JPS63288967A - Tough ceramic material for gas turbine and its production - Google Patents

Abstract

PURPOSE:To obtain the titled material which has excellent strength, toughness, heat resistance, corrosion resistance and wear resistance by mixing high-purity Si3N4 powder and high-purity SiC whiskers, molding the mixture, and adding and incorporating silica to and into the mixture, then forming a capsule on the molding and sintering the molding with a hot isostatic press. CONSTITUTION:(B) 10-40% SiC whiskers contg. <=0.5% metal impurities is mixed with (A) Si3N4 powder contg. <=0.5% metal impurities except Si and having <=1mu average grain size and SiO2 is added to this mixture or is subjected to high-temp. oxidation at 900-1,500 deg.C in an O2-contg. atmosphere or air or the like to form an SiO2 (C) film on the surfaces of the respective components A, B. This mixture is then molded to a desired shape by slurry cast molding, injection molding or extrusion molding. The molding is then vacuum-sealed into a silica glass tube in which, for example, BN is packed to form the capsule. The capsule is thereafter treated at 1,650-2,200 deg.C under 100-2,000atm. pressure by the hot isostatic press. The capsule is removed after cooling and the molding is heat-treated at, for example, about 1,400 deg.C in an H2 atmosphere. The tiled material consisting of Si3N4 having 0 open pore rate and having the thin layer of 3-20% silica at the boundary is thus obtd.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to materials for gas turbines used for vehicles, aircraft power generation, etc., and particularly relates to materials with improved heat resistance, corrosion resistance, and wear resistance. The present invention relates to a strong ceramic material for gas turbines and a method for manufacturing the same.

[Conventional technology] Silicon nitride, which has been considered for use in gas turbines, stationary blades, combustors, etc. for vehicles, aircraft, marine power generators, etc., has a relatively strong strength among ceramics. Although it has high thermal shock resistance, it is characterized by low toughness as a general characteristic of ceramics. For this reason, attempts have been made to toughen ceramics by mixing silicon nitride with ceramic whiskers such as silicon carbide.

The conventional method for manufacturing whisker-reinforced silicon nitride is to use silicon nitride powder and alumina as a sintering aid.

A few percent or more of other metal compounds such as ittria are mixed in, whiskers are further mixed in, molded, and pressure sintered.

[Problems to be solved by the invention] However, in order to make the silicon nitride matrix denser, a large amount of sintering aid is mixed, which causes the molded sintered body to have a high temperature (1,000°C or higher). There is a problem that the strength of the steel is reduced, and the corrosion resistance against high-temperature combustion gas is also reduced.

Also, if no sintering aid is mixed, the above heat resistance will not be achieved.

Although high corrosion resistance can be achieved, there are problems with sintering and difficulty in improving the density of the matrix.

The present invention has been made in consideration of the above circumstances, and is a strong ceramic material for gas turbines that has good high-temperature strength and corrosion resistance and has a dense matrix. The purpose is to provide materials and methods for producing the same.

[Means and effects for solving the problems] In order to achieve the above-mentioned objects, the present invention provides silicon carbide whiskers with 10
~40%, the content of metals other than silicon is 0.5% or less, and the open porosity is substantially O. The matrix is silicon nitride, and the boundary is a layer mainly composed of silica or silicon oxynitride. It is a strong ceramic material for gas turbines with a metal impurity content of 0.5% or less and an average grain size of 1μ.
Metal impurity content ω0.5 for silicon nitride powder of m or less
% or less of silicon carbide whiskers to be 10 to 40%, molding is performed using this mixture, and the mixture or molded product contains 3 to 20% silica by adding silica or high temperature oxidation. Then, capsules are formed on the molded body and heated to 1.650 to 2,200°C.
This is a method for producing a strong ceramic material for gas turbines, which is sintered by hot isostatic pressing.

The strong ceramic material according to the present invention is a ceramic having a silicon nitride matrix and reinforced with silicon carbide whiskers, and has the following characteristics in particular compared to conventional materials.

a. The content of metals other than silicon, which cause a decrease in high-temperature strength and corrosion resistance of silicon nitride, was set to 0.5% or less.

b. In order to deal with the accompanying difficulty in sinterability of the matrix, 3 to 20% of silica was added to create a dense structure.

C1 carbonization [Also forms a thin layer of silica at the interface between the nitride whiskers and the silicon nitride matrix, improving strength and toughness compared to when the whiskers and matrix are directly bonded.

[Embodiments] Hereinafter, preferred embodiments of the present invention will be described based on the attached drawings.

The accompanying drawings show the manufacturing process of a strong ceramic material for gas turbines.

In the figure, silicon carbide whiskers 1 as raw materials and silicon nitride powder 2 as a matrix are mixed 3 and then molded 4.
do. Containing silica 5 in this mixture or molded body,
A capsule is formed 6 in the molded body. Thereafter, sintering (densification) is performed using a hot isostatic press (HIP) 7 to obtain a tough ceramic material 8.

Since it is difficult to manufacture the above-mentioned tough ceramic materials using conventional manufacturing methods, the present invention has made it possible to manufacture such materials using the following manufacturing method.

The manufacturing method will be explained below for each step.

■ Raw Materials The tough ceramics of the present invention use silicon nitride as a matrix, but in order to facilitate densification in the subsequent sintering stage, silicon nitride powder with an average particle size of 1 μm or less is preferable. must be 0.5 μm or less.

With silicon nitride powder having an average particle size of 1 μm or more, it is difficult to set the open porosity of the matrix to O.

The silicon nitride powder also needs to have a content of metal impurities other than silicon of 0.5% or less, preferably 0.2% or less. If the metal impurities exceed this, the high temperature strength and corrosion resistance of the silicon nitride matrix will decrease, especially against corrosion in high temperature and high speed as gas turbines.
This is important.

It is necessary to uniformly form silica on the surface of this silicon nitride powder. This makes it possible to densify the silicon nitride powder at the stage of hot isostatic press sintering of the pattern. The appropriate amount of silica is in the range of 3 to 20% relative to silicon nitride.

If the amount of silica is less than this, it will have little effect on sintering silicon nitride, and if the amount of silica exceeds this, it will become densified, but
The high temperature strength of the matrix is reduced.

Silicon carbide whiskers are mixed with the above silicon nitride powder to strengthen the matrix. The reason for selecting silicon carbide whiskers is that silicon carbide, like silicon nitride, has excellent high-temperature strength at temperatures of several hundred degrees Celsius or more and high corrosion resistance, and also has a higher modulus of elasticity than silicon nitride. This is because it has a coefficient of thermal expansion of , which reduces the tensile stress load on the silicon nitride matrix and has a large effect on toughening it.

It is also necessary to form a homogeneous silica film on the surface of these silicon carbide whiskers. After sintering,
This is because a thin layer of silica or silicon oxynitride is formed between the silicon nitride matrix and the silicon carbide whiskers, producing the above-mentioned toughening effect.

The thickness of this silica film is 1/1 of the diameter of the whisker.
100 or more and about 1/20 is good. If it is less than this, it is insufficient to prevent direct and strong bonding between the matrix and the whiskers, and if it is more than this, the strength of the material will decrease at high temperatures. This value corresponds to about 3 to 20% by weight of the surface silica relative to the whisker.

The method for forming silica on the surface of the silicon nitride powder and silicon carbide whiskers described above is as follows:
The method may be performed by surface oxidation in an oxygen-containing atmosphere such as air at a high temperature, or by a method in which a silica film is deposited on the surface using a silica sol solution. Also, Si
The silica film may be formed by a CVD method using Cρ4+H2+H20 gas or the like. - In addition, this silica formation may be performed initially at the stage of each raw material, at the stage of mixing silicon nitride powder and silicon carbide powder, or at a stage after molding the mixture.

■ Mixing nitriding (mixing the silicon carbide whiskers with the silicon carbide whiskers, but preferably dispersing them in a liquid such as water, a non-aqueous solvent, molten wax, or molten resin will improve the dispersion effect) be able to.

The amount of silicon carbide whiskers to be mixed with the silicon nitride powder is selected so that the latter is 10 to 40%.

The reason for this is that below 10%, the toughening effect is low;
This is because it becomes difficult to increase the packing density during mixing and molding.

■ Molding The above mixture is molded into a predetermined shape using methods such as plaster casting, injection molding, extrusion molding, and isostatic press molding.

■ Capsule formation and hot isostatic press sintering The compact obtained through the above steps is sintered and densified by hot isostatic press.

The temperature for hot isostatic pressing must be selected at a temperature at which the silica contained in the silicon nitride powder softens or melts and sintering progresses, and must be 1,650°C or higher.

Even under the high pressure of hot isostatic pressing, if the temperature exceeds 2,200°C, silicon nitride etc. will decompose, and silicon carbide whiskers will further deteriorate, so the upper limit for sintering is 2,20°C.
It is necessary to set the temperature to 0°C. The higher the pressure, the greater the effect.
Pressure range of normal hot isostatic press, i.e. 100~2,
A pressure of about 0,000 atmospheres is sufficient.

Hot isostatic pressing under such conditions is carried out by the capsule method, that is, after forming an airtight and flexible capsule on the molded article. Regarding this capsule molding method, if the shape of the molded product is relatively simple, vacuum sealing in a silica glass tube filled with nitrided boron powder may be used, but if the shape of the molded product is complex, In this case, the method for which the present inventors have previously filed a patent application (Japanese Patent Laid-Open No. 15902/1983
No., JP-A-61-17902) is effective.

If the methods of these earlier applications are applied to the present invention, the first layer on the molded body is a layer of nitrided boron powder as a release agent, and the second layer is a pressure transmitting material during hot isostatic pressing. As a third layer, a low melting point glass layer such as silica glass + alumina powder, such as Vilex, is formed to make the capsule airtight, and after making the capsule airtight by heating in a vacuum, etc., it is heated to the required temperature and pressure. Hot isostatic press sintering is performed.

After hot isostatic press sintering, the capsule is removed, and the result is a dense silicon nitride matrix reinforced with silicon carbide whiskers and containing almost no metal impurities, with silica or silicon oxide at the matrix-whisker interface. A gas turbine component is obtained consisting of a strong ceramic material with a thin layer of material.

Next, the present invention will be explained in more detail using specific examples.

Example 1 Silicon nitride powder with metal impurities of 0.1% or less and an average particle size of 0.3 μI was oxidized in dry air at 1,300°C to reduce the concentration to about 5%.
of silica was formed. Similarly, silicon carbide whiskers having an average diameter of 2 .mu.m were oxidized in dry air at 1,400.degree. C. to form a silica layer of approximately 0.0000000000000000000000000000000000000000000000000 on the surface. This silicon nitride powder and silicon carbide whiskers are 70%:3
0%, and kneaded at 100° C. in a molten material II mainly consisting of wax. Thereafter, it was molded into the shape of a gas turbine movie using an injection molding machine. Layers were formed on this molded body, with the first layer being nitrided boron powder, the second layer being silica and alumina mixture powder, and the third layer being Vycol powder, and heated to form an airtight capsule. After, 1.90
Hot isostatic pressing was performed at 0°C and 2,000 atm. After removing the capsule, a test piece was taken, and it was found to be a strong ceramic material with an apparent porosity of 0.2% or less, O2 metal impurities, and silica as a main component at the whisker-matrix boundary.

For comparison, silicon nitride powder and silicon carbide powder, in which no silica layer was formed by high-temperature oxidation, were molded in the same way and subjected to hot isostatic press sintering, and results were found.

