Note: Descriptions are shown in the official language in which they were submitted.
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Description
Overlay Coat;ngs With High Yttrium Contents
Technical Field
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~ This invention relates to improved NiCrAlY or
NiCoCrAlY coatings which are particularly useful in the
circumstance where the coating is applied to a hafnium
free superalloy substrate. The essence of the invention
is to increase the yttrium level in the coating to a level
in excess of that previously used.
.,
- 10 Background Art
NiCrAlY and NiCoCrAlY type coatings derive their
protective capability from their ability to form (and
repeatedly reform) a thin layer of alumina on their outer
surface. It is this alumina layer which resists oxida-
- 15 tion and hot corrosion attack. Oxygen active elements
: ~
such as yttrium are added to the coatings to promote ad-
herence of the alumina and to retard its tendency to
spall. Hafnium can also be added to improve alumina ad-
herence.
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-- 20 The prior art did not anticipate this invention,
appaxently because of the sequence of developments in
the superalloy and coatings field. In the beginning,
supexalloys were used in the so called equiaxed form and
were used either uncoated or coated with the simple coat-
ings such as aluminide coatings. As performance require-
ments increased, there were parallel improvements in the
superalloy and coating areas1 in the superalloy area
substantial improvements were obtained through the de-
: EH-7174 ~d~
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velopment of directional solidificat;on in which the
superalloy is caused to solidify with elongated grains
which are crystallographically oriented~ This is de-
scribed in U.S. patent 3,26~,505 which is af,signed to
the assignee of the present application. In the coating
area at about the same time the MCrAlY type overlay coat~
ings were developedO Such coatings are substantially
more protective than the previously used aluminide coat-
ingsO U.S. patents 3,542,530, 3,676,085, 3,754,303 and
3,928,026 relate to MCrAlY overlay coatingsO
Despite the prom~se of directional solidification,
significant problems were encountered in the area of
transverse ductility. It was found that the grain
boundaries separating the orientated elongated grains
were notably weak in the direction transverse to the
axis of elongation. This problem was overcome through
the addition of small amounts of hafnium to the alloy
(.5-2~ Hf) as described in U.S. patent 3,711,337 which
is also assigned to the present assignee.
Because of the devalopment of hafnium modified
directional solidified articles and MCrAlY overlay coat~
ings in the same laboratory at about the same time, it
was natural that they would be used in combination with
each other. It is also natural that highly demanding
applications, specifically first stage turbine blades,
would involve the use of both hafnium modified directional
solidified substrates and MCrAlY overlay coatings.
The MCrAlY coatings were optimized, as to composition,
on directionally solidified hafnium contAin;ng columnar
grain substrates. It was found that yttrium in levels
greater than about .5 or .7 percent by weight produced
undesirable low melting point phases at the coating
substrate interface, apparently phases based on yttrium
and hafnium.
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An advancement beyond columnar grain articles is
the use of similar ~ut more ~dvanced solidi~icatisn
techniques to produce single crystal articles as de-
scribed in U.S. patent 3,4Y4,70~. Such single crystal
articles are free from internal grain boundaries and
hence do not suffer from the transverse ductility prob~
lem encountered .in the case of columnar grain articles~
Consequently, there is no need to add hafnium to the
alloys for ductility improvements and in fact it has
been found desirable to elim.inate hafnium so as to im-
prove the heat treatability of single crystal superalloy
articles. This subject is discussed in U.S. patent
4,116,723 which is assigned to the present assignee~
The present invention arises ~rom the belated ap~
- 15 preciation that the hafnium in the substrate material
played a s.ignificant role in enhancing coating perform-
ance as well as in improving transverse ductility and
that the previously developed NiCrAlY and MiCoCrAlY
coatings can be significantly improved.
Disclosure of Invention
The coating compositions of the present invention
have the following broad composition ranges: 10-35%
chromium, 8-20% aluminum, 0~30~ chromium, 1.1-3.0~
yttrium, balance essentially nickel. Through the use
of higher yttrium levels than those previously used in
this type of coating, improved oxidation resistance re
sults. The coatings have particular utility on sub-
strates which are free from intentional additions of
hafnium~
Other features and advantages will be apparent from
the specification and claims and from the accompanying
drawings which illustrate an em~odiment of the invention.
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Brief Description of Drawings
~ The figures shows coating life as a function o~
yttrium level for two different coat~ngs.
Best Mode For Carrying Out The Invention
The basis of this invention is the discovery that
under certain circumstances, increasing the amount of
yttrium in a NiCoCrAlY coating over that taught in the
prior art can be beneficial. The circumstance ~n ~hich
this high yttrium NiCoCrAlY coating is particularly
beneficial is when the coating is applied to hafnium free
substrates. Table 1 lists composition ranges for the
coatings of the present invention.
