JP3131008B2 - Method for producing ceramics having oriented long-axis crystals - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel ceramic sintered body having an oriented (particularly two-dimensionally oriented) long-axis crystal (for example, a columnar or needle-like crystal or a plate-like crystal having a long axis) and its production. In particular, with respect to a method for producing such a long-axis two-dimensionally oriented crystal in a sintering process (in-situ), an ultra-tough and ultra-high-strength material that can withstand a severe use environment is used. To provide.

[0002]

[Technical Background] In recent years, the need for ultra-high-strength materials that can withstand harsh use environments has been increasing, and great expectations have been placed on improving the toughness and reliability of ceramics. In particular, ceramics are highly promising as on-vehicle engine parts and members because they have higher strength than metals and are excellent in heat resistance.

[0003] However, ceramics have a very low fracture toughness value as compared with metals and are brittle in fracture, so that the problem of still lacking reliability has not been completely overcome.

That is, since ceramics are inherently brittle, to improve their toughness, a method of dispersing or precipitating a heterogeneous phase as a source of energy dissipation in a material,
That is, it has been considered that compounding is basically effective. Various mechanisms for improving toughness by particle dispersion or fiber dispersion have been proposed so far, and research to find conditions that can achieve high strength and high toughness by fundamentally pursuing each mechanism has been conducted. It has been done energetically.

One example of a high-strength material that can withstand use at high temperatures is silicon nitride-based ceramics. Silicon nitride-based ceramics are attracting attention for their excellent strength properties, and are being studied as structural materials. Some have been put to practical use. However, silicon nitride, like other ceramic materials, has a major problem as a structural material that lacks reliability in strength properties (has large variations) and has low toughness. Therefore, its practical application has not yet taken a step further. It is the current situation.

[0006]

2. Description of the Related Art A report by Ken Kawashima et al.
In ₃ N ₄ diversification of Ceramics "silicon nitride ceramic (2) pp. 135-146, cited drawbacks of such a silicon nitride ceramics, the results of attempts of improvement by HIP method have been reported. That is, the flexural strength is 680 to 1079 M by Sinter (gas pressure sintering).
953-1230MPa with Pa, Sinter + HIP
a, Data of 1000 to 1227 MPa in Sinter / HIP (gas pressure sintering to HIP continuous treatment) and fracture toughness values of 5.3, 5.2 and 5.7 MPa · m ^1/2 are reported ( 135-138, Table).

[0007]

[0006] However, when obtaining the intended high flexural strength, K _IC is reduced, the bending strength to be obtained having a high K _IC conversely has a tendency to become low. That is, the bending strength and the fracture toughness are opposite to each other, and it is considered that the combination thereof is extremely difficult (No. 143).
See FIG. 15 to FIG. 17).

[0008] Therefore, as a method for improving both of the above characteristics, there is a method of “gas pressure sintering (Sinter) + HI.
P ”or“ Gas pressure sintering / HIP ”(from gas pressure sintering to H
(A continuous process up to IP), and it is considered that there is still a great difficulty in practical use. Although some improvement was achieved by this method, the bending strength was 1074 MPa, and the K _{IC was} 6.7 MPm.
The combination of ^1/2 was the best, and trying to raise one of the characteristic values above this would cause the other to decrease, so there was no doubt about the compromise between the two characteristic values.

On the other hand, as a conventional method for increasing strength and toughness, a method based on so-called fiber reinforcement by adding and dispersing fibers in a ceramic matrix has been mainly used. In the case of this method, it is difficult to uniformly disperse the fibers in the matrix, it is difficult to ensure matching or bonding between the fibers and the matrix phase, and furthermore, the strength of the fibers deteriorates during the sintering process. There are various problems, such as the need to carefully select fibers and sintering conditions that do not occur.

Accordingly, a first object of the present invention is to provide a novel manufacturing method capable of orienting a long axis crystal of a ceramic sintered body.

[0011] The present invention, in the second, and combines high strength and high toughness, and also aims to provide a manufacturing method of the novel sintered ceramic.

[0012]

Means for Solving the Problems According to the Invention The first object of the present invention is to
Pre-sintered body containing long axis crystal or long axis crystal nucleus
In the temperature range where the plastic deformation under pressure is possible.
Axial pressure compression sufficient to orient the axial crystal
At the same time as the compression axis.
Oriented columns characterized by expanding and deforming in the direction
For producing ceramic sintered body having a needle-like or needle-like crystal
Achieved by

[0013] As a result, the ceramic sintered body comprising a substantially oriented major axis crystal in a one-dimensional or two-dimensional direction is obtained, to combine high strength and high toughness, orienting controlling the crystal Becomes possible.

Claim 2 The following dependent claims show more preferred specific developments of the production method of the present invention.

That is, the method further comprises a crystal growth step of growing the long-axis crystals (or long-axis crystal nuclei) under control in a state of being oriented following the above-mentioned orientation step or at the same time as the orientation step. The orientation state and the crystal length (or the degree of crystal growth, and in some cases, the aspect ratio adjustment control in consideration of the growth of the thickness) can be achieved (claim 2).

