Note: Descriptions are shown in the official language in which they were submitted.
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Title
PULSE COLD GAS DYNAMIC SPRAYING APPARATUS.
Field of the invention
This invention relates to improvements in the spraying method to apply a
coating using high kinetic
energy of sprayed materials, in particular the Cold Gas Dynamic Spray (CGDS)
process.
Background of the invention
Prior art has already disclosed various processes and methods for spray
coatings. The methods used
vary as in the type of coating and its purpose. Coatings are most easily
grouped according to their
primary function, although a given coating may provide more than one basic
function. The most
important functional applications are for: thermal insulation, wear
resistance, corrosion and
chemical resistance, electrical conductivity or resistance, radioactive
shielding, provision of
dimensionally restorative coating and cosmetic repair.
Several techniques have been used to implement spray coatings, including
widely used gas flame-
spraying, arc spraying, plasma spraying, detonation spraying and high-velocity
flame spraying. Of
the multiple thermal processes, the detonation spraying process produces the
highest quality
coatings. More recently, a method called CGDS was developed. CGDS is a coating
process
whereby coatings can be produced without significant heating of the sprayed
materials. In contrast
to flame, arc and plasma spraying processes, with CGDS there is no melting of
particles prior to
impact with the substrate. The adhesion of particles in this process is
secondary to their kinetic
energy upon impact. In this process, very high particle velocities (300 to
1200 m/s) are obtained by
accelerating an expanding gas stream to supersonic speeds through the use of a
converging-
diverging De Laval-type nozzle. The gas and particle temperatures remain well
below the melting
temperature of the sprayed material. As compared to the conventional thermal
spray techniques, the
distinguishing features of the CGDS process are as follows: the temperature of
the sprayed material
is always below the melting point and all sprayed materials remain in a solid
state throughout the
spraying process. This solid-state processing has several unique advantages,
including: avoiding
undesirable chemical changes (e.g. oxidation) and microstructure changes (e.g.
grain growth) during
the deposition process, and producing minimal or even compressive residual
stresses. Therefore
CGDS is ideally suited for depositing oxygen-sensitive materials (e.g. Al, Mg,
Ti, Cu, etc.),
temperature-sensitive materials (e.g. nano-structured and amorphous materials)
and phase-sensitive
materials (e.g. carbide composites).
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CGDS was introduced to the field through work done by Alkimov et al. as
disclosed in U.S. Pat. No.
5,302,414 and by Kashirin et al. as disclosed in U.S. Pat. No. 6,402,050.
These two patents describe
two different types of CGDS: an upstream-sprayed particle feeding technique
(by Alkimov) and a
downstream-sprayed particle feeding technique (by Kashirin).
The upstream-sprayed particle feeding CGDS system introduces the sprayed
particles into the flow
of gas at the converging portion of the nozzle co-axially. It uses high
pressure and high temperature
gases (e.g. helium) and has high deposition efficiencies. However, a common
problem encountered
in the operation of this system is nozzle-clogging, especially at the nozzle
throat between the
converging and diverging sections. Other disadvantages of this CGDS system are
high operational
and equipment costs.
In comparison, the downstream-sprayed particle feeding CGDS system introduces
the sprayed
particles into the flow of gas at the diverging section of the nozzle
radially. It uses lower pressure
gases (e.g. air, nitrogen), is portable and is less expensive. Due to the
lower particle velocities that
can be reached with this system, only a limited number of materials can be
deposited and the
deposition efficiencies are much lower than the upstream-sprayed particle
feeding CGDS system.
Summary of the invention
The ultimate objective of this invention is to provide an improved Cold Gas
Dynamic Spray
(CGDS) apparatus capable of discharging powdered materials at greater
velocities with higher
deposition efficiency, as well as a greater variety of materials (e.g.
nanopowders). This objective is
achieved by attaching a specialized device to an existent CGDS system. The
device is a pulse-
generator and is attached behind the converging portion of the De Laval-type
nozzle. This device
significantly increases the efficiency of the spray process in three ways.
