U.S. patent application number 09/758195 was filed with the patent office on 2001-07-19 for ceramic heat barrier coating having low thermal conductivity, and process for the deposition of said coating.
This patent application is currently assigned to SOCIETE NATIONAL D'ETUDE ET DE CONSTRUCTION DE MOTEURS D'AVIATION "SNECMA". Invention is credited to Alperine, Serge Alexandre, Huchin, Jean-Pierre Julien Charles, Jaslier, Yann Philippe, Malie, Andre Hubert Louis, Portal, Romain.
Application Number | 20010008708 09/758195 |
Document ID | / |
Family ID | 9526988 |
Filed Date | 2001-07-19 |
United States Patent
Application |
20010008708 |
Kind Code |
A1 |
Jaslier, Yann Philippe ; et
al. |
July 19, 2001 |
Ceramic heat barrier coating having low thermal conductivity, and
process for the deposition of said coating
Abstract
A ceramic heat barrier coating is deposited on a substrate so
that the coating has a columnar growth pattern which is interrupted
and repeated a number of times throughout its thickness by
successive regermination of the ceramic deposit. The regermination
is obtained by a vapour phase deposition process wherein a
polluting gas is introduced intermittently during the deposition of
the ceramic. The resulting ceramic coating has a lower thermal
conductivity than conventional columnar ceramic coatings.
Inventors: |
Jaslier, Yann Philippe;
(Melun, FR) ; Malie, Andre Hubert Louis; (Targe,
FR) ; Huchin, Jean-Pierre Julien Charles;
(Chatellerault, FR) ; Alperine, Serge Alexandre;
(Paris, FR) ; Portal, Romain; (Evry, FR) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
SOCIETE NATIONAL D'ETUDE ET DE
CONSTRUCTION DE MOTEURS D'AVIATION "SNECMA"
|
Family ID: |
9526988 |
Appl. No.: |
09/758195 |
Filed: |
January 12, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09758195 |
Jan 12, 2001 |
|
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09325042 |
Jun 3, 1999 |
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Current U.S.
Class: |
428/623 ;
427/255.28; 428/472; 428/629; 428/632; 428/633; 428/701 |
Current CPC
Class: |
C23C 14/30 20130101;
Y10T 428/12618 20150115; C23C 28/345 20130101; C23C 28/3455
20130101; Y10T 428/24926 20150115; C23C 28/3215 20130101; Y10T
428/12549 20150115; Y10T 428/1259 20150115; C23C 28/04 20130101;
Y10T 428/12611 20150115; C23C 14/083 20130101; Y10T 428/24942
20150115 |
Class at
Publication: |
428/623 ;
427/255.28; 428/629; 428/632; 428/633; 428/701; 428/472 |
International
Class: |
C23C 016/00; B32B
015/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 4, 1998 |
FR |
98.06986 |
Claims
1. A ceramic heat barrier coating deposited on a substrate, said
coating comprising a columnar growth pattern interrupted and
repeated a plurality of times throughout the thickness of said
coating as a result of successive regermination of the ceramic
deposit.
2. A ceramic heat barrier coating according to claim 1, wherein
said coating comprises a plurality of ceramic layers having the
same structure and the same composition as one another, and
interfaces which separate said ceramic layers and which are
parallel to said substrate, each ceramic layer corresponding to a
zone of germination and competitive growth.
3. A ceramic heat barrier coating according to claim 2, wherein
each ceramic layer has a fibrous structure in which each fibre is
oriented substantially perpendicularly to said substrate.
4. A ceramic heat barrier coating according to claim 3, wherein
said fibres have a maximum diameter of not more than 5
micrometers.
5. A ceramic heat barrier coating according to claim 2, wherein
each of said ceramic layers has a thickness of not more than 150
micrometers.
6. A ceramic heat barrier coating according to claim 5, wherein
each layer has a thickness of between 1 and 20 micrometers.
7. A process for vapour phase deposition of a ceramic heat barrier
coating wherein germination and growth of the coating are effected
in a deposition chamber by vapour condensation on a substrate to be
covered said process including the step of intermittently
introducing a polluting gas into the deposition chamber during
deposition in order to produce successive regerminations of the
ceramic during said deposition.
8. A process according to claim 7, wherein said polluting gas
interacts with chemical components present in the vapour phase in
said deposition chamber to cause regermination of the ceramic
during deposition.
9. A process according to claim 7, wherein said polluting gas
interacts with the surface of the deposited ceramic to cause
regermination of the ceramic during deposition.
10. A process according to claim 9 wherein said polluting gas
interacts chemically with the surface of the deposited ceramic.
11. A process according to claim 10, wherein said polluting gas
comprises at least atoms selected from the elements C, N, O, H, Si,
Cl, Br, F and I.
12. A process according to claim 10 wherein said polluting gas is
air.
13. A process according to claim 9, wherein said polluting gas
interacts physically by a process of adsorption on the surface of
the deposited ceramic.
14. A process according to claim 13, wherein said polluting gas is
selected from the rare gases Xe, Kr, Ar and He, mixtures of said
rare gases, and carbon monoxide.
15. A process for vapour phase deposition of a ceramic heat barrier
coating wherein germination and growth of the coating are effected
in a deposition chamber by vapour condensation on a substrate to be
covered, said process including the steps of: placing two
vaporization crucibles in said deposition chamber, one of said
crucibles containing a ceramic material and the other of said
crucibles containing a pollutant; continuously vaporizing said
ceramic material; and, intermittently vaporizing said pollutant so
as to produce successive regerminations of the ceramic during the
condensation thereof.
16. A ceramic heat barrier coating according to claim 1, wherein
said substrate is a metal article and said coating covers at least
part of the surface of said article.
17. A ceramic heat barrier coating according to claim 16, wherein
there is an aluminoforming alloy sublayer between the surface of
said metal article and said ceramic coating.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to a ceramic heat barrier coating
having low thermal conductivity, a process for depositing such a
ceramic coating, and to metal articles protected by the coating.
The invention is particularly applicable to the protection of hot
superalloy components of turbomachines, such as the turbine blades
or diffusers.
[0003] 2. Summary of the Prior Art
[0004] The manufacturers of turboengines, whether for use on land
or in aeronautics, face constant demands to increase engine
efficiency and reduce fuel consumption. One way of addressing these
demands is to increase the burnt gas temperature at the turbine
inlet. However, this approach is limited by the ability of the
turbine components, such as the diffusers and moving blades of the
high pressures stages, to withstand high temperatures. Refractory
metallic materials known as superalloys have been developed to make
such components. These superalloys, which are nickel or cobalt or
iron based, give the component mechanical strength at high
temperature (creep resistance). The maximum temperature at which
these superalloys can be used is 1100.degree.C., which is well
below the temperature, typically 160.degree.C., of the burnt gases
at the turbine inlet. The blades and diffusers are therefore
provided with internal cavities and are cooled by convection by the
introduction of air into these internal cavities taken at a
temperature of 600.degree.C. from the compressor stages. Some of
this cooling air flowing in the internal channels of the components
discharges through ventilation apertures in the wall to form a film
of cool air between the surface of the component and the hot
turbine gases. To obtain significant temperature gains at the
turbine inlet it is known to deposit a heat barrier coating on the
components.
[0005] Heat barrier technology consists of coating the components
with a thin insulating ceramic layer varying in thickness from a
few tens of micrometers to a few millimeters. The ceramic layer
typically consists of zirconia stabilised with yttrium and has the
advantages of low thermal conductivity and the good chemical
stability necessary in the severe conditions experienced during
turbine operation. A bonding sublayer of an aluminoforming metal
alloy can be interposed between the superalloy and the ceramic
layer and serves to boost the adhesion of the ceramic layer while
protecting the substrate from oxidation.
