JPH0633171B2 - Silicon nitride sintered body and method for manufacturing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a high-density silicon nitride sintered body excellent in mechanical strength and toughness, and a method for producing the same.

2. Description of the Related Art Since silicon nitride is a substance having a strong covalent bond and it is difficult to sinter by itself, it is performed by adding various additives. For example, in a system to which yttrium oxide and aluminum oxide are added, excellent thermal shock resistance is obtained, but in some cases, heat resistance, mechanical strength, and toughness are inferior.

JP-A-62-2077 for the purpose of improving heat resistance
A system containing yttrium oxide + aluminum oxide + aluminum nitride disclosed in Japanese Patent Laid-Open No. 66-20, JP-A-62-20
A system containing yttrium oxide + cerium oxide + magnesium oxide, which is disclosed in Japanese Patent No. 7765, has been attempted, and it is known that it is effective in improving heat resistance.

Further, in Japanese Patent Application Laid-Open No. 58-41770, a silicon nitride sintered body is produced in which the mechanical strength is small even in a high temperature oxidizing atmosphere in a system containing yttrium oxide + aluminum oxide + aluminum nitride + various silicides. Are trying.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, in the above materials, although the thermal shock resistance and the heat resistance are excellent, the mechanical strength and the toughness have not been drastically improved. When applied to the resulting structural member, there were problems such as lack of reliability. Therefore, in addition to improving heat resistance,
A material having improved mechanical strength and toughness is desired.

The present invention has been made to solve the above problems. An object of the present invention is to provide a silicon nitride sintered body having high density, high mechanical strength and high toughness, and having heat resistance such as a small decrease in mechanical strength even under a high temperature oxidizing atmosphere, and It is to provide the manufacturing method.

Means for Solving the Problems The silicon nitride sintered body of the present invention comprises at least 1 to 15% by weight of one or more rare earth oxides, 1 to 8% by weight of magnesium oxide, at least 1 selected from titanium silicide and zirconium silicide. It is characterized in that the seed silicide is 0.1 to 3% by weight and the balance is silicon nitride.

The sintered body of the present invention contains at least one kind of rare earth oxide. Examples of the rare earth oxide of the present invention include yttrium oxide, cerium oxide, lanthanum oxide and the like.

Oxides of rare earth elements have the function of promoting the crystal phase transition from the α phase to the β phase in the melt during the sintering of silicon nitride,
Further, high temperature strength is improved by generating columnar crystal grains of silicon nitride.

If the total amount of these components exceeds 15% by weight, the mechanical strength of the obtained sintered body at high temperature decreases, so the content is preferably 15% by weight or less. On the other hand, if it is less than 1% by weight, the melt is insufficient and sufficient densification cannot be achieved, which is not preferable. Therefore, the amount added is preferably in the range of 1 to 15% by weight, but more preferably 3 to 10% by weight in order to obtain sufficiently high mechanical strength and toughness.

Magnesium oxide forms a liquid phase at the time of sintering with the above rare earth oxide, but has the effect of lowering the melting point thereof as compared with the case of the rare earth oxide alone, further promoting the crystal phase transition and further promoting the densification. Have the effect of Further, when magnesium oxide is contained, the aspect ratio (ratio of major axis and minor axis) of columnar particles generated in the sintering process becomes large and the diameter of the minor axis becomes large, so that the toughness is improved. . The sintered body of the present invention is composed of 1 to 8% by weight of magnesium oxide, but if it exceeds 8% by weight, the homogeneity of the structure is impaired, and if it is less than 1% by weight, a sufficient aspect ratio cannot be obtained. To obtain a sintered body with a particularly high toughness, 1
More preferably, it is in the range of ˜6% by weight.

Titanium silicide and zirconium silicide are both considered to act as nuclei for the transition of α-phase to β-phase of silicon nitride in the melt formed by the rare earth oxide and magnesium oxide at the time of sintering. It accelerates and contributes to homogenization of the structure, and a stable and homogeneous sintered body can be obtained especially when a large-sized sintered body is fired. The sintered body of the present invention contains 0.1 to 3% by weight of at least one kind of silicide selected from titanium silicide and zirconium silicide, but if it is added in excess of 3% by weight, the mechanical strength at high temperature decreases, and
If it is less than 0.1% by weight, sufficient contribution to homogenization of the structure is not recognized.

The silicon nitride sintered body of the present invention comprises 1 to 15% by weight of one or more kinds of rare earth oxides, 1 to 8% by weight of magnesium oxide, at least one kind of silicide selected from titanium silicide and zirconium silicide. Although 3% by weight is contained, this problem was achieved only by combining these compositions.

The method for producing a silicon nitride sintered body according to the present invention comprises 1 to 15% by weight of one or more rare earth oxides, 1 to 8% by weight of magnesium oxide, at least one silicide selected from titanium silicide and zirconium silicide. A mixed powder composed of 0.1 to 3% by weight and the balance of silicon nitride is molded, and the molded body is sintered in an atmosphere containing nitrogen gas.

The silicon nitride powder used in the present invention is preferably a silicon nitride powder having an α-type crystal structure from the viewpoint of sinterability,
It may contain β-type or amorphous silicon nitride powder. Fine particles having an average particle size of 5 μm or less are used in order to obtain a sufficiently high bulk density during sintering. The rare earth oxide, magnesium oxide, and various silicides added as sintering aids have an average particle size of 10 μm in order to obtain a homogeneous and high-density sintered body.
The following fine particles are preferable.

In the method of the present invention, these components are mixed in a planetary mixer using a solvent such as acetone or n-hexane and using a pot and a ball of silicon nitride or silicon carbide. The mixed powder thus adjusted is pressure-molded to obtain a molded product having a predetermined shape. As a molding method, a known molding method is used. For example, uniaxial molding pressure 10 to 10
Mold at 0MPa, secondary hydrostatic molding pressure of 150-700MPa.

This molded body is heated and sintered at 1600 to 2000 ° C. to obtain a sintered body. As the sintering method, any of an atmospheric pressure sintering method, a hot press sintering method, a gas pressure sintering method and a hot isostatic pressing sintering method can be used. It is also possible to combine the methods.

The atmosphere at the time of sintering is preferably an atmosphere containing a nitriding gas for suppressing decomposition of silicon nitride at a high temperature. Since silicon nitride decomposes at about 1800 ° C or higher under 1 atmosphere of nitrogen gas, when sintering at 1800 ° C or higher, the nitrogen gas pressure at the sintering temperature is set to be equal to or higher than the decomposition critical pressure of silicon nitride at that temperature. There is a need to.

During the sintering, the liquid phase of the rare earth oxide, magnesium oxide and titanium silicide, zirconium silicide and the like is kept at 1500 to 1600 ° C. for 30 minutes or more in order to uniformly distribute the liquid phase, and the silicon nitride is sufficiently wetted. Further, at a temperature of 1600 to 2000 ° C., silicon nitride is dissolved and reprecipitated in the above liquid phase to cause a crystal phase transition, and it is densified and sintered. During this dissolution / reprecipitation process, there is a limit of solid solution in the melt, so it is necessary to hold it for 30 minutes or longer.

Action The silicon nitride sintered body of the present invention contains at least one kind of silicide selected from one or more kinds of rare earth oxides, magnesium oxide, titanium silicide and zirconium silicide. The obtained sintered body has a uniform structure in which columnar crystal grains are entangled with each other, and has a high bending strength of 1000 MPa or more at room temperature and a toughness of 6.5 MPa m1 / 2 or more. In addition, since it has heat resistance such as a small decrease in mechanical strength even in a high temperature oxidizing atmosphere, it is highly reliable as a structural material.

In order to obtain a sintered body with a particularly high mechanical strength, the hot pressing method gives a bending strength of 1500 M at room temperature.
An extremely high sintered body with a degree of Pa is obtained. Further, in order to obtain a sintered body having a complicated shape, it is preferable to use an atmospheric pressure sintering method, a gas pressure sintering method, or a hot isostatic pressing sintering method, but a sintered body having particularly high toughness is obtained. Therefore, it is desirable to use the gas pressure sintering method. In the case of subjecting to gas pressure sintering after normal pressure sintering, columnar grains develop well, so a sintered body with a toughness value K _IC of 8.6 MPa m1 / 2 is extremely high.

Examples Next, examples of the present invention will be described together with comparative examples.

