Note: Descriptions are shown in the official language in which they were submitted.
CA 02601189 2013-11-21
P.7599/Ke/Pa
Sulzer Metco AG, CH-5610 Wohlen, (Switzerland)
A method for the manufacture of a coating having a columnar
structure .
The invention relates to a method for the manufacture of a coating
having a columnar structure
as well as to a spray powder with which the method can be carried
out.
Large areas can be provided uniformly with thin films using a special
LPPS process ("low pressure plasma spraying process"), that is an
LPPS TF process ("LPPS thin film process"). This process, known for
example from US-A- 5,853 815, is a plasma spraying process. The
manufacture of a uniform coating is achieved by a spray gun having a
geometrically suitable design. A substrate to be coated is put into a
process chamber in which a pressure lower than 10 kPa is
established, whereas a pressure of, for example, around 100 kPa
(approximately ambient pressure) is present in the spray gun. The
pressure drop between the interior of the spray gun and the process
26 chamber has the effect that the thermal process beam expands to a
broad beam in which the material to be sprayed is unifoi July
distributed: it is a thermal process beam which is generated by the
flame of a defocusing plasma beam. A dense layer can be deposited
over a relatively large area in a single passage by means of a process
beam widened in this manner. Coatings having special properties can
be generated by multiple deposition of such layers. Coating
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thicknesses in the micrometre range can be in particular be
generated.
In a special LPPS TF process, a hybrid coating is carried out using the
thermal process beam. This process, which is known from EP-A 1 034
843 (= P.7192) or from EP-A- 1 479 788 (= P.7328), permits a thermal
spraying to be combined with a vapour phase deposition and so to
unify the possibilities of both methods. The properties of the process
beam are determined by controllable process parameters, in particular
by the parameters of pressure, enthalpy, composition of a process gas
mixture and composition and form of application of the material to be
sprayed. A thermal barrier coating (TBC) with a columnar
microstructure can be manufactured using the hybrid coating
method. This coating or layer is approximately composed of cylindrical
or spindle-like particles whose central axes are aligned perpendicular
to the substrate surface. This columnar layer with an anisotropic
microstructure is stretch tolerant with respect to a thermal strain
variation, i.e. to changing strains, which result from repeatedly
occurring temperature changes. The coating reacts to the changing
strains in a largely reversible manner, i.e. without any formation of
cracks, so that its service life is considerably extended in comparison
with the service life of a coating which does not have a columnar
microstructure.
The described plasma spraying method is a preferred coating method.
Instead of a plasma beam, another thermal process beam could also
be used to manufacture a coating having a columnar structure as well
as a dense structure, if the coating material can be vaporised using
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such a thermal process beam. The invention described in the following
includes this generalisation. Examples for further thermal process
beams are: electron beams, flames of reactive gas mixtures, electrical
arcs, laser beams.
It is the object of the invention to provide a method for the
manufacture of a coating having a columnar structure in which a
necessary vaporisation of coating material and deposition can be
carried out more efficiently than in the known methods.
In the method for the manufacture of a coating (10) having a
columnar structure, preferably a dense structure, a coating material
in the form of primary corpuscles (1) is injected with a carrier gas into
a thermal process beam. The coating material is transformed in the
process beam into a vapour phase and is deposited as condensate in
the form of a columnar coating on a substrate (100). The primary
corpuscles are liquid droplets or they are in each case formed by an
agglomerate of particles which are held together by cohesive forces of
a connecting medium or by adhesive forces. The liquid droplets
include a chemical precursor of the coating material in the form of a
salt solution and are transformed by thermal action in the process
beam into secondary corpuscles containing particles (2). The primary
or secondary corpuscles are disintegrated in the process beam by
mechanical and thermal interaction. In this connection, the particles
are dispersed so that coating material is vaporised fully or partly by
thermal action on the individual particles.
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Dependent claims 2 to 8 relate to advantageous embodiments of the
method in accordance with the invention. A spray powder for this
method is the subject of claims 9 to 14.
