A PART COMPRISING A COATING ON A SUPERALLOY METAL
SUBSTRATE, THE COATING INCLUDING A METAL UNDERLAYER
The invention relates to a part comprising a coating
on a substrate, the coating including a metal underlayer
5 covering said substrate.
Such a part is in particular a metal part that is
required to withstand high levels of mechanical and
thermal stress in operation, and in particular a part
with a superalloy substrate. Such a thermomechanical
10 part constitutes in particular a part of an aviation or
terrestrial turbine engine. Said part may in particular
constitute a blade or a vane or a nozzle for a turbine of
a turbine engine, and in particular of a turbojet or a
turboprop for an airplane.
15 The search for increased efficiency in turbine
engines, in particular in the field of aviation, and the
search to reduce fuel consumption and polluting emissions
of gas and non-burned residues have led to coming closer
to stoichiometric combustion of the fuel. This situation
20 is accompanied by an increase in the temperature of the
gas leaving the combustion chamber on its way to the
turbine.
At present, the temperature limit for using
superalloys is about 1100°C, with the temperature of the
25 gas at the outlet from the combustion chamber or at the
inlet to the turbine possibly being as high as 1600°C.
Consequently, it has been necessary to adapt the
materials of the turbine to this high temperature by
improving techniques for cooling turbine blades and vanes
30 (hollow blades and vanes) and/or improving the hightemperature
strength properties of such materials. This
second technique, in combination with using superalloys
based on nickel and/or on cobalt, has led to several
solutions including depositing a thermally insulating
35 coating referred to as a thermal barrier that is made up
of a plurality of layers on the superalloy substrate.
^
Over the last thirty years, the use of thermal
barriers in aeroengines has become general practice and
it enables the gas inlet temperature to turbines to be
increased, the flow rate of cooling air to be reduced,
5 and thus the efficiency of engines to be improved.
This insulating coating serves to create a
temperature gradient through the coating on a part that
is being cooled during steady operating conditions, with
the total amplitude of the temperature gradient possibly
10 exceeding 100°C for a coating having a thickness of about
150 micrometers (pm) to 200 pm and presenting
conductivity of 1.1 watts per meter per kelvin (W.m"i.K~^) .
The operating temperature of the underlying metal forming
the substrate for the coating is thus decreased by the
15 same gradient, thereby leading to significant savings in
the volume of cooling air needed and in the specific
consumption of the turbine engine, and also leading to a
longer lifetime for the part.
It is known to have recourse to using a thermal
20 barrier that comprises a layer of ceramic based on
yttrium oxide stabilized zirconia, i.e. yttria-stabilized
zirconia having a molar content of yttrium oxide lying in
the range 4% to 12% (and in particular 6% to 8%),
presenting a coefficient of expansion that is different
25 from that of the superalloy constituting the substrate,
and presenting thermal conductivity that is quite low.
The stabilized zirconia may also sometimes contain at
least one oxide of an element selected from the group
constituted by the rare earths, and preferably from the
30 following subgroup: Y (yttrium), Dy (dysprosium), Er
(erbium), Eu (europium), Gd (gadolinium), Sm (samarium),
Yb (ytterbium), or a combination of an oxide of tantalum
(Ta), and at least one rare earth oxide, or with a
combination of an oxide of niobium (Nb) and at least one
35 rare earth oxide.
Among the coatings used, mention is made of the
fairly generalized use of a layer of ceramic based on
^
zirconia that is partially stabilized with yttrium oxide,
e.g. ZrQ_92Yo_o8'-'i.96-
In order to anchor this ceramic layer, a metal
underlayer having a coefficient of expansion that ideally
5 is close to that of the substrate, is generally
interposed between the substrate of the part and the
ceramic layer. In this way, the metal underlayer serves
firstly to reduce stresses due to the difference between
the coefficients of thermal expansion of the ceramic
10 layer and of the substrate-forming superalloy.
This underlayer also provides adhesion between the
substrate of the part and the ceramic layer, it being
understood that adhesion between the underlayer and the
substrate of the part takes place by inter-diffusion,
15 while adhesion between the underlayer and the ceramic
layer takes place by mechanical anchoring and by the
propensity of the underlayer at high temperature to
develop a thin oxide layer at the ceramic and underlayer
interface, which oxide layer serves to provide chemical
20 contact with the ceramic.
In addition, this metal underlayer provides the
superalloy of the part with protection against corrosion
and oxidation phenomena (the ceramic layer is permeable
to oxygen).
25 In particular, it is known to use an underlayer
constituted by a nickel aluminide including a metal
selected from platinum, chromium, palladium, ruthenium,
iridium, osmium, rhodium, or a mixture of these metals,
and/or a reactive element selected from zirconium (Zr),
30 cerium (Ce), lanthanum (La), titanium (Ti), tantalum
(Ta), hafnium (Hf), silicon (Si), and yttrium (Y).
For example, a coating of the (Ni,Pt)Al type is used
in which the platinum is in insertion in the nickel
lattice of the p-NiAl intermetallic compounds. The
35 platinum is deposited electrolytically prior to
thermochemical aluminization treatment.
0
Under such circumstances, this metal underlayer may
be constituted by a platinum-modified nickel aluminide
NiPtAl using a metal comprising the following steps:
preparing the surface of the part by chemical etching and
5 sand blasting; electrolytically depositing a platinum
(Pt) coating on the part; optionally applying heat
treatment to the resulting assembly to cause the Pt to
diffuse into the part; depositing aluminum (Al) by
chemical vapor deposition (CVD) or by physical vapor
10 deposition (PVD); optionally heat treating the resulting
assembly to cause Pt and Al to diffuse into the part;
preparing the surface of the resulting metal underlayer;
and depositing a ceramic coating by electron beam
physical vapor deposition (EB-PVD).
15 In conventional manner, said underlayer is
constituted by an alloy suitable for forming a protective
alumina layer by oxidation: in particular, using a metal
underlayer that includes aluminum gives rise by natural
oxidation in air to a layer of alumina AI2O3 that covers
20 all of the underlayer. The purity and the growth rate of
the oxide layer at the interface is a parameter that is
very important in controlling the lifetime of the thermal
barrier system.
Usually, the ceramic layer is deposited on the part
25 to be coated either by a spray technique (in particular
plasma spraying) or by physical or chemical vapor
deposition, i.e. by evaporation (e.g. using EB-PVD to
form a coating deposited in an evacuated evaporation
enclosure under electron bombardment).
30 With a spray coating, a zirconia-based oxide is
deposited using plasma spray type techniques under a
controlled atmosphere, thus leading to a coating being
formed that is constituted by a stack of molten droplets
that have been impact-quenched, flattened, and stacked so
35 as to form an imperfectly-densifled deposit of thickness
generally lying in the range 50 pm to 1 millimeter (mm).
A coating deposited by a physical technique, e.g. by
electron beam evaporation, gives rise to a coating made
up of an assembly of columns that are oriented
substantially perpendicularly to the surface for coating,
5 over a thickness lying in the range 20 ]im to 600 pm.
Advantageously, the space between the columns enables the
coating to compensate effectively the thermomechanical
stresses that, at operating temperatures, are due to the
differential expansion relative to the substrate.
10 Parts are thus obtained that present lifetimes that
are long while they are being subjected to hightemperature
thermal fatigue.
Conventionally, such thermal barriers thus
constitute a thermal conductivity discontinuity between
15 the outer coating of the mechanical part, which forms the
thermal barrier, and the substrate of the coating, which
forms the material constituting the part.
