Silicon nitride sintered body and method of producing the same

Abstract: An excellent silicon nitride sintered body having a high strength at high temperatures as well as at room temperature can be provided by the method of the present invention, comprising, preparing a raw material consisting of Si₃N₄ powder, a rare earth element oxide powder, and SiC powder, and at least one of a W compound powder and a Mo compound powder, forming the raw material into a shaped body, and then firing the shaped body in N₂ atmosphere to substantially crystallize the grain boundary phase of the Si₃N₄ grains. The silicon nitride sintered body consists essentially of Si₃N₄ as a main component, and the remainder of a rare earth element compound, SiC and at least one of a W compound and a Mo compound, the grain boundary phase of Si₃N₄ grains consisting substantially of crystal phases. The silicon nitride sintered body is dense and thin in color so that uneven coloring thereof can be decreased.

Description: The present invention relates to silicon nitride sintered bodies having high strength at high temperatures, and a method of producing the same. Heretofore, for a silicon nitride sintered body containing an oxide of a IIIa Group element including rare earth elements, a method of producing the sintered body has been disclosed wherein 85 mole % or more of Si₃N₄ is mixed with 15 mole % or less of at least one oxide of IIIa series elements, and the mixture is shaped and sintered in a non-oxidizing atmosphere, as described in Japanese Patent publication No. 48-7,486. Also silicon nitride sintered body has been disclosed which consists of at least 50 wt% of Si₃N₄, not over than 50 wt% of Y₂O₃ or at least one oxide of La series elements, and 0.01-20 wt% of Al₂O₃, as described in Japanese Patent Publication No. 49-21,091. However, there are problems in that the mere addition of a rare earth element to silicon nitride cannot produce a silicon nitride sintered body having high strength at high temperatures, and that the addition of Al₂O₃ results in low softening point and hence remarkably decreased high temperature strength of the crystal grain boundary phase, though the addition of Al₂O₃ improves densification of the silicon nitride sintered body. In order to solve the problem of high temperature strength, the applicant disclosed a technique in Japanese Patent Application Laid-open No. 63-100,067 wherein a desired composition and a desired quantity ratio of rare earth element is added to Si₃N₄ powder to specify the crystal phase of the sintered body so as to achieve high strength at high temperatures. However, the silicon nitride sintered body disclosed in the Japanese Patent Application Laid-open No. 63-100,067 has a problem in that, though it achieves high strength to a certain extent at high temperatures, the strength is inferior to room temperature strength thereof. This is considered to be due to a small amount of glass phase remaining in the composition even after the crystallization of the grain boundary phase. In order to decrease the remaining glass phase, a method may be considered of calculating the whole amounts of oxygen contained in the raw materials of silicon nitride into SiO₂ amount by conversion, and enlarging the quantity ratio of rare earth element oxide to SiO₂ in the raw materials so as not to leave glass phase in the sintered body as far as possible. However, the method has a problem in that the densified sintered body is difficult to produce. An object of the present invention is to obviate the above problems and to provide a silicon nitride sintered body having high strength at temperatures from room temperature to high temperatures as well as a method of producing the same. The present invention provides a silicon nitride sintered body as set out in claim 1. The present invention also provides a method of producing a silicon nitride sintered body, as set out in claim 5. The inventors have found that a silicon nitride sintered body having a Si₃N₄ crystal grain boundary phase (intergranular phase) consisting substantially of one or more crystal phases can be obtained by adding SiC, and a W compound such as WC, and/or a Mo compound such as Mo₂C to a Si₃N₄ powder containing a desired rare earth element compound, such as rare earth element oxide, mixing, and firing the mixture in N₂ atmosphere, and that the silicon nitride sintered body can achieve high strength at temperatures from room temperature to high temperatures. Namely, the addition of a W compound and/or a Mo compound functions with the rare earth element compound to accelerate the densification of the sintered body so as to mainly improve the strength at room temperature, while the addition of SiC is effective to accelerate the densification of the sintered body as well as the crystallization of the grain boundary phase so as to mainly improve the strength at high temperatures. The addition of SiC, and at least one of a W compound and a Mo compound functions synergistically to obtain a silicon nitride sintered body having high strength at temperatures from room temperature to high temperatures. Also, the addition of a W compound and/or a Mo compound has an effect of decreasing the frequent peculiar coloring of the Si₃N₄ sintered body due to the addition of a rare earth element. Moreover, though the coloring of the sintered body due to a rare earth element is not preferable because the coloring changes when exposed to an oxygenic atmosphere, the color changing can be neglected owing to the decrease of the coloring due to the above effect. Furthermore, the addition of a W compound and/or a Mo compound functions synergistically with the addition of SiC to decrease uneven coloring of the sintered body even when the sintered body has a thick wall thickness. The amount of addition of the rare earth element oxide is preferably 2.7-10 mol %. If it is less than 2.7 mole %, a sufficient liquid phase for the densification of the sintered body can not be obtained, while if it exceeds 10 mole %, the densification can hardly be obtained even when SiC, and at least one of a W compound and a Mo compound are added to the Si₃N₄ powder containing a desired rare earth element compound. More preferably, it is 4.7-9 mole %. Generally, the optimum amount of the rare earth element oxide varies depending on the used raw material silicon nitride, and is in a range of 2.7-10 mole %. Rare earth element oxide other than Y₂O₃ and Yb₂O₃, such as Lu₂O₃, Tm₂O₃ or Er₂O₃ may be used. Mole % used herein is calculated by rare earth element oxide mole/{rare earth element oxide mole + Si₃N₄ mole}. As regards the raw material Si₃N₄, those having large α content is preferable from a viewpoint of sintering property. Desirably, oxygen content thereof is 1-3 wt%. The amount of addition of SiC is desirably in a range of 0.5-11 wt% relative to the reciped amount of silicon nitride and rare earth element oxide. If it is less than 0.5 wt%, a sufficient densification effect and crystallization acceleration effect can not be obtained. If it exceeds 11 wt%, SiC prevents the densification of the sintered product. More preferable amount is 1-5 wt%. Any SiC of α type, β type or amorphous type may be used. The amount of addition of a W compound and/or a Mo compound is desirably in a range of 0.5-3 wt% relative to the reciped amount of silicon nitride and the rare earth element oxide. If it is less than 0.5 wt%, a sufficient effect of improving the strength at room temperature may not be exhibited. More preferable amount is 1-2 wt%. In the preferred method of the invention, at first a mixture of the raw material silicon nitride powder, a rare earth element oxide powder, SiC, and at least one of a W compound and a Mo compound is prepared. Then, the resultant mixture is formed to a desired shape to obtain a shaped body. Thus obtained shaped body is fired at 1,700-2,100°C, preferably at 1,900-2,000°C in N₂ atmosphere under room pressure or under pressure, and crystallized by a temperature-decreasing process or a reheating treatment process. Impurities, particularly cationic elements, such as Al, Fe, Mg, Ca, Na or K, in the raw materials to be used, are desirably not over than 0.5 wt%, more preferably not over than 0.1 wt%. Particle diameter of the raw materials is preferably as small as possible from a viewpoint of sintering property and desirably not over than 2 µm, more preferably not over than 1 µm. As regards the crystallization of the grain boundary phase, SiC has an effect of accelerating the crystallization of the grain boundary phase, so that the temperature-decreasing process can sufficiently crystallize the grain boundary phase, if the temperature-decreasing rate down to 1,000°C is not more than 100°C/min. If the crystallization of the grain boundary phase is not sufficient due to a faster temperature-decreasing rate than the above or a some other cause, a reheating treatment process may be effected to perform crystallization. Also, the reheating treatment process may be effected for a purpose of removing a residual stress in the sintered body, or the like purpose. The reheating treatment process is preferably effected at 1,300-1,500°C. As a W compound and/or a Mo compound, WC or Mo₂C is mentioned, but metallic W or Mo, silicides or oxides of W or Mo may also be used. If the raw materials are heat treated at 1,000-1,500°C in an oxidizing atmosphere so as to form a surface layer substantially consisting of SiO₂ and a compound having a Re-Si-O structure (wherein Re represents a rare earth element) on the sintered body, the sintered body can exhibit a further high strength. For a better understanding of the present invention, reference is made to the accompanying drawings, in which; Fig. 1 is a characteristic graph of an X-ray diffractionetry of an example of the silicon nitride sintered body of the present invention. Hereinafter, the present invention will be explained in more detail with reference to examples. Example 1 A raw material powder of silicon nitride having a purity of 97 wt%, a cationic element impurity (Al, Fe, Mg, Ca, Na and K) content of total of not more than 0.1 wt%, an oxygen content of 2.2 wt%, an average particle diameter of 0.6 µm, a BET specific surface area of 17 m²/g, and an α content of 0.95; a rare earth element oxide having a purity of 99 wt%, an average particle diameter of 0.3-2.0 µm, and characteristic properties as shown in Table 1; SiC having a purity of 99 wt%, an average particle diameter of 0.4 µm, a BET specific surface area of 20 m²/g; and either one of a W compound having a purity of 99 wt%, an average particle diameter of 0.4-3 µm, and a BET specific surface area of 0.5-10 m²/g and Mo₂C having a purity of 99 wt%, a particle diameter of 0.5-4 µm, and a BET specific surface area of 0.3-10 m²/g; are reciped in the reciping ratios as shown in Table 1, and ground in a wet type mill. Then, the ground mixture is freed of water by evaporation, granulated to particles of a diameter of 150 µm to obtain a shaping powder. Thereafter, the shaping powder is formed into shaped bodies of 50×40×6 mm, fired at firing conditions as shown in Table 1 to obtain silicon nitride sintered bodies Nos. 1-20 of the present invention. SiC used in the sintered body is α type for No. 7, amorphous for No. 11, and β type for the other sintered bodies. Also, the same raw materials are used and reciped in the reciping ratios as shown in Table 1, ground, granulated and formed in the same manner as described above, and fired at the firing conditions as shown in Table 1 to obtain silicon nitride sintered bodies Nos. 21-32 of comparative examples. Temperature-decreasing rate at the firing is fundamentally 100°C/min down to 1,000°C, except for No. 9 which used a temperature-decreasing rate of 110°C/min from a heat treatment temperature of 1,400°C for 6 hrs in nitrogen. These sintered bodies are measured on relative density, crystal phases of grain boundary phase, and four-point bending strength at room temperature and 1,400°C in the manners as described below, and the results are shown in Table 1. Relative density of the sintered bodies is determined by measuring bulk density of the sintered body based on the Archimedean principle, and calculating the ratio of the bulk density to the theoretical density of the sintered body. The theoretical density is calculated from the composition of the reciped powders and the densities of the reciped powders. The densities of the reciped powders are Si₃N₄: 3.2 g/cm³, Y₂O₃: 4.8 g/cm³, Yb₂O₃: 9.2 g/cm³, Tm₂O₃: 8.8 g/cm³, Lu₂O₃: 9.4 g/cm³, Er₂O₃: 8.6 g/cm³, SiC: 3.2 g/cm³, WC: 15.7 g/cm³, Mo₂C: 9.2 g/cm³, W: 19.2 g/cm³, Mo: 10.2 g/cm³, WO₃: 7.2 g/cm³, MoO₃: 4.7 g/cm³, WSi₂: 9.4 g/cm³, and MoSi₂: 6.3 g/cm³. Four-point bending strength is measured according to JIS R-1601 "Method of Testing a Bending Strength of Fine Ceramics". Crystal phases of grain boundary phase are determined by an X-ray diffraction analysis using CuK α-ray. In Table 1, J is a crystal of caspedian structure, H is a crystal of apatite structure, K is a crystal of wollastonite structure, L is Ln₂SiO₃ (Ln: rare earth element), and S is a crystal expressed by Ln₂SiO₇. A result of an X-ray diffraction analysis of the silicon nitride sintered body of No. 6 of the present invention is shown in Fig. 1, wherein a represents β-Si₃N₄, b represents J phase which is a crystal of caspedian structure, c represents H phase which is a crystal of apatite structure, d represents WSi₂, and e represents β-SiC. As a result of chemical analyses of the sintered bodies, the chemical analyses coincided with compositions of the sintered bodies calculated from the reciping ratios of the raw materials. As clearly apparent from the above Table 1, the sintered bodies Nos. 1-20 containing a rare earth element oxide, SiC and at least one of a W compound and a Mo compound have high strength both at room temperature and 1,400°C. In contrast, the sintered bodies of Nos. 21 and 24-26 containing merely a rare earth element and SiC and the sintered bodies of Nos. 22, 23 and 27-32 containing merely a rare earth element and either one of a W compound and a Mo compound, have low strength. Example 2 The same raw materials as those of Example 1 are used and reciped in reciping ratios as described in Table 2, mixed and ground in a wet type mill. Thereafter, the ground mixture is freed of water by evaporation, granulated to particles of a diameter of 150 µm to obtain a shaping powder. Then, the shaping powder is formed into shaped bodies of a size of 50×40×6 mm, and fired at firing conditions as described in Table 2 to obtain the silicon nitride sintered bodies of Nos. 33-51 of the present invention. The silicon nitride sintered bodies of Nos. 48-51 are respectively heat treated in an oxidizing atmosphere at 1,000°C × 5 hrs, 1,200°C × 2 hrs, 1,300°C × 1 hr, or 1,500°C × 1 hr. Also, the same raw materials are used and reciped in the reciping ratios as shown in Table 2, ground, granulated and formed in the same manner as described above, and fired at the firing conditions as described in Table 2 to obtain silicon nitride sintered bodies of Nos. 52-55 of comparative examples. SiC used is β type. Temperature-decreasing rate from the firing temperature down to 1,000°C is 100°C/min. These sintered bodies are measured on bulk density, crystal phases and four-point bending strength at room temperature and 1,400°C. The results are shown in Table 2. As clearly apparent from the above Table 2, the effect of addition of WC or Mo₂C is respectively seen particularly in the increase of strength of the sintered bodies at room temperature, viewed from a comparison of Example No. 34 and Comparative Example No. 52 and a comparison of Example No. 45 and Comparative Example No. 52. As apparent from the foregoing explanations, according to the method of producing a silicon nitride sintered body of the present invention, SiC and at least one of a W compound and a Mo compound are added to Si₃N₄ powder containing a desired rare earth element oxide, and fired in N₂ atmosphere, so that the grain boundary phase of Si₃N₄ grains can be substantially made of crystal phases and hence a silicon nitride sintered body can be obtained having a high strength at temperatures from room temperature to high temperatures.

Claims: A silicon nitride sintered body essentially consisting of Si₃N₄ as a main component and the remainder being (a) at least one rare earth element compound, (b) SiC and (c) at least one of a W compound and a Mo compound in a form effective as an agent for accelerating densification of the sintered body during sintering, the grain boundary phases between the Si₃N₄ grains consisting substantially of crystal phases. A silicon nitride sintered body of claim 1, wherein the rare earth element or elements of the rare earth element compound(s) is Y and/or Yb. A silicon nitride sintered body of claim 1 or claim 2, wherein as a W compound there is present a silicide of W. The silicon nitride sintered body of any one of claims 1 to 3, wherein as a Mo compound there is present a silicide of Mo. A method of producing a silicon nitride sintered body, comprising preparing a raw material consisting of Si₃N₄ powder, a rare earth element oxide powder, SiC powder, and at least one of W powder, a W compound powder, Mo powder and a Mo compound powder, forming the raw material into a shaped body, firing the shaped body in N₂ atmosphere and substantially crystallizing the grain boundary phase between the Si₃N₄ grains with the W and/or Mo or compound thereof acting as densification acceleration agent during sintering. A method of claim 5, wherein the rare earth element or elements of the rare earth element oxide powder is Y and/or Yb. The method of claim 5 or claim 6, wherein W compound and/or the Mo compound is present and is or are selected from carbides of W, carbides of Mo, oxides of W, oxides of Mo, silicides of W and silicides of Mo.

Inventors: ISOMURA MANABU, SAKAI HIRAOKI, ISOMURA, MANABU, SAKAI, HIRAOKI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1991
Main classification: C04B35
IPC codes: C04B35