JPS6389462A - Manufacture of silicon nitride base sintered body - Google Patents

Abstract

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

Description

Detailed Description of the Invention (Field of Industrial Application) This invention can be used as a material for various structural parts used in a wide range of fields such as automobiles, mechanical equipment, chemical equipment, aerospace equipment, etc., and has particularly excellent high-temperature strength. The present invention relates to a method for producing a silicon nitride sintered body suitable for obtaining a fine ceramic material having the following properties.

(Prior art) Sintered bodies whose main component is silicon nitride are scientifically stable at room and high temperatures and have high mechanical strength, so they are suitable for sliding parts such as bearings, engine parts such as turbocharger rotors, etc. It is a suitable material as

However, since it is difficult to sinter silicon nitride alone, a method is usually used in which sintering is performed by adding a sintering aid such as MgO, 8lz(h-Yz(h), etc.). Furthermore, a method is known in which a mixture of Si powder to which YgO:+ is added is nitrided and then fired for the purpose of improving the melting point of the glass phase remaining in the sintered body and reducing the amount of the glass phase.

(Problems to be Solved by the Invention) However, in the conventional sintering method described above, sintering is thought to be based on liquid phase sintering mediated by the liquid phase generated during sintering. The phase remains in the sintered body as a glass phase. On the other hand, the properties of the sintered body, such as creep resistance, high temperature strength, and oxidation resistance, are greatly affected by the glass phase remaining in the sintered body. In particular, there is a problem in that the presence of a large amount of glass phase with a low softening temperature is undesirable because it significantly deteriorates the high-temperature mechanical properties of the silicon nitride sintered body.

In addition, the above-mentioned method of firing after nitriding not only requires time for nitriding because the nitriding reaction rate is slow, but also has the problem that it cannot be applied to large-sized parts because the nitriding efficiency decreases in the case of thick-walled parts. Ta.

(Means for Solving the Problems) The present invention was made by focusing on the above-mentioned conventional problems. This invention was achieved by developing a method for manufacturing the body with high productivity.

The method for producing a silicon nitride sintered body of the present invention includes silicon powder,
Periodic Table Group Ua, Group IIIa, 'lrs Aj! A raw material mixed powder is obtained by adding and mixing an oxide of at least one element selected from the group consisting of: Treated at a temperature in the range of 1500°C and then heated to 1600°C under a nitrogen gas atmosphere of 1 atm or more.
It is characterized by processing at a temperature in the range of ~2200°C.

In one embodiment of the present invention, the molded body is nitrided at a temperature in the range of 1000 to 1500°C in a nitrogen atmosphere of 10 atm or more, and then heated at 1600 to 2200°C in a nitrogen gas atmosphere of 1 atm or more but less than 500 atm. treated at temperatures ranging from 1,600 to 2
The sintering process can be carried out at temperatures in the range of 200°C.

The starting material used in this invention is Si powder to which an oxide and/or oxide precursor of at least one element selected from the group consisting of Group IIa, Group IVa, Zr, and AI of the periodic table is added. It is a mixed powder. These powders are preferably sufficiently ground and mixed to have a particle size of several μm. Further, as the oxide, it is also possible to use an oxide that changes into an oxide when heated, such as carbonate. In the present invention, oxide precursors refer to substances that are converted into oxides by heating.

Group Ua, Group IIIa, Zr of the periodic table mentioned above,
The amount of the oxide and/or oxide precursor of at least one element selected from the group consisting of Al added to the Si powder is such that after sintering, all of the oxide precursor becomes an oxide and all of the Si powder is nitrided. In the case of silicon, the weight of the oxide is approximately equal to the total weight of the oxide of one or more elements selected from the group consisting of Group IIa, Group IIIa, Zr, and Al of the periodic table and silicon nitride. Approximately 5 to 3
It is determined to be within the range of 0% by weight. If it is less than about 5% by weight, it will not function as an auxiliary agent, and if it is more than about 30% by weight, the proportion of silicon nitride powder will decrease and the properties of silicon nitride will deteriorate.

The above mixed powder is formed into a molded body using a suitable molding means. The molding means is not particularly limited, but can be selected from conventional ceramic molding methods such as die press molding, rubber press molding, and injection molding, depending on the shape of the intended product.

In this invention, the nitriding step is carried out in a nitrogen gas atmosphere of 10 atm or more at a temperature in the range of 1000 to 1500°C, preferably 1
It is carried out at a temperature in the range of 200-1450°C. The nitriding reaction is promoted by performing the nitriding reaction under high nitrogen gas pressure. In addition, as the nitriding reaction progresses, the number of open pores decreases as the density increases, so gas supply to the interior is inhibited in nitrogen at normal pressure, making it difficult to assemble large-sized parts, but in nitrogen at high pressure, By nitriding, the amount of gas supplied to the inside increases, so the nitriding reaction can proceed sufficiently to the inside. When the temperature of the nitriding reaction is below 1000°C, the reaction is slow, and when it is above 1500°C, Si melts and the shape cannot be maintained. Therefore, 1000-1500℃
It is desirable to gradually increase the temperature within the range of . Further, the time for the nitriding treatment depends on the size of the particles used and the amount used, and cannot be given in general terms, but in short, it is sufficient to carry out the nitriding treatment until the nitriding is completed.

Next, the sintering process is performed under a nitrogen gas atmosphere of 1 atm or more.
It is carried out at a temperature in the range of 00 to 2200°C. If the pressure is below 1 atm, 5iJa decomposes violently and a dense sintered body cannot be obtained.

The gas pressure required to suppress decomposition is determined by the firing temperature; higher temperatures require higher gas pressures. Furthermore, if the firing temperature is 1600° C. or lower, a sufficient amount of liquid phase will not be generated, so that densification will not occur. At temperatures above 2200°C, grain growth becomes intense and the strength of the sintered body decreases. This sintering process is continued until a dense sintered body is obtained.

When the amount of the above-mentioned additive auxiliary agent is small, or when the sinterability is poor, such as when a high melting point auxiliary agent is used, the firing process is preferably performed by the following two steps.

First, the nitrided molded body is treated in a nitrogen atmosphere of 1 atm or more and less than 500 atm at a temperature in the range of 1600 to 2200°C. The treatment time is preferably 10 minutes or more.

In this step, the compact is heated to a temperature in the range of 1,600 to 2,200° C., whereby the oxide-based auxiliary agent forms a liquid phase, and densification progresses by the mechanism of liquid phase sintering.

At this time, the atmosphere is set to 1 atm or more and less than 500 atm in a nitrogen atmosphere because if it is less than 1 atm, silicon nitride will decompose and will not become densified, and if it is over 500 atm, high pressure nitrogen gas will be trapped in the sintered body. This is because the density is only increased to about 90% of the theoretical density. In this process, the theoretical density is 90
% or more (the remainder has closed pores). Next, the thus treated sintered body is further treated at a temperature in the range of 1,600 to 2,200° C. in a nitrogen atmosphere of 500 atmospheres or more. In this step, the remaining closed pores disappear by a mechanism similar to that of normal HIP processing, and a dense sintered body is obtained.

In these two steps, the processing temperature is 16oO to 220o.
The reason for setting the temperature in the range of 0°C is that below 1600°C, the sinterability is poor and it is not sufficiently densified, resulting in a decrease in high temperature strength.
This is because at 2200° C. or higher, grain growth of silicon nitride becomes large and the strength at room temperature and high temperature decreases. These sintering steps are preferably carried out in one process by controlling the temperature and pressure, but may be carried out in two steps. Then, through the sintering process, the molded body is made into 5i3
Nt+ Si:+N4+510z+ BIJ+//lN
It may be covered with powder such as Further, although the atmospheric gas is preferably 100% nitrogen gas, other inert gases such as Ar may be added.

(Effects of the Invention) As described above, according to the present invention, a dense and high-strength silicon nitride sintered body can be obtained.

(Example 1) A mixed powder was obtained by densely mixing St powder with a concentration of 90% in terms of SiffN and yttrium oxide with a concentration of 10% in terms of SiffN. 200 kgf/am of this mixed powder
Rubber press molded with a pressure of φ50 x 50v
A molded body having the shape of s was formed. This compact was heated to 2,000 ml under the conditions of the nitriding-firing treatment schedule shown in Fig. 1.
From about 1200℃ to about 1500℃ under nitrogen atmosphere at atmospheric pressure
Nitriding was carried out by heating to a temperature of
It was heated at a temperature of 0° C. for about 30 minutes, and then the atmospheric pressure was gradually increased to 2000 atm for about 1.5 hours to perform baking. A sintered body having a density of 3.30 g/c+n' was obtained.