[0006] However, the application of a ceramic coating to a metal
article poses the problem of differential expansion of the metal
and the ceramic during thermal cycling. The thermal expansion
coefficient of zirconia-based ceramics, although relatively high,
is still appreciably below that of metals. The microstructure of
the coating must therefore be controlled so as to be able to
withstand, without flaking, the heat deformations caused by the
metal substrate.
[0007] Heat spraying and physical deposition in the vapour phase of
an electron beam, called EB-PVD (electron beam physical vapour
deposition) for short, are the two industrial processes used to
deposit the heat barriers. For application to the aerodynamic part
of the blades and diffusers the EB-PVD method is preferred to heat
spraying, mainly because it gives a coating with a better surface
texture and reduces obstruction of the ventilation apertures. Also,
the EB-PVD process helps to provide the layer with a microstructure
in the form of microcolumns perpendicular to the article surface.
The microstructure enables the coating to deal with thermal and
mechanical deformations in the plane of the substrate. For this
reason EB-PVD heat barriers have a thermomechanical fatigue life
which is considered to be better than that of plasma-sprayed
ceramic layers.
[0008] In vapour deposition processes the coating is the result of
vapour condensing on the article to be covered. There are two
categories of vapour phase processes--physical processes (PVD) and
chemical processes (CVD). In physical vapour phase processes the
coating vapour is produced by vaporization of a solid material,
also called the target. Vaporization can be produced by evaporation
caused by a heat source or by cathodic atomization, a process in
which the material is atomized by ionic bombardment of the target.
In chemical vapour phase processes the coating vapour is the result
of a chemical reaction between the gaseous components, which occurs
either in the vapour phase or at the coating/gas interface. The
vapour phase deposition processes are carried out in a controlled
atmosphere to prevent contamination or pollution of the deposits by
reaction with unwanted gas components. To this end, the deposition
chamber is preliminarily exhausted to a secondary vacuum (between
10.sup.-6 Torr and 10.sup.-4 Torr) and baked. An inert or reactive
working gas can be introduced in a controlled manner during
deposition.
[0009] The evaporation of refractory and ceramic materials requires
intense heating means. Accordingly, electron beam heating is used.
The ceramic material to be evaporated is in the form of sintered
bars whose surface is swept by a focused electron beam. Some of the
kinetic energy of the beam is converted into heat on the bar
surface. A particular feature of the EB-PVD process is that the
working pressure is reduced so as to facilitate evaporation of the
bars and the transfer of coating vapour from the target to the
substrate. Also, electron guns require pressures of less than
10.sup.-4 Torr if they are to operate (arcing problems) which means
that the electron gun must be pumped separately from the pumping of
the chamber.
[0010] During the EB-PVD deposition of heat barriers the articles
are heated to a high temperature of around 1000.degree.C. by
radiant heating of the bars. The surface temperature thereof is
estimated by be 3500.degree.C. At this temperature some of the
zirconia molecules from the bar surface are dissociated in the
reaction:
ZrO.sub.2.fwdarw.ZrO+1/2O.sub.2
[0011] Some of the oxygen thus dissociated from the zirconium oxide
molecules is lost as a result of the pumping of the chamber, with
the consequence that the zirconia deposits are rendered
substoichiometric (oxygen depleted). This effect can be countered
by the introduction of an oxygen-rich gas (typically a mixture of
argon and oxygen) at a pressure of a few milli-Torr into the
chamber during the deposition. The effect can also be corrected
ex-situ when no reactive gas is introduced into the chamber during
deposition. The stoichiometry of the coating is then restored by
subjecting the coated articles to a simple annealing in air at a
temperature of 700.degree.C. for 1 hour. The introduction of oxygen
into the EB-PVD chamber also helps to preoxidise the articles in
situ before the ceramic deposition. The alumina film thus formed on
the surface of the bonding sublayer provides satisfactory adhesion
of the ceramic layer. In the industrial EB-PVD process only those
article surfaces facing the vaporization source are coated. To
cover an article of a complex geometrical shape, such as a rotor
blade or a diffuser, the article must be rotated in the flow of
coating vapour.
