JP2600116B2 - Superplastic silicon nitride sintered body and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superplastic silicon nitride sintered body, a method for producing the same, and a superplastically processed silicon nitride sintered body. More specifically, the present invention relates to a new silicon nitride sintered body capable of manufacturing, by plastic working, ceramic mechanical parts and the like useful in the fields of semiconductor manufacturing equipment, chemical plants, non-ferrous metal manufacturing machines, welding robots, and the like. It is about.

[0002]

2. Description of the Related Art Ceramics have excellent properties such as high strength and high hardness at low and high temperatures which are not found in other materials. In particular, silicon nitride ceramics have been put to practical use as mechanical parts used under severe conditions. It is evolving, and it is expected that the field of application will be further expanded if the price of parts is reduced. On the other hand, ceramics have a problem that processing is difficult when manufacturing parts. In the case of metal materials, forming and cutting by rolling and casting are easy, and the processing cost is low.However, cutting is not possible with ceramics, and diamond tools are required for grinding and polishing. There is a restriction that the cost is extremely high, and in some cases, it is more than half of the component price.

As described above, in order to reduce the processing cost of ceramics having a problem in workability, it is necessary to approach the final shape at the time of manufacturing a component and minimize the processing amount. Therefore, in order to realize this purpose, research on near net shape forming using superplasticity to obtain a desired shape has been conducted. In this case, superplasticity is a phenomenon in which metals and ceramics (zirconia, mullite, apatite) composed of ultrafine particles are easily plastically deformed under high-temperature external stress at a high temperature. A material having a complicated shape can be easily manufactured from the above material. Therefore, even ceramics can be processed to the final part by the same means as metal, and it is expected that the processing cost can be reduced. In fact, practical materials have been developed for oxide ceramics.

[0004] However, silicon nitride requires a high temperature for sintering, and conventionally commercially available raw materials cannot provide sintered bodies composed of fine and uniform particles. Therefore, as a remedy, a method has been proposed in which silicon carbide particles are dispersed to suppress the growth of silicon nitride grains, thereby producing a sintered body composed of fine particles. The composite sintered body obtained by this method has a stress of 200 to 500 kg / cm ² and a strain rate on the order of 10 ^-5 / sec at a temperature of 1650 ° C.
However, it is not practical because the deformation speed is too low and the processing takes a long time. In addition, since a high temperature is required, there is a problem that the surface is thermally decomposed during processing.

Further, a composite sialon sintered body comprising particles of two phases, α-sialon and β-sialon, which are solid solutions of silicon nitride, was reported in J. Am. Ceram. Soc., Vol. 76, p. 1073 (1992). ing. α-sialon and β-sialon are obtained by dissolving different metals in α-type and β-type silicon nitride, respectively, to stabilize the structure. This composite sintered body shows a strain rate of 7 × 10 ⁻⁴ / sec under a compressive stress of 600 kg / cm ² at 1550 ° C., and each sintered body has at least two phases of crystalline solid. There is. In addition, low-temperature stable type silicon nitride powder mainly composed of α is used as a raw material and sintered at a low temperature in order to suppress grain growth.

However, in this case, as sintering proceeds, a considerable part of the particles undergoes a phase transition to the β type, and at this time, some of the particles grow abnormally. For this reason, a uniform structure cannot be obtained, and grain growth occurs during superplastic working. Then, work hardening occurs in which the strain rate decreases as the processing proceeds. For these reasons, a superplastic sintered body at a practical level has not yet been obtained.

In order to solve such problems, the inventors of the present invention have previously made a sintered body made of uniform and ultrafine β-type silicon nitride using β-type silicon nitride powder of a high-temperature stable type as a raw material. Was developed. Since the silicon nitride sintered body is composed of fine and uniform particles, it is possible to solve the problem relating to the workability of superplastic working. However, this sintered body has a disadvantage that the fracture toughness is low even if the strength is high, reflecting the characteristics of the structure that is fine and uniform, and therefore, cracks grow by impact and fatigue, and are easily broken. There was a problem. In other words, although the workability problem was solved,
As a result, the mechanical properties of the obtained parts are lower than those of conventional materials.

