Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
The present invention relates in general to the coating
arts and, more particularly, to the production of coatings
by thermal spray techniques.
Three thermal spray techniques are utilized at the
present time in the production of coatings, viz., flame,
plasma and detonation spray processes. These processes all
depend upon the generation of a hot stream of gases which
are used to heat and propel a finely-divided coating material
to a surface to be coated. ;~
In the flame spray process the combustion of a gas
mixture such as oxygen and acetylene provides the necessary
heat. The plasma spray technique does not depend upon a
combustion process. Instead an inert gas, usually argon,
is electrically excited resulting in a high temperature
plasma. In the detonation spray process a controlled ex-
plosion of gases, such as a mixture of oxygen, acetylene
and nitrogen takes place within the detonation gun, the
powders being driven therefrom on a shock wave,
Flame gun gas temperatures are, of course, determined
and limited by those attainable in the process of combus-
tion. In the detonation spray processes, temperatures of
about 6000F. and exit gas velocities on the order of
2500 feet per second are typical. Plasma gas temperatures,
however, are extremely high, reaching 20a000F. and very
high gas velocities are attainable.
Because of the significant differences in the respec-
tive process parameters, particularly temperature and gas
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velocity, significant differences may be seen in the de-
posited coatings depending upon the process utilized and
despite the fact that powders of the same composition are
- being sprayed. The higher temperatures in the plasma spray
gun results, in some coating systems, in a deposited coating
of different phase structure than that achieved with the
detonation gun. This difference in phase structure results
in a difference in the physical properties of the coating,
the difference in many instances being sufficient to result
in acceptability or unacceptability of the coating for the
purpose intended.
One coating composition adapted to deposition by spray
techniques, usually plasma or detonation spray, is a nickel/
aluminum material. This is utilized to provide fretting and
gall resistance to certain titanium alloy gas turbine engine
parts. Experience has shown that many coatings deposited by ~ -
conventional plasma spray techniques are of good theoretical
density but are highly stressed and, hence, prone to cracking
and spalling, particularly under conditions involving thermal
shock. Those resultant from a detonation spray with lower
formation temperatures are more satisfactory from a cracking
and spalling viewpoint, but are less than optimum, particu-
larly in tenms of masking requirements and economy.
With existing processes and apparatus it is impossible,
in many, many circumstances at leasta to generate spray
coatings having not only the requisite composition and
metallurgical structure but also the desired coating adher-
ence and density. It is also desirable to minimize some of
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the production problems incident to the conventional spray-
ing operations. Detonation processes, for example, are for
safety reasons usually conducted with the operator situated
in a position remote from the coating operation itself, re-
quiring careful prepositioning of the part to be coated
and/or remote control. Conventional plasma spray techniques
require careful surface preparation and masking and may pose
some danger in overheating the substrate.
SUMMARY OF THE INVENTION
The present invention is directed to improvements in
thermal spray coating apparatus and methods These improve-
ments not only eliminate the majority of the drawbacks
associated with existing apparatus and methods, but also
provide a number of ancillary benefits as well
A major advantage of the invention results from an ~-
ability to generate optimum coating structures, in a variety
of coating systems if desired, with excellent adherence and
density. This advantage is, moreover, achieved with con-
current improvements in process economy and safety.
The invention contemplates the provision of an elongated
passageway for defining the hot gas stream in thermal spray
apparatus and into which the powders to be sprayed may be
injected into a cooled gas stream at a predetermined
location or locations, the residence time of the coating
particles being such as to provide the desired heating and
velocity to such particles.
In one embodiment of the invention, a cooled nozzle
extension assembly adapted to mate with conventional plasma
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spray equipment is fabricated with an aerodynamically efficient
passageway through which the hot plasma may be passed and into
which the coating powders may be introduced at a selected loc-
: ation or locations along the passageway. In operation thereof,
helium is utilized as the plasma gas.
