JPH0321502B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of industrial application) The present invention is applied to the field of engineering ceramics,
More specifically, it relates to high-temperature, high-strength silicon nitride ceramics used as high-temperature structural materials in parts that require high-temperature strength, oxidation resistance, and corrosion resistance, such as engine parts, heat-resistant jigs and tools for steel manufacturing, gas turbine blades, etc., and methods for producing the same. It is something. (Conventional technology) In recent years, high-temperature structural materials have become important for the development of equipment that operates at high temperatures, such as high-temperature gas turbines, diesel engines, and MHD power generation, as well as for use as high-hardness tools and corrosion-resistant materials. However, silicon nitride (Si ₃ N ₄ ) is attracting attention as one of the most important materials as it has sufficient strength at high temperatures, is chemically stable, and is resistant to thermal shock. has been done. By the way, the above-mentioned Si ₃ N ₄ has excellent physical properties as mentioned above, but this is because Si ₃ N ₄ is silicon (Si).
This originates from the fact that it is a compound consisting of a strong covalent bond between nitrogen (N) and nitrogen (N). At the same time, this means that _{Si 3} N ₄ alone is difficult to sinter, making it difficult to manufacture high-density, high _- strength products. Addition of sintering aids such as Y ₂ O ₃ , Al ₂ O ₃ , MgO, etc. is required. However, these sintering aids generally cause vitreous formation at the interface of Si ₃ N ₄ powder particles.
Although it is possible to obtain a dense sintered body by adding a sintering aid because it has the role of bonding Si ₃ N ₄ particles to each other, on the other hand, the added aid will form at high temperatures of 1000℃ or higher. The grain boundary phase (glass phase) of the sintered body is softened, so the high-temperature strength decreases to less than half of the room-temperature strength. In addition, _{Y2O3 -} added system has relatively excellent high-temperature strength.
Si ₃ N ₄ materials have a problem in that oxidation occurs through grain boundaries in the temperature range of 900 to 1000°C, resulting in significant property deterioration. Therefore, it has been considered to obtain a sintered body without the addition of sintering aids, and since the strength of a sintered body generally tends to be proportional to its density, hot isostatic pressing (hot isostatic pressing), which has recently attracted attention, has been An attempt was made to sinter Si ₃ N ₄ without the addition of a sintering aid under an inert gas at high temperature and pressure using the HIP method (hereinafter abbreviated as HIP). The result was obtained by HIP sintering without additives.
Si ₃ N ₄ showed no decrease in strength up to 1300℃, but
It showed a decreasing trend at 1400℃. However, the strength level was low, ranging from 20 to 40 Kg/ ^mm2 . Therefore, when we tried to investigate the cause of the low strength level, we found that the reason was that it was contained in the raw materials.
Although the amount of impurities such as Fe, Ni, As, and Sb (~200PPm) is not a problem at all in the usual additive-added Si ₃ N ₄ system, these impurities aggregate during the HIP process and become the starting point of fracture. It was found that this was due to the formation of particles, and it was found that this was a phenomenon unique to Si ₃ N ₄ without additives. It was also found that the strength deterioration at 1400°C was due to the use of raw materials with a large amount of _O2 . (Problems to be Solved by the Invention) The present invention addresses the above-mentioned actual situation and solves the problem that although the strength of Si ₃ N ₄ without additives does not decrease at high temperatures up to 1300°C, the reason for its low strength level is due to impurities in the raw material. (Fe, Ni, etc. form FeSi, FeSi ₂ , NiSi, etc.) and the knowledge that strength deterioration at 1400℃ is due to the use of raw materials with a large amount of O ₂ . For this purpose, iron group metals such as Fe, Ni, Cr, etc.
We focused on the need to remove impurity elements such as Va group metals such as As and Sb, and the need to control the amount of _O2 in the raw material to maintain strength up to 1400℃. By using Si ₃ N ₄ as a raw material, we can create an ideal high-temperature, high-strength Si ₃ N ₄ sintered body, especially the material that will be needed in future gas turbine components.
The aim is to obtain a Si ₃ N ₄ sintered body that does not lose strength up to 1400°C. (Means for Solving the Problems) That is, the characteristics of the present invention are the removal of the above impurity elements and the regulation of O ₂ materials in the raw materials. This is a Si ₃ N ₄ HIP sintered body with no sintering aid added, with a group metal element of 50 PPm or less and an oxygen content of _{2 %} or less. A high-temperature, high-strength sintering process is performed in which the impurity iron group elements and Va group metal elements contained are reduced to 50 PPm or less and the oxygen content is reduced to 2% or less by heat treatment and/or vacuum heat treatment, followed by HIP treatment. A method for producing Si ₃ N ₄ ceramics without binder additives. The details of the present invention will be further explained below. First, the Si ₃ N ₄ powder raw material used in the present invention is metal Si
In addition to those obtained by the nitriding method, SiCl _{4 and} Si
Those manufactured from (NH) ₂ are used. However, as mentioned above, this Si ₃ N ₄ powder raw material has
It contains impurity elements such as iron group elements such as Fe, Ni, and Cr, and Va group metal elements such as As and Sb. It also contains oxygen _O2 . Therefore, it is necessary to remove these impurity elements and regulate the amount of _O2 , and for this reason, the former is particularly subjected to heat treatment in dry chlorine and/or vacuum heat treatment. In this case, the amount of impurity elements contained is:
Although it is not particularly critical to set the amount of O ₂ to 50 PPm at the maximum and 2% or less, it is preferable based on the experimental results shown in the Examples below. After highly purifying the Si ₃ N ₄ raw material powder in this way,
Sintering is performed by HIP processing, which is a known method, and is usually performed at temperatures of 1600°C or higher, preferably 1700 to 2000°C, under an inert atmosphere such as Ar gas or _N2 gas.
℃ and above 500 atm, preferably 700
It is carried out under pressure greater than atmospheric pressure. Hereinafter, specific examples will be given to clarify how superior the present invention is compared to each comparative example. (Example) First, a general table will be shown.

[Table] Next, each example will be sequentially explained based on the above table. Example 1 After press-forming raw material A in the above table into a size of 50 x 50 x 20 mm, the compact was placed in a tubular furnace for impurity purification treatment, and the temperature was gradually raised while dry chlorine was flowed twice per minute. , and held at 900°C for 2 hours. The analysis results of the sample after treatment were as shown as A′ in the previous table, and a decrease in impurity Fe was observed.
The amount of impurity elements was below 50PPm. This sample was then glass-sealed using a press-sealing method using Vycor glass powder.
HIP treatment was performed under the conditions of ℃, 150 MPa, and 2 hours, and sintered. As shown in the table above, the bending strength of the obtained sintered body is 66Kg/mm ² at room temperature, 78Kg/mm 2 at 1200℃, and even 1400Kg/mm ² at room temperature.
The strength was 75Kg/mm ² at ℃, which is an epoch-making high-temperature strength for a Si ₃ N ₄ sintered body. In addition, Weibull coefficient of room temperature strength (number of samples 15)
m=12. (Comparative example) Sample C, which is Si ₃ N ₄ powder produced by a commercially available silicon nitriding method, was press-molded, and without impurity purification treatment, it was glass-sealed by a press-sealing method using Vycor glass powder.
HIP treatment was performed under the conditions of ℃, 150 MPa, and 2 hours. As shown in the table above, the bending strength of the obtained sintered body was 23Kg/mm ² at room temperature, 48Kg/mm ² at 1200℃, and further increased at 1400℃.
It was 32 Kg/mm ² , which was far inferior to that of Example 1. Moreover, in the sintered body of Comparative Example 1, as a result of observing the fragments of the bending test piece, all the fractures were found to originate from inclusions dispersed in the sintered body, and X-ray analysis revealed that the inclusions were Fe silicides. It turns out that there is something. Further, in this comparative example, although the Weibull coefficient was apparently high, the strength value itself was low and was not suitable for practical use. In addition, the strength decreased at 1400°C. The attached drawing is a chart comparing the above-mentioned Example 1 and Comparative Example 1 for the purpose of comparison, and the difference between the two can be clearly seen. Comparative Example 2 Next, Si ₃ N ₄ raw material A with the same composition as in the above example synthesized by silicon diimide pyrolysis method was press-molded, and a press seal using Vycor glass powder was made without impurity purification treatment. The glass was sealed using the method and HIP sintered under the conditions of 2000℃, 150MPa, and 2 hours. Although the bending strength of the obtained sintered body was improved compared to that of Comparative Example 1, aggregates of inclusions were occasionally present in the sintered body, and inclusions were especially present near the surface on the tensile stress side of the test piece. exist,
When this becomes the starting point of fracture, it shows an extremely low value, so the overall average value is relatively low, and the strength varies widely, making it a somewhat unreliable material. Example 2 As in Example 1, raw material B shown in the table above was prepared in a 50×50
After press molding to a size of 20 mm, the molded product was placed in a tube furnace for impurity purification treatment, and the temperature was gradually raised while dry chlorine was flowed twice per minute, and the temperature was maintained at 900° C. for 2 hours. The sample after treatment was analyzed and found to be free of impurities, especially
A large decrease in As and Sb impurity elements was observed. Next, the sample body was glass-sealed using a press sealing method using Vycor glass powder, and then
When HIP sintered under the conditions of 2000℃, 150MPa, and 2 hours, the bending strength of the obtained sintered body was 70Kg/at room temperature.
mm ² and 72Kg/mm ² at 1200°C, it showed excellent high-temperature strength as a Si ₃ N ₄ sintered body. Further, the Weibull coefficient of the room temperature strength value was 8. Example 3 Next, the B raw material in Example 2 was molded and impurity purified in the same manner as in Example 1, and the temperature was further raised to 1200°C and vacuumed to 10 ^-3 Troo.
It was held for 1 hour. When the sample after treatment was analyzed, it was found that B'' in the above table.
As shown in Figure 2, the purity was even higher than in Example 2, and in particular Va group elements were effectively removed. This sample was then HIP sintered under the same conditions as in Examples 1 and 2. As a result, the bending strength of the HIP sintered body was further improved compared to Example 2, as shown in the table above, and the Weibull coefficient was also improved. . Comparative Example 3 The Si ₃ N ₄ synthesized by the Si ₃ N ₄ gas phase reaction method
Using raw material B, HIP sintering was performed in the same manner as in Comparative Example 2 without impurity purification treatment. The obtained sintered body was found to be much worse in bending strength than those of Examples 2 and 3. Comparative Example 4 Raw material A' in the table above was treated in the atmosphere at 800 degrees,
The surface oxidation of Si ₃ N ₄ powder was promoted to produce raw material D with an oxygen content of 2.7%, and the raw material D was prepared in the same manner as in Example 1 above.
A HIP sintered body was obtained. When the bending strength of this sintered body was measured, it was 82Kg/mm ² at room temperature, and the Weibull coefficient was 13.
Although a better result was obtained than A' (Example 1), 1200
℃, shows a rapid decrease in strength at 1400℃,
This was not desirable as a high-temperature structural material. (Effects of the Invention) The present invention is made of high-purity Si ₃ N ₄ from which impurity elements have been removed and iron group elements and Va group elements are 50 PPm or less and oxygen content is 2% or less. Due to the reduction of elements, inclusions that cause fracture are not formed, and it has the remarkable effect of realizing a high-temperature, high-strength material that has never been reported as a Si ₃ N ₄ material without the addition of a sintering aid. Moreover, by HIPing and sintering under high temperature and high pressure without adding any sintering aids, the intertwining of Si ₃ N ₄ particles is made stronger, and the heat resistance and strength of Si ₃ N ₄ itself can be expressed as is. This makes it possible to provide a sintered body that is ideal as an engineering ceramic material that is expected to be developed.

[Brief explanation of drawings]

The figure shows the present invention and the conventional method without the addition of sintering aids.
It is a comparative chart of temperature changes in bending strength of Si ₃ N ₄ .

Claims (1)

[Scope of Claims] 1. A sintered body obtained by hot isostatic pressing Si ₃ N ₄ without the addition of sintering aids, which contains impurity elements of iron group and Va group metal elements of 50 PPm or less, High-temperature, high-strength silicon nitride ceramics characterized by an oxygen content of 2% or less. 2 A Si ₃ N ₄ raw material without the addition of sintering aids or a molded body thereof is heat treated in dry chlorine and/or vacuum heat treated to remove impurities of iron group metals and Va group metals contained in the raw material or molded body. A method for producing high-temperature, high-strength silicon nitride ceramics, which comprises reducing the element content to 50 PPm or less and the oxygen content to 2% or less, followed by hot isostatic pressing and sintering. 3 Place the Si ₃ N ₄ molded body without the addition of sintering aids into a tube furnace, gradually raise the temperature while flowing dry chlorine, and heat to 900 ml.
3. The method for manufacturing high-temperature, high-strength silicon nitride ceramics according to claim 2, wherein impurity elements are reduced by holding the ceramic at ℃ for 2 hours. 4 Place the Si ₃ N ₄ molded body without the addition of sintering aids into a tube furnace, gradually raise the temperature while flowing dry chlorine, and heat to 900 ml.
℃ for 2 hours, the temperature is further increased to 1200℃, evacuated to 10 ^-3 Torr, and held for 1 hour to reduce impurity elements. Production method.