JPH09183676A - Silicon nitride sintered body, manufacturing method thereof, and heat engine parts - Google Patents

Abstract

(57) Abstract: A grain boundary phase could be crystallized into a YAM phase or an apatite phase to enhance mechanical strength at high temperatures, but its long-term oxidation resistance was insufficient. SOLUTION: This is mainly composed of silicon nitride or sialon, contains an oxide of a group 3a element of the periodic table and silicon oxide, and contains SiO ₂
/ RE ₂ O _{3 having} a molar ratio of 2 or less contains a grain boundary crystal such as apatite, YAM, wollastonite or melilite in the grain boundary obtained by firing at 1700 to 2000 ° C in a nitrogen atmosphere. Zn, Na, Pb, Al,
SiO ₂ containing two or more cations of Fe, Ca, B and Mg in total of 20,000 ppm or more in terms of oxide
Apply a paste consisting of glass powder consisting mainly of
Heat-treated in an oxidizing atmosphere at 00 to 1500 ° C. for 0.5
A glass layer mainly composed of SiO ₂ of ˜100 μm is formed and used as a component for a heat engine.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is excellent in corrosion resistance from room temperature to high temperature, and is particularly used for parts for heat engines such as parts for automobiles and parts for gas turbine engines such as scrolls and combustors. The present invention relates to a high quality sintered body and a manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, a silicon nitride sintered body has been known to have heat resistance,
Due to its excellent thermal shock resistance and oxidation resistance, it is being applied to engineering ceramics, especially for heat engines such as turbo-borers. This silicon nitride sintered material is generally used for Y ₂ O ₃ , Al ₂ O _{3 and} silicon nitride.
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 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 cycle of Y ₂ O ₃ or the like with respect to silicon nitride is deteriorated. In a sintered body having a simple ternary system (Si ₃ N ₄ —SiO ₂ —RE ₂ O ₃ ) composition consisting of a Group 3a element (RE) of the table and silicon oxide, the grain boundaries of the sintered body are Si
YAM phase, apatite phase, or silicon oxynitride (Si ₂ N ₂ O) composed of -RE-O-N composition
It has been proposed to increase the melting point of grain boundaries or to improve oxidation resistance by precipitating a crystalline phase such as a phase or a disilicate (RE ₂ Si ₂ O ₇ ) phase.

[0004]

Although it is considered that crystallization of grain boundaries is effective in improving the high temperature characteristics as described above, the crystal phase of the crystal phase crystallized in the grain boundaries is considered to be effective. Oxynitride such as melilite, which has extremely poor oxidation resistance, may be present in some cases, and it is observed that countless cracks are generated on the surface of the sintered body after the oxidation due to the volume expansion caused by the oxidation.

For this reason, it has been proposed to crystallize the crystallizing phase at the grain boundary into a crystal having good oxidation resistance such as a silicon oxynitride phase or a disilicate phase. The composition was difficult to sinter, and it was uneconomical because it required a high sintering temperature.

Therefore, the surface of the sintered body is subjected to heat treatment again after sintering to form a film strong against oxidation on the surface, and the surface of the sintered body is resistant to oxidation by chemical and physical vapor deposition methods. It has been proposed to form an excellent film of
The former has the drawback that the film tends to crystallize after being used for a long period of time and cracks or peels off, while the latter has the problem of being expensive because it requires a special device for film formation, so it is still satisfactory. The current situation is that no has been obtained.

[0007]

Means for Solving the Problems The inventors of the present invention have focused on and investigated a method of forming a film having excellent oxidation resistance on the surface of a sintered body, and as a result, burned a powder having a specific component mixture. It is possible to easily form a strongly adhered film with excellent oxidation resistance on the surface of the sintered body by applying heat treatment again under prescribed conditions after coating on the surface of the sintered body. It was found that the chemical conversion property and the corrosion resistance can be greatly improved, and the present invention has been completed.

