Manufacturing method for composite ceramic materials

Abstract

To provide ceramic matrix composites that have a higher specific modulus relative to silicon carbide and enable the molding of an article of a complex shape.SOLUTION: A method of producing ceramic matrix composites includes a mixing step, a molding step, a heat treatment step, an infiltration step, and a reactive sintering step. In the mixing step, boron carbide powder, a matrix filler, and a binder resin are mixed to obtain a mixed raw material. In the molding step, the mixed raw material is fed into a mold, heated and pressurized to obtain a molded body. In the heat treatment step, the molded body is heat-treated to obtain a fired body. In the infiltration step, metallic silicon or a silicon alloy is melted and infiltrated into the fired body. In the reactive sintering step, the metallic silicon or a silicon alloy infiltrated into the fired body is reacted with the matrix filler and carbon to produce silicon carbide and to sinter the boron carbide powder. The matrix filler is carbon milled fiber. The carbon milled fiber does not remain in the sintered body in the reactive sintering step.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing a composite ceramic material that is composed of boron carbide as a main component and inorganic materials containing silicon carbide and silicon. [Background technology]

[0002] In response to rising fuel prices and demands for reducing carbon dioxide (CO2) emissions, there are increasingly stringent demands for energy conservation in air conditioners, refrigeration equipment, automobiles, aircraft, and other devices equipped with combustion engines. In air conditioners and refrigeration equipment, compressors must be made more efficient to save energy, and this requires lightweight or highly rigid drive components. In combustion engines, gas turbine engines or turbine generators must be made more efficient, and this requires raising the operating temperature or reducing the weight of the turbine.

[0003] Research into materials that satisfy these requirements is ongoing. While the development of iron-, nickel-, and cobalt-based alloys has led to advances in high-temperature performance, weight reduction remains insufficient, and alternative materials are being considered. Lightweight ceramic materials have also attracted attention as heat-resistant materials due to their high heat resistance and lower density compared to metallic materials. However, due to their relatively brittle nature, ceramic composites (Ceramic Matrix Composites (CMCs)) that incorporate fibers and other materials have been gaining attention as materials suitable for structural components. Generally, CMCs are composites of a ceramic matrix and reinforcing fibers. To date, SiC (silicon carbide) has been used as the matrix and SiC fibers as the reinforcing fibers. SiC / SiC, a combination of SiC and SiC fibers, has been developed as a CMC and is now being put to practical use.

[0004] In recent years, boron carbide (B4C), which is even lighter and more rigid than SiC, has attracted attention, and B4C / Si composites, which are matrix composites of boron carbide reinforcement and metal silicon (Si), are manufactured by infiltrating metal silicon into a boron carbide preform. The Young's modulus of boron carbide is approximately 460 GPa at maximum, and the density is 2.52 g / cm3. 3 From these, the specific modulus of elasticity, which indicates the elastic modulus per unit density of boron carbide, is 182.5 GPa / (g / cm 3 ) This specific elastic modulus of boron carbide is superior to that of silicon carbide. However, boron carbide is generally produced by a reaction under high temperature and high pressure. Under high temperature and high pressure, sintered boron carbide is hard and has very poor workability, making it difficult to mold and manufacture complex shapes. For this reason, a production method at lower temperatures and pressures is needed. Accordingly, a method has been proposed in which boron carbide is impregnated or pressure-infiltrated with metals such as metallic silicon or aluminum (Al). Patent Document 1, noting that the reaction between silicon and boron carbide during impregnation is one cause of cracking, discloses a method in which molten metallic silicon and a boron carbide-containing material are mixed and pre-melted, and the pre-melted material is impregnated into a porous preform containing boron carbide. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2007-513854 Summary of the Invention [Problem to be solved by the invention]

[0006] When the number of voids in a preform is small and the void diameter is small, voids remain if the pressure applied during impregnation or pressure infiltration of metals such as silicon metal or aluminum is weak, as in the conventional technology described above. Such preforms do not have sufficient properties as a reinforcing material and are difficult to mold into complex shapes. Conversely, when the number of voids in a preform is large, metals such as silicon metal or aluminum can be easily filled into the preform, but the amount of reinforcing material in the preform is reduced, and therefore sufficient properties as a reinforcing material cannot be obtained. In other words, when using a preform molded from boron carbide powder using a binder or the like, in order to obtain the properties of boron carbide as a reinforcing material, it is necessary to increase the filling rate of boron carbide. However, increasing the filling rate of boron carbide requires high pressure to mold the preform, making it difficult to mold complex shapes and requiring high pressure to infiltrate metals such as silicon metal or aluminum into the voids in the preform. Furthermore, poor filling or cracks are more likely to occur. Conversely, if the filling rate of boron carbide in the preform is reduced, the moldability and manufacturability improve, but the sufficient properties of boron carbide as a reinforcing material cannot be obtained. Thus, when using a preform formed by molding boron carbide powder with a binder or the like, it is difficult to achieve both moldability and manufacturability and the property as a reinforcing material, namely, a specific elastic modulus superior to that of silicon carbide.

[0007] The present disclosure has been made in view of the above, and aims to provide a method for manufacturing a composite ceramic material that has a specific elastic modulus higher than that of silicon carbide and can be formed into complex shapes. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems and achieve the object, the method for producing a composite ceramic material according to the present disclosure includes a mixing step, a molding step, a heat treatment step, an infiltration step, and a reaction sintering step. 76.2% by weight or more and 80.0% by weight or less Boron carbide powder and 7.9% by weight or more and 12.5% ​​by weight or less Matrix filler and 9.4% by weight or more and 15.9% by weight or lessThe mixed raw material is mixed with a binder resin to obtain a mixed raw material. In the molding process, the mixed raw material is placed in a mold and heated and pressurized to harden the binder resin and obtain a molded body. In the heat treatment process, the molded body is heated in an inert atmosphere or a vacuum atmosphere to carbonize the binder resin and obtain a fired body. In the infiltration process, metal silicon or a silicon alloy is brought into contact with the fired body and heated in an inert atmosphere or a vacuum atmosphere to melt the metal silicon or silicon alloy and infiltrate the fired body. In the reactive sintering process, the metal silicon or silicon alloy infiltrated into the fired body reacts with the matrix filler and the binder resin contained in the fired body to carbonize the carbon, generating silicon carbide, and sintering the boron carbide powder to obtain a sintered body. The matrix filler is carbon milled fiber. In the molding process, heating and pressure are applied so that the void ratio of the compact is 10% or more. In the heat treatment process, heat treatment is performed so that the void ratio of the fired body is 20% or more but less than 40%, producing a fired body with a network structure in which boron carbide particles are connected by carbon milled fibers or carbon particles formed by carbonizing the binder resin. In the reactive sintering process, the carbon milled fibers react with the silicon metal or silicon alloy to form silicon carbide, and no carbon milled fibers remain in the sintered body. [Effects of the Invention]

