FIELD OF THE INVENTION

This invention relates to the technology of applying coatings to the surfaces, and in particular, to gas-dynamic methods of applying coatings with the use of an inorganic powder. It can be used in different branches of mechanical engineering, particularly for the restoration of the shape and dimension of metal parts, for the manufacturing and repair of metal parts to improve their impermeability or corrosion resistance or heat resistance or other property.

BACKGROUND OF THE INVENTION

Gas-dynamic spray methods are the effective techniques for producing metal and mixed metal — ceramic coatings by the treating the substrate by a high - velocity jet of fine solid particles. In these methods the particles are accelerated in the high velocity gas stream by the drag effect. Only compressed gases, predominantly air, are used for particle acceleration without using any combustible.

Known in the art is a method and apparatus for applying coatings [U.S. Pat. No. 5,302,414 issued 1994]. With this method, a coating is applied by introducing metal powders into a compressed gas flow, accelerating the gas- powder mixture in a supersonic nozzle (a de Laval type nozzle) and directing the accelerated powder particles to the substrate. The accelerated particles impinge on the substrate while having kinetic energy sufficient for adhering to the substrate surface. The coatings are produced with powder particles

having a particle size of from 1 to 50 microns. The powder particles neither melt nor begin to soften prior to impingement on the substrate and adhere to the substrate when their kinetic energy is transformed to a sufficient mechanical deformation.

The improvement of this method and apparatus [U.S. Pat. No. 6,139,913 issued 2000 and U.S. Pat. No. 6,283,386 issued 2001] includes the proper choice of the gas flow cross-section to allow for the formation of coatings by particles having a particle size up to 106 microns.

The main disadvantages of these methods are due to a powder is injected into the heated compressed gas flow prior to passage through the de Laval nozzle throat. Because the heated main gas flow (gas stream) is under high pressure, an injection of the powder requires expensive and complicated high pressure powder delivery (powder supply) systems. The powder particles and heated main gas both must pass through the throat of the nozzle, and the particles often stick to the walls of a diverging portion and a throat of the nozzle and clog the nozzle. This requires a complete shutdown of the system and cleaning of the nozzle. As a result, the gas temperature must be sufficiently low - such that no softening and sticking of the particles to the nozzle walls take place. That temperature often turns out to be insufficient for effective coating. Besides, when using the powders with hard particles, a considerable wear of the nozzle throat occurs, causing the early destroying of the nozzle.

The further prior art methods of coating [U.S. Pat. No. 6,756,073 issued 2004; RU 2205897, 2001; RU 2100474, 1997; U.S. Pat. No. 6,402,050 issued 2002] are free ,©f these drawbacks. These inventions use a supersonic nozzle, and a preheated compressed gas is supplied to this nozzle. The gas, when passing through a converging portion, a throat and a diverging portion of the nozzle, accelerates and forms a supersonic flow in the nozzle. At the point

following the nozzle throat (downstream of the throat), the powder particles are introduced into said flow. They are accelerated by the supersonic gas flow and directed to a surface of the base (substrate). In these methods the powder particles do not pass through the nozzle throat. This aljtaws the gas temperature to be increased with no fear that the particles will stick to the walls of the nozzle and clog or plug the nozzle throat. Since the velocity of the gas flow accelerating the powder particles is roughly proportional to the square root of the gas temperature, an increase of the gas temperature results in an increase of the velocity gained by the powder particles in the nozzle, and so, in an increase of the probability of their adherence to the substrate surface upon impingement. Thus, it has been possible to increase the efficiency of particle deposition.

But, due to the fact that the powder is introduced only downstream of the nozzle throat, the entire length of the nozzle portion available for the powder partiple acceleration is considerably reduced. The reduced accelerating distance diminishes the growth of coating deposition efficiency which could be achieved owing to the increase in the gas temperature.

The most similar to the claimed solution is an apparatus and method reported in CA 2270260, 2004. The apparatus comprises a compressed gas heater; a supersonic nozzle (the de Laval nozzle) directly connected to the compressed gas heater and comprising a throat positioned between a converging portion and a diverging portion of the nozzle; a unit for supplying powders into the nozzle, the powder being introduced (injected) into the nozzle downstream of the nozzle throat.

In this apparatus, the powder particles do not pass through the nozzle throat, and hence, they do not wear its walls. This allows the use of the powders with hard ceramic particles. Moreover, since in the supersonic portion (positioned downstream of the throat), the gas temperature is

85 significantly lower than in the subsonic portion (positioned in front of the throat) and in the nozzle throat, the apparatus allows to increase the compressed gas temperature without nozzle clogging by the particle sticking to the nozzle walls.

