JPH0840774A - Silicon nitride sintered product - Google Patents

Abstract

PURPOSE:To obtain a silicon nitride sintered product having high oxidation resistance in a high temperature oxidizing atmosphere, having a high strength from room temperature to a high temperature, and excellent in creep-resistant characteristics. CONSTITUTION:The characteristics of the silicon nitride sintered product comprise having a main crystal phase comprising silicon nitride, at least a disilicate phase represented by a formula: RE2Si2O7 (RE is the group 3a element in the periodic table) on the grain boundary of the main crystal phase, and the multicomponent oxide phase of at least one element selected from the group of Al, Ga, Ti, Hf, Zr, Nb, Ta, V, W, Mo, Cr and Fe and having a melting point of >=1500 deg.C with the group 3a element in the periodic table, and further having a silicon oxynitride phase in the grain boundary.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is excellent in oxidation resistance at high temperatures and creep resistance at high temperatures, and particularly in automobiles such as pistons, cylinders, valves, cam rollers, rocker arms, piston rings, piston pins and the like. Parts, turbine rotors, turbine blades, nozzles, combustors, scrolls, nozzle supports, seal rings,
The present invention relates to a silicon nitride sintered body that is preferably used for gas turbine engine parts such as spring rings, diffusers, and ducts.

[0002]

2. Description of the Related Art Conventionally, silicon nitride sintered bodies have high toughness,
Due to its excellent heat resistance, thermal shock resistance, and oxidation resistance, engineering ceramics, especially turbines
Applications for heat engines such as This silicon nitride sintered material is generally used for Y ₂ O ₃ , A
High-density and high-strength characteristics are obtained by adding a sintering aid such as l ₂ O ₃ or MgO. Further improvement in strength and oxidation resistance at high temperatures is demanded for such silicon nitride sintered bodies when the operating conditions thereof further increase. In order to meet such demands, various improvements have been attempted so far, such as examination of sintering aids and improvement of firing conditions.

Among them, silicon nitride, which has been conventionally used as a sintering aid, deteriorates the high temperature characteristics from the viewpoint that an oxide forming an amorphous phase or a low melting point crystal phase such as Al ₂ O ₃ deteriorates high temperature characteristics. On the other hand, a simple ternary system (Si) composed of a Group 3a element (RE) such as Y ₂ O ₃ and silicon oxide is used.
₃ N ₄ —SiO ₂ —RE ₂ O ₃ ) in a sintered body having a composition such as a YAM phase composed of Si—RE—O—N, an apatite phase, and a silicon oxynitride phase at the grain boundaries of the sintered body. It has been proposed to increase the melting point and stabilize the grain boundary phase by precipitating a crystal phase. Among these, the sintered body in which the silicon oxynitride phase and / or the disilicate phase is deposited on the grain boundary has excellent oxidation resistance at high temperatures.

[0004]

However, by precipitating a silicon oxynitride phase and / or a disilicate phase at the grain boundaries, the grain boundaries are more amorphous than those having an amorphous or low melting point crystalline phase. The high temperature characteristics are improved to some extent, but the disilicate itself is 1400 ° C.
Since it plastically deforms at a high temperature, it has been a factor that hinders the improvement of the high temperature strength of the entire sintered body. Therefore, it is still insufficient for practical use of a silicon nitride sintered body having a disilicate phase at the grain boundary, and further improvement in high temperature strength is required.

Therefore, an object of the present invention is to provide a silicon nitride sintered body which is excellent in oxidation resistance from a low temperature to a high temperature and is excellent in bending strength from a room temperature to a high temperature of 1400 ° C. Is.

[0006]

[Means for Solving the Problems] With respect to the above problems, the present inventors have conducted extensive studies to improve the high temperature strength of a sintered body in which a disilicate phase is precipitated at grain boundaries, and as a result, the die silicate is obtained. The inventors have found that by dispersing and depositing a specific composite oxide in the phase, the deformation resistance of the disilicate itself can be increased and the high temperature strength can be increased, and the present invention has been completed.

That is, the silicon nitride sintered body of the present invention has a main crystal phase made of silicon nitride and at least RE ₂ Si ₂ O ₇ (RE is an element of Group 3a of the periodic table) at the grain boundaries of the main crystal phase. )
A disilicate phase represented by the following formula and Al, Ga, Ti, Hf, Zr, Nb, Ta, V having a melting point of 1500 ° C. or higher.
It is characterized in that a complex oxide phase of at least one element selected from the group of W, Mo, Cr and Fe and an element of Group 3a of the periodic table is present.

The present invention will be described in detail below. The overall structure of the silicon nitride sintered material of the present invention has a silicon nitride crystal phase as a main phase, and this main phase has an average grain size of 1 to 10
It consists of a β-Si ₃ N ₄ phase of μm. Further, at least the Group 3a element of the periodic table, Si (silicon), O (oxygen) and / or N (nitrogen) are present in the grain boundary of the main phase. What is important in the constitution of the present invention is that, firstly, a disilicate phase is present as a grain boundary crystal phase. This disilicate phase is represented by RE ₂ Si ₂ O ₇ (RE is an element of Group 3a of the periodic table), but Si ₂ N ₂ O may be precipitated in some cases.

The silicon nitride-based sintered body of the present invention is characterized in that the complex oxide phase is further dispersed in the grain boundary phase. This complex oxide phase has a melting point of 1500 ° C. or higher Al, Ga, Ti, Hf, Zr, Nb, Ta, V,
It is a composite oxide of at least one element selected from the group of W, Mo, Cr and Fe and an element of Group 3a of the periodic table, and has the general formula RExMyOz (M is Al, Ga, Ti,
Hf, Zr, Nb, Ta, V, W, Mo, Cr and F
At least one element selected from the group e, RE is an element of Group 3a of the periodic table). Table 1 shows a typical composite oxide.

[0010]

[Table 1]

Among these composite oxides, the melting point is especially 2
Those having a temperature of 000 ° C. or higher are preferably used.

More preferably, as the Group 3a element of the periodic table, especially Sc, Y, Eu, Gd, Tb, Dy, Ho,
Since rare earth elements such as Er, Tm, Yb, and Lu have a small ionic radius, the bond strength of crystals is high, and the strength of the grain boundary phase is further enhanced, which is desirable because they form a complex oxide with a Group 3a element of the periodic table. Among the above metals, Zr, Cr, Hf, Ta, and W are most preferable among the above metals because they do not undergo phase transformation in the temperature range from room temperature to 1500 ° C.

This composite oxide phase is present in the grain boundaries of the silicon nitride main crystal grains in an amount of 0.05 at the grain boundaries together with the disilicate phase.
Although they are scattered in a size of ˜2 μm, they may be scattered in the disilicate grains in some cases.

According to the present invention, other independent phases, for example, metals of Groups 4a, 5a and 6a of the Periodic Table and their carbides, nitrides and silicides are provided as long as they do not deteriorate the structure and properties. Alternatively, SiC or the like may be present as dispersed particles or whiskers.

Next, a specific method for manufacturing the silicon nitride sintered body of the present invention will be described. According to the present invention, silicon nitride powder is used as a main component as a starting material, and oxide oxide powder, Group 3a element oxide oxide powder of the Periodic Table, silicon oxide powder, Group 3a element of the Periodic Table and Al, Ga are added as additive components. T
A composite oxide powder of at least one element selected from the group of i, Hf, Zr, Nb, Ta, V, W, Mo, Cr and Fe is used. This complex oxide powder is composed of a Group 3a element oxide of the periodic table and Al, Ga, Ti, Hf, Zr, N.
b, Ta, V, W, Mo, Cr, and an oxide of at least one metal selected from the group of Fe are used to prepare the composition of RExMyOz described above. It is obtained by calcination at a temperature equal to or lower than the melting point, and crushed to have an average particle size of 0.3 to 2 μm.

Further, as a form of addition of the Group 3a element oxide of the Periodic Table or the silicon oxide powder, a compound consisting of the Group 3a element oxide of the Periodic Table and SiO ₂ or silicon nitride and the Group 3a element oxide of the Periodic Table. It is also possible to use a complex oxide powder of a substance and SiO ₂ .

The silicon nitride powder used may be either α-type or β-type, and the particle size thereof is 0.4 to 0.4.
1.2 μm and an amount of impurity oxygen of 0.8 to 2.0% by weight are suitable, and any production method such as a direct nitriding method or an imide decomposition method may be used.

According to the present invention, using the above-mentioned raw material powder, 1 to 15 mol%, especially 3 to 8 mol%, and 3 to 8 mol% of SiO ₂ are used on the basis of three components, the oxide of the Group 3a element of the periodic table. ~ 3
0 mol% balance Si ₃ N _4, and 1 to 20% by weight, especially 3 to 1% by weight of the complex oxide in the weight ratio with respect to the above three components.
Add at a rate of 0% by weight.

In the above composition, if the group 3a oxide of the periodic table as a sintering aid is less than 1 mol%, it is difficult to densify, and if it is more than 15 mol%, the absolute amount of the grain boundary phase increases and the sintering increases. The high temperature strength of the body tends to decrease.

If the amount of the composite oxide is less than 1% by weight, the strengthening effect cannot be obtained, and if it is more than 20% by weight, the sintering process is hindered.

Further, according to the present invention, the molar ratio represented by SiO ₂ / RE ₂ O ₃ of the Group 3a oxide of the periodic table and the silicon oxide containing the impurity oxygen component in the silicon nitride powder is 2
It is controlled so as to be ~ 20. This is necessary for controlling the crystal phase in the grain boundary phase as described above, and if this ratio is less than 2, a crystal phase such as a disilicate phase is not generated and an apatite phase, a YAM phase, or the like is generated. This is because precipitation tends to occur as the main crystal phase of the grain boundary, and if it exceeds 20, the sinterability decreases.

After weighing each powder in the above proportions, a vibration mill,
After thoroughly mixing with a rotary mill, a barrel mill or the like, the mixed powder 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.

Thereafter, the compact is densified by a known firing method such as a hot pressing method, a normal pressure firing method, or a nitrogen gas pressure firing method. Furthermore, densification can be enhanced by performing a hot isostatic pressing (HIP) method after the baking. As another method, the molded body or sintered body may be sealed with glass and fired by the HIP method.

Specific firing conditions in these firings are firing at a temperature of 1500 to 1900 ° C. under a nitrogen pressure of 1 to 100 atm, which does not decompose silicon nitride. In addition,
When the firing temperature exceeds 1900 ° C., the silicon nitride crystal causes grain growth and causes strength deterioration.

Next, after firing, heat treatment is performed in a cooling process, or the obtained sintered body is subjected to 10 times in a non-oxidizing atmosphere.
By performing the heat treatment at a temperature of 00 to 1600 ° C., the disilicate phase or the disilicate phase and the silicon oxynitride phase can be precipitated in the grain boundary phase, and the composite oxide phase can be dispersed.

According to the above manufacturing method, the case where the composite oxide powder is used as the starting material has been described, but as another method, Al, Ga, Ti, Hf, Zr, Nb, Ta,
It is also possible to use a single metal oxide powder of at least one element selected from the group consisting of V, W, Mo, Cr and Fe. In that case, it is slowly cooled in the cooling process after firing and the RE ₂ Si
₂ O ₇ (RE: Group 3a element of the periodic table) contains Cr, Hf, Nb, Zr and Ta in RE in the disilicate phase.
And the like, and then heat-treated again in the temperature range of 1000 to 1600 ° C., so that the supersaturated solid-soluted metal element is mixed in the RE ₂ Si ₂ O ₇ grains as RExMyOz complex oxide in the disilicate phase. By finely precipitating into the grain boundary phase, a nanocomposite structure can be formed in the grain boundary phase, whereby the high temperature strength of the grain boundary phase and the high temperature strength of the sintered body can be further improved.

[0027]

It is known that the silicon nitride sintered body undergoes deformation and destruction under load applied at a temperature higher than 1400 ° C. The resistance of the sintered body to deformation is greatly affected by the Young's modulus of the grain boundary phase at high temperatures and the high temperature plastic deformation resistance. A silicon nitride sintered body having a disilicate phase of a Group 3a element of the periodic table at the grain boundary has very excellent oxidation resistance even in a high temperature oxidizing atmosphere, but the disilicate phase itself has an ionic bond. Since it is a type crystal, it tends to be plastically deformed by dislocation at high temperature. Therefore, the silicon nitride-based sintered body having this in the grain boundary phase has low deformation resistance at high temperatures.

According to the present invention, in order to improve the high temperature strength, particles which are stable at high temperature are dispersed in the structure, particularly in the grain boundary phase, to suppress the plastic deformability. Therefore, the periodic table 3a having a melting point of 1500 ° C. or higher
By using a composite oxide of a group element and a predetermined metal element, by suppressing the dislocation at high temperature of the disilicate phase by dispersing it in the grain boundary, and acting as a composite strengthening particle of the grain boundary, in a high temperature oxidizing atmosphere A product having high strength at high temperature while retaining excellent oxidation resistance can be obtained.

[0029]

EXAMPLES Silicon nitride powder (BET specific surface area 10 m ² / g, α ratio 95%, oxygen amount 1.2% by weight, imide thermal decomposition raw material) as raw material powder and periodic table of purity 99.9% or more Group 3a oxide powder (RE ₂ O ₃ ) and purity 99.9%
The above silicon oxide powder was prepared.

On the other hand, as a complex oxide, various periodic table group 3a element oxides and Al, Ga, Ti, Hf, and Z are included.
A mixture of oxides of metals of r, Nb, Ta, V, W, Mo, Cr and Fe mixed at a predetermined ratio shown in Table 1 is heat treated at 1000 to 1500 ° C. for 5 hours. After that, it is crushed and the average particle size is 0.3 to 2μ.
m powder was obtained.

Next, the above raw material powders, which were weighed at the molar ratio shown in Table 1, were mixed and pulverized in a vibration mill for 72 hours using isopropyl alcohol as a solvent and silicon nitride balls. 60mm, thickness 10m
m was press-molded with a pressure of 3 ton / cm ² .

Then, the molded body was fired under the conditions shown in Table 2 and cooled, and then further heated in nitrogen at a temperature of 1400 ° C. for 2 hours.
Heat treated for 4 hours.

The theoretical density ratio of the obtained sintered body was calculated from the specific gravity by the Archimedes method, and 3 × 4.
JISR, cut and polished into a 40 mm test piece shape
A 4-point bending bending strength test was carried out at room temperature and 1500 ° C. based on 1601, and the average value of the test results of 10 pieces was taken as the oxidation resistance property of the sintered body at 100 ° C. in the atmosphere of 1400 ° C.
The weight increase after holding for a period of time was measured and is shown in Table 2. Further, the bending test piece was supported in the same manner as in the three-point bending test of JIS R1601, a load of 400 MPa was applied, and the test piece was held at 1400 ° C. for a maximum of 100 hours, and the time until failure was measured.
Furthermore, the grain boundary crystal phase was identified by measuring the X-ray diffraction of the surface of the sintered body. The results of these measurements are shown in Table 2.

[0034]

[Table 2]

As is clear from Table 2, in samples No. 20 and 22 in which no composite oxide was added, the creep resistance was low, and in sample No. 21 in which disilicate was not precipitated in the grain boundary crystals. The oxidation resistance was low and the creep resistance was also low.

In contrast to these comparative examples, the products of the present invention other than the above all have a bending strength of 820 MPa or more at room temperature,
At 1400 ° C., 660 MPa or more, the increase in oxidized weight is 0.
It showed excellent characteristics of 1 mg / cm ² or less and creep resistance of 3.2 hr or more.

[0037]

As described in detail above, according to the present invention,
It is possible to improve high temperature strength and creep resistance at high temperature while maintaining oxidation resistance at high temperature. As a result, high reliability can be imparted even when used for automobile parts, gas turbine parts, and the like.

Claims (2)

[Claims] 1. A main crystalline phase made of silicon nitride, a disilicate phase represented by at least RE ₂ Si ₂ O ₇ (RE is an element of Group 3a of the periodic table) in a grain boundary of the main crystalline phase, and a melting point. Is 1500 ° C or higher Al, Ga, Ti, Hf, Zr, N
Silicon nitride based sintering characterized by the presence of a complex oxide phase of at least one element selected from the group consisting of b, Ta, V, W, Mo, Cr and Fe and an element of Group 3a of the periodic table. body. 2. The silicon nitride sintered body according to claim 1, further comprising a silicon oxynitride phase in the grain boundary phase.