JPH02107567A - Production of high-density silicon nitride sintered body - Google Patents

Abstract

PURPOSE:To obtain the title high-density silicon nitride sintered body wherein the pinholes deteriorating the high-temp. characteristic are not left even when the sintered body is secondarily calcined at a high gas pressure by preforming silicon nitride and a sintering assistant, and secondarily calcining the primarily calcined presintered body at specified temp. and pressure. CONSTITUTION:The silicon nitride powder and sintering assistant are preformed into a specified shape. The preform is primarily calcined in a gaseous nitrogen atmosphere, and then secondarily calcined in a compressed inert gas atmosphere to produce a densified silicon nitride sintered body. The secondary calcination is carried out by the following schedule. (1) The preform is heated to the temp. at which a liq. phase is formed in the sintered body at <=300atm, and (2) heated at a temp. higher than the temp. at which the liq. phase is formed while gradually increasing the pressure from 300atm to a specified pressure. In this production process, the condition that an open cell is not formed in the primarily sintered body must be fulfilled. When the open cell is formed, pinholes are left remaining even by the above-mentioned schedule.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for manufacturing a high-density silicon nitride sintered body, and more particularly to a manufacturing method for obtaining a sintered body in which no pinholes remain inside.

<Prior Art> In recent years, there has been active development of equipment that operates at high temperatures, such as high-temperature gas turbines and diesel engines, with the aim of improving thermal efficiency, saving fuel, reducing pollution, and reducing weight. Among ceramic materials, silicon nitride is attracting attention as a material used in these devices used at high temperatures.

This silicon nitride (Si3Ng) has excellent physical and chemical properties, but it has been difficult to densify it by blowing. Methods for manufacturing such a Si3N4 sintered body include: (1) gas pressure sintering method; (2) sintering aid (8β201+ MgO);

There are four methods, including a normal pressure sintering method in which sintering is performed under atmospheric pressure by adding YzOz (a sintering aid such as a rare earth oxide), (1) a hot press method, and (2) a hot isostatic press method.

Among these methods, the hot press method produces a sintered body with relatively high density and high strength, but it is difficult to obtain a sintered body with a complicated shape and costs are inevitably increased.

The pressureless sintering method is a method of sintering Si, N, and powder that has been formed into an arbitrary shape in advance, so it is easy to manufacture products with complex shapes, but it requires a relatively large amount of sintering aid, so As the glass content of the glass increases, it is difficult to obtain a sintered body with excellent high-temperature properties. In this respect, the gas pressure sintering method and the hot isostatic pressing method can be densified with a small amount of sintering aid, and 5iJ
This method is useful as a method for producing 4 sintered bodies.

An example is Japanese Patent Publication No. 61-46430, which outlines that SiJ powder is formed into a predetermined shape, pre-sintered to a relative density of 92% or more to close the pores, and then
This is a method of secondary sintering using hot isostatic pressing (HIP) at a temperature of 500°C or higher and a nitrogen partial pressure of 500 atm or higher, and the resulting sintered body has a density of 98% or higher, but the internal A sintered body in which pinholes have not disappeared is obtained, and there is no mention of the relationship between the refinement of the temperature and pressure schedule during secondary firing. In addition, the density of the preliminary sintered body is 97
% or less, it is difficult to obtain a pinhole-free sintered body using the above method.

<Problems to be Solved by the Invention> In the above sintering method, the present inventors obtained a sintered body with a relative density of 98.4χ by performing secondary firing under a nitrogen gas pressure of 11,000 atm. 2500℃ 2 hours in atmosphere
When the heat treatment was carried out under the conditions of r, cranks were generated inside the sintered body, which is presumed to be caused by the generation of excessive tensile stress. This is because the gas used as a pressure medium (
It is presumed that this is because nitrogen gas (such as nitrogen gas) entered the sintered body and was trapped under high pressure in the pinholes remaining in the sintered body.

In other words, if high-pressure atmospheric gas is trapped in the pinholes remaining inside the sintered body after secondary firing under high gas pressure, its properties will inevitably deteriorate when used as a high-temperature structural material. It means that.

In view of this situation, the present invention has been developed to
The object of the present invention is to provide a sintering method for obtaining a sintered body that does not have pinholes that cause deterioration of properties at high temperatures even after subsequent firing.

<Means for Solving the Problems> The present inventors have developed a sintered body that does not leave internal pinholes that would cause the aforementioned problems in a firing method in which a preform is subjected to secondary firing under a gas pressure of 300 atm or more. After careful consideration of the sintering method used to obtain this material, it was found that the temperature and pressure schedule of the secondary firing had a large effect on whether or not pinholes remained.

Based on this knowledge, the present invention was able to solve this problem by the following means.

Preform silicon nitride powder and sintering aid into a predetermined shape,
A schedule for primary firing in an N2 atmosphere and then secondary firing in a pressurized atmosphere of an inert gas such as N2 or A7,
(a) The temperature up to the liquid phase formation temperature in the sintered body is 300a.
A method for producing a high-density silicon nitride sintered body by pressurizing and heating at a temperature below tm and increasing the pressure to a predetermined pressure exceeding 300 atm below the liquid phase formation temperature in the (Tl) sintered body. It is.

In the present invention, it is necessary that there be no open pores in the primary sintered body, and if there are open pores, pinholes will remain even if the above schedule is followed.

The presence or absence of open pores in the primary sintered body can be determined by the presence or absence of mercury intrusion at an injection pressure higher than atmospheric pressure using the mercury intrusion method, and if mercury intrusion is not observed, open pores are not present. First, if mercury intrusion is observed, it can be determined that open pores exist.

Here, the liquid phase generation temperature in the sintered body can be determined as the temperature at which rapid firing shrinkage begins when the molded body is heated.

However, since this method causes a slight error in the temperature at which the liquid phase formation temperature is determined, the pressure increase is preferably performed at a temperature higher than this temperature by 30° C. or more, more preferably at a temperature higher than this temperature by 50° C. or more.

<Function> As mentioned above, the presence or absence of open pores in the primary sintered body is important in order to obtain a high-density sintered body. That is,
If there are no open pores, it is generally unlikely that atmospheric gas during secondary sintering will enter the sintered body. However, in gas pressure sintering, the pressure of the atmospheric gas is increased to perform firing.
It is thought that gas pressure acts on the sintered body as an external force and is influenced by this. The inventor studied this point and found that if the pressure is increased to over 300 atm at a temperature where no liquid phase is generated in the primary sintered body, even if the primary sintered body has no open pores, It was found that pinholes remained after the subsequent sintering, and that the gas used as the pressure medium existed at high pressure in the pinholes.

However, the liquid phase formation temperature is 300a tm or less, and 3
It was found that no pinholes were present in the secondary sintered body when the pressure was raised to over 00 atm at a temperature higher than the liquid phase formation temperature.

The reason for this is thought to be the difference in the state of the grain boundary phase in the primary sintered body. Above the liquid phase formation temperature, the grain boundary phase is in a liquid phase and is sufficiently wet with the silicon nitride grains, so even if gas pressure acts and shear force is generated between the two, there is no gas between them. No penetrable voids are created.

At temperatures where no liquid phase is generated, silicon nitride grains and the grain boundary phase are insufficiently wetted, and when shear force from pressurization acts between them, tiny voids are created between them through which gas can enter. As a result, the gas used as a pressure medium enters the primary sintered body and is trapped under high pressure, causing cranking due to heat treatment. However, the pressure is 30
When the temperature is 0 atm or less, there is little effect as an external force, and even if the temperature is such that no liquid phase is generated, no voids are created in the sintered body through which gas can enter.

Therefore, by using a schedule in which the gas pressure is maintained at 300 atm or less at temperatures where no liquid phase is generated in the primary sintered body, and gradually increased to a predetermined pressure above 300 atm at temperatures where liquid phases are generated, The sintered body can be fired under high pressure without allowing pressure medium gas to enter the sintered body, and as a result, a sintered body with no remaining pinholes can be obtained.

<Example> 5iJn powder 9 with a specific surface area of 12rn''/g and an α rate of 92%
3 layers% and a specific surface area of 8 m/g to 2□0. Powder triplicate %
, 3% by weight of Y2O3 powder with a specific surface area of 10 m/g, and 1% by weight of Mgo powder with a specific surface area of 20 m1B were mixed in ethanol, then dried and granulated.

After pressing this powder into a mold of tox 5 x 30m, it was heated to 1500.1550.16 in a latm N2 atmosphere.
Each sample was held at 0.001650°C for 2 hours to obtain four types of preliminary sintered bodies shown in the table.

The presence or absence of open pores in this pre-sintered body was measured by mercury intrusion method. According to this, the intrusion of mercury was observed in the material with a density of 88.7%, and the porosity was 4.99%.

Those with a density of 91.1 to 97.8% are 3000% below atmospheric pressure.
Even at a pressure of 0 psi, no intrusion was observed and no open pores were present. Also, the relationship between firing temperature and shrinkage is 8×8×
The linear shrinkage rate in the longitudinal direction of the molded product of No. 4011 was measured. The results are shown in FIG. According to FIG. 1, the liquid phase formation temperature (temperature at which contraction begins rapidly) of this sample was found to be approximately 1370°C. Next, based on this data, 2000a was calculated using the four schedules shown in Figure 2.
Secondary firing was performed at tm -1800°C for 2 hours. T represents the temperature during secondary firing, P represents the pressure, and the horizontal axis represents the time. FIGS. 2A to 2C show comparative examples, and FIG. 2D shows the present invention.

Note that the pressure medium gas is nitrogen (N2) gas.

The resulting secondary sintered body was examined for density, internal pinholes, and the presence or absence of cranks inside the sintered body after being treated at 1,500° C. for 2 hours in a 1 atmNz atmosphere.

The results are shown in the table.

<Effects of the Invention> According to the present invention, if the primary sintered body has no open pores, even if the density of the primary sintered body is 97% or less, it will be completely densified by secondary firing, and A stable sintered body with no change in the sintered body was obtained.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the thermal shrinkage rate and firing temperature of a silicon nitride molded body. FIG. 2 is a graph showing a schedule of temperature T and pressure P during secondary firing.

Claims (1)

[Claims] Preforming silicon nitride powder and a sintering aid into a predetermined shape,
In the method for manufacturing a silicon nitride sintered body, which is first fired in a nitrogen (N_2) gas atmosphere and then secondarily fired in a pressurized inert gas atmosphere to become dense, the second firing is (a) sintering. The temperature up to the liquid phase formation temperature in the compact is 300at.
(b) 300a above the liquid phase formation temperature in the sintered body.
A method for manufacturing a high-density silicon nitride sintered body, which involves gradually increasing the pressure beyond tm to a predetermined pressure and heating under pressure.