Process for producing nonequiaxed silicon aluminum oxynitride

A .beta.'SiAlON produced from initial reactant materials comprising Al.sub.2 O.sub.3, and carbon that produces a .beta.'SiAlON material having a strength greater than about 50 ksi at approximately 1000.degree. C. In a preferred embodiment of the invention the .beta.'SiAlON material has a strength greater than about 50 ksi at approximately 1200.degree. C. In a second embodiment of the present invention is a low-cost process for producing unsintered high purity alpha silicon nitride structured SiAlON by carbothermic reaction from initial reactant materials comprising Al.sub.2 O.sub.3, SiO.sub.2 and carbon.

TECHNICAL FIELD 
This invention relates to a process for making nonequiaxed silicon aluminum 
oxynitride material and, more particularly, relates to a process for 
producing an unsintered silicon aluminum oxynitride by carbothermic 
reaction without the need to use contaminating impurities to increase the 
rate of reaction. 
BACKGROUND ART 
Silicon aluminum oxynitride refractory/ceramic materials, and more 
particularly materials in the Si.sub.3 N.sub.4 --AlN--Al.sub.2 O.sub.3 
--SiO.sub.2 system, are of ever-increasing interest for refractory/ceramic 
applications. For ease of identification, compositions within this system 
are referred to as SiAlON, and a number of different phases of SiAlON have 
been produced and identified. For example, Jack et al U.S. Pat. No. 
3,991,166 describes one phase and methods of making it, the phase having 
the general formula Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z where z is 
greater than zero and less than or equal to five. Various compositions 
within the bounds of the general formula taught by Jack et al may be 
produced, and each has a crystalline structure similar to beta-Si.sub.3 
N.sub.4 and is consequently identified as beta'-SiAlON (.beta.'-SiALON). 
.beta.'-SiAlON can be defined as a solid solution of Al.sub.2 O.sub.3 
within a matrix of Si.sub.3 N.sub.4. The compositional limits of 
reactants, referred to as effective reactants, to produce .beta.'-SiAlON 
may be seen by referring to FIG. 2. The compositional amounts of Si.sub.3 
N.sub.4, AlN and Al.sub.2 O.sub.3 for any .beta.'-SiAlON formulation may 
be determined by referring to line AB which is a plot of the compositions 
of the aforesaid compounds to produce a .beta.'-SiAlON having the general 
formula Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z where z is greater than zero 
and less than or equal to five. 
Another phase, known as y-phase SiAlON represented by the formula 
SiAl.sub.4 O.sub.2 N.sub.4, is described in an article entitled "Review: 
SiAlONs and Related Nitrogen Ceramics", published in Journal of Material 
Sciences, 11 (1976) at pages 1135-1158. Compositions of SiAlON within a 
given phase and from phase to phase demonstrate varying characteristics, 
for example, variances in density, which effect their preferential use in 
a given application. 
A number of processes for making silicon aluminum oxynitride refractories 
and technical ceramics have been suggested. Weaver U.S. Pat. No. 3,837,871 
describes a method for producing a product having a substantial amount of 
what the patentee believes to be the quaternary compound silicon aluminum 
oxynitride which has a structure similar to that of beta Si.sub.3 N.sub.4 
but with an expanded lattice structure. Weaver's method of making the 
described product is hot pressing Si.sub.2 ON.sub.2 (silicon oxynitride) 
in the presence of varying amounts of aluminum. 
Kamigaito et al U.S. Pat. No 3,903,230 describes a method of making a 
silicon aluminum oxynitride ceramic by sintering or hot pressing a mixture 
of finely divided powders of silicon nitride, alumina and aluminum 
nitride. 
Cutler U.S. Pat. No. 3,960,581 describes a process for producing SiAlON by 
reacting silicon and aluminum compounds in the presence of carbon and 
nitrogen. Cutler teaches and stresses the importance of using a reactant 
material having the silicon and aluminum compounds intimately combined 
prior to nitriding in order that aluminum oxide is intimately dispersed 
throughout silicon nitride in the final product. Suggested reactant 
materials are clay, rice hulls having a solution containing a dissolved 
aluminum salt absorbed therein, and a precipitate of aluminum and silicon 
salts. In each case Cutler emphasizes that the silicon and aluminum 
compound reactants are intimately combined prior to nitriding to produce 
SiAlON. Further, in the process as taught by Cutler, excess carbon and 
unreacted silicon dioxide must be removed from the mixture after the 
mixture is nitrided. 
Maeda U.S. Pat. No. 4,172,108 describes a process for production of SiALONs 
which involves heating a mixture containing a silicon nitride precursor 
having at least one silicon-nitrogen bond and an alumina precursor having 
at least one aluminum-oxygen bond to at least 1000.degree. C. 
Inoue U.S. Pat. No. 4,680,278 describes a process for preparing aluminum 
nitride powder having small particle size and small particle size 
distribution and also having a uniform shape of particles, at a lower 
temperature and in a shorter period of time. Inoue teaches that the 
aluminum nitride powder can be mixed in predetermined amounts with silicon 
carbide and silicon nitride to form SiAlON. 
Thus far, of all the SiAlON materials, the .beta.'-SiAlONs have generated 
the greatest interest because their refractory properties and corrosion 
resistance characteristics are comparable to other nitride refractories 
such as silicon nitride and silicon oxynitride. More recently, the 
.beta.'-SiAlONs have generated a great deal of interest as technical 
ceramics, i.e. monolithic engineered ceramics. 
.beta.'-SiAlON compositions offer a distinct advantage over silicon nitride 
and silicon oxynitride for making a refractory because some of the 
compositions of .beta.'-SiAlON material can be used to produce a high 
density refractory by conventional sintering techniques. To produce high 
density ceramics from silicon nitride or silicon oxynitride requires the 
use of pressure sintering techniques. 
Jack et al U.S. Pat. No. 3,991,166 describes a .beta.'-SiAlON product 
produced by sintering a mixture of alumina or a compound which decomposes 
to produce alumina and silicon nitride. Another method of producing 
.beta.'-SiAlON as described by Jack et al is nitriding silicon powder in 
the presence of alumina powder. 
Demit U.S. Pat. No. 4,147,759 describes a method of manufacturing 
.beta.'-SiALON compounds. The method involves reacting silicon nitride and 
aluminum oxynitride in the presence of an agent which generates gaseous 
silicon monoxide. 
It may be noted that several of the foregoing processes employ silicon 
nitride or silicon oxynitride as reactants. Neither of these compounds is 
found in nature and they are relatively expensive to produce. The 
production of SiAlON from discrete particles of an SiO.sub.2 source and 
discrete particles of an Al.sub.2 O.sub.3 source requires a catalyst to 
enhance both reaction rates and stoichiometry so as to make the process 
inexpensive and economically attractive. 
Phelps et al U.S. Pat. No. 4,499,193 describes a process for 
carbothermically producing an unsintered refractory material comprising 
essentially .beta.'-SiAlON wherein the initial reactants include discrete 
particles of an SiO.sub.2 source and discrete particles of an Al2O3 
source. Phelps discloses that it is advantageous to add iron in the form 
such as Fe.sub.2 O.sub.3 as a catalyst in promoting the formation of 
.beta.'-SiAlON. The .beta.'-SiAlON material produced according to the 
teachings of this patent have the microstructure of .beta.'-Si.sub.3 
N.sub.4 and sinter into a material that has an equiaxed microstructure. 
Phelps et al U.S. Pat. No. 4,511,666 describes a process for 
carbothermically producing an unsintered refractory material comprising 
essentially .beta.'-SiAlON wherein initial reactants include discrete 
particles of an SiO.sub.2 source, discrete particles of an Al.sub.2 
O.sub.3 source and discrete particles of silico alumina compounds. The 
initial reactants are nitrided for sufficient times and temperatures to 
convert at least a portion of the initial reactants to at least a portion 
of effective reactants, and the effective reactants are then further 
heated to produce an essentially .beta.'-SiAlON refractory material. 
Phelps discloses that it is advantageous to add iron in the form such as 
Fe.sub.2 O.sub.3 as a catalyst in increasing the rate of reaction and 
promoting the formation of .beta.'-SiAlON. Phelps also discloses that 
oxides of other transitional metals such as nickel, chrome or manganese, 
for example, may also be used as catalysts. The .beta.'-SiAlON material 
produced according to the teachings of this patent have the microstructure 
of .beta.-Si.sub.3 N.sub.4 and sinter into a material that has an equiaxed 
microstructure. 
Generally, only a small percentage of catalyst, such as 2% or less Fe.sub.2 
O.sub.3, for example, is added to increase the rate of carbothermic 
reaction and reduce the length of time needed to form SiAlON. However, 
when Fe.sub.2 O.sub.3 is used as a catalyst, the iron reacts with silica 
to produce an FeSi phase which is present as a contaminant in the final 
sintered product. FeSi forms flaw sites in the SiAlON which initiate 
fractures and lowers its room temperature strength. Furthermore, when the 
SiAlON is used in high temperature (1200.degree.-1300.degree. C.) 
applications, the FeSi oxidizes and further reduces the materials 
strength. 
The use of .beta.'-SiAlONs as refractory/ceramic materials is the result of 
their ability to maintain superior strength, hardness, creep resistance 
and resistance to chemical attack at elevated temperatures (above 
1000.degree. C.). Co-pending U.S. Ser. No. 351,660 discloses a process for 
producing an unsintered SiAlON material by carbothermic reaction without 
the use of contaminating transition metal oxides to increase the rate of 
reaction. The process includes providing small quantities of SiAlON 
crystals which seed the reaction. The absence of FeSi in the 
.alpha.'SiAlON produced a material with an increased the high temperature 
strength. 
Increasing demands in the refractory/ceramic industry as creating a need 
for materials with increased strength at high temperatures. It would be 
advantageous, therefore, to provide a process whereby readily available 
and relatively inexpensive initial reactant materials comprising Al.sub.2 
O.sub.3 and SiO.sub.2 are nitrided to make silicon aluminum oxynitride 
materials having a strength greater than 50 ksi at 1000.degree. C. 
Another object of the present invention is to provide a low-cost process 
for producing SiAlON from initial reactant materials comprising Al.sub.2 
O.sub.3, SiO.sub.2 and carbon that does not require the addition of 
transition metals such as iron, nickel, chrome or manganese, to be used as 
catalysts to increase the rate of reaction. 
A further object of the present invention is to provide a process for 
producing unsintered .beta.'-SiALON powder by carbothermic reaction. 
These and other objects and advantages will be more fully understood and 
appreciated with reference to the following description. 
SUMMARY OF THE INVENTION 
A .beta.'SiAlON produced from initial reactant materials comprising 
Al.sub.2 O.sub.3, SiO.sub.2 and carbon that produces a .beta.'SiAlON 
material having a strength greater than about 50 ksi at approximately 
1000.degree. C. In a preferred embodiment of the invention the 
.beta.'SiAlON material has a strength greater than about 50 ksi at 
approximately 1200.degree. C. 
A second embodiment of the present invention is a low-cost process for 
producing unsintered high purity alpha silicon nitride structured 
.beta.'SiAlON by carbothermic reaction from initial reactant materials 
comprising Al.sub.2 O.sub.3, SiO.sub.2 and carbon. 
A third embodiment of the present invention is a nonequiaxed .beta.'SiAlON 
material. The nonequiaxed .beta.'SiAlON is produced by a process which 
comprises the steps of (1) providing a mixture of initial reactant 
materials as sources of SiO.sub.2, Al.sub.2 O.sub.3 and C in a reactor; 
(2) adding into said mixture crystals selected from the group of 
.alpha.-Si.sub.3 N.sub.4 and unsintered .beta.'-SiALON having the crystal 
structure of .alpha.-Si.sub.3 N.sub.4 ; (3) nitriding said mixture at 
temperatures between 1200.degree. C. and 1450.degree. C. for a time 
sufficient to convert at least a portion of said initial reactants to at 
least a portion of effective reactants; and (4) heating said effective 
reactants in the presence of nitrogen at temperatures from 1400.degree. to 
1530.degree. C. for a time sufficient to convert said effective reactants 
to an essentially .beta.'-SiAlON refractory material having the crystal 
structure of .alpha.-Si.sub.3 N.sub.4. This .beta.'-SiAlON can then be 
sintered form .beta.'-SiALON having the crystal structure of 
.beta.-Si.sub.3 N.sub.4. This sintered .beta.'-SiAlON has a microstructure 
that has acicular (needlelike crystals) throughout in addition to the 
equiaxed crystals.

DESCRIPTION OF A PREFERRED EMBODIMENT 
As has been noted previously, .beta.'-SiAlON may be defined as a solid 
solution of Al.sub.2 O.sub.3 within an Si.sub.3 N.sub.4 matrix and is 
represented by the general formula Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z 
where z is greater than zero and less than or equal to five. Heretofore, 
.beta.'SiAlONs are equiaxed and have a microstructure that is equiaxed and 
have a crystal structure of expanded .beta.'Si.sub.3 N.sub.4. 
The carbothermic formation of .beta.'-SiAlON by the process of the present 
invention can be represented by the following equation: 
##STR1## 
where X is selected from the group of .alpha.-Si.sub.3 N.sub.4 and 
unsintered .beta.'-SiALON having the microstructure of .alpha.-Si.sub.3 
N.sub.4. 
.alpha.-Si.sub.3 N.sub.4 differs from .alpha.-Si.sub.3 N.sub.4 in that the 
crystal lattice of the .alpha.-Si.sub.3 N.sub.4 in the c direction is 
approximately twice the length of the crystal lattice of .beta.-Si.sub.3 
N.sub.4. 
To produce .beta.'-SiAlON by a process of this invention, initial reactants 
Al.sub.2 O.sub.3, SiO.sub.2 and C are provided in compositional ratios as 
indicated by the line AB in FIG. 1. To produce a .beta.'-SiAlON when z=2 
with a formula of Si.sub.2 AlON.sub.3, for example, would require 23% by 
weight Al.sub.2 O.sub.3, 24% by weight C and 53% by weight SiO.sub.2. 
