JP4511987B2 - Thermal barrier coating material - Google Patents

Description

  The present invention relates to a thermal barrier coating material applicable to equipment parts used in a high temperature environment such as a moving blade, a stationary blade, a combustor, and a jet engine of a power generation gas turbine.

  In order to improve the efficiency of gas turbines, jet engines, etc., the combustion gas has been getting hotter. Therefore, in order to protect the metal part from high temperature (for example, the maximum surface temperature of the blade is about 1350 ° C. in a 1700 ° C. class gas turbine), the surface of the part is provided with a thermal barrier coating (TBC). As a material for the thermal barrier coating, low thermal conductive ceramics such as rare earth stabilized zirconia such as yttria stabilized zirconia (YSZ) are used (for example, see Patent Document 1). The thermal barrier coating is applied by atmospheric pressure plasma spraying on a base material that is a metal part, after a metal bonding layer is applied by low pressure plasma spraying or the like.

  Thermal barrier coatings applied to metal parts by atmospheric pressure plasma spraying have many pores inside rather than a dense structure. FIG. 1 shows a schematic diagram of the structure of the thermal barrier coating. As shown in FIG. 1, in the structure of the thermal barrier coating material 1, there are atmospheric pores 2 having a diameter of several tens of μm, small pores 3 having a diameter of several μm, linear pores 4 and 5 having a narrow width, and the like. There are pores of various shapes. At the same time as the thermal barrier coating 1 itself is a low thermal conductive ceramic, the heat insulating performance of the material is maintained by such a large number of pores 2 to 5 existing inside, and the high temperature of the metal part as the base material is high. It can be used in an environment.

Zirconia when used as a high-temperature structural material, including thermal barrier coatings, is not a single composition (ZrO ₂ ), but a state where a few mole percent of a rare earth oxide such as yttria is added as a stabilizer (partially stabilized zirconia) Used in In pure zirconia (ZrO ₂ ) with no added stabilizer,
~ 1000 ° C 2370 ° C
Monoclinic crystal ← → Tetragonal crystal ← → There are two phase transitions such as cubic crystal, and by itself, sudden volume change accompanying monoclinic and tetragonal phase transition occurs when the temperature rises and falls. It cannot be used as a high-temperature structural material. Therefore, it is necessary to add a few mole% of rare earth oxide or the like to stabilize the tetragonal phase, which is the operating temperature range phase, even at a low temperature so that no monoclinic phase is generated. Even with partially stabilized zirconia that controls the amount of stabilizer and stabilizes the tetragonal phase, there is also a report that the monoclinic phase gradually precipitates during the heat cycle of repeated use at high temperature and temperature rise and fall, This is an important problem when using zirconia as a thermal barrier coating.

There is also a report that a material having a cubic pyrochlore structure such as La ₂ Zr ₂ O ₇ is applied as a thermal barrier coating material replacing zirconia (see Patent Documents 2 to 4). According to the literature, La ₂ Zr ₂ O ₇ is said to be suitable as a thermal barrier coating material because its thermal conductivity is smaller than that of zirconia and its oxygen permeability is smaller than that of zirconia. . However, La ₂ Zr ₂ O ₇ is also known to have a phase transition between fluorite and pyrochlore, and when sprayed, cracking occurs when changing from a high-temperature phase to a low-temperature phase, and the film strength is weak.
JP 2003-026475 A Japanese Patent Laid-Open No. 10-212108 European Patent No. 0848077 US Pat. No. 6,117,560

  Thus, it is indispensable for the zirconia-based thermal barrier coating material to ensure phase stability. The material used for the thermal barrier coating is required to have a high melting point, a low thermal conductivity, and no phase transition from high temperature to room temperature so as not to reduce the film strength.

  The present invention has been made to solve the above-mentioned problems, has a high melting point equivalent to YSZ, has no problems such as phase transition, has good phase stability in the operating temperature range, The object is to provide a novel thermal barrier coating material having a conductivity lower than that of zirconia.

  As described above, in order to provide a material having good phase stability in the operating temperature range, low thermal conductivity, and no phase transition as a novel thermal barrier coating material replacing zirconia, the present inventors have We searched for materials using one-principles calculations. The “first-principles calculation” is a method for obtaining various physical property values on the nanometer scale by changing the conditions for solving the basic equations of quantum mechanics.

