JPH04231381A - Silicon nitride-silicon carbide composite sintered body and its production - Google Patents

Abstract

PURPOSE:To obtain a composite sintered body excellent in room temp. strength, high-temp. strength, and hardness by adding a specified amt. of silicon carbide to silicon nitride containing oxides of group 3a elements so that alpha-Si3N4 remains in the sintered body. CONSTITUTION:A mixture powder is prepared from 90-99.5mol% of silicon nitride containing impurity oxygen and 10-0.5mol% oxides of group 3a elements (e.g. Y2O3). To 100 pts.wt. of this mixture powder, 1-100 pts.wt. of silicon carbide powder is added, mixed and molded into a desired shape by press molding, injection molding, etc. Then a sealing material such as glass is applied on the surface of the molded body. Then >=50MPa pressure is given to the molded body through the glass layer and the body is calcined at 1450-1900 deg.C. Thereby, silicon nitride-silicon carbide composite sintered body having the alpha-Si3N4/beta-Si3N4 ratio from 0.01 to 1 is obtd. This composite sintered body is suitable as a structural material of heat engine, or other heat-resistant material or wear-resistant material.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to a silicon nitride-silicon carbide composite sintered body mainly composed of silicon nitride and silicon carbide, and a method for manufacturing the same. , relates to a sintered body with excellent high-temperature strength and hardness, and a method for producing the same.

0002

BACKGROUND OF THE INVENTION Silicon nitride sintered bodies have been used as engineering ceramics, particularly as structural materials for heat engines, because of their excellent strength, sophistication, and thermal and chemical stability.

[0003] As a method for obtaining such a silicon nitride sintered body, a sintering aid such as an oxide of a group 3a element of the periodic table is added to and mixed with silicon nitride powder, and after molding, a non-oxidizing It is obtained by firing in an atmosphere at a temperature of 1500 to 2000°C.

However, although silicon nitride sintered bodies have excellent properties, they have a problem in that their strength etc. decrease at high temperatures. Up until now, efforts have been made to address this problem of deterioration of high-temperature strength by improving sintering aids and changing the firing atmosphere and firing pattern.
At present, no definitive countermeasures have been taken.

[0005]

[Problems to be solved by the invention] Part 1 of this deterioration of strength
One factor is that in the firing process, for example, α-Si
3N4 undergoes a phase transition to β-Si3N4 at around 1600℃ and grain growth of silicon nitride particles occurs, so acicular crystals with relatively large grain sizes precipitate, and these large particles become a source of destruction and deteriorate properties. It is possible to do so. Since such sintering behavior is an unavoidable factor in the sintering of silicon nitride, the strength of the silicon nitride sintered body itself is approximately 1000 MPa at room temperature and approximately 700 MPa at 1400°C.
is the limit.

[0006] Also, from the viewpoint of hardness, there is a limit to the desire for high hardness because the grain size tends to vary due to the above-mentioned microstructure.

[0007] In response to this phenomenon, there have been attempts to suppress the grain growth of silicon nitride by adding silicon carbide or the like to silicon nitride, but from the standpoint of overall properties such as strength and hardness, At present, it has not been studied and is not fully effective.

[0008]

[Means for solving the problem] The present inventors conducted a detailed study on the above problem and found that a system in which an oxide of a group 3a element of the periodic table is added to silicon nitride. By adding a specific amount of silicon carbide and firing in the sintering process, silicon nitride is densified while suppressing the phase transition from α-Si3 N4 to β-Si3 N4,
It has been found that by leaving α-Si3N4 in the sintered body, grain growth of silicon nitride crystals can be effectively suppressed, and a sintered body having a fine structure and excellent strength and hardness can be obtained.

That is, the present invention contains 90 to 99.5 mol % of silicon nitride containing impurity oxygen and 0.5 to 10 mol % of Group 3a elements of the periodic table in terms of oxide. A composite sintered body containing 1 to 100 parts by weight of a silicon carbide component dispersed in 100 parts by weight of a silicon nitride component, wherein α-Si in the silicon nitride crystal in the sintered body is
3N4/β-Si3N4 ratio is 0.01 to 1
It is characterized by:

[0010] Furthermore, in the method for producing the composite sintered body of the present invention, silicon nitride containing impurity oxygen is contained in an amount of 90 to 99.5 mol %, and a Group 3a element of the periodic table is contained in an amount of 0.5 to 99.5 mol% in terms of oxide. 1
Silicon nitride component 100 each contained in a proportion of 0 mol%
A molded product obtained by adding a silicon carbide component at a ratio of 1 to 100 parts by weight to parts by weight is heated to 50 MPa through a glass layer.
It is characterized by firing at a temperature of 1450 to 1900°C under the above pressure.

