U.S. patent application number 14/653031 was filed with the patent office on 2015-11-19 for process for coating a substrate with an abradable ceramic material, and coating thus obtained.
The applicant listed for this patent is COMMISSARIAT L'ENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES. Invention is credited to Luc Bianchi, Aurelie Quet.
Application Number | 20150329954 14/653031 |
Document ID | / |
Family ID | 48521042 |
Filed Date | 2015-11-19 |
United States Patent
Application |
20150329954 |
Kind Code |
A1 |
Quet; Aurelie ; et
al. |
November 19, 2015 |
PROCESS FOR COATING A SUBSTRATE WITH AN ABRADABLE CERAMIC MATERIAL,
AND COATING THUS OBTAINED
Abstract
The invention relates to the field of the coating of substrates
with an abradable material. More specifically it relates to a
method for coating at least one surface of a substrate with at
least one ceramic compound, as well as to a thereby obtained
coating. It also relates to a substrate having at least one surface
coated with such a coating. It further relates to a device for
applying the coating method. Applications: fields of mechanical
engineering and aeronautical design.
Inventors: |
Quet; Aurelie; (Tours,
FR) ; Bianchi; Luc; (Artannes Sur Indre, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
COMMISSARIAT L'ENERGIE ATOMIQUE ET AUX ENERGIES
ALTERNATIVES |
Paris |
|
FR |
|
|
Family ID: |
48521042 |
Appl. No.: |
14/653031 |
Filed: |
December 17, 2013 |
PCT Filed: |
December 17, 2013 |
PCT NO: |
PCT/EP2013/076934 |
371 Date: |
June 17, 2015 |
Current U.S.
Class: |
428/220 ;
106/122; 106/286.8; 118/47; 427/446; 427/452; 427/453 |
Current CPC
Class: |
C23C 4/11 20160101; C23C
4/129 20160101; C23C 4/134 20160101; C23C 4/123 20160101; B05D 1/34
20130101 |
International
Class: |
C23C 4/12 20060101
C23C004/12; C23C 4/10 20060101 C23C004/10 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 18, 2012 |
FR |
1262250 |
Claims
1. A method for coating at least one surface of a substrate with at
least one layer comprising at least one ceramic compound, the
method comprising: a) simultaneously injecting: solid particles of
n ceramic compounds S.sub.1, . . . , S.sub.n through a first
injection means, n being an integer greater than or equal to 1, and
at least 90 percent (%) by a number of the solid particles of the n
ceramic compounds S.sub.1, . . . , S.sub.n having a greatest
dimension of more than 5 micrometers (.mu.m); and a liquid phase
through a second injection means, the liquid phase comprising a
solvent, solid particles of p ceramic compounds L.sub.1, . . . ,
L.sub.p and/or at least one precursor of the solid particles of the
p ceramic compounds L.sub.1, . . . , L.sub.p, p being an integer
greater than or equal to 1, and at least 90% by number of the solid
particles of the p ceramic compounds L.sub.1, . . . , L.sub.p
having a greatest dimension of less than or equal to 5 .mu.m, into
a thermal jet, whereby a mixture of the solid particles of the n
ceramic compounds S.sub.1, . . . , S.sub.n and of the liquid phase
is obtained in the thermal jet (35); and then b) projecting the
thermal jet, which contains the mixture of the solid particles of
the n ceramic compounds S.sub.1, . . . , S.sub.n and of the liquid
phase, on said surface of the substrate, whereby the layer
comprising at least one ceramic compound is formed on said
surface.
2. The method according to claim 1, wherein each of the n ceramic
compounds S.sub.1, . . . , S.sub.n and of the p ceramic compounds
L.sub.1, . . . , L.sub.p includes at least one element selected
from the Periodic Classification of the Elements from among
transition elements, metalloids and lanthanides.
3. The method according to claim 2, wherein each of the n ceramic
compounds S.sub.1, . . . , S.sub.n and of the p ceramic compounds
L.sub.1, . . . , L.sub.p is selected from oxides, silicates and
zirconates of at least one element selected from the Periodic
Classification of the Elements from among transition elements,
metalloids and lanthanides.
4. The method according to claim 3, wherein each of the n ceramic
compounds S.sub.1, . . . , S.sub.n and of the p ceramic compounds
L.sub.1, . . . , L.sub.p is selected from simple oxides, silicates
and zirconates of at least one element selected from among
aluminum, silicon, titanium, strontium, zirconium, barium, hafnium,
scandium, yttrium and lanthanides.
5. The method according to claim 4, wherein each of the n ceramic
compounds S.sub.1, . . . , S.sub.n and of the p ceramic compounds
L.sub.1, . . . , L.sub.p is selected from the following ceramic
compounds: a simple oxide of an element selected from zirconium,
hafnium, scandium, yttrium and lanthanides, simple oxides of
zirconium and hafnium which may be stabilized by an yttrium oxide;
a silicate of at least one element selected from among aluminum,
yttrium, scandium and lanthanides, the silicate may be doped with
at least one oxide of at least one element of the second column of
the Periodic Classification of the Elements; a zirconate of at
least one element selected from among yttrium, scandium and
lanthanides, the zirconate being selected from among those which
crystallize according to a pyrochlore or perovskite structure; and
mixtures of these ceramic compounds.
6. The method according to claim 1, wherein at least 90% by number
of the solid particles of the n ceramic compounds S.sub.1, . . . ,
S.sub.n have a greatest dimension of more than 5 .mu.m and less
than 100 .mu.m.
7. The method according to claim 1, wherein the liquid phase is a
colloidal aqueous solution of the solid particles of the p ceramic
compounds L.sub.1, . . . , L.sub.p and/or of at least one precursor
of the solid particles of the p ceramic compounds L.sub.1, . . . ,
L.sub.p.
8. The method according to claim 1, wherein the n ceramic compounds
S.sub.1, . . . , S.sub.n are all identical with the p ceramic
compounds L.sub.1, . . . , L.sub.p.
9. The method according to claim 1, wherein n and p are both equal
to 1, and the ceramic compounds S.sub.1 and L.sub.1 are both
mullite.
10. The method according to claim 1, wherein: the injecting of the
solid particles of the n ceramic compounds S.sub.1, . . . , S.sub.n
is carried out with an angle .alpha..sub.S formed by the directions
of the tilt axis of the means for injecting the solid particles of
the n ceramic compounds S.sub.1, . . . , S.sub.n and of the
longitudinal axis of the thermal jet, comprised between 75 and 105
degrees (.degree.); and the injecting of the liquid phase is
carried out with an angle .alpha..sub.L formed by the directions of
the tilt axis of the means for injecting the liquid phase and of
the longitudinal axis of the thermal jet, comprised between
75.degree. and 105.degree..
11. The method according to claim 1, wherein the liquid phase is
injected into the thermal jet at a distance from the substrate
which is less than or equal to the distance from the substrate at
which the solid particles of the n ceramic compounds S.sub.1, . . .
, S.sub.n are injected into the thermal jet.
12. The method according to claim 1, wherein deposition of the
layer is achieved with a blown arc plasma projection method by
means of a plasma-forming gas.
13. The method according to claim 12, wherein the plasma-forming
gas is selected from argon, helium, dinitrogen, dihydrogen, binary
mixtures of the latter, and the ternary mixtures of the latter.
14. The method according to claim 13, wherein the plasma-forming
gas is an argon-helium-dihydrogen ternary mixture.
15. The method according to claim 1, wherein said layer or each of
the layers comprising at least one ceramic compound has a thickness
ranging from 10 .mu.m to 2 mm.
16. The method according to claim 1, wherein the volume proportion
of solid particles of the p ceramic compounds L.sub.1, . . . ,
L.sub.p and/or of precursors of said ceramic compounds in the
liquid phase is comprised between 2% and 20%.
17. The method according to claim 1, wherein the ratio of the
volume of the solid particles of the n ceramic compounds S.sub.1, .
. . , S.sub.n to the volume of the solid particles of the p ceramic
compounds L.sub.1, . . . , L.sub.p is comprised in an interval
ranging from 0.4 to 3.
18. The method according to claim 1, wherein the flow rate with
which the liquid phase is injected into the thermal jet, is
(0.05.+-.0.03) liters per minute (L/min).
19. The method according to claim 1, wherein the sequence of the
steps a) and b) is repeated once or several times.
20. The method according to claim 1, wherein said layer or each of
the layers comprising at least one ceramic compound has a porosity
at least equal to 20%.
21. An abradable coating (R.sub.m) comprising at least one layer of
at least one ceramic compound, said layer or each of said layers
having a porosity at least equal to 20%, said layer comprising: a
plurality of solid particles of n ceramic compounds S.sub.1, . . .
, S.sub.n, n being an integer greater than or equal to 1, and at
least 90% by number of the solid particles of the n ceramic
compounds S.sub.1, . . . , S.sub.n having a greatest dimension of
more than 5 .mu.m; and a plurality of solid particles of p ceramic
compounds L.sub.1, . . . , L.sub.p, p being an integer greater than
or equal to 1, and at least 90% by number of the solid particles of
the p ceramic compounds L.sub.1, . . . , L.sub.p having a greatest
dimension of less than or equal to 5 .mu.m.
22. The coating according to claim 21, wherein each of the n
ceramic compounds S.sub.1, . . . , S.sub.n and of the p ceramic
compounds L.sub.1, . . . , L.sub.p is selected from among simple
oxides, silicates and zirconates of at least one element selected
from among aluminum, silicon, titanium, strontium, zirconium,
barium, hafnium, scandium, yttrium and lanthanides.
23. The coating according to claim 21, wherein n and p are both
equal to 1, and the ceramic compounds S.sub.1 and L.sub.1 are both
mullite.
24. The coating according to claim 21, wherein said layer or each
of the layers comprising at least one ceramic compound has a
thickness ranging from 10 .mu.m to 2 mm.
25. The coating according to claim 21, wherein said layer or each
of the layers comprising at least one ceramic compound has a
plurality of pores having a size comprised between 0.001 and 50
.mu.m, the plurality of pores comprising: a network of micropores
having a size comprised between 0.001 and 1 .mu.m, which network of
micropores is defined by the solid particles of the p ceramic
compounds L.sub.1, . . . , L.sub.p for which at least 90% by number
have a greatest dimension of less than or equal to 5 .mu.m, wherein
said network of micropores is included within a network of
macropores having a size comprised between 1 and 50 .mu.m, which
network of macropores is defined by the solid particles of the n
ceramic compounds S.sub.1, . . . , S.sub.n, for which at least 90%
by number have a greatest dimension of more than 5 .mu.m.
26. The coating according to claim 21, wherein said layer or each
of said layers of the coating always has a porosity at least equal
to 20% after submitting the latter to a temperature above
1,000.degree. C.
27. A substrate having at least one surface on which was carried
out the deposition of a coating (R.sub.m) as defined in claim
21.
