JP3207940B2 - 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 relates to a silicon nitride sintered body having high toughness and high strength, and excellent in impact resistance and thermal shock resistance, and can be suitably used particularly for tools, machine parts, structural parts and the like. .

[0002]

2. Description of the Related Art Although a silicon nitride sintered body is excellent in bending strength, especially high temperature strength, it has a problem that it is inferior in toughness to a zirconia sintered body. Therefore, various means for increasing the toughness have been proposed, for example, by including hard particles having a larger coefficient of thermal expansion than silicon nitride,
Technology for applying compressive stress to silicon nitride particles (Japanese Patent Laid-Open No. 3-1
No. 93667), a technique of adding a component serving as a nucleus of a β-type silicon nitride powder to a raw material powder (JP-A-3-29037).
No. 0), a technique in which a non-oxide powder other than silicon nitride is contained in the raw material composition (JP-A-2-31366), and a technique for controlling the pore diameter and pore distribution in a sintered body (JP-A-Hei. No. 4-26553) is known.

[0003]

SUMMARY OF THE INVENTION Among the above prior arts,
The former three focus on raw material composition or additive components, but since toughness is a characteristic of a sintered body, it controls the microscopic structure of the sintered body regardless of the raw material etc. It is important to do.

However, the silicon nitride sintered body disclosed in Japanese Patent Application Laid-Open No. 4-26553 contains relatively large pores in the sintered body, and since they exist as aggregates, the reliability of the material is reduced. Or the strength may be reduced.

An object of the present invention is to solve the above-mentioned problems and to provide a silicon nitride sintered body having excellent strength, toughness, and excellent thermal shock resistance.

[0006]

Means for solving the problems are as follows. In a silicon nitride sintered body substantially consisting of a sintering aid and Si ₃ N ₄ or Sialon particles, a columnar silicon nitride particle and a grain boundary phase are formed. 3μm or less in width and 0.5μ in length
The sintered compact contains micro crack-like voids having at least one corner having a radius of curvature of not less than m and not more than 20 μm and not more than 0.5 μm in the sintered body at a rate of not less than 5000 / mm ^2. A silicon nitride-based sintered body.

In this means, a desirable silicon nitride-based sintered body is one in which the majority of minute crack-like voids are located at the tip of the long axis of the columnar silicon nitride particles. Similarly, a desirable silicon nitride-based sintered body has a columnar silicon nitride particle having a minor axis of 1 μm or more and an aspect ratio of 5 or more is 1000 particles / mm.
There are ^two or more.

The sintered body of the present invention is obtained, for example, by mixing a silicon nitride powder with a powder of a rare earth oxide, magnesium oxide, alumina or the like as a sintering aid and molding the mixture to obtain a compact.
The primary baking is performed at a high temperature exceeding 0 ° C. and a nitrogen atmosphere of 10 atm or less, and further under a nitrogen atmosphere of 10 atm or more.
It can be obtained by performing secondary firing at a temperature of 00 ° C. or higher. In the cooling process of primary and secondary firing of this manufacturing process,
The crack can be formed between the columnar particle and the grain boundary phase from the difference in thermal expansion between the a-axis length and the c-axis length of the columnar particle.
Preferably, the cooling step is performed at a rate of 10 ° C./min. It is better to perform at the above cooling rate. More preferably, in the primary firing, the heating rate is 5 ° C. /
min. It is better to bake it. By increasing the heating rate, more columnar particles can be generated.

[0009]

[Action]

(1) Necessity of Cracks and Reasons for Limiting the Cracks The reason why the fine crack-like voids are necessary in the present invention is that when cracks exist at the interface between the columnar particles and the grain boundary phase, these fine crack-like voids are nucleated during stress loading. As a result, many microcracks are generated and toughness occurs, and furthermore, the interaction between the cracks induced by the stress easily causes crack deflection and particle bridging effect to increase the toughness. Further, these minute crack-like voids also reduce stress generated at the time of thermal shock and improve thermal shock resistance.

[0010] More preferably, the location of the minute crack-like voids is located at the tip of the major axis of the columnar particles. The reason is that the β-type silicon nitride crystal grows in the c-axis direction during firing and becomes columnar, which causes anisotropy in the thermal expansion of the particles. That is, the thermal expansion coefficient of the β-silicon nitride particles c-axis direction is 2.4 × 10 ^-6 / ℃, a a-axis direction is 3.4 × 10 ^{over 6} / ° C., for example an aspect ratio (c
When (length in the axial direction / length in the a-axis direction) is 10, the amount of thermal expansion in the c-axis direction is about seven times that in the a-axis direction. For this reason, the thermal stress is large in the c-axis direction, so that the crack is located at the tip of the long axis of the columnar particle, and the effect of relaxing the thermal stress is higher.

[0011] The reason for limiting the shape of the cracks is that when the cracks become spherical pores, they become defects at the time of destruction and cause deterioration in strength and decrease in strength reliability. Inability to induce microcracks. Therefore, the crack is 0.5μ.
It is necessary to have at least one corner having a radius of curvature of not more than m. Further, it is preferable that the distribution of cracks is uniform in the sintered body. When present as an aggregate, cracks are coalesced and act as a large defect at the time of destruction, causing deterioration in strength.

The reason for limiting the crack length is 0.5
If the thickness is less than μm, no effect can be obtained, and if it is more than 20 μm, it acts as a defect and causes a decrease in strength. Also,
If the width exceeds 3 μm, the pores have a detrimental property. The cracks have various shapes such as an elliptical shape, a crescent shape, and a U-shape. Here, the crack length means a maximum diameter or a maximum diagonal length.
On the other hand, the reason for limiting the abundance of minute crack-like voids is
If the number is less than 5000 / mm ² , the effect of cracks is small.

[0013] (2) destruction of necessity the sintered and an aspect ratio in the minor axis is 1μm or more in the body of 5 or more columnar silicon nitride particles 1000 / mm ² or more with the silicon nitride sintered body of the columnar particles The toughness value is further improved and is preferred. The reason is that the presence of the columnar grains grown by the grains causes the microcracks induced from the microcrack-like voids to interact with the silicon nitride grains grown by the grains, deflecting the cracks,
This is because a toughening mechanism such as particle crosslinking works more effectively.
The reason that the number of the columnar particles is 1,000 or more per mm ² is that if the number is less than 1,000, the above effect cannot be obtained.

EXAMPLES The sintering aids shown in Tables 1 and 2 were added to α-type silicon nitride powder having a BET specific surface area of 10 m ² / g and wet-mixed with ethanol. 1.5ton / c dried compound powder
Press molding was performed at a hydrostatic pressure of m ² and sintering was performed under the conditions shown in Tables 1 and 2 to obtain a silicon nitride sintered body.

With respect to the obtained sintered body, general characteristics and microstructure characteristics of the sintered body were determined by the following methods. First, as for the material strength, the bending strength was measured by a three-point bending test according to JIS R1601. The fracture toughness value was measured by the SEPB method of JIS R1607. Thermal shock test is 4mm
A test piece of 3 mm x 40 mm was heated to a predetermined temperature, dropped into water, quenched, and then subjected to a bending test at room temperature. The thermal shock temperature was determined from the temperature at which the strength deteriorated due to the thermal shock.

Confirmation of existence of minute crack-like voids, crack density, width, length and radius of curvature of corners were obtained by mirror-polishing the sintered body by lap polishing, followed by SEM observation and photograph analysis of the photograph. It measured by doing. The analyzed area is 1000 μm ² . The above measurement results are shown in Tables 1 and 2.

[0016]

[Table 1]

[Table 2] As can be seen from the table, those having microstructure characteristics falling within the range of the present invention satisfied a fracture toughness value of 10 MPam ^0.5 or more and a thermal shock temperature of 1100 ° C. or more. On the other hand, No. 3 which does not belong to the scope of the present invention. The sintered bodies of Nos. 8 to 11 exhibited the same bending strength as the sintered body of the present invention, but the fracture toughness did not reach 10 MPam ^0.5 , and the thermal shock temperature was 1000 ° C. or lower.

[0017]

As described above, the silicon nitride sintered body of the present invention has a high fracture toughness value and excellent thermal shock resistance as compared with other sintered bodies as shown in the examples, As a structural heat-resistant material such as a rolling roller, reliability and life can be greatly improved.

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C04B 35/584-35/596 C04B 35/599

Claims (3)

(57) [Claims] 1. A silicon nitride sintered body consisting essentially of a sintering aid and Si ₃ N ₄ or Sialon (Sialon) particles, wherein the width of the interface between the columnar silicon nitride particles and the grain boundary phase is 3 μm or less. In the sintered body, micro-crack-shaped voids having a length of 0.5 μm or more and 20 μm or less and having at least one corner having a radius of curvature of 0.5 μm or less are provided at a rate of 5000 / mm ² or more. A silicon nitride-based sintered body characterized in that it is substantially uniformly contained. 2. The majority of the minute crack-like voids are located at the tip of the major axis of the columnar silicon nitride particles.
3. The silicon nitride based sintered body according to 1.). 3. A minor 1μm or more, the silicon nitride sintered body of claim 1, an aspect ratio of 5 or more columnar silicon nitride particles are present 1000 / mm ² or more.