A tough Si.sub.3 N.sub.4 ceramic composite is provided by incorporating a ternary oxide additive containing hafnia, titania and zirconia. The ternary oxide additive consists essentially of 60 to 85 mole percent hafnia, 10 to 30 mole percent titania and 10 to 30 mole percent zirconia, and is added to the Si.sub.3 N.sub.4 in a proportion ranging from about 10 to about 50%, by volume of ternary oxide additive, based on the overall composition. The ceramic composites are prepared by providing a mixture of the Si.sub.3 N.sub.4 and the ternary oxide additive, forming the mixture into a predetermined shape, e.g. by cold pressing, and sintering the shape at an appropriate temperature, to solidify and densify the shape.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the field of ceramics, and particularly to the 
field of low thermal expansion Si.sub.3 N.sub.4 composites. 
2. Description of the Prior Art 
Ceramic compositions produced from silicon nitride and silicon carbide are 
the two most advanced ceramic materials presently employed. Silicon 
nitride has an advantage over silicon carbide in that the former has a 
lower thermal expansion coefficient than the latter. 
Si.sub.3 N.sub.4 can be made tougher by adding a second phase. Thus, for 
example zirconium oxide, ZrO.sub.2, has been added to Si.sub.3 N.sub.4, 
but since Si.sub.3 N.sub.4 has a low thermal expansion coefficient while 
ZrO.sub.2 has a relatively high thermal expansion coefficient, the thermal 
expansion coefficient of the resulting Si.sub.3 N.sub.4 /ZrO.sub.2 
composites is increased over that of Si.sub.3 N.sub.4 alone. 
Si.sub.3 N.sub.4 has also been toughened by incorporating fiber. However, 
fiber-reinforced Si.sub.3 N.sub.4, for example, suffers from (1) the 
difficulty in orienting and dispersing the fibers or whiskers, and (2) 
reactions between the fibers and Si.sub.3 N.sub.4 matrix. Dispersion 
toughened Si.sub.3 N.sub.4 has not shown significant improvements because 
the dispersoids (a) were too large, (b) were poorly dispersed, or (c) had 
a higher coefficient of thermal expansion (COTE) than the Si.sub.3 N.sub.4 
matrix. A higher COTE may actually hinder toughening mechanisms. Cracks 
prefer to propagate normal to tensile stresses and parallel to compressive 
stresses. Thus, cracks are deflected around particles in hydrostatic 
tension (COTE dispersoid &gt;COTE Si.sub.3 N.sub.4) but attracted into 
particles under hydrostatic compression (COTE dispersoid &lt;COTE Si.sub.3 
N.sub.4). When cracks propagate around particles, the particles accomplish 
little in toughening the matrix. When cracks propagate into particles, on 
the other hand, the particles can accomplish a high degree of toughening 
by dissipating energy and blunting the crack. 
Accordingly, past attempts to produce toughened Si.sub.3 N.sub.4 composites 
have met with only limited success. 
U.S. Pat. No. 3,657,063 to S. D. Brown et al, and assigned to the same 
assignee as the present application, discloses a high thermal shock 
resistant composite formed of a first layer of a composition of low 
expansion particulate oxide ultimately bonded to a second layer of a low 
expansion preformed silica material. The particulate oxide layer consists 
essentially of hafnia, zirconia and titania. 
SUMMARY OF THE INVENTION 
There is provided according to the basic concept of the present invention, 
a silicon nitride ceramic composite having enhanced toughness by 
incorporating a ternary oxide additive containing hafnia, titania and 
zirconia. Such hafnia-rich mixed oxide composition described in greater 
detail below, has a lower thermal coefficient of expansion (COTE) than 
Si.sub.3 N.sub.4 and when intimately mixed with the silicon nitride, 
induces toughness into the resulting sintered silicon nitride matrix 
ceramic composite. 
Other features of the invention include the method of completely dispersing 
the ternary oxide toughening phase into the silicon nitride matrix 
material, and the method for forming the resulting uniform mixture of 
silicon nitride matrix material and ternary oxide mixture, e.g. by 
injection molding, prior to sintering to produce the tough Si.sub.3 
N.sub.4 ceramic composite of the present invention. 
Thus, a toughened ceramic structural material can be fabricated according 
to the invention by adding to Si.sub.3 N.sub.4 the above ternary oxide 
composition having a high melting temperature and a COTE lower than 
Si.sub.3 N.sub.4, particularly in conjunction with wet mixing techniques 
which ensure complete dispersion of the toughening phase into the Si.sub.3 
N.sub.4. 
OBJECTS OF THE INVENTION 
It is accordingly an object of the present invention to provide improved 
low thermal expansion Si.sub.3 N.sub.4 composite ceramics. 
