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A METHOD OF FABRICATING A THERMAL BARRIER COVERING A
SUPERALLOY METAL SUBSTRATE, AND A THERMOMECHANICAL PART
RESULTING FROM THIS FABRICATION METHOD
The invention relates to a method of fabricating or
repairing a thermal barrier covering a superalloy metal
substrate, and also to the thermomechanical part that
results from this fabrication method.
The search for increasing efficiency in
turbomachines, in particular in the field of aviation,
and the search for reducing fuel consumption and
polluting emissions of gas and combustion residues have
led to getting closer to stoichiometric combustion of
fuel. This situation is accompanied by an increase in
the temperature of the gas leaving the combustion chamber
__ and f~owing towards the turbine.
Nowadays, the limiting temperature of use for
superalloys is about 1100°C, while the temperature of the
gas leaving the combustion chamber or entering the
turbine may be 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 the blades of turbines
(hollow blades) and/or by improving the abilities of
these materials to withstand high temperatures. This
second approach, in combination with the use of
superalloys based on nickel and/or cobalt, has led to
several solutions including depositing a thermally
insulating coating on the superalloy substrate, which
coating is referred to as a thermal barrier and is made
up of a plurality of layers.
The use of thermal barriers in aeroengines has
become widespread over the past twenty years and enables
the temperature of the gas admitted into turbines to be
increased, enables the flow of cooling air to be reduced,
and thus enables engine efficiency to be improved.
The insulating coating serves to create a
temperature gradient through the coating on a part that
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is cooled under continuous operating conditions, and the
total amplitude of the gradient may exceed 100°C for a
coating having a thickness of about 150 micrometers (pm)
to 200 pm and presenting thermal conductivity of
1.1 watts per meter and per kelvin (W.m-l.K-1). The
operating temperature of the underlying metal forming the
substrate for the coating is reduced by the same
gradient, thereby leading to large savings in the volume
of cooling air that is needed, in the lifetime of the
part, and in the specific consumption of the turbine
engine.
It is known to have recourse to using a thermal
barrier including a ceramic layer based on zirconia
stabilized with yttrium oxide and presenting a
coefficient of expansion that is different from that of
the superalloy constituting the substrate, together with
thermal conductivity that is quite low. 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 subgroup constituted
by: Y (yttrium); Dy (dysprosium); Er (erbium); Eu
(europium); Gd (gadolinium); Sm (samarium); Yb
(ytterbium); or a combination of an oxide of Ta
(tantalum) and at least one rare earth oxide; or with a
combination of an oxide of Nb (niobium) and at least one
rare earth oxide.
In order to anchor this ceramic layer, a metallic
underlayer having a coefficient of expansion close to
that of the substrate is generally interposed between the
substrate of the part and the ceramic layer. This
underlayer provides adhesion between the substrate of the
part and the ceramic layer, it being understood that the
adhesion between the underlayer and the substrate of the
part is provided by interdiffusion, and that the adhesion
between the underlayer and the ceramic layer is provided
by mechanical anchoring and by the propensity of the
underlayer to develop, at high temperature and at the
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ceramic/underlayer interface, a thin layer of oxide that
provides chemical contact with the ceramic. In addition,
this metal underlayer protects the part against corrosion
phenomena.
Amongst the coatings used, mention is made of the
quite widespread use of a layer of ceramic based on
zirconia that is partially stabilized with yttrium oxide,
e.g. Zro.92Yo.os01.96"
In particular, in known methods (air plasma spray,
very low pressure plasma spray), it is known to make use
of an underlayer formed of an alloy of the MCrAlY type,
where M is a metal selected from nickel, cobalt, iron, or
a mixture of these metals, and that constitutes a gammagamma
prime matrix of nickel cobalt with, in solution
therein, chromium containing NiAl p precipitates.
It is also known to make use of an underlayer, e.g.
constituted by nickel aluminides, that includes 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),
cerium (Ce), lanthanum (La), titanium (Ti), tantalum
(Ta), hafnium (Hf), silicon (Si), and yttrium (Y). For
example, a coating of the Ni (1-xl PtxAl type is used in
which the platinum is inserted in the nickel lattice.
The platinum is deposited electrolytically before the
aluminization thermochemical treatment.