[0012] EB-PVD ceramic layers may have undeniable advantages for use
on turbine blades, but they suffer from the major disadvantage of a
thermal conductivity (typically from 1.4 to 1.9 W/mK) which is
twice that of plasma sprayed heat barriers (from 0.5 to 0.9 W/mK).
This difference in thermal conductivity is associated with the
morphology of the deposits. The ceramic microcolumns perpendicular
to the article surface which are found in EB-PVD depositions offer
little hindrance to heat transfer by conduction and by radiation,
whereas plasma sprayed depositions have a network of micro cracks
which extend substantially parallel to the plane of the deposit,
usually in the form of incomplete joints between the ceramic
droplets which are crushed in the spraying. These micro cracks are
much more effective in preventing heat conduction through the
deposit. The insulation provided by a ceramic layer is proportional
to its conductivity and thickness. For a given insulation level,
halving the thermal conductivity of the ceramic layer would enable
the coating thickness to be approximately halved--a considerable
advantage when used on rotor blades subjected to centrifugal
force.
[0013] WO 96/11288 describes a composite laminated heat barrier
coating consisting of a stack of nanometric layers of a thickness
between 0.001 and 1 micrometer and of a different nature (typically
zirconia/alumina). The reduced thermal conductivity associated with
such a structure is attributed to the dispersion of the phonons at
the interfaces between the layers, the phonons being largely
responsible for conductive transfer in dielectrics. The
aforementioned document describes multilayer coatings of small
thickness of the order of 4 to 5 micrometers which have a thermal
conductivity half the value calculated from a law of the mixture.
The reduction in thermal conductivity provided by this coating
results from the creation of interfaces between two layers of
different kinds. However, a sandwich structure of this kind
consisting of nanometric layers suffers from thermal instability.
During long spells at the high temperatures (about 1100.degree.C.)
characteristic of the operating conditions experienced in turbines
the fine layers may interdiffuse and homogenize the material--i.e.,
the interfaces responsible for reducing the thermal conductivity
disappear.
[0014] WO 93/18199 and EP 0705912 disclose a heat barrier
comprising a ceramic coating which consists of a number of layers
of different structure. Adjacent layers have a different structure
from one another in order to be able to produce an interface
between each layer. The columnar morphology of the coating through
its thickness is retained, this being considered to be a basic
property for withstanding thermal cycling. In this coating the
multilayer structure is obtained by intermittent ionic bombardment
of the layer surface in conjunction with vapour condensation. The
ionic bombardment is produced by polarising the article at a high
negative voltage so that the article becomes the cathode of a gas
discharge. The effect of the intermittent bombardment on the
morphology of the resulting ceramic layer leads to the creation of
relatively dense ceramic layers. However, a layer of this kind is
not suitable for retaining a low thermal conductivity since heat
ageing leads of course to densification of the columns and reduces
the density spread between the various layers. Also, the
association of a high voltage with the high temperatures
(1000.degree.C.) required for EB-PVD deposition greatly complicates
industrial implementation of the method.
[0015] As a rule, heat barriers having a laminated microcomposite
structure (more commonly called a multilayer structure)--i.e., a
microstructure based on the presence of interfaces to increase the
resistance to heat flow--are unsuitable for high-temperature use
because of the instability of the interfaces in operation. Because
of diffusion at high temperatures an interface between two
materials having a different composition or structure from one
another breaks down in a graduated zone. This implies the
disappearance of the interface and its associated heat
resistance.
SUMMARY OF THE INVENTION
[0016] It is an object of the invention to provide a ceramic heat
barrier coating which has a heat resistance equivalent to that of
conventional ceramic coatings and a thermal conductivity at least
half that of coatings obtained by the conventional EB-PVD
processes, the thermal conductivity not degrading with age during
operation and even improving.
[0017] To this end, the invention provides a ceramic heat barrier
coating deposited on a substrate, said coating comprising a
columnar growth pattern interrupted and repeated a plurality of
times throughout the thickness of said coating as a result of
successive regermination of the ceramic deposit.
