JP3124864B2 - 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 used for automobile parts and gas turbine engine parts having 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 and a method for producing the same.

[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 has been applied to engineering ceramics, especially for heat engines such as turbo rotators. Recently, as a sintering aid, Y ₂ O ₃ , Sc
Addition of a rare earth element oxide such as ₂ O ₃ or aluminum oxide is disclosed in JP-B-52-3649, JP-B-58-5-5.
No. 190 is proposed.

Further, in a system containing aluminum oxide, it is known that β-sialon is formed by a reaction with silicon nitride, and by obtaining a sintered body containing β-sialon as a main component, corrosion resistance is increased. It has also been proposed to improve.

[0004]

However, when aluminum oxide is used in combination with Y ₂ O ₃ or the like as a sintering aid, its sinterability can be enhanced, thereby increasing room temperature or high temperature. A high-density sintered body with excellent strength is obtained,
Although it is being used as a structural material, its use conditions have become stricter and it is necessary to increase the durability against heat such as thermal shock resistance, and further improvement in properties is desired to increase the reliability as a material. .

Accordingly, the present invention is excellent in mechanical properties sufficient for use in automobile parts and gas turbine engine parts from room temperature to high temperature, and particularly excellent in flexural strength from room temperature to high temperature of 1000 ° C. Another object of the present invention is to provide a silicon nitride based sintered body having high thermal conductivity and excellent thermal shock resistance and a method for producing the same.

[0006]

Means for Solving the Problems In order to improve the mechanical and thermal characteristics of the sintered body, the inventors of the present invention have proposed a method for improving the mechanical and thermal characteristics of the sintered body. After repeated studies based on the importance of controlling phases,
Of the rare earth element compounds used as auxiliary components, L
By using a lutetium compound such as u ₂ O _3, it is possible to increase the strength as compared with Y ₂ O _{3 and the} like which have been widely used so far, and to make it exist in the grain boundary phase. -Sialon (Si _6-Z Al _z O _z N _8-z ) is constituted as a main crystal phase, and its z value is controlled to a specific range, and a sintered body having a specific thermal conductivity equal to or more than 10
It has been found that it is excellent in flexural strength up to a high temperature of 00 ° C. and can enhance thermal shock resistance.

That is, the silicon nitride sintered body of the present invention has a β-
Sialon (Si _6-Z Al _z O _z N _8-z , 0.05 ≦ z
≦ 0.35) A sintered body comprising a crystal phase as a main phase, and at least a lutetium-containing group 3a element of the periodic table containing silicon, aluminum, oxygen and nitrogen in its grain boundary phase. The total amount of Group 3a elements of the periodic table is 0.1 to 10 mol% in terms of oxide, and the thermal conductivity at room temperature is 1
It is not less than 5 W / m · K.

[0008] In the production method of the present invention, an oxide of a Group 3a element in the periodic table containing at least lutetium oxide, silicon oxide and aluminum oxide, or a compound thereof, is added to a main component consisting of silicon nitride powder. The total amount of the Group 3a elements of the periodic table is 0.1 to 10 mol% in terms of oxide, and the amount of aluminum oxide is based on the total amount of the main component and aluminum in terms of oxide. Is formed in a non-oxidizing atmosphere at 1600 to 1900 ° C. after molding, or a mixture of silicon powder or silicon powder and silicon nitride powder. With respect to the components, an oxide of a Group 3a element of the periodic table containing at least lutetium oxide, silicon oxide and aluminum oxide, or a compound thereof is used as an auxiliary component. Wherein the total amount of the Group 3a elements in the periodic table is 0.1 to 10 mol% in terms of oxide, and the aluminum is based on the total amount of silicon nitride and aluminum oxide in the main component. After forming a mixture having a ratio of 1.2 mol% to 8.4 mol% in terms of oxide of the above, heat treatment is performed in a nitrogen-containing atmosphere at 800 ° C. to 1500 ° C. to nitride the silicon. 1
By firing in a non-oxidizing atmosphere at 900 ° C., β
-Sialon (Si _6-Z Al _z O _z N _8-z , 0.05 ≦
(z ≦ 0.35) A sintered body comprising a crystal phase and a grain boundary phase containing lutetium, silicon, aluminum, oxygen and nitrogen is obtained.

Hereinafter, the present invention will be described in detail. The silicon nitride based sintered body of the present invention has a structure of β-sialon (Si _6-Z Al
_z O _z N _8-z ) is a main crystal phase and is composed of a grain boundary phase containing at least lutetium and silicon, aluminum, oxygen and nitrogen.

According to the present invention, first, β phase which is the main crystal phase
- it is important value of z in the composition formula of Sialon _{(Si 6-Z Al z O} z N 8-z) is 0.05 to 0.35. This is because if z is less than 0.05, the excellent features of β-sialon cannot be obtained, and if z exceeds 0.35, the thermal conductivity of the sintered body decreases.

