JP3124865B2 - Silicon nitride sintered body and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applicable to parts for automobiles and gas turbine engines, which have excellent strength characteristics from room temperature to high temperature, high thermal conductivity, and excellent thermal shock resistance. The present invention relates to a silicon nitride sintered body used and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, silicon nitride sintered bodies have excellent properties such as heat resistance, thermal shock resistance and oxidation resistance, and are high in strength and relatively lightweight. Applications as parts for various heat engines, such as turbines, are being promoted.

In general, the above-mentioned silicon nitride sintered body has been intended to realize high density and high strength by adding aluminum oxide or magnesium oxide as a sintering aid to silicon nitride raw material powder.

[0004] In recent years, however, the operating conditions for silicon nitride sintered bodies have become higher, and further improvements in strength and oxidation resistance at high temperatures have been required.

Therefore, in order to produce a high-density and high-strength silicon nitride sintered body in response to such demands, a rare earth element oxide (R) such as yttria is added to a silicon nitride raw material as a sintering aid.
The addition of E ₂ O ₃ ) or aluminum oxide has been proposed in Japanese Patent Publication No. 52-3649 and Japanese Patent Publication No. 58-5190.

[0006]

However, as described above, the density is increased by using a rare earth element oxide (RE ₂ O ₃ ), aluminum oxide or the like as a sintering aid for the silicon nitride raw material, and the high strength at a high temperature is obtained. In order to realize the oxidation, the addition amount of the sintering aid is reduced as much as
At a high temperature of 0 to 1850 ° C., high-pressure atmospheric pressure sintering has to be performed, and there has been a problem that the manufacturing cost is significantly increased.

On the other hand, if the firing temperature is lowered to realize low-pressure atmosphere firing in order to reduce the manufacturing cost, the amount of the sintering aid added must be increased.
There was a problem that the desired strength and thermal shock resistance could not be obtained, and it was not practical.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has as its object to enable low-temperature and low-pressure atmosphere firing, and to provide components for automobiles and gas turbine engines from room temperature to high temperature. Mechanical properties sufficient for use as a material, that is, a strength from room temperature to 800 ° C. can be ensured at a practical level, and a silicon nitride-based sintered body having high thermal conductivity and excellent thermal shock resistance and production thereof It is to provide a method.

[0009]

Means for Solving the Problems The silicon nitride sintered body and the method of manufacturing the same according to the present invention are intended to improve the mechanical and thermal properties of the sintered body by using Si _{6 -Z} Al _Z O _Z N _8. Β expressed by _-Z
The crystal phase of sialon as the main crystal phase and its grain boundary phase as light rare earth element, heavy rare earth element, silicon, aluminum,
Composed of oxygen and nitrogen, z in the composition formula of β-sialon
Is specified in the range of 0.05 to 0.35, and exists in the main crystal phase of the sintered body and the grain boundaries of the crystal phase so that the thermal conductivity at room temperature is 15 W / m · K or more. It is characterized by controlling the sub-phase.

[0010] The production method comprises the steps of preparing silicon nitride prepared so that the molar fraction of the amount of aluminum oxide to the total amount of silicon nitride and aluminum oxide is 1.2 to 8.4; R synthesized from rare earth element oxide and silicon oxide and / or total rare earth element oxide and silicon oxide
A powder mixture composed of E ₂ Si ₂ O ₇ (RE: rare earth element) and aluminum oxide is molded and then sintered.

[0011] Alternatively, silicon and silicon nitride prepared so that the amount of silicon oxide in terms of silicon nitride and the mole fraction of aluminum oxide with respect to the total amount of silicon nitride and aluminum oxide are 1.2 to 8.4, A powder mixture comprising RE ₂ Si ₂ O ₇ (RE: rare earth element) synthesized from rare earth element oxide, heavy rare earth element oxide, silicon oxide and / or all rare earth element oxide and silicon oxide, and aluminum oxide was formed. Thereafter, the silicon is nitrided in a nitrogen-containing atmosphere at 800 ° C. to 1500 ° C., and then sintered.

[0012]

According to the above construction, since the grain boundary phase is formed by using both the light rare earth element oxide and the heavy rare earth element oxide, the difference in ionic radius between the two rare earth elements is large, and the eutectic reaction can be prevented. This raises the melting point of the liquid phase formed in the gaps between the silicon nitride particles, and the densification by rearrangement proceeds, so that a dense sintered body can be obtained at a low temperature.

Further, by controlling the value of z in the composition formula of β-sialon to be in the range of 0.05 to 0.35, high heat conductivity can be maintained while maintaining the excellent characteristics of β-sialon. Rate can be realized, and as a result, the thermal shock fracture resistance coefficient increases and the thermal shock resistance increases.

On the other hand, in the sintering process, silicon nitride particles pass through a liquid phase composed of a light rare earth element, a heavy rare earth element, silicon, aluminum, oxygen and nitrogen, and dissolve and re-precipitate with aluminum element in the liquid phase. Sintering proceeds while taking in the oxygen element, and the composition formula is Si _6-Z Al _Z O _Z N _8-Z
Β-sialon represented by

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the silicon nitride sintered body of the present invention and a method for producing the same will be described in detail with reference to embodiments. The silicon nitride based sintered body according to the present invention has a composition of Si _6-Z Al _Z O _Z N
Β- Sialon represented by _8-Z as a main component, has a grain boundary phase composed of light rare earth element, heavy rare earth element, silicon, aluminum, oxygen and nitrogen.In the present invention, light rare earth element and It is characterized in that it contains both heavy rare earth elements, and the light rare earth elements are atomic number 5
7 to 63 lanthanide elements, specifically La,
Ce, Nd, Sm, etc., and particularly Sm, which has a small hygroscopic property of the oxide, are desirable. On the other hand, heavy rare earth elements are Y having an atomic number of 39 and a lanthanide element having an atomic number of 64 to 71. Is Er, Yb, Lu, etc., but Y is generally preferable.

That is, in the present invention, it is necessary to use a light rare earth element and a heavy rare earth element simultaneously. For example, when light rare earth element oxides are used or heavy rare earth element oxides are used, each rare earth element is used. Since the ionic radii of the two are close to each other, a significant decrease in the melting point cannot be expected only by forming a solid solution between the rare earth element oxides.

Therefore, by using the light rare earth element oxide and the heavy rare earth element oxide at the same time, a eutectic reaction is caused to lower the melting point, and the densification by rearrangement proceeds to suppress the grain growth of the silicon nitride particles. As a result, a dense and high-strength sintered body can be obtained at a low firing temperature.

Since the eutectic composition of the light rare earth element oxide and the heavy rare earth element oxide is in the vicinity of 50 mol%, the composition ratio of both is preferably in the vicinity of 1.

If the total amount of the oxides of the light rare earth element and heavy rare earth element in the sintered body exceeds 10 mol%, the volume fraction of the grain boundary phase in the sintered body increases. Since the high-temperature strength and the thermal conductivity may be reduced, the total amount in terms of oxide is preferably 1 to 10 mol%, more preferably 3 to 6 mol%.

Next, if the value of z in the composition formula of β-sialon is less than 0.05, the excellent characteristics of β-sialon cannot be brought out. Since the thermal conductivity is reduced, the value of z is 0.05 to
It needs to be 0.35.

That is, ceramics is a polycrystal composed of a crystal and a grain boundary phase, and there are many factors of phonon scattering that lower the thermal conductivity. The thermal conductivity of the β-sialon sintered body largely depends on the amount of impurities remaining inside the grains and grain boundaries, the integrity of the crystal, and the microstructure. The thermal conductivity decreases as the amount of solution increases.

Further, the thermal shock fracture resistance coefficient can be obtained from the following equation. R ′ = S (1-ν) k / Eα (R ′ is thermal shock resistance, S is bending strength, E is Young's modulus, ν is Poisson's ratio, k is thermal conductivity, and α is thermal expansion coefficient. Here, the Young's modulus, Poisson's ratio, and coefficient of thermal expansion of the ceramic sintered body are almost constant unless the amount of the grain boundary phase becomes extremely large. However, the bending strength and the thermal conductivity vary greatly depending on the sintered body. In particular, in a high-strength material, as is clear from the above equation, it is possible to increase the thermal shock resistance by improving the thermal conductivity.
Thermal shock resistance can be increased.

Next, a method for producing the silicon nitride sintered body of the present invention will be described. First, a silicon nitride powder or a silicon powder and a silicon nitride powder as main components as raw material powders,
In addition to using light rare earth element oxides, heavy rare earth element oxides, silicon oxide and aluminum oxide powders as additive components, or light rare earth element oxides, heavy rare earth element oxides, silicon oxide and / or all rare earth element oxides RE ₂ Si ₂ O ₇ synthesized from silicon and silicon oxide (RE: rare earth element)
And aluminum oxide powder can also be used.

The silicon nitride powder comprises α-Si ₃ N ₄ , β
—Si ₃ N ₄ , and the average particle size is 0.4
The average particle diameter of the silicon powder is preferably 10 μm or less, particularly preferably 3 μm or less in order to facilitate nitriding.

When silicon powder is used as the main component, the weight is increased without dimensional change in the nitriding step to be described later, so that the density of the molded body is improved and the dimensional shrinkage amount during firing can be reduced. Dimensional accuracy can be improved. However, if a large amount of silicon powder is contained, it is difficult to nitride all of the added silicon powder in the nitriding step described later, and on the contrary, the properties of the silicon nitride-based sintered body may be deteriorated. It is preferable that the ratio of the amount of silicon powder in terms of silicon nitride to the amount of silicon nitride powder be 1 or less.

The formation reaction of β-sialon of the above composition formula by the reaction between silicon nitride and aluminum oxide according to the present invention is considered as follows.

[2- (z / 4)] Si ₃ N ₄ + (z / 2) Al ₂ O ₃ → Si _{6 -z} Al _Z O _Z N _8-Z + (z / 4) SiO ₂ In order to control the value of z in the range of 0.05 to 0.35, the amount of aluminum oxide equivalent to the total amount of silicon nitride equivalent and / or the sum of silicon nitride equivalent and aluminum oxide equivalent is calculated. From 1.2 to
It is necessary to prepare and mix to 8.4.

The mixed powder thus obtained is molded into a desired shape by a known molding method, for example, press molding, casting molding, extrusion molding, injection molding, cold isostatic pressing and the like.

Next, when the compact contains silicon powder, the compact is placed in a nitrogen-containing atmosphere at 800 ° C. to 150 ° C.
Nitriding is performed at a temperature of 0 ° C. to obtain silicon nitride. In order to nitride all of the silicon powder contained by the nitriding treatment, the temperature is gradually increased within the temperature range,
It is preferable to gradually nitride.

Thereafter, for example, a hot pressing method, normal pressure firing, pressure firing using nitrogen gas, and H
A dense sintered body is obtained by a known firing method such as IP firing or glass seal HIP firing.

At this time, if the firing temperature is too high, the β-sialon crystal, which is the main phase, grows and the strength decreases,
The sintering temperature is 1850 ° C. or less, especially 1600 to 175
It is desirable to set the temperature to 0 ° C. and to fire in a non-oxidizing atmosphere containing nitrogen gas.

Further, a silicon nitride sintered body can be obtained by dispersing particles or whiskers of a metal of Group 4a, 5a, or 6a of the periodic table or a carbide, nitride, silicide, or silicon carbide thereof by a known technique. It is also possible to improve the properties of the silicon nitride-based sintered body by compounding them by being present in them.

(Example 1) BET specific surface area 9 m ² / g,
Silicon nitride (Si ₃ N) having an α ratio of 98% and an oxygen amount of 1.2% by weight
₄ ) and various light rare earth element oxides (L-RE ₂ O ₃ ),
Heavy rare earth element oxide (H-RE ₂ O ₃ ), silicon oxide (S
iO _2), using powder of aluminum oxide (Al ₂ O _3), after formulated so as to have the composition shown in Table 1, 1t / cm
Molding was performed at a molding pressure of ² .

[0034]

[Table 1]

Next, these compacts are placed in a silicon carbide sagger, and baked at 1700 ° C. for 10 hours in a nitrogen atmosphere at normal pressure while controlling the atmosphere in order to reduce composition fluctuation. did.

The sintered body thus obtained was subjected to JIS-R16
Samples for evaluation were prepared by polishing to predetermined dimensions and shapes according to the 01 standard.

Using the evaluation sample, specific gravity was measured based on the Archimedes method, the theoretical density ratio was calculated, and the z value was determined from the peak shift of the β-silicon nitride crystal by X-ray diffraction measurement. Further, a four-point bending strength test was performed at room temperature and 800 ° C. based on JIS-R1601 standard, and the thermal conductivity was measured by a laser flash method. Table 2 shows the above results.

[0038]

[Table 2]

According to the results of Tables 1 and 2, the composition formula is S
z value of _{i 6-Z Al Z O Z} N 8-Z in β- sialon crystal phase represented is, Sample No. 1 of less than 0.05 is insufficient densification, z value exceeds 0.35 Sample No. 19 had a low thermal conductivity. Further, Sample Nos. 4, 8, and 15 in which neither light rare earth elements nor heavy rare earth elements were used were all insufficiently densified. Further, Sample No. 25, in which the amount of light rare earth element and heavy rare earth element in terms of oxide exceeds 10 mol%, the thermal conductivity was reduced.

On the other hand, all of the present inventions had sufficient bending strength and high thermal conductivity at practical levels.

Example 2 The average particle size was 3 μm and the amount of oxygen was 1.
1% by weight of silicon (Si) and a BET specific surface area of 9 m ² / g,
Silicon nitride (Si ₃ N) having an α ratio of 98% and an oxygen amount of 1.2% by weight
₄ ), various light rare earth element oxides (L-RE ₂ O ₃ ),
Heavy rare earth element oxide (H-RE ₂ O ₃ ), silicon oxide (S
iO ₂ ), aluminum oxide (Al ₂ O ₃ ), or RE partially synthesized from a rare earth element oxide and silicon oxide powder
After mixing using ₂ Si ₂ O ₇ and aluminum oxide (Al ₂ O ₃ ) powder to obtain the composition shown in Table 3, 1 t / c
Molding was performed at a molding pressure of m ² .

[0042]

[Table 3]

The obtained molded product was placed in a nitrogen gas stream at 12
The nitriding treatment was performed at a temperature of 00 ° C. for 5 hours and further at a temperature of 1400 ° C. for 10 hours. At this time, the rate of weight increase was measured, and the rate of weight increase was divided by the rate of weight increase when the added silicon was converted to 100% silicon nitride, thereby obtaining the nitriding rate of silicon.

Next, the nitride was placed in a silicon carbide sagger and fired at 1700 ° C. for 10 hours in a nitrogen atmosphere at normal pressure under controlled atmosphere in order to reduce composition fluctuation.

The sintered body thus obtained was treated in the same manner as in Example 1, and the same items were evaluated. The results shown in Table 4 were obtained.

[0046]

[Table 4]

According to the results of Tables 3 and 4, the composition formula is S
i z value of _{6-Z Al Z O Z N} 8-Z in β- sialon crystal phase represented is, Sample No. 1, 5 less than 0.05 is insufficient densification, z value 0.35 The sample No. 10 exceeding had a low thermal conductivity. Further, Sample Nos. 8 and 9 in which both the light rare earth element and the heavy rare earth element were not used were insufficiently densified. Further, Sample No. 11 in which the amount of light rare earth element and heavy rare earth element in terms of oxide exceeded 10 mol% had a low thermal conductivity.

On the other hand, all of the present invention had sufficient bending strength and high thermal conductivity at practical levels.

[0049]

According to the silicon nitride sintered body and the method of manufacturing the same of the present invention, low firing temperature and low pressure atmosphere firing can be performed, and from room temperature to high temperature, such as automobile parts and gas turbine engine parts. It is possible to obtain a silicon nitride sintered body having sufficient mechanical properties to be used at a practical level and having high thermal conductivity and excellent thermal shock resistance.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-53-115712 (JP, A) JP-A-58-185484 (JP, A) JP-A-4-321562 (JP, A) JP-A-6-185484 305839 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C04B 35/599

Claims (3)

(57) [Claims] The present invention provides a β-sialon crystal phase represented by a composition formula of Si _6-Z Al _Z O _Z N _8-Z as a main phase, and its grain boundary phase is a light rare earth element, a heavy rare earth element, silicon, It is composed of aluminum, oxygen and nitrogen, and the value of z in the above composition formula is 0.1.
And the thermal conductivity at room temperature is 15 W /
A silicon nitride based sintered body having a m · K or more. 2. RE ₂ Si ₂ O ₇ (RE: rare earth element) synthesized from silicon nitride, light rare earth element oxide, heavy rare earth element oxide, silicon oxide and / or all rare earth element oxide and silicon oxide, Forming a powder mixture composed of aluminum oxide and having a molar fraction of aluminum oxide with respect to the total amount of silicon nitride and aluminum oxide of 1.2 to 8.4, followed by sintering; Method for producing high quality sintered body. 3. RE ₂ Si ₂ O ₇ synthesized from silicon and silicon nitride, light rare earth element oxide, heavy rare earth element oxide, silicon oxide and / or all rare earth element oxide and silicon oxide.
(RE: rare earth element), a powder mixture composed of aluminum oxide and having a silicon nitride equivalent amount and a molar fraction of aluminum oxide amount to the total amount of silicon nitride and aluminum oxide of 1.2 to 8.4. After molding, 800
A method for producing a silicon nitride-based sintered body, comprising nitriding the silicon in a nitrogen-containing atmosphere at a temperature of from 1500C to 1500C and thereafter sintering the silicon.