[0018] The morphology of the ceramic coating of the invention is
different from the conventional columnar structures in which
microcolumns are continuous throughout the thickness of the
deposit. In contrast, the morphology of the ceramic coating of the
invention comprises a pattern of columnar growth which is
interrupted and repeated in controlled fashion throughout its
thickness, called repeat germination morphology. The coating
comprises a fibrous microstructure which is finer than conventional
columnar deposits.
[0019] The invention also provides a process for vapour phase
deposition of a ceramic heat barrier coating wherein germination
and growth of the coating are effected in a deposition chamber by
vapour condensation on a substrate to be covered, the process
including the step of intermittently introducing a polluting gas
into the deposition chamber during deposition so as to interact
with the surface of the ceramic being deposited and/or with
chemical components present in the vapour phase in the chamber in
order to produce successive regermination of the ceramic during
condensation. The term "polluting gas" denotes a gas which causes a
rupture of the crystallographic growth pattern of the coating
during deposition without damaging the mechanical integrity of the
article.
[0020] The invention also relates to a metal superalloy article
whose surface is at least partly coated by a ceramic coating in
accordance with the invention.
[0021] Other preferred features and advantages of the invention
will become apparent from the following detailed, but
non-limitative, description of the invention and preferred
embodiments with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a diagram showing the morphology throughout the
thickness of a conventional columnar ceramic coating deposited by a
conventional EB-PVD process;
[0023] FIG. 2 shows an example of the localized variation of the
thermal conductivity of a conventional columnar ceramic coating
plotted against its thickness;
[0024] FIG. 3 is a diagram showing the morphology throughout the
thickness of an example of a repeat germination ceramic coating in
accordance with the invention;
[0025] FIG. 4a is a photograph showing the fibrous morphology of a
repeated germination ceramic coating in accordance with the
invention before thermal ageing;
[0026] FIG. 4b is a photograph showing the fibrous morphology of
the repeated germination ceramic coating after ageing;
[0027] FIG. 5a illustrates the thermal conductivity change of a
conventional ceramic coating plotted against coating thickness;
[0028] FIG. 5b illustrates the thermal conductivity of a repeated
germination ceramic coating plotted against coating thickness;
[0029] FIG. 6 is a diagram showing a first example of a polluting
gas introduction cycle in a process in accordance with the
invention; and
[0030] FIG. 7 is a diagram showing a second example of a polluting
gas introduction cycle in a process in accordance with the
invention.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
[0031] The invention is based on a finding that a vapour phase
deposited ceramic coating has a morphology 1 which changes over its
thickness such as shown in FIG. 1. This morphology gradient is
marked in particular by a density of mircocolumns 1 which decreases
as a function of deposit thickness. The microcolumns are very fine
at the interface with the substrate and can be likened to fibres,
but tend to flare out in the outer region of the ceramic layer. One
of the consequences of the change in the morphology of the ceramic
coating with its thickness is that the thermal conductivity of the
coating increases with its thickness (see FIG. 2). The outer region
of the ceramic coating has a thermal conductivity which is higher
than the coating region adjacent the substrate. This effect has
been noticed in the case of materials which are very good heat
conductors, such as diamond deposited by CVD, and in the case of
poorer conductors such as yttriated zirconia. The morphology of a
vapour phase deposited layer with a columnar structure and
thickness e can be modelled as consisting of a layer of thickness
Z.sub.1 adjacent the substrate in which the average diameter of the
columns is low, and an outer layer of thickness e-Z.sub.1 in which
the average diameter of the columns is high. The thickness of the
layer adjacent the substrate with the thin columns corresponds to
the germination and competitive growth steps of the microcolumns of
the deposit. The outer zone of the coating corresponds to the
microcolumns which have passed through the selective competitive
growth step. For given deposition conditions the thickness of the
competitive growth zone is fixed and does not usually exceed about
ten micrometers, whereas the outer growth zone has no theoretical
limit and increases with deposition time.
