U.S. patent application number 09/952438 was filed with the patent office on 2002-03-07 for thermal barrier layer and process for producing the same.
Invention is credited to Beele, Wolfram.
Application Number | 20020028344 09/952438 |
Document ID | / |
Family ID | 7814221 |
Filed Date | 2002-03-07 |
United States Patent
Application |
20020028344 |
Kind Code |
A1 |
Beele, Wolfram |
March 7, 2002 |
Thermal barrier layer and process for producing the same
Abstract
A product which is to be exposed to a hot gas and has a thermal
barrier layer, such as a component for hot gas ducts (turbine
blades, heat shields, etc.), has a metallic base body made of a
superalloy based on nickel, cobalt or iron. An adhesion promoter
layer also serves to form aluminum oxide/chromium oxide. A thermal
barrier layer is formed of a ternary or pseudoternary oxide having
a pyrochlore or perovskite structure. The oxide is stable with
respect to phase between room temperature and melting temperature
even in the absence of a phase stabilizer. A process is provided
for producing components of that type by atmospheric plasma
spraying or electron beam PVD methods.
Inventors: |
Beele, Wolfram; (Ratingen,
DE) |
Correspondence
Address: |
Siemens Corporation
Intellectual Property Department
186 Wood Avenue South
Iselin
NJ
08830
US
|
Family ID: |
7814221 |
Appl. No.: |
09/952438 |
Filed: |
September 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09952438 |
Sep 13, 2001 |
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09329760 |
Jun 10, 1999 |
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6319614 |
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09329760 |
Jun 10, 1999 |
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PCT/DE97/02769 |
Nov 26, 1997 |
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Current U.S.
Class: |
428/632 ;
427/446; 428/702 |
Current CPC
Class: |
C23C 4/11 20160101; C23C
14/08 20130101; Y10T 428/12611 20150115 |
Class at
Publication: |
428/632 ;
428/702; 427/446 |
International
Class: |
B32B 015/04; B05D
001/08 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 10, 1996 |
DE |
196 51 273.5 |
Claims
I claim:
1. A product to be exposed to a hot gas, comprising a metallic base
body to which a ceramic thermal barrier layer formed with a ternary
or pseudoternary oxide is bonded, in which said oxide has a
pyrochlore crystal structure of the structural formula
A.sub.2B.sub.2O.sub.7.
2. The product according to claim 1, in which a bonding layer
having a bonding oxide is formed between the base body and the
thermal barrier layer.
3. The product according to claim 1, which, between the base body
and the thermal barrier layer has an adhesion promoter layer
forming a bonding oxide.
4. The product according to claim 2, in which the bonding oxide is
at least one of aluminum oxide and chromium oxide.
5. The product according to claim 1, in which the oxide undergoes
no phase transition between room temperature and a maximum
permissible working temperature which is higher than 1250.degree.
C.
6. The product according to claim 1, in which the oxide has a
melting temperature above 2150.degree. C.
7. The product according to claim 1, in which the thermal barrier
layer has pores or other voluminous defects.
8. The product according to claim 1, in which the thermal barrier
has a columnar microstructure, the axial direction of the
crystallites being perpendicular to the surface of the base
body.
9. The product according to claim 1, which is a component of a heat
engine.
10. The product of claim 9, in which the heat engine is a gas
turbine.
11. The product of claim 1, in which the oxide is a metal hafnate,
a metal zirconate, a metal cerate or a mixed form of these
oxides.
12. The product of claim 11, in which the element A of the oxide is
at least one of lanthanum, aluminum, and cerium.
13. The product of claim 12, in which the oxide comprises lanthanum
hafnate.
14. A product to be exposed to a hot gas, comprising a metallic
base body to which a ceramic thermal barrier layer formed with a
ternary or pseudoternary oxide is bonded, in which said oxide has a
perovskite crystal structure of the structural formula ABO.sub.3,
in which A is calcium or ytterbium, and when A is calcium B, is
hafnium and when A is ytterbium, B is hafnium or zirconium.
15. The product according to claim 14, in which a bonding layer
having a bonding oxide is formed between the base body and the
thermal barrier layer.
16. The product according to claim 14, which, between the base body
and the thermal barrier layer has an adhesion promoter layer
forming a bonding oxide.
17. The product according to claim 15, in which the bonding oxide
is at least one of aluminum oxide and chromium oxide.
18. The product according to claim 14, in which the oxide undergoes
no phase transition between room temperature and a maximum
permissible working temperature which is higher than 1250.degree.
C.
19. The product according to claim 14, in which the oxide has a
melting temperature above 2150.degree. C.
20. The product according to claim 14, in which the thermal barrier
layer has pores or other voluminous defects.
21. The product according to claim 14, in which the thermal barrier
has a columnar microstructure, the axial direction of the
crystallites being perpendicular to the surface of the base
body.
22. The product according to claim 14, which is a component of a
heat engine.
23. The product according to claim 22, in which the heat engine is
a gas turbine.
24. In a process for producing a product to be exposed to hot gas
and having a metallic base body, the improvement which comprises
applying a ceramic thermal barrier layer formed with a ternary or
pseudoternary oxide having a pyrochlore crystal structure of the
structural formula A.sub.2B.sub.2O.sub.7) to the base body by
plasma spraying or by a PVD method.
25. A process according to claim 24, in which the PVD method
comprises an electron beam PVD method.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of copending
International Application No. PCT/DE97/02769, filed Nov. 26, 1997,
which designated the United States.
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
[0002] The invention relates to a ceramic-coated product, in
particular a ceramic coated component, for use in a hot gas duct,
especially in industrial gas turbines. The invention furthermore
relates to a process for producing a product having a thermal
barrier layer.
[0003] A product of that type has a base body of a metal alloy
based on nickel, cobalt or iron. Products of that type are
primarily used as a component of a gas turbine, in particular as
gas turbine blades or heat shields. The components are exposed to a
hot gaseous flow of aggressive combustion gases. They must
therefore be capable of withstanding very heavy thermal stresses.
It is furthermore necessary for those components to be resistant to
oxidation and corrosion.
[0004] Primarily for moving components, e.g. gas turbine blades,
but also for static components, there are also mechanical
requirements. The power and the efficiency of a gas turbine in
which components that can be subjected to hot gas are used, rise
with increasing operating temperature. In order to achieve high
efficiency and high power, those parts of the gas turbines which
are especially subjected to the high temperatures are coated with a
ceramic material. The latter acts as a thermal barrier layer
between the hot gas flow and the metallic substrate.
[0005] The metallic base body is protected from the aggressive hot
gas flow by coatings. That being the case, modern components
usually have a plurality of coatings, each of which fulfils
specific requirements. A multilayer system is thus involved.
[0006] Since the power and efficiency of gas turbines rise with
increasing operating temperature, efforts are constantly being made
to achieve higher gas turbine performance by improving the coating
system. A first approach with a view to this improvement is in
optimizing the adhesion layer. U.S. Pat. No. 4,321,310 discloses
the application of an McrAlY adhesion layer in such a way that it
has a low degree of surface roughness. A layer of aluminum oxide is
then formed thereon in order to achieve thereby a substantial
improvement in the adhesion of the thermal barrier layer
[0007] U.S. Pat. No. 4,880,614 discloses incorporation of a
high-purity aluminum layer between the MCrAlY adhesion layer and
the metallic base body. That aluminum is used to form a dense
Al.sub.2O.sub.3 layer on the adhesion layer in order to increase
the life of the coated component.
[0008] U.S. Pat. No. 5,238,752 discloses an adhesion layer of
nickel aluminides or platinum aluminides. A layer of aluminum oxide
is formed on that adhesion layer. The thermal barrier layer is
applied thereon.
[0009] U.S. Pat. No. 5,262,245 discloses that the aluminum oxide
layer is formed as an oxidation layer from the material of the base
body. For that purpose, the base body has a nickel-based alloy
which has strongly oxide-forming alloy constituents.
[0010] U.S. Pat. No. 4,676,994 discloses the application of a layer
that forms aluminum oxide to a base body. Aluminum oxide is formed
on the surface of this layer. A dense ceramic layer is applied
thereon by evaporation coating. This ceramic layer is formed of a
dense substoichiometric ceramic material. It may be an oxide,
nitride, carbide, boride, silicide or a different refractory
ceramic material. A thermal barrier layer is applied to that
ceramic layer.
[0011] The great majority of the above U.S. patents indicate that
the thermal barrier layer has a columnar microstructure in which
the crystallite columns of the columnar microstructure extend
perpendicular to the surface of the base body. Stabilized zirconium
oxide is indicated as the ceramic material. Suitable stabilizers
include calcium oxide, magnesium oxide, cerium oxide and,
preferably, yttrium oxide. The stabilizer is needed in order to
prevent a phase transition from the cubic to the tetragonal and
then monoclinic crystal structure. In essence, the tetragonal phase
is stabilized to about 90%.
[0012] In U.S. Pat. No. 4,321,311, voluminous defects are provided
in the thermal barrier layer in order to reduce stresses which are
produced in the thermal barrier layer when the temperature changes,
as a result of the fact that the base body and the thermal barrier
layer have different coefficients of thermal expansion. The thermal
barrier layer has a columnar structure with gaps between the
individual columns of the coating of zirconium oxide stabilized
with yttrium oxide.
[0013] Another proposal for solving the problem of stress when
confronted with temperature variation is indicated in U.S. Pat. No.
5,236,787. There, an intermediate layer of a metal/ceramic mixture
is interposed between the base body and the thermal barrier, in
which the metallic proportion of this intermediate layer increases
in the direction of the base body and to decrease in the direction
of the thermal barrier layer. Conversely, the ceramic proportion
should be low close to the base body and high close to the thermal
barrier layer. The thermal barrier layer proposed is a zirconium
oxide stabilized with yttrium oxide and having some proportion of
cerium oxide. The thermal barrier layers are deposited on the base
body by plasma spraying or PVD methods. The proportion of the
yttrium oxide stabilizer is from 8 to 20% by weight.
[0014] U.S. Pat. No. 4,764,341 discloses the bonding of a thin
metal layer to a ceramic. Nickel, cobalt, copper and alloys of
these metals are used for the metal layer. In order to bond the
metal layer to the ceramic substrate, an intermediate oxide such as
aluminum oxide, chromium oxide, titanium oxide or zirconium oxide
is applied to the ceramic substrate. At a sufficiently high
temperature, this intermediate oxide forms a ternary oxide through
oxidation by incorporating an element from the metallic
coating.
SUMMARY OF THE INVENTION
[0015] It is accordingly an object of the invention to provide a
product to be exposed to a hot gas and having a base body of metal
and bonded thereto a thermal barrier layer, and a process for
producing the same, which overcome the disadvantages of the
heretofore-known products and processes of this general type.
[0016] With the foregoing and other objects in view there is
provided, in accordance with the invention, a product to be exposed
to a hot gas and having a metallic base body to which a ceramic
thermal barrier layer formed with a ternary or pseudoternary oxide,
is bonded, in which said oxide has a pyrochlore crystal structure
of the structure formula A.sub.2B.sub.2O.sub.7.
[0017] With the objects of the invention in view, there is also
provided a product to be exposed to a hot gas and having a metallic
base body to which a ceramic thermal barrier layer formed with a
ternary or pseudoternary oxide, is bonded, in which that oxide has
a perovskite crystal structure of the structure formula ABO.sub.3
in which A is calcium or ytterbium, and when A is calcium, B is
hafnium and when A is ytterbium, B is at least one of zirconium and
hafnium.
[0018] The invention is based on the fact that, until now,
materials for thermal barrier layers have predominantly been
pseudobinary ceramics, that is ceramic materials having a general
structural formula which can be represented as AB.sub.2 or
A.sub.2B.sub.3. In this case, a material based on zirconium oxide
has proved most advantageous. However, from as little as
900.degree. C., zirconium oxide displays evidence of aging. This is
caused by the zirconium oxide thermal barrier layer sintering. As a
result, the pores and the voluminous defects in the thermal barrier
layer undergo progressive diminishment, and the stresses caused by
the different thermal expansion coefficients of the material
forming the thermal barrier layer and the material forming the base
body are reduced less and less well. This sintering process is
reinforced by material impurities. It is further reinforced by the
interaction of the thermal barrier layer with hot gas constituents,
with materials in the base body and the material of the adhesion
layer. Above all, the yttrium oxide used as a stabilizer promotes
aging. Since it is desirable to have a long service life of gas
turbines operating under full load, for example 10,000 hours, the
permissible surface temperature of components having thermal
barrier layers made of zirconium oxide is limited to 1250.degree.
C. This maximum permissible surface temperature dictates and limits
the power and efficiency of gas turbines.
[0019] According to the invention, in contrast thereto, the product
has a ceramic thermal barrier layer with a ternary or pseudoternary
oxide. The oxide preferably has a pyrochlore or perovskite
structure as defined. The material of the thermal barrier layer
preferably has no phase transition from room temperature to its
melting temperature. It is then not necessary to add a stabilizer.
The melting temperature depends on the respective chemical compound
and is preferably above 2150.degree. C.
[0020] According to a particular feature of the invention, a
bonding layer having a bonding oxide is disposed between the base
body and the thermal barrier layer. This layer can, for example, be
produced by applying an oxide. Preferably, however, the bonding
layer forms an adhesion promoter layer by oxidation, which adhesion
promoter layer is disposed between the thermal barrier layer and
the base body. The oxidation of the adhesion promoter layer can
take place before application of the thermal barrier layer, or
alternatively during use of the product in an oxygen-containing
atmosphere. In this case, the adhesion promoter layer preferably
contains a metallic element that forms an oxide. It is likewise
possible for the bonding layer to be formed directly by oxidation
of the alloy of the metallic base body. For this purpose, the alloy
of the base body has a corresponding metallic element. The bonding
oxide is preferably chromium oxide and/or aluminum oxide.
[0021] According to a further feature of the invention, the product
is preferably a component of a heat engine, for example a gas
turbine blade, a heat shield part of a combustion chamber of a gas
turbine or a component of a combustion engine. Such gas turbine
components, e.g. turbine blades or heat shields, preferably have a
base body which is formed of a superalloy based on nickel, chromium
or iron. On this base body there is, in particular, an MCrAlY
adhesion promoter layer. It also serves as an oxidation protection
layer since, in air or virtually any other oxygen-containing
environment (i.e. at least when the component is used, if not
earlier) part of the aluminum and/or chromium is converted into
oxide. On this adhesion promoter layer is the thermal barrier layer
which is formed of a ternary or pseudoternary oxide having a
pyrochlore or perovskite structure. The term ternary oxide defines
a substance which is formed of atoms of three different chemical
elements. The term pseudoternary oxide defines a substance which
contains atoms of more than three different chemical elements, but
these atoms belong to only three different element groups, the
atoms of the individual elements in each of the three different
element groups being equivalent in terms of crystallography.
[0022] These ceramic substances have the low thermal conductivity
required of thermal barrier layers. The thermal conductivity is, in
particular at higher temperatures, comparable with that of
zirconium oxide. Furthermore, the ceramic substances of the thermal
barrier layer have a coefficient of thermal expansion which is
compatible with the coefficient of thermal expansion of the
material of the base body. The coefficient of thermal expansion is
about 9.times.10.sup.-6/K. The ceramic substances of the thermal
barrier layer which contain ternary oxides are preferably phase
stable between room temperature and melting temperature. This
obviates the need for a stabilizer, whose presence promotes aging.
They are furthermore sure to adhere stably to the base body through
the use of the MCrAlY adhesion promoter layer. It should
furthermore be emphasized that the rates of evaporation of the
ceramic substances of the thermal barrier layer are very low. As an
order of magnitude, for example, the evaporation rate of lanthanum
hafnate is 0.4 .mu.m per 1000 hours at 1600.degree. C.
[0023] With the objects of the invention in view, there is
additionally provided a process for applying the thermal barrier
layers in which the coating takes place with a ternary oxide, in
particular a pyrochlore ceramic through atmospheric plasma spraying
or a PVD method, for example an EB-PVD (Electron Beam Physical
Vapor Deposition) method. In the case of both methods, a layer
having the desired porosity can be introduced by suitable choice of
the process parameters. It is also possible to produce a columnar
microstructure. It is in this case not absolutely necessary for the
starting material used for the coating to already have the same
chemical and crystallographic composition as the material of the
finished coating. Above all in the case of the lanthanum hafnate,
it is possible to use a powder mixture, being formed of two binary
oxides, for the starting material of the coating process. The mass
ratio of the two powders corresponds in this case to the
stoichiometric composition of the thermal barrier layer then formed
on the component by the coating process. By way of example, a
thermal barrier layer made of lanthanum hafnate can be produced by
using a mixture of hafnium oxide and lanthanum oxide as starting
material in an EB-PVD process. In this case, the molar ratio of
hafnium oxide to lanthanum oxide is 1.29.
[0024] Other features which are considered as characteristic for
the invention are set forth in the appended claims.
[0025] Although the invention is illustrated and described herein
as embodied in a product to be exposed to a hot gas and having a
thermal barrier layer, and a process for producing the same, it is
nevertheless not intended to be limited to the details shown, since
various modifications and structural changes may be made therein
without departing from the spirit of the invention and within the
scope and range of equivalents of the claims.
[0026] The construction and method of operation of the invention,
however, together with additional objects and advantages thereof
will be best understood from the following description of specific
embodiments when read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a plan view of a plane of a pyrochlore
structure;
[0028] FIG. 2 is a view of a portion of an elementary cell of a
pyrochlore structure;
[0029] FIG. 3 is a view of a unit cell of a perovskite
structure;
[0030] FIG. 4 is a view of a unit cell of the perovskite structure,
in which the unit cell has been shifted by 1/2, 1/2, 1/2 relative
to the one in FIG. 1; and
[0031] FIG. 5 is a fragmentary, diagrammatic, cross-sectional view
of a turbine blade.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] Referring now to the figures of the drawings in detail, it
is noted that in a thermal barrier layer made of a ceramic
substance being formed of a ternary oxide having pyrochlore
structure, the crystal structure has 88 atoms per unit cell. The
general chemical structural formula of ternary oxides of this type
is A.sub.2B.sub.2O.sub.7, in which "A" and "B" are metal ions and
"O" is oxygen.
[0033] The pyrochlore structure is described below with regard to
FIG. 1. The relatively small B cations coordinate with oxygen atoms
in the form of an octahedron. These octahedra form a
three-dimensional network in which the neighboring octahedra each
share an oxygen atom. The relatively large A cations in this case
are situated in a hexagonal ring formed by the oxygen atoms of the
coordination octahedra of the B cations. At right angles to the
surface of the ring, above and below the respective A cation, there
is an oxygen atom having a bond length which in this case is
somewhat shorter than the bond length to the oxygen atoms of the
ring. An A cation therefore coordinates with oxygen in the form of
a hexagonal double pyramid.
[0034] Another description of the pyrochlore structure shown in
FIG. 2 is that the structure is composed of two types of
coordination polyhedra for the cations. In this case, six
equidistant oxygen atoms in the form of trigonal antiprisms
coordinate the relatively small B cations. The larger A cation is
coordinated by six equidistant oxygen atoms and two additional
oxygen atoms with somewhat shorter bond length.
[0035] These eight oxygen atoms form a twisted cube around the A
cation.
[0036] Difficulties arise in describing the structure, in
particular because the coordination polyhedra become distorted when
there are different bond lengths between the cations and oxygen
atoms, depending on which chemical elements are actually present
for the A and B cations. It therefore seems that powder
diffractometry measurements do not permit reliable conclusions
regarding mutual coordination of the various atoms. To this extent,
it is necessary and, for characterizing the polychlore structure,
it is sufficient to characterize it through the use of the 2.theta.
values from the powder diffractometry measurement. The following
table gives the 2.theta. values for intensities characteristic of
polychlore, and the associated hkl values.
1 2.theta. hkl 29.2 111 33.2 200 47.8 220 56.7 311 59.4 222 69.8
400 77.2 331 79.6 420 89.0 511/311
[0037] Due to impurities in the powder which is being examined,
slight deviations from the 2.theta. values may occur in the first
decimal place. Systematic errors may also occur in powder
diffractometry measurement. Errors of this type can basically
affect the measured 2.theta. values in two ways: on one hand, the
measured 2.theta. values may as a whole be shifted to larger or
smaller 2.theta. values. In this case, however, the difference
between two consecutive 2.theta. values remains the same. On the
other hand, it may happen that the intensities as a whole appear
stretched or squashed over the 2.theta. value range. Nevertheless,
the ratio of the distances between consecutive 2.theta. values for
the sample which is measured is equal to the ratio of the
corresponding distances between consecutive 2.theta. values in the
table given above.
[0038] The A and B cations in the general chemical structural
formula preferably stand for rare earth metals and aluminum
(generally: A.sup.3+ cations) and hafnium, zirconium and Ce
(generally: B.sup.4+ cations)
[0039] In order to provide a thermal barrier layer with a ternary
oxide, in particular with pyrochlore structure, the following
substances are preferably suitable: lanthanum hafnate
(La.sub.2Hf.sub.2O.sub.7), lanthanum zirconate
(La.sub.2Zr.sub.2O.sub.7), aluminum hafnate
(Al.sub.2Hf.sub.2O.sub.7), cerium hafnate
(Ce.sub.2Hf.sub.2O.sub.7), cerium zirconate
(Ce.sub.2Zr.sub.2O.sub.7), aluminum cerate
(Al.sub.2Ce.sub.2O.sub.7) and lanthanum cerate
(La.sub.2Ce.sub.2O.sub.7).
[0040] Suitable coating materials with pyrochlore structure also
include pseudoternary oxides. These can, for example, have the
structural formula La.sub.2(HfZr)O.sub.7 or (CeLa)Hf.sub.2O.sub.7.
Compounds having fractional indices can also be considered, for
example La.sub.2 (Hf.sub.1.5Zr.sub.0.5) O.sub.7. It is also
possible for both the A ions and the B ions to include a plurality
of elements at the same time. These compounds are distinguished in
that, in comparison with the elements which constitute them, they
have a solubility range of several mol %. The formation of deposits
with heavily superstoichiometric or substoichiometric composition
is thereby avoided. They are furthermore distinguished in that they
are stable with respect to phase over a broad temperature range.
This means that the pyrochlore structure is maintained in the
temperature range relevant to operation in hot gas ducts. Thus,
La.sub.2Hf.sub.2O.sub.2 and La.sub.2Zr.sub.2O.sub.7 do not change
their crystal structure until above 1500.degree. C. This also
obviates the need to add a stabilizer. The stabilizer's effect of
promoting aging of the material is consequently eliminated, and the
permissible operating temperature can consequently be raised to
higher values.
[0041] Coating materials with a perovskite structure have the
general chemical structural formula ABO.sub.3. Compounds with
perovskite structure differ from those with ilmenite structure,
which also have the general chemical formula ABO.sub.3, in that the
A ions are relatively small compared to the B ions.
[0042] Crystallographically, the perovskite structure is described
reliably enough. It is substantially smaller than the pyrochlore
structure. The perovskite structure has four atoms in the unit
cell. FIG. 3 shows a unit cell of the perovskite structure. FIG. 4
shows a unit cell of the perovskite structure, which is shifted by
1/2, 1/2, 1/2 relative to the unit cell in FIG. 3. The smaller A
cations are represented as solid circles, the larger B cations as
shaded circles and the oxygen anions as empty circles. As can be
seen from FIGS. 3 and 4, the perovskite structure is a cubic
structure. In this structure, the larger B ions occupy the corners
of the unit cube, the smaller A ions occupy the center and the O
ions occupy its surface centers (FIG. 4). The structure can also be
described in that the larger B ions and the O ions together form a
cubic close-packed system where 1 in 4 of the octahedral sites are
occupied with A ions. The B ions are each coordinated with 12 O
ions in the form of a cubo-octahedron, and each O ion has four
neighboring B ions and two neighboring A ions.
[0043] The following oxide compounds having perovskite structures
are preferably used as the material for thermal barrier layers:
ytterbium zirconate (YbZrO.sub.3), ytterbium hafnate (YbHfO.sub.3),
calcium zirconate (CaZrO.sub.3) and calcium hafnate (CaHfO.sub.3).
Ytterbium zirconate and ytterbium hafnate are particularly
preferred in this case.
[0044] Also in the case of materials with perovskite structure for
thermal barrier layers, it is not necessary for all of the cations
of the A group the B group to be the same element. Here again,
pseudoternary oxide compounds, for example with structural formula
Yb(Zr.sub.0.5Hf.sub.0.5)O.- sub.3 etc. are possible.
[0045] Like coating materials with pyrochlore structure, materials
with perovskite structure also exhibit no phase transition from
room temperature to high temperatures, if not to the melting
temperature. For this reason, they are as advantageous as coating
materials with pyrochlore structure.
[0046] FIG. 5 represents a portion of a non-illustrated gas turbine
blade or a heat shield element of a combustion chamber of a gas
turbine. An adhesion promoter layer 2 is applied to a base body 1
which is formed of a superalloy, in particular based on nickel,
cobalt or iron. The adhesion promoter layer 2 is formed of a
metal/chromium/aluminum/yttrium (MCrAlY) alloy. The adhesion
promoter layer 2 serves to ensure adhesion between a thermal
barrier layer 4 and the base body 1. The aluminum and/or chromium
contained in the adhesion promoter layer serves to form aluminum
oxide/chromium oxide. A bonding layer 3, in particular a dense
passive layer, of aluminum oxide or chromium oxide, respectively,
is formed and protects the base body 1 from oxidation by acting as
an oxygen barrier. The thermal barrier layer 4 is applied to the
base body 1 preferably by atmospheric plasma spraying or by a PVD
method, e.g. EB-PVD method. If atmospheric plasma spraying is used
as the application method, the process parameters can be chosen in
such a way as to set up the desired porosity in the thermal barrier
layer 4. When an EB-PVD method is used, a columnar structure can be
produced in the thermal barrier layer 4. In this case, the
crystallite columns extend perpendicular to the surface of the base
body 1. The thermal barrier layer 4 is formed, for example, of
lanthanum hafnate. The thermal barrier layer 4 has a relatively
loose microstructure. It contains pores or other voluminous
defects. Even as a columnar microstructure, it is characterized by
a relatively loose structural configuration. This loose structural
configuration exhibits some degree of susceptibility to erosion in
a hot gas flow. In order to provide protection against erosion
phenomena of this type, the surface of the thermal barrier layer 4
may be fused to form a dense and compact protective layer 5, as
represented herein. The protective layer 5 may, for example, be
produced by laser fusion. It is likewise possible to apply the
thermal barrier layer 4 directly to the base body 1. In this case,
the alloy of the base body 1 will already be constructed in such a
way that it is suitable for forming a bonding oxide, for example
chromium oxide and/or aluminum oxide. This bonding oxide then forms
the bonding layer 3.
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