Introduction
The drive to improve engine combustion efficiency while reducing emissions has meant that the operating temperatures within the turbine section of gas-turbine engines, whether aero, industrial, or ma-rine, have increased significantly during the last 30 years. For aeroengines, the mean blade temperatures are now around 1050ЊC,with peak temperatures in excess of 1150ЊC.1,2Under industrial and marine service conditions, temperatures are slightly lower (800–950ЊC), but more severe envi-ronments are encountered.3,4This trend of increasing temperature is continuing, as il-lustrated in Figure 1, which highlights the efficiency improvement (12% since 1970)associated with raising the turbine entry temperature to 2000K (1723ЊC) and the overall pressure ratio (the ratio of the stag-nation pressure at the compressor stage out-let to the pressure at the fan inlet) to 50 in Rolls-Royce aero-derivative gas turbines.5,6
This improved performance has been achieved by novel material design, im-proved cooling technologies, and better
manufacturing methods.2,3,7Thus, uncooled blades have been surpassed by cooled blades that, in turn, have been replaced by coated, cooled blades. The development of nickel-based superalloys has similarl
y progressed from wrought alloys through cast alloys to single-crystal alloys, with the latest developments focused on fourth-generation single-crystal alloys.
As a result of these improvements, the need for coating systems to bestow ade-quate oxidation and corrosion resistance upon the component is now essential, an inevitable result of the necessary reduc-tion in chromium content in the advanced alloys to achieve the required improve-ment in mechanical properties at higher operating temperatures. Diffusion alu-minide coatings were the first of these environmental-protection coatings (EPCs)introduced into service around 1960.8–14Aluminide coatings were formed using chemical vapor deposition (CVD) meth-ods and heat treatment to diffuse alu-minum into the component surface,
forming a layer of -NiAl on the compo-nent surface capable of forming a stable,slow-growing Al 2O 3scale (see the sections on Diffusion Coating Processes and Mod-ified Aluminide Coatings for more de-tails). Some 10–12 years later, in the early 1970s,4,8,10–13overlay coatings were intro-duced. These offered the capability of custom designing corrosion-resistant alloys—the MCrAlY series of alloys,where M is Ni, Co, or a mixture of Ni and Co—which were overlaid on the superal-loy substrates using vacuum deposition,flame/plasma spraying, or, more recently ,electroplating technologies, as discussed in the Overlay Coatings section.
Most recently , thermal-barrier coatings (TBCs) have been introduced on hot-gas-path components to lower metal surface temperatures. These ceramic overlay coat-ings work in conjunction with cooling technologies to provide a low-thermal-conductivity surface (the thermal barrier)in contact with the hot gases. Plasma-sprayed TBCs were first introduced in the mid- to late 1970s on static components.Rotating components were only recently introduced into service, following the de-velopment of the electron-beam physical vapor deposition (EB-PVD) process dis-cussed in detail in the section on the His-torical Development of Thermal-Barrier Coating Technologies.4,7,11,13–16
High-Temperature Surface Protection
The current drive for energy savings by improving efficiency while minimizing the emission of pollutants places a heavy de-mand on materials, since inevitably these requirements mean higher operating tem-peratures and in some cases exposure to more aggressive environments. Thus, oxi-dation and hot-corrosion processes have become significant, life-limiting, materials degradation processes, often limiting the upper service temperatures and having an associated impact both on engine efficiency and reliability . (For details on oxidation and hot corrosion, there are several key references and books on the topic.17–24) As a consequence, any development in su-peralloy technology has seen a parallel de-velopment in coating systems and surface protection.
Conventionally cast, directionally solidi-fied, and first-generation single-crystal materials can be protected against oxida-tion and corrosion to give adequate high-pressure turbine blade life using diffusion aluminide coatings and MCrAlY overlay coatings.4,8,10–13As the turbine entry tem-perature has increased, platinum alu-minides have replaced conventional aluminides,3,10,11,24–26and improved MCrAlY
A dvances in Coating
Design for High-Performance Gas Turbines
J.R.
