SI3N4-AL2O3 COMPOSITE SINTERED BODIES AND METHOD OF PRODUCING THE SAME

Abstract: An Si₃N₄-Al₂O₃ composite sintered body suitable for use in high-temperature structural materials consists of α-Al₂O₃ and at least one crystal phase of Si₃N₄ and sialon and is produced by sintering a shaped body of a particular Si₃N₄-Al₂O₃ mixed powder at 1,500-1,900°C. The body has high strength at high temperature and excellent oxidation resistance.

Description: This invention relates to Si₃N₄-Al₂O₃ composite sintered bodies and a method of producing the same, and more particularly to Si₃N₄-Al₂O₃ composite sintered bodies suitable for use as structural materials having a high strength at high temperature and a method of producing the same. Al₂O₃ is widely used as a substrate or a package for integrated circuits, a chip for cutting tool or a refractory material. However, the strength of Al₂O₃ at a temperature of not lower than 1,000°C is low as compared with a silicon nitride sintered body and a silicon carbide sintered body, so that there is a restriction in its use as a structural material for engine parts and the like. In order to increase the strength and toughness of Al₂O₃, it is proposed to disperse SiC whiskers into Al₂O₃. For example, the increase of the toughness in the Al₂O₃ sintered body through the dispersion of SiC whiskers is disclosed in American Ceramics Society Bulletin, 64[2], 298-304 (1985). However, when this sintered body is heated in air, the oxidation of SiC occurs violently, lowering strength, and the sintered body cannot be used at a high temperature for a long time. Further, JP-A-55-126,574, JP-A-59-78,972 and JP-A-62-187,174 disclose that when Si₃N₄ is added to Al₂O₃, the strength, toughness and hardness are increased in the resulting Al₂O₃ sintered body with Si₃N₄ particles dispersed therein or in the resulting sintered body in which Al₂O₃ particles are dispersed into a sialon matrix. For these sintered bodies, however, the high temperature properties are not mentioned at all, or the satisfactory increase of the strength and toughness cannot be achieved, or the high-temperature strength cannot be expected due to the presence of sintering aid such as spinel or the like. EP-A-141 366 describes a ceramic composition consisting of α-Al₂O₃, Si₃N₄ and sialon, obtained by sintering at 1775°C a powder mixture consisting of Al₂O₃ and Si₃N₄. The powder mixture may contain 20% Si₃N₄. It is an object of the invention to solve the aforementioned drawbacks of the conventional technique and to provide Si₃N₄-Al₂O₃ composite sintered bodies, which have a high strength from room temperature to higher temperature and an excellent oxidation resistance and are suitable for use as structural materials such as engine parts and the like, and a method of producing the same. According to a first aspect of the invention, there is provided an Si₃N₄-Al₂O₃ composite sintered body as set out in claim 1. According to a second aspect of the invention, there is provided a method of producing an Si₃N₄-Al₂O₃ composite sintered body, as set out in claim 2. The Si₃N₄-Al₂O₃ composite sintered body according to the invention and the production method thereof will be described in detail below. As a starting mixed powder, Si₃N₄ powder and Al₂O₃ powder preferably have a high purity and are fine. Further, the content of metallic elements other than Si and Al should be not more than 0.5% by weight in the mixed powder because the metallic element impurity remains in a vitreous state in the sintered body to degrade the high-temperature properties. Moreover, the Si₃N₄ powder preferably has an oxygen content of not more than 2% by weight and a particle size of not more than 5 µm. As the starting Al₂O₃ powder, γ-Al₂O₃ or the like may be used if it is converted into α-Al₂O₃ after the firing. The starting Si₃N₄ powder may be α-type or β-type. Thus, there is provided a mixed powder for subsequent shaping comprising 25-45 mol% of the above Si₃N₄ powder and the balance being Al₂O₃ powder, in which a content of metallic element other than Si and Al is not more than 0.5% by weight. In this case, the sintering aid should not be added because it remains in a vitreous state in the sintered body to degrade the high-temperature properties like metal impurity. The mixing is carried out under a wet or dry state by means of a ball mill or the like. The resulting mixed powder is shaped into a desired form by dry pressing, injection molding or the like. The shaped body made from the mixed powder of Si₃N₄ powder and Al₂O₃ powder is densified by pressureless sintering, hot pressing, or hot isostatic pressing (HIP), during which a part or a whole of Si₃N₄ reacts with Al₂O₃ to form sialon. On the other hand, a part of Al₂O₃ reacts with Si₃N₄ as mentioned above, but the remaining portion of Al₂O₃ remains as α-Al₂O₃. Thus, Si₃N₄-Al₂O₃ composite sintered body consisting of α-Al₂O₃ and at least one crystal phase of Si₃N₄ and sialon according to the invention is obtained by these reactions. The sintering temperature is preferably 1,500-1,900°C for these reactions and for densification. When the sintering temperature is lower than 1,500°C, the sintered body is not sufficiently densified, while when it exceeds 1,900°C, the grain growth of Si₃N₄ and Al₂O₃ and the evaporation reaction become violent to cause the reduction of strength and bulk density and the degradation of high-temperature properties. In the mixed powder, the content of Si₃N₄ powder should be within a range of 25-45 mol%. When this content exceeds 45 mol%, the whole of the sintered body is changed into sialon, losing the composite effect. As the firing atmosphere, an inert gas atmosphere such as nitrogen, argon or the like is preferable for preventing the oxidation of Si₃N₄. In the hot isostatic pressing, there may be performed a method wherein a presintered body having less open pore is previously produced by pressureless sintering or hot pressing and then subjected to hot isostatic pressing, or a method wherein the shaped body is airtightly sealed with a metal, glass or the like and then subjected to hot isostatic pressing. The invention will be described with reference to the following examples. Example α-Si₃N₄ powder having a content of metallic element impurity other than Si and Al of 0.2% by weight and a purity of 98% and α-Al₂O₃ powder having an average particle size of 0.5 µm and a purity of not less than 99.8% were mixed at a mixing ratio as shown in the following Table 1 and further mixed in a polyethylene container containing iron balls each coated with polyethylene under a wet state of acetone for 10 hours. The thus obtained mixed powder was previously shaped into a size of 50 mm in diameter and 10 mm in thickness and pressed under a pressure of 200 MPa. Then, the shaped body was hot pressed at a temperature shown in Table 1 under a pressure of 30 MPa to obtain Si₃N₄-Al₂O₃ composite sintered bodies of Examples 1-3 according to the invention and Comparative Examples 1-4. Further, the same procedure as mentioned above was repeated, except that Y₂O₃ or MgO having a purity of not less than 99% was further added to the mixed powder, to obtain the composite sintered bodies of Comparative Examples 5 and 6. All of these sintered bodies were dense sintered bodies having a porosity of not more than 1%. In Table 1 are shown results on the crystal phase observed by an X-ray diffraction method in Examples 1-3 and Comparative Examples 1-6. Further, in Examples 1-3 and Comparative Examples 1-6, the three-point flexural strengths at room temperature and 1,300°C were measured according to a method of JIS R1601 to obtain results as shown in Table 1. Moreover, the increase of weight per unit surface area in Examples 1-3 and Comparative Examples 1-6 was measured after the heating at 1,300°C for 100 hours to obtain results as shown in Table 1. As is seen from the above results, in the Si₃N₄-Al₂O₃ composite sintered bodies according to the invention, the three-point flexural strength at 1,300°C is not less than 550 MPa, so that there is no lowering of the strength from the strength value at room temperature, and also the increase of weight through oxidation at 1,300°C for 100 hours is not more than 0.1 mg/cm². That is, the sintered bodies according to the invention have high strength at high temperature and excellent oxidation resistance as compared with the comparative examples. As mentioned above, the Si₃N₄-Al₂O₃ composite sintered body according to the invention consists of α-Al₂O₃ and at least one crystal phase of Si₃N₄ and sialon and can have a strength of not less than 550 MPa over a temperature range of room temperature to 1,300°C and an excellent oxidation resistance, so that it is applicable to high-temperature structural materials such as engine parts and the like.

Claims: An Si₃N₄-Al₂O₃ composite sintered body, consisting of α-Al₂O₃ and at least one of the crystal phases Si₃N₄ and sialon obtained by sintering at 1500-1900°C a mixed powder comprising 25-45 mol% of Si₃N₄ and the balance Al₂O₃ powder, in which the content of metallic elements other than Si and Al is not more than 0.5% by weight. A method of producing an Si₃N₄-Al₂O₃ composite sintered body, which comprises shaping a mixed powder comprising 25-45 mol% of Si₃N₄ and the balance Al₂O₃ powder, in which the content of metallic elements other than Si and Al is not more than 0.5% by weight, and then sintering the shaped powder at 1500-1900°C.

Inventors: HIRAI, TOSHIO, NAKAHIRA, ATSUSHI 407 DOKUSHIN-RYO, NIIHARA, KOICHI 1-2-2 HASHIRIMIZU-SHUKUSHA
Applicants: HIRAI, TOSHIO, NGK INSULATORS, LTD., NIIHARA, KOICHI
Application year: 1988
Main classification: C04B35
IPC codes: C04B35