Abrasive tool making process – material – or composition – With inorganic material – Metal or metal oxide
Patent
1990-12-07
1992-06-23
Group, Karl
Abrasive tool making process, material, or composition
With inorganic material
Metal or metal oxide
501 89, 501 91, 501 92, 501 95, 501 96, C04B 3510
Patent
active
051239353
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention concerns Al.sub.2 O.sub.3 composites having high strength and toughness, process for producing them and throw-away tips made of Al.sub.2 O.sub.3 composites.
BACKGROUND ART
Al.sub.2 O.sub.3 composites are excellent in corrosion resistance, oxidation resistance and wear resistance and they have been known, together with Si.sub.3 N.sub.4 ceramic, etc., as structural materials suitable to machinery parts, jigs, tools, etc., i.e., as engineering ceramics. Although Al.sub.2 O.sub.3 composites are poor in the strength and fracture toughness, as compared with Si.sub.3 N.sub.4 ceramic, since they are sinterable at low temperature and highly pure Al.sub.2 O.sub.3 powder can be available easily at a reduced cost they are outstandingly advantageous in view of the economy or mass production and various studies have been made for improving the foregoing subject in view of the characteristic of Al.sub.2 O.sub.3 composites.
For instance, U.S. Pat. No. 4,554,445 discloses Al.sub.2 O.sub.3 composites in which acicular SiC whiskers are dispersed in an Al.sub.2 O.sub.3 series matrix. The Al.sub.2 O.sub.3 ceramics in which SiC whiskers are dispersed can be produced by dry-mixing a powder of Al.sub.2 O.sub.3 with an average grain size of 0.3 .mu.m and SiC whiskers and then sintering them under uniaxial pressing under the condition of 1850.degree. C., 41 MPa for 45 min and it is described that the fracture toughness is improved by about 8 to 9 kg/mm.sup.3/2 as compared with conventional Al.sub.2 O.sub.3 composites and the fracture strength can also be improved up to 800 MPa (81.6 kg/mm.sup.2).
However, even the Al.sub.2 O.sub.3 composites having SiC whiskers dispersed therein are still poor in view of the strength and the toughness as compared with Si.sub.3 N.sub.4 ceramic. Further, in a case where Al.sub.2 O.sub.3 composites partially contain coarse crystals with the reason as described later, the strength and the toughness are further lowered than those of the conventional Al.sub.2 O.sub.3 composites, and the relatively high strength or toughness as described above can not easily be reproduced.
That is, coarse crystals are grown in the production process as described above, because the sintering temperature has to be increased owing to the addition of SiC, etc. and, for example, the sintering temperature increases as high as 1850.degree. C. Since SiC whiskers have an effect of suppressing crystal grain growth, no abnormal growth in the crystal grains is caused during sintering if the SiC whiskers are uniformly dispersed. Abnormal growth of the crystal grains occurs in a case where SiC whiskers are localized to result in portions lacking in SiC whiskers.
In view of the above, as a method of suppressing the formation of such coarse crystals, it may be considered a method of at first adding SiC whiskers to a solvent to form a slurry-like mixture (hereinafter simple referred to as a slurry) and then mixing the slurry with an Al.sub.2 O.sub.3 powder for uniformly dispersing the SiC whiskers. However, it is extremely difficult to uniformly disperse the SiC whiskers in the solvent and uniformly disperse the SiC whiskers in the solvent and uniform dispersion of the SiC whiskers in the slurry requries long time mixing. On the other hand, since the long time mixing breaks the acicular SiC whiskers to remarkably lower the aspect ratio, the SiC whiskers can no more contribute to the improvement of the strength and the toughness in the above-mentioned method, by which no improvement can be expected for the strength and the toughness of the sintering product. In addition, in the method as disclosed in the U.S. patent, the mixture of the Al.sub.2 O.sub.3 powder and the SiC whiskers are sintered under uniaxial pressing and the molding is applied only by the press-sintering. Accordingly, the density of the ceramics is relatively low in the production method described above and since it is difficult to increase the relative density to greater than 95%, there is also a problem that products of high densi
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Kanamaru Moriyoshi
Ohyama Noboru
Tatsuno Tsuneo
Group Karl
Kabushiki Kaisha Kobe Seiko Sho
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