Abstract:
Disclosed is a method for repairing defects in kinetically sprayed surfaces. The typical defects comprise isolated or connected conical shaped holes in the kinetic spray coating. The repair involves thermally spraying a molten material into the defective area to fill in the cone followed by continued kinetic spraying to complete the coating.

Description:
TECHNICAL FIELD 
     The present invention is related to a kinetic spray process and, more particularly, to a method for healing defective kinetically sprayed surfaces. 
     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. 
     BACKGROUND OF THE INVENTION 
     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 of the main gas 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 not only fill the voids between previous particles bound to the substrate but also engage in particle to particle bonds. The particles also break any oxide shells on previously bonded particles. 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 without a phase transition. 
     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. 
     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. 
     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 bond 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 bonding. 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. 
     The kinetic spray process has been used to create very thick layers of several centimeters in thickness or more. In addition, the process has been used to create tooling because of its versatility and ability to rapidly build thick layers. One difficulty that can occur in layers of any thickness, but that can be quite noticeable in layers that are 5 millimeters or thicker, is the formation of defects. These defects typically have the shape of right conical cones. Once they begin to develop they are stable and can not be corrected by the kinetic spray process. Continued kinetic spraying leads to an enlarging of the defect. The defects are normal to the surface being sprayed and they have a near constant slant height S described by the equation:
 
 S= ( R   2   +H   2 ) 0.5 
 
Wherein R is the radius of the cone defect and H is the height of the cone. In the past, these defects required discarding of the kinetically sprayed surface because they could not be repaired. This leads to costly operations and time delays, particularly if the defect is not observed immediately. It would be advantageous to develop a method for repairing these defective surfaces that once applied would allow for continued kinetic spraying of the repaired surface.
 
     SUMMARY OF THE INVENTION 
     In one embodiment, the present invention is a method for repairing a defect in a kinetically sprayed surface comprising the steps of providing a kinetically sprayed surface having a defect in the surface, applying a repair coating to the defect by thermally spraying a molten material on the defect, thereby filling the defect and repairing the defect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic layout illustrating a kinetic spray system for performing the method of the present invention; 
         FIG. 2  is an enlarged cross-sectional view of a kinetic spray nozzle used in the system; 
         FIG. 3  is photograph of a kinetically sprayed surface showing a large conical defect; 
         FIG. 4  is a photograph of a kinetically sprayed surface showing a string of isolated conical defects; 
         FIG. 5  is a photograph of a kinetically sprayed surface showing a merged string of defects that form a U-shaped channel; and 
         FIG. 6  is a photograph of the defects shown in  FIG. 4  after repair of a portion according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention comprises a method for repairing a defective kinetically sprayed surface. The method combines the use of a thermal spray process, which is known in the art, with the relatively new technology of the kinetic spray process. The kinetic spray process used is generally described in U.S. Pat. Nos. 6,139,913, 6,283,386 and the two 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. 
     Referring first to  FIG. 1 , a kinetic spray system for use 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 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 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 high 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 temperature of the main gas varies from 100 to 3000° C., depending on the powder or powders being sprayed. The pressure of the main gas and the powder feeder varies from 200 to 500 psi. The powder feeder  30  mixes particles of a powder or a powder mixture of particles with unheated high-pressure air and supplies the mixture to a supplemental inlet line  48  of the nozzle  34 . The particles are described below and may comprise a metal, an alloy, a ceramic, or mixtures thereof. As known to those of ordinary skill in the art an alloy is defined as a solid or liquid mixture of two or more metals, or of one or more metals with certain nonmetallic elements, as in carbon containing steel. 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 . As would be understood by one of ordinary skill in the art, the system  10  can include multiple powder feeders  30 , all of which are connected to supplemental feedline  48 . For clarity only one powder feeder  30  is shown in  FIG. 1 . Having multiple powder feeders  30  allows one to spray mixtures, or to rapidly switch between spraying one particle population to spraying a multiple of particle populations. Thus, an operator can form zones of two or more types of particles that smoothly transition to a single particle type and back again. 
       FIG. 2  is a cross-sectional view 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 . 
     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. The predetermined diameter can range from 0.40 to 3.00 millimeters. Preferably it ranges from 0.40 to 0.90 millimeters in diameter. The tube  50  has a central axis  52  which 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 . 
     Mixing chamber  42  is in communication with the de Laval type nozzle  54 . The nozzle  54  has an entrance cone  56  that decreases in diameter to a throat  58 . Downstream of the throat is 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. 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 distance from the throat  58  to the exit end  60  may vary from 60 to 400 millimeters. 
     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 . 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. 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 . These temperatures and the exposure time of the particles are kept low enough that the particles are always at a temperature below their melting temperature so 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 exiting the nozzle  54  are directed toward a surface of a substrate to coat it. 
     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 include a metal, all the particles striking the substrate surface fracture the oxidized surface layer and the metal particles 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. 
     EXAMPLES 
       FIGS. 3-6  show copper coatings on copper substrates wherein the coatings are applied by a kinetic spray process and there are defects in the coating. In all the examples the copper particles were applied using a kinetic spray process with the following parameters: particle sizes were from 50 micron to less than 106 micron, main gas pressure 300 pounds per square inch, powder feed pressure 350 pounds per square inch, main gas temperature 900° F., traverse rate 0.25 inches per second, and standoff distance of approximately 1 inch. 
     In  FIG. 6  half of the defective surface has been repaired using a thermal spray process according to the present invention. Specifically, the thermal spray was applied using a wire arc thermal spray process with the following parameters: arc gun TAFA 8835, wires Tafa Monel wire type 70T a nickel/copper alloy, 31 volts and 200 amps for the arc, air pressure of 130 pounds per square inch for atomization and 90 pounds per square inch for cooling, traverse speed of 100 millimeters per second, and a standoff distance of 9 inches. 
     In  FIG. 3  an example of a kinetically sprayed copper surface exhibiting a large conical defect is shown at  100 . The cone is 1.3 inches high and at a height of 0.95 inches the diameter of the defect is about 0.95 inches. 
     In  FIG. 4  an example of a string series of defects in a kinetically sprayed copper surface is shown at  106 . The multiple defects are separated, but if the kinetic spray were continued they would eventually merge. 
     In  FIG. 5  an example were a series of defects have merged into a U-shaped channel is shown at  110 . 
     In  FIG. 6  the sample from  FIG. 4  was taken and a portion  112  was thermally sprayed with monel as described above. One can see that the defects have been fully repaired. It is now possible to continue the kinetic spray application to complete the kinetic spray coating without further defects. 
     The repair can be made using any thermal spray process. For example, a plasma gas thermal spray process, a High Velocity Oxy-Fuel combustion (HVOF) thermal spray process, a wire arc thermal spray, an air plasma thermal spray, a vacuum plasma, a flame spray, or radio frequency plasma thermal spray. These general processes are known in the art, but have not been utilized to repair kinetically sprayed surfaces. Any of these processes are suitable for applying a thermal sprayed layer to correct the defect. 
     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.