In the crystal growth step, a first predetermined curve showing a change in toughness with an increase in the aspect ratio when the aspect ratio of the long-axis crystal is oriented, and a change in strength with the increase in the aspect ratio. according to a second predetermined curve showing, until a predetermined aspect ratio, and more and this to control to grow, are managed in advance test is designed to industrial technology, the products of high yield uniform quality are obtained ( (See FIG. 5).

It is preferable for the orientation that the amount of expansion deformation is at least equivalent to the amount of firing shrinkage of the pre-sintered body from the green compact (claim 3 ).

The expansion deformation is performed in at least a one-dimensional direction (preferably, a two-dimensional direction) orthogonal to the pressure-compression axis (claims 4 and 5 ).

A pre-sintering step of sintering the pre-sintered body without substantially applying compressive pressure (that is, hot press pressure or a force for forcibly compressing a material to be fired such as HIP) is further provided. The pre-sintered body in the fired and shrunk state is loaded into a hot press apparatus with a play gap, and hot-pressed under a pressure sufficient for substantial orientation of the long-axis crystal, thereby forming an oriented long-axis crystal. Continuing hot press pressing for a time sufficient to grow until a predetermined aspect ratio is reached.
According to 6 ), the pre-sintered body can be pre-sintered (for example, by gas pressure sintering or other ordinary sintering) in, for example, a hot press apparatus or without substantially applying hot press pressure. At the same time, the growth of long-axis crystals after orientation can be controlled according to the purpose. If pre-sintering is performed using a hot press device (but without HP treatment), the compression deformation is started from the production of the pre-sintered body (the process from cold press forming to pre-sintered body or pre-sintering). Up to the crystal orientation step by using a single hot press device. In addition, instead of a hot press device, the crystal orientation by compression deformation can be performed by hot pressing a sheet-like pre-sintered body (for example, hot rolling with a roll),
Other means can be used as long as they are plastic deformation means by hot pressing. In the case of hot pressing, there is an advantage that high density can be achieved by growing crystals under pressure after orientation.

[0020] comprise a step of crystal growth to reach a predetermined aspect ratio by further HIP treatment the sintered ceramic That secondary sintered body having an oriented major axis crystals (claim 7) by hot press pressurization , The strength and the toughness are further increased by the growth of the long-axis crystal without deteriorating the strength.

By making the pre-sintered body contain a glass phase together with the long-axis crystal (or its nucleus, which may be in a non-oriented state), it is possible to obtain an orientation treatment as easily as possible. A pre-sintered body having sufficient high strength and high toughness can be obtained.

The term "long-axis crystal" includes not only columnar or needle-like crystals (naturally also including rod-like or fibrous crystals) but also plate-like crystals having a long axis. It is important that the crystal contains at least a crystal that can be further grown in a processing step after the pre-sintering, and a crystal nucleus may be used as long as the crystal can be oriented.

In the oriented long-axis crystals (particularly, columnar or needle-like crystals), an increase in toughness by deflection strengthening can be achieved without adding fibers or whiskers. However, the present invention
Further reinforcement by compounding (fiber or whisker reinforcement,
It does not exclude particle dispersion enhancement).

The long axis of example and to the pillar-shaped crystals barrel crystals, by employing silicon nitride, the preferred sintered body is obtained (claim 8). As long-axis crystal ceramic materials capable of crystal orientation, besides silicon nitride, SbSI, SbSOI, PbBi ₂ N
b ₂ O ₉ , Bi ₄ Ti ₃ O ₁₂ , (Pb, K) Nb ₂ O ₆ , (Sr,
Ba) Nb ₂ O ₆ , (Pb, Ba, La) Nb ₂ O _6, etc. (more than columnar or needle-like), and other cordierite (more than long axis flat plate).

In the case of silicon nitride, the columnar crystal is β-Si
Grows as ₃ N ₄ , but the starting powder is α-Si ₃ N ₄ or
A composition (or crystal nucleus) capable of producing β-Si ₃ N ₄ during pre-sintering may be used. The same applies to other types of ceramic materials, and it is not always necessary to use a long-axis crystal as a starting powder in the pre-sintering step, and a material suitable for pre-sintering can be selected.

According to the production method of the [0026] present invention, it is produced containing choking ceramic sintered body oriented substantially in two dimensions at least the length axis crystal orientation by hot deform under pressure
Can also at least a long axis crystals oriented by hot deform under pressure comprising substantially oriented in one-dimensional direction, wherein the oriented major axis crystal aspect ratio of 4 or more columnar silicon nitride crystal der it is possible to produce a Ruse ceramic sintered body.

The ceramic sintered body containing the oriented columnar or needle-like crystals as a main phase can provide a predetermined high strength without necessarily relying on a fiber-reinforced phase added separately from the main phase from the beginning. · Ru can achieve high toughness.

The oriented long-axis crystals may be included as a dispersed phase with respect to a matrix constituent phase.

[0029] the oriented major axis crystal by the two-dimensional directions including substantially aligned, Ru higher strength and higher toughness ceramic sintered body is obtained.

The oriented long-axis crystal is a silicon nitride crystal.
(Especially columnar silicon nitride crystals)
Silicon nitride sintered body having a high toughness Ru obtained.

The aspect ratio of the oriented long axis crystal is 4
It preferred to be equal to or greater than.

Typically, according to the invention, a strength of 900M
A ceramic containing columnar silicon nitride crystals substantially equal to or greater than Pa and having a fracture toughness of 4 MPam ^1/2 or greater and substantially two-dimensionally oriented.
Mick sintered body (in particular, silicon sintered body nitride) is Ru obtained.
Furthermore, strength 1100MPa or more, fracture toughness value K _IC about 7M
Pam ^1/2 or more, maximum strength 1200MPa or less
Above (in particular, more than 1200～1300MPa), fracture toughness value K _IC 8MPam ^1/2 or more (in particular, 8～9MPam on ^1/2 or more <br/>) is reached in a combination of unprecedented value of. It has a long-axis crystal substantially two-dimensionally oriented and has an aspect ratio of 8 to 1
With 2, Ru ceramic sintered body of ultra-high strength and ultra high fracture toughness is obtained.

In other words, by including a long-axis crystal which is substantially two-dimensionally oriented and isotropically controlled in growth at a predetermined aspect ratio, both strength and toughness are theoretically predicted from the material. the highest level is Ru been achieved.

[0034]

DETAILED DESCRIPTION OF THE INVENTION Hereinafter, silicon nitride will be described as a typical example of a material for producing a typical long-axis crystal.

Factors for improving the fracture toughness value can be roughly classified into two. (1) Direct interaction between the main crack and the dispersed particles (2) Mechanism for forming a process zone near the tip of the main crack (1) includes a curvature of the leading edge of the main crack and a deflection of the main crack surface. And (2) include microcracking, stress-induced phase transition, and fracture surface crosslinking.

In silicon nitride capable of forming columnar crystals in the sintering process, crack deflection is considered to be a major factor as a mechanism for increasing toughness. According to the calculations of Faber et al., A columnar crystal dispersion with an aspect ratio of 12 has a fracture toughness (theoretical value) of about four times that of the matrix, but the measured values are smaller. (KT Faber et al., Acta
Metall. 31 , 565, 577 (1983),
"Ceramic reinforced composite ceramics", see Industrial Technology Service Center (1991), pp. 68-72).

However, it is not known how to produce such a sintered body. Therefore, in the present invention, it is possible to obtain a higher fracture toughness value than before by orienting the long-axis (columnar or rod-shaped) crystal of silicon nitride two-dimensionally or one-dimensionally, or by controlling the crystal growth. It was done. In order to realize the orientation of the long-axis crystal, a pre-sintered body (or primary sintered body) containing the long-axis crystal is compressed and deformed with respect to one axis at a predetermined high temperature, and at the same time, is deformed differently (particularly orthogonal). Expansion) in one or two dimensions about the axis (ie hot plastic deformation)
By doing so, it became clear that the crystal can be oriented one-dimensionally or two-dimensionally. as a result,
For example, the fracture toughness value ranges from 3 MPam ^{1/2 in} a non-oriented state to about 4 MPam ^1/2 to about 8 corresponding to an aspect ratio of 4 to 12.
Those that reach MPam ^1/2 or more are obtained.

It is considered that the following two mechanisms are acting on the present invention. (1) Orientation of long axis crystal (2) Growth control of oriented long axis crystal

(1) <Mechanism of Increase in Fracture Toughness Due to Orientation> Crack deflection is the most important factor in increasing the toughness of silicon nitride, which forms long-axis crystals during the sintering process. For that purpose, it is essential to control the long axis crystal.
In addition, it is considered that if both the orientation control and the growth control of the long-axis crystal are possible, it is possible to obtain the maximum fracture toughness value exhibited by silicon nitride (this also applies to other ceramic materials). Valid). In general, when the orientation control and the growth control are described below, the maximum crack deflection occurs and a high fracture toughness value is exhibited. Preferably, the greatest effect is obtained by realizing both the orientation control and the growth control. Both controls are based on a gradual basic process (long axis crystal formation by pre-sintering, followed by crystal orientation by hot pressing plastic deformation), and by adding subsequent HIP,
It is surprising that it provides a dramatic improvement in both properties.

As shown in FIG. 3, when a crack develops in the sintered body, the two-dimensional alignment material shown in ii) is clearly in a direction substantially orthogonal to the three-dimensional alignment material shown in i). Propagating cracks are less likely to develop. Therefore, it is considered that the two-dimensionally oriented material is more advantageous for increasing the toughness against cracks in a specific direction.