Nicholls
compositions offering better oxidation re-sistance have been developed.11,26,27 Second-generation single-crystal alloys, although stronger, are susceptible to hot corrosion, particularly high-temperature sulfidation, and a wide range of alterna-tive coating strategies was proposed to address these issues.11,26,28–30 The latest de-velopment in this drive for improved sur-face protection, high temperatures, and better performance is TBCs applied to tur-bine aerofoil surfaces and other hot-gas-path components.
Growth of a Stable Oxide17–20,23
At high temperatures within gas-turbine engines, coatings that provide protection against oxidation grow a com-pact, adherent oxide scale (usually alu-mina) that provides a barrier between the high-temperature gases and the underly-ing metal. Growth of the oxide (and there-fore the corrosion rate) is controlled by the rate of species transport across the scale. This is diffusion-controlled (Wagner’s model) and results in parabolic or sub-parabolic growth rates. Thus, without the protective scale, the coating and ulti-mately the substrate are subject to rapid attack, often following linear kinetics. Chromia
scales (Cr2O3) are also used but offer less protection than alumina (Al2O3), especially at temperatures above 850–900ЊC, because chromia scales tend to sublime (through the formation of volatile
CrO3) above these temperatures. How-
ever, for temperatures between 600ЊC and
750ЊC, where acid fluxing can occur,
chromia-forming coatings are preferred.
The growth rates of oxide scales gener-
ally determine life for coatings, since this
controls key element depletion from the
coating in forming the scale. Spallation
rates due to mismatch stresses also de-
pend on scale thickness, with the growth
of a 10–15 m scale corresponding to the
effective coating life.3
The type of scale formed and scale
growth rates can be controlled by small
additions of reactive elements such as
yttrium, cerium, lanthanum, hafnium, or
zirconium (yttrium is widely used in over-
lay coatings as an addition, usually
0.3–0.5 wt%). Silicon has also been found
to be beneficial, especially under Type II
hot-corrosion conditions (a particularly
severe form of hot corrosion that occurs at
intermediate temperatures, 600–750ЊC,
due to SO3-induced acidic fluxing; see the
section on Phenomenology of Hot Corro-
sion). Precious-metal additions, such as
platinum, palladium, ruthenium, and rhe-
nium, are also known to improve the
oxidation/corrosion resistance of high-
temperature coatings.
Thus, protection of material exposed to
aggressive environments at high tempera-
tures depends on the properties of the
formed corrosion product (oxide or corro-
sion scale), which should act as a diffusion
barrier to prevent further attack by the at-
mosphere. The layer should be chemically
stable in the environment and prevent dif-
fusion of both the reacting gases (inward)
and the metal species (outward). Finally,
in order to maintain its effectiveness as a
diffusion barrier, the layer should be me-
chanically stable (i.e., it should not crack
or spall).
Phenomenology of Hot Corrosion
Hot salt corrosion is a form of high-
temperature attack on metals and alloys
that occurs in the presence of a molten de-
posit; metalwork in power station boilers,
waste incinerators, gas turbines, and diesel
engines is susceptible to this degradation
process. Most research in this area has
been related to the gas turbine, but the un-
derlying principles can be readily extended
to other spheres. Two types of attack are
observed, depending on the blade tem-
perature. Figure2a maps temperature
profiles measured on a turbine blade
under marine service conditions. Similar
temperature distributions can be found on
cooled industrial turbine blades. At
800–950ЊC, a high-temperature condition
occurs known as Type I hot corrosion,
characterized by the formation of a thick,
porous, outer oxide layer, an intermediate
layer of internal oxide particles mixed
with depleted alloy, and an inner region
containing internal sulfide particles that
lead the attack. Figure2b illustrates this
type of attack. Internal oxides and sulfides
are formed by reaction within the alloy be-
tween various alloying additions and oxy-
gen or sulfur that has dissolved into the
alloy as a result of the corrosion process.It
is generally observed that a maximum at-
tack rate occurs at about 850–900ЊC; at
higher temperatures, the deposition of
molten salts is reduced since the vapor
pressure is insufficient for condensation to
occur.
