JP2946593B2 - Silicon nitride sintered body and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the invention]

(Industrial application field) The present invention can be used as a material for various structural components used in a wide range of fields such as automobiles, mechanical devices, chemical devices, and aerospace devices. Hot press suitable for obtaining (1
Axial pressure sintering) The present invention relates to a silicon nitride sintered body and a method for producing the same. (Prior Art) A sintered body containing silicon nitride as a main component is chemically stable at normal and high temperatures and has high mechanical strength. Therefore, sliding members such as bearings and engine members such as turbocharger rotors are used. Is a suitable material. However, since silicon nitride is difficult to sinter alone, MgO, Al ₂ O ₃ , Y ₂ O ₃
A method of sintering by adding a sintering aid such as this is used (as a method for producing this kind of silicon nitride sintered body,
JP-A-49-63710, JP-A-54-15916, JP-A-60-13
There are many things disclosed in, for example, 7873. (Problems to be Solved by the Invention) However, MgO, Al ₂ O ₃ ,
A conventional silicon nitride-based sintered body obtained by adding and sintering a large amount of an oxide such as Y ₂ O ₃ contains a low-melting glass phase at a grain boundary in the sintered body. Therefore, there is a problem that high-temperature characteristics such as creep resistance, high-temperature strength, and oxidation resistance of the sintered body are deteriorated. On the other hand, the present inventor has found that the high-temperature strength is improved by reducing the oxygen content in the sintered body, and has been disclosed in Japanese Patent Application Nos. 63-199709 and 1-55923. No. is proposed. However, in the above sintered body, there is a composition that is difficult to sinter depending on the ratio of the oxide auxiliary and SiO ₂ , and there is a problem that it is difficult to achieve the densification by the gas pressure sintering method. (Object of the Invention) The present invention has been made in view of the conventional problems as described above, and has excellent strength characteristics not only at ordinary temperatures but also at high temperatures, and has a small decrease in strength at high temperatures. It is an object of the present invention to provide a body and a method of manufacturing the same.

