JPH04103303A - Molding method for composite ceramic - Google Patents

Abstract

PURPOSE:To enhance strength, fracture toughness value and resistance to heat by superplastic deforming a silicon nitride-silicon carbide composite sintered body composed of regular particles under tensile or compression stress, and then heat treating the same in the non-oxidizing atmosphere, under the normal pressure or pressurized state and at the specified temperature. CONSTITUTION:A silicon nitride-silicon carbide composite sintered body composed of regular particles mainly is molded into the given shape by applying stress in the temperature zone showing superplasticity. Torsion speed under tensile strength or compression stress is made optimum by the molding temperature. The constitution of the superplasticized sintered body is mainly composed of regular particles same as before the superplasticizing and can be precision formed into the given shape without passing through the complicated molding and other processes. Heat treatment is applied under the non-oxidizing atmosphere under the normal pressure or pressurized state and at 1000-2300 deg.C after superplasticizing to change the constitution of a sintered body and enhance strength and modify a grain boundary phase to a composition of high resistance to heat, transform a glass phase to a crystalline phase and enhance resistance to heat.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a method for forming composite ceramics, and more specifically, a silicon nitride-silicon carbide composite sintered body mainly composed of equiaxed particles is subjected to tensile stress or The present invention relates to a method for forming composite ceramics, which is characterized by carrying out heat treatment after superplastically deforming and forming under the action of compressive stress (hereinafter sometimes referred to as superplastic working).

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

Silicon nitride and silicon carbide have begun to be used because of their heat resistance, thermal shock resistance, wear resistance, and corrosion resistance, but since they are so-called brittle materials, their moldability is extremely poor. Generally, silicon nitride and silicon carbide members are manufactured by molding raw material powder by casting, molding, injection molding, etc., and processing after sintering. In addition, although conventional molding methods can give quite complex shapes, dimensional shrinkage occurs during sintering, so for parts that require precision, this molded product must be processed through primary sintering, cutting, and secondary sintering. After bonding, the final product had to be obtained by grinding and polishing. The hard or brittle nature of ceramics, which should be an inherent advantage, results in the production of ceramics requiring a great deal of labor in the molding and processing steps. In addition, such poor processing efficiency causes an increase in manufacturing costs, which is a major obstacle to mass production of ceramic members.

On the other hand, metal materials exhibiting high ductility are manufactured efficiently and inexpensively by so-called plastic working, and this good workability is a major factor in the widespread use of metal materials. If this kind of plastic working can be applied to non-oxide ceramics such as silicon nitride and silicon carbide,
Manufacturing costs are significantly lower than with conventional manufacturing methods, and it is expected to lead to mass production of ceramics for structural materials and a dramatic expansion of applications.

However, conventional silicon nitride and silicon carbide exhibit brittle fracture at room temperature, and although they begin to exhibit plastic deformation even in a high temperature range of 1200° C. or higher, the amount of deformation is extremely small. For example, the amount of deformation to creep rupture in the -axial tensile creep test is 8% for hot-pressed silicon nitride;
In pressureless sintered silicon nitride, it is less than 3%, and breaks without stable deformation. For this reason, it has been impossible to apply processing by plastic deformation to silicon nitride or silicon carbide.

By the way, some metal alloys are difficult to plastically work, and these metal materials are formed and processed by causing so-called superplastic deformation using controlled strain rates in high temperature ranges. There is. This superplastic deformation occurs under stress far below the normal yield point without constriction, and depending on the type of material, it deforms by several hundred percent.
Many variations are possible. It is known that by such a method, a member having a complex shape can be manufactured at a relatively low cost even if the metal alloy is difficult to plastically work.

The present inventors previously demonstrated that a silicon nitride-silicon carbide composite sintered body consisting of fine, mainly equiaxed particles exhibited superplasticity in a uniaxial tensile test at controlled temperature and strain rate, and that the superplastic deformation We found a way to mold it by

(Japanese Patent Application No. 1-335063) However, in this method of forming by superplastic working, the sintered body used must be mainly composed of equiaxed and fine particles, and the process of superplastic working requires Because microscopic cavities are likely to be formed in the process, molded products obtained by superplastic processing cannot be said to have sufficient strength, fracture toughness, or heat resistance to be used as structural members as they are.

Therefore, the present inventors investigated this moldability and found that
It has been found that by heat-treating a workpiece after superplastic working, the strength, fracture toughness, or heat resistance of the resulting workpiece is significantly improved compared to a molded product immediately after superplastic working.

In other words, the present invention provides a method of producing a processed product at a low cost and with even better strength, fracture toughness, and heat resistance than after superplastic processing by superplastically processing non-oxide ceramics and heat-treating the same. It is about providing.

