JPH04317469A - Production of silicon nitride sintered body - Google Patents

Abstract

PURPOSE:To simply produce a silicon nitride sintered body having excellent oxidation resistance. CONSTITUTION:Starting material for a silicon nitride sintered body is molded, this molded body is heated in an oxidizing atmosphere to form a silicon oxide- based surface layer on the surface of the molded body and then the molded body is fired. A silicon nitride sintered body having excellent oxidation resistance can simply be produced.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a silicon nitride sintered body.

0002

[Prior Art] Silicon nitride (Si3N4) sintered bodies have excellent high-temperature strength, thermal shock resistance, and corrosion resistance.
It is used as gas turbine engine parts, heat exchanger materials, high-temperature bearings, and high-temperature roll materials for steel manufacturing. However, this sintered body has low oxidation resistance and is oxidized when used at high temperatures.

[0003] For this reason, various methods have been considered to improve the oxidation resistance, one of which is to coat the surface with an oxide. To coat this oxide to form a surface layer, (1) yttrium oxide (Y2O3) and aluminum oxide (Al2O3), which serve as sintering aids, are used.
3), or a method in which a slurry of silicon oxide (SiO2) is impregnated into a molded body of a sintered body raw material, and then the molded body is pre-fired to form an oxide layer on the surface, or (2) the above-mentioned Y2O3 A method of generating a gas containing a sintering aid such as from a packing powder placed outside a molded body of a sintered body raw material and adsorbing it to the surface of the molded body to form an oxide-rich surface layer; (3) A method is known in which the surface of a sintered body is coated with an oxide by a chemical vapor phase method or the like.

0004

[Problem to be solved by the invention] Among the above methods, (
If a surface layer rich in Y2 O3 or Al2 O3 is formed in 2), a large amount of glass phase or crystal phase with low oxidation resistance will be formed on the surface of the sintered body during sintering.

[0005] Furthermore, when a coating layer is applied to the surface of the sintered body in (3), there is no strong chemical bond with the Si3N4 sintered body that is the base, and furthermore, due to distortion due to the difference in coefficient of thermal expansion, , the surface layer may peel off. Therefore, coating with a surface oxidation layer does not necessarily improve oxidation resistance.

[0006] Furthermore, a sintered body obtained by applying an SiO2 component to the surface of a molded body and firing the same becomes a sintered body having excellent oxidation resistance. However, the means for applying the SiO2 component is complicated to work with, such as applying a slurry or using packing powder, and when manufacturing products with complex shapes, the thickness of the surface layer tends to be uneven. It has the disadvantage of poor reproducibility.

The present invention has been made in view of the problems of the prior art described above, and it is an object of the present invention to provide a method for easily producing a silicon nitride sintered body having excellent oxidation resistance.

[0008]

[Means for Solving the Problems] The present invention involves forming a surface layer mainly composed of silicon oxide on the surface of a molded product obtained by molding a raw material for a silicon nitride sintered body by heating the molded product in an oxidizing atmosphere. A method for producing a silicon nitride sintered body is characterized by comprising a first step and a second step of forming a silicon nitride sintered body by firing a molded body having the surface layer.

[0009]

[Operation] In the first step of the present invention, at least a portion of the silicon nitride present on the surface of the molded body is oxidized by heating in an oxidizing atmosphere and changed to silicon oxide, and in the second step, the silicon oxide is When fired, it forms on the surface of the sintered body as a layer that is integral with the matrix.

0010

According to the present invention, a silicon nitride sintered body having excellent oxidation resistance can be easily produced. The surface layer of the obtained sintered body is formed integrally with the matrix, so
Adhesion between the surface layer and the sintered body matrix is good. This further prevents the surface layer from peeling or cracking.

[0011]

【Example】

(Specific Examples of the Present Invention) Hereinafter, more specific examples of the present invention will be described.

[0012] The method for producing a silicon nitride sintered body according to the present invention involves heating a molded body obtained by molding a raw material for a silicon nitride sintered body in an oxidizing atmosphere so that silicon oxide is mainly formed on the surface of the molded body. A surface layer is formed (first step), and the molded body is fired (second step).

[0013] In the present invention, the silicon nitride sintered body produced is one made of silicon nitride (Si3N4), or a substance such as sialon in which other elements are dissolved in a part of silicon nitride (silicon nitride). related substances).