The SiO.sub.2, Al.sub.2 O.sub.3 and C initial reactants, together with one 
micron crystals selected from the group of .alpha.-Si.sub.3 N.sub.4 and 
unsintered .beta.'-SiALON having the microstructure of .alpha.-Si.sub.3 
N.sub.4, are mechanically mixed by any suitable mixing method to uniformly 
blend the particles, if necessary. The particles are then combined with 
enough liquid vehicle, such as water, by mixing either during blending or 
subsequent thereto, preferably subsequent thereto, to render the mixture 
plastic for extruding or other molding methods familiar to one skilled in 
the art to produce a pellet suitable for nitriding. The particle size of 
the reactants may vary, but generally, the smaller the particle size, the 
more complete the reaction when fired, as will be discussed later. The 
preferred median particle size of Al.sub.2 O.sub.3 is less than 3.5 
microns and more preferably less than 1 micron. The preferred SiO.sub.2 
source is fumed silica having a median particle size of 0.1 micron, 
however the silica source may have a particle size which is as large as 
about 25 microns. 
After mixing and molding the initial reactants into pellets, the pellets 
are dried at a low temperature, such as 110.degree. C., for example, to 
drive off any excess moisture. The pellets are then charged into a 
reaction chamber adapted to nitride and heat the pellets in a two-stage 
heating cycle. Nitrogen may be provided as a gas or a compound, such as 
ammonia, for example, that will reduce to nitrogen gas at the reaction 
temperature. It is preferred that the nitrogen be provided continuously 
under a positive pressure to insure that the nitrogen will uniformly 
contact all of the reactants during the reaction cycle. A suitable reactor 
to accomplish the above purposes is a fluid bed reactor or packed bed 
reactor provided with a nitrogen gas dispersing means near the bottom of 
the reactor and a nitrogen and off-gas outlet near the top. After charging 
a first charge of pellets into the reactor into an upper heat zone to form 
a suitable bed, nitrogen is dispersed through the bed under a positive 
pressure to purge the reactor of its normal atmosphere. 
After establishing a nitrogen atmosphere within the reactor, temperature of 
the reactants is elevated by a suitable heating means to a temperature of 
at least 1200.degree. C., preferably at least 1400.degree. C., in the 
upper heating zone of the reactor. It is believed that by maintaining the 
reactants at a given temperature of at least 1200.degree. C. for a 
sufficient period of time, a portion of the initial reactants are reduced 
to a portion of the effective reactants necessary for producing an 
unsintered .beta.'-SiAlON having microstructure of .alpha.-Si.sub.3 
N.sub.4. The material has a composition that falls within the 
.beta.'SiAlON phase. However, x-ray diffraction patterns of the material 
is similar to .alpha.-Si.sub.3 N.sub.4. 
The period of time required to accomplish this initial reaction will vary 
with the temperature employed. It has been discovered that although no 
catalyst has been added to the initial mixture heating the mixture at a 
temperature of 1200.degree.-1400.degree. C. for 1-3 hours, for example, is 
sufficient to accomplish the initial reaction in the process when it is 
seeded with .beta.'-SiALON having the microstructure of .alpha.-Si.sub.3 
N.sub.4. It has also been discovered that when the reactant temperatures 
exceed 1450.degree. C., the nitridation of silica will cease and silicon 
carbide will form as the preferred species. In addition, Al.sub.2 O.sub.3 
and carbon have been found to react at an appreciable rate when the 
reactant temperatures exceed 1450.degree. C. to 1480.degree. C. Therefore, 
if the nitrification is not completed before beginning the second higher 
temperature heating, silicon carbide will be present in the product. 
It is believed that the above-described initial nitriding step yields 
Si.sub.3 N.sub.4, traces of AlN which are not normally detectable by x-ray 
diffraction procedures and CO as off-gas. The reactions may be represented 
by the equations: 
##STR2## 
where X is selected from the group of .alpha.-Si.sub.3 N.sub.4 and 
unsintered .beta.'-SiALON having the microstructure of .alpha.-Si.sub.3 
N.sub.4. 
It may be noted that in addition to Si.sub.3 N.sub.4 and AlN, Al.sub.2 
O.sub.3 is also required as an effective reactant in producing 
.beta.'-SiAlON having the microstructure of .alpha.-Si.sub.3 N.sub.4. 
Al.sub.2 O.sub.3 is provided in a quantity in excess of the amount needed 
for production of the necessary AlN so that a portion of the Al.sub.2 
O.sub.3 remains as an effective reactant after the initial reaction. It is 
also to be noted that some conversion of the effective reactants begins to 
occur during the first heating step at temperatures as low as 1200.degree. 
C. 
Following the above-described initial nitriding step, the first charge of 
pellets is moved to a second heat zone and the reactant temperature is 
increased to a maximum of 1650.degree. C., preferably within a range of 
1450.degree. to 1530.degree. C., and maintained within that temperature 
range for a time sufficient to convert the effective reactants to an 
unsintered .beta.'-SiAlON powder having the microstructure of 
.alpha.-Si.sub.3 N.sub.4. Concurrently with the movement of the first 
charge of pellets into the second heat zone, additional initial reactants 
are charged into the first heat zone. As previously stated, it is believed 
that some conversion of the effective reactants begins to occur at 
temperatures as low as 1200.degree. C., but it has been discovered that if 
the temperature is increased, less time is required to effect an 
essentially complete conversion of the effective reactants to unsintered 
.beta.'-SiAlON powder having the microstructure of .alpha.-Si.sub.3 
N.sub.4. Within a range of 1450.degree. to 1530.degree. C., a heating time 
of 2 hours is sufficient to yield an essentially single phase 
.beta.'-SiAlON material having the microstructure of .alpha.-Si.sub.3 
N.sub.4. Thus, the time of residence of the reactants in each heat zone 
can be controlled to be essentially the same and the process can be 
operated on a continuous or batch-by-batch basis. 
In an alternate method of operating the process continuously, the initial 
reactants may be fed into the first heat zone at a rate suitable to 
traverse the first heat zone and effect the conversion to effective 
reactants. The effective reactants then move continuously into the second 
heat zone and traverse the second zone in a sufficient length of time to 
allow the reactants to convert to an essentially .beta.'-SiAlON material 
having the microstructure of .alpha.-Si.sub.3 N.sub.4. 
It may be seen that the extent of the heat zones may be adjusted to insure 
that the pellets remain in each heat zone a sufficient length of time as 
they advance at a uniform rate. Although raising the temperature in the 
final nitriding step is advantageous in effecting a conversion of the 
transitional or effective reactants into an essentially single phase 
.beta.'-SiAlON having the microstructure of .alpha.-Si.sub.3 N.sub.4, 
raising the temperature above approximately 1650.degree. C. promotes the 
formation of other SiAlON phases which may be detrimental to the purposes 
of the end product formed by the method of the invention. 
During the final heating step after nitriding, a nitrogen atmosphere is 
maintained in the reactor to preserve a stoichiometric balance as 
expressed in the equation: 
##STR3## 
Once again, it is to be noted that some conversion of the effective 
reactants begins to occur during the first heating step at temperatures as 
low as 1200.degree. C., but if the temperature is increased, less time is 
required to effect an essentially complete conversion of the effective 
reactants to .beta.'-SiAlON having the crystal structure of 
.alpha.-Si.sub.3 N.sub.4. One skilled in the art will appreciate that 
equations (a), (b) and (c) above are taking place simultaneously (although 
not at the same rates) and are theoretical tools for understanding the 
carbothermic reaction of equation (a). 
In the foregoing description, the two-step nitriding and heating cycle of 
the reactants is accomplished successively. The reactions may be 
represented by the equations: 
##STR4## 
Surprisingly, it has been found that if .alpha.-Si.sub.3 N.sub.4 crystals 
are used in place of the .beta.'-SiAlON crystals, the structure of the 
resulting .beta.'SiAlON is similiar to .alpha.-Si.sub.3 N.sub.4. 
.beta.'-SiAlON refractory material having the crystal structure of 
.alpha.-Si.sub.3 N.sub.4 can then be sintered form .beta.'-SiALON having 
the crystal structure of .beta.-Si.sub.3 N.sub.4. Prior to sintering the 
.beta.'-SiAlON refractory material having the crystal structure of 
.alpha.-Si.sub.3 N.sub.4 is ground to a powder and then compacted. The 
sintering of the .beta.'-SiAlON refractory material may be expressed in 
the equation: 
##STR5## 
As noted above, the sintered .beta.'-SiAlON has a microstructure that is 
similar to .alpha.-Si.sub.3 N.sub.4 and has acicular (needlelike) crystals 
throughout in addition to the equiaxed crystals. 
The formation of acicular .beta.'-SiAlON is surprising. Acicular 
.beta.'-SiAlON is a new form of .beta.'-SiAlON. Heretofore, .beta.'-SiAlON 
produced by carbothermic reaction has crystal structure of .beta.-Si.sub.3 
N.sub.4 and sintered into an equiaxed material. 
For convenience, the term alpha SiAlON(.alpha.-SiAlON) is used herein to 
describe .alpha.-Si.sub.3 N.sub.4 structured .beta.'SiAlON with crystal 
structure similar to .alpha.-Si.sub.3 N.sub.4. The formation of alpha 
SiAlON may be represented by the equations: 
##STR6## 
Heretofore, the carbothermic reaction approach has always produced a 
SiAlON material that when sintered produced an equiaxed .beta.'-SiAlON. 
The alpha phase of the SiAlON was never produced by carbothermic reaction 
of discrete particles of an SiO.sub.2 source and discrete particles of an 
Al.sub.2 O.sub.3 source. 
The following examples are offered to illustrate the production of 
unsintered .beta.'SiAlON having the microstructure of .alpha.-Si.sub.3 
N.sub.4 and nonequiaxed .beta.'SiAlON by the process of this invention. 
