SILICON NITRIDE SINTERED BODIES AND PROCESS FOR MANUFACTURING THE SAME

Abstract: Silicon nitride sintered bodies of high strength at high temperature comprise not less than 70% by weight of Si₃N₄ and essentially consist of Si, O, N and a remainder being at least two of the rare earth elements Y, Er, Tm, Yb and Lu. The ratio of the molar amount of all the rare earth elements when calculated as Ln₂O₃ in which Ln is Y, Er, Tm, Yb or Lu to the molar amount of oxygen when calculated as SiO₂ is in a range from 0.4 to 1.3. The amount of oxygen when calculated as SiO₂ is obtained by subtracting the amount of oxygen in all the Ln₂O₃ from the amount of oxygen in the sintered body and expressing the remaining amount of oxygen as SiO₂ . Each of the rare earth elements is not more than 95 mol% of the sum of all the rare earth elements in the sintered body. The intergranular phase of the Si₃N₄ grains is substantially a crystalline phase of which not less than 50% is a crystalline phase of an apatite structure. Such bodies can be made by pressureless sintering or sintering in a pressurized atmosphere.

Description: The present invention relates to silicon nitride sintered bodies having high strength at high temperatures and a process for manufacturing such sintered bodies through pressureless sintering or sintering under a pressurized atmosphere. Silicon nitride sintered bodies containing at least one oxide of an element in Group III a (including a rare earth oxide or oxides)as well as processes for manufacturing such sintered bodies have been proposed. For instance, Japanese Patent Publication No. 48-7,486 discloses a process for manufacturing silicon nitride sintered bodies, which comprises mixing not less than 85 mol% of Si₃N₄ and not more than 15 mol% of at least one kind of an oxide selected from oxides of elements in Group III a , molding the resulting mixture and then sintering the molding in a non-oxidative atmosphere. Japanese Patent Publication No. 49-21,091 discloses silicon nitride sintered bodies consisting of at least 50% by weight of Si₃N₄, not more than 50% by weight of at least one kind of an oxide selected from Y₂O₃, and oxides of La series elements, and from 0.01 to 20% by weight of Al₂O₃. However, silicon nitride sintered bodies having high strength at high temperatures cannot be obtained merely by adding a rare earth element or elements into silicon nitride. Although densification is promoted by the addition of Al₂O₃ to Si₃N₄, an intergranular phase has a low softening point so that strength at high temperatures is largely lowered. As processes for producing silicon nitride sintered bodies having high strength at high temperatures, for instance, Japanese Patent Publication No. 59-35,867 discloses a method for producing silicon nitride sintered bodies in which not less than 2% by weight of La₂O₃ and not less than 2% by weight of Y₂O₃ are added in a total amount of not less than 10% by weight. Japanese Patent Applioation Laid-open No. 55-3,397 discloses a process for manufacturing silicon nitride sintered bodies in which an intergranular phase is crystallized by adding SiO₂ and an oxide or oxides of a rare earth element or elements. Further, U.S. Patent 4,234,343 discloses silicon nitride sintered bodies which contain Yb, Y, Er or Dy and Sc, Al, Cr, Ti, etc. and in which an intergranular phase is crystallized into Si₂N₂O and M₂Si₂O₇ (in which M is Yb, Y, Er, Dy, Sc, Al, Co or Ti). As silicon nitrides sintered bodies having high density and high strength which can be produced by a pressureless sintering process enabling sintering of complicated shapes of green bodies, for instance, Japanese Patent Application Laid-open No. 59-174,577 discloses silicon nitride sintered bodies to which from 5 to 30% by weight of two or more kinds of oxides selected from oxides of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu are added. However, in any of the above-mentioned published techniques, although silicon nitride sintered bodies having high strength at high temperatures can be obtained by hot pressing, silicon nitride sintered bodies having such high strength at high temperatures as obtained in the case of the hot pressing cannot be obtained by pressureless sintering or sintering under pressurized atmosphere and high strength exceeding 500 MPa at high temperatures of around 1,400°C cannot be expected. It is an object of the present invention to obviate the above-mentioned problems, and to provide silicon nitride sintered bodies having high strength even at high temperatures (1,400°C) and a process for manufacturing such silicon nitride sintered bodies through pressureless sintering or sintering under a pressurized atmosphere which enables green bodies of complicated shapes to be fired. According to an aspect of the present invention, there is provided a silicon nitride sintered body comprising not less than 70% by weight of Si₃N₄ and essentially consisting of Si, O, N and a remainder being at least two kinds of rare earth elements selected from Y, Er, Tm, Yb, and Lu, wherein (1) a ratio of a molar amount of all the rare earth elements contained in said sintered body when calculated as Ln₂O₃ in which Ln is a rare earth element selected from Y, Er, Tm, Yb, and Lu with respect to a molar amount of oxygen when calculated as SiO₂ is in a range from 0.4 to 1.3, said molar amount of oxygen when calculated as SiO₂ being obtained by subtracting an amount of oxygen contained in all the Ln₂O₃ obtained by expressing all the rare earth elements contained in said sintered body as Ln₂O₃ from an amount of oxygen contained in said sintered body and converting a remaining amount of oxygen into SiO₂; (2) any one of the rare earth elements contained in said sintered body is not more than 95 mol% of the sum of all the rare earth elements contained in the sintered body; and (3) an intergranular phase of Si₃N₄ grains contained in said sintered body is substantially of a crystalline phase and not less than 50% of the crystalline phase of the intergranular phase is a crystalline phase of an apatite structure. According to another aspect of the present invention, there is provided a process for manufacturing silicon nitride sintered bodies, which comprises preparing a mixed powder consisting of a raw material powder of silicon nitride and two or more kinds of rare earth element oxides selected from Y₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, and Lu₂O₃, and molding said mixed powder, and then firing a thus obtained molding at 1,700°C to 2,100°C pressurelessly or under a pressurized N₂ atmosphere, wherein any one of said selected rare earth element oxides is not more than 95 mol% of the total addition amount of the oxides of the rare earth elements and a ratio of the total addition amount of the selected oxides of the rare earth elements to a molar amount of SiO₂ which is obtained by calculating all oxygen contained in said sintered body (excluding oxygen introduced into the sintered body from the added rare earth element oxides) as SiO₂ is in a range from 0.4 to 1.3. According to the present invention, the silicon nitride sintered bodies having high strength of not less than 500 MPa at high temperatures (1,400°C) can be obtained through pressureless sintering or sintering under a pressurized atmosphere. These and other optional features and advantages of the invention will be appreciated upon reading of the following description of the invention when taken in conjunction with attached drawings, with the under­ standing that modifications, variations and changes of the same could be made by the skilled person in the art to which the invention pertains without departing from the spirit of the invention. For a better understanding of the invention, reference is made to the attached drawings, wherein: Fig. 1 is a diagram showing a diffraction pattern of Sample No. 3 of the silicon nitride sintered bodies according to the present invention in an X-ray diffraction analysis by using a Cu-Kα line; and Figs. 2(a) and 2(b) are microphotographs of a transmission electron microscope showing crystalline structures of Sample No. 3 of the silicon nitride sintered bodies according to the present invention. In the above-mentioned construction, the densification of the silicon nitride sintered body is promoted by incorporating at least two kinds of oxides of rare earth elements having smaller cation radii (Er₂O₃, Y₂O₃, Tm₂O₃, Yb₂O₃, and Lu₂O₃) among the rare earth elements, preferably 50 to 95 mol% of Yb₂O₃ with respect to the total addition amount of oxides of the rare earth elements, more preferably Y₂O₃ and Yb₂O₃, to the silicon nitride sintered body. Although some densification-promoting effect is obtained even when a rare earth element or elements having larger cation radii are used, strength at high temperatures (1,400°C) is not high. This is because the crystalline intergranular phase of the apatite structure containing the rare earth element or elements having the larger cation radii have different properties from those of the crystalline intergranular phase of the apatite structure containing the rare earth elements having smaller cation radii. The reason why any one of the oxides of the rare earth elements is restricted to not more than 95 mol% of the total addition amount of the oxides of the rare earth elements is that a liquid phase obtained by mixing two kinds of the rare earth element oxides outside the above restriction range does not exhibit densification effect through a decreased eutectic point, an increased wettability, and a lowered viscosity, so that dense silicon nitride sintered bodies cannot be obtained. The silicon nitride sintered bodies preferably contain at least Yb₂O₃, and the addition amount of Yb₂O₃ is preferably from 50 to 95 mol% with respect to the total addition amount of the oxides of the rare earth elements. More preferably,the amount of Y₂O₃ and Yb₂O₃ is in a molar ratio of Y₂O₃/Yb₂O₃=50/50 to 5/95. The reason why the total addition molar amount of the oxides of the rare earth elements is determined such that the ratio of the total addition molar amount of the oxides of the rare earth elements to the molar amount of SiO₂ which is calculated by converting oxygen contained in the sintered body (excluding oxygen introduced thereinto through the addition of the oxides of the rare earth oxides) as SiO₂ may be 0.4 to 1.3 is that when the oxides or the rare earth elements are added to give the above range, the intergranular liquid phase having a densifying effect can be obtained and the intergranular phase can be substantially converted to the crystalline phase having the apatite structure. If the ratio exceeds 1.3, the intergranular liquid phase having the densifying effect cannot be obtained and much Ln₄Si₂O₇N₂ (wherein Ln denotes a rare earth element) is contained in the crystalline phase. On the other hand, if the ratio is less than 0.4, although the intergranular liquid phase having the densifying effect can be obtained, Ln₂Si₂O₇ (Ln: a rare earth element) is contained in the crystalline phase, so that sintered bodies having high strength at high temperatures cannot be obtained. In order that the sintered bodies may be densified to have high strength at high temperatures, it is necessary that at least the intergranular phase of the Si₃N₄ is substantially composed of a crystalline phase and at least 50% of the intergranular phase is a crystalline phase of the apatite structure. Preferably, it is desirable that substantially all the intergranular phase is the crystalline phase of the apatite structure. More preferably, the above molar ratio is from 0.5 to 1.2. The crystalline phase of the apatite structure is a crystalline phase having the same crystalline structure as that of hexagonal system crystals having a chemical formula: Y₁₀(SiO₄)₆N₂ represented by a JCPDS card 30-1462. In the case of sintered bodies to which not less than two kinds of rare earth elements are added and mixed, lattice sites occupies by Y ions in the Y₁₀(SiO₄)₆N₂ crystals are replaced by rare earth element ions. Said not less than two kinds of rare earth elements are completely solid solved. It may be that lattice sites of N are replaced by O and the lattice sites of Y are vacant to maintain electrical neutrality. Fig. 1 shows an X-ray diffraction pattern of a sintered body obtained according to the present invention. In Fig. 1, diffraction peaks shown by "a" are attributable to β-Si₃N₄, and diffraction peaks shown by "b" are those of a crystalline phase of the apatite structure as the intergranular crystalline phase. In the crystalline phase of the apatite structure, depending upon the size of the cations, the volume of a lattice varies and the locations of the X-ray peaks shift, but the crystalline phase is identified by the X-ray diffraction pattern. The amount of SiO₂ must be determined under consideration of an amount of oxygen contained in a powdery raw material of silicon nitride and an amount of oxygen introduced through oxidation of Si₃N₄ during the preparation of the sintered body. The amount of SiO₂ is an amount obtained by determining the amount of oxygen in the sintered body through a chemical analysis and subtracting an amount of oxygen which is incorporated into the sintered body through the addition of the oxides of the rare earth elements from the thus determined oxygen amount, and converting an amount of a remaining oxygen to an amount of SiO₂. For this reason, when a silicon nitride raw material powder containing a smaller amount of oxygen, a silicon nitride raw material powder to which SiO₂ is separately added, or a powder in which an amount of oxygen is increased by calcining a raw material powder is used, or when an amount of oxygen introduced is increased or decreased by modifying the preparation method, the sintered bodies according to the present invention can be obtained by increasing or decreasing the addition amount of the oxides of the rare earth elements depending upon the amount of SiO₂. The amount of SiO₂ is preferably from 1.0 to 5.0% by weight to obtain such an amount as attaining the densification. If it is less than 1.0% by weight, the total addition amount of the oxides of the rare earth elements is so small that the amount of a liquid phase may be insufficient for densification. On the other hand, if it is more than 5.0% by weight, the intergranular phase may be in excess so that the strength at high temperatures becomes extremely lowered and the intrinsic strength characteristics of the silicon nitride sintered bodies disappear due to a smaller amount of silicon nitride. One manufacturing method according to the present invention is as follows: First, a mixture is obtained by preparing and mixing a powdery raw material of Si₃N₄ and an additive consisting of not less than two kinds of oxides of rare earth elements selected from Y₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, and Lu₂O₃ as desired. If necessary, the silicon nitride raw material powder is oxidized to increase the amount of SiO₂ or a separately prepared powder of SiO₂ is added thereto. Needless to say, it is preferable that the silicon nitride raw material powder is of a high purity containing not more than 0.5% by weight of impurities of metallic or cationic elements. Impurity metallic elements such as Al, Fe, Mg, etc. interrupt the crystallization of the intergranular phase and decreases the strength at high temperatures. At that time, the composition of the mixture is adjusted such that any one of the selected oxides of the rare earth elements is not more than 95 mol% of the total addition amount of the oxides of the rare earth elements and that the total addition amount of the selected oxides of the rare earth elements is 0.4 to 1.3 times the molar amount of SiO₂ which is obtained by converting oxygen contained in the sintered body (excluding the oxygen introduced thereinto through the addition of the oxides of the rare earth elements). It may be that the oxides of the rare earth elements, Y₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃ and Lu₂O₃ may be added in a form of a solid solution such as (Y,Yb)₂O₃. Alternatively, they may be added in a form of a compound such as a hydroxide, a nitrate, a nitride or the like which is converted into an oxide until the sintering and densification are started through firing. However, they may be preferably added in a form of an oxide because the densification is interrupted by the generation of a volatile component or the control of the amount of oxygen contained in the sintered body becomes unstable. Then, a molding is obtained by molding the thus obtained mixture in a desired shape according to a slip casting, an injection molding, a press molding or the like. The thus obtained molding is fired at a temperature from 1,700 to 2,100°C and preferably from 1,900 to 2,000°C under atmospheric pressure or in a pressurized N₂ atmosphere. The reason why the firing temperature is limited to 1,700°C to 2,100°C is that 1,700°C or more is necessary to obtain a sufficient densification required for aquiring the intergranular phase having a high melting point, while if it exceeds 2,100°C, a specific component is conspicuously evaporated from the sintered body even under the pressurized N₂ atmosphere. Further, in order to restrain the decomposition of silicon nitride with increase in the sintering temperature, it is better that the N₂ pressure in the firing atmosphere is increased. It is preferable that the atmosphere is not less than 2 atm, not less than 7 atm, and not less than 20 atm at 1,800°C, 1,900°C and 2,000°C, respectively. Alternatively, it is possible to fire the molding in a powder bed. Preferably, firing is carried out at a temperature from 1,900°C to 2,000°C in the N₂ atmosphere of 9 to 100 atm. Further, an excellent fired surface of the sintered body can be obtained by increasing an oxygen partial pressure with addition of CO or CO₂ to the atmospheric N₂. During the temperature-descending step in this firing, the intergranular phase is substantially crystallized, so that the silicon nitride sintered crystalline body can be obtained in which not less than 50% of the intergranular phase, preferably substantially all the intergranular phase, is converted to a crystal­ line phase of the apatite structure. In addition to the above crystallization during the temperature-descending step, the crystallization may be performed through reheating, if necessary. In this case, the reheating is preferably done at a temperature from 1,200°C to 1,500°C. It goes without saying that the silicon nitride sintered body having the same composition as in the present invention may be densified through a hot pressing, or a hot isostatic pressing to obtain silicon nitride sintered bodies having high strength at high temperatures. Further, it is possible that the silicon nitride sintered bodies having the same composition as in the present invention may be obtained by firing at atmospheric pressure or in a pressurized N₂ atmosphere, densifying, promoting densifying through hot isostatic pressing without crystallization during the temperature-­ descending step in the firing, and crystallizing the intergranular phase during the temperature-descending step in the hot isostatic pressing or during reheating, thereby improving the characteristics. However, the present invention is characterized in that the densification is effected by sintering under the atmospheric pressure or in the pressurized N₂ atmosphere so that the sintered bodies having high strength at high temperatures can be obtained. As having been mentioned in the above, according to the silicon nitride sintered bodies and the manufacturing process thereof in the present invention, silicon nitride sintered bodies having high strength at high temperatures are obtained by sintering under atmospheric pressure or the pressurized atmosphere through controlling the oxides of the rare earth elements in a range in which the molar ratio between the rare earth element oxides and SiO₂ is specified and by controlling the densification due to the intergranular phase having the limited composition and the crystalline phase during the crystallization. In the following, specific examples of the present invention will be explained below. These examples are merely given in illustration of the invention, and should not be interpreted to limit the scope of the invention. Example 1 A raw material powder of Si₃N₄ containing 0.5% by weight of a metallic or cationic element impurities such as C, Al, Fe, Mg, Ca, etc. and 1.7% by weight of oxygen and having an average grain size of 0.6 pm and a BET specific surface area of 22 m²/g and an additive shown in Table 1 having a purity of 99.9% by weight and an average grain size of 0.3 to 2 µm were mixed at a recipe shown in Table 1. Then, by using a nylon vessel of an inner volume of 1.2 liters, 200 g of a thus formulated raw material were added with 1.2 kg of grinding media made of a silicon nitride base porcelain and 500 mℓ of water, which was milled at a vibration frequency of 1,200 times/minutes for 100 hours in a vibration mill. Thereafter, the water was evaporated and a thus milled powder was granulated into a grain size of 100 µm, thereby obtaining a molding powder. Then, the molding powder was isostatically pressed at a pressure of 3 tons/cm² to obtain a molding of 60x60x6 mm. The molding was fired under firing conditions shown in Table 1, and then crystallized at 1,400°C for 6 hours. In this way were produced each of the sintered bodies Nos. 1 through 9 according to the present invention. In the meanwhile, the same raw material was used, and an additive in Table 1 was added thereinto at a recipe as shown in Table 1. A thus formulated powder was similarly subjected to milling, granulating, molding, firing under firing conditions in Table 1, and crystallization, thereby producing similary each of the crystallized silicon nitride sintered bodies Nos. 10 to 17 as Comparative Examples. With respect to the above sintered bodies Nos. 1-17, the content of oxygen in sintered body was determined by a chemical analysis, and an amount of oxygen introduced into the sintered body by addition of the additive of the oxides of the rare earth elements was subtracted from the thus determined content of oxygen. A remaining amount of oxygen was determined as a molar amount of SiO₂, and a molar ratio between the addition amount of the rare earth element oxides and the amount of SiO₂ are shown in Fig. 1. As a result of the chemical analysis, it was revealed that the analyzed amounts of the rare earth elements did not differ from the addition amount thereof. The bulk densities of the sintered bodies, the crystalline phase of the intergranular phase, and four point bending strengths at room temperature and 1,400°C were shown in Table 1. The bulk densities of the sintered bodies were measured according to Archimedean method. The four point bending strengths were measured according to "Fine ceramic bending strength-testing method" specified in JIS R-1601. The intergranular crystalline phase was determined from results of the X-ray diffraction using the Cu-Kα line. In Table 1, H and S mean the crystals of an apatite structure and Ln₂Si₂O₇ (Ln is a rare earth element), respectively. A diffraction pattern in the X ray diffraction analysis by using the Cu-Kα line is shown in Fig. 1 with respect to the silicon nitride sintered body No. 3 according to the present invention. Among diffraction peaks, those shown by "a" are attributable to β-Si₃N₄ and the peaks represented by "b" are those of the crystalline phase of the apatite structure. Transmission electron microscope photographs of the silicon nitride sintered body No. 3 according to the present invention are shown in Figs. 2(a) and 2(b), in which C and D-zones show grains of Si₃N₄ and an intergranular phase, respectively. As obvious from Table 1, not less than two kinds of the rare earth element compounds of Y₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, and Lu₂O₃ are added as the additive in the present invention such that any one of the selected oxides of the rare earth elements as the additives is not more than 95 mol% that is, 95/5 to 5/95 molar ratio of the total addition amount of the rare earth element oxides; the total molar addition amount of the selected rare earth element oxides is from 0.4 to 1.3 times the molar amount of SiO₂ which is obtained by converting the content of oxygen contained in the sintered body to SiO₂; as shown in the photograph of Fig. 2, the intergranular phase is substantially crystallized, and is composed of the crystalline phase of the apatite structure; and the silicon nitride sintered bodies have high strength at 1,400°C. In the case of all the silicon nitride sintered bodies outside the scope of the present invention, that is, the silicon nitride sintered bodies (Nos. 10 to 13) using other compounds as the rare earth elements additives than Y₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃ and Lu₂O₃, the sintered bodies (Nos. 14 to 15) added with a single rare earth element oxide,and the sintered bodies (Nos. 16 to 17) in which the total addition molar amount of the rare earth element oxides to the molar amount of SiO₂ is outside the scope of the present invention, the strength at 1,400°C was low or sufficient densification was not obtained. The silicon nitride sintered body Nos. 1-9 according to the present invention each containing Y₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃ and/or Lu₂O₃ in the range limited in the present invention had the intergranular phase composed of the apatite structure crystalline phase, and exhibited high strength at 1,400°C being not less than 500 MPa. Example 2 A raw material powder of Si₃N₄ containing 0.1% by weight of metallic or cationic element impurities of C, Al, Fe, Mg, Ca, etc. and 1.7% by weight of oxygen and having an average grain size of 0.5 µm and a BET specific surface area of 15 m²/g, and a Y₂O₃-Yb₂O₃ additive shown in Table 2 and having a purity of 99.9% by weight and an average grain size of 0.3 to 2 µm or an Er₂O₃-Yb₂O₃ additive shown in Table 3 were mixed. By using a nylon vessel having an inner volume of 1.2 liters, a thus formulated raw material were added with 1.2 kg of grinding media made of silicon nitride base porcelain and 500 ml of water, which was milled at a vibration frequency of 1,200 times/min in a vibration mill for 10 hours. Then, the water was evaporated and the milled raw material was granulated to a grain size of 100 µm as a molding powder. Next, the thus granulated molding powder was isostatically pressed at a pressure of 3 tons/cm² to prepare a molding of 60x60x6 mm, which was fired at 1,900°C in an N₂ atmosphere of 10 atm for 2 hours. Further, the fired product was crystallized at 1,400°C for 6 hours, thereby obtaining each of the sintered bodies Nos. 21 to 28 and 31 to 37 according to the present invention. In the meanwhile, the same raw material was used, and an additive in Table 2 or 3 was added thereinto at a recipe shown therein. The thus obtained powder was similarly subjected to milling, granulating, molding and firing in the same firing conditions as in the present invention, thereby obtaining each of the sintered bodies Nos. 29, 30 and 38 as Comparative Examples. With respect to these sintered body Nos. 21 to 38, the content of oxygen in the sintered body was determined by a chemical analysis, and an amount of oxygen incorporated into the sintered body through addition of the additive of the rare earth element oxides was subtracted therefrom. Then, a remaining amount of oxygen was converted into a molar amount of SiO₂. The ratio between the total addition molar amount of the oxides and the molar amount of SiO₂ was shown in Tables 2 and 3. As in Example 1, the bulk densities of the sintered bodies, the crystalline phase of the intergranular phase, and four point bending strengths at room temperature and 1,400°C were determined as in Example 1 and shown in Tables 2 and 3. A ratio of H phase in Tables 2 and 3 means a relative integrated intensity of the crystalline phase having the apatite structure to a sum of integrated intensities of largest peaks of intergranular crystalline phases of Yb₂SiO₅:(204) face, Ln₂Si₂O₇:(021) face, Ln₄Si₂O₇N₂:(131) face, and apatite:(211) face excluding β-Si₃N₄. Ln means a rare earth element. As obvious from Tables 2 and 3, the strength having not less than 500 MPa at 1,400°C could be obtained in Sample Nos. 21 to 28 and 31 to 37 in which any one kind of the oxides of the rare earth elements included in the scope of the present invention is not more than 95 mol%, that is, from 95/5 to 5/95 at the molar ratio with respect to the total addition amount of the oxides of the rare earth elements. Further, Samples Nos. 25 to 28 and Nos. 34 to 37 in which Yb₂O₃ is from 50 to 95 mol% could acquire a strength of not less than 550 MPa at 1,400°C. A strength of not less than 570 MPa at 1,400°C could be obtained in Sample Nos. 25 to 28 in which Y₂O₃/Yb₂O₃=50/50 to 5/95. The silicon nitride sintered body Nos. 25 to 28 according to the present invention have excellent oxidation resistance in that the weight increment