The present inventors have found that the above requirement can be satisfied in a cubic structure derived from pyrochlore, for example, a structure represented by A ₃ BO ₇ . Since these materials have sites where element A and element B are in solid solution, it can be expected to show low thermal conductivity. In addition, by selecting Nb or Ta as the element B, it can be expected to have a high dielectric constant and lower thermal conductivity, and is considered suitable as a new thermal barrier coating material.

As a result of further research, many of the oxides (Ln ₃ NbO ₇ ) in which the element A found in the above research is rare earth and the element B is Nb (Ln ₃ NbO ₇ ) have no phase transition from high temperature to room temperature. It was revealed that the phase stability at 1 and the thermal conductivity was low. Similarly, a system in which a part of Nb is substituted with Ta, that is, an Ln ₃ Nb _1-x Ta _x O ₇ system oxide similarly exhibits low thermal conductivity and also has good phase stability in the operating temperature range. It became clear that.

That is, the thermal barrier coating material according to the reference example of the present invention has a composition formula (1):
_{Ln 3 Nb 1-x Ta x} O 7
(However, 0 ≦ x ≦ 1, Ln represents one or more elements selected from the group consisting of Sc, Y and lanthanoid elements)
Including the in represented composition mainly.
In the composition represented by the composition formula (1), _two tetravalent Zr in a composition having a conventional cubic pyrochlore structure including La ₂ Zr ₂ O ₇ are composed of trivalent Ln. The structure is replaced with one and pentavalent Nb and / or Ta. Therefore, in the composition represented by the composition formula (1), it is considered that Ln and Nb and / or Ta are in a solid solution state at random. Therefore, heat is easily scattered in the composition represented by the composition formula (1), and thus the composition has low thermal conductivity.
Furthermore, in the composition of the composition formula (1), Ln and Nb and / or Ta are solid-solved, so that phase transition hardly occurs, and even if thermal spraying is performed, the formed film is hardly cracked.

In the composition formula (1), the atomic ratio of Ln / (Nb _1-x Ta _x ) is about 3. However, these compounds form a solid solution, so the atomic ratio is about 2.5 to 3.5. It has a solid solution width of That is, the thermal barrier coating material according to the reference example of the present invention has a composition formula (2):
Ln _{3 + a} Nb _1-ax Ta _x O _7-a
(However, −0.5 ≦ a ≦ 0.5, 0 ≦ x ≦ 1-a, Ln represents one or more elements selected from the group consisting of Sc, Y and lanthanoid elements)
It may contain the composition represented by these as a main component.

In the composition formula (1) or (2), Ln is not limited to one element, and may be two or more elements selected from the group consisting of Sc, Y, and a lanthanoid element. For example, the thermal barrier coating material of the present invention has a composition formula (3) in which Ln is a composite of two elements A and B:
_{A 3-y B y Nb 1} -x Ta x O 7
(However, 0 ≦ x ≦ 1, 0 <y <3, A and B are different types of elements selected from the group consisting of Sc, Y and lanthanoid elements)
Or a thermal barrier coating material mainly comprising a composition represented by formula (4):
_{A 3 + a-y B y} Nb 1-a-x Ta x O 7-a
(However, −0.5 ≦ a ≦ 0.5, 0 ≦ x ≦ 1-a, 0 <y <3 + a, A and B are different elements selected from the group consisting of Sc, Y and lanthanoid elements)
The thermal barrier coating material which mainly contains the composition represented by these may be sufficient.
The present invention provides a thermal barrier coating material mainly comprising a composition represented by the composition formula (1) or (2), wherein the Ln represents a mixed element of Yb and Nd.

The thermal barrier coating material of the present invention has a high melting point equivalent to YSZ. In particular, the thermal barrier coating material of the present invention using Ta has a higher melting point than YSZ. This corresponds to an improvement in melting point of 400 to 700 ° C. as compared with the conventional strontium-based material. The thermal barrier coating material of the present invention exhibits sufficiently low thermal conductivity even when used alone. Moreover, even if the thermal barrier coating material of the present invention is used in combination with zirconia-based materials such as current rare earth stabilized zirconia, as shown in the compounding formula described in the examples below, Since the low thermal conductivity is not impaired, it is suitable as a thermal barrier coating material.
Further, the thermal barrier coating material of the present invention is suitable as a thermal barrier coating material without deteriorating its low thermal conductivity even when used as a multilayer structure together with zirconia-based materials such as current rare earth stabilized zirconia. .

