Note: Descriptions are shown in the official language in which they were submitted.
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Background o~ the Invention
This invention relates primarily to metallic coatings
and coated articles and, more particularly, to me~allic coatings
applied t o m~tal articles for high temperature use.
As modern power generation apparatus, such as the gas
turbine engine, has evolved, the environmental operating temperatures in
its hotter sections have increased. Althoug4 metallurgists have developed
; improved alloys from which metallic components can be made, some are
subject to surface deterioration such as through oxidation or hot corroLsion,
to a degree greater than that which iB desirable. ~herefore, concurrently
with .he evolution of such apparatus has been the development of high
temperature operating surface treatments and coatings.
From the literature, it can be seen that a large number of
such coatings involve the use of aluminum as an important ingredient in the
coating. Earlier methods involved applying aluminum metal to the surface
directly such as through dipping in molten aluminum or spraying molten
aluminum onto the surface of an article. Such methods resulted in an
.
increase in article dimensions. Therefore, in order to retain the criSical
dimensions of an article such as for use in gas turbines, the pack diffusion
process was developed. C)ne example of such a pack process is represen~ed
by U. S. 3, 667, 985 - Levine et al issued June 6, 1972. Vapor deposition
of high temperature coatings, including aluminum as an important ingredient,
is shown in one form in U. S. 3, 528, 861 - ~lam e. al issued September 15J
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1970. Another method for vapor depositing coatings on a
substrate is shown in U.S. Patent 3,560,252 - Kennedy, issued
February 2, 1971.
Although a number of methods, compositions and
mixtures have been developed for the purpose of inhibiting or
retarding surface deterioration of articles exposed to the
environment at elevated temperatures, each has its limitation in
respect to the length of time it can afford protection.
Summary of the Invention
It is a principal object of the present invention to
provide an improved surface barrier including a system which
is applicable to a variety of coating methods and materials,
and which provides improved oxidation and sulfidation resistance
to a metallic article with which it is associated.
Another object is to provide a metallic article having
a surface portion of improved resistance to oxidation and
sulfidation and capable of being applied in a variety of ways.
Still another object is to provide an improved coating
material which can be used in improved methods for providing
an article with an oxidation and sulfidation resistant barrier.
These and other objects and advantages will be more
clearly understood from the following detailed description,
the examples and the drawings, all of which are intended to
be t~pical of rather than in any way limiting on the scope ¢
the present invention.
The metal article associated with the present
invention is provided with improved oxidation and sulfidation
resistance through application of a metallic coating which in-
cludes, as one coating ingredient, the element hafnium in the
range of 0.1 - 10 weight percent. In respect to the method
associated with the present invention, the element Hf can be app~d
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in a variety of ways. For example, the Hf can be applied to the article
surface before coating or it can be applied to the coated surface after coating.In addition, it can be included in or with the coating material or ingredients,
generally in powder form, from which the coating is generated. Thus,
; 5 associated with the present invention is a novel coating powder and coating
mixture material which can be used in the method to generate the article
- associated with the present invention.
Brief Description of the Drawings
FIG. 1 is a photomicrograph at 500 magnifications of an
aluminide coating including the element Hf, according to the present invention,
after 850 hours in a 2100~F (1150C) dynamic oxidation test;
FIG 2 is a photomicrograph at 500 magnifications of the same - ~ -
coating as in FIG. 1, applied in the same way to the same substrate but not
including the element Hf in the surface portion, after 400 hours in the 2100F
( 11 50C) dynamic oxidation test; and
FIG 3 is a graphical comparison of oxidation data of an
aluminide coating on separate specimens of the same Ni-base superalloy,
with and without the presence of Hf in the coating.
Description of the Preferred Embodiments
2a The degree to which an aluminide-type coating can protect
a metal surface, for example a nickel or cobalt base superalloy surface,
;~ depends on the coatingls ability to generate a dense, adhesive Al2O3 layer.
