JPH0829923B2 - Silicon nitride powder - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a silicon nitride powder capable of forming a sintered body which is easily sintered and is excellent in bending strength and toughness. Silicon nitride is used as a high temperature structural material for gas turbine members, nozzles, bearings and the like.

[Conventional technology]

Conventionally, as a method for producing a silicon nitride powder, there are known (1) metal silicon direct nitriding method, (2) silica reduction nitriding method, and (3) silicon halide method. Probably because the powders produced by these methods have different production histories, even if the amount of metal impurities, the amount of oxygen, the particle size, and the specific surface area are the same, the sinterability of the powder and the sintered body after sintering are There is a big difference in characteristics such as bending strength.

Generally, the powder produced by the method (1) is easily sinterable but has a low bending strength, the powder produced by the method (2) is hardly sintered, and the powder produced by the method (3). Is said to exhibit intermediate performance.

Regarding the amount of solid solution oxygen, the powder of the method (1) undergoes a pulverizing step, and therefore, it is often more than 2% by weight, and at least 1.5% by weight is contained. When the acid treatment or the like is carried out through the steps for removing impurities by the method (1), the amount of dissolved oxygen is reduced, but it is still difficult to make it less than 1.0% by weight. On the other hand, even in the case of the powder of the method (2), since silica powder is used as a raw material, silica remains, and the amount of dissolved oxygen is usually over 2% by weight.

Regarding the crystal morphology, there are two types of α phase and β phase, but the general sintering mechanism of silicon nitride, that is, α phase once dissolves in the liquid phase, then becomes supersaturated and precipitates as β phase From the viewpoint, it is said that a higher α phase content is desirable. However, there is a drawback that the columnar crystal growth is not uniform during the precipitation of the columnar crystal accompanying the α → β transition, and abnormal grain growth occurs to prevent fine densification.

Japanese Unexamined Patent Publication No. 1-145380 proposes the relationship between the β conversion rate and the particle size in the range of 30 to 100% β phase. However, in this method, since the oxygen content is high,
The α → β transition is more likely to occur at low temperatures during sintering, and the amount of grain boundary phase formed by the sintering aid also increases to change the solubility of silicon nitride, resulting in a β-columnar shape with a very small aspect ratio. Since only crystals are generated, there is a problem that toughness is not improved.

[Problems to be Solved by the Invention] An object of the present invention is to provide a silicon nitride powder that solves the above problems.

[Means for solving the problem]

That is, the present invention provides a silicon nitride powder having an α phase of 70% or less and a β phase of 30% or more, a solid solution oxygen amount of less than 0.5% by weight, and a fine powder amount of 0.2 μm or less of 6% by volume or less. It is a characteristic silicon nitride powder.

 Hereinafter, the present invention will be described in more detail.

First, in the silicon nitride powder of the present invention, the proportion of α phase is 70% or less and the proportion of β phase is 30% or more. If the proportion of α phase exceeds 70%, abnormal grain growth occurs during the precipitation of columnar crystals accompanied by α → β transition, the columnar crystals become non-uniform, and those with a large aspect ratio crystallize, preventing densification. To be done.

The ratio of the α phase in the present invention was calculated by the following formula by _obtaining the peak intensities of I ₁₀₂ and I ₂₁₀ as the α phase and I ₁₀₁ and I ₂₁₀ as the β phase by X-ray diffraction.

Next, the amount of dissolved oxygen of the silicon nitride powder of the present invention is less than 0.5% by weight. The amount of dissolved oxygen is less than 0.5% by weight.
When the amount of solid solution oxygen is 0.5% by weight or more, the α → β transition is likely to occur at a low temperature during sintering, and the amount of the grain boundary phase formed by the sintering aid is increased, so that the solubility of silicon nitride is increased. The sinterability is improved but the toughness is not improved because only β columnar crystals having a small aspect ratio are formed.

Further, in the present invention, when the amount of fine powder of 0.2 μm or less exceeds 6% by volume, the aspect ratio of the silicon nitride particles of the sintered body becomes small, and the bending strength and toughness improving effects cannot be recognized.

The silicon nitride powder of the present invention has an average particle size of 1.2 to 0.4 μ.
m, specific surface area 6 to 14 m ² / g, metallic impurities such as Fe, Al and
It is desirable that the total amount of Ca is 2000 ppm or less in order to achieve the intended purpose.

Next, a method for producing the silicon nitride powder of the present invention will be described.

The silicon nitride powder of the present invention can be produced by various methods, but the characteristics such as the α phase ratio (hereinafter referred to as α fraction), the amount of dissolved oxygen, the average particle size and the specific surface area can be freely adjusted. The silicon halide method which can be used is most suitable. For example, as a seed with an α fraction of 10% or less, a specific surface area of 12 m ² / g
Silicon nitride powder, producing about 100 silicon nitride powder
It can be produced by adding 3 to 20 parts by weight per part by weight to the intermediate imide, adjusting the oxygen partial pressure to 10 ^-5 atm or less, and crystallizing at a temperature of 1500 to 1600 ° C.

In the direct nitriding method of metallic silicon, for example, 5 to 15 parts by weight of silicon nitride powder having an α fraction of 10% or less and a specific surface area of about 12 m ² / g is added and mixed per 100 parts by weight of metallic silicon powder of 88 μm or less, and nitriding is performed. Nitrid the bulk density of the specimen to about 0.7 to 1.2. Nitriding is performed in a mixed gas atmosphere of nitrogen and ammonia,
The nitrogen partial pressure is adjusted to 0.9 atm or more, the temperature is raised at a rate of 20 to 50 ° C / h, and the temperature is 1400 to 1500 ° C. It should be noted that during nitriding, one or more selected from alkali metal and alkali metal halides are continuously, intermittently or temporarily supplied in a gaseous state in a gaseous state, so that the solid solution oxygen is lower and the toughness is lower. It is possible to manufacture a high-quality silicon nitride powder.

Furthermore, conventional silicon nitride powder obtained by the direct nitriding method of metallic silicon (for example, "SN-G2" manufactured by Denki Kagaku Kogyo Co., Ltd.) is heat-treated at a temperature of 1600 to 1700 ° C under a nitrogen atmosphere, or The silicon nitride powder of the present invention can also be manufactured by mixing silicon nitride powders having different α fractions.

〔Example〕

Hereinafter, more detailed description will be given with reference to Examples and Comparative Examples.

Examples 1 to 9 Comparative Examples 1 to 3 Silicon tetrachloride and ammonia were reacted at a molar ratio of 1: 6 at a temperature of 200 ° C. or lower to synthesize an intermediate containing silicon diimide and ammonium chloride. As shown in Table 1, to the intermediate, silicon nitride powder having a specific surface area of 12 m ² / g with different α fraction was added as a seed powder at different addition amounts, and the powder was put into a silicon nitride crucible and nitrogen was added. Under a gas flow, the temperature was kept at 500 ° C. for deammonium chloride treatment.

Next, it is heated to a temperature of 1500 ° C. or higher to decompose the silicon diimide into silicon nitride powder, at which time,
By changing the kind of seed powder (α fraction), the amount of seed added to 100 parts by weight of generated silicon nitride (parts by weight), and the oxygen partial pressure in the atmosphere during decomposition, as shown in Table 1, Powders having different rates, specific surface areas, amounts of dissolved oxygen and particle sizes (average particle size and fine powder amount under 0.2 μm) were produced.

Commercially available average particle size of 1.2μ
m Y ₂ O ₃ and Al ₂ O ₃ having an average particle size of 1.4 μm were added in an amount of 5% by weight and 2% by weight, respectively, 1.1.1-trichloroethane was further added, and the mixture was wet mixed in a ball mill for 4 hours, After drying, it was molded with a molding pressure of 100 kg / cm ² into a shape of 6 × 10 × 60 mm, and then CIP molded with a molding pressure of 2700 kg / cm ² . These compacts were kept at a temperature of 1800 ° C. for 4 hours under a normal pressure of a nitrogen atmosphere to produce a sintered compact.

The obtained sintered body was cut into a size of 3 × 4 × 40 mm, and the density of the sintered body, the three-point bending strength at room temperature, and the toughness value were measured. The results are shown in Table 1.

Examples 10 to 17 Comparative Examples 4 to 9 10 parts by weight of silicon nitride powder having a Si purity of 99.9% by weight and a particle size of 88 μm and 100 parts by weight of metal silicon powder having an α fraction of 10% and a specific surface area of 12 m ² / g. Is added and mixed, and it is used to
A nitriding specimen having a bulk specific gravity shown in the table and a length of 150 mm × width of 150 mm and a thickness shown in Table 2 was molded and nitrided in an electric furnace. The nitriding conditions are as shown in Table 2, and the temperature rising temperature is a value at a temperature of 1100-1450 ° C.

The obtained silicon nitride ingot was crushed by a jaw crusher and a top grinder to a size of 0.2 mm, and it was crushed in a ball mill with an internal volume of 1 50 g, 4φ Fe ball 0.5, water.
100 g was put and wet pulverized for another 20 hours, then treated with hydrochloric acid and hydrofluoric acid, filtered, dried and crushed to produce a silicon nitride powder.

With the obtained silicon nitride powder, a sintered body was produced in the same manner as in Example 1, and the physical properties were measured. The results are shown in Table 2.

Examples 18 to 23 Comparative Examples 10 to 12 Commercially available silicon nitride powder (“SN-G2” manufactured by Denki Kagaku Kogyo Co., Ltd.) obtained by a direct metal nitriding method was used to change the heat treatment temperature and time to obtain powder characteristics. Of various powders were produced. Table 3 shows those characteristics. The obtained silicon nitride powder was used to produce a sintered body in the same manner as in Example 1,
Physical properties were measured. The results are shown in Table 3.

Examples 24 to 32 Comparative Examples 13 to 15 As shown in Table 4, silicon nitride powder A having a low α fraction and silicon nitride powder β having a high α fraction were mixed to produce silicon nitride powder. Table 4 shows the powder characteristics. With the obtained silicon nitride powder, a sintered body was produced in the same manner as in Example 1, and the physical properties were measured. The results are shown in Table 4.

The physical property values shown in each example were measured by the following methods.

(1) Solid solution oxygen amount (wt%): LECO Co. TC-
A 136-type O / N simultaneous analyzer was used. The calculation method is JP-A-1-18
According to Japanese Patent No. 3407.

(2) Specific surface area (m ² / g): According to Cantersorb Jr BET 1-point method manufactured by Yuasa Ionics.

(3) Average particle diameter ([mu] m): manufactured by HORIBA, Ltd. C _A C _A -70
Depends on 0.

(4) α separation (%): By X-ray diffraction of Geiger flux RAD-II B type manufactured by Rigaku Denki KK

(5) Sum of Fe, Al and Ca (ppm): In accordance with JIS-G-1322.

(6) Sintered body density (g / cm ³ ): By Archimedes method.

(7) Three-point bending strength (MPa): Measured using Shimadzu's Autograph AG-2000A.

(8) Toughness (MPa ・ m ^1/2 ): Indentation Fraccture
Depends on Method.

(K _1C φ / Ha ^1/2 ) (H / E φ) ^2/5 = 0.129 (c / a) ^−3/2 9 E …… Young's modulus, H …… Hardness, a …… Indentation length, c… …
Crack length, φ = 3, load is 20 kg, load application time is 15 seconds [Effect of the invention] Since the silicon nitride powder of the present invention is excellent in sinterability, bending strength at room temperature (bending strength) of 900 MPa or more , Toughness value 8.5M
It is possible to obtain a sintered body having a Pa · m ^1/2 or more.

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Claims (1)

[Claims] 1. A silicon nitride powder having an α phase of 70% or less and a β phase of 30% or more, wherein the amount of dissolved oxygen is less than 0.5% by weight and the amount of fine powder of 0.2 μm or less is 6% by volume or less. And then it is silicon nitride powder.