JPS6054271B2 - Method for manufacturing silicon nitride-based sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a silicon nitride-based sintered body having excellent strength even under high temperatures.

Silicon nitride-based sintered bodies are being developed as structural material parts that require high stress resistance at high temperatures.

However, in this type of sintered body, for example, magnesium oxide is added in consideration of sinterability, but generally a decrease in mechanical strength is observed at high temperatures, for example, about 1000°C. As a solution to these problems, a manufacturing method has been tried in which a rare earth oxide molded product such as silicon nitride-yttrium oxide is heat-treated or sintered in the coexistence of aluminum nitride. According to this method, a silicon nitride-based sintered body exhibiting high strength at high temperatures can be obtained, but there is a disadvantage that the heat treatment and sintering treatment require a relatively long time. The present inventors have conducted studies to improve the above-mentioned disadvantages, and as a result, when nitrides such as titanium, zirconium, hafnium, vanadium, niobium, and chromium are co-existing, a relatively short heat treatment and sintering treatment are required. It has been found that a high-temperature, high-strength sintered body can be obtained. Based on this knowledge, the present invention aims to provide a manufacturing method suitable for mass production of silicon nitride-based sintered bodies with high strength under high temperatures.

To explain the present invention in detail below, the present invention heat-treats an unfired body containing rare earth element oxides and silicon nitride as essential components in the coexistence of titanium, zirconium, hafnium, vanadium, niobium, or chromium nitride. a silicon nitride-based sintered body comprising: a step of generating an oxynitride consisting of a rare earth element, silicon, oxygen and nitrogen; and a step of sintering the oxynitride obtained in the above step. For example, the manufacturing method is as follows.

First, a mixed powder of a rare earth element oxide and silicon nitride is prepared as a raw material, and a powder compact of a predetermined shape is made. Thereafter, this powder compact is heated to a temperature that produces an oxynitride in the coexistence of a nitride such as titanium, zirconium, hafnium, vanadium, niobium, tantalum, or chromium, for example, 165°C.
Heat treatment is performed at a temperature of about 0 to 1850°C. In the powder compact during this heat treatment process, the oxide is converted into an oxynitride (crystallized) by an oxynitriding reaction between the rare earth element, silicon, nitrogen, and oxygen. After converting the oxide component in the powder compact into an oxynitride, the
By heating the material to a temperature of approximately .degree. C. and subjecting it to normal pressureless sintering, hot-pressing, and other sintering treatments, a silicon nitride-based sintered body exhibiting excellent mechanical strength even at high temperatures can be obtained. In the present invention, the coexistence of nitrides such as titanium, zirconium, hafnium, vanadium, niobium, tantalum, and chromium refers to silicon oxide components generated by decomposition and evaporation from silicon nitride in the unfired body during heat treatment. This means that the nitride is provided in an adjacent position where it can react with the nitride. The specific methods of heat treatment in the coexistence of nitrides include A), a method of embedding (covering) nitride powder in a container such as a crucible, and heating it; and B), a method of heating a nitride sintered body. One example is a method in which the unsintered body is placed in a manufactured container such as a crucible and heated. Further, in the present invention, silicon nitride, which is a component of the starting material, may be either α-type silicon nitride or β-type silicon nitride, and the powder particle size is preferably 5 μm or less.

On the other hand, other rare earth elements that make up the composition include yttrium, lanthanum, cerium, and plastin. These include theodium, europium, neodymium, desprosium, promethium, terbium, samarium, gadolinium, holmium, erbium, thulium, ytterbium, ruthenium, scandium, etc. One or more of these!
It may be used in a mixed system. However, the composition ratio of the rare earth element oxide may be selected to be generally 15% by weight or less, preferably about 5% by weight or less in the rare earth element oxide silicon mononitride. In the present invention, the starting material may be only a two-component system of the tetraoxide of the rare earth element and silicon nitride, but a part of the silicon nitride component (up to 7% by weight of the silicon nitride component) may be replaced with silicon carbide, for example. , carbides such as aluminum carbide, and oxides such as aluminum oxide and silicon oxide may be substituted as appropriate.

According to the method of the present invention as described above, an oxynitride containing no amorphous phase can be formed by a relatively short treatment time, and a temperature of 100'C
A silicon nitride-based sintered body that maintains high mechanical strength even at higher temperatures can be easily obtained. For example, it took two hours to heat-treat a powder compact obtained from a yttrium oxide-silicon nitride mixture at 1750°C in the coexistence of aluminum nitride to generate the desired oxynitride, whereas the method of the present invention In the case of 0.5 to 1. Even just a few hours was enough. Moreover, the sintered body obtained by the sintering treatment after oxynitriding maintains and exhibits a strength of about 85% of the value at room temperature even at a high temperature of, for example, 1200'C. In this respect, for example, a sintered body made by sintering yttrium oxide-silicon nitride without taking the step of coexisting other nitrides is
Maintains a value of approximately 90% at 0C and approximately 36% at 1200℃ (all values relative to room temperature),
It is a remarkable difference compared to the fact that it is only a demonstration. By the way, according to the method of the present invention, the reason why high mechanical strength is maintained and exhibited even at high temperatures (no significant decrease in mechanical strength occurs even at high temperatures) is considered to be as follows.

That is, silicon nitride powder as a raw material usually contains impurities such as silicon oxide, aluminum oxide, and iron oxide, and a metal oxide is appropriately added as a sintering aid. However, these impurities and sintering aids form an amorphous phase during the sintering process. In other words, the obtained sintered body is composed of a crystalline phase of silicon nitride and an amorphous phase mainly composed of oxides (additives), and the amorphous phase softens at high temperatures. This softening causes deterioration of high temperature strength. However, according to the present invention, heat treatment is first performed in the coexistence of titanium nitride, etc., and most of the oxide components in the powder compact are converted into oxynitrides during this heat treatment process. The sintered body finally obtained in this way is composed of silicon nitride monooxynitride (crystalline) containing almost no amorphous phase, so it is hardly affected by the amorphous phase even at high temperatures. It is thought that it maintains excellent mechanical strength. Next, examples of the present invention will be described.

Example 1 97 parts by weight of silicon nitride powder, 90% of which is α-type silicon nitride, and 3 parts by weight of yttrium oxide were mixed in an alumina pot and an alumina ball in the coexistence of n-butanol for 10 minutes.
A mixed powder having an average particle size of 0.5 μm was prepared by time-pulverization and mixing.

Using the mixed powder prepared above as raw material, 500k9/Clt
The powder compacts were molded at a pressure of 30 x 30 x 10 in the order of 30 x 30 x 10 as green bodies. This powder molded body was placed in a graphite crucible, the periphery of the molded body was coated with titanium nitride powder, and heat treated by holding the powder at 1750° C. for three times in a nitrogen gas atmosphere.
After that, this heat-treated molded body was heated at 1780°C and 400k.
g/C7l! A sintered body was obtained by hot pressing under the following conditions. This sintered body was dense, and when a part of it was scraped off, crushed, and subjected to X-ray diffraction, it was found that β-Si3N4 and -S
It exhibited a crystalline phase with i3N4.Y2O3. Furthermore, the bending strength of the above sintered body was measured using the three-point bending method (span 2i1 load application acceleration 0.5?/Min).
It was as shown in the figure. Example 2 50 parts by weight or 3 parts by weight of silicon nitride powder, 90% of which is α-type silicon nitride, 50 parts by weight or 7 parts by weight of aluminum oxide powder, and 5 parts by weight of yttrium oxide were used as starting materials and pulverized. A mixed powder was prepared, and a powder compact as an unsintered body was produced in the same manner as in Example 1.

This powder compact was heat-treated under the same conditions as in Example 1, except that the temperature was 1700°C and the time was 2 hours.
Next, hot pressing was performed at 1750° C. and 300 k9/Clt to obtain a sintered body. The mechanical strength of the thus obtained sintered body at 1200°C is 90k9/i or 85k9/i, and X-ray diffraction of the sintered body shows that the grain boundary phase is completely crystallized and other constituent phases are not present. β''-S formed by solid solution of aluminum oxide in β-Si3N4 as
i3N4 was also recognized. Furthermore, according to X-ray diffraction of the above sintered body, aluminum oxide is solidly dissolved in both oxynitride and silicon nitride, and even if the sintering temperature is selected to be relatively low, a dense sintered body cannot be obtained. Ta.

Example 3 Silicon nitride (Si3N4) powder, yttrium oxide (Y
2O3), samarium oxide (Srn2O3), dysprosium oxide (Dy2O3), lanthanum oxide (La2O3)
) and aluminum oxide (Al.O3) were selected at the composition ratios (parts by weight) shown in the table, and three pulverized mixtures were prepared in the same manner as in Example 1, including a comparative example.

These adjusted powders are press-molded to produce powder compacts as unsintered bodies, and these compacts are buried (covered) in titanium nitride powder or the like and housed in a graphite crucible (nitride powder). Coexistence means A) The above-mentioned molded bodies were housed in a crucible made of a titanium nitride sintered body or the like (nitride coexistence means B) and were heat-treated in a nitriding gas atmosphere. Thereafter, the molded bodies in which the oxide components in the molded bodies had been converted into oxynitrides by the heat treatment were sintered by hot-pressing or pressureless sintering. About the sintered body thus obtained
Each constituent phase is detected by line diffraction and the intensity at room temperature and 1200'C (K9/Wrlt)
The results obtained are combined with the above heat treatment conditions and sintering treatment conditions. The results are shown in the table below.

In the table, the constituent phase (a) of the sintered body is β-Si3N4+S.
l3N4・Y2O3 phase, (b) is β-Si3N4+S
i3N4・Sm2O3 phase, (c) is β-Sl3N4+
Si3N4・Dy2O3 phase, (d) is β″-Sl3N
4+Si3N4・Y2O3 phase, (e) is β)-Sl3
N4+Sl3N4・La2O3 phase, (f) is β-Si
3N4+Sl3N4・Y2O3+amorphous phase, also (g
) indicate β-Sl3N4+ amorphous phase, respectively.

As is clear from the above examples, according to the present invention, even if the sintering time in the sintering process is shortened compared to Comparative Example 1, a sintered body with high strength can be obtained at high temperatures.

The figure also shows temperature and strength Kf for the sintered body of Comparative Example 3.
/Td is also shown. As described above, the silicon nitride-based sintered body obtained by the present invention maintains excellent strength even at high temperatures. Therefore, the present invention can be said to be a method suitable for coating members that require high stress resistance under high temperatures, such as gas turbine materials.

[Brief explanation of the drawing]

The figure is a curve diagram showing the relationship between temperature and strength for silicon nitride-based sintered bodies produced by the method of the present invention and a conventionally known method.

Claims (1)

[Claims] 1 Unfired bodies containing rare earth element oxides and silicon nitride as essential components are processed into titanium, zirconium, hafnium,
A step of heat-treating in the coexistence of vanadium, niobium, tantalum, or chromium nitride to produce an oxynitride consisting of rare earth elements, silicon, oxygen, and nitrogen, and a sintering treatment of the oxynitride obtained in the above step. A method for producing a silicon nitride-based sintered body, comprising the steps of: