JPH04321562A - Sintered silicon nitride - Google Patents

Abstract

PURPOSE:To obtain a sintered silicon nitride having a strength higher than that of fiber-reinforced sintered material by mixing a sintering assistant such as rare-earth metal to Al2O3 etc., and sintering the mixture to get a composition falling within a prescribed range. CONSTITUTION:Si3N4 raw material powder produced by imide decomposition process, etc., is mixed with 5-15wt.% of a sintering assistant such as Y2O3 and Al2O3 and the mixture is sintered to obtain the objective sintered silicon nitride having a composition falling within the range encircled by lines connecting the points A, B, C, D and E of the diagram (excluding the composition on the line D-E). The ordinate of the diagram is the weight ratio of the rare-earth metal and lanthanide metal oxide to Al2O3 and AlN in the sintering assistant and the abscissa is the ratio of measured Z value to the theoretical Z value (Z value is a measure of the amount of substituted solid solution of Al and oxygen element in the crystal of beta-silicon aluminum oxynitride [beta'- Si6-zAlzOzN8-z (0<=z<=4.2)] of the obtained sintered material).

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength Si3N4 sintered body, which is attracting attention as a structural ceramic.

0002

2. Description of the Related Art Generally, the strength of ceramics is affected by porosity, crystal grain size, and surface condition. These factors also govern the strength of Si3N4-based sintered bodies, which are attracting attention as structural ceramics.

In an attempt to improve the strength of Si3N4-based sintered bodies, efforts have been made to develop sintering aids and sintering methods. For example, in hot press sintering, Am. Ceram.
Soc. Bull. , 52 (1973) pp560 ~
100 Kg/m2 (bending strength), and ~100 Kg/m2 (bending strength) in pressureless sintering is reported in Proceedings of the 1981 Ceramics Association Annual Meeting, 178 (1981). In both cases, strength is improved by minimizing porosity.

[0004] Also, a method for manufacturing a Si3N4-Y2O3-Al2O3-based silicon nitride sintered body using Y2O3 as the main sintering aid is disclosed in Japanese Patent Publications No. 49-21091 and Japanese Patent Publication No. 48
-38448.

As shown in the relevant patent specification, these Si3N4 in the β crystal lattice have a maintenance-like structure,
Since this is dispersed in the matrix phase, it is thought that strength and toughness can be improved. In other words, this is β
The crystal Si3N4 lattice is hexagonal and it actively utilizes the fact that it grows anisotropically in the C-axis direction, and is particularly published in Special Publication No. 1983-38448 and Ceramics Association Journal Vol. 94, pp. 96.
(1986), fibrous β-crystalline Si3N4
In some cases, crystal grains of 10-odd micrometers or more grow in the longitudinal direction.

On the other hand, in Japanese Patent Publication No. 56-51153, β-phase S
A sintered body containing a so-called Si-Al-O-N lattice in which part of the Si in the i3N4 lattice is replaced by Al and part of the N by O has excellent strength, thermal shock resistance, and oxidation resistance. Discloses that it is a thing. Here, raw materials Si3N4 and Al2
The β-phase silicon aluminum oxynitride lattice is formed by mixing and sintering O3 powders.

[0007]

[Problems to be Solved by the Invention] However, in the above technology, since the growth of fibrous tissue is used as a strengthening mechanism, it is considered to be an effective material strengthening mechanism in itself, but if the growth is not sufficiently controlled, abnormalities may occur. This may involve grain growth or the generation of pores, so it is not necessarily considered effective for improving strength.

In particular, hot press sintering may be used to obtain the fibrous structure as shown in Japanese Patent Publication No. 49-21091, or as described in Ceramic Industry Association Journal Vol. 94, pp. 167 (
This is especially true when the growth of this fibrous structure is actively utilized by adding a β-crystalline Si3N4 raw material that has been heat-treated to grow into a fibrous structure, as shown in (1986).

On the other hand, regarding the β'-sialon structure ceramic material disclosed in Japanese Patent Publication No. 56-51153, various experimental results by the inventors show that the amount of solid solution of Al and O in the β' phase silicon aluminum oxynitride lattice increases. On the other hand, the ratio of long axis grain size/short axis grain size (=aspect ratio) of the crystal grains becomes smaller, the ratio of fibrous structure decreases, and a decrease in nitrogen temperature bending strength and toughness is observed accordingly. It became clear.

[0010] For similar content, see Ziegle.
r, G et al. (Science of Ceramics
vol 12 (1984) pp361) is Al2O
Y in Si3N4 materials with 3 and Y2O3 as auxiliary agents.
This shows that the aspect ratio decreases by increasing the amount of Al2O3 added relative to the amount of 2O3, and is thought to indirectly indicate the findings of the inventors. From the above content, it is considered that in order to improve the strength and toughness of the Si3N4-based sintered body, there is a causal relationship between controlling the morphology of the fibrous Si3N4 crystal grains. We have come to the idea that it is important to optimize the amount of solid solution of Al and O in the crystal lattice.

In the present invention, instead of dispersing long fibrous crystals in the matrix as described above, the particles are fine grains, have a high aspect ratio, are uniform, and contain Al and O elements dissolved in the crystal grains. It has been discovered that a sintered body whose matrix phase is composed of β crystals having a low amount of solid solution has higher strength than the fiber-reinforced sintered body described above.

0012

[Means for solving the problem] As mentioned above, Al and O
It has been found that the structure is formed by precipitating β' crystals with a low amount of dissolved elements within a certain composition range. The composition range is shown in FIG. Here, the horizontal axis shows the ratio between the measured Z value and the theoretical Z value. Here, the Z value is a value corresponding to Z of the β-sialon crystal structure represented by S6-ZAlZOZN8-Z, and indicates the amount of Al and O substituted solid solution. On the other hand, the measured Z value is determined from the X-ray diffraction results of the sintered body, and the theoretical Z value is the value of the added A.
This is the Z value when it is assumed that all l is substituted and dissolved in the Si3N4 lattice. Moreover, the vertical axis of FIG. 1 shows the ratio of Al2O3 added as a raw material to rare earth metals and lanthanide metal oxides in addition weight ratio. In this Figure 1, fine grains with a low solid solution amount of Al and O elements are found in the shaded area surrounded by A, B, C, D, and E (excluding the straight line connecting DE). It has become clear that the matrix phase is composed of highly uniform β-crystals with a high aspect ratio, and that a sintered body with high strength and reliability can be obtained. In order to make the present invention more effective, it is desirable that the following sintering conditions be satisfied.

(1) The Si3N4 raw material powder is manufactured by an imide decomposition method, and 60% or more of the oxygen impurities contained in the raw material powder are present on the surface of the raw material powder. (2) Heat treatment at 1350 to 1650°C for 2 hours or more in an N2 gas atmosphere. (3) After the heat treatment in (2) above, the temperature is 1700℃ or higher19
Final sintering shall be performed in a N2 gas atmosphere at 00°C or less for 2 hours or more.

The conditions (1) to (3) above were derived from the inventors' investigation of the process of substitution solid solution of Al and O in the crystal lattice of Si3N4, which revealed that this process is divided into three processes. This is a condition that has been rejected. i.e.

001
5] (First step) This is the initial step of heat treatment at 1350° C. to 1650° C. in an N2 gas atmosphere for 2 hours or more, during which the added α-Si3N4 transforms into high-temperature type β crystals. Here, the β crystal once replaces Al and O as a solid solution, and the Z value/
Shows a value higher than the theoretical Z value. That is, as is well known, this is a process that involves a crystal transformation from α to β through a liquid phase, with substitutional solid solution of Al and O.

(Second process) This is the latter stage of the first process, and is a process in which the transformed β crystals serve as nuclei and β crystallization progresses. In this process, when 100% β crystallization is completed, the Z value of the sintered body approximately approximates the theoretical Z value.

(Third process) This is a sintering process at 1700° C. or higher for 2 hours or more in an N2 gas atmosphere. During this time, the Z value of the sintered body becomes less than the theoretical Z value, and the β crystal grains become different. Growth of highly oriented crystals with a large aspect ratio is performed. During this period, the sintered body achieves densification to 95% or more of the theoretical density.

Through the above-mentioned three processes, the inventors discovered that Al and O, which had once been substituted and dissolved in the β-crystal, are precipitated again from within the crystal lattice, and at this time, the crystal grows with high anisotropy and a high aspect ratio. I discovered that. Furthermore, it has been found that β-crystals that do not contain solid solutions are thought to have high cleavage strength, and thus contribute to improving the strength of the sintered body itself. Although the detailed mechanism of this phenomenon is not clear, Al2O3
By using rare earth metals and lanthanide metal oxides in combination, the rate of crystal transformation is accelerated by substitutional solid solution of Al and O, and the rare earth and lanthanide metal oxides are reacted with Al and O during the final densification process of the sintered body. It is thought that Al and O, which were once solid-dissolved, re-precipitate because of their high properties.

[0019] Here, it is not essential that Al2O3, rare earth metal oxides, and lanthanide metal oxides be added as raw material powders, as long as they each have the form of oxide powder immediately before sintering. In other words, for example, when it is added as an alkoxide salt such as Al or Y, or when it is added as an organic salt such as Al or Y, and after going through an oxidation treatment process immediately before sintering, it ends up having the above oxide form immediately before sintering. It fully achieves inventiveness. As a result, the sintered body is densified, and many anisotropic β crystals with high cleavage strength are precipitated, resulting in a bending strength of 10
A sintered body of silicon nitride and β'-sialon that stably has characteristics of 0 kg/mm 2 or more is obtained.

On the other hand, the total amount of Al2O3 and rare earth and lanthanide metal oxides is 5% by weight.
If it is less than 15% by weight, the sinterability will not be sufficient and densification will not proceed sufficiently, and if it exceeds 15% by weight, the grain growth of the crystal grains will be non-uniform and strength deterioration will occur, which satisfies the present invention. The range of the amount of the auxiliary added is 5% by weight or more and 15% by weight or less.

[0021]

[Example] Si3N4 raw material powder manufactured by imide decomposition method (average particle size 0.5 μm, alpha crystallization rate 96.5%), commercially available Y2O3 (average particle size 0.9 μm) and Al2O3
(average particle size: 0.4 μm) were mixed in a wet ball mill in ethanol for 100 hours using the formulation shown in Table 1. This 2
After removing foreign matter and agglomerated particles with a 0 μm mesh, it was dried, and then CIP molded (CI
After heating at 1550°C for 6 hours under 2 atmospheres of N2 gas, the product was further sintered at 1750°C for 6 hours under 2 atmospheres of N2 gas. H
IP processed. From the obtained sintered body, JISP, 1601
After cutting out a sintered body of compliant size 3 x 4 x 35 mm3, grinding and processing, 0.2 mm with diamond paste #3000.
Table 1 shows the results obtained by subjecting the finished product to S to a predetermined evaluation test. Regarding the Z value of the obtained sintered body, the results obtained as the amount of solid solution of Al2O3 in accordance with Japanese Patent Publication No. 56-51153 are also shown in the same table. Regarding the evaluation of the structure of each sintered body, see Ziegler, G et al.
ence of Ceramics vol.
12 (1984) pp361) 2
The average crystal grain size and aspect ratio were determined for 100 to 300 samples each from the structure obtained on the dimensional cross section, and the results are shown in the table.

[0022]

[Table 1]

[0023]

Effects of the Invention In the present invention, long fibrous crystals are not dispersed in the silicon nitride matrix, but fine grains, high aspect ratio, and uniform grains are formed, and Al and A sintered body whose matrix phase is composed of β crystals with a low solid solution amount of O element has higher strength than a fiber-reinforced sintered body.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the composition range of the present invention.

Claims (2)

[Claims] Claim 1: The weight ratio of one or more rare earth metals and lanthanide metal oxides added as sintering aids to one or two aluminum oxides and aluminum nitrides is plotted on the vertical axis. β'-Silicon aluminum oxynitride [β'-Si6-ZAlZOZN8-Z (0≦Z≦4
．． 2)] Measured z value indicating the amount of substituted solid solution of Al and O elements in the crystal phase, and one or two types of Al elements of added aluminum oxide and aluminum nitride are all β'-silicon aluminum oxynitride crystals. A straight line connecting the points A, B, C, D, and E shown in Figure 1 with the horizontal axis representing the ratio to the theoretical z value calculated as a solid solution by substitution in the phase, but does not include the line DE. A silicon nitride-based sintered body characterized by being represented by a range surrounded by . [Claim 2] The total amount of sintering aids added is 5% by weight or more1
The silicon nitride-based sintered body according to claim 1, wherein the content is 5% by weight or less.