JP3124882B2 - Silicon nitride sintered body and method for producing the same - 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 a high temperature and particularly used for parts for automobiles and parts for gas turbine engines.

[0002]

2. Description of the Related Art Conventionally, silicon nitride sintered bodies have been applied to engineering ceramics, especially for heat engines such as tarbor heaters because of their excellent heat resistance, thermal shock resistance and oxidation resistance. ing. This silicon nitride-based sintered body is generally made of Y ₂ O ₃ , A
By adding a sintering aid such as l ₂ O ₃ or MgO, high density and high strength characteristics are obtained. When such a silicon nitride sintered body is used under higher temperature conditions, further improvement in strength and oxidation resistance at high temperatures is required. 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 low melting point oxides such as Al ₂ O _3, which have been conventionally used as a sintering aid, deteriorate the high-temperature characteristics, the periodicity of Y ₂ O ₃ or the like with respect to silicon nitride is considered. Simple ternary system (Si ₃ N ₄ —SiO ₂ —RE) composed of Group 3a element (RE) and silicon oxide
In the sintered body having a composition of ₂ O _3), YAM phase consisting Si-RE-O-N-based composition in the grain boundary of the sintered body,
Apatite phase, silicon oxynitride (Si ₂
It has been proposed to increase the melting point of a grain boundary or to achieve oxidation resistance by precipitating a crystal phase such as an N ₂ O) phase and a disilicate (RE ₂ Si ₂ O ₇ ) phase.

[0004]

Although crystallization of grain boundaries is considered to be effective in improving high-temperature characteristics, silicon nitride and sintering aid are contained in the grain boundary phase. The presence of metal oxides that are inevitably mixed in other than the components makes it impossible to completely crystallize the grain boundaries, and conversely, oxides that did not contribute to crystallization form low-melting-point substances and have high-temperature properties. Was sometimes deteriorated.

Therefore, it is conceivable to use a high-purity raw material having a small amount of impurities as a raw material to be used. However, such a high-purity raw material has poor moldability, and is very expensive, and the production cost is high. There is a problem.

[0006] In addition, there is a sialon sintered body or the like as a sintered body in which the grain boundary phase component is dissolved in silicon nitride crystal to reduce the amount of the grain boundary phase. Such a sintered body has a fracture toughness. However, at present, there is no satisfactory product in terms of characteristics.

[0007]

Means for Solving the Problems The inventors of the present invention have focused on the behavior of the impurities and have repeatedly studied them. As a result, the grains are crystallized by firing under predetermined conditions, and at the same time, the impurities are removed from the grain boundaries. The present inventors have found that a solid solution can be formed in the crystal phase, thereby reducing the influence of impurities on the properties of the sintered body, and exhibiting the original effect of crystallization of the grain boundaries, and have reached the present invention.

That is, the silicon nitride sintered body of the present invention has at least one selected from apatite, YAM, and wollastonite having silicon nitride as a main crystal phase and containing a Group 3a element of the periodic table at its grain boundary. A sintered body containing 10 to 10,000 ppm of Al, Fe, Ca, and Mg cation impurities in the sintered body, and containing the cation impurity in the grain boundary crystal phase. It is characterized in that a part or the whole is in solid solution, and further, as a method for producing such a sintered body, mainly silicon nitride,
It contains a Group 3a element oxide (RE ₂ O ₃ ) and silicon oxide (SiO ₂ ) in the periodic table and has a molar ratio represented by SiO ₂ / RE ₂ O ₃ of 2 or less.
10 to 100 cationic impurities of Fe, Ca and Mg
A molded body containing 00 ppm was placed in an atmosphere containing nitrogen for 170 minutes.
After firing for 3 hours or more at a temperature of 0 to 2000 ° C., the temperature is gradually cooled from the firing temperature to 800 ° C. at a rate of 15 ° C./hr or less.

Hereinafter, the present invention will be described in detail. The silicon nitride sintered body of the present invention is mainly composed of silicon nitride in composition, and contains, as additional components, a Group 3a element of the periodic table and excess oxygen. Here, the excess oxygen means that when a 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. This is the amount of oxygen remaining except for the oxygen contained therein, most of which is mixed as oxygen contained in the silicon nitride raw material or added silicon oxide. In the present invention, it is considered that all are present as SiO ₂ .

The silicon nitride-based sintered body of the present invention is structurally mainly composed of a silicon nitride crystal phase, most of which is composed of β-Si ₃ N ₄ and has an average grain size of about 1 to 10 μm. Exists in diameter. In addition, the grain boundary has
At least a group a element and excess oxygen (which is considered to exist as silicon oxide) are present, but Al, Fe, Ca, Mg as unavoidable impurities are present in the grain boundaries.
And the like, and these cationic impurities may be 10 to 10,000 pP depending on the raw material used.
pm.

A major feature of the present invention is that the cationic impurities are present in the crystal phase existing in the grain boundary phase. Thereby, the content of Al, Fe, Ca, Mg, etc. in the glass in the grain boundary phase is reduced, and the melting point of the glass is increased, and the high-temperature characteristics of the sintered body are improved.

According to the study of the present inventors, the crystal phases capable of forming a solid solution of cationic impurities as described above are limited to apatite, YAM, and wollastonite. Oxynitride (Si ₂ N
₂ O) phase, the disilicate (RE ₂ Si ₂ O ₇₎ phase was confirmed that no solid solution. Therefore, it is necessary that at least one crystal phase selected from apatite, YAM, and wollastonite exists in the grain boundary phase.

In order to precipitate a crystal phase such as apatite, YAM, wollastonite or the like at the grain boundaries, an oxide of an element of Group 3a of the Periodic Table in the amount of excess oxygen in the sintered body in terms of silicon oxide (SiO ₂ ). (RE ₂ O ₃ ) SiO ₂ /
The molar ratio represented by RE ₂ O ₃ is 2 or less, particularly 1.0 to
It is necessary to control the composition to 2.0, and if this molar ratio exceeds ₂ , Si ₂ N ₂ O or RE ₂ Si ₂ O ₇
And the like, and the high-temperature characteristics are not improved.

However, if the amount of cationic impurities in the sintered body exceeds 10,000 ppm, the solid solution of the impurity component in the grain boundary crystal phase becomes saturated, and the incorporation into the grain boundary glass phase becomes remarkable. The effect of the present invention cannot be expected.
Therefore, in the present invention, the amount of these cationic impurities is 10 to
It is desirable that the content be 10,000 ppm, particularly 100 to 6000 ppm or less.

The periodic table 3a used in the present invention
Examples of group elements include Y and lanthanoid elements, and among them, Yb, Er, Dy, and Lu are particularly desirable, and among these, Lu is most desirable. It is desirable to control these amounts in the range of 3 to 10 mol%, particularly 3 to 7 mol%, and if it is less than 3 mol%, it is difficult to densify, and if it exceeds 10 mol%, the absolute amount of grain boundaries increases. And the high-temperature characteristics are likely to deteriorate.

In the silicon nitride sintered body of the present invention, any component which does not inhibit the above-mentioned behavior at the grain boundary may be used.
It is also possible to add a small amount, for example, periodic table 4a,
It is of course possible to improve the properties as a composite material by adding an appropriate amount of a 5a or 6a group metal or a carbide, nitride, silicide or SiC thereof as dispersed particles or whiskers.

Next, a method for producing the silicon nitride sintered body of the present invention will be described. According to the present invention, a silicon nitride powder is used as a main component as a starting material, and an oxide of a Group 3a element of the periodic table, optionally a silicon oxide powder is added as an additional component. As an addition form, a compound composed of an oxide of a Group 3a element of the periodic table and silicon oxide, or a compound powder of silicon nitride, an oxide of an element of the Group 3a of the periodic table and silicon oxide can be used.

The silicon nitride powder used may be either α-type or β-type, and has a particle diameter of 0.4
１．２1.2 μm is appropriate.

According to the present invention, using these powders,
SiO ₂ / RE ₂ O between Group 3a element oxide (RE ₂ O ₃ ) of the periodic table and excess oxygen in terms of silicon oxide (SiO ₂ )
_It is prepared so that the molar ratio with ₃ is 2 or less. At this time, it is appropriate that the silicon nitride powder is 70 to 97 mol%, and the oxide of a group 3a element of the periodic table (RE ₂ O ₃ ) is 3 to 10 mol%. Here excess oxygen is the total amount of SiO ₂ powder impurities oxygen was added to the amount SiO ₂ in terms contained in the silicon nitride powder.

If the amount of the Group 3a element oxide in the periodic table is less than 3 mol%, the sinterability tends to decrease, and if it exceeds 10 mol%, the amount of the grain boundary component tends to increase and the high-temperature strength tends to decrease. . On the other hand, if the molar ratio exceeds 2, the formation of the crystalline phases of apatite, YAM and wollastonite cannot be expected as described above.

The mixed powder mixed in the above ratio is formed into an arbitrary shape by a desired molding means, for example, a die press, a casting molding, an extrusion molding, an injection molding, a cold isostatic pressing and the like. In the compact at this time, according to the impurities in each raw material and the impurities mixed in the process, 10 to 1
0000 ppm, particularly 100 to 6000 ppm.

Next, the molded product was heated to 1600
It is fired in a non-oxidizing atmosphere at 1950 ° C. Specifically, nitrogen gas is pressurized to 1.5 atm or more according to the firing temperature, and the conditions are such that silicon nitride is not decomposed. Accordingly, as the firing method, normal pressure firing, nitrogen gas pressure firing, hot isostatic pressure firing, and the like can be used. In addition, normal pressure firing, nitrogen gas pressure firing, hot isostatic pressure firing, or molding The body can also be embedded or sealed in glass and hot isostatically fired in a high pressure gas.

In the above-mentioned production method, in order to precipitate a specific crystal phase at the grain boundary and to dissolve the cation impurity in the grain boundary crystal phase as described above, it is necessary to place the cation impurity in an atmosphere containing nitrogen at 1700 to 1700. 2000 ° C, especially 1800-1
After baking at a temperature of 950 ° C. for 3 hours or more, particularly 4 to 12 hours, the temperature is further increased from the baking temperature to 800 ° C. by 15 ° C./m
It is preferable to gradually cool at a rate of 12 ° C./min or less, particularly at 12 ° C./min. Firing time of 3 hours or more, cooling rate of 15 ° C / mi
The reason for limiting to n or less is that if any of these deviates from the above range, the solid solution of impurities into the grain boundary crystal phase becomes insufficient and the effect of the present invention is not exhibited.

[0024]

The metal such as Al, Fe, Ca and Mg is inevitably contained in the silicon nitride raw material and the like, and exists at the grain boundary in the sintered body. When the grain boundary is crystallized, these components usually form a glass phase together with components such as SiO ₂ and group 3a element oxides of the periodic table that do not contribute to crystallization separately from the crystal phase. This glass phase is composed of Al, Fe, Ca and M
The inclusion of g lowers the melting point, which degrades the high-temperature characteristics of the sintered body.

Therefore, according to the present invention, YA is contained in the grain boundaries.
By precipitating crystals of M, apatite and wollastonite and dissolving cationic impurities such as Al, Fe, Ca and Mg in the crystals, Al, Fe,
Since Ca and Mg are fixed and the mixing of these components into the glass phase such as SiO ₂ can be suppressed, it is possible to prevent the melting point of the glass from being lowered.

As a result, the high-temperature characteristics of the sintered body for suppressing the formation of the low-melting glass in the grain boundary phase can be improved, and in particular, the high-temperature strength and the creep resistance can be improved.

[0027]

EXAMPLES Several kinds of silicon nitride powders having different amounts of cationic impurities (BET specific surface area: 7 to 15 m ² /
g, α ratio 94-99%, oxygen amount 0.9-1.2% by weight)
And various types of oxide powders of the Group 3a element of the periodic table and silicon oxide powders, and Si ₃ N ₄ , RE ₂ O ₃ , SiO _2
2 were mixed so as to be as shown in Table 1, and mixed and pulverized with a vibration mill for 72 hours using silicon nitride balls with isopropyl alcohol as a solvent, and after drying the slurry, the diameter was 60 m.
m and a thickness of 10 mm were subjected to rubber press molding under a pressure of 3 t / cm ² . The amount of Al, Fe, Ca and Mg as the amount of cationic impurities was quantitatively analyzed by ICP emission spectrometry for the obtained molded body, and the total amount is shown in Table 1. The sintered body was fired under the conditions shown in Table 1. Then, the obtained sintered body was subjected to X-ray diffraction measurement to obtain S
Crystal phases other than i ₃ N ₄ were detected and are shown in Table 2.

Further, the sintered body was placed in a Teflon pressurized container, and was placed in a 5: 1 (volume ratio) water: sulfuric acid solution at 200 ° C.
Decompose under pressure for 1.5 hours to elute only the amorphous phase (glass phase) in the sintered body.
e, the amounts of Ca and Mg were measured, and the amount subtracted from the previously measured total amount of Al, Fe, Ca and Mg in the compact was determined as the amount of impurities dissolved in the grain boundary crystals, and the amount was expressed as a table. 2 is shown.

As for the mechanical properties, the sintered body is 3 × 4
Cut and polished into a test piece shape of × 40mm, JIS-R
Based on 1601, a four-point bending strength test at room temperature and 1400 ° C. was performed. In addition, as creep resistance properties,
Based on the above four-point bending test, 400 MPa at 1400 ° C
Was held in a state where a load was applied, the time until breakage was measured, and held for a maximum of 10 hours. The results are shown in Table 2.

[0030]

[Table 1]

[0031]

[Table 2]

According to the results of Samples Nos. 1 to 5 in Tables 1 and 2, the solid solution of impurities in the grain boundary crystal phase is promoted by increasing the firing time and gradually cooling after firing. I understand. Accordingly, all of the samples Nos. 2 to 5 of the present invention were excellent in high-temperature strength and improved in creep resistance as compared with the sample No. 1 of the conventional method.

According to the results of Samples Nos. 6 and 7, when the amount of impurities in the compact exceeds 10,000 ppm, even if the solid solution of the impurities into the grain boundary crystal phase progresses, the characteristics are sufficiently improved. Is not obtained and the total amount of cationic impurities is 10,000 ppm
It turns out that it is necessary to:

In Sample No. 21 in which the grain boundary crystal phase was composed of silicon oxynitride or disilicate, no solid solution was recognized in the grain boundary crystal phase, and the grain boundary crystal phase was apatite, YAM, wollastonite. Is important.

In Table 1, all of the products of the present invention in which a large amount of cationic impurities were dissolved in the grain boundary crystal phase were found to have high high-temperature strength and excellent creep resistance.

[0036]

As described in detail above, according to the present invention,
The deterioration of high-temperature characteristics due to cationic impurities such as Al, Ca, Fe, and Mg is reduced, and a sintered body excellent in high-temperature strength and high-temperature creep resistance can be obtained. Also,
According to the present invention, even if a raw material containing a certain amount of cationic impurities is used, the influence of the impurities can be reduced, so that it is not necessary to use a high-purity raw material, so that the production can be performed at low cost.

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-4-243972 (JP, A) JP-A-4-65362 (JP, A) JP-A-2-97465 (JP, A) (58) Field (Int. Cl. ⁷ , DB name) C04B 35/584

Claims (2)

(57) [Claims] 1. A sintered body comprising silicon nitride as a main crystal phase and at least one grain boundary crystal phase selected from apatite, YAM and wollastonite containing a Group 3a element of the periodic table at its grain boundary. In addition, the sintered body contains 10 to 10000 ppm of Al, Fe, Ca and Mg cationic impurities, and some or all of the cationic impurities are dissolved in the grain boundary crystal phase. A silicon nitride based sintered body characterized by the following. 2. An oxide of a Group 3a element of the periodic table (RE ₂ O ₃ ) and a silicon oxide (Si) mainly composed of silicon nitride.
O ₂₎ comprises, together with the molar ratio represented by SiO ₂ / RE ₂ O ₃ is made of the ratio of 2 or less, Al, Fe, a shaped body comprising 10~10000ppm cation impurities of Ca and Mg, nitrogen 1700-2000 in an atmosphere containing
A method for producing a silicon nitride-based sintered body, characterized in that after firing at a temperature of 3 ° C. for 3 hours or more, the temperature is gradually cooled from the firing temperature to 800 ° C. at a rate of 15 ° C./hr or less.