A DENSE, SELF-REINFORCED SILICON NITRIDE CERAMIC PREPARED BY PRESSURELESS OR LOW PRESSURE GAS SINTERING

Abstract: A dense, self-reinforced silicon nitride ceramic prepared by pressureless sintering or low pressure gas sintering. The composition comprises (a) silicon nitride, at least 20 percent of which is in the form of beta -silicon nitride whiskers having a high average aspect ratio, (b) from 2 to 10 weight percent of a glassy grain boundary phase containing magnesium oxide, yttrium oxide, silica, zirconia, and optionally, one oxide selected from the group consisting of calcium, gallium, indium and hafnium oxides, and (c) a second crystalline phase in an amount ranging from 0.5 to 5.0 weight percent comprising zirconium oxide, and (d) crystalline phases of metal zirconium silicide and/or metal zirconium silicon nitride in a combined amount from 0.1 to 3.0 weight percent, wherein the metal is tantalum, calcium, hafnium, gallium or indium, and wherein, when the metal is calcium, hafnium, gallium or indium, the same metal occurs as the metal oxide in glassy phase. The ceramic exhibits high fracture toughness and high fracture strength and has a density of at least 98 percent theoretical.

Description: -1 - A DENSE, SELF-REINFORCED SILICON NITRIDE CERAMIC PREPARED BY PRESSURELESS OR LOW PRESSURE GAS SINTERING This invention pertains to a silicon nitride (Si3N ) ceramic body and a process for preparing the same. 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. Typically, the densification of silicon nitride requires the presence of a densification aid, such as MgO, Y2O3, AI2O3, Ceθ2> Siθ2 > or Zrθ2- A powder mixture is usually prepared comprising silicon nitride and one or more of such densification aids and heated under conditions described hereinafter. The densification aids form a liquid phase into which the silicon nitride is dissolved. Thereafter the dissolved silicon nitride coalesces to form a densified silicon nitride body. Typically, the densification of the aforementioned powder mixture is carried out by one of four general methods: hot pressing (HP), hot isostatic pressing (HIP), pressureless sintering, or low pressure gas sintering. Hot pressing involves the simultaneous application of heat and mechanical pressure to the powder mixture at temperatures high enough to cause densification. Typical hot pressing conditions include a nitrogen atmosphere, a temperature within a range of from 1650°C to 1900°C, and a pressure within a range of from 2000 psig to 5,000 psig (13-8 MPa to 34.5 MPa). The pressure is usually applied to the powder mixture by means of a uniaxial ram press. In the hot isostatic pressing method, the powder mixture is placed in a non-permeable, deformable container to which heat and pressure are applied. In this method, pressure within a range of from 10,000 psig to 30,000 psig (68.9 MPa to 206.8 MPa) is applied equally to all faces of the powder compact, usually by means of a pressurized gas. The temperature of this method typically ranges from 1800°C to 2100°C. Pressureless sintering generally connotes a process of thermally densifying pre-pressed powder compacts without using a container for the compact and without externally applying high pressure to the compact. Thus, the powder mixture is pre-pressed into a desired near net shape and then heated to a high temperature, typically within a range of from 1650°C to 1800°C, under a flow of inert gas such as nitrogen, at one atmosphere pressure. Low pressure gas sintering is similar to pressureless sintering with the exception -3- that low over-pressures of the inert gas up to about 150 psig (1.0 MPa) are applied. Densification of silicon nitride alone normally does not go to completion in the absence of high pressure. For example, the density of a silicon nitride ceramic body might only reach 80 or 90 percent of its theoretical value. A density of 98 percent or more is required in order to achieve a ceramic having excellent mechanical and physical properties, such as high fracture strength and high fracture toughness. In addition, at high temperatures and low pressures, silicon nitride decomposes into elemental silicon and nitrogen. Thus, the commercial need for fully densified silicon nitride ceramics having excellent fracture strength and fracture toughness is currently met predominantly by the hot pressing or hot isostatic pressing of silicon nitride with densification aids. Disadvantageously, however, the hot pressing and hot isostatic pressing methods require complicated high pressure equipment. Moreover, only a ceramic having a simple shape can be prepared. The shape must thereafter be diamond ground into a more complicated net shape. One aspect of this invention is a process of preparing a dense, self-reinforced silicon nitride ceramic body by pressureless or low pressure gas sintering. The process comprises subjecting a powder mixture comprising: (a) silicon nitride in an amount suffi¬ cient to provide a ceramic body; (b) magnesium oxide in an amount suffi¬ cient to promote densification of the powder; (c) yttrium oxide in an amount sufficient to promote the essentially complete conversion of the starting silicon nitride to β-silicon nitride; (d) a catalytic amount of zirconium oxide; and (e) w whisker growth enhancing compound in an amount sufficient to promote the formation of β-silicon nitride whiskers, said compound being an oxide selected from calcium, tantalum, gallium, hafnium or indium; to ^ pressure in the range from atmospheric (1 atmosphere) to 100 atmospheres and to conditions of temperature sufficient to provide for densification to a value which is at least 98 percent of theoretical. In addition, the conditions of temperature and pressure are sufficient to provide in situ formation of β-silicon nitride whiskers having a high average aspect ratio. For the purposes of this invention a "high" average aspect ratio means an average aspect ratio of at least 2.5. In this manner a dense, self-reinforced silicon nitride ceramic body having a Palmqvist toughness greater than 37 kg/mm. Advantageously, the process of this invention provides a nearly fully densified or fully densified silicon nitride ceramic body by pressureless or low pressure sintering. "Nearly fully densified" means that the density of the sintered body is at least 98 percent of the theoretical value. "Fully densified" means that the density of the sintered body is at least 99.5 percent of the theoretical value. The pressureless process of this invention reduces the need for high pressure equipment, eliminates the need -for diamond grinding, and readily provides complicated near net shapes in large numbers. A second aspect of this invention is a silicon nitride ceramic body having a density which is at least 98 percent of the theoretical value and a Palmqvist toughness greater than 37 kg/mm. This silicon nitride comprises: (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; (b) a glassy grain boundary phase, in an amount ranging from 2 weight percent to 10 weight percent of total body weight, comprising magnesium oxide, yttrium oxide, silica, and zirconia, and one oxide selected from calcium oxide, gallium oxide, hafnium oxide or indium oxide; (c) a second crystalline phase comprising zirconia, in an amount ranging from 0.2 weight percent to 3«0 weight percent of total body weight; and (d) at least one crystalline phase comprising metal zirconium silicide and/or metal zirconium silicon nitride, in an amount ranging from 0.1 weight percent to 3-0 weight percent of total body weight, wherein the metal is selected from tantalum, calcium, gallium, hafnium or indium, and wherein when the metal is calcium, gallium, hafnium or indium, the same metal occurs as the oxide in the glassy phase (b) hereinabove. In a third aspect, this invention is a cutting tool comprising the above-identified silicon nitride ceramic body. The silicon nitride ceramic body of this invention exhibits a significantly higher fracture toughness than conventional monolithic silicon nitride ceramics. 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, 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 (alpha/beta) weight ratio. The starting powder desirably contains no greater than 20 weight percent β-silicon nitride. The β-silicon nitride content is preferably no greater than 10 weight percent, more pref¬ erably no greater than 6 weight percent. Generally, the higher the purity of the start¬ ing 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. Although some impurities may be tolerated in the powder, they are preferably minimized as much as possible. Carbon, for example, which is likely to form silicon carbide during sintering, is tolerable in small amounts. Oxygen, present to some extent in the form of silica (Siθ2) > is commmonly 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 amount typically ranges from 1.0 to 5.0 weight percent, based on the total weight of the powder mixture. The amount preferably ranges from 3.0 weight percent to 4.0 weight percent. The silicon nitride starting powder can be of any size or surface area provided that the dense, self- reinforced ceramic body of this invention is obtained by pressureless or low pressure gas sintering. The sizes specified in U.S.-A 4,883,776 at column 4, lines 33-50 provide satisfactory results. The silicon nitride is suitably present in an amount within a range from 90 weight percent to 98 weight percent, based on the total weight of the powder mixture. The range is desirably from 90 to 98 weight percent for low pressure gas sintering and from 92 to 97 weight percent, for pressureless sintering. The range is preferably from 92 to 97 weight percent for low pressure gas sintering and from 93 to 96 weight percent for pressureless sintering. Raw silicon nitride powders cannot be densified to high densities in the absence of densification aids. Thus, a densification aid must be admixed with the silicon nitride starting powder to promote densification of the silicon nitride during processing. The densification aid is suitably magnesium oxide. Magnesium-containing glass forms a liquid phase at a temperature between 1300°C and 1500°C into which the α-silicon nitride dissolves. The rate of mass transport of α-silicon nitride is usually quite rapid in the magnesium-based liquid phase. Any amount of magnesium oxide is acceptable which produces by pressureless or low pressure gas sintering the dense, self-reinforced silicon nitride ceramic body of the invention. The amount is suitably within a range of from 0.5 to 3-0 weight percent based on the total weight of the powder mixture. The range is desirably from 0.7 weight percent to 2.5 weight percent, preferably from 0.9 weight percent to 2.0 weight percent. In addition to a densification aid, powder mixtures that result in the dense, self-reinforced silicon nitride ceramic bodies of the present invention must contain a conversion aid. The conversion aid is suitably yttrium oxide. The conversion aid forms a glassy grain boundary 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 β-silicon nitride, in the form of elongated, single crystal whiskers or grains, 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 synonymous and may be used interchangeably. Any amount of yttrium oxide 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 dense, self-reinforced silicon nitride ceramic body of the invention by pressureless or low pressure gas sintering. The amount is desirably within a range of from 1.0 weight percent to 6.0 weight percent based on the total weight of the powder mixture. The range is preferably from 2 weight percent to 4 weight percent. Surprisingly, the weight ratio of conversion aid (Y2O3) to densification aid (MgO) has been found to affect (a) the density of the ceramic body, and (b) the fracture toughness of the ceramic body provided that the whisker growth enhancing compound is also present in the powder mixture. Any weight ratio of conversion aid to densification aid is acceptable providing (a) the density achieved is at least 98 percent of theoretical, and (b) the fracture toughness shows an improvement over the fracture toughness value of 5 MPa (m)* for non- reinforced, monolithic silicon nitride. Typically, the weight ratio of yttria to magnesia is suitably from 5:1 to 1:2. The weight ratio is preferably within a range of from 4:1 to 1:1. 