In accordance with an embodiment, a thermal powderspray apparatus comprises: means for providing a gas at high
temperature; an elongated nozzle forming an exit for the gas
and through which the gas may be passed as a high velocity
stream; means for cooling the gas to provide a colled gas
stream in at least a portion of the nozzle: and means for inject-
ing powder to be sprayed into the cooled gas stream upstream
of the downstream end of the nozzle at an injection location pro-
viding sufficient powder residence time in the cooled gas stream
to impart a desired level of heating and a high velocity to the
powder.
In accordance with a further embodiment, a plasma
powder spray apparatus comprises: means for generating a plasma:
an elongated nozzle forming an exit form the plasma generating
means and through which the plasma may be passed as a high
velocity stream, means for cooling the plasma to provide a cooled
gas stream in at least a portion of the nozzle, and means for
injecting powder to be sprayed into the cooled gas stream upstream
of the downstream end of the nozzle at an injection location
providing sufficient powder residence time in the cooled gas stream
to impart a desired level of heating and a high velocity to the
powder.
In accordance with a still further embodiment, a plasma
powder spray apparatus comprises: a plasma gun having a plasma
generating chamber, a nozzle having an elongated passageway there-
through communicating with the chamber: means for cooling the
nozzle: and access means through which powder to be sprayed may
be injected into the passageway intermediate its ends at an
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injection location selected to impart a desired level of heating
and a high velocity to the powder.
From a different aspect, there is provided, in accordance
with the invention a plasma powder spray apparatus including a
plasma generating chamber, the improvement which comprises:
a nozzle having an elongated passageway therethrough communicating
with and forming an exit for the plasma generating chamber,
means for cooling a plasma traversing the passageway, and access
means for injecting powder to be sprayed into the passageway
intermediate its ends and downstream of the initial source of
plasma cooling.
In accordance with a still further aspect, an embodi-
ment of the invention comprises, a nozzle extension assembly for
a plasma spray gun comprising: an inner member having an elong-
ated passageway extending therethrough, one end of the inner -
member being adapted to mate with gas exit opening of the plasma
gun, the passageway being aligned with said port' coolant jacket
surrounding the inner member, the coolant jacket cooperating with
the inner member to form a coolant chamber therebetween, means
for admitting a coolant to the coolant chamber, access means
communicating with the passageway intermediate its ends for
admitting a powder to the passageway; and means for connecting
the nozzle extension assembly to the plasrna gun in position.
From a still further aspect, a plasma powder spray
method comprises: generating a plasma, passing the plasma at high
velocity through an elongated nozzle, cooling the plasma, forming
a cooled plasma gas stream' admitting coating powder to the cooled
gas stream in the nozzle, providing sufficient powder residence
time in the cooled gas stream to plasticize the powder and impart
a high velocity thereto, and directing the plasticized powder to
a surface to be coated and effecting a coating buildup thereon
to the desired thickness.
In accordance with a still further embodiment, a plasma
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powder spray method comprises: generating a helium plasma athigh temperature, passing the plasma at high velocity through
an elongated nozzle, cooling the nozzle and the plasma passing
therethrough formlng a cooled plasma gas stream, admitting coating
powder to the cooled gas stream in the nozzle, plasticizing the
powder and imparting a high velocity thereto, and directing the
plasticized powder to a surface to be coated and effecting
a coating buildup thereon to the desired thickness.
BRIEF DESCRIPTION OF THE DRAWING
The drawing depicts plasma spray apparatus according
to the teachings of this invention.
DESCRIPTION OF THE_PREFERRED EMBODIMENTS
The plasma spray apparatus shown in the drawing corres-
ponds to that which has been actually used in the deposition
of coatings according to the present invention.
A spray nozzle extension assembly 2 is adapted to fit
around the nozzle 4 of a standard plasma spray gun 6, such as
the METCO 3MB Plasma Gun with GP Nozzle. The nozzle extension
assembly comprises a tubular finned mernber 8 having a passageway
10 extending therethrough. As shown, the finned member is formed
of a material of high therrnal conductivity, such as copper and
is surrounded by a steel water jacket 14 having a cooling water
inlet 16 and outlet 1~. The cooling fluid passing through the
water chamber 19 cools the finned member preventing melting or
other heat damage due to the hot plasma flowing through the
passageway 10 during operation of the apparatus. In its traverse
through the passageway in the cooled finned member, the hot
plasma itself undergoes substantial cooling.
