JP2731333B2 - Silicon nitride sintered body, method of manufacturing the same, silicon nitride powder 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 sintered body, and more particularly, to a thick, high-strength, high-density silicon nitride sintered body, a method for producing the same, and a silicon nitride powder used for the same.

[0002]

2. Description of the Related Art Conventionally, silicon nitride sintered bodies have been attracting attention because of their high strength at high temperatures, chemical stability, etc., and have been applied to structural materials for heat engines such as diesel and gas turbines. Have been studied in various ways. In general, since silicon nitride is a hard-to-sinter body, in order to impart strength to the obtained sintered body,
A silicon nitride powder, MgO, by firing molded by the addition of Y ₂ 0 sintering aid such as _3, are manufacturing silicon nitride sintered body.

In such a method for producing silicon nitride,
When the generation temperature of the liquid phase generated during firing is high, a liquid phase is generated, a phase transformation from α-Si ₃ N ₄ to β-Si ₃ N ₄ occurs, and β-Si to be controlled crystal growth and densification of ₃ N _4, it is known that not be obtained a high-density high-strength sintered body. In such a situation, for a thin product, a sintered body having a high density and a high strength is obtained by controlling the firing temperature, the heating rate, the cooling rate, the firing atmosphere, and the like to a high degree.

[0004]

However, for thick-walled products, the liquid phase generation speed and the phase transformation speed are different due to differences in atmosphere, temperature, and the like between the surface and the inside of the molded product. There is a problem that a sintered body having a thickness of 20 mm or more cannot be obtained at a high density even by using the material preparation technology and the firing control technology that have been used. The present invention has been made in view of such problems of the related art, and an object thereof is to provide a high-density, high-strength, thick silicon nitride sintered body, a method of manufacturing the same, and a method of manufacturing the same. An object of the present invention is to provide a silicon nitride powder and a method for producing the same.

[0005]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and as a result, have found that the behavior of SiO ₂ involved in liquid phase generation and phase transformation of silicon nitride can be measured in the vicinity of the surface of silicon nitride powder. The inventors have found that the above object can be achieved by adjusting and controlling the amount of oxygen, and have completed the present invention.

Accordingly, the silicon nitride sintered body according to the present invention has one or two kinds of grains selected from the group consisting of a rare earth element silicate, silicate nitride and silicon dioxide, wherein the grain boundary phase does not contain an Al component. A silicon nitride sintered body containing the above composition,
The oxygen component conversion amount calculated by the molar ratio between the vicinity of the inner center and the vicinity of the surface of the sintered body is represented by the following equation: 0.1 ≦ (SiO ₂ / Re ₂ O ₃ ) c− (SiO ₂ / Re ₂ O ₃ ) s ≦ 0.5 (where Re is a rare earth element, (SiO ₂ / Re ₂ O ₃ ) c is the oxygen component conversion amount near the inner center, (SiO ₂ / Re
₂ O ₃ ) s indicates an oxygen component conversion amount near the surface. ) Is satisfied.

Further, according to the method for producing a silicon nitride sintered body of the present invention, in producing a silicon nitride sintered body, the oxygen content of the silicon nitride powder is adjusted to 0.5 to 3.0% by weight. The surface of the powder is represented by the following formula: Os / Ss = 30 to 100 (where Os is the amount of oxygen (wt%) on the powder surface and Ss is the specific surface area (m ² / mg) of the powder). Adjusted to satisfy the oxygen content, mixed the obtained silicon nitride powder with a sintering aid containing no Al component , molded,
It is characterized by firing.

[0008]

Further, according to the method for producing silicon nitride powder of the present invention, the oxygen content is adjusted to 0.5 to 3.0% by weight,
In addition, the following formula Os / Ss = 30 to 100 (Os in the formula indicates the amount of oxygen (wt%) on the surface of the powder, and Ss indicates the specific surface area (m ² / mg) of the powder). A method for producing a silicon nitride powder adjusted to satisfy the amount, wherein the silicon nitride powder has a content of 30 to 80% by weight.
Water is added and mixed to obtain a slurry, and the temperature of the slurry is set to 3
It is characterized by mixing and stirring for 1 to 5 hours while maintaining the temperature at 0 to 80 ° C.

Further, according to the method for producing silicon nitride powder of the present invention, the oxygen content is adjusted to 0.5 to 3.0% by weight, and Os / Ss = 30 to 100 (where Os is The amount of oxygen (wt%) on the surface of the powder, and Ss indicates the specific surface area (m ² / mg) of the powder.) There is a temperature of 70-99 ° C and a humidity of 90-9
A container controlled to 9% is filled with silicon nitride powder and exposed in this state for 10 to 500 hours.

Further, according to the method for producing silicon nitride powder of the present invention, the oxygen content is adjusted to 0.5 to 3.0% by weight, and the following formula Os / Ss = 30 to 100 (where Os is The amount of oxygen (wt%) on the surface of the powder, and Ss indicates the specific surface area (m ² / mg) of the powder.) In addition, a furnace controlled at a temperature of 300 to 800 ° C. is filled with a silicon nitride powder and flowed for 1 to 20 hours.

[0012]

In the silicon nitride sintered body of the present invention, the ratio between SiO ₂ and the sintering aid present in the grain boundary phase does not change much near the surface of the sintered body and near the center of the inside. Therefore, SiO ₂
And the sintering aid (actually, a compound derived from the sintering aid) are uniformly dispersed from the vicinity of the surface of the sintered body to the vicinity of the inner center thereof. This is considered to mean that the β-Si ₃ N ₄ particles constituting the sintered body have a uniform particle size, small pores, and few firing defects. Therefore, the sintered body of the present invention has a small difference in strength between the vicinity of the surface and the vicinity of the inner center, and can be applied to thick products, particularly products having a thickness of 20 mm or more.

In the present invention, the grain boundary phase of the sintered body is controlled by controlling the oxygen content of the silicon nitride powder, particularly the oxygen content near the surface (for example, a region from the surface to several nm). The following reasons can be considered as the reason why the structure is uniform from the surface to the inside of the body.

That is, in the sintering process of the silicon nitride sintered body, first, a sintering aid (metal oxide) acts to generate a liquid phase (glass phase). Next, α-Si ₃ N ₄ is dissolved in this liquid phase, and is precipitated as β-Si ₃ N ₄ . In order for this dissolution-precipitation reaction to occur uniformly inside the sintered body, it is essential to generate a uniform liquid phase over a wide area.

The formation of the liquid phase is affected not only by the sintering aid but also by the SiO ₂ present on the surface of the Si ₃ N ₄ powder. For example, in sintering of Si ₃ N ₄ containing a sintering aid having a low melting point such as MgO or CaO, a sintering aid (metal oxide) and SiO ₂ react at a considerably low temperature to form a liquid phase. I do.

On the other hand, in an auxiliary mainly composed of a rare earth element oxide, the rare earth element oxide and SiO ₂
Eutectic temperature of the composite oxide is higher than the melting point of SiO _2, SiO ₂ amount present on the surface of the Si ₃ N ₄ powder is a large effect in the liquid phase product of該助agent with. Therefore,
It is considered that the existence of Si ₃ N ₄ particles having an appropriate SiO ₂ coating is important in achieving uniform liquid phase generation in a wide area.

Next, the silicon nitride sintered body of the present invention will be described. This onset Ming of the sintered body, including the grain boundary phase is Al component
And contains a rare earth element silicate, silicate nitride, silicon dioxide or a composition thereof. These silicates and silicate nitrides are caused by sintering aids,
Examples of the sintering aid include rare earth element oxides. In addition, as rare earth elements, Y, Lu, Er, Y
b, Tm, Sc, and Nd can be exemplified.

Further, as the sintering aid, not only the above oxides but also silicides and carbides may be used. Examples of the silicide include silicides such as Mo, W, Nb, and Ta. Further, as the carbide, Si, Mo,
And carbides such as W, Nb and Ta. In addition, these sintering aids are not limited to one kind, and may be used as needed.
Species or more may be used in combination.