The other form of attack is known as
low-temperature hot corrosion, or Type II
hot corrosion, and is usually observed in
marine or industrial gas turbines in the
temperature range of 650–750ЊC. In this
type of attack, large oxide- and sulfide-
filled pits form, but there are no internal
sulfide particles. Figure2c illustrates this
form of attack. Due to the large tempera-
ture gradients that exist along the length
of a turbine blade, it is possible for both
types of hot corrosion to occur on the same
component.
In addition to understanding oxidation
and corrosion resistance, any interactions
between the coating system and the sub-
strate through interdiffusion must also be
Figure1.Improvement in efficiency with increasing turbine operating temperatures and pressures for a family of Rolls-Royce engines developed during the period 1970–2000.5,6 SFC is specific fuel consumption.Overall pressure ratio is the ratio of compressor exit pressure relative to ambient.T emperatures are turbine entry gas temperatures, measured in K.Data points are engine operating conditions during the test.A “civil type test”is an engine performance evaluation criterion applied to civil aircraft engines.
understood, as they can further degrade coating performance and also modify the substrate’s mechanical behavior. Coating Developments:A Historical Perspective
In general, the development of manu-facturing processes for high-temperature coatings has paralleled the evolution of gas-turbine materials and turbine compo-nent design. Early coatings involved sur-face modification of a component by diffusion, and this was first applied to tur-bine airfoils in 1957.9Such diffusion coat-ings are still in wide use today. Later, in response to the demand for high perform-ance, surface-engineered turbine compo-nents with modified diffusion coatings and overlay coatings were introduced.4,8–13 This occurred in the early to mid-1970s. More recently, the drive for higher turbine entry temperatures has seen the introduc-tion of TBCs on nozzle guide vanes and turbine blades as well as within combus-tor systems.14–16,31–34
Diffusion-Coating Processes Diffusion-coating processes have been applied for many years to improve the en-vironmental resistance of a base alloy by enriching the surface in Cr, Al, or Si. Pack chromizing, in which the chromizing reac-tants are in the form of a powder pack placed around the component to be coated,
was in widespread use in the early 1950s35
to increase the oxidation/corrosion resis-
tance of low-alloy steels. In the 1960s, alu-
minizing was first used for the protection
of superalloy gas-turbine airfoils.8–11There
was renewed interest in the 1970s in sili-
conizing,36,37and later in silicon-modified
aluminides,38–40when novel solutions
were being sought to the low-temperature
hot-corrosion problems associated with
contaminants in industrial and marine
turbine plants burning impure, high-sulfur
fuels.24,37,40,41
Diffusion coatings can be applied to
hot-gas components using a range of tech-
niques, including pack cementation, slurry
cementation, overpack CVD, and vapor-
phase CVD. Comprehensive reviews of
the deposition methods of diffusion coat-
ings are given in the references.9–11,42–46
In pack aluminizing, overpack alu-
minizing, and vapor-phase aluminizing,
the deposition rate and morphology of the
coating depend on the aluminum activity
in the gas phase, processing time, and tem-
perature. Coatings are classified as either
low-activity, when outward diffusion of
nickel occurs, or high-activity, when in-
ward diffusion of aluminum occurs. In the
latter case, a surface layer of Ni2Al3forms
and a further heat treatment is required to
convert this brittle surface layer to NiAl.
This step is usually combined with a heat
treatment required to recover substrate
properties. Figure3illustrates a typical
aluminide coating deposited on a nickel-
based superalloy using a high-activity, pack
coating process. The coating was deposited
using an aluminizing pack containing
2.2% Al at 900ЊC, and was heat-treated for
2 h at 1120ЊC, then 24 h at 845ЊC. Figure4
illustrates a similar aluminide coating pro-
duced using vapor-phase aluminizing.
The properties of the aluminide coating
(or any diffusion coating) depend upon
the process methodology, the substrate
composition, and the subsequent heat
treatment. Typically, aluminide coatings
contain in excess of 30 wt% Al and are de-
posited in thicknesses between 30 m and
100 m, depending on the type of alu-
minide formed. They offer satisfactory per-
formance for many aviation, industrial,
and marine engine applications. Under se-
vere hot-corrosion conditions, or at tem-
peratures above 1050ЊC, aluminide coatings
offer only limited protection. To address
these issues, modified aluminide coatings
and overlay coating (MCrAlY) technolo-
gies were developed in the 1970s, and de-
Figure2.Hot corrosion in industrial and marine gas turbines.(a)Schematic side view of a
gas turbine blade, showing isotherms equivalent to full-power operation.(b), (c)Micrographs of the surface of a turbine blade, showing (b)T ype I hot corrosion (900ЊC) and (c)T ype II hot corrosion (700Њ
C).
Figure3.Fully processed pack
aluminide coating (high-activity) on
IN738.phase is an intermetallic
compound based on NiAl, formed by
diffusing aluminum into the surface of a
nickel-based alloy.
Figure4.Vapor-phase aluminized
coating (low-activity) on IN738.
velopment in these two areas continues today to combat the increased demands placed on the modern gas-turbine power plants.
Modified Aluminide Coatings Modified aluminide coatings are fabricated9–11,37–40,42–46by depositing an in-terlayer (e.g., 7 m of platinum, by elec-troplating or PVD to subsequently form a platinum aluminide) prior to the aluminiz-ing process, by pretreating the superalloy before aluminizing (e.g., chromizing prior to pack aluminizing), by codepositing ele-ments from a pack or slurry, or by blending them in the vapor phase—Sermaloy J and Sermaloy 1515 are slurry codeposited silicon-aluminide coatings.38–40
Alloying coating additions include Cr, Si, Ta, various rare-earth elements, and pre-cious metals, with many of these coatings now commercially available. To improve the high-temperature oxidation perform-ance of aluminides, a significant advance was made with the development of platinum-modified aluminides. Platinum aluminide diffusion coatings are now an accepted industrial standard, outperform-ing conventional aluminides under high-temperature oxidation, cyclic oxidation, and hot-corrosion conditions.4,3,8,11,25,26,45–60 Much work is continuing in this important area, both as EPCs and as TBC bond coats—an environmental coating, specifi-cally designed to grow alumina, which acts as an underlayer, bonding the ceramic top coat to a superalloy.
Platinum Aluminide Coatings
Most commercial platinum aluminides are manufactured by first electroplating 5–10 m of platinum, then heat-treating to diffuse the platinum into the nickel-based superalloy prior to aluminizing. Both high-activity and low-activity alu-minizing have been used, as have pack ce-mentation, overpack CVD, and gas-phase CVD processes. Figure5illustrates a high-activity platinum aluminide produced by
pack cementation. Clearly, variations in
platinum thickness, heat treatment, alu-
minizing processes, and the substrate
result in a wide range of coatings/
microstructures broadly known as plati-
num aluminides.
Despite numerous studies of platinum-
modified aluminides,60,61the exact mecha-
nism by which platinum improves the
coating performance is still a subject of
open discussion.
Well documented since the mid-
1970s,61–64platinum additions improve
scale adhesion and this must, in part, ac-
count for the improved cyclic oxidation
resistance of the platinum aluminide bond
coats. It has been proposed that platinum
additions may reduce the growth rate of
interfacial voids,47–49interact with indige-
nous sulfur,48,50,51and limit the outward
diffusion of minor alloying elements (e.g.,
Ti, Ta, etc.) from the superalloy,51,52all of
which would improve scale adhesion. The
role the platinum coating addition plays
in scale formation and growth is an area of
much discussion when considering plat-
inum aluminide diffusion coatings. This is
reactive metaldue in part to the varying coating mor-
phologies produced during the manufac-
ture of platinum aluminide coatings, the
influence of the substrate, how coating
microstructure changes with oxidation/
aging, and the consequential influence of
these three effects on oxide morphologies
and growth rates.54–60,65,66Most researchers,
however, concur that platinum promotes
the selective oxidation of aluminum, and
this results in the formation of a purer,
slower-growing alumina scale.