28. A device for applying the method as defined in claim 1, the
device comprising: a torch capable of producing a thermal jet; a
projection gas reservoir; a first reservoir, which contains the
solid particles of the n ceramic compounds S.sub.1, . . . ,
S.sub.n; a second reservoir, which contains the liquid phase; a
means for fixing and positioning the substrate with respect to the
torch; an injection system independently connecting the first
reservoir and a first injection means provided at its end with a
nozzle for injecting the solid particles of the n ceramic compounds
S.sub.1, . . . , S.sub.n; and the second reservoir and a second
injection means provided at its end with a nozzle for injecting the
liquid phase, wherein the injection system allows simultaneous
injection of the solid particles of the n ceramic compounds
S.sub.1, . . . , S.sub.n and of the liquid phase into the thermal
jet generated by the torch; and a pressure reducer, which allows
adjustment of the pressure inside the second reservoir.
29. The method according to claim 13, wherein the plasma-forming
gas is an argon-helium mixture or an argon-dihydrogen mixture.
30. The method according to claim 20, wherein said layer or each of
the layers comprising at least one ceramic compound has a porosity
at most equal to 40%.
31. The coating according to claim 21, wherein said layer or each
of the layers comprising at least one ceramic compound has a
porosity at most equal to 40%.
Description
TECHNICAL FIELD
[0001] The invention relates to a method for coating at least one
surface of a substrate with at least one ceramic compound.
[0002] The invention also relates to the thereby obtained
coating.
[0003] The invention further relates to a substrate having at least
one surface coated with such a coating.
[0004] Finally, the invention relates to a device for applying said
coating method.
[0005] The technical field of the invention may be defined notably
as that of the coating of substrates with an abradable material,
and more particularly of the coating of substrates with an
abradable ceramic material.
[0006] Coatings made of an abradable ceramic material mainly find
their usefulness in devices in which the mobile parts have to be as
close as possible to fixed parts.
[0007] Thus, the deposition of a coating made of an abradable
ceramic material as the one produced according to the invention,
gives the possibility, when the coating is contacted with a mobile
part, of wearing away said coating in preference rather than the
mobile part.
[0008] Consequently, the invention may find its application,
generally in the field of mechanical engineering, and more
particularly in the field of aeronautic design, such as for example
for protecting the integrity of the surface condition of fixed
parts of turbine engines, such as low and high pressure
compressors, turbines or even stators.
[0009] The references located between ([ ]) refer back to the list
of bibliographic references which is presented subsequently to the
detailed discussion of a particular embodiment of the
invention.
STATE OF THE PRIOR ART
[0010] Generally, when an apparatus is operating, sometimes certain
apparatus elements are caused to come into contact accidentally and
at a non-negligible velocity, then causing these elements to be
worn, for example by abrasion, or even by making the latter and the
apparatus unusable. These elements may for example be a fixed
element or a mobile element, or further two mobile elements and
each in motion.
[0011] In order to remedy this problem, the deposition on a
substrate of a coating comprising at least one layer of an
abradable material, or more simply the deposition of an
<<abradable coating>>is a frequently applied technique
in fields such as mechanical engineering and aeronautical
design.
[0012] By abradable coating, abradable material, is generally meant
that this coating or material preferentially wears away relatively
to the part located facing it, and may be easily machined by mobile
parts.
[0013] Such coatings for example are used within automotive
turbochargers, or further at the walls of land-based turbines and
of gas turbines of aeronautical engines.
[0014] In this latter case, the function of the abradable coating
is to form dynamic joints, which give the possibility of minimizing
the play existing between the tip of the rotary blades and the case
of the charger or of the turbine ring.
[0015] Thus, the coating, which is deposited on a fixed turbine
element, the stator, is worn away upon contact with the top of the
blades, whether the latter occurs during running of revolutions of
the rotor or else in the event of accidental contact during
operation. The existence of this coating then gives the possibility
of promoting optimum operation of the turbine engines, with reduced
play and without damaging the structure of the blades.
[0016] In order to be able to be applied on such elements, a
coating should meet the following requirements: [0017] a capability
of being easily worn, which is expressed by low structural
cohesion, for example in order not to damage the top of the blades;
[0018] a gradual wear mechanism in order to provide long service
life of the coating; [0019] a resistance to erosion generated by
the high pressure gas flows, for example the combustion gas flows,
and the flows of particles circulating at high velocities; [0020] a
resistance to high temperatures, typically above 1,000 degrees
Celsius (.degree. C.), with preservation of the mechanical
properties of the coating, but also resistance to chemical
phenomena such as oxidation and corrosion.
[0021] A certain number of techniques are known giving the
possibility of making coatings having such properties.
[0022] These properties may notably be obtained, according to
documents [1] of Cowden et al. and [2] of Rigney et al., by
associating within the coating, elements such as: [0023] a metal
matrix, for example made in a super alloy such as CoNiCrAlY (which
is obtained by associating a nickel-chromium alloy and a
cobalt-aluminum-yttrium alloy), or a ceramic matrix, such as
zirconium oxide (IV) stabilized with yttrium (III) oxide which is
further noted as YSZ (or <<yttria-stabilized
zirconia>>). This matrix gives resistance to oxidation and
mechanical integrity at the high temperatures defined above, which
thus gives the possibility of ensuring a compromise between
abradability and resistance to erosion; [0024] a significant
porosity localized in the external portion of the abradable
coating, able to interact with the tops of the blades. A
significant porosity gives the possibility of making the coating
sufficiently friable so as to promote wear of the latter at the
moment of contact with the top of the blades. Let us recall that
the porosity of the coating is defined by the percentage of the
coating volume occupied by voids or <<pores>>; and
[0025] optionally, a solid ceramic lubricant, such as boron nitride
(BN) or further graphite, in order to limit heating generated
during the passing of the blades.
[0026] A technique often used for making abradable coatings is
thermal projection. Several thermal projection methods are notably
used in research laboratories and in industry for producing, on
very diverse substrates in terms of nature and shape, deposits of
ceramic, metal, polymeric materials, but also combinations
thereof.
[0027] The coatings produced by thermal projection may be obtained
from compounds to be deposited or from precursors of compounds to
be deposited, these compounds or precursors may appear: [0028] in
solid form, for example in the form of solid particles which have
an average particle size typically comprised between 5 and 100
micrometers (.mu.m), or further agglomerated particles at a
nanometric scale; or else [0029] in gas form or in liquid form, for
example in the form of solutions, suspensions, or further colloidal
sols of the compounds or precursors of compounds as described in
document [3].
[0030] In this technique, the compounds or precursors of compounds
entering the formation of the coating are injected into a heat
source which is produced by a projection gas, for example a mixture
of a combustible gas and of an oxidizer gas or an ionized gas of
the plasma type. The solid particles which are introduced or
generated inside the flame are partly or totally melted, and then
accelerated towards a substrate in order to form, on the surface of
the latter, a coating by stacking of solid particles and of molten
particles also called <<lamellas>>(or
<<splats>>).
[0031] By applying thermal projection techniques to the making of
abradable coatings, it is possible to generate two types of
coating.
[0032] A first type of coating, highly porous, may be made by
including non-molten particles in the coating.
[0033] However, this type of coating, which proves to be difficult
to reproduce, does not have satisfactory properties for a use as an
abradable coating, i.e. proper mechanical strength and porosity
greater than or equal to 20 percent (%). Indeed, thermal projection
of solid particles which have an average particle size of more than
5 .mu.m, for example by plasma projection, only gives the
possibility of conventionally attaining porosities comprised
between 5 and 20%.
[0034] A second more dense type of coating may be obtained, the
porosity is then generated by introducing sacrificial solid
particles of organic or ceramic nature within the coating.
[0035] Thus, the document [4] of Clingman et al. describes a method
for producing an abradable coating for elements of turbine engines,
such as a compressor or turbine shroud. The coating consists of a
matrix of zirconium (IV) oxide stabilized with an oxide selected
from yttrium (III) oxide (Y.sub.2O.sub.3), magnesium oxide (MgO)
and calcium oxide (CaO), in which particles of a crystalline
aromatic polyester are dispersed which may be easily decomposed at
a temperature above about 500.degree. C. The porosity of the
obtained coating with this method is evaluated to be between 20 and
33%.
[0036] Similarly, the document [5] of Vine et al. describes the
possibility of associating, within an YSZ matrix, solid particles
of poly (methyl methacrylate) (PMMA) and particles of a solid
lubricant, such as silicon carbide (SiC) or boron nitride, for
designing an abradable coating having a porosity comprised between
20 and 35%.
[0037] The document [6] of Rangaswamy et al., as for it, describes
an abradable coating for gas turbine elements, comprising a matrix
formed with a metal or a mixture of metals selected from aluminum,
cobalt, copper, iron, nickel and silicon, a solid lubricant such as
calcium fluoride (CaF.sub.2), molybdenum disulfide (MoS.sub.2) or
boron nitride, and a porogenic agent appearing in the form of solid
particles of graphite or of a polymer, such as an aromatic
polyimide or a polyester selected from a homopolyester of
p-oxy-benzoyl and an ester of poly(p-oxybenzoylmethyl).
[0038] The porosity within coatings of the second type may further
be generated by combining the inclusion of ceramic particles and
the generation of a network of cavities on the surface of the
coating after thermal projection.
[0039] Thus, the document [7] of Le Biez et al. described an
abradable coating for gas turbine elements, comprising a matrix of
a nickel-chromium-aluminum alloy in which hollow beads in a
silico-aluminous material are dispersed. A network of cavities is
machined on the surface of the coating, which then has a porosity
at least equal to 40%.
[0040] If the methods described in documents [4], [5], [6] and [7]
give the possibility of obtaining coatings having porosities of
more than 20%, and which may therefore be used as abradable
coatings, these methods however apply joint thermal projection of
materials with very different thermal properties. For example, in
document [4], the melting temperature of zirconium(IV) oxide is
2,715.degree. C. at room temperature, while that of the polymer
forming the solid particles which are dispersed in the coating is
of about 500.degree. C. at the same pressure, then causing
inhomogeneity of the produced coating.
[0041] Document [8] of Pettit, Jr. et al. proposes for solving this
inhomogeneity problem, a method for producing an abradable coating
intended for turbine engine elements, which applies plasma
projection and which makes use of the different temperature zones
of the thermal jet: [0042] into the central portion of the thermal
jet are injected particles of a nickel-chromium or MCrAlY alloy, M
being selected from nickel, cobalt, iron and mixtures thereof; and
[0043] into the peripheral portion of the thermal jet are injected
solid particles of an organic polymer such as PMMA (Lucite.RTM.,
DuPont),
[0044] the temperature inside the peripheral portion of the thermal
jet being much lower than that inside the central portion of the
jet.
[0045] Methods for coating with an entirely ceramic abradable
material have further been developed.
