U.S. patent application number 13/715361 was filed with the patent office on 2013-06-20 for reactive gas shroud or flame sheath for suspension plasma spray processes.
The applicant listed for this patent is Albert Feuerstein, Don J. Lemen, Thomas F. Lewis, III, Christopher A. Petorak. Invention is credited to Albert Feuerstein, Don J. Lemen, Thomas F. Lewis, III, Christopher A. Petorak.
Application Number | 20130156968 13/715361 |
Document ID | / |
Family ID | 47520274 |
Filed Date | 2013-06-20 |
United States Patent
Application |
20130156968 |
Kind Code |
A1 |
Petorak; Christopher A. ; et
al. |
June 20, 2013 |
REACTIVE GAS SHROUD OR FLAME SHEATH FOR SUSPENSION PLASMA SPRAY
PROCESSES
Abstract
A system and method for producing thermal spray coatings on a
substrate from a liquid suspension is disclosed. The disclosed
system and method include a thermal spray torch for generating a
plasma and a liquid suspension delivery subsystem for delivering a
flow of liquid suspension with sub-micron particles to the plasma
to produce a plasma effluent. The liquid suspension delivery
subsystem comprises an injector or nozzle which can produce a
reactive gas shroud surrounding the plasma effluent. A flame
envelope can also be used to isolate injection of the liquid
suspension. The shroud or flame envelope can retain the sub-micron
particles entrained within the plasma effluent and substantially
prevent entrainment of ambient gases into the plasma effluent. The
liquid suspension delivery subsystem can be arranged as an axial
injection system, a radial internal injection system or an external
radial injection system.
Inventors: |
Petorak; Christopher A.;
(Carmel, IN) ; Lemen; Don J.; (Indianapolis,
IN) ; Feuerstein; Albert; (Carmel, IN) ;
Lewis, III; Thomas F.; (Zionsville, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Petorak; Christopher A.
Lemen; Don J.
Feuerstein; Albert
Lewis, III; Thomas F. |
Carmel
Indianapolis
Carmel
Zionsville |
IN
IN
IN
IN |
US
US
US
US |
|
|
Family ID: |
47520274 |
Appl. No.: |
13/715361 |
Filed: |
December 14, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61570516 |
Dec 14, 2011 |
|
|
|
61570532 |
Dec 14, 2011 |
|
|
|
Current U.S.
Class: |
427/446 ;
239/290 |
Current CPC
Class: |
C23C 4/134 20160101 |
Class at
Publication: |
427/446 ;
239/290 |
International
Class: |
C23C 4/12 20060101
C23C004/12 |
Claims
1. A thermal spray system for producing coatings on a substrate
from a liquid suspension comprising: a thermal spray torch for
generating a plasma; a liquid suspension delivery subsystem for
delivering a flow of the liquid suspension with sub-micron
particles; and a nozzle assembly for delivering the plasma from the
thermal spray torch to the liquid suspension to produce a plasma
effluent, the nozzle assembly adapted for producing a reactive gas
shroud substantially surrounding said plasma effluent; the reactive
gas shroud configured to substantially retain entrainment of the
sub-micron particles in the plasma effluent and substantially
inhibit gases from entering and reacting with the plasma effluent;
wherein the reactive gas shroud reacts with the plasma effluent to
enhance fragmentation of the suspension droplets and create
evaporative species of the sub-micron particles within the plasma
effluent.
2. The thermal spray system of claim 1, wherein the shroud extends
from the substrate surface to the nozzle assembly.
3. The thermal spray system of claim 1, wherein the shroud is a
laminar flowing shield.
4. The thermal spray system of claim 1, wherein the shroud has an
axial distance less than a distance from the nozzle to the
substrate surface.
5. The thermal spray system of claim 1, further comprising an inert
gas shroud disposed about the reactive gas shroud.
6. The thermal spray system of claim 1, further comprising a first
reactive gas shroud and a second reactive gas shroud.
7. The thermal spray system of claim 1, further comprising an
injector adapted to produce a flame envelope surrounding the flow
of the liquid suspension.
8. The thermal spray system of claim 1, wherein the liquid
suspension system is configured internal to the nozzle.
10. The thermal spray system of claim 1, wherein the liquid
suspension system is configured internal to the nozzle so as to
deliver an axial flow of the liquid suspension.
11. The thermal spray system of claim 1, wherein the liquid
suspension system is configured external to the nozzle.
12. A method of producing coatings on a substrate using a liquid
suspension with sub-micron particles dispersed therein, the method
comprising the steps of: generating a plasma from a thermal spray
torch; delivering a flow of liquid suspension with sub-micron
particles dispersed therein to the plasma or in close proximity
thereto to produce a plasma effluent stream; surrounding the plasma
effluent with a reactive gas shroud to keep the sub-micron
particles entrained within the plasma effluent and substantially
prevent entrainment of ambient gases into the plasma effluent;
reacting the shroud gas with the plasma effluent to enhance
fragmentation of the suspension droplets and create evaporative
species of the sub-micron particles within the plasma effluent; and
directing the shrouded plasma effluent with the sub-micron
particles contained therein towards the substrate to coat the
substrate.
13. The method of claim 12, further comprising the step
of-substantially preventing entrainment of gases into the shrouded
effluent.
14. The method of claim 12, further comprising the step of
fragmenting droplets of the liquid suspension across the reactive
shroud.
15. The method of claim 12, further comprising the step of
introducing an inert gas shroud substantially surrounding the
effluent.
16. The method of claim 12, further comprising the step of
introducing a second reactive shroud gas substantially surrounding
the effluent.
17. The method of claim 12, further comprising the step of
introducing a flame envelope surrounding the liquid suspension.
18. The method of claim 12, further comprising injecting the liquid
suspension external to the nozzle.
19. The method of claim 12, further comprising injecting the liquid
suspension internal to the nozzle.
20. A coating deposited on the substrate prepared according to the
process of claim 12.
Description
[0001] The present application claims priority from U.S.
application Ser. No. 61/570,532, filed Dec. 14, 2011, which is
incorporated by reference herein in its entirety, and U.S.
application Ser. No. 61/570,516, filed Dec. 14, 2011, which is
incorporated by reference herein in its entirety.
FIELD OF INVENTION
[0002] The present invention relates to suspension plasma sprays,
and more particularly to methods and systems for the shrouding of
suspension plasma spray effluents and/or sheathing the injection of
liquid suspensions using a reactive gas and/or flame envelope to
facilitate and control the effluent and suspension
interactions.
BACKGROUND
[0003] Conventional plasma spray technology primarily uses powder
feeders to deliver powdered coating material into a plasma jet of a
plasma spray gun. However, this technology is typically limited to
the use of particles of at least +350 mesh (i.e., a median particle
size of approximately of 45 microns in which 50 percent of
particles are smaller than the median size and the other 50 percent
of the particles are larger than the median size) or larger. As
particle size decreases below +325 mesh, introducing powdered
coating material directly into the plasma jet becomes progressively
more difficult. Fine particles tend to pack tightly and
agglomerate, increasing the likelihood of clogging in conventional
powder feed systems.
[0004] In addition to clogging, conventional plasma spray
technology is also ill-suited to the use of fine particles for
other reasons. Because of the low mass of fine particles, combined
with the extreme velocities of the plasma jet, fine particles tend
to be deflected away from a boundary layer of the plasma jet
without penetrating the boundary layer during radial injection. The
velocity necessary for penetration of the fine coating particles is
too large to physically be accomplished without disturbing the
effluent itself. Practical limitations exist to increase velocity
to this degree.
[0005] The need for coating finer particles is desired for use in
thermal barrier coatings. The finer particles typically result in
denser coatings and finer microstructural features, including for
example, smaller lamellar splats and grains. The finer particles
also tend to produce coated parts with improved microstructure.
Fine particles are also easier to melt because of its large surface
area relative to its small mass.
[0006] Suspension plasma spray (SPS) has emerged as a means for
depositing finer particles. SPS is a relatively new advancement in
plasma spray techniques which utilizes a liquid suspension of
sub-micron size particles of the coating constituents or
particulates materials, rather than a dry powder, as the coating
media. The liquid serves as a carrier for the sub-micron size
particles that would otherwise tend to agglomerate restricting or
eliminating powder flow to the torch. The liquid also has been
shown to function as a thermally activated solution that
precipitates solids or reacts with suspended particles. Due
primarily to the use of very small particles suspended in the
liquid carrier, the suspension plasma spray process has
demonstrated the ability to create unique coating microstructures
with distinctive properties. The liquid droplets also provide the
additional mass to impart the momentum necessary for entrainment by
radial injection.
[0007] Notwithstanding the improvements of SPS over conventional
plasma spray technology, current SPS systems and processes continue
to suffer from a variety of drawbacks. For instance, conventional
SPS typically produce coatings having uncontrolled microstructure
grain size and/or lack of directional orientation growth, both of
which can result in poor coating properties. To further compound
the microstructural problem, adverse chemical reactions can occur
between the substrate and the deposited coating materials.
[0008] Further, longer stand-off distances between the nozzle
location and the deposition point may be required to adequately
coat complicated geometries such as turbine blades. However, the
longer stand-off distances may provide the coating constituents
excessive dwell or residence time, thereby causing cooling and
resolidifcation of coating constituents prior to reaching the
substrate. Reducing the stand-off distance can cause insufficient
heating such that the particulates are never able to absorb enough
heat and fully melt. In both cases, the end result is lack of
particulate adhesion to the substrate, thereby reducing deposition
efficiency of the material. The finer particulate size of the
coating constituents have increased surface areas that can rapidly
heat up and cool down at faster rates than typically encountered in
standard plasma technology. Accordingly, the increased surface area
of the finer particulates creates unprecedented challenges to
optimizing the correct stand-off distance.
[0009] Still further, turbulent flow of the plasma gas effluent
emerges from the nozzle of the torch. The turbulent interaction of
the plasma effluent with the atmosphere imparts rapid decreases in
effluent temperature and rapid directional flow changes that result
in the ejection of the coating particulates from the flow path
directed to the substrate. As a result, the ejected particulates
result in decreased deposition efficiency.
[0010] The above problems are only a few examples of the types of
new challenges posed by the utilization of SPS systems and
processes to deposit ever increasingly finer coating media
constituents. In view on the on-going challenges, there is a need
to improve upon the current suspension plasma spray processes and
systems.
SUMMARY OF INVENTION
[0011] As described in more detail below, the present embodiments
of the invention addresses some of the disadvantages and provides
techniques to control the aforementioned interactions through use
of a reactive gas shroud surrounding the plasma effluent stream and
liquid suspension contained therein (collectively, referred to as
"effluent," "effluent stream," "plasma," or "plasma effluent," or
"plasma effluent stream" herein and throughout the specification).
The present invention uniquely combines a reactive gas shroud with
a plasma spray process using submicron particles delivered via
liquid suspension to improve current suspension plasma spray
capabilities and create new coating microstructure possibilities
through controlling the suspension injection and fragmentation as
well as the interactions between the effluent and suspensions.
[0012] The invention may include any of the following aspects in
various combinations and may also include any other aspect
described below in the written description or in the attached
drawings.
[0013] The present invention may be characterized as a thermal
spray system for producing coatings on a substrate from a liquid
suspension comprising: a thermal spray torch for generating a
plasma; a liquid suspension delivery subsystem for delivering a
flow of the liquid suspension with sub-micron particles; and a
nozzle assembly for delivering the plasma from the thermal spray
torch to the liquid suspension to produce a plasma effluent, the
nozzle assembly adapted for producing a reactive gas shroud
substantially surrounding said plasma effluent; the reactive gas
shroud configured to substantially retain entrainment of the
sub-micron particles in the plasma effluent and substantially
inhibit gases from entering and reacting with the plasma effluent;
wherein the reactive gas shroud reacts with the plasma effluent to
enhance fragmentation of the suspension droplets and create
evaporative species of the sub-micron particles within the plasma
effluent.
