JPH0421570A - Sialon sintered compact and production thereof - Google Patents

Abstract

PURPOSE:To obtain the title sintered compact excellent in mechanical strength and fracture toughness by phase transition of an alpha-type silicon nitride powder- contg. stock powder into beta-type under specified conditions along with making needle crystal growth followed by sintering at the temperature for SIALON synthesis. CONSTITUTION:A stock powder containing an alpha-type silicon nitride powder and satisfying SIALON composition is held at the alpha-beta transition temperature for a specified time to effect phase transition of the alpha-type to beta-type along with making the needle crystal growth of the beta-type silicon nitride. The resulting powder is then sintered at the temperature for SIALON synthesis, thus giving the objective SIALON sintered compact containing needle crystal grains 3-100mum in means size and (1:3)-(1:20) in aspect ratio. In the present sintered compact, alpha-type-derived granular crystal grains and beta-type-derived needle crystal grains exist in a mixed state.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a sialon sintered body having excellent mechanical strength and fracture toughness, and a method for producing the same.

(Prior art) Sialon whose main constituent element is 5i-AI-0-N is
It has a small coefficient of thermal expansion and has excellent heat resistance, oxidation resistance, and corrosion resistance, and is applied to various high-strength heat-resistant parts and high-temperature corrosion-resistant parts, including engine parts, along with Si3N4 sintered materials and SiC sintered materials. An attempt is being made to do so.

Such sialons include α-type sialon, which has the same structure as α-type Si3N+, and β-type sialon, which has the same structure as β-type Si3N4.
) type Sialon is manufactured as follows.

In other words, the sialon sintered body has higher sinterability than the Si3N4 sintered body, so the β-type sialon has a higher sinterability than Si3N4 sintered body.
N4-AIN-AI203, Si3N4-A
IN-3jO;+, Si3Ns-AIN-A
Using a predetermined amount of mixed powder such as I203-8iO,+ as a starting material, a molded body is produced by a press molding method, etc., and this molded body is subjected to normal pressure sintering or atmospheric pressure sintering in an inert gas atmosphere. It is manufactured by applying

As described above, β-type sialon sintered bodies have relatively high sinterability, so dense sintered bodies can be easily obtained by normal pressure sintering method or atmosphere pressure sintering method, and they are also acid resistant. It has excellent properties such as excellent corrosion resistance and corrosion resistance, and there is almost no decrease in mechanical strength even at high temperatures exceeding 1400°C. At present, the absolute value of the mechanical strength of the compact itself is inferior to that of Si3N4 sintered compacts and SiC sintered compacts, and the fracture toughness value has not yet reached a fully satisfactory value.

Therefore, as an attempt to improve the mechanical strength and fracture toughness of such β-type sialon sintered bodies, we have added ceramic whiskers and other materials to reinforce them with fibrous materials, and the like Attempts have been made to add additives such as rare earth oxides to densification through liquid phase sintering.

(Problem to be solved by the invention) However, among the above-mentioned strength improvement methods, for example, adding ceramic whiskers has poor compatibility with β-sialon powder depending on the type of whiskers, resulting in a decrease in sinterability and a problem in obtaining good results. The density of the resulting sintered body decreases, and sufficient improvement effects such as strength and thermal shock resistance cannot be obtained. Furthermore, the addition of rare earth oxides and the like has the problem of deteriorating high-temperature properties such as oxidation resistance and strength at high temperatures.

On the other hand, the hot press method produces products that are somewhat dense and have excellent mechanical strength, but the hot press method is limited in the shapes that can be obtained and cannot take full advantage of its relatively high sinterability. There were also problems such as high manufacturing costs.

The present invention was made to address such problems, and an object of the present invention is to provide a sialon sintered body having higher toughness and a method for manufacturing the same.

[Structure of the Invention] (Means for Solving the Problems) The sialon sintered body of the present invention has an average grain size of 3 μm or more and 100 μm or less as a whole of the sialon sintered body, and an aspect ratio of 1:3 to 1. It is characterized by containing acicular crystal grains in the range of :20.