Example 1 A rare earth oxide powder, a magnesium oxide powder, and various silicide powders were added to silicon nitride powder (average particle diameter 0.5 μm, α conversion rate of 97% or more) in a predetermined amount (% by weight) shown in Table 1, Acetone was used as a solvent and kneaded in a silicon nitride ball mill for 24 hours.

The rare earth oxide powder used was Y ₂ O ₃ powder (average particle size 0.3 μm) as yttrium oxide powder, CeO ₂ powder (average particle size 1.0 μm) as cerium oxide powder, and La ₂ O ₃ as lanthanum oxide powder. It is a powder (average particle size 1.0 μm).
Magnesium oxide powder is MgO powder (average particle size 0.2 μm)
Is. The various silicides are TiSi ₂ powder (average particle diameter 2 μm) as titanium silicide and ZrSi ₂ powder (average diameter 2 μm) as zirconium silicide.

Then, the obtained mixed powder was dried, molded, and then sintered. Molding conditions were a uniaxial mold forming pressure of 100 MPa and a cold hydrostatic pressure of 700 MPa to obtain a plate-like body of 50 mm × 50 mm × 10 mm. The atmospheric pressure sintering conditions are as follows: holding in a nitrogen gas atmosphere at atmospheric pressure at a temperature of 1600 ° C. for 2 hours and then at a temperature of 1750 ° C. for 5 hours.

In the case of gas pressure sintering, after performing the above-mentioned atmospheric pressure sintering in advance, it was performed under a pressure of 4 MPa in a nitrogen gas atmosphere at a temperature of 2000 ° C. and a holding time of 1 hour.

Further, in the case of hot isostatic pressing, the same atmospheric pressure sintering as in the case of gas pressure sintering is performed in advance, and then 10 minutes in a nitrogen gas atmosphere.
It was conducted under the conditions of a pressure of 0 MPa and a temperature of 1800 ° C. and a holding time of 1 hour.

The characteristics of each sintered body obtained according to the present invention are shown in Table 1 together with the addition amount of the sintering aid and the sintering conditions. Regarding mechanical strength, a three-point bending test was performed at room temperature and a nitrogen atmosphere at 1200 ° C. in accordance with JIS R1601, and the bending strength was measured. Regarding toughness, the fracture toughness value K _IC was measured by the SEPB (Single Edge Pre-cracked Beam) method.

As shown in Table 1, the samples according to the examples of the present invention are excellent in both bending strength and toughness, but the samples corresponding to the comparative examples have particularly room temperature bending strength and toughness as compared with the examples of the present invention. It was confirmed to be inferior.

Example 2 A mixed powder was produced in the same manner as in Example 1, and a sintered body was produced by hot press sintering. The hot press conditions are a graphite die in a nitrogen gas atmosphere and a temperature of 40 MPa.
The range was 1700 to 1750 ° C, and the holding time was 1 hour. Example 1
Similarly to the above, the characteristics of the sintered body are shown in Table 2 together with the addition amount of the sintering aid and the hot pressing conditions.

Similar to Example 1, the characteristics of the sintered body according to the present invention are bending strength,
It was confirmed that the samples corresponding to the comparative examples were excellent in both toughness, but particularly inferior in room temperature transverse strength and toughness as compared with the examples of the present invention.

EFFECTS OF THE INVENTION According to the present invention, it has become possible to further improve the mechanical strength and toughness of a silicon nitride sintered body having sufficient heat resistance as described above. This makes it possible to produce a silicon nitride sintered body with extremely excellent reliability,
Its industrial utility is enormous.

Claims (2)

[Claims] 1. 1 to 15% by weight of one or more rare earth oxides,
1 to 8% by weight of magnesium oxide, at least one silicide selected from titanium silicide and zirconium silicide 0.1
A silicon nitride-based sintered body, characterized in that it is made up of 3% by weight and the balance silicon nitride. 2. One to 15% by weight of one or more rare earth oxides,
1 to 8% by weight of magnesium oxide, at least one silicide selected from titanium silicide and zirconium silicide 0.1
A method for producing a silicon nitride-based sintered body, which comprises molding a mixed powder of 3% by weight and the balance silicon nitride, and sintering the molded body in an atmosphere containing nitrogen gas.