[0046] Thus, document [9] of Lima et al. describes a method for
preparing a coating for elements such as compressors or combustion
chambers, which comprises thermal projection of ceramic YSZ
particles appearing as agglomerates of nanometric size. In this
document, the projection parameters are controlled so that the
particles, once they are deposited on the substrate to be coated,
form porous agglomerates of micrometric size and consisting of
non-molten YSZ particles and included in a matrix of molten YSZ
particles.
[0047] Document [10] of Allen, as for it, describes a method for
producing an abradable coating for elements such as turbine shroud
segments. This method comprises thermal projection of an aqueous
suspension comprising a precursor of a ceramic material, for
example YSZ, and a lubricant appearing in solid form, selected from
boron trichloride, urea, guanidine and other organic
nitrogen-containing compounds.
[0048] As compared with documents [4] to [8], the methods described
in documents [9] and [10] give the possibility of getting rid of
the inhomogeneity due to materials having very different melting
temperatures, and of suppressing the final heat treatment which is
required for removing the polymeric and organic particles used for
generating porosity.
[0049] Furthermore, a coating in an entirely ceramic abradable
material gives the possibility of attaining high operating
temperatures, typically above 1,000.degree. C., which are
frequently attained in fields such as aeronautics.
[0050] However, certain limitations may be noted, such as for
example: [0051] the requirement of accurate control of the
projection parameters, like temperature, in the method described in
document [9], in order to obtain a bimodal structure associating
non-molten and molten solid ceramic particles; or further [0052]
the requirement of using a lubricant appearing in solid form, like
boron nitride in the method described in document [10], in order to
reduce the friction coefficient within the coating.
[0053] Therefore, the inventors set the goal of developing a method
for preparing a coating which fits the criteria listed above in
order to be able to be used as an abradable coating, i.e. notably:
a capability of being easily abraded while having a slow wear
mechanism as well as resistance to erosion and to high temperatures
while preserving suitable mechanical properties. Typically, such a
coating should have a porosity greater than or equal to 20%, while
having a homogeneous thickness and structure.
[0054] The goal of the present invention is also to provide such a
method which is simple, reliable, easy to apply and notably avoids
the use of additives.
[0055] The goal of the present invention is further to provide a
method for preparing an abradable coating which does not have the
drawbacks, defects and disadvantages of the methods of the prior
art and which solves the problems of the methods of the prior
art.
DISCUSSION OF THE INVENTION
[0056] These goals and further other ones are achieved by the
invention which firstly proposes a method for coating at least one
surface of a substrate with at least one (abradable) layer
comprising at least one ceramic compound, said method being
characterized in that it comprises the following steps:
[0057] a) simultaneous injection: [0058] of solid particles of n
ceramic compounds S.sub.1, . . . , S.sub.n through a first
injection means, n being an integer greater than or equal to 1, and
at least 90% by number of the solid particles of n ceramic
compounds S.sub.1, . . . , S.sub.n having a greatest dimension of
more than 5 .mu.m; and [0059] of a liquid phase through a second
injection means, the liquid phase comprising a solvent, solid
particles of p ceramic compounds L.sub.1, . . . , L.sub.p and/or at
least one precursor of the solid particles of the p ceramic
compounds L.sub.1, . . . , L.sub.p, p being an integer greater than
or equal to 1, and at least 90% by number of the solid particles of
the p ceramic compounds L.sub.1, . . . , L.sub.p having a greatest
dimension of less than or equal to 5 .mu.m,
[0060] into a thermal jet, whereby, a mixture of the solid
particles of the n ceramic compounds S.sub.1, . . . , S.sub.n and
of the liquid phase is obtained in the thermal jet; and then
[0061] b) projection of the thermal jet, which contains the mixture
of the solid particles of the n ceramic compounds S.sub.1, . . . ,
S.sub.n and of the liquid phase, onto said surface of the
substrate, whereby the layer comprising at least one ceramic
compound is formed on said surface.
[0062] Thus, the method of the invention is based on the
observation of the inventors, according to which thermal projection
of a mixture obtained by simultaneous injection into the thermal
jet of: [0063] n ceramic compounds S.sub.1, . . . , S.sub.n which
appear as solid particles (having a suitably selected particle
size) of these compounds; and [0064] p ceramic compounds L.sub.1, .
. . , L.sub.p which appear as solid particles (also having a
suitably selected particle size and different from that of the
solid particles of the n ceramic compounds S.sub.1, . . . ,
S.sub.n) comprised in a liquid phase,
[0065] gives the possibility of obtaining a coating which has
optimum properties, notably in terms of porosity, for use as an
abradable coating.
[0066] The method of the invention is distinguished from the prior
art since it combines the advantages provided by the injection via
a dry route of solid particles of n ceramic compounds S.sub.1, . .
. , S.sub.n into a thermal jet on the one hand and by simultaneous
injection of a liquid phase carrying solid particles of p ceramic
compounds L.sub.1, . . . , L.sub.p and/or at least one precursor of
the solid particles of the p ceramic compounds L.sub.1, . . . ,
L.sub.p. The general and preferred operating conditions of the
method of the invention are discussed hereafter.
[0067] The definition of certain of the terms used for describing
the invention is also specified in the following.
[0068] According to the invention, the substrate may be organic,
inorganic or mixed, i.e. a same surface of the substrate, notably
the surface to be coated by the method according to the invention,
may both be organic and inorganic.
[0069] Advantageously, the substrate supports the operating
conditions of the method of the invention.
[0070] Advantageously, the substrate consists of at least one
material selected from semi-conductors, such as silicon; organic
polymers such as polymethyl methacrylates (PMMA), polycarbonates
(PC), polystyrenes (PS), polypropylenes (PP) and polyvinyl
chlorides (PVC); metals such as aluminum, titanium, nickel,
tungsten, molybdenum; metal alloys such as NiAl, TiAl, TiAlV,
steels, superalloys such as MCrAlY alloys (with M=Fe, Ni, Co,
Ni/Co); glasses; mineral oxides, for example as layers, such as for
example silica (SiO.sub.2), alumina (Al.sub.2O.sub.3), zirconium
(IV) oxide (ZrO.sub.2), titanium (IV) oxide (TiO.sub.2), tantalum
(V) oxide (Ta.sub.2O.sub.5) or further magnesium oxide (MgO);
carbides, borides, nitrides; carbonaceous substrates; and composite
or mixed materials comprising several of these materials.
[0071] Still better, the substrate consists of a TiAlV alloy (an
alloy of titanium, aluminum and vanadium), for example TiAl.sub.6V
(an alloy consisting of 90% by mass of Ti, 6% by mass of aluminum
and 4% by mass of vanadium).
[0072] Before coating the substrate with at least one layer with
the method according to the invention, the surface of the substrate
which is intended to be coated is optionally prepared and/or
cleaned in order to remove organic and/or inorganic contaminants
which might prevent deposition, or even binding, of the coating on
the surface, and in order to improve the adherence of the
coating.
[0073] The method for preparing the surface may consist in
generating surface roughness by sandblasting.
[0074] The cleaning method used depends on the nature of the
substrate and may be achieved with one or several techniques
selected from physical, chemical and mechanical techniques known to
one skilled in the art.
[0075] In a non-limiting way, the cleaning method may be achieved,
for example with a technique selected from among immersion in an
organic solvent, detergent cleaning, acid etching, and the
combination of two or more of these techniques, this or these
techniques may further be assisted with ultrasonic waves.
[0076] The cleaning may optionally be followed by rinsing with tap
water, and then by rinsing with deionized water, the rinses being
optionally followed by drying with a technique selected from among
the lift-out technique, alcohol spraying, a compressed air jet, a
hot air jet, or infrared rays.
[0077] Within the scope of the present invention, it is specified
that the expression <<chemical element>>designates an
element of the Periodic Table of the Chemical Elements, further
known under the names of Periodic Classification of the Elements or
Mendeleev Table, while the expression <<chemical
compound>>designates a molecule or an ionic compound formed
with at least two different chemical elements.
[0078] Within the scope of the present invention, the definition of
the expression <<ceramic compound>>is not recalled and
is well known to the man skilled in the art.
[0079] From among the ceramic compounds which may enter the
composition of the layers prepared by the method according to the
invention, mention may notably be made of: [0080] oxides, such as
simple metal oxides (for example, an aluminum oxide or further a
zirconium oxide) or further mixed metal oxides (for example, a
metal silicate or further a metal zirconate); [0081] non-oxides,
such as for example, carbides, borides, nitrides, of metals such as
tungsten, magnesium, platinum, silicon, zirconium, hafnium,
tantalum or further titanium; or further [0082] composite ceramics,
generally defined as being a combination of one or several oxides
and of one or several non-oxides, such as those mentioned
above.
[0083] In this respect, it is specified that the terms of metal and
metallic refer to elements which are conventionally considered as
metals in the Periodic Classification of the Elements, in
particular the transition elements (such as for example, titanium,
zirconium, niobium, yttrium, vanadium, chromium, cobalt and
molybdenum), the other metals (such as aluminum, gallium, germanium
and tin), the lanthanides and actinides. These terms also refer to
metalloid elements such as for example silicon.
[0084] According to the invention, the method comprises in step a),
the simultaneous injection of solid particles of n suitably
selected ceramic compounds S.sub.1, . . . , S.sub.n, and of a
liquid phase comprising a solvent, solid particles of p ceramic
compounds L.sub.1, . . . , L.sub.p and/or at least one precursor of
the solid particles of the p suitably selected ceramic compounds
L.sub.1, . . . , L.sub.p.
[0085] Thus, advantageously, each of the n ceramic compounds
S.sub.1, . . . , S.sub.n and of the p ceramic compounds L.sub.1, .
. . , L.sub.p includes at least one element selected from the
Periodic Classification of the Elements from among transition
elements, metalloids and lanthanides.
[0086] Still more advantageously, each of the n ceramic compounds
S.sub.1, . . . , S.sub.n, and of the p ceramic compounds L.sub.1, .
. . , L.sub.p is selected from oxides, silicates and zirconates of
at least one element selected from the Periodic Classification of
the Elements from among transition elements, metalloids and
lanthanides.
[0087] Still better, each of the n ceramic compounds S.sub.1, . . .
, S.sub.n, and of the p ceramic compounds L.sub.1, . . . , L.sub.p
is selected from simple oxides, silicates and zirconates of at
least one element selected from aluminum, silicon, titanium,
strontium, zirconium, barium, hafnium and elements of the
<<rare earth>>family as defined by the International
Union of Pure and Applied Chemistry (IUPAC) (cf. [11]), i.e.
scandium, yttrium and the lanthanides.
[0088] Advantageously, each of the n ceramic compounds S.sub.1, . .
