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Method for electrochemical coating

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Method for electrochemical coating


In a method for coating a work piece, a layer is electrochemically produced from a first material. In order to generate an inhomogeneous expansion behavior of the layer, a thermal spraying, in particular a cold gas spraying, achieves that specific zones are created in the layer from a material having a different thermal expansion behavior. These zones expand more laterally than in the direction of the layer thickness so that directed internal stresses occur in the layer upon heating or cooling of the component, which can be specifically utilized depending on the design conditions of the component.

Browse recent Siemens Aktiengesellschaft patents - München, DE
Inventors: Jens Dahl Jensen, Oliver Stier, Gabriele Winkler
USPTO Applicaton #: #20120269982 - Class: 427448 (USPTO) - 10/25/12 - Class 427 
Coating Processes > Spray Coating Utilizing Flame Or Plasma Heat (e.g., Flame Spraying, Etc.) >Nonuniform Or Patterned Coating

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The Patent Description & Claims data below is from USPTO Patent Application 20120269982, Method for electrochemical coating.

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CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to International Application No. PCT/EP2010/067830 filed on Nov. 19, 2010 and German Application No. 10 2009 060 937.7 filed on Dec. 22, 2009, the contents of which are hereby incorporated by reference.

BACKGROUND

The invention relates to a process for coating a workpiece on which a layer is produced electrochemically.

A process of the type mentioned at the outset is described, for example, in DE 602 25 352 T2. This process makes it possible to coat the surface of a workpiece electrochemically, for example by brush plating. Here, a nonwoven, open-pored sponge or a brush is used as transferer in order to transfer an electrolyte onto the surface to be coated. There, a metallic material is deposited on the surface from the electrolyte by application of an electric potential between the substrate and an electrode arranged in the region of the transferer for the electrolyte.

From WO 2006/061081 A2, it is also known that electrochemical deposition of metal can also be carried out using ionic liquids which replace an aqueous electrolyte. The use of ionic liquids, i.e. salt melts, which are in liquid form in the range below 100° C., preferably even at room temperature, has the advantage that their use gives larger process windows for the deposition of metals which, owing to their position in the electrochemical series of metals, cannot be deposited or can be deposited only with difficulty by aqueous electrolytes. An example of such a metal is Ta. It should be noted that the metal ions deposited from the salt melt onto the surface to be coated have to be replaced by fresh metal ions introduced into the salt melt in order for the deposition process not to come to a halt. A method of keeping the concentration of metal ions constant is described, for example, in DE 43 44 387 A1.

SUMMARY

It is one possible object to improve an electrochemical coating process so that the electrochemically deposited layers display an inhomogeneous expansion behavior.

The inventors propose for a second material having a coefficient of thermal expansion α which differs from that of the first material to be applied to the workpiece using a thermal spraying process and subsequently being embedded in the layer by electrochemical coating. This embedding can be carried out in such a way that the zones still form part of the resulting surface of the coated component, so that embedding occurs only on the lateral flanks of the zones. As an alternative, it is also possible to embed the zones in the layer in such a way that they are fully enclosed by the first material. For the purposes of this discussion, zones are subvolumes of the layer whose lateral dimension (i.e. dimension viewed in the direction parallel to the surface to be coated) is greater than their thickness dimension (i.e. dimension measured perpendicular to the surface to be coated). This leads to the thermal expansion behavior of the zones being more noticeable in the lateral direction of the layer than perpendicular to this direction. This causes, according to the proposal, the inhomogeneous expansion behavior of the layer produced.

For example, the second material can have a greater coefficient of thermal expansion a than the first material. In this case, the expansion of the zones leads to additional compressive stresses being formed in the regions of the layer adjacent to the zones. These can be used for stabilizing the microstructure of the layer if this were to react to tensile stresses by, for example, formation of cracks.

An inhomogeneous expansion behavior of the layer, which can be matched to different structural requirements for the component to be coated, can advantageously be produced by a suitable combination of the first material and the second material and by suitable geometric configuration of the zones. The zones can also be produced from a material which has a lower coefficient of thermal expansion α than the first material. In this case, additional compressive stresses would be generated in the first material of the layer when the component bearing the layer is cooled. This could, for example, be advantageous when the first material of the layer tends to display cold embrittlement and therefore has to be protected from occurrence of tensile stresses at low temperatures.

