JP2687632B2 - Method for producing silicon nitride sintered body - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a silicon nitride sintered body that can be used as a heat-resistant structural material for gas turbine parts, diesel engine parts, and the like.

[Prior Art] Conventionally, a silicon nitride sintered body is molded by adding fine powder of a sintering aid such as yttria, spinel and yttria, yttria and alumina to silicon nitride powder, and the resulting compact is hot pressed or hot pressed. It is manufactured by sintering in a nitrogen atmosphere under pressure.

For example, in Japanese Patent Laid-Open No. 57-77072, spinel is added to silicon nitride powder as a sintering aid to be molded, and sintered in a nitrogen atmosphere higher than normal pressure to achieve high density and high strength sintering. There is a disclosure that the body can be obtained.

However, the above-mentioned sintering aid forms a liquid phase at the time of sintering to promote the sintering of silicon nitride, but after the sintering, it forms a glass phase and remains at the grain boundaries in the sintered body. . Therefore, when the sintered body is exposed to a high temperature, the sintering aid component existing in the grain boundaries again forms a liquid phase, which lowers the strength of the sintered body, and therefore is preferably used as a component used at high temperature. Absent.
That is, in the silicon nitride-spinel system sintered body, the grain boundary phase forms an amorphous phase. Therefore, when used at 1200 ° C. or higher, the strength of the sintered body cannot be prevented from decreasing unless the amorphous phase is crystallized.

Gas turbine engine parts made of ceramics have a high operating temperature range of 1300 to 1400 ° C. For this reason
It is necessary to manufacture a silicon nitride sintered body whose strength is not significantly reduced even at 1400 ° C.

[Problems to be Solved by the Invention] The present invention has been made in view of the above circumstances.
It is an object of the present invention to provide a silicon nitride sintered body whose strength is not significantly reduced even in the temperature range of ° C.

[Means for Solving the Problems] A method for manufacturing a silicon nitride sintered body according to the present invention comprises a forming step of forming a formed body from a mixed powder of silicon nitride powder and a sintering aid powder, and In a method for producing a silicon nitride sintered body, which comprises a sintering step of sintering in an oxidizing atmosphere to obtain a sintered body, in the molding step, when the mixed powder is 100% by weight, the silicon nitride powder 99 The ratio of ytterbium oxide (Yb ₂ O ₃ ) powder to spinel (MgAl ₂ O ₄ ) powder is up to 93% by weight.
1 to 7% by weight of the sintering aid powder mixed in a ratio of 5/2 to 2/2
Is mixed to obtain the mixed powder.

The method for producing a silicon nitride sintered body of the present invention is a method in which silicon nitride powder is mixed with a sintering aid fine powder having a specific composition ratio to form a molded body, and then the molded body is sintered.

The silicon nitride powder used for this production has an average particle size of 1.0μ.
It is preferable to use a fine powder of m or less in order to improve the sinterability and form a dense sintered body. The amount of silicon nitride powder is 99
~ 93% by weight. If the amount of silicon nitride exceeds 99% by weight, the amount of the sintering aid is insufficient, the sinterability is not improved, and the grain boundary crystal phase cannot be generated by the sintering aid, which is not preferable. If the amount of silicon nitride is less than 93% by weight, the sintering aid will be too much to increase the sinterability and the relative density will be high, but the grain boundaries of the amorphous phase in the sintered body will be high and the high density. It is not preferable because the strength in the temperature range decreases.

The sintering aid is formed of a mixture of ytterbium oxide (Yb ₂ O ₃ ) powder and spinel (MgAl ₂ O ₄ ) powder. The mixing ratio is ytterbium oxide: spinel in the range of 5: 2 to 2: 2. If the amount of ytterbium oxide is large and the ratio thereof exceeds 5: 2, the relative density decreases and the strength at high temperature decreases, which is not preferable. Further, if the amount of ytterbium oxide is small and the ratio is smaller than 2: 2, the strength at high temperature is further lowered, which is not preferable. Further, it is necessary for the total amount of the mixed powder of the sintering aid to be in the range of 1 to 7% by weight in order to maintain the strength of the sintered body at high temperatures. If the amount of the sintering aid is less than 1% by weight, Yb ₄ Si ₂ N ₂ will be present at the grain boundaries.
The grain boundary crystal phase of O ₇ is not generated, and if it exceeds 7% by weight, the amount of the grain boundary phase is increased and an amorphous phase is generated, so that the bending strength is rather lowered, which is not preferable. This sintering aid is preferably a fine powder having an average particle size of 0.5 μm or less in order to improve sinterability.

The sintering step can be performed under normal conditions, for example, in a non-oxidizing nitrogen atmosphere at 1750 to 1850 ° C. under normal pressure or pressure. If the sintering temperature is less than 1750 ° C., the sintered body will not be sufficiently densified, and if it exceeds 1850 ° C., the grain size will not be refined due to abnormal grain growth and bending strength will decrease, which is not preferable.

The obtained sintered body was subjected to a static oxidation test (1400 ° C, in air,
In 300 hr.), The increase in oxidation is small, and the oxidation resistance is improved as compared with the conventional silicon nitride sintered body. The reason for this has not been fully clarified, but it is presumed that the amorphous phase Si ₃ N ₄ is hard to be oxidized because Yb ₄ Si ₂ N ₂ O _7, which is a crystalline phase of the grain boundary, is generated. To be done.

[Operation] In the method for manufacturing a silicon nitride sintered body of the present invention, silicon nitride is sintered using a mixed powder obtained by mixing ytterbium oxide and spinel in a specific ratio as a sintering aid. At the grain boundaries of the resulting sintered body, Yb ₄ Si ₂ N ₂ O ₇ formed by the sintering aid and silicon nitride is present, so that the liquid phase portion is reduced and the sintering in the high temperature range occurs. The decrease in strength of the knot is reduced.

Further, since the crystalline phase of Yb ₄ Si ₂ N ₂ O ₇ is present at the grain boundaries, the oxidation resistance of silicon nitride is improved.

[Examples] Hereinafter, specific examples will be described.

In the molding process, high-purity silicon nitride powder with an average particle size of 0.5 μm and ytterbium oxide (Yb ₂ O ₃ ) and spinel (MgAl ₂ O ₄ ) powder with an average particle size of 0.2 μm were used as sintering aids. The mixed powders weighed in the ratios shown in the table were added, ethanol was further added, and mixing was performed in a ball mill. The ethanol-removed and dried mixed powder was molded into a square rod shape with a mold. The obtained molded body was subjected to secondary molding to apply a hydrostatic pressure of 3000 kg / cm ² to prepare a molded body (5 × 4 × 50 mm).

In the sintering process, the above-mentioned compact was pressed under a nitrogen atmosphere at a pressure of 9 kgf.
Sintered body was prepared by sintering at 1750 to 1850 ° C./cm ² .

In the table, the composition ratio of the raw material of each sample, the sintering temperature, the relative density of the sintered body, and the four-point bending strength (room temperature, 1200 ° C, 1400 ° C) are shown.

The relative density was measured by the Archimedes method.

4-point bending strength is based on JIS standard bending test pieces (3 x 4 x 36 m
Then, the bending strength at room temperature, 1200 ° C and 1400 ° C was measured according to JIS standard.

As a result, in Nos. 1 to 10 of the examples, the relative density was 98.0% TD.
Above, bending strength is 865 ~ 790 (MPa) at room temperature, 1
690-750 (MPa) at 200 ℃, 585-635 at 1400 ℃
(MPa), the degree of decrease is extremely small. In particular, No. 5 is a sintered body whose strength is not significantly reduced by temperature.

On the other hand, in Comparative Example No. 1, since the amount of spinel is small, the strength is significantly reduced at 1400 ° C, and the relative density is 97.0%, which is poor in compactness. On the contrary, No. 2 has a large amount of spinel, and the strength decrease at 1400 ° C is large. No. 3 is a case where the sintering aid is more than the range of the present invention, and the strength at 1400 ° C is also low. No. 4 is 1200 ° C when conventional spinel and yttria are used as sintering aids.
Has low strength. Nos. 5 and 6 are the cases where the amount and the ratio of the conventional sintering aid are changed, and the strength at high temperature is largely reduced.

[Effect] According to the method for producing a silicon nitride sintered body of the present invention, by using a mixture of ytterbium oxide and spinel in a specific composition ratio as a sintering aid, Yb ₄ Si ₂ N ₂ O ₇ can be added to grain boundaries. Since the crystalline phase is formed, it is possible to suppress the strength reduction of the sintered body at high temperature. In addition, since the amount of increased oxidation is smaller than that of the conventional sintered body and the decrease in strength at high temperature is small, it can be used as an engine part.

Claims (1)

(57) [Claims] 1. A molding step of molding a molded body from a mixed powder of silicon nitride powder and a sintering aid powder, and a sintering step of sintering the molded body in a non-oxidizing atmosphere to form a sintered body. In the method for producing a silicon nitride sintered body consisting of, in the molding step, when the mixed powder is 100% by weight,
The ratio of ytterbium oxide (Yb ₂ O ₃ ) powder to spinel (MgAl ₂ O ₄ ) powder was 5/9 in the silicon nitride powder of 99 to 93% by weight.
A method for producing a silicon nitride sintered body, characterized in that 1 to 7% by weight of the sintering aid powder mixed in a ratio of 2 to 2/2 is blended to obtain the mixed powder.