Chromium and Silicon Aluminides
To combat hot corrosion, additions of
chromium42,45,67–72and silicon36–40,46were
researched in the 1970s and 1980s. Al-
though no better than standard aluminides
under high-temperature oxidation condi-
tions, chromium-rich diffusion coatings
offered improved performance in indus-
trial and marine turbines burning high-
sulfur fuels.67,72Silicon additions were also
shown to improve the hot-corrosion resis-
tance of -NiAl.46With about 10 wt% silicon
in the diffusion-formed aluminide, this sil-
icon aluminide coating proved uniquely
resistant to Type I and Type II hot corro-
sion,38–40although it was somewhat brit-
tle.68This research was the basis for
Sermaloy J (Figure6), a CrSi2-dispersed -
NiAl diffusion coating.38–40
Later, as engine technologies advanced
and directionally solidified or single-
crystal alloys were more widely adopted,
less chromium was available within the
alloy to form CrSi2, a critical component
within Sermaloy J.73Joint research between
Sermatech and Rolls-Royce led to the de-
velopment of Sermaloy 1515, a layered-
structure slurry aluminide containing
bands of CrSi2capable of providing hot-
corrosion resistance to low-Cr-containing,
single-crystal alloys.40,41,73,74
Overlay Coatings
Diffusion coatings, by the nature of
their formation, imply a strong interde-
pendence on substrate composition in de-
termining both their corrosion resistance
and mechanical properties; hence, the pos-
sibility of depositing a more nearly ideal
coating, with a good balance between oxi-
dation, corrosion, and ductility, has stimu-
lated much research interest since the early
1970s. The early MCrAlY coatings were al-
loys based on cobalt containing 20–40%
chromium additions, 12–20% aluminum
additions, and yttrium levels around
0.5%, with the most successful coating
being Co-25Cr-14Al-0.5Y.12The most re-
cent coatings are more complex and are
based on the MCrAlX system, where M is
Ni, Co, Fe, or a combination of these, and
X is an oxygen-active element, such as Y,
Si, Ta, or Hf,29,45,46,75–78or a precious metal,
such as Pt, Pd, Ru, or Re.27,75,79–82The com-
position of the MCrAl part of the system is
selected to give a good balance between
corrosion resistance and coating ductility,
while the active element additions are in-
tended to enhance oxide-scale adhesion
and decrease oxidation rates. Current
thinking suggests that a combination of
active elements is beneficial in reducing
coating degradation by means of their
synergistic interaction.
Overlay coatings have been deposited
using a range of techniques. The earliest
production method was EB-PVD.9How-
ever, because of the high capital cost in
Figure5.
Platinum aluminide coating.
Figure6.Silicon aluminide coating
(Sermaloy J).
setting up a commercial EB-PVD plant,plasma-spray methods have found wide acceptance, particularly the argon-shrouded and vacuum plasma-spray processes.3,29,67,82–84 More recently, high-velocity oxyfuel spraying processes,29,85–87composite electroplating,77,88,89 and auto-catalytic electroless deposition 30methods have been used to deposit overlay coating systems. However, coatings produced by EB-PVD processes are still considered the commercial standard against which other process routes are compared. Figure 7is a micrograph of an EB-PVD CoCrAlY (ATD5B) coating on MarM002, a nickel-based superalloy . Figure 8is an argon-shrouded, plasma-sprayed NiCoCrAlY coating, and Figure 9is a NiCoCrAlY coat-ing produced by the composite electro-plate method.
Overlay coatings of classic design, with 18–22% Cr and 8–12% Al, generally per-form better at higher temperatures (above 900ЊC) where oxidation is the dominant failure mode, reflecting the good adherence of the thin alumina scales promoted by the presence of active elements such as yttrium.Generally , NiCrAlY and NiCoCrAlY coat-ings outperform the cobalt-based systems
under these high-temperature oxidizing conditions.70,71
Methods have been investigated to im-prove the traditional MCrAlY perform-ance. New MCrAlXY alloys have been developed with multiple additions of ac-tive elements (e.g., Ti, Zr, Hf, Ta, and Si have been researched 45,46,75–78) or with the incorporation of precious-metal additions (Pt, Pd, Re, and Ru).27,75,79–82Surface modi-fications have also been examined using CVD,90–92PVD,75,76,93 electroplating,94–98and reprocessing with high-energy beams.45,78,99–102 An interesting proposal from such recent processing work is the possibility of depositing a single-crystal MCrAlY alloy coating onto a single-crystal superalloy .100,101The single-crystal epitaxial coating was produced using laser-cladding technology , with controlled solidification of the melt pool.
Such duplex coating processes result in the formation of layered or graded structures that can result in improved per-formance. For example, aluminizing a CoNiCrAlY alloy , as demonstrated for the commercial GE29+ and GE34+ processes 91and earlier published work on pulse alu-minizing 90can improve hot-corrosion re-sistance.68Similarly , silicon modifications to the surface of a CoCrAlY 76 coating have been demonstrated to improve resistance to low-temperature hot corrosion. Recently ,surface modifications were extended fur-ther by grading not only the aluminum profile, but also that of chromium to further enhance hot-corrosion resistance while maintaining excellent high-temperature behavior under oxidizing conditions. These smart coating concepts 28,103,104are discussed in the Smart Overlay Coatings section.
Modified Overlay Coatings
Recent work has focused on the modifi-cation of the surface of MCrAlY alloys with platinum and other precious met-als 94–98,105,106to improve their performance
under high-temperature cyclic oxidation conditions.94–96,105,106Such treatments were first proposed in the early 1980s as a method of improving the resistance of the MCrAlY alloy to corrosion by aggressive deposits from a range of alternate fuel sources.75The need for improved cyclic oxidation resistance driven by the pursuit of bond coats for advanced TBCs has re-newed interest in the performance of platinum-modified MCrAlYs, both as EPCs and as bond coats. Platinum modifi-cation of the surface of the MCrAlY 94–96,105or just the superalloy without depositing on the MCrAlY 97,98,106,107improves the cyclic oxidation performance. Figure 10il-lustrates the patented 97,98platinum diffu-sion treatment to the surface of a superalloy ,resulting in a ␥ϩ␥’ two-phased coating.This advantage of the platinum surface treatment relates to the ability of the platinum-diffused layer to provide a smoother, stronger surface that is more defect-free. Both strategies are being used for TBC bond coats and will be discussed in the section on the Historic Development of Thermal-Barrier Coating Technologies.Smart Overlay Coatings
Smart overlay coatings 28,103,104are func-tionally graded coating systems designed to provide high-temperature corrosion protection over a wide range of operating conditions. The SMARTCOAT design con-sists of an MCrAlY base enriched first in chromium, then aluminum, to provide a chemically graded structure. At tempera-tures above 900ЊC, the coating oxidizes to produce a protective alumina scale. How-ever, at lower temperatures, this alumina scale does not reform rapidly enough to confer protection under Type II hot-corrosion conditions. The SMARTCOAT is therefore designed with an intermediate
Figure 7.CoCrAlY (A TD5B) coating deposited by electron-beam physical vapor deposition (EB-PVD) on a
nickel-based superalloy (MarM002).
Figure 8.Argon-shrouded, plasma-sprayed MCrAlY (M NiCo) coating on
a nickel-based superalloy.
Figure 9.TM963 NiCoCrAlY coating,produced by the composite electroplate process, deposited onto a nickel-based
superalloy.
Figure 10.A platinum-diffused ␥ϩ␥’coating on CMSX4.98,99
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