JPH0544429B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to mullite composite ceramics containing silicon carbide fibers. Prior art and its problems Sintered bodies of alumina, zirconia, silicon nitride, silicon carbide, etc. have high strength and hardness, and also have excellent heat resistance, corrosion resistance, abrasion resistance, etc., and are used as structural materials. It is seen as a promising material, and attempts are being made to use it in engine parts, etc. However, oxide-based sintered bodies such as alumina and zirconia have poor high-temperature strength and thermal shock resistance, and silicon nitride and silicon carbide have poor toughness and oxidation resistance, so they are widely used as high-temperature structural materials. However, it has not yet been fully utilized. Therefore, in recent years, in order to improve the mechanical properties of ceramics, research has been carried out on improving the toughness by incorporating silicon carbide (SiC) fibers into single-phase ceramics such as alumina, zirconia, and silicon nitride. Among these, alumina-SiC ceramics have been recognized to have an effect of improving toughness by combining SiC fibers, but have insufficient mechanical properties such as high-temperature strength, and silicon nitride-SiC ceramics have excellent high-temperature strength. However, there are difficulties in improving the strength at low temperatures, and the oxidation resistance at high temperatures is also poor, and these reasons are preventing the expansion of its use in high-temperature structural materials. Means for Solving the Problems In view of the current situation as described above, the present inventor has conducted extensive research in order to find a ceramic material that has excellent properties as a high-temperature structural material. As a result, ceramics, which are dense sintered bodies made of mullite as a base material and uniformly dispersed SiC fibers, contain a specific amount of glassy phase, and have a density above a specific value, can be produced at room and high temperatures. It was discovered that this material has high strength and toughness, and excellent heat resistance, and the present invention has now been completed. That is, the present invention provides a sintered body containing 5 to 30 volume % of silicon carbide fibers and 0.5 to 8 volume % of a glassy phase, with the remaining 95 volume % or more being mullite crystal, and having a theoretical density of 90% or more. The present invention relates to a mullite composite ceramic containing hydrocarbon silicon fibers. The composite ceramic of the present invention has a structure in which SiC fibers, which have high strength up to high temperatures, are uniformly dispersed in a matrix mainly composed of mullite, which is an oxide with excellent heat resistance. According to the research conducted by the present inventors, it has been found that in order to effectively exhibit the effects of combining SiC fibers, it is important to appropriately control the microstructure of ceramics. In particular, it has been found that it is necessary to have a glassy phase present in a specific range of amount, which was conventionally thought to cause a decrease in high-temperature strength and oxidation of SiC. Specifically, the ceramics of the present invention must satisfy the following conditions. (a) Part excluding silicon carbide fibers and glassy phase
More than 95% by volume is mullite crystal. In the present invention, mullite, which is an oxide with excellent heat resistance, is used as the ceramic base material. In the present invention, mullite crystals are
As well as mullite crystals represented by 3Al ₂ O ₃ 2SiO ₂ (71.8% by weight of Al ₂ O ₃ and 28.2% by weight of SiO ₂ ),
It also includes solid solutions of mullite. Furthermore, up to about 30% of the Al in the mullite may be replaced with Cr as a solid solution. In particular, a sintered body in which a part of Al is replaced with Cr as a solid solution contributes to improvements in toughness, corrosion resistance, etc. The average particle size of the mullite crystals is preferably 8 μm or less, more preferably 5 μm or less. If the grain size of the mullite crystals is too large, the strength of the base material itself will be low, and the width of the glassy phase formed at the grain boundaries will become large, making it easy to reduce the high-temperature strength, which is not preferable. In the ceramics of the present invention, mullite crystals account for 95% or more by volume of the area excluding silicon carbide fibers and the glassy phase. If it is less than 5% by volume,
Al ₂ O ₃ , Cr ₂ O ₃ , ZrO _{2 ,} etc. (according to powder X-ray diffraction method) may be present, but if too much, the mullite crystals become granular crystals and the high temperature strength decreases. For this reason, it is preferable to have a single phase of mullite crystal. (b) The amount of silicon carbide fiber is 5 to 30% by volume. The ceramic of the present invention has a structure in which silicon carbide fibers (SiC whiskers, fibers made of SiC polycrystals, etc.) are uniformly dispersed in the ceramic in the form of fibers. Silicon carbide fibers are dispersed two-dimensionally or three-dimensionally in a sintered body to improve strength,
Improves toughness etc. If the fibers are too large, even in a dense sintered body, the grain boundaries between the fibers and mullite become a stress concentration source for fracture, resulting in a decrease in strength. In addition, the aspect ratio of the fiber (length/
Although it is desirable that the diameter (diameter) is large, if it is too large, it will hinder the densification of the base material. For this reason, the average diameter of the fibers should be approximately 0.05 to 20 μm,
Preferably 0.1 to 8μ, which is the same as mullite crystal grains.
The thickness is preferably about 0.3 to 4 μm. Further, the average length of the fibers is preferably 2
~30μm, more preferably 3~15μm
degree. The amount of silicon carbide fiber in ceramics is 5 to 30
% by volume, preferably 10-25% by volume.
If the amount of fibers is less than 5% by volume, the effect of improving strength and toughness will be small, while if it exceeds 30% by volume, the fibers will inhibit mass transfer during the sintering process, resulting in poor sinterability of the base material, Due to the presence of pores, the density of the sintered body is low, and as a result, the strength at room temperature is also low. (c) The glassy phase is 0.5 to 8% by volume. In the present invention, the glassy phase refers to an amorphous phase existing at grain boundaries of mullite crystals, silicon carbide fibers, and the like. The glassy phase is present in ceramics.
It is necessary to be present at 0.5 to 8% by volume, and 1
More preferably, it is present in an amount of ~5% by volume. Due to the presence of the glassy phase, liquid phase sintering occurs in addition to diffusion sintering of mullite, making it possible to sinter ceramics densely at low temperatures. This prevents the decomposition of mullite as a base material and the oxidation of silicon carbide fibers during the sintering process. Further, the glassy phase has the effect of suppressing grain growth of mullite, and functions to form a base material consisting of dense and homogeneous mullite polycrystals. Furthermore, it also contributes to columnarization of mullite crystals, improving strength and toughness. If the amount of glassy phase is too small, the above effects will not be sufficiently achieved, and the difference in thermal expansion between the base material mullite and silicon carbide will be small, so the stress effect from the base material will be applied to the fibers in the sintered body. It contributes only a small amount and is prone to fiber pull-out in the tensile stress field.
The strength improvement effect becomes smaller. On the other hand, if there is too much vitreous phase, the plastic deformation and viscosity reduction of the vitreous phase in the high-temperature stress field will easily cause the propagation of fracture and the pulling out of fibers from the vitreous phase, resulting in a rapid decrease in strength. Easy to do. The glassy phase is not particularly limited, and may be, for example, a Na ₂ O−Al ₂ O ₃ −SiO ₂ type, but
at least one compound of K, Li, B, Mg, Cr and Y, such as their carbonates, hydroxides,
Compounds that become oxides in the firing process, such as chlorides and oxides, are added to form a glassy phase consisting mainly of these components, Al, Si, and O, which are the constituent components of mullite, and Si of silicon carbide fibers. It is preferable to form. In addition to the sintering effect, the above components have the following properties: K improves high-temperature strength by increasing the viscosity of the glassy phase, Li improves the thermal shock resistance of the glassy phase, and B
improves the sinterability of SiC and mullite, Mg, Cr,
Y contributes to improving toughness. (d) The theoretical density of the sintered body is 90% or more. In the present invention, the theoretical density refers to the theoretical density of a 100% dense sintered body with no pores, and the density of the sintered body is expressed as a percentage of this theoretical density. . The theoretical density is
If it is less than 90%, there are many pores in the sintered body, forming continuous pores, which causes a decrease in strength and corrosion resistance. However, when the pores are minute and exist in a small volume, the pores contribute to the stress dispersion effect in the stress field. Preferably, the theoretical density is
Must be 92% or more. The composite ceramics of the present invention can be produced, for example, by the following method. Mullite is Si, Al in solution or sol form.
After mixing the containing compounds so that the Al/Si ratio has the composition of mullite, the solvent is scattered, and the material is fired.
It is preferable to use mullite that has been crystallized in advance, but it can also be formed by a reaction sintering method in which mullite is reacted and precipitated during firing of ceramics using kaolin (Al ₂ O ₃ 2SiO ₂ 2H ₂ O) and alumina. You may let them. The compound for forming a glassy phase is preferably added when forming mullite crystals in advance, but may be added when adding SiC fibers to the mullite raw material. It is preferable to loosen the silicon carbide fibers in a wet process and remove any fiber clumps, and then gradually add the silicon carbide fibers to a mixture consisting of a mullite raw material and a compound that forms a glassy phase and mix them in a wet process. It may be mixed together with the raw material and the compound for forming the glassy phase by wet grinding. After preparing the mixture with the necessary raw materials in this way, it is either wet molded or dried in the atmosphere or under reduced pressure so that the silicon carbide fibers do not separate, and the raw material for molding is then molded according to a conventional method. death,
The composite ceramic of the present invention is then obtained by firing the molded body. In the ceramics of the present invention, since the glassy phase contributes to densification of the ceramic, not only pressure sintering method but also _normal pressure sintering method is possible. Alternatively, it may be fired in these atmospheres or in a non-oxidizing atmosphere using a hot press, a hot isostatic press, or the like. The firing conditions may be approximately 1500 to 1700°C for approximately 0.5 to 10 hours. Effects of the Invention The composite ceramics of the present invention has a structure in which high-strength silicon carbide fibers are wrapped in a dense sintered body of mullite, and its strength at room temperature and high temperature of about 1400°C is as high as 30 Kgf/mm ² or more. In addition, it has high toughness due to fiber reinforcement. Furthermore, since the silicon carbide fibers are wrapped in mullite, which is stable in a high-temperature oxygen atmosphere, the ceramic as a whole has good oxidation resistance. The ceramics of the present invention have such excellent properties and can be used in parts that require durability at high temperatures, such as engine parts, heat exchanger parts, high-temperature conveying rolls, burner nozzles, soaking tubes, and heat treatment parts. It is extremely useful as a EXAMPLES The present invention will be described in detail with reference to Examples below. Example 1 The raw materials shown in Table 1 below, excluding SiC fibers, were wet-pulverized for 24 hours in a ball mill and mixed. As mullite, (Al ₂ O ₃ + SiO ₂ ) min 99.99
%, a specific surface area of 5 m ² /g, and a crystal phase consisting of mullite. As a glassy phase formation product,
A 1:2 (mole ratio) mixed powder of Na and K carbonates was used. While stirring the obtained mixed slurry with a mixer, β-SiC whiskers having an average diameter of 0.4 μm and an average length of 15 μm were gradually added and mixed in the proportions shown in Table 1. Next, this mixed slurry was dried and granulated using a spray dryer, and then granulated using a carbon mold at each temperature shown in Table 1.
A plate-shaped composite ceramic was obtained by hot press sintering at Kg/cm ² . Table 2 shows the amount of mullite, the amount of glassy phase, and the amount of SiC in the obtained ceramics. Also, JIS-R
Table 2 shows the results of measuring the three-point bending strength using -1601 and the toughness value (KIC) using the MI method.

【table】

【table】

[Table] As is clear from Table 2, the ceramics of the present invention exhibit high values of strength and toughness at room and high temperatures. On the other hand, ceramics containing less than 5% SiC fibers (comparative products 1 and 2) have a toughness value of 3.0.
In addition, when the SiC fiber content exceeds 30% (comparative product 3) and when the glassy phase content is less than 0.5% (comparative product 4), the sinterability is poor and the theoretical density is low. Strength at high temperatures decreases.
When the glassy phase exceeds 8% (comparative product 5), the high temperature strength becomes particularly low.

Claims (1)

[Claims] 1 5-30% by volume of silicon carbide fibers and glassy phase
A sintered body containing 0.5 to 8% by volume, with the remaining 95% or more by volume being mullite crystals, and having a theoretical density of 90
% or more of mullite composite ceramics containing silicon carbide fibers.