First, it creates a higher
pressure environment that introduces a wave of pulses to the exit stream,
thereby increasing the
speed of any introduced powder materials. Second, the addition of this device
introduces steam to
the overall process which also increases the speed of the exit stream. Third,
the introduction of
steam into the process also increases the drag co-efficient of any injected
powders, thereby once
again contributing to an increase in the velocity of the exit stream. This
improved design and the
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resulting increased efficiency of the CGDS process also permits a greater
choice in the type of
powdered materials that may be used.
The pulse-generator generates detonation-type impulse waves by injecting hot
compressed water
into the low-pressure (vacuum) and high-temperature discharge chamber. The
vacuum or low-
pressure environment in the discharge chamber provides a greater pressure
gradient for the creation
of detonation-type impulse waves. The injections are repeated in a cyclical
manner at a pre-
determined frequency.
The injected water rapidly becomes a high-pressure impulse steam wave. These
impulse steam
waves accelerate when discharged through the pulse-generator and through the
converging-
diverging, De Laval-type nozzle, ultimately creating a greater velocity when
reaching the exit point
of the nozzle. These impulse steam waves are dragging any introduced powder
materials and
accelerating them towards a substrate. These impulse steam waves reach speeds
of Mach 2 and
higher in the diverging portion of the nozzle. For sphere-shaped, sprayed
particles, the speed of the
stream at Mach 2 and higher is a milestone achievement for optimizing the
value of the drag co-
efficient (Cd).
In addition, at various points between the nozzle throat and the substrate
(depending upon various
parameters) the pressure and the temperature drop create a saturated impulse
steam wave, thereby
causing portions of the steam in the wave to revert to a liquid state. The
reintroduction of water in a
liquid state at the last stage of the process does three things. It promotes
agglomeration of the
powdered particles in the exit stream. It causes water-hammering of the
substrate which improves
adhesion of the sprayed materials to the substrate. Finally, it cools down the
nozzle which
minimizes nozzle-clogging. The improved agglomeration of powdered particles is
a critical/key
benefit because it will permit the deposition of nanopowders; a hitherto
unattainable objective in the
prior art CGDS process.
Brief description of the drawings
Figs. 1 and 3 are sectional views of a known, prior art CGDS apparatus.
Figs. 2 and 4 are sectional views of the CGDS apparatus embodying the present
invention.
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Detailed description of the preferred embodiments
Fig.1 shows the prior art CGDS apparatus (1), where the low pressure (80 to
120 psi) gas supply
(usually air or nitrogen) is heated in an electrical heater (19) to
temperatures from 80 - 550 C and
forced through the passage (11) into the back-cover (12) and the converging
portion (14) of the
nozzle (13). The back pressure at the nozzle discharge is lower than the
critical pressure of the
supplied gas and the Mach number of the gas stream at the throat (15) is Mach
1 and the flow is
sonic. In the diverging portion (16) of the nozzle (13) the gas stream
accelerates to supersonic
velocities. When the required temperature of the supplied gas is reached
(regulated by the control
(18)), the powder materials are introduced to the stream from the powder
feeder (17) and injected
into this high velocity gas stream at the diverging portion (16) of the nozzle
(13). At this point they
are dragged by the gas stream and accelerated towards the substrate (5) to
form a coating (6).