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 powder mixture must also contain zirconium oxide, a component that serves several functions. First, zirconium oxide catalyzes the formation of elongated β-silicoπ nitride whiskers. In this sense, it acts similarly to a fourth ingredient, the whisker growth enhancing compound. Second, zirconium oxide acts as a densification aid. Third, the addition of zirconium oxide broadens the compositional range of the invention wherein full density can be achieved. Fourth, zirconium oxide acts as a getter for metallic impurities, thereby cleaning the glassy phase. For example, iron and chromium react with zirconia to form complex structures which separate as crystalline phases. In the absence of zirconia, metallic impurities remain in the glassy phase thereby lowering the melting point and the fracture strength of the ceramic. Because such a multiplicity of functions are performed by the zirconium oxide component, it is therefore referred to as a "catalyst" for the purposes of this invention. Any catalytic amount of zirconium oxide is acceptable provided it is sufficient to achieve the functions noted hereinbefore and promote formation of the dense, self-reinforced silicon nitride composition of this invention by pressureless or low pressure gas sintering. The amount suitably ranges from 0.2 weight percent to 5.0 weight percent based on the total weight of the powder mixture. The amount preferably ranges from 0.5 weight percent to 1.0 weight percent. U.S.-A 4,891,342 teaches that zirconia is converted on sintering in a nitrogen atmosphere to zirconium nitride. The presence of zirconium nitride in a silicon nitride ceramic disadvantageously lowers its high temperature physical properties. The lowering is related to the re-conversion of zirconium nitride to zirconium oxide in an oxidizing atmosphere. The re¬ conversion is accompanied by an increase of about 30 percent in the volume of the ceramic. Therefore, sintered bodies containing zirconium nitride are easily cracked. Unexpectedly, it has now been discovered, in the process of this invention, that zirconium oxide is stabilized by the glass formed from magnesia, yttria, silica, and a whisker growth enhancing compound. Therefore, essentially no undesirable zirconium nitride is formed during the sintering process of this invention. Consequently, the composition of this invention possesses excellent physical properties, specifically fracture strength and fracture toughness. The magnesium oxide and zirconia are present in amounts sufficient to provide a magnesium oxide to zirconia weight ratio that produces the dense, self- reinforced silicon nitride composition of this invention. The weight ratio suitably ranges from 15:1 to 1:10, desirably from 10:1 to 1:1, preferably from 5:1 to 1:1. The fourth component of the powder mixture is a whisker growth enhancing compound. This compound helps provide a ceramic body of superior fracture toughness and fracture strength. Any amount of the whisker growth enhancing compound in the starting powder is acceptable provided it promotes the formation of β-silicon nitride whiskers and produces the dense, self-reinforced silicon nitride ceramic body of this invention by pressureless or low pressure gas sintering. The amount is suitably within a range of from 0.01 weight percent to 3.0 weight percent, based on the total weight of the powder mix¬ ture. The range is desirably from 0.1 weight percent to 1.0 weight percent, preferably from 0.15 weight percent to 0.5 weight percent. U.S.-A 4,883,776 teaches that calcium, particularly calcium oxide, provides advantages as a whisker growth enhancing compound when silicon nitride powder compositions are hot-pressed into finished ceramic bodies. Calcium oxide is also beneficially employed in the pressureless sintering or low pressure gas sintering processes of this invention. Silicon nitride powders doped with calcium oxide in amounts ranging from 0.01 weight percent to 3.0 weight percent yield desirable results. Commercial silicon nitride powders are believed to contain only 100 ppm or less of calcium oxide. Elements other than calcium also promote growth of β-silicon nitride whiskers and provide, by pressureless sintering or low pressure gas sintering, silicon nitride ceramic bodies having a fracture toughness (Palmqvist) greater than about 37 kg/mm. Satisfactory results are obtained with tantalum oxide, hafnium oxide, gallium oxide or indium oxide, also in amounts ranging from 0.01 weight percent to 3.0 weight percent. The whisker growth enhancing compound(s) and oxides of magnesium, yttrium, and zirconium are desirably in the form of powders that are pure and sufficiently small in size. Commercially available materials identified for use as whisker growth enhancing compounds, densification aids and conversion aids, particularly the oxide powders, generally contain less than 20 ppm each of assorted impurities. These levels of impurities are tolerable. Levels of impurities of 0.5 weight percent or more are not recommended as they may cause a change in the final ceramic composition and properties. Small powder particles disperse more readily than large powder particles. The oxide powders used herein desirably have an average particle size no greater than 5 μm in diameter. In the process of this invention, the starting silicon nitride powder is blended with a combination of the densification aid, conversion aid, zirconium oxide catalyst, and whisker growth enhancing compound to obtain a powder mixture which is used in preparing the densified, tough silicon nitride ceramic body of this invention. Ordinarily, the total quantity of magnesium oxide, yttrium oxide, zirconium oxide, whisker growth enhancing compound is no greater than 10 weight percent of the total weight of the powder mixture. The total quantity will depend, however, on probable end use applications for sintered ceramics prepared from the powder mixture. Preferably, however, the total quantity ranges from 3 weight percent to 7 weight percent. The powder mixture containing silicon nitride, magnesium oxide, yttrium oxide, zirconium oxide, and a whisker growth enhancing compound is suitably prepared by conventional methods. Ball-milling of the components in powder form is one acceptable method. Alternatively, powdered zirconia need not be added to the powder mixture, but can be obtained from the use of zirconia balls during attritor mixing of the other powder components. The preferred method of preparing the powder mixture comprises using an attrition mixer with zirconia balls to prepare a finely-divided suspension of powder mixture components 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. Preparation of the finely-divided suspension requires no particular order of addition of the compo¬ nents. For example, powdered components other than silicon nitride can be added either individually or simultaneously 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 is employed. The preparation is typically conducted in a large vessel at room temperature (taken as 23°C) under air with vigorous stirring. If desired, an ultrasonic vibrator may be used in a supplementary manner to break down smaller agglomerates. The carrier medium may be any inorganic or organic compound that is a liquid at room temperature and atmospheric pressure and in which ceramic powders can be dispersed. Examples of suitable carrier media include water; alcohols, such as methanol, ethanol or isopropanol; ketones, such as acetone or methyl ethyl ketone; aliphatic hydrocarbons, such as pentane or hexane; and aromatic hydrocarbons, such as benzene and toluene. The carrier medium is desirably an organic liquid, preferably 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. The quantity of carrier medium desirably provides a solids content in a range of from 15 volume percent to 40 volume percent. The range is preferably from 20 volume percent to 30 volume percent. Below 15 volume percent, the viscosity of the solid suspension may be too low and the deagglomeration mixing may be ineffective. Above 40 volume percent, the viscosity may be too high, and the deagglomeration mixing may be difficult. To aid in dispersing the powder mixture components, 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. 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 is in a range of from 0.01 to 1.0 weight percent of the powder mixture. The finely-divided suspension is converted into greenware by conventional techniques such as slip- -casting. Alternatively, the suspension can be dried into B. powder, ground, and poured into a mold of the desired shape. Drying is accomplished by conventional means, such as spray-drying or oven drying under a nitrogen purge. The admixture of the powder mixture and the attritor balls is preferably dried in an oven under a nitrogen purge after removal of excess carrier medium. Temperatures used in drying depend on the boiling point of the carrier medium employed. Drying is typically conducted at a temperature just below the boiling point of the carrier medium under atmospheric pressure. Preferably, the carrier medium is methanol and the temperature of drying is about 60°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 60 mesh (Tyler equipment) (250 μm) ; more preferably, less than 80 mesh (Tyler equipment) (180 μm). The powder obtained on sieving is the powder mixture suitable for use in pressureless or low pressure gas sintering. After preparing the dried powder mixture, it is then dry pressed at room temperature into the desired greenware in near net shape. Any conventional dry pressing equipment is suitable, such as a hydraulic press having a ram moving uniaxially and perpendicular to the die plates or any common isostatic press. Thereafter, the compacted powder is removed from the dry pressing equipment and is pressureless sintered or low pressure gas sintered in accordance with the process of this invention. Any conventional equipment suitable for pressureless sintering or low pressure gas sintering is acceptable provided the dense, self-reinforced silicon nitride ceramic of this invention is formed. Typically, the compacted powder is placed into a graphite crucible that is positioned in a silicon nitride powder bed, or, preferably, a powder bed of the same composition as the powder mixture. The bed is maintained under an inert atmosphere, preferably nitrogen gas, to prevent the oxidation and decomposition of silicon nitride at high temperatures. The inert atmosphere may be static or flowing, preferably static. If a flow is employed, it is suitably very low, for example in a range of from 0.01 1/min to 0.1 1/min. Typically, the pressure during sintering ranges from atmospheric (1 atmosphere (atm)) to 100 atm. The pressure desirably ranges from 1 atm to 10 atm. The pressure is preferably about 1 atm. At atmospheric pressure ( 1 atm) , the process is referred to as "pressureless sintering." At superatmospheric pressures ranging up to 10 atm, or at most 100 atm, the process is referred to as "low pressure gas sintering." Any sintering temperature will suffice provided the dense, self-reinforced silicon nitride ceramic of this invention is formed. Typically, however, the temperature must be carefully controlled for two reasons. First, the elongated β-silicon nitride whiskers are found to form in a narrow temperature range. Second, silicon nitride is prone to decompose at high temperatures and low pressures. The temperature is desirably maintained within a range of from 1650°C to 1825°C. A preferred temperature range is from 1700°C to 1750°C. Below 1650°C, the formation of elongated β-silicon nitride whiskers may be retarded. Above 1825°C, the silicon nitride may decompose. Accurate measurement of high temperatures, such as 1650°C to 1825°C, is technically difficult. Some variation in the preferred temperature range may be observed depending on the method employed in measuring the temperature. The temperatures are desirably measured using a tungsten- -rheniu thermocouple, obtained from and calibrated by the Omega Company. The amount of time that greenware is sintered depends upon the specific powder composition. It should, however, be sufficient to bring the greenware to nearly full or full densification. For the purposes of this invention, "nearly full densification" refers to a density which is at least 98 percent of the theoretical value, whereas "full densification" refers to a density which is at least 99.5 percent of the theoretical value. Typically, it takes from 2 hours to 