JPH0118031B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of manufacturing a silicon nitride sintered body with high strength and a complex shape, and more specifically, a silicon nitride pre-sintered body pre-sintered into an arbitrary shape is prepared by hot isostatic pressing (hereinafter referred to as HIP method). This invention relates to a method for advantageously increasing the density by reducing the loss of thermal energy. In recent years, there has been active development of equipment that operates at high temperatures, such as high-temperature gas turbines, diesel engines, and MHD power generation, with the aim of improving thermal efficiency, saving fuel, reducing pollution, and reducing weight. However, the development of these devices is focused on the development of high-temperature structural materials, and the formation of these materials is attracting attention, but conventional heat-resistant metals cannot necessarily be used under the high temperatures required. In order to conserve heat-resistant metal materials, which do not have sufficient mechanical strength and are scarce in resources, ceramics made from materials such as Si, Al, C, and N, which are relatively abundant on the ground, are used as high-temperature structural materials. Development is progressing. In particular, among these ceramic high-temperature structural materials, silicon nitride is a material that has sufficient strength at high temperatures, is chemically stable, and is resistant to thermal shock compared to conventional ceramics mainly made of alumina (Al ₂ O ₃ ). (Si ₃ N ₄ ) is attracting attention as one of the most promising. As mentioned above, this Si ₃ N ₄ has superior physical properties compared to conventional oxide ceramics mainly made of alumina, but this is mainly due to the fact that Si ₃ N ₄ is composed of silicon (Si) and nitrogen (N). This is because it is a compound consisting of a strong covalent bond with. On the other hand, this means that it is very difficult to manufacture products with the desired shape, and in fact, most of the research in this field in recent years has focused on how to make high-strength Si ₃ N ₄ molded products. However, it cannot be said that a completely satisfactory manufacturing technology has been developed yet. That is, at present, when high density and high strength are achieved, the molded body is inevitably limited to a simple shape, and in order to obtain a complex shape, the strength must be sacrificed. Conventionally, the generally well-known method for manufacturing such Si ₃ N ₄ compacts is the CVD method, which adds a sintering aid to Si ₃ N ₄ powder and heats it under atmospheric pressure or about 10 atm.
Method of sintering under _N2 atmosphere, hot press method,
These are four methods of nitriding reaction sintering of Si. Among these, the hot pressing method yields molded bodies with relatively high density and high strength, but there are problems such as difficulty in obtaining molded bodies with complex shapes and high cost. On the other hand, the nitriding reaction sintering method uses Si powder as a raw material, so it has the advantage of being easy to mold into complex shapes using existing methods, but the density of the obtained sintered body is low and the A major problem is that it is not possible to obtain strong Si ₃ N ₄ sintered bodies, and currently the density of Si ₃ N ₄ sintered bodies obtained by this method is only a little over 80%; The lack of density hinders the improvement of the strength of the sintered body. Moreover, one of the major problems is that the nitriding reaction requires an extremely long time, for example, the reaction treatment time can be as short as 2 days, or as long as 10 days or more. On the other hand, the pressureless sintering method is a method of sintering compressed silicon nitride powder that has been formed into an arbitrary shape in advance, so it is relatively easy to manufacture products with complex shapes, but it has to be of low density. The highest rate is around 95%. In recent years, as a way to improve this method, Japanese Unexamined Patent Publication No. 52
-47015 and No. 53-102320, a method has been proposed in which a green compact preformed into an arbitrary shape is sintered in an N ₂ gas atmosphere of several atm to several tens of atm. According to the research, a sintered body with a maximum density of 98% has been obtained. However, in none of these methods has there been a method that simultaneously satisfies molding of complex shapes and high density. Generally, when sintering Si ₃ N ₄ , MgO, SiO ₂ , Al ₂ O ₃ , etc. are usually added as sintering aids.
It is thought that these are scattered during sintering due to the following reaction with Si ₃ N ₄ . Si ₃ N ₄ +3MgO→3SiO↑+3Mg↑×2N ₂ ↑ Si ₃ N ₄ +3SiO ₂ →6SiO↑+2N ₂ ↑ Si ₃ N ₄ +Al ₂ O ₃ →2AlN+3SiO↑+N ₂ ↑ On the other hand, Si ₃ N ₄ itself also It is known that thermal decomposition occurs. Si ₃ N ₄ →3Si→2N ₂ ↑ These thermal decomposition reactions during sintering cause weight loss of the sintered body, and sometimes the weight loss due to thermal decomposition is greater than the density increase rate due to shrinkage of the sintered body due to sintering. It has been reported that the weight loss rate can sometimes reach 50%.
One of the methods for suppressing this thermal decomposition is the aforementioned N ₂ gas atmosphere sintering method, but a weight reduction of several percent is still observed. According to the research conducted by the present inventors, as described later, the weight loss due to pyrolysis is closely related to the density before sintering, and when the initial density is low, the weight loss due to pyrolysis is significant, and when the initial density is high, the weight loss due to pyrolysis is significant. There is also a tendency for the decrease to become smaller. From this, in the conventional _N2 gas atmosphere sintering method, the density of the green compact before sintering is about 60% at most, and the pores are completely open, so thermal decomposition during sintering takes place. It is thought that the product diffuses from the inside to the outside due to the difference in partial pressure, and the decomposition reaction progresses until it reaches the inside of the molded article. On the other hand, in the _N2 gas atmosphere sintering method, increasing the _N2 gas partial pressure is one way to suppress the thermal decomposition mentioned above.
Although this is an effective means, it must be said that the N ₂ gas partial pressure of several tens of atmospheres as in the conventional method is extremely insufficient thermodynamically to effectively suppress thermal decomposition. Therefore, based on the current situation as described above, the present inventors first formed silicon nitride powder into a predetermined shape, pre-sintered it to a relative density of 92% or more, and then heated the pre-sintered body to a temperature of 1500°C. As described above, we have proposed a method for producing a high-density sintered body with a relative density of 98% or more by directly applying high-temperature, high-pressure gas with a nitrogen partial pressure of 500 atm or more to HIP treatment. Of course, HIP processing is a well-known method for densifying Si ₃ N ₄ , but the conventional method
This is a normal HIP method using Ar gas, and uses a container such as glass, in which Si ₃ N ₄ powder,
Alternatively, a common method is to encapsulate and seal the sintered body and perform HIP treatment. However, when this method is put into practical use, it is difficult to obtain a container with a complicated shape, and it is difficult to uniformly fill the Si ₃ N ₄ inside the container.
There are many problems to be solved, such as means for preventing reaction between Si ₃ N ₄ and the container and means for releasing Si ₃ N ₄ from the mold, and it is not practical. There is also a method of pre-sintering and sealing the surface to some extent and HIP treatment, but in this case,
Densification has not been achieved because thermal decomposition of Si ₃ N ₄ causes significant weight loss. Therefore, the above-mentioned proposal uses the HIP method, makes the relative density of the pre-sintered body 92% or more, and
High density was achieved by HIP treatment under N ₂ gas partial pressure, but even with such a method of utilizing HIP treatment, there are the following problems when attempting to commercialize it. In other words, HIP treatment requires a high temperature of 1000°C or higher, especially higher for Si ₃ N ₄ , whereas when charging a pre-sintered body into a HIP furnace, after pre-sintering, the temperature is The pre-sintered body is charged into the HIP furnace in a state where the sintering temperature has decreased. Of course, this is unavoidable since it is charged into the HIP furnace in batches, but considering that a certain degree of temperature rise occurs during preliminary sintering, the loss of thermal energy is considerable. It cannot be denied that it is a thing. Moreover, when the surface of a sintered body pre-sintered at high temperatures is rapidly cooled, or when it is rapidly cooled and then reheated, it is likely to generate minute cracks and cracks, which not only inevitably leads to a decrease in strength, but also causes problems in terms of quality. cause problems. The present invention deals with the above-mentioned situation and the actual state of HIP processing of Si ₃ N ₄ , and provides a suitable and advantageous manufacturing method for high-strength silicon nitride sintered bodies based on the various experiments described above, and also provides a method for producing high-strength silicon nitride sintered bodies. The purpose is to provide an economical and rational method that takes into account the loss of thermal energy in the process and adapts to current energy conservation measures. Therefore, the sintering method of the present invention, which is suitable for such purposes, utilizes HIP treatment in an N ₂ -gas atmosphere, and at the same time is based on the continuity of the process with _preliminary sintering _. After adding and mixing a sintering aid and forming it into a predetermined shape, it is pre-sintered at 1000 to 1800°C in a non-oxidizing gas atmosphere, and the temperature of the pre-sintered body is maintained at 500°C or higher.ゝImmediately charge it into a HIP furnace that has been preheated to 500℃ or higher and heat it to 1500℃ or higher.
HIP treatment is performed under a high temperature, high pressure _N2 gas atmosphere of 2000℃ and 500atm or higher, and after the treatment, the sintered body is taken out of the HIP furnace at a temperature of 500℃ or higher, and then placed in a heat treatment furnace maintained at 500℃ or higher. It is characterized in that it is charged and subjected to heat treatment in a non-oxidizing gas atmosphere. Hereinafter, the sintering method of the present invention will be explained in detail according to its steps. First, the method of the present invention is carried out in three steps: preliminary sintering, HIP treatment, and heat treatment. Among these, the first preliminary sintering involves adding and mixing Si ₃ N ₄ powder with a sintering aid, molding it into a predetermined shape, and then sintering it for 1000 minutes in a non-oxidizing gas, preferably N ₂ gas atmosphere. It is sintered at a high temperature of ℃~1800℃. The Si ₃ N ₄ powder used here can be obtained by nitriding metal Si, gas phase reaction method, etc.
From the viewpoint of the bending strength of the sintered body, the gas phase reaction method is particularly effective.
Those made from SiCl ₄ and NH ₄ are most preferred. Furthermore, the sintering aid added to the Si ₃ N ₄ powder is
Any oxide or nitride of Y, Al, Mg, Ti, etc., or a mixture thereof, especially Y ₂ O _{3
-Al2O3 -} MgO based sintering aids are the most common and effective. The amount of these sintering aids added is usually 30% or less based on the Si ₃ N ₄ powder.
Preferably it is 5 to 15%. Next, known molding means can be used to mold the Si ₃ N ₄ powder mixed with the sintering aid, and various methods such as compression molding, injection molding, and hydrostatic molding are used as appropriate. In this way, the molded article formed into a predetermined shape as described above is then heated with a non-oxidizing gas, preferably a non-oxidizing gas.
It is usually subjected to preliminary sintering under _N2 gas atmosphere.
This sintering is performed at a high temperature of 1000 to 1800°C. As the preliminary sintering means, conventionally known means such as the normal pressure sintering method and the hot pressing method are used. Note that the temperature of the preliminary sintering is not necessarily constant depending on the type and amount of the sintering aid added, but it is effective to maintain the temperature range of 1000 to 1800°C. be. The surface of the sintered body that has been pre-sintered in this way is sealed to some extent, and the pores in the sintered body are not connected to the surface, which will be described later.
High densification can be easily achieved through HIP processing. In this case, considering the bending strength of the Si ₃ N ₄ sintered body, it is preferable that the α-type silicon nitride in the raw material powder be 80% or more. The Si ₃ N ₄ pre-sintered body sintered as described above has completed the first stage, and is then cooled down to room temperature and placed in a HIP furnace while maintaining the temperature above 500°C without lowering it. and subjected to HIP treatment. Since a HIP furnace usually performs HIP processing under high temperature and high pressure, it is generally preheated and maintained at a high temperature of 500°C or higher, and therefore the pre-sintered body is processed as a continuous process.
It can be designed to be charged into a HIP furnace. After this preliminary sintering, it is not possible to charge the product into a HIP furnace without first lowering the temperature and perform the HIP treatment.
The sintering method using HIP saves the thermal energy required to raise the temperature compared to the case where the preliminary sintered body is taken out, cooled, and then charged into the HIP furnace, and the HIP treatment cycle can be shortened. Of course, there is an advantage in preventing the occurrence of cracks caused by rapid cooling when taking out the pre-sintered body or rapid heating when charging the pre-sintered body into a HIP furnace. HIP treatment is carried out directly in a known HIP furnace without using a container, but Si ₃ N ₄
Because it can prevent the decomposition reaction of and increase the density of
It is necessary to carry out under _N2 gas atmosphere. The treatment temperature is 1500 to 2000°C, preferably higher than the pre-sintering temperature. Of course, this HIP
Naturally, the temperature must be below the decomposition temperature of Si ₃ N ₄ , and the decomposition temperature also increases as the HIP pressure increases, but it must be at least 100°C lower than the decomposition temperature at the pressure during HIP treatment. It is preferable to do so. On the other hand, the HIP pressure is usually over 500 atm, and if it is less than this, the HIP process will take a long time, and
Since the decomposition reaction amount of Si ₃ N ₄ increases in proportion to time, it not only causes a decrease in the weight of the sintered body, but also makes it difficult to achieve high density. Therefore HIP pressure is at least
Advantageously, the pressure is above 500 atmospheres, preferably above 700 atmospheres. In addition, the higher the HIP pressure, generally
Although the decomposition reaction of Si ₃ N ₄ is suppressed and high density is easily achieved, it is not practical because it takes time to increase the pressure and the HIP processing equipment such as the pressure increase compressor and the main pressure vessel become large. Therefore industrially, 2500
It is desirable to HIP under pressure up to atmospheric pressure. In this case, the processing time is 5 minutes or more, usually around 30 minutes. The sintered body that has been densified by the above HIP treatment is then taken out from the HIP furnace at a temperature of at least 500℃ or higher, and then charged into a heat treatment maintained at 500℃ or higher and heat treated in a non-oxidizing gas atmosphere. Can be applied to. In the conventional case, the HIP-treated sintered body is then
It was cooled to room temperature and used as a product, but
The present invention further provides an increase in strength and improves the properties of the product by applying heat treatment. However, this heat treatment also has to avoid thermal decomposition.
It is important to perform this under a non-oxidizing gas atmosphere such as N ₂ gas. As with the HIP process described above, the heat treatment temperature is not constant depending on the type of sintering aid in the pre-sintered body, the amount mixed, or the HIP conditions, but a temperature of 500°C or higher is effective in increasing the strength of the sintered body. It is true.
However, the heat treatment time does not need to be very long;
About 10 minutes is enough. In this way, a Si ₃ N ₄ sintered body is manufactured through the first, second, and third stages, but the resulting sintered body is not rapidly cooled or heated during the process, so cracks may occur. Made of high-quality, high-density material with high strength
It becomes a Si ₃ N ₄ sintered body. Also, by this heat treatment
Further strength improvement can be achieved by crystallizing the sintering aid glass phase at the Si ₃ N ₄ grain boundaries.
Moreover, even during this heat treatment, the sintered body after HIP treatment is taken out while still at 500°C or higher and charged into a heat treatment furnace which is also maintained at 500°C or higher, so it is possible to reduce the loss of thermal energy. As mentioned above, according to the sintering method of the present invention,
Si ₃ N ₄ powder is pre-sintered at a high temperature of 1000-1800℃, and then heated to 500℃ without lowering it to room temperature.
Preheated to 500℃ or higher while maintaining the temperature above 500℃.
The sintered body is charged into a HIP and subjected to densification treatment by HIP treatment, and the sintered body after HIP treatment is kept at a temperature of 500℃ or higher without being taken out into the air. Since the heat treatment is carried out by charging the material into the subsequent heat treatment furnace, the generation of cracks due to thermal shock during rapid cooling or rapid heating as in the past is completely prevented, resulting in high strength and high density.
Si ₃ N ₄ sintered body can be easily obtained, and
By strictly regulating the temperature maintenance through the preliminary sintering, HIP treatment, and heat treatment, it is hoped that each of these processes will be continuous, and the loss of thermal energy consumed will be reduced, thereby achieving the energy saving that is currently required. It is a sintering method that is highly anticipated in the future as an economical and practical manufacturing process, as it is compatible with various demands, contributes to improving production efficiency by shortening the cycle, and is an economical and practical manufacturing process. Hereinafter, specific embodiments of the method of the present invention will be further explained with reference to Examples. Example Y _{2 O 3 powder, Al 2 O 3 powder and} MgO were added as sintering aids to commercially available Si ₃ N ₄ powder obtained by nitriding Si powder.
Various mixed powders made by mixing various amounts of sintering aids made of powder were press-molded at a pressure of 500 kg/cm ² , and then the green compacts were molded under the conditions shown in Table 1. Pre-sintered in an _N2 gas atmosphere,
This pre-sintered body was then heated to each temperature shown in Table 1.
Charge the HIP furnace and apply the HIP treatment conditions listed below.
The HIP treatment was performed, and the sintered body after the HIP treatment was taken out at the temperature shown in Table 1, and the heat treatment shown in the procedure was performed to produce a Si ₃ N ₄ sintered body. When the density and bending strength of these Si ₃ N ₄ sintered bodies were measured, the results shown in Table 1 were obtained.

【table】

[Table] As is clear from Table 1, the cooling rate of the sintered body after HIP treatment is high in Comparative Example No. 1, which does not undergo heat treatment after HIP treatment, and Comparative Example No. 5, in which the heat treatment temperature is low at 400°C. Therefore, despite the high density, the bending strength is low, whereas the method of the present invention has a low bending strength.
No. 2, No. 4, and No. 6 are all sintered bodies with high density and high bending strength, and it is recognized that the heat treatment effect is large. This is because in Comparative Examples No. 1 and No. 5, fine cracks were generated due to thermal shock due to rapid cooling after HIP treatment, whereas in the method of the present invention, fine cracks were generated due to thermal shock caused by rapid cooling after HIP treatment.
This is thought to be because in No. 2, No. 4, and No. 6, the sintered bodies were not rapidly cooled due to the heat treatment, which prevented the occurrence of cracks. In addition, heat treatment
No. 7, which was subjected to heat treatment for a long time of 5 hours, had a considerably higher bending strength than the other samples because the glass phase at the grain boundaries of Si ₃ N ₄ was crystallized by the heat treatment. In addition, Comparative Example No. 3, in which the temperature at the time of charging into the HIP furnace was as low as 300°C, had low bending strength and was considerably inferior to that obtained by the method of the present invention, but this was due to thermal shock caused by rapid heating during HIP treatment. It is thought that the bending strength of the sintered body decreased as a result of defects such as cracks occurring. In addition, samples No. 9 and No. 9, in which the amount of sintering aid added was changed.
The same tendency as above was observed for No. 10, No. 11, and No. 12, and No. 9 and No. 10 produced by the method of the present invention had a density lower than that of the sintered body only pre-sintered without HIP treatment. It was found that both the bending strength and the bending strength were improved, and both were good. Regarding the notation method, it is clear that maintaining the temperature at each stage above the required temperature is consistent with the viewpoint of energy saving and contributes to significant energy saving. From the above results, it can be confirmed that the method of the present invention is extremely effective industrially as a method for producing a high-strength silicon nitride sintered body. In particular, complex shapes can be created by applying HIP processing without using containers.
It is expected that this method will be put to practical use in the future as a method for producing Si ₃ N ₄ high-density sintered bodies.

Claims (1)

[Claims] 1. A sintering aid is added to and mixed with silicon nitride powder, which is formed into a predetermined shape, and then pre-sintered at 1000 to 1800°C in a non-oxidizing gas atmosphere. Immediately heat the body to 500℃ while maintaining the body temperature above 500℃.
The sintered body is charged into a hot isostatic pressing furnace that has been preheated to a temperature of 1500 to 2000°C and subjected to hot isostatic pressing in a high-temperature, high-pressure nitrogen gas atmosphere of 500 atm or more. of silicon nitride, which is removed from a hot isostatic press furnace at a temperature of 500°C or higher, then charged into a heat treatment furnace maintained at 500°C or higher, and heat-treated in a non-oxidizing gas atmosphere. Sintering method. 2. The method for sintering silicon nitride according to claim 1, wherein the amount of α-type silicon nitride in the raw material powder is 80% or more. 3. The method for sintering silicon nitride according to claim 1 or 2, wherein the sintering aid is _{Y2O3 - Al2O3 -} MgO-based. 4. The method for sintering silicon nitride according to any one of claims 1 to 3, wherein the grain boundaries of silicon nitride are crystallized by heat treatment in the final step.