EP2066828B1 - Method for feeding particles of a coating material into a thermal spraying process - Google Patents

Description

The invention relates to a method for injecting particles of a layer material in a cold gas injection process, in which the particles are passed through a feed line and fed via the mouth of the feed line to a carrier gas stream, wherein the carrier gas stream transporting the particles to a surface to be coated Component serves. For this purpose, the carrier gas stream is passed through a stagnation chamber into which also opens the feed line, and then accelerated to the surface to be coated through a nozzle.

Thermal spraying is generally used to inexpensively create layers on components to be coated or to provide them with different non-producing properties. For this purpose, the layer material must be fed into the injection process, this usually being in the form of particles. These particles are passed through a supply line, which they leave through an orifice to be detected by a carrier gas stream, which is directed for the purpose of coating on the component to be coated. In order for the particles to adhere to the component to be coated, it must be imprinted with an amount of energy that depends on the coating method and the material, which leads to adhesion of the particles on the component to be coated. This energy input can be done for example by heating the particles during spraying or by an acceleration of the particles. In cold gas spraying, however, the kinetic energy introduced by acceleration into the process becomes upon impact of the particles on the component to be coated in a deformation or heat converted. A heating of the particles leads to a softening or even to a melting of the particles with a sufficient input of energy, whereby an adhesion of the particles striking the component to be coated is facilitated.

In the case of cold gas spraying, an energy input in the form of kinetic energy is in the foreground, wherein an additional heating of the particles can take place, but usually does not lead to a melting or melting of the particles. Due to the high kinetic energy of the particles they deform plastically when hitting the surface to be coated, with a simultaneous deformation of the surface causes adhesion of the particles. Furthermore, with the high-speed flame spraying, for example, a thermal spraying process is made available in which both the kinetic energy and the thermal energy of the particles impinging on the surface to be coated play a significant role in the formation of the layer. The cold gas spraying finds for example in the DE 197 47 386 A1 Mention.

In order to achieve a high-quality coating result, it is of particular importance that the particles provided for the coating can be supplied to the carrier gas stream in a well-defined manner. To ensure this, in particular agglomeration of the particles must be suppressed so that they can be fed as uniformly as possible and not as large clusters in the carrier gas stream. Again US 6,715,640 B2 can be seen, the reduction or cancellation of agglomeration of the coating particles, for example, by mechanical means. The particles are in a funnel-shaped container stocked and taken from this in the required quantity. By vibration and stirring, the withdrawn amount can be treated such that a separation of the particles takes place and they can be supplied to a transport gas. This results in a particle-gas mixture which can be fed through a supply line to the carrier gas stream of a thermal spraying process.

Out A. Killinger et al., "High Velocity Suspension Flame Spraying (HVSFS), A New Approach to Spraying Nanoparticles with Hypersonic Speed", Surface & Coatings Technology 201 (2006) 1922-1929 as well as the US 6,579,573 B2 , of the US Pat. No. 6,491,967 B1 , of the EP 1 134 302 A1 and the DE 103 92 691 T5 thermal coating methods are known in which the energy input into the beam with the coating particles by a flame such. B. a plasma flame takes place. In these flame spray coating methods, the adhesion of the coating particles to the substrate to be coated with the flame is ensured as a relatively high energy density energy source. This energy source is in the form of a flame in the center of a coating nozzle, so that coating particles in the form of a liquid dispersion can be fed directly to the flame. The high energy density of the flame guarantees a complete evaporation of the dispersion medium, whereby the amount of energy necessary for the evaporation can be made available by suitable regulation of the energy supply for the flame. The flame can readily provide the amount of energy necessary for the evaporation of the dispersant due to the high energy density.

According to the WO96 / 06957 It is described that the supply of liquid suspension is also used in a plasma spraying application can find to protect the particles from the thermal attack of the plasma flame by means of the liquid dispersant.

According to the WO2005 / 061116 A1 It is known that in a cold gas spraying process also particles can be used which represent a solid dispersion in their structure. These may, for example, consist of cobalt, it being possible for tungsten carbide particles, which form a dispersed phase, to be embedded in the matrix of the cobalt material. These particles can be deposited on a substrate, which then has the same structure as the particles. This means that the cobalt matrix is also deposited.

According to the US 5,833,891 It is also known that liquid dispersions can be deposited from a drug by means of acoustic sound waves on substrates such as glass beads.

The object of the invention is to specify a method for feeding particles into a cold gas injection process, with which a comparatively accurate metering of the particles is made possible.

This object is achieved according to the invention with the method given in claim 1 according to the invention in that the particles are dispersed in the supply line prior to introduction, the additive being transferred to the gaseous state after leaving the mouth of the feed line in the carrier gas stream. According to the invention, it is thus provided that the particles of the layer material are not transported or handled as pure powder, but rather that the particles are finely distributed in a liquid or solid additive. This additive has the advantage of being easier to handle as such is, as the present as a dry powder particles. As a result, a simpler and, in particular, more accurate metering can advantageously take place, so that a method for feeding these particles can profit from this. However, since the thermal spraying process requires that the particles in the carrier gas stream be in pure state again at the latest when the component surface is reached, it is further provided according to the invention for the additive to assume a gaseous state in the carrier gas stream after it leaves the mouth of the feed line. In this way, it is advantageously achieved that the material of the additive does not form a particulate or droplet-shaped phase, but only contributes a Patialdruck to the carrier gas. By transfer of the additive in the gaseous state, ie by evaporation of a liquid additive or by sublimation or melting and evaporation of a solid so the separation of the particles in the carrier gas stream is enforced by the additive. On the other hand, the particles are advantageously prevented from clumping by the solid or liquid additive during transport through the supply line.

Advantageously, the carrier gas stream is passed through a stagnation chamber and then accelerated through a nozzle. This process control for the thermal spraying process is particularly necessary if the injection process is to take place while introducing a significant amount of kinetic energy into the particles, as is necessary in the already mentioned methods of cold gas spraying. Due to the fact that the carrier gas stream is previously passed through a stagnation chamber, advantageously the residence time of the molecules of the carrier gas flow in the thermal spraying device can be increased. This facilitates the supply of thermal energy, these preferably during the residence time of the molecules of the carrier gas flow in the stagnation chamber is transferred. The stagnation chamber here is to be understood as meaning a line structure for the carrier gas flow which is widened in cross-section compared to the nozzle. However, the cross-sectional expansion does not cause stagnation in the strict sense, but merely reduces the flow velocity of the carrier gas flow, so that the residence time of the gas molecules in the stagnation chamber is increased compared to the nozzle.

The transfer of heat energy into the stagnation chamber can be done by all known energy sources. For example, the wall of the stagnation chamber can be heated, so that the thermal energy is radiated into the interior of the stagnation chamber, or is transferred to abutting gas molecules of the carrier gas flow. Furthermore, it is possible to make an energy input into the volume of the stagnation chamber. This can be done for example by ignition of an arc inside the stagnation chamber, by electromagnetic induction or by laser irradiation. Furthermore, it is also possible to heat the nozzle next to the stagnation chamber. The energy input into the thermal Spraying device is necessary so that a transfer of the additive takes place in the gaseous state. It must absorb thermal energy to change its state of aggregation.

According to a particular embodiment of the invention it is provided that the carrier gas stream is heated prior to feeding to the nozzle such that condensation (and thus solidification) and / or resublimation of the additive is prevented, in particular in the nozzle. When dimensioning the amount of heat supplied to the carrier gas stream, it must be taken into account that the approximately adiabatic expansion of the carrier gas behind the nozzle throat causes it to cool strongly. This cooling can also lead to a resublimation or condensation and solidification of the additive in extreme cases. In this way, new particles or droplets can form from the additive, which, together with the particles to be deposited, strike the surface to be coated. Here, the additive may lead to undesirable contamination of the layer. However, if sufficient heating of the carrier gas, the molecules of the mixed with this additive remain in the gaseous state, so that they can not be deposited or only in negligible amount in the forming layer.

Near the nozzle exit of the thermal spraying device, the most critical conditions generally prevail with regard to resublimation or condensation or solidification of the additive since, in addition to a negative pressure relative to the environment, there also occurs a temperature minimum of the carrier gas flow. For the design of the least necessary heating of the carrier gas flow is, however, ultimately the state of the carrier gas flow when hitting the component to be coated is decisive and not the state in the nozzle.

Under certain conditions, it may also be desirable to resublimate or condense or solidify the additive. In this case, the additive consists of a material which is to be deposited in the forming layer and possibly to react with the deposited particles. The possibly necessary energy is also obtained from the thermal energy supplied to the carrier gas stream.

When choosing the additive, it must be taken into account that it must not cause explosive exothermic reactions in the carrier gas stream. This would be the case in particular if the sublimation or evaporation produces a gas mixture with the carrier gas which contains oxygen and an easily oxidizable, ie flammable substance. It is irrelevant which of these substances are contributed by the carrier gas and which of the substances by the additive. The heating and pressure increase before the nozzle exit would quickly lead to uncontrollable explosive phenomena in the presence of a potentially explosive gas mixture. On the other hand, however, a controllable reaction in the carrier gas stream could provide additional energy for the coating, or, in a reaction with the particles intended for coating, also directly affect the chemical composition of the coating to be formed in a desired manner.

According to a particular embodiment of the invention is for obtaining the additive a gaseous at room temperature and atmospheric pressure starting material by increasing the pressure and / or cooling solidifies or liquefies. An additive obtained in this way has the advantage that it becomes gaseous again under normal conditions, which normally prevail outside the thermal spraying device. Therefore, such an additive at the outlet from the nozzle opening of the thermal spray device is advantageously also very easy to convert into a gaseous state.

However, prevail in the thermal spray device temperatures that are above the standard conditions. Therefore, water can also be used as an additive according to another embodiment of the invention. The prerequisite for this is, however, that the temperature at the nozzle outlet at least not significantly below a temperature of 100 ° C, since a formation of water droplets could not be prevented in this case. The use of water as an additive has the particular advantage that this liquid is chemically relatively stable at a relatively low boiling point and therefore fails to react with most provided for coating particle types. In addition, water is also in the event of an exit into the environment as unproblematic in terms of its environmental impact to evaluate.

In the case where the additive is used in the liquid state, it is advantageous to prepare and stock a suspension with stirring. This suspension can then be fed into the feed line, it being possible to fall back on the already proven technology for the metering of the particles for the purpose of metering the particles. As a result, the suspended particles can advantageously be metered in a simple manner by handling the additive. The metering of the particles for the injection process can be carried out in particular taking into account the particle concentration in the Suspension by adjusting the volume flow in the supply line done. It is of great importance that the concentration of particles is kept constant by stirring or moving the suspension, so that it can be fed directly into the feed line with known volume flow.

If a solid additive is used, it is advantageous to disperse the particles disperse therein and carry out a conditioning, in particular grinding or atomization, whereby the solid additive is processed into a powder. This results in a powder, which is generally coarser than the particles themselves and which is easier to guide and dose due to its properties than the particles themselves. Since the additive should not be deposited in the layer to be formed, must in the choice of additive the layering process itself is not taken into account. Therefore, optimized additives can be selected for the line and dosage, which compensate for any dosing problems of the particles provided for coating. The powder can therefore be added metered without any problems to a gas stream conducted through the feed line, it being possible to choose the metering taking into account the film formation process during thermal spraying.

The production of a suspension or a powder with finely divided particles for coating has the advantage that, in addition to a larger variety of particle materials, finer particles can also be used. These would no longer be transported without clogging when added directly to a gas stream. However, the assistance of a liquid or solid additive simplifies the Transport in the supply line and thus also the dosage in the thermal spraying process.

Figur 1

eine Kaltgasspritzpistole, die für ein Ausführungsbeispiel des erfindungsgemäßen Verfahrens geeignet ist, im Längsschnitt und

Figur 2

schematisch eine thermische Spritzvorrichtung, die zur Durchführung des erfindungsgemäßen Verfahrens geeignet ist, als Blockschaltbild.

Further details of the invention will be described below with reference to the drawings. Identical or corresponding elements in the individual figures are each provided with the same reference numerals and will only be explained several times as far as there are differences between the individual figures. Show it

FIG. 1

a cold gas spray gun, which is suitable for an embodiment of the method according to the invention, in longitudinal section and

FIG. 2

schematically a thermal spray device, which is suitable for carrying out the method according to the invention, as a block diagram.

A cold gas spray gun 11 according to FIG. 1 represents the core of a thermal spray device 12 according to FIG. 2 dar. The cold gas spray gun 11 according to FIG. 1 consists essentially of a formed in a single housing 13 Laval nozzle 14 and stagnation 15. In the region of the stagnation chamber 15, a heating coil 16 is embedded in the wall of the housing 13, which causes the heating of a carrier gas, which through an inlet 17 of the stagnation 15 is supplied.

The carrier gas passes through the inlet 17 first into the stagnation chamber 15 and leaves it through the Laval nozzle 14. In this case, the carrier gas in the stagnation chamber can be heated up to 800 ° C. By a supply line 18, the mouth 19 is arranged in the stagnation chamber 15 and Laval nozzle 14, for example, a liquid additive fed with the particles provided for coating. By a relaxation of the charged with the particles and the additive carrier gas flow through the Laval nozzle 14, a cooling of the carrier gas stream is effected, which has temperatures below 300 ° C in the region of the nozzle opening. This temperature reduction is due to a substantially adiabatic expansion of the carrier gas, which has, for example, a pressure of 30 bar in the stagnation chamber and is relieved to atmospheric pressure outside the nozzle opening.

In FIG. 2 is shown schematically as a cold spray gun 11 according to FIG. 1 to a thermal spray device 12 can be completed. The thermal spray gun 11 is arranged in a housing space 20, not shown, in which a component to be coated 21 can be arranged, which has a surface to be coated 22 to the nozzle opening of the cold spray gun 11. Furthermore, the carrier gas stream 23 is indicated by an arrow, it being clear that the carrier gas stream is aligned with the surface 22 and impinges there to form a layer 24 which is formed from the particles 25 present in the carrier gas stream. Instead of a heating coil 16 according to FIG. 1 are arranged on the cold spray gun 11 different energy sources for heat supply. A microwave generator 26 is suitable for heating the carrier gas contained in the stagnation chamber 15 as well as the particles and the additive by electromagnetic induction. Furthermore, two lasers 27 are mounted on the cold spray gun 11, which radiate a laser beam into the interior of the stagnation chamber 15, which intersect just before the mouth of the feed line 18. As a result, a targeted energy input in the additive provided with the particles is possible, wherein by transfer of the additive in the gaseous state, this energy is absorbed and the thermal load of the particles 25 is limited so.

Furthermore, a reservoir 28 is provided for the carrier gas used, which can be supplied via a line 29 of a preheating unit 30 and then the inlet 17 to the stagnation chamber 15. A regulation of the gas flow is possible via throttle valves, not shown.

Furthermore, storage containers are provided for the particles, which can be charged alternatively. A storage hopper 31 may contain a suitably conditioned powder of an additive in whose powder particles the particles provided for the coatings are finely dispersed. The powder is conditioned so that the supply into the supply line 18 can be done without problems. In this case, a gas stream is passed through the feed line to which the powder particles are added. Furthermore, a storage tank 32 is provided, in which a suspension of a liquid additive and dispersed therein particles for coating can be stored. In this, a stirring device 33 is provided, which ensures the homogeneity of the dispersion. The storage hopper 31 and the storage tank 32 are surrounded by a thermal insulation 34, which allows the economic use of cooled additives, for example, gaseous substances present at room temperature.

Claims (8)

1. Method for the feed of particles (25) of a layer material into a cold-gas spraying process, in which the particles (25) are conducted through a supply line (18) and are delivered to a carrier gas stream (23) via the mouth (19) of the supply line (18), the carrier gas stream (23) serving for transporting the particles (25) to a surface (22), to be coated, of a component (21) and, for this purpose, being routed through a stagnation chamber (15) and subsequently accelerated through a nozzle (14), the particles (25), before being introduced into the supply line (18), being dispersed in a liquid or solid additive,
   characterized in that
   such an amount of heat energy is transmitted into the stagnation chamber that the additive absorbs such an amount of thermal energy that, after leaving the mouth (19) of the supply line (18), said additive changes into the gaseous state in the carrier gas stream (23).
2. Method according to Claim 1,
   characterized in that
   the carrier gas stream (23), before being delivered to the nozzle (14), is charged with an amount of heat which is great enough that the cooling of the carrier gas in the nozzle does not lead to condensation and solidification and/or resublimation of the additive.
3. Method according to either one of Claims 1 and 2,
   characterized in that
   the carrier gas stream is heated in the stagnation chamber (15).
4. Method according to one of Claims 1 to 3,
   characterized in that
   water is used as an additive.
5. Method according to one of the preceding claims,
   characterized in that
   a suspension is produced from the liquid additive and the particles (25) by agitation and is stored.
6. Method according to Claim 5,
   characterized in that
   the metering of the particles (25) for the spraying process takes place, taking into account the particle concentration in the suspension, by setting the volume flow in the supply line (18).
7. Method according to one of Claims 1 to 3,
   characterized in that
   the solid additive in which the particles (25) are distributed dispersedly is processed into a powder by means of conditioning, in particular grinding or atomization.
8. Method according to Claim 7,
   characterized in that
   the powder is added, metered, to a gas stream conducted through the supply line (18).

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