after an oxidizing test at 1,400°C in open air for 100 hours is from 0.1 to 0.2 mg/cm², and they also have excellent creep resistance in that a creep rate at 1,400°C in open air under a load stress of 200 MPa is 1 to 3x10⁻⁵h⁻¹. Example 3 A raw material powder of Si₃N₄ containing 0.1% by weight of metallic or cationic element impurities such as C, Al, Mg, Fe, Ca, etc. and 1.7% by weight of oxygen and having an average grain size of 0.5 µm and a BET specific surface area of 15 m²/g and a Y₂O₃-Yb₂O₃ additive shown in Table 4 having a purity of 99.9% by weight and an average grain size of 0.3 to 2 µm were mixed together. By using a nylon vessel having an inner volume of 1.2 liters, 200 g of the formulated raw material were added with 1.2 kg of grinding media made of silicon nitride base porcelain and 500 ml of water. Then, a resulting mixture was milled in a vibration mill at a vibration frequency of 1,200 times per minute for 10 hours. The water was evaporated and the thus milled powder was granulated to a grain size of 100 µm, thereby obtaining a molding powder. Next, a molding of 60x60x6 mm was prepared in an isostatic press under pressure of 3 ton/cm². Samples shown in Table 4 were fired at 1,900°C in N₂ atmosphere of 10 atm for 2 hours, and then crystallized at 1,400°C for 6 hours, thereby obtaining silicon nitride sintered bodies Nos. 41 to 43 according to the present invention. The same material was used, and an additive shown in Table 4 was mixed thereto at a recipe shown therein. Then, the thus formulated powder was similarly subjected to the milling, granulating and molding. Then, the molding was fired under the same firing conditions as in the above products according to the present invention and similarly crystallized, thereby obtaining silicon nitride sintered bodies Nos. 44 and 45 as Comparative Examples. With respect to the sintered body Nos. 41 to 45, the content of oxygen in the sintered body was determined by a chemical analysis, an amount of oxygen incorporated into the sintered body through addition of the additive of the oxides of the rare earth elements was subtracted therefrom, thereby determining the residual oxygen content as an amount of SiO₂. The ratio between the total addition molar amount and the molar amount of SiO₂ was shown in Table 4. The bulk densities of the sintered bodies, the crystalline phase of the intergranular phase, four point bending strengths at room temperatures and 1,400°C were determined similarly to Example 1, and the ratio of the H phase was determined similarly to Example 2. Results were shown in Table 4. As obvious from Table 4, a strength of not less than 500 MPa at 1,400°C could be obtained in Sample Nos. 41 to 43 according to the present invention in which the ratio of the total addition molar amount of the oxides of the rare earth elements to the molar amount of SiO₂ which was obtained by converting the amount of oxygen contained in the silicon nitride sintered body (excluding the oxygen introduced thereinto through the addition of the rare earth element oxides) is from 0.4 to 1.3. To the contrary, Comparative Example Nos. 44 and 45 in which the molar ratio is outside the scope of the present invention has the ratio of the H phase being not more than 50% and low strength at high temperatures. Example 4 A low oxygen content raw material powder of Si₃N₄ containing 0.1% by weight of metallic or cationic element impurities such as C, Al, Fe, Ca, etc. and 1.0% by weight of oxygen and having an average grain size of 0.6 µm and a BET specific surface area of 10 m²/g and an additive shown in Table 5 having a purity of 99.9% by weight and an average grain size of 0.3 to 2 µm were mixed together at a recipe of Table 5, and 1.2 g of grinding media made of silicon nitride base porcelain and 500 ml of water were added to 200 g of the thus mixed raw material by using a 1.2 liter inner volume vessel made of nylon, which was milled in a vibration mill at a vibration frequency of 1,200 times/minute for 10 hours. Then, the water was evaporated and the milled powder was granulated to a grain size of 100 µm, thereby obtaining a molding powder. Next, a molding of 60x60x6 mm was prepared by isostatically pressing the molding powder under pressure of 3 ton/cm². The molding was fired at 1,900°C in an N₂ atmosphere of 10 atm for 6 hours and further crystallized at 1,400°C for 6 hours, thereby obtaining each of the sintered bodies Nos. 51 and 52 according to the present invention. In the meanwhile, the same raw material was used, and an additive shown in Table 5 was mixed thereinto at a formulating recipe in Table 5, followed by similarly milling, granulating, and molding and then firing in the same firing conditions as in the products according to the present invention, thereby obtaining similarly each of crystallized silicon nitride sintered bodies Nos. 53 and 54 as