JPH05201767A - Silicon nitride-based sintered compact and its production - Google Patents

Abstract

PURPOSE:To provide a silicon nitride-based sintered compact which undergoes slight deterioration of the strength in the temp. range from room temp. to a high temp. and has excellent oxidation resistance. CONSTITUTION:A compact consisting of 70-97mol% silicon nitride and 3-30mol%, in total, of the oxide of a group IIIa element of the periodic table and silicon oxide in 1:>=2 molar ratio is fired in a nonoxidizing atmosphere and the resulting sintered compact is held at a temp. between the softening temp. of glass formed at the grain boundaries of the sintered body and the crystallization temp. of the glass which is crystallized to y-RE2Si2O7 crystals (RE is the above- mentioned group IIIa element). The sintered compact is then held at a temp. between the crystallization temp. of the glass and the transition temp. of y- RE2Si2O7 which undergoes transition to beta-RE2Si2O7 to deposit y-RE2Si2O7 at the crystal boundaries of a silicon nitride crystal phase as the principal phase.

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 which is excellent in strength characteristics from room temperature to high temperature and is particularly used for automobile parts, gas turbine engine parts and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, silicon nitride sintered bodies are excellent in heat resistance, thermal shock resistance and oxidation resistance, and therefore, their application has been promoted for engineering ceramics, especially for heat engines such as turbochargers. ing. 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. Further improvement in strength and oxidation resistance at high temperature is required for such a silicon nitride-based sintered body when the operating conditions thereof further increase in temperature. In order to meet such demands, various improvements have been attempted so far, such as examination of sintering aids and improvement of firing conditions.

In view of the fact 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, rare earth elements such as Y ₂ O ₃ are used for silicon nitride. A sintered body having a simple ternary composition of elements and silicon oxide has been proposed. In addition, Y composed of Si ₃ N ₄ —RE ₂ O ₃ —SiO _{2 is} added to the grain boundary of the sintered body.
It has been proposed to increase the melting point and stabilize the grain boundaries by precipitating a crystal phase such as an AM phase, an apatite phase, a wollastonite phase, a silicon oxynitride phase, or a disilicate phase.

[0004]

However, by crystallizing the grain boundary, high temperature characteristics are improved as compared with the case where the grain boundary is amorphous, but a stable crystal phase is generated. In addition, a grain boundary phase having a low melting point is formed by a component that does not contribute to crystallization at the same time when a predetermined crystal phase is precipitated, and thus a sufficient effect due to crystallization is not obtained. Was the current situation.

Therefore, it is still insufficient for practical use of such a sintered body, and further improvement in strength and improvement in oxidation resistance are required.

From the standpoint of oxidation resistance, the present invention has sufficient strength characteristics to be used in automobile parts, gas turbine engine parts and the like from room temperature to high temperature, particularly from room temperature to 14
It is an object of the present invention to provide a silicon nitride sintered body which is excellent in bending strength up to a high temperature of 00 ° C. and is also excellent in oxidation resistance characteristics from a low temperature to a high temperature.

[0007]

In order to improve the strength characteristics and the oxidation resistance characteristics of the sintered body, the present inventor has determined the composition of the sintered body and the sub-phase existing in the grain boundary of the silicon nitride phase. As a result of repeated studies based on the viewpoint that it is important to control, 70 to 97 mol% of silicon nitride, 3 to 30 mol% of the oxide of group 3a element of the periodic table and silicon oxide in total, and After a molded body having a composition in which the molar ratio of silicon oxide to the Group 3a element oxide of the periodic table is 2 or more is fired in a non-oxidizing atmosphere, the softening of the glass formed in the grain boundaries of the sintered body. The temperature is kept between the temperature Tg and the crystallization temperature Tc from the glass to the y-type RE ₂ Si ₂ O ₇ crystal, and then the crystallization temperature Tc and the y-type RE ₂ Si ₂ O ₇ to the β-type RE ₂ Si.
_The temperature is maintained between the transition temperature Tt at which it transforms to ₂ O ₇ , and the silicon nitride crystal phase is the main phase, and y-type RE ₂ Si ₂ O is present at the grain boundaries.
₇ (RE is an element of Group 3a of the Periodic Table), which suppresses the formation of an amorphous layer in the grain boundaries, has excellent strength characteristics from room temperature to high temperature, and from 140 ° C to 140 ° C.
It was found that a sintered body having excellent oxidation resistance up to 0 ° C can be obtained.

