Note: Descriptions are shown in the official language in which they were submitted.
2 ~7 rJOC3~ 13DV-8682
SUPERALLOY COMPONENT WITH DISPERSION-CONTAINING
PROTECTIVE COATING, AND METHOD OF PREPARATION
CROSS REFERENCE TO RELATED_APPLICATION
This application is related to concurrently filed and
commonly assigned application Serial No. (Attorney
Docket 13DV-8335), the disclosure of which is hereby
incorporated by referen~e.
BACKGROUND O~ THE INVENTION
This invention relates to the protection of
superalloys to be used at elevated temperatures, and, more
particularly, to superalloy articles protected by
coatings.
This invention was made with Government support under
Contract No. N-0019-80-C-0017 awarded by the Naval Air
Propulsion Center.
One of the most demanding materials applications in
current technolo~y is found in ths hot-stage components
used in aircraft jet engines. The higher the operating
temperature of an engine, the greater its efficiency, and
the more power it can produce from each gallon of fuel.
There is therefore an incentive to operate such engines at
as high a te~perature as possible. The primary limitation
on the operating temperatures of a jet engine is the
material-~ used in the hottect regions of the engine, such
as gas turbine blades and vanes.
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2~7~oQ~1 13DV-8682
There has been much research to develop materials that
can be used in high temperature engine applications. The
currently most popular and successful of such materials
are the nickel-base superalloys~ which are alloys of
S nickel with additions of a number of other elements such
as, for example, chromium, cobalt, aluminum, and tantalum.
The compositions of these superalloys are carefully
engineered to maintain their strength and other mechanical
properties even during use at the high temperature of
engine operation, which is in the neighborhood of 2000-F
or more.
The materials used in the jet engines must operate at
high temperatures, but additionally are subjected to
oxidative and corrosive conditions. Oxidation of
materials such as nickel and many of its alloys is rapid
at engine operating temperatures. The engine components
are also subjected to corrosive attack by chemicals in the
burned fuel, as well as ingested agents such as salt that
might be drawn into the engine as it operates near an
ocean. The materials that have the best mechanical
properties at high te~peratures often are not as resistant
to oxidation and corrosion as other materials, and there
is an ongoing search for materials that offer a compromise
between the best mechanical properties and the best
oxidation and corrosion resistance.
High operating temperatures can also be achieved by
other techniques not related directly to the alloy
compositions used in the components. For example, control
of grain structures and use of single crystals can result
in improved properties. Cooling passages may be provided
in the components, and cooling air passed through them to
lower their actual operating temperature.
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In another approach which is the primary focus of the
present invention, a thin protective metallic coating is
deposited upon the component. The coatinq protects the
substrate from oxidation and corrosion damage. The
coating must be adherent to the superalloy substrate and
must remain adherent through many cycles of heating to the
operating temperature and then cooling back to a lower
temperature when the engine is idling or turned off.
Because materials of different compositions have different
coefficients of thermal expansion, differential strains
develop between the coating and the component.
To accommodate the strains imposed by the thermal
cycling, the thin coatings have been made of materials
that are relatively weak and ductile. Such a coating can
plastically deform either in tension or compression to
remain adherent to the surface of the substrate as the
substrate is heated and cooled. Most coatings for
nickel-base superalloys have been made of alloys of
nickel, chromium, aluminum, and yttrium, which are termed
NiCrAlY alloys, and nickel, cobalt, chromium, aluminum,
and yttrium, which are termed NiCoCrAlY alloys. The term
MCrAlX, where M represents nickel, cobalt, iron or some
combination thereof and X represents yttrium, hafnium,
tantalum, silicon or some combination thereof, is a widely
used generic description for this type of alloy. While
such alloys contain many of the same elements as the
substrate materials, the proportions of those elements
have been adjusted to enhance oxidation and corrosion
resistance rather than mechanical properties. They
therefore lack the strength to serve as the structural
components themselves, but serve well as protective
coatings.
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There remains an ongoing need for improved metallic
coating materials that can protect the substrates for
extended periods of time, against oxidation and corrosion
damage. The present invention fulfills this need, and
further provides related advantages.
SUMMARY OF T~E INVENTION
The present invention provides a class, and specific
alloy compositions, of metallic coating materials useful
in protecting high-temperature superalloys. The
compositions are compositionally and structurally
compatible with the superalloy substrates, protect against
oxidation and corrosion damage, and remain adherent,
crack-free, and protective for greater periods of time
than prior metallic superalloy coatings. The coatings are
formulated and applied by conventional techniques~
In accordance with the invention, a coated superalloy
component comprises a substrate article formed of a
superalloy and an adherent coating over the substrate.
The coating is a nickel-base superalloy additionally
containing at least about 0.3 ~olume percent of dispersed
particles of an oxide of yttrium, hafnium, a rare earth,
or combinations thereof.
It is well known to provide yttrium and/or rare earth
elements in superalloys and superalloy coatings to improve
their resistance to oxidation. However, these elements
are present in s~all amounts or are provided in such a
compositional and formation context that a high fraction
of their oxide dispersoids is not formed. The approach of
the present invention intentionally provides a high
fraction of dispersoids distributed in such a way as to
improve the properties of the coating.
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13DV-8682
The coatings of the invention represent a significant
departure from conventional thinking in the metallic
coating area. Heretofore, metallic superalloy coatings
were made weak and ductile, to accommodate the strains
imposed by the substrate as the component was repeatedly
heated and cooled. The coating is deformed in complex
planar strain conditions that are dictated by the
deformation of the more massive substrate. The coating
must deform plastically and/or in creep to a new set point
during the temperature and load cycling of the engine, and
a weak coating was deemed most desirable to operate under
these constraints.
It was observed in the research underlying this
invention that metallic coatings tend to fail in thermal
fatigue, and that the weak coatings did not offer
sufficient mechanical resistance to such fatigue failure.
The present invention therefore provides an
oxide-dispersion containing coating, a coated article, and
a method for preparation thereof. The oxide-containing
coating is more resistant to thermal fatigue damage than
the NiCrAlY or NiCoCrAlY alloys conventionally used as
metallic coating materials, without sacrificing oxidation
and corrosion resistance.
The present invention provides an important advance in
the art of superalloys, as well as a departure from the
conventional thought in the field. The coating of the
invention permits a controllable improvement to the
properties of the coating through selection of the
fraction of dispersoid, while retalning the chemical
components that lead to oxidation and corrosion
resistance. Other features and advantages of the
invention will be apparent from the following mor~
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detailed description of the preferred embodiment, taken in
conjunction with the accompanying drawings which
illustrate, by way of example, the principles of the
invention.
BRIEF DESCRI~TION OF THE DRAWINGS
Figure 1 is a perspective view of a turbine blade
having a metallic protective coating; and
Figure 2 is an enlarged sectional view of the turbin~
blade of Figure 1, taken along lines 2-2.
DETAILED ~ESCRIPTION OF THE PREFERRED EMBODIMENT
The coating of the invention is pre~erably used with
nickel-base superalloys, in applications such as a jet
engine gas turbine blade 10 illustrated in Figure 1, or a
gas turbine vane which has a similar appearance in -
relevant respects. The blade 10 may be formed of any
suitable superalloy such as Rene' 80, a well known
nickel-base superalloy which has a nominal composition, in
weight percent, of 14 percent chromium, 9.5 percent
cobalt, 5 percent titanium, 4 percent tungsten, 4 percent
molybdenum, 3 percent aluminum, 0.17 percent carbon, 0.06
percent zirconium, 0.015 percent boron, and the balance
nickel. Another example is a more advanced nickel-base
superalloy such as Rene' N4, having a composition, in
weight percent, of 7.5 cobalt, 9.O chromium, 3.7 aluminum,
4.2 titanium, 1.5 percent molybdenum, 4.0 percent
tantalum, 6.0 percent tungsten, 0.5 percent niobium, and
balance nickel. These substrate superalloys are presented
as exa~ples, and the coatings are not limited for use with
these substrates.
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13DV-8682
Such a blade 10 includes an airfoil section 12 against
which hot combustion gases are directed when the engine
operates, and whose surface is subjected to severe
oxidation and corrosion attack during service. If the
surface of the airfoil section 12 is not protected against
oxidation and corrosion in some fashion, it will normally
last at most only a-few cycles of operation. The airfoil
section 12 is anchored to a turbine dis~ (not shown)
through a root section 14. In some cases, cooling
passages 16 are present in the airfoil section 12, through
which cool bleed air is forced to remove heat from the
blade 10. The blade 10 is normally prepared by a casting
and solidification procedure well known to those skilled
in the art, such as investment casting or, more
preferably, directional solidification or single crystal
growth.
According to the present invention, the airfoil
section 12 is protected by a metallic protective coating
20, as illustrated in detail in Figure 2, which depicts an
enlargement of a section throuqh the surface portion of
the blade 10. The nickel-base superalloy of the blade 10
(or of a gas turbine vane or other superalloy component)
forms a substrate 22 upon which and over which the coating
20 is deposited. The coating 20 contains at least about
0.3 percent by volume of a dispersion of oxide particles
24 formed by the oxidation of yttrium and/or rare earth
elements. These particles are typically equiaxed or
roughly spherical in shape, but they could have an
elongated shape. (In Figure 2, both the volume fraction
of the dispersoid and it~ size are exaggerated for
purposes of clarity of illustration.)
2~ Q~l. 13DV-8682
detailed description of the preferred embodiment, taken in
conjunction with the accompanying drawings which
illustrate, by way of example, the principles of the
invention.
S BRIEF DESCRIPTION OF THE DRAWINGS
Figure l is a perspectivQ view of a turbine blade
having a metallic protective coating; and
Figur~ 2 is an enlarged sectional view of the turbine
blade of Figure 1, taken along lines 2-2.
DETAILED DESCRIPTION OF ~E PREFERRED EMBODIMENT
The colting of the invention is pre~erably used with
nickel-base superalloys, in applications such as a jet
engine gas turbine blad~ 10 illustrated in Fiqure l, or a
gas turbine vane which has a similar appearance in
relevant respects. The blade 10 may be formed of any
suitable superalloy such as Rene' 80, a well known
nicXel-base superalloy which has a nominal composition, in
weight percent, of 14 percent chromium, 9.5 percent
cobalt, 5 percent titanium, 4 percent tungsten, 4 percent
molybdenum, 3 percent aluminum, 0.17 percent carbon, 0.06
perc~nt zirconium, 0.015 percent boron, and the balance
nickel. Another example is a more advanced nickel-base
superalloy such as Rene' N4, having a composition, in
wQight percent, o~ 7.5 cobalt, 9.0 chromium, 3.7 aluminum,
4.2 titanium, 1.5 percent molybdenum, 4.0 percent
tantalum, 6.0 percent tung~ten, 0.5 pQrcent niobium, and
balance nickel. Thes~ substrate superalloys are presented
as examples, and the coating~ are not limlted ~or USQ with
thes~ substrate~.
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In accordance with another approach for preparing the
coated component, a method for preparing a coated
superalloy component comprises the steps of providing a
substrate article formed of a nickel-base superalloy; and
applying an adherent coating to the article by a thermal
spray process such as vacuum or air plasma spray, the
coating being a nickel-base superalloy additionally
containing a sufficient amount of an element selected from
the group consisting of yttrium, hafniu~, and the rare
earths, and combinatione thereof, to form at least about
0.3 volume percent of dispersed oxide particles upon
subsequent oxidation heat treatment. In this approach, a
metallic alloy is applied to the surface. The superalloy
of the coating can be any coating designed for protection
of the substrate against oxidation and/or corrosion, but
additionally has a sufficient excess amount of yttrium,
hafnium or a rare earth to form the oxide dispersoids
during a subsequent heat treatment. After the coating is
applied, the coated component is heat treated in an
oxidizing atmosphere to form the oxide dispersoids within
the superalloy matrix. The oxides can be simple oxides or
complex oxides containing one or more of the elements
yttrium, hafnium and the rare earths.
The coating must contain at least about 0.3 volume
percent of the oxide dispersoids, or the individual
dispersoid particles will be too widely spaced to have a
significant effect on reduction of cracking of the coating
during thermal fatigue cycling. Larger amounts are
acceptable, as long as they do not lead to brittleness of
the coating. From about O.5 to about l.O volume percent
of the dispersoids is preferred.
2 ~ 7~ 13DV-8682
As used herein, the term "the rare earths" comprehends
those elements of the lanthanide series of the periodic
table, atomic numbers 57-71 inclusive. These elements
include lanthanum, cerium, praseodymium, neodymium,
prometheum, samarium, europium, gadolinium, terbium,
dysprosium, holmium, erbium, thuli~m, ytterbium, and
lutetium. Although yttrium, atomic number 39, and
hafnium, atomic number 72, are sometimes grouped together
with the rare earths, they are separately identified
herein, and not included among the rare earths.
In a preferred embodiment of the invention, a
conventional NiCoCrAlY coating material was modified by
adding 0.5 volume percent yttrium oxide and also alloying
elements to strengthen the grain boundary regions of the
coating. A standard NiCoCrAlY coating alloy has a
composition, in weight percent, of about 20-23 percent
cobalt, 18 percent chromium, 12.5 percent aluminum, 0~3
percent yttrium, and balance nickel. A satisfactory
coating alloy is obtained by leaving the major alloying
elements substantially as they are, and adding carbon,
boron, or zirconium to strengthen the grain boundaries of
the coating, and sufficient yttrium oxide to produce a
dispersion of about 0.5 volume percent yttrium oxide
(Y203) particles throughout the coating. The grain
boundary strengthener is preferably up to about 0.07
weight percent carbon, up to about 0.030 weight percent
zirconium, and up to about 0.030 percent boron, or
combinations thereof. The preferred minimum contents are
about 0.01 percent carbon, about 0.005 percent zirconium,
and about 0.005 percent boron. Amounts below those
indi~ated do not strengthen the grain boundaries to any
appreciable degree, which may lead to premature failure of
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the coating due to grain boundary creep. Amounts above
the maximums can lead to grain boundary embrittlement,
also a cause of premature failure.
A powder is formed from material of this composition
by any convenient known method, such as mechanical
alloying, and then applied to the component substrate by
vacuum plasma spraying.
In accordance with this aspect of the invention, a
coated superalloy component comprises a substrate article
formed of a superalloy; and an adherent coating over the
substrate, the coating being a nickel-base superalloy
containing at least one gamma phase ~rain boundary
strengthener selected from the group consisting of up to
about 0.07 percent carbon, up to about 0.030 percent
zirconium, and up to about 0.030 percent boron, and
additionally containing at least about 0.3 volume percent
of dispersed oxide particles formed of an element selected
from the group consisting of yttrium, hafnium, the rare
- earths, and combinations thereof.
The presently most preferred coating according to the
invention has a metallic matrix composition, in weight
percent, of 20 percent cobalt, 18 percent chromium, 12
percent aluminum, 0.95 percent carbon, 0.015 percent
boron, 0.015 percent zirconium, 0.3 percent yttrium
(present in the metallic ~orm), and 1.0 percent silicon.
About 0.5 percent by volume of yttrium oxide particulate
material was mixed with the metallic matrix composition by
mechanical alloying. (Yttrium is present in this coating
as a metallic alloying element, below its solubility
limit, and also as the oxide dispersoid yttriun oxide.)
This coating may be applied by any type of plasma
spraying, but preferably by vacuum plasma spraying, which
was employed in the examples described herein.
2~75~1. . 13DV-8682
The coating of the preceding paragraph was applied to
a simulated gas turbine blade made of the Rene' N4
superalloy discussed previously. These simulated blades
were comparatively tested against identical simulated
S blades of the same Rene' N4 superalloy, except having a
CODEP coating prepared by the pack-diffusion process
disclosed in U.S. Patent No. 3,540,878. In a burner rig
thermal fatigue test, the coated blades were cycled
between 970-F and 1800-F for 5000 cycle~, and inspected.
The blades coated with the dispersion-containing coating
exhibited approximately the same number of cracks as for
the CODEP coated blades, but the severity of the cracks
was much less for the dispersion-containing coatings.
In an accelerated burner rig oxidation test at 2075-F
and Mach 1 gas velocity, a set of test specimens with the
dispersion-containing coating had an average lifetime of
585 hours, as compared with 125 hours for identical
CODEP-coated specimens. In a hot corrosion test at 1700~F
and a 5 ppm (parts per million) salt environment, the
~o dispersion-containing coating had an average life of at
least 1600 hours (at which time the test was
discontinued), compared to only 550 hours for the
CODEP-coated blade.
Thus, the dispersion-containing coating of the
invention produces improved results in simulated operating
environments as compared with state-of-the-art CODEP
coatings.
Coatings of the present invention are typically
stronger than conventional NiCoCrAlY coatings. For
example, the rupture life of a conventional NiCoCrAlY
coating, tested at 1600-F and 3,000 pounds per square inch
stress, is about 13 hours. As deposited, the coating
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13DV-8682
described above has a rupture life under the same test
conditions of about 23 hours. After heat treating the
coating 2 hours at 2310-F, the rupture life was increased
to 506 hours. The increased strength is believed to
contribute to the observed reduction in severity of cracks
in the coating.
Thus, the present approach provides an advancement in
the protection of superalloy substrates, and more
particularly nickel-base superalloy substrates by metallic
protective coatings. Although the present invention has
been described in connection with specific examples and
embodiments, it will be understood by those skilled in the
arts involved, that the present invention is capable of
modification without departing from its spirit and scope
as represented by the appended claims.
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