Silicon nitride ceramic and process for forming the same

Details Silicon nitride ceramic and process for forming the same Silicon nitride ceramic and process for forming the same

1996-04-10

1998-06-16

Group, Karl

Compositions: ceramic

Ceramic compositions

Refractory

501 972, 501 973, 501 981, 501 982, 264665, 264683, C04B 35584, C04B 35599

Patent

active

057670269

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a silicon nitride (encompassing "sialon") ceramic excellent in mechanical properties especially at ordinary temperatures which ceramic can be obtained by applying superplastic deformation to a silicon nitride (encompassing "sialon") sintered body applicable to machine parts including a valve train of an internal combustion engine, and a process for forming a silicon nitride ceramic according to which process a silicon nitride sintered body can be formed into a desired shape through superplastic deformation thereof.


BACKGROUND ART

For the purpose of attaining lighter weights and miniaturization of machine parts including a valve train of an internal combustion engine as well as improving the durability thereof, there have recently been attempts to use ceramic materials excellent in abrasion resistance and sliding properties as materials of parts. Among such ceramics, silicon nitride (Si.sub.3 N.sub.4, encompassing "sialon") ceramics have attracted particular attention because of their light weights and high strengths.
In order to put silicon nitride ceramics into practical use as parts of a valve train, etc., however, there is need for not only an improvement in the mechanical strengths of the materials thereof but also development of a forming process excellent in aspects of productivity and cost.
Specifically, since silicon nitride is such a brittle material as to be extremely poor in processability, a method wherein a starting powder material is molded according to casting, molding with a mold, injection molding or the like, and then sintered is adopted for production of a complicatedly shaped article. Since shrinkage is involved in sintering according to the foregoing method, however, subsequent mechanical working such as cutting, grinding and/or polishing is necessary in the field where an accuracy is required, whereby an increase in production cost is incurred to become a hindrance to mass production.
On the other hand, plastic working of some kinds of ceramics through utilization of superplasticity thereof has recently attracted attention. For example, Japanese Patent Publication No. 5,282/1991 proposes a forming process wherein the superplasticity of an oxide ceramic (especially ZrO.sub.2) is utilized. On the other hand, Japanese Patent Laid-Open No. 197,003/1991 discloses that a silicon nitride-silicon carbide composite sintered body having fine and isometric grains can be formed at controlled temperature and strain rate by virtue of the superplasticity thereof.
In such plastic working through utilization of superplasticity, a ceramic material can be formed into a predetermined shape with a good accuracy, without any conventional mechanical working step, to attain mass production with a good productivity. However, the present status of art is that oxide ceramics such as ZrO.sub.2 account for an overwhelming majority of ceramic materials capable of exhibiting superplastic deformation, while only composite materials containing at least two kinds of non-oxide ceramics, such as Si.sub.3 N.sub.4 --SiC ceramics, have been reported as non-oxide ceramic materials.
Since there are no examples of silicon compound ceramics capable of superplastic deformation except for peculiar composite materials containing two mutually different silicon compounds (Si.sub.3 N.sub.4 --SiC) as described above, there has been a demand for materialization of a simple silicon nitride (encompassing "sialon") ceramic capable of superplastic deformation.
Under the foregoing circumstances of prior art, an object of the present invention is to find simple sintered bodies of silicon nitride and sialon capable of exhibiting superplastic deformation as has hitherto been unknown, provide a forming process wherein the superplastic deformation thereof is utilized, and provide a silicon nitride ceramic, which may be a sialon ceramic, formed by the foregoing forming process and having excellent mechanical properties especially at ordinary temperatures.


DISCLOSURE OF INVENTION

In order to

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