JPS62207769A - Manufacture of silicon nitride sintered body - Google Patents

Abstract

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

Description

[Detailed description of the invention] [Object of the invention] (Industrial application field) This invention can be used as a material for various structural parts used in a wide range of fields such as automobiles, 411 machinery, chemical equipment, aerospace equipment, etc. The present invention relates to a method for producing a silicon nitride sintered body suitable for obtaining a fine ceramic material having particularly excellent high-temperature strength.

(Conventional technology and its problems) Sintered bodies mainly composed of silicon nitride are chemically stable at room and high temperatures and have high mechanical strength, so they are used in sliding parts such as bearings, turbocharger rotors, etc. This material is suitable for engine parts, etc.

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 or Al2O2-Y2O2. These sinterings are thought to be caused by liquid phase sintering mediated by a liquid phase generated during sintering, and after sintering, the liquid 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 influenced by the second phase, that is, the glass phase, remaining in the sintered body. In particular, the presence of a glass phase with a low softening temperature is undesirable because it greatly reduces the high-temperature mechanical properties of the silicon nitride sintered body.

On the other hand, among the silicon nitride sintered bodies, there is a silicon nitride sintered body having an α-sialon composition, which has particularly excellent properties. This silicon nitride sintered body called α-Sialon has Li, Mg, Ca
, Y, etc. are in interstitial solid solution, and at the same time, St
It is a substitution type solid solution with An at the position and 0 at the N position, and in addition to the characteristics such as high temperature and high strength inherent to silicon nitride sintered compacts, the thermal conductivity decreases due to the solid solution of many atoms. Due to these characteristics, it is attracting attention as a material for ceramic engines that require insulation.

As a method for manufacturing this α-SiAlON type silicon nitride sintered body, a method of manufacturing a mixed powder of Si3N, AiN, and metal oxide by hot pressing (Japanese Patent Application Laid-Open No. 56-129
667), a method of manufacturing by pressureless sintering method (Japanese Patent Application Laid-open No. 5
7-3769), a method of manufacturing by gas pressure sintering method (
JP-A No. 60-86075) is known.

However, although the hot press method has excellent sinterability and can produce a dense sintered body, it can only produce products with a simple shape. In addition, although the pressureless sintering method is applicable to products with complex shapes, a high-strength sintered body cannot be obtained because sintering progresses slowly and several percent of pores remain in the sintered body. Furthermore, although the gas pressure sintering method can be applied to products with complex shapes and improves strength due to lower porosity than the pressureless sintering method, some pores remain in the sintered body and the strength is reduced. Not above. In addition, upward densification in these three methods requires the inclusion of large amounts of AiN and metal oxides, which generates a glass phase in the sintered body, reducing its strength, especially at high temperatures. There was a problem.

The present invention was made in view of the above-mentioned conventional problems, and an object of the present invention is to provide a silicon nitride sintered body that has particularly excellent strength at high temperatures.

[Structure of the Invention] (Means for Solving the Problems) A method for producing a silicon nitride sintered body according to the present invention includes silicon nitride powder containing silicon nitride powder (FLA group 1 of the periodic table).

After mixing raw material powder to which at least one oxide of an element selected from the group consisting of Li, Ca, and Mg is added and molding to obtain a molded body, the molded body is heated at a pressure of 1 atm or more to 500 atm. 1600℃ or more in a nitrogen atmosphere of less than 22
It is characterized in that the treatment is carried out at a temperature of 00°C or less, and then the treatment is carried out at a temperature of 1600°C or more and 2200°C or less in a nitrogen atmosphere of 500 atmospheres or more.

In one embodiment of the present invention, when producing a silicon nitride sintered body, silicon nitride powder is added with one powder from periodic table 111a.
After mixing raw material powder to which two or more oxides selected from group element oxides are added and molding to obtain a molded body, 1.
1600°C or more and 2200°C or less, more preferably 1800°C or less, in a nitrogen atmosphere of at least 500 atm.
The treatment is carried out at a temperature of 2200° C. or lower, and then at a temperature of 1600° C. or higher and 2200° C. or lower, more preferably 180° C. or higher and 2200° C. or lower, in a nitrogen atmosphere of 500 atmospheres or higher, and more preferably, the Two or more oxides are (Y203) (M203) 1-x...(1
) (where M is Nd, Sm, and 0.2≦X≦
0.8 (representing a molar ratio).

In another embodiment of the present invention, when producing a silicon nitride sintered body, aluminum nitride (elements of group Ila of the periodic table, Li, Ca, etc.) is added to the silicon nitride powder.
and an oxide of an element selected from the group consisting of Mg (S i , AfL) 12 (0, N) 1
B (0 x ≦ 2) ... (2) (where M is the metal element of the above oxide) After mixing into the composition shown by the formula and molding to obtain a molded body, the mixture is heated at 1 atm or more to 5
(1600℃ or less in a nitrogen atmosphere of less than 00 atmospheres) 2200
℃ or less, and then treated at a temperature of 1600°C or more and 2200°C or less in a nitrogen atmosphere of 500 atmospheres or more, and more preferably, the range of X in the above formula is 0.05≦X. It is characterized in that it is ≦0.25.

The present inventors conducted research to obtain a silicon nitride sintered body with excellent high-temperature mechanical properties, and found that when sintering silicon nitride powder with at least an oxide-based sintering aid added, As physical sintering aids (elements of group 111a of the periodic table, Li, Ca.

Using one or more oxides of elements selected from the group consisting of Mg), a mixed powder of silicon nitride powder and at least the oxide sintering aid added thereto is molded, and then the molding is performed. Body under 1 atm, less than 11m500 atm,
More preferably, the treatment is carried out at a temperature of 1600°C or more and 2200°C or less in a nitrogen atmosphere of 2 atm or more and less than 500 atm, and then at a temperature of 1600°C or more and 2200°C or more in a nitrogen atmosphere north of 500 atm.
It has been found that a sintered body with excellent high-temperature mechanical properties can be obtained by processing at a temperature of 200°C or lower.

That is, the present invention solves the above problem by focusing on increasing the softening temperature of the second phase in the sintered body. Since the oxides of group 111a of the periodic table and Li, Ca, and Mg used as sintering aids in this invention have high melting points, they cannot be used as a single component in normal pressure sintering or gas pressure sintering. Freezing temperature (1600'C or more 2200'
C! In the range below F), a sufficient amount of low-viscosity liquid phase is not generated, resulting in poor sinterability, and conventionally it has been used when pressure is applied from outside, such as in a hot press. In one embodiment of the invention, the melting point of the oxide can be lowered by using a mixture of Group 111a oxides, in particular two stubs, and the type of oxide and the ratio of two stubs can be reduced. Sintering temperature can be increased to 2200°C or more by selecting the appropriate value.
A sufficient amount of liquid phase was obtained below.

Moreover, this liquid phase is similar to conventional MgO, A1203 Y2
Since the melting point is higher than that of the liquid phase when 03 or the like is used as an auxiliary agent, it is thought that even if it remains in the sintered body after sintering, the strength will not decrease at high temperatures.

Among silicon nitride sintered bodies, in order to obtain a silicon nitride sintered body having an α-sialon composition which is dense and has high strength and whose strength decreases little at high temperatures, it is necessary to add silicon nitride powder (group 1ea of the periodic table). , Li.

A molded body of a raw material mixed powder obtained by mixing at least one oxide of an element selected from the group consisting of
1600℃ or more 2200℃ under nitrogen atmosphere below 00atm
It has been found that a dense and high-strength sintered body can be obtained by processing at a temperature of 1600°C or higher and 2200°C or lower in a nitrogen atmosphere of 500 atmospheres or higher after the treatment at a temperature of 500°C or lower.

That is, as mentioned above, the sintering process is divided into two stages, first at 1600°C in a nitrogen atmosphere of 1 atm or more and less than 500 atm.
By processing at a temperature of 2200°C or lower, densification is promoted while suppressing the decomposition of the raw material powder and α-sialon, and the theoretical density ratio is 90% or more (the remainder is closed pores).
A sintered body is obtained. Next, by increasing the nitrogen atmosphere to 500 atmospheres or more, external force is applied to the surface of the sintered body to promote densification. As a result, closed pores remaining inside the sintered body disappear, and a sintered body with low porosity can be obtained. A particular feature of this method is that it is not necessary to obtain a sintered body completely free of pores in the first stage of sintering, and it is only necessary that most of the pores become closed pores. Therefore, even when the value of X in the above-mentioned formula (2) is small, a sufficiently dense sintered body can be obtained.

