Abstract:
Disclosed is a system and a method for applying both a kinetic spray applied coating layer and a thermal spray applied layer onto a substrate using a single application nozzle. The system includes a higher heat capacity gas heater to permit oscillation between a kinetic spray mode wherein the particles being applied are not thermally softened and a thermal spray mode wherein the particles being applied are thermally softened prior to application. The system increases the versatility of the spray nozzle and addresses several problems inherent in kinetic spray applied coatings.

Description:
TECHNICAL FIELD  
         [0001]    The present invention is a method and an apparatus for applying a coating to a substrate, and more particularly, to a method and an apparatus for applying both a kinetic spray coating and a thermal spray coating from the same nozzle.  
         BACKGROUND OF THE INVENTION  
         [0002]    A new technique for producing coatings on a wide variety of substrate surfaces by kinetic spray, or cold gas dynamic spray, was recently reported in articles by T.H. Van Steenkiste et al., entitled “Kinetic Spray Coatings,” published in Surface and Coatings Technology, vol. 111, pages 62-71, Jan. 10, 1999 and “Aluminum coatings via kinetic spray with relatively large powder particles” published in Surface and Coatings Technology 154, pages 237-252, 2002. The articles discuss producing continuous layer coatings having low porosity, high adhesion, low oxide content and low thermal stress. The articles describe coatings being produced by entraining metal powders in an accelerated air stream, through a converging-diverging de Laval type nozzle and projecting them against a target substrate. The particles are accelerated in the high velocity air stream by the drag effect. The air used can be any of a variety of gases including air or helium. It was found that the particles that formed the coating did not melt or thermally soften prior to impingement onto the substrate. It is theorized that the particles adhere to the substrate when their kinetic energy is converted to a sufficient level of thermal and mechanical deformation. Thus, it is believed that the particle velocity must be high enough to exceed the yield stress of the particle to permit it to adhere when it strikes the substrate. It was found that the deposition efficiency of a given particle mixture was increased as the inlet air temperature was increased. Increasing the inlet air temperature decreases its density and increases its velocity. The velocity varies approximately as the square root of the inlet air temperature. The actual mechanism of bonding of the particles to the substrate surface is not fully known at this time. It is believed that the particles must exceed a critical velocity prior to their being able to bond to the substrate. The critical velocity is dependent on the material of the particle and to a lesser degree on the material of the substrate. It is believed that the initial particles to adhere to a substrate have broken the oxide shell on the substrate material permitting subsequent metal to metal bond formation between plastically deformed particles and the substrate. Once an initial layer of particles has been formed on a substrate subsequent particles bind not only to the voids between previous particles bound to the substrate but also engage in particle to particle bonds. The bonding process is not due to melting of the particles in the air stream because while the temperature of the air stream may be above the melting point of the particles, due to the short exposure time the particles are never heated to a temperature above their melt temperature. This feature is considered critical because the kinetic spray process allows one to deposit particles onto a surface with out a phase transition.  
           [0003]    This work improved upon earlier work by Alkimov et al. as disclosed in U.S. Pat. No. 5,302,414, issued Apr. 12, 1994. Alkimov et al. disclosed producing dense continuous layer coatings with powder particles having a particle size of from 1 to 50 microns using a supersonic spray.  
           [0004]    The Van Steenkiste articles reported on work conducted by the National Center for Manufacturing Sciences (NCMS) and by the Delphi Research Labs to improve on the earlier Alkimov process and apparatus. Van Steenkiste et al. demonstrated that Alkimov&#39;s apparatus and process could be modified to produce kinetic spray coatings using particle sizes of greater than 50 microns.  