[Means for Solving the Problems] The present invention provides a processed product with excellent strength and fracture toughness by deforming a silicon nitride-silicon carbide composite sintered body under the action of stress in a superplastic temperature range and then heat-treating it. This paper relates to a method for forming composite ceramics that provides the following properties. Specifically, after a silicon nitride-silicon carbide composite sintered body mainly consisting of equiaxed particles is superplastically deformed under the action of tensile stress or compressive stress, it is deformed in a non-oxidizing atmosphere at normal pressure or under pressure.
The present invention relates to a method for forming composite ceramics, which is characterized by performing heat treatment at 1000 to 2300°C.

In the present invention, silicon nitride used for superplastic working is
It is necessary that the silicon carbide composite sintered body has a fine grain size and is mainly equiaxed. If the grain size of the silicon nitride-silicon carbide composite sintered body used is 2 μm or more, or if the grain size is not equiaxed but has a structure in which columnar grains are entangled, This makes it difficult to carry out the superplastic forming process of the invention. This is because cavities are generated when stress is applied in the temperature range where deformation occurs, and the strength and other mechanical properties of the resulting molded product are significantly deteriorated. For example, conventionally used silicon nitride has a large particle size and a structure in which columnar particles are intertwined, so the amount of deformation is small even in high temperature ranges, and stable deformation does not occur and may cause rupture or cavities. This results in significant deterioration of mechanical properties.

Therefore, in the present invention, first, a silicon nitride-silicon carbide composite sintered body consisting of fine and mainly equiaxed particles,
A predetermined shape is formed by applying stress in a temperature range that exhibits superplasticity.

This temperature range is usually 1400 to 1700°C, preferably 1
The temperature is 450-1650°C. In a temperature range higher than this, thermal alteration of the silicon nitride-silicon carbide composite sintered body occurs, making it difficult to cause stable deformation.

Furthermore, in a temperature range lower than this, the molding speed becomes slow, which is economically unfavorable.

In the present invention, the strain rate under the action of tensile stress or compressive stress is optimized by the molding temperature, but if molding is performed at an excessively high speed, cavities due to grain boundary burrs may be formed in the material. , these may link together to form large defects, lowering the strength of the molded product or making it porous, which may have an adverse effect on the mechanical properties.

Furthermore, if the molding speed is too low, the sintered body will be exposed to high temperatures for a long time, making it more susceptible to thermal deterioration.

Therefore, the strain rate during molding is 10-'5ec
-' or more, 10-'5ec-' or less, preferably 10
-' It is desirable to carry out at 5ec-' or less.

The atmosphere may be either an oxidizing atmosphere or a non-oxidizing atmosphere, but it is preferably carried out in a non-oxidizing atmosphere. This is to prevent the silicon nitride-silicon carbide composite sintered body from undergoing deterioration due to oxidation during superplastic processing, and an oxidizing atmosphere may be used when forming is completed in a short time, but In such cases, it is preferable to carry out the process in a non-oxidizing atmosphere.

The structure of the sintered body subjected to superplastic processing in this manner is mainly composed of equiaxed particles, as before processing. Therefore, the fracture toughness value is lower than that of conventional silicon nitride with developed columnar particles.

In addition, superplastic working tends to produce minute cavities inside and on the surface of the sintered body, and the strength after working tends to be lower than before working. Therefore, it is difficult to say that the strength and fracture toughness values of molded products obtained by superplastic working are sufficient for use as structural members, etc. as they are.

Therefore, in the present invention, heat treatment is performed after superplastic working. One purpose of this heat treatment is to improve fracture toughness and strength by changing the structure of the sintered body, or to increase strength by eliminating or reducing cavities that have formed through heat treatment. be.

Another purpose is to improve heat resistance by transforming the grain boundary phase into a composition with high heat resistance or by changing the glass phase into a crystalline phase.

The heat treatment in the present invention to achieve this purpose is carried out in a non-oxidizing atmosphere at normal pressure or under pressure.
It is carried out at a temperature of 0 to 2300°C.

When this heat treatment is performed at normal pressure, the temperature is 1800℃
The following is preferable, and when carried out under pressure such as gas pressure or HIP, the temperature is preferably within a range where silicon nitride does not decompose, that is, 2300° C. or lower. The heat treatment time varies depending on the composition and shape of the molded article or the heat treatment temperature, but is usually about 1 to 24 hours.

Through this heat treatment, the structure of the sintered body changes from an equiaxed and fine structure to a structure consisting of columnar grains and equiaxed grains with grain growth, and this microstructural change causes the fracture toughness value and strength to improve compared to immediately after processing. This will lead to a significant improvement. Furthermore, the strength of the sintered body is improved by reducing or eliminating cavities formed in the sintered body. In particular, when this heat treatment is carried out under pressure such as gas pressure or HIP, it is more effective because decomposition of silicon nitride is suppressed and cavities in the molded article are more likely to be reduced or eliminated. Furthermore, this heat treatment changes the grain boundary phase to a composition with high heat resistance and crystallizes it, thereby improving fracture toughness and heat resistance.

This heat treatment may be carried out successively after superplastic working without first cooling. After carrying out superplastic working, the sintered body is cooled to room temperature, and then the temperature is raised to a predetermined temperature and heat treatment is carried out. Good too.

In this way, in the present invention, by first forming in the superplastic temperature range, for example, a thin plate can be formed by compression, a thin plate can be bent, or precisely formed in a precision mold processed into a predetermined shape. Can be molded. Furthermore, by performing heat treatment according to the present invention, the strength, fracture toughness, or heat resistance of the resulting molded product is significantly improved compared to a molded product immediately after superplastic processing. Further, after the heat treatment according to the present invention, further heat treatment may be performed in an oxidizing atmosphere. By performing heat treatment in such an oxidizing atmosphere, the surface condition of the molded product can be improved.

Next, examples of the present invention will be shown. The examples shown below are merely examples of the present invention, and are not intended to be limiting unless they go beyond the gist of the present invention.

Example 1 Y
After adding 6% by weight of 2O3 and 220.2% by weight of Aj, wet-mixing in ethanol and drying, the mixture was filled into a graphite die with a diameter of 50M, and hot-press baked at 1650℃ for 1 hour at a pressure of 350Kg/cm3 in nitrogen gas. I made a conclusion. The obtained sintered body had a density of 3.2 g/ra3, and was a silicon nitride-silicon carbide composite sintered body mainly composed of equiaxed particles.

A specimen having a cylindrical part with a diameter of 3 InIll and a length of 10 mm was made from this sintered body, and heated at 1600°C in a nitrogen atmosphere.
In a high temperature furnace set at a strain rate of 4 x 1O-
5SeC-' was applied with tensile stress until the length of the cylindrical part became 14.4 mm, and the diameter was 2.5 m [1
Processed to 1. Fracture toughness value after superplastic processing is 5.4MP
a-m”2, and the room temperature bending strength was 520 MPa.

Next, this superplastically worked test piece was held at 1750°C in a nitrogen atmosphere at normal pressure for 2 hours, and the fracture toughness value and bending strength were measured. As a result, the fracture toughness value was 6.4 MPa-
The room temperature bending strength was improved to 850 MPa.

Example 2-5 A test piece (35x
35 x 5 mm) were placed through a graphite plate at 1600° C. in nitrogen. This is 8 x 1O-5sec
-' compression deformation (superplastic working) was performed to obtain a thin plate with a thickness of 4 mm. No cracks or alterations were observed in this thin plate, and the fracture toughness and bending strength after superplastic processing were each 5.3 Mp.
a-ml/2.620 MPa.

Next, this superplastic processed product was heat treated under the conditions shown in Table 1, and as a result, in all cases, physical properties superior to those before the heat treatment in fracture toughness and strength were obtained.

Table 1 Example 6 Y2O was added to an amorphous powder of 1 μm or less consisting of silicon, carbon, nitrogen, and oxygen with a carbon content of 9.6% by weight.

After adding 6% by weight and 2032% by weight of AA and drying by wet mixing in ethanol, it was filled into a graphite die with a diameter of 501nIn, and hot press sintered at 1650°C for 1 hour at a pressure of 350Kg/cm' in nitrogen gas. I did it. The obtained sintered body had a density of 3.268/CJ 3 and was a silicon nitride-silicon carbide composite sintered body mainly composed of equiaxed particles.

This sintered body was superplastically deformed under the same conditions as in Example 2 to create a four stroke thick test piece (superplastic processed product).

The fracture toughness value of this test piece is 5-2MP a-m+ /2
The bending strength at room temperature is 740 MPa, and the bending strength is 1
The bending strength at 300° C. was 370 MPa, and it was recognized that plastic deformation had occurred. Next, after holding the molded product after superplastic processing at 1750°C for 4 hours in nitrogen, the fracture toughness value, room temperature strength, and high temperature strength were measured, and the fracture toughness value was 6.2 MPa-m'''. The bending strength at room temperature was 890 MPa.The bending strength at 1300°C was 700 MPa, and no plastic deformation was observed at this time. (In other words, an improvement in the heat resistance of the molded product was observed.) [Effects] As described above, by superplastically processing a silicon nitride-silicon carbide composite sintered body having a microstructure as shown in the present invention, conventional silicon nitride sintered body silicon nitride-silicon carbide composite sintered body It is possible to perform precision molding into a predetermined shape without going through the complicated molding and processing steps that are used in molding processes, and further improves strength and strength by performing heat treatment after processing.
It can improve fracture toughness or heat resistance, and can be used efficiently for high-temperature, high-strength parts such as gas turbines and engines that require dimensional accuracy and excellent mechanical properties, insulation parts, wear-resistant materials, cutting tools, etc. It can be manufactured at low cost.

Patent applicant: Institute of Technology Director Mitsubishi Gas Chemical Co., Ltd.

Claims (1)

[Claims] A silicon nitride-silicon carbide composite sintered body mainly consisting of equiaxed particles is superplastically deformed and molded under the action of tensile stress or compressive stress, and then heated to a temperature of 1000 to 2300 in a non-oxidizing atmosphere at normal pressure or under pressure. A molding method for composite ceramics characterized by heat treatment at ℃.