Abstract:
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.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    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 
       [0002]    The invention relates to a process for coating a workpiece on which a layer is produced electrochemically. 
         [0003]    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. 
         [0004]    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 
       [0005]    It is one possible object to improve an electrochemical coating process so that the electrochemically deposited layers display an inhomogeneous expansion behavior. 
         [0006]    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. 
         [0007]    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. 
         [0008]    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. 
         [0009]    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. 
         [0010]    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. 
         [0011]    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. 
         [0012]    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. 
         [0013]    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. 
         [0014]    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. 
         [0015]    How zones can be formed geometrically is described in detail below. 
         [0016]    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. 
         [0017]    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. 
         [0018]    As an alternative, it is also possible for the zones to be arranged as rectangles in a two-dimensional array on the workpiece. 
         [0019]    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. 
         [0020]    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. 
         [0021]    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. 
         [0022]    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. 
         [0023]    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. 
         [0024]    The process can be employed particularly advantageously for working rollers of a roll mill. These serve, inter alia, to transport the material to be rolled, e.g. a metal sheet, whose wall thickness, for example, is to be reduced by being conveyed between the working rollers. The working rollers of a roll mill are therefore subject to tremendous wear. This can be reduced by the coatings applied according to the proposal when particles of a hard material are preferably embedded in the zones. These can be, for example, oxides of Al, Co, Mg, Ti, Si or Zr, nitrides of Al, B or Si or carbides of B, Cr, Ti, Si or W or else carbonitrides. Carbon as graphite, diamond, DLC (diamond-like carbon) or glassy carbon or mixtures of all the materials mentioned can also be used. Particularly preferred hard materials are the following: TiC, B 4 C, Cr 3 C 2 , SiC, WC, TiN, MoB, TiB 2 , Al 2 O 3 , Cr 2 O 3 , TiO 2 . Particles of cemented hard metals (WC, TiC or TiN in a proportion of ≧80% by weight in a matrix of Co, Ni, Cr, Fe) can also be used. 
         [0025]    The hard materials mentioned can be deposited together with particles of a matrix material as second material in the zones. The first material can be selected with a lower coefficient of thermal expansion than the second material in order to generate compressive stresses in the zones which, owing to the proportion of hard materials, have to be reinforced against the occurrence of tensile stresses in the microstructure on heating of the roller surface. Comparatively high concentrations of hard material particles can then be realized in the zones. 
         [0026]    The hard materials used in the zones of the layer produced firstly advantageously reduce the abrasion thereof, so that the wear resistance thereof increases. Furthermore, the hard materials also serve the purpose of increasing the surface roughness of the layer, which is necessary to transmit the torque of the working rollers to the metal sheet to be rolled. If the hard materials are provided by the multilayer structure of the roller over the entire layer thickness, it is also advantageously ensured that the surface roughness of the roller is maintained even in the event of abrasion of the layer with progressive wear as a result of continual exposure of fresh hard material particles. This means that a component which fully meets the surface roughness requirements over its entire intended life is advantageously created. 
         [0027]    The advantages of the process will be summarized once more at this point. Electrochemical deposition of even electrochemically base metals such as Ti, Ta, Si, Al or Mg is possible when an ionic liquid is selected as electrolyte. Inexpensive deposition is possible, in particular by selection of the brush plating process since comparatively rapid layer growth can be achieved here. Introduction of particles into the zones being formed from the second material is possible and high particle concentrations in the layer can be achieved. The process is also partially applicable to large workpieces since these do not have to be dipped into an electrolyte in the case of brush plating. In particular, the process can also be employed for repair purposes since the coating system (including a cold gas spray gun and a transferer for brush plating) is transportable and can therefore also be used, for example, at the place of use of the workpiece to be repaired. 
       First Example 
       [0028]    Surface cleaning and activation is firstly carried out on the workpiece to be coated. This can be carried out, for example, by brush cleaning using an alkaline and/or cyanide-containing electrolyte and brush etching using an acidic electrolyte, e.g. hydrochloric acid or sulfuric acid. The first coating step in which a ductile base material such as nickel or nickel-cobalt is deposited as first material is then carried out. This process is carried out by brush plating. As electrolyte, it is possible to use, for example, a Watts electrolyte. The transferer for brush plating, which can be a felt or sponge soaked with the electrolyte, is moved over the surface to be coated. An anode in the form of a rod, wire braid or composed of spheres can be present in the transferer. The material of the anode is either the base material of the layer to be deposited, in which case this then dissolves and has to be replaced at regular intervals, or an inert anode, for example a platinum anode. 
         [0029]    Depending on the workpiece geometry, the further coating step can be carried out subsequently to electrochemical coating or simultaneously at another place. Here, zones of a second material having a different coefficient of thermal expansion are applied by thermal spraying, preferably cold gas spraying, with the particles intermeshing mechanically with the surface and therefore adhering. In cold gas spraying, the surface is advantageously subject to barely any thermal stress. This can therefore immediately be fed once again to the electrochemical coating step. A tight sequence of electrochemical and thermal coating steps can be realized. As a result, rapid layer buildup is possible, which advantageously improves the economics of the parts produced. 
       Second Example 
       [0030]    Coating is firstly carried out in a nonaqueous electrolyte. Surface cleaning and activation of the workpiece to be coated is carried out in the above-described way by brush cleaning and brush etching. After drying at 100° C., the first coating step, in which a metallic layer of, for example, titanium is deposited, is carried out. This process is carried out by brush plating. The electrolyte used for the deposition of titanium as first material is 1-butyl-3-methylimidazolium tetrafluoroborate in which titanium tetrafluoroborate is dissolved as ion carrier. A felt or sponge is soaked with this electrolyte and moved over the area to be coated on the component. The transferer formed by the felt or sponge is equipped with an electrode in the above-described manner. This can be formed of titanium or an inert material such as platinum. 
         [0031]    Depending on the workpiece geometry, the second coating step can be carried out alternately with electrochemical coating or else simultaneously at a place at which electrochemical coating is not being carried out at the particular time. Here, zones composed of, for example, aluminum as second material are produced by the abovementioned cold gas spraying. In the subsequent electrochemical treatment step, the zones are then incorporated into the metal matrix in the above-described manner by electrochemically depositing titanium again. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
           [0033]      FIG. 1  is an example of the proposed process for coating a workpiece, 
           [0034]      FIGS. 2 and 3  show a movement pattern as can be traced by the cold gas spraying nozzle as per  FIG. 1  and 
           [0035]      FIGS. 4 to 11  show layer structures which can be produced by examples of the process. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0036]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
         [0037]    In the example of the proposed process as per  FIG. 1 , a roller-shaped workpiece  11  is provided with an antiwear layer. The workpiece  11  is mounted so as to be rotatable about its central axis  12 , with the axis of rotation  13  being identical to the central axis  12 . A bearing  14  is schematically shown, with the workpiece  11  being turned at constant speed by a drive (not shown) during coating. 
         [0038]      FIG. 1  shows a plan view of the workpiece  11  from above looking vertically downward. During coating, a transferer  15  which is formed of a sponge having open pores  16  is brought into contact with the workpiece from the one side. An electrolyte is applied by this via a feed system  17  in a manner not shown in more detail to the surface  18  of the workpiece which moves along under the transferer. Electrochemical coating takes place during this process, and the workpiece  11  and the transferer are for this purpose connected to a voltage source  19 . 
         [0039]    At the same time, cold gas spraying takes place on the opposite side of the workpiece. A cold gas spraying nozzle  20  is for this purpose directed at the surface  18  of the workpiece and moved stepwise over the surface approximately in the direction of the axis of rotation  13 . At the pause locations  26  (shown in  FIG. 3 ) of the cold gas jet, small depots  27  (shown in  FIG. 4 ) of a material having a coefficient of expansion other than that of the layer material applied by electrochemical coating are formed. When the cold gas jet is moved between the pause locations, individual particles from the cold gas jet  21  remain adhering to the surface and are subsequently incorporated into the layer matrix in the layer then formed at the transferer  15  due to rotation of the workpiece. However, these have a negligible effect compared to the depots  27  because of their small size. 
         [0040]    It can also be seen in  FIG. 1  that a movement region  22  of the cold spraying nozzle  20  is somewhat smaller than the length of the workpiece since, for example, in the case of working rollers of roll mills as workpieces to be coated each end face region does not participate in the rolling process and is therefore also not subjected to the severe wear stress. If the movement region  22  of the cold gas spraying nozzle  20  is selected so that it does not extend to the edge of the workpiece to be coated, this has advantages in terms of carrying out the process. The movement pattern of the cold spraying nozzle is shown in  FIG. 2 . This is a path corresponding to a figure eight, with the continual movement  24  of the workpiece owing to the rotation being taken into account. A line  25  as per  FIG. 3  is traced on the surface  18  of the workpiece  11  due to the figure eight-shaped path, so that uniform loading of the surface with particles occurs. The pause locations  26  of the cold gas jet, which lead to a buildup of depots  27  in the layer material  28  as shown in  FIG. 4  having a chessboard-like layer structure are also shown in  FIG. 3 . 
         [0041]      FIG. 4  shows a plan view onto the layer surface. It can be seen that the depots  27  are embedded in the first material  28  of the layer so as to form part of the layer surface. In  FIG. 5 , on the other hand, the depots  27  are completely surrounded by the material  28  of the layer. This can be achieved by an electrochemical coating step with the first material  28  of the layer being carried out after application of the depots  27  without the second material being applied once more. A layer  29  formed in this way thus has three coats  30  of which only the middle coat is provided with the depots  27 . 
         [0042]      FIG. 6  once again shows the layer surface with exposed strips  31  of the second material which are embedded at the side flanks in the first material  28 . Another embodiment is obtained when rectangles  32  are produced instead of the strips  31 , as is shown in  FIG. 7 . These are also exposed at the top, so that they can be seen in the layer surface, while their sides are embedded in the first material. 
         [0043]      FIG. 8  shows how a bending beam  33  can be integrated into the layer  29  on the component  11 . To ensure that this can be produced in a cantilever fashion, wax as sacrificial material  34  is applied in a prescribed shape to the component  11  so that the sacrificial material also closes an orifice  35  in the component  11  and thus prevents this orifice from being closed by the coating process. Above the sacrificial material  34 , the first material  28  is firstly deposited electrochemically; for this purpose, the sacrificial material has to be provided with an electrically conductive initial layer. A zone  36  is subsequently produced on the first material by cold gas spraying and the flanks  36   a  thereof are subsequently embedded in the first material  28 . To prevent the zone  36  itself from being coated by the first material  28 , this zone is electrically insulated (for example by a protective surface coating). In this way, a two-layer composite which bends in the event of temperature changes owing to the inhomogeneous expansion behavior and can in this way also close the orifice  35  is formed at least in the middle part of the bending beam  33 . To ensure this bending and closure function the sacrificial material  34  is removed by, for example, melting after the bending beam  33  has been produced. 
         [0044]    Another example can be seen in  FIGS. 9 and 10 ; here, the section planes of the other figures are drawn in appropriately (section X-X corresponds to the section plane in  FIG. 10  and section IX-IX corresponds to the section plane in  FIG. 9 ).  FIG. 9  shows a beam  37  which is integrated into the layer  29 . One end of the beam  37 , which is formed entirely of the second material, is embedded in the first material  28  (cf.  FIG. 10 ) and thus fixed in the region of the layer  29 . A hollow space is defined by the sacrificial material  34  and leads to the beam  37  being arranged in a cantilever fashion in the layer. 
         [0045]    Heating causes the beam  37  to expand and when a sufficient increase in length has occurred, a spacing a is bridged so that the beam  37  contacts a cross strut  38  made of the first material  28 . This likewise spans, in a cantilever fashion, an equilibration orifice  39 , so that on further heating and expansion of the beam  37 , the cross strut  38  can deform elastically. It can be seen from  FIG. 10  that the sacrificial material underneath the beam  37  and the cross strut  38  also ensures that there is no connection to the component  11 . After production of the layer  29  is complete, the sacrificial material has to be removed. 
         [0046]    In addition,  FIG. 9  shows the places  40  at which electrodes could contact the surface in order to detect a change in the electrical resistance in the case of contact of the beam  37  with the cross strut  38 . This can, in particular, be measured when the beam  37  has a lower electrical resistance than the first material  28 . 
         [0047]      FIG. 11  depicts a component  11  which is configured as a shaft and is shown in cross section. The layer is formed of the first material  28 , with strips  31  running in the axial direction being provided in the layer. Viewed from the outside, the component  11  thus has a layer pattern as is shown in  FIG. 6 . 
         [0048]    The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in  Superguide  v.  DIRECTV,  69 USPQ2d 1865 (Fed. Cir. 2004).