JP2863569B2 - Silicon nitride sintered body and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a silicon nitride sintered body having high strength even at a high temperature exceeding 1000 ° C. and excellent in heat resistance, and a method for producing such a silicon nitride sintered body. It relates to a method for stable production.

[Problems to be solved by conventional technology and invention]

Silicon nitride has excellent heat resistance, thermal shock resistance, oxidation resistance,
It has chemical resistance and toughness, and is promising as a structural ceramic. Recently, the use of silicon nitride for engine parts has been considered, and various research and development have been conducted.

By the way, since silicon nitride is a compound having strong covalent bonding, it is difficult to sinter and mold only with silicon nitride powder,
Usually, oxides such as MgO, Y ₂ O ₃ , and Al ₂ O ₃ are added as sintering aids for sintering.

In sintering, the sintering aid and silicon nitride first form a liquid phase, from which silicon nitride is reprecipitated to obtain a sintered body. This oxide forms an amorphous phase at the grain boundaries of silicon nitride because it can only form a solid solution in the crystal lattice. For this reason, the obtained sintered body has a structure having a two-phase mixed structure of a columnar crystal silicon nitride phase and an amorphous phase surrounding the same.

Therefore, the heat resistance and high temperature strength of the obtained sintered body depend on the characteristics of the amorphous phase rather than the silicon nitride itself. This is because, when the sintered body is applied to a portion where the temperature becomes high, first, the amorphous phase which is a grain boundary phase is softened. In addition, when a stress is applied to the amorphous phase at a high temperature, voids may be generated, which causes cracks. Further, cracks develop along the amorphous phase, which leads to destruction of the sintered body.

Therefore, in order to improve the heat resistance and high-temperature strength of the silicon nitride sintered body, the amorphous phase, which is a grain boundary phase, is crystallized, and a method of performing a long-time heat treatment at 1300 to 1400 ° C. is adopted. Have been. However, in this method, although a sialon-based sintered body can provide a reasonable result, a silicon nitride-based sintered body does not achieve desired improvements in heat resistance and high-temperature strength.

The sintering of silicon nitride, as described above, but using a sintering aid composed of an oxide, in a sintered body having excellent heat resistance is generally Y ₂ O ₃ -Al ₂ O ₃ system bake Binders are used. However, it is known that a sintered body obtained using a Y ₂ O ₃ —Al ₂ O ₃ sintering aid shows a sharp decrease in strength at a temperature exceeding 1000 ° C. Furthermore, even if the mixing ratio of Y ₂ O ₃ and Al ₂ O ₃ in the sintering aid is specified, the heat resistance and high-temperature strength of the obtained sintered body vary, and it is difficult to keep the product quality constant. Was. This is considered to be due to the presence of SiO _2, which is an unavoidable component, in silicon nitride as a raw material. That is, when Y ₂ O ₃ and Al ₂ O ₃ are added as sintering aids, what actually acts as an aid in sintering of silicon nitride is a SiO ₂ —Y ₂ O ₃ —Al ₂ O ₃ system. Component. If the content of SiO ₂ in silicon nitride is different for each raw material used, the composition of the Y ₂ O ₃ -Al ₂ O ₃ system component that substantially acts as an auxiliary is not constant, and as a result, the heat resistance of the sintered body The properties and high-temperature strength vary.

Therefore, an object of the present invention is to solve the above problems, and
An object of the present invention is to provide a silicon nitride sintered body having excellent heat resistance that does not cause a decrease in strength even at a high temperature exceeding 00 ° C., and a method for stably producing such a sintered body.

[Means for solving the problem]

The present inventors have conducted intensive studies to achieve the above object, and as a result, added as a sintering aid to the amount of oxygen (in terms of SiO ₂ amount) inevitably contained in the silicon nitride powder as the raw material. If the amounts of Y ₂ O ₃ and Al ₂ O ₃ are properly specified and the sintering temperature is set appropriately, the grain boundary phase can be easily crystallized, and thus has excellent heat resistance and high-temperature strength. The present inventors have found that a silicon nitride sintered body can be obtained, and have reached the present invention.

That is, in the silicon nitride sintered body of the present invention obtained by adding Y ₂ O ₃ and Al ₂ O ₃ as sintering aids, the compounding amount of the sintering aid is such that silicon nitride and the sintering aid are mixed. 15% by weight or more of the total, and the ratio of the oxygen content (expressed by the amount of SiO ₂ ) of the silicon nitride to the amounts of Y ₂ O ₃ and Al ₂ O ₃ is SiO ₂ −Y ₂ O ₃ −Al ₂
In a triangular diagram (expressed by weight ratio) showing the O ₃ ternary system, point A (0, 80, 20), point B (9, 73, 18), point C (9, 5)
5, 36) and D (0, 60, 40).

