WO1993000310A1 - Improved sialon composition - Google Patents

Abstract

A composition containing at least 70 weight percent of a beta-prime sialon compound and at least about 27 weight percent of a solid solution is disclosed. A method of heat-treating said composition is also disclosed. Silica is charged via line (12) to mill (10) to be comminuted until the oxygen content is between 44-47 weight percent. The comminuted silica is dicharged via line (22) to mill (24) to be comminuted with remaining materials until the mixture is single phase. The single phase material is discharged to the pellitizer (38) then to the reactor (42) where they are ignited, cooled, and passed to crusher (46). The crushed material can be used as is or sieved (50).

Description

IMPROVED SIA ON COMPOSITION

Technical Field

An improved sialon composition with superior mechanical and thermal properties is disclosed. Background Art

United States patent 5,030,600 of George T. Hida et al. discloses a composite material containing at least 70 weight percent of single-phase beta sialon and at least 20 weight percent of alpha-alumina. Although the composite material of this patent has excellent mechanical and thermal proper¬ ties, its properties are not good enough for use in severe environments requiring a particular combination of excellent thermal and excellent mechanical properties.

It is an object of this invention to provide a sialon mate¬ rial which a hardness at least 19 GigaPascals, a strength of at 1,000 megaPascals (at room temperature), a fracture toughness of at least 6 megaPascals, and a strength decay at 1,350 degrees Celsius of less than 25 percent. Disclosure of Invention

In accordance with this invention, there is disclosed a sialon composition which consists essentially of at least 70 weight percent of beta-prime-sialon, and at least 27 weight percent of a solid solution containing silicon nitride, alumina, and aluminum nitride. Brief Description of the Dr.-.'ing

The present invention will be more fully understood by reference to the following detailed description thereof, when read in conjunction with the attached drawings, wherein like reference numerals refer to like elements and wherein: Figure 1 is a flow sheet of a preferred embodiment of the process of this invention. Best Mode for Carrying Out the Invention

The composite material produced by the process of this invention preferably consists -essentially of two phas¬ es, a beta-prime sialon compound and a sialon solid solu¬ tion.

The beta-prime sialon compound is primarily synthe¬ sized. The term "primarily synthesized" is described in United States patent 5,030,600, the entire disclosure of which is hereby incorporated by reference into this specifi¬ cation. Thus the beta-prime-sialon compound preferably has no replaceable cations. In the majority of crystal lattices of this sialon compound, only silicon, aluminum, oxygen, and nitrogen atoms preferably appear.

In one preferred embodiment, the beta-prime-sialon compound is of the formula Sig__.Al_.0_.Ng__, (wherein z is from about 2.5 to about 3.5).

The material produced by the process of this inven¬ tion comprises at least about 70 weight percent of said beta-prime-sialon compound, wherein z is from about 2.5 to about 3.5. It is preferred that the material comprise at least about 72 weight percent of said beta-prime-sialon compound. It is most preferred that the material contain at least 73 weight percent of the beta-sialon material.

Without wishing to be bound to any particular theory, applicants believe that the presence of the beta-sialon compound in their material tends to produce a superior material because such compound does not contain replaceable cations, such as the yttrium, zirconium and magnesium ca¬ tions which are commonly present in prior art beta-sialon materials (which are solid solutions).

The material of this invention preferably contains at least two phases. In this embodiment, in addition to con¬ taining at least 70 weight percent of said beta-prime-sialon co pound, such material also contains at least 27 weight percent of a specified sialon solid solution.

The sialon solid solution which is present in appli¬ cant's material contains alpha-silicon nitride, aluminum nitride, and alpha-alumina.

In general, such sialon solid solution contains from about 2 to about 50 weight percent, by weight of solid solution, of alpha-silicon nitride and from about 0.5 to about 14 percent of aluminum nitride, by weight of said solid solution.

The sialon solid solution also contains from about 97.5 to about 36 weight percent of alpha alumina, by weight of said solid solution.

The material of this invention may be sintered with¬ out pressure and without the use of sintering aids. It is preferred, however, to mix such material with a binder prior to forming it. Thus, one may use the binders described in United States patent 5,030,600.

The material of this invention, when formed into a green body and sintered in accordance with a test of sin¬ tered cylinders described in United States patent 5,030,600, has certain specified properties. Thus, the sintered cylin¬ ders: (l)have a density of at least 98.5 percent of its theoretical density (and, in general, have a density of at least 3.23 grams per cubic centimeter), (2)have an elastic modulus of at least about 300 gigaPascals (as tested in accordance with A.S.T.M. Standard Test C-848-78), (3)have a flexural strength of at least about 1000 megaPascals, as tested in accordance with A.S.T.M. Standard Test F-417-78, (4)have a wear resistance (as expressed as a loss of materi¬ al volume) of at least about 20 cubic millimeters per 100 revolutions, as tested in accordance with A.S.T.M. Stan¬ dard Test B-611, and (5)have a porosity of less than about 1.0, as determined in accordance with A.S.T.M. Standard Test C-373-72 (water absorption) . Furthermore, the sintered cylinders produced by the aforementioned treatment, when the sintering temperature is 1,680 degrees centigrade, and when the sintering time is 3.0 hours, will have a density of at least about 98.5 percent of theoretical (calculated) densi¬ ty. The density of said cylinders is determined in accor¬ dance with A.S.T.M. Standard Test C-20-63.

Furthermore, when the material of this invention is tested in accordance with the aforementioned procedure, it will have a hardness of at least 19 GigaPascals and a frac¬ ture toughness of at least about 6 megaPascals/meter- 5 (as determined by the notched beam test) .

The material of this invention retains substantially more of its strength under adverse conditions than do many prior art materials. Thus, for example, when it is sintered in accordance with the aforementioned procedure and its strength is determined at time zero, it will not thereafter lose a substantial portion of such strength. When such sintered material is thereafter subjected to a temperature of 1,350 degrees Celsius for 100 hours, it will still have a strength of at least 75 percent of its time zero strength.

The process of applicants' invention produces a pre¬ cursor which, after heat-treatment, produces a composition containing the beta-prime-sialon compound. The beta-prime sialon compound in applicants' precursor material contains from about 35 to about 25 percent of silicon, from about 24 to about 33 percent of aluminum, from about 14 to about 20 percent of oxygen, and from about 27 to about 22 percent of nitrogen, all as determined by microprobe analysis. Conven¬ tional means of making such microprobe analysis are de¬ scribed in United States patent 5,030,600.

In one preferred embodiment, the beta-prime-sialon compound in applicants' precursor material contains from about 33 to about 27 percent of silicon, from about 26 to about 31 percent of aluminum, from about 15 to about 19 percent of oxygen, and from about 26 to about 23 percent of nitrogen.

In an even more preferred embodiment, the beta-prime sialon compound in applicants' precursor material contains from about 31 to about 29 percent of silicon, from about 28 to about 29 percent of aluminum, from about 16 to about 18 percent of oxygen, and from about 25 to about 24 percent of nitrogen.

In the most preferred embodiment, the beta-prime- sialon compound in applicants' precursor material contains about 29.7 percent of silicon, about 28.6 percent of aluminum, about 17.0 percent of oxygen, and about 24.7 percent of nitrogen. The preferred process of applicants' invention

In the process of this invention, an "activated" set of reactants is preferably provided for a controlled combus¬ tion-synthesis reaction. The term "controlled combustion- synthesis" ("CCS") refers to a self-sustained, highly ex¬ othermic direct synthesis, often referred to aε "SHDS," or "metallothermic reduction and synthesis," often referred to as "MTRS."

"SHDS," a self-sustained, highly exothermic process of direct synthesis from elements, is a self-propagating, high temperature form of controlled combustion-synthesis; it is a chemical process involving an exothermic reaction between a metal and a nonmetal powder mixture.

"MTRS," a metallo-thermic reduction and synthesis process involving thermite-type reactions, is another form of controlled combustion synthesis; it is a chemical process involving the reduction of oxides or other compounds of metals by igniting mixtures of these compounds with powdered metals, followed by in situ synthesis of new compounds using the energy released luring the reduction sequence. Both SHDS and MTRS reactions are surface-type reac¬ tions in which there is minimal diffusion of the reactants through the product layers during the high-temperature stage of the process. In order to maximize the yield obtained with these reactions, the amount of contact between the reagents has to be maximized.

One means of maximizing contact between the reagents is to increase the density of contact points (the number of contact points per unit volume) . This may be achieved by intensive mechanochemical treatment of a specified powder mixture.

The contact points between the reagents should be intimate; there must be intimate contact as close as possi¬ ble to the molecular level. Intensive mechanochemical treatment helps insure penetration of the reagent particles into each other's structure and increases the number of intimate contact points.

In one preferred embodiment of applicants' process, it is preferred to conduct the mechanochemical activation treatment while the sample is contacted with flowing nitro¬ gen-containing gas at a flow rate of from about 0.1 to about 0.5 gallons per minute.

Figure 1 illustrates one preferred embodiment of the process of this invention.

In the first step of the process of Figure 1, an oxy¬ gen-deficient silica material is provided. The oxygen- deficient silica material may be provided by comminuting silica in nitrogen environment and/or under vacuum environ¬ ment. This comminution may be conducted in any of the rotary mills or vibration mills described elsewhere in this specification until a single-phase silica material with an oxygen content of from about 44 to about 47 weight percent is obtained. Thus, referring to Figure 1, to mill 10 is charged the silica via line 12, surfactant via line 14, and either nitrogen-containing gas and/or a source of vacuum via line 16.

In one preferred embodiment, from about 0.1 to about 1.0 percent of surfactant, by total weight of silica and surfactant, is added to the mill 10 by line 14. As used in this specification, the term "surfactant" refers to a sur¬ face active agent, i.e., a substance that alters energy relationships at interfaces.

In another embodiment, no surfactant is added to mill 10.

If a surfactant is added to mill 10, it is preferred to use a surfactant which will be volatilized at a temperature of 1,200 degrees centigrade. The use of such a surfactant guarantees that substantially no surfactant residue will be present in the sintered product.

Silica is added to the mill 10 via line 12. After the silica has been comminuted in mill 10, from about 30 to about 45.5 weight percent of such silica, by total weight of aluminum material, aluminum nitride, silicon nitride, surfactant, metal fluoride, and silica, is added to the second mill described below.