[0008] As described above, the sintered body having superplasticity is inherently low in toughness, and cannot achieve mechanical properties of a practical level as it is, so that the range of application must be limited. There is a point. The present invention has been made in view of the above circumstances, and solves the problems of conventional superplastic ceramics, is capable of superplastic working, and has a new superplasticity having excellent mechanical properties. It is an object of the present invention to provide a silicon nitride sintered body, a method for producing the same, and a silicon nitride sintered body obtained by superplastic working.

[0009]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems by providing β-type silicon nitride particles of 85% by volume to 98% by volume and a grain boundary phase of 2% by volume to 15% by volume. A silicon nitride sintered body having a constituted relative density of 95% or more, wherein the silicon nitride particles have an average particle diameter of 0.
93% by weight or more 9% by weight of matrix particles of 3 microns or less 9
9.5% by weight or less and 2 of the average particle size of the matrix particles.
0.5% by weight of core particles having an average particle diameter of 2 times or more and 7 times or less
A superplastic silicon nitride sintered body (Claims 1, 2, and 3) characterized by being comprised of at least 7% by weight or less.

Further, the present invention provides a method for producing the above sintered body, wherein the matrix has an average particle size of 0.35 μm or less, a specific surface area of 20 m ² / g or more, and a β ratio of 85% or more. Core nitride powder having an average particle diameter of 2 to 7 times the average particle diameter of the matrix powder, from 0.4 to 6% by weight, from 79.1% to 97.5% by weight of silicon nitride fine powder for use. 0.9% by weight or less, and rare earth element oxides or Al ₂ O ₃ , MgO, CaO and SiO
_At least one of 2 is added and mixed as a sintering aid in an amount of 2% by weight or more and 15% by weight or less under a N ₂ atmosphere at a temperature of 1500 ° C. or more and 1850 ° C. or less, 100-1000 kg / cm.
A method (claim 4) characterized by hot pressing at a pressure of ² or less is also provided.

The present invention further provides a silicon nitride sintered body (claims 5 and 6) obtained by superplastic working and heat treatment from the above superplastic sintered body.

[0012]

In other words, the present invention will be described in more detail from the background. First, as is well known, a technique utilizing superplasticity for working was first developed for a metal material. But,
A material that has just been subjected to superplastic processing has low strength and creep resistance because the particles remain small, and is not practical. Therefore,
In the case of metal, a material with an ultrafine structure is prepared, first subjected to superplastic processing, and then to the recrystallization temperature (higher than the processing temperature)
Heat treatment at higher temperatures changes the constituent phases and texture of the material to improve its mechanical properties. However, mechanical properties of ceramic materials are not improved by simply heat-treating them.

Therefore, a small amount of large particles outside the particle size distribution is added to fine and uniform particle size powder and sintered, and nuclei for grain growth are added to a sintered body having a uniform structure. When a larger β-type particle is added as a grain growth nucleus to a silicon nitride sintered body having a fine and uniform structure and heat treatment is performed at a high temperature, the nucleus grows preferentially. Therefore, a composite structure similar to that of the whisker-reinforced silicon nitride sintered body is developed, and a high-toughness ceramic is obtained. The inventors of the present invention have already proposed such toughness, high strength, and high reliability by adding such a nucleus and controlling the structure using the nucleus. Therefore, if the sintered body containing the nucleus is heat-treated at a temperature higher than the processing temperature after the superplastic working, the nucleus grows preferentially and becomes large columnar particles. The size and shape of the columnar particles depend on the size and amount of the added core particles, the type and amount of the sintering aid, the sintering temperature, and the like. In general, since the substance is concentrated and diffused into the nucleus, the growth rate is larger than that of the fine matrix particles, and the columnar anisotropic growth occurs. By such a method, a composite structure in which large columnar particles are dispersed between fine, uniform and nearly spherical matrix particles is obtained, and a sintered body having high fracture toughness is obtained. Therefore, if a core particle having an amount and a size that does not impair the superplasticity is introduced when producing a superplastic sintered body, a high-toughness and high-strength sintered body should be finally obtained by heat treatment.

The present invention has been completed based on the above findings. That is, as described above, in the present invention, 79.1% by weight of a silicon nitride fine powder for a matrix having an average particle size of 0.35 μm or less, a specific surface area of 20 m ² / g or more and a β ratio of 85% or more. More than 9
An average particle size of 7.5% by weight or less
0.4 to 6.9% by weight of powder for nuclear particles which is twice or more and seven times or less, and a rare earth element oxide or Al
_One or more of ₂ O ₃ , MgO, CaO, and SiO ₂ are added as sintering aids in an amount of 2% by weight to 15% by weight, and after mixing, are mixed at 1500 ° C. to 1850 ° C. in a nitrogen atmosphere.
By a manufacturing method in which hot pressing is performed at a temperature of 100 to 1000 kg / cm ² at a temperature of not more than 85% by volume and not more than 98% by volume and a relative phase composed of a grain boundary phase of not less than 2% by volume and not more than 15% by volume. A silicon nitride sintered body having a density of 95% or more, wherein the silicon nitride particles have an average particle size of 0.
Core particles having an average particle size of 93% by weight or more and 99.5% by weight or less and 3% or less and a maximum particle size of not more than twice the average particle size and not more than 2 times the average particle size of the matrix. Is obtained from 0.5% by weight or more to 7% by weight or less.

[0015] The superplastic sintered body is then
Under a compressive or tensile stress of 00 kg / cm ² ,
After superplastic working in a temperature range of 350 to 1650 ° C. at a strain rate of 10 ⁻⁴ / sec or more, in nitrogen at 1 to 100 atm,
The particles are grown by heating to 1650 to 2000 ° C., the matrix has an average particle size of 0.5 μm or less, and the average particle size of the particles grown from the nucleus in a columnar form is 1 μm or more and 5 μm or less and the total amount is 5% by weight or more of silicon particles 5
A sintered body having a content of 5% by weight or less and a fracture toughness of 4.5 MPa · m ^1/2 or more is obtained. Production of Superplastic Sintered Body In the method for producing a superplastic silicon nitride sintered body of the present invention, the matrix raw material powder having an average particle diameter of 0.35 μm or less and a specific surface area of 20 m ² / g or more is uniform and This is because it is necessary to be fine. A powder having an average particle size exceeding 0.35 μm and / or a specific surface area of 20 m ² / g or less, which deviates from these conditions, has a nucleus for grain growth and abnormally grows during heating. Desirably, the average particle size is 0.1 to 0.2 microns and the specific surface area is 25 to 35 m ² / g. In addition, even if the powder having a high α ratio is a uniform and fine powder as described above, a phase transition of α → β occurs during sintering, and during this process, some of the particles undergo abnormal grain growth and become uniform. No organization is obtained. Therefore, in addition to the above conditions, the β ratio needs to be 85% or more. To this powder, β-type powder serving as a nucleus for grain growth is added in an amount of 0.5% by weight or more and 7% by weight or less based on the total amount of silicon nitride. The average particle size of this powder is at least twice and at most 7 times the value of the matrix powder. If the average particle size is smaller than twice, a considerable portion of the particle size distribution of the matrix powder overlaps, and the effect as a nucleus for grain growth is not sufficient. On the other hand, when the ratio exceeds 7 times, the growth particles become large, and the strength of the sintered body decreases. Desirably, it is 2 times or more and 7 times or less and 0.4 microns or more and 1.0 microns or more. If the amount of the nuclei is less than 0.5% by weight, the effect of the addition is not sufficient, and if it exceeds 7% by weight, superplastic working becomes difficult. Desirably, it is in the range of 1.0 to 3.0% by weight.

