JP7774516B2 - Manufacturing method for composite ceramic materials - Google Patents

Description

This disclosure relates to a method for producing a composite ceramic material composed primarily of boron carbide and inorganic materials containing silicon carbide and silicon.

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 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.

Research is ongoing into materials that meet these requirements, and while the development of iron-, nickel-, and cobalt-based alloys has led to improvements in high-temperature performance, weight reduction remains insufficient, and alternative materials are being considered. Lightweight ceramic materials are also attracting attention as heat-resistant materials due to their high heat resistance and lower density compared to metallic materials. However, because ceramic materials are relatively brittle, ceramic matrix composites (CMCs), which are composites of ceramic materials with fibers, are gaining attention as materials suitable for structural components. Generally, CMCs are materials that combine a ceramic matrix with 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.

In recent years, boron carbide ( _B4C ), which is even lighter and more rigid than SiC, has attracted attention. _B4C /Si composites, which are matrix composites of boron carbide reinforcement and silicon metal (Si), have been produced by infiltrating a boron carbide preform with silicon metal. Boron carbide has a maximum Young's modulus of approximately 460 GPa and a density of 2.52 g/ ^cm3 . Therefore, the specific modulus of boron carbide, which indicates the elastic modulus per unit density, is 182.5 GPa/(g/ ^cm3 ). This specific modulus of boron carbide is superior to that of silicon carbide. However, boron carbide is generally produced by reaction under high temperature and high pressure. Under high temperature and high pressure, sintered boron carbide is hard and has very poor processability, making it difficult to mold and manufacture complex shapes. Therefore, a manufacturing method at a lower temperature and pressure is required. Accordingly, a method of impregnating or pressure-infiltrating boron carbide with metals such as metallic silicon or aluminum (Al) has been proposed. Patent Document 1 focuses on the fact that the reaction between silicon and boron carbide during impregnation is one cause of cracking, and 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 preform, which is a porous body containing boron carbide.

Special Publication No. 2007-513854

When a preform contains few voids and their diameters are 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 lack sufficient reinforcing properties and are difficult to mold into complex shapes. Conversely, when a preform contains many voids, 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, resulting in insufficient reinforcing material properties. In other words, when using preforms molded with boron carbide powder using a binder or other material, a high boron carbide filling rate is required to maximize the properties of boron carbide as a reinforcing material. However, increasing the boron carbide filling rate requires higher pressure to mold the preform, making it difficult to mold complex shapes and requiring higher pressure to infiltrate metals such as silicon metal or aluminum into the voids in the preform. Furthermore, poor filling or cracking is more likely to occur. Conversely, if the filling rate of boron carbide in the preform is reduced, formability and manufacturability improve, but the sufficient properties of boron carbide as a reinforcing material cannot be obtained. As such, when using preforms made by molding boron carbide powder with a binder, it is difficult to achieve both formability and manufacturability while also achieving the reinforcing material properties of having a specific elastic modulus superior to that of silicon carbide.

The present disclosure has been made in light 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 molded into complex shapes.

To solve the above-mentioned problems and achieve the object, the present disclosure provides a method for producing a composite ceramic material, which includes a mixing step, a molding step, a heat treatment step, an infiltration step, and a reaction sintering step. The mixing step involves mixing 76.2 wt % to 80.0 wt % of boron carbide powder, 7.9 wt % to 12.5 wt % of a matrix filler, and 9.4 wt % to 15.9 wt % of a binder resin to obtain a mixed raw material. The molding step involves placing the mixed raw material in a mold and applying heat and pressure to harden the binder resin to obtain a molded body. The heat treatment step involves heat-treating the molded body in an inert atmosphere or a vacuum atmosphere to carbonize the binder resin and obtain a sintered body. The infiltration step involves contacting metallic silicon or a silicon alloy with the sintered body and heating it in an inert atmosphere or a vacuum atmosphere to melt the metallic silicon or silicon alloy and infiltrate the sintered body. In the reactive sintering process, the metal silicon or silicon alloy infiltrated into the sintered body reacts with the matrix filler and carbon carbonized from the binder resin contained in the sintered body to produce silicon carbide, and the boron carbide powder is sintered to produce a sintered body. The matrix filler is carbon milled fiber. In the molding process, heating and pressure are applied to the molded body so that the void ratio is 10% or more. In the heat treatment process, heat treatment is performed so that the void ratio of the sintered body is 20% or more but less than 40%, producing a sintered body having a network structure in which boron carbide particles are connected by carbon milled fiber or carbon particles carbonized from the binder resin. In the reactive sintering process, the carbon milled fiber reacts with the metal silicon or silicon alloy to completely convert into silicon carbide, and no carbon milled fiber remains in the sintered body.