The invention will be explained in the following with reference to the
drawings. There are shown:
Fig. 1 a section of a coating which was manufactured using the
method in accordance with the invention;
Fig. 2 a schematic representation of an agglomerate which is a
corpuscle composed of particles;
Fig. 3 an illustration for the disintegration of the
agglomerate;
Fig. 4 a measured size distribution of the corpuscles of a
spray
powder in accordance with the invention;
Fig. 5 a segment of a turbine having two turbine vanes; and
Fig. 6 a section through the segment in Fig. 4 parallel to the
base plate.
In the example of a coating 10 which is manufactured in accordance
with the invention using a plasma spraying method and which is
shown as a narrow section in Fig. 1, the coating 10 applied to a
substrate 100 has a thickness of approximately one third of a
millimetre. The section shown is drawn in the manner of a
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micrograph. Slit-like gaps 13, which are characteristic for a columnar
structure, are disposed in the direction of the axes between elongate
zones 12 whose axes are substantially perpendicular to the substrate
100. The coating material is in a state between the gaps 13 which is
dense thanks to a low porosity. The porosity in relation to the total
coating 10 is lower than 10 to 15%.
This coating 10 is generated with a spray powder which has the size
distribution of corpuscles 1 documented in Fig. 4. The corpuscles 1
are composed of particles 2 to form an agglomerate such as is shown
schematically in Fig. 2. The illustration shows a two-dimensional
model of a corpuscle 1. The particles 2 drawn as polygons represent
grains which have been generated, for example, from homogeneous
solids by breaking and milling. The corpuscles 1 are each formed by
an agglomerate of particles 2 which are held together by cohesive
forces 4 of a connection medium 3 (binder). The medium 3 is present
on the surfaces of the particles 2 as a thin film and as bridges
communicating cohesive forces 4 between the particles 2.
In the plasma spraying process for the manufacture of the coating 10
having a columnar structure as well as a dense structure, the thermal
process beam is generated by a flame of a defocusing plasma beam (cf.
EP-A- 1 034 843). A coating material in the form of the corpuscles 1 is
injected with a carrier gas into the plasma beam. The properties of the
plasma beam are determined by adjustable process parameters, in
particular by the parameters of process pressure, enthalpy and
composition of a process gas mixture. The method in accordance with
the invention can also be carried out using other thermal process
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beams than the plasma beam. A further generalisation relates to the
shape of the corpuscles 1. Primary corpuscles are distinguished from
secondary ones:
The primary corpuscles 1 are formed by liquid droplets or in each case
by an agglomerate of particles 2 which are held together by cohesive
forces 4 of a connecting medium 3 (binder) or by adhesive forces. The
adhesive forces are exerted directly between the particles without any
binder, with a mechanical inter-engagement of surface structures (for
example dendrites) being able to effect these forces. Binder-free
connections between the particles 2 can also be generated by
calcination at a low calcinating temperature and/or with a short
treatment period. At this temperature or with this period, weak
sintered connections are formed at contact points between the
particles 2 which can also be understood as adhesive forces.
The droplet-like corpuscles 1 contain a chemical precursor of the
coating material in the form of a salt solution and are transformed by
thermal action in the process beam into the secondary corpuscles
containing particles. Such a salt is, for example, zirconium nitrate
which is transformed into zirconium oxide while splitting off nitric
oxides.
The primary or secondary corpuscles are disintegrated in the process
beam by mechanical and thermal interaction and the particles are
dispersed so that coating material is melted and vaporised fully or
partly by thermal action on the individual particles. A thermal
decomposition or vaporisation of the connecting medium 3 results
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,
from the thermal interaction. The disintegration of the corpuscles 1 is
illustrated by Fig. 3. The medium 3 is thermally eliminated at the
surfaces 20 of the peripheral particles 2 so that the particles 2 are
separated by mechanical interaction and can be efficiently vaporised
as dispersed particles 2* by the flame. The mechanical interaction
results due to the viscosity of the plasma by shear forces between the
plasma and the corpuscles 1.
The spray powder of the plasma spraying method is an aggregate of
the primary corpuscles 1 which are formed in each case by an
agglomerate of the particles 2. These primary corpuscles 1 can be
generated by spraying of a slurry or slip. The slurry is made from the
particles 2, from a liquid, a binder and, where necessary, a dispersing
agent. The sprayed slurry is dried and the spray-dried material is
used as a spray powder (with a post-treatment as a rule being
necessary by further communition and sifting). The binder has been
dissolved in the liquid of the slurry at a high dilution so that the
cohesive forces 4 generated by the binder after the drying only effect a
minimal holding together of the particles. The disintegration capability
of the corpuscles 1 in the process beam can thereby be implemented.
Spray-dried spray powder can be manufactured as follows, for
example. Powder-form coating material is slurried with a suitable
liquid, preferably deionised water, and an organic binder to form a
slurry. CMC (carboxy methyl cellulose) is a preferred binder. However,
other known binders can also be used, for example PVA (polyvinyl
alcohol) or MC (methyl cellulose). In a preferred embodiment of the
slurry, the binder portion therein amounts to between approximately
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0.5 and 5% by weight with respect to the dry weight of the coating
material. The viscosity of the slurry influences the size of the spray-
dried corpuscles so that this size can be varied simply by changing the
liquid portion. The slurry is a suspension, which can be stabilised
using a dispersing agent, for example using "Nopcosperse" (around 2%
by weight).
The powder-form coating material can be gained by breaking of a
material present in block form which is manufactured, for example,
from a powder generated chemically and by precipitation and
subsequently sintered. The powder gained in this manner consists of
edged grains which are substantially more compact than the particles
of the original powder. The compaction can be carried out in an
induction oven or in an electrical arc.
The size distribution of the primary corpuscles 1 in the diagram of Fig.
4 can be represented by a bell curve. (X = cumulated volume portions;
AX = differential volume portions). This curve can be characterised by
three parameters, namely the maximum diameters Di (i = 10, 50, 90)
for the volume portions X10%, X50% and X90% which each include
10, 50 or 90% by volume of the primary corpuscles 1.
The following ranges apply to the diameters of a spray powder in
accordance with the invention and to the corresponding volume
portions:
for X10%: D10 < 10 pm; for X50%: D50 < 20 pm; for X90%:
Dgo < 40 pm.
In Fig. 4, D10 = 1.83 pm, D50 = 4.94 pm and DgO = 12.60 pm.
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It is important for the invention that the primary corpuscles 1 are not
too large. The corpuscles 1 may also not be too small so that the
injecting into the process beam can take place without complications.
The diameters should be less than 35 pm - corresponds to around
- 400 mesh - and should be larger than around 5 pm. The diameters
for the particles 2 of the primary corpuscles 1 lie in the range between
0.1 and 5 pm. In the extreme case, a primary corpuscle 1 can
comprise only one particle 2.
Three options are distinguished on how the coating material can be
injected into the process beam: I) in the form of slurry droplets; II) in
the form of droplets of the salt solution explained above; and III) as
spray powder in the form of solid agglomerates. It in particular
applies:
I) In the case of slurry droplets, capillary forces of a liquid form the
cohesive forces. In this connection, this liquid, which as a rule
contains a dispersing agent (but not a binder), has been used for the
slurrying of the particles and for the generation of the slurry. The
spraying of the slurry is carried out directly before the entry into the
plasma beam.
II) The spraying of the salt solution is carried out directly before the
entry into the process beam so that the particles of the secondary
corpuscles are generated in the process beam.
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III) If the coating material is used as a spray powder, the binder
portion after the drying of the slurry droplets should amount to a
maximum of 5% by weight and a minimum of 0.5% by weight. The
binder portion preferably lies in the range between 1 and 2% by
weight.
The following materials can be used, for example, for the slurry:
as the liquid, demineralised water or an organic solvent, in particular
an alcohol;
as the dispersing agent, polycarbonic acid, a polycarboxylate
compound or a polymetacarboxylate compound, polyethyleneimines or
an amino alcohol;
and as the binder, polyvinyl alcohol, polyvinyl pyrolidine,
polysaccharide, acrylic polymers and copolymers, starch, polyvinyl
propylene, polyethylene glycols or a cellulose compound, for example
carboxy methyl cellulose, methyl cellulose or hydroxyethylcellulose.