Another object of the invention is the production of Si.sub.3 N.sub.4 
ceramic composites containing a second phase which effects substantial 
toughening of the resulting silicon nitride ceramic composite.C 
Yet another object is the provision of Si.sub.3 N.sub.4 ceramics having a 
low thermal expansion coefficient provided by incorporation of an additive 
having a high melting temperature and a COTE lower than silicon nitride. 
A specific object of the invention is to provide Si.sub.3 N.sub.4 ceramic 
composites having enhanced toughness by incorporation of a hafnia-rich 
mixed oxide composition having a lower COTE than Si.sub.3 N.sub.4. 
An additional object is the provision of procedure for producing such 
improved toughened Si.sub.3 N.sub.4 ceramic composites. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The hafnia-rich mixed oxide composition employed in the subject invention 
consists essentially of commercially available hafnium oxide, titanium 
oxide and zirconium oxide. These oxides are generally known as hafnia 
(HfO.sub.2), titania (TiO.sub.2) and zirconia (ZrO.sub.2) The ternary 
oxide composition as employed herein generally consists essentially of 60 
to 85 mole percent hafnia, 10 to 30 mole percent titania and 10 to 30 mole 
percent zirconia, with the preferred mole ratio dependent upon the thermal 
expansion characteristics desired in the resulting Si.sub.3 N.sub.4 
ceramic composite. 
In preferred practice, the three components of the ternary oxide 
composition are intimately mixed using colloidal processing techniques 
utilizing individual powders of submicron particle size. This is 
accomplished by mixing the three components in colloidal solution and then 
heating the mixed powders to pre-react them and form a solid solution of 
such components, and then grinding the reacted composition to produce an 
intimately mixed powder of the three oxide components, for admixture with 
the Si.sub.3 N.sub.4 matrix material. 
More specifically, each of the three oxide components can be dispersed in 
water at a pH required to produce a good dispersion. The respective 
dispersed powders are separately sedimented by conventional techniques 
such as centrifuging to eliminate relatively large particles and 
agglomerates greater than 1 micron size to obtain submicron material. 
Mixing of the resulting three suspensions is then carried out to obtain a 
uniform three phase aqueous dispersion containing the proper amount of 
particulate oxide component in each phase in colloidal state. If desired, 
the three oxide components, in appropriate proportions, can be dispersed 
initially in water and the relatively large particles greater than 1 
micron removed by sedimentation. 
The multiphase dispersion of all of the powder components can be flocced, 
as by a change in pH, to prevent segregation and retain an intimate 
mixture. The dispersion can then be filtered as by slip casting, and then 
dried, either at ambient temperature or elevated temperature, e.g. about 
50.degree. C. The dried composite powder is then heated to temperature 
ranging from about 1700 to about 1850.degree. C., e.g. 1700.degree. C. 
for approximately 4 hours, to react the three oxide components and obtain 
a solid solution thereof. The resulting material is then crushed to form 
an intimate mixture of the three oxide components in the hafnia-rich 
ternary oxide composition. 
The reground solid solution of the ternary oxide composition can then be 
dispersed in water to form an aqueous dispersion which is again sedimented 
as by centrifuging to eliminate relatively large particles and 
agglomerates to obtain particles of 1 micron or less in size. 
Silicon nitride in conventional powder form is dispersed in water at a pH 
required to produce a good dispersion and is treated by sedimentation 
techniques noted above to remove relatively large particles, i.e. 
particles greater than one micron size, and provide an aqueous dispersion 
of silicon nitride particles of micron or submicron size. 
The aqueous dispersion of silicon nitride is then mixed with the aqueous 
dispersion of the hafnia-rich ternary oxide composition, in proportions 
such that the ternary oxide composition comprises about 10 to about 50%, 
preferably about 15 to about 30%, by volume, based on the overall 
composition of silicon nitride and ternary oxide mixture. If desired, a 
small amount of sintering aid, e.g. about 2 to 4% of alumina by weight of 
the total solid composition, can be added to the aqueous dispersion or 
slurry. The resulting aqueous dispersion of the silicon nitride and 
ternary oxide composition containing, e.g. 15 to 30% by volume of the 
oxides in the solid composition, can then be formed into a desired shape 
prior to sintering. 
Various forming techniques can be employed to form a shape and consolidate 
it. Thus, for example, the Si.sub.3 N.sub.4 /ternary oxide composition can 
be subjected to cold pressing, isostatic pressing, extrusion, injection 
molding or slip casting. Cold pressing can be carried out in a suitable 
die under a suitable pressure, e.g. 15,000 psi. In injection molding a 
plasticizer is generally added with the Si.sub.3 N.sub.4 /oxide 
composition, and following injection molding to produce the desired shape 
the plasticizer is removed. Slip casting is generally carried out by 
introducing the aqueous dispersion of the Si.sub.3 N.sub.4 /ternary oxide 
composition into a plaster of paris moId where the water is removed 
leaving a dry layer of powder. 
The techniques of press forming, injection molding and slip casting of the 
Si.sub.3 N.sub.4 /ternary oxide composition are preferred. 