The above-mentioned mixed powder of silicon nitride of 85% to 98% by weight contains a rare earth element oxide or Al ₂
One or more of O ₃ , MgO, CaO, and SiO ₂ are added as a sintering aid in an amount of 2% by weight to 15% by weight and sintered. The sintering aid forms a liquid phase at the sintering temperature,
Promotes diffusion at grain boundaries. In order to obtain a sintered body while suppressing grain growth, it is necessary to increase the density by sintering at a low temperature and for a short time. Therefore, an auxiliary system that generates a liquid phase at a low temperature is desirable. For this purpose, the sintering aid is added in an amount of 2 to 15% by weight. If it is less than 2% by weight, the effect of promoting sintering is insufficient.
A high density sintered body cannot be obtained. If it is more than 15% by weight, sintering is easy, but the mechanical properties of the sintered body deteriorate. Sintering is performed in a nitrogen atmosphere. However, if the temperature is lower than 1500 ° C., the grain boundary diffusion rate is low and sintering is insufficient. If the temperature exceeds 1850 ° C., grain growth occurs and a fine structure cannot be obtained. 1500 ° C
A temperature of at least 1750 ° C. is desirable. In order to complete sintering at a low temperature, it is necessary to apply a pressure of 100 to 1000 kg / cm ² . If the pressure is lower than this range, there is no pressurizing effect, and if it exceeds the upper limit, the apparatus becomes expensive. Superplastic silicon nitride sintered body A superplastic silicon nitride sintered body has a relative density of 95% or more composed of 85% to 98% by volume of silicon nitride particles and 2% to 15% by volume of a grain boundary phase. Is a silicon nitride sintered body. This is within this range when the above-mentioned raw materials and sintering aids are used. Since the silicon nitride sintered body must have superplasticity and include nuclei that grow in the subsequent heat treatment, the silicon nitride particles have an average particle size of 0.3 μm or less and a maximum particle size not exceeding twice the maximum. And a core having an average particle size of 2 to 7 times the average particle size of the matrix. The reason that the matrix particles are in the above range is that abnormal grain growth does not occur during superplastic working and heat treatment. Outside this range, significant grain growth also occurs from the matrix, making it difficult to control the structure. The number of core particles for grain growth is as small as possible from the point of superplasticity and preferably small. Otherwise, plastic working becomes difficult. On the other hand, in terms of grain growth in the heat treatment step, it is necessary that the amount is large and large. Otherwise, it will not act as a nucleus. Therefore, 0.5% by weight of core particles having an average particle size of 2 to 7 times the average particle size of the matrix.
It is necessary to add at least 7% by weight.

In order to measure the particle size of the silicon nitride sintered body, the sample is cut and polished, treated with microwave plasma of CF ₄ gas, and observed with a scanning electron microscope (SEM). The silicon nitride particles are removed by this treatment, and the oxide glass at the grain boundaries remains, so that the shape of the particles can be easily observed. Using 500 or more particles from the SEM photograph, the diameter, length and area of the particles are calculated by an image processing method. The diameter of the particles is the shortest diameter of the particles on the polishing surface. The average particle diameter (D ₅₀ ) is the number average of a large number of measured diameters. Superplastic working and heat treatment plastic working are generally performed under a compressive or tensile stress of 50 to 2000 kg / cm ² . If the strain rate is 10 ^-4 / sec or more, the processing speed is sufficient for ceramics. Within the temperature range of 1350-1650 ° C,
The sintered body having the composition and structure of the present invention can be subjected to superplastic working under the above conditions. If the temperature is higher than 1650 ° C., the processing speed increases but the grain growth becomes remarkable, and thermal decomposition occurs from the surface. At a temperature lower than 1350 ° C., an extremely high processing pressure is required, which is not practical.
After processing, 1650-2000 in nitrogen at 1-100 atm
Heat to ° C. to grow the particles into columns. In particular, it is important to grow nuclei selectively. At lower temperatures, the nuclei do not grow, and at higher temperatures, grain growth is too fast to control the structure. Within an appropriate temperature range, lower temperatures require longer treatment. Desirable heat treatment temperature is 1700-185
The temperature is 0 ° C., 3 to 20 hours at 1700 ° C., and 1 to 5 hours at 180 ° C. Heat treatment under pressurized nitrogen is effective for the purpose of thermally decomposing silicon nitride at a high temperature and preventing the sintering aid from being lost in the atmosphere during the heat treatment. 1
At 700 ° C., the maximum pressure may be about 10 atm. Higher pressures are also effective, but in practice, at most 100 atm is sufficient because the equipment is expensive.
Although the matrix also grows by heat treatment, it is necessary to suppress the average particle size to 0.5 μm or less. This is 0.5
This is because particles exceeding a micron are likely to become nuclei for abnormal growth. The average particle diameter of the particles grown from the nucleus in a columnar form is 1 μm or more and 5 μm or less and the amount is 5% by weight or more and 55% by weight or less of the total silicon nitride particles. It is also possible to control the size of the particles. Less than 1 micron,
If the amount is 5% by weight or less, the effect as a reinforcing material is not sufficient. If it is 5 μm or more, or 55% by weight or more, large particles come into contact with each other, and there is a problem in that the mechanical properties vary. If the grain growth is controlled by the heat treatment in this manner, the nuclei grow selectively and uniformly distributed. Therefore, the fracture toughness of the superplastic sintered body has a uniform structure (2.5
３3.5 MPa · m ^1/2 ) to the value of the heterogeneous structure (4.5).
(MPa · m ^1/2 or more).

