JPH057348B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to an improvement in a method for manufacturing a ceramic sintered body containing silicon nitride as a main component, and more specifically, to provide a ceramic sintered body with little deformation and high strength. . [Prior art] Magnesium oxide (hereinafter referred to as magnesia) and aluminum oxide (hereinafter referred to as magnesia) are used as sintering aids.
Conventionally, a (white) structurally heterogeneous layer is formed on the surface of a ceramic sintered body obtained by molding and sintering a ceramic powder containing silicon nitride as the main raw material and adding alumina. Are known. This heterogeneous layer is porous and significantly weakens the strength of the sintered body. Therefore, in the method of molding and sintering ceramic powder having the above composition, in practical use, processing such as scraping off the white porous layer portion is required, or it is difficult to obtain a sintered body of a molded body having a thin wall portion. Since this method tends to cause deformation, it was considered to be inappropriate as a method for manufacturing a sintered body with a particularly complex shape. [Problems to be Solved by the Invention] The present invention has been devised in view of the above circumstances, and has high strength by thinning and reducing the thickness of the white porous portion. Moreover, the present invention provides a method for manufacturing a ceramic sintered body that does not cause deformation even when manufacturing a complex-shaped object. [Means for Solving the Problems] The manufacturing method according to the present invention uses silicon nitride (Si ₃ N ₄ )
A ceramic molded body made by molding ceramic powder into a predetermined shape using magnesium oxide (MgO) and aluminum oxide (Al ₂ O ₃ ) as main raw materials and adding sintering aids as yttrium oxide (Y ₂ O ₃ )
It is characterized by sintering while being exposed to gas. The configuration requirements will be explained below. The silicon nitride powder contained in the raw material powder of the ceramic molded body (silicon nitride powder as the ball raw material, and powder consisting of magnesia and alumina) has a weight ratio of:
90% or more, and magnesia powder and alumina powder are blended and mixed so that each of them is 5% or less. In addition, the average particle size of silicon nitride powder is 2 μm.
(hereinafter simply referred to as μ) or less, and magnesia powder and alumina powder may preferably have a value of 1 μ or less. Note that it is desirable that the silicon nitride powder and the sintering aid powder be mixed as uniformly as possible. For this,
For example, silicon nitride powder and sintering aid powder are thoroughly mixed using a ball mill or other known appropriate method. In the process of manufacturing a ceramic molded body from this raw material powder, the conventional manufacturing process of ceramic molded bodies can be used as is. For example, a ceramic injection molding method can be used to produce a ceramic molded body with a complicated shape. In the ceramic injection molding method, raw material powder is mixed with resin as a binder to form a certain kneaded product, and then
Injection molding is carried out in the same manner as ordinary plastic injection molding, and the molded body is heated to remove the resin.
This is a ceramic molded body. It is also possible to produce a compacted ceramic molded body by putting raw material powder into a mold and compressing it with a press or the like. As for the heating method and the like in the sintering process, the sintering process of the conventional method for manufacturing a ceramic sintered body can be adopted. That is, the above ceramic molded body is heated at 1650 to 1850°C in a non-oxidizing atmosphere such as nitrogen gas.
It is heated for 1 to 4 hours to form a silicon nitride sintered body. A feature of the present invention is that the ceramic molded body is sintered while being exposed to itria gas. By exposing it to itria gas, the thickness of the (white) porous layer on the surface of the ceramic molded body can be reduced.
Formed thin and reduced. [Operations and Effects of the Invention] In the present invention, ceramic powder made of silicon nitride as a main raw material and magnesia and alumina added as sintering aids is molded and then sintered while being exposed to ytria gas. Itria gas densifies and reduces the white porous layer portion formed on the surface when the ceramic molded body is sintered. Therefore, it is possible to prevent a decrease in bending strength and deformation of thin parts due to the presence of the white porous layer,
This is particularly effective when manufacturing complex-shaped objects. Moreover, according to the present invention, since itria gas is exposed to the surface of the ceramic molded body, compared to the case where itria is included as a sintering aid in advance,
A predetermined effect can be achieved more easily. That is, in order to prevent deformation and improve the bending strength of a ceramic sintered body made of silicon nitride as the main raw material and magnesia and alumina added as sintering aids, it is possible to prevent the deformation and improve the bending strength by exposing the surface of the ceramic molded body to itria gas. , can be fully achieved. Furthermore, when firing a ceramic sintered body, the method of disposing itria powder in a firing furnace is simpler than the method of pre-containing itria as a sintering aid. Furthermore, by adjusting the amount of ittria powder, it is easy to control the strength of the surface layer of the ceramic sintered body. Furthermore, by adjusting the amount of Itria powder, it is possible to create a strength gradient from the surface of the ceramic sintered body to the inside. [Example] (First example) (1) Deformation test 96 wt% of silicon nitride powder (trade name SN9S, manufactured by Denki Kagaku Kogyo Co., Ltd.) with an average particle size of 1.4 μ as silicon nitride powder, and as a sintering aid, Average particle size
0.015μ magnesia powder (trade name #100, manufactured by Ube Industries, Ltd.) 1wt% and alumina powder with an average particle size of 0.02μ (Iwatani Chemical Co., Ltd., B type)
3wt% was obtained or adjusted. These powders were mixed in a ball mill for 96 hours using reagent grade ethanol to obtain a raw material powder. 18 wt % of an organic binder for injection molding was added to the raw material powder and kneaded, and injection molded into the shape of an impeller (radial turbine wheel) of a gas turbine engine part to obtain a preform. This preformed body was heated from room temperature to 500°C at a heating rate of 3°C/hour in a nitrogen gas atmosphere to degrease the organic binder and obtain a ceramic molded body. FIG. 1 is a longitudinal sectional view schematically showing a firing furnace (first firing furnace) for firing this ceramic molded body.
The firing furnace has a furnace chamber 3 made of graphite material, a heating device (not shown), and an intake/exhaust system (not shown) that sets the pressure inside the furnace chamber 3 and feeds nitrogen gas. Further, inside the furnace chamber 3, there is a shelf 5 on which a plurality of molded bodies 1 of turbine wheels can be placed;
Ittria powder 6 is placed at the bottom of the furnace chamber 3 at a rate of 0.2 g per volume of the furnace chamber. In this firing furnace, the turbine wheel 1 was fired at 1760° C. for 4 hours at 9.5 atmospheres and in a nitrogen gas atmosphere. (Second Example) In the second example, the shelf 5 of the first example was removed, and the same turbine wheel molded body 1 as in the first example was replaced with 85 wt of boron nitride (BN) (hereinafter referred to as boron nitride) powder. % and 15 wt % of ittria powder, and fired under the same conditions as in the first example. (First Comparative Example) In the first comparative example, itria powder was not placed in the furnace chamber 3 of the firing furnace, and everything else was fired in the same manner as in the first example, and the turbine wheel of the first comparative example was I got it. (Evaluation) Relative bulk densities (expressed as %TD with respect to theoretical density) were measured for the sintered bodies obtained in the first and second examples and the first comparative example. Further, the degree of deformation (degree of deformation of the turbine wheel before and after firing) was measured according to the following criteria. FIG. 2 is a schematic diagram for measuring the degree of deformation of a turbine wheel. FIG. 2a shows a ceramic molded body before firing, and FIG. 2b shows a ceramic sintered body after firing.

【table】

[Table] As shown in FIG. 2, the degree of deformation was measured using the blade 2 of the turbine wheel before firing as the reference line and the angle (°) of the blade 2 hanging down from the reference line. The results are shown in Table 1. As is clear from the table, no measurable deformation was observed in the turbine wheel according to the example. This is thought to be due to the fact that ittria gas is acting effectively and being absorbed from the surface of the molded article. (2) Bending strength test (Third example) In the third example, the same raw material powder as in the first example was used, preformed at a pressure of 200 kg/cm ² , and finally subjected to hydrostatic pressure at 3 ton/cm ² . Form into 5x4x4x5
A rectangular rod-shaped ceramic molded body of mm was molded.
FIG. 3 is a longitudinal sectional view schematically showing a firing furnace (second firing furnace) for firing this ceramic molded body. The difference from the first firing furnace of the first embodiment is as follows.
Only the internal structure of the furnace. A silicon nitride container 9 was placed in this second firing furnace, and 200 square bar-shaped ceramic molded bodies 7 were housed therein, and a lid 10 was placed on the container.
Also, in a container 9 without a lid 10, add 10 pieces of ittria powder.
g and baked under the same conditions as in the first example. (Fourth Example) FIG. 4 is a vertical cross-sectional view schematically showing a firing furnace according to a fourth example. That is, in the fourth embodiment, the silicon nitride container 9 is removed from the third embodiment, and a square bar-shaped ceramic similar to that of the third embodiment is placed in the mixed powder 4 of 75 wt% boron nitride powder and 25 wt% ittria powder. The molded body 7 was buried and fired under the same conditions as in the third example. (Fifth Example) FIG. 5 is a vertical sectional view schematically showing a firing furnace according to a fifth example. In the fifth embodiment,
The angular molded body 7 is stored in a container 8 made of Ittria with a lid made of Ittria. In this embodiment, the angular molded body 7 is heated through a container 8 made of Yttoria, and its surface is exposed to Yttoria gas generated by heating the container and the lid. (Second Comparative Example) In a second comparative example, the square bar-shaped compact 7 was fired in the same manner as in the third example except that no ittria powder was used, and a sintered body according to the second comparative example was obtained. I got it. (Evaluation) Regarding the sintered bodies obtained in the third, fourth, and fifth examples and the second comparative example, the thickness of the white porous layer on the surface of the sintered body after firing and the surface of the sintered body were evaluated. 1mm
Three-point bending strength (according to JIS-R-1601 "Bending strength testing method for fine ceramics") was measured at room temperature before and after grinding. In addition, the measured values are

[Table] Average values for each 15 experiments. The results are shown in Table 2. As is clear from Table 2, the thickness of the porous layer on the surface of the sintered body in Examples is thin, and it can be seen that the bending strength of the sintered body is improved even before grinding. This is considered to be because itria gas is effectively absorbed from the surface of the molded article. Therefore, by exposing the surface of the ceramic molded body to itria gas, high strength can be achieved even if surface grinding after firing is omitted. Furthermore, the method of exposing the surface of the ceramic molded body to yttria gas is simpler than the method of pre-containing yttoria as a sintering aid in the ceramic molded body, and a sufficient effect can be obtained in practical use.

[Brief explanation of the drawing]

FIG. 1 is a vertical sectional view schematically showing a firing furnace for firing a ceramic molded body in the first embodiment. FIG. 2 is a schematic diagram for measuring the degree of deformation of a turbine wheel. FIG. 3 is a vertical sectional view schematically showing a firing furnace for firing a ceramic molded body in the third embodiment. FIG. 4 is a vertical sectional view schematically showing a firing furnace for firing a ceramic molded body in the fourth embodiment. FIG. 5 is a vertical sectional view schematically showing a firing furnace for firing a ceramic molded body in the fifth embodiment.

Claims (1)

[Claims] 1. Ceramic powder made of silicon nitride (Si ₃ N ₄ ) as a main raw material and with magnesium oxide (MgO) and aluminum oxide (Al ₂ O ₃ ) added as sintering aids is molded into a predetermined shape. A method for producing a ceramic sintered body, which comprises sintering a ceramic molded body while exposing it to yttrium oxide (Y ₂ O ₃ ) gas. 2. The ceramic sintered body according to claim 1, wherein the yttrium oxide gas is generated by placing yttrium oxide powder together with the ceramic molded body in a sintering furnace and heating it. Production method. 3. The method for manufacturing a ceramic sintered body according to claim 2, characterized in that boron nitride (BN) powder is mixed into the yttrium oxide powder. 4. The method for manufacturing a ceramic sintered body according to claim 2, characterized in that the ceramic molded body is embedded in the yttrium oxide powder. 5. The method of manufacturing a ceramic sintered body according to claim 1, wherein the ceramic sintered body is a turbine wheel.