JP3124862B2 - Method for producing silicon nitride based sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a silicon nitride sintered body having excellent strength characteristics from room temperature to high temperature and particularly used for parts for automobiles and parts for gas turbine engines.

[0002]

2. Description of the Related Art Conventionally, a silicon nitride sintered body has been known to have heat resistance,
Because of its excellent thermal shock resistance and oxidation resistance, it is being applied to engineering ceramics, especially for heat engines such as turbo rotators. This silicon nitride sintered body is generally made of Y ₂ O ₃ , Al ₂ O ₃
Alternatively, by adding a sintering aid such as MgO, high density and high strength characteristics are obtained. As such silicon nitride sintered bodies are required to be further improved in strength and oxidation resistance at high temperatures when their use conditions are further increased. In response to such demands, various improvements have been attempted, for example, by studying sintering aids and improving firing conditions.

Among them, from the viewpoint that a low melting point oxide such as Al ₂ O ₃ which has been conventionally used as a sintering agent deteriorates high-temperature characteristics, the periodicity of Y ₂ O ₃ or the like with respect to silicon nitride is considered. in the sintered body having a composition of table group 3a element (RE) and a simple ternary system consisting of a silicon oxide _{(Si 3 N 4 -SiO 2 -RE} 2 O 3), the grain boundary of the sintered body Si
It has been proposed to increase the melting point and stabilize the grain boundary by precipitating a crystal phase such as a YAM phase and an apatite phase composed of -RE-ON.

[0004]

However, when the grain boundary phase is crystallized, the high-temperature characteristics are improved to some extent as compared with the case where the grain boundaries are amorphous, but the oxidation resistance at high temperatures is poor. Since it is not possible to produce a stable crystalline phase with sufficient mechanical properties, the practical use of such a sintered body is hindered, and further improvements in strength and oxidation resistance are required. Have been.

Further, as a silicon nitride source, only silicon nitride powder is used, and a sintering aid such as Y ₂ O ₃ is used.
In a system using a group III element oxide and silicon oxide, as liquid phase sintering progresses, firing shrinkage occurs, so that shrinkage is large when manufacturing a complicated shape product requiring high dimensional accuracy after firing. It is difficult to control to set dimensions,
Or there is a problem that a polishing process becomes complicated. Therefore, a method of increasing the density of a compact by adding silicon powder as a starting material and nitriding the silicon in a nitrogen atmosphere before firing has been proposed, but an appropriate composition to cope with such a method has been studied. It has not been.

Accordingly, the present invention has excellent resistance to oxidation from low to high temperatures, and has sufficient strength characteristics for use in automobile parts and gas turbine engines from room to high temperatures, particularly from room temperature to 1500. It is an object of the present invention to provide a method for producing a silicon nitride sintered body which is excellent in bending strength up to a high temperature of ° C. and is applied to a part requiring high dimensional accuracy.

[0007]

Means for Solving the Problems In order to have high dimensional accuracy and to enhance the high-temperature characteristics of the sintered body, the present inventors have reduced the amount of shrinkage during firing and reduced the composition of the sintered body. As a result of repeated studies based on the viewpoint that it is important to control the grain boundary crystal phase in the sintered body, Lu ₂ O ₃ , or Lu ₂ O ₃ and Lu ₂ O _3, except for Lu ₂ O ₃ 0.1 to 10 mol% of one or more of the group 3a element oxides, silicon oxide (SiO ₂ ) in such an amount that the molar ratio to the total amount of the group 3a element oxides in the periodic table becomes 2 or more, and the balance Is a molded body made of silicon or silicon and silicon nitride at 800 to 1500
After nitriding the silicon by heat treatment in a nitrogen-containing atmosphere at
Further, after firing in a non-oxidizing atmosphere containing nitrogen, at least RE ₂ Si ₂ O ₇ (RE is a Group 3a element of the periodic table) crystal is precipitated in the grain boundary phase by a cooling process after firing or a heat treatment after firing. By doing so, it was found that the above-mentioned object was achieved.

Hereinafter, the present invention will be described in detail. According to the method for producing a silicon nitride-based sintered body of the present invention, silicon as a starting material,
It contains silicon and silicon nitride as main components, and as an additive component, Lu ₂ O ₃ or Lu ₂ O ₃ and Lu
_It contains one or more oxides of Group 3a elements of the periodic table other than ₂ O ₃ and excess oxygen. Here, the excess oxygen means that when the group 3a element of the periodic table other than Si in the sintered body forms an oxide stoichiometrically from the total amount of oxygen in the sintered body, it binds to the element. Is the amount of oxygen remaining, excluding the oxygen
Most of them are mixed as oxygen contained in the silicon nitride raw material or added silicon oxide, and in the present invention, all of them are considered as existing as SiO ₂ .

In the starting material, as an additive component, Lu ₂ O ₃ , or a compound composed of silicon oxide and one or more of Lu ₂ O ₃ and at least one oxide of a Group 3a element of the periodic table other than Lu ₂ O ₃ , Or silicon nitride and Lu ₂ O ₃ , or L
It is also possible to use a compound powder consisting of silicon oxide and one or more oxides of group 3a elements of the periodic table other than u ₂ O ₃ and Lu ₂ O ₃ .

The silicon powder used has an average particle size of 10 μm or less, particularly 3 μm, in order to facilitate nitriding.
m or less are desirable. The silicon nitride powder is blended to improve the sintering characteristics, but at the same time, the firing deformation increases. Therefore, the addition amount of the silicon nitride powder is desirably 75 mol% or less of the whole. . In this case, the powder used may be either α-type or β-type, and the particle size is suitably from 0.4 to 1.2 μm.

According to the production method of the present invention, it is important to precipitate RE ₂ Si ₂ O ₇ (RE is an element of Group 3a of the periodic table) in the grain boundaries of the final sintered body using the above-mentioned starting materials. It is. That, RE ₂ Si with a melting point of the crystal itself is increased by including Lu in the crystal of RE ₂ Si ₂ O _7
The eutectic temperature of the binary system of ₂ O ₇ and Si ₃ N ₄ is increased, and the high-temperature strength can be increased. In the sintering process, this crystal phase exists as a liquid phase due to the reaction with the silicon nitride particles, and enhances the sinterability. However, if the crystal phase remains as a glass phase in the grain boundary phase as it is after cooling, the high-temperature strength is reduced. It degrades oxidation resistance. Therefore, by precipitating the RE ₂ Si ₂ O ₇ crystal phase by a predetermined cooling process or heat treatment, the high-temperature strength can be increased and the oxidation resistance can be increased.

From this viewpoint, according to the present invention, Lu ₂
O ₃ , or at least one of Group 2a oxides of the periodic table other than Lu ₂ O ₃ and Lu ₂ O ₃ contains 0.1 to 10 mol%, particularly 0.3 to 5 mol%, of silicon oxide. Depending on the compounding, the periodic table 3a containing the amount of excess oxygen in terms of silicon oxide and Lu ₂ O ₃
SiO ₂ / RE with the total amount of group oxides (RE ₂ O ₃ )
It is necessary to prepare and mix such that the molar ratio represented by ₂ O ₃ becomes 2 or more. At this time, the amount of excess oxygen in terms of silicon oxide means that the amount of impurity oxygen contained in the silicon nitride powder is
Silicon oxide powder is added with O ₂ in terms amounts or oxidation silicofluoride which the silicon-containing compound, is in the above ratio is less than 2, Ya RE ₁₀ Si ₂ O ₂₃ N ₄ in the grain boundary phase than RE ₂ Si ₂ O ₇ R
An apatite phase such as E ₁₀ (SiO ₄ ) ₆ N ₂ starts to exist, and when the ratio further decreases, the apatite phase alone or a YAM phase represented by RE ₄ Si ₂ O ₇ N ₂ precipitates, and the acid resistance is reduced. This is because the oxidation characteristics, particularly the oxidation resistance around 900 ° C., are deteriorated.

The content of Lu or one or more elements of Group 3a of the periodic table other than Lu and Lu is 0.1 to 10 mol%, especially 0.3 to 5 mol% in terms of oxide. This is because, if it exceeds 10 mol%, the volume fraction of the grain boundary phase in the sintered body increases, and the high-temperature strength of the sintered body deteriorates. In a composite system of Lu and another element of Group 3a of the periodic table, it is necessary that Lu does not fall below 0.1 mol%, particularly 0.3 mol% in terms of oxide.

The Group 3a element of the periodic table other than Lu used in the present invention includes Y and lanthanoid elements, and Yb and Er are particularly preferable.

According to the present invention, Al ₂ O ₃ , Mg
The presence of low melting point metal oxides such as O and CaO inhibits crystallization of the grain boundary phase and deteriorates high-temperature strength and high-temperature oxidation resistance. It is desirable to control as follows.

On the other hand, the metals of Groups 4a, 5a and 6a of the Periodic Table, and their carbides, nitrides, silicides, SiC, etc., have characteristics even when present in the sintered body of the present invention as dispersed particles or whiskers. Since the influence of deterioration is small, it is of course possible to improve the properties as a composite material by adding an appropriate amount of these based on a known technique.

Next, the mixed powder obtained by mixing the above powders in a predetermined range is molded into a desired shape by a known molding method, for example, press molding, casting molding, extrusion molding, injection molding, cold isostatic pressing and the like. .

Next, the compact is heat-treated at a temperature of 800 to 1500 ° C. in an atmosphere containing nitrogen to nitride silicon contained in the compact to produce silicon nitride. There is no dimensional change during the conversion to silicon nitride, and the density increases because the weight increases. It is desirable to control the ratio of the theoretical density of the compact after the nitriding treatment to 60% or more. In order to nitride all of the silicon contained in this nitriding treatment, it is desirable to gradually nitride while increasing the temperature in multiple steps within the above-mentioned temperature range. There is. The nitriding can be promoted by treating the atmosphere in a nitrogen pressurized atmosphere of 1 to 50 atm. In addition, Lu ₂ added to the system in this nitriding step
RE ₂ Si ₂ O ₇ is produced by the reaction of silicon oxide with O ₃ , or one or more of Lu ₂ O ₃ and at least one oxide of a Group 3a element of the periodic table other than Lu ₂ O ₃ .

Next, the high-density molded body obtained as described above is fired by a known firing method, for example, a hot pressing method, a normal pressure firing method, a nitrogen gas pressurizing firing method, and a heat treatment after firing. Intermediate hydrostatic pressure treatment (HIP treatment) and HIP treatment after glass sealing are performed to obtain a dense sintered body having a theoretical density ratio of 95% or more. If the temperature at this time is too high, the silicon nitride crystal grains grow and the strength is reduced. Therefore, the temperature is desirably 1600 to 2000 ° C, particularly 1650 to 1900 ° C.

Next, RE ₂ Si ₂ O ₇ crystals are precipitated at the grain boundaries by a cooling process in the firing step, a temporary holding in the cooling step, or a heat treatment after the end of the firing step. Specifically, cooling from the firing temperature is performed at 200 ° C./hr.
The heat treatment may be performed as follows: slow cooling, a step of lowering the temperature from the firing temperature, or after the completion of firing, at 1000 to 1600 ° C. in a nitrogen atmosphere.

The silicon nitride-based sintered body thus obtained has a silicon nitride crystal phase as a main phase, and most of it is composed of β-Si ₃ N ₄ . In addition, at least Lu, Si, N, O, and the like are present at the grain boundaries of the main phase. Lu, or one or more elements of Group 3a of the periodic table other than Lu and Lu, and excess oxygen (as silicon oxide) Is considered to exist), and RE ₂ Si ₂
A sintered body in which O ₇ (RE: Group 3a element of the periodic table) is precipitated.

[0022]

The mechanical properties and oxidation resistance of a silicon nitride sintered body are determined by the grain boundary phase.

The oxidation resistance can be improved by crystallizing the grain boundary crystal phase into RE ₂ Si ₂ O ₇ (RE is an element of Group 3a of the periodic table). Among the elements of Group 3a of the periodic table, the smaller the ionic radius, the smaller the diffusion of oxygen and the better the oxidation resistance. At the same time, the melting point of RE ₂ Si ₂ O _{7 and} the melting point of silicon nitride and RE ₂ Si ₂ O ₇ The melting point of a two-component system can be improved. Among the lanthanide compounds, Lu has the smallest ionic radius, and by using the Lu element, excellent oxidation resistance and mechanical properties can be imparted from room temperature to high temperatures, particularly from 1500 ° C.

By using a silicon powder or a silicon powder and a silicon nitride powder as a main component and nitriding the silicon before firing to produce silicon nitride, there is no dimensional change and the molding is performed to increase the weight. Body density can be improved. Therefore, the amount of shrinkage in the firing stage is reduced, and a sintered body with a small amount of deformation and excellent dimensional accuracy can be obtained.

[0025]

EXAMPLES As raw material powder, silicon powder having an average particle diameter of 3 μm and oxygen content of 1.1% by weight, silicon nitride powder (BET specific surface area 8 m ² / g, α rate of 98%, oxygen content of 1.2% by weight, metal Lu ₂ O ₃ powder, Lu
_{Using an} oxide powder of a Group 3a element of the periodic table other than ₂ O ₃ , or a powder of Lu ₂ O ₃ or another RE ₂ Si ₂ O ₇ powder synthesized from an oxide powder of the Group 3a of the periodic table and a silicon oxide powder. (Sample No. 9) to prepare the composition shown in Table 1. And add a binder to the formulation,
The blade was formed by casting. The obtained molded body was placed in nitrogen at 1200 ° C. for 5 hours, and further at 1400 ° C. for 10 hours.
Time nitriding treatment. At this time, it was confirmed from the weight increase rate that all the added silicon was nitrided in all the samples.

Thereafter, the obtained nitride was placed in a silicon carbide sagger, and the atmosphere was controlled to reduce composition fluctuations, and fired at 1850 ° C. for 4 hours in a nitrogen gas stream of 10 atm. . The sintered body obtained in order to sufficiently crystallize the grain boundary phase was heat-treated in a nitrogen stream at 1400 ° C. for 24 hours.

The dimensions of the obtained sintered body were measured,
The shrinkage relative to the dimensions of the molded body was measured. Also, the blade shape of the blade was measured, and the maximum deformation from the design value was measured. Next, the obtained sintered body was cut, processed, and polished into a shape specified by JISR1601, to prepare a sample. Specific gravity measurement based on the Archimedes method for this sample, JI
A four-point bending test at room temperature and 1500 ° C. based on SR1601 was performed.

Further, the sample was placed in air at 900 ° C.
It was exposed to air at 00 ° C. for 100 hours, and the weight change per unit surface area was determined from the weight increase and the surface area of the sample. Further, the crystal phase of the grain boundary phase in the sintered body was identified by X-ray diffraction measurement. The results are shown in Table 2. For the composition of the sintered body, the sample was pulverized, the amount of oxygen was determined by an infrared absorption method after conversion with final carbon dioxide, and the amount of nitrogen was determined by the thermal conductivity according to the 3a of the periodic table containing silicon and Lu. Group elements were measured by ICP emission spectroscopy, and the composition of the sintered body was determined from these.

[0029]

[Table 1]

[0030]

[Table 2]

According to Tables 1 and 2, the samples Nos. 6 to 8 to which other Group 3a element oxides of the periodic table which do not contain Lu ₂ O ₃ alone were added had lower high-temperature strengths, SiO ₂
In Sample No. 14 having a / RE ₂ O ₃ ratio of 1.5, apatite was crystallized at the grain boundaries, and the oxidation resistance was significantly deteriorated.

Sample No. 19, in which Lu ₂ O ₃ or one or more of Lu ₂ O ₃ and at least one oxide of a Group 3a element of the periodic table other than Lu ₂ O ₃ exceeds 10 mol%, is resistant to oxidation. Had deteriorated. Further, the sample No, 1 to which no silicon powder was added as a raw material had a large shrinkage rate of 21% and a deformation amount of 1%.
It was more than 00 μm.

In contrast to these comparative examples, in all of the other samples according to the present invention, precipitation of RE ₂ Si ₂ O ₇ or RE ₂ Si ₂ O ₇ crystal and Si ₂ N ₂ O crystal was recognized at the grain boundary. All showed excellent bending strength and oxidation resistance. In addition, the shrinkage is 15% or less and the deformation is 100
It was very excellent in dimensional accuracy of not more than μm.

[0034]

As described in detail above, according to the present invention,
Shrinkage and deformation during the sintering process can be reduced, and a sintered body with high dimensional accuracy can be manufactured. Lu ₂ O ₃ ,
Alternatively, periodic table 3a other than Lu ₂ O ₃ and Lu ₂ O ₃
RE ₂ Si ₂
O ₇ hot from room by precipitating crystals, particularly 1
It is possible to provide a silicon nitride sintered body that has small strength deterioration up to 500 ° C. and has excellent oxidation resistance. As a result, it can be applied to various structural materials that require high dimensional accuracy used at room temperature and high temperature, including structural materials for heat engines such as ceramic gas turbine parts and ceramic turbo rotors made of silicon nitride sintered body. can do.

Continued on the front page (72) Inventor Kenichi Tajima 1-4-4 Yamashita-cho, Kokubu-shi, Kagoshima Prefecture Inside Kyocera Research Institute (72) Inventor Tomohiro Iwaida 1-4-4 Yamashita-cho, Kokubu-shi, Kagoshima Inside Kyocera Research Institute Examiner Yuichi Fukakusa (56) References JP-A-54-11114 (JP, A) JP-A-2-311364 (JP, A) JP-A-5-163066 (JP, A) JP-A-6-234571 (JP) , A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C04B 35/584

Claims (1)

(57) [Claims] (1) Lu ₂ O ₃ , or Lu ₂ O ₃ and Lu ₂
Oxidation in an amount of 0.1 to 10 mol% of one or more oxides of the Group 3a element of the periodic table other than O ₃ , and a molar ratio of 2 or more to the total amount of the group 3a element oxides of the periodic table. A molded body made of silicon (SiO ₂ ) and the balance silicon or silicon and silicon nitride is heat-treated in a nitrogen-containing atmosphere at 800 to 1500 ° C. to nitride the silicon, and then in a non-oxidizing atmosphere containing nitrogen. After firing, at least RE ₂ Si is added to the grain boundary phase by a cooling process after firing or a heat treatment after firing.
_A method for producing a silicon nitride-based sintered body, wherein ₂ O ₇ (RE is an element of Group 3a of the periodic table) is precipitated.