[0032] FIG. 3 illustrates an example of a ceramic coating in
accordance with the invention. The thick coating comprises a
structure which reproduces the germination and competitive growth
zone of vapour phase deposits several times throughout the
thickness of the coating. However, the germination of layers
deposited by conventional vapour phase processes occurs naturally
only once, namely at the interface with the substrate. The present
invention resides in producing a repeated germination coating. The
morphology of a coating produced in accordance with the invention
has a columnar growth pattern which consists of the repetition of
the structure of the layer adjacent the substrate throughout the
thickness of the deposit. To achieve this the columnar growth
pattern is interrupted and repeated several times through the
thickness of the deposit by successive regermination of the deposit
of the ceramic layer. The ceramic coating comprises interfaces
2,3,4 and 5 which are parallel to the deposit plane, these
interfaces separating ceramic layers of thickness Z.sub.1, Z.sub.2,
. . . Z.sub.5 having the same structure and same composition as one
another, each layer corresponding to the germination zone and
competitive growth zone typical of vapour phase deposited columnar
structures. The morphology of the coating is fibrous rather than
columnar, the fibres being oriented substantially perpendicularly
to the coating plane. The diameter of the fibres does not exceed 5
micrometers. The thickness of each layer is less than 150
micrometers and is preferably between 1 and 10 micrometers. The
thicknesses of the successive layers can be different from one
another.
[0033] The repeated germination ceramic layer concept is different
from the concept of laminated microcomposites in that the layers
adjacent one another are the same as regards composition and
microstructure. However, the thickness of the layers may vary.
[0034] Also, for the use of the coatings as a heat barrier, the
concept of a repeated germination ceramic layer is different from
the concept of laminated microcomposites to the extent that the
formation of an interface between two adjacent layers is intended
not to reduce heat flow but to control the structure of the layer
which it is required to repeat. Also, it is the structure of each
regerminated layer, and not the interfaces between each layer,
which helps to reduce the thermal conductivity of the layer.
[0035] The ceramic obtained and shown in FIGS. 4a and 4b has a fine
structure of fibrous morphology rather than of columnar morphology,
which gives such deposits a high surface density and an associated
high surface energy making it particularly sensitive to sintering
phenomena at high temperature. In other words, thermal ageing in
operation may cause a great change in the morphology of the coating
due to the sintering phenomena. The interfaces between each zone of
regerminated ceramic are caused to disappear. The fibres weld
together to form a fine dispersion of substantially spherical
porosities which is very effective in reducing the thermal
conductivity of the layer. This change reduces rather than
increases the thermal conductivity of the layer.
[0036] FIGS. 5a and 5b diagrammatically show the change in thermal
conductivity of a conventional ceramic coating (FIG. 5a) and the
change in thermal conductivity of a repeated germination ceramic
coating (FIG. 5b) plotted against coating thickness.
[0037] The thermal conductivity of a repeated germination ceramic
coating does not increase with coating thickness and its value is
near the value obtained for a thin conventional ceramic
coating.
[0038] The invention also relates to a metal article whose surface
is at least partly coated by such a ceramic coating. The metal
article may be made of a superalloy. Before the ceramic layer is
applied, the article surface may be coated with an aluminoforming
alloy sublayer. This sublayer may belong to the class of MCrAIY
deposits or diffused deposits which consist of aluminides
consisting partly of nickel and of precious metals such as elements
of the platinum group. Preferably, the sulphur content of the base
alloy and sublayer is less than 0.8 ppm by weight. The application
of the ceramic layer may be preceded by the formation of an alumina
film adhering to the surface of the sublayer or to the surface of
the superalloy without a sublayer.
[0039] Regermination of the ceramic layer is not a direct process.
Intermittent deposition tests with the EB-PVD process using a
shutter for the vaporization source showed that regermination of
the ceramic layer cannot be produced just by interrupting and then
resuming deposition. Similarly, intermittent deposition on an
article surface by rotation of the article in the ceramic vapour
flow, which is typical of the industrial EB-PVD process, does not
produce regermination of the ceramic layer. However, we have found
that regermination of the ceramic layer can be obtained when
ceramic deposition is resumed on the surface of a ceramic layer
formed previously in an earlier deposition batch. Our
interpretation of this result is that once the vacuum is broken and
the coated article has left the EB-PVD chamber the surface of the
ceramic deposit becomes polluted. The ceramic molecules condensing
on the surface of a previously polluted ceramic layer no longer
recognise the crystallographic planes of the ceramic and cannot
therefore establish an epitaxial relationship with the surface.