The silicon nitride-based sintered body of the present invention contains silicon nitride as a main component, and contains an element of Group 3a of the periodic table and excess oxygen as additional components. Here, excess oxygen means the amount of total oxygen in the sintered body from the periodic table 3a in the sintered body.
When the group element forms an oxide stoichiometrically, it is the remaining oxygen amount excluding oxygen bound to the element, most of which is oxygen contained in the silicon nitride raw material or SiO ₂
And the like are mixed as additions, and in the present invention, all S
Consider as existing as iO ₂ .

The sintered body of the present invention structurally has a silicon nitride crystal phase as a main phase, most of which is β-Si.
_It consists of ₃ N ₄ . Further, a Group 3a element of the Periodic Table and excess oxygen (which is considered to exist as silicon oxide) are present in the grain boundary of the main phase. According to the present invention, y-type RE ₂ is present in this grain boundary phase. It is important to deposit Si ₂ O ₇ . During the sintering process, this crystal phase exists as a liquid phase with a low melting point due to the reaction with silicon nitride particles and enhances the sinterability, but if it remains as a glass phase in the grain boundary phase after cooling, the high temperature strength will decrease. At the same time, the oxidation resistance is deteriorated. Therefore, by precipitating the y-type RE ₂ Si ₂ O ₇ crystal phase by a predetermined cooling process or heat treatment, it is possible to enhance the high temperature strength and at the same time the oxidation resistance.

Further, in order to precipitate the above crystal phase, the amount of excess oxygen in the sintered body converted to silicon oxide (SiO ₂ ) (SiO ₂ ) is converted to the amount of oxide of the group 3a element (RE) of the periodic table (RE).
₂ O ₃ molar ratio (SiO ₂ / RE ₂ O ₃ against)) is required to be 2.0 or more. This is because when the above ratio is less than 2.0, the grain boundary phase contains RE other than RE ₂ Si ₂ O _7.
A trace amount of a glass layer made of —Si—O—N is likely to be present, which lowers high temperature strength and deteriorates oxidation resistance. Moreover, even if completely crystallized, RE ₁₀ Si ₂ O ₂₃ N ₄
This is because the apatite phase described in RE ₁₀ (SiO ₄ ) ₆ N ₂ or the like and the YAM phase described in RE ₄ Si ₂ O ₇ N ₂ are precipitated and deteriorate the oxidation resistance characteristics.

Further, in order to precipitate the above crystal phase, the amount of excess oxygen in the sintered body in terms of silicon oxide (SiO ₂ ) in terms of silicon oxide (RE) in terms of oxide of group 3a element (RE) in the periodic table (RE) is calculated.
₂ O ₃₎ molar ratio (SiO ₂ / RE ₂ O ₃₎ of 2.0 or more with respect, it is necessary that especially 2.0 to 5.0. This is because when the above ratio is less than 2.0, RE ₂ is contained in the grain boundary phase.
_{Si 2 O 7 RE-Si-} O-N glass layer traces tends to exist consisting of the addition deteriorates the oxidation resistance with decreasing the high-temperature strength. Even if it is completely crystallized, R
The apatite phase described by E ₁₀ Si ₂ O ₂₃ N ₄ and RE ₁₀ (SiO ₄ ) ₆ N ₂ and the YAM phase described by RE ₄ Si ₂ O ₇ N ₂ are precipitated to improve the oxidation resistance. This is because it deteriorates.

According to the present invention, y-type RE ₂ Si ₂ is formed at the grain boundary.
O ₇ crystals are deposited, but the molar ratio (SiO ₂ / RE ₂
O ₃ ) is 2.0 or more and about 2.5 or less, the crystalline phase precipitates only the y-type RE ₂ Si ₂ O ₇ crystal, but when the molar ratio is more than 2.5, other than RE ₂ Si ₂ O ₇ crystal. Although a slight amount of Si ₂ N ₂ O may be precipitated, there is no problem from the viewpoint of oxidation resistance. However, when a crystal phase mainly composed of Si ₂ N ₂ O is precipitated, there arises a problem that the fracture toughness value is lowered. Therefore, the molar ratio is preferably 2.0 to 2.5.

The periodic table No. 3a used in the present invention is as follows.
Examples of the group element include Y and lanthanoid elements, but Yb, Er and Dy are particularly preferable.

According to the silicon nitride sintered body of the present invention, the presence of a low-melting metal oxide such as Al ₂ O ₃ , MgO, or CaO hinders the crystallization of the grain boundaries and increases the high temperature strength. The total amount of these oxides is 0.5 for deterioration.
It is desirable to control the content to be not more than weight%.

Next, a method for producing a silicon nitride sintered body according to the present invention will be described. First, silicon nitride powder is the main component as a raw material powder, and the periodic table 3a is an addition component.
In addition to the addition of the group oxide oxide and the silicon oxide powder, or the compound powder consisting of the group 3a oxide of the group 3a and silicon oxide as an addition component, or silicon nitride and the periodic table 3
A compound powder composed of a group a element oxide and silicon oxide can also be used.

The silicon nitride powder used is itself α-Si.
Either ₃ N ₄ or β-Si ₃ N ₄ may be used, and their particle size is preferably 0.4 to 1.2 μm.

According to the invention, using these powders,
Silicon nitride is 70 to 97 mol%, the total amount of periodic table group 3a element oxide (RE ₂ O ₃ ) and excess oxygen (SiO ₂ conversion amount) is 3 to 30 mol%, and SiO ₂ / RE ₂ O _{3 is used.} Are adjusted and mixed so that the molar ratio represented by is 2.0 or more. The excess oxygen (SiO ₂ conversion amount) at this time is the sum of the amount of impurity oxygen contained in the silicon nitride powder converted to SiO ₂ and the added silicon oxide powder, or the silicon oxide conversion amount of the silicon-containing compound. is there.

The mixed powder thus obtained was molded into a desired shape by a known molding method such as press molding, cast molding, extrusion molding, injection molding, cold isostatic molding, and then obtained. A known firing method for the molded body, for example, a hot pressing method, normal pressure firing, nitrogen gas pressure firing,
Further, HIP calcination after these calcinations, glass seal HIP calcination and the like are carried out to obtain a dense sintered body.
If the firing temperature at this time is too high, the silicon nitride crystal grains grow and the strength decreases.
A non-oxidizing atmosphere containing nitrogen at ˜1850 ° C. is desirable.

Next, after firing, heat treatment is performed in the cooling process, or the obtained sintered body is heat treated in a non-oxidizing atmosphere. At this time, according to the conventional heat treatment method, another RE ₂ S
The i ₂ O ₇ crystal is precipitated and a large amount of an amorphous phase remains between the silicon nitride crystal grains and the crystal phase of the grain boundary, resulting in deterioration of high temperature strength.

Therefore, according to the present invention, as the heat treatment method, first, the softening temperature Tg of the glass formed at the grain boundaries of the sintered body and the crystallization from the glass to the y-type RE ₂ Si ₂ O ₇ crystal are performed. Y type RE ₂ Si ₂ O
Generate ₇ crystal nuclei. Then, the crystallization temperature Tc and y
Y by holding between the mold RE ₂ Si ₂ O ₇ crystals of transition temperature Tt to metastasize to β-type RE ₂ Si ₂ O ₇ crystals
By performing heat treatment while growing the crystal nuclei of the RE ₂ Si ₂ O ₇ -type RE ₂ Si
It is possible to form a solid solution at the ₂ O ₇ crystal interface and to suppress the formation of an amorphous layer between the silicon nitride crystal grains and the crystal phase of the grain boundaries.

The softening temperature Tg, the crystallization temperature Tc, and the transition temperature Tt can be determined by firing in the same manner as described above and then rapidly cooling to room temperature to obtain a sintered body having a glass phase as a grain boundary phase. A thin piece is cut out from this sintered body, and the glass composition of this grain boundary phase is determined by the UTW-EDX method using an analytical electron microscope. Next, a mixed powder prepared to have the same composition as this glass composition is molded, fired and rapidly cooled to prepare a glass, and a softening temperature Tg of this glass and a y-type RE ₂ Si ₂ O ₇ crystal are obtained by the DTA method. Crystallization temperature Tc, and y-type RE ₂ Si ₂ O ₇ crystal to β-type RE ₂
The transition temperature Tt to the Si ₂ O ₇ crystal can be obtained.

According to experiments conducted by the present inventors, the softening temperature Tg of a glass composed of various silicon nitrides and RE ₂ Si ₂ O ₇ is about 950, Tc is about 1100 ° C., and T
t is a temperature around 1300 ° C. Therefore, as the heat treatment temperature, the temperature of the first step is set to around 1050 ° C,
It is preferable to set the temperature of the second stage to around 1150 ° C. By prolonging the holding time of the second stage, the phase transition can no longer occur even if the temperature is further raised.

[0023]

The crystal represented by RE ₂ Si ₂ O ₇ (RE: rare earth element) has various polymorphs (α, β, γ, δ), but especially y-type RE ₂ Si ₂ O It is important that it crystallizes to ₇ . This y-type RE ₂ Si ₂ O ₇ crystal has a unique property that it can form a solid solution with a metal element such as Ca or Fe as compared with other RE ₂ Si ₂ O ₇ crystals.

Metal elements such as Ca and Fe are unavoidably present in the silicon nitride raw material, and are present at three grain boundaries between the silicon nitride crystal grains in the sintered body, and the silicon nitride crystal and the grain boundary crystal phase are present. It remains as an amorphous layer at the interface with and deteriorates the high temperature characteristics, especially the stress rupture characteristics. While solid-dissolving elements such as Ca and Fe depending on heat treatment conditions, y
Crystallizing the grain boundary phase in RE ₂ Si ₂ O ₇ of the mold,
This crystal phase can be stabilized by heat treatment for a long time. Therefore, according to the present invention, an impurity metal such as Ca or Fe can be solid-dissolved in the crystal phase of the grain boundary to fix the element, thereby forming an amorphous layer due to the presence of these impurity elements. Can be suppressed.

This makes it possible to reduce strength deterioration from room temperature to high temperature and to impart excellent oxidation resistance from room temperature to high temperature.

[0026]

Example As raw material powder, silicon nitride powder (BET specific surface area 8 m2 / g, α ratio 98%, oxygen amount 1.2% by weight, cation metal impurity amount 0.03% by weight) and various periodic table 3a. RE ₂ synthesized from group 3 element oxides, silicon oxide powders, or group 3a element oxides of periodic table and silicon oxide powders
Si ₂ O ₇ powder was used (Sample No. 19, 19) to prepare the composition shown in Table 1, and the mixture was molded with 1 t / cm ² .

In the table, the molded bodies of Samples No. 1 to No. 12 were placed in a silicon carbide sagger and the atmosphere was controlled to reduce the compositional fluctuation, and the atmosphere was controlled under a nitrogen gas stream of 10 atm 185.
Firing was performed under the conditions of 0 ° C. and 4 hours. Further, some of the samples were subjected to heat treatment during cooling under the conditions shown in Table 1. Further, some of the samples were heat-treated under the conditions shown in Table 1 in a nitrogen gas stream at normal pressure.

As for the molded bodies of Sample Nos. 13 to 19, sintered bodies were prepared by the seal HIP method. Specifically, first, prior to firing, the molded body obtained by the above-described method is treated with BN powder or the like for the purpose of preventing reaction with the glass or the like as a seal HIP material in the firing step. A powder having poor wettability with glass is made into a slurry and applied to a molded body, or the above slurry is spray-applied. Next, the molded body coated with BN was encapsulated in a glass capsule, and a sintered body was produced by the HIP method at 1700 ° C. for 1 hour. Some of the samples were heat-treated under atmospheric pressure in a nitrogen gas stream under the conditions shown in Table 1 to obtain sintered bodies.

The obtained sintered body was ground to a shape specified in JIS-R1601 to prepare a sample. Specific gravity measurement based on Archimedes method, JIS-
A four-point bending transverse strength test was carried out at room temperature and 1000 ° C. based on R1601. Further, the sample was exposed to 900 ° C. air or 1400 ° C. air for 100 hours, and the weight change per unit surface area was determined from the weight increase amount and the surface area of the sample. In addition, the crystal of the grain boundary phase in the sintered body was identified by X-ray diffraction measurement. The results are shown in Table 2. The composition of the sintered body was crushed from a sample, the amount of oxygen was converted into final carbon dioxide, and quantified by an infrared absorption method. The amount of nitrogen was measured by thermal conductivity. It was obtained by analysis and obtained from these. The results are shown in Table 2.

[0030]

[Table 1]

[0031]

[Table 2]

According to the results of Table 1 and Table 2, SiO ₂
Sample No. 1 having a / RE ₂ O ₃ ratio of 1.0 has a reduced strength due to insufficient densification, and Samples No. 2, No. 3, No. 13 having a higher density in the range where this ratio is less than 2, In No. 14, precipitation of a crystal phase mainly composed of YAM or YAM and apatite was observed, and the strength showed a somewhat high value,
It was inferior in oxidation resistance at high temperature. SiO ₂ /
Even if RE ₂ O ₃ is 2 or more, the heat treatment conditions are not appropriate and the grain boundary phase is not crystallized into y-type RE ₂ Si ₂ O ₇ , No. 4,
The samples of No. 8, No. 9 and No. 15 had deteriorated high temperature strength.

Sample No, 20 in which the total amount of the oxide of Group 3a element of the periodic table and silicon oxide is less than 3 mol% cannot be densified, and Sample No, 2 in excess of 30 mol% cannot be obtained.
In No. 1, deterioration of strength was recognized.

In contrast to these comparative examples, all the other samples according to the present invention have y-type RE ₂ Si ₂ O ₇ ,
Or y-type RE ₂ Si ₂ O ₇ crystal and slightly Si ₂ N
Precipitation of ₂ O crystals was observed, and excellent bending strength
It showed oxidation resistance.

[0035]

As described in detail above, according to the present invention,
By subjecting a specific crystal phase to precipitation at grain boundaries by heat treatment under predetermined conditions, it is possible to provide a silicon nitride-based sintered body having little strength deterioration from room temperature to high temperature and having excellent oxidation resistance.

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[Procedure amendment]

[Submission date] May 13, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0029

[Correction method] Change

[Correction content]

The obtained sintered body was ground to a shape specified in JIS-R1601 to prepare a sample. Specific gravity measurement based on Archimedes method, JIS-
A four-point bending transverse strength test was carried out at room temperature and 1400 ° C. according to R1601. The sample was exposed to 900 ° C. air or 1400 ° C. air for 100 hours, and the weight change per unit surface area was determined from the weight increase amount and the surface area of the sample. In addition, the crystal of the grain boundary phase in the sintered body was identified by X-ray diffraction measurement. The results are shown in Table 2. The composition of the sintered body was crushed from a sample, the amount of oxygen was converted into final carbon dioxide, and quantified by an infrared absorption method. The amount of nitrogen was measured by thermal conductivity. It was obtained by analysis and obtained from these. The results are shown in Table 2.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0031

[Correction method] Change

[Correction content]

[0031]

[Table 2]

Claims (3)

[Claims] 1. A silicon nitride crystal phase as a main phase, with y at its grain boundaries.
A silicon nitride sintered body, characterized in that a type RE ₂ Si ₂ O ₇ (RE is a Group 3a element of the periodic table) crystal is deposited. 2. A group 3a element (RE) of the periodic table in the sintered body.
_{2. The} molar ratio represented by SiO ₂ / RE ₂ O ₃ is 2.0 or more in terms of the oxide conversion amount (RE ₂ O ₃ ) and the excess oxygen conversion amount of silicon oxide (SiO ₂ ). Silicon nitride sintered body. 3. A total of 3 to 30 mol% of silicon nitride 70 to 97 mol%, a group 3a element oxide of the periodic table and silicon oxide, and a silicon oxide of the group 3a element oxide of the periodic table. After firing a molded product having a composition with a molar ratio of 2 or more to a non-oxidizing atmosphere, the softening temperature Tg of the glass formed at the grain boundaries of the sintered product and y-type RE ₂ Si from the glass.
After holding once during the crystallization temperature Tc for ₂ O ₇ crystal,
From the crystallization temperature Tc and y-type RE ₂ Si ₂ O ₇ crystal, β
A method for producing a silicon nitride-based sintered body, which is maintained at a transition temperature Tt at which a type RE ₂ Si ₂ O ₇ crystal is transformed.