Technical Field
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The present invention relates to the field of applying
and producing coatings from powder materials and more
particularly to a method of producing a coating from powder
materials and to a device for carrying out said method.
Background of the Invention
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Known in the art is a method of producing a coating from
powder materials, which comprises introducing a starting
powder material into a gas stream which accelerates material
particles, shaping a high-velocity gas-and-powder jet, and
applying the powder material to the surface of an article
(Inventor's Certificate RU No. 1618778, Cl. C23C 4/00, 1991).
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This method suffers from the following disadvantages:
particles of a coating material cannot be accelerated by a
gas stream up to velocities close to those of the gas
accelerating thereof, and the process of forming a coating
cannot be effected with relatively not high parameters of the
working gas (air) with the pressure P < 10*105 N/m2.
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Said disadvantages restrict the technological
possibilities of the method and involve considerable power
inputs.
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Also known in the art is a method of producing a coating
from powder materials on the surface of an article whose
material is selected from the group consisting of metals,
alloys, dielectrics, which method comprises the steps of
forming an accelerating flow of a working carrier gas,
introducing particles of a powder material into this flow,
feeding the resulting gas-and-powder mixture into an
accelerating supersonic nozzle, and applying the powder
material to the surface of an article by the gas flow
(International Application WO 91/19016, Cl. C23C 4/00, B05B
7/24, B05C 19/00, 1991).
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For carrying this method into effect, a device is used,
comprising a spraying unit made as a supersonic nozzle with a
subsonic converging and a supersonic parts and an
intermediate nozzle, a means for feeding a compressed working
carrier gas, and a means for introducing a gas-and-powder
mixture into the spraying unit (International Application WO
91/19016, Cl. C23C 4/00, B05B 7/24, B05C 19/00, 1991).
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The disadvantages of the known method and device are as
follows: limited possibilities of producing an adequate-quality
coating material due to the presence of a superficial
oxide film on particles of the employed powder materials and,
as a result, the presence of oxides in the structure of the
coating material; an insufficient effectiveness of the
particle acceleration process due to the deceleration of
particles in the compressed wall layer of gas at the surface
of the article being treated; insufficiently high
physicochemical properties of produced coatings; limited
possibilities of controlling the velocity of the gas flow and
the particles of the powder material from which the coating
is formed; the presence of friction and of the deceleration
of particles along the walls of the acceleration channel of
the supersonic nozzle, which lead to reducing the
effectiveness of the coating application process and the
service life of the spraying system. Said disadvantages
interfere with producing coatings having high physico-mechanical
properties, restrict the technological
possibilities of producing a high-quality coating from powder
materials, and also interfere with the provision of a high
effectiveness of the coating application process and with the
provision of a long service life of the spraying system.
Disclosure of the Invention
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The problem underlying the described invention is the
provision of a method of preparing a coating from powder
material and a device for carrying this method into effect,
which would make it possible to broaden the technological
possibilities of producing coatings from various powder
materials and from their mixtures, to improve the
physicochemical properties of the resulting coating material,
to raise the effectiveness of the process of applying and
forming the coating, and to prolong the service life of the
spraying system.
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The indicated technical result is attained owing to the
use of method of producing a coating from powder materials
and of a device for carrying this method into effect, the
essence of which is as follows:
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The herein-proposed method of producing a coating from
powder materials, comprising the steps of forming an
accelerating flow of a carrier gas, introducing particles of
a powder material thereinto, feeding the resulting gas-and-powder
mixture into an accelerating supersonic nozzle, and
applying the powder material by means of the gas flow to the
surface of an article, envisages that before feeding the gas-and-powder
mixture into the supersonic nozzle the mixture is
pre-accelerated by a gas inert with respect to the starting
powder material to a velocity defined by the number 0.3 ≤ M ≤
1.0, where M is the Mach number, and accelerated additionally
by introducing the gas-and-powder mixture into the core of
the accelerating flow of the carrier gas, and that before
applying the coating powder material to the surface of an
article the powder particles are separated from the gas. The
accelerating flow of the working carrier gas is formed in
accordance with the square law of variation of the profile of
the accelerating supersonic nozzle area. This provides a
possibility for a more effective utilization of the gas flow
energy.
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It is reasonable to effect feeding of the gas-and-powder
mixture into the accelerating nozzle with the cumulative flow
of the accelerating carrier gas and of the gas inert to the
powder material set in accordance with the rated conditions
of the accelerating nozzle.
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The method envisages the use of air or of a mixture of
gases as the working carrier gas and the use of a gas or a
mixture of gases not reacting with the powdered components of
the powder material as the gas inert with respect to this
material.
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Before the preliminary acceleration of the powder
material, it is subjected to mechanical, electrochemical or
chemical treatment in a gaseous medium inert to the starting
powder material, and the powder material is supplied in a
flow of this medium into a metering feeder.
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It is desirable that a gas with the temperature T ≤ 300
K should be used as the gas inert to the powder material.
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The gas-and-powder mixture may be introduced into the
core of the accelerating flow from the metering feeder in a
pulsed mode.
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When applying the powder material to the surface of an
article, the latter may be brought in vibratory motion
coaxially to the incident two-phase supersonic flow.
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For enhancing the plasticity of the surface to be
coated, the article is subjected to superficial heating.
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In the course of applying a coating, a potential may be
supplied to the article to be coated, the sign of this
potential being opposite to that of the particles in the two-phase
flow of the gas-and-powder mixture.
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The essence of the invention, as regards the herein-proposed
device, is that the device for producing coatings
from powder materials, comprising a spraying unit made as an
accelerating supersonic nozzle with a subsonic converging and
a supersonic parts and an intermediate nozzle, a means for
feeding a compressed working carrier gas, and a means for
introducing a gas-and-powder mixture into the spraying unit,
and a metering feeder, is provided with a means for supplying
an additional compressed gas inert to the powder material,
the intermediate nozzle is made as a supersonic one, with the
diameter of the nozzle exit section dsect. smaller than the
diameter of the critical section Dcrit. of the accelerating
nozzle, and is arranged coaxially with the possibility of
translational displacement in the subsonic converging part,
the accelerating supersonic nozzle having at the outlet of
the supersonic part a linear portion which passes into a
portion with a curvilinear surface of a radius R.
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The device is provided with a unit for treating the
powder material with a view to activating and cleaning the
surface the particles of the coating material, with a gas
pulser connected to the means for introducing the gas-and-powder
mixture, with a vibrator for imparting vibrations to
the article being coated, and with a power supply source for
supplying a potential to the article.
Brief Description of Drawings
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The invention is further explained by a particular
example of its embodiment and by the accompanying drawings,
in which:
- Fig. 1 shows a general view of a device for producing a
coating from powder materials;
- Fig. 2 shows a spraying unit of the proposed device with
a means for introducing a gas-and-air mixture and separating
a two-phase flow into fractions.
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Best Mode of Carrying the Invention into Effect
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The device of the invention comprises a means 1 for
feeding a compressed gas inert to a coating material (to a
mixture of powdered components), a shutoff valve 2, a unit 3
for the pretreatment of a powder material to remove an oxide
film from and to activate particles of the powder material, a
unit 4 for collecting the oxide film and the fine-dispersed
fraction of the powder, a unit 5 for feeding the treated
powder material to a metering feeder 6, a shutoff valve 7, a
pulser 8 of the gas inert to the powder material, a means 9
for feeding a working carrier gas (for instance, air), a
system 10 for controlling the working process of applying
coatings, a heater 11 of the working carrier gas, a unit 12
for the provision of translational axial displacement of an
intermediate supersonic nozzle 13, a chamber 14 for adjusting
the carrier-gas flow, a centering perforated sleeve 15, an
accelerating nozzle 16 with a linear portion 17 which passes
into a portion with a curvilinear surface 18 for varying the
direction of motion of the gas flow, a means 19 for turning
the gas flow away from the article being coated 20, a means
21 for securing and rotating the article being coated, a unit
22 for the translational displacement of the article being
coated, a vibrator 23, a power supply source 24, a tube 25
for feeding the gas-and-powder mixture, a tube 26 for feeding
the working carrier gas.
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The method of the invention is carried into effect and
the device of the invention operates in the following manner.
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A gas inert to the coating material (to a mixture of
powdered components) is fed under a pressure, with the
shutoff valve open, from the means 1 to the unit 3 for the
pretreatment of the powder coating material, said unit being
preliminarily filled with a required amount of a powder (a
mixture of powders). When the unit 3 for the pretreatment of
the powder material is thrown into action, oxide film is
removed from the surface of the powder material particles
therein by a mechanical or any other method. After the
pretreatment, waste substances in the form of oxide film
particles and non-calibrated fine-dispersed fraction of the
powder come to the unit 4. Then the particles of the powder
coating material are activated by an electromagnetic,
thermal, chemical or any other method, depending on the
chemical composition and physical properties of the powder
material (the mixture of powdered components). The activated
particles are supplied with the help of the unit 5 into the
metering feeder 6. After filling the metering feeder 6 with
the powder material and with the gas inert to the powder
material, the units 3, 4, 5 are switched off, and the shutoff
valve 2 is closed, whereby supplying the gas inert to the
powder material is stopped.
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For the process of producing a coating from powder
materials, a working carrier gas (for instance, air,
nitrogen) is supplied under a pressure from the feeding means
9 to the system 10 for controlling the working process of
applying coatings, wherein the pressure is reduced to the
required working value. The gas whose pressure reduced to the
required value comes to the heater 11 of the gas, where the
gas is heated to a temperature corresponding to the rated
operating conditions. The heated working carrier gas comes
along the tube 26 for feeding the working carrier gas to the
chamber 14 for adjusting the carrier-gas flow, and then
through profiled apertures in the centering sleeve the
working carrier gas is fed to the accelerating supersonic
nozzle 16, wherein the carrier gas is accelerated to the
velocity required for applying and forming a coating. When
the required parameters of the working carrier gas are
reached, the shutoff valve 7 is opened, whereby feeding of
the gas inert to the powder material is effected, which gas
first comes to the pulser 8, where it acquires a pulsating
component having a required frequency. From the pulser 8 the
gas inert to the powder material with the required frequency
comes to the metering feeder 6 wherein, as the gas under
variable pressure is mixed with the particles of the powder
coating material, a gas-and-powder mixture is formed, in
which the powder particles are in the form of a suspension.
The resulting gas-and-powder mixture is fed in the form of a
two-phase flow through the tube 25 into the intermediate
supersonic nozzle 13 and then into the core of the
accelerating flow of the carrier gas into the area of the
critical section of the accelerating supersonic nozzle 16.
The required rated thermo- and gasdynamic parameters of the
working carrier gas and of the pulsating gas-and-powder
mixture in the area of the critical section of the
accelerating nozzle 16 are attained by displacing the
intermediate supersonic nozzle 13. This displacement is
performed in a translatory manner along the axis of the
nozzles, with the help of the unit 12 for the provision of
translational axial displacement, for a distance depending on
the velocity of flow and the frequency of pulsations of the
gas-and-powder mixture in the area of the critical section of
the accelerating supersonic nozzle 16. When the required
displacement is achieved, the unit 12 for the provision of
axial displacement of the intermediate supersonic nozzle 13
is switched off. The pulsating mixture of particles with the
gas inert to them, issuing from the intermediate supersonic
nozzle 13, having the initial axial velocity 0.3 ≤ M ≤ 1.0
(where M is the Mach number), is mixed in the area of the
critical section of the accelerating supersonic nozzle 16
with the working carrier gas, and in this area the main
acceleration of the gas-and-powder mixture takes place.
Having acquired the required velocity along the channel of
the accelerating supersonic nozzle 16, the flow of the gas-and-powder
mixture adjusts the direction of its motion within
the linear portion 17, then the main part of the gas flow
turns within the portion with a curvilinear surface 18 and
gets into the entrance portion of the means 19 for turning
off the gas flow. The powder particles of the coating
material, having a large mass and sluggishness, continue
their rectilinear motion till they collide with the surface
of the article being coated. When the necessary thermo- and
gas-dynamic conditions for applying the coating are attained,
the article 20 to be coated, which is secured in the means 21
for securing the article, is set in (back-and-forth or
rotary) motion. Then the vibrator 23, connected with the unit
22 for the displacement of the article, and the power supply
source 24, which feeds to the article 20 a potential whose
sign is opposite to that of the moving particles, are
switched on. This is how the process of applying a coating to
and forming a coating on the surface of an article occurs.
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As the gas inert to the powder material, a gas (a
mixture of gases) may be chosen, which does not react
chemically with the powder material of the coating, for
instance, nitrogen, argon, helium, krypton, and others. The
choice is determined by particular requirements to the
properties of the resulting coating, its structure, and the
composition of the material of the latter.
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Removal of the oxide film from the surface of the powder
particles of the coating material, activation of the
particles, and their preliminary acceleration by the gas
inert to the particles, prevent oxidation of the coating
material and make it possible, when applying and forming a
coating, to obtain chemically pure materials, without oxides
in their structure, to improve substantially their structure,
physicochemical and technological properties.
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The coefficient of utilization of the coating material
being applied, the adhesion, cohesion and its structure
depend on the impact velocity of the coating particles being
applied with the surface of the material.
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Preliminary acceleration of the particles of the coating
material by the gas inert to them at a temperature T ≤ 300 K,
with the velocity of the accelerating gas of 0.3 ≤ M ≤ 1.0,
and introducing the gas-and-powder mixture into the core of
the carrier gas accelerating the particles make it possible
to increase substantially the velocity of the particles of
the coating material to a velocity close to that of the
accelerating gas and thereby to make maximum use of the
energy of the gas jet accelerating the particles. Introducing
the powder mixture into the core of the accelerating flow of
the carrier gas eliminates the effect of deceleration of the
particles by the walls of the flow-through part of the
supersonic accelerating nozzle, prolongs the service life of
the device, increases the coefficient of utilization of the
coating material being applied. For eliminating the effect of
decelerating the coating particles in the compressed
near-wall gas layer, which originates as the supersonic gas
jet falls upon the surface of the article, in the method of
the invention the gas flow is turned and moved away from the
surface of the article being treated. Turning of the gas flow
is brought about by the physical effect which originates as
the plane-parallel flow of gas flows around the curvilinear
surface of radius R. Thus, turning of the gas flow and moving
it away from the surface of the surface of the article being
treated eliminate the appearance of the compressed near-wall
gas layer on the surface of the article, and the coating
particles continue their rectilinear motion, reach the
surface of the article with the velocity which they acquired
in their interaction with the gas flow accelerating them.
Thereby, it becomes possible, with smaller energy parameters
of the gas flow, to apply particles of the of the coating
material with higher impact velocities with the surface of
the material of the article. As a result, the power inputs
are reduced, the coefficient of utilization of the coating
material is increased, and the structure, quality and
properties of the coating material are improved. Setting the
cumulative rate of flow of the working carrier gas and of the
gas inert to the powder coating material in accordance with
the calculated conditions of the gas flow issue from the
accelerating nozzle is the regime in which the pressure in
the exit section of the accelerating supersonic nozzle
corresponds to the pressure of the ambient medium. Under this
condition the velocity of the gas flow issuing from the
supersonic nozzle will be maximum. Any changes in the
parameters of the carrier gas, such as temperature, pressure,
kind of gas, lead to off-design conditions of the issue,
i.e., to a loss of the gas flow velocity and, consequently,
of the velocity of particles of the coating material being
applied.
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Vibratory motions of the article being treated in the
case of synchronous introducing in the pulsed mode of the
gas-and-powder mixture into the core of the accelerating flow
of the carrier gas are effected in such a manner that feeding
the gas-and-powder mixture occurs at the moment when the
article being treated moves in a direction opposite to that
of the flow of particles of the coating material. In this
case the velocity of moving particles of the coating material
and the velocity of displacing the article being treated are
added together. As a result, the velocity of collision of the
particles with the surface increases. This leads to an
increase in the depth of penetration of the particles into
the surface of the material of the article, in the extent of
their plastic deformation, in the coefficient of utilization
of the coating material, to an improvement in the structure
of the coating and its technological properties.
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In order to intensify the course of mechanical and
chemical processes in the superficial layer of the material
of the article on collision of particles of the coating
material with the surface of the article in the method of the
invention, the article is subjected to surface heating to a
temperature at which the plasticity of the surface of the
material of the article is close to the plasticity of the
coating material. This makes it possible to apply coatings
having a smaller hardness to harder and less plastic surfaces
of articles. In this case the properties of the transition
zone (material of the particles and material of the article)
change sharply, the structure of this zone and, as a
consequence, adhesion of the coating, becomes improved. In
the transition zone there is also formed an intermetallic
compound on application of a metal to a metal, consisting of
the material of the coating particles being applied and of
the material of the article. Solid particles, while moving in
the gas flow, acquire a charge having a definite value and
sign due to friction with gas, against each other, and on the
walls of the flow-through part of the gas channel. The value
and sign of the charge depend on the material of the
particles. When in the method of the invention an opposite
sign potential is fed to the surface of the article being
coated, there occurs an increase in the velocity of the
charged particles as they approach to the surface of the
article, and a micro-arc discharge originates on collision.
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Acting on the moving flow of charged particles by
different electromagnetic methods and by the value of the
potential fed to the article, it is possible to regulate the
process of applying and forming the coating.
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This makes it possible to vary the structure, properties
and quality of the coating in the course of applying thereof.
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The structural feature of the means for introducing the
gas-and-powder mixture make it possible to effect axial
introducing of the coating particles with the starting
velocity into the core of the working carrier-gas flow
accelerating said particles, obviating the interaction of
particles of the coating material with the walls of the
accelerating nozzle 16 under the conditions of dsect. ≤ Dcrit.
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Thereby it becomes possible to prolong the service life
of the device, to make maximum use of the energy of the gas
flow for increasing the kinetic energy of the particles of
the coating material, to carry into effect the process of
coating formation with relatively not high starting
parameters of the carrier gas, to increase the coefficient of
utilization of the material being applied.
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The device of the invention may be used for producing
multifunctional coatings and materials from various powdered
components selected from the group comprising metals, alloys
or mechanical mixtures thereof, as well as dielectrics and
organic compounds.
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Owing to the provision of conditions which allow
activating the particles and rule out the presence of an
oxide film on their surface, the range of materials used for
the application of coatings by the method of the invention
may be substantially broadened.
-
So, the described method and device for producing
coatings from powder materials make it possible to intensify
the process of applying and forming coatings from various
powders and their mixtures, to obtain a coating material
without the presence of oxide inclusions in its structure, to
improve the structure and physicochemical properties of the
coating material, to increase the coefficient of utilization
of the coating material, to reduce the energy parameters of
the gas flow, to broaden the possibility of controlling the
process of applying and forming coatings, to obtain materials
in coatings with unique properties, to broaden the
technological and functional possibilities of the process for
producing coatings.
Industrial Applicability
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The present invention may be used in metallurgy,
mechanical engineering, radio- and electronic engineering and
other industries for improving the technological and
physicochemical properties of articles, for restoring various
worn-out parts, and for imparting specific properties to the
surface of articles.