Resuming deposition on a polluted surface therefore leads to
regermination of the ceramic layer. However, a process for
producing a heat barrier in which thin elementary layers are
deposited in a number of different charges in order to make them
regerminate one on top of the other is industrially
impractical.
[0040] The invention therefore also relates to an industrially
viable vapour phase deposition process for forming repeated
germination ceramic layers. The process consists of producing
regermination in situ of the EB-PVD ceramic layer without
interrupting deposition. To this end, we exploit the fact that the
coating surface on which the ceramic molecules condense is
particularly reactive during deposition because of the free bonds
of the surface atoms and the oxygen depletion, the method involving
polluting the coating surface in situ during deposition. The
surface may be polluted by a variety of methods.
[0041] A first pollution method, which may be called the in situ
chemical pollution method, is to introduce a reactive polluting gas
intermittently into the deposition chamber, the polluting gas
interacting with the ceramic layer surface during deposition and/or
with the chemical components present in vapour form in the chamber
so as to produce regermination of the ceramic material during
condensation. The brief introduction of a reactive gas leads to the
partial formation of a compound other than zirconia on the coating
surface during deposition. The formation of a surface compound
amounts to in situ pollution of the ceramic layer surface during
deposition and produces its regermination without interrupting
deposition. The surface compound can be discontinuous, its
distribution on the surface of the ceramic being sufficient to
produce regermination of the ceramic layer. Among the reactive
gases envisaged in the case where the ceramic consists of oxides,
the gases preferred are those which lead to partial surface
nitridation or carburization of the ceramic layer. In the case
where the ceramic coating consists of carbides, the preferred
reactive gases are those which lead to partial surface nitridation
or oxidation of the ceramic layer. In the case where the ceramic
coating consists of nitrides the preferred gases are those which
lead to partial surface carburization or oxidation of the ceramic
layer. Typically, the polluting gas is atomic or molecular and
consists partly of atoms of elements selected from C, N, O, H, Si,
Cl, Br, F and I. As a rule, reactive gases containing nitrogen or
ammonia or hydrocarbons or carbon oxides or hydrogen or even
halogenated gases can be used.
[0042] A second polluting method called in situ physical pollution
involves producing interaction between the surface of the articles
undergoing deposition and a non-reactive gas which can be adsorbed
on the surface of the deposition without giving rise to a chemical
reaction. The adsorption of gas molecules on the surface of the
deposit disturbs the condensation of ceramic molecules on clearly
defined crystallographic planes. The brief introduction of a gas
which is not reactive but which is adsorbed very readily on the
surface of the ceramic layer during deposition may interrupt the
epitaxial relationship, leading in fact to regeneration of the
ceramic layer. The non-reactive gases which may be used for
producing an in situ physical pollution may be selected from the
inert gases and any other non-reacting gas very likely to be
adsorbed on the surface of the deposited ceramic material, such as,
for example, the rare gases Xe, Kr, Ar and He, mixtures of rare
gases, and carbon monoxide.
[0043] A third method of preparing a repeated regeneration ceramic
layer is to intermittently condense a material different from the
material used for the ceramic layer in conjunction with continuous
condensation of ceramic vapour. The brief condensation of a
material other than that of the ceramic layer during deposition
constitutes pollution, and can be achieved by a flash of the vapour
of the polluting material simultaneously with continuous
vaporization of the ceramic material. The polluting material may be
a metal or a ceramic material whose composition is other than that
of the deposited ceramic layer. This process can be performed
industrially by using two vaporizing crucibles, one containing the
ceramic material and the other the polluting material, the two
materials being vaporized individually by an electron beam. The
ceramic material in the first crucible is evaporated continuously,
whereas the polluting material in the second crucible is evaporated
intermittently by a separate electron beam.
