U.S. patent application number 14/360487 was filed with the patent office on 2014-11-06 for method and apparatus for depositing stable crystalline phase coatings of high temperature ceramics.
This patent application is currently assigned to NATIONAL RESEARCH COUNCIL OF CANADA. The applicant listed for this patent is Cristian Victor Cojocaru, Rogerio Soares Lima, Christian Moreau. Invention is credited to Cristian Victor Cojocaru, Rogerio Soares Lima, Christian Moreau, Youliang Wang.
Application Number | 20140329021 14/360487 |
Document ID | / |
Family ID | 48468930 |
Filed Date | 2014-11-06 |
United States Patent
Application |
20140329021 |
Kind Code |
A1 |
Lima; Rogerio Soares ; et
al. |
November 6, 2014 |
Method and Apparatus for Depositing Stable Crystalline Phase
Coatings of High Temperature Ceramics
Abstract
Producing high temperature coatings on ceramics using a high
enthalpy air plasma spray torch and supplying in at least 3 passes
to deposit the coating is advantageous for spraying high
temperature ceramics while avoiding formation of undesirable
phases, if the stand off distance is chosen such that a width of a
bead produced from a single spray pass on the substrate at
.about.800.degree. C. is less than 70% of a diameter of a plume of
the torch at the stand off, and neither assisted heating, nor
forced cooling, nor subsequent heat treatment is used. The rapid
cooling endemic to thermal spray that leads to amorphous,
metastable and other undesirable phases of alkaline earth
aluminosilicate (e.g., barium-strontium aluminosilicate (BSAS)),
rare earth silicates (RESs), mullite, etc. can be mitigated
sufficiently by the close stand off and high enthalpy torch to
provide highly crystalline and stable phase coatings.
Inventors: |
Lima; Rogerio Soares;
(Boucherville, CA) ; Cojocaru; Cristian Victor;
(Longueuil, CA) ; Moreau; Christian;
(Boucherville, CA) ; Wang; Youliang; (Milpitas,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lima; Rogerio Soares
Cojocaru; Cristian Victor
Moreau; Christian |
Boucherville
Longueuil
Boucherville |
|
CA
CA
CA |
|
|
Assignee: |
NATIONAL RESEARCH COUNCIL OF
CANADA
Ottawa
ON
|
Family ID: |
48468930 |
Appl. No.: |
14/360487 |
Filed: |
November 25, 2011 |
PCT Filed: |
November 25, 2011 |
PCT NO: |
PCT/CA2011/001285 |
371 Date: |
May 23, 2014 |
Current U.S.
Class: |
427/452 ;
118/723ME; 427/446 |
Current CPC
Class: |
C23C 4/134 20160101;
C04B 41/009 20130101; C04B 41/009 20130101; C23C 4/12 20130101;
F01D 5/288 20130101; C04B 41/85 20130101; C04B 41/009 20130101;
C04B 41/009 20130101; C04B 41/4527 20130101; C23C 4/067 20160101;
C23C 4/11 20160101; F05D 2230/311 20130101; C04B 41/4527 20130101;
C04B 41/5024 20130101; C04B 41/5037 20130101; F01D 5/284 20130101;
C04B 35/565 20130101; C04B 41/4527 20130101; C04B 35/00 20130101;
C04B 35/806 20130101; C04B 35/806 20130101; C04B 35/584
20130101 |
Class at
Publication: |
427/452 ;
427/446; 118/723.ME |
International
Class: |
C23C 4/12 20060101
C23C004/12; C23C 4/06 20060101 C23C004/06 |
Claims
1. A method of producing a high temperature coating on a ceramic
substrate, the method comprising: operating a high enthalpy plasma
spray torch and supplying the torch with a feedstock powder
comprising a high temperature ceramic with a melting point above
1600.degree. C., forming undesirable solid phases when rapidly
cooled below about 900.degree. C.; spraying the ceramic substrate
with the torch in at least 3 passes to deposit the coating; wherein
a stand-off distance is maintained during the deposition such that
a width of a bead produced from a single spray pass on the
substrate at .about.800.degree. C. is less than 70% of a diameter
of a plume of the torch at the stand-off, whereby the coating
contains less undesirable solid phase without requiring assisted
heating, forced cooling or subsequent heat treatment.
2. The method according to claim 1 wherein the substrate surface
temperature centred on the plume during deposition exceeds
1000.degree. C. because of the proximity of the torch, and the
absence of assisted heating and forced cooling.
3. The method according to claim 1 wherein the substrate surface
temperature is raised prior to deposition by scanning with the
torch over the substrate, immediately prior to deposition, to raise
the surface temperature above .about.500.degree. C., more
preferably from .about.800-1000.degree. C.
4. The method according to claim 1 wherein spraying the ceramic
substrate with the torch in at least 3 passes; comprises moving a
surface of the substrate to be coated in a direction perpendicular
to the spray jet at a speed of 0.1-5 m/s; or exposes each region of
the substrate to the plume for about 8 s per coating.
5. (canceled)
6. The method according to claim 1 wherein the stand-off distance
is: chosen where the width of a single spray bead on a substrate at
.about.800.degree. C. is 20-70% of the diameter of the plasma plume
at the same stand-off distance; chosen where the width of a single
spray bead on a substrate at .about.800.degree. C. is 35-65% of the
diameter of the plasma plume at the same stand-off distance; chosen
where the width of a single spray bead on a on substrate at
.about.800.degree. C. is 40-60% of the diameter of the plasma plume
at the same stand-off distance; chosen where the width of a single
spray bead on a on substrate at .about.800.degree. C. is from
.about.0.5 cm to .about.2.5 cm, and the plume diameter is from
.about.1 cm to .about.5 cm, the former being at most 70% of the
latter; or chosen where the width of a single spray bead on a on
substrate at 800.degree. C. is from .about.0.5 cm to .about.2.5 cm,
and the plume diameter is from -1 cm to .about.5 cm, the former
being at most 40-60% of the latter.
7. The method according to claim 1 wherein operating the high
enthalpy torch comprises: supplying a power of 60-200 kW; supplying
a power of 80-150 kW; supplying a power of 10-140 kW; supplying a
total plasma gas flow at 100-500 slpm; supplying a total plasma gas
flow at 150-400 slpm; supplying a total plasma gas flow at 250-310
slpm; supplying plasma gas including N.sub.2; supplying plasma gas
including 10-90% N.sub.2; supplying plasma gas consisting of 10-80%
Ar, 5-40% H.sub.2, and/or He, and 30-80% N.sub.2; or supplying
plasma gas consisting of 0-25% H.sub.2, 25-45% Ar and 45-50%
N.sub.2.
8. The method according to claim 1 wherein the feedstock powder:
consists of 80% or more of the high temperature ceramic with a
melting point above 1600.degree. C.; consists essentially of the
high temperature ceramic with a melting point above 1600.degree.
C.; consists of 80% or more of the high temperature ceramic with a
melting point above 1600.degree. C., the high temperature ceramic
having a preferred phase preferentially formed only at a
temperature above 900.degree. C.; consists essentially of the high
temperature ceramic with a melting point above 1600.degree. C., the
high temperature ceramic having a preferred phase preferentially
formed only at a temperature above 900.degree. C., consists of 80%
or more of the high temperature ceramic with a melting point above
1600.degree. C., the high temperature ceramic having a preferred
phase preferentially formed only at temperatures between
900-2000.degree. C.; consists essentially of the high temperature
ceramic with a melting point above 1600.degree. C., the high
temperature ceramic having a preferred phase preferentially formed
only at temperatures between 900-2000.degree. C.; consists of 80%
or more of the high temperature ceramic with a melting point above
1600.degree. C., the high temperature ceramic having a preferred
phase preferentially formed only at temperatures between
900-1600.degree. C.; or consists essentially of the high
temperature ceramic with a melting point above 1600.degree. C., the
high temperature ceramic having a preferred phase preferentially
formed only at temperatures between 900-1600.degree. C.
9. A method of producing a coating of an alkaline earth
aluminosilicate or rare earth silicate (RES) on a ceramic
substrate, the method comprising: providing a feedstock of an
alkaline earth aluminosilicate or RES powder and the ceramic
substrate; plasma spraying the feedstock onto the ceramic substrate
with a high enthalpy plasma spray torch to deposit the coating in
at least 3 passes; wherein a stand-off distance is maintained
during the deposition such that the width of a bead produced from
single spray pass on the substrate at .about.800.degree. C. is less
than 70% of a diameter of a plume of the torch at the stand-off,
whereby the coating consists essentially of a desirable stable
crystalline phase without assisted heating, forced cooling, or
subsequent heat treatment.
10. The method according to claim 9 wherein the substrate surface
temperature centred on the plume during deposition exceeds
1000.degree. C. because of the proximity of the torch, and because
of the absence of assisted heating and forced cooling.
11. The method according to claim 9 wherein the substrate surface
temperature is raised prior to deposition by scanning with the
torch over the substrate, immediately prior to deposition, to raise
the surface temperature above .about.500.degree. C., more
preferably from .about.800-1000.degree. C.
12. The method according to claim 9 wherein depositing the coating
in at least 3 passes comprises moving a surface of the substrate to
be coated in a direction perpendicular to the spray jet at a speed
of 0.1-5 m/s.
13. The method according to claim 9 wherein the stand-off distance
is: chosen where the width of a single spray bead on a substrate at
.about.800.degree. C. is 20-70% of the diameter of the plume at the
stand-off distance, chosen where the width of a single spray bead
on a on substrate at .about.800.degree. C. is 35-65% of the
diameter of the plume of the plume at the stand-off distance,
chosen where the width of a single spray bead on a on substrate at
.about.800.degree. C. is 40-60% of the diameter of the plume at the
stand-off distance, chosen where the width of a single spray bead
on a on substrate at .about.800.degree. C. is from .about.0.5 cm to
.about.2.5 cm, and the plume diameter at the stand-off distance is
from .about.1 cm to .about.5 cm, the former being at most 70% of
the latter, or chosen where the width of a single spray bead on a
on substrate at .about.800.degree. C. is from .about.0.5 cm to
.about.2.5 cm, and the plume diameter at the stand-off distance is
from .about.1 cm to .about.5 cm, the former being 40-60% of the
latter.
14. The method according to claim 9 wherein plasma spraying
comprises: supplying a power of 60-200 kW; supplying a power of
80-150 kW; supplying a power of 110-140 kW; supplying a total
plasma gas flow at 100-500 slpm; supplying a total plasma gas flow
at 150-400 slpm; supplying a total plasma gas flow at 250-310 slpm;
supplying plasma gas including N.sub.2; supplying plasma gas
including 10-90% N.sub.2; supplying plasma gas consisting of 10-80%
Ar, 5-40% H.sub.2, and/or He, and 30-80% H>; or supplying plasma
gas consisting of 10-25% H.sub.2, 25-45% Ar and 45-50% N.sub.2.
15. The method according to claim 9 wherein the alkaline earth
aluminosilicate or RES powders: consists of 80% or more of the
alkaline earth aluminosilicate or RES and has a melting point above
1600.degree. C.; consists essentially of the alkaline earth
aluminosilicate or RES and has a melting point above 1600.degree.
C.; consists of 80% or more of the RES and has a melting point
above 1600.degree. C., the alkaline earth aluminosilicate or RES
having a preferred phase preferentially formed only at a
temperature above 1000.degree. C.; consists essentially of the
alkaline earth aluminosilicate or RES and has a melting point above
1600.degree. C., the alkaline earth aluminosilicate or RES having a
preferred phase preferentially formed only at a temperature above
1000.degree. C.; consists of 80% or more of the alkaline earth
aluminosilicate or RES and has a melting point above 1600.degree.
C., the RES having a preferred phase preferentially formed only at
temperatures between 1000-2000.degree. C.; consists essentially of
the alkaline earth aluminosilicate or RES and has a melting point
above 1600.degree. C., the alkaline earth aluminosilicate or RES
having a preferred phase preferentially formed only at temperatures
between 1000-2000*0; consists of 80% or more of the alkaline earth
aluminosilicate or RES and has a melting point above 1600.degree.
C., the alkaline earth aluminosilicate or RES having a preferred
phase preferentially formed only at temperatures between
1200-1600.degree. C.; consists of the alkaline earth
aluminosilicate or RES and has a melting point above 1600.degree.
C., the alkaline earth aluminosilicate or RES having a preferred
phase preferentially formed only at temperatures between
1200-1600.degree. C.; consists of 80% or more of BSAS; or consists
essentially of BSAS.
16. A method of producing a mullite coating on a ceramic substrate,
the method comprising: providing a feedstock of mullite powder and
the ceramic substrate; and plasma spraying the feedstock onto the
ceramic substrate with a high enthalpy plasma spray torch to
deposit the coating in at least 3 passes; wherein a stand-off
distance is maintained during the deposition such that a width of a
bead produced from a single spray pass on the substrate at
.about.800.degree. C. is less than 70% of a diameter of a plume of
the torch at the stand-off, whereby the desirable stable
crystalline phase of the coating is maintained without requiring
assisted heating, forced cooling or subsequent heat treatment.
17. The method according to claim 16 wherein the localized
temperature of the substrate surface within the plasma plume spot
during deposition exceeds 1000.degree. C. because of the proximity
of the torch in the absence of assisted heating and forced
cooling.
18. The method according to claim 16 wherein the substrate surface
temperature is raised prior to deposition by scanning with the
torch over the substrate, immediately prior to deposition, to raise
the surface temperature above .about.500.degree. C., more
preferably from .about.800-1000.degree. C.
19. The method according to claim 16 wherein depositing the coating
in at least 3 passes comprises moving a surface of the substrate to
be coated in a direction perpendicular to the spray jet at a speed
of 0.1-5 m/s.
20. The method according to claim 16 wherein the stand-off distance
is: chosen where the width of a single spray bead on a substrate at
.about.800.degree. C. is 20-70% of the diameter of the plume at the
stand-off distance, chosen where the width of a single spray bead
on a on substrate at .about.800.degree. C. is 35-65% of the
diameter of the plume at the stand-off distance, chosen where the
width of a spray bead on a on substrate at .about.800.degree. C. is
40-60% of the diameter of the plume at the stand-off distance,
chosen where the width of a spray bead on a on substrate at
.about.800.degree. C. is from .about.0.5 cm to .about.2.5 cm, and
the plume diameter at the stand-off is from .about.1 cm to .about.5
cm, the former being at most 70% of the latter, or chosen where the
width of a spray bead on a on substrate at .about.800.degree. C. is
from .about.0.5 cm to .about.2.5 cm, and the plume diameter at the
stand-off is from .about.1 cm to .about.5 cm, the former being
40-60% of the latter.
21. The method according to claim 16 wherein plasma spraying
comprises: supplying a power of 60-200 kW; supplying a power of
80-150 kW; supplying a power of 110-140 kW; supplying a total
plasma gas flow at 100-500 slpm; supplying a total plasma gas flow
at 150-400 slpm; supplying a total plasma gas flow at 250-3 0 slpm;
supplying plasma gas including N.sub.2; supplying plasma gas
including 10-90% ivfe; supplying plasma gas consisting of 10-80%
Ar, 5-40% H.sub.2, and/or He, and 30-80% N.sub.2; or supplying
plasma gas consisting of 10-25% H.sub.2, 25-45% Ar and 45-50%
N.sub.2.
22. The method according to claim 16 wherein the mullite powder:
consists of 80% or more of the mullite; or consists essentially of
the mullite.
23. A system for thermal spraying to produce a high temperature
coating on a ceramic substrate, the system comprising: a high
enthalpy air plasma spray torch; a feed supply containing a
feedstock powder comprising a high temperature ceramic that has a
melting point above 1600.degree. C., the feed supply operably
coupled to the torch; a plasma gas supply operably coupled to the
torch; an electrical power supply operably coupled to the torch; a
ceramic substrate; and a support for controlling a stand-off
distance between the ceramic substrate and torch, while permitting
a spray jet of the torch to scan across a surface of the substrate
to be coated, wherein the stand-off distance is chosen so that the
width of a single bead on a substrate at .about.800.degree. C. is
less than 70% of a diameter of a plume of the torch.
24. The system according to claim 23 wherein the localized
temperature of the substrate surface within the plasma plume spot
during deposition exceeds 1000.degree. C. because of the proximity
of the torch, and because of an absence of assisted heating or
forced cooling.
25. The system according to claim 23 wherein the support permitting
scanning comprises a computer numerical controlled subsystem for
spraying the ceramic substrate with the torch in at least 3 passes
by moving a surface of the substrate to be coated in a direction
perpendicular to the spray jet at a speed of 0.1-5 m/s.
