U.S. patent application number 15/211588 was filed with the patent office on 2017-01-19 for high temperature protective coating.
This patent application is currently assigned to ANSALDO ENERGIA SWITZERLAND AG. The applicant listed for this patent is ANSALDO ENERGIA SWITZERLAND AG. Invention is credited to Piero-Daniele GRASSO, Johannes Clemens SCHAB, Alexander STANKOWSKI, Julien Rene Andre ZIMMERMANN.
Application Number | 20170016123 15/211588 |
Document ID | / |
Family ID | 54007472 |
Filed Date | 2017-01-19 |
United States Patent
Application |
20170016123 |
Kind Code |
A1 |
SCHAB; Johannes Clemens ; et
al. |
January 19, 2017 |
HIGH TEMPERATURE PROTECTIVE COATING
Abstract
The invention relates to a high temperature protective coating
based on MCrAlY coating, with M at least one element out of the
group of Ni, Co and Fe, for a component of a turbo machine,
especially a gas turbine, the coating containing at least at least
1.75 vol.-% chromium borides and the coating consisting of the
following chemical composition (in wt.-%): 10-27 Cr; 3-12 Al; 1-4
Si; 0.1-3 Ta; 0.01-3 Y; 0.1-3 B; 0-7 M, with M being a different
element out of said group compared to the remainder, and the
remainder being M and inevitable impurities. A preferred embodiment
is a coating with the following chemical composition: 10-27 Cr;
3-12 Al; 1-4 Si; 0.1-3 Ta; 0.01-3 Y; 0.1-3 B; 0-7 Co and the
remainder being Ni and inevitable impurities.
Inventors: |
SCHAB; Johannes Clemens;
(Untersiggenthal, CH) ; ZIMMERMANN; Julien Rene
Andre; (Neuenhof, CH) ; STANKOWSKI; Alexander;
(Wurenlingen, CH) ; GRASSO; Piero-Daniele;
(Niederweningen, CH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ANSALDO ENERGIA SWITZERLAND AG |
Baden |
|
CH |
|
|
Assignee: |
ANSALDO ENERGIA SWITZERLAND
AG
Baden
CH
|
Family ID: |
54007472 |
Appl. No.: |
15/211588 |
Filed: |
July 15, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23F 11/02 20130101;
F05D 2300/175 20130101; C23C 30/00 20130101; C23C 4/073 20160101;
C22C 19/058 20130101; F01D 5/288 20130101; C22C 19/07 20130101;
F05D 2300/132 20130101; F05D 2230/31 20130101 |
International
Class: |
C23F 11/02 20060101
C23F011/02; C22C 19/05 20060101 C22C019/05; C22C 19/07 20060101
C22C019/07 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 17, 2015 |
EP |
15177229.0 |
Claims
1. High temperature protective coating based on MCrAlY coating,
with M at least one element out of the group of Ni, Co and Fe, for
a component of a turbo machine, especially a gas turbine, the
coating containing at least 1.75 vol.-% chromium borides and
consisting of the following chemical composition (in wt.-%): 10-27
Cr; 3-12 Al; 1-4 Si; 0.1-3 Ta; 0.01-3 Y; 0.1-3 B; 0-7 M, with M
being a different element out of said group compared to the
remainder; the remainder being M and inevitable impurities.
2. The coating according to claim 1, wherein the coating consists
of the following chemical composition (in wt.-%): 10-27 Cr; 3-12
Al; 1-4 Si; 0.1-3 Ta; 0.01-3 Y; 0.1-3 B; 0-7 Co; the remainder
being Ni and inevitable impurities.
3. The coating according to claim 1, wherein the coating consists
of the following chemical composition (in wt.-%): 10-27 Cr; 3-12
Al; 1-4 Si; 0.1-3 Ta; 0.01-3 Y; 0.1-3 B; 0-7 Ni; the remainder
being Co and inevitable impurities.
4. The coating according to claim 1, wherein the Cr content is
21-25 wt.-%, preferred 22-25 wt.-%.
5. The coating according to claim 1, wherein the Al content is 4-6
wt.-%.
6. The coating according to claim 1, wherein the Si content is
1.5-2.6 Si wt.-%.
7. The coating according to claim 1, wherein the Ta content is
1.5-3 wt.-%.
8. The coating according to claim 1, wherein the Y content is
0.01-1 wt.-%.
9. The coating according to claim 1, wherein the B content is 0.1-1
wt.-%.
10. The coating according to claim 1, wherein the M content, with M
being a different element out of said group compared to the
remainder, is 0-1 wt.-%.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to the technology of
turbomachines, especially gas turbines. It refers to an advanced
high temperature protective coating based on a MCrAlY coating
(M=Ni, Co, Fe or combinations thereof) for a component of a
turbomachine.
PRIOR ART
[0002] MCrAlY coatings are commonly applied on hot gas paths
components of modern gas turbines. In general, MCrAlY coatings are
either applied as an overlay or as a bond coat for thermal barrier
coating systems (TBC).
[0003] The main target of an overlay is to protect the Ni-/Co-base
superalloy substrate from oxidation and hot corrosion. Furthermore,
the mechanical integrity of the coating system and of the
corresponding base material shall be ensured.
[0004] During engine service, the boundary conditions (like e.g.
temperature, mechanical stresses, etc.) are different for each
component (per stage and even locally on the component). Some
components or some specific component areas are prone to fatigue
(cyclic loading), while others face increased creep, oxidation
and/or hot corrosion impact (base-load).
[0005] On one hand, the modern energy market demands for industrial
gas turbine (IGT)-engines running in base-load modus, on the other
hand an increasing number of engines is running in (high-) cyclic
modus. As a matter of fact, the mechanical and thermal loading of
MCrAlY coatings used in engines running in (high-) cyclic modus
differs significantly from the ones running in base-load.
[0006] Mechanical properties, like ultimate tensile strength,
ductility or plastic energy, are strongly dependent on the coating
composition and the related microstructure. In order to answer the
requirements of modern engine operation and the related distress
modes, it is of strong interest to be able to produce coatings with
advanced flexibility and adjustable properties. Such a modular
coating concept is for example disclosed in document EP 2 781 616
A1.
[0007] Most of the so far known MCrAlY, especially NiCrAlY coatings
have been designed for answering the base load operation demand:
strong oxidation and corrosion resistance. However, in (high-)
cyclic operating gas turbines, the failure mode of the parts is
more likely triggered by thermo-mechanic fatigue (TMF). Standard
coatings usually have poor TMF resistance due to their lack of
ductility at low temperatures (<500.degree. C.) and strength at
high temperature (>500.degree. C.).
[0008] The lack of ductility at low temperature is caused by the
large amount of fine .gamma.', .beta.-(NiAl), and .alpha.-Cr
precipitates (coming from the high content of Al and Cr) limiting
the dislocation propagation.
[0009] The lack of strength at high temperature is caused by the
partial dissolution of the .gamma.', .beta.-(NiAl), and .alpha.-Cr
precipitates into the .gamma. matrix, leading to a softening effect
and loss of strength.
[0010] Furthermore, when a large amount of .beta.-(NiAl) is present
this phenomenon is even increased due to the ductile to brittle
transition temperature of the body centered cubic (bcc) phase.
[0011] In .gamma./.gamma.' coatings, the transformation of .gamma.'
into .beta.-(NiAl) when increasing the temperature is also an
issue, as this is causing a large thermal expansion, leading to
stress build-up when used as bond coat and eventually to TBC
spallation. In addition, this is leading to stress accumulation in
the coating (overlay) and earlier cracking. This phenomenon is
limiting the maximum working temperature of the coating and/or
leading to early failure in cyclic operation.
[0012] FIG. 1 shows an overview about prior art MCrAlY alloy
classes and their oxidation resistance and hot corrosion
resistance. This well-known figure is disclosed in: Eskner, M.:
Mechanical behaviour of gas turbine coatings. Stockholm: Kungl.
Tekniska hogskolan., 2004, p. 3, and shows very clearly that
NiCrAlY coatings have a high oxidation resistance, but as a
disadvantage only a low hot corrosion resistance.
[0013] Several NiCrAlY alloys are for example described in the
following documents: WO 03/060194 A1, U.S. Pat. No. 3,620,693, U.S.
Pat. No. 4,477,538, U.S. Pat. No. 4,537,744, U.S. Pat. No.
3,754,903, U.S. Pat. No. 4,013,424, U.S. Pat. No. 4,022,587 and
U.S. Pat. No. 4,743,514.
[0014] Document WO 03/060194 A1 describes that most of NiCrAlY
alloys suffer from formation of undesirable phases, like .sigma.
and/or .beta.-(NiAl), which are detrimental if present in higher
volume-fractions. Therefore, there is proposed to avoid the
presence of .beta.-(NiAl) by using a coating comprised of .gamma.,
.gamma.', .alpha.-Cr and a negligible content of orthorhombic
M.sub.2B (<1% volume fraction). The coating contains between 23
and 27 wt. % Cr, between 4 and 7 wt. % Al, between 0.1 and 3 wt. %
Si, between 0.1 and 3 wt. % Ta, between 0.2 and 2 wt. % Y, between
0.001 and 0.01 wt. % B, between 0.001 and 0.01 wt. % Mg and between
0.001 and 0.01 Ca, with Ni and inevitable impurities making up the
remainder. But although the formation of .beta.-(NiAl) could be
prevented, the coating still suffers from the ductile to brittle
transition (DBTT) if operated at elevated temperatures.
[0015] Document US 2010/0330295 A1 describes an improvement of the
coating ductility by obtaining a predominantly .gamma.' structure
that is modified with a platinum group metal in order to avoid the
formation of the .beta.-(NiAl) phase which is brittle at low
temperature.
[0016] Document US 2012/0128525 A1 describes the optimization of
the composition of a bond coat. The .gamma. to .gamma.' transition
temperature shall be increased by addition of Tantalum
(preferentially without Re). Tantalum stabilizes the formation of a
three phase system (.beta.-(NiAl), .gamma., .gamma.') with an
increased .gamma./.gamma.' transition temperature (higher than the
coating service temperature), allowing reducing the local
stresses.
SUMMARY OF THE INVENTION
[0017] It is an object of the present invention to provide an
advanced high temperature protective MCrAlY coating for a component
of a turbomachine, which coating has improved properties compared
to known MCrAlY coatings, especially higher coating ductility at
lower operation temperatures (<500.degree. C.) and significantly
increased tensile strengths (at comparable strain) at elevated
operation temperatures (.gtoreq.500.degree. C.). As a consequence,
the plastic energy is increased for the entire working temperature
range and crack initiation is avoided, or at least significantly
reduced, leading to increased service lifetime in (high-) cyclic
operation modus.
[0018] These objects are obtained by a coating according to claim
1.
[0019] The inventive advanced high temperature protective MCrAlY
coating wherein M is at least one element out of the group of Ni,
Co and Fe, for a component of a turbo machine, especially a gas
turbine, contains at least 1.75 vol.-% chromium borides and
consists of the following chemical composition (in wt.-%): 10-27
Cr; 3-12 Al; 1-4 Si; 0.1-3 Ta; 0.01-3 Y; 0.1-3 B; 0-7 M, with M
being a different element out of said group compared to the
remainder and the remainder being M and inevitable impurities.
[0020] According to an embodiment of the invention the coating
consists of the following chemical composition (in wt.-%): 10-27
Cr; 3-12 Al; 1-4 Si; 0.1-3 Ta; 0.01-3 Y; 0.1-3 B; 0-7 Co and the
remainder being Ni and inevitable impurities.
