Abstract:
A new gas collimator for use in a kinetic spray system is disclosed. The collimator reduces turbulence of the main gas and results in significant increases in the amount of particles deposited on a substrate using the system. Kinetic spray nozzles incorporating the new collimator also have significantly higher deposition efficiencies. The new collimator enables the main gas temperature to be reduced while permitting much higher depositions and deposition efficiencies compared to the prior art collimator. Also disclosed is a low pressure injection method for a kinetic spray system. The coaxial, low pressure injection method enables the use of low pressure powder feeders, which are low cