Gas-dynamic spraying method for applying a coating

A cold gas-dynamic spraying method for applying a coating to an article introduces into a gas particles of a powder of a metal, alloy, polymer or mechanical mixture of a metal and an alloy, the particles having a particle size of from about 1 to about 50 microns. The gas and particles are formed into a supersonic jet having a temperature considerably below a fusing temperature of the powder material and a velocity of from about 300 to about 1,200 m/sec. The jet is directed against an article of a metal, alloy or dielectric, thereby coating the article with the particles.

The present invention relates to metallurgy, more specifically, a method of 
and an apparatus for applying a coating. 
BACKGROUND ART 
Protection of structures, equipment, machines, and mechanisms made of 
ferrous metals from corrosion and effect exerted by aggressive media, an 
improvement of specifications of materials, including obtaining materials 
with prescribed properties, and development of resource-saving processing 
technologies are important scientific, technical and practical problems. 
These problems can be solved by various methods, including applying powder 
coatings with widely usable gas flame-spray, electric arc, detonation and 
plasma methods. 
The gas flame-spray method is based on gas combustion products used at 
1000.degree. to 3000.degree. C., creation of a flow of these gases in 
which powder particles being applied are fused. A velocity of 50 to 100 
m/s is imparted to said particles, and the surface is treated with the gas 
and powder flow containing the fused particles. This treatment results in 
a coating. The low values of velocity and temperature of the applied 
particles substantially limit application of this method. 
The explosive method eliminates these disadvantages in part, according to 
which the energy of detonating gases at 2000.degree. to 3500.degree. C. is 
used owing to which fact the velocity of the particles is substantially 
increased up to 400 to 700 m/s and their temperature is increased up to 
2000.degree. to 3500.degree. C. to ensure application of coatings of 
powders of metals, alloys, and dielectrics. This method is highly 
disadvantageous in low productivity explained by the impact acceleration 
process of deposition: a resulting shock wave and a gas flow following it 
cause a high level of a thermal and dynamic pulse effect produced upon the 
product and also of acousting noise which restricts the possibilities of 
application of this method. 
The most promising is a method of plasma deposition consisting in 
application of a powder coating to the surface of a product with a 
high-temperature gas jet (5000.degree. to 3000.degree. C.). 
Known in the art is a method for applying coatings to the surface of a 
product whose material is selected from the group consisting of metals, 
alloys, and dielectrics, said method comprising introducing into a gas 
flow a powder of the material selected from the group consisting of 
metals, alloys, their mechanical mixtures or dielectrics to form a gas and 
powder mixture to be directed onto the surface of the product (the book V. 
V. Kudinov, V. M. Ivanov. Nanesenie Plazmoi Tugoplavkikh Pokryty 
/Application of Refractory Coatings with Plasma/. Mashinostroenie 
Publishing House, Moscow. 1981, pp.9 to 14). 
The prior art method is characterized in that powder particles of a size of 
from 40 to 100 .mu.m are introduced into a high-temperature gas flow 
(5000.degree. to 3000.degree. C.) in the form of a plasma jet. Said powder 
particles are heated to the melting point or higher, the powder particles 
are accelerated by the plasma jet gas flow and directed to the surface 
being coated. Upon impingement, the powder particles interact with the 
surface of a product thus forming the coating. In the prior art method, 
the powder particles are accelerated by a high-temperature plasma jet and 
transferred, in molten state, to the product being coated; as a result, 
the high-temperature jet runs in the product to exert a thermal and 
dynamic effect upon its surface, i.e., causes local heating, oxidation and 
thermal deformations. For instance thin-walled products are heated up to 
550.degree. C., oxidized and twisted while the coating peels off. 
The high-temperature jet flowing into the surface of a product intensifies 
chemical and thermal processes, causes phase transformations and 
appearance of oversaturated and non-stoichiometric structures, and hence, 
the structural changes in the material. Also the high level of a thermal 
effect on the coating results in hardening heated melts and gas liberation 
during crystallization which bring about the formation of evolved porosity 
and appearance of microcracks, i.e., impairs specifications of the 
coating. 
It is known that, with an increase in the temperature of a plasma jet, 
plasma density in comparison with gas density under normal conditions 
linearly decreases, i.e., at 1000.degree. C., density of the jet becomes 
scores of times a factor that results in a lower resistance coefficient of 
particles. To sum up given a plasma jet velocity of 1000 to 2000 m/s 
(which is about equal to, or slightly below then, the sonic velocity), the 
particles are accelerated up to 50 to 200 m/s (even up to 350 m/s at 
best), i.e., the process of acceleration is not efficient enough. 
As is known with a decrease in a size of powder particles heating, melting, 
and overheated thereof in a plasma jet are enhanced. As a result, the, 
fine fractions of powder of a size from 1 to 10 .mu.m are heated to a 
temperature above the melting point, and their material intensively 
evaporates. For this reason, the plasma deposition of particles having a 
size below 20 to 40 .mu.m causes great difficulties and particles of a 
size from 40 to 100 .mu.m are normally used for this purpose. 
It should also be noted that the prior art method makes use of plasma jets 
of energy-consuming diatomic gases which call for application of high 
power which explains stringent requirements imposed upon the structure of 
apparatuses. It is only natural that limitations of the method of 
deposition on small-size objects are rather essential and can be 
eliminated through the complete removal of energy applied by cooling or 
providing a dynamic vacuum, i.e., by evacuation of high-temperature gases 
which requires high power consumption. 
Therefore, the prior art method has the following disadvantages: the high 
level of thermal and dynamic effect on the surface being coated; 
substantial changes in properties of the material being applied during the 
coating application, such as electrical conductance, heat conductance, and 
the like; changes in the structure of material as a result of phase 
transformations and appearance of oversaturated structures following from 
the chemical and thermal effect of the plasma jet and the hardening of 
overheated melts; ineffective acceleration of powder particles resulting 
from low density of plasma; intensive evaporation of fine powder fractions 
of a size from 1 to 10 .mu.m; stringent requirements imposed upon the 
structure of apparatus in view of hightemperature processes of the prior 
art method. 
Known in the art is an apparatus for carrying out the prior art method for 
applying coatings to the surface of a product, comprising a metering 
feeder having a casing accommodating a hopper for a powder communicating 
with a means for metering the powder formed as a drum having depressions 
in its cylindrical surface, a mixing chamber and also provided with a 
nozzle for accelerating powder particles communicating with the mixing 
chamber, a source of compressed gas, and a means connected thereto to 
supply the compressed gas to the mixing chamber (in the book V. V. 
Kudinov, V. M. Ivanov, Nanesenie Plazmoi Tugoplavkikh Pokryty /Application 
of Refractory Coatings with Plasma/. Mashinostroenie Publishing House, 
Moscow. 1981, pp.20 to 21, FIG. 11; p.26, FIG. 13). 
The prior art apparatus is characterized by a plasma sprayer (plasmotron), 
comprising a cylindrical (subsonic) nozzle having passages for supplying a 
plasma-forming gas and water for cooling thermally stressed units of the 
plasma sprayer (namely, the nozzle) in which refractory materials are 
used. Powder particles are introduced from the metering feeder at the edge 
of the nozzle. 
Since energy for forming plasma jet is applied in the form of an arc in the 
passage of a plasmotron nozzle, the nozzle is subjected to intensive 
electric erosion and high-temperature exposure. As a result, a rapid 
erosion wear of the nozzle occurs, the service life of which is 15 to 20 
hours. With the sophisticated construction and use of refractory materials 
and water cooling service life can be prolonged to 100 hours. 
The introduction of the particles at the edge of a nozzle and erosion of 
the inner duct of the nozzle lower the efficiency of acceleration of the 
powder particles. Thus, in combination with a low density of plasma, the 
prior art apparatus ensures a velocity of powder particles of up to 300 
m/s with a gas escape velocity of up to 1000 m/s. 
As a result of the powder getting into the space between moving parts of 
the metering feeder (e.g., between the drum and casing), the drum tends to 
be jammed. 
Therefore, the prior art apparatus has the following disadvantages: short 
service life which is mainly determined by the service life of a nozzle of 
15 to 100 hours and which is associated with the high density of a heat 
flux in the direction towards the plasmotron nozzle and erosion of the 
electrodes a factor that makes one to use expensive, refractory, and 
erosion-resistant materials; the inefficient acceleration of the deposited 
particles because the nozzle design is not optimal and is subjected to 
changes entailed by the electrical erosion of the inner duct; unreliable 
operation of the metering feeder of is the drum type which is caused by 
the powder getting into the space between the moving parts thus causing 
their jamming. 
DISCLOSURE OF THE INVENTION 
It is the principal object of the present invention to provide a method of 
and apparatus for applying a coating to the surface of a product, which 
allow the level of thermal and dynamic and thermal and chemical effect 
exerted the surface being coated and upon powder particles to be 
substantially lowered and initial structure of the powder material be 
substantially preserved, without phase transformations, appearance of 
oversaturated structures, and hardening during the application and 
formation of coatings, efficiency of acceleration of applied powder 
particles be enhanced, evaporation of fine fractions of the powder with a 
particle size from 1 to 10 .mu.m be eliminated, a lower level of thermal 
and erosion exposure of the components of an apparatus be ensured, the 
service life of the apparatus being prolonged up to 1000 hours without 
using expensive, refractory and erosion-resistant materials, the operation 
of the duct for powder particles acceleration being improved and operation 
reliability of the metering feeder being enhanced, even in metering fine 
powder fractions. 
The problem set forth is accomplished by providing a method for applying a 
coating to the surface of a product made of a material selected from the 
group consisting of metals, alloys, and dielectrics, comprising 
introducing into a gas flow a powder of the material selected from the 
group consisting of metals, alloys, mechanical mixtures thereof or 
dielectrics to form the gas and powder mixture which is directed onto the 
surface of the product, wherein, according to the invention, the powder 
used has a particle size of from 1 to 50 .mu.m in an amount ensuring a 
mass flow rate density of the particles between about 0.05 and about 17 
g/s.multidot.cm.sup.2, a supersonic velocity being preset to the gas flow, 
and a supersonic jet of the predetermined profile being formed to assure a 
velocity of the gas powder mixture powder particles of from 300 to 1200 
m/s. 
Owing to the fact that the powder is used with a particle size of from 1 to 
50 .mu.m, denser coatings are produced, filling of the coating layer and 
its continuity are improved, the volume of microvoids decreases, and the 
structure of the coating becomes more uniform, i.e., its corrosion 
resistance, hardness, and strength are enhanced. 
The density of a mass flow rate of the particles of between about 0.05 and 
about 17 g/s.multidot.cm.sup.2 increases the utilization factor of the 
particles, hence, productivity of application. With a flow rate of 
particles below 0.05 g/s.multidot.cm.sup.2, the utilization factor tends 
to zero, and with that of above 17 g/s.multidot.cm.sup.2, the process 
becomes economically ineffective. 
The presence of supersonic velocity ensures acceleration of the powder in a 
gas flow and lowers temperature of the gas flow owing to gas expansion 
with its supersonic escape. The formation of a supersonic jet of the 
predetermined profile with a high density and a low temperature, due to 
increasing resistance coefficient of particles with an increase in gas 
density and a decrease in temperature, contributes to a more efficient 
acceleration of powder particles and a decrease in the thickness of the 
compressed gas layer upstream of the product being coated, and hence, a 
lower decrease in velocity of the particles in the compressed gas layer, a 
decrease in the level of thermal and dynamic and thermal and chemical 
effect on the surface being coated and the powder particles being applied, 
elimination of evaporation of particles having a size of from 1 to 10 
.mu.m, preservation of the initial structure of powder material and 
elimination of a hardening process of the coating and thermal erosion 
effect on the apparatus components. 
Imparting acceleration to the gas - powder mixture from 300 to 1200 m/s 
ensures a high level of kinetic energy to the powder particles which upon 
impingement of the particles against the surface of a product is 
transformed into plastic deformation of the particles with a bond formed 
with the product. 
Therefore, the invention, which makes use of finely-divided powder 
particles of a size of from 1 to 50 .mu.m with a density of mass flow rate 
of from 0.05 to 15 g/s.multidot.cm.sup.2 and imparting acceleration to the 
powder particles through a supersonic jet of the predetermined profile 
with a high gas density and a low gas temperature to a velocity of from 
300 to 1200 m/s substantially lower the level of thermal and dynamic and 
thermal and chemical effect on the surface being coated and enhances 
efficiency of particle acceleration which provides for the production of 
denser coatings, reduces the volume of microvoids therein and improves the 
filling of the coating layer and its continuity. This results in a uniform 
structure of the coating with the substantially preserved formation of the 
powder material without phase transformations and hardening, i.e., the 
coatings applied do not crack, their corrosion resistance, microhardness, 
and cohesion and adhesion strength are enhanced. 
It is preferred that a supersonic jet of the predetermined profile be 
formed through gas expansion in accordance with linear principles. Such a 
solution provides ease of maintenance and economy of the manufacture of an 
apparatus for the realization of this process. 
It is preferred that the gas flow use a gas having a pressure of from about 
5 to about 20 atm. and is a temperature below the melting point of the 
powder particles. This solution promotes the efficient acceleration of 
powder particles on account of high density of the gas, reduces thermal 
and dynamic and thermal and chemical effect and also contributes to ease 
of maintenance and economy in the manufacture of the apparatus realizing 
this method. 
Air can be used as the gas for forming a gas flow. This ensures the 
acceleration of the powder particles to a velocity of up to 300 to 600 m/s 
and the economy of the coating process. 
It is preferred that helium be used as the gas for forming a gas flow. This 
imparts a velocity of from 1000 to 1200 m/s to the powder particles. 
It is preferred that an air/helium mixture be used as the gas for forming a 
gas flow. The mixture concerned makes it possible to regulate the velocity 
of powder particles within the range of from 300 to 1200 m/s. 
As a possible variant of controlling the velocity of particles from 300 to 
1200 m/s it is technologically and economically justifiable if the gas is 
heated to from 30.degree. to 400.degree. C. which effects a saving in the 
application of coatings inasmuch as air is used here and also enables one 
to regulate the velocity of particles within wide limits. 
The above problem is also solved by providing an apparatus for carrying out 
the method for applying a coating comprising a metering feeder having a 
casing incorporating a hopper for a powder communicating with a means for 
metering the powder formed as a drum having depressions in its cylindrical 
surface, and a mixing chamber and provided with a nozzle for accelerating 
powder particles communicating with the mixing chamber, a source of 
compressed gas, and a means connected thereto for supplying the compressed 
gas to the mixing chamber, and which, according to the invention, 
comprises a powder particle flow controller which is mounted in a spaced 
relation to the cylindrical surface of the drum, with a space ensuring the 
necessary flow rate of the powder, and an intermediate nozzle connected to 
the mixing chamber and communicating, via an inlet pipe thereof, with the 
means for supplying compressed gas, the metering feeder having a baffle 
plate mounted on the bottom of the hopper and being adjacent to the 
cylindrical surface of the drum which has its depressions extending along 
a helical line, the drum being mounted horizontally in such a mariner that 
one portion of its cylindrical surface defines the bottom of the hopper 
and the other part thereof defines the generant of the mixing chamber, 
particles acceleration nozzle being substantially a supersonic and having 
a profile passage. 
The provision of the powder particle feed controller ensures the desired 
flow rate of the powder during coating application. 
The provision of the baffle plate mounted on the hopper bottom prevents 
powder particles from getting into the space between the drum and the 
casing of the metering feeder thus preventing the drum from being jammed. 
The provision of the depression on the cylindrical surface of the drum 
extending along a helical line lowers fluctuations of the flow rate of 
particles on metering. 
The provision of a portion of the drum functioning as the hopper bottom and 
of the other portion of the drum functioning as the generant of a mixing 
chamber ensures the uniform filling of depressions with the powder and 
also reliable admission of the powder to the mixing chamber. 
The provision of the supersonic nozzle having a profiled passage allows a 
supersonic velocity to be imparted to the gas flow and a supersonic jet of 
the predetermined profile to be formed with high density and low 
temperature so as to ensure acceleration of the powder particles of a size 
of from 1 to 50 .mu.m to a velocity of from 300 to 1200 m/s. 
