JP2687633B2 - 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 having excellent heat resistance is formed by adding sintering aids such as yttria, spinel and yttria, yttria and alumina to silicon nitride powder, and then forming the obtained molded body into a hot body. It is manufactured by sintering in a nitrogen atmosphere under press or pressure.

For example, in Japanese Unexamined Patent Publication No. 1-188471, mullite and yttrium oxide are added as sintering aids (0.8 to 6% by weight) to silicon nitride powder (94 to 99.2% by weight) and the mixture is molded in a non-oxidizing atmosphere. There is a disclosure of a method for manufacturing a sintered body that is sintered in the above.

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 boundary again forms a liquid phase. As a result, the strength of the sintered body is reduced, so that it is not preferable to use it as a component used at a high temperature. That is, in the silicon nitride-mullite-yttria type sintered body, it has been confirmed by an electron microscope analysis or the like that the grain boundary phase usually forms an amorphous phase. Therefore, when used at 1200 ° C. or higher, the strength cannot be prevented from decreasing unless the amorphous phase is crystallized (high melting point crystal).

As a ceramic gas turbine engine part,
The temperature during use is in the high temperature range of 1300 to 1400 ℃. For this reason, it is necessary to obtain 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 to The sintering aid powders 1 to 6 mixed with 94% by weight of the ytterbium oxide (Yb ₂ O ₃ ) powder and the mullite (3Al ₂ O ₃ .2SiO ₂ ) powder in a ratio of 3/1 to 1/1. It is characterized in that the mixed powder is blended in a weight percentage 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 fine powder of a sintering aid having a specific composition ratio to form a molded body, and then the molded body is sintered. .

As the silicon nitride powder, it is preferable to use a fine powder having an average particle size of 1.0 μm or less in order to enhance the sinterability and form a dense sintered body. The amount of silicon nitride powder is 99-94% by weight.
When the amount of silicon nitride exceeds 99% by weight, the amount of the sintering aid is insufficient, the sinterability does not increase, and the grain boundary crystal phase cannot be generated, which is not preferable. If the amount of silicon nitride is less than 94% by weight, the sintering aid will be too much and the sinterability will be high, and the relative density will be high, but the grain boundary phase will be abundant and the amorphous phase will be abundant in the high temperature range. It is not preferable because the strength at the point of time decreases.

The sintering aid is formed of a mixture of ytterbium oxide (Yb ₂ O ₃ ) powder and mullite powder. The mixing ratio is ytterbium oxide: mullite in the range of 3: 1 to 1: 1. If the amount of ytterbium oxide is large and the ratio thereof exceeds 3: 1, 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 less than 1: 1, the strength at high temperature is further lowered, which is not preferable. Further, it is necessary that the total amount of the sintering aids is in the range of 1 to 6% by weight in order to maintain the strength at high temperature. If the amount of the sintering aid is less than 1% by weight, the grain boundary crystal phase of Yb ₄ Si ₂ N ₂ O ₇ cannot be formed, and if it exceeds 6% by weight, the amount of the grain boundary phase is large. It is not preferable because the amorphous phase is easily generated and the high temperature strength is lowered. 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.
Also, the gas pressure of the atmosphere is set to increase the compactness of the sintered body.
It is preferably carried out in a nitrogen gas of 5 kg f / cm ² or more.

The obtained sintered body was subjected to a static oxidation test (1400 ° C, in air,
After 300 hours, the increase in oxidation was small and the oxidation resistance was improved compared to the conventional silicon nitride sintered body. The reason for this has not been fully clarified, but Yb ₄ Si ₂ N ₂ O _7, which is the crystalline phase of the grain boundary, is generated, and Si ₃ N _{4 in the} amorphous boundary is more difficult to be oxidized. Guessed.

[Operation] In the method for producing a silicon nitride sintered body of the present invention, silicon nitride is sintered using a mixed powder obtained by mixing ytterbium oxide and mullite at 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, so that the temperature range is high. The decrease in strength of the sintered body is small.

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] Specific examples will be described below.

Molding step, the average particle size of 0.5μm high purity silicon nitride powder of an average particle diameter as a sintering aid is 0.2μm ytterbium oxide (Yb ₂ O ₃₎ and mullite _{(3Al 2 O 3 · 2SiO 2} ) The mixed powder was weighed in the ratio shown in the table, ethanol was further added, and the mixture was mixed 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 step, the gas pressure of the above molded body under a nitrogen atmosphere is maintained.
Sintering was carried out at 1750 to 1850 ° C at 10 kg f / 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, Nos. 1 to 9 of the examples have a density of 98.0% TD or more, a bending strength of 800 (MPa) or more at room temperature, 690 to 790 (MPa) at 1200 ° C, and 605 to 670 at 1400 ° C. (MPa), the degree of decrease is extremely small. In particular, No. 5 is a sintered body whose strength is not significantly reduced by temperature.

In Comparative Examples Nos. 1 and 2, since the amount of the sintering aid was large, the strength at 1400 ° C. was significantly reduced to 450 and 300 (MPa). However, the relative densities are 99.2 and 99.5%, and the density is sufficient even if the sintering temperature is 1750 ° C. Has the property. No. 3 is a case where the amount of mullite is small, and the relative density is 97.0% even if the sintering temperature is high, and it is not densified. And the decrease in strength at 1200 ° C and 1400 ° C is
Although it is less than in the case of Nos. 1 and 2, the value is lower than that of the example. No. 4 is the reverse of No. 3 and has a lot of mullite. The strength at room temperature is higher than that of No. 3, but it is lower than that of No. 3 at 1200 ℃ and 1400 ℃. Nos. 5 and 6 are the cases where the conventional spinel and yttria were used as the sintering aids and the amounts and proportions thereof were changed, and the decrease in strength at high temperature was large.

[Effect] According to the method for manufacturing a silicon nitride sintered body of the present invention, the grain boundary in the sintered body is obtained by sintering ytterbium oxide and mullite as a sintering aid with a mixture having a specific composition ratio. To
Since the crystalline phase of Yb ₄ Si ₂ N ₂ O ₇ is formed, it is possible to suppress the strength reduction of the sintered body at high temperature. In addition, the silicon nitride sintered body produced by this method can be used as an engine part because the amount of increase due to oxidation is small.

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 sintering, in which 99 to 94% by weight of the silicon nitride powder is mixed with the ytterbium oxide (Yb ₂ O ₃ ) powder and the mullite (3Al ₂ O ₃ .2SiO ₂ ) powder in a ratio of 3/1 to 1/1. A method for producing a silicon nitride sintered body, characterized in that 1 to 6% by weight of auxiliary powder is blended to obtain the mixed powder.