In this invention, the raw material powders include (1) silicon nitride, (2) an oxide of an element selected from among elements of group 11a of the periodic table, Li, Ca, and Mg, and (2) aluminum nitride as necessary. Here, the silicon nitride is preferably an alpha-type powder, but a beta-type or amorphous powder may also be used. Furthermore, the elements of Group 1a of the periodic table are Sc, Y, La, and Ce.

Pr, Nd, Pm, Sm, Eu, Gd, Tb.

The elements are Dy, Ho, Er, Tm, Yb, and Lu. These elements used as sintering aids and Li, Ca, M
The oxide (g) is preferably a fine powder, but oxides such as hydroxides and carbonates of these may be produced by heat treatment.

In addition, these oxides may be used alone or in combination with the two sintering tools E. In this case, the two oxide sintering aids may be used in combination.
The combinations and ratios thereof can be arbitrarily selected from among the oxides listed in L, but it is more preferable to use the following criteria.
When 20% by weight of auxiliary agents are added, the combination and ratio of auxiliary agents is such that densification does not occur at sintering temperatures below 1600°C and sintering occurs only at temperatures above 1600°C.

For example, when using Y2O3 and Nd2O3, (Y2
Sintering is possible with O3) (Nd203)1-X (see formula (1) above) in the range of 0.2≦X≦0.8 (molar ratio). Also, when using Y2O3 and Sm2O3,
(Y2O3)XSm203) 1-x, 0.2≦X≦0.
Sintering is possible within a range of 8 (molar ratio).

In addition, when the silicon nitride sintered body has a sialon composition,
The mixing ratio of the three types of powders, silicon nitride, aluminum nitride, and the oxide, is expressed by the general formula, M(Si, A-cross)12(
"N) 113...(3) (M is an element of the L oxide) It is more preferable to select the value so as to express the following. In this case, the value of However, if a sintered body with little strength loss at high temperatures is required, it is preferable that 0.05≦X≦0.25, that is, X is 0.05≦X≦0.25.
If X is less than 0.25, the effect of promoting sintering is small and it is difficult to obtain a high-strength sintered body. If As a result, high-temperature strength tends to decrease.

The method for molding these mixed powders is not particularly limited, but for example, any conventional ceramic molding method such as mold press molding, rubber press molding, or injection molding can be selected depending on the shape of the intended product.

Next, sintering is performed in two steps. First, a powder compact is treated in a nitrogen atmosphere of 1 atm or more and less than 500 atm at a temperature of 1600° C. or more and 2200° C. or less. The treatment time is preferably 10 minutes or more. In this step, the compact is heated to a temperature of 1,600° C. or more and 2,200° C. or less, so that the oxide-based auxiliary agent forms a liquid phase, and densification progresses by the mechanism of liquid phase sintering. In addition, if aluminum nitride is included, AfLN and oxide auxiliary agents and Si3N4
reacts, and denseness progresses. At this time, the atmosphere is set to 1 atm or more and less than 500 atm in a nitrogen atmosphere because silicon nitride decomposes and does not become dense when it is less than 1 atm.
This is because if the pressure is lower than 0 atm, high-pressure nitrogen gas is trapped in the sintered body, and the sintered body is only densified to about 90% of the theoretical density. In this step, a sintered body having a theoretical density of 90% or less L (the remainder is closed pores) is obtained. Next, the sintered body is treated at a temperature of 1,600° C. or more and 2,200° C. or less 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 1600℃ or higher2
The reason why the temperature is 200° C. or lower is that if the temperature is lower than 1600° C., the sinterability is poor and densification does not occur in 1 minute, resulting in a decrease in high-temperature strength. Further, if the temperature exceeds 2200°C, the 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 they may also be carried out in two steps.
It may be covered with powder such as S, i3N4+5i02, BN, AfLN, etc. Also, the atmosphere gas may be nitrogen gas loo
%, but other inert gases such as Ar may be added.

(Example 1) Silicon nitride powder 81.8% by weight, yttrium oxide 7.3%
A mixed powder consisting of neodymium oxide (wt%) and neodymium oxide (10.9 v [uit%) was molded in a mold at a pressure of 200 kgf/am2, and then rubber pressed at a pressure of 2000 kgf/am2 (7) to form a plate shape of 5 x 6 x 50 mm. When this was heat-treated under the schedule conditions shown in FIG. 1 in a nitrogen atmosphere, a dense sintered body with a density of 3.24 g/cm' was obtained. Next, the surface of this sintered body is ground, and
After processing into a shape of ×4 × 40 mm, span 3
When a bending test was conducted by three-point bending at 0 mm, the average strength of the five pieces at a test temperature of 1300°C was 55 kgf/mm2 as shown in Table 1.