In this apparatus in the interest of maintaining a
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high gas velocity the passageway 10 has been shaped for
aerodynamic efficiency, utilizing an inlet portion 20, a
nozzle portion 22 and an outlet portion 24. The particular
assembly shown is 6.3 inches in length with an inlet por-
tion 4 x 0.215 inches, a nozzle portion about 0.25 inch
long having a throat diameter of 0 14 inch, and an outlet
portion having a diameter of 0.15 inch. Thus, the nozzle
is convergent/slightly divergent.
Typically, it is desirable to provide the powders to
the surface to be coated, not only at high velocity and ,
heated, but in a plastic rather than molten condition. As
the plasma gas traverses the passageway it is cooled and,
accordingly, introduction of the powders at a downstream
location will generally result in a reduced heating of the
powders because the temperature is lower than the upstream
temperature. Accordingly, the nozzle is fabricated to pro-
vide sufficient length to substantially reduce the plasma
temperature in the nozzle. Thus, as the word "elongated"
is used herein, it will be understood to mean sufficient
length to provide substantial cooling of the plasma. From
the foregoing, it will be seen that in the present invention
the coating powders are exposed in a relatively low temper-
ature/long time cycle as contrasted with a high temperature/
short time cycle in conventional plasma spray operations.
The nozzle extension assembly is provided with an
access port or ports, 40 and 42 in the drawing, through
which powder may be introduced into the plasma gas stream.
The location of these powder access ports will depend upon
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the powders being sprayed and the particular process para-
meters and apparatus being utilized. Basically, however,
the location is selected to provide the correct heating of
the powders.
In the spraying of nickel/aluminum in the apparatus
described, the powders are admitted in an inert carrier gas
through access port 42 which is a 1/16 inch hole located
about 3.5 inches downstream from the nozzle extension inlet
or just upstream of the nozzle portion.
One or more access ports can be utilized for the intro-
duction of differing powder compositions where such powders
are to be sprayed concurrently or sequentially, or for the
introduction of powders of the same composition where the
processing parameters are to be changed. The formation of
graded coatings by gradually phasing in one composition while
phasing out another thereby eliminating a planar interface
between the compositions is readily achieved.
As has been previously discussed, powder temperature
can be readily controlled in a given system by careful
selection of the axial location along the passageway where
the powders are admitted to the hot gas stream. The apparatus
is also readily adaptable to other means of powder tempera-
ture control. Access port 40 or some other port can, for
example, be utilized for the admission of a temperature-
modifying gas to the plasma stream. This temperature-
modifying gas may simply be a cold gas stream of the plasma
gas composition or may be one which alters the heat transfer
characteristics or some other property of the plasma.
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As shown, the nozzle extension assembly comprises
apparatus distinct from the plasma gun itselfO This parti-
cular construction was selected for reasons of practicality
to permit utilization of the present invention with existing
plasma equipment. There is, of course, no reason why the
extended nozzle cannot be integral with the gun itself.
Also although the finned member 8 is shown formed as a single
piece, various portions thereof may preferably be formed as
separate members either to permit adaption of the assembly
to alternative coating operations or equipment, or simply
to facilitate repairs or replacement of parts as they wear
in use.
Usually to develop the optimum phase structure in the
applied coating it is advantageous to have the powder parti~
cles impacting the surface to be coated in a plastic condi-
tion, but at as low a temperature as possible. However,
the cooler the particles the higher the impact velocity
must be to generate the maximum density and adherence.
Thus, there is a considerable advantage to be gained through
the provision of a capability of providing a high coating
particle velocity.
Particle velocities are inherently limited by the gas
velocity in the particular system being employed. In de-
tonation spray processes, the particles are typically
limited to shock wave velocities on the order of 2500 feet
per second. Plasma spray guns, using argon as recommended
by the manufacturers, may reach gas velocities up to 4000
feet per second. In the preferred embodiments of the
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present invention gas velocities of up to 12,000 feet per
second or higher are possible.
Contrary to the usual industry practice, the use of
helium as the plasma gas is preferred in the present inven-
tion. Although helium is known to have possible use in
- plasma spray operations, its light weight and poor heat
transfer characteristics have resulted in industry dis-
couraging its use in conventional plasma spray equipment.
In the present invention its use is not only possible but
advantageous.
In conventional equipment the gases exiting the plasma
gun quickly disperse. Powders injected into such a stream
- reside therein for only a very short period of time. In
these short residence times, the use of helium with its
poor heat transfer capabilities, rather than argon, would
increase the difficulty of imparting proper heat to the
powders. This same short residence time and rapidly dis-
persing gas also aggravate the problem of providing the
velocity component to the powders.
The preferred use of helium in the present invention
provides controlled heating and a high velocity capability.
In addition there are other advantages. With every coating
process, it is essential to consider not only the effect of
coating components and process parameters on the coating
per se, but also their effect on the substrate being coated.
Often the character of the substrate is such that certain
;~ temperatures of the substrate not be exceeded. The rela-
tively poor heat transfer qualities of helium, as compared
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to argon for example, inherently result in a reduced heat
transfer to the substrate
` In the conventional plasma spray operations, the dis-
persion of the heated gases results in a fairly large sub-
- ~ strate area receiving h~at, particularly areas where no
coating is desired and which may be masked. In the present
invention, there is a much greater degree of focus in the
stream. Thus, smaller areas of the substrate are usually
exposed at any one time to the hot gases and, hence, with
a greater heat sink substrates remain cooler. As an addi-
tional benefit, it has been found that because of greater
deposition area control the necessity and extent of masking
is minimized; variations in coating structure and thickness
are more controlled; and there is less powder waste, pro-
moting economy.
Coating operations are also facilitated in another way
through use of this invention. In use of a detonation gun
operations are usually conducted with the operator posi-
tioned remote from the coat~ng operation for safety reasons.
With conventional plasma spray guns the exiting gas is at
such a high temperature that eye damage from ultra-violet
radiation can quickly occur and suitable eye protection is
required. In the present invention, exit gas temperatures
are reduced and the possibility of eye damage is lessened -
although, of course, suitable safety measures should be ob-
sér~ed in any event.
In a conventional process, a part is typically prepared
for coating by, first, masking to leave exposed only the
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areas to be coated; second, grit blasting; third, a cleanup
to remove the effects of the grit blasting; and finally, a
remasking. The present invention eliminates the need for
many of these conventional steps in many cases Since
focusing is vastly improved the extent of masking is much
reduced. Further, because particle velocities are very
high, it has been found possible to eliminate the grit
blasting operation and the masking and cleanup associated
therewith. A simple surface wipe for degreasing with
Freon has been found to be sufficient.
Example
Apparatus ..
Plasma Gun - METC0 3MB with GP Nozzle
Power Supply - PLASMADYNE
350 D.C. arc amps
50-56 D.C. arc volts
Powder Feeder - S.S. AIRABRASIVE unit
- (miniature grit blaster)
powder feed rate .357 lbs./hr.
Nozzle Extension
Assembly - per drawing
Powder ~METC0 450)
Composition (wt.%)-95 percent nickel
5 percent aluminum
Particle size -170 + 325 mesh (ASTM B214)
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Process Parameters
Plasma Gas - helium
Gas Rate - 275 ft.3/min.
~un to Substrate
Distance - 2-3 inches
Size of Focus - 3/8 inch
Substrate - titanium alloy
Coating area - flat washer
Deposition
Using the hand held coating gun with attached
nozzle extension assembly, a coating .008-.010 inch in
thickness was applied for galling and fretting resistance
to one surface of the flat washer.
Results
A coating density of well over 99 percent
of theoretical density was achieved. This is in excess
of that attainable in any conventional plasma processO
Adherence was excellent. Repeated thermal shocking
from high temperature resulted in no evidence whatsoever
of cracking or flaking.
Although this invention has been described in detail
with reference to certain examples and preferred embodiments
for the sake of illustration, the invention in its
broader aspects is not limited to such specific details
but departures may be made from such details without
departing from the principles of the invention and without
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sacr;ficing its chief advantages.
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