[0014] The present invention may also be characterized as a method
of producing coatings on a substrate using a liquid suspension with
sub-micron particles dispersed therein, the method comprising the
steps of: generating a plasma from a thermal spray torch;
delivering a flow of liquid suspension with sub-micron particles
dispersed therein to the plasma or in close proximity thereto to
produce a plasma effluent stream; surrounding the plasma effluent
with a reactive gas shroud to keep the sub-micron particles
entrained within the plasma effluent and substantially prevent
entrainment of ambient gases into the plasma effluent; reacting the
shroud gas with the plasma effluent to enhance fragmentation of the
suspension droplets and create evaporative species of the
sub-micron particles within the plasma effluent; and directing the
shrouded plasma effluent with the sub-micron particles contained
therein towards the substrate to coat the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The above and other aspects, features, and advantages of the
present invention will be more apparent from the following, more
detailed description thereof, presented in conjunction with the
following drawings, wherein:
[0016] FIG. 1 is a schematic illustration of a prior art suspension
plasma spray process employing an axial injection of the liquid
suspension;
[0017] FIG. 2 is a schematic illustration of a prior art suspension
plasma spray process employing an internal radial injection of the
liquid suspension;
[0018] FIG. 3 is a schematic illustration of a prior art suspension
plasma spray process employing an external radial injection of the
liquid suspension;
[0019] FIG. 4 is a schematic illustration of a reactive gas shroud
of a suspension plasma spray process employing an axial injection
of the liquid suspension in accordance with an embodiment of the
present invention;
[0020] FIG. 5 is a schematic illustration of a reactive gas shroud
of a suspension plasma spray process employing an internal radial
injection of the liquid suspension in accordance with another
embodiment of the present invention;
[0021] FIG. 6 is a schematic illustration of a reactive gas shroud
of a suspension plasma spray process employing an external radial
injection of the liquid suspension in accordance with yet another
embodiment of the present invention;
[0022] FIG. 7 shows yet another embodiment of the present invention
employing a dual gas shroud consisting of an inner reactive gas
layer and an outer inert gas shield surrounding a suspension plasma
spray process;
[0023] FIG. 8 shows yet another embodiment of the present invention
employing a dual gas shroud consisting of a first reactive gas
layer and a second reactive gas layer surrounding a suspension
plasma spray process;
[0024] FIG. 9 is a schematic illustration of an suspension plasma
spray process employing a gas shrouded or gas sheathed axial
injection of the liquid suspension in accordance with an embodiment
of the present invention;
[0025] FIG. 10 is a schematic illustration of a suspension plasma
spray process employing a gas shrouded or gas sheathed internal
radial injection of the liquid suspension in accordance with
another embodiment of the present invention; and
[0026] FIG. 11 is a schematic illustration of a suspension plasma
spray process employing a gas shrouded or gas sheathed external
radial injection of the liquid suspension in accordance with yet
another embodiment of the present invention.
DETAILED DESCRIPTION
[0027] The present disclosure relates to a novel SPS system and
process for the deposition of coating material. The SPS system and
process of the present invention is particularly suitable for
deposition of sub-micron particles. The disclosure is set out
herein in various embodiments and with reference to various aspects
and features of the invention.
[0028] The relationship and functioning of the various elements of
this invention are better understood by the following detailed
description. The detailed description contemplates the features,
aspects and embodiments in various permutations and combinations,
as being within the scope of the disclosure. The disclosure may
therefore be specified as comprising, consisting or consisting
essentially of, any of such combinations and permutations of these
specific features, aspects, and embodiments, or a selected one or
ones thereof.
[0029] The present invention recognizes the shortcomings of current
SPS systems and processes. These shortcomings can be better
identified by referring to FIGS. 1-3. FIGS. 1-3 show several
schematic illustrations of prior art suspension plasma spray
systems and processes 100, 200 and 300 employing an axial injection
of the liquid suspension; internal radial injection of the liquid
suspension and external radial injection of the liquid suspension,
respectively. In each of these prior art systems, numerous physical
and chemical interactions are occurring, many of which are
uncontrolled. For example, FIGS. 1 and 2 show fragmentation of the
liquid carrier occurs at regions 110 and 201 in an undesirable
random-like manner due to the turbulent flow in the effluent. The
fragmentation occurs soon after the plasma effluent and liquid
suspension are in contact. As used herein, the term "effluent" and
"plasma effluent" will be used interchangeably and are intended to
refer to any combination of the plasma gas, coating constituents or
particles and liquid carrier, each of which is flowing from the
outlet of a torch nozzle. For example, at the immediate outlet of
each of nozzles 105, 205 and 305 of their respective torches, the
effluent 140, 240 and 340 will more than likely consist of plasma
(i.e., hot primary torch gas ionized by virtue of being exposed to
an arc generated between the cathode and anode) and droplets of
liquid carrier containing coating particles (i.e., liquid
suspension 109, 209 and 309). However, within the vicinity of the
substrate 108, 208 and 308, the effluent 140, 240 and 340 will
primarily consist of the coating particulates and a potentially
significantly cooler effluent 140, 240 and 340, as substantially
all of the liquid carrier has evaporated by this stage of the SPS
coating process 100, 200 and 300.
[0030] FIGS. 1 and 2 also show that a portion of the fragmented
droplets of the liquid suspension 109 and 209 are ejected from the
effluent 140 and 240 at regions 110 and 210, respectively.
[0031] FIGS. 1-3 further show atmospheric entrainment 122, 222 and
322 into the plasma effluent 140, 240 and 340 in a region that is
in close proximity to the outlet of the torch nozzle 105, 205 and
305. The infiltration of atmospheric gases, including oxygen,
results in accelerated combustion of the entrained atmosphere with
flammable liquid carriers (e.g., ethanol). In addition, FIG. 1
shows there is evaporation of the liquid carrier, as shown by
representative region 105, causing many of the sub-micron solid
particles to coalesce and melt. Where ideal thermal conditions
within the effluent 140, 240 and 340 exist, a percentage of the
sub-micron or very fine particles transform into an evaporative
species, thereby resulting in lowered deposition efficiency and
inadequate coating of the substrate 108, 208 and 308.
[0032] These fragmented droplets, melted particles and evaporated
species of the suspension 109, 209 and 309 along with the
combustion by-products resulting from atmospheric entrainment are
carried along the effluent stream 140, 240 and 340 towards the
substrate 108, 208 and 308, during which time additional
suspension-particle chemical reactions occur including unwanted
reactions such as particle oxidation, as depicted at regions 105,
205 and 305. Also during the transit of the effluent 140, 240 and
340, many fragmented droplets and particles continue to be ejected
from the suspension 109, 209 and 309, thereby further lowering
deposition efficiency.
[0033] FIGS. 1-3 further show that as the effluent stream 140, 240
and 340 approaches the substrate 108, 208 and 308 to be coated, the
temperature profile within the effluent stream 140, 240 and 340
changes resulting in some re-solidification of cooler particles and
condensing of entrained evaporated species. Upon reaching the
substrate 108, 208 and 308, the coating material in the various
physical states impact the substrate and form a coating 106, 206
and 306, including the physical bonding of coating material to the
substrate. Adverse chemical reactions between the substrate 108,
208 and 308 and the coating materials can occur.
[0034] Current suspension plasma spray systems suffer from the
disadvantage of not adequately controlling these physical and
chemical interactions during the three key phases of the suspension
plasma spray process, namely: (i) suspension injection and
fragmentation; (ii) effluent and suspension interactions; and (iii)
substrate interactions with effluent and coating buildup.
[0035] As will be discussed in FIGS. 4-11, the present embodiments
of the invention address many of the aforementioned disadvantages
shown in FIGS. 1-3. The present invention provides techniques to
control the aforementioned adverse interactions through use of a
reactive gas shroud and/or sheath surrounding the effluent stream
and/or injection location for liquid suspension.
[0036] Turning now to FIGS. 4 through 6, there are shown schematic
illustrations of different embodiments of the present invention,
namely depictions of suspension plasma spray systems and processes
400, 500 and 600, respectively. SPS system and process 400 employs
an axial injection of the liquid suspension 409 with an extended
reacted gas shroud 401 surrounding the effluent 440 (i.e., plasma
and liquid suspension 409). Any suitable reactive gas may be used
to create the reacted gas shroud 401, such as, for example, oxygen,
hydrogen carbon dioxide; hydrocarbon fuels and in some instances
nitrogen or combinations thereof. Through use of reactive gas
shroud 401, the effluent 440 and suspension 409 interaction can be
more precisely controlled to create new coating microstructure
possibilities as a result of the chemical reactions occurring
between the suspension 409 and the reactive gas shroud 401.
[0037] FIG. 4 shows that the shroud 401 is created by flowing
reacted gas at a predetermined flow rate through an outer nozzle
that surrounds an inner nozzle through which the liquid suspension
409 and primary torch gas 416 can sequentially or co-flow relative
to each other. The shroud 401 is oriented around the flow of
effluent 402, thereby forming a protective envelope of reactive gas
that surrounds the effluent 440. FIG. 4 shows that the shroud 401
extends continuously from within the nozzle 405 of the torch to the
substrate surface 408 to create a completed envelope of the
effluent 440 contained therein.
[0038] Prior to the liquid suspension 409 emerging from the outlet
of nozzle 405, a plasma 419 is created as primary torch gas 416
flows between a cathode 412 and anode 413 into a region where an
arc is generated. The carrier gas transports the liquid suspension
409 and is shown flowing with the liquid suspension 409 through the
center of the nozzle 405. An arc is generated between the cathode
412 and anode 413. The primary torch gas 416 passes through the arc
region and ionizes into a hot plasma 419 of gaseous ions and/or
radicals within the nozzle 405. The plasma 419 provides the thermal
energy source required to evaporate the liquid carrier and melt the
coating constituents 415 of liquid suspension 409 as the effluent
440 flows towards the substrate surface 408. The plasma 419 also
provides the energy source to provide sufficient momentum to
accelerate the coating constituents or particles 415 towards the
substrate surface 408.
[0039] After the plasma 419 is created, the liquid suspension 409
(i.e., liquid carrier droplets with coating constituents 415
contained therein) and plasma 419 emerge from the outlet of the
nozzle 405 as an effluent 440. The shrouded gas 401 converges
within a throat section of the nozzle 405 and thereafter emerges
from the nozzle 405. It should be understood that the terms
"shroud" and "shrouded gas" have the same meaning and will be used
herein and throughout the specification interchangeably.
[0040] In a preferred embodiment, the reactive gas shroud 401 is an
oxygen-containing gas, such as, for example, oxygen gas or an
oxygen diluted mixture of gases. The oxygen-containing reactive gas
shroud 401 can be used to control or increase the degree of mixing
and spatial location of the mixing of the reactive gas 401 with the
effluent 440, thus more precisely controlling the degree and
location of the combustion with the effluent 440 and resulting
thermal energy profile. Enhanced combustion or other thermal
reactions also can improve the fragmentation of the droplets of the
liquid suspension 409 as well as evaporation of the sub-micron
coating particles 415 within the suspension 409. The
oxygen-containing reactive gas shroud 401 can be used with a fuel
based liquid carrier to produce more complete combustion which can
be initiated or effected further upstream or closer to generation
of the plasma source 419 than would occur with a non-shrouded spray
process or traditional inert gas shroud around the plasma spray
effluent. The embodiment of FIG. 4 demonstrates that advancing the
combustion process further upstream toward the plasma source 419
would enable use of lower power plasma torches to both melt and
evaporate the sub-micron particles 415 within the liquid carrier
through more efficient use of the plasma stream's thermal
energy.
[0041] The reactive gas shroud 401 is configured to flow at a
sufficient flow rate relative to that of the effluent 440 so as to
form a continuous envelope about the effluent 440. The effluent 440
is characterized as having a trajectory or flow path of the liquid
suspension 409 defined, at least in part, from the outlet of the
nozzle 405 to the substrate surface 408, whereby the flow path is
partially or fully enveloped by the reactive shroud 401. As shown
in the embodiment of FIG. 4, the length of the reactive shroud 401
extends from the outlet of the nozzle 405 to the substrate surface
408 to fully surround the effluent 440. The continuous envelope of
the shroud 401 creates a thermal envelope that acts as an effective
insulator to retain heat in the effluent stream 440 across longer
flow path distances from the outlet of the nozzle 405 to the
surface of the substrate 408. The controlled temperature from the
outlet of the torch 405 to the substrate 408 enables evaporation of
the liquid carrier of the liquid suspension 409. After evaporation
of the liquid carrier, the heat used to evaporate the liquid
carrier is now realized by the coating constituents 415 generally
contained within the droplets of the liquid suspension 409, which
are now free floating and travelling towards the substrate surface
408. The coating constituents 415 partially or substantially melt
without undergoing significant cool down as they flow towards the
surface of the substrate 408. The molten coating constituents 415
impact the substrate surface 408 to deposit as a coating 403. In
this manner, the improved thermal envelope therefore improves
deposition efficiency. Further, the retention of heat within the
effluent 440 creates improved uniformity in temperature
distribution that can decrease stand-off working sensitivity. As
such, the present invention as shown in the embodiment of FIG. 4
allows a unique SPS system and process 400 for coating complicated
geometries at father stand-off distances than previously attainable
with conventional SPS, without incurring substantial solidification
of the coating constituents 415 as they impact the substrate
surface 408.
[0042] While enhanced combustion resulting from use of the oxygen
containing gas and a fuel based liquid carrier is one embodiment of
the present system and method, other chemical reactions may be
facilitated with the use of a reactive shroud gas that will react
with various elements or compounds in the liquid medium resulting
in a chemical reaction that occurs spontaneously or occurs due to
the thermal energy of the plasma effluent. Such chemical reactions
can be designed and controlled to yield improvements in the coating
chemical composition, physical property or microstructure,
including for example the formation of oxides, carbides or nitrides
of the particles.
[0043] Advantageously, the use of the reactive gas shroud 401
around the plasma effluent 440 operates to create and/or retain
more heat in the effluent 440 providing a larger operation envelope
for the coating process. The larger operational envelope translates
to longer working distances between torch nozzle 405 and substrate
408 as well as better thermal treatment of the sub-micron particles
415. In other words, the sub-micron particles 415 along its flow
path trajectory are maintained at the prescribed operating
temperatures for longer residence times resulting in improved
melting and an increase in the evaporative species of the particles
within the plasma effluent 440. Use of reactive gas shroud 401 also
facilitates control of the environment and temperatures near the
substrate surface 408.
[0044] The use of a reactive gas shroud 401 surrounding a
suspension plasma spray effluent 440 opens up numerous
possibilities to develop new liquid carriers for such sub-micron
particle containing suspensions 409 or solutions.
[0045] In each of the embodiments of the present invention, the
reactive gas shroud can be configured in a controlled manner. The
most likely means of control involve adjusting or manipulating the
flow characteristics of the reactive gas shrouds, including the
volumetric flow rate and/or velocity of the gas shroud as well as
concentrations of the reactive elements in the reactive gas
shrouds. In addition, the turbulence and dispersion characteristics
of the reactive gas shroud may also be controlled. Many of these
flow characteristics are dictated by the geometry and configuration
of the nozzle or nozzles used to form the reactive gas shrouds as
well as the reactive shroud gas supply pressures and
temperatures.
[0046] The embodiment of FIG. 4 shows that the shrouded gas 401 is
configured to flow in a laminar flow rate regime. The controlled
and lowered velocity of the laminar flowing shroud 401 can enable
the fragmentation phenomena of the droplets of the liquid
suspension 409 across the shroud 401 to occur in a more controlled
manner compared to conventional SPS systems and processes 100, 200
and 300 of FIGS. 1-3. The fragmented droplets of liquid suspension
409 therefore attain an improved uniformity in size distribution.
As a result, the coating constituents 415 deposit on the substrate
surface 408 to form a coating 403 having a more controlled particle
size distribution.
[0047] The shroud 401 also counteracts any tendency for droplets of
the liquid suspension 409 to eject from the effluent 440. Generally
speaking, in the absence of the shroud 401, the effluent 440 is in
a turbulent flow regime which may be sufficient to break up liquid
droplets into smaller droplets, and in the process of doing so,
undesirably impart excessive momentum to at least some of the
droplets to eject them from the effluent stream 440. Employing the
shroud 401 can facilitate the retention of the droplets of the
liquid suspension 409 and coating constituents 415 within the
effluent 440. As a result, increased utilization of the coating
constituents 415 is attained.
[0048] The combination of the aforementioned process benefits can
produce a coating 403 deposited onto the substrate surface 408
having a microstructure with grain orientation and sufficiently
small particle size distribution. The favorable microstructural
possibilities are controllable and reproducible by virtue of the
innovative SPS system and process 400.
