Method for manufacturing silicon nitride sintered bodies

Abstract

The present invention provides a method for manufacturing silicon nitride sintered bodies that reduces the effort required to process the silicon nitride sintered body into the desired shape, and enables the production of sintered bodies with high dimensional accuracy while suppressing the formation of large voids on the surface. [Solution] Preferably, a β-type silicon nitride powder with 60% or more mass is used, and a green body containing the silicon nitride powder and a sintering aid is fired to obtain a pre-sintered body. Then, the pre-sintered body is subjected to a removal process using an abrasive material containing abrasive grains and a carbon content of 50% by mass or less, and then the sintering is completed.

Description

[Technical Field]

[0001] This invention relates to a novel method for manufacturing silicon nitride sintered bodies. More specifically, it provides a method for manufacturing silicon nitride sintered bodies that reduces the effort required for processing into the desired shape, has high dimensional accuracy, and produces sintered bodies with excellent surface properties. [Background technology]

[0002] In recent years, silicon nitride sintered bodies, possessing high strength, high hardness, high thermal conductivity, and high toughness, have been used as components in various industrial equipment such as circuit boards for power semiconductors, jigs for semiconductor manufacturing equipment, and bearing balls, due to their resistance to cracking and other characteristics.

[0003] On the other hand, in the manufacturing method of molded bodies made of silicon nitride sintered bodies, the high hardness of silicon nitride sintered bodies means that it takes a great deal of time to process them into molded bodies with the desired shape and dimensions, which is a bottleneck and leads to increased manufacturing costs.

[0004] Generally, silicon nitride sintered bodies are manufactured by molding a raw material mixture of silicon nitride powder, a sintering aid, and a binder to produce a green body, then decomposing and removing the binder from the green body (degreasing), and finally firing it.

[0005] Conventionally, several methods have been proposed to solve the problems in the processing of sintered bodies in the above manufacturing method. For example, in a method for manufacturing a silicon nitride sintered body that is a wafer fork, a method is used in which a green body is laser-processed to the desired shape, and then degreased and fired (see Patent Document 1); in a method for manufacturing a silicon nitride sintered body that is a ball, a method is used in which a green body is polished to approximate a perfect sphere, and then degreased and fired (see Patent Document 2); and in a method for manufacturing an assembly in which a sintered body consists of multiple parts, the primary molded bodies of each part are manufactured by cutting molded bodies that have been hardened by reducing the plasticizer content of the green body, and then assembled and fired (see Patent Document 3).

[0006] The conventional manufacturing method described above involves processing the green body or degreased body before firing in order to easily obtain the desired shape. However, since the green body and degreased body have low strength, they are prone to breakage during processing, raising concerns about a decrease in product yield. Furthermore, applying external force during processing may cause dents and density unevenness. In addition, density differences exist in different parts of the green body and degreased body, leading to significant distortion of the sintered body. Therefore, it is difficult to predict such dimensional changes during processing before firing, requiring a considerable margin in the dimensions after firing, and necessitating extensive processing to correct these margins after firing.

[0007] To address the above problem, it is considered that the time required for the removal process can be significantly reduced by stopping the firing process midway through the firing process to obtain a pre-sintered body that is easy to remove by polishing with an abrasive material containing abrasive particles, while also ensuring sufficient strength during processing. This pre-sintered body can then be subjected to polishing for removal, and then fired again to produce a sintered body. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 6-53303 [Patent Document 2] Japanese Patent Application Publication No. 1-130908 [Patent Document 3] Japanese Patent Application Publication No. 6-116007 [Overview of the project] [Problems that the invention aims to solve]

[0009] However, it was found that when the removal process in the aforementioned pre-sintered body is carried out using an abrasive material containing diamond abrasive grains, which are widely used in polishing, large voids tend to form on the surface of the sintered body obtained by further firing, resulting in a problem of reduced surface properties.

