JP2012180235A - Method for producing silicon nitride-based ceramic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a silicon nitride-based ceramic, which suppresses spurt of metal Si in the process of nitriding metal Si.SOLUTION: The method for producing a silicon nitride-based ceramic includes: in a method for producing a silicon nitride-based ceramic by nitriding metal Si, a pore forming step in which a green compact of a mixture comprising metal Si, a sintering aid and a pore-forming agent which forms pores on heating is heated to form pores between metal Si particles in the green compact; a nitriding step in which a porous compact, obtained in the pore-forming step and comprising metal Si with formed pores and the sintering aid, is fired in a nitrogen atmosphere to obtain a reaction sintered compact; and a densification step in which the reaction sintered compact is fired at a temperature exceeding the nitriding temperature at which the reaction sintered compact is obtained in the nitriding step, whereby the reaction sintered compact is densified to obtain a finished sintered compact.

Description

The present invention relates to a method for producing silicon nitride (Si ₃ N ₄ ) -based ceramics by a post reaction sintering method.

  Since silicon nitride ceramics have excellent characteristics such as heat resistance, high strength, wear resistance, and thermal shock resistance, they are attracting attention as engineering ceramics and are useful materials. Until now, it has been put into practical use as engine parts materials, bearing materials, tool materials, molten metal parts, bearing members, various roll materials for rolling, vanes for compressors, turbo rotors, cutting tools and the like.

There are mainly two methods for producing silicon nitride ceramics. The first method is a normal pressure sintering method in which Si ₃ N ₄ powder is used as a raw material and a sintering aid is added to the powder to sinter it. The second method is a reactive sintering method in which metal Si powder is used as a raw material and is nitrided at a temperature of 1500 ° C. or lower.

In the normal pressure sintering method, a dense sintered body can be easily obtained, but in order to obtain a high-quality sintered body, it is necessary to use a fine and low-impurity Si ₃ N ₄ powder. There is a problem that ₃ N ₄ powder is very expensive. On the other hand, in the reactive sintering method, since metal Si powder is used, a product can be manufactured at a relatively low cost, but the relative density is about 70 to 80% as compared with a sintered body obtained by the atmospheric pressure sintering method. Thus, a dense sintered body cannot be obtained.

  Therefore, in recent years, a post-reaction sintering method (also referred to as a two-stage sintering method) in which an inexpensive metal Si powder is used as a raw material, and a sintering aid is added and reaction sintering and densification sintering are used in combination. Has been proposed. In the post-reaction sintering method, a sintering aid is added to metal Si powder, which is the starting material for reaction sintering, and after performing a nitriding step for nitriding metal Si, the densification is achieved by raising the firing temperature further. The process is performed. Note that silicon nitride ceramics manufactured by the post reaction sintering method are disclosed in Patent Documents 1 and 2.

JP 2007-197226 A JP 2008-24579 A

  However, the method of nitriding the metal Si has a problem that the metal Si is ejected during the manufacturing process.

  Specifically, in order to obtain silicon nitride ceramics, metal Si is nitrided with nitrogen under high temperature conditions, but in order to obtain high-strength silicon nitride ceramics, nitridation is just before the melting of metal Si. It is preferable to carry out with. For this reason, nitridation is performed under high temperature conditions for metal Si, and metal Si is easily ejected. If metal Si is ejected, the yield of the final fired body is reduced and the manufacturing cost is increased, so the significance of using metal Si as a raw material is diminished in the first place.

  The present invention has been made in view of the above-described conventional problems, and an object of the present invention is to provide a method for producing silicon nitride-based ceramics that suppresses ejection of metal Si in the nitridation process of metal Si.

  In order to solve the above problems, a method for producing a silicon nitride ceramic according to the present invention is a method for producing a silicon nitride ceramic for nitriding metal Si. In the method for producing silicon nitride ceramics, pores are formed by heating metal Si, a sintering aid, and heating. Heating the powder compact of the mixture containing the pore-forming agent to be formed to form pores between the metal Si in the powder compact, and the metal with pores formed in the pore-forming process A nitriding process in which a porous body containing Si and a sintering aid is fired in a nitrogen atmosphere to obtain a reaction sintered body, and a reaction firing at a temperature exceeding the nitriding temperature at which the reaction sintered body in the nitriding process is fired And a densification step of densifying the sintered body to obtain a final sintered body.

  In the manufacturing method, pores are formed between the metal Si in the hole making step. And since many pores are formed in the porous body obtained at the pore making process, the specific surface area of the porous body is large. Furthermore, in the nitriding step according to the present invention, nitrogen is introduced into the pores, so that the metal Si and nitrogen can be contacted in a wide range. Under these conditions, the metal Si and nitrogen can react uniformly, and a portion of the metal Si in the porous body that is overheated without being nitrided is less likely to occur. Finally, the reaction sintered body obtained in the nitriding step is fired to obtain a final sintered body. According to the manufacturing method, metal Si can be prevented from being ejected in the porous body, yield can be increased, and manufacturing cost can be suppressed.

