Patent Publication Number: US-2006013962-A1

Title: Deposition of high melting temperature and variable resistance metal materials on plastic and metal surfaces using a combination of kinetic and thermal spray processes

Description:
INCORPORATION BY REFERENCE  
      U.S. Pat. No. 6,139,913, “Kinetic Spray Coating Method and Apparatus,” and U.S. Pat. No. 6,283,386 “Kinetic Spray Coating Apparatus” are incorporated by reference herein. 
    
    
     TECHNICAL FIELD  
      The present invention is directed to a method for deposition of high melting temperature and variable resistance metal material onto either metal or plastic surfaces and use of the same to create long length strain gauges.  
     RELATED APPLICATIONS  
      NONE.  
     BACKGROUND OF THE INVENTION  
      At the present time there are no efficient methods for depositing high melting temperature metals onto plastics. The plastic materials of interest can be formed objects or surfaces formed from a plastic or a plastic layer over a metal substrate. Thermal spray processes are commonly used to deposit high melting temperature metals onto other metals, however they have been unable to deposit these metals onto plastics. The metals do not adhere to the plastics and cause physical and thermal damage to the plastics. To date methods have included bonding a metal surface to the plastic material and then applying the metal layer to the bonded surface using thermal spray. Alternatively, a series of steadily increasing melting temperature metals are deposited as a series of layers to eventually achieve a high melting temperature metal layer on the plastic material.  
      Another difficulty is detecting stress in structural beams, pipes and conduits, especially if they are buried or covered by other building materials. At the present time there are no satisfactory methods for detecting such stresses so repairs can be accomplished readily.  
      It would be advantageous to develop a simple and rapid method for depositing high melting temperature metals onto plastic materials. In addition, it would be advantageous to develop long length strain gauges to detect stresses in covered materials to allow for easier repair.  
     SUMMARY OF THE INVENTION  
      In one embodiment, the present invention is a method of forming a strain gauge on a metal surface comprising the steps of: applying an electrically insulative layer to a metal surface; kinetically spraying a discontinuous non-electrically conductive pattern of powder particles onto the electrically insulative layer, the particles adhering to the insulative layer and a majority of the particles partially protruding there from; and thermally spraying an electrically conductive metal layer having a variable resistance onto the pattern of powder particles, the metal layer only adhering to the powder particle pattern and the resistance of the metal layer varying as a function of stress in the metal surface.  
      In another embodiment, the present invention is a method of forming a strain gauge on a plastic material surface comprising the steps of: kinetically spraying a discontinuous non-electrically conductive pattern of powder particles onto a surface of a plastic material, the particles adhering to the surface and a majority of the particles partially protruding there from; and thermally spraying an electrically conductive metal layer having a variable resistance onto the pattern of powder particles, the metal layer only adhering to the pattern of powder particles and the resistance of the metal layer varying as a function of stress in the plastic material surface.  
      In another embodiment the present invention is a method of forming strain gauge on a plastic material surface comprising the steps of: applying a discontinuous non-electrically conductive pattern of powder particles onto a surface of a plastic material and applying a compressive force to the pattern of particles, the particles adhering to the surface and a majority of the particles partially protruding there from; and thermally spraying an electrically conductive metal layer having a variable resistance onto the pattern of powder particles, the metal layer only adhering to the pattern of the powder particles and the resistance of the metal layer varying as a function of stress in the plastic material surface.  
      In another embodiment, the present invention is a method of forming a high melting temperature metal layer on a plastic material surface comprising the steps of: kinetically spraying a pattern of powder particles onto a surface of a plastic material, the particles adhering to the surface and a majority of the particles partially protruding there from; and thermally spraying a high melting temperature metal layer onto the pattern of powder particles, the metal layer only adhering to the pattern of powder particles in the plastic material surface and having a melting temperature of at least 400° F.  
      In another embodiment, the present invention is a method of forming a high melting temperature metal layer on a plastic material surface comprising the steps of: applying a pattern of powder particles onto a surface of a plastic material and applying a compressive force to the pattern of particles, the particles adhering to the surface and a majority of the particles partially protruding there from; and thermally spraying a high melting temperature metal layer onto the pattern of powder particles, the metal layer only adhering to the pattern of powder particles in the plastic material surface and having a melting temperature of at least 400° F.  