           [0005]    The modified process and apparatus for producing such larger particle size kinetic spray continuous layer coatings are disclosed in U.S. Pat. Nos. 6,139,913, and 6,283,386. The process and apparatus described provide for heating a high pressure air flow and combining this with a flow of particles. The heated air and particles are directed through a de Laval-type nozzle to produce a particle exit velocity of between about 300 m/s (meters per second) to about 1000 m/s. The thus accelerated particles are directed toward and impact upon a target substrate with sufficient kinetic energy to impinge the particles to the surface of the substrate. The temperatures and pressures used are sufficiently lower than that necessary to cause particle melting or thermal softening of the selected particle. Therefore, as discussed above, no phase transition occurs in the particles prior to impingement. It has been found that each type of particle material has a threshold critical velocity that must be exceeded before the material begins to adhere to the substrate by the kinetic spray process.  
           [0006]    One difficulty associated with all of these prior art kinetic spray systems arises from defects in the substrate surface. When the surface has an imperfection in it the kinetic spray coating may develop a conical shaped defect over the surface imperfection. The conical defect that develops in the kinetic spray coating is stable and can not be repaired by the kinetic spray process, hence the piece must be discarded. A second difficulty arises when the substrate is a softer plastic or a soft ceramic composite. These materials can not be coated by a kinetic spray process because the particles being sprayed bury themselves below the surface rather than deforming and adhering to the surface.  
         SUMMARY OF THE INVENTION  
         [0007]    In one embodiment, the present invention is a method of coating a substrate comprising the steps of: providing particles of a material to be sprayed; providing a supersonic nozzle having a throat located between a converging region and a diverging region, directing a flow of a gas through the nozzle, and injecting the particles into the nozzle and entraining the particles in the flow of the gas; maintaining the gas at a temperature insufficient to heat the particles to a temperature at or above their melting temperature in the nozzle and accelerating the particles to a velocity sufficient to result in adherence of the particles on a substrate positioned opposite the nozzle; and maintaining the gas at a temperature sufficiently high to heat the particles to a temperature at or above their melting temperature in the nozzle thereby melting the particles and entraining the molten particles in the flow of the gas and directing the entrained molten particles at a substrate positioned opposite the nozzle. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0009]    [0009]FIG. 1 is a generally schematic layout illustrating a kinetic spray system for performing the method of the present invention;  
         [0010]    [0010]FIG. 2 is an enlarged cross-sectional view of one embodiment of a kinetic spray nozzle used in the system; and  
         [0011]    [0011]FIG. 3 is an enlarged cross-sectional view of an alternative embodiment of a kinetic spray nozzle used in the system. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]    The present invention comprises an improvement to the kinetic spray process as generally described in U.S. Pat. Nos. 6,139,913, 6,283,386 and the articles by Van Steenkiste, et al. entitled “Kinetic Spray Coatings” published in Surface and Coatings Technology Volume III, Pages 62-72, Jan. 10, 1999, and “Aluminum coatings via kinetic spray with relatively large powder particles” published in Surface and Coatings Technology 154, pages 237-252, 2002 all of which are herein incorporated by reference.  
         [0013]    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.  
         [0014]    The spray system  10  further includes an air compressor  24  capable of supplying air pressure up to 3.4 MPa (500 psi) to a high pressure air ballast tank  26 . The air ballast tank  26  is connected through a line  28  to both a powder feeder  30  and a separate air heater  32 . The air heater  32  supplies high pressure heated air, the main gas described below, to a kinetic spray nozzle  34 . The powder feeder  30  mixes particles of a spray powder with unheated air and supplies the mixture to a supplemental inlet line  48  of the nozzle  34 . A computer control  35  operates to control both the pressure of air supplied to the air heater  32  and the temperature of the heated main gas exiting the air heater  32 . The main gas can comprise air, argon, nitrogen helium and other inert gases.  
         [0015]    [0015]FIG. 2 is a cross-sectional view of one embodiment of the nozzle  34  and its connections to the air heater  32  and the supplemental inlet line  48 . A main air passage  36  connects the air heater  32  to the nozzle  34 . Passage  36  connects with a premix chamber  38  which directs air through a flow straightener  40  and into a mixing chamber  42 . Temperature and pressure of the air or other heated main gas are monitored by a gas inlet temperature thermocouple  44  in the passage  36  and a pressure sensor  46  connected to the mixing chamber  42 .  
         [0016]    This embodiment of the nozzle  34  requires a high pressure powder feeder  30 . With this nozzle  34  and supplemental inlet line  48  set up the powder feeder  30  must have pressure sufficient to overcome that of the heated main gas. The mixture of unheated high pressure air and coating powder is fed through the supplemental inlet line  48  to a powder injector tube  50  comprising a straight pipe having a predetermined inner diameter. When the particles have an average nominal diameter of from 50 to 106 microns it is preferred that the inner diameter of the tube  50  range from 0.4 to 3.0 millimeters. When larger particles of 106 to 250 microns are used it is preferable that the inner diameter of the tube 50 range from 0.40 to 0.90 millimeters. The tube  50  has a central axis  52  that is preferentially the same as the axis of the premix chamber  38 . The tube  50  extends through the premix chamber  38  and the flow straightener  40  into the mixing chamber  42 .  
         [0017]    Mixing chamber  42  is in communication with the de Laval type supersonic nozzle  54 . The nozzle  54  has an entrance cone  56  that forms a converging region which decreases in diameter to a throat  58 . Downstream of the throat is a diverging region that ends in an 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 portion 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. When particles of from 50 to 106 microns are used the length from the throat  58  to the exit end  60  can range from 60.0 to 80.0 millimeters, however, when particles of from 106 to 250 microns are used then preferably the distance from the throat  58  to the exit end  60  ranges from 200.0 to 400.0 millimeters. 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.  
         [0018]    As disclosed in U.S. Pat. Nos. 6,139,913 and 6,283,386 the powder injector tube  50  supplies a particle powder mixture to the system  10  under a pressure in excess of the pressure of the heated main gas from the passage  36  using the nozzle  54  shown in FIG. 2. 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 .  
         [0019]    [0019]FIG. 3 is a cross-sectional view of another embodiment of the nozzle  34  and its connections to the air heater  32  and the powder feeder  30 . A main air passage  36  connects the air heater  32  to the nozzle  34 . Passage  36  connects with a premix chamber  38  that directs air through a flow straightener  40  and into a chamber  42 . Temperature and pressure of the air or other 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 .  
         [0020]    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 a 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.  
         [0021]    The de Laval nozzle  54  of FIG. 3 is modified from the embodiment shown in FIG. 2 in the diverging region. In this embodiment, a mixture of heated or unheated low pressure air and coating powder is fed from the powder feeder  30  through one of a plurality of supplemental inlet lines  48  each of which is connected to a powder injector tube  50  comprising a tube having a predetermined inner diameter, described above. For simplicity the actual connections between the powder feeder  30  and the inlet lines  48  are not shown. The injector tubes  50  supply the particles to the nozzle  54  in the diverging region downstream from the throat  58 , which is a region of reduced pressure, hence, in this embodiment the powder feeder  30  can be a low pressure powder feeder, discussed below. The length of the nozzle  54  from the throat  58  to the exit end can vary widely and typically ranges from 100 to 400 millimeters.  
         [0022]    As would be understood by one of ordinary skill in the art the number of injector tubes  50 , the angle of their entry relative to the central axis  52  and their position downstream from the throat  58  can vary depending on any of a number of parameters. In FIG. 3 ten injector tubes  50  are show, but the number can be as low as one and as high as the available room of the diverging region. The angle 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. As for the embodiment of FIG. 2, the inner diameter of the injector tube  50  can vary between 0.4 to 3.0 millimeters. The use of multiple injector tubes  50  in this nozzle  54  permits one to easily modify the system  10 . One can rapidly change particles by turning-off a first powder feeder  30  connected to a first injector tube  50  and the turning on a second powder feeder  30  connected to a second injector tube  50 . Such a rapid change over is not easily accomplished with the embodiment shown in FIG. 2. For simplicity only one powder feeder  30  is shown in FIG. 1, however, as would be understood by one of ordinary skill in the art, the system  10  could include a plurality of powder feeders  30 . The nozzle  54  of FIG. 3 also permits one to mix a number of powders in a single injection cycle by having a plurality of powder feeders  30  and injector tubes  50  functioning simultaneously. An operator can also run a plurality of particle populations, each having a different average nominal diameter, with the larger population being injected closer to the throat  58  relative to the smaller size particle populations and still get efficient deposition. The nozzle  54  of FIG. 3 will permit an operator to better optimize the deposition efficiency of a particle or mixture of particles. For example, it is known that harder materials have a higher critical velocity, therefore in a mixture of particles the harder particles could be introduced at a point closer to the throat  58  thereby giving a longer acceleration time.  
         [0023]    Using a de Laval nozzle  54  like that 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 . These results show why one can use much lower pressures to inject the powder when the injection takes place after the throat  58 . The low pressure powder feeder  30  that can be used with the nozzle  54  of FIG. 3 has a cost that is approximately ten-fold lower than the high pressure powder feeders  30  that need to be used with the nozzle  34  of FIG. 2. Generally, the low pressure powder feeder  30  is used at a pressure of 100 psi or less. All that is required is that it exceed the main gas pressure at the point of injection.  
         [0024]    The system  10  of the present invention differs from the prior art systems because it can operate in two modes. In a first mode it operates as a typical kinetic spray system. In a second mode it operates as a thermal spray system. This dual mode capacity is made possible by using an air heater  32  that is capable of achieving higher temperatures than a typical kinetic spray system. This higher capacity air heater  32  may require that the main air passage  36 , supplemental inlet lines  48 , tubes  50  and nozzle  34  be made of high heat resistant materials.  
         [0025]    When operating in the kinetic spray mode the computer control  35  and the thermocouple  44  interact to monitor and maintain the main gas at a temperature that is always insufficient to cause melting in the nozzle  34  of any particles being sprayed. Even in this mode, the main gas temperature can be well above the melt temperature of the particles and may range from at least 300 to at least 3000 degrees Celsius. 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.  
         [0026]    Since in the kinetic mode the temperature of the particles is always less than the melting point of the particles, even upon impact on a substrate placed opposite the nozzle  34 , 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.  
         [0027]    Upon striking a substrate opposite the nozzle  54  the kinetic sprayed particles flatten into a nub-like structure with an aspect ratio of generally about 5 to 1. When the substrate is a metal and the particles are a metal the particles striking the substrate surface fracture the oxidation on the surface layer and subsequently form a direct metal-to-metal bond between the metal particle and the metal substrate. Upon impact the kinetic sprayed particles transfer substantially all of their kinetic and thermal energy to the substrate surface and stick if their yield stress has been exceeded. As discussed above, for a given particle to adhere to a substrate during the kinetic spray mode it is necessary that it reach or exceed its critical velocity which is defined as the velocity where at it will adhere to a substrate when it strikes the substrate after exiting the nozzle. This critical velocity is dependent on the material composition of the particle. In general, harder materials must achieve a higher critical velocity before they adhere to a given substrate. It is not known at this time exactly what is the nature of the particle to substrate bond; however, it is believed that a portion of the bond is due to the particles plastically deforming upon striking the substrate.  
         [0028]    As disclosed in U.S. Pat. No. 6,139,913 the substrate material may be comprised of any of a wide variety of materials including a metal, an alloy, a semi-conductor, a ceramic, a plastic, and mixtures of these materials. Other substrates include wood and paper. All of these substrates can be coated by the process of the present invention in either mode of operation. The particles used in the present invention may comprise any of the materials disclosed in U.S. Pat. Nos. 6,139,913 and 6,283,386 in addition to other known particles. These particles generally comprise metals, alloys, ceramics, polymers, diamonds and mixtures of these. Preferably the particles used have an average nominal diameter of from 60 to 250 microns. Mixtures of different sized or different material compositions of particles can also be used in the system  10  either by providing them as a mixture or using multiple tubes  50  and the nozzle  54  shown in FIG. 3.  
         [0029]    When the system  10  is operating in the thermal spray mode the computer control  35  and the thermocouple  44  interact to monitor and maintain the main gas at a temperature that is always sufficient to cause melting in the nozzle  34  of any particles being sprayed. Thus, the particles exit the nozzle  34  in a molten state and strike the substrate while molten. After striking the substrate the molten particles flatten and adhere to the substrate. The system  10  allows one to thermally spray the same types of particles onto the same types of substrates. During a given coating operation the system  10  can be oscillated between the two modes as desired. Preferably when in the thermal spray mode the system  10  heats the particles to a temperature of from the melting point of the particles to 400 degrees Celsius above the melting point of the particles, more preferably from the melting point of the particles to 200 degrees Celsius above the melting point of the particles, and most preferably from the melting point of the particles to 100 degrees Celsius above the melting point of the particles. To accomplish this the air heater  32  is selected to have a higher heating capacity. The air heater  32  can comprise any of a number of designs including a thermal plasma heater, it may include a combustion chamber, and it may be a high temperature resistive heater element. All of these systems are known in the art. The air heater  32  just needs to be able to oscillate between the kinetic spray mode and the thermal spray mode and to be able to heat the particles to temperatures above their melt points during their passage through the nozzle  34  for the thermal spray mode.  
         [0030]    The system  10  permits a user to solve two difficulties with conventional kinetic spray systems, namely healing defective kinetic spray coatings and permitting kinetic spray coatings on softer materials. As discussed in the background above, one problem with kinetic spray systems is that if the substrate surface has any defects or imperfections these can cause conical defects in the kinetic spray applied coating. The defects appear as a right circular cone. This defect is stable in that with continued kinetic spray application the defect just becomes more evident. With a typical kinetic spray system the coating would have to be discarded and a new one begun. With the present system  10  this problem can be solved in two ways. First, the substrate can be sprayed initially in the thermal spray mode to provide a thin coating that covers the surface defects and provides a better surface, which allows kinetically sprayed particles to plastically deform and bond to the better surface, then the system  10  can be switched into the kinetic spray mode to build up a kinetic spray coating on the substrate. Second, should defects become evident during the coating process while the system  10  is operating in the kinetic spray mode, the system  10  can be oscillated into the thermal spray mode to “heal” the defect by filling it in and then the system  10  can be returned to the kinetic spray mode. In this fashion, because the time in the thermal spray mode is relatively short, the substrate is not subjected to the large thermal stresses that can occur with prolonged thermal spray application. Some of this thermal stress would be relieved by the subsequent peening effect of the kinetically sprayed particles.  
         [0031]    The system  10  also allows a user to apply a kinetic spray coating to soft materials. Such materials may comprise certain plastics and ceramic composites. With a conventional kinetic spray system some of these materials can not be coated because the particles tend to bury themselves below the surface of the substrate rather than plastically deforming and coating the substrate. With the present system  10  a user initially applies a thin coating of the particles in the thermal spray mode and then oscillates to the kinetic spray mode to complete the coating.  
         [0032]    An additional advantage of the nozzle  54  shown in FIG. 3 is that by injecting the particles after the throat  58  the potential for plugging the throat  58  is avoided. Plugging of the throat  58  can occur with the nozzle  54  design shown in FIG. 2.  
         [0033]    While the preferred embodiment of the present invention has been described so as to enable one skilled in the art to practice the present invention, it is to be understood that variations and modifications may be employed without departing from the concept and intent of the present invention as defined in the following claims. The preceding description is intended to be exemplary and should not be used to limit the scope of the invention. The scope of the invention should be determined only by reference to the following claims.