JPH07223866A - Silicon nitride-based composite ceramic and production thereof - Google Patents

Abstract

PURPOSE:To obtain a silicon nitride-based composite ceramic having high tenacity by mixing silicon nitride powder and a sintering assistant with a specific metallic powder, making the mixture into specific flat plates, molding the flat plates, and sintering the molded article to react the metallic powder with the silicon nitride. CONSTITUTION:Silicon nitride powder and a sintering assistant are compounded with metallic powder of at least one kind selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, Al and their alloys. These are mixed with a ball mill, etc. This mixture is made into flat plates having d/t>4 where (d) is the minimum diameter of the flat plates and (t) is their thicknesses. These flat plates are molded and sintered at 1500-2000 deg.C. The metallic powder reacts with silicon nitride and is led into its nitride and silicide. This process works to produce a silicon nitride-based composite ceramic having a strengthened phase consisting of the metallic nitride and silicide and having flat shapes in which the minimum diameter (d) and the depth (t) have the relation of d/t>4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride-based composite ceramic and a method for producing the same for the purpose of increasing toughness.

[0002]

2. Description of the Related Art Silicon nitride has been put into practical use as engineering ceramics until now, but its toughness is not sufficient, and its use in a wide range is hindered. Therefore, compounding has been attempted for the purpose of improving toughness, and various techniques have been disclosed. For example, a silicon nitride sintered body in which the binder phase is a nitride and / or a silicide in which a metal phase (W, Mo, Cr, Al, Ta, Ti) is transformed by heating at high temperature (Japanese Patent Laid-Open No. 58-58).
-194775), Ti, V, Cr, etc. IVa, V
Silicon nitride composites reinforced with particles of carbides, nitrides, silicides, oxides, and oxynitrides of the a and VIa group elements (Japanese Patent Laid-Open No. Sho 60-62).
-180962, 61-31358), a plate-like particle having a specific shape of a silicide and / or a carbide (d ₁ / d when the major axis of the major surface of the particle is d ₁ and the minor axis is d ₂ ). ₂ <
10 and the thickness thereof is ⅕ or less of d ₂ ) reinforced silicon nitride sintered body (JP-A-61-53167).
No.), a silicon nitride composite reinforced with whiskers such as silicon carbide and long fibers (JP-A-61-291463 and 61-61).
No. 227969, No. 62-12671) and the like are disclosed.

However, in the method of using a nitride and / or a silicide in which the metal phase is transformed by high temperature heating as a binder phase, a high density as in the case of using an oxide type auxiliary agent known as a binder phase is used. It is difficult to obtain the above sintered body, and as a result, sufficient characteristics cannot be obtained. Also,
In the particle strengthening of, the toughness is not sufficiently improved, and in the method of strengthening with the plate-like particles having the specific shape, the toughness is improved as compared with the method of, but it is not yet sufficient. Moreover, a step of producing a plate-like powder having a specific shape is required. Further, in the strengthening with whiskers, the toughness improvement as much as that of the plate-like powder has been achieved, but the whiskers are expensive and it is difficult to uniformly disperse them in the matrix, so the whiskers become lumpy. This serves as a destruction source and deteriorates the properties of the composite. On the other hand, with the reinforcement by long fibers, high toughness is obtained in the direction perpendicular to the fiber orientation. However, long fibers are very expensive, the manufacturing process is complicated, and as a result, the cost is very high.

[0004]

The object of the present invention is to solve the above problems,
Provided are a silicon nitride-based composite ceramic having good productivity and significantly improved toughness, and a method for producing the same.

[0005]

SUMMARY OF THE INVENTION The present invention is directed to a silicon nitride-based composite ceramic in which Ti, Zr, Hf, V, N
b, Ta, Cr, Mo, Al and nitrides and silicides of one or more metals selected from their alloys as a strengthening phase,
The reinforced phase has a flat shape, and the relationship between the minimum diameter d of the flat surface and the thickness t is d / t> 4. Ti, Zr, Hf, V, Nb, Ta, Cr, M as auxiliary agents
One or more metal powders selected from o, Al and their alloys are added and mixed so that the relationship between the minimum diameter d of the flat surface and the thickness t satisfies d / t> 4. After flattening and molding the mixed powder,
The present invention relates to a method for producing a silicon nitride-based composite ceramic, which comprises sintering at a temperature of 2000 ° C. and reacting the metal powder with silicon nitride to transform it into a nitride and a silicide.

The silicon nitride-based composite ceramic of the present invention is
Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, Al
And nitrides and silicides of one or more metals selected from their alloys as the strengthening phase. The reinforced phase has a flat shape, and the relationship between the minimum diameter d of the flat surface and the thickness t is d
It was found that the toughness was significantly improved by setting / t> 4. Such a silicon nitride based composite ceramic is
For the silicon nitride powder and the sintering aid, Ti, Zr, Hf, V,
The metal powder is flattened by adding and mixing one or more metal powders selected from Nb, Ta, Cr, Mo, Al and alloys thereof. Sintering at a temperature, reacting the metal powder with silicon nitride to convert it to a nitride and a silicide,
It can be produced by using these nitrides and silicides as a strengthening phase. This is because it was found that the morphology of the strengthening phase strongly depends on the deformation state of the added metal powder during mixing. That is, the additive metal powder is flattened by mechanical mixing with a mixing medium such as a ball at the time of mixing, but the shape of the nitride and the silicide formed by the reaction with the silicon nitride at the time of sintering is flat. It was found that the shape of the converted metal powder hardly changed. Therefore, if the additive metal powder is flattened so as to satisfy d / t> 4 by controlling the mixing conditions, it is possible to make the strengthening phase to be flat particles that also satisfy d / t> 4. It was.

The silicon nitride powder used in the present invention is as follows:
Although not particularly limited, a silicon nitride powder obtained by firing an amorphous silicon nitride powder and / or a nitrogen-containing silane compound in a nitrogen-containing inert gas atmosphere or a nitrogen-containing reducing gas atmosphere is preferably used. As a sintering aid, Al ₂
Known oxide-based sintering aids such as O ₃ and Y ₂ O ₃ are used. If the sintering aid is not added, sufficient densification cannot be achieved. The metal powder to be added may be one kind of the metal, as long as it produces both nitride and silicide to the same extent by the reaction with silicon nitride at the sintering temperature, but it is usually a metal that mainly produces nitride. At least one kind of powder and a metal powder that mainly forms a silicide are added. For example, Ti is T
Since iN, Mo mainly produce MoSi ₂ , and Ta mainly produces TaSi ₂ , the kind and proportion of the additive metal may be selected so that both the nitride and the silicide are produced to the same extent. If the production is biased to either the nitride or the silicide, it causes gas defects due to residual N ₂ gas, toughness due to residual Si, and further lowers strength, so that sufficient characteristics cannot be obtained.

The method for mixing the silicon nitride powder and the sintering aid with the metal powder is not particularly limited, and both wet and dry methods can be adopted. Ethanol, methanol or the like is generally used as a solvent in the case of wet mixing. As the mixing device, a ball mill, a vibration mill, an attritor, a planetary ball mill or the like can be used. By mixing, the metal powder deforms from a spherical shape to a flat shape, but the mixing time,
Since the amount of deformation changes depending on conditions such as the number of revolutions, the mixing conditions may be controlled so that the flattened shape satisfies d / t> 4. Further, in this mixing process, since the silicon nitride powder adheres to the surface of the flattened metal powder, the granulation of the metal particles during sintering can be prevented and the reaction between the metal and silicon nitride is promoted. Although undeformed particles remain after mixing depending on the particle size of the metal powder used, if the amount of flattened particles is appropriate, the toughness is improved. Usually, it is desirable to use a powder having a particle size of 1 to 100 μm in order to easily promote flattening. The particle size of the silicon nitride powder is not particularly limited, but an average particle size of 1 μm or less with good sinterability is desirable.

After the obtained mixed powder is molded into a desired shape, it is heated to 150 in a non-oxidizing gas atmosphere such as nitrogen or argon.
Sinter at a temperature of 0-2000 ° C. As a sintering method, C
The flattened particles are three-dimensionally randomly oriented in the process of sintering the IP-molded compact under normal pressure and further densifying it by HIP. Since the converted particles are two-dimensionally oriented in the direction perpendicular to the pressing direction, the characteristics of the sintered body can be made anisotropic. The volume ratio of the strengthening phase can be selected in a wide range of 0.1 to 95% depending on the blending ratio of silicon nitride and the added metal, but in order to improve the toughness, the volume ratio of particles satisfying d / t> 4 is set to 10%. It is preferably 60%.

[0010]

According to the present invention, the strengthening phase has a flattened form, and at least one of silicon nitride and at least one nitride and silicide having different mechanical properties and thermal properties from each other are contained. Deflection of cracks in the vicinity of particles becomes more complicated at the time of fracture than in the case of grain strengthening, as well as strengthening with either nitride or silicide, and as a result, the energy required for fracture is large. As a result, the toughness can be significantly improved. Further, since nitrides generally have high hardness and silicides have excellent oxidation resistance, it is possible to improve hardness and oxidation resistance at the same time. Furthermore, since the morphology of the strengthening phase can be flattened by utilizing the deformation during the mixing of the silicon nitride powder and the additive metal powder, it is easy to control the shape of the strengthening phase, and the strengthening phase is Since it is generated in-situ, it has good compatibility with silicon nitride and does not introduce manufacturing defects such as pores at the interface. In addition, the manufacturing process of the irregular-shaped reinforcing particles is not required, and the manufacturing process is the same as that of the silicon nitride single phase, so that the cost increase due to the compounding can be suppressed. Further, as described above, if the reinforcing phase is two-dimensionally oriented, the toughness in the direction perpendicular to the orientation direction can be further improved.

[0011]

EXAMPLES The present invention will be described more specifically by showing Examples and Comparative Examples below. Example 1 Silicon nitride powder (E-10, manufactured by Ube Industries, Ltd.), titanium, and molybdenum powder were reacted with silicon nitride, titanium, and molybdenum to obtain a silicon nitride volume ratio of 50%, and molybdenum silicide (MoSi ₂ ). And titanium nitride (TiN) have a volume ratio of 28% and 22%, respectively.
Titanium: molybdenum was weighed so as to be 2.8: 1.3: 1, and alumina and yttria were added as a sintering aid in a weight ratio of 3% to silicon nitride. These mixed powders were mixed in ethanol solvent using silicon nitride balls to
Ball mill mixing was performed for 00 hours. 1 and 2 show a scanning electron microscope image and an optical microscope image of a cross-sectional structure of the appearance of the mixed powder after the ball milling. From these, it can be seen that the metal powder is flattened by the ball mill mixing and further the silicon nitride powder is attached to the surface.

This mixed powder was placed in a graphite mold and hot-pressed at 1750 ° C. under a pressure of 200 kg / cm ² in argon for 1 hour for sintering. Figure 3
An optical microscope image of a cross-sectional structure of the obtained sintered body is shown. It can be seen that the reinforcing phase is two-dimensionally oriented and satisfies d / t> 4. As a result of X-ray diffraction, strong peaks of β-silicon nitride, titanium nitride (TiN) and molybdenum silicide (MoSi ₂ ) were obtained, and the strengthening phase was mainly titanium and titanium nitride (TiN).
It was confirmed that the result is that molybdenum was generated by reaction with molybdenum silicide (MoSi ₂ ) in N). The fracture toughness of this sintered body was measured by SEPB method to be 12MP.
A high value of a√m was obtained.

Comparative Example 1 A mixed powder having the same composition as in Example 1 was used, and the ball mill time was 1
Sintering was performed under the same conditions as in Example 1 with 0 hours and 24 hours. The shape of the strengthening phase of the obtained sintered body was d /
t was 1-2, and d / t was 2-4 in 24 hours. When the fracture toughness of this sintered body was measured by the SEPB method,
The values were 7.5 MPa√m and 9 MPa√m, respectively, which were higher than 6 MPa√m obtained for the silicon nitride single-phase sintered body sintered by the same method for comparison. But,
It was confirmed that the toughness was significantly improved by setting the shape of the strengthening phase to d / t> 4, which is lower than 12 MPa√m of Example 1.

Example 2 A sintered body was manufactured under the same conditions as in Example 1, except that the volume ratio of the strengthening phase (TiN and MoSi ₂ ) was changed. The results of measuring the fracture toughness of the obtained sintered body by the SEPB method are shown in FIG.
It was found that a high toughness value was obtained when the volume ratio of the strengthening phase was 10 to 60%, but the toughness was lowered when it was less than 10% or more than 60%.

Example 3 and Comparative Example 2 A sintered body was manufactured under the same conditions as in Example 1, except that the kind of metal powder was changed. Table 1 shows the results obtained by measuring the fracture toughness of the obtained sintered body by the SEPB method. High toughness was exhibited when both nitride and silicide were produced to the same extent, but toughness was low when nitride or silicide was used alone due to gas defects due to residual N ₂ gas and residual Si. It became a value.

[0016]

[Table 1]

[Brief description of drawings]

FIG. 1 is a scanning electron microscope photograph as a substitute for a drawing showing the particle structure of a mixed powder after a ball mill of Example 1 of the present invention.

FIG. 2 is an optical micrograph as a substitute for a drawing showing the particle structure of the mixed powder after the ball mill of Example 1 of the present invention.

FIG. 3 is an optical micrograph, which is a drawing and shows the structure of the ceramic material of the sintered body obtained in Example 1 of the present invention.

FIG. 4 is a diagram showing the relationship between the volume fraction of the strengthening phase and the fracture toughness of the sintered body obtained in Example 2 of the present invention.

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Claims (2)

[Claims] 1. In a silicon nitride-based composite ceramic,
Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, Al
And nitrides and silicides of one or more metals selected from these alloys as a strengthening phase, the form of the strengthening phase is flat, and the relationship between the minimum diameter d of the flat surface and the thickness t is d / t. > 4, a silicon nitride-based composite ceramics. 2. A silicon nitride powder and a sintering aid containing Ti, Z
By adding and mixing one or more metal powders selected from r, Hf, V, Nb, Ta, Cr, Mo, Al and alloys thereof, the metal powders having a flat surface minimum diameter d and thickness t Flattened so that the relationship satisfies d / t> 4, and after molding the mixed powder, sinter at a temperature of 1500 to 2000 ° C. to react the metal powder with silicon nitride to form a nitride and a silicide. The method for producing a silicon nitride based composite ceramic according to claim 1, wherein the method is modified.