JP2972836B2 - Forming method of composite ceramics - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for forming a composite ceramic, and more particularly to a method for forming a silicon nitride-silicon carbide composite sintered body using superplasticity.

[Conventional technology and its problems]

Silicon nitride and silicon carbide have recently attracted attention and are used in various fields as so-called non-oxide ceramics for structural materials.

Such silicon nitride and silicon carbide have begun to be used due to their heat resistance, thermal shock resistance, abrasion resistance, and corrosion resistance. However, since they are so-called brittle materials, their workability is extremely poor. The silicon member is manufactured by molding a raw material powder by casting, die molding, injection molding, or the like, and after sintering. In addition, although a conventional molding method can give a considerably complicated shape, dimensional shrinkage occurs at the time of sintering. Therefore, in a member requiring accuracy, this molded product is subjected to primary sintering, cutting, and secondary sintering. After tying, the final product had to be obtained by grinding and polishing. The property of being hard or brittle, which should be the original advantage of ceramics, has resulted in the production of ceramics requiring a great deal of labor in molding and processing steps. In addition, such inferior processing efficiency causes an increase in manufacturing cost, and is a major obstacle to mass production of ceramic members.

On the other hand, in the case of metal materials that exhibit large ductility, they are manufactured efficiently and inexpensively by so-called plastic working.
This good workability is a major factor in widespread use of metal materials. If such plastic working can be applied to non-oxide ceramics such as silicon nitride and silicon carbide, the production cost will be much lower than in conventional production, and the mass production of ceramics for structural materials or the dramatic expansion of applications will increase. Expected.

However, conventional silicon nitride and silicon carbide show brittle fracture at room temperature, and in a high temperature region of 1200 ° C. or more, for example, the deformation amount until creep rupture in a uniaxial tensile creep test is 0.8% in hot-pressed silicon nitride.
%, 3% or less in normal pressure sintered silicon nitride, and breaks without stable deformation. For this reason, it was impossible to apply plastic working to silicon nitride or silicon carbide.

By the way, it is difficult to plastically process certain metal alloys, and in such metal materials, it is possible to generate fine crystal grains at a controlled strain rate and temperature range, and to form and process by causing so-called superplastic deformation. Is being done. This superplastic deformation causes deformation under stress much lower than the normal yield point without causing constriction, and can be as much as several hundred percent depending on the kind of material. It is known that a member having a complicated shape can be manufactured relatively inexpensively by such a method, even if the metal alloy is difficult to plastically process.

This phenomenon of superplastic deformation has not yet been found in non-oxide ceramics such as silicon nitride and silicon carbide.

The present inventors have thought that such superplastic deformation is necessarily present in non-oxide ceramics such as silicon nitride and silicon carbide, and as a result of intensive studies, as a result, silicon nitride having fine and equiaxed grains- In a uniaxial tensile test at a controlled temperature and strain rate, the silicon carbide composite sintered body was found to exhibit large ductility without forming necking, that is, exhibit superplasticity. An object of the present invention is to provide a method for forming a non-oxide ceramic which has not been known hitherto, by utilizing superplastic deformation.

[Means for solving the problem]

The present invention relates to a method for forming a composite ceramics characterized by deforming a silicon nitride-silicon carbide composite sintered body under a stress action in a superplastic temperature range, and forming the composite ceramics. Silicon nitride-silicon carbide composite sintered body having a microstructure having an average particle size of 2 μm or less, wherein silicon carbide is mainly composed of β phase, the content thereof is 10 to 50% by volume, and silicon nitride has at least β phase A silicon nitride-silicon carbide composite sintered body mainly containing equiaxed particles having a phase of 40% or more and an average particle diameter of 2 µm or less is applied in a superplastic temperature range of 1400 ° C to 1700 ° C. The present invention relates to a method for forming a composite ceramic which is deformed and formed at a strain rate of 10 ^-1 sec ^-1 or less.