[0009] The method for producing a composite ceramic material according to the present disclosure has the effect of providing a specific elastic modulus higher than that of silicon carbide, and enabling the formation of complex shapes. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional view schematically showing an example of the configuration of a composite ceramic material manufactured by the manufacturing method of a composite ceramic material according to the first embodiment. [Figure 2] FIG. 1 is a cross-sectional view schematically showing an example of the configuration of a preform before reaction sintering of a composite ceramic material manufactured by the manufacturing method of a composite ceramic material according to the first embodiment. [Figure 3] 1 is a flowchart showing an example of a procedure for a method for manufacturing a composite ceramic material according to the first embodiment. [Figure 4] 1 is a flowchart showing an example of a procedure for a method for manufacturing a composite ceramic material according to a second embodiment. [Figure 5]FIG. 1 is a diagram showing an example of how fired bodies are bonded together using a bonding binder to form a structure. [Figure 6] FIG. 10 is a side view schematically illustrating an example of the state of a joint between fired bodies. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, a method for manufacturing a composite ceramic material according to an embodiment of the present disclosure will be described in detail with reference to the drawings.

[0012] Embodiment 1 Fig. 1 is a cross-sectional view schematically showing an example of the configuration of a composite ceramic material manufactured by the manufacturing method for a composite ceramic material according to embodiment 1. Fig. 2 is a cross-sectional view schematically showing an example of the configuration of a preform before reaction sintering of the composite ceramic material manufactured by the manufacturing method for a composite ceramic material according to embodiment 1. The configuration of composite ceramic material 1 will be described with reference to Figs. 1 and 2.

[0013] As shown in FIG. 1, composite ceramic material 1 manufactured by the manufacturing method of composite ceramic material 1 according to the first embodiment includes boron carbide (BC) particles 11, silicon carbide (SiC) particles 12, and silicon (Si) particles 13. Here, silicon particles 13 are made of metallic silicon or a silicon alloy. Composite ceramic material 1 has a structure in which silicon carbide particles 12 and silicon particles 13 fill voids between boron carbide particles 11. The main component of composite ceramic material 1 is boron carbide. Note that the composite ceramic material according to the first embodiment is a combination of multiple ceramic materials and is not composited with reinforcing fibers.

[0014] In the composition of composite ceramic material 1, the main component is boron carbide, and the volume ratio of boron carbide is 50% or more. Composite ceramic material 1 also contains silicon carbide and silicon as other components. If composite ceramic material 1 contains a large amount of silicon or silicon carbide, the apparent specific elastic modulus decreases. For this reason, the volume ratio of boron carbide, which has the highest specific elastic modulus as a single element, in composite ceramic material 1 is set to 50% or more, and by making boron carbide the main component of composite ceramic material 1, the specific elastic modulus of composite ceramic material 1 is made larger than a specified value.

[0015] The Young's modulus of boron carbide, the main component of composite ceramic material 1, is 450 GPa. The density of boron carbide is 2.52 g / cm. 3 Therefore, the specific elastic modulus of boron carbide is 178.6 GPa / (g / cm 3 ) Compared to silicon carbide, boron carbide has a higher elastic modulus, a lower density, and a higher specific elastic modulus. In other words, boron carbide, the main component of composite ceramic material 1, is a material that can be expected to improve the specific elastic modulus.

[0016] As described above, the silicon particles 13 are metallic silicon or a silicon alloy. The reason silicon is included in the composite ceramic material 1 is that it is difficult to reduce the silicon content to zero and fill it with other components during the manufacturing process. It is possible to replace the silicon particles 13 with voids 25. However, the voids 25 correspond to defects in the material and reduce the material properties. For this reason, filling the material with silicon particles 13 improves the material properties and increases the specific elastic modulus, rather than replacing the silicon particles 13 with voids 25 to create empty spaces.

[0017] In the configuration of the composite ceramic material 1 of the present disclosure, the silicon carbide filling rate is 15% or more and 35% or less by volume. As described below, the composite ceramic material 1 is produced by infiltrating metal silicon or a silicon alloy into a preform 20 shown in FIG. 2 and then reactive sintering. In reactive sintering, a carbon component is essential to react with the metal silicon or silicon alloy to form silicon carbide. As shown in FIG. 2, the preform 20 before reactive sintering is a porous sintered body containing boron carbide particles 11, carbon milled fibers 21, and carbon (C) particles 22. However, the carbon particles 22 are carbonized binder resins, which will be described later. The carbon milled fibers 21 contained in the preform 20 react with the metal silicon or silicon alloy during the silicon infiltration and silicon carbide reactive sintering processes, which will be described later, and all of them become silicon carbide. By setting appropriate reaction conditions in the silicon infiltration and silicon carbide reaction sintering steps, the carbon milled fibers 21 do not remain in the sintered body after the silicon infiltration and silicon carbide reaction sintering steps. In other words, the carbon milled fibers 21 are replaced by silicon carbide particles 12.

[0018] The boron carbide particles 11 in the porous preform 20 shown in FIG. 2 have a network structure cross-linked by carbon particles 22 formed by carbonizing the binder resin and carbon milled fibers 21. In other words, the boron carbide particles 11 are connected to each other by the carbon milled fibers 21 or carbon particles 22. Networking the boron carbide particles 11 with the carbon milled fibers 21 and carbon particles 22 facilitates the creation of voids 25 and ensures sufficient strength for the porous preform 20. The use of carbon milled fibers 21 and carbon particles 22 reduces the filling ratio of components other than the boron carbide particles 11. This allows the proportion of voids 25 in the porous preform 20 to be increased without reducing the filling ratio of the boron carbide particles 11, thereby controlling the volume expansion due to reactive sintering. This enables reactive sintering of a composite ceramic material 1 with a filling ratio of boron carbide particles 11 of 50% or more without generating cracks. At this time, the volume ratio of silicon carbide produced by the reaction is preferably 15% or more and 35% or less.

[0019] If the volumetric ratio of silicon carbide produced by the reaction is greater than 35%, cracks are more likely to occur. Reducing the filling rate of boron carbide particles 11 to prevent cracks results in an increased filling rate of silicon carbide. This means that the proportion of boron carbide particles 11 decreases and the proportion of silicon carbide increases, undesirably lowering the specific elastic modulus of the composite ceramic material 1. On the other hand, if the reaction with infiltrated silicon is suppressed and the volumetric ratio of silicon carbide produced by the reaction is less than 15%, the proportion of unreacted silicon increases, undesirably lowering the specific elastic modulus of the composite ceramic material 1. For these reasons, it is desirable to set the volumetric ratio of silicon carbide produced by the reaction to between 15% and 35%.

[0020] Next, a method for manufacturing the composite ceramic material 1 according to the first embodiment will be described. Fig. 3 is a flowchart showing an example of the steps of the method for manufacturing the composite ceramic material according to the first embodiment. The method for manufacturing the composite ceramic material 1 includes a raw material mixing step (step S1), a molding step (step S2), a heat treatment step (step S3), a silicon infiltration and silicon carbide reaction sintering step (step S4), and a finish processing step (step S5). Each step will be described below.

[0021] The raw material mixing step S1 is a step of uniformly mixing the raw material boron carbide powder, matrix filler, and binder resin (carbon precursor) at a predetermined mixing ratio to produce a mixed raw material. The average particle size distribution of the powder raw materials in the mixed raw material may be a single level distribution, but it is preferable that it be a distribution with two or more different levels.

[0022] Examples of boron carbide powders used as powder raw materials are boron carbide F500 and F150 manufactured by 3M. The average particle size of F500 is approximately 15 μm, and the average particle size of F150 is 84.5 μm. In this manner, in the first embodiment, boron carbide powders are used so that the average particle size distribution has two or more levels. The reason for using two or more levels of powder is that combining two or more levels of powders with different average particle sizes makes it easier to increase the filling rate of boron carbide than using a single level of powder. In this case, the difference in average particle size of the combined powder raw materials is preferably three times or more, and more preferably five times or more.

[0023] The boron carbide powder may be uncoated, or may be pre-coated with carbon or boron nitride (BN). The use of boron carbide powder coated with carbon or boron nitride makes it possible to realize a composite ceramic material 1 with low density and high elasticity, which can reduce cracking during the silicon infiltration and silicon carbide reaction sintering processes.

[0024] The matrix filler used in the mixed raw materials is preferably a raw material containing carbon, which will become silicon carbide in the subsequent silicon infiltration and silicon carbide reaction sintering processes. In one example, the matrix filler is carbon milled fiber 21. An example of carbon milled fiber 21 is cut or chopped fiber, which is not a continuous long fiber, but is cut or chopped into short lengths, or milled fiber, which is cut or chopped fiber ground in a grinder and milled. Carbon milled fiber 21 is a fiber with a length of less than 1 mm. By using carbon milled fiber 21 with a length of less than 1 mm, no fibers remain in the final sintered material. However, very finely ground particles such as granular carbon black, i.e., particles ground to the point where they are no longer fibrous, are not suitable as matrix fillers.

[0025] The matrix filler serves to create a network structure that secures the boron carbide particles 11 at appropriate intervals. The carbon milled fibers 21 secure the boron carbide particles 11 to form and maintain appropriate voids 25 during the subsequent molding and heat treatment processes. When molding using powdered raw materials and a binder resin without using a matrix filler, the remaining voids 25 are smaller than the powder size of the raw materials. If the voids 25 are small, the size of the sintered compact will only increase slightly during the subsequent silicon infiltration process, resulting in poor sintering of the interior. Therefore, it is believed that large raw material powders are necessary to form large voids 25 that are easy for silicon to penetrate. However, using large raw material powders results in a coarse and inhomogeneous structure of the material. This reduces the strength of the composite ceramic material 1 and makes it difficult to form a smooth surface during surface processing. As described above, if the powder raw materials consist only of boron carbide powder and binder resin, it is not possible to produce a homogeneous composite ceramic material 1 that has voids 25 of a desired size and a desired strength.

[0026] Therefore, in the first embodiment, by using carbon milled fiber 21 as a matrix filler, it becomes possible to easily form voids 25 of a size equal to or larger than that of the raw material powder. Furthermore, even if the number of voids 25 is increased, the material strength of the preform 20 and the substrate formed by bonding multiple preforms 20 described in the second embodiment with a bonding binder and then sintering can be sufficiently ensured compared to when molding using only raw material powder. This makes handling easier in subsequent processes, reduces the occurrence of cracks and breakage caused by the occurrence of cracks, and enables processing of complex shapes before sintering.

[0027] As an example, the matrix filler may be milled fiber, which is fiber obtained by pulverizing Mitsubishi Chemical Corporation's Dialead (registered trademark) K63712 or the like to a length of less than 1 mm. The type of carbon fiber is not limited to pitch-based carbon fiber, and may be polyacrylonitrile (PAN)-based carbon fiber or organic fiber that is carbonized by heat treatment. The average length of the short fibers constituting the carbon milled fiber 21 is preferably four or more times the average particle size of the powder raw material. This is because large voids 25 cannot be formed if the average length of the short fibers is less than four times the average particle size of the powder raw material. Note that in the first embodiment, the matrix filler does not contain silicon carbide powder.

[0028] The binder resin is a component used in molding the preform 20. The binder resin is preferably a material that serves as a carbon precursor, and in one example, a thermoplastic powdered resin is desirable. One example of a powdered resin is powdered phenolic resin, which is a powdered phenolic resin. Using powdered phenolic resin as the binder resin facilitates homogenization during the mixing process of the raw materials. In addition, it is possible to reduce the filling rate of the binder resin in the compact, resulting in a sintered body that is easy to infiltrate with silicon. By sintering such a sintered body, a composite ceramic material 1 that can be made low-density and highly elastic can be realized.

[0029] Although powdered phenolic resin has been cited as an example of a binder resin, the binder resin may be any resin that can be carbonized by heat treatment. Liquid resins other than powdered phenolic resins may also be used, such as furan resin, epoxy resin, and imide resin. In this case, the surface of boron carbide powder is coated with a liquid resin. Furthermore, the binder resin may be an insolubilized thermoplastic resin. An example of a powdered resin that can be used as a carbon precursor is PG-9400 manufactured by Gun-ei Chemical Industry Co., Ltd.

[0030] In the molding process of step S2, the mixed raw materials obtained in the raw material mixing process are placed in a mold, which is a molding die, and heated and pressurized to harden the binder resin and obtain a molded body of the desired shape. Using boron carbide powder with two or more levels of average particle size reduces the pressure required during molding, making it easier to mold the molded body. Furthermore, homogenization is possible, which homogenizes the composite ceramic material 1 and improves its properties.

[0031] In the molding process, it is desirable to set the void fraction of the compact to 10% or more. The void fraction is the ratio of voids 25 to the volume determined from the outer shape of the compact. By setting the void fraction of the compact to 10% or more, the pressure applied during molding of the compact can be reduced, making it easier to mold the compact. Furthermore, by setting the void fraction of the compact to 10% or more, the occurrence of cracks in the subsequent heat treatment process can be reduced. The void fraction of the compact depends on the lower limit of the boron carbide fraction after sintering. For example, when the volume fraction of boron carbide is 50% of the lower limit, the upper limit of the void fraction of the compact is 20% or less. The void fraction also depends on the relationship between the amount of silicon carbide generated by sintering and the amount of reacted carbon. The boron carbide powder after being formed into a compact in the molding process is called boron carbide particles 11.

[0032] The heat treatment step of step S3 involves heat-treating the compact in an inert atmosphere or a vacuum to carbonize the binder resin in the compact and obtain a fired body. At this time, the binder resin, which serves as a carbon precursor in the compact, is carbonized to form carbon particles 22. The resulting compact is the preform 20 shown in FIG. 2. As shown in FIG. 2, the preform 20 includes boron carbide particles 11, carbon milled fibers 21, and carbon particles 22. The boron carbide powder may be uncoated or may be coated with a coating layer 111. Examples of the coating layer 111 include carbon and boron nitride. As described above, by including the carbon milled fibers 21 and the binder resin in the mixed raw material, the boron carbide particles 11 are fixed to the carbon milled fibers 21 by the carbon particles 22, forming a network of connections between the boron carbide particles 11 and forming voids 25.

[0033] The void fraction of the sintered body obtained in the heat treatment step should be 20% or more and less than 40%. By making the void fraction of the sintered body 20% or more and less than 40%, it becomes possible for silicon to penetrate deep into the sintered body during silicon infiltration in the subsequent step. Furthermore, by making the void fraction of the sintered body 20% or more and less than 40%, it becomes possible to absorb the volume expansion of silicon carbide produced by reaction during silicon carbide reaction sintering in the subsequent step. This makes it possible to perform reaction sintering without generating cracks.

[0034] The silicon infiltration and silicon carbide reaction sintering process in step S4 involves heating and infiltrating the sintered body with silicon (metallic silicon or a silicon alloy) in an inert atmosphere or a vacuum, reacting the silicon with the carbon in the sintered body to form silicon carbide, while simultaneously sintering the boron carbide particles 11 to produce the composite ceramic material 1. The process of infiltrating the sintered body with silicon and the process of reacting the carbon and silicon in the sintered body to form the boron carbide particles 11 proceed simultaneously. Therefore, it is difficult to strictly separate these two processes. Therefore, in the following, the process of infiltrating the sintered body with silicon in the silicon infiltration and silicon carbide reaction sintering process will be referred to as the infiltration process, and the process of reacting the carbon and silicon in the sintered body to form the boron carbide particles 11 will be referred to as the reaction sintering process. In the reaction sintering process, all of the carbon milled fibers 21 react with silicon to form silicon carbide. That is, the carbon milled fibers 21 become silicon carbide particles 12. Therefore, no carbon milled fibers 21 remain in the composite ceramic material 1. In the silicon infiltration and silicon carbide reaction sintering steps, the heat treatment temperature and heat treatment time are set so that all of the carbon milled fibers 21 react with silicon and become silicon carbide particles 12.

[0035] The finishing process in step S5 is a process of finishing the composite ceramic material 1 obtained through the silicon infiltration and silicon carbide reaction sintering processes into a final part shape. This results in the production of a part made of the composite ceramic material. This completes the manufacturing method of the composite ceramic material 1.

[0036] In the first embodiment, carbon milled fibers 21 are used as the raw material for reactive sintering. The carbon milled fibers 21 improve the dispersion of the boron carbide particles 11, which are the main component. Without the carbon milled fibers 21, the boron carbide particles 11 would be in contact with each other, resulting in too small gaps between the boron carbide particles 11 in the compact. By interposing the carbon milled fibers 21 between the boron carbide particles 11, the spacing between the boron carbide particles 11 in the compact can be increased. In other words, the fibers can bridge and secure the boron carbide particles 11. Because the carbon milled fibers 21 are included, the compact formed from the mixed raw material hardly shrinks, even when the binder resin decomposes and shrinks in volume. Consequently, a sintered body with more voids 25 than the compact can be obtained. Furthermore, in the molded body, the boron carbide particles 11 are bridged with the carbon milled fibers 21, so larger voids 25 can be achieved compared to when the boron carbide particles 11 are molded together without using the carbon milled fibers 21.

[0037] The sintered body formed from such a compact is porous and is a composite material containing carbon milled fibers 21, and therefore has good processability. Furthermore, the sintered body formed in this manner has greater strength than a sintered body obtained by sintering powder, allowing complex shapes to be realized. Generally, during a reactive sintering process, volume expansion occurs when the internal carbon reacts to form silicon carbide. However, the sintered body produced by the manufacturing method for the composite ceramic material 1 according to embodiment 1 hardly experiences the dimensional change that occurs with sintering ordinary ceramics, because the volume expansion is offset by the voids 25 already present in the sintered body. This allows sintered bodies of complex shapes to be sintered as is.

[0038] In the first embodiment, a preform 20 formed from boron carbide powder, milled carbon fibers 21, and a resin binder is carbonized and fired to form a fired body. The fired body is then infiltrated with silicon, converting the carbon particles 22 of the carbonized binder resin and the milled carbon fibers 21 into silicon carbide with the silicon infiltrated therein, while simultaneously sintering the boron carbide particles 11. This results in a composite ceramic material 1 having a B4C / SiC / Si composition. This manufacturing process enables the production of a composite ceramic material 1 with a low density and high elastic modulus at a lower temperature than conventional sintering processes and without the application of pressure. Furthermore, articles with uneven thickness and complex shapes can be manufactured more easily than before.

[0039] The molded preform 20 has a network structure with voids 25, in which boron carbide particles 11 and carbon milled fibers 21 are fixed and bonded by a binder resin. In this way, the preform 20 molded from boron carbide particles 11 and a binder resin makes it easy to increase the void ratio. Because the boron carbide particles 11 are networked by the carbon milled fibers 21, it is also possible to increase the strength of the preform 20. As a result, even if the filling rate of boron carbide is reduced, the shape can be maintained and moldability can be improved. Specifically, it becomes possible to reduce the pressure applied during molding, making it possible to manufacture products with more complex shapes.

[0040] Furthermore, when the molded body is heat-treated and carbonized, its high void ratio suppresses shrinkage during sintering due to the effect of the carbon milled fibers 21. Since shrinkage during sintering is suppressed, the occurrence of cracks in the sintered body is eliminated. The sintered body carbonized by heat treatment has a network structure in which boron carbide particles 11 are fixed by carbon milled fibers 21 and carbon particles 22 formed by carbonizing the binder resin. Therefore, the preform 20 has a larger pore distribution and is stronger than a preform 20 composed of boron carbide particles 11 without carbon milled fibers 21. This allows for easy shaping before silicon infiltration. Furthermore, shaping before sintering, i.e., before silicon infiltration, significantly reduces the amount of processing required in the hard, difficult-to-process state after sintering. As a result, complex-shaped products can be manufactured more quickly and more easily than before.

[0041] The fired body before silicon infiltration contains boron carbide particles 11, carbon milled fibers 21, and carbon particles 22 formed by carbonizing the binder resin. Therefore, during silicon infiltration, the infiltrated silicon reacts with the carbon milled fibers 21 and carbon particles 22 to form silicon carbide. As a result, voids 25 in the fired body are filled with silicon carbide produced by the reaction and unreacted silicon, and the voids 25 in the fired body are almost completely eliminated.

[0042] When the sintered body has a network structure of boron carbide particles 11, there is almost no carbon to react with the infiltrating silicon, so most of the filling area is filled with the infiltrated silicon. Because silicon has a lower elastic modulus than boron carbide, increasing the silicon filling amount causes a decrease in properties. Furthermore, decreasing the void fraction and increasing the boron carbide filling fraction makes it difficult to mold and sinter the preform 20. Furthermore, when the voids 25 become smaller, it becomes more difficult for silicon to infiltrate.

[0043] On the other hand, the manufacturing method of composite ceramic material 1 according to embodiment 1 makes it possible to fill voids 25 of the fired body mainly with silicon carbide particles 12, not just silicon particles 13, without forcibly increasing the filling rate of boron carbide. Silicon carbide has a higher elastic modulus than silicon and is equivalent to that of boron carbide, so a decrease in rigidity can be reduced. As described above, the manufacturing method of composite ceramic material 1 according to embodiment 1 has an effect of realizing molding of complex shapes because the specific elastic modulus is higher than that of silicon carbide.

[0044] Embodiment 2 Fig. 4 is a flowchart showing an example of the procedure of a method for manufacturing a composite ceramic material according to embodiment 2. Note that the same steps as those in Fig. 3 are assigned the same step numbers, and their explanations will be omitted. The method for manufacturing a composite ceramic material 1 according to embodiment 2 includes a shape processing step (step S11) after the heat treatment step of step S3 and before the silicon infiltration and silicon carbide reaction sintering step of step S4.

[0045] The shape processing step in step S11 is a step in which the sintered body obtained in the heat treatment step is processed into a part shape as a substrate. The shape processing step is performed as needed. In the shape processing step, the carbonized sintered body is machined into a shape close to the final part shape, and the machined sintered bodies are bonded together using a bonding binder to form a substrate. In other words, the substrate is formed by bonding multiple sintered bodies with a bonding binder and processing them into a shape.

[0046] The sintered body has a porous structure in which the boron carbide particles 11 in the preform 20 are fixed to the carbon milled fibers 21 by the carbon particles 22 formed by carbonizing the binder resin, forming a network structure. In other words, because the sintered body is a composite material containing the carbon milled fibers 21, which are carbon fibers, it has sufficient strength compared to a porous body made only of powder of boron carbide particles 11. This makes it easy to machine, and complex or hollow shapes can also be realized.

[0047] To achieve complex or hollow shapes, fired bodies are processed into individual sections and then bonded together using a bonding binder to form a new shape. FIG. 5 is a schematic diagram illustrating an example of how fired bodies are bonded together using a bonding binder to form a structure. FIG. 6 is a schematic side view illustrating an example of the state of the bonded portion between fired bodies. FIG. 6 is an enlarged side view of region R in FIG. 5. This example illustrates the assembly of a substrate 55, a structure having a partition member inside a hollow rectangular box. As shown in FIG. 5, a rectangular tubular fired body 51 and two plate-shaped fired bodies 52 and 53 of different sizes are formed through the molding process and heat treatment process. These three fired bodies 51, 52, and 53 are used to assemble the substrate 55. Specifically, the plate-shaped fired body 53 is bonded to one end face of the rectangular tubular fired body 51 via a bonding binder 57. This results in a rectangular box-shaped member with one open face. Furthermore, a plate-shaped fired body 52 is bonded to the inside of the rectangular box-shaped member via a bonding binder 57. This forms a substrate 55. As shown in Fig. 6, the bonding binder 57 is interposed at the joint between the rectangular cylindrical fired body 51 and the plate-shaped fired body 52. ​​In this way, the bonding binder 57 is used when assembling a structure using a plurality of fired bodies 51, 52, and 53.

