JPH0456791B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD This invention relates to a toughened hot-pressed sintered body of silicon carbide ceramic. Conventional technology Silicon carbide-based sintered bodies have high strength over a wide temperature range up to high temperatures, high hardness and excellent wear resistance, and are chemically stable and have good oxidation resistance. It is a material that is expected to be used as a structural material. Silicon carbide sintered bodies are generally manufactured by vapor phase method, reaction sintering method,
Manufactured by pressureless sintering, hot pressing, etc. Among these manufacturing methods, the gas phase method is mainly used for thin film manufacturing. Furthermore, it is said that it is difficult to obtain a high-density body using the reaction sintering method. In the pressureless sintering method, Prochatka et al. (Journal of
The American Ceramic Society, vol. 58, p.72,
(1975), a material with a relative density of 96.4% was obtained by adding boron and carbon. However, the sintered body obtained by pressureless sintering has a limit to the density that can be achieved, has low toughness, and has a K _IC value of 2.0 to 2.0.
It is about 2.5MPam ^1/2 , and for application to structural materials,
Further improvement in toughness is desired. In the hot pressing method, a dense sintered body with a relative density of 99% or more is obtained. For example, Alliegro et al. (Journal of The American Ceramic
Society, vol. 39, p. 386-389, 1956). Moreover, in Japanese Patent Publication No. 57-41538, dense silicon carbide having a relative density of 99% or more and a transverse rupture strength of more than 70.31 Kg/mm ² is prepared by adding 0.5 to 5% by weight of aluminum to α-silicon carbide powder. A sintered body and a method for manufacturing the same are disclosed. However, like pressureless sintered bodies, these hot-pressed sintered bodies have low toughness, and the K _IC value is low.
It is about ^1/2 of 2.5 MPam, and cannot be said to fully meet the requirements as a structural material. Silicon carbide-based sintered bodies are thermally and chemically stable at high temperatures and have excellent mechanical properties as high-temperature structural materials. Despite this, it has not yet been put into practical use at present. This is thought to be caused solely by the low toughness of conventional silicon carbide-based sintered bodies. Regarding toughness in conventional technology, when boron and carbon are used as sintering aids, intragranular fracture occurs almost over the entire surface of both the pressureless sintered body and the hot press sintered body, as shown in Figure 2. The K _IC value is as low as 2.0 to 2.5 MPam ^1/2 . Second
The figure is an optical micrograph (1500x) of the grain structure of a silicon carbide sintered body to which boron and carbon are added as auxiliaries. A crack was introduced and the surface was etched. Furthermore, in Japanese Patent Publication No. 57-41538, since an aluminum compound is added as an auxiliary agent to the silicon carbide sintered body, grain boundary fracture is possible. but,
In general, hot-pressed sintered bodies are equiaxed grains, and the sintered body disclosed in Japanese Patent Publication No. 57-41538 is no exception, and is equiaxed grains. Therefore, even if grain boundary fracture occurs, crack deflection does not occur and toughening cannot be expected, so the K _IC value is 2.5 MPam ^1/2
The value is relatively low. Problems to be Solved by the Invention The biggest drawback that hinders the practical application of silicon carbide sintered bodies to high-temperature structural materials is their low toughness. Therefore, the present invention is intended to improve the toughness of a silicon carbide ceramic sintered body. Means for Solving the Problems That is, the present invention provides: (1) A mixed powder containing silicon carbide powder as a main component and 0.5 to 40% by weight of at least one selected from aluminum or aluminum compounds,
A silicon carbide-based ceramic sintered body produced by hot press sintering and characterized in that the sintered body contains 10% by volume or more of silicon carbide particles with an aspect ratio of 2 or more, and (2) an aluminum compound oxidized. The sintered body according to claim 1, which is aluminum, and (3) the silicon carbide is β-SiC, the aluminum compound is Al ₂ O ₃ , and the sintering temperature is 1800 to 2300°C.
The sintered body according to claim 1, wherein the sintering time is 15 minutes or more. Function The silicon carbide powder used as the raw material can be used in either the α phase or the β phase. Aluminum or an aluminum compound acts as a sintering aid during hot press sintering, and also produces silicon carbide particles with a large aspect ratio through sintering, causing intergranular fracture along them, and sintering Added to serve as a crack deflection site for the body. As the aluminum or aluminum compound, metal aluminum, aluminum oxides, carbides, nitrides, and other compounds that decompose at high temperatures to produce aluminum or aluminum compounds can also be used. Examples include metal aluminum, aluminum oxide, aluminum carbide, aluminum nitride, and aluminum-containing organic compounds, and it is necessary to add at least one of these. In the present invention, in order to improve the toughness of the silicon carbide sintered body, aluminum or an aluminum compound is added to the silicon carbide powder and sintered so that crack deflection occurs effectively when the crack propagates inside the sintered body. The sintering process is carried out to create a state in which grain boundary fracture is likely to occur, and the sintering process is performed at a higher sintering temperature and for a longer period of time than is generally used, thereby causing the growth of silicon carbide particles and sintering particles with a large aspect ratio. The crack deflection is caused by being present in the structure. Figure 1 shows silicon carbide powder added with 2% by weight of aluminum oxide at 1900℃ x 20 hours x 40MPa.
This is an optical micrograph (1500x magnification) of the grain structure of a sintered body created by hot press sintering under these conditions. The K _IC value of this sintered body was 5.3 MPam ^1/2 . Cracks were introduced into the silicon carbide sintered body by the IM method, and the photograph shows the etched surface. The drawings show that there are many grains with large aspect ratios in the sintered body, and that the crack form is intergranular fracture, resulting in crack deflection. On the other hand, FIG. 2 is an optical micrograph of a conventionally known sintered silicon carbide body to which boron and carbon are added as auxiliary agents for comparison. This sintered body
The K _IC value was 1.9 MPam ^1/2 . As can be seen by comparing Figure 1 (present invention) and Figure 2 (comparative example), although both contain many particles with large aspect ratios, the conventional method (Figure 2) It can be seen that in the case of the present invention (FIG. 1), crack deflection does not occur due to intragranular fracture, whereas in the present invention (FIG. 1), crack deflection occurs due to intergranular fracture. or,
This shows that crack deflection helps build toughness. That is, in the conventional method of silicon carbide sintered bodies using boron and carbon as auxiliaries, although silicon carbide particles with a large aspect ratio are obtained, crack deflection does not occur due to intragranular fracture. Furthermore, in the silicon carbide sintered bodies using aluminum or an aluminum compound as an auxiliary agent, which have been known so far, there are no particles with a large aspect ratio, and crack deflection does not occur. As a result of the present invention, a sintered body containing silicon carbide particles with a large aspect ratio and a fracture path of intergranular fracture can be obtained. As a result, crack deflection can be effectively caused, and silicon carbide sintered bodies produced by conventional methods can be produced. compared to 2
A silicon carbide sintered body with a K _IC value more than double that was obtained. In the present invention, the total concentration of aluminum or aluminum compound is preferably 0.5 to 40% by weight. The reason for this is that if it is less than 0.5% by weight, sintering will not proceed sufficiently, and if it exceeds 40% by weight, the high temperature strength will deteriorate significantly. Further, since aluminum or an aluminum compound also acts as a grain growth inhibitor for silicon carbide, the amount thereof is desirably in the range of 0.5 to 20% by weight. A suitable sintering temperature is 1800 to 2300°C.
This is because sintering does not proceed sufficiently at temperatures below 1800°C, and thermal decomposition of silicon carbide begins at temperatures above 2300°C. Among these, a range of 1900 to 2150°C is preferable.
The sintering time is preferably 15 minutes or more. This is because particles with a large aspect ratio, which is the objective of the present invention, cannot be obtained if the heating time is less than 15 minutes. In addition, the hot press pressure is preferably 10 MPa or more, and if the pressure is lower than this, it is difficult to obtain a dense sintered body. More preferably it is 20 MPa or more. The upper limit of the hot press pressure depends on the pressure resistance of the die used, but is approximately 50 MPa for commonly used graphite dies. Among the above conditions of sintering temperature, sintering time, and press pressure, in order to generate particles with a large aspect ratio, the press pressure does not have much influence, but the sintering temperature and the sintering time are important. Furthermore, the higher the sintering temperature and the longer the sintering time, the more likely grain growth will occur and the easier it will be to obtain particles with a large aspect ratio. For example, in a silicon carbide sintered body to which 2% by weight of aluminum oxide is added, in order to obtain a K _IC value of 4 MPam ^1/2 or more, the sintering time must be at least 10 hours at the sintering temperature of 1900°C; At 2100℃ the sintering time is 30
It is desirable that the duration be at least 1 minute. Incidentally, as the atmosphere during hot press sintering, a vacuum, an inert gas such as argon, nitrogen gas, etc. are used. By performing hot pressing under the above conditions, particles with a large aspect ratio are obtained and the K _IC value increases. Specifically, in order for crack deflection to work effectively to strengthen toughness, particles with an aspect ratio of 2 or more must be present in an amount of 10% or more by volume.
In particular, the K _IC value is improved to more than twice that of conventional silicon carbide sintered bodies. In other words, in order to stably express 4 MPam ^1/2 or more, it is desirable that particles with an aspect ratio of 3 or more account for 15% by volume or more. The aspect ratio and the abundance of particles with a large aspect ratio were measured by photographing the grain structure of the etched surface of the silicon carbide sintered body using an optical microscope or a scanning electron microscope. The aspect ratio of the silicon carbide particles in the present invention was defined as the ratio of the vertical and horizontal lengths of the particles in the photograph. The abundance of particles with a large aspect ratio was defined as the ratio of the area occupied by particles with a predetermined aspect ratio or higher to the area of the microstructure photograph. Regarding bending strength, toughening occurs as the sintering temperature and sintering time increase, but grain growth also becomes significant. It becomes an upwardly convex curve with the maximum value. Examples of the present invention will be shown below. Examples Example 1 Silicon carbide powder with an average particle size of 0.3 μm is
2% by weight of 0.2 μm aluminum oxide was added.
The mixed powder was milled in a ball mill using hexane as a solvent for 24 hours.
A mixed powder was obtained by kneading for hours. After removing the solvent, the mixed powder was placed in a graphite die and heated at a temperature of 1900℃ and a press pressure of 40MPa.
Hot press sintering was performed in an argon gas atmosphere to obtain a silicon carbide sintered body. The sintering time was 30 minutes and 20 hours. The bending strength of the obtained sintered body was measured based on JIS-R-1601, and the fracture toughness K _IC value was measured using the SEPB method. Furthermore, each sample was polished, etched, and observed with a scanning electron microscope. From the scanning electron micrograph, the abundance (volume %) and average aspect ratio of particles with an aspect ratio of 2 or more were measured. The SEPB method is an application of the method of stopping brittle crack propagation used in hard and brittle metal materials to ceramics, and it allows the crack tip to have a radius of curvature ρ0, which is equivalent to a fatigue pre-crack in metal materials. This method introduces a pre-crack and is a simple and valid method for evaluating fracture toughness values based on linear fracture mechanics. The results obtained are shown in Table 1. From Table 1, it can be seen that as the sintering time becomes longer, the proportion of particles with a larger aspect ratio increases and toughening occurs. A silicon carbide sintered body to which boron and carbon were added as a comparative example was prepared by a hot pressing method, and the conditions were a temperature of 2150° C., a sintering time of 30 minutes, and a pressing pressure of 40 MPa. Example 2 A mixed powder having the same composition as in Example 1 was hot-press sintered at 1900° C. and 2100° C. for 30 minutes under a press pressure of 40 MPa in an argon gas atmosphere to obtain a silicon carbide sintered body. The obtained sintered body was evaluated in the same manner as in Example 1, and the obtained results are shown in Table 2. From this, it can be seen that the higher the sintering temperature, the higher the proportion of particles with a large aspect ratio, and the higher the toughness. Example 3 A sintered silicon carbide body was obtained from silicon carbide powder to which 10% by weight of aluminum oxide was added in the same manner as in Example 1, and evaluated. The results are shown in Table 3. As in Example 1, it can be seen that the longer the sintering time, the higher the toughness. However, since the aluminum oxide content is higher than in Example 1, grain growth is slightly suppressed.
The increase in K _IC value is smaller than in Example 1. Example 4 A sintered body was obtained from silicon carbide powder to which 10% by weight of aluminum oxide was added in the same manner as in Example 2, and evaluated. The results are shown in Table 4. As in Example 2, it can be seen that the higher the sintering temperature, the higher the toughness. However, as in Example 3, the aluminum oxide content is higher than in Example 2, so the increase in K _IC value is smaller than in Example 2. Example 5 Silicon carbide powder with an average particle size of 0.3 μm was
2% by weight of 1.8 μm aluminum nitride was added.
The mixed powder was milled in a ball mill using hexane as a solvent for 24 hours.
A mixed powder was obtained by kneading for hours. After removing the solvent, the mixed powder was placed in a graphite die and 1950
Hot press sintering was performed in a vacuum atmosphere at a temperature of 40 MPa and a press pressure of 40 MPa to obtain a silicon carbide sintered body. The sintering time was 30 minutes and 20 hours. The obtained sintered body was evaluated in the same manner as in Example 1. The results are shown in Table 5. As in Example 1, it can be seen that as the sintering time becomes longer, the proportion of particles with a larger aspect ratio increases and toughening occurs. Example 6 Aluminum powder with an average particle size of 10 μm was ground for 48 hours in a silicon carbide ball mill. Average particle size 0.3μm
The aluminum powder after crushing is added to the silicon carbide powder.
% by weight and kneaded in the same manner as in Example 1.
Sintered and evaluated. The results are shown in Table 6. As in Example 1, it can be seen that the longer the sintering time, the more the proportion of particles with a large aspect ratio increases, and toughening occurs. Example 7 Silicon carbide powder with an average particle size of 0.3 μm is made into a slurry in sufficiently dehydrated isopropyl alcohol, heated and boiled, and then mixed in a ball mill for about 10 hours. 2 for silicon carbide in terms of aluminum oxide
Prepare aluminum isopropoxide ((i-C ₃ H ₇ O) ₃ Al) with a purity of 99% or more so that the weight percentage is 99% by weight, dissolve it in sufficiently dehydrated isopropyl alcohol, heat and boil it, and then boil it for about 10 hours. Stirred. After that, the silicon carbide slurry and aluminum isopropoxide solution were mixed and stirred for about 10 hours.
Water adjusted to pH 2 was added to the mixed slurry in an amount 100 times the mole of aluminum isopropoxide, and stirring was performed again for about 10 hours. After the mixing was completed, the mixed solution was dried by spray drying. The obtained dry powder was heated in argon gas for 1200 mL.
After heat treatment at .degree. C. for 1 hour, silicon carbide and aluminum oxide phases were detected by X-ray diffraction. This mixed powder was sintered in the same manner as in Example 1.
We conducted an evaluation. The results are shown in Table 7. As in Example 1, it can be seen that as the sintering time becomes longer, the proportion of particles with a larger aspect ratio increases and toughening occurs.

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[Table] _Effects of the Invention According to the present invention, it has become possible to raise the K _IC value of a silicon carbide sintered body to a value similar to or higher than that of a silicon nitride sintered body. It seems that silicon nitride-based sintered bodies were one step ahead in application to high-temperature structural materials, but it is clear that silicon carbide sintered bodies can have the same toughness as silicon nitride sintered bodies, thanks to the present invention. Therefore, it is expected that the application of silicon carbide sintered bodies to high-temperature structural materials will significantly advance. In addition, in terms of quality assurance, doubling the K _IC value quadruples the allowable defect size, which has a huge advantage in terms of non-destructive testing, and contributes in this respect as well. However, it is large.

[Brief explanation of the drawing]

Figure 1 shows the silicon carbide sintered body (2% by weight) of the present invention.
This is an optical microscopic photograph (1500x magnification) of the grain structure of the part containing cracks in the case of aluminum oxide (addition of aluminum oxide). FIG. 2 is an optical microscopic photograph (1500x magnification) of the grain structure of a portion containing cracks in a silicon carbide sintered body containing boron and carbon as auxiliaries (comparative example).

Claims (1)

[Scope of Claims] 1. A mixed powder containing silicon carbide powder as a main component and 0.5 to 40% by weight of at least one selected from aluminum or an aluminum compound is produced by hot press sintering, and the powder is mixed into a sintered body. A silicon carbide ceramic sintered body characterized by containing 10% or more by volume of silicon carbide particles having an aspect ratio of 2 or more. 2. The sintered body according to claim 1, wherein the aluminum compound is aluminum oxide. 3. The sintering method according to claim 1, wherein the silicon carbide is β-SiC, the aluminum compound is Al ₂ O ₃ , the sintering temperature is 1800 to 2300°C, and the sintering time is 15 minutes or more. Body.