When the particle size of the silicon nitride-silicon carbide composite sintered body used in the present invention is 2 μm or more, and the particle size is not equiaxed, and has a structure in which, for example, columnar particles are intertwined. Cannot apply the molding method of the present invention.
This is because cavities are formed when a stress is applied in a temperature range where deformation occurs, and the strength and other mechanical properties of the obtained molded body are significantly deteriorated. For example,
Conventionally used silicon nitride has a large particle size and a structure in which columnar particles are entangled, so the amount of deformation is small even at high temperatures, and it breaks without stable deformation and generates cavities This will cause significant deterioration of mechanical properties.

Further, the content of silicon carbide is suitably from 10 to 50% by volume, and if the content is less than this, grain growth occurs in the deformation process to hinder deformation, or conversely, if the amount of silicon carbide is large, In such a case, it becomes difficult to obtain a dense sintered body. Furthermore, at least the β phase of the silicon nitride formation phase is 40
%. When the β phase is small, α-β transition occurs in the molding process of the present invention, and grain growth and columnarization are promoted, so that stable deformation cannot be caused.

The silicon nitride-silicon carbide composite sintered body suitable for the molding method of the present invention has a composition comprising silicon, carbon, nitrogen and oxygen, has a carbon content of 3.0 to 15.0% by weight, and has an average particle size of 1 μm. It is obtained by mixing the following amorphous powder with a sintering aid that generates a liquid phase in the sintering process, and sintering at a sintering temperature of 1500 to 2000 ° C.

The amorphous powder having the above composition is disclosed in, for example, JP-A-60-200812, JP-A-60-20013, JP-A-60-221311, JP-A-60-235707, Kaisho
A powder comprising amorphous silicon, carbon, nitrogen and oxygen obtained by a method disclosed in JP-A-61-117108 or the like is used. This powder already has silicon carbide and silicon nitride well mixed at the raw material stage, and is suitable for obtaining a sintered body having a uniform microstructure suitable for the present invention by uniformly dispersing silicon carbide. is there. The silicon carbide present in the raw material powder serves as a nucleus for crystallization of silicon nitride during the sintering process and acts to suppress the growth of silicon nitride grains. The microstructure of the silicon-silicon carbide composite sintered body will be different.

That is, when the amount of silicon carbide is small, nitrogen silicon has a structure with many columnar particles, and when the amount of silicon carbide is large, silicon nitride suppresses grain growth, so that equiaxed particles are formed. Easy to have many structures.

Therefore, in order to obtain a sintered body suitable for the growth method of the present invention, the carbon content of the raw material powder needs to be 3.0 to 15.0% by weight. If the carbon content is less than this, columnarization and grain growth of silicon nitride occur, and the molding method of the present invention cannot be applied. If the amount of carbon is larger than this, it is difficult to obtain a dense sintered body.

A sintering aid that generates a liquid phase during sintering of the raw material powder of the present invention having an appropriate carbon content, or a molding binder when molding is performed, is added and mixed well.

As the sintering aid used to obtain the sintered body used in the present invention, any of those conventionally used for silicon nitride and silicon carbide can be used. For example, Mg
O, Al ₂ O ₃ , Y ₂ O ₃ , AlN, SiO ₃ and lanthanum-based oxides are exemplified, and these can be used alone or in combination.

The amount of these sintering aids used is usually in the range of 0.1 to 20% by weight. This mixing method may be any of the conventional dry or wet mixing methods.

Next, except for the case of directly sintering from powder such as hot pressing or HIP, a predetermined shape is given in advance by a method such as mold molding, casting molding or injection molding.

As the sintering method, a conventional method such as ordinary atmospheric sintering, hot pressing, gas pressure sintering, or HIP can be applied as it is. The sintering temperature depends on the sintering method, but is suitably 1500 to 2000 ° C. Usually, the temperature and time during which the grain growth of the composition is suppressed and the columnarization of silicon nitride does not progress are selected. The sintering conditions are also affected by the composition of the raw material powder used. For example, if 10
In the case of sintering the powder containing weight% by a typical hot press method, 1600 to 1750 ° C, 200 to 400 Kg / cm ² , 1 to 1
Sintered for 3hrs.