3 hours to heat the sample up to the sintering temperature. At the desired temperature the sample is sintered for a time preferably within a range of from 3 hours to 36 hours, preferably from 12 hours to 15 hours. Afterwards, the sample is cooled to room temperature over a period of 2 hours. The pressureless sintering and low pressure gas sintering methods, described hereinbefore, allow for the formation of silicon nitride ceramic articles which can be used as cutting tools. A variety of shapes, such as, flat plates or tubes can be prepared. These plates may range in size from 6 inches (15.2 cm) in length by 6 inches (15.2 cm) in width by 0.7 inch (1.8 cm) in thickness to 16 inches (40.6 cm) in length by 16 inches (40.6 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 compacting plaques. Tubes may range from 4 inches (10.2 cm) in length by 0.5 inch (1.3 cm) in diameter to 8 inches (20.3 cm) in length by 1.5 inches (3.8 cm) in diameter. Cutting tools can be fabricated by slicing and grinding these plates and tubes into a variety of cutting tool shapes. The pressureless and low pressure sintering methods of this invention typically allow for formation of near net shapes. The silicon nitride ceramic body produced by the process of this invention is a dense material having no significant porosity. Densification desirably proceeds to greater than 98 percent of the theoretical value, preferably to greater than 99 percent of the theoretical value, and more preferably to greater than 99.5 percent of the theoretical value. Those skilled in the art will recognize that the theoretical density will vary depending upon the composition of the ceramic. As measured by X-ray diffraction, the silicon nitride is present in the beta crystalline form. This indicates essentially complete alpha to beta conversion during processing. Quite unexpectedly, the β-silicon nitride is present predominately as single crystal, "needle- -like" whiskers or elongated grains, 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 a range of from 1 μm to 20 μm in length with a mean diameter of from 0.2 μm to 1.5 μm. A preferred size is from 3 μm to 10 μm in length with a mean diameter from 0.3 μm to 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 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 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 2.5; preferably, at least about 4.0. 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 silicon nitride crystalline phase, the composition of this invention contains a glassy grain boundary phase in an amount ranging from 2 weight percent to 10 weight percent of the total weight of the ceramic. The amount preferably ranges from 5 weight percent to 7 weight percent of the total weight. The glassy phase comprises magnesium oxide, yttrium oxide, silica and zirconia. The amount of zirconia may range from traces to a significant portion of all of the zirconia available. The glassy phase also contains one additional oxide selected from calcium oxide, gallium oxide, hafnium oxide or indium oxide. The composition of this invention also contains a second crystalline phase in an amount ranging from 0.5 weight percent to 5.0 weight percent of the total weight of the ceramic. The second crystalline phase is zirconium oxide. Two other crystalline phases may be present in a combined one or amounts totaling not more than 3.0 weight percent. Specifically, these other crystalline phases comprise metal zirconium silicide and metal zirconium silicon nitride, wherein the metal is selected from tantalum, calcium, gallium, hafnium or indium. When the metal is calcium, gallium, hafnium or indium, the same metal occurs in the glassy phase as the metal oxide. When the metal is tantalum, the glassy phase does not contain a detectable amount of tantalum oxide. Specific compositions covered by this invention comprise the aforementioned beta-silicon nitride crystalline phase and further comprise a glassy phase and additional crystalline grain boundary phases selected from: (A) A glass grain boundary phase containing magnesium oxide, yttrium oxide, silica, and zirconia; a second crystalline phase of zirconia; and no metal zirconium silicide or metal zirconium silicon nitride phases being present; (B) A glassy grain boundary phase containing magnesium oxide, yttrium oxide, silica and zirconia; a second crystalline phase of zirconia; and one or two additional phases selected from tantalum zirconium silicide or tantalum zirconium silicon nitride; (C) A glassy grain boundary phase containing magnesium oxide, yttrium oxide, silica, zirconia and calcium oxide; a second crystalline phase of zirconia; and one or two additional phases selected from calcium zirconium silicide or calcium zirconium silicon nitride; (D) A glassy grain boundary phase containing magnesium oxide, yttrium oxide, silica, zirconia and gallium oxide; a second crystalline phase of zirconia; and one or two additional phases selected from gallium zirconium silicide or gallium zirconium silicon nitride; (E) A glassy grain boundary phase containing magnesium oxide, yttrium oxide, silica, zirconia and hafnium oxide; a second crystalline phase of zirconia; and one or two additional phases selected from hafnium zirconium silicide or hafnium zirconium silicon nitride; and (F) A glassy grain boundary phase containing magnesium oxide, yttrium oxide, silica, zirconia and indium oxide; a second crystalline phase of zirconia; and one or two additional phases selected from indium zirconium silicide or indium zirconium silicon nitride. Preferably, the composition comprises the aforementioned beta-silicon nitride crystalline phase, a glassy grain boundary phase containing magnesium oxide, yttrium oxide, silica and zirconia; a second crystalline phase of zirconia; and one or two additional phases selected from tantalum zirconium silicide or tantalum zirconium silicon nitride. The mechanical properties of the self-rein¬ forced silicon nitride ceramic body are readily measured by use of standard tests. In particular, fracture toughness (Kic) 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) mea¬ sures the resistance of the material to fracture under a load. Fracture strength is defined as the maximum unit stress at which fracture occurs. Test bars are prepared and tested as described in U.S.-A 4,883,776 at column 10, lines 61-68. Typically, the fracture strength at room temperature is at least 95 ksi (650 MPa). Preferably, the fracture strength at room temperature ranges from 120 ksi (825 MPa) to 150 ksi (1050 MPa). At 1200°C, the fracture strength is typically at least 65 ksi (450 MPa). Toughness is defined and measured as described in U.S.-A 4,883,776 at column 11, lines 14-27. The room temperature (taken at 23°C) fracture toughness of the silicon nitride ceramic body of this invention is greater than 6 MPa (m)*. The room temperature fracture toughness is desirably greater than 7 MPa (m)*, preferably greater than 8 MPa (m)*. The room temperature fracture toughness more preferably ranges from 8 MPa (m)* to 11 MPa (m)*. 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. Samples are prepared and tested as described in U.S.-A 4,883,776 at column 11, lines 44-64. The Vickers hardness number of the silicon nitride ceramic of this invention is at least 1400 kg/mm2 at room temperature. The Vickers hardness number desirably ranges from 1425 kg/mm 2 to 1700 kg/mm 2 at room temperature; preferably from 1550 kg/mm 2 to 1700 kg/mm 2 . Palmqvist toughness testing is accomplished as described in U.S.-A 4,883,776 at column 12, lines 3-13. The silicon nitride ceramic body of this invention benefically exhibits a Palmqvist toughness at room temperature of at least 37 kg/mm. The Palmqvist toughness at room temperature is desirably within a range of from 37 kg/mm to 52 kg/mm, preferably from 40 kg/mm to 46 kg/mm. The following examples serve to illustrate the novel dense, self-reinforced silicon nitride composition of this invention and the method of preparing the novel silicon nitride ceramic. The examples are not intended to be limiting of the scope of this invention. All percentages are weight percent unless otherwise noted. The silicon nitride powder, commercially available from Ube Industries, Ltd. under the trade designation SN-E10, contained 1.6 percent oxygen, less than 100 ppm Cl, less than 100 ppm Fe, less than 50 ppm Ca, and less than 50 ppm Al. It had a crystallinity of greater than 99.5 percent, an alpha-SigNj j content greater than 95 percnet, and a surface area of 11.2 m2/g. Moly Corp supplied yttria, and Baker Incorporated supplied magnesia and calcia. Zirconia, in the form of zirconia balls, was available from Union Process. Zirconia powder was available from Toyo Soda (type TZ- 3Y20A). Tantalum oxide, gallium oxide, hafnium oxide and indium oxide were available from Alfa Products. Example 1 (a) Powder Preparation A series of dried powder mixtures was prepared according to the following general method: Desired amounts of silicon nitride, magnesia, yttria, and tantalum oxide were 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 350 revolutions per minute (rpm) for a mixing time of 1 hour. Methanol was used as the mixing medium in an amount sufficient to provide 20 weight percent solids. After agitation, excess methanol was removed by vacuum filtration. The mixture was then dried under a flow of dry nitrogen gas. After drying, the mixture was separated from the zirconia balls using a 30 mesh (U.S. Series) (600 μm) stainless steel sieve and subsequently passed through 40 mesh (U.S. Series) (425 μm) and 60 mesh (U.S. Series) (250 μm) stainless steel sieves to provide the dried powder mixtures set forth in Table I. TABLE I* * Percentages of components are based on weight percent in the powder mixture. Powder mixtures were sintered for 12 hr at 1750°C and 1 atm. (b) Dry Pressing and Pressureless Sintering The dried powder mixtures prepared hereinabove were dry pressed and pressureless sintered according to the following general procedure: A sample of the dried powder mixture (80 g) was poured into a stainless steel die 2 inches (5.1 cm) in diameter by 0.5 inch (1.3 cm) in thickness. The powder was dry pressed at room temperature under a pressure of 5 psi (35 kPa) to form a disc having a density of about 49-50 percent of the theoretical value. The disc was then isostatically pressed at room temperature under a pressure of 45 ksi (310.3 MPa) to achieve a density of about 55 percent of theoretical. The resulting pressed disc was placed in a graphite crucible. The crucible was placed in a silicon nitride powder bed of the same composition as the compacted powder mixture. The disc was then sintered for 12 hours in a 4 inch (10.1 cm) ASTRO graphite furnace at 1750°C under a static nitrogen atmosphere of 1 atm pressure. The density of the silicon nitride ceramic discs, prepared hereinabove, was 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 with the results shown in Table I. The powder compositions of Example 1 comprising silicon nitride, magnesium oxide, yttrium oxide, zirconium oxide and tantalum oxide can be pressureless sintered to form ceramic bodies having densities at least 98 percent of theoretical. (c) Phases and Morphology A dried powder mixture was prepared as described hereinabove containing 94.8 percent silicon nitride, 2.9 percent yttrium oxide, 1.6 percent magnesium oxide, 0.17 percent tantalum oxide, and 0.52 percent zirconium oxide. The powder was dry pressed and pressureless sintered to achieve a ceramic having a density that was 100 percent of the theoretical value. Silicon nitride was present essentially in the β crystalline phase, as determined by X-ray diffraction. The icrostructure of the ceramic was determined by scanning electron microscopy (SEM), as viewed in a plane. About 35 volume percent of the silicon nitride appeared in the form of elongated whiskers or grains having an average diameter between 0.5 μm and 1.0 μm and having an average aspect ratio above 2.5. The bulk chemical composition of the ceramic was determined by neutron activation analysis, and was found to contain about 92 percent silicon nitride, about 6 percent glassy grain boundary phase, and about 2 percent other crystalline phases. The glassy grain boundary phase had an average glass composition, as measured by TEM, as follows: 47 ± 10 percent yttrium oxide, 32 ± 11 percent silica, 17 ± 4 percent magnesia, and 4 percent nitrogen. Traces of calcium oxide impurity, coming from the magnesium oxide, and traces of zirconium oxide were detected in the glassy grain boundary phase. A second crystalline phase of zirconium oxide was found in a concentration of 0.1 percent. In addition, two minor crystalline phases comprising zirconium tantalum silicide and zirconium tantalum silicon nitride were found in a total concentration of 2.0 percent based on total weight. At room temperature, the ceramic had a fracture toughness, as measured by the Palmqvist technique, of 43.3 kg/mm; a Kic fracture toughness, as measured by the Chevron test, of 7.8 MPa«m^; a flexural strength of 120 ksi (827 MPa); and a Vickers hardness of 1550 kg/mm2. At 1200°C the fracture strength was 79 ksi (543 MPa). Example 2 A silicon nitride powder mixture containing silicon nitride, magnesium oxide, yttrium oxide, zirconium oxide, and tantalum oxide was prepared and pressureless sintered as in Example 1(c), with the exception that the die was in the form of a plate having the dimensions 7 inches (17.8 cm) in length by 7 inches (17.8 cm) in width by 1.1 inch (2.8 cm) in thickness. A ceramic body was obtained with a density that was 99.99 percent of the theoretical value. In addition, the ceramic contained elongated grains of β-silicon nitride having an average aspect ratio about 2.5 , a second crystalline phase of zirconium oxide, two minor crystalline phases comprising tantalum zirconium silicide and tantalum zirconium silicon nitride, and a glassy grain boundary phase containing magnesium oxide, yttrium oxide, and silica. This example shows that the powder composition of Example 1(c) can be pressureless sintered into large, fully densified ceramic plates. Example 3 A silicon nitride powder mixture containing 94.8 percent silicon nitride, 2.9 percent yttrium oxide, 1.6 percent magnesium oxide , 0.5 percent zirconium oxide, and 0.2 percent tantalum oxide was prepared and dry pressed into discs as described in Example 1. The discs were pressureless sintered under 1 atmosphere of flowing nitrogen with the results shown in Table II. TABLE II* * Samples were heated over a 2 hr period to the sintering temperature, sintered for time shown, and cooled to room temperature over a period of 1.5 hours. The silicon nitride powder composition of Example 3 was pressureless sintered into ceramic bodies having densities of at least 98 percent of theoretical. The operative sintering temperature ranged between 1700°C and 1800°C. Palmqvist toughness values ranged between 38 kg/mm and 45 kg/mm and Vickers hardness values ranged between 1420 kg/mm2 and 1580 kg/mm2 are observed. Example 4 A powder mixture was prepared according to the procedure of Example 1, with the exception that the composition comprised 95.8 percent silicon nitride, 2.3 percent yttria, 1.3 percent magnesia, 0.5 percent zirconia and 0.1 percent calcium oxide. The powder mixture was dry pressed into discs which were pressureless sintered at a variety of sintering temperatures and times, as in Example 1. The resulting densities are set forth in Table III. TABLE III Samples were heated over a 2 hr period to the sintering temperature, sintered under 1 atm flowing N2 at temperature and time shown, and cooled to room temperature over a period of 1.5 hours. The data shown in Table III demonstrates that the powder compositions of Example 4 can be pressureless sintered into ceramic bodies having full densification. The optimum sintering conditions comprise a temperature in the range from 1700°C to 1750°C and sintering time of at least 12 hours, and preferably, at least 18 hours. X-ray diffraction indicates that α-silicon nitride is completely converted to β-silicon nitride, the latter forming elongated grains with average diameter of about 0.7 μm. The Palmqvist toughness, measured at room temperature, of samples E-4(a-e) is 41 kg/mm, while the measured Vickers hardness falls in a range of from 1450 kg/mm2 to 1500 kg/mm2. Example 5 A series of powder mixtures was prepared as in Example 1, with the exception that the composition comprised 95.3 percent silicon nitride, 2.9 percent yttria, 1.6 percent magnesia, 0.2 percent tantalum oxide, and a concentration of zirconia as shown in Table IV. TABLE IV The powder mixtures were pressed into discs and pressureless sintered at 1750°C for 12 hours as in Example 1 with the results set forth in Table IV. The data in Table IV show that silicon nitride ceramic bodies having a sintered density of at least about 98 percent of theoretical are prepared by pressureless sintering the powder mixtures of Example 5. Moreover, zirconia is required in an amount ranging from 0.2 percent to 0.5 percent in order to achieve a sintered density above 99.0 percent of theoretical. Example 6 A powder mixture was prepared, acording to the procedure of Example 1, having a composition comprising silicon nitride (94.8 percent), yttria (2.9 percent), magnesia (1.6 percent), zirconia (0.5 percent) and indium oxide (0.2 percent). The powder was pressureless sintered at 1750°C for 12 hours. A silicon nitride ceramic body was obtained having a sintered density of 99.7 percent of theoretical value and having a fracture strength, at room temperature, of 115 ksi (793 MPa), a Vickers harness of 1580 kg/mm2, and a Palmqvist toughness of 41.2 kg/mm. Example 7 A powder mixture was prepared, according to the procedure of Example 1, having a composition comprising silicon nitride (94.8 percent), yttria (2.9 percent), magnesia (1.6 percent), zirconia (0.5 percent) and gallium oxide (0.2 percent). The powder was pressureless sintered at 1750°C for 12 hours. A silicon nitride ceramic body was obtained having a sintered density of 100.0 percent of theoretical and having a Palmqvist toughness, at room temperature, of 40.2 kg/mm, a Vickers hardness of 1575 kg/mm2, and a fracture strength of 120 ksi (827 MPa). Example 8 A powder mixture was prepared, according to the procedure of Example 1, with a composition comprising silicon nitride (94.8 percent), yttria (2.9 percent), magnesia (1.6 percent), zirconia (0.5 percent) and hafnium oxide (0.2 percent). The powder was pressureless sintered at 1750°C for 12 hours. A silicon nitride ceramic body having a sintered density of 98.1 percent of theoretical was obtained.

