JPH03174364A - Silicon nitride-based sintered body - Google Patents

Abstract

PURPOSE:To enhance deflection strength and toughness by constituting the sintered body of Si3N4, oxide of rare earth elements and (excess O2/oxide of rare earth elements) which have a specified molar ratio and incorporating fine Fe powder at a low content in the sintered body. CONSTITUTION:Si3N4 powder having 3-20m<2>/g BET specific surface area, >=95% alpha-rate of change and 0.8-1.4wt.% O2 content is washed by acid such as hydrochloric acid. This treated treated Si3N4, rare earth elements and SiO2 are weighted at prescribed amount and mixed. Thereafter a binder is added and the mixture is granulated and molded. Then the molded body is heated to remove the binder thereafter calcined at 1200-1600 deg.C in an inert gas atmosphere. Furthermore powder such as BN powder which is bad in wetting to glass is applied to the surface of the molded body at about 1-10mm. Then the impurities in BN powder are removed by heat-treating the molded body at 1200-1450 deg.C at a reduced pressure. Thereafter glass powder is applied to the surface and the molded body is roasted at 1450-1800 deg.C in the gaseous N2 atmosphere by an HIP method. An Si3N4-based sintered body is obtained which incorporates 70-99mol% (hereinafter shown in %) Si3N4, 0.1-5% oxide of rare earth elements, <=25% excess O2 (expressed in terms of SiO2), 2-55 molar ratio (excess O2/oxide of rare earth elements) and <=50ppm Fe content and has <=7mum mean particle diameter.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a silicon nitride sintered body that is suitable for heat engines such as gas turbines and turbo rotors and has excellent bending strength and toughness at high temperatures.

(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.

Generally, in order to manufacture these silicon nitride sintered bodies, silicon nitride itself is difficult to sinter, so various sintering aids such as rare earth element oxides are added, and hot press method or atmospheric pressure sintering is performed. method or gas pressure firing method, etc. are adopted. Recently, 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 is fired under high pressure (hereinafter referred to as seal HIP) to achieve high density,
A method of obtaining a high-strength sintered body has been proposed.

On the other hand, from the point of view of &II power, as mentioned above, Y z 0
In addition to oxides of rare earth elements such as 3 and 3, oxides such as Ah(h) and MgO are most commonly used as sintering aids.
If sintered bodies contain oxides such as MgO or MgO, low melting point substances will be generated at the grain boundaries of the sintered body, resulting in a decrease in high temperature strength and high temperature oxidation resistance. 5i3Na-REzOx (rare earth oxide)-5i
A simple ternary composition system of n, has been studied.

In addition, from the perspective 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 strength and high-temperature properties, and the grain boundary phase has been developed to improve the strength of the grain boundary phase itself. Attempts have been made to substantially crystallize the phase. Therefore, recently, for the above simple ternary composition, Si has been added to the grain boundaries by examining the firing conditions or by heat treating the sintered body.
Various crystal phases consisting of 3N, -RE, 0, (rare earth oxide) to SiO□, such as apatite, YAM, wollastonite, etc., have also been precipitated.

However, although the crystallization of the grain boundary phase has some effect on high-temperature strength, it is extremely difficult to stably precipitate only a specific crystal phase at the grain boundary, and in some cases, the grain boundary phase In some cases, a glass phase with a low melting point is formed in addition to the crystalline phase, resulting in deterioration of the properties.

Therefore, the present inventors proposed that S in the above ternary system M
By firing a composition containing an excessive amount of Sin with a molar ratio of iO proposed that a sintered body composed of oxygen and nitrogen with excellent room temperature strength, high temperature strength, and oxidation resistance could be obtained.

(Problems to be solved by the invention) However, although the above-mentioned sintered body has superior properties in terms of bending strength and oxidation resistance compared to conventional products, the dispersion of properties has not yet been resolved, and production However, it has the disadvantage that it is difficult to obtain a sintered body with stable properties.

(Objective of the Invention) Therefore, an object of the present invention is to provide a silicon nitride sintered body that has the above-mentioned excellent strength and oxidation resistance and is excellent in mass production.

