JPH04154666A - Silicon nitride sintered compact and its production - Google Patents

Abstract

PURPOSE:To obtain the title sintered compact improved in high-temperature strength and oxidation resistance by heat treatment of a specific Si3N4 sintered compact in a non-oxidative atmosphere containing SiO to effect depositing silicon oxynitride crystal phase on said compact's surface. CONSTITUTION:A mixture comprising (A) 85-99mol% of Si3N4, (B) 0.5-5mol% of an oxide (RE2O3) of group III a elements and (C) <=10mol%, in terms of SiO2, of excess oxygen with the molar ratio SiO2/RE2O3 of <=2 is formed and then calcined in a non-oxidative atmosphere at 1600-2000 deg.C into a Si3N4 sintered compact with the ratio of density to its theoretical density of >=95%. Hence, this sintered compact is heat-treated at 1300-1900 deg.C in a non-oxidative atmosphere containing SiO to effect depositing SiO2 or silicate glass on the surface layer of the sintered compact and also depositing silicon oxynitride crystal phase on the surface layer 1-100mum thick.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a silicon nitride sintered body with excellent high-temperature properties useful as a structural component of a heat engine such as a gas turbine, and a method for manufacturing the same.

(Prior art) Silicon nitride sintered bodies have long attracted attention as materials with excellent strength, hardness, and thermal and chemical stability at high temperatures, and their application as engineering ceramics, particularly as structural materials for heat engines, has been progressing. There is.

Generally, silicon nitride itself is difficult to sinter, so it is necessary to add oxides of group IIIa elements of the periodic table such as Y2O5 and Ah03 as sintering aids. In addition, when producing such a sintered body, after adding and mixing a predetermined amount of the above-mentioned sintering aid powder to silicon nitride powder, this mixture is molded by a known molding method to form a non-oxidizing body at 1500 to 2000°C. A high-density sintered body is obtained by firing in an atmosphere.

From the viewpoint of characteristics, such a silicon nitride sintered body is desired to have a uniform structure or composition as a whole.

In addition, silicon nitride sintered bodies, especially when used as structural materials for heat engines such as turbo rotors and gas turbine rotors,
It is required to have high bending strength at high temperatures, excellent oxidation resistance, and little deterioration in strength from room temperature to high temperature.

Therefore, an oxide of a Group IIIa element in the periodic table (REzOs) and silicon oxide (SiOz) are added to silicon nitride (SisNn), which has excellent compositional properties at high temperatures. StJ, -REzO, -
A glass having a high melting point of 5 tO□ and a sintered body in which a crystalline phase is precipitated have been proposed in JP-A-55-3397.

In addition, the present applicant previously proposed in Japanese Patent Application No. 124663/1982 that high-temperature strength can be increased by reducing the molar ratio of Sing/REz (h to 2 or less in the above ternary system, and also In Ganhei 02-61781, Si
It was proposed that the oxidation resistance at high temperatures could be greatly improved by making the molar ratio of O□/REz (h larger than 2).

(Problems to be Solved by the Invention) However, although these sintered bodies certainly have excellent properties in either high-temperature strength or oxidation resistance, they are insufficient in the other property. There is also a problem that the characteristics are unstable.

In addition, the above properties, especially the bending strength, are the strengths when the surface of the sintered body is mirror-polished. For example, when creating a sintered body with a complicated shape, it is necessary to completely polish the entire surface. If this is not possible, there is a problem that the properties will be greatly deteriorated due to roughness on the surface of the sintered body, and
Even when the surface of a sintered body is ground to improve dimensional accuracy, there is a problem in that the processing scratches become a source of destruction and reduce the strength.

For example, for roughness and processing scratches on the surface of the sintered body,
A known method is to heat-treat a sintered body in an oxidizing atmosphere such as the air to oxidize the silicon nitride on the surface, thereby producing a film made of silicon oxide to heal scratches and roughness on the surface of the sintered body. . However, this method has the effect of annealing the processing scratches on the machined surface of a sintered body whose surface has been processed, and can improve the internal strength of the sintered body. For sintered bodies that cannot be processed in any way after firing, the strength of the fired surface cannot be recovered, and the oxidation resistance of the sintered body itself depends on the type of sintering aid and sintering conditions. Even if the same oxidation treatment is performed, variations in strength are likely to occur, and depending on the oxidation treatment conditions, the surface may become rough.

(Means for Solving the Problems) As a result of considering the above-mentioned problems, the present inventor has determined that a sintered body having a specific composition is heat-treated in a nitrogen atmosphere containing SiO. SiO□ from the atmosphere on the surface of the sintered body
By precipitating the crystal phase represented by Si and N20 on the surface layer, the surface of the sintered body can be homogenized without variation, and the sintered body has excellent high-temperature strength and oxidation resistance. We found that it is possible to obtain

That is, the silicon nitride sintered body of the present invention has a silicon nitride content of 85
~99 mol%, and oxides of group 1 elements of the periodic table (R
EzOs) from 0.5 to 5 mol% and excess oxygen.

The equivalent amount is 10 mol% or less, and SiO2/RE2O
In a silicon nitride sintered body having a molar ratio represented by 3 of 2 or less, a crystal phase represented by Si, N, and 0 is contained only in the surface layer part of 1 to 100 μm from the surface of the sintered body. In addition, the manufacturing method includes 85 to 99 mol% of silicon nitride and 0.5 to 5 mol% of the oxide of Group Ia element of the periodic table (REzO:+). , the amount of excess oxygen converted to SiO□ is 10 mol% or less, and SiO□
A silicon nitride sintered body having a molar ratio of /RE, O, of 2 or less containing SiO 1300 to 190
Heat treatment is performed in a non-oxidizing atmosphere at 0°C to precipitate silicon oxide or silicate glass on the surface layer of the sintered body, and Si, N,
It is characterized in that a crystal phase represented by O is precipitated.

The present invention will be explained in detail below.

The silicon nitride sintered body of the present invention is composed mainly of silicon nitride, and contains at least 111a of the periodic table as a sintering aid.
It contains group elements. In addition, impurity oxygen, which is inevitably mixed in from the silicon nitride raw material, also exists in the sintered body. The overall composition of the sintered body including these is silicon nitride of 8
5 to 99 mol%, Group IIIa element (RE) of the periodic table is 0.5 to 5 mol% in terms of oxide (REzOs), excess oxygen is 10 mol% or less in terms of SiO□, and Si
It is important that the molar ratio expressed by O□/REzOx is 2 or less.

Here, excess oxygen refers to the amount of oxygen remaining after subtracting oxygen mixed as a sintering aid such as oxides of Group A elements of the periodic table (RE2O3) from the total amount of oxygen per volume in the sintered body. It is.

The composition of the sintered body is limited as above when the silicon nitride content is less than 85 mol%, or the oxide of Group IIIa element of the periodic table exceeds 5 mol%, or the excess oxygen exceeds 10 mol%. In both cases, the high-temperature strength decreases, and conversely, silicon nitride exceeds 99 mol%, or
If the content of the group element oxide is less than 0.5 mol %, a dense sintered body cannot be obtained, and the properties are significantly deteriorated. Furthermore, if the molar ratio of 5it)□/RE2O3 in the overall composition of the sintered body exceeds 2, the high temperature strength will decrease.

Normally, a sintered body having the above composition is structurally composed of main crystal grains made of silicon nitride and a grain boundary phase existing between the main crystal grains. It contains a Group IIIa element oxide and silicon oxide, and these exist as a glass phase, a crystalline phase, or a mixed phase thereof, but in order to improve high-temperature properties, it is desirable that the grain boundaries be crystallized.

Therefore, the silicon nitride sintered body of the present invention is characterized in that a silicon oxynitride crystal phase represented by 5izN,O is precipitated only in the surface layer.

This silicon oxynitride crystal phase represented by 5i2NzO is a very stable crystal phase that does not oxidize even at high temperatures exceeding 1000°C, so it is possible that it exists in the surface layer of the sintered body. This makes it possible to greatly improve the oxidation resistance of the sintered body in a high temperature range.

Furthermore, it is important that the precipitated region of this Si, N, 0 crystal phase is only in the surface layer.
m, particularly preferably 5 to 50 μm.

This is because if the thickness of the surface layer is thinner than 1 μm, there is no effect of improving oxidation resistance in the high temperature range, and if the thickness exceeds 100 μm, the amount of oxygen in the sintered body itself becomes large and the strength in the high temperature range deteriorates. This is to do so.

In order to control the crystal phase precipitated in the sintered body as described above, the amount of excess oxygen in the surface layer in terms of SiO□ and the SiO2/
It is most preferable to control the molar ratio expressed by RE2O3. That is, 5 lots/RE! When the 03 molar ratio is 2 or less, RE4SI zOJz (Y A M ) phase and S
Although crystal phases such as apatite phase represented by i, N4・RE, and 03 (melilite) phases are likely to precipitate, SiO□
When the /REz03 molar ratio exceeds 2, 5tzN, 0 crystal phases are mainly precipitated, and in addition, a disilicate crystal phase represented by REzSizOl is precipitated.

Therefore, according to the sintered body of the present invention, the surface layer of SiO□
It is desirable to control the molar ratio of /RE2O3 to exceed 2.

In order to manufacture such a sintered body, first, silicon nitride, an oxide of a group IIIa element of the periodic table, and silicon oxide are made of the above-mentioned composition, and the composition is substantially homogeneous inside and outside, and the theoretical density ratio is 95%. A high-density sintered body having the above density is obtained.

As a method for obtaining such a sintered body, silicon nitride powder, an oxide powder of an element of group I [[[[A] of the periodic table], and optionally a silicon oxide powder are used, and the silicon nitride powder is 85 to 99 mol%, Table Group IIIa element oxides (RE.

0. ) powder is 0.5 to 5 mol%, and silicon oxide (SiO□
) the powder consists of 10 mol% or less, and 5iO1/RE
They are weighed so that the molar ratio represented by z03 is 2 or less. Note that the silicon nitride powder used here is α
Either type or β type can be used, and the average particle size is preferably 0.1 to 1.0 μm, and the amount of impurity oxygen unavoidably contained is 0.5 to 3.0 μm.
Preferably, it is % by weight. At this time, it is necessary to convert the impurity oxygen into silicon oxide and adjust the amount of silicon oxide powder added so that it falls within the above range.

Next, the mixture having the above composition is molded into a predetermined shape by a known molding method such as press molding, extrusion molding, injection molding, cast molding, or cold isostatic pressing, and then fired.

Firing is performed in a non-oxidizing atmosphere at 1,600 to 2,000°C.
This can be carried out by a firing method such as normal pressure firing, hot press firing, nitrogen gas pressure firing, or hot isostatic pressure firing.

In addition, as another method for obtaining a high-density sintered body, powders of oxides of group IIIa elements of the periodic table and silicon oxide, and if desired, silicon nitride powder are added to metal silicon, and then after molding, This is treated in a nitrogen atmosphere to nitride the metal silicon, and then 1. A high-density sintered body is obtained by firing in the same manner as the firing method described above.

Next, this sintered body is heat-treated at a temperature of about 1300 to 1900° C. in a non-oxidizing atmosphere such as nitrogen or argon containing SiO. In such an atmosphere, for example, when a mixture of metal Si and silicon oxide is placed in a heating furnace at a weight ratio of 0.1 to 100, both metal Si and silicon oxide decompose and react at high temperatures, forming SiO. Gas is generated.

According to this heat treatment, the generated SiO gas condenses on the surface of the sintered body at a high temperature or during the cooling process from a high temperature, and precipitates as SiO□ on the surface of the sintered body.

At the same time, the periodic table 1 [1a
A silicate glass layer is formed on the surface of the sintered body while partially reacting with the group element oxide.