In an advantageous embodiment, cold gas spraying is employed as thermal spraying process. This is a process in which the coating particles remain adhering to the surface primarily as a result of their high kinetic energy. It is therefore also referred to as kinetic spraying. The kinetic energy is generated by a cold gas injection nozzle, a convergent-divergent nozzle, in a gas jet, with heating of the particles not occurring or occurring to only a small extent. In this case, the increase in temperature is not sufficient, as in the case of other thermal spraying processes, to melt the particles. The advantage of the use of cold gas spraying is therefore that the integrity of the microstructure of the particles used is not impaired by the cold gas spraying. In addition, this process has the advantage that, particularly in the case of a soft electrochemically produced layer matrix of the previous coat, the particles penetrate into the layer, as a result of which better distribution of the particles in the layer formed is achieved.

In a further embodiment, the layer is produced in a plurality of coats by carrying out the thermal spraying process and electrochemical coating alternately a plurality of times. This makes it possible, as indicated at the outset, to produce a layer structure in which the zones are completely embedded in the layer, i.e. no proportion of them forms on the surface. This is particularly advantageous when the material of the zones has to be, for example, protected against corrosive attack. In addition, the complete embedding of the zones allows particularly effective transmission of tensile or compressive stresses into the surrounding microstructure matrix of the first material.

In a particular embodiment, the thermal spraying and electrochemical coating are carried out simultaneously but each at different places on the workpiece. This allows, advantageously, a particularly high efficiency to be achieved in coating of the workpiece. A prerequisite is that the workpiece has to be coated only partially and simultaneously (at different places) by each of the two coating processes. In the case of thermal spraying, this is necessary in any case because coating always occurs only at the point of impingement of the coating jet. In electrochemical coating, it is necessary to select a coating process in which partial coating of the component is possible, i.e. in which the entire component does not dip into the electrolyte. This is preferably possible when employing brush plating, with only the subregion of the workpiece which is in contact with the transferer of the electrolyte being electrochemically coated at a particular time.

Simultaneous coating of the workpiece by the two coating processes can particularly preferably be employed when a cylindrical body, in particular a working roller for roll mills, is coated as workpiece, with this being set into rotation about its central axis and electrochemical coating being carried out at one place on its circumference and thermal spraying being carried out at another place on its circumference. This can be effected, for example, by only part of the circumferential area of the cylindrical workpiece being dipped into the electrolyte. Uniform coating is then ensured by uniform rotation of the cylindrical workpiece by which the entire outer surface can be gradually coated. Thermal coating can be carried out in the region which does not dip into the electrolyte. Rotation of the roller is also very advantageous when employing brush plating. The transferer for brush plating then only has to be brought into contact with the workpiece, with relative motion between the workpiece and the transferer being brought about by the continual rotation of the cylindrical workpiece.

In a particular embodiment, an ionic liquid is used as electrolyte for electrochemical coating. This has the advantage that even relatively base metals can be deposited from a nonaqueous medium, namely the salt melt of ionic coating. Ionic liquids are organic liquids which are formed of a cation such as an alkylated imidazolium, pyridinium, ammonium or phosphonium ion and an anion such as simple halides, tetrafluoroborates or hexafluorophosphates, bi(trifluoromethylsulfonyl)imides or tri(pentafluoroethyl)trifluorophosphates.

Since ionic liquids also have a high electrochemical stability, it is advantageously possible to deposit, inter alia, Ti, Ta, Al and Si which cannot be deposited from aqueous electrolytes because of the strong evolution of hydrogen. Suitable metal salts, which are also mentioned in the abovementioned WO 2006/061081 A2, are, for example, halides, imides, amides, alkoxides and salts of monobasic, dibasic or polybasic organic acids, e.g. acetates, oxalates or tartrates. The metals which are to be electrochemically deposited are brought into the suitable ionic liquid by anodic dissolution. A soluble electrode is used as counterelectrode to the component to be coated. This soluble electrode is formed of the metal which is to be applied as a coating. As an alternative, the metal to be deposited can also be added as salt to the ionic liquid. Then, a platinum electrode, for example, can be used as counterelectrode to the substrate. In this case, it has to be ensured that the concentration of the metal ions to be deposited in the ionic liquid is maintained, which is described in more detail in, for example, the abovementioned DE 43 44 387 A1. In addition, the metals can also be deposited as nanocrystalline layers when using ionic liquids. For this purpose, suitable cations, e.g. pyrrolinium ions, which are surface-active and therefore act as grain refiners in electrochemical deposition have to be added to the ionic liquid. It is advantageous that the addition of wetting agents or brighteners can frequently be dispensed with under these conditions.

How zones can be formed geometrically is described in detail below.

In an embodiment, the zones can be distributed as island-like depots in a regular pattern on the workpiece. A lower limit to the size of these island-like depots is imposed purely by the gas jet of the cold gas spraying process employed producing an impingement spot having certain dimensions on the component to be coated. This gives the smallest possible size of the depot. If the depot is to be larger, the cold gas jet has to be conducted in a suitable way during production of the depot. It is advantageous to produce depots having a round base area, but other geometries can also be realized. The production of comparatively small depots is advantageous because a dense change between the first material and the second material in the layer can be realized thereby. Stress peaks in the microstructures of the first material and of the second material can in this way be kept low as soon as these are formed as a result of the inhomogeneous expansion behavior of the layer.

Another possibility is to arrange the zones as strips on the workpiece. This makes it possible to produce an inhomogeneous expansion behavior which differs not only in respect of the expansion behavior of the layer perpendicular to the surface of the workpiece but also in respect of the lateral expansion behavior in different directions in the layer.

As an alternative, it is also possible for the zones to be arranged as rectangles in a two-dimensional array on the workpiece.

It is particularly advantageous for the layer to be produced in the region of at least one zone on a sacrificial material, e.g. wax, which is removed to form a hollow space, for example by melting, after production of the layer. In this way, cantilever structures which, owing to their inhomogeneous expansion behavior, can be used as mechanical adjusting elements can advantageously be formed from the zones of the second material and the layer composite of the first material surrounding these zones. The driving force for actuation of the adjusting elements is accordingly temperature differences during operation of the coated component.

For example, it is possible for the zone formed by the second material together with the first material of the remaining layer to be configured so as to give a multilayer, cantilevered bending beam. At its one end, the bending beam is then joined to the remaining layer composite. Underneath the bending beam, there is the abovementioned hollow space, with the other end of the bending beam being freely movable. As a result of the different expansion behavior of the two materials, which are preferably arranged in two adjoining layers, the beam bends by the mechanism which is known, for example, from bimetallic strips. The adjusting element is realized in this way.

A bending beam configured in this way can be produced with its free end above, for example, an orifice in the surface of the workpiece. This orifice can, for example, serve for introduction of a cooling medium. The bending beam can be configured so that the orifice is opened only when a particular temperature is exceeded, so that the coolant is introduced only in the case of a threatening overheating of the component. A temperature-controlled valve is advantageously realized in this way. Throttling of the coolant flow can also be achieved.

In another embodiment, the zone as cantilevered beam is produced from the second material. This has a greater coefficient of thermal expansion α than the first material. The bending beam is joined at its one end to the remaining layer composite and its other end is at a defined spacing from the remaining layer composite. The beam formed in this way preferably has no component of the first material. This structure can, for example, be used as thermal switch. When the component is heated, the beam expands as a result of the greater coefficient of thermal expansion α of the beam and at a particular temperature bridges the defined distance to the remaining layer composite. This produces a contact which requires electrical conductivity of at least the second material and leads to a change in the electrical behavior of the layer. This can be measured and used as a switching signal. If the first material is an electric insulator, a suitable configuration of input leads, for example composed of the first material, also enables an electric switch to be realized by the beam.

In the case of components having an axis of rotation, which are preferably cylindrically symmetric, it is particularly advantageous for the parts of the layer provided with zones to alternate with parts of the layer without these zones in the circumferential direction relative to the axis of rotation. In this way, it is, as indicated above, advantageously possible to produce a compressive stress in the circumferential direction in the component owing to the inhomogeneous expansion behavior. This can be particularly advantageous when the component is unintentionally subjected to tensile stress in the zones in the peripheral region, for example because of high rotational speeds and the resulting centrifugal forces.



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stats Patent Info
Application #
US 20120269982 A1
Publish Date
10/25/2012
Document #
13518627
File Date
11/19/2010
USPTO Class
427448
Other USPTO Classes
International Class
23C4/12
Drawings
4



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