[0049] In accordance with another embodiment of the present
invention, FIG. 5 shows an SPS system and process 500 in which the
liquid suspension 509 is internally injected within the torch
nozzle 505. The internal injection of the liquid suspension 509 can
occur in a substantially radial direction at an orthogonal
orientation with respect to the axis of the plasma 519 that is
generated within the nozzle 505 between the cathode 512 and anode
513. It should be understood that the angle of injection of the
liquid suspension 509 relative to the plasma 519 may be varied.
[0050] FIG. 5 shows that the primary torch gas 516 passes through
the arc region and ionizes into a hot plasma state 519 of gaseous
ions within the nozzle 505. The liquid suspension 509 is internally
injected into the plasma region 519 It should be understood that
injection of suspension 509 can occur downstream of the plasma 519
within the anode, which may represent a region where the torch gas
516 has cooled down from the plasma state to a superheated gas. The
turbulent flow of the plasma 519 fragments and/or atomizes the
liquid carrier droplets of suspension 509 within the nozzle 505 and
also at the outlet of the nozzle 505.
[0051] As shown in the embodiment of FIG. 5, the length of the
reactive shroud 501 extends in a continuous manner from the outlet
of the nozzle 505 to the substrate surface 508. The shroud 501
provides heat retention to create a continuous thermal envelope and
also prevents ejection of the droplets of suspension 509 from the
effluent 540. The embodiment of FIG. 5 shows that the shrouded
reactive gas 501 is configured to flow in a laminar flow rate
regime. The controlled and lowered velocity of the laminar flowing
shroud 501 can enable the fragmentation phenomena of the droplets
of the liquid suspension 509 across the shroud 501 to occur in a
more controlled manner compared to conventional SPS systems and
processes 100, 200 and 300 of FIGS. 1-3. The fragmented droplets of
liquid suspension 509 therefore attain an improved uniformity in
size distribution. As a result, the coating constituents 515
deposit on the substrate surface 508 to form a coating 503 having a
more controlled particle size distribution. It should be understood
that certain coating applications may not require substantial
fragmentation of the droplets of liquid suspension 509. As such, in
another embodiment of the present invention, the shroud 501 can be
configured to not fragment the droplets yet still achieve the other
benefits of utilizing a shroud 501 that have been mentioned
above.
[0052] Other injection locations of the liquid suspension are
contemplated in accordance with the principles of the present
invention. For instance, FIG. 6 shows an SPS system and process 600
in which the liquid suspension 609 is injected externally to the
torch nozzle 605. The external injection of the liquid suspension
609 can occur in a substantially radial direction at an orthogonal
orientation with respect to the axis of the plasma effluent 640. It
should be understood that the angle of injection of the liquid
suspension 609 relative to the plasma effluent 640 may be varied.
Similar to FIG. 5, the reactive shrouded gas 601 is configured to
flow in a laminar flow rate regime to produce more uniform
fragmentation of the droplets of the liquid suspension 609.
[0053] Other variations for the reactive gas shroud are
contemplated by the present invention. For example, FIG. 7 is a
schematic illustration of another embodiment of the present
invention employing a dual gas shroud consisting of an inner
reactive gas shroud layer 701 and an outer inert gas shield 702
surrounding a suspension plasma spray process 700. The inner
reactive gas shroud layer 701 is preferably laminar flowing, as
shown in FIG. 7. Use of the dual shroud in this specific
arrangement may further improve heat retention within the region
that the effluent 740 flows within, particle fragmentation of the
droplets and temperature uniformity along the substrate 708. The
dual shroud also can improve confinement of the coating
particulates 715 within the effluent 740 along the flow path,
thereby substantially reducing or eliminating coating particulate
715 ejection from the effluent 740. As a result, increased
deposition efficiency on the substrate 708 is attained.
[0054] In yet another design variation of the reactive gas shroud,
FIG. 8 shows a dual reactive gas shroud consisting of a first inner
reactive gas shroud layer 802 and a second outer reactive gas
shroud layer 801 surrounding a suspension plasma spray process 800.
The first inner reactive gas shroud layer 802 is preferably laminar
flowing, as shown in FIG. 8. Unlike FIG. 7, the dual reactive gas
shroud has two reactive shrouds. Each of the reactive gas shrouds
801 and 802 is independently controlled (e.g., the flow rates are
independently controlled). The gases used for the reactive gas
shrouds 801 and 802 may be the same or different. The presence of
two reactive shrouds or shields that are independently controlled
can help improve combustion reactions along the flow path of the
effluent 840. In addition to enhanced combustion resulting from use
of a dual reactive gas shroud system and process 800, other
chemical reactions may be facilitated with the use of a dual
reactive shroud gas in which each of the reactive gas shrouds 801
and 802 preferentially react with specific elements or compounds in
the liquid suspension 809 resulting in a chemical reaction that
occurs spontaneously or occurs due to the thermal energy of the
plasma effluent 840. Such chemical reactions can be designed and
controlled to yield improvements in the chemical composition,
physical property or microstructure of the deposited coating
803.
[0055] Where dual layer shrouds or mixed shrouds are employed using
both reactive gases and inert gases, the inert gases typically
include argon, nitrogen, and helium or combinations thereof.
[0056] Other variations of the reactive gas shroud can be employed.
In one example, two or more reactive gas shrouds can be configured,
preferably independent of each other, to surround an effluent. In
another example, two or more reactive gas shrouds in combination
with an inert gas shroud can be employed. The inert gas shroud can
be configured between the reactive gas shrouds. Alternatively, the
inert gas shroud can be arranged so as to surround all of the
reactive gas shrouds. Still, as a further design variation, the
inert gas shroud or shield can be positioned within each of the
reactive gas shrouds. In another embodiment, a reactive gas shroud
may also be selectively configured so as to only surround only a
portion of the effluent along its flow path towards the
substrate.
[0057] The process benefits, some of which have been mentioned
above, can translate into more controlled microstructures of
deposited coatings. The present invention recognizes that
parameters which determine the microstructure and properties of the
coatings include the temperature, size and velocity of the coating
constituents or particles and the extent to which the particles
have reacted with or exposed to the surrounding environment during
deposition. In the present invention, the reactive gas shroud can
retain heat and create a more uniform temperature and controlled
temperature distribution as the coating particles impact the
substrate surface. Additionally, the laminar flow reactive gas
shrouds can help create more uniformly fragmented coating
particles. The shrouded effluent therefore creates an improved
microstructure.
[0058] Additional factors impacting the microstructure and
properties of the deposited coatings include the rate of
deposition, angle of impact, and substrate properties, each of
which can be controlled to a greater degree, by virtue of the
shroud. Since the coating constituents or particles are heated and
accelerated by the gaseous effluent of the plasma, the temperature
and velocity of the coating particles are a function of the
physical and thermal characteristics of the effluent stream and the
standoff distance between the exit of the plasma spray device and
the substrate. By controlling the properties of the effluent stream
by use of the shroud, the temperature and velocity of the coating
particles can be controlled with greater precision to improve
coating adhesion and coating microstructure.
[0059] A specific type of reactive gas shroud which can be employed
in the present invention is a flame envelope surrounding the liquid
suspension at or near the injection point. Turning now to FIGS. 9
through 11 there are shown schematic illustrations of different
embodiments of the configuration of a flame envelope, namely
depictions of suspension plasma spray systems and processes
employing a flame envelope shrouding the axial injection of the
liquid suspension; a flame envelope shrouding an internal radial
injection of the liquid suspension; and a flame envelope shrouding
an external radial injection of the liquid suspension,
respectively. The term "flame envelope" as used herein and
throughout the specification means a combusting flow formed by the
combustion of a fuel and an oxidant which extends along the axis of
the injected suspension stream.
[0060] FIG. 9 shows a suspension plasma spray system and process
900 employing a flame envelope 910 shrouding the axial injection of
the liquid suspension 909. The flame envelope 910 extends from the
distal end of the injection nozzle 905 or nozzle face up to a point
where the plasma 919 is generated between the cathode 912 and the
anode 913. It should be understood that the flame envelope 910 can
extend the entire length of the suspension stream being injected
from out of the nozzle 905(i.e., extends from the nozzle face to
the entry point in the plasma effluent). The flame envelope 910 can
provide sufficient thermal energy to evaporate the liquid droplets
prior to exiting nozzle 905. As such, dry sub-micron coating
particulates 915 can be introduced as the effluent 940 without
agglomeration and without clogging in the injector. The flame
envelope 910 can also provide sufficient kinetic energy to improve
fragmentation of the droplets of the suspension 909 and coating
particle 915 size distribution.
[0061] FIG. 10 shows an alternative suspension plasma spray system
and process 1000 employing a flame envelope 1010 shrouding the
radial injection of the liquid suspension 1009. The flame envelope
1010 extends along the injector of the liquid suspension 1009 and
can evaporate the liquid droplets prior to being introduced into
the effluent 1040. The flame envelope 1010 can also impart
sufficient kinetic energy to the droplets of suspension 1009,
thereby improving fragmentation and coating particle 1015 size
distribution.
[0062] The flame envelope may also be configured external of the
nozzle as shown in FIG. 11. FIG. 11 shows a suspension plasma spray
system and process 1100 employing a flame envelope 1110 shrouding
the radial injection of the liquid suspension 1109. The flame
envelope 1110 extends along the injector of the liquid suspension
1109 and can evaporate the liquid droplets prior to being
introduced into the plasma effluent 1119. The flame envelope 1110
can also impart sufficient kinetic energy to the droplets of
suspension 1109, thereby improving fragmentation and coating
particle 1015 size distribution.
[0063] As shown in the illustrated embodiments of FIGS. 9-11, the
flame envelope 910, 1010 and 1110 serves several functions. For
example, the flame envelope 910, 1010 and 1110 can function as a
shroud for the liquid suspension 909, 1009 and 1109 that prevents
the entrainment of ambient gases into the injected suspension
stream 909, 1009 and 1109 and thereby inhibits unwanted physical
and chemical reactions such as oxidation of the sub-micron
particles contained within the suspension 909, 1009 and 1109.
Preventing entrainment of ambient gases also inhibits velocity
decay of the suspension injection and allows the liquid suspension
909, 1009 and 1109 with the sub-micron particles contained therein
to penetrate into the plasma 919, 1019 and 1119 with substantial
retention of the injection velocity.
[0064] Furthermore, the flame envelope 910, 1010 and 1110 also
functions as a reactive shroud or partially reactive shroud that,
when properly controlled, can initiate desired reactions within
their respective liquid suspensions 909, 1009 and 1109 or between
the suspension 909, 1009 and 1109 and the shroud gases at or near
the point of injection. For example, where the liquid carrier is a
fuel, such as ethanol, the flame envelope initiates the combustion
reaction of the liquid carrier which increases both the thermal and
kinetic energy of the injection event proximate the entry to the
plasma effluent. This additional thermal and kinetic energy causes
improved fragmentation of the droplets as well as enhanced melting
or evaporation of the sub-micron particles in the suspension before
they reach the plasma effluent. In applications where the liquid
carrier is not a fuel, the flame envelope provides an energy source
to evaporate the liquid carrier and melt, partially melt or even
evaporate the suspended particles prior to entrainment into the
plasma effluent.
[0065] Generally speaking, by shrouding the liquid suspension in
the flame envelope as it is directed towards the plasma effluent,
the process characteristics of the overall suspension plasma spray
(SPS) system are radically altered. In short, use of the flame
envelop or similar reactive shroud surrounding the injection stream
effectively separates the control of the delivery of the sub-micron
coating particles to the SPS system, which is accomplished via
suspension from a supply vessel, from the control of the
entrainment of the sub-micron coating particles into the plasma,
which can be in suspension or non-suspension form.
[0066] For example, using the disclosed flame envelope surrounding
the suspension injection stream enables an SPS system employing
delivery of a suspension but entrainment or injection of a dry
submicron particle into the plasma effluent similar to APS powder
injection, but at the sub-micron particle size. Alternatively, the
flame envelope surrounding the suspension injection stream enables
an SPS system employing delivery of a suspension but entrainment or
injection of melted sub-micron particles into the plasma effluent,
the injection of evaporated species of the sub-micron particles
into the plasma effluent. Still further, the disclosed flame
envelope surrounding the suspension injection stream enables an SPS
system employing delivery of a liquid suspension with entrainment
or injection of highly fragmented suspension droplets into the
plasma effluent. Finally, if properly designed and controlled, the
disclosed flame envelope or reactive shroud surrounding the
injection stream enables delivery of a liquid suspension wherein
the sub-micron particles are reacted in-situ to form the desired
ceramic or cermet coating materials which are entrained into the
plasma effluent.
[0067] In addition, each of the above-described delivery, injection
and entrainment techniques allows more precise control of the
average particle size and particle size distribution injected or
entrained into the effluent and subsequently impacting the
substrate to provide the desired coating microstructures. The use
of a flame envelope or reactive shroud surrounding the suspension
injection enables new choices or design options for the composition
of the SPS liquid suspensions, including make-up of the liquid
carriers and particle characteristics.
[0068] Finally, since the flame envelope or reactive sheath/shroud
surrounding the liquid suspension injection in reference to the
Figures has the potential to provide additional thermal and kinetic
energy to the SPS spray process, the present system and method
would enable use of lower power plasma torches in an SPS process
and a more efficient use of the thermal energy in the plasma
stream. Also, the use of the presently disclosed flame envelope or
reactive sheath/shroud surrounding the liquid suspension injection
provides opportunities to further control and enhance the entire
SPS process including: delivery or handling of the suspension;
creating of the plasma jet; injection or entraining the coating
materials into the plasma jet; and delivery/impact of the coating
materials onto the substrate to be coated.
[0069] Through the use of the present flame envelope or reactive
sheath/shroud and the additional kinetic energy associated
therewith, the injection of the coating materials into the plasma
jet is preferably controlled so as to reach the optimized location
within the effluent and with reduced interaction by effluent flow
at the point of injection. For example, a portion of effluent at or
near the point of suspension injection can be deflected to allow
the sub-micron particles in either dry powder for, partially melted
form, melted form and/or evaporative form to extend further into
the effluent stream in a controlled and uniform manner
[0070] Alternatively, where the flame envelope or sheath/shroud is
employed as part of the SPS process merely to inhibit entrainment
of ambient gases into the injected suspension and to allow the
liquid suspension to penetrate deeply into the plasma effluent
stream, the sheath/shroud is likely to promote further
fragmentation of the suspension into droplets in controlled manner
and location. By fragmenting the droplets, the flame envelope or
reactive gas shroud aids in the control of the droplet size and
droplet size distribution of the suspension being injected into the
plasma effluent. In this manner, there is less fragmentation
occurring in the plasma effluent and droplet size and droplet size
distribution will be generally independent of spatial and temporal
changes occurring as the plasma effluent moves toward the substrate
to be coated. In other words, the droplet size and droplet size
distribution is more precisely controlled resulting in improved
plasma spray process control and improved coating microstructures
FIG. 9 shows another embodiment of the present invention employing
a combustion flame shroud surrounding a suspension plasma spray
process.
[0071] It should be appreciated that the use of dual gas shrouds
depicted in FIG. 7 and FIG. 8 as well as the use of a combustion
flame shroud surrounding the effluent depicted in FIG. 9 can be
equally applied to suspension plasma spray systems utilizing
internal radial injection configurations, external radial injection
configurations and axial injection configurations.
[0072] As indicated above, the typical reactive gases used for the
reactive gas shroud include oxygen, hydrogen, carbon dioxide;
hydrocarbon fuels, and nitrogen or combinations or combinations
thereof.
[0073] It is to be noted that the present invention is capable of
depositing a wide array of fine particulate sizes in the sub-micron
range, previously not possible by coating technologies, including
that of conventional plasma spraying. For example, in one
embodiment, the SPS system and process of the present invention can
deposit coating particulates below 100 nm. In another embodiment,
the present invention can deposit coating particulates 10 .mu.m or
lower, without incurring undesirable agglomeration of the fine
particulates as typically encountered in conventional spray systems
and processes.
[0074] Advantageously, the SPS system described herein can be
prepared utilizing suitable torch and nozzle assemblies that are
commercially available, thus enabling and simplifying the overall
fabrication process. Aspects of plasma generation can be carried
out using standard techniques or equipment.
[0075] Any suitable liquid suspension delivery subsystem can be
employed for delivering a flow of the liquid suspension with
sub-micron particles dispersed therein to the plasma. The liquid
suspension source is a dispenser for the liquid suspension. The
source typically includes a reservoir, transport conduit (e.g.,
tubing, valving, and the like), and an injection piece (e.g.,
nozzle, atomizer and the like). In addition, the liquid suspension
delivery subsystem may contain measurement feedback of the process
(e.g., flow rate, density, temperature) and control methods such
as, for example, pumps and actuators that can work in conjunction
or independently from one another. The system may also contain
additional flushing or cleaning systems, mixing and agitation
systems, heating or cooling systems as known in the art.
[0076] From the foregoing, it should be appreciated that the
present invention thus provides a system and method for reactive
gas shrouding of suspension plasma sprays and/or flame sheathing of
liquid suspensions. While the invention herein disclosed has been
described by means of specific embodiments and processes associated
therewith, numerous modifications and variations can be made
thereto by those skilled in the art without departing from the
scope of the invention as set forth in the claims or sacrificing
all of its features and advantages.
* * * * *