JP3236733B2 - Silicon nitride sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention has excellent strength characteristics from room temperature to high temperature and also has excellent creep resistance. In particular, the present invention relates to automotive parts such as pistons, cylinders, valves, cam rollers, rocker arms, piston rings, piston pins and the like. , Turbine rotor, turbine blade, nozzle, combustor, scroll, nozzle support, seal ring,
The present invention relates to a silicon nitride sintered body suitably used for a gas turbine engine component such as a spring ring, a diffuser, and a duct.

[0002]

2. Description of the Related Art Conventionally, a silicon nitride sintered body has been known to have heat resistance,
Because of its excellent thermal shock resistance and oxidation resistance, it is being applied to engineering ceramics, especially for heat engines such as turbo rotators. This silicon nitride sintered body is generally made of Y ₂ O ₃ , Al ₂ O ₃
Alternatively, by adding a sintering aid such as MgO, high density and high strength characteristics are obtained. As such silicon nitride sintered bodies are required to be further improved in strength and oxidation resistance at high temperatures when their use conditions are further increased. In response to such demands, various improvements have been attempted, for example, by studying sintering aids and improving firing conditions.

Among them, from the viewpoint that oxides such as Al ₂ O ₃ and MgO, which have been conventionally used as sintering aids, deteriorate the high-temperature characteristics, the period of silicon nitride, such as Y ₂ O ₃ Simple ternary system (Si ₃ N ₄ —SiO ₂ —RE ₂ O ₃ ) composed of Group 3a element (RE) and silicon oxide
In the sintered body having the composition of
It has been proposed to increase the melting point and stabilize the grain boundaries by precipitating a crystal phase such as a YAM phase, an apatite phase, and a silicon oxynitride phase composed of i-RE-ON. For example, Japanese Patent Application Laid-Open No. 63-1000067 discloses a silicon nitride having a crystalline phase having an apatite structure at grain boundaries formed of a sintered body containing two or more rare earth elements of Y, Er, Tm, Yb, and Lu. Quality sintered bodies have been proposed.

[0004]

Problems to be Solved by the Invention When a conventionally proposed sintered body containing various rare earth elements is actually used for a gas turbine or an automobile part, the sintered body may be long under a high temperature and a high load. You may be exposed for a time,
There is also a problem that the sintered body is gradually deformed, so-called creep occurs. The occurrence of such creep deformation has been a major factor impeding practical application even if the strength is high.

In Japanese Patent Application Laid-Open No. 63-1000067, the apatite phase is precipitated in an amount of 50% or more to improve the high-temperature strength. However, when the apatite phase is large, the oxidation resistance is lowered. If silicon oxynitride (Si ₂ N ₂ O) is precipitated in a large amount to improve the oxidation resistance, the oxidation resistance is good, but the fracture toughness value decreases, and the surface of the sintered body during firing is reduced. There is a problem that silicon oxynitride is decomposed at the outer peripheral portion of a few mm to roughen the surface and a color tone difference occurs.

Accordingly, the present invention has sufficient high-temperature strength and high-temperature oxidation resistance from room temperature to high
Excellent creep resistance and high fracture toughness,
It is an object of the present invention to provide a silicon nitride-based sintered body in which a decomposition phase or the like does not occur on the outer periphery of the sintered body.

[0007]

As described above, the present inventors have determined that the oxides of Group 3a element of the periodic table used as factors affecting the high-temperature strength, oxidation resistance and creep resistance of a sintered body. That the type of and the type of crystal phase at the grain boundary are important, and as a result of repeated investigations based on this finding, it was determined that at least Lu was selected as a Group 3a element of the periodic table, It has been found that the above-mentioned object is achieved by the fact that the ratio of impurity oxygen to the element of Group 3a of the periodic table is in a specific range, and the presence of a disilicate phase and a monosilicate phase at the grain boundaries of the sintered body. The present invention has been reached.

