JPH0513905B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for manufacturing silicon nitride ceramics. More specifically, the present invention relates to a method for producing a silicon nitride ceramic sintered body useful as a high-temperature structural material and excellent in high-temperature strength and high-temperature stability. BACKGROUND TECHNOLOGY In general, ceramics are brittle materials and have almost no elongation when broken, so they are used in a low stress state or with various measures taken to relieve stress. Furthermore, high heat resistance is also an important property of ceramics, but its application has been limited due to strength limitations. However, if high-temperature strength and high-temperature stability can be imparted to ceramics, it will be possible to significantly expand its application fields, and since it is an abundant resource, it will be possible to save energy and reduce the cost of various structural materials. becomes possible. Therefore, silicon carbide-based and silicon nitride-based ceramics have recently attracted attention. In particular, silicon nitride ceramics have excellent high-temperature strength and oxidation resistance, and are expected to be used as high-temperature structural materials for engine parts and other applications, and their development is progressing. By the way, as a method for manufacturing SiO ₃ N ₄ sintered body,
Various methods are known, such as a reaction sintering method, a post reaction sintering method, an ordinary pressure sintering method, an atmosphere pressure sintering method, a HIP method, an ultra-high pressure sintering method, and a hot press method. However, sintering alone is extremely difficult, and it is necessary to add a sintering aid to obtain a dense sintered body. Such sintering aids include MgO, Al ₂ O ₃ ,
Oxides such as Y ₂ O ₃ and BeO are mainly used,
A substantially dense sintered body has been obtained by sintering a mixture of these and SiO ₃ N ₄ by, for example, a normal pressure pressing method or a hot pressing method. However, when using a sintering aid,
A liquid phase is generated during sintering, which remains in the sintered body as a glass phase after densification. Therefore, there was a serious problem that the high temperature strength was reduced and the original strength of silicon nitride could not be maintained. For example, when MgO is used as a sintering aid, it is known that the strength at 1300°C decreases to about half of the room temperature strength. Under these circumstances, the following various methods have been proposed to improve high-temperature strength. (i) First, J.Amer.Ceram.Soc., 1975, 58 [7/
8], 323 uses Y ₂ O ₃ as a sintering aid and heat treatment before sintering to generate Si ₃ N ₄ and Y ₂ O ₃ crystals and prevent the formation of a glass phase. A method is disclosed. In addition, ()Al ₂ O ₃
(or BeO) as a sintering aid to form a solid solution of SiO ₃ N ₄ to prevent the glass phase from remaining;
() A method of reducing the amount of glass phase produced by adding carbon and reducing SiO ₂ , () Using SiO ₃ N ₄ raw material with low oxygen content and limiting the amount of sintering aid used. Examples include a method of reducing the glass phase by However, the above method (i) requires the use of a hot press sintering method, and since Y ₂ O ₃ is expensive, there are drawbacks such as high raw material costs. In addition, in methods () and (),
Sintering did not progress sufficiently, and therefore a high-strength sintered body was not obtained. Furthermore, method (2) also had drawbacks such as high raw material cost and difficulty in sintering. Problems to be Solved by the Invention As mentioned above, SiO ₃ N ₄ , which is attracting attention as a high-temperature structural material, is a material with high covalent bonding properties, so it is difficult to use various materials when producing sintered bodies. It was necessary to use a sintering aid, but the aid remains as a glass phase during sintering and reduces high temperature strength. Various countermeasures have been proposed, but all of them are insufficient, and so far no method has been known to achieve a satisfactory high-temperature strength improvement effect. Therefore, developing such a method has great significance in expanding new application fields for ceramics and reducing manufacturing costs for various structural materials, and there are strong expectations from various fields. There is. The purpose of the present invention is to provide a new method for manufacturing silicon nitride ceramics that can improve such high-temperature strength, and also has greatly improved high-temperature strength and high-temperature stability, and can be used as a high-temperature structural material. It is also an object of the present invention to provide extremely useful silicon nitride ceramics. Means for Solving the Problems The present inventors have discovered that silicon nitride-based ceramic materials,
In particular, in light of the above-mentioned current state of improvement in high-temperature structural materials, we have conducted various studies and found that MgO as a sintering aid has been developed.
By using Al ₂ O ₃ and SiO ₂ and heat-treating the resulting sintered body under optimal conditions, SiO ₃ N ₄ with high high temperature strength, little change over time at high temperatures, and excellent stability can be obtained. discovered that a system sintered body could be obtained,
The invention has been completed. That is, the method for producing silicon nitride ceramics of the present invention includes 60 to 99% by weight of silicon nitride and a total of 40 to 99% by weight of magnesium oxide, aluminum oxide, and silicon oxide.
Contains 1.0% by weight, molar ratio MgO/Al ₂ O ₃ is 0.1-3,
Sintering a raw material mixture in which MgO/SiO ₂ is 2 or more,
The sintered body obtained is maintained in a non-oxidizing atmosphere at a temperature of 1150°C to 1500°C, and spinel is precipitated in the sintered body. In the method of the present invention, the raw material mixture of silicon nitride ceramics is processed according to the general steps of molding and sintering, and the sintering is performed by various conventionally known methods, such as in a non-oxidizing atmosphere as shown below. Examples include a pressureless sintering method within a chamber, a hot press method, and a high temperature isostatic pressure method (HIP method). The non-oxidizing atmosphere includes not only an inert gas atmosphere but also a reducing atmosphere,
