JP2746760B2 - Silicon nitride-silicon carbide composite sintered body and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride-silicon carbide composite sintered body mainly composed of silicon nitride and silicon carbide, and a method for producing the same. And a sintered body having excellent high-temperature strength and hardness and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, silicon nitride-based sintered bodies have been applied to engineering ceramics, particularly as structural materials for heat engines, because of their excellent strength, advanced properties, and excellent thermal and chemical stability.

[0003] As a method of obtaining such a silicon nitride sintered body, a sintering aid such as an oxide of an element of Group 3a of the periodic table is added to silicon nitride powder, mixed, molded, and then non-oxidized. It is obtained by firing at a temperature of 1500 to 2000 ° C. in an atmosphere.

[0004] However, while the silicon nitride sintered body has excellent characteristics, it has a problem that its strength and the like are reduced at high temperatures. Up to now, the problem of deterioration of the high-temperature strength has been improved by improving the sintering aid and changing the firing atmosphere and the firing pattern.
At present, no definitive measures have been taken.

[0005]

Problems to be Solved by the Invention
One factor is that, for example, α-Si
₃ N ₄ in order to produce the grain growth of silicon nitride grains as well as phase transition to Beta─Si ₃ N ₄ at around 1600 ° C., a relatively large needle-like crystals of a particle size of precipitated this large particles becomes broken source The characteristics may be degraded. Since such sintering behavior is an inevitable factor in sintering silicon nitride, the strength of the silicon nitride sintered body itself is limited to about 1000 MPa at room temperature and about 700 MPa at 1400 ° C.

[0006] Also, from the viewpoint of hardness, there is also a limit to the desire for high hardness because the grain size tends to vary systematically for the above reasons.

[0007] In response to such a phenomenon, there has been an attempt to suppress the grain growth of silicon nitride by adding silicon carbide or the like to silicon nitride, but from the viewpoint of overall characteristics such as strength and hardness. At present, it has not been studied and has not been sufficiently effective.

[0008]

Means for Solving the Problems The present inventors have examined the above problems in detail, and found that a system obtained by adding a Group 3a element oxide of the periodic table to silicon nitride was used. Silicon carbide is added in a specific amount, and sintering is performed in the sintering process to convert silicon nitride from α-Si ₃ N ₄ to β-
Densification while suppressing phase transition to Si ₃ N ₄ , leaving α-Si ₃ N ₄ in the sintered body, thereby effectively suppressing the grain growth of silicon nitride crystals and the strength of a fine structure And that a sintered body excellent in hardness can be obtained.

That is, the present invention contains 93 to 99.5 mol% of silicon nitride containing impurity oxygen and 0.5 to 7 mol% of a Group 3a element of the periodic table in terms of oxide. A composite sintered body in which a silicon carbide component is dispersed and contained in a proportion of 1 to 100 parts by weight with respect to 100 parts by weight of a silicon nitride component, wherein the silicon nitride has an average particle diameter of 1 μm or less, and the silicon carbide has an average Α-Si ₃ N ₄ / β-S in the silicon nitride crystal exists as crystals each having a particle size of 1 μm or less.
i ₃ ratio of N ₄ is 0.01, and Al in the sintered body, Ca, is the total amount of an oxide in terms of Mg 0.
It is characterized by being at most 5% by weight.

The method for producing a composite sintered body according to the present invention is characterized in that silicon nitride containing impurity oxygen is 93 to 99.5 mol% and Group 3a element of the periodic table is an oxide of 0.5 to 7%. The silicon carbide component is dispersed and contained at a ratio of 1 to 100 parts by weight with respect to 100 parts by weight of the silicon nitride component respectively contained at a ratio of mol%, and the total amount of Al, Ca and Mg in terms of oxide is 0. The sintered body having a weight ratio of not more than 0.5% by weight is fired at a temperature of 1450 to 1900 ° C. through a glass layer under a pressure of 50 MPa or more, so that silicon nitride has an average particle size of 1 μm or less and silicon carbide has an average particle size of 1 μm or less. Each exists as a crystal,
Α-Si ₃ N ₄ / β-Si ₃ in the silicon nitride crystal
It is characterized in that a sintered body having a N ₄ ratio of 0.01 to 1 is produced.

According to the silicon nitride-silicon carbide composite sintered body of the present invention, it is composed mainly of a silicon nitride component and a silicon carbide component in terms of composition. The silicon carbide component basically means only silicon carbide particles, while the silicon nitride component comprises a system containing a sintering aid component in a sintered body, including silicon nitride containing impurity oxygen.

The silicon nitride component contains 93 to 99.5 mol%, particularly 95 to 99.5 mol% of silicon nitride containing impurity oxygen, and 0.5 to 7 mol of a Group 3a element of the periodic table in terms of oxide. %, Especially 1 to 5 mol%. The Group 3a element of the periodic table acts as an oxide to increase the sinterability of the entire system. If the amount is less than 0.5 mol%, the sinterability is reduced and a dense sintered body is obtained. The properties are deteriorated, and if it exceeds 7 mol%, the high temperature strength is deteriorated.

In the silicon nitride component, the impurity oxygen contained in the silicon nitride also contributes to the improvement of sinterability as, for example, SiO ₂ , similarly to the Group 3a element of the periodic table. Table 3a Group element oxide equivalent (RE
₂ O ₃₎ and SiO and SiO ₂ in terms of the impurity oxygen ₂ /
When the molar ratio represented by RE ₂ O ₃ is 0.5 to 10, particularly 1 to
4 is desirable.

According to the present invention, such a silicon nitride component 10
Silicon carbide component is added at a ratio of 1 to 100 parts by weight with respect to 0 parts by weight. The amount of the silicon carbide component is limited to the above range. When the silicon carbide component is less than 1 part by weight, there is no effect of suppressing the grain growth of silicon nitride crystal by adding silicon carbide,
The dislocation of silicon nitride from α-Si ₃ N ₄ to β-Si ₃ N ₄ progresses, and high strength and high hardness cannot be obtained. If it exceeds 100 parts by weight, sinterability is reduced and strength is deteriorated. is there. From the viewpoint of characteristics, the amount of the silicon carbide component is desirably 30 to 70 parts by weight based on 100 parts by weight of the silicon nitride component.

According to the composite sintered body of the present invention, the silicon nitride crystal has α-Si ₃ N ₄ and β-Si ₃ N ₄ coexisting in the sintered body, and α-Si ₃ N ₄ / β- When the ratio of Si ₃ N ₄ is 0.
It is important that it is from 01 to 1, especially from 0.05 to 0.5. This is because when the ratio is smaller than 0.01, α-Si
_Since it is considered that the dislocation from ₃ N ₄ to β-Si ₃ N ₄ has almost completely progressed, the average grain size of the silicon nitride crystal increases, and abnormal grains easily occur. This is because high hardness cannot be obtained. Conversely, if it exceeds 1, it means that sintering has not progressed, and the density of the sintered body is low, and both the strength and the hardness are greatly reduced.

Further, from the above viewpoint, the silicon nitride crystal and the silicon carbide crystal have an average particle diameter (minor diameter) of 1 μm or less,
In particular, 0.8 μm or less, present in the form of particles having an aspect ratio represented by a major axis / minor axis of 2 to 10, particularly 3 to 9 on average,
The silicon carbide crystal itself has a granular shape, and has an average particle size of 1 μm in the grain boundary of the silicon nitride crystal or in the silicon nitride crystal grain.
It is desirable that the particles be present as particles having a particle size of at most m, particularly at most 0.8 μm. Most of the silicon carbide crystals are β-type, but α-type may exist in some cases.

As described above, the shapes of the silicon nitride crystal and the silicon carbide crystal are limited as described above. If the silicon nitride crystal is larger than 1 μm, it easily becomes a source of fracture of the sintered body and causes deterioration in strength. If the aspect ratio is smaller than 2, the entanglement between the particles becomes small, so that the particles easily move at a high temperature and the creep characteristics are deteriorated. On the other hand, when the particle size of the silicon carbide crystal is larger than 1 μm, the sinterability is deteriorated, and the needle-like formation of the silicon nitride crystal is hindered, and the characteristics are deteriorated.

Further, as a component constituting a grain boundary, for example, Al ₂ O ₃ , Ca
It is known to use O, MgO, etc., but if these oxides are present at the grain boundary or partially dissolved in the silicon nitride crystal, the melting point of the grain boundary is lowered and the viscosity at high temperature is also lowered. Therefore, the creep characteristics are deteriorated together with the high temperature strength. Therefore, it is desirable to control the metal elements of Al, Ca, and Mg to be 0.5% by weight or less, particularly 0.3% by weight or less in the total amount in terms of oxide.

The elements of Group 3a of the periodic table used in the present invention include Y, Sc, Er, Yb, Ho,
Among them, Y is easily aggregated in the sintered body and abnormal grain growth is apt to occur.
Yb is particularly desirable.

Next, the method for producing the silicon nitride-silicon carbide composite sintered body of the present invention will be described. First, as starting materials, silicon nitride powder, silicon carbide powder, 3a of the periodic table
Group oxides and, in some cases, silicon oxide powders are used.