Claims: CLAIMS : 1. A process of preparing a dense, self- reinforced silicon nitride ceramic body by pressureless or low pressure gas sintering, the process comprising subjecting a powder mixture comprising: (a) silicon nitride in an amount suffi¬ cient to provide a ceramic body; (b) magnesium oxide in an amount suffi¬ cient to promote densification of the powder; (c) yttrium oxide in an amount sufficient to promote the essentially complete conversion of the starting silicon nitride to β-silicon nitride; (d) a catalytic amount of zirconia; and (e) a whisker growth enhancing compound in an amount sufficient to promote the formation of β-silicon nitride whiskers, said compound being an oxide selected from calcium oxide, tantalum oxide, gallium oxide, hafnium oxide or indium oxide; to a pressure in the range from atmospheric (1 atmosphere) to 100 atmospheres and to conditions of temperature sufficient to provide for densification to a value which is at least 98 percent of theoretical and sufficient to provide in situ formation of β-silicon nitride whiskers having a high average aspect ratio, such that the ceramic has a Palmqvist toughness greater than 37 kg/mm. 2. A process as claimed in Claim 1 wherein the amount of silicon nitride ranges from 90 to 98 weight percent, the amount of magnesium oxide ranges from 0.5 to 3.0 weight percent, the amount of yttrium oxide ranges from 1.0 to 6.0 weight percent, the amount of zirconium oxide ranges from 0.2 to 5.0 weight percent, and the amount of whisker growth enhancing compound ranges from 0.01 to 3.0 weight percent, all weight percentages being based on total weight of the powder mixture. 3. A process as claimed in any one of the preceding claims wherein the temperature is in a range from 1650°C to 1825°C and the pressure is within a range of from 1 atm to 10 atm. 4. A process as claimed in any one of the preceding claims wherein yttrium oxide and magnesium oxide are present in a weight ratio of yttrium oxide or magnesium oxide within a range of from 5:1 to 1:2 and the magnesium oxide and zirconium oxide are present in a weight ratio of magnesium oxide or zirconium oxide within a range of from 15:1 to 1:10. 5. A silicon nitride ceramic body having a density which is at least 98 percent of the theoretical value and a toughness greater than 37 kg/mm, as measured by the Palmqvist technique, 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; (b) a glassy grain boundary phase, in an amount ranging from 2 to 10 weight percent of the total weight, comprising magnesium oxide, yttrium oxide, silica, zirconia and an oxide selected from calcium oxide, gallium oxide, hafnium oxide or indium oxide; (c) a second crystalline phase comprising zirconia, in an amount ranging from 0.5 to 5.0 weight percent of the total weight; and (d) one or two crystalline phases, in an amount ranging from 0.1 to 3.0 weight percent, comprising metal zirconium silicide and/or metal zirconium silicon nitride wherein the metal is selected from tantalum, calcium, gallium, hafnium or indium, and wherein, when the metal is calcium, gallium, hafnium or indium, and wherein, when the metal is calcium, gallium, hafnium, or indium, the same metal occurs as the oxide in the glassy phase (b) hereinabove. 6. A composition as claimed in Claim 5 wherein the glassy grain boundary phase contains magnesium oxide, yttrium oxide, silica and zirconia and wherein two additional crystalline phases of tantalum zirconium silicide and tantalum zirconium silicon nitride are present. 7. A composition as claimed in Claim 5 wherein the glassy grain boundary phase contains magnesium oxide, yttrium oxide, silica, zirconia and calcium oxide, and wherein two additional crystalline phases of calcium zirconium silicide and calcium zirconium silicon nitride are present. 8. A composition as claimed in Claim 5 wherein the glassy grain boundary phase contains magnesium oxide, yttrium oxide, silica, zirconia and gallium oxide, and wherein two additional crystalline phases of gallium zirconium silicide and gallium zirconium silicon nitride are present. 9. A composition as claimed in Claim 5 wherein the glassy grain boundary phase contains magnesium oxide, yttrium oxide, silica, zirconia and hafnium oxide, and wherein two additional crystalline phases of hafnium zirconium silicide and hafnium zirconium silicon nitride are present. 10. A composition as claimed in Claim 5 wherein the glassy grain boundary phase contains magnesium oxide, yttrium oxide, silica, zirconia and indium oxide, and wherein two additional crystalline phases of indium zirconium silicide and indium zirconium silicon nitride are present. 11. A composition as claimed in any one of Claims 5-10 wherein the Vickers hardness is at least 1400 kg/mm2, the flexural strength is at least 95 ksi (650 MPa) and the fracture toughness, as measured by the Chevron technique, is at least 6 MPa«ml/2. 12. A cutting tool prepared from the composition of anyone of Claims 5-11.

Inventors: PYZIK ALEKSANDER J, PYZIK, ALEKSANDER, J.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1992
Main classification: C04B35
IPC codes: C04B35