JPS62223067A - Manufacture of silicon nitride base sintered body - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a method for manufacturing a silicon nitride sintered body, and more particularly, to a method for manufacturing a silicon nitride sintered body that has high strength at high temperatures and excellent creep resistance. .

(Prior art and its problems) Sintered bodies made of silicon nitride mainly consist of covalent bonds, and have excellent properties in terms of strength, hardness, and thermal and chemical stability, so they are used as engineering ceramics, especially , and its application to heat engines such as gas turbines is progressing.

In recent years, as heat engines have become more efficient, the operating temperature of heat engines can reach 1400° C. or higher, and materials that can be used under these conditions are desired. BACKGROUND ART In addition to hot press methods and atmospheric pressure sintering methods, gas pressure sintering methods have been studied in recent years as sintering methods for producing silicon nitride.

These methods aim to cause the added composition to remain at grain boundaries as a glass phase or crystalline phase after sintering, or to remain as a solid solution in the crystalline phase of silicon nitride, without being decomposed.

If the composition added in this manner remains in the sintered body, the excellent properties of high temperature strength and creep resistance due to the unique coexistence bonding properties of silicon nitride will be suppressed. That is, when a sintering aid mainly of oxide type is added, the aid reacts with silicon nitride, and a metal compound mainly composed of silicon acid and nitride is generated at the grain boundaries. Such acids and nitrides promote sintering densification and improve sinterability during sintering, but because they themselves have strong ionic bonding properties, they tend to deteriorate the properties of the sintered body at high temperatures. be.

In response to this tendency, by using Zr0z as a sintering aid and precipitating ZrO□ in the grain boundary phase of the sintered body, we have utilized the high melting point of ZrO2 to obtain a sintered body with excellent high-temperature properties. Attempts are being made to do so. However, in producing such a sintered body, hot pressing method,
In any of the firing methods, such as the non-pressure firing method and the gas pressure firing method, ZrO reacts with Si and N4, and ZrN is generated in the sintered body.

This ZrN undergoes a volumetric expansion of about 30χ during the reaction process from ZrN to ZrO in a high-temperature oxidizing atmosphere, so cranks tend to occur in the sintered body during oxidation (the sintered body itself containing ZrN is susceptible to high-temperature oxidation). In the atmosphere, it was extremely easy to oxidize, and its properties were insufficient as a material for high temperatures.

(Means for solving the problem) The present inventor conducted research on the above problem and found that when firing a system containing ZrO□ in silicon nitride, a metal Si/SiO2 mixture was placed in a nitrogen atmosphere. However, by controlling the atmosphere, it is possible to suppress the generation of ZrN in the sintered body, thereby preventing ZrO from entering the grain boundary phase.
It was discovered that the precipitation of □ can be promoted and a silicon nitride sintered body with excellent high-temperature strength can be obtained.

That is, in the present invention, after molding a mixed powder consisting of 1 to 20% by weight of ZrO□, 1 to 10% by weight of Y2O:l, and the balance mainly consisting of silicon nitride, an atmosphere is A method for manufacturing a silicon nitride sintered body is provided, which is characterized by firing at a firing temperature of 1700 to 2000°C in a controlled nitrogen atmosphere.

The present invention will be explained in detail below.

In a system containing ZrO2 as a sintering aid for silicon nitride, when firing is performed based on a known firing method such as hot press method, non-pressure firing method, gas pressure firing method, etc., ZrO□
partially causes a reversible reaction with silicon nitride as shown in equation (1) below.

4Si3N4+6Zr(h:6ZrN+12SiOi
+5N2 ↑...(1) Through this reaction, ZrN is generated in the sintered body, and SiO gas and N2
Gas is generated. When the generated ZrN is exposed as a sintered body to a high-temperature oxidizing atmosphere, it is oxidized to ZrO□, which causes volume expansion. This volumetric expansion tends to cause cranking in the sintered body, which causes a decrease in high-temperature strength. Therefore, ZrN should not be present in the sintered body.

The manufacturing method of the present invention allows the reaction (1) described above to proceed in the right direction by setting the atmosphere during firing in a system containing Zr0t to a mixed atmosphere of SiO and N2 controlled at a constant pressure. It is an attempt to control the

According to the production method of the present invention, first 1 to 20% by weight, preferably 5'h to 10% by weight of Zr0z and 1 to 10% by weight are added.
A mixed powder is prepared containing Y2O3 in a weight percent, preferably 1 to 5 weight percent, and the balance mainly consisting of silicon nitride.

The mixed powder is molded into a desired shape by a known molding method such as press molding, casting molding, extrusion molding, injection molding, etc., and then transferred to a firing process.

Next, the firing process will be explained with reference to FIG. 1, using a gas pressure firing method as an example.

