Ceramic composite and flying object radome

Abstract

To obtain a ceramic composite capable of achieving both thermal shock resistance improvement and grinding processability improvement.SOLUTION: A ceramic composite 1 used for a flying object radome includes a main phase including Si3N42 and BN3, and a sub-phase including R2Si3O3N44. The total content of Si3N42 and BN3 relative to the total amount of the ceramic composite 1 is 80 mass% or more and 96 mass% or less. The ratio of the mass of BN3 relative to Si3N42 is between 2 / 98 and 10 / 90. BN3 includes agglomerated particles.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a ceramic composite and a radome for an air vehicle in which the ceramic composite is used. [Background technology]

[0002] Conventionally, there are known flying objects that fly by radio wave guidance toward a set target. The flying object is equipped with electronic devices such as an antenna that measures the distance and direction to the target, and a radome is installed at the tip of the flying object to protect these electronic devices from aerodynamic forces during flight.

[0003] Because projectiles fly at supersonic speeds, they are subjected to rapid heating (aerodynamic heating) due to friction with the air during flight. In particular, the radome installed at the tip of the projectile is heated to temperatures of over 1000°C due to aerodynamic heating. For this reason, the radome must be heat-resistant and resistant to thermal shock. Furthermore, in order for the projectile to measure the distance and direction to the target during flight, it must send and receive radio waves using an antenna via the radome, so the radome must also be radio-transparent.

[0004] To meet the performance requirements of such radomes, oxide ceramics (fine ceramics) with a thermal expansion coefficient of 3 ppm / °C or less, such as cordierite and fused silica, are generally used as radome materials. However, when these oxide ceramics are used in the radomes of flying objects that fly at higher speeds than conventional ones, the radomes do not have sufficient thermal shock resistance, and there is a possibility that the radomes will crack due to thermal stress caused by aerodynamic heating. Hereinafter, ceramic radomes will be referred to as "ceramic shaped objects."

[0005] Therefore, for example, Patent Document 1 discloses a ceramic shaped product made of Si3N4 (silicon nitride), which has higher mechanical strength and better thermal shock resistance than oxide ceramics. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2013 / 124871 Summary of the Invention [Problem to be solved by the invention]

[0007] However, while Si3N4 has high mechanical strength and excellent thermal shock resistance, it also has high hardness. Therefore, when Si3N4 is used as a material for ceramic objects as disclosed in Patent Document 1, the grinding process required to manufacture the ceramic objects becomes difficult.

[0008] The present disclosure has been made in view of the above, and has an object to obtain a ceramic composite that can achieve both improved thermal shock resistance and improved grinding workability. [Means for solving the problem]

[0009] As a result of intensive research aimed at solving the above-mentioned problems, the inventors have found that the thermal shock resistance of ceramic materials, such as ceramic composites used in ceramic shaped articles, is related to the thermal expansion coefficient, thermal conductivity, mechanical strength (mainly bending strength), and elastic modulus of the ceramic material, and that the higher the thermal conductivity and mechanical strength of the ceramic material and the lower the thermal expansion coefficient and elastic modulus, the better the thermal shock resistance. Furthermore, they have found that the grindability of ceramic materials is related to the elastic modulus of the ceramic material, and that the lower the elastic modulus of the ceramic material, the better the grindability. Furthermore, they have found that Si3N4 itself has a high elastic modulus, which causes a decrease in the thermal shock resistance and grindability of ceramic materials. Therefore, the inventors discovered that by using agglomerated particles of BN (boron nitride), which has high thermal conductivity and low elastic modulus, together with Si3N4, and combining Si3N4 and BN at a specific content and mass ratio to form a composite, and further adding R2Si3O3N4 (rare earth oxynitride) as a subphase, which strengthens the bonds between Si3N4 particles and improves mechanical strength, it is possible to achieve both improved thermal shock resistance and improved grinding workability, and have completed the present disclosure.

[0010] That is, in order to solve the above-mentioned problems and achieve the object, the ceramic composite according to the present disclosure is a ceramic composite used in an airborne radome, and contains a main phase containing Si3N4 and BN and a subphase containing R2Si3O3N4. The total content of Si3N4 and BN relative to the total amount of the ceramic composite is 80 mass% or more and 96 mass% or less. The mass ratio of BN to Si3N4 is between 2 / 98 and 10 / 90. The BN contains agglomerated particles. [Effects of the Invention]

[0011] The ceramic composite according to the present disclosure has the effect of achieving both improved thermal shock resistance and improved grinding workability. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a cross-sectional view showing a ceramic composite according to a first embodiment of the present invention; [Figure 2] FIG. 1 is a cross-sectional view showing an example of contact between Si3N4 and R2Si3O3N4 according to the first embodiment. [Figure 3] FIG. 10 is a cross-sectional view showing another example of how Si3N4 and R2Si3O3N4 are in contact with each other in the first embodiment. [Figure 4] FIG. 1 is a cross-sectional view showing an example of an aggregated particle of BN according to the first embodiment. [Figure 5] FIG. 1 is a cross-sectional view showing another example of an agglomerated particle of BN according to the first embodiment. [Figure 6] FIG. 1 is a cross-sectional view showing an example of a BN agglomerate particle in a space formed by being surrounded by Si 3 N 4 particles according to the first embodiment. [Figure 7] FIG. 1 is a cross-sectional view showing an example of an agglomerated particle of BN in a space formed by being surrounded by particles of Si3N4 and particles of R2Si3O3N4 according to the first embodiment. [Figure 8] FIG. 10 is a cross-sectional view showing another example of a BN agglomerate particle in a space formed by being surrounded by Si 3 N 4 particles according to the first embodiment; [Figure 9] FIG. 10 is a cross-sectional view showing another example of a BN agglomerate particle in a space formed by being surrounded by Si3N4 particles and R2Si3O3N4 particles according to the first embodiment. [Figure 10] Schematic cross-sectional view showing a flying object according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A ceramic composite and a radome for a flying object according to embodiments will be described in detail below with reference to the accompanying drawings.

[0014] Embodiment 1 FIG. 1 is a cross-sectional schematic diagram showing a ceramic composite 1 according to a first embodiment. As shown in FIG. 1, the ceramic composite 1 contains a main phase containing Si3N42 and BN3 and a subphase containing R2Si3O3N44. In this specification, Si3N4 refers to Si3N4 crystal particles, BN refers to BN crystal particles, and R2Si3O3N4 refers to R2Si3O3N4 crystal particles, but these are abbreviated as Si3N4, BN, and R2Si3O3N4, respectively. The ceramic composite 1 is used in an airborne radome 9 (see FIG. 10) according to a second embodiment, which will be described later.