[0021] According to a further embodiment of the invention the
coating consists of the following chemical composition (in
wt.-%):
[0022] 10-27 Cr; 3-12 Al; 1-4 Si; 0.1-3 Ta; 0.01-3 Y; 0.1-3 B; 0-7
Ni and the remainder being Co and inevitable impurities.
[0023] Preferred other embodiments of the invention are disclosed
in the dependent claims.
[0024] The invention describes an advanced MCrAlYB coating class
containing the element boron in higher amount. The respective
material composition is disclosed as well as the application of
MCrAlYB and/or Cr.sub.2B containing coatings. Key advantages are
the higher coating ductility at lower operation temperatures
(<500.degree. C.) and significantly increased tensile strengths
(at comparable strain) at elevated operation temperatures
500.degree. C.). As a consequence, the plastic energy, toughness
respectively, is increased for the entire working temperature
range. Crack initiation is avoided, or at least significantly
reduced, leading to increased service lifetime in (high-) cyclic
operation modus. An increased ductility level is promoted at
different temperatures, whereas the detrimental influence of
.beta.-(NiAl) formation and dissolution is avoided. Increased high
temperature strength, ensures creep resistance in base-load
operation.
[0025] The strengthening effect, resulting from the presence of CrB
and/or Cr.sub.2B precipitates, is independent of any phase
transition of e.g. .gamma., .gamma.', .beta.-(NiAl), .alpha.-Cr or
.sigma. and can easily be adjusted by the added quantity of boron.
The high temperature stability of CrB and/or Cr.sub.2B ensures a
stable strengthening effect until melting of the coating matrix
(e.g. .gamma.-phase). The presence of CrB and/or Cr.sub.2B reduces
the chromium depletion rate, which is not the case for regular
coatings containing only .alpha.-Cr or .alpha.-Cr phase. In case of
chromium depletion in surface near regions during operation due to
oxide formation, the CrB and/or Cr.sub.2B precipitates will
progressively dissolve and release the chromium needed to form a
protective chromium-oxide-scale increasing the coating service
lifetime in base-load operation with respect to hot corrosion.
Furthermore, the advanced coating promotes formation of highly
protective alumina scales which increases the coating service
lifetime in base-load operation with respect to oxidation.
[0026] The influence of the several alloying elements to the
properties of the coating according to the invention is the
following:
Chromium:
[0027] A sufficient Chromium (>10 wt.-%, preferred: >22
wt.-%) content is needed in order to form borides (Cr.sub.2B) which
deliver high temperature strength and ensure proper protection
against high temperature corrosion by the formation of a protective
Cr.sub.2O.sub.3 scale. However, the Chromium content should not
exceed the upper limit of 27 wt.-% (preferred: 25 wt.-%) in order
to avoid a high volume fraction of the brittle .alpha.-Cr phase
present at lower temperatures, which decreases the cyclic lifetime
(crack initiation due to low ductility). Furthermore, the formation
of brittle carbides (type: M.sub.6C) is promoted by a high Chromium
content. In order to avoid intense carbide formation, it is
recommended that the Cr content shall not exceed the upper limit of
27 wt.-% (preferred: 25 wt.-%).
Aluminium:
[0028] In order to ensure a proper oxidation resistance (stable
.alpha.-Al.sub.2O.sub.3 scale formation) and to reach a sufficient
coating lifetime, the original Aluminium content of the coating
should not be lower than 3 wt.-% (preferred: 4 wt.-%).
[0029] The formation of the brittle .gamma.' phase (Ni.sub.3Al),
which delivers the main strengthening effect, is dependent on the
Al content of the coating. For optimized mechanical properties
(ductility at low temperature and strength at high temperature),
the Aluminium content should be in the range of 3-12 wt.-%
(preferred: 4-6 wt.-%).
[0030] The Aluminium content should not exceed the upper limit of
12 wt.-% (preferred: 6 wt.-%) in order to avoid a high volume
content of brittle intermetallic .beta.-(NiAl) phase which
decreases cyclic lifetime and causes large thermal expansion
stresses during thermal cycling (risk of TGO/TBC spallation).
Silicon:
[0031] Silicon is acting as melting point depressant (increased
ductility), promotes the formation of brittle silicates, is
effective against low temperature hot corrosion and increases the
oxidation resistance by increasing the activity of oxide scale
formers like Al, Cr and Y. The Silicon content shall not exceed the
upper limit of 4 wt.-% (preferred: 2.6 wt.-%) in order to avoid the
formation of a high volume fraction of brittle silicates. For an
increased oxidation resistance and optimized coating lifetime, the
coating shall at least contain 1 wt.-%, preferred: 1.5 wt.-%
Si.
Tantalum:
[0032] Tantalum promotes the formation of the .gamma.' phase
(increases strength), improves the oxidation resistance and is
known to form carbides. In order to avoid a high volume fraction of
brittle carbides, the Tantalum content shall not exceed the upper
limit of 3 wt.-%. Optimized mechanical properties (with respect to
tensile testing, see FIG. 4) have been found when 0.1-3 wt.-%
(preferred: 1.5-3 wt.-%) of Ta are added to the alloy.
Cobalt:
[0033] This element is a solid solution strengthening element and
substitutes Ni in the .gamma. matrix and to some extend also in the
.gamma.' lattice. Furthermore, it has an influence on the .gamma.'
morphology, promotes TCP (topologically close-packed phase)
formation and can decrease the high temperature corrosion
resistance. The Cobalt content (in a Ni base alloy shall) not
exceed the upper limit of 7 wt.-% (favoured 1 wt.-%) in order to
avoid the formation of the brittle .sigma.-phase (Co, Cr rich)
which decreases the coating plasticity and cyclic lifetime
respectively. Optimized properties as result of tensile testing
have been found, when 0-1 wt.-% Cobalt are added to the alloy
(favoured composition).
Yttrium:
[0034] Yttrium is added in order to increase the oxidation
resistance of the coating material. Transient oxidation promotes
the selective oxidation of Al and thereby a stable formation,
growth and extended high temperature stability of the protective
.alpha.-Al.sub.2O.sub.3 scale. The adherence of alumina and chromia
scales on Ni and Co substrates is increased by additions of Y.
Furthermore, Yttrium generally reduces the chromia oxidation rate.
The Yttrium content shall not exceed the upper limit of 3 wt.-%
(preferred: 1 wt.-%) in order to avoid intense formation of
non-stable and inhomogeneous growing Y.sub.2O.sub.3 scales due to
the high oxygen affinity of Yttrium. Increased oxidation resistance
and a stable formation of a protective .alpha.-Al.sub.2O.sub.3
scale are ensured when 0.01-3 wt.-% (preferred: 0.01-1 wt.-%) Y are
added to the alloy.
Boron:
[0035] This element is added in order to form borides (Cr.sub.2B)
which are thermodynamically stable within the entire coating
operation temperature range. If less than 0.1 wt.-% Boron are
added, the volume fraction of borides is too low and the
strengthening effect is not present.
[0036] However, if more than 3 wt.-% (preferred: 1 wt.-%) Boron are
added, a high volume fraction of brittle borides is formed and the
toughness (plastic energy), cyclic lifetime respectively, decreases
again.
[0037] During service, the borides (Cr.sub.2B) act as Cr reservoir
releasing Cr to the depleted .gamma. matrix which can then diffuse
towards the coating-environment-interface to form a protective
Cr.sub.2O.sub.3 scale.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] The present invention is now to be explained more closely by
means of different embodiments and with reference to the attached
drawings.
[0039] FIG. 1 shows in a schematic overview MCrAlY alloy classes
(according to the known prior art) and their oxidation resistance
and hot corrosion resistance;
[0040] FIG. 2 shows the calculated phase fracture dependent on
temperature in the range between 600.degree. C. and 1400.degree. C.
for an advanced NiCrAlSiTaCoBY coating according to an embodiment
of the invention;
[0041] FIG. 3 shows the dependence between the boron content and
the Cr.sub.2B volume fraction of the standard MCrAlY (0 wt.-% B)
and 4 different advanced metallic coating systems according to
embodiments of the present invention and
[0042] FIG. 4 shows tensile test results at ambient temperature
(left part) and at 600.degree. C. (right part) for common state of
the art NiCrAlY coatings and for NiCrAlSiTaCoBY coatings according
to several embodiments of the present invention.
DETAILED DESCRIPTION OF DIFFERENT EMBODIMENTS OF THE INVENTION
[0043] The invention describes an advanced high temperature
resistant MCrAlYB coating class containing--as the main factor--the
element boron, leading to the formation of chromium-borides, in
higher amount (at least 1.75 vol.-% chromium-borides) compared to
similar state of the art coatings. M is at least one element out of
the group of Ni, Co and Fe. In addition Si and Ta are alloying
elements in said MCrAlYB coating according to the invention.
[0044] Some examples of preferred embodiments are coatings
consisting of the following elements (given in wt.-%), wherein the
balance is always Ni and inevitable impurities:
TABLE-US-00001 TABLE 1 Chemical composition of several exemplary
embodiments of the coating according to the invention Element
Coating Ni Cr Al Si Ta Co B Y AC-I Balance 24.02 5.3 2.34 1.24 1.02
0.23 0.45 AC-II Balance 23.04 5.1 2.08 1.48 2.04 0.46 0.40 AC-III
Balance 22.07 4.9 1.82 1.72 3.06 0.69 0.35 AC-IV Balance 21.08 4.7
1.56 1.96 4.08 0.92 0.30
[0045] The coating is applied onto the surface of a metallic
component, for example a gas turbine blade made of a Ni-base
superalloy.
[0046] The application is done under air, vacuum or inert gas by
one of the following thermal spray processes: [0047] Low pressure
plasma spray (LPPS) [0048] Vacuum plasma spray (VPS) [0049]
Atmospheric plasma spray (APS) [0050] High velocity oxygen fuel
(HVOF) [0051] Physical vapour deposition (PVD) [0052] Chemical
vapour deposition (CVD) [0053] Electrochemical deposition or by any
other suited application process which is state of the art.
[0054] The coating microstructure (phase distribution), at
thermodynamic equilibrium, was calculated using Thermo-Calc method.
The results for coating composition AC-III (see Table 1) are shown
in FIG. 2. The Cr.sub.2B volume fraction is constant over the
complete test temperature range, while the .alpha.-Cr fraction is
decreasing with increasing the temperature and does not more exist
at temperatures above about 760.degree. C. In addition, the volume
fraction of .gamma.' is significantly decreasing with increasing
temperature.
[0055] FIG. 3 shows the dependence between the boron content and
the Cr.sub.2B volume fraction of the standard MCrAlY (0 wt.-%
B--see Table 2) and the four different advanced metallic coating
systems according to the invention with their chemical composition
(given in wt.-%) described in Table 1.
TABLE-US-00002 TABLE 2 Chemical composition of the tested reference
standard coating Element Coating Ni Cr Al Si Ta Co B Y MCrAlY
Balance 25.0 5.5 2.6 1.0 0.5 -- 0.5
[0056] The nominal content of Ni, Ta, Co, and B in the four samples
of the embodiments according to the invention was increased,
whereas the Cr, Al, Si and Y content was decreased. The adjustment
of the coating microstructure is simple, as the volume fraction of
borides is linearly increasing with the boron content.
[0057] The advanced NiCrAlSiTaCoBY coating microstructure is
comprised of a .gamma.-matrix which contains .gamma.', .alpha.-Cr
and Cr.sub.2B precipitates. Formation of undesirable phases like
.alpha.-Cr or .beta.-(NiAl), which have a significant influence on
the ductile to brittle temperature (DBTT) and on the coefficient of
thermal expansion, is avoided. The risk of stress accumulation in
the coating (overlay) leading to surface cracking and stress
build-up when used as bond coat eventually causing TBC spallation
is significantly reduced.
[0058] Main hardening effect for NiCrAlY alloys is precipitation
hardening. With increasing temperature, the volume-fraction of
.gamma.' and .alpha.-Cr precipitates is significantly decreasing
(see FIG. 2). In consequence, mechanical properties change and e.g.
ultimate tensile strengths is significantly decreased. If compared
to common NiCrAlY alloys, the NiCrAlSiTaCoBY coating has increased
high temperature strength due to precipitation hardening by
thermodynamically stable CrB and/or Cr.sub.2B precipitates.
[0059] Tensile test results for various NiCrAlSiTaCoBY coating
compositions (embodiments of the present invention) in comparison
to a known state of the art NiCrAlY coating compositions (as
reference material) are shown in FIG. 4.
[0060] NiCrAlSiTaCoBY coatings offer higher tensile ductility at
lower temperatures and higher tensile strengths at comparable
strain (<6%) for higher temperatures.
[0061] As a matter of fact, the disclosed advanced coating class
according to the invention does perform much better in cyclic
loading. Enhanced tensile strength, respectively creep resistance,
at elevated temperature and less crack probability and severity due
to increased ductility at low temperature do lead to a
significantly extended lifetime of the high temperature protective
layer.
[0062] The hot corrosion resistance will be increased, due to a
diffusion-controlled dissolution of the CrB and/or Cr.sub.2B phase
which is acting as a chromium reservoir during long term
service.
[0063] Boron is known to be a fast diffusing element. In case of
chromium depletion in surface near regions during operation due to
oxide formation, the CrB and/or Cr.sub.2B precipitates will
dissolve and progressively release chromium which is needed to form
a protective chromium-oxide-scale. Furthermore, the advanced
coating promotes formation of highly protective alumina scales
which increases the coating service lifetime in base-load operation
with respect to oxidation.
TABLE-US-00003 TABLE 3 Chemical composition of several exemplary
embodiments of the coating according to the invention (in wt.-%)
Element Coating Ni Cr Al Si Ta Co B Y AC-V Balance 10.0 3.0 1.0 0.1
-- 0.1 0.01 AC-VI Balance 27.0 12.0 4.0 3.0 7.0 3.0 3.0
[0064] Coatings with a chemical composition at the lower specified
range (see embodiment AC-V in Table 3) show a significant
ductility, toughness respectively, increase. These coatings are
especially optimized for application in high-cyclic operation with
less oxidation and corrosion attack. On the other hand, coatings
with a chemical composition at the upper specification range (see
embodiment AC-VI, in Table 3) deliver best protection from
oxidation and hot corrosion at increased ductility (compared to
standard MCrAlY). These coatings are especially optimized for
cyclic and base-load mode with extended service lifetime intervals
(compared to the current state-of-the-art MCrAlY's).
[0065] The key advantages of the present invention are: [0066] High
temperature protective coating with increased lifetime in
(high-)cyclic operation mode and at least same lifetime in
base-load modus [0067] High temperature protective coating with
increased ductility at low operation temperatures (T<500.degree.
C.) due to an optimized microstructure with reduced volume fraction
of brittle phases like .alpha.-Cr, or .gamma.' [0068] High
temperature protective coating with increased tensile strength,
respectively creep resistance, at higher operation temperatures
(T.gtoreq.500.degree. C.) due to dispersion strengthening effect of
CrB and/or Cr.sub.2B precipitates [0069] High temperature
protective coating with increased (w.r.t. standard MCrAlY)
oxidation/hot corrosion properties due to presence of CrB and/or
Cr.sub.2B precipitates acting as chromium reservoirs in depleted
areas [0070] Adjustable strengthening and reservoir effect due to
linear relation between boron content and volume-fraction of CrB
and/or Cr.sub.2B precipitates
* * * * *