When the obtained molded body 1 is placed in a firing furnace 2, a mixture 3 made of metal Si and 5i02 is placed in the firing furnace 2 at the same time. Thereafter, the temperature of the firing furnace 2 is raised by the heater 4, and the firing temperature is maintained at 1700 to 2000°C. At this time, N2 gas is introduced into the firing furnace 2. The N2 gas pressure is set to be higher than the equilibrium nitrogen pressure of silicon nitride at the firing temperature, thereby suppressing the decomposition of silicon nitride. At this time, the following reaction (2) progresses in the mixture 3 placed in the firing furnace 2 in advance.

St + 5iOz::2SiO↑ (2) SiO gas is generated by this reaction (2), and the atmosphere is controlled to the equilibrium SiO pressure at the firing temperature of reaction (2).

By performing the firing in a controlled atmosphere in this manner, the progress of the reaction (1) described above can be suppressed and the generation of ZrN can be prevented.

This means that the equilibrium SiO gas pressure in reaction (2) at the Nt gas pressure and calcination temperature is the same as in reaction (1).
It is presumed that this is because the z gas pressure is higher than the SiO gas pressure.

Since the reaction of the mixture of metal St and Sin used in the firing process proceeds with the same molar ratio as shown in (2), it is desirable that the mixture is Si/5iOz #1 (molar ratio), and SiO If the amount of □ is large, a large amount of oxygen will be generated in the process of reaction (2), which is not preferable.

On the other hand, when the amount of Si is large, the generation of StO is small, and the reaction (1
) cannot be prevented from moving to the right.

According to the present invention, in addition to the gas pressure firing method described above, it is also possible to apply a non-pressure firing method, a hot isostatic pressing method, and a hot pressing method.

In addition, when the composition in the present invention is outside the above-mentioned range,
That is, if the amount of ZrO□ is less than 1% by weight, the high temperature strength deteriorates significantly, and if it exceeds 20% by weight, the fracture toughness is poor.

On the other hand, if the amount of v203 is less than 1 ff 11%, the sinterability will decrease, and if it exceeds 10% by weight, the oxidation resistance will decrease.

In addition to adding ZrO□ and YzOz as powders, it is also possible to add a powder in which Yz(h) is dissolved in ZrO in advance to form stabilized or partially stabilized zirconia.

In the silicon nitride sintered body obtained by the manufacturing method of the present invention, ZrN is not generated during the firing process, and ZrO□
is precipitated as a crystalline phase in the grain boundary phase of the sintered body. This crystal phase is Y2O, partially stabilized zirconia (PSZ) in solid solution state.
Alternatively, it is stabilized zirconia and has a very high melting point, so even if it is used in a high-temperature oxidizing atmosphere, there will be no strength deterioration due to softening of the grain boundary phase, and the oxidized surface is 5iO
Since a dense film of z is formed, oxidation weight gain is also small.

The invention is illustrated by the following example.

Examples After molding a mixed powder having the composition shown in Table 1, it was fired under the firing conditions shown in Table 1. The obtained sintered body is J
According to the 4-point bending method of ISR1601, room temperature, 200℃,
The strength at 1400°C was measured. In addition, the weight gain due to oxidation at 1400°C was achieved by placing the test piece in the atmosphere for 24 hours at 1400°C.
It was evaluated by the increase in weight per unit area of 1'i.

Note that the metal Si/5int mixture is installed within a range of about ±100g relative to the weight of the molded body, and in this example, each
0g was set.

As is clear from Table 1, Si: +N4-ZrOz-Y
If the sintering of the ZOz system is performed simply in a nitrogen atmosphere or an ^r atmosphere (from 隘7 to Nll0), the reaction between 5iJ4 and ZrOz will proceed, ZrN will be produced, and oxidation will be severe, and the bending strength of 1400°C will eventually decrease. Also 20Kg/c
It was less than 4.

In contrast to these comparative examples, the samples of the present invention (numbers 1 to 5)
and l1hll to l1hll to 14) all exhibit high strength at high temperatures, and the decrease in bending strength is small compared to the bending strength at room temperature, that is, they have excellent creep resistance.

Moreover, it has excellent oxidation resistance, with an oxidation weight gain of 0.1mg/cf.
It was below fl.

However, from a compositional standpoint, if the amount of Y2O exceeds 10% by weight (11 kL6), the creep resistance and oxidation resistance tend to decrease.

(Effect of the invention) As described above, the manufacturing method of the present invention
When firing an O2+YtO* composition, a mixture of metal Si/5iOz is placed in a firing furnace and fired in N2, resulting in a silicon nitride with high strength at high temperatures and excellent creep resistance without producing ZrN. A high-quality sintered body can be obtained, and it can be applied to heat engines such as turbines as well as glow plugs used in high-temperature atmospheres.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the firing step of the manufacturing method of the present invention. 1... Molded body 2... Firing furnace 3... Mixture 4... Heater

Claims (1)

[Claims] 1 to 20% by weight ZrO_2 and 1 to 10% by weight
After molding a mixed powder consisting of Y_2O_3 and the remainder mainly silicon nitride, the mixture of metal Si and SiO_2 is heated to
A method for producing a silicon nitride sintered body, the method comprising firing at a firing temperature of 0°C.