Example 2 Silicon nitride powder with metal impurities of 0.1% or less and an average particle size of 0.4 μm and silicon carbide whiskers with metal impurities of 0.1% or less were each dispersed in an aqueous solution and then mixed. The shape of the gas turbine stationary blade was then fabricated by performing mud casting using an unglazed mold. This molded body was impregnated with a 20% aqueous solution of silica sol and dried at 1,000°C.
The molded body was heated to about 17% silica. This was vacuum-sealed into a Vycor glass tube filled with boron nitride to form a capsule, then processed in a hot isostatic press at 2,000°C and 1,500 atm, and after cooling, After removing the Vycor glass capsule,
Further, heat treatment was performed at 1,400° C. in a nitrogen atmosphere. As a result of investigating the microscopic structure of the obtained gas turbine stationary blade parts,
Habo Theory W! It was confirmed that the i-degree dense polycrystalline silicon nitride was reinforced with silicon carbide whiskers, and a layer mainly made of silicon oxynitride was formed at the boundary between the matrix and the whiskers.

For comparison, gas turbine stationary blade parts were fabricated using silicon nitride powder to which 1% of aluminum oxide and yttrium oxide were added using the same manufacturing process. When exposed to the high-temperature, high-speed gas flow at the rear of the combustor, the surface temperature of the parts was 1,200℃ when placed together with the stator vane parts without metal oxide addition.
, at a gas flow rate of 200 m/s for 10 hours, the parts to which aluminum oxide and yttrium oxide were added were significantly corroded, whereas the parts to which no metal oxide was added were hardly corroded.

Example 3 Silicon nitride powder with metal impurities of 0.1% or less and an average particle size of 0.6 μm was dispersed in an aqueous solution and ball milled for 60 hours, resulting in an average particle size of 0.4 μm.

The amount of produced silica was 5%. This silicon nitride powder is mixed with silicon carbide whiskers at a concentration of 40%, and colloidal silica is added at a concentration of 3% of the silicon nitride powder and silicon carbide whiskers to ensure thorough mixing and dispersion. went. This slurry was spray-dried, the resulting granules were filled into a rubber mold, and a part shape of a combustor liner was manufactured by isostatic press molding.

The obtained molded body was coated with boron nitride powder, placed in a graphite mold filled with silica/glass powder, heated and pressed at 2,000°C and 500 direction f/C12 using a hot press, Hot equal partial pressure pressing was performed using fused silica glass as a capsule. The resulting combustor liner part was dense and had sufficient shape accuracy with only slight surface machining.

[Effects of the Invention] As is clear from the above explanation, the present invention exhibits the following excellent effects.

■ The silicon nitride matrix is reinforced with silicon carbide whiskers, so it has higher strength and toughness than regular silicon nitride.

■ Since the silicon nitride powder is uniformly covered with silica using silica sol, sinterability is improved and the matrix becomes denser by HIP sintering.

■ Because the silicon carbide whiskers are homogeneously covered with silica using silica sol, sintering corrosion is also prevented due to the If of silica at the interface between the silicon nitride matrix and the silicon carbide whiskers.
is formed, the strength of the boundary becomes appropriate, and the strength and toughness are improved.

■ It contains almost no metal compounds other than silicon as sintering aids, so it has high high temperature strength and high corrosion resistance.

■ Since it is a strong ceramic material with the above-mentioned characteristics, it can be applied to gas turbine parts, especially moving blades and stationary blades.

[Brief explanation of the drawing]

The accompanying drawings are diagrams showing the manufacturing process of the tough ceramic material for gas turbines of the present invention. In the figure, 1 is silicon carbide whisker, 2 is silicon nitride powder,
5 is a silica-containing material, and 8 is a strong ceramic material.

Claims (2)

[Claims] (1) The matrix is silicon nitride containing 10 to 40% silicon carbide whiskers, the content of metals other than silicon is 0.5% or less, and the open porosity is essentially 0, and silica or A strong ceramic material for gas turbines that has a layer mainly composed of silicon oxynitride. (2) Average particle size of 1 μm with metal impurity content of 0.5% or less
Metal impurity content 0.5% for the following silicon nitride powder
The following silicon carbide whiskers are mixed to a concentration of 10 to 40%, and this mixture is used for molding, and 3 to 20% of silica is added to the mixture or molded product by silica addition or high temperature oxidation. A method for producing a strong ceramic material for a gas turbine, comprising: forming a capsule on a molded body and sintering it by hot isostatic pressing at 1,650 to 2,200°C.