If the method in accordance with the invention is carried out as a
plasma spraying process, this is done in accordance with the following
specifications:
a) a value is selected for the process pressure between 50 and 5,000
Pa, preferably between 100 and 500 Pa. The specific enthalpy of the
plasma beam is generated by delivering an effective power which is to
be determined empirically and which lies, according to experience, in
the range from 20 to 100 kW, preferably 40 to 80 kW.
b) The process gas includes a mixture of inert gases, in particular a
mixture of argon Ar and helium He, and furthermore, optionally,
hydrogen, nitrogen and/or a reactive gas, with the volume ratio of Ar
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to He advantageously lying in the range from 2: 1 to 1 :4 and the total
gas flow lying in the range from 30 to 150 SLPM.
c) The primary corpuscles 1 are injected at a conveying rate between 5
and 60 g/min, preferably between 10 and 40 g/min.
d) The substrate is preferably moved relative to a cloud of the
vaporised coating material during the material application, in
particular by rotary or pivot movements and/or by translatory
movements.
A coating material is used whose portion which can be vaporised
amounts to at least 70%. The plasma beam is generated with a
sufficiently high specific enthalpy so that at least 5% of the coating
material, preferably at least 50%, is transformed into the vapour
phase during vaporisation.
The particles 2 form a homogeneous or heterogeneous mixture with
materials which are the same or different in the primary corpuscles 1.
The particles 2 can form a mixture of materials which react chemically
in the process beam after the vaporisation at least partly with one
another or with a reactive gas of the process gas mixture. The reaction
products are condensed out during coating.
In the manufacture of a TBC coating having a columnar structure, an
advantageous connection of the coating to the substrate arises onto
which the columnar coating is applied. Whereas a large-area peeling
of the coatings caused by thermal strain variation is observed with
non-columnar coatings, the same thermal strain variation results in
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milder damage with the columnar coating: A dandruff like
precipitation of relatively small-area coating islands is created.
In a preferred process management, regions of the substrate 100 are
coated which are located in the geometrical shadow of the process
beam.
Thermal spray processes are usually so-called "line-of-sight
processes", that is the substrate 100 is only coated where the thermal
process beam impacts directly. Regions which are located in the
geometrical shadow, that is are not directly exposed to the process
beam, are not coated in such processes.
It is, however, also possible with the method in accordance with the
invention to coat regions of the substrate 100 which are not directly
exposed - in the geometrical sense - to the process beam. That is,
"non-line-of-sight" coating can also be carried out with the method in
accordance with the invention. Coating can take place so-to-say
around the corner. This should be explained in the following with
respect to Fig. 5 and to Fig. 6.
Fig. 5 shows, in a very simplified representation, a segment of a
turbine which is designated in total by the reference numeral 50. Fig.
6 shows this segment 50 in a sectional presentation, with the cut
taking place parallel to a base plate designated by 51 in Fig. 5.
The turbine, for example, a gas turbine, usually includes a plurality of
rotating impellers and stationary guide elements. Both the impellers
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and the guide elements each include a plurality of turbine vanes 52.
The turbine vanes 52 can each be mounted individually at their foot to
a common axle of the turbine or they can be provided in the form of
segments which each include a plurality of turbine vanes 52. This
configuration is frequently called a cluster-vane segment or,
depending on the number of turbine vanes, a double-vane segment, a
triple-vane segment, etc.
In Fig. 5 and in Fig. 6, there is shown in a very simplified
representation such a segment 50 of a gas turbine which includes two
turbine vanes 52 which each extend from the base plate 51 up to a
cover plate 53. The segment 50 can be in one piece or consist of a
plurality of individual parts. The presentation of details known per se
such as cooling air bores or cooling air passages has been omitted in
Figs. 5 and 6 for reasons of better clarity.
In such substrates 100 such as the segment 50, geometrical shadow
regions or hidden or covered regions exist which cannot be acted on
directly - in the geometrical sense- by the process beam. Such regions
are designated by the reference symbol B in Fig. 6. It is frequently the
case that such regions B can also not be reached due to a rotation of
the substrate 100 in the process beam or due to another relative
movement between the process beam and the substrate.
A coating can also be manufactured using the method in accordance
with the invention in such regions B which are located in the
geometrical shadow of the process beam, that is not in the line-of-
sight of the process beam. It is consequently possible with the method
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in accordance with the invention to coat around corners, edges and
rounded portions.
This is in particular advantageous for the coating of turbine vanes of
gas turbines and specifically for segments of such turbines which
include two or more turbine vanes.
