Abstract:
For improving the variability in the coating of substrates a coating apparatus is proposed having a plasma generator for generating a plasma jet which exits from a coating head of the plasma generator. A first particle reservoir and a second particle reservoir are provided. The particles from the first particle reservoir and the second particle reservoir are supplied to the plasma jet as a particle mixture via a transport pipe. A supply control device is provided for setting the amount of particles from the first particle reservoir fed into the transport pipe relative to the amount of particles from the second particle reservoir.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from German Patent Application No. 10 2012 108 919.1, filed on Sep. 21, 2012, which application is incorporated herein by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a coating apparatus for coating a substrate with a plasma generator. 
         [0003]    Furthermore the invention relates to a method for coating a. 
       BACKGROUND OF THE INVENTION 
       [0004]    The complex requirements of modern engineering and material challenges implies, to a growing extent, the use of material combinations, amongst them compound materials and layered systems. Such layered systems may for example be used as protective or functional layers on objects against corrosive, thermal, chemical, or biological stresses in many ways. For making such material or layer compounds currently various technologies are employed. Therein often chemical vapor deposition (CVD) or physical vapor deposition (PVD) are used, Further established methods are soldering, diffusion welding, or powder metallurgical compound pressing with possible subsequent smithing. Therein the layers are either applied onto a compact substrate via the melted phase (thermal spraying) or via the vapor (PVD) or gas (CVD) phase, or are directly connected with a compact substrate material as compact parts by means of an auxiliary substance (soldering) or by simultaneous application of pressure and temperature (diffusion welding). 
         [0005]    These known techniques have method specific limitations, however. Unfavorable layer properties like for example open porosity and cracks in the layer reduce the protective effect against reactive media. Due to temperature gradients between the materials during production of the layer compounds often stresses remain in the thermally affected regions of the parts. Therefore often laborious additional processes are required. 
         [0000]    These disadvantages can often be reduced or completely eliminated by the direct application of layers by means of a plasma jet to which powder is supplied. Such a method for example is known from U.S. Pat. No. 5,853,815. In this document it is proposed to homogeneously coat a substrate with a plasma stream covering the entire width of the substrate. A particle reservoir is directly connected with a plasma generator via a pipe. A large pressure difference between the plasma gun and the plasma generator creates a shock pattern, causing the coating stream to fan out widely and also resulting in a thorough distribution of the coating material in the plasma stream. 
         [0006]    Various material combinations can be applied onto a substrate in this way. To this end for example a powder consisting of a mixture of several types of material is used. In this way many material combinations can be applied even on substrates of complex shape, given a corresponding control of the nozzles. For example, in this way a very wear resistant but brittle material can be embedded into an elastic matrix. It is furthermore possible to sinter powders comprising a mixture of plural fine grained metallic components during the coating process. 
         [0007]    From DE 199 58 473 A1 a method and an apparatus are known wherein by means of a plasma a multilayered structure is applied onto a substrate. Therein the properties of the individual layers can be chosen from within a wide range. To this end it is proposed to supply to the plasma jet exiting from the plasma generator the species forming the layer, so called precursor materials, in the form of powder, gases, or liquids, which then are chemically or physically changed in the plasma in such a way that they are deposited as a cluster in the nano or microscale range on the substrate. In this way a composite layer system can be applied if precursor materials with different properties are supplied to the plasma jet at different locations. A disadvantage of this method of applying layers to substrates is that the property of the layer to be applied is fixed in the process. 
         [0008]    The international application PCT/DE2006/000638, published as WO 2006/108395 A1 describes an apparatus and a method for plasma coating. A plasma generator with plural expansion stages is disclosed, wherein each expansion stage exhibits an inlet for a coating material. Downstream from the expansion stages a mixing chamber is provided, in which the coating materials are mixed with each other and with the plasma. 
         [0000]    The German patent document DE 10 2008 053 640 B3 discloses a coating method in which a layer is sprayed onto an object. The spray material is melted from wires by an electric arc. A filler material can be supplied to the spray jet via an injector. 
       SUMMARY OF THE INVENTION 
       [0009]    The object of the present invention is to provide an apparatus for coating a substrate, wherein the properties of the coating to be applied are changeable during the coating process. 
         [0010]    According to the invention this object is achieved by a coating apparatus tier coating a substrate, comprising
       a plasma generator for generating a plasma jet; which exits from   a coating head of the plasma generator from which the plasma jet exits;   a first particle reservoir connected with a transport pipe for supplying particles stored in the first particle reservoir to the plasma jet:   at least a second particle reservoir is provided and configured to supply particles from the second reservoir via the transport pipe to the plasma jet in a particle mixture with the particles from the first particle reservoir; and   a supply control device for setting an amount of the particles fed from the first particle reservoir into the transport pipe relative to the amount of the particles fed from the second particle reservoir into the transport pipe.       
 
         [0016]    A further object of the invention is to provide a method by which the possibilities of coating substrates become more varied. 
         [0017]    With respect to the method the object is achieved by a method for coating a substrate comprising the following steps:
       generating a plasma jet with a plasma generator having at least one coating head from which the plasma jet exits;   feeding particles from at least a first particle reservoir and from at least a second particle reservoir via a transport pipe to a supply control device in which they are mixed;   supplying a particle mixture of particles from the first particle reservoir and of particles from the second particle reservoir from the supply control device to the plasma jet via the transport pipe; and   directing the plasma jet together with the particle mixture onto a surface of the substrate in order to form the coating.       
 
         [0022]    A coating apparatus for coating a substrate is proposed. The coating apparatus comprises a plasma generator for generating a plasma jet, wherein the plasma jet exits from a coating head of the plasma generator. Particles from a first particle reservoir can be supplied to the plasma jet via a transport pipe. A second particle reservoir is provided from which particles can also be supplied via the transport pipe to the plasma jet. A supply control device in the transport pipe allows setting the amount of particles from the first particle reservoir relative to the amount of particles from the second particle reservoir. Advantageously this ratio of amounts of particles can be varied even during the coating process. This also makes possible the generation of a changing layer profile on the surface of the substrate. 
         [0023]    In a preferred embodiment of the coating apparatus a controller for controlling the amount of particle mixture supplied to the plasma jet is provided. Therein the controller may be configured in such a way that the amount of supplied particles can be varied over a wide range, even during the coating process. Moreover, the controller may be a switch or configured to exhibit a switching function so that by this switch the supply of particles to the plasma jet may be allowed or interrupted. 
         [0024]    In a further embodiment of the invention a plurality of particle reservoirs is provided. The particle reservoirs therein may be mixed with respect to their relative amounts by a common supply control device or may be applied onto the surface of the substrate with corresponding separate coating heads. 
         [0025]    Preferentially for each particle reservoir there is provided at least one separate process by which a fluidized powder is generated from the particle reservoirs. The particle reservoir and the corresponding process gas form a respective particle supply unit. The particle supply unit may comprise a process gas control unit for controlling the mixing relation between the particles and the process gas. 
         [0026]    In a further embodiment of the invention the coating apparatus may comprise at least a second coating head and a further particle supply unit corresponding to the second coating head. The particle supply unit therein exhibits a further particle reservoir, a process gas, and a process gas control unit. With this embodiment of the invention it is also possible to provide a plurality of coating heads and respectively corresponding particle supply units, 
         [0027]    In the method for coating a substrate in a first embodiment the coating is done with a coating apparatus having a plasma generator for generating a plasma jet and also having a coating head, from which the plasma jet exits. For coating the substrate particles are supplied to the plasma jet from a first particle reservoir via a transport pipe. Also, particles from a second particle reservoir are mixed with those from the first particle reservoir by a supply control device and then fed into the transport pipe together and supplied to the plasma jet as a particle mixture. The plasma jet, together with the particle mixture, is then directed onto the surface of the substrate for forming the coating. Therein the particles from the first particle reservoir may be fluidized with a first process gas and the particles from the second particle reservoir may be fluidized with a second process gas. The fraction of particles from the first particle reservoir within the mixture can be set between 10% and 90%, and the fraction of particles from the second particle reservoir can be set between 10% and 90%. Furthermore it is possible to vary the amount of particles from the first particle reservoir relative to the amount of particles from the second particle reservoir during the coating of the substrate by changing the mixing ratio between the first and second particles during the application. 
         [0028]    In a further embodiment of the method according to the invention the coating is done with a coating apparatus having a plasma generator for generating a plasma jet and also having a coating head from which the plasma jet exits. Therein the substrate is coated by supplying particles from a first particle reservoir via a transport pipe to the plasma jet at a first supply location and supplying particles from a second particle reservoir to the plasma jet at a second supply location in such a way that on the substrate a first layer of particles from the first particle reservoir and a second layer of particles from the second particle reservoir are formed. As an alternative, the first and second supply location may also be chosen in such a way that a gradient layer or a compound layer is formed on the substrate. 
         [0029]    The second layer or gradient layer or compound layer in a further embodiment of this method is covered with a further layer, wherein particles from a third particle reservoir are fed into a further transport pipe, then are supplied to the second plasma jet of a second coating head, and then are applied onto the second layer of particles from the second particle reservoir or onto the gradient layer or onto the compound layer. 
         [0030]    With the method according to the invention and the apparatus according to the invention the properties of the layer to be applied may be varied over a wide range. By specific controlled supply of coating materials into the plasma coating process functional compound layers may be applied. The thickness and the composition of the compound layer therein may be controlled in such a way that the desired electrical, mechanical and chemical properties can be tailored. Also plural layers, including with different properties, and gradient layers may be generated on the substrate. 
         [0031]    According to an additional object of the invention a coating apparatus for coating a substrate is provided. The coating apparatus has at least a first plasma generator and at least a second plasma generator each of which generating a plasma jet. The first plasma generator has a coating head from which the plasma jet exits. The second plasma generator has a coating head from which the plasma jet exits. A first particle reservoir is connected with a transport pipe for supplying particles stored in the first particle reservoir to the plasma jet of the first plasma generator. At least a second particle reservoir is provided and configured to supply particles from the second reservoir via the transport pipe to the plasma jet of the first plasma generator in a particle mixture with the particles from the first particle reservoir. A least a third particle reservoir is connected with a transport pipe for supplying particles stored in the third particle reservoir to the plasma jet of the second plasma generator. A supply control device is provided for setting an amount of the particles fed from the first particle reservoir into the transport pipe to a first injector relative to the amount of the particles fed from the second particle reservoir into the transport pipe to a second injector, wherein the first injector and the second injector are arranged in relation to the plasma jet of the first plasma generator. A further supply control device is provided for setting an amount of the particles fed from the third particle reservoir into the transport pipe to a third injector arranged in relation to the plasma jet of the second plasma generator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    Additional advantages and advantageous embodiments are presented in the subsequent figures and pertaining description, where 
           [0033]      FIG. 1  is a schematic coating apparatus with a plasma module for providing a plasma jet; 
           [0034]      FIG. 2  is a schematic further embodiment of a coating apparatus with two plasma modules, wherein each of which provides a plasma jest; 
           [0035]      FIG. 3   a  through c are examples of layers that may be formed with the coating apparatus, in schematic representation; 
           [0036]      FIG. 4  is a schematic representation of a possible layered structure on a substrate after a coating; 
           [0037]      FIG. 5  is a schematic representation of the principle of a gradient layer by a depth profile; and 
           [0038]      FIG. 6  is a schematic representation of an example of a conductive coating formed with the coating apparatus. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0039]      FIG. 1  schematically shows a coating apparatus  10  for coating a substrate The coating apparatus  10  has a plasma module with a coating head  26 , a source for a plasma process gas  56  and a power supply  58 . 
         [0040]    The coating head  26  has a plasma chamber  60  in which an electric arc  20  is started between two electrodes  62  and  64 . Electrical energy is supplied to this electric arc  20  from the power supply  58  for sustaining it, so that, depending on the modulation of the power supply  58 , a continuous plasma jet  22  or a pulsed plasma jet  22  is generated, which exits on the exit side  26 A of the coating head  26 . At the feed side  26 E of the coating head  26  a plasma process gas  56  may be supplied, so that the plasma process gas  56  streams through the plasma chamber  60  in a controlled manner. A mixture of process gas  30 ,  32  and particles may be supplied to the plasma jet  22  via an injector  66 , which here is shown as an external injector. The particles may be partially molten by the high energy density in the plasma jet  22 . In this way they can be deposited on the surface  12   a  of the substrate  12  as first layer  50 . As the substrate  12  and the coating head  26  are moveable relative to each other, a continuous layer  50  can be formed on the substrate  12 . 
         [0041]    The particle mixture supplied to the injector  66  in the embodiment of the invention shown in  FIG. 1  is provided by a first particle supply unit  34  and a second particle supply unit  36 . A process gas control unit  38 ,  42  is provided in the particle supply units  34 ,  36 , respectively. By the process gas control unit the fractions of particles in the respective process gas  30 ,  32  can be controlled independently of each other. if necessary different process gases  30 ,  32  may be used in each particle supply unit  34 ,  36 , the process gases being adapted to the particles in the particle reservoirs. From the mixture of particles and process gases  30 ,  32  fluids are generated, which can be mixed in varying amounts relative to each other by a supply control device  18 . The mixture depends on the layer  50  desired on the substrate  12 . Usually the mixing ratio of the particles is chosen such that the fraction of the particle mixture with particles from the first particle reservoir  14  is set between 10% and 90%, and that the fraction of particles from the second particle reservoir  16  is set between 10% and 90%. 
         [0042]    The supply control device  18  therein is configured such that a ratio which is constant in time between the amount of particles from the first particle reservoir  14  and the amount of particles from the second particle reservoir  16  can be set for the particle mixture. Furthermore also supply control devices  18  may be employed by which in addition or exclusively a time-varying mixing relation can be set. During the supply of particles it is also possible, at least temporarily, to set the relative amount of one of the particle types to  0 , so that for a specific part of the surface of the substrate  12  the applied first layer  50  contains only particles from one particle reservoir. 
         [0043]    The supply control device  18  may for example be media adder. Therein two fluids may be supplied as two or more partial streams to one or more mixing chambers within the media adder, in which the mixing occurs. The mixing reaction may be controlled, wherein also a time-varying mixing ratio can be set, The mixture is then usually released through an opening in the bottom or top of the mixing chamber and supplied to the transport pipe  24 , which for example may be a system of hoses. For the transport pipe  24  also materials different from hoses can be used, like for example metal pipes, depending on the particles which are to be used for coating the substrate  12 . Via the transport pipe  24  the particle mixture reaches the injector  66 . Upstream of the injector  66  a controller  28  may be provided, by which the amount of particle mixture supplied to the injector  66  is controlled. Control may include a throttling of the particle stream or a dynamical switching process, i.e. controlled blocking and opening of the path to the transport pipe  24  in the controller  28 . 
         [0044]    With this apparatus dynamically changeable layers  50  may be applied. Thickness and material composition can be dynamically set via the supply rates of the particle supply units  34 ,  36  and the controller  28 . In this way the composition of a layer may also be dynamically changed during an active coating process. 
         [0045]      FIG. 2  schematically shows a further embodiment of the apparatus for coating a substrate  12 . According to this embodiment of the invention plural, in the example shown two, injectors  66 ,  68  correspond to the coating head  26 . Again the particles from the particle supply units  34 ,  36  are fluidized in the desired fractions. Afterwards the particles from the particle supply unit  34  are separately supplied to a first injector  66  and enter the plasma jet  22  at a first supply location  46 . The particles from the particle supply unit  36  are supplied to a second injector  68  and enter the plasma jet  22  at a second supply location  48 . Upstream from the injectors  66 ,  68  respective supply control devices  18  may be provided, the action of which has already been described in the context of  FIG. 1 . Through this arrangement two separate layers  50 ,  52  (double layer), independent of each other, can be generated on the surface  12   a  of the substrate  12 , the properties of which may be different (see  FIG. 6 ). 
         [0046]    There is also the possibility to form a so called gradient layer  54  (see  FIG. 3   c ) with this apparatus. This is particularly advantageous, as both the double layer and the gradient layer  54  can be applied onto the substrate  12  in one process step. Depending on the arrangement of the injectors  66 ,  68  and therefore depending on the position of the supply locations  46 ,  48  relative to the plasma jet  22  a wide range of effects can be achieved. These depend on the injection taking place in different regions of the plasma jet  22 . These regions differ by jet velocity, temperature, and plasma composition. Depending on the fluid dynamical mixing of the material streams, multi layers or mixed layers result ( FIG. 3 ). 
         [0047]    In  FIG. 2  there is furthermore schematically shown that the process carried out with the coating head  26  can be extended, To this end a further coating head  27  can be added to the coating apparatus  10 . In the simplest case a plasma process gas  56  and a power supply  58  are provided for this coating head  27  on its feed side  27 E. Furthermore there corresponds to it a third particle supply unit  37 , which in turn has a particle reservoir  15  and a process gas  33 . With the process gas control unit  44  the ratio of process gas  33  and particles from the particle reservoir  15  can be set. By means of an already described supply control device  18  the amount of particles from the particle reservoir  15  can be controlled. Thus a third layer  53  can be deposited onto the second layer  52 . 
         [0048]    In order to form a layer system with more than three layers or a layer system with two or more gradient layers, the coating apparatus  10  may be provided with a further coating head  26  and two injectors  66 ,  68 , which correspond to the one described above, instead of the simple coating head  27  described. 
         [0049]      FIG. 3   a  schematically shows a layered structure which may be formed with a coating apparatus  10  according to  FIG. 2 . Therein a first layer  50 , a second layer  52 , and a third layer  53  have been applied onto the substrate  12 . 
         [0050]      FIG. 3   b  schematically shows a so called compound layer  55 , which may be formed with a coating apparatus  10  according to  FIG. 1  or  2 . Therein the particles from the particle reservoirs  14 ,  16  are mixed by a mixing process ( FIG. 1 ) or by an adequate choice of the supply locations  46 ,  48  in such a way that an as homogeneous as possible distribution of the particle types within the volume of the applied compound layer  55  results. 
         [0051]      FIG. 3   c  schematically shows a gradient layer  54  which can be formed with the coating apparatus  10  according to  FIG. 2 . Therein the supply locations  46 ,  48  are chosen in such a way that the amount of particles in y-direction decreases or increases, respectively. 
         [0052]      FIG. 4  schematically shows that it is possible to create various transitions in the sequence of layers to be applied onto the substrate  12 . To this end the shown sequence of layers is formed during a single coating run through a suitable configuration of the coating apparatus  10 . 
         [0053]    In segment A three different materials with the particles r, s, t are deposited with a fixed ratio onto the substrate as a layer. In segment B, later in time during the same coating process, the layer thickness of the compound layer  55  is reduced continuously, and a cover layer of phase u applied on the compound layer  55 . in segment C the layer thickness of the entire multilayer is reduced, until in segment D the layer is interrupted completely and thus the substrate  12  is not covered by a layer at this location. In segment E the layer thickness of the phase u is increased continuously and in regions F transitions into a gradient layer  54 , in which at the surface of the phase u the material r is embedded at the highest concentration. 
         [0054]      FIG. 5  schematically shows the principle of the design of a gradient layer by means of a depth profile. The material composition starts from a layer material S 1  having the highest concentration at the transition point to the substrate  12 . Towards the surface the layer material S 1  decreases continuously, reaching essentially the value zero at the surface. The layer material S 2  essentially has the value  0  at the transition point to the substrate  12  and continuously increases towards the surface. In the example shown there is a transition region U, in which the layer material S 1  and the layer material S 2  have an essentially equal concentration. 
         [0055]      FIG. 6  shows a particular application of the coating apparatus  10  according to the invention and the method according to the invention for coating a substrate  12  with the example of a conductive layer  74  and an insulating layer  72 . Both layers are applied onto a substrate  12  with the coating apparatus  10 . Therein the conductive layer  74  is applied onto the substrate  12  as a strip-like structure. The conductive strip formed this way is to be protected towards the outside by an insulating layer  72  in the region KO. Therein the insulating layer may be interrupted in the regions K 1  and K 2  to facilitate the formation of a contact. 
         [0056]    The invention has been described with reference to preferred embodiments. It is obvious for the skilled person that changes and modifications can be made to the invention without leaving the scope of the subsequent claims. 
       LIST OF REFERENCE SIGNS 
       [0057]      10  coating apparatus 
         [0058]      12  substrate 
         [0059]      12   a  surface of the substrate 
         [0060]      14  particle reservoir 
         [0061]      15  particle reservoir 
         [0062]      16  particle reservoir 
         [0063]      18  supply control device 
         [0064]      20  electric arc 
         [0065]      22  plasma jet 
         [0066]      23  second plasma jet 
         [0067]      24  transport pipe 
         [0068]      25  second transport pipe 
         [0069]      26  coating head 
         [0070]      26 A exit side 
         [0071]      26 E feed side 
         [0072]      27  second coating head 
         [0073]      27 E feed side 
         [0074]      28  controller 
         [0075]      30  process gas 
         [0076]      32  process gas 
         [0077]      33  process gas 
         [0078]      34  first particle supply unit 
         [0079]      36  second particle supply unit 
         [0080]      37  third particle supply unit 
         [0081]      38  process gas control unit 
         [0082]      40  particle supply unit 
         [0083]      42  process gas control unit 
         [0084]      44  process gas control unit 
         [0085]      46  first supply location 
         [0086]      48  second supply location 
         [0087]      50  first layer 
         [0088]      52  second layer 
         [0089]      53  third layer 
         [0090]      54  gradient layer 
         [0091]      55  compound layer 
         [0092]      56  plasma process gas 
         [0093]      58  power supply 
         [0094]      60  plasma chamber 
         [0095]      62  electrode 
         [0096]      64  electrode 
         [0097]      66  first injector 
         [0098]      68  second injector 
         [0099]      70  third injector 
         [0100]      72  insulating layer 
         [0101]      74  conductive layer 
         [0102]    A, B, C, D, E, F segments of a layer 
         [0103]    r, s, t particles 
         [0104]    S 1  layer material 
         [0105]    S 2  layer material 
         [0106]    U transition region 
         [0107]    K 1  region 
         [0108]    K 2  region 
         [0109]    K 3  region