Silicon nitride sintered body

Abstract: A silicon nitride sintered body having a composition of 1.5-3.0 weight % of Y₂O₃, 0.1-1.0 weight % of Al₂O₃ and the balance being substantially of Si₃N₄, the weight ratio of Y₂O₃/Al₂O₃ being 2.5 or more, the body having a density of 3.0 g/cm³ or more and having a structure in which the minor axes of grains are substantially 6 µm or less. This silicon nitride sintered body is produced by preparing a powder mixture containing Y₂O₃ powder, Al₂O₃ powder and silicon nitride powder in the above proportions, the silicon nitride powder having an oxygen content of 1.2 weight % or less, a specific surface area of 9-11 m²/g and a metal impurity content of 200 ppm or less; and sintering the powder mixture at a temperature of 1900-2100°C and a pressure of 5 kg/cm² G or more in a nitrogen atmosphere. The sintered body is useful as a ceramic material for use in engineering.

Description: The present invention relates to a silicon nitride sintered body having excellent high-temperature strength and oxidation resistance. Silicon nitride sintered bodies are generally used as structural ceramics for gas turbine parts, etc. under severe conditions at high temperatures, because of their high strength, heat resistance, thermal shock resistance, wear resistance, oxidation resistance etc. However since silicon nitride itself is a material which cannot be easily sintered, various sintering aids are added to increase its density. The sintering aids tend to form glass phases of low-melting points at the boundaries of silicon nitride grains resulting in a deterioration of high-temperature strength of the silicon nitride. Accordingly, the following attempts have been proposed: (1) Adding oxides of rare earth elements such as yttrium oxide. (2) Adding oxides of rare earth elements such as yttrium oxide together with other oxides such as aluminum oxide (for instance, Japanese Patent Publication No. 49-21091). (3) Adjusting the formulation of yttrium oxide, aluminum oxide and silicon nitride to improve the oxidation resistance at 1400°C (Japanese Patent Laid-Open No. 63-185863). However, since these methods suffer from the following problems, they are not practical methods. Specifically speaking, the method (1) of adding yttrium oxide solves the above problem (formation of low-melting point glass phases). In fact, since the boundaries of silicon nitride grains are bonded together by high-viscosity glass or crystalline compositions such as Si₃N₄·Y₂O₃, the high-temperature strength and high-temperature creep resistance of the silicon nitride sintered body are not likely to deteriorate as a consequence. In this respect, this method is effective to some extent. However, the resulting silicon nitride sintered body shows insufficient high-temperature strength etc. and sintering of the silicon nitride is difficult, so that pressureless sintering cannot be effected. In the method (2) of adding yttrium oxide and aluminum oxide, sintering is accelerated, and high-density, high-strength sintered bodies can be obtained even by pressureless sintering. However since the resulting silicon nitride sintered bodies suffer from a severe decrease in high-temperature strength, dense sintered bodies having excellent high-temperature strength cannot be obtained without using special sintering methods such as hot pressing after a crystallization treatment. With respect to sintered bodies obtained by the method (3) of adjusting the formulation of yttrium oxide, aluminum oxide and silicon nitride to improve the oxidation resistance at 1400°C, the amounts of additives are extremely small. Accordingly, to achieve high density, a sintering temperature of 1850°C or higher is needed, resulting in extraordinary growth of crystal grains in the structure of the resulting sintered bodies. Therefore, although the oxidation resistance and strength at 1400°C are not lowered, the bodies do not have sufficiently improved strength at room temperature. Therefore, an object of the present invention is to provide a silicon nitride sintered body having excellent oxidation resistance and high strength at a high temperature. Another object of the present invention is to provide a method of producing such a silicon nitride sintered body. As a result of intense research in view of the above objects, the present inventors have investigated method (3) above and found that by controlling the grain growth in the structure of the sintered body such that the crystal grains in the structure have minor axes which are at most substantially 6 µm or less, silicon nitride ceramics having improved strength not only at room temperature but also at 1400°C can be obtained. The present invention is based upon this finding. Thus, according to one aspect the present invention provides a silicon nitride sintered body comprising 1.5-3.0 weight % of Y₂O₃, 0.1-1.0 weight % of Al ₂O₃ and the balance being substantially of Si₃N₄, the weight ratio of Y₂O₃/Al ₂O₃ being 2.5 or more, said Si₃N₄ having been added as silicon nitride having an oxygen content of less than 1.2 wt %, a specific surface area of 9-11 m²/g and a metal impurity content of 200 ppm or less, said body having a density of 3.0 g/cm³ or more and having a structure in which the minor axes of grains are substantially 6 µm or less. According to another aspect the present invention provides a method of producing a silicon nitride sintered body having a structure in which the minor axes of grains are substantially 6 µm or less, comprising the steps of preparing a powder mixture comprising 1.5-3.0 weight % of Y₂O₃ powder, 0.1-1.0 weight % of Al ₂O₃ powder and balance substantially silicon nitride powder, a weight ratio of Y₂O₃/Aℓ₂O₃ being 2.5 or more, the silicon nitride powder having an oxygen content of 1.2 weight % or less, a specific surface area of 9-11 m²/g and a metal impurity content of 200 ppm or less; and sintering the powder mixture at a temperature of 1900-2100°C and a pressure of 0,5 MPa (5 kg/cm²) G or more in a nitrogen atmosphere. The present invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a scanning-type electron photomicrograph showing the microstructure in the rupture surface of the silicon nitride sintered body in Example 2; and Fig. 2 is a scanning-type electron photomicrograph showing the microstructure in the rupture surface of the silicon nitride sintered body in Comparative Example 2. In the present invention, the content of Y₂O₃ is 1.5-3.0 weight %, preferably 2.0-3.0 weight %, more preferably 2.0-2.8 weight %. When the content of Y₂O₃ is higher than 3.0 weight %, the resulting sintered body has a low oxidation resistance at a high temperature. On the other hand, when the content of Y₂O₃ is lower than 1.5 weight %, the resulting sintered body cannot be made fully dense, failing to meet the requirements of oxidation resistance and strength. The content of Aℓ₂O₃ is 0.1-1.0 weight %. When it is higher than 1.0 weight %, the resulting sintered body has poor oxidation resistance and high-temperature strength. On the other hand, when it is lower than 0.1 weight %, the resulting sintered body is not fully dense like in the case of Y₂O₃, failing to achieve the desired oxidation resistance and strength. The preferred content of Aℓ₂O₃ is 0.4-1 wt %, more preferably 0.4-0.8 wt %. With respect to the weight ratio of Y₂O₃/Aℓ₂O₃, it should be 2.5 or more. When this weight ratio is lower than 2.5, the resulting sintered body does not have a sufficient oxidation resistance at 1400°C. The silicon nitride sintered body of the present invention should have a density of 3.0 g/cm³ or more, which corresponds to about 93% or more of the theoretical density. When the density is lower than this level, sufficient strength cannot be achieved. When extraordinary grain growth takes place such that the minor axes of grains become substantially more than 6 µm in the sintered body structure, the ununiformness of the structure increases, resulting in an increase in the strain of the sintered body. As a result, the residual stress increases in the sinter body matrix. The residual stress generated in the matrix accelerates the propagation of cracks in the sintered body, resulting in a decrease in the mechanical strength of the sintered body. Thus, by suppressing the extraordinary grain growth in the structure, thereby providing the sintered body with a fine structure, the sintered body can have a high strength not only at room temperature but also at a high temperature. To prevent the extraordinary grain growth, Y₂O₃ and Aℓ₂O₃ should be added such that the above conditions are met, and the silicon nitride powder used as a starting material should meet the following conditions: (a) Oxygen content of 1.2 weight % or less, preferably 0.8-1.2 wt %, particularly 0.8-1.1 wt %. (b) Specific surface area of 9-11 m²/g and (c) Metal impurity content of 200 ppm or less, preferably ≦ 150 ppm, particularly 50 ppm. It is necessary to sinter the powder mixture