. , S.sub.n, and of the p ceramic compounds L.sub.1, . . . ,
L.sub.p is selected from ceramic compounds which are usually used
in the composition of thermal barriers such as for example: [0089]
a simple oxide of an element selected from zirconium (for example
zirconium(IV) oxide (ZrO.sub.2)), hafnium (for example hafnium(IV)
oxide (HfO.sub.2)), scandium (for example scandium(III) oxide
(Sc.sub.2O.sub.3)), yttrium (for example yttrium(III) oxide
(Y.sub.2O.sub.3)) and the lanthanides, simple oxides of zirconium
and hafnium which may be stabilized with an yttrium oxide (for
example Y.sub.2O.sub.3, which allows preparation of the YSZ oxide
already mentioned above in the presence of ZrO.sub.2); [0090] a
silicate of at least one element selected from aluminum (for
example mullite), yttrium, scandium and the lanthanides, the
silicate may be doped with at least one oxide of at least one
element of the second column of the Periodic Classification of the
Elements (or element of the earth-alkaline family); [0091] a
zirconate of at least one element selected from yttrium, scandium
and the lanthanides, the zirconate being selected from those which
crystallize according to a pyrochlore structure (for example
lanthanum zirconate (La.sub.2Zr.sub.2O.sub.7), gadolinium zirconate
(Gd.sub.2Zr.sub.2O.sub.7), niobium zirconate
(Nb.sub.2Zr.sub.2O.sub.7)) or according to a perovskite structure
(for example strontium zirconate (SrZrO.sub.3) and barium zirconate
(BaZrO.sub.3)); [0092] and mixtures of these ceramic compounds.
[0093] It is specified that by the expression <<solid
particle>>, is designated a particle appearing in solid form,
at ambient pressure and temperature, the ambient or room
temperature being defined as being the temperature at which the
particle is located when the latter is neither subject to cooling,
nor to any heating. Room temperature is generally from 15 to
30.degree. C., for example from 20 to 25.degree. C.
[0094] Preferably, the solid particles of the n ceramic compounds
S.sub.1, . . . , S.sub.n are particles which may be of any shape,
but for which at least 90% by number have a greatest dimension of
more than 5 .mu.m and less than 100 .mu.m.
[0095] It is specified that the greatest dimension of a particle
corresponds to the diameter of the latter when it is established,
for example by reproducible grain size analysis, that the particle
has or substantially has the shape of a sphere.
[0096] Advantageously, the liquid phase results from the contact
with a solvent, of solid particles of the p ceramic compounds
L.sub.1, . . . , L.sub.p and/or of at least one precursor of the
solid particles of the p ceramic compounds L.sub.1, . . . ,
L.sub.p.
[0097] By the term of <<precursor>>, is generally meant
at least one chemical compound used in any of the chemical
reactions by which the p ceramic compounds L.sub.1, . . . , L.sub.p
(which appear as solid particles) are obtained.
[0098] Thus, the liquid phase may advantageously result from
putting into solution or alternatively suspending, in a solvent,
solid particles of the p ceramic compounds L.sub.1, . . . , L.sub.p
and/or of at least one precursor of solid particles of the p
ceramic compounds L.sub.1, . . . , L.sub.p, it being specified that
at least 90% by number of the solid particles of each of the p
compounds L.sub.1, . . . , L.sub.p has a greatest dimension of less
than or equal to 5 .mu.m.
[0099] In the case of putting into suspension (suspending), the
obtained liquid phase may be a real, true, solution or
alternatively a colloidal solution of the solid particles of the p
ceramic compounds L.sub.1, . . . , L.sub.p and/or of at least one
precursor of the solid particles of the p ceramic compounds
L.sub.1, . . . , L.sub.p.
[0100] It is considered that a chemical compound, and in
particular, a ceramic compound or a precursor of a ceramic
compound, is soluble in a solvent when it is able to form a real
solution or a colloidal solution with this solvent. One refers to a
real solution when the solute is a molecule of small size, while
one rather refers to a colloidal solution when the solute is a
macromolecule (a size ranging from 5 nanometers (nm) to 1 .mu.m,
cf. [12]).
[0101] Advantageously, the solvent is selected from water, organic
solvents (for example, ethanol), mixtures of water and of at least
one organic solvent miscible with water (for example, a
water-ethanol mixture) and mixtures of organic solvents miscible
with each other.
[0102] Still better, the liquid phase is a colloidal aqueous
solution of the solid particles of the p ceramic compounds L.sub.1,
. . . , L.sub.p and/or of at least one precursor of the solid
particles of the p ceramic compounds L.sub.1, . . . , L.sub.p.
[0103] According to the invention, the integers n and p, either
identical or different, are selected independently of each other.
These integers n and p are selected in a range from 1 to 10, still
better, in a range from 1 to 5, all the intermediate values
comprised in the thereby defined ranges being considered.
[0104] According to a first alternative, the n ceramic compounds
S.sub.1, . . . , S.sub.n may all be identical with the p ceramic
compounds L.sub.1, . . . , L.sub.p, and the integer n is then equal
to the integer p.
[0105] In other words, the n ceramic compounds S.sub.1, . . . ,
S.sub.n injected through the first injection means are exactly the
same as the p ceramic compounds L.sub.1, . . . , L.sub.p which are
injected through the second injection means, or which are obtained
in the thermal jet after the chemical reaction(s) for forming the p
ceramic compounds L.sub.1, . . . , L.sub.p (in the case when
precursors of these p ceramic compounds L.sub.1, . . . , L.sub.p
are injected through the second injection means).
[0106] In particular, n and p are both equal to 1, and the ceramic
compounds S.sub.1 and L.sub.1 are both mullite. This is a
crystalline aluminosilicate existing in the form of a solid
solution of composition Al.sub.2[Al.sub.2+2xSi.sub.2-2x]O.sub.10-x
with 0.17.ltoreq.x.ltoreq.0.5. The composition of this
aluminosilicate may thus change between the <<mullite
3:2>>(3 Al.sub.2O.sub.3.2 SiO.sub.2) and <<mullite
2:1>>(2 Al.sub.2O.sub.3.SiO.sub.2) forms, the different
stoichiometries being obtained by substituting silicon atoms with
aluminum atoms within the crystal.
[0107] In this case, the liquid phase is a colloidal aqueous
solution of mullite, which may for example be prepared by
suspending solid particles of aluminum nitrate, an aqueous
suspension of colloidal silicon particles and deionized water.
[0108] According to a second alternative, the n ceramic compounds
S.sub.1, . . . , S.sub.n may be partly or totally different from
the p ceramic compounds L.sub.1, . . . , L.sub.p, the integer n
then not being necessarily equal to the integer p. Thus, the
association of ceramic compounds having various intrinsic
properties may be achieved for optimization purposes of the
behavior in situ of the coating obtained by the method of the
invention (for example, by imparting mechanical strength properties
at high temperatures i.e. typically above 1,000.degree. C.).
[0109] According to the invention, the injection of step a) is
achieved in a thermal jet, whereby a mixture of the solid particles
of the n ceramic compounds S.sub.1, . . . , S.sub.p and of the
liquid phase is obtained in the thermal jet.
[0110] The thermal jet may consist of a gas (also called a
<<projection gas>>) or of a mixture of gases and acts
as an enthalpy source, which allows: [0111] an increase in the
temperature of the solid particles of the n ceramic compounds
S.sub.1, . . . , S.sub.n, optionally up to the melting point of the
latter, the n ceramic compounds S.sub.1, . . . , S.sub.n then
appearing as partly or totally molten solid particles in the
thermal jet on the one hand; and [0112] vaporization of the solvent
of the liquid phase, increase in the temperature of the solid
particles of the p ceramic compounds L.sub.1, . . . , L.sub.p,
optionally up to the melting point of the latter and/or increase in
the temperature of the precursor(s) of the p ceramic compounds
L.sub.1, . . . , L.sub.p in order to allow for the chemical
reaction(s) leading to the synthesis of the p ceramic compounds
L.sub.1, . . . , L.sub.p, the p ceramic compounds L.sub.1, . . . ,
L.sub.p then appearing as partly or totally molten solid particles
in the thermal jet on the other hand.
[0113] The nature of the projection gas is selected depending on
the projection technique of the thermal jet which is used. The
projection gas may be a mono-, poly-atomic gas or further a mixture
of gases, as defined hereafter.
[0114] The simultaneous injection of the solid particles of the n
ceramic compounds S.sub.1, . . . , S.sub.n and of the liquid phase
may be achieved with any suitable means for injecting solids and
liquids.
[0115] Thus, for example, a first injection means may be connected
to reservoir(s) containing the solid particles of the n ceramic
compounds S.sub.1, . . . , S.sub.n, while a second injection means
may be connected to reservoir(s) containing the liquid phase.
[0116] Still for example, the solid particles of the n ceramic
compounds S.sub.1, . . . , S.sub.n may be injected into the thermal
jet as a jet of these particles, and the liquid phase may be
injected as a jet or droplets, preferably with suitable momentum so
as to be substantially identical with that of the thermal jet.
[0117] Advantageously, the injection of the solid particles of the
n ceramic compounds S.sub.1, . . . , S.sub.n and of the liquid
phase is achieved with an angle .alpha. (for example from
75.degree. to 105.degree., notably 90.degree.) relatively to the
longitudinal axis of the thermal jet. In other words: [0118] the
injection of the solid particles of the n ceramic compounds
S.sub.1, . . . , S.sub.n is achieved advantageously, with an angle
.alpha..sub.S formed by the directions of the tilt axis of the
means for injecting the solid particles of the n ceramic compounds
S.sub.1, . . . , S.sub.n and of the longitudinal axis of the
thermal jet, comprised between 75 and 105 degrees)(.degree.) (for
example 90.degree.); and [0119] the injection of the liquid phase
is advantageously achieved with an angle .alpha..sub.L formed by
the directions of the tilt axis of the means for injecting the
liquid phase and of the longitudinal axis of the thermal jet,
comprised between 75.degree. and 105.degree. (for example
90.degree.).
[0120] Moreover, during their work, the inventors were able to show
an influence: [0121] of a distance D.sub.S comprised between the
substrate and the injection point for the solid particles of the n
ceramic compounds S.sub.1, . . . , S.sub.n in the thermal jet; and
[0122] of a distance D.sub.L comprised between the substrate and
the injection point for the liquid phase in the thermal jet.
[0123] Indeed, the inventors noticed that the porosity level may be
adjusted by varying the distance D.sub.S-D.sub.L. Mobilization of
the energy of the thermal jet is greater for the vaporization of
the liquid phase than for the melting of the solid particles of the
n ceramic compounds S.sub.1, . . . , S.sub.n.
[0124] Also, preferably, the liquid phase is injected into the
thermal jet at a distance from the substrate which is less than or
equal to the distance of the substrate at which the solid particles
of the n ceramic compounds S.sub.1, . . . , S.sub.n are injected
into the thermal jet. In other words, the injection distances in
the thermal jet are preferably selected so as to satisfy the
following inequality (inequation): D.sub.S.gtoreq.D.sub.L.