Nevertheless, due to the shift of the powder injection point downstream

90 of the nozzle throat (i.e. powder introduction inside the nozzle rather than in front of it), the length of the nozzle portion available for particle acceleration is reduced. As a result, the final powder particle velocity reduces followed by the decrease of sprayed powder deposition efficiency.

95 SUMMARY OF THE INVENTION

The object of present invention is an increase of sprayed powder deposition efficiency with the retention of the possibility to increase the compressed gas temperature and to use powders with hard particles.

100 The given object is accomplished by the fact that in the prior art apparatus for gas-dynamic applying coatings, comprising a compressed gas heater, a supersonic nozzle (the de Laval nozzle), directly connected to the gas heater and having a throat positioned between a converging portion and a diverging portion, a unit for supplying powders into the nozzle, wherein the

105 powder injection components are placed downstream of the nozzle throat, the unit for supplying powders into the nozzle has one or more powder feeders connected through conduits to the components for injection of one or more powders into the nozzle, and a nozzle portion positioned downstream of the powder injection components and intended for acceleration of the powders is

110 made having parameters to suit the following relation: 0.015 < B ^• ( Sout / Sinj - 1) / L < 0.03 where

Sout - the area of the nozzle cross-section at the outlet;

Sinj - the area of the nozzle cross-section at the location of the powder 115 injection components;

L - the length of the nozzle portion intended for acceleration of the powders;

B - the minimal dimension of the nozzle cross-section at the location of the powder injection components.

120 Depending on a shape and composition of the surface being treated and on the task to be accomplished upon coating, the nozzle can have a round or rectangular cross-section.

For the convenience during practical application of the apparatus, the nozzle portion intended for powder acceleration (acceleration portion) can be

125 made as a replaceable element, hi this case, it can be continuously divergent or have one or more cylindrical sections. The components for injection powders into ψe nozzle can be made as an orifice (orifices) in the nozzle wall or in the form of the tubes passing through the nozzle throat with the outlets being positioned downstream of (behind) the throat; with this, two or more

130 components for injection powders can be made so as to ensure the powder supply equidistant from the nozzle throat

To provide an easy change in the composition of a powder being sprayed, each feeder can be connected to its component for injection the powder into the nozzle. To simplify the construction, two or more feeders can 135 be connected to the same component for injection the powder into the nozzle. For the convenience of apparatus practical use the compressed gas heating can be provide^ by electric heater.

The comparative analysis has shown that the claimed solution is distinguished from the prototype by the fact that a nozzle portion positioned

140 downstream of the powder injection components and intended for acceleration of the powders has parameters to suit the following relation: 0.015 < B ^■ ( Sout / Sinj - 1) / L < 0.03 where

Sout - the area of the nozzle cross-section at the outlet;

145 Sinj - the area of the nozzle cross-section at the location of the powder injection components;

L - the length of the nozzle portion intended for acceleration of the powders;

B - the minimal dimension of the nozzle cross-section at the location 150 of the powder injection components, which makes it possible to judge the conformity of the invention to the criterion of novelty.

The given object can also be accomplished if in the prior art method of gas-dynamic applying coatings, comprising heating a compressed gas;

155 supplying it into a supersonic nozzle (the de Laval nozzle) having a throat positioned between a converging portion and a diverging portion; forming a supersonic gas flow in the nozzle; injection a powder into the supersonic gas flow downstream of the nozzle throat (behind the throat); accelerating the powder by the gas flow in the nozzle; directing said accelerated powder to the

160 substrate surface; and forming a coating, the powder is injected into the supersonic gas flow downstream of the throat, said powder comprising the particles of one or more substances, one of which being a metal and/or an alloy, the gas flow downstream of the nozzle throat being formed to suit the following relation:

165 0.015 < B ^• (Sout/Sinj - 1) / L < 0.03, where

Sout - the area of the gas flow cross-section at the nozzle outlet;

Sinj - the area of the gas flow cross-section at the point of powder injection;

170 L - the length of the gas flow in the nozzle from the powder injection point to the nozzle outlet;

B - the minimal dimension of the gas flow cross-section at the point of powder injection.

Depending on the coating properties required, a metal powder, and/or a

175 mechanical mixture of ceramic and metal powders is employed as a powder for forming the coating, or several powders of different hardness are injected into the supersonic flow at the same time, a ceramic powder being employed as one of the powders. The particle size of the powders used, both metal and ceramic ones, ranges from 1 to 100 micrometers.

180 The gist of the present invention resides in the following.

Upon gas-dynamic spraying of powder materials, a coating is formed by the separate particles, which upon impingement on the base are adhered to its surface basically due to the transformation of their kinetic energy to bonding energy.

185 Therefore, the possibility of the particle adherence to the surface depends mainly on their velocity. The higher is the velocity of each particular particle, the higher is the possibility of its adherence to the substrate surface, and hence, the higher is powder spraying efficiency (deposition efficiency) as a whole.

190 In all apparatuses for gas-dynamic applying coatings of powder materials the particles are accelerated in the high velocity gas flow by the drag effect. Accelerating Stokes force is proportional to the difference of the velocity of gas flow and that of the particles. For a limited acceleration time the particles never run up to the velocity of a gas flow and always lag behind

195 it. The longer the particle is in the gas flow, the less is its lag, i.e. a velocity of the particle approaches that of gas.

It would seem obvious that one should elongate, as far as possible, a nozzle portion intended for acceleration of powder particles (an acceleration portion which is a nozzle portion extending from the point of powder

200 injection up to the nozzle outlet). In that case, the particles move in the gas flow longer, and hence, they are accelerated to higher velocity.

In practice, it turned out to be not nearly so. A gradual increase in the length of the nozzle acceleration portion initially results in an increase in the particle velocity and greater efficiency of spraying the powder onto the

205 substrate. However, on further increase in the length of the acceleration portion, the decrease of particle deposition efficiency is observed.

At first sight, this could be attributed to retardation of the gas flow due to the gas flow friction on the nozzle walls. In fact, in the apparatuses for gas- dynamic spraying, highly extended nozzles are commonly used whose length

210 is dozens of times greater than the nozzle cross-sectional dimensions. In this case, retardation of gas in the nozzle can be sufficient, and under gas deceleration below the particle velocity, the particles start to be retarded instead of being accelerated.

However, in actual practice it has been found that with increasing the

215 length of the nozzle acceleration portion, the decrease of the deposition efficiency begins well before sufficient deceleration of gas in the nozzle. That is, with some extension of the nozzle acceleration portion, the velocity of gas in the nozzle remains much above that of the particles. So with such extension of the nozzle acceleration portion the powder particles in the nozzle must

220 acquire higher velocity. But in practice, the actual spraying efficiency unexpectedly decreases.

This effect can be explained by the following.

The powder particles injected into a gas flow necessarily have a velocity component directed across the flow. This velocity component arises

225 both straight on introduction of the particles into the flow and in subsequent stages of particle trajectory evolution in the flow due to collision of the particles and tjieir scattering by discontinuities of the flow. The acceleration of the particles in the nozzle is effected by a high-velocity gas flow directed along the nozzle axis. Therefore, practically straight after introduction of the

230 powder particles into the accelerated gas flow, a transverse component of powders velocity becomes much less than the longitudinal one (directed along the gas flow). However, it exists and is assumed by the authors to be of considerable importance. The point is that the particles whose velocity is not directed strictly along the nozzle axis can impinge on the nozzle walls, and

235 naturally, lose some of their longitudinal velocity. Besides, near the nozzle walls there is always a boundary gas layer the velocity of which is sufficiently lower than that of the main gas flow. The particles having a transverse velocity component can get into this boundary layer and slow down therein.

At the same statistical dispersion of transverse velocities of the

240 particles, a probability that the particles get into a near-wall area of the nozzle increases with reducing the nozzle cross-section and increasing its length. As a consequence, the observed effect turned out to be connected not only with the nozzle length but with the cross section of its acceleration portion and with the degree of its divergence (increase of cross-sectional area of the

245 nozzle in the direction of the gas flow).

Thus, with extending the nozzle acceleration portion, two processes take place at pne time. Firstly, there is an increase in the velocity of the particles that have not impinged on the nozzle walls. Secondly, there is an increase in the number of the particles that have reached the near- wall area

250 and partially lost the velocity upon the impingement on the nozzle walls or upon deceleration in the gas boundary layer.

As a result, with increasing the length of the nozzle, the maximal velocity of the particles in the nozzle increases, whereas the ratio of these high-velocity particles in the overall flow of the particles goes down.

255 Consequently, as the nozzle is made longer, first, an average velocity of the particles increases and then it slows down.

In practice, this effect manifests itself through a variation in powder deposition efficiency. In the process, the powder deposition efficiency varies only slightly over a definite range of the nozzle acceleration portion length. In 260 this range, the processes of the particle acceleration by the gas flow and deceleration in the near-wall area of the nozzle are approximately equalized, and so, there is only slight variation in the powder deposition efficiency.