When the obtained sintered body was cut and observed, no unreacted portion was present inside it. Further, a test piece having a shape of 3×4×4Qw was cut out from the sintered body and subjected to a three-point bending test at 1200° C. with a span of 301 m.

The results are shown in Table 1. In this case, the average value measured for five test pieces is shown.

From these tests, it was possible to obtain a dense and high-strength silicon nitride sintered body according to the present invention.

(Comparative Example 1) As shown in Table 2, a molded article was made from the same composition as in Example 1 and subjected to the same molding treatment as tested in Example 1. This molded body was processed into 1
heating at a heating rate of 50'C/hour for about 6 hours under a nitrogen atmosphere at atmospheric pressure, then heating to a temperature of 1900C for about 30 minutes, The pressure of the nitrogen atmosphere was increased to 2 for about 2 hours.
The mixture was heated at a pressure of 1,000 atm and then fired at 1,900°C for about 1°30 hours in this nitrogen atmosphere.

When the thus obtained sintered body was tested in the same manner as described in Example 1, it was densified only to a density of 2.70 g/cm3, indicating that there was an unreacted portion of Si inside the sintered body. Ta.

(Examples 2 to 4) The composition of St powder and oxide powder is shown in Table 1 (Si powder is S
i3N4 equivalent values are shown) and mixed at a ratio of 200
Mold molding is performed at a pressure of 2000 kgf/cm”.
Rubber press at a pressure of 25 kg f/am
A plate-shaped molded body with dimensions of X 6 X 50 mm was molded. Each of these molded bodies was heat treated under a nitrogen atmosphere under the conditions of Schedule I shown in FIG. 1 in the same manner as described in Example 1.

Next, the surface of each obtained sintered body was ground and 3×4×4
It was processed into a test piece with a shape of Qmm. Each of these test pieces was subjected to a three-point bending test at 1200° C. with a density and a span of 30 mm. These results are shown in Table 2 as the average value measured for each of the five test pieces. As shown in Table 2, according to the present invention, a dense and high-strength silicon nitride sintered body could be obtained.

(Comparative Examples 2 to 4) Si3N powder and oxide powder were mixed in the composition and ratio shown in Table 2, and each molded body was molded in the same manner as described in Example 2. Each of these compacts was heated under the conditions of treatment schedule ① shown in Figure 3 in a nitrogen atmosphere of 1 atm.
The temperature was increased to 1900° C. over a period of time, heated at this temperature for about 30 minutes, then the nitrogen atmosphere pressure was increased to 2000 atm over about 1.30 hours, and further heat treated under this pressure for about 1 hour.

Next, test pieces were made from each of the obtained sintered bodies in the same manner as described in Example 2, and a bending test was performed. Table 2 shows the average values of density and strength measured for each of the five test pieces. As shown in Table 2, it can be seen that when the amount of oxide powder added is small, the density becomes low, and when the amount added is large, the density becomes high but the high temperature strength becomes low. Thus, in these comparative examples, a sintered body with high density and high strength at high temperature could not be obtained.

[Brief explanation of the drawing]

FIG. 1 is a curve diagram showing the schedule (■) for nitriding and firing the compacts in Example 1 and Examples 2 to 4, FIG. 2 is a curve diagram showing the schedule (■) for treating the compacts in Comparative Example 1, and FIG. 3 is a curve diagram showing schedule (2) for processing molded bodies in Comparative Examples 2 to 4. Patent Applicant: Nissan Motor Co., Ltd. Representative Patent Attorney Akatsuki Sugimura Examiner: Patent Attorney Oki Sugimura Time (C) for Figure 1 and Figure 2)

Claims (1)

[Claims] 1. Silicon powder contains Group IIa and Group IIIa of the periodic table,
An oxide of at least one element selected from the group consisting of Zr and Al and/or an oxide precursor is added and mixed to obtain a raw material mixed powder, and a molded body made of this raw material powder is
A silicon nitride sintered material characterized by being treated at a temperature in the range of 1000 to 1500°C under a nitrogen atmosphere of 0 atm or more, and then at a temperature in the range of 1600 to 2200°C in a nitrogen gas atmosphere of 1 atm or more. Method for producing solids.