JP3207044B2 - Silicon nitride sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has excellent strength characteristics from room temperature to high temperature and also has excellent creep resistance. In particular, the present invention relates to automotive parts such as pistons, cylinders, valves, cam rollers, rocker arms, piston rings, piston pins and the like. , Turbine rotor, turbine blade, nozzle, combustor, scroll, nozzle support, seal ring,
The present invention relates to a silicon nitride sintered body suitably used for a gas turbine engine component such as a spring ring, a diffuser, and a duct.

[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 oxides such as Al ₂ O ₃ and MgO, which have been conventionally used as sintering aids, deteriorate the high-temperature characteristics, the period of silicon nitride, such as Y ₂ O ₃ Simple ternary system (Si ₃ N ₄ —SiO ₂ —RE ₂ O ₃ ) composed of Group 3a element (RE) and silicon oxide
In the sintered body having the composition of
It has been proposed to increase the melting point and stabilize the grain boundaries by precipitating a crystal phase such as a YAM phase and an apatite phase composed of i-RE-ON.

[0004] Specifically, Japanese Patent Application Laid-Open No. 63-1000067 discloses a sintered body containing two or more rare earth elements of Y, Er, Tm, Yb and Lu, and a crystal phase having an apatite structure at a grain boundary. Has been proposed.

[0005]

However, the sintered bodies containing various rare earth elements which have been conventionally proposed are:
It has a certain degree of high temperature strength and oxidation resistance. However, when such a conventional silicon nitride-based sintered body is actually used for a gas turbine or an automobile part, it may be exposed for a long time in a state where a high load is applied in a high temperature state. There is a problem that the sintered body is gradually deformed, that is, so-called creep occurs. The occurrence of such creep deformation has been a major factor impeding practical application even if the strength is high.

Further, as described in Japanese Patent Application Laid-Open No. 63-1000067, when the grain boundary phase is formed of a composite oxide with two or more rare earth elements, the eutectic point is lowered, and the creep resistance at high temperatures is also low. Was something.

Accordingly, the present invention provides a silicon nitride sintered material having sufficient strength characteristics for use in automobile parts and gas turbine engines from room temperature to high temperature, and excellent in creep resistance at 1500 ° C. It is intended to provide the body.

[0008]

Means for Solving the Problems The present inventors have studied the factors that influence the creep resistance and found that the type of rare earth element used and its composition are important for the creep resistance. As a result of repeated examinations based on this, it was found that Lu was used as a rare earth element, that other rare earth elements were not contained as much as possible, and that the ratio with impurity oxygen present in the sintered body was controlled to a specific range. It has been found that creep resistance can be improved while maintaining high-temperature strength, and the present invention has been accomplished.

That is, in the silicon nitride sintered body of the present invention, silicon nitride is at least 70 mol%, Group 3a element of the periodic table is 1 to 10 mol% in terms of oxide, and impurity oxygen is in terms of SiO ₂ . 1 to 20 mol%, wherein the group 3a element of the periodic table is mainly composed of Lu, and the amount of the group 3a element of the periodic table other than Lu is 7 mol% or less of the total amount of the group 3a element of the periodic table. Yes,
Further, the molar ratio of the amount of the impurity oxygen in terms of SiO ₂ to the amount in terms of the oxide of the Group 3a element of the periodic table is less than 2.

Hereinafter, the present invention will be described in detail. The silicon nitride-based sintered body of the present invention is composed mainly of silicon nitride in composition, and further contains an element of Group 3a of the periodic table and impurity oxygen as additional components. Here, the impurity oxygen is the remaining oxygen amount obtained by subtracting the oxygen amount stoichiometrically contained in the Group 3a element compound as an additive from the total oxygen amount in the sintered body, Most of them are mixed as oxygen contained in the silicon nitride raw material or added silicon oxide, and all of them are considered to contain Si—O chemical bonds.

The specific composition of the silicon nitride-based sintered body of the present invention is as follows: silicon nitride is 70 mol% or more, particularly 85 to 98 mol%, and the Group 3a element of the periodic table is 1 to 15 mol in terms of oxide. %, Particularly 1 to 7 mol%, and it is important that impurity oxygen is contained in a ratio of 1 to 20 mol%, particularly 1 to 12 mol% in terms of SiO ₂ . This is because if the amount of silicon nitride is less than 70 mol%, the high temperature strength is not exhibited, and the periodic table 3a
If the amount of the group element is 1 mol% or less, the densification is insufficient.
If it exceeds 0 mol%, high-temperature strength and high-temperature creep resistance deteriorate. Further, when the amount of impurity oxygen is less than 1 mol%, a compound having low high-temperature oxidation resistance such as melilite, which is a compound of silicon nitride and an oxide of an element of Group 3a of the periodic table, is likely to be formed at the grain boundary. No, 20 mol%
This is because, when the ratio exceeds 1, the volume fraction of the grain boundary phase increases, and the high-temperature characteristics deteriorate.

