JPH08104570A - Production of silicon nitride-based sintered compact - Google Patents

Abstract

PURPOSE: To reduce the harmful effect of the reactions of a tungsten compd. with silicon nitride and silicon oxide on a sintered compact, to finely disperse WSi2 in the sintered compact and to produce a sintered compact having high strength over the temp. range from room temp. to 1,000 deg.C with low-cost starting materials. CONSTITUTION: A compact based on silicon nitride, contg. at least oxide of a group IIIa element of the Periodic Table and aluminum oxide as additive components and further contg. a tungsten compd. of <=3μm average particle diameter other than tungsten silicide by 0.5-10wt.% (expressed in terms of WSi2 ) of the total amt. is heated at 800-1,400 deg.C under a reduced pressure of <=10Torr to form WSi2 of <=3μm average particle diameter and then the compact is fired in a nonoxidizing atmosphere.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a silicon nitride sintered body used for automobile parts, gas turbine engine parts, etc., which has excellent strength characteristics from room temperature to high temperature and excellent thermal shock resistance. Regarding

[0002]

2. Description of the Related Art Conventionally, silicon nitride sintered bodies have
Due to its excellent thermal shock resistance and oxidation resistance, it is being applied to engineering ceramics, especially for heat engines such as turbochargers.

Generally, since it is difficult to sinter a silicon nitride sintered body by using silicon nitride alone, as a sintering aid, an oxide of a Group 3a group 3a element such as Y ₂ O ₃ or Al _{2 is used.} O _{3 and the} like are added to achieve high densification (Japanese Examined Patent Publication No. 52).
-3649, Japanese Patent Publication No. 58-5190).

[0004]

However, when a rare earth element oxide and aluminum oxide are used as the sintering aid, the sinterability is enhanced, so that the densification can be achieved and the room temperature strength is improved. Although it becomes a high sintered body,
In terms of high temperature characteristics of 1000 ° C., the grain boundary phase composed of rare earth elements, silicon, aluminum, oxygen, and nitrogen itself has a low melting point, and is easily softened at high temperatures, so that the high temperature characteristics of the entire sintered body are practical. However, its use conditions were limited to the low temperature range. Therefore, it is required to improve the high temperature characteristics, especially the high temperature strength and the thermal shock resistance while maintaining such sinterability.

Therefore, as a method for strengthening the grain boundary, M
It is known to disperse fine chemical stable compounds such as silicides, carbides, borides of o, W, Ta and Ti.
No. 194038 and the like.

However, according to such a method, such dispersed particles themselves are expensive, and when such a compound is added to the starting material, it reacts with silicon nitride or silicon oxide in the sintering process to change the volume. There is a problem that the characteristics of the sintered body may be adversely affected by the generation of gas or the generation of gas accompanying the reaction, and there is a problem that the strength improving effect due to the dispersion of fine particles cannot be sufficiently obtained.

[0007]

[Means for Solving the Problems] As a result of repeated studies on the above problems, the present inventors have found that fine WSi is contained in the sintered body.
_{When 2} is dispersed, an inexpensive W compound having a small particle size is blended as a starting material, and this is heat treated once to generate WSi ₂ before firing, and then main firing is performed to obtain another WSi without reacting with the components
The inventors have found that the grain size of ₂ can be easily controlled, the strength can be remarkably increased, and that the production can be performed at low cost, and the present invention has been completed.

That is, the method for producing a silicon nitride sintered body of the present invention contains silicon nitride as a main component, contains at least an oxide of a Group 3a element of the periodic table and aluminum oxide as an additive component, and further has an average particle size of 10 torr of a compact containing a tungsten compound of 3 μm or less (excluding W silicide) in a proportion of 0.5 to 10% by weight in terms of WSi ₂
It is characterized in that it is heated at a temperature of 800 ° C. to 1400 ° C. in the following reduced pressure to generate WSi ₂ having an average particle size of 3 μm or less, and then baked in a non-oxidizing atmosphere.

According to the production method of the present invention, first, silicon nitride powder is mainly used as a starting material, and oxide powder and aluminum oxide powder of Group 3a element of the Periodic Table, and optionally silicon oxide powder are used as additive components. These additional components are added in an amount suitable for enhancing the sinterability of silicon nitride. Specifically, 3 to 15% by weight of Group 3a element oxide of the periodic table, 1 to 10% by weight of aluminum oxide, and 0 to 10% by weight of silicon oxide are mixed. The silicon nitride powder used as a starting material is itself α-
Either Si ₃ N ₄ or β-Si ₃ N ₄ can be used, and their particle size is preferably 0.4 to 1.2 μm.

According to the present invention, a tungsten compound having an average particle size of 3 μm or less is further added to the above composition. Examples of tungsten compounds other than WSi ₂ include W
O ₃ , WC, WB and the like can be mentioned. Since this tongue-ten compound finally becomes WSi ₂ , it is added in a proportion of 0.5 to 10% by weight, especially 1 to 5% by weight in terms of WSi ₂ . The reason why the average particle size of the W compound is limited to 3 μm or less is that when the W compound is large, WSi ₂ in the final sintered body is limited.
This is because the grain size of 3 exceeds 3 μm, which on the contrary becomes a fracture source, and the effect of improving strength cannot be exhibited. Further, the amount is limited to the above range, because if it is less than 0.5% by weight, the effect of preventing grain boundary softening by WSi ₂ dispersion is not sufficient,
If it exceeds 10% by weight, the amount of dissolution in the grain boundary phase increases and the melting point is lowered, so that the desired strength cannot be obtained.

Then, the mixture to which the W compound is added as described above is molded into an arbitrary shape by a desired molding means such as a die press, a cold isostatic press, an extrusion molding, and the like.
800 ° C to 1400 ° C under reduced pressure of 10 torr or less
Heat treatment at the temperature of. By this heat treatment, the W compound is reacted with silicon nitride or silicon oxide to generate WSi ₂ . By reducing the pressure during the heat treatment at 10 torr or less, the reaction between the W compound and silicon nitride or silicon oxide is promoted, and the silicon monoxide and nitrogen generated during the reaction are rapidly discharged to the outside of the compact. To prevent changes in the composition of the grain boundary phase and formation of voids. Therefore, if the pressure exceeds 10 torr or the processing temperature is lower than 800 ° C., the generated gas is not sufficiently discharged and voids remain in the sintered body, so that the desired strength cannot be obtained. Further, when the processing temperature exceeds 1400 ° C., silicon nitride itself begins to decompose.

After the heat treatment as described above, known firing methods such as hot pressing, atmospheric pressure firing, nitrogen gas pressure firing, and hot isostatic firing (HIP) firing after these firing, And a dense sintered body is obtained by firing with glass seal HIP firing or the like. If the firing temperature at this time is too high, the silicon nitride crystal that is the main phase grows grains and the strength decreases.
It is desirable that the nitrogen gas-containing non-oxidizing atmosphere is 00 ° C., particularly 1650 to 1850 ° C. By this firing, silicon nitride is β-S regardless of whether the raw material is α or β.
i ₃ N ₄ .

By this firing, finally, a β-silicon nitride main crystal phase and a grain boundary phase containing a Group 3a element of the periodic table, aluminum, oxygen, silicon and nitrogen are contained, and WSi ₂ is contained in the grain boundary phase. It is possible to obtain a sintered body having a particle size of 3 μm or less.

The periodic table No. 3a used in the present invention is as follows.
As the group element, Y, Er, Yb and Lu are desirable. Although there are few significant differences in characteristics among these elements, Y is the most desirable because it can be obtained at low cost.

[0015]

The mechanical and thermal properties of the silicon nitride sintered body are determined by the β-silicon nitride phase which is the main crystal phase and its grain boundary phase. The grain boundary phase is composed of a Group 3a element of the periodic table, silicon, aluminum, oxygen, and nitrogen, and fine WSi ₂ crystals having an average grain size of 3 μm or less are dispersed to form fine WSi ₂ as pinning. As a result of preventing the softening of the field phase, the high temperature characteristics of the sintered body can be improved.

Moreover, according to the present invention, commercially available inexpensive W
Since a compound is used for heat treatment before firing to form WSi ₂ and then firing, it is possible to prevent adverse effects caused by a reaction of the W compound with silicon nitride or silicon oxide during firing.
The dispersion effect of Si ₂ can be maximized. Thereby, excellent mechanical properties can be imparted from room temperature to a high temperature of 1000 ° C.

[0017]

【Example】

Example 1 Silicon nitride powder (BET specific surface area 9 m ² / g, α ratio 98
%, Oxygen amount 1.2% by weight) and various periodic table group 3a element oxide powders, aluminum oxide powders, silicon oxide powders,
Tungsten oxide powder (average particle size 1 μm) was used to prepare the composition shown in Table 1, and the mixture was molded with 1 t / cm ² . The obtained compact was heat-treated under the conditions shown in Table 1 to generate WSi ₂ . The molded body after this treatment was put in a silicon carbide sagger and fired under the conditions of Table 1 in a nitrogen gas stream at normal pressure.

The obtained sintered body was ground to a shape specified in JIS-R1601 to prepare a sample. For this sample, room temperature and 10 based on JIS-R1601
A 4-point bending bending strength test at 00 ° C was performed. Further, the size of WSi ₂ particles was measured by SEM observation of the sample subjected to mirror finishing. The measurement results are shown in Table 1.

[0019]

[Table 1]

According to the results shown in Table 1, Sample Nos. 8 and 9 in which the amount of WSi ₂ dispersed is less than 0.5% by weight or more than 10% by weight have low high-temperature strength, and therefore, the reduced pressure during the heat treatment results. Processing temperature is lower than 800 ℃ 140
Samples Nos. 15, 18, and 20 which were subjected to a temperature higher than 0 ° C. or a reduced pressure of 10 torr or more did not have the desired properties because the strength properties were low.

In contrast to these comparative examples, all the other samples according to the present invention were excellent in flexural strength, and achieved a strength of 900 MPa or more at room temperature and 1000 ° C.

Example 2 In Example 1, several kinds of powders having different particle diameters were prepared as the tungsten compound, and the powder was mold-molded at 1 t / cm ² so as to have the composition shown in Table 2. Then, this molded body was treated under a reduced pressure of 1 torr at 1100 ° C. for 5 hours,
Further, it was calcined in nitrogen at 1800 ° C. for 5 hours under normal pressure. The characteristics of the obtained sintered body were evaluated in the same manner as in Example 1.

The molded body was placed in a silicon carbide sagger and the atmosphere was controlled at 1100 ° C. to reduce composition fluctuation.
Firing was performed under the conditions of time (1 torr) 1800 ° C. for 5 hours.

The obtained sintered body was ground to a shape specified in JIS-R1601 to prepare a sample. About this sample, room temperature and 1000 based on JIS-R1601
A 4-point bending bending strength test at 0 ° C was performed. Further, the size of WSi ₂ particles was measured by SEM observation of the sample subjected to mirror finishing. The results are shown in Table 2.

[0025]

[Table 2]

According to the results shown in Table 2, when the average particle size of the W compound exceeds 3 μm, the average particle size of WSi ₂ tends to exceed 3 μm. As a result, the average particle size of the W compound is 3 μm.
Nos. 25 to 28 exceeding m were all low in strength. Therefore, it is important that the average particle size of the W compound is 3 μm or less.

[0027]

As described above in detail, according to the method for producing a silicon nitride sintered body of the present invention, the adverse effect on the sintered body due to the reaction of the W compound with silicon nitride or silicon oxide is reduced, and the sintering is performed. WSi ₂ can be finely dispersed in the bonded body, and a low-strength raw material can be used to produce a high-strength sintered body from room temperature to 1000 ° C.

Claims (1)

[Claims] 1. A tungsten compound containing silicon nitride as a main component, at least an oxide of a Group 3a element of the periodic table as an additive component, and aluminum oxide, and having an average particle diameter of 3 μm or less (however, a silicide of W is excluded). ) In an amount of 0.5 to 10% by weight based on the total amount of WSi ₂ in an amount of 10 t.
After heating at a temperature of 800 ° C. to 1400 ° C. under reduced pressure of orr to produce WSi ₂ having an average particle size of 3 μm or less,
A method for producing a silicon nitride-based sintered body, which comprises firing this in a non-oxidizing atmosphere.