The silicon nitride powder contains 95% of α-Si ₃ N ₄ because of its low sinterability due to the fact that silicon carbide is contained in the system.
It is preferred that it be present in the above ratio, and those having an average particle size of 1 μm or less and an impurity oxygen content of 2% by weight or less are suitably used. As the silicon carbide powder, any of α-type and β-type can be used, and a powder having an average particle diameter of 1 μm or less and an impurity oxygen amount of 2% by weight or less is used. These silicon nitride powder and silicon carbide powder are each present as individual powders,
Powder obtained by compounding silicon nitride and silicon carbide at a predetermined ratio can also be used.

Next, using the above powder, a nitride containing 92 to 99.5 mol% of silicon nitride containing impurity oxygen and 0.5 to 10 mol% of an oxide of a Group 3a element of the periodic table. 1 to 10 parts by weight of silicon carbide component per 100 parts by weight of silicon component
After being weighed to 0 parts by weight and thoroughly mixed, the mixture is molded into a desired shape by a known molding method, for example, a molding method such as press molding, injection molding, extrusion molding, casting molding, or cold isostatic pressing. I do. It should be noted that in the above process, the mixing of Al, Mg, and Ca elements contained in the compact from each step was performed so that the total amount in terms of oxide would be 0.5% by weight or less. avoid. For example, it is necessary to use raw materials having a small amount of these elements, or to consider the material of the balls used in, for example, ball mill mixing or the like when mixing, and to limit mixing from mixing.

Next, the compact obtained by the above method is fired at a firing temperature of 1450 to 1900 ° C. As the firing means, normal pressure firing, hot press firing,
Nitrogen gas pressure firing (QPS firing), hot isostatic pressure firing (H
IP baking) and the like, and in some cases, baking can be performed in combination with these. However, from α-Si ₃ N ₄ to β-
An optimal method of firing while suppressing dislocation to Si ₃ N ₄ is to apply a sealing material made of glass or the like to the surface of the molded body obtained as described above, and to fire at a high temperature and a high pressure.
The so-called seal HIP method is adopted.

As a specific method, first, a glass such as BN powder or the like is used for the purpose of preventing the molded body obtained by the above-mentioned method from reacting with glass or the like as a sealing material in the firing step before firing. A slurry having poor wettability with the slurry is applied to a molded body, or the slurry is spray applied.

Next, for the molded body to which BN has been applied,
A glass powder which forms a seal at the time of firing is applied to the surface thereof, or the above-mentioned molded body is encapsulated in a glass capsule. As another method, the molded body is buried in a container filled with glass powder.

Thereafter, 1450 to 190 by the HIP method.
HIP baking is performed at a temperature of 0 ° C. and a pressure of 50 MPa or more. The sintering is performed by first introducing a nitrogen gas having a temperature equal to or higher than the softening point of the glass present on the surface of the molded body and equal to or higher than the decomposition equilibrium pressure of silicon nitride at the temperature by about 0.01 to 0.2 MPa. It is softened to form an impermeable film made of glass on the surface of the molded body. After the gas-impermeable membrane is completely formed, the temperature and pressure are increased until the pressure in the furnace can be sufficiently densified. The pressure medium at this time is nitrogen,
Use an inert gas such as argon. Thereafter, after the sintering has sufficiently proceeded, the temperature and pressure are reduced to end the sintering.

The firing temperature was set to 1450 to 1900.
The reason why the firing temperature is limited to 1 ° C. is that if the firing temperature is higher than 1900 ° C., the dislocation of silicon nitride from α-type to β-type proceeds in the sintered body and crystals grow and the grain size increases, thereby increasing the strength. Is deteriorated.

The obtained sintered body is
The grain boundary of the sintered body is crystallized by heat treatment in a non-oxidizing atmosphere at 0 to 1700 ° C., for example, Si ₃ N ₄ —R
High temperature properties can be improved by precipitating a known crystal phase of E ₂ O ₃ (RE: Group 3a element of the periodic table) -SiO ₂ system.

[0029]

EXAMPLE The raw material powder had an average particle size of 0.3 μm and α-S
Silicon nitride powder having an i ₃ N ₄ content of 98% and oxygen content of 1.3% by weight, silicon carbide powder having an average particle diameter of 0.3 μm, and Y ₂ O ₃ and Sc having an average particle diameter of 0.5 μm ₂ O ₃ , E
Using each powder of r ₂ O ₃ , Yb ₂ O ₃ , Ho ₂ O ₃ , Dy ₂ O ₃ and silicon oxide powder, they were weighed and mixed so that the composition became the ratio of Table 1, and this was mixed with a binder. It was mixed and ground in methanol. After drying and granulating the obtained slurry, it was press-molded at a pressure of 1 ton / cm ² .

BN powder (particle size: 1)
５5 μm) after applying the paste with a thickness of 1 to 10 mm,
Glass having SiO ₂ as a main component was applied in a thickness of 1 to 10 mm. This molded body was fired at 1750 ° C. under a 196 MPa nitrogen pressurized atmosphere for 1 hour under hot isostatic pressure. In addition,
Sample No. 14 in Table 1 was fired at a firing temperature of 1950 ° C.

For the obtained sintered body, the relative density was determined by Archimedes method, the four-point bending strength at room temperature and 1400 ° C. based on JISR1601, and the average of silicon nitride crystal and silicon carbide crystal was determined from an electron micrograph. The particle size was measured. Further, Vickers hardness under a load of 20 kg was measured.

For the sintered body, the intensity of (101) and (210) of β-Si ₃ N _{4 was determined} from the X-ray diffraction curve to L (10).
1), L (210), (102), (210) of α-Si ₃ N ₄
When the peak intensity of each crystal phase is l (102) and l (210),
The α / β ratio was determined by the following equation: α / β = Σ [L (101) + L (210)] / Σ [l (102) + l (210)]. The results are shown in Table 2.

[0033]

[Table 1]

[0034]

[Table 2]

According to the results shown in Tables 1 and 2, in Sample No. 14, in which no α-Si ₃ N ₄ was left in the sintered body, only a strength of 63 MPa at 1400 ° C. was obtained. By lowering the sintering temperature and leaving α-Si ₃ N ₄ by adding an appropriate amount of SiC, Si ₃ N ₄ and SiC can be present as fine particles, and the strength at 1400 ° C. is 800 MPa.
a and a hardness of 19 GPa or more.

However, the periodic table 3a in the silicon nitride component
In the sample in which the amount of the group element exceeds 7 mol% in terms of the oxide, the high-temperature characteristics are greatly deteriorated, and when it is less than 0.5 mol%, the sinterability is greatly reduced, and a high-density sintered body cannot be obtained.

[0037]

As described in detail above, according to the present invention,
Silicon carbide is added to silicon nitride containing a Group 3a element oxide of the periodic table, and α-Si ₃ N ₄ is left in the sintered body, so that each of the silicon nitride crystal and the silicon carbide crystal is finely divided. Room temperature and 1400
It has excellent strength at high temperatures of ° C. and can provide high hardness. Thereby, the composite sintered body can be used for heat engine structures such as gas turbines and turbo rotors, or as other heat-resistant and wear-resistant materials, and can be used in a wide range of applications.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hideki Uchimura 1-4-4 Yamashita-cho, Kokubu-shi, Kagoshima Inside the Kyocera Research Institute (72) Inventor Masahito Nakanishi 1-4-4 Yamashita-cho, Kokubu-shi, Kagoshima Kyocera Corporation Examiner in the Research Institute Yoichi Gotani (56) References JP-A-1-275470 (JP, A) JP-A-58-74570 (JP, A)

Claims (2)

(57) [Claims] 1. A method according to claim 1, wherein the silicon nitride containing impurity oxygen is 93 to 99.
5 mol%, and 0.3% of an element of Group 3a of the periodic table in terms of oxide.
Silicon nitride component 1 contained at a ratio of 5 to 7 mol%
A composite sintered body in which a silicon carbide component is dispersed and contained at a ratio of 1 to 100 parts by weight with respect to 00 parts by weight, wherein the silicon nitride has an average particle diameter of 1 μm or less, and the silicon carbide has an average particle diameter of 1 μm. The following crystals are present, respectively, the ratio of α-Si ₃ N ₄ / β-Si ₃ N _{4 in} the silicon nitride crystal is 0.01 to 1, and Al, C in the sintered body
(a) A silicon nitride-silicon carbide composite sintered body, wherein the total amount of Mg in terms of oxide is 0.5% by weight or less. 2. The method according to claim 1, wherein the silicon nitride containing impurity oxygen is 93 to 99.
5 mol%, and an oxide of the Group 3a element of the periodic table in an oxide of 0.5 to
Silicon nitride components 100 each containing 7 mol%
A molded article containing a silicon carbide component dispersedly in a proportion of 1 to 100 parts by weight with respect to parts by weight and having a total amount of 0.5% by weight or less in terms of oxides of Al, Ca, and Mg is a glass layer. From 1450 to 1900 under a pressure of 50 MPa or more
Sintering at a temperature of 0 ° C., the silicon nitride has an average particle size of 1 μm or less,
Silicon carbide exists as crystals each having an average particle size of 1 μm or less, and α-Si ₃ N ₄ / β-
A method for producing a silicon nitride-silicon carbide composite sintered body, which comprises producing a sintered body having a Si ₃ N ₄ ratio of 0.01 to 1.