In the first step, the raw material for the silicon nitride sintered body may be the silicon nitride or silicon nitride-related substance itself, or the silicon nitride or silicon nitride-related substance is produced by sintering in the second step. It may also be a composition.

For example, Sialon, which is a silicon nitride-related substance, has the α type (general formula Mx (Si, Al)12(O,
N) 16 (0<x≦2, M is at least one of Li, Mg, Ca, Y, etc.); α′-Si3N
4) or β type (general formula Si6-
ZAlZOZN8-Z (denoted by 0<z≦4.2; also called β'-Si3N4), etc.

[0016] As a composition for producing α-sialon in which Y is dissolved in solid solution by sintering in the second step, Si3N4-
AlN-Y2O3, Si3N4-AlN-Al
2 O3 -Y2 O3 , Si3 N 4-Al2
Examples include mixtures such as O3 -Y2 O3 -YN. In addition, the compositions that generate β-sialon by sintering in the second step include Si3N4-Al2O3, S
i3N4-AlN-Al2O3, Si3N
Examples include mixtures such as 4-Al2O3-SiO2. In addition, in the composition that produces α-sialon,
By controlling the blending ratio of the composition, α-sialon and β-sialon can be generated at the same time. For example, α-sialon and β-sialon containing Y as a solid solution
When trying to form SiAlON at the same time, Si3
Y2 O3 −AlN, Y2 O3 − for N 4
AlN-Al2 O3 or YN-Al2 O3
By controlling the blending ratio of , α-sialon and β-sialon can be generated simultaneously. This is because Al and O form a solid solution in Si3N4 to form β-sialon. In this case, even if the mixture is the same, the production ratio of α-sialon and β-sialon differs depending on the firing temperature and firing time, so it is possible to obtain a sintered body with the desired production ratio by controlling the firing conditions. can.

[0017] Although there are no restrictions on the particle size of silicon nitride as a raw material, it is a high-purity raw material produced by the imide method that has a low content of calcium (Ca), an impurity that significantly accelerates oxidative deterioration. It is better to use

It is preferable to add a sintering aid to the raw material for the silicon nitride sintered body.

As the sintering aid, commercially available high-purity oxide synthetic powders such as Y2 O3, Al2 O3, AlN, etc. may be used. There is no limit to the amount of the sintering aid added, but in order to obtain a dense sintered body, the amount of the sintering aid added is at least 0.1.
It is preferable to add more than % by weight (wt%). Furthermore, if too much sintering aid is added, the properties of the glass phase formed will be dominated by the sintering aid and the oxidation resistance will deteriorate, so the amount of sintering aid added should be 8 wt% or less. If the content is good, more preferably 5 wt% or less, the oxidation resistance will be significantly improved.

Further, carbides, nitrides, etc. may be added to the raw material for the purpose of improving strength and toughness.

[0021] The average particle size of the raw material powder is approximately 15 μm.
The following range is desirable. If the average grain size is larger than this, grain growth will be significant and strength will likely decrease.

In the first step, in order to mold the raw material for the silicon nitride sintered body, after thoroughly mixing the raw material powder,
Molding is performed using a molding method such as a uniaxial molding method, a rubber press method, a slip casting method, or an injection molding method.

When an organic binder is added for molding, the binder is sufficiently removed before the next oxidation step. When a molded article containing a large amount of organic binder used in injection molding or the like is heated in the air, it may crack or break. In addition, if the strength of the molded body is weak and it will break due to thermal stress associated with oxidation treatment, the molded body may be heated to a temperature of 1100 to 1300 in an inert atmosphere.
It is preferable to perform heat treatment in the temperature range of 0.degree. C., so-called calcination, to increase the strength of the compact.

The atmosphere in which the oxidation treatment is carried out is not particularly limited as long as it contains oxygen, but it is preferable to use the atmosphere because there are fewer restrictions on the equipment and running costs are lower.

[0025] Since oxidation of Si3N4 powder progresses at a temperature of 600°C or higher, it is preferable to heat the molded body under conditions such that the surface temperature of the molded body becomes 600°C or higher in the oxidation treatment. However, if the temperature is too high, not only the surface but also the inside of the molded body will be oxidized, resulting in a Si3N4 sintered body.
Original properties such as strength change. For this reason, it is preferable to carry out the treatment within a range where the surface temperature of the molded body is 1500°C or less. Further, as a method of creating a temperature gradient inside the molded body and selectively oxidizing only the surface, a method that utilizes heat transfer by radiation instead of heat transfer by gas convection may be adopted.

