JP2021179041A - Silicon carbide fiber and manufacturing method thereof - Google Patents

Abstract

To provide a silicon carbide fiber having excellent creep resistance and a manufacturing method thereof.SOLUTION: A silicon carbide fiber 1 satisfies, when observing its cross section, a relation of (d1ave/d2ave)<0.90 between d1ave, which is an average value of particle diameters of silicon carbide crystal 11 in a region 0.5 μm from its surface toward its center (surface layer region) and d2ave, which is an average value of particle diameters of silicon carbide crystal 13 in a circular area of 0.4 μm in radius from the center. Preferably, a surface of the silicon carbide fiber 1 includes nitrogen atom-containing silicon carbide wherein some of carbon atoms in silicon carbide are replaced by nitrogen atoms.SELECTED DRAWING: Figure 1

Description

The present invention relates to a silicon carbide fiber having excellent creep resistance and a method for producing the same.

Conventionally, a ceramic matrix composite material (CMC), which contains a reinforcing material made of ceramic fiber or carbon fiber and whose density is increased by a matrix made of ceramic, is lighter than a heat-resistant metal material and is lighter than a monolithic ceramic material. It is known that the destructive energy is large. As a reinforcing material, a fiber-reinforced SiC / SiC composite material using silicon carbide fiber, which is lightweight, has high strength, and has excellent heat resistance, is used as a structural member for an aerospace engine such as a combustor in an aircraft jet engine. It is applied to high-temperature parts in gas turbines for power generation and other high-temperature parts in various plants. The following techniques are known as such silicon carbide fibers.

Patent Document 1 discloses a silicon carbide fiber in which silicon carbide constitutes a portion of the fiber in an amount of more than 90% by weight and is substantially homogeneous over the front surface of the cross section of the fiber.
Further, in Patent Document 2, the density is 2.7 to 3.2 g / cm ³ , Si: 50 to 70% by weight, C: 28 to 45% by weight, Al: 0.05 to 3.8% by weight. , O: 0.05 to 1.5% by weight, and B: 0.05 to 0.5% by weight, which is a crystalline silicon carbide-based fiber having a SiC sintered structure and has a surplus C in the fiber cross section. A crystalline silicon carbide-based ceramic fiber having a surplus C occupancy rate of 0.05% to 10%, which is indicated by the area ratio of the above, is disclosed.

Japanese Unexamined Patent Publication No. 2-133617 Japanese Unexamined Patent Publication No. 2016-188439

J. J. In the literature of Sha et al. (See Journal of Natural Materials 329-333 (2004) 592-596), "when the silicon carbide fiber is exposed to a high temperature uniform environment such as, for example, 1800 ° C., as shown in FIG. 13, for example. , Silicon carbide crystals 7 are obtained in which the silicon carbide crystals are uniformly grown and the pores (defects) inherent in the crystals are also grown in proportion to the crystal grain size. As a result, coarse defects near the fiber surface act as a fracture starting point. Therefore, the fiber strength (tensile strength) is significantly reduced. On the other hand, the coarsening of the crystal particles is effective in suppressing high-temperature deformation due to grain boundary slip, and the stress is smaller than the fiber strength (tensile strength). The creep resistance is improved in the region. " From this, in order to suppress the decrease in fiber strength (tensile strength) as much as possible and improve creep resistance, the silicon carbide crystal near the fiber surface is kept small and the silicon carbide crystal inside the fiber is enlarged. Silicon carbide fibers with different specific structures are considered to be effective.
An object of the present invention is to provide a silicon carbide fiber having excellent creep resistance, a method for producing the same, and a composite material containing the silicon carbide fiber.