Nevertheless, standard present-day thermal-barrier
systems present certain limits, including the following:
20 • because the oxidation resistance of firstgeneration
substrates of the AMI and/or AM3 type is not
optimized in terms of the ability of the thermal-barrier
system to withstand spalling, it is necessary to use an
attachment underlayer that withstands high temperature
25 oxidation under thermomechanical cycling conditions. A
first-generation superalloy of the "AMI" type presents
the following composition in percentages by weight: 5% to
8% Co; 6.5% to 10% Cr; 0.5% to 2.5% Mo; 5% to 9% W; 6% to
9% Ta; 4.5% to 5.8% Al; 1% to 2% Ti; 0 to 1.5% Nb; C, Zr,
30 B, each less than 0.01%: the balance to 100% being
constituted by Ni;
• the relative fragility of the metal underlayer as
from a certain temperature (e.g. the p-(Ni,Pt)Al metal
underlayer presents a ductile-brittle phase transition at
35 a temperature of about 700°C): for high levels of
mechanical stress, premature cracking occurs in the
underlayer, which then propagates into the substrate and
leads to the part deforming, or indeed to the part
breaking;
• the lack of microstructure stability in the
attachment underlayer during use at high temperature.
5 Interdiffusion between the underlayer and the superalloy
leads to the p-(Ni,Pt)Al coating being transformed into
martensite and then into y-Ni and y'^NijAl.
In the prior art, in order to improve the ability of
the thermal-barrier system to withstand oxidation,
10 proposals have been made to add hafnium (Hf) in the
substrate or directly in the composition of the metal
underlayer. It is known that hafnium improves the
ability of the system to withstand oxidation, but that it
also serves to reduce significantly damage at the
15 interface between the metal underlayer and the substrate
(reference: "Effect of Hf, Y and C in the underlying
superalloy on the rumpling of diffusion aluminide
coatings" - Acta Materialia, Volume 56, Issue 3, February
2008, pp. 489-499, V.K. Tolpygo, K.S. Murphy,
20 D.R. Clarke). Nevertheless, although it has proved to be
effective, adding hafnium presents a significant risk
since precipitates may form in the metal underlayer
during deposition such that the hafnium can no longer
perform its role of providing protection against
25 oxidation. Furthermore, it should be observed that
depositing hafnium by physical vapor deposition
techniques presents a relatively high cost.
In the prior art, in order to improve the
thermomechanical strength of the part, proposals have
30 been made to vary the chemical composition of the
substrate, in particular by adding several percent of Re
(Rhenium), in particular in the range 3% to 6%.
Efforts have been devoted mainly to chemical
optimization of the metal substrate and very little work
35 has been carried out simultaneously on the substrate and
metal underlayer pair.
Thus, until now no solution has made it possible to
improve both the ability of the substrate to withstand
oxidation and also the thermomechanical strength of the
part, without the improvement in one of these aspects
5 being detrimental to the other aspect.
An object of the present invention is to provide a
coating that makes it possible to overcome the drawbacks
of the prior art, and in particular that provides the
possibility of improving the thermomechanical strength of
10 the metal underlayer of the thermal barrier.
In addition, when the coating includes a ceramic
layer on the metal underlayer, the lifetime of the
thermal barrier with respect to spalling should also be
improved by reinforcing the oxidation-withstanding
15 properties of the metal underlayer and by conserving a
low-roughness surface state for longer during thermal
cycling.
To this end, the present invention provides a part
comprising a coating on a superalloy metal substrate, the
20 coating comprising a metal underlayer covering said
substrate, the part being characterized in that said
metal underlayer contains a base of nickel aluminide and
also contains 0.5 atomic percent (at%) to 0.95 at% of one
or more stabilizer elements M from the group formed by Cu
25 and Ag for stabilizing the gamma and gamma prime phases.
It can thus be understood that in the invention
provision is made for a total presence lying in the range
0.5 at% to 0.95 at% of one or more stabilizer elements M
for stabilizing the gamma and gamma prime phases, these
30 elements being selected from the group formed by Cu and
Ag, i.e. 0.5 at% to 0.95 at% of Cu only, or of Ag only,
or of a mixture of both.
The inventors have found that with such a
modification for the composition of the metal underlayer,
35 a metal underlayer is obtained that is much more stable
over time (withstands oxidation better and maintains its
microstructure better), that is a better crystallographic
match with the superalloy substrate (y and y' phases of
the metal underlayer), and with a coefficient of thermal
expansion that is closer to that of the superalloy, and
that is less subjected to interdiffusion.
5 This solution also presents the additional advantage
of reducing the rate at which the underlayer oxidizes.
Furthermore, it is found that by means of this
composition, the metal underlayer is less subjected to
the formation of defects and thus conserves for longer a
10 surface state with low roughness at its top surface or
surface forming an interface with the ceramic layer,
thereby contributing to increasing the lifetime of the
coating.
Overall, by means of the solution of the present
15 invention, it is possible to make a coating that presents
a longer service lifetime.
Preferably, said metal underlayer includes as its
stabilizing element M only Ag in the range 0.5 at% to
0.95 at%. Preferably, this single stabilizer element Ag
20 is present at a content lying in the range 0.6 at% to
0.9 at%, and preferably at a content lying in the range
0.7 at% to 0.85 at%.
Preferably, said metal underlayer includes as its
stabilizing element M only Cu in the range 0.5 at% to
25 0.95 at%. Preferably, this single stabilizer element Cu
is present with a content in the range 0.6 at% to
0.9 at%, and preferably with a content in the range
0.7 at% to 0.85 at%.
In another preferred provision, said metal
30 underlayer also contains platinum group elements in the
range 2 at% to 30 at%, and preferably in the range 15 at%
to 25 at%, so as to form a metal underlayer with an
NiPtAl type base.
The term "platinum group metal" is used to mean
35 platinum, palladium, iridium, osmium, rhodium, or
ruthenium.
Preferably, said metal underlayer also contains at
least one of the reactive elements RE making up the
following reactive elements of the rare earth type: Hf,
Zr, Y, Sr, Ce, La, Si, Yb, Er, and the reactive element
5 Si, with each reactive element being at a content lying
in the range 0.05 at% to 0.25 at%.
Furthermore, and preferably, the metal underlayer is
of the NiAl(Pt)MRE type (where Pt is a platinum group
element) or of the NiAlMRE type (without any element Pt
10 of the platinum group).
Preferably, said metal underlayer also contains as
reactive element(s) (RE): 0.05 at% < Hf < 0.2 at% and/or
0.05 at% < Y < 0.2 at% and/or 0.05 at% < Si < 0.2 at%.
More precisely, the metal underlayer contains an
15 NiPtAl type base, as its stabilizer element M only Ag in
the range 0.75 at% to 0.9 at%, and as reactive elements
0.08 at% < Hf < 0.20 at% and/or 0.10 at% < Y < 0.20 at%
and/or 0.15 at% < Si < 0.25 at%. Under such
circumstances, the metal underlayer is of the NiPtAlM(RE)
2 0 type.
Furthermore, the following provision may
advantageously be adopted:
• said metal underlayer also contains in the range
5 at% to 36 at% of Al (aluminum), and preferably in the
25 range 8 at% to 25 at% of Al; if the metal underlayer is
of the NiPtAlM(RE) type, then it preferably contains in
the range 15 at% to 25 at% of Al.
Advantageously, said metal layer presents thickness
of less than 20 pm, and preferably of less than 15 jim.
30 Preferably, said metal underlayer includes a nickel
aluminide base and further includes a metal selected from
platinum, chromium, palladium, ruthenium, iridium,
osmium, rhodium, or a mixture of these metals, and/or one
or more reactive elements selected from zirconium (Zr),
35 cerium (Ce), lanthanum (La), strontium (Sr), hafnium
(Hf), silicon (Si), ytterbium (Yb), erbium (Er), and
yttrium (Y).
10
In another preferred provision, said metal substrate
of the part is made of a nickel-based superalloy.
In particular, said metal substrate is made of a
nickel-based superalloy of the AMI (NTaSCKWA) type.
5 The invention is not limited to parts with a
substrate made of a nickel-based superalloy: a part made
of a superalloy based on cobalt may also carry a coating
with the composition in accordance with the invention.
The invention also relates to a coating that further
10 comprises a ceramic layer covering said metal underlayer,
in order to form a thermal barrier.
In particular, the part of the present invention may
form a turbine part for a turbine engine.
In another aspect of the present invention, the part
15 forming a part of a turbine engine is a blade or a vane,
in particular a turbine blade or vane, a portion of a
nozzle, a portion of an outer shroud or of an inner
shroud of a turbine, or a portion of a wall of a
combustion chamber.
20 Other advantages and characteristics of the
invention appear on reading the following description
made by way of example and with reference to the
accompanying drawings, in which:
• Figure 1 is a diagrammatic section view showing a
25 portion of a mechanical part coated in a coating;
• Figure 2 is a diagrammatic section view showing a
portion of a mechanical part coated in a coating forming
a thermal barrier;
• Figures 3 and 4 are micrograph sections at two
30 different magnifications showing the various layers of
the thermal barrier at the surface of the part, after a
cyclic oxidation-resistance test, and with a prior art
metal underlayer;
• Figure 5 shows the composition profile of the
35 metal underlayer of the part of Figures 3 and 4, as a
function of depth;
11
• Figures 6 and 7 are micrograph sections at two
different magnifications showing the various layers of
the thermal barrier at the surface of the part after a
cyclic oxidation-resistance test, and with a metal
5 underlayer of the invention;
• Figure 8 shows the composition profile of the
metal underlayer of the part of Figures 6 and 7, as a
function of depth; and
• Figures 9 and 10 show the ability of various
10 samples to withstand spalling when subjected to thermal
cycling (cyclic oxidation at 1100°C in air).
In a first embodiment, the metal part shown in a
fragmentary view in Figure 1 comprises a coating 11
deposited on a superalloy substrate 12, e.g. a superalloy
15 based on nickel and/or on cobalt. The coating 11
comprises a metal underlayer 13 deposited on the
substrate 12. An interdiffusion zone 16 situated at the
surface of the substrate 12 is modified in operation by
certain elements of the metal underlayer 13 diffusing
20 into the substrate 12.
The bonding underlayer 13 is a metal underlayer
constituted by or including a nickel aluminide base
optionally containing a metal selected from: platinum,
chromium, palladium, ruthenium, iridium, osmium, rhodium,
25 or a mixture of these metals, and/or a reactive element
selected from zirconium (Zr), cerium (Ce) , strontium
(Sr), titanium (Ti) , tantalum (Ta), hafnium (Hf), silicon
(Si), and yttrium (Y), in particular a metallic
underlayer constituted by NiAlPt.
30 Such a coating 11 is a protective coating used
against phenomena of hot oxidation and of corrosion.
In a second embodiment, said coating 11 also
comprises a ceramic layer 14 covering said metal
underlayer 13.
35 This is a mechanical part shown partially in
Figure 2 and it has a thermal barrier coating 11
deposited on the superalloy substrate 12, e.g. a
12
superalloy based on nickel and/or on cobalt. The thermal
barrier coating 11 comprises a metal underlayer 13
deposited on the substrate 12, and a ceramic layer 14
deposited on the underlayer 13.
5 The ceramic layer 14 is constituted by an yttriumstabilized
zirconia base having a molar content of
yttrium oxide lying in the range 4% to 12% (partiallystabilized
zirconia). Under such circumstances, the
stabilized zirconia 14 may also contain at least one
10 oxide of an element selected from the group constituted
by the rare earths, and preferably from the following
subgroup: Y (yttrium), Dy (dysprosium), Er (erbium), Eu
(europium), Gd (gadolinium), Sm (samarium), Yb
(ytterbium), or a combination of an oxide of tantalum
15 (Ta) and at least one rare earth oxide, or with a
combination of an oxide of niobium (Nb) and at least one
rare earth oxide.
During fabrication, the bonding underlayer 13 is
oxidized prior to depositing the ceramic layer 14, giving
20 rise to the presence of an intermediate layer 15 of
alumina between the underlayer 13 and the ceramic layer
14.
In the view of Figure 2, there can be seen the
various layers mentioned above, with a column structure
25 that is typical for the ceramic layer 14 present on the
surface.
After being used in service, the part (e.g. a
turbine blade or vane) will have been subjected to
hundreds of high temperature cycles (at about 1100°C),
30 and it will present a thermal barrier of morphology that
has changed and that ends up by becoming damaged and
spalling so that the substrate is no longer protected.
With reference to Figures 3 to 5, the structure of
the thermal barrier 11 is shown after 300 one-hour
35 thermal cycles at 1100°C in air, in order to illustrate
the behavior of a prior art thermal barrier when
subjected to cyclical oxidation.
13
This thermal barrier 11 in Figures 3 and 4 was
deposited on a substrate 12 made of a nickel-based alloy
of the AMI or NTaSGKWA type, and it comprises a metal
underlayer 13 of beta phase (Ni,Pt)Al (i.e. p-(Ni,Pt)Al),
5 surmounted by an intermediate layer 15 of alumina (AlgOj) ,
itself covered in the layer of stabilized zirconia
ceramic 14.
Black residues of sand-blasting alumina can be seen
in the bottom portion of the metal underlayer 13. This
10 interdiffusion zone 16 situated in contact with the
substrate 12 is characterized by precipitates of heavy
elements and by topologically close-packed (TCP) phases
(pale precipitates of globular and needle shapes). It
should be recalled that TCP phases are constituted by
15 precipitates of heavy elements that appear at locations
where a large amount of material has diffused, in the
interdiffusion zone between the metal underlayer and the
substrate.
At higher magnification (Figure 4), it can be seen
20 that the surface of the metal underlayer 13 is highly
irregular. There can also be seen delamination or loss
of adhesion at the interface formed between the
intermediate alumina layer 15 (or thermally grown oxide
(TGO)) and the zirconia layer (outer ceramic layer 14).
25 Furthermore, the beginning of a beta to gamma prime
phase transformation (P —> y') can be seen in the P metal
underlayer 13 after 300 cycles (Figure 3), located at the
joints of the P grains. This transformation tends to
induce changes of volume and thus make the coating 11
30 brittle.
Furthermore, it can be seen from the profile of the
composition of the metal underlayer 13 (Figure 5), that
the aluminum of the intermediate alumina layer 15 has
diffused into the metal underlayer 13, with a significant
35 proportion of aluminum (more than 30 at%) being found at
depths in the range 10 ^m to 20 ]im.
f^
14
Reference is now made to Figures 6 to 8 which
correspond respectively to views similar to those of
Figures 3 to 5, for a coating 11 presenting a metal
underlayer 13' and a ceramic layer 14. The only
5 difference lies in the fact that the metal underlayer 13'
has the composition of the present invention.
In particular, in this example, it is a metal
underlayer 13' of the y/y' NiPtAl type (i.e. the
gamma/gamma prime NiPtAl type) that has been doped with
10 Hf (0.13 at%), Y (0.15 at%). Si (0.22 at%), and Ag
(0.83 at%) .
For this purpose, tests were performed using the
spark plasma sintering (SPS) technique with foils of pure
aluminum and of pure platinum that were stacked on one
15 another. More precisely, the following were stacked on
the AMI substrate one on another and in the following
order:
• a 50 nanometer (nm) layer of Si deposited by the
high frequency physical vapor deposition (PVD-HF)
20 technique lying directly on the AMI substrate;
• a 150 nm layer of the element Y that was deposited
by the PVD-HF technique;
• a 90 nm layer of the element Hf that was deposited
by the PVD-HF technique;
25 • a 220 nm layer of the element Ag that was
deposited by the conventional PVD-HF technique;
• a 10 |jm foil of platinum (element Pt) ; and
• a 2 ]im foil of aluminum (element Al) .
Thereafter, the stack was subjected to the SPS step
30 that serves not only to consolidate the assembly but also
produce interdiffusion of the elements, and then
homogenizing annealing was performed for 10 hours (h) at
1100°C.
That was sample E4 in Table 1 below, which gives the
35 compositions of various samples, E3 and E4 being doped
with Ag as the stabilizer element M, while El and E2
constitute reference samples without a stabilizer element
15
M and with a standard p-(NiPt)Al underlayer. The
performance of these four samples was tested under cyclic
oxidation over 1000 cycles at 1100°C in air, and the
results are shown in Figures 8 and 9.
Table 1
Sample
El
E2
E3
E4
Pt
pm
7
7
4
10
Al
pm
not
measured
not
measured
0
2
Hf at%
(nm)
<0.05
<0.05
0.11
(50)
0.13
(90)
Y at%
(nm)
0
0
0.07
(45)
0.15
(150)
Si at%
(nm)
0
0
--
0.22
(50)
Ag at%
(nm)
0
0
1.62
(275)
0.83
(220)
As can be seen in Figures 9 and 10, the ability of
samples E3 and E4 of the invention to withstand spalling
10 is significantly improved under thermal cycling, since
with reference samples El and E2 without the stabilizing
element, spalling was total after 1000 cycles, whereas
for sample E3, 50% of the surface had not yet spalled and
for sample E4, 100% of the surface had not yet spalled.
15 It can be seen that this coating 11 in accordance
with the invention does not have TCP phases, with the
absence of an interdiffusion zone with numerous
precipitates implying a reduction in mechanical stresses
in operation.
20 Furthermore, this coating 11 in accordance with the
invention does not have any P ^^ y' (i.e. beta to gamma
prime) phase transformation in the metal underlayer 13'.
Other comparisons were made between the (Ni,Pt)Al
beta type metal underlayer 13 and the gamma/gamma prime
25 NiPtAl type metal underlayer 13' presenting the
composition in accordance with the invention.
16
Table 2 shows the contents of platinum and aluminum
found in the oxide layer 15 in the metal underlayer 13 or
13' at the specified depths:
Table 2
[ P t ]
[Al]
P metal underlayer 13
(E2)
3 at% to 5 at% (y' or P
phase) in the range
0 to 30 ]im
18 at% to 30 at% (y' or
P phase) in the range
0 to 30 ]im
y-y' metal underlayer
13' (E4)
5 at% at 8 \im
12 at% at 8 pm
It can thus be seen that using a metal underlayer
13' with a composition in accordance with the invention
prevents the metal underlayer 13' being depleted of
10 aluminum by diffusion to the substrate.
Thus, in the coating 11 in accordance with the
invention, after cyclic oxidation at high temperature, it
can be seen (see also Figure 8), that there occurs less
interdiffusion of the metal underlayer 13' into the
15 superalloy substrate.
Both metal underlayers 13 and 13' are aluminaforming
(Figures 4 and 7).
Furthermore, the roughness Ra of the samples in the
micrographs in section of the coatings has been
20 calculated and is given in Table 3.
Ra (pm)
Before cycling
After 1000 cycles
Table 3
P metal
underlayer 13
(E2)
0.54
6.6
y-y' metal
underlayer 13'
(E4)
0.515
2
17
The roughness of the metal underlayer 13 increases
after 1000 thermal cycles and reveals complete spalling.
The roughness of the metal underlayer 13' in accordance
with the invention varies little, thereby ensuring that
5 the ceramic layer is well anchored on the underlayer.
The metal underlayer 13' in accordance with the
present invention may be made using various deposition
techniques.
In particular, it is possible to use various
10 techniques involving one or more steps.
The metal underlayer 13' may be deposited in a
single step using the following alternative techniques:
• physical vapor deposition (PVD) from a target
having the composition desired for the metal underlayer
15 13';
• deposition of the SPS type from a powder
presenting the composition desired for the metal
underlayer 13' or foils of pure metals, or a foil of the
matching composition; and
20 • deposition by plasma spraying (e.g. low pressure
plasma spraying (LPPS)) using a powder presenting the
composition desired for the metal underlayer 13'.
It is also possible to make the metal underlayer 13'
using the techniques of the prior art while adding the
25 additional element(s) thereto in one or more additional
steps.
In one possible solution, the stabilizer elements M
(Cu and/or Ag) are deposited together with any reactive
elements RE (Hf, Zr, Y, Sr, Ce, Sr, Si, Er, Yb) by PVD or
30 by SPS, and where applicable platinum group elements
(PGE) are deposited electrolytically.
Under such circumstances, it should be understood
that all of the additives (RE, M, Pt, Al) should be added
before the SPS step. The stack of superposed layers is
35 then subjected to interdiffusion by SPS prior to
homogenizing heat treatment.

1 4fe
10
18
CLAIMS
1. A part comprising a coating (11) on a superalloy metal
substrate (12), the coating comprising a metal underlayer
(13) covering said substrate (12), the part being
characterized in that said metal underlayer (13) contains
a base of nickel aluminide and also contains 0.5 at% to
0.95 at% of one or more stabilizer elements M from the
group formed by Cu and Ag for stabilizing the gamma and
gamma prime phases.
2. A part according to claim 1, characterized in that
said metal underlayer includes as its stabilizing element
M only Ag in the range 0.5 at% to 0.95 at%.
15 3. A part according to claim 1, characterized in that
said metal underlayer includes as its stabilizing element
M only Cu in the range 0.5 at% to 0.95 at%.
4. A part according to any preceding claim, characterized
20 in that said metal underlayer (13) also contains platinum
group elements in the range 2 at% to 30 at% so as to form
a metal underlayer with an NiPtAl type base.
5. A part according to any preceding claim, characterized
25 in that said metal underlayer (13) also contains at least
one of the reactive elements (RE) including the following
reactive elements of the rare earth type: Hf, Zr, Y, Sr,
Ce, La, Si, Yb, Er, and the reactive element Si, with
each reactive element (RE) being at a content lying in
30 the range 0.05 at% to 0.25 at%.
6. A part according to any preceding claim, characterized
in that said metal underlayer (13) also contains as
reactive element(s) (RE): 0.05 at% < Hf < 0.2 at% and/or
35 0.05 at% < Y < 0.2 at% and/or 0.05 at% < Si < 0.25 at%.
19
7. A part according to any preceding claim, characterized
in that said metal underlayer (13) contains an NiPtAl
type base, as its stabilizer element M only Ag in the
range 0.75 at% to 0.9 at%, and as reactive elements
5 0.08 at% < Hf < 0.20 at% and/or 0.10 at% < Y < 0.20 at%
and/or 0.15 at% < Si < 0.25 at%.
8. A part according to any preceding claim, characterized
in that said metal substrate (12) is made of a nickel-
10 based superalloy.
9. A part according to any preceding claim, characterized
in that said coating further comprises a layer of ceramic
(14) covering said metal underlayer (13).
15
10. A part according to any preceding claim.
Characterized in that it forms a turbine part for a
turbine engine.
20 11. A part according to any preceding claim,
characterized in that it constitutes a turbine engine
blade or vane.