High temperature ceramic composite

A composite ceramic structure which does not fail catastrophically and thus is useful as a ceramic rolling contact bearing assembly is disclosed. The structure is a ceramic monolith bonded through an interlayer to a fiber-reinforced ceramic body. The structure is useful at elevated temperatures.

TECHNICAL FIELD 
The present invention is directed to composite ceramic structures which (i) 
have surface properties of ceramic monoliths, (ii) are not subject to the 
catastrophic failure as are ceramic monoliths, and (iii) can withstand 
extended exposure to temperatures up to 500.degree. C. or higher. The 
composite structures are produced by a chemical attachment of a ceramic 
monolith to a fiber-reinforced ceramic. The attachment causes any cracks 
generated in the monolith to be arrested by the fiber-reinforced ceramic. 
BACKGROUND OF THE INVENTION 
A major impediment to the use of ceramic monoliths in certain applications 
is that when they fail, they do so catastrophicly without any warning of 
the impending failure. Previous attempts at overcoming the problem of 
catastrophic failure have entailed reinforcing the ceramic monolith with 
other materials, i.e. refractory fibers. While the addition of the 
refractory fibers generally does toughen a ceramic monolith, prevent the 
catastrophic failure, and allow the ceramic to be used at conventional 
elevated temperatures, it also destroys some of the desirable properties 
of the ceramic monolith such as wear resistance and fatigue resistance. 
Moreover, the incorporation of the fibers creates an entirely new material 
having substantially different surface and other properties from the base 
ceramic from which it was prepared. Thus, while the fiber-reinforced 
materials possess increased toughness and non-catastrophic failure 
characteristics, they are unsuitable for applications requiring the 
surface integrity and properties of a monolithic ceramic. And, as new 
materials, they must undergo extensive evaluation before even being 
considered for commercial use. 
Ceramic monoliths having the same chemical composition have been joined to 
each other in the past, particularly to produce complex shapes which could 
not be molded as a single body. The joined bodies, however, have suffered 
from the same catastrophic failure problems as single monoliths. 
Also ceramic monoliths have been joined to metals, particularly for the 
purpose of putting a wear surface on the metal. Although beneficial for 
wear performance, these materials still generally exhibit catastrophic 
failure behavior. Moreover, the vast difference in the coefficients of 
thermal expansion between metals and ceramics results in high stresses in 
the structures which (i) can lead to fracture during thermal excursions in 
use and manufacture and (ii) precludes the use of such structures at 
temperatures greater than about 400.degree. C. for copper-silver braze 
connections. 
Ceramic light weight armor for helicopter seats and the like has been 
produced by joining silicon carbide or boron carbide bodies to a polymeric 
composite of epoxy with Kevlar fibers. The function of the polymeric 
composite is to stop the shrapnel and hold the ceramic pieces together 
after fracture from a projectile. The resultant structure is only useful 
at relatively low temperature due to the presence of the polymeric 
composite. 
Thus if a composition could be developed having (i) the surface and other 
characteristics of a ceramic monolith, (ii) the non-catastrophic failure 
characteristics of ductile engineering materials and (iii) the capacity 
not to deteriorate when exposed to elevated temperatures for extended 
periods of time, it would find utility in many future applications. 
Particularly, such a material is needed to produce rolling contact 
bearings for use in future aircraft engine components. Also it should find 
use in automobile engine components, aerospace control surfaces, leading 
edges in aircraft or missile application, turbine engine components, high 
temperature enclosures, space application including the National Aerospace 
plane (NASP) engine, as well as in various structural applications. 
It is an object of the present invention to produce such a composition 
which heretofore has not existed. 
SUMMARY OF THE INVENTION 
The composite ceramic structures according to the present invention are 
characterized in that they have at least one surface with the properties 
of a ceramic monolith, are not subject to the usual catastrophic failure 
of ceramic monoliths, and do not fail when exposed to elevated 
temperatures for an extended period of time. The structures comprise a 
dense ceramic monolith bonded to a fiber-reinforced ceramic body by means 
of a high temperature resistant interlayer.

DETAILED DESCRIPTION OF THE INVENTION 
The ceramic monoliths useful in the present invention are dense bodies of 
ceramic materials such as silicon nitride and silicon carbide. For bearing 
applications the currently preferred monolith is silicon nitride due to 
its excellent rolling contact fatigue behavior (30,000,000 cycles at 
1.times.10.sup.6 psi contact stress) which makes it particularly 
advantageous when the composite structure is used as a bearing ring. The 
silicon nitride monoliths, for example, are generally prepared by 
sintering silicon nitride powder in combination with one or more sintering 
aids. Any sintering aid may be used so long as it can make the silicon 
nitride dense, strong, and hard. However, it is preferable to use MgO, 
Al.sub.2 O.sub.3, Y.sub.2 O.sub.3, or other rare earth oxides, and/or a 
compound of Mg, Al, Y or other rare earths which is converted to the 
appropriate oxide by heating. When an Mg compound is used, it should be 
present in an amount of about 0.5 to 15% by weight when calculated as MgO. 
When an Al compound is used, it should be present in an amount of about 2 
to 15% by weight when calculated as Al.sub.2 O.sub.3. When a Y or other 
rare earth compound is used, it should be present in an amount of about 1 
to 15% by weight when calculated as the oxide. 
Bonded to the monoliths to provide catastrophic failure resistance to the 
monoliths are fiber-reinforced ceramic bodies. To prepare the desired 
joined product, the ceramic matrix composite must exhibit non-catastrophic 
failure. This is generally exhibited by materials that have fracture 
toughnesses greater than about 20 MPam.sup.1/2. The bodies may be prepared 
from a variety of ceramic fibers including alumina, carbon, silica, 
silicon carbide, and the like. The specific fiber used will depend upon 
the matrix, but more preferably it will be silicon carbide. The fiber may 
be either continuous or discontinuous, but preferably is continuous. When 
discontinuous fibers are used they must have sufficient length to result 
in fiber-reinforced ceramic bodies which do not fail catastrophically. 
Generally, a fiber length of 10 mm or more may be used, preferably at 
least 30 mm, with longer fibers preferred. The ceramic matrix which is 
reinforced by the fiber may be silicon nitride, silicon carbide, alumina, 
a silicate glass, a silicate-based glass ceramic, and the like. Preferably 
the ceramic matrix will be silicon carbide, silicon nitride, or a 
silicate-based glass ceramic. Most preferably the ceramic matrix will be a 
silicate-based glass ceramic. For the fiber-reinforced ceramic body to not 
fail catastrophically, there must be a "debonding" layer between the fiber 
reinforcement and the matrix. Suitable debonding layers include such as 
carbon, boron nitride, as well as the presence of regions of high 
porosity, as described in U.S. Pat. No. 4,885,199. Preferably the 
debonding layer is carbon. The debonding layer material may be coated onto 
the fiber before incorporation into the matrix or may be formed in situ by 
reaction between the fiber and the matrix during preparation of the 
ceramic body. The currently most preferred fiber-reinforced ceramic body 
is a calcium aluminosilicate glass ceramic with continuous silicon carbide 
yarn imbedded therein and having a carbon debonding layer formed in situ. 
Such material is available from Corning Glass Works as "CAS/Nicalon 
Experimental 1." 
The ceramic monolith must be bonded to the fiber-reinforced ceramic body so 
that the composite structure can not fail catastrophically, i.e. even 
after a crack develops in the monolith the bonding must be sufficiently 
strong to prevent the monolith from breaking away from the 
fiber-reinforced ceramic body. Any suitable bond, i.e. one not only 
holding the solid monolith to the ceramic body but also holding a cracked 
monolith thereto, such as diffusion bonding with an active or reactive 
metal interlayer, reactive metal brazes, active metal brazes, or 
glass/ceramic diffusion bonding may be used. Preferably, however, an 
active metal braze is used since it permits bond formation generally with 
the lowest amount of exposure of the materials being joined to elevated 
temperatures which could detrimentally affect one or more of the 
components, e.g. extensive exposure of the composite to temperatures above 
about 600.degree. C. in an oxidative atmosphere can cause oxidation of the 
debonding layer and thus eliminate the non-catastrophic failure 
characteristics of the composite structure. Bonding methods which use 
deformable metal interlayers may be used when the difference in thermal 
expansion coefficients between the monolith and the fiber-reinforced 
composite are large. 
In the bonding process the monolith and the fiber-reinforced ceramic body 
are joined, generally through an intermediate cohesive interlayer, 
generally at temperatures of about 500.degree. C. or higher, with the 
specific temperature and heating cycle being dependent upon the specific 
materials being joined as well as the joining method and materials. The 
interlayer serves to bond the monolith to the fiber-reinforced ceramic 
body and hold both a solid and a cracked monolith to the fiber-reinforced 
ceramic body. Examples of suitable interlayer materials include very thin, 
i.e. about 0.001 to about 0.005 inch, preferably about 0.002 to 0.004 
inch, copper, nickel, aluminum, and similar metals; active metal braze 
compositions such as those based upon copper, silver, gold, palladium, 
titanium, and the like; and sputter coatings of active metals such as 
titanium, nickel, and the like. These interlayer materials are all well 
known in the art of joining and further details thereon may be found in 
the literature. The interlayer is placed between the surfaces to be joined 
and the composite structure is heated for a controlled period until the 
joining has been completed. While the principles of joining the ceramic 
materials used herein are well known, it is also known that a specific 
joining system must be developed for each pair of ceramics to be joined. 
Variations in the joining material as well as the time and temperatures 
used to perform the join can be determined by routine experimentation. A 
particularly suitable, relatively low temperature, active metal braze 
composition for joining a silicon nitride monolith to a calcium 
aluminosilicate glass is one containing about 59% silver, 27% copper, 13% 
indium, and 1% titanium. When this braze was used to join a silicon 
carbide monolith to the same glass with the same brazing cycle of Example 
I, the join was not successful. Changing the heating cycle or the braze 
composition will produce a successful join. 
When an active metal braze is used, a braze in foil form is generally used. 
Foil brazes have very limited ductility and cannot be molded easily but 
they do ensure coverage of the entire surface being joined. Alternatively, 
paste brazes may be used when the joint does not lend itself to a flat 
interface, e.g. rough or multiple curvatures. Preferably, the brazing will 
be performed under high vacuum conditions, i.e. an initial vacuum of at 
least about 10.sup.-5 Torr. To ensure good brazing results, both 
components being joined need to reach the brazing temperature at the same 
time during the heat up. If one material were to heat faster during 
furnace heating (e.g. due to higher thermal conductivity) a non-uniform 
heat distribution could occur at the braze joint resulting in poor braze 
character. 
As in most joining procedures, it is essential that the surfaces being 
joined be smooth, dry and clean of all dirt and oily residues. Generally, 
the surfaces will be polished to sufficient smoothness, e.g. a 5.0 
microinch RMS finish or better. 
One product which may be prepared using the composite technology of this 
invention is a rolling contact bearing assembly. One type of bearing 
assembly which may be formed according to this invention is shown in FIG. 
1. The bearing assembly 10 comprises a ceramic outer ring 12 formed from a 
ceramic monolith member 14 attached to a fiber-reinforced member 16 and a 
ceramic inner ring 18 formed from a ceramic monolith member 20 attached to 
a fiber-reinforced member 22. Within the cavity formed by the inner and 
outer rings is positioned a ceramic ball rolling element 24 which is 
retained in position by a retainer 26. Normally a plurality of balls are 
used in a single assembly. The tolerances are designed so that the ceramic 
balls may freely rotate in the cavity. The ceramic monolith members and 
the ball 24 may be made of any suitable ceramic material as previously 
described herein, but preferably are silicon nitride. The fiber-reinforced 
members may also be made of any suitable materials as previously described 
herein. The retainer is preferably manufactured of heat-resistant 
ceramics, but may be manufactured of other suitable materials such as 
metal, metal alloys, and composites. The outer bearing ring generally has 
dimensions of about 4 to 500 mm and the ball 24 generally has a diameter 
of about 1/2 to 50 mm. 
The rolling contact bearing is constructed by first forming the individual 
components, then attaching the fiber-reinforced members to the ceramic 
monolith members by means of a suitable active metal braze to form inner 
and outer bearing rings, and then assembling the complete bearing. 
FIG. 2 (like parts have the same reference numerals) shows another rolling 
contact bearing assembly 28 which is similar to the bearing of FIG. 1, 
except that the geometry of the inner ring 30 is slightly different from 
the geometry of the inner ring 18 of the bearing of FIG. 1. In this 
embodiment the inner ring 30 comprises a ceramic monolith member 32 and a 
fiber reinforcement member 34. 
An example of a roller style bearing is shown in FIG. 3. As shown, the 
bearing 35 comprises a ceramic outer ring 36 formed from a ceramic 
monolith member 38 attached to a fiber-reinforced member 40 and a ceramic 
inner ring 42 formed from a ceramic monolith member 44 attached to a 
fiber-reinforced member 46. In the cavity formed between the inner and 
outer rings is a cylindrical roller 48 positioned within the retainer 50. 
As in the previous designs, normally a plurality of rollers are present in 
a single assembly. The rings are free to rotate with respect to one 
another. The materials used to construct the individual members and the 
method of construction is as described with reference to FIG. 1. 
In use, the ceramic composite bearing assemblies may be subjected to a 
variety of stresses from both steady applied loads and impact. In many 
cases the bearing races also serve as structural elements in the assembly 
which uses the bearing. While previous work has shown catastrophic failure 
of the rings to be unlikely from rolling fatigue sources, brittle failure 
is possible from impact, thermal excursions, and unexpected structural 
loads. 
The fiber-reinforced ceramic reinforcing portions keep the monolithic race 
surfaces in rigid registration when a structural failure occurs in either 
the inner or outer ring. This interrupted failure will allow the bearing 
to function, albeit with increased vibration, until such time as the 
bearing can be replaced. The high wear resistance of the ceramic monolith 
will prevent rapid degradation of the fractured edges of the race surface. 
This invention eliminates catastrophic failure in these critical 
components, overcoming a severe impediment to the use of ceramic monolith, 
i.e. silicon nitride, bearings in commerce and industry. 
In the following non-limiting examples all parts and percents are by weight 
unless otherwise specified. 
EXAMPLE I 
A 2.times.2.times.0.1 inch silicon nitride tile containing 1% magnesia 
sintering aid is prepared for joining by polishing to a 5.0 microinch rms 
finish with diamond paste. An identical sized tile of a calcium 
aluminosilicate ceramic glass reinforced with 35 volume % uniaxially 
aligned silicon carbide fiber (CAS/Nicalon Experimental #1 from Corning 
Glass Works) is prepared for joining by grinding the surface flat and also 
polishing to a 5.0 microinch rms finish. Both polished surfaces are 
cleaned of all dirt and oily residues. A 0.002 inch thick piece of active 
metal brazing foil having a composition of 23.5% copper, 60.75% silver, 
1.25% titanium, and 14.5% indium (Incusil-ABA from GTE-WESCO) is placed 
between the two polished surfaces. 
The tiles are fixed together to prevent sideways movement by placement in a 
graphite fixture which had been previously outgassed to 1200.degree. C. in 
vacuum to reduce any absorbed gases. The fixture had been machined with a 
relief around the brazed area to prevent contact between the braze alloy 
and the graphite. A cold wall metallic element vacuum furnace which pulled 
an initial vacuum of greater than 10.sup.-5 Torr is used. The tiles are 
heated to 590.degree. C. at 50.degree. C. per minute, held for 20 minutes, 
heated to 770.degree. C. at 15.degree. C. per minute, held for 10 minutes, 
and then the furnace is cooled to room temperature. 
The joined composite structure is cut into test specimens 0.231" 
wide.times.0.194" thick.times.2" long. The specimens are cut so that the 
fibers are parallel to the length of the specimen. The specimens are 
tested in a four point flexure configuration using a support span of 40 mm 
and a load span of 20 mm. The silicon nitride monoliths crack at an 
average stress of 69,300 psi with a strain at maximum stress of 0.0081. 
The cracked composite is shown in FIG. 4. As can be seen, the crack is 
through the monolith but stops at the braze which continues to hold the 
monolith in place. The fiber-reinforced ceramic body bonded to the 
monolith prevents catastrophic failure of the monolith. Deformation beyond 
the 0.0081 strain is within the fiber-reinforced ceramic material without 
catastrophic failure. 
EXAMPLE II 
The procedure of Example I is repeated except that the braze is replaced by 
a 0.004" thick active metal brazing foil having the composition 27.5% 
copper, 70.5% silver, and 2% titanium (Cusil from GTE/WESCO) and the high 
temperature brazing step is performed at 860.degree. C. rather than 
770.degree. C. 
The composite structure is tested in the same manner as Example I. The 
silicon nitride monoliths crack at an average stress of 67,250 psi with a 
strain at a maximum stress of 0.00765. The cracked composite is 
essentially identical as in FIG. 4 with the crack extending through the 
monolith but stopping at the braze which continues to hold the monolith in 
place. The fiber-reinforced ceramic body bonded to the monolith again 
prevents catastrophic failure of the monolith. 
COMATIVE EXAMPLE A 
The procedure of Example I is repeated except that the braze is omitted and 
direct bonding of the monolith to the fiber-reinforced ceramic body is 
attempted by hot pressing at temperatures of 1200.degree., 1400.degree., 
and 1500.degree. C. for times ranging from 0.5 to 4 hours. The bodies do 
not join. 
COMATIVE EXAMPLE B 
The procedure of Comparative Example A is repeated except that an 
aluminosilicate glass is placed between the monolith and the 
fiber-reinforced ceramic body. A very weak join is formed, but the join 
does not survive machining into test specimens. Catastrophic failure of 
the monolith occurs during a stress-strain test. 
COMATIVE EXAMPLE C 
In an attempt to join silicon carbide fibers to a dense silicon nitride 
body, i.e. to use only the reinforcing fibers but without the ceramic body 
of Example I, by using nitrogen glasses, the following is done: a 
2.times.2.times.1/8 inch tile of Si.sub.3 N.sub.4 with 1% MgO was used as 
the monolith. SiC filaments (AVCO monofilaments) were cut and layered on 
the Si.sub.3 N.sub.4 plate. On top of the fibers a powder mixture of a 
nitrogen glass forming composition (20 mole % AlN, 25 mole % Y.sub.2 
O.sub.3, 55 mole percent SiO.sub.2) was applied. This was fired at 
1520.degree. C. for 2 hours in 1 atm of nitrogen. The result was brittle 
failure since the combination of filament plus glass did not form a 
composite structure having the necessary non-catastrophic failure 
characteristics. 
COMATIVE EXAMPLE D 
The same braze composition, braze conditions, and composite of Example III 
were used for joining to Al.sub.2 O.sub.3 and ZrO.sub.2 ceramic monoliths. 
The joinings were unsuccessful. Because of the larger coefficient of 
thermal expansion for Al.sub.2 O.sub.3 and ZrO.sub.2 than for silicon 
nitride and silicon carbide, they were put in tension during cooling, 
which caused the Al.sub.2 O.sub.3 to crack and the braze to fail in the 
ZrO.sub.2 case. The use of ductile metal interlayers along with the brace 
are necessary to join Al.sub.2 O.sub.3 and ZrO.sub.2 to the specific 
fiber-reinforced ceramic used.