JPH0782018A - Zirconia - Google Patents

Abstract

PURPOSE:To obtain a zicronia with secured mechanical strength and stably prevent from corrosion due to steam by incorporating mainly tetragonal zicronia with rare earth metal oxide(s) so as to come to specified concentration(s). CONSTITUTION:A zicronia >=90% in the content or tetragonal crystal is incorporated with at least one kind of rare earth metal oxide so as to come to 2.5-6.0mol% of its average concentration and 2.3-6.6mol% of its concentration in a microregion. By this design, because of specifying the average concentration(s) and the concentration if the microregion(s) of the rare earth metal oxide(s) as stabilizer(s), production of the aimed zicronia excellent in mechanical strength as well as highly excellent in corrosion resistance to steam becomes possible.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to zirconia having high strength and excellent steam corrosion resistance.

[0002]

2. Description of the Related Art Since zirconia is superior in toughness among ceramic materials, it is used as a ceramic blade.
Since it has a high melting point and excellent high temperature strength in addition to toughness, it is widely used as a high temperature member such as a solid electrolyte or refractory brick, or as a heat insulating member such as a chemical plant or smelting. In particular, in addition to excellent mechanical properties, it has the advantages of low thermal conductivity and a coefficient of thermal expansion close to that of metal, and recently it has been used to shield heat from aircraft jet engines, gas turbine dynamic and stationary blades, combustors, etc. Promising as a coating material.

Incidentally, zirconia has three crystal structures of monoclinic crystal, tetragonal crystal and cubic crystal, and the tetragonal crystal is the best from the viewpoint of mechanical properties. Pure zirconia (Zr
O ₂ ), the monoclinic crystal is a stable phase at room temperature, but when the temperature is increased, it becomes about 23 ° C from a monoclinic crystal to a tetragonal crystal at about 1000 ° C.
Reversible phase transition from tetragonal to cubic at around 00 ℃. The occurrence of this phase transition and the accompanying volume change are obstacles for the practical use of pure zirconia. That is, during the cooling process in the sintering process of zirconia, zirconia undergoes a phase transition from tetragonal to monoclinic, which is inferior in mechanical properties, and at that time, a large volume expansion occurs, which causes self-destruction in combination with reduced mechanical properties. I am afraid. In order to eliminate the phase transition in this cooling process, compounds such as Y ₂ O ₃ and CaO are usually used to carry tetragonal and cubic crystals with excellent mechanical properties to room temperature, that is, to stabilize the high temperature phase. Used by adding. As a result, the tetragonal to monoclinic phase transition that occurs during the cooling process can be suppressed, and a tetragonal or cubic crystal having excellent mechanical properties from room temperature to high temperature can be obtained.

However, the tetragonal zirconia stabilized with Y ₂ O ₃ also has a drawback that when exposed to an atmosphere of 80 to 300 ° C. containing water vapor, a phase transition different from the phase transition due to thermal history occurs. Remain. For example, when a stabilized zirconia is used for the thermal barrier coating on a moving blade or a stationary blade used in an atmosphere containing water vapor such as a gas turbine, or for the thermal barrier coating of a combustor, the operating temperature is 1100 to 1300. Although there is no problem at ℃, in the temperature range of 80 ~ 300 ℃, which is passed or held at the time of starting and stopping the gas turbine, stabilized zirconia has the drawback that it undergoes a phase transition from tetragonal to monoclinic (below, Called steam corrosion). Due to this steam corrosion, the zirconia coating layer is cracked and deteriorated, resulting in a marked decrease in durability.

In order to prevent steam corrosion as described above,
A method of adding 3 mol% Y ₂ O ₃ as a stabilizer and further adding 10 mol% CeO (J.Sato et.al. J. Mater.Sc
i.20 (1985) 3988-3992) and a method of dispersing alumina (K.
Tsukuma et.al.J.Mater.Sci.lett.4.857-861 (1985)),
Alternatively, a method of reducing the crystal grain size of zirconia to 1 μm or less (J.Sato et.al. Zirconia Ceramics 10 157-17
4, Uchida Old Tsuruga, etc.), a method of precipitating a phase that is hard to undergo phase transition by high temperature treatment (M. Yoshimura et.al. Zirconia Ceramics 10 185-194, Uchida Old Tsuruhaku), etc. There is. However, although partial success has been achieved in reducing steam corrosion, complete prevention of steam corrosion has not yet been realized. In particular, in the case of tetragonal zirconia, which has excellent mechanical properties, steam corrosion is not completely prevented even with zirconia containing 2 to 6 mol% of Y ₂ O ₃ , which is widely used as a stabilizer.
This is a constraint for practical use as a high temperature member.

[0006]

As described above, various attempts have been made to prevent steam corrosion in conventional zirconia to which Y ₂ O ₃ or the like was added as a stabilizer. Has not been completely prevented. Therefore, in systems exposed to an environment of 80 to 300 ° C. where water vapor exists, such as gas turbines and chemical plants, zirconia that secures mechanical properties and does not cause water vapor corrosion is desired.

The present invention has been made to solve the above problems, and provides zirconia capable of stably preventing steam corrosion while securing mechanical strength. Has an aim.

[0008]

Means and Actions for Solving the Problems The inventors of the present invention have conducted extensive studies on the concentration and distribution of a stabilizer in zirconia in order to achieve the above object, and as a result, have excellent mechanical properties of zirconia. It has been found that it is important to regulate and control the bias of the elements, which is inevitable in materials, especially ceramic materials in which the elements do not easily move, in order to secure the above-mentioned properties and prevent the steam corrosion. That is, conventionally, it was not possible to achieve complete prevention of steam corrosion with tetragonal zirconia excellent in mechanical properties,
For example, because the amount of Y ₂ O ₃ added was controlled only by the average concentration, in order to prevent steam corrosion,
This is because it has not been found that controlling the concentration distribution of Y ₂ O ₃ is extremely important.

The present invention defines the average concentration of the stabilizer in order to make the zirconia a tetragonal crystal structure excellent in mechanical strength, and the concentration distribution of the stabilizer in order to prevent steam corrosion of zirconia. By stipulating it, both mechanical properties that could not be achieved hitherto and complete prevention of steam corrosion are achieved. The concentration of the stabilizer here is a value obtained by converting the concentration of the stabilizing element dissolved in zirconia into the oxide concentration of the stabilizing element.

Conventionally, the control of the concentration of the stabilizer in zirconia has been carried out by the average concentration of the stabilizer in the raw material powder, but in ceramics such as zirconia, the concentration of the stabilizer in the raw material powder is distributed. That is, it is inevitable that there is a bias. Nevertheless, conventional zirconia has been managed as a material based only on the average concentration. For example, when preparing zirconia with 4 mol% Y ₂ O ₃ added, pure ZrO ₂ and pure Y ₂ O ₃ are mixed so that the average concentration of Y ₂ O ₃ is 4 mol% and the raw material powder is mixed. A method of sintering this raw material powder at a high temperature or coating it by a thermal spraying method, PVD, CVD or the like has been adopted. In this case, the mixed raw material powder is used in order to move and homogenize the Y element during calcination at high temperature or during sintering, etc. It is not possible to completely homogenize the concentration. In particular, when coating the raw material powder in a molten or semi-molten state by passing it through a plasma state at a high speed, as in the thermal spraying method, the homogenization time is extremely short, so the Y in the formed coating layer is The concentration of ₂ O ₃ is distributed at a concentration that deviates considerably from the average concentration.

Since steam corrosion is extremely sensitive to the amount of Y ₂ O _3, etc., it is significantly affected by such concentration distribution in the formed sintered member or coated member. On the other hand, Y ₂
A method of using a raw material powder in which O ₃ is previously solid-dissolved has also been adopted, but in this case as well, since the raw material powder is controlled by the average concentration, Y is more preferable than the case of using the mixed powder as described above.
Although the concentration distribution decreased, steam corrosion could not be completely prevented because the distribution was not controlled.

The present invention was made based on the above-mentioned findings, and the zirconia of the present invention is at least
One kind of rare earth element oxide is added, and in zirconia containing 90% or more of tetragonal crystal, the rare earth element oxide is
The average concentration in the zirconia is 2.5 to 6.0 mol%
, And the concentration in the micro area is 2.3 to 6.
It is characterized by being distributed between 6 mol%.

The zirconia of the present invention contains at least one selected from oxides of rare earth elements as a stabilizer. Examples of such rare earth oxides include yttrium oxide, cerium oxide, scandium oxide, praseodymium oxide, neodymium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium oxide, and oxide. Ytterbium, lutetium oxide, etc. are illustrated. Among the oxides of these rare earth elements, the trivalent rare earth elements are preferable to the tetravalent rare earth elements in that when added in the same amount, oxygen vacancies that act favorably against steam corrosion are generated.

Particularly, yttrium oxide is preferable because it is effective for producing stabilized zirconia having excellent strength and toughness. It is also effective to use yttrium oxide in combination with other rare earth oxides. For example, when a rare earth element having a larger ionic radius than that of yttrium is used, the lower limit value of the concentration distribution necessary for suppressing the phase change to monoclinic crystal due to the water vapor atmosphere can be made small, which is advantageous in structural control. . As such a rare earth oxide, neodymium oxide, samarium oxide, praseodymium oxide, dysprosium oxide or the like is preferably used.

The zirconia of the present invention contains 90% or more of tetragonal ZrO ₂ in order to secure the required mechanical strength, and in order to achieve this, the average concentration of rare earth oxide is 2.5 to 6.0 mol%. And the range. When the average concentration of rare earth oxides exceeds 6.0 mol%, the amount of cubic ZrO ₂ increases and it becomes impossible to obtain sufficient mechanical strength. Further, if the average concentration of the rare earth oxide is less than 2.5 mol%, it becomes impossible to suppress the decrease in mechanical strength due to steam corrosion. The constituent phases other than tetragonal crystal are
It may be a monoclinic crystal, a cubic crystal, or a mixture thereof.

Since the above-mentioned average concentration defines the average value of a wide range of rare earth oxide concentrations, the actual concentration of rare earth oxides in zirconia simply defining the average concentration is It permits the existence of a portion having a concentration significantly lower than the lower limit value of 2.5 mol% to a portion having a concentration significantly higher than the upper limit value of 6.0 mol%. On the other hand, from the viewpoint of preventing steam corrosion, even with the same tetragonal crystal structure,
Although it is necessary to set the concentration distribution of the rare earth oxide in an appropriate range, zirconia defined only by the average concentration includes a concentration outside the optimum concentration range. On the other hand, in order to obtain zirconia having a high mechanical strength, a tetragonal crystal having a rare earth oxide concentration lower than that of cubic ZrO ₂ having a high rare earth oxide concentration (average concentration of 6.0 mol% or more).
ZrO ₂ is preferred.

Therefore, in the zirconia of the present invention,
Along with the average concentration of the above rare earth oxides, the concentration distribution of the rare earth oxides in the minute region is specified in the range of 2.3 to 6.6 mol%. The concentration distribution of this rare earth oxide is intended to suppress the decrease in mechanical strength due to steam corrosion for the lower limit value, and for the upper limit value it should be composed of tetragonal ZrO ₂ having high mechanical strength. It was intended. Therefore, in order to prevent the steam corrosion while ensuring the mechanical strength, it is necessary to set the concentration distribution of the rare earth oxide in the minute region to the range of 2.3 to 6.6 mol%.

That is, the reason why the lower limit value of the concentration distribution of the rare earth oxide in the minute region is defined as 2.3 mol% is that when the concentration of the rare earth oxide is biased by less than 2.3 mol%, for example, the atmospheric atmosphere at 200 ° C. (Steam partial pressure P
_H2O = 1.7 × 10 ^-2 atm), the phase transition occurs in less than 2.3 mol% and monoclinic crystals increase. As a result, the amount of tetragonal crystals necessary for ensuring mechanical strength cannot be obtained, and what is important is that the number of unstable tetragonal crystals causing steam corrosion rapidly increases. The reason for setting the upper limit of the distribution of rare earth oxides to 6.6 mol% is
When the deviation exceeds 6.6 mol%, it is advantageous from the viewpoint of preventing steam corrosion, but cubic crystals increase rapidly and the amount of tetragonal crystals required to secure mechanical strength cannot be obtained.

When only yttrium oxide is used as the stabilizer, the average concentration is preferably in the range of 3.0 to 5.0 mol%. The concentration distribution of yttrium oxide in the minute region is preferably in the range of 2.8 to 5.6 mol%. Further, in order to completely suppress the phase transition to the monoclinic crystal in the water vapor atmosphere, it is desirable that the lower limit value of the yttrium oxide concentration distribution is 3.5 mol%.

In the zirconia of the present invention, the concentration distribution of the rare earth oxide in the minute region is particularly important. However, in general wet chemical analysis, the average concentration of rare earth oxides can be obtained, but the concentration distribution in a minute region cannot be known. Therefore, in the zirconia of the present invention, it is assumed that the concentration distribution of the minute regions of the zirconia cross section is measured by X-ray diffraction, EPMA, analytical electron microscope, etc., and the concentration distribution of these minute regions is within the above range. Here, since the accuracy of the distribution density increases as the area to be measured becomes smaller, the density when specified in the measurement area with a diameter of 5 μm or less may be within the range of the distribution specified in the present invention. preferable. More desirably, the concentration obtained in a microscopic area with a diameter of 1.0 μm or less is
It is desirable that all are within the specified distribution range.

The zirconia of the present invention can be used not only as a sintered body but also as a coating material on metal or ceramics. The zirconia sintered body or coating layer to which the present invention is applied can be produced with good reproducibility by applying, for example, the following production method.

That is, a rare earth oxide powder as a stabilizer and zirconia powder are mixed to prepare a stabilized zirconia powder. In this mixing step, the stabilizer powder and the zirconia powder are mixed by a general method.

First, a method of manufacturing a zirconia sintered body will be described. When obtaining a zirconia sintered body, first, a stabilized zirconia powder is obtained from a stabilizer and a zirconia powder by a coprecipitation method or a sol-gel method. By applying such a method, powder in a uniform state can be produced. Further, a sintered body is produced by firing this powder. At this time, it is preferable to perform firing at a low temperature for a short time so as to prevent grain growth and not impair the uniformity of the concentration distribution. For example, it is preferable to perform firing at a temperature of about 1350 to 1500 ° C. for about 0.5 to 4 hours. By suppressing the grain growth in the sintering stage in this way, it is possible to prevent the element transfer accompanying the grain growth.
Thereby, the concentration distribution of the rare earth oxide can be made uniform. Next, the case of forming a zirconia coating layer will be described. First, it is preferable to mix a rare earth oxide powder as a stabilizer and a zirconia powder, and then to prepare a raw material powder for a zirconia coating layer by an electrofusion method. That is, the above-mentioned mixed powder is electrofused and then rapidly cooled to be powdered. At this time, it is desirable that the heating temperature and the heating time during electrofusion be as large as possible. For example, it is preferable to heat the raw material powder at a temperature equal to or higher than the melting point (about 2400 ° C.) and to heat for 1 hour or more.

The coating method of zirconia is not particularly limited by the method of forming it, and the thermal spraying method and PV
Any method such as D or CVD can be applied.
However, it is preferable to apply a PVD method such as a thermal spraying method or EB-PVD to parts that are mass-produced in a complicated shape, such as a power generation plant member such as a gas turbine or a chemical plant member,
Further, it is preferable to apply the CVD method or the PVD method to thin film parts or minute parts such as an electrolyte member or an electronic member of a solid oxide fuel cell.

[0025] For example, in the thermal spraying method, the raw material powder is melted by moving it in plasma, but since such a melting cannot take a sufficient homogenizing time, the concentration of the rare earth oxide concentration in the coating layer is distributed. However, there is a portion that is extremely deviated from the average density. Therefore, as described above, it is preferable to perform the melting process in advance at the stage of the raw material powder, and thereby the concentration distribution of the rare earth oxide can be made uniform uniformly, that is, within the range of the present invention. After that, as in the case of the normal thermal spraying method, the raw material that has undergone the melting process is moved in plasma and deposited on the thermal sprayed member, whereby the coating layer of zirconia of the present invention can be stably obtained.

[0026]

EXAMPLES Examples of the present invention will be described below.

Examples 1 to 20, Comparative Examples 1 to 21 23.8 wt% Co-16 on a 70 × 50 × 30 mm SUS 304 substrate.
An alloy layer having a composition of 7wt% Cr-13.0wt% Al-0.6wt% Y-balNi was formed by the atmospheric spraying method, and each zirconia (Examples 1 to 20) shown in Table 1 was sprayed thereon, A coating layer was formed. The thermal sprayed layer of each zirconia in these examples was prepared by mixing zirconia and a rare earth oxide so as to have a predetermined composition, and then mixing this powder in a crucible 2500.
After electromelting under the condition of ° C x 2 hours, the powder was rapidly crushed, pulverized and stirred, and was subjected to atmospheric spraying under the conditions shown in Table 1 to be formed.

For comparison with the present invention, Table 2 was prepared on a substrate made of SUS 304 on which an alloy layer having the same composition as that of the example was formed.
Each zirconia shown in (Comparative Examples 1 to 21) was sprayed to form a coating layer. The sprayed layer of each zirconia in these comparative examples was prepared by mixing zirconia and a rare earth oxide so as to have a predetermined composition based on the conventional method and adding 2% P
After granulating with an VA aqueous solution, the powder was fired in hydrogen gas, and the powder was formed by performing atmospheric spraying under the same conditions as in the example. In addition, each of the spray-coated layers according to the examples and the comparative examples was composed of 90% or more of tetragonal ZrO ₂ .

[0029]

[Table 1] Part of each zirconia layer obtained in this way and the comparative example was taken, the average concentration of the stabilizer was examined by wet analysis, and the concentration of the stabilizer in the micro area of 1 μm in diameter was measured by EPMA at 50 points. The concentration distribution was measured and examined.
The results are also shown in Tables 2 and 3.

[0030]

[Table 2]

[Table 3] As is clear from Table 2, the zirconia layers according to the examples all have an average concentration within the range of 3.5 to 5.0 mol% or within the range of 2.5 to 6.0 mol%, and the concentration distribution in the minute region is within 2.8 to It is within the range of 5.6 mol% or within the range of 2.3 to 6.6 mol%. On the other hand, the zirconia layer according to the comparative example is
Within the range of 2.5 to 6.0 mol% (Comparative Example 11 only: 2.5 mol%
However, the concentration in the micro area is 2.8 mol.
There is a region of less than or equal to%, further less than 2.3 mol%, and a region of greater than or equal to 5.6 mol% and further greater than or equal to 6.6 mol%, and it can be seen that the concentration distribution varies widely.

Next, the zirconia layers according to the examples and the comparative examples were held in the atmospheric atmosphere at 200 ° C. (steam partial pressure = 0.02 atm) and steam at 15 atm for 200 hours respectively, and then the zirconia layer surface was removed. The crystal structure was examined by X-ray diffraction and the amount of phase transition was evaluated. At the same time, the peel strength test was conducted to examine the adhesion strength of these zirconia layers. Tables 4 and 5 show the evaluation results after the treatment in the air atmosphere, and Tables 6 and 7 show the evaluation results after the treatment in the steam.

[0032]

[Table 4]

[Table 5]

[Table 6]

[Table 7] As is clear from Tables 4 and 6, each zirconia layer according to the example has suppressed or prevented phase transition even after the treatment in steam, and has excellent resistance to steam corrosion. I understand. On the contrary,
As is clear from Tables 5 and 7, it can be seen that each zirconia layer according to the comparative example undergoes a large amount of phase transition after the treatment in water vapor, and the durability and the like are significantly reduced due to water vapor corrosion. .

[0033]

As described above, according to the present invention,
Since the concentration of the stabilizer is controlled together with the average concentration by the concentration distribution in a minute region, it becomes possible to provide zirconia having excellent mechanical strength and extremely excellent steam corrosion resistance.

[0034]

Claims (1)

[Claims] 1. In zirconia containing 90% or more of tetragonal crystals by adding at least one rare earth element oxide, the average concentration of the rare earth element oxide in the zirconia is in the range of 2.5 to 6.0 mol%. Zirconia characterized by being present and having a concentration in the minute region distributed between 2.3 and 6.6 mol%.