JPH048396B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a high-strength, high-toughness ceramic sintered body suitable for use as a high-temperature structural material for gas turbine blades, various engine parts, and the like. BACKGROUND OF THE INVENTION Heat-resistant alloys have traditionally been used for parts exposed to high temperatures and harsh environments, such as gas turbine blades. However, in recent years, higher temperature use has been desired in order to improve performance, and heat-resistant alloys are reaching their limits of use. Therefore, as materials to replace these heat-resistant alloys, silicon carbide, silicon nitride, and sialon (Si ₆ -
zAlzOzN ₃ −z: 0<z<4) is attracting attention as a high-temperature structural material. However, as is well known, these materials have the major drawback of being brittle, and have not been put into full-scale practical use to date. Methods for strengthening ceramics by compounding them with other substances include (1) dispersing particles of metals and metal carbides, nitrides, or silicides, and (2) dispersing whiskers and fibers. Conceivable. However, in any of the above methods, when a metal and a metal carbide, nitride, or silicide are combined, the toughness at room temperature is improved, but since it contains a metal phase, there is a problem in oxidation resistance at high temperatures. Yes, the life of parts is short when used at high temperatures, making it impractical. Therefore, active research is being carried out on composite ceramics in which ceramic particles, whiskers, fibers, etc. are dispersed, and which can maintain excellent oxidation resistance and strength even at high temperatures. especially,
Whiskers and fibers are showing promise. In other words, when a crack occurs in ceramics, it is thought that the force to absorb the progress of the crack is greater than in the case of particle dispersion. An example of using whiskers is JP-A-59-54680. In this case, a ceramic is obtained in which SiC whiskers are composited into Si ₃ N ₄ and the strength does not deteriorate even at high temperatures. However, it is impossible to significantly improve the toughness of ceramics using whiskers alone. The main reason for this is 25
If the content exceeds volume %, whisker dispersion becomes extremely non-uniform, the density decreases, and a normal sintered body cannot be obtained. In addition, the diameter of commonly available SiC whiskers is very thin at 0.5 to 1.0 μm, which is almost the same as the matrix particle size, and because the whiskers are in close contact with the matrix, their ability to absorb the progress of cracking is weak. It's for a reason. On the other hand, regarding fibers, some FRMs based on combinations of metal-metal fibers and metal-ceramic fibers have been put into practical use.
However, FRC of ceramics-ceramic fibers
Although there have been various discussions about this, it has not yet been put into practical use. [Object of the Invention] An object of the present invention is to significantly improve the toughness of silicon nitride and sialon, and to provide a silicon nitride sintered body or a sialon sintered body that has high strength up to high temperatures and excellent toughness. [Summary of the Invention] To summarize the present invention, the present invention relates to fiber-reinforced composite ceramics, and the present invention relates to fiber-reinforced composite ceramics, which is a sintered body in which silicon nitride or sialon is used as a matrix and ceramic fiber is composited therein. In, the ceramic fiber is
Made of carbon containing SiC on its surface, thickness 0.1~
A surface layer of 5 μm is provided, and the length of the ceramic fiber is at least 10 times the diameter including the surface layer. In order to significantly improve the toughness of ceramics with fibers, the compatibility of the ceramic-fiber interface becomes an issue. Even if the elastic modulus and breaking stress of the fiber are high, the effect will not necessarily be exhibited when composited with ceramics. That is, the effect varies greatly depending on the reactivity and thermal expansion coefficient during sintering of ceramics and fiber.
Even if a matrix ceramic is combined with a fiber of a substance that reacts with each other during sintering, the effect of the fiber will not be exhibited, and on the contrary, the properties of the ceramic itself will deteriorate. Toughening of ceramics by fibers is achieved by pulling the arrayed fibers out of the ceramics with a moderate force. However, if the surface layer or all of the fiber reacts with the ceramic and becomes integrated, this pull-out effect will not work and the toughness will not improve. On the other hand, if the thermal expansion coefficients of non-reactive substances differ significantly, cracks will occur in the ceramics or fibers, making it impossible to obtain a sound sintered body. Furthermore, even if no cracks occur, the sintered body is highly distorted, and a gap is created between the ceramic and the fiber, resulting in extremely weak pulling force, which does not lead to improvement in toughness. According to the invention, this pull-out force is between 0.1 and 15
It was found that the toughness was greatly improved when the amount was in the range of Kg/mm ² . Fracture toughness value KIC
All those showing values of 10 MN/m ^3/2 or more were within this range. In consideration of the above, the present inventors have studied how to strengthen ceramics using fibers. As mentioned above, SiC,
Si ₃ N ₄ and Sialon are promising because they have excellent heat resistance, thermal shock resistance, and corrosion resistance at high temperatures.