[0010] Therefore, the object of the present invention is to provide a manufacturing method for a silicon nitride sintered body in which a removal process is performed on the pre-sintered body using an abrasive material containing abrasive particles, and the sintering is completed by firing, and which enables the surface properties of the obtained silicon nitride sintered body to be maintained to a high degree. [Means for solving the problem]

[0011] The inventors of the present invention conducted diligent studies to solve the above-mentioned problems and found that when removing the pre-sintered body using an abrasive material containing abrasive grains, the amount of carbon in the abrasive material has a significant effect on the sintering of the surface of the resulting sintered body. Further studies revealed that by using an abrasive material containing abrasive grains with a limited carbon content below a specific value, the amount of carbon present on the surface of the pre-sintered body after removal can be reduced. This makes it less likely for large voids to form on the surface of the resulting sintered body, and a sintered body with a highly maintained surface sintering state can be obtained, thus completing the present invention.

[0012] In other words, the present invention provides a method for manufacturing a silicon nitride sintered body, characterized by firing a green body containing silicon nitride powder and a sintering aid to obtain a pre-sintered body in a state where sintering is not yet complete, then performing a removal process on the pre-sintered body using an abrasive material containing abrasive grains and having a carbon content of 50% by mass or less, and then firing the pre-sintered body after the removal process to complete the sintering.

[0013] Making 60% or more of the silicon nitride powder β-type silicon nitride powder is particularly advantageous because it reduces the generation of voids due to phase transitions.

[0014] Considering the processability of the pre-sintered body and the strength of the sintered body, the density of the pre-sintered body is 2.3 to 2.9 g / cm³. 3 The density of the sintered body is 3.0 g / cm³. 3 It is preferable that the above conditions are met.

[0015] Further, it is preferable that the pressure during firing of the green compact and the sintered compact is 1.0 MPa or less.

Advantages of the Invention

[0016] According to the manufacturing method of the present invention, by the removal process by polishing in the state of the green compact, it has workability comparable to that of the green compact in the conventional method, and the sintered compact obtained by further firing the green compact has a small change rate from the green compact, so it is possible to manufacture a silicon nitride sintered compact with extremely excellent surface smoothness, dimensional accuracy, etc. Moreover, in the present invention, in the polishing, by using a specific abrasive with a small carbon content, the sintered compact obtained by firing the green compact after the removal process has no large pores and can maintain high surface physical properties.

Embodiments for Carrying Out the Invention

[0017] The manufacturing method of the present invention includes firing a green compact containing silicon nitride powder and a sintering aid to obtain a green compact in a state where sintering is not completed, and then performing a removal process on the green compact using an abrasive containing abrasive grains and having a carbon content of 50% by mass or less, and then firing the green compact after the removal process to complete sintering. Note that the silicon nitride sintered compact manufactured by the manufacturing method of the present invention may sometimes be simply referred to as a sintered compact.

[0018] In the present invention, the green compact basically contains silicon nitride powder and a sintering aid, and it is also possible to use additives such as a binder for improving the strength of the green compact in combination.

[0019] Silicon nitride powder can be any known one without particular limitation. However, when α-type silicon nitride powder is used as the raw material, it is difficult to suppress surface irregularities and voids that occur when α-phase silicon nitride transforms into β-phase during sintering. To avoid this, a powder containing 60% by mass or more, preferably 80% by mass or more of β-type silicon nitride powder, with the balance being α-type silicon nitride powder, is preferably used. Also, the average particle size of the silicon nitride powder is preferably 0.4 to 1.6 μm, particularly 0.5 to 1.2 μm. The average particle size is the median diameter D50 in the volume-based particle size distribution and can be measured by the laser diffraction scattering method. Furthermore, the content of metal impurities in the silicon nitride powder is preferably such that the contents of Al and Fe are each 75 ppm or less, particularly 50 ppm or less, and the content of Ca is 100 ppm or less, particularly 70 ppm or less. The content of metal impurities can be measured by ICP emission spectrometry.

[0020] Also, sintering aids can be any known ones without particular limitation. For example, Al2O3, MgO, Y2O3, ZrO2, etc. can be mentioned, and these can be used alone or in combination as needed. Also, metals such as titanium, tungsten, molybdenum, and iron, or oxides, oxynitrides, and nitrides can be added as colorants.

[0021] Furthermore, binders can be any known ones without particular limitation. For example, organic polymer-based ones (such as acrylic resin and polyvinyl alcohol, etc.) can be mentioned, and these can be used alone or in combination as needed.

[0022] The amounts of the sintering aid, binder, and colorant used, if any, with respect to the silicon nitride powder are not particularly limited, and known amounts can be adopted without particular limitation. Specifically, with respect to 100 parts by mass of the silicon nitride powder, the sintering aid is preferably 4 to 15 parts by mass, particularly 6 to 12 parts by mass, the binder is preferably 1 to 30 parts by mass, particularly 1 to 6 parts by mass, and the colorant is preferably 0 to 3 parts by mass.

[0023] The shape of the aforementioned green body is not particularly limited, as it will be formed into the shape of the target sintered body by the removal process of the pre-sintered body described later. However, in order to reduce the amount of pre-sintered body to be removed, it is preferable to form it into a shape that approximates the shape of the sintered body. That is, taking into account the shrinkage due to sintering, it is preferable to form it into a shape close to the shape of the target silicon nitride sintered body. Specifically, if the target silicon nitride sintered body is plate-shaped, it is preferable to form it into a plate shape, and if it is spherical, it is preferable to form it into a spherical or cubic shape. Furthermore, if a complex shape is desired, it is preferable to form it into a cube, rectangular prism, or other shape that includes that complex shape.

[0024] The method for forming the green body can be any known method without particular limitation. For example, in the case of a plate, methods such as the doctor blade method, the T-die method (extrusion molding), and the roll compaction method can be used, while in the case of a sphere, cube, or rectangular prism, methods such as press molding, injection molding, and CIP molding can be used. The green body may be formed by a single method or by a combination of multiple methods.

[0025] Furthermore, if organic matter is used as a binder in the green material, degreasing is generally performed. The degreasing conditions are not particularly limited and any known method for decomposing the binder can be used. For example, one method is to decompose and remove the binder at 500-600°C in the presence of oxygen, such as air.

[0026] The calcination conditions for the green body or degreased body can be any known calcination conditions without particular limitation. Specifically, it is preferable to carry out the calcination in the presence of nitrogen, and although the suitable temperature needs to be adjusted depending on the particle size of the raw materials and the amount of additives, the calcination temperature should be 1350 to 1650°C, particularly 1365 to 1450°C, because if the temperature is too high, sintering will proceed too much and polishing will become difficult. Furthermore, the pressure can be reduced, atmospheric pressure, or increased pressure without particular limitation. The pressure during calcination of the pre-sintered body can be, for example, 10 MPa or less, preferably 1.0 MPa or less.

[0027] In the present invention, the pre-sintering is performed by interrupting the firing process before sintering is complete and removing the pre-sintered body. The state before sintering is complete is preferably one in which removal processes such as polishing with an abrasive material containing abrasive particles are easily performed, strength during processing is ensured, and furthermore, a shrinkage rate during subsequent firing is small. Specifically, the density of the pre-sintered body is 2.3 to 2.9 g / cm³. 3 Preferably, 2.4 to 2.8 g / cm³ 3 It is preferable to perform the pre-sintering in such a way that the density of the pre-sintered body is 2.3 g / cm³. 3 Preferably 2.4 g / cm³ 3 That's all, and 2.9 g / cm³ 3 The following, preferably 2.8 g / cm³ 3 The following applies:

[0028] The pre-sintered body having the aforementioned density has a lower Vickers hardness than a sintered body that has completed sintering, while having a higher Vickers hardness than a general green body or degreased body. For example, a pre-sintered body is preferable if its Vickers hardness (Hv) measured with an indentation weight of 20 kgf is preferably 800 or less, more preferably 200 to 700, as this makes it easy to remove. That is, the Vickers hardness (Hv) of the pre-sintered body measured with an indentation weight of 20 kgf is preferably 800 or less, more preferably 700 or less, and preferably 200 or more.

[0029] Since the pre-sintered body having the aforementioned density is almost completely sintered, the sintered body obtained by further firing it has an extremely small rate of change from the pre-sintered body. Therefore, it is possible to manufacture a silicon nitride sintered body with remarkably superior surface smoothness and dimensional accuracy.

[0030] In the aforementioned pre-sintering, the means for adjusting the density of the pre-sintered body are not particularly limited, but it is preferable to adjust the density of the sintered body by first confirming the relationship between the sintering temperature and time through experiments, and then removing the pre-sintered body from the firing furnace after firing at the temperature and time that result in the desired density.

[0031] In the present invention, the removal of the temporary sintered body can be carried out using any known processing method that uses an abrasive material containing abrasive particles, such as a grinding wheel or abrasive liquid, without any particular limitations. Specifically, this can include surface grinding, lapping, polishing, barrel grinding, cylindrical grinding, and grinding using a grinding wheel. Furthermore, when the removal process is carried out wet, an abrasive liquid containing abrasive particles can be used as the abrasive material.

[0032] In the present invention, it is important to use an abrasive material with a carbon content of 50% by mass or less, preferably 45% by mass or less, and more preferably 42% by mass or less, for the removal process of the pre-sintered body. This suppresses the formation of voids on the surface of the sintered body that occur when abrasive particles remain on the surface of the pre-sintered body, compared to when using a grinding wheel containing diamond abrasive particles (carbon content of about 90% by mass), which are typical abrasive particles used in polishing silicon nitride sintered bodies. This effect is even more pronounced in the method of the present invention, which can reduce the amount of polishing after the final firing. The lower limit of the carbon content in the abrasive material is not particularly limited, but may be 0% by mass, 10% by mass or more, or 20% by mass or more. The carbon content of abrasive materials can be measured using SEM / EDS (Scanning Electron Microscope / Energy Dispersive X-ray Spectrometer).

[0033] As for the grinding wheels mentioned above, grinding wheels containing alumina abrasive grains (alumina grinding wheels) and grinding wheels containing GC (green carborundum), which is silicon carbide, as abrasive grains (GC grinding wheels) are preferably used because they contain the same elements as the sintered body and / or sintering aid and do not inhibit sintering.

[0034] In this invention, the carbon content of an abrasive material containing abrasive grains is calculated based on the proportion of carbon in the solid components that affect the amount of carbon remaining on the surface after processing of a pre-sintered body. For example, in the case of a grinding wheel in which abrasive grains are fixed by a binder, the carbon content is calculated by combining the amount of carbon in the abrasive grains and the carbon content of the binder. Therefore, in grinding wheels that use a binder, it is preferable to use an inorganic binder that does not contain carbon. Furthermore, when using a grinding fluid in which abrasive grains are dispersed in a dispersion medium such as oil or a surfactant, the carbon content can be calculated from the amount of carbon in the solid abrasive grains contained in the grinding fluid.

[0035] This is presumably because the pre-sintered body does not have a sufficiently high density, resulting in many voids. Solid components, including abrasive grains, can become deeply embedded in these voids, making complete removal difficult even with ultrasonic cleaning. The abrasive grains remaining in these voids then affect the sinterability during the subsequent sintering process. On the other hand, the dispersion medium in the abrasive fluid is easily removed by cleaning and evaporates before reaching the sintering temperature, so it is not thought to affect the sinterability.

[0036] In the present invention, a typical embodiment of the removal process is shown below.

[0037] (1) When manufacturing a plate-shaped silicon nitride sintered body A method of producing a plate-like body of a predetermined thickness as a preliminary sintered body, taking into account the removal allowance and shrinkage allowance due to the final firing, and then removing material from one or both sides of the plate-like body.

[0038] - A method of manufacturing a rectangular or cubic block as a pre-sintered body, cutting it into a plate-like body of a predetermined thickness, taking into account the material removal allowance and shrinkage allowance during the final firing, using milling, turning, etc., to form the target silicon nitride sintered body, and then further removing material from one or both sides using a grinding wheel as needed.

[0039] (2) When manufacturing a ball-shaped silicon nitride sintered body · As a green compact, a spherical body of a predetermined size is manufactured in consideration of the machining allowance and the shrinkage allowance due to the final firing for the target silicon nitride sintered body, and machining is performed to approximate it to a perfect sphere.

[0040] · As a green compact, a rectangular parallelepiped or cubic block is manufactured, cut into a spherical body of a predetermined size in consideration of the machining allowance and the shrinkage allowance due to the final firing by lathe machining, and machining is performed using a grindstone to approximate it to a perfect sphere.

