Method for producing fiber-reinforced composite material

Abstract

This method for producing a fiber-reinforced composite material involves: introducing, into an atmosphere-controllable reaction furnace, reinforcing fiber fabric including reinforcing fibers interwoven with each other and including voids having an aspect ratio of 35 or more between the reinforcing fibers; flowing a raw material gas including hydrogen and methyltrichlorosilane into the reaction furnace; and heating the reaction furnace so as to keep at least the reinforcing fiber fabric at a temperature exceeding 1000°C but not exceeding 1100°C while maintaining the reaction furnace at a pressure lower than atmospheric pressure to cause a reaction to form a coating film including silicon carbide in the voids, thereby bonding the reinforcing fibers to each other with the coating film.

Description

Method for producing fiber-reinforced composite material

[0001] The following disclosure relates to a method for producing a fiber-reinforced composite material by forming a matrix in the narrow gaps between reinforcing fibers by vapor phase synthesis.

[0002] Although ceramics have high strength and heat resistance, many ceramics suffer from the drawback of being brittle. To overcome this brittleness, attempts have been made to combine ceramics as a matrix with reinforcing fibers made of inorganic materials such as SiC. For example, a fiber-reinforced composite can be produced by weaving a reinforcing fiber bundle consisting of several hundred reinforcing fibers into a fabric and filling the voids between the fibers with ceramics by chemical vapor infiltration (CVI).

[0003] The CVI method is expected to form a matrix in narrow voids compared to other methods, such as liquid phase impregnation (e.g., polymer melt impregnation pyrolysis (PIP)), solid phase impregnation (SPI), and melt impregnation (MI). However, since the matrix growth rate is particularly slow compared to other methods, production using the CVI method alone is inefficient. Attempts have been made to apply the CVI method to very narrow voids and other methods to wider voids.

[0004] Patent Documents 1 and 2 disclose related techniques.

[0005] International Patent Application Publication WO98 / 21163A1 International Patent Application Publication WO2015 / 129772A1

[0006] In order to increase the growth rate by the CVI method, various parameters have been investigated and the configuration of the reaction apparatus has been devised. Although these efforts have produced some success, if the improvement in growth rate is traded off for insufficient impregnation into narrow voids, the advantages of the CVI method may ultimately be lost. The technology disclosed below has been devised to overcome this problem.

[0007] A method for producing a fiber-reinforced composite material according to the present disclosure includes introducing a reinforcing fiber fabric, which includes reinforcing fibers woven together and which includes voids between the reinforcing fibers, the voids having an aspect ratio of 35 or greater, into a reactor capable of controlling the atmosphere; flowing a raw material gas containing hydrogen and methyltrichlorosilane into the reactor; and, while maintaining the reactor at a pressure reduced from atmospheric pressure, heating the reactor so as to maintain at least the reinforcing fiber fabric at a temperature exceeding 1000°C and not exceeding 1100°C, thereby causing a reaction that produces a coating made of silicon carbide within the voids, thereby bonding the reinforcing fibers to each other by the coating.

[0008] A method is provided that can form a matrix of silicon carbide with high efficiency even when the voids are relatively narrow.

[0009] FIG. 1 is a schematic perspective view of a reinforcing fiber bundle formed by bundling a plurality of reinforcing fibers. FIG. 2 is a schematic perspective view of a woven fabric made of reinforcing fiber bundles. FIG. 3 is a diagram schematically illustrating an apparatus for producing a fiber-reinforced composite material by vapor-phase synthesis. FIG. 4 is a schematic cross-sectional view of a substrate having a void and a coating formed in the void. FIG. 5 is an SEM image of a cross section of a silicon carbide coating formed in a void at 1050°C. FIG. 6 is an SEM image of a cross section of a silicon carbide coating formed in a void at 1075°C. FIG. 7 is an SEM image of a cross section of a silicon carbide coating formed in a void at 1100°C. FIG. 8 is a graph showing the temperature dependence of the growth rate. FIG. 9 is a graph showing the temperature dependence of the growth rate resolved for each film-forming species. FIG. 10 is a graph showing the temperature dependence of the ratio of the growth rate to the growth rate.

[0010] Some exemplary embodiments are described below with reference to the accompanying drawings.

[0011] Referring to FIG. 1, reinforcing fibers 1 are long fibers made of an inorganic material such as silicon carbide and having a diameter of several μm to several tens of μm. Typically, several hundred reinforcing fibers 1 are bound together using an appropriate sizing (bundling) agent to form fiber bundles 3. Instead of or in addition to a sizing agent, the reinforcing fibers 1 may be bound together using other appropriate fibers. As illustrated in FIG. 2, multiple fiber bundles 3 are arranged vertically and horizontally and woven together so as to be intertwined with each other to form a reinforcing fiber fabric 7. In the example of FIG. 2, the fiber bundles 3 are stacked while remaining generally straight and are bound together by other fibers or fiber bundles 5 running in a direction penetrating them. However, the fiber bundles 3 themselves may meander and be intertwined with each other. In the example of FIG. 2, the fiber bundles 3 run either parallel or perpendicular to each other, but they may also intersect at an angle. Furthermore, the fabric 7 may be a two-dimensional fabric (not shown) or a three-dimensional fabric (as illustrated in FIG. 2). The woven fabric 7 is cut or molded into an appropriate shape to form a preform 100, which is then subjected to the matrix forming process described below.

[0012] As can be easily seen from Figures 1 and 2, the woven fabric 7 contains various voids. For example, the voids G1 between adjacent fibers 1 in individual fiber bundles 3 are very small in width, e.g., about 1 μm. Meanwhile, the length of the voids G1 is very long compared to the width, e.g., several mm or more. That is, the aspect ratio of the voids G1 can exceed 1,000. Meanwhile, the voids G3 between the fiber bundles 3 may be, for example, about 10 μm in width and have an aspect ratio of several tens to several hundreds. In areas where the fiber bundles 3 do not contact each other, the width of the voids G5 may be even larger and the aspect ratio may be even smaller. As will be described in more detail below, the manufacturing method according to this embodiment can be used to form a matrix in any of these voids, but is suitable for voids with a relatively high aspect ratio. That is, in a shape such as void G3, the relatively high aspect ratio makes it difficult to grow a matrix near the bottom of the void, but the relatively large width requires a high growth rate for the matrix to fill the void. This embodiment is particularly suitable for such gaps, and of course would also be suitable for gaps such as G1.

[0013] 3, the manufacturing apparatus for a fiber-reinforced composite material according to this embodiment generally includes a supply system 20 for supplying a raw material gas, and a reactor 30 for causing a gas-phase reaction while controlling the atmosphere. The raw material gas is supplied from the supply system 20 to the reactor 30 via a flow path 39, and is exhausted from the reactor 30 via an exhaust path 41.

[0014] The supply system 20 includes a source gas supply system and two or more auxiliary gas supply systems. The source gas supply system supplies a main source such as methyltrichlorosilane (MTS) and may include, for example, a tank 21a, a valve 21b, and a mass flow controller 21c. MTS is stored in a liquid state in the tank 21a and gradually vaporizes at room temperature or by appropriate heating. The vaporized MTS is supplied to the reaction chamber 33 at a controlled flow rate via the valve 21b and the mass flow controller 21c. The auxiliary gas supply systems are used to supply auxiliary sources or carrier gases such as hydrogen, hydrogen chloride, and helium. These gases are stored in compressed states in cylinders 22a, 23a, and 24a, respectively, and supplied to the reaction chamber 33 via valves 22b, 23b, and 24b and mass flow controllers 22c, 23c, and 24c. Needless to say, by adjusting these appropriately, the composition of the gas to be supplied can be adjusted.

[0015] The furnace 30 includes a reaction chamber 33 for accommodating the preforms 100 and a heater 35 disposed therein. The reaction chamber 33 may be, for example, a quartz tube with both ends open, but is not limited thereto. The reaction chamber 33 may be long enough along the gas flow direction to accommodate multiple preforms 100 simultaneously. The preforms 100 may be loaded and unloaded in batches, or may be processed sequentially by gradually transporting them from an inlet to an outlet. The heater 35 is an appropriate heating means, such as a carbon heater, and heats the gas flow and preforms 100 inside the reaction chamber 33 through the wall and controls their temperatures. The furnace 30 may include a thermocouple or other thermometer for temperature measurement. Needless to say, the heater 35 may be feedback-controlled based on the output of the thermometer.

[0016] A pump 45 is connected to the exhaust line 41, and the pressure inside the reaction furnace 30 can be adjusted by balancing the supply by the supply system 20 and the exhaust by the pump 45. A vacuum gauge 31 can be used to check the pressure, and the supply amount and the exhaust amount may be feedback-controlled based on the output of the vacuum gauge 31. A valve 43 may be provided in the exhaust line 41 to adjust the exhaust capacity and close the reaction furnace 30.

[0017] The preform 100 is loaded into the reaction chamber 33 and fixed appropriately. MTS is mixed with hydrogen, or further mixed with hydrogen chloride, and further diluted with helium, and fed into the furnace 30 at a controlled flow rate. The pressure inside the furnace 30 is controlled by balancing these feeds with evacuation by the pump 45. Typically, the furnace 30 is maintained at a pressure lower than atmospheric pressure throughout the reaction process. In parallel, the gas flow and the preform 100 are heated by the heater 35, which decomposes the MTS on the surface of the reinforcing fibers 1 or on the surface of the coating already applied, and a coating made of silicon carbide grows. The coating fills the voids G1, G3, and G5, becoming a matrix that ultimately bonds the reinforcing fibers 1 together.

[0018] Typical reaction conditions are, for example, a total pressure of 5 to 760 Torr, a temperature of 800 to 1100° C., and a total flow rate of the raw material and auxiliary gases of 100 to 1000 sccm. Among the raw material and auxiliary gases, the ratio of MTS to hydrogen is 0.2 to 3, with the remainder being an inert gas such as helium or an inert gas and hydrogen chloride.

[0019] The growth rate of the coating depends on the total pressure. Averaged over the entire preform, the growth rate of the coating generally tends to increase with increasing total pressure up to at least about 50 Torr. However, the growth rate tends to decrease above 80 Torr. These trends do not appear to be very dependent on temperature. The growth rate of the coating also depends on temperature, increasing with temperature up to at least 850°C, but rather decreasing above 900°C. That is, the total pressure can preferably be 20 Torr or more and atmospheric pressure or less, and more preferably 50 Torr or more and 300 Torr or less, and the temperature can be 850 to 900°C.

[0020] On the other hand, the thicker the preform, the more likely it is that the coating will not grow sufficiently in the center. The growth rate distribution in the thickness direction depends on the total pressure and temperature, and generally, the higher the pressure and the higher the temperature, the greater the difference in the thickness direction. Therefore, the total pressure and / or temperature may be reduced depending on the thickness of the preform. For example, if the preform is particularly thick, the total pressure may be set to 50 Torr or less or 850°C or less to prioritize uniformity in the thickness direction.

[0021] The above-mentioned growth conditions take into account the average film growth across the entire preform. The preferred conditions are different when considering micro-growth conditions, such as voids in the preform. MTS itself is one of the film-forming species that adheres to the reaction surface and decomposes to produce silicon carbide. In addition, highly active intermediates, such as radicals derived from MTS, are also film-forming species, and the adhesion probability of the latter is significantly higher than that of the former. When film-forming species penetrate into the interior through the opening of a void and adhere to active points on the film-growing surface, highly active intermediates preferentially adhere to the active points near the opening, rather than MTS. As a result, as illustrated in Figure 4, the film growth rate generally increases closer to the opening of the void. In other words, the greater the growth contribution of the highly active intermediate, the earlier the area near the opening becomes blocked, inhibiting matrix formation inside the void. Therefore, a balance with the growth contribution of MTS is particularly important for film growth into voids with an aspect ratio above a certain level.

[0022] It is possible to predict that the higher the temperature, the more non-uniform the film formation in the voids will be. Contrary to this prediction, as will be described in detail later with reference to test results, temperatures exceeding 1000°C can be used to form a matrix in the voids. However, taking into account the heat resistance of the furnace, the temperature can be limited to, for example, 1100°C or less. The ratio of MTS to hydrogen in the raw material and auxiliary gases is 1 or more, and the total pressure is 50 to 80 Torr.

[0023] The reaction conditions do not have to be the same throughout the matrix formation process, and can be changed as appropriate. For example, in the first stage, first reaction conditions suitable for voids G1 with a large aspect ratio can be used, in the second stage, second reaction conditions suitable for voids G3 with a medium aspect ratio can be used, and in the third stage, third reaction conditions suitable for voids G5 with a small aspect ratio can be used. Furthermore, in one or more of the multiple stages, another method can be used instead of the CVI method. These steps can be performed batchwise as independent multiple stages, or as a series of processes in which the conditions are continuously changed.

