JPH064515B2 - High toughness silicon nitride sintered body and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a high-toughness silicon nitride composite sintered body suitable for use in gas turbine blades, various engine parts, and the like.

[Background of the Invention]

Conventionally, heat-resistant alloys have been used for parts exposed to high temperatures or adverse environments such as gas turbine blades. However, in recent years, the operating temperature of the heat-resistant alloy has reached the critical temperature for use due to higher performance. Therefore, as a material replacing these heat-resistant alloys, attention has been paid to a silicon nitride sintered body having high heat resistance, oxidation resistance and heat shock resistance. However, as is well known, silicon nitride ceramics have the drawback of being brittle, and have not yet been put to practical use in earnest.

As a ceramic toughening material, for example, a cermet for a cutting tool disclosed in Japanese Examined Patent Publication No. 59-26977, in which various carbides and oxides are added to a metal as a bonding layer, is well known. These thermites are ceramics bonded with a metal phase such as Co or Ni. Since the brittleness of the ceramic is covered with the metal phase, the strength and toughness at room temperature are improved, but the heat resistance is poor at high temperatures because the metal phase is contained, and the strength and hardness at high temperatures are reduced. Therefore, active research has been conducted on composite ceramics in which whiskers or fibers of ceramics, which can maintain excellent oxidation resistance and strength even at high temperatures, are dispersed. That is, when cracks occur in the ceramics, the progress thereof is blocked by whiskers or fibers. For example, in JP-A-59-54680, a silicon nitride ceramics sintered body in which Sic whiskers are dispersed and reinforced is reported. However, it is difficult to significantly improve the toughness of the ceramics by only adding the whiskers in a dispersed manner. The reason is that silicon nitride and SiC whiskers do not react much even at high temperatures during sintering, and they exist in the silicon nitride matrix as whiskers in a dispersed state, but the sintered body receives cracks due to external force. Even if it occurs, the diameter of the dispersed whiskers is as thin as 0.5 to 1.0 μm (according to commonly available gas phase method synthesis), and it is considered that the ability to prevent the progress of cracks is small. It is possible to add a large amount of whiskers, but this is not preferable because the sintered density decreases only.

On the other hand, in JP-A-58-41770, various kinds of silicides of 5% by weight or less are added to silicon nitride to improve thermal shock resistance and improve mechanical strength at high temperature, but it is still not sufficient. This is, for example, in X-ray diffraction of a Si ₃ N ₄ sintered body to which CrSi ₂ is added, CrSi, Cr ₃ Si, Cr ₂ N, C
rN was detected and formed from particles of a few different phases as shown in FIG. As a result of cracking and microscopic examination, the crack 1 could not easily progress through the grain boundaries of the grains 2 and 3 and prevent the sintered body from cracking.

[Object of the Invention]

It is an object of the invention to provide a silicon nitride sintered body in which the toughness of a ceramic containing silicon nitride as a main component is significantly improved, and a method for producing the same.

[Outline of Invention]

The present inventors have noticed that silicon nitride and SiC whiskers are difficult to react with each other, but SiC whiskers and metal silicide are easily reacted with each other. Then, it was thought that silicon nitride, SiC whiskers and metal silicide could be firmly integrated.

As a result of conducting the test, it was possible to obtain a silicon nitride-based sintered body having excellent toughness than expected. As a result of a microscopic examination of this sintered body, it was found that it had a structure as shown in FIG.

That is, in the silicon nitride matrix 5, the metal silicide 2 and S
A reaction phase 7 with the iC whiskers 6 is formed. In some cases, the metal silicide 4 may be formed by reacting the metal silicide 2 with silicon nitride. These are formed close to each other. As shown in the figure, the progress of the crack 1 bypasses when it hits the reaction phase 7, and the effects of this reaction phase are as follows: (1) The particles of the metal silicide and the SiC whiskers pass through the reaction phase. Strongly adheres to the base material silicon nitride, which results in a strong bond between the SiC whisker and silicon nitride, and also strengthens the whisker itself, improving toughness, (2) Since the SiC silicide whiskers bind the metal silicide phase through the reaction phase, the strength of the particles that are aggregates of these phases increases and it becomes difficult for the cracks to penetrate grain boundaries. (3) The reaction phase The mechanical strength is high, which may prevent the progress of cracking.

From the above, the reaction phase is particularly preferably a phase having a large mechanical strength, and specifically a solid solution of SiC and a metal carbide is preferable.

Generally, when a crack is generated in a ceramic, the existence of strong particles hinders the progress of the crack, so that the crack bypasses and energy is consumed thereby, which improves the toughness. The effect of the present invention is also close to this theory. As shown in Fig. 1, the crack 1 is the reaction phase 7
When it hits, the energy is absorbed and the crack stops. The SiC whiskers that do not encounter the metal silicide exist in the silicon nitride as they are. The same applies to metal silicides.

In the present invention, the amount of SiC whiskers added is 5 to 25.
Vol% is preferred. Even if it is more than this, sufficient toughness cannot be obtained. Particularly, when it exceeds 25 vol%, the density is drastically reduced, which is not preferable as a sintered body.

The metal silicide of the present invention preferably has a strong affinity with carbon. In this case, a reaction phase composed of a solid solution of SiC in which a part of Si is replaced with a metal and a metal silicide is easily generated between the SiC whiskers and the metal silicide or the metal nitride. These include Ti, Zr, Hf,
There are V, Nb, Ta, Cr, Mo, and W silicides, and Ti, V, and Cr silicides are particularly preferable because they have excellent oxidation resistance at high temperatures.

Zr and Hf silicides easily change to nitrides during sintering,
There is no change in achieving the object of the present invention.

In the present invention, the silicon nitride content is preferably 60 vol% or more.

Further, as a sintering aid, MgO, which is generally used,
It is preferable to add 0.1 to 20 vol% of at least one selected from Al ₂ O ₃ , Sc ₂ O ₃ , Y ₂ O ₃ and rare earth oxides.

Next, a method for manufacturing the high toughness silicon nitride sintered body of the present invention will be described.

Silicon nitride 60vol% or more and SiC whiskers 5 to 25vo
1%, the metal silicide 5 to 25% by volume, the sintering aid 0.
It is obtained by mixing 1 to 20 vol% in a mixer, molding, and hot-pressing in vacuum or in an inert gas at 1600 ° C to 1900 ° C.

Example of Invention

 Examples of the present invention will be described below.

Example 1 A predetermined amount of a sintering aid was added to Si ₃ N ₄ (particle size: 1 to ₃ μm) powder and metal silicide (particle size: 530 μm) powder, and they were sufficiently mixed with a raker machine. There are various sintering aids, but Al
_A suitable range is 0.1 to 20 vol% of ₂ O ₃ and Y ₂ O ₃ . In this case, it has been confirmed that particularly high-temperature strength ceramics can be obtained. In this example, ₂ Al ₂ O ₃ was used.
vol% and Y ₂ O ₃ were 3 vol%, but 0.1 to 20 vol
It has been confirmed that similar results are obtained in the addition range of%.

Since the SiC whiskers are entwined with each other, they were weighed so that the final sintered body would have a predetermined vol%, and ultrasonic waves were applied to disperse the whiskers in ethyl alcohol. The SiC whiskers used in this example have a thickness of 0.3 to 1.0 μm and a length of 20 to 5
It is 0 μm. Then, the mixture powder was gradually added while stirring with a stirrer for uniform dispersion. After that, the mixture is mixed again using a ladle machine, and then the particles are sized by a 16-mesh sieve and pressed to 400 kg / cm ² with a press to a thickness of 6 m.
A grimbody with m ^t and a diameter of 60 mm was produced. This was set in a hot press using a graphite die and sintered in N ₂ gas under a pressure of 300 kg / cm ² at a maximum heating temperature of 1800 ° C.

According to the above method, first, Si ₃ N ₄ is used as a mother phase, Sic whiskers are kept at 10 vol% and TiSi ₂ is 0, 2.5, 5, 5.
Sintered bodies added with 10, 20, 25, 30 and 35 vol% were prepared. A test piece of 3 mm × 4 mm × 36 mm was sampled from this sintered body. After polishing this, 0.01m wide in the center
Make a notch with a depth of 0.3 mm and a depth of 0.3 mm, and
Ge Notched Beam), a test piece was prepared and the fracture toughness value K _1C was determined.

The results are shown in FIG. The Si ₃ N ₄ value of the simple substance is also shown, but K _1C with addition of whiskers is 7.3 MN / m ^3/2
And the increase is small. However, the addition of TiSi ₂ further increases K _1C , especially in the range of 5 to 25 vol%. However, if 30vol% or more is added, K
_1C decreases. This is due to the density of the sintered body. 25 vo
This is because the relative density of the sintered body of 1% or less is 99% or more, whereas the relative density of the sintered body added of 30 vol% or more is reduced to 95% or less. Moreover, since K _1c does not increase so much up to 2.5 vol%, the appropriate amount of TiSi ₂ addition is 5
~ 25 vol%. On the other hand, the 20 vol% added sample was mirror-polished and observed with an optical microscope and a scanning electron microscope.

The added TiSi ₂ particles are Si ₃ N ₄ and N _{2 in the} atmosphere.
It was confirmed that the particles reacted with the gas to form particles in which TiN, Ti ₂ N and a solid solution of Ti and Si were mixed. As described above, in the aggregate of different phases, which is the conventional method in which only the silicide is added, the interface of the different phases makes the progress of cracking a capacity and is not recognized as an improvement in K _1c .
However, as is clear in this example, a reaction phase (Si, Ti) C formed by the reaction between the SiC whiskers and the metal silicide was recognized at a part of the grain interface. This reaction phase is Ti
The particles were firmly adhered to the solid solution of N, Ti ₂ N and Ti, Si, and the particles and Sic whiskers were firmly adhered.

When a crack was generated by a Vickers hardness tester,
It was found that the cracks progressed while curving because their progress was hindered in these reaction phases. Due to this, it is estimated that K _1c has improved.

Example 2 By the same method as in Example 1, the amount of TiSi ₂ was set to 2 in the mother phase Si ₃ N _4.
SiC whiskers at 0, 2.5,
Sintered bodies of 5, 10, 20, 25 and 30 vol% were produced. The result of measuring K _1C by the SENB method in the same manner as in Example 1 is shown in FIG. When the SiC whiskers are 2.5 vol%, the effect by addition is small and the increase of K _1C is small. Again 30 vo
If 1% or more is added, the density decreases and K _1C decreases. From these results, K of the Si ₃ N ₄ —TiSi ₂ sintered body was measured.
₅ to 25 vol% of SiC whiskers is suitable for improving _1C , and the addition of about 10 vol% has the greatest effect.

Comparative Example In the same manner as in Example 2, with the amount of TiSi ₂ in the mother phase Si ₃ N ₄ being constant at 20 vol%, SiC having a particle size of 0.5 to 10 μm was used.
A sintered body was prepared by adding 5 to 25 vol% of particles. The K _1C of these sintered bodies was 7 to 8 MN / m ^3/2, which was smaller than that of the above-mentioned addition of SiC whiskers. This is because the used SiC particles have an aspect ratio of about 1 to 3,
It is considered that this is because the strength of the aggregate with the metal silicide phase and / or the metal nitride phase is small and the effect of preventing cracking is small as compared with the case where whiskers are used.

Example 3 ZrSi ₂ , HfSi ₂ , VS in the same manner as in Example 1
i ₂ , NbSi ₂ , TaSi ₂ , CrSi ₂ , MoSi ₂ ,
A Si ₃ N ₄ -metal silicide-SiC whisker sintered body containing WSi ₂ was prepared. K _1C was measured for these sintered bodies. The results are shown in Table 1.

Example 4 In the same manner as in Example 1, a composite sintered body in which CrN was solely added to Si ₃ N ₄ was prepared and studied. As a result, the same result as the addition of CrSi ₂ was obtained. That is, the added CrN reacts with Si in Si ₃ N ₄ to produce CrSi ₂ , Cr ₃ Si, C
R ₂ N and CrN were produced, and the particles were mixed with different phases, and K _1C was as small as about 7 MN / m ^3/2 . On the other hand, if SiC whiskers are further added to this, K _1C will be about 1
It was remarkably increased to 0 MN / m ^3/2 .

〔The invention's effect〕

 The toughness of the silicon nitride sintered body of the present invention is remarkably excellent.

Also, because it has excellent resistance to oxidation and thermal shock,
It is suitable as a material for high temperature equipment such as gas turbines and various engines.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a grain structure and a fracture mode of a sintered body according to the present invention, and FIG. 2 is a schematic diagram showing a grain structure and a fracture mode of a conventional sintered body. 3 and 4 are curve diagrams showing changes in the fracture toughness value of the sintered body of the present invention. 1 ... crack, 2,3 ... metal silicide, 4 ... metal nitride,
5 ... Silicon nitride matrix, 6 ... Silicon carbide whiskers,
7 ... Reaction phase.

Claims (7)

[Claims] 1. A metal silicide phase containing silicon nitride as a main component,
A high toughness silicon nitride sintered body containing silicon carbide whiskers and containing a reaction phase of a metal silicide and silicon carbide whiskers. 2. The metal of the metal silicide phase according to claim 1, wherein the metal of the metal silicide phase is Ti, Zr, Hf, V, Nb, Cr, Mo.
And at least one selected from W, a high-toughness silicon nitride sintered body. 3. The silicon carbide whiskers according to claim 2, wherein the reaction phase comprises at least one metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Cr, Mo and W, so that a part of Si in the silicon carbide whiskers is A high toughness silicon nitride sintered body, which is a solid solution phase of a substituted one and a metal silicide, and the reaction phase is welded to the metal silicide phase. 4. The method according to claim 1, wherein the silicon nitride is 60 vol% or more and the silicon carbide whiskers are 5 to 25 vol.
%, Metal silicide 5 to 25 vol%, a high toughness silicon nitride sintered body. 5. The method according to claim 2, wherein the silicon nitride is 60 vol% or more and the silicon carbide whiskers are 5 to 25 vol.
%, Metal silicide 5 to 25 vol%, a high toughness silicon nitride sintered body. 6. The method according to claim 3, wherein the silicon nitride is 60 vol% or more and the silicon carbide whiskers are 5 to 25 vol.
%, Metal silicide 5 to 25 vol%, a high toughness silicon nitride sintered body. 7. Silicon nitride 60 vol% or more, silicon carbide whiskers 5 to 25 vol%, Ti, Zr, Hf, V, N.
At least one metal silicide selected from b, Cr, Mo and W 5 to 25 vol%, M of 0.1 to 20 vol%
gO, Al ₂ O ₃ , Sc ₂ O ₃ , Y ₂ O ₃ and at least one kind selected from rare earth oxides are mixed and molded, and hot pressed and sintered at 1600 ° C to 1900 ° C in a vacuum or an inert gas. A method for producing a high toughness silicon nitride sintered body, which comprises: