Compositions: ceramic – Ceramic compositions – Carbide or oxycarbide containing
Patent
1993-05-25
1995-08-08
Bell, Mark L.
Compositions: ceramic
Ceramic compositions
Carbide or oxycarbide containing
C04B 3551
Patent
active
054398533
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a mixed oxide ceramic sintered body having high strength, excellent toughness and high reliability and no defect such as pore, microcrack and the like as well as a method of producing the same, and particularly to (Ln--Al--Si) oxide composite ceramics (Ln is one or more rare earth elements and a mixture thereof) having a uniform structure and a technique for producing the same.
BACKGROUND ART
In general, ceramic materials are large in the strength at high temperature, and excellent in the heat resistance, oxidation resistance and corrosion resistance, so that they are particularly desirable as a structural material. Especially, a great interest is taken in the application of such ceramics as a structural material used in a temperature region exceeding a use limit of a metal. Among these ceramics, silicon carbide (abbreviated as SiC hereinafter) or silicon nitride (abbreviated as Si.sub.3 N.sub.4 hereinafter) is excellent in the heat resistance and oxidation resistance, so that it is very desirable as a structural material capable of using at a high temperature.
However, SiC or Si.sub.3 N.sub.4 is hardly sintered, and is a material that it is very difficult to provide a dense sintered body without adding an assistant. For this end, the firing has hitherto been conducted by adding Al.sub.2 O.sub.3 as a sintering aid to SiC powder, or by adding Y.sub.2 O.sub.3, CeO.sub.2, Al.sub.2 O.sub.3, AlN, MgO or the like as a sintering aid to Si.sub.3 N.sub.4 powder.
Therefore, the SiC sintered body obtained by using Al.sub.2 O.sub.3 or the like as a sintering aid becomes dense through liquid-phase sintering, but pores are apt to be generated owing to the reaction between Al.sub.2 O.sub.3 and SiC. As a result, the strength is not more than 600 MPa and the toughness value at break is 5 MPa.m.sup.1/2, and a m-value of Weibble distribution as an indication showing the reliability of ceramics is not more than 10, so that it could be said to be a poor reliability material.
On the other hand, the Si.sub.3 N.sub.4 sintered body obtained by using Y.sub.2 O.sub.3, CeO.sub.2, Al.sub.2 O.sub.3, AlN, MgO or the like as a sintering aid is dense and large in the strength, but is low in the toughness as compared with zirconia ceramics. As a result, the Si.sub.3 N.sub.4 sintered body having a large strength at high temperature, an excellent oxidation resistance and the like is a material having a poor reliability in use as a structural material because the toughness is still low.
On the contrary, a part of the inventors has previously proposed a technique that the strength and toughness value and reliability of the SiC sintered body are improved by composing SiC with other ceramics. For example, a technique of producing a dense SiC-rare earth oxide-alumina composite sintered body through pressureless sintering is proposed by Omori, Takei and so on in J. Am. Ceram. Soc., 65 (1982) C-92. Further, there is proposed a technique of producing a dense SiC-rare earth oxide-alumina composite sintered body by raising the firing temperature in the pressureless sintering to 2150.degree. C. to form Al metal and Si semiconductor (see J. Mater. Sci., 23 (]988) 3744-3749 by Omori and Takei).
As a means for increasing the toughness of the Si.sub.3 N.sub.4 sintered body, a technique of improving the strength and toughness value and reliability of the Si.sub.3 N.sub.4 sintered body by anisotropically growing Si.sub.3 N.sub.4 crystal is proposed by Kawashima et al (see Takeshi Kawashima, Hiromi Okamoto, Hideharu Yamamoto and Akira Kitamura, Silicon Nitride Ceramics 2, Uchidarokauho, p135-146). That is, such a proposed technique is a method of improving the toughness of Si.sub.3 N.sub.4 -rare earth oxide-alumina composite sintered body using Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 as a sintering aid by placing a green shaped body in a capsule and sintering it under a gas pressure to anisotropically grow Si.sub.3 N.sub.4 crystal.
In the SiC sintered body synthesized by the conventional technique, small defects are
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Communications of the American Ceramic Society, Pressureless Sintering of SiC, Mamoru Omori and Humihiko Takei, C-92 Jun. 1982.
Journal of Materials Science, Preparation of pressureless-sintered SiC-Y.sub.2 O.sub.3 -Al.sub.2 O.sub.3, Mamoru Omori and Humihiko Takei, 3744-3749 (23) 1988, no month.
Hirai Toshio
Omori Mamoru
Bell Mark L.
Wright A.
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