26. The system according to claim 23 wherein the stand-off distance
is: where the width of a single bead on a on substrate at
.about.800 C is 20-70% of the diameter of the plume at the
stand-off, where the width of a single bead on a on substrate at
.about.800.degree. C. is 35-65% of the diameter of the plume at the
stand-off, where the width of a single bead on a on substrate at
.about.800.degree. C. is 40-60% of the diameter of the plume at the
stand-off, where the width of a single bead on a on substrate at
.about.800.degree. C. is from .about.0.5 cm to .about.2.5 cm, and
the plume diameter is from .about.1 cm to .about.5 cm at the
stand-off, or where the width of a single bead on a on substrate at
-800 0 is from .about.0.5 cm to .about.2.5 cm, and the plume
diameter is from .about.1 cm to .about.5 cm at the stand-off, the
former being 40-60% of the latter.
27. The system according to claim 23 wherein: the electrical supply
applies a power of 60-200 kW; the electrical supply applies a power
of 80-150 kW; the electrical supply applies a power of 110-140 kW;
the plasma gas supply supplies a total plasma gas flow at 100-500
slpm; the plasma gas supply supplies a total plasma gas flow at
150-400 slpm; the plasma gas supply supplies a total plasma gas
flow at 250-310 slpm; the plasma gas supply supplies plasma gas
including N.sub.2; the plasma gas supply supplies plasma gas
including 10-90% N.sub.2; the plasma gas supply supplies plasma gas
consisting of 10-80% Ar, 5-40% H.sub.2, and/or He, and 30-80%
N.sub.2; or the plasma gas supply supplies plasma gas consisting of
10-25% H.sub.2, 25-45% Ar and 45-50% N.sub.2.
28. The system according to claim 23 wherein the feedstock powder:
consists of 80% or more of a alkaline earth aluminosilicate or rare
earth silicate; consists essentially of a alkaline earth
aluminosiiicate or rare earth silicate; consists of 80% or more of
BSAS; consists essentially of BSAS; consists of 80% or more of
mullite; consists essentially of mullite; consists of 80% or more
of the high temperature ceramic with a melting point above
1600.degree. C.; consists essentially of the high temperature
ceramic with a melting point above 1600.degree. C.; consists of 80%
or more of the high temperature ceramic with a melting point above
1600.degree. C., the high temperature ceramic having a preferred
phase preferentially formed only at a temperature above 90
Q.degree. C.; consists essentially of the high temperature ceramic
with a melting point above 1600.degree. C., the high temperature
ceramic having a preferred phase preferentially formed only at a
temperature above 900.degree. C.; consists of 80% or more of the
high temperature ceramic with a melting point above 1600.degree.
C., the high temperature ceramic having a preferred phase
preferentially formed only at temperatures between 900-2000.degree.
C.; consists essentially of the high temperature ceramic with a
melting point above 1600.degree. C., the high temperature ceramic
having a preferred phase preferentially formed only at temperatures
between 900-2000.degree. C.; consists of 80% or more of the high
temperature ceramic with a melting point above 1600.degree. C., the
high temperature ceramic having a preferred phase preferentially
formed only at temperatures between 900-1600.degree. C., or
consists essentially of the high temperature ceramic with a melting
point above 1600.degree. C., the high temperature ceramic having a
preferred phase preferentially formed only at temperatures between
900-1600.degree. C.
29. A method of producing a high temperature coating on a ceramic
substrate, the method comprising operating a high enthalpy air
plasma spray torch and supplying the torch with a feedstock powder
comprising a high temperature ceramic with a melting point above
1600.degree. C., that forms undesirable solid phases when rapidly
cooled below about 900.degree. C., wherein a cumulative deviation
from the following norms is less than 20%:
|[N.sub.2]|=47.5.+-.2.5%, |[H.sub.2]|=17.5+7.5%, |[Ar]|=35.+-.10%,
|TGF|=280.+-.30 slpm, |P|=110.+-.23 kW, |D|=8.+-.2 cm, where
[N.sub.2], [H.sub.2], and [Ar] are respectively the percentages of
N.sub.2, H.sub.2, and Ar in the plasma plume, TGF is the total gas
flow, P is the torch power, and D is the stand-off distance
measured in cm.
30. The method according to claim 29 wherein the cumulative
deviation is less than 10%.
31. (canceled)
32. The method according to claim 29 wherein the cumulative
deviation is less than 1%.
33. The method according to claim 29 wherein: operating the torch
comprises spraying the ceramic substrate with at least 3 passes, by
relative movement of a surface of the substrate to be coated
perpendicular to the spray jet at a speed of 0.1-5 m/s: spraying
the ceramic substrate with the torch in at least 3 passes exposes
each region of the substrate to the plume for about 8 s per
coating; or the substrate surface temperature is raised prior to
deposition by scanning with the torch over the substrate,
immediately prior to deposition, to raise the surface temperature
above .about.500.degree. C., more preferably from
.about.800-1000*C.
34. (canceled)
35. (canceled)
36. The method according to claim 29 wherein the feedstock powder:
consists of 80% or more of the high temperature ceramic with a
melting point above 1600.degree. C.; consists essentially of the
high temperature ceramic with a melting point above 1600.degree.
C.; consists of 80% or more of the high temperature ceramic with a
melting point above 1600.degree. C., the high temperature ceramic
having a preferred phase preferentially formed only at a
temperature above 900.degree. C.; consists essentially of the high
temperature ceramic with a melting point above 1600.degree. C., the
high temperature ceramic having a preferred phase preferentially
formed only at a temperature above 900.degree. C.; consists of 80%
or more of the high temperature ceramic with a melting point above
1600.degree. C., the high temperature ceramic having a preferred
phase preferentially formed only at temperatures between
900-2000.degree. C.; consists essentially of the high temperature
ceramic with a melting point above 1600.degree. C., the high
temperature ceramic having a preferred phase preferentially formed
only at temperatures between 900-200 Q.degree. C.; consists of 80%
or more of the high temperature ceramic with a melting point above
1600.degree. C., the high temperature ceramic having a preferred
phase preferentially formed only at temperatures between
900-1600.degree. C.; or consists essentially of the high
temperature ceramic with a melting point above 1600.degree. C., the
high temperature ceramic having a preferred phase preferentially
formed only at temperatures between 900-1600.degree. C.
Description
FIELD OF THE INVENTION
[0001] The present invention relates in general to thermal spray
arts, and in particular to techniques for producing stable
crystalline phase coatings of such materials as alkaline earth
aluminosilicates (e.g., barium-strontium aluminosilicate (BSAS)),
rare earth silicates (RESs), and mullites, and in particular to a
low cost coating process that is phase pure, without assisted
heating of a substrate.
BACKGROUND OF THE INVENTION
[0002] To increase the efficiency of a gas turbine engines for
aerospace applications and power generation, higher operating
temperatures are needed. In the same time the durability of the
engine components must correspondingly sustain the temperature
increase. While metal super alloys are widely used in the hot
sections of gas turbines, there is currently a temperature limit
(.about.1200.degree. C.) for the usage of these metals. New
materials, and in particular silicon (Si) based ceramics (e.g., SiC
and Si.sub.3N.sub.4) in the form of ceramic matrix composites
(CMCs) have emerged for high temperature applications 200.degree.
C.) due to their high mechanical strengths at high temperatures and
density values in the order of 1/3 of those of metallic superalloys
[1, 2, 3, 4, 5]. These same material characteristics can be applied
to engineer the external structural body of the next generation of
hypersonic vehicles.
[0003] For most of these applications (gas turbines and hypersonic
aircraft) protective coatings are beneficial and even required.
Layered refractory oxide coatings offering a suitable
thermal-insulation reduce the thermal gradient through the
structural component and increase its durability. A coating for
components formed of Si-based materials (CMCs) must simultaneously
fulfill a second role namely environmental protection, like for
instance, in the high temperature environments containing water
vapour in the hot sections of gas turbines (e.g., blades, vanes and
combustion chambers) or in low O.sub.2 atmospheric pressures at
high altitudes for hypersonic aircraft. Thus these coatings have
been named environmental barrier coatings (EBCs), and they are
expected to work at temperatures higher than 1300.degree. C.
[0004] Regarding gas turbines, a low permeability for oxidant
species is a critical feature for an EBC system so as to inhibit
the major degradation mechanism of the Si-based substrate that is
formation of silicon hydroxide (Si(OH).sub.4) gas by the reaction
of the water vapour originating from the jet fuel combustion with
the SiO.sub.2 scale formed on the Si-based substrate [1, 2, 3, 4,
5].
[0005] The future hypersonic vehicles flying in low Earth orbit
will also employ Si-based materials on its external body structure.