Configuration of the Invention

(Means for Solving the Problems) Following the above proposal, the present inventor conducted intensive studies on the effects of sintering aids, sintering conditions, and the like on the high-temperature characteristics of the silicon nitride sintered body. It has been found that even a composition that is difficult to sinter by the method can be densified by the hot press method, and that the sintered body obtained by this method can further improve the high-temperature strength. The present invention is based on the above findings, and is based on Si ₃ N ₄ −
_{a · Re 2 O 3 -b ·} SiO 2 ( provided that, Re ₂ O ₃ is one also oxides of two or more of the periodic table III a group element) is represented by, a 0.5
mo% or more and 5.0mo% or less, b is 0.5mo% or more and 5.0mo
% And b / (a + b) is 0.50 or more and 0.66 or less,
The hot-pressed silicon nitride-based sintered body is characterized in that the oxygen content in the sintered body is 2% by weight or less and the bulk density of the sintered body is 95% or more of the theoretical density. In producing a hot-pressed silicon nitride sintered body, one or more oxides of Group IIIa element of the periodic table are added to a silicon nitride powder containing 0.3% by weight or more and 4.0% by weight or less of silicon oxide. And sintered under a nitrogen atmosphere of 2 to 500 atm at a temperature of 1800 to 2200 ° C and a pressure of 100 kgf / cm ² or more by hot press (uniaxial pressure sintering). And a method for producing such a hot-pressed silicon nitride-based sintered body by means of a method for solving the above-mentioned conventional problems. It is characterized by having. (Operation) Hereinafter, the operation of each component in the present invention will be described in more detail together with the reasons for limiting the numerical values and the like. As described above, the hot-pressed silicon nitride-based sintered body according to the present invention has a total content of the oxide of the Group IIIa element in the periodic table IIIa added as a sintering aid and carried over into the sintered body by 0.5. mo% or more and 5.0mo% or less, the content of silicon oxide is set to 0.
5 mol% to 5.0 mol%, the molar ratio of silicon oxide to all oxides in the sintered body is in the range of 0.50 to 0.66, the oxygen content in the sintered body is 2 wt% or less, and the bulk density is Is set to 95% or more of the theoretical density to thereby provide excellent high-temperature characteristics. Here, the oxygen in the sintered body is derived from the oxide added as a sintering aid and the silicon oxide contained as an impurity in the silicon nitride powder as a starting material. If the content exceeds 2% by weight in the sintered body, the high-temperature characteristics of the sintered body are deteriorated. Therefore, it is necessary to limit the oxygen content in the sintered body to 2% by weight or less. In addition, the ratio of the bulk density to the theoretical density of the sintered body is 95%
The reason for the above is that the sintered body having the ratio of less than 95% lacks denseness and the strength at room temperature and high temperature is reduced. The oxides of Group IIIa elements of the Periodic Table are those in which the sintering aid is carried over to the sintered body, and the total content of the sintered body after sintering is 0.5 mol% or more and 5.0 mol% or less. Must be added to and mixed with the starting materials. This is because when the content of the oxide in the sintered body is less than 0.5% by mole, the effect as a sintering aid cannot be exerted and the sinterability deteriorates, resulting in a dense sintering. If a sintered body is not obtained, and if it exceeds 5.0% by mole, sinterability is good, but the high temperature characteristics of the sintered body are deteriorated. As these elements, Y, La, Nd, Sm, etc. are usually used alone or in combination as described later. Furthermore, silicon oxide (Si) for all oxides in the sintered body
The molar ratio of O ₂ ), that is, the value of b / (a + b), determines the amount of excessive oxidation in the grain boundary phase. When the molar ratio is less than 0.50 or more than 0.66, the sinterability is low. Although good, a high-temperature strength is not sufficient because a liquid phase having a low melting point is generated. And, in the composition having a molar ratio of 0.50 or more and 0.66 or less, although a high melting point liquid phase is generated, sinterability is reduced, but a sufficiently dense sintered body is obtained according to the production method of the present invention,
It is possible to further improve the high-temperature strength. In the silicon nitride-based sintered body according to the present invention, as long as the conditions such as the oxygen content and the bulk density in the above-described sintered body, and the contents and the component ratios of the respective components are satisfied, the starting material, the forming, and the firing are performed. Although there is no particular limitation on the conditions for the production method such as, for example, as described above, one kind of silicon nitride powder containing 0.3% by weight or more and 4.0% by weight or less of silicon oxide is used as described above. Alternatively, two or more kinds of oxides of Group IIIa elements are added, and a temperature of 1800 ° C. to 2200 ° C. and 100 kgf / cm ^{2 in} a nitrogen atmosphere of 2 to 500 atm.
By performing hot pressing (uniaxial pressure sintering) at the above pressure, a manufacturing method in which the sintered body is fired until the bulk density becomes 95% or more of the theoretical density can be adopted. Of these, impurity silicon oxide (SiO ₂ ) in silicon nitride
Must be determined to satisfy the condition of the final composition because it affects the amount of excess oxygen in the sintered body. However, the silicon oxide initially contained in the silicon nitride became
Since there is a portion that scatters as O gas, and the amount of the scatter varies depending on the firing conditions, it must be determined according to the firing process.
The following ranges can be said to be desirable. Since the silicon oxide content in silicon nitride is low, the final composition of SiO ₂
When the content condition is not satisfied, silicon oxide can be added as a starting material. Further, Sc, Y, lanthanoids (atomic numbers)
57-71), but usually Y, La, Nd, Sm
Are easily used due to the price, availability, and handling convenience. In this case, one kind of the oxide may be added alone, but particularly when the amount of the auxiliary agent is small, it is better to use a mixture of two or more kinds in order to improve the sinterability. Next, the mixture is fired by hot pressing. The hot pressing conditions used in this case are hot pressing (uniaxial pressure sintering) at a temperature of 1800 ° C to 2200 ° C and a pressure of 100 kgf / cm ² or more in a nitrogen atmosphere of 2 to 500 atm. By doing so, the bulk density of the sintered body becomes 95
Perform until the concentration reaches at least%. At this time, if the nitrogen atmosphere is less than 2 atm, a dense sintered body cannot be obtained because silicon nitride is decomposed by a reaction of Si ₃ N ₄ → 3Si + 2N ₂ . If the pressure exceeds 500 atm, high-pressure gas is confined in the closed pores, so that final sintering is hindered and a dense sintered body cannot be obtained. Therefore, the pressure in a nitrogen atmosphere is set to 2 atm or more and 500 atm or less, and the pressure at the time of hot pressing (uniaxial pressure sintering) is set to 100 kgf / cm ² or more. At this time, if the pressure is lower than 100 kgf / cm ² , the effect of accelerating sintering is small, which is not preferable. In addition, the temperature at this time is 1800 ° C or more and 2200 ° C or less, but the sintering temperature is 18
If the temperature is lower than 00 ° C., densification is not promoted, and if it exceeds 2200 ° C., the strength is lowered due to grain growth, which is not preferable. (Examples) Examples 1 to 5 Periodically, a silicon nitride powder having an average particle size of 1.0 μm and a silicon oxide content of 1.5% by weight was to have a mixture composition shown in Examples 1 to 5 in Table 1. Table III Addition of an oxide of Group a element as a sintering aid, ball mill mixing in ethanol for 24 hours, drying, placing in graphite mold and hot pressing under hot pressing conditions also shown in Table 1 Was done. Table 2 shows the composition of the sintered body obtained here. Each of the sintered bodies was ground to a shape of 3 × 4 × 40 mm with a diamond wheel, and the density of each sintered body was measured. At the same time, the strength was measured by performing a three-point bending test with a span of 30 mm at room temperature and 1400 ° C. did. These results are also shown in Table 2. As shown in Table 2, each of the sintered bodies obtained in this example had a density of 95% or more, had high strength at room temperature, and had excellent high-temperature characteristics in which the strength at high temperature was desired to decrease. It was confirmed that it was a sintered body. Comparative Examples 1 and 2 In Comparative Examples 1 and 2, as shown in the columns of Comparative Examples 1 and 2 in Table 1, the total of the oxides of the Group IIIa element in the periodic table was 0.5 mol.
%, And mixed in the same manner as in Example 1, except that the mixture was hot-pressed under the hot-press conditions shown in Comparative Examples 1 and 2 in Table 1. Table 2 shows the composition of the sintered body obtained here. Further, the density, room temperature strength, and high temperature strength of each sintered body were examined in the same manner as in the above example, and the results are shown in Table 2. As shown in Table 2, in the sintered bodies of Comparative Examples 1 and 2, the ratio of silicon oxide to the total oxides in the sintered bodies was too large.
It was found that the strength decreased at high temperatures. Comparative Example 3 A mixed powder having the mixture composition shown in the column of Example 1 in Table 1 was subjected to die molding at a pressure of 200 kgf / cm ² , and then 2,000 kg.
A molded pair was prepared by performing rubber press molding at a pressure of f / cm ² , and then the molded body was heated at 1900 ° C. under a nitrogen gas pressure of 10 atm.
For 1 hour. The bulk density of the obtained sintered body is 90% of the theoretical density.
And the room temperature strength was 450 MPa. When the high-temperature strength at 1400 ° C. was examined, it was 320 MPa. Thus, a dense sintered body could not be obtained by the ordinary gas pressure sintering method in the composition of 0.50 ≦ b / (a + b) ≦ 0.66. Comparative Example 4 A mixed powder having the mixture composition shown in the column of Example 1 in Table 1 was placed in a graphite mold and hot-pressed at 1900 ° C. under a nitrogen gas pressure of 1 atm under a pressure of 200 kgf / cm ² for 1 hour. Performed to obtain a sintered body. The sintered body obtained here did not densify due to the generation of metallic Si due to thermal decomposition.

