U.S. patent number 3,839,618 [Application Number 05/351,814] was granted by the patent office on 1974-10-01 for method and apparatus for effecting high-energy dynamic coating of substrates.
This patent grant is currently assigned to Geotel, Inc.. Invention is credited to Erich Muehlberger.
United States Patent |
3,839,618 |
Muehlberger |
October 1, 1974 |
METHOD AND APPARATUS FOR EFFECTING HIGH-ENERGY DYNAMIC COATING OF
SUBSTRATES
Abstract
An electrical plasma-jet spray torch is adapted to effect spray
coating of substrates in a reduced-pressure chamber, at super-sonic
plasma velocities, thereby achieving extremely dense coatings of
high-purity material. The spray powder is preheated to a
predetermined temperature before entering the plasma, and is
delivered simultaneously to the plasma from a plurality of powder
sources. Both the plasma and a transferred arc are employed to
preheat the substrate to a predetermined temperature, before the
spraying operation commences. Furthermore, the plasma is employed
to effect post-annealing and stress-relieving functions. The
transferred arc power source is connected between the plasma torch
and the substrate, to add energy to the powder while in flight and
to provide a fusion bond between the coating and the substrate. The
gaseous environment in the spray chamber, and in the powder-inlet
passages, is carefully selected and controlled to achieve desired
results.
Inventors: |
Muehlberger; Erich (Costa Mesa,
CA) |
Assignee: |
Geotel, Inc. (Long Island,
NY)
|
Family
ID: |
26909146 |
Appl.
No.: |
05/351,814 |
Filed: |
April 17, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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214584 |
Jan 3, 1972 |
|
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Current U.S.
Class: |
219/121.47;
219/121.5; 219/121.51; 219/121.52; 427/450; 219/76.16 |
Current CPC
Class: |
H05H
1/42 (20130101); C23C 4/134 (20160101); B05B
7/168 (20130101); B05B 7/226 (20130101); B05B
13/0278 (20130101); C23C 4/137 (20160101) |
Current International
Class: |
B05B
13/02 (20060101); B05B 7/16 (20060101); B05B
7/22 (20060101); C23C 4/12 (20060101); H05H
1/42 (20060101); H05H 1/26 (20060101); B23k
009/04 () |
Field of
Search: |
;219/76,121P,75,70,77
;313/231 ;117/93.1PF,105.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Reynolds; Bruce A.
Attorney, Agent or Firm: Rothenberg; Allan Gausewitz;
Richard L.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application constitutes a continuation-in-part of my copending
patent application Ser. No. 214,584, filed Jan. 3, 1972, for an
"Apparatus and Method for Plasma Spraying", now abandoned and
containing subject matter incorporated in a continuation
application, Ser. No. 372,260, filed June 21, 1973, and the
disclosure thereof is disclosures incorporated by this reference as
though fully set forth herein.
Claims
1. Apparatus for effecting spray-coating of a substrate, which
comprises:
a. means to define an environmental chamber,
b. means to reduce the pressure in said chamber to a value below
atmospheric pressure,
c. an electrical plasma-jet torch positioned to direct a jet of
plasma into said chamber, against a substrate disposed in said
chamber,
d. means to supply spray powder to said plasma jet for heating
therein and subsequent impingment against said substrate, and
e. means to effect preheating of said spray powder to an elevated
temperature prior to the time said spray powder reaches said
plasma,
said preheating means cooperating with said plasma-jet torch to
effect a high degree of heating of said powder prior to the time
that said powder
2. The invention as claimed in claim 1, in which said torch
supplies said
3. The invention as claimed in claim 1, in which a source of
electrical power is connected between said torch and said
substrate, to supply additional electrical energy to said plasma
while it travels toward said
4. The invention as claimed in claim 1, in which said powder-supply
means recited in clause (d) comprises at least one tube supplied by
a source of spray powder and carrier gas, said tube being adapted
to conduct said spray powder and carrier gas to plasma generated by
said torch, and in which said preheating means recited in clause
(e) comprises means to
5. The invention as claimed in claim 4, in which said means to heat
said spray powder while in said tube comprises means to effect
heating of said
6. The invention as claimed in claim 5, in which said tube is
electrically conductive, and in which said means to effect heating
of said tube
7. The invention as claimed in claim 6, in which said tube is
formed of
8. The invention as claimed in claim 1, in which said powder supply
means recited in clause (d) comprises a plurality of sources of
spray powder and carrier gas, and means to effect simultaneous flow
of powder and gas from
9. The invention as claimed in claim 1, in which said plasma-jet
torch comprises a back electrode and a nozzle electrode, means to
maintain a high-current electric arc between said back and nozzle
electrodes, and means to supply arc gas to the space between said
electrodes for heating by said arc and passing through said nozzle
electrode into said chamber in
10. The invention as claimed in claim 9, in which said means to
effect preheating of said spray powder effects supplying of said
spray powder to
11. The invention as claimed in claim 10, in which said means to
effect preheating of said spray powder effects supplying of said
spray powder to the nozzle passage through said nozzle electrode,
and comprises at least one powder-introduction passage
communicating transversely with said
12. The invention as claimed in claim 11, in which said
powder-introduction passage delivers powder to said nozzle passage
in a generally upstream direction relative to the direction of
plasma flow through said nozzle
13. The invention as claimed in claim 12, in which said nozzle
passage includes a throat and an end portion diverging from said
throat to an exit, said powder introduction passage including a
section extending along
14. Apparatus for effecting spray-coating of a powder onto a
substrate, which comprises:
a. an electrical plasma torch comprising a back electrode and a
nozzle electrode having a nozzle passage,
b. means to maintain a high-current electric arc between said
electrodes,
c. means to introduce arc gas into the space between said
electrodes,
whereby said gas is heated by said arc and then passes outwardly
through the nozzle passage of said nozzle electrode,
d. means to introduce spray powder into said nozzle passage,
said spray powder means comprising at least one powder-introduction
passage extending through the side wall of said nozzle pasage,
and
e. means to effect a substantial amount of preheating of said spray
powder prior to the time it passes through said nozzle passage wall
and into the hot gas in said nozzle passage,
said hot gas combining with said preheating means to achieve a
highly
15. The invention as claimed in claim 14, in which said preheating
means comprises a powder tube communicating with said
powder-introduction passage, said powder tube being formed of metal
having a substantial electrical resistance, and means to pass a
high current through said powder tube to heat the same and thus the
spray powder flowing
16. The invention as claimed in claim 15, in which said means to
introduce spray powder further comprises means to introduce spray
powder and carrier
17. Apparatus for effecting spray-coating of a powder onto a
substrate, which comprises:
a. an electrical plasma torch comprising a back electrode and a
nozzle electrode having a nozzle passage,
b. means to maintain a high-current electric arc between said
electrodes,
c. means to introduce arc gas into the space between said
electrodes,
whereby said gas is heated by said arc and then passes outwardly
through the nozzle passage of said nozzle electrode,
d. means to introduce spray powder into said nozzle passage,
said spray powder means comprising at least one powder-introduction
passage extending through the side wall of said nozzle passage,
and
e. means to effect a substantial amount of preheating of said spray
powder prior to the time it passes through said nozzle passage wall
and into the hot gas in said nozzle passage,
said hot gas combining with said preheating means to achieve a
highly effective heating of said spray powder, said preheating
means comprising a powder tube communicating with said
powder-introduction passage, said powder tube being formed of metal
having a substantial electrical resistance, and means to pass a
high current through said powder tube to heat the same and thus the
spray powder flowing therethrough, said means to pass current
through said powder tube comprising a powder-preheat power source
one terminal of which is connected to said tube and the other
terminal of which is connected to said nozzle electrode, and means
to
18. The invention as claimed in claim 17 in which said means to
maintain a high-current electric arc between said electrodes
recited in clause (b) comprises a DC power source the negative
terminal of which is connected to said back electrode and the
positive terminal of which is connected to said nozzle electrode,
and in which said powder preheat power source comprises a second DC
power source the negative terminal of which is connected to said
tube and the positive terminal of which is connected to
19. The invention as claimed in claim 17, in which said means to
connect said nozzle electrode to said powder tube comprises a split
electrically conductive clamp ring connected to said powder tube,
and means to clamp
20. A method of applying a spray-coating onto a substrate, which
comprises:
a. providing an environmental chamber,
b. reducing the pressure in said chamber to a value far below
atmospheric pressure,
c. directing an electrically generated plasma jet of
high-temperature gas into said chamber at a gas velocity in the
range of Mach 1 to Mach 5,
d. entraining particles of spray material in said gas jet,
e. causing said jet and said spray material to impinge against a
substrate to be coated, and
21. The invention as claimed in claim 20, in which said method
further comprises causing the pressure in said chamber to be below
one-half
22. The invention as claimed in claim 21, in which said method
further comprises causing the pressure in said chamber to be below
40 mm of
23. The invention as claimed in claim 20, in which said method
further comprises so adjusting the temperature of said jet and
other parameters that said particles are soft, but not molten, at
the instant when they
24. The invention as claimed in claim 20, in which said step of
preheating comprises effecting preheating of said particles of
spray material prior
25. The invention as claimed in claim 20, in which said method
further comprises passing an electrict current through said jet to
said substrate prior to commencement of said entraining step
recited in clause (d),
26. The invention as claimed in claim 20, in which said method
further comprises directing said jet against said substrate after
cessation of said entraining step recited in clause (d), whereby to
post-anneal said
27. The invention as claimed in claim 20, in which said method
further comprises generating said gas jet by means of a
high-current electrical
28. The invention as claimed in claim 27, in which said substrate
is electrically conductive, and including the step of passing a
large electrical transfer current through said jet by means of a
circuit which
29. The invention as claimed in claim 28, in which said transfer
current is caused to be sufficiently large to effect localized
fusion of said
30. A method of spray-coating a substrate, which comprises:
a. providing an electrical plasma-jet spray torch comprising a back
electrode and a nozzle electrode,
b. maintaining a high-current electric arc between said
electrodes,
c. passing gas between said electrodes and then through the nozzle
passage in said nozzle electrode, whereby said gas is heated by
said arc and emanates from said nozzle passage in the form of a
high-temperature, high-velocity jet,
d. preheating particles of spray material and then causing said
preheated particles to be entrained in said jet, and
e. directing said jet containing said particles against a
substrate,
whereby said particles adhere to said substrate and form a coating
thereon.
31. The invention as claimed in claim 30, wherein said step of
preheating particles and cuasing them to be entrained includes the
step of flowing the particles through a heating tube and then
through the wall of the nozzle electrode in a direction that
extends rearwardly at an acute angle
32. The invention as claimed in claim 30, in which said method
further comprises so regulating the amount of said preheating
effected by step (d), and so regulating the temperature of said
jet, that said particles are soft but not liquid at the instant of
impingement against said
33. The invention as claimed in claim 30, in which said method
further comprises causing the velocity of the gas in said jet to
have a velocity
34. The invention as claimed in claim 33, in which said method
further comprises causing said substrate to be disposed, during
performance of said spray-coating method, in an environmental
chamber the pressure in
35. The invention as claimed in claim 30, in which said method
further comprises introducing said spray material into said gas
through the wall of said nozzle passage, and effecting said
preheating by passing said spray material through a hot conduit
which communicates with said nozzle
36. A method of forming a dense, high-purity coating on a
substrate, which comprises:
a. providing an environmental chamber,
b. reducing the pressure in said chamber to a value far below
atmospheric pressure,
c. disposing a substrate in said chamber,
d. employing an electrical plasma-jet spray torch to generate a jet
of high-temperature gas in said chamber,
e. causing the gas velocity in said jet to be in the range of Mach
1 to Mach 5,
f. effecting preheating of a spray powder,
g. introducing said preheated spray powder into said jet for
acceleration thereby, and
h. directing said powder-containing jet against said substrate to
thus
37. The invention as claimed in claim 36, in which said method
further comprises causing the pressure in said chamber to be below
one half atmosphere, introducing said spray powder into said jet by
passing said powder and a carrier gas through the wall of the
nozzle passage in said spray torch, and preheating said spray
powder by passing the same through a hot elongated tube prior to
said passing of said powder through the wall
38. The invention as claimed in claim 37, in which said method
further comprises passing a large electric current through said jet
between said
39. The invention as claimed in claim 38, in which said method
further comprises employing said large electric current to preheat
said substrate prior to said introduction of spray powder, and
employing said jet to post-anneal said substrate subsequent to
cessation of said introduction of
40. A method of applying a spray coating onto a substrate which
comprises:
a. establishing and maintaining an electrically generated plasma
arc,
b. expanding the plasma arc within and projecting it from a nozzle
as a super-sonic stream at a velocity in the range of Mach 1 to
Mach 5,
c. entraining preheated particles of spray material in said plasma
stream before the plasma is projected from the nozzle,
d. causing said plasma stream and spray material to impinge against
the substrate to be coated, and
e. heating said spray material during its flight from said nozzle
to said substrate by passing an electric current from the nozzle to
the substrate
41. The method of claim 40 including the step of heating said
spray
42. The method of claim 41 including the steps of providing an
environmental chamber, reducing and maintaining the pressure in
said chamber to a value far below atmospheric pressure, mounting
the substrate to be coated within said chamber, and projecting said
super-sonic plasma stream with the particles of spray material
entrained therein into said
43. The method of spray coating a substrate with high velocity
particles comprising the steps of
a. establishing and maintaining an electrically generated plasma
arc,
b. expanding said plasma arc as a plasma stream from the throat to
the exit of a super-sonic nozzle having a ratio of exit area to
throat area sufficient to achieve a plasma stream exit velocity
between Mach 1 and Mach 5,
c. injecting into said plasma stream a stream of particles to be
projected, said particles having a diameter not greater than about
44 microns, and
d. heating said particles to a temperature near to but less than
their melting point, said last-mentioned step comprising the step
of applying additional heat to said particles before heat is
imparted thereto by said
44. The method of claim 43 including the step of heating said
particles in flight by passing an electrical current from said
nozzle to a substrate to be coated by said particles, said
electrical current passing through said
45. The method of claim 44 wherein said step of passing an
electrical current from said nozzle to said substrate is initiated
before the step of injecting a stream of particles into the plasma
stream so that the substrate is preheated before the particles
impinge thereon.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of methods and apparatus for
effecting plasma spray-coating of various substances onto
substrates.
2. Description of Prior Art
Plasma-spray coating of substrates in reduced-pressure chambers is
taught by Ducati U.S. Pat. No. 3,010,009, issued Nov. 21, 1961.
However, such patent does not disclose super-sonic plasma
velocities, nor powder or substrate preheat, nor various other
important matters which permit achievement of surprisingly dense,
pure, and bonded coatings.
Plasma spraying at super-sonic plasma velocities has been effected
in the atmosphere, for more than a year prior to the filing date of
the present application, as taught by my copending patent
application Ser. No. 143,956, filed May 17, 1971, now abandoned,
for "Coating Heat Softened Particles By Projection in a Plasma
Stream of Mach 1 to Mach 3 Velocity," and also by my copending
patent application Ser. No. 133,126, filed Apr. 12, 1971, now Pat.
No. 3,740,522, for a "Plasma Torch, and Electrode Means Therefor."
The disclosures of both of these patent applications are
incorporated by this reference as though fully set forth herein.
The use of super-sonic wind tunnels of the plasma type for chemical
synthesis and other purposes, is taught by U.S. Pat. No. 3,360,682,
issued Dec. 26, 1967.
Relative to preheating of the spray powder prior to introduction
into plasma employed for coating a substrate, applicant knows of no
prior art. Patent No. 3,598,944 relates to heating of particulate
material before it is introduced into a plasma heating zon in a
device for creating spherical granules of nuclear fuel (as
distinguished from in a plasma spray torch). Preheating the
substrate in an inert atmosphere at atmospheric pressure has been
described for achieving metallurgical bonding of tungsten coatings
on polished tungsten substrates.
The use of a plurality of power sources, for spraying at sub-sonic
plasma velocities, is taught by Pat. No. 3,183,337, issued May 11,
1965. The connection of an arc power source between the torch and
workpiece, in plasma spraying at atmospheric pressure, is taught by
Pat. No. 3,179,783. Prior transferred arc plasma guns have relied
solely on thermal effects of plasma and of the transferred arc for
providing a hard surfacing. This has been achieved by melting the
substrate together with an additional material in the form of a
welding rod or powder over a small, localized spot of high
temperature and to a relatively great depth below the surface of
the substrate.
SUMMARY OF THE INVENTION
The present method and apparatus make possible and practical the
spray-coating onto various substrates of numerous metals, alloys,
oxides, etc., to create high-purity coatings the densities and bond
strengths of which approach theroetical maximums. The method and
apparatus relate to the directing into a reduced-pressure
environmental chamber of a plasma jet wherein the plasma velocity
is in the range of Mach 1 to Mach 5, introducing spray powder into
the jet, and causing the spray powder to soften and then impinge
against a substrate. The pressure in the environmental chamber
should be below one-half atmosphere and is preferably very much
lower. Although the chamber pressure is thus low, the stagnation
pressure immediately adjacent the substrate is high, and this aids
in creating dense and well-bonded coatings. Several methods are
employed to impart additional heat energy to the particles.
Before the introduction of spray powder is effected, the plasma jet
is directed against the substrate for a time period sufficient to
preheat the substrate to a desired temperature. Furthermore, for
spraying of many types of powders, an arc power source is connected
between torch and substrate, thus adding heat energy to the
particles as they fly toward the substrate. The energy thus added
may be made sufficiently great to effect localized fusion at the
substrate surface, thereby enhancing the quality of the bond
between substrate and coating. The arc power source also may be
used for preheating of the substrate.
The spray powder particles are of small size, and are preheated to
a predetermined temperature before reaching the plasma. Such
preheating is accomplished by means of an electrical-resistance
tube through which the powder and its carrier gas are passed. The
hot carrier gas is adapted to create desired chemical effects, for
example reducing effects, on the spray powder. Furthermore, the
composition of the plasma-forming gas may be such that desired
chemical effects occur in the environmental chamber.
Spray powder is delivered to the plasma from a plurality of sources
which operate simultaneously. The powder and its gas should be
introduced into the super-sonic nozzle of the plasma torch in the
region of the nozzle throat. The direction of powder introduction
is prefereably upstream, to increase the dwell time of the spray
particles in the jet. In certain arrangements, the powder
introduction conduit extends through the nozzle wall at an acute
angle relative to the surface of the nozzle passage.
After the spray powder is turned off, the plasma jet is continued
in operation for a time period sufficiently long to post-anneal the
coating, thereby relieving thermal and other stresses.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of the apparatus for effecting
high-energy dynamic coating of substrates;
FIG. 2 is a longitudinal central sectional view of the plasma torch
portion of the coating apparatus; and
FIG. 3 is a transverse sectional view on line 3--3 of FIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
As stated in my copending application for "Coating Heat Softened
Particles by Projection in a Plasma Stream of Mach 1 to Mach 3
Velocity," Ser. No. 143,956, it has been known that coating
qualities including characteristics of bonding strength, coating
density and coating uniformity show marked improvement with
increasing velocity of the impinging particles. This is due in part
to the fact that the mechanical working effected by impingement of
such high velocity particles enhances the bonding. Accordingly, as
set forth in such application, Ser. No. 143,956, the powder to be
spray coated is entrained in a super-sonic stream of plasma so as
to maximize particle velocity and thus achieve the highly desirable
qualities resulting from such high velocity. However, for certain
particles, and in particular for those requiring high heat inputs
to attain the desired near-melted (but not molten) state, certain
characteristics inherent in the high-velocity mechanism create
obstacles to optimum spray coating of such particles at high
velocity. High velocity of the super-sonic plasma stream works to
cool the particles that are initially heated within the plasma
torch nozzle, or at least to significantly decrease the continued
transfer of heat to these particles during the flight from the
nozzle to the workpiece substrate. As is well known, the
super-sonic stream is rapidly expanded to attain its high velocity
and, concomitantly, its temperature drops significantly. In fact,
in some high-velocity plasma torches, temperature in the arc
chanber may be as high as 25,000.degree. F. whereas temperature of
the arc after it has passed some distance from the nozzle exit, may
drop by a factor of two or three so that the plasma arc may have a
temperature in the order of only 12,000.degree. F. as it nears the
workpiece. Since super-sonic spray coating involves injecting
particles into the plasma stream within the nozzle at a point
within or close to the nozzle throat, the heating effect upon the
particles due solely to temperature difference is maximum within
the nozzle. As the plasma stream expands and cools, this
temperature difference decreases and the heat imparted to the
particles likewise decreases. Accordingly, in some situations,
although the particles to be spray coated may acquire the desired
high kinetic energy, it is possible that they may not be heated to
the desired value on impingement at the substrate.
A significant feature of the present invention comprises methods
and apparatus for adding thermal energy to particles to be spray
coated at very high velocity. In accordance with one feature of the
invention, the particles are preheated in elongated powder
injection tubes as they are being transported to the nozzle and to
the plasma stream therein. In accordance with a second feature of
the invention, heat energy is added to the particles after they
leave the nozzle and while they are in flight. This is in addition
to the heat energy that is added by the plasma stream in which the
particles are entrained. The added heat is imparted to the
particles in flight by means of a transferred arc that flows an
electrical current from the nozzle to the substrate. The
transferred arc flows both through the plasma and through the
particles entrained therein during flight of the particles. It
provides electrical heating of the particles while they are in
flight.
The use of the transferred arc for this additional heating of high
kinetic energy particles has a number of additional advantages, all
of which combine to improve the spray coating process. The
transferred arc is a considerably more efficient source of heat for
the substrate as compared with the heat provided to the substrate
by the impinging plasma stream. Approximately only 30 percent of
the plasma arc power source is transferred by the plasma arc to the
substrate as heat, whereas 90 percent or more of the transferred
arc power source is transferred to the substrate as heat.
At least in part due to the very high efficiency of heating and the
rapid heating effect of the transferred arc, the heat introduced
into the substrate thereby is concentrated at the surface of the
substrate where the actual bonding takes place. This effect is even
further enhanced when the process is employed for spray coating of
a substrate contained in a low pressure or substantially evacuated
chamber. In such a case, the plasma arc, together with the
particles therein, are diffused, spreading over a relatively larger
area and may impinge upon substrate area as great as five inches in
diameter when the latter is sixteen inches from the plasma torch,
for example. As the plasma stream and entrained particles are
diffused, so too, the current path of the transferred arc is
diffused. Accordingly, the transferred arc will impinge upon an
enlarged area of the substrate, substantially equivalent to the
area of impingement of the plasma stream. This diffusing effect
further concentrates the transferred arc heating at the substrate
surface, and avoids any significant deep penetration of the heating
effect below the substrate surface. The arrangement substantially
eliminates localized spot puddling effects of the prior art wherein
melting of the substrate occurs to significant depth.
The above-described features of the transferred arc, namely the
faster and more efficient heating of the substrate and the diffused
or enlarged substrate heating area additionally provide for
significantly increased speed of preheating of the substrate.
Accordingly, use of the transferred arc allows much more rapid
heating of larger areas of a massive substrate. It does not require
heating of the entire substrate (in depth) whereby it is possible
to keep the back side or other portions of a substrate at a
relatively low temperature even though the surface thereof to be
coated is heated to temperatures sufficient to achieve a proper
metallurgical bond.
Referring now to FIG. 1, an exemplary apparatus for carrying out
principles of the present invention comprises an elongated tank 10
which defines a sealed environmental chamber 11 containing a
substrate 12 to be spray-coated. In the illustrated apparatus,
substrate 12 is disposed in a vertical plane transverse to the
longitudinal extent of tank 10, being mounted by a bracket 13 on a
truck 14 which rolls along a track 16. The track 16 is oriented
longitudinally of the tank 10, so that the truck may be moved
towards and away from the plasma torch 17 located at the left end
of the tank as viewed in FIG. 1. The truck 14 may contain a motor
and may be operated by control means exterior to the tank, so that
movement of the substrate 12 may be effected automatically by
remote control.
Means are provided to reduce greatly the pressure in chamber 11, to
a value much less than atmospheric. This is illustrated
schematically to comprise a pump means 18 which is connected
through a conduit 19, a gate valve 20, and a filter 21 to the
environmental chamber 11. The pump means 18 exhaust to the
atmosphere, or may be adapted to recirculate gas back to the plasma
torch 17. Pump means 18 may incorporate a diffusion pump and a
mechanical pump, in series-relationship to each other, the
diffusion pump being upstream relative to the mechanical pump.
Filter 21 is adapted to remove from the gas emanating from chamber
11 any particles of spray powder or other material which do not
adhere to the substrate 12. Mounted upstream from filter 21,
between such filter and the substrate 12, is a baffle plate 22
adapted to block excessively high-velocity flows of gas into the
filter. Baffle plate 22 may be cooled by suitable means, for
example water-cooling coils, not shown.
The plasma torch 17 is mounted in a sealed re-entrant outer housing
23 which is sealingly related to the end wall of tank 10. The
entire outer housing 23 is shown in FIG. 1, and the right end
portion of the outer housing 23 is shown in FIG. 2. The plasma
torch 17 is illustrated as being of the general type described in
detail in my copending patent application Ser. No. 133,126, and in
my copending application Ser. No. 214,584, cited above, the
specifications and drawings of which are incorporated by reference
herein as if set forth in full. Thus, the torch 17 comprises two
annular housing elements 24 and 25 formed of synthetic resin, and
which nest with each other and with an annular front housing
element 26 of highly conductive metal. A rear electrode assembly 27
projects forwardly through housing elements 24, 25, terminating at
the front end thereof in a tungsten slug 28. Tungsten slug 28 is
disposed concentricaly of a super-sonic nozzle element 29 which is
formed of metal and is seated coaxially in the front housing
element 26.
Nozzle element 29 has a relatively large-diameter rear portion 31
in which the tungsten slug 28 is concentrically disposed, and also
has a somewhat smaller-diameter central portion 32 adapted to
effect seating of the forward foot point of the electric arc.
Portion 32 communicates coaxially with the cylindrical portion 33
which constitutes the throat of the nozzle, the portion 33 having a
diameter smaller than that of the central portion 32. Th forward
end of throat 33 communicates with a forwardly-divergent conical
portion 34 which communicates with the environmental chamber
11.
The divergent portion 34, and a part of throat or cylindrical
portion 33, are disposed in a forwardly-protuberant region of
element 29 which extends forwardly of front housing element 26,
being sealingly associated with the cylindrical wall of an opening
in outer housing 23. Furthermore, portion 36 is adapted to provide
an entrance region for spray powder as stated in detail
hereinafter. The nozzle is arranged to provide the
convergent-divergent configuration for producing the expanded
super-sonic plasma flow to be described below.
The tungsten slug 28 is connected coaxially to a copper base 37
which, in turn, is threadedly related to a slug holder portion 38
of the rear electrode assembly 27.
Means are provided to introduce arc gas around copper base 37 and
thence forwardly around slug 28, for flow through the nozzle
portion 31, 32, 33 and 34 into the chamber 11. This comprises a
suitable source 39 of high-pressure gas and which is connected
through a conduit 41 to an annular chamber 42 which surrounds an
annular gas-injector ring 43 formed of ceramic or other
heat-resistant insulating material. The conduit 41 is illustrated
schematically only, since it actually extends through housing
elements 24 and 25 in the manner described in the cited patent
application Ser. No. 214,584. From chamber 42, the gas flows
inwardly through passages in gas-injector ring 43 to the space
surrounding the forwardly-protuberant cylindrical portion of slug
holder 38, following which it flows forwardly around the copper
base 37 and thence around tungsten slug 28. There may be a
multiplicity of passages through the gas-injector ring 43, and
these may be inclined in various directions, one preferred manner
of introduction being tangential in order that the inflowing gas
will pass vortically around the slug 28.
A water and current-carrying conduit 45 is provided as shown in the
lower portion of FIG. 2, connecting to the front housing element
26. Water flows inwardly through conduit 45 from a suitable water
source 46. It then flows upwardly into housing element 46 in
encompassing relationship to the super-sonic nozzle element 29.
More specifically, the water flows around a generally cylindrical
baffle 47, then flows forwardly around the front end of the baffle
47, then flows rearwardly between the baffle and the exterior
surface of nozzle 29, then flows rearwardly out the read end of the
baffle 47, and then enters a chamber 48 defined between housing
elements 25 and 26. Suitable spacer means 49 are provided to mount
the baffle 47 in outwardly-spaced relationship from the wall of the
nozzle 29.
From chamber 48, the water flows rearwardly through passage means
51 to an annular groove 52 in rear electrode assembly 27, following
which it flows into passage means 53 in the assembly 27 and thence
out through a conduit 54 which conducts both water and electricity.
The conduit 54 connects to a suitable drain 55.
Means are provided to feed spray powder, at high rates, to the
plasma. More specifically, means are provided to feed powder to the
plasma which is still in the nozzle passage in nozzle 29. Thus,
bores 56 and 57 are provided in diametrically opposite portions of
nozzle element 29 as shown in FIG. 2, the outer end of each bore
being plugged, the inner end being in communication with throat 33
of the nozzle passage. The bores 56 and 57 are inclined relative to
the axis of the torch and extend at a relatively small acute angle
with respect to the surface of the conical divergent portion of the
nozzle passage. This arrangement causes the powder to be introduced
into the nozzle throat, flowing in a generally upstream direction
relative to the plasma, whereby to increase the dwell time of the
spray powder in the plasma. Introduction of the powder into the
nozzle throat accomplishes maximum transfer of kinetic energy to
the powder because the plasma has maximum density at the throat.
Bores 56 and 57 communicate, respectively, with passages 58 and 59
(FIGS. 2 and 3) which, in turn, connect to elongated electrically
conductive powder-conductor tubes 60-61.
The powder tubes 60-61 are quite long, and pass through sealed
couplings 62 (FIG. 1) which permit passage through the wall of the
sealed tank 10. The tubes 60-61 communicate, respectively, with
powder feed sources 63-64. Each source 63-64 may be of the general
type described in U.S. Pat. No. b 3,517,861 for Positive-Feed
Powder Hopper and Method.
Referring to FIGS. 2 and 3, the ends of powder tubes 60-61 are
electrically and mechanically connected to the exterior of nozzle
element 29 by first and second semicircular split-ring elements
66-67. Each split ring 66-67 is preferably formed of metal, and has
a metal coupling element 68 fixedly secured therein, as by
press-fitting and/or brazing. The outer ends of element 68 connect
to powder tubes 60-61. The inner ends of element 68 are beveled and
seat forcibly against conical connecting portions of passages 58-59
to couple the split rings 66-67 to nozzle 29, with the inner ends
of coupling element 68 seated in the conical connecting portions of
passages 58-59. Screws 69 are inserted through opposed portions of
the split rings and are suitably tightened to secure the ring and
tube assembly to the nozzle.
The simultaneous use of a plurality of sources of spray powder and
spray-powder carrier gas, is also shown and disclosed in the
above-cited patent application Ser. No. 214,584.
Proceeding next to a description of the various electrical
connections in the system, an arc power supply is schematically
represented at 71, having its negative terminal connected to
conduit 54 and its positive terminal connected to conduit 45. Thus,
the negative terminal of source 71, which is a DC power source,
connects to the rear electrode assembly 27 and thus to tungsten
slug 28. Correspondingly, the positive terminal of source 71
connects to the front housing element 26 and thus to the
super-sonic nozzle element 29 which also operates as the anode of
the torch.
The present system also incorporates a high-current DC power supply
72 (FIG. 1), which constitutes the transferred-arc power supply and
is connected between nozzle element 29 and the substrate 12. Staged
more definitely, the negative terminal of power supply 72 is
connected to the positive terminal of arc power supply 71 and also
to conduit 45. The positive terminal of source 72 is connected to
substrate 12, the connection being through a coupling 76 and
flexible conduit 74 (FIG. 1) and also through the truck 14 and
bracket 13.
The apparatus further comprises a powder preheat power supply 76,
which is a DC source the negative terminal of which is connected to
each of the two electrically-conductive powder tubes 60-61,
preferably at or near outer ends thereof. The positive terminal of
source 76 connects to the negative terminal of source 72 and to the
positive terminal of source 71.
DESCRIPTION OF THE METHOD
The first step in the method comprises operating the pump means 18
in order to reduce the pressure in environmental chamber 11 to a
value far below atmospheric. The pressure in chamber 11 should be
below one-half atmosphere, and is preferably caused to be a very
small fraction of atmospheric. For example, the pressure in chamber
11 may be reduced to 150 microns of mercury before the arc power
and arc gas are turned on.
The truck 14 is moved to properly position the substrate 12. The
distance between the substrate 12 and the protuberant portion 36 of
the nozzle 29 may be on the order of about 16 inches, although
other distances may be employed. A precise distance may be
determined empirically in order to achieve optimum spray
conditions.
After the chamber is thus substantially evacuated, the arc gas
source 39 and arc power source 71 (FIG. 2) are turned on and an arc
is started between tungsten slug 28 and the wall of nozzle portion
32. Starting may be by high frequency or other means. The arc
current is caused to be very high, for example 800 amperes, at a
voltage of 90 volts. The gas delivered from source 39 is preferably
argon or helium, although various other gases may be employed, for
example to provide desired environmental conditions in the chamber
11. The pressure in the arc chamber is high, for example 7
atmospheres.
The vacuum pumping is continued throughout the process to maintain
a pressure of about 40 mm of mercury within the environmental
chamber 11.
The described high-current electric arc, gas flow, and arc chamber
pressure cooperate with the super-sonic nozzle and the low pressure
in chamber 11 to create a very long and rapidly-diverging
super-sonic plasma jet which is indicated at 77 in FIG. 1. The jet
77 will cover a relatively large area of substrate 12, which
operates (in combination with the rapid introduction of spray
powder, as described below) to effect rapid formation on the
substrate of a layer of coating material.
The plasma torch 17 is operated to direct the jet 77 against
substrate 12 for a time period sufficiently long to effect
preheating of the substrate 12 to a desired temperature. Such
temperature, and the duration of the preheating step, are
determined empirically for various substrates, various powers, etc.
It is emphasized, however, that the use of the plasma jet 77 to
effect preheating of the substrate cuases the preheating operation
to be accurate, effective and fast.
The substrate preheating operation is preferably augmented by
turning on the transferred arc power supply 72, which applies a
voltage between the nozzle 29 and the substrate 12. The result is
the flow of high current through the plasma 77, which acts as a
conductor, resulting in a corona discharge around the substrate 12
and adding energy to the system whereby preheating occurs more
rapidly than would otherwise be the case. The heating effect of the
transferred arc provides additional advantages. It is more
efficient, since as much as 90 percent of the transferred arc power
source may appear as heat at the substrate whereas only about 30
percent of the arc power source is transferred as heat to the
substrate by the plasma stream. Furthermore, the heating effect of
the transferred arc is concentrated at the surface of the substrate
where the bonding is to take place. Thus, a large massive substrate
need not have all of its mass heated to proper bonding
temperatures.
At an appropriate time during the preheating step, the powder feed
sources 63-64 are caused to supply powder-conducting gas (but not
the powder itself) to tubes 60-61 and thus to the plasma. The
composition of the gas may be argon, helium, etc., although it is
emphasized that the invention comprises employing gas which will
adapt to predetermined chemical reactions with the powder being
supplied to the torch. As an example, the gas passed through tubes
60-61 may be hydrogen, in order to effect reduction of oxides which
are supplied from sources 63-64 to the torch.
After the flow of powder-conducting gas (but now powder) through
tubes 60-61 has been initiated, the powder preheat power supply 76
is applied to thus effect electrical-resistance heating of tubes
60-61. Such tubes are formed of electrically resistive material,
for example stainless steel or inconel, and are heated to high
temperatures ranging up to 1,000.degree. - 2,000.degree. F. In an
exemplary embodiment, each tube is about 6 feet in length, having
an inside diameter of about one-sixteenth of an inch and a wall
thickness of 0.030 inches. The temperatures of tubes 60-61 are
monitored by means of suitable thermocouples (chromel-alumel) or by
an optical pyrometer (not shown). The thermocouples, when employed,
should be fastened to one or both of tubes 60-61 a short distance
(for example about 1 inch) from the split rings 66-67.
After the substrate preheating has been effected, and after the
powder tubes 60-61 are at the desired temperatures and there is
powder-conducting gas flowing therethrough, then the powder sources
63-64 are operated to deliver powder to the powder-conducting gas
so that spray powder flows through tubes 60-61 to the plasma. Such
spray powder is first preheated in the tubes 60-61, then is
introduced into the plasma, preferably by passing into the throat
of the nozzle element 29 through passages 56, 57 in the upstream
directions described relative to FIG. 2, and then is further heated
both by the plasma stream and by transferred arc current while
traveling with the jet 77 to the substrate 12. The combined heating
steps cause the powder particles to be brought to a high
temperature which should be just below their melting points, so
that the particles are soft but are not molten. The particles are
thus in a near molten state and at nearly maximum velocity in
flight within the projected plasma stream at a point between the
nozzle exit and the substrate. The particles thus impinge against
substrate 12 in soft condition and at great velocities, which is an
important factor tending to cause formation of a highly dense
coating of powder material on the substrate 12. It is pointed out
that different powders may be employed in sources 63-64, in order
that alloying will occur as desired. Although the described powder
injection angle and position is preferred for maximized transfer of
kinetic energy, other powder injection combinations may be employed
for use with particles requiring less heating.
The velocity of the plasma emanating from the torch is very high,
for example 13,000 feet per second. Furthermore, the spray powder
is in the plasma for a long distance, for example 16 inches.
Although the spray powder does not move nearly as fast as does the
plasma iteslf, the spray powder is nevertheless accelerated to high
velocities which (when combined with the soft condition of the
particles) cause a very dense coating to occur on the substrate 12.
As indicated in the specific examples described below, the method
and apparatus of this invention will form non-porous, high density
coatings metallurgically bonded to the substrate.
The particle size of the powder fed to the torch is small. The
maximum diameter of each particle should not be above 44 microns
and is preferably much less.
When the substrate 12 is an electrical conductor, the transferred
arc power supply 72 may be employed to increase the energy imparted
to the particles and, if desired, to cause a localized fusion to
occur between the inner surface of the coating and the outer
surface of the substrate. Power supply 72 may deliver a very high
current, for example 100 amperes at 130 volts, which current
becomes smaller when electrically conductive powder is introduced
into the plasma jet 77.
Although the pressure in environmental chamber 11 is maintained at
a low value, it is emphasized that there is a high stagnation
pressure immediately adjacent the coating on substrate 12. This
stagnation pressure may be several atmospheres, and results from a
very high velocity of the jet 77 which immediately is reduced to
zero as the substrate 12 is struck. The high stagnation pressure
against substrate 12 is a factor causing working or "hammering" of
the coating by the gas, thereby tending to increase the density of
the coating.
Not only is the coating very dense, but it is caused to be pure due
to the environmentally desirable conditions present not only in
chamber 11 but in the powder tubes 60-61. Thus, for example,
coating can be caused to be substantially 100 percent free of
oxides, if desired.
During continuance of the spraying operation, the pump means 18
continues to operate in order to maintain the pressure in chamber
11 at the desired low value. The plate 22 prevents the plasma from
entering the filter 21 at an excessive velocity, and the filter 21
removes any excess spray powder from the gas being passed through
the gate valve 20 and conduit 19 to the pump means 18 and thus to
the atmosphere (or back through the gas-introduction conduit or
tube 41 [FIG. 2] for recirculation to the plasma torch 17). It is
pointed out the suitable means may be provided to effect water
cooling of the holder or support-plate for the substrate which is
sprayed in accordance with the present method. Such cooling may be
employed in conjunction with various types of substrates,
particularly those which tend to melt at relatively low
temperatures. Thus, even though part (the back side, for example)
of the substrate is cooled, the surface heating added by the
transferred arc helps maintain the substrate surface conditions
required for good quality coating.
After a coating of the desired thickness is caused to deposit on
substrate 12, powder sources 63-64 are operated to shut off the
flow of powder to the torch. However, operation of the torch (and
the transferred arc, if desired) is continued for a desired time
period in order to effect post-heating and heat-treating of the
substrate and its coating. The post-heating causes stress relief in
the coating, with attendant desired results. The present method
therefore comprises the simultaneous and/or sequential spraying and
heat-treating of the substrate, followed by desired annealing
steps. As previously mentioned, alloying operations may be effected
by simultaneously delivering different powders from the sources
63-64. Because a plurality of powder feed sources 63-64 are
employed simultaneously, the rate of deposition of powder on
substrate 12 is greatly augmented.
The various parameters, power, temperature, arc and environmental
chamber pressures, and nozzle ratio of exit to throat area, are so
adjusted that the gas emanating from plasma torch 17, as the plasma
jet 77, has a velocity in the range Mach 1 to Mach 5. Desirably, a
velocity of Mach 3 is particularly practical to employ.
The present method therefore comprises a high-energy (both kinetic
and thermal) dynamic metallurgical process which may be employed to
coat numerous conductive and nonconductive substrates 12, for
example with various metals and oxides, with numerous types of pure
and/or alloyed coatings of metals, ceramics and other materials.
The coatings are applied extremely rapidly, to accurately control
the thicknesses, and are pure, extremely dense and non-porous. The
metallurgical treating steps which are effected both before and
after application of powder are factors tending to increase the
quality of the finished product. True metallurgical bonding is
accomplished.
Specific Examples
The following common (to all examples) conditions were employed in
each specific example described below. Arc power supply 71 was 800
amps at 90 volts. The work chamber was initially evacuated to 150
microns of mercury and thereafter, the vacuum pumping was continued
to maintain a pressure of 40 millimeters of mercury during the
preheat and coating process. The arc gas employed was a mixture of
400 standard cubic feed per hour of argon and 107 standard cubic
feed per hour of helium. Powder gas was employed at a rate of 50
standard cubic feed per hour of argon. The spraying distance, that
is, the distance between the torch exit nozzle and the substrate
was 16 inches, plus or minus about 0.25 inches.
Example I
An "inner metallic" of tungsten carbide/cobalt powder (WC 88
percent and 12 percent Co) having a particle size of from 5 to 20
microns was coated upon a substrate of 410 stainless steel with the
above described common conditions. In this example, the substrate
was not preheated and no transferred arc power was employed.
However, the powder tubes were preheated by supply 76, employing 50
amps per powder heating tube at 30 volts. A coating of 10 mil
thickness was achieved in 25 seconds. This was a good, dense
coating, but a metallurgical bond was not achieved.
Example II
The materials employed in Example I, both powder and substrate,
were used with the above-identified common conditions. The
substrate was preheated with both the transferred arc and the
plasma jet to a temperature of approximately 2,000.degree. F.
Transferred arc power from supply 72 was employed at 100 amps and
130 volts. The same powder and powder preheat power was employed as
in Example I. It may be noted that the transferred arc current
increases as the powder particles are fed into the stream since
these particles participate in conducting the trnasferred arc
current to the substrate. During preheat, the temperature of the
substrate rose to 2,080.degree. F. and then stabilized at
2,050.degree. F. during the coating. A coating of 9 mils thickness
was achieved in 20 seconds. Upon analysis, this coating exhibited
an exceedingly high density, with no porosity. A metallurgical bond
was established.
Example III
A metal alloy, nickle chromium powder of 15 to 44 microns in size,
was sprayed upon a preheated substrate of 304 stainless steel
employing the above-identified common conditions. Powder tube
preheating was not employed. A transferred arc of 90 amps at 130
volts power was employed for assisting (the plasma stream itself)
in preheating of the substrate and in enhancing the coating
processes. A coating of 12 mils in thickness was achieved in 25
seconds having an exceedingly high density. A metallurgical bond
was established.
Example IV
Employing the above-identified common conditions, particles of a
pure ceramic, aluminum oxide, of 5 to 37 microns in size were
sprayed upon a substrate of 410 stainless steel without employing
any transferred arc. Powder tube preheating was employed as
described in Examples I and II using 50 amps per tube at about 30
volts, but without preheating of the substrate. A coating of 10
mils in thickness was achieved in 80 seconds. The coating was
observed to be slightly grey in color, of high density and without
porosity. Electrical resistance of the coating was found to have an
unexpectedly high value of 350 volts per mil. This is an unusually
high quality dielectric coating having a resistance value
considerably higher than many previously attainable.
The foregoing detailed description is to be clearly understood as
given by way of illustration and example only, the spirit and scope
of this invention being limited solely by the appended claims.
* * * * *