TABLE 1
Broad ~ Prè~fer~ea
Nickel Balance Balance
Cobalt 0~30 15-25
Chromium 10-35 15-25
Aluminum 8-20 10-20
Yttrium 1.1-3.0 1~1-3O0
2~
These co~positions ~ide Pnh~n~ ooating n~;~A~;~n
resistance and ~re simil~r to the NiCoCr~1~ composition
~escribed in U.S. patent 3,928',026 except for the higher
yttrium levels, The i`nvention com,positions are suited
' 5 for the protection of nlckeI base superalloy articles.
It is known in the art that under certain circumstances
other additions to NiCoCrAlY coatings can be beneficial.
, For example, U.S. patent 4,034,142 suggests the addition
::
of silicon to MCrAlY coatings while U.S. patent 3,918,139
suggests the addition of platinum and similar noble metals
'' to NiCoCrAlY coatingsO The present invention can be
',: applied to the modified NiCoCrAlY coatings described in
these references~ It is also envisioned that tantalum
(0-10%) can be aaded to the high yttrium MCrAlY coatings
lS for improved properties without significantly affecting
the beneficial effect supplied ~y the high yttrium con~
tent of the present invention~
The coating compositions previously described may
, be applied to a variety of substrate types using dif-
': 20 ferent application techniques. Various considerations
affect the selection of substrate and coating technique
and different benefits can result. The greatest benefit
,, results when the high yttrium NiCoCrAlY coatings of the
invention are applied to substrates which are free from
- 25 intentional additions of hafnium. Such substrates may
, be either of conventionally cast polycrystalline form or
,'' directionally soli~ified columnar grain form, or di-
'' rectionally solidified single crystal form. The columnar
grain form is not likely because, to date, hafnium has
' 30 been found to be necessary to promote adequate transverse
,' grain boundary ductility in columnar grain articles.
" Nonetheless if one had a ~afnium free columnar grain
'' article, the invention coating would be highly pro~
', tective to it. High yttrium coatings will protect ha~ni-
35 num free substrates independent of coating deposition
technique employed.
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The figure shows the effect of yttrium content on
oxidation life of an NiCoCr~lY coating material as well
as a silicon modified Ni~CoCrAlY coating material D The
nom;nal composition by weight of the NiCoCrAlY material
was 22% cobalt, 18% chrom~um, 12.5% aluminum, varying
amounts of yttrium and balance nickel and the nominal
composition by weight of the modified silicon NiCoCrAlY
composition was 22% cobalt, ~8% chromium, 12~5~ aluminum,
1.5% silicon, varying amounts of yttrium and balance
nickel. All tests were run on a single crystal substrate
having a nominal composition of by weight 10% chromium,
5~ cobalt, 4~ tungsten, 1.4% titanium, 12% tantalum, 5
aluminum balance essentially nickel.
The coatings were applied by electron beam vapor
physical deposition process. Testing was performed in
a cyclic burner rig operated at a peak temperature of
2100F. The coating life results were ad~usted to re-
flect differences in coating th~ckness.
The effect of increased yttrium is seen to be a
significant increase in coating life. For example in
the NiCoCrAlY case, increa~ing the yttrium content from
1% to 2% increases the coating life from about 825 hours
to about 1290 hours, an increase of more than 50%.
~nother class of substrates to which the invention
coating can be applied are those substrates which con-
tain hafnium. If the invention coatings are applied to
sub~trates which contain hafnium, the formation o low
melting phases at the substrate-coating interface may be
a problem. This can be min;mi2ed if the coating is
applied by a plasma spray process. Plasma spray deposit-
ed coatings invariably contain a sufficient quantity of
oxygen to oxidize the substrate hafnium which is near
the coating-substrate interface~ Once the hafnium is
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oxidized, it is stable and will not form the deleterious
low melting point phase. Thus the deposition o~ the in-
vention coatings ~y plasma spraying onto hafnium con
taining substrates does not present melting point prob-
lems.
Wherl MCrAlY coatings are applied by ~apor deposition
to hafnium cont~;n;ng substrates, hafnium from the sub-
strate diffuses into the coating and improves its per-
formance. However, when MCrAlY coatings are applied by
plasma spraying to hafnium contA;ning substrates, the
oxygen which is present as yttrium and aluminum oxides
in the coating prevents the hafnium from di~fusing suf-
ficiently to the surface of the coating to improve its
performance. Instead, the diffusing hafnium reduces the
o~her oxides, ~alumina and yttria) and forms stable haf-
nium oxides, and as a result, no coating improvement re-
sults.
By using the yttrium-rich c~ating of the present
invention, it is possible to compensate for the loss of
the substrate hafnium "effect" on plasma sprayed coat~
ings. The increased ~ttrium substitutes for the missing
hafnium in promoting good adherence of the coatings'
protective alumina scale. Thus plasma spray coating
durability is enhanced even on hafnium-containing sub
stratesO
It should be understood that the invention is not
limited to the particular em~odiments shown and described
herein, but that various changes and modifications may
be made without departing from the spirit and scope of
this novel concept as defined by the following claims~