Of these, SiC has minimal strength loss even at high temperatures.
is particularly promising as a high-temperature material. However, a sintered body cannot be produced unless the sintering temperature of SiC is extremely high, at 2000°C or higher. Therefore, the heat resistance of the fibers that are composited to make them tougher becomes a problem. i.e.
This is because there is currently no material that can withstand the sintering temperature of SiC without reaction. Therefore, we selected silicon nitride or sialon, which can be sintered at 1500 to 1800°C, as the matrix ceramic, and attempted to strengthen it with fibers. The present inventors previously proposed a SiC fiber composite sintered body in which a carbon film is provided on Si ₃ N ₄ or Sialon as a matrix ceramic. The key point is to provide a thin C film on the surface of the SiC fiber so that it can be easily pulled out. The fracture toughness value K _IC of this sintered body was 12 MN/m ^3/2 or more, indicating an excellent toughness value. The present invention has superior characteristics over the previous inventions. In other words, the previous invention is SiC
A carbon film is provided on the surface of the fiber, which allows it to withstand use at low temperatures for a sufficiently long time. However, when used in air at high temperatures for a long period of time, the carbon film on the fiber surface exposed on the surface of the sintered body burns and the drawing effect is lost. Therefore, as a result of studying this problem, we have arrived at the present invention. That is, by providing a carbon film containing SiC on the fiber surface, we were able to obtain a composite sintered body that can withstand high temperatures in air. The components of this film exhibit excellent properties when containing 10 to 50 mol% of SiC. In other words, if it exceeds 50 mol%, the film will react with the silicon nitride or SiAlON matrix during sintering, and there will be no pull-out effect, and if it is less than 10 mol%, there will be too much carbon and it will not be able to withstand long-term use at high temperatures. . Further, good results can be obtained when the film thickness is 0.1 to 5 μm. If the membrane is too thin, it will have no effect, and if it is too thick, the fiber reinforcement effect will be lost. The amount of SiC in the film can be easily analyzed by, for example, the micro-Ausier method, by measuring the ratio of the C peak to the Si peak in the film.
In one analysis example, the peak ratio of C to Si (C/Si) for a 100% SiC film was 0.416, and the peak ratio (C/Si) for a mixed film of C and SiC was 1.923. This corresponds to approximately 22 mol% of SiC in the mixed film when converted to mol% of SiC. The fibers used include SiC and Si ₃ N ₄ whiskers with a diameter of about 0.1 to 20 μm, and CVD onto C fibers.
A fiber consisting of a layer of SiC can be used. The latter fiber has a diameter of 100 to 140 μm.
are commercially available. The diameter of the fiber, including the surface film, is preferably at least five times the grain size of the silicon nitride and sialon. With a diameter of this order, the fiber has sufficient mechanical strength and is less likely to break when pulled out, which has a great effect on improving toughness. Furthermore, if the diameter is too large, cracks tend to occur at the interface between the fiber and the base material due to the difference in thermal expansion, which is undesirable. Specifically, a range of 10 to 200 μm is particularly good. In addition, in order for the fiber to exhibit a sufficient pull-out effect, its length must be at least 10 times the diameter including the surface film. In addition, the amount of fiber added is 15
It exhibits excellent properties in the range of ~40% by volume. Further, in the case of a random arrangement, 20 to 50% by volume is good; too few fibers will have no effect on improving toughness, and too many fibers will cause a decrease in toughness and strength. The density of silicon nitride or sialon as the matrix used in the present invention is preferably 90% or more, and 95%
The above is more preferable. [Examples of the Invention] Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples. Example 1 A predetermined amount of Si ₃ N ₄ (average particle size: 2 μm) powder and sintering aid powder were weighed and thoroughly mixed in a sintering machine. There are various well-known sintering aids, but as a result of our research, we found that the appropriate amount of Al ₂ O ₃ and Y ₂ O ₃ is in the range of 0.1 to 20 volume %, and that ceramics with especially high high temperature strength can be obtained at this time. It was confirmed. In this example, Al ₂ O ₃ (average particle size 0.5 to 1.0 μm) 2% by volume, Y ₂ O ₃
(Average particle size 2 to 5 μm) Although the amount was kept constant at 3% by volume, it was confirmed that similar results could be obtained within the above addition range. After adding an appropriate amount of 5% polyvinyl alcohol aqueous solution to this mixed powder, the powder was sieved using a 16-mesh sieve. On the other hand, SiC fiber is first deposited on 35 μm C fiber.
A CVD film with a diameter of 140 μm and a length of about 60 mm was used. This SiC fiber was immersed in a solvent containing appropriate amounts of polycarbosilane, an organic high-molecular silicon compound, and phenol resin. This was heat-treated in a vacuum, and a film containing 22 mol% SiC in C was formed on the surface using the micro-Ausier method described above.
Composited with Si ₃ N ₄ . The fiber is made by unidirectionally arranging long fibers and composing 20% by volume, with a thickness of 6 mm and a diameter of 60 mm.
A green body of mm was fabricated. This was set in a hot press using a well-known graphite die and heated to a maximum heating temperature of 1800°C under a pressure of 300 kg/cm ² in nitrogen gas.
Sintered with Using the same method, we also fabricated a sintered body in which a SiC fiber without a film on the fiber surface was composited into Si ₃ N ₄ . These sintered bodies were cut parallel to the fibers to obtain test pieces having a width of 3 mm, a thickness of 4 mm, and a length of 36 mm. After polishing this, the center part has a width of 0.1
A test specimen using the SENB (Single Edge Notched Beam) method was prepared by making a cut of 0.5 mm and 0.5 mm in depth at right angles to the fiber, and the fracture toughness value K _IC was determined. Figure 1 shows K _IC
A graph comparing (MN/m ^3/2 , vertical axis) is shown.
As shown in Figure 1, there is no film when using SiC fiber coated with a mixed film of C and SiC.
It showed superior toughness values compared to SiC fiber. Figure 2 shows the effect of the amount of fiber added. That is, FIG. 2 is a graph showing the relationship between the amount of SiC fiber added (volume %, horizontal axis) and K _IC (MN/m ^3/2 , vertical axis). As shown in FIG. 2, a remarkable effect was observed when the amount was 15 to 40% by volume. Similarly, fibers with diameters of 5, 10, 50, 200 and 300 μm were tested. As a result, the 300 μm fiber was too thick to be effective. Meanwhile, 10~200μ
When m, the same effect as in FIGS. 1 and 2 was observed. Moreover, K _IC at 5 μm was 10 MN/m ^3/2 . Figures 3 and 4 show the K _IC measurement results when the film composition and film thickness were varied. The film composition was controlled by changing the mixing ratio of polycarbosilane and phenolic resin. In other words, Figure 3 shows the relationship between SiC (molar ratio, horizontal axis) and
This is a graph showing the relationship between the film thickness (μm, horizontal axis) and _{K IC (} MN/m ^3/2 , vertical axis).
m ^3/2 , vertical axis). Third
The figure shows the case where the film thickness is 1.0μm, and Fig. 4 shows the case where the film thickness is 1.0μm.
This is the case of SiC20 and C80 (molar ratio). As a result,
The film thickness is 0.1 to 5 μm, and the film composition has a SiC content of 50 mol%.
The toughness is significantly improved in the following cases. Example 2 Similar to Example 1, 20% SiC was applied to the surface of the SiC fiber.
A uniaxially oriented green body was prepared by coating a 1 μm thick C film containing mol %. SiN ₄ − with different densities was created by changing the sintering conditions.
A SiC fiber composite sintered body was produced, and the fracture toughness value K _IC was determined in the same manner as in Example 1. Figure 5 shows the results. That is, FIG. 5 is a graph showing the relationship between relative density (%, horizontal axis) and K _CIC (MN/m ^3/2 , vertical axis). The toughness of sintered bodies with a relative density of 95% or more is significantly improved, but the effect is small with relative densities below that. That is, in order to increase the toughness by using fibers, it is preferable that the density of the composite sintered body is 95% or more. Furthermore, the fifth
In the figure, the bending strength of a sample with a relative density of 100% is from room temperature to
It was 80Kg/mm2 ^or more in the range of 1100℃. Example 3 Heat resistance, which is a feature of the present invention, was investigated.