This metal underlayer may also be constituted by a
platinum-modified nickel aluminide (Ni, Pt)Al using a
method that comprises the following steps: preparing the
surface of the part by chemical cleaning and sand
blasting; electrolytically depositing a coating of
platinum (Pt) on the part; optionally heat treating the
result to cause the Pt to diffuse in the part; depositing
aluminum (Al) by chemical vapor deposition (CVD) or by
physical vapor deposition (PVD); optionally heat treating
the result in order to cause Pt and Al to diffuse into
the part; preparing the surface of the resulting metal
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underlayer; and depositing a ceramic coating by electron
beam physical vapor deposition (EB-PVD).
Finally, the underlayer may correspond to a coating
solely of diffused platinum that consists in a gammagamma
prime matrix of nickel cobalt with Pt in solution.
In order to obtain a coating and/or the coating
underlayer, a step is sometimes also implemented that
consists in modifying the surface of the superalloy part
by depositing a layer of platinum that is more than 10 pm
thick and then in performing diffusion heat treatment.
Thus, the Applicant company makes use of a
thermochemical coating known as ClA that is formed by a
chromium-modified aluminide coating and that results from
successively implementing two vapor deposition steps: a
first step of depositing a 2 pm to 6 pm thick layer of
chromium, followed by an aluminization step.
Such a coating is used more as a coating for
protecting parts from oxidation or high temperature
corrosion, or optionally as an underlayer for a thermal
barrier.
In traditional manner, the use of a metal underlayer
including aluminum generates a layer of alumina Al20 3 by
natural oxidation in air, which layer covers the entire
underlayer.
Usually, the ceramic layer is deposited on the part
for coating either by a spray technique (in particular a
plasma spray technique), or by physical vapor deposition,
i.e. by evaporation (e.g. by electron beam physical vapor
deposition (EB-PVD) in which a coating is formed by
deposition in an evacuated evaporation enclosure under
electron bombardment).
With a sprayed coating, a zirconia-based oxide is
deposited by plasma spray type techniques in a controlled
atmosphere, thereby leading to the formation of a coating
that is constituted by a stack of molten droplets that
are quenched by shock, flattened, and stacked so as to
build up a deposit that is imperfectly densified to a
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thickness generally lying in the range 50 pm to
1 millimeter (mm) .
A physically-deposited coating, e.g. using
evaporation under electron bombardment, gives rise to a
coating that is made up of an assembly of small columns
that are directed substantially perpendicularly to the
surface for coating, over a thickness lying in the range
20 pm to 600 pm. Advantageously, the space between the
columns enables the coating to be effective in
compensating the thermomechanical stresses that, at
operating temperatures, are due to the differential
expansion relative to the superalloy substrate.
Thus, parts are obtained having lifetimes that are
long in terms of high temperature thermal fatigue.
Conventionallyr such thermal barriers thus create a
discontinuity in thermal conductivity between the outer
coating on the mechanical part, forming the thermal
barrier, and the substrate of the coating that forms the
material constituting the part.
Usually, it is found that thermal barriers that give
rise to a significant discontinuity in thermal
conductivity also give rise to a significant risk of
delamination between the coating and the substrate, or
more precisely at the interface between the underlayer
and the ceramic layer. This situation leads to flaking
of the ceramic layer, such that the substrate is locally
no longer protected by the layer of insulating ceramic,
and is subjected to higher temperatures, so it becomes
damaged very quickly.
This damage results in part from the phenomenon
commonly known as "rumpling" that occurs during cycles
involving large variations in the temperature to which
the materials are subjected once the engines are put into
service, with this applying in particular to turbine
blades.
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This phenomenon leads to deformation of the
underlayer and results from various parameters. Rumpling
may be explained by:
the initial surface state that has a major role
concerning the adhesion of the ceramic in service;
· the difference of the coefficients of expansion
between the underlayer and the superalloy, which leads to
progressive deformation of the coating during successive
cycles at high temperature;
· the p-(Ni,Pt)Al~y'-Ni 3Al phase transformation and
interdiffusion phenomenon between the metal substrate and
the coating;
· the martensitic transformation of the p-(Ni,Pt)Al
phase that occurs on cooling at aluminum contents of less
. than 37% atomic;
growth stresses in the alumina layer; and
the chemical composition of the substrate (effect
of reactive elements).
In the literature, it is accepted that the rumpling
phenomenon is a degradation mechanism that is inevitable
for thermal barrier systems. Thus, the article
"Temperature and cycle-time dependence of rumpling in
platinum-modified diffusion aluminide coatings"
(V.K. Tolpygo and D.R. Clarke, Scripta Materialia 57
(2007), pp. 563-566) shows clearly the effects of
temperature, frequency, and duration of thermal cycles,
these parameters being significant factors in the
progress of the rumpling phenomenon at high temperature.
According to the authors, this phenomenon of underlayer
deformation is associated directly with temperature and
remains inevitable at temperatures higher than 1100°C.
Numerous attempts in the prior art at avoiding or
retarding the appearance of the rumpling phenomenon are
based on modifying the chemical composition of the
superalloy substrate. Thus, the article "Effect of Hf,
Y, and C in the underlying superalloy on the rumpling of
diffusion aluminide coatings", by V.K. Tolpygo et al.
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Acta Materialia, 56 (2008), pp. 489-499, presents the
decohesion of the thermal barrier that results from the
rumpling phenomenon as being inevitable and observes a
modification of the time at which it appears as a
function of the content of hafnium and carbon in the
superalloy.
In the same manner, Spitsberg et al. in the article
"On the failure mechanisms of thermal barrier coatings
with diffusion aluminide bond coatings", Materials
Science and Engineering, A 394 (2005), pp. 176-191 show
that the use of a substrate enriched in rhenium can
modify lifetime in terms of flaking for identical surface
treatment. The effect of rhenium appears to modify the
time for the rumpling phenomenon to appear, but it cannot
be--eliminated completely under any circumstances.
An object of the present invention is thus to
propose a method of fabricating a thermal barrier and a
thermal barrier structure resulting from said method that
prevent or retard the appearance of the rumpling
phenomenon, or that minimize its magnitude.
Another object of the invention is to provide a
superalloy thermomechanical part that results from said
fabrication method and that limits damage to the
underlayer resulting from the rumpling phenomenon while
the part, in particular, a blade, is in operation at high
temperature, and to do in such a manner as to increase
significantly the flaking lifetime of the thermal barrier
system.
To this end, according to the present invention, the
fabrication method is characterized in that the following
step is implemented: the surface state of the underlayer
is smoothed by at least one physicochemical and/or
mechanical process prior to depositing the ceramic layer
in such a manner that the number of defects presenting a
peak-to-peak difference (between the bottom of a valley
and the top of a peak) greater than or equal to 2 pm is
at
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most five over any distance (pitch or extent) of 50 ~m,
and then depositing the ceramic layer.
In this way, it can be understood that the
conditions to be satisfied in order to achieve this
object correspond to combining the following two
conditions:
· a surface state of the underlayer that presents
controlled roughness with a limited density of "large
defects" per unit area; and
· the presence of a ceramic layer on the underlayer
(directly on the underlayer or with an interposed alumina
layer) .
With roughness that satisfies the conditions set out
in the present patent application, and in the presence of
a ceram-ic- laye-r, the Applicant has found that the
rumpling phenomenon is non-existent or in any event
greatly limited, even though there used to be a prejudice
against being able to escape from the rumpling phenomenon
in particular by having recourse to modifying the surface
state of the underlayer or to modifying the chemical
composition of the underlayer.
The explanations that the Applicant suggests
concerning the unexpected performance of thermal barriers
obtained by the fabrication method in accordance with the
invention lie in particular in the fact that an effect of
synergy is obtained: the optimized surface state of the
underlayer makes it possible firstly to achieve good
adhesion of the ceramic layer, and secondly to limit the
number of occurrences of large-amplitude defects
(indentations or peaks) both over the surface of the
underlayer and over the surface of the ceramic layer,
thereby avoiding creating centers for delamination, and
indeed the ceramic layer stiffens the thermal barrier and
guarantee·s high-temperature protection for the layers of
material situated under it. The presence of the ceramic
layer prevents any deformation of the metal underlayer,
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if and only if the surface state is optimized in
compliance with the parameters given below.
Overall, the solution of the present invention makes
it possible to increase the lifetime of the thermal
barrier and of the part coated with the thermal barrier
by inhibiting the rumpling phenomenon while the part is
in service.
This solution also presents the additional advantage
of being easy to implement and to reproduce.
The solution of the present invention goes against a
prejudice relating to the impossibility of avoiding the
rumpling phenomenon, and this result is made possible by
determining conditions that need to be satisfied for the
assembly constituted by the underlayer and the ceramic
layer, without being limited to the characteristics of
the underlayer alone or of the ceramic alone.
