JP2000154058A - High strength ceramics and method for evaluating defect in ceramics - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain ceramics which attain high strength by utilizing a method for evaluating defects in ceramics applicable even to ceramics sintered by HIP and the principle of such an evaluating method. SOLUTION: Ceramics sintered by hot isostatic pressing after pressure sintering are heat-treated at a temperature in the range from (the normal sintering temperature) -200 deg.C to (the normal sintering temperature) +100 deg.C and defects are observed by a light transmission observation method. When alumina or zirconia ceramics sintered by hot isostatic pressing after normal sintering are heat-treated at nine temperatures at 50 deg.C intervals in the range of 1,200-1,600 deg.C for 0.5-4 hr each and defects are observed by the method, the average defect size is <=70 μm at the heat treatment temperature at which the average defect size is observed to be maximum.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to high wear resistant ceramics for cutting tools and pump pulp parts, automotive parts such as turbochargers and pulp, and implant materials for gas turbines and artificial bones. The present invention relates to a high-strength ceramic and a method for evaluating a defect of such a ceramic.

[0002]

2. Description of the Related Art Ceramics have excellent corrosion resistance, heat resistance and wear resistance, and are widely used as materials for machine parts, jigs and tools. However, there is a defect that the fracture toughness of ceramics is very low (3 to 10 MPam ^1/2 ) and the fracture strength is low as compared with the fracture toughness of metal materials. For these reasons, many attempts have been made to improve the fracture toughness and fracture strength of ceramics. As such a technique, a method of adding SiC whiskers (for example, US Pat. No. 4,543,345), a so-called nanocomposite material in which nano-order fine particles are dispersed in ceramic crystal grains, and the like have been proposed.

[0003] With the development of these techniques, the fracture toughness value is more than doubled as compared with ceramics before that. However, even with these techniques, it is difficult to say that the breaking strength is improved as expected. It is considered that such a situation occurs because, in each of the above-mentioned technologies, similarly to the fracture toughness value, the defects that govern the fracture strength of ceramics cannot be controlled to be small.

On the other hand, as a technique for eliminating defects, particularly pores, in ceramics, hot isostatic pressing (hereinafter, sometimes abbreviated as "HIP") in which sintering is performed in a high-temperature, high-pressure gas.
Sintering is known. It is said that when ceramics are manufactured by HIP sintering, the pores after sintering become closed pores and complete densification of the ceramics can be achieved.

[0005] However, assuming that the densification is complete and the defects are completely removed, a ceramic of theoretical strength should be obtained. However, the actual strength after HIP sintering is about 1/10 of that. It remains at a low value. Also,
Compared with the strength before HIP sintering, the improvement range is 20 to 3
It is about 0%, and the expected strength has not been achieved.

[0006] On the other hand, as a method for evaluating the defects of ceramics, for example, “Academic Transactions of the Ceramic Society of Japan” [98 (5), p515-516 (1990)]
Or “FC Report” [9 (1991) No. 5, p1
87], there is a method of thinning a ceramic sintered body and directly observing light transmission. However, in a ceramic material that has been HIPed for high density and high strength, the residual pores are crushed by a high-pressure gas of 1000 atm or more, so that defects cannot be observed by a normal light transmission method. Further, the ultrasonic deep wound method and the X-ray transmission method are not effective means for evaluating such defects. As described above, there are defects in the ceramics that cannot be detected by the conventional defect evaluation method, and if such defects can be evaluated, it can be expected that means to bring the ceramic closer to the theoretical strength can be expected. The fact is that it cannot be evaluated.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation,
An object of the present invention is to provide a ceramic defect evaluation method applicable to IP-sintered ceramics, and a ceramic capable of achieving high strength by utilizing the principle of such an evaluation method.

[0008]

According to the present invention which has solved the above-mentioned problems, a ceramic which has been subjected to hot isostatic pressing after normal pressure sintering has a temperature of +100 with respect to the temperature at the time of normal pressure sintering. ° C
This is a defect evaluation method for ceramics, which has a gist in that a heat treatment is performed at a temperature within a temperature range of -200 ° C and the defect is observed by a light transmission observation method. The ceramics to be evaluated in the evaluation method of the present invention include alumina, zirconia, silicon carbide, silicon nitride, mullite, and all composite ceramics containing at least two of them.

Further, the high-strength ceramic of the present invention which has solved the above-mentioned problems is alumina or zirconia ceramic which is sintered under normal pressure and then hot isostatically pressurized. 0.5 to 4 at 9 temperatures at 50 ° C intervals within the temperature range
The gist is that the average defect size is 70 μm or less at the heat treatment temperature at which the largest average defect size is observed when each part is heat-treated and light transmission observation is performed.

[0010]

DETAILED DESCRIPTION OF THE INVENTION It is known that the progress of sintering of ceramics is limited by grain boundary diffusion. And HIP
In the case of sintering, the diffusion of grain boundaries is promoted by a gas pressure of 1000 atm or more, and the grain boundary diffusion speed is about 100 times that of normal pressure sintering.
It is said to double. However, the diffusion distance in the HIP processing time of about 2 hours is at most about 10 μm, and pores and pore aggregates having a radius larger than this diffusion distance may be affected by mass transfer during HIP sintering as described above. It cannot be completely filled.

Accordingly, pores and pore aggregates having a size of 20 μm or more remain, and are crushed by the HIP gas pressure. In the crushed pores, a gas of about the same level as in the HIP sintering (about 1000 atm) is cooled while being confined, so that a large tensile stress (residual tensile stress) is generated in the vicinity thereof. Will do. At first glance, the state in which the pores are crushed is such that the pores which become defective due to the HIP treatment disappear, but the strength of the ceramic having many such portions is not improved as expected. For these reasons, HI
In order to evaluate the P-treated ceramic, it is necessary to quantify the portion where the residual stress remains.

The present inventors have paid attention to the tensile residual stress as described above, and found that the crushed pores can be reproduced by heat-treating the sample after the HIP treatment again to soften the grain boundaries. As a result of a detailed study of the temperature, it has been found that the above object can be achieved satisfactorily by adopting the above configuration, and the present invention has been completed.

[0013] The heat treatment temperature at which the crushed pores can be reproduced (hereinafter sometimes referred to as the "reproduction temperature") varies depending on the type of ceramics. It has a relationship with the sintering temperature, and the normal pressure sintering temperature is + 100 ° C to -200 ° C.
Within the temperature range. A preferable range of the reproduction temperature is about ± 0 ° C. to −100 ° C. with respect to the normal pressure sintering temperature. If the crushed pore is reproduced in this way,
These pores can be easily observed by a light transmission method. Note that ordinary normal pressure sintering is performed at a temperature at which a relative density of 95% or more can be obtained by raising the temperature at 100 ° C./h and maintaining the temperature for about 2 hours.
It may be about 4 hours.

The strength of the ceramic is proportional to the fracture toughness value and inversely proportional to the square of the defect size which is the starting point of the fracture. As described above, many attempts have been made to improve the fracture toughness value in order to improve the strength of ceramics, but little consideration has been given to reducing the defect size. This is considered to be largely due to the fact that there was no effective manual throw for quantitatively evaluating defects in ceramics. In particular, HIP
The treated ceramic sintered body was apparently densified to the true density, and it was thought that pores did not remain like the sample after normal pressure sintering. Had been.

The inventors have noticed that if the defects were completely removed by HIP sintering, the strength would have been further improved, but the actual strength improvement range was at most about 20 to 30%. Although it appeared that the defect was completely removed by the HIP sintering, it was considered that the defect actually remained in the ceramics. The pores removed by the grain boundary diffusion promoted by the HIP process are at most 10
A pore having a size of about μm and a size larger than that will be crushed by high pressure during HIP sintering.

As described above, since one atmosphere is naturally contained in the pores, the pores crushed by the HIP sintering have the same pressure as the HIP treatment (for example, H
If the IP processing pressure is 1500 atm, a high pressure atmosphere of about 1500 atm is left. It is considered that, due to the confined high-pressure atmosphere, a tensile stress acts near the crushed pore, and this remains as a weak portion (defect in the HIP sintered body). "Defects" as described above are not observable by conventional methods,
According to the evaluation method of the present invention, observation becomes possible. Next, each requirement specified in the present invention will be described.

First, as described above, the heat treatment temperature of the ceramics (the above-mentioned reproduction temperature) is + 100 ° C. to −200 ° C. with respect to the temperature during normal pressure sintering, but in the case of ceramics such as alumina and zirconia. 1200 ° C
The pores can be reproduced by heat treatment in a temperature range of の 1600 ° C.

Therefore, the above-mentioned temperature range is set to 9 at 50 ° C. intervals.
Two temperatures were selected and heat treatment was performed at each temperature, and the relationship between the heat treatment temperature and ceramic strength was further studied. As a result, it has been found that if the average defect size is set to 70 μm or less at the heat treatment temperature at which the average defect size is observed to be the largest, a high-strength ceramic can be obtained.

A specific procedure for obtaining the high-strength ceramic of the present invention will be described. First, in the production of alumina ceramics, alumina having a purity of 99.99% or more is used as a main raw material, and this is mixed with MgO having a purity of 99.5% or more.
One or more oxides such as CaO and Y ₂ O ₃ are added as a sintering aid up to about 0.5% by weight. Here, if the addition amount of this oxide exceeds 0.5% by weight, segregation is likely to occur in the subsequent mixing step or a glass phase is likely to occur in the sintering step.

Next, 0.5 to 4.0% by weight of an organic solvent such as HPC (hydroxypropylcellulose), PVA (polyvinyl alcohol) or acrylic is used as a binder. At this time, water is used as the solvent. If necessary, 0.1 to 0.5% by weight of ammonium polycarboxylate or the like is contained as a dispersant.

As described above, the main raw material powder, the binder, the dispersant and the like are mixed. At this time, it is preferable that the slurry concentration is 50 to 65% by weight. If the solute concentration is less than 50% by weight, it is difficult to obtain solid granules in the subsequent granulation and drying step, and if it exceeds 65% by weight, deformed granules other than spherical particles may be formed. As a mixing method for mixing them, a slurry is obtained according to a conventional method. At this time, if the slurry is foaming, about 0.01 to 1.5% by weight of an antifoaming agent is added.

Next, the particles are granulated and dried by a spray drier to obtain secondary particles containing a sintering aid, and are brought into a preferable state in terms of workability in work and prevention of component deposition. In order to obtain granules that are as solid, spherical and crushable as possible, special attention must be paid to the conditions of the spray dryer in granulation drying. For example, the number of rotations of the disk above the chamber is 100
The particle size is made as small as possible by increasing the speed to about 00 rpm. Further, if the granules are rapidly dried, the binder is concentrated on the surface. Therefore, the temperature of the hot air is preferably set to about 110 to 150 ° C. without increasing the boiling point of the solvent so much.

Subsequently, the alumina secondary particles containing the sintering aid are formed into a desired shape. The molding method at this time is not particularly limited, but the isostatic pressing method,
An injection molding method, a uniaxial molding method, or the like can be applied. After such forming, it may be further formed by machining, or may be formed into a simple floc shape, and then formed into a desired shape by machining.

The compact obtained as described above is first sintered under normal pressure, and the atmosphere of the normal pressure sintering at this time is sufficiently atmospheric. And the sintering temperature is 1250-15
Perform at 50 ° C. By performing such a treatment, a normal pressure sintered body having a relative density of 95% or more can be obtained. If the sintering temperature at this time is too low, the density of the normal pressure sintered body does not sufficiently increase, and the sintered body is not densified by pressure sintering (HIP sintering) described later. On the other hand, if it is too high, the growth of the crystal grains proceeds remarkably, and the strength decreases. The sintering time depends on the amount and shape, but in most cases, about 0.5 to 4 hours is sufficient. Even if this sintering time is extended, the grain growth will rather proceed. Become.

Next, the obtained normal pressure sintered body is subjected to HIP sintering. The normal pressure sintered body is embedded in a refractory powder such as alumina, zirconia, titania, and the like, and argon,
The main sintering is performed in a pressurized atmosphere of an inert gas such as nitrogen. However,
As long as the above atmosphere is achieved, it is not necessary to embed the normal-pressure sintered body in a refractory powder such as alumina, zirconia, or titania.

The pressure sintering conditions in this HIP sintering are as follows:
Temperature: 1200-1450 ° C, pressure: 1000-200
It is preferably 0 kgf / cm ² . This temperature is 1
Below 200 ° C., densification is sufficiently achieved, 1450
If the temperature exceeds ℃, densification proceeds, but growth of crystal grains is remarkable, and strength is reduced. When the pressure is less than 1000 kgf / cm ^{2, the} effect of pressurizing for densification is not so high,
Even if the pressure exceeds 000 kgf / cm ² , the effect of pressurization is not so obtained, and the cost of the apparatus is rather increased.
It is sufficient that the pressure sintering time is maintained for about 0.5 to 3 hours. Even if this time is too long, the effect does not change much.

Next, in the production of zirconia ceramics, one or more oxides such as MgO, CaO and Y ₂ O ₃ are dissolved in zirconia powder having an average particle diameter of 1.0 μm or less, and The amount of the solid solution oxide is Y ₂ O
2.0 to 4.0 mol% in ₃ and 3.0 to 5.0 in MgO
mol%, and 3.0 to 6.0 mol% in CaO. When these oxides are added in combination, the total amount may be 3.0 to 9.0 mol%. Then, a binder may be added to the zirconia powder in which the oxide is dissolved, and a mixing method for mixing these may be in accordance with a conventional method. still,
It is preferable in terms of wet handling, drying and granulation to form sintering aid-containing secondary particles in terms of workability in operation and prevention of component segregation.

The reason for using the zirconia powder in which the oxide is dissolved is to form a large number of tetragonal crystals of the zirconia crystal phase to be described later at a use temperature (= room temperature) to obtain partially stabilized zirconia. As the crystal structure of zirconia,
Monoclinic crystals in the low temperature range, tetragonal crystals in the high temperature range, and cubic crystals in the high temperature range. If the amount of oxide added is small, the tetragonal / monoclinic transformation occurs at a high temperature during cooling after sintering, and if the amount is large, the cubic crystal becomes stable up to room temperature during cooling after sintering. It is necessary to dissolve the substance in an appropriate amount.

Subsequently, the zirconia powder to which the oxide is dissolved and the binder is added is formed into a desired shape. The molding method at this time is not particularly limited, but an isostatic pressing method, an injection molding method, a one-touch molding method, or the like can be applied. After this molding, it may be further formed by machining, or may be formed into a simple floc shape and then formed into a desired shape by machining.

The obtained compact is first sintered under normal pressure. Atmosphere for atmospheric pressure sintering at this time is sufficient. In addition, the rate of temperature rise is 50 to 100 on average to the holding temperature.
Sintering is performed at 1250 to 1550 ° C at a rate of ° C / h, and cooling is performed at an average cooling rate of 50 to 150 ° C up to room temperature.
Thus, a normal pressure sintered body having a relative density of 95% or more is obtained. If the sintering temperature at this time is too low, the density of the normal pressure sintered body does not sufficiently increase, and the sintered body is not densified by pressure sintering described below. On the other hand, if it is too high, the growth of crystal grains will proceed remarkably, and the strength will decrease. The sintering time depends on the amount and shape, but in most cases, about 0.5 to 4 hours is sufficient. Even if the sintering time is lengthened, the grain growth will rather proceed.

After the above sintering, it is important that the tetragonal content is 80% by volume or more. Partially stabilized zirconia has high strength and high toughness. It is explained that the onset is due to the above-described tetragonal / monoclinic transformation at the crack tip in the stress field and the absorption of fracture energy. Therefore, it is necessary that a large number of tetragons are formed after sintering.
It is important that the content is 0% by volume or more.

Next, the obtained atmospheric pressure sintered body is HIP sintered. Then, the normal pressure sintered body is embedded in a refractory powder such as alumina, zirconia, or titania, and the main sintering is performed in an inert gas pressurized atmosphere such as argon or nitrogen. At this time, similarly to the above, it is not always necessary to embed the normal-pressure sintered body in a refractory powder of alumina, zirconia, titania or the like in the above atmosphere.

The pressure sintering conditions are as follows: the heating rate is 200 to 500 ° C./h on average to the holding temperature;
Sintering at 50 ° C and cooling rate of 50 to 3 on average to room temperature
500 C / h. And the relative density is about 100
% HIP sintered body is obtained. The holding pressure at this time is 10
It is preferably from 00 to 2000 kgf / cm ² .

If the sintering temperature is lower than 1200 ° C., the densification does not proceed sufficiently. If the sintering temperature exceeds 1450 ° C., the densification proceeds, but the growth of crystal grains is remarkably reduced in strength. Holding pressure is 1000
If the pressure is less than kgf / cm ² , the effect of pressurizing for densification is not so much obtained, and if it exceeds 2000 kgf / cm ² , the effect of pressurization is not so obtained, and the cost of the apparatus is rather increased. It is sufficient to maintain the pressure sintering time for about 30 minutes to 3 hours, and the effect does not change much even if the time is increased.
It is important that the tetragonal content is 80% by volume or more even after the HIP sintering.

The method for measuring the crystal structure of zirconia employed in the present invention is as follows. Rotating anti-cathode type X
Ray diffraction device RU-300 (trade name: manufactured by Rigaku Corporation)
Was used for qualitative and quantitative analysis of the crystal structure. The tube used in this apparatus is Cu (monochromatic by curved monochrome crystal: Kα ray by Cu), the tube voltage is 50 kV, the tube current is 200 mA, and qualitative analysis is performed by obtaining multiple records of the X-ray diffraction profile by measurement. Was done. Then, the measured diffraction peak of each crystal structure was precisely measured again, and the integrated intensity was calculated after each peak separation. Quantitative planning was performed for the value, and the volume ratio was calculated. However, in the obtained quantitative value, the total of each component was taken as 100%.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples are not intended to limit the present invention, and any change in the design based on the above and following points is not limited to the present invention. It is included in the technical range of.

[0037]

EXAMPLES A ceramic according to the present invention was manufactured by the following procedure. First, an alumina powder having a purity of 99.9% or more is used as a main raw material, and an oxide of MgO, CaO, Y ₂ O ₃ or the like is added thereto.
The seeds were added in an amount of 0.0 to 0.5% by weight as a sintering aid. An organic binder such as HPC, PVA or acrylic was contained in an amount of 2 to 5% by weight, and as a dispersant, ammonium polycarboxylate or the like was contained in an amount of 0.1 to 0.5% by weight. At this time, water or ethanol was used as the solvent, and the concentration of the solute was 55% by weight. These raw materials were wet-mixed in a pot mill or a ball mill for 20 hours to form a slurry. After mixing, 0.01 to 1.5% by weight of an antifoaming agent was added to the slurry in which bubbles were generated. The mixed balls used at this time were made of alumina having a purity of about 99.5% in order to avoid mixing of impurities.

Thereafter, the hot air temperature of the spray drier is set at 70 to 170 ° C., and the number of rotations of the disc is set at 7000 to 150 ° C.
Granulated and dried at 00 rpm, average particle size: 60 to 200
The resulting particles were alumina secondary particles containing a sintering aid of μm. The alumina secondary particles having different particle diameters and shapes can be obtained depending on the conditions of the sintering aid, binder, and spray dryer.

Next, the alumina secondary particles were formed at a molding pressure of 1 to 1.
It was molded by a hydrostatic pressing method at 3 ton / cm ² . Six types of alumina compacts having different defects and different sizes obtained by the above-mentioned manufacturing method were sintered under normal pressure at 1500 ° C. for 2 hours.
The obtained sintered body was heated at a temperature of 1400 ° C. and a pressure of 1500.
HIP sintering was performed in an Ar atmosphere of kgf / cm ² for 2 hours. When the density of the obtained sintered body was measured by the Archimedes method, it was confirmed that the sintered body was densified to a true density (a target of 3.98 g / cm ³ ).

Thereafter, the surface was machined to a thickness of about 0.1 mm with a surface grinder, and the upper and lower surfaces were mirror-polished with 3 μm diamond abrasive grains. When the sample thus obtained was observed through a transmission optical microscope for light transmission, no image identified as a defect was found. The structure of this sintered body is shown in FIG. 1 (micrograph as a substitute for a drawing).

These samples were prepared at 1200 ° C. and 1250 ° C.
° C, 1300 ° C, 1350 ° C, 1400 ° C, 1450
C., 1500.degree. C., 1550.degree. C., and 1600.degree. C., and a thin piece having a thickness of 0.1 mm was prepared.
In the range of ° C., many black image portions were observed in the ceramics. FIG. 2 (micrograph instead of drawing) shows 145
The results of light transmission observation when heat-treated at 0 ° C. are shown. The portion of the black image shown in FIG. 2 corresponds to a defect, and indicates that the presence of the defect scatters light and does not transmit light.

When the heat treatment temperature is lower than 1300 ° C., the grain boundaries are not sufficiently softened and the defects cannot be reproduced. On the other hand, when the heat treatment temperature exceeds 1600 ° C., sintering proceeds, and defects disappear and disappear. In addition, the thickness of the flakes when such observation is performed is preferably about 0.01 to 0.5 μm. If the thickness exceeds 0.5 μm, sufficient transmitted light cannot be obtained and the tissue is not clearly observed. If the thickness is less than 0.01 mm, the strength is insufficient and handling becomes difficult.

The size of the black portion was quantified by the Fulman method, and the numerical value was defined as the average defect size. In addition, these six types of alumina ceramics are used in 3 × 4 × 50 (m
m) and processed into a three-point bending test with a span of 30 mm. FIG. 3 shows the relationship between the average defect size and strength in ceramics.

As is apparent from FIG. 3, when the defect size in the HIP treated body reproduced by the heat treatment is 70 μm or less, the three-point bending strength shows a high strength of 600 MPa or more.

[0045]

As described above, when the defect size quantified by (heat treatment + transmission observation method) is 70 μm or less, a ceramic having a high breaking strength of 600 MPa or more can be realized. Such high-strength ceramics include various wear-resistant ceramics such as cutting tools and pump valve parts, automotive parts such as turbochargers and pulp, industrial machine parts such as gas turbines, and implant materials such as artificial bones. Suitable for use.

[Brief description of the drawings]

FIG. 1 is a micrograph instead of a drawing showing an example of the structure of a ceramic sintered body according to the present invention.

FIG. 2 is a drawing-substituting micrograph showing an example of a structure observed by light transmission when heat-treated at 1450 ° C.

FIG. 3 is a graph showing the relationship between the defect size and the strength of the HIP-treated alumina ceramic.

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Moriga Kanamaru 1-5-5 Takatsukadai, Nishi-ku, Kobe F-term in Kobe Steel Research Institute Kobe Research Institute 3C046 PP00 4G030 AA17 AA36 BA18 BA19 BA20 CA07 GA28 GA29 GA33 4G031 AA12 AA29 BA18 BA19 BA20 CA07 GA12 GA16

Claims (2)

[Claims] 1. A ceramic which has been subjected to hot isostatic pressing after normal pressure sintering at a temperature within a temperature range of + 100 ° C. to −200 ° C. with respect to the sintering temperature during the normal pressure sintering. A defect evaluation method for ceramics, which comprises performing a heat treatment and observing defects by a light transmission observation method. 2. An alumina or zirconia ceramic sintered under normal isostatic pressure after normal pressure sintering, wherein said ceramic is sintered at nine different temperatures at intervals of 50 ° C. within a temperature range of 1200 to 1600 ° C. A high-strength ceramic, wherein the average defect size is 70 μm or less at a heat treatment temperature at which the largest average defect size is observed when each part is subjected to a heat treatment for 0.5 to 4 hours and light transmission observation is performed on each part.