JPH0224788B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a high-density and high-strength silicon nitride (Si ₃ N ₄ ) sintered body having good heat resistance. Conventionally, high-density silicon nitride sintered bodies with excellent heat resistance have been produced using small amounts of sintering aids, namely MgO,
Add Al ₂ O ₃ , Y ₂ O ₃ , AlN, etc. and heat to 1700-1800℃
It is obtained by hot-pressing at a temperature range of 100 mL or by sintering in a pressurized nitrogen atmosphere. Although these sintering aids promote high density through liquid-phase sintering, they remain in the sintered body as a glass phase even after sintering, causing deterioration of high-temperature strength. The characteristics are insufficient, and improvement in this respect is desired. On the other hand, a ternary oxide mixture in which silicon oxide, aluminum oxide, calcium oxide, etc. were added as liquid phase constituents was selected, and a composite sintering aid containing titanium oxide as a nucleating agent was used to create a sintered body. Some improvements have been made, such as trying to maintain high-temperature strength by crystallizing the glass phase remaining in the silicon nitride during the cooling process, but the drawback is that it is susceptible to thermal shock due to the mismatch in thermal expansion properties between the precipitated crystal phase and silicon nitride. It was hot. In the present invention, bismuth oxide and alkali metal aluminate are used together as sintering aids, and the sintered body is heat-treated at a predetermined temperature, thereby creating anisotropic properties in the residual glass phase in the sintered body. The object of the present invention is to provide a method in which strong layered or fibrous crystals are precipitated, thereby making it possible to obtain a heat-resistant and high-strength sintered body. That is, the manufacturing method of the present invention uses silicon nitride powder and 0.5 to 15% by weight of bismuth oxide as a sintering aid.
and at least one of lithium aluminate, sodium aluminate, and potassium aluminate
0.1 to 5% by weight, a binder is added to this, molded and sintered, and then the sintered body is heat-treated at a temperature of 600 to 1200°C in a non-oxidizing atmosphere. It consists of crystallizing the residual glass phase. The sintering aid used in the present invention contains bismuth oxide and at least one of alkali metal aluminates, namely lithium aluminate, sodium aluminate, and potassium aluminate (hereinafter simply referred to as metal aluminate). Consisting of By using bismuth oxide and metal aluminate together, alkali metal silicates and bismuthates precipitate, and these crystallize in highly anisotropic layered or fibrous forms at grain boundaries, making it difficult to sinter. Improves the body's heat resistance and high strength properties. The blending amount of bismuth oxide is 0.5 to 15% by weight,
Preferably it is 5 to 10% by weight. The amount of metal aluminate is 0.1 to 5% by weight, preferably 0.5 to 2% by weight. Aluminate metal salts can be used alone or in combination of two or more. The silicon nitride powder used in the present invention is commonly used. Further, the binder may be any commonly used binder, but organic binders such as polyvinyl alcohol and stearic acid are particularly preferred. After mixing silicon nitride powder and a sintering aid in a predetermined ratio and adding a binder, a molded body is obtained according to a conventional method. The obtained molded body is heat treated as required, and then sintered according to a conventional method. For example, normal pressure sintering or pressure sintering is performed at 1600 to 1900°C in a nitrogen atmosphere. In the case of pressure sintering, it is carried out under a nitrogen atmosphere of 2 to 500 atmospheres, preferably 200 atmospheres or more.
Moreover, if the compact is encapsulated in an appropriate pressure medium and subjected to so-called hot isostatio pressing, a product with an even higher degree of sintering can be obtained. Following the sintering operation, the sintered body is heat treated at a temperature of 600-1200°C. This heat treatment crystallizes the glass phase in the sintered body into a fibrous or layered form. 600℃
If it is less than 1,200°C, the glass phase will not be crystallized, and if it exceeds 1200°C, a crystal with strong anisotropy will not be obtained, both of which are unfavorable. The non-oxidizing atmosphere is preferably a nitrogen gas atmosphere. The method of the present invention will be explained below based on Examples and Comparative Examples. In addition, in the following examples and comparative examples, % means weight %. Example 1 High purity silicon nitride (α rate 78%) with bismuth oxide 8
% and 0.6% of lithium aluminate were added, and pulverization and mixing were carried out in a wet process for 36 hours in a rubber-lined ball mill using ethyl alcohol as a solvent. Polyvinyl alcohol is added as a binder to the mixed powder after drying, and it is molded under a pressure of 800 kg/cm ² .
A test piece of 35 x 35 x 10 mm was obtained. The green density of the specimen was 54% of the theoretical density. This test piece was slowly heated to 400°C and kept in this state for 5 hours to volatilize and remove organic matter.
Then, at a temperature of 1700 °C under a nitrogen atmosphere of 200 atm 80
The mixture was held for a minute to perform sintering. Further, this sintered body was kept at a temperature of 900° C. for 2 hours to crystallize the residual glass phase. The density of the obtained sintered test piece A was 97% of the theoretical density. In addition, when the above sintered test piece A was crushed and the generated phases were identified by X-ray diffraction, in addition to silicon nitride, petalite: LiAlSi ₄ O ₁₀ and bismuthate: Si ₂ Bi ₂₄ O ₄₀ and Al ₂ Bi ₂₄ O ₃₉ were detected. Further, when specimen A was observed under an electron microscope, it was found that layered or fibrous crystals were precipitated at the grain boundaries. Comparative Example High-purity silicon nitride (α rate 78%) and 5% magnesium oxide, 1% aluminum oxide, and 2% titanium oxide were mixed and sintered in the same manner as in Example 1. The obtained sintered body is designated as test piece B. Test Example The sintered test pieces A and B obtained in Example 1 and Comparative Example were measured for bending strength and subjected to a thermal cycle test, respectively. The heating and cooling cycle test was conducted by heating between 60℃ x 10 minutes and 1200℃ x 10 minutes.
This is done by heating and cooling repeatedly at intervals of 2 minutes. Table 1 shows the results for each sintered test piece A and B.

