A self-reinforced silicon nitride ceramic body and a method of preparing the same

Abstract: Self-reinforced silicon nitride ceramic bodies of high fracture toughness are prepared by hot-pressing a powder mixture containing silicon nitride and three other components under conditions such that densification and the in situ formation of ß-silicon nitride whiskers having a high aspect ratio occur. The other components are silicon dioxide as a densification aid, a conversion aid such as yttrium oxide and a compound, such as calcium oxide, which enhances growth of ß-silicon nitride whiskers. The resultant silicon nitride ceramic has a ß-silicon nitride crystalline phase, at least 20 volume percent of which is in the form of whiskers or elongate grains having an average aspect ratio of at least 2.5, and a glassy second phase. The glassy phase contains the silicon dioxide, the conversion aid and the compound which enhances growth of ß-silicon nitride whiskers. The glassy phase may also include a minor amount of aluminum nitride or boron nitride.

Description: This invention pertains to a silicon nitride (Si₃N₄) 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. Fracture strength is directly proportional to the fracture toughness and inversely proportional to the square root of the flaw size. Monolithic silicon nitride, however, has a relatively low fracture toughness of about 5 MPa (m) ½ . It is therefore desirable to provide a silicon nitride ceramic having high fracture toughness combined with small flaw size that includes β-silicon nitride whiskers to reinforce the silicon nitride body. US-A-4609633 discloses silicon nitride sintered bodies comprising: (a) 0.5 to 25 weight percent of at least one oxide, nitride or oxynitride of a rare earth element selected from scandium, yttrium and the lanthanides; (b) 0.2 to 30 weight percent of at least one oxide, nitride or oxynitride of a Group IIa element (viz. beryllium, magnesium, calcium, strontium, barium, and radium) or a nitride or oxynitride of a Group IIIb element (viz. boron, aluminum, gallium, indium, and thallium); and (c) silicon nitride and inevitable impurities. It is stated that the body cannot have combinations of a rare earth oxide (a) with a Group IIa oxide (b) (see column 2, lines 17/20) but it is apparent from the exemplified bodies (see Samples 1 to 4 and 15 to 18) that both a rare earth oxide (a) and a Group IIa oxide (b) can be present if a nitride or oxynitride (a) or (b) also is present. It is stated that the presence of the nitride or oxynitride in the sintering auxiliary heightens the bonding strength between second phases chiefly comprising the sintering auxiliary and the silicon nitride rigid phases after sintering and thereby prevents, inter alia, abnormal growth of silicon nitride grains (see column 2, lines 36/52). It is further stated that grains, as distinct from rods or needle-like structures, having a small aspect ratio provide excellent thermal shock resistance (see column 6, lines 40/50). The composition of US-A-4609633 can additionally include one or more carbides or nitrides of a Group IVa, Va, or VIa element to enhance wear resistance and an oxide, nitride or oxynitride of a Group Ia element to enhance sinterability and densification. There is no exemplification of such compositions. It is stated in US-A-4609633 that SiO₂ in an amount of not more than 1.2% by weight causes a decomposition temperature inherent in the silicon nitride to drop and thereby lowers the temperature at which the silicon nitride reacts with sintering auxiliaries. The SiO₂ is introduced from the reactor vessel or the balls during mixing and grinding of the starting material powder and there is no disclosure of the deliberate addition of SiO₂ as a distinct component of said powder. WO-A-8907093 discloses a process for preparing a silicon nitride ceramic body, comprising preparing a powder mixture comprising (a) silicon nitride in an amount sufficient to provide a ceramic body; (b) a source of magnesium in an amount sufficient to promote densification of the powder; (c) a source of yttrium 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 derivative of an element selected from, inter alia, calcium, sodium, potassium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, strontium, zirconium, niobium, barium, lanthanum, gallium, indium, hafnium, and tantalum. and hot-pressing the powder mixture or greenware thereof under conditions such that densification and in situ formation of β-silicon nitride whiskers having an average aspect ratio of at least 2.5 occur and that said silicon nitride ceramic body is predominately β-silicon nitride. The only exemplified sources of magnesium or yttrium in WO-A-8907093 are the oxides. It is stated in WO-A-8907093 that silicon nitride usually contains oxygen in the form of Si₂O and that oxygen contents below 5 wt% are preferred with oxygen contents below 2 wt% being most preferred. US-A-5021372 (published June 4th, 1991) discloses the preparation of self-reinforced silicon nitride ceramic bodies having a fracture toughness greater than 6 MPa (m) ½ , and containing predominately β-silicon nitride whiskers having a high average aspect ratio by subjecting 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 (i) a Group Ia element selected from lithium, sodium, potassium, rubidium, cesium, and francium; or (ii) a Group IIa element selected from beryllium, calcium, strontium, barium, and radium, 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 (i) a Group IIIa element selected from scandium, actinium, and lanthanum; or (ii) a Group Ia element selected from 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 (i) a Group Ia element selected from sodium and potassium; copper; (ii) a Group IIa element selected from strontium and barium; (iii) zinc; (iv) a Group IIIa element selected from scandium and lanthanum; (v) a Group IVa element selected from titanium and zirconium; (vi) a Group Va element selected from vanadium and niobium; (vii) chromium; (viii) manganese; (ix) a Group VIII element selected from iron, cobalt, and nickel; or mixtures thereof, or an oxide of a Group IIIb element selected from indium, gallium and boron; hafnium; or tantalum; (b), (c) and (d) being derived from three different elements, to conditions of temperature and pressure sufficient such that densification and in situ formation of β-silicon nitride whiskers having an average aspect ratio of at least 2.5 and such that the silicon nitride ceramic body has a fracture toughness greater than 6 MPa (m) ½ is formed. It is stated in US-A-5021372 that the powder mixture can further comprises silica in an amount of from 2.3 to 6 weight percent based upon total weight of the powder mixture to improve fracture strength of resultant hot-pressed bodies. At the lower silica concentrations, it is present as impurity in commercially available silica nitride but at higher concentrations silica is added to the powder mixture. EP-A-0472256 (published February 26th, 1992 and having a filing date of February 13th, 1991 and a priority date of August 24th, 1990) discloses essentially the same process as US-A-5021372 but includes the possibility that the densification aid is a source of (iii) zinc; (iv) a Group IIIb element selected from indium and gallium; (v) silicon; (vi) a Group IVa element selected from hafnium and titanium; or (vii) tantalum. 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 and having a fracture toughness greater than 6 MPa (m) ½ , as measured by the Chevron notch technique at 23°C. The process comprises subjecting a powder mixture comprising: (a) silicon nitride in an amount sufficient to provide a ceramic body; (b) free silicon dioxide in an amount sufficient to promote densification of the powder; (c) a conversion aid in an amount sufficient to attain essentially complete conversion of the starting silicon nitride to β-silicon nitride, the conversion aid being a source of (i) a Group IIIa element selected from yttrium, scandium, actinium, and lanthanum; or (ii) a Group Ia element selected from lithium, sodium, potassium, rubidium, cesium and francium; and (d) at least one whisker growth enhancing compound in an amount sufficient to promote formation of β-silicon nitride whiskers, the compound being a source of (i) a Group Ia element selected from sodium and potassium; copper; (ii) a Group IIa element selected from calcium, strontium and barium; (iii) zinc; (iv) a Group IIIa element selected from scandium and lanthanum; (v) a Group IVa element selected from titanium and zirconium; (vi) a Group Va element selected from vanadium and niobium; (vii) chromium; (viii) manganese; (ix) a Group VIII element selected from iron, cobalt, and nickel; or mixtures of two or more thereof, or an oxide of a Group IIIb element selected from indium, gallium and boron;, hafnium; or tantalum, provided, however, that the element used as a whisker growth enhancing compound differs from that used as a conversion aid and that, when the powder mixture contains a source of a said Group Ia element or a said Group IIa element, the total silicon dioxide content is greater than 6 weight percent based on the total weight of 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. In this manner a self-reinforced silicon nitride ceramic body having a fracture toughness greater than 6 MPa (m) ½ , as measured by the Chevron notch technique described hereinbelow, is formed. The process may include a preliminary step of preparing the powder mixture. For the purposes of the present invention, a "high" average aspect ratio means an average aspect ratio of at least 2.5. The powder mixture optionally includes aluminum nitride or boron nitride in an amount of 0.01 to five weight percent, based upon total powder mixture weight. Any means may be used to apply pressure and temperature so long as sufficient densification and in situ whisker formation occur. Application of pressure and temperature beneficially occurs by hot-pressing or by hot isostatic pressing, preferably by hot-pressing. In a second aspect, this invention is a silicon nitride ceramic body having a fracture toughness greater than 6 MPa (m) ½ , as measured by the Chevron notch technique at 23°C, comprising: (a) a crystalline phase of β-silicon nitride of which at least 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 2.5; and (b) a glassy phase, in an amount not greater than 35 weight percent of total body weight, comprising (1) silicon dioxide, (2) a conversion aid and (3) a β-silicon nitride whisker growth enhancing compound, said conversion aid and whisker growth enhancing compound being as defined above. The glassy phase optionally contains an amount, e.g., 0.01 to 15.0 percent by weight of the glassy phase, of aluminum nitride or boron nitride. In a third aspect, this invention is a cutting tool comprising the above-identified silicon nitride ceramic body. In a fourth aspect, the invention is the use of silicon dioxide as the densification aid in the preparation of a silicon nitride ceramic body having a fracture toughness greater than 6 MPa (m) ½ , as measured by the Chevron notch technique at 23°C, by subjecting a powder mixture comprising (a) silicon nitride in an amount sufficient to provide a ceramic body; (b) a densification aid in an amount sufficient to promote densification of the powder, said amount being from 1 to 28 weight percent based on the total weight of the powder mixture; (c) a conversion aid in an amount sufficient to attain essentially complete conversion of the starting silicon nitride to β-silicon nitride, said amount being from 0.2 to 29.5 weight percent based on the total weight of the powder mixture, the conversion aid being a source of (i) a Group IIIa element selected from yttrium, scandium, actinium, and lanthanum; or (ii) a Group Ia element selected from lithium, sodium, potassium, rubidium, cesium and francium, and (d) at least one whisker growth enhancing compound different from said conversion aid in an amount sufficient to promote formation of β-silicon nitride whiskers, the amount being from 0.01 to 5 weight percent based upon total weight of the powder mixture, said compound being a source of (i) a Group Ia element selected from sodium and potassium; copper; (ii) a Group IIa element selected from calcium, strontium and barium; (iii) zinc; (iv) a Group IIIa element selected from scandium and lanthanum; (v) a Group IVa element selected from titanium and zirconium; (vi) a Group Va element selected from vanadium and niobium; (vii) chromium; (viii) manganese; (ix) a Group VIII element selected from iron, cobalt, and nickel; or mixtures of two or more thereof, or an oxide of a Group IIIb element selected from indium, gallium and boron; hafnium; or tantalum, to conditions of temperature of at least 1750°C and pressure of at least 20 MPa (3000 psig) sufficient to provide for densification of the powder mixture and in situ formation of β-silicon nitride whiskers having an average aspect ratio of at least 2.5 in an amount of at least 20 volume percent, as measured by viewing one plane of the silicon nitride ceramic body by scanning electron microscopy. The silicon nitride ceramic body of this invention exhibits a significantly higher fracture toughness than the monolithic silicon nitride ceramics of the prior art. 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. The silicon nitride starting material used in preparing the ceramic body of this invention can be any silicon nitride powder. Illustrative powders include the crystalline forms of α-silicon nitride and β-silicon nitride, or the noncrystalline amorphous forms of α-silicon nitride and β-silicon nitride, or mixtures thereof. The silicon nitride powder desirably contains a predominant amount of the α-crystalline form, or the amorphous form or mixtures thereof. The starting silicon nitride preferably contains a predominant amount of the alpha crystalline form. The starting powder advantageously possesses a high α/β weight ratio, e.g., that attained with a β-silicon nitride content of 20 weight percent or less. The β-silicon nitride content is beneficially less than 10 weight percent, desirably less than 6 weight percent. The silicon nitride starting material beneficially constitutes 80 to 97 percent by weight of the total powder mixture weight. An increase in purity of the starting silicon nitride powder generally results in improved properties of the resultant ceramic body. Commercially available silicon nitride powders may, however, contain nonmetallic impurities. Although some impurities may be tolerated, they should be minimized as much as possible. The level of oxygen, present in the form of a silica coating on the surface of silicon nitride particles, may, for example, be reduced by leaching the powder to remove at least a portion of the coating. Elemental silicon, another impurity, is usually present in amounts ranging up to 0.5 weight percent. These amounts are not deleterious and can be tolerated. 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 which yields the ceramic body of this invention via hot-pressing. Certain sizes, however, pose some difficulties. For example, large particles (those having an average diameter in the range 15 to 50 µm) may be present as hard agglomerates which cannot be easily broken. Powders containing such agglomerates make poor ceramics. On the other hand, very fine powders (those having an average diameter less than about 0.2 µm) pose processing problems. The starting powder beneficially has an average diameter in the range from 0.2 µm to 10.0 µm. The average diameter is desirably 0.2 µm to 3.0 µm. The surface area of the silicon nitride particles is suitably in the range of 5 m²/g to 15 m²/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. The surface area is beneficially in the range of 8 m²/g to 15 m²/g. Raw silicon nitride powders cannot be densified to high densities in the absence of a densification aid. 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 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. Silicon dioxide or silica (SiO₂) is used as the densification aid for all aspects of the present invention. It is present in an amount sufficient to promote densification as described herein and yield a silicon nitride ceramic body having a fracture toughness greater than 6 MPa (m) ½ . The sum of said amount and any silicon dioxide present in the silicon nitride is 1 to 28 weight percent, preferably 3 to 10 weight percent, based on total weight of the powder mixture. 