(Example 2) When a mixed powder consisting of 81.5% by weight of silicon nitride, 7.3% by weight of yttrium oxide, and 11.2% by weight of samarium oxide was molded and sintered under the same conditions as in Example 1, the density was 3.
．． A dense sintered body of 29 g/cm3 was obtained.
When the bending strength at 300°C was measured, as shown in Table 1, an average of 50 kgf/mm2 was obtained for the five pieces.

(Comparative Example 1) Example 1. When the molded body having the composition of Example 2 was sintered at 1700°C for 4 hours in a nitrogen atmosphere of 1 atm, the bulk density of the sintered body was 2.15 g/cm' and 1.9, respectively.
5g7cm' and did not densify.

(Comparative Example 2) Silicon nitride powder 90% by weight, magnesium oxide 10% by weight
When a mixed powder consisting of was sintered under the conditions of Comparative Example 1,
As shown in Table 1, the bending strength at 1300°C was 12 kgf/m 2 as an average of the five pieces. Note that the density of the sintered body was 3610 g/cm'.

Note that raw material powder (specifically, SiN,
: 85% by weight, Y2O3: 10% by weight, A 203:5
As shown in Example 1.2, oxides of Group 1a elements of the periodic table were added to the silicon nitride powder as shown in Example 1.2. It has been experimentally confirmed that a sintered body obtained by adding two or more kinds of, pressurizing and heating under Scheduling conditions has a higher bending strength at 1300''C and is more desirable.

Table 1 (Examples 3 to 5) Silicon nitride powder, aluminum nitride powder, and oxide powder were mixed in the composition shown in Table 2, molded at a pressure of 200 kgf/cm2, and then molded at a pressure of 2000 kgf/cm2. It was rubber pressed under pressure and molded into a plate shape of 5 x 6 x 50 mm. This was heat treated in a nitrogen atmosphere under the schedule conditions shown in Figure 1.Then, the surface of the sintered body was ground and
After processing into a shape of X 4 X 40 mm, a three-point bending test was conducted at 1200° C. with a density and a span of 30 mm. The average of the measurements of each of the five trees is shown in Table 2. As shown in Table 2, according to the present invention, a dense and high-strength sialon sintered body can be obtained.

(Comparative Examples 3 to 6) The molded bodies having the compositions shown in Table 2 were heat-treated under the conditions of Schedule 2 and Schedule 3 shown in Table 2 and Figures 2 and 3. The results are also shown in Table 2. , Thus, it was confirmed that according to schedules ■ and ■, densification did not progress sufficiently and the strength was low.

Table 2 [Effects of the Invention] As explained above, in the method for producing a silicon nitride sintered body according to the present invention, silicon nitride powder is
A raw material powder containing at least one oxide of an element selected from the group consisting of Group 1a, Li, Ca, Mg) is mixed and molded to obtain a molded body, and then the molded body is heated to 1 atm. 1600℃ under nitrogen atmosphere below 500atm
Treated at a temperature of over 2200″C, then 500%
Since the treatment is carried out at a temperature of 1,600° C. or higher and 2,200° C. or lower in a nitrogen atmosphere of pressure or higher, a very excellent effect can be obtained in that a silicon nitride sintered body with extremely high strength at high temperatures can be obtained. .

[Brief explanation of drawings]

Figures 1, 2, and 3 show the time schedules (I, II, l) of the sintering conditions used in the examples of this invention.
It is an explanatory view showing i). Patent Applicant: Patent Attorney, Nissan Motor Co., Ltd., Patent Attorney, Fumi Oshio Figure 1 Processing time (hours) Figure 2 Figure 3

Claims (1)

[Claims] (1) Silicon nitride powder (group IIIa of the periodic table, Li,
After mixing raw material powder to which at least one oxide of an element selected from the group consisting of Ca, Mg) is mixed and molded to obtain a molded body, the molded body is heated to
1600℃ or more 2200℃ under nitrogen atmosphere below 0 atmospheres
1. A method for producing a silicon nitride sintered body, which comprises treating at a temperature of 1,600° C. or more and 2,200° C. or less in a nitrogen atmosphere of 500 atmospheres or more.