JP3520998B2 - Heat-resistant silicon nitride sintered body and method for producing 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 structural ceramic material, in particular, a heat-resistant silicon nitride sintered body which can be directly immersed in a molten metal of aluminum, zinc, copper, iron, and alloys of these metals, and a sintered body thereof. It relates to a manufacturing method. The silicon nitride sintered body of the present invention means a generic name of both silicon nitride and sialon.

[0002]

2. Description of the Related Art For example, in the process of melting and casting aluminum, a stalk for supplying the molten aluminum into the mold, a heater tube for heating and holding the molten aluminum, and a thermocouple protection tube for measuring the temperature of the molten aluminum. Etc. are used by being directly immersed in molten aluminum.
For these members, cast iron coated with ceramic powder having heat resistance has been conventionally used.

Since the heat-resistant ceramic coating does not adhere to the cast iron with sufficient adhesion, it is easily peeled off. When peeled off, the cast iron tends to dissolve in the molten aluminum, which causes a problem that the quality of the molten aluminum deteriorates. For this reason, the coating must be repaired in daily inspection, resulting in poor work efficiency. Further, since it is made of cast iron, there is a problem that it is relatively heavy and not easy to handle. Recently, in order to solve such a problem, a method of using high-density and high-strength silicon nitride or sialon instead of the cast iron member has been proposed in Japanese Patent Publication No. 4-44628.

[0004]

When a heat-resistant silicon nitride sintered material such as silicon nitride or sialon is used, the quality of the molten metal is improved because almost no impurities are dissolved in the molten aluminum. . However, when a heat-resistant silicon nitride sintered material such as silicon nitride or sialon is used for melting aluminum, it is eroded by the flux containing NaF and NaCl as the main components, and aluminum and slag adhere. For this reason, there arises a new problem that the high temperature strength and the thermal shock resistance which these sintered bodies originally have are impaired, and in the case of stalk, the flow rate of the molten aluminum becomes uneven.

Therefore, the object of the present invention is to modify the surface layer of a heat-resistant silicon nitride sintered material such as silicon nitride or sialon to reduce the erosion due to the flux and to prevent the adhesion of metal such as aluminum or slag. Another object of the present invention is to provide a heat-resistant silicon nitride-based sintered body that is unlikely to occur and is suitable for a member for molten metal.

[0006]

DISCLOSURE OF THE INVENTION The inventors of the present invention have searched for a material in which molten aluminum or slag does not adhere in an aluminum molten metal having a flux interposed therebetween, and as a result, the aluminum adhesion of a material containing magnesium oxide as a main component has been found. I found that the sex was extremely small. That is, the heat-resistant silicon nitride-based sintered body according to the present invention has a heat resistance of 60% by weight or more of silicon nitride particles or sialon particles and a grain boundary phase composed of Si, Al, Y, O, N and unavoidable impurities. It is a sintered body, and has a thickness of 0. 1 which is adhered by a mixed layer of a grain boundary phase component and magnesium oxide formed on the surface of a base sintered body.
It is characterized in that a surface layer containing magnesium oxide of 1 μm or more and less than 10 μm as a main component is formed.

The reason why magnesium oxide is the main component is as follows. When the silicon nitride sintered body is immersed in a molten metal such as aluminum in an oxidizing atmosphere, SiO ₂ which is an acidic oxide is formed on the surface of the silicon nitride sintered body. By the acid-base reaction between the SiO ₂ and basic oxides such as NaO and NaAlO ₂ produced from the metal-dissolving flux, deposits containing aluminum as a main component are formed.

Therefore, if a layer of magnesium oxide, which is a basic oxide, is formed on the surface of the silicon nitride sintered body,
It is possible to prevent the reaction between the molten metal and the silicon nitride sintered body via the flux.

Most of the surface layer containing magnesium oxide as a main component is made of MgO, and as another component, an oxide of an element contained in the underlying silicon nitride sintered body, for example, SiO.
_A small amount of ₂ , Al ₂ O ₃ , Y ₂ O _3, etc. is mixed. And
A thickness of at least 0.1 μm is required to prevent oxidation of the underlying silicon nitride sintered body. However, if the thickness is 10 μm or more, cracks or peeling may occur due to the difference in thermal expansion coefficient with the underlying silicon nitride sintered body.

The underlying silicon nitride sintered body contains 60% by weight or more of silicon nitride particles or sialon particles, and an element constituting the underlying silicon nitride sintered body, such as Si, Al, Y, O,
The grain boundary phase composed of N or the like needs to be less than 40% by weight. When the content of silicon nitride particles or sialon particles is less than 60% by weight, the heat resistance and thermal shock resistance originally possessed by the silicon nitride sintered material are deteriorated, and it becomes unusable as a member for molten metal such as aluminum.

The surface layer containing magnesium oxide as a main component and the underlying silicon nitride sintered material are firmly bonded to each other. This is because a layer in which the grain boundary phase component and magnesium oxide are mixed is formed on the surface of the underlying silicon nitride sintered body by heat treatment (described later). As described above, since it is not merely mechanical adhesion, it does not easily peel off even when used as a member for molten metal, and it can be sufficiently used.

Further, in order to enhance the reaction inhibiting effect between the molten metal and the silicon nitride sintered body, boron nitride (BN) is mainly used as magnesium oxide for the purpose of reducing the wettability between the molten metal and the silicon nitride sintered body. It can also be spray-coated on the component surface layer.

Although aluminum has been described above as an example,
In molten metals of other metals and alloys, similar reactions occur between the metal oxides and SiO ₂ formed on the surface of the silicon nitride sintered body. However, as in the case of aluminum, this reaction can be prevented by the surface layer containing magnesium oxide as the main component.

Next, in the method for producing a heat-resistant silicon nitride sintered body of the present invention, a magnesium compound slurry or solution is applied to the surface of the silicon nitride sintered body, dried and then heated in an oxidizing atmosphere. As a result, a surface layer containing magnesium oxide as a main component is formed.

As a method for forming a surface layer containing magnesium oxide as a main component, there are methods such as physical vapor deposition (PVD) and chemical vapor deposition (CVD) for forming a thin film form from a vapor phase, but an expensive apparatus is used. Since it is necessary, it is disadvantageous in terms of cost. Therefore, in the present invention, a method of heating after coating is adopted. Here, the magnesium compound slurry or solution is, for example, a slurry in which magnesium oxide is dispersed in alcohol, an alcohol solution of magnesium nitrate, an alcohol solution of magnesium chloride, an alcohol solution of magnesium alkoxide, or the like. The concentration of the magnesium compound in these solutions is about 5 to 30% by weight.

The heating temperature is 800 ° C to 1400 ° C.
If the temperature is lower than 800 ° C., the strength for the applied magnesium compound to firmly adhere to the underlying silicon nitride sintered body is not secured. When the temperature exceeds 1400 ° C, the oxidation reaction on the surface of the underlying silicon nitride sintered body becomes intense, and the surface composition becomes Si.
O ₂ becomes the main component, and the acid-base reaction between this acidic oxide SiO ₂ and the flux causes vigorous aluminum adhesion.

As another method for producing the heat-resistant silicon nitride-based sintered body of the present invention, a slurry or solution of a magnesium compound is applied to the surface of a silicon nitride-based molded body, and after drying, a silicon nitride-based molded body is obtained. By sintering, a surface layer containing magnesium oxide as a main component may be formed simultaneously with sintering.

[0018]

The heat-resistant silicon nitride-based sintered body of the present invention has a layer containing magnesium oxide, which is a basic oxide, as a main component on the surface thereof. Almost no reaction with basic oxides such as NaAlO ₂ . For this reason, even when immersed in a flux-treated metal melt such as aluminum for a long time, erosion and the formation of deposits can be reduced.

[0019]

EXAMPLES Example 1 Si ₃ N ₄ powder 87% by weight, Al ₂ O ₃ powder 5% by weight, 21
3% by weight of R solid solution powder and 5% by weight of Y ₂ O ₃ powder are mixed,
After molding at a pressure of 1000 kgf / cm ² , a molded product having a diameter of 12 mm and a length of 122 mm was obtained by processing. The obtained compact was put in a silicon nitride crucible and sintered at 1750 ° C. for 5 hours in a N ₂ gas stream at atmospheric pressure using a carbon heater to obtain a sialon sintered body having a density of 3.24 g / cm ^3. It was A liquid mixture of 20% by weight magnesium oxide powder and alcohol was applied to the surface of this sintered body with a brush to a thickness of about 50 μm, dried and then heat-treated. The heat treatment conditions were heating to 980 ° C. at a heating rate of 200 ° C. per hour, holding for 1 hour there, and then gradually cooling.

The unreacted magnesium oxide powder on the surface of the obtained sintered body was mechanically removed, and the thickness was 2 μm containing magnesium oxide firmly fixed to the sialon sintered body as a main component.
A surface layer of was obtained. About this sialon sintered body,
1 in 800 ° C molten aluminum fluxed
A test of soaking for 0 hour was conducted. After the test, the weight and area ratio of aluminum and slag attached to the sialon sintered body were measured. The results are shown in Table 1.

[0021]

[Table 1]

Example 2 A commercially available boron nitride spray (a lubricant release agent L-BN manufactured by Showa Denko) was applied to the surface of the sialon sintered body having the surface layer containing magnesium oxide as the main component obtained in Example 1. A coated product was prepared. About this sialon sintered body,
The immersion test was performed in the same manner as in Example 1, and Table 1 shows the weight and area ratio measurement results of aluminum and slag attached to the sialon sintered body.

Example 3 90% by weight of Si ₃ N ₄ powder, ₄ % by weight of Al ₂ O ₃ powder, Y ₂
After mixing 6% by weight of O ₃ powder, 1000 kgf / cm ²
After molding under pressure, the diameter is 12mm and the length is 1 by processing.
A 22 mm compact was obtained. The obtained molded body was placed in a silicon nitride crucible and a carbon heater was used to perform normal pressure N ₂
Sintered in a gas stream at 1750 ° C. for 5 hours to give a density of 3.20.
A g / cm ³ silicon nitride sintered body was obtained. An alcohol solution of 20% by weight magnesium nitrate was applied to the surface of this sintered body, dried and then heat-treated. The heat treatment conditions are the same as in Example 1. The thickness of the magnesium oxide surface layer obtained by this treatment was 1.5 μm. The immersion test was performed in the same manner as in Example 1, and Table 1 shows the weight and area ratio measurement results of aluminum and slag attached to the silicon nitride sintered body.

Example 4 A commercially available boron nitride spray (a lubricant release agent L-BN manufactured by Showa Denko) was applied to the surface of the silicon nitride sintered body having the surface layer containing magnesium oxide as the main component obtained in Example 3. Was applied. The immersion test was performed in the same manner as in Example 1, and Table 1 shows the weight and area ratio measurement results of aluminum and slag attached to the silicon nitride sintered body.

Comparative Examples 1, 2 In Examples 1 and 3, the ceramics sintered bodies were not subjected to the magnesium oxide surface layer treatment, the same immersion test as in Example 1 was carried out, and aluminum adhered to the silicon nitride sintered bodies was tested. In addition, Table 1 shows the weight and area ratio measurement results of Noro.

From Table 1, it can be seen that the surface layer containing magnesium oxide as the main component according to the present invention significantly reduces the adhesion of aluminum to the silicon nitride sintered body. Further, it can be seen that spraying boron nitride onto the surface layer containing magnesium oxide as the main component further reduces and improves aluminum adhesion.

Example 5 The same treatment as in Example 1 was carried out by applying the same magnesium oxide alcohol slurry as in Example 1 to the sialon sintered body shown in Example 1, followed by drying, and then heating at 1000 ° C. for 1 hour. It was As a result, a surface layer containing magnesium oxide as a main component was formed in a thickness of 8 μm on the surface of the sialon sintered body. Regarding the sialon sintered body having this surface layer, as in Example 1, 1
As a result of a test of soaking for 0 hour, the amount of aluminum adhered was 3.5 mg / cm ² , and it was found that the amount of adhered aluminum was extremely small.

Comparative Example 3 In the same manner as in Example 5, the application of the magnesium oxide alcohol slurry was repeated 7 times and then heated at 1000 ° C. for 1 hour. As a result, a surface layer containing magnesium oxide as a main component was formed in a thickness of 13 μm on the surface of the sialon sintered body. However, since the interface between the surface layer and the sialon sintered body was cracked, the surface layer was partly peeled off, and the underlying sialon sintered body was exposed. As a result of performing a test of immersing the sialon sintered body in this state in the molten aluminum as in Example 1, the amount of deposited aluminum was 25.4.
It was mg / cm ² , and it was found that there was remarkable adhesion.

Example 6 The sialon compact shown in Example 1 was spray-coated with an alcohol solution of 20% by weight magnesium nitrate, dried, and then sintered at 1700 ° C. for 10 hours. As a result, a surface layer containing magnesium oxide as a main component was formed. As with Example 1, the result of the adhesion test was 4.9 mg.
/ Cm ² was attached, which was almost the same as in Example 1.

[0027]

EFFECTS OF THE INVENTION By forming the surface layer containing magnesium oxide as a main component of the present invention, the high temperature characteristics such as high temperature strength and thermal shock resistance inherent to a silicon nitride sintered body such as silicon nitride or sialon are impaired. Without this, it is possible to reduce the adhesion and slag adhesion due to the erosion of the molten metal such as aluminum.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsuhiko Furushiro 5200 Mikashiri, Kumagaya-shi, Saitama, Hitachi, Ltd. Magnetic Materials Research Laboratory (56) Reference JP-A-3-106558 (JP, A) JP-A-58- 208182 (JP, A) JP-B 4-44628 (JP, B2) (58) Fields investigated (Int.Cl. ⁷ , DB name) C04B 41/80-41/91

Claims (4)

(57) [Claims] 1. A heat resistant sintered body comprising silicon nitride particles or sialon particles in an amount of 60% by weight or more and a grain boundary phase composed of Si, Al, Y, O, N and inevitable impurities. Heat resistance characterized by forming a surface layer mainly composed of magnesium oxide having a thickness of 0.1 μm or more and less than 10 μm, which is formed by adhering with a mixed layer of a grain boundary phase component formed on the surface of the body and magnesium oxide Silicon nitride sintered material. 2. The heat-resistant silicon nitride sintered material according to claim 1, which is used for a member for molten metal. 3. A heat-resistant sintered body comprising 60% by weight or more of silicon nitride particles or sialon particles and a grain boundary phase composed of Si, Al, Y, O, N and inevitable impurities. After applying magnesium compound on the surface of and drying,
By heat treatment at a temperature of 800 to 1400 ° C,
A method for producing a heat-resistant silicon nitride-based sintered body, which comprises forming a surface layer containing magnesium oxide as a main component. 4. A heat resisting method, characterized in that a magnesium compound is applied to the surface of a silicon nitride molded body, and after drying, a surface layer containing magnesium oxide as a main component is formed at the same time when the silicon nitride molded body is sintered. For manufacturing a crystalline silicon nitride sintered body.