(Means for Solving the Problems) The present inventors have investigated the cause of the variation in the structure of test pieces that showed low strength in the 5iJn-REzOs (rare earth oxide)-5iO□ system sintered bodies. When we observed the fracture sources, it was found that abnormal grain growth existed in most of the structures. Furthermore, when we analyzed the structure of this abnormal grain growth area, we found that iron (Fe) was present, and a low melting point substance was produced by the reaction with the sintering aid.
This revealed that silicon nitride had abnormal grain growth.

Therefore, it was discovered that by minimizing the amount of iron (Fe) mixed in the raw materials or in the manufacturing process, abnormal grain growth hardly occurs and variations in properties can be eliminated, leading to the present invention.

That is, in the present invention, silicon nitride is 70 to 99 mol%, rare earth element oxide is 0.1 to 5 mol%, and excess oxygen is Si
n, calculated as 25 mol% or less, the (excess oxygen/rare earth element oxide) molar ratio is greater than 2 and is in the range of 25 or less, and the average crystal grain size of the silicon nitride crystal is 7 μm or less @fine Iron (Fe) in a silicon nitride sintered body consisting of a
The above objective is achieved by reducing the content to 50 ppm or less.

The present invention will be explained in detail below.

The major feature of the present invention is that its composition is silicon nitrideff1
70-99 mol%, especially 80-93.5 mol% and rare earth element oxides 0.1-5 mol%, especially 0.5-4 mol%
, 25 mol% or less in excess oxygen (calculated as SiO□), especially 6
The molar ratio (excess oxygen/rare earth element oxide) is greater than 2 and less than 25, particularly from 3 to 20. 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. impurity oxygen,
Alternatively, it is composed of oxygen added as Sin, and in the present invention, all values are shown in converted amounts as Sin.

The present invention focuses on the fact that when the above-mentioned composition is fired at a relatively low temperature to obtain a sintered body having a fine microstructure, iron (Fe) as an impurity has a large influence on the properties of the sintered body. It is something.

Although the reason is not clear, the present inventors consider the following.

When this composition system is used to perform high-temperature firing, such as gas pressure firing, sintering progresses as a liquid phase with low viscosity is generated by the auxiliary component, but at this time, the sintered body Since most of the impurity metals are dispersed in grain boundaries with low viscosity, it is thought that the impurities have almost no effect on the properties.

However, when the above composition system is fired at a low temperature using a method such as hot isostatic pressure firing, although a liquid phase is generated by the auxiliary components during the sintering process, the viscosity remains extremely high and sintering is not possible. proceed. At this time, if impurity metals such as iron are present in the grain boundaries, low melting point substances are generated by reaction with auxiliaries, etc., and low melting point substances are generated due to the high viscosity of other grain boundaries themselves. It is presumed that this is because only the substance moves within the grain boundaries and aggregates, and the silicon nitride grows abnormally in the agglomerated areas, becoming a source of destruction.

In the present invention, based on the above knowledge, we investigated the amount of iron (Fe) and found that the amount was 50pp in the sintered body.
m or less, particularly 30 ppm or less, thereby suppressing the effects on the properties as described above and reducing variations in the properties of the sintered body.

The method for manufacturing the silicon nitride sintered body having the above structure is based on firing the above-mentioned composition at a low temperature, and it is necessary to avoid iron contamination as much as possible. As a specific example, the seal HIP method is suitable. From now on, Seal H
This will be explained using the IP law as an example.

First, as raw material powders, silicon nitride powder, rare earth element oxide powder, and optionally Sin powder are used. Silicon nitride powder has a BET specific surface area of 3 to 3 to promote sinterability.
20 m"/g and a pregelatinization rate of 95% or more. In addition, commercially available products generally have an oxygen content of 0.8 to 1.
The content is about 4% by weight, but it can be adjusted arbitrarily by adding Sin.

First, most of the iron mixed into the system is impurities in the raw material powder. Generally, 60 ppm or more of iron exists as a metal impurity in silicon nitride powder.According to the present invention, it is most efficient to first remove iron from this silicon nitride powder.

As a method for removing iron in the raw material, for example, iron can be easily removed by washing the raw material with an acid such as hydrochloric acid or nitric acid.

Next, the thus treated powder is weighed and mixed with the above-mentioned U-kai, a binder is added thereto, and after granulation, it is molded. A well-known method can be used for the molding, and specifically, press molding, extrusion molding, cast molding, injection molding, etc. can be used.

After removing the binder from the molded body, a powder having poor wettability with glass, such as BN powder, is applied to the surface of the molded body in order to prevent reaction with glass, which is a sealing material, during the firing process. To apply a powder such as BN that has poor wettability with glass to the surface of the molded body, the powder such as BN may be made into a slurry and applied to the molded body, or the slurry may be spray-coated. Note that the amount of coating applied to the surface of the molded product is preferably such that the thickness thereof is about 1 to 10 mm.

A drying process is required after applying the above-mentioned powder such as BN, and since cracks are likely to occur in the molded product at this time, the molded product is temporarily calcined in an inert gas atmosphere at 1200 to 1600"C before BN is applied. This is desirable.

In addition, impurities such as BzO□ are present in the BN powder,
During firing, this gets mixed into the sintered body and forms grain boundaries with a low melting point, deteriorating the high-temperature properties of the sintered body.
It is desirable to remove these impurities by heat treatment under reduced pressure at 200 to 1450°C, and the conditions at this time must be such that silicon nitride does not decompose.

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 sealed in a glass capsule. Alternatively, the molded body may be buried in a heat-resistant container filled with glass powder.

Thereafter, the molded body is fired under high temperature and high pressure using the HIP method.

In firing by the HIP method, the temperature in the furnace is raised under reduced pressure to remove moisture contained in the molded body, and then the temperature is raised to a firing temperature higher than the softening point of the glass present on the surface of the molded body, and at the same time, While introducing nitrogen gas at a partial pressure equal to the decomposition equilibrium pressure of silicon nitride at this temperature or about 0.01 to 0.2 MPa, the glass is softened to form a gas-impermeable glass film on the surface of the molded body. The firing temperature at this time is 145
It is set to 0 to 1800'C1, especially 1500 to 1750°C. After the gas-impermeable film is completely formed on the surface of the compact, the furnace pressure is increased to a pressure of 50 MPa or higher, for example, under conditions that can sufficiently densify the furnace pressure using an inert gas such as nitrogen or argon as a pressure medium. let At this stage, a liquid phase is generated by the rare earth oxide, Sin, and silicon nitride, the firing progresses, and the densification is almost completed. Thereafter, the crystal is grown by holding it at the maximum temperature and pressure for a predetermined period of time, and then the temperature and pressure are lowered to complete the firing.

In the sintered body thus obtained, an α-silicon nitride crystal phase remains together with a fine β-silicon nitride crystal phase, and silicon, rare earth elements, oxygen, and nitrogen are present at the grain boundaries of this crystal phase. do. According to the above manufacturing method, in addition to the high melting point glass phase, a silicon oxynitride crystal phase represented by 5izNzO or REzOx 2 S i 0x (R
A guisilicate crystal phase represented by E (rare earth element) may also be formed. Such a grain boundary structure has the advantage that it has superior properties in terms of high temperature strength and oxidation resistance compared to various conventionally known crystal phases, and can be produced stably.

In addition, due to the reduced iron content, abnormal grain growth of silicon nitride is suppressed, and a sintered body with excellent stability of properties can be obtained.

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

In the present invention, the composition of the sintered body is limited to the above-mentioned proportions, all of which are important factors for obtaining excellent properties. If the molar ratio (excess oxygen/rare earth element oxide) is less than 2, the oxidation resistance at high temperatures tends to deteriorate, and if it exceeds 25, glass with a low melting point will be formed. This is because the high-temperature characteristics deteriorate. Furthermore, the reason why the average crystal grain size is set to 7 μm or less is to increase the room temperature strength and high temperature strength, and because grain growth is promoted and if the grain size exceeds 7 μm, the desired strength cannot be obtained.

Note that Y is the most common rare earth element used in the present invention, but Er5Yb, H.

Heavy rare earth elements such as , Dy, and Gd are preferable from the viewpoint of stability of properties.

The invention will now be explained with the following examples.

(Example) As raw material powder, silicon nitride powder CBET specific surface area 5 m"
7g, gelatinization rate 99%, impurity oxygen amount i, o weight%, F
e content 62 ppm) and various rare earth oxides or S
iO□ powder was used.゛The silicon nitride powder was subjected to acid cleaning treatment using hydrochloric acid to reduce the iron content.

Using these powders, the results are shown in Table 1! 11 After blending and mixing, press molding at It/cd to 140
It was calcined at 0''C.

After applying a paste of 8N powder with a particle size of 1 to 5 μm to a thickness of 1=10 mm to the obtained molded body, 0.2 To
Impurities were removed by heat treatment at 1350° C. under reduced pressure of rr.

After that, 1~10mm of glass mainly composed of 5iOz
It was applied to a thickness of .

The molded body thus treated was placed in a hot isostatic pressure firing furnace and fired under the firing conditions shown in Table 1.

Next, the iron content of the sintered body after glass removal was determined by IC.
In addition to quantifying P by analysis, 14 test pieces were cut out from each sintered body and heated at room temperature according to JISRI601.
The average values of four-point bending strength at 1200°C and 1400°C and oxidation weight gain at 1400″C were determined.

In addition, the source of fracture in eight bending test pieces for each sample was observed, and the number of specimens in which the source of fracture was abnormal grain growth due to iron was determined.

Furthermore, the average particle size of silicon nitride crystal particles was calculated from an electron micrograph of the sintered body.

The results are shown in Table 1.

In addition, as a comparative example, molded bodies were produced using two types of raw materials with different amounts of iron, and were fired under the conditions shown in Table 1 using the gas pressure firing method, and the properties were evaluated in the same manner.

(The following is a blank space) In Table 1, FIG. 1 shows an electron micrograph of the structure of the abnormal grain growth area that was the source of fracture in the bending test piece for the Nα4 sample.

According to Table 1, Na with iron content exceeding 50 ppm
Sample No. 4 has Fe and S from the center of the fracture source in Figure 1.
i is detected, and it is understood that there is abnormal grain growth around it. In addition, there were variations in characteristics, and the average value was also low.

On the other hand, most of the products of the present invention in which the iron content was suppressed to 50 ppm or less showed grain boundary fracture, almost no abnormal grain growth, and had good properties.

On the other hand, the oxidation resistance of N117 samples with a (excess oxygen/rare earth element oxide) molar ratio lower than 2 is poor, and the oxidation resistance of N117 samples with a molar ratio exceeding 25 is 140.
Low strength at 0°C.

In addition, as a comparative example, in the samples of Na17.18, which were prepared by firing a composition within the scope of the present invention at high temperature under nitrogen gas pressure, there was no abnormal grain growth due to iron on the structure, and the source of fracture was intergranular fracture. It was found that there was almost no effect of iron. However, both had large crystal grain sizes and were inferior to the products of the present invention in terms of characteristics.

(Effects of the Invention) As detailed above, according to the silicon nitride sintered body of the present invention, the amount of excess oxygen can be increased in a simple ternary composition of 5iJ4REzO* (rare earth oxide)-5iO□ (excess oxygen). By reducing the iron content in the sintered body, it is possible to reduce the variation in properties of the sintered body, which has excellent room temperature and high temperature strength.

This makes it possible to further promote mass production of the silicon nitride sintered body as a high-temperature structural material for heat engines, etc., as various mechanical structural parts, and to improve the reliability of its characteristics.

[Brief explanation of the drawing]

FIG. 1 is an electron micrograph showing the Mi woven structure of the abnormal grain growth area due to iron.

Claims (1)

[Claims] 70 to 99 mol% silicon nitride and 0 rare earth element oxides
．． 1 to 5 mol% and excess oxygen (SiO_2 equivalent amount) 25
Consisting of mol% or less (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, and an average grain size of silicon nitride crystals of 7 μm or less, wherein the iron (Fe) content in the sintered body is 5
A silicon nitride sintered body characterized by having a content of 0 ppm or less.