JPH0826829A - Silicon nitride sintered body - Google Patents

Abstract

PURPOSE:To produce a silicon nitride sintered compact having a sufficient strength and durability as a structural material used at about 1500 deg.C at the maximum, on the other hand, a relatively high strength even at ambient temperature. CONSTITUTION:This silicon nitride sintered compact comprises silicon nitride grains and a grain boundary containing R (R is a rare earth element), Si, O and N. In the sintered compact, a melilite phase (R2Si3N4O3) is present only in the surface layer part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride sintered body suitable for a heat engine such as a gas turbine and having excellent strength and creep resistance.

[0002]

2. Description of the Related Art Since a silicon nitride sintered body is excellent in mechanical properties, heat resistance, corrosion resistance, etc., it has been attempted to be applied to structural materials used in heat engines such as automobile engines and gas turbine engines. There is.

Since this silicon nitride has a high covalent bond, it is difficult to sinter, and Al ₂ O ₃ and MgO are required for sintering.
And a sintering aid such as a rare earth element oxide is required. However, the strength of the thus-obtained sintered body decreases at high temperatures, and various attempts have been made to study sintering aids and crystallize grain boundaries. For example, in JP-A-64-56368, the grain boundaries are apatite or disilicate, in JP-A-64-61357 the grain boundaries are wollastonite or caspadein, and in JP-A-64-61358. Has crystallized grain boundaries into melilite to achieve high temperature and high strength. In addition, an attempt to increase the amount of oxygen in the surface layer of the sintered body from the inside in order to prevent deterioration of strength due to deterioration of the surface condition,
It is disclosed in Japanese Patent Application Laid-Open No. 4-154667.

[0004]

As described above, conventionally, in the silicon nitride sintered body, high strength has been achieved mainly by examining the composition and sintering conditions. However, even though these sintered bodies have high strength at room temperature, they are still subject to large strength deterioration at high temperature or poor creep resistance, and are subjected to grain boundary crystallization treatment to maintain strength at high temperature. As a result, there is a problem that the strength level at room temperature is lowered.

An object of the present invention is to provide a silicon nitride sintered body which has sufficient strength and durability as a structural material used at a maximum temperature of about 1500 ° C. and has a relatively high strength even at room temperature. is there.

[0006]

In order to achieve the object, the silicon nitride sintered body of the present invention comprises silicon nitride particles and R
(R is a rare earth element), a sintered body composed of a grain boundary phase containing Si, O and N is characterized in that a melilite phase (R ₂ Si ₃ N ₄ O ₃ ) is present only in the surface layer portion thereof. .

The rare earth element is Sc of the 3A group of the periodic table,
Either Y or a lanthanoid element, but especially Y,
Er and Yb are preferred. Here, the melilite phase is JC
PDS card No. It is a crystal phase having the same X-ray diffraction pattern as 28-1457. A silicon nitride sintered body having a surface layer portion having the melilite phase having a thickness of 10 to 500 μm is preferable.

More preferably, the rare earth element in the silicon nitride sintered body is converted into R ₂ O ₃ molar amount and the oxygen amount in the sintered body is subtracted from the residual oxygen amount contained in R ₂ O ₃ . SiO ₂ that the amount of oxygen is calculated from the molar amount in terms of SiO ₂ /
The value of (R ₂ O ₃ + SiO ₂ ) is 0.5 to 0.8.

[0009]

In general, when the same sintering aid is used in the silicon nitride sintered body, the strength deterioration at high temperature is smaller when the grain boundary phase is crystallized than when the grain boundary phase is the glass phase. The room temperature strength has a low value. According to the present invention, the grain boundary of the surface layer of the sintered body is substantially crystallized in the melilite phase, and the residual glass is very small, so that the high temperature strength is excellent (15
450 MPa or more at 00 ° C, more preferably 500 M
Pa or more). In addition, since only the surface layer portion is crystallized to melilite, a difference in thermal expansion occurs between the surface layer portion grain boundary and the internal grain boundary, and residual stress occurs in the sintered body at a temperature near room temperature. The strength is relatively high as a heat resistant material (700 MPa or more, more preferably 800 MPa).
MPa or higher).

The thickness of the surface layer portion where the melilite phase is present is 1
When the thickness is less than this range, the melilite phase is changed to SiO ₂ and R ₂ Si ₂ O ₇ due to oxidation at around 1000 ° C.
This is because high strength at high temperature cannot be achieved. On the other hand, the above-mentioned residual stress becomes maximum near the interface between the inside and the surface layer portion and becomes smaller as the distance from the interface increases, so that if the thickness exceeds this range, the residual stress in the surface layer portion will decrease and the room temperature strength will decrease. . A particularly preferable range of the thickness is 50 to 300 μm.

Regarding the sintered body structure, in the case of a uniform structure of fine equiaxed silicon nitride particles, it exhibits high strength at room temperature, but is greatly deteriorated at high temperature, and its creep resistance is also low. In order to maintain strength and durability at high temperatures, it is advantageous to have a composite structure of equiaxed fine particles and columnar coarse particles.

SiO ₂ / (R ₂ O ₃ + SiO ₂ ) in the sintered body
The value of 0.5 to 0.8 is within this range (particularly preferably 0.55 to 0.75), but the sintered body structure is a composite structure of fine particles and coarse particles, If this value is smaller than 0.5, the structure of the sintered body becomes coarse as a whole to lower the room temperature strength. If it is larger than 0.8, it is difficult to densify and it is difficult to obtain a high-strength sintered body. is there.

The state of the internal grain boundary phase differs depending on the composition of the sintering aid, the sintering conditions and the heat treatment conditions after sintering, but SiO ₂ / (R ₂ O ₃ + SiO ₂ ) in the sintered body. When the value of) is within the above range, the glass phase or the crystal phase of R ₄ Si ₂ N ₂ O ₇ or R ₂ Si ₂ O ₇ is obtained.

[0014]

[Examples] Si ₃ N ₄ powder having an average particle size of 0.6 μm and an α ratio of 97%, a rare earth oxide having an average particle size of 1 to ₃ μm and an average particle size of about 1
Each powder _{SiO 2, V 2 O 5,} MoO 3 and WO ₃ in μm were blended in the composition shown in Table 1, was wet-mixed by a silicon nitride-made ball mill, the powder obtained by drying 55x55x25
mm at a pressure of 4 ton / cm ² for isostatic pressing, normal pressure sintering,
A sintered body is obtained by gas pressure sintering and hot isostatic pressing or a sintering method combining these. The sintered body of the present invention can be obtained by a reheating treatment, but the temperature at this time is a relatively high temperature of 1600 to 1800 ° C., and it is in a nitrogen atmosphere containing a slight amount of CO. Is desirable.

[0015]

[Table 1] Table 2 shows various characteristics of the obtained sintered body. Among the characteristics shown in Table 2, the density of the sintered body was expressed as a ratio to the theoretical density measured by the Archimedes method and calculated by the mixing rule. The amount of oxygen contained in the sintered body was measured by an oxygen amount analyzer, and the amount of rare earth element was quantified by ICP emission spectrometry. The grain boundary phase was identified using an X-ray diffractometer, and the strength was measured by a four-point bending test according to JIS R 1601 and R 1604. The thickness of the surface layer portion in which the melilite phase was present was determined by electron microscope observation after mirror-polishing the cross section of the sintered body. Further, in the grain boundary phase, a silicide (VS, such as V, W, Mo) added as a sintering aid (VS
i ₂ , V ₅ Si ₃ , MoSi ₂ , WSi ₂ , W ₅ Si ₃ ),
The presence of these solid solutions was observed in some cases, but they were omitted because they were uniform throughout the sintered body.

[0016]

[Table 2] As is clear from Table 2, according to the sintered body belonging to the scope of the present invention, high strength (450 MPa or more) can be obtained without impairing room temperature strength (700 MPa or more) and at a high temperature of 1500 ° C. However, No. 6 is SiO ₂ /
Since the value of (R ₂ O ₃ + SiO ₂ ) is a little too small, No. 1
It is considered that the room temperature strength was slightly lower than that of -5.
In addition, No. It is considered that since the value of No. 7 was slightly excessive, the density was low and the high temperature strength was also slightly low.

On the other hand, No. The sintered bodies of Nos. 8 to 10 have low strength at 1500 ° C because no melilite phase is present in the surface layer. That is, No. Since the sintered bodies of Nos. 8 and 10 produced a single crystal phase as a whole, the room temperature strength was also low, and NO. Since the grain boundary phase of the sintered body of 9 was a glass phase, the room temperature strength was relatively high, but the 1500 ° C. strength was remarkably low. No. In No. 11, the melilite phase was present in the surface layer, but the thickness was less than 10 μm, so the strength at 1500 ° C. was low. Conversely, No.
In No. 12, the room temperature strength was low because the melilite phase was present in the entire sintered body (corresponding to a thickness of 500 μm or more).

[0018]

As is clear from the above description, according to the crystal structure of the present invention, it is possible to obtain a silicon nitride sintered body having a high room temperature strength and little strength deterioration at high temperatures, which is suitable as a high temperature structural member. You can

Claims (3)

[Claims] 1. Silicon nitride particles and R (R is a rare earth element),
In a sintered body composed of a grain boundary phase containing Si, O and N, a melilite phase (R ₂ Si ₃ N ₄ O ₃ ) is present only in the surface layer portion.
A silicon nitride sintered body characterized by being present. 2. The silicon nitride sintered body according to claim 1, wherein the surface layer portion in which the melilite phase is present has a thickness of 10 to 500 μm. 3. The residual oxygen amount obtained by subtracting the oxygen amount contained in R ₂ O ₃ from the molar amount of the rare earth element in the sintered body converted to R ₂ O ₃ and the oxygen amount in the sintered body is SiO 2. _{SiO 2 / (R 2 O 3} + SiO 2) silicon nitride sintered body according to claim 1 or 2 value is 0.5 to 0.8 of which is calculated from the molar amount in terms of _2.