For example, N ₂ , H ₂ and the like can be cited as typical examples. In the method of the present invention, the silicon nitride ceramic raw material composition mainly contains silicon nitride and contains MgO, Al ₂
Contains _O3 and _SiO2 as sintering aids. Silicon nitride is mainly α type, and fine powder with a particle size of -325 mesh is used. On the other hand, MgO, Al ₂ O ₃ and SiO ₂ can be used as oxide powders or compounds that can be easily added to oxides by heating etc., such as carbonates, and the total amount of these in terms of oxides is 1.0～
Used in an amount of 40% by weight. If the sintering aid is used in an amount less than 1.0% by weight, the densification of the sintered body will be insufficient, and if it is used in an amount exceeding 40% by weight, the excellent properties of silicon nitride itself will be impaired. Both are undesirable because they cause Furthermore, MgO and Al ₂ O ₃ are used in a mixed manner so that the molar ratio is in the range of 0.1 to 3, and more favorable results can be obtained by setting this molar ratio in the range of 0.5 to 2. . That is, the molar ratio is 3
Based on the above, if MgO is excessive, the desired spinel formation will be insufficient and the residual ratio of glass phase will be high.On the other hand, if MgO is less than 0.1 and Al ₂ O ₃ is excessive, the sintering will be poor. The progress of sintering is insufficient, and a high-strength sintered body cannot be obtained. MgO and SiO ₂ are blended in a molar ratio of 2 or more. This is because if the molar ratio is less than 2, the spinel that has precipitated will be dissolved by heat treatment below 1150°C, and therefore the molar ratio MgO/SiO ₂
must be 2 or more. For example, sintering is performed at 1600° to 1900°C in the pressureless sintering method;
Preferably at a temperature within the range of 1650° to 1800°C,
It is carried out in a non-oxidizing atmosphere (usually an _N2 gas atmosphere). This sintering temperature is also a critical condition,
Sintering does not proceed sufficiently at temperatures below 1600℃, and as a result, a high-strength sintered body cannot be obtained;
Sintering at a temperature exceeding 100% causes significant decomposition of SiO ₃ N ₄ and grain boundary growth, resulting in deterioration of various properties. Note that good results can also be obtained by pressure sintering such as hot press, and even higher densification can be achieved by HIP method. The sintered body thus obtained is then sintered and purified in a non-oxidizing atmosphere to precipitate a spinel structure. The sintering temperature is within the range of 1150° to 1500°C, and this sintering temperature is critical, and spinel will not precipitate if it is outside the range. That is, it is known that at temperatures below 1150°C, MgOSiO ₂ precipitates, which does not improve the strength of the sintered body but rather reduces it, and at temperatures above 1500°C, crystallization from the glass phase does not occur. Therefore, we cannot expect the effect of sintering (see attached Figure 1). According to another aspect of the method of the present invention, after the sintering treatment of the raw material composition powder, by controlling the cooling rate and maintaining the temperature within the above temperature range for at least 1 hour, i.e. temperature range during cooling
A similar effect can be achieved by adjusting the residence time of the sintered body at 1150 to 1500°C to be 1 hour or more. Function Thus, by sintering a raw material composition of a predetermined composition and then subjecting it to a sintering treatment according to the method of the present invention, it is possible to greatly improve the high-temperature strength of a silicon nitride ceramic sintered body. Become. This effect becomes even more remarkable when the high-pressure sintering method and HIP method, which are advantageous in terms of densification, are used. Such an effect is based on the fact that the glass phase caused by the sintering aid disappears during the sintering process, and on the contrary, spinel (MgAl ₂ O ₄ ), which is advantageous in terms of strength, is formed. Therefore, selection of the sintering temperature is very important. Conventionally, when used at 1000 to 1150°C, there has been a problem that MgOSiO ₂ precipitates and the strength of the SiO ₃ N ₄ sintered body is significantly impaired. However, by sintering and purifying according to the method of the present invention, the formation of MgOSiO ₂ can be almost completely prevented. Further, although there are α-type and β-type SiO ₃ N ₄ , the α-type is advantageous in terms of strength, and the method of the present invention mainly uses α-type SiO ₃ N ₄ . Examples Hereinafter, the method of the present invention will be described in more detail with reference to Examples. However, these Examples are for illustrating the present invention and are not intended to limit the scope of the present invention in any way. Example 1 SiO ₃ N ₄ powder containing 90 wt% α-SiO ₃ N ₄ , MgO powder, Al ₂ O ₃ powder, and SiO ₂ powder with an average particle size of 0.5 μ were mixed as shown in Table 1 below. Mixed at a ratio of
Thirteen kinds of raw materials were obtained by mixing in a pole mill. Note that MgO/SiO ₂ was adjusted to be constant at 2.5. After the thus obtained mixed powder was pressed, it was heated at 1750°C under 2 atmospheres of N ₂ gas atmosphere.
Sintered for hours. Next, it was cooled to 1200° C. in a furnace and kept at this temperature for 5 hours under a nitrogen gas atmosphere to perform sintering purification. It was further cooled to room temperature in the furnace. For comparison, sintering was omitted for some samples, and the samples were cooled to room temperature as they were after sintering. From the obtained product, dimensions 3 mm x 4 mm x
A 40 mm test piece was cut out and subjected to a bending test. The presence or absence of spinel formation was also investigated. The results are shown in Table 1.