That is, the silicon nitride sintered body of the present invention is selected from apatite, YAM, wollastonite, and melilite, which have silicon nitride or sialon as a main crystal phase, and whose grain boundaries contain a Group 3a element of the periodic table. Zn, Na, Pb, Al, on the surface of the sintered body containing at least one kind of grain boundary crystal phase
Si containing a total of 20000 ppm or more of two or more kinds of cations selected from Fe, Ca, B and Mg in terms of oxide.
A glass layer containing O ₂ as a main component is coated, and as a method for producing such a sintered body, silicon nitride or sialon is mainly used, and an oxide of a group 3a group 3a (RE ₂ ) of the periodic table is used. O ₃ ) and silicon oxide (SiO ₂ )
And a SiO ₂ / RE ₂ O ₃ molar ratio represented by a ratio of 2 or less in a nitrogen-containing atmosphere.
Zn, Na, Pb, Al, Fe, Ca, B is formed on the surface of the sintered body obtained by firing at a temperature of 00 to 2000 ° C. for 3 hours or more.
After applying a paste composed of glass powder mainly containing SiO ₂ containing a total of 20000 ppm or more of two or more cations selected from Mg and Mg in terms of oxide, 700 to
Heat treatment is performed in an oxidizing atmosphere at 1500 ° C to remove SiO ₂
It is characterized in that a glass layer mainly composed of is formed.

[0009]

[Operation] Usually, in producing a silicon nitride sintered body,
Although cation impurities such as Al, Fe, Ca, and Mg are inevitably contained in the silicon nitride powder, most of these impurity ions are burnt between the silicon nitride crystal grains in the sintered body. It exists at the grain boundaries in the body. And
When the grain boundaries are crystallized, usually, apart from the crystal phase, these components form a glass phase together with the components such as SiO ₂ and Group 3a element oxide of the periodic table which do not contribute to crystallization. This glass phase has a low melting point because it contains Al, Fe, Ca, and Mg, which deteriorates the high-temperature characteristics of the sintered body.

Therefore, crystals of YAM, apatite, wollastonite are precipitated in the grain boundaries, and cation impurities such as Al, Fe, Ca, and Mg are dissolved in the crystals to form Al, Fe, Since Ca and Mg can be fixed and the mixture of these components into the glass phase such as SiO ₂ can be suppressed, lowering of the melting point of the glass phase can be prevented. As a result, the high temperature characteristics of the sintered body can be improved because the generation of the low melting point glass in the grain boundary phase is suppressed, and especially the high temperature strength and the creep resistance characteristics can be improved.

However, the oxidation resistance of this sintered body largely depends on the volume change due to the oxidation of crystals such as YAM, apatite, and wollastonite existing in the grain boundaries, and the grain boundary phase is generally amorphous. In many cases, it has poorer oxidation characteristics than other sintered bodies, which may cause a problem in long-term reliability in an oxidizing atmosphere.

On the other hand, according to the present invention, an amorphous (glass) layer having a low melting point and composed of a composite oxide is formed on the above-mentioned sintered body which is excellent in high temperature strength and creep resistance. Oxidation resistance can be improved. In particular, Zn, N
a total of 20000 p of two or more kinds of cations selected from a, Pb, Al, Fe, Ca, B and Mg in terms of oxide.
When a paste containing glass powder mainly containing SiO ₂ containing pm or more is applied to the surface of the sintered body and then heat-treated to form a glassy thin layer, it is possible to perform treatment at a relatively low temperature. Since it has a large adhesive force with the silicon nitride sintered body as a base, even if it is used for a long period of time in an actual engine operating environment, a minute fracture such as a crack does not occur.

Therefore, since the oxidation is governed only by the diffusion through the protective film, cracks are generated on the surface of the sintered body after the oxidation due to the rapid progress of the oxidation, and further the cracks are generated through the cracks. The rapid progress of oxidation can be suppressed. Thus, by forming the glass layer, the glass layer functions as an oxidation protection film, and the long-term reliability of the component can be improved.

As a result, the silicon nitride sintered material of the present invention can be used in scrolls, combustors, nozzles, gas ducts,
By applying it as a part exposed to a high temperature oxidizing atmosphere such as various gas turbines such as a shroud, the durability and stability of long-term operation of the part can be realized.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The silicon nitride sintered body of the present invention is composed of silicon nitride or sialon as a main component in terms of composition, and contains an element of Group 3a of the periodic table and excess oxygen as additive components. . Here, excess oxygen means that when a group 3a element of the periodic table other than Si in the sintered body forms a stoichiometric oxide from the total amount of oxygen in the sintered body, it is bonded to the element. The remaining oxygen amount excluding oxygen, most of which is mixed as oxygen contained in the silicon nitride raw material or added silicon oxide.
Consider as existing as O ₂ .

In the present invention, the silicon nitride-based sintered body as a substrate has a silicon nitride crystal phase or a sialon crystal phase as a main phase in terms of structure, and most of them are β-Si ₃
Consists of N ₄ or β-sialon, approximately 1-50 μ
It exists in a particle size of m. In addition, at the grain boundaries,
At least the group a element and the excess oxygen (though believed to exist as silicon oxide) are present, but these elements are crystallized into at least one crystal phase selected from apatite, YAM, wollastonite and melilite. These crystal phases are Al contained in the silicon nitride raw material,
Cationic impurities such as Fe, Ca, Mg, etc. can be dissolved in the crystal, whereby Al, Fe, Ca, M
The content of g in the grain boundary phase in the glass is reduced, the melting point of the glass is increased, and the high temperature characteristics of the sintered body are improved.

In order to precipitate a crystal phase of apatite, YAM, wollastonite, melilite or the like at the grain boundary, the excess oxygen in the sintered body is converted into silicon oxide (SiO ₂ ) in the amount of silicon oxide (SiO ₂ ) in the periodic table 3a It is necessary to control the composition so that the molar ratio represented by SiO ₂ / RE ₂ O ₃ with respect to the oxide (RE ₂ O ₃ ) conversion amount is 2 or less, particularly 0.7 to 2.0. When it exceeds 2, Si ₂ N ₂ O or RE ₂ S is formed in the grain boundary.
Since a crystal phase such as i ₂ O ₇ is precipitated, it has no effect of forming a solid solution with impurities, and the high temperature characteristics are not improved when a raw material containing a large amount of impurities is used.

The periodic table 3a used in the present invention
Examples of the group element 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 the amount of these in the range of 2 to 10 mol%, particularly 3 to 7 mol%, and if it is less than 2 mol%, it becomes difficult to densify, and if it exceeds 10 mol%, the absolute amount of grain boundaries increases. However, the high temperature characteristics are likely to deteriorate. In the silicon nitride sintered body of the present invention, in addition to the above components, a trace amount of a component that does not hinder the above behavior at grain boundaries can be added, for example, a metal of Group 4a, 5a, 6a of the periodic table. Of course, it is also possible to improve the characteristics of the composite material by adding an appropriate amount of these oxides, carbides, nitrides, silicides or SiC as dispersed particles or whiskers.

According to the present invention, the surface of the silicon nitride sintered material having the above-mentioned properties is mainly composed of SiO ₂ and
As components other than O ₂ , elements called low melting point glass components, Zn, Na, Pb, Al, Fe, Ca, B and M
There are two or more kinds selected from g, and preferably three or more kinds. These elements facilitate the vitrification of SiO ₂ on the silicon nitride sintered body, and can lower the heat treatment temperature for forming a film. Further, the presence of these elements in the glass layer prevents crystallization of the coated glass layer when the coated sintered body is used in a high temperature environment (for example, a gas turbine engine operating temperature range). It is possible to improve long-term stability of oxidation resistance. The number of components other than SiO ₂ is set to 2 or more because the effect of suppressing the crystallization of the glass layer is poor and the crystallization easily occurs during long-term use when only one type is used.

The thickness of the coated glass layer is 0.5 to 10
A thickness of 0 μm, preferably 2-60 μm is suitable.
This is because when the thickness is less than 0.5 μm, the coating effect cannot be expected and the oxidation easily proceeds. When the thickness is more than 100 μm, the coefficient of thermal expansion of the silicon nitride sintered body as the base and the coating glass layer is increased. This is because the difference may cause the glass layer to peel easily in an environment accompanied by thermal shock.

Next, a method of manufacturing the silicon nitride sintered body of the present invention will be described. First, in the production of a silicon nitride-based sintered body as a substrate, silicon nitride powder or sialon powder as a starting material is used as a main component, and an oxide of a Group 3a element of the periodic table, and in some cases silicon oxide and aluminum oxide powder are added and mixed. To do. In addition, as the addition form, the third
Compound consisting of Group a element oxide and silicon oxide, Compound consisting of silicon oxide and aluminum oxide, Compound consisting of Group 3a element oxide and aluminum oxide of the periodic table, or Silicon nitride and Group 3a element oxide of the periodic table It is also possible to use a compound powder of silicon oxide and silicon oxide.

The silicon nitride powder or sialon powder used may be either α type or β type,
The particle size is preferably 0.4 to 1.2 μm.

According to the invention, using these powders,
SiO ₂ / RE ₂ O _{3 of} Group 3a element oxide (RE ₂ O ₃ ) of the periodic table and silicon oxide conversion of excess oxygen (SiO ₂ ).
It is prepared so that the molar ratio thereof is 2 or less. At this time, the addition amount of the Group 3a element oxide (RE ₂ O ₃ ) of the periodic table is appropriately 2 to 10 mol%. The excess oxygen here is the total amount of the impurity oxygen contained in the silicon nitride powder converted to SiO ₂ and the added SiO ₂ powder.

When the amount of the Group 3a element oxide of the periodic table is less than 2 mol%, the sinterability tends to decrease, and when it exceeds 10 mol%, the amount of grain boundary components tends to increase and the high temperature strength tends to decrease. . Further, if the above molar ratio exceeds 2, it is not possible to expect formation of apatite, YAM, and wollastonite crystal phases as described above.

The mixed powder mixed in the above proportions is molded into a desired shape by a desired molding means such as a die press, a cast molding, an extrusion molding, an injection molding, a cold isostatic pressing and the like. In the molded body at this time, the cation impurities are
Impurities in each raw material and impurities mixed in the process are combined to be 10 to 10000 ppm, especially 100 to 6000 p.
It is included in the range of pm.

Next, this molded body is filled with nitrogen containing 1600 to 1600.
Baking is performed in a non-oxidizing atmosphere at 1950 ° C. Specifically, the nitrogen gas is pressurized to 1.5 atm or more depending on the firing temperature, and the silicon nitride is not decomposed. Therefore, as the firing method, normal pressure firing, nitrogen gas pressure firing, hot isostatic firing, etc. can be used. In addition, normal pressure firing, nitrogen gas pressure firing and hot isostatic firing or molding It is also possible to embed or seal the body in glass and perform hot isostatic pressing in high pressure gas.

In the above-mentioned manufacturing method, in order to precipitate a specific crystal phase at the grain boundary and to cause the cation impurities to form a solid solution in the grain boundary crystal phase as described above, 1700 to 1700 in an atmosphere containing nitrogen. 2000 ° C, especially 1800-1
Baking at a temperature of 950 ° C. for 3 hours or more, particularly 4 to 12 hours.

On the surface of the sintered body thus obtained, Z
200 in total of two or more kinds of cations selected from n, Na, Pb, Al, Fe, Ca, B and Mg in terms of oxide.
SiO containing more than 00ppm, especially more than 30,000ppm
₂ was added and mixed with glass powder Solvent consisting mainly of a paste prepared is coated and dried the paste surface of the sintered body.

At this time, the above-mentioned glass powder contains a total of 20 oxide powders of two or more kinds selected from Zn, Na, Pb, Al, Fe, Ca, B and Mg with respect to SiO ₂ powder.
After adding and mixing in an amount of 000 ppm or more, 800-
It is obtained by heating and melting at 1500 ° C., quenching, and crushing it.

Thereafter, in a temperature range of 700 to 1500 ° C., preferably 800 to 1350 ° C.,
Heat treatment is performed for about 1 to 4 hours in an oxidizing atmosphere such as air to form a coated glass layer mainly composed of SiO ₂ and having a thickness of 0.5 to 100 μm, preferably ₂ to 60 μm.
When the temperature at the time of heat treatment is lower than 700 ° C., a strong glass layer is not formed and the adhesion to the silicon nitride sintered body substrate is low.
Further, when the heat treatment temperature exceeds 1500 ° C., volatilization of components in the coated glass layer actively occurs, and it becomes extremely difficult to control the composition. The thickness of the coated glass layer can be easily controlled by the coating amount when the powder or paste substance having the composition to be glass is coated on the sintered body substrate.

[0031]

【Example】

Example 1 As a raw material powder, a silicon nitride powder having a cationic impurity amount of 4300 ppm (BET specific surface area 7 to 15 m ² / g, α ratio 9
4 to 99%, oxygen amount 0.9 to 1.2% by weight) and various periodic table group 3a oxide (RE ₂ O ₃ ) powders and silicon oxide (SiO ₂ ) powders are used to produce Si ₃ N ₄ , RE ₂ O
_3, SiO ₂ (SiO impurity oxygen silicon nitride in the raw material ₂
(Including the converted amount) is mixed as shown in Table 1, isopropyl alcohol is used as a solvent in a vibration mill for 72 hours using silicon nitride balls, and the slurry is dried and then dried into a shape having a diameter of 60 mm and a thickness of 10 mm. Rubber press molding was performed at a pressure of 3 ton / cm ² . The obtained molded body was fired under the conditions shown in Table 1.

The obtained sintered body was subjected to X-ray diffraction measurement to identify a crystal phase other than β-Si ₃ N ₄ , that is, a grain boundary crystal phase, and the results are shown in Table 1.

On the other hand, SiO ₂ powder having an average particle size of 2 μm,
ZnO powder, Na ₂ O powder, PbO powder, Al ₂ O ₃ powder, Fe ₂ O ₃ powder, CaCO ₃ powder, BaCO ₃ powder and MgO powder were mixed in the composition shown in Table 2 and then mixed at 1250 ° C. After melting by heating, rapid cooling, and pulverization, glass powder was obtained. Then, α-terpineol was added to this glass powder, and the mixture was kneaded for 1 hour and then passed through a three-roll mill three times to prepare a paste.

Next, the above-mentioned paste was applied to the surface of the previously obtained sintered body, dried and further treated under the heat treatment conditions shown in Table 2 to form a glass layer on the surface of the sintered body.

The glass layer-covered silicon nitride-based sintered body thus obtained was heated to 100 ° C. in an atmospheric furnace maintained at 1000 ° C.
After exposure for a period of time, changes in mechanical properties before and after exposure were evaluated.

The mechanical properties of the sintered body are 3
A test piece shape of × 4 × 40 mm was cut and polished, and a 4-point bending transverse strength test was performed at room temperature and 1400 ° C. according to JIS-R1601 with the surface of the test piece as a tensile surface. Further, as the creep resistance property, based on the above-mentioned four-point bending test, the load was kept at 1400 ° C. under a load of 400 MPa, and the time until the fracture was measured.
Hold for hours. Table 3 shows the results.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

As is clear from the results shown in Tables 1 to 3, even the sample not having the glass layer formed thereon showed a high strength before the exposure test, but the strength largely decreased after the exposure test. However, according to the present invention, the samples with the glass layer formed each show a decrease in strength of 50 MP even after the exposure test.
It is within a and it is understood that the durability is improved. The same result was obtained in the creep test.

However, the sintered body h in which the crystal phase of the grain boundary phase of the sintered body is made of silicon oxynitride or disilicate or the sintered body i made of an amorphous material has a low strength per se, and the glass is The formation of the layers did not change the properties.
In addition, the components other than SiO ₂ in the glass layer are 20,000 p
Sample Nos. 20 and 21 having less than pm did not have the effect of improving the durability, and the characteristics were largely deteriorated after the exposure test. Furthermore, the sample N containing only one component other than SiO ₂ in the glass layer
There was no effect even at o.22, and characteristic deterioration was observed after the exposure test.

Example 2 Sample Nos. 2 and 5 in Table 2 in Example 1 and, for comparison, Sample No. 8 in which no glass layer was formed on the surface.
Was used to prepare a scroll. Then, the scroll was incorporated into a gas turbine engine and operated for 100 hours under the condition that the scroll was exposed to an oxidizing high temperature gas of about 1000 ° C.

After the operation, the scroll was taken out, and the surface thereof was observed. As a result, Sample Nos. 2 and 5 having the glass layer according to the present invention showed no change with the surface exhibiting gloss. However, in sample No. 8 in which no glass layer was formed, the surface was discolored white, and it was confirmed that the surface was deteriorated by oxidation.

[0044]

As described in detail above, according to the present invention, by forming a specific amorphous (glass) layer composed of a complex oxide with a low melting point while being excellent in high temperature strength and creep resistance. The oxidation resistance can also be improved. As a result, it is possible to improve long-term reliability as a heat engine component such as a gas turbine component or an automobile component.

Claims (3)

[Claims] 1. A main crystal phase of silicon nitride or sialon, and at least one kind of grain boundary crystal phase selected from apatite containing a Group 3a element of the periodic table, YAM, wollastonite, and melilite in the grain boundary. Z surface of the sintered body
200 in total of two or more kinds of cations selected from n, Na, Pb, Al, Fe, Ca, B and Mg in terms of oxide.
A silicon nitride-based sintered body characterized by being coated with a glass layer mainly containing SiO ₂ containing at least 00 ppm. 2. A mole mainly composed of silicon nitride or sialon, containing an oxide (RE ₂ O ₃ ) of Group 3a element of the periodic table and silicon oxide (SiO ₂ ), and represented by SiO ₂ / RE ₂ O _3. Zn, Na, Pb, Al, on the surface of the sintered body obtained by firing a molded body having a ratio of 2 or less at a temperature of 1700 to 2000 ° C. for 3 hours or more in an atmosphere containing nitrogen.
Si containing a total of 20000 ppm or more of two or more kinds of cations selected from Fe, Ca, B and Mg in terms of oxide.
After applying a paste made of glass powder mainly containing O ₂ , heat treatment is carried out in an oxidizing atmosphere at 700 to 1500 ° C. to form a glass layer mainly containing SiO ₂ and a silicon nitride fired product. A method for producing a bound body. 3. A heat engine part comprising the silicon nitride sintered body according to claim 1.