EXAMPLE 1 
.beta.'-SiAlON with z=0.5 was prepared from a mixture of 156 grams Al.sub.2 
O.sub.3 (5.2%), 2019 grams silica (67.4%) and 825 grams carbon(27.5%). 
This mixture was blended with 450.0 grams of 1 micron .beta.'-SiAlON seed 
crystals(15 wt. % addition to the mixture on a dry basis). The final blend 
had a final composition of 4.51 wt. % Al.sub.2 O.sub.3, 58.53 wt. % 
SiO.sub.2, 23.91 wt. % carbon and 13.04 wt. % .beta.'-SiAlON seed. The 
stoichiometric reaction may be represented by the following equation: 
##STR7## 
The blend was charged into a 1.3 gal ceramic ball mill where the materials 
were uniformly mixed. The resultant powder was blended with H.sub.2 O and 
extruded as pellets. The pellets were dried at 110.degree. C. to drive off 
the water and was charged into an enclosed reactor vessel provided with an 
inlet below the pellet bed to permit uniform circulation of gaseous 
nitrogen through the pellets and an outlet near the top of the vessel to 
permit discharge of nitrogen and reaction gas products. 
The vessel having the pellets therein was enclosed in a heating chamber and 
nitrogen was charged into the vessel at a pressure sufficient to maintain 
a flow of nitrogen through the vessel throughout the subsequent heating 
cycles. 
When it was determined that the reaction vessel had been purged of air, 
temperature within the heating chamber was increased an amount necessary 
to raise the temperature of the pellets to 1400.degree. C. and that pellet 
temperature was maintained for 3 hours. 
The pellet temperature was then increased to 1500.degree. C. and maintained 
there for 2 hours. The pellets were then cooled to room temperature and 
analyzed for composition. It was determined by x-ray diffraction that the 
processed material comprised: .beta.'-SiAlON as a major phase, possible 
trace of SiC and possible trace Fe. Chemical analysis was as follows: 51.7 
wt. % Si; 5.52 wt. % Al; O (NA); N (NA); and 0.2 wt. % Fe impurities. Note 
iron levels of 1-2% are typical when Fe.sub.2 O.sub.3 has been added as a 
catalyst. 
The material was ground and pressed into billets and sintered at 
temperatures between 1650.degree.-1750.degree. C., then machined into type 
"B" bars. The bars were then tested in accordance with Military Standard 
1942. The strength of the bars was tested at room temperature and average 
strength was found to be 77.3 ksi with a range of strengths from 71.4 to 
87.5 ksi. 
FIG. 3 is a photomicrograph of the material of Example 1 (prior art) 
equiaxed .beta.'-SiAlON that has been sintered and etched. The 
photomicrograph is at at 4,940 times magnification. 
In addition, the material was sintered into bars having the following 
dimensions: 1.875 inches.times.0.25 inches.times.0.125 inches. The 
strength of the bars was tested by 4 point bending at room temperature, 
1093.degree. C., 1204.degree. C. and 1260.degree. C. The strengths are are 
recorded on Table 1 below. 
TABLE 1 
______________________________________ 
Strength (ksi) 
Temp. .degree.C. 
Average Range 
______________________________________ 
25 57.6 46.4-65.7 
1093 50.4 49.2-53.3 
1204 39.2 35.1-44.1 
1260 28.8 25.1-33.7 
______________________________________ 
EXAMPLE 2 
Example 1 was repeated except that no SiAlON crystals were added to the 
mixture before firing. In addition, no iron catalyst was added. It was 
determined by x-ray diffraction that the processed material was comprised 
of .beta.'-SiAlON as a major phase, Si.sub.2 ON.sub.2 as a major phase and 
SiC as a minor phase. Chemical analysis was as follows: 50.9 wt. % Si; 
5.45 wt. % Al and 0.1 wt. % Fe impurities. 
The x-ray results of Example 2 reveal that the nitriding reaction was not 
completed at 1400.degree. C. as evidenced by the oxynitride and relative 
large quantity of silicon carbide. 
EXAMPLE 3 
Example 1 was repeated except that alpha Si.sub.3 N.sub.4 crystals instead 
of .beta.'-SiAlON crystals were added to the mixture before firing. 
Yttrium was added to the mix as a sintering aid. 
It was determined by x-ray diffraction that the processed .beta.'SiAlON 
material comprised .alpha.-SiAlON with crystal structure similar to 
.alpha.-Si.sub.3 N.sub.4 as a major phase, possible trace of SiC and 
possible trace Fe. The material was sintered into military type "B" bars 
and tested in accordance with Military Standard 1942. The strength of the 
bars was tested at room temperature and average strength was found to be 
79.5 ksi with a range of strengths from 72.3 to 90.5 ksi. 
FIG. 4 is a photomicrograph of material of Example 3 that has been sintered 
and etched. The photomicrograph is at 4,940 times magnification. 
Surprisingly, the sintered material contained acicular .beta.'-SiAlON 
crystals throughout. Heretofore, SEM's of .beta.'SiAlONs were always 
revealed an equiaxed material. The x-ray pattern of the sintered material 
was found to be similar to .beta.'-Si.sub.3 N.sub.4. 
In addition, the material was sintered into bars having the following 
dimensions: 1.875 inches.times.0.25 inches.times.0.125 inches. The 
strength of the bars was tested by 4 point bending at room temperature, 
1093.degree. C., 1204.degree. C. and 1260.degree. C. The strengths are are 
recorded on the following Table 2. 
TABLE 2 
______________________________________ 
Strength (ksi) 
Temp. .degree.C. 
Average Range 
______________________________________ 
25 68.1 59.6-72.6 
1093 61.3 51.3-68.5 
1204 65.4 61.1-72.9 
1260 59.8 53.3-64.8 
______________________________________ 
It is contemplated that different forms of SiO.sub.2 and Al.sub.2 O.sub.3 
may be used in practicing the process of the present invention. Thus for 
example, some forms of SiO.sub.2 which can be used as the initial reactant 
are quartz, cristabolite, tridymite and amorphous silica. Some suitable 
forms of Al.sub.2 O.sub.3 which can be used as the initial reactant are 
.alpha.-Al.sub.2 O.sub.3, .alpha.-Al.sub.2 O.sub.3, .kappa.-Al.sub.2 
O.sub.3 and other phases of Al.sub.2 O.sub.3, oxides and hydroxides of 
aluminum, aluminum carbonate, aluminum nitrate and gibbsite. One skilled 
in the art will appreciate that the choice of SiO.sub.2 and Al.sub.2 
O.sub.3 which is actually used will be made on cost considerations and 
that many other forms of SiO.sub.2 and Al.sub.2 O.sub.3 than those 
specifically listed can be used. In a preferred form of the present 
invention the preferred median particle size of the Al.sub.2 O.sub.3 
initial reactant is less than 3.5 microns. 
It is also contemplated that different weight percents of seed material may 
be used in practicing the present invention. Thus for example, other than 
15 wt. % of on a dry basis seed material may be used. A preferred range 
for the weight percent of seed material is 4 to 25%. One skilled in the 
art will appreciate that the higher the weight percent of seed material 
that is actually used the faster will be the rate of reaction. Although 
weight percents as high as 50% may be used, the upper limit of the actual 
percent of seed material is not critical to practicing the invention. The 
real upper limit to the percentage of seed material which is actually used 
will be determined by cost considerations. At the lower limit of seed 
material used in practicing the present invention, the rate of reaction 
will be greatly reduced when the percentage of material is below 1%. 
It is further contemplated that seed crystals used to produce the 
.beta.'SiAlON material of the present invention can be can also be made 
using .alpha.-Si.sub.3 N.sub.4 as the seed material. Once the unsintered 
.beta.'SiAlON powder having the crystal structure of .alpha.-Si.sub.3 
N.sub.4 is made in accordance with the present invention, part of the 
.beta.'SiAlON material produced can be used as seed material of future 
.beta.'SiAlON material. Thus, after the initial .beta.'SiAlON having the 
microstructure of .alpha.-Si.sub.3 N.sub.4 has been produced there is no 
need to purchase additional .beta.'SiAlON or .alpha.-Si.sub.3 N.sub.4 for 
use as seeding material. 
It is further contemplated that the unsintered .beta.'SiAlON material made 
in accordance with the present invention can be readily ground to any 
desired degree of fineness to make a highly reactive grain or powder for 
subsequent processing. The unsintered .beta.'SiAlON powder can then be 
molded into a desired shape and sintered to produce a high density, high 
strength part such as a component of an internal combustion engine or a 
high temperature gas turbine. In another application, the unsintered 
SiAlON powder might be combined with a suitable carrier, applied as a 
layer to a substrate and affixed thereto by heating to a temperature 
sufficient to fuse or bond the SiAlON to the substrate. Those skilled in 
the art will recognize that the particulate .beta.'SiAlON of the present 
invention is easier to grind or mill to a desired grain or powder than 
sintered .beta.'SiAlON. To the extent that sintered .beta.'SiAlON could be 
ground, the resultant powder would not be desirable for use in resintering 
to make a shape or part. It would have a limited sintering capability, and 
to the extent it could be resintered, the resintered product would be 
substantially inferior to a product made from the unsintered .beta.'SiAlON 
material of the present invention. 
It is further contemplated that the unsintered .beta.'SiAlON material 
having the microstructure of .alpha.-Si.sub.3 N.sub.4 may also be made by 
other than carbothermic reaction. Thus for example, the seeding method 
disclosed in the present application may be used in conjunction with the 
method disclosed in U.S. Pat. No. 4,172,108. The use of the present 
invention in this process would include that seed material be mixed with a 
silicon nitride precusor having at least one silicon-nitrogen bond and 
which is at least one substituted amino or imino silane and an alumina 
precusor having a least one aluminum-oxygen bond, is at least one member 
selected from the group consisting of trailkoxyaluminums, 
tiracyloxyaluminums and polyaluminoxanes in an organic solvent, removing 
the organic solvent by evaporation to provide a SiAlON precursor 
composition, and effecting conversion of the SiALON precursor composition 
into SiALON by heating the SiALON precursor composition at a temperature 
of not lower than 1000.degree. C. at a rate of elevating the temperature 
of at most 400.degree. C. per hour either in an atmosphere of an ammonia 
or inert gas under reduced pressures. 
It is further contemplated that the unsintered .beta.'SiAlON material 
having the microstructure of .alpha.-Si.sub.3 N.sub.4 may also be made 
according to the method disclosed in U.S. Pat. No. 4,438,051. The use of 
the present invention in this process would include a translucent 
.beta.'SiAlON sintered product produced by (1) mixing fine powders of (a) 
silicon nitride, and aluminum nitride having a high purity of at least 99% 
and a particle size of at most 200 microns, (b) fine powders of aluminum 
oxide and silicon oxide having a high purity of at least 99% in such a 
proportion as to form .beta.'SiAlON of the formula Si.sub.6-z Al.sub.z 
O.sub.z N.sub.8-z where z is from 1 to 4.2, and (c) crystals selected from 
the group of .alpha.-Si.sub.3 N.sub.4 and unsintered .beta.'-SiALON having 
the microstructure of .alpha.-Si.sub.3 N.sub.4 ; and (2) hot pressing the 
mixture in a nitrogen atmosphere at a temperature of from 1500.degree. to 
1850.degree. C. under pressure to form 10 to 1500 kg/cm.sup.2. 
It is further contemplated that the unsintered .beta.'SiAlON material 
having the microstructure of .alpha.-Si.sub.3 N.sub.4 may also be made 
according to the method disclosed in U.S. Pat. No. 4,685,607. The use of 
the present invention in this process would include a nitride 
ceramic-metal complex material produced by (a) bringing a metallic 
material selected from the group consisting of: (i) a metal rich in 
reactivity with a precursor to nitride (ii) an alloy of said metal into 
contact with the surface of a nitride ceramic material; and (iii) crystals 
selected from the group of .alpha.-Si.sub.3 N.sub.4 and unsintered 
.beta.'-SiALON having the microstructure of .alpha.-Si.sub.3 N.sub.4 ; and 
(b) heating under vacuum the nitride ceramic material which is kept in 
contact with the metallic material so as to dissociate the surface of the 
nitride ceramic material into nitrogen and a precursor to nitride, thereby 
alloying the dissociated precursor to nitride to react with the metal or 
alloy thereof so as to achieve bonding between the nitride ceramic 
material and the metallic material. 
It is further contemplated that the unsintered .beta.'SiAlON material 
having the microstructure of .alpha.-Si.sub.3 N.sub.4 may also be made 
according to the method disclosed in U.S. Pat. No. 4,731,236. The use of 
the present invention in this process would include a SiAlON powder, which 
is produced by a process comprising the steps of: (1.) introducing (a) a 
decomposable silicon compound, (b) decomposable aluminum compound, (c) a 
decomposable carbon compound and (d) seed crystals selected from the group 
of .alpha.-Si.sub.3 N.sub.4 and unsintered .beta.'-SiALON having the 
microstructure of .alpha.-Si.sub.3 N.sub.4, into a steam-containing hot 
gas to decompose the decomposable silicon compound, aluminum compound and 
carbon compound in the hot gas into silicon oxide, aluminum oxide and 
elemental carbon, respectively, thereby producing a fine solid particle 
mixture consisting essentially of silicon oxide, aluminum oxide, elemental 
carbon and seed crystals dispersed in gas; and (2.) collecting the fine 
solid particle mixture dispersed in the gas from the gas phase by a 
solid-gas separating technique to obtain a carbon-containing composition 
consisting essentially of silicon oxide, aluminum oxide and elemental 
carbon, and calcining the composition in a nitrogen-containing gas 
atmosphere. 
It is further contemplated that the unsintered .beta.'SiAlON material 
having the microstructure of .alpha.-Si.sub.3 N.sub.4 made in accordance 
with the present invention can be readily ground to any desired degree of 
fineness to make a highly reactive grain or powder for subsequent 
processing. 
While the invention has been described in terms of preferred embodiments, 
it is intended that all matter contained in the above description or shown 
in the accompanying drawings shall be interpreted as being illustrative. 
The present invention is indicated by the broad general meaning of the 
terms in which the appended claims are expressed.