JPS61174165A - Alumina-silicon carbide heat-resistant composite sintered body and manufacture - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION A. Field of Industrial Application The present invention relates to an alumina-silicon carbide heat-resistant composite sintered body and a method for manufacturing the same.
Excellent mechanical strength at high temperatures above room temperature and 1000℃
The present invention relates to the alumina-silicon carbide heat-resistant composite sintered body having high high-temperature hardness and suitable as a material for high-temperature structures, and a method for manufacturing the same.

BACKGROUND OF THE INVENTION Alumina is most widely used among ceramics, such as in substrates and packages for integrated circuits, and chips for cutting tools. However, although alumina is a chemically extremely stable substance, its mechanical strength decreases at temperatures above around 800°C, which is relatively low for ceramics, so it is not satisfied as a ceramic for high-temperature structures, and is not used in that field. Silicon nitride and silicon carbide are used. Ceramics mainly composed of alumina are easier to sinter than silicon nitride and silicon carbide, and if their mechanical properties at high temperatures are improved, their range of applications will be further expanded.

An alumina-zirconia composite sintered body is known as an example of improving the mechanical properties of alumina ceramics by combining a second phase. Japanese Patent Publication No. 59-25748 discloses that metastable tetragonal zirconia is allowed to remain at room temperature, and residual compression is caused by approximately 4% volume expansion due to tetragonal-monoclinic transformation induced by stress at the crack tip. Due to stress,
Techniques are disclosed that significantly improve mechanical properties at room temperature. However, at high temperatures above about 900° C., which is the temperature of the above-mentioned transformation, the above-mentioned effects cannot be expected.

In addition, as an example of improving the mechanical strength of alumina ceramics at room temperature, Wahi et al.
of Materials 5science 15,8
75-885.1980, there is an example in which titanium carbide is combined.

In addition, examples of alumina ceramics in which the mechanical strength is not improved but the fracture toughness is improved include Mc, (:
Ceramic Engineer by auley
ing ScienceProceedings 2,
639-660. As reported in 1982, B.
CeramicE by a-Mica and Rice
ngineering 5science Procee
As reported in dings 2 +639-660° 1982, there is an example of a composite of plate-shaped boron nitride, but none of these papers mention mechanical properties at high temperatures.

C. Purpose of the Invention The present invention has been made in view of the above circumstances, and includes:
It is an object of the present invention to provide an alumina-based heat-resistant sintered body that has excellent mechanical strength and high hardness not only at room temperature but especially at high temperatures of 1000° C. or higher, and a method for manufacturing the same.

2. Structure of the invention, that is, the first invention of the present invention has a structure in which 2 to 30% by volume of fine silicon carbide is substantially separated from each other and dispersed in an alumina matrix, and The present invention relates to an alumina-silicon carbide heat-resistant composite sintered body that has excellent mechanical strength and high high-temperature hardness.

In addition, the second invention of the present invention provides fine silicon carbide 2 to 30
The alumina-silicon carbide heat-resistant composite sintering according to the first invention, which comprises molding a mixed powder whose volume percentage is essentially alumina powder, and sintering the molded body at a temperature within the range of 1400 to 1800°C. It pertains to a method for producing a solid.

Fracture of alumina ceramics at high temperatures was reported by Evans et al.
Ramics+ 6 +423-448, 1983, cavitation under stress (nucleation of the crank) and until a critical crank size is reached (s
However, in both cases, preferential diffusion at grain boundaries and local stress states are the subject of consideration.

As a result of intensive research, the inventor discovered that silicon carbide particles with an average particle size of 3 μm or less, or silicon carbide fibers (whiskers) with a diameter of 1 μm or less and a length of 2011 m or less, were incorporated into an alumina matrix independently (separated from each other). ) improves the mechanical properties at high temperatures by dispersing, imparting localized residual stress to the alumina matrix grain boundaries, or by providing conditions that resist atomic diffusion and crank growth at the grain boundaries. We found that it is possible to improve the

The above mechanism will be explained in detail below.

A case in which the silicon carbide particles are smaller than the alumina crystal grains and a case in which the silicon carbide particles are approximately the same size will be considered.

Furthermore, in the former case, we will consider cases in which silicon carbide particles are present at grain boundaries in the alumina matrix and cases in which silicon carbide particles are present within crystal grains.

When silicon carbide particles are smaller than alumina crystal grains and exist at grain boundaries in the alumina matrix, as schematically shown in FIG. Give a tensile residual stress St perpendicular to the plane (this is because silicon carbide has a smaller coefficient of thermal expansion than alumina),
This is unfavorable for cavitation and crank growth, but more importantly, under high-temperature stress, the diffusion path of atoms in the grain boundary 3 phase is narrowed, slowing down the overall diffusion rate, and silicon carbide particles By forming a bridge at the surface and acting as a resistance to crack growth,
Improves mechanical properties at high temperatures. From the viewpoint of forming crosslinks at grain boundaries, fibrous silicon carbide has a greater effect than silicon carbide particles.

When silicon carbide particles exist within the crystal grains of the alumina matrix, as schematically shown in FIG. Relieves local stress levels and improves mechanical properties at high temperatures.

When the silicon carbide particles have the same size as the alumina crystal grains, as schematically shown in FIG. 3, the silicon carbide particles 2
A compressive residual stress Sc is applied to the three grain boundaries of the alumina crystal grains 1 perpendicular to the external tensile stress So above and below to improve mechanical properties at high temperatures. In addition, the left and right grain boundaries 3a of silicon carbide particles 2
It imparts tensile residual stress St to the surface, which is unfavorable for cavitation and crank growth, but moreover,
The silicon carbide particles act to prevent the left and right cracks from coalescing, improving mechanical properties at high temperatures. Note that the maximum compressive residual stress exists at the silicon carbide particle/alumina crystal grain interface, and crack growth along this interface is difficult, and failure does not occur along this interface.

Above, we have explained the case where silicon carbide is particles, but
The principle is the same as above when silicon carbide is in the form of fibers.

It will be understood that this improves the mechanical properties at high temperatures.

Residual stress near the silicon carbide particles or fibers is caused by the difference in thermal expansion coefficients between silicon carbide and alumina. The former has a smaller coefficient of thermal expansion than the latter.

Silicon carbide, which has higher high-temperature hardness than alumina, contributes to the hardness at high temperatures. In the case of silicon carbide fibers, the improvement in high temperature hardness is particularly remarkable.

The present invention has been made based on the above findings.

Next, each phase constituting the sintered body, sintering conditions, etc. will be explained.

The α-type alumina constituting the matrix can be conveniently used, but other crystal forms such as the γ-type may also be used. The purity is preferably 98% or more, and the average particle size is preferably 5 μm or less, more preferably 1 μm or less.

The silicon carbide to be dispersed in the matrix may be in the α-type or β-type crystal form, with a purity of preferably 98% or higher, and its shape may be equiaxed (particle), but it may also be in the form of fibers, which can be micro-cranked when discarded. The mechanical strength at room temperature decreases. In the case of fibrous silicon carbide, the diameter is 1 μm or less and the length is 20 μm or less. If the diameter exceeds 1 μm, microcracks are likely to occur as in the case of the particles mentioned above, and if the length exceeds 20 μm, the silicon carbide fibers become entangled and cannot be separated from each other (particularly at room temperature). Strength begins to decrease.

The proportion of silicon carbide in the matrix is 2 to 30% by volume of the entire sintered body (the area ratio under a microscope is also 2 to 30%). If this amount is less than 2% by volume, the effect of improving high temperature strength will not be noticeable, and if this amount exceeds 30% by volume, there will be many areas where silicon carbide comes into contact with each other, and the mechanical strength at room temperature and high temperature will deteriorate. begins to decline. When silicon carbide is fibrous, the upper limit of the amount of dispersion is preferably 20% by volume.

Based on the present invention, the sintered body is composed of two components, alumina and silicon carbide, but there may be a small amount of other components contained as impurities in the raw materials. As in the base sintered body, a small amount of magnesia (Mg) is added to suppress the growth of alumina grains during sintering.
It is preferable to include O).

As the molding method, all customary molding methods such as press molding, slurry casting, injection molding, and extrusion molding can be employed. Of course, hot pressing can also be used.

Sintering in vacuum or non-oxidizing atmosphere
Perform at a temperature within the range of °C. If the sintering temperature is lower than 1400°C, the density of the obtained sintered body will be low;
At high temperatures exceeding I, abnormal growth of alumina crystal grains occurs, and in both cases, mechanical strength and hardness at room temperature and high temperature decrease.

The optimum sintering temperature varies depending on pressureless sintering, hot pressing and amount of silicon carbide. When the amount of silicon carbide is 2 to 15% by volume in normal pressure sintering, the temperature is preferably 1600 to 1700 °C,
For 15 to 30% by volume, the temperature is preferably 1700 to 1800°C. 1 when the amount of silicon carbide is 2 to 15% by volume in hot press
400-1600°C is preferred, and 15-30% by volume is preferably 1600-1700°C.

E, Example Specific examples of the present invention will be described below.

Alpha-type alumina powder with a purity of 99% or more and an average particle size of 0.7 μm is blended with silicon carbide powder or fibers as shown in Table 1 below, and mixed with ethyl alcohol in a ball mill using a plastic container and alumina balls. The mixture was wet-mixed as a liquid for 1 hour and dried to form a mixed powder.

After press-molding these mixed powders, a compact of approximately 20 x 50 x 15 m was formed using an isotropic water pressure of 1500 kg/cd, and this compact was sintered in an argon gas stream under the sintering conditions shown in Table 1.

(Margins below, continued on next page) Using a diamond grindstone and diamond blade, bending test pieces of 3 x 4 x 36 m were made from these sintered bodies.
A 3x4x4m hardness test piece was taken, and the bending test piece was
00 diamond polishing wheel, 1μ for the hardness test piece.
A mirror finish was applied using a diamond whetstone. Note that the density of the hardness test piece was measured before the hardness test.

The bending test was conducted with an outer fulcrum distance of 30 m and an inner fulcrum distance of 10 m.
The hardness test was carried out using a 4-point bending test method with a crosshead speed of 0.5 m/l 1 in, using a Vickers hardness needle under an argon flow with a load of 1 kg. The test temperatures for both tests were room temperature, 1000°C, and 1200°C.

The test results are also listed in Table 1.

From the same table, the alumina-silicon carbide heat-resistant composite sintered body based on the present invention has the same bending strength and hardness at room temperature as the conventional single-phase alumina sintered body, and has a bending strength of 11 at high temperatures. It can be seen that both the hardness and hardness are significantly improved. In addition, the sintered body in which fibrous silicon carbide is dispersed has higher hardness at both room temperature and high temperature than the sintered body in which granular silicon carbide is dispersed, and the bending strength is further improved, especially at a high temperature of 1200°C. has been done.

From the above results, the alumina-silicon carbide heat-resistant composite sintered body based on the present invention can be expected to significantly improve performance when applied, for example, to tips for cutting tools. It can now be used in fields such as gas turbine parts, which were previously not possible, and we can expect the field of use to expand.

As described in detail of the invention, the alumina-silicon carbide heat-resistant composite sintered body based on the first invention has 2 to 30% by volume of fine silicon carbide that is substantially separated from each other and forms an alumina matrix. Because it has a dispersed structure, it has significantly high mechanical strength and hardness at high temperatures, and is expected to improve the performance of alumina-based sintered bodies and expand the range of applications. Further, as described above, the manufacturing method based on the second invention requires only the use of ordinary ceramic manufacturing equipment and does not particularly require special equipment, so the manufacturing cost is also low.

[Brief explanation of the drawing]

The drawings are diagrams schematically showing the microscopic structure of the alumina-silicon carbide heat-resistant composite sintered body according to the present invention and the state of residual stress in the vicinity of silicon carbide particles. Figure 2 is a schematic diagram when silicon carbide particles exist within alumina crystal grains. Figure 3 is a diagram when silicon carbide particles and alumina crystal grains are of similar size. FIG. In addition, in the symbols shown in the drawings, 1...Alumina crystal grain 2...Silicon carbide particle 3...Alumina crystal Grain boundary 3a...
...... Alumina grain boundary St in contact with silicon carbide particles ...... Residual tensile stress Sc ...... Residual compressive stress SO ...... External It is tensile stress. Agent Patent Attorney Hiroshi Aisaka 10th 20th 30th

Claims (1)

[Claims] 1. It has a structure in which 2 to 30% by volume of fine silicon carbide is substantially separated from each other and dispersed in the alumina matrix, and has excellent mechanical strength at high temperatures and high temperature hardness. High alumina-silicon carbide heat-resistant composite sintered body. 2. Silicon carbide exhibits a granular shape with an average particle size of 3 μm or less,
An alumina-silicon carbide heat-resistant composite sintered body according to claim 1. 3. The alumina according to claim 1, wherein the silicon carbide has a fibrous shape with a diameter of 1 μm or less and a length of 20 μm or less.
Silicon carbide heat-resistant composite sintered body. 4 A mixed powder consisting of 2 to 30% by volume of fine silicon carbide and the remainder substantially alumina powder is molded, and this molded body is heated to 14% by volume.
A method for producing an alumina-silicon carbide heat-resistant composite sintered body having excellent mechanical strength at high temperatures and high high-temperature hardness, which is sintered at a temperature within the range of 00 to 1800°C. 5 Silicon carbide exhibits a granular shape with an average particle size of 3 μm or less,
A method for producing an alumina-silicon carbide heat-resistant composite sintered body according to claim 4. 6. The alumina according to claim 4, wherein the silicon carbide has a fibrous shape with a diameter of 1 μm or less and a length of 20 μm or less.
A method for manufacturing a silicon carbide heat-resistant composite sintered body.