The present invention is shown below.
1. 1. When looking at the cross section of the silicon carbide fiber containing silicon carbide, the particle size of the silicon carbide crystal in the region from the surface to the center of the silicon carbide fiber up to 0.5 μm (hereinafter, also referred to as “surface layer region”). Between the average value d1 _{ave and the average value d2 ave} of the particle size of the silicon carbide crystal in the circular region (hereinafter, also referred to as “central region”) having a radius of 0.4 μm from the center of the silicon carbide fiber (d1 _ave /). A silicon carbide fiber having a relationship of d2 _{ave) <0.90.}
2. 2. Item 2. The silicon carbide fiber according to Item 1, wherein the average value d1 _{ave of the particle size is 35 nm or less.}
3. 3. Item 2. The silicon carbide fiber according to Item 1 or 2, wherein the surface of the silicon carbide fiber contains a nitrogen atom-containing silicon carbide in which a part of carbon atoms in the silicon carbide is replaced with a nitrogen atom.
4. Item 3. The silicon carbide fiber according to Item 3, wherein the nitrogen atom content is 0.1 to 5 atomic%.
5. Item 2. The silicon carbide fiber according to any one of Items 1 to 4, wherein the silicon carbide is β-SiC and the content ratio of the β-SiC is 50% by volume or more with respect to the total of the silicon carbide fibers. ..
6. A silicon carbide fiber comprising a nitrogen atom-containing silicon carbide in which a part of carbon atoms in silicon carbide is replaced with a nitrogen atom on the surface.
7. The method for producing a silicon carbide fiber according to any one of Items 1 to 6 above, which comprises a modification step of irradiating the fiber containing silicon carbide with a laser under a nitrogen atmosphere at 0 ° C. or lower. A characteristic method for producing silicon carbide fibers.
8. A composite material comprising the silicon carbide fiber according to any one of Items 1 to 6 above.

According to the silicon carbide fiber of the present invention, since the average particle size d1 _ave of the silicon carbide crystal in the surface layer region _{is smaller than the average particle size d2 ave} of the silicon carbide crystal in the surface layer region, the creep resistance is excellent. When the silicon carbide fiber of the present invention is dispersed in a matrix made of silicon carbide ceramics to obtain a silicon carbide fiber reinforced composite material, sufficient strength and creep resistance are obtained at a high temperature of, for example, 1400 ° C. be able to. This is because at such a high temperature, the grain growth of the crystals constituting the silicon carbide fiber contained in the silicon carbide fiber reinforced composite material is suppressed.
According to the method for producing a silicon carbide fiber of the present invention, a silicon carbide fiber having excellent creep resistance can be efficiently produced.
The composite material of the present invention can be, for example, a silicon carbide fiber reinforced composite material in which the silicon carbide fibers of the present invention are dispersed in a matrix made of ceramics, and is contained in a matrix made of silicon carbide ceramics. When dispersed to form a silicon carbide fiber reinforced composite material, it is suitable as a raw material for articles having excellent durability in terms of structural stability, strength, creep resistance and the like at high temperatures such as 1400 ° C.

It is a schematic diagram which shows the cross-sectional structure of the silicon carbide fiber of this invention. It is a schematic diagram which shows the silicon carbide fiber manufacturing apparatus used in an Example. It is a schematic diagram which shows the cross-sectional structure of the silicon carbide fiber before and after the production in the silicon carbide fiber production method of this invention. It is a graph which shows the particle size distribution of the silicon carbide crystal in the surface layer region of the silicon carbide fiber (before treatment) used in [Example]. It is a graph which shows the particle size distribution of the silicon carbide crystal in the central region of the silicon carbide fiber (before treatment) used in [Example]. It is a graph which shows the particle size distribution of the silicon carbide crystal in the surface layer region of the silicon carbide fiber obtained in Example 1. FIG. It is a graph which shows the particle size distribution of the silicon carbide crystal in the central region of the silicon carbide fiber obtained in Example 1. FIG. It is a graph which shows the particle size distribution of the silicon carbide crystal in the surface layer region of the silicon carbide fiber obtained in Example 2. FIG. It is a graph which shows the particle size distribution of the silicon carbide crystal in the central region of the silicon carbide fiber obtained in Example 2. FIG. It is a graph which shows the particle size distribution of the silicon carbide crystal in the surface layer region of the silicon carbide fiber obtained in the comparative example 1. FIG. It is a graph which shows the particle size distribution of the silicon carbide crystal in the central region of the silicon carbide fiber obtained in the comparative example 1. FIG. It is a schematic diagram which shows the test piece used for measuring the tensile strength of a silicon carbide fiber. It is a schematic diagram which shows the cross-sectional structure of the silicon carbide fiber before and after the heat treatment when the conventional method is applied.

In FIGS. 1, 3 and 13, since substantially circular crystals are drawn side by side, fibers having voids between the crystals are shown, but in the actual fibers, the crystals are densely arranged inside. It is a fiber with no voids.

