Compositions: ceramic – Ceramic compositions – Refractory
Patent
1990-03-02
1992-03-03
Dixon, Jr., William R.
Compositions: ceramic
Ceramic compositions
Refractory
501 98, C04B 3558
Patent
active
050932900
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a silicon nitride-type material with reduced deterioration of strength at high temperature suitable for super-high speed cutting tools or high temperature structural material.
BACKGROUND OF THE INVENTION
Heretofore, as Si.sub.3 N.sub.4 material, those described in, for example, Japanese Patent Unexamined Publication (KOKAI) Sho 49-21091, Sho 57-95873, Sho 59-182276, etc. have been known.
As production processes for them, there are a production method for .beta.-Si.sub.3 N.sub.4 in a Si.sub.3 N.sub.4 -Y.sub.2 O.sub.3 -Al.sub.2 O.sub.3 system by adding Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 or in a Si.sub.3 N.sub.4 -Y.sub.2 O.sub.3 -MgO-Al.sub.2 O.sub.3 system by adding Y.sub.2 O.sub.3 -MgO-Al.sub.2 O.sub.3 as a sintering aid, as well as a production method for .alpha.'-Si.sub.3 N.sub.4 and .beta.-Si.sub.3 N.sub.4 in a Si.sub.3 N.sub.4 -Y.sub.2 O.sub.3 -AlN series .alpha.-sialon or Si.sub.3 N.sub.4 -Y.sub.2 O.sub.3 -Al.sub.2 O.sub.3 -AlN series by adding Y.sub.2 O.sub.3 -AlN.
However, a glass phase comprising Si, N and additive ingredients is present between Si.sub.3 N.sub.4 particles, which bind crystal grains of Si.sub.3 N.sub.4 in any of the sintered bodies.
The glass phase is softened at high temperature to reduce the strength. That is, deterioration of the strength is remarkable at a temperature higher than 800.degree. C. and the strength is reduced to about one-half, in relation to normal temperature bending strength, at a temperature higher than 1200.degree. C.
For preventing the strength deterioration at high temperature, a Si.sub.3 N.sub.4 series material free from any glass phase, for example, a solid-solution-type Si.sub.3 N.sub.4 such as .beta.-sialon or a reaction-sintered Si.sub.3 N.sub.4 compound of Si.sub.3 N.sub.4 alone may be satisfactory. However, although these materials show less deterioration of strength at high temperature the strength itself is low at normal temperature and they are not suitable for structural material and tool material.
As for other means, although it is possible to prevent softening at high temperature by crystallizing the glass phase, this requires heat treatment in addition to sintering, making the steps complicated.
The principal object of the present invention is to obtain silicon nitride material having bending strength at normal temperature comparable with that of conventional silicon nitride having a glass phase but, in addition having much less deterioration of strength at high temperature.
SUMMARY OF THE INVENTION
According to the present invention, the foregoing object has been attained by using an Y.sub.2 O.sub.3 -MgO-AlN system as a sintering aid and controlling the stoichiometrical ratio of Y, Mg, Al in the sintered body within a specific range.
That is, in accordance with the present invention, it is possible to obtain silicon nitride material having a bending strength at normal temperature of not less than 100 kg/mm.sup.2, which is comparable with that of conventional .beta.-Si.sub.3 N.sub.4 having a glass phase, and with much less deterioration of the strength, such as deterioration of strength at high temperature of not greater than 10% at 1000.degree. C. and 20% at 1200.degree. C. as compared with conventional material, by controlling the mol number of Y, Mg, and Al present in the sintered body of the Si.sub.3 N.sub.4 -Y.sub.2 O.sub.3 -MgO-AlN series.
As specific conditions in the stoichiometrical ration of Y, Mg and Al in the sintered body described above, the following three conditions are to be satisfied: ##EQU3## where A.sub.1, B.sub.1 and C.sub.1 represent the atomic mol numbers of Y, Mg and Al, respectively, per one mol of Si.sub.3 N.sub.4.
Stated in other terms, the stoichiometrical ratio of Y.sub.2 O.sub.3, MgO and AlN in the sintered body described above must satisfy the following four conditions: ##EQU4## wherein A.sub.2, B.sub.2 and C.sub.2 represent the molecular mol numbers of Y.sub.2 O.sub.3, AlN and MgO, respectively, per one mol of Si.sub.3 N.sub.4.
In the silicon nitride-type sintered body accordi
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Furukawa Mitsuhiko
Nagano Mitsuyoshi
Shiroyama Masaharu
Takano Yasumi
Dixon Jr. William R.
Green Anthony J.
Nippon Tungsten Co. Ltd.
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