JP7347045B2 - BN-coated SiC fiber and method for producing SiC-based composite material - Google Patents

Description

The present invention relates to a BN-coated SiC fiber used in a SiC-based composite material in which a matrix mainly composed of SiC is formed between fibers mainly composed of SiC, and a method for producing the SiC-based composite material.

In response to the growing global interest in environmental conservation and the soaring price of crude oil, efforts are being made to increase the combustion efficiency of jet engines to improve fuel efficiency. In order to increase combustion efficiency, it is necessary to raise the temperature of the combustion gas to higher temperatures, and ceramic matrix composite materials are attracting attention as new materials for parts exposed to high temperatures, along with highly heat-resistant Ni-based superalloys. .

As such a ceramic matrix composite material, a SiC matrix composite material in which a matrix mainly composed of SiC is formed between fibers mainly composed of SiC is known. As a SiC-based composite material, for example, Patent Document 1 discloses that by forming an interphase layer of boron nitride on fibers made of silicon carbide on the surface and forming a matrix using this, a strong boron nitride layer is formed between the fibers and the matrix. It is disclosed that an interphase layer, that is, a BN interface layer, can be formed. According to this, the BN interface layer increases the bonding strength between the BN interface layer/fibers and between the BN interface layer/matrix, making it possible to create a composite material with higher strength than the matrix material.

Special Publication No. 2001-505864

However, in the composite material of Patent Document 1, although the strength is higher than that of the matrix material, the elongation is small, and a SiC-based composite material with even higher strength has been desired.

Therefore, the present invention provides a SiC-based composite material with high ductility and strength, a BN-coated SiC fiber used in the SiC-based composite material, and a method for producing the SiC-based composite material.

The BN-coated SiC fiber of the present invention includes a first BN coating layer serving as the outermost layer and a second BN coating layer inside the first BN coating layer on the surface of the fiber mainly composed of SiC, The amount of oxygen contained in the first BN coating layer is higher than the amount of oxygen contained in the second BN coating layer.

Further, in the BN-coated SiC fiber of the present invention, it is preferable that the first BN coating layer has a lower crystallinity than the second BN coating layer.

Further, in the BN-coated SiC fiber of the present invention, it is preferable that the first BN coating layer contains oxygen amount of 3 atomic percent or more.

Further, the method for producing SiC of the present invention includes a step of producing the BN-coated SiC fiber, and a step of forming a crystalline carbon layer while forming a matrix mainly composed of SiC on the BN-coated SiC fiber. It is characterized by having the following.

Further, in the SiC manufacturing method of the present invention, it is preferable that the method for forming the matrix containing SiC as a main component is a chemical vapor impregnation (CVI) method.

According to the present invention, a SiC-based composite material with high ductility and strength can be obtained.

FIG. 1 is a schematic diagram of a SiC-based composite material according to a first embodiment of the present invention. In the SiC matrix composite material according to the first embodiment of the present invention, a region containing a crystalline carbon layer is formed between the SiC fiber, the SiC matrix, the crystalline BN layer formed on the surface of the SiC fiber, and the matrix. (a) A cross-sectional TEM image, (b) a selected area electron diffraction pattern of a crystalline BN layer and a crystalline carbon layer. They are (a) a cross-sectional TEM image and (b) a high-resolution TEM image of a region including SiC fibers and a crystalline BN layer in the SiC-based composite material according to the first embodiment of the present invention. They are (a) a cross-sectional TEM image of a region containing a crystalline carbon layer and a SiC matrix in a SiC-based composite material according to a first embodiment of the present invention, and (b) a high-resolution TEM image thereof. FIG. 2 is a schematic diagram showing a typical displacement-load curve of the SiC-based composite material of the present invention obtained in a single fiber pullout test using a nanoindenter, and the arrangement of an indenter, fibers, and matrix. These are the results of investigating the dependence of the interfacial shear strength between the fiber and the matrix on the thickness of the BN coating layer before matrix formation using CVI. FIG. 2 is a schematic diagram of a cross section of a BN-coated SiC fiber according to a second embodiment of the present invention. (a) Cross-sectional TEM image of the BN-coated SiC fiber according to the second embodiment of the present invention, (b) High-resolution TEM image of the first BN coating layer, and (c) High-resolution TEM image of the second BN coating layer. It is. FIG. 2 is a schematic diagram of a continuous film forming apparatus. These are (a) a cross-sectional TEM image of a BN-coated SiC fiber formed on a SiC fiber of a comparative example, (b) a selected area electron diffraction pattern of a BN layer, and (c) a high-resolution TEM image of a cross-section of a BN layer. (a) Cross-sectional TEM image near the SiC fiber/interface layer/SiC matrix interface of the SiC-based composite material of the comparative example, (b) selected area electron diffraction pattern of the crystalline BN layer, (c) crystalline BN layer and matrix This is a high-resolution TEM image of the area containing the image. 2 is a room temperature bending stress-displacement curve of the SiC-based composite material of the present invention and a comparative sample. It is a SEM image of the fracture surface of the SiC-based composite material of the present invention and a comparative sample after a bending test. 2 is a room temperature tensile stress-strain curve of the SiC-based composite material of the present invention and a comparative sample. It is a SEM image of the fracture surface of the SiC-based composite material of the present invention and a comparative sample after a tensile test.

