JPH02175661A - Calcined silicon nitride-based compact and production thereof - Google Patents

Abstract

PURPOSE:To obtain a calcined compact, having a crystalline structure and grain boundaries consisting of an amorphous substance and excellent in stability, high-temperature strength and oxidation resistance by calcining a mixture of Si3N4 with a rare earth element oxide of a specific composition prepared by excessively adding SiO2 thereto at a low temperature and quenching the calcined mixture. CONSTITUTION:The above-mentioned calcined Si3N4 compact is a calcined compact of a composition system consisting of 77-90mol% Si3N4, 0.1-5mol% rare earth element oxide and <=25mol% excess oxygen at >2 to <=25 molar ratio of excess oxygen/rare earth oxide. The average grain diameter of the Si3N4 crystals is <=7mu and the grain boundary phase thereof is an amorphous substance composed of Si, rare earth element, O and N. The aforementioned calcined compact is produced by providing a gas-impermeable sealing on the surface of the compact consisting of the above-mentioned composition, calcining the compact at 1450-1800 deg.C in a high-pressure gas and then cooling the calcined compact at >=1000 deg.C/hr rate.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a silicon nitride sintered body having excellent bending strength and oxidation resistance at high temperatures, and a method for producing the same.

(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.

Silicon nitride itself is difficult to sinter because it consists of covalent bonds, so various sintering aids such as rare earth element oxides, alkaline earth metal oxides, or l:Al2O5 are added to form the silicon nitride. , 1600 to 20 depending on firing means such as normal pressure firing, hot press firing, gas pressure firing, hot isostatic pressure firing, etc.
It is obtained by firing at a temperature of 00°C.

In such a silicon nitride sintered body, the grain boundary phase of the sintered body has attracted attention as a factor determining high-temperature characteristics, and attempts have been made to improve the strength of the grain boundary phase. Therefore,
Recently, by applying heat treatment to sintered bodies, grain boundaries have been changed to silicon nitride (Si3N4), rare earth element oxides (R
Various crystal phases consisting of E2O3) and 5iO7, such as melilite, apatite, YAM, and wollastonite, have been precipitated.

[Problem that the invention seeks to solve]

However, although high-temperature strength can be improved to some extent by crystallizing grain boundaries, it is difficult to stably generate the desired crystalline phase because the composition of the grain boundaries changes depending on various conditions, and Because the properties differ depending on the type of crystalline phase, it lacks stability, and it is difficult to crystallize all grain boundaries. Therefore, there was a problem in that the desired high temperature characteristics could not be obtained.

[Purpose of the invention]

An object of the present invention is to solve the above-mentioned problems and provide a sintered silicon nitride element having excellent stability and high-temperature properties, and a method for manufacturing the same.

[Means for solving problems]

As a result of repeated studies to address the above-mentioned problems, the present invention was developed by adding an excessive amount of 5iO7 as an additive component, firing it at a low temperature, and rapidly cooling it to form grain boundaries with silicon, rare earth elements, oxygen,
By forming a high melting point glass phase composed of nitrogen, a fine crystal structure can be formed and the grain boundary phase can be composed of a uniform structure, resulting in excellent stability and improved high temperature strength and oxidation resistance. I found out that.

That is, the present invention uses silicon nitride (hereinafter referred to as SiN4) 70
~99 mol% Rare earth element oxide 0.1 to 5 mol% and excess oxygen/Rare earth element oxide (molar ratio) from 25 mol% or less to more than 2 and 25 or less. The average particle size of the 5iJa crystal is 7μ
"It is made into an amorphous body whose grain boundary phase is composed of silicon, rare earth elements, oxygen, and nitrogen, and its manufacturing method is to..." It is obtained by providing a permeable seal, firing under high pressure at a temperature of 1400 to 1800°C, and then rapidly cooling at a rate of 1000°/hr or more.

The present invention will be explained in detail below.

The main purpose of the present invention is to form the grain boundaries as a uniform single phase and to make the grain boundaries amorphous in order to eliminate the non-uniformity of the grain boundary phase caused by conventional crystallization. The purpose is to increase the viscosity.

The basic composition for this purpose is 70 to 99 mol%, especially 80 to 93.5 mol% of 5iJ4, 0.1 to 5 mol%, especially 0.5 to 4 mol% of rare earth element oxide, and 25% of excess oxygen. It consists of a proportion of less than mol %, especially 6 to 20 mol %.

Note that excess oxygen is the amount of oxygen excluding oxygen mixed in as rare earth element oxides from the total amount of oxygen contained in the entire system.
Specifically, impurity oxygen in the 5iJa raw material or St
It is composed of oxygen added as ow,
In the present invention, the amount is expressed in terms of 5 iOz. The major compositional feature of the present invention is that the excess oxygen/rare earth element oxide (mole ratio) is greater than 2 and less than 25, especially 3 to 20.
This means that there is a large amount of excess oxygen, which is an important factor in making the grain boundaries amorphous. The reason for limiting this molar ratio to the above range is
If this molar ratio is less than 2, crystals will tend to precipitate at the grain boundaries, resulting in a lack of stability and the object of the present invention will not be achieved.On the other hand, if it exceeds 25, a glass with a low melting point will be easily formed, and the grain boundaries will become unstable. This is because the melting point of the sintered body decreases too much and the high temperature strength of the sintered body decreases. Moreover, in the above composition range, even if either Si3N4 or the rare earth element oxide deviates from the above range, the strength will decrease.

In the grain boundaries of the sintered body of the present invention, due to the rare earth element oxygen and excess oxygen, which are additive components, and some solid solution of Si3N4,
It consists of an amorphous substance consisting essentially of silicon, rare earth elements, oxygen, and nitrogen, and because there is no crystalline precipitation like in the conventional case, uniform grain boundaries are formed.

One of the concrete ways to judge whether the grain boundary phase becomes amorphous is by the presence or absence of the silicon oxynitride phase (Si2N20 phase), which is the crystalline phase that is most likely to precipitate in the above-mentioned composition system. In the case, in the X-ray diffraction chart, 5iJzO(020) plane peak intensity/(α-3iJt(
210) plane peak intensity factor β-5 + J4 (210) plane peak intensity)
If the precipitation of the 5iJzO phase is suppressed at a level of 5iJzO phase or less, it is said that the grain boundaries have become substantially amorphous.

According to the present invention, in order to increase the melting point of this grain boundary, it is desirable to use an element that can form a high melting point silicate as the rare earth element. Such elements include Yb, E
Examples include r, Dy, and Ho.

In addition, according to the sintered body of the present invention, it is important that the average grain size of the 5iJ4 crystal grains is 7 μm or less, particularly 5 μm or less, and by forming such a fine crystal structure, mechanical strength can be improved. can be improved.

According to the method for manufacturing a silicon nitride sintered body of the present invention, silicon nitride powder, rare earth element oxide powder, and in some cases 5iO8 powder are used as raw material powders, but the silicon nitride powder is BET in order to promote sinterability. Specific surface area is 3-20m27
g. It is desirable that the gelatinization rate is 95% or more.

Using these powders, silicon nitride 70 to 99 mol%, especially 80 to 93.5 mol%, rare earth element oxides 0.1 to 5 mol%, especially 0.5 to 4 mol%, excess oxygen (SiOz equivalent amount) ) 25 mol% or less, especially 6 to 20 mol%, and prepared and mixed so that the excess oxygen/rare earth element oxide (molar ratio) is greater than 2 and 25 or less, especially in the range of 3 to 20. do.

The thus obtained mixed powder is molded into a desired shape by a known molding method, such as press molding, casting molding, extrusion molding, infusi-Fsilane molding, etc., and then transferred to firing.

According to the present invention, since the above composition contains a large amount of a component that easily volatilizes during firing, that is, excess oxygen, it is necessary to select a firing method that sufficiently suppresses such volatilization.

Therefore, according to the present invention, firing is performed under high pressure gas while the surface of the molded body is completely sealed with a gas-impermeable seal made of glass or the like. According to this method, the composition of the molded body does not change due to the presence of a sealing material between the firing atmosphere and the molded body, and is almost the same as the composition of the sintered body, so mass production is possible without the need for any atmosphere control. It also has the advantage of being excellent in properties. Specifically, after applying a mold release agent such as BN to the surface of the molded body, if desired, glass is further applied. This molded body is placed in, for example, a hot isostatic pressure firing furnace and heated, and after sealing of the glass on the surface of the molded body is completed, the temperature is further raised and pressure is applied with a gas such as N2 or argon.

The final firing temperature is 1450-1800℃, especially 1500℃.
Pressure 500~2000a tn+ at low temperature ~1750℃
to hold. After firing, the grain boundaries are made amorphous by setting the cooling rate to 1000° C./hr or more, particularly 1100° C./hr or more.

Alternatively, a molded body coated with a mold release agent or the like may be immersed in a glass bath, and fired using the same method, pressure, and temperature settings as described above.

According to the present invention, under the above conditions, the cooling rate is particularly important for making grain boundaries amorphous, and the cooling rate is 1000°C/
If it is less than hr, crystal phases tend to precipitate at grain boundaries, making it impossible to achieve the object of the present invention.

Note that when the above-mentioned firing method is adopted, there is generally a problem that glass components infiltrate into the molded body and affect the properties of the sintered body, but in the composition of the present invention, SiO
Since it contains a large amount of □, it also has the unique property that even if glass components enter, it has almost no effect on the properties of the sintered body. This has the advantage that there is almost no need for various measures to prevent glass intrusion.

Furthermore, the manufacturing method of the present invention is characterized by the fact that the entire system is easily sinterable due to the large amount of excess oxygen in the composition. The advantage is that it can be fired at a relatively low temperature of 1800°C.

According to such low temperature firing, βSi3 of α-3iJ4
Since the grain growth accompanying the transition to N4 is suppressed, a fine structure can be obtained as a sintered body structure, and in particular, high strength can be achieved by setting the average grain size of the 5iJ4 crystals to 7 μm or less. At the same time, because the firing temperature is low, α-
Since the transition of 5iJa to β-3i3N is difficult to occur, the proportion of α-3i3N4 in the sintered body is smaller than α-3i.
lN4. /α-8i+Na +β5iJ4 ratio is 5%
This is also a major feature, and the room temperature strength can be increased by increasing α-3iJn.

Furthermore, although the silicon nitride sintered body of the present invention has the characteristics described above, for example, inorganic fibrous structures (whiskers) are sintered to this sintered body for the purpose of improving toughness. It can also be dispersed throughout the body.

Specifically, silicon carbide whiskers or silicon nitride whiskers having a length of 20 μm or less are uniformly dispersed in a volume of 5 to 30 volumes, and the toughness can be improved by the whisker pulling effect or the deflection of the crank by the whiskers.

The invention will now be explained with the following examples.

〔Example〕

Silicon nitride powder (BET specific surface area: 5 m27) was used as the raw material powder.
g, gelatinization rate 95χ, oxygen content 1.0% by weight) and various rare earth oxide powders or 5i02 powder to form the composition shown in Table 1. After mixing, 1 t/cm"
It was press-molded and calcined at 1400°C.

After applying BN powder to a thickness of 1 to 10 mm and further applying glass to a thickness of 1 to 10 mm, the obtained molded body was heat treated at 1350'c under reduced pressure to remove impurities. Ta.

The thus-treated molded body was placed in a hot isostatic pressure firing furnace and heated to the softening temperature of glass in a N2 gas atmosphere of 1 atm, and then the temperature and pressure were increased to meet the predetermined conditions shown in Table 1. Hot isostatic pressure firing was performed.

After that, the sintered body obtained by removing the glass on the surface was heated at room temperature, 1200°C, 1400°C according to JISR1601.
4-point bending strength at °C and 1000 °C x 24
The oxidation weight gain during 24 hours at 1400°C was investigated.

In addition, for the sintered body, the amorphization of the grain boundary phase was determined by the 5izNzO (020) plane peak/(α5i
J4 (210) plane peak)'X100 (χ) was determined based on the intensity ratio, and those with this value of 30 (χ) or less were determined to be products of the present invention. Furthermore, the content of α-3iJ4 and the average particle diameter of the Si Ja crystal particles were measured using SEM photographs.

[Margin below]

As is clear from Table 1, in mlO,16 where the excess oxygen/rare earth element oxide ratio is 2 or less, the room temperature strength is low and the strength at 1400°C is also low. On the other hand, in floor 6 where this ratio exceeds 25, a large amount of 5iJ20 phase is precipitated, and although the room temperature strength is high, the high temperature strength is poor.

In addition, N1112 where the firing temperature of old P exceeds 1800℃
, 13.17 had a large average particle size due to the grain growth of Si:+1L, and therefore the bending strength was low in both cases.

Furthermore, the cooling rate is slower than 1000℃/hr.
In both samples ll and 18, an apatite crystal phase was precipitated, and the object of the present invention was not achieved.

In contrast to these comparative examples, other products of the present invention had a room temperature strength of 10
50MPa or more, 1200℃ strength 800MPa or more, 1
A strength of 600 MPa or more at 400°C was achieved, and the oxidation weight gain was 0.1 mg/cm” or less at 1000°C, 1400°C.
It exhibited excellent properties with O of 1 mg/cm2 or less.

Furthermore, as a result of TEM analysis of the grain boundaries of these products of the present invention, silicon, rare earth elements, oxygen, and nitrogen were detected in all of them.

〔Effect of the invention〕

As detailed above, the silicon nitride sintered body of the present invention has a fine crystal structure and grain boundaries are amorphous, so that the grain boundaries are uniform and have a high melting point. A sintered body with excellent high-temperature strength and oxidation resistance can be obtained.

Claims (2)

[Claims] (1) Consisting of 70 to 99 mol% silicon nitride, 0.1 to 5 mol% of rare earth element oxide, and 25 mol% or less of excess oxygen,
A silicon nitride sintered body in which the excess oxygen/rare earth element oxide (molar ratio) is in the range of more than 2 and less than 25, and the average particle size of silicon nitride crystals in the sintered body is 7 μm or less, and A silicon nitride sintered body characterized in that the interfacial phase consists of an amorphous substance consisting essentially of silicon, rare earth elements, oxygen, and nitrogen. (2) Silicon nitride 70-99 mol%, rare earth element oxide 0
．． Gas impermeability on the surface of a molded body with a composition of 1 to 5 mol%, excess oxygen (SiO_2) 25 mol% or less, and excess oxygen/rare earth element oxide (molar ratio) greater than 2 and 25 or less 1450-1800 with sex stickers
After firing under high pressure gas at a temperature of
A method for producing a silicon nitride sintered body, characterized in that the silicon nitride sintered body is made into an amorphous body composed of oxygen and nitrogen.