The silicon carbide fiber of the present invention is generally a fiber having a circular (including substantially circular) cross section and a diameter (fiber diameter) of 10 to 15 μm, and has, for example, the cross-sectional structure shown in FIG. _{In the present invention, the average value d1 ave} of the particle size d1 of the silicon carbide crystal in the region (surface layer region) from the surface to the center of the fiber to 0.5 μm, and the circular region having a radius of 0.4 μm from the center of the fiber. The following formula (1) is satisfied between _{the average value d2 ave} of the particle size d2 of the silicon carbide crystal in (central region). The measuring method of the average value d1 _ave and the average value d2 _ave is shown in [Example].
(D1 _ave / d2 _ave ) <0.90 (1)

The particle size d1 of the silicon carbide crystal in the surface layer region is preferably 1 to 100 nm, more preferably 2 to 50 nm. The average particle size d1 _ave is preferably 35 nm or less, more preferably 20 to 30 nm.
The particle size d2 of the silicon carbide crystal in the central region is d2> d1, preferably 2 to 200 nm, and more preferably 5 to 100 nm. The average particle size d2 _ave is preferably 25 nm or more, more preferably 25 to 40 nm.
As described above, the silicon carbide fiber of the present invention has a structure in which the silicon carbide crystal in the surface layer region is smaller than the silicon carbide crystal in the surface layer region. The silicon carbide fiber of the present invention tends to be slightly inferior in tensile strength as compared with the silicon carbide fiber in which the size of the silicon carbide crystal is substantially uniform as a whole and the particle size of the silicon carbide crystal is the same as the particle size d1. However, better creep resistance can be obtained. _{The d1 ave} / d2 _ave exhibiting such performance is preferably 0.5 to 0.9, and more preferably 0.7 to 0.9.

The constituent material of the silicon carbide fiber of the present invention is not particularly limited as long as it is at least 50% by volume of silicon carbide with respect to the whole. That is, the silicon carbide fiber of the present invention may be composed of only silicon carbide, or may be composed of silicon carbide and other components. Examples of other components include carbon, boron, Al, Ti, Zr and the like.
As the silicon carbide, α-SiC and β-SiC are known, but β-SiC is particularly preferable from the viewpoint of exhibiting the flexibility of the fiber. When the silicon carbide fiber of the present invention is mainly composed of β-SiC, the lower limit of the content ratio is preferably 50% by volume, more preferably 80% by volume.

It is preferable that at least a part of the surface of the silicon carbide fiber of the present invention contains nitrogen atom-containing silicon carbide in which some carbon atoms in silicon carbide are replaced with nitrogen atoms. That is, the surface layer region preferably contains silicon carbide containing a nitrogen atom.
When the silicon carbide fiber of the present invention contains a nitrogen atom-containing silicon carbide, the content ratio of the nitrogen atom derived from the nitrogen atom-containing silicon carbide is preferably 0.1 atom% or more, more preferably 0.1 atom% or more, based on the whole fiber. It is 0.1 to 5 atomic%.

The other silicon carbide fiber 1 of the present invention is a fiber having a nitrogen atom-containing silicon carbide 3 on the surface in which a part of carbon atoms in silicon carbide is replaced with a nitrogen atom (see FIG. 1).
_{When the cross section of the silicon carbide fiber is viewed, the average value d1 ave} of the particle size of the silicon carbide crystal in the region from the surface of the fiber to 0.5 μm toward the center and the radius from the center of the fiber are 0. It is preferable to have a relationship of (d1 _ave / d2 _{ave ) <0.90 between the average value d2 ave} of the particle size of the silicon carbide crystal in the circular region of 4 μm.

When a ceramic matrix composite material or a member containing the ceramic matrix composite material is manufactured using a silicon carbide fiber having a surface containing silicon carbide containing a nitrogen atom, for example, even if it is treated at a temperature of 1200 ° C to 1400 ° C, the surface of the fiber can be treated. The presence of nitrogen atoms can suppress the grain growth of silicon carbide crystals. Further, even when the member containing the ceramic matrix composite material is used under the above-mentioned high temperature conditions, it is possible to suppress the grain growth of crystals in the contained silicon carbide fibers.

In the present invention, the method for producing a silicon carbide fiber satisfying the above formula (1) or a fiber having a nitrogen atom-containing silicon carbide on which a part of carbon atoms in the silicon carbide is replaced with a nitrogen atom is 0 ° C. It is provided with a modification step of irradiating a fiber containing silicon carbide with a laser under the following nitrogen atmosphere.

In the production method of the present invention, the article to be irradiated with the laser is a fiber containing silicon carbide, which may be a single fiber or a fiber bundle (hereinafter, these are referred to as "silicon carbide fiber raw materials"). ). The constituent material of the fiber is preferably silicon carbide in an amount of at least 50% by volume based on the whole. That is, the fiber may be composed of only silicon carbide, or may be composed of silicon carbide and other components. A particularly preferred silicon carbide is β-SiC. The fiber may be a fiber having a core portion mainly composed of silicon carbide crystals and a coating portion to which an inorganic substance is further attached to a part of the surface of the core portion. This inorganic substance is not particularly limited, and may consist of, for example, boron nitride, carbon, a silicate containing a rare earth element, or the like. Further, the adhered form of the inorganic substance can be, for example, a film, a plate, a granular substance, or the like.

In the reforming step according to the present invention, the silicon carbide fiber raw material is irradiated with a laser under a nitrogen atmosphere at 0 ° C. or lower. The laser used is not particularly limited, and it is preferable to use a laser having a wavelength that does not absorb nitrogen, for example, a laser having a wavelength of 0.65 to 2 μm. In the present invention, a fiber laser, an Nd: YAG laser, an Nd: YVO laser, an Nd: YLF laser, a titanium sapphire laser and the like can be used.
The output of the laser is preferably 1000 to 3000 W / cm ² , more preferably 1500 to 2500 W / cm ² . Further, the irradiation time is not particularly limited. As will be described later, the silicon carbide fiber raw material can be irradiated with a laser while scanning. In this case, the sweep rate is preferably 0.5 to 5 mm / s, more preferably 1.0 to 3 mm / s. Is.

Further, the nitrogen atmosphere in the reforming step is formed by a gas or liquid having an extremely low oxygen partial pressure, and the nitrogen is a gas or a liquid. In the present invention, the upper limit of the temperature at the time of laser irradiation is 0 ° C., preferably −20 ° C., more preferably −180 ° C. In the present invention, liquid nitrogen is preferable, and it is particularly preferable to irradiate the laser with the silicon carbide fiber raw material immersed in the liquid nitrogen.

FIG. 2 is an explanatory diagram showing a typical example of the apparatus used in the manufacturing method of the present invention.
FIG. 2 is a schematic view of a manufacturing apparatus 10 that irradiates a silicon carbide fiber raw material with a laser 18 in a state where the silicon carbide fiber raw material (fiber 5 or fiber bundle 11) is immersed in liquid nitrogen 12. The irradiation method of the laser 18 is appropriately selected depending on the size of the silicon carbide fiber raw material and the like. For example, when modifying most of the silicon carbide fiber raw material, the laser is used with the silicon carbide fiber raw material fixed. It is preferable to apply a method of irradiating while scanning or changing the optical path through a light diffusing lens, or a method of irradiating a laser having a fixed optical path while moving the silicon carbide fiber raw material. In these methods, the laser may be irradiated while rotating the silicon carbide fiber raw material in the radial direction. The manufacturing apparatus 10 of FIG. 2 is configured to irradiate the laser 18 oscillated from the fiber laser 14 while scanning it through the galvanometer 15 with the silicon carbide fiber raw material fixed. It is not limited to the device.

As other manufacturing equipment (not shown), a chamber in which the silicon carbide fiber raw material is placed in a closed system, a cooling means for cooling the silicon carbide fiber raw material to 0 ° C. or lower in the chamber, and nitrogen for supplying nitrogen gas to the chamber. A device including a gas supply means and a laser irradiation means can also be used.

In the reforming step, since the silicon carbide fiber raw material is exposed to a nitrogen atmosphere of 0 ° C. or lower, the above-mentioned preferable wavelength, that is, a laser having a wavelength of 0.65 to 2.0 μm irradiates the silicon carbide fiber raw material. Then, the laser permeates through the nitrogen and instantly heats the inside of the silicon carbide fiber raw material to 1500 ° C. or higher (estimated). On the other hand, since the surface of the silicon carbide fiber raw material is exposed to a nitrogen atmosphere of 0 ° C. or lower, the temperature rise near the surface is suppressed, and as a result, the grain growth near the surface of the silicon carbide fiber raw material is higher than that inside. Is suppressed (see FIG. 3).

In the above-mentioned reforming step, when the silicon carbide fiber raw material is irradiated with a laser under a nitrogen atmosphere of 0 ° C. or lower, a fiber having a nitrogen atom-containing silicon carbide in which some carbon atoms in the silicon carbide are replaced with nitrogen atoms is provided on the surface. Can be manufactured. As the silicon carbide fiber raw material, a fiber having a core portion mainly composed of silicon carbide crystals and a coating portion having an inorganic substance such as boron nitride attached to a part of the surface of the core portion was subjected to a modification step. In some cases, it is possible to obtain a fiber having nitrogen atom-containing silicon carbide in which some carbon atoms in silicon carbide are replaced with nitrogen atoms on the surface of the exposed core portion in which the coating portion is not present. In this case, since the coating portion is rarely modified or altered by laser irradiation, the obtained fiber includes a portion constituting the silicon carbide fiber of the present invention and a coating portion on a part of the surface thereof. It can be a composite material of the invention.

The composite material of the present invention contains the above-mentioned silicon carbide fibers of the present invention, and a suitable example is a composite material in which the silicon carbide fibers are dispersed and contained in a matrix made of ceramics or a resin. The material constituting the matrix is particularly preferably ceramics, and may be silicon carbide, alumina, titanium carbide, vanadium carbide, chromium carbide, scandium carbide, zirconium carbide, niobium carbide, molybdenum carbide, hafnium carbide, tantalum carbide and the like. Can be done. The form of the silicon carbide fiber contained in the composite material of the present invention is not particularly limited, and may be either one in which the fibers are in contact with each other (woven fabric or the like) or one in which the fibers are in a non-contact state.
The content ratio of the silicon carbide fiber of the present invention in the composite material using the ceramic as a matrix is preferably 20 to 80 parts by mass, more preferably 30 to 60 parts by mass with respect to 100 parts by mass of the ceramics constituting the matrix. Is.

In the composite material using the ceramics as a matrix, when a woven fabric (plain weave, multiple weave, three-dimensional weave, etc.) is used to form a member having a predetermined shape, silicon carbide is filled in the voids between the fibers of the preform made of the woven fabric. A manufacturing method for forming is applied, and specific examples are shown below.
(1) A silane compound such as _{SiCl 4 and a hydrocarbon such as C 3} H ₈ are supplied into a preform as a raw material gas for producing silicon carbide, and a silicon carbide matrix is formed by a thermal decomposition reaction of these raw material gases. Method of forming (2) A method of impregnating a mixture of metallic silicon powder and carbon powder into a preform and then reacting them to form a silicon carbide matrix (3) Impregnating the preform with a silicon carbide precursor polymer. After that, a method of calcining and ceramicizing to form a silicon carbide matrix (4) After impregnating a preform with a mixture of silicon carbide powder and carbon powder, the silicon and carbon powder are injected by injecting molten silicon. (5) A method of impregnating a preform with silicon carbide powder and a sintering aid to prepare a preformed body, and then pressure-stituting to form a silicon carbide matrix. Method

In the present invention, as a composite material using ceramics as a matrix, for example, when the silicon carbide fiber of the present invention is dispersed in a matrix made of silicon carbide ceramics to obtain a silicon carbide fiber reinforced composite material, the temperature is about 1400 ° C. Structures of aerospace engine structural members such as combustors in aircraft jet engines, high-temperature parts in power generation gas turbines, and other high-temperature parts in various plants, which are required to have strength and toughness at high temperatures. Suitable as a material. When conventionally known silicon carbide fibers are dispersed in a matrix made of silicon carbide ceramics to obtain a silicon carbide fiber reinforced composite material, grain growth of crystals constituting the silicon carbide fibers occurs at the above-mentioned high temperature. Although it was remarkable and sufficient strength and creep resistance could not be obtained, when the composite material containing the silicon carbide fiber of the present invention was used, the grain growth of the crystals constituting the silicon carbide fiber was carried out at the above-mentioned high temperature. It is possible to obtain a member having excellent structural stability, strength and creep resistance.

Hereinafter, embodiments of the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.

1. 1. Silicon Carbide Fiber Raw Material The silicon carbide fiber raw material used in Examples and Comparative Examples is obtained by immersing silicon carbide continuous fiber "Hi-Nicalon TypeS" (trade name) manufactured by NGS Advanced Fiber Co., Ltd. in distilled water at 80 ° C. for 30 minutes. It is a dessized fiber bundle. This product is a fiber bundle having a length of 200 mm and consisting of 500 fibers made of β-SiC. The cross-sectional shape of each fiber is circular, and the diameter (fiber diameter) is 10 μm, but the fiber diameter after desizing is also substantially the same.

2. 2. Production and Evaluation of Modified Silicon Carbide Fiber The above-mentioned raw material of silicon carbide fiber was subjected to heat treatment to produce modified silicon carbide fiber, and the following evaluation was performed.
_{(1) Calculation of the average value d1 ave} of the particle size of the silicon carbide crystal in the surface layer region _{and the average value d2 ave and} (d1 _ave / d2 _ave ) of the particle size of the silicon carbide crystal in the central region in the cross section of the modified silicon carbide fiber. Cross sections of the modified silicon carbide fiber before and after production were photographed with a transmission electron microscope to obtain images containing silicon carbide crystals in the surface layer region and the central region. The crystal grain size measurement target in the surface layer region is a silicon carbide crystal in the region from the surface of the fiber to 0.5 μm toward the center, and the crystal grain size measurement target in the central region is a circle with a radius of 0.4 μm from the center of the fiber. Silicon carbide crystals in the region were used. The area was calculated for each of the observed silicon carbide crystals, and the particle size of one silicon carbide crystal was obtained by converting the area value into the diameter assuming that the silicon carbide crystal was circular. From the particle size of 100 or more silicon carbide crystals in each region, a graph showing the area ratio to the particle size is created, the average values d1 _ave and d2 _ave are calculated, and (d1 _ave / d2 _ave ) is calculated from these values. Calculated.
The measurement beginning of (2) tensile strength, relative to each measuring fiber one, was measured diameter (d _f) by a laser diffraction method. From the obtained d _f , the cross-sectional area (A _f ) of the fiber was calculated.
Next, as shown in FIG. 12, the upper and lower parts of the measurement fiber are fixed with an adhesive 22 so as to be arranged in the center of the cutout portion of the mount 21 in which the center is cut out, and then both ends are cut and pulled. A test piece 20 for strength measurement was prepared. This test piece 20 was set on an Instron "5940 single column tabletop tester" (trade name), and was tensilely fractured under the conditions of a constant displacement speed of 1 mm / min and a distance between gauge points of 25 mm to maximize the tensile load. The value (F _f ) was obtained.
From the above A _f and F _{f , the tensile strength (σ f} ) was calculated using the following equation (2).
σ _f = F _f / A _f (2)

FIG. 4 is a graph showing the area ratio with respect to the crystal grain size from the image including the silicon carbide crystals in the surface layer region and the central region in the cross section of the silicon carbide fiber 11x (before laser irradiation) taken out from the dessized fiber bundle 11. FIG. 5 was created and (d1 _ave / d2 _ave ) = 0.90 was obtained. The tensile strength was measured and found to be 4.16 GPa (see Table 1).

Example 1
For the production of the modified silicon carbide fiber, the modified silicon carbide fiber production apparatus 10 of FIG. 2 was used. In the device 10 of FIG. 2, the dessized ^{fiber bundle 11 and the liquid nitrogen 12 are placed in a glass container having an internal volume of 1200 cm 3} , and a laser 18 oscillated from a fiber laser 14 manufactured by SPI Lasers is used as a galvanometer 15. It is a device that irradiates and heat-treats the fiber bundle 11 through the fiber bundle 11. The temperature of the fiber bundle at the time of laser irradiation was measured with a radiation thermometer 16 manufactured by Chino Corporation.
In the configuration of FIG. 2, a 60 mm long portion of the fiber bundle 11 ^{was irradiated with a laser (wavelength: 1070 ± 10 nm, output: 2000 W / cm 2} ) while being swept at 1.1 mm per second. The number of sweeps was one. Further, the temperature of the laser irradiation portion in the fiber bundle 11 was 1800 ° C.

Modified silicon carbide fibers (fibers mainly composed of β-SiC) were taken out from the fiber bundle after laser irradiation, and the above (1) and (2) were evaluated. The results are shown in Table 1. Graphs showing the area ratio with respect to the crystal grain size obtained from the images containing the silicon carbide crystals in the surface layer region and the central region are shown in FIGS. 6 and 7, respectively.