【table】

[Table] As is clear from Table 1, test piece A (product of the present invention) has almost the same bending strength at room temperature as test piece B (comparative product), and there is little decrease in strength at high temperatures. In addition, in the thermal cycle test, test piece B cracked after 26 cycles, while test piece A
It does not break even after 400 cycles and has excellent durability in cold and hot cycles. Example 2 High purity silicon nitride (α rate 81%) with bismuth oxide 8
%, 0.3% lithium aluminate, and 0.3% potassium aluminate, and molded and sintered under the same conditions as in Example 1, followed by heat treatment at 1000° C. in a nitrogen atmosphere. The obtained test piece also had excellent thermal shock resistance. Furthermore, when the test piece was observed under an electron microscope, a fibrous or layered crystal phase was precipitated at the grain boundaries. Example 3 Using raw materials having the same composition as in Example 1, molding and sintering were carried out in the same manner. Sintering was carried out at 1800° C. under normal pressure in a nitrogen atmosphere. Sintered body under nitrogen atmosphere 900℃
Heat treatment was carried out at ℃ for 2 hours. The obtained test piece had excellent thermal shock resistance. Example 4 A gas turbine engine blade was prototyped using the methods of Examples 1 and 2. Each blade was subjected to a hot spin test (105,000 rpm, every 5 minutes).
After 200 cycles of 500°C and 1100°C, no cracks appeared on any of the blades. As is clear from the above description, in the silicon nitride sintered body obtained by the method of the present invention, the glass phase existing at the grain boundaries in the sintered body has a highly anisotropic shape such as a fibrous or layered shape. Since it is crystallized, the main grain boundaries are strongly connected to each other, and therefore it has excellent thermal shock resistance and high heat-resistant strength.

Claims (1)

[Claims] 1. Mix silicon nitride powder with 0.5 to 15% by weight of bismuth oxide and 0.1 to 5% by weight of at least one of lithium aluminate, sodium aluminate, and potassium aluminate as a sintering aid, and add a binder to this. silicon nitride sintered body, which is formed by molding, sintering, and then heat-treating the sintered body at a temperature of 600 to 1200°C in a non-oxidizing atmosphere to crystallize the residual glass phase in the sintered body. manufacturing method.