JPH0971471A - Silicon nitride sintered compact and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a silicon nitride sintered compact excellent in oxidation resistance at high temperatures. SOLUTION: This silicon nitride sintered compact comprises a sintering assistant which is distributed in a surface layer part to 500μm depth from the surface of the silicon nitride sintered compact at a concentration of <=10% maximum value of that of an additive distributed in a part at >=500μm depth from the surface thereof. The sintered compact is formed from a silicon nitride powder prepared by blending the sintering assistant therein, covering the silicon nitride sintered compact with an inorganic oxide powder which is a solid at <=1,700 deg.C and capable of forming a compound with the sintering assistant and heating the covered silicon nitride sintered compact at 1,300-1,700 deg.C temperature for >=0.5hr. Thereby, the concentration of the sintering assistant in the surface layer part of the silicon nitride sintered compact is reduced and the oxidation resistance at high temperatures is improved.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a ceramic material,
In particular, the present invention relates to a silicon nitride sintered body which has excellent mechanical properties such as strength and toughness and also has oxidation resistance.

[0002]

2. Description of the Related Art Conventionally, ceramic materials have been widely used in various fields in which strength, insulation, heat resistance, etc. are required, and have been improved and specialized as the demand for materials has become higher. There is.

Among various ceramic materials, a silicon nitride material has high strength and toughness, excellent mechanical properties, and also has heat resistance, so that it is widely applied as a mechanical component. However, the heat resistance of the silicon nitride material is such that it can withstand temperatures up to about 1000 ° C., and if it is used under high temperature conditions of 1200 ° C. or higher such as gas turbine parts, the oxidation resistance and corrosion resistance are reduced. Therefore, it is insufficient to be applied to such a field.

In order to enable the use at high temperatures as described above, Japanese Patent Laid-Open No. 61-55301 discloses that a sialon (Si-Al-ON) layer is formed on the surface of a silicon nitride material. Proposed. Sialon is formed by sintering a mixed powder of silicon nitride powder and alumina powder, and alumina forms a solid solution with silicon nitride in sialon, so that it hardly forms a grain boundary phase. As a result, the oxidation resistance of the silicon nitride material provided with the sialon layer on the surface is maintained up to about 1500 ° C. However, if the oxidation resistance is improved by such a method, the strength of the material as a whole is lowered.

As a method for improving the strength of a silicon nitride sintered body at high temperature, Japanese Patent Laid-Open No. 6-100387 discloses a method.
It has been proposed to remove the surface layer of the sintered body after heat-treating the silicon nitride sintered body containing the sintering aid in an oxygen-containing atmosphere. However, since this method has a step of removing the surface layer of the sintered body, it is difficult to apply it to a sintered body having a complicated shape, and the oxidation resistance at high temperature is insufficient.

[0006]

As described above, the conventional silicon nitride material has a problem that it is not possible to obtain a material having strength and sufficient oxidation resistance at high temperature.

The present invention has been made in order to solve the problems of the prior art as described above, and retains the oxidation resistance at high temperature, strength and toughness, and is applicable to products of any shape. It is intended to provide a material and a manufacturing method thereof.

[0008]

In order to achieve the above object, the inventors of the present invention have conducted extensive studies and found that good results can be obtained by covering a silicon nitride sintered body with a coating powder and performing heat treatment. The present invention has led to the invention of the silicon nitride sintered body of the present invention and a method of manufacturing the same.

The silicon nitride sintered body of the present invention is a yttria,
A silicon nitride sintered body containing a sintering aid selected from alumina, spinel, titania, zirconia, and hafnia, wherein the depth from the surface of the silicon nitride sintered body is 500 μm.
The concentration of the sintering aid distributed up to the surface layer portion is 10% or less of the maximum value of the concentration of the additive distributed in the portion where the depth from the surface is 500 μm or more.

Further, the method for producing a silicon nitride sintered body of the present invention comprises a step of forming a silicon nitride sintered body from a silicon nitride powder containing a sintering aid, and a step of forming the silicon nitride sintered body at 1700 ° C. The silicon nitride sintered body is covered with an inorganic oxide powder capable of forming a compound with a binder, and heated at a temperature of 1300 to 1700 ° C. for 0.5 hours or more, and the concentration of the sintering agent in the surface layer of the silicon nitride sintered body is increased. And the step of reducing.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail below.

Since silicon nitride has poor sinterability, a dense body cannot be obtained when a sintered body is manufactured from silicon nitride alone.
An oxide such as alumina or yttria is added to the silicon nitride powder as a sintering aid to manufacture a silicon nitride sintered body. Such a sintering aid exists as a second component in the grain boundaries of granular silicon nitride unless a specific solid solution is formed.

