JPS6149270B2 - - Google Patents

Description

[Detailed Description of the Invention] Silicon nitride sintered bodies have a low coefficient of thermal expansion and therefore have excellent thermal shock resistance, and are used in some thermocouple protection tubes, and in the future will also be used in automobile engine parts and turbine engine blades. It has the potential to be used. However, the biggest problem at present is that it is difficult to manufacture products with complex shapes at high density and at low cost. For example, the method of adding a sintering aid and hot press molding requires the addition of several percent of Al ₂ O ₃ , Y ₂ O ₃ , MgO, etc., but even in this case it is difficult to reduce the porosity to 0.5% or less. At the same time, the thermal expansion coefficient increases due to the addition of the above-mentioned compound, which has the disadvantage that the original advantage of good thermal shock resistance is lost. Recently, the raw materials for silicon nitride powder have been reviewed, and by using fine-grained silicon nitride powder with good sinterability (for example, powder produced by plasma method), it is possible to produce products without sintering aids or hot press molding. Attempts have also been made to obtain a dense silicon nitride sintered body by a manufacturing method in which cold press molding is followed by sintering by a normal sintering method.

As the sintering of silicon nitride becomes easier,
What is also required is an improvement in the tensile fracture strength or bending fracture strength of the sintered body. The present inventor has also confirmed that the strength of these materials is proportional to the reciprocal of the square root of the particle size, similar to that of alumina sintered bodies. In other words, even with high density, silicon nitride sintered bodies with coarse grains cannot solve the problem of insufficient strength, and the development of fine-grained, high-density silicon nitride sintered bodies will likely become more important in the future. . The present invention provides a manufacturing method to meet the above objectives. The feature of the present invention method is that a SiO ₂ film is formed on the surface of a silicon nitride sintered body that has been prefired to an appropriate density, and then densification treatment is performed in a hot isostatic press (HIP) device. It will be done. The appropriate density here is 93% or more and 96%.
This corresponds to 7% or less and 4% or more in terms of porosity. When the porosity exceeds 7%, oxidation progresses to the inside of the sintered body due to the subsequent oxidation treatment, and the high temperature strength of the final sintered body decreases (due to the SiO ₂ film remaining at the grain boundaries). If it is less than 4%, it is possible to increase the density by HIP treatment without oxidation treatment, but after performing cold press molding, sintering is performed by the normal sintering method, or by hot pressing. 1750 to reduce the porosity to less than 4% using the method
Since a high temperature (pressure) sintered body at a temperature of ℃ or higher is required, crystal grains grow significantly and become a coarse grained sintered body.
Next, the surface of the pre-sintered body is left to form a non-porous SiO ₂ film, and then it is consolidated in a HIP device. HIP processing conditions are 1350℃ or higher and 1700℃
Below, a pressure of 500 atmospheres or more is practically required.
If the temperature is lower than 1350℃ and the pressure is lower than 500 atmospheres, it is not practical because compaction requires a long time (>10 hours). If the processing temperature exceeds 1700℃
Grain growth occurs during the HIP process, resulting in only coarse-grained sintered bodies. The SiO ₂ film formed on the surface has an appropriate viscosity at 1350°C to 1700°C as an air-permeable pressure transmission medium film, so it is suitable for high-temperature and high-pressure gases (inert gases such as Ar and N ₂ ) in the HIP equipment. It has been found that pressure can be effectively transmitted to the sintered body. Furthermore, the presence of the non-porous SiO ₂ coating has the advantage of preventing N ₂ from dissociating from the silicon nitride sintered body. The present invention will be explained below with reference to Examples.

Example 1 Commercially available β・Si ₃ N ₄ powder (average particle size 1μ) was hot pressed (1700°C x 1hr, 280Kg/cm ² ) to give an air light rate of 7.
% sintered body was obtained. This sintered body was heated in the atmosphere at 1300°C for 3 hours to oxidize the surface. Thereafter, the material was compacted using Ar gas in a HIP device at 1450°C for 1 hour and 2000Kg/cm ² . The surface of the obtained final sintered body is covered with a transparent glassy film, and the density of the sintered body after removing the glassy substance is 99.5%.
The average particle size was 2-3 microns. The bending strength was measured to be 70Kg/mm ² at room temperature (25°C) and 68Kg/mm ² at 1200°C, showing good strength properties.

Example 2 Si ₃ N ₄ powder (average particle size 0.3μ) 98% by weight, Y ₂ O ₃ 1% by weight, prepared by plasma dissolution spraying.
Paraffin was added to 1% by weight of Al ₂ O ₃ and cold pressed at a pressure of 2 t/cm ² . After deparaffinization in a H ₂ stream at 1000°C for 1 hr, it was baked at 1700°C for 1 hr under a partial pressure of N ₂ of 0.2 atm. A sintered body with a porosity of 5% was obtained. The surface of this sintered body was oxidized at 1200°C x 1/2 hr in an oxygen gas stream (O ₂ partial pressure ≒ 1 atm), and then at 1600°C x 1/2 hr.
HIP treatment was performed at 700 atm for 2 hours. The surface of the final sintered body obtained was covered with a glassy film. When the glass coating was removed with hydrofluoric acid and the density and bending strength of the sintered body were measured, the density was 99.8% and the bending strength was 85.
Kg/mm ² (room temperature). The average grain size was as small as 1.8μ, and it was found that this sintered body was a silicon nitride sintered body with unprecedented fine grains and high density.

Claims (1)

[Claims] 1 Si ₃ N ₄ powder or Si ₃ N ₄ and Al ₂ O ₃ , Y ₂ O ₃ ,
After cold-pressing the powder containing one or more types of MgO, it is sintered under ordinary pressure or pre-fired by hot pressing, and then oxidized in an oxidizing atmosphere to form a SiO ₂ film on the surface of the pre-fired product. 1. A method for producing high-density silicon nitride parts, which is characterized in that the coated silicon nitride parts are re-fired under pressure in a hot isostatic pressure device using an inert gas as a pressure medium.