For this type of application, a silicon monoxide (SiO) scale will
tend to be formed on the surface of these Si-based materials due to
the low O.sub.2 atmospheric pressures at high altitudes and the
high temperatures caused by the friction with the atmosphere during
ascending and re-entry. The SiO scale will tend to sublime under
these conditions. Therefore, EBCs will be needed to protect the
exposed structure of hypersonic aircraft [6].
[0006] Thermal spraying is the state-of-the-art processing method
to deposit EBCs on Si-based materials. All ceramics and cermets are
not equally easy to apply by thermal spraying. Some are relatively
simple: as long as the feedstock reaches a melting temperature
during the spraying, and is accelerated to a speed within a given
range, it can be deposited reasonably efficiently. Some are more
complex in that overheating of the feedstock above a temperature
far in excess of the melting point, leads to reactions between the
feedstock and ambient gases that lead to impurities in the coating.
But a large and growing class of materials that are desired to be
thermally sprayed are quite challenging. Specifically the alkaline
earth aluminosilicate Barium Strontium AluminoSilicate (BSAS),
i.e.; Ba.sub.1-xSr.sub.xAl.sub.2Si.sub.2O.sub.8, 0<x<1, and
the aluminosilicate mullite (Al.sub.6Si.sub.2O.sub.13), which are
highly regarded in the field of high temperature thermal coating
industries, and solidify in various phases [1, 2, 5]. The phase
constitution of such a coating is telling as to the thermal history
of the coating, as each phase is formed preferentially at different
temperatures and pressures. Only one phase of the BSAS or mullite,
is desired. Other alkaline earth aluminosilicates, like barium
aluminosilicate (SAS) and strontium aluminosilicate (SAS) can also
be applied as EBCs. RESs have also been studied and considered as
potential candidates for EBC applications, including but not
limiting: Y.sub.2SiO.sub.5, Er.sub.2SiO.sub.2, Yb.sub.2SiO.sub.5,
Lu.sub.2SiO.sub.5 and Yb.sub.2Si.sub.2O.sub.7 [7].
[0007] Currently BSAS and mullite are state-of-the-art materials
thermal sprayed to produce EBCs. Thermal spraying is a group of
processes wherein a feedstock material (usually in a powder, but
also in the form of a wire or rod, or sometimes a precursor
solution or suspension) is injected into a plume (typically a
plasma or a combustion flame) of a thermal spray torch, where the
feedstock is heated and propelled as a jet of individual molten or
semi-molten particles, toward a substrate surface. At impact with
the substrate/previous deposition, the at least semimolten
particles flatten and form think lamellae (splats) that conform and
adhere to the irregular substrate/previous deposition, and/or to
one another. At impact the splats cool down very rapidly
(10.sup.5-10.sup.7.degree. C./s) and resolidify, generally before
the arrival of the next splat, resulting in a distinctive lamellar
structure.
[0008] Due to the high quenching (cooling/solidification) rates,
the atoms of the molten particles may not have enough time and/or
energy (nucleation kinetics) to achieve their most desired stable
crystalline form. It is very common for thermal sprayed coatings to
include multiple crystalline, metastable and/or amorphic zones.
[0009] The desire for pure Celsian (monoclinic) phase of BSAS is
well known. The metastable Hexacelsian (hexagonal) phase is stable
only within the temperature range of .about.1600-1760.degree. C.,
whereas, the Celsian phase is stable from room temperature up to
.about.1600.degree. C. The coefficients of thermal expansion (CTEs)
of the Hexacelsian phase (.about.8.times.10.sup.-6.degree.
C..sup.-1), the Celsian phase (.about.5.times.10.sup.-6.degree.
C..sup.-1), and a typical Si-based substrate (SiC, or
Si.sub.3N.sub.4) used in high temperature applications, is
.about.4-5.times.10.sup.-6.degree. C..sup.-1, attest to the
compatibility of the Celsian phase with the substrate. This coupled
with the fact that the Celsian phase provides a high resistance to
water vapour attack [5, 7] explains why the Celsian phase is so
valuable for high temperature applications. The matching of the
expansion reduces stresses during thermal cycling (heating and
cooling) increasing a lifetime and performance of the coating.
[0010] When BSAS is deposited under regular thermal spray
conditions, i.e., when the substrate temperature is below
.about.500.degree. C., the coating is amorphous and/or crystallizes
in the metastable Hexacelsian phase due to the rapid solidification
of the BSAS particles. These two phases will transform into Celsian
at temperatures.gtoreq.1200.degree. C. Thus the inclusion of
amorphous and hexacelsian phases occur to the detriment of high
temperature coating performance in two ways. The difference in
thermal expansion of the hexacelsian phase with respect to the
Celsian phase and Si-based substrate will result in stress changes
during heating, so that if enough Hexacelsian phase is present, the
coating will crack and spall. Secondly, operation at temperatures
of 1300-1500.degree. C. will cause phase transformations that lead
to stresses due to dimensional changes that result from the removal
of amorphous/Hexacelsian phase in favour of the Celsian phase. If
sufficient inclusions are present, the coating will likely crack
and spall, as ceramics have very low plasticity. Therefore it is
imperative that a pure Celsian phase is formed for like high
temperature applications.
[0011] Given the control over the temperature that is required to
selectively form the Celsian phase, and the transient nature of
heating with thermal spray torches, conventional wisdom in the art
dictates that BSAS is deposited at a relatively low temperature so
that it includes substantial (or exclusively) amorphous phase on
thermal spray deposition, followed by heating (annealing) the
coating for an extended period at a temperature of over
1200.degree. C. to effectively transform the amorphous and
Hexacelsian phase into the Celsian phase [9]. Prior art references
[10], [11], [12] teach methods of producing predominantly Celsian
phase coatings, each requiring annealing at 1250.degree. C. for 24
h, and none claims to produce an essentially pure Celsian phase.
None of these teach use of a high enthalpy plasma thermal spray
torch. One drawback to the teachings of [10], [11], is that the
whole substrate has to be kept at high temperatures
(>870.degree. C.). It is difficult to maintain these
temperatures on large and complex turbine parts during thermal
spraying. In any case, this generally requires that the thermal
spraying be performed in a furnace.
[0012] Similarly, mullite is known to form a mixture of amorphous
and crystalline phases during regular thermal spray deposition,
i.e., when the substrate temperature is kept below
.about.500.degree. C. Mullite crystallizes at .about.1000.degree.
C., leading to the cracking and failure of the coating (when
operating at T.gtoreq.1000.degree. C.) due to the lack of
plasticity of this material. Therefore, it is known in the art to
spray mullite in an oven at .about.1000.degree. C. ([13][14]) which
does provide highly crystalline mullite coatings, that exhibit
excellent performance as intermediate layers in EBCs.
[0013] US patent application 2011/0033630 to Naik et al. teaches
deposition of BSAS on a mullite interlayer with the substrate at a
temperature of less than 150.degree. C. when the BSAS is first
deposited. This application teaches that the temperature of the
substrate during deposition may be less than 200.degree. C., i.e.,
(15-200.degree. C. or 20-150.degree. C.), presumably with forced
cooling. The power of the torch, plasma gas flow rates, plasma
constituents (especially the concentration of N.sub.2), and the
stand-off between the plume is nowhere disclosed. As a result, it
is impossible to reproduce, verify or compare coatings according to
this invention.
[0014] All air plasma spray (APS) torches are not created equal.
When not specified, a typical conventional plasma spray torch would
be expected to have a power of 20-60 kW, and use Ar/H.sub.2 as
plasma gases at total gas flow rates in the order of .about.20-80
standard litres per minute (slpm). Other possible combination of
plasma gases for these types of equipment operating at these ranges
of power and total gas flow are Ar/N.sub.2, Ar/He, N.sub.2/H.sub.2,
N.sub.2/He, and on a lesser scale, a tertiary combination of
thereof. In contrast, a high enthalpy APS torch for thermal spray
processing, while generally providing the same mechanism, is
typically designed for .about.60-150 kW power, .about.100-400 slpm
total plasma gas flow delivery, and typically uses a tertiary
plasma gas combination that includes N.sub.2.
[0015] There is tremendous interest in next generation high
temperature coatings such as EBCs, to contain high energy
reactions, for aerospace turbines, power generation and hypersonic
aircraft external structure. Several groups well versed in the art
have tried to solve the problem of avoiding back-side heating or
furnace-based deposition, but have failed to produce phase purity,
have cracked the substrate, or have failed to produce optimized
coatings. For example, according to NASA researchers (reported K.