The present invention applies not only when making a
thermal barrier for initial fabrication of a
thermomechanical part, but also for repairing a thermal
barrier. When performing a repair, the method described
herein is performed beforehand, the ceramic layer is
removed, and optionally the underlayer is removed, and
then a new underlayer is deposited.
Under such circumstances, the surface portions that
have been repaired in application of the conditions
determined by the present invention benefit from
increased lifetimes of the thermal barrier recreated in
this way.
Such a repair may be found to be necessary on
particular wear zones of certain parts, in particular the
leading edges and trailing edges of blades in the field
of aviation, be they fan blades, compressor blades,
and/or turbine blades of a turbine engine.
The invention is preferably applied to
thermomechanical parts presenting a nickel-based
superalloy substrate, in particular monocrystalline
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turbine blades that are cooled by air flowing in internal
channels.
The invention applies to thermomechanical parts
presenting a substrate made of any type of superalloy, in
particular one based on nickel and/or on cobalt and/or on
Fe.
Concerning the conditions that need to be satisfied
for the surface state of the underlayer, the Applicant
has found various ways of characterizing them. Thus, one
or another or several of the following provisions are
applicable:
· the physicochemical and/or mechanical process
gives rise to a surface state of the underlayer such that
the number of defects (indentations or peaks) presenting
an amplitude greatBr than 1 pm relative to the mean
position of the top face of the underlayer (mean profile
or theoretical surface line) is at most five over any
distance (pitch or extent) of 50 pm;
· the physicochemical and/or mechanical process
gives rise to a surface state of the underlayer such that
the roughness Ra of the underlayer lies in the range
0.05 pm to 3 pm, and preferably in the range 0.05 pm to
1 pm, where the roughness Ra is the mean difference: this
is the arithmetic mean of the differences relative to the
mean line or the integral mean of all of the differences
in absolute value;
· the physicochemical and/or mechanical process
gives rise to a surface state of the underlayer such that
the roughness Rz of the underlayer is less than 10 pm,
where roughness Rz is regularity: this is the mean of the
total differences of roughness "Rt" observed over five
lengths, where "Rt" is the total difference that
corresponds to the greatest difference in level between
the top of the highest peak and the bottom of the deepest
indentation;
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· the physicochemical and/or mechanical process
gives rise to a surface state of the underlayer such that
at least one of the following criteria is satisfied:
0 pm < Rk < 5 pm;
0 pm < Rvk < 3 pm;
0 pm < Rpk < 3 pm;
-1 < Sk < 1; and
1 < Ek < 10;
where the parameters Rk, Rpk, and Rvk are calculated on
the basis of an Abott curve, Rk being the depth of the
peak-limited profile that represents the depth of the
central roughness of the profile, Rvk being the depth of
the valleys that are eliminated and represents the mean
depth of the valleys exceeding the central portion of the
·~· profi-le, and Rpk being the height of the peaks that ha-v&
been eliminated and represents the mean height of the
peaks exceeding the central portion of the profile, and
where Sk corresponds to the symmetry of the amplitude
distribution curve and Ek to the overall reference trace.
The physicochemical and/or mechanical process that
enables the looked-for surface state to be obtained
preferably forms part of the group comprising: dry sand
blasting, wet sand blasting, mechanical polishing,
electrolytic polishing, and tribofinishing.
For example, "tribofinishing" is used to mean
processes that incorporate the techniques of polishing,
deburring, deoxidizing, smoothing, degreasing,
These processes use abrasive media (ceramic, porcelain,
plastics, metals), chemical additives, and equipment that
generates movement (vibrators, centrifuges, ... ), in a
controlled chemical environment.
The present invention also provides a
thermomechanical part obtained by the above-described
fabrication method.
In particular, the present invention provides a
thermomechanical part made on a superalloy metal
substrate and covered in a thermal barrier including at
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least an underlayer and a ceramic layer, in which one or
more of the following provisions have been implemented:
· the underlayer is a metal underlayer constituted
by nickel aluminide optionally containing 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),
cerium (Ce), lanthanum (La), titanium (Ti), tantalum
(Ta), hafnium (Hf), silicon (Si), and yttrium (Y), in
particular a metal underlayer constituted of NiAlPt, or
or a metal underlayer of the MCrAlY type, where M is a
metal selected from nickel, cobalt, iron, or a mixture of
these metals, or based on Pt. Finally, the metal
underlayer may correspond to a coating of platinum
diffused on its own and constituting a gamma-gamma prime
matrix of nickel cobalt with platinum (Pt) in solution;
said underlayer is constituted by an alloy
suitable for forming a protective layer of alumina by
oxidation; and
· said ceramic layer is based on stabilized
zirconia, i.e. yttrified zirconia presenting a molar
content of yttrium oxide lying in the range 4% to 12%.
