Compositions: ceramic – Ceramic compositions – Carbide or oxycarbide containing
Patent
1992-12-14
1994-10-04
Bell, Mark L.
Compositions: ceramic
Ceramic compositions
Carbide or oxycarbide containing
501 97, 501 99, 501154, 264 65, 264 66, C04B 3558
Patent
active
053526414
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a structural ceramic to be used for automobile parts, wear-resistant tools, etc., and more particularly to a method of increasing the strength and toughness of a silicon nitride ceramic.
BACKGROUND ART
Silicon nitride is a material well balanced in strength, fracture toughness and resistances to-corrosion, abrasion, thermal shock and oxidation, etc., and has become the center of attraction recently as an engineering ceramic for structural members at room temperature and high temperature. However, in order to use silicon nitride ceramics in fields requiring high reliable materials, for example, in automobile parts, etc., it is indispensable to further improve the fracture toughness of the ceramics to overcome the brittleness and increase the strength thereof. An increase in the strength of a ceramic that is a polycrystalline material has heretofore been contrived by refining the individual crystal grain thereof, but this method lowers the fracture toughness of the material which makes it more brittle. Japanese Patent Publication No. 265173/1987 discloses a technique for improving the fracture toughness of a ceramic material by combining a silicon nitride matrix with silicon carbide wherein the whiskers are dispersed in the matrix. It is thought that according to the above-mentioned method the fracture toughness of the ceramic material is improved as the cracks, which may progress and expand during the fracture, are deflected by the whiskers or because extraction or crosslinking of the whiskers takes place. The fracture toughness of the ceramic material is therefore improved by the combination with the whiskers. However, it is difficult to completely remove the agglomerates of the whiskers by a mechanical means and when the size thereof is of the order of 1 to 10 .mu.m, such whiskers or agglomerates, like the coarse grain, form breaking points, thereby decreasing the strength of the ceramic material. Such a decrease in strength is also observed in a composite material of long fibers. In addition, the composite material of the particle dispersion type that is formed by mechanically mixing the particles having a diameter on the order of microns with the matrix and firing the mixture cannot exhibit the remarkable compounding effect of the dispersed particles in both strength and toughness.
According to the conventional process for producing a composite sintered body such as a process in which silicon nitride, acting as the host phase, is mechanically mixed with silicon carbide in a dispersed phase to form a mixture which is then sintered, it is substantially impossible to form the dispersed particles having a sufficiently small average particle size in the sintered body, since the average particle size of the raw powder material is of the order of one micron or at least several hundred nanometers. Accordingly one cannot expect to increase the strength of the composite sintered body by the above conventional process.
Further Japanese Patent Laid-Open No. 159256/1988 discloses a process in which silicon carbide particles having an average particle size of 1 .mu.m or smaller are homogeneously dispersed in silicon nitride particles and the silicon nitride particles are subjected to grain growth to form columnar crystals. However, even with the aforementioned composite sintered body of silicon nitride-silicon carbide, a lower proportion of silicon carbide tends to form silicon nitride of columnar crystal, thus minimizing the improvement in strength in spite of some improvement in fracture toughness, whereas a higher proportion of silicon carbide suppresses the formation of silicon nitride of columnar crystal, thereby lowering the fracture characteristics, thereby decreasing the strength.
Under such circumstances, in order to obtain a composite material with fine dispersed particles as a sintered body, it is effective to adopt a process for producing dispersed particles in situ during the sintering by compositing the raw powder materials themselves. For example, as
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Matsui Jin-Joo
Yamakawa Akira
Bell Mark L.
Bierman Jordan B.
Jones Deborah
Sumitomo Electric Industries Ltd.
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