Abstract:
A method for producing a metal foam includes depositing metal-containing particles and blowing-agent-containing particles by cold spraying. A layer formed in this manner is formed from metallic particles and particles containing blowing agent, wherein the blowing agent forms the core of coated particles. The shell of said particles is likewise metallic, such that it is easier to deposit said particles together with the metallic particles. Therefore, a higher concentration of blowing agent can advantageously be produced in the layer. Greater possibilities are advantageously produced thereby to provide the porous layers with required pore profiles.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage Application of International Application No. PCT/EP2014/059000 filed May 2, 2014, which designates the United States of America, and claims priority to DE Application No. 10 2013 210 198.8 filed May 31, 2013, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to a method for producing a metal foam, in which metal-containing particles together with particles containing a solid blowing agent are deposited as a layer on a substrate by cold spraying. Thereafter, the blowing agent is activated, the pores of the metal foam forming in the layer. Activation of this type can be performed, for example, by means of a heat treatment. In this process, the layer is heated to such an extent that the blowing agent changes from the solid state to a gaseous state. The increase in pressure associated therewith leads to the formation of pores in the metallic matrix. The heat treatment is carried out in temperature ranges in which the metallic matrix is softened to such an extent that the pores can form. 
         [0003]    Moreover, the invention relates to a method for producing coated particles. 
       BACKGROUND 
       [0004]    The production of metal foams by cold spraying is known from U.S. Pat. No. 6,464,933 B1 and U.S. Pat. No. 7,402,277 B2. These production methods involve processing particles of the metal which is to form the metallic matrix of the metal foam. Particles of the blowing agent are admixed to said material, these being deposited in the metallic matrix. However, the rate of deposition of the blowing agent is limited by virtue of the fact that the properties of said blowing agent mean that it is suitable in principle only to a limited extent for deposition by cold spraying. It is generally known that brittle materials, like the materials of the blowing agent, can be deposited in the metallic matrix formed by the substantially more ductile metallic particles only to the extent to which the deformation of the particles upon impact allows for the more brittle particles to be incorporated. Therefore, the pore density is limited in the deposition of the particles of the blowing agent. 
       SUMMARY 
       [0005]    One embodiment provides a method for producing a metal foam, comprising depositing metal-containing particles together with particles containing a solid blowing agent as a layer on a substrate by cold spraying, and activating the blowing agent, with the pores of the metal foam forming in the layer, wherein coated particles having a core consisting of the blowing agent and a metallic shell are used as particles which contain the blowing agent. 
         [0006]    In a further embodiment, aluminum, copper, nickel, iron, steel or silver are used as materials for the shell. 
         [0007]    In a further embodiment, the material of the shell is the same as the material of the exclusively metal-containing particles. 
         [0008]    In a further embodiment, the material of the core is a metal hydride, in particular magnesium hydride or titanium hydride, or a carbonate, in particular calcium carbonate or magnesium carbonate. 
         [0009]    In a further embodiment, the mixing ratio between the particles containing the blowing agent and exclusively metal-containing particles is varied during the coating process. 
         [0010]    In a further embodiment, a gradient layer with a variable density of the pores is produced. 
         [0011]    In a further embodiment, the density of the pores in the boundary layer adjacent to the substrate and/or in the boundary layer close to the surface is reduced to zero by processing only the exclusively metal-containing particles there. 
         [0012]    In a further embodiment, the density of the pores in the boundary layer adjacent to the substrate is maximized by processing only the particles containing blowing agent. 
         [0013]    In a further embodiment, the metal foam is produced by a heat treatment of the layer which follows the conclusion of the coating process. 
         [0014]    In a further embodiment, the energy input during the cold spraying and/or an energy input into the substrate are chosen to be so high that the particles containing the blowing agent are heated to a temperature above the reaction temperature of the blowing agent when they impact on the substrate. 
         [0015]    Another embodiment provides a method for producing coated particles, wherein said particles are produced from cores consisting of a blowing agent by surrounding these cores with shells consisting of a metal. 
         [0016]    In a further embodiment, an electroless electrochemical method is used for coating the cores with the shell. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Example embodiments of the invention are described below with reference to the drawings, in which: 
           [0018]      FIG. 1  schematically shows an exemplary embodiment of the method according to the invention, 
           [0019]      FIGS. 2 to 9  show various exemplary embodiments of the porous layers, in each case before and after activation of the blowing agent, and 
           [0020]      FIG. 10  shows an exemplary embodiment of the method for producing the coated particles. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Embodiments of the invention provide a method for producing a metal foam by cold spraying and also a method for producing suitable particles for the cold spraying method, which produce metal foams having a pore density and pore type which are variable over the greatest possible spectrum. 
         [0022]    Some embodiments provide a method in which coated particles having a core consisting of the blowing agent and a metallic shell are used as particles which contain the blowing agent. These can then be deposited together with the metal-containing particles, the metal-containing particles containing in particular exclusively metal. The advantage of coating the blowing agent with a metallic material lies in the fact that these coated particles have a behavior similar to the particles consisting of a metal during processing. Upon impact of the coated particles, the shells provide a reserve of ductility with respect to the behavior of the coated particles, and therefore the deformation of the shells ensures improved incorporation of the particles into the layer which forms. This has the advantage that the concentration range of the particles containing the blowing agent can be varied within a relatively large range in the layer which forms. In this respect, how high the proportion of particles containing the blowing agent should be in the spray powder depends primarily on the thickness of the shells, the method parameters and the desired layer result. In this regard, it is possible to set concentrations of the particles containing the blowing agent of between 0 and 100% (the percentage indicated denotes the numerical proportion of the particles). In this respect, it is also possible to process 100% of the particles containing the blowing agent, if the shell consisting of the metal has a sufficient thickness. In this case, what is thus deposited is only one type of particles, which have both the property of the particles containing blowing agent and the property of the metal-containing particles. 
         [0023]    Cold spraying is a method which is known per se, in which particles intended for the coating are accelerated, preferably to supersonic speed, by means of a convergent-divergent nozzle, so that they remain adhering to the surface to be coated on account of their impressed kinetic energy. In this respect, the kinetic energy of the particles is utilized, leading to plastic deformation thereof, with the coating particles being melted merely at their surface during impact. Therefore, this method, compared to other thermal spraying methods, is referred to as cold spraying because it is carried out at relatively low temperatures, at which the coating particles remain substantially solid. A cold spraying plant having a gas heating device for heating a gas is preferably used for the cold spraying, which is also termed kinetic spraying. A stagnation chamber is connected to the gas heating device and, at the outlet side, is connected to the convergent-divergent nozzle, preferably a Laval nozzle. Convergent-divergent nozzles have a convergent partial portion and also a divergent partial portion, these being connected by a nozzle neck. At the outlet, the convergent-divergent nozzle generates a powder jet in the form of a gas stream with particles located therein at high speed, preferably supersonic speed. 
         [0024]    According to one embodiment, it is provided that aluminum, copper, nickel, iron, steel or silver are used as materials for the shell. These materials are advantageously very ductile, and therefore when selecting these materials the proportion of particles containing the blowing agent can advantageously be chosen to be very high. In the case of less ductile materials, too, it is possible according to the invention for the range of variation of the possible proportion of particles containing the blowing agent to be increased, a range of variation of between 0 and 100% not being achieved in these cases. With the available ductile materials, however, it is already possible to manage diverse design tasks. By way of example, aluminum can be used to produce metal foams for lightweight construction. This metal has a low density in any case, but can be used, through the formation of pores, for producing still lighter components. Copper and silver advantageously have a high thermal and electrical conductivity. These materials can therefore be used, for example, as heat exchanger materials, the pores being suitable for forming inner channel structures for the passage of a fluid which is to be heated or cooled. Iron and steel are common construction materials which are additionally inexpensive. They can be used to produce various structural components, it being possible for the lubricating properties to be improved, for example, by a lubricant in the pore systems which have formed. 
         [0025]    According to another embodiment, it is provided that the material of the shell is the same as the material of the exclusively metal-containing particles. This produces a metal foam which forms a metallic matrix only of a metallic material. The material of the shell is, as it were, incorporated into said metallic matrix. This is not absolutely necessary, however. By way of example, it would also be possible for the material of the shells to be a catalytically active material, which is deposited into the pore surfaces during the formation of the metal foam. Then, the exclusively metal-containing particles would be merely the support for such a catalytic structure. This structure could be utilized catalytically for the formation of an open metal foam (more details are provided herein below in relation to the possibilities for producing open and closed metal foams). 
         [0026]    According to another embodiment, it is provided that the material of the core is a metal hydride, in particular magnesium hydride or titanium hydride, or a carbonate, in particular calcium carbonate or magnesium carbonate. These blowing agents are common blowing agents for producing metal foams. They provide an expedient temperature range when they are activated, and therefore it is possible to select a suitable blowing agent depending on the softening temperatures of the metallic matrix. This is possible with the aid of general knowledge in the art and, for example, in consultation with the aforementioned documents U.S. Pat. No. 6,464,933 B1 and U.S. Pat. No. 7,402,277 B2. 
         [0027]    Furthermore, it is advantageous if the mixing ratio between the particles containing the blowing agent and exclusively metal-containing particles is varied during the coating process. In this respect, it is possible to take different tasks of the metal foam to be formed into consideration. It is possible to produce gradient layers with a variable density of the pores. It is also possible that the density of the pores by suitably setting the mixing ratio in the boundary layer adjacent to the substrate, i.e. the layer proportion lying directly on the substrate, and/or in the boundary layer close to the surface, i.e. the layer proportion which is involved in the formation of the surface, only the metal-containing particles are processed. This has the advantage that a pore-free surface of the layer can be produced and also a boundary layer to the substrate with few pores can be produced. As a result, the surface properties of the layer can be influenced in the desired way and it is possible to maximize the adhesion between the layer and the substrate. 
         [0028]    It is also advantageously possible that the density of the pores in the boundary layer adjacent to the substrate is maximized by processing only the particles containing blowing agent. This has the effect that, during the formation of the metal foam, the adhesion of the layer on the substrate is largely abolished. After the metal foam has been produced, the layer can therefore be detached from the substrate without any problems. In this case, the substrate makes its surface available merely as the basis for producing an independent component in the form of the layer. A self-supporting, independent component is from the layer. By way of example, this can be a heat-exchanger plate, the mass of which can advantageously be optimized by detaching the substrate after the metal foam has been produced. 
         [0029]    It is furthermore advantageous if the metal foam is produced by a heat treatment of the layer which follows the conclusion of the coating process. In other words, the layer is firstly finished and then a heat treatment is carried out, during which the metal foam is formed. The energy input of the cold spraying is thus too low, and therefore it is not sufficient for activating the blowing agent during the deposition of the layer. This method is preferably suitable for producing closed metal foams. Even given a large proportion of particles containing blowing agent, walls will form between the individual pores on account of the deformation of the shells. 
         [0030]    Another possibility consists in the fact that the energy input during the cold spraying and/or an energy input into the substrate are chosen to be so high that the particles containing the blowing agent are heated to a temperature above the reaction temperature of the blowing agent when they impact on the substrate. The energy input during cold spraying can be increased, for example, by preheating the blowing agent. This is then transferred to the particles to different degrees by a different residence period of the particles in the stagnation chamber connected upstream of the cold spraying nozzle. If the heat input into the particles containing the blowing agent and into the particles consisting exclusively of metal is to be different, it is thus possible to choose the feed-in points of these particles in the stagnation chamber to be at a different distance from the cold spraying nozzle. Two different feed-in points for the particles containing the blowing agent and the exclusively metallic particles is advantageous in any case if the concentration of the particles containing the blowing agent is to be varied. For this purpose, an independent feed of both particle types is namely advantageous, since powders with various mixing ratios do not have to be attained. 
         [0031]    The energy input into the substrate can be effected, for example, by preheating the latter. Another possibility consists in locally heating the substrate (and the substrate provided with the layer located in the structure) by irradiating the point of impact of the cold gas jet with a laser beam. In any case, the additional energy input has the effect that the blowing agent is already activated upon impact of the particles containing the blowing agent on the substrate. The metal pores form, as it were, in situ and increase the porosity of the substrate, i.e. the formation of pores, at the points of impact of said particles. Since, in this case, the pores are formed at the time of great plastic deformation of the particles, the shells of the particles containing the blowing agent are destroyed during the reaction. In this way, it is also possible to produce open-pore metal foams, if the concentration of particles containing the blowing agent is chosen to be sufficiently high so that sufficient particles containing the blowing agent are deposited directly adjacent in the layer which builds up, and in this way a connection between the pores which form is ensured. 
         [0032]    Other embodiments provide a method for producing the coated particles, wherein said particles are produced from cores consisting of a blowing agent by surrounding these cores with shells consisting of a metal. The coating of the cores with the shell can advantageously be carried out using an electroless electrochemical method. These methods are generally known. 
         [0033]    As shown in  FIG. 1 , a substrate  11  is coated by means of a cold spraying installation  12 . Of the cold spraying installation, only a stagnation chamber  13  and a convergent-divergent nozzle  14  connected to the stagnation chamber are shown by way of example. The nozzle  14  produces a cold gas jet  15 , with which particles  16  are deposited on the substrate  11 , as a result of which the layer  17  is formed. In the meantime, the substrate  11  is preheated by a heating system  18 . In addition, a laser  21  directs a laser beam  19  onto the point of impact of the cold gas jet  15 . It is also possible for the carrier gas to be preheated in the stagnation chamber  13  by means of a heating system  20 . 
         [0034]      FIG. 2  shows the layer  17  in a cut form. It can be seen that particles  22  consisting exclusively of a metal and also coated particles  23  comprising a core  24  consisting of a blowing agent and a shell consisting of the metal are deposited on the substrate  11 . After the cold spraying of the layer, the coated substrate is subjected to a heat treatment, during which the blowing agent of the cores  24  is activated. The layer which results after the heat treatment can be seen in  FIG. 3 . It can be seen that pores  26  have formed in the metallic matrix of the layer  17 . This gives rise to a metal foam. 
         [0035]    In the layer  17  shown in  FIG. 2 , the particles  23  containing the blowing agent are distributed uniformly over the entire layer thickness. As can be gathered from  FIG. 3 , pores are also formed here in the marginal region of the layer. This means that pores can form an interface with the surface of the substrate  11 , as can be identified in the case of the pore  26   a.  It is also possible for the pores to be open with respect to the surface  27  of the layer, as can be identified in the case of the pore  26   b.    
         [0036]    As shown in  FIG. 4 , the layer  17  is deposited in three stages. In stage I, only metallic particles  22  are deposited. In stage II, a mixture of particles  23  containing the blowing agent and metallic particles  22  is deposited. In stage III, only metallic particles  22  are again deposited. 
         [0037]    As can be gathered from  FIG. 5 , this produces a layer  17  having pores  26  which are formed only in the interior of the layer. The surface  27  and also an interface  28  with the substrate  11  are pore-free owing to deposition phases I and III. The adhesion of the layer  17  on the substrate  11  is therefore not impaired by the pores  26 , just like the surface quality of the surface  27  of the layer  17 . 
         [0038]    In phase II as shown in  FIG. 4 , particles  23  containing blowing agent have been deposited in such a low concentration that the pores  26  which form are self-contained, i.e. that these pores are surrounded completely by the matrix material of the layer  17 . This gives rise to a closed-pore metal foam. 
         [0039]    As shown in  FIG. 6 , the layer  17  is deposited in two phases I, II, which correspond to phases I, II as shown in  FIG. 4 . Phase III is dispensed with, however, and therefore the surface of the layer deposited as shown in  FIG. 6  is partially also formed by particles  23  containing the blowing agent. In addition, the particles containing the blowing agent are larger than the metallic particles  22 . If one considers, then, the resulting layer as shown in  FIG. 7  after the heat treatment, it can be seen that the particles  23  containing the blowing agent have had the effect, on account of their higher concentration and their size, that common pores have been formed. The shells  25  have been destroyed in the process. Since phase III has been dispensed with in the deposition, the openings of the pores  26  are also located in part in the surface  27 , giving rise to an open-pore metal foam in the layer  17 , the channels being accessible from the surface  27 . A liquid lubricant can be introduced into these pores, for example. Another possibility would be the introduction of catalytic particles (not shown). 
         [0040]    As shown in  FIG. 8 , the layer  17  is deposited in four phases. The concentration of the particles  23  containing the blowing agent is shown schematically in hatched form over the layer thickness. In phase I, exclusively particles containing blowing agent are deposited (concentration=100%). In phase II, exclusively metallic particles are deposited (concentration of the particles containing blowing agent=0%). In phase III, a gradient layer is produced by increasing the concentration of the particles  23  containing blowing agent from 0 to 80% and then reducing it back to 0%. In phase IV, exclusively metallic particles are deposited again (concentration of the particles containing blowing agent=0%). 
         [0041]    The layer which results after the heat treatment can be seen in  FIG. 9 . The deposition of particles containing blowing agent in phase I has the effect that the layer  17  detaches from the substrate  11  during the heat treatment. This gives rise, as it were, to a single large pore between the layer and the substrate. The layer  17  thus represents an independent plate-shaped component after the heat treatment. This could be used, for example, as a heat-exchanger plate. Phases II and IV have the effect that this plate-shaped component has a closed surface. The end concentration of 80% of particles containing blowing agent in the gradient layer deposited in phase III has the effect that an open-pore channel system has formed in the interior of the layer  17 , this channel system more or less providing a cohesive cavity and being supported by columnar structures  29 . This channel system could be used for the passage of a fluid which is to exchange heat with another fluid beyond the surfaces  27  of the layer  17 . 
         [0042]    As shown in  FIG. 10 , a particle  23  is produced, by way of example, in a bath  30  by the electroless deposition of the shell  25  on the core  24 . The electroless deposition of metals is known per se. By way of example, copper or nickel can be deposited by an electroless method.