Patent Publication Number: US-5021372-A

Title: Method of preparing a self-reinforced silicon nitride ceramic of high fracture toughness

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation-in-Part of copending application Ser. No. 07/297,627, filed Jan. 13, 1989 now abandoned which is a Continuation-in-Part of copending application Ser. No. 07/148,748, filed Jan. 27, 1988 now U.S. Pat. No. 4,883,776. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention pertains to a silicon nitride (Si 3  N 4 ) ceramic body and a process for preparing the ceramic body. 
     Silicon nitride ceramics are recognized for their excellent mechanical and physical properties, including good wear resistance, low coefficient of thermal expansion, good thermal shock resistance, high creep resistance and high electrical resistivity. In addition, silicon nitride ceramics are resistant to chemical attack, particularly to oxidation. Because of these attributes, silicon nitride is useful in a variety of wear and high temperature applications, such as cutting tools and parts in pumps and engines. 
     Failure of silicon nitride ceramics is generally associated with brittleness and flaws. The object therefore is to prepare a silicon nitride ceramic with high fracture toughness (KIC) and strength. Fracture strength is directly proportional to the fracture toughness and inversely proportional to the square root of the flaw size. High fracture toughness combined with small flaw size is therefore highly desirable. Monolithic silicon nitride, however, has a relatively low fracture toughness of about 5 MPa (m) 1/2 . 
     U.S. Pat. No. 4,543,345 teaches that the addition of silicon carbide whiskers to ceramic materials can result in an increase in the fracture toughness. Silicon carbide whiskers described in this patent have a single crystal structure and are in a size range of about 0.6 μm in diameter and about 10 μm to about 80 μm in length. This technique, however, does not provide significant toughening in the case of silicon nitride ceramics. Moreover, the use of silicon carbide whiskers is associated with serious processing problems. The whiskers have a tendency to agglomerate and settle. It is difficult to deagglomerate the whiskers without significantly destroying the whiskers&#39; length. In addition, the whiskers are difficult to manufacture; thus, they display inconsistent properties and are costly. It would be highly desirable to have a silicon nitride ceramic of high fracture toughness which does not require the presence of silicon carbide whiskers. 
     U.S. Pat. No. 4,669,890 discloses a highly dense silicon nitride sintered body containing predetermined amounts of yttria (Y 2  O 3 ), magnesia (MgO), and ceria (CeO) and a method for preparing such a body. The amounts are, based upon weight of the sintered body, 2-15% by weight of Y as Y 2  O 3 , 1-10% by weight of Ce as CeO, 1-10% by weight of Mg as MgO and 75-95% by weight of Si 3  N 4 . The bodies have, as shown in Table I, a demonstrated room temperature mechanical strength (four point bending test) of 670-750 MPa. It would be very desirable to have a silicon nitride ceramic with a fracture strength (room temperature) of 825 MPa or higher and a fracture toughness greater than about 6 MPa (m) 1/2 . 
     U.S. Pat. No. 4,025,351 and related U.S. Pat. No. 4,004,937 disclose a silicon nitride-based ceramic formed from a powder mixture containing 60-92 mole percent silicon nitride with the balance being metal oxide. The metal oxide component consists of at least one of MgO, ZnO and NiO and at least one of Al 2  O 3 , Cr 2  O 3 , Y 2  O 3 , TiO 2  and SnO 2 . The mole ratio of the first group to the second group ranges from 1:9 to 9:1. In one embodiment, the mixed metals are heated to between 1600 and 1800° C. for 2 to 3 hours. In a second embodiment, the mixed metals are first converted to a spinel which is then ground, mixed with the silicon nitride and sintered. A small quantity of CaO or CoO may also be included in the silicon nitride-based ceramic. 
     U.S. Pat. No. 4,406,668 discloses a coated ceramic cutting tool comprising a densified silicon nitride substrate body having at least one hard, adherent coating layer consisting essentially of a refractory metal nitride. The substrate body consists essentially of a first phase of silicon nitride and a second refractory phase comprising silicon nitride and an effective amount of a single densification additive. The densification additive is selected from the group consisting of silicon dioxide, aluminum oxide, magnesium oxide, yttrium oxide, zirconium oxide, hafnium oxide, the lanthanide rare earth oxides, and mixtures thereof. The coating is a nitride of titanium, vanadium, chromium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten or mixtures thereof. 
     It is known that the high temperature strength of hot-pressed silicon nitride ceramics can be increased by crystallization of the grain-boundary glass phase (second phase). This has been demonstrated in a hot-pressed composite containing beta(β)-silicon nitride and a crystalline second phase of Si 3  N 4  ·Y 2  O 3 , as reported by Akihiko Tsuge et al. in the Journal of the American Ceramics Society, 58, 323-326 (1975). However, the fracture toughness of this silicon nitride is only 5-6 MPa (m) 1/2 . 
     It is also known that the presence of single crystal β-silicon nitride whiskers with a high aspect ratio can increase the fracture toughness of silicon nitride ceramics, as reported by F. F. Lange, in the Journal of the American Ceramics Society, 62 (12), 1369-1374, (1983). &#34;Aspect ratio&#34; is defined as the ratio of length to diameter or width. Thus, single crystal whiskers with a high aspect ratio are fibrous in nature. If such whiskers are also strong, crack propagation must take a tortuous path around the whiskers, thereby leading to high fracture toughness. The transformation of alpha(α)-silicon nitride to β-silicon nitride takes place above 1600° C.; however, crystals of the beta phase precipitate usually as a mixture of equiaxed grains and elongated grains with a low aspect ratio. Reproducible control of the aspect ratio is a difficult problem. 
     Typically, the prior art is silent with regard to aspect ratio and fracture toughness of silicon nitride ceramics. U.S. Pat. No. 4,279,657, for example, discloses a powder dispersion containing silicon nitride and magnesium oxide which is hot-pressed to form a light-transmitting silicon nitride ceramic. The ceramic is disclosed to comprise more than 50 weight percent β-silicon nitride ranging in grain size from 1 μm to about 10 μm, but typically less than 5 μm. This patent teaches that impurities, such as calcium, in a total amount greater than about 0.1 weight percent are undesirable. U.S. Pat. No. 4,227,842 discloses a cutting tool consisting essentially of beta phase silicon nitride and yttrium oxide. The tool is prepared by hot-pressing a powder mixture of c-silicon nitride and yttrium oxide to obtain a ceramic of 100 percent theoretical density. U.S. Pat. No. 4,652,276 teaches a cutting tool comprising a granular phase consisting essentially of β-silicon nitride and an intergranular amorphous phase consisting essentially of magnesium oxide from about 0.5 to about 10 weight percent, yttrium oxide from about 2.5 to about 10 weight percent, silicon oxide in an amount-less than about 2.5 weight percent, and the balance less than 5 weight percent impurities such as aluminum. The tool is prepared by hot-pressing. 
     It would be very desirable to have a silicon nitride ceramic of high fracture toughness and high fracture strength. It would be advantageous if such a strong silicon nitride ceramic could be prepared without silicon carbide reinforcing whiskers. Moreover, it would be highly desirable to have a process which would be reproducible, inexpensive, and amenable to industrial scale-up for preparing such a tough and strong silicon nitride ceramic. 
     SUMMARY OF THE INVENTION 
     In a first aspect of this invention is a process for preparing a self-reinforced silicon nitride ceramic body containing predominately β-silicon nitride whiskers having a high average aspect ratio. The process comprises preparing or providing a powder mixture comprising: 
     (a) silicon nitride in an amount sufficient to provide a ceramic body; 
     (b) a densification aid, said densification aid being a non-oxide derivative of magnesium or a source of an element selected from the group consisting of beryllium, calcium, strontium, barium, radium, lithium, sodium, potassium, rubidium, cesium and francium, said source being present in an amount sufficient to promote densification of the powder; 
     (c) a conversion aid, said conversion aid being a non-oxide derivative of yttrium or a source of an element selected from the group consisting of scandium, actinium, lanthanum, lithium, sodium, potassium, rubidium, cesium and francium, said source being present in an amount sufficient to promote the essentially complete conversion of the starting silicon nitride to β-silicon nitride; and 
     (d) at least one whisker growth enhancing compound in an amount sufficient to promote the formation of β-silicon nitride whiskers, said compound being a non-oxide derivative of calcium, or a derivative of an element selected from the group consisting of sodium, potassium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, strontium, zirconium, niobium, barium and lanthanum, and mixtures thereof, or an oxide of an element selected from the group consisting of gallium, indium, hafnium, tantalum and boron, 
     and subjecting the powder mixture to conditions of temperature and pressure sufficient to provide for densification and in situ formation of β-silicon nitride whiskers having a high average aspect ratio occur. In this manner a self-reinforced silicon nitride ceramic body having a fracture toughness greater than about 6 MPa (m) 1/2 , as measured by the Chevron notch technique described hereinbelow, is formed. For the purposes of the present invention a &#34;high&#34; average aspect ratio means an average aspect ratio of at least about 2.5. The powder mixture optionally includes aluminum nitride or boron nitride in an amount of from about 0.01 to about five weight percent, based upon total powder mixture weight. Any means may be used to apply pressure and tempeature so long as sufficient densification and in situ whisker formation occur. Application of pressure and temperature beneficially occurs by hot-pressing or hot isostatic pressing, preferably by hotpressing. 
     In a second aspect, this invention is a silicon nitride ceramic body having a fracture toughness greater than about 6 MPa (m) 1/2 , as measured by the Chevron notch technique described hereinbelow, comprising: 
     (a) a crystalline phase of β-silicon nitride of which at least about 20 volume percent, as measured by viewing one plane of the silicon nitride ceramic body by scanning electron microscopy, is in the form of whiskers having an average aspect ratio of at least about 2.5; and 
     (b) a glassy phase, in an amount not greater than about 35 weight percent of the total weight, comprising a densification aid, e.g., a non-oxide derivative of magnesium, a conversion aid, e.g., a non-oxide derivative of yttrium, silica, and a beta-silicon nitride whisker growth enhancing compound, said compound being a non-oxide derivative of calcium, a derivative of an element selected from the group consisting of sodium, potassium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, strontium, zirconium, niobium, barium and lanthanum, or an oxide of an element selected from the group consisting of gallium, indium, hafnium, tantalum and boron. The glassy phase optionally contains an amount, e.g., up to about 15.0 percent by weight of the glassy phase, of aluminum nitride or boron nitride. The densification aid and the conversion aid are the same as those detailed above in the first aspect. 
     In a third aspect, this invention is a cutting tool comprising the above-identified silicon nitride ceramic body. 
     Unexpectedly, the silicon nitride ceramic body of this invention exhibits a significantly higher fracture toughness than the monolithic or whisker-reinforced silicon nitride ceramics of the prior art. Moreover, if the fracture toughness of the silicon nitride ceramic of this invention is normalized with respect to density, the normalized fracture toughness and fracture strength are among the highest known for any ceramic material. Advantageously, the silicon nitride ceramic of this invention is self-reinforced and does not require the presence of expensive single crystal raw material whisker additives such as silicon carbide whiskers. More advantageously, the process for preparing the novel, self-reinforced silicon nitride ceramic body of this invention is reproducible, amenable to industrial scale-up, and less expensive than processes using silicon carbide whisker reinforcement. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The silicon nitride starting material used in preparing ceramic body of this invention can be any silicon nitride powder, including the crystalline forms of α-silicon nitride and β-silicon nitride, or noncrystalline amorphous silicon nitride, or mixtures thereof. Preferably, the silicon nitride powder is predominately in the alpha crystalline form or the amorphous form, or mixtures thereof. More preferably, the starting silicon nitride is predominately in the alpha crystalline form. It is also advantageous if the preferred starting powder possesses a high α/β weight ratio. Preferably, the starting powder contains no greater than about 20 weight percent β-silicon nitride; more preferably, no greater than about 10 weight percent β-silicon nitride; most preferably, no greater than about 6 weight percent β-silicon nitride. 
     Generally, the higher the purity of the starting silicon nitride powder, the better will be the properties of the finished ceramic body. Depending on the source, however, the silicon nitride powder may contain nonmetallic impurities. Some impurities may be tolerated in the powder, although it is preferred to minimize these as much as possible. Oxygen, for example, is present to some extent in the form of silica, SiO 2 , which usually is found as a coating on the surface of the silicon nitride particles. The amount of silica varies according to the purity of the starting silicon nitride powder and its method of manufacture. The silica content may be reduced by leaching or increased by adding free silica in order to attain a desired total silica content. In addition to oxygen, elemental silicon is usually present in amounts ranging up to about 0.5 weight percent. These amounts of elemental silicon are not deleterious and can be tolerated. Other nonmetals, such as carbon which is likely to form silicon carbide during hot-pressing or sintering, are tolerable in small amounts. 
     The silicon nitride starting powder can be of any size or surface area provided that the ceramic body of this invention is obtained by hot-pressing. Large particles having an average diameter in the range from about 15 μm to about 50 μm, for example, may be in the form of hard agglomerates which cannot be easily broken. Powders containing such agglomerates make poor ceramics. On the other hand, very fine powders having an average diameter less than about 0.2 μm are difficult to obtain uniformly and to process. Preferably, the particles have an average diameter in the range from about 0.2 μm to about 10.0 μm; more preferably, from about 0.5 μm to about 3.0 μm. Preferably, the surface area of the silicon nitride particles is in the range from about 5 m 2  /g to about 15 m 2  /g, as determined by the Brunauer-Emmett-Teller (BET) method of measuring surface area, which is described by C. N. Satterfield in Heterogeneous Catalysis in Practice, McGraw-Hill Book Company, 1980, pp. 102-105. More preferably, the surface area is in the range from about 8 m 2  /g to about 15 m 2  /g 
     Raw silicon nitride powders cannot be densified to high densities in the absence of densification aids. Thus, at least one densification aid is admixed with the silicon nitride starting powder in a manner described hereinbelow for the purpose of promoting densification of the silicon nitride during processing. The densification aids form a liquid phase into which the α-silicon nitride dissolves. The liquid phase forms at a temperature or over a temperature range which varies with the densification aid. The rate of mass transport of the α-silicon nitride is usually quite rapid in the liquid phase; thus, the silicon nitride density increases until a critical mass is reached and precipitation occurs. 
     Copending application, Ser. No. 07/148,748, filed Jan. 27, 1988, teaches the use of magnesium oxide as a densification aid. Copending application, Ser. No. 07/297,627, filed Jan. 13, 1989, expands the scope of densification aids to include non-oxide derivatives of magnesium. Further work demonstrates that sources of beryllium, calcium, strontium, barrium, radium, lithium, sodium, potassium, rubidium, cesium and francium also promote densification of silicon nitride. The source is suitably an oxide, but acceptable results are obtained with non-oxide derivatives such as borides or nitrides. 
     Any amount of a densification aid which promotes densification as described herein and produces the tough silicon nitride ceramic body of the invention is acceptable. The densification aid is beneficially a non-oxide derivative of magnesium, beryllium oxide, calcium oxide, strontium oxide, barium oxide or radium oxide. The densification aid is desirably calcium oxide or strontium oxide and is present in an amount in the range from about 0.04 weight percent to about 27.0 weight percent based on the total weight of the powder mixture. The amount of densification aid is desirably in the range from about 0.5 weight percent to about 9.8 weight percent: and preferably, from about 0.9 weight percent to about 4.7 weight percent. 
     In addition to a densification aid, the powder mixture must contain a conversion aid. The conversion aid forms a glassy phase through which mass transport is, in general, considerably slower than in the densification aid. Thus, α-silicon nitride dissolves in the conversion aid on heating, but is not readily densified. Advantageously, however, the conversion aid promotes the rapid, essentially complete conversion of α-silicon nitride to β-silicon nitride. This conversion is most desirable because the β-silicon nitride in the form of elongated, single crystal whiskers is responsible for the high fracture toughness and high fracture strength of the silicon nitride ceramic body of this invention. All references hereinafter to silicon nitride whiskers, single crystal whiskers and single crystal silicon nitride whiskers are intended to be synonomous and may be used inerchangeably. Any amount of conversion aid can be employed in the starting powder providing the quantity is sufficient to cause the essentially complete conversion of the starting silicon nitride to β-silicon nitride, and is sufficient to produce the tough silicon nitride ceramic body of the invention. Preferably, the amount of conversion aid employed is in the range from about 0.2 weight percent to about 29.5 weight percent based on the total weight of the powder mixture. More preferably, the amount of conversion aid employed is in the range from about 1.0 weight percent to about 10.0 weight percent; most preferably, from about 1.7 weight percent to about 8.5 weight percent. 
     The conversion aid is suitably a non-oxide derivative of yttrium or a source or derivative of an element selected from the group consisting of scandium, actinium, lanthanum, lithium, sodium, potassium, rubidium, cesium and francium. The conversion aid is beneficially scandium oxide, lanthanum oxide, actinium oxide or sodium oxide. The conversion aid preferably is lanthanum oxide or sodium oxide. 
     Surprisingly, the weight ratio of conversion aid to densification aid has been found to affect the fracture toughness of the finished ceramic, providing the whisker growth enhancing compound is also present in the powder mixture. Any weight ratio of conversion aid to densification aid is acceptable providing the fracture toughness shows an improvement over the prior art fracture toughness value of 5 MPa (m) 1/2   for nonreinforced, monolithic silicon nitride. The weight ratio is suitably from about 0.25 to about 8. The weight ratio is beneficially in the range from about 0.5 to about 5: desirably, from about 1 to about 35 and, preferably, from about 1 to about 1.8. In the absence of a whisker growth enhancing compound, the conversion aid/densification aid weight ratio has no significant effect on the fracture toughness. 
     The third component required to be present in the powder mixture is a whisker growth enhancing compound. This compound helps to provide a ceramic body of superior fracture toughness and high strength. Just how the whisker growth enhancing compound contributes to the excellent physical properties which are observed in the silicon nitride ceramic body of this invention is not completely understood. It is possible that the whisker growth enhancing compound improves the viscosity of the glassy phase thereby facilitating the nucleation of elongated whiskers of β-silicon nitride; the latter being primarily responsible for the improved fracture toughness. The aforementioned theory is presented with the understanding that such a theory is not to be binding or limiting of the scope of the invention. Any amount of the compound in the starting powder is acceptable providing the amount is sufficient to promote the formation of β-silicon nitride whiskers, described hereinbelow, and sufficient to produce the tough silicon nitride ceramic body of this invention. Preferably, the amount of the compound employed is in the range from about 0.01 weight percent to about five weight percent based on the total weight of the powder mixture. More preferably, the amount of whisker growth enhancing compound employed is in the range from about 0.1 weight percent to about 1.0 weight percent; most preferably, from about 0.2 weight percent to about 0.5 weight percent. 
     In copending application, Ser. No. 07/148,748, filed Jan. 27, 1988, the presence of calcium, particularly calcium oxide, was found to provide advantages when silicon nitride powder compositions were formed by hot-pressing into finished ceramic bodies. Silicon nitride powders doped with up to about 5.3 weight percent calcium oxide were found to be desirable. It was believed that commercial silicon nitride powders contained only 100 ppm or less of calcium oxide. 
     Copending application, Ser. No. 07/297,627, filed Jan. 13, 1989, included three discoveries. First, non-oxide derivative of magnesium worked as densification aids. Second, non-oxide derivative of yttrium provided satisfactory results when used as conversion aids. Third, elements of The Periodic Table of the Elements other than calcium also promoted growth of β-silicon nitride whiskers and provided hot-pressed silicon nitride ceramic bodies with a fracture toughness greater than about 6 MPa (m) 1/2 , as measured by the Chevron notch technique and a fracture strength in excess of 120 ksi (825 MPa). Satisfactory results were obtained when the element was selected from the group consisting of sodium, potassium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, strontium, zirconium, niobium, barium, lanthanum and mixtures thereof. The elements were beneficially selected from the group consisting of barium, sodium, potassium, titanium, vanadium, strontium, zirconium, niobium, lanthanum, tungsten and mixtures thereof. Desirable results followed when the element was titanium, niobium, strontium or a mixture of two or more of such elements. The foregoing elements, or mixture thereof, were suitably added in the form of a derivative, e.g., an oxide, a boride, a carbide, a carbonitride, a nitride or an oxynitride, rather than in their elemental form. 
     It has now been discovered that additional substitutions may be made for calcium oxide without sacrificing either fracture toughness or fracture strength. Particularly suitable results are obtained with an oxide of an element selected from the group consisting of gallium, indium, hafnium, tantalum and boron. 
     Acceptable results are believed to be attainable with derivatives of lithium, beryllium, magnesium, silicon, germanium, selenium, rubidium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium, tin, antimony, tellurium, cesium, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, tungsten, rhenium, osmium, iridium, platinum, gold, thallium, lead, bismuth, polonium, francium, radium, thorium, protactinium, neptunium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, and lawrencium. Non-oxide derivatives of gallium, indium, hafnium, tantalum and boron may also produce satisfactory results. Skilled artisans recognize that elements 58-71 and 90-103 of the Periodic Table of the Elements, commonly referred to as &#34;the Lanthanides&#34;, are not readily available in their pure form. They are, however, available as mixtures of two or more of such elements. Such mixtures are also believed to be suitable for purposes of the present invention. Skilled artisans also recognize that certain of the elements and their derivatives may be less desirable than the others based upon considerations such as availability and cost. 
     Suitable results are also obtained with non-oxide derivatives of calcium. Illustrative non-oxide derivatives include borides, carbides, carbonitrides, nitrides and oxynitrides. 
     Skilled artisans will recognize that results will vary depending upon both the element(s) of choice and the particular derivative(s) of that element or mixture of elements. The whisker growth enhancing aid is beneficially a source or derivative of niobium or an oxide of an element selected from the group consisting of potassium, sodium, strontium, barium, scandium, lanthanum, titanium, zirconium, vanadium, chromium, tungsten, manganese, iron, cobalt, nickel, copper, zinc, calcium, gallium, indium, hafnium, tantalum and boron. The whisker growth enhancing aid is desirably niobium stannate, niobium galliate, niobium boride, or an oxide of potassium, barium, scandium, niobium, titanium, chromium, tungsten, manganese, cobalt, nickel, zinc, calcium, gallium, indium, hafnium, tantalum. The whisker growth enhancing aid is preferably an oxide of an element selected from the group consisting of gallium, indium, hafnium, tantalum and boron. 
     It is desirable to use whisker growth enhancing compounds and derivatives of magnesium and yttrium in the form of powders which are pure and sufficiently small in size. Purity is not typically a problem, because commercially available whisker growth enhancing compounds and derivatives of magnesium and yttrium, particularly the oxide powders, generally contain less than 20 ppm each of assorted impurities. These levels of impurities are tolerable. Larger amounts of impurities, as for example in the 0.5 weight percent range, are not recommended as they may cause a change in the final ceramic composition and properties. A small powder particle size is favored, because dispersion is enhanced by smaller particles. Preferably, the oxide powders have an average particle size no greater than about 5 μm in diameter. 
     In the process of this invention, it is required to mix the starting silicon nitride powder, described hereinabove, with a combination of a densification aid, a conversion aid, and a whisker growth enhancing compound to obtain a powder mixture, which is used in preparing the tough silicon nitride ceramic body of this invention. Suitable densification aids, conversion aids and whisker growth enhancing components are disclosed herein. Ordinarily, the total quantity of densification aid, conversion aid and the whisker growth enhancing compound is no greater than about 35 weight percent of the total weight of the powder mixture. The total quantity will depend, however, on probable end use applications for fired ceramics prepared from the powder mixture. For some applications, total quantities in excess of 35 weight percent will provide acceptable results. Preferably, however, the total quantity is in the range from about 5 weight percent to about 35 weight percent for medium temperature and/or the highest fracture toughness applications. By &#34;medium temperature&#34;, it is meant temperatures in the range from about 900° C. to about 1200° C. Ceramic cutting tools are an example of a medium temperature and very high fracture toughness application. Preferably, the total quantity is in the range from about 0.25 weight percent to about 5 weight percent for high temperature and/or moderately high fracture toughness applications. By &#34;high temperature&#34;, it is meant temperatures from about 1200° C. to about 1400° C. Parts for ceramic engines are an example of a high temperature and moderately high fracture toughness application. 
     The preparation of the powder mixture containing silicon nitride, densification aid(s), conversion aid(s) and whisker growth enhancing compound(s), is accomplished in any suitable manner. Ball-milling of the components in powder form is one acceptable manner of preparation. 
     The preferred method of preparing the powder mixture comprises use of an attritor with zirconia balls to prepare a finely-divided suspension of silicon nitride and a powdered combination of the densification aid(s), the conversion aid(s) and the whisker growth enhancing compound(s) in a carrier medium, drying an admixture of the suspension and the attritor balls, beneficially after removing, by filtration or otherwise, excess carrier medium, and thereafter separating the attritor balls to obtain the powder mixture. 
     The preparation of the finely-divided suspension of silicon nitride and the combination of the densification aid(s), the conversion aid(s) and the whisker growth enhancing compound(s) in a carrier medium requires no particular order of addition of the components. For example, it is possible to add the powdered combination or powdered components thereof to a colloidal suspension of silicon nitride in a carrier medium or vice versa. Alternatively, all components of the powder mixture may be added simultaneously to the carrier medium prior to attritor milling. The latter method is preferred, particularly when an organic carrier medium such as toluene or an alcohol is employed. 
     The carrier medium may be any inorganic or organic compound which is a liquid at room temperature and atmospheric pressure. Examples of suitable carrier media include water; alcohols, such as methanol, ethanol and isopropanol; ketones, such as acetone and methyl ethyl ketone; aliphatic hydrocarbons, such as pentanes and hexanes and aromatic hydrocarbons, such as benzene and toluene. The carrier medium is desirably an organic liquid, preferably toluene or an alcohol such as methanol. The function of the carrier medium is to impart a viscosity suitable for mixing to the solid powders. Any quantity of carrier medium which achieves this purpose is sufficient and acceptable. Preferably, a quantity of carrier medium is employed such that the solids content is in the range from about 20 weight percent to about 50 weight percent. More preferably, a quantity of carrier medium is employed such that the solids content is in the range from about 35 weight percent to about 45 weight percent. Below the preferred lower limit the viscosity of the solid suspension may be too low and the deagglomeration mixing may be ineffective. Above the preferred upper limit the viscosity may be too high, and the deagglomeration mixing may be difficult. 
     The components of the powdered combination are added to the carrier medium in any manner which gives rise to a finely dispersed suspension of the components. Typically, the process is conducted in a large vessel at room temperature (taken as 23° C.) under air with vigorous stirring. Any common stirring means is suitable, such as a ball-milling device or an attrition mixer. An ultrasonic vibrator may be used in a supplementary manner to break down smaller agglomerates. The attrition mixer is preferred. 
     To aid in the dispersion of components of the powder mixture, optionally one or more surfactants or dispersants can be added to the suspension. The choice of surfactant(s) or dispersant(s) can vary widely as is well-known in the art. 
     If the carrier medium is toluene, a coupling agent, such as an aluminate coupling agent commercially available from Kenrich Petrochemicals under the trade designation KEN-REACT KA 322, may be used to aid in forming a suspension. When using an alcohol such as methanol, a dispersant such as a polyethyleneimine may be used to facilitate mixing and a flocculant such as oleic acid may be used to ease recovery of the powder mixture. 
     Any amount of surfactant or dispersant is acceptable providing dispersion of powder mixture components is improved. Typically, the amount of surfactant is in the range from about 0.01 to 1.0 weight percent of the powder mixture. 
     The finely-divided suspension is now ready for processing into greenware. For example, the suspension can be slip-cast by techniques well-known in the art for eventual sintering. Alternatively, the suspension can be dried into a powder and ground for use in hot-pressing processes. Drying is accomplished by standard drying means, such as by spray-drying or oven drying under a nitrogen purge. Preferably, drying of the admixture of the powder mixture and the attritor balls is accomplished in an oven under a nitrogen purge after removal of excess carrier medium. During the drying process, additional free carrier medium is removed. The temperature of the drying depends on the boiling point of the carrier medium employed. Typically, the drying process is conducted at a temperature just below the boiling point of the carrier medium under atmospheric pressure. Preferably, the carrier medium is toluene or an alcohol and the temperature of drying is about 50° C. After drying, the resulting powder is separated from the attritor balls and sieved through a screen to obtain a powder having a maximum agglomerate diameter of about 100 μm. The screen size is usually less than about 60 mesh; more preferably, less than about 80 mesh. The powder which is obtained on sieving is the powder mixture which is used in the hot-pressing process of this invention. 
     The preferred method of processing the powder mixture is by hot-pressing, which comprises heating the powder under pressure to obtain the densified ceramic body. Any standard hot-pressing equipment is acceptable, such as a graphite die equipped with a heating means and a hydraulic press. The hot-pressing is conducted under an inert atmosphere, such as nitrogen, to prevent the oxidation and decomposition of silicon nitride at high temperatures. The direction of pressing is uniaxial and perpendicular to the plane of the plates. 
     Any processing temperature and pressure will suffice providing the novel silicon nitride ceramic of this invention, described hereinbelow, is obtained. Typically, however, the temperature must be carefully controlled, because the elongated β-silicon nitride whiskers are found to form in a narrow temperature range. Preferably, the temperature is maintained during pressurizing in the range from about 1750° C. to about 1870° C. More preferably, the temperature is maintained in the range from about 1800° C. to about 1850° C. Most preferably, the temperature is maintained in the range from about 1820° C. to about 1840° C. Below the preferred lower temperature limit the formation of elongated β-silicon nitride whiskers may be retarded. Above the preferred upper temperature limit the silicon nitride may decompose, and special pressure equipment may be required to conduct the densification. Skilled artisans will recognize, however, that the use of high pressure techniques such as hot isostatic pressing may allow use of higher temperatures, e.g. up to 2000° C. or even 2100° C. It is noted that the accurate measurement of high temperatures, such as those quoted hereinabove, is technically difficult. Some variation in the preferred temperature range may be observed depending on the method employed in measuring the temperature. The preferred temperatures of this invention are measured by use of a tungsten-rhenium thermocouple, obtained from and calibrated by the Omega Company. 
     While the pressure during hot-pressing is important, it is not quite as critical a parameter as temperature. Typically, the pressure should be sufficient to cause densification of the green body. Preferably, the pressure is in the range from about 3000  psig to about 6000 psig; more preferably, from about 4000 psig to about 5500 psig; most preferably, about 4500 psig to about 5200 psig. Below the preferred lower pressure limit the powder will not be sufficiently densified. Above the preferred upper pressure limit the powder will densify in a shorter time and at a lower temperature. Although less rigorous processing conditions seem on the surface to be desirable, the formation of elongated p-silicon nitride crystals may be inhibited at lower temperatures and shorter pressing times. 
     The amount of time that the powder mixture is heated under pressure should be sufficient to bring the powder to essentially complete densification. Generally, ram movement is a good indicator of the extent of densification. As long as the ram continues to move, the densification is incomplete. When the ram has stopped moving for at least about 15 minutes, the densification is essentially complete at about 99 percent or greater of the theoretical value. Thus, the time required for hot-pressing is the time during ram movement plus about an additional 15 to 30 minutes. Preferably, the time is in the range from about 15 minutes to about 2 hours; more preferably, from about 30 minutes to about 90 minutes; most preferably, about 45 minutes to about 75 minutes. 
     The hot-pressing method of densification, described hereinbefore, allows for the formation of silicon nitride ceramic articles which can be used as cutting tools. A variety of shapes can be made by hot-pressing, one common shape being a flat plate. These plates may range in size from about 2 inches in length by about 1.5 inches in width by about 0.45 inch in thickness to about 16 inches in length by about 16 inches in width by about 1.0 inch in thickness. Smaller and larger plates can also be fabricated, as determined by the size of the hot-pressing plaques. Cutting tools can be fabricated by slicing and grinding these plates into a variety of cutting tool shapes. 
     The silicon nitride ceramic body which is produced by the hot-pressing process of this invention is a dense material having no significant porosity. Preferably, densification proceeds to greater than 95 percent of the theoretical value; more preferably, to greater than 97 percent of the theoretical value; most preferably, to greater than 99 percent of the theoretical value. Moreover, as measured by X-ray diffraction, the silicon nitride is present in the beta crystalline form, indicating essentially complete alpha to beta conversion during processing. Quite unexpectedly, the β-silicon nitride is present predominately as single crystal, &#34;needle-like&#34; whiskers, as determined by both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The size of the hexagonal β-silicon nitride grains is usually in the range from about 3 μm to about 20 μm in length with a mean diameter of from about 0.5 μm to about 1.5 μm; preferably from about 10 μm to about 20 μm in length with a mean diameter from about 0.5 μm to about 1.0  μm. 
     Since the whiskers are oriented randomly, it is difficult to determine exactly the percentage of silicon nitride which exists as whiskers, as opposed to equiaxed particles. The measurement is made by viewing one plane of the silicon nitride ceramic in a scanning electron microscope (SEM) and measuring the percentage by volume occupied by whiskers having an aspect ratio between 2 and 16. It is observed that the whiskers are homogeneously distributed and randomly oriented throughout the ceramic body, and that the volume occupied by the whiskers is approximately the same in all planes. Typically, the percentage of silicon nitride whiskers having an aspect ratio of between 2 and 16 is at least about 20 volume percent as measured in a plane. Preferably, the percentage of silicon nitride whiskers having an aspect ratio between 2 and 16 is at least about 35 volume percent as measured in a plane. Unexpectedly, the whiskers are found to have a high average aspect ratio. Typically, the average aspect ratio of the silicon nitride whiskers is at least about 2.5; preferably, at least about 5.5. It is noted that because the aspect ratio is measured in a plane, the average aspect ratio is a lower bound. For example, a whisker which is perpendicular to the plane may have an apparent aspect ratio of less than 2; whereas the true aspect ratio may be very much greater than 2. 
     In addition to the β-silicon nitride phase, the ceramic body of this invention contains a glassy second phase, which constitutes no greater than about 35 weight percent of the total weight. The glassy second phase has a bulk chemical composition consisting essentially of from about 8 weight percent to about 60 weight percent of a densification aid, from about 15 weight percent to about 64 weight percent of a conversion aid, from about 7 weight percent to about 77 weight percent silica, and from about 0.1 weight percent to about 25 weight percent of at least one whisker growth enhancing compound, as determined by neutron activation analysis; and wherein the conversion aid to densification aid weight ratio is in the range from about 0.25 to about 8. 
     Small quantities of silicon carbide and two unknown phases may be present in a total amount not exceeding about 10 weight percent. One of the unknown phases, phase &#34;X&#34;, possesses a fiber-like, layered and ordered structure. The typical size of the particles of this phase is about 500 Å in width by about 0.7 μm to 1.0 μm in length. This phase, being distributed throughout the glassy phase, connects and bridges Si 3  N 4  whiskers. In other words, unique microstructures can be created with small needles or fibers of phase X situated between comparatively large Si 3  N 4  whiskers or particles. 
     The mechanical properties of the self-reinforced silicon nitride ceramic body are readily measured by use of standard tests. In particular, fracture toughness (K IC ) is measured according to the Chevron notch and the Palmqvist methods described hereinafter. Fracture strength (modulus of rupture) is measured according to the Military Standard 1942b test. Hardness is measured according to the Vickers indentation test. 
     Fracture strength (modulus of rupture) measures the resistance of the material to fracture under a steady load. Fracture strength is defined as the maximum unit stress which the material will develop before fracture occurs. Test bars are prepared by cutting rectangular bars (45 mm×4 mm×3 mm) of the silicon nitride ceramic in a plane perpendicular to the pressing direction. The bars are ground on the surfaces parallel to the pressing plates using a 500 grit grinding wheel (Military Standard 1974). The fracture strength is measured at room temperature using a 4-point bend test with 20 mm span and crosshead speed of 0.5 mm/min. Typically, the fracture strength at room temperature is at least about 650 MPa. Preferably, the fracture strength at room temperature ranges from about 825 MPa to about 1250 MPa; more preferably, from about 1000 MPa to about 1150 MPa. High temperature strength is measured using a 3-point bend test with 20 mm span and crosshead speed of 0.5 mm/min. Typically, at 1000° C. the fracture strength is at least about 550 MPa. Typically, at 1300° C. the fracture strength is at least about 300 MPa. 
     Toughness measures the resistance of the material to fracture under a dynamic load. More specifically, fracture toughness is defined as the maximum amount of energy which a unit volume of material will absorb without fracture. In the present invention two methods are employed to measure fracture toughness. The first of these is the Chevron notch test. Test bars are prepared as described hereinabove, and additionally scored with a Chevron notch. The test bars are then subjected to a 3-point bend test with 40 mm span and crosshead speed of 0.5 mm/min. Typically, the fracture toughness of the silicon nitride ceramic body of this invention, as measured at room temperature (taken as 23° C.) by the Chevron notch technique, is greater than about 6 MPa (m) 1/2 . Preferably, the room temperature fracture toughness is greater than about 7 MPa (m) 1/2  : more preferably, greater than about 8 MPa (m) 1/2 . Most preferably, the room temperature fracture toughness ranges from about 9 MPa (m) 1/2   to about 14 MPa (m) 1/2 . Preferably, at 1000° C. the fracture toughness is greater than about 6 MPa (m) 1/2 . More preferably, at 1000° C. the fracture toughness ranges from about 7 MPa (m) 1/2   to about 12 MPa (m) 1/2 . 
     In the evaluation of cutting tool materials it is useful to measure the Palmqvist toughness and the Vickers hardness. Both measurements can be made simultaneously on one test sample, and therefore these tests are very convenient. 
     The Vickers hardness test measures the resistance of the ceramic material to indentation. A sample, approximately 1 cm in length by 1 cm in width by 1 cm in height, is placed on a flat surface, and indented with a standard Vickers diamond indentor at a crosshead speed of 0.02 in/min. The Vickers hardness number is calculated from the applied load, in this case 14 kg, and the cross-sectional area of the indentation. Prior to making the test, the test sample is polished in a special manner. First, the sample is cleaned and rough spots are flattened by use of a 220-grid diamond wheel. Next, a 45-micron diamond wheel is used to start the polishing. Next, the sample is treated to a series of polishings at 30 psi and 200 rpm in the following consecutive manner: three five-minute intervals with 30 -micron diamond paste, three five-minute intervals with 15-micron diamond paste, three five-minute intervals with 6-micron diamond paste, two five-minute intervals with 1-micron diamond paste, and one five-minute interval with 0.25-micron diamond paste. Between each interval the sample is thoroughly cleansed by washing with water and sonicating for two minutes. The Vickers hardness number of the silicon nitride ceramic of this invention is at least about 1325 kg/mm 2  at room temperature. Preferably, the Vickers hardness number ranges from about 1340 kg/mm 2  to about 1600 kg/mm 2  at room temperature; more preferably, from about 1450 kg/mm 2  to about 1600 kg/mm 2 . 
     The Palmqvist toughness test is an extension of the Vickers test. (See S. Palmqvist in Jerndontorets Annalen, 141 (1957), 300, for a description of the Palmqvist toughness test.) The test sample is prepared and indented as in the Vickers test, but the 14-kg load is additionally held for 15 seconds. The sample cracks. The measurements of the indented diagonals and the crack lengths are made on a Nikon UM2 microscope at 1000×magnification. The Palmqvist toughness (W) is directly proportional to the applied load (P) and inversely proportional to the crack length (L). Preferably, the silicon nitride ceramic body of this invention exhibits a Palmqvist toughness at room temperature of at least about 37 kg/mm. Preferably, the silicon nitride ceramic body of this invention exhibits a Palmqvist toughness at room temperature in the range from about 37 kg/mm to about 52 kg/mm: more preferably, from about 45 kg/mm to about 52 kg/mm. 
     Illustrative Embodiments 
     The following examples serve to illustrate the novel self-reinforced silicon nitride composition of this invention, the method of preparing the novel silicon nitride ceramic, and the utility of the composition as a cutting tool. The examples are not intended to be limiting of the scope of this invention. All percentages are weight percent unless otherwise noted. 
    
    
     EXAMPLE 1 
     Materials: Silicon nitride (KemaNord P95-H) is employed containing 1.81 percent oxygen, 0.6 percent carbon, and the following major metallic impurities 641 ppm iron, 315 ppm Al, and 25 ppm Ti. The silicon nitride is present in the alpha and beta crystalline forms in an α/β weight ratio of 95/5. The BET surface area of the silicon nitride powder is 10.15 m 2  /g and the average particle size is about 1 μm in diameter. Magnesium oxide (J. T. Baker) is employed containing less than 5 ppm each of boron, zinc, and iron. Greater than 80 percent of the MgO particles range in size from 0 2 μm to 0.7 μm in diameter. Yttrium oxide (Molycorp) is employed containing less than 10 ppm each of sodium and iron. The Y 2  O 3  particles range in size from 2 μm to 5 μm in diameter. Calcium oxide (Aldrich Chemical Co.) is employed containing less than 0.002 percent each of lead and iron. The average CaO particle size is about 3 μm in diameter. 
     The above-identified magnesium oxide (4.7 g) and calcium oxide powders (0.2 g) are suspended in 80 ml of water, and agitated at room temperature under air by means of a mechanical stirrer to form a two-oxide suspension. The pH of the suspension is adjusted to 11.35 by the addition of aqueous sodium silicate (7 drops). After adjustment of the pH, the suspension is ultrasonicated for 30 seconds to break down fine agglomerates. After sonication the pH is observed to drop. The pH is readjusted to 11.5 by adding 5 drops of 5 M sodium hydroxide. The suspension is mixed for about 30 minutes. Yttrium oxide powder (8.5 g), described hereinabove, is added to the suspension, and the suspension is sonicated for 30 seconds and mixed with a mechanical stirrer for 30 minutes. Silicon nitride powder (86.6 g), described hereinabove, is added to the suspension, and the suspension is mixed in an attrition mixer for about 30 minutes to ensure complete dispersion of all components. The resulting suspension is poured through a 100 mesh nylon sieve. The pH is adjusted to 9.8 by adding 10 ml of 50 percent nitric acid to increase the flocculation slightly. The finely divided suspension is dried in an oven at 90° C. for a period of 12 hours under a flow of dry nitrogen gas. After drying, the resulting powder mixture is passed through a 60 mesh sieve. The powder mixture is composed of 86.6 percent silicon nitride, 4.5 percent magnesium oxide, 8.5 percent yttrium oxide, and 0.2 percent calcium oxide. 
     The powder mixture (80 g), described hereinabove, is poured into a graphite die in the shape of plates measuring 2 inches in length by 1.5 inches in width by 0.5 inches in depth. A pressure of 1000 psig is applied to the die, while the temperature is raised from ambient to about 1200° C. in about 30 minutes. At about 1200° C. the pressure is gradually increased to 5000 psig and maintained thereat. The temperature is then increased to 1825° C. over a 40-minute period. The die is maintained at 1825° C. and a pressure of 5000 psig for 45 minutes. Afterwards the die is cooled over a 2 hour period to 100° C. At 1500° C. the pressure is slowly released. When the die reaches room temperature, it is opened, and a silicon nitride ceramic body in the shape of a plate having the above-identified dimensions is retrieved. 
     The density of the silicon nitride ceramic body, prepared hereinabove, is measured by the water immersion method, as described in &#34;Modern Ceramic Engineering&#34; by D. W. Richerson, Marcel Dekker, 1982, and by stereology analysis from SEM photomicrographs. The density is essentially 100 percent of theoretical, and therefore the material is essentially nonporous. Silicon nitride is present essentially in the β crystalline phase, as determined by X-ray diffraction. The bulk chemical composition of the ceramic is determined by neutron activation analysis, and is found to contain 77.2 percent silicon nitride, 20.4 percent glassy second phase, and 2.4 percent silicon carbide. The glassy second phase is found to consist of 32.4 percent magnesium oxide, 42.2 percent yttrium oxide, 2.5 percent calcium oxide, and 23.0 percent silicon oxide. Two unidentified phases are found. The first is present in a quantity of 3.9 percent, and possesses a composition of 9 percent magnesium, 59 percent silicon and 32 percent nitrogen. The second is present in a quantity of 1 percent, and possesses a fiber-like, layered and ordered structure typically 500 Å in width and 0.7 μm in length. The microstructure of the silicon nitride ceramic, prepared hereinabove, is determined by scanning electron microscopy (SEM), as viewed in a plane. About 35 volume percent of the silicon nitride appears in the form of elongated whiskers having an aspect ratio ranging from 2 to 16. The average aspect ratio is 5.6. 
     The fracture strength of the above-identified silicon nitride ceramic body, measured by the 4-point bend test described hereinbefore, is 130 ksi (890 MPa) at room temperature and 90 ksi (616 MPa) at 1000° C. The fracture toughness measured by the Chevron notch technique is 13.9 MPa (m) 1/2   at room temperature and 11.5 MPa (m) 1/2   at 1000° C. The Vickers hardness measured at room temperature and under a 14-kg load ranges from 1350 kg/mm 2  to 1400 kg/mm 2  and averages 1375 kg/mm 2 . The Palmqvist toughness measured at room temperature ranges from 49.3 kg/mm to 51.1 kg/mm. It is seen that the fracture toughness of this silicon nitride ceramic body is very high. 
     EXAMPLES 2 (a-n) 
     A series of hot-pressed silicon nitride ceramic compositions is prepared according to the procedure of Example 1, except that the composition of the powder mixture is varied as described in Table I. The Vickers hardness and the Palmqvist toughness measured at room temperature are presented in Table I for each composition. 
     
                                           TABLE I*                                
__________________________________________________________________________
                             Palmqvist                                    
                                   Vickers                                
                             Toughness                                    
                                   Hardness                               
Ex. 2                                                                     
   % Si.sub.3 N.sub.4                                                     
        % MgO                                                             
             % Y.sub.2 O.sub.3                                            
                  % CaO                                                   
                       Y.sub.2 O.sub.3 /MgO                               
                             (kg/mm)                                      
                                   (kg/mm.sup.2)                          
__________________________________________________________________________
a  86.10                                                                  
        2.65 11.20                                                        
                  0.05 4.23  36.9  1461                                   
b  86.45                                                                  
        3.50 10.00                                                        
                  0.05 2.85  40.2  1403                                   
c  86.65                                                                  
        4.30 9.00 0.05 2.10  39.6  1353                                   
d  86.75                                                                  
        4.70 8.50 0.05 1.81  42.2  1442                                   
e  87.05                                                                  
        5.40 7.50 0.05 1.39  40.6  1392                                   
f  87.25                                                                  
        6.10 6.60 0.05 1.10  42.4  1379                                   
g  87.65                                                                  
        7.20 5.10 0.05 0.70  38.9  1324                                   
h  86.40                                                                  
        3.50 10.00                                                        
                  0.10 2.85  44.5  1392                                   
i  86.70                                                                  
        4.70 8.50 0.10 1.81  47.5  1401                                   
j  87.60                                                                  
        7.20 5.10 0.10 0.70  40.2  1385                                   
k  86.60                                                                  
        4.70 8.50 0.20 1.81  51.0  1391                                   
l  86.60                                                                  
        4.70 8.20 0.50 1.82  47.6  1380                                   
m  86.30                                                                  
        4.50 8.20 1.00 1.82  44.2  1375                                   
n  85.30                                                                  
        4.50 8.20 2.00 1.82  40.1  1370                                   
__________________________________________________________________________
 *Percentages of components are based on weight percent in the powder     
 mixture. Toughness and hardness values are measured at room temperature. 
 
    
     The data show that the Palmqvist toughness and the Vickers hardness vary as a function of the calcium oxide concentration and the Y 2  O 3  /MgO weight ratio in the powder mixture. For example, it is seen in Examples 2(d,i,k,l,m,n) that as the calcium oxide concentration increases at constant Y 2  O 3  /MgO ratio, the fracture toughness passes through a maximum value of 51 kg/mm at a calcium oxide concentration of 0.20 weight percent. As the calcium oxide concentration increases at constant Y 2  O 3  /MgO ratio, the hardness decreases. 
     Comparative Experiments 1(a-d) 
     Four hot-pressed silicon nitride ceramic bodies are prepared as in Example 1, except that calcium oxide is omitted from the preparation. The powder compositions are listed in Table II. The Vickers hardness and the Palmqvist toughness are measured as described in Example 2, and the values are tabulated in Table II. 
     
                                           TABLE II*                               
__________________________________________________________________________
                         Palmqvist                                        
Comp.                    Toughness                                        
                               Hardness                                   
Ex. 1                                                                     
    % Si.sub.3 N.sub.4                                                    
         % MgO                                                            
              % Y.sub.2 O.sub.3                                           
                   Y.sub.2 O.sub.3 /MgO                                   
                         (kg/mm)                                          
                               (kg/mm.sup.2)                              
__________________________________________________________________________
a   86.40                                                                 
         3.60 10.00                                                       
                   2.78  36.6  1407                                       
b   86.80                                                                 
         4.70 8.50 1.81  36.1  1377                                       
c   87.75                                                                 
         7.20 5.05 0.70  36.1  1438                                       
d   93.20                                                                 
         2.42 4.38 1.81  35.8  1395                                       
__________________________________________________________________________
 *Percentages of components are based on weight percent in the powder     
 mixture. Toughness and hardness values are measured at room temperature. 
 
    
     When Comparative Experiments 1(b,d) are compared with Examples 2(d,i,k,l,m,n) it is seen that the self-reinforced silicon nitride ceramic body of this invention possesses a significantly higher Palmqvist toughness than the samples which do not contain calcium oxide. The same conclusion holds on comparing Comparative Experiment 1(a) with Examples 2(b and h). Even at a low Y 2  O 3  /MgO ratio and a low calcium oxide concentration, the improvement in the ceramic body of this invention is noticeable, as seen in the comparison between Comparative Experiment 1(c) and Examples 2(g and j). 
     EXAMPLES 3(a-c) 
     Three hot-pressed silicon nitride compositions are prepared according to the procedure of Example 1, except that the powder compositions are varied as described in Table III. The Y 2  O 3  /MgO ratio in these powder compositions is 1.82. The Palmqvist toughness and the Vickers hardness are measured at room temperature according to the procedure in Example 2. The values obtained are presented in Table III. 
     
