Abstract:
A new kinetic spray process is disclosed that enables the coating to withstand severe bending and stress without delamination. The method includes use of a low pressure kinetic spray supersonic nozzle having a throat located between a converging region and a diverging region. A main gas temperature is raised to from 1000 to 1300 degrees Fahrenheit and the coating particles are directly injected into the diverging region of the nozzle at a point after the throat. The particles are entrained in the flow of the gas and accelerated to a velocity sufficient to result in partial melting of the particles upon impact on a substrate positioned opposite the nozzle and adherence of the particles to the substrate. The coating also has a desirable shinny surface. The method finds special application in coating substrates for use in formation of electrical connections.

Description:
TECHNICAL FIELD  
         [0001]    The present invention is directed toward a method for producing an electrical contact using a kinetic spray process, and more particularly, toward a method that includes selective melting of kinetically sprayed particles.  
         INCORPORATION BY REFERENCE  
         [0002]    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, pp. 237-252, 2002, all of which are herein incorporated by reference.  
         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 two articles by T. H. Van Steenkiste et al. The first was entitled “Kinetic Spray Coatings,” published in Surface and Coatings Technology, vol. 111, pages 62-71, Jan. 10, 1999 and the second was entitled “Aluminum coatings via kinetic spray with relatively large powder particles”, published in Surface and Coatings Technology 154, pp. 237-252, 2002. The articles discuss producing continuous layer coatings having high adhesion, low oxide content and low thermal stress. The articles describe coatings being produced by entraining metal powders in an accelerated gas stream, through a converging-diverging de Laval type nozzle and projecting them against a target substrate. The particles are accelerated in the high velocity gas stream by the drag effect. The gas 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 exceed a critical velocity 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 thus 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. The critical velocity is dependent on the material of the particle. 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 particles because the temperature of the particles is always below their melting temperature.  
           [0004]    One aspect of the technique is that the particles are entrained in the converging side of the nozzle, pass through a narrow throat and then are expelled from the diverging section of the nozzle onto a substrate. One difficulty that can arise is that with certain particles sizes the throat can rapidly become plugged. In a recent related United States application, filed Apr. 5, 2002 and assigned Ser. No. 10/117,385 this was addressed through a modification of the kinetic spray technique that involves injection of the particles into the diverging region of the nozzle and then entraining them in the accelerated gas stream. The technique removes clogging of the nozzle throat as a limitation and reduces the wear on the nozzle.  
           [0005]    Using the basic technique attempts were made to coat electrical contact substrates with tin particles. The particles adhered to and coated the electrically conductive substrates. During impact fracturing occurs in the particles as they plastically deform and adhere to a substrate and other particles. It was found, however, upon subsequent bending of the coated substrates to form them into the required terminal shape the particles broke internally along these fracture lines and left a fragment of the original tin particle at the break on the substrate. These broken particles negatively affect the substrate surface. The present invention is directed to a method of overcoming the particle fracturing behaviour and to design a coating that could withstand severe bending without damage.  
         SUMMARY OF THE INVENTION  
         [0006]    In one embodiment the present invention is a method of kinetic spray coating a substrate comprising the steps of: providing particles of a tin 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 of from 1000 to 1300 degrees Fahrenheit; 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 partial melting of the particles upon impact on a substrate positioned opposite the nozzle and adherence of the particles to the substrate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0008]    [0008]FIG. 1 is a generally schematic layout illustrating a kinetic spray system for performing the method of the present invention;  
         [0009]    [0009]FIG. 2 is an enlarged cross-sectional view of a kinetic spray nozzle used in the system;  
         [0010]    [0010]FIG. 3 is a graph of the effect of main gas temperature on cohesive and adhesive forces of a tin coating according to the present invention on a brass substrate;  
         [0011]    [0011]FIG. 4 is a scanning electron photomicrograph of a tin particle bonded to a brass alloy substrate not according to the present invention showing fracture regions;  
         [0012]    [0012]FIG. 5 a  is a scanning electron photomicrograph of a tin particle bonded to a brass alloy substrate not according to the present invention that has not been bent;  
         [0013]    [0013]FIG. 5 b  is a scanning electron photomicrograph of a region adjacent to that show in FIG. 5 a  which has been bent at 90 degrees;  
         [0014]    [0014]FIG. 6 a  is a scanning electron photomicrograph of tin particles bonded to a brass alloy substrate according to the present invention;  
         [0015]    [0015]FIG. 6 b  is a scanning electron photomicrograph of tin particles bonded to a brass alloy substrate according to the present invention of FIG. 6 a  at a higher magnification;  
         [0016]    [0016]FIGS. 7 a  and  7   b  are scanning electron photomicrographs of tin particles bonded to a brass alloy substrate according to the present invention;  
         [0017]    [0017]FIG. 7 c  is a schematic diagram of what may be occurring when particles are sprayed according to the present invention;  
         [0018]    [0018]FIGS. 8 a  and  8   b  are scanning electron photomicrographs of cross-sections of tin particles prior to their being sprayed;  
         [0019]    [0019]FIGS. 9 a  and  9   b  are scanning electron photomicrographs of cross-sections of a tin particle sprayed according to the present invention; and  
         [0020]    [0020]FIG. 10 is a scanning electron photomicrograph of a cross-section of a tin particle sprayed according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    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 work holder  18  is preferably designed to feed a substrate material past a nozzle  34  at traverse speeds of from 20 to 400 feet/minute, more preferably at speeds of from 30 to 50 feet/minute. 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.  
         [0022]    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, more preferably from 300 to 400 psi. The low pressure powder feeder  30  mixes particles of a spray powder and supplies the mixture of particles to the nozzle  34 . Preferably the particles are fed at a rate of from 20 to 80 grams per minute 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 .  
         [0023]    [0023]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 within the nozzle  34  of any particles being sprayed. 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 . As discussed below, for the present invention it is preferred that the main gas temperature range from 1000 to 1300° F., and more preferably from 1100 to 1300° F. 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.  
         [0024]    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.  
         [0025]    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.  
         [0026]    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 .  
         [0027]    Using a nozzle  54  having a length of 300 millimeters from throat  58  to exit end  60 , a throat of 2.8 millimeters, an exit end  60  with a rectangular opening of 5 by 12.5 millimeters, main gas pressure of 300 psi, main gas temperature of 700° F., and an injector tube  50  angle of 45 degrees, the pressure drops quickly as one goes downstream from the throat  58 . The measured pressures were: 14 psi at 1 inch after the throat  58 ; 10 psi at 2 inches from the throat  58 ; 20 psi at 3 inches from the throat  58 ; 22 psi at 4 inches from the throat  58 ; 22 psi at 5 inches from the throat  58  and below atmospheric pressure beyond 6 inches from the throat  58 . For the present invention it is preferred that the injector tube  50  be located a distance of from 0.5 to 5 inches from the throat, more preferably from 0.5 to 2 inches, and most preferably from 0.5 to 1 inches. 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, most preferably from 5 to 60 psi. All that is required is that it exceed the main gas pressure at the point of injection.  
         [0028]    The nozzle  54  preferably produces an exit velocity of the entrained particles of from 300 meters per second to 800 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 . The importance of the main gas temperature is discussed more fully below.  
         [0029]    It is preferred that the exit end  60  of the nozzle  54  have a standoff distance of from 10 to 40 millimeters, more preferably from 15 to 30 millimeters, and most preferably from 15 to 20 millimeters from the surface of the substrate. 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. Preferably the particles have an average nominal diameter of from 60 to 90 microns.  
       EXPERIMENTAL DATA  
       [0030]    It was initially believed that the present system could be used to coat brass substrates with tin using the standard main gas temperatures of from 600 to 700° F. to coat the substrate. In the data reported below the nozzle  54  is 300 millimeters long, has a throat  58  with a diameter of 2.8 millimeters, and an exit end  60  of 12.5 millimeters by 5 millimeters. The main gas pressure is 300 psi, the main gas temperatures are as noted below, the standoff distance was 20 millimeters, and the injector tube  50  was at an angle of 45 degrees. The particles had a nominal average size of from 63 to 90 microns. The substrates were either C26000 ½ hard cartridge brass or C42500 extra spring tin brass. The C26000 has a Rockwell B hardness of 68, a yield strength of 51 ksi, and a tensile strength of 62 ksi. The C42500 is a copper alloy having a Rockwell B hardness of 92, a yield strength of 90 ksi, and a tensile strength of 92 ksi.  
         [0031]    Several continuous tin coatings were produced on C26000 brass substrate for adhesion testing and failure mode analysis. The substrates were coated at a traverse rate of 400 feet per minute and a particle feed rate of 73 grams per minute. Adhesion measurements were made using a Romulus adhesion tester from Quad Group. Pull studs are attached onto the tin surface with epoxy, mounted in the machine and tested until failure. FIG. 3 is a graph showing the force required to break the studs free as a function of main gas temperature used during the coating process. The failure mode was either in the coating, C in the figure, or at the coating/substrate interface, CS in the figure. For main gas temperatures below 400° F. the failure mode was observed to occur at the coating/substrate interface, these are adhesive forces. For main gas temperatures above 400° F. the failure mode was observed in the coating itself, cohesive forces. FIG. 3 shows that the force required to remove the pull studs increases with increasing main gas temperature.  
         [0032]    It was surprisingly found that when these same spray parameters were used to attempt to coat the C42500 substrate, the coating failed. Specifically, the coating adhered to the substrate when the substrate was flat, however, when the substrate was stamped into the desired electrical terminal shape the coating failed. The typical electrical terminal is stamped out in a die that introduces several 90 degree bends into the substrate. FIG. 4 is a black and white scanning electron micrograph photo (SEM) of a tin particle bonded to the C42500 surface. The operating spray parameters were a traverse rate of 400 feet/min, a main gas temperature of 700° F., and a particle feed rate of 22 g/min. The figure shows a region from the substrate surface to approximately half way through the particle, see the dotted lines in the figure, where a zone of fractured, broken looking material is present, labeled the fracture region in the figure. The top surface of the particle appears to be intact and undamaged.  
         [0033]    To test the adhesion of the tin particles to the C42500 substrate surface the substrate was bent 90 degrees and examined with the SEM. FIG. 5 a  is a photo of the tin particles adhered to the substrate, where no bending of the substrate has occurred. The particles appear well-rounded and adhered to the surface. FIG. 5 b  is an SEM of a region where the substrate was bent 90 degrees. In FIG. 5 b  the tin particles appear to have partially delaminated from the surface. A portion of the particle remains attached to the C42500 substrate surface. The increased material properties of the C42500, such as higher hardness, increased yield strength, and increased tensile strength appear to have caused the tin particles to internally fracture on impact. A part of the lower portion of the particles appears to be well bonded and remains attached even under severe distortion of the substrate.  
         [0034]    It has been surprising found in the present invention that increasing the main gas temperature to a temperature of from 1000 to 1300° F. results in a superior bonding to C42500 and prevents delamination even upon severe bending. Additionally, the traverse speed was lowered to 30 to 50 feet/min and the feed rate was lowered to 20 to 30 grams/min. In part the harder surface of the C42500 is requiring more initial particle impacts to prepare the substrate surface for the subsequent arriving particles. If this were the only requirement, however, then one would assume that increasing the feed rate would compensate for the surface preparation. This was not observed to be true. Increasing the feed rate did not increase the number of adhered tin particles on the substrate surface. Instead the higher feed rates produced excess tin powder, which stuck to the oil layer on the substrate or went into the dust collector. The higher feed rates may also contribute to mass loading of the high velocity gas stream resulting in lower actual particle velocities.  
         [0035]    [0035]FIGS. 6 a - b  are SEM taken of tin particles sprayed at a traverse feed rate of 40 feet/min, a main gas temperature of 1040° F., and a feed rate of 22 grams/min. FIG. 6 a  shows tin particles on the C42500 surface standing proud with the typical hemispherical appearance. One also observes in FIG. 6 b , which is at a higher magnification, that unlike previous coatings the tin particles have both a shiny and smooth upper surface appearance. Surprisingly, when these surfaces were subjected to severe bending of 90 degrees there was no delamination of the tin particles from the substrate. Adhesion testing using a dimple punch to compound stretch the substrate in multiple directions revealed very strong bonding of the coating to the substrate. The bonding is even stronger than that observed using the previous parameters of a main gas temperature of 600 to 700° F. The particles themselves plastically deformed without debonding from the substrate surface. This is unexpected because the particles were traveling at higher particle velocities as a result of the higher main gas temperature and should have a higher degree of fracturing resulting in an increase in the number of tin fragments on the substrate after adhesion testing.  
         [0036]    In FIGS. 7 a - b  individual tin particles are shown that clearly show retention of the rounded shape and the smooth shiny surfaces. In FIG. 7 c  a schematic showing one possible explanation for these new coatings is presented. While not wishing to bound by any single theory it is believed that the higher main gas temperature raises the particle temperature higher and increases the particle velocity to a level such that when combined with the kinetic energy released by the plastic deformation, strain and heat on impact it causes a partial melting of parts of the outer layer of the particle. It is believed that a partial melting is occurring at the upper surface region as well as at the interface between the particle and the substrate. It is obvious that the particles are not melting as in thermal spray methods. The evidence for this conclusion is two fold. First, the tin particles are still standing proud above the substrate surface and appear similar in shape to the earlier coatings, had the tin particles been molten this structure would have been destroyed before impact and thin splats similar to those observed with thermal spray would have formed after impact. Second, after severe substrate deformation no evidence is found for tin fragments on the substrate surface suggesting that the fragmented zone observed previously in FIG. 4 is not present in these new coatings. A resolidified zone of metal restoring the structural integrity of the tin particles would have replaced this fragmented region seen in FIG. 4 if a partial melting has occurred.  
         [0037]    [0037]FIGS. 8 a - b  are SEM of etched cross-sections of tin particles from the initial starting powders. One can clearly distinguish the internal grain boundaries and structures of the particles before spraying. Comparing these photos with the SEMs in FIGS. 9 a - b  and  10  of the particles after impact with the substrate surface we observe several different structures not present in the initial powders.  
         [0038]    [0038]FIG. 9 a  is an SEM of a tin particle sprayed using the new high temperature method described above onto C42500. In FIG. 9 a  a thin solid looking layer at the arrow with no observable grain structure is located between the substrate and the particle. The central core of the particle appears to be composed of regions with microstructure similar to those shown in FIGS. 8 a - b . FIG. 9 b  is a magnified image of the region noted in FIG. 9 a . Again a thin quenched layer is present between the substrate and the particle, a thin layer having a different grain structure from the interior of the particle (see arrow in FIG. 9 b ) on the outer particle surface, a layer of plastically deformed internal grain boundaries, and an internal core region with a microstructure similar to the original particles.  
         [0039]    [0039]FIG. 10 is an SEM of another high temperature sprayed tin particle. Again note the thin rapidly quenched layer between the substrate and the particle and the outer edge of the particle (see arrows). Also note the thicker layer with the different microstructure on the upper surface (see dashed lined area). The SEMs in FIGS. 9 a - b  and  10  suggest that there may be selective area melting of the particles at high main gas temperatures. This selective melting presumably is responsible for the high adhesion between the substrate and the particles.  
         [0040]    To further enhance the present invention it is possible to pre-heat the particles in the powder feeder  30 . Preferably the particles are heated to within 100° F. of their melting point. Because the particles are being injected after the throat  58  these higher temperatures are possible without causing clogging of the nozzle  54 .  
         [0041]    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.