JPH03218969A - Silicon nitride-based body - Google Patents

Abstract

PURPOSE:To improve the oxidation resistance and strength of an Si3N4-rare earth metal oxide-SiO2 ternary sintered body contg. excess oxygen at high temp. by depositing silicon oxynitride and disilicate phases on the grain boundaries in the sintered body. CONSTITUTION:Silicon oxynitride and disilicate phases as crystal phases are deposited on the grain boundaries in a silicon nitride-based sintered body consisting of 70-99mol% Si3N4, 0.1-5mol% rare earth metal oxide (RE2O3) and <=25mol% (expressed in terms of SiO2) excess oxygen in 2-25 molar ratio of excess oxygen to RE2O3.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is a silicon nitride sintered material that has excellent bending strength and oxidation resistance at high temperatures suitable for heat engines such as gas turbines and turbo engines. Concerning a method of manufacturing a body.

(Prior Art) Silicon nitride sintered bodies have traditionally attracted attention as engineering ceramics, particularly as materials for heat engines, because of their excellent strength, hardness, and thermal and chemical stability at high temperatures. Specific examples of heat engines include parts for turbo rotors and gas turbines, and when applied to these, the temperature range for sintered bodies is from room temperature to about 1200°C (7), and for some parts it is 1400°C (7). 7) Excellent mechanical properties are required at high temperatures, and recently it has been desired to improve properties at 1500°C.

Generally, as a method for manufacturing these silicon nitride sintered bodies, since silicon nitride itself is difficult to sinter, various sintering aids such as rare earth element acids (arsenic compounds) are added, and the honto press method, Normal pressure firing method, gas pressure firing method, etc. are employed. Recently, with the aim of increasing density and strength, a method has been developed in which an impermeable seal made of glass or the like is formed on the surface of a silicon nitride molded body having a desired composition, and then fired under high pressure.
(hereinafter referred to as seal HTP) is also being researched.

? On the other hand, in terms of composition, in addition to rare earth element oxides such as Y203 as mentioned above, oxides such as Alz(h) and MgO are most commonly used as sintering aids; Considering the high-temperature properties of A103 and Ago, the above oxidation Si3N4-RE203 substantially free of substances
A composition consisting of a simple ternary system of (rare earth oxide) -SiO2 is also being considered.

In addition, from the perspective of the structure of a sintered body, the grain boundary phase in the sintered body is attracting attention as a factor that determines high-temperature properties, and the grain boundary phase is used to improve the strength of the grain boundary phase itself. Attempts have been made to substantially crystallize it. Therefore, recently, for the above-mentioned simple ternary composition, StJ4 has been added to the grain boundaries by examining the firing conditions or by heat treating the sintered body.
Attempts have also been made to improve the high temperature properties by precipitating various crystal phases consisting of -REzOz (rare earth oxide) -SiO2, such as melilite, apatite, YAM or wollastonite.

? Problems to be Solved by the Invention) However, although the sintered body in which the various crystal phases described above are precipitated at the grain boundaries has a certain degree of improvement in strength or oxidation resistance at room temperature and at high temperatures of 1400°C, It has the disadvantage that its properties, especially its oxidation resistance, deteriorate significantly at a high temperature of 1500°C, making it almost unusable at a temperature of 1500°C.

Therefore, the present applicant has particularly proposed that compared to conventional sintered bodies,
As a sintered body with excellent oxidation resistance at a high temperature of 500'C, a system containing a large amount of excess oxygen, specifically, 70 to 99 mol% silicon nitride and 0.1 to 5 mol% of rare earth element oxide is used.
mol% and excess oxygen (SiO2 equivalent amount) of 25 mol% or less, and the molar ratio (excess oxygen/rare earth element oxide) is 2.
We have proposed a sintered body in which silicon nitride grain boundaries are made amorphous or silicon oxynitride O crystals are precipitated by firing a composition in the range of 25 or less. It lacks stability in properties and has insufficient strength especially at 1400°C. Ta.

(Object of the invention) The object of the invention is to maintain excellent high-temperature oxidation resistance while
The object of the present invention is to provide a silicon nitride ceramic sintered body with improved bending strength at a high temperature of 400°C.

(Means for Solving the Problems) As a result of research into the above problems, the present inventors have developed a composition of Si3N4-REzoz (rare earth oxide)-SiO■ system containing a large amount of excess oxygen, In particular, silicon nitride is 70 to 99 mol % and rare earth element oxide is 0. It consists of 1 to 5 mol% and 25 mol% or less of excess oxygen (SiO2 equivalent amount), and has a composition in which the (excess oxygen/rare earth element oxide) molar ratio is greater than 2 and 25 or less, By making the silicon oxynitride phase and the disilicate phase simultaneously exist at the grain boundaries of the silicon nitride crystal grains in the sintered body, and by setting the silicon nitride crystal grain size to less than 10μ, It has been found that high temperature strength can be greatly improved.

? The invention will be described in detail below.

According to the present invention, first, the composition of the sintered body is silicon nitride 70-9
9 mol %, especially 80 to 93.5 mol %, and 0.9 mol % of the rare earth element oxide. 1 to 5 mol%, especially 0.5 to 3 mol%, excess oxygen of 25 mol% or less, especially 6 to 20 mol%, and (excess oxygen/rare earth element oxide) molar ratio greater than 2 and 25 Hereinafter, it is particularly important that the ratio be 3 to 200.

Note that excess oxygen is the amount of oxygen excluding the stoichiometric amount of oxygen mixed in as rare earth element oxides from the total amount of oxygen contained in the entire system of the sintered body, and specifically, the amount of oxygen in the silicon nitride raw material. It is composed of impurity oxygen or oxygen added as SiO2, and both values are expressed in terms of SiO2.

The reason why the composition of the sintered body is limited to the above range is because the room temperature strength and high temperature strength will deteriorate even if any of silicon nitride, rare earth oxide, and excess oxygen deviates from the above range. The reason why the molar ratio of excess oxygen and rare earth element oxide is limited to the above range is that if this molar ratio is less than 2, silica will form at the grain boundaries. This is because precipitation of oxynitride phase and disilicate phase crystals becomes difficult, and oxidation resistance at high temperatures of 1,500°C tends to deteriorate.On the other hand, when the temperature exceeds 25, glass with a low melting point tends to be formed, and high-temperature properties deteriorate. Because it does.

According to the present invention, a major feature is that both a silicon oxynitride phase and a disilicate phase are present at the grain boundaries of silicon nitride crystal grains of the sintered body having the above composition. The silicon oxynitride phase is a crystalline phase consisting of silicon, nitrogen, and oxygen, and is generally represented by the chemical formula of Si2NzO. On the other hand, the disilicate phase is a crystalline phase consisting of rare earth elements, silicon, and oxygen, and is generally represented by the chemical formula REzOi.2SiO■. In addition, if the grain boundaries contain only silicon oxynitride phase or only disilicate phase, 1
Although the oxidation resistance at 500°C is excellent, the high temperature strength is insufficient, and the object of the present invention cannot be achieved.

Next, as a specific method for manufacturing the silicon nitride sintered body of the present invention, first, it consists of the above-mentioned composition, and silicon oxynitride is present at the grain boundaries. By preparing a sintered body containing a mixture of carbon and glass phases and heat-treating it under specific conditions, it is possible to generate a disilicate phase along with a silicon nitrite phase at grain boundaries, and this method is the most productive. It is suitable from the viewpoint of stability of properties.

As a method for creating a sintered body having a silicon oxynitride crystal phase at grain boundaries, silicon nitride powder, rare earth element oxide powder, and in some cases silicon oxide powder are used as raw material powder, and silicon nitride, rare earth element oxide, Excess oxygen (SiO
(2) Weigh and mix so that the amount (converted amount) becomes a composition containing a large amount of excess oxygen as described above. At this time, the silicon nitride powder has a BET specific surface area of 3 to 20 m"/g to promote sinterability.
, it is desirable that the gelatinization rate is 95% or more. In addition, silicon nitride powder generally has an impurity oxygen content of 0.8 to 1.5.
Although it is contained in about % by weight, the total amount of oxygen can be arbitrarily adjusted to 1 by adding silicon oxide.

A binder is appropriately added to the above mixed powder, the mixture is granulated, and then molded. Well-known methods can be used for molding, specifically press molding, extrusion molding, and casting. Alternatively, injection molding or the like can be used.

The molded body thus obtained is fired after removing the binder.

Firing is performed in a non-oxidizing atmosphere at 1450 to 2000°C, depending on the firing method. As the firing means, gas pressure firing method, hot isostatic pressure firing method, etc. are suitable.

According to the gas pressure firing method, the firing temperature is set at 1700-200°C.
Controlled at 0°C, nitrogen gas was added to the atmosphere at a gas pressure of 1.5 to 1.
Introduced and fired at 00 atm. Since the composition of the present invention contains a large amount of excess oxygen, in order to suppress decomposition fluctuations in the amount of excess oxygen, SiO2 powder or a mixed powder of Si02, Si, and N4 is placed in the furnace to generate additional oxygen in the atmosphere. It is desirable to leave it.

On the other hand, according to the hot isostatic pressure firing method, a so-called seal HIP method is preferably employed in which a sealing material made of glass or the like is coated on the surface of the molded body and fired at high temperature and high pressure. In this specific method, first, prior to firing, the molded body obtained by the above-mentioned method is coated with wettable glass such as BN powder in order to prevent the reaction with glass, which is a sealing material, in the firing process. Either the bad powder is made into a slurry and applied to the molded body, or the slurry is spray applied.

゛Please note that the amount of BH applied to the surface of the molded product is
Approximately 10 mm is desirable.

Next, glass powder that forms a seal during firing is applied to the surface of the molded body coated with BN, or the molded body is encapsulated in a glass capsule. As another method, the molded body may be buried in a lupus filled with glass powder. Thereafter, it is fired at high temperature and pressure using the HIP method.

Firing is performed by first raising the temperature to a firing temperature that is above the softening point of the glass present on the surface of the molded body, and at the same time raising the temperature to a temperature equal to or 0.01 to 0 lower than the decomposition equilibrium pressure of silicon nitride at that temperature.
．． While introducing nitrogen gas at a high pressure of about 2 MPa, the glass is softened to form a gas-impermeable glass film on the surface of the molded body.

After the gas-impermeable film is completely formed on the surface of the compact, the pressure inside the furnace can be sufficiently densified, for example? ,50
Increase the pressure to M Pa or higher. The pressure medium used at this time is an inert gas such as nitrogen or argon. At this stage, a liquid phase is generated by the rare earth oxide, SiO2, and silicon nitride, the firing progresses, and the densification is almost completed. After that, both the temperature and pressure are lowered and firing is completed.

The firing temperature is 1450°C-1800°C1, especially l
The temperature is set at 450°C to 1730°C.

By the above-mentioned gas pressure sintering method or seal HIP method, it is possible to obtain a sintered body in which the grain boundaries are composed of a silicon oxynitride phase alone or a non-qualified sum of a silicon oxynitride crystal phase, silicon, rare earth elements, oxygen, and nitrogen. .

Next, the sintered body obtained by the above method was
Crystallization of the grain boundaries is further advanced by heat treatment in an oxidizing atmosphere of 0''C to 1600 degrees C or a non-oxidizing atmosphere such as nitrogen for about 3 to 100 hours, and disilicate is formed at the grain boundaries along with the silicon oxynitride phase. A phase can be precipitated.Furthermore, at this time, the grain boundaries may contain an amorphous phase as well as the above-mentioned crystalline phase.

If the heat treatment temperature at this time is lower than 1300°C, crystallization of grain boundaries will not proceed, so a disilicate phase will not be formed, and l
If the temperature is higher than 600°C, the surface of the sintered body tends to decompose and its strength deteriorates.

According to the above manufacturing method, a sintered body consisting of a fine structure with an average grain size (major axis) of silicon nitride crystal particles of 10 μl or less and an aspect ratio of 3 or more can be obtained. In the isostatic pressure firing method, the firing temperature is set at 1450℃.
By setting the temperature to a low temperature of ~1730°C, grain growth of silicon nitride particles is suppressed, so it is possible to promote the refinement of silicon nitride crystal grains.
A sintered body with a microstructure of micrometers or less can be obtained.

Thereby, the room temperature strength and high temperature strength of the sintered body can be improved.

According to the present invention, in order to improve the crystallinity of grain boundaries in a sintered body, oxides that exist in grain boundaries and easily form a glass phase, specifically oxides such as AlzOz, CaO, MgO, Fe403, etc. Should it be less than 0.05% by weight based on the total amount of the sintered body? Delicious. In addition, Y20 is common as a rare earth element oxide, but YbzO+, ErzO=, Ho
It is preferable to use a heavy rare earth element oxide such as z03 because it prevents the occurrence of color unevenness and allows a sintered body with stable characteristics to be obtained.

The invention will now be explained with the following examples.

(Example 1) As a raw material powder, silicon nitride powder (BET specific surface area 5 m"
/g, gelatinization rate 95%, impurity oxygen content 10% by weight) and various rare earth oxides or SiO■ powder to form the composition shown in Table 1. After mixing, It/cjl
Press molded.

The obtained molded body was placed in a furnace containing SiO powder.
It was fired at the temperature shown in Table 1 under a nitrogen gas atmosphere of tm.

Next, the obtained sintered body was further heat-treated under the conditions shown in Table 1.

For each sintered body after heat treatment, according to JISR1601,
Four-point bending strength at room temperature, 1400°C and 1
An oxidation resistance test was conducted at 500"C x 24 hours, and the oxidation weight increase after the test was measured.

Furthermore, crystal phases other than silicon nitride were identified from the X-ray diffraction curve of the sintered body after heat treatment.

The results are shown in Table 1.

(Example 2) Using the same raw material powder as in Example 1, the mixture was prepared and mixed in the proportions shown in Table 2, press-molded at 1t/c1il, and calcined at 1400'C.

A paste of BN powder (particle size 1 to 5 μm) was applied to the obtained molded body to a thickness of 1 to 10 mm, and then glass containing SiO2 as a main component was applied to a thickness of 1 to 10 mm.

The molded bodies thus treated were placed in a hot isostatic pressure firing furnace and fired under various conditions to produce several types of samples with different properties as shown in Table 2.

Next, these samples were heat treated under the conditions shown in Table 1.

The strength, oxidation resistance, and crystal phase of the sintered body after the heat treatment were determined in the same manner as in Example 1.

The results are shown in Table 1.

According to Table 1, a sintered body (sample 8.19) in which the grain boundaries are composed of silicon oxynitride phase or this and glass particles
) have excellent oxidation resistance at 1500°C, but low bending strength at 1400'C. In addition, sintered bodies with a (excess oxygen/rare earth element oxide) molar ratio of 2 or less and grain boundaries containing glass or YAM crystals (Samples 11 and 22
) has excellent strength at 400°C, but poor oxidation resistance at 15”00°C, and no improvement in oxidation resistance was observed even after heat treatment (Samples 12 and 23).

Furthermore, when heat treating a sintered body in which a silicon oxynitride phase has been formed at the grain boundaries, if the temperature is low (Samples 9 and 20), the formation of a disilicate phase is not observed, and there is almost no improvement in high-temperature strength. .

In addition, if the processing temperature is too high (sample 10, 21),
In addition to the silicon oxynitrite phase and the disilicate phase, other crystal phases such as apatite were observed, but the surface of the sintered body was decomposed in both cases, and the properties were satisfactory.

In contrast, the samples of the present invention are all silicon? A xynitride phase and a disilicate phase are precipitated,
In terms of characteristics, oxidation weight increase is 0. It exhibited excellent oxidation resistance of 12 mg/cm2 or less and excellent high-temperature strength of 500 MPa or more.

Note that the two types of crystal phases at the grain boundaries in the sample of the present invention both have silicon oxynitride as a main component, and the average particle diameter (longer diameter) of the silicon nitride particles in the sintered body is 10 μm. or less, and the average aspect ratio is 3 or more, especially when the hot isostatic firing method shown in Table 2 is used, the average aspect ratio is 7 μ■
The sintered body had the following fine structure.

In addition, the X-ray diffraction chart of sample No. 2 is
Shown in the figure.

(Effects of the Invention) As detailed above, according to the silicon nitride sintered body of the present invention, in the simple ternary system of SiJ REz03 (rare earth oxide) -SiO■ containing a large amount of excess oxygen, By precipitating the silicon oxynitride phase and the disilicate crystal phase, high-temperature bending strength can be improved while maintaining oxidation resistance at a high temperature of 1500°C.

Therefore, the application of the silicon nitride sintered body to industrial parts, particularly as parts for heat engines, can be further expanded.

[Brief explanation of drawings]

FIG. 1 is an X-ray diffraction chart of the silicon nitride sintered body of the present invention.

Claims (2)

[Claims] (1) Consisting of 70 to 99 mol% silicon nitride, 0.1 to 5 mol% rare earth element oxide, and 25 mol% or less of excess oxygen (SiO_2 equivalent amount), expressed as excess oxygen/rare earth element oxide A silicon nitride sintered body having a molar ratio of more than 2 and less than or equal to 25, characterized in that grain boundary crystals in the sintered body are silicon oxynitride and disilicate. Quality sintered body. (2) The average grain size of the silicon nitride crystal grains of the sintered body is 1
The silicon nitride sintered body according to claim 1, which has a diameter of 0 μm or less.