As in Example 1, a unidirectionally aligned Si ₃ N ₄ --SiC fiber composite sintered body was produced using a 140 μm SiC fiber coated with a C film containing 22 mol % of SiC. From now on, we will collect a test piece for the SENB method,
It was left in air at 1000℃ for a maximum of 2500 hours. After that, the K _IC was measured, and it was found that the value was almost the same as before the oxidation test, indicating that it had excellent heat resistance. The results are shown in Figure 6. That is, FIG. 6 is a graph showing the relationship between time (hours, horizontal axis) and K _IC (MN/m ^3/2 , vertical axis). The molar amount of SiC in the C film is 10~
When the amount of SiC was 50 mol %, excellent oxidation resistance was similarly exhibited, but when the amount of SiC was 0 or 5 mol %, the film burned during heat treatment, resulting in a decrease in K _IC . Example 4 The toughness value of Si ₃ N ₄ or Sialon with SiC fibers is particularly high when the fibers are arranged at right angles to the direction of crack propagation. However, in the present invention, the effect is not lost even if they are mixed randomly. 7th
The figure shows a C film coated with 10 mol% SiC.
A graph showing the relationship between the addition amount (%, horizontal axis) and fracture toughness value (MN/m ^3/2 , vertical axis) when SiC whiskers with a diameter of 0.5 to 5 μm (aspect ratio of 10 or more) are arranged randomly. be. If the amount added is less than 15% by volume, the effect will be small, but if it is more than 20% by volume, the toughness will be significantly improved. When added in excess of 60% by volume, its contribution to toughness is small. From this, the appropriate amount for random compounding is 20 to 50% by volume, especially 30 to 40% by volume.
Volume % has a large effect. At this time, the bending strength is also 40
Kg/ ^mm2 or more. Figure 8 shows Si ₃ N ₄ -20 volume%
These are the measurement results of K _IC (MN/m ^3/2 , vertical axis) of SiC whiskers with and without a C film containing 10 mol% of SiC. Even when whiskers were added randomly, the film showed an improvement in toughness. Similar results were obtained for Si ₃ N ₄ whiskers. Example 5 Similar to Example 1, Sialon was used as the matrix. In other words, 1μ of C film containing 30 mol% of SiC is applied to SiAlON with a composition of Si ₅ AlON ₇ .
A SiAlON-SiC fiber composite sintered body was prepared using SiC fibers with a diameter of 5 to 300 μm coated on the surface. The K _IC was measured by the SENB method. The results are shown in Table 1.

〔Effect of the invention〕

As explained above, the fiber-reinforced composite ceramics of the present invention has excellent heat resistance, and has high strength and toughness even at high temperatures, so it has the remarkable effect of being extremely reliable as a structural material. be.

[Brief explanation of drawings]

Figure 1 is a graph comparing fracture toughness values with and without a film provided on the surface of the fibers in composite ceramics using fibers, and Figure 2 shows changes in fracture toughness values depending on the amount of fiber added in composite ceramics. Figure 3 is a graph showing the change in fracture toughness due to the difference in the amount of C and SiC in the film coated on the SiC fiber surface of composite ceramics.
A graph showing the relationship between the thickness of the film coated on the surface of the SiC fiber and the fracture toughness value. Figure 5 is a graph showing the relationship between the relative density and fracture toughness value of composite ceramics. Graph showing fracture toughness values after thermal history at 1000℃, No. 7
The figure is a graph showing the change in fracture toughness value depending on the whisker addition amount in composite ceramics in which SiC whiskers with a surface film are randomly added.
A graph comparing fracture toughness values with and without a film provided on the whisker surface in composite ceramics using SiC whiskers. Figure 9 shows a part of the structure of an example of a structural component using the composite ceramics of the present invention. FIG. 3 is a perspective view showing a broken surface.

Claims (1)

[Scope of Claims] 1. In a fiber-reinforced composite ceramic which is a sintered body in which silicon nitride or sialon is used as a matrix and ceramic fiber is composited therein, the ceramic fiber has a thickness of carbon containing SiC on its surface. A surface layer with a thickness of 0.1 to 5 μm is provided,
A fiber-reinforced composite ceramic, characterized in that the length of the ceramic fiber is 10 times or more the diameter including the surface layer. 2. The fiber-reinforced composite ceramic according to claim 1, wherein the surface layer has a SiC content of 10 to 50 mol%. 3 The amount of the fibers is 15 to 40% by volume when the fibers are arranged side by side in one, two or three dimensions, and 20 to 50% by volume when the fibers are randomly oriented.
A fiber-reinforced composite ceramic according to claim 1 or 2. 4 The fiber is SiC CVD on carbon fiber.
The fiber-reinforced composite ceramic according to any one of claims 1 to 3, which is composed of layers. 5. The fiber-reinforced composite ceramic according to any one of claims 1 to 3, wherein the core of the fiber is a SiC or Si ₃ N ₄ whisker.