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, although α-silicon nitride dissolves in the conversion aid on heating, it is not readily densified. The conversion aid does, however, promote the rapid, essentially complete conversion of α-silicon nitride to β-silicon nitride. This conversion is desirable because the β-silicon nitride, in the form of elongate, single crystal grains or whiskers, is believed to be responsible for the high fracture toughness and high fracture strength of the silicon nitride ceramic bodies of this invention. All references to silicon nitride whiskers, single crystal elongate grains or whiskers and single crystal silicon nitride whiskers are intended to be synonymous and may be used interchangeably. Any amount of conversion aid can be included in the starting powder so long as it is sufficient to cause essentially complete conversion of the starting silicon nitride to β-silicon nitride and produce the tough silicon nitride ceramic body of the invention. The amount of conversion aid is, based on total weight of the powder mixture, in a range which is beneficially 0.2 weight percent to 29.5 weight percent. The range is desirably 1.0 to 10.0 weight percent and preferably 1.7 to 8.5 weight percent. The conversion aid is a source of yttrium, scandium, actinium, lanthanum, lithium, sodium, potassium, rubidium, cesium and/or francium. The conversion aid is beneficially yttrium oxide, scandium oxide, lanthanum oxide, actinium oxide or sodium oxide. Yttrium oxide is a preferred conversion aid. The amount of conversion aid should also be sufficient to provide a weight ratio of conversion aid to silicon dioxide which results in a fracture toughness which is greater than 5 MPa (m) ½ , the toughness for non-reinforced, monolithic silicon nitride. The weight ratio is suitably 0.25 to 8, beneficially 0.25 to 5, desirably 0.25 to 3 and preferably 0.25 to 1.8. 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, albeit via a mechanism which is not completely understood. Any amount of the compound in the starting powder is acceptable providing it is sufficient to promote the formation of β-silicon nitride whiskers, described hereinbelow, and produce the tough silicon nitride ceramic body of this invention. The amount is suitably in a range of 0.01 to five weight percent based on the total weight of the powder mixture. The range is beneficially 0.1 to 1.0 weight percent and desirably 0.2 to 0.5 weight percent. The whisker growth enhancing compound is suitably a source or derivative of calcium, sodium, potassium, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, strontium, zirconium, niobium, barium, lanthanum, or mixtures thereof, or an oxide of gallium, indium, hafnium, tantalum or boron. The whisker growth enhancing compound is beneficially an oxide of calcium, tantalum, hafnium, niobium or mixtures thereof. Certain elements, e.g., scandium, lanthanum, sodium and potassium, may be used either as a conversion aid or as a whisker growth enhancing compound. These elements may not, however, simultaneously serve in both capacities. In other words, if an element is used as a conversion aid, it may not be used as a whisker growth enhancing compound in the same powder mixture. Whisker growth enhancing compounds are desirably in the form of powders which are pure and sufficiently small in size. Purity is not typically a problem. Commercially available materials used as whisker growth enhancing compounds, particularly the oxide powders, generally contain less than 20 ppm each of assorted impurities. These levels of impurities are tolerable. Larger amounts of impurities, e.g., 0.5 weight percent or more, are not recommended as they may cause a change in the final ceramic composition and properties. A small powder particle size is favored because it disperses readily. The oxide powders have an average particle size which is preferably less than 5 µm in diameter. The powder mixture used to prepare the silicon nitride bodies of the present invention include silicon nitride, silicon dioxide, a conversion aid and at least one whisker growth enhancing compound. All components of the powder mixture and suitable amounts of each component are disclosed herein. The amounts of all components other than silicon nitride, when added together, typically result in a sum equal to no more than 35 weight percent of total powder mixture weight. The sum will, however, depend on probable end use applications for ceramic bodies prepared from the powder mixture. For some applications, a sum in excess of 35 weight percent will provide acceptable results. For applications such as ceramic cutting tools requiring resistance to medium temperatures, e.g., those in a range of 900°C to 1200°C., or high fracture toughness, e.g., greater than 7 MPa (m) ½ , or both, a sum within a range of 4 to 35 weight percent provides satisfactory results. The sum is desirably within a range of 4 to 12 weight percent. A sum within a range of from 0.25 to 4 weight percent provides acceptable results for applications such as ceramic engine parts which typically require resistance to high temperatures, e.g., those in a range of 1200°C to 1400°C., or moderate fracture toughness, e.g., 6 to 7 MPa (m) ½ or both. The powder mixture may be prepared in any suitable manner. Ball-milling of the components in powder form is one acceptable manner of preparation. An attritor with zirconia balls is desirably used to prepare a finely-divided suspension of powder mixture components in a carrier medium. Excess carrier medium is beneficially removed via filtration or otherwise to leave an admixture of residue from the suspension and the attritor balls. The admixture is dried prior to separating the attritor balls from the, residue or powder mixture. The powder mixture components need not be added in any particular order. For example, powdered components other than the silicon nitride may be added either singly or in combination to a colloidal suspension of silicon nitride in a carrier medium or vice versa. As an alternative, 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 viscosity is sufficient and acceptable. The quantity suitably yields a solids content of 20 to 50 volume percent, based upon total volume of solids and carrier medium. The solids content range is beneficially 35 to about 45 volume percent. A solids content of less than 20 volume percent provides in a viscosity which is too low for effective mixing of powder mixture components. Similarly, a solids content in excess of 50 volume percent translates to a viscosity which is so high that mixing is difficult, if not impractical. One or more surfactants or disperssants can be added to the suspension to aid in dispersing components of the powder mixture. 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 polyethylene amine, 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 as long as it improves dispersion of powder mixture components. A typical amount of surfactant is in a range from 0.01 to 1.0 weight percent, based upon powder mixture weight. The finely-divided suspension is now ready for processing into greenware. 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. Drying of the admixture of the powder mixture and the attritor balls is desirably accomplished in an oven under a nitrogen purge after removal of excess carrier medium. During the drying process, additional free carrier medium is removed. Temperatures used in of the drying depend 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. The carrier medium is beneficially 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 100 µm. The screen size is usually less than 60 mesh (250 µm); preferably, less than 80 mesh (180 µm). The powder obtained on sieving is the powder mixture used in processing via hot-pressing. Hot-pressing, or heating a powder under pressure, is a preferred method of converting the powder mixture into the silicon nitride ceramic bodies of the present invention. Any standard hot-pressing equipment, such as a graphite die equipped with a heating means and a hydraulic press, is acceptable. Hot-pressing is usually 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 press plates. Any combination of processing temperature and pressure will suffice if it produces the novel silicon nitride ceramic of this invention. Typically, however, the temperature must be carefully controlled because the elongate β-silicon nitride whiskers are found to form in a narrow temperature range. The pressing temperature is beneficially maintained within a range of from 1750°C to 1870°C. The temperature range is desirably 1800°C to 1850°C, preferably from 1820°C to 1840°C. Temperatures of less than 1750°C may retard formation of elongated β-silicon nitride whiskers. At temperatures in excess of 1870°C, silicon nitride may decompose or special pressure equipment may be required to conduct the densification. Skilled artisans 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. The accurate measurement of temperatures as high as 2100°C is technically difficult and some variation may be observed depending on the measurement technique. Temperatures are beneficially measured by use of a tungsten-rhenium thermocouple such as that 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. The pressure is beneficially 3000 to 6000 psig (20 to 40 MPa). The range is desirably 4000 to 5500 psig (27 to 38 MPa) and preferably 4500 to 5200 psig (31 to 36 MPa). A pressure of less than 3000 psig (20 MPa) will not lead to sufficient densification. Although a pressure in excess of 6000 psig (40 mPa) will yield substantially complete densification in a shorter time and at a reduced pressure, such a pressure may inhibit the formation of elongated β-silicon nitride crystals. Time at pressure should be sufficient to bring the powder to essentially complete densification. Ram movement is an acceptable indicator of the extent of densification. As long as the ram continues to move, densification is incomplete. When the ram stops moving for 15 minutes or more, densification is essentially complete, e.g., 99 percent or more of the theoretical value. A halt in ram movement of less than 15 minutes may be an acceptable indicator for a skilled press operator. Thus, the time required for hot--pressing is the time during ram movement plus a safety margin of 10 minutes or more, desirably 15 to 30 minutes. The time is suitably 15 minutes to 2 hours. The range is beneficially 30 to 90 minutes, desirably 45 to 75 minutes. Densification via hot-pressing facilitates formation of silicon nitride ceramic articles which can be used as cutting tools. Hot-pressing lends itself to fabrication of a variety of shapes, one of which is a flat plate. Such a plate may range in size from 2 inches (5 cm) in length by 1.5 ( 4 cm) inches in width by 0.45 inch (1.1 cm) in thickness to 16 inches (40 cm) in length by 16 inches (40 cm) in width by 1.0 inch (2.5 cm) 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 prepared in accordance with this invention is a dense material having no significant porosity. The body has a density which is suitably greater than 95 percent of the theoretical value, beneficially greater than 97 percent of the theoretical value, and desirably greater than 99 percent of the theoretical value. X-ray diffraction analysis of the silicon nitride body showsthat silicon nitride is present in its beta crystalline form, indicating essentially complete alpha to beta conversion during processing. Quite unexpectedly, the β-silicon nitride is predominately present as single crystal, "needle-like" whiskers, as determined by both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The size of the hexagonal β-silicon nitride grains or whiskers is usually in a range of 1 µm to 20 µm in length with a mean diameter of from 0.2 µm to 0.5 µm; desirably in a range of 5 µm to 10 µm in length with a mean diameter 0.3 µm to 0.5 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. Observations of the silicon nitride bodies of the present invention show that the whiskers are homogeneously distributed and randomly oriented throughout the body. The observations also show that the volume occupied by the whiskers is approximately the same in all planes. Using this measurement criterion, the whiskers desirably occupy at least 20 volume percent, preferably at least 35 volume percent, of the total body volume. The whiskers found in the silicon nitride bodies have an unexpectedly high average aspect ratio (ratio of length to diameter). The average aspect ratio is typically at least 2.5, desirably at least 5.5. Because the aspect ratio is measured in a plane, the average aspect ratio is actually 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 35 weight percent of total body weight. The glassy second phase has a typical bulk chemical composition, as determined by neutron activation analysis consisting essentially of 8 to 80 weight percent of silicon dioxide, from 20 weight percent to 90 weight percent conversion aid and 0.1 to 25 weight percent whisker growth enhancing compound. 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 Palmqvist methods. Fracture strength (modulus of rupture) is measured according to the Military Standard 1942b test. Hardness is measured according to the Vickers indentation test. These tests and test sample preparation techniques are described in US-A-4,919,689 at column 10, line 58 through column 12, line 20. The fracture strength at room temperature is suitably at least 650 MPa, desirably 750 MPa to 1250 MPa. At 1000°C, the fracture strength is suitably at least 550 MPa. 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 suitably greater than 6 MPa (m) ½ , beneficially greater than 7 MPa (m) ½ , and preferably greater than 8 MPa (m) ½ . The Vickers hardness number (at room temperature) of the silicon nitride ceramic of this invention is beneficially at least 1325 kg/mm², desirably from 1340 kg/mm² to 1600 kg/mm² and preferably from 1450 to 1650 kg/mm². The silicon nitride ceramic body of this invention exhibits a Palmqvist toughness at room temperature which is beneficially at least 37 kg/mm; desirably 37 to 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. All percentages are weight percent unless otherwise noted. Examples 1-6 A series of six compositions containing 88.0 percent silicon nitride, varying amounts of yttria and silica and 0.3 percent of calcium oxide (CaO), tantalum oxide (Ta₂O₅), hafnium oxide (HfO₂) or niobium oxide (Nb₂O₅) are converted to densified materials by procedures described hereinafter and subjected to Vickers Hardness (kg/mm²), Palmqvist Toughness (kg/mm) and flexure strength (MPa) testing. The contents of each composition are shown in the Table together with the ratio of conversion aid (yttria) to silicon dioxide and test results The silicon nitride (UBE-SN-E-10) contains 1.20 percent oxygen, 0.08 percent carbon, and the following major metallic impurities: 49 parts per million (ppm) iron, 21 ppm Al, and 10 ppm Cr, Ni and Ca. 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 m²/g and the average particle size is about 0.2 µm in diameter. The silica (Cab-O-Sil, Lot IF 054, Grade L-90 (Cabot Corporation)) has a BET surface area of 100 m²/g and an average particle diameter of 0.027 µm. The silica contains the following major metallic impurities: < 2 ppm Al, < 5 ppm Zr, and < 2 ppm of V, Ti and Ca combined. The yttrium oxide (Molycorp) contains less than 10 ppm each of sodium and iron. The Y₂O₃ particles range in size from 2 µm to 5 µm in diameter. The calcium oxide (Aldrich Chemical Co.) contains less than 0.002 percent each of lead and iron. The average CaO particle size is about 3 µm in diameter. The tantalum oxide (Matheson, Coleman and Bell) has a 99.5% purity. The principal impurities are less than 100 ppm niobium and less than 10 ppm each of aluminum, magnesium, iron, copper and tin. Hafnium dioxide (Morton Thiokol, Inc (Alfa Products)) is 99.9% pure. Its main impurity is about 100 ppm zirconium. Niobium oxide (Morton Thiokol, Inc.) is 99.9% pure. Its main impurity is 50 ppm of tantalum. Each of the above-identified compositions is suspended in 160 ml of methanol and agitated at room temperature under air by means of a mechanical attritor for 1 hour to form a slurry of mixed powders. Attrition takes place in a 750 cc tank containing 1220 grams of 3/16 inch (0.48 cm) zirconia balls. The slurry has a solids content of 25 volume percent, based upon total slurry volume. The attritor operates at a speed of 200-250 revolutions per minute (rpm) (21-26 radians per second) for the first six minutes and 300 rpm (31 radians per second) for the last 54 minutes. Following attrition, the slurry is poured through a 100 mesh (150 µm) nylon sieve. Oleic acid (fourteen drops) is added to flocculate the compositions or mixed powders out of suspension. The flocculated powders are dried in an oven at 70°C for a period of 12 hours under a flow of dry nitrogen gas. After drying, the mixed powders are passed through a 60 mesh (250 µm) sieve. The resulting powder mixture contains 88.0 percent silicon nitride, 9.27 percent silica, 2.43 percent yttrium oxide, and 0.3 percent whisker growth aid (tantalum oxide, calcium oxide, hafnium oxide or niobium oxide). A 170g quantity of the resulting powder mixture is poured into a graphite die in the shape of a plate measuring 3 inches (7.6 cm) in length by 2.5 inches (6.4 cm) in width by 0.4 inches (1 cm) in depth. A pressure of 1000 psig (6.9 MPa) 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 (34.5 MPa) 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 (34.5 MPa)for 60 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 "Modern Ceramic Engineering" by D. W. Richerson, Marcel Dekker, 1982, and by stereology analysis from SEM photomicrographs. The density is essentially 100 percent of theoretical. The material is therefore essentially nonporous. Silicon nitride is present essentially in the β crystalline phase, as determined by X-ray diffraction. 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 (based on the total volume of the ceramic body) 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 about 3.0. The fracture strength (Military Standard 1942b) of Example 4 is 128 ksi (882 MPa) at room temperature. The fracture toughness measured by the Chevron notch technique is 7.2 MPa (m) ½ at room temperature. Similar results are expected for Examples 1-3, 5 and 6. The data show that the silicon nitride materials all have satisfactory properties in conjunction with fracture toughness values greater than 6 MPa(m) ½ . Similar results are expected with other compositions disclosed herein.

Claims: A process for preparing a silicon nitride ceramic body having a fracture toughness greater than 6 MPa (m) ½ , as measured by the Chevron notch technique at 23°C, comprising: subjecting a powder mixture comprising (a) silicon nitride in an amount sufficient to provide a ceramic body; (b) free silicon dioxide in an amount sufficient to promote densification of the powder, the sum of said amount and any silicon dioxide present in the silicon nitride being from 1 to 28 weight percent based on the total weight of the powder mixture; (c) a conversion aid in an amount sufficient to attain essentially complete conversion of the starting silicon nitride to β-silicon nitride, said amount being from 0.2 to 29.5 weight percent based on the total weight of the powder mixture, the conversion aid being a source of (i) a Group IIIa element selected from yttrium, scandium, actinium, and lanthanum; or (ii) a Group Ia element selected from lithium, sodium, potassium, rubidium, cesium and francium, and (d) at least one whisker growth enhancing compound in an amount sufficient to promote formation of β-silicon nitride whiskers, the amount being from 0.01 to 5 weight percent based upon total weight of the powder mixture, said compound being a source of (i) a Group Ia element selected from sodium and potassium; copper; (ii) a Group IIa element selected from calcium, strontium and barium; (iii) zinc; (iv) a Group IIIa element selected from scandium and lanthanum; (v) a Group IVa element selected from titanium and zirconium; (vi) a Group Va element selected from vanadium and niobium; (vii) chromium; (viii) manganese; (ix) a Group VIII element selected from iron, cobalt, and nickel; or mixtures of two or more thereof, or an oxide of a Group IIIb element selected from indium, gallium and boron; hafnium; or tantalum, provided, however, that the element used as a whisker growth enhancing compound differs from that used as a conversion aid and that, when the powder mixture contains a source of a Group Ia element or a Group IIa element, the total silicon dioxide content is greater than 6 weight percent based on the total weight of the powder mixture, to conditions of temperature of at least 1750°C and pressure of at least 20 MPa (3000 psig) sufficient to provide for densification of the powder mixture and in situ formation of β-silicon nitride whiskers having an average aspect ratio of at least 2.5 in an amount of at least 20 volume percent, as measured by viewing one plane of the silicon nitride ceramic body by scanning electron microscopy. A process of Claim 1, wherein the conversion aid is an oxide of yttrium, scandium, lanthanum, actinium or sodium and the whisker growth enhancing compound is an oxide of calcium, tantalum, hafnium, niobium or mixtures thereof. A process of Claim 2, wherein the conversion aid is yttrium oxide. A process of any one of the preceding claims, wherein the silicon dioxide and the conversion aid are present in a ratio of conversion aid to total silicon dioxide of 0.25 to 8. A process of Claim 4, wherein said ratio is 0.25 to 1.8. A process of any one of the preceding claims, wherein the amount of silicon nitride is 80 to 97 weight percent based upon