  In the method for producing a silicon nitride ceramic according to the present invention, the ratio of the pore former to the total volume of the metal Si and the sintering aid is preferably 30% by volume or more and 60% by volume or less.

  Thereby, the pore formed by heating a pore making agent becomes large. As a result, the metal Si and nitrogen can be contacted more uniformly, and nitriding can be suitably performed.

  Moreover, in the manufacturing method of the silicon nitride ceramics of this invention, it is preferable that the ratio of the pore making agent with respect to the total volume of the said metal Si and a sintering aid is 45 volume% or more and 60 volume% or less.

  Thereby, metal Si and nitrogen can be contacted very uniformly, and a silicon nitride-based ceramic can be obtained from a reaction sintered body containing more α phase advantageous for formability.

  Moreover, in the manufacturing method of the silicon nitride ceramics of this invention, it is preferable that the temperature which heats the green compact of the mixture in the said hole making process is 500 degreeC or more and 1000 degrees C or less.

  Thereby, it is difficult to leave the carbide modified with the pore former on the metal Si or the sintering aid in the compact. Since the carbide can be a starting point for causing cracks in the final sintered body, it is possible to provide high-quality silicon nitride-based ceramics that are unlikely to crack.

  In order to solve the above problems, a method for producing a silicon nitride ceramic according to the present invention is a method for producing a silicon nitride ceramic for nitriding metal Si. In the method for producing silicon nitride ceramics, pores are formed by heating metal Si, a sintering aid, and heating. Heating the powder compact of the mixture containing the pore-forming agent to be formed to form pores between the metal Si in the powder compact, and the metal with pores formed in the pore-forming process It is preferable to include a nitriding step of firing a porous body containing Si and a sintering aid in a nitrogen atmosphere to obtain a reaction sintered body.

  In the manufacturing method, pores are formed between the metal Si in the hole making step. And since many pores are formed in the porous body obtained at the pore making process, the specific surface area of the porous body is large. Furthermore, in the nitriding step according to the present invention, nitrogen is introduced into the pores, so that the metal Si and nitrogen can be contacted in a wide range. Under these conditions, the metal Si and nitrogen can react uniformly, and a portion of the metal Si in the porous body that is overheated without being nitrided is less likely to occur. According to the manufacturing method, the ejection of metal Si in the porous body can be suppressed, the yield can be increased, and the manufacturing cost can be suppressed.

  As described above, the method for producing a silicon nitride ceramic of the present invention heats a powder compact of a mixture containing metal Si, a sintering aid, and a pore-forming agent that forms pores by heating. A pore-forming step of forming pores between the metal Si in the compact and a porous body containing the metal Si and pores formed in the pore-forming step and a sintering aid in a nitrogen atmosphere Nitriding process to obtain a sintered body by firing and densification to obtain a final sintered body by firing and densifying the reaction sintered body at a temperature exceeding the nitriding temperature at which the reaction sintered body was fired in the nitriding process A process comprising the steps of:

  Therefore, there is an effect that it is possible to provide a method for producing a silicon nitride ceramic that suppresses the ejection of metal Si in the nitriding process of metal Si.

It is a flowchart which shows the flow of the process of the manufacturing method which concerns on this invention. 6 is a photographic view showing a final sintered body obtained in Comparative Example 1. FIG. 2 is a photographic view showing a final sintered body obtained in Example 1. FIG. It is a SEM photograph figure which shows each final baking body obtained in Examples 1-3 and the comparative example 1. FIG. 6 is a photographic diagram showing the reaction sintered bodies obtained in Example 5 and Comparative Example 1. FIG. It is a SEM photograph figure which shows the reaction sintered compact obtained in Example 4, 5, and 6, respectively. 6 is a graph showing changes in shrinkage rate of silicon nitride ceramics due to temperature changes in Example 5, Comparative Example 1 and Comparative Example 2. It is the figure which observed the fracture | rupture source after measuring the bending strength of the final sintered body of Example 12 by EDS.

  An embodiment of the present invention will be described with reference to FIG. The method for producing a silicon nitride ceramic according to the present invention is a method for producing a silicon nitride ceramic for nitriding metal Si. When obtaining a final sintered body (silicon nitride ceramic), a pore forming process, a nitriding process and a dense process are performed. Conversion step. In the manufacturing method, metal Si is used as a starting material, and this is nitrided to obtain a silicon nitride ceramic. Therefore, silicon nitride ceramics can be produced at a lower cost without using silicon nitride as a starting material.

