JP3213797B2 - Silicon nitride sintered body and method for producing 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 sintered body having excellent strength suitable for a structural member for a heat engine such as a diesel engine or a gas turbine, and a method for producing the same.

[0002]

2. Description of the Related Art Silicon nitride sintered bodies are excellent in mechanical properties, heat resistance, corrosion resistance, etc., and have been applied to structural materials used in heat engines such as automobile engines and gas turbine engines. ing. Here, silicon nitride is difficult to sinter because of its high covalent bonding property.
Sintering aids such as l ₂ O ₃ , MgO and rare earth oxides are required. Further, the sintering temperature is 1500 ° C. or higher, and in some cases, the sintering is performed at 1800 ° C. or higher in a nitrogen pressurized atmosphere.

[0003] When graphite is used as a heating element of a silicon nitride firing furnace, it is considered that the firing atmosphere is a reducing atmosphere containing trace amounts of CO and / or C. The sintered body obtained under such sintering conditions has a rough baked surface, which is one of the causes of a decrease in mechanical strength. For this reason, various studies have been made to improve the baked surface strength. For example, in Japanese Patent Application Laid-Open No. 6-219840, by using an appropriate amount of β-Si ₃ N ₄ powder mixed with α-Si ₃ N ₄ powder as a silicon nitride raw material, the pore size of the burnt surface is reduced, A technique for reducing the baked surface strength to 70% or more of the internal strength is disclosed. In JP-A-7-41367, the fracture toughness value of a polished surface polished to a depth of 50 μm from a burnt surface is 5.5 MPa · m ¹ by limiting appropriate sintering aids and firing conditions. ^By setting it to ^{/ 2} or more, the strength on the baked surface is improved.

[0004]

Since the silicon nitride sintered body is fired at a high temperature in an inert atmosphere containing a small amount of reducing gas as described above, the burning surface of the obtained sintered body is usually It often becomes rough, and its mechanical strength is usually lower than that obtained by polishing the surface of the sintered body. For this reason, particularly when commercializing products requiring high mechanical strength, it has been necessary to polish the surface of the sintered body. However, in the case of a structural member having a complicated shape, polishing is difficult, and even in a simple shape, the cost required for polishing is very high. I have.

[0005] In view of the fact that the cause of the low burnt surface strength is that concave portions such as open pores existing in the burnt surface act as defects, the present inventors consider that the burnt surface has a concave portion. Have also found that the mechanical strength of the sintered body can be improved by forming a specific crystal therein. That is, the present invention provides a silicon nitride sintered body of the present invention and a method for producing the same, for the purpose of improving the strength of the burnt surface even if there are recesses on the burnt surface.

[0006]

According to a first aspect of the present invention, a needle-like crystal of β-silicon nitride is formed in a concave portion present on the surface of a silicon nitride sintered body. A silicon nitride sintered body having at least 30% by volume, wherein the
The gist of the present invention is a silicon nitride sintered body characterized in that there is no grain boundary phase between the needle-like crystals of β-silicon nitride.

According to a second aspect of the present invention, the silicon nitride sintered body according to claim 1, wherein the silicon nitride sintered body has a compound containing a rare earth element, Si, O and N as a grain boundary phase. Is the gist.

According to a third aspect of the present invention, the maximum length of the recess is 100 μm or less.
The gist is the silicon nitride sintered body described in any one of 2.

A fourth aspect of the present invention is a mixing and molding step of mixing a raw material powder containing silicon nitride powder and a rare earth element oxide powder as main components to form a compact, and forming the compact into an inert atmosphere of 10 atm or less. A pre-sintering step of firing at 1750 to 1950 ° C. to form a pre-sintered body;
sintering at 1750 to 1950 ° C. under an inert atmosphere of at least tm or gas pressure firing or HIP firing to form a silicon nitride sintered body.
A method for producing a silicon nitride sintered body, characterized in that the main firing step is performed in a silicon nitride case or a non-oxide case in which silicon nitride is coated on the inner surface. And

Here, a rare earth element, Si,
A silicon nitride sintered body containing a compound containing O and N means that a rare earth element compound or the like as a sintering aid becomes a liquid phase at the time of firing, and this is an R-Si-ON (R: rare earth element) -based material after cooling. Glass or R-Si-ON-based compound crystal exists at the grain boundary of silicon nitride particles.

[0011] In the recesses present on the burning surface of the sintered body, β-
Having a silicon nitride needle crystal of 30% by volume or more of the concave portion means the following. That is, the pre-sintered body is subjected to gas pressure firing or HIP firing in a silicon nitride case or a non-oxide case in which silicon nitride is coated on the inner surface to thereby obtain a pre-sintered body. This refers to a state in which a large number of needle-like crystals of β-silicon nitride have been formed in concave portions present on the burning surface of the sintered body. Further, 30% by volume or more of the concave portion means that the space inside the opening portion of the concave portion is regarded as the space of the concave portion, and the volume of the needle crystal of β-silicon nitride occupying the space is 30% or more. . When the needle-shaped crystal plays the role of a beam, the stress in the concave portion can be reduced or the strength at the time of starting breaking can be improved. Here, since there is no grain boundary phase between the acicular crystals of β-silicon nitride in the concave portions, it is necessary that the acicular crystals be denser. The effect of improvement can be obtained.

On the other hand, since the recess itself is one of the origins of destruction, it is natural that a smaller and smaller one is preferable. When the maximum length of the concave portion existing on the burnt surface of the silicon nitride sintered body is larger than 100 μm, the β-
Even if needle-like crystals of silicon nitride are present in the recesses, the effect is reduced. The maximum length of the recess is defined as the width of the recess in the outer shell of the low-density portion which is recognized as being substantially composed of needle-like crystals and / or voids of β-silicon nitride without substantially having a grain boundary phase. Say the maximum length.

In the manufacturing method according to the second aspect of the present invention, the pre-firing step is performed at 1750 to 1950 ° C. in an inert atmosphere of 10 atm or less.
The reason is that if the atmospheric pressure is further increased, sintering is hindered. Regarding the firing temperature,
When the temperature is lower than 1750 ° C., the density of the sintered body cannot be sufficiently increased, and as a result, it is difficult to obtain a completely densified sintered body after gas pressure firing or HIP firing. In the case of firing at a temperature higher than 1950 ° C., although the density of the sintered body is sufficiently increased, the burnt surface is particularly roughened, and recesses such as open pores formed on the burnt surface of the pre-sintered body become large. This is not preferred. Note that the inert atmosphere can be obtained by supplying an inert gas such as nitrogen or argon, and a nitrogen atmosphere is particularly preferable.

The gas pressure firing or HIP firing in the main firing step is performed under an inert atmosphere of 50 atm or more at 1750 to 195.
The reason why it is performed at 0 ° C. is that if it is less than 50 atm, a dense sintered body cannot be obtained. Also, if the temperature is lower than 1750 ° C., a dense sintered body cannot be obtained.
On the other hand, when sintering is performed at a temperature higher than 1950 ° C., although a dense sintered body can be obtained, the baked surface is particularly rough, which is not preferable.

The reason why the gas pressure firing or HIP firing, which is a feature of the present invention, is performed in a silicon nitride case or a non-oxide case having silicon nitride coated on its inner surface. Is as follows. That is, reducing gas such as C and CO generated from graphite which is a material of a heater of a firing furnace by firing in the above-mentioned case is blocked by a case made of silicon nitride, or a reducing gas. And the partial pressure of SiO generated from the pre-sintered body and the case can be increased. Then, the β-silicon nitride particles are likely to grow in a vapor phase on the surface of the pre-sintered body that is in contact with this atmosphere. As a result, a large number of needle-like crystals of β-silicon nitride grow in vapor phase in concave portions such as open pores existing on the burnt surface of the pre-sintered body.

The case may be made of any material that can form an SiO atmosphere under the sintering conditions described in claim 4. For example, the above-mentioned case made of silicon nitride can be used. The term "made of silicon nitride" means that the material contains silicon nitride as a main component, and may contain a rare earth element compound or the like as an auxiliary component as a sintering aid. Alternatively, silicon nitride may be coated on the inner surface of a non-oxide case such as graphite or BN (boron nitride).

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described below by way of examples within the scope of the present invention, and comparative examples outside the scope of the present invention.

[0018]

【Example】

Examples 1 to 5 Si ₃ N ₄ powder having an average particle diameter of 0.6 μm and an α ratio of 97%, a rare earth element oxide powder, SiO ₂ and V ₂ O as sintering aids
₅ , MoO ₃ , MoSi ₂ , and WO ₃ powders were prepared at the composition ratios shown in Table 1, and these were wet-mixed and ground in a silicon nitride ball mill. Note The average particle size powders of sintering _{aids, Y 2 O 3: 1.0μm,} Yb 2 O 3: 1.0
μm, Er ₂ O ₃ : 1.0 μm, SiO ₂ : 1.3 μm, Mo
Si ₂ : 3.0 μm, V ₂ O ₅ : 2.5 μm, MoO ₃ : 2.
Those having a thickness of 0 μm and WO ₃ : 0.3 μm were used.

[0019]

[Table 1]

Next, drying is performed, and the obtained mixed powder is dried for 2 tons.
Isostatic pressing with on / cm ² pressure 55 × 55 ×
A 25 mm compact was obtained. Next, the compact was preliminarily fired under the conditions shown in Table 1. The obtained preliminary sintered body, Quai inner space a material shown in Table 1 is about 100φ × 50 ^t mm - housed in the scan, gas pressure firing or H under the conditions of Table 1
IP sintering was performed to obtain a silicon nitride sintered body.

[0021] The sintered body density, baking surface strength, polished surface strength, room temperature strength ratio, the maximum length of the recessed portion, and the volume of the β-silicon nitride needle crystal occupying the recessed portion of the obtained silicon nitride sintered body. The proportions are shown in Table 2.

[0022]

[Table 2]

The density of the sintered body was measured by the Archimedes method and expressed as a relative density to the theoretical density calculated by the mixing rule.

The strength is measured according to JIS R 1601 and JIS
In accordance with R 1604, it was measured by a four-point bending strength measuring method. The test piece had a surface roughness of 0.8 S or less according to JIS B 601. The burnt surface strength was measured using the burnt surface side as a tensile surface, and the polished surface strength was measured using the polished surface side as a tensile surface. Room temperature and 1
Two conditions of 250 ° C. were set. The room temperature strength ratio indicates the value of (baked surface room temperature strength / polished surface room temperature strength).

The length of the concave portion on the surface of the baked skin is
A cross section including a concave portion that is considered to be a fracture origin after the measurement of the baked surface strength or an arbitrary cross section was observed by SEM and measured.

The volume ratio of the needle crystal of β-silicon nitride occupying the concave portion can be determined by observing the cross section of the sintered body including the concave portion by SEM observation.
This was determined from the ratio of the occupied area of the concave portion and the needle crystal measured on the M observation photograph.

FIG. 1 is a SEM observation photograph of a fracture surface near the origin of fracture of the sample of Example 3. As can be seen from the photograph, it is considered that the destruction originated from the depressed portion of approximately 70 μm in size composed of voids and needle-like crystals of β-silicon nitride of about 10 μm in dark gray in the photograph. The volume occupied by the needle-like crystals of β-silicon nitride in the recess was 70% in terms of the area ratio of the SEM observation photograph.

As is evident from Table 2, the sintered bodies of Examples 1 to 5 belonging to the scope of the present invention have a room temperature strength ratio of 0.8 or more and a high baked surface strength. The mechanical strength of the sintered body itself was sufficiently high as indicated by the polished surface strength, and a high-strength silicon nitride sintered body could be obtained without polishing the surface of the sintered body.

Comparative Examples 1 and 2 In Comparative Examples 1 and 2, a silicon nitride sintered body was obtained in the same manner as in Examples 1 and 3, except that the case in gas pressure firing or HIP firing was made of graphite. Things.
Each of the obtained sintered bodies has a baked surface strength of 55 at room temperature.
0 MPa, 540 MPa, strength ratio at room temperature 0.71, 0.
It was as low as 67. The proportions of the acicular crystals of β-silicon nitride in the recesses considered to be the origin of the fracture were 5, respectively.
10% by volume was insufficient. FIG. 2 is an SEM observation photograph of a fracture surface near the fracture origin of the sample of Comparative Example 2. As you can see from the photo, the gap and the photo are light gray,
It is probable that a depressed portion of about 60 μm wide composed of β-silicon nitride needle crystals of about 10 μm size was the origin of the destruction.

Comparative Example 3 In Comparative Example 3, a silicon nitride sintered body was obtained in the same manner as in Example 3 except that the case in the HIP firing was made of BN. The obtained sintered body has a baked surface strength of 51 at room temperature.
At 0 MPa, the room temperature strength ratio was as low as 0.64.
In addition, the needle-like crystals of β-silicon nitride occupied 20% by volume in the concave portions considered to be the origin of the fracture.

Comparative Example 4 Comparative Example 4 was performed except that the preliminary firing was performed at 2000 ° C.
A silicon nitride sintered body was obtained in the same manner as in Example 1. The obtained sintered body had a baked surface strength as low as 460 MPa at room temperature. In addition, the length of the recess, which is considered to be the origin of the fracture, was 150 μm, which was out of the range of the present invention.

Comparative Example 5 In Comparative Example 5, a silicon nitride sintered body was obtained in the same manner as in Example 1 except that the HIP baking was performed at 2000 ° C. The obtained sintered body has a baked surface strength of 530M at room temperature.
Pa, which was much lower than the polished surface strength of 850 MPa.

[0033]

As is apparent from the above, according to the present invention, the gas pressure is increased in a case made of silicon nitride or a case made of non-oxide having silicon nitride coated on the inner surface. By firing or HIP firing, a large number of needle-like crystals of β-silicon nitride are generated in concave portions such as open pores present on the burnt surface of the silicon nitride sintered body, and the sintered body is brought into this state. Can be improved to a value close to the polished surface strength. For this reason, even if the sintered body has a complicated shape, high strength can be obtained without polishing the surface of the sintered body.

[Brief description of the drawings]

FIG. 1 is an SEM observation photograph showing a crystal structure of a fracture surface of a fracture origin part of a sample of Example 3.

FIG. 2 is an SEM observation photograph showing a crystal structure of a fracture surface of a fracture origin part of a sample of Comparative Example 2.

Continuation of the front page (56) References JP-A-7-187799 (JP, A) JP-A-6-92738 (JP, A) JP-A-5-105520 (JP, A) JP-A-3-275565 (JP) , A) JP-A-7-330436 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C04B 35/584-35/596

Claims (4)

(57) [Claims] 1. A silicon nitride sintered body having needle-like crystals of β-silicon nitride in a concave portion present on the burnt surface of the silicon nitride sintered body in an amount of 30% by volume or more of the concave portion. Β
-A silicon nitride sintered body characterized in that no grain boundary phase exists between needle-like crystals of silicon nitride. 2. The silicon nitride sintered body according to claim 1, wherein the silicon nitride sintered body has a compound containing a rare earth element, Si, O and N as a grain boundary phase. 3. The silicon nitride sintered body according to claim 1, wherein the maximum length of the recess is 100 μm or less. 4. A mixing and forming step of mixing a raw material powder containing a silicon nitride powder and a rare earth element oxide powder as main components to form a compact, and the compact is subjected to an inert atmosphere of 10 atm or less at 1750 to 1950 ° C. A pre-sintering step of sintering the pre-sintered body into a pre-sintered body by gas pressure sintering at 1750 to 1950 ° C. or H under an inert atmosphere of 50 atm or more.
In the method for producing a silicon nitride sintered body having a main firing step of performing IP firing to form a silicon nitride sintered body, the main firing step is performed in a case made of silicon nitride or by coating silicon nitride on the inner surface. A method for producing a silicon nitride sintered body, which is performed in a non-oxide case.