[0048] The bonding binder 57 includes a binder resin and at least one of boron carbide powder and silicon carbide powder. The bonding binder 57 may further include carbon milled fiber 21. The binder resin content in the bonding binder 57 is preferably greater than that in the mixed raw material. The binder resin used here may be the same as or different from the binder resin added to the mixed raw material. An example of a binder resin is powdered phenolic resin. By combining the sintered bodies 51, 52, and 53 using the bonding binder 57 and then performing a heat treatment, a substrate 55 with a complex shape can be manufactured. By mixing the same powder contained in the mixed raw material into the bonding binder 57, the bonding binder 57 will have a structure similar to that of the other parts after re-firing. In other words, after sintering in the silicon infiltration and silicon carbide reaction sintering process in step S4, the bonding binder 57 will have a structure similar to that of the other parts and will be less likely to form singularities. As a result, by bonding the processed sintered bodies 51, 52, 53 together with a bonding binder 57 containing a powdered phenolic resin as a binder resin and at least one of boron carbide powder and silicon carbide powder, it is possible to realize hollow structures or complex shapes including internal structures such as ribs and beams, which are difficult to achieve through post-processing.

[0049] In the subsequent silicon infiltration and silicon carbide reaction sintering process in step S4, the shaped, sintered substrate 55 is infiltrated with silicon (metallic silicon or silicon alloy) by heating in an inert or vacuum atmosphere. The silicon reacts with the carbon in the substrate 55 to form silicon carbide, simultaneously sintering the boron carbide particles 11, producing the composite ceramic material 1. Furthermore, the bonded portions formed using the bonding binder 57 are also converted to silicon carbide by the silicon infiltration, resulting in an integrated structure with bond strength equivalent to that of the base materials. Furthermore, because the bonding binder 57 contains at least one of boron carbide powder and silicon carbide powder, the bonded portions are also reactively sintered in the silicon infiltration and silicon carbide reaction sintering process, resulting in a composition equivalent to that of the base materials. In other words, a complex, integrated composite ceramic structure can be realized.

[0050] In the second embodiment, in the shape processing step of step S11, the part is processed into a shape close to the final part shape before sintering, which significantly reduces the amount of finishing processing required after sintering. With ordinary ceramic materials, after sintering they become hard and very difficult to process, making it difficult to create complex shapes, but the process described above makes it possible to create products with complex shapes.

[0051] In the second embodiment, a shaping process is performed in which multiple component-shaped sintered bodies 51, 52, and 53 are bonded together via a bonding binder 57 to form a substrate 55. This process is followed by a silicon infiltration and silicon carbide reaction sintering process. The bonding binder 57 includes a binder resin and at least one of boron carbide powder and silicon carbide powder. The substrate 55 before silicon infiltration contains boron carbide particles 11, milled carbon fibers 21, and carbon particles 22 formed by carbonizing the binder resin. Therefore, the silicon infiltrated in the silicon infiltration process reacts with the milled carbon fibers 21 and the carbon particles 22 to form silicon carbide. As a result, the voids 25 in the substrate 55 are filled with the silicon carbide produced by the reaction and the unreacted silicon, resulting in the voids 25 in the substrate 55 being almost completely eliminated.

[0052] When the substrate 55 has a network structure formed by boron carbide particles 11, there is almost no carbon present to react with the infiltrating silicon, so most of the filling area is filled with the infiltrated silicon. Silicon has a lower elastic modulus than boron carbide, so increasing the silicon filling rate can cause a decrease in properties. Furthermore, decreasing the void fraction and increasing the boron carbide filling rate makes it difficult to mold and sinter the preform 20. Furthermore, smaller voids 25 make it more difficult for silicon to infiltrate.

[0053] On the other hand, in the manufacturing method of composite ceramic material 1 according to embodiment 2, when substrate 55 is used, it is possible to fill voids 25 of substrate 55 mainly with silicon carbide, not just silicon, without forcibly increasing the filling rate of boron carbide. Silicon carbide has a higher elastic modulus than silicon and is equivalent to that of boron carbide, so it is possible to reduce the decrease in rigidity. [Example]

[0054] The method for producing the composite ceramic material 1 of the present disclosure will be described in detail below with reference to examples and comparative examples.

[0055] Example 1 The boron carbide powder used was 3M's boron carbide F150 and F500, the matrix filler was Mitsubishi Chemical's milled fiber K7351M, and the binder resin was Gun-ei Chemical Industry's powdered phenolic resin PG-9400. These materials were weighed out and uniformly mixed in a weight ratio of 40:8:5:7 to form a mixed raw material. The average particle size of the boron carbide F150 was 84.5 μm, and the average particle size of the boron carbide F500 was approximately 15 μm. Thus, two levels of boron carbide powder with different average particle sizes were used.

[0056] The mixed raw materials are poured into a mold, and the binder resin is cured for 2 hours under conditions of a molding pressure of 5 MPa and a curing temperature of 150°C to obtain a molded body, which has a void ratio of 18%.

[0057] The compact is then heat-treated in an inert atmosphere at 800°C to carbonize the binder and obtain a fired body, which has a void ratio of 20%.

[0058] The sintered body and silicon metal are then placed in a boron nitride-coated graphite jig and heated to 1500°C in a vacuum furnace. This melts the silicon metal and impregnates the sintered body, causing the silicon to react with the carbon inside the sintered body to form silicon carbide, while sintering the boron carbide powder. After sintering, the body is allowed to cool naturally and removed from the vacuum furnace to obtain the sintered composite ceramic material 1.

[0059] After removing unreacted silicon adhering to the surface of the composite ceramic material 1, the material is processed into a specified shape and its specific gravity and Young's modulus are measured. The composite ceramic material 1 manufactured by the method of Example 1 has a specific gravity of 2.7 and a Young's modulus of 416 GPa. The cross-sectional structure was observed using a laser microscope, and component analysis was performed using energy dispersive X-ray spectroscopy (EDX) and a laser microscope. The proportions of B4C, SiC, and Si were found to be 55%, 33%, and 12%, respectively. The proportions of each component were determined from cross-sectional observation and bulk specific gravity. Because the structure is granular, the proportions of each component in any cross-section are assumed to be uniform and equivalent, and the area ratio is considered to be equal to the volume ratio. In subsequent examples, similar cross-sectional observation and component analysis were performed to determine the proportions of each component.

[0060] Example 2 In Example 2, composite ceramic material 1 was prepared in the same manner as in Example 1, except that boron carbide F150, boron carbide F500, matrix filler, and binder resin were mixed in a weight ratio of 48:0:5:10. In other words, boron carbide powder with an average particle size of level 1 was used in Example 2. Furthermore, in Example 2, the void fraction of the compact was 20%, and the void fraction of the fired body after heat treatment was 23%.

[0061] Unreacted silicon adhering to the surface of the sintered composite ceramic material 1 is removed, and the specific gravity and Young's modulus are measured. The specific gravity of the composite ceramic material 1 produced by the method of Example 2 is 2.67, and the Young's modulus is 392 GPa. Observation of the cross-sectional structure reveals that the proportions of B4C, SiC, and Si are 50%, 30%, and 20%, respectively. When the boron carbide powder is at a level of 1, the proportion of boron carbide decreases compared to Example 1, and the amount of unreacted silicon increases, which is thought to result in a lower Young's modulus compared to Example 1.

[0062] (Comparative Example 1) In Comparative Example 1, a composite ceramic material 1 was prepared in the same manner as in Example 1, except that boron carbide F150, boron carbide F500, matrix filler, and binder resin were mixed in a weight ratio of 40:8:5:12, respectively. In Comparative Example 1, the weight ratio of the binder resin was increased compared to Example 1. In Comparative Example 1, the void fraction of the green body was 9%, and the void fraction of the fired body after heat treatment was 14%. Furthermore, cracks occurred in the fired body. Therefore, the desired composite ceramic material 1 could not be obtained under the conditions of Comparative Example 1, so measurements of the specific gravity and Young's modulus, and observation of the cross-sectional structure were not performed. In Comparative Example 1, the void fraction was smaller than in Examples 1 and 2. As a result, it is believed that the voids 25 cannot absorb the volumetric shrinkage caused by the decomposition of the binder resin during heat treatment, resulting in the generation of cracks.

[0063] Example 3 In Example 3, composite ceramic material 1 was prepared in the same manner as in Example 1, except that boron carbide F150, boron carbide F500, matrix filler, and binder resin were mixed in weight ratios of 40:10:8:6. In Example 3, the proportion of matrix filler in the mixed material was higher than in Examples 1 and 2. In Example 3, the void fraction of the green body was 11%, and the void fraction of the fired body after heat treatment was 15%.

[0064] Unreacted silicon adhering to the surface of the sintered composite ceramic material 1 is removed, and the specific gravity and Young's modulus are measured. The specific gravity of the composite ceramic material 1 manufactured by the method of Example 3 is 2.65, and the Young's modulus is 424 GPa. Observation of the cross-sectional structure reveals that the proportions of B4C, SiC, and Si are 65%, 25%, and 10%, respectively.

[0065] (Comparative Example 2) In Comparative Example 2, a composite ceramic material 1 was prepared in the same manner as in Example 1, except that boron carbide F150, boron carbide F500, matrix filler, and binder resin were mixed in a weight ratio of 10:40:5:10. In Comparative Example 2, the proportion of boron carbide F150 with a large average particle size was smaller than that of boron carbide F500 with a small average particle size. In Comparative Example 2, the void fraction of the compact was 9%, and the void fraction of the fired body after heat treatment was 12%. Furthermore, cracks occurred in the fired body. For this reason, measurements of specific gravity and Young's modulus, and observation of cross-sectional structure were not performed. When using two or more levels of boron carbide powder, increasing the proportion of boron carbide powder with a large average particle size compared to boron carbide powder with a small average particle size reduces the spacing between boron carbide particles 11 and the void fraction. As a result, it is believed that the voids 25 cannot absorb the volumetric shrinkage caused by the decomposition of the binder resin during heat treatment, resulting in the occurrence of cracks.

[0066] Example 4 In Example 4, three fired bodies 51, 52, and 53 shown in FIG. 5 were produced using the same weight ratios of raw materials as in Example 1 and in the same manner as in Example 1. As shown in FIG. 5, the three fired bodies 51, 52, and 53 were combined using a bonding binder 57 to form a substrate 55, and then heat treatment was performed again. The bonding binder 57 was prepared by adding powdered phenolic resin to the mixed raw materials prepared for molding, and then adding an organic solvent such as alcohol to dissolve the resin and form a paste. Instead of adding powdered resin and an organic solvent, a liquid resin may be added to form a paste. The type of resin is not limited to phenolic resin; it may be a type such as furan resin or epoxy resin, in which the organic components are decomposed by heat treatment, leaving carbon behind.

[0067] The substrate 55 and silicon metal are then placed in a BN-coated graphite jig and heated to 1500°C in a vacuum furnace. This melts the silicon metal and infiltrates it into the substrate 55, causing the silicon to react with the carbon inside the substrate 55 to form silicon carbide, and sintering the boron carbide powder. After sintering, the material is allowed to cool naturally and removed from the vacuum furnace, yielding a sintered composite ceramic material 1 with an integrated hollow structure.

[0068] When the cross section of the joint was examined, it was found to have the same structure as the base material, with no clear boundary visible. In addition, no cracks or other defects were found in the joint.

[0069] In this way, before silicon infiltration, the sintered body is processed into parts, and the parts are bonded together using a bonding binder 57 to form a substrate 55. Thereafter, silicon infiltration is performed on the substrate 55, followed by reactive sintering, thereby realizing a composite ceramic structure with a complex structure or a hollow structure, which was difficult to achieve with conventional ceramic materials.

[0070] The configurations shown in the above embodiments are merely examples, and may be combined with other known technologies, or different embodiments may be combined with each other. It is also possible to omit or modify parts of the configurations as long as they do not deviate from the gist of the invention.

[0071] Various aspects of the present disclosure are summarized below as appendices.

[0072] [Appendix 1] a mixing step of mixing boron carbide powder, a matrix filler, and a binder resin to obtain a mixed raw material; a molding step of charging the mixed raw material into a mold and applying heat and pressure to harden the binder resin to obtain a molded body; a heat treatment step of heat-treating the compact in an inert atmosphere or a vacuum atmosphere to carbonize the binder resin and obtain a fired body; an infiltration step of bringing metallic silicon or a silicon alloy into contact with the sintered body and heating it in an inert atmosphere or a vacuum atmosphere to melt the metallic silicon or the silicon alloy and infiltrate the sintered body; a reactive sintering step in which the metallic silicon or the silicon alloy infiltrated into the sintered body reacts with the matrix filler and the carbon carbonized from the binder resin contained in the sintered body to generate silicon carbide, and the boron carbide powder is sintered to obtain a sintered body; Including, the matrix filler is carbon milled fiber; In the reaction sintering step, the carbon milled fibers react with the metallic silicon or the silicon alloy to become silicon carbide, and no carbon milled fibers remain in the sintered body. [Appendix 2] 2. The method for producing a composite ceramic material according to claim 1, wherein the boron carbide powder having two or more levels of average particle size is used in the mixing step. [Appendix 3] 3. The method for producing a composite ceramic material according to claim 1, wherein in the mixing step, the binder resin is a powdered phenolic resin. [Appendix 4] 4. The method for producing a composite ceramic material according to any one of claims 1 to 3, wherein in the mixing step, the surface of the boron carbide powder is coated with carbon or boron nitride. [Appendix 5] 5. The method for producing a composite ceramic material according to any one of claims 1 to 4, wherein in the forming step, the formed body has a void ratio of 10% or more. [Appendix 6] 6. The method for producing a composite ceramic material according to any one of claims 1 to 5, wherein in the heat treatment step, the void ratio of the fired body is 20% or more and less than 40%. [Appendix 7] The method further includes a shape processing step of processing the fired body into a part shape, or bonding the fired bodies together with a bonding binder to obtain a substrate of a predetermined shape, In the infiltration step, the metal silicon or the silicon alloy is brought into contact with the substrate to infiltrate the metal silicon or the silicon alloy into the substrate; 6. The method for producing a composite ceramic material according to any one of claims 1 to 5, wherein in the reactive sintering step, the metallic silicon or the silicon alloy infiltrated into the substrate is reacted with the matrix filler and the carbon carbonized by the binder resin contained in the substrate to produce silicon carbide, and the boron carbide powder is sintered to obtain a sintered body. [Appendix 8] 8. The method for producing a composite ceramic material according to claim 7, wherein in the shaping step, the bonding binder contains the binder resin and at least one of boron carbide powder and silicon carbide powder. [Explanation of symbols]

[0073] 1 Composite ceramic material, 11 Boron carbide particles, 12 Silicon carbide particles, 13 Silicon particles, 20 Preform, 21 Carbon milled fiber, 22 Carbon particles, 25 Voids, 51, 52, 53 Sintered body, 55 Substrate, 57 Bonding binder, 111 Coating layer.

Claims

1. A mixing step of mixing 76.2% by weight or more and 80.0% by weight or less of boron carbide powder, 7.9% by weight or more and 12.5% ​​by weight or less of a matrix filler, and 9.4% by weight or more and 15.9% by weight or less of a binder resin to obtain a mixed raw material; a molding step of charging the mixed raw material into a mold and applying heat and pressure to harden the binder resin to obtain a molded body; a heat treatment step of heat-treating the compact in an inert atmosphere or a vacuum atmosphere to carbonize the binder resin and obtain a fired body; an infiltration step of bringing metallic silicon or a silicon alloy into contact with the sintered body and heating it in an inert atmosphere or a vacuum atmosphere to melt the metallic silicon or the silicon alloy and infiltrate the sintered body; a reactive sintering step in which the metallic silicon or the silicon alloy infiltrated into the sintered body reacts with the matrix filler and the carbon carbonized from the binder resin contained in the sintered body to generate silicon carbide, and the boron carbide powder is sintered to obtain a sintered body; Including, the matrix filler is carbon milled fiber; In the molding step, heating and pressure are applied to the molded body so that the void ratio of the molded body is 10% or more, In the heat treatment step, the heat treatment is performed so that the void ratio of the sintered body is 20% or more and less than 40%, thereby producing the sintered body having a network structure in which boron carbide particles are connected to each other by the carbon milled fiber or carbon particles obtained by carbonizing the binder resin, In the reaction sintering step, the carbon milled fibers react with the metallic silicon or the silicon alloy to become silicon carbide, and no carbon milled fibers remain in the sintered body.

2. 2. The method for producing a composite ceramic material according to claim 1, wherein the boron carbide powder used in the mixing step has two or more levels of average particle size.

3. 2. The method for producing a composite ceramic material according to claim 1, wherein in said mixing step, said binder resin is a powdered phenolic resin.

4. 2. The method for producing a composite ceramic material according to claim 1, wherein in the mixing step, the surface of the boron carbide powder is coated with carbon or boron nitride.

5. The method further includes a shape processing step of processing the fired body into a part shape, or bonding the fired bodies together with a bonding binder to obtain a substrate of a predetermined shape, In the infiltration step, the metal silicon or the silicon alloy is brought into contact with the substrate to infiltrate the metal silicon or the silicon alloy into the substrate; 2. The method for producing a composite ceramic material according to claim 1, wherein in the reactive sintering step, the metallic silicon or the silicon alloy infiltrated into the substrate is reacted with the matrix filler contained in the substrate and carbon carbonized by the binder resin to produce silicon carbide, and the boron carbide powder is sintered to obtain a sintered body.

6. 6. The method for producing a composite ceramic material according to claim 5, wherein in the shaping step, the bonding binder contains the binder resin and at least one of boron carbide powder and silicon carbide powder.

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