JPS589787B2 - Manufacturing method for high-strength ceramics - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing high-strength ceramics mainly composed of silicon nitride.

As the conditions for high-temperature use of gas turbine parts and high-temperature roll materials for steelmaking become more severe, materials are being changed from metal-based materials to ceramic-based materials, with high strength and high heat resistance. Silicon nitride is attracting attention as a material due to its impact resistance and high wear resistance.

However, silicon nitride has drawbacks such as being a material that is difficult to sinter when used alone and that it is difficult to obtain high strength, and various methods have been proposed to solve these problems.

The main ones are: (1) Adding oxides of alkaline earth elements such as magnesium oxide.

(2) Add aluminum oxide.

(3) Adding an oxide of a rare earth element such as yttrium oxide.

(4) Adding non-oxides such as aluminum nitride.

There are other methods.

However, these methods have the following drawbacks and are still problematic from a practical standpoint.

(1) The method of adding magnesium oxide is an effective method in terms of promoting sintering of silicon nitride, but magnesium oxide reacts with silicon oxide formed on the surface of silicon nitride grains, and has a low melting point. Since a glass phase is produced, the high temperature strength or creep resistance of the silicon nitride body as a whole is impaired, and there is a problem in its application to members that are subjected to loads at high temperatures.

(2) The method of adding aluminum oxide has the unique effect of accelerating sintering as aluminum oxide forms a solid solution in silicon nitride grains, but high-temperature strength and high-temperature creep resistance are still not necessarily sufficient. do not have.

(3) The method of adding yttrium oxide is (1) (
This method improves the drawback of 2), and it is true that the grain boundaries of silicon nitride grains are bonded with high viscosity glass or a crystalline composition such as Si3N4/Y2O3, which impairs high-temperature strength and high-temperature creep resistance. This is rare, and in this respect it is effective.

However, even with this method of adding yttrium oxide, high-temperature strength and the like are still insufficient depending on the application, and furthermore, it does not satisfy the demand for further lowering the firing temperature.

(4) The method of adding aluminum nitride together with alumina, etc., has the effect of improving hardness, oxidation resistance, etc., but can only provide an insufficient strength.

The present invention provides a method for solving all of these drawbacks, and the gist thereof is to add one or more of yttrium oxide, cerium oxide, and magnesium oxide in an amount of 0.5 to
30% by weight, and 0.1 to 15% by weight of metal aluminum,
A method for producing high-strength ceramics, which comprises molding a compound in which the remainder is silicon nitride particles and firing it in a non-oxidizing atmosphere, or hot pressing the compound in a non-oxidizing atmosphere. .

Generally, various additives are used to impart sinterability, but many of these additives tend to form a glass phase or soften at high temperatures, and this is due to nitridation. This is thought to be due to a glass phase or a low melting point crystalline phase produced as a result of the reaction of the silicon oxide film formed on the surface of the silicon grains with these additives.

It is true that yttrium oxide, cerium oxide, magnesium oxide, magnesium oxide, aluminum oxide, etc. are additive components that should have a considerable effect, but as long as they are used alone, the effect seems to be due to the above reasons. As mentioned above, only unsatisfactory effects were obtained.

Therefore, the inventors of the present invention deeply investigated these points, and as a result, they were fully satisfied by further adding metals such as metal aluminum, metal zirconium, and metal titanium, which have a higher affinity for oxygen than metal silicon, especially metal aluminum. It has been found that results can be obtained.

The reason why the effects of the present invention are obtained by further adding metal aluminum has not yet been fully elucidated, but it is thought to be as follows.

That is, it is presumed that the sinterability is due to the heat generated by the thermite reaction of metal aluminum, but as shown in the examples below, it is possible to lower the sintering temperature by about 100°C. became.

In addition, regarding the positive effect on high-temperature strength, metal aluminum removes oxygen from the silicon oxide film formed on the surface of silicon nitride grains in the firing temperature range, and some of it becomes Al203 and some of it becomes nitrided and becomes AlN. By reducing the glass phase or low melting point crystal phase that would occur if metallic aluminum is not added, high strength at high temperatures can be maintained without impairing the effects of additives such as yttrium oxide. it is conceivable that.

Therefore, although adding alumina or aluminum nitride instead of metallic aluminum has the effect of promoting sintering to some extent, it does not have the effect of eliminating the silicon oxide film on the surface of silicon nitride grains, and therefore, adding metallic aluminum Neither the remarkable sintering effect nor the effect of increasing high temperature strength as in the case was observed.

