U.S. patent application number 09/746457 was filed with the patent office on 2001-07-12 for thermal barrier coating systems and materials.
This patent application is currently assigned to United Technologies Corporation. Invention is credited to Maloney, Michael J..
Application Number | 20010007719 09/746457 |
Document ID | / |
Family ID | 25070681 |
Filed Date | 2001-07-12 |
United States Patent
Application |
20010007719 |
Kind Code |
A1 |
Maloney, Michael J. |
July 12, 2001 |
Thermal barrier coating systems and materials
Abstract
A new family of ceramic materials is identified having
particular utility as thermal insulating or thermal barrier
coatings on metallic substrates. The ceramic materials have a
pyrochlore structure and are typified by the composition
A.sub.2B.sub.2O.sub.7 where A and B are various ions and O is
oxygen. A may have a positive charge of 3.sup.+or 2.sup.+and B may
have a positive charge of 4.sup.+or 5.sup.+. These materials are
characterized by having chemical stability, thermal stability and
thermal insulating properties superior to those of currently used
thermal barrier ceramics. An example pyrochlore material is
lanthanum zirconate.
Inventors: |
Maloney, Michael J.; (Port
St. Lucie, FL) |
Correspondence
Address: |
Pratt & Whitney
Patent Department
Mail Stop 132-13
400 Main Street
East Hartford
CT
06108
US
|
Assignee: |
United Technologies
Corporation
|
Family ID: |
25070681 |
Appl. No.: |
09/746457 |
Filed: |
December 21, 2000 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09746457 |
Dec 21, 2000 |
|
|
|
09436212 |
Nov 8, 1999 |
|
|
|
6231991 |
|
|
|
|
09436212 |
Nov 8, 1999 |
|
|
|
08764419 |
Dec 12, 1996 |
|
|
|
6117560 |
|
|
|
|
Current U.S.
Class: |
428/633 ;
416/241B; 416/241R; 427/248.1; 428/472; 428/623; 428/629;
428/701 |
Current CPC
Class: |
C23C 28/3215 20130101;
C23C 28/321 20130101; F05D 2300/21 20130101; C23C 4/11 20160101;
C23C 14/083 20130101; F05D 2300/2118 20130101; Y10T 428/12549
20150115; C23C 28/345 20130101; F23R 3/002 20130101; Y10T 428/12618
20150115; Y10T 428/1259 20150115; F05D 2230/90 20130101; C23C
28/3455 20130101; Y02T 50/60 20130101; F01D 5/288 20130101; C23C
30/00 20130101; F02K 1/822 20130101 |
Class at
Publication: |
428/633 ;
428/629; 428/623; 428/472; 428/701; 416/241.00R; 416/241.00B;
427/248.1 |
International
Class: |
B32B 015/04 |
Claims
I claim:
1. A metallic article comprising a metallic substrate, said
substrate having a ceramic coating on its surface wherein said
ceramic coating has a cubic pyrochlore structure.
2. An article as in claim 1 wherein said metallic substrate is
selected from the group consisting of steels, superalloys, titanium
alloys and copper alloys.
3. An article as in claim 1 wherein said pyrochlore has the formula
A.sub.2B.sub.2O.sub.7 and the A and B species ionic radii fall
approximately within the cubic field in FIG. 2.
4. An article as in claim 1 wherein the B constituent comprises Hf,
Ti, Zr, and single phase mixtures thereof.
5. An article as in claim 1 wherein the A constituent comprises La,
Gd, Zr, and single phase mixtures thereof.
6. An article as in claim 1 wherein said coated article is adopted
to be used in environments where the free surface of the pyrochlore
coating will be heated and the free surface of the substrate will
be cooled, whereby the pyrochlore coating will reduce heat
flow.
7. An article as in claim 1 wherein said pyrochlore has a columnar
microstructure.