[0125] The vaporization of a solvent actually mobilizes a
significant amount of the energy of the jet and promotes more rapid
extinction of the plasma jet, i.e., the length of the plasma jet
decreases (variable depending on the nature of the solvent, ethanol
mobilizing less energy than water for example). If the injection of
the liquid phase is accomplished upstream, it does not have
sufficient available energy for melting the solid particles
downstream. By introducing the solid particles upstream or at the
same distance as the liquid phase, a sufficient amount of energy is
available for ensuring melting of the solid particles, which is
required for the cohesion of the deposit. A sufficient amount of
energy remains available downstream for vaporization of the solvent
and treatment of the liquid phase.
[0126] Moreover, the temperature of the solid particles of the n
ceramic compounds S.sub.1, . . . , S.sub.n during their injection
into the thermal jet may be room temperature as already defined
above, for example 20.degree. C. Advantageously, it is possible to
control and modify the temperature of these particles for their
injection into the thermal jet, for example so that it is comprised
in a range from 20 to 150.degree. C.
[0127] It is possible to preheat in particular the solid particles
before the injection in order to get rid of possible problems of
relative humidity which may cause agglomeration of the solid
particles and decrease the flowability of the powder.
[0128] Further, the temperature of the liquid phase during its
injection into the thermal jet may for example range from room
temperature, for example 20.degree. C., up to a temperature below
the boiling temperature of this liquid phase. Advantageously, it is
possible to control and modify the temperature of the liquid phase
for its injection into the thermal jet, for example for it being
from 1 to 99.degree. C. Depending on the imposed temperature, the
liquid phase then has a different surface tension which causes a
more or less rapid and efficient fragmentation mechanism when it
arrives into the thermal jet. The temperature may therefore have an
effect on the quality of the obtained coating.
[0129] According to the invention, the method also comprises a step
b), in which a projection of the thermal jet, which contains the
mixture of the solid particles of the n ceramic compounds S.sub.1,
. . . , S.sub.n and the liquid phase, is achieved on the substrate
whereby a layer comprising at least one ceramic compound is formed
on the substrate.
[0130] As mentioned above, the projection of the thermal jet, or
<<thermal projection>> ("thermal spraying"), groups the
whole of the methods by which solid constituents of a material (or
<<filler material>>), here the solid particles of the n
ceramic compounds S.sub.1, . . . , S.sub.n and optionally those
suspended in the liquid phase, are melted or brought to a plastic
condition by means of a source of heat or a source of enthalpy. The
mixture formed in the thermal jet is then projected (sprayed) onto
the substrate to be coated, on which it mechanically adheres and
solidifies (without generating any melting phenomenon of the
substrate).
[0131] Depending on the nature of the ceramic compound(s) comprised
in the mixture, the latter may be deposited on the substrate as a
layer by applying thermal projection methods as stated
hereafter.
[0132] According to a first alternative, the deposition may be
carried out with a flame projection method with a projection
gas.
[0133] Advantageously, the flame projection method is selected from
a flame-powder projection method and a hypersonic flame projection
method, with continuous or discontinuous firing (HVOF or
<<High Velocity Oxy Fuel>>method, HVAF or <<High
Velocity Air Fuel>>method).
[0134] Advantageously, the projection gas used in a flame
projection method is selected from acetylene, propylene,
hydrocarbons (for example propane) and ternary mixtures such as:
[0135] an ethylene-acetylene-propylene mixture (for example
Crylene.RTM., which is a mixture of these gases in the volume
proportions 73/22/5); or further [0136] a
methylacetylene-propadiene-hydrocarbon mixture (for example
Tetrene.RTM., which is a mixture consisting in volume proportions,
of 39% of a mixture of methylacetylene and of propadiene, 44% of
propylene and 17% of a mixture of butane, propane and unsaturated
derivatives of both of these alkanes).
[0137] Advantageously, the projection gas is brought to a
temperature comprised between 3,000 and 3,500 Kelvin (K).
[0138] According to a second alternative, the deposition may be
achieved with a blown arc plasma projection method by means of a
plasma-forming gas.
[0139] In this alternative, the thermal jet, which is then a plasma
jet, may be generated by a plasma-forming gas which is
advantageously selected from argon, helium, dinitrogen, dihydrogen,
binary mixtures thereof, such as an argon-helium mixture or an
argon-dihydrogen mixture, and ternary mixtures thereof, such as an
argon-helium-dihydrogen mixture, the latter mixture being most
particularly preferred.
[0140] Advantageously, the method for generating the plasma is
selected from an arc plasma either blown or not, and inductive or
radiofrequency plasma, for example in a supersonic mode. The
generated plasma may operate at atmospheric pressure or at a lower
pressure.
[0141] Advantageously, the device which is used for generating the
plasma is an arc plasma torch.
[0142] Advantageously, the projection gas is brought to a
temperature comprised between 5,000 and 15,000 K.
[0143] Advantageously, the projection gas has a viscosity ranging
from 10.sup.-4 to 5.10.sup.-4kilograms by meter second (kg/ms).
[0144] Advantageously, the deposit is made by a blown arc plasma
projection method.
[0145] Thus, during the application of the thermal projection
method, the solid particles of the n ceramic compounds S.sub.1, . .
. , S.sub.n and the liquid phase simultaneously penetrate into the
thermal jet.
[0146] The kinetic and thermal energies of the thermal jet are used
for partly or totally melting the solid particles of the n ceramic
compounds S.sub.1, . . . , S.sub.n on the one hand, and for
fractionating the liquid phase into a plurality of droplets under
the effect of the shear forces of the thermal jet, vaporizing the
solvent of the liquid phase and leading to the obtaining of solid
particles of the p ceramic compounds L.sub.1, . . . , L.sub.p which
are partly or totally melted on the other hand.
[0147] Once the core of the thermal jet is attained, as the latter
is a high temperature (for example, from 6,000 to 14,000 K for a
blown arc plasma projection) and high velocity medium, the mixture
formed by the partly or totally molten solid particles of the
ceramic compounds S.sub.1, . . . , S.sub.n, L.sub.1, . . . ,
L.sub.p and the solvent droplets of the liquid phase is accelerated
in order to be collected on the substrate, in the form of a deposit
which constitutes the coating.
[0148] It is specified that the temperature of the thermal jet is
selected depending on the chemical nature of the species which
compose the mixture and on the desired coating. The temperature may
be selected so as to be placed in a partial melting configuration
of the solid particles of the mixture, in order to preserve at best
the initial properties within the layer(s) which compose(s) the
coating.
[0149] For example, it may be interesting to retain partial melting
in the case of mullite in order to retain a crystallized condition
(the passing of the powder in the plasma jet producing a portion of
amorphous mullite phases), in the case of yttriated zirconia, total
melting of the particles gives the possibility of obtaining a
non-transformable therefore stable phase, generally of high
interest for the targeted applications.
[0150] The substrate to be coated is for obvious reasons
preferentially positioned relatively to the thermal jet so that the
projection of the mixture is directed onto the surface to be
coated. The positioning is adjusted for each application, depending
on the selected projection conditions and on the desired
microstructure of the deposit.
[0151] Thus, said or each of the layers comprising at least one
ceramic compound which may be deposited by the method of the
invention may have a thickness ranging from 10 .mu.m to 2 mm.
[0152] Moreover, the inventors were able to show that the mixture
obtained within the thermal jet by simultaneous injection of the
solid particles of the n ceramic compounds S.sub.1, . . . , S.sub.n
and of the liquid phase gave the possibility of generating, after
impact on the substrate to be coated, a structured deposit at two
scales and having non-zero porosity, the deposit associating:
[0153] a first network comprising solid particles of the n ceramic
compounds S.sub.1, . . . , S.sub.n, in molten form, and laid out as
lamellas; and [0154] a second network comprising solid particles of
the p ceramic compounds L.sub.1, . . . , L.sub.p, in molten or
non-molten form, which has low mechanical integrity, which is
articulated around the solid particles of the first network, and
which plays the role of a perturbing element of the lamellar layout
of the first network by generating porosity within the deposit.
[0155] The inventors also noticed that the porosity of the
deposited layer(s) was closely related to parameters relating to
the liquid phase, such as the volume proportion of solid particles
of the p ceramic compounds L.sub.1, . . . , L.sub.p and/or of
precursors of these ceramic compounds in the liquid phase, or
further the flow rate with which the liquid phase is injected into
the thermal jet.
[0156] Advantageously, the volume proportion of solid particles of
the p ceramic compounds L.sub.1, . . . , L.sub.p and/or of
precursors of these ceramic compounds in the liquid phase is
comprised between 2% and 20%.
[0157] Advantageously, the ratio of the volume of the solid
particles of the n ceramic compounds S.sub.1, . . . , S.sub.n to
the volume of the solid particles of the p ceramic compounds
L.sub.1, . . . , L.sub.p is comprised in an interval ranging from
0.4 to 3.
[0158] Advantageously, the flow rate with which the liquid phase is
injected into the thermal jet is (0.05.+-.0.03) liters per minute
(L/min).
[0159] Advantageously, said or each of the layers comprising at
least one ceramic compound has a plurality of pores having a size
comprised between 0.001 and 50 micrometers. The physico-chemical
characteristics of the plurality of pores are described later
on.
[0160] The inventors further noticed that by submitting a coating
as the one obtained by the method of the invention, to temperatures
above 1,000.degree. C., typically operating temperatures of the
devices to which are integrated these coatings, the porosity of the
coating was not reduced.
[0161] The inventors actually noticed that consolidation of the
coating was observed at such temperatures. In reality, the
consolidation which is caused by sintering and coalescence
phenomena of the solid particles comprised in the deposit and pores
formed within the deposit, is a reorganization of the material
areas and of the porous areas, without reduction of the total pore
volume.
[0162] The method of the invention thus gives the possibility of
obtaining an erosion-resistant coating, while retaining highly
appreciable mechanical properties at high temperatures. Further, it
gives the possibility of obtaining a coating with controlled
porosity greater than or equal to 20%, which allows the use of the
latter as an abradable coating.
[0163] However, it should be noted that, preferably, the overall
porosity of the coating (i.e. the porosity of the layer(s)
comprising at least one ceramic compound, which is/are deposited by
applying the method of the invention) should not be much greater
than 20%, since a coating having a too high porosity is subject to
too rapid wear of the ceramic abradable material deposit and is
only with difficulty a durable solution for use as an abradable
coating in the aforementioned fields.
[0164] Advantageously, said or each of the layers comprising at
least one ceramic compound has a porosity at least equal to 20%;
preferably at least equal to 20%, and at most equal to 40%, for
example 35%.
[0165] Let us specify that in the case of a multilayer deposit,
each of the layers should have a porosity at least equal to 20%;
and preferentially comprised between 20% and 40%, for example 35%,
so that the whole may be used as an abradable coating.
[0166] Further, it is possible to contemplate alternation of the
deposit of a layer of suitable porosity for an abradable
application, i.e. at least equal to 20%, preferably from 20% to
40%, and of the deposit of a porosity layer not adapted for use as
an abradable coating (for example a porosity of 5%).
[0167] The method of the invention further gives the possibility of
obtaining a structured coating by advantageously controlling other
properties, such as thickness of the homogenous deposit on a
substrate with a complex shape, or further the possibility of a
deposit on any type of substrate, regardless of their nature and
their roughness.
[0168] In the particular case when the ceramic compounds S.sub.1
and L.sub.1 are both mullite, an evaluation of the properties of a
coating R.sub.m obtained by applying the method of the invention
was carried out within the scope of a comparative test with
coatings based on mullite prepared by methods according to the
prior art.
[0169] Thus, for example, a coating R.sub.1 is made on a substrate
consisting of TiAlV (alloy of titanium, aluminum and vanadium) by
blown arc plasma projection of solid mullite particles, but without
any injection of any liquid phase, all the other parameters
remaining moreover identical with those used for making
R.sub.m.
[0170] For example, a coating R.sub.2 is made on a substrate
consisting of TiAlV (alloy of titanium, aluminum, and vanadium) by
blown arc plasma projection of a colloidal aqueous solution
containing precursors of solid particles of mullite, but without
any injection of solid particles of mullite.
[0171] Still for example, a coating R.sub.3 is made on a substrate
consisting of TiAlV (alloy of titanium, aluminum, and vanadium) by
blown arc plasma projection of a mixture made by simultaneous
injection, into the plasma jet, of solid mullite particles on the
one hand and of deionized water on the other hand containing
neither solid mullite particles nor precursors of solid mullite
particles, the injection of the water into the plasma jet being
carried out at a distance D.sub.L from the substrate such that the
following inequality is satisfied: D.sub.S.gtoreq.D.sub.L.
[0172] Comparison of the properties of the coatings R.sub.1,
R.sub.2, R.sub.3 and R.sub.m is carried out and discussed in the
discussion of a particular embodiment of the invention appearing
hereafter.
[0173] The invention is not limited to the embodiment of the method
of the invention which has just been described. Thus, the method of
the invention may be applied several times on a same substrate, the
simultaneous injection into the thermal jet then involving: [0174]
solid particles of n ceramic compounds S.sub.1, . . . , S.sub.n
which may be of different nature, in terms of composition and/or of
greater particle dimensions; for example, n has the value 2, and
the ceramic compounds S.sub.1 and S.sub.2 are mullite and YSZ
oxide; and [0175] a liquid phase comprising a solvent and solid
particles of p ceramic compounds L.sub.1, . . . , L.sub.p and/or of
at least one precursor of the solid particles of the p ceramic
compounds L.sub.1, . . . , L.sub.p which may be of different
nature, in terms of composition and/or of greatest dimension of the
particles,
[0176] and this, in order to make successive layers which comprise
different ceramic materials or else deposits with composition
gradients. These successive depositions of layers are useful for
example in applications such as layers with a heat (conductive and
insulating) property, diffusion barrier layers and/or layers with
controlled porosity.
[0177] Thus, advantageously, the sequence of steps a) and b) of the
method of the invention is repeated once or several times.
[0178] Thus, for example, a coating R.sub.4 is produced by
depositing on the surface of a substrate consisting of TiAlV (alloy
of titanium, aluminum and vanadium), a first layer having the
composition R.sub.1, and then a second layer having the coating
composition R.sub.m according to the invention. Just like for
R.sub.1, R.sub.2, R.sub.3 and R.sub.m, the evaluation of the
properties of R.sub.4 is carried out and discussed in the
discussion of a particular embodiment of the invention appearing
hereafter.
[0179] The projection (spraying) method of the present invention
may easily be industrialized since its specificity and its
innovative nature notably lie in the injection system, which may be
adapted on all thermal projection machines already present in the
industry; in the nature of the species which are simultaneously
injected into the thermal jet; but also in the selection of the
operating conditions imposed to the thermal jet, for obtaining a
structured coating which has the properties of the ceramic
compound(s) composing it.
[0180] Also, the object of the invention is an abradable coating
comprising at least one layer of at least one ceramic compound,
said or each of said layer(s) having a porosity at least equal to
20%, preferably at least equal to 20% and at most equal to 40%,
said layer comprising: [0181] solid particles of n ceramic
compounds S.sub.1, . . . , S.sub.n, n being an integer greater than
or equal to 1, and at least 90% by number of the solid particles of
the n ceramic compounds S.sub.1, . . . , S.sub.n having a greatest
dimension of more than 5 .mu.m; and [0182] solid particles of p
ceramic compounds L.sub.1, . . . , L.sub.p, p being an integer
greater than or equal to 1, and at least 90% by number of the solid
particles of the p ceramic compounds L.sub.1, . . . , L.sub.p
having a greatest dimension of less than or equal to 5 .mu.m.
[0183] The essential point of the characteristics of this coating
has already been described during the description of the method
allowing it to be obtained.
[0184] However, it is recalled that advantageously, each of the n
ceramic compounds S.sub.1, . . . , S.sub.n and of the p ceramic
compounds L.sub.1, . . . , L.sub.p includes at least one element
selected from the Periodic Classification of the Elements from
among transition elements, metalloids and lanthanides.
[0185] Still more advantageously, each of the n ceramic compounds
S.sub.1, . . . , S.sub.n and of the p ceramic compounds L.sub.1, .
. . , L.sub.p is selected from among simple oxides, silicates and
zirconates of at least one element selected from the Periodic
Classification of the Elements from among transition elements,
metalloids and lanthanides.
[0186] Still better, each of the n ceramic compounds S.sub.1, . . .
, S.sub.n and of the p ceramic compounds L.sub.1, . . . , L.sub.p
is selected from among simple oxides, silicates and zirconates of
at least one element selected from among aluminum, silicon,
titanium, strontium, zirconium, barium, hafnium and the elements of
the "rare earth" family as defined by the International Union of
Pure and Applied Chemistry, i.e. scandium, yttrium and
lanthanides.
[0187] Advantageously, each of the n ceramic compounds S.sub.1, . .
. , S.sub.n and of the p ceramic compounds L.sub.1, . . . , L.sub.p
is selected from among ceramic compounds which are customarily used
in the composition of thermal barriers and which have been
mentioned previously in the description of the method of the
invention.
[0188] Advantageously, said or each of the layers comprising at
least one ceramic compound which is/are comprised in the coating
according to the invention has a thickness ranging from 10 .mu.m to
2 mm.
[0189] Advantageously, said or each of the layers comprising at
least one ceramic compound has a plurality of pores having a size
comprised between 0.001 and 50 micrometers, the plurality of pores
being described more specifically as comprising: [0190] a network
of micropores having a size comprised between 0.001 and 1 .mu.m,
said micropore network is defined by the solid particles of the p
ceramic compounds L.sub.1, . . . , L.sub.p for which at least 90%
by number have a greatest dimension of less than 5 .mu.m, [0191]
and which micropore network is included within a network of
macropores having a size comprised between 1 and 50 .mu.m, which
macropore network is defined by the solid particles of the n
ceramic compounds S.sub.1, . . . , S.sub.n for which at least 90%
by number have a greatest dimension greater than or equal to 5
.mu.m.
[0192] Advantageously, said or each of said layers of the coating
as defined earlier always has a porosity at least equal to 20%,
preferably at least equal to 20% and at most equal to 40%, for
example 35%, after submitting the latter to a temperature above
1,000.degree. C.
[0193] The object of the invention is also a substrate having at
least one surface on which deposition of a coating as defined
earlier has been carried out.
[0194] The object of the invention is further a device for applying
the method as defined earlier, the device comprising: [0195] a
torch capable of producing a thermal jet; [0196] a projection gas
reservoir; [0197] a first reservoir, which contains the solid
particles of the n ceramic compounds S.sub.1, . . . , S.sub.n;
[0198] a second reservoir which contains the liquid phase; [0199] a
means for fixing and positioning the substrate with respect to the
torch; [0200] an injection system allowing simultaneous injection
of the solid particles of the n ceramic compounds S.sub.1, . . . ,
S.sub.n and of the liquid phase into the thermal jet generated by
the torch, which injection system independently connects: [0201]
the first reservoir and a first injection means provided at its end
with a nozzle for injecting the solid particles of the n ceramic
compounds S.sub.1, . . . , S.sub.n; and [0202] the second reservoir
and a second injection means provided at its end with a nozzle for
injecting the liquid phase; and [0203] a pressure reducer, which
allows adjustment of the pressure inside the second reservoir.
[0204] Advantageously, the torch is a plasma torch and the thermal
jet is a plasma jet. Examples of plasma-forming gases are given
hereinabove, the reservoirs of these gases are available
commercially. The reasons of these advantageous selections have
been discussed earlier
[0205] Advantageously, the plasma torch is capable of producing a
plasma jet having a temperature ranging from 5,000 to 15,000K.
[0206] Advantageously, the plasma torch is capable of producing a
plasma jet having a viscosity ranging from 10.sup.-4 to 5.10.sup.-4
kg/ms.
[0207] Advantageously, the device of the invention comprises two
reservoirs, the first one containing the solid particles of the n
ceramic compounds S.sub.1, . . . , S.sub.p, the second one
containing the liquid phase being pressurized and comprising the
solid particles of the p ceramic compounds L.sub.1, . . . , L.sub.p
and/or at least one precursor of the solid particles of the p
ceramic compounds L.sub.1, . . . , L.sub.p.
[0208] Advantageously, the device of the invention further
comprises a cleaning reservoir containing a solution for cleaning
the piping and the injection means. Thus, the piping and the
injection means may be cleaned between each application of the
method of the invention.
[0209] The injection system comprises pipes allowing the solid
particles of the n ceramic compounds S.sub.1, . . . , S.sub.n to be
conveyed from the first reservoir to the first injection means. The
same applies for conveying the liquid phase from the second
reservoir to the second injection means.
[0210] The first reservoir which contains the solid particles of
the n ceramic compounds S.sub.1, . . . , S.sub.n is connected to a
carrier gas, which is for example argon, under the effect of which
these particles are conveyed as far as the first injection
means.
[0211] Advantageously, the reservoir which contains the liquid
phase is connected to a compressed air network by means of pipes
and to a compression gas source, for example of compressed air. A
pressure reducer allows adjustment of the pressure inside the
reservoir of the liquid phase, generally at a pressure of less than
or equal to 600 kilopascals (kPa). A pump may also be used. Under
the effect of the pressure, the liquid phase is conveyed as far as
the second injection means through pipes and then leaves the second
injection means, for example as a liquid jet which is mechanically
fragmented as droplets.
[0212] The flow rate and the momentum of the liquid phase at the
outlet of the second injection means notably depends on the
pressure in the reservoir used and/or of the pump, on the
characteristics of the dimensions of the nozzle of the injections
means, and on the rheological properties of the liquid phase (for
example, the mass proportion of solid particles of the p ceramic
compounds L.sub.1, . . . , L.sub.p and/or of precursors of these
ceramic compounds).
[0213] Both injection means allow injection of the solid particles
of the n ceramic compounds S.sub.1, . . . , S.sub.n and of the
liquid phase into the thermal jet.
[0214] According to the invention, the device may be provided with
a number of injection means of more than two, for example depending
on the amounts or on the composition of the solid particles of the
n ceramic compounds S.sub.1, . . . , S.sub.n and of liquid phase to
be injected.
[0215] Advantageously, the injection of the solid particles of the
first ceramic compound and of the liquid phase is carried out with
an angle .alpha. with respect to the longitudinal axis of the
thermal jet. In other words, and advantageously, the angles
.alpha..sub.S and .alpha..sub.L as defined earlier in connection
with the method are comprised between 70.degree. and 105.degree.,
for example 90.degree..
[0216] According to the invention, the line for injecting the solid
particles of the first ceramic compound and the liquid phase may be
thermostated so as to control, and optionally modify, the injection
temperature of the latter. This control of the temperature and this
modification may be achieved at the pipes and/or at the reservoirs
(or compartments).
[0217] According to the invention, the device may comprise a means
for attachment and displacement of the substrate with respect to
the torch.
[0218] This means may consist in clamps, screws, adhesives or an
equivalent system allowing the substrate to be attached and
maintained during the thermal projection in a selected position,
and in a means giving the possibility of displacing in rotation and
in translation the surface of the substrate facing the thermal jet
and in the longitudinal direction of the plasma jet. Thus, it is
possible to optimize the position of the surface to be coated, with
respect to the thermal jet, in order to obtain a homogenous
coating.
[0219] Thus, the invention gives the possibility of carrying out
direct and simultaneous injection by means of a well adapted
injection system, for example by using the device of the invention,
for solid particles of the first ceramic compound and a liquid
phase containing at least one second ceramic compound, the nature
of the injected elements and the simultaneity of the injections
contributing to the formation of a ceramic coating having a
porosity of more than 20%.
[0220] Other characteristics and advantages of the invention will
become apparent from the additional description which follows,
which relates to an exemplary embodiment of the method of the
invention and to tests for evaluating the properties of a coating
R.sub.m according to the invention, this additional description
referring to the appended figures.
[0221] It is obvious that these examples are only given as an
illustration of the objects of the invention and by no means are a
limitation of these objects.
[0222] For the sake of clarity, the dimensions of the different
elements illustrated in FIGS. 1, 3 and 12 are not in proportion
with their actual dimensions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0223] FIG. 1 shows a simplified diagram of a device for applying
the method of the invention allowing simultaneous injection of the
solid particles of at least one first ceramic compound and the
liquid phase into a plasma jet, with a schematic illustration of
the plasma torch.
[0224] FIG. 2 illustrates the grain size analysis of the solid
mullite particles as used in a particular embodiment of the method
according to the invention, the cumulative refusal "RC" versus the
aperture O.
[0225] FIG. 3 is a schematic illustration of the microscopic
structure of a section of a coating according to the invention and
not subject to a heat treatment after the thermal projection, this
section being made according to a plane perpendicular to the
surface of the coating.
[0226] FIG. 4 is a micrograph obtained by optical microscopy (OM)
of a polished section of a coating according to the invention and
not subject to a heat treatment after thermal projection; this
section is made along a plane perpendicular to the surface of the
coating.
[0227] The scale plotted on FIG. 4 represents 100 .mu.m.
[0228] FIG. 5 is an enlarged micrograph by OM of the micrograph of
FIG. 4.
[0229] The scale plotted on FIG. 5 represents 50 .mu.m.
[0230] FIG. 6 is a micrograph obtained by scanning electron
microscopy (SEM) with a detector of backscattered electrons of a
polished section of a coating according to the invention, and
produced along a plane perpendicular to the surface of the
coating.
[0231] FIG. 7 is a micrograph obtained by OM of a polished section
of a coating R.sub.1 as described earlier, and produced along a
plane perpendicular to the surface of the coating.
[0232] The scale plotted on FIG. 7 represents 50 .mu.m.
[0233] FIG. 8 is a micrograph obtained by SEM of a fracture of the
coating R.sub.1 as described earlier.
[0234] The fracture is a section obtained by brittle fracture of
the coating, it allows observation of the microstructure in a
section without any polishing.
[0235] FIG. 9 is a micrograph obtained by SEM of a fracture of a
coating R.sub.2 as described earlier.
[0236] FIG. 10 is an enlarged micrograph by SEM of the micrograph
of FIG. 9.
[0237] FIG. 11 is a micrograph obtained by OM of a polished section
of a coating R.sub.4 as described earlier, and produced along a
plane perpendicular to the surface of the coating.
[0238] The scale plotted on FIG. 11 represents 50 .mu.m.
[0239] FIG. 12 is a schematic illustration of the microscopic
structure of a section of a coating according to the invention
after having been subject to heat treatment at a temperature of
1,300.degree. C. after thermal projection, this section being
produced along a plane perpendicular to the surface of the
coating.
[0240] FIGS. 13, 14 and 15 are micrographs obtained by OM (for
FIGS. 13 and 14) or by SEM (for FIG. 15) of polished sections of
the coatings respectively presented in FIGS. 4, 5 and 6 subject to
heat treatment at a temperature of 1,300.degree. C. carried out
after thermal projection; these sections are produced along a plane
perpendicular to the surface of each of the coatings.
[0241] The scale plotted on FIG. 13 represents 100 .mu.m.
DETAILED DESCRIPTION OF A PARTICULAR EMBODIMENT
[0242] The following numbered sections relate to the description of
a particular embodiment of the invention.
[0243] In a first phase, it is proceeded with the description of an
embodiment of the method of the invention, and with the making of a
coating R.sub.m made of an abradable ceramic material according to
the invention.
[0244] It is then proceeded with the comparison of the porosity of
the coating R.sub.m with those of the coatings R.sub.1, R.sub.2 and
R.sub.3 prepared in accordance with methods according to the prior
art.
[0245] Finally it is proceeded with the evaluation of the stability
of the coating R.sub.m after having been subject to heat treatment
at a temperature of 1,300.degree. C.
[0246] 1. Method of the Invention and Production of a Coating
R.sub.m According to the Invention
[0247] In a particular embodiment of the invention, solid mullite
particles and a liquid phase appearing as a colloidal aqueous
solution comprising precursor compounds of solid mullite particles
are injected simultaneously into a blown arc plasma of an
argon-helium-dihydrogen ternary mixture, the composition of which
is specified hereafter.
[0248] 1.1. Method of the Invention
[0249] First of all reference is made to FIG. 1, which
schematically illustrates the experimental assembly which gave the
possibility of producing mullite deposits. This assembly consists
of: [0250] a DC current plasma torch Sulzer Metco F4VB.RTM.
equipped with an anode of an internal diameter of 6 mm, 10; [0251]
a TiAlV substrate, 11; [0252] a device 12 allowing attachment and
displacement of the substrate 11 to be coated with respect to the
plasma torch 10 at a given distance; and [0253] a system 13 for
injecting solid particles of mullite and a colloidal aqueous
solution comprising precursor compounds of solid mullite
particles.
[0254] Firstly, the injection system 13 involves a first reactor 14
consisting of the solid mullite particles 15 which are from the
reservoir 17. The assembly formed with the reactor 14 and the
reservoir 17 is of the type of the one of distributors of solid
particles which are marketed by Sulzer-Metco.
[0255] The grain size analysis of the solid mullite particles 15 is
conducted by laser grain size measurement by means of a Mastersizer
2000 apparatus (Malvern), and is illustrated in FIG. 2.
[0256] As this may be determined by reading the data of FIG. 2, the
cumulated refusals relating to a greatest dimension of the
particles of 49.0; 27.6 and 10.5 .mu.m respectively have the values
of 10; 50 and 90%. In other words, 10%, 50% and 90% by number of
the solid mullite particles 15 respectively have a greatest
dimension of more than 49.0; 27.6 and 10.5 .mu.m.
[0257] During the tests, the solid mullite particles 15 are driven
out of the reactor 14 under the effect of a carrier gas flow, in
this case argon, with a flow rate of 4-10.sup.-3 cubic metres cubes
per minute (m.sup.3/min), the provision of which is ensured via a
supply pipe 19. The solid mullite particles 15 are then conducted,
via an outlet pipe 20, from the reactor 14 to a first injection
means 21 which has an injection nozzle 22 at its end.
[0258] Secondly, the injection system 13 involves a second reactor
23, intended for mixing a liquid phase which comprises precursor
compounds of solid mullite particles. The liquid phase is in this
case a colloidal aqueous solution 24 comprising precursor compounds
of solid mullite particles.
[0259] A colloidal aqueous sol of mullite is prepared.
[0260] The colloidal aqueous solution 24 which is placed in the
reactor 23 has a mass proportion of precursor compounds of solid
mullite particles with a value of 15%. It is then homogenized by
means of a magnetic stirring device 25.
[0261] The second reactor 23 is also equipped with a pressure
reducer 26 which allows adjustment of the pressure inside the
latter, and which is connected to a compression gas, here
compressed air, the supply of which is ensured via a pipe 27.
[0262] The second reactor 23 is further equipped with a valve 28,
as well as with a pipe 29 connecting the inside of the reactor 23
to a reservoir 30 containing a cleaning liquid 31, here deionized
water.
[0263] During the tests, the valve 28 is closed and the colloidal
aqueous solution 24 is driven out of the reactor 23 under the
effect of a pressure of 300 kPa which is imposed by the pressure
reducer 26 and the compression gas circulating via the pipe 27. The
colloidal aqueous solution 24 is then conducted, via an outlet pipe
32, from the reactor 23 to a second injection means 33 which has an
injection nozzle 34 at its end.
[0264] The simultaneous injection of the solid mullite particles
15, and of the colloidal aqueous solution 24 is achieved in a
plasma jet 35, generated by a blown arc plasma at an intensity of
650 amperes (A) and stemming from the plasma torch 10 through the
projection nozzle 36, the latter being located at a distance D of
100 millimeters (mm) with respect to the substrate 11.
[0265] The plasma-forming gas from which the plasma jet 35 is
generated, is a ternary mixture consisting in volume proportions of
50.8% of argon, 23% of helium and 8% of dihydrogen.
[0266] On the one hand, the injection of the solid particles of
mullite 15 into the thermal jet 35 is produced via the outlet
orifice of the injection nozzle 22 of the first injection means 21,
with a diameter of 1.5 mm, which implies, upon considering the
previous data, a flow rate of solid mullite particles 15 of 15
grams per minute (g/min). This injection is carried out with an
angle .alpha..sub.S formed by the directions of the tilt axis of
the first injection means 21 and of the longitudinal axis of the
plasma jet 35, having the value 90.degree., and at a distance
D.sub.S of 94 mm with respect to the substrate 11.