[0024] To investigate the conditions for matrix formation, a silicon carbide coating formation test was conducted on a substrate having voids, as shown in Figure 4. The substrate 11 was made of silicon and had a trench G11 with a width W and a depth D (aspect ratio D / W). The trench G11 simulated the voids between fibers. Using the apparatus shown in Figure 3, the substrate 11 was introduced into a furnace 30, and a silicon carbide coating growth test was conducted. A total of 640 sccm of source gas was flowed into the furnace 30, with a flow rate of 320 sccm for MTS, 80 sccm for hydrogen, and the remainder being helium. The total pressure was adjusted to 60 Torr (the partial pressures of MTS and hydrogen were 30 Torr and 7.5 Torr, respectively). The substrate 11 was heated to a temperature of 925 to 1100°C using a heater 35 to form a silicon carbide coating.

[0025] After the coating was formed, the substrate 11 was removed and cut, and the cross sections were observed using a scanning electron microscope (SEM). The thicknesses t1, t2, ... tn of the coating 13 were measured at regular intervals from the opening A11 of the trench G11 toward the bottom B11. Examples of cross-sectional SEM images are shown in Figures 5 to 7, and the thickness measurement results for these examples are shown in Table 1. The aspect ratio of the trench was 43 in the example of Figure 5 and 35 in the examples of Figures 6 and 7.

[0026]

[0027] For films grown at either temperature, the film thickness was larger near the opening, but was generally constant at distances of 60 μm or more from the opening. Assuming that the film growth was a combination of growth contributed by MTS and growth contributed by highly active species, the contribution of each film-forming species was estimated by fitting the following growth rate prediction formula using the sticking probability η as a variable.

[0028]

[0029] In the formula, η is the adhesion probability of each film-forming species, W is the width of the opening, and L is the length of the trench.

[0030] The film growth rate due to the contribution of MTS increases with increasing temperature, as shown in Figure 8. The film growth rate due to the contribution of highly active species also increases with increasing temperature, as shown in Figure 9, but the increase is steeper below 1000°C and more gradual above 1000°C. As can be seen from Figure 10, the ratio of the growth rate due to the contribution of MTS to the total growth rate is minimal at 1000°C and increases with temperature above 1000°C.

[0031] From the above test results, it is understood that temperatures above 1000°C increase the contribution of MTS, which is advantageous for growing a coating uniformly in the voids and is also advantageous in terms of growth rate. It is clear that this effect can be obtained when the aspect ratio of the voids is 35 or more.

[0032] Although there are no test results available at temperatures above 1100°C, by extrapolating Figures 8 to 10, it is expected that the contribution of MTS will further increase and the contribution of highly active species will further decrease, resulting in a greater total growth rate and more uniform growth of the film.

[0033] Although several embodiments have been described, modifications or variations of the embodiments can be made based on the above disclosure.

[0034] A method is provided that can form a matrix of silicon carbide with high efficiency even when the voids are relatively narrow.

Claims

1. A method for producing a fiber reinforced composite material, comprising: introducing a reinforcing fiber fabric, the reinforcing fiber fabric including interwoven reinforcing fibers and including voids between the reinforcing fibers, the voids having an aspect ratio of 35 or greater, into a reactor capable of controlling the atmosphere; flowing a feed gas including hydrogen and methyltrichlorosilane into the reactor; and, while maintaining the reactor at a reduced pressure below atmospheric pressure, heating the reactor so as to maintain at least the reinforcing fiber fabric at a temperature exceeding 1000°C and not exceeding 1100°C, thereby inducing a reaction which produces a coating of silicon carbide within the voids, thereby bonding the reinforcing fibers to each other by the coating.

2. The method of claim 1, wherein in said source gas, the ratio of methyltrichlorosilane to hydrogen is 1 or greater.

3. The method of claim 1, wherein during said reacting step, the interior of said reactor is maintained at a pressure of at least 20 Torr but not greater than atmospheric pressure.

4. The method of claim 1, wherein said reacting step continues until said voids are at least partially filled by said coating.

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