Numerous experiments have shown that this effect of equalizing the particle acceleration and deceleration processes is achieved by the choice of a 265 definite geometric parameters of the nozzle acceleration portion, and namely, Numerous experiments have shown that this effect of equalizing the particle acceleration and deceleration processes is achieved if a definite relation between the basic geometric parameters of the nozzle acceleration portion is ensured, and namely, 270 0.pl5 < B ^• (Sout/Sinj - 1) / L < 0.03, where .

Sout - the area of the gas flow cross-section at the nozzle outlet; Sinj - the area of the gas flow cross-section at the point of powder injection;

275 L - the length of the gas flow in the nozzle from the powder injection point to the nozzle outlet;

B - the minimal dimension of the gas flow cross-section at the point of powder injection.

When the nozzle's parameters were beyond the bounds of the said

280 limits, the decrease of the deposition efficiency was observed. In particular, with fixed values of the minimal cross-sectional dimension and cross- sectional area of the nozzle, excessively short and excessively long nozzles offered lower Reposition efficiency than those whose length was kept within the said range.

285 The indicated features have not been revealed in other engineering solutions on studying the prior art. Thus, the claimed solution meets the inventive criterion and inventive step.

290

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in the accompanying drawings, in which Fig.l is a structural arrangement of the claimed apparatus and Fig.2 is a

295 schematic illustration of the supersonic nozzle. The apparatus comprises a compressed gas heater 1, a nozzle 2 with a nozzle throat 3, a powder supply unit comprising powder feeders 4 and powder injection components 5 connected to the feeders by means of pipes 6, a nozzle acceleration portion 7 positioned downstream of the powder injection components up to the nozzle

300 outlet and made, for instance, as a replaceable element 8 (Fig.2) also comprising a cylindrical section 9 (Fig.2).

In operation, a compressed gas is delivered to the heater 1 to be heated to the required temperature. The heated gas enters the supersonic nozzle 2, wherein it sequentially passes through a converging portion, the throat 3 and a

305 diverging portion of the nozzle, and accelerates up to a supersonic velocity.

The powders to be sprayed are introduced into this supersonic gas flow through the powder injection components 5. The powder particles are accelerated by a high-velocity gas flow at the nozzle acceleration portion 7 and then they are directed to the substrate surface. 310 Depending on a shape and composition of the surface being treated and on the task to ,be accomplished upon coating, the nozzle can have a round or rectangular cross-section.

For the use of the powders of hard substances (in particular, ceramic particles), the nozzle acceleration portion can be made, in full or in part, as a 315 replaceable element 8 (Fig.2). In this case, the nozzle portion worn by the hard particles can be easily replaced.

To compensate for retardation of the gas flow against the nozzle walls, the nozzle acceleration portion can be made, in full or in part, divergent.

For simplification of the nozzle construction, its acceleration portion 320 can have one or more cylindrical sections 9 (Fig.2).

Depending on the particular structure of the nozzle, one or more components for powder injection can be made as orifices (Fig.l) in the nozzle wall or as the tubes passing through the nozzle throat (Fig.2). Two or more powder injection components can be made so as to ensure the powder supply 325 equidistant from the nozzle throat (Fig.1 ).

To provide an easy change of the powder being sprayed, each feeder can be connected to separate powder injection component. Two or more powder feeders can be connected to the same powder injection component to simplify the apparatus structure (Fig.l). 330 For the convenience of apparatus practical use the compressed gas heater can be electrical.

SPECIFIC EXAMPLES OF THE INVENTION

335 The present invention is illustrated by the following specific examples given in Table 1 and Table2 below.

Table 1 presents the results of coating weights measurements. The coatings were sprayed with the round nozzles of different length at constant values of B=3.6 mm, Siηj = 10 mm" and Sout = 18 mm ² . The temperature of 340 compressed air was 370 ⁰ C. In all the cases, the same quantity of the powder was used, comprising: a) aluminum (60%, wt.) and aluminum oxide (40%, wt.) particles, b) copper (70%, wt.) and aluminum oxide (30%, wt.) 345 particles, c) zinc (60%, wt.) and aluminum oxide (40%, wt.) particles.

Table 1

350

Table 2 presents other results of coating weights measurements. The coatings were sprayed with rectangular nozzles of different length with constant values of B = 3.6 mm, Sinj = 15 mm ² and Sout = 30 mm ² . In all the cases, the same quantity of the powder was used, comprising the particles of aluminum (60%, wt.) and aluminum oxide (40%, wt). The temperature of compressed gas was as follows: a) 37O ⁰ C, b) 450 ⁰ C, and c) 520 ⁰ C.

Table 2

In both cases compressed gas was an air under pressure of 7 bars.

Both tables demonstrate that as the dimensions relation approaches the limiting values, the coating mass diminishes, indicating the decrease of the powder deposition efficiency.