However, the sintering aid required for improving the sinterability, on the other hand, reduces the oxidation resistance of the silicon nitride sintered body. However, it is not all of the sintering aids that causes the oxidation resistance to decrease, but the sintering aids that are in the grain boundary phase near the surface of the sintered body. The oxidation or corrosion of silicon nitride due to the invading oxygen is promoted. Therefore, if the amount of the sintering aid in the surface layer portion of the sintered body that is easily penetrated by outside air is small, the oxidation resistance of the sintered body at high temperature can be improved. That is, in order for the silicon nitride sintered body to be dense and have excellent heat resistance at high temperatures,
It is important to reduce the concentration of the sintering aid to about 1% or less in the surface layer portion of the sintered body, particularly in the region from the surface to a depth of 500 μm.

When a silicon nitride sintered body is manufactured by a usual method, the sintering aid is distributed substantially uniformly throughout the sintered body.
When such a sintered body is heated, particularly when heated to a temperature of 1300 ° C. or higher, the sintering aid of the grain boundary phase near the surface of the sintered body tends to move and diffuse toward the outside of the sintered body. . The degree of this movement depends on the heating temperature and the type of sintering aid, and among them, the amount of yttria movement is larger than that of other sintering aids. However, the moving amount of the sintering aid due to heating is not so large as to reduce the concentration of the sintering aid to the above-mentioned level, and furthermore, when the heat treatment is performed in an atmosphere containing oxygen, the amount of Oxygen penetrates into the sintered body and oxidizes the silicon nitride matrix, causing embrittlement.

On the other hand, when the surface of the sintered body is covered with a powder of alumina or the like and subjected to heat treatment, the progress of oxidation due to the infiltration of oxygen into the sintered body can be reduced. If a powder of a specific type is used for coating depending on the type of the agent, the concentration of the sintering aid in the surface layer portion of the sintered body can be significantly reduced. This is because the powder covering the surface of the sintered body and the sintering aid form a compound, whereby the movement of the sintering aid is promoted. Further, the movement of the sintering aid to the outside of the sintered body is also promoted by the physical absorption between the particles of the alumina powder, and if the coating powder is removed after the heat treatment, the sintering assistant that has moved to the outside of the sintered body is removed. Does not remain on the surface, and a sintered body having a low concentration of the sintering aid in the surface layer can be easily obtained. or,
Cooling after the heat treatment prevents the sintering aid from returning to the inside of the sintered body.

Therefore, the raw material powder in which the sintering aid is mixed with the silicon nitride powder is compacted into a desired shape, sintered, and then the sintered body is embedded in the coating powder and heat-treated to obtain the surface. A silicon nitride sintered body having a low concentration of the sintering aid can be obtained. Preparation of the raw material powder, compaction molding and sintering may be performed according to a conventional method. For example, the raw material powder is prepared by mixing the sintering aid and the silicon nitride powder in a ball mill. As the sintering aid used,
IIA group element oxides such as yttria, magnesia, spinel, alumina, zirconia, titania, hafnia, II
Group IA element oxides Group VA element oxides and group IIIB element oxides can be used, among which yttria is preferable, when a combination of yttria and alumina is used as a sintering aid, and alumina is used as the coating powder. 90% or more of yttria in the surface layer of the sintered body easily moves to the outside.
In this case, yttria with alumina 0.5: 1 to 5: 1
Particularly preferable results can be obtained by mixing them at a ratio of and used as a sintering aid.

The amount of the sintering aid compounded into the raw material powder is appropriately determined as needed, but generally it is used in an amount of about 3 to 10% by weight based on the total amount of the raw material powder. It is desirable to use a silicon nitride powder having a particle size of about 0.1 to 5.0 μm. The particle size of the sintering aid is preferably about 0.05 to 1.0 μm. A powder having a too small particle size is difficult to handle, and when a powder having an excessively large particle size is used, the surface of the sintered body after the sintering aid in the surface layer portion is reduced becomes rough and brittle.

In the compacting, the raw material powder is put into a mold and compacted by cold pressing or the like to obtain a compact.
A sintered compact is obtained by heating the obtained compact to 1600 to 1800 ° C. and sintering. More preferably,
The compact is pressure-sintered under a pressure of about 20 to 50 MPa. Sintering is performed in a non-oxidizing atmosphere such as nitrogen gas.

The obtained sintered body is embedded in the coating powder and heat-treated. As the coating powder, a powder of an inorganic oxide that does not melt at the heat treatment temperature of the sintered body such as alumina, yttria, magnesia, and titania is used, and among these, it is preferable to use a powder that forms a compound with a sintering aid. . Examples of combinations of the coating powder and the sintering aid that form such a compound include yttria and alumina, alumina and magnesia, titania and zirconia, and the like. Therefore, for example, when yttria or magnesia is used as a sintering aid, it is suitable to use alumina as the coating powder.

The longer the heating time of the sintered body and the higher the heating temperature, the more the amount of the sintering aid moved, resulting in 1300 ° C.
By heating at the above temperature for 0.5 hour or more, preferably 1 hour or more, a sintered body having sufficiently improved oxidation resistance at high temperature can be obtained. However, if the heating temperature is too high, the coating powder adheres to the sintered body and cannot be removed. For example, when the alumina powder becomes 1800 ° C. or higher, it adheres to the sintered body and cannot be removed. Therefore, the heating temperature is preferably 1300 to
The temperature is set to 1700 ° C, more preferably 1500 ° C. It is preferable to use a coating powder having a particle size of 0.1 to 3.0 μm. If the particle size is small, it is difficult to handle, and if the particle size is too large, it cannot be brought into close contact with the sintered body. Since the sintered body is surrounded by the coating powder and is not exposed to the atmosphere, it is preferable to use a non-oxidizing atmosphere such as nitrogen gas in order to surely prevent the sintered body from being oxidized from the oxidizing atmosphere. Preferably, the sintered body is embedded in the coating powder in a reduced pressure non-oxidizing atmosphere and heat treatment is performed. When the heat treatment is performed in an oxidizing atmosphere, the oxidation of silicon nitride in the surface layer of the sintered body progresses before the sintering aid moves to the outside of the sintered body, resulting in embrittlement. Further, the amount of the sintering aid that has moved is small, and the sintering aid that has moved out of the sintered body forms a glassy film on the surface of the sintered body.

The silicon nitride sintered body after the heat treatment is taken out from the coating powder, and a clean silicon nitride sintered body can be obtained after the wiping operation of the coating powder. If necessary, it may be further processed into a desired shape.

By the heat treatment as described above, the concentration of the sintering aid in the surface layer portion from the surface of the silicon nitride sintered body to a depth of 500 μm can be reduced to 10% or less of the internal concentration, The concentration of the sintering aid in the surface layer portion can be reduced to 1% or less by the heat treatment of the silicon nitride sintered body containing the normally used amount of the sintering aid. Such a sintered body has sufficient oxidation resistance even under heating at about 1500 ° C. and is extremely dense.

[0023]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples.

Example 1 Yttria powder (average particle size 1.0 μm) 5% by weight, alumina powder (average particle size 0.0
5 μm) 2% by weight and silicon nitride powder (average particle size 0.5 μ
m) A 93% by weight mixture was well stirred using a ball mill to prepare a raw material powder.

This raw material powder was filled in a carbon mold in a reduced pressure nitrogen atmosphere of 0.1 MPa, and a compact was formed by cold pressing. Further, the molded body in the carbon mold was pressure-sintered for 1 hour by hot pressing at 1800 ° C. and a pressing pressure of 30 MPa to obtain a silicon nitride sintered body.

On the other hand, in a reduced pressure nitrogen atmosphere of 0.1 MPa, alumina powder (average particle size 0.5 μm) was placed in a carbon crucible.
m) was filled, and the sintered body obtained above was embedded in this alumina powder and heated at 1500 ° C. for 1 hour. The sintered body after heating is taken out from the alumina powder, 4 mm x 3 mm x 40
Three mm test pieces were cut. Using one of the test pieces, a three-point bending test is performed according to JIS 1601,
The strength at room temperature was determined. Also, another test piece 1300
A three-point bending test was performed in the atmosphere of ° C to determine the strength at high temperature. The third test piece was subjected to an oxidation resistance test in which it was exposed to the atmosphere of 1500 ° C. for 1000 hours, and the weight change due to the oxidation of the test piece was measured. Table 1 shows the results.

Further, the yttria distribution state of the cross section of the obtained silicon nitride sintered body was measured by EPMA. The relationship between the depth from the surface of the sintered body and the yttria concentration obtained from the result is shown by the line E1 in FIG. In FIG. 1, the vertical axis represents the ratio of the yttrium concentration at each depth to the maximum yttrium concentration in the sintered body (yttrium concentration ratio), and the horizontal axis represents the depth from the surface of the sintered body.

(Examples 2 to 7, Comparative Example 1) The heat treatment temperature and heat treatment time of the sintered body were changed as shown in Table 1,
The same operation as in Example 1 was repeated to obtain a test piece by subjecting the silicon nitride sintered body to heat treatment, and the strength of the test piece, the weight change by the oxidation resistance test, and the yttria distribution state of the cross section of the sintered body were measured. These results are also shown in Table 1 and line E of FIG.
2 to 6 (Examples 2 to 6) and C1 (Comparative Example 1).
In Example 7, the alumina powder adhering to the surface of the sintered body after the heat treatment could not be removed, and the measurement could not be performed.

(Comparative Example 2) A silicon nitride sintered body was prepared in the same manner as in Example 1 and was not embedded in alumina powder,
Heated at 500 ° C. for 1 hour. From the sintered body after heating, three test pieces were cut and processed in the same manner as in Example 1, and the strength at normal temperature and high temperature and the weight change due to oxidation were measured. Moreover, the yttria distribution state of the cross section of the sintered body was measured by EPMA.

[0030]

[Table 1] ------------------ Heat treatment Sintered body after heat treatment Heating time The heating temperature bending strength (MPa) weight by oxidation (hr) (℃) ambient temperature hot change (mg / cm ²⁾ ------------------------- ---------- Example 1 1 1500 1140 630 0.8 Example 2 0.5 0.5 1500 1090 520 2.5 Example 3 0.3 1500 1500 100 340 7.8 Example 4 1 1200 1120 330 9.2 Example 5 1 1300 1100 580 1.8 Example 6 1 1700 1080 610 1.3 Example 7 1 1800 --- Comparative Example 1-1120 310 10.7 Comparative Example 2 1 1500 1070 430 6 . 9 −−−−−−−−−−−−−−−−−−− As is clear from FIG. 1, the yttrium concentration in the surface layer of the sintered body is reduced by subjecting the silicon nitride sintered body to the heat treatment. . That is, the sintering aid is moved by the heat treatment. Further, it is clear that the lower the yttrium concentration in the surface layer portion in FIG. 1, the smaller the change in weight of the sintered sample piece due to oxidation and the better the oxidation resistance. The surface was brittle and easily broken.

From the results of EPMA analysis, it is clear that the heat treatment of Comparative Example 2 does not sufficiently reduce the concentration of the sintering aid. Further, the weight change due to oxidation also shows a large value that is hardly said to be sufficient in oxidation resistance. The reason why the weight change is smaller than that in Comparative Example 1 is that oxidation is progressing during the heat treatment.

(Example 8) As raw material powder, yttria (average particle size 1.0 μm) 5% by weight, magnesia 5% by weight
And a silicon nitride powder (average particle size 1.0 μm) of 90% by weight were used to manufacture a silicon nitride sintered body in the same manner as in Example 1, and heat treatment was performed to cut out a test piece, The strength change, the weight change due to the oxidation resistance test, and the magnesium distribution state on the cross section of the sintered body were measured. As a result, the three-point bending strength was 950 MPa at room temperature and 490 at 1300 ° C.
MPa, weight change due to oxidation was 1.7 mg / cm ² . Further, the concentration of magnesium in the surface layer portion within a depth of 500 μm from the surface of the sintered body was 0.9% of the concentration inside the depth of 500 μm or more.

As described above, the heat treatment in the coating powder can reduce the concentration of the sintering aid in the surface layer portion of the sintered body, whereby the oxidation resistance of the silicon nitride sintered body at high temperature can be reduced. Is clearly improved.

[0034]

As described above, the silicon nitride sintered body of the present invention has excellent oxidation resistance at high temperatures,
It is of high quality. Further, the method for producing a silicon nitride sintered body of the present invention is simple and can be applied to a wide range of applications, and its industrial value is extremely large.

[Brief description of drawings]

FIG. 1 is a graph showing a concentration distribution of yttrium in a silicon nitride sintered body.

[Explanation of symbols]

 E1 Example 1 E2 Example 2 E3 Example 3 E4 Example 4 E5 Example 5 E6 Example 6 C1 Comparative Example 1 C2 Comparative Example 2

Claims (2)

[Claims] 1. A silicon nitride sintered body containing a sintering aid selected from yttria, alumina, spinel, titania, zirconia and hafnia, wherein the depth from the surface of the silicon nitride sintered body is up to 500 μm. The concentration of the sintering aid distributed in the surface layer part of is not more than 10% of the maximum value of the concentration of the additive distributed in the part where the depth from the surface is 500 μm or more. Union. 2. A step of forming a silicon nitride sintered body from a silicon nitride powder containing a sintering aid, and an inorganic oxide powder which is solid at 1700 ° C. or lower and is capable of forming a compound with the sintering aid. And covering the silicon nitride sintered body with 1300 to 1700
A temperature of 0.5 ° C. or more for 0.5 hour or more to reduce the concentration of the sintering aid in the surface layer portion of the silicon nitride sintered body, the method for producing a silicon nitride sintered body.