Further, the method for producing a sialon sintered body of the present invention uses α-type silicon nitride powder as a starting material for the silicon nitride component,
A raw material powder containing this α-type silicon nitride powder is prepared so as to satisfy the sialon composition, and this raw material powder is held at the α-β transition temperature for a predetermined period of time to convert a part of the α-type silicon nitride into the β-type. While undergoing a phase transition to silicon nitride, this β-type silicon nitride is grown into needle-shaped crystals, and then the starting material is sintered at a sialon synthesis temperature to form granular crystals originating from the particle shape of the α-type silicon nitride powder. It is characterized in that grains and acicular crystal grains derived from the shape of the β-type silicon nitride are mixed together.

Further, the second manufacturing method of the present invention uses an αβ mixed powder obtained by mixing an α-type silicon nitride powder and a β-type silicon nitride powder with acicular crystals as a starting material for the silicon nitride component, and contains this αβ mixed powder. Then, a raw material powder is prepared to satisfy the Sialon composition, and the raw material powder is prepared to contain four elements of aluminum, oxygen, and nitrogen, and this raw material powder is sintered at the Sialon synthesis temperature to form the α-type silicon nitride. It is characterized in that granular crystal grains originating from the particle shape of the powder and acicular crystal grains originating from the shape of the β-type silicon nitride are mixed together.

In the sialon sintered body of the present invention, the acicular crystal grains contained in the sintered body hinder the development of cracks due to their elongated shape, and the average grain size of the sintered body as a whole including these acicular crystals The diameter is 3μ1 or more and 100μm or less.

Here, the average grain size is the average value of the width of each grain on the cut line in a randomly selected cross section of the sialon sintered body.

If the average grain size is less than the 3 μm mark, the crystal grains are too fine and it will not be possible to bend the growing crack away from the straight direction, and if it exceeds 100 μm, the crystal grains will be too large and will easily become the starting point of fracture. This is because it causes a decrease in strength.

Further, the aspect ratio of the acicular crystal grains contained in such a sialon sintered body is in the range of 1=3 to 1:20.

If the aspect ratio is less than 1:3, the elongated shape will not be fully utilized and the effect of inhibiting crack growth will not be exhibited. On the other hand, if the aspect ratio exceeds 1.20, the crystal grains will be too long and thin, so it will be impossible to crack or cut the crystal grains and move straight ahead in the knitting direction.

A sialon sintered body containing such acicular crystal grains can be obtained by the manufacturing method of the present invention.

The first manufacturing method is to use α-type silicon nitride powder as a starting material for the silicon nitride component side, and mix other predetermined components such as aluminum oxide as a raw material powder for sialon synthesis.
This raw material powder is held at the α-β transition temperature for a predetermined time to
This is a method in which a part of type silicon nitride undergoes a phase transition to β-type silicon nitride, and at the same time, this β-type silicon nitride is grown into needle-shaped crystals. By changing the time in the range of 0.2 to 10 hours, the average grain size and aspect ratio of the obtained sialon sintered bodies are controlled.

The α-type silicon nitride powder used here has an average powder particle size of 0
．． It is preferably about 2 to 1.5 μD.

This powder particle size is the particle size as a powder, and was measured using a microtrack measuring device.

That is, the average particle size in the raw material powder and the average particle size in the Sialon sintered body described above are different in meaning.

In the second method for producing a ceramic sintered body of the present invention, an αβ mixed powder obtained by mixing an α-type silicon nitride powder and a β-type silicon nitride powder having acicular crystals is used as a starting material as a silicon nitride component.

Among these starting materials, α-type silicon nitride is not acicular and is a granular crystal with an aspect ratio of approximately 1:1, and a mixed powder of α-type silicon nitride and acicular β-type silicon nitride is mixed with aluminum oxide. By sintering it with other components such as, β'-type sialon containing a mixture of granular crystals and needle-like crystals is obtained.

In this method, the α-type component in the starting material is sintered by direct heating to the sialon synthesis temperature without being specially held at the α-β transition temperature, so that the α-type component in the starting material is converted into the β-type component without changing its granular shape. After passing through silicon nitride, it remains as granular crystals in the β' sialon, and since the other β-type component uses twisted needle crystals, it remains in this needle shape as a needle crystal in the β' sialon.

