibration. 
m=constant depending on the mode of vibration. 
T=shape factor, which depends upon the shape, size, and Poisson's ratio of 
the specimen and the mode of vibration. 
l=length of the specimen 
.rho.=density

The objects and advantages of this invention are further illustrated by 
example, but it should be understood that the particular material used in 
these examples, as well as amounts thereof, and the various conditions and 
other details described, should not be construed to unduly limit this 
invention. Percents and parts are by weight unless otherwise specified. 
The examples below describe adding silicon carbide, under various 
conditions, to two different host ceramic matrices. The matrices were 
3:1:2 alumina-boria-silica and 1:1 zirconia:silica. The elastic moduli 
reported in the examples for the control samples, i.e. those without 
silicon carbide, were 165 GPa (24.times.10.sup.6 psi) for 
alumina-boria-silica, and 90 GPa (13.times.10.sup.6 psi) for 
zirconia-silica. These values correspond to published values of 151 GPa 
(22.times.10.sup.6 psi) for alumina-boria-silica [Properties of 3M 
Nextel.TM. 312 Ceramic Fibers, 3M Ceramic Fiber Products, St. Paul, MN 
(1986)] and 76-104 GPa (11-15.times.10.sup.6 psi) for zirconia-silica [J. 
F. Lynch et al., Engineering Properties of Selected Ceramic Materials, 
American Ceramic Society (1966) pp. 5.5.1-12]. 
EXAMPLE 1 
Alumina-Boria-Silica having 3:1:2 molar ratio, with SiC 
The silicon carbide dispersion was prepared by sonifying (Branson.TM. 
Sonifier.TM. 350 Smith Kline Co., Shelton, Conn.) 1.7 grams of SiC (Los 
Alamos National Laboratory, hereinafter LANL) in 30 cc acetone for 10 
minutes with cooling by dry ice. The dispersion was mixed with 100 cc 
distilled water containing 0.05 gram anionic surfactant (Lomar PWA.TM., 
Diamond Shamrock Corp.) and sonified for 10 minutes more. This dispersion 
was mixed with 144 g of a 17% solids 3:1:2 molar ratio 
alumina-boria-silica precursor liquid and sonified for another 10 minutes. 
The fiber precursor material was made according to the procedure of 
Example 3 in U.S. Pat. No. 4,047,965, excepting for the greater amount of 
boric acid for the B.sub.2 O.sub.3 in the 3:1:2 molar ratio in the 
composition; the aluminum formoacetate was made by the digestion of 
aluminum metal in formic and acetic acids [aluminum powder (120 grams) was 
dissolved in a 90.degree. C. solution of 2200 grams water, 236 grams 
formic acid, and 272 grams acet5ic acid, over a period of eight hours]. 
The resulting precursor liquid was concentrated in a rotary evaporation 
flask (Buchi, Switzerland) operating at 35.degree.-45.degree. C. and 736 
mm Hg and the volatiles were removed until the viscosity was greater than 
75,000 mPa sec. Fibers were produced from the viscous sol by extruding 
through forty 102-micrometer diameter orifices and by collecting the 
fibers on a wheel turning at 30 meters per minute. The fibers were divided 
into two batches with one fired in air at 850.degree. C. for 15 minutes 
and the second batch at 950.degree. C. for 15 minutes. The heat-rise 
schedule was about 7.degree. C./min. with a 15 minute pause-soak at 
430.degree. C. and the black fibers were removed promptly when 950.degree. 
C. was attained. 
The fibers were oval shaped with a major axis of about 22 micrometers and a 
minor axis of about 11 micrometers. 
The average tensile strength of the 950.degree. C. fiber was 1035 MPa 
(150,000 psi) and for the 850.degree. C. fibers 1200 MPa (175,000 psi) 
(Basis: 1 psi=6900 Pa). The 850.degree. C. and 950.degree. C. fibers 
moduli of elasticity measured as 180 GPa (26.times.10.sup.6 psi) and 172 
GPa (25.times.10.sup.6 psi), respectively. The 3:1:2 alumina:boria:silica 
control fiber (Nextel.TM., 3M, St. Paul, Minnesota) had a tensile strength 
of 1550 MPa (225,000 psi) and a modulus of elasticity of 165 GPa 
(24.times.10.sup.6 psi). 