That is, the silicon nitride sintered body of the present invention is mainly composed of silicon nitride and contains at least Lu.
A sintered body containing a Group 3 element and impurity oxygen, wherein the Group 3a element of the periodic table is contained in a proportion of 1 to 10 mol% of the total amount in terms of oxide, and the impurity oxygen SiO ₂ The molar ratio (SiO ₂ / RE) of the conversion amount to the oxide conversion amount (RE ₂ O ₃ ) of the Group 3a element of the periodic table.
₂ O ₃ ) is 1.6 to 3.5, and the grain boundary phase of the sintered body contains a disilicate phase and a monosilicate phase as crystal phases.

Hereinafter, the present invention will be described in detail. The silicon nitride-based sintered body of the present invention contains silicon nitride as a main component, and contains, as additional components, a Group 3a element of the periodic table and impurity oxygen. Here, the impurity oxygen is the remaining oxygen amount obtained by subtracting the oxygen amount stoichiometrically contained in the Group 3a element compound as an additive from the total oxygen amount in the sintered body, Most of the oxygen contained in the silicon nitride raw material,
Alternatively, these are mixed as silicon oxide to be added, all of which contain a chemical bond of Si—O, for example, S
It is considered to exist as iO ₂ .

The silicon nitride-based sintered body of the present invention is structurally mainly composed of a silicon nitride crystal phase, and most of the sintered body is composed of β-Si ₃ N ₄ having an average particle size of 1 to ₃₀ μm. Become. In addition, at least grain boundaries of the periodic table
RE ₂ S consisting of group a elements, Si (silicon) and O (oxygen)
Disilicate phase represented by i ₂ O ₇ and RE ₂ SiO ₅
It is a great feature that it contains a monosilicate phase represented by In the grain boundary phase, oxynitrite glass containing a nitrogen component may be present when crystallization is not sufficient other than the above-mentioned crystal phase. The presence of such a glass phase may slightly lower the mechanical properties at high temperatures, but has a small effect on oxidation resistance and fracture toughness. Therefore, the aforementioned 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 specific composition of the sintered body according to the present invention is as follows: silicon nitride is 70 mol% or more, particularly 85 to 99 mol%, and Group 3a element of the periodic table is 1 to 10 mol% in terms of oxide, especially 1 to 10 mol%. It is preferable that 1 to 7 mol% and impurity oxygen be contained in a ratio of 1 to 15 mol%, particularly 1 to 12 mol% in terms of SiO ₂ . This is because when the amount of silicon nitride is less than 70 mol%, high-temperature strength is not exhibited, and when the amount of group 3a element in the periodic table is less than 1 mol%, densification is insufficient, and when it exceeds 10 mol%, high-temperature strength is not obtained. And the high temperature creep resistance deteriorates. Further, when the amount of impurity oxygen is less than 1 mol%, a compound having low high-temperature oxidation resistance such as melilite, which is a compound of silicon nitride and an oxide of an element of Group 3a of the periodic table, is likely to be formed at the grain boundary. On the other hand, if it exceeds 15 mol%, the volume fraction of the grain boundary phase increases and the high temperature characteristics deteriorate.

A major feature of the composition in the present invention is that it contains at least Lu (lutetium) as a Group 3a element of the periodic table. This Lu is 1% of the total amount in terms of oxide.
Desirably, it is present in an amount of at least mol%. In the present invention, as elements other than Lu, Group 3a elements of the periodic table, Y,
Yb, Er, Dy, Ho, Tb, Sc and Tm.

Furthermore, according to the present invention, the molar ratio (SiO ₂ / RE ₂ O ₃ ) of the amount of impurity oxygen in terms of SiO _{2 with} respect to the oxide amount (RE ₂ O ₃ ) of the group 3a element of the periodic table is described. )
Is from 1.6 to 3.5, especially from 1.6 to 2.7. This means that if the molar ratio is less than 1.6,
Precipitation of RE ₂ Si ₂ O ₇ phase and RE ₂ SiO ₅ phase cannot be expected. If it is larger than 3.5, precipitation of Si ₂ N ₂ O phase increases,
This is because a decomposition layer is formed on the surface, which causes deterioration of characteristics.