This causes a compositional change in the surface layer portion of the sintered body and the inside of the sintered body. Specifically, among the silicon, nitrogen, oxygen, and group IIIa elements of the periodic table that constitute the sintered body, silicon and oxygen are present more in the surface layer than in the interior, and nitrogen and
Group IIa elements tend to decrease.

According to the present invention, the heat treatment time, temperature, and SiO concentration in the atmosphere are controlled so that the precipitation of SiO□ at this time is within 1=lOOl1m of the surface layer of the final sintered body, and the sintered body is The molar ratio of Sing/REz03 in the surface layer of the sample is controlled to exceed 2.

According to this treatment, the surface layer of the sintered body is gradually cooled after the treatment or heat treated in a non-oxidizing atmosphere at 1300 to 1900°C to crystallize the grain boundaries of the sintered body. A silicon oxynitride crystal phase represented by Si, N, and 0 is precipitated in this area. Furthermore, the aforementioned YAM phase or crystal phases such as apatite, wollastonite, and apatite are precipitated inside the sintered body.

Incidentally, the oxides of Group IV a elements of the periodic table used in the present invention include Y2O3, yb, o, Er, 03
Examples include Dyto, HozDz 5ctOz, etc. Considering the stability of the properties of the sintered body, Ybz03,
Erz03 is preferred.

Further, according to the present invention, other additives can be added within a range that does not adversely affect the effects of the present invention described above.

Specifically, A1□03, -03, NbzO5, Cr2
Examples include O3, Mo0i, Zr0z, and SrO, but especially A1□03 inhibits crystallization of grain boundaries, so it is desirable to suppress it to 1% by weight or less.

(Function) According to the present invention, the surface layer contains more silicon and oxygen than the interior, and also contains Si and N20 crystal phases that have excellent chemical stability at high temperatures. Deterioration in a harsh atmosphere can be prevented. Also,
High-temperature strength can be increased by controlling the amount of oxygen and silicon in the interior. By combining these two types of sintered bodies with different properties and providing the above-mentioned surface layer with a specific thickness, the sintered body has excellent oxidation resistance in a high-temperature oxidizing atmosphere and high high-temperature strength. The body can provide.

In addition, when a sintered body is heat-treated in an SiO-containing atmosphere during the manufacturing process, even if the sintered body is a ground product or an unburned product, SiO□ precipitates on the surface, causing the surface layer to become re-fired. As a result, the surface layer portion is modified and homogenized, thereby eliminating the decrease in strength of the sintered body due to roughness of the surface of the sintered body, and the original strength of the sintered body can be exhibited.

The invention will now be explained with the following examples.

(Example) Silicon nitride powder (gelatinization rate 90%, impurity oxygen content 2.0% by weight, average particle size 0.5 μm), oxide powder of group IIIa element of the periodic table, and silicon oxide powder are shown in Table 1. After weighing and mixing in the proportions shown, press molding into 4 x 5
After molding into a shape of x50mm, 1950℃ nitrogen gas pressure 9
It was fired in an ATM atmosphere.

After heat treating each sample under the conditions shown in Table 1,
A heat treatment was performed for 2 hours in a nitrogen gas atmosphere at 1400° C. to crystallize grain boundaries. Note that when heat treatment was performed in a nitrogen atmosphere containing SiO, a mixed powder of metal Si and SiO□ was placed in the furnace.

The flexural strength of the treated sintered body was measured at room temperature and 1400°C in accordance with JISR1601, and further 1
The weight increase per unit area after being held in an oxidizing atmosphere at 400° C. for 24 hours was measured.

In addition, we compared the compositional changes in the amount of silicon and oxygen between the surface layer and the inside of each sintered body by EPMA analysis, and also determined the thickness of the surface layer where the amount of oxygen and silicon is higher than the inside. Examined. Furthermore, the above surface layer was quantitatively analyzed by EPMA analysis, and the molar ratio of the oxide of Group IIIa element of the periodic table (uEzoi) and silicon oxide (SiO□) was determined (
Sift/REvOs) was calculated.

In addition, X-ray diffraction measurements were performed on the surface layer and the center of the sintered body, and the existing crystal phases were identified.

These results are shown in Table 1.

(Left below) According to the results in Table 1, SiO
Sample N11l, which is a conventional sintered body whose molar ratio is 2 or less and whose surface layer is also 2 or less, has precipitated YAM crystal phases and has excellent high-temperature strength; The oxidation resistance is insufficient at 0.2g/c++''.In addition, in sample N112 where the SiO2/RE2O3 molar ratio exceeds 2 in both the internal and surface parts, the crystalline phase is 5izN, O crystalline phase inside and outside. It was precipitated and had excellent oxidation resistance at high temperatures, but its strength at high temperatures was 30 kg/lll.
1z is low.

In addition, sample N in which the amount of added oxide of group IIIa element of the periodic table exceeds 5 mol % (Li2 also has low high temperature strength, and sample NcL14 in which the amount of silicon oxide exceeds 10 mol % also has characteristics) It was bad.

On the other hand, in accordance with the present invention, when a sintered body having a predetermined composition ratio is treated in a non-oxidizing atmosphere containing SiO, the amount of oxygen and silicon in the surface layer is lower than that in the inside. Si, N,
An O crystal phase was detected, and an internal YAM phase was detected in most of the samples.

In addition, in terms of characteristics, high temperature oxidation resistance is 0.1+ag/c+a" or less between samples, the same level as 2, and high temperature strength is 1 between samples.
A sintered body with excellent properties of 70 kg/am2 or more, which is the same level as that of the previous example, was obtained.

However, the processing time in the SiO atmosphere is long and the thickness of the surface layer exceeds 100 ttra for sample N1.
No. 115 had excellent high-temperature oxidation resistance, but a decrease in strength was observed. Furthermore, even in the presence of SiO in nitrogen, the surface of the sintered body decomposed in Sample No. 7, where the treatment temperature was high, and the high-temperature strength was significantly reduced.

Furthermore, in terms of composition, the surface layer of Sample 16, which was simply treated in the atmosphere, had a higher amount of oxygen than the inside, and a disilicate phase had precipitated, resulting in poor characteristics. .

(Effects of the Invention) As detailed above, according to the present invention, the surface of the sintered body is made of a material with excellent oxidation resistance, and the inside is made of a material with excellent high-temperature strength. A sintered body with excellent oxidation resistance and strength can be provided.

Further, according to the manufacturing method of the present invention, it is possible to substantially modify the deteriorated surface after baking or grinding, thereby recovering the strength, and improving oxidation resistance and A sintered body with high strength can be provided.

Therefore, it is especially effective when applied to complex-shaped products such as turbo rotors and gas turbines, and various other structural materials that require high-temperature properties.

Claims (3)

[Claims] (1) Silicon nitride is 85 to 99 mol% and is found in III of the periodic table.
Group a element oxide (RE_2O_3) is 0.5 to 5 mol%
In a silicon nitride sintered body in which the SiO_2 equivalent amount of excess oxygen is 10 mol% or less and the molar ratio expressed by SiO_2/RE_2O_3 is 2 or less, a thickness of 1 A silicon nitride sintered body characterized in that a silicon oxynitride crystal phase exists only in the surface layer of ~100 μm. (2) The silicon nitride sintered body according to claim 1, wherein a YAM, apatite, wollastonite, or melilite crystal phase is present inside the sintered body. (3) Silicon nitride is 85 to 99 mol%, and it is found in III of the periodic table.
Group a element oxide (RE_2O_3) is 0.5 to 5 mol%
Then, a silicon nitride sintered body with an excess oxygen equivalent to SiO_2 of 10 mol% or less and a molar ratio of SiO_2/RE_2O_3 of 2 or less is heated to a non-oxidized state at 1300 to 1900°C containing SiO. heat treatment in a neutral atmosphere to precipitate silicon oxide or silicate glass on the surface layer of the sintered body, and precipitate a silicon oxynitride crystal phase only in the surface layer with a thickness of 1 to 100 μm from the surface of the sintered body. A method for producing a silicon nitride sintered body, characterized by: