U.S. patent application number 14/721171 was filed with the patent office on 2015-12-17 for flame-assisted flash sintering.
This patent application is currently assigned to NGIMAT CO.. The applicant listed for this patent is NGIMAT CO.. Invention is credited to Andrew Tye Hunt, Stephen Johnson, Ganesh Venugopal.
Application Number | 20150361561 14/721171 |
Document ID | / |
Family ID | 53494711 |
Filed Date | 2015-12-17 |
United States Patent
Application |
20150361561 |
Kind Code |
A1 |
Hunt; Andrew Tye ; et
al. |
December 17, 2015 |
FLAME-ASSISTED FLASH SINTERING
Abstract
The present disclosure is directed to an apparatus and method of
sintering inorganic powder coatings on substrates, and includes a
flame and an electric plasma. The method is capable of being used
in an open atmospheric environment. The substrate is electrically
conductive and is used as one electrode while the flame is used as
the other electrode that is moved over the areas of the powder
coating to be sintered. An electrical current is used to cause a
plasma produced through the flame, resulting in a combined energy
and temperature profile sufficient for inorganic powder-powder and
powder-substrate bonding. This method is referred to as
"flame-assisted flash sintering" (FAFS).
Inventors: |
Hunt; Andrew Tye; (Atlanta,
GA) ; Johnson; Stephen; (Georgetown, KY) ;
Venugopal; Ganesh; (Johns Creek, GA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NGIMAT CO. |
Norcross |
GA |
US |
|
|
Assignee: |
NGIMAT CO.
Norcross
GA
|
Family ID: |
53494711 |
Appl. No.: |
14/721171 |
Filed: |
May 26, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14169470 |
Jan 31, 2014 |
|
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14721171 |
|
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61759074 |
Jan 31, 2013 |
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Current U.S.
Class: |
427/535 ; 118/47;
315/111.21; 427/533 |
Current CPC
Class: |
H01J 37/32027 20130101;
H01J 37/32449 20130101; H01J 37/32036 20130101; H01J 2237/332
20130101; H01J 37/32064 20130101; C23C 24/087 20130101; H01J
2237/336 20130101; C23C 24/082 20130101; H05H 1/48 20130101; H01J
37/248 20130101; H01L 21/0237 20130101 |
International
Class: |
C23C 24/08 20060101
C23C024/08; H01J 37/32 20060101 H01J037/32 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with government support under
Contract No. F121-181-0680, awarded by the United States Air Force.
The Government has certain rights in this invention.
Claims
1. A method of manufacturing a coating, the method comprising: a.
providing a substrate having an exposed first surface, b. providing
a powder having a plurality of particles, c. disposing said powder
to said first surface of said substrate to form a particle layer,
d. creating a flame near or on a first electrode so that a high
voltage current can pass from the flame, through the powder layer
and substrate to a second electrode connected to said substrate, e.
electrically energizing said electrodes causing a current flow
through said flame and particle layer, and f. consolidating or
sintering said particles to some degree on said substrate in said
current flow area.
2. The method of claim 1 wherein the substrate is a metal, a
semiconductor, or composite containing a metal or a
semiconductor.
3. The method of claim 1 wherein the particles are a ceramic,
metalloid, or semiconductor.
4. The method of claim 1 wherein the particles have an electrical
conductivity less than that of said substrate.
5. The method of claim 1 wherein the flame assembly is electrically
energized by means of an AC or DC power supply and creates
electrical plasma in the flame.
6. The method of claim 1 further comprising applying from 300 V to
2000 V of electrical potential between said first electrode and a
second electrode attached to said substrate and having at least 1
mA of current.
7. The method of claim 1 wherein said current flow is between 2 mA
and 30 mA.
8. The method of claim 1 further comprising applying from 200 V to
1500V of electrical potential between said first electrode and a
second electrode attached to said substrate and having at least 2
mA of current.
9. The method of claim 1 wherein said electrical potential is
introduced into the flame by an electrode that is adjacent to or in
the flame.
10. The method of claim 1 wherein said flame is in the temperature
range of 1000.degree. C. to 3000.degree. C. and produces chemically
and thermally generated ions as constituents of a plasma.
11. The method of claim 1 wherein said flame produces chemically
and thermally generated ions as constituents of a flame plasma and
the electrical potential creates an arc-like plasma in the
flame.
12. The method of claim 1 wherein the current flow occurs at a
voltage at least less than one-quarter of that possible without a
flame in similar gas composition and distance but with less fuel
gas so is not ignitable.
13. A device for sintering a powder coating on to a substrate
comprising: a. at least one fuel source capable of supplying a
flammable gas fuel, b. a fuel delivery means such as a control
valve, mass-flow controller or rotameter, capable of delivering at
least one gaseous fuel to a torch, c. said torch capable of
producing a flame of sufficient temperature to produce chemically
and thermally generated ions as constituents of a flame plasma, d.
an electrical circuit configured to apply at least part of the
range of 100 V to 2000 V of electrical potential or a flow of
current at least part of the range of 1 mA to 100 mA through said
flame plasma and create an arc plasma, and e. a controller or
electrical circuit capable of supplying said current or said
voltage.
14. The device of claim 13 further comprising a resistor of 20 k to
300 k Ohm in the electrical circuit.
15. The device of claim 13 wherein said traversing means is a
robotic arm with multiple degrees of motion freedom so that the
torch can be maintained near the same angle and distance to the
substrate even when the substrate is a complex shape.
16. The device of claim 13 further comprising a heating system that
brings the coating and substrate up to a desired initial
temperature for processing.
17. A process for sintering a coating onto a substrate with a
combination of a flame and an electric current wherein at least 1
mA of current is passed through the flame and coating to yield a
higher degree of sintering to the coating than if the flame alone
was used.
18. The process of claim 17 wherein the coating has a resistivity
higher than that of the material it is on.
19. The process of claim 17 wherein said current flow is between 5
mA and 100 mA.
20. The method of claim 19 wherein the particles have an electrical
conductivity less than that of said substrate.
Description
CROSS REFERENCE TO RELATED CASES
[0001] This application is a continuation of U.S. Non-Provisional
application Ser. No. 14/169,470, filed on Jan. 31, 2014. The
entirety of that application is hereby incorporated. This
application claims the benefit of priority of U.S. Provisional
Application Ser. No. 61/759,074, filed on Jan. 31, 2013. The
entirety of that provisional application is hereby
incorporated.
BACKGROUND
[0003] 1. Field of the Disclosure
[0004] The present disclosure is directed to an apparatus and
method of sintering coatings onto substrates, and includes the use
of a flame with an electric field plasma. The method is capable of
being used in an open atmospheric environment. An electrical
voltage is used to generate an electric plasma produced through the
flame, resulting in a combined energy profile sufficient for
powder-powder sintering and powder-substrate bonding. This method
is referred to as "flame-assisted flash sintering" (FAFS).
[0005] 2. Background of the Disclosure
[0006] Ceramic coatings on metallic substrates serve myriad
purposes in a number of applications because the ceramics provide
desirable wear, hardness, chemical, appearance, wetting, thermal,
or electrical properties. One very important area of use is for
heat exchangers, in which the ceramic coating typically serves to
shield the underlying metal from unwanted effects due to extreme
heat or chemical corrosion. A pure metal is desirable for the most
efficient exchange of heat, but modern air-conditioning and energy
recovery systems can generate temperatures in excess of 500.degree.
C. that can lead to decreased performance and longevity, because of
corrosion and oxidation of the metal. Because ceramic materials
generally have superior temperature and corrosion resistance,
compared with metals, ceramics can extend the life of heat
exchangers operating in extreme environments, albeit with some
reduction in operating efficiency.
[0007] Ceramic coatings are also essential to performance and
longevity in thermal barrier coatings (TBC) for gas-turbine
engines, among other applications. The hot gas streams in
gas-turbine engines can reach temperatures well in excess of
1000.degree. C. and a barrier coating is thus necessary to protect
the underlying metal from corrosion and, for TBC, thermally
insulating coatings are helpful.
[0008] Numerous other applications are known to benefit from
ceramic coatings onto metals, including fuel cells,
battery-electrode coatings, wire-insulation coatings, wear and
abrasion surfaces, cookware, engines, exhaust shields, power plants
of various types, biomedical implants, and aerospace
applications.
[0009] Two common methods to deposit ceramics onto metals are air
plasma spraying (APS) and electron-beam physical vapor deposition
(EB-PVD). In APS, ceramic powder is injected into an
acetylene-oxygen flame nozzle that contains a plasma arc formed by
a voltage and the high temperatures generated from the combustion
process. As the powder feedstock is injected through this hot
region (>2500.degree. C.), the powder melts and some
consolidates into large droplets that are then conveyed to the
metal substrate where they splat-impact, cool, and resolidify. This
method is widely used to make thick porous films of ceramics, but
is not suitable for making very thin, smooth films with high
density and low porosity.
[0010] The porosity and smoothness issue is improved when using
EB-PVD, where an intense beam of electrons melts and vaporizes a
solid ceramic target inside a vacuum chamber. As a melt is formed,
vapor-phase material is generated within the low-pressure chamber
and a uniform coating is deposited on a nearby substrate. Although
this process deposits films that are generally superior to APS, the
method is costly, because it is slower and requires expensive
vacuum chambers, source targets, and power supplies for beam
generation and steering. Moreover, in any vapor-phase deposition, a
large percentage of the target material becomes wasted and
deposited on the surrounding chamber walls and the substrate must
be manipulated in the vacuum chamber to coat all the surfaces.
Thus, cost is a limiting issue with EB-PVD and it is only used in
the most demanding applications. Plasma-enhanced chemical vapor
deposition (PECVD) is a similar technique in that it is a
low-pressure vapor deposition process, but suffers from some of the
same cost issues as EB-PVD.
[0011] Various techniques exist that use electric fields to sinter
ceramic materials. Such techniques are collectively referred to as
"field-assisted sintering" (FAST), and include spark plasma
sintering (SPS), pulsed electric current sintering (PECS), and
flash sintering. In all of these methods, an electric field is
applied across a green body material and resistive heating caused
by current flow consolidates the powder material. Traditional SPS
applies uniaxial pressure to a ceramic green body sample that is
sandwiched between two conductive graphite dies that generate the
electric field. Commercial versions of such systems exist, but they
are not well-suited to handle large-area thin films or complex
shapes, and typically require a vacuum atmosphere. Published
information shows such electric field-induced sintering has been
applied to ceramic parts but not to coatings of ceramic on metals
or other conductive substrates.
[0012] In a variation on SPS, several publications have
demonstrated that so-called "flash sintering" can be used to
consolidate ceramics at moderately low temperatures without the
need for external pressure or a vacuum. Flash sintering uses an
external heating source to bring the ambient temperature of the
ceramic to a baseline temperature (for example, as low as
.about.850-1000.degree. C. for YSZ), and an electrical current
flowing through the sample then consolidates the powder in a matter
of seconds. Reduced sintering temperatures and times present a
major opportunity for cost savings in materials processing. The
actual temperature at which sintering occurs, and the speed of
sintering was shown to be controlled by the electric field
strength. In each of the above-mentioned field-assisted processes,
the physical restriction of having two conductive electrodes limits
the geometries of the ceramic parts being sintered.
[0013] Although common applications of ceramic coating may be
satisfied by the various ceramic coating processes described, there
is a continuing need for a method of ceramic coating that produces
very little waste in terms of coating material, that works well for
large or contoured parts, and that can be applied under atmospheric
conditions, free of the burdens of traditional vacuum chambers.
SUMMARY OF THE INVENTION
[0014] The present invention comprises a method and its apparatus
for sintering powder coatings onto electrically conductive
substrates, including the use of a flame with an electric plasma to
sinter a powder, or layer applied to a substrate surface. The
method is capable of being used in an open atmospheric environment.
The substrate is electrically conductive and is used as one
electrode while the flame is used as the other electrode that is
moved over the areas of the powder coating to be sintered. An
electrical voltage is used to generate an electric plasma within
the flame, resulting in a combined temperature and energy profile
sufficient for powder-powder sintering and powder-substrate
bonding. This method is referred to as "flame-assisted flash
sintering" (FAFS).
[0015] Powders may include metals, semiconductors, ceramics, and
composites. Suitable examples of metals include base metals and
alloys, such as those listed in the ASTM database as well as other
publications. Highly conductive materials, such as aluminum,
copper, silver, or precious metals, will be more difficult to
sinter unless the connection between the grains is poor so that the
powder layer is more resistive prior to FAFS processing. Examples
of semiconductors include those listed in various semiconductor
databases and numerous publications, and include pure materials and
mixed valence materials. Suitable ceramics include metal oxides or
metalloid oxides and most compounds in publications including the
PDF file or ceramic phase diagram databases. Composite examples
include a combination of any of the above metals, semiconductors,
and/or ceramics, such as stainless steel mixed with YSZ or alumina
to better match thermal expansion coefficient or improve the bond
strength to that of the substrate. Coatings may be composed of
powders, binders, and coating-stabilizing additives. The binder may
be an organic material, such as a polymer, that is volatilized
either before or during the FAFS process. Alternatively, the binder
may be an inorganic material, such as alumina, that could be
integrated into the ceramic structure during the sintering process.
Substrates may include metals, semi-conductors, composites,
metal-coated insulators, and ceramics, so long as they conduct
electricity better than the powder coating layer at sintering
temperatures. Examples of suitable substrates include all the
semiconductors and metals above, with common ones including various
grades of steel, titanium, aluminum, silver, precious metals, and
superalloys.
[0016] Coatings may be deposited by a variety of methods, including
Meyer Rod drawing, doctor-blade coating, dip-coating, spin-coating,
aerosol-jet printing, inkjet printing, and electrophoretic
deposition.
[0017] Advantages of the present invention include that it enables
a lower cost and non-contact method of electric field sintering
powder coatings, decreases sintering times, enables applications
not suitable for vacuum chambers, is amenable to large and complex
shapes, and has the potential to control the degree of sintering
and grain growth through judicious selection of process
parameters.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1a is a schematic showing various features of the
disclosed apparatus. The features shown in FIGS. 1a and 1b
illustrate key elements of the inventive device.
[0019] FIG. 1b shows a detailed view, taken from FIG. 1a, showing
additional features of the disclosed apparatus at greater
magnification. Also shown are additional features not shown in FIG.
1a for clarity.
[0020] FIG. 1c shows a detailed view, taken from FIG. 1a, showing
alternate features of the disclosed apparatus. FIG. 1c is a
detailed view taken from FIG. 1a, showing features in greater
magnification.
[0021] FIG. 1d shows a representative electrical circuit capable of
creating conditions suitable for sintering. A circuit is made by
electrically connecting a voltage V 140 to the torch and substrate.
A ballast resistor Rb 280 is connected anywhere in the series
circuit. Between the torch and the substrate electrode, there is an
inherent electrical resistance in the flame, Rf 275, as well as the
substrate and coating, Rsc 285. Ground 290 is relative and simply
denotes the common voltage used to create the voltage used in the
circuit.
[0022] FIG. 2 shows a chart describing current as a function of
voltage through a flame. Specifically, FIG. 2 shows a chart
describing current as a function of voltage V 140 through a flame
120. The fuel mixture used for FIG. 2 includes propane for Gas 1
130, and oxygen for Gas 2 135. Substrate 160 is stainless steel,
coated with a 20-25 micron-thick powder layer 180 of 8YSZ
ceramic.
[0023] FIGS. 3a-c describe the relationship of a combined flame and
plasma heat source in combination with travel speed for a spot size
on a substrate. Each figure shows a top view including a flame
energy spot 300 and a plasma energy spot 310, with a plasma energy
spot 310 having a smaller diameter than the flame energy spot 300.
Each figure shows the relationship of the plasma energy spot 310
relative to the flame energy spot 300 based on traverse speed S 245
of the torch 100 with a vector Tx 235. Just below, a relative
temperature/energy profile of the substrate at a given position in
time is illustrated, each of which corresponds to the top view.
[0024] In FIG. 3a, the traverse speed S 245 is "near zero,"
resulting in the plasma energy spot 310 generally centered within
the flame energy spot 300 in concentric circles.
[0025] FIG. 3b shows a similar illustration to FIG. 3a for a "slow"
traverse speed S 245 (but greater than near zero), wherein the
plasma energy spot 310 is shown offset from the flame energy spot
300 in a direction of travel of velocity Tx 235.
[0026] FIG. 3c shows a case having a "fast" traverse speed S 245
(greater than "slow"), wherein the plasma temperature rise TRP1 355
is experienced even more quickly due to a further decrease in time
dt 370.
[0027] FIGS. 3d and 3e show different paths that a torch 100 may
traverse to consolidate an area of green powder 180 onto substrate
160. A rectangular pattern, as shown in FIG. 3d, may result in an
ideal traverse path, having no overlaps. In contrast, FIG. 3e shows
an irregular pattern having an area of overlap.
[0028] FIGS. 4a-c show images of actual test results according to
one aspect of the present disclosure.
[0029] FIG. 4a is an image of actual test results from the
experimental parameters and conditions shown in Table 1.
[0030] FIG. 4b is a line drawing reproduced from a section of FIG.
4a for clarity. Lines illustrate ceramic particles of 8%
yttria-stabilized zirconia ("8YSZ") that were sintered as the
plasma within the flame traversed a path over the surface. The
flame path is indicated by the vertical arrow shown.
[0031] FIG. 4c shows a magnified view of FIG. 4a including the
scale of the image.
[0032] FIGS. 5a-b show scanning electron microscope images of
sintered features at various test conditions. FIGS. 5a and 5b
correspond to the Table 2 parameters.
[0033] FIG. 5a illustrates a continuously sintered line roughly
three times the width of the sintered features shown in Example 1,
but the change in contrast is more subtle, with grain features
still evident.
[0034] FIG. 5b shows three scanning electron microscope (SEM)
images, corresponding to the three power levels.
[0035] FIG. 6 shows scanning electron microscope images of sintered
features having varying numbers of passes. FIG. 6 shows the
parameters of Table 2 but includes multiple FAFS passes.
[0036] FIGS. 6a and 6b show a level of consolidation of particles
with one pass, although some porosity is visible.
[0037] FIGS. 6c and 6d show an increase to three passes with a
corresponding increase in particle consolidation due to increased
grain growth.
[0038] In FIGS. 6e and 6f, yet a further increase in consolidation
is shown.
[0039] FIGS. 7a-b show scanning electron microscope images
contrasting the scratch resistance of unsintered and sintered test
results. FIGS. 7a and 7b each show SEMs of two samples stitched
together in the same image. The top image shows an unsintered
region (no arc plasma), while the bottom image shows a sintered
region (with arc plasma).
[0040] In FIG. 7a, a scratch is clearly shown in the unsintered
region, but is essentially invisible in the sintered region,
confirming a bond between the ceramic and the substrate. The
scratch pressure was not measured, but was consistent.
[0041] FIG. 7b is a magnified view of FIG. 7a. The scratch is
clearly seen in the unsintered region and a mark is seen in the
sintered region, but ceramic was not removed from the
substrate.
[0042] FIGS. 8a-b show a photograph and scanning electron
micrograph, respectively, of LSM ceramic particles that were
densified using the present invention. The FAFS process was
optimized to demonstrate a region of sintered surface LSM that was
achieved by running the FAFS equipment in a defined pattern. The
plasma arc path on the LSM coatings was mostly continuous and
well-defined straight line that moved with the flame path.
[0043] In FIG. 8a, the FAFS device was rastered with small offsets
in the regions that exhibited a lighter, more reflective
surface.
[0044] The color change is indicative of material densification, as
shown in FIG. 8b, which is a higher magnification scanning electron
microscope image of the same surface shown in 8a.
DETAILED DESCRIPTION OF THE INVENTION
[0045] The features shown in FIGS. 1a and 1b illustrate key
elements of the inventive device. FIG. 1b is a detailed view, taken
from FIG. 1a, showing features at greater magnification. Also shown
are additional features not shown in FIG. 1a for clarity.
[0046] A flame means, such as, for example, a commercial jeweler's
torch, torch 100, is supplied a fuel mixture by pressure means
capable of producing a flame 120. The fuel mixture may include
argon, nitrogen, hydrogen, propane, methane, butane, acetylene,
oxygen, air, and the like. Fuel, oxidizer, or other gas may be
provided in tanks such as Gas 1 130 or Gas 2 135, wherein the fuel,
oxidizer or other gas is stored in a compressed state, at
substantially greater than atmospheric pressure. An oxidizer
capable of sustained burning can entrain air. Normally, delivery of
fuel is enabled passively by a valve and regulator system (not
shown), creating a pressure difference between tanks such as Gas 1
130 or Gas 2 135 and the atmosphere. Alternatively, delivery of
fuel may be provided by a gas pump (not shown) capable of
pressurizing gas on demand. The flame 120, when sufficient
conditions, such as voltage 140 are applied, is capable of
conducting current through the flame that usually will take the
form of a plasma 150. The type of fuel and the fuel mixture can be
optimized for the type of powder to be sintered and other factors,
such as the metal or other suitable substrate 160 to be coated, the
plasma 150 required, and desired temperature profile 170. The
plasma 150, in combination with the flame shown 120, will produce a
spot-sized temperature profile 170, capable of forming a bonded
area 250 of powder 180 to a first surface 190 of substrate 160 in
seconds in an open atmosphere. Many shapes and forms of flame are
possible, and a wide range of burners are applicable to this
technique, to enhance the powder processing and area coverage.
[0047] Optionally, the substrate 160 may be preheated. Preheating
may be performed in a wide range of known ways, including in an
oven (not shown), then removed prior to sintering. Preheating may
be performed by positioning a second surface 200 of substrate 160
in contact with, or near, a hotplate 220. Hotplate 220 may be any
shape, from a traditional flat "plate" to a contoured shape
configured to approximate the shape of a second surface 200 of
substrate 160. The hotplate 220 enables heat within predefined
limits to increase the temperature of the powder 180 and the first
surface 190 of substrate 160, as needed, to improve the sintering
process conditions. Preheating may also be performed locally with a
torch 100, flame or other means, such as IR lamps.
[0048] Torch 100 is attached to a traversing means, simply shown as
an arrow, indicating a velocity vector, velocity Tx 235. The
traversing means, such as velocity Tx 235, is capable of travelling
a predefined path or is capable of responding to an automatic
control system (not shown). An automatic control system is capable
of detecting a reference, such as an edge or edges, a protrusion,
feature, or fiducial, then traversing a path relative to the
reference, such as parallel, orthogonal, at an angle, or other
linear or non-linear path. The traversing means may travel at a
constant or variable rate specified by a user, or determined by a
control system. The control system may, for example, automatically
sense a surface temperature then iteratively adjust the rate to
maintain the surface temperature within predetermined limits. The
path may be limited to a single plane, commonly described as
two-dimensional ("2D"), a single plane with up-down capability,
commonly described as two-and-a-half dimensional ("21/2 D"), or any
space within two orthogonal planes, commonly described as
three-dimensional ("3D"). Robotics systems can also be used to move
the torch, the substrate, or both, as needed, to more effectively
process various shapes and sized parts.
[0049] The substrate is positioned to receive the flame 120 and
plasma 150, preferably at an angle 110 of roughly 90.degree. to the
flame to achieve the most centered flame and plasma profile. There
may be occasions, however, when an angle substantially greater than
or less than 90.degree. is preferred. For example, while traversing
a complex 3D shape, an angle 110 of 90.degree. may cause physical
interference that can be corrected by changing the angle 110 of the
flame to the substrate. Instead, an angle 110 of, say, 105.degree.
or 75.degree. may be preferred, but resulting in an uncentering of
plasma and flame or a loss of energy to the surface. A control
system can compensate for this energy loss by changing the rate of
traverse or the distance from the surface as well. Similarly,
traversing a corner, such as shown in FIGS. 3d and 3e, at a
constant rate may increase the energy at the surface relative to
the energy provided on a straight path. A control system may be
used to automatically sense a surface temperature then adjust the
rate to maintain surface conditions within predetermined
limits.
[0050] The torch 100 may traverse in a 3D space, such as any
combination of the common x, y, and z coordinates. The torch 100
may traverse in a z-direction towards the substrate 160, thereby
decreasing distance H 210. This provides a means of warm-up at the
first point of sintering. The traverse of torch 100 is shown by
velocity Tz 240.
[0051] FIGS. 1b and 1c show the flame 120 positioned a distance H
210 from a first surface 190 of the substrate 160. It is known that
the first surface 190 of substrate 160 will experience a flame 120
having increased temperature closer to the torch 100, such as when
the distance H 210 is small. Likewise, it is known that that the
first surface 190 of substrate 160 will experience decreased
temperature as the distance H 210 increases.
[0052] FIG. 1c is a detailed view taken from FIG. 1a, showing
features in greater magnification. Also shown is the electric
plasma 150, an optional means of introducing dopant 260 to the
plasma 150, dopant dispenser 270, and consolidation zone 250,
wherein sintering of powder 180 to powder 180 and powder 180 to
substrate 160 occurs. These features are not shown in FIG. 1a for
clarity.
[0053] Introduction of a dopant 260 is used to increase plasma
electrical conductivity due to an increase in ionic activity. One
form of dopant is a chemical dissolved in a precursor solution or
as a vapor in a gas stream, as described in U.S. Pat. No.
6,193,911, hereby incorporated herein by reference. In FIG. 1c,
dopant dispenser 270 dispenses in-line with the flame 120. An
alternate means of dispensing could dispense outside the flame 120,
but directed towards the flame 120. Argon or another gas that will
readily support a plasma, could be used to dope the gaseous fuel
mixture but would not precipitate or condense in the coating. Most
metal or metalloid elements when used as a dopant, however, could
be deposited into the coating.
[0054] As described previously, dopants may be added to the flame
120 to affect the plasma temperature. It is preferred that dopants
do not introduce contaminants into the powder 180 layer; thus, an
element that results in a similar or complimentary constituent is
preferred. By way of example, for a yttria-stabilized zirconia
(YSZ) ceramic powder, a dopant containing yttrium nitrate, upon
heating, will produce a byproduct of yttrium oxide,
Y.sub.2O.sub.3.
[0055] FIG. 1d shows a representative circuit for creating
conditions suitable for sintering. A circuit is made by
electrically connecting a voltage V 140 to the torch and substrate.
A ballast resistor Rb 280 is connected anywhere in the series
circuit. Between the torch and the substrate electrode, there is an
inherent electrical resistance in the flame, Rf 275, as well as the
substrate and coating, Rsc 285. Ground 290 is relative and simply
denotes the common voltage used to create the voltage used in the
circuit.
[0056] At sufficient flame temperatures, a high density of ionized
particles are present in the flame 120, resulting in a normal flame
plasma. With an electrical bias applied, such as voltage V 140,
these charged particles will move, creating a current. At
sufficient levels of ionization, caused by high temperatures and
sufficient electrical current, there is dielectric breakdown when
the plasma 150 discharge occurs, the flame resistance Rf 275 can
drop by over 90%. Without the flame present, such an electrically
generated discharge plasma is not possible at such low voltages or
currents. It is such an electrical plasma in a flame that is
important in the method of the present invention. A passive ballast
in series, such as a ballast resistor 280, can be used to prevent
current spikes. Alternatively, a ballast capacitor may be preferred
in some applications or a ballast coil when alternating current is
used. An active circuit can be used for Rb, such as an
appropriately designed power supply, providing similar ballast
function during plasma discharge, may also be used to regulate the
circuit resistance during discharge.
[0057] FIG. 1b shows an alternative means of connecting voltage V
140. Instead of connecting voltage V 140 to the torch 100, an
independent flame electrical input 230 may be used. This provides
an alternate means of creating an electrical bias to enable an
electrical plasma 150 in the flame 120. A material that is both
electrically conductive and able to withstand high temperatures,
such as tungsten, tungsten alloy, or platinum, is introduced into
the flame 120, as shown in FIG. 1c. The combined energy of the
flame 120 and plasma 150 result in a high energy consolidation zone
250, wherein bonding of powder 180 to powder 180 and powder 180 to
substrate 160 occurs.
[0058] The substrate 160 may be a metal, semiconductor, ceramic,
composite, metal-coated insulator, or other substrate having high
temperature consolidation requirements but should have higher
electrical conductivity than the coating. The powder 180 may be a
metal or ceramic or mixture also having high temperature
consolidation requirements. The surface of the substrate may be
pre-treated by physical or chemical means to improve adhesion with
the powder in its green and/or sintered states. Physical means
include sand-blasting, mechanical abrasion, polishing, and the
like. Chemical means include bond-coating, chemical etching, and
the like. One preferred embodiment includes a substrate 160 of
stainless steel and a powder 180 of ceramic. The powder 180,
typically having a desired distribution of particle sizes, is
applied to the substrate. Because the powder is not yet sintered,
it is described as "green" or powder in a "green state." The powder
may be provided with a binder as a preformed shape, provided in a
flexible tape form, or compacted directly onto the substrate. The
powder in green state may also contain sintering aids to provide
improved sintering of the powder.
[0059] Additional variables can be controlled instead of, or in
addition to, a traversing rate to maintain proper surface
conditions suitable for creating an effective consolidation zone
250 that traverses the powder and forms a continuous sintered
coating. For example, energy from flame 120 can be controlled by
the amount, mixture, or type of gas supplied to the flame, such as
Gas 1 130 and Gas 2 135. Energy from the plasma 150 can be
controlled by controlling the current or input voltage V 140, type
of gas or gas mixture or by the introduction of a dopant 260 to the
flame, as described previously.
[0060] FIG. 2 shows a chart describing current as a function of
voltage V 140 through a flame 120. The fuel mixture used for FIG. 2
includes propane for Gas 1 130, and oxygen for Gas 2 135. Substrate
160 is stainless steel, coated with a 20-25 micron-thick powder
layer 180 of 8YSZ ceramic. Under these conditions, 400 V produced a
measurable current of 0.07 mA and a dramatic brightening in the
flame, indicating electrical plasma formation in the flame. A
nearly linear increase in current occurred with an increase in
voltage. Testing extended up to 1200 V, resulting in a current of
over 8 mA, created through the flame.
[0061] FIGS. 3a-c describe the relationship of a combined flame 120
and plasma 150 heat/energy effects in combination with travel speed
for a spot size on a substrate as measured at the substrate at a
given point in time. Each figure shows a top view including a flame
energy spot 300 and a plasma energy spot 310, with a plasma energy
spot 310 having a smaller diameter than the flame energy spot 300.
Each figure shows the relationship of the plasma energy spot 310
relative to the flame energy spot 300 based on traverse speed S 245
of the torch 100 with a vector Tx 235. Just below, a relative
temperature/energy profile of the substrate at a given position in
time is illustrated, each of which corresponds to the top view.
[0062] In FIG. 3a, the traverse speed S 245 is "near zero,"
resulting in the plasma energy spot 310 generally centered within
the flame energy spot 300 in concentric circles. To simplify the
following discussion temperature is taken to mean energy available
to perform the desired sintering and can be in various forms such
as electrical current, plasma, thermal, or chemical sources. The
corresponding total temperature rise TRT 360 includes a preheat
temperature rise TRPp 345 from a preheat temperature profile 320, a
flame temperature rise TRF 350 from a flame temperature profile
330, and a plasma temperature rise TRP1 355 from a plasma
temperature profile 340. In these examples, a preheat temperature
profile 320 exists. If not, the total temperature rise TRT 360
would likely be increased, because the consolidation temperature
required for effective consolidation would remain unchanged. As the
flame traverses, at least the first surface 190 of substrate 160
will experience a flame temperature rise TRF 350 from the flame
energy spot 300, then a plasma temperature rise TRP1 355 from the
plasma energy spot 310 after a period of time. The rate of
temperature rise would be the temperature rise divided by time dt
370. For example, a rate of total temperature rise would be (total
temperature rise TRT 360/time dt 370).
[0063] FIG. 3b shows a similar illustration to FIG. 3a for a "slow"
traverse speed S 245 (but greater than near zero), wherein the
plasma energy spot 310 is shown offset from the flame energy spot
300 in a direction of travel of velocity Tx 235. In this case, the
rate of total temperature rise TRT 360 would be greater than in
FIG. 3a due to a decrease in the denominator, time dt 370, for
essentially the same numerator. Thus, at least the first surface
190 of substrate 160 will experience a more rapid temperature rise
than in FIG. 3a.
[0064] FIG. 3c shows a case having a "fast" traverse speed S 245
(greater than "slow"), wherein the plasma temperature rise TRP1 355
is experienced even more quickly due to a further decrease in time
dt 370. A rate of total temperature rise that occurs too quickly
may cause shock to the powder 180, substrate 190, or both. However,
a rate of total temperature rise TRT 360 that is too slow may cause
increased ceramic grain growth or oxidation of at least the first
surface 190 of substrate 160. Grain growth may be preferred in some
applications, but oxidation of the substrate is generally avoided
during surface bonding.
[0065] FIGS. 3d and 3e show different paths that a torch 100 may
traverse to consolidate an area of green powder 180 onto substrate
160. A rectangular pattern, as shown in FIG. 3d, may result in an
ideal traverse path, having no overlaps. In contrast, FIG. 3e shows
an irregular pattern having an area of overlap. At least a first
surface 190 of substrate 160 would experience a total temperature
rise TTR 360 more than once in specific areas. Due to thermal
cycling and differences in thermal expansion possibly causing
cracking or spalling, it might be best to minimize any overlaps.
Additionally, the flame 120 is not a binary device that may be
switched off then on again at will. An inventive solution includes
switching off the plasma 150 but not the flame 120, reducing the
total temperature rise TRT 360 of the surface to within acceptable
limits for many applications. If a further reduction of energy is
needed the flame 120 may be raised, resulting in an increased
distance H 210, thereby further reducing the temperature of at
least the first surface 190 of substrate 160 that had previously
been sintered. These methods may also be used to preheat a region
prior to sintering.
[0066] Although flame-assisted flash sintering is capable of being
used in a vacuum environment, with flames being stable to at least
15 torr, it is practically preferred for use in non-vacuum
environments, enabling in-place applications, such as very large
components, repair applications, and applications requiring
challenging orientations, such as vertical or overhead surface
coatings.
[0067] Additionally, although FAFS was developed for coating
metals, it is applicable to any substrate having electrically
conductive properties, and other desired properties.
[0068] Flame-assisted flash sintering may also be used for bonding
or welding of material(s) to conductive surfaces. In this case, the
material could be in a green, partially sintered, or fully sintered
state. During bonding of the material, the material may also
undergo partial or full sintering or grain growth. The material to
be welded may be in the form of a green-state coating, as a tape or
sheet, or a solid, shaped to conform to the substrate surface.
[0069] It is also possible to sinter just desired areas with the
FAFS process. If the material is in coating form, specific areas of
the coating may be welded and sintered to the substrate by FAFS,
and the unwelded and unsintered ceramic could be removed to expose
the substrate in areas where no coating is desired. Unsintered
material can be removed by many different processes, including
washing, scrubbing, blowing, vibration, or other known cleaning or
removal methods. The FAFS process can be localized and it may be
easier to define shapes and areas for the coating to remain than to
mask or otherwise limit where the material is to be applied to the
substrate.
[0070] For the examples described, the following preparations were
made. A slurry was made for coating metal substrates. The slurry or
paste can be made in many ways, or purchased. The following is
simply the method used and does not limit the FAFS process.
[0071] Oxide powder is added to a solvent and dispersed with an
ultrasonic probe (e.g., Hielscher UIP100 hd). Slurries were
sonicated for .about.10 min at .about.75% amplitude while manually
stirred in an ice bath to minimize solvent evaporation. Slurries
were cooled to room temperature via the ice bath prior to use. Once
settling in the slurry begins to occur (typically .about.30-60 min
after sonication), slurries were resonicated and cooled to room
temperature again before further use. The end fractional amounts
are approximate because some solvent is lost.
Example slurry recipes:
YSZ
[0072] 44.3 g Tosoh TZ-8YS YSZ powder 78 wt % powder of slurry 56.7
g n-butanol solvent 9.7 vol % powder of slurry
LSM
[0073] 40.35 g n-butanol solvent 55 wt % powder of slurry 22.15 g
nGimat-produced LSM powder 6.4 vol % powder of slurry
MCO
[0074] 22.15 g nGimat-produced MCO 32 wt % powder of slurry 48.28 g
n-butanol solvent 21 vol % powder of slurry
[0075] The metal substrate was prepared as follows. After cutting
to size and removal of masking adhesive, 0.075'' grade-430
stainless steel substrates (McMaster Can #1292T26) were cleaned
with distilled ethanol in an ultrasonic bath cleaner for .about.15
min to remove any residual adhesive remaining on the substrate
surface. After cleaning, substrates were rinsed in reverse osmosis
or distilled water and sprayed dry with compressed air. Care was
taken to keep dust particles and drying marks to a minimum.
[0076] The slurry was applied as a coating onto the metal substrate
as follows. Clean substrates were placed onto flattened sheets of
aluminum foil and then onto the glass coating plate of a bench top
automated coating system. A size #70 wound-wire Meyer rod was
cleaned by bath sonication in distilled ethanol and sprayed dry
with compressed air. Cleaning cycles with ethanol were continued
until the rod was completely clear of debris. With both substrate
and coating rod cleaned, the rod was inserted into the holder and
lowered onto the substrate. Slurry was pipetted onto the substrate
and the coating rod was drawn across. After coating, wet samples
were transferred to a hot plate and dried for .about.5 min at
.about.130.degree. C. Once dry, coated substrates were inspected
manually for defects and any excess coating was removed from the
substrate back with a dust-free wipe.
[0077] Typical coating thicknesses for LSM and MCO samples were
.about.12-15 .mu.m, while YSZ samples typically had a dried
thickness of .about.25-30 .mu.m. For the listed examples, the
following equipment was used, but these items could be replaced
with other equipment or set of components that perform similar
functions: [0078] 1. The flame equipment used was a Smith Little
Torch with #5 tip (jeweler's torch) [0079] 2. The voltage or
current supplies used were a Stanford PS300 high voltage power
supply, an Acopian P01HP60 high voltage power supply, and a Hoefer
PS2500 high voltage power supply; they were used interchangeably.
[0080] 3. Alicat mass flow controllers, 0.5 SLPM and 2.0 SLPM
(propane and O.sub.2, respectively) [0081] 4. Omega OMEGALUX.RTM.
infrared radiant panel heater [0082] 5. Standard (industrial) grade
propane and oxygen gases [0083] 6. Custom-made substrate chuck,
made from type 309 stainless steel of dimensions
3''.times.6''.times.1/4''
[0084] Using the above equipment and prepared materials, the
examples listed below were made with the following process.
Single-sided coated substrates were clamped into place in the
substrate chuck. The clamping ensured there was good electrical
contact between the sample and the chuck, which is connected to
electrical ground through a 100 k.OMEGA. ballast resistor. The
substrate chuck was positioned atop the substrate heater such that
the chuck rested only on the ceramic surface of the heater and did
not physically touch the metallic body of the heater. Electrical
grounding issues may occur if the metallic chuck does touch the
metallic heater body, which is in electrical contact with
essentially all components of the FAFS system (enclosure, motor
drives, etc.). The ballast resistor is connected in series with the
negative side of the power supply and serves to restrict the
maximum current in the circuit. The ballast resistor is
intentionally placed on the negative side of the circuit so that
the positive voltage applied to the torch is not attenuated through
additional resistance before any plasma is ignited. Note that the
ballast resistor must be of a sufficient wattage rating to handle
the power delivered to it: in these experiments, a 25-W ballast
resistor of 100-150 k.OMEGA. resistance was used. The resistor was
found to help stabilize the power flow, but other means to finely
control the electrical power, such as different circuitry or power
supplies, can replace this or alter its value.
[0085] The substrate heater was driven by a PID temperature
controller and set to a temperature between 300 and 800.degree. C.
A separate thermocouple device monitored the surface temperature of
the substrate chuck (the PID temperature controller reads the
temperature inside the heater box, not on the surface) and
typically reads 100-200.degree. C. lower than the heater set-point.
Thus, the baseline temperature of the substrates was between 200
and 700.degree. C. rather than 300-800.degree. C. The entire heater
assembly was mounted on a single-axis linear motion stage.
[0086] The torch is clamped by an electrically insulating fixture
onto a two-axis linear motion stage above, in the vicinity of the
substrate heater and coated substrate. It is important that the
torch be clamped using electrically insulating materials to prevent
high voltage from being transferred to the motion system and thus
the rest of the assembly. This is important both for operator
safety and practical purposes, to avoid shorting the power supply
to ground. The high voltage is supplied to the torch by means of an
electrical spade lug that is silver-soldered to the body of the
electrically conductive torch tip. A matching spade connector
crimped onto the end of a cable (capable of withstanding high
voltages) mates to the lug; this cable is connected to the positive
terminal of the power supply.
[0087] A motion trajectory for the torch is determined and
programmed into software that controls the motion of the entire
three-axis system. It is useful to define a three-axis Cartesian
coordinate system consisting of x, y, and z axes, such that the
z-axis is parallel to the common understanding of vertical (up and
down) movement, and the x-y plane is orthogonal to the z-axis. The
trajectory used in all experiments to date consisted of holding the
torch at a fixed height (z position) above the substrate surface
while rastering along at a fixed speed in the x-y plane. At the end
of each raster line (assuming rastering along the major axis, x),
the substrate position is indexed in y and the torch returns to the
initial x position. This pattern is repeated a number of times
until the desired number of scan lines have been executed.
Practical values used in our example experiments are shown in the
table below.
TABLE-US-00001 z height 0.10-0.15'' z trajectory speed 10-12''/min
x trajectory speed 1-10''/min y trajectory speed 1-10''/min x scan
length 1-3'' y index position length 0.05-0.1''
[0088] Before electrically energizing the circuit, combustible
gases are delivered to the torch and the flame is lit. Successful
methods of gas delivery in these experiments included manual
rotameter flow devices as well as electronic mass flow controllers
designed to deliver precise amounts of gas. The latter has the
advantage of creating a very stable flame, which is required to
support a stable plasma. Fuel and oxidizing gases were delivered
through separate mass flow controllers and premixed within the
torch assembly. Propane and oxygen were used as the primary fuel
gases in these experiments in amounts of 100 sccm and 375 sccm of
flow, respectively. Other fuel gases that were tested included
methane and acetylene, but it was determined that the former burned
too cool and the latter burned too hot for the specific
experimental conditions desired. Air, and argon mixed with oxygen,
were demonstrated to be functional with the FAFS process but were
not used in these examples. Various gases (or other fuel gases,
such as butane and hydrogen) may be used once appropriate
experimental conditions are ascertained.
[0089] By setting a voltage on the power supply, the FAFS circuit
was energized. All experiments to date were performed as described
above with the torch at a positive electrical potential with
respect to the substrate chuck, and, by extension, the substrate.
It may be that reversing the polarity of this voltage may show
comparable or even greater success than the present configuration.
Changing the placement of the ballast resistor to the positive side
of the circuit is also a modification that may be contemplated with
the experimental parameters. It is noted that the torch is only
electrically energized after lighting the combustible gases for
safety reasons.
[0090] Voltages between 500 and 1100 V were applied to the torch
(with respect to the substrate) to achieve currents ranging from 1
to 5 mA. The optimal configuration to date has been to run the
power supply in constant voltage mode, meaning that the current
responds to a fluctuating resistance while the voltage at the torch
remains constant. An alternative method is to run the power supply
in constant current mode, in which the voltage responds to
fluctuations in resistance while the current remains constant. In
theory, constant current mode should be preferable because the
temperature increase due to the electrical current within the
ceramic is proportional to power, and power is proportional to the
square of the current multiplied by the ceramic resistance. As the
ceramic resistance remains mostly constant, a change in current has
a significant effect on the deposited power, and thus the
temperature increase, within the ceramic. In practice, however, we
found that a constant current mode was more difficult to implement
than a constant current mode due to the finite response time of the
power supply. However, this may simply be an equipment limitation
because we do not have the ideal power supply.
[0091] Once a flame is lit and the torch is electrically energized,
the scanning motion trajectory begins, with the torch descending in
the z-axis until it reaches the fixed height at which it will begin
the x-y scanning motion. A typical value for this height is 0.15'',
which provides enough space for stable combustion of the fuel-gas
mixture before the primary combustion zone contacts the substrate
surface. The z-height is an important parameter in the FAFS
process, because the hottest section of the flame can reach
temperatures in excess of 2,000.degree. C., sufficient to oxidize,
damage, or even melt the ceramic coating or metal substrate. For
the flame device, use at a height of 0.1'' may damage the coating
due to extreme heat stress, while a height of 0.2'' may be too far
away from the surface to generate a stable plasma arc using the
current torch apparatus.
[0092] The nature of the FAFS process differs substantially between
the two ceramic materials most studied and successfully
demonstrated thus far, 8YSZ and LSM. In the case of 8YSZ, an
extremely bright plasma was ignited as the torch approached a
height of 0.15''. Using a voltage of 850 V, the current generated
was 2.5-3.5 mA. The substrate heater was set to 800.degree. C. for
8YSZ and lower temperatures tended to cause coating spalling or
delamination. The plasma arc, which extended visibly from the torch
tip to the substrate, moved rapidly and sporadically within the
lateral extent of the combustion zone. For a x-y scanning speed of
1''/min, the 0.1-0.2 mm diameter plasma arc moved in such a way as
to expose 50-80% of the ceramic coating within the lateral extent
of the combustion zone.
[0093] LSM, on the other hand, displayed a dull-glowing plasma arc
that existed primarily on the periphery of the combustion zone and
did not move sporadically, but instead more readily stayed in one
location, relative to the torch tip. The result was a straight
sintered line, generated by the moving torch tip, as opposed to a
network of irregularly shaped lines formed on the 8YSZ coatings.
The experimental parameters used for the LSM experiments were a
heater set-point of 350.degree. C., power supply voltage of 1100 V,
current of 3.0-4.0 mA, and x-y scanning speed of 10''/min. It was
found that higher substrate heater temperatures were detrimental to
the LSM coatings, unlike 8YSZ.
[0094] Once the scanning trajectory was complete, samples were
either allowed to cool slowly to room temperature while residing on
the substrate heater, or were instantly removed for examination.
There was no noticeable difference observed between the two
different cooling rates, although one may be preferable to the
other upon closer examination in the future.
Example 1
TABLE-US-00002 [0095] TABLE 1 Experimental parameters for Example
1. Flame + Heater ceramic SP Voltage Current Flame Tz Traverse
resistance (.degree. C.) (V) (mA) (in/min) speed (k.OMEGA.) 700 750
2.5 3-12 1-10 100-200 Electrical Plasma input Nozzle Propane
O.sub.2 Powder arc power height H flow flow size diameter (W) (mm)
(sccm) (sccm) (nm) (.mu.m) 1.6 2.25 100 370 200 70-100
[0096] FIG. 4a is an image of actual test results from the
experimental parameters and conditions shown in Table 1.
[0097] FIG. 4b is a line drawing reproduced from a section of FIG.
4a for clarity. Lines illustrate ceramic particles of 8%
yttria-stabilized zirconia ("8YSZ") that were sintered as the
plasma within the flame traversed a path over the surface. The
flame path is indicated by the vertical arrow shown. Note the
interrupted sintering pattern shown. Instead of a continuous
vertical line, a series of discontinuous lines, angled (+/-)
slightly off-vertical (flame direction) occur. These measure about
0.9 mm in length, and include a separation distance of roughly 0.2
mm. Additionally, random sintering lines extend out from each side
of the vertical path at a greater angle, and are also separated by
a distance of roughly 0.2 mm from the vertical sintered line
segments. This pattern roughly simulates typical arrow fletching.
Such patterns are more commonly formed at larger nozzle heights
which requires higher voltage, perhaps resulting in a plasma energy
that is too high, with multiple electrical plasma streams occurring
with a somewhat repeatable, but irregular, fletching pattern.
[0098] Tests were performed over a range of traverse speeds and
flame warm-up as the torch approached the substrate (Tz).
[0099] FIG. 4c shows a magnified view of FIG. 4a including the
scale of the image. It is clear that sintering has formed grains at
a width of at least 60 microns, which is hundreds of times greater
than the original grain size. The change in contrast of the
sintered feature shows the ceramic to be fully consolidated into a
single surface with little to no visible porosity, suggesting a
fully densified region.
Example 2
[0100] Compared with Example 1, voltage is set at three discrete
levels, resulting in total electrical input powers of 1.0 W, 1.5 W,
and 1.9 W. The ceramic material is 8YSZ. The nozzle height was
reduced 0.5 mm, down to 1.75 mm.
TABLE-US-00003 TABLE 2 Experimental Parameters for Example 2 Linear
Flame + Heater Flame flame ceramic SP Voltage Current approach
speed resistance (.degree. C.) (V) (mA) speed (in/min) (k.OMEGA.)
700 650, 940, 2, 3, 4 3-12 1-10 125-250 1080 Plasma Electrical
Propane O.sub.2 Ceramic Arc input Nozzle flow flow Particle
diameter power height (sccm) (sccm) size (.mu.m) 1.0, 1.5, 1.75 100
370 200 200-300 1.9
[0101] FIGS. 5a and 5b correspond to the Table 2 parameters. FIG.
5a illustrates a continuously sintered line roughly three times the
width of the sintered features shown in Example 1, but the change
in contrast is more subtle, with grain features still evident. FIG.
5b shows three scanning electron microscope (SEM) images,
corresponding to the three power levels. Grain growth of the
ceramic particles is clearly evident, but full consolidation, as
shown in FIG. 4c, was not achieved. This level of sintering may be
ideal for some applications, such as thermal insulation or abrasion
resistance, but may not be adequate where a sealed surface is
required, such as for corrosion resistance or fuel cell
membrane.
[0102] FIG. 6 shows the parameters of Table 2 but includes multiple
FAFS passes.
[0103] In these examples, the effects of particle consolidation are
shown based on the number of times the FAFS process passes over the
same region. FIGS. 6a and 6b show a level of consolidation of
particles with one pass, although some porosity is visible. FIGS.
6c and 6d show an increase to three passes with a corresponding
increase in particle consolidation due to increased grain growth.
Yet a further increase in consolidation is shown in FIGS. 6e and
6f.
Example 3
TABLE-US-00004 [0104] TABLE 3 Experimental Parameters for Example
3. Flame + Heater Traverse ceramic SP Voltage Current Flame Tz
speed resistance (.degree. C.) (V) (mA) (in/min) (in/min)
(k.OMEGA.) 350 1050 4.0-4.5 10-12 10 100-200 Electrical input
Nozzle Propane O.sub.2 Powder Plasma power height H flow flow size
Arc (W) (mm) (sccm) (sccm) (nm) diameter 3-5 3.71 100 370 40
100-150
[0105] Example 3 demonstrates the applicability of the FAFS
invention to densify a different ceramic material, lanthanum
strontium manganite (LSM). FIGS. 8a-b show a photograph and
scanning electron micrograph, respectively, of LSM ceramic
particles that were densified using the described invention. In
FIG. 8a, the FAFS device was rastered with small offsets in the
regions that exhibited a lighter, more reflective surface. This
color change is indicative of material densification, as shown in
FIG. 8b, which is a higher magnification scanning-electron
microscope image of the same surface shown in 8a. To yield
crack-free LSM coatings it was determined experimentally that lower
heater temperatures were needed than YSZ, which corresponds well
with YSZ having a much higher melting point than LSM. The FAFS
process was optimized to demonstrate a region of sintered surface
LSM that was achieved by running the FAFS equipment in a defined
pattern (FIG. 8). The plasma arc path on the LSM coatings was
mostly continuous and well-defined straight line that moved with
the flame path. LSM represents a different class of materials from
8YSZ in that it differs in crystal structure, melting point and
exhibits much higher electrical conductivity.
Example 4
[0106] The preferred embodiment in Example 4 is suitable for YSZ or
LSM on a 430 grade ferritic stainless steel substrate. The FAFS
method and device is useable for a wide range of coating materials
and substrates, with appropriate adjustments to achieve the desired
coating density and sintering. LSM coatings used low temperature
for the preheat and this will likely be appropriate for lower
melting point materials. More insulating materials will likely
behave more like YSZ, with the arc wandering more than was the case
with the less resistive LSM. One skilled in art of materials
processing will understand the properties of the coating
composition, substrate properties, and limitations and choose
appropriate conditions to achieve the desired end coating
properties.
Experimental Parameters for Example 4.
TABLE-US-00005 [0107] Flame Linear Flame + Heater approach flame
ceramic SP Voltage Current speed speed resistance (.degree. C.) (V)
(mA) (in/min) (in/min) (k.OMEGA.) 300-800 300-1100 1.5-4.0 3-10
2-10 100-200 Power in Plasma flame + Nozzle Propane O.sub.2
Substrate Arc ceramic height flow flow thickness diameter (W) (mm)
(sccm) (sccm) (mm) (.mu.m) 1.5-5 2.5-4.0 100 370 1-4 75-300
[0108] Bonding of the ceramic to the substrate was confirmed by
scratch testing using a steel pick. FIGS. 7a and 7b each show SEMs
of two samples stitched together in the same image. The top image
shows an unsintered region (no arc plasma), while the bottom image
shows a sintered region (with arc plasma). In FIG. 7a, a scratch is
clearly shown in the unsintered region, but is essentially
invisible in the sintered region, confirming a bond between the
ceramic and the substrate. The scratch pressure was not measured,
but was consistent. FIG. 7b is a magnified view of FIG. 7a. The
scratch is clearly seen in the unsintered region and a mark is seen
in the sintered region, but ceramic was not removed from the
substrate. Energy-dispersive X-ray spectroscopy (EDX) was used to
determine that this mark was a deposit of stainless steel from the
scratch tool. Thus, the sintering and ceramic-to-substrate bond
remained strong even with scratch pressure sufficient to abrade the
stainless steel pick.
[0109] The results achieved differ widely from those achieved by
flame or arc plasma alone. On both YSZ and LSM coatings, flame-only
processing was performed and nominal sintering was achieved and the
adhesion was poor. A TIG welder was tried with the YSZ coating and
the arc would jump from spot to spot where, it is believed, there
was a lower electrical resistance to the powder coating. The
TIG-treated material could be readily removed from the surface and
the plasma arc could be scanned continuously over the surface.
[0110] The FAFS process uses a flame to define a path where the
plasma arc is restricted and then the flame can be traversed or
moved relatively over the area to be treated. Additionally, the
flame has some conductivity and can support a lower resistance path
so that lower power plasma arc can exist versus non-flame-based
plasma arcs. The plasma is a composite of both a flame plasma and
an electric arc plasma, which enables a lower current flow than is
required to sustain a pure electric arc so that the right amount of
energy to properly sinter, without damaging the powder coating, can
be achieved more readily. Less than one-quarter the `normal`
current of a `pure` arc plasma is desirable. The current and
voltage required to form an arc plasma is known to vary with the
composition of the gas medium. Thus the one-quarter comparison is
for a generally similar gas composition with simply a reduction of
the fuel component so that a flame cannot be ignited. Of course,
any air that might be entrained should be included in the gas mix.
Furthermore, the flame helps to bring the coating material up to a
temperature where electric current sintering can be effective.
[0111] The powder coating should be of good quality without coating
material lacking in the area of processing. While the flame does
control the zone of the electric plasma are, if there are holes or
cracks in the coating, the arc will try to move to these areas of a
lower resistance path and will jump over or move quickly by areas
where the coating has significantly higher resistance.
[0112] Coating contaminants should be minimized, as is the case for
most coating methods. Conductive contaminants, which approach size
of the coating thickness, should also be eliminated as much as
possible because these can also act as grounds for the arc to jump
to. Some contaminants might dramatically alter the melting point or
resistance of the coating and result in different coating
morphologies or properties as well as difficult to control currents
or voltages. As with many processes, cleaner or more consistent
properties are better.
Embodiments of the Present Invention Include
[0113] 1. A method of manufacturing a coated substrate, the method
comprising: [0114] a. providing a substrate having an exposed first
surface, [0115] b. providing a powder having of a plurality of
articles, [0116] c. disposing said powder to said first surface of
said substrate to form a powder layer [0117] d. providing a
flammable gas capable of creating a flame, [0118] e. providing an
orifice capable of dispensing said flammable gas toward said powder
layer on said substrate, [0119] f. creating a flame that connects a
first electrode to the flame so that a high voltage current can
pass from the flame, through the powder layer and substrate to a
second electrode connected to said substrate, [0120] g.
electrically energizing said electrodes causing a current flow
through said flame and powder layer, [0121] h. wherein said flame
produces chemically and thermally generated ions as constituents of
a flame plasma and the electrical potential creates an arc-like
plasma in the flame wherein the arc-like plasma occurs at a voltage
and current at least one-quarter of that possible without a flame,
and [0122] i. consolidating said powder on said substrate in said
current flow area,
[0123] 2. A device for sintering a powder coating on to a substrate
comprising: [0124] a. at least one fuel source capable of supplying
a fuel [0125] b. a fuel delivery means, capable of delivering at
least one fuel to a torch [0126] c. said torch capable of producing
a flame of sufficient temperature to produce a plasma [0127] d. an
electrical circuit configured to flow current through said plasma
[0128] e. a controller or electrical circuit capable of controlling
current [0129] f. a traversing means capable of traversing said
torch while said plasma is energized with current.
[0130] 3. A FAFS method of manufacturing a coated substrate, the
method comprising: [0131] a. providing a substrate having an
exposed first surface [0132] b. providing a powder having of a
plurality of particles. [0133] c. disposing said powder to said
first surface of said substrate to form a powder layer [0134] d.
providing a flammable gas capable of creating a flame [0135] e.
providing a flame head, burner or torch capable of dispensing said
flammable gas toward said powder layer on said substrate [0136] f.
creating a flame near or on a first electrode so that a high
voltage current can pass from the flame, through the powder layer
and substrate to a second electrode connected to said substrate.
[0137] g. electrically energizing said electrodes causing a current
flow through said flame and powder layer [0138] h. consolidating or
sintering said powder on said substrate in said current flow
area.
[0139] 4. The method in 3. (above) wherein the substrate is a
metal, a semiconductor, or composite containing a metal or a
semiconductor.
[0140] 5. The method in 3. (above) wherein the powder is a ceramic,
metalloid, or semiconductor.
[0141] 6. The method in 3. (above) wherein the powder has an
electrical conductivity less than that of said substrate.
[0142] 7. The method in 3. (above) where the powder has an
electrical conductivity less than that of said substrate.
[0143] 8. The method in 3. (above) where the flame head, burner or
torch capable is electrically energized by means of an AC or DC
power supply and creates electrical plasma in the flame.
[0144] 9. The method in 3. (above) further comprising applying from
100 V to 1500 V of electrical potential between said electrode and
a second electrode attached to said substrate and having at least 1
mA of current.
[0145] 10. The method in 3. (above) wherein said current flow is
between 1 mA and 100 mA.
[0146] 11. The method in 3. (above) wherein said current flow is
between 2 mA and 30 mA.
[0147] 12. The method in 3. (above) further comprising applying
from 200 V to 800 V of electrical potential between said electrode
and a second electrode attached to said substrate and having at
least 2 mA of current.
[0148] 13. The method in 3. (above) where the said electrical
potential is introduced into the flame by an electrode that is
adjacent to or in the flame.
[0149] 14. The method in 3. (above) wherein said flame is in the
temperature range of 1000.degree. C. to 3000.degree. C. and
produces chemically and thermally generated ions as constituents of
a plasma.
[0150] 15. The method in 3. (above) wherein said flame produces
chemically and thermally generated ions as constituents of a flame
plasma and the electrical potential creates an arc-like plasma in
the flame.
[0151] 16. The method in 15. (above) wherein the arc-like plasma
occurs at a voltage and current at least less than one-quarter of
that possible without a flame in similar gas composition with less
fuel gas so is not ignitable.
[0152] 17. The method in 3. (above) wherein the electric arc is
traversed over select areas where coating material is desired for
the product being made and subsequently the FAFS-treated powder
layer is removed when the substrate is subject to a cleaning or
unsintered powder removal method.
[0153] 18. The method in 3. (above) wherein the FAFS process is
repeated at least twice over the coating material.
[0154] 19. A device for sintering a powder coating on to a
substrate comprising: [0155] a. at least one fuel source capable of
supplying a flammable gas fuel. [0156] b. a fuel delivery means
such as a control valve, mass-flow controller or rotameter, capable
of delivering at least one gaseous fuel to a torch [0157] c. said
torch capable of producing a flame of sufficient temperature to
produce chemically and thermally generated ions as constituents of
a flame plasma [0158] d. an electrical circuit configured to apply
100 V to 2000 V of electrical potential and control a desired flow
of current of 1 mA to 100 mA through said flame plasma and create
an arc plasma. [0159] e. a controller or electrical circuit capable
of controlling said current or said voltage [0160] f. a traversing
means capable of traversing said torch relative to the substrate
while said plasma is energized with current.
[0161] 20. The device in 19. (above) further comprising a resistor
of 20 k to 300 k Ohm in the electrical circuit.
[0162] 21. The device in 19. (above) further comprising a resistor
of 40 k to 150 k Ohm in the electrical circuit.
[0163] 22. The device in 19. (above) wherein said traversing means
is a robotic arm with multiple degrees of motion freedom so that
the torch can be maintained near the same angle and distance to the
substrate even when the substrate is a complex shape.
[0164] 23. The device in 19. (above) further comprising a substrate
heating system that brings the coating and substrate up to a
desired initial temperature for processing.
[0165] Unless otherwise indicated, all numbers expressing
quantities of ingredients, properties such as molecular weight,
reaction conditions, and so forth used in the specification and
claims are to be understood as being modified in all instances by
the term "about." Accordingly, unless indicated to the contrary,
the numerical parameters set forth in the following specification
and attached claims are approximations that may vary depending upon
the desired properties sought to be obtained by the present
invention. At the very least, and not as an attempt to limit the
application of the doctrine of equivalents to the scope of the
claims, each numerical parameter should at least be construed in
light of the number of reported significant digits and by applying
ordinary rounding techniques.
[0166] While the invention has been particularly shown and
described with reference to a preferred embodiment, it will be
understood by those skilled in the art that various changes in form
and detail may be made therein without departing from the spirit
and scope of the invention.
[0167] All documents, books, manuals, papers, patents, published
patent applications, guides, abstracts, and other references cited
herein are incorporated by reference in their entirety. Other
embodiments of the invention will be apparent to those skilled in
the art from consideration of the specification and practice of the
invention disclosed herein. It is intended that the specification
and examples be considered as exemplary only, with the true scope
and spirit of the invention being indicated by the following
claims.
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