JPH11139877A - Heat insulating silicon nitride-base sintered compact and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a heat insulating silicon nitride-base sintered compact having high thermal shock resistance and denseness and excellent in durability as well as in heat insulating property. SOLUTION: The sintered compact is based on Si3 N4 having <=100 nm average grain diameter, contains a Ti compd. as dispersed particles, has <=5 W/mK heat conductivity and preferably has >=600 deg.C thermal shock resistance measured by an underwater rapid cooling method. The sintered compact is produced as follows; Si3 N4 powder and metal Ti powder are mixed and comminuted to <=50 nm average particle diameter by a mechanical alloying method and the resultant composite powder is densified and sintered to <=100 nm average grain diameter of Si3 N4 and >=93% relative density.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention is useful as a mechanical structural member of an automobile part or the like, and has a lower thermal conductivity than a known silicon nitride sintered body and excellent heat insulating properties. The present invention relates to a binder and a method for manufacturing the same.

[0002]

2. Description of the Related Art Silicon nitride (Si ₃ N ₄ ) is a material that is balanced in strength, fracture toughness, corrosion resistance, abrasion resistance, thermal shock resistance, oxidation resistance, and the like. Etc. are used in a wide range. In particular, recently, it has attracted attention as a structural material for automobile engines and gas turbines.

In particular, as a dense ceramic, a silicon nitride sintered body has a high thermal shock resistance, and is therefore useful as a heat resistant material. However, silicon nitride has a thermal conductivity of 20 W
/ MK, which is relatively high, so that it was not necessarily suitable as a component requiring heat insulation.

As a heat insulating ceramic material, zirconium oxide having a low thermal conductivity has been generally used, but has a problem that its reliability is low due to poor thermal shock resistance. Further, porous ceramics are excellent in both heat insulating properties and thermal shock resistance, but are considered to have low reliability as structural materials because they cannot provide airtightness or have low strength durability.

[0005]

As described above, a ceramic material having excellent heat insulating properties is demanded as a structural material for an automobile engine or the like, but at present, a ceramic material having heat insulating properties and excellent thermal shock resistance is required. As a result, in many cases, a member or equipment having low thermal efficiency or low reliability has to be used.

In view of the above circumstances, the present invention provides a silicon nitride-based sintered body which is excellent in heat insulation, has high thermal shock resistance, is dense, and has excellent durability, and a method for producing the same. The purpose is to:

[0007]

In order to achieve the above object, a heat-insulating silicon nitride-based sintered body provided by the present invention comprises a silicon compound having an average particle diameter of 100 nm or less as a main component, and a titanium compound as a dispersed particle. And a thermal conductivity of 5 W / m
K or less. Further, the heat-insulating silicon nitride-based sintered body preferably has a thermal shock resistance of 600 ° C. or higher as measured by a quenching method in water.

According to the method of the present invention for producing a heat-insulating silicon nitride sintered body having a thermal conductivity of 5 W / mK or less, a silicon nitride powder and a metal titanium powder are subjected to mechanical alloying so that the average particle size of silicon nitride is 50 nm. Mix and crush until it is less than
The average particle diameter of silicon nitride was 100 nm.
Or less and the relative density is 93% or more.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The silicon nitride based sintered body of the present invention
It is made of silicon nitride composed of crystal grains having an average particle size of 100 nm or less. At the same time, the average particle size of silicon nitride is 1
By controlling the thickness to be equal to or less than 00 nm, the thermal conductivity of the silicon nitride-based sintered body can be reduced to 5 W / mK or less.
The structural ceramic material has a thermal conductivity of 5 W.
By setting it to / mK or less, the heat insulation required in many cases can be secured, and the thermal efficiency can be improved as a heat insulating member or a heat insulating device.

When the average particle size of silicon nitride exceeds 100 nm, the thermal conductivity increases, and it becomes difficult to obtain necessary heat insulating properties. Even in such a case, in order to suppress the thermal conductivity to 10 W / mK or less, it is necessary to increase the porosity of the sintered body or add a large amount of additives such as a glass component having a low thermal conductivity. All of these methods should be avoided because they will lower the thermal shock resistance inherent in the silicon nitride sintered body, and will also reduce properties such as wear resistance and mechanical strength.

In order to control the average particle size of silicon nitride to 100 nm or less, a sintered body is manufactured using a composite powder of silicon nitride and metallic titanium as described later. This allows
Metal titanium becomes crystal nuclei at the same time as sintering, and fine silicon nitride particles precipitate, and fine titanium nitride precipitates around silicon nitride particles, suppressing the growth of silicon nitride grains during the sintering process. can do. If the amount of titanium in the sintered body is less than 5% by weight with respect to silicon nitride, it does not precipitate as fine crystal grains during sintering, and if it exceeds 50% by weight, the strength and thermal shock resistance of the sintered body decrease. So 5-5
It is preferred to be in the range of 0% by weight.

Further, the heat-insulating silicon nitride-based sintered body of the present invention can maintain excellent thermal shock resistance inherent in silicon nitride.
The higher the thermal shock resistance, the higher the reliability in the case of a component, but the thermal shock resistance when measured by the underwater quenching method, in which a heated sintered body is put into water and the change in strength is examined, is 600.
If the temperature is at least ° C, there is no practical problem. For example, it has been clarified that even a heat-resistant ceramic part exposed to 1000 ° C. does not cause a problem due to thermal shock if it can withstand heating at 600 ° C. by a quenching method in water.

