JPS60186491A - Silicon nitride sintered body - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a silicon nitride sintered body I having high mechanical reliability.

[Technical background of the invention and its problems]

Silicon nitride ceramics have high bond strength.

Due to its excellent heat resistance, it is expected to be used as a strong material or wear-resistant material used at high temperatures. When used for such purposes.

High mechanical properties and reliability are required for the material.

Generally, the strength of a material is determined by the density of the material itself, and the size of defects or pores existing inside the material influences the strength of the material. Therefore, such materials are strongly required to have a dense and homogeneous structure without large defects or pores, and require precise quality and process control from the raw material stage. However, since defects or pores are what govern material strength, it is almost impossible to completely eliminate them from sintered bodies, so it is important to reduce the size of each defect in order to achieve the above objective. It becomes technology.

For example, the development of homogeneous powder raw materials, the search for effective sintering additives, advanced control of the manufacturing process, and even HIP after sintering.
(Hot isostatic pressing) treatment is an important research item as a means of increasing the reliability of sintered bodies.

The fracture strength of brittle materials such as ceramics is generally explained by the quadratic equation.

σf: Strength of material γ: Surface energy E: Young's modulus C: Size of defect This is a formula derived from Griffith's fracture model, and the key is to reduce the size of the defect represented by C in the formula. This shows that the material strength is improved. In addition, the cause of strength variation is a phenomenon caused by the fact that the size of defects contained in each material is different.5 It is thought that the variation will theoretically disappear if the size of defects becomes constant. It will be done. Possible defects here include pores, foreign matter, crystal grains, and scratches on the surface of the sintered body due to grinding or the like.

Based on this idea, as a specific method for reducing the strength variation by uniformizing the size of defects in ceramic materials, when manufacturing a sintered body25, in advance, other defects that can be the size of defects are It is known to add substances to the temperature. For example, a method is known in which by adding whiskers or short fibers and sintering at °C, these added substances remain in the sintered body as they are and act as defects, thereby reducing variations in material strength. In this case, silicon carbide is selected as the material to be added as it is a substance that easily reacts with silicon nitride and iron. However, not reacting with the silicon nitride of the base material is necessary for it to exist as a defect, but from the viewpoint of material strength, it is not compatible with the base material, so whiskers or short fibers can be removed without a drop. Although the variation in strength itself is reduced, the strength is extremely low, which has been considered a problem when used for structural purposes.

[Purpose of the invention]

The present invention has been made to solve the above problems, and aims to provide a highly reliable and high-strength silicon nitride sintered body.

[Summary of the invention]

The present inventors used "seeds" as starting materials in the silicon nitride synthesis reaction by silica reduction of silicon dioxide-carbon-nitrogen system.
By mixing the substances in advance [From the second point, we focused on the fact that silicon nitride is formed mainly from the ′′ ribs ′′ and using silicon carbide whiskers (including short fibers) as a ``seed'' material, silicon nitride with good adhesion was formed. It was discovered that silicon carbide whiskers (including short fibers) coated with

When this silicon carbide whisker (including short fibers) was added to the silicon nitride-rare earth oxide system and sintered, densification was achieved with the rare earth oxide, and the defect size in the sintered body was reduced by the added silicon carbide whiskers (including short fibers). Because the surface of the material (fibers) is coated with silicon nitride, it has a strong bond with the base material.

It was found that the average strength was improved.

Specifically, the silicon nitride sintered body of the present invention has the surface of the added silicon carbide whiskers (including short fibers) densely coated with silicon nitride, which is preferably synthesized by a silica reduction method. As a result,
It has the characteristics of reducing variations in strength and at the same time providing a high-strength material.

In the present invention, silicon carbide whiskers (including short fibers) are essential components that act as defects that control material strength. The silicon carbide whiskers (including short fibers) are preferably of high purity, and usually have a purity of 97% or higher.

However, if the size is too small, it will not act as a defect, so it is necessary to select an appropriate size.

The method for producing silicon carbide whiskers (including short fibers) coated with two-phase silicon nitride according to the present invention is the synthesis of silicon nitride powder by a silica reduction method.

This method applies the principle that silicon nitride synthesized by pre-existing fine silicon nitride in the starting raw material grows using silicon nitride particles present in the mixed powder as nuclei.

In this method [2], silica powder is a necessary component as a silicon source which is a constituent element of the silicon nitride layer.

The silica powder used is 1, for example, pure quartz or silica.

Powders of high-silica-containing substances such as silica sand, colloidal silica, and organosilicon compounds that convert into silica at high temperatures (but at temperatures below 1550°C), such as methyl silicic acid (CH
Silica precursor materials such as 1lS#02) can be mentioned.

The particle size of the silica powder is preferably 1 μm or less.

When the particle size exceeds 1 μm, the reactivity of the silica powder decreases.

Carbon powder is a necessary component as a reducing agent for silica.

Examples of the carbon powder used include carbon black,
Examples include powders of carbon precursor materials such as graphite, lamp black carbon, and organic compounds that convert to carbon at high temperatures (but below 1550° C.), such as phenolic resins.

The particle size of this carbon powder is preferably 0.2 μm or less. This is because if the particle size exceeds 0.2 μm, the ash-like properties of the carbon powder will decrease.

The reason why silicon nitride-coated silicon carbide whiskers (including short fibers) are formed is thought to be as follows.

That is, as a −order reaction sto, -1-c, sio 10c.

A reaction proceeds. This reaction is a solid phase reaction and SiO
and N, or NH, easily react in the gas phase to form S in the gas phase.
Generate i,N. However, in the present invention, since silicon carbide coexists in advance, St existing in the gas phase
, N4 is rapidly deposited with SIC as its core.

It grows and forms a surface layer centered exactly at 810. In this way, the whiskers (including short fibers) are densely coated with silicon nitride. the result.

Even if silicon nitride is added, it is considered that the fibers are less likely to come out of the crucible as in the case of ordinary fiber-reinforced materials I because of their close contact with the base material, and as a result, the strength of the material is improved.

[Embodiments of the invention]

The present invention will be further explained below with reference to Examples.

Example 1 Consisting of 1 part by weight of silica powder with an average particle size of 0.05 μm, 0.5 part by weight of lamp black carbon with an average particle size of 0.02 μm, and 81C Wiss force weight part with a diameter of about 2 μm and a length of 15 μm. The mixed powder was heated to 1450°C in nitrogen gas.
, and baked for 5 hours. Thereafter, this fired product was placed in the atmosphere and heated at 700° for 0.5 hour to perform a decarburization treatment to obtain silicon nitride-covered silicon carbide whiskers.

Next, 90% silicon nitride with an average grain size of 1.01 m, yttrium oxide 5% with an average grain size of 0.8 μm, and an average grain size of 0.3 μm.
A mixed powder of 2% aluminum oxide (m) and 3% silicon carbide whiskers was prepared and sintered at 1780'O in nitrogen gas under normal pressure or hot press. Obtained: 3 from the sintered body
Cut out a test piece of X3X30M.

Three-point bending strength (KP/if) was measured. The results are shown in the table below.

As a table comparison, silicon carbide whiskers not coated with silicon nitride were evaluated using the same method, and although no large differences in strength were observed, the average strength was approximately 37.
It was found that Ky/- was very low.

〔Effect of the invention〕

As detailed above, according to the present invention, a silicon nitride sintered body having high strength and extremely high reliability can be obtained.

Agent: Patent attorney Kensuke Norichika and 1 other person

Claims (1)

[Claims] A silicon nitride sintered body having a silicon nitride-rare earth oxide-silicon carbide whisker system, characterized in that the surface of the silicon carbide whisker is coated with silicon nitride.