Fig.2 shows the CGDS apparatus (2) of the present invention whereby a pulse-
generator (20) is
inserted into the existing CGDS apparatus of the prior art (1). The pulse-
generator body (21) is
embedded and securely connected between the nozzle (13) and the back-cover
(12). The injector
(23) is inserted within the pulse-generator body (21) and together with the
converging portion (14)
of the nozzle (13) creates the pulse-generator chamber (22). The shapes of the
conical portion (30)
of the injector (23) and the converging portion (14) of the nozzle (13) make
this portion of the pulse-
generator chamber (22) into a converging or converging-diverging passage. The
pulse-generator
chamber (22) is embedded and securely connected with a passage (26) to the
water heater (24). The
passage (26) has an in-line solenoid valve (27) and an in-line check valve
(29). The injector (23)
links the supplied gas between the back-cover chamber (10) of the back-cover
(12) and the
converging portion (14) of the nozzle (13). The low pressure (80 to 200 psi)
supplied gas (e.g. air,
nitrogen or helium or any mixture thereof) is heated to temperatures of 400 -
700 C in the electrical
heater (19) and is forced through the passage (11) into the back-cover chamber
(10) where it passes
the injector (23) through the pulse-generator chamber (22), the throat (15)
and the diverging portion
(16) of the nozzle (13). As the heated supplied gas passes through, it heats
the chamber (22) and the
overall apparatus (20). At the throat (28) of the injector (23) the supplied
gas has a sonic velocity
and is accelerated through the diverging portion (16) of the nozzle (13). As
this high velocity gas
stream passes through the pulse-generator chamber (22) it creates a vacuum
therein. The supplied
gas is regulated by the control (18) and the temperature in the pulse-
generator chamber (22) is
regulated by the control (25). In order to reach the required temperature of
the supplied gas
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(400 - 700 C) and to keep the CGDS apparatus portable and light, a secondary
heater (not shown in
the drawing) has to preheat the supplied gas before it reaches the primary
heater (19). To achieve
better control of the temperature in the pulse-generator chamber (22), a small
band heater (not
shown in the drawing) can be assembled around the pulse-generator body (21).
Temperature and
pressure of the water in the water heater (24) is regulated by the control
(25). To avoid boiling the
water in the water heater (24), the water temperature is set to 5 ¨ 10 C
below the water saturation
temperature at the given pressures (e.g. for compressed water at 87 psi the
water temperature should
be 148 - 153 C). The control (25) also regulates the water injections via the
solenoid valve (27).
These injections of water are repeated in a cyclical manner at a pre-
determined frequency range (e.g.
1-20 injections per second) and strictly correlate with the injection of the
discharged powder
materials from the powder feeder (17). When the desired temperatures in both
the pulse-generator
chamber (22) and in the water heater (24) are reached, one drop of the hot
compressed water is
injected into the pulse-generator chamber (22). Instantaneously, a portion of
the injected water (up
to 30%) becomes flash steam. The remaining 70%+ of the water also rapidly
converts to steam.
This rapid vaporization of the injected water generates detonation-type
impulse steam waves. These
impulse steam waves accelerate through the converging or the converging-
diverging portion of the
pulse-generator chamber (22) and through the diverging portion (16) of the
CGDS nozzle (13).
These impulse steam waves reach speeds of Mach 2 and higher in the diverging
portion (16) of the
nozzle (13). For sphere-shaped, sprayed particles, the speed of the stream at
Mach 2 and higher is a
milestone achievement for optimizing the value of the drag co-efficient (Cd).
These impulse steam
waves accelerate the powder materials injected into the high velocity gas
stream at the diverging
portion (16) of the nozzle (13) towards the substrate (5) to form a coating
(6). If, in between the
nozzle throat (15) and the substrate (5), the pressure and the temperature of
the impulse steam waves
drop to the saturation point then these waves become saturated and a portion
of the steam waves
revert to the liquid state as they move through the diverging portion (16) of
the nozzle (13). Any
steam reverting to a liquid state will promote water hammering of the sprayed
materials for better
adhesion of the sprayed particles to the subtrate (5) and, in addition, will
cool down the nozzle (13).
This will also promote agglomeration of the powder particles and increase the
efficiency of the
spray process, as well as permit the deposition of nanopowders; a hitherto
unattainable objective in
the prior art CGDS process.
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Fig.3 shows the prior art CGDS apparatus (3), where the high pressure (80 to
300 psi) gas supply
(usually helium) is heated in an electrical heater (19) to temperatures from
80 - 550 C and forced
through the passage (11) into the back-cover (12) and the mixing chamber (10).
When the required
temperature of the supplied gas is reached (regulated by the control (18)),
the powder materials are
introduced to the mixing chamber (10) from the powder feeder (17) where they
are mixed with the
supplied gas and forced through the converging portion (14) of the nozzle
(13). The back pressure at
the nozzle discharge is lower than the critical pressure of the supplied gas
and the Mach number of
the mixed stream at the throat (15) is Mach 1 and the flow is sonic. In the
diverging portion (16) of
the nozzle (13) the mixed stream accelerates to supersonic velocities towards
the substrate (5) to
form a coating (6).
Fig.4 shows the CGDS apparatus (4) of the present invention where a pulse-
generator (20) is
inserted into the existing CGDS apparatus of the prior art (3). The pulse-
generator body (21) is
embedded and securely connected between the nozzle (13) and the back-cover
(12). The injector
(23) is inserted within the pulse-generator body (21) and together with the
converging portion (14)
of the nozzle (13) creates the pulse-generator chamber (22). The shapes of the
conical portion (30)
of the injector (23) and the converging portion (14) of the nozzle (13) make
this portion of the pulse-
generator chamber (22) into a converging or converging-diverging passage. The
pulse-generator
chamber (22) is also embedded and securely connected with the passage (26) to
the water heater
(24). The passage (26) has an in-line solenoid valve (27) and an in-line check
valve (29). The
injector (23) links the stream of the supplied gas and powder materials
between the back-cover
chamber (10) of the back-cover (12) and converging portion (14) of the nozzle
(13). The high
pressure (80 to 300 psi) supplied gas (e.g. helium) is heated to temperatures
of 400 - 700 C in the
electrical heater (19) and is forced through the passage (11) into the back-
cover chamber (10) where
it is mixed with the powder materials and forced through the injector (23) and
the pulse-generator
chamber (22), the throat (15) and the diverging portion (16) of the nozzle
(13) towards the substrate
(5) to form a coating (6). As the heated mixture of the supplied gas and the
powder materials pass
through the pulse-generator chamber (22), the mixture heats the chamber (22)
and the overall
apparatus (20). At the throat (28) of the injector (23) the mixture stream has
a sonic velocity and is
accelerated through the diverging portion (16) of the nozzle (13). As this
high velocity mixture
passes through the pulse-generator chamber (22) it creates a vacuum therein.
The supplied gas is
regulated by the control (18), and the temperature in the pulse-generator
chamber (22) is regulated
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by the control (25). In order to reach the required temperature of the
supplied gas (400 - 700 C) and
to keep the CGDS apparatus light, a secondary heater (not shown in the
drawing) has to preheat the
supplied gas before it reaches the primary heater (19). To achieve better
control of the temperature
in the pulse-generator chamber (22), a small band heater (not shown in the
drawing) can be
assembled around the pulse-generator body (21). Temperature and pressure of
the water in the
water heater (24) are regulated by the control (25). To avoid boiling the
water in the water heater
(24), the water temperature is set to 5 ¨ 10 C below the water saturation
temperature at the given
pressures (e.g. for compressed water at 87 psi the water temperature should be
148 - 153 C). The
control (25) also regulates the water injections via the in-line solenoid
valve (27). The water
injections are repeated in a cyclical manner at a pre-determined frequency
range (e.g. 1-20 injections
per second). When the desired temperatures in both the pulse-generator chamber
(22) and in the
water heater (24) are reached, one drop of the hot compressed water is
injected into the pulse-
generator chamber (22). Instantaneously, a portion of the injected water (up
to 30%) becomes flash
steam. The remaining 70%+ of the water also rapidly converts to steam. This
rapid vaporization of
the injected water generates detonation-type impulse steam waves. These
impulse steam waves
accelerate through the converging or the converging-diverging portion of the
pulse-generator
chamber (22) and through the diverging portion of the CGDS nozzle (13). These
impulse steam
waves reach speeds of Mach 2 and higher in the diverging portion of the
nozzle. For sphere-shaped,
sprayed particles, the speed of the stream at Mach 2 and higher is a milestone
achievement for
optimizing the value of the drag co-efficient (Cd). If, in between the nozzle
throat (15) and the
substrate (5), the pressure and the temperature of the impulse steam waves
drop to the saturation
point then these waves become saturated and a portion of the steam waves
revert to the liquid state
as they move through the diverging portion (16) of the nozzle (13). Any steam
reverting to a liquid
state will promote water hammering of the sprayed materials for better
adhesion of the sprayed
particles to the subtrate (5) and, in addition, will cool down the nozzle
(13). This will also promote
agglomeration of the powder particles and increase the efficiency of the spray
process, as well as
permit the deposition of nanopowders; a hitherto unattainable objective in the
prior art CGDS
process.