[0041] In addition, according to the above-mentioned machining for manufacturing a plate-like silicon nitride sintered body, not only adjustment of dimensional accuracy and the like but also reduction of the warp of the plate-like body can be achieved by setting a large amount of grinding.

[0042] In the present invention, the slag generated by the machining can be pulverized, treated such as decarbonization and re-nitriding as necessary, and reused as a raw material for the sintered body, thereby improving the utilization rate of the silicon nitride powder.

[0043] In the present invention, the green compact after the grinding is further fired to obtain the target silicon nitride sintered body. The firing conditions for the final firing are such that the density of the obtained sintered body is 3.0 g / cm 3 Above, particularly, 3.2 g / cm 3 Above, it is preferable to perform firing. The upper limit value of the density of the sintered body is not particularly limited, but for example, it is 3.5 g / cm 3 Below. The conditions for this main firing may be the same as those for the initial firing, but in order to maximize the density of the resulting sintered body, it is preferable to fire at a higher temperature than that used for the initial firing. Specifically, it is preferable to fire at a temperature 5°C or more, preferably 10°C or more, higher than the temperature of the initial firing, and at a temperature that does not exceed 1800°C, the decomposition temperature of silicon nitride, under normal pressure, and at a temperature below 1900°C even under pressure, so as not to generate voids due to the evaporation of the sintering aid. Furthermore, in this invention, it is not necessary to perform sintering at high pressure as in HIP (Hot Isostatic Pressing), and it is possible to suppress voids even at pressures of, for example, 10 MPa or less, preferably 1.0 MPa or less, when forming the sintered body. In this invention, it is possible to obtain a sintered body with excellent surface properties without performing sintering by HIP, but sintering by HIP may be performed as appropriate if necessary, from the viewpoint of further increasing the density of the sintered body or further reducing voids.

[0044] The Vickers hardness of the silicon nitride sintered body is greater than that of the pre-sintered body described above. The Vickers hardness (Hv) measured with an indentation weight of 20 kgf is, for example, 1200 or higher, preferably 1300 or higher, more preferably 1340 or higher, and for example, 1450 or lower.

[0045] In the present invention, the silicon nitride sintered body obtained after the final firing may be subjected to a removal process or polishing process in order to fine-tune the dimensions to the desired size or to further improve the smoothness of the surface. [Examples]

[0046] The following examples illustrate the present invention in more detail, but the present invention is not limited to these examples. Representative examples include plate-shaped bodies and spherical bodies.

[0047] Example 1 (plate-like body) Beta-type silicon nitride powder with an average particle size of 0.7 μm, along with yttrium oxide powder and aluminum oxide powder as sintering aids, were prepared. A raw material slurry was prepared by adding a water-soluble acrylic binder, a dispersant, and water to this raw material powder (mixture).

[0048] Next, the prepared raw material slurry is granulated by spray drying, and the resulting granules are pressure-molded using a mold molding machine to produce a product measuring 20mm in length, 20mm in width, 8mm in thickness, and with a density of 2.3g / cm³. 3 A plate-like green body was obtained. The green body was degreased to obtain a degreased body.

[0049] Furthermore, the degreased body was placed in the firing furnace, the furnace was replaced with nitrogen gas, and then the degreased body was fired at 1365°C for 10 hours under a pressure of approximately 0.8 MPa to obtain a pre-sintered body with the dimensions, density, and Vickers hardness (Hv) shown in Table 1.

[0050] The obtained pre-sintered body was subjected to a removal process (grinding) only in the thickness direction using an alumina grinding wheel (carbon content 30%). The grinding target value was set considering the shrinkage rate, with a final target thickness of 6.50 mm. Water was used as the grinding fluid. Table 1 shows the dimensions (thickness) of the pre-sintered body obtained by the removal process and the time required for the removal process.

[0051] After the grinding process described above, the parts were ultrasonically cleaned with pure water to remove grinding dust, and then dried.