This 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
subgroup: Y (yttrium); Dy (dysprosium); Er (erbium); Eu
(europium); Gd (gadolinium); Sm (samarium); Yb
(ytterbium); or a combination of an oxide of Ta
(tantalum) and at least one rare earth oxide; or with a
combination of an oxide of Nb (niobium) and at least one
rare earth oxide.
The present invention also provides a
thermomechanical part for a turbomachine, and in
particular a combustion chamber, a turbine blade, a
turbine distributor, or any thermomechanical part
suitable for being coated in a thermal barrier system.
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Other advantages and characteristics of the
invention appear on reading the following description
given by way of example and made with reference to the
accompanying drawings, in which:
· Figure 1 is a diagrammatic section view showing a
portion of a mechanical part coated in a thermal barrier;
· Figure 2 is a micrographic section showing the
various layers of the thermal barrier on the surface of
the part;
· Figure 3 is a view analogous to Figure 2 for a
part that has suffered damage to the thermal barrier in
service;
· Figures 4A, 48, and 4C show different roughness
profiles corresponding to different surface states of the
underlayer7
· Figures SA and S8 are micrographic sections at
different magnifications showing a prior art thermal
barrier before service, and Figure SC shows the roughness
profile of the corresponding surface of the underlayer
prior to being put into service;
· Figures 6A, 68, and 6C are views in the new state,
prior to service and at different magnifications, that
are similar respectively to the views of Figures SA, S8,
and SC for a first implementation of the method in
accordance with the invention;
· Figures 7A, 78, and 7C are views in the new state,
prior to service and at different magnifications, that
are similar respectively to the views of Figures SA, S8,
and SC for a second implementation of the method in
accordance with the invention;
· Figures SA and 88 are micrographic sections
showing respectively a prior art thermal barrier after
service and a thermal barrier that results from the
second implementation of the method in accordance with
the invention, likewise after service, and Figure 8C is a
chart showing the flaking lifetimes of the various
thermal barriers
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· Figures 9A and 9B are micrographic sections at
different magnifications showing, after service, a
thermal barrier resulting from an implementation of the
method in accordance with the invention;
Figures lOA and lOB are micrographic sections at
different magnifications showing an implementation of the
method of the invention presenting a zone of the ceramic
layer that has flaked; and
· Figure 11 illustrates the rumpling phenomenon.
The mechanical part shown in part in Figure 1 has a
thermal barrier coating 11 deposited on a superalloy
substrate 12, such as a superalloy based on nickel and/or
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.
The bonding underlayer 13 is a metal underlayer
constituted by nickel aluminide, optionally containing 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), cerium (Ce), lanthanum (La), titanium
(Ti), tantalum (Ta), hafnium (Hf), silicon (Si), and
yttrium (Y), in particular a metal underlayer constituted
of NiAlPt, or a metal underlayer of the MCrAlYPt type,
where M is a metal selected from nickel, cobalt, iron, or
a mixture of these metals, or else based on Pt. Finally,
the bonding underlayer 13 may correspond to a coating of
platinum diffused on its own and constituting a gammagamma
prime matrix of nickel cobalt with platinum (Pt) in
solution.
The ceramic layer 14 is constituted by yttrified
zirconia having a molar content of yttrium oxide lying in
the range 4% to 12% (partially stabilized zirconia). The
stabilized zirconia 14 may also sometimes contain at
least one oxide of an element selected from the group
constituted by the rare earths, and preferably from the
subgroup: Y (yttrium); Dy (dysprosium); Er (erbium); Eu
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(europium); Gd (gadolinium); Sm (samarium); Yb
(ytterbium); or a combination of an oxide of Ta
(tantalum) and at least one rare earth oxide; or with a
combination of an oxide of Nb (niobium) and at least one
rare earth oxide.
During fabrication, the bonding underlayer 13 is
oxidized prior to the ceramic layer 14 being deposited,
thereby giving rise to the presence of an intermediate
layer of alumina 15 between the underlayer 13 and the
ceramic layer 14.
Figure 2 shows the various above-described layers,
with a typical column structure of the ceramic layer 14
present at the surface.
After service, in which the part (e.g. a turbine
blade) has been subjected to hundreds of cycles at high
temperature (about 1100°C), the morphology of the thermal
barrier layer becomes modified as shown in Figure 3:
damage has appeared at the interface 16 between the
underlayer 13 and the ceramic layer 14 that presents a
rupture, this loss of bonding between the underlayer 13
and the ceramic layer 14 inevitably leading to
delamination and flaking, i.e. to loss of the ceramic
layer 14.
In the context of the present invention, the
Applicant has analyzed various roughness profiles of the
underlayer 13 as obtained after different surface
treatments (prior art standard and optimized ranges in
accordance with the present invention), and also the
consequences in terms of flaking lifetime when the
underlayer is coated in a ceramic layer 14.
Thus, the curve 20 in Figure 4A corresponds to the
roughness profile of the underlayer 13 after standard
prior art sand blasting treatment prior to depositing the
ceramic layer: there are numerous departures of the
surface level about the mean profile with several "large"
defects 21 presenting a departure between peaks (distance
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between the bottom of a furrow and the top of a ridge) of
the order of 4 pm.
The curve 22 in Figure 4B corresponds to the
roughness profile of the underlayer 13 as it results from
a first implementation of the method in accordance with
the invention making use of a first physicochemical
and/or mechanical process serving to modify the surface
state prior to depositing the ceramic layer. This
process is dry sand blasting for several minutes at a
pressure of a few bars. As can be seen from curve 22,
the departures of the surface level about the mean
profile are smaller, and in general of the order of 1 pm,
at most.
Curve 24 in Figure 4C corresponds to the roughness
pk0£ile of the underlayer 13 as it results from a se~ond
implementation of the method in accordance with the
invention using a second physicochemical and/or
mechanical process that serves to modify the surface
state prior to depositing the ceramic layer. This
process is mechanical polishing. As can be seen in
Figure 24, the departures of surface level around the
mean profile are much smaller, and in general about
0.5 pm, at most.
By correlating the surface state of the underlayer
13 with the appearance of the rumpling phenomenon in the
thermal barrier 11 comprising both the underlayer 13 and
the ceramic layer 14, the Applicant has managed to
establish various roughness criteria that need to be
satisfied by the surface state of the underlayer 13 prior
to depositing the ceramic layer in order to ensure that
the rumpling phenomenon in the thermal barrier 11
comprising both the underlayer 13 and the ceramic layer
14 is very greatly delayed and/or completely inhibited.
When the presence of large amplitude defects is
avoided, then the presence of crack initiation points and
of privileged zones for harmful deformations, in
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particular the rumpling phenomenon, is avoided, in
particular concerning the underlayer 13.
Thus, for example, the Applicant has established a
first condition that consists in limiting the number of
defects presenting a peak-to-peak difference that is
greater than or equal to 2 pm and that is no more than
5 pm over any distance of 50 pm, the peak-to-peak
difference being measured between the bottom of a valley
and the top of a peak.
Figures SA, 5B, and 5C show a prior art thermal
barrier in which the surface state of the underlayer 13
does not satisfy the above first condition. Figure 5C
shows more than five defects presenting a peak-to-peak
difference of more than 2 pm (specifically six "large
defects" identified by arrows in Figure 5B).
Figures 6A, 6B, and 6C show a thermal barrier
obtained by the first implementation of the method in
accordance with the invention using the first
physicochemical and/or mechanical process and presenting
a surface state for the underlayer 13 that satisfies said
first condition: in Figure 6C, there can be seen only two
defects presenting a peak-to-peak difference of more than
2 pm (and thus fewer than five such defects).
Figures 7A, 7B, and 7C show a thermal barrier
obtained by the second implementation of a method in
accordance with the invention using the second
physicochemical and/or mechanical process and presenting
a surface state for the underlayer 13 that likewise
satisfies said first condition: the surface state visible
in Figure 7A is even more regular and close to a straight
line than in Figure 6A. In Figure 7C, there can be seen
no defect presenting a peak-to-peak difference of more
than 2 pm (so the number of such defects is less than
five).