According to the present invention, it is important to use at least a lutetium compound as a sintering aid component.
By including lutetium in the grain boundary phase, the melting point of the grain boundary phase increases, and the high-temperature strength can be increased. This lutetium can naturally be contained in combination with one or more other group 3a elements of the periodic table.

The lutetium oxide equivalent or the total amount of lutetium and one or more oxides of other elements of Group 3a of the periodic table is 0.1 to 10 mol%, especially 0.1 to 10 mol%. Desirably, the content is 0.3 to 5 mol%. If the amount is less than 0.1 mol%, the sinterability decreases, and if it exceeds 10 mol%, the volume fraction of the grain boundary phase in the sintered body increases, and the high-temperature strength decreases. This is because the thermal conductivity is reduced. When lutetium is used in combination with other Group 3a elements of the periodic table, lutetium is 0.1 mol% or more in terms of oxide,
In particular, 20% of the oxide equivalent of Group 3a element of the whole periodic table
The lutetium addition effect is exhibited by being present in the above ratio.

As the Group 3a element of the periodic table other than lutetium used in the present invention, Y and lanthanoid elements are mentioned, and Yb and Er are particularly preferable.

Next, a method for producing the silicon nitride sintered body of the present invention will be described. First, silicon nitride powder or silicon powder and / or silicon nitride powder is used as a main component powder. Silicon nitride powder itself is α-Si
Either ₃ N ₄ or β-Si ₃ N ₄ can be used, and the particle diameter thereof is preferably 0.4 to 1.2 μm.

When silicon powder is used as a starting material, there is no dimensional change and the weight increases in the nitriding step described later, so that the density of the compact is improved, and the amount of dimensional shrinkage during firing can be reduced. Dimensional accuracy can be improved. Silicon powder having an average particle diameter of 10 μm or less, particularly 3 μm or less is suitable for easy nitriding. However, when a large amount of silicon powder is contained in the starting material, it is difficult to convert all of the added silicon powder into silicon nitride in the nitriding step described later, and conversely, the characteristics may be deteriorated. It is preferable to use the silicon powder and the silicon nitride powder together in such a ratio that the ratio of the silicon powder in terms of silicon nitride to the silicon nitride powder is 1 or less.

Next, at least lutetium oxide or at least one of lutetium oxide and an oxide of a Group 3a element of the periodic table, and silicon oxide or aluminum oxide powder are added as auxiliary components to the main component. Alternatively, a compound powder of these components is used. Here, silicon oxide indicates the total amount of oxygen inevitably contained in the silicon nitride raw material or an additive such as SiO ₂ .

According to the present invention, β-sialon (Si ₆ -Z Al _z O _z N) obtained by the reaction between silicon nitride and aluminum oxide is used.
The formation reaction of _8-z ) is shown below.

[0018]

Embedded image

(ME MILBERG and
WM MILLER, J. Am. Ceram. Soc., 61 3-4 179 (197
8)).

Therefore, the value of z is set to 0.05 to 0.3.
In order to control the amount in the range of 5, the molar fraction of the aluminum oxide equivalent to the total amount of silicon nitride equivalent to silicon as the main component composition, the total amount of silicon nitride, and the total amount of aluminum oxide equivalent is required. It is necessary to prepare and mix to be 1.2 mol% to 8.4 mol%.

On the other hand, the total amount of lutetium oxide or lutetium oxide and other group 3a element oxides of the periodic table added as auxiliary components is 0.1 to 10 mol%,
Particularly, it is prepared and mixed so as to be 0.3 to 5 mol%.
When an oxide of a Group 3a element of the periodic table other than lutetium is used in combination, the content of lutetium oxide must be 0.1 mol% or more, particularly 0.3 mol% or more.

The mixed powder thus obtained is formed into a desired shape by a known molding method, for example, press molding, cast 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 1500 ° C.
Is heat-treated at the temperature described above to nitride the silicon powder contained in the compact to produce silicon nitride. In this nitriding treatment, in order to nitride all of the contained silicon powder, it is preferable to gradually increase the temperature within the above temperature range while gradually increasing the temperature.

Thereafter, the obtained molded body is fired by a known firing method, for example, a hot press method, normal pressure firing, nitrogen gas pressure firing, and after these firing, HIP firing or glass seal HIP firing. , To obtain a dense sintered body.
In this sintering process, silicon nitride particles are dissolved and reprecipitated in a liquid phase composed of lutetium, or lutetium and one or more of the elements of Group 3a of the periodic table, aluminum, oxygen, and nitrogen. Sintering proceeds while taking in the aluminum element and oxygen element in the liquid phase, and β-
It is believed that a sialon main crystal phase is formed.

