JP2694369B2 - Silicon nitride sintered body - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a silicon nitride sintered body which is suitable for a heat engine such as a gas turbine or a turbo rotor and which has excellent bending strength and toughness at high temperatures.

(Prior Art) Conventionally, silicon nitride-based sintered bodies have attracted attention as engineering ceramics, particularly as materials for heat engines, because of their excellent strength, hardness, and thermochemical stability at high temperatures.

Generally, in order to manufacture these silicon nitride sintered bodies, since silicon nitride itself is difficult to sinter, various sintering aids such as rare earth element oxides are added, and hot pressing and normal pressure firing are performed. Method or gas pressure firing method is adopted. Also,
Recently, a high-density material has been formed by forming an opaque seal made of glass or the like on the surface of a silicon nitride molded body having a desired composition and firing it under high pressure (hereinafter referred to as seal HIP).
A method for obtaining a high-strength sintered body has been proposed.

On the other hand, from the viewpoint of the composition, as described above, in addition to rare earth element oxides such as Y ₂ O _3, oxides such as Al ₂ O ₃ and MgO are most commonly used as sintering aids. However, considering the high temperature characteristics of the sintered body, when Al ₂ O ₃ or MgO is contained in the sintered body, a low melting point substance is generated at the grain boundary of the sintered body, so high temperature strength and high temperature acid resistance. From the standpoint that the chemical conversion property is lowered, a simple ternary composition system of Si ₃ N ₄ —RE ₂ O ₃ (rare earth oxide) -SiO ₂ which does not substantially contain the above oxide is being studied.

Further, from the viewpoint of the structure of the sintered body, the grain boundary phase in the sintered body has been attracting attention as a factor that determines the high temperature characteristics, and the grain boundary phase is changed for the purpose of improving the strength of the grain boundary phase itself. Attempts have been made to substantially crystallize. Therefore, recently, for the above-mentioned simple ternary composition, Si ₃ N ₄ -RE ₂ O _{3 was} added to the grain boundaries by studying the firing conditions or heat treatment of the sintered body.
Various crystal phases composed of (rare earth oxide) -SiO ₂ such as apatite, YAM, wollastonite, etc. are also deposited.

However, although crystallization of the grain boundary phase has a certain effect on the high-temperature strength, it is very difficult to stably precipitate only a specific crystal phase at the grain boundary. In some cases, a low melting point glass phase other than the crystalline phase is formed, thus deteriorating the characteristics.

Therefore, the present inventors have considered that the above ternary composition
The composition of SiO ₂ / RE ₂ O ₃ molar ratio exceeding ₂ and 25 or less containing excess SiO ₂ is fired at a low temperature by the seal HIP method to form a fine silicon nitride grain structure and a grain boundary phase of silicon. It was proposed that a sintered body composed of a rare earth element, oxygen and nitrogen and having excellent room temperature strength, high temperature strength and oxidation resistance can be obtained.

(Problems to be Solved by the Invention) However, although the above-described sintered body has excellent characteristics in bending strength and oxidation resistance as compared with conventional products, it is due to the fact that the structure is a fine grain structure. Then, it has been found that there is a drawback that the so-called sintered body itself tends to deteriorate in the toughness because it does not have sufficient effect against the development of cracks from the outside.

(Object of the Invention) Accordingly, the object of the present invention is to provide a sintered body having excellent toughness while maintaining the above-mentioned excellent strength and oxidation resistance.

(Means for Solving Problems) As a result of examining the above problems, the present inventors have previously proposed Si ₃ N ₄ -RE ₂ O ₃ (rare earth oxide) -SiO ₂
By using the composition of the system to form the microstructure of acicular particles in the structure of the sintered body, it is possible to increase the toughness (K _1c ) while maintaining excellent room-temperature bending strength and high-temperature bending strength. I found that I could do it.

That is, the present invention comprises 70 to 99 mol% of silicon nitride, 0.1 to 5 mol% of rare earth element oxide, and 25 mol% or less of excess oxygen in terms of SiO ₂ (excess oxygen / rare earth element oxide. ) A silicon nitride sintered body having a molar ratio of more than 2 and in the range of 25 or less, wherein the average aspect ratio of silicon nitride sintering of the sintered body is 3 or more, and the average particle diameter (major axis) is 15 μm or less. And a toughness (K _1c ) of 6 MPa · m ^1/2 or more.

 Hereinafter, the present invention will be described in detail.

The composition of the sintered body of the present invention has a silicon nitride content of 70 to 99.
Mol%, especially 80-93.5 mol%, and rare earth element oxide 0.1
.About.5 mol%, especially 0.5 to 4 mol%, and excess oxygen is 25 mol% or less, especially 6 to 20 mol%, in terms of SiO ₂ . The excess oxygen is the amount of oxygen excluding the amount of oxygen mixed in as a rare earth element oxide in the stoichiometric amount from the total amount of oxygen contained in the entire system of the sintered body. Of the impurity oxygen or oxygen added as SiO ₂ , and in the present invention, both show the equivalent amount of SiO ₂ .

The compositional feature of the present invention is that the (excess oxygen / rare earth element oxide) molar ratio is greater than 2 and is 25 or less, particularly 3 to
It consists of 20 points. The reason why the molar ratio is limited to the above range is that when the molar ratio is 2 or less, the oxidation resistance at high temperature tends to deteriorate, and conversely, when the molar ratio exceeds 25, a glass having a low melting point tends to be produced and the high temperature characteristics deteriorate. Further, even if any one of silicon nitride, rare earth oxide, and excess oxygen deviates from the above range, room temperature strength and high temperature strength deteriorate, and the object of the present invention cannot be achieved.

According to the present invention, in the above specific composition system, the average aspect ratio of the silicon nitride crystal particles in the sintered body is 3 or more, particularly 5 or more, and the average particle diameter (major axis) is 15 μm or less, particularly 10 μm or less. By forming the structure, the toughness of the sintered body can be significantly improved while maintaining the above-described excellent strength and oxidation resistance. If the average aspect ratio is less than 3, desired toughness cannot be obtained, and if the average particle size exceeds 15 μm, these particles are likely to become a fracture source and the strength decreases.

As a method for producing the silicon nitride sintered body having the above structure, various production methods can be mentioned, but the seal HIP method is suitable for obtaining particularly excellent characteristics. Therefore, the seal HIP method will be described here as an example.

First, a silicon nitride powder, a rare earth element oxide powder, and optionally a SiO ₂ powder are used as raw material powders. Silicon nitride powder has a BET specific surface area of 3 to ²⁰ m ² /
g, desirably at least 95%. The oxygen content is generally but contained about 0.8 to 1.4% by weight commercially available, it can be arbitrarily adjusted by the addition of SiO _2.

These powders are weighed and mixed with the above-mentioned composition, a binder is added and granulated, and then molded. A well-known method can be adopted for molding, specifically, press molding, extrusion molding,
Cast molding, injection molding, etc. can be adopted.

The molded body obtained in this manner is debindered, and then powder such as BN powder having poor wettability is formed on the surface of the molded body in order to prevent reaction with the glass or the like as a sealing material in the firing step. Apply. BN on surface of compact
For the coating of powder having poor wettability with glass, etc., powder of BN or the like may be made into a slurry and applied to the molded body, or the slurry may be spray-applied. The coating amount on the surface of the molded body is preferably about 1 to 10 mm.

A drying step is required after powder application of the above BN etc., and cracks are likely to occur in the molded body at this time. Therefore, the molded body before BN application should be calcined once in an inert gas atmosphere at 1200 to 1600 ° C. Is desirable.

In addition, impurities such as B ₂ O ₃ are present in the BN powder and are mixed into the sintered body during sintering to form low melting point grain boundaries and deteriorate the high temperature characteristics of the sintered body. Before firing, 1200 ~
It is desirable to perform a heat treatment under a reduced pressure of 1450 ° C. to remove the impurities. Note that it is necessary to perform the conditions at this time under conditions that do not cause decomposition of silicon nitride.

Next, a glass powder that forms a seal at the time of firing is applied to the surface of the molded body coated with BN, or is encapsulated in a glass capsule. As another method, the molded body can be buried in a heat-resistant container filled with glass powder. Thereafter, the molded body is fired under high temperature and high pressure by the HIP method.

Next, FIG. 1 shows a specific firing pattern for obtaining the sintered body of the present invention. According to FIG. 1, first, points AB
In (1), the temperature in the furnace is raised under reduced pressure to remove the water contained in the compact.

Next, at the point B-C, the temperature is raised to a firing temperature equal to or higher than the softening point of the glass present on the surface of the molded body, and at the same time, the decomposition equilibrium pressure of silicon nitride at that temperature or equal
While introducing nitrogen gas with a high partial pressure of about 0.01 to 0.2 MPa, the glass is softened to form a gas impermeable film of glass on the surface of the molded body.

After the gas impermeable membrane is completely formed on the surface of the molded body,
At the points C-D, the pressure in the furnace is increased to a pressure of 50 MPa or higher under the condition that the pressure can be sufficiently densified. At this time, an inert gas such as nitrogen or argon is used as the pressure medium. At this stage, a liquid phase is produced by the rare earth oxide, SiO ₂ and silicon nitride, and firing proceeds, and at point D, the densification is almost completed.

After that, at point D-E, the crystal is grown by maintaining it at the maximum temperature and pressure for a predetermined time, and after point E, the temperature and the pressure are both lowered to complete the firing. The maximum temperature at this time is 14
It is set to 50 to 1800 ° C, especially 1500 to 1750 ° C.

In the present invention, in order to obtain a sintered body having the above-mentioned excellent properties, for example, a large amount of α-Si ₃ N ₄ is left in the sintered body at the stage where the densification is almost completed at point D,
In that state, the silicon nitride is made needle-like by maintaining it for a predetermined time.
Usually, the acicularization of silicon nitride occurs at a temperature at which a liquid phase is formed accompanying the transition from α-Si ₃ N ₄ to β-Si ₃ N ₄ , but at points C-D
If the α-β transition progresses too much in the interval, the state becomes similar to that when a high β-Si ₃ N ₄ containing raw material is used as a starting raw material, and the effective acicularization aimed at by the present invention is hindered.

Therefore, the amount of α-Si ₃ N ₄ in the sintered body at the point D can be increased by increasing the rate of pressure increase between the points C and D to 50 MPa / Hr or more.

The firing temperature was set to 1450 as the firing condition during HIP.
The reason for limiting the temperature range to 1800 ° C is that a fine needle-like structure is formed in this temperature range. Densification is not achieved below 1450 ° C, and abnormal grain growth easily occurs above 1800 ° C. No improvement in mechanical properties can be expected. Further, pressure is an essential factor for densification, and a densified body cannot be obtained at less than 50 MPa. In addition, between the points D and E, it is held for a sufficient time for progressing the acicularization of the tissue, specifically, about 1 to 5 hours. Further, the cooling rate after the point E is preferably 500 to 1500 ° C./Hr.

The sintered body thus obtained contains needle-shaped β-silicon nitride crystal phase and a small amount of α-silicon nitride crystal phase in some cases, and silicon, rare earth elements, oxygen, and nitrogen at grain boundaries of the crystal phase. According to the manufacturing method as described above, a silicon oxynitride crystal phase represented by Si ₂ N ₂ O or RE ₂ O ₃ · 2SiO ₂ (RE: rare earth element) is present in addition to the high melting point glass phase. In some cases, a disilicate crystal phase is generated. Such a grain boundary structure has advantages in that it has excellent properties in high-temperature strength and oxidation resistance as compared with conventionally known various crystal phases and can be manufactured stably.

In addition, the characteristics can be improved by subjecting this sintered body to heat treatment under a predetermined condition, for example, at a temperature of 1200 to 1700 ° C. in a non-oxidizing atmosphere, or heat treatment in an oxidizing atmosphere.

 Hereinafter, the present invention will be described with reference to the following examples.

(Example) Silicon nitride powder (BET specific surface area 5 m ² /
g, α conversion rate 99%, impurity oxygen amount 1.0% by weight) and various rare earth oxides or SiO ₂ powders are mixed and mixed to have the composition shown in Table 1, and then pressed at 1 t / cm ² . 14 after molding
It was calcined at 00 ° C.

A paste of BN powder having a particle size of about 1 to 5 μm was applied to the obtained molded body in a thickness of 1 to 10 mm, and then heat-treated at 1350 ° C. under a reduced pressure of 0.2 Torr to remove impurities.

Thereafter, a glass containing SiO ₂ as a main component was applied in a thickness of 1 to 10 mm.

The molded body thus treated was placed in a hot isostatic press furnace and fired based on the firing pattern shown in FIG. First, after removing water at 500 ° C. under reduced pressure, nitrogen gas 0.1
In an atmosphere of MPa, the temperature in the furnace was raised to the softening point temperature of the glass or higher, and further raised to each firing temperature shown in Table 1 at a heating rate of about 900 ° C / Hr. After confirming that the glass seal was completed on the surface of the molded body, the pressure in the furnace was increased at a pressure increasing rate shown in Table 1 using Ar as a pressure medium, and firing was performed under predetermined conditions. After firing, each was cooled at a cooling rate of 1000 ° C./Hr.

Next, with respect to the sintered body after glass removal, K _1c was measured according to JIS R1601 by room temperature, 1200 ° C. and 1400 ° C. 4-point bending bending strength, and IM method. Furthermore, the increase in the weight of oxidization at 1400 ° C was investigated.

Further, the average aspect ratio and the average grain size (major axis) of the silicon nitride crystal grains were measured from the mirror surface photograph of each sintered body.

 The results are shown in Table 1.

According to Table 1, samples 9 and 14 having a (excess oxygen / rare earth element oxide) molar ratio of less than 2 have low room temperature strength and strength of 1400 ° C. Further, Sample 5 having a molar ratio of more than 25 has low strength. Further, Sample 10 in which the firing temperature was set high had abnormal grain growth and was similarly low in strength.

Furthermore, Samples 11 and 15 in which the pressurization rate was set slow during firing
All had a small aspect ratio, and the effect of improving toughness was not observed.

Compared to these comparative examples, the sample of the present invention has a small particle size, a large aspect ratio, and characteristically 1100MP.
It is understood that it has a bending strength of a or more, an excellent toughness of 6.0 MPa m ^1/2 or more, and an excellent property of 0.1 mg / cm ² or less in high temperature oxidation resistance.

(Effect of the Invention) As described in detail above, according to the silicon nitride sintered body of the present invention, Si ₃ N ₄ -RE ₂ O ₃ (rare earth oxide) -SiO ₂ (excessive oxidation)
In a simple ternary composition system containing a large amount of excess oxygen, by forming a fine structure of acicular silicon nitride crystal grains on the structure, excellent strength is maintained at room temperature and high temperature, and high toughness is maintained. Can be given.

As a result, it is possible to further expand its use as various mechanical structural parts, including high-temperature structural materials such as heat engines for silicon nitride sintered bodies.

[Brief description of the drawings]

FIG. 1 shows a specific firing pattern for obtaining the silicon nitride crystalline material of the present invention.

Claims (1)

(57) [Claims] 1. Silicon nitride 70 to 99 mol%, rare earth element oxide 0.1 to 5 mol%, and excess oxygen (SiO ₂ conversion amount) 25% mol or less, and (excess oxygen / rare earth element oxide) mol A silicon nitride sintered body having a ratio of more than 2 and in the range of 25 or less, in which the silicon nitride crystals of the sintered body have an average aspect ratio of 3 or more and an average particle diameter (major axis) of 15 μm or less,
A silicon nitride sintered body having a toughness (K _1c ) of 6 MPa · m ^1/2 or more.