Since the mixing chamber and the intermediate nozzle connected thereto 
communicate with the means for supplying compressed gas through the inlet 
pipe of the intermediate nozzle, the metering feeder can be supplied from 
different compressed gas sources including portable and stationary gas 
facilities which can be installed for away from the metering feeder. 
It is preferred that the passage of a supersonic nozzle for acceleration of 
particles have one dimension of its flow-section larger than the other, 
with the ratio of the smaller dimension of the flow-section at the edge of 
the nozzle to the length of the supersonic portion of the passage ranging 
from about 0.04 to about 0.01. 
This construction of the passage allows a gas and powder jet of the 
predetermined profile to be formed, ensures an efficient acceleration of 
the powder, and lowers velocity loss in the compressed gas layer in front 
of the surface being coated. 
A turbulence nozzle for a gas flow leaving the compressed gas supply means 
may be provided on the inner surface of the intermediate nozzle, at the 
outlet thereof in the mixing chamber, which device agitates the flow of 
gas directed from the intermediate nozzle to the cylindrical surface of 
the drum thus assuring the effective removal of the powder and formation 
of the gas and powder mixture. 
It is preferred that the intermediate nozzle be mounted in such a manner 
that its longitudinal axis extend at an angle from 80 to 850 with respect 
to a normal to the cylindrical surface of the drum. When the gas flow runs 
in the cylindrical surface of the drum, a recoil flow is formed and as a 
consequence of the effective mixing of the powder and gas. 
It is preferred that the apparatus comprise a means for supplying 
compressed gas to depressions in the cylindrical surface of the drum and 
to the upper part of the hopper to balance pressures in the hopper and the 
mixing chamber. This solution eliminates the effect of pressure on the 
metering of the powder. 
It is preferred that the means for gas supply be provided in the casing of 
a metering feeder in the form of a passage communicating the interior 
space of the intermediate nozzle to the interior space of the hopper and 
also comprise a tube connected to the intermediate nozzle and extending 
through the hopper, the top part of the tube being bent at an angle of 
180.degree.. This simplifies the design, promotes reliability in 
operation, and prevents the powder from getting into the passage during 
loading the powder into the hopper. 
It is preferred that the apparatus comprise a means for heating compressed 
gas having a gas temperature control system for controlling the velocity 
of a gas and powder mixture with the supersonic jet. Such solution ensures 
gas escape velocity control by varying its temperature and accordingly the 
velocity of powder particles is also controlled. 
To enhance heat transfer from a gas heater, the inlet of compressed means 
gas heating may be connected, through a pneumatic line to the mixing 
chamber of the metering feeder and the outlet can be connected to the 
nozzle for acceleration of powder particles. 
For applying coatings of polymeric materials, it is advisable that the 
apparatus comprise a premix chamber at the inlet of the nozzle for 
acceleration of powder particles, the inlets of the means for gas heating 
and of the inlet pipe of the intermediate nozzle of the metering feeder 
being connected by means of individual pneumatic lines to a compressed gas 
supply and their outlets being connected to the premix chamber by means of 
other individual pneumatic lines. 
It is preferred that the heating means be provided with a heater element 
made of a resistor alloy. This allows the overall dimensions of the 
heating means and its weight to be reduced. 
To lower heat losses and enhance economic effectiveness of the apparatus, 
it is preferred that the heater element be mounted in a casing 
accommodating a heat insulator. 
To make the heating means compact and ensure heating with low temperature 
differentials between the gas and heater element, the latter may be made 
in the form of a spiral of a thin-walled tube, with the gas flowing 
therein. 
To ensure a substantial reduction of the effect of the gas supplied to the 
gas and powder mixture from the metering feeder on operation of the 
supersonic nozzle, it is preferred that the premix chamber have a 
diaphragm mounted in its casing and having ports for equalizing the gas 
flow over the cross-section and a branch pipe coaxially mounted in the 
diaphragm for introducing powder particles, the cross-sectional area of 
the branch pipe being substantially 5 to 15 times as small as the 
cross-sectional area of the pneumatic line connecting the gas heating 
means to the premix chamber. 
To diminish wear of the drum, alterations of its surface, and reduce 
jamming, the drum may be mounted for rotation in a sleeve made of a 
plastic material, which adjoins the cylindrical surface of the drum. 
The plastic material of the sleeve may be in the form of fluoroplastic 
(TEFLON). This allows the shape of the drum to be retained owing to 
absorption of the powder particles by the material of said sleeve.

The invention considers a method for applying a coating to the surface of a 
product. The material of the product is selected from the group consisting 
of metals, alloys and dielectrics. In this case the material may be in the 
form of metal, ceramic or glass. The method consists in that a powder of a 
material selected from the group consisting of metals, alloys or their 
mechanical mixtures, and dielectrics is introduced into a gas flow to form 
the gas and powder mixture which is directed onto the surface of the 
product. According to the invention, the powder has particles of a size of 
from 1 to 50 .mu.m in an amount ensuring a density of mass flow rate of 
the particles between 0.05 and 17 g/s.multidot.cm.sup.2. Supersonic 
velocity is imparted to the gas flow, and a supersonic jet is formed with 
the predetermined profile with high density and a low temperature. The 
resulting gas and powder mixture is introduced into the supersonic jet to 
impart thereto acceleration to ensure a velocity of the powder particles 
ranging from 300 to 1200 m/s. 