8. A metallic article as in claim 1 wherein said metallic article
has an oxide scale on its outer surface, said oxide consisting
essentially of alumina, and whereas said cubic pyrochlore ceramic
coating is bonded to said oxide scale.
9. An article as in claim 8 wherein said metallic substrate is
selected from the group consisting of steels, superalloys, titanium
alloys and copper alloys.
10. An article as in claim 8 wherein said pyrochlore has the
formula A.sub.2B.sub.2O.sub.7 and the A and B species ionic radii
fall approximately within the cubic field in FIG. 2.
11. An article as in claim 8 wherein the B constituent comprises
Hf, Ti, Zr, and single phase mixtures thereof.
12. An article as in claim 8 wherein the A constituent comprises
La, Gd, Zr, and single phase mixtures thereof.
13. An article as in claim 8 wherein said coated article is adopted
to be used in environments where the free surface of the pyrochlore
coating will be heated and the free surface of the substrate will
be cooled, whereby the pyrochlore coating will inhibit heat
flow.
14. A metallic article comprising a metallic substrate, said
substrate having an alumina forming coating on its surface and a
pyrochlore coating bonded to said alumina forming coating.
15. An article as in claim 14 wherein said substrate coating
comprises an alumina forming metallic overlay coating.
16. An article as in claim 14 wherein said substrate coating
comprises a diffusion aluminide coating.
17. An article as in claim 14 wherein the B constituent comprises
Hf, Ti, Zr, and single phase alloys thereof.
18. An article as in claim 14 wherein the A constituent comprises
La, Gd, Zr, and single phase alloys thereof.
19. An article as in claim 14 wherein said coated article is
adopted to be used in environments where the free surface of the
pyrochlore coating will be heated and the free surface of the
substrate will be cooled, whereby the pyrochlore coating will
inhibit heat flow.
20. An article as in claim 14 wherein said pyrochlore has a
columnar microstructure.
21. In a superalloy gas turbine component which operates in an
environment with gas temperatures in excess of 1,000.degree.C.,
said component having internal cooling passages, the improvement
which comprises a coating of a ceramic having a pyrochlore crystal
structure, said coating being located to reduce heat flow into said
component.
22. A method of thermally insulating a metallic substrate which
comprises: applying a cubic pyrochlore ceramic coating to at least
a portion of said substrate by electron beam physical vapor
deposition.
23. A method as in claim 22 wherein said substrate is selected from
the group consisting of steels, superalloys, titanium alloys and
copper alloys.
24. An article as in claim 22 wherein said pyrochlore has the
formula A.sub.2B.sub.2O.sub.7 and the A and B species ionic radii
fall approximately within the cubic field in FIG. 2.
25. An article as in claim 22 wherein the B constituent comprises
Hf, Ti, Zr, and single phase alloys thereof.
26. An article as in claim 22 wherein the A constituent comprises
La, Cd, Zr, and single phase alloys thereof.
27. An article as in claim 22 wherein said coated article is
adopted to be used in environments where the free surface of the
pyrochlore coating will be heated and the free surface of the
substrate will be cooled, whereby the pyrochlore coating will
inhibit heat flow.
28. An article as in claim 22 wherein said pyrochlore has a
columnar microstructure.
29. A metallic article as in claim 22 wherein said metallic article
has an oxide scale on its outer surface, said oxide consisting
essentially of alumina, and wherein said pyrochlore ceramic coating
is bonded to said oxide scale.
30. A method of thermally insulating a metallic substrate which
comprises: applying a cubic pyrochlore ceramic coating to at least
a portion of said substrate by thermal spray deposition.
31. An article as in claim 30 wherein said metallic substrate is
selected from the group consisting of steels, superalloys, titanium
alloys and copper alloys.
32. An article as in claim 30 wherein said pyrochlore has the
formula A.sub.2B.sub.2O.sub.7 and the A and B species ionic radii
fall approximately within the cubic field in FIG. 2.