Comparative Examples. With respect to the sintered bodies Nos. 51 to 54, the content of oxygen in the sintered body was determined by a chemical analysis and an amount of oxygen incorporated into the sintered body through addition of the additive of the oxides of the rare earth elements was subtracted therefrom. A remaining content of oxygen was determined as a molar amount of SiO₂. The ratio between the total addition molar amount and the molar amount of SiO₂ was shown in Table 5. The bulk densities of the sintered bodies, the crystalline phase of the intergranular phase, and four point bending strengths at room temperature and 1,400°C were determined similarly to Example 1 and the ratio of the H phase was obtained similarly to Example 2. Results were shown in Table 5. As obvious from Table 5, as in the same with Example 3, strength of not less than 500 MPa at 1,400°C could be obtained in Sample Nos. 51 and 52 according to the present invention in which the molar ratio is from 0.4 to 1.3 and the Si₃N₄ raw material having a low content of oxygen was used. To the contrary, in Comparative Example Nos. 53 and 54 having the molar ratio outside the scope of the present invention, the ratio of the H phase was not more than 50% and strength at high temperatures was low. The present invention is not limited to the above­ mentioned Examples, but a variety of modifications and changes could be made. For instance, although the rare earth elements are added in a form of an oxide in the above-mentioned Examples, the rare earth elements may be added in a form of a nitride, a chloride, a hydroxide, a carbonates, a sulfates, a nitrate or the like so long as the intergranular phase is crystallized into the apatite structure at a substantially the same mixing ratio of the rare earth element oxides as in the present invention. As evident from the above detailed explanation, according to the silicon nitride sintered bodies and the manufacturing method thereof in the present invention, a specific additive which is adjusted at a specified molar ratio is used so that the silicon nitride sintered bodies having a high strength of not less than 500 MPa at high temperatures (1,400°C) could be obtained according to the pressureless or pressurized atmosphere sintering method. Therefore, the present invention can be favorably applied to structural materials for such as gas turbine parts, etc. which have complicated shapes.

Claims: 1. A silicon nitride sintered body comprising not less than 70% by weight of Si₃N₄ and essentially consisting of Si, O, N and a remainder being at least two rare earth elements selected from Y, Er, Tm, Yb and Lu, wherein (1) the ratio of the molar amount of all the rare earth elements in said sintered body when calculated as Ln₂O₃ in which Ln is Y, Er, Tm, Yb or Lu with respect to the molar amount of oxygen when calculated as SiO₂ is in the range from 0.4 to 1.3, said amount of oxygen when calculated as SiO₂ being obtained by subtracting the amount of oxygen in Ln₂O₃ obtained by expressing all the rare earth elements present as Ln₂O₃ from the amount of oxygen in said sintered body and converting the remaining amount of oxygen into Si02 (2) any one of the rare earth elements in said sintered body is not more than 95 mol% of the sum of all the rare earth elements in the sintered body and (3) the intergranular phase of the Si₃N₄ grains in said sintered body is substantially a crystalline phase of which not less than 50% is a crystalline phase of an apatite structure. 2. A silicon nitride sintered body according to claim 1, wherein Yb is present in an amount of 50 to 95 mol% of all the rare earth elements. 3. A silicon nitride sintered body according to claim 2, wherein the rare earth elements comprise Y and Yb. 4. A process for manufacturing a silicon nitride sintered body, which comprises preparing a mixed powder consisting of a raw material powder of silicon nitride and two or more rare earth element oxides selected from Y₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃ and Lu₂O₃ (or compounds convertible into two or more of those oxides), molding said mixed powder, and then firing a thus obtained molding at 1,700°C to 2,100°C pressurelessly or in a pressurized N₂ atmosphere, wherein any one of said selected rare earth element oxides is not more than 95 mol% of the total addition amount of the oxides of the rare earth elements and the ratio of the total addition molar amount of the selected oxides of the rare earth elements to the molar amount of SiO₂ which is obtained by calculating all oxygen contained in said sintered body (excluding oxygen introduced into the sintered body from the added rare earth element oxides) as SiO₂ is in a range from 0.4 to 1.3.

Inventors: SOMA, TAKAO, TAKAHASHI, TOMONORI
Applicants: NGK INSULATORS, LTD.
Application year: 1987
Main classification: C04B35
IPC codes: C04B35