[Table] * indicates a reference example. As is clear from the results in Table 1, MgO + Al ₂ O ₃
When the concentration is less than 1.0 wt% (specimen 1), no spinel precipitation is observed even after sintering and even if the conditions regarding the molar ratio MgO/Al ₂ O ₃ are met, and the bending strength is low. is low. Furthermore, MgO + Al ₂ O ₃
When the molar ratio MgO/Al ₂ O ₃ exceeds 40 wt% (test piece 13), even if the sintering of the present invention is performed,
Even if the above conditions are satisfied, the bending strength is low, making it impossible to obtain the desired high-strength ceramics. In addition, in specimens 8 and 9 where the molar ratio MgO/Al ₂ O ₃ exceeds 3, no spinel precipitation is observed and the bending strength is extremely low, and furthermore, in the case where the molar ratio is less than 0.1 (test specimen 6), Although spinel precipitation is observed, the bending strength is insufficient. It is therefore clear that the amount of sintering aid as well as the molar ratio MgO/Al ₂ O ₃ are critical conditions in the method of the invention. Furthermore, as seen in Test Piece 7, when no sintering treatment is performed, no spinel precipitation is observed even if other conditions are satisfied, and this makes it possible to understand the importance of sintering. Example 2 Contains 90wt% α-SiO ₃ N ₄ as in Example 1
_{SiO3N4 powder} , MgO powder with average particle size 0.5μ, _Al2
Nine types of raw materials were obtained by blending O ₃ powder and SiO ₂ powder at the blending ratios shown in Table 2 and mixing them in a ball mill. Furthermore, the MgO/Al ₂ O ₃ ratio is
It was set as 0.8 constant. After the thus obtained mixed powder was pressed, it was heated at 1750°C under a 2 atm _N2 gas atmosphere.
It was sintered for 2 hours. Next, it was cooled to 1200° C. in a furnace and kept at this temperature for 5 hours under a N ₂ gas atmosphere for sintering and purification. It was further cooled to room temperature in the furnace. The obtained molar ratio MgO/SiO ₂ is 2
Precipitation of spinel was confirmed in the above sintered body.
Thereafter, it was heated again to 1050° C. in an N ₂ gas atmosphere and held for 5 hours for re-sintering, and then cooled to room temperature in the furnace. A test piece with dimensions of 3 mm x 4 mm x 40 mm was cut out from the obtained product, and a bending test was performed on it. The presence or absence of spinel formation was also investigated. The results are shown in Table 2.