This protective oxide scale can separate and leave the surface, such as by
- spalling when stress due to thermal cycling is imposed, by mechanical
erosion or by fluxing due to the presence of corrosive molten salts. Such
removal of A12O3 scale will lead to the depletion of Al and therefore the
relatively rapid failure of the coating. It has been recognized through the
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present invention that the inclusion of hafnium in the coating
can change the morphology of the Al203 formed and result in better
oxide scale adherence and stability of the oxide scale in the
presence of molten salts. The improvement in adherence is brought
about by the hafnium oxide (HfO2) causing keying of the oxide
surface, such as through interlocking fingers, with the under-
lying balance of the coating. Thus, the presence f HfO2 in-
creases the stability of the A1203 generally resulting in at least
a two-fold improvement in coating life.
The type of keying or interlocking arrangement which
results from the use of hafnium in connection with the present
invention is shown by the typical photomicrograph in FIG. 1 at
500 magnifications after 850 hours exposure at 2100F (1150C)
in air. That portion of the coating generally indicated as A is
the outer surface portion or oxide scale, with B bing the
aluminide coating portion of the type described in the above-
mentioned patent 3,667,985 diffused into C, the substrate portion
of a Ni-base superalloy, sometimes referred to as ~ene'* 120 alloy,
and consisting nominally, by weight, of 0.17~ C, 9% Cr, 4% Ti,
0.015% B, 4.3% Al, 7~ W, 2~ Mo, 10% Co, 3.8~ Ta, 0.08% Zr with
the balance essentially Ni and incidental impurities. The
irregular, interlocking relationship between the oxide scale
portion A and the aluminide coating portion B can be seen at the
interface between those two portions. Referring to FIG. 2 in
which like, primed letters identify like portions, the same
aluminide coating, but without the inclusion of the element
Hf as in the coating in FIG. 1, after only 400
hours exposure at 2100F (11~0C) in air, results
in relatively smooth interface between oxide scale A'
and the aluminide B'. The significantly lower adherence of
the oxide scale A' in FIG. 2, resulting from the less desirable
mechanical inte~bck~ng between the oxide scale and the underlying
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aluminide coating, leads to a significantly lc~wer surface protection capabilit~y
cornpared with the system shown in FIG, 1.
During the evaluation of the present invention, represented
by the following typical examples, it has been recognized that the inclusion
of Hf as an ingredient in a metallic coating, within the range of about 0.1 -
10 wt. %, provides the unusual adherence and stability characteristics of
the basic Al2O3 scale, discussed in connection with FIGS, 1 and 2. However,
below about 0. 1 wt. ~o there has been found to be too little difference in the
- coating morphology to result in any significant change. Above about 10 wt. %
Hf can be detrimental to the coating because HfO2 is relatively porous; thus,
when it is present in too great an amount, it allows the conduction of oxygen
through the coating. Therefore, such large amounts of Hf in the coating will
make the coating oxidize faster and fail more quickly than if no Hf were
present.
Although there are a number of coatings which include Al and
with which the present invention can be associated, the present invention has
beer~ extensively evaluated in connection with a diffusion aluminide coating
method and material sometimes referred to as CODEP coating and described
in above-mentioned U. S. Patent 3, 667, 985. This type of coating is generated
through the use of a coating source metal powder, which includes the element
Al in an Al-Ti-C alloy, and a halide salt which will react with the coating
powder at the coating temperature, generally in the range of 1200 - 2100F
(650 - 1150C), to produce a metal halide from which the aluminum is
deposited on an article surface to be coated. Such surface can be embedded
in the c:oating powder, generally mixed with the halide salt and an inert
extender, such as A12O3 powder, or it can be held within a container
including such a mixture so that the metal halide generated can contact the
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article surface to provide the coating. That form of such
method in which the article to be coated is embedded in such
a powder mixture is widely used commercially and is
frequently referred to as the pack diffusion coating method.
xamples 1 - 6
The above-described type of pack diffusion coating
process was used to apply an aluminide coating to a nickel-
base superalloy, sometimes referred to as Rene' 80 alloy, and
consisting nominally, by weight, of 0.15~ C, 14% Cr, 5% Ti,
0.015% B, 3% Al, 4% W, 4% Mo, 9.5% Co, 0.06% Zr, with the
balance Ni and incidental impurities. Two types of pack
mixtures were prepared. A first, called Pack A in the following
Table, used the Al-Ti-C ternary alloy employed and claimed
in U.S. Patent 3,540,878 - Levine et al issued November 17,
1970 within the range, by weight, of 50 - 70% Ti, 20-48% Al
and 0.5 - 9% combined C. Such a pack included 4 wt. % of such
alloy in powder form along with 0.2 wt ~ NH4F, various amounts
of hafnium powder from which the examples of the following
Table were selected, the balance of the mixture being A1203.
- 2Q A second pack, called Pack B in the Table substituted 4%
of an iron-aluminum powder for the Al-Ti-C alloy powder as
. . .
the coating source. In this Pack B, the alloy consisted
essentially of, by weight, 51 - 61% Al, with the balance
; Fe and was further characterized by being in the form of a
; 25 two-phase structure of Fe2A15 and FeA13.
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TABLE
COATING COMPOSITION VS. COATING LIFE
Hf (wt. ~o)2100F Dynamic Oxidation
Example Pack in Pack in Coatin~ ~life in hr/mil)
1 1~ 0.2 2 250
2 A 0.35 5-8 300
3 A 2. 20 50
4 A 0 0 150 .
B 2 2 250
6 B 3 5-8 300
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~lthough in these examples Hf was added as Hf powder, it
should be understood that other convenient forms for addition of Hf to the
pack include use of a hafnium halide, for example HfF4, HfC14, etc. or an
alloy or other compound including Hf.
One group of specimens of the above-described Rene' 80
alloy were embedded in Pack A, another group in Pack B and all were
processed in the range of 1900 - 1950F (1038 - 1066C) in hydrogen for
about four hours in a series of evaluations to generate an aluminide coating,
including varying amounts of Hf, diffused into the surface of the specimen.
~he above Table includes selected examples typical of results obtained from
inclusion of Hf as a powder in the packs. It should be understood that the
amount of Hf in the coating is unique to the coating process and the ingredients, of the pack, for example, as shown by a comparison of E:xamples 1 and 5,
2 and 6, and 3 and 5. The unique result according to the present invention
is the presence of Hf in the coating, in or on the article surface, in the
range of 0.1 - 10 wt. %. As will be shown in connection with other examples,
this level of Hf in such coating can be achieved in a variety of ways.
Because the amount of Hf in the coating resulting from
Example 3 was at about 20 wt. %, outside the scope of the present invention,
the coating was unsatisfactory because the high volume fraction of HfO2 in
the protective oxide produced on this specimen allowed rapid diffusion of
oxygen through the protective layer causing premature failure of the coating,
even earlier than the specimen of Example 4 with no Hf. The absence of Hf,
as shown by Example 4, results in a coating life significantly lower than the
coating associated with the present invention and represented by Examples
1, 2, 5 and 6.
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Example 7
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Comparison of 2100F (1150C) cyclic dynamic oxidation test
data for specimens of the above-described Rene1 120 alloy is shown in the
graphical presentation of FIG. 3. Specimens of such alloy were processed
in Pack A and in Pack B as in Examples 1 - 6 and in the Table to result in
the same coating content, As can be seen from a vertical comparison of
life at any thickness of the additive layer of the aluminide coating, the life
of the coating associated with the present invention is about twice that of the
same coating applied to the same substrate with the same thickness but
without Hf. From these data, the significant effect of Hf on this type of
coating is easily seen. As will be shown in subsequent examples, Hf has a
similar effect on other types of metal coatings.
Example 8
The coating procedure used in applying the coatings from
Pack A described abore was repeated on specimens of the Rene' 120 alloy
except that HfF4 halide salt was substituted for the Hf metal powder as the
source of hafnium. In this particular example, HfF4 powder was included
in the amount of 0. 2 wt. % in the pack to result in 2% Hf in the resulting
aluminide coating. Dynamic oxidation testing at 2100F (1150C) in air of
such a coating showed it to have about twice the life time of the above-
described Pack A aluminide coating without Hf.