  According to the present invention, since the thermal barrier coating material mainly contains the composition represented by the above composition formula (1) or (2), it has a high melting point equivalent to YSZ, A material suitable for use in a thermal barrier coating film exhibiting a lower thermal conductivity can be provided. The thermal barrier coating material of the present invention is suitably used on the surface thereof to protect metal parts of, for example, a 1700 ° C. class gas turbine from high temperatures.

  Furthermore, according to the present invention, the thermal barrier coating material includes a material obtained by combining the composition represented by the above-described composition formula (1) or (2) and a known zirconia-based material. It is possible to provide a thermal barrier coating material in which the thermal conductivity is further controlled within an appropriate range.

  Embodiments of the present invention will be described below.

[ Reference Example 1]
It is generally known that the thermal conductivity of a material greatly depends on its crystal structure, and that it has a lower thermal conductivity by taking a complicated structure. Therefore, we studied the complication of the crystal by bringing substituted solid solution of other elements in the Yb ₃ NbO _7. Nb constituting Yb ₃ NbO ₇ is a pentavalent element, and Ta belonging to the same group 5A in the periodic table was selected as a substitution element. This is because the ionic radius of Ta ⁵⁺ is 0.68 あ り, which is almost the same as that of Nb ⁵⁺ (0.69 Å), so that it is considered that it can be easily dissolved.

A material obtained by replacing part or all of Nb of Yb ₃ NbO ₇ with Ta, Yb ₃ Nb _0.75 Ta _0.25 O ₇ , Yb ₃ Nb _0.5 Ta _0.5 O ₇ , Yb ₃ Nb _0.25 ta _0.75 O _7, and to produce a _Yb 3 TaO _7. Yb ₂ O ₃ , Nb ₂ O ₅ , Ta ₂ O ₅ or the like was selected as a starting material, weighed to a predetermined ratio, and solid phase mixed using a ball mill. After drying the mixed powder, it was calcined at 1400 ° C. When the calcined powder was identified by powder X-ray diffraction, it was confirmed that no unreacted raw material components remained and that all samples were in a single phase.
These samples were fired at 1600 ° C., a disk-shaped sample having a diameter of 10 mmφ and a thickness of 1 mm was cut out from the sintered body, and the thermal conductivity was measured by a laser flash method. Table 1 shows the values of thermal conductivity at room temperature. In addition, the value (1000 degreeC, literature value) of the thermal conductivity of 3YSZ was also described in the table | surface as a comparative material.

As shown in Table 1, the oxide Yb ₃ NbO ₇ containing Yb and Nb has a melting point of 2150 ° C., a thermal conductivity of 1.21 [W / mK], and an oxide Yb containing Yb and Ta. ₃ TaO ₇ has a melting point of 2450 ° C. and a thermal conductivity of 1.21 [W / mK], all of which show suitable physical property values as a thermal barrier coating. All of the prepared samples have a thermal conductivity smaller than that of 3YSZ. This suggests that when these materials are used as thermal barrier coatings, the temperature of the underlying metal part can be lower than the current 3YSZ.
As a result of Ar + H ₂ atmospheric pressure plasma spraying using the present compound, the thermal conductivity of the thermal spray coating is reduced further to about 3-6 percent around a value shown in Table 1, obtained thermal barrier effect is high film It was.

[ Reference Example 2]
Although Yb constituting Yb ₃ NbO ₇ shown in Reference Example 1 is a trivalent element, the thermal conductivity was measured by replacing Yb with Er or the like belonging to the same rare earth element. The ionic radius of Yb ³⁺ is 1.01 Å, which is close to that of Er ³⁺ (1.03 Å). For Er ₃ NbO ₇ , Er ₃ Nb _0.75 Ta _0.25 O ₇ , Er ₃ Nb _0.5 Ta _0.5 O ₇ , Er ₃ Nb _0.25 Ta _0.75 O ₇ , Er ₃ TaO ₇ , The sample was fired at 1600 ° C., a disk-shaped sample having a diameter of 10 mmφ and a thickness of 1 mm was cut out from the sintered body, and the thermal conductivity was measured by a laser flash method. Table 2 shows the values of thermal conductivity at room temperature. In addition, the value (1000 degreeC, literature value) of the thermal conductivity of 3YSZ was also described in the table | surface as a comparative material.

As shown in Table 2, oxide _Er 3 NbO ₇ containing Er and Nb has a melting point of 2100 ° C., the thermal conductivity is 1.23 [W / mK], also oxides containing Er and Ta Er ₃ TaO ₇ has a melting point of 2150 ° C. and a thermal conductivity of 1.23 [W / mK], all of which show suitable physical property values as a thermal barrier coating. All of the prepared samples have a thermal conductivity smaller than that of 3YSZ. This suggests that when these materials are used as thermal barrier coatings, the temperature of the underlying metal part can be lower than the current 3YSZ.

[ Reference Example 3]
Yb constituting Yb ₃ NbO ₇ shown in Reference Example 1 is a trivalent element, but the thermal conductivity was measured by replacing Yb with Y or the like belonging to the same group 3A element. The ionic radius of Yb ³⁺ is 1.01 Å, which is close to that of Y ³⁺ (1.04 Å). Y ₃ NbO ₇ , Y ₃ Nb _0.75 Ta _0.25 O ₇ , Y ₃ Nb _0.5 Ta _0.5 O ₇ , Y ₃ Nb _0.25 Ta _0.75 O ₇ , Y ₃ TaO ₇ The sample was fired at 1600 ° C., a disk-shaped sample having a diameter of 10 mmφ and a thickness of 1 mm was cut out from the sintered body, and the thermal conductivity was measured by a laser flash method. Table 3 shows the values of thermal conductivity at room temperature. In addition, the value (1000 degreeC, literature value) of the thermal conductivity of 3YSZ was also described in the table | surface as a comparative material.

As shown in Table 3, the oxide Y ₃ NbO ₇ containing Y and Nb has a melting point of 2150 ° C., a thermal conductivity of 1.25 [W / mK], and an oxide Y containing Y and Ta. ₃ TaO ₇ has a melting point of 2450 ° C. and a thermal conductivity of 1.25 [W / mK], all of which show suitable physical property values as a thermal barrier coating. All of the prepared samples have a thermal conductivity smaller than that of 3YSZ. This suggests that when these materials are used as thermal barrier coatings, the temperature of the underlying metal part can be lower than the current 3YSZ.
As a result of performing Ar + H ₂ atmospheric pressure plasma spraying using this compound, the thermal conductivity as the sprayed coating is further reduced to about 30 to 60% of the value shown in Table 3, and a film having a high heat shielding effect is obtained. It was.

[Example 1 ]
Yb constituting Yb ₃ NbO ₇ shown in Reference Example 1 is a trivalent element, and Nb and the like belonging to the same group 3A element are mixed with Yb to complicate the crystal structure. The ionic radius of Yb ³⁺ is 1.01 Å, which is close to that of Nd ³⁺ (1.23 Å). Yb _1.5 Nd _1.5 NbO ₇ , Yb _1.5 Nd _1.5 Nb _0.75 Ta _0.25 O ₇ , Yb _1.5 Nd _1.5 Nb _0.5 Ta _0.5 O ₇ , Yb _1.5 Nd _1.5 Nb _0.25 Ta _0.75 O ₇ and Yb _1.5 Nd _1.5 TaO ₇ were fired at 1600 ° C., and the sintered body was disk-shaped with a diameter of 10 mmφ and a thickness of 1 mm. A sample was cut out and the thermal conductivity was measured by a laser flash method. Table 4 shows the values of thermal conductivity at room temperature. In addition, the value (1000 degreeC, literature value) of the thermal conductivity of 3YSZ was also described in the table | surface as a comparative material.

As shown in Table 4, the oxide Yb ₂ NdNbO ₇ containing Yb, Nd, and Nb has a melting point of 2050 ° C., a thermal conductivity of 0.92 [W / mK], and Yb, Nd, and Ta are included. The oxide Yb ₂ NdTaO ₇ contained has a melting point of 2050 ° C. and a thermal conductivity of 0.92 [W / mK], and all of them show suitable physical property values as a thermal barrier coating. All of the prepared samples have a thermal conductivity smaller than that of 3YSZ. This suggests that when these materials are used as thermal barrier coatings, the temperature of the underlying metal part can be lower than the current 3YSZ.
As a result of Ar + H ₂ atmospheric pressure plasma spraying using this compound, the thermal conductivity of the sprayed coating is further reduced to about 30 to 60% of the value shown in Table 4, and a film having a high heat shielding effect is obtained. It was.

[Example 2 ]
FIG. 2 is a phase diagram of the Y ₂ O ₃ —Nb ₂ O ₅ system.
From this phase diagram, the range of Y / Nb = 2.5 to 3.5 which is the range of the present invention, that is, the region where Nb ₂ O ₅ / (Y ₂ O ₃ + Nb ₂ O ₅ ) is about 22 to 29 mol%. Thus, it can be seen that there is no phase transition from the low temperature region to the high temperature region, and the low temperature phase remains.
It should be noted that substantially the same phase diagram was obtained for a system using other rare earth elements instead of Y and a system in which part or all of Nb was replaced with Ta. It can be seen that there is no phase transition to the phase and it remains in the low temperature phase.

[Example 3 ]
The composite of the low thermal conductivity material according to the present invention with zirconia was studied.
When two types of ceramics having different thermal expansion coefficients are combined, the thermal expansion coefficient (α _c ) of the composite material is expressed by the following equation (1) called the Turner equation. In the equation (1), α is a coefficient of thermal expansion, K is a bulk modulus, V is a volume fraction, a subscript m is a matrix, and a subscript p is a phase to be added.

_{α c = (α p V p} K p + α m V m K m) / (V p K p + V m K m) ··· (1)

The thermal conductivity (λ _c ) of the composite material is expressed by the following equation (2) called the Maxwell-Eucken equation. In the equation (2), λ is thermal conductivity, V is volume fraction, subscript m is a matrix, and subscript p is a phase to be added.

λ _c = λ _m (1 + 2V _p Φ) / (1-V _p Φ) (2)

However, Φ is expressed by the following equation (3).

Φ = (1-λ _m / λ _p ) / (2λ _m / λ _p +1) (3)

By using these equations (1), (2) and (3), when two materials having different thermal expansion coefficients and thermal conductivities are combined, the composite thermal expansion coefficient and the composite thermal conductivity are calculated. Can be calculated.
Consider, for example, the thermal conductivity when 3YSZ and Y ₃ TaO ₇ are compounded 1: 1. Assuming that 3YSZ is a matrix and Yb ₃ NbO ₇ is added, λ _m = 2.2, λ _p = 1.21, and V _p = 0.5 from Table 1, and these are (2) and ( 3) Substituting into the equation, the composite thermal conductivity (λ _c ) is λ _c = 1.7.
It becomes. This is a value suitable as a thermal barrier coating.

  Thus, by combining the material of the present invention exhibiting low thermal conductivity with zirconia, the thermal conductivity of zirconia can be lowered. It is considered that the thermal conductivity can be similarly controlled for other materials of the present invention.

  According to the thermal barrier coating material of the present invention, it is possible to provide a thermal barrier coating film having a low thermal conductivity. It is possible to obtain a high-performance gas turbine that can obtain heat resistance and durability and can easily cope with a high temperature of the combustion gas.

It is a schematic diagram of the structure | tissue of a thermal barrier coating. Y ₂ O ₃ is a state diagram of -Nb ₂ O ₅ system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Thermal barrier coating material 2 Atmosphere hole of several dozen micrometer diameter 3 Small pore of several micrometer diameter 4 Linear pore 5 Linear pore

Claims (6)

Composition formula (1):
_{Ln 3 Nb 1-x Ta x} O 7
(However, 0 ≦ x ≦ 1, Ln represents a mixed element of Yb and Nd)
A thermal barrier coating material mainly comprising a composition represented by Composition formula (2):
Ln _{3 + a} Nb _1-ax Ta _x O _7-a
(However, −0.5 ≦ a ≦ 0.5, 0 ≦ x ≦ 1-a, Ln represents a mixed element of Yb and Nd)
A thermal barrier coating material mainly comprising a composition represented by   The thermal barrier coating material according to claim 1, wherein x = 0. The thermal barrier coating material according to claim 1, wherein x = 1 in the composition formula (1). The thermal barrier coating material according to claim 2, wherein x = 1−a in the composition formula (2).   A thermal barrier coating material in which the composition according to any one of claims 1 to 5 and the zirconia-based material are combined or have a multilayer structure.

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