33. An article as in claim 30 wherein the B constituent comprises
Hf, Ti, Zr, and single phase alloys thereof.
34. An article as in claim 30 wherein the A constituent comprises
La, Gd, Zr, and single phase alloys thereof.
35. An article as in claim 30 wherein said coated article is
adopted to be used in environments where the free surface of the
pyrochlore coating will be heated and the free surface of the
substrate will be cooled, whereby the pyrochlore coating will
inhibit heat flow.
36. An article as in claim 30 wherein said pyrochlore has a
columnar microstructure.
37. A metallic article as in claim 30 wherein said metallic article
has an oxide scale on its outer surface, said oxide consisting
essentially of alumina, and wherein said pyrochlore ceramic coating
is bonded to said oxide scale.
38. A gas turbine engine component which comprises a superalloy
substrate having a layer consisting essentially of lanthanum
zirconate on at least a portion of it's external surface.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a class of ceramic materials for
thermal barrier coatings, to thermal barrier coatings made of such
materials, and to metallic parts having such thermal barrier
coatings. The thermal barrier coatings have particular utility in
gas turbine engines. The ceramic materials are a family of ceramics
having a pyrochlore structure.
BACKGROUND OF THE INVENTION
[0002] Gas turbine engines are well developed mechanisms for
converting chemical potential energy, in the form of fuel, to
thermal energy and then to mechanical energy for use in propelling
aircraft, generating electric power, pumping fluids etc. At this
time the major available avenue for improved efficiency of gas
turbine engines appears to be the use of higher operating
temperatures. However the metallic materials used in gas turbine
engines are currently very near their upper limits of thermal
stability. In the hottest portion of modern gas turbine engines,
metallic materials are used at gas temperatures above their melting
points. They survive because they are air cooled. But excessive air
cooling reduces engine efficiency.
[0003] Accordingly, there has been extensive development of thermal
barrier coatings for use with cooled gas turbine aircraft hardware.
By using a thermal barrier coating, the amount of cooling air
required can be substantially reduced.
[0004] Such coatings are invariably based on ceramics; mullite,
alumina, etc. have been proposed but zirconia is the current
material of choice. Zirconia must be modified with a stabilizer to
preserve its cubic crystal structure at elevated temperatures,
typical stabilizers include yttria, calcia, ceria and magnesia.
[0005] Generally speaking, metallic materials have coefficients of
thermal expansion which exceed those of ceramic materials,
consequently one of the problems that must be addressed in the
development of successful thermal barrier coatings is to match the
coefficient of thermal expansion of the ceramic material to the
metallic substrate so that upon heating, when the substrate
expands, the ceramic coating material does not crack. Zirconia has
a high coefficient of thermal expansion and this is a primary
reason for the success of zirconia as a thermal barrier material on
metallic substrates.
[0006] Thermal barrier coatings have been deposited by several
techniques including thermal spraying (plasma, flame and HVOF),
sputtering and electron beam physical vapor deposition (EBPVD). Of
these techniques, electron beam physical vapor deposition is
currently a preferred technique for demanding applications because
it produces a unique coating structure. Electron beam physical
vapor deposited ceramic materials, when applied according to
certain parameters, have a columnar grain microstructure consisting
of small columns separated by gaps which extend into the coating.
These gaps allow substantial substrate expansion without coating
cracking and/or spalling see U.S. Pat. No. 4,321,311. According to
U.S. Pat. No. 5,073,433 a similar structure (comprising
segmentation cracks), although on a larger scale, can be obtained
by plasma spray techniques.
[0007] Despite the success with the current use of electron beam
physical vapor deposited zirconia base coatings there is a
continuing desire for improved coatings which exhibit superior
thermal insulation capabilities, especially improved in insulation
capabilities when normalized for coating density. Weight is always
a critical factor when designing gas turbine engines, particularly
in rotating parts. Ceramics thermal barrier coatings are not load
supporting materials, consequently they add weight without
increasing strength. There is a strong desire for a ceramic thermal
barrier material which adds the minimum weight while providing the
maximum thermal insulation capability. In addition there are
obviously the normal desires for long life, stability, economy
etc.
[0008] Although this coating was developed for application in gas
turbine engines, the invention clearly has utility in other
applications where high temperatures are encountered such as
furnaces.
DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1a depicts the crystal structure of lanthanum
zircolate, a pyrochlore.
[0010] FIG. 1b depicts the crystal structure of zirconia, a
fluorite structure.
[0011] FIG. 2 shows the relationship between the A and B
constituent ionic sizes necessary to produce a pyrochlore
structure.
[0012] FIG. 3a depicts a ceramic coating directly on a metallic
substrate.
[0013] FIG. 3b depicts a ceramic coating on a metallic substrate
with an intermediate bond coat.
[0014] FIG. 3c depicts an expanded view of the interface between
the bond coat and the ceramic layer in FIG. 3b.
[0015] FIG. 4 shows the ZrO.sub.2-La.sub.2O.sub.3 phase
diagram.
[0016] FIG. 5 shows the thermal conductivity of several ceramic
materials.
[0017] FIG. 6 shows the coefficient of thermal expansion for
several ceramic materials.
SUMMARY OF THE INVENTION
[0018] The essence of the present invention arises from the
discovery that a class of ceramic materials has great utility as
thermal barrier coatings on metallic substrates. These materials
have a pyrochlore crystal structure.
[0019] The term pyrochlore is used to identify an ore of tantalum
found in Canada. The term more generally describes a ceramic
structure of the composition A.sub.2 B.sub.2O.sub.7 where A can
have valance of 3.sup.+or 2.sup.+and B can have a valance of
4.sup.+or 5.sup.+and wherein the sum of the A and B valences is 7.
The oxygen can be replaced by in part by sulfur or fluorine.
Typical pyrochlores which we believe to have potential as thermal
barrier coatings are those in which A is selected from the group
consisting of lanthanum, gadolilniunm and yttrium and mixtures
thereof and B is selected from the group consisting of zirconium,
hafnium and titanium and mixtures thereof. Many other pyrochlores
exist which also have potential as thermal barrier materials. See
"Oxide Pyrochlores--A Review" by M. A. Subramanian et al, Progress
in Solid State Chemistry, vol 15, pp 55-143, 1983 (incorporated
herein by reference) for a full description of pyrochlores.
[0020] We have found that on a density adjusted basis pyrochlores
which we have investigated have thermal insulating properties which
exceed those of the more commonly used zirconia based thermal
barrier materials. Additionally many of the pyrochlore materials
leave a phase relationship in which the pyrochlore structure is
phase stable up to the melting point. Consequently stabilizing
additions are not required. Most of the pyrochlores which we have
investigated leave melting points of more than 3000.degree.F.
(1650.degree.C.) and generally more than 4000.degree.F.
(2200.degree.C.). Additionally these materials adhere to alumina.
These are all properties which are useful in thermal barrier
coatings.
[0021] The invention coating materials and coatings will usually be
used to protect a superalloy substrate from excess temperatures.
Superalloys are metals, usually based on iron, nickel or cobalt and
containing chromium and aluminum and usually titanium and
refractory metals, and having useful properties above
1200.degree.F. (650.degree.C.). Other substrates, including steels,
copper alloys and titanium alloys may be protected. Table I
describes exemplary substrate materials.
1TABLE I (wt % Exemplary Superalloy Compositions) Cr Co W Cb Ti Al
B Hf C Ni Ta Mo Zr Re PWA1422 9 10 12 1 2 5 .015 1.6 .14 Bal -- --
-- -- PWA1426 6.4 12.6 6.4 -- -- 5.9 0.012 1.5 -- Bal 3.0 1.7 .08
.3 PWA1480 10 5 4 -- 1.5 5 -- -- -- Bal 12 -- -- -- IN 792 12 9 3.8
-- 4.1 3.5 .015 0.5 .12 Bal 3.9 1.9 .12 --
[0022] As in other ceramic thermal barrier coatings, adherence of
the pyrochlore ceramic to the alloy substrate is critical.
[0023] It is known from prior zirconia thermal barrier coatings
that a metallic bond coat (sometimes described as an overlay
coating) such as a MCrAlY is a superior bond coat for oxide ceramic
coatings. It is also known that aluminide coatings are useful bond
coats through generally not as durable as MCrAlY bond coats. The
common feature of overlay coatings and aluminide coatings is that
they both form adherent alumina surface films or scales.
[0024] A broad composition range for MCrAlY materials is 10-25% Cr,
5-15 Al, 1-1.0 Y balance selected from Fe, Ni, and Co and mixtures
of Ni and Co. Additions of up to 5% each of Hf, Ta or Re, up to 1%
of Si and up to 3% each of Os, Pt, Pd, or Rh may also be made.
Table II describes exemplary MCrAlYs that can be applied by thermal
spray processes, by EDPVD processes, and by electroplating.
2TABLE II (wt % Exemplary MCrAlY Compositions) Ni Co Cr Al Y Hf Si
NiCrAlY Bal -- 19.5 12.5 .45 -- -- CoCrAly -- Bal 18 11 .45 -- --
NiCoCrAlY Bal 23 18 12.5 .3 -- -- NiCoCrAlY Bal 22 17 12.5 .6 .25
.4
[0025] An alternate bond coat is a diffusion aluminide formed by
diffusing aluminum into the substrate surface. Diffusion aluminides
are well known and may be applied using a mixture (termed a pack)
containing an aluminum source, such as an aluminum alloy or
compound, an activator (usually a halide compound such as NaF) and
an inert material such as alumina. The part to be coated is buried
in the pack and heated to 1500-2000.degree.F. while a carrier gas,
such as hydrogen, is flowed through the pack out of pack processes
wherein the part is not buried in the pack are also known. The
incorporation of precious metals such as Pt, Rh, Pd and Os into
aluminide coatings is known. See U.S. Pat. No. 5,514,482 for a
description of aluminide coating processes.
[0026] Combinations of overlay and aluminide coatings are also
possible. See U.S. Pat. No. 4,897,315 for a description of a system
having an inner MCrAlY overlay coating and an outer aluminide
coating. See U.S. Pat. No. 4,005,989 for a description of the
reverse combination, an inner aluminide coating and an outer
overlay coating.
[0027] The common feature of these bond coats and bond coat
combinations is that they form an adherent layer of alumina on
their outer surface. The invention thermal barrier coating has
limited solubility in alumina but bonds firmly to the alumina.
[0028] In certain cases, superalloys may form sufficiently perfect
and adherent alumina layers that ceramics may adhere without a
separate bond coat. See U.S. Pat. Nos. 5,262,245, 4,895,201,
5,034,284, 5,346,563, and 5,538,796.
[0029] To date all successful applications of ceramic coatings to
superalloy has included oxide layer (usually alumina, rarely
silica) between the bond coat (or substrate) and the ceramic
coating.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0030] The pyrochlore structure is a complex structure which can be
described in a variety of ways, as a fluorite structure derivative,
or as a network of octamedal linked corner to corner with cations
filling the interstices.
[0031] FIG. 1a is a depiction of a cubic pyrochlore crystal
structure. Regardless of structure description the pyrochlore
structure has a chemical composition of A.sub.2B.sub.2O.sub.7 or
occasionally A.sub.2B.sub.2O.sub.6 or AB.sub.2O.sub.6 with the
latter two being referred to as defect pyrochlores. FIG. 1a
illustrates lanthiainum zirconate having an A.sub.2B.sub.2O.sub.7
chemistry. FIG. 1b shows a cubic fluorite structure, the structure
of stabilized zirconia. A comparison of FIGS. 1a and 1b shows both
similarities and differences between the two structures. Both FIGS.
1a and 1b are views down the <100> crystal axis. Visually the
pyrochlore structure appears to be less regular than the fluorite
structure.
[0032] The A and B ions can have different valances as long as the
sum of the A and B valences adds up to 7, in the case of the
A.sub.2, B.sub.2O.sub.7 or 6 in the case of A.sub.2B.sub.2O.sub.6
structures.
[0033] The pyrochlore structure forms only for certain
relationships of A and B ionic radii. FIG. 2 illustrates this
relationship indicating the general combination of A and B ionic
radii which produce cubic pyrochlores. We have found that the
boundaries of this diagram are somewhat uncertain and we believe,
based on our investigations, that lanthanum titanate [La.sub.2
Ti.sub.2O.sub.7] has a stable cubic pyrochlore structure.
[0034] Noncubic pyrochlores are known but for the purpose of this
invention we prefer to use ceramics which have a cubic pyrochlore
structure.
[0035] As shown in FIG. 2, the formation of the desired cubic
pyrochlore crystal structure is controlled by the relative ionic
radii of the A and B constituents. It is possible to use a mixture
of elements for the A and/or B constituents to provide an average
ionic radius which will cause formation of a cubic pyrochlore
structure. By way of example, from FIG. 2 it can be seen that both
Gd.sub.2Ti.sub.2O.sub.7 and Y.sub.2Zr.sub.2O.sub.7 will have cubic
pyrochlore structures. As a general rule, compounds of the formula
(Gd.sub.xY.sub.y)(Ti.sub.aZr.sub.b- )O.sub.7, where x+y=2 and a+b
=2, will also have a cubic pyrochlore structure.
[0036] Further, a compound such as ln.sub.2Zr.sub.2O.sub.7 which is
not cubic could likely be rendered cubic through partial
substitution of (for example) Nd for In and/or Ti for Zr, to bring
the average A and B ionic radii into the cubic pyrochlore region
shown in FIG. 2.
[0037] We have worked with the type of pyrochlores denoted by the
A.sub.2B.sub.2O.sub.7 formula and of these we prefer to use
gadolinium, lanthanum or yttrium for the A ionic species and
hafnium, titanium or zirconium for the B ionic species. Lanthanum
zirconate seems particularly useful because lanthanum and zirconium
have similar vapor pressures, making vapor deposition more readily
possible. We have used only the materials of the
A.sub.2B.sub.2O.sub.7 structure and have not attempted the use of
the known pyrochlores which have either fluorine or sulfur to
substitute a portion of the oxygen but we do not believe that there
is any reason to exclude the sulfur and fluorine substituted
compositions from the present invention. We have also not
experimentally evaluated the A.sub.2B.sub.2O.sub.6 and
AB.sub.2O.sub.6 structures but believe that they also may have
utility in thermal barrier coatings.
[0038] Ti, Zr and Hf all display complete solid solubility in each
other and we believe that any combination of Ti+Zr+Hf can be used
as the B ionic species. Similarly, Gd, La and Y have substantial
solid solubilities (Gd-La has complete solubility). Any combination
of Gd+La+Y which does not form a second phase can be used as the A
ionic species. These alloys of the A and B species must satisfy the
criteria of FIG. 2 and possess a pyrochlore structure.
[0039] The low thermal conductivity of oxide pyrochlore compounds
can be rationalized by consideration of crystallographic and
chemical effects on thermal conductivity. The thermal conductivity
of dielectric solids at elevated temperature is determined by
phonon scattering by crystal imperfections and other phonons. Oxide
pyrochlore compounds exhibit many of the features associated with
low thermal conductivity materials. The pyrochlore crystal
structure has a high intrinsic defect concentration. It has been
experimentally established that as the difference in atomic mass
between constituents in a compound increases, the thermal
conductivity of that compound tends to decrease. Although the
pyrochlore and fluorite structure are closely related, substitution
of a high concentration of high atomic mass atoms (lanthanum,
gadolinium and yttrium) into the fluorite structure provides a
means to lower thermal conductivity that does not readily exist
with stabilized zirconia compounds. It should be noted that, for
thermal barrier applications, the benefits obtained from the
reduction in thermal conductivity resulting from the use of high
atomic mass elements must out weigh the (debit incurred from higher
density.
[0040] Reduction in thermal conductivity has also been associated
with increasing complexity of crystallographic structure. As shown
in FIG. 1a, the pyrochlore structure exhibits a greater degree of
complexity than the fluorite structure shown in FIG. 1b. The cubic
pyrochlore structure is similar to the cubic fluorite structure but
with a large number of the oxygen atoms displaced (and one in eight
missing).
[0041] Thermal barrier coatings are typically applied by thermal
spray processes, such as plasma spray, in air (APS) or in low
pressure (LPPS) by high velocity oxygen fuel processes (HVOF) or by
detonation guns (D Gun). Electron beam physical vapor deposition
(EBPVD) and sputtering are other techniques. Electron beam physical
vapor deposition is a favored process. Depending upon the
application and circumstances, each process has particular
advantages. All of these processes can be readily used to apply
oxide pyrochlore thermal barrier coatings. As previously discussed,
the EBPVD process offers advantages since it develops a structure
suited for extreme temperature applications and is therefore be
most suitable for coating hot section turbine components. Thermal
spray processing offers advantages in coating large components of
complex shape and would be most suitable in coating components such
as combustors.
[0042] FIGS. 3a, b and c illustrates variants of the thermal
barrier coatings of the present invention. FIG. 3a depicts a coated
article which comprises a superalloy substrate 10 having a
pyrochlore top coat 20 on its outer surface 21. In gas turbine
applications the backside 11 of the superalloy substrate 10 will be
cooled by cooling air (not shown) and the outside front surface 21
of the pyrochlore will be exposed to elevated temperatures. There
may also be holes between the outer surface and the backside
permitting cooling air to flow from the backside to the outer
surface. Angled and shaped cooling holes in combination with
flowing hot gases on the outer surface can produce film cooling in
which a layer of cool air separates the outside surface from the
hot gases to further reduce heat flow. Heat will flow from the
front surface 21 to the cooled surface 11 and the quantity of the
heat flow will be substantially reduced by the pyrochlore layer. As
previously mentioned the pyrochlore may be applied by various
methods and the macrostructure of the pyrochlore layer will be
largely a function of the deposition process. The most basic
invention embodiment is a pyrochlore layer adhered to a substrate
which reduces heat flow in the presence of a thermal gradient.
[0043] FIG. 3b illustrates a preferred construction which employs a
bond coat 15 between the substrate 10 and the pyrochlore 20. The
bond coat 15 improves adherence and provides oxidation protection
for the substrate. FIG. 3c is an expanded view of the interlayer 16
between the bond coat 15 and the pyrochlore layer 20. An oxide
layer 22, principally of alumina, exists at this interlayer and is
believed to be principally responsible for the adherence of the
pyrochlore.
[0044] It is known to augment the naturally occurring alumina layer
on the bond coat by sputtering alumina onto the bond coat, in the
case of zirconia thermal barrier coatings, and the use of a
separately applied alumina layer (rather than a thermally grown
oxide layer) is also an embodiment of this invention.
[0045] In a further embodiment another ceramic layer may be applied
to the free surface of the pyrochlore. This added layer can be
selected to reduce oxygen diffusion, to provide erosion and
abrasion resistance, or to provide a desired thermal emissivity
characteristic, or some combination of these characteristics.
EXAMPLE I
[0046] The use of the La.sub.2Zr.sub.2O.sub.7 (lanthanum zirconate)
pyrochlore oxide compound as a EBPVD applied thermal barrier
coating will be illustrated. The advantageous properties of the
La.sub.2Zr.sub.2O.sub.7 pyrochlore oxide relative to stabilized
zirconia for thermal barrier coating include thermal conductivity,
thermal expansion, density, and phase stability. FIG. 4 shows the
La.sub.2O.sub.3 - ZrO.sub.2 phase diagram with the pyrochlore phase
field labeler P. It can be seen that the pyrochlore structure (at
about 35 mol % La.sub.2O.sub.3) is stable up to the melting point
at about 2300.degree.C. (4172.degree.F.).
[0047] FIG. 5 shows thermal conductivity of La.sub.2Zr.sub.2O.sub.7
compared to the thermal conductivity of cubic zirconia as a
function of temperature. At ica thermal barrier coating use
temperatures, the pyrochlore compound exhibits a thermal
conductivity which is about 50% that of stabilized zirconia. The
density of the La.sub.2Z.sub.2O.sub.7 pyrochlore compound is
approximately the same as stabilized zirconia (approximately 6
gr/cm.sup.3) so on a weight corrected basis, the thermal
conductivity benefit is also about 50%.
[0048] To illustrate the benefit, the 50 percent reduction thermal
conductivity allows the coating thickness to be reduced by 50
percent for the same degree of thermal protection. Decreasing the
coating mass by 50 percent on a typical turbine blade will lower
the blade pull at the blade root by approximately 1,500 pounds (680
Kg), at typical operating conditions, which results in a
significant increase in blade life and permits a reduction in mass
of the disk to which the blades are attached. If the thermal
barrier coating were held at the same thickness, and the cooling
air flow held constant, the substrate temperature would be reduced
by about 100.degree.F. (55.degree.C.), giving increased substrate
ife. Keeping the coating thickness constant and reducing the
airflow would increase engine efficiency.
[0049] FIG. 6 shows the mean coefficient of thermal expansion of
La.sub.2Br.sub.2O.sub.7 compared to that of cubic stabilized
zirconia as a function of temperature. The thermal expansion of
La.sub.2Zr.sub.2O.sub.7 thermal barrier coating can be seen to be
similar to that of the zirconia thermal barrier coating. This means
that La.sub.2Zr.sub.2O.sub.7 will behave similarly to zirconia
during thermal cycling.
EXAMPLE II
[0050] Lanthanum zirconate was applied to a substrate by electron
beam vapor deposition (EBPVD), in a controlled atmosphere chamber.
The coating was applied to a single crystal substrate (of the
nominal composition of PWA 1480 (see Table II)). The coating
process was carried out in a vacuum of 3.2.times.10.sup.-4 Torr
with an oxygen flow rate of 50 sccm. Oxygen was added to ensure
pyrochlore oxygen stoichiometry, see U.S. Pat. No. 5,087,477. The
substrate temperature was 1840.degree.F. during deposition with a
substrate to source distance of 5.25 inches. The source pyrochlore
ceramic was evaporated with an electron beam run at 0.8 A and
10,000 V. The source oxide was a.sub.2Zr.sub.2O.sub.7 powder. The
coating exhibited the favorable columnar grain structure typical of
electron beam physical vapor deposited cubic zirconia thermal
barrier coatings that provides strain relief and improved
durability over plasma sprayed coatings.
[0051] FIG. 7 shows an X-ray diffraction scan obtained from the
surface of the coating. The diffraction peaks have been indexed to
the pyrochlore crystal structure which demonstrates that the
pyrochlore structure was formed in the deposited thermal barrier
coating.
[0052] Although this invention has been shown and described with
respect to detailed embodiments thereof, it will be understood by
those skilled in the art that various changes, omissions and
additions in form and detail thereof may be made without departing
from the spirit and scope of the claimed invention.
* * * * *