As will be understood by those skilled in the metallurgical
and metal coating arts, conduct of a coating process at a lower temperature
than that included in the present examples will result in a slower and less
efficient deposition rate. Thus, if lower temperatures are used, the amount
of Hf available to react with the coating source metal can be adjusted to
provide the desired amount of Hf in the coating, within the scope of the
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present invention. However, it has been recognized that inclusion of greater
than about 10 wt. % Hf with the coating source material, irrespective of the
form in which the Hf is used (for example as Hf powder, as a Hf compound
such a halide, as an alloy including Hf, etc.), is more detrimental than
5 beneficial. This is shown by a comparison of Examples 3 and 4 in the Table.
Thus, one form of the pack or coating mixture associated with the present
invention includes Hf in the coating source in an amount of from a small but
. effective amount up to 10 wt, % Hf, which provides in a resulting coating the
element Hf in the range of 0.1 - 10 wt, %.
Example 9
The coating associated with the present invention can be
attained by first sputtering, according to the well-known, commercially
used process, a thin layer of Hf metal on the surface of an article to be
protected and then aluminide coating, for example as has been described in
15 previous examples. In one series of examples, such application of Hf to a
thickness of about 0. 02 - 0. 04 mils, followed by aluminiding in accordance
with Pack A described above resulted in 4 - 8 wt. % Hf in the coating. The
same dynamic oxidation testing showed the coating life and resistance to be
equivalent to that of coatings prepared as in Examples 1, 2, 5 and 6.
The present invention has been used in conjunction with a
variety of coatings which can be applied in a number of ways and with the
same beneficial results. For example, in commercial use are a group of
coating alloys based on an element selected from Fe, Co or Ni and including
such elements as Cr, ~1 and Y. One such system evaluated in connection
25 with the present invention is described in the above-mentioned U. S. Patent
3, 528, 861. Such a coating can be applied by physical vapor deposition, ion
plating, sputtering, plasma spraying, etc. In addition, multiple, alternating
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layers of Fe, Co or Ni with Cr can be applied to the surface of an article to
be protected, followed by the application of Al and Hf according to the present
invention.
Example 10
The above-described Rene' 80 nickel-base superalloy was
electroplated with two alternating coatings of Cr and Ni, the layers having
a thickness of 0.1 and 0, 2 mils, respectively. The surface thus coated was
; placed in a Pack A type mixture similar to that described in connection with
the processing of the examples in the above Table,except that the ingredients
of the pack in this example consisted essentially of, by weight, 40% of the ~ -
ternary AlTiC coating source powder, 0. 35% Hf powder, 0. 2% NH4F with the
balance of the pack being A12O3. After processing for about 4 hours in the
range of 1900 - 1950~F (1038 - 1066C) in hydrogen, the surface was diffused
and alloyed into a Ni-20%Cr-20~oA1-5%Hf coating. After 600 hours in the
dynamic oxidation test described above, it was concluded from weight gain
data and microstructural examinations that the coating prepared in this
example would protect the Rene' 80 alloy specimen between 1-1/2 and 2 times
longer than a similar coating without Hf.
. From these examples, which are meant to be typical of rather
than in any way limiting on the scope of the present invention, it will be
- readily recognized by those skilled in the art the variety of modifications
and variations of which the present invention is capable, for example in
. respect to the compositions of alloys,packs, methods of application, etc.
One unique feature of the present invention is that it provides for the forma-
tion of a composite surface oxide more stable than A12O3 alone. Thus, the
combination of aluminum and hafnium oxides of the present invention provides
generally double or more the coating life for coatings with which it is formed.
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This is due at least partially to the unique keying arrangemellt of the coating's
oxide scale with the underlying portion of the coating as a result of the
combination of hafnium and aluminum oxides in the scale. It has been
found that an element such as Zr, which also forms oxides more stable than
Al203, does not provide such keying relationship.
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