                                           TABLE III*                              
__________________________________________________________________________
                            Palmqvist                                     
                       % Glass                                            
                            Toughness                                     
                                  Hardness                                
Ex. 3                                                                     
   % Si.sub.3 N.sub.4                                                     
        % MgO                                                             
             % Y.sub.2 O.sub.3                                            
                  % CaO                                                   
                       Content                                            
                            (kg/mm)                                       
                                  (kg/mm.sup.2)                           
__________________________________________________________________________
a  93.15                                                                  
        2.40 4.35 0.10 6.85 39.9  1371                                    
b  86.70                                                                  
        4.70 8.50 0.10 13.30                                              
                            46.2  1396                                    
c  80.40                                                                  
        6.90 12.60                                                        
                  0.10 19.60                                              
                            48.3  1389                                    
__________________________________________________________________________
 *Percentages of components are based on weight percent in the powder.    
 Toughness and hardness values are measured at room temperature.          
 
    
     The data show that as the glass content increases, the Palmqvist toughness also increases; whereas the Vickers hardness varies in a non-linear fashion. 
     EXAMPLE 4 
     The hot-pressed silicon nitride ceramic body of Example 1 is diamond ground into a cutting tool insert. The cutting tool insert is made according to the ANSI standards in the SNG 433 style. The cutting edge is chamfered at a 20° angle by 0.008-inch width. The insert is tested in a face milling application using a 40 HP Cincinnati #5 single spindle, knee and saddle, vertical milling machine with a 5 HP variable speed table. The work material is a nodular &#34;ductile&#34; cast iron measuring 2 inches in diameter and having a measured hardness of 207 BHN. A milling cutter having a 6-inch diameter is used with a -5° axial rake and a -5° radial rake. A 15° lead angle is employed. The machine is run at a cutting speed of 1360 surface feet per minute, a 0.060-inch depth of cut, and a feed rate of 0.005 inch per revolution (or tooth). The center line of the cutter and the center line of the workpiece are coincident. No cutting fluid is used. Successive passes are taken on the cast iron, and the cutting edge is examined for flank wear and chippage after every 8 passes. Testing is terminated when the flank wear or chippage exceeds 0.015 inch in depth as measured with a 30-power microscope. It is found that an average of 26.0 passes are achieved prior to failure. The flank wear is uniform. 
     COMPARATIVE EXPERIMENT 2 (a-b) 
     A commercial silicon nitride ceramic body is obtained from each of the following sources (a) Boride Products (Product No. US-20) and (b) GTE Valeron Corporation (Product No. Q6). Each sample is diamond ground into a cutting tool in the manner described in Example 4. The cutting tools are used to cut nodular &#34;ductile&#34; cast iron in the manner described in Example 4. It is found that an average of 13.5 passes of the Boride Products sample are achieved prior to failure, and an average of 10.5 passes of the GTE sample are achieved prior to failure. In both cases the flank wear is uniform. When Comparative Experiments 2(a and b) are compared with Example 4, it is seen that the silicon nitride ceramic body of this invention significantly outperforms the commercial products. 
     Examples 5(a-w)--Effect of Various Whisker Growth Enhancing Compounds Upon the Properties of Self-Reinforced Si 3  N 4   
     A series of hot-pressed silicon nitride compositions is prepared using a variation of the procedure described in Example 1 as well as a different silicon nitride powder and a variety of elements to be evaluated for use as whisker growth enhancing compounds. The elements and the resultant physical properties are listed in Table IV. The physical properties include fracture strength (modulus of rupture) measured according to Military Standard 1942b, fracture toughness (Chevron notch technique) and Hardness (Vickers indentation test). 
     The silicon nitride powder is commercially available from Ube Industries, Ltd. under the trade designation SN-E10. It contains 1.46 percent oxygen, less than 100 ppm Cl, less than 100 ppm Fe, less than 50 ppm Ca, and less than 50 ppm Al. It has a crystallinity of greater than 99.5 percent, a ratio of α/(α+β) of less than five and a surface area of 11.2 m 2  /g. 
     K 2  CO 3  and Na 2  CO 3 , both commercially available from Fisher Scientific, are suitable sources, respectively for K 2  O and Na 2  O. Fisher Scientific also supplies La 2  O 3 , ZrO 2 , Cr 2  O 3 , WO 3 , MnO 2  and Fe 2  O 3 . Morton Thiokol supplies SrO and V 2  O 4 . Matheson, Coleman and Bell supply BaO. Baker Incorporated supplies TiO 2 , NiO and CaO. Alfa Products supplies Sc 2  O 3 , NbB 2 , CoO, ZnO, Ga 2  O 3 , In 2  O 3 , HfO 2 , Ta 2  O 5 , B 2  O 3  and CuO. 
     The materials are mixed in an attritor (Union Process batch attritor, Model 01HD - 750 cc capacity with polytetrafluoroethylene coated tube and stirrer) containing zirconia balls with a stirring rate of 330 revolutions per minute (rpm). A mixture of 225 ml toluene and 25 drops of an aluminate coupling agent, commercially available from Kenrich Petrochemicals under the trade designation Kenreact KA 322, is used as a mixing medium. The silicon nitride powder (86.3 g), the magnesium oxide powder (4.7 g), the yttrium oxide powder (8.5 g) and the powdered elemental derivative identified in Table IV (0.5 g) are added to the mixing medium and agitation is started. After about thirty minutes of mixing, an additional 100 ml of toluene is added and the agitation stirring rate is increased to 630 rpm for a period of two minutes to thin the mixture before removing it from the attritor together with the zirconia balls. Excess toluene is removed by vacuum filtration. The mixture is then dried under a flow of dry nitrogen gas. After drying, the mixture is separated from the zirconia balls using a 30 mesh stainless steel sieve and subsequently passed through 40 mesh and 60 mesh stainless steel sieves to provide a dried powder mixture composed of 86.3 percent silicon nitride, 4.7 percent magnesium oxide, 8.5 percent yttrium oxide and 0.5 percent derivative. The powder is then hot-pressed into plates and subjected to physical property testing as described in Example 1. 
     
                       TABLE IV                                                    
______________________________________                                    
                 Fracture  Fracture Vickers                               
      Additive/  Strength  Toughness                                      
                                    Hardness                              
Ex. 5 Derivative (MPa)     (MPa · m1/2)                          
                                    (kg/mm.sup.2)                         
______________________________________                                    
a     K.sub.2 O  1062      8.43     1516                                  
b     Na.sub.2 O 869       8.65     1555                                  
c     SrO        869       8.56     1604                                  
d     BaO        917       8.83     1486                                  
e     Sc.sub.2 O.sub.3                                                    
                 896       8.36     1520                                  
f     La.sub.2 O.sub.3                                                    
                 862       7.71     1515                                  
g     TiO.sub.2  1145      10.32    1526                                  
h     ZrO.sub.2  972       8.0      1512                                  
i     V.sub.2 O.sub.4                                                     
                 1069      8.36     1514                                  
j     NbB.sub.2  1034      8.21     1500                                  
k     Cr.sub.2 O.sub.3                                                    
                 1034      8.1      1512                                  
l     WO.sub.3   958       7.54     1482                                  
m     MnO.sub.2  972       --       1528                                  
n     Fe.sub.2 O.sub.3                                                    
                 841       7.74     1519                                  
o     CoO        972       7.75     1496                                  
p     NiO        979       7.39     1538                                  
q     CuO        855       7.05     1496                                  
r     ZnO        945       6.83     1548                                  
s     CaO        896       9.5      1430                                  
t     Ga.sub.2 O.sub.3                                                    
                 986       10.16    --                                    
u     In.sub.2 O.sub.3                                                    
                 1015      9.92     --                                    
v     HfO.sub.2  1005      9.85     --                                    
w     Ta.sub.2 O.sub.5                                                    
                 1075      10.86    --                                    
x     B.sub.2 O.sub.3                                                     
                 915       9.93     --                                    
______________________________________                                    
 -- means not measured                                                    
 
    
     The data presented in Table IV demonstrate that a number of elemental derivatives, when used as whisker growth enhancing compounds, provide self-reinforced silicon nitride compounds with satisfactory physical properties. Similar results are expected with other compositions which are disclosed herein. 
     EXAMPLE 6(a-g)--Effect of Adding Aluminum Nitride or Boron Nitride 
     A series of hot-pressed silicon nitride ceramic compositions is prepared according to the procedure of Example 5 except, in the case of Examples 6a-6d, for a variation in the mixing technique due to a change in mixing medium from toluene to methanol and the substitution of a small amount (see Table Va) of aluminum nitride (Example 6a) or boron nitride (Example 6b) for silicon nitride in the compositions shown respectively for Examples 5c and 5j. 
     About 80 ml of methanol are added to the same attritor as used in Example 5 with stirring at a rate of 200 rpm. The powder components are added slowly over a period of about 18 minutes with additional methanol (about 50 ml) as needed to wash the powder components from the walls of the attritor. If desired for more uniform and rapid dispersion, a dispersant, such as a 50% mixture of polyethyleneimine in methanol, commercially available under the trade designation Corcat P-12 from Virginia Chemicals, may be added in an amount of, for example, 26 drops. After mixing for an hour at 220 rpm, a small amount (14 drops) of oleic acid is added to flocculate the slurry. The flocculated slurry is then recovered as described in Example 5. 
     Composition and physical property data for Examples 6a-6h as well as 5c and 5j are summarized in Tables Va-Vc. 
     