【table】

[Table] * indicates reference example Example 3 Similar to Example 1, SiO ₃ containing 90wt of α-SiO ₃ N _4
N4 powder, MgO powder with average particle size 0.5μ, _{Al2O3 powder} and _SiO2 powder 2wt%, 6wt% and 1wt respectively
% (molar ratio MgO/Al ₂ O ₃ = 0.8, MgO/SiO ₂ =
3) and pulverized and mixed with a ball mill, 1750
Sintered at ℃ for 2 hours. Next, a large number of products were obtained which were sintered at various temperatures in the range of 800°C to 1600°C for 3 hours each, and a bending test was performed on these products in the same manner as in Example 1. The results of examining the presence or absence of spinel precipitation are attached. It is shown graphically in Table 1. From the results shown in FIG. 1, it can be seen that excellent results are obtained at a sintering temperature of 1150° to 1500°C, and this condition is critical in the method of the present invention. still,
The results shown in Figure 1 show great strength even at temperatures above 1500°C and below 1000°C. However, when the product is used at room temperature, the strength decrease is small, but when the product is used at high temperatures (1000°C to 1150°C), precipitation of MgOSiO ₂ as mentioned above is unavoidable, resulting in a large strength decrease. It will be done. Furthermore, 1000°
Even at temperatures up to 1150°C, the strength of the product decreases significantly. Effects of the Invention As described in detail above, according to the method of the present invention, MgO and Al ₂ O ₃ are used in combination as sintering aids, the amount and molar ratio are limited to a predetermined range, and By performing sintering treatment at a predetermined temperature, spinel, that is, MgAl ₂ O ₄ is precipitated, and the glass phase grain boundaries in the sintered body are reduced. As a result, the high-temperature strength of the ceramic product obtained can be greatly improved, and the precipitation of MgOSiO ₂ , which becomes a problem when used at 1000°C to 1150°C, can be suppressed. Therefore, silicon nitride ceramic products obtained by the method of the present invention can be used as high-temperature structural materials, such as gas turbine engine parts (blades, rotors, combustion tubes, etc.), diesel engine parts (liners, pistons, cylinders, etc.), It is extremely effective as a heat-resistant, high-strength material for rocket parts (radomes, etc.) and other materials.

[Brief explanation of the drawing]

Attached FIG. 1 is a graph plotting the relationship between sintering temperature and bending strength and the presence or absence of spinel precipitation.

Claims (1)

[Claims] 1. 60 to 99% by weight of silicon nitride, and a total of 1.0 to 99% of magnesium oxide, aluminum oxide, and silicon oxide.
Contains 40% by weight, molar ratio MgO/Al ₂ O ₃ is 0.1 to 3,
Sintering a raw material mixture in which MgO/SiO ₂ is 2 or more,
The obtained sintered body is heated at a temperature of 1150°C to 1500°C.
1. A method for producing silicon nitride ceramics, which comprises annealing under a non-oxidizing atmosphere to precipitate spinel in the sintered body. 2. The method according to claim 1, wherein the molar ratio MgO/ _Al2O3 is within the range of 0.5 to ₂ . 3. The method according to claim 1 or 2, characterized in that the sintering is performed for one hour or more. 4. Claim 1, wherein the sintering is carried out by controlling the cooling rate after the completion of sintering and holding the product at a temperature of 1150 to 1500°C for 1 hour or more.
or the method described in paragraph 2.