[0014] Si ₂ N ₂ O phase crystal has a characteristic similar to Si ₃ N ₄ crystal, Si ₃ N ₄ if its amount is increased
In the form of needles, thereby lowering the fracture toughness value. Also, when the partial pressure of SiO gas is low during firing in a non-oxidizing atmosphere, the Si ₂ N ₂ O crystals on the outer peripheral portion of the sintered body are easily decomposed, and a decomposition phase is generated in a portion of about several mm from the surface layer, resulting in color tone. There is a difference. Therefore, by increasing the partial pressure of SiO gas,
It is possible to suppress the decomposition of ₂ N ₂ O crystal and eliminate the color tone difference.

In the sintered body of the present invention, TiN, TiC, TaC, TaN, VC, NbC,
Periodic tables 4a, 5a such as WC, WSi ₂ , Mo ₂ C,
Group 6a metals and their carbides, nitrides, silicides or S
Even if iC or the like is present as an independent phase in the sintered body of the present invention as dispersed particles or whiskers, the effect of deteriorating the properties is small, and therefore, based on known techniques,
It is of course possible to improve the properties of the composite material by adding an appropriate amount.

However, metals such as Al, Mg, Ca, and Fe form oxides having a low melting point, thereby inhibiting crystallization of grain boundaries and deteriorating high-temperature strength. %, Particularly preferably 0.5% by weight or less, more preferably 0.1% by weight or less.

Next, a method for producing the silicon nitride sintered body of the present invention will be described. According to the present invention, a powder of a Group 3a element of the periodic table containing a silicon nitride powder as a main component as a starting material and at least Lu ₂ O ₃ as an additional component, or a silicon oxide powder as the case may be. Further, as an addition form, a compound composed of Lu ₂ O ₃ and SiO ₂ , or a compound powder of silicon nitride, Lu ₂ O ₃ and SiO ₂ can be used. The silicon nitride powder used is α-type,
Any of the β-forms can be used, the particle size is 0.4 to 1.2 μm, the amount of cationic impurities is 1% by weight or less, particularly 0.5% by weight or less, and the amount of impurity oxygen is 0.5 to 5%.
2.0% by weight is appropriate, and any method such as a direct nitriding method or an imide decomposition method may be used.

According to the present invention, using these powders,
It is prepared so as to satisfy the above-mentioned composition. When controlling SiO ₂ / RE ₂ O ₃ ratio described above in the formulation, Lu considering the oxygen partial or the like to be adsorbed oxygen unavoidably contained in the silicon nitride in the SiO ₂ minutes or manufacturing process
_The amount of an oxide of an element of Group 3a of the periodic table such as ₂ O ₃ is determined. In some cases, it may be prepared by adding SiO ₂ powder.

After weighing each powder at the above ratios,
After sufficiently mixing with a rotary mill, a barrel mill, or the like, the mixed powder is formed into an arbitrary shape by a desired molding means, for example, a die press, a casting molding, an extrusion molding, an injection molding, a cold isostatic pressing, or the like.

Next, the compact can be densified by a known firing method, for example, a hot pressing method, a normal pressure firing method, a nitrogen gas pressure firing method, or the like. Furthermore, densification can be improved by performing treatment by hot isostatic pressing (HIP) after the calcination. As another method, the above-mentioned molded body or sintered body can be glass-sealed and fired by the HIP method. Specific firing conditions are 1500-2 under nitrogen pressure under which silicon nitride does not decompose.
Fire at a temperature of 000 ° C. When the firing temperature is 2000
When the temperature exceeds ℃, the silicon nitride crystal causes grain growth and causes strength deterioration.

Further, by gradually cooling in the cooling step after the above-mentioned sintering, or by subjecting the sintered body to a heat treatment at 1000 to 1700 ° C., the grain boundaries can be crystallized and the characteristics can be further improved.

If high dimensional accuracy is required, a part of the silicon nitride powder is replaced with Si powder to produce a compact, which is then placed in a nitrogen-containing atmosphere at 800 to 1500.
After the heat treatment at a temperature of ° C., the Si powder is nitrided and converted into Si ₃ N ₄ to increase the density of the compact, and then fired under the above-described firing conditions, the shrinkage during firing can be reduced.

[0023]

According to the present invention, at least Lu is selected as an element belonging to Group 3a of the periodic table constituting the grain boundary of the silicon nitride based sintered body, thereby attaining the other elements of the conventional periodic table 3a. High-temperature characteristics can be improved as compared with group-group elements. The mechanism by which such excellent effects are exerted is that Lu has the smallest ionic radius in the lanthanoid system and has a large bonding force with other elements, so that mechanical strength at high temperatures and grain size, which 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 of the periodic table.

Further, the oxidation 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 RE ₂ Si ₂ O ₇ phase and the RE By precipitating the ₂ SiO ₅ phase, since these crystal phases are very stable even in a high-temperature oxidizing atmosphere, excellent oxidation resistance is exhibited.

As a crystal phase stable in a high-temperature oxidizing atmosphere, there is a Si ₂ N ₂ O crystal. However, the crystal of the Si ₂ N ₂ O phase inhibits the needle-like formation of Si ₃ N ₄ when its amount increases. Therefore, as a result, the fracture toughness value of the sintered body is reduced, and a decomposition phase is generated in the surface layer unless the atmosphere during firing is controlled to a high SiO gas atmosphere. In contrast, the crystals of the RE ₂ Si ₂ O ₇ phase and the RE ₂ SiO ₅ phase
₃ without inhibiting the needle of N _4, also in order to be stably generated without being affected by the atmosphere during sintering, the sintered body surface improves the fracture toughness while maintaining high oxidation resistance Decomposition can be suppressed.

[0026]

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) and a purity of 99% or more or 96% Lu
_A proper amount of Si ₃ N ₄ , RE ₂ O ₃ , SiO ₂ is prepared by using ₂ O ₃ powder, an oxide powder of Group 3a of the periodic table having a purity of 99% or more, and a silicon oxide powder having a purity of 99.9% or more. And
Using a silicon nitride ball with methanol as a solvent, 120
After mixing and crushing with a rotary mill and drying the slurry,
Rubber press molding was performed at a pressure of 3 t / cm ² into a shape having a thickness of 0 mm and a thickness of 20 mm.

The compact was fired by various firing methods shown in Table 1. In the table, GPS indicates that the molded product is 160
It is fired at 0 to 1950 ° C. under a nitrogen partial pressure of 1 to 10 atm for 20 hours. Hot press is 1700-1900
Calcination was performed at a temperature of 36.3 MPa for 1 hour. HIP
Is to bury the molded body in Vycor glass and apply Ar gas at a temperature of 1750 ° C. at which Vycor glass melts for 2000 times.
It was fired for one hour with atm applied. Further, after firing under the above GPS conditions, GPS + HIP
Hot isostatic firing at 00 ° C. under a nitrogen pressure of 2000 atm for 1 hour. Atmospheric firing is 1600-1850 ° C,
It is calcined under 1 atm of nitrogen for 50 hours. In order to suppress decomposition during firing, the furnace input during firing is 300 g.
/ Liter.

Further, in Table 1, Samples Nos. 25 and 26 are a part of silicon nitride powder (No. 25 is 51 mol% in terms of silicon nitride).
Equivalent to 70 mol% for No. 26) was replaced with silicon powder at 1200 ° C. under a nitrogen pressure of 10 atm for 2 hours.
1 hour at 1400 ° C, nitrogen pressure 1
After nitriding in a 2 hour pattern at atmospheric pressure, the above GPS
It baked on condition of.

With respect to the obtained sintered body, the ratio of the theoretical density to the theoretical density was calculated from the specific gravity according to the Archimedes method.
Cutting and polishing into a test piece shape of × 40 mm, JIS-
Based on R1601, a 4-point bending strength test was performed at room temperature and 1500 ° C., and the average value of 10 test results was
The sintered body was placed in an atmosphere of 1500 ° C.
The weight increase after holding for 0 hours was measured and the results are shown in Table 2. Further, the crystal of the grain boundary phase of the sintered body was identified by X-ray diffraction measurement, and the main phase and the subphase were determined. The amount of oxygen in Table 1 is obtained by pulverizing the sintered body to obtain the amount of oxygen by chemical analysis, and calculating the amount of oxygen excluding the amount of oxygen in the added Group 3a element oxide of the periodic table in terms of SiO _2. It was done. Further, the bending test piece was supported in the same manner as in the four-point bending test of JISR1601, a load of 400 MPa was applied, the test piece was held at 1500 ° C. for a maximum of 100 hours, and the time until breakage was measured.

FIG. 1 shows a chart of X-ray diffraction measurement of Sample No. 6 in the table as a typical silicon nitride sintered body of the present invention.

[0031]

[Table 1]

[0032]

[Table 2]

As is clear from the results of Tables 1 and 2, Samples Nos. 1, 2, and 3 using Y, Yb, Er, etc. as Group 3a elements of the periodic table and Sample No. 6 using Lu. , 16-
As compared with No. 20, the one containing Lu was higher in the high temperature strength, and showed a very excellent result in the creep resistance. Furthermore, in Samples Nos. 4 to 9 in which the amount of Group 3a element oxide in the periodic table was changed, Sample No. 4 in which the amount was less than 1 mol% was insufficiently densified, and Samples in which the amount exceeded 10 mol%. In No. 6, the high temperature strength and the creep resistance were reduced.

In Sample Nos. 10 to 15 in which the molar ratio of SiO ₂ / RE ₂ O ₃ was changed, when the SiO ₂ / RE ₂ O ₃ ratio was 1.6 or more, the disilicate phase and the monosilicate phase It can be seen that the crystals precipitate and the oxidation resistance is improved. However, Sample No. consisting of only the disilicate phase or consisting of 13 and 14 show a decrease in fracture toughness,
Further, the sample N having an SiO ₂ / RE ₂ O ₃ ratio of more than 3.5
In o.15, Si ₂ N ₂ O was precipitated, the fracture toughness value was reduced, and a decomposition phase was generated on the outer peripheral portion, and a difference in color tone was observed inside and outside the sintered body.

Sample No. 21 using another firing method was used.
Satisfactory characteristics were also obtained in Sample Nos. 25 and 26, which were subjected to nitriding treatment in which the silicon nitride was replaced with silicon.

[0036]

As described in detail above, according to the present invention,
Creep resistance at high temperature, oxidation resistance,
It is possible to improve the fracture toughness and obtain a sintered body having no color difference. Thereby, high reliability can be imparted even when used for automobile parts, gas turbine parts, and the like.

[Brief description of the drawings]

FIG. 1 is a chart of an X-ray diffraction measurement of a silicon nitride based sintered body of the present invention.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-100067 (JP, A) JP-A-4-292466 (JP, A) JP-A-64-56368 (JP, A) JP-A-5-56 163066 (JP, A) JP-A-4-6160 (JP, A) JP-A-6-234571 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C04B 35/584-35 / 596

Claims (1)

(57) [Claims] 1. A sintered body mainly composed of silicon nitride, containing a Group 3a element of the Periodic Table containing at least Lu and impurity oxygen, wherein the Group 3a element of the Periodic Table is contained in a total amount in terms of oxide. 1 to 10 mol%, and the periodic table 3a in terms of the amount of the impurity oxygen in terms of SiO ₂ .
Molar ratio (S) relative to the oxide equivalent of group element (RE ₂ O ₃ )
iO ₂ / RE ₂ O ₃ ) is 1.6 to 3.5, and the grain boundary phase of the sintered body contains a disilicate phase and a monosilicate phase as crystal phases. Union.