[0015] Si3N42 is mainly composed of needle-shaped crystal particles, which bond together through sintering, thereby improving the mechanical strength of the ceramic composite 1. In other words, Si3N42 improves the thermal shock resistance of the ceramic composite 1. The average grain size (average major axis) of the Si3N42 crystal particles in the longitudinal direction is not particularly limited, but is preferably 2 μm or more and 30 μm or less, and more preferably 5 μm or more and 20 μm or less. Hereinafter, the Si3N42 crystal particles will be referred to as particles 2a.

[0016] BN3, when present in the space 5 surrounded by the Si3N42 particles 2a or the space 5 surrounded by the Si3N42 particles 2a and the R2Si3O3N44 particles 4a, reduces the elastic modulus of the ceramic composite 1 and improves the thermal shock resistance and grinding processability of the ceramic composite 1.

[0017] The total content of Si3N42 and BN3 relative to the total amount of ceramic composite 1 in this embodiment is 80% by mass or more and 96% by mass or less, preferably 81% by mass or more and 95% by mass or less, more preferably 82% by mass or more and 94% by mass or less, and even more preferably 83% by mass or more and 93% by mass or less. If the total content of Si3N42 and BN3 relative to the total amount of ceramic composite 1 is less than 80% by mass or exceeds 96% by mass, the thermal shock resistance of ceramic composite 1 is not sufficiently improved. If the total content of Si3N42 and BN3 relative to the total amount of ceramic composite 1 exceeds 98% by mass, ceramic composite 1 is not sufficiently densified, and the mechanical strength of ceramic composite 1 is reduced.

[0018] The mass ratio of BN3 to Si3N42 in the ceramic composite 1 of this embodiment is between 2 / 98 and 10 / 90, preferably between 4 / 96 and 6 / 94. If the proportion of BN3 is too low, the effect of reducing the elastic modulus of the ceramic composite 1 is not sufficiently obtained. As a result, when the ceramic composite 1 is used in a radome 9 for an airborne vehicle, the thermal stress caused by the temperature difference between the surface and the interior of the radome 9 increases, and the radome 9 may break or crack. In other words, the thermal shock resistance of the ceramic composite 1 is not sufficiently improved. On the other hand, if the proportion of BN3 is too high, the mechanical strength of the ceramic composite 1 is significantly reduced, and the thermal shock resistance of the ceramic composite 1 is not sufficiently improved.

[0019] The rare earth oxynitride R2Si3O3N44 strengthens the bonds between the Si3N42 particles 2a, thereby enhancing the mechanical strength of the ceramic composite 1. Specifically, R2Si3O3N44 enhances the thermal shock resistance of the ceramic composite 1. Therefore, the presence of R2Si3O3N44 in contact with the Si3N42 particles 2a facilitates further enhancing the mechanical strength of the ceramic composite 1. The proportion of R2Si3O3N44 in contact with the Si3N42 particles 2a is preferably 70% or more, more preferably 80% or more, and even more preferably 90% or more. FIG. 2 is a cross-sectional view showing an example of the contact between Si3N42 and R2Si3O3N44 according to the first embodiment. FIG. 3 is a cross-sectional view showing another example of the contact between Si3N42 and R2Si3O3N44 according to the first embodiment. R2Si3O3N44 may be in contact with only one particle 2a of Si3N42 as shown in FIG. 2, but it is preferable that R2Si3O3N44 be in contact with a plurality of particles 2a of Si3N42 as shown in FIG.

[0020] The rare earth oxynitride of this embodiment is produced by a reaction between a sintering aid for densifying Si3N42, which is a constituent of the main phase, and Si3N42 during firing. For example, when Y2O3 (yttrium oxide) is used as a sintering aid, the produced rare earth oxynitride is Y, which is the H phase. 10There are four types of rare earth oxynitrides: (SiO4)6N2, K-phase YSiO2N, J-phase Y4Si2O7N2, and M-phase R2Si3O3N44. Among these four types, the H-phase and K-phase have low melting points, making it difficult to obtain sufficient mechanical strength for applications exposed to high temperatures, such as the airborne radome 9. Furthermore, the J-phase cannot be formed in coexistence with Si3N42 in a thermodynamic equilibrium state, and therefore cannot be incorporated as a subphase of Si3N42. For this reason, it is preferable to use the M-phase R2Si3O3N44, which can be incorporated as a subphase of Si3N42 and has a high melting point, as the rare earth oxynitride. The rare earths constituting the M-phase R2Si3O3N44 are not particularly limited, but Y2Si3O3N4, which is produced by using Y2O3, which is suitable as a sintering aid for Si3N42, is the most preferred. As the sintering aid, in addition to the rare earth oxide Y2O3, for example, oxides of aluminum, titanium, magnesium or silicon, nitrides of aluminum or titanium, etc. may be used in combination with the rare earth oxide.

[0021] The content of the sintering aid relative to the total amount of the ceramic composite 1 of this embodiment is not particularly limited, but is preferably 2% by mass to 20% by mass, more preferably 3% by mass to 19% by mass, even more preferably 4% by mass to 18% by mass, and most preferably 5% by mass to 17% by mass. If the content of the sintering aid is less than 2% by mass, the ceramic composite 1 may not be sufficiently densified. On the other hand, if the content of the sintering aid exceeds 20% by mass, the contents of Si3N42 and BN3 relative to the total amount of the ceramic composite 1 become small, and the thermal shock resistance of the ceramic composite 1 may not be sufficiently improved.

[0022] FIG. 4 is a cross-sectional view schematically illustrating an example of a BN3 agglomerated particle 3a according to the first embodiment. FIG. 5 is a cross-sectional view schematically illustrating another example of a BN3 agglomerated particle 3a according to the first embodiment. BN3 according to the present embodiment includes the agglomerated particles 3a shown in FIG. 4 or 5. Agglomerated particles 3a of BN3 are softer (have a lower apparent modulus of elasticity) than single BN3 particles of the same size. Therefore, the inclusion of agglomerated particles 3a in BN3 can effectively reduce the modulus of elasticity of ceramic composite 1. The shape of BN3 agglomerated particles 3a is not particularly limited and may be a roughly spherical (circular) shape as shown in FIG. 4 or an angular shape as shown in FIG. 5. The average particle size of BN3 agglomerated particles 3a is 30 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less, from the viewpoint of uniformly dispersing BN3 agglomerated particles 3a among SiN particles 2a. If the average particle size of the BN3 agglomerated particles 3a exceeds 30 μm, it may be difficult to obtain a state in which the BN3 agglomerated particles 3a are uniformly dispersed among the Si3N42 particles 2a. As a result, portions rich in BN3 (hereinafter referred to as "BN-rich portions") and portions rich in Si3N42 (hereinafter referred to as "Si3N4-rich portions") may be unevenly formed within the ceramic composite 1. This may cause the BN-rich portions of the ceramic composite 1 to start to reduce the mechanical strength of the ceramic composite 1, which may reduce the thermal shock resistance of the ceramic composite 1.

[0023] The particle size of the primary particles 3b constituting the aggregated particles 3a of the BN3 of this embodiment is preferably fine to facilitate aggregation of the primary particles 3b. The particle size of the primary particles 3b is preferably 1 μm or less, more preferably 0.05 μm to 0.9 μm, even more preferably 0.1 μm to 0.8 μm, and most preferably 0.2 μm to 0.5 μm. Furthermore, for example, hexagonal BN or turbostratic BN is used for the primary particles 3b of this embodiment. In particular, turbostratic BN is preferably used for the primary particles 3b because it is easy to obtain primary particles 3b with a fine particle size. Here, the average particle size of the primary particles 3b in the ceramic composite 1 can be obtained by cutting the ceramic composite 1, magnifying the cross section with a scanning electron microscope (SEM) at, for example, 15,000 times, measuring the major axis of at least 20 primary particles 3b, and averaging the measured values. The crystal structure of the primary particles 3b in the ceramic composite 1 can be identified from a diffraction pattern obtained by measuring the surface of the ceramic composite 1 by X-ray diffraction.

[0024] Fig. 6 is a cross-sectional view showing an example of a BN3 agglomerate particle 3a in a space 5 surrounded by Si3N42 particles 2a according to the first embodiment. Fig. 7 is a cross-sectional view showing an example of a BN3 agglomerate particle 3a in a space 5 surrounded by Si3N42 particles 2a and R2Si3O3N44 particles 4a according to the first embodiment. Fig. 8 is a cross-sectional view showing another example of a BN3 agglomerate particle 3a in a space 5 surrounded by Si3N42 particles 2a according to the first embodiment. Fig. 9 is a cross-sectional view showing another example of a BN3 agglomerate particle 3a in a space 5 surrounded by Si3N42 particles 2a and R2Si3O3N44 particles 4a according to the first embodiment. In this embodiment, BN3 exists so as to fill the space 5 formed by being surrounded by the SiN particles 2a shown in FIGS. 6 and 8, or the space 5 formed by the SiN particles 2a and the RSiON particles 4a shown in FIGS. 7 and 9.

[0025] As shown in Figures 6 and 7, a single agglomerated particle 3a of BN3 may be present so as to fill the space 5, or as shown in Figures 8 and 9, a plurality of agglomerated particles 3a of BN3 may be present so as to fill the space 5. Alternatively, agglomerated particles 3a of BN3 and non-agglomerated particles 3c of BN3 may be present (coexist) so as to fill the space 5. From the viewpoint of effectively reducing the elastic modulus of ceramic composite 1 and improving the thermal shock resistance of ceramic composite 1, it is preferable that 60% or more of the BN3 present in space 5 be agglomerated particles 3a of BN3, and it is more preferable that all of the BN3 present in space 5 be agglomerated particles 3a.

[0026] 6 and 8, the mechanical strength of the ceramic composite 1 is affected by the size of the spaces 5 formed by the Si3N42 particles 2a and the minor diameter of the Si3N42 particles 2a surrounding the spaces 5. For example, when the size of the spaces 5 formed by the Si3N42 particles 2a is large and the minor diameter of the Si3N42 particles 2a surrounding the spaces 5 is small, the mechanical strength of the ceramic composite 1 tends to decrease. For this reason, the ratio of the average size of the spaces 5 formed by the Si3N42 particles 2a to the average minor diameter of the Si3N42 particles 2a surrounding the spaces 5, i.e., the value obtained by dividing the average size of the spaces 5 by the average minor diameter of the Si3N42 particles 2a (average size of spaces 5 / average minor diameter of Si3N42 particles 2a), is preferably 15 or less, and more preferably 10 or less. Here, the average size of the spaces 5 formed by being surrounded by the Si3N42 particles 2a can be obtained by cutting the ceramic composite 1, magnifying the cross section with a scanning electron microscope, for example, 15,000 times, measuring the size of at least 20 spaces 5 (the major axis of the spaces 5 if the spaces 5 have an aspect ratio), and averaging the measured values. Also, the average minor axis of the Si3N42 particles 2a surrounding the spaces 5 can be obtained by cutting the ceramic composite 1, magnifying the cross section with a scanning electron microscope, for example, 15,000 times, measuring the minor axis of the Si3N42 particles 2a surrounding each of the at least 20 spaces 5, and averaging the measured values.

[0027] The finer the average size of the spaces 5, the easier it is to improve the mechanical strength of the ceramic composite 1, regardless of the minor axis of the Si3N42 particles 2a surrounding the spaces 5. For this reason, the average size of the spaces 5 formed by being surrounded by the Si3N42 particles 2a is preferably 30 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less. By setting the average size of the spaces 5 in this range, the elastic modulus of the ceramic composite 1 can be reduced without reducing the mechanical strength of the ceramic composite 1, and the thermal shock resistance of the ceramic composite 1 can be improved.

