JP3322033B2 - Crystalline silicon nitride powder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a crystalline silicon nitride powder suitable as a raw material for producing a silicon nitride sintered body useful as a high-temperature structural material.

[0002]

2. Description of the Related Art Silicon nitride sintered bodies have attracted attention as high-temperature structural materials for gas turbine parts, automobile engine parts and the like in recent years because of their high strength and excellent corrosion resistance and thermal shock resistance. . Such a silicon nitride sintered body is
Normally, a sintering aid is mixed with silicon nitride powder, press molding,
It is manufactured by forming a molded body by injection molding, extrusion molding, or the like, and sintering the molded body under normal pressure or pressure. In each of these methods, when producing a silicon nitride-based sintered body, it is known that the powder properties of the silicon nitride powder as a raw material, particularly the oxygen content greatly affects the sinterability and the properties of the sintered body. ing. That is, in general, silicon nitride powder having a high oxygen content is easy to sinter, but does not provide sufficient high-temperature strength. Further, silicon nitride powder having a low oxygen content is difficult to sinter, and it is difficult to sufficiently densify it.

It is generally known that if oxygen contained in silicon nitride powder is divided into surface oxygen and internal oxygen, the surface oxygen particularly greatly affects sinterability. Tokuhei 5-
No. 87441 discloses that the surface oxygen is 0.3 to 0.6 wt.
%, And an internal oxygen content of 0.5 to 1.1 wt%. It is not clear whether the present invention is in the form of surface oxygen, that is, whether surface oxygen is present in the oxide layer or the oxynitride layer. It is also unknown where the boundary between surface oxygen and internal oxygen exists on the particle surface. Oxides and oxynitrides have different reactivity with oxide sintering aids used during sintering, such as yttrium oxide (Y ₂ O ₃ ) and aluminum oxide (Al ₂ O ₃ ), and the formation temperature of the glass phase is different. There is a problem that it changes greatly.

[0004]

An object of the present invention is to solve the above problems,
An object of the present invention is to provide a crystalline silicon nitride powder capable of obtaining a silicon nitride sintered body having excellent sinterability and excellent sintered body characteristics such as high-temperature strength.

[0005]

As a result of various studies on the oxygen content, oxygen form and distribution of oxygen in the silicon nitride powder, the present inventors have shown the distribution of surface oxygen in the silicon nitride powder as shown in FIG. It turns out that it can be represented by the model shown. In FIG. 1, a silicon oxide layer, a silicon oxynitride layer existing subsequent to the silicon oxide layer, and internal silicon nitride exist from the outermost surface of the particle. The oxygen distribution in the silicon oxynitride layer is based on the composition ratio O / (O) of oxygen atoms [O] and nitrogen atoms [N].
+ N) has a composition gradient that monotonically decreases from the connection point with the silicon oxide layer to the inside. Furthermore, as a result of various examinations on the effect of the oxygen content, the oxygen form, and the distribution state of oxygen in the silicon nitride powder on the sinterability and the properties of the sintered body, particularly on the high-temperature strength, it was found that the sinterability was high, the high-temperature high strength, etc. A silicon nitride powder capable of obtaining a silicon nitride sintered body having excellent sintered body characteristics has been found. That is, the present invention is a crystalline silicon nitride powder having a silicon oxide layer and a silicon oxynitride layer on the particle surface,
Oxygen present in the outermost silicon oxide layer is 0.1 wt% or less in terms of oxygen amount, and oxygen present in the silicon oxynitride layer following the silicon oxide layer is 0.4 to 0.4 wt.
The present invention relates to a crystalline silicon nitride powder characterized by being 1.2 wt%.

In the present invention, the distribution of oxygen in the silicon nitride powder is based on the oxygen existing in the silicon oxide layer on the outermost surface of the particles (hereinafter referred to as oxygen in the surface silicon oxide layer) and the oxygen existing after the silicon oxide layer. Oxygen present in the silicon nitride layer (hereinafter referred to as
Oxygen in the surface silicon oxynitride layer), oxygen (hereinafter referred to as surface oxygen) which is a combination of oxygen in the surface silicon oxide layer and oxygen in the surface silicon oxynitride layer, and exists as a solid solution in the silicon nitride crystal inside the particles. It is defined by oxygen (hereinafter referred to as internal oxygen) and total oxygen content (sum of surface oxygen content and internal oxygen content).