(2) <Mechanism of Increase in Fracture Toughness by Controlling Growth of Long-Axis Crystal> As shown in FIG. 4, even when only the two-dimensionally oriented material is considered, (i) compared to the case where the long-axis crystal is short ( ii) When the long-axis crystal is long, crack deflection is large, and the crack in the direction crossing the long-axis crystal is less likely to develop. Alternatively, it can be seen that the optimum thickness should exist without being too thick or too thin. Therefore, to show the maximum crack deflection,
There should be an optimal aspect ratio for it. From this viewpoint, the present invention has conducted experiments and confirmed its effectiveness.

The sintered body of silicon nitride having the strength and toughness suitable for the purpose of use can be obtained by the synergistic effect of the two mechanisms (1) and (2). It is possible to obtain controlled sintered bodies.

Based on this viewpoint, the orientation of the long-axis crystal was controlled for the purpose of improving the fracture toughness of silicon nitride which forms the long-axis crystal during the sintering process.

In the conventional sintering method, long-axis crystals of silicon nitride are three-dimensionally non-oriented (in a random state) and have a fracture toughness of about 3 to 5. Therefore, in the present invention, with the aim of improving the fracture toughness value, a pre-sintered body of silicon nitride containing a long-axis crystal together with a glass phase is uniaxially press-compressed and deformed by hot pressing at a temperature at which pressurized plastic deformation is possible. At the same time, this was expanded and deformed in the other two axial directions orthogonal to each other to orient the long-axis crystal two-dimensionally (hereinafter referred to as “HP processing”).
As a result, the fracture toughness value could be improved to about 8 MPam ^1/2 . The bending strength is 800
What was about MPa was improved to about 1200 MPa.

In the present invention, “substantially oriented” refers to
A state in which the effect is significantly exhibited by the orientation of the long-axis crystal in a predetermined direction. Further, in the case of silicon nitride, the oriented long axis crystal ranges from an aspect ratio of 4 or more to 12 or more, preferably about 6.5 to 12 (more preferably 8 to 12).
To 12).

In the case of a silicon nitride sintered body, it is sufficient that the HP treatment is performed basically within the sintering temperature range. The conditions of the HP treatment are as follows:
It is preferable that the temperature is in the range of 0 ° C., the pressure is 100 to 500 kg / cm ² , and the holding time is 0.5 to 3 hours. Pressure is 1
If it is less than 00 kg / cm ² , the orientation is insufficient, and
If it exceeds kg / cm ² , the crystal growth rate in the oriented state tends to be rather slow.

The optimum conditions for the orientation by the HP treatment are as follows: the temperature is about 1750 ° C., the pressure is about 3
30 ± 50 kg / cm ² , holding time about 1-2 hours.

The firing shrinkage of the pre-sintered body from the state before sintering (cold press molding volume or green or dry molded body) is about 10 to 20% (vol%) in the case of silicon nitride. It is preferable that the amount of expansion deformation in the non-compression axis direction during the HP treatment for orientation be at least an amount corresponding to the firing shrinkage ratio.

The pre-sintering is carried out until a sufficiently high temperature is reached, and until long-axis crystals are generated and grown to a degree sufficient for orientation. It is not preferable to apply a compression action by external pressurization such as a hot press before reaching such a state.

The sintering method by hot pressing is known. In this case, hot pressing is generally started from the temperature at which sintering starts (about 1500 ° C. for silicon nitride). However, in the present invention, about 17% of silicon nitride is used.
It is preferable to sufficiently perform pre-sintering until the temperature reaches 00 ° C. or more for free long-axis crystal growth.

For orientation by hot press plastic deformation,
A long-axis crystal (or its nucleus) is prepared in the pre-sintered body so that it can be oriented. Therefore, (1) the aspect ratio of the long axis crystal (or nucleus) of the pre-sintered body is preferably 1.5 or more (more preferably 2-3),
(2) For plastic deformation, a predetermined flowable phase (generally, a glass phase or a liquid phase) must be present. In order to ensure sufficient orientation possibility, the pre-sintered body is subjected to a direct compression action such as hot pressing etc.
It is preferable to produce crystals by sintering in a natural state without subjecting the crystals to a natural state.

The silicon nitride sintered body preferably contains a predetermined sintering aid in addition to silicon nitride. First, Y ₂ O ₃ will be indispensable for forming a glass phase at the grain boundary and then a specific crystal phase (Y ₂ Si ₃ O ₃ N ₄ ). Further sintering aids include Al ₂ O ₃ , La ₂ O ₃ , Nd ₂ O ₃ , CeO
It is preferable to use one or more rare earth oxides such as ₂ together with Y ₂ O ₃ .

The ratio of silicon nitride: sintering aid = 98: 2-9
The content is preferably in the range of 0:10 mol%, and Y ₂ O ₃ is preferably 1 mol% or more. When the amount of the sintering aid (or the components other than the components constituting silicon nitride during sintering) exceeds 10 mol, the generation and growth of β-Si ₃ N _{4 to} be a long-axis crystal tend to be inhibited. Be careful as there are.

As a result of such an HP treatment, a combination of high strength and high toughness having a bending strength of about 1100 Pa or more and a fracture toughness value KIC of about 6.7 MPam ^1/2 is obtained. Was. FIG. 5 shows sample N of Table 1 described below.
o. 1 to 4 show measured values.

It should be noted that the HP treatment causes long axis crystals (columnar crystals) to grow together with (or after) the orientation, and to be substantially linear (or slightly curved) as shown in FIG. The two intersecting curves indicate strength (flexural strength) and fracture toughness. Assuming that the aspect ratio is x, the toughness y is y = 2.8 + 0.54x
(MPam ^1/2 ) (formula I), the strength y ′ is y ′ = 1445−
52x (MPa) (Formula II).

According to these formulas I and II, HP treatment is performed until a predetermined aspect ratio is reached, and a sintered body having desired strength and toughness can be obtained. This fact depends on the material design,
Extremely useful in fabrication.

That is, the toughness value K _IC has an aspect ratio of 4 to 10.
A curve of about 5 to about 8 MPam ^1/2 , corresponding to an intensity of about 1250 to 950 M for a similar aspect ratio.
It shows the value of Pa.

This is an effect of the HP treatment (that is, by the oriented growth). According to the present invention, the controlled oriented growth of the long-axis crystal (its state and process) is further controlled by growth as described later. It was found to be extremely useful as a basis for

(2) <Growth Control of Long-Axis Crystal> When sintering silicon nitride, a sintering aid is generally mixed, but depending on the type of the aid, growth of long-axis (columnar) crystal during HP sintering is performed. There are considerable differences between degrees. In the present invention, La ₂ O ₃ —Y ₂ O ₃ system, Al ₂ O ₃ —Y ₂ O ₃ system, Nd
₂ O ₃ -Y ₂ O ₃ system, results of testing for CeO ₂ -Y ₂ O ₃ system, a difference occurs in the growth of the long axis crystal, Nd ₂ generates the greatest long axis crystal O ₃ -Y ₂ O It has been found that the fracture toughness value of the ₃ type silicon nitride reaches 8 MPam ^1/2 by the HP treatment. In addition, as a sintering aid for silicon nitride, Al
Rare earth oxides including ₂ O ₃ and Y ₂ O ₃
A known material such as O can be used.

Further, a high pressure isotropic pressure is applied at a high temperature to the silicon nitride (secondary sintered body) obtained by subjecting the pre-sintered body to HP once (HIP
Treatment), the long-axis crystal grows further, and the temperature,
By changing the pressure, it is possible not only to control the degree of growth but also to prevent the strength from decreasing with the increase in the aspect ratio as seen in the HP treatment, and not only toughness but also strength It was also found that can be further increased.

(2-1) Specific example of growth control of long-axis crystal (growth control by changing sintering aid and sintering conditions) According to differences in sintering aid and sintering conditions during hot pressing, There was a difference in the long axis (columnar) crystal.

[0062]

[Table 1]

As described above, the growth control of columnar crystals (thickness and length,
In particular, it can be understood that aspect ratio control becomes possible. The conditions of pre-sintering to obtain a pre-sintered body also affect the aspect ratio, but pre-sintering includes moderate long-axis crystal growth achieved by gas pressure sintering and the like, and is as high as possible. A sintering state is achieved.

It should be noted that, if the raw material of the pre-sintered body is molded and sintered (gas pressure sintering or the like) by using long axis (columnar) crystal silicon nitride (β type) or the like, Is very low (and therefore bulky), that is, sintering does not proceed sufficiently, and therefore all of the density, strength and fracture toughness values are amorphous (generally rounded particles). It is far inferior to those molded and fired using finely divided starting particle powder.

This fact indicates that long-axis crystals (columnar or rod-shaped crystals) are effective for silicon nitride in addition to being generated (in-situ) from the above-mentioned fine powder starting particles during the sintering process. No method has been found at this time.

The sintering of the pre-sintered body is preferably performed in the same HP apparatus for performing the HP processing (orientation processing) in the subsequent stage from the viewpoint of work efficiency, but it is also possible to perform pre-sintering in another furnace or HP apparatus. It is possible if needed.

(B) (2-2) <Strength based on growth control by HIP processing−
Toughness Value Control> Curves A and B indicated by arrows in FIG. 5 show the effects of the HIP processing, and represent curves (first and second) of the toughness value and the strength value, respectively, which change with an increase in the aspect ratio. . Curves A and B represent both values A of the state after the HP treatment (secondary sintered body).
Starting from 1B1 (Sample No. 2 in Table 1), 1-5
The figure shows changes in the values of toughness and strength when HIP treatment for 1 hour is performed at 1800 ° C. × 100 kg / cm ² .
A temperature substantially equal to or higher than the temperature of this HIP treatment (preferably, at a higher temperature of 10 to 50 ° C, for example,
(At 0 ° C., preferably at a pressure of 100 to 300 atm) at a pressure of 100 to 300 atm.

Quite surprisingly, the tendency of the strength to decrease due to the continuation of the HP treatment after orientation is interrupted and both curves show an increase. With the continuation of the HIP treatment, the toughness value K _IC increases uniformly with increasing aspect ratio, reaching a maximum of about 9.7 MPam ^1/2 . On the other hand, the strength increases to more than 1300 MPa with the increase of the aspect ratio of 7 to 9, and after drawing the top with the aspect ratio of 9 to 10,
Although it slightly decreases, about 1240 MPa is shown even when the aspect ratio is 11 or more.

The data shows that 2
Long-axis (columnar) crystals that are dimensionally oriented and grown to a predetermined aspect ratio are grown by HIP processing while being controlled in the direction of increasing the aspect ratio, thereby achieving high strength and high toughness, which were conventionally extremely difficult. The combination of values is to be achieved in a very controlled (controlled) manner.

This finding is extremely important not only for practical application in the future, but also as a model mechanism for increasing the strength-toughness not only for silicon nitride but also for ceramics generally producing long-axis crystals. It is no exaggeration to say that it is a breakthrough.

[0071]

EXAMPLES Examples of orientation control of long-axis crystals (two-dimensional orientation control by HP treatment) will be described below.

(1) Preparation of pre-sintered body i) The following materials were used as raw materials. Silicon nitride: Ube Industries, Ltd. E-10 (average particle size 0.25 μm) (α type) Hermann C. Starck LC12-S (average particle size 0.77 μm) Electrochemical Co., Ltd. SN-P21B (average particle size) Diameter 0.80 μm), SN-P21C (average particle size 0.69 μm), SN-P21C3 (average particle size 0.52 μm) Sintering aid: (A): Y ₂ O ₃ (Mitsubishi Chemical Corporation) (average particle size) (B): Nissan Rare Element Chemical Co., Ltd. La ₂ O ₃ (average particle size 0.5 μm), Nd ₂ O ₃ (average particle size 7.3 μm), CeO ₂ (average particle size 6. (B): Sumitomo Chemical Co., Ltd. Al ₂ O ₃ (average particle size: 0.37 μm) Solvent: Toagosei Co., Ltd. Trichlorethylene (trichlene R) (125 wt% based on solid content) Dispersant: Ajinomoto AL-M (organic) (2 wt% based on solid content)

Ii) Mixing ratio of raw materials Si ₃ N ₄ : Sintering aid B: Y ₂ O ₃ = 92: 5: 3 mol% As sintering aid B, La ₂ O ₃ , Al ₂ O ₃ , Nd ₂ O ₃ ,
Any one of CeO ₂ was used.

Iii) Clay was prepared in the following steps (a) to (f). (A) Weighing Si ₃ N ₄ + sintering aid B + Y ₂ O ₃ solvent, dispersant, cobblestone (Si ₃ N ₄ ) (b) Wet mixing 16 hours (ball mill) (c) Drying oven 100 ° C. × 18 hours ( d) Dry grinding 48 hours (ball mill) (e) Sieving (150 μm mesh through) (f) Clay

Iv) Pre-sintering The clay is filled in a hot press device (inner diameter: 280 mm) equipped with a carbon mold coated with a release agent (BN) on the inner surface and cold to a height of 12 mm at a pressure of 330 kg / cm ² . It was compression molded. Then, the mixture was heated under N ₂ gas pressure of 9 kg / cm ² to 13
The temperature was increased to 1750 ° C. at a rate of temperature increase of 1 ° C./min, and the temperature was maintained for 1 or 2 hours to perform preliminary sintering, thereby obtaining a pre-sintered body (columnar β-silicon nitride). The presintered body had a density of 2.5 g / cm ³ (70% of the theoretical density ratio), and the columnar crystal had an average aspect ratio of 3 (2
To 5).

This process is schematically shown in FIGS. 1 (1) to (2a), wherein (2a) shows that the pre-sintered body is in a state of natural shrinkage due to sintering in a hot press apparatus (shrinkage ratio about 20). %).

(2) Long-axis crystal orientation control (two-dimensional orientation control by HP treatment) The pre-sintered body was then hot-pressed to 330 kg / cm
Predetermined time under ² pressure (1 hour or 2 hours), N ₂ gas圧常pressure (1 kg / cm ²⁾ and held in an atmosphere, carried out the orientation of the long axis (columnar) crystals and crystal growth in an aligned state Was done. See (3) and (3a) in FIG. (3a) shows the result of hot pressing (secondary sintered body). FIG. 2 shows the pre-sintered body (2a) and the secondary sintered body (HP-processed product) (3) in FIG.
The state of the crystal orientation in the state a) is schematically shown.

By subjecting the silicon nitride pre-sintered body to the HP treatment, the long axis (columnar) crystal can be oriented two-dimensionally, and the fracture toughness and strength have been greatly improved as described below. Here, No. Nos. 1 to 4 correspond to the samples shown in Table 1, and Nos. 4 are non-oriented comparative examples corresponding to Nos. 1 to 4, and No. 1 was obtained by gas pressure sintering.
It was obtained by sintering under the same conditions as in Nos. 1 to 4 (including the HP times of Nos. 1 to 4 without performing hot pressing) for the same time.

[0079]

[Table 2]

The strength and toughness values in Table 2 are based on JIS, respectively.
Three-point bending method and single edge V-notch beam method (S
EVNB method). 1 to
The measured value of the anisotropic sintered body of No. 4 showed a value about 10% lower in the direction (weak direction) orthogonal to the two-dimensional orientation direction. In addition, No. Nos. 1 to 4 and Comparative Example Nos. The densities of 1a to 4a are 3.3 to 3.5 and 2.5 to 2.7 g / cm ^{3, respectively.}
Met.

(3) The effect of controlling the growth of the long axis (columnar) crystal is as shown in Table 1 and FIG. Table 1 shows the relationship among the length, thickness, aspect ratio, HP treatment conditions, and sintering aid components of the long axis (columnar) crystal.

(3-1) That is, by the HP treatment after the pre-sintering, the fracture toughness value increases as the aspect ratio increases from 4 to 10, but the strength decreases according to a tendency almost inversely proportional thereto. It became clear. However, considering that this aspect ratio allows control of the growth of long-axis crystals by the present invention, it is possible to achieve high fracture toughness while maintaining high strength by selecting the aspect ratio in a preferable range. Indicates that is possible. That is, the fracture toughness value is about 6 at the aspect ratio of 6 to 8.
An excellent combination of ７7 MPam ^1/2 and a strength of about 1100 MPa or more is achieved. According to the present invention, there is an extremely excellent advantage in that the control of the orientation of the long-axis crystal and the control of the aspect ratio can be simultaneously achieved. Of course, it is also possible to obtain one of high strength or high toughness particularly intentionally.

(3) <HIP Treatment After Orientation> Further, the secondary sintered body (H
By performing the HIP treatment on the (P-treated body), a further increase in both strength and toughness can be achieved on the basis of the secondary sintered body without lowering the strength. Also, by controlling the aspect ratio in this way, the strength,
The highest level of both toughness is ensured.

That is, as shown in FIG. 2 (Aspect ratio 6.8) 1800 ° C x 1
The HIP treatment was performed at a pressure of 100 kg / cm ² in an N ₂ gas atmosphere for 5 hours. As a result, the strength of the secondary sintered body was B
_It increases along with the increase of the aspect ratio in the direction of the arrow along the curve B from _one point, and increases around the aspect ratio of 9 to 130.
It showed a peak exceeding 0 MPa. This is in contrast to the case where the aspect ratio is increased by continuing the hot pressing. Further, the toughness further increased along the curve A from the point A _{1 of the} secondary sintered body in the direction of the arrow along with the increase of the aspect ratio, and showed a peak (9.7 MPam ^1/2 ) at the aspect ratio of 11.

That is, by combining the growth of a fixed long-axis (columnar) crystal in a two-dimensionally oriented state under hot pressing with the crystal growth under isotropic pressure by HIP treatment, the aspect ratio can be theoretically determined. The maximum toughness value was increased to near the value (12) that resulted in the highest expected toughness, thereby achieving the highest toughness value with a further increase in strength rather than with strength degradation.

[0086]

According to the one-dimensional or two-dimensional orientation treatment (so-called HP treatment) of the present invention, the long-axis crystals contained in the presintered body are oriented in a predetermined direction and controlled to a predetermined aspect ratio. As a result, either or both of the fracture toughness value and the strength are easily improved. (Claim 1)

Further, by controlling the growth of the long-axis crystal of the present invention (the aspect ratio can also be controlled), it is possible to quantitatively increase both the fracture toughness and the strength without incurring a significant decrease in either one. became. As a result, a long-axis crystal-based sintered body having desired strength and fracture toughness can be obtained.
(Claim 1)

According to the features of the respective dependent claims 2 to 8 , the intended high-strength and high-toughness ceramic sintered body can be obtained in a specific and prescribed manner. In particular, by further applying HIP to the secondary sintered body (result of HP processing),
The aspect ratio can be increased (long-axis crystal growth) without lowering the strength, and an unprecedented ultrahigh toughness and ultrahigh strength ceramic sintered body can be obtained. (Claim 8)

In concrete implementation of the present invention, by combining crystal orientation and growth control, a ceramic sintered body having both high strength and high toughness, which has been regarded as difficult in the past, can be obtained. This has led to a dramatic progress in the development of ceramic materials, and further development in various directions is expected based on this.

Although the embodiments have been described by taking silicon nitride as an example, the present invention can be naturally applied to a ceramic material capable of forming a long-axis crystal in a sintering process and a composite material thereof.

In the examples, the fibers such as whiskers were not dispersed and strengthened, but the present invention can be similarly applied to a composite material to which these fiber reinforcements are added according to the present invention. It is no wonder that the strength action by the compounding of can be further added.

In the present invention, a columnar (including a rod-shaped) crystal has been described as an embodiment, but a needle-shaped crystal or a long plate-shaped crystal is naturally included as one mode of the long-axis crystal, and the orientation and aspect ratio are naturally included. Can be controlled.

Silicon nitride is an optimal material for embodying the present invention in that it has heat resistance, produces long-axis (columnar) crystals, and is capable of controlling growth. In particular, the powder starting particles are granular (α-Si ₃ N _{4 in} the case of silicon nitride) or starting component particles that generate silicon nitride in the sintering process, and long-axis (columnar) crystals are generated during pre-sintering. Is advantageous in the present invention, and the present invention is applicable to similar ceramic materials.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing one embodiment of a manufacturing method of the present invention step by step.

FIG. 2 is a schematic diagram showing an orientation state of a major axis (columnar) crystal of a sintered body in the states (2a) and (3a) of FIG. 1;

FIG. 3 is a schematic diagram showing a relationship between deflection and orientation by a long axis (columnar) crystal during crack propagation,

FIG. 4 is a schematic diagram showing the state of crack deflection with respect to the short- and long-thickness of an oriented long-axis (columnar) crystal;

FIG. 5 shows the relationship between the aspect ratio, strength, and fracture toughness HP.
3 shows graphs showing processing and HP processing + HIP processing, respectively.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor, Tsuguo Ito 5 Shintocho-cho, Seto City, Aichi Prefecture (72) Inventor Yoshiki Kato 3-18, Ushinomaki-cho, Mizuho-ku, Nagoya-shi, Aichi Prefecture 5 beef winding complex 701 Room No. 72 (72) Inventor Seio Ito 15-3 Nakamichi, Kugozaki-cho, Okazaki-shi, Aichi Prefecture (72) Inventor Misao Iwata 1-601, Tenpaku-cho, Tenpaku-ku, Nagoya-shi, Aichi No.2, No. 206, No. 2 Person Kiyoshi Sato 1-3-1, Nishitsurugaoka, Oi-machi, Iruma-gun, Saitama Prefecture In-house Research Institute of Tonen Co., Ltd. (72) Inventor Nao Suzuki 1-3-1, Nishitsurugaoka, Oi-machi, Iruma-gun, Saitama Prefecture 1-1 of Tonen Research Laboratory (72) Inventor Takeshi Isoda 1-3-1 Nishitsurugaoka, Oi-machi, Iruma-gun, Saitama Prefecture Tonen Co., Ltd. (56) References JP-A-3-8771 (JP, A) JP-A-63-63 159259 (JP, A) JP-A-2-311376 (JP, A) JP-A-63-101519 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C04B 35/64 C04B 35 / 00

Claims (8)

(57) [Claims] 1. A ceramic presintered body containing a long-axis crystal or a long-axis crystal nucleus is applied in one axial direction in an amount sufficient to orient the long-axis crystal within a temperature range in which the plastic deformation is possible under pressure. A ceramic sintered body having a substantially oriented long-axis crystal, which comprises a crystal orientation step of compressing and compressing and deforming in the direction of the compression axis, and simultaneously expanding and deforming in a direction different from the compression axis. Production method. 2. The method according to claim 1, further comprising a crystal growth step of controlling and growing a long-axis crystal in an oriented state following or simultaneously with the orientation step. The amount of wherein the expansion deformation process according to one of claims 1-2 which is an amount corresponding at least to the sintering shrinkage of the green compact of the prebaked sintered body. Wherein said expansion deformation process according to one of claims 1 to 3, which substantially takes place in the one-dimensional direction perpendicular to the pressing axis of compression. Wherein said expansion deformation process according to one of claims 1 to 3, which substantially takes place in a two-dimensional direction perpendicular to the pressing axis of compression. 6. A pre-sintering step of sintering said pre-sintered body without substantially applying compressive pressure, wherein said pre-sintered body in a fired and shrunk state is inserted into a hot press apparatus with a play space. And hot-pressing under a pressure sufficient for substantial orientation of the long-axis crystals, and continuing hot-pressing for a time sufficient to grow the oriented long-axis crystals until reaching a predetermined aspect ratio. claim 1-5, characterized in that
The production method according to one of the above. 7. claimed in one of claims 1 to 6 comprising the step of crystal growth to reach a predetermined aspect ratio by further HIP treatment the sintered ceramic That secondary sintered body having an oriented major axis crystals Manufacturing method. 8. A process according to one of claims 1 to 7 which is a plate-like crystals the major axis crystal having a columnar or needle-like crystal or long axis.