When a powder having a carbon content of 6% by weight is used as a raw material, suitable conditions are 1500-1700 ° C., 200-400 kg / cm ² , and 0.5-5.0 hrs. It goes without saying that such conditions are used and are affected by the sintering aid.

The silicon nitride-silicon carbide composite sintered body thus obtained is a sintered body in which silicon carbide is mainly composed of a β phase and silicon nitride contains at least a β phase of 40% or more,
The constituent particles have an average particle size of 2 μm or less and are mainly equiaxed. The relative density must be at least 90% or more.

On the other hand, in the case of HIP or gas pressure sintering, the decomposition temperature of silicon nitride can be increased, so that the sintering temperature can be increased. By such a sintering method, it is also possible to change silicon carbide in the silicon nitride-silicon carbide composite sintered body into a phase rich in α phase.

 Next, the molding method of the present invention will be specifically described.

The silicon nitride-silicon carbide composite sintered body manufactured as described above is formed into a predetermined shape by applying a stress in a temperature range showing superplasticity. This temperature range is usually 1400-1700
° C, preferably 1450-1650 ° C. In a temperature range higher than this, thermal deterioration of the silicon nitride-silicon carbide composite sintered body occurs, and the physical properties of the obtained molded body deteriorate. Further, in the temperature range below this, the molding speed becomes slow, which is not economically preferable. The strain rate is optimized by the molding temperature.However, if molding is performed at an excessively high rate, cavities due to grain boundary sliding are generated in the material, and these are connected to form a large defect, resulting in a large defect. The strength may be reduced or the mechanical properties may be adversely affected as the material becomes more porous.
Therefore, the strain rate during molding is 10 ^-1 sec ^-1 or less,
It is preferable to carry out at 10 ⁻³ sec ⁻¹ or less.

The atmosphere at the time of formation may be either air or non-oxidizing atmosphere, but it is preferable to carry out in a non-oxidizing atmosphere. This is to prevent the silicon nitride-silicon carbide composite sintered body from being deteriorated by oxidation due to oxidation.When molding is completed in a short time, an oxidizing atmosphere may be used. Is preferably performed in a non-oxidizing atmosphere.

The jig used for this molding is made of ceramics, graphite, or a super heat-resistant alloy, and is selected according to the atmosphere during molding and the molding temperature.

By molding in such a superplastic temperature range, for example, a thin plate can be formed by compression, a thin plate can be bent, or precision molding can be performed in a precision mold processed into a predetermined shape. . In addition, this molding method can process only a specific portion that requires particularly high accuracy, and can improve manufacturing efficiency and cost. As described above, according to the present invention, it is possible to guide from a sintered body having a simple shape to a sintered body having a complicated shape.

〔The invention's effect〕

The average particle size of 2μ thus produced by liquid phase sintering
m, a silicon nitride-silicon carbide composite sintered body having a microstructure of not more than m is deformed under the action of stress in a superplastic temperature range and molded, thereby performing a predetermined molding without complicated molding and processing steps as in the prior art. Precision molding can be performed on the shape.

Further, according to the present invention by superplastic deformation, a complicated shape can be provided from a silicon nitride-silicon carbide composite sintered body having a simple shape. Further, in the present invention, it is possible to achieve mass production of ceramic members by using many molds to be used in molding of the same shape.

Therefore, high-temperature high-strength members and heat insulating members such as gas turbines and engines required for dimensional accuracy by the present invention,
Alternatively, it is possible to efficiently produce wear-resistant materials and cutting tools.

Next, examples of the present invention are shown together with comparative examples. The embodiments described below are merely examples of the present invention, and the present invention is not limited thereto without departing from the gist of the present invention.

Examples 1 and 2 An amorphous powder composed of silicon, carbon, nitrogen and oxygen containing 9.6% by weight of carbon and having an average particle size of 1 μm or less was mixed with Y ₂
After adding O ₃ , 6 wt% and Al ₂ O ₃ 2 wt%, wet-mixing in ethanol and drying, filling in a graphite die with a diameter of 50 mm, hot pressing at 1700 ° C. for 1 hr at 350 kg / cm ^{3 in} nitrogen gas Press sintering was performed. The obtained sintered body has a density of 3.28 g / cm ³ , a β conversion rate of nitrogen silicon of 96%, and a silicon nitride-silicon carbide composite sintered body mainly composed of equiaxed particles having an average particle diameter of 1 μm or less. Met.

From this sintered body, a specimen having a cylindrical portion having a diameter of 3 mm and a length of 10 mm was prepared, and in a high-temperature furnace set at 1650 ° C. in a nitrogen atmosphere, a strain rate of 8 × 10 ^-5 sec ^-1 and 4 × 10 ^-5 s
Tensile deformation was performed at ec ^-1 . The deformation resistance was 20 to 50 MPa, and the nominal strain after deformation was 110% in each case. Also,
No constriction due to deformation was observed, and uniform elongation was observed.

Example 3 The same sintered body (5 × 5 × 3 mm) as in Examples 1 and 2 was set in argon at 1650 ° C. via a graphite plate. This was compression-deformed at 8 × 10 ⁻⁵ sec ⁻¹ to obtain a thin plate having a thickness of 2 mm.
No cracks or alteration were found in this sheet.

Examples 4 to 6 An amorphous powder having a carbon content of 6.2% by weight consisting of silicon, carbon, nitrogen and oxygen having a particle size of 1 μm or less was mixed with 6% by weight of Y ₂ O ₃ and Al.
₂ O ₃ 2wt% was added, wet-mixed in ethanol and dried, then filled into a 50mm diameter graphite die, 350K in nitrogen gas
Hot press sintering was performed at 1650 ° C. for 1 hour under a pressure of g / cm ³ .

The obtained sintered body has a density of 3.29 g / cm ³ , a β conversion ratio of nitrogen silicon of 73%, and a silicon nitride-silicon carbide composite sintered body mainly composed of equiaxed particles having an average particle diameter of 1 μm or less. Met.

Specimens similar to those of Examples 1 and 2 were prepared from this sintered body, and the strain rate was 8 × 10 ⁻⁵ sec ⁻¹ and 4 × 10 ⁻⁵ sec at 1600 ° C.
As a result of conducting a tensile test under the conditions of ^-1 and 2 × 10 ^-5 sec ^-1 , the nominal strains after deformation were 56%, 105% and 210%, respectively.
Met.

Examples 7 to 9 From the same raw material powders as in Examples 4 to 6, a predominantly equiaxed silicon nitride-silicon carbide composite sintered body having an average particle size of 2 µm or less shown in Table 1 was obtained by hot press sintering.

Specimens similar to those of Examples 1 and 2 were prepared from this sintered body, and subjected to tensile tests under various temperature conditions at a strain rate of 8 × 10 ⁻⁵ sec ⁻¹ , and the results shown in Table 1 were obtained.

Comparative Example 1 A commercially available silicon nitride sintered body in which columnar particles were entangled was subjected to a strain rate of 8 × 10 ⁻⁵ sec ⁻¹ and a deformation temperature of 1600 ° C. in the same manner as in Example 4.
When a tensile test was performed, the sample was broken at a strain of 5%.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yinmasa Kawakami Niigata City Niigata City Niigata 182 Shinwari Sanritsu Gas Chemical Co., Ltd. Niigata Research Laboratories (56) References JP-A-62-278169 (JP, A JP, A 62-52191 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C04B 35/56-35/58 C04B 41/80

Claims (1)

(57) [Claims] 1. An average particle size of 2 μm produced by liquid phase sintering.
m, a silicon nitride-silicon carbide composite sintered body having a microstructure of not more than m, wherein silicon carbide is mainly composed of a β phase and the content thereof is 10 to 50% by volume. Containing, the average particle diameter of the particles constituting the sintered body is 2μm or less mainly silicon nitride-silicon carbide composite sintered body consisting of equiaxed particles, superplastic temperature range 1400 ~ 17
10 ^-1 under the action of tensile or compressive stress at 00 ° C
A forming method of a composite ceramics, characterized by forming by deforming at a strain rate of sec ^-1 or less.