Further, the method of the present invention for producing a silicon nitride sintered body using Y ₂ O ₃ powder and Al ₂ O ₃ powder as a sintering aid, the compounding amount of the sintering aid, the silicon nitride powder and the The raw material powder comprising the sintering aid is not less than 15% by weight, and the oxygen content (expressed in terms of the amount of SiO ₂ ) in the silicon nitride powder, the amount of Y ₂ O _{3 and the} amount of Al ₂ O ₃ Is a point A (0, 80, 2) in a triangular diagram (expressed in terms of weight ratio) showing a ternary system of SiO ₂ —Y ₂ O ₃ —Al ₂ O ₃ .
0), point B (9, 73, 18), point C (9, 55, 36) and D
The sintering aid is blended and sintered in a region surrounded by four points (0, 60, 40).

 Hereinafter, the present invention will be described in detail.

The silicon nitride raw material powder used in the present invention is desirably of high purity, and impurity elements such as Fe, Ca, Al, and Cl are
It is necessary to keep it below 100 ppm. In addition to the above, O is inevitably present as an impurity element,
This can be considered to exist substantially in the form of SiO ₂ .

The silicon nitride powder is desirably a fine powder, and desirably has a narrow particle size distribution. When the particle diameter is small, the number of nucleation sites increases when reacting with the sintering aid and dissolving and reprecipitating, and as a result, the structure is refined. In addition, if the particle diameters are not uniform and there are some coarse particles, the structure of that part becomes coarse during sintering and becomes a source of destruction. BET as a guide for particle size
The value must be at least 1 m ² / g, preferably 10 m
² / g.

Silicon nitride has α-type and β-type crystal systems, and silicon nitride powder containing a large amount of α-type is used as a raw material powder. It is due to the columnar crystallization of the crystal that silicon nitride exhibits high toughness. This columnar crystallization is caused when α-type silicon nitride powder reacts with a sintering aid to form a liquid phase and reprecipitates. This is because the α-form is promoted by the phase transformation to the β-form. In order for the sintered body to have high toughness, at least 60% or more of the silicon nitride powder needs to be α-type, preferably 90% or more.

Y ₂ O _{3 to be} added as a sintering aid needs to be of high purity like silicon nitride powder. Ce, as an impurity element
Pr, Nd, Sm, Tb, Dy, Ca, Fe and the like can be considered, but it is desirable to suppress each of them to 100 ppm or less, preferably 50 ppm or less. Further, the particle diameter needs to be fine powder equivalent to or more than the silicon nitride powder. Therefore, the BET value needs to be 1 m ² / g or more, preferably 10 m ² / g or more.

Also, Al ₂ O ₃ , another component of the sintering aid, needs to be similarly high in purity. As impurity elements,
Na, Ca, Mg, Fe, Si, Ga, Cr and the like are conceivable, and they are each set to 100 ppm or less, preferably 50 ppm or less. Al ₂ O ₃ includes α-type and γ-type, and either of them may be used. However, the α-type has a BET value of up to about 10 m ² / g, while the γ-type has a BET value of 100 m ² / g or more. Suitable as an auxiliary. Either of these uses a BET value of 1m ²
/ g, preferably 10 m ² / g or more.

The amount of Y ₂ O ₃ and Al ₂ O ₃ added as a sintering aid is 15% by weight or more based on the entire raw material powder to which the silicon nitride powder is added. When the amount of the sintering aid is less than 15% by weight, crystallization of the grain boundary phase does not occur, and improvement in high-temperature strength is not observed.

The reason why the grain boundary phase is crystallized when the content of the sintering aid is 15% by weight or more is considered to be as follows. That is, whether or not the grain boundary phase is crystallized depends on whether or not crystal nuclei are generated in the liquid phase forming the grain boundary during solidification after liquid phase sintering. The free energy change ΔG at this time is as follows, assuming that the nucleus is a sphere. Here, r is the radius of the spherical nucleus, ΔGn is the change in the free energy of the nucleus per unit volume, and γ _s is the change in the surface energy per unit area during nucleation.

In order to generate nuclei in the liquid phase, ΔG ≦ 0 (2) must be satisfied. Substituting (2) into (1) and rearranging Holds. Here the 3γ _s / ΔGn = r _c Then _{r ≧ r c ... (4)} . In other words, in order to generate a crystal nucleus, there must be a nucleus generation site called r _c which is larger than the critical nucleus size.
It is considered that by setting the total amount of the sintering aid to 15% by weight or more, the amount of the grain boundary phase is increased, and the nucleation portion is increased, whereby crystallization of the grain boundary phase is facilitated.

By the way, as described above, SiO ₂ is unavoidably present on the silicon nitride surface. In the present invention, the amount of compound oxygen in silicon nitride used in advance is determined by analysis, and from this value, the content of SiO _{2 in} silicon nitride is determined. The amount can be estimated.
Since the compound oxygen contained in silicon nitride can be considered to be substantially due to SiO ₂ , the amount of SiO ₂ can be calculated from the oxygen amount X (% by weight) obtained by the analysis according to the following formula.

Knowing the oxygen content in silicon nitride, this and Y ₂ O ₃
It is important to define the relationship with the amount of Al ₂ O ₃ .
This is because not only the added Y ₂ O ₃ and Al ₂ O ₃ actually work as an aid to the sintering reaction, but also the SiO _{2 in} silicon nitride.
This is because it is considered to be a ternary substance of SiO ₂ —Y ₂ O ₃ —Al ₂ O ₃ to which is added.

In the present invention, the amount of SiO ₂ is SiO _2, Y ₂ O ₃ and Al ₂ O ₃
When the amount exceeds 9% by weight with respect to the total amount of, the grain boundary phase becomes difficult to crystallize, and the high-temperature strength and heat resistance become poor. Therefore, the amount of SiO ₂ needs to be 9% by weight or less.

FIG. 1 shows a graph of Si calculated from the oxygen content in silicon nitride.
SiO ₂ -Y ₂ O showing the range of O ₂ amount, Y ₂ O ₃ and Al ₂ O ₃ and the mixing ratio
_{It is} a triangular diagram (expressed by weight ratio) of _3- Al ₂ O ₃ ternary system. In the present invention, the weight ratio of the oxygen content (expressed in terms of the amount of SiO ₂ ) of silicon nitride to the Y ₂ O ₃ and Al ₂ O ₃ added as sintering aids is represented by the point A (0, 80, 20). ), Point B (9, 73, 18), point C (9, 55, 36) and D (0, 60, 40) (coordinates are
(In the order of SiO ₂ , Y ₂ O ₃ , and Al ₂ O ₃ ). First, the line segment BC indicates a point where the SiO ₂ content is 9% by weight. The ratio between Y ₂ O ₃ and Al ₂ O ₃ is defined as a region between the line segment AB and the line segment CD (including the region on both line segments). In a region on the left side of the line segment AB, Y ₂ O ₃ becomes excessive, and crystallization of the grain boundary phase becomes difficult. On the other hand, in the composition in the region on the right side of the line segment CD, Al ₂ O ₃ becomes excessively large, making it difficult to crystallize the grain boundary phase, and there is no improvement in high-temperature strength and heat resistance.

Next, a method for manufacturing a silicon nitride sintered body of the present invention will be described.

First, silicon nitride powder and Y ₂ O ₃ and Al ₂ O ₃ powders as sintering aids are mixed.

It is reliable and inexpensive to perform the mixing by a ball mill method. The longer the kneading time by the ball mill method is, the more preferable it is 10 hours or more. Desirably mix for at least 78 hours. Thus, by mixing both well, a uniform fine structure can be obtained.

It is preferable that the pot and the ball used for kneading are composed of one or a combination of two or more components of the raw material powder. Specifically, it is optimal to use one made of silicon nitride. Since pots and balls made of silicon nitride are expensive, pots and balls made of alumina may be used. Thereby, mixing of different components into the raw material due to abrasion of the pot or ball can be prevented.

In addition, as a dispersion medium at the time of kneading, methyl alcohol,
Ethyl alcohol, acetone, water and the like can be used.

After kneading, the slurry is dried and granulated. The dry granulation method is roughly classified into two methods. The first method is a method of removing the solvent by drying (thermally or removing the solvent by a microwave oven or the like), and then granulating by sieving. The second method is a method using a spray drier, in which drying and granulation are performed simultaneously. In the present invention, either method may be used.

Next is molding, press molding, rubber press molding,
Injection molding, casting and the like can be used.

 Next, sintering will be described.

In the present invention, in order to make the liquid phase sintering method of silicon nitride effective, for example, sintering can be performed using a firing program shown in FIG.

First, as a first step, the compact is heated at 300-600 ° C for 30-
Heat for 60 minutes to remove moisture and alcohol adhering to the surface of the sintered powder.

Next, as a second step, the temperature is raised to 800-1200 ° C. and held for 15-40 minutes. This is for the purpose of equalizing the temperature of the molded body, and is performed for the purpose of preventing generation of cracks and internal stress during sintering.

In the third step, raise the temperature to 1700-1750 ° C and
Hold for 10 minutes. This is also performed for soaking the molded body, but in the molded body having the above composition, the liquid crystallinity temperature is from 1600 to
Since it is considered to be around 1700 ° C., a liquid phase is formed at this stage.

As a fourth step, the temperature is raised to 1800 to 1950 ° C. and held for 1 to 30 minutes. By setting the temperature higher by about 100 to 250 ° C. than the liquid phase crystallization temperature, liquid phase sintering can be completed in a short time. If the liquid crystal sintering time is long, the mechanical properties of the sintered body obtained by coarsening the crystal structure are reduced, so that the time is set to 30 minutes or less. If the heating and holding time is less than 1 minute, the sintering is not completed, which is not preferable. If the temperature is set higher within the above range, the holding time of the temperature can be shortened.However, since sublimation or decomposition of silicon nitride may occur, in this case, sintering is performed under N ₂ gas of less than 10 atm. Perform a knot.

Finally, the compact is kept at 1700 to 1750 ° C. for 30 to 60 minutes to eliminate pores in the grain boundary phase remaining at the latter stage of sintering, thereby obtaining a sintered body.

The heating rate and the cooling rate during the firing program are:
The temperature is preferably about 40 ° C./min and 1000 ° C./min, respectively.

The silicon nitride sintered body of the present invention can be obtained by the firing program described above, but may be subjected to HIP processing to increase the strength of the sintered body.

The present invention will be described in more detail with reference to the following specific examples.

〔Example〕

Example 1, Comparative Examples 1-6 Y ₂ O ₃ powder, Al ₂ O ₃ powder, and silicon nitride powder in the proportions shown in Table 1 were all charged in an Al ₂ O ₃ pot having an internal volume of 2, respectively. And put it to 200g. The oxygen content of the used silicon nitride powder was 1.26% by weight.

Add ethanol 1 to each pot and add 1 at 20φ.
3 kg of alumina balls were put therein, and mixed in a wet ball mill for 96 hours.

The analysis values of the used silicon nitride powder, Y ₂ O ₃ and Al ₂ O ₃ powder are shown in Tables 2, 3 and 4, respectively.

After completion of the kneading, the alcohol was removed by a microwave oven, and the mixture was granulated by a sieve (<# 60) to obtain a final raw material powder.

Next, 10 cc of alcohol was added to 50 g of the raw material powder, and the mixture was stirred well, and then pressed into a 60φ disk at a pressing pressure of 1.5 tons. Further, rubber pressing was performed at a pressure of 3.5 ton / cm ² to obtain a molded body.

The obtained compact was buried in a mixed powder of a silicon nitride powder and a boron nitride powder, and was subjected to normal pressure sintering in a nitrogen gas atmosphere. The sintering temperature followed a program as shown in FIG. Here, the first step is 30 minutes at 500 ° C., the second step is 40 minutes at 1190 ° C., the third step is 10 minutes at 1750 ° C., the fourth step is 10 minutes at 1850 ° C., and the fifth step is 40 minutes at 1750 ° C. Min, and the temperature rise and fall rates are 40 ° C / min and 1000, respectively.
° C / min.

After firing, grinding and cutting were performed to produce a test piece of 4.0 mm × 3.0 mm × 36 mm.

The density, hardness, fracture toughness, and bending strength of the obtained test pieces were measured. The results are shown in Table 9.

In order to confirm the presence or absence of a crystal phase in the grain boundary phase, X-ray diffraction and observation of the fracture surface by a scanning electron microscope were performed. In the X-ray diffraction of the test piece of Example 1, a plurality of peaks independent of the silicon nitride crystal were observed. This confirmed that the grain boundary phase was crystallized.

FIG. 3 shows SiO ₂ and Y ₂ O ₃ in Examples and Comparative Examples.
And the component ratio of Al ₂ O ₃ . Here, black circles (•) indicate Examples, white circles (O) indicate Comparative Examples, and the numbers after each circle indicate Example No.

Examples 2 and 3, and Y ₂ O ₃ powder in the proportions shown in Comparative Examples 7-11 Table 5, using the Al ₂ O ₃ powder and silicon nitride powder, the Likewise silicon nitride sintered body as in Example 1 Manufactured. The same Y ₂ O ₃ and Al ₂ O ₃ powder as used in Example 1 was used. The oxygen content of the silicon nitride powder used was 0.
It was 8% by weight. Table 6 shows the analysis values of the silicon nitride powder.

The density, hardness, fracture toughness, and bending strength of the obtained test pieces were measured. The results are shown in Table 9.