It is preferred that substantially all of the silica used in mill 10 be smaller than about 53 microns. In one embodiment, substantially all of the silica particles used in mill 10 are smaller than about 15 microns. One may use any commercially available silica with the correct particle size distribution.

The silica used in the process should be substantial¬ ly dry, i.e., it should contain less than about 0.1 percent of moisture.

Referring again to Figure 1, mill 10, which may be a high-speed rotary ball mill, is used to activate the silica. One may also use a vibratory ball mill to comminute and activate the mixture. Additionally, or alternatively, one may use comminution means such as jet mills, attrition mills, high speed hammer mills, and the like. These mills are described in United States patent 5,030,600.

In the process of this invention, when mill 10 is a vibratory ball mill, it is preferred to comminute silica in it for a period of at least about 8 hours and, more prefer¬ ably, from about 8 to about 24 hours; in one preferred embodiment, the mixture is comminuted in the vibratory mill for from about 12 to about 18 hours.

High-intensity mill 10 may be a high-speed cen¬ trifugal mill. When high-intensity mill 10 is a centrifugal mill, it is preferred to operate it at a speed of at least 500 revolutions per minute for at least about 2 hours. In one embodiment, the mill is operated for from about 2 to about 6 hours at a speed of from about 550 to about 850 revolutions per minute. In another embodiment, the mixture is comminuted in the mill for from about 3 to about 4 hours.

When one uses either a high-speed ball mill or a vibratory mill in the process of this invention, a specified grinding media preferably should be used. The grinding media should have a density of at least about 6.0 grams per cubic centimeter. Thus, e.g., one may use zirconia media, stainless steel media, tungsten carbide media, and the like.

When a high-speed ball is used, the weight/weight ratio of grinding media/material to be ground should be at least about 6/1. With a vibratory ball mill, said weight/weight ratio should be at least about 8/1. The volume/volume ratio of the grinding media/material to be ground should be no more than about 3/1 in the ball mill and no more than about 4/1 in the vibratory mill.

The silica is comminuted in mill 10 until it prefer ably has an oxygen content of from about 44 to about 47 weight percent. Samples of the silica may be periodically removed via line 18 to laboratory 20, wherein such silica may be subjected to conventional elemental analysis. Thus, for example, one may use an oxygen determinator, such as model no. RO-416DR sold by Leco Corporation of 300 Lakeview Avenue, Saint Joseph, Michigan 49085-2396, to conduct such oxygen analysis. Alternatively, or additionally, one may use conventional spectral analysis.

The comminuted silica, with the desired oxygen-cont¬ ent, may be discharged via line 22 to mill 24, which mill 24 will be used to comminute the silica, aluminum, aluminum nitride, silicon nitride, and metal fluoride. This mill 24 can be a centrifugal mill, a vibratory mill, a planetary mill, or any of the mills described above. In one embodi¬ ment, the same type of mill is used for both mill 10 and mill 24.

Into high-intensity mill 24 is charged nitrogen-con¬ taining gas via line 26, aluminum material via line 28, silicon nitride via line 29, aluminum nitride via line 30, and metal fluoride via line 32. The mill 24 is then operat¬ ed until the mixture is substantially single phase and the nitrogen content of the mixture has increased at least about 2.0 weight percent.

The reaction mixture is comminuted until analysis reveals that it is comprised of substantially a single phase material, as shown by scanning electron microscopy. A sample of the reaction mixture in mill 24 may be periodical¬ ly removed via line 33 to laboratory 34 wherein it may be subjected to analysis by a scanning electron microscope to determine whether it consists essentially of only one phase. The reaction mixture is also comminuted until analysis in laboratory 34 reveals that the nitrogen content of the mixture has increased at least about 2.0 weight percent over the nitrogen-content of the starting mixture.

When mill 24 is a centrifugal mill, at least about 2 hours of comminution at a rate of at least 500 revolutions per minute is usually required to form a one-phase material. When mill 24 is a vibratory mill, at least about 6 hours of comminution is generally required to form a one-phase material.

The nitrogen-containing gas used in the process may be either pure nitrogen, a mixture of nitrogen and inert gas, a mixture of nitrogen and hydrogen, nitrogen-compounds in gaseous form (such as ammonia), and the like. Any material which is in gaseous state at the conditions used for the comminution (which may, e.g., be conducted under elevated temperature), and which will furnish combined or uncombined nitrogen to the material being comminuted, may also be used.

It is preferred that the nitrogen-containing gas be comprised of at least 90 volume percent of combined and/or uncombined nitrogen. It is more preferred that the nitrogen-containing gas consist essentially of nitrogen.

The aluminum material used in the process may be either pure aluminum, aluminum alloy, or mixtures thereof. Suitable aluminum alloys include, for example, alloys of aluminum with lithium, silicon, magnesium, and the like.

In one preferred embodiment, the aluminum material is aluminum. This aluminum is charged to high intensity mill 24 via line 28. The aluminum used in this, process is pre¬ ferably a powder which contains less than about 1.0 percent, by weight, of combined oxygen. The term "combined oxygen" refers to oxygen which is combined with aluminum and, usual¬ ly, is in the form of a metal oxide. The amount of combined oxygen may be determined by evaluating the total oxygen present by conventional means such as, e.g., spectral analy¬ sis. The aluminum or aluminum alloy should be in powder form and will preferably have a specific surface area of from about 0.2 to about 1.1 square meters per gram.

In one embodiment, the aluminum material used is in the form of fine aluminum flakes.

In the aluminum material used in the process of this invention, the specified weight ratio is based upon the con¬ centration of pure aluminum. When an aluminum alloy materi¬ al is used, or when a mixture of aluminum or aluminum alloy material is used, the aluminum content of the alloy is first calculated, and this aluminum content is then used to deter¬ mine the aluminum content of the material.

The particle size distribution of the aluminum material will preferably range from about 4 to about 28 microns and, more preferably, from about 6 to about 12 microns; substan¬ tially 100 percent of the aluminum particles will be within the specified size range. In one preferred embodiment, at least about 80 weight percent of the aluminum particles are between from about 6 to about 12 microns in size.

Particle size analysis may be conducted by means well known to those skilled in the art. Thus, e.g., one can use the procedures described on pages 8-2 to 8-8 of Perry and Chilton's "Chemical Engineers' Handbook," Fifth Edition (McGraw-Hill Book Company, New York, 1973).

The aluminum material used in the process is prefer¬ ably substantially dry, i.e., it contains less than about 0.1 percent, by weight, of moisture.

From about 23 to about 34 percent of the aluminum material, by total weight of aluminum material, aluminum nitride, silicon nitride, surfactant, comminuted silica, and metal fluoride, is charged to the mill 24 via line 28.

Aluminum nitride is used in the process of the inven¬ tion. The aluminum nitride is charged to the mill 24 via line 30. From about 0.5 to about 8 weight percent of the aluminum nitride, by total weight of the aluminum, aluminum nitride, silicon nitride, surfactant, oxygen-deficient silica, and metal fluoride, is fed to mill 24 via line 30. One may use commercially available aluminum nitride products. Thus, e.g., one may use the high density aluminum nitride available from ICD Group, Inc. (641 Lexington Avenue, New York, N.Y.), product no. 06 26 561000 available from Hermann C. Starck, Inc. (280 Park Avenue, New York, N.Y. ) , Aldrich product number 24,193-0, and the like.

The aluminum nitride which may be used in the process of this invention preferably contains less than about 2.0 weight percent of combined oxygen, has substantially all of its particles smaller than about 325 mesh (53 microns), and is substantially dry, containing less than about 0.1 percent of moisture. The reaction mixture comprised of the aluminum nitride also should be substantially dry.

Silicon nitride is also used in the process of the invention. Such silicon nitride is added to mill 24 via line 29. The silicon nitride used in the process preferably has a nitrogen, content of. at least. about 35 weight percent, an oxygen content of less than about 2.0 weight percent, a free silicon content of less than 1.0 weight percent, a particle size such that substantially all of its particles are smaller than about 10 microns, and a purity of at least 99.5 percent. From about 2 to about 25 weight percent (by total weight of all material charged to mill 24) of silicon nitride is charged via line 29.

It is preferred that the weight of the silicon ni¬ tride added to mill 24 be from about 3.0 to about 4.0 times the weight of the aluminum nitride charged to mill 24. It is even more preferred that the weight/weight ratio of silicon nitride/aluminum nitride charged to mill 24 be from about 3.2 to about 3.7. In an even more preferred embodi¬ ment, said weight/weight ratio is from about 3.3 to about 3.5.

In one preferred embodiment, from about 0.1 to about 1.0 percent of surfactant, by total weight of aluminum material, silicon nitride, aluminum nitride, surfactant, metal fluoride, and silica, is added to the mill 24 by line 31. One may use the same surfactant as described above for use with the comminution of the silica. Alternatively one may use different surfactant.

Oxygen-deficient silica is added to the mill 24 via line 22. From about 30 to about 47 weight percent of such silica, by total weight, is added to mill 24.

A fluoride compound may be, but need not be, used in the process. From about 0 to about 5 weight percent (by total weight) of a fluoride compound is charged to mill 24 via line 32. This fluoride compound is preferably a metal fluoride which is selected from the group consisting of sodium fluoride, sodium hydrogen fluoride, potassium fluo¬ ride, potassium hydrogen fluoride, lithium fluoride, hydro¬ gen fluoride, potassium tetrafluoroaluminate, aluminum fluoride, potassium hexafluoroaluminate, sodium tetrafluor¬ oaluminate, sodium hexafluoroaluminate, and mixtures there¬ of. It is preferred that all of the fluoride compound parti¬ cles be less than about 100 mesh (150 microns).

After the aluminum material, the aluminum nitride, the silicon nitride, the surfactant, the fluoride compound, and the silica have been charged to mill 24, the reaction mix¬ ture is milled under nitrogen-containing gas until a sub¬ stantially single phase composition has been produced. Samples of the mixture may be periodically withdrawn from the mill 24 via line 33 and subjected to analysis in labora¬ tory 34.

Once a substantially one-phase material has been formed in mill 24, the reaction mixture is discharged via line 36; thereafter, in one embodiment (illustrated in Figure 1), it is pelletized in pelletizer 38.