[0044] A fourth method of preparing a repeated germination ceramic
layer is to exploit the reactivity of the ceramic vapour in the
vaporization zone. Because of the thermal excitation and the
interaction with the primary and secondary electrons originating
from the electron bombardment, the ceramic molecules may be in a
dissociated, excited and/or ionized state. These various states of
excitation of the vapour phase components boosts their chemical
interaction with a reactive gas. The condensation of the product of
the chemical reaction between a polluting gas and the ceramic
vapour on the surface of the coating during deposition provides the
pollution. To increase the efficiency of this method the polluting
gas may be introduced locally near the vaporization zone.
[0045] FIG. 6 shows a first example of a polluting gas introduction
cycle. The article is placed in the deposition chamber at a high
temperature in the presence of a partial oxygen pressure. The
oxygen present in the initial phases of the deposition serves to
promote the formation of an alumina film on which the ceramic
vapour will condense. Some of the partial oxygen pressure is
produced in the EB-PVD chamber by the dissociation of the
refractory oxide during vaporization. In addition, the deposition
chamber can be supplied with an oxygen-rich gas during the
introduction of the articles into the preheating chamber and during
the initial deposition phase. This gas is non-polluting. After a
deposition time T.sub.1, during which the pressure and rate of flow
of the non-polluting gas remain constant, a polluting gas is
introduced into the chamber for a period T.sub.2 which is shorter
than the time T.sub.1. The polluting gas can be introduced in
association with a constant rate of flow of non-polluting gas.
Preferably, the introduction of the polluting gas is abrupt (square
signal). The rate of flow of the polluting gas is therefore
subjected to a pressure control in which the reference value is a
high-pressure value P.sub.1 (see FIG. 5). After a time T.sub.2 the
rate of polluting gas flow is interrupted and the pumping speed of
the chamber is controlled on the basis of a low-pressure reference
value P.sub.2. The pumping speed is controlled, for example, by the
opening of a diaphragm disposed at the inlet of the pumps.
Alternatively, and as shown in FIG. 7, the polluting gas can be
introduced into the chamber at a constant total pressure therein by
reducing the delivery of non-polluting gas. Whichever configuration
is chosen, the partial pressure of the polluting gas must be
sufficient to react with the surface of the ceramic layer during
deposition in order to produce the regermination. This partial
threshold pressure of the polluting gas used depends upon the
nature of the gas. Selection of the polluting gas depends upon its
chemical or physical reactivity with the ceramic material used for
the heat barrier which it is required to deposit. The times T.sub.1
and T.sub.2 are chosen in dependence upon the ceramic deposition
rate. Typically, the thickness of ceramic deposited during the
period T= T.sub.1+T.sub.2 does not exceed 50 micrometers.
Preferably, the thickness of ceramic deposited during this period
T=T.sub.1+T.sub.2 does not exceed 20 micrometers.
[0046] The vapour phase deposition process can be a chemical (CVD)
process or a physical (PVD) process. Preferably, the vaporization
is effected by electron beam heating.
[0047] If the ceramic layer consists of an oxide or a mixture of
oxides, the compounds which are the product of the reaction between
the ceramic and the polluting gas (oxynitrides, carbides,
carbonitrides and so on) tend to disappear during operation due to
high-temperature oxidation. This is not a disadvantage to the
extent that the sole reason for their presence is to produce
regermination of the ceramic layer. They do not need to have
post-deposition thermal stability. If the regermination of the
ceramic layer is the result of intermittently introducing a
non-reactive highly adsorbent gas on the surface of the ceramic
layer during deposition, the gases thus trapped tend to be resorbed
after annealing at temperatures above the deposition temperature.
This is not a disadvantage because the sole reason for their
presence is to produce regermination of the ceramic layer. A final
advantage of the invention that it can readily be carried out with
existing industrial vapour phase deposition installations.
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