N. Lee at al. New Generation of Plasma-Sprayed Mullite Coatings on
Silicon Carbide, J Am, Ceram. Soc. 78(3) (1995) 705-710):
"Precipitation of amorphous mullite can be prevented by decreasing
the cooling rate. One approach to decreasing the cooling rate is to
heat the substrate during the spraying. A first attempt was made to
heat the substrate with a plasma torch. However, it was not
possible to attain a uniform and stable substrate temperature with
the plasma torch heating. A severe cracking in the substrate was
observed when mullite was sprayed on a plasma-torch-heated SiC.
Amorphous mullite was still observed, indicating that substrate was
heated below the crystallization temperature . . . . The reason for
the substrate-cracking when underheated is not understood at this
point." It is noted that the substrate used was the SiC "Hexoloy
SA.TM." from Saint-Gobain, Niagara Falls, N.Y., USA.
[0016] As mullite is a lower crystallization temperature material
than BSAS and other RESs, as the prior art all teaches low
temperature amorphous thermal spray deposition, it follows that all
of the problems with mullite will be as great or greater for BSAS.
More recently, a paper to Harder et al. entitled Chemical and
Mechanical Consequences of Environmental Barrier Coating Exposure
to Calcium-Magnesium-Aluminosilicate J. Am. Ceram. Soc. 94 [S1]
S178-S185 (2011), teaches: "In room-temperature plasma-sprayed
systems, BSAS is deposited as an amorphous phase and crystallizes
in the metastable Hexacelsian phase (hexagonal) when heated.
Heating the substrate to 1200.degree. C. during the plasma-spray
process is sufficient to deposit BSAS in the Hexacelsian phase.
This Hexacelsian phase then undergoes a phase transformation at
temperatures over 1200.degree. C. into the stable Celsian phase
(monoclinic). This transformation is accompanied by a 0.5% volume
reduction and drastically changes the elastic modulus and
coefficient of thermal expansion (GTE) of the topcoat . . . ."
Harder et al. does not disclose the details of the plasma spraying
used, but does indicate that the coatings applied were sprayed in a
furnace "where the substrate was heated to 1200.degree. C. in order
to avoid depositing amorphous material that would cause
catastrophic failure upon crystallization."
[0017] This crystallization issue also happens for RES materials
during plasma spraying for EBC applications [7]. In this reference
[7], Lee et al. state that "the details of the coating parameters
are described elsewhere", which is another paper from Lee et al.
[14]. In this reference [14], Lee et al. described that highly
crystalline mullite was achieved when the coatings were air plasma
sprayed on SiC substrates that were kept inside a box furnace at
1050.degree. C. during the deposition process.
[0018] A faster and lower cost solution for producing coatings of a
stable crystalline phase are in demand for coating ceramics for
high temperature application, such as EBCs, especially for alkaline
earth aluminosilicates (e.g., BSAS), RESs, and mullite. New and
innovative solutions that reduce equipment requirements for
producing the coatings are required.
SUMMARY OF THE INVENTION
[0019] Applicant has found, contrary to what has been established
in the prior art, that by 1) employing a high enthalpy APS torch,
2) using a fairly short stand-off distance, and 3) applying the
coating in at least 3 passes, as-sprayed coatings of BSAS, mullite,
and the like, can be deposited without assisted heating or cooling,
and without heat treatment, to produce coatings with levels of a
high-temperature stable crystalline phase that are higher than that
possible in accordance with the prior art.
[0020] In accordance with the present invention, a method of
producing a high temperature coating on a ceramic substrate is
provided. The method comprising: operating a high enthalpy plasma
spray torch and supplying the torch with a feedstock powder
comprising a high temperature ceramic with a melting point above
1600.degree. C., forming undesirable solid phases when rapidly
cooled below about 900.degree. C.; and spraying the ceramic
substrate with the torch in at least 3 passes to deposit the
coating; wherein a stand off distance is maintained during the
deposition such that a width of a bead produced from a single spray
pass on the substrate at .about.800.degree. C. is less than 70% of
a diameter of a plume of the torch at the stand off. Doing so, the
coating contains less undesirable solid phase without requiring
assisted heating, forced cooling or subsequent heat treatment.
[0021] The substrate surface temperature centred on the plume
during deposition preferably exceeds 1000.degree. C. because of the
proximity of the torch, and the absence of assisted heating or
forced cooling. Spraying the ceramic substrate with the torch in at
least 3 passes may comprise moving a surface of the substrate to be
coated in a direction perpendicular to the spray jet at a speed of
0.1-5 m/s. The stand-off distance may be chosen where the width of
a single spray bead on a substrate at .about.800.degree. C. is
20-70%, more preferably 35-65%, more preferably 40-60% of the
diameter of the plasma plume at the same stand-off distance, and
the width of a single spray bead on a on substrate at
.about.800.degree. C. may be from .about.0.5 cm to .about.2.5 cm,
and the plume diameter is from .about.1 cm to .about.5 cm.
[0022] Operating the high enthalpy torch may comprise supplying:
electrical power of 60-200 kW, more preferably 80-150 kW, more
preferably 110-140 kW; and plasma gas with a total flow rate of
100-500 slpm, more preferably 150-400 slpm; more preferably 250-310
slpm. The plasma gas may include N.sub.2, such as 10-90% N.sub.2
and may consist of 10-80% Ar, 5-40% H.sub.2, and/or He, and 30-80%
N.sub.2; or 10-25% H.sub.2, 25-45% Ar and 45-50% N.sub.2.
[0023] The feedstock powder may consist of 80% or more, or consist
essentially of the high temperature ceramic with a melting point
above 1600.degree. C., such as a high temperature ceramic having a
preferred phase preferentially formed only at a temperature above
900.degree. C. such as at temperatures between 900-2000.degree. C.,
or 900-1600.degree. C.
[0024] The method may be applied to an alkaline earth
aluminosilicate, or rare earth silicate, or mullite, as well as an
alumina and a titania. In particular in deposition annealing of
BSAS and mullite have been demonstrated.
[0025] Also accordingly a system for thermal spraying to produce a
high temperature coating on a ceramic substrate is provided. The
system comprises; a high enthalpy air plasma spray torch; a feed
supply containing a feedstock powder comprising a high temperature
ceramic that has a melting point above 1600.degree. C., the feed
supply operably coupled to the torch; a plasma gas supply operably
coupled to the torch; an electrical power supply operably coupled
to the torch; a ceramic substrate; and a support for controlling a
stand-off distance between the ceramic substrate and torch, while
permitting a spray jet of the torch to scan across a surface of the
substrate to be coated. In such a system, the stand-off distance is
chosen so that the width of a single bead on a substrate at
.about.800.degree. C. is less than 70% of a diameter of a plume of
the torch.
[0026] The support and torch are preferably operable so that a
localized temperature of the substrate surface within the plasma
plume spot during deposition exceeds 1000.degree. C. because of the
proximity of the torch, and because of an absence of assisted
heating or forced cooling. The support permitting scanning may
comprises a computer numerical controlled subsystem for spraying
the ceramic substrate with the torch in at least 3 passes by moving
a surface of the substrate to be coated in a direction
perpendicular to the spray jet at a speed of 0.1-5 m/s. The
stand-off distance is preferably where the width of a single bead
on a on substrate at .about.800.degree. C. is 20-70%, more
preferably 35-65%, more preferably 40-60% of the diameter of the
plume at the stand off, and the width of the bead at
.about.800.degree. C. may be from .about.0.5 cm to .about.2.5 cm,
and the plume diameter at the stand of may be from .about.1 cm to
.about.5 cm.
[0027] The electrical supply applies a power of 60-200 kW, more
preferably 80-150 kW, more preferably 110-140 kW. The plasma gas
supply supplies a total plasma gas flow at 100-500 slpm, more
preferably 150-400 slpm, more preferably 250-310 slpm. The plasma
gas includes N.sub.2, for example, consisting of 10-90% N.sub.2.
The plasma gas may consist of 10-80% Ar, 5-40% H.sub.2, and/or He,
and 30-80% N.sub.2; or 10-25% H.sub.2, 25-45% Ar and 45-50%
N.sub.2.
[0028] The feedstock powder may consist of 80% or more of, or
substantially consist of an alkaline earth aluminosilicate or rare
earth silicate, such as BSAS, or mullite.