                       TABLE Va                                                    
______________________________________                                    
                               Fracture                                   
                               Tough- Vickers                             
                      Fracture ness   Hard-                               
Ex.  Additives (wt-%) Strength (MPa ·                            
                                      ness                                
No.  SrO    NbB.sub.2                                                     
                    AIN  BN   (MPa)  m1/2)  (kg/m.sup.2)                  
______________________________________                                    
5c   0.5    0.0     0.0  0.0   869   8.56   1604                          
6a   0.5    0.0     1.0  0.0  1096   10.26  1541                          
5j   0.0    0.5     0.0  0.0  1034   8.21   1512                          
6b   0.0    0.5     0.0  1.0  1076   8.72   1482                          
______________________________________                                    
 
    
     
                       TABLE Vb                                                    
______________________________________                                    
                  Fracture  Fracture                                      
                                    Vickers                               
Ex.  Additives (wt-%)                                                     
                  Strength  Toughness                                     
                                    Hardness                              
No.  ZnO    AIN    BN   (MPa)   (MPa · m1/2)                     
                                        (kg/m.sup.2)                      
______________________________________                                    
6c   0.5    1.0    0.0  1014    8.44    1496                              
6d   0.5    0.0    1.0   917    8.52    1537                              
______________________________________                                    
 
    
     
                       TABLE Vc                                                    
______________________________________                                    
               Fracture    Fracture                                       
                                   Vickers                                
Ex.    BN      Strength    Toughness                                      
                                   Hardness                               
No.    (wt-%)  (MPa)       (MPa · m1/2)                          
                                   (kg/m.sup.2)                           
______________________________________                                    
5c     0.0     869         8.56    1604                                   
6e     0.5     896         9.45    1450                                   
6f     2.0     931         9.39    1365                                   
6g     3.0     958         8.76    1377                                   
______________________________________                                    
 
    
     A review of the data presented in Tables Va-Vc highlights several points. First, the substitution of a minor amount of aluminum nitride or boron nitride for a corresponding amount of silicon nitride provides improvements in both fracture strength and fracture toughness at the expense of a limited reduction in hardness (Table Va). Second, the trend shown in Table Va wherein AlN appears to provide a greater improvement in fracture strength and fracture toughness than BN does not hold true for all compositions. Table Vb shows that the reverse is true when the whisker growth enhancing compound is ZnO. Third, substantial increases in the amount of BN give limited improvement in fracture strength at the expense of both fracture toughness and hardness. In other words, large amounts of AlN or BN are unnecessary. In addition, a certain amount of physical property tailoring is possible with limited compositional variation. Similar results are attained with other compositions of the present invention. 
     Examples 7a-7e--Replication of Example 6b with Varying Amounts of the Glassy Phase 
     Using the procedure of Example 6, the components of Example 6b (Si 3  N 4 , Y 2  O 3 , MgO, NbB 2  and BN) and the component ratios of components other that silicon nitride, a series of four hot-pressed silicon nitride ceramic bodies are prepared with varying percentages of the Si 3  N 4  phase (components other than silicon nitride). The component ratios are 1.8 (Y 2  O 3  /MgO), 14.7 (total glassy phase/BN) and 29.4 (total glassy phase/NbB 2 ). The Si 3  N 4  content and physical properties are summarized in Table VI. 
     
                       TABLE VI                                                    
______________________________________                                    
               Fracture   Fracture                                        
                                  Vickers                                 
Ex.    Si.sub.3 N.sub.4                                                   
               Strength   Toughness                                       
                                  Hardness                                
No.    (wt-%)  (MPa)      (MPa · m1/2)                           
                                  (kg/m.sup.2)                            
______________________________________                                    
7a     80       993       9.17    1404                                    
7b     85      1076       8.72    1482                                    
7c     90      1069       9.92    1526                                    
7d     95      1096       9.47    1548                                    
7e     97       875       8.45    1601                                    
______________________________________                                    
 
    
     The data presented in Table VI suggest that the Si 3  N 4  content may be varied considerably without undue adverse effects upon physical properties of the resultant hot-pressed silicon nitride ceramic bodies. Similar results are attainable with other compositions of the present invention. 
     Examples 8a-8d--Replication of Example 6a with Varying Amounts of the Glassy Phase 
     Using the procedure of Example 6, the components of Example 6a (Si 3  N 4 , Y 2  O 3 , MgO, SrO and AlN) and the component ratios of components other that silicon nitride, a series of four hot-pressed silicon nitride ceramic bodies are prepared with varying percentages of the Si 3  N 4  phase (components other than silicon nitride). The component ratios are 1.8 (Y 2  O 3  /MgO), 14.7 (total glassy phase/AlN) and 29.4 (total glassy phase/SrO). The Si 3  N 4  content and physical properties are summarized in Table VII. 
     
                       TABLE VII                                                   
______________________________________                                    
               Fracture   Fracture                                        
                                  Vickers                                 
Ex.    Si.sub.3 N.sub.4                                                   
               Strength   Toughness                                       
                                  Hardness                                
No.    (wt-%)  (MPa)      (MPa · m1/2)                           
                                  (kg/m.sup.2)                            
______________________________________                                    
8a     85      1089       9.3     1591                                    
8b     90      1125       8.7     1622                                    
8c     95      1102       8.6     1631                                    
8d     97       993       8.8     1651                                    
______________________________________                                    
 
    
     The data presented in Table VII suggest that the trend in physical property changes observed in Examples 7a-7e as a result of an increase in Si 3  N 4  content holds true when AlN is substituted for BN and NbB 2  is replaced by SrO. Similar results are attainable with other compositions of the present invention. 
     Comparative Examples 3a-3e--Effect of Omitting One or More Components of the Composition of Example 5g 
     Using the procedure of Example 5, a series of hot-pressed silicon nitride ceramic bodies are prepared by omitting one or more components of the composition of Example 5g and adjusting the remaining components as shown in Table VIII. Table VIII also shows the resultant physical properties. Example 5g is included in Table VIII for ease of comparison. 
     
                                           TABLE VIII                              
__________________________________________________________________________
Ex/                                                                       
Comp                                                                      
    Composition   Fracture                                                
                        Fracture                                          
                              Vickers                                     
Ex  (wt-%)        Strength                                                
                        Toughness                                         
                              Hardness                                    
No. Si.sub.3 N.sub.4                                                      
        Y2O.sub.3                                                         
            MgO                                                           
               CaO                                                        
                  (MPa) (MPa · m1/2)                             
                              (kg/m.sup.2)                                
__________________________________________________________________________
5g  86.3                                                                  
        8.5 4.7                                                           
               0.5                                                        
                  896   9.5   1430                                        
3a  86.8                                                                  
        8.5 4.7                                                           
               0.0                                                        
                  607   5.0   1480                                        
3b  86.3                                                                  
        0.0 13.2                                                          
               0.5                                                        
                  386   5.57  1449                                        
3c  86.8                                                                  
        0.0 13.2                                                          
               0.0                                                        
                  352   4.65  1450                                        
3d  86.3                                                                  
        13.2                                                              
            0.0                                                           
               0.5                                                        
                  510   3.83  1461                                        
3e  86.8                                                                  
        13.2                                                              
            0.0                                                           
               0.0                                                        
                  572   5.54  1534                                        
__________________________________________________________________________
 
    
     The data presented in Table VIII clearly demonstrate the effect of omitting one or more components, as in Comparative Examples 3a-3e, from the composition of Example 5g. Although the data contained in this table relates to the use of a calcium derivative as the additive component, similar results are attainable without undue experimentation using other derivatives of elements listed in The Periodic Table of the Elements and included within the scope of the present invention. 
     Examples 9(a-c)--Effect of Varying the Source of Niobium 
     Using the procedure of Example 6, a series of three hot-pressed silicon nitride ceramic bodies are prepared using sources of niobium other than that used in Example 5j. The sources of niobium are as follows: 
     Example 9a--Nb 3  Sn 
     Example 9b--Nb 3  Ga 
     Example 9c--NbO 
     Physical property data for the three bodies and for Example 5j are summarized in Table IX. 
     
                       TABLE IX                                                    
______________________________________                                    
         Fracture    Fracture  Vickers                                    
Example  Strength    Toughness Hardness                                   
No.      (MPa)       (MPa · m1/2)                                
                               (kg/m.sup.2)                               
______________________________________                                    
5j       1034        8.21      1500                                       
9a       1000        8.03      1529                                       
9b        938        8.20      1538                                       
9c        896        7.26      1533                                       
______________________________________                                    
 
    
     The data presented in Table IX demonstrate that non-oxide derivatives perform as well as, if not better, than the oxides. Similar results are attainable with other compositions of the present invention without undue experimentation. 
     Examples 10(a-b)--Effect of Replacing MgO with SrO or CaO 
     Using the procedure of Example 5, two hot-pressed silicon nitride ceramic bodies are prepared by substituting equal weight percentages of strontium oxide (SrO) (Example 10a) or calcium oxide (CaO) (Example 10b) for the magnesium oxide (MgO) used in Example 5. Physical property data for the two bodies and for Examples 5c and 5s are summarized in Table X. CaO is the whisker growth enhancing compound for Examples 10a and 5s. SrO is the whisker growth enhancing compound for Examples 10b and 5c. 
     
                       TABLE X                                                     
______________________________________                                    
         Fracture    Fracture  Vickers                                    
Example  Strength    Toughness Hardness                                   
No.      (MPa)       (MPa · m1/2)                                
                               (kg/m.sup.2)                               
______________________________________                                    
10a      1027        6.67      1634                                       
 5s       896        9.5       1430                                       
10b      1014        6.61      1556                                       
 5c       869        8.56      1604                                       
______________________________________                                    
 
    
     The data presented in Table X demonstrate that strontium oxide and calcium oxide are suitable substitutes for magnesium oxide. Similar results are expected with other oxides from Group IIA of the Periodic Table of the Elements, namely beryllium oxide, barium oxide and radium oxide. 
     Example 11--Effect of Using Magnesium Oxide as a Whisker Growth Enhancing Compound. 
     Using the procedure of Example 10 and the composition of Example 10b, save for the substitution of 0.5 weight percent MgO for a like amount of SrO, a hot-pressed silicon nitride ceramic body is prepared. Physical property data for the body are as follows: 
     Fracture Strength (MPa)-800 
     Fracture Toughness (MPa.m1/2)-7.94 
     Vickers Hardness (kg/m 2 )-1545 
     The data demonstrate the suitability of MgO as a whisker growth enhancing compound. 
     Example 12(a-b)--Effect of Substituting SrO for MgO in Compounds Containing a Small Amount of Aluminum Nitride 
     Using the procedure of Example 6, two hot-pressed silicon nitride ceramic bodies are prepared from the compositions shown in Table XIA. Physical property data for the bodies are shown in Table XIB. 
     
                       TABLE XIA                                                   
______________________________________                                    
Example                                                                   
No.      Si.sub.3 N.sub.4                                                 
                   Y.sub.2 O.sub.3                                        
                           SrO    AIN  TiO.sub.2                          
______________________________________                                    
12a      96        2.32    1.28   0.27 0.13                               
12b      97        1.74    0.96   0.2  0.1                                
______________________________________                                    
 
    
     
                       TABLE XIB                                                   
______________________________________                                    
         Fracture    Fracture  Vickers                                    
Example  Strength    Toughness Hardness                                   
No.      (MPa)       (MPa · m1/2)                                
                               (Kg/mm.sup.2)                              
______________________________________                                    
12a      820         8.52      1650                                       
12b      770         8.29      1640                                       
______________________________________                                    
 
    
     The data shown in Table XIB, like that of Table IX, demonstrate the suitability of SrO as a substitute for MgO in the preparation of self-reinforced silicon nitride bodies. Similar results are expected with other oxides from Group IIA of the Periodic Table of the Elements. 
     Evaluation of Cutting Tool Effectiveness 
     The hot-pressed silicon nitride body of Example 12 is diamond ground according to American National Standards Institute (A.N.S.I.) standards into an SNG 434 style cutting tool insert. The cutting edge is chamfered at a 30° angle with a 0.006 inch (0.015 cm) width. 
     The insert is tested in a face milling application using a 40 Horsepower Cincinnati #5 single spindle, knee and saddle, vertical milling machine with a 5 Horsepower variable speed table. The work material is Class 30 grey cast iron, four inches (10.16 cm) wide and 12 inches (30.48 cm) long with a measured hardness of 170 BHN (220 kg/mm 2 ). A milling cutter having a 12 inch (30.48 cm) diameter, one tooth milling cutter, is used with a -5° axial rake and a -5° radial rake. A 15° lead angle is employed. The machine is run at a cutting speed of 3000 surface feet per minute (914 meters per minute), a 0.060 inch (0.152 cm) depth of cut, and a feed rate of 0.013 inch (0.330 mm) per revolution (or tooth). The center line of the cutter and the center line of the workpiece are coincident. No cutting fluid is used. 
     Successive passes are taken on the cast iron work material. The cutting edge is examined for flank wear and chippage after each pass. Testing is terminated when the flank wear or the chippage exceeds 0.010 inch (0.025 cm) in depth as measured with a 30-power microscope. The insert removes about 590 cubic inches (9668 cm 3 ) of iron prior to failure. An SNG 434 style cutting tool insert prepared from a nominal silicon nitride material (actually a silicon-alumina-nitride or sialon) marketed by Kennametal Corporation under the trade designation Kyon™ 3000 removes 340 cubic inches (5572 cm 3 ) of iron prior to failure. An SNG 434 style cutting tool insert prepared from a silicon nitride material marketed by Boride Products under the trade designation US-20 removes 311 cubic inches (5096 cm 3 ) of iron prior to failure. 
     The foregoing illustration demonstrates the suitability of the self-reinforced silicon nitride material prepared in Example 12b in cutting tool applications. Similar results are expected with other compositions disclosed herein. 
     Example 13--Substitution of Lanthanum Oxide for Yttrium Oxide 
     Using the procedure of Example 5, and a composition identical to that of Example 5s, save for the substitution of lanthanum oxide for yttrium oxide, a hot-pressed silicon nitride body is prepared. Physical property data for the body are as follows: 
     Fracture Strength (MPa)--930 
     Fracture Toughness (MPa.m1/2)--9.0 
     Vickers Hardness (kg/m 2 )--1526 
     The data demonstrate the suitability of lanthanum oxide as a conversion aid. Similar results are expected with other oxides of elements from Group IIIA of the Periodic Table of the Elements. Satisfactory results are also expected with non-oxide sources of the Group IIIA elements as well as sources of Group IIA elements. 
     Example 14(a-b)--Substitution of Sodium Oxide for Yttrium Oxide or Magnesium Oxide 
     Using the procedure of Example 5, and a composition identical to that of Example 5s, save for the substitution of sodium oxide for yttrium oxide in Example 14a and the substitution of sodium oxide for magnesium oxide in Example 14b, two hot-pressed silicon nitride bodies are prepared. Physical property data for the bodies are shown in Table XII. 
     
                       TABLE XII                                                   
______________________________________                                    
              Fracture Fracture                                           
Example       Strength Toughness                                          
No.           (MPa)    (MPa · m1/2)                              
______________________________________                                    
14a           925      8.54                                               
14b           720      7.4                                                
______________________________________                                    
 
    
     The data presented in Table XII demonstrate that sodium oxide is an effective substitute either as a densification aid (Example 14b) or a conversion aid (Example 14a). Similar results are expected with other sources of sodium as well as sources of other elements of Group IA of the Periodic Table of the Elements. 
     Example 15(a-h)--Variation in the Ratio of Conversion Aid to Densification Aid 
     A series of hot-pressed silicon nitride ceramic compositions is prepared according to the procedure of Example 6, except that the ratio of conversion aid (yttrium oxide) to densification aid (magnesium oxide) is varied as shown in Table XIII. The basic composition of the powder mixture is 86.3 percent Si 3  N 4 , 0.5 percent CaO and 13.2 percent of a combination of MgO and Y 2  O 3 . The fracture toughness and the flexure strength are shown in Table XIII for each composition. 
     
                       TABLE XIII*                                                 
______________________________________                                    
                     Fracture   Fracture                                  
Ex.                  Toughness  Strength                                  
15     Y.sub.2 O.sub.3 /MgO                                               
                     (MPa · m1/2)P                               
                                (MPa)                                     
______________________________________                                    
a      8:1           7.15       827                                       
b      5:1           10.02      944                                       
c      3:1           9.53       1006                                      
d      1.8:1         10.37      1034                                      
e      1:1           10.50      1034                                      
f      1:2           9.46       930                                       
g      1:3           7.68       703                                       
h      1:4           6.54       701                                       
______________________________________                                    
 *Percentages of components are based on weight percent in the powder     
 mixture.                                                                 
 
    
     The data presented in Table XIII demonstrate that a wide range of compositional variation produces hot-pressed silicon nitride ceramic bodies having a fracture toughness in excess of 6 MPa.m1/2 and a flexure strengths in excess of 700 MPa. Similar results are expected with other compositions disclosed herein. 
     Example 16(a-f)--Effect of Hot-Pressing Temperature Upon Physical Properties 
     A series of hot-pressed silicon nitride ceramic compositions is prepared according to the procedure of Example 6, save for one modification. The modification is the maximum temperature of 1825° C. used in Example 1 is varied as shown in Table XIV. All compositions are identical to that of Example 15d. The Palmqvist Toughness of each composition is also shown in Table XIV. 
     
                       TABLE XIV                                                   
______________________________________                                    
                        Palmqvist                                         
Example      Temperature                                                  
                        Toughness                                         
Number       (°C.)                                                 
                        (kg/mm)                                           
______________________________________                                    
16a          1650       36.9 ± 1.7                                     
16b          1700       40.1 ± 1.5                                     
16c          1750       42.4 ± 1.8                                     
16d          1785       44.4 ± 1.8                                     
16e          1825       44.9 ± 1.9                                     
16f          1850       43.2 ± 0.6                                     
______________________________________                                    
 The data presented in Table XIV demonstrate that acceptable Palmqvist
 Toughness is attainable over a relatively broad temperature range. Similar
 results are expected with other physical properties such as fracture
 strength and fracture toughness as well as with other compositions
 disclosed herein.
 
    
     Example 17(a-c)--Effect of Hot-Pressing Time Upon Physical Properties 
     A series of hot-pressed silicon nitride ceramic compositions is prepared according to the procedure of Example 16e, save for one modification. The sole modification is a change in the hot-pressing time from minutes to that shown in Table XV. Accordingly, all hot-pressing is done at a temperature of 1825° C. The Palmqvist Toughness of the compositions is also shown in Table XV. The Palmqvist Toughness and hot-pressing time for Example 16e is shown for comparison. 
     
                       TABLE XV                                                    
______________________________________                                    
                       Palmqvist                                          
Example       Time     Toughness                                          
Number        (minutes)                                                   
                       (kg/mm)                                            
______________________________________                                    
16e           60       44.9 ± 1.9                                      
17a            5       40.4 ± 1.2                                      
17b           15       41.3 ± 1.5                                      
17c           120      39.6 ± 1.7                                      
______________________________________                                    
 
    
     The data presented in TabIe XV demonstrate that considerable latitude in terms of time produces acceptable Palmqvist Toughness values. Similar results are expected with other physical properties such as fracture strength and fracture toughness as well as with other compositions disclosed herein. 
     Example 18(a-g)--Effect of Silica (SiO 2 ) Content Upon Physical Properties 
     A series of hot-pressed silicon nitride ceramic compositions is prepared using the procedure and composition of Example 1 with modifications of the silica (SiO 2 ) content of the composition. Silica (SiO 2 ) is inherently present in Si 3  N 4 . The actual SiO 2  content of a particular powder depends upon variables such as particle size and method of preparation. Typical commercially available Si 3  N 4  powder has an oxygen content in excess of two percent by weight of powder which equates to an SiO 2  content of 3.75 percent by weight of powder or more. The Si 3  N 4  powder used in this example is commercially available from UBE Industries America under the trade designation UBE-SN-10 and has an oxygen content of 1.3 percent by weight of powder which equates to a silica content of 2.43 percent by weight of powder. Free silica is added to increase the silica content and thereby the oxygen content of a given composition. The total silica content (weight percent), fracture strength and fracture toughness for each composition is shown in Table XVI. 
     
                       TABLE XVI                                                   
______________________________________                                    
        Total        Fracture Fracture                                    
Ex.     SiO.sub.2    Strength Toughness                                   
No.     (wt-%)       (MPa)    (MPa · m1/2)                       
______________________________________                                    
18a     2.34          890     9.1                                         
18b     2.59         1048     9.6                                         
18c     2.84         1082     9.4                                         
18d     3.09         1254     9.7                                         
18e     3.59         1254     9.8                                         
18f     4.09         1034     --                                          
18g     5.09         1069     --                                          
______________________________________                                    
 -- means not measured                                                    
 
    
     The data presented in Table XVI demonstrate that control of composition oxygen content, at least in the form of silica, has a marked effect upon fracture strength of resultant hot-pressed bodies. Similar results are expected with other compositions disclosed herein.