      In another embodiment, the present invention comprises a method of forming a long length strain gauge comprising the steps of: providing an electrically insulative layer on a surface the surface comprising one of a ceramic surface, a metal surface, or a mixture thereof; kinetically spraying a continuous, electrically conductive layer of powder particles onto the electrically insulative layer, the particles adhering to the insulative layer and a majority of the particles partially protruding there from and the electrical resistance of the powder particles layer varying as a function of stress in the surface.  
      In another embodiment, the present invention comprises a method of forming a strain gauge on a plastic material surface comprising the steps of: kinetically spraying a continuous electrically conductive pattern of powder particles onto a surface of a plastic material, the particles adhering to the surface and a majority of the particles partially protruding there from and the electrical resistance of the powder particles layer varying as a function of stress in the plastic material surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic diagram of a kinetic spray system according to the present invention;  
       FIG. 2  is a cross-sectional view of one embodiment of a supersonic nozzle for use in the kinetic spray system of  FIG. 1 ;  
       FIG. 3  is a cross-sectional view of another embodiment of a supersonic nozzle for use in the kinetic spray system of  FIG. 1 ;  
       FIG. 4  is a schematic diagram illustrating deposition of powder particles onto a plastic material according to the present invention;  
       FIG. 5  is a schematic diagram of one embodiment of the present invention;  
       FIG. 6  is a schematic diagram of another embodiment of the present invention;  
       FIG. 7  is a photomicrograph of a lexan sheet coated according to the present invention;  
       FIG. 8  is a photomicrograph of a lexan sheet coated according to the present invention;  
       FIG. 9  is a schematic diagram of a device for testing the electrical resistance as a function of applied stress of a coating prepared according to the present invention; and  
       FIG. 10  is a graph illustrating the results of a series of tests conducted using a coating prepared according to the present invention and the device shown in  FIG. 9 . 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT  
      Referring first to  FIG. 1 , a kinetic spray system according to the present invention is generally shown at  10 . System  10  includes an enclosure  12  in which a support table  14  or other support means is located. A mounting panel  16  fixed to the table  14  supports a work holder  18  capable of movement in three dimensions and able to support a suitable workpiece formed of a substrate material to be coated. The enclosure  12  includes surrounding walls having at least one air inlet, not shown, and an air outlet  20  connected by a suitable exhaust conduit  22  to a dust collector, not shown. During coating operations, the dust collector continually draws air from the enclosure  12  and collects any dust or particles contained in the exhaust air for subsequent disposal.  
      The spray system  10  further includes a gas compressor  24  capable of supplying gas pressure up to 3.4 MPa (500 psi) to a high pressure gas ballast tank  26 . The gas ballast tank  26  is connected through a line  28  to powder feeder  30  and a separate gas heater  32 . The powder feeder  30  can either be a high pressure powder feeder or a low pressure feeder as described below. The gas heater  32  supplies high pressure heated gas, the main gas described below, to a kinetic spray nozzle  34 . It is possible to provide the nozzle  34  with movement capacity in three directions in addition to or rather than the work holder  18 . The pressure of the main gas generally is set at from 150 to 500 psi. The powder feeder  30  mixes particles of a spray powder with the gas at a desired pressure and supplies the mixture of particles to the nozzle  34 . A computer control  35  operates to control both the pressure of gas supplied to the gas heater  32  and the temperature of the heated main gas exiting the gas heater  32 . Useful gases include air, nitrogen, helium and others.  
       FIG. 2  is a cross-sectional view of one embodiment of the nozzle  34  and its connections to the gas heater  32  and a high pressure powder feeder  30 . A main gas passage  36  connects the gas heater  32  to the nozzle  34 . Passage  36  connects with a premix chamber  38  that directs the main gas through a flow straightener  40  and into a chamber  42 . Temperature and pressure of the heated main gas are monitored by a gas inlet temperature thermocouple  44  in the passage  36  and a pressure sensor  46  connected to the chamber  42 . The main gas has a temperature that is always insufficient to cause melting in the nozzle  34  of any particles being sprayed. The main gas temperature can range from 200 to 3000° F. The main gas temperature can be well above the melt temperature of the particles. Main gas temperatures that are 5 to 7 fold above the melt temperature of the particles have been used in the present system  10 . What is necessary is that the temperature and exposure time to the main gas be selected such that the particles do not melt in the nozzle  34 . The temperature of the gas rapidly falls as it travels through the nozzle  34 . In fact, the temperature of the gas measured as it exits the nozzle  34  is often at or below room temperature even when its initial temperature is above 1000° F.  
      Chamber  42  is in communication with a de Laval type supersonic nozzle  54 . The nozzle  54  has a central axis  52  and an entrance cone  56  that decreases in diameter to a throat  58 . The entrance cone  56  forms the converging region of the nozzle  54 . Downstream of the throat  58  is an exit end  60  and a diverging region is defined between the throat  58  and the exit end  60 . The largest diameter of the entrance cone  56  may range from 10 to 6 millimeters, with 7.5 millimeters being preferred. The entrance cone  56  narrows to the throat  58 . The throat  58  may have a diameter of from 3.5 to 1.5 millimeters, with from 3 to 2 millimeters being preferred. The diverging region of the nozzle  54  from downstream of the throat  58  to the exit end  60  may have a variety of shapes, but in a preferred embodiment it has a rectangular cross-sectional shape. At the exit end  60  the nozzle  54  preferably has a rectangular shape with a long dimension of from 8 to 14 millimeters by a short dimension of from 2 to 6 millimeters. The diverging region can have a length of from about 100 millimeters to about 400 millimeters.  
      In this embodiment the injector tube  50  is aligned with the central axis  52 . An inner diameter of the injector tube  50  can vary between 0.4 to 3.0 millimeters. The nozzle  54  produces an exit velocity of the entrained particles of from 300 meters per second to as high as 1200 meters per second. The entrained particles gain kinetic and thermal energy during their flow through this nozzle  54 . It will be recognized by those of skill in the art that the temperature of the particles in the gas stream will vary depending on the particle size and the main gas temperature. The main gas temperature is defined as the temperature of heated high-pressure gas at the inlet to the nozzle  54 . Since these temperatures are chosen so that they heat the particles to a temperature that is less than the melting temperature of the particles, even upon impact, there is no change in the solid phase of the original particles due to transfer of kinetic and thermal energy, and therefore no change in their original physical properties. The particles themselves are always at a temperature below their melt temperature. The particles exiting the nozzle  54  are directed toward a surface of a substrate to coat it.  
       FIG. 3  is a cross-sectional view of another embodiment of the nozzle  34  and its connections to the gas heater  32  and a low pressure powder feeder  30 . This nozzle  34  differs from that in  FIG. 2  in two ways. First, it is connected to a low pressure powder feeder  30  rather than a high pressure one. Second, the supplement inlet line  48  connects to an injector tube  50  that supplies the particles to the nozzle  54  in the diverging region downstream from the throat  58 , which is a region of reduced main gas pressure. The main gas passage  36  connects the gas heater  32  to the nozzle  34 . Passage  36  connects with a premix chamber  38  that directs the main gas through a flow straightener  40  and into a chamber  42 . Temperature and pressure of the heated main gas are monitored by the gas inlet temperature thermocouple  44  in the passage  36  and the pressure sensor  46  connected to the chamber  42 . The main gas has a temperature that is always insufficient to cause melting in the nozzle  34  of any particles being sprayed. The main gas temperature can range from 200 to 3000° F. The main gas temperature can be well above the melt temperature of the particles. Main gas temperatures that are 5 to 7 fold above the melt temperature of the particles have been used in the present system  10 . What is necessary is that the temperature and exposure time to the main gas be selected such that the particles do not melt in the nozzle  34 . The temperature of the gas rapidly falls as it travels through the nozzle  34 . In fact, the temperature of the gas measured as it exits the nozzle  34  is often at or below room temperature even when its initial temperature is above 1000° F.  
      Chamber  42  is in communication with the de Laval type supersonic nozzle  54 . The nozzle  54  has a central axis  52  and an entrance cone  56  that decreases in diameter to a throat  58 . The entrance cone  56  forms the converging region of the nozzle  54 . Downstream of the throat  58  is the exit end  60  and a diverging region is defined between the throat  58  and the exit end  60 . The largest diameter of the entrance cone  56  may range from 10 to 6 millimeters, with 7.5 millimeters being preferred. The entrance cone  56  narrows to the throat  58 . The throat  58  may have a diameter of from 3.5 to 1.5 millimeters, with from 3 to 2 millimeters being preferred. The diverging region of the nozzle  54  from downstream of the throat  58  to the exit end  60  may have a variety of shapes, but in a preferred embodiment it has a rectangular cross-sectional shape. At the exit end  60  the nozzle  54  preferably has a rectangular shape with a long dimension of from 8 to 14 millimeters by a short dimension of from 2 to 6 millimeters.  
      The angle of the injector tube  50  relative to the central axis  52  can be any that ensures that the particles are directed toward the exit end  60 , basically from 1 to about 90 degrees. It has been found that an angle of 45 degrees relative to central axis  52  works well. An inner diameter of the injector tube  50  can vary between 0.4 to 3.0 millimeters.  
      Using a nozzle  54  as shown in  FIG. 3  having a length of 300 millimeters from throat  58  to exit end  60 , a throat of 2 millimeters and an exit end  60  with a rectangular opening of 5 by 12.5 millimeters the pressure drops quickly as one goes downstream from the throat  58 . The measured pressures were: 14.5 psi at 1 inch after the throat  58 ; 20 psi at 2 inches from the throat  58 ; 12.8 psi at 3 inches from the throat  58 ; 9.25 psi at 4 inches from the throat  58 ; 10 psi at 5 inches from the throat  58  and below atmospheric pressure beyond 6 inches from the throat  58 . The rate at which the main gas pressure decreases is a function of the cross-sectional area of the throat  58  and the cross-sectional area of the diverging region at the point of injection. With a larger throat  58  and the same cross-sectional area of the diverging region the main gas pressure stays above atmospheric for a longer distance. What is necessary is that the powder particles be injected at a point located between the throat  58  and the position in the diverging region where the main gas pressure is at atmospheric pressure so one always uses a positive pressure in the powder feeder  30 . This embodiment allows one to use much lower pressures to inject the powder when the injection takes place after the throat  58 . The low pressure powder feeder  30  of the present invention has a cost that is approximately ten-fold lower than the high pressure powder feeder used with the nozzle  34  of  FIG. 2 . Generally, the low pressure powder feeder  30  is used at a pressure of 100 psi to 5 psi. All that is required is that it exceeds the main gas pressure at the point of injection and that the main gas pressure be above atmospheric.  
      The nozzle  54  produces an exit velocity of the entrained particles of from 300 meters per second to as high as 1200 meters per second. The entrained particles gain kinetic and thermal energy during their flow through this nozzle  54 . It will be recognized by those of skill in the art that the temperature of the particles in the gas stream will vary depending on the particle size and the main gas temperature. The main gas temperature is defined as the temperature of heated high-pressure gas at the inlet to the nozzle  54 . Since these temperatures are chosen so that they heat the particles to a temperature that is less than the melting temperature of the particles, even upon impact, there is no change in the solid phase of the original particles due to transfer of kinetic and thermal energy, and therefore no change in their original physical properties. The particles themselves are always at a temperature below their melt temperature. The particles exiting the nozzle  54  are directed toward a surface of a substrate to coat it.  
      The powder particles used for kinetic spraying in accordance with the present invention generally comprise metals, alloys, ceramics, diamonds and mixtures of these particles. The particles may have an average nominal diameter of from greater than 50 microns to about 200 microns. Preferably the particles have an average nominal diameter of from 50 to 180 microns.  
      Preferably the main gas pressure using either embodiment of the nozzle  34  is set at from 200 to 400 psi and the main gas temperature is preferably from 200 to 3000° F. Preferably when using the nozzle  34  shown in  FIG. 2  the pressure of gas used in the high pressure powder feeder  30  is from 25 to 75 psi above the main gas pressure as measured at the pressure sensor  46 . The stand off distance between the exit end  60  and the substrate is preferably from 0.5 to 12 inches, more preferably from 0.5 to 7 inches and most preferably from 0.5 to 3 inches. The traverse rate of the nozzle  34  and the substrate relative to each other is preferably from 25 to 2500 millimeters per second, more preferably from 25 to 250 millimeters per second, and most preferably from 50 to 150 millimeters per second. Preferably the powder particles are feed to the nozzle  34  at a rate of from about 10 to 60 grams per minute. The preferred particle velocities range from about 300 to 1200 meters per second.  
      In the present invention the kinetic spray process as described above is used in combination with thermal spray technology to coat materials with high temperature metals or ceramics that are difficult or impossible to coat using thermal spray technology alone. Thermal spray systems are well know in the art and will not be described in detail. The key difference between thermal spray and kinetic spray is that in all thermal spray systems the particles emerge from the thermal spray system in a molten state prior to striking a substrate. Upon striking the substrate these molten particles splat as they strike the substrate and stick under the proper conditions. One of the drawbacks with thermal spray technology has been the inability to use it to coat plastic materials with high melting temperature metals or ceramics. The reason is that the molten particles damage the plastic material and generally do not adhere. As used in the present specification and claims a plastic material is broadly defined as polymers that can be formed or molded under heat or pressure and may be either a thermosetting plastic or a thermoforming plastic. Such materials include, but are not limited to: fluorocarbon resins, nylons, phenolics, polyimides, silicones, cellulosics, polyethylenes, polypropylenes, polybutylenes, polyarcrylics, polymethacrylics, polystyrenes, polyurethanes, acetals, polycarbonates, acrylonitrile-butadiene-styrenes, polyvinychlorides, epoxies, and terephthalates.  
      A significant advantage of the present invention is that the thermally sprayed metals will only stick to the kinetically sprayed particles that adhere to the substrate and not directly to the substrates of interest, thus one does not require a mask or extensive post coating modifications. This is important because applying masking materials can be a costly process. Use of the present invention allows one to coat plastic materials with high melting temperature metals or ceramics that have melting temperatures of 400° F. or greater. For the present invention twin wire arc thermal spray processes are especially useful, although other thermal spray processes such as plasma thermal spray, flame spray and high velocity oxy-fuel spray can be used. These high melting temperature materials can also be used to form long length strain gauges as described below.  
       FIG. 4  is a schematic diagram showing deposition of powder particles onto a plastic material by a kinetic spray process. The plastic material substrate is shown at  100 . The particles  102  were kinetically sprayed at too high of a particle velocity and thus most penetrated through a surface  104  of the plastics material  100 , this is not according to the present invention. The particles  106  were kinetically sprayed at too low of a particle velocity and the majority do not adhere to the plastics material  100 . The particles  108  were kinetically sprayed according to the present invention and show the proper penetration and adherence to the surface  104 . The particles  108  penetrate the plastic material  100  sufficiently to adhere while still protruding from the surface  104 . Importantly, the particles  108  do not penetrate all the way through the plastic material  100 , but only partially so a good portion of the particle  108  remains above the surface  104 . The required particle velocity is determined by the material of the particle powder, their average nominal diameter, and the hardness of the plastic material.  
       FIG. 5  is a schematic diagram of one embodiment of the present invention. In this embodiment a plastic material  100  has been initially coated with powder particles  108  by a kinetic spray process. Then a twin wire arc thermal spray system is used to apply a high melting temperature metal or ceramic layer  110 . The layer  110  can comprise a metal, an alloy, a ceramic or mixtures thereof. An optional final layer is an outer protective layer  112 . The outer protective layer can also be a plastic material, insulation or other coating.  
       FIG. 6  is a schematic diagram of another embodiment of the present invention. In this embodiment a surface  120  comprising a metal or a ceramic material is initially coated with an insulative layer  122 , which may be a plastic material. Then powder particles  108  are deposited using a kinetic spray process. After the kinetic spray process the powder particles  108  are covered with a high melting temperature metal or ceramic layer  110  via a thermal spray process. Finally, an optional layer is an outer protective layer  112 .  
      The two embodiments shown in  FIGS. 5 and 6  are meant to generally illustrate the invention. The surface  120  can be formed from a metal, an alloy, a ceramic, or a surface that is a mixture of these. The insulative layer  122  is preferably an electrically insulative layer formed from a plastic material or a ceramic such as a plasma thermal spray applied layer of alumina. The invention finds at least two principal uses: as a method of creating long length strain gauges; and as a method for coating plastics with high melting temperature metals or ceramics.  
      In one embodiment the invention can be used to form long length strain gauges that can be use to monitor stress in surfaces. By selecting as the material for the layer  110  a metal that has a variable electrical resistance a strain gauge can be created. In this use the plastic material  100  or the electrically insulative layer  122  is first coated with powder particles  108  by a kinetic pray process, examples below. The kinetic spray parameters are adjusted to provide for a discontinuous non-electrically conducting distribution of the powder particles  108  onto the plastic material  100  or the insulative layer  122 . The kinetic spray process can spray lines that have a width of as little as 2 millimeters. Then a thermal spray process, preferably twin wire arc, is used to deposit a metal layer  110  using a metal that has a variable electrical resistance onto the powder particles  108 . Examples of these metals include copper, copper alloys, nickel chrome alloys and others. A unique feature of the present invention is that the thermally sprayed particles only adhere where the powder particles  108  have been deposited. The thermally sprayed metal layer  110  will not adhere to bare plastic material  100  or the bare insulative layer  122 . This eliminates the need for masking. As shown below, stress in the plastic material  100  or the metal surface  120  caused by bending can lead to a measurable change in the electrical resistance and electrical conductance of the thermal sprayed metal layer  110  and can be correlated with a stress value. Thus by continuously or periodically measuring the conductance or resistance of the layer  110  one can detect stress in the plastic material  100  or layer  120 . This can find special use in very long length strain gauges such as for pipes, conduits, structural beams, support structures and other metal or ceramic surfaces  120  and plastic materials  100 . When used on pipes, for example, it can be used to detect stress over long distances of 50 feet or more even if the pipe is buried. On structural beams it can be used to detect stress in buildings and bridges and other structures. As can be seen irrespective of whether the surface is a plastic material  100  or a metal or a ceramic layer  120  the present invention can be used to create unique long length strain gauges. Preferably, when coating pipes or beams the kinetic spray pattern and thermal spray pattern is helical around the pipe or beam.  
      As an alternative to kinetic spray applying the powder particles  108  one can also deposit the powder particles  108  onto the plastic material  100  or the insulative layer  122  in the desired pattern and then subject it to compressive force of from about 2000 to 5000 pounds. The particles  108  then adhere to the plastic material  100  or insulative layer  122  and can then be coated with the metal layer  110 . It can be advantageous to pre-heat the plastic material  100  or insulative layer  122  prior to application of the powder particles  108 . Again the thermally sprayed metal layer  110  only adheres to the pattern of the powder particles  108 .  
      A third method for creating a long length strain gauge is to increase the density of the kinetically sprayed powder particle  108  deposit until it is continuous and electrically conductive and to use a powder particle  108  material that has a variable electrical resistance. In this embodiment there is no need for a second thermally sprayed layer. One begins with a plastic material  100  or an insulative layer  122  on a metal or a ceramic and then applies powder particles  108  using a metal, an alloy, or a mixture thereof onto the plastic material  100  or insulative layer  122 . The density can be increased by slowing the transverse rate, increasing the powder feed rate, or increasing the number of deposit passes.  
      In a second embodiment of the present invention it can be used to deposit high melting temperature metals or ceramics onto plastic materials  100  or insulative layers  122 . The same deposition techniques described above can be used for the powder particles  108 , namely either kinetic spray or compression. The difference is that the powder particle  108  layer does not necessarily need to be discontinuous and electrically non-conductive, but it can be. Then the layer  110  of metal or ceramic or a mixture thereof is deposited using a thermal spray process. Preferably the process is used to deposit metals or ceramics having melting temperatures of 400° F. or greater. Such deposits using thermal spray were not previously obtainable on plastic materials or insulative layers.  
      These deposited metal layers  110  can be used to form heating elements by connecting the metal layer  110  to an electrical source. Examples included forming a heated steering wheel, by coating a plastic wheel shell with copper by a kinetic spray process and then covering this with nickel chrome using a thermal spray process. This allows for a heated steering wheel. Similarly a plastic panel can be coated with copper using a kinetic spray process followed by a deposit of nickel chrome using a thermal spray process to form a heater panel.  
       FIG. 7  is a photomicrograph showing an example of the present invention. A lexan plastic material  130  was initially sprayed with copper by a thermal spray process using a nozzle  34  like that shown in  FIG. 2 . The kinetic spray parameters were as follows: a main gas pressure of 300 psi, powder particle gas pressure of 350 psi, main gas temperature of 800° F., traverse speed 100 millimeters per second, powder feed rate of 12 grams per minute, powder particle nominal average diameter of from greater than 50 to 106 microns, and a stand off distance of 1 inch. The deposited copper powder particles  132  formed a discontinuous non-conducting pathway on the lexan  130  with well defined edges. The twin wire arc thermal spray was used to deposit a metal layer  134  over the powder particles  132 . The twin wire arc parameters were as follows: nickel chrome wires were used, arc voltage of 32 V, arc current of 140 A, traverse speed of 1200 millimeters per second, stand off distance of 8 inches, atomizing air of 130 psi and cooling air of 90 psi. If copper is used to form the metal layer  134  then the only changes to the thermal spray parameters are to reduce the arc voltage to 29 V and to increase the current to 200 A. The thermal spray was passed over the lexan  130  for either 3 or 4 passes as noted. Several important findings emerge. First, the metal layer  134  is continuous, electrically conductive and has variable resistance. Second, the metal layer  134  only adhered to the powder particle  132  pathway and no where else on the lexan  130 .  
       FIG. 8  is an example of the present invention using a nozzle as shown in  FIG. 3 . A lexan plastic material  140  was initially sprayed with tin powder particles  142 . The kinetic spray parameters were as follows: a main gas pressure of 300 psi, powder particle gas pressure of 50 psi, main gas temperatures of 800° F., traverse speed of 100 millimeters per second, powder feed rate of 54 grams per minute, powder particle nominal average diameter of greater than 63 to 90 microns or 100 to 177 microns, and a stand off distance of 1 inch. The deposited tin powder particles  142  formed a discontinuous non-conducting pathway on the lexan  140  with defined edges. The twin wire arc thermal spray was used to deposit a metal layer  144  over the powder particles  132 . The twin wire arc parameters were as follows: nickel chrome wires were used, arc voltage of 32 V, arc current of 140 A, traverse speed of 1200 millimeters per second, stand off distance of 8 inches, atomizing air of 130 psi and cooling air of 90 psi. If copper is used to form the metal layer  144  then the only changes to the thermal spray parameters are to reduce the arc voltage to 29 V and to increase the current to 200 A. The thermal spray was passed over the lexan  130  for either 3 or 4 passes as noted. Several important findings emerge. First, the metal layer  144  is continuous, electrically conductive and has variable resistance. Second, the metal layer  144  only adhered to the powder particle  142  pathway and no where else on the lexan  140 .  
      In  FIG. 9   a  three point bend stress/strain device is shown generally at  150 . A metal plate  151  is supported on two supports  152 . A plastic plate  154 B is attached with epoxy to a bottom side of the metal plate  151  and another plastic plate  154 T is attached with epoxy to a top side of the metal plate  151 . Powder particles  156  are applied in a discontinuous non-electrically conducting array by the kinetic spray process described above to each plastic plate  154 B and  154 T. A thermal spray process is been used to deposit a metal layer  158 B and  158 T over the powder particles  156  on the plates  154 B and  154 T respectively. A protective film  112  is applied to each metal layer  158 B and  158 T. An applied downward force  164  puts the top metal coating  158 T in compression and the bottom metal coating  158 B in tension. Spaced apart electrical probes  160 T and  162 T are place on the top metal layer  158 T. Spaced apart electrical probes  160 B and  162 B are also placed on the bottom metal layer  158 B. The electrical resistance or alternatively the electrical conductance between the two top probes  160 T and  162 T and the electrical resistance or alternatively the electrical conductance between the two bottom probes  160 B and  162 B are measured as the force  164  is applied. The force  164  introduces both stress and strain into the layers  158 B and  158 T.  
       FIG. 10  is a graph of results obtained using the device  150  shown in  FIG. 9 . An aluminum plate had a lexan substrate epoxyed to its top and bottom sides. Each of the lexan substrates were kinetic spray coated with copper powder particles as described above. Then a metal layer of nickel chrome was deposited using a twin wire arc thermal spray process onto the powder particles. For each side the distance between the electrical probes was 70 millimeters. Reference line  172  shows the electrical resistance of the top metal layer, which is in compression, as the force was increased. The results show that as the applied force increased the electrical resistance decreased. Reference line  174  shows the electrical resistance of the bottom metal layer, which is in tension, as the force was increased. The results show that as the applied force increased the electrical resistance increased. Reference line  175  shows a plot of the measured strain while reference line  176  is a linear fit to the data shown in reference line  175 .  
      The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and do come within the scope of the invention. Accordingly, the scope of legal protection afforded this invention can only be determined by studying the following claims.