[0029] Also accordingly, a method of producing a high temperature
coating on a ceramic substrate is provided. The method comprising
operating a high enthalpy air plasma spray torch and supplying the
torch with a feedstock powder comprising a high temperature ceramic
with a melting point above 1600.degree. C., that forms undesirable
solid phases when rapidly cooled below about 900.degree. C.,
wherein a cumulative deviation from the following norms is less
than 20%: |[N.sub.2]|=47.5.+-.2.5%, |[H.sub.2]|=17.5.+-.7.5%,
|[Ar]|=35.+-.10%, |TGF|=280.+-.30 slpm, |P|=110.+-.23 kW,
|D|=8.+-.2 cm, where [N.sub.2], [H.sub.2], and [Ar] are
respectively the percentages of N.sub.2, H.sub.2, and Ar in the
plasma plume, TGF is the total gas flow, P is the torch power, and
D is the stand off distance measured in cm. The cumulative
deviation may be less than 10%, more preferably less than 5% or 1%.
Operating the torch may comprise spraying the ceramic substrate
with at least 3 passes, by relative movement of a surface of the
substrate to be coated perpendicular to the spray jet at a speed of
0.1-5 m/s. The feedstock powder may consist of 80% or more, or
substantially consist of a ceramic with a melting point above
1600.degree. C., such as a ceramic that preferentially forms a
preferred phase only at a temperature above 900.degree. C., such as
at temperatures between 900-2000.degree. C., or 900-1600.degree.
C.
[0030] Further features of the invention will be described or will
become apparent in the course of the following detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] In order that the invention may be more clearly understood,
embodiments thereof will now be described in detail by way of
example, with reference to the accompanying drawings, in which:
[0032] FIGS. 1a,b are schematic illustrations of a high enthalpy
air plasma spray apparatus showing a typical suitable stand-off
distance for thermal spraying in accordance with the prior art, and
an embodiment of the invention, respectively;
[0033] FIG. 2 is a graph of X-ray diffraction (XRD) spectra of high
enthalpy, and conventionally sprayed mullite coatings deposited on
a ceramic substrate;
[0034] FIG. 3 schematically illustrates differential thermal
analysis (DTA) of the mullite coatings of FIG. 2;
[0035] FIG. 4 is a scanning electron micrograph (SEM) image of BSAS
powder particles for use as a feedstock in plasma spraying;
[0036] FIG. 5 is a scanning electron micrograph (SEM) image of a
cross-section of a barrier system, including a silicon (Si) bond
coat, a mullite interlayer and a BSAS top coat sprayed in
accordance with an embodiment of the present invention;
[0037] FIG. 6 is a panel showing cross-sectional SEM image of the
barrier system after high temperature exposure in water vapour
environment, a high magnification image of the bond coat/substrate
interface, and an unprotected substrate subjected to the same;
[0038] FIG. 7 plots X-ray diffraction spectra of BSAS powder, the
as sprayed coating, and the coating after exposure to the heat
treatment
[0039] FIG. 8 is an SEM top view image of a BSAS spray bead showing
the importance of multiple passes of the spray torch;
[0040] FIG. 9 is an XRD spectrum of the bead shown in FIG. 8,
confirming presence of an amorphous phase in the BSAS coating when
only applied in a single pass; and
[0041] FIG. 10 is an image of the temperature profile of a
SiC-based CMC substrate surface immediately after being heated by
the air plasma spray torch for 5 seconds.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0042] The present invention was discovered with experiments using
a high enthalpy APS torch. Given the problems reported in the prior
art with cracking of substrates, it is surprising that using a much
higher enthalpy torch, and a much shorter stand off distance, no
substrate cracking is observed, and given the absence of any
reproducible report of thermal sprayed coatings deposited without
assisted heating, the very high purity observed was remarkable.
[0043] FIGS. 1a,b schematically illustrate a high enthalpy air
plasma spray apparatus respectively showing a stand-off distance
typically used for thermal spraying in accordance with the prior
art, and an embodiment of the invention. FIG. 1a, because of the
larger stand off, illustrates less of the apparatus, however
similar supply equipment is required for a high enthalpy torch 10,
in either case, with the proviso that prior art systems typically
do not spray the same ceramics that are an integral part of the
spray system in FIG. 1b. As shown in FIG. 1b, the apparatus
includes a hopper 13 connected to the torch by a feed supply 12.
The hopper 13 contains a high temperature ceramic powder 14,
composed of a ceramic that melts above .about.1600.degree. C., that
is delivered to the torch 10 by the feed supply 12 in a manner
known in the art. The ceramic used in accordance with the present
invention tends to form amorphous, metastable, or otherwise
undesirable phases if rapidly cooled below a temperature of about
900.degree. C., and the invention mitigates the formation of the
undesirable phases. For example, the ceramic may be an alkaline
earth aluminosilicate (e.g., BSAS), RES, mullite, or an alumina or
titania (doped or pure). A plasma gas supply 15 (partially shown)
is coupled to the torch 10 to supply plasma gas. Preferably the
plasma gas comprises a mix of plasma gasses that together provide
1) sufficient thermal conductivity to the ceramic particles, 2)
sufficient enthalpy to heat the ceramic particles to a desired
temperature, for example, above 2000.degree. C., and 3) a low
enough threshold for ionization to readily initiate plasma
formation and low enough enthalpy to avoid melting the electrodes
of the torch 10. Conveniently 1)-3) may be ensured by providing:
sufficient concentrations of H.sub.2 (e.g., 5-40%, more preferably
10-25%), N.sub.2 (e.g., 30-80%, more preferably 45-50%), and Ar
plasma gas (e.g., 10-80%, more preferably 25-45%), respectively,
although other plasma gasses (e.g., He) could be added or
substituted to some degree. The torch 10 includes an electrical
power supply 19 (in partial view) for ionizing the gas to form the
plasma, which defines a plume 16, which is typically relatively
stable in shape and size during constant supply conditions. The
particles of the ceramic powder 14, entrained in the plume 16,
melt, at least at their surface, and are accelerated by the plume
16, to be ejected as a spray jet 17. The parameters of the spray
jet 17 (velocity distribution, diameter, temperature distribution,
size distribution, etc.) are not as uniform as the envelope of the
plume 16, which can typically be maintained in a steady state, and
keep a constant shape.
[0044] The spray jet 17 is rapidly cooling, and gradually
decelerating in flight, until it strikes a coating surface of a
substrate 18. Substrate 18 is formed of a ceramic, such as a
Si-based ceramic matrix composite (including SiC, and
Si.sub.3N.sub.4). In accordance with the present invention, the
substrate 18 is positioned a relatively short stand off distance D
from the torch 10. This obviates the need for assisted heating. The
stand off distance D is chosen so that the diameter of the plume 16
at D is appreciably larger than the main core of spray jet 17,
which forms the individual spray beads. The substrate 18 is
typically supported by a sample holder 18 in a manner that permits
the spray jet 17 to scan across the coating surface, either by
motion of the torch 10 or of the sample holder 18 (i.e., the motion
normal to the direction of spraying, such as shown by arrow
11).
[0045] For a stand off distance D shorter than the plume length 16,
the shorter the stand off, the less dwell time the particles have
within the plume 16, and accordingly the less energy is absorbed
and the slower and colder spray jet 17 particles are, but the heat
transfer from the plasma plume to the substrate is maximized.
Conversely, the greater the stand off (until a distance beyond the
plume's length), the greater the temperature and speed imparted to
the spray jet 17 (up to a critical point), and also a wider and
less focused spray jet 17 is at contact with the surface. The
greater the stand off, the lower the heat transfer from the plasma
plume to the substrate. The less focused the spray jet 17, results
in a greater variability of the deposition conditions (particle
angle, local surface temperature, particle temperature and
velocity, etc.), leading to a less definite and compact deposition
trace (continuous sequence of deposition produced by the scanning)
known as a bead. Applicant postulates that if the stand-off
distance D is chosen so that a width of a single bead on a
substrate at .about.800.degree. C. is less than 70% of a diameter
of the plume 16 of the torch 10 at the same stand off distance, and
the coating is produced with at least three passes of the spray jet
17, substantially less of the undesired phase is produced, as
sufficient substrate heating and dwell time of the particles are
provided.
[0046] The bead width is a sensitive parameter, and may vary widely
with changes in the spray jet conditions and deposition conditions.
The stand off where the bead has a certain width may reliably be
determined by providing stable feed conditions, and spraying the
substrate while holding a temperature of the substrate constant,
such as at 800.degree. C. This not to be understood as a
requirement for the substrate to be at 800.degree. C. during
coating deposition, but merely as one possible manner of reliably
determining a bead width as a function of stand off. Coating
deposition in accordance with the present invention is provided
without assisted heating or cooling, and without post spray heat
treatment or annealing.
[0047] In sum, it is believed that by providing a thin bead in a
wide plume, the heat required for gradual enough cooling to provide
favourable nucleation kinetics (e.g., sufficient duration at or
above 1000.degree. C.) is provided, to avoid deposition of
undesirable phases. In all aspects of the invention, however,
Applicant does not wish to be limited by the foregoing theory.
[0048] Alternatively viewed, the invention comprises operating the
high enthalpy air plasma spray torch 10 with the ceramic powder 14
to produce a coating on substrate 18 using a set of deposition
conditions that substantially conform with the following norms:
|[N.sub.2]|=47.5.+-.2.5%, |[H.sub.2]|=17.5.+-.7.5%,
|[Ar]|=35.+-.10%, |TGF|=280.+-.30 slpm, |P|=110.+-.23 kW,
|D|=8.+-.2 cm where [N.sub.2], [H.sub.2], and [Ar] are respectively
the percentages of N.sub.2, H.sub.2, and Ar in the plasma plume,
TGF is the total gas flow, P is the torch power, and D is the stand
off distance measured in cm. Strict adherence to these limits are
not believed to be required, as taking a slightly higher N.sub.2 or
H.sub.2 fraction or lower Ar fraction may be mitigated by higher
coolant rates, and/or slightly lower total gas flow rates, for
example. Accordingly, a cumulative deviation from these norms of
less than 20%, more preferably of less than 10%, and more
preferably of less than 5%, or 1% are likely to be operable and
provide the advantages of the present invention, subject to the
limits on safe use of the high enthalpy air plasma spray torch, and
an acceptable deposition efficiency.
[0049] Although not necessarily mandatory, pre-heating of the
surface of the substrate using the torch can be employed. The
typical pre-heating temperatures are above .about.500.degree. C.,
more preferably .about.800-1000.degree. C.
EXAMPLE 1
Mullite
[0050] The same mullite powder (SG #1020, d.sub.10=15
.mu.m/d.sub.50=30 .mu.m/d.sub.90=50 .mu.m, Saint-Gobain,
Worchester, Mass., USA) was sprayed using a conventional 40 kW APS
torch (F4-MB, Sulzer Metco, Westbury, N.Y., USA), and a
high-enthalpy 120 kW APS torch (Axial III, Northwest Mettech, North
Vancouver, BC, Canada). In the case of conventional spraying, a
cooling system consisted of air lets was used to cool the front
size of the substrate to limit the maximum coating surface
temperature reached during deposition to .about.100.degree. C.
High-enthalpy spraying was performed without assisted heating or
cooling, and the maximum coating surface temperature during
deposition reached values of .about.1000.degree. C.
[0051] A carrier gas (Ar at 9 slpm) was used to feed the mullite
powder in a conventional manner, delivering the powder at a rate of
approximately 4 g/min. Specifically, the high enthalpy torch spray
parameters were as follows: torch plume gasses were: N.sub.2 45%,
Ar 45%, and H.sub.2 10%, total gas flow 280 slpm, power .about.120
kW, and a stand off distance was 6 cm. The diameter of the torch
nozzle was 0.5'' (1.27 cm). The plume had a length greater than 6
cm. The substrate was SiC Hexaloy SA.TM. (Saint-Gobain, Niagara
Falls, N.Y., USA). Pre-heating of the substrate with the torch was
not employed in this specific case.
[0052] Both coatings were analyzed via XRD, and the results are
shown in FIG. 2. The XRD technique was employed as an attempt to
compare the degree of crystallinity of the coatings. The ratio of
peak areas between 20.degree. and 40.degree. to the total
integrated area in this region was measured and defined as a
crystallinity index (CI). The CI value is proportional to the
degree of crystallinity of the coating, at least on its surface.
The high-enthalpy spraying produced highly crystalline mullite
coatings (B: crystallinity index CI.gtoreq.0.95). These highly
crystalline coatings also did not exhibit signs of debonding or
delamination when deposited on SiC-based substrates. In comparison,
the CI of the conventional sprayed mullite was about 0.25, and the
XRD spectrum shows the typical amorphous hump across the
20-40.degree. range (A).
[0053] Because X-rays used in regular XRD analysis penetrate the
sample to a depth of few microns below its surface, and as the
coating temperature was raised during deposition from room
temperature, it might be hypothesized that some amorphous residual
mullite phase may have been formed in the initial few layers
(passes) of coating deposition. This residual amorphous mullite, if
formed, is unlikely to be detected via regular XRD probing. In
order to investigate whether amorphous mullite was present in the
microstructure of the as-sprayed coatings, they were heat-treated
at temperatures above the known crystallization point of mullite
(i.e., .about.1000.degree. C.) and examined using differential
thermal analysis (DTA). The temperature was raised at a rate of
10.degree. C./min up to 1200.degree. C. in an N.sub.2 atmosphere.
The results of DTA are shown in FIG. 3. A shows a strong peak of
the conventional APS sprayed coating, whereas B shows no peak at
989.degree. for the high enthalpy APS sprayed coating.
[0054] In order to perform a valid comparison, the mass of each
coating evaluated via DTA was similar (.+-.30 mg). The DTA
exhibited an exothermic phase transformation peak at 989.degree. C.
for the low crystallinity coating (CI=0.25), very similar to that
observed by Lee et al. [12] for a highly amorphous mullite thermal
spray coating, which corresponds to the crystallization temperature
of the mullite. The coating deposited with the high-enthalpy APS
torch, did not exhibit any peaks at this region, confirming the
mullite coating contained no amorphous phase through its overall
microstructure.
EXAMPLE 2
BSAS
[0055] FIG. 4 is a SEM image of a spray-dried BSAS powder (Amperit
870.084, d.sub.10=20 .mu.m/d.sub.50=53 .mu.m/d.sub.90=86 .mu.m, H.
C. Starck, Newton, Mass., USA) employed in this invention.
[0056] This powder was fed to a high enthalpy APS torch. The
coating was applied overtop of an interlayer of mullite (deposited
as described above). The powder feed was assisted by a carrier gas
(Ar at 9 slpm) at a powder feed rate of .about.4 g/min.
[0057] Table 1 lists spray parameters used to produce BSAS coatings
and mullite coatings with the Axial III APS torch (torch nozzle
diameter 0.5'' (1.27 cm)). The first three rows are better suited
to BSAS deposition, and the last two sets are better suited to
mullite deposition. Specifically, the density of BSAS coating
produced with the deposition conditions according to bottom two
rows were found to be too dense, precluding the easy formation of
vertical cracks, and the deposition conditions according to the
first 3 rows produce mullite with lower density than is desired.
Many other thermal spray conditions could be varied that would have
an expected effect on the thermal spray conditions resulting in
similar coatings, as is well known in the art, and thus features
such as feedstock characteristics (e.g. morphology and porosity),
feedstock particle size distribution, and feedstock powder feed
rate, inter alia can have some impact on the optimal settings.
TABLE-US-00001 TABLE 1 Total gas flow Stand off N2 % Ar % H2 %
(slpm) Power (kW) distance (cm) 45 45 10 280 87 6 45 45 10 280 113
6 45 45 10 280 118 6 50 25 25 280 127 10 50 25 25 280 133 10
[0058] The BSAS powder was sprayed using a high-enthalpy APS torch
(Axial III, Northwest Mettech, North Vancouver, BC, Canada). The
coating deposition occurs via a scan pattern of the APS torch on
the substrate surface, which sprays successive layers. The
temperature of the surface of the coating was monitored with a
pyrometer, which is placed behind the APS torch. It is important to
point out that the pyrometer measures the temperature aimed over a
specific point (target) on the coating surface, i.e., the overall
surface coating temperate and the substrate temperature are not
measured.
[0059] The maximum measured coating surface temperature at the
target of the pyrometer was at least .about.1000.degree. C. when
the high-enthalpy torch was operated at .about.113-118 kW, and at
least 800.degree. C. when operated at .about.87 kW both at a stand
off distance of 6 cm. The coating surface temperature is
challenging to measure, due to the fact that this event occurs when
the high energetic plasma plume and spray jet is over the specific
pyrometer target. Thus there is a spatial and/or temporal lag
between when the surface is maximally heated and when the
temperature is measured. Therefore, the real maximum coating (or
substrate) surface temperature is higher than measured via a
pyrometer or infra-red camera.
[0060] FIG. 5 is a SEM image showing an as-sprayed barrier system
on the SiC substrate, the barrier system having the Si bond coat,
the mullite interlayer and a BSAS top coat deposited on a SiC
Hexoloy SA.TM. substrate. The microstructure in FIG. 5 shows that
the Si bond coat appears as a dense and crack-free layer showing
good adhesion to the substrate (i.e., no gaps or spallation).
Likewise good adhesion is shown between the interfaces of the Si
bond coat, the mullite interlayer and the near-crack free BSAS top
coat. This appears to be the first report of an as-sprayed
crystalline BSAS coating (Celsian phase) produced without assisted
heating or post spray annealing, and the first report of an
as-sprayed crystalline mullite coating produced without assisted
heating or cooling or post spray annealing.
[0061] As the standard attempt to simulate the lean environment of
a gas turbine [2], the barrier system was subjected to a thermal
treatment at 1300.degree. C. (1 atm) in a continuous flow
(.about.3.5 cm/s) of water vapour (90% H.sub.2O, 10% air) inside a
tube furnace for up to 500 h. FIG. 6 includes an image of the same
barrier system, showing the effects of heat treatment at
1300.degree. C. for 500 h in a water vapour environment (left). An
enlargement of the SiC substrate/bond coat interface (bottom right)
shows no appreciable difference before and after the heat
treatment. An image of an unprotected SiC Hexoloy SA.TM. substrate
after the same type of heat treatment is shown at the top right.
After 1300.degree. C./500 h in water vapour, a SiO.sub.2 scale
loosely adhered to the substrate was formed over the SiC surface.
Severe through-cracks were formed throughout this silica layer.
Therefore, the mullite and BSAS coatings deposited in accordance
with an embodiment of the present invention effectively protected
the Si-based ceramics substrate against oxidation and water vapour
attack.
[0062] The BSAS coating composition was analyzed in several
manners. FIG. 7 shows X-ray diffraction (XRD) spectra of BSAS
powder (Amperit 870.084, H. C. Starck, Newton, Mass., USA), the
as-sprayed coating, and the coating after exposure to a high
temperature, aggressive heat treatment. Both which substantially
consist of the Celsian monoclinic phase
(Ba.sub.0.75Sr.sub.0.25Al.sub.2Si.sub.2O.sub.8), as identified
based on the JCPDF #38-1451. The as-sprayed coating basically
exhibits the same crystalline structure, i.e., the diffraction
peaks overlap those of the powder. No evident amorphous hump is
observed. Therefore, this as sprayed coating appears to be highly
crystalline, exhibiting predominantly the celsian phase. After heat
treatment at 1300.degree. C. for 500 h, substantially the same XRD
pattern is produced.
[0063] Harder and Faber [8] studied the
amorphous/Hexacelsian-to-Celsian phase transformation kinetics of
BSAS. They subjected samples to heat treatments between 1100 and
1400.degree. C. from 30 min to 40 h. At 1100.degree. C., there was
no discernable transformation up to 40 h of heat treatment. Phase
transformation to Celsian began to be observed at temperatures of
1200.degree. C. This explains why the prior art typically requires
annealing at temperatures above 1250.degree. C.
[0064] The exclusivity of the Celsian phase in the as-sprayed
coating can be explained by positing that the real maximum coating
surface temperature at the plasma jet spot was higher than
1200.degree. C. By using a spray jet diameter that is smaller than
the plume diameter at the surface, the region under the spot is
subjected to an intense heat provided by the plume, providing for
annealing in real time of the coating during its deposition.
[0065] FIG. 8 is a scanning electron micrograph showing in plan
view, a single bead of BSAS sprayed at .about.120 kW (SiC Hexoloy
SA.TM. substrate surface pre-heated with the torch at
.about.800.degree. C.). Arrows point to inclusions that were
partially molten in deposition. About half of the surface of the
bead is covered by such semi-molten regions. This considerable
amount of semi-molten particles will likely influence the energy
necessary to nucleate Celsian from amorphous and metastable
Hexacelsian (Harder and Faber [8]) during the concurrent annealing
and deposition performed by the high enthalpy APS torch.
[0066] FIG. 9 is a XRD spectrum of the single BSAS bead of FIG. 8
and the XRD spectrum of a BSAS coating (.about.180 .mu.m thick/SiC
substrate surface pre-heated with the torch at .about.700.degree.
C.) sprayed at .about.85 kW (maximum coating temperature
.about.800.degree. C.), using the same APS torch, plasma gas
composition and spray distance (6 cm). The XRD spectrum shows that
in spite being deposited at similar surface temperature conditions,
the bead exhibits a significant amount of amorphous phase,
characterized by the amorphous hump between the angles of
20.degree. and 40.degree.. On the contrary, the coating exhibits a
highly crystalline Celsian phase. Based on this information, it is
advisable to deposit the BSAS in at least 3 and preferably in at
least 5 passes to perform annealing in real time. The conditions
employed in high-enthalpy APS torches, such as, higher torch power
and total gas flow than those of conventional ones, help to explain
the fact that other inventions had to employ holding time and/or
post-spray annealing to produce BSAS coatings exhibiting at least
50 vol % of celsian phase. The higher power levels and total gas
flows, in addition to the high enthalpy N.sub.2 gas, provided by
the high enthalpy APS torch create the conditions for annealing the
sample in real time during spraying, thereby inducing the formation
of the celsian phase in as-sprayed coatings, without the necessity
of a holding time and/or post-spray annealing. This annealing in
real time is carried out throughout the successive deposit of spray
beads, layers and torch passes over the substrate surface, because
one single pass forming a single bead does not promote the
formation of a highly crystalline celsian phase.
[0067] Harder and Faber [8] studied the transformation kinetics of
BSAS coatings produced via APS from a fully crystalline celsian
powder. As-sprayed coatings exhibited a mixture of amorphous,
metastable Hexacelsian and Celsian BSAS phases. The presence of the
Celsian phase was attributed to the presence of semi-molten
particles embedded in the coating microstructure, which was
estimated to be .about.10 wt %. By employing electron backscattered
diffraction (EBSD) it was observed that the transformations from
amorphous/metastable Hexacelsian phase to the Celsian phase at high
temperatures were strongly influenced by the Celsian phase. The
semi-molten BSAS celsian particles embedded in the as-sprayed
coating acted as seeds for the nucleation of the Celsian phase
transformation from the amorphous and metastable hexacelsian, upon
annealing. Harder and Faber concluded that the energy necessary to
transform APS BSAS coatings will be heavily influenced by the
semi-molten particles [8], i.e., the higher the amount of
semi-molten BSAS Celsian particles embedded in the coating
microstructure, the lower the amount of energy required to nucleate
the amorphous/Hexacelsian BSAS into stable crystalline Celsian.
[0068] Based on the equations determined by Harder and Faber [8],
it is possible to calculate that it is necessary to reach a
temperature of 1850.degree. C. for .about.8 seconds to induce the
phase transformation of 50% of Celsian phase from metastable
Hexacelsian. Over a 4 cm.times.4 cm substrate area, the .about.2.5
cm diameter plasma plume spot stays .about.8 seconds over the
surface of the coating during one complete set of torch passes
(single beads) to produce a single layer.
[0069] In order to obtain an approximate effect of the substrate
surface heating by the spray torch, the following experiment was
carried out. The high enthalpy APS torch was set up at the same
conditions to deposit a BSAS coating and placed standing still in
front of a SiC-based CMC substrate for 5 seconds, but no BSAS
powder was fed. After this interval, the torch was set aside in a
rapid thrust to open a line of sight for the infra-red (IR) camera
to immediately measure the substrate surface temperature. The
temperature profile can be seen in FIG. 10.
[0070] The IR camera used can measure temperatures from 200.degree.
C. to 1600.degree. C. When the torch is shifted off the substrate
surface, and the IR camera comes in line of sight with the
substrate surface, the measured temperature increases abruptly to
values higher than 1600.degree. C. (i.e., the limit of the
equipment). The temperature is not registered for .about.1.5 s,
until it cools down to 1600.degree. C. and below. Therefore, during
a continuous deposition when multiple torch scans are employed to
build the coating on the substrate surface, using the conditions
described in this invention, the coating surface temperature levels
under the plasma plume spot likely reach values higher than
1600.degree. C. When the BSAS coating surface is exposed to these
temperature levels for some seconds, the thermodynamic conditions
required to induce the amorphous/Hexacelsian phase transformation
to stable crystalline Celsian seem to occur.
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[0085] Other advantages that are inherent to the structure are
obvious to one skilled in the art. The embodiments are described
herein illustratively and are not meant to limit the scope of the
invention as claimed. Variations of the foregoing embodiments will
be evident to a person of ordinary skill and are intended by the
inventor to be encompassed by the following claims.
* * * * *
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