The oxygen present in the surface silicon oxide layer of the silicon nitride powder in the present invention is in a range of 0.1 wt% or less in terms of oxygen amount, and the oxygen present in the surface silicon oxynitride layer is in terms of oxygen amount. It is in the range of 0.4 to 1.2 wt%. When the amount of oxygen present in the surface silicon oxide layer exceeds 0.1 wt%, the oxidation resistance of the silicon nitride powder is significantly reduced, and the oxidation easily proceeds during a wet ball mill with a water solvent,
As a result, the surface oxygen content before sintering exceeds 1.2 wt%. The amount of oxygen present in the surface silicon oxynitride layer is 0.4
If the amount is less than wt%, the densification rate decreases, and a high-density sintered body cannot be obtained. On the other hand, if the oxygen content exceeds 1.2 wt%, a high-density sintered body can be obtained, but the strength characteristics (strength, toughness, etc.) of the obtained sintered body deteriorate.

The surface oxygen content is 0.4 to 1.2 wt%.
And the total oxygen content is preferably in the range of 0.6 to 2.0 wt%. If the surface oxygen content is less than 0.4 wt%, the densification rate is reduced, and a high-density sintered body cannot be obtained. When the surface oxygen content exceeds 1.2 wt%, a dense sintered body can be obtained, but the total oxygen content of the silicon nitride powder increases, and the amount of the grain boundary phase in the sintered body increases. Sintered body characteristics such as high-temperature strength are reduced. If the total oxygen content is less than 0.6 wt%, the surface oxygen content is also small, so that it is difficult to densify. When the surface oxygen content is 0.4 wt% or more, a dense sintered body can be obtained, but the obtained sintered body has poor oxidation resistance and cannot be used for high-temperature mechanical parts and the like. On the other hand, if the total oxygen content exceeds 2.0% by weight, the characteristics of the sintered body such as high-temperature strength deteriorate. In particular, when the surface oxygen amount is 1.2 wt% or less, the internal oxygen amount is 0.8 wt%.
% Or more, the fracture toughness of the obtained sintered body is reduced, and the sintered body is brittle, so that it is difficult to use it as a mechanical part.

Further, the internal oxygen content of the silicon nitride powder in the present invention is preferably 0.8 wt% or less, particularly preferably 0.6 wt% or less. If the internal oxygen content exceeds 0.8 wt%, the composition of the grain boundary phase changes when the silicon nitride particles dissolve in the grain boundary phase made of a sintering aid or the like during the sintering process of silicon nitride. Precipitation and growth of columnar crystals are hindered, and sufficient development of sintered body properties, particularly fracture toughness, cannot be expected. Also, the high-temperature strength is reduced.

The composition ratio O / (O + N) between oxygen atoms [O] and nitrogen atoms [N] in the silicon oxynitride layer is 0.95 to 1 at the connection point with the outermost silicon oxide layer. 0 to 0.05 at the connection point with the internal silicon nitride crystal portion, and has a composition gradient of O / (O + N) that monotonically decreases from the connection point with the silicon oxide layer to the inside in the silicon oxynitride layer Is preferred. If the composition ratio is out of the range, the structure of the sintered body becomes heterogeneous due to abnormal grain growth or the like, and sufficient strength characteristics cannot be exhibited.

[0011] The specific surface area of the silicon nitride powder in the present invention is preferably in the range of 5 to 30 m ² / g, it is preferably in the range of particularly 7~15m ² / g. Furthermore, when the specific surface area of the silicon nitride powder is [S] (m ² / g) and the surface oxygen amount is [O] (wt%), the O / S is in the range of 0.04 to 0.15. Is preferred. If the specific surface area is smaller than 5 m ² / g, the densification rate is reduced, and a high-density sintered body cannot be obtained. Also, the specific surface area is
If it exceeds 30 m ² / g, the bulk density of the molded body is reduced by a usual molding method, and microcracks, voids, etc. are generated after sintering. Further, in the sintering of a product having a complicated shape, cracks (crack aggregates) occur at corners and the like of the sintered body.

The amount of oxygen in the surface silicon oxide layer according to the present invention,
The oxygen content and the internal oxygen content in the surface silicon oxynitride layer are the oxygen content measured by the variable concentration chemical analysis method. The measuring method by the variable concentration chemical analysis method will be described below. The model of the oxygen distribution on the surface of the silicon nitride particles is as shown in FIG.
The value actually obtained from the analysis result is an integrated value. FIG. 2 shows a conceptual diagram of the analysis results together with the abbreviations shown below. First, measured by the total oxygen content O _t of the silicon nitride powder oxygen nitrogen simultaneous analyzer (e.g., LECO Corporation TC-136 Model). Next, 1 g of the silicon nitride powder was weighed, and 50 ml of a hydrofluoric acid solution having a concentration of 0.0001 to 10 wt% was added.
Stir for 0 hours. The treatment liquid is filtered using a Teflon membrane filter having an opening diameter of 0.5 μm or less. Atomic absorption spectrometer or ICP analyzer, N
H ₄ ⁺ is quantified by using indophenol blue absorption spectrophotometry or ion chromatography. Next, the eluted Si ₃ N ₄ corresponding to the eluted NH ₄ ⁺ amount was calculated, and the eluted S ₃ was calculated from the value obtained by subtracting the Si amount corresponding to the eluted Si ₃ N ₄ from the eluted Si amount.
to calculate the iO _2.

The eluted Si thus determined_ThreeN_FourQuantity (below
Bottom, Y_SNAnd eluted SiO_TwoQuantity (hereinafter, Y_SOOf)
The sum is defined as the total elution amount (hereinafter referred to as X) (these X, Y
_SN, Y _SOIs the integrated value from the particle surface). Hydrofluoric acid
Each X and Y measured by changing the concentration of the solution and the stirring time_SNThe
When lots are produced, as X increases from 0, Y_SNIs real
Qualitatively 0 (measured value is 0.02 wt% or less)
Area (X = 0 to X₁, Hereinafter referred to as a surface oxide layer region), Y
_SNA region in which the increase is proportional to the square of the X increase (X = X₁~
X_Two, Hereinafter referred to as a surface silicon oxynitride layer region), Y_SNincrease
(X = X)_Two~ Inside, below, inside
Area). Therefore, in each area
X and Y_SN, Y_SOIs expressed by the following equation.

[0014]

(A) Surface oxide layer region (X = 0 to X ₁ ) Y _SN = 0 Formula (1) Y _SO = X Formula (2) (b) Surface oxynitride layer region (X = X _{1 to} X) ₂ ) Y _SN = a (XX ₁ ) ² Equation (3) Y _SO = X-a (XX ₁ ) ² Equation (4) (c) Internal area (X = X ₂ to internal) Y _SN = a (X ₂ −X ₁ ) ² + (XX ₂ ) Formula (5) Y _SO = X ₂ −a (X ₂ −X ₁ ) ² Formula (6)

Here, a is a composition gradient in the silicon oxynitride layer, which is smaller than a constraint condition (a condition where the gradient of Y _SN becomes 1) at the boundary X ₂ between the surface silicon oxynitride layer and the inside.

[0016]

2a (X ₂ −X ₁ ) = 1 Equation (7)

## EQU1 ## Surface oxygen amount in the surface silicon oxide layer of oxygen content in Y _SO at the boundary X ₁ of the oxide layer region and the surface oxynitride layer region (hereinafter, referred to as O ₁₎ corresponds to. Y _{SO at} boundary X ₂ between surface silicon oxynitride layer region and internal region
The amount of oxygen contained is the amount of surface oxygen (hereinafter referred to as O _S )
Is equivalent to The value obtained by subtracting the oxygen amount O ₁ in the surface silicon oxide layer from the surface oxygen amount O _S corresponds to the oxygen amount (hereinafter referred to as O ₂ ) in the surface silicon oxynitride layer. The internal oxygen amount (hereinafter, referred to as O _i ) is obtained by subtracting the surface oxygen amount O _S from the total oxygen amount O _t . After all, these oxygen contents are expressed by the following equations.

[0018]

## EQU3 ## Oxygen content in surface oxide layer O ₁ = X ₁ × 2O / SiO ₂ formula (8) Surface oxygen content O _S = {X ₂ −a (X ₂ −X ₁ ) ² } × 2O / SiO ₂ formula (9) surface silicon oxynitride layer oxygen amount O ₂ = O _S - O ₁ equation (10) internal oxygen O _i = O _t - O _S equation (11) where, O is the atomic weight of oxygen, SiO ₂ is It is the molecular weight of silicon dioxide.

The method for producing silicon nitride powder in the present invention is not particularly limited. For example, in the imide thermal decomposition method, when firing the amorphous silicon nitride powder and / or the nitrogen-containing silane compound under a nitrogen-containing inert gas atmosphere or a nitrogen-containing reducing gas atmosphere, the amorphous silicon nitride powder and / or It can be obtained by controlling the amount of oxygen in the nitrogen-containing silane compound and / or the oxygen partial pressure in the firing atmosphere. As the nitrogen-containing silane compound, silicon diimide, silicon tetraamide,
Silicon nitrogen imide, silicon chlorimide and the like can be mentioned. These are known methods, for example, a method in which a silicon halide such as silicon tetrafluoride, silicon tetrachloride, silicon tetrabromide, and silicon tetraiodide is reacted with ammonia in a gas phase; It is produced by a method of reacting with ammonia.

The amorphous silicon nitride powder can be prepared by a known method, for example, a method of thermally decomposing the nitrogen-containing silane compound in a nitrogen or ammonia atmosphere at a temperature in the range of 600 to 1200 ° C., silicon tetrafluoride, Silicon tetrachloride, silicon tetrabromide,
Examples include those produced by a method of reacting silicon halide such as silicon tetraiodide with ammonia at a high temperature. In the direct nitriding method, by controlling the oxygen partial pressure in the nitriding atmosphere, the heat treatment temperature of the bulk silicon nitride after nitriding, the pulverization conditions of the bulk silicon nitride after the heat treatment, and the mixed acid treatment conditions of the silicon nitride powder after the pulverization, A desired silicon nitride powder can be obtained.

[0021]

The present invention will be described more specifically with reference to the following Examples and Comparative Examples. Example 1 A crystalline silicon nitride powder was produced by using the silicon diimide powder shown in Table 1 as a raw material and performing thermal decomposition and firing under the conditions shown in Table 1. Stirring treatment was performed with 1 g of the obtained silicon nitride powder and 50 g of a hydrofluoric acid aqueous solution having the concentration shown in Table 2, and the mixture was filtered through a Teflon membrane filter. The Si concentration and NH ₄ ⁺ concentration in the filtrate were measured by an ICP analyzer (ICAP-575, manufactured by Nippon Jarrell Ash) and an ion chromatograph analyzer (IC-7, manufactured by Yokogawa Electric Corporation).
000S). Quantitative results and elution S
Table 2 shows the calculation results of the i ₃ N ₄ amount, the eluted SiO ₂ amount, and the total eluted amount, and FIG. 3 shows the plot of the total eluted amount and the eluted Si ₃ N ₄ amount. The boundary X ₂ boundary X ₁ and the surface silicon oxynitride layer region and the inner region of the surface oxide layer region and the surface silicon oxynitride layer region from FIG. 3 obtained in the nonlinear least squares method. The obtained X ₁ and X ₂
Are shown in Table 3. Table 2 shows the total oxygen content, the oxygen content in the surface oxide layer, the oxygen content in the surface silicon oxynitride layer, and the internal oxygen content calculated from X ₁ and X ₂ . The density of the surface oxide layer is 2.33 g / c
Table 3 shows the thickness of each layer when m ³ and the density of the surface silicon oxynitride layer were 2.69 g / cm ³ . Table 3 shows the specific surface area measured by the BET one-point method by the nitrogen gas adsorption method.

Next, the obtained crystalline silicon nitride powder 93w
5% by weight of yttria and aluminum as sintering aid
The mixed powder to which 2 wt% is added is used as a medium for ethanol.
Using a ball mill for 48 hours, and then dry under reduced pressure.
Dried. 500 kg / cm of the resulting mixture^TwoMolding pressure
After molding into a 6 × 45 × 75 mm shape with
0kg / cm^TwoCIP molding was performed at a molding pressure of The obtained result
Fill the form into a silicon nitride crucible and place in a nitrogen gas atmosphere.
Sintered at 0 ° C. for 2 hours. The obtained sintered body is cut out and ground.
After cutting, relative density, 4-point bending strength, fracture toughness K ₁C, wa
Ible coefficient was measured. Table 4 shows the results. phase
The pair density is 3.265 g / cm of the theoretical density.^ThreeToshi Archime
According to Death method, 4-point bending strength is based on JISR-1601 method
From the fracture toughness value K₁C is based on JIS R-1607 method
I asked. The Weibull coefficient is 30 room bending test.
It calculated from the result of.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

[0026]

[Table 4]

Examples 2 to 9 and Comparative Examples 1 to 7 Using various silicon diimide powders and amorphous silicon nitride powders shown in Table 1 as raw materials, thermal decomposition and calcination were carried out under various conditions shown in Table 1 to obtain crystalline materials. A silicon nitride powder was manufactured. Table 3 shows the results of measuring the powder characteristics of the obtained crystalline silicon nitride powder in the same manner as in Example 1. Next, after using the obtained crystalline silicon nitride powder as a raw material, a sintered body was manufactured under the same conditions as in Example 1, and the characteristics of the sintered body were measured. Table 4 shows the results.

Comparative Example 8 The crystalline silicon nitride powder obtained in Example 4 was aged for 3 months in a thermo-hygrostat at 50 ° C. and 60% relative humidity. Table 3 shows the results of measuring the powder characteristics of the obtained powder in the same manner as in Example 1. As shown in FIG. 4, the oxygen distribution of this powder is such that O / (O + N) in silicon oxynitride does not decrease monotonically and rises upward (the SiO ₂ r in comparison with the linear composition gradient on the surface layer side).
ich). Next, after using the obtained crystalline silicon nitride powder as a raw material, a sintered body was manufactured under the same conditions as in Example 1, and the characteristics of the sintered body were measured. Table 4 shows the results.

[0029]

According to the present invention, it is possible to obtain a silicon nitride powder capable of obtaining a silicon nitride sintered body excellent in sinterability and excellent in sintered body characteristics such as high-temperature strength.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a model of oxygen distribution on the surface of a silicon nitride powder.

FIG. 2 is a conceptual diagram showing an analysis result of oxygen distribution on the surface of a silicon nitride powder.

FIG. 3 is a diagram showing a plot of the total elution amount and the elution amount of Si ₃ N ₄ in the analysis of the oxygen distribution of the silicon nitride powder obtained in Example 1.

FIG. 4 is a view showing an oxygen distribution of a silicon nitride powder obtained in Comparative Example 8.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C01B 21/068 C01B 21/082 C04B 35/626 CA (STN)

Claims (1)

(57) [Claims] 1. A crystalline silicon nitride powder having a silicon oxide layer and a silicon oxynitride layer on the surface of particles and having a specific surface area of 7 to 15.3 m ² / g, and a total oxygen content of 0.6 to 2. 0w
t% , oxygen present in the outermost silicon oxide layer is 0.1 wt% or less in terms of oxygen amount, and oxygen present in the silicon oxynitride layer existing subsequent to the silicon oxide layer is 0.4 wt% in terms of oxygen amount. ~
O / (O) in the silicon oxynitride layer.
+ N) The atomic ratio is 0.1 at the connection point with the outermost silicon oxide layer.
95-1 at the connection point with the internal silicon nitride crystal part.
05 value and silicon nitride bonding from the connection point with the silicon oxide layer
A crystalline silicon nitride powder, which monotonically decreases toward a connection point with a crystal part .