total weight of the powder mixture. A process of any one of the preceding claims, wherein the silicon nitride contains less than 10 weight percent β-silicon nitride. A process of any one of the preceding claims, wherein the total amount of silicon dioxide is 3 to 10 percent; the amount of conversion aid is 1.0 to 10.0 percent; and the amount of whisker growth enhancing compound is 0.1 to 1.0 percent, all said percentages being based on the total weight of the powder mixture. A process of Claim 8, wherein the amount of conversion aid is 1.7 to 8.5 percent and the amount of whisker growth enhancing compound is 0.2 to 0.5 percent, both said percentages being based on the total weight of the powder mixture. A process of any one of the preceding claims, wherein the powder mixture further comprises aluminum nitride or boron nitride in an amount of 0.01 to 5 weight percent based upon total weight of the powder mixture. A process of any one of the preceding claims, wherein the temperature is 1750° C to 1870°C and the pressure is 20 MPa (3000 psig) to 40 MPa (6000 psig). A process of Claim 11, wherein the temperature is 1800°C to 1850°C and the pressure is 27 MPa (4000 psig) to 38 MPa (5500 psig). A process of Claim 12, wherein the temperature is 1820°C to 1840°C and the pressure is 31 MPa (4500 psig) to 36 MPa (5200 psig). A silicon nitride ceramic body having a fracture toughness greater than 6 MPa (m) ½ , as measured by the Chevron notch technique at 23°C, obtainable by a process as claimed in Claim 1 and comprising: (a) a crystalline phase of β-silicon nitride of which at least 20 volume percent (based on the total volume of the ceramic body) 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 2.5; and (b) a glassy phase, in an amount not greater than 35 weight percent of total body weight, comprising (1) silicon dioxide, (2) a conversion aid and (3) a β-silicon nitride whisker growth enhancing compound, said conversion aid and whisker growth enhancing compound being as defined in Claim 1. A body of Claim 14, wherein the composition thereof is as defined in any one of Claims 2 to 10. A body of Claim 14 or Claim 15, wherein the glassy phase further comprises aluminum nitride and/or boron nitride in an amount of 0.01 to 15.0 weight percent based upon total glassy phase weight. A body of any one of Claims 14 to 16, wherein the glassy phase comprises 8 percent to 80 percent silicon dioxide, 20 to 90 percent conversion aid, and 0.1 to 25 percent β-silicon nitride whisker growth enhancing compound; the weight ratio of conversion aid to silicon dioxide is 0.25 to 8; and not greater than 10 weight percent of total body weight is present as phases other than said crystalline and glassy phases. A body of any one of Claims 14 to 17, wherein the said aspect ratio is at least 5.5. A body of any one of Claims 14 to 18, wherein the said whiskers are present in an amount of at least 35 volume percent. A cutting tool fabricated from a body of any one of Claims 14 to 19. The use of silicon dioxide as the densification aid in the preparation of a silicon nitride ceramic body having a fracture toughness greater than 6 MPa (m) ½ , as measured by the Chevron notch technique at 23°C, by subjecting a powder mixture comprising (a) silicon nitride in an amount sufficient to provide a ceramic body; (b) a densification aid in an amount sufficient to promote densification of the powder, said amount being from 1 to 28 weight percent based on the total weight of the powder mixture; (c) a conversion aid in an amount sufficient to attain essentially complete conversion of the starting silicon nitride to β-silicon nitride, said amount being from 0.2 to 29.5 weight percent based on the total weight of the powder mixture, the conversion aid being a source of (i) a Group IIIa element selected from scandium, actinium, and lanthanum; or (ii) a Group Ia element selected from lithium, sodium, potassium, rubidium, cesium and francium, and (d) at least one whisker growth enhancing compound different from said conversion aid in an amount sufficient to promote formation of β-silicon nitride whiskers, the amount being from 0.01 to 5 weight percent based upon total weight of the powder mixture, said compound being a source of (i) a Group Ia element selected from sodium and potassium; copper; (ii) a Group IIa element selected from calcium, strontium and barium; (iii) zinc; (iv) a Group IIIa element selected from scandium and lanthanum; (v) a Group IVa element selected from titanium and zirconium; (vi) a Group Va element selected from vanadium and niobium; (vii) chromium; (viii) manganese; (ix) a Group VIII element selected from iron, cobalt, and nickel; or mixtures of two or more thereof, or an oxide of a Group IIIb element selected from indium, gallium and boron; hafnium; or tantalum, to conditions of temperature of at least 1750°C and pressure of at least 20 MPa (3000 psig) sufficient to provide for densification of the powder mixture and in situ formation of β-silicon nitride whiskers having an average aspect ratio of at least 2.5 in an amount of at least 20 volume percent, as measured by viewing one plane of the silicon nitride ceramic body by scanning electron microscopy. A use as claimed in Claim 21, wherein the powder mixture is as defined in any one of Claims 2 to 10.

Inventors: PYZIK ALEKSANDER J, PYZIK BARBARA M, PYZIK, ALEKSANDER J., PYZIK, BARBARA M.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1991
Main classification: C04B35
IPC codes: C04B35