JPH06305839A - Sintered silicon nitride and its production - Google Patents

Abstract

PURPOSE:To obtain a sintered material composed of beta-sialon crystal phase and a grain boundary phase containing lutetium, etc., and having improved bending strength from room temperature to high temperature and thermal shock resistance by baking a formed material containing silicon nitride as a main component and a specific amount of an assistant at a prescribed temperature. CONSTITUTION:A main component consisting of powdery silicon nitride is mixed with an assistant component consisting of the group 3a element oxide including lutetium oxide, silicon oxide and aluminum oxide or their compounds to obtain a mixture containing 0.1-10mol% of the group 3a elements in total and 1.2-8.4mol% of aluminum oxide based on the sum of the main component oxide and the aluminum oxide. The mixture is baked in a non-oxidizing atmosphere at 1600-1900 deg.C to obtain a sintered material composed of a beta-sialon crystal phase and a grain boundary phase containing group 3 elements including lutetium in combination with silicon, aluminum, oxygen and nitrogen and having a thermal conductivity of >=15 w/m.K at room temperature.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for automobile parts, gas turbine engine parts, etc. 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 and a method for manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, silicon nitride sintered bodies have
Due to its excellent thermal shock resistance and oxidation resistance, it is being applied to engineering ceramics, especially for heat engines such as turbochargers. Recently, Y ₂ O ₃ and Sc are used as sintering aids for producing high density and high strength sintered bodies.
_The addition of rare earth element oxides such as ₂ O ₃ and aluminum oxide is disclosed in Japanese Examined Patent Publication Nos. 52-3649 and 58-5.
No. 190 proposed.

It is known that β-sialon is formed by the reaction with silicon nitride in a system containing aluminum oxide. Corrosion resistance is obtained by obtaining a sintered body containing β-sialon as a main component. It is also proposed to improve.

[0004]

However, when Y ₂ O ₃ or the like and aluminum oxide are used in combination as the sintering aid, the sinterability can be increased, which results in room temperature or high temperature. A high density sintered body with excellent strength is obtained,
Although it is being used as a structural material, it is necessary to increase its durability against heat such as thermal shock resistance as the usage conditions become stricter, and further improvement of its characteristics is desired in order to increase its reliability as a material. .

Therefore, the present invention is excellent in mechanical properties sufficient to be used in automobile parts, gas turbine engine parts and the like from room temperature to high temperature, and particularly excellent in bending strength from room temperature to high temperature of 1000 ° C. Further, it is to provide a silicon nitride sintered body having a high thermal conductivity and an excellent thermal shock resistance, and a method for producing the same.

[0006]

In order to improve the mechanical and thermal characteristics of the sintered body, the inventors of the present invention have found that the sub-crystals existing in the grain boundaries of the main crystal phase and the silicon nitride phase of the sintered body. As a result of repeated studies based on the viewpoint that it is important to control the phase,
Of the rare earth element compounds used as auxiliary components, L
By using a lutetium compound such as u ₂ O ₃ or the like, it is possible to increase the strength as compared with Y ₂ O ₃ or the like which has been widely used so far, and to make it exist in the grain boundary phase, and β -Sialon (Si _6-Z Al _z O _z N _8-z ) is used as a main crystal phase, and the z value thereof is controlled within a specific range, and a sintered body having a specific thermal conductivity of 10% or more is selected from room temperature to 10%.
It has been found that not only the bending strength up to a high temperature of 00 ° C is excellent, but also the thermal shock resistance can be enhanced.

That is, the silicon nitride sintered body of the present invention is β-
Sialon (Si _6-Z Al _z O _z N _8-z , 0.05 ≦ z
≦ 0.35) A sintered body containing a crystal phase as a main phase, and a grain boundary phase containing at least lutetium, an element of Group 3a of the periodic table, silicon, aluminum, oxygen and nitrogen, wherein 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 characterized by being 5 W / m · K or more.

Further, in the production method of the present invention, 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 agent with respect to the main component composed of silicon nitride powder. As a component, the total amount of the Group 3a elements of the Periodic Table is 0.1 to 10 mol% in terms of oxide, and the oxide equivalent of aluminum relative to the total amount of the main component and the oxide equivalent of aluminum. Is molded in a non-oxidizing atmosphere at 1600 to 1900 ° C. after molding a mixture having a ratio of 1.2 mol% to 8.4 mol%, or is composed mainly 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. And the total amount of the Group 3a elements of the Periodic Table is 0.1 to 10 mol% in terms of oxide, and the total amount of silicon nitride in the main component and the amount of oxide of aluminum in the main component is the total amount of aluminum. After molding a mixture having an oxide conversion amount of 1.2 mol% to 8.4 mol%, the mixture is heat treated in a nitrogen-containing atmosphere at 800 ° C. to 1500 ° C. to nitride the silicon, and then 1600 to 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) and a grain boundary phase containing lutetium, silicon, aluminum, oxygen and nitrogen are obtained.

The present invention will be described in detail below. The silicon nitride sintered body of the present invention has a structure of β-sialon (Si ₆ -Z Al
_z O _z N _8-z ) as 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, firstly, the main crystalline phase β
It is important that the 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 characteristics of β-sialon cannot be taken out, and if z exceeds 0.35, the thermal conductivity of the sintered body decreases.

Further, 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 can be increased and the high temperature strength can be increased. It should be noted that this lutetium can naturally be contained in combination with one or more other elements of Group 3a of the periodic table.

Further, the total amount of these lutetium in terms of oxide or the total amount of lutetium and one or more of lutetium and one or more of the other elements in Group 3a of the Periodic Table is 0.1 to 10 mol%, particularly It is preferably 0.3 to 5 mol%. This is because if the above 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 and other elements of Group 3a of the periodic table are used in combination, lutetium is 0.1 mol% or more in terms of oxide,
In particular, 20% in the oxide conversion amount of Group 3a element of the entire periodic table
The lutetium addition effect is exerted by making it exist in the above ratio.

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

Next, a method for manufacturing 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 as a raw material powder. The silicon nitride powder itself is α-Si.
Either ₃ N ₄ or β-Si ₃ N ₄ can be used, and their particle size is preferably 0.4 to 1.2 μm.

When silicon powder is used as the starting material, there is no dimensional change in the nitriding step described later and the weight increases, so that the density of the molded body is improved and the dimensional shrinkage during firing can be reduced, and the sintered body can be reduced. The dimensional accuracy of can be improved. In order to facilitate nitriding, it is suitable that the silicon powder has fine particles having an average particle size of 10 μm or less, particularly 3 μm or less. However, when the starting raw material contains a large amount of silicon powder, it becomes difficult to convert all of the added silicon powder into silicon nitride in the nitriding step described later, and conversely, the characteristics may deteriorate. It is preferable to use the silicon powder and the silicon nitride powder together in such a ratio that the silicon nitride conversion amount of the silicon powder and the ratio of the silicon nitride powder are 1 or less.

Then, at least lutetium oxide, or at least one of lutetium oxide and one or more oxides of Group 3a elements of the periodic table, and silicon oxide and aluminum oxide powder are added as auxiliary components for the above main components. Alternatively, compound powders of these components are used. Here, the total amount of silicon oxide added is oxygen, which is inevitably contained in the silicon nitride raw material, or an additive such as SiO ₂ .

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

[0018]

[Chemical 1]

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

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

On the other hand, the total amount of lutetium oxide or lutetium oxide and other oxides of Group 3a of the periodic table added as an auxiliary component is 0.1 to 10 mol%,
In particular, 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 together, lutetium oxide needs to be 0.1 mol% or more, and particularly 0.3 mol% or more.

The mixed powder thus obtained is molded into a desired shape by a known molding method such as press molding, cast molding, extrusion molding, injection molding or cold isostatic pressing.

Next, when the compact contains silicon powder, the compact is in a nitrogen-containing atmosphere at 800 ° C. to 1500 ° C.
The silicon powder contained in the molded body is nitrided by heat treatment at the temperature of 1 to generate silicon nitride. In this nitriding treatment, in order to completely nitrid the contained silicon powder, it is preferable to gradually nitridize the temperature within the above temperature range while raising the temperature in multiple stages.

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

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

According to the present invention, in addition to the above composition,
Metals of Group 4a, 5a, and 6a elements of the periodic table and their carbides, nitrides, silicides, or SiC have characteristics even if they exist in the sintered body of the present invention as dispersed particles or whiskers. Since there is little influence such as deterioration, it is naturally possible to improve the characteristics as a composite material by adding an appropriate amount thereof based on a well-known technique.

[0027]

The mechanical and thermal properties of the silicon nitride sintered body are determined by the β-sialon particles and the grain boundary phase, and the thermal shock fracture resistance coefficient of ceramics is usually expressed by the following formula 1.

[0028]

[Equation 1]

It is obtained from In the formula, R'is a thermal shock fracture resistance coefficient, S is a flexural strength, E is a Young's modulus, ν is a Poisson's ratio, k is a thermal conductivity, and α is a thermal expansion coefficient. Young's modulus of a sintered body, Poisson's ratio The coefficient of thermal expansion is almost constant unless the amount of grain boundary phase is extremely large, but the transverse rupture strength and the coefficient of thermal conductivity greatly change depending on the sintered body. In particular, in the case of a high-strength material, by increasing the thermal conductivity, the coefficient of thermal shock fracture resistance can be increased, and as a result, the thermal shock resistance of the sintered body can be increased, as is clear from Equation 1.

Ceramics is a polycrystalline body composed of crystals and grain boundary phases. Therefore, there are many factors of phonon scattering that reduce the thermal conductivity. The thermal conductivity of the β-sialon sintered body largely depends on the amount of impurities remaining inside the grains and the grain boundaries, the crystal perfection, and the microstructure. In particular, the thermal conductivity decreases as the solid solution amount of Al and O in the crystal increases. According to the present invention, based on the above viewpoint, as a result of examining the relationship between the thermal conductivity and the solid solution amounts of Al and O, the thermal conductivity is maximized in the range of z of 0.05 to 0.35. It is possible to increase the thermal shock resistance of the sintered body.

Further, lutetium is an element having the smallest ionic radius among the elements of Group 3a of the periodic table, and since it can raise the melting point and glass transition temperature of the grain boundary phase, it must improve the mechanical strength from room temperature to high temperature. You can

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. By containing oxygen and nitrogen, excellent mechanical properties from room temperature to 1000 ° C. and excellent thermal conductivity can be imparted, and thereby thermal shock resistance at high temperatures can be significantly improved. .

[0033]

【Example】

Example 1 Silicon nitride powder (BET specific surface area 9 m ² / g, α ratio 98
%, Oxygen content 1.2% by weight) and lutetium oxide, or lutetium oxide and various oxides of Group 3a 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
O ₇ powder was used (Sample No. 11) to prepare the composition shown in Table 1, and the mixture was molded with 1 t / cm ² .

The molded body was placed in a silicon carbide mortar and fired at 1800 ° C. for 10 hours in a pressurized nitrogen gas stream under controlled atmosphere in order to reduce composition fluctuation.

The obtained sintered body was ground to a shape specified in JIS-R1601 to prepare a sample. For this sample, the z value 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, and further at room temperature and 1 based on JIS-R1601.
A 4-point bending bending strength test at 000 ° C. was performed. Also,
The 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, 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. 1 is insufficiently densified, and No. 1 having a z value exceeding 0.35. No. 14 had lower thermal conductivity. Sample No. containing no lutetium It can be seen that 3, 4, and 5 have low high-temperature strength as compared with the product of the present invention containing lutetium. Further, the lutetium and Group 3a elements of the Periodic Table have oxide conversion amounts exceeding 10 mol%. Sample No. 19 had low thermal conductivity. In contrast to these comparative examples, all the other samples according to the present invention were at room temperature and 1000
It was excellent in bending strength at ° C and had high thermal conductivity.

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

The obtained molded body was heated at 1200 ° C. in a nitrogen stream.
Nitriding treatment was performed for 5 hours at 1400 ° C. for 10 hours. At this time, the nitriding rate of silicon is calculated from

[0041]

[Equation 2]

It was determined by

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

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, No. 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 is less than 0.05. No. 22 is insufficiently densified, and No. 22 having a z value exceeding 0.35. No. 26 had low thermal conductivity. No. containing no lutetium No. 24 had low high temperature strength. Furthermore, the oxide conversion of lutetium exceeds 10 mol%. Sample No. 27 had low thermal conductivity. In contrast to these comparative examples, all the other samples according to the present invention were excellent in bending strength up to high temperature and had high thermal conductivity.

[0048]

As described in detail above, according to the present invention,
In the system with β-sialon as the main phase, room temperature to 10
The strength up to 00 ° C. can be increased and the thermal shock resistance can be improved, and it can be put to practical use as parts for automobiles, parts for gas turbine engines and the like.

Front page continued (72) Inventor Takehiro Oda 1-4 Yamashita-cho, Kokubun-shi, Kagoshima Prefecture Kyocera Stock Company Research Institute (72) Kenichi Tajima 1-4-4 Yamashita-cho, Kokubun-shi, Kagoshima Kyocera Stock Company In the laboratory (72) Inventor Tomohiro Iwata 1-4 Yamashita-cho, Kokubun-shi, Kagoshima Kyocera Stock Company Research Institute

Claims (3)

[Claims] 1. β-sialon (Si _6-Z Al _z O _z N
_8-z , 0.05 ≦ z ≦ 0.35) Periodic table containing a crystal phase as a main phase and at least lutetium in the grain boundary phase.
A sintered body containing a group a element, silicon, aluminum, oxygen and nitrogen, wherein the total amount of the group 3a elements of the periodic table is 0.1 to 10 mol% in terms of oxide and at room temperature. Has a thermal conductivity of 15 W / m · K or more. 2. An oxide of Group 3a element of the periodic table containing at least lutetium oxide, silicon oxide and aluminum oxide, or a compound thereof as an auxiliary component with respect to the main component of silicon nitride powder, and Table 3a
The total amount of the group elements is 0.1 to 10 mol% in terms of oxide, and the amount of aluminum in terms of oxide is 1.2 mol% with respect to the total amount of the main component and the amount of oxide in aluminum.
To 8.4 mol% of the mixture, after molding,
Firing 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) Silicon nitride based sintering characterized by obtaining a sintered body comprising a crystal phase and a grain boundary phase containing at least lutetium group 3a element in the periodic table, silicon, aluminum, oxygen and nitrogen How to make a body. 3. An oxide of a group 3a group 3a element of the periodic table containing at least lutetium oxide, silicon oxide and aluminum oxide, or a compound thereof with respect to the main component of silicon powder or silicon powder and silicon nitride powder. The total amount of the elements of Group 3a of the Periodic Table is 0.1 to 10 mol% in terms of oxide, and the sum of the amount of silicon nitride in the main component and the amount of oxide in aluminum is included as an agent component. 800 ° C. after molding a mixture having a ratio of aluminum to oxide of 1.2 mol% to 8.4 mol% relative to
After heat-treating in a nitrogen-containing atmosphere at ˜1500 ° C. to nitride the silicon, it is fired in a non-oxidizing atmosphere at 1600˜1900 ° C. to obtain β-sialon (Si ₆ -Z Al _z O _z N
_8-z , 0.05 ≦ z ≦ 0.35) to obtain a sintered body composed of a crystal phase and a grain boundary phase containing at least lutetium and a Group 3a element of the periodic table, silicon, aluminum, oxygen and nitrogen A method for producing a silicon nitride-based sintered body characterized by the above.