[0267] On the other hand, the injection of the colloidal aqueous
solution 24 into the thermal jet 35 is carried out via the outlet
orifice of the injection nozzle 34 of the second injection means
33, with a diameter of 250 .mu.m. This injection is carried out
with an angle .alpha..sub.L formed by the directions of the tilt
axis of the second injection means 33 and of the longitudinal axis
of the plasma jet 35, having the value 90.degree., and at a
distance D.sub.L of 80 mm with respect to the substrate 11.
[0268] 1.2. Coating R.sub.m According to the Invention
[0269] By applying the method according to the invention as
described in point 1.1., a coating R.sub.m according to the
invention and based on mullite is obtained.
[0270] The coating R.sub.m is obtained on a substrate 11 consisting
of TiAlV, which is located both: [0271] at a distance D of 100 mm
from the projection nozzle 36 of the plasma torch 10; [0272] at a
distance D.sub.S of 94 mm from the injection point of the solid
mullite particles 15 into the plasma jet 35; and [0273] at a
distance D.sub.L of 80 mm from the injection point of the colloidal
aqueous solution 24 into the plasma jet 35.
[0274] Depending on the duration of the plasma projection, the
thickness of the obtained deposits is comprised between 50 and
1,000 .mu.m.
[0275] FIG. 3 is a schematic illustration of the structure of the
coating R.sub.m, which includes solid mullite particles 37 defining
a network 38 of macropores with a size comprised between 1 and 50
.mu.m and said macropores being at least partly occupied by solid
mullite particles which are generated within the plasma jet 35 from
mullite precursors contained in the colloidal aqueous solution 24,
and which define a network 39 of micropores with a size comprised
between 0.001 and 1 .mu.m.
[0276] The micrographs shown in FIGS. 4, 5 and 6 show the
microstructure of the coating R.sub.m according to the
invention.
[0277] In particular, the micrograph of FIG. 6 produced by SEM
allows observation of a structured deposit with two networks of
pores (macro- and micro-pores) like those having just been
described for making comments on FIG. 3.
[0278] The network 39 of micropores has low mechanical integrity,
perturbs the layout of the particles 37 and significantly
contributes to the overall porosity of the coating R.sub.m.
[0279] 2. Evaluation of the Properties of R.sub.m as Compared with
Those of R.sub.1, R.sub.2 and R.sub.3
[0280] 2.1. Comparison of the Properties of R.sub.1, R.sub.2 and
R.sub.3 with Those of R.sub.m
[0281] Three mullite-based coatings R.sub.1, R.sub.2 and R.sub.3
prepared by applying methods of the prior art, in order to compare
the properties of these coatings with those of the coating R.sub.m
according to the invention, notably in terms of porosity.
[0282] Preparation of R.sub.1, R.sub.2 and R.sub.3
[0283] The plasma projection parameters which are used for
producing R.sub.1, R.sub.2 and R.sub.3 are identical with those
used for producing R.sub.m. The only modified parameter is the
nature of the compounds which are injected into the plasma jet 35,
before impact on the substrate 11 on which the coating is
applied.
[0284] Thus, R.sub.1 is produced by blown arc plasma projection of
solid mullite particles 15, but without any injection of a liquid
phase into the plasma jet 35.
[0285] R.sub.2 is produced by blown arc plasma projection of a
colloidal aqueous solution 24 which contains precursors of solid
mullite particles, but without any injection of solid mullite
particles 15 into the plasma jet.
[0286] R.sub.3 is produced by blown arc plasma projection of a
mixture obtained within the plasma jet 35, by simultaneous
injection of solid mullite particles 15, and of deionized water
containing neither mullite solid particles, nor precursors of solid
mullite particles.
[0287] The injection of deionized water into the plasma jet 35 is
produced at a distance D.sub.L from the substrate such that the
following inequality is satisfied: D.sub.S.gtoreq.D.sub.L.
[0288] The overall porosity of the coatings R.sub.1, R.sub.2,
R.sub.3 and R.sub.m is determined by the hydrostatic thrust method,
according to the NF EN 623-2 standard (entitled <<Advanced
technical ceramics--Monolithic ceramics--General and textural
properties>>, in particular the vacuum method no. 1 of the
part 2 entitled: <<Determination of the density and of the
porosity>>).
[0289] The results of the global porosity measurements are shown in
Table 1.
TABLE-US-00001 TABLE 1 Coating R.sub.1 R.sub.2 R.sub.3 R.sub.m
Global porosity 7 -- 13 35
[0290] Overall Porosity of R.sub.1
[0291] The overall porosity of 7% measured for R.sub.1 is low and
characteristic of a coating obtained by plasma projection of solid
particles on a substrate, without any liquid phase injection.
[0292] This relatively low global porosity is expressed in the
coating by a dense distribution of solid mullite particles 15 in
the molten state, as observed on the micrograph obtained by OM
which is shown in FIG. 7. The lamellar and compact geometry of the
solid mullite particles is particularly visible in SEM (mark 40,
FIG. 8).
[0293] Overall Porosity of R.sub.3
[0294] The overall porosity measured for R.sub.3 is 15%, i.e.
nearly twice that of R.sub.1.
[0295] The deionized water which is injected into the plasma jet 35
seems to form a perturbing element of the lamellas of solid mullite
particles 15 which are deposited on the substrate 11. The
perturbation is then a factor which increases the overall porosity
of the coating.
[0296] Overall Porosity of R.sub.2
[0297] The coating R.sub.2 which is obtained is finely structured
as a highly porous network.
[0298] Overall Porosity of R.sub.m
[0299] The overall porosity of the coating R.sub.m according to the
invention is 35%, and is thus even more significant than those of
R.sub.1, and R.sub.3.
[0300] In the same way as for R.sub.3, the elements of the mixture
obtained within the plasma jet 35 seem to form perturbing elements
of the network of lamellas of solid mullite particles 15 found
within the coating R.sub.m, these elements being: [0301] deionized
water which is a solvent of the colloidal aqueous solution 24;
[0302] but also the solid mullite particles conveyed by the
colloidal aqueous solution 24 and generating a porous network 39
with low mechanical cohesion.
[0303] The micrographs of FIGS. 4 to 6 actually show that the
overall porosity of the coating R.sub.m and, for the abradable
nature of this coating, are in majority or even exclusively
generated by the network 39 of micropores, while the solid mullite
particles 37 which are from the first injection means define a
network of macropores 38 with a greater size.
[0304] 2.2. Coating R.sub.m Comprising at Least One Layer of a
Ceramic Material
[0305] A mullite-based coating R.sub.4, the micrograph of which
obtained by OM is shown in FIG. 11, is prepared: [0306] by
depositing a first ceramic layer 41 comprising solid mullite
particles, the layer having the composition of the coating R.sub.1
and being made by a technique as described above; and then [0307]
by depositing on the first ceramic layer 41, a second ceramic layer
42 comprising solid mullite particles, the layer having the
composition of the coating R.sub.m according to the invention and
being made by applying the method according to the invention.
[0308] A coating with "hybrid" properties is thus produced, the
latter associating: [0309] a layer 41, which has a very compact
distribution of solid mullite particles in the molten state, as
observed within R.sub.1 (FIG. 8); and [0310] a layer 42, which is
characterized by the existence of a much more porous structure,
consisting of solid mullite particles of different dimensions and
as observed within R.sub.m (FIGS. 4 to 6).
[0311] 3. Evaluation of the Properties of R.sub.m after Heat
Treatment at 1,300.degree. C.
[0312] It is proceeded with evaluating the stability of R.sub.m at
high operating temperatures of the devices including a substrate on
which the coating is deposited, the relevant temperatures being
typically above 1,000.degree. C.
[0313] To do this, the coating R.sub.m applied on a substrate
consisting of TiAlV is subject to a 24 hour heat treatment at a
temperature of 1,300.degree. C.
[0314] FIG. 12 is a schematic illustration of the microstructure of
the coating R.sub.m after thermal treatment, which includes a first
network of pores 44, formed within the stack of the solid mullite
particles in molten form 43. Around pores 44, is organized a
network 45 of pores, of smaller size, which stems from the
reorganization, at the end of the heat treatment, of the pore
network 39 (FIG. 3).
[0315] The micrographs shown in FIGS. 13, 14 and 15, which
correspond to the structures shown in FIGS. 4, 5 and 6 respectively
after thermal treatment, show the reorganized microstructure of
R.sub.m.
[0316] The micrograph of FIG. 15 (produced by SEM) allows
observation of a structured deposit with two networks of pores
(macro- and micro-pores), which includes solid mullite particles in
molten form 43 defining a network of macropores 44 and said
macropores being at least partly occupied by solid mullite
particles which are generated within the plasma jet 35 from
precursors of mullite contained in the colloidal aqueous solution
24, and which define a network 45 of micropores.
[0317] The network 45 of micropores has low mechanical integrity,
perturbs the layout of the network of macropores 44 and
significantly contributes to the overall porosity of the coating
R.sub.m.
[0318] By comparing the micrographs of FIGS. 6 and 15, it is noted
that the reorganization of the structure of R.sub.m at the end of
the heat treatment is expressed by coalescence and/or crushing of
solid mullite particles 43, of the macropores 44 and of the network
45 of micropores within the coating.
[0319] This being the case, the determination of the overall
porosity of R.sub.m after heat treatment does not give the
possibility of detecting any significant phenomenon of
densification of the coating, the overall porosity of R.sub.m
remains unchanged and has the value 35%.
[0320] Thus, it emerges from these studies that the consolidation
of the coating, via sintering mechanisms beneficial to the increase
in the erosion resistance, does not generate any reduction of the
overall porous volume and allows the abradable nature specific to a
coating R.sub.m according to the invention to be preserved.
QUOTED REFERENCES
[0321] [1] U.S. Pat. No. 3,084,064. [0322] [2] U.S. Pat. No.
3,879,831. [0323] [3] Patent applications FR-A1-2877015 and
US-A1-2008/0090071 [0324] [4] U.S. Pat. No. 4,269,903. [0325] [5]
U.S. Pat. No. 4,936,745. [0326] [6] U.S. Pat. No. 5,434,210. [0327]
[7] Patent application FR-A1-2 832 180. [0328] [8] U.S. Pat. No.
4,696,855. [0329] [9] US Patent application 2008/0167173. [0330]
[10] US Patent application 2010/0015350. [0331] [11] "Red Book"
terminological base of the International Union of Pure and Applied
Chemistry, section IR-3.5:
http://old.iupac.org/publications/books/rbook/Red_Book.sub.--2005.pdf.
[0332] [12] Kirk-Othmer, Concise Encyclopedia of Chemical
Technology, 1985, Wiley-Interscience.
* * * * *
References