EXAMPLE 2 
A 20 wt % of silicon carbide in a matrix of alumina-boria-silica (3:1:2) 
was prepared as follows: 
Step 1: 1.7 grams of silicon carbide (Los Alamos National Laboratory) were 
sonified (Branson.TM. Sonifier.TM. 250, Smith Kline Co., Shelton, Conn.) 
in 40 cc acetone for 10 minutes. 
Step 2: The dispersion was slowly mixed with 60 cc distilled water 
containing 0.1 gram anionic surfactant (Lomar PWA.TM., Diamond Shamrock, 
Morristown, N.J.) and sonified for another ten minutes. 
Step 3: The mixture was placed on a rotating flask (Rotovapor.sup.TM, 
Buchi, Switzerland) until the acetone was removed. 
Step 4: Repeat step (1) for another 1.7 grams of SiC. 
Step 5: The dispersion from step (4) was then mixed with the mixture from 
step (3). At this point the sold contained 3.4 grams SiC, 40 cc acetone, 
0.1 gram anionic surfactant and 60 cc water. 
Step 6: The resulting sol from step 5 was then added to 72 grams of a 17% 
solids 3:1:2 molar ratio alumina-boria-silica precursor liquid and 
sonified again for another 10 minutes. 
Step 7: The resulting precursor liquid was concentrated in a rotary 
evaporator flask as described in Example 1. 
Fibers were produced from the viscous sol by extruding it through a 
spinnerette having forty-102 micrometer diameter holes and collecting the 
fibers on a wheel at a linear speed of 30 meters/min. 
The fibers were fired in air for 15 minutes at 950.degree. C. after heating 
at a rate of 7.5.degree. C. per minute from room temperature. 
The fired black fibers were characterized for tensile strength and modulus 
of elasticity. 
Three separate batches of fiber were prepared by this procedure. The 
tensile strengths and moduli of elasticity data were as follows: 
______________________________________ 
Number of 
Tensile strength: measurements 
______________________________________ 
Run 1 965 MPa (140 .times. 10.sup.3 psi) 
10 
Run 2 724 MPa (105 .times. 10.sup.3 psi) 
5 
Run 3 1014 MPa (147 .times. 10.sup.3 psi) 
7 
______________________________________ 
No. of Number of 
Modulus of elasticity 
fibers measurements 
______________________________________ 
Run 1 269 GPa = (39 .times. 10.sup.6 psi) 
5 24 
Run 2 324 GPa = (47 .times. 10.sup.6 psi) 
5 30 
Run 3 324 GPa = (47 .times. 10.sup.6 psi) 
7 27 
______________________________________ 
EXAMPLE 3 
This example was prepared as in Example 2 except 51. g of SiC, 120 cc 
acetone and 90 grams of alumina-boriasilica precursor liquid were used. 
Two separate batches of black fibers were made and the properties were as 
follows: 
______________________________________ 
Number of 
Tensile strength: measurements 
______________________________________ 
Run 1 800 MPa (116 .times. 10.sup.3 psi) 
7 
Run 2 807 MPa (117 .times. 10.sup.3 psi) 
11 
______________________________________ 
No. of Number of 
Modulus of elasticity 
fibers measurements 
______________________________________ 
Run 1 290 GPa (42 .times. 10.sup.6 psi) 
11 43 
Run 2 359 GPa (52 .times. 10.sup.6 psi) 
6 25 
______________________________________ 
EXAMPLE 4 
This example used silicon carbide obtained from cabothermal synthesis 
according to the following procedure. 
The raw materials for the SiC was a 3:1 raio of carbon black (Monarch.TM. 
1100 by Cabot) and silica (Nalco 2327), according to the reaction: 
EQU SiO.sub.2 +3C.fwdarw.SiC+2CO 
The carbon black was dispersed into the silica sol, dried, crushed and 
vacuum fired in an Astro.TM. furnace (Astro Industries, Inc., Santa 
Barbar, CA, U.S.A.) at 1400.degree. C. for 5 hours. 
This SiC powder had a particle size range of 600-900 Angstoms. Coarse 
particles had been separated from the powder by ball milling in acetone 
solvent for 20 hours. The dispersion was filtered through a No. 4 Whatman 
filter and refiltered through a Balston filter tube grade CO. The acetone 
was evaporated. This powder was dispersed into a 1:1 molar ratio 
zirconia-silica precursor. 
A zirconia silica precursor liuqid was prepared by mixing 302.84 grams 
zirconium acetate (Harshaw/Filtrol Partnership, Elyria, Ohio) into 110.6 
grams silica sol (Nalco.TM.-1034A, Nalco Chemical Company, Oak Brook, 
ILL.). This gave 1:1 molar ratio of zirconia-silica precursor of 27.3 wt % 
calcined solids. 
The dispersion of silicon carbide in zirconia-silica precursor liquid was 
prepared by ball milling 10 grams of the SiC powder, as described in 
Example 5, into 179 grams of zirconia precursor for 60 hours. The 
dispersion was filtered through Whatman.TM. No. 4 filter paper, then 
filtered again through Whatman.TM. No. 54. Twenty grams of lactic acid (85 
wt % aqueous solution) and 6.7 grams of formamide were added to the above 
filtered dispersion. 
The resulting precursor liquid was concentrated under vacuum in a rotary 
evaporation flask (Buchi, Switzerland) partly submerged in a water bath at 
temperatures of 35 to 45.degree. C. until it was viscous enough to enable 
the pulling of fibers with a glass rod. Fibers were spun from viscous sol 
with a 75 micrometer orifice spinnerette and 1.2 MPa (175 psi) extrusion 
pressure. The fibers were fired in air in an electric furnace (Lemont.TM. 
KHT, Lemont Scientific, State College, PA.), at 950.degree. C. for 15 
minutes after heating at a rate of 7.75.degree. C./minute. 
The average tensile strength of the resulting fibers was 703 MPa (102,000 
psi). The average modulus of elasticity for 10 fibers was 124 GPa 
(18.times.10.sup.6 psi) (a 3.6 density was used based on weight percent of 
silicon carbide in the matrix). 
The average tensile strength of the zirconiz silica control was 1014 MPa 
(147,000 psi). 
The average modulus of elasticity of the zirconia silica control was 90 GPa 
(13.times.10.sup.6 psi). 
EXAMPLE 5 
Two grams of SiC (LANL) were partially oxidized at 600.degree. C. for three 
hours in air in a Lindberg.TM. furnace (Lindberg Furnace C., Watertown, 
Wis.). The partially oxidized SiC was mixed in 47 grams of a 17 % solids 
3:1:1.3 molar ratio alumina-boria-silica precursor, sonified for 10 
minutes and fulterred through a No. 54 Whatman filter paper. The resulting 
prevurosr liquid was concentrated in a rotary evaporator flash as 
described in example 1. 
The concentrated sol was extruded using a spinnerette with 40 holes of 76 
micrometer diameter each and an extrusion pressure of 1.4 MPa. The 
continuous fibers obtained were dark brown in color and were fired in an 
electric tube furnace (KHT 250, Lemont Scientific State College, PA) to 
1300.degree. C. and held for 15 minutes. The furnace used a rate of 
heating of 7.4.degree. C. per minute. The fibers were black. The oxidized 
SiC powder contained about 42 wt % silica as measured by carbon analysis. 
This caused the resulting composite fiber to be 12 wt % silicon carbide in 
a matrix of alumina:boria:silica: 3:1:2 (mole raio). 
The 1300.degree. C. fired fibers had an average tensile strength of 932 MPa 
(135.times.10.sup.3 psi) and and average modulus of elasticity of 200 GPa 
(29.times.10.sup.6 psi). 
Various modifications and alterations of this invention will become 
apparent to those skilled in the art without departing from the scope and 
spirit of this invention, and it should be understood that this invention 
is not to be unduly limited to the illustrative embodiments set forth 
herein.