The α-type silicon nitride powder used here as a starting material preferably has an average particle size of about 0.2 to 15 μm, and the acicular β-type silicon nitride powder preferably has an average particle size of about 0.5 to 3.5 μm.

This average particle diameter was determined by the same method as the average particle diameter of the starting material in the first production method described above.

The α-type:β-type ratio of the starting material is preferably about 99.1=595, and the acicular shape of the β-type silicon nitride powder preferably has an aspect ratio of 1:3 to 1.20.

If the aspect ratio is less than 1 to 3, the elongated shape will not be fully utilized and the effect of inhibiting crack growth will not be exhibited. On the other hand, when the aspect ratio exceeds 1=20, the crystal grains are too long and slender, so the crystal grains are cut through cracks and travel straight, and cannot be released laterally.

Using such starting materials, this mixed powder is molded into a desired shape by a known molding method such as a press molding method or a slip casting method.

Then, in an inert gas atmosphere, at 1700-1800°C
Bake at a temperature of about

At this time, sintering and sialon synthesis of one of the starting raw materials, α-type silicon nitride powder, proceed simultaneously and transform into β-type without much change in particle shape, and β-type nitride of the other acicular crystals. Silicon is sintered while leaving its needle-like shape intact, resulting in a β'-type sialon with a structure in which needle-like crystals are intertwined.

Note that ceramic sintered bodies that are dense, have excellent mechanical strength, and have high fracture toughness values can also be obtained by the normal pressure sintering method or the atmospheric pressure sintering method, but these sintering methods are limited. Instead, a hot press molding method, a hot isostatic sintering method (HIP), or the like may be applied.

The ceramic sintered body of the present invention obtained in step 2 becomes a β'-type sialon in which spherical products and needle-like crystals are mixed.

(Function) According to the present invention, the Sialon sintered body has an acicular shape in which the average grain size of the sintered body as a whole is 3 μm or more and 100 μm or less, and the aspect ratio is in the range of 1 = 3 to 1.20. contains crystal grains.

Here, the acicular crystal grains form an entangled structure, and the cracks propagating in the sintered body are bent from the direction of propagation to a different direction along the twisting grain boundaries of the crystal grains, thereby preventing the propagation of the cracks. hinder.

In other words, since conventional Sialon has fine crystal grains of about 2 μm, cracks that once occurred would propagate straight through the sintered body and destroy the sintered body. In a sintered body, the presence of needle-shaped crystal grains makes the shape of the crystal grains in the sintered body irregular, forming an intertwined structure to improve mechanical strength, and depending on the length of the crystal grains, a crack propagation path is created. Either way, they are blocked and have no choice but to flee to the side.

Therefore, the development of cracks can be stopped midway, and toughness can be improved.

(Example) Next, examples of the present invention will be described.

Examples 1 to 4 Silicon nitride powder with an average particle size of 0.4 μm and aluminum oxide powder IO with an average particle size of 0.9 μm by weight
and 24 hours using polmir using ethanol as a dispersion medium.
A raw material powder was prepared by mixing for a period of time.

Next, a predetermined amount of binder was added to 100 parts by weight of this raw material powder, and a length of 5 was added under a molding pressure of about 1000 kg/cJ.
A rod-shaped molded body having a width of 0III1×width of 50 mmΔ and a thickness of 5a++n was produced.

Next, this molded body was heated to 700°C in a nitrogen atmosphere to degrease it, and then held at 1700°C in a nitrogen gas atmosphere for varying times from 1 hour to 10 hours, followed by sialon synthesis. Various Sialon sintered bodies were produced by sintering at different temperatures, as shown in Table 1, which differed in the average grain size of the sintered body as a whole and the average aspect ratio of the needle-like crystals in the sintered body.

Note that the average grain size of the sintered body as a whole was determined by randomly cutting two parts of the produced sintered body and observing it with a microscope at a magnification of 1000 times.

Further, the average aspect ratio was determined by calculating the average ratio of the short axis to the long axis of 50 particles randomly selected on the photograph.

After mirror-finishing the sialon sintered body thus obtained, the fracture toughness value was measured. These results are shown in Table 1.

Comparative Examples 1 to 2 Using the raw material powder used in Example 1, firing was performed for 0.1 hour and 15 hours at the same holding temperature as in Example 1, and Sialon firing was performed with different average particle diameters and aspect ratios. A concretion was produced.

Then, the fracture toughness value was measured under the same conditions as in Example 1.

The results are shown in Table 1 together with the results of Examples.

The surface fracture toughness value was measured by the microindentation method, and the unit is MPaJi.

Further, the unit of the average particle diameter is μm.

As is clear from the results in Table 1, the sialon sintered bodies of the examples contain acicular crystal grains that have the most effective average grain size for preventing crack propagation and have a predetermined aspect ratio. By doing so, we were able to significantly improve toughness.

Examples 5 to 8 Next, examples of a method for producing a sialon sintered body using a mixed powder of α-type silicon nitride and β-type silicon nitride as the silicon nitride component will be described.

First, α-type silicon nitride powder and an aspect ratio of I:20 are used.
A starting material was prepared by mixing the following acicular β-type silicon nitride in the proportions shown in Table 2.

Furthermore, the general formula of β-sialon expressed by the following formula: Si
At ON (Z-0~4,2)s-z
A raw material powder was prepared by adding At2o3 and ^IN so that the odor was Z-1.

Sialon sintered bodies were produced using such raw material powders under the same conditions as in Example 1.

Regarding these sialon sintered bodies, the fracture toughness values were measured in the same manner as in Example], and the bending strength was also examined. The results are shown in Table 2.

Comparative Examples 3 to 6 Starting materials were prepared by mixing α-type silicon nitride powder and acicular β-type silicon nitride in the proportions shown in Table 2.

Further, the aspect ratios of the acicular β-type silicon nitride used here are as shown in Table 2.

Regarding these sialon sintered bodies, the fracture toughness values were measured in the same manner as in Example 1, and the bending strength was also examined. The results are shown in Table 2.

(Left below) As is clear from the results in Table 2, when the amount of β-type silicon nitride powder mixed is small, as in the comparative example, or when the aspect ratio of the mixed β-type silicon nitride is not an appropriate value. However, the strength and toughness of the Sialon sintered body are not improved.

In contrast, the sialon sintered body produced by the method of the example had improved both strength and toughness due to the entanglement of particles within the sintered body.

[Effects of the Invention] As explained above, according to the present invention, acicular crystal particles of a predetermined size are present in the sialon sintered body and form an entangled structure, allowing cracks to escape. Major developments can be prevented.

Therefore, it is possible to obtain a sialon sintered body of higher quality with excellent mechanical strength and fracture toughness values.

Applicant: Toshiba Corporation

Claims (3)

[Claims] (1) The average grain size of the sialon sintered body as a whole is 3 μm
A Sialon sintered body characterized by containing acicular crystal grains having a diameter of 100 μm or less and an aspect ratio of 1:3 to 1:20. (2) Using α-type silicon nitride powder as a starting material for the silicon nitride component, preparing a raw material powder that contains this α-type silicon nitride powder and satisfying the SiAlON composition, and heating this raw material powder at the α-β transition temperature. Hold it for a specified period of time,
Part of the α-type silicon nitride undergoes a phase transition to β-type silicon nitride, and the β-type silicon nitride is grown into needle-shaped crystals, and then the raw material powder is sintered at a sialon synthesis temperature, and the α-type silicon nitride is sintered at a sialon synthesis temperature. A method for producing a Sialon sintered body, comprising mixing granular crystal grains derived from the particle shape of silicon nitride powder and acicular crystal grains derived from the shape of the β-type silicon nitride. (3) As a starting material for the silicon nitride component, an αβ mixed powder made by mixing α-type silicon nitride powder and β-type silicon nitride powder with acicular crystals is used, and the raw material is made to contain this αβ mixed powder and satisfy the SiAlON composition. A powder is prepared, and this raw material powder is sintered at a sialon synthesis temperature to form granular crystal grains originating from the particle shape of the α-type silicon nitride powder and acicular crystal grains originating from the shape of the β-type silicon nitride powder. A method for producing a sialon sintered body, characterized by mixing the sialon sintered bodies.