Abstract:
Disclosed is a method and a nozzle for a kinetic spray system that uses much lower powder pressures than previously used in kinetic spray systems. The method permits one to significantly decrease the cost of the powder delivery portion of the system, to run the system at higher temperatures for increased deposition efficiency and to eliminate clogging of the nozzle. The nozzle is a supersonic nozzle having a throat located between a converging region and a diverging region, with the diverging region defined between the throat and an exit end. At least one injector is positioned between the throat and the exit end with the injector in direct communication with the diverging region. The powder particles to be sprayed are injected through the at least one injector and entrained in a gas flowing through the nozzle. The entrained particles are accelerated to a velocity sufficient to cause them to adhere to a substrate positioned opposite the nozzle.

Description:
TECHNICAL FIELD  
         [0001]    The present invention is directed to a method and nozzle for producing a coating using a kinetic spray system with much lower powder pressures than previously used. The invention permits one to significantly decrease the cost of the powder delivery system, to run the system at higher temperatures for increased deposition efficiency and to eliminate clogging of the nozzle.  
         INCORPORATION BY REFERENCE  
         [0002]    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.  
         BACKGROUND OF THE INVENTION  
         [0003]    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 an article by T. H. Van Steenkiste et al., entitled “Kinetic Spray Coatings,” published in Surface and Coatings Technology, vol. 111, pages 6271, Jan. 10, 1999. The article discusses producing continuous layer coatings having low porosity, high adhesion, low oxide content and low thermal stress. The article describes 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. It is believed that the initial particles to adhere to a metal or alloy 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 the subsequent particles both bind to the voids between previously bound particles and also engage in particle to particle bonds. The bonding process is not due to melting of the particles in the air stream because the temperature of the air stream and the time of exposure to the heated air are selected to ensure that the temperature of the particles is always below their melting temperature.  
           [0004]    That work had 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.  
           [0005]    The Van Steenkiste article reported on work conducted by the National Center for Manufacturing Sciences (NCMS) 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 and up to about 106 microns.  
           [0006]    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 provide for heating a high pressure air flow up to about 650° C. 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 lower than that necessary to cause particle melting or thermal softening of the selected particle. Therefore, 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. The disclosed method did not disclose the use of particles in excess of 106 microns.  
           [0007]    There are several difficulties associated with current kinetic spray systems. First, the powder is injected into the heated main gas stream prior to passage through the de Laval nozzle. Because the heated main gas stream is under high pressure injection of the powder requires high pressure powder delivery systems, which are quite expensive. Second, the powder particles and heated main gas both must pass through the throat of the nozzle and the particles frequently plug a portion of the diverging section and the nozzle throat, which requires a complete shutdown of the system and cleaning of the nozzle. Finally, for a given material the main gas temperature must be sufficiently low that it does not result in melting of the particles and significant plugging of the nozzle, which may not be an ideal temperature for efficient deposition.  
         SUMMARY OF THE INVENTION  
         [0008]    In one embodiment the present invention is a method of kinetic spray 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, the gas having a temperature insufficient to cause melting of the particles in the nozzle; and injecting the particles directly into the diverging region of the nozzle at a point after the throat, entraining the particles in the flow of the gas and accelerating the particles to a velocity sufficient to result in adherence of the particles on a substrate positioned opposite the nozzle.  
           [0009]    In another embodiment the present invention is a supersonic nozzle for a kinetic spray system comprising: a throat located between a converging region and a diverging region, the diverging region defined between the throat and an exit end; and at least one injector positioned between the throat and the exit end, the injector in direct communication with the diverging region.  
           [0010]    In yet another embodiment the present invention is a kinetic spray system comprising: a supersonic nozzle comprising a throat located between a converging region and a diverging region, the diverging region defined between the throat and an exit end; at least one injector positioned between the throat and the exit end, the injector in direct communication with the diverging region; a low pressure powder feeder connected to the at least one injector; and a high pressure source of a heated main gas connected to the nozzle. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    In the drawings:  
         [0012]    [0012]FIG. 1 is a generally schematic layout illustrating a kinetic spray system for performing the method of the present invention; and  
         [0013]    [0013]FIG. 2 is an enlarged cross-sectional view of a kinetic spray nozzle used in the system. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]    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 article by Van Steenkiste, et al. entitled “Kinetic Spray Coatings” published in Surface and Coatings Technology Volume III, Pages 62-72, Jan. 10, 1999, all of which are herein incorporated by reference.  
         [0015]    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.  
         [0016]    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 low pressure 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 pressure of the main gas generally is set at from 150 to 500 psi. The low pressure powder feeder  30  mixes particles of a spray powder and supplies the mixture of particles to 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 .  
         [0017]    [0017]FIG. 2 is a cross-sectional view 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 . 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 generally ranges 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.  
         [0018]    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.  
         [0019]    The de Laval nozzle  54  is modified from previous systems in the diverging region. In the present invention a mixture of 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. 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. 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.  
         [0020]    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. 2 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. An inner diameter of the injector tube  50  can vary between 0.4 to 3.0 millimeters. The use of multiple injector tubes  50  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 prior systems. 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 system 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 present system  10  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.  
         [0021]    Using a nozzle  54  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 that 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  of the present invention has a cost that is approximately ten-fold lower than the high pressure powder feeders that have been used in past systems. 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.  
         [0022]    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.  
         [0023]    Upon striking a substrate opposite the nozzle  54  the 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 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  54 . 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.  
         [0024]    As disclosed in U.S. Pat. No. 6,139,913 the substrate material useful in the present invention 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. All of these substrates can be coated by the process of the present invention. 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 know particles. These particles generally comprise metals, alloys, ceramics, polymers, diamonds and mixtures of these. The particles may have an average nominal diameter of from 1 to 110 microns. Preferably the particles have an average nominal diameter of from 50 to 90 microns.  
       EXAMPLES  
       [0025]    In a first example a system and nozzle designed according to U.S. Pat. No. 6,139,913 was used to spray tin particles having an average nominal diameter of 60 to 90 microns onto a substrate. The substrate was not sandblasted prior to attempts to coat it. The nozzle had a length of 80 millimeters from throat to exit end, a throat of 2.8 millimeters, and an injector tube that injected the particles under a high pressure of approximately 300 to 350 psi into the chamber. The maximal main gas temperature that could be used without clogging of the nozzle in that system was 300° F.  
         [0026]    In a second series of examples a system  10  designed according to the present invention was used. The nozzle  54  had a length from throat  58  to exit end of 300 mm with a rectangular exit of 5 by 12.5 millimeters and a throat  58  of 2.8 millimeters. A total of eleven injector tubes  50  were positioned into the nozzle  54  after the throat  58 . The injector tubes  50  were spaced apart by one inch and set at an angle of 45 degrees with respect to the central axis  52 . Using this nozzle  54  tin particles of 60 to 90 microns could be sprayed at a main gas temperature of up to 1000° F. without clogging of the nozzle  54 . In separate experiments the tin particles were sprayed through injector tubes  50  at one, seven and eight inches downstream from the throat  58 . The injection pressures ranged from just over positive pressure at both seven and eight inches from the throat to 20 psi at one inch from the throat  58 . Thus, using the nozzle  54  of the present invention a powder can be sprayed at over a three-fold higher temperature and a sixteen-fold lower pressure compared to prior kinetic spray systems.  
         [0027]    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.