Here, the silicon nitride-based ceramic is a polycrystalline body mainly composed of silicon nitride, and includes various sintering aids such as Y ₂ O ₃ and MgO. In addition, ceramics (for example, sialon) in which part of silicon and nitrogen in silicon nitride are replaced with different atoms (for example, aluminum and oxygen) are also included in the silicon nitride-based ceramics. FIG. 1 is a flowchart showing the flow of steps of the manufacturing method according to the present invention.

(S1. Raw material preparation step)
First, metal Si, a sintering aid, and a pore former, which are raw materials for a silicon nitride ceramic, are prepared.

  Metal Si, which is the main raw material of silicon nitride, can be a commercially available product. For example, it is a low-purity raw material or a pulverized powder of a low-purity silicon wafer at the time of producing a silicon wafer used for semiconductor applications. A powder equivalent to 600 is exemplified. The purity of the metal Si may be, for example, 96.0% or more and 98.5% or less.

The method for producing a silicon nitride ceramic according to the present invention is a post-reaction sintering method, and uses metal Si as a raw material, but part of the amount of metal Si used may be replaced with Si ₃ N ₄ . For example, the proportion the _Si 3 _{N 4} with respect to the total amount of the metal Si and _Si 3 _{N 4} 0 wt% or more, may be 50 wt% or less. However, since increasing the proportion of Si ₃ N ₄ is disadvantageous in terms of cost, the proportion of Si ₃ N ₄ is preferably 0% by weight or more and 30% by weight or less, preferably 0% by weight or more and 10% by weight or less. More preferably, it is most preferably 0% by weight.

  The shape of the metal Si may be granular, rod-like, fibrous, or crushed. Since the metal Si is mixed with at least a sintering aid and a pore-forming agent, it is preferable that the average particle diameter is relatively small so that uniform mixing can be performed. However, since workability | operativity will fall when too small, practically the average particle diameter of metal Si is 0.5 micrometer or more and 40 micrometers or less, Preferably it is 0.5 micrometer or more and 10 micrometers or less.

  Sintering aid is mixed with metal Si and pore former, and after metal Si is nitrided, it melts at high temperature and densifies silicon nitride by forming a liquid phase in the gap between silicon nitrides It is. These are constituents of the grain boundary phase in the obtained silicon nitride ceramics.

As the sintering aid, known materials can be used, for example, oxides of rare earth elements (lanthanoid elements including yttrium) are used, and oxides such as yttrium (Y), ytterbium (Yb), and erbium (Er) are used. It is preferable to do. It is also possible to use MgO, and _Al 2 _{O 3.} These sintering aids may be used alone or in combination of two or more. As a sintering aid, a rare earth element or alkaline earth element compound (such as carbonate) that becomes an oxide during sintering may be used.

  In addition to the above-mentioned sintering aid, it is also effective to use aluminum compounds such as aluminum nitride and aluminum oxide; and oxides or nitrides of titanium, zirconium and hafnium as a part of the sintering aid. It is. The aluminum compound contributes to strengthening the bonding force between the silicon nitride crystal grains. These compounds may be added as oxides or nitrides, or compounds that become oxides or nitrides during sintering may be added.

  The ratio of the sintering aid described above is preferably in the range of 2 to 17 parts by weight with respect to 100 parts by weight of metal Si. If the amount of the sintering aid is less than 2 parts by weight, the silicon nitride ceramic may not be sufficiently densified. On the other hand, when the amount of the sintering aid exceeds 17 parts by weight, the amount of segregation of the grain boundary phase and the sintering aid component increases more than necessary, thereby reducing the wear resistance and strength of the silicon nitride ceramic. There is a risk of inviting.

  The pore-forming agent is mixed together with the metal Si and the sintering aid, and is included as one component of the mixture containing the metal Si, the sintering aid, and the pore-forming agent. In the pore-forming step, pores are formed by heating the pore-forming agent in the powder compact of the mixture. When the average particle diameter of the metal Si is small (for example, 0.5 μm or more and 40 μm or less), the metal Si is in close contact with each other, so that pores are hardly formed. However, according to the present invention, since pores can be easily formed even when such metal Si is used, particularly when the average particle diameter of metal Si is small, the use significance of the pore former is great. I can say that.

  Any known pore-forming agent can be used as long as it melts or vaporizes below the nitriding temperature of metal Si. That is, the pore-forming agent is heated to form pores, but the pore-forming agent may be one that forms pores by melting or one that forms pores by vaporization.

  For example, resins such as polyethylene, polypropylene, polymethyl acrylate, polymethyl methacrylate, polyvinyl alcohol, polyphenol, and paraffin; plant materials such as starch, nut shell, walnut shell, and corn; carbon materials such as graphite and carbon fiber Can be mentioned. When the pore-forming agent is a resin, it normally melts at 100 ° C. or more and 500 ° C. or less, and when it is a plant-based material or carbon-based material, oxidation to carbon monoxide and carbon dioxide occurs at about 400 ° C. Usually vaporizes.