[0052] Subsequently, the pre-sintered body was placed in the firing furnace, and the furnace was purged with nitrogen gas. The temperature was then raised to 1750°C under a pressure of approximately 0.8 MPa and maintained for 10 hours to perform the final firing, thereby producing a plate-shaped sintered body. The thickness (final target thickness: 6.5 mm), density, and surface hardness (Vickers hardness (Hv)) of the obtained sintered body were measured, and the results are shown in Table 1.

[0053] Furthermore, after polishing the surface of the sintered body to a mirror finish with a surface roughness of 0.005 μm, observation with a 100x video microscope revealed no large pores larger than 20 μm on the surface of the sintered body. Table 1 shows the maximum pore diameter confirmed by the video microscope observation.

[0054] Examples 2-4 (plate-like bodies) In Example 1, sintered bodies were prepared in the same manner as in Example 1, except that the density of the pre-sintered body was changed by varying the amount of sintering aid and the temperature during pre-sintering, as shown in Table 1. The density and dimensions (thickness) of the obtained sintered bodies were measured, and the results are shown in Table 1. Furthermore, as shown in Table 1, there were no large voids in the obtained sintered bodies, and there were no problems with physical properties such as surface hardness.

[0055] Comparative Example 1 (Plate-like body) In Example 1, a sintered body was prepared in the same manner, except that the grinding wheel used for polishing the pre-sintered body was replaced with a diamond grinding wheel (carbon content 87% by mass). The density and dimensions (thickness) of the obtained sintered body were measured, and the results are shown in Table 1. Furthermore, as shown in Table 1, the obtained sintered body had large voids, and a decrease in surface hardness was observed.

[0056] Examples 5 and 6 (spherical bodies) Granules were prepared in the same manner as in Example 1, and the resulting granules were pressure-molded using a mold molding machine to produce granules with a diameter of 15 mm and a density of 2.2 g / cm³. 3 A roughly spherical green body was obtained. This green body was degreased to obtain degreased bodies with the densities shown in Table 1.

[0057] Next, the degreased body was placed in the firing furnace, the furnace was replaced with nitrogen gas, and then pre-sintered under the conditions shown in Table 1 to obtain a pre-sintered body. The density at this time is shown in Table 1.

[0058] The obtained pre-sintered body was subjected to a complete removal process (grinding / polishing) using a GC grinding wheel (carbon content 41% by mass). The grinding / polishing target value was set considering the shrinkage rate, with a final target diameter of 13.0 mm. Water was used as the grinding / polishing fluid. Table 1 shows the dimensions (diameter) of the pre-sintered body obtained by the removal process and the time required for grinding / polishing. After the grinding / polishing process, ultrasonic cleaning with pure water was performed to remove grinding / polishing residue, and then the body was dried.

[0059] Subsequently, the pre-sintered bodies were placed in the firing furnace, and the furnace was purged with nitrogen gas. Then, spherical sintered bodies were produced by firing under the conditions shown in Table 1. The density, dimensions (diameter, final target diameter: 13.0 mm), and surface hardness of the obtained sintered bodies were measured, and the results are shown in Table 1.

[0060] As demonstrated in this embodiment, by obtaining a pre-sintered body, performing a removal process, and then carrying out the final firing, it became possible to easily obtain a sintered body close to the target dimensions in a short time. Furthermore, as shown in Table 1, no large voids were present on the surface of the obtained sintered body.

[0061] [Table 1]

Claims

1. A method for producing a silicon nitride sintered body, characterized by firing a green body containing silicon nitride powder and a sintering aid to obtain a pre-sintered body in an incomplete state, then subjecting the pre-sintered body to a removal process using an abrasive material containing abrasive grains and having a carbon content of 50% by mass or less, and then firing the pre-sintered body after the removal process to complete the sintering.

2. The method for producing a silicon nitride sintered body according to claim 1, wherein 60% by mass or more of the silicon nitride powder is β-type silicon nitride powder.

3. The density of the aforementioned pre-sintered body is 2.3 to 2.9 g / cm³. ３ The density of the sintered body is 3.0 g / cm³. ３ The method for producing a silicon nitride sintered body according to claim 1, as described above.

4. The method for producing a silicon nitride sintered body according to claim 1, wherein the pressure during firing of the pre-sintered body and the sintered body is 1.0 MPa or less.

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