JPH10212167A - Silicon nitride-base composite sintered compact and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a silicon nitride-base composite sintered compact having superior high-temp. strength and toughness in the temp. range from room temp. to 1,500 deg.C and excellent in high-temp. strength and creep resistance. SOLUTION: This composite sintered compact contains 0.5-25 pts.wt. hard particles of at least one selected from among silicides of Ta, Nb, Mo and W and SiC in a dispersed state to 100 pts.wt. silicon nitride compsn. consisting of 70-99mol% silicon nitride, 0.5-10mol% (expressed in terms of oxide) Lu and impurity-like oxygen as the balance. The silicon nitride exists as crystal grains having <=5μm average grain diameter and an average aspect ratio of >=5 and the hard particles exist as crystal grains having 1-5μm average grain diameter. The impurity-like oxygen and group IIIa elements on the periodic Table including Lu exist chiefly as a crystal phase of one or more selected from among apatite, YAM and wollastonite in the grain boundaries of the silicon nitride grains and hard grains.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is excellent in strength characteristics from room temperature to high temperature, excellent in fracture toughness, and excellent in creep resistance, especially for pistons, cylinders, valves, cam rollers, rocker arms, piston rings, piston pins, etc. Automotive parts, turbine rotors, turbine blades, nozzles, combustors, scrolls, nozzle supports, seal rings, spring rings, diffusers,
The present invention relates to a silicon nitride composite sintered body suitably used for a gas turbine engine component such as a duct, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, silicon nitride-based sintered bodies have been applied as engineering ceramics, particularly as heat engine structural materials, because of their excellent strength, hardness and thermal and chemical stability. Such a silicon nitride-based sintered body is obtained by adding and mixing a sintering aid such as an oxide of a Group 3a element in the periodic table to silicon nitride powder, forming the mixture, and then forming the mixture at 1500 to 2000 ° C. in a non-oxidizing atmosphere. At the following temperature. However, while the silicon nitride sintered body has excellent characteristics, it has a problem that strength and creep strength are reduced at high temperatures. Up to now, the problem of deterioration of the high-temperature properties has been improved by improving the sintering aid and changing the firing atmosphere and firing pattern, but no definitive measures have been reached. Is the current situation.

[0003] On the other hand, a silicon carbide-based sintered body has an excellent property of hardly deteriorating at a high temperature as in a silicon nitride-based sintered body, but has a low absolute strength and a low toughness. There is a problem of scarcity.

Therefore, recently, a composite sintered body obtained by adding silicon carbide to silicon nitride and firing the same has been disclosed in Japanese Patent Laid-Open No. 3-53.
74, and Japanese Patent Application Laid-Open No. 3-131571. In addition, since this composite sintered body contains silicon carbide in a dispersed manner, the sinterability of the system is reduced, and therefore, usually Y ₂
Enhance sinterability by the addition of Al ₂ O ₃ or the like in addition to the O rare earth oxide such as _3, thereby achieving a high density.

[0005]

According to the above prior art, by adding silicon carbide to silicon nitride,
The toughness can be increased as compared with the silicon nitride sintered body. However, the present inventors have examined the high-temperature characteristics of the above sintered body in more detail, and found that there is a problem that the creep resistance at high temperatures is poor. When a sintered body having such characteristics is applied to, for example, a turbine rotor, it is deformed or broken by operating for a long time under a stress load, which is a factor greatly hindering practical use.

This is thought to be largely due to the state of the grain boundaries in the sintered body, but there is no specific countermeasure, and the effect of improving the characteristics by combining silicon nitride and silicon carbide is sufficiently exhibited. There was no current situation.

Further, as described in JP-A-63-100067, when a grain boundary phase is formed of a composite oxide with two or more kinds of rare earth elements, the eutectic point is lowered, so that the creep resistance at high temperatures is also low. Was something.

Accordingly, the present invention has high-temperature strength characteristics and fracture toughness sufficient for use in automobile parts from room temperature to high temperature, for gas turbine engines, etc., and at 1500
An object of the present invention is to provide a silicon nitride composite sintered body having excellent creep resistance at a temperature of ° C and a method for producing the same.

[0009]

Means for Solving the Problems As described above, the present inventors have examined the strength at room temperature and high temperature, the fracture toughness, and the creep resistance. As a result of finding that it is greatly involved in the characteristics and further study, it was found that silicon nitride, Ta, N
b, Mo, W, at least one type of hard particles selected from silicides and SiC, and further containing Lu ₂ O ₃ as an oxide of a Group 3a element of the periodic table as a sintering aid. In the sintered body, the silicon nitride crystal has an average particle size of 5 μm.
m or less, as crystal grains having an average aspect ratio of 5 or more,
The hard particles are present as crystal particles having an average particle size of 1 to 5 μm, and the impurity oxygen and Lu are mainly apatite at grain boundaries of the silicon nitride crystal particles and the hard particles.
It has been found that by having at least one crystal phase selected from the group of YAM and wollastonite, a sintered body having excellent creep resistance at high temperatures and high strength can be obtained.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The silicon nitride composite sintered body of the present invention contains a silicon nitride component and at least one kind of hard particles selected from silicides of Ta, Nb, Mo, W and SiC. Lu in the silicon nitride component as a sintering aid
Contains (lutetium) compound and impurity oxygen. Here, the term "impurity oxygen" refers to the remaining oxygen obtained by subtracting the stoichiometric oxygen contained in the Group 3a element compound of the periodic table such as Lu ₂ O ₃ as an additive from the total amount of oxygen in the sintered body. Quantity
Most of them are mixed as oxygen contained in the silicon nitride raw material or added silicon oxide, and all of them are considered to exist as, for example, SiO ₂ containing a chemical bond of Si—O.

The silicon nitride sintered body of the present invention is structurally composed mainly of a silicon nitride crystal phase, and has an average particle size of 5 μm or less, particularly 3 μm or less, and an average aspect ratio of 5 or more. In particular, β-Si _{3 of} 7 or more needle-like and fine crystals
Consisting of N _4. In addition to the silicon nitride crystal phase, a hard particle crystal phase exists as crystal particles having an average particle size of 1 to 5 μm. In addition, grain boundaries exist between these crystal phases. As described above, by allowing the silicon nitride crystal phase to exist as fine needle-like crystals, the strength and toughness of the sintered body can be increased, and the average grain size is larger than 5 μm, or the average aspect ratio is larger than 5. If it is too small, the effect of improving high-temperature strength and toughness is not sufficiently exhibited. Further, when the average particle size of the hard particle crystal phase is smaller than 1 μm, the effect of improving fracture toughness is small, and when the average particle size is larger than 5 μm, inconveniences such as impairing sintering and causing insufficient sintering occur.

Further, at the grain boundaries of the silicon nitride crystal and the hard particle crystal phase, a crystal (hereinafter referred to as Lu-Si) composed of at least Lu, Si (silicon), O (oxygen) and N (nitrogen).
—ON-based crystal), specifically, Lu ₄ Si ₂
At least one crystal phase of YAM represented by O ₇ N ₂ , apatite represented by Lu ₁₀ Si ₇ O ₂₃ N ₄ , and wollastonite represented by LuSiO ₂ N is present. In addition, a glass phase containing the above element may be present in the grain boundary phase when the crystallization is not sufficient other than the above crystal phase. The presence of such a glass phase may slightly decrease the mechanical properties at high temperatures, but has little effect on oxidation resistance and fracture toughness, so that the above-mentioned crystal phase is clearly detected by X-ray diffraction or the like. There is no particular problem as long as it is at the required level.

The composite sintered body of the present invention is characterized in that, based on the composition, a silicide of Ta, Nb, Mo, W and 100 parts by weight of a silicon nitride component comprising silicon nitride, Lu ₂ O ₃ and impurity oxygen are used. At least one kind of hard particles selected from SiC is contained in a ratio of 0.5 to 25 parts by weight. The reason why the amount of the hard particles is limited to the above range is that when the amount of the hard particles is less than 0.5 part by weight, the effect of improving the high temperature properties, fracture toughness and creep resistance by combining silicon nitride and the hard particles is obtained. Is not expected, and if it exceeds 25 parts by weight, sinterability is reduced and strength is deteriorated. In terms of characteristics, the amount of the hard particle component is 1 to 100 parts by weight of the silicon nitride component.
Desirably, the amount is from 10 to 10 parts by weight.

The composition of the silicon nitride component is as follows: 70 to 99 mol%, particularly 85 to 97 mol%, of silicon nitride;
is 0.5 to 10 mol%, particularly 1 to 7 mol% in terms of Lu ₂ O _3.
Mol%, and the balance consists of impurity oxygen. Particularly, impurity oxygen is 1 to 20 mol%, particularly 2 to 14 mol% in terms of silicon oxide.
Mol%. If the amount of silicon nitride is less than 70 mol%, high-temperature strength is not exhibited. If the amount of Lu is less than 0.5 mol%, densification is insufficient. The creep property deteriorates. On the other hand, if the amount of impurity oxygen is less than 1 mol%, a compound having low resistance to high-temperature oxidation such as melilite, which is a compound of silicon nitride and Lu ₂ O ₃ , is likely to be formed at the grain boundary.
If the content exceeds 20 mol%, the volume fraction of the grain boundary phase increases, and the high-temperature characteristics tend to deteriorate.

Further, according to the present invention, in the silicon nitride component, the molar ratio of the amount of impurity oxygen in terms of SiO _{2 with} respect to the oxide (RE ₂ O ₃ ) of a Group 3a element of the periodic table containing Lu (RE ₂ O ₃ ). (SiO ₂ / RE ₂ O ₃ ) is preferably less than 2, especially 1 to 1.9, more preferably 1.2 to 1.8. This is because when the molar ratio is 2 or more, the YAM,
This is because precipitation of crystal phases such as apatite and wollastonite cannot be expected, and as a result, high-temperature strength is reduced and creep resistance is also deteriorated.

Further, according to the present invention, it is most preferable that the element of Group 3a of the periodic table is composed of only Lu and that there is no other Group 3a element of the periodic table other than Lu. This is because Lu has the smallest ionic radius among Group 3a elements of the periodic table.
As a result, the melting point of the crystal phase precipitated in the grain boundary phase can be increased, and as a result, the high-temperature strength and the creep resistance can be dramatically improved as compared with other Group 3a elements of the periodic table. it can. However, in reality, there is some mixing of Group 3a elements of the periodic table other than Lu. In that case,
Lu preferably occupies 93 mol% or more, particularly 97 mol% or more, of the Group 3a elements of the entire periodic table. This is because, when other Group 3a elements of the periodic table other than Lu coexist, the eutectic point is lower than that of Lu alone, and as a result,
This is because the creep resistance of the sintered body deteriorates. In addition,
As elements other than Lu in Group 3a of the periodic table, Y, Yb,
Er, Dy, Ho, Tb, Tm, Sc and the like. As described above, other elements belonging to Group 3a of the periodic table other than Lu may be contained in the above ratio. In this case, the amount of Lu in the total amount of the sintered body is 0.5 mol% in terms of oxide. It is necessary to control so as not to fall below.

According to the present invention, Al, Mg, Fe
Oxides form a low melting point oxide at the grain boundaries, which hinders the crystallization of the grain boundaries and degrades the high-temperature strength and creep resistance properties. %, Particularly preferably 0.5% by weight or less, more preferably 0.1% by weight or less.

Next, the method for producing the silicon nitride composite material of the present invention will be described. First, as a starting material for forming a silicon nitride component, silicon nitride powder, Lu ₂ O ₃
A powder, in some cases, a powder of an element oxide of a Group 3a element of the periodic table other than Lu, or a silicon oxide powder in some cases. In addition, a compound composed of Lu ₂ O ₃ and SiO ₂ can be used as an addition form. The silicon nitride powder to be used may be any of α-type and β-type, the particles of which are 0.4 to 1.2 μm, the amount of cationic impurities is 1% by weight or less, particularly 0.5% by weight or less, The amount of impurity oxygen is suitably 0.5 to 2.0% by weight, and any method such as a direct nitriding method or an imide decomposition method may be used.

The above-mentioned raw materials are composed of 70 to 99 mol% of silicon nitride, 0.5 to 10 mol% of Lu in terms of oxide, and the balance of impurity oxygen. Lu may be replaced with another Group 3a element of the periodic table so that Lu satisfies the ratio of 93% or more, particularly 97% or more, of the group 3a elements of the entire periodic table. Then, the molar ratio (SiO ₂ / RE ₂ O) of the amount of the impurity oxygen in terms of SiO _{2 to the} amount in terms of the oxide of the group 3a element of the entire periodic table (RE ₂ O ₃ ) (RE ₂ O ₃ ).
₃ ) is less than 2, especially 1 to 1.9, more preferably 1.2 to 1.
Adjust so that it becomes 8. At this time, SiO ₂ / RE ₂ O
_{The 3} molar ratio is controlled in consideration of impurity oxygen inevitably contained in the silicon nitride powder, oxygen adsorbed in the production process, and the like, and may be adjusted by adding SiO ₂ powder in some cases.

Further, at least one type of hard particle powder selected from silicides of Ta, Nb, Mo and W having an average particle diameter of 1 to 5 μm and silicon carbide is added to 100 parts by weight of the silicon nitride component. 0.5 to 25 parts by weight. Incidentally, as the silicide, a compound such as an oxide capable of forming a silicide in the sintering process may be added. As the hard particle powder, specifically, TaSi ₂ , NbSi ₂ , MoS
i ₂ , WSi ₂ , SiC and the like.

After the silicon nitride component and the hard particles are sufficiently mixed by a vibration mill, a rotary mill, a barrel mill, or the like, the mixed powder is molded by a desired molding means, for example, die pressing, casting, sludge molding, extrusion molding, or the like. It is formed into an arbitrary shape by injection molding, cold isostatic pressing or the like.

Next, the molded body is primarily held at 1600 to 1800 ° C. in a non-oxidizing atmosphere containing nitrogen to columnarize the silicon nitride crystal, and then higher than 1800 ° C.
Desirably, sintering is performed at a temperature of 2000 ° C. or less. At this time, silicon nitride crystal grains having an average particle diameter of 5 μm or less and an average aspect ratio of 5 or more can be generated by primary holding of silicon nitride. Therefore, if this primary holding is not performed, the average grain size exceeds 5 μm, or the silicon nitride becomes a granular silicon nitride crystal having an average aspect ratio smaller than 5, and high strength and toughness cannot be obtained.

At this time, it is necessary to set the firing atmosphere to a nitrogen gas pressure at which the silicon nitride does not decompose at the firing temperature. For this purpose, the nitrogen gas pressure in the atmosphere is set to the silicon nitride decomposition equilibrium pressure. As described above, it is desirable to set the primary holding pressure to 0.5 to 5 atm and to set the pressure to more than 5 atm during the secondary holding. Further, in addition to the above nitrogen gas in the atmosphere, the SiO gas is generated. It is desirable to keep. This SiO gas can be generated by disposing SiO ₂ powder or the like in a firing furnace.
This is because the surface of the sintered body is easily decomposed if no SiO gas is present at the time of sintering, so that melilite is generated and the oxidation resistance of the sintered body is deteriorated.

Further, the sintered body produced as described above is subjected to nitrogen and nitrogen for the purpose of reducing voids.
It is also possible to perform a hot isostatic firing (HIP) process of firing at a temperature of 1600 to 1900 ° C. under a pressure of 1000 atm or more using a gas such as argon.

Further, in the cooling process after the above-mentioned calcination, the temperature is slowly cooled at a rate of 300 ° C./hr or less, or the sintered body is heat-treated in a non-oxidizing atmosphere at 1000 to 1400 ° C. to reduce the grain boundary. The crystallization can be enhanced and the characteristics can be improved.

In the above manufacturing method, when high dimensional accuracy is required for a product, a part of or all of the silicon nitride powder is replaced with silicon powder to produce a compact, which is then placed in a nitrogen-containing atmosphere. After the nitriding treatment at 800 to 1500 ° C. to convert into α-silicon nitride to increase the density of the compact, and then firing under the above-described firing conditions, shrinkage during firing can be reduced.

According to the present invention, at least Lu is used as a Group 3a element of the periodic table constituting the grain boundary of the silicon nitride based sintered body.
By selecting, it is possible to improve high-temperature characteristics as compared with other conventionally used Group 3a elements of the periodic table. The mechanism by which such an excellent action is exhibited is that Lu has the smallest ionic radius in the lanthanoid system and has a large bonding force with other elements, so that the mechanical properties at high temperatures and the granularity that is the main cause of creep are high. Since the slip phenomenon of the field is small and the diffusion of oxygen is small, it is considered that the oxidation resistance is superior to other Group 3a elements in the periodic table.

The creep resistance of the sintered body is determined by the characteristics of the grain boundary of the grain boundary phase of the sintered body. According to the present invention, the material is selected from YAM, apatite and wollastonite. By precipitating at least one type of crystal phase, these crystal phases are very stable even in a high-temperature atmosphere, so that excellent high-temperature strength and creep resistance are exhibited.

Further, the hard particles have an effect of appropriately suppressing the grain growth, and by dispersing these crystal particles as fine particles, respectively, the large crystal particles in a normal silicon nitride sintered body are dispersed. The phenomenon of destruction due to the presence can be reduced as much as possible, and the transverse rupture strength at high temperatures can be greatly improved. Further, the hard particles have an effect of inhibiting the progress of cracks, and contribute to the improvement of fracture toughness.

[0030]

EXAMPLES Silicon nitride powder (BET specific surface area 9 m ² / g, α rate 98% or more, oxygen content 1.1% by weight,
Amount of cationic metal impurities such as Al, Mg, Ca, Fe, etc. 3
0 ppm or less), the hard particles of Table 1 having an average particle size of 0.5 to 7 μm, and Lu ₂ O having a purity of 99% or more or 96%.
₃ Powder and Periodic Table 3a other than Lu with a purity of 99% or more
A powder of a Group III element oxide and a silicon oxide powder having a purity of 99.9% or more were weighed and mixed so as to have a molded body composition shown in Table 1, and were rotated for 120 hours using silicon nitride balls with methanol as a solvent. After mixing and pulverizing, the slurry is dried, and 3 t / cm in a shape having a diameter of 60 mm and a thickness of 20 mm.
Rubber press molding was performed at a pressure of ² , and the molded body was fired under nitrogen gas pressure (GPS) under the firing conditions shown in Table 1.
In the GPS firing, the primary firing temperature was maintained at 1 atm of N ₂ for 10 hours, and the secondary firing temperature was maintained at 10 atm of N ₂ for 5 hours. Further, the GPS + HIP is obtained by hot isostatic firing at 1700 ° C. and a nitrogen pressure of 2000 atm for 1 hour after the above GPS firing.

Samples Nos. 36 to 39 used silicon powder having an average particle diameter of 3 μm and an oxygen content of 1.1% by weight. The amount of the ratio in () was blended. And in nitrogen,
A nitriding treatment was performed at 1300 ° C. for 10 hours. It was confirmed by X-ray diffraction measurement that no silicon remained in the sintered body after nitriding. Thereafter, the nitride was fired under the firing conditions shown in Table 1. In addition, the amount of Si in Table 1 is a weight ratio after nitriding.

For each of the obtained sintered bodies, a theoretical density ratio was calculated from the specific gravity by the Archimedes method, and 3 ×
Cut and polished into a 4 x 40 mm test piece shape, JIS-
Based on R-1601, a four-point bending strength test was performed at room temperature and 1500 ° C., and the average of the results of the ten tests is shown in Table 2. Further, the crystal of the grain boundary phase of the sintered body was identified by X-ray diffraction measurement. In addition, the amount of impurity oxygen in Table 1 is obtained by pulverizing a sintered body and obtaining the total oxygen amount by chemical analysis.
The oxygen content excluding the oxygen content in the added Group 3a element oxide of the periodic table is converted into SiO ₂ . Further, the bending test piece was supported in the same manner as in the four-point bending test of JIS-R-1601, and a load of 450 MPa was applied, and a maximum of 2
The sample was held for 00 hours, and the time until destruction was measured. The fracture toughness was measured by the IM method.

[0033]

[Table 1]

[0034]

[Table 2]

As is clear from the results shown in Tables 1 and 2, all of the samples of the present invention had a room temperature strength of 900 MPa or more.
500 ° C strength of 600 MPa or more, toughness of 7.5 MPa · m
Excellent high-temperature properties of ^1/2 or more and 200 hours or more of creep resistance were exhibited.

[0036]

As described in detail above, the silicon nitride composite sintered body of the present invention is prepared by mixing specific hard particles and using a Lu compound as a sintering aid to increase the temperature from room temperature to 1500 ° C. At high temperatures, it has excellent high-temperature strength and toughness, as well as excellent creep resistance, and as a result, its durability can be significantly improved as a component for an automobile or a component for a gas turbine engine.

Claims (4)

[Claims] 1. A silicon nitride component comprising 70 to 99 mol% of silicon nitride, 0.5 to 10 mol% of Lu in terms of oxide, and a silicon nitride component comprising the remainder of impurity oxygen and 100 parts by weight of the silicon nitride component. On the other hand, at least one hard particle component selected from silicides of Ta, Nb, Mo, and W and SiC is added in an amount of 0.5%.
A composite sintered body that is dispersed and contained at a ratio of ２５25 parts by weight, wherein the silicon nitride has a mean particle size of 5 μm or less and an average aspect ratio of 5 or more, and the hard particles have an average particle size of 1 to 5. The group 3a element of the periodic table containing the impurity oxygen and Lu, which is present as 5 μm crystal grains, is mainly selected from the group consisting of apatite, YAM, and wollastonite at the grain boundaries of silicon nitride crystal grains and hard grains. A silicon nitride-based composite sintered body characterized by being present as at least one or more crystal phases. 2. The silicon nitride component according to claim 1, wherein the molar ratio of the amount of the impurity oxygen in terms of SiO _{2 to the} amount in terms of the oxide of a Group 3a element of the periodic table is less than 2. Silicon nitride composite sintered body. 3. A silicon nitride 70 to 99 mol%, and 0.5 to 10 mol% Lu in terms of oxide, the balance being impurities oxygen, wherein the SiO ₂ equivalent amount of the impurity oxygen L
a silicon nitride component having a molar ratio of less than 2 to the oxide equivalent of a Group 3a element of the periodic table containing u,
Ta, N having an average particle size of 1 to 5 μm with respect to 0 parts by weight.
b, Mo, W A molded body containing at least one hard particle component selected from the group consisting of silicides of Si and SiC at a ratio of 0.5 to 25 parts by weight in a non-oxidizing atmosphere containing nitrogen. The method of manufacturing a silicon nitride-based composite sintered body characterized in that, after holding at 1600 to 1800 ° C. to form the silicon nitride crystal into a column, then sintering at a temperature higher than 1800 ° C. 4. Silicon or silicon and silicon nitride in an amount of 70 to 99 mol% in terms of nitride, and Lu in an amount of 0.5 to 0.5% in terms of oxide.
A silicon nitride component comprising 10 mol% and the balance being impurity oxygen, wherein the molar ratio of the impurity oxygen in terms of SiO _{2 to} the oxide equivalent of the Group 3a element of the periodic table containing Lu is less than 2; With respect to 100 parts by weight of the silicon nitride component,
Molded product comprising at least one hard particle component selected from the group consisting of silicides of Ta, Nb, Mo, W having an average particle diameter of 1 to 5 μm and SiC in a proportion of 0.5 to 25 parts by weight. Is heat-treated in a nitrogen-containing atmosphere at 800 to 1500 ° C. to nitride the silicon, and is further maintained at 1600 to 1800 ° C. in a non-oxidizing atmosphere containing nitrogen to form a columnar silicon nitride crystal. And then sintering at a temperature higher than 1800 ° C.