Figures 8A and 8B show respectively a prior art
thermal barrier after service (1000 cycles at 1100°C) in
which the surface state of the underlayer 13 does not
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comply with the first condition, and a thermal barrier
obtained by the second implementation of the method in
accordance with the invention using the second
physicochemical and/or mechanical process and presenting
a surface state for the underlayer 13 that satisfies said
first condition.
The flaking lifetimes were measured for prior art
thermal barriers that do not comply with the surface
state conditions for the underlayer 13 present under the
ceramic layer, and for thermal barriers obtained by
implementing the fabrication method of the invention:
Figure 8C shows the results for cycles of one hour at
1100°C in air.
The first test (on the left in Figure 8C) relates to
a sample having a prior art thermal barrier (as shown in
Figures SA and 5B) and it withstood about 600 cycles.
The second test A (in the middle of Figure 8C)
relates to a sample having a thermal barrier similar to
the above thermal barrier except for the fact that it was
obtained by the first implementation of the method in
accordance with the invention, using the first
physicochemical and/or mechanical process (as shown in
Figures 6A and 6B) so as to present a surface state for
the underlayer 13 that complies with said first
condition. This thermal barrier withstood about 800
cycles, giving a lifetime that is about 30% longer.
The third test B (on the right in Figure 8C) relates
to a sample having a thermal barrier similar to that of
the first test except for the fact that it was obtained
by the second implementation of the method in accordance
with the invention using the second physicochemical
and/or mechanical process (as shown in Figures 7A and 7B)
so as to present a surface state for the underlayer 13
that complies with said first condition. This thermal
barrier withstood about 1100 cycles, giving a lifetime
that was increased by about 85%.
19
In order to avoid or delay the appearance of the
rumpling phenomenon, the Applicant has shown the
important role of the surface state of the pnderlayer 13
in the presence of the ceramic layer 14 in forming an
assembly that constitutes a thermal barrier suitable for
withstanding the rumpling phenomenon.
Thus, as can be seen in Figures 9A and 9B, which are
micrographic views of the thermal barrier after service
at different magnifications and obtained using the second
implementation of the method in accordance with the
invention using the second physicochemical and/or
mechanical process.
In Figure 9A there can be seen no rumpling damage
has appeared at the interface 16 between the underlayer
13 and the ceramic layer 14.
In Figure 9B, it can be seen that the alumina layer
15 remains dense, homogenous, and adherent, in spite of
its considerable thickness.
Figures lOA and lOB show a thermal barrier obtained
by the second implementation of the method in accordance
with the invention using the second physicochemical
process (polishing) so as to present a surface state for
the underlayer 13 that satisfies said first condition.
In order to show the essential role of the ceramic layer
14, these Figures lOA and lOB at two different
magnifications show the effect of having no ceramic layer
14 in the middle zone of Figure lOA and in the right-hand
portion of Figure lOB: at the end of high temperature
cycling, undulations appeared at the location of the
underlayer 13 that was not coated in the ceramic layer
14, whereas such undulations are completely absent from
the zones coated in the ceramic layer 14.
In this way, it can be understood that the
conditions that need to be satisfied in order to achieve
this object corresponding to combining the following two
conditions:
20
· the surface state of the underlayer 13 must
present controlled roughness with a limited density of
"large defects" per unit area; and
there must be the ceramic layer 14 on the
underlayer 13 (directly on the underlayer 13 or with an
interposed layer of alumina 15).
Figure 11 shows the rumpling phenomenon for a zone
of the underlayer 13 that is not coated in the ceramic
layer 14: if there is initially a ~urface defect of size
greater than the critical size, then after aging in
service at high temperatures, the shape of the defect
becomes more accentuated, thereby leading to undulation,
which causes a rupture at the interface 16 between the
underlayer 13 and the ceramic layer 14. In particular,
with surface defects in the underlayer 13 of a size
greater than the critical size:
· such large-sized defects are to be found in the
ceramic layer 14 (defects in the columns), thereby
weakening the mechanical strength of the ceramic layer
and its ability to withstand high temperatures;
· such locations are privileged places for
metallurgical phase transformations within the thermal
barrier; and
· such locations constitute zones that encourage the
initiation of cracks.
Thus, it can be seen that the ceramic layer 14 is
essential to avoid very rapid degradation of the thermal
barrier 11, and that it serves simultaneously to stiffen
the stack and to protect the underlayer 13, thereby
inhibiting the rumpling phenomenon when the initial
surface state of the underlayer 13 satisfies the
conditions determined by the Applicant.