As described above, according to the present invention, a superplastic silicon nitride sintered body containing a nucleus for grain growth is produced, and the nucleus is grown by heat treatment after processing to obtain a high toughness sintered body. .

[0019]

The present invention will be described below more specifically with reference to examples and comparative examples. Examples 1 to 3 Commercially available β-type fine powder (SN-P21 manufactured by Denki Kagaku)
From FC), fine powder and coarse powder were separated by sedimentation and centrifugal classification. The fine powder is a powder for a matrix and has an average particle size of 0.
25 microns, specific surface area 23.7 m ² / g, β ratio 93%
Met. The coarse powder (core powder) had an average particle size of 0.76 μm, a specific surface area of 2.4 m ² / g, and a β ratio of 96%. These powders and the sintering aid were weighed in the proportions shown in Table 1. The powder was dispersed in hexane as a solvent and mixed in a silicon nitride planetary ball mill for 2 hours. After the mixture was dried, about 2.5 g was weighed and filled in a graphite mold coated with BN powder having a diameter of 10 mm. A pressure of 200 kg / cm ² was applied to this powder, the temperature was raised to a predetermined temperature shown in Table 1 at 30 ° C./min in a nitrogen atmosphere, and the power was turned off without holding and cooled.

The density was measured from the size and weight of the sample. The relative density was calculated as the theoretical density of the sum of the contributions of the respective raw materials. The sintered body was cut and polished, and plasma etching was performed using CF ₄ gas. In this process, the silicon nitride particles are thinly removed, and the glass phase at the grain boundaries remains. When this sample is viewed with a scanning microscope, the grain boundaries are clearly identified, so that the structure can be easily observed. From the photograph, the particle size and area of about 1000 particles were measured by image analysis. Since the matrix and the core peak are separated in the particle size distribution, they can be easily identified. The results are shown in Table 1.
Although the particles are growing during sintering, the average particle size of the matrix and nuclei is shown to be smaller than that of the raw powder. This is because the average particle size of the raw material is a particle size corresponding to 50% volume of the particle size distribution curve, whereas the number average is employed in the sintered body.

[0021]

[Table 1] The obtained sintered body was subjected to superplastic working by compression under the conditions shown in Table 2. This workpiece was heat-treated in nitrogen under the conditions shown in Table 2. The structure of the sintered body after the heat treatment was evaluated in the same manner as the raw material sintered body. Table 3 also shows the volume content (%) of the columnar particles grown from the nucleus. The sample was polished with 1 micron diamond abrasive grains, and the fracture toughness was determined by the indentation press-fit method of JIS R1607. Vickers indenter load is 20
kg. For comparison, the values of the fracture toughness of the sintered body before processing are also shown. The structure and fracture toughness of the sintered body after superplasticity were almost the same as before the working. As a result, a high-toughness silicon nitride sintered body could be manufactured by a method of performing heat treatment after superplastic working of a sintered body containing nuclei for grain growth.

[0022]

[Table 2]

[0023]

[Table 3] Comparative Example 1 The β-type fine powder (for matrix) used in Example was heated to 1750 ° C. under the same conditions as in Example 1 and cooled to produce a pre-sintered body. The structure of the sintered body was fine and uniform, and the fracture toughness was 2.4 MPa · m ^1/2 . This is 300
When processed at 1550 ° C. under a compressive stress of kg / cm ² , superplastic deformation occurred at a deformation rate of 32 × 10 ⁻⁴ / sec. When this material was heated at 1750 ° C. for 1 hour in nitrogen at 10 atm, a structure having a uniform grain size but a slightly columnar grain growth was obtained. However, particles grown to 1 micron or more were not observed. The fracture toughness of this sintered body is 3.9 MPa · m
^As low as ^1/2 , the effect of the heat treatment was not sufficient. Comparative Example 2 Commercially available high-purity α-type fine powder (manufactured by Ube Industries, E-1
0) was heated to 1750 ° C. under the same conditions as in Example 1 and cooled to produce a pre-sintered body. The texture is a fine and uniform matrix with an average particle size of 0.25 microns and an average particle size of 0.7
It consists of a 4 micron core. The nuclei were formed when a part of the α-form of the raw material was phase-transformed to β-form. This sintered body was subjected to a compression test under the same conditions as in Comparative Example 1 to give 7.8 × 10 ⁻⁵ /
The deformation speed was low in seconds and was not suitable for superplastic working.

[0024]

According to the present invention, as described in detail above, a superplastic silicon nitride sintered body that can be made into a highly plastic sintered body by superplastic working is provided.

Claims (6)

(57) [Claims] 1. A silicon nitride sintered body having a relative density of 95% or more composed of β-type silicon nitride particles of 85% to 98% by volume and a grain boundary phase of 2% to 15% by volume. And core particles in which silicon nitride particles have an average particle diameter of 93% by weight or more and 99.5% by weight or less and an average particle diameter of 2 times or more and 7 times or less the average particle diameter of the matrix particles. Is 0.5% by weight or more and 7% by weight
A superplastic silicon nitride sintered body comprising: 2. The method according to claim 1, wherein the silicon nitride particles have a mean particle size of 0.3 μm or less and a maximum particle size of not more than twice the mean particle size. 2. The superplastic silicon nitride sintered body according to claim 1, wherein said superplastic silicon nitride has an average particle diameter of 0.4 to 1.0 micron. 3. The superplastic nitriding material according to claim 1, wherein said superplastic nitriding material is deformed at a strain rate of 10 ^-4 / sec or more within a temperature range of 1350 to 1650 ° C. under a compressive or tensile stress of 50 to 2000 kg / cm ^2. Silicon sintered body. 4. A silicon nitride fine powder for a matrix having an average particle diameter of 0.35 μm or less, a specific surface area of 20 m ² / g or more and a β ratio of 85% or more, 79.1% by weight or more and 97.5% by weight. % Or less, 0.4% by weight or more and 6.9% by weight or less of core particle powder having an average particle size of 2 times or more and 7 times or less of the average particle size of the matrix powder, and a rare earth element oxide or Al ₂ O _3. , MgO, CaO and SiO ₂
Singly or two or more of them are added and mixed as a sintering aid in an amount of 2% by weight or more and 15% by weight or less,
100 ° C. to 1850 ° C., 100 to 1000 kg /
4. The method for producing a superplastic silicon nitride sintered body according to claim 1, wherein hot pressing is performed under a pressure of cm ² . 5. The matrix particles have an average particle size of 0.5 μm or less, and the particles grown in a columnar form from the nucleus have an average particle size of 1 μm or more and 5 μm or less and the amount is 5% by weight or more of the total silicon nitride particles. A silicon nitride sintered body characterized by being 55% by weight or less and having a fracture toughness of 4.5 MPa · m ^1/2 or more. 6. The superplastic silicon nitride sintered body according to claim 1, 2 or 3, under a compressive or tensile stress of 50 to 2000 kg / cm ² , in a temperature range of 1350 to 1650 ° C. and 10 ^-4 / The method for producing a silicon nitride sintered body according to claim 5, wherein the silicon nitride sintered body is deformed at a strain rate of not less than sec and heated at 1650 to 2000 ° C. in nitrogen at 1 to 100 atm.