The resulting composition shape is then sintered for solidification and 
densification at sufficiently high temperature, e.g. about 1700.degree. to 
about 1850.degree. C. Thus, such sintering can be carried out in an air 
furnace at 1700.degree. C., e.g. for about an hour, or the sintering can 
be carried out in nitrogen at higher temperatures, e.g. about 1800.degree. 
C.

The following are examples of practice of the invention, but it will be 
understood that such examples are only illustrative and not limitative of 
the invention. 
EXAMPLE I 
A composition containing 60 m/o (mole percent) hafnia, 20 m/o zirconia and 
20 m/o titania is dispersed in water and particles greater than 1 micron 
are removed by sedimentation. The resulting suspension is mixed in 
colloidal state assisted by the use of ultrasonic energy. The powder 
suspension is then flocculated by adjusting the pH by adding hydrochloric 
acid, to retain an intimate mixture and prevent segregation. 
The suspended powder mixture is formed into a disc shape by press filtering 
through a metal filter disc under pressure, and the resulting shape is 
dried at 50.degree. C. overnight. The dried composition is then heated at 
about 1700.degree. C. for four hours in air to form a solid solution. The 
resulting solid solution is crushed and then ball milled using tungsten 
carbide, zirconium oxide or aluminum oxide balls. 
An aqueous dispersion of the crushed solid solution powder is made and 
particles of 1 micron size or less are recovered by sedimentation. 
Less than 1 micron size Si.sub.3 N.sub.4 powder and ternary oxide solid 
solution powder of less than 1 micron size are mixed while suspended in an 
aqueous slurry, employing 70 v/o (volume percent) Si.sub.3 N.sub.4 and 30 
v/o ternary oxide mixture. 4 w/o (weight percent) Al.sub.2 O.sub.3 
sintering aid is added to the slurry. The resulting slurry is press 
filtered to form a disc-shaped sample of the powder mixture, and the 
resulting powder mixture is dried, as noted above, and the dried powder 
mixture is sintered in air or nitrogen at 1700.degree. C. for one hour. 
The composite powders are sintered in an aluminum oxide or Si.sub.3 
N.sub.4 crucible. 
A tough silicon nitride ceramic composite is obtained of about 98% 
theoretical density, high strength of about 650 MPa (mega Pascals) and 
high toughness of 6-8 MPa.multidot.m.sup.1/2. 
EXAMPLE II 
The procedure of Example I is substantially followed except employing 1 to 
5 micron particle size of ternary oxide mixture and less than 1 micron 
size Si.sub.3 N.sub.4 powder, and a proportion of 80 v/o Si.sub.3 N.sub.4 
and 20 v/o ternary oxide mixture, and forming the resulting mixture by hot 
pressing at 1700.degree. C. for 1 hour in a graphite die. 
Results similar to Example I are obtained. 
EXAMPLE III 
The procedure of Example I is followed except that the zirconia, hafnia and 
titania are coprecipitated in the proper molar ratios, and flocculated to 
prevent separation. 
The solids are then calcined and reacted to form a solid solution. 
The Si.sub.3 N.sub.4 powder and the crushed ternary oxide solid solution 
powder are mixed in the proportions of 75 v/o Si.sub.3 N.sub.4 and 25 v/o 
ternary oxide mixture, to which is added 2 w/o silica to enhance 
sintering, and the mixture is formed as a colloidal aqueous suspension. 
The aqueous suspension is filtered and the resulting powder mixture is 
mixed with 50 v/o of a plastic binder and the resulting composition is 
injection molded to form a shaped sample, followed by drying and sintering 
at 1700.degree. C. for 1 hour in air. The resulting composition is then 
subjected to hot isostatic pressing at 1600.degree. C. and 30,000 psi to 
achieve a ceramic composite having near theoretical density. 
The novel Si.sub.3 N.sub.4 /ternary oxide ceramic composite materials of 
the present invention can be employed to manufacture various items of 
commerce and science. Thus, for example the composites can be employed for 
the fabrication of advanced heat engine structural components such as 
Diesel engine components and in other structural applications requiring a 
tough refractory material having a low coefficient of thermal expansion 
and high thermal shock resistance, such as for nozzle applications in high 
performance rocket engines, and in the fabrication of combustion chambers. 
From the foregoing, it is seen that according to the invention, tough 
Si.sub.3 N.sub.4 ceramic composites having lower thermal expansion than 
the Si.sub.3 N.sub.4 matrix material can be produced by incorporating the 
above hafnia-rich ternary oxide composition as additive, in conjunction 
with various processing techniques to obtain ceramic composites having a 
variety of important applications. 
It is to be understood that what has been described is merely illustrative 
of the principles of the invention and that numerous arrangements in 
accordance with this invention may be devised by one skilled in the art 
without departing from the spirit and scope thereof.