Note: Descriptions are shown in the official language in which they were submitted.
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Reactive Gas Shroud or Flame Sheath for Suspension Plasma Spray Processes
Field of Invention
[0001] 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
[0002] 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 +325mesh, 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.
[0003] 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.
[0004] 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.
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[0005] 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.
[0006] 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.
[0007] 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.
[0008] Still further, turbulent flow of the plasma gas effluent emerges
from the
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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.
[0009] 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
[00010] 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.
[00011] 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.
[00012] 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
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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.
[00013] 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
[00014] 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:
[00015] Fig. 1 is a schematic illustration of a prior art suspension plasma
spray
process employing an axial injection of the liquid suspension;
[00016] Fig. 2 is a schematic illustration of a prior art suspension plasma
spray
process employing an internal radial injection of the liquid suspension;
[00017] Fig. 3 is a schematic illustration of a prior art suspension plasma
spray
process employing an external radial injection of the liquid suspension;
[00018] 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;
[00019] Fig. 5 is a schematic illustration of a reactive gas shroud of a
suspension
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plasma spray process employing an internal radial injection of the liquid
suspension in
accordance with another embodiment of the present invention;
[00020] 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;
[00021] 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;
[00022] 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;
[00023] 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;
[00024] 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
[00025] 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
[00026] 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.
[00027] 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
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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.
[00028] The present invention recognizes the shortcomings of current SPS
systems
and processes. These shortcomings can be better identified by referring to
Figures 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, Figures 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.
[00029] Figures 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.
[00030] Figures 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
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oxygen, results in accelerated combustion of the entrained atmosphere with
flammable
liquid carriers (e.g., ethanol). In addition, Figure 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.
[00031] 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.
[00032] Figures 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
substrate108, 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.
[00033] 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.
[00034] As will be discussed in Figures 4-11, the present embodiments of
the
invention address many of the aforementioned disadvantages shown in Figures 1-
3. The
present invention provides techniques to control the aforementioned adverse
interactions
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through use of a reactive gas shroud and/or sheath surrounding the effluent
stream and/or
injection location for liquid suspension.
[00035] 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
effluent440 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.
[00036] Figure 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.
Figure 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.
[00037] 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
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accelerate the coating constituents or particles 415 towards the substrate
surface 408.
[00038] 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.
[00039] 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 Figure 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.
[00040] 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 Figure 4, the length of the reactive shroud
401
extends from the outlet of the nozzle 405 to the substrate surface 408 to
fully surround
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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 Figure 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.
[00041] 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.
[00042] 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-
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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.
[00043] 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.
[00044] 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.
[00045] The embodiment of Figure 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.
[00046] 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
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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.
[00047] 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.
[00048] In accordance with another embodiment of the present invention,
Figure 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.
[00049] Figure 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.
[00050] As shown in the embodiment of Figure 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 Figure 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
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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.
[00051] Other injection locations of the liquid suspension are contemplated
in
accordance with the principles of the present invention. For instance, Figure
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
Figure 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.
[00052] 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 Figure 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.
[00053] 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
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process 800. The first inner reactive gas shroud layer 802 is preferably
laminar flowing,
as shown in Figure 8. Unlike Figure 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.
[00054] 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.
[00055] 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.
[00056] 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
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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.
[00057] 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.
[00058] 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.
[00059] 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
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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.
[00060] 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.
[00061] The flame envelope may also be configured external of the nozzle as
shown in Figure 11. Figure 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.
[00062] 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-
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micron particles contained therein to penetrate into the plasma 919, 1019 and
1119 with
substantial retention of the injection velocity.
[00063] 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.
[00064] 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.
[00065] 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
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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.
[00066] 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.
[00067] 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.
[00068] 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
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[00069] 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.
[00070] 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.
[00071] 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.
[00072] 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 m or lower, without incurring undesirable
agglomeration of the fine
particulates as typically encountered in conventional spray systems and
processes.
[00073] Advantageously, the SPS system described herein can be prepared
utilizing
suitable torch and nozzle assemblies that are commercially available, thus
enabling and
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simplifying the overall fabrication process. Aspects of plasma generation can
be carried out
using standard techniques or equipment.
[00074] 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.
[00075] 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.