Embodiments of the present invention will be described in detail below with reference to the drawings.

(First embodiment)
First, as a first embodiment of the present invention, a SiC-based composite material 100 shown in FIG. 1 will be described.
The SiC-based composite material 100 includes fibers 10 mainly composed of SiC and a matrix 40 mainly composed of SiC, and includes a crystalline BN layer 20 formed on the surface of the SiC fibers 10 and the matrix 40. , a crystalline carbon layer 30 is included between them. That is, the crystalline carbon layer 30 is formed between the crystalline BN layer 20 and the matrix 40.

FIG. 2 shows the relationship between the fibers 10 mainly composed of SiC, the matrix 40 mainly composed of SiC, the crystalline BN layer 20 formed on the surface of the SiC fibers 10, and the matrix 40 in the SiC-based composite material 100. 3 is a TEM image of a region including the crystalline carbon layer 30. From the electron diffraction pattern of the crystalline BN layer 20 shown in FIG. 2(b)(1), Debye rings consisting of minute spots due to hexagonal BN appear, indicating a high degree of crystallinity. Furthermore, B and N are mainly detected from the EDX elemental analysis regions 2 and 3 of the crystalline BN layer 20 in Table 1 below. In addition, in the electron diffraction pattern of the crystalline carbon layer 30 shown in FIG. 2(b)(2), Debye rings consisting of fine spots due to hexagonal graphite (graphite) appear, indicating a high degree of crystallinity. I understand that. Furthermore, as shown in Table 1 below, C was mainly detected from the EDX elemental analysis region 4 of the crystalline carbon layer 30.

Here, the SiC fiber 10 is a fiber containing SiC as a main component and having a diameter of about 7 to 15 μm. It contains 80 to 99.8 mass percent of SiC as a main component, and may also contain oxygen, metal components such as Ti, Zr, and Al, or ceramic components such as TiB ₂ and B ₄ C. Note that from the viewpoint of mechanical properties and heat resistance, it is preferable that the SiC content is high, the oxygen content is low, and the material is crystalline. Further, it is usually used in the form of a fiber bundle of about 500 to 1,600 single fibers.

Further, the SiC matrix 40 is a bulk body mainly composed of SiC, and has a porosity of about 2 to 20 volume percent. The main component is 80% by mass or more of SiC, and may also contain impurities such as oxygen, carbon, and silicon. Note that, from the viewpoint of mechanical properties and heat resistance, it is preferable that the SiC content is high and that it is crystalline. In particular, it is preferable that the SiC matrix near the fibers has a high degree of crystallinity.

Further, the crystalline BN layer 20 exhibits a turbostratic structure consisting of string-like crystals with a width of several nanometers as shown by the dotted line in FIG. 3(b). The turbostratic structure is a structure that is two-dimensionally ordered but three-dimensionally disordered (the stacked structure in the C-axis direction is disordered) due to the translation and rotation of the BN hexagonal basal plane in the monoatomic plane. The axial spacing is slightly larger than that of normal hexagonal BN.

In addition, the crystalline carbon layer 30 is made of string-like crystals with a width of several nanometers as shown in FIG. It is thought that the laminated structure of the layer is disordered, resulting in a turbostratic structure.

In the SiC-based composite material 100 of the first embodiment, an interface layer 50 is formed, which is composed of a crystalline BN layer 20 and a crystalline carbon layer 30, which have lower shear strength than the SiC matrix 20. When tensile stress is applied to the SiC-based composite material 100, the interfacial layer 50 is preferentially destroyed and the SiC fibers 10 are pulled out from the SiC matrix 40. As a result, brittle fracture of the SiC fibers 10 is suppressed, and a SiC-based composite material 100 with high ductility and strength can be obtained.

The pulling mechanism of this SiC fiber 10 will be explained in detail. When tensile stress is applied to the SiC-based composite material 100, cracks occur in the matrix, and if the fibers 10 pass through without being broken, a situation called bridging occurs in which the fibers connect the cracks. In this case, if the interface between the fibers 10 and the matrix 40 is strongly bonded, fiber breakage will occur during crack propagation, leading to brittle fracture. On the other hand, if interfacial separation of a certain length occurs at the interface between the fibers 10 and the matrix 40 during crack propagation, slippage occurs between the fibers 10 and the matrix 40, and bridging of the fibers 10 occurs, resulting in ductility. . Therefore, it is important to reduce the peel resistance (shear strength) at the interface and optimize the slip resistance. Recently, a BN layer formed by thermal CVD has been used as this interface layer. According to US Pat. No. 5,302,300, a BN interphase layer formed by an array of nanometers exhibiting slight anisotropy (exhibiting randomly arranged anisotropic regions with a size of less than 15 nm) is disclosed.

However, the present inventors conducted various studies on BN interphase layers and found that BN interphase layers with slight anisotropy have high bonding strength within the interphase layer, between interphase layers/fibers, and between interphase layers/matrix. When fracture occurs under tensile stress, it is difficult for fracture to occur within the interphase layer, between the interphase layer/fibers, or between the interphase layer/matrix, and with the BN interphase layer alone, fiber pulling out from the matrix is insufficient, and sufficient ductility and It was found that no strength was obtained.

Therefore, the interface layer 50 has a two-layer structure of the crystalline carbon layer 30 and the crystalline BN layer 20. The shear strength of the crystalline carbon layer 30 is lower than that of the crystalline BN layer 20, and when tensile stress is applied to the SiC-based composite material 100, the crystalline carbon layer 30 or the crystalline carbon layer 30 and the crystalline BN layer 20 It is thought that the interface between the two is preferentially destroyed. For this reason, it is presumed that fiber pullability is superior to that of the SiC-based composite material 100 having the interface layer 50 consisting only of the crystalline BN layer 20.

In order to verify the fiber pullability of the SiC-based composite material 100 of the present invention, a single fiber pullout test was conducted using a nanoindenter. Using a Shimadzu dynamic ultra-microhardness tester (DUH-211S), a diamond triangular pyramidal Berkovich indenter (edge angle 115°) was pressed into the fiber cross section as shown in Figure 5, and the displacement-load curve of the indenter was measured. Obtained. In region I of the displacement-load curve, the indenter 60 is pushed into the fiber 10, and in region II the fiber 10 is pulled out of the matrix 40. Thereafter, in region III, the ridge line of indenter 60 is pushed into matrix 40 to the maximum set load, and then the load is unloaded in region IV. Here, the interfacial shear strength τ necessary for pulling out the fiber 10 from the matrix 40 is calculated by the following formula.

Formula 1

Here, F is the load required for fiber pulling occurring in region II of the displacement-load curve shown in FIG. 5, r is the radius of the fiber 10, and t is the sample thickness.

FIG. 6 shows the dependence of the interfacial shear strength between the fibers 10 and the matrix 40 on the thickness of the BN coating layer before matrix formation by CVI. When the thickness of the BN coating layer became 130 nm or more, the interfacial shear strength decreased rapidly compared to the case without the BN coating, reached a minimum at a thickness of 230 nm, and increased again at a thickness beyond that. It was confirmed that the fiber pull-out property was good when the BN coating layer thickness was approximately 130 to 230 nm.

As described above, the SiC-based composite material 100 has the crystalline carbon layer 30 formed between the crystalline BN layer 20 formed on the surface of the SiC fiber 10 and the matrix 40 mainly composed of SiC. By doing so, it is possible to obtain a SiC-based composite material with high ductility and strength and good fiber pull-out property.

(Second embodiment)
Next, as a second embodiment of the present invention, a BN-coated SiC fiber 200 that can easily produce the SiC-based composite material 100 of the first embodiment, that is, a SiC-based composite material with high ductility and strength, is easily produced. A possible BN-coated SiC fiber 200 and a method for manufacturing the SiC-based composite material 100 using the same will be described.

As shown in FIG. 7, the BN-coated SiC fiber 200 has a first BN coating layer (1) 202 (hereinafter referred to as BN layer (1)) which is the outermost layer on the fiber 201 whose main component is SiC. and a second BN coating layer (2) 203 (hereinafter sometimes referred to as BN layer (2)) inside the BN coating layer (1) 202. ing. Further, it is preferable that the amount of oxygen contained in the BN layer (1) 202 is higher than the amount of oxygen contained in the BN layer (2) 203. Further, it is preferable that the crystallinity of the BN layer (1) 202 is lower than the crystallinity of the BN coating layer of the BN layer (2) 203.

A cross-sectional TEM image of the BN-coated SiC fiber 200 is shown in FIG. 8(a). In the high-resolution TEM image (b) of the BN coating layer (1), lattice fringes appear here and there in the areas indicated by arrows, but most of them are amorphous. On the other hand, in the high-resolution TEM image of (c) the BN coating layer (2), it exhibits a highly crystalline turbostratic structure (dotted line in the figure) as described above. Furthermore, the EDX composition analysis results shown in Table 2 below show that the oxygen content of the BN coating layer (1) is about four times higher than that of the BN coating layer (2).

Furthermore, such a BN coating layer (1) and a BN coating layer (2) can be formed using, for example, a continuous film forming apparatus 301 as shown in FIG. After the fiber bundle wound around the bobbin is unwound from the unwinding chamber 302, the sizing agent adhering to the fibers is removed by heating in a sizing removal chamber 303 in which a heating heater 308 is installed. Thereafter, while the fiber bundle passes through a film forming chamber 304 comprising a CVD (Chemical Vapor Deposition) furnace 309, the fiber surface is coated with a BN layer, and the fiber bundle is wound around a bobbin in a winding chamber 305. Note that the CVD process gas is a B ₂ H ₆ /NH ₃ /H ₂ mixed gas, a BCl ₃ /NH ₃ /Ar mixed gas, etc., and is supplied from the process gas inlet 307 and is not consumed in the film forming chamber 304. The remaining gas is discharged from the furnace through the exhaust port 306.

Here, the mechanism by which the BN coating layer becomes a two-layer structure consisting of the first BN coating layer (1) 202 and the second BN coating layer (2) 203 will be considered. The crystallinity of the BN film depends on the film formation temperature, and the lower the film formation temperature, the lower the crystallinity. The BN coating layer (2) 203 with high crystallinity is formed on the fiber surface while passing through the CVD furnace 309 of the film forming chamber 304 in the continuous film forming apparatus 301, and the BN coating layer (2) with low crystallinity ( 1) It is thought that 202 is formed outside the CVD furnace 309 where the temperature is lower than inside the furnace (between the end of the CVD furnace 309 on the process gas introduction side and the process gas introduction port 307).

By using the BN-covered SiC fiber 200 described above, the SiC-based composite material described in the first embodiment can be manufactured, for example, as follows.
First, 200 bundles of BN-coated SiC fibers produced by the film-forming method described above are woven to produce a two-dimensional or three-dimensional fabric. Next, a matrix containing SiC as a main component is formed between the BN-covered SiC fibers 200 constituting the fabric.

An example in which a CVI (Chemical Vapor Infiltration) method is used as a method for forming a matrix containing SiC as a main component will be described below. A batch type CVI furnace is used, and a mixed gas of SiCl ₄ /CH ₄ (C ₃ H ₈ )/H ₂ is used as the process gas. In the case of a two-dimensional fabric, CVI is performed with the fabric sheet laminate partially temporarily fixed.

Alternatively, after forming a certain amount of matrix by CVI, the remaining SiC matrix may be formed by PIP (Polymer Impregnation and Pyrolysis), which forms a SiC matrix by thermal decomposition of polycarbosilane, which is a SiC precursor.

By such a manufacturing method of the BN-coated SiC fiber 200 and the SiC matrix composite material, when forming the SiC matrix 40 by CVI, a matrix mainly composed of SiC, the SiC matrix 40 or the process gas and the oxygen content are It is considered that the highly crystalline carbon layer 30 is formed by the reaction with the high BN coating layer (1) 202.

To explain this reaction in detail, a chemical reaction between the SiC matrix 40 or the CVI process gas and oxygen contained in the BN coating layer (1) 202 forms the B-Si-O glass and the highly crystalline carbon layer 30. It is thought that it will be done. It is presumed that the B--Si--O glass was halogenated and volatilized by reaction with HCl, which is a thermal decomposition product of the process gas, during CVI, leaving only the highly crystalline carbon layer 30.

As described above, by using the BN-covered SiC fiber 200 and the above manufacturing method, the SiC-based composite material 100 having the crystalline BN layer 20 and the crystalline carbon layer 30 can be easily manufactured. That is, a SiC-based composite material with high ductility and strength can be easily produced.

Note that, in the BN-covered SiC fiber 200, the crystallinity of the BN layer (1) 202 is preferably lower than the crystallinity of the BN layer (2) 203. The lower the crystallinity, the easier the BN layer will take in more oxygen. By controlling the crystallinity of the BN layer in this way, the BN layer (1) 202 and the BN layer (2) 203 with different oxygen contents can be easily produced, making it possible to create a SiC-based composite material with high ductility and strength. can be easily produced.

Further, it is preferable that the amount of oxygen contained in the BN layer (1) 202 in the BN-coated SiC fiber 200 is 3 atomic percent or more. At an oxygen content of less than 3 atomic percent, no crystalline carbon layer 30 is formed after matrix formation by CVI.

(Example)
A SiC-based composite material 100 was produced based on the second embodiment, and its bending properties and tensile properties at room temperature were evaluated.

(Example 1)
The sample was a three-dimensional fabric (width 70 mm x length 70 mm x thickness 4 mm, XY direction fiber arrangement pitch: 3 mm, fiber orientation ratio X:Y:Z = 1:1.1:0.2, fiber volume ratio = 29%), and a SiC matrix 20 was formed to obtain a SiC-based composite material 100. Note that the BN layer (1) 202 and the BN layer (2) 203 of the BN-coated SiC fiber 200 were formed by the continuous film forming apparatus 301 described above. The thickness of the entire BN layer was set to about 400 nm or less, and the relationship between the thickness of the entire BN layer, bending properties, and tensile properties was investigated.

(Comparative example)
Further, as a comparative sample, a SiC-based composite material 400 consisting of SiC fibers and a SiC matrix 40 coated with only the BN layer (2) 203 with high crystallinity and low oxygen content was produced. Here, since only the BN layer (2) 203 with high crystallinity and low oxygen content is formed on the SiC fiber surface, the formation of the BN layer (1) 202 with low crystallinity and high oxygen content is prevented. did. Specifically, as shown in FIG. 9, a protective tube 310 is provided between the end of the CVD furnace 309 on the process gas introduction side and the process gas introduction port 307 to prevent the fibers from being exposed to the process gas. The film was formed using Next, fiber bundles made of SiC fibers coated with only a BN layer with high crystallinity and low oxygen content are woven into a three-dimensional fabric (width 70 mm x length 70 mm x thickness 4 mm, fiber arrangement pitch in the XY direction: 3 mm, fiber orientation ratio X:Y:Z = 1:1.1:0.2, fiber volume ratio = 29%), and a SiC matrix 20 was formed between the fiber bundles and in the gaps between the fibers by the CVI method, It was set as SiC-based composite material 400. The thickness of the entire BN layer was set to about 200 nm or less, and the relationship between the thickness of the entire BN layer, bending properties, and tensile properties was investigated.

(Observation and composition evaluation of BN-coated fibers prepared for comparison samples)
FIG. 10 shows a cross-sectional TEM image and EDX component analysis results of the SiC fiber coated with only the BN layer (2) 203 used in comparative sample 400. A clear lattice image appears in the (c) high-resolution TEM image of the 203 region of the BN layer (2), and Debye rings consisting of minute spots also appear in the selected area electron diffraction pattern of the BN layer (b). It was confirmed that the crystallinity was high. Further, as shown in Table 3, it was confirmed from the EDX component analysis results of the BN layer (2) 203 region that the oxygen content was sufficiently low at less than 3 atomic percent.

(Observation and composition evaluation of comparative samples)
FIG. 11 shows a TEM image of the SiC fiber 10/crystalline BN layer 20/SiC matrix 40 interface in comparative sample 400. Further, the results of EDX component analysis are shown in Table 4. (c) A clear lattice image appears in the high-resolution TEM image of the crystalline BN layer 20 region, and (b) Debye rings consisting of minute spots also appear in the selected area electron diffraction pattern of the crystalline BN layer 20. This fact confirmed that high crystallinity was maintained even after matrix formation. On the other hand, the presence of a crystalline carbon layer as shown in FIG. 2 was not observed between the SiC matrix 40 and the crystalline BN layer 20.

(Room temperature bending property evaluation)
The SiC-based composite material 100 of the present invention and the comparative sample 400 were evaluated in accordance with the test standard ASTM C1341-13 using SHIMADZU AG-IS 10KN. The room temperature bending stress-displacement curve is shown in FIG. Note that the total thickness of the BN layer before forming the SiC matrix was approximately 200 nm. It was confirmed that the SiC-based composite material 100 of the present invention had a bending strength at break of 533 MPa, which was higher than that of the comparative sample 400, which was 297 MPa. Furthermore, SEM images of the fracture surfaces of each composite material after the bending test are shown in FIG. On the fracture surface (a) of the SiC-based composite material 100 of the present invention, a large number of "pulling out fibers", which is evidence of slipping between the material and the SiC matrix 30, was confirmed. This slippage confirmed that the SiC-based composite material 100 had high strength and elongation. On the other hand, in the fracture surface (b) of the comparative sample, "pulling out of fibers" did not occur much. In particular, almost no fiber pull-out was observed in the area surrounded by the dotted line.

(Room temperature tensile property evaluation)
The SiC-based composite material 100 of the present invention and the comparative sample 400 were evaluated using INSTRON 5982 according to the test standard ASTM C1275-16. The room temperature tensile stress-strain curve is shown in FIG. The total thickness of the BN layer before forming the SiC matrix was approximately 200 nm, and the test piece size was 70 mm x 10 mm x 1 mm. The SiC-based composite material 100 of the present invention has a tensile strength of 249 MPa and a strain at break of 0.5%, which is higher than that of the comparative sample, which has a tensile strength of 55 MPa and a strain at break of 0.04%. confirmed. Furthermore, SEM images of the fracture surfaces of each sample after the tensile test are shown in FIG. On the fracture surface (a) of the SiC-based composite material 100 of the present invention, a large number of "pulling out fibers", which is evidence of slipping between the material and the SiC matrix 30, was confirmed. This slippage confirmed that the SiC-based composite material 100 had high strength and elongation. On the other hand, in the fracture surface (b) of the comparative sample, "pulling out of fibers" did not occur much.

(Relationship between BN film thickness and room temperature mechanical properties)
Table 5 summarizes the relationship between the overall thickness of the BN layer before forming the SiC matrix, the interfacial shear strength between the fiber and the matrix, the bending strength, and the tensile properties in the SiC-based composite materials of the present invention and comparative samples. In the SiC-based composite material of the present invention, when the BN film thickness was about 230 nm, the interfacial shear strength between the fibers and the matrix became the minimum, and the bending strength and tensile strength became the maximum.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and can be modified within the technical scope of the claims.

202: First BN layer 203: Second BN layer 10, 201: SiC fiber 20: Crystalline BN layer 30: Crystalline carbon layer 40: SiC matrix 50: Interface layer 60: Nanoindenter 100: SiC base composite Material 200: BN-coated SiC fiber 400: Comparative sample SiC-based composite material 301: Continuous film forming apparatus 302: Unwinding chamber 303: Sizing removal chamber 304: Film forming chamber 305: Winding chamber 306: Exhaust port 307: Process Gas inlet 308: Heater 309: CVD furnace

Claims (5)

A first BN coating layer serving as the outermost layer and a second BN coating layer inside the first BN coating layer are provided on the surface of the fiber mainly composed of SiC, and the first BN coating layer contains A BN-coated SiC fiber characterized in that the amount of oxygen contained in the second BN coating layer is higher than the amount of oxygen contained in the second BN coating layer. The BN-coated SiC fiber according to claim 1 , wherein the first BN coating layer has a lower crystallinity than the second BN coating layer. 3. The BN-coated SiC fiber according to claim 1 , wherein the first BN coating layer contains at least 3 atomic percent of oxygen. Forming a crystalline carbon layer between the step of producing the BN-coated SiC fiber according to any one of claims 1 to 3 and forming a matrix mainly composed of SiC on the BN-coated SiC fiber. The process of
A method for producing a SiC-based composite material, comprising: 5. The method for producing a SiC-based composite material according to claim 4 , wherein the method for forming the matrix containing SiC as a main component is a chemical vapor impregnation (CVI) method.