Example 2
A modified silicon carbide fiber (a fiber mainly composed of β-SiC) was obtained by performing the same operation as in Example 1 except that the number of times of sweeping the laser was set to 5. Table 1 shows the results obtained by performing the evaluations of (1) and (2) above. Graphs showing the area ratio with respect to the crystal grain size obtained from the images containing the silicon carbide crystals in the surface layer region and the central region are shown in FIGS. 8 and 9, respectively.

Comparative Example 1
The dessized fiber bundle is placed in a carbon heater furnace, the inside is an argon gas atmosphere, the total pressure is 1 atm, and the temperature is 30 ° C / min from room temperature to 1700 ° C, and 10 from 1700 ° C to 1800 ° C. The heating was performed at each heating rate of ° C./min, and after reaching 1800 ° C., the heating was stopped. Table 1 shows the results obtained by evaluating the above-mentioned (1) and (2) with respect to the obtained modified silicon carbide fiber (fiber mainly composed of β-SiC). Graphs showing the area ratio with respect to the crystal grain size obtained from the images containing the silicon carbide crystals in the surface layer region and the central region are shown in FIGS. 10 and 11, respectively.

From the above experiments, the modified silicon carbide fiber obtained in Comparative Example 1 had a tensile strength of 3.02 GPa, and the tensile strength of the untreated fiber was significantly reduced from the tensile strength (4.16 GPa). On the other hand, the tensile strengths of the modified silicon carbide fibers obtained in Examples 1 and 2 were 3.92 GPa and 3.47 GPa, respectively, and the decrease in strength was suppressed.

Since the silicon carbide fiber of the present invention is lightweight, has high strength, and has excellent heat resistance, it is suitable as a raw material for producing a composite material in which the silicon carbide fiber is dispersed and contained in a matrix made of ceramics or a resin. In particular, composite materials dispersed in a matrix made of ceramics include structural members for aerospace engines such as combustors in jet engines for aircraft, high-temperature parts in gas turbines for power generation, and other high-temperature parts in various plants. Suitable as a structural material.

1: Silicon carbide fiber 2: Silicon carbide crystal in the surface layer region 3: Silicon carbide crystal containing nitrogen atom 4: Silicon carbide crystal in the central region 5: Silicon carbide fiber raw material (fiber)
7: Uniformly heated silicon carbide fiber 10: Silicon carbide fiber manufacturing apparatus 11: Silicon carbide fiber raw material (resizing fiber bundle)
11x: Silicon carbide fiber raw material (dessized fiber)
12: Liquid nitrogen 13: Glass container 14: Fiber laser 15: Galvanometer 16: Radiation thermometer 18: Laser 20: Test piece for measuring tensile strength 21: Mount 22: Adhesive

Claims (8)

_{When looking at the cross section of the silicon carbide fiber containing silicon carbide, the average value d1 ave} of the particle size of the silicon carbide crystal in the region from the surface to the center of the silicon carbide fiber and the silicon carbide. A silicon carbide fiber having a relationship of (d1 _ave / d2 _{ave ) <0.90 between the average value d2 ave} of the particle size of the silicon carbide crystal in a circular region having a radius of 0.4 μm from the center of the fiber. The silicon carbide fiber according to claim 1, wherein the average value d1 _{ave is 35 nm or less.} The silicon carbide fiber according to claim 1 or 2, wherein the surface of the silicon carbide fiber contains a nitrogen atom-containing silicon carbide in which some carbon atoms in the silicon carbide are replaced with nitrogen atoms. The silicon carbide fiber according to claim 3, wherein the nitrogen atom content is 0.1 to 5 atomic%. The silicon carbide fiber according to any one of claims 1 to 4, wherein the silicon carbide is β-SiC and the content ratio of the β-SiC is 50% by volume or more with respect to the whole of the silicon carbide fiber. .. A silicon carbide fiber comprising a nitrogen atom-containing silicon carbide in which a part of carbon atoms in silicon carbide is replaced with a nitrogen atom on the surface. The method for producing a silicon carbide fiber according to any one of claims 1 to 6, further comprising a modification step of irradiating the fiber containing silicon carbide with a laser under a nitrogen atmosphere at 0 ° C. or lower. A characteristic method for producing silicon carbide fibers. A composite material comprising the silicon carbide fiber according to any one of claims 1 to 6.

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