[0028] 7 and 9, the mechanical strength of the ceramic composite 1 is affected by the size of the space 5 formed by the Si3N42 particles 2a and the R2Si3O3N44 particles 4a and the minor axis of the Si3N42 particles 2a surrounding the space 5. For example, if the size of the space 5 formed by the Si3N42 particles 2a and the R2Si3O3N44 particles 4a is large and the minor axis of the Si3N42 particles 2a surrounding the space 5 is small, the mechanical strength of the ceramic composite 1 tends to decrease. For this reason, the ratio of the average size of spaces 5 formed by being surrounded by Si3N42 particles 2a and R2Si3O3N44 particles 4a to the average minor axis of the Si3N42 particles 2a surrounding these spaces 5, i.e., the value obtained by dividing the average size of spaces 5 by the average minor axis of the Si3N42 particles 2a (average size of spaces 5 / average minor axis of Si3N42 particles 2a), is preferably 15 or less, and more preferably 10 or less. The methods for determining the average size of spaces 5 formed by being surrounded by Si3N42 particles 2a and R2Si3O3N44 particles 4a and the average minor axis of the Si3N42 particles 2a surrounding these spaces 5 are the same as those for determining the average size of spaces 5 and the average minor axis of the Si3N42 particles 2a when spaces 5 are formed by being surrounded by Si3N42 particles 2a.

[0029] The finer the average size of the spaces 5, the easier it is to improve the mechanical strength of the ceramic composite 1, regardless of the minor axis of the Si3N42 particles 2a surrounding the spaces 5. For this reason, the average size of the spaces 5 formed by the Si3N42 particles 2a and the R2Si3O3N44 particles 4a is preferably 30 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less. By setting the average size of the spaces 5 in this range, the elastic modulus of the ceramic composite 1 can be reduced without reducing the mechanical strength of the ceramic composite 1, and the thermal shock resistance of the ceramic composite 1 can be improved.

[0030] Returning to FIG. 1 , the porosity of the ceramic composite 1 of this embodiment is related to the mechanical strength and thermal shock resistance of the ceramic composite 1. That is, if the porosity of the ceramic composite 1 is too high, voids may connect within the ceramic composite 1, resulting in a decrease in the mechanical strength of the ceramic composite 1. On the other hand, if the porosity of the ceramic composite 1 is too low, the elastic modulus of the ceramic composite 1 may increase, resulting in a decrease in the thermal shock resistance of the ceramic composite 1. Therefore, from the viewpoint of obtaining the desired mechanical strength and thermal shock resistance, the porosity of the ceramic composite 1 of this embodiment is preferably 30 vol% or less, more preferably 5 vol% to 29 vol%, and even more preferably 7 vol% to 28 vol%.

[0031] Here, in this specification, the porosity of the ceramic composite 1 is calculated by the Archimedes method. Specifically, the porosity of the ceramic composite 1 is calculated by the following formula (1) using the measured values ​​of the mass and dimensions of a rectangular parallelepiped test piece cut out from the ceramic composite 1. The dimensions of the rectangular parallelepiped test piece are the length, width, and height. Porosity={1-[W dry / (L×W×T) / ρ theory ]}×100···(1)

[0032] In formula (1), W dryis the mass (g) of the test piece dried at 150°C for 2 hours. In addition, in formula (1), L, W, and T are the length, width, and height dimensions (cm) of the rectangular parallelepiped test piece, respectively. ρ theory is the theoretical density of the specimen (g / cm 3 )

[0033] The ceramic composite 1 of this embodiment is composed of an insulator. Furthermore, since the ceramic composite 1 contains Si3N42 and BN3, which have low dielectric constant properties, as its main phase, the ceramic composite 1 has a low dielectric constant. Specifically, the ceramic composite 1 has a dielectric constant of 8.0 or less. Therefore, the ceramic composite 1 also has excellent radio wave transmittance.

[0034] Next, an example of a method for manufacturing the ceramic composite 1 according to this embodiment will be described. The method for manufacturing the ceramic composite 1 includes a preparation step, a granulation step, a molding step, a degreasing step, a firing step, and a grinding step.

[0035] The preparation step involves mixing Si3N42 powder, BN3 agglomerated powder, sintering aid, dispersant, binder, and water to prepare a slurry. The average particle sizes of the Si3N42 powder and sintering aid are not particularly limited, but are preferably 1 μm or less, more preferably 0.8 μm or less, and even more preferably 0.5 μm or less. The average particle size of the BN3 agglomerated powder is 30 μm or less, more preferably 20 μm or less, and even more preferably 10 μm or less. If the average particle size of the BN3 agglomerated powder exceeds 30 μm, it may be difficult to obtain a uniform dispersion of the BN3 agglomerated powder among the Si3N42 particles 2a. Furthermore, the particle size of the primary particles 3b constituting the BN3 agglomerated particles 3a shown in FIG. 4 is not particularly limited, but is preferably 1 μm or less, more preferably 0.9 μm or less, even more preferably 0.8 μm or less, and most preferably 0.5 μm or less. If the particle size of the primary particles 3b exceeds 1 μm, the aggregated particles 3a of BN3 tend to break apart easily, making it difficult to maintain the shape of the aggregated particles 3a. It is preferable to use a rare earth oxide (R2O3) as the sintering aid. Among rare earth oxides, it is particularly preferable to use Y2O3. In addition to the rare earth oxides listed above, oxides of aluminum, titanium, magnesium, or silicon, or nitrides of aluminum or titanium may also be used in combination with the rare earth oxides as the sintering aid.

[0036] The dispersant is not particularly limited in type as long as it can be used in aqueous slurries, and may be appropriately selected from known dispersants. Examples of dispersants include anionic surfactants such as alkyl sulfate ester salts, polyoxyethylene alkyl ether sulfate ester salts, alkylbenzene sulfonates, reactive surfactants, fatty acid salts, and naphthalene sulfonate-formaldehyde condensates; cationic surfactants such as alkylamine salts, quaternary ammonium salts, and amphoteric surfactants such as alkylbetaine and alkylamine oxide; and nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyalkylene derivatives, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene sorbitol fatty acid esters, glycerin fatty acid esters, polyoxyethylene fatty acid esters, polyoxyethylene fatty acid castor oil, polyoxyethylene alkylamines, and alkylalkanolamides. These dispersants may be used alone or in combination of two or more.

[0037] The binder is not particularly limited and may be appropriately selected from known binders. Examples of binders include acrylic, cellulose, polyvinyl alcohol, polyvinyl acetal, urethane, and vinyl acetate resins. These binders may be used alone or in combination of two or more.

[0038] The water is not particularly limited and may be appropriately selected from known waters, such as pure water, RO water, and deionized water.

[0039] The mixing method for preparing the slurry is not particularly limited and may be appropriately selected from known mixing methods, such as those using a kneader, a ball mill, a planetary ball mill, a kneading mixer, or a bead mill.

[0040] The granulation step is a step of granulating the slurry to prepare a granulated powder. The granulation method is not particularly limited and may be appropriately selected from known granulation methods. Examples of the granulation method include spray drying using a spray dryer or the like. The conditions for spray drying may be appropriately adjusted depending on the equipment used and are not particularly limited.

[0041] The molding process is a process in which the granulated powder is molded into a predetermined shape to produce a powder molded body. In the molding process, the granulated powder is filled into a mold having a predetermined shape. Subsequently, in the molding process, the granulated powder is pressed and solidified. This produces a powder molded body by pressure molding the granulated powder. The pressure molding method is not particularly limited and may be appropriately selected from known pressure molding methods. Examples of pressure molding methods include cold isostatic pressing (CIP) molding, warm isostatic pressing (WIP) molding, and uniaxial pressing. The pressure during pressure molding may be adjusted appropriately depending on the type of granulated powder, the apparatus used, etc., and is not particularly limited, but is generally 30 MPa or more and 500 MPa or less.

[0042] The debinding process is a process of debinding the powder compact. By performing the debinding process, the binder in the powder compact is removed. The debinding method is not particularly limited and may be appropriately selected from known debinding methods. For example, the debinding method may be a method of heat-treating the powder compact in an air atmosphere. The heating temperature is not particularly limited as long as it is a temperature at which the binder can be thermally decomposed, but is generally 300°C or higher and 800°C or lower.

[0043] The firing step is a step of firing the powder compact after degreasing. A ceramic sintered body is produced by performing the firing step. The firing method is not particularly limited and may be appropriately selected from known firing methods. For example, the firing method may involve firing the powder compact after degreasing in a nitrogen atmosphere. The pressure of the nitrogen gas during firing may be atmospheric pressure, but is preferably 0.2 MPa to 1.0 MPa to suppress thermal decomposition of Si3N42. The firing temperature is not particularly limited, but is preferably 1700°C to 2100°C, preferably 1750°C to 2050°C, and more preferably 1800°C to 2000°C to promote the reaction between Si3N42 and the rare earth oxide sintering aid and promote the formation of the subphase R2Si3O3N44.

[0044] The grinding process is a process of grinding the surface of the sintered powder compact, i.e., the ceramic sintered body. By performing the grinding process, the shape of the ceramic sintered body is adjusted. The grinding method is not particularly limited, and may be appropriately selected from known grinding methods. For example, a grinding method using a diamond bit is used. The ceramic composite 1 is obtained by the above process.

[0045] Next, the effects of the ceramic composite 1 according to this embodiment will be described.

[0046] Generally, the thermal shock resistance of a ceramic material is related to its coefficient of thermal expansion, thermal conductivity, mechanical strength (mainly bending strength), and elastic modulus. The higher the thermal conductivity and mechanical strength, and the lower the coefficient of thermal expansion and elastic modulus, the better. Therefore, in this embodiment, as shown in FIG. 1 , ceramic composite 1 contains a main phase containing Si3N42 and BN3. By combining BN3, which has high thermal conductivity and low elastic modulus, with Si3N42, the thermal conductivity of ceramic composite 1 can be increased and the elastic modulus of ceramic composite 1 can be reduced while maintaining a low coefficient of thermal expansion. Furthermore, in this embodiment, ceramic composite 1 contains a subphase containing R2Si3O3N44, which strengthens the bonds between particles 2a of the Si3N42 main phase, thereby increasing the mechanical strength of ceramic composite 1. Furthermore, in this embodiment, as shown in FIGS. 4 to 9 , BN3 contains agglomerated particles 3a, which reduces the elastic modulus of ceramic composite 1. This improves the thermal shock resistance of ceramic composite 1.

[0047] In general, the grindability of a ceramic material is related to its elastic modulus, and the lower the elastic modulus, the better. Therefore, in this embodiment, as shown in FIG. 1, ceramic composite 1 contains a main phase containing Si3N42 and BN3. By combining BN3, which has a low elastic modulus, with Si3N42, which has a high elastic modulus, the elastic modulus of ceramic composite 1 can be reduced. This improves the grindability of ceramic composite 1. In other words, this embodiment provides ceramic composite 1 that can achieve both improved thermal shock resistance and improved grindability.

[0048] In this embodiment, the total content of Si3N42 and BN3 relative to the total amount of ceramic composite 1 is 80 mass% or more and 96 mass% or less, thereby improving the thermal shock resistance of ceramic composite 1 without reducing the mechanical strength of ceramic composite 1.

[0049] In this embodiment, the mass ratio of BN3 to Si3N42 is between 2 / 98 and 10 / 90, which reduces the elastic modulus of the ceramic composite 1 and suppresses a decrease in the mechanical strength of the ceramic composite 1, thereby improving the thermal shock resistance of the ceramic composite 1.

[0050] 4 to 9, the average particle size of the BN3 agglomerated particles 3a is 30 μm or less, which allows the BN3 agglomerated particles 3a to be uniformly dispersed among the Si3N42 particles 2a. This increases the mechanical strength of the ceramic composite 1, thereby further improving the thermal shock resistance of the ceramic composite 1.

[0051] In this embodiment, primary particles 3b constituting aggregated particles 3a of BN3 shown in FIGS. 4 to 9 have an average particle size of 1 μm or less, which makes it easier to aggregate primary particles 3b.

[0052] In this embodiment, the proportion of R2Si3O3N44 in contact with Si3N42 shown in Figure 1 is 70% or more, which can increase the mechanical strength of the ceramic composite 1 and further improve the thermal shock resistance of the ceramic composite 1.

[0053] In this embodiment, the value obtained by dividing the average size of the spaces 5 formed by the Si3N42 particles 2a and the R2Si3O3N44 particles 4a shown in Figures 7 and 9 by the average minor axis of the Si3N42 particles 2a surrounding the spaces 5 is 15 or less, which further increases the mechanical strength of the ceramic composite 1 and further improves the thermal shock resistance of the ceramic composite 1.

[0054] In this embodiment, the average size of the spaces 5 formed by the Si3N42 particles 2a and the R2Si3O3N44 particles 4a is 30 μm or less, and therefore the elastic modulus of the ceramic composite 1 can be reduced without reducing the mechanical strength of the ceramic composite 1, thereby further improving the thermal shock resistance of the ceramic composite 1.

[0055] In this embodiment, the porosity of the ceramic composite 1 shown in FIG. 1 is 5 volume % or more and 30 volume % or less, thereby suppressing a decrease in the mechanical strength of the ceramic composite 1 and further improving the thermal shock resistance of the ceramic composite 1.

[0056] In this embodiment, the BN3 shown in FIGS. 4 to 9 contains turbostratic BN, which makes it easier to obtain primary particles 3b with a fine particle size.

[0057] Embodiment 2 Next, a flying object 6 according to a second embodiment will be described with reference to Fig. 10. Fig. 10 is a cross-sectional schematic diagram showing a flying object 6 according to the second embodiment. Note that in the second embodiment, parts that overlap with those in the first embodiment will be given the same reference numerals and descriptions thereof will be omitted.

[0058] The flying object 6 flies by radio wave guidance toward a set target. The flying object 6 includes a body 7, an antenna 8, and a flying object radome 9. The flying object radome 9 includes a ceramic shaped object 10 and a radome ring 11. The ceramic shaped object 10 is made of the ceramic composite 1 according to the first embodiment. That is, the ceramic shaped object 10 of the flying object radome 9 uses the ceramic composite 1 according to the first embodiment.

[0059] The airframe 7 is the main body of the flying object 6. The airframe 7 is formed in a cylindrical shape with a bottom. The material of the airframe 7 is a metal having a higher thermal expansion coefficient than the material of the ceramic object 10. Examples of such metals include iron and aluminum. The tip of the airframe 7 that faces the airframe radome 9 is open. The airframe 7 is not particularly limited and may be appropriately selected from known airframes 7.

[0060] The antenna 8 is an electronic device that is provided inside the aircraft body 7 and measures the distance and direction to a target.

[0061] The ceramic object 10 is a hollow member that is installed at the tip of the airframe 7 via a radome ring 11 to protect the antenna 8. The ceramic object 10 is installed so as to cover an opening in the airframe 7. The ceramic object 10 forms the outer shell of the tip of the flying object 6. The ceramic object 10 is formed in a streamlined shape that tapers away from the airframe 7 in the axial direction.

[0062] The radome ring 11 is a component that connects the fuselage 7 and the ceramic object 10. The radome ring 11 is formed in a cylindrical shape that is open at both axial ends. When viewed along the axial direction, the radome ring 11 has a thick circular shape. From the viewpoint of improving the heat resistance and thermal shock resistance of the radome ring 11, it is preferable to use a metal with a low thermal expansion coefficient as the material for the radome ring 11. Examples of such metals include invar, super invar, and titanium. From the viewpoint of reducing thermal stress generated in the radome ring 11, it is preferable to use super invar, which has a lower thermal expansion coefficient. The radome ring 11 may be formed by stacking multiple sheets of fiber reinforced plastics (FRP). The radome ring 11 is not particularly limited and may be appropriately selected from known radome rings 11.

[0063] The ceramic object 10 and the radome ring 11 may be bonded to each other with a resin adhesive or an inorganic adhesive. Alternatively, the ceramic object 10 and the radome ring 11 may be fixed to each other by mechanical fastening stress such as a spring. The radome ring 11 and the fuselage 7 are bonded to each other with metal bolts or the like.

[0064] Next, the effects of the flying object 6 according to this embodiment will be described.

[0065] In the present embodiment, the flying object 6 includes the ceramic shaped object 10 made of the ceramic composite 1 according to the first embodiment, which achieves both improved thermal shock resistance and improved grinding workability, thereby improving the thermal shock resistance and grinding workability of the flying object 6. Furthermore, in the present embodiment, the improved grinding workability can improve the productivity of the flying object radome 9.

[0066] Next, the effects of the present disclosure will be further described using examples and comparative examples.

[0067] Table 1 shows the composition ratios and blending amounts of semilac composites in Examples and Comparative Examples, as well as the structure and properties of sintered ceramic objects. The properties include four evaluation items: mechanical strength, elastic modulus, thermal shock resistance, and grindability. Table 2 shows the types of BN agglomerated particles. Here, BN agglomerated particles are classified into six types, A to F, based on differences in the average particle size of the agglomerated particles and the average particle size of the primary particles.

[0068] [Table 1]

[0069] [Table 2]

[0070] Example 1 As shown in Table 1, the raw materials for the ceramic composite mixed powder were Si3N4 powder, BN powder, YO3 powder (a sintering aid), and Al2O3 (aluminum oxide) powder (a sintering aid). The BN powder was BN agglomerated particles C, as shown in Table 2. Although not shown in Table 1, the average particle size of the Si3N4 powder was 0.1 μm. The powder composition was 81 wt% Si3N4 powder, 9 wt% BN powder, 9 wt% YO3 powder, and 1 wt% Al2O3 powder. To 100 wt% of the mixed powder raw materials, 2 parts by mass of a dispersant, 1 part by mass of polyvinyl alcohol (a binder), and 50 parts by mass of water were added and mixed in a ball mill for approximately 5 hours to prepare a slurry. The slurry was then spray-dried using a spray dryer to obtain a granulated powder for ceramic molding.

[0071] Next, the granulated powder for the ceramic shaped product was filled into a mold having the shape of the ceramic shaped product, and a powder compact was obtained by CIP molding using a cold isostatic press. The pressure was 98 MPa.

[0072] Next, the obtained powder compact was heat-treated in an air atmosphere at 600° C. for 2 hours, thereby carrying out a debinding treatment of the powder compact.

[0073] Next, the degreased powder compact was fired in a nitrogen atmosphere at 1900°C for 6 hours. The nitrogen gas pressure during firing was 0.9 MPa. Through the above steps, a ceramic shaped article according to Example 1 was obtained.

[0074] Example 2 The procedure was the same as in Example 1, except that a ceramic shaped object was obtained by mixing 86 wt% Si3N4 powder, 9.6 wt% BN powder, and 3 wt% Y2O3 powder.

[0075] Example 3 The procedure was the same as in Example 1, except that a ceramic shaped object was obtained using a blending amount of Si3N4 powder of 77 wt%, a blending amount of BN powder of 8.5 wt%, and a blending amount of Y2O3 powder of 14 wt%.

[0076] Example 4 The procedure was the same as in Example 1, except that a ceramic shaped object was obtained using a blending amount of Si3N4 powder of 72 wt%, a blending amount of BN powder of 8 wt%, and a blending amount of Y2O3 powder of 19 wt%.

[0077] Example 5 The procedure was the same as in Example 1, except that a ceramic shaped object was obtained by changing the blending amount of Si3N4 powder to 86 wt% and the blending amount of BN powder to 4.5 wt%.

[0078] Example 6 The procedure was the same as in Example 1, except that a ceramic shaped object was obtained by changing the blending amount of Si3N4 powder to 88 wt% and the blending amount of BN powder to 1.8 wt%.

[0079] Example 7 The procedure was the same as in Example 1, except that the BN powder used was the agglomerated BN particles C and non-agglomerated BN particles shown in Table 2, and the blending amount of the agglomerated BN particles C was 5.4 wt % and the blending amount of the non-agglomerated BN particles was 3.6 wt % to obtain a ceramic object.

[0080] Example 8 The procedure was the same as in Example 1, except that the BN powder used was the agglomerated BN particles C and non-agglomerated BN particles shown in Table 2, and the blending amount of the agglomerated BN particles C was 3.6 wt % and the blending amount of the non-agglomerated BN particles was 5.4 wt %.

[0081] Example 9 The procedure was the same as in Example 1, except that the BN powder used was the agglomerated BN particles C and non-agglomerated BN particles shown in Table 2, and the blending amount of the agglomerated BN particles C was 1.8 wt % and the blending amount of the non-agglomerated BN particles was 7.2 wt %.

[0082] Example 10 The procedure was the same as in Example 1, except that the BN powder used was the BN agglomerated particles A shown in Table 2, and the blending amount of the BN agglomerated particles A was 9 wt % to obtain a ceramic shaped article.

[0083] Example 11 The procedure was the same as in Example 1, except that the BN powder used was the BN agglomerated particles B shown in Table 2, and the amount of BN agglomerated particles B mixed was 9 wt % to obtain a ceramic shaped article.

[0084] Example 12 The procedure was the same as in Example 1, except that the BN powder used was BN agglomerated particles D shown in Table 2, and the amount of BN agglomerated particles D mixed was 9 wt % to obtain a ceramic shaped article.

[0085] Example 13 The procedure was the same as in Example 1, except that the BN powder used was BN agglomerated particles E shown in Table 2, and the blending amount of BN agglomerated particles E was 9 wt % to obtain a ceramic shaped article.

[0086] Example 14 The procedure was the same as in Example 1, except that the BN powder used was the BN agglomerated particles F shown in Table 2, and the blending amount of the BN agglomerated particles F was 9 wt % to obtain a ceramic shaped article.

[0087] (Comparative Example 1) The procedure was the same as in Example 1, except that the blending amount of Si3N4 powder was 90 wt % and a ceramic shaped article was obtained without adding BN powder.

[0088] (Comparative Example 2) The procedure was the same as in Example 1, except that a ceramic shaped object was obtained by mixing 68 wt% Si3N4 powder, 7.5 wt% BN powder, and 24 wt% Y2O3 powder.

[0089] (Comparative Example 3) The procedure was the same as in Example 1, except that a ceramic shaped object was obtained by mixing 88 wt% Si3N4 powder, 9.8 wt% BN powder, and 1 wt% Y2O3 powder.

[0090] Comparative Example 4 The procedure was the same as in Example 1, except that non-agglomerated BN particles were used as the BN powder and the blending amount of the non-agglomerated BN particles was set to 9 wt % to obtain a ceramic shaped article.

[0091] (Comparative Example 5) The procedure was the same as in Example 1, except that no Y2O3 powder was added and the amount of Al2O3 powder mixed was 10 wt % to obtain a ceramic shaped object.

[0092] (Test Method) The ceramic shaped articles of Examples 1 to 14 and Comparative Examples 1 to 5 were evaluated for (1) thermal shock resistance, (2) grindability, and (3) porosity.

[0093] (1) Thermal shock resistance A salt bath test was conducted to simulate aerodynamic heating during flight. For the salt bath test, a molten solution of barium chloride melted at 1200°C was used. To evaluate the thermal shock resistance after the salt bath test, the three-point bending strength of the ceramic shaped object was measured before the salt bath test (before the high-temperature arc exposure test) and after the salt bath test (after the high-temperature arc exposure test). The three-point bending strength ratio before and after the salt bath test was calculated using the following formula (2), and is shown in Table 1. Thermal shock resistance of ceramic molded object = [3-point bending strength after salt bath test] / [3-point bending strength before salt bath test] (2)

[0094] (2) Grindability To evaluate the grindability of the ceramic shaped objects, the grinding speeds of the ceramic shaped objects of each Example and Comparative Example were measured. The grinding speed measurement results are shown in Table 1 as relative values ​​of the grinding speeds of the ceramic shaped objects of each Example or Comparative Example to the grinding speed of the ceramic shaped object of Comparative Example 1, with the grinding speed of the ceramic shaped object of Comparative Example 1 being set to 1 (value of [grinding speed of the ceramic shaped object of each Example or Comparative Example] / [grinding speed of the ceramic shaped object of Comparative Example 1]).

[0095] (3) Porosity The porosity was calculated using the Archimedes method described above.

[0096] The presence or absence of Y2Si3O3N4 in the ceramic shaped articles of each Example and Comparative Example was measured by X-ray diffraction, and the results are shown in Table 1. The average particle size of the BN agglomerated particles and the average particle size of the primary particles constituting the BN agglomerated particles were also measured, and the results are shown in Table 1. The average particle size of the BN agglomerated particles and the average particle size of the primary particles constituting the BN agglomerated particles were determined by magnifying the cross section of a test piece cut out from each ceramic shaped article by 15,000 times using a scanning electron microscope, measuring the long diameter of 20 randomly selected agglomerated particles and the long diameter of 20 randomly selected primary particles, and averaging the measured values.