In the sintered body of the present invention, the converted value of the oxygen component in the vicinity of the center of the inside of the sintered body and the converted value of the oxygen component in the vicinity of the surface of the sintered body are represented by the following formula (SiO ₂ / Re ₂ O). _{3) c- (SiO 2 / Re} 2 O 3) s ≦
0.5 (Re in the formula is a rare earth element, (SiO ₂ / Re ₂ O ₃ ) c is an oxygen component conversion amount near the inner center, and (SiO ₂ / Re
₂ O ₃ ) s indicates an oxygen component conversion amount near the surface. ) Is satisfied. Here, the “oxygen component conversion value” is
Silicon nitride sintered body analysis value of the oxygen content contained in, SiO ₂
And a sintering aid (typically, Re ₂ O ₃ , where Re indicates a rare earth element).

Next, a method for producing a sintered body according to the present invention will be described. In the production method of the present invention, first, silicon nitride powder (substantially α-Si ₃ N ₄ ) has an oxygen content of 0.5 to 3.0% by weight and the following formula Os / Ss = 30 -100 (Os in the formula represents the amount of oxygen (% by weight) on the powder surface, and Ss represents the specific surface area of the powder (m ² / mg)). Deviating from the range of the above formula is not preferable for the following reasons.

That is, when Os / Ss is less than 30, S
As i ₃ N ₄ powder, Os is small or Ss is large, which means that Os per surface area of Si ₃ N ₄ powder is large.
, A uniform liquid phase cannot be formed in a wide range, and the dissolution / precipitation reaction of the Si ₃ N ₄ particles proceeds only in the vicinity of the Si ₃ N ₄ powder (particles) where the sintering aid exists. Will do.

On the other hand, when Os / Ss exceeds 100, Os / Ss
Is high or Ss is small, and the amount of oxygen per surface area of the Si ₃ N ₄ particles is too large. For this reason, the generation of the liquid phase proceeds more than necessary, and the shrinkage of the molded body locally proceeds by the dissolution / precipitation reaction. If such a phenomenon occurs in the process of firing the thick molded body, the shrinkage proceeds in the vicinity of the surface having a high temperature, which hinders the shrinkage inside. Furthermore, β-
Since Si ₃ N ₄ does not precipitate uniformly from the liquid phase, a small β-
Abnormal grain growth occurs with Si ₃ N ₄ as a nucleus. This is because this causes the generation of pores in the vicinity of the abnormally grown Si ₃ N ₄ particles.

Further, oxygen content and surface effective oxygen amount, respectively, 1.0 to 2.5% by weight, more preferably controlled to be 50 to 70. The adjustment of the oxygen amount and the surface effective oxygen amount can be performed by appropriately controlling the humidity, temperature, and the like of the silicon nitride powder. Specifically, as a wet adjustment method, water is added to and mixed with silicon nitride powder to obtain a water content of 30 to
An 80% by weight slurry is prepared and the temperature of the slurry is 30 to 8
A method in which the mixture is kept at 0 ° C. and mixed and pulverized for 1 to 5 hours can be exemplified. As the mixing and crushing machine used in this method, a mixing and crushing machine with a temperature adjusting mechanism, for example, an attritor mill is preferable.

As a method of controlling the humidity, silicon nitride powder is heated at a temperature of 70 to 99 ° C. in a container and a humidity of 90 to 99 ° C.
%, And then exposing for 10 to 500 hours. During this exposure, the silicon nitride powder may be caused to flow. Further, as a heat treatment method, a method of flowing silicon nitride powder in a heating furnace controlled at 300 to 800 ° C. for 1 to 20 hours can be exemplified.

Then, the silicon nitride powder whose oxygen content and surface effective oxygen content have been adjusted as described above is used without containing an Al component.
Mixed with a sintering aid, and molded and fired by a conventional method to obtain a silicon nitride sintered body.

[0026]

EXAMPLES The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited to these examples. (Examples 1 to 15, Comparative Examples 1 to 4) As shown in Table 1,
Predetermined moisture was added to the silicon nitride powder and heated to a predetermined temperature. Next, a cobblestone made of Si ₃ N _{4 having} a diameter of ₅ mm was added and mixed and pulverized for a predetermined time by an attritor mill. A predetermined sintering aid was added to the obtained slurry and mixed for 3 hours. And
The obtained mixture was granulated by a spray drying method to obtain a molding powder. At this time, the specific surface area (BET
Specific surface area), the amount of oxygen and the amount of available oxygen on the surface were measured.

Here, the oxygen content and the surface effective oxygen content were analyzed and measured according to the following procedure. First, silicon nitride powder was added to 11
After drying at 0 ° C. for 2 hours, the mixture was gradually cooled to room temperature in a desiccator, 1 g of the obtained analysis sample was evaluated, and a sealable container made of polytetrafluoroethylene (for example, HU-1 manufactured by San-ai Kagaku)
(Teflon inner cylinder of a 00 type pressure decomposition container). Then, a hydrofluoric acid solution (hydrofluoric acid: water = 1: 9 (weight ratio)) 50
Then, the mixture was sealed, and stirred at 70 ° C. for 1 hour. After cooling with water to room temperature, the mixture was immediately filtered through a 0.65 μm membrane filter. The obtained residue by filtration is washed with 5 ml of water,
This washing was repeated several times. The washed residue was placed in a dryer and dried at 110 ° C. for 2 hours.

Next, the analysis sample before the treatment with the hydrofluoric acid solution and a part of the dried residue by filtration are subjected to oxygen analysis using an oxygen analyzer, for example, an ECM-2800 nitrogen / oxygen analyzer manufactured by Dig Seisakusho. The minute was quantified. The quantitative value of oxygen from the analysis sample before the hydrofluoric acid treatment is the oxygen amount (total oxygen amount), and the quantitative value from the dried residue is the internal oxygen amount. The surface effective oxygen amount (Os) was determined as the difference between the total oxygen amount and the internal oxygen amount.

Next, the molding powder obtained as described above was subjected to die pressing and CIP molding to obtain a diameter of 80 mm ×
A disc-shaped molded body having a thickness of 8 mm or a diameter of 80 mm x a thickness of 25 mm was produced. This molded body was treated with 9 kg / cm ² of N ₂
It was fired in an atmosphere at 1900 ° C. for 3 hours to obtain a silicon nitride sintered body. A test piece having a thickness of 4 mm × 3 mm × 40 mm was cut out from the center (of the thickness) of the obtained sintered body, and this test piece was used at room temperature and 1400 ° C. according to JIS R 1601.
A four-point bending strength test was performed in accordance with the standards described in Table 2, and the obtained results are shown in Table 2. In addition, the relative density of each test piece is also described.

(Examples 16 to 21, Comparative Examples 5 to 8) As shown in Table 3, silicon nitride powder was charged into a high-temperature and constant-humidity dryer controlled at a predetermined temperature and humidity, and exposed for a predetermined time. Next, except that a sintering aid was added and mixed using a cobblestone,
The same operation as in Examples 1 to 15 was performed. Table 4 shows the obtained results. (Examples 22 to 29, Comparative Examples 9 to 12) As shown in Table 5, the silicon nitride powder was caused to flow in a rotary kiln maintained at a predetermined temperature for a predetermined time, and then a sintering aid was added thereto and a cobblestone was used. The same operations as in Examples 1 to 15 were performed except for mixing, and the obtained results are shown in Table 6.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

[0035]

[Table 5]

[0036]

[Table 6]

As is evident from Tables 1 to 3, excellent strength characteristics were obtained in Examples 1 to 28 in which the amount of oxygen and the effective surface oxygen were adjusted and which were within the scope of the present invention. Therefore,
According to this embodiment, excellent strength characteristics can be imparted to a silicon nitride sintered body product having a thickness of about 20 mm.

[0038]

As described above, according to the present invention,
Since the behavior of SiO ₂ involved in the liquid phase generation and the phase transformation of silicon nitride is controlled by adjusting the amount of oxygen near the surface of the silicon nitride powder, high-density, high-strength silicon nitride sintering is performed. The present invention can provide a body, a method for producing the same, a silicon nitride powder used for the same, and a method for producing the same.

Continuation of front page (56) References JP-A-1-313308 (JP, A) JP-A-3-290369 (JP, A) JP-A-63-218584 (JP, A) JP-A-5-170542 (JP) , A)

Claims (10)

(57) [Claims] 1. A silicon nitride firing method in which the grain boundary phase does not contain an Al component and contains one or more compositions selected from the group consisting of rare earth element silicates, silicate nitrides and silicon dioxide. In the sintered body, the oxygen component conversion amount calculated by the molar ratio between the vicinity of the inner center of the sintered body and the vicinity of the surface is represented by the following equation: 0.1 ≦ (SiO ₂ / Re ₂ O ₃ ) c− (SiO ₂ / Re ₂ O ₃ ) s ≦ 0.5 (where Re is a rare earth element, (SiO ₂ / Re ₂ O ₃ ) c is the oxygen component conversion amount near the inner center, (SiO ₂ / Re
₂ O ₃ ) s indicates an oxygen component conversion amount near the surface. A silicon nitride sintered body, which satisfies the relationship represented by (1). 2. The method according to claim 1, wherein the rare earth element is Y, Lu, Er, Yb,
The sintered body according to claim 1, wherein the sintered body is one or more elements selected from the group consisting of Tm, Sc, and Nd. 3. The method according to claim 2, wherein the grain boundary phase comprises Mo, W, Nb and T.
a) a silicide of one or more elements selected from the group consisting of a and / or a carbide of one or more elements selected from the group consisting of Si, Mo, W, Nb and Ta. The sintered body according to claim 1 or 2, wherein 4. The sintered body according to claim 1, wherein the thickness of the sintered body is 20 mm or more. 5. When producing a silicon nitride sintered body, the oxygen content of the silicon nitride powder is adjusted to 0.5 to 3.0% by weight,
The powder is represented by the following formula: Os / Ss = 30 to 100 (Os in the formula represents the amount of oxygen (wt%) on the surface of the powder, and Ss represents the specific surface area (m ² / mg) of the powder). Adjusted to satisfy the surface effective oxygen amount, the obtained silicon nitride powder was mixed with a sintering aid containing no Al component , and molded,
A method for producing a silicon nitride sintered body, characterized by firing. 6. The silicon nitride powder has an oxygen content of 1.0 to 2.5.
The process according to claim 5, wherein the wt% to adjust the surface effective oxygen amount 50-70. 7. The oxygen content is adjusted to 0.5 to 3.0% by weight, and the following formula: Os / Ss = 30 to 100 (where Os is the oxygen content (% by weight) on the powder surface, Ss Is a specific surface area (m ² / mg) of the powder.) A method for producing a silicon nitride powder which is adjusted so as to satisfy a surface effective oxygen amount represented by the following formula: %
Water is added and mixed to obtain a slurry, and the temperature of the slurry is set to 3
A method for producing silicon nitride powder, comprising mixing and grinding for 1 to 5 hours while maintaining the temperature at 0 to 80 ° C. 8. The oxygen content is adjusted to 0.5 to 3.0% by weight, and the following formula: Os / Ss = 30 to 100 (where Os is the oxygen amount (% by weight) on the powder surface, Ss Is a specific surface area (m ² / mg) of the powder.) A method for producing silicon nitride powder, which is adjusted to satisfy the surface effective oxygen amount represented by the following formula: ~ 9
A method for producing silicon nitride powder, characterized in that a container controlled to 9% is filled with silicon nitride powder and exposed in this state for 10 to 500 hours. 9. The method for producing silicon nitride powder according to claim 8, wherein the silicon nitride powder is caused to flow. 10. The oxygen content is adjusted to 0.5 to 3.0% by weight, and the following formula: Os / Ss = 30 to 100 (where Os is the oxygen content (% by weight) on the powder surface, Ss Is a specific surface area (m ² / mg) of the powder.) This is a method for producing a silicon nitride powder adjusted to satisfy the surface effective oxygen amount represented by the following formula, and the temperature is controlled at 300 to 800 ° C. A method for producing silicon nitride powder, characterized by filling a furnace with silicon nitride powder and flowing it for 1 to 20 hours.