The manufacturing method for the composite ceramic material disclosed herein has the advantage of having a higher specific elastic modulus than silicon carbide, making it possible to mold complex shapes.

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. 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. 1 is a flowchart showing an example of a procedure for a method for manufacturing a composite ceramic material according to Embodiment 1. 1 is a flowchart showing an example of a procedure for a method for manufacturing a composite ceramic material according to a second embodiment. FIG. 1 is a diagram showing an example of how fired bodies are bonded together using a bonding binder to form a structure. FIG. 10 is a side view schematically illustrating an example of the state of a joint between fired bodies.

Below, a method for manufacturing a composite ceramic material according to an embodiment of the present disclosure is described in detail with reference to the drawings.

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 of 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 of 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 .

As shown in FIG. 1 , composite ceramic material 1 manufactured by the manufacturing method of composite ceramic material 1 of embodiment 1 includes boron carbide (B ₄ C) 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, which are spaces between boron carbide particles 11. The main component of composite ceramic material 1 is boron carbide. Note that the composite ceramic material according to embodiment 1 is a combination of multiple ceramic materials and is a ceramic material that is not composited with reinforcing fibers.

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 greater than a specified value.

The Young's modulus of boron carbide, the main component of composite ceramic material 1, is assumed to be 450 GPa. Furthermore, 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 ), which is the Young's modulus divided by the density. Compared to silicon carbide, boron carbide has a higher elastic modulus, a lower density, and a higher specific elastic modulus. In other words, in composite ceramic material 1, boron carbide, the main component, is a material that can be expected to improve the specific elastic modulus.

As mentioned above, the silicon particles 13 are metallic silicon or a silicon alloy. The reason silicon is included in the composite ceramic material 1 is because 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 are equivalent to defects in the material and reduce the material's properties. For this reason, filling the material with silicon particles 13 improves the material's properties and increases the specific elastic modulus, rather than replacing the silicon particles 13 with voids 25 to create empty spaces.

In the configuration of the composite ceramic material 1 disclosed herein, 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 metallic silicon or a silicon alloy into the preform 20 shown in FIG. 2 and then reactive sintering. In reactive sintering, a carbon component is essential to react with the metallic 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 are described below. The carbon milled fibers 21 contained in the preform 20 react with the metallic silicon or silicon alloy during the silicon infiltration and silicon carbide reactive sintering processes, described below, and are all converted into silicon carbide. By setting appropriate reaction conditions in the silicon infiltration and silicon carbide reaction sintering processes, no carbon milled fibers 21 remain in the sintered body after the silicon infiltration and silicon carbide reaction sintering processes. In other words, the carbon milled fibers 21 are replaced by silicon carbide particles 12.

The boron carbide particles 11 in the porous preform 20 shown in Figure 2 have a network structure cross-linked by carbon particles 22 (carbonized binder resin) and carbon milled fibers 21. In other words, the boron carbide particles 11 are connected to each other by carbon milled fibers 21 or carbon particles 22. Networking the boron carbide particles 11 with carbon milled fibers 21 and carbon particles 22 makes it easy to ensure voids 25 and achieves sufficient strength for the porous preform 20. The use of carbon milled fibers 21 and carbon particles 22 allows for a lower filling ratio of materials other than the boron carbide particles 11. This allows for an increased proportion of voids 25 in the porous preform 20 without reducing the filling ratio of the boron carbide particles 11, thereby controlling volume expansion due to reactive sintering. This enables reactive sintering of a composite ceramic material 1 with a boron carbide particle 11 filling ratio of 50% or more without generating cracks. In this case, the volume ratio of silicon carbide produced by the reaction is preferably 15% or more and 35% or less.

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 reduces the proportion of boron carbide particles 11 and increases the proportion of silicon carbide, undesirably lowering the specific elastic modulus of the composite ceramic material 1. On the other hand, suppressing the reaction with the infiltrating silicon and reducing the volumetric ratio of silicon carbide produced by the reaction to less than 15%, undesirably increasing the proportion of unreacted silicon, thereby 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%.

Next, a method for manufacturing the composite ceramic material 1 according to embodiment 1 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 embodiment 1. The method for manufacturing the composite ceramic material 1 includes a raw material mixing process (step S1), a molding process (step S2), a heat treatment process (step S3), a silicon infiltration and silicon carbide reaction sintering process (step S4), and a finishing process (step S5). Each process will be described below.

The raw material mixing process in step S1 is a process in which the raw material boron carbide powder, matrix filler, and binder resin (carbon precursor) are uniformly mixed in 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.

Examples of the boron carbide powder used as the powder raw material 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 way, in embodiment 1, boron carbide powder is 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 powder 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, it is desirable for the average particle sizes of the combined powder raw materials to differ by three times or more, and more desirably by five times or more.

The boron carbide powder may be uncoated, or may be pre-coated with carbon or boron nitride (BN). By using boron carbide powder coated with carbon or boron nitride, a composite ceramic material 1 can be realized that has low density and high elasticity, and that reduces cracking during the silicon infiltration and silicon carbide reaction sintering processes.

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. One example of 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 pieces, or milled fiber, which is cut or chopped fiber ground in a grinder to form a milled shape. Carbon milled fiber 21 is a fiber with a length of less than 1 mm. By using carbon milled fiber 21, which is a fiber less than 1 mm in length, no fibers remain in the final sintered material. However, very finely ground particles such as granular carbon black, i.e., particles ground until they are no longer fibrous, are not suitable as matrix fillers.

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 a matrix filler, the remaining voids 25 are smaller than the powder size of the raw materials. If the voids 25 are small, even a slight increase in size during the subsequent silicon infiltration process results in the sintered compact, with silicon only penetrating the exterior, resulting in poor sintering of the interior. For this reason, it is believed that large raw material powders are necessary to form large voids 25 that are easily penetrated by silicon. 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 such, 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 the desired strength while also having voids 25 of the desired size.

In this regard, in embodiment 1, by using carbon milled fiber 21 as a matrix filler, it is 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 sintering multiple preforms 20 bonded with a bonding binder as described in embodiment 2 can be sufficiently ensured compared to molding using raw material powder alone. This makes handling easier in subsequent processes, reduces cracking and breakage caused by cracking, and enables complex shaping before sintering.

As an example, the matrix filler may be milled fiber, which is fiber obtained by milling 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; it may also be polyacrylonitrile (PAN)-based carbon fiber or organic fiber that is carbonized by heat treatment. It is desirable that the average length of the short fibers that make up the carbon milled fiber 21 be at least four 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. In the first embodiment, the matrix filler does not contain silicon carbide powder.

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 one example is a thermoplastic powdered resin. One example of a powdered resin is powdered phenolic resin. Using powdered phenolic resin as the binder resin makes it easier to homogenize the raw materials during mixing. It also makes it possible to reduce the filling rate of the binder resin in the molded body, resulting in a sintered body that is easy to infiltrate with silicon. Sintering such a sintered body makes it possible to realize a composite ceramic material 1 that can be made low-density and highly elastic.

Although powdered phenolic resin has been mentioned as an example of a binder resin, any binder resin that can be carbonized by heat treatment can be used. In addition to powdered phenolic resin, liquid resins such as furan resin, epoxy resin, and imide resin can also be used. 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. One example of a powdered resin that can be used as a carbon precursor is PG-9400 manufactured by Gunei Chemical Industry Co., Ltd.

The molding process in step S2 involves pouring the mixed raw materials obtained in the raw material mixing process into a mold, where they are 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.

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 external shape of the compact. By setting the void fraction of the compact to 10% or more, the pressure applied during molding 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, if the volume fraction of boron carbide is 50%, the upper limit of the void fraction of the compact will be 20% or less. The void fraction also depends on the relationship between the amount of silicon carbide produced by sintering and the amount of reacted carbon. The boron carbide powder after being formed into a compact in the molding process is referred to as boron carbide particles (11).

The heat treatment process of step S3 involves heating the molded body in an inert atmosphere or a vacuum atmosphere to carbonize the binder resin in the molded body and obtain a fired body. At this time, the binder resin, which serves as a carbon precursor in the molded body, is carbonized to form carbon particles 22. The resulting molded body is the preform 20 shown in FIG. 2. As shown in FIG. 2, the preform 20 contains 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 carbon milled fibers 21 and a 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, resulting in network-like connections between the boron carbide particles 11 and the formation of voids 25.

The void fraction of the sintered body obtained in the heat treatment process should be between 20% and 40%. By ensuring that the void fraction of the sintered body is between 20% and 40%, it becomes possible for silicon to penetrate deep into the sintered body during silicon infiltration in a subsequent process. Furthermore, by ensuring that the void fraction of the sintered body is between 20% and 40%, it becomes possible to absorb the volume expansion of the silicon carbide produced by the reaction during silicon carbide reaction sintering in a subsequent process. This makes it possible to perform reaction sintering without generating cracks.

In step S4, the silicon infiltration and silicon carbide reaction sintering process involves heating and infiltrating the sintered body with silicon (metallic silicon or a silicon alloy) in an inert atmosphere or vacuum, causing the silicon to react with the carbon in the sintered body to form silicon carbide, while simultaneously sintering the boron carbide particles 11, thereby producing 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 sinter the boron carbide particles 11 proceed in unison. For this reason, it is difficult to strictly separate these two processes. Therefore, hereinafter, 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 sinter 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. As a result, no carbon milled fibers 21 remain in the composite ceramic material 1. In the silicon infiltration and silicon carbide reaction sintering processes, 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.

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

In the first embodiment, carbon milled fiber 21 is used as the raw material for reactive sintering. The carbon milled fiber 21 serves to improve the dispersion of the main component, boron carbide particles 11. If the carbon milled fiber 21 were not present and the boron carbide particles 11 were in contact with each other, the gaps between the boron carbide particles 11, i.e., voids 25, would be too small in the compact. On the other hand, by interposing the carbon milled fiber 21 between the boron carbide particles 11, it is possible to increase the spacing between the boron carbide particles 11 in the compact. In other words, the fibers can bridge and fix the boron carbide particles 11. Thus, because the carbon milled fiber 21 is 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, which allows for larger voids 25 to be achieved compared to molding boron carbide particles 11 together without using the carbon milled fibers 21.

The sintered body formed from such a compact is porous and, because it is a composite material containing carbon milled fibers 21, is easy to process. Furthermore, sintered bodies formed in this manner are stronger than sintered bodies made by sintering powder, making it possible to create complex shapes. Generally, during the 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 composite ceramic material 1 according to embodiment 1 experiences almost no dimensional change, which 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.

In the first embodiment, a preform 20 formed from boron carbide powder, carbon milled 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 carbon milled 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.

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, a 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, improving moldability. Specifically, it becomes possible to reduce the pressure applied during molding, enabling the production of products with more complex shapes.

Furthermore, when the compact is heat-treated and carbonized, its high void ratio suppresses shrinkage during firing due to the mixing effect of the carbon milled fibers 21. Suppressing shrinkage during firing eliminates cracks in the fired body. The fired 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 from carbonized binder resin. Therefore, it has a larger pore distribution and higher strength 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-work state after sintering. As a result, it becomes possible to manufacture complex-shaped products more quickly and more easily than before.

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, the voids 25 in the fired body are filled with the silicon carbide produced by the reaction and the unreacted silicon, resulting in the voids 25 in the fired body being almost completely eliminated.

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

On the other hand, the manufacturing method of composite ceramic material 1 according to embodiment 1 makes it possible to fill the 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 the effect of having a higher specific elastic modulus than silicon carbide, making it possible to mold complex shapes.

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 composite ceramic material 1 according to embodiment 2 includes a shaping 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.

The shaping process in step S11 is a process in which the sintered body obtained in the heat treatment process is used as a substrate to be machined into a part shape. The shaping process is performed as needed. In the shaping process, 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 shaping them.

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 composed only of powdered boron carbide particles 11. This makes it easy to machine, and complex or hollow shapes can also be realized.

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. Figure 5 is a schematic diagram illustrating an example of how fired bodies are bonded together using a bonding binder to form a structure. Figure 6 is a schematic side view illustrating an example of the state of the bonded portion between fired bodies. Figure 6 is an enlarged side view of region R in Figure 5. This example illustrates the assembly of substrate 55, a structure having a partition member inside a hollow rectangular box. As shown in Figure 5, the molding and heat treatment processes form rectangular tubular fired body 51 and two plate-shaped fired bodies 52 and 53 of different sizes. These three fired bodies 51, 52, and 53 are used to assemble substrate 55. Specifically, plate-shaped fired body 53 is bonded to one end face of rectangular tubular fired body 51 via bonding binder 57. This forms a rectangular box-shaped member with one open side. 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 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 multiple fired bodies 51, 52, and 53.

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 the binder resin content 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 of 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 using a bonding binder 57 containing a powdered phenolic resin 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 ribs, beams, etc., which are difficult to achieve through post-processing.

In the subsequent silicon infiltration and silicon carbide reaction sintering process in step S4, the shaped, sintered substrate 55 is infiltrated with silicon, either metallic silicon or a 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 portions bonded using the bonding binder 57 are converted to silicon carbide by the silicon infiltration, and the bonded components are simultaneously converted to silicon carbide, resulting in an integrated structure bonded with the same strength as 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.

In embodiment 2, 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 work required after sintering. With ordinary ceramic materials, after sintering they become hard and very difficult to process, making it difficult to create complex shapes. However, the process described above makes it possible to create products with complex shapes.

In the second embodiment, after a shaping process in which multiple component-shaped sintered bodies 51, 52, and 53 are bonded together via a bonding binder 57 to form a substrate 55, silicon infiltration and silicon carbide reaction sintering processes are carried out. The bonding binder 57 includes a binder resin and at least one of boron carbide powder and silicon carbide powder. Because 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, the silicon infiltrated in the silicon infiltration process reacts with the milled carbon fibers 21 and 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.

When the substrate 55 has a network structure made up of boron carbide particles 11, there is almost no carbon present to react with the infiltrating silicon, so most of the filled area is filled with the infiltrated silicon. Silicon has a lower elastic modulus than boron carbide, so increasing the amount of silicon filled can cause a decrease in properties. Furthermore, decreasing the void rate 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.

On the other hand, in the manufacturing method of composite ceramic material 1 according to embodiment 2, when using substrate 55, it is possible to fill voids 25 in substrate 55 primarily with silicon carbide, rather than 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, which reduces the loss of rigidity.

The manufacturing method for the composite ceramic material 1 of the present disclosure is described in detail below using examples and comparative examples.

Example 1
A mixed raw material is used, in which 3M boron carbide powders F150 and F500, Mitsubishi Chemical Corporation's milled fiber K7351M, and Gun-ei Chemical Industry Co., Ltd.'s powdered phenolic resin PG-9400 are used as the binder resin, and the boron carbide powders are weighed out and uniformly mixed in a weight ratio of 40:8:5:7. The average particle size of the boron carbide F150 is 84.5 μm, and the average particle size of the boron carbide F500 is approximately 15 μm. Thus, two levels of boron carbide powder with different average particle sizes are used.

The mixed raw materials are poured into a mold, and the binder resin is cured for two hours under conditions of a molding pressure of 5 MPa and a curing temperature of 150°C to obtain a molded body. The void ratio of the molded body at this stage is 18%.

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

The sintered body and metallic silicon are then placed in a boron nitride-coated graphite jig and heated to 1500°C in a vacuum furnace. This melts the metallic silicon and impregnates the sintered body, causing the silicon to react with the carbon inside the sintered body to form silicon carbide, and 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.

After removing unreacted silicon adhering to the surface of the composite ceramic material 1, the composite ceramic material 1 is processed into a predetermined shape and its specific gravity and Young's modulus are measured. The specific gravity of the composite ceramic material 1 manufactured by the method of Example 1 is 2.7, and its Young's modulus is 416 GPa. The cross-sectional structure is observed using a laser microscope, and component analysis is performed using energy dispersive X-ray spectroscopy (EDX) and a laser microscope. The proportions of _B4C , SiC, and Si are 55%, 33%, and 12%, respectively. The proportions of each component are 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, cross-sectional observation and component analysis are similarly performed to determine the proportions of each component.

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. That is, in Example 2, boron carbide powder having an average particle size of level 1 was used. In addition, in Example 2, the void fraction of the compact was 20%, and the void fraction of the fired body after heat treatment was 23%.

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 level 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.

(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 due to the decomposition of the binder resin during heat treatment, resulting in the generation of cracks.

Example 3
In Example 3, composite ceramic material 1 was produced 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, respectively. In Example 3, the ratio 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%.

The unreacted silicon adhering to the surface of the sintered composite ceramic material 1 was removed, and the specific gravity and Young's modulus were measured. The specific gravity of the composite ceramic material 1 manufactured by the method of Example 3 was 2.65, and the Young's modulus was 424 GPa. Observation of the cross-sectional structure revealed that the proportions of _B4C , SiC, and Si were 65%, 25%, and 10%, respectively.

(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. Therefore, 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 relative to that of 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.

Example 4
In Example 4, three sintered 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 sintered 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 in which the organic components are decomposed by heat treatment, leaving carbon behind, such as furan resin or epoxy resin.

The substrate 55 and metallic silicon are then placed in a BN-coated graphite jig and heated to 1500°C in a vacuum furnace. This melts the metallic silicon 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.

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. Furthermore, no cracks or other defects were found in the joint.

In this way, before silicon infiltration, the sintered body is processed into parts, and a bonding binder 57 is used to bond the parts together to form a substrate 55. Substrate 55 is then silicon infiltrated and reactively sintered, making it possible to create composite ceramic structures with complex or hollow structures, which were difficult to achieve with conventional ceramic materials.

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. Parts of the configuration may also be omitted or modified without departing from the spirit of the invention.

The various aspects of this disclosure are summarized below as appendices.

[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 claim 1, wherein in the mixing step, the boron carbide powder has a surface coated with carbon or boron nitride.
[Appendix 5]
5. The method for producing a composite ceramic material according to claim 1, wherein in the forming step, the void ratio of the formed body is 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 sintered 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 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 and the carbon carbonized from 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 includes the binder resin and at least one of boron carbide powder and silicon carbide powder.

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 (6)

a mixing step of mixing 76.2 wt % or more and 80.0 wt % or less of boron carbide powder, 7.9 wt % or more and 12.5 wt % or less of a matrix filler, and 9.4 wt % or more and 15.9 wt % 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. The method for manufacturing a composite ceramic material described in claim 1, characterized in that the boron carbide powder used in the mixing process has two or more levels of average particle size. The method for manufacturing a composite ceramic material described in claim 1, characterized in that in the mixing process, the binder resin is powdered phenolic resin. The method for producing a composite ceramic material described in claim 1, characterized in that, in the mixing process, the surface of the boron carbide powder is coated with carbon or boron nitride. 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. 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.