The composition ratio of SiO ₂ , Y ₂ O ₃ and Al ₂ O ₃ in each example is shown in the triangular diagram of FIG.

Examples 4 and 5, Comparative Examples 12 to 16 and Y ₂ O ₃ powder in the proportions shown in Table 7, Al ₂ O ₃ powder, and using a silicon nitride powder, the same way silicon nitride sintered body as in Example 1 Was manufactured. The same Y ₂ O ₃ powder and Al ₂ O ₃ powder as those used in Example 1 were used. The oxygen content of the silicon nitride powder is 0.4
% By weight. Table 8 shows the analysis values of silicon nitride.

The density, hardness, fracture toughness, and bending strength of the obtained test pieces were measured. The results are shown in Table 9. The composition ratio of SiO ₂ , Y ₂ O ₃ and Al ₂ O ₃ in each example is shown in the triangular diagram of FIG.

(Effects of the Invention) As described in detail above, in the present invention, the compounding amount of the sintering aid composed of Y ₂ O ₃ and Al ₂ O ₃ is set to a specific amount or more,
The amount of inevitably SiO ₂ in the presence of silicon nitride powder was estimated, since also defines the relationship between the Y ₂ O ₃ amount and the amount of Al ₂ O ₃ sintering aid added with the amount of SiO _2, sintered Can easily crystallize the grain boundary phase formed by the above, and thus a sintered body excellent in high-temperature strength and heat resistance can be obtained. As described above, since the composition of the component actually acting as a sintering aid is defined, the grain boundary phase can be reliably crystallized, and variations in high-temperature strength and heat resistance do not occur for each product.

Further, in the present invention, the sintering includes a step of holding at a temperature higher than the liquid phase crystallization temperature for a short time, and since the crystal grain size of silicon nitride is formed small, the sintered body having excellent mechanical strength is formed. It can be.

Such a silicon nitride sintered body has the same strength even at a high temperature of about 1200 ° C. as at room temperature, and can be used as a structural ceramic in a portion exposed to a high temperature.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the oxygen content (Si) in silicon nitride according to the present invention.
O ₂ ), and Y ₂ O ₃ and Al ₂ O ₃ added as sintering aids.
FIG. 2 is a triangular diagram of a ternary system of SiO ₂ —Y ₂ O ₃ —Al ₂ O ₃ showing the range of the weight ratio of the amount, and FIG. 2 is used when producing a silicon nitride sintered body by the method of the present invention. FIG. 3 is a graph showing an example of a firing program. FIG. 3 shows SiO ₂ , Y ₂ O ₃ and Al ₂ O in Examples and Comparative Examples.
₃ is a triangular diagram showing a component ratio of _3. FIG.

Continuation of front page (58) Field surveyed (Int. Cl. ⁶ , DB name) C04B 35/584

Claims (3)

(57) [Claims] 1. A silicon nitride sintered body obtained by adding Y ₂ O ₃ and Al ₂ O ₃ as sintering aids, wherein the amount of said sintering aid is such that silicon nitride and said sintering agent are mixed. with 15 wt% or more of total auxiliaries, the ratio between the amount of the oxygen content of the silicon nitride and (expressed in SiO ₂ equivalence), Y ₂ O ₃ and Al ₂ O ₃ is, SiO ₂ - Y ₂ O ₃
In a triangular diagram (expressed in terms of weight ratio) showing the ternary system of —Al ₂ O ₃ , points A (0, 80, 20), points B (9, 73, 18), and points C
(9, 55, 36) and a point D (0, 60, 40) in a region surrounded by four points, a silicon nitride sintered body. 2. A method for producing a silicon nitride sintered body using a Y ₂ O ₃ powder and an Al ₂ O ₃ powder as sintering aids, wherein the compounding amount of the sintering aids is The raw material powder comprising the sintering aid is 15% by weight or more, and the oxygen content (expressed in terms of the amount of SiO ₂ ) in the silicon nitride powder and Y ₂ O ₃
And the ratio of the amount of Al ₂ O ₃ is in a triangular diagram showing the _{SiO 2 -Y 2 O 3 -Al 2} O 3 ternary system (expressed by weight), the point A (0,80,2
0), point B (9, 73, 18), point C (9, 55, 36) and D
A method for producing a silicon nitride sintered body, comprising mixing and sintering the sintering aid so as to be in a region surrounded by four points (0, 60, 40). 3. The method according to claim 2, wherein the raw material powder is kneaded for at least 10 hours by a ball mill method and then molded.
A method for producing a silicon nitride sintered body, comprising sintering by a firing program including a step of firing at a temperature of at least 1800 to 1950 ° C. for 1 to 30 minutes.