According to the present invention, the Group 3a element of the periodic table is preferably composed of only Lu, and it is most preferable that there is no Group 3a element other than Lu other than Lu. There may be some mixing. Therefore, in the present invention, the Group 3a element of the periodic table other than Lu is preferably 7 mol% or less, particularly 5 mol% or less, more preferably 3 mol% or less based on the total amount of the Group 3a elements of the periodic table. . this is,
By selecting Lu as a Group 3a element of the periodic table, the high-temperature characteristics are dramatically improved as compared with other Group 3a elements of the periodic table, and other Group 3a elements of the periodic table other than Lu are not included. This is because, if present, the eutectic point is lower than in a single system, and as a result, the creep resistance of the sintered body is deteriorated.
In the present invention, the elements of Group 3a of the periodic table other than Lu include Y, Yb, Er, Dy, Ho, Tb, and Tm.
And Sc.

Further, according to the present invention, the ratio (SiO ₂ / RE) of the amount of impurity oxygen in terms of SiO _{2 to the} amount of oxide of the group 3a element in the periodic table (RE ₂ O ₃ ) is referred to.
It is important that ₂ O ₃ ) is less than 2. This is a major factor in determining the composition of the grain boundary, and if this ratio is 2 or more, the creep resistance is deteriorated.

Further, the silicon nitride crystal phase is structurally the main phase, most of which is composed of β-Si ₃ N ₄ and has an average particle size of about 1 to 30 μm. Also,
Lu and oxygen are present at least at the grain boundaries. This grain boundary may be a glass phase, but is crystallized in consideration of high-temperature strength, and is composed of Si ₃ N ₄ —RE ₂ O ₃ —SiO ₂ such as an apatite phase, a YAM phase, and a wollastonite phase.
It is desirable to consist of a crystal phase of

[0015] In the sintered body, other than the above composition, Ti
N, TiC, TaC, TaN, VC, NbC, WC, W
Periodic tables 4a, 5a such as Si ₂ , Mo ₂ C, WO _3, etc.
The group 6a metal compound or SiC has a small effect of deteriorating its properties even when present in the sintered body of the present invention as dispersed particles or whiskers. Naturally, it is also possible to improve the characteristics.

However, metals such as Al, Mg and Ca form oxides having a low melting point, which hinders crystallization of grain boundaries and deteriorates high-temperature strength. In particular, it is desirable to control the amount to 0.5% by weight or less, more preferably 0.1% by weight or less.

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 starting material as a main component, and a purity of 95% is used as an additional component.
The above-mentioned Lu ₂ O ₃ powder or, if necessary, silicon oxide powder is added. Lu ₂ O ₃ and S
It is also possible to use a compound composed of iO ₂ or a compound powder of silicon nitride, Lu ₂ O ₃ and SiO ₂ . The silicon nitride powder to be used may be either α-type or β-type, and has a particle diameter of 0.4 to 1.2 μm and a cation impurity amount of 1% by weight or less, particularly 0.5% by weight or less. The amount of impurity oxygen is suitably 0.5 to 2.0% by weight, and any method such as a direct nitriding method or an imide decomposition method may be used.

According to the present invention, using these powders,
It is prepared so as to satisfy the above-mentioned composition. When controlling SiO ₂ / RE ₂ O ₃ ratio described above in the formulation, Lu considering the oxygen partial or the like to be adsorbed oxygen unavoidably contained in the silicon nitride in the SiO ₂ minutes or manufacturing process
_The amount of an oxide of an element of Group 3a of the periodic table such as ₂ O ₃ is determined. In some cases, it may be prepared by adding SiO ₂ powder.

The mixed powder mixed in the above ratio is formed into a desired shape by a desired molding means, for example, a die press, a casting molding, an extrusion molding, an injection molding, a sludge molding, a cold isostatic pressing and the like. .

Next, the compact can be densified by a known firing method, for example, a hot pressing method, a normal pressure firing method, a nitrogen gas pressure firing method, or the like. Furthermore, densification can be improved by performing treatment by hot isostatic pressing (HIP) after the calcination. As another method, the above-mentioned molded body or sintered body can be glass-sealed and fired by the HIP method. Specific firing conditions are 1500-2 under nitrogen pressure under which silicon nitride does not decompose.
Fire at a temperature of 000 ° C. When the firing temperature is 2000
When the temperature exceeds ℃, the silicon nitride crystal causes grain growth and causes strength deterioration.

Further, by gradually cooling in the cooling step after the above-mentioned calcination, or by subjecting the sintered body to a heat treatment at 1000 to 1800 ° C., the grain boundaries can be crystallized and the characteristics can be further improved.

[0022]

According to the method of the present invention, among the oxides of Group 3a elements of the periodic table, the following components are added to silicon nitride:
Simultaneously satisfying the condition of selecting Lu and adding it with the other Group 3a element of the periodic table without using as much as possible, and controlling the ratio to impurity oxygen (SiO ₂ / RE ₂ O ₃ ) to less than 2. By doing so, it is possible to exhibit high creep resistance properties without deformation of the sintered body even when the sintered body is maintained at a high temperature, particularly 1500 ° C. under a high load state for a long time, while maintaining a high high-temperature strength of the sintered body.

The reason for this is that Lu has the smallest ionic radius among the lightertanoid elements, and its viscosity at high temperatures as a glass or crystal phase with other grain boundary components such as Si ₃ N ₄ and SiO ₂ is greatly increased. When kept at a high temperature, one of the creep mechanisms, Si ₃
This is considered to be because slippage of the N ₄ particles hardly occurs. In addition, it is considered that the crystallization of the grain boundary can further increase the viscosity of the grain boundary, so that the creep resistance is further improved.

[0024]

EXAMPLES Silicon nitride powder (BET specific surface area 10 m ² / g, α rate 95%, oxygen content 1.2% by weight, cation metal impurity content 0.03% by weight, imide decomposition raw material) as raw material powder
If, Lu ₂ O ₃ powder having a purity of 91% to 99% (most of the impurity components made by Yb ₂ O ₃ and Er ₂ O _3.)
And silicon oxide powder having a purity of 99.9% or more,
An appropriate amount of i ₃ N ₄ , RE ₂ O ₃ , and SiO ₂ are prepared, mixed and pulverized with a vibration mill for 72 hours using silicon nitride balls with isopropyl alcohol as a solvent, and the slurry is dried. Rubber press molding was performed at a pressure of 3 t / cm ² .

The compact was fired by various firing methods shown in Tables 1 and 2. In the table, GPS is obtained by firing the molded body at 1700 ° C. at a nitrogen pressure of 1 atm for 10 hours, and further firing at 1900 ° C. at a nitrogen pressure of 10 atm for 5 hours. Hot press is 36.3MPa at 1800 ℃
For 1 hour. HIP embeds the molded body in Vycor glass and melts the Vycor glass.
This was fired for 1 hour at a temperature of 50 ° C. with an Ar gas applied at 2000 atm. In addition, GPS + HIP
Is fired at 1750 ° C. and a nitrogen pressure of 20
Hot isostatic firing at 00 atm for 1 hour.

In order to suppress decomposition during firing, the furnace input during firing was kept constant at 300 g / liter. In the table, the samples other than Sample No. 17 were heat-treated at 1400 ° C. for 24 hours for crystallization.

The theoretical density ratio of the obtained sintered body was calculated from the specific gravity by the Archimedes method, and 3 × 4
Cut and polished into a test piece shape of × 40mm, JIS-R
A four-point bending strength test was performed at room temperature and 1500 ° C. based on 1601 and the average value of the ten test results was shown in Table 1.
And Table 2. Further, the crystal of the grain boundary phase in the sintered body was identified by X-ray diffraction measurement. SiO in the composition of the sintered body
_The amount of ₂ is obtained by pulverizing the sintered body and obtaining the amount of oxygen by chemical analysis, and converting the amount of oxygen of the added rare earth element oxide (RE ₂ O ₃ ) from the oxygen content to SiO ₂ .
Further, the bending test piece was supported in the same manner as in the four-point bending test of JISR1601, and a load of 450 MPa was applied to the test piece at 1500 ° C.
For a maximum of 100 hours, and the time until destruction was measured.

[0028]

[Table 1]

[0029]

[Table 2]

As is clear from the results shown in Tables 1 and 2, Samples Nos. 22, 23 and 24 using Y, Yb, Er and the like as Group 3a elements of the periodic table and Sample N using Lu.
o. When compared with 3, 7 to 10, Lu at high temperature strength
Was higher, showing very good results in creep resistance. Further, in Sample No. 11 in which the amount of Group 3a element of the periodic table other than Lu exceeded 7 mol%, no effect of improving the characteristics was observed. Samples Nos. 15 and 16 having an SiO ₂ / RE ₂ O ₃ ratio of 2 or more had low creep resistance.

Further, Sample No. 1 having a small amount of RE ₂ O ₃
Is insufficient in densification, and in sample No. 6 exceeding 10 mol%, both the bending strength at room temperature and 1500 ° C. and the creep resistance are reduced. Satisfactory characteristics were obtained for each sample of the present invention even when other firing methods were used.

From these results, it was found that the use of Lu and the SiO ₂ / RE ₂ O ₃ ratio were important factors for improving the creep resistance.

[0033]

As described in detail above, according to the present invention,
The creep resistance at high temperatures can be improved together with the high temperature characteristics. Thereby, high reliability can be imparted even when used for automobile parts, gas turbine parts, and the like.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-100067 (JP, A) JP-A-4-6160 (JP, A) JP-A-6-271357 (JP, A) JP-A 8- 48565 (JP, A) JP-A-7-330435 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C04B 35/584-35/596

Claims (1)

(57) [Claims] 1. A method according to claim 1, wherein said silicon nitride is at least 70 mol%,
It contains a group a element in an amount of 1 to 10 mol% in terms of oxide and an impurity oxygen in a proportion of 1 to 20 mol% in terms of SiO ₂ , and the group 3a element of the periodic table is mainly composed of Lu, and a period other than Lu is used. The amount of Group 3a element in the periodic table is 7 mol% or less of the total amount of Group 3a element in the periodic table, and the molar amount of the impurity oxygen in terms of SiO _{2 with} respect to the oxide in terms of the group 3a element in the periodic table. A silicon nitride sintered body having excellent creep resistance, wherein the ratio is less than 2.