In order to produce the silicon nitride-based sintered body of the present invention, a silicon nitride powder and a metal titanium powder are mixed and pulverized by a mechanical alloying method to obtain a composite powder having an average particle diameter of silicon nitride particles of 50 nm or less. And When the average particle size of the silicon nitride particles exceeds 50 nm, it is difficult to reduce the crystal particle size of the silicon nitride after sintering to 100 nm or less, and it is also difficult to make the silicon nitride compact. In order to obtain the average particle diameter of 50 nm or less, it is preferable that the acceleration during mixing be in the range of 50 to 200 G.

Next, the composite powder is sintered in a nitrogen atmosphere and densified so that the average particle size of silicon nitride is 100 nm or less and the relative density is 93% or more. At the time of this sintering, as described above, metallic titanium changes into nitride and precipitates, thereby suppressing grain growth of silicon nitride. When the average particle size of silicon nitride exceeds 100 nm, the thermal conductivity of the sintered body is reduced by 5%.
W / mK or less is difficult, and it is also difficult to obtain a sintered body having a thermal shock resistance of 600 ° C. or more by a quenching method in water.

[0015]

EXAMPLE An α-type Si ₃ N ₄ powder having an average particle size of 0.3 μm was
Al ₂ O ₃ and Y ₂ O ₃ were added as a sintering aid in an amount of 1% by weight, respectively, and a metal Ti powder having an average particle size of 5.0 μm in an amount shown in Table 1 below was added, followed by ZrO ₂ lining. Table 1 shows the results of a ball mill using a pot and ZrO ₂ balls.
Under the mixing conditions shown in Table 1. For each of the obtained composite powders, the particle size of the Si ₃ N ₄ particles was evaluated using TEM, and the average particle size is shown in Table 1. Next, each composite powder was sintered under pressure at a sintering temperature shown in Table 1 in a nitrogen atmosphere at 1 atm, thereby densifying.

[0016]

[Table 1] Ti content Ball mill mixing conditions Composite powder Sintering temperature sample (wt%) Acceleration (G) Time (hr) Si ₃ N ₄ (nm) (° C) 1 *-120 6 200 1400 2 5 120 6 40 1400 3 20 120 6 10 1400 4 50 120 6 10 1400 5 70 120 6 60 1400 6 * 20 2 4 400 1600 7 20 50 8 10 1300 8 * 20 150 4 10 1900 9 * 20 100 8 15 1700 10 * 20 100 8 15 1200 (Note) Samples marked with * in the table are comparative examples.

For each of the obtained Si ₃ N ₄ sintered bodies, a thin film specimen was prepared by ion etching, and the average particle size of the Si ₃ N ₄ elementary particles in the matrix was evaluated using a transmission electron microscope. In addition, the relative density, thermal conductivity, and thermal shock resistance of the underwater quenching method of each Si ₃ N ₄ sintered body were evaluated, and the results are shown in Table 2 below.

[0018]

[Table 2] (Note) Samples marked with * in the table are comparative examples.

As can be seen from the above results, the sample 1 of the comparative example has no added Ti powder, the sample 6 has a large Si ₃ N ₄ particle size of the composite powder, and the samples 8 and 9 have a sintering temperature of Since they are high, the grain size of Si ₃ N ₄ in each sintered body is large, and the thermal conductivity is 10 W / mK or more. Also,
Sample 10 of the comparative example has a low sintering temperature, so that the density of the sintered body has decreased, and the thermal shock resistance has also slightly decreased.

On the other hand, each sample of the present invention is obtained by
₃ The average particle size of N ₄ is less and fine 100 nm, both with showing the following thermal conductivity of 5W / mK, and is excellent in thermal shock resistance. However, in Sample 5, the amount of added metallic Ti was large, and the particle size of the Si ₃ N ₄ particles in the composite powder was slightly large, so that the thermal shock resistance of the final sintered body was slightly reduced.

[0021]

According to the present invention, a nitride having a low thermal conductivity required as a heat insulating ceramic material for a structure such as an automobile engine, a high thermal shock resistance, a dense structure and an excellent durability. A silicon-based sintered body can be provided.

Claims (3)

[Claims] 1. A heat-insulating silicon nitride-based sintered body comprising silicon nitride having an average particle diameter of 100 nm or less as a main component, a titanium compound as dispersed particles, and a thermal conductivity of 5 W / mK or less. . 2. The thermal shock resistance by quenching in water is 600 ° C.
The heat-insulating silicon nitride-based sintered body according to claim 1, wherein: 3. An average particle size of silicon nitride of 50 nm is obtained by mechanically alloying silicon nitride powder and metal titanium powder.
The resulting composite powder was mixed and pulverized until the average particle size of silicon nitride was 100 nm or less and the relative density was 93% or less.
%. A method for producing a heat-insulating silicon nitride-based sintered body having a thermal conductivity of 5 W / mK or less, wherein the sintered body is densified and sintered to at least 5%.