  The ratio of the pore former is preferably 10% by volume or more and 60% by volume or less with respect to the total volume of the metal Si and the sintering aid. This is because the amount of pores formed is less than 10% by volume. On the other hand, if it exceeds 60% by volume, it becomes difficult to obtain an effect proportional to the amount of pore-forming agent used, and the pore-forming agent is unnecessarily consumed.

  Further, the ratio of the pore former is more preferably 30% by volume or more and 60% by volume or less with respect to the total volume of the metal Si and the sintering aid. By setting the lower limit to 30% by volume, pores formed by melting of the pore-forming agent are increased. More preferably, it is 40 volume% or more and 60 volume% or less. Thereby, metal Si and nitrogen can be made to contact more uniformly, and nitriding can be performed suitably.

More preferably, the ratio of the pore former is 45% by volume or more and 60% by volume or less with respect to the total volume of the metal Si and the sintering aid. By setting the lower limit of the ratio of the pore forming agent to 45% by volume or more, the metal Si and nitrogen can be contacted very uniformly, and nitriding from a reaction sintered body containing more α phase advantageous for formability. Silicon-based ceramics can be obtained. Even if an amount exceeding 60% by volume is used, it is difficult to obtain the effect of increasing the proportion of the α phase in the reaction sintered body. In addition, the weight fraction of the metal Si: α-Si ₃ N ₄ : β-Si ₃ N ₄ after the nitriding step is obtained from the peak intensity of X-ray diffraction after pulverizing the reaction sintered body after the nitriding step.

  The shape of the pore former is not particularly limited, and may be spherical, granular, rod-like, fibrous, or crushed.

  The above mixture may contain various additives as other components. Examples thereof include carbon nanotubes and carbon nanocoils for improving electrical conductivity; silicon carbide and metal particles for improving mechanical properties. The addition amount of the additive is appropriately set according to the purpose of use.

  Each component contained in the mixture is desirably mixed in a fine state. By mixing in such a state, pores by the pore-forming agent are uniformly formed, metal Si and nitrogen can be brought into contact in a more uniform state, and nitriding can be suitably performed. Further, since the sintering aid is more uniformly dispersed, densification of silicon nitride described later is more suitably performed. The preferable average particle diameter of each component is 3 μm or more and 20 μm or less, preferably 5 μm or more and 15 μm or less. When the average particle size of each component is less than 3 μm, the average particle size is too small, and it becomes difficult to handle each component, so that workability is lowered. On the other hand, when the average particle diameter exceeds 20 μm, it becomes difficult to uniformly mix the components. In addition, the average particle diameter indicates an average value of the shortest side when the shape of each component is not spherical but is a rod shape, a fiber shape, or the like. That is, when the shape is a rod shape, the average particle diameter is not a length or a long diameter, but shows an average value of the short diameter.

(S2. Grinding step)
When the average particle diameter of each component is larger than a desired value, it is desirable to pulverize each component to reduce the size (grinding step). A ball mill, a bead mill, a roll mill, or the like can be used for pulverization, and a silicon nitride ball mill or bead mill is preferably used from the viewpoint of preventing impurities from being mixed.

  The mixing of each component may be performed dry without adding a liquid, or may be performed wet by adding a liquid. Examples of the liquid added in the case of wet mixing include organic solvents such as ethanol and xylene in addition to water. As a usage-amount of a liquid, what is necessary is just to be 5 volume% or more and 50 volume% or less with respect to the total volume of each component of a mixture, for example. In the case of wet mixing, it is preferable to add various dispersants to the slurry mixture to control the slurry viscosity.

  The ground metal Si preferably has a small particle size and is uniform. Specifically, the median diameter of the metal Si after pulverization (particle diameter (d50) when the cumulative percentage in the volume-based cumulative particle size distribution is 50%) is preferably 1.7 μm or less, and more preferably. Is 1.1 μm or less. Further, the difference between the particle size (d90) when the cumulative percentage is 90% and the particle size (d10) when the cumulative percentage is 10% is preferably 2.0 μm or less, and more preferably 1.2 μm or less. The median diameters (d50), d90 and d10 are values measured by a laser diffraction particle size distribution measuring apparatus (manufactured by Shimadzu Corporation, SALD-7000, light source 405 nm, set refractive index 4.0-0.01i).

  By setting the median diameter (d50) of metal Si to 1.7 μm or less, the surface area can be increased, and in the nitriding step, nitriding can be promoted and abnormal grain growth can be prevented. Moreover, when the difference between d90 and d10 is 2.0 μm or less, the particle size of the metal Si powder is made uniform, and each particle can be nitrided uniformly.

(S3. Mixing and granulating process)
Next, the metal Si, the sintering aid, and the pore former are mixed so that the ground metal Si, the sintering aid, and the pore former have a predetermined composition ratio (mixing step). As a mixing apparatus used in the mixing step, for example, a ball mill can be used. In that case, mixing may be performed for 24 hours using a Si ₃ N ₄ ball having a ball diameter of 10 mm. Then, the obtained mixture is dried and granulated (granulation process). At this time, a binder resin or the like may be appropriately added to form granules. Thereby, it becomes easy to granulate the mixture. Examples of the granulating agent include paraffin, which may be dissolved in an organic solvent such as cyclohexane and added to the mixture.

(S4. Molding process)
Subsequently, the granulated product is filled into a mold having a predetermined shape, and pressure-molded to form a green compact. A known molding method can be applied as the pressure molding method, and a uniaxial press molding method, a cold isostatic pressing method (CIP) method, or the like can be used. Moreover, after temporary molding by a uniaxial press molding method, the main molding may be performed using CIP.

(S5. Hole making process)
In the hole making step, the green compact of the above mixture is heated to melt or vaporize the pore forming agent to form pores between the metal Si (hole forming step). Various heating methods such as atmospheric pressure heating, pressure heating (hot press), atmospheric pressure heating, HIP (hot isostatic pressing: hot isostatic press) heating can be applied to the formation of pores. In the hole forming step, the powder body containing metal Si, a sintering aid, and a hole forming agent is heated to form pores between the metal Si.

  The temperature at which the compact is heated in the pore making process may be any temperature at which the pore former melts or vaporizes. Usually, when the pore forming agent is a resin, the pore forming agent can be melted at 100 ° C. or more and 500 ° C. or less. When the pore forming agent is a plant-based material or a carbon-based material, 800 ° C. or more and 900 ° C. It is possible to vaporize the pore former below. The green compact obtained in the molding process contains organic substances such as a dispersant added in the pulverization process and a binder resin added in the granulation process. Since the compact is heated in the pore forming process, the degreasing process for removing these organic substances is performed at the same time.

  When carbide with modified pore former remains on metal Si or sintering aid, the carbide is the starting point of cracks in the silicon nitride ceramics (reaction sintered body and reaction sintered body) finally obtained. There is a risk of becoming. Therefore, in order to gasify and remove the carbide, the temperature at which the powder compact of the mixture is heated in the pore forming process is preferably 500 ° C. or higher and 1000 ° C. or lower. Thereby, since it becomes difficult to produce a crack in silicon nitride ceramics, high quality silicon nitride ceramics can be provided.

  The pore forming step may be performed in an air atmosphere, but when the subsequent nitriding step is continuously performed, it is preferably performed in a nitrogen atmosphere. Usually, the heating time of the compact in the pore making process is 1 hour or more and 5 hours or less. By the heating in this pore forming process, pores are formed between the metal Si, and a porous body which is an intermediate material before nitriding is obtained. By forming pores due to the pore-forming agent in the porous body, the specific surface area of the metal Si is increased, and nitrogen can be brought into contact with the metal Si more entirely in the subsequent nitriding step. That is, it becomes possible to make metal Si and nitrogen contact more uniformly.

(S6. Nitriding process (reaction sintering process))
The nitriding step is a reaction sintered body (post-reaction sintered body) which is a silicon nitride ceramic by firing the porous body containing metal Si having pores obtained in the pore forming step in a nitrogen atmosphere. ). Since the heat treatment is performed in the hole forming step, the hole forming step and the nitriding step can be performed continuously by raising the temperature. Note that the nitriding step may be performed once the porous body is taken out.

  The nitriding process can be performed by the same heating method as the hole making process, such as atmospheric pressure heating, pressure heating (hot press), atmospheric pressure heating, HIP (hot isostatic pressing: hot isostatic press) heating, etc. Various heating methods can be applied.

  The temperature for firing the porous body in the nitriding step may be any temperature at which metal Si in the porous body is nitrided. Usually, the temperature is 1200 ° C. or more and 1450 ° C. or less, but it is desirable to perform nitriding at a high temperature from the viewpoint of promoting nitriding. Conventionally, from the above viewpoint, since nitriding has been performed at a high temperature at which silicon does not melt, metal Si is overheated and jetting occurs. As described above, an object of the present invention is to suppress the ejection. In this specification, the term “spout” refers to a state in which molten Si flows out of the sample.

  In the manufacturing method of the present invention, pores are formed between the metal Si in the hole making step. And since many pores are formed in the porous body obtained at the pore making process, the specific surface area of the porous body is large. In the nitriding step according to the present invention, since nitrogen is introduced into the pores, the metal Si and nitrogen can be contacted in a wide range. Under these conditions, the metal Si and nitrogen can react uniformly, and a portion of the metal Si in the porous body that is overheated without being nitrided is less likely to occur. As a result, ejection of metal Si in the porous body can be suppressed, yield can be increased, and manufacturing cost can be suppressed.

The reaction fired body has a large number of pores, and it is preferable that the reaction fired body has a small relative density. Since the pores formed by the pore-forming agent have a relatively uniform size, a space in which grain growth accompanying a phase change from α-Si ₃ N ₄ to β-Si ₃ N ₄ occurs in the densification step is preferable. This is because it is secured. From this viewpoint, the relative density of the reaction fired body is preferably 75% or less, more preferably 60% or less, and particularly preferably 55% or less. The lower limit is not particularly limited, but is generally 45% or more.

  In this production method, pores are formed using a pore-forming agent, but after the nitriding step, a densification step is performed at a temperature higher than the nitriding temperature as will be described later. In the densification step, the reaction sintered body is fired to increase the density. For this reason, the setting in each process is made on the assumption that a high-density silicon nitride-based ceramic is finally obtained also in the previous process of the densification process.

  In view of this point, in the present invention, a porous body having pores is formed by a pore-forming agent, and the formation of pores in the raw material can cause a decrease in the density of the resulting silicon nitride ceramics. It is usually expected. That is, in the raw material processing process, a process for reducing the density of the raw material is once performed, and at first glance, a process opposite to the purpose of densification is performed. Therefore, the manufacturing method according to the present invention invites the expectation that a final fired body having a low density is obtained contrary to densification, and it can be said that the manufacturing method is special compared to the conventional method. It is none other than what was discovered by earnest examination.

(S7. Densification step)
The densification step is a step of firing and densifying the reaction sintered body at a temperature exceeding the nitriding temperature in the nitriding step, and a final sintered body that is a silicon nitride ceramic is obtained by this step.

  The densification process can be performed by the same heating method as the hole making process, and normal pressure heating, pressure heating (hot press), atmospheric pressure heating, HIP (hot isostatic pressing: hot isostatic press) heating. Various heating methods such as these can be applied.

  The temperature for firing the reaction sintered body is usually 1600 ° C. or higher, preferably 1750 ° C. or higher. The upper limit is not particularly limited, but it is preferably 1950 ° C. or lower from the viewpoint of preventing the final sintered body from being damaged. Further, the firing time is usually 1 hour or more and 10 hours or less.

  Since the ejection of metal Si is suppressed in the nitriding process of the previous process, the quality of the reaction sintered body to be fired in the densification process is suitable without being impaired. Further, if metal Si is ejected, it is necessary to clean the firing furnace and the like, and the tact time is increased as well as the manufacturing cost is lowered, but such a possibility is also reduced. Originally, the post-reaction sintering method using metal Si as a raw material is advantageous in terms of cost compared with the atmospheric pressure sintering method. According to this manufacturing method, silicon nitride ceramics can be produced at low cost and with a short tact time. Can be manufactured.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and the technical means disclosed in different embodiments can be appropriately combined. Such embodiments are also included in the technical scope of the present invention.

  Although this invention is demonstrated more concretely based on an Example and a comparative example and FIGS. 1-8, this invention is not limited to this. Those skilled in the art can make various changes, modifications, and alterations without departing from the scope of the present invention. In addition, each physical property in the following examples and comparative examples was evaluated as follows.

(1) Relative density The relative density of the compact, the reactive sintered body, and the final sintered body was determined by calculating the ratio of the measured density by the Archimedes method to the theoretical density.

(2) Vickers hardness, fracture toughness value and bending strength A test piece was cut out from the final sintered body, and the bending strength was measured according to the method specified in JIS R1603. The final sintered body after the bending test is mirror-polished, and the Vickers hardness is measured at a load of 20 kg according to the method specified in JIS R160X. The fracture toughness value (MPa · m ^1/2 ) Was calculated.

(3) Weight fraction of silicon nitride phase By X-ray diffraction, the weight fraction of Si, α-Si ₃ N ₄ , and β-Si ₃ N _{4 in} the reaction fired body was determined from the X-ray diffraction peaks of the respective components.

(4) Ejection of Si Whether or not ejection of metal Si occurred in the nitriding step was judged by visual observation.

(5) Shrinkage rate The shrinkage rate of silicon nitride ceramics (fired from the reaction sintered body to the final sintered body) in the nitriding process to the densification process was measured using a micrometer (No.293-661, manufactured by Mitutoyo Corporation). -10N).

(6) Observation of reaction sintered body by thin piece translucent method The test piece was processed into a thickness of about 100 μm using a polishing machine, and the thin piece was observed in a transmission mode of an optical microscope.

[Examples 1 to 12]
(S1. Raw material preparation step)
The following were used as the starting metal Si, sintering aid and pore former.
・ Metal Si
# 600 powder manufactured by Yamaishi Metal Co., Ltd., average particle size 12.6 μm
Sintering aid Y ₂ O ₃ : “RU-P” manufactured by Shin-Etsu Chemical Co., Ltd., average particle size 1.1 μm
MgO: “500A” manufactured by Ube Industries, Ltd., average particle size 0.05 μm
ZrO ₂ : “TZ-O” manufactured by Tosoh Corporation, average particle size 0.07 μm
・ Dispersant: “Floren G” manufactured by Kyoeisha Chemical Co., Ltd.
・ Poreforming agent: “Bellpearl R200” (phenol resin) manufactured by Air Water Bellpearl Co., Ltd.
(S2. Grinding step)
Grinding was performed under the following grinding conditions. In addition, let the obtained metal Si be a bead mill ground material.

(Crushing conditions) “Minizea” manufactured by Ashizawa Finetech was used as the bead mill, and 290 g of Si ₃ N ₄ beads having a bead diameter of 0.5 mm were used as the grinding media. Then, a slurry obtained by mixing 1200 g of metal Si powder and 4000 g of ethanol was passed through a bead mill for 10 passes and pulverized. The rotation speed of the bead mill is 3000 rpm.

With respect to the starting materials blended so that the weight ratio of the bead mill pulverized product (metal Si powder): Y ₂ O ₃ : MgO: ZrO ₂ and the dispersant is 93: 2: 5: 1: 3, Ball mill wet mixing was performed using a certain volume of pearl so that the ratio shown in Table 1 was obtained. Specifically, various sintering aids and dispersants are blended in the above weight ratio with respect to 50 g of the ball mill pulverized product, this starting material is added to 400 ml of ethanol, and a Si ₃ N ₄ ball having a ball diameter of 10 mm. Was used to perform ball mill mixing for 24 hours.

  Then, 35 ml of paraffin, DOP (dioctyl phthalate) and cyclohexane were added to the starting material and granulated.

(S4. Molding process)
Thereafter, using a mold having a cylindrical hollow portion having an outer diameter of 15 mm, molding was performed by applying a pressure of 50 MPa for 30 seconds by a uniaxial press molding method. Furthermore, it was molded by applying a pressure of 200 MPa for 60 seconds by a cold isostatic press (CIP) method. Thereby, the compacted green compact was obtained.

(S5. Hole making step) to (S7. Densification step)
The obtained green compact was held at 500 ° C. for 3 hours to perform pore formation and degreasing. Then, in each Example, the nitriding process and the densification process were performed on the baking conditions of Table 1. In the nitriding step, firing was performed in a nitrogen atmosphere. In the densification step, firing was performed while flowing 0.9 MPa of nitrogen at a rate of 4 l (4 liters) / min.

(Evaluation)
Table 2 shows the relative density, Vickers hardness, fracture toughness, bending strength, and Si ejection results of the compacts, reaction sintered bodies, and final sintered bodies for Examples 1 to 3. Moreover, the relative density of the compact, the reaction sintered body, and the final sintered body in Examples 4 to 12, the weight fraction of Si, α-Si ₃ N ₄ and β-Si ₃ N _{4 in} the reaction sintered body, and Table 2 shows the bending strength of the final sintered body as a result of the ejection of Si.

[Comparative Example 1]
A final sintered body was obtained in the same manner as in Example 1 except that Belpearl was not used. Table 2 shows the relative density, Vickers hardness, fracture toughness, bending strength, and Si ejection results of the compact, reaction sintered body, and final sintered body for Comparative Example 1.

[Comparative Example 2]
In Comparative Example 1, a final sintered body was obtained using Si ₃ N ₄ powder (“SN-E10” manufactured by Ube Industries, Ltd.) instead of metal Si.

(Examination of results of Examples and Comparative Examples)
As shown in Table 1, in Comparative Example 1 in which the pore forming agent Bell Pearl was not used, metal Si was ejected to the reaction sintered body in the nitriding step, and the relative density of the final sintered body, Vickers hardness Although the influence on the thickness and fracture toughness was good, the product value was low as the final sintered body. Also, it took time to clean the reactor. A photograph of the final sintered body obtained in Comparative Example 1 is shown in FIG.

  On the other hand, in the reaction sintered bodies in Examples 1 to 12, no metal Si was ejected, and the final sintered body was not damaged by the metal Si ejection. This is a result resulting from the formation of pores by the pore-forming agent. A photograph of the final sintered body obtained in Example 1 is shown in FIG.

  The final fired bodies in Comparative Example 1 and Examples 1 to 3 all have a good relative density. The SEM (scanning electron microscope) photograph figure of each final sintered body is shown to Fig.4 (a)-(d). Particularly in Example 3 (FIG. 4D) using paraffin as a pore-forming agent, a final fired body having a high relative density of 95.8% could be obtained.

  Moreover, the photograph figure of the reaction sintered compact by a thin piece translucent method is shown in FIG. FIG. 5A is a photographic view showing a reaction sintered body according to Example 5, and FIG. 5B is a photographic view showing a reaction sintered body according to Comparative Example 1. In FIG. 5 (a), uniform pores (white portions in the figure) are observed, and since pores are not observed in FIG. 5 (b), pores are formed due to the pore-forming agent. Was confirmed.

  Furthermore, the SEM photograph figure of the reaction sintered compact obtained in each of Example 4, 5, and 6 is shown in FIG. (B), (d), and (f) shown in the lower part of FIG. 6 are enlarged views of the upper parts (a), (c), and (d), respectively. As shown in FIG. 6, when the pore forming agent of Example 4 is 25 vol%, the size of the pores formed is small, but when the pore forming agent is used at 30 vol% as in Example 5, It can be seen that larger pores can be formed. Further, when 35 vol% of the pore-forming agent was used as in Example 6, the photographing cross-section of the pores was about twice that of Example 5, and it was clear that the pores were suitably formed. Yes.

Further, FIG. 7 shows changes in the shrinkage rate of the silicon nitride ceramics due to temperature changes in Example 5 (30% pore-forming agent), Comparative Example 1 (no pore-forming agent) and Comparative Example 2. In Comparative Example 2, since Si ₃ N ₄ powder is used as a raw material instead of metal Si, the ejection of metal Si does not naturally occur, and the shrinkage rate gradually increases as silicon nitride ceramics become denser. On the other hand, in Comparative Example 1, the shrinkage rate does not change much up to around 1800 ° C., but the reaction fired body rapidly contracts at the processing temperature of 1850 ° C. in the densification step.

  In the manufacturing process of the final fired body, by gently densifying, the growth of coarse particles can be suppressed and uniform densification is achieved. In Example 5 according to the present invention, it is clear that densification is performed more slowly than in Comparative Example 1, and it is expected that abnormal grain growth is suppressed in the obtained final fired body. .

In Examples 4 to 12, 25% by volume or more of bell pearl is used. When 45% by volume of Belpearl was used as in Example 8, the proportion of α-Si ₃ N ₄ was rapidly increased as compared with the case of 40% by volume of Bell Pearl of Example 7. I understand. Also, it increased the amount of BELLPEARL to Example 12, the proportion of α-Si ₃ N ₄ did not increase much in proportion to the amount of use. Considering the effect on the usage amount, the usage amount of Belpearl is determined to be 60% by volume.

  Finally, the fracture source after measuring the bending strength of the final fired body of Example 12 was analyzed by EDS (energy dispersive X-ray spectroscopy). An EDS observation diagram of the final fired body is shown in FIG. FIG. 8A is a view showing a source of destruction of the final fired body, FIG. 8B is a view showing a distribution of silicon, and FIG. 8C is a view showing a distribution of carbon.

  It was found that carbon remains in the destruction source of FIG. 8A and this carbon is the destruction source. For this reason, it is expected that the final fired body is less likely to be destroyed if the powder body is heated at a higher temperature in the hole making step to reduce the residual amount of carbon.

  The present invention is used for engine parts materials, bearing materials, tool materials, molten metal parts, bearing members, various roll materials for rolling, nitriding used for various structural materials such as compressor vanes, turbo rotors and cutting tools. It can be used for silicon-based ceramics.

Claims (5)

In the method for producing silicon nitride ceramics for nitriding metal Si,
Forming a pore between the metal Si in the powder body by heating the powder body of the mixture containing the metal Si, the sintering aid, and a pore-forming agent that forms pores by heating; and
A nitriding step for obtaining a reactive sintered body by firing a porous body containing metal Si having pores formed therein and a sintering aid obtained in the pore forming step in a nitrogen atmosphere;
A densification step in which the reaction sintered body is fired and densified at a temperature exceeding the nitriding temperature at which the reaction sintered body in the nitriding step is fired, and a final sintered body is obtained;
A method for producing silicon nitride ceramics, comprising:   The method for producing a silicon nitride-based ceramic according to claim 1, wherein the ratio of the pore former to the total volume of the metal Si and the sintering aid is 30% by volume or more and 60% by volume or less.   The method for producing a silicon nitride-based ceramic according to claim 2, wherein the ratio of the pore former to the total volume of the metal Si and the sintering aid is 45% by volume or more and 60% by volume or less.   The method for producing a silicon nitride ceramic according to any one of claims 1 to 3, wherein a temperature for heating the green compact of the mixture in the pore forming step is 500 ° C or higher and 1000 ° C or lower. . In the method for producing silicon nitride ceramics for nitriding metal Si,
Forming a pore between the metal Si in the powder body by heating the powder body of the mixture containing the metal Si, the sintering aid, and a pore-forming agent that forms pores by heating; and
A nitriding step for obtaining a reactive sintered body by firing a porous body containing metal Si having pores formed therein and a sintering aid obtained in the pore forming step in a nitrogen atmosphere;
A method for producing silicon nitride ceramics, comprising: