JPH0772102B2 - Method for manufacturing zirconia sintered body - Google Patents

Description

The present invention relates to a method for producing a zirconia sintered body having high strength, high toughness, thermal stability, and excellent mechanical strength.
More specifically, a zirconia sintered body having a tetragonal crystal as a crystal phase is obtained by subjecting a pre-sintered body of a zirconia powder containing a stabilizer to a tetragonal crystal as a crystal phase to a main sintering under high temperature and high pressure gas pressure. Manufacturing method.

A conventional zirconia sintered body in which stabilizers such as yttria (Y ₂ O ₃ ), calcia (CaO) and magnesia (MgO) are added to zirconia (ZrO ₂ ) is a fully stabilized zirconia sintered body consisting of only cubic crystals. A body and a partially stabilized zirconia sintered body composed of cubic crystals and monoclinic crystals are known, and these sintered bodies are used as heat-resistant materials, solid electrolytes and the like. However, although this completely stabilized zirconia sintered body is stable in the temperature range from normal temperature to 1500 ° C, it has low strength and is vulnerable to thermal shock, so it is easily damaged and is suitable for use in oxygen sensors and structural parts in exhaust gas. There was a limit. Also,
The partially stabilized zirconia sintered body consisting of cubic crystals and monoclinic crystals has higher strength and better thermal shock resistance than the fully stabilized zirconia sintered body, but structural materials, dies and cutting and cutting tools It cannot be said that the strength is sufficient to be used as, and further, there is a defect that the strength is lowered and the material is broken when it is used for a long time in a specific temperature range of 200 ° C to 300 ° C. For example, in the case of ZrO ₂ -Y ₂ O ₃ system partially stabilized zirconia, at around 1500 ° C, which is the sintering temperature, it is composed of cubic- and tetragonal mixed-phase crystal grains. In the cooling process to 1, the transition occurs to a monoclinic crystal which is a stable phase at about 500 ° C or less, and becomes a sintered body composed of cubic crystal and monoclinic crystal. If the amount of this monoclinic crystal that has been transformed increases, many cracks will occur in the sintered body, leading to destruction.

However, in recent years, zirconia sintered bodies mainly composed of tetragonal crystals or tetragonal crystals and cubic crystals have been reported even at room temperature (Gupta et al., TKGUPTA et al, Journal of Materials Science, 1
2,2421 (1977)). This tetragonal-containing zirconia sintered body that is metastable at room temperature (hereinafter abbreviated as meta-stabilized zirconia sintered body) has a lower concentration of yttria than the fully stabilized zirconia sintered body made of cubic crystal. It is a metastabilized ZrO ₂ -Y ₂ O ₃ system sintered body, which is clearly different from the partially stabilized zirconia sintered body composed of cubic crystal and monoclinic crystal. The meta-stabilized zirconia sintered body containing this tetragonal crystal has high strength and high toughness and excellent thermal shock resistance as compared with the fully stabilized zirconia sintered body and the partially stabilized zirconia sintered body. Therefore, it is possible to prevent deterioration over time, which is a problem, in the temperature range of 200 to 300 ° C. The reason why the meta-stabilized zirconia sintered body exhibits excellent mechanical properties has not been clarified yet, but at least the following is considered. The transition between tetragonal and monoclinic in zirconia is a martensitic transformation that occurs thermally or by stress such as pressure from the surroundings.
Then, the transition from the tetragonal system to the monoclinic system causes expansion of a volume of several%. Therefore, at a temperature of about 500 ° C. or less, that is, a metastable zirconia sintered body containing a tetragonal crystal that exists metastable in the stable region of a monoclinic crystal, due to the stress at the tip of the crack when a crack occurs in the sintered body, The tetragonal grains undergo martensitic transformation and are transformed into monoclinic crystals, and at the same time, volume expansion occurs, and compressive stress due to volume expansion occurs at the tip of the crack, which hinders the progress of the crack. As a result, the meta-stabilized zirconia sintered body has excellent high strength,
It is considered to exhibit high toughness. ZrO ₂ -CaO system, ZrO
_{Even in the 2-} MgO system sintered body, Garby et al. (Journal of Aus
t Ceram Soc., 13 (5), 8 (1977)) found a meta-stabilized zirconia sintered body containing calcia and magnesia in which tetragonal crystals exist at room temperature. Furthermore, according to Clausen et al. (Ceramic Bulletin, 56 (6), 559 (1977)), by allowing unstable zirconia particles to exist in a tetragonal crystal form in a member other than zirconia such as an alumina sintered body,
We have obtained a sintered body with high strength and high toughness. However, in order to use it as a high-grade structural material such as a mechanical part, the life and reliability of the part are very important, and therefore these meta-stabilized zirconia sintered bodies are not yet sufficient.

On the other hand, non-oxides such as Si ₃ N ₄ and SiC are expected to have high strength, but it is difficult to sinter to high density under normal pressure. A hot isostatic pressing method (hereinafter abbreviated as HIP method) of sintering such a hardly sinterable substance under high temperature and high pressure conditions to increase the density has been proposed. However, in this HIP method,
Regarding the application in the production of oxide-based sintered bodies, there are applications to alumina-based cutting tools, PZT, ferrite, etc., as well as the ZrO ₂ sintered body die disclosed in JP-A-57-130717. There is. However, the purpose of applying the HIP method in the publication is to remove pores in the sintered body die and to improve the smoothness of the die surface, and contains a tetragonal crystal as in the present invention. This is completely different from the method aimed at significantly improving the bending strength of the zirconia sintered body.

In view of these circumstances, the present inventors have earnestly studied a highly reliable ceramic sintered body having excellent strength and thermal stability. As a result, a zirconia presintered body containing a tetragonal crystal is further subjected to high temperature and high pressure gas pressure. It was found that a meta-stabilized zirconia sintered body in which tetragonal crystals having remarkably high bending strength as compared with the pre-sintered body remain can be obtained by the main sintering, and the present invention was achieved.

That is, the present invention includes yttria having a Y ₂ O ₃ / ZrO ₂ molar ratio of 5/95 to 7/93, magnesia having a MgO / ZrO ₂ molar ratio of 5/95 to 10/90, and C
aO / ZrO ₂ molar ratio 6 / 94-11 / 89 calcia or CeO ₂ / ZrO ₂
Pre-calcined mainly with tetragonal or tetragonal and cubic crystal phases obtained by pre-sintering a compact made of zirconia powder containing ceria with a molar ratio of 7/93 to 12/88 at 1000 to 1700 ° C. Mainly sintering tetragonal crystal or tetragonal crystal and cubic crystal by performing main sintering under high temperature and high pressure gas pressure of 100 atm or more and 1200 to 1800 ° C, which is not lower than 150 ° C lower than the pre-sintering temperature. And a method for producing a zirconia sintered body, which comprises obtaining a sintered body having a mean tetragonal crystal grain size of 2 μm or less.

The zirconia sintered body obtained by the present invention is a meta-stabilized zirconia sintered body containing a tetragonal crystal, which is different from the conventional fully stabilized zirconia sintered body and partially stabilized zirconia sintered body in strength and thermal stability. It is particularly excellent in sex.
In order to obtain those having such excellent properties, a pre-sintered body containing tetragonal crystals is obtained by controlling the kind and content of the stabilizer, the particle size of the zirconia powder, the pre-sintering conditions, etc. Furthermore, it is essential to perform the main sintering by the HIP method under specific conditions.

Hereinafter, the present invention will be described in more detail.

The raw material powder used for producing the sintered body of the present invention includes zirconia powder and yttria, magnesia, calcia or ceria as a stabilizer. When yttria is used, the ratio of zirconia to yttria is Y ₂
The O ₃ / ZrO ₂ molar ratio is in the range of 5/95 to 7/93. Here, when the amount of yttria added is 5 mol% or more, the amount of monoclinic crystals in the sintered body is small, cracks are less likely to occur in the sintered body, and the specific temperature of 200 to 300 ° C. Even if it is kept in the area for a long time, strength does not deteriorate. However, when the amount of yttria added exceeds 7 mol%, a cubic crystal-only sintered body is formed, and the tetragonal transition effect does not occur.

Further, in the powder, it is preferable that yttria is uniformly dissolved in zirconia as a solid solution or uniformly dispersed. For that purpose, fine powder of zirconia and yttria with an average particle size of 1 μm or less is directly mixed by a ball mill or the like,
Calcination may be performed in the range of 500 to 1100 ℃, but zirconium halides, zirconium oxyhalides, zirconium nitrates, zirconium compounds such as carbonates, and yttrium compounds such as yttrium halides should be mixed uniformly in solution. After or after precipitating a precipitate by hydrolysis or alkali from the homogeneous mixed solution, the method of removing the solvent, calcining only the solid at 500 ~ 1100 ℃, or a solution containing a zirconium compound and yttria or After uniformly mixing the zirconia and the solution containing the yttrium compound, the solvent is removed and only the solid matter is removed.
The raw material powder is preferably obtained by a method of calcination at 500 to 1100 ° C.

The powder thus obtained is pulverized by wet pulverization and dried to obtain a molding powder. The average particle size of this powder is 0.1
The specific surface area is about 5 to ²⁰ m ² / g, though the specific surface area is about 5 to 50 m ² / g. Next, a molded body having a predetermined shape is obtained by various molding methods such as mold pressing, isostatic pressing, mud casting, and extrusion injection molding. In this case, depending on the molding method, the powder may be added with a molding aid ordinarily used to carry out the treatment. 1000 times this molded body in an atmosphere of air, oxygen, or an inert gas.
Sintering at ˜1700 ° C. gives a zirconia pre-sintered body mainly composed of tetragonal crystals or tetragonal crystals and cubic crystals. What is important in the sintered body before the HIP method treatment is that the transverse strength is increased by the HIP method treatment described later. To this end, in the pre-sintered body before the HIP method treatment, it is preferable that the average particle size of the tetragonal crystal particles is 2 μm or less, as in the present sintered body.

The zirconia pre-sintered body thus obtained has a temperature range of 1200 to 1800 ° C. under a pressure of 100 atm or more in an inert gas atmosphere such as argon or nitrogen gas, and ZrO ₂ —Y ₂ O ₃
In the case of the sintered body of the system, the HIP method treatment is preferably carried out in the temperature range of 1200 to 1650 ° C., but at a temperature not lower than 150 ° C. lower than the pre-sintering temperature in any case. The pressure and temperature of this HIP process are determined by the density of the pre-sintered body, the crystal phase, etc., but when the temperature of the HIP process is less than 1200 ° C or less than 150 ° C lower than the pre-sintering temperature, or When the pressure is less than 100 atm, improvement in properties such as transverse rupture strength of the sintered body by the HIP method treatment cannot be recognized. Further, in the ZrO ₂ —Y ₂ O ₃ system sintered body, when the temperature is 1800 ° C. or higher, the average particle diameter of the tetragonal crystal particles in the sintered body becomes 2 μm or more, which is not preferable. This is because a sharp decrease in strength is not observed, but if held in a specific temperature range of 200 to 300 ° C for a long time, minute cracks due to the formation of monoclinic crystals tend to occur and the phenomenon of strength decrease easily occurs. Is.

The average particle size of the tetragonal particles of this sintered body can be determined as follows from the observation by an electron micrograph of a mirror-polished surface of the zirconia sintered body which is etched with hydrofluoric acid. That is, when the sintered body is composed only of tetragonal crystals, the average area s () of one tetragonal particle obtained from the number n of tetragonal particles existing in a certain area S containing 100 or more tetragonal particles The diameter d of a circle having an area equal to s = S / n) was calculated by the formula d = (4s / π) ^{1/2, and} was taken as the average sintered particle diameter of the tetragonal particles of the sintered body. In addition, even if it is not composed only of tetragonal crystals, the tetragonal crystal content is f, and the number of tetragonal crystal particles contained in a certain area S is n (even monoclinic crystal particles that cannot be distinguished from tetragonal crystal particles in the photograph are also included. Counted as tetragonal grains Monoclinic grains are produced by a phase transition of some tetragonal grains during the cooling process, and the size difference between tetragonal grains is small, and they are regarded as tetragonal grains. On the other hand, cubic grains are larger than tetragonal grains and can be clearly distinguished.) If the average area per grain is s, then s = Sf / Then, the average crystal grain size of the tetragonal grains of the sintered body can be obtained in the same manner as the system consisting of only tetragonal crystals.

In this HIP method treatment, if the theoretical density of the sintered body before treatment is low and some pores in the sintered body are open holes that communicate with the surface, it is possible that the HIP method is used to close the pores. Since only the part will be consumed and the open part will remain as it is, the sintered body with low theoretical density is covered with oxides or nitrides such as Si and A1 and sealed by HIP method. Treatment is preferred. Generally, if the porosity is 5% or less, this sealing treatment is not necessary.

The zirconia sintered body that has been subjected to the HIP method treatment as described above is mainly composed of tetragonal or tetragonal and cubic crystal phases. It is important that the abundance of each is, as mol%, tetragonal crystal = 10 to 100 monoclinic crystal = 0 to 40 cubic crystal = 100− (tetragonal crystal + monoclinic crystal). Further, it is preferable that the tetragonal crystal content is 50 mol% or more and the monoclinic crystal content is 10 mol% or less in order to obtain high strength. Here, if the content of the tetragonal crystal is less than 10 mol%, the strengthening mechanism due to the transition of the tetragonal crystal hardly acts, and the effect of the HIP method treatment becomes small. Also, if the monoclinic crystal exceeds 40 mol%, fine cracks in the sintered body are likely to occur, resulting in a decrease in strength or 200
The stability at ~ 300 ° C also deteriorates, making it practically unusable as a machine part. Here, the contents of tetragonal crystals, monoclinic crystals and cubic crystals were calculated by the following formula from the intensity obtained by analyzing the sintered body by the X-ray diffraction method and integrating the peaks of the diffraction pattern.

C _M = {M / (M + T <111> + C <111>)} × 100 C _T = {(T <220> + T <004>) / (C <400> + T <2
20> + T <004>)} × (100−C _M ) × 100 C _C = 100− (C _M + C _T ) where C _M : monoclinic zirconia content (mol%) C _T : tetragonal System zirconia content (mol%) C _C : Cubic zirconia content (mol%) M: Sum of <111> and <111> diffraction line intensities of monoclinic zirconia T <111>, T <220>, T <004>: Intensity of diffraction lines of <111>, <220>, and <004> of tetragonal zirconia C <111>, C <400>: <111 of cubic zirconia >,
Intensity of Diffraction Lines of <400> As described above, the zirconia sintered body containing yttria as a stabilizer has been described as an example.
A similar zirconia sintered body can be obtained by using calcia or ceria. In this case, the content of the stabilizer is preferably 5 to 10 mol% in magnesia, 6 to 11 mol% in calcia, and 7 to 12 mol% in ceria. Moreover, it does not matter even if two or more kinds of these stabilizers are mixed.

As described above, according to the method of the present invention, in comparison with the one not subjected to the HIP method treatment, the bending strength is usually 1.2 times or more, and in some cases, 2.7 times or more is obtained as a zirconia sintered body. be able to. Further, according to the present invention, it becomes possible to manufacture a sintered body having a complicated shape by using the pressure of the high temperature and high pressure gas, and the ceramic sintered body is used as a structural material such as a high performance machine part. Is made possible.

The ceramic sintered body exhibits oxygen ion conductivity at high temperatures and is excellent in thermal shock strength and mechanical strength, and thus can be used for various purposes. For example, it can be used in a very wide range such as solid electrolytes for oxygen concentration batteries, internal combustion engine parts, cutting tools, dies, blades, and other parts such as sliding parts for industrial machines.

Hereinafter, the method of the present invention will be described more specifically with reference to Examples, but the present invention is not limited thereto.

(Example 1) An aqueous solution of zirconium oxychloride and yttrium chloride was prepared so that the molar concentration of yttrium oxide was 6 mol% with respect to the total molar amount of zirconium oxide and yttrium oxide, and the mixture was heated for 60 hours and evaporated. This was dried and calcined at 900 ° C. to obtain zirconia powder having an average particle diameter of 0.35 μm in which 6 mol% of yttria was dissolved or dispersed.

This powder was pulverized with ethanol in a ball mill, dried and sized to obtain a molding powder.

This powder was cold isostatically pressed and then fired in the atmosphere for 2 hours to obtain a zirconia pre-sintered body. Further, this pre-sintered body was subjected to HIP method treatment to obtain a zirconia sintered body according to the method of the present invention. Pre-sintering temperature, HIP process condition,
Tables 1 and 2 show the density of the sintered body, the contents of tetragonal and cubic crystals, the average particle size of tetragonal crystals, and the bending strength. For comparison, the bending strength of the pre-sintered body without HIP treatment is also shown. The bending strength is measured by cutting out a 3 × 4 × 40 mm test piece from the sintered body, grinding and finishing it with a # 400 wheel, and measuring it by 3-point bending with a span of 30 mm and a crosshead speed of 0.5 mm / min. did.

(Example 2) Three kinds of powders were synthesized in the same manner as in Example 1, except that calcium chloride, magnesium chloride or cerium chloride was used instead of yttrium chloride. Pre-sintering and HIP method treatment were performed using these powders to produce zirconia sintered bodies, and their characteristics were measured. The conditions and the results are shown in Tables 1 and 2. At the same time, as a comparison, the one not subjected to the HIP method treatment is also shown.

Claims (2)

[Claims] 1. Y ₂ O ₃ / ZrO ₂ molar ratio yttria of 5/95 to 7/93, MgO / ZrO ₂ molar ratio 5/95 to 10/90 magnesia, CaO / ZrO.
₂ molar ratio 6 / 94-11 / 89 calcia or CeO ₂ / ZrO ₂ molar ratio
Pre-sintered body having mainly tetragonal or tetragonal and cubic crystal phases obtained by pre-sintering a compact made of zirconia powder containing 7/93 to 12/88 ceria at 1000 to 1700 ° C. By main sintering under high temperature and high pressure gas pressure of 100 atm or more and 1200 to 1800 ° C. and not lower than 150 ° C. lower than the pre-sintering temperature to mainly form tetragonal crystals or tetragonal and cubic crystals and Tetragonal average crystal grain size is 2μ
A method for producing a zirconia sintered body, characterized in that a sintered body having a size of m or less is obtained. 2. A zirconia sintered body having a crystal phase abundance ratio of mol%: tetragonal crystal = 10 to 100 monoclinic crystal = 0 to 40 cubic crystal = 100− (tetragonal crystal + monoclinic crystal) 2. A method for producing a zirconia sintered body according to claim 1.