Thus, the method of using yttrium oxide, cerium oxide, and magnesium oxide as additives, which have been proposed as promising additives but have been difficult to put into practical use, can be improved by further adding metal aluminum, as in the present invention. This makes it possible to provide a truly practical silicon nitride body.

In the present invention, the reason for adding 0.5 to 30% by weight of one or more of yttrium oxide, cerium oxide, and magnesium oxide depends on the amount of metal aluminum added, but if it is 30% by weight or more, The strength of the silicon nitride body as a whole will decrease, and in some cases, the low expansion property of the silicon nitride body will be lost.On the other hand, if it is less than 0.5% by weight, the silicon nitride body may not be sintered sufficiently. By becoming a thing.

The reason why the amount of metallic aluminum is limited to 0.1 to 15% by weight, preferably 0.5 to 10% by weight is that if 15% by weight or more is added, unreacted metallic aluminum will remain at the grain boundaries, causing the formation of silicon nitride bodies. The strength of 0 will decrease, and 0
．． If it is less than 1% by weight, it is not sufficient to eliminate the silicon oxide film on the surface of the silicon nitride grains, and the high temperature strength of the silicon nitride body as a whole cannot be sufficiently improved, and the present invention claims to improve the sinterability. due to failure to fully achieve one of the objectives of

The particle size of the metallic aluminum that can be used in the present invention is not particularly limited, and a normal particle size is sufficient.

Naturally, if the particle size is too large, the reaction with the silicon oxide film will be insufficient, and unreacted metal aluminum will remain.If the particle size is too small, it will be difficult to obtain the raw material metal aluminum ( From an economic point of view, the optimum thickness is about 1 to 500 μm, preferably about 5 to 200 μm.

As explained above, the present invention is characterized by the addition of metallic aluminum, and additives as sintering aids may be used in addition to yttrium oxide, cerium oxide, and magnesium oxide, which are limited to the scope of the present invention. too,
For example, commonly known sintering aids such as beryllium oxide, titanium oxide, and zirconium oxide have certain effects.

(However, in the case of these additives, the absolute value effect as an additive is smaller than that of yttrium oxide, etc. within the scope of the present invention, so although a corresponding effect can be recognized relatively by adding metal aluminum, For example, the absolute value level of high temperature strength etc. does not become large.

) Next, in the present invention, yttrium oxide, cerium oxide, and magnesium oxide do not simply mean those in the oxide state, but include all of the above-mentioned oxides that can be converted into the above oxides by heat treatment. Specifically, oxides of yttrium, cerium, and magnesium, as well as halides, carbonates, sulfates, nitrates, organic acids, and the like of these metal elements can be effectively used.

In addition, if impurities (such as alumina) contained in the additives within the scope of the present invention such as silicon nitride particles, metal aluminum, and yttrium oxide or mixed in during the grinding and blending process of raw materials are 5% by weight or less, the present invention also applies. does not impair the effect of

Examples Various raw materials were prepared as shown in Table 1, and samples were prepared by molding and usually firing or hot pressing.

The manufacturing conditions for each are as follows.

Normal firing: Raw materials prepared to a predetermined composition were mixed and pulverized for 5 days using a silicon nitride ball in a silicon nitride pot mill.

This mixture was then heated at a pressure of 200 kg/cm2 for 35 min.
It was pressed into a molded body of 12 x 13 mm, placed in a silicon nitride box material, and fired in a nitrogen stream at a temperature in the range of 1700 to 1800°C for 1 hour.

Hot pressing: The process is the same as the above normal firing until the mixture is obtained, and this mixture is heated in a graphite mold at 1750°C and weighed 350 kg.
/cm2 for 1 hour (in a nitrogen atmosphere) to obtain a sample of 60φ x 12mm.

The properties of these samples are also listed in Table 1.

As can be seen from Table 1, the effect of adding metallic aluminum within the range of the present invention is large (comparison between Examples 1 to 7 and Examples 9 to 16), and when yttrium oxide is outside the range of the present invention, No effect of adding metal aluminum was observed (see Examples 8 and 17).

Further, in Example 18, metallic titanium was added instead of metallic aluminum, and although a considerable effect can be seen, the effect is not even close to that of metallic aluminum.

A comparison of Examples 2 and 9 reveals that although they have substantially the same properties, there is a 100° C. difference in firing temperature between the two.

Claims (1)

[Claims] 1 A mixture of 0.5 to 30% by weight of one or more of yttrium oxide, cerium oxide, and magnesium oxide, 0.1 to 15% by weight of metal aluminum, and the remainder silicon nitride particles is molded to form a non-oxidizing mixture. firing in an atmosphere;
Alternatively, a method for producing high-strength ceramics, which comprises hot pressing the blend in a non-oxidizing atmosphere.