Because of this optimized surface state satisfying
one or more of the conditions as determined by the
Applicant, the following results are obtained:
· an alumina layer 15 is grown that is dense,
regular, and that adheres at all points to the underlayer
21
13, thereby providing complete physical protection for
the underlayer 13 by means of the alumina layer 15 and
the ceramic layer 14; and
· a limit on the number of defects in the ceramic
layer 14.
The examples described relate to nickel-based
substrates coated in an underlayer 13 of NiAlPt type and
covered in an alumina layer 15, itself surmounted by a
ceramic layer 14 that is constituted by yttrified
zirconia.

22
CLAIMS
1. A fabrication method of fabricating a thermal barrier
(11) covering a superalloy metal substrate, said thermal
barrier (11) comprising at least an underlayer (13) and a
ceramic layer (14), the method being characterized in
that the following step is performed:
· the surface state of the underlayer (13) is
smoothed by at least one physicochemical and/or
mechanical process prior to depositing the ceramic layer
(14) in such a manner that the number of defects
presenting a peak-to-peak difference greater than or
equal to 2 pm is at most five over any distance of 50 pm,
and then depositing the ceramic layer (14).
2. A fabrication method according to claim 1,
characterized in that the physicochemical and/or
mechanical process gives rise to a surface state of the
underlayer (13) such that the number of defects
presenting an amplitude greater than 1 pm relative to the
mean position of the top face of the underlayer (13) is
at most five over any distance of 50 pm.
3. A fabrication method according to either preceding
claim, characterized in that the physicochemical and/or
mechanical process gives rise to a surface state of the
underlayer (13) such that the roughness Ra of the
underlayer (13) lies in the range 0.05 pro to 3 pm.
4. A fabrication method according to any preceding claim,
characterized in that the physicochemical and/or
mechanical process gives rise to a surface state of the
underlayer (13) such that the roughness Ra of the
underlayer (13) lies in the range 0.05 pm to 1 pm.
5. A fabrication method according to any preceding claim,
characterized in that the physicochemical and/or
mechanical process gives rise to a surface state of the
23
underlayer (13) such that the roughness Rz of the
underlayer (13) is less than 10 pm.
6. A fabrication method according to any preceding claim,
characterized in that the physicochemical and/or
mechanical process gives rise to a surface state of the
underlayer (13) such that at least one of the following
criteria is satisfied:
0 pm < Rk < 5 pm;
0 pm < Rvk < 3 pm;
0 pm < Rpk < 3 pm;
-1 < Sk < 1; and
1 < Ek < 10.
7. A fabrication method according to any preceding claim,
characterized in that the physicochemical and/or
mechanical process forms part of the group comprising dry
sand blasting, wet sand blasting, mechanical polishing,
electrolytic polishing, and tribofinishing.
8. A superalloy thermomechanical part including a thermal
barrier (11) obtained by the method according to any
preceding claim.
9. A superalloy thermomechanical part according to claim
8, characterized in that the underlayer (13) is a metal
underlayer (13) constituted by nickel aluminide
optionally containing 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), cerium (Ce), lanthanum
(La), titanium (Ti), tantalum (Ta), hafnium (Hf), silicon
(Si), and yttrium (Y), or a metal underlayer (13) of the
MCrAlY type, where M is a metal selected from nickel,
cobalt, iron, or a mixture of these metals, or based on
Pt, or indeed a metal underlayer (13) corresponding to a
coating of platinum diffused on its own and consisting in
24
a gamma-gamma prime matrix of nickel cobalt with platinum
(Pt) in solution.
10. A superalloy thermomechanical part according to claim
8 or claim 9, characterized in that said underlayer (13)
is constituted by an alloy suitable for forming a
protective layer of alumina by oxidation.
11. A superalloy thermomechanical part according to any
one of claims 8 to 10, characterized in that said ceramic
layer (14) is based on yttrified zirconia presenting a
molar content of yttrium oxide lying in the range 4% to
12%.
12. A superalloy thermomechanical part according to any
one of claims 8 to 11, characterized in that said part is
a combustion chamber, a turbine blade, a turbine
distributor, or any thermomechanical part suitable for
being coated in a thermal barrier system.
Dated this 26/07/2011