EP1073778B1 - Method for producing an openly porous sintered metal film - Google Patents

Abstract

A method for producing a thin openly porous metal film from a metal powder that can be sintered. The metal powder is suspended in a carrier fluid with a specific size distribution of particles, the suspension is applied to a supporting material in at least one thin film and dried, and the green layer thus formed is sintered. The thickness of the layer formed by suspension thus applied corresponds at least to the thickness (s) of the metal film to be produced after sintering, whereby (s) is at least three times the value of the diameter (D) of the powder particles, D=1-50 urn and the maximum thickness of the finished metal film is 500 mum.

Description

The invention relates to a method for producing a thin metal layer with open porosity from a sinterable Metal powder.

In technology are used for a wide variety of applications porous body needed by a flowing medium are flowed through, either supporting reactive processes should be or in a fluid medium contained solid particles, d. H. filtered should be. Filter body made of ceramic material must be made relatively thick due to the risk of breakage Filter body made of pressed and sintered metal powders are proportionate for manufacturing reasons thick. Because of the thickness that cannot be reduced, especially with fine-pored material, correspondingly large flow resistances on. The use of plastics as Filter material is limited by its lower strength and the low temperature resistance. One use of metallic materials as a porous layer known in the form of fabrics made from metal fibers or fleeces.

In the case of such a porous medium through which a medium flows Layer there is a need to create undesirable flow resistances to minimize so that layer thicknesses as thin as possible are to be aimed for. Leave out of metallic fabric or fleece accordingly thin layers, for example in one Produce a thickness of approximately 100 µm. However, these are few dimensionally stable, have relatively large pores and in terms of the porosity has large tolerances. As for the production such fabrics and fleeces are accordingly thin and therefore even expensive wires that have to be used are those made from them manufactured fabrics and nonwovens correspondingly expensive.

From US 5 592 686 is a process for the production of porous Metal parts with micro and / or macro porous properties known for which powder particles with a diameter of maximum 300µm with a binder in a solvent be suspended, from this green slice layers in one Thicknesses of 0.05 to 2 mm are made, several of these Layers on top of each other, laminated under pressure and then be sintered.

EP-B-0 525 325 describes a method for producing porous, metallic sintered workpieces known in the first a metal powder is suspended in a carrier liquid, which consists of a binder dissolved in a solvent and which is adjusted so that the suspension is pourable is. This suspension is poured into a mold. Subsequently the solvent is evaporated off so that the remaining binder the metal powder in the through the Form predetermined geometry is solidified and a manageable Green body forms. After separating from the mold the green body is sintered in the usual way. This previously known Process is preferably proportional to the production Thick-walled sintered parts are provided due to their geometry better than a casting process in the conventional method by pressing a metal powder have it made into a mold. Thin-layered, open, Porous parts cannot be produced with this process.

The invention is based on the object, the known To improve the process so that even thin, porous, and provided required, self-supporting metal layers are also produced can be.

This object is achieved by the method according to the invention solved that the sinterable powder with a predetermined Size distribution of the powder particles in a carrier liquid, formed by one with an evaporable solvent liquefied binder that is suspended Suspension in at least one thin layer on a carrier body applied, dried and the green layer thus formed is sintered, the layer thickness of the applied Suspension at least the thickness s of the to be generated Corresponds to the metal layer after sintering, where s is at least corresponds to 3 times the average diameter D of the powder particles, with D = 1µm to 50µm, the layer thickness of the finished metal layer is a maximum of 500 microns. This is used to advantage that at Sinter the individual powder particles firmly together connect, but free spaces remain between the powder particles, the one in relation to the thickness of the metal layer open porosity result, so that the metal layer for flowing Media becomes permeable. The size of the porosity can influenced by the particle size of the metal powder used be so that there are very thin porous metal layers have the pore size specified. Because in the making Inhomogeneities and voids can occur the layer thickness is at least 3 times the diameter D of the Correspond to powder particles. By the ratio mentioned between the layer thickness s and the particle diameter D. ensured that there were always several "layers" of powder particles are stacked and continuous "holes" that larger than the desired porosity can be avoided. It is particularly expedient here if the layer thickness s 5 to 15 times, preferably 10 to 15 times Diameter D of the powder particles. "Through holes" can be avoided.

Diameter D is the mean particle diameter to understand the metal powder used. Powder in The meaning of the invention is not only powders made of pure metals, but also powders from metal alloys and / or powder mixtures made of different metals and metal alloys to understand. In particular, this includes steels, preferably Chrome-nickel steels, bronzes, nickel-based alloys such as Hastalloy, Inconel or the like, where powder mixtures can also contain high-melting components, such as platinum or the like. The one to use Metal powder and its particle size is different from each Depending on the application.

The consistency of the to be set via the carrier liquid Suspension essentially depends on how the suspension is applied to the support body. When pouring, if necessary with subsequent wiping of an excess from the cast suspension layer, the suspension can set in a somewhat viscous consistency become. In a so-called. Pouring foil or spraying a fluid consistency must be specified. Around the carrier body with the applied green layer to be able to handle drying, it is also useful here that the carrier liquid through a with an evaporable Solvent liquefied binder is formed. This ensures that the green layer as a result the adhesion of the individual powder particles to one another has sufficient strength over the binder.

In a particularly expedient embodiment, it is provided that that the suspension in several thin sub-layers one after the other is applied to the carrier body. Here you can the individual sub-layers each from an identical Suspension are built up. It is in a further embodiment the invention but also possible for the individual Sub-layers of suspensions with different size distributions for the metal powder used and / or different Use metal powders. This allows it for example, on the one hand to use metal powder that the Sintered metal layer has a particularly good porosity on the other hand, it is also possible to have at least one metal layer manufacture that in their metal composition has particularly favorable properties for the application, for example, has catalytic properties.

It is expedient if the partial layer applied in each case at least dried before applying the next sub-layer becomes. This ensures that the first applied sub-layer is sufficiently solidified so that it by the application method, for example by spraying the next sub-layer is not deformed. on the other hand is due to the remaining solvent content in the previously applied, dried partial layer represents that the next sub-layer is reliable and is connected with the same packing density and the finished Green layer has the desired strength.

In another embodiment of the invention, that the respective sub-layer before applying the next Partial layer is sintered. This procedure is special then advantageous if different in a multilayer structure Metal powders are used that are highly divergent Need sintering temperatures. This makes it possible that first the partial layer is applied to the carrier body which is the metal powder with the highest sintering temperature contains, and after sintering the first metal layer the next following ones in the corresponding order Partial layers with the lower sintering temperatures can be applied and sintered. This has the advantage that the desired through the individual sintering steps The porosity of the individual sub-layers is retained would be lost if the suspension with such a Apply heterogeneous powder mixture in one layer and in would sinter one step. This would be due to the necessary high sintering temperatures for only a portion in the Powder mixture the remaining, low sintering powder components inside the seal so that the porosity would be largely lost.

In a preferred embodiment of the invention, that the suspension as a layer on a flat, flexible Carrier body applied and after drying as a green layer separated from the carrier body and separated into one membrane-like, porous finished part is sintered. The advantage this procedure is that first a proportionate large green layer can be produced can, from which after drying by punching or cutting Pieces of film and green sheet in the desired shape can be produced. With these sections peeled off the green sheet from the support body and then sintered as an independent part. Can as a carrier plastic or metal foils are used here. The Carrier body is expedient before the suspension is applied coated with a release agent.

In a particularly advantageous embodiment of the Invention is provided that the suspension as a layer a high-temperature resistant, preferably flat support body is applied, dried on this, sintered and then as a membrane-like, porous, metallic finished part is removed from the carrier body. Used as a carrier body one is a material that does not bond during sintering with the green layer on the support body, as is the case, for example, with ceramic materials If this is the case, this procedure offers the possibility of membrane-like metallic porous finished parts industrially with a low Share of manual work with extensive automation to manufacture. The particular advantage here is that the dry, still sensitive green layer to carry out the sintering process is not lifted off the carrier body and must be handled here, but that they only after the Sintering is removed. This reduces the number of rejects and also given the opportunity for the carrier liquid a lower proportion of binder to form the suspension to be provided, since only so much binder is to be added to a safe handling of the carrier body after spraying the Ensure layer until insertion into the sintering furnace.

In this process too, the suspension can be poured or spraying can be applied to the carrier body. To a Cutting or punching the green sheet with the support body or to avoid the finished porous metal membrane, it is useful if in an embodiment of the invention before application a contour mask of the suspension on the carrier body is launched. This makes it possible to apply the suspension to apply the carrier already in the intended final contour, so that a subsequent cutting process is omitted. On Another advantage of using a contour mask is in that the applied in particular by a spraying process Suspension also in that limited by the contour mask Edge area has the predetermined layer thickness. It exists even the possibility of a corresponding additional Spray run in which only the edge area in an overflow is sprayed with suspension, the finished porous To give the membrane a slightly greater thickness in the edge area, so that here a better stiffness and a sufficient volume of deformation is present if, for example porous membrane to be clamped on the edge.

When using the above-described invention Process for producing such sintered metal layers in the form of a thin, porous membrane attached to the Place of fabrics or fleeces can be used it has surprisingly been shown that the sintered membrane ductile, mechanically stable and within certain limits is also elastic, with the particular advantage being given here is that such a membrane with a tight tolerance defined porosity and low flow resistance can be produced, the porosity essentially by specifying the particle size and the flow resistance by the thickness and particle size of the sintered Metal layer is determined. By selecting the ones to be used Metals, metal alloys and / or the metal powder mixtures for the metal powder practically any Requirement with regard to mechanical, thermal and / or chemical resistance.

In an advantageous further embodiment of the invention The method also provides that the finished sintered porous membrane is calibrated by rolling. By this measure can be set a defined thickness and the Smooth the surface. Furthermore, the pore size in the The metal layer defines shrink because of the small Thick not only the surface areas, but the metal layer overall "deformed". But that's also given the opportunity to start with something greater thickness and a somewhat coarser and therefore cheaper Produce metal powder and then by the rolling process to reproducibly reduce the pore size.

If the carrier body is also part of the finished part and accordingly the metal layer is solid with it is to be connected is in a different embodiment provided that the suspension on at least one surface of a metallic carrier body applied, dried and the green layer then sintered firmly onto the carrier body becomes. The carrier body can in turn be a Sintered molded part, also a porous sintered molded part with coarser Pore structure. The suspension can in turn be cast by thin film, Spray or dip on the surface of the Carrier body are applied. The metal layer can ever depending on the intended use on the outer wall and / or the inner wall be applied.

The metallic carrier body is replaced by a tubular one Carrier body formed, then is in an embodiment of the invention Process provided that when applying the Suspension and at least during part of the dry season the carrier body is rotated about the pipe axis. This is ensures that the layer thickness until the solidification Get suspension as a green layer on the support body remains. It is useful here, especially when thin-film casting and when spraying when the suspension exits defined in addition to the rotation relative to the surface is moved.

Fig. 1

einen Verfahrensablauf, bei dem das Teil durch einen Stanzschritt geformt wird,

Fig. 2

einen Verfahrensablauf, bei dem das Teil durch einen Spritzvorgang geformt und eigenständig gesintert wird,

Fig. 3

einen Verfahrensablauf, bei dem das Teil durch einen Spritzvorgang geformt und mit Hilfe eines Trägerkörpers gesintert wird.

Porous membranes produced as a finished part or porous metal layers applied to a porous carrier body are particularly suitable for use as a filter and, with a corresponding adjustment of the porosity of the metal layer, also as a microfilter. In the case of impermeable support bodies, such a component can also be used as catalysts with a suitable composition with regard to the metal powder used and with a corresponding porosity. The method according to the invention is explained in more detail below with the aid of schematic flow diagrams for the application in the production of thin, porous metal layers which can be used as an independent part. Show it:

Fig. 1

a process sequence in which the part is formed by a stamping step,

Fig. 2

a process sequence in which the part is molded by a spraying process and sintered independently,

Fig. 3

a process flow in which the part is molded by a spraying process and sintered with the aid of a carrier body.

In the method shown in FIG. 1, one is made from one sinterable metal powder and a carrier liquid formed suspension with the help of a spray or pouring head 1 on a carrier body 2 in the form of a larger film section from a plastic or metal foil as a thin Layer 3 is applied. The one with a thin suspension layer 3 coated carrier body 2 is here by a larger film section then in a drying facility 4 guided, in the heat of the carrier liquid, evaporated, for example, ethanol or isopropanol becomes. A binder dissolved in the carrier liquid remains in the thin to increase the green strength Layer.

The so dried and now with a solid green layer 3.1 provided film section is then a punching device 5 supplied in the with the help of a punch knife 6 a part 7 in the desired outer contour together with the is stamped out as a carrier body 2 adhering film part. For simplification, only part 7 is punched out here shown. There is, however, the possibility in one or more in successive punching steps Parts 7 from the section of film provided with the green layer punch.

In a subsequent separation step 8, the die-cut is also punched out Part 2.1 of the carrier film is peeled off from the green sheet 3.1, which are then introduced as green compact 3.2 into a sintering furnace 9 and there among those for the respective powder composition specified conditions is sintered. From the Sintering furnace 9 can then produce the finished part 3.3 in the form of a solid thin metal layer with open porosity removed become.

2 is based on a flexible, in remaining, however, dimensionally stable support body 2.2, for example made of a silicone rubber, a mask 10, which with a cutout 11 is provided which corresponds to the desired final contour corresponds to the porous metal layer part to be produced. Then - as described with reference to FIG. 1 - the support body 2.2 provided with a corresponding mask With the help of a spray or pouring head 1 with the metal suspension sprayed so that on the support body 2.2 through the cutout 11 the mask 10 delimited a corresponding area, thin suspension layer 3 is applied. Here too with a corresponding area size of the carrier body 2.2 Mask 10 with a corresponding plurality of cutouts 11 be provided.

In a next step, the mask 10 is removed, so that the carrier body 2.2 with the remaining thin Suspension layer 3 are introduced into the drying oven 4 can, in which the carrier liquid is evaporated.

In a subsequent separation step 8, the carrier body 2.2 the green layer 3 removed, which is shown schematically here a bending of the carrier body 2.2 on the edge of a cutting edge 12 is indicated, so that subsequently the isolated green body is sintered again in the sintering furnace 9. From the sintering furnace 9 can then the finished part 3.3 in the form of a solid, thin Metal layer with open porosity can be removed. In this procedure, the stamping step 5 is omitted because through the mask 10 with its cutout 11 the required contour already exists. The carrier body remains intact and can be used again.

The method shown schematically in FIG. 3 corresponds in its sequence up to the process step of drying in Drying oven 4 the method described with reference to FIG. 2, so that reference is made to the previous description can. The only difference is that the carrier body 2.2 made of a high temperature resistant material there is no connection with the on the sintering Carrier body located green layer 3 comes in as this for example in the case of a ceramic material.

In contrast to the method according to FIG. 2, the carrier body 2.2 with the green compact 3.2 on it in the sintering furnace 9 introduced and also together with the carrier body 2.2 removed from the sintering furnace 9 again. Only that done sintered, porous metal layer part 3.3. is then from the carrier body 2.2 decreased.

With all processes it is possible to use several spray or Pouring overflows with differently structured suspensions a multilayer structure for the metal layer part to be produced to realize.

The method described with reference to FIG. 3 also offers furthermore the advantage that initially only one layer the temperature-resistant support body 2.2 applied and on the Carrier body is sintered. Then the ready sintered, still on the carrier body 2.2 porous metal layer, a further layer of an optionally differently composed suspension applied then - as described above - dried and sintered. The sintering process results in a firm bond between the first and the second as well as each further, in metal layer applied in this way. The advantage is there in doing so in that the second and each additional one still to be applied Metal layer in terms of its different composition also sintered under other temperature conditions can be. For example, it is possible in one first layer a high powder metal composition Sintering temperature as a porous layer and then sintering metal powder compositions in the second and each additional layer to sinter as a porous layer, which is due to their compositions at lower temperatures have to be sintered. This ensures that the desired by adapted sintering conditions Porosity of the individual layers is preserved.

By the used in the method according to FIGS. 2 and 3 Contour masks are also given the option of applying the suspension in the spray process after each Form the main layer additional overflows in the edge area perform so as to have a porous metal layer part reinforced edge.

Claims (13)

1. A process for producing a thin, open-pore metal layer from a sinterable powder consisting of pure metals, metal alloys and/or powder mixtures of different metals and metal alloys, characterised in that the metal powder is suspended with a given size distribution of powder particles in a carrier liquid, formed by a binder liquefied with an evaporable solvent, in that the suspension is applied in at least one thin layer onto a support and dried and the green layer thus formed is sintered, wherein the layer thickness of the suspension applied corresponds at least to the thickness s of the metal layer to be produced after sintering, wherein s corresponds to at least 3 times the average diameter D of the powder particles, with D = 1µm to 50µm, wherein the layer thickness of the finished metal layer amounts to at most 500µm.
2. A process according to claim 1, characterised in that the suspension is applied to the support in a plurality of successive thin sub-layers.
3. A process according to one of claims 1 or 2, characterised in that suspensions with different size distributions of the metal powder and/or different metal powders are used for the individual sub-layers.
4. A process according to one of claims 1 to 3, characterised in that the respective sub-layer is at least partially dried before application of the next sub-layer.
5. A process according to one of claims 1 to 4, characterised in that the respective sub-layer is sintered before application of the next sub-layer.
6. A process according to one of claims 1 to 5, characterised in that the suspension is applied as a layer to a flat, flexible support, from which it is separated in the form of a green layer after drying and sintered separately to yield a membrane-type porous finished part.
7. A process according to one of claims 1 to 6, characterised in that the suspension is applied as a layer on a high temperature-resistant, preferably flat, support, dried and sintered thereon and then removed from the support as a membrane-type, porous, metallic finished part.
8. A process according to one of claims 1 to 7, characterised in that an outline mask is laid upon the support prior to application of the suspension.
9. A process according to one of claims 1 to 8, characterised in that the suspension is applied to at least one of the walls of a tubular metallic support and dried and the green layer thus formed is then sintered firmly onto the support.
10. A process according to claim 9, characterised in that the tubular support is rotated about the tube axis during application of the suspension and at least during part of the drying time.
11. A process according to one of claims 1 to 10, characterised in that the suspension is applied to the support by thin layer casting, spraying or dipping.
12. A process according to one of claims 1 to 11, characterised in that the suspension outlet is moved relative to the support during application of the suspension.
13. A process according to one of claims 1 to 12, characterised in that the ready-sintered porous membrane is sized by rolling.

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