【The invention's effect】

As described above, the hot-pressed silicon nitride-based sintered body according to the present invention is formed of Si ₃ N ₄ -a.Re ₂ O ₃ -b.SiO
₂ (where Re ₂ O ₃ is one or more oxides of Group IIIa elements of the Periodic Table III), and a is 0.5 mol% or more and 5.0 mol% or more.
% Or less, b is 0.5mo% or more and 5.0mo% or less and b / (a +
b) is 0.50 or more and 0.66 or less, the oxygen content in the sintered body is 2% by weight or less, and the bulk density of the sintered body is 95% or more of the theoretical density. Without
It is a fine ceramic material that has a small decrease in strength at high temperatures and has excellent high temperature properties such as creep resistance, high temperature strength, heat resistance, and oxidation resistance, and is suitable as a material for various structural articles used at high temperatures. It greatly contributes to weight reduction of these various structural articles.

Claims (3)

(57) [Claims] 1. The following formula: Si ₃ N ₄ -a.Re ₂ O ₃ -b.SiO ₂ (where Re ₂ O ₃ is one or more oxides of Group IIIa elements of the periodic table) Where a is 0.5mo% or more and 5.0mo% or less,
b is 0.5mo% or more and 5.0mo% or less and b / (a + b) is
A hot-pressed silicon nitride sintered body characterized in that the sintered body has an oxygen content of 0.5% or more and 0.66 or less, the oxygen content in the sintered body is 2% by weight or less, and the bulk density of the sintered body is 95% or more of the theoretical density. 2. The hot-pressed silicon nitride sintered body according to claim 1, wherein the sintered body has an oxygen content of 1.40% by weight or less. 3. A method according to claim 1, wherein said silicon nitride powder contains 0.3% by weight or more and 4.0% by weight or less of silicon oxide.
100 kgf at a temperature of 1800 ° C or more and 2200 ° C or less under a nitrogen atmosphere of 2 atm or more and 500 atm or less by adding an oxide of a group a element
^2. The hot-pressed silicon nitride sintered body according to claim 1, wherein the sintered body is fired by hot pressing at a pressure of not less than / cm ² until the bulk density of the sintered body becomes 95% or more of the theoretical density. Method.