JPS6337064B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a method for manufacturing silicon nitride ceramics. [Technical Background of the Invention] Sintered bodies containing silicon nitride as a main component (silicon nitride ceramics) have excellent heat resistance, being able to withstand high temperatures of around 1900°C, and have a low coefficient of thermal expansion. It also has impact resistance.
Attempts are being made to apply silicon nitride ceramics to structural parts that require high strength at high temperatures, such as diesel engines and gas turbines, as well as corrosion-resistant and wear-resistant parts. By the way, conventional methods for manufacturing silicon nitride ceramics include adding auxiliary agents such as rare earth element oxides and magnesia as additives, and using atmospheric pressure sintering, hot pressing, etc., since silicon nitride is difficult to sinter by itself. A method of densification sintering such as HIP is used. [Problems with Background Art] When silicon nitride ceramics are used as a structural material, a high degree of homogenization is required as well as strength at room and high temperatures. however,
It has been extremely difficult to produce highly homogenized silicon nitride ceramics using the conventional methods described above. [Object of the Invention] The present invention aims to provide a method for producing silicon nitride ceramics which not only have high heat resistance and high strength but also have uniform strength. [Summary of the Invention] Through the research described below, the present inventors have developed a method for producing highly homogenized silicon nitride ceramics. It is known that when α-silicon nitride (α-Si ₃ N ₄ )-rare earth element oxide system is sintered, the crystal grains grow into long columnar shapes during the conversion from α to β, resulting in improved strength. However, it is very difficult to control the aspect ratio during the growth of such crystal grains, and since the strength is determined particularly by the crystal grain size in the longitudinal direction, variations inevitably occur. Based on this, the present inventors have determined that α-Si ₃ N ₄
α→β by calcining a mixture of rare earth element oxide and
After this conversion was performed to grow crystal grains in the form of long columns, this calcined product was crushed to make the aspect ratio uniform. However, it has not always been possible to obtain highly homogenized silicon nitride ceramics by simply sintering calcined powders with the same aspect ratio. Therefore, the present inventors conducted a thorough study on the grain growth of the crushed calcined material, and found that the α/β ratio at the time of calcination has a significant effect, and based on this, the α/β ratio was determined. 1 to 0.1, and by re-sintering the pulverized powder, it was found that highly homogeneous silicon nitride ceramics with no variation in strength could be obtained. Hereinafter, the manufacturing method of the present invention will be explained in detail. First, silicon nitride powder having an α content of 80% or more, preferably 85% or more is prepared. This silicon nitride powder has an average particle size of 2μm or less, preferably 0.1~
It is desirable to choose one with a diameter of 1 μm. Subsequently, additives are added to this silicon nitride powder to prepare raw material powder. Additives include rare earth element oxide alone, or rare earth element oxide and alumina, magnesia, aluminum nitride, iron oxide, titanium oxide,
A mixture with at least one selected from zirconium oxide and molybdenum carbide is used. The amount of these additives added to silicon nitride powder is 15
It is preferable that the amount of rare earth element oxide is 8% by weight or less, especially when using the above mixture. Note that the particle size of the additive may be 1 μm or less. Next, the raw material powder is calcined to convert the α/β ratio to 1 to 0.1. The reason for limiting the α/β ratio during calcination is that if the ratio exceeds 1, long columnar grain growth tends to occur during resintering due to the conversion of remaining α-Si ₃ N ₄ to β. The variation in strength increases. On the other hand, if the ratio is less than 0.1, the strength of the obtained silicon nitride ceramics will decrease. The calcination conditions were in a nitrogen stream,
It is desirable to set the temperature at 1600 to 1850°C, preferably 1700 to 1800°C. If the temperature during calcination is less than 1600℃, the conversion of α-Si ₃ N ₄ to β-Si ₃ N ₄ will be slow, and the grain growth will also be slow; however, if the temperature exceeds 1850℃, Si ₃ N ₄ begins to decompose. Next, the calcined product is pulverized. The particle size at this time is preferably 1 to 50 μm. The reason for this is that if the particle size of the pulverized powder is less than 1 μm, the aspect ratio will become small, making it difficult to improve the strength, whereas if the particle size exceeds 50 μm, defects will become large and the strength will likely deteriorate. Next, pressureless sintering or hot pressing is performed using the pulverized powder of the calcined product to produce silicon nitride ceramics. 1700~ for this resintering
It is preferable to carry out at a temperature of 1820℃, 1700℃.
Below 1820°C, sintering does not proceed sufficiently, while above 1820°C, decomposition of Si ₃ N ₄ begins to occur. Note that when HIP or atmospheric pressure is used for resintering, processing at even higher temperatures is possible. [Embodiments of the Invention] Next, embodiments of the present invention will be described. Example 1 First, Si ₃ N ₄ with an average particle size of 1.0 μm and an α content of 92%
A raw material powder was prepared by mixing 92% by weight of powder, 5% by weight of Y ₂ O ₃ powder with an average particle size of 0.9 μm, and 3% by weight of Al ₂ O ₃ powder with an average particle size of 0.3 μm. Subsequently, this raw material powder was calcined at a temperature of 1700° C. for 30 minutes in a nitrogen stream. This calcined product had an α/β ratio of 0.8. Next, after pulverizing the calcined material to an average particle size of 5.8μm, this pulverized powder was heated at 1780℃ and 300Kg/
Silicon nitride ceramics were produced by hot pressing at cm ² for 90 minutes. The resulting silicon nitride ceramic has a density of 3.24
g/cc. In addition, silicon nitride ceramics of 3 x 4 x 36 mm
The test piece was cut out to the dimensions of , and the three-point bending strength (σ), (m) _was measured.
Kg/mm ² , Weibull coefficient (m) = 21, and σ ₁₂₀₀ =
It was found to have extremely high strength of 82Kg/mm ² and m=27, and high homogeneity. Example 2 The same raw material powder as in Example 1 was calcined under different temperature conditions to produce calcined products with α/β ratios of 1, 0.6, 0.4, 0.2, and 0.1, and then these calcined products were used in Example 1. Five types of silicon nitride ceramics were manufactured by performing the same treatment as described above. The three-point bending strength of the obtained ceramics was measured, and the results are shown in the table below.

〔Effect of the invention〕

As detailed above, according to the present invention, high heat resistance,
It is possible to provide a method for producing silicon nitride ceramics which not only have high strength but also have uniform strength and are effective as structural materials for diesel engines, gas turbines, etc.

Claims (1)

[Claims] 1. In the production of silicon nitride ceramics made of silicon nitride-additive-based sintered bodies, the α content is
A process of mixing additives with 80% or more silicon nitride, and calcining this mixture to achieve an α/β ratio of 1 to 0.1.
1. A method for producing silicon nitride ceramics, comprising the steps of: converting the calcined product into powder; and pulverizing the calcined material and then resintering it. 2. The method for producing silicon nitride ceramics according to claim 1, wherein the additive is a rare earth element oxide. 3. Claim No. 3, characterized in that the additive is a mixture of a rare earth element oxide and at least one selected from alumina, magnesia, aluminum nitride, iron oxide, titanium oxide, zirconium oxide, and molybdenum carbide. A method for producing silicon nitride ceramics according to item 1. 4. The method for producing silicon nitride ceramics according to claim 1, characterized in that the calcination is carried out in a nitrogen stream at a temperature of 1,600 to 1,800°C. 5. The method for producing silicon nitride ceramics according to claim 1, characterized in that the calcined product is pulverized to a particle size of 1 to 20 μm. 6. The method for producing silicon nitride ceramics according to claim 1, wherein the resintering is pressureless sintering. 7. The method for producing silicon nitride ceramics according to claim 1, wherein the resintering is hot pressing.