If finely divided powder particles are used with the above-mentioned 
density of their mass flow rate, and if acceleration is imparted to the 
powder particles by means of a supersonic jet of the predetermined 
profile, which has high density and low gas temperature to a velocity 
ranging from 300 to 1200 m/s, a substantial decrease in the level of 
thermal and dynamic and thermal and chemical effection the surface being 
coated is ensured, and the efficiency of acceleration of the powder 
particles is enhanced, which results in denser coatings being produced, 
with a lower volume of microvoids and with enhanced continuity. The 
coating structure is uniform with retention of substantially the initial 
structure of the powder material, without phase transformations, i.e., the 
coatings do not crack, their corrosion resistance, microhardness, cohesive 
and adhesive strength are enhanced. 
In accordance with the invention, the gist of the method resides in that 
application of coating by spraying is effected by a high-velocity flow of 
powder which is in solid state, i.e., at a temperature which is much lower 
than the melting point of the powder material. The coating is thus formed 
owing to the impact and kinetic energy of particles which is spent for 
high-speed plastic deformation of the interacting bodies in microvolumes 
which are commensurable with a particle size and also for local heat 
liberation and cohesion of particles with the surface being coated and 
therebetween. 
The formation of a supersonic jet of the predetermined profile is carried 
out by expanding the gas according to a linear law, which renders the 
process simple and economical. 
For a gas flow, use is made of gas which is under a pressure of from about 
5 to about 20 atm. and at a temperature below the melting point of the 
powder particles, which ensures the efficient acceleration of the powder 
particles owing to a high density of the gas and to a lower thermal and 
dynamic and thermal and chemical effect. 
Acceleration is imparted to the powder particles to a velocity ranging from 
about 300 to about 600 m/s by using air as the gas for forming a gas flow. 
To impart to the powder particles a velocity ranging from 1000 to 1200 m/s, 
helium is used, and to impart a velocity ranging from 300 to 1200 m/s a 
mixture of air and helium is used. 
For accelerating various materials of powder, gases are used which have 
different sound velocities at constant temperature, which can impart 
different velocities to the powder particles. For such powders as tin, 
zinc, aluminium, and the like, use can be made of air, an air/helium 
mixture in various proportions may be used for nickel, iron, cobalt, and 
the like. By changing the percentage of components, the velocity of a gas 
jet, and, accordingly the velocity of powder particles, can be varied. 
Another option for controlling the velocity of particles between 300 and 
1200 m/s is a change in the initial gas temperature. It is known that with 
an increase in gas temperature sound velocity in the gas increases. This 
allows the jet velocity, and accordingly the velocity of the deposited 
powder particles to be controlled by a weak underheating of the gas at 
30.degree. to 400.degree. C. During expansion of the gas, when the 
supersonic jet is formed, the gas temperature decreases substantially 
which permits maintaining the thermal effect on powder particles at low 
level, a factor that is important in the application of polymeric coatings 
to products and apparatus components. 
An apparatus for applying coatings to the surface of a product comprises a 
metering feeder (FIG. 1) having a casing 1' which accommodates a hopper 2 
for a powder having a lid 2' mounted by means of thread 2", a means for 
metering the powder, and a mixing chamber 3, all communicating with one 
another. The apparatus also has a nozzle 4 for accelerating powder 
particles in communication with the mixing chamber 3, a compressed gas 
supply 5 and a means connected thereto for supplying the compressed gas to 
the mixing chamber 3. The compressed gas supply means is in the form of a 
pneumatic line 6 which connects, via a shut-off and control member 7, the 
compressed gas supply 5 to an inlet pipe 8 of metering feeder 1. A powder 
metering means is in the form of a cylindrical drum 9 having on its 
cylindrical surface 9' depressions 10 and communicating with the mixing 
chamber 3 and with the particle acceleration nozzle 4. 
According to the invention, the apparatus also comprises a powder particle 
flow controller 11 which is mounted in a spaced relation 12 relative to 
the cylindrical periphery 9' of the drum 9 so as to ensure the desired 
mass flow rate of the powder during coating, and an intermediate nozzle 13 
positioned adjacent the mixing chamber 3 and communicating, via the inlet 
pipe 8, with the compressed gas supply means and with the compressed gas 
supply 5. 
To prevent powder particles from getting into a space 14 between the drum 9 
and casing 1' of the metering feeder 1 thus to avoid the jamming of the 
drum 9, a baffle plate 15 is provided on the hopper bottom which 
intimately engages the cylindrical surface 9' of the drum 9. 
To ensure a uniform filling of depressions 10 with the powder and its 
reliable admission to the mixing chamber 3, the drum 9 is mounted to 
extend horizontally in such a manner that one portion of its cylindrical 
surface 9' is used as a bottom 16 of hopper 2 and the other portion forms 
a wall 17 of mixing chamber 3. Depressions 10 in the cylindrical surface 
9' of the drum 9 extend along a helical line (FIG. 2), which lowers 
fluctuations of the flow rate of powder particles during metering. To 
impart to a gas flow supersonic velocity with the predetermined profile, 
with high density and low temperature, and also to ensure acceleration of 
powder particles to a velocity ranging from 300 to 1200 m/s, nozzle 4 for 
acceleration of the powder particles is made supersonic and has a passage 
18 of profiled cross-section (FIG. 3). The passage 18 of the nozzle 4 has 
one dimension "a" of its flow-section which is larger than the other 
dimension "b", and the ration of the smaller dimension "b" of the 
flow-section at an edge 19 of nozzle 4 (FIG. 1) to length "1" of a 
supersonic portion 20 of passage 18 ranges from about 0.04 to about 0.01. 
This construction of passage 20 allows a gas and powder jet of the 
predetermined profile to be formed, ensures efficient acceleration of the 
powder, and lowers velocity loss in the compressed gas layer upstream of 
the surface being coated. 
A turbulence nozzle 21 of a compressed gas flow admitted to a nozzle 13 
through the pipe 8 and leaving the means for compressed gas supply is 
provided on the inner surface of the intermediate nozzle 13, at the outlet 
thereof in mixing chamber 3. This turbulence nozzle 21 ensures an 
effective removal of powder and formation of a gas and powder mixture. To 
provide a recoil flow and ensure an effective mixing of powder and gas 
when the gas flow runs in the portion of the cylindrical surface 9' of 
drum 9 forming wall 17 of the mixing chamber 3, intermediate nozzle 13 is 
mounted in such a manner that its longitudinal axis 0--0 extends at an 
angle of from 80.degree. to 85.degree. with respect to a normal "n--n" 
drawn to the cylindrical surface 9' of drum 9. 
The apparatus for applying a coating to the surface of a product also 
comprises a means for supplying compressed gas to depressions 10 in the 
cylindrical surface 9' of drum 9 and to a top part 22 of the hopper 2 to 
balance the pressure in the hopper 2 and the mixing chamber 3. The 
provision of such means removes the pressure exerts on the metering of the 
powder. 
The means for gas supply is in the form of a passage 23 in the casing 1' of 
the metering feeder 1 which communicates an interior space 24 of 
intermediate nozzle 13 with the top part 22 of hopper 2 and has a tube 25 
which is connected to the intermediate nozzle 13, extends through the 
hopper 2 and is bent, at its top part, at an angle of 180.degree.. 
The means constructed as described above ensures reliable operation and 
prevents powder from getting into the passage 23 on loading the powder 
into the hopper 2. 
To assure control of gas escape velocity by varying its temperature, and 
according the velocity of powder particles, another embodiment of the 
apparatus has a means 27 (FIG. 4) for preheating the compressed gas and a 
gas temperature control system which allows gas and powder mixture 
velocity to be controlled when it moves through the nozzle 4 for 
acceleration of the powder particles. 
The gas temperature control system has a power supply 28 which is 
electrically coupled, via terminals 29, by means of cables 30, to a gas 
heating means, a temperature indicator 31, and a thermocouple 32 
engageable with the body of nozzle 4. 
Gas heating means 27 is connected in series with metering feeder 1. 
To enhance heat transfer from the heater to gas, an inlet 33 of means 27 
for heating compressed gas is connected, by means of a pneumatic line 34, 
to the mixing chamber 3 of metering feeder 1, and its outlet 35 is 
connected, by means of a pneumatic line 36, to the nozzle 4 for 
acceleration of the powder particles. 
If a coating is applied with polymeric materials, the apparatus is provided 
with a premix chamber 37 (FIG. 5) mounted at the inlet of nozzle 4 for 
acceleration of powder particles. The inlet 33 of means 27 for heating the 
compressed gas and an inlet 38 of metering feeder 1 are connected by means 
of individual pneumatic lines 39 to the compressed gas supply 5, and their 
outlets 35 and 40 are connected, by means of other pneumatic lines 41, to 
the premix chamber 37. This embodiment of the apparatus has the parallel 
connection of said means 27 for gas heating to the metering feeder 1. 
Means 27 has a casing 42 (FIG. 4) which has an inner heat insulator 43. 
The casing 42 accommodates a heater element 44 made of a resistor alloy in 
the form of a spiral of a thin-walled tube in which the gas flows. 
To reduce the effect of the gas supplied from the metering feeder 1 on 
operation of the supersonic nozzle 4, the premix chamber 37 has a 
diaphragm 45 (FIG. 5) mounted therein and having ports 46 for equalizing 
gas velocity over the cross-section, and a branch pipe 47 mounted in the 
premix chamber 37 coaxially with diaphragm 45 for introducing powder 
particles from the metering feeder 1. The crosssectional area of branch 
pipe 47 is substantially 5 to 15 times as small as the cross-sectional 
area of the pneumatic line 41 connecting the means 27 for gas heating to 
the premix chamber 37. 
The drum 9 is mounted for rotation in a sleeve 48 (FIG. 6) made of plastic 
material and being engaged with the cylindrical surface 9' of the drum 9. 
The plastic material of sleeve 48 is a fluoroplastic TEFLON which ensures 
the preservation of the shape of drum 9 by absorbing the powder particles. 
The provision of sleeve 48 lowers wear of drum 9 and reduces alterations of 
its surface 9', and also eliminates its jamming. 
The apparatus for applying a coating shown in FIG. 1 functions in the 
following manner. A compressed gas from the gas supply 5 is supplied along 
the pneumatic line 6, via shut-off and control member 7, to the inlet pipe 
8 of metering feeder 1, the gas being accelerated by means of intermediate 
nozzle 13 and directed at an angle of between 80.degree. and 85.degree. to 
impinge against the cylindrical surface 9' of drum 9 which is stationary 
and then gets into the mixing chamber 3 from which it escapes through the 
profiled supersonic nozzle 4. Supersonic nozzle 4 is brought to operating 
conditions (5 to 20 atm.) by means of the shut-off and control member 7 
thus forming a supersonic gas jet at a velocity ranging from 300 to 1200 
m/s. 
The powder from the hopper 2 gets to the cylindrical surface 9' of drum 9 
to fill depressions 10 and, during rotation of the drum, the powder is 
transferred into the mixing chamber 3. The gas flow formed by the 
intermediate nozzle 13 and turbulized by the turbulence nozzle 21 blows 
the powder off the cylindrical surface 9' of the drum 9 into the mixing 
chamber 3 wherein a gas and powder mixture is formed. The flow rate of the 
powder in an amount between 0.05 and 17 g/s.multidot.cm.sup.2 is preset by 
the number of revolutions of the drum 9 and space 12 between the drum 9 
and powder flow controller 11. The baffle plate 15 prevents the powder 
from getting into the space 14 between the casing 1' and drum 9. The gas 
from intermediate nozzle 13 is additionally separated along passages 23 to 
be admitted into the space 12 between the drum 9 and the casing 1' to 
purge and clean it from is the remaining powder, and through the tube 25, 
the gas gets into the top part 22 of the hopper 2 balances the pressure in 
the hopper 2 and mixing chamber 3. The gas and powder mixture from the 
mixing chamber 3 is accelerated in the supersonic portion 20 of the 
passage 18. A high-speed gas and powder jet is thus formed which is 
determined by the cross-sectional configuration of the passage 18 with the 
velocity of particles and density of their flow rate necessary for the 
formation of a coating. For the given profile of the supersonic portion 20 
of passage 18, the density of mass flow rate of powder particles is 
specified by the metering feeder 1, and the velocity of particles is 
prescribed by the usable gas. For example, by varying the percentage of 
helium in a mixture with air between 0% and 100%, the velocity of powder 
particles can be varied between 300 and 1200 m/s. 
The apparatus for applying a coating shown in FIG. 4 functions in the 
following manner. 
The compressed gas from gas supply 5 is fed, via pneumatic line 6 and 
shut-off and control member 7 which adjusts the required pressure between 
5 and 20 atm in the apparatus, to the metering feeder 1 whose drum 9 is 
stationary. The gas then flows through metering feeder 1 and to be 
admitted, via pneumatic line 34, to a heater element 44 of gas heating 
means 27 to be heated therein to a temperature between 30 and 400.degree. 
C., which is specified by the gas temperature control system. The heated 
gas is supplied through pneumatic line 36 to the profiled supersonic 
nozzle 4 and escapes therefrom due to gas expansion, the gas temperature 
being dropped when the apparatus is brought to the preselected jet escape 
conditions the drum 9 of metering feeder 1 is brought to rotation and the 
desired concentration of powder particles is specified by means of the 
powder flow controller 11 and by the speed of the drum 9, and the velocity 
of the powder particles accelerated in the supersonic nozzle 4 is preset 
by varying the gas heating temperature. 
In depositing the polymeric powders, the apparatus is used (FIG. 5) in 
which the powder from metering feeder 1 is fed directly through the branch 
pipe 41 to the premix chamber 37, and the gas heated in the heating means 
27 passes through the ports 46 of diaphragm 45 to transfer the powder into 
the supersonic nozzle 4 in which the necessary velocity is imparted to the 
particles. 
PRACTICAL EXAMPLES 
EXAMPLES 1 
The apparatus shown in FIG. 1 was used for coating application. 
Working gas - air. Air pressure - 9 atm., flow rate--0.05 kg/s, 
deceleration temperature --7.degree. C. Mach number at the nozzle edge 
-2.5 to 4. The material of products--steel and brass. 
An aluminium powder particle size--from 1 to 25 .mu.m, a density of flow 
rate of the powder--between 0.01 and 0.3 g/s.multidot.cm.sup.2, a velocity 
of particles of from 300 to 600 m/s. 
Coating conditions are given in Table 1. 
TABLE 1 
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Flow rate Treat- Coating 
Change in temperature 
density, ment thickness, 
of heat-insulated 
No. g/s .multidot. cm.sup.2 
time, T .mu.m support, .degree.C. 
______________________________________ 
1 0.01 1000 -- 2 
2 0.05 20 8 6 
3 0.05 100 40 6 
4 0.10 100 90 14 
5 0.15 100 150 20 
6 0.3 100 390 45 
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It can be seen from the Table 1 that the coating is formed with a flow rate 
density of powder from 0.05 g/s.multidot.cm.sup.2 and up. With an increase 
in density of a powder flow rate up to 0.3 g/s.multidot.cm.sup.2, the 
temperature of a heat insulated support increases up to 45.degree. C. It 
follows from the above that coatings can be applied under the 
above-mentioned conditions, and products have a minimum thermal effect. 
Examples 2, 3, 4, 5 and 6. 
The apparatus shown in FIG. 1 was used for coating application. 
The material of deposited powders--copper, aluminium, nickel, vanadium, an 
alloy of 50% of copper, 40% of aluminium, and 10% of iron. 
The support material--steel, DURALUMIN, brass, and bronze, ceramics, glass: 
supports were used without heat insulation. 
______________________________________ 
gas pressure 15 to 20 atm.; 
gas deceleration temperature 
0 to 10.degree. C.: 
Mach number at the nozzle edge 
2.5 to 3; 
working gas - mixture of air and 
helium with 50% of helium; 
gas flow 20 to 30 g/s; 
particle flow rate density 
0.05 to 17 g/s .multidot. cm.sup.2. 
______________________________________ 
The velocity of particles was determined by the method of laser Doppler 
anemometry, and the coefficient of utilization of particles was determined 
by the weighing method. The results are given in Table 2. 
TABLE 2 
______________________________________ 
Ex- 
ample Particle Particle Particle Coefficient of par- 
No. material size, .mu.m 
velocity, m/s 
ticle utilization % 
______________________________________ 
2 copper 1-40 650 .+-. 10 
10 
800 .+-. 10 
30 
900 .+-. 10 
40 
1000 .+-. 10 
80 
3 aluminium 1-25 650 .+-. 10 
40 
1000 .+-. 10 
60-70 
1200 .+-. 10 
80-90 
4 nickel 1-40 800 .+-. 10 
10 
900 .+-. 10 
40 
1000 .+-. 10 
80 
5 vanadium 1-40 800 .+-. 10 
10 
900 .+-. 10 
30 
1000 .+-. 10 
60 
6 alloy 1-100 700 .+-. 10 
10 
800 .+-. 10 
20 
900 .+-. 10 
50 
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It can be seen from Table 2 that with an increase in velocity of particles 
for all materials, the coefficient of utilization increases, but its 
values differ for different materials. The support temperature in all 
cases did not exceed 50.degree. to 70.degree. C. 
After a prolonged operation with application of coatings, with the time of 
operation of the apparatus of at least 1000 hours, various components of 
the apparatus have been inspected and it has been revealed that the nozzle 
profile did not have any marked alterations. Thin powder material coating 
films were found in the area of critical cross section and the supersonic 
portion thereof as a result of friction with the nozzle walls during 
movement. These films did not have any effect on operating conditions of 
the nozzle. The individual occlusions of particles being deposited have 
been found in the fluoroplastic sleeve of the metering feeder, but the 
configuration of the drum and depressions of its cylindrical surface is 
remained substantially unchanged. 
Therefore, the service life of reliable operation of the apparatus was at 
least 1000 hours. The absence of energy-stressed components makes the 
upper limit of productivity substantially unlimited. 
EXAMPLE 7 
The apparatus shown in FIG. 4 used for application of coatings had the 
following parameters: Mach number at the edge of the 
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nozzle 2.5 to 2.6; 
gas pressure 10 to 20 atm; 
gas temperature 30 to 400.degree. C.; 
working gas air; 
gas flow rate 20 to 30 g/s; 
powder flow consumption 
0.1 to 10 g/s; 
powder particle size 1 to 50 .mu.m. 
______________________________________ 
The coatings were applied with particles of aluminium, zinc, tin, copper, 
nickel, titanium, iron, vanadium, cobalt to metal products, and the 
coefficient of utilization of the powder was measured (in percent) versus 
air heating temperature and related powder particles velocity. 
The results are given in Table 3. 
TABLE 3 
______________________________________ 
Powder Air temperature, .degree.C. 
material 
10 20 100 200 350 400 
______________________________________ 
aluminium 
0.1-1% 1-1.5 10 30-60 90-95 
zinc 1-2 2-4 10 50-80 
tin 1-30 80-40 40-60 
copper 10-20 50 80-90 90 
nickel 20 50-80 80-90 
titanium 50-80 -- -- 
iron 20-40 60-70 80-90 
vanadium 20 40-50 60-70 
cobalt 20 40-50 50-60 
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It can be seen from Table 3 that when air is used as working ga at room 
temperature, high-quality coatings can be produced from powders of such 
plastic metals as aluminium, zinc, and tin. Slight air heating to 
100.degree.-200.degree. C. resulting in an increase in particle velocity 
allows coatings to be produced from the majority of the above-mentioned 
metals. The product temperature does not exceed 60.degree. to 100.degree. 
C. 
EXAMPLE 8 
The apparatus shown in FIG. 5 was used for coating application. 
______________________________________ 
the nozzle 1.5 to 2.6; 
gas pressure 5 to 10 atm; 
gas temperature 30 to 180.degree. C.; 
working gas air; 
gas flow rate 18 to 20 g/s; 
powder flow rate 0.1 to 1 g/s; 
powder particle size 20 to 60 .mu.m. 
______________________________________ 
A polymer powder was applied to products of metal, ceramics, and wood. A 
coating thickness was from 100 to 200 .mu.m. Further thermal treatment was 
required for complete polymerization. 
It can be seen from the above that the invention makes it possible to: 
apply coatings from several dozens of microns to several millimeters thick 
from metals, their mechanical mixtures, alloys, and dielectrics to 
products of metals, alloys, and dielectrics, in particular, to ceramics 
and glass with a low level of thermal effect on the products; 
apply coatings with fine fraction powders, with a particle size between 1 
and 10 .mu.m without phase transformations, appearance of oversaturated 
structures, and hardening during coating formation; 
enhance the efficiency of acceleration of the powder by using high-density 
compressed gases; 
substantially lower thermal effect on apparatus components. 
The construction of the apparatus ensures its operation during at least 
1000 hours without employment of expensive erosion-resistant and 
refractory materials, high throughput capacity which is substantially 
unlimited because of the absence of thermally stressed components which 
enables one to incorporate apparatus into standard flow lines to which it 
can be readily matched as regards throughput capacity, e.g., in a flow 
line for the manufacture of steel pipes having protective coatings of 
zinc, aluminium and stainless steel. 
Industrial Applicability 
The invention can be most advantageously used, from the manufacturing and 
economic point of view in restoring the geometrical dimensions of worn 
parts, in increasing wear resistance, in protecting of ferrous metals 
against corrosion. 
The invention may be most advantageously used in metallurgy, mechanical 
engineering, aviation, ship building, agricultural machine building, in 
the automobile industry, in the instrument making and electronic 
technology for the application of corrosion-resistant, electrically 
conducting, antifriction, surface-hardening, magnetically conducting, and 
dielectric coatings to parts, structures, and equipment which are 
manufactured, in particular, of materials capable of withstanding a 
limited thermal effect and also to large-size objects such as sea-going 
and river vessels, bridges, and large diameter pipes. 
The invention may also find application for producing multiple-layer 
coatings and combined (metalpolymer) coatings as part of comprehensive 
manufacturing processes for producing materials with expected properties.