as a starting material in a nitrogen atmosphere under the following conditions: (d) A temperature of 1900-2100°C, preferably 1950-2050°C, and (e) A pressure of 0,5 MPa (5 kg/cm²) G or more. When a gas pressure sintering method is utilized, a pressure of 1-2 MPa (10-20 kg/cm²) G is preferred. When the oxygen content of the silicon nitride powder exceeds 1.2 weight %, high-oxygen content phases are formed in the grain boundaries. When the total content of metal impurities exceeds 200 ppm, impurity phases appear in the grain boundaries, so that the sintered body becomes easily softened at a high temperature. Thus, the sintered body has a low oxidation resistance. It should be noted that the metal impurity content may be as low as is theoretically possible. Further, even within the composition range of the present invention, when the specific surface area of the silicon nitride powder exceeds 11 m²/g, the extraordinary grain growth takes place such that the minor axes of grains in the sintered body structure exceed substantially 6 µm. On the other hand, when the specific surface area is lower than 9 m²/g, sufficient density cannot be achieved in the resulting sintered body. In the method of the present invention, the sintering temperature should be 1900-2100°C. When the sintering temperature is too low, the sintered body cannot be made fully dense. On the other hand, when the sintering temperature is too high, the silicon nitride undesirably tends to be decomposed. The sintered body of the present invention may generally be produced by the following method: First, Si₃N₄ powder, Y₂O₃ powder and Aℓ₂O₃ powder meeting the above conditions are mixed in proper proportions. It is preferable that the Si₃N₄ powder has an average particle size of about 0.3-0.4 µm, that the Y₂O₃ powder has an average particle size of about 0.5-2 µm, and that the Aℓ₂O₃ powder has an average particle size of about 0.4-0.5 µm. The mixing of these powders may be conducted using a ball mill, a dispersing machine, etc. The resulting powder mixture is compressed to form a green body or a pressed body by a die press, a cold isostatic press (CIP), etc. In the process of pressing, pressing aids, such as a polyvinyl alcohol solution, etc. may be added, if necessary. The sintering of the green body can be conducted properly by a known high pressure method. For instance, a gas pressure sintering method, a hot isostatic pressing method (HIP), etc. can be used. The silicon nitride sintered body of the present invention thus obtained has a density of 3.0 g/cm³ or more, preferably 3.10-3.25 g/cm³, and the minor axes of grains in the structure are at most substantially 6 µm or less. It is not necessarily clear why the effects of the present invention can be obtained by a silicon nitride sintered body meeting the above conditions, but it may be as follows: With respect to the composition, even though the content of aluminum oxide is 0.1-1.0 weight %, the high-temperature strength of the sintered body is deteriorated when the content of yttrium oxide exceeds 3.0 weight %. This presumably is due to the fact that since the grain boundaries containing these additives are not crystallized but made glassy, oxidation proceeds through the grain boundaries, resulting in a decrease in the high-temperature strength. In addition, the extraordinary grain growth in the sintered body structure and the absolute value of the residual stress in the sintered body have a close correlation. When there is the extraordinary grain growth in the sintered body structure, the absolute value of the residual stress exceeds 45 MPa, resulting in the acceleration of propagation of cracks in the sintered body. As a result, the mechanical strength of the sintered body cannot be fully improved. Incidentally, even though the contents of the sintering aids are within the above ranges the absolute value of the residual stress of the sintered body becomes too large when the silicon nitride powder has an excessive specific surface area. Thus, an uneven stress field is generated in the grain boundaries, inducing an unevenness in the sintered body structure. For the above reasons, in the ranges of the present invention to which yttrium oxide, aluminum oxide and silicon nitride are restricted the dense sintered body has crystallized grain boundaries and a fine structure. As a result, the sintered body has a high strength and a high creep resistance even at a high temperature of 1400°C. The present invention will be described referring to the following non-limiting Examples: Examples 1-5, Comparative Examples 1-7 Silicon nitride powder synthesized by a thermal decomposition method of silicon imide (manufactured by Tosoh Corporation, oxygen content: 0.9-1.3 weight %, total amount of metal impurities: 80 ppm, BET specific surface area: 8-12 cm²/g), yttrium oxide powder (manufactured by Mitsuibishi Chemical, Ltd., fine powder grade), and aluminum oxide powder (manufactured by Sumitomo Chemical, Co., Ltd., AKP 30) were used, and they were mixed in proportions shown in Table 1 in a pot mill made of silicon nitride for 24 hours. Each mixture thus obtained was subjected to cold isostatic pressing at a pressure of 14715 MPa (1500 kg/mm²) to form a green body of 50 mm x 30 mm x 5 mm. The green body was embedded in boron nitride powder and sintered at 1950°C in Examples 1-5 and Comparative Examples 1-6, and 1850°C in Comparative Example 7, each in a nitrogen atmosphere for 4 hours. In all Examples sintering was achieved using a gas pressure sintering method using a pressure of 1,0 MPa (10 kg/cm²) G. With respect to each sintered body thus produced, a 3-point bending test was conducted according to JIS R 1601-1981 at room temperature and 1400°C. Further, by measuring a weight gain by oxidation at 1400°C for 100 hours, its oxidation resistance was evaluated. The results are shown in Table 1. As examples of the sintered body structure, Figs. 1 and 2 respectively show scanning-type electron photomicrographs of rupture surfaces of the sintered bodies in Example 2 and Comparative Example 2. It is clear from Fig. 1 that the silicon nitride sintered body of the present invention has minor axes of grains which are substantially 6 µm or less. As described above in detail, since the silicon nitride sintered body of the present invention has extremely high oxidation resistance and strength at such a high temperature as 1400°C, it can be used in wider applications than the conventional silicon nitride sintered bodies. In particular, the silicon nitride sintered body of the present invention is an engineering ceramic material with excellent high-temperature strength, which is suitable for structural members such as turbine parts, etc.

Claims: A silicon nitride sintered body comprising 1.5-3.0 weight % of Y₂O₃, 0.1-1.0 weight % of Al ₂O₃ and the balance being substantially of Si₃N₄, the weight ratio of Y₂O₃/Al ₂O₃ being 2.5 or more, said Si₃N₄ having been added as silicon nitride having an oxygen content of less than 1.2 wt %, a specific surface area of 9-11 m²/g and a metal impurity content of 200 ppm or less, said body having a density of 3.0 g/cm³ or more and having a structure in which the minor axes of grains are substantially 6 µm or less. A body as claimed in claim 1 wherein the content of Y₂O₃ is 2.0-3.0 wt %. A body as claimed in claim 1 or claim 2 wherein the content of Al ₂O₃ is 0.4-1.0 wt %. A body as claimed in any of the preceding claims having a density of 3.10-3.25 g/cm³. A method of producing a silicon nitride sintered body having a structure in which the minor axes of grains are substantially 6 µm or less, said method comprising the steps of preparing a powder mixture comprising 1.5-3.0 weight % of Y₂O₃ powder, 0.1-1.0 weight % of Al ₂O₃ powder and the balance being substantially of silicon nitride powder, the weight ratio of Y₂O₃/Al ₂O₃ being 2.5 or more, said silicon nitride powder having an oxygen content of 1.2 weight % or less, a specific surface area of 9-11 m²/g and a metal impurity content of 200 ppm or less, and sintering said powder mixture at a temperature of 1900-2100°C and a pressure of 0,5 MPa (5 kg/cm²) G or more in a nitrogen atmosphere. A method as claimed in claim 5 wherein the silicon nitride powder has an oxygen content of 0.8-1.2 wt %. A method as claimed in claim 5 or claim 6 wherein the silicon nitride powder has a metal impurity content of 150 ppm or less. A method as claimed in any one of claims 5 to 7 wherein sintering of said powder mixture is carried out at 1950-2050°C. A method as claimed in any one of claims 5 to 8 wherein sintering of said powder mixture is carried out at a pressure of 1-2 MPa (10-20 kg/cm²) G.

Inventors: ARAKAWA TOSHIHIKO, HAMAZAKI KAGEHISA, KAWAKAMI YASUNOBU, MORI TOSHIYUKI, ARAKAWA, TOSHIHIKO, HAMAZAKI, KAGEHISA, KAWAKAMI, YASUNOBU, MORI, TOSHIYUKI
Applicants: HONDA MOTOR CO LTD, HONDA GIKEN KOGYO KABUSHIKI KAISHA
Application year: 1989
Main classification: C04B35
IPC codes: C04B35