According to the silicon nitride-silicon carbide composite sintered body of the present invention, the composition is mainly composed of a silicon nitride component and a silicon carbide component. The silicon carbide component basically means only silicon carbide particles, while the silicon nitride component consists of a system including silicon nitride containing impurity oxygen and a sintering aid component in the sintered body.

The silicon nitride component contains 90 to 99.5 mol%, particularly 95 to 99 mol% of silicon nitride containing impurity oxygen, and 0.5 to 10 mol% of Group 3a elements of the periodic table in terms of oxides.
, especially in a proportion of 1 to 5 mol%. periodic table 3
Group A elements act as oxides to improve the sinterability of the entire system, and if the amount is less than 0.5 mol%, the sinterability decreases, making it impossible to obtain a dense sintered body and deteriorating the properties. However, if it exceeds 10 mol%, high temperature strength deteriorates.

[0013] In the silicon nitride component, the impurity oxygen contained in the silicon nitride also contributes to improving the sinterability as, for example, SiO2, like the elements of group 3a of the periodic table, and more preferably Oxide equivalent amount of Group 3a elements (R
E2O3 ) and the SiO2 equivalent amount of impurity oxygen
The molar ratio expressed as O2 /RE2 O3 is 0.5 to 1
It is preferably 0, especially 1 to 4.

According to the present invention, such silicon nitride component 10
The silicon carbide component is added at a ratio of 1 to 100 parts by weight relative to 0 parts by weight. The reason for limiting the amount of silicon carbide component to the above range is that if the silicon carbide component is less than 1 part by weight, the addition of silicon carbide will not have the effect of suppressing grain growth of silicon nitride crystals.
This is because dislocation of silicon nitride from α-Si3 N4 to β-Si3 N4 progresses, making it impossible to obtain high strength and high hardness, and if it exceeds 100 parts by weight, sinterability decreases and strength deteriorates. From the viewpoint of properties, the amount of silicon carbide component is preferably 30 to 70 parts by weight based on 100 parts by weight of the silicon nitride component.

According to the composite sintered body of the present invention, in the silicon nitride crystal, α-Si3 N4 and β-Si3 N4 coexist in the sintered body, and α-Si3 N4 /β-Si3 N
It is important that the ratio of 4 is between 0.01 and 1, especially between 0.05 and 0.5. This is because when the above ratio is smaller than 0.01, the dislocation from α-Si3 N4 to β-Si3 N4 is considered to have progressed almost completely, so the average grain size of silicon nitride crystals increases and abnormal grains also occur. This is because the desired high strength and hardness cannot be obtained because the sintered body tends to have a low density and hardness. Significant decline.

Furthermore, from the above point of view, silicon nitride crystals and silicon carbide crystals have an average grain size (minor diameter) of 1 μm or less, particularly 0.8 μm or less, and an average aspect ratio expressed by major axis/breadth axis of 2. ~10, especially 3 to 9 grains, and the silicon carbide crystal itself exists in a grain shape at the grain boundary of the silicon nitride crystal or within the silicon nitride crystal grain with an average grain size of 1 μm or less, particularly 0.8 μm or less. It is desirable that the particles exist as particles. Note that although most of this silicon carbide crystal is of the β type, the α type may exist in some cases.

The reason why the shapes of the silicon nitride crystal and silicon carbide crystal are limited as described above is that if the silicon nitride crystal is larger than 1 μm, this easily becomes a source of destruction of the sintered body and causes deterioration of the strength. This is because if the aspect ratio is smaller than 2, the particles become less entangled with each other, making it easier for the particles to move at high temperatures and deteriorating the creep characteristics. On the other hand, if the grain size of the silicon carbide crystal is larger than 1 μm, the sinterability deteriorates and the acicular formation of the silicon nitride crystal is inhibited, resulting in deterioration of the characteristics.

[0018] In addition, as a component constituting grain boundaries, for example, Al2 O3, C
It is known to use aO, MgO, etc., but if these oxides exist at the grain boundaries or as a solid solution in some silicon nitride crystals, the melting point of the grain boundaries will lower and the viscosity at high temperatures will also decrease. As a result, not only the high-temperature strength but also the creep properties deteriorate. Therefore, the metal elements such as Al, Ca, and Mg should be 0.5% by weight or less, especially 0.3% by weight in the total amount in terms of oxides.
It is desirable to control it so that it is as follows.

[0019] The elements of Group 3a of the periodic table used in the present invention include Y, Sc, Er, Yb, Ho,
Among them, Y is easily agglomerated in the sintered body and tends to cause abnormal grain growth, so Er,
Yb is particularly preferred.

Next, the method for producing a silicon nitride-silicon carbide composite sintered body according to the present invention will be explained. First, silicon nitride powder, silicon carbide powder, and a material from periodic table 3a are used as starting materials.
Group element oxides, optionally silicon oxide powders are used.

The silicon nitride powder contains silicon carbide and has low sinterability, so α-Si3 N4 is
Often present in a proportion of 5% or more, with an average particle size of 1 μm
Hereinafter, those having an impurity oxygen content of 2% by weight or less are preferably used. Further, as the silicon carbide powder, either α type or β type can be used, and a powder having an average particle size of 1 μm or less and an impurity oxygen content of 2% by weight or less is used. These silicon nitride powder and silicon carbide powder may be present as individual powders, or may be a composite powder of silicon nitride and silicon carbide in a predetermined ratio.

Next, using the above powder, nitride containing 92 to 99.5 mol % of silicon nitride containing impurity oxygen and 0.5 to 10 mol % of oxide of Group 3a element of the periodic table was prepared. 1 to 10 parts of silicon carbide component per 100 parts by weight of silicon component
After weighing to give 0 parts by weight and thoroughly mixing, mold into a desired shape by a well-known molding method such as press molding, injection molding, extrusion molding, cast molding, cold isostatic pressing, etc. do. In addition, in the above process, the elements Al, Mg, and Ca contained in the molded product are mixed in from each step so that the total amount in terms of oxide amount is 0.5% by weight or less. avoid. For example, it is necessary to use raw materials with a small amount of these elements, or to take into consideration the material of the balls used in, for example, ball mill mixing, to limit contamination from mixing.

Next, the molded body obtained by the above method is fired at a firing temperature of 1450 to 1900°C. In addition, firing methods include normal pressure firing, hot press firing, nitrogen gas pressure firing (QPS firing), hot isostatic pressure firing (HI
P firing) etc. are adopted, and in some cases these can be combined for firing, but from α-Si3 N4 to β
The optimal method for firing while suppressing the dislocation to -Si3N4 is the so-called seal HIP, in which the molded body obtained as described above is coated with a sealing material made of glass or the like on the surface and then fired under high temperature and high pressure. law is adopted.

[0024] In this specific method, first, prior to firing, a glass such as BN powder is applied to the molded body obtained by the above-mentioned method in order to prevent reaction with the sealing material such as glass during the firing process. A powder with poor wettability is made into a slurry and applied to the molded body, or the slurry is applied by spraying.

Next, for the molded body coated with BN,
Glass powder that forms a seal upon firing is applied to its surface, or the molded body is encapsulated in a glass capsule. Another method is to bury the molded body in a container filled with glass powder.

[0026] After that, 1450 to 190
HIP firing is performed at a temperature of 0° C. and a pressure of 50 MPa or more. Firing is performed by first heating the glass while introducing nitrogen gas at a temperature equal to or higher than the decomposition equilibrium pressure of silicon nitride at a temperature equal to or higher than the softening point of the glass existing on the surface of the molded body by about 0.01 to 0.2 MPa. The glass is softened to form an impermeable film on the surface of the molded product. After the gas-impermeable membrane is completely formed, the temperature and pressure within the furnace are increased to a level that allows for sufficient densification. The pressure medium at this time is nitrogen,
Use an inert gas such as argon. Thereafter, after the firing has sufficiently progressed, the temperature and pressure are lowered and the firing is completed.

[0027] The temperature during firing is 1450 to 1900.
The reason for limiting the temperature to 1900°C is that if the firing temperature is higher than 1900°C, the dislocation of silicon nitride from α type to β type will progress in the sintered body, and the crystal grains will grow and the grain size will increase, which will reduce the strength. This is because it deteriorates.

[0028] Also, for the obtained sintered body, 130
The grain boundaries of the sintered body are crystallized by heat treatment in a non-oxidizing atmosphere at 0 to 1700°C, for example, Si3N4
-RE2 O3 (RE: Group 3a element of the periodic table) -S
By precipitating the well-known iO2-based crystal phase, high-temperature properties can be improved.

[0029]

[Example] As raw material powder, the average particle size is 0.3 μm, α-
Si3 N4 content 98%, oxygen content 1.3% by weight
silicon nitride powder, silicon carbide powder with an average particle size of 0.3 μm, and Y2O3 with an average particle size of 0.5 μm,
Sc2 O3 , Er2 O3 , Yb2 O3 , H
O2 O3, Dy2 O3 powders, and silicon oxide powder were weighed and mixed so that their compositions would be in the proportions shown in Table 1, and mixed and ground together with a binder in methanol. After drying and granulating the obtained slurry, 1
Press molding was performed at a pressure of ton/cm2.

[0030] BN powder (particle size 1
After coating the paste with a thickness of 1 to 10 mm (~5 μm), glass containing SiO2 as a main component was coated with a thickness of 1 to 10 mm. This molded body was heated to 196MP at 1750°C.
Hot isostatic firing was carried out for 1 hour in a nitrogen pressurized atmosphere (a). For sample number 14 in Table 1, the firing temperature was 1950.
It was heated to ℃ and fired.

For the obtained sintered body, the relative density was determined by the Archimedes method, the 4-point bending strength at room temperature and 1400° C. was determined based on JISR1601, and the average of silicon nitride crystals and silicon carbide crystals was determined from electron micrographs. Particle size and average aspect ratio were measured. Also, load 2
Vickers hardness under 0 kg was measured.

For the sintered body, the (101) and (210) intensities of β-Si3 N4 are determined from the X-ray diffraction curve by L(101), L(210), and (
The peak intensity of each crystal phase of (102) and (210) is expressed as l(1
02), l(210), the following formula α/β=Σ[L(101) +L(210)]/Σ[
l(102)+l(210)] The α/β ratio was determined. The results are shown in Table 2.

[0033]

[Table 1]

[0034]

[Table 2]

According to the results in Tables 1 and 2, sample No. 1 in which no α-Si3 N4 remains in the sintered body
4, only a strength of 63 MPa can be obtained at 1400°C, but the present invention lowers the firing temperature and increases the strength of Si.
By adding an appropriate amount of C and allowing α-Si3N4 to remain, Si3N4 and SiC can be present as fine particles, and a strength at 1400°C of 800 MPa or more and a hardness of 19 GPa or more were achieved.

However, the periodic table 3a in the silicon nitride component
Samples in which the group element content exceeds 10 mol % exhibit significant deterioration in high-temperature properties, while those in which the group element content is less than 0.5 mol % greatly reduce sinterability, making it impossible to obtain a high-density sintered body.

[0037]

[Effects of the Invention] As detailed above, according to the present invention,
By adding silicon carbide to silicon nitride containing an oxide of a Group 3a element of the periodic table and leaving α-Si3 N4 in the sintered body, silicon nitride crystals and silicon carbide crystals are each made into fine particles. at room temperature and 14
It has excellent strength at a high temperature of 00°C and can be imparted with high hardness. Thereby, it is possible to promote the practical use of this composite sintered body for heat engine structures such as gas turbines and turbo rotors, or as other heat-resistant materials and wear-resistant materials, and to expand its uses.

Claims (3)

[Claims] [Claim 1] Silicon nitride containing impurity oxygen is 90 to 9
For 100 parts by weight of a silicon nitride component containing 9.5 mol% and a Group 3a element of the periodic table at a ratio of 0.5 to 10 mol% in terms of oxide, 1 to 100 parts by weight of a silicon carbide component.
A composite sintered body containing dispersed parts in parts by weight,
α-Si3 N4 / in silicon nitride crystal in sintered body
A silicon nitride-silicon carbide composite sintered body, characterized in that the proportion of β-Si3 N4 is 0.01 to 1. 2. The silicon nitride-carbide according to claim 1, wherein the silicon nitride crystals are present as particles with an average grain size of 1 μm or less and an average aspect ratio of 2 to 10, and the silicon carbide is present as particles with an average grain size of 1 μm or less. Silicon composite sintered body. [Claim 3] Silicon nitride containing impurity oxygen is 90 to 9
For 100 parts by weight of a silicon nitride component containing 9.5 mol% and a Group 3a element of the periodic table at a ratio of 0.5 to 10 mol% in terms of oxide, 1 to 100 parts by weight of a silicon carbide component.
Manufacture of a silicon nitride-silicon carbide composite sintered body, characterized in that a molded body containing dispersed particles in parts by weight is fired at a temperature of 1450 to 1900°C under a pressure of 50 MPa or more through a glass layer. Method.