[0026] As a heating means, it is common to use an ordinary resistance heating type heater such as a Kanthal furnace or a SiC heating element furnace. A method of partially heating may also be used.

[0027] Through this first step, S near the surface of the molded body
A part of i3N4 is oxidized to form silicon oxide (SiO2
) is formed. Further, if a substance other than Si3N4 is contained in the molded article, it will be oxidized to produce an oxide, and the oxide may be contained in the surface layer. The thickness of the surface layer is preferably in the range of 2 to 500 μm. When the surface layer is thinner than 2 μm, its function as an antioxidant layer is insufficient, and when it is thicker than 500 μm, the surface layer may peel off.

The firing method in the second step includes normal pressure sintering, gas pressure sintering, hot isostatic pressing (HIP) sintering, and the like. Among these, it is effective to perform pre-sintering under conditions that eliminate open pores, and then perform post-HIP sintering in which the pre-sintered body is HIPed to obtain a dense sintered body, in order to increase the material strength. The firing temperature is preferably in the range of 1500 to 1900°C. When performing pressure sintering such as HIP, the pressure is preferably in the range of 100 to 2000 atm, and the processing time is preferably in the range of 1 to 4 hours. If the pressure and temperature exceed this range, a dense sintered body cannot be obtained, especially if the sintering aid is small.

Furthermore, in HIP sintering, the molded body and the surface layer become denser and the adhesion between the molded body and the surface layer is also improved. In addition, since a surface layer is formed on the molded body in the first step, HIP can be applied without the need for glass encapsulation.
Can be sintered.

[0030] After sintering, depending on the sintering conditions, SiO2 may remain in the surface layer, or SiO2 may become Si3N.
There are cases where SiO2 reacts with SiO2 and the like and changes into SiAlON, etc., leaving no SiO2.

According to the present invention, a silicon nitride sintered body having improved oxidation resistance, high density, and high strength can be produced.

(Example) Examples of the present invention will be described below.

(Example 1) 5 wt% of Y2O3 and 1% of Al2O3 were added to silicon nitride powder produced by the imide method.
wt% was added and wet-mixed in a polypot using ethanol as a medium. After drying the mixed powder, it was molded into pellets with a diameter of 20 mm and a thickness of 10 mm by uniaxial molding. The following four types of treatments (a) to (d) are applied to this molded pellet.
I did it. (a) The molded product pellets were inserted into an atmospheric furnace heated to 1000° C., heated for 30 seconds, and then taken out of the furnace and allowed to cool. This operation was repeated 5 times. (b) The molded product pellets were heated for 30 seconds in an air furnace heated to 1200°C, and then quickly taken out and allowed to cool at room temperature. (c) No treatment was applied to the molded pellets. (d) The formed pellets were heated in a vacuum furnace at 1200° C. for 10 minutes, cooled to room temperature, and then taken out of the furnace.

The sample subjected to the above treatment was sintered at 1750° C. for 2 hours. In order to test and evaluate the oxidation resistance of the obtained sintered body, the sintered body was left in the atmosphere at 1200°C for 1000 hours, and after cooling, the weight increase due to oxidation was measured.
Furthermore, the surface condition of the sintered body was observed. The results are shown in Table 1.

[Table 1]

As is clear from Table 1, the sintered body of the comparative example had a large amount of weight gain due to oxidation and oxidation pits, whereas the sintered body of this example had little weight gain due to oxidation. Furthermore, no oxidation pits were generated, indicating that the oxidation properties were excellent.

(Example 2) 4 wt% of Y2 O3 and 1 wt of Al2 O3 were added to the same silicon nitride powder as in Example 1.
%, mixed and molded in the same manner as in Example 1 to produce molded pellets. The following three types of treatments (e) to (g) were performed on the molded pellets. (e) The molded product pellets were heated for 1 minute in an atmospheric furnace heated to 1200°C, and then promptly taken out from the furnace and allowed to cool in the atmosphere. Thereafter, it was sintered at 1750°C for 4 hours in a nitrogen atmosphere. (f) The molded pellets were sintered as they were at 1750° C. for 4 hours in a nitrogen atmosphere. Thereafter, silicon carbide was deposited on the surface of the obtained sintered body by CVD. (g) The shaped pellets were sintered as they were at 1750° C. for 4 hours in a nitrogen atmosphere. Thereafter, SiO2 was deposited on the surface of the obtained sintered body by CVD.

[0037] In order to test and evaluate the oxidation resistance of the obtained sintered body, the sintered body was inserted into an atmospheric furnace heated to 1400°C, held for 100 hours, and then cooled in the furnace. When the sample was taken out after the temperature reached room temperature and the surface of the sintered body was observed, it was found that an oxide layer mainly composed of SiO2 was formed on the surface of the sample treated in (e). Further, in the sample subjected to the treatment (f), peeling of the silicon carbide vapor deposited film was observed at the corner portions, and the underlying silicon nitride sintered body was locally oxidized to generate oxidized pits. In the sample treated in (g), cracks occurred on the entire surface of the sintered body, and peeling occurred in some parts.

From this, the sintered body of this example ((e)
It can be seen that the sample subjected to the above treatment has excellent oxidation resistance, with no peeling or cracking occurring on the surface compared to the comparative examples (samples treated under (f) and (g)).

(Example 3) Si3 N4 94.5wt
%, Y2 O3 2.0 wt%, and AlN 3.5 wt%, and the obtained molded body was heated in oxygen at 1%.
It was heated at 300°C for 1 hour. As a result, a dense surface layer with a thickness of about 400 μm mainly composed of SiO2 was formed on the surface of the molded body. This surface layer has a porosity of approximately 0.7%
It is very dense, and the porosity inside is about 30.
% and was porous. This molded body was immersed in N2 for 1
HIP treatment was performed at 800° C. and 2000 atm for 2 hours. As a result, a dense sintered body with a porosity of about 0.1% both in the surface layer and inside was obtained. The crystal phase in the surface layer of this sintered body is 100% β'-Si3 N4, and the internal crystal phase is approximately 87% β'-Si3 N4 and α'-Si3
N4 It was about 13%.

The four-point bending strength of this sintered body is 13 at room temperature.
0 kg/mm2, and 70 kg/mm2 at 1400°C. Further, the weight increase when oxidized at 1300° C. for 100 hours was 0.05 mg/cm 2 or less, indicating that the oxidation resistance was improved.

For comparison, a compact of the above mixed powder was fired at 1300° C. for 6 hours in N 2 and then subjected to HIP at 1800° C. and 2000 atm for 4 hours in N 2 . However, the porosity of the obtained sintered body was 9.0%, and a dense sintered body could not be obtained.

(Example 4) A compact of the same mixed powder as in Example 3 was heated in air at 1450° C. for 1 hour. As a result, a dense surface layer with a thickness of about 500 μm mainly composed of SiO2 was formed on the surface of the molded body. This surface layer was very dense with a porosity of about 0.8%, and the inside was porous with a porosity of about 25%. This molded body was heated in N2.
HIP treatment was performed at 1850° C. and 2000 atm for 2 hours. As a result, a dense sintered body with a porosity of 0.2% in both the surface layer and the interior was obtained. The crystal phase in the surface layer of this sintered body is 100% β'-Si3 N4, and the internal crystal phase is approximately 90% β'-Si3 N4 and α'-Si3
N4 It consisted of about 10%.

The four-point bending strength of this sintered body is 12 at room temperature.
0 kg/mm2, and 71 kg/mm2 at 1400°C. Further, the weight increase when oxidized at 1300° C. for 100 hours was 0.05 mg/cm 2 or less, indicating that the oxidation resistance was improved.

For comparison, a compact of the above mixed powder was HIPed in N2 at 1850°C and 2000 atm for 4 hours.
was applied. However, the porosity of the obtained sintered body was 8.6%.
Therefore, it was not possible to obtain a dense sintered body.

Claims (1)

[Claims] 1. A first step of forming a surface layer mainly composed of silicon oxide on the surface of the molded body by heating a molded body formed from a raw material of a silicon nitride sintered body in an oxidizing atmosphere;
A method for manufacturing a silicon nitride sintered body, comprising a second step of forming a sintered body by firing a molded body having the surface layer.