The material discharged via line 36 may be pelletized in pelletizer 38 by pressing it into small pellets whose minimum size is at least about 1.0 inches. The pellets may be in the shape of a cylinder, a bar, etc. The pressure used in the pelletizing should be sufficient to confer enough strength to the pellets so that they can be handled without disintegrating. The porosity of the pellets should be at least about 40 percent.

It is preferred to press the pellets at a pressure of at least about 100 pounds per square inch. Any conventional press may be used to pelletize the powder from mill 24. Thus, e.g., one may use a hydraulic press. In one embodi¬ ment, a hydraulic press is used which is comprised of hy¬ draulic pumps which are connected to plungers.

The pressed pellets may then be discharged via line 40 to reactor 42, in which they are ignited. Nitrogen- containing gas may be flowed into the reactor prior to and during ignition; alternatively, an air environment may be used during ignition and combustion.

In one embodiment, the pellets from line 40 are self ignited in reactor 42. The material to be ignited is placed in reactor 42 and is heated to a temperature of about 760 degrees centigrade until ignition occurs. Thus, e.g., the material to be ignited can be charged to a graphite boat, and the graphite boat assembly can be inserted into the reactor and heated until ignition occurs. In one aspect of this embodiment, the material from line 36 may be formed into a shaped body and heated to cause it to reaction sinter, thereby simultaneously forming the shaped body.

In another embodiment, the pellets are ignited by contacting them with a source of heat at a temperature of at least about 800 degrees centigrade such as, e.g., a torch, a hot wire, a spark, a laser beam, etc. The combusted material is then allowed to cool to ambient temperature. In one embodiment, the ignited materi¬ al is discharged to a refractory tray, wherein it is al¬ lowed to cool to ambient.

The cooled, combusted material then is crushed. Referring again to Figure 1, the cooled, combusted material may be passed via line 44 to crusher 46. In one embodiment, cooled, reacted pellets are crushed, preferably in a roller mill. The pellets are crushed so that substantially all of their particles are within the range of about 0.1 to about 0.5 millimeters. Any suitable roll crusher may be used. Thus, e.g., one may use a roll crusher having smooth rolls. Medium-peripheral speed mills (such as ring-roll mills, bowl mills, roll mills, etc.) may be used; see, e.g., pages 8-22 of said Perry and Chilton's book. The rolls in the crusher preferably are smooth.

Referring to Figure 1, the crushed material may be discharged via line 48. Thereafter, it may either be used as is or subject to further size reduction in siever 50.

The crushed matter from line 48 consists essentially of least about 16 weight percent of material of the beta- prime sialon compound of the formula Si-__Al_zO_.Ng__z, from about 54 to about 28 weight percent of alumina, from about 0.5 to about 8 weight percent of aluminum nitride, from about 2 to about 25 weight percent of silicon nitride, and from about 30 to about 24 weight percent of free silicon.

The crushed matter is a precursor material which may be used to create a sialon bond phase through reaction bonding for oxide and nonoxide refractory materials.

The silicon material in the precursor material may be in the form of powder and/or whiskers. In general, the use of relatively short comminution times and self-ignition of powder will tend to produce silicon in the powder form. On the other hand, the use of relatively long comminution times and the ignition of pellets will tend to produce silicon whiskers.

The term "whisker" refers to a needle-like, single crystal. This term is discussed in United States patent 4,990,295.

In one embodiment, at least about 90 weight percent of the silicon material is in whisker form. If one desires powdered silicon, he may either modify the comminution time and/or the ignition mode, and/or he may mill the composite material to destroy the whiskers.

The silicon whisker produced in applicants' process generally has a diameter of from about 3 to about 7 microns and a length of from about 50 to about 300 microns. It con¬ sists essentially of silicon.

The density of the silicon whiskers is about 2.3 grams per cubic centimeter, which is substantially lower than the density of the alumina. Thus, it is relatively easy to separate the silicon whiskers from alumina by standard flotation processes.

The precursor material of this invention may be used to create a bond phase by mixing it with oxide or nonoxide refractory material and, thereafter, heating the mixture to a temperature of at least about 1250 degrees Centigrade while blanketing the material with nitrogen-containing gas.

The precursor material of this invention is a- highly activated silicon/alumina composite powder material suitable for fast oxynitride bonding of refractory oxides, carbides, and nitrides. Thus, one may use it together with such inorganic materials as silicon carbide, silicon nitride, titanium nitride, boron nitride, and the like.

In general, in order to cause the reaction bonding to occur, one should preferably mix at least about 5 weight percent (and, preferably, from about 5 to about 30 weight percent) of the precursor material with the oxide or nonox- ide refractory material. The oxide or nonoxide refractory may be in the form of powder and/or whiskers. Furthermore, in applicants' precursor, the silicon may be in the form of powder and/or whiskers.

Instead of using silicon to produce silicon nitride reaction bonded silicon carbides, one may use applicants' precursor material. Because, in this precursor, the silicon part of the composite has a high nitrogen sorption potential at elevated temperatures, high bond rates are obtained with no free silicon left in the product; and the bond phase created is a sialon material (instead of silicon nitride) . The bonded refractory material produced by applicants ' process exhibits improved corrosion resistance when immersed in certain molten metals, and improved resistance to chemi¬ cal attack.

By way of further illustration, one may add from about 5 to about 30 weight percent of such precursor material to silicon nitride powders and thereafter heat the mixture under nitrogen to cause reaction bonding to occur. One may add from about B to about 30-weight percent of said precur¬ sor material to refractory grade silicon carbide powder, form such mixture into a into a desired shape, and heat the green shape under nitrogen atmosphere. In another embodi¬ ment, the precursor material is mixed with boron carbide powder. In yet another embodiment, the refractory material used is boron nitride.

In general, in order to form a reaction-bonded product between the precursor material and the ceramic material, the two materials are mixed (preferably wet-mixed in a slurry) , dried (if necessary), formed into a green shape, and then heated while under nitrogen-containing gas (such as nitro¬ gen) to a temperature sufficient to transform the precursor into a sialon phase. In general, the temperature used will be form about 1,450 to about 1,650 degrees Centigrade. Size reduction of the precursor material

In one preferred embodiment, the crushed material from line 42 is passed via line 49 to mill 51, wherein it is milled until at least about 95 percent of its particles are smaller than 325 mesh (53 microns). In one aspect of this embodiment, the crushed material from line 49 is passed to ball mill 51 where it is milled in two stages. In the first stage of this embodiment, the material is milled until substantially all of its particles are smaller than about 150 microns (100 mesh). Thereafter, the -100 mesh material is preferably mixed with from 0.1 to about 1.0 weight per¬ cent of surfactant (such as the unsaturated fatty acid[s] described elsewhere in this specification) which may be charged via line 53, and this mixture is then further milled until at least about 95 percent of the material passes 325 mesh (and is less than 53 microns). Samples of the mixture may periodically be removed from mill 51 via line 46 to laboratory 57, wherein sieve analysis of the material may be conducted. In determining how much material tested in laboratory.57 is smaller than 53 microns, water at a pres¬ sure of from about 5 to about 15 pounds per square inch is used to attempt to force the particles through the 325 mesh sieve.

In another embodiment, the crushed material from crush¬ er 46 is passed via line 48 to a two-deck vibratory sieve 50. This vibratory sieve preferably has at least two sieves which sequentially screen and/or treat the crushed material. The first screen in the vibratory sieve, into which the crushed material is first discharged, is 150 mesh. Material which passes through this sieve is then passed to the second vibratory sieve, which is 325 mesh. Heat treatment of the sieved, precursor material

In one preferred embodiment, the novel precursor mate¬ rial of this invention, which preferably has been sieved as described above, is heat-treated. Precursor material may be passed via line 59 and/or 61 and/or 63 to furnace 52, in which the material may be heat-treated.

In applicants* heat-treatment process, it is preferred to use impure, intermediate product in which substantially all of the particles are smaller than 325 mesh (53 microns) . Thus, for example, the material from siever 50 which passes through the 325 mesh may be passed via line 59 to furnace 52.

It is preferred to conduct the thermal conditioning treatment in a high-temperature furnace able to reach a temperature in excess of 1,600 degrees centigrade. Such furnaces are well known to those in the art and are de¬ scribed, e.g., on pages 606-612 of Volume 5 of the McGraw- Hill Encyclopedia of Science & Technology.

In one preferred embodiment, the furnace 52 is a graphite furnace containing a graphite heating element; and the thermal conditioning treatment is conducted under nitro¬ gen atmosphere. Such a furnace, e.g., is the graphite ele¬ ment furnace available as model number 1000-45120-FP30 from Thermal Technology, Inc., Astro Division, of Santa Barbara, California.

In another embodiment, the furnace 52 is a rotary furnace. Thus, e.g., one may use a rotary furnace obtainable from Advanced Vacuum Systems of Ayer, Ma. 01432.

Referring to Figure 1, the sieved material may be passed via line 59 to furnace 52. A nitrogen source 65 sup¬ plies a nitrogen or nitrogen-containing atmosphere via line 67 to the furnace after the material has been loaded into the furnace.

The sieved material may be placed into a refractory container prior to being placed into the furnace; the use of such a container will prevent the material from being contaminated by carbon from the graphite heating element. It is preferred that such container substantially encapsulate the sieved material.

In one embodiment, the refractory container is a high-temperature furnace tube which is obtainable from, e.g., the McDanel Refractory Company of Beaver Falls, .Pa. The material is charged into such a tube, the tube is then placed into the furnace, and nitrogen is caused to flow from one end of the tube to another through the material.

Once the sieved material is in the furnace, the furnace door is closed, and the furnace is purged with nitrogen or nitrogen-containing gas. Thereafter, the temper¬ ature of the furnace is raised from ambient to a temperature of from about 800 to about 1,000 degrees centigrade. It is preferred to raise the temperature from ambient to from about 850 to about 950 degrees centigrade. It is even more preferred to use a temperature of about 900 degrees centi¬ grade.

It is preferred to raise the temperature of the fur¬ nace from ambient at a rate of from about 5 to about 15 degrees per -minute and, preferably, from about 8 to about 12 degrees per minute. It is most preferred to use a rate of about 10 degrees per minute.

Nitrogen or nitrogen-containing gas is flowed over the sieved material as its temperature is being raised. It is preferred to use a sufficient amount of nitrogen gas so that it flows through the sieved material at a rate of from about 4 to about 10 liters per minute and exerts a pressure upon said material of at least 2 pounds per square inch gravity. It is preferred that the gas flow rate be such that the gas exerts a pressure upon such material of from about 2 to about 7 pounds per square inch gravity.

Once the sieved material has reached the temperature of from about 800 to about 1,000 degrees centigrade, it is maintained at this temperature for from about 15 to about 60 minutes while it is contacted with a sufficient amount of nitrogen so that the nitrogen gas flows through the material at a flow rate of from about 4 to about 10 liters per minute and exerts a pressure upon said material of at least about 2 p.s.i.g. It is preferred that the gas exert a pressure upon said material of from 2-7 p.s.i.g.

After this initial heat treatment, the temperature of furnace is then raised to from about 1240 to about 1260 de¬ grees centigrade at a rate of from about 5 to about 15 degrees per minute. It is preferred to raise the tempera¬ ture to 1,250 degrees centigrade at a rate of about 8 to about 12 degrees per minute. During this temperature in¬ crease, the sieved material is contacted with a sufficient amount of nitrogen or nitrogen-containing gas so that the gas flows through the material at a flow rate of from about 1 to about 4 liters per minute and exerts a pressure upon said material of at least about 14 (and preferably from about 14 to about 16) p.s.i.g.

Once the material has reached the temperature of from about 1240 to about 1260 degrees centigrade, it is main¬ tained at this temperature for at least about 30 (and pre¬ ferably from about 30 to about 90) minutes while being contacted with a sufficient amount of nitrogen so that the gas exerts a pressure upon said material of at least about 14 p.s.i.g. (and, preferably, from about 14 to about 16 p.s.i.g); in one embodiment, the material is maintained at the desired temperature for from about 45 to about 75 minutes. During this time, the nitrogen or nitrogen- containing gas flow rate is preferably less than about 1 liter per minute, preferably being substantially 0 liters per minute.

After the material has been maintained at 1240-1260 degrees centigrade for at least 30 minutes, its temperature is then raised to from about 1400 to at ut 1500 αegrees centigrade at a rate of from about 5 to about 15 degrees per minute while being contacted with a sufficient amount of nitrogen or nitrogen-containing gas so that the nitrogen gas flows through the material at a flow rate of from about 1 to about 4 liters per minute and exerts a pressure upon said material of at least about 14 p.s.i.g. (and, preferably, from about 14 to about 16 pounds per square inch gravity) . It is preferred to raise the temperature of the material to from about 1430 to about 1470 degrees centigrade. It is even more preferred to raise the temperature of the material to about 1450 degrees centigrade.

Once the material has reached the temperature of 1400-1500 degrees centigrade, it is maintained at such temperature for at least about 30 minutes (and, preferably, from about 30 to about 90 minutes while) being contacted with a sufficient amount of nitrogen or nitrogen-containing gas so that the nitrogen gas flows through the material at a flow rate of at least about 1 liter per minute and exerts a pressure upon said material of at least about 14 p.s.i.g.

Thereafter, the material is cooled to a temperature of from about 1000 to about 800 degrees centigrade while being contacted with a sufficient amount of nitrogen gas so that the nitrogen gas flows through the material at a flow rate of at least about 1 liter per minute and exerts a pressure upon said material of at least about 14 p.s.i.g.; the rate of cooling is from about 5 to about 15 degrees per minute and, preferably, is from about 8 to about 12 degrees per minute.

Once the material has been cooled to a temperature of from about 1,000 to about 800 degrees centigrade, it is then allowed to cool to ambient without any external cooling means; however, during this cooling, it is still maintained under a sufficient amount of nitrogen gas so that the nitrogen flows through the material at a flow rate of from at 1 st about 1 liter per minute and exerts a pressure upon said material of at least about 2 pounds per square inch.

The heat-treated material may be discharged from fur¬ nace 52 and passed via line 54 to crusher 56, wherein it may be crushed so that substantially all of its particles are smaller than 325 mesh. Thereafter, the crushed powder may be passed via lie 58 to siever 60, which may be similar to siever 50. The material which passes through the bottom of siever 60 will have all of its particles smaller than 53 microns.

Referring again to Figure 1, material which is dis¬ charged from siever 60, via line 62, generally contains at least 70 weight percent (and preferably at least 72 weight percent) of the beta-prime-sialon compound of the formula Sig__zAl_zO_zNg__z. In one preferred embodiment, the material contains at least about 73 weight percent of such beta-prime-sialon compound.

The material which is discharged from siever 60 pre¬ ferably has a particle size distribution such that at least 90 weight percent of its particles are smaller than 53 microns, at least about 50 percent of its particles are smaller than 25 microns, and at least about 10 percent of its particles are smaller than 15 microns. This material preferably has a specific surface area of at least about 0.8 square meters per gram.

The material which is discharged from siever 60 may then be passed via line 62 to mill 64, in which its parti¬ cles may be deagglomeated. Milling will occur in mill 64 until analysis reveals that at least about 90 percent of the particles are smaller than 5.0 microns, at least 50 percent of the particles are smaller than 3.0 microns, and at least 10 percent of the particles are smaller than 2.0 microns; particles of the material to be milled may periodically be withdrawn via line 66 to laboratory 68, wherein it can be subjected to particle size analysis. This milling prefer¬ ably occurs in the presence of either silicon nitride and/or sialon grinding media, although other grinding media may be used.

The specific surface area of the deagglomerated parti¬ cles milled in mill 64 should be at least 3.0 square meters per gram. It is preferred that the material in mill 64 be wet milled.

In one embodiment, the particles are wet milled in the presence of isopropyl alcohol; in this embodiment, the spe¬ cific surface area of the milled material is at least about 3.0 square meters per gram. In another embodiment, which is preferred, the milling occurs in an aqueous slurry, such as a water slurry; in this embodiment, the specific surface area is at least about 5.0 square meters per gram and often is at least about 7 square meters per gram.

In another embodiment, illustrated in Figure 1 the material from mill 64, which preferably is in aqueous media, is discharged via line 70 to. attritor 72, wherein it is further milled until it has a specific surface area of at least about 12 square meters per gram and a particle size distribution such that at least about 90 weight percent of its particles are smaller than 1.0 micron and at least about 50 weight percent of its particles are smaller than about 0.5 microns.

In one preferred embodiment, attritor 72 is a batch attritor such as, e.g., the heavy-duty 0-1 Batch Attritor produced by the Union Process Company of 1925 Akron-Peninsu¬ la Road, Akron, Ohio.

In one embodiment of the grinding process, the aqueous media used is water, such as, e.g., deionized water. Thus, one may charge to attritor 72 eighty volume percent of grinding media, about 10 volume percent of the beta-sialon material, and about 10 volume percent of deionized water.

The material from mill 64 and/or attritor 72 may then be passed to drier 74, via line 76 or 78. The material in drier 74 is dried to a moisture content of less than 2.0 percent.

Any of the driers known to those skilled in the art may be used as drier 74. Thus, for example, one may use the driers described on pages 20-3 to 20-64 of said "Chemical Engineers Handbook," supra.

In one preferred embodiment, drier 74 is a spray drier. As is known to those skilled in the art, spray drying is accomplished by atomizing feed liquid into a drying chamber where the small droplets are subjected to a stream of hot air and are converted to powder particles.

One preferred spray drier which may be used is the model "Type HT" spray drier which is sold by Niro Atomizer Inc. of 9165 Rumsey Road, Columbia, Maryland 21045. Preparation of a reinforced body

The aforementioned material which contains the beta- prime-sialon compound may be. reinforced with oxide material and/or nonoxide material such as, for example, aluminum oxide material, silicon carbide material, boron carbide material, boron nitride material, aluminum nitride material, titanium diboride material, and mixtures thereof. In one preferred embodiment, a reinforced sialon material is pro¬ duced by a process in which a sialon powder (all of whose particles are preferably smaller than about 5 microns) is wet milled with reinforcing agent, and the slurry thus produced is then formed into a green body and dried. In this embodiment, the materials may be milled in aqueous media (such as water), nonaqueous media (such as isopropyl alcohol) , and the like.

By way of illustration, the material of this inven¬ tion may be reinforced with aluminum oxide which is either in powder and/or whisker and/or short fibrous form. In this embodiment, it is preferred to use at least 10 volume per¬ cent (and, more preferably, at least 20 volume percent) of the alumina material.

By way of further illustration, the material may be reinforced with silicon carbide which is either in powder and/or whisker and/or short fibrous form. In this embodi¬ ment, it is preferred to use from about 10 to about 50 volume percent of the silicon carbide material. In one aspect of this embodiment, 20 volume percent of the silicon carbide material is used. In another aspect of this embodi¬ ment, 40 volume percent of the silicon carbide material is used.

By way of further illustration, the material may be reinforced with boron carbide material, which preferably is in powder form. In this embodiment, it is preferred to use at least 10 volume percent of the boron carbide material.

By way of further illustration, the material may be reinforced boron nitride material, which preferably is in powder .form. In this embodiment, it is preferred to use at least 10 volume percent of the boron nitride material.

By way of further illustration, the material may be reinforced with aluminum nitride material, which is either in powder and/or whisker form. In this embodiment, it is preferred to use at least 10 volume percent of the aluminum nitride material.

By way of further illustration, the material may be reinforced with titanium diboride, which preferably is in powder form. In this embodiment, it is preferred to use at least 10 volume percent of the titanium diboride material.

Mixtures of the aforementioned reinforcing agents may be used. In general, it is preferred to wet-mix the rein¬ forcing agent and applicants' material in the presence of aqueous media. However, when the reinforcing agent is degraded by aqueous media (as is, e.g., aluminum nitride), then a nonaqueous media (such as isopropyl alcohol) may be used.

The material made in applicant's process may be used to prepare a green body. Thus, e.g., one may use the crushed material discharged via line 58. Alternatively, or additionally, one may use the ground material prepared in mill 64, 95 percent of which is smaller than 5 microns. Thus, one may prepare a material with other particle size(s) for use in the green body.

The green body preferably contains at least one material selected from the group consisting of binders, lubricants, and mixtures thereof. Typical binders and lubricants which may be used include unsaturated fatty acids (such as, e.g., stearic or oleic acid), organic materials, starches, gums, waxes, dextrine, corn flower, polyvinyl alcohol, marine derivatives, lignin extracts, methyl cellu¬ lose, and the like. It is preferred that the binder and/or lubricant used burn out of the ceramic body at temperatures below 400 degrees centigrade so that they will not interfere with ceramic reactions which occur at higher temperatures.

One preferred class of binders are the vinyl binders which are soluble in water. These binders are preferably selected from the group consisting of polyvinyl acetate, polyvinyl alcohol, polacrylamide, polvinyl pyrolidone, and mixtures thereof.

In one preferred embodiment, a green body is formed by an injection molding process. In this process, it is preferred to mix from about 75 to about 92 parts of applic¬ ants' composition, with a particle size such that at least about 95 percent of its particles are smaller than 5 mi¬ crons, with from about 25 to about 8 parts of thermoplastic binder.

The injection molding process is well known to those skilled in the art and is described, e.g., on pages 590 to 592 of Brage Golding's "Polymers and Resins" (D. Van Nos- trand Company, Inc., Princeton, New Jersey, 1959. Injec¬ tion molding of ceramic materials is described on pages 373 to 377 of James S. Reed's "Introduction to the Principles of Ceramic Processing" (John Wiley & Sons, New York, 1988).

In this embodiment, in addition to using thermoplas¬ tic binder, one may also mix applicants' composition with a surfactant such as, e.g., oleic acid. In general, from about 75 to about 92 percent of the composition is dispersed in from about 8 to about 25 percent of the binder. Thereaft¬ er, the mixture is injection molded into a green body at a preferred pressure of from about 4 to about 8 atmospheres.

Applicants* composition may be mixed with one or more of said binders and/or lubricants and formed into green bodies by conventional means. Thus, by way of illustration, one may dry press the mixture at a pressure of from between about 35 to about 200 megapascals, one may use hot pressing, one may use isostatic pressing, and one may use any of the conventional means known to those skilled in the ceramic art for forming green bodies from powder mixtures.

In one preferred embodiment, the binder used in the mixture to be pressed is a polyvinyl alcohol aqueous solu¬ tion which contains no more than about 8 percent, by weight of solution, of polyvinyl alcohol. From about 8 to about 25 parts, by weight, of this solution are preferably mixed with from about 75 to about 82 parts of applicant's composition, preferably with a composition which contains particles at least about 95 percent of which are smaller than 53 microns.

After the green body comprised of applicant's compo¬ sition has been formed, it is preferred to subject it to conditions to insure that it has substantially no moisture and substantially no organic matter in it prior to the time it densified. It is also preferred that, prior to sinter- ing, the green body contain no more than about 0.1 weight percent of residual carbon.

The green body will preferably be densified by heating it to a temperature of from about 1,650 to about 1,700 degrees centigrade, preferably for from about 1 to about 4 hours. It is preferred that, prior to densifica- tion, the green body contain less than about 0.1 percent of moisture. Thus, one may dry the green body by conventional means until it has the desired moisture content.

The temperature and time required for drying will vary with conditions such as, e.g., the thickness of the green body, the humidity, the air velocity, etc. By way of illus¬ tration, in one preferred embodiment, in which the green body contains about 8 parts by weight of a 5.0 percent solution of polyvinyl alcohol, the green body is dried at the rate of about 0.5 inch per hour by being subjected to a temperature of from about 105 to about 150 degrees centi¬ grade; in this embodiment, the drying also serves to remove the polyvinyl alcohol from the green body.

It is also preferred prior to densification to remove substantially all of the organic material in the green body from the green body so that the green body contains less than about 0 1 weight percent of such organic matter. This debindering _an also be accomplished by conventional means, by heating the green body until the organic matter in it is burned out. As is known to those skilled in the art, the debindering conditions will vary with factors such as the type(s) and concentration(s) of the organic material in the green body, the thickness of the green body, etc. By way of illustration, in one embodiment in which the green body con¬ tains about 8 weight percent of an organic binder, the green body is heated over a period of from about 8 to about 24 hours, in various steps, to temperatures of 150-180, 250, 400-800, and 800-1100 degrees centigrade. It is often required to heat the green body to a temperature of at least about 800 degrees centigrade to burn out the hydrocarbon matter in it.

It is preferred to provide a green body for densifying which contains less than about 0.1 percent moisture, less than about 0.1 percent of organic matter, and less than about 0.1 percent of residual carbon. This debindered green body is preferably densified by heating it under nitrogen to a temperature of from about 1,650 to about 1,750 degrees centigrade for from about 1 to about 4 hours. The densified body formed by this process contains at least 70 weight percent of a compound of the formula Si-__.Al_.O_zNg__z, wherein z is preferably about 3.0, and at least about 27 weight percent of the solid solution described elsewhere in this specification.

The green-body of this invention, which is comprised of applicants* beta-prime-sialon compound, may be densified without pressure. Thus, in one preferred embodiment, the pressure on the system is substantially ambient, ranging from about 760 to about 800 Torr.

The green body of this invention may be densified without the use of a sintering aid.

By way of illustration, in one experiment the composi¬ tion containing the beta-prime-sialon compound was mixed with "METHOCEL" binder and pressed into a pallet. The pallet was packed in a powdered mixture of 40 percent boron nitride/60 percent sialon and was sintered at 1,700 degrees Centigrade for 3.0 hours under flowing nitrogen at at¬ mospheric pressure. In this experiment, the sintered mate¬ rial had a density of 3.27 grams per cubic centimeter, a flexural strength of 1,000 megapascals, an elastic modulus of 300 gigapascals, a hardness of 19 gigapascals, a fracture toughness of 6, and a thermal shock resistance of 1,100 degrees Centigrade. In one preferred embodiment, the debindered body is sintered with a specified sintering cycle. In this cycle, the body may be sintered while exposed to a nitrogen envi¬ ronment, without being contacted with any other material except the support on which it rests. Alternatively, the body may be sintered after it has been packed with powder.

In one embodiment, it is preferred to sinter the beta- sialon debindered body after it has been packed in a ceramic powder. It is preferred, in this embodiment, to pack the debindered green body in a nitride powder such as, e.g., boron nitride powder, and/or silicon nitride powder, and/or sialon powder, and/or mixtures thereof. In general, the packing powder should have a particle size such that sub¬ stantially all of its particles are smaller than 53 microns. In one aspect of this embodiment, boron nitride comprises part of all of the packing powder(s); the use of such boron nitride helps prevent the agglomeration of the packing powder.

In this aspect of this embodiment, it is preferred to pack the green body so that substantially none of its sur¬ face is directly exposed to the atmosphere in the furnace.

The atmosphere used during sintering is preferably nitrogen-containing gas and will comprise or consist, e.g., of nitrogen, mixtures of nitrogen and inert gas (such as argon), mixtures of nitrogen with hydrogen, and the like. In those embodiments wherein mixtures of nitrogen are used with one or more other gases, it is preferred that such mixtures contain at least about 75 volume percent of nitro¬ gen.

In the sintering process, the debindered green body may be placed into a furnace. Thereafter, after the furnace has been flooded with the nitrogen-containing gas, the temperature of the furnace is raised from ambient to a temperature of from about 1230 to about 1,270 degrees centi- grade at a rate of from about 10 to about 20 degrees per minute. It is more preferred to raise the temperature to about 1250 degrees centigrade at a rate of from about 15 to about 20 degrees centigrade. During this cycle, the nitro¬ gen-containing gas is flowed over the debindered green body at a flow rate of from about 2 to about 12 liters per minute.

After the material has been raised to its temperature of from about 1230 to about 1270 degrees centigrade, it is held at this temperature for at least about 30 minutes while the nitrogen-containing gas continues to flow over the green body. Thereafter, it is raised from its temperature of 1230-1270 to a higher temperature of from about 1650 to about 1700 degrees centigrade at a rate of from about 5 to about 10 degrees centigrade per minute. Once it reaches this higher temperature, it is maintained within this tem¬ perature range for at least about 1.0 hour while being contacted with flowing nitrogen. However, in this stage, it is preferred to reduce the flow rate of the nitrogen so that such flow rate will not exceed about 4 liters per minute and, preferably, will be at least about 10 percent lower than the flow ate used in the first part of the process.

After the material has been heated at its temperature of 1650 to 1700 degrees Centigrade, it is then allowed to cool while being contacted with flowing nitrogen. It pre¬ ferably is cooled to ambient at a rate of no greater than 25 degrees Centigrade per minute.

The properties of the sintered material obtained by the process of this invention are improved. Thus, for example, the sintered body produced from a green body packed in ceramic powder will generally have a bulk density of from about 3.23 to about 3.28 grams per cubic centimeter, a hard¬ ness of from about 16 to about 19 gigapascals, an elastic modulus of from about 260 to about 300 gigapascals, a flex- ural strength of from about 600 to about 1,000 megapascals, a fracture toughness (indentation) of from about 5.2 to about 6, a coefficient of thermal expansion of from about 3.04 to about 3.08 x 10~"/degrees centigrade, a thermal conductivity of from about 21 to about 23 watts/meter de¬ grees Kelvin, a thermal shock resistance of 1,100 degrees centigrade, and a maximum use temperature in air of 1,370 degrees centigrade.

In one embodiment, the oxidation resistance of the sintered material will be such that, after it has been heated to a temperature of at least 1200 degrees Centigrade for 100 hours in air, the mechanical properties of the material will be at least about 90 percent as great as the mechanical properties of the unheated, unaged material.

The material produced by the process of this inven¬ tion, preferably after it has been heat-treated by the heat- treatment process of the invention, may be used as a sinter¬ ing aid with various ceramic materials to produce sintered products with improved properties.

In the sintering process of this invention, it is preferred to mill the applicants' material until substan¬ tially all of its particles are smaller than 5.0 microns. If necessary, the milled beta sialon particles are then dried until they contain less than about 2.0 weight percent of moisture.

The milled and dried particles may then be added to the ceramic material to be sintered. In general, the sin¬ tering aid is intimately mixed with the ceramic material.

In general, from about 1.0 to about 25 weight percent of the aforementioned sintering aid is mixed with the ceram¬ ic material to be sintered.

In one preferred embodiment, wet-mixing is used to combine the sintering aid with the ceramic material to be sintered; this is feasible inasmuch as the beta-sialon sin- tering aid is not hydrolyzable in water. Thus, one may ball mill an aqueous slurry comprised of the sintering aid, water, and the ceramic material to be sintered. After such slurry has been well mixed, one may form a green body from it by conventional means and, thereafter, sinter the green body.

In one preferred embodiment, the ceramic material to be sintered is cordierite, which is a silicate of the formu¬ la 2Mg0.2 l₂0₃.5Siθ2. In this embodiment, the addition of from about 5 to about 15 weight percent of the sintering aid produces a sintered composite material with substantially improved refractoriness, thermal shock resistance, and mechanical strength.

In another preferred embodiment, the ceramic material to be sintered is mullite, which is a refractory aluminosi- licate of the formula 3AI₉O3.2Si0 . In this embodiment, the addition of from about 2 to about 8 weight percent of the sintering aid produces a sintered material with substantial¬ ly improved creep resistance, thermal shock resistance, and microhardness.

In yet another preferred embodiment, the ceramic mate¬ rial is alumina. The addition of from about 2 to about 10 weight percent of the sintering aid produces an alumina with increased microhardness and thermal shock resistance.

In yet another preferred embodiment, the ceramic-mate¬ rial to be sintered is zirconia which, preferably, has been partially stabilized with stabilizers such as alumina, magnesisa, calcia, yttria, ceria, zirconium nitride, and the like. The addition of from about 5 to about 15 weight percent of the sintering aid produces a sintered zirconia body with substantially diminished porosity, increased bulk density, and improved microhardness. It should be noted that, in this embodiment, the sintering aid not only acts as a sintering aid but also acts to stabilize the zirconia. In yet another embodiment, the ceramic material to be sintered is magnesia. In this embodiment, the addition of from about 1 to about 5 weight percent of the sintering aid substantially improves the properties of the sintered body.

In yet another embodiment, the ceramic material to be sintered is silicon carbide powder. In this embodiment, the use of from about 10 to about 20 weight percent of the sintering aid results in a sintered product with higher bulk density and lower apparent porosity. The use of substan¬ tially the same concentration of sintering aid with other carbide materials (such as titanium carbide, tungsten car¬ bide, and the like) produces substantially similar and advantageous results.

In yet another preferred embodiment, the ceramic mate¬ rial to be sintered is silicon nitride. In this embodiment, the use of from about 5 to about 20 weight percent of the sintering aid produces a sintered material with improved mechanical and thermal properties. It will be appreciated that the sintering aid may be used at 5-20 weight percent concentration with other nitridi -, such as titanium nitride, tantalum nitride, aluminum nitride, and the like.

Unlike prior art sintering aids (such as yttria/alumi- na) , applicants' sintering aid is relatively inexpensive and produces a sintered product whose properties are not degrad¬ ed by the presence of the sintering aid.

In one embodiment of this invention, the crystal growth of applicants' laterial is suppressed during sinter¬ ing. One may suppress such crystal growth by well known techniques. Thus, for example, one may add a crystal growth suppressant to the milled material. Alternatively, one may add a crystal growing supplier to the unignited pellet and/or to the material used to form such pellet so that, during combustion, a crystal growth suppressor is synthe- sized in situ and homogeneously dispersed throughout the combusted material.

In one embodiment, the crystal growth suppressor is an oxide or a mixture of oxides, such as, e.g., magnesium oxide, magnesium silicate, magnesium aluminum silicate, magnesium aluminate, and the like.

In another embodiment, the crystal growth suppressor is a non-oxide material such as, e.g., silicon carbide, titanium diboride, boron carbide, mixtures thereof and the like.

In yet another embodiment, mixtures of oxide and nonoxide materials may be used.

In this embodiment, the material which is formed into a green body and sintered has superior mechanical proper¬ ties.

The following examples are presented to illustrate the claimed invention and are not to be deemed limitative thereof. Unless otherwise specified, all parts are by weight, and all temperatures are in degrees centigrade. Examples Example 1

47 grams of silica (catalog number 13024, Johnson Matthey/AESAR Group) were charged to a stainless steel vessel with a capacity of about 250 cubic centimeters (part 27-17-229-6, Brinkman Instruments Company, Cantiague Road, Westbury, New York 11590) along with grinding media consist¬ ing of 18 tungsten carbide cycloids having a 8 millimeter diameter, a 16.0 millimeter length, and a total weight of 306 grams. The vessel was then covered with a stainless steel lid, and the covered vessel was then introduced into a Brinkman centrifugal ball mill (Centrifugal Ball Mill S-l, Brinkman Instruments Company, see page 38 of the Rl catalog) and comminuted for about 3.0 hours at a speed of 540 revolu¬ tions per minute. After such comminution, the lid was removed, and 35 grams of aluminum powder (type 1401T, Alumi¬ num Company of America) were added. The vessel was then covered with an aluminum aeration cover (part number 27-16- 219-3, Brinkman Instruments Company). This aluminum cover permitted the introduction of nitrogen gas during comminu¬ tion.

The mixture was then again comminuted in the ball mill while being contacted with flowing nitrogen at a flow rate of about 2.0 liters per hour; the comminution occurred over a period of about 3.0 hours at a speed of 540 revolutions per minute.

The comminuted material was discharged from the vessel, and pellets of 1.125 inch diameter were pressed in a cylindrical die with a Carver Lab Press 13-872 (see page 897 of the Fisher '88 Catalog, Fisher Scientific Company); a pressure of 3 tons was used.

The pellets were charged into a graphite boat. The boat was then placed into a graphite reactor and, while under a nitrogen atmosphere, was heated to a temperature of about 700 degrees centigrade, at which temperature igni¬ tion occurred.

The combusted pellets were cooled by allowing the reactor to return to ambient temperature. The pellets were then removed from the reactor and crushed until their aver¬ age size was less than about 2.0 millimeter.

The crushed pellets were then milled in a four-liter jar mill operated at a speed of 60 revolutions per minute for 8 hours; zirconia cylinders were used as the grinding media. Thereafter, this material was sieved on a 150 mesh sieve; substantially all of the material passed through the sieve.

The material which passed through the 150 mesh sieve was mixed with 0.05 weight percent of oleic acid, and this mixture was then ground in a four-liter jar mill operated at 60 r.p.m. with zirconia cylinders for about 24 hours. The material produced by this grinding was then sieved with a 325 mesh sieve; substantially all of the material passed through this sieve.

A portion of the material passing through the 325 mesh sieve was subjected to X-ray diffraction analysis. Such analysis indicated that the material contained alumina in excess of 50 weight percent. Two minor phases also were revealed. One of them was silicon, which was present in a concentration in excess of 30 weight percent. The other of them was a beta-prime-sialon compound of the formula Si^Al-O^N-., which was present in a concentration of in excess of 15 weight percent.

A portion of the sieved material was subjected to analysis by Scanning Electron Microscope and Energy Disper¬ sive Spectrometry, using the methods and apparatuses de¬ scribed in the specification. Such analysis indicated that the silicon phase in the material was associated with a content of aluminum, oxygen, and nitrogen.

The sieved material was charged into a 2.5 inch dia¬ meter furnace tube obtained from the McDanel Refractory Company of Beaver Falls, Pennsylvania. The furnace tube/sieved material assembly was then placed into a graph¬ ite element furnace, available as model number 1000-45120- FP30 from Thermal Technology, Inc., Astro Division, of Santa Barbara, California. This furnace was equipped with a nitrogen source.

The furnace door was closed, the furnace was purged with nitrogen, and nitrogen was caused to flow from one end of the tube to another through the material. The tempera¬ ture of the furnace was raised from ambient to a temperature of 900 degrees centigrade at a rate of 10 degrees per minute.

Nitrogen was flowed over the sieved material as its temperature was being raised. The nitrogen flow was about 4 liters per minute.

Once the sieved material had reached the temperature of 900 degrees centigrade, it was maintained at this temper¬ ature for 30 minutes while the nitrogen flow was reduced to 1 liter per minute and the nitrogen pressure was increased to 15 p.s.i.g. After this initial heat treatment, the temperature of furnace was then raised to 1,250 degrees centigrade at a rate of 10 degrees per minute. During this temperature increase, the nitrogen pressure and flow rate was maintained as before.

Once the material had reached the temperature of 1250 degrees centigrade, it was maintained at this temperature for 45 minutes. During this time, the nitrogen vent was closed so that the flow rate was substantially zero; the pressure in the reactor was about 16 p.s.i.g.

After the material had been maintained at 1250 de¬ grees centigrade for 45 minutes, its temperature was then raised to 1,450 degrees centigrade at a rate of 10 degrees per minute. The nitrogen flow rate was 1 liter per minute, and the pressure was 15 p.s.i.g.

Once the material had reached the temperature of 1450 degrees centigrade, it was maintained at such temperature for 90 minutes. The nitrogen flow rate was 1 liter per minute, and the pressure was 15 p.s.i.g.

Thereafter, the material was cooled to a temperature of 900 degrees centigrade. During this cooling, the nitro¬ gen flow rate was 1 liter per minute, *?-_d the pressure was 15 p.s.i.g. The rate of cooling was 10 dt-jrees per minute.

Once the material id been cooled to a temperature of 900 degrees centigrade it was then allowed to cool to ambient without any external cooling means; cooling was allowed to occur over a period of about 60 minutes. During this cooling, the nitrogen flow rate was 4 liters per minute, and the pressure was about 2 p.s.i.g.

The cooled material was removed from the furnace and crushed so that substantially all of its particles were smaller than 0.5 millimeters. Thereafter, the crushed material was passed through a twin-deck vibratory sieve whose first sieve was 150 mesh and whose second sieve was 325 mesh.

The heat-treated material passing through the 325 mesh screen was then analyzed to determine its composition

X-ray diffraction analysis indicated that material contained two major phases. The aforementioned beta-prime- sialon compound was present as a major phase at a concen¬ tration in excess of 70 weight percent. The aforementioned alpha-alumina was present as a major phase at a content in excess of 25 weight percent. Example_2

2,714 grams of the heat-treated, sieved material pro¬ duced in Example 1 were mixed in a one-gallon plastic bottle with 13.5 grams of oleic acid (catalog number OL 103, Spec¬ trum Chemical Manufacturing Company, Gardena, California) and 1,837 grams of 0.5 inch diameter zirconia media for 10 hours in a roller mill at a speed of 190 revolutions per minute. The mixed material was discharged from the plastic bottle and then pressed in a cylindrical die (1.125 diamet¬ er) with a Carver Laboratory Press, model number 13-872, available from the Fisher '88 Catalog (Fisher Scientific Company) ; a pressure of 7 tons was used. Each of the pellets so produced weighed about 10 grams.

The pellets were then debindered. They were placed into a Neytech 85N muffle furnace (available from the NEY Company, Barkmeyer Division, Yucaipa, California) and heated to a temperature of 400 degrees centigrade; they were then maintained at this temperature for 2 hours.

The debindered pellets were then measured and, after being measured, were charged into the Thermal Technology, Inc. graphite furnace. The pellets were first placed onto a refractory plate made of alumina, and the plate containing the pellets was then placed into the aforementioned 2.5 inch McDanel furnace tube.

Nitrogen was flowed through the ends of the furnace tube at a rate of 4 liters per minute, and the nitrogen pressure on the pellets was 5 p.s.i.g.

The temperature was raised from ambient to 1,200 degrees centigrade at a rate of 15 degrees per minute while maintaining the same nitrogen flow rate and pressure. Thereafter, the 1,200 degree centigrade temperature was maintained for 30 minutes while the nitrogen flow was re¬ duced to 1 liter per minute; the nitrogen pressure was increased to 15 p.s.i.g.

Thereafter, the temperature was increased to 1,650 degrees centigrade at a rate of 10 degrees per minute while the nitrogen flow rate and pressure were maintained as before.

Once.the pellets reached a temperature of 1,650 degrees centigrade, they were held at this temperature for 90 minutes while the nitrogen flow rate and pressure were maintained as before.

Thereafter, the furnace was cooled to 900 degrees centigrade at a rate of 15 degrees per minute; during this cooling, the nitrogen pressure was 15 p.s.i.g., and the nitrogen flow rate was increased to 4 liters per minute.

Once the temperature had reached 900 degrees centi¬ grade, the furnace was allowed to cool to ambient over a period of about 60 minutes; during this cooling, the nitro¬ gen pressure was reduced to 4 p.s.i.g. and the nitrogen flow rate was maintained at 4 liters per minute.

The cooled pellets were then measured to determine their shrinkage. The difference in the maximum dimension of each pellet was used to determine shrinkage. The average linear shrinkage of the pellets was 18 percent (in diameter) and 17 percent (in thickness). The volume shrinkage was 40 percent.

The density and water absorption of the pellets also were determined by immersing them in water and boiling them in water for one hour. The average density was 3.19 grams per cubic centimeter (which was 97 percent of the pellets' theoretical, calculated density); the average water absorp¬ tion was 1.6 percent.

The elastic modulus of the pellets was determined in accordance with A.S.T.M. test C-848-78. The average elastic modulus of the pellets was 291 gigaPascals.

The flexural strength of the pellets was determined in accordance with A.S.T.M. test F-417-78. The average flexural strength was 486 megaPascals.

The hardness and fracture toughness of the pellets were determined by indentation. The average hardness was 14 gigaPascals. The average fracture toughness was 3.9 mega¬ Pascals per/meter⁰*⁵.

The wear resistance of the pellets were determined by a wet test in accordance with A.S.T.M. B-611. The pellets lost about 24 cubic millimeters of material after 100 revo¬ lutions. Example 3

The procedure of Example 2 was substantially followed, with the exceptions the pellets were sintered at a final temperature at 1,680 degrees centigrade for 180 minutes, the nitrogen flow during the entire sintering cycle was 4 liters per minute, and the nitrogen pressure during such cycle was 2 p.s.i.g.

The cooled pellets were measured and evaluated in accordance with the procedure of Example 2.

The average linear shrinkage of the pellets was 17 cent (in diameter) and 16 percent (in thickness). The volume shrinkage was 40 percent.

The density and water absorption of the pellets also were determined. The average density was 3.22 grams per cubic centimeter; the average water absorption was 0.8 percent.

The average elastic modulus of the pellets was 318 GigaPascals. The average flexural strength was 623 mega¬ Pascals. The average hardness was 16 GigaPascals. The average fracture toughness was 5.6 megaPascals per/meter 0 5.

The wear resistance test indicated that the pellets lost about 22 cubic millimeters of material after 100 revolu¬ tions. Example 4

The procedure of Example 1 was substantially followed, with the exception that 42.0 grams of silicon dioxide were charged to the mill together with 8.2 grams of silicon nitride and 2.4 grams of aluminum nitride After the first comminution in the ball mill, the reaction vessel was opened, ad 31.4 grams of aluminum powder were added.

X-ray diffraction analysis of the combusted material which had not yet been heat-treated revealed the presence of alumina in excess of 45 weight percent, silicon in excess of 25 weight percent, the aforementioned beta-prime-sialon com¬ pound in excess of 15 weight percent, and silicon nitride in a concentration of about 5.0 weight percent; traces of aluminum nitride also were present.

X-ray analysis of the heat-treated material revealed the presence of the aforementioned beta-prime-sialon com¬ pound in excess of 70 weight percent, and a solid solution containing silicon nitride, aluminum nitride, and alumina in excess of 25 weight percent. Example 5

The procedure of Example 1 was substantially followed, with the exception that 37.8 grams of silicon dioxide were charged to the mill together with 14.7 grams of silicon ni¬ tride and 4.3 grams of aluminum nitride After the first comminution in the ball mill, the reaction vessel was opened, and 28.4 grams of aluminum powder were added.

X-ray analysis of the heat-treated material revealed the presence of the aforementioned beta-prime-sialon com¬ pound in excess of 70 weight percent, and a solid solution containing silicon nitride, aluminum nitride, and alumina, in excess of 25 percent. Example 6

The procedure of Example 1 was substantially followed, with the exception that 34.6 grams of silicon dioxide were charged to the mill together with 20.2 grams of silicon ni¬ tride and 6.0 grams of aluminum nitride After the first comminution in the ball mill, the reaction vessel was opened, and 26.0 grams of aluminum powder were added.

X-ray analysis of the heat-treated material revealed the presence of the aforementioned beta-prime-sialon com¬ pound .in excess of 70 weight percent, in excess of 25 weight percent of a solid solution containing silicon ni¬ tride, aluminum nitride, and alumina. Example 7

The procedure of Example 2 was substantially followed, with the exceptions that the composite material which was sintered was the composition of Example 5 and the pellets were sintered at a final temperature at 1,680 degrees centi¬ grade for 180 minutes.

The cooled pellets were measured and evaluated in accordance with the procedure of Example 2.

The average linear shrinkage of the pellets was 17 percent (in diameter) and 17 percent (in thickness). The volume shrinkage was 40 percent.

The density and water absorption of the pellets also were determined. The average density was 3.19 grams per cubic centimeter; the average water absorption was about 1 percent.

The average elastic modulus of the pellets was about 300 GigaPascals. The average flexural strength was 550 megaPascals. The average hardness was 15 GigaPascals. The average fracture toughness was greater than 6.0 megaPascals per/meter^0,5. The wear resistance test indicated that the pellets lost about 24 cubic millimeters of material after 100 revolutions. Example 8

The heat-treated and sieved material of Example 5 was subjected to particle size analysis. Ten percent of its particles were smaller than 5.46 microns, 50 percent of its particles were smaller than 16.06 microns, 90 percents of its particles were smaller than 53.04 microns, the median particle size was 25.06 microns, and the material had a specific surface area of 1.65 square meters per gram.

1,400 grams of the heat-treated and sieved material of Example 5 were charged to a 1.6-gallon capacity jar mill along with 10,800 grams of yttria-stabilized grinding media having a 5.0 millimeter diameter and 1,000 milliliters of deionized water. The mill was run at a speed of 50 revo¬ lutions per minute for 24 hours.

After the wet-milling of the material, slurry was discharged to a porcelain capsule and dried at a temperature of about 60 degrees centigrade until its moisture content was less than 2.0 weight percent.

The dried, milled material was then subjected to particle size analysis. Ten percent of its particles were smaller than 1.15 microns, 50 percent of its particles were smaller than 2.31 microns, 90 percent of its particles were smaller than 3.51 microns, the median particle size was 2.32 microns, and the material had a specific surface area of 6.85 square meters per gram. Example 9

The procedure of Example 8 was repeated with the exception that isopropyl alcohol was used instead of deion- ized water. The specific surface area of the dried, milled powder obtained in this experiment was 3.0 square meters per gram.

It is to be understood that the aforementioned de¬ scription is illustrative only and that changes can be made in the apparatus, the ingredients and their proportions, and in the sequence of combinations and process steps as well as in other aspects of the invention discussed herein without departing from the scope of the invention as defined in the following claims.

Claims

We claim:

1. A composition comprised of at least about 70 weight percent of a beta-prime sialon compound and at least about 27 weight percent of a solid solution, wherein:

(a) said beta-prime-sialon compound is of the formula Sig__zAl_z0_,Ng__z, wherein z is from about 2.5 to about 3.5;

(b)said beta-prime-sialon compound is comprised of from about 35 to about 25 weight percent of silicon, from about 24 to about 33 weight percent of aluminum, from about 14 to about 20 weight percent of oxygen, and from about 27 to about 22 weight percent of nitro¬ gen; and

(c)said solid solution is comprised of from about 2 to about 50 weight percent of alpha-silicon nitride, from about 0.5 to about 14 weight percent of aluminum ni¬ tride, and from about 97.5 to about 36 weight percent of alpha-alumina.

2. A precursor.material comprised of at least 16 weight percent of a beta-prime-sialon compound of the formula Sig_ _zAl_zO_zNg__z, from about 52 to about 28 weight percent of alumina, from about 0.5 to about 8 weight percent of alumi¬ num nitride, from about 2 to about 25 weight percent of silicon nitride, and from about 29 to about 24 weight per¬ cent of free silicon, wherein:

(a)said beta-prime-sialon compound is comprised of from about 35 to about 25 weight percent of silicon, from about 24 to about 33 weight percent of alumi¬ num, from about 14 to about 20 weight percent of oxygen, and from about 27 to about 22 weight percent of nitrogen,

(b)said z is from about 2.5 to about 3.5, (c) said aluminum nitride is comprised of less than about 2.0 weight percent of combined oxygen, has substantially all of its particles smaller than about 53 microns, and comprises less than about 0.1 percent of moisture,

(d) said silicon nitride is comprised of at least about 35 weight percent of nitrogen, less than about 2.0 weight percent of oxygen, and less than about 1.0 weight percent of free silicon, and

(e) substantially all of the particles of said sili¬ con nitride are smaller than about 10 microns.

3. The precursor material as recited in claim 2, wherein at least about 90 weight percent of said free silicon is in the form of a silicon whisker.

4. The precursor material as recited in claim 3, wherein said silicon whisker has a diameter of from about 3 to about 7 microns and a length of from about 50 to about 300 microns.

5. The precursor material as recited in claim 2, wherein substantially all of the particles of said precursor mate¬ rial are from about 0.1 to about 0.5 millimeters in size.

6. A mixture consisting essentially of from about 5 to about 30 weight percent of the precursor material recited in claim 2 and an inorganic material selected from the group consisting of silicon carbide, silicon nitride, titanium nitride, boron nitride, boron carbide, and mix¬ tures thereof.

7. A mixture comprised of from about 30 to about 45.5 weight percent of oxygen-deficient silica, from about 23 to about 34 weight percent of aluminum material, from about 0.5 to about 8 weight percent of aluminum nitride, from about 2 to about 25 weight percent of silicon nitride, and from about 0 to about 5 weight percent of a metal fluoride compound, wherein:

(a)said oxygen-deficient silica is comprised of from about 44 to about 47 weight percent of oxygen and from about 56 to about 53 weight percent of silicon; (b)said aluminum material is selected from the group consisting of aluminum, aluminum alloy and mixtures thereof, wherein:

(a)said aluminum material comprises less than about 1.0 percent of combined oxygen and less than about 0.1 percent of moisture, and

(b)said aluminum material has a specific surface area of from about 0.2 to about 1.1 square meters per gram, and the particles of said material range in size from about 4 to about 28 microns;

2. said aluminum nitride is comprised of less than about 2.0 percent of combined oxygen and less than about 0.1 percent of moisture;

3. said silicon nitride is comprised of at least about 35 weight percent of nitrogen, less than about 2.0 weight percent of oxygen, and less than about 1.0 weight percent of free silicon;

4. substantially all of the particles of said silicon nitride are smaller than about 10 microns; and

5. said silica is comprised of less than about 0.1 percent of moisture, and substantially all of the particles of said silica are smaller than about 53 microns.

8. A pellet with a porosity of at least about 40 percent and a size of at least about 1.0 inch, wherein said pellet is comprised of a substantially single phase material which comprises from about 30 to about 45.5 weight percent of oxygen-deficient silica, from about 23 to about 34 weight percent of aluminum material, from about 0.5 to about 8 weight percent of aluminum nitride, from about 2 to about 25 weight percent of silicon nitride, and from 0 to about 5 weight percent of a fluoride compound, and wherein said oxygen-deficient silica, said aluminum material, said alumi¬ num nitride, and said silicon nitride are as recited in claim 7.

9. A mixture comprised of a major amount of the material recited in claim 1 and from about 10 to about 50 volume percent of an inorganic material selected from the group consisting of silicon carbide, boron carbide, boron nitride, aluminum oxide, aluminum nitride, titanium diboride, and mixtures thereof.

10. A process for heat-treating a composite material, comprising the steps of:

(a)providing the precursor material recited in claim 2;

(b)heating said material to a temperature of from about 800 to about 1,000 degrees centigrade for from about 15 to about 60 minutes while contacting it with a sufficient amount of nitrogen so that the nitrogen flows through said material at a rate of from about 4 to about 10 liters per minute; (c) )thereafter .raising the. temperature of the mate¬ rial to from about 1,240 to about 1,260 degrees centigrade at a rate of from about 5 to about 15 degrees per minute while contacting it with a suffi¬ cient amount of nitrogen so that the nitrogen flows through said material at a rate of from about 1 to about 4 liters per minute;

(d)thereafter heating said material at said tempera¬ ture of from about 1,240 to about 1,260 degrees centigrade for at least about 30 minutes while con¬ tacting it with a sufficient amount of nitrogen so that the nitrogen flows through said material at a rate of least 10 liters per minute and experts a pressure upon said material of at least about 14 pounds per square inch gravity; (e)thereafter raising the temperature of said mate¬ rial to from about 1,400 to about 1,500 degrees centigrade at a rate of from about 5 to about 15 degrees per minute while contacting it with a suffi¬ cient amount of nitrogen so that the nitrogen flows through said material at a rate of from about 1 to about 4 liters per minute; and

(f)thereafter heating said material to a temperature of from about 1,400 to about 1,500 degrees centi¬ grade while contacting it for at least about 30 minutes with a sufficient amount of nitrogen so that the nitrogen flows through said material at a rate of at least 1 liter per minute and exerts a pressure upon said material of at least 14 pounds per square inch gravity.

11. A process for heat-treating a composite material, comprising the steps of:

(a)providing the precursor material recited in claim 2;

(b)heating said material to a temperature of from about 800 to about 1,000 degrees centigrade for from about 15 to about 60 minutes while contacting it with a sufficient amount of nitrogen-contain¬ ing gas so that the nitrogen-containing gas flows through said material at a rate of from about 4 to about 10 liters per minute and exerts a pressure upon said material of at least about 2 pounds per square inch gravity, wherein said nitrogen-con¬ taining gas consists essentially of at least about 95 volume percent of nitrogen and from about 2 to about 5 volume percent of hydrogen; (c)thereafter raising the temperature of the mate¬ rial to from about 1,240 to about 1,260 degrees centigrade at a rate of from about 5 to about 15 degrees per minute while contacting it with a sufficient amount of said nitrogen-containing gas so that the nitrogen-containing gas flows through said material at a rate of from about 1 to about 4 liters per minute;

(d)thereafter heating said material at said tem¬ perature of from about 1,240 to about 1,260 de¬ grees centigrade for at least about 30 minutes while contacting it with a sufficient amount of nitrogen gas so that the nitrogen flows through said material at a rate of no less than about 10 liters per minute;

(e)thereafter raising the temperature of said material to from about 1,400 to about 1,500 de¬ grees centigrade at a rate of from about 5 to about 15 degrees per minute while contacting it with a sufficient amount of nitrogen so that the nitrogen flows through said material at a rate of from about 1 to about 4 liters per minute; and (f)thereafter heating said material for at least about 30 minutes to a temperature of from about 1,400 to about 1,500 degrees centigrade while contacting it with a sufficient amount of nitrogen so that the nitrogen flows through said material at a rate of at least 1 liter per minute and exerts a pressure upon said material of at least 14 pounds per square inch gravity.

12. A process for preparing the precursor material recit¬ ed in claim 2, comprising the steps of:

(a)providing a mixture comprising from about 23 to about 34 weight percent (by weight of aluminum material, silicon nitride, aluminum nitride, and silica) of aluminum material, from about 0.5 to about 8 weight percent of aluminum nitride, from about 2 to about 25 weight percent of silicon ni¬ tride, and from about 30 to 45.5 weight percent of silica, wherein:

1. said aluminum material is selected from the group consisting of aluminum, aluminum alloy and mixtures thereof, wherein:

(a)said aluminum material comprises less than about 1.0 percent of combined oxygen and less than about 0.1 percent of mois¬ ture, and

(b)said aluminum material has a specific surface area of from about 0.2 to about 1.1 square meters per gram, and the parti¬ cles of said material range in size from about 4 to about 28 microns;

2. said aluminum nitride is comprised of less than about 2.0 percent of combined oxygen and less than about 0.1 percent of moisture;

3. said silicon nitride is comprised of at least about 35 weight percent of nitrogen, less than about 2.0 weight percent of oxygen, and less than about 1.0 weight percent of free silicon,

4. substantially all of the particles of said silicon nitride are smaller than about 10 mi¬ crons, 5. said silica is comprised of less than about 0.1 percent of moisture, and substan¬ tially all of the particles of said silica are smaller than about 53 microns;

(b)thereafter comminuting said mixture while con¬ tacting it with nitrogen until said mixture, as revealed by scanning electron microscopy, comprises substantially a single phase material; and (c)thereafter heating said single-phase material to a temperature of at least about 760 degrees centi¬ grade until ignition occurs.

13. A process for preparing the precursor material recit¬ ed in claim 2, comprising the steps of:

(a)providing an oxygen-deficient silica which con¬ tains from about 44 to about 47 weight percent of oxygen and from about 56 to about 53 weight percent of silicon;

(b)comminuting a mixture comprised of from about 30 to about 45.5 weight percent of said oxygen-defi¬ cient silica, from about 23 to about 34 weight percent of aluminum material, from about 0.5 to about 8 weight percent of aluminum nitride, from about 2 to about 25 weight percent of silicon ni¬ tride, and from 0 to about 5 weight percent of a fluoride compound while blanketing said mixture with a nitrogen-containing gas until a substantially single-phase composition is produced, wherein said aluminum material, said aluminum nitride, said silicon nitride, and said silica are as recited in claim 13;

(c)thereafter pressing said mixture into pellets, each of which have a porosity of at least about 40 percent and a size of at least about 1.0 inches; and (d) )thereafter subjecting at least one of said pel¬ lets to a temperature of at least about 760 degrees centigrade, thereby igniting and combusting said pellet.

14. A composite material comprised of from about 80 to about 94 weight percent of the material recited in claim 1, and from about 20 to about 6 weight percent of whisker material selected from the group consisting of titanium carbide whiskers, silicon carbide whiskers, silicon nitride whisk¬ ers, aluminum nitride whiskers, and mixtures thereof.

15. A process for providing a composite material whose particles have a specific surface area of at least 5.0 square meters per gram, comprising the steps of:

(a)providing a composite material comprised of the material described in claim 1,

(b)mixing said composite material with aqueous media to produce an aqueous slurry, and

(c)wet milling said aqueous slurry until at least about 90 weight percent of its particles are smaller than 1.0 micron and at least 50 weight percent of its particles are smaller than 0.5 microns.

16. A sintering aid which consists essentially of the mate¬ rial recited in claim 9, wherein at least about 95 weight percent of the particles of such material are smaller than 5.0 microns, and said material contains less than about 2.0 weight percent of moisture.

17. A sinterable ceramic composition comprised of from about 1 to about 25 weight percent of the sintering aid of claim 18 and from about 75 to about 99 weight percent of a ceramic material selected from the group consisting of cordierite, mullite, alumina, zirconia, magnesia, silicon carbide, titanium carbide, tungsten carbide, silicon ni¬ tride, titanium nitride, tantalum nitride, aluminum nitride, and mixtures thereof.