JPH04154668A - Production of silicon nitride sintered compact - Google Patents

Abstract

PURPOSE:To obtain the title sintered compact improved in high-temperature strength by calcining a specific form or its calcinated form in a non-oxidative atmosphere. CONSTITUTION:A mixture of (A) Si3N4 powder 0.1-1.0mum in mean particle diameter and >=1wt.% in impurity oxygen content and (B) an oxide (e.g. Y2O3) of group IIIa elements with 0.1-3mum in mean particle diameter is formed and then calcinated at 1200-1600 deg.C in a non-oxidative atmosphere containing, if needed, N2 into a calcinated form having <=1.5wt.% in the level of the impurity oxygen present on the surface of the Si3N4 powder and 0.1-3wt.% in the whole impurity oxygen content. Then, this form or calcinated form is calcined at 1600-2000 deg.C for 0.5-10hr in a non-oxidative atmosphere.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to a method for manufacturing a silicon nitride sintered body, and more specifically to a method for manufacturing a silicon nitride sintered body with improved high-temperature properties as a heat engine structural material. Concerning a method of manufacturing a body.

(Prior art) Silicon nitride sintered bodies have been used as engineering ceramics as materials with excellent strength, hardness, and thermal and chemical stability, and in particular, are being applied to heat engines such as turbo rotors and gas turbines. .

As a method for obtaining such a silicon nitride sintered body, in addition to Y2O and rare earth element oxides as sintering aids, Ah
By adding Ox, MgO, SrO, etc., and firing this in a non-oxidizing atmosphere at 1500 to 2000°C using methods such as normal pressure firing, hot press firing, nitrogen gas pressure firing, and hot isostatic pressure firing. It has been obtained.

On the other hand, although silicon nitride sintered bodies have excellent properties,
Its properties tend to deteriorate significantly at high temperatures exceeding 200°C. This is because the sintered body obtained by the method described above is composed of silicon nitride crystal grains and a grain boundary phase, and a low melting point glass phase consisting of sintering aids and impurities is generated in this grain boundary phase. It is believed that.

Therefore, the presence of impurity oxygen in silicon nitride raw material powder has been cited as one of the factors that promotes the formation of low-melting point glass and deteriorates the high-temperature strength of sintered bodies, and attempts have been made to remove this impurity oxygen. Attempts are being made to do so. Specifically, it has been proposed that during sintering, the compact is buried in silicon nitride, boron nitride, or aluminum nitride powder and fired, the impurity oxygen is extracted from the compact, and the oxygen is fixed by the filling powder. There is. It has also been proposed to define the amount of impurity oxygen, which is determined by subtracting the amount of oxygen mixed as a sintering aid, from the total amount of oxygen in the sintered body to a specific range.

(Problems to be Solved by the Invention) However, in the above-mentioned conventional technology, the method of filling the sintered body during firing cannot remove the impurity oxygen to the extent that the high-temperature strength can actually be improved; There is a problem in that the surface of the sintered body must be polished because the surface becomes rough, and the method of controlling the total amount of impurity oxygen in the compact or sintered body does not necessarily have the characteristics of the sintered body as described above. The characteristics were unstable because they were not determined by the amount of impurity oxygen calculated by the method.

(Means for Solving the Problems) Therefore, the present inventors conducted a detailed study on the causal relationship between the amount of impurity oxygen and high-temperature characteristics. In general, silicon nitride raw material powder produced by imide decomposition method, silica reduction method, gas phase method, etc. contains at least 1% by weight of impurity oxygen regardless of the production method. internal oxygen present inside the particles;
Oxygen can be broadly divided into surface oxygen present on the particle surface, and impurity oxygen usually contains about 0.1 to 3% of impurity inside the powder. According to the knowledge of the present inventors, in such a raw material powder, it is thought that impurity oxygen present in the surface portion greatly contributes to the high temperature characteristics of the sintered body, and this surface oxygen is reduced by 1. It was discovered that by setting the content to 5% by weight or less, a silicon nitride sintered body having excellent properties could be stably produced, and the present invention was achieved.

That is, the present invention provides silicon nitride powder and silicon nitride powder
1 molded body or calcined body containing group element oxide powder
When firing in a non-oxidizing atmosphere at 600 to 2000°C, the amount of oxygen present on the surface of the silicon nitride powder in the molded body or calcined body is 1.5% by weight or less, and the total amount of impurity oxygen is 0.5% by weight. Below, the present invention will be described in detail.

According to the manufacturing method of the present invention, first, silicon nitride powder is used as a raw material powder, and at least an oxide powder of a group IIIa element of the periodic table is used as a sintering aid.

Silicon nitride powder is α type with an average particle size of 0.1 to 1.0 μm,
Either β type can be used. Further, this silicon nitride powder inevitably contains impurity oxygen in a proportion of 1% by weight or more.

On the other hand, Y2
O1, ErzOz, YbzOz, Dyz03, HO
z03 and the like, and those having an average particle size of 0.1 to 3 μm are preferably used.

Next, the raw material powders are mixed in a predetermined ratio and then molded into a desired shape by known molding means such as press molding, extrusion molding, injection molding, cast molding, cold isostatic pressing, etc.

According to the present invention, the amount of surface impurity oxygen in the silicon nitride powder constituting the molded body or its calcined body is 1.5% by weight.
It is important to set the following.

That is, when the amount of oxygen on the surface exceeds 1.5% by weight, this oxygen remains at the grain boundaries in the final sintered body and forms a low melting point glass phase, resulting in a decrease in high temperature strength. The amount of oxygen on this surface can be controlled by various methods described below, but if this amount of oxygen on the surface is completely eliminated, crystals such as melilite will easily precipitate in the final sintered body.
Although this crystal has excellent high-temperature strength, it has poor oxidation resistance. Therefore, the amount of surface oxygen is desirably set to 0.1 to 1.2% by weight. On the other hand, the total amount of impurity oxygen including surface oxygen affects the high temperature strength, and the amount is 0.1 to 3% by weight.
It is necessary that the total impurity oxygen content is 3% by weight.
This is because the high-temperature strength decreases when the temperature exceeds . Note that it is almost difficult to reduce the total amount of impurity oxygen to less than 0.1% by weight. It is preferably 0.1 to 2% by weight.

As a method for producing such a molded body, it is preferable to use a silicon nitride raw material powder whose amount of each impurity oxygen satisfies the above range as a starting material, but low-purity silicon nitride powder containing a large amount of impurity oxygen However, it can be reduced by, for example, heat-treating the raw material powder with carbon to remove the impurity oxygen content on the powder surface, or heat-treating the raw material powder in a nitrogen atmosphere at 1200 to 1600°C to replace oxygen with nitrogen. You can also do it.

However, even if the amount of impurity oxygen is controlled at the raw material powder stage,
For example, oxygen is adsorbed on the surface of the raw material due to the mixing process with other auxiliary agents or the like or due to moisture in the air. Therefore, it is preferable to heat the molded product at a temperature of 1200 to 1600°C before firing it.
It is preferable to perform a heat treatment in a nitrogen atmosphere to replace impurity oxygen with nitrogen, and adjust the amount of surface oxygen to the above-mentioned range.

The molded body or calcined body in which the amount of impurity oxygen is controlled in this manner is fired for 0.5 to 10 hours in a non-oxidizing atmosphere containing nitrogen at 1700 to 2000°C. As the firing means, normal pressure firing, hot press firing, nitrogen gas pressure firing, hot isostatic pressure firing (HIP method), etc. are employed.

The sintered body obtained in step 2 is composed of silicon nitride crystal grains and a grain boundary phase. At this time, if the grain boundary phase is made of glass or contains glass, Crystallization of grain boundaries can be achieved by heat treating the compact at a temperature of 1300 to 1900° C. in a non-oxidizing atmosphere.

Further, according to the present invention, in addition to the oxide of the Group IIIa element of the periodic table as a sintering aid, AhO+, MgO1S
rO, WC, WOz, etc. can also be used, but from the viewpoint of high temperature strength of the sintered body, the amount of Ah03 and MgO may be 0.00% because they may form a glass phase with a low melting point at the grain boundaries of the sintered body.
It is desirable to suppress it to 2% by weight or less.

In addition, even if other elements such as 10 and WCh are added in a proportion of 2% by weight or less, the properties are not significantly affected.

The invention will now be explained with the following examples.

(Example) As raw material powder, several types of silicon nitride raw material powders with an average particle size of 0.5 μm and different amounts of impurity oxygen and an average particle size of 1.5 μm were used.
Using desired oxides of Group IIIa elements of the periodic table having a diameter of 0 μm, these were mixed in the proportions shown in Table 1. Next, this mixed powder was press-molded to create a molded body.

This molded body was heat-treated in a nitrogen atmosphere at 1600° C. to replace surface oxygen with nitrogen, and by controlling the treatment time, several types of molded bodies with different amounts of surface oxygen were created. At this time, the amount of surface oxygen in each molded article was measured using a form-based oxygen analyzer.

Next, this molded body was placed in an atmosphere of nitrogen gas pressure of 9 atm for 2 hours.
It was baked at 000°C for 2 hours.

The bending strength of the obtained sintered body at room temperature and 1400° C. was measured based on JISR1601.

The results are shown in Table 1.

(Less than one margin) Table 1 Note: * indicates samples outside the scope of the present invention.

According to Table 1, samples Nα1 and 10 in which the amount of surface oxygen in the silicon nitride in the compact exceeds 1.5% by weight have insufficient high temperature strength, and the samples in which the amount of total impurity oxygen exceeds 3% by weight Similarly, the strength of Nα18 and Nα19 also significantly deteriorated.

On the other hand, all the samples of the present invention showed high strength, with a room temperature strength of 950 MPa or more and a 1400°C strength of 600 Mpa.
Pa or higher was achieved.

(Effects of the Invention) As detailed above, according to the manufacturing method of the present invention, by controlling the amount of oxygen on the surface of the silicon nitride powder in the compact or calcined body before firing, the sintered product has excellent high-temperature strength. The body can be stably produced.

Therefore, it is possible to expand its use as various high-temperature structural materials including materials for heat engines such as gas turbines and turbo rotors.

Claims (1)

[Claims] A molded body or a calcined body containing silicon nitride powder and an oxide powder of a Group IIIa element of the periodic table is heated to 1600 to 20
In the method for manufacturing a silicon nitride sintered body which is fired in a non-oxidizing atmosphere at 00°C, the amount of impurity oxygen present on the particle surface of the silicon nitride powder in the compact or calcined body is 1.
A method for producing a silicon nitride sintered body, characterized in that the amount of impurity oxygen is 5% by weight or less and the total amount of impurity oxygen is 0.1 to 3% by weight.