[0097] As evident from Table 1, in Examples 1 to 14, in which the total content of Si3N4 and BN relative to the total ceramic composite was 80% by mass or more and 96% by mass or less, the mass ratio of BN to Si3N4 was between 2 / 98 and 10 / 90, and the ceramic composite contained a subphase containing Y2Si3O3N4, which is formed by the reaction of the sintering aids Y2O3 and Si3N4 during sintering, and BN agglomerates, the ceramic composites exhibited minimal deterioration in three-point bending strength and improved thermal shock resistance even after thermal shock in a salt bath test. In particular, in Example 12, in which the primary particles constituting the BN agglomerates had a fine particle size of 0.5 μm, and in Examples 13 and 14, in which the average particle size of the BN agglomerates was small, the ceramic composites exhibited excellent effects in reducing the elastic modulus, thereby further improving the thermal shock resistance of the ceramic composites. Furthermore, comparing Example 1 with Examples 7 to 9, Example 1, which contained only BN agglomerates, exhibited the highest thermal shock resistance. The ceramic shaped article of Example 7, which had the highest content of BN agglomerates relative to the total amount of BN among Examples 7 to 9, had the next highest thermal shock resistance. Furthermore, the ceramic shaped articles of Examples 1 to 14 also had good grindability.

[0098] On the other hand, in Comparative Example 1, where BN was not contained, the elastic modulus of the ceramic composite was very high, resulting in a significant decrease in the thermal shock resistance of the ceramic composite and poor grindability. In Comparative Example 2, where the total content of Si3N4 and BN relative to the total weight of the ceramic composite was 75 wt%, the thermal shock resistance of the ceramic composite was significantly low. In Comparative Example 3, where the total content of Si3N4 and BN relative to the total weight of the ceramic composite was 98 wt%, the porosity of the ceramic composite was very high, resulting in a decrease in the mechanical strength (bending strength) of the ceramic composite and a significantly low thermal shock resistance of the ceramic composite. In Comparative Example 4, where BN agglomerate particles were not contained, the effect of reducing the elastic modulus of the ceramic composite was weak, and the thermal shock resistance of the ceramic composite was not improved. Furthermore, in Comparative Example 5, where a subphase containing Y2Si3O3N4 was not contained, the mechanical strength (bending strength) of the ceramic composite was low, resulting in a lack of improvement in the thermal shock resistance of the ceramic composite.

[0099] The results of Examples 1 to 14 and Comparative Examples 1 to 5 show that the thermal shock resistance and grindability of the ceramic shaped product are closely related to the composition ratio and blending amount of the ceramic composite used in the ceramic shaped product. Furthermore, by adjusting the composition ratio and blending amount of the ceramic composite, it is possible to provide a radome for an air vehicle that achieves both improved thermal shock resistance and improved grindability.

[0100] 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.

[0101] Various aspects of the present disclosure are summarized below as appendices.

[0102] (Appendix 1) A ceramic composite used in an airborne radome, comprising: a main phase including Si3N4 and BN; a subphase comprising R2Si3O3N4; Contains the total content of the Si3N4 and the BN relative to the total amount of the ceramic composite is 80 mass% or more and 96 mass% or less, the mass ratio of the BN to the Si3N4 is between 2 / 98 and 10 / 90; The ceramic composite is characterized in that the BN contains agglomerated particles. (Appendix 2) 2. The ceramic composite according to claim 1, wherein the average particle size of the BN agglomerates is 30 μm or less. (Appendix 3) 3. The ceramic composite according to claim 1, wherein the primary particles constituting the agglomerated particles of BN have an average particle size of 1 μm or less. (Appendix 4) 4. The ceramic composite according to claim 1, wherein the proportion of the R2Si3O3N4 in contact with the Si3N4 is 70% or more. (Appendix 5) The ceramic composite according to any one of claims 1 to 4, wherein the average size of the spaces formed by the Si3N4 particles and the R2Si3O3N4 particles divided by the average minor axis of the Si3N4 particles surrounding the spaces is 15 or less. (Appendix 6) 6. The ceramic composite according to any one of claims 1 to 5, wherein the average size of the spaces formed between the Si3N4 particles and the R2Si3O3N4 particles is 30 μm or less. (Appendix 7) 7. The ceramic composite according to any one of claims 1 to 6, wherein the porosity of the ceramic composite is 5% by volume or more and 30% by volume or less. (Appendix 8) 8. The ceramic composite according to any one of claims 1 to 7, wherein the rare earth component of R2Si3O3N4 is Y2Si3O3N4. (Appendix 9) 9. The ceramic composite of claim 1, wherein the BN comprises turbostratic BN. (Appendix 10) A radome for an airborne vehicle, comprising a ceramic shaped object made of the ceramic composite according to any one of appendices 1 to 9. [Explanation of symbols]

[0103] 1 Ceramic composite, 2 Si3N4, 2a, 4a particles, 3 BN, 3a agglomerated particles, 3b primary particles, 3c non-agglomerated particles, 4 R2Si3O3N4, 5 space, 6 projectile, 7 airframe, 8 antenna, 9 radome for projectile, 10 ceramic shaped object, 11 radome ring.

Claims

1. A ceramic composite used in an airborne radome, comprising: Si 3 N 4 and a main phase comprising BN; R 2 Si 3 O 3 N 4 and a subphase comprising Contains The Si content relative to the total amount of the ceramic composite 3 N 4 and the total content of BN is 80% by mass or more and 96% by mass or less, The Si 3 N 4 the mass ratio of the BN to the The ceramic composite is characterized in that the BN contains agglomerated particles.

2. 2. The ceramic composite according to claim 1, wherein the average particle size of the agglomerated particles of BN is 30 μm or less.

3. 2. The ceramic composite according to claim 1, wherein the primary particles constituting the agglomerated particles of BN have an average particle size of 1 μm or less.

4. The Si 3 N 4 The R 2 Si 3 O 3 N 4 2. The ceramic composite according to claim 1, wherein the ratio of is 70% or more.

5. The Si 3 N 4 Particles of and the R 2 Si 3 O 3 N 4 The average size of the space formed by the particles of Si is 3 N 4 2. The ceramic composite according to claim 1, wherein the value obtained by dividing the average minor axis of the particles by the average minor axis of the particles is 15 or less.

6. The Si 3 N 4 Particles of and the R 2 Si 3 O 3 N 4 2. The ceramic composite according to claim 1, wherein the average size of the spaces formed by the particles is 30 μm or less.

7. 2. The ceramic composite according to claim 1, wherein the porosity of the ceramic composite is 5% by volume or more and 30% by volume or less.

8. The R 2 Si 3 O 3 N 4 The rare earth elements that make up 2 Si 3 O 3 N 4 2. The ceramic composite of claim 1 , wherein:

9. 2. The ceramic composite of claim 1, wherein the BN comprises turbostratic BN.

10. A radome for an air vehicle, comprising a ceramic shaped article made of the ceramic composite according to any one of claims 1 to 9.

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