If the firing temperature at this time is too high, the β-sialon crystal, which is the main crystal phase, grows and the strength decreases, so that a nitrogen gas-containing non-oxidizing atmosphere of 1900 ° C. or less, particularly 1600 to 1850 ° C. It is desirable that

According to the present invention, in addition to the above composition,
Elemental metals of Groups 4a, 5a, and 6a of the Periodic Table, and their carbides, nitrides, silicides, and SiC have properties even when present in the sintered body of the present invention as dispersed particles or whiskers. Since there is little influence of deterioration, it is of course possible to improve the properties as a composite material by adding an appropriate amount of these based on well-known techniques.

[0027]

The mechanical and thermal properties of a silicon nitride sintered body are determined by the β-sialon particles and the grain boundary phase. Generally, the thermal shock resistance of ceramics is expressed by the following equation (1).

[0028]

(Equation 1)

Is required. In the formula, R 'is the thermal shock fracture resistance coefficient, S is the bending strength, E is the Young's modulus, ν is the Poisson's ratio, k is the thermal conductivity, and α is the thermal expansion coefficient. Young's modulus of the sintered body, Poisson's ratio The coefficient of thermal expansion is almost constant unless the amount of the grain boundary phase is extremely large, but the transverse rupture strength and the thermal conductivity vary greatly depending on the sintered body. Particularly, in the case of a high-strength material, as is apparent from Equation 1, by increasing the thermal conductivity, it is possible to increase the thermal shock fracture resistance coefficient, and thus to increase the thermal shock resistance of the sintered body.

Ceramics are polycrystals composed of crystals and grain boundary phases. Therefore, there are many factors of phonon scattering that lower the thermal conductivity. The thermal conductivity of the β-sialon sintered body greatly depends on the amount of impurities remaining inside the grains and grain boundaries, the integrity of the crystal, and the microstructure. In particular, the thermal conductivity decreases as the amount of solid solution of Al and O in the crystal increases. According to the present invention, the relationship between the thermal conductivity and the amount of solid solution of Al and O is examined based on the above viewpoint, and as a result, the thermal conductivity is maximized when z is in the range of 0.05 to 0.35. Thus, the thermal shock resistance of the sintered body can be increased.

Lutetium is the element having the smallest ionic radius among the Group 3a elements of the periodic table, and can increase the melting point and the glass transition temperature of the grain boundary phase, so that the mechanical strength can be improved from room temperature to a high temperature. Can be.

According to the present invention, as described above, the z value of the β-sialon (Si ₆ -Z Al _z O _z N _8-z ) crystal phase is controlled, and at least lutetium, silicon and aluminum are contained in the grain boundary phase. And oxygen and nitrogen, it is possible to impart excellent mechanical properties and excellent thermal conductivity from room temperature to 1000 ° C., thereby greatly improving thermal shock resistance at high temperatures. .

[0033]

【Example】

Example 1 Silicon nitride powder (BET specific surface area 9 m ² / g, α rate 98
%, Oxygen content 1.2% by weight) and lutetium oxide, or lutetium oxide and various group 3a element oxides of the periodic table,
Lu ₂ Si ₂ partially synthesized from lutetium oxide and silicon oxide powder using silicon oxide powder and aluminum oxide powder
Using O ₇ powder (Sample No. 11), the mixture was blended to have the composition shown in Table 1, and then molded at 1 t / cm ² .

The compact was placed in a silicon carbide sagger, and calcined in a pressurized nitrogen gas stream at a temperature of 1800 ° C. for 10 hours in order to reduce composition fluctuation.

The obtained sintered body was polished to the shape specified in JIS-R1601 to prepare a sample. The z value of this sample was determined from the specific gravity measurement based on the Archimedes method and the peak shift of the β-silicon nitride crystal by X-ray diffraction measurement.
A four-point bending strength test at 000 ° C. was performed. Also,
Thermal conductivity was determined by the laser flash method. The results are shown in Table 2.

[0036]

[Table 1]

[0037]

[Table 2]

According to the results shown in Tables 1 and 2, No. 3 having a z value of less than 0.05 in the β-sialon (Si ₆ -Z Al _z O _z N _8-z ) crystal phase was used. No. 1 is insufficiently densified, and No. 1 has a z value exceeding 0.35. In No. 14, the thermal conductivity decreased. Sample No. containing no lutetium 3, 4, and 5 show that the high-temperature strength is lower than that of the product of the present invention containing lutetium. In addition, the oxides of lutetium and Group 3a element of the periodic table in terms of oxide amount exceeded 10 mol%. The 19 samples had reduced thermal conductivity. In contrast to these comparative examples, the samples according to the present invention were all room temperature and 1000
It was excellent in flexural strength at ℃ and high thermal conductivity.

Example 2 Silicon powder and silicon nitride powder having an average particle diameter of 3 μm and an oxygen content of 1.1% by weight (BET specific surface area 9 m ² / g,
α rate 98%, oxygen content 1.2% by weight) and lutetium oxide,
Group 3a element of the periodic table An oxide powder, a silicon oxide powder, and an aluminum oxide powder were blended so as to have a composition shown in Table 3, and then molded at 1 t / cm ² .

The obtained molded body was placed in a nitrogen stream at 1200 ° C.
For 5 hours and further at 1400 ° C. for 10 hours. At this time, from the weight increase rate, the silicon nitridation rate

[0041]

(Equation 2)

[0042]

The obtained nitride was placed in a silicon carbide sagger, and the atmosphere was controlled in order to reduce the composition fluctuation, and calcined at a temperature of 1800 ° C. for 10 hours in a stream of pressurized nitrogen gas. did.

The evaluation of the obtained sintered body was performed in the same manner as in Example 1. The results are shown in Table 4.

[0045]

[Table 3]

[0046]

[Table 4]

According to the results shown in Tables 3 and 4, the z-value in the β-sialon (Si ₆ -Z Al _z O _z N _8-z ) crystal phase was less than 0.05. No. 22 is insufficiently densified, and No. 22 has az value exceeding 0.35. Sample No. 26 had a low thermal conductivity. No. containing no lutetium. In No. 24, the high temperature strength was reduced. Further, the lutetium oxide equivalent of more than 10 mol% of No. 1 Sample No. 27 had a low thermal conductivity. In contrast to these comparative examples, all of the other samples according to the present invention were excellent in flexural strength up to high temperatures and had high thermal conductivity.

[0048]

As described in detail above, according to the present invention,
In a system with β-sialon as the main phase, room temperature
The strength up to 00 ° C. can be increased and the thermal shock resistance can be improved, so that it can be put to practical use for parts for automobiles, parts for gas turbine engines, and the like.

Continuing from the front page (72) Inventor Kenichi Tajima 1-4, Yamashita-cho, Kokubu-shi, Kagoshima Inside Kyocera Research Institute Inc. Examiner Yuichi Fukakusa (56) References JP-A-53-115712 (JP, A) JP-A-63-233082 (JP, A) JP-A-53-63413 (JP, A) JP-A-53-115712 (JP) JP-A-58-185484 (JP, A) JP-A-4-321562 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C04B 35/599

Claims (3)

(57) [Claims] (1) β-sialon (Si ₆ -Z Al _z O _z N)
_8-z , 0.05 ≦ z ≦ 0.35) The third phase of the periodic table containing a crystal phase as a main phase and at least lutetium in its grain boundary phase.
A sintered body containing a group a element, silicon, aluminum, oxygen and nitrogen, wherein the total amount of the group 3a element of the periodic table is 0.1 to 10 mol% in terms of oxide, and Characterized by having a thermal conductivity of 15 W / m · K or more. 2. The method according to claim 1, further comprising, as an auxiliary component, an oxide of a Group 3a element of the periodic table containing at least lutetium oxide, silicon oxide and aluminum oxide, or a compound thereof based on the main component consisting of silicon nitride powder. Ritsuyo 3a
The total amount of group-group elements is 0.1 to 10 mol% in terms of oxide, and the amount of aluminum oxide is 1.2 mol% based on the total amount of the main component and the amount of aluminum oxide.
After shaping a mixture consisting of
Baking in a non-oxidizing atmosphere at 00 to 1900 ° C., β-sialon (Si ₆ -Z Al _z O _z N _8-z , 0.05 ≦ z ≦
0.35) A silicon nitride based sintering characterized by obtaining a sintered body comprising a crystal phase and a grain boundary phase containing at least lutetium, ie, a Group 3a element of the periodic table, silicon, aluminum, oxygen and nitrogen. Body making. 3. An oxide of a Group 3a element of the periodic table containing at least lutetium oxide, silicon oxide and aluminum oxide, or a compound thereof, to silicon powder or a main component consisting of silicon powder and silicon nitride powder. The total amount of the Group 3a element of the periodic table is 0.1 to 10 mol% in terms of oxide, and the total amount of the amount in terms of silicon nitride and the amount in terms of oxide of aluminum in the main component is contained. After forming a mixture having a ratio of 1.2 mol% to 8.4 mol% in terms of aluminum oxide with respect to
After nitriding the silicon and the heat treatment in to 1500 ° C. in a nitrogen-containing atmosphere, and fired in a non-oxidizing atmosphere at 1600 to 1900 ° C., beta-sialon _{(Si-6 Z Al z O} z N
_8-z , 0.05 ≦ z ≦ 0.35) Obtain a sintered body composed of a crystal phase and a grain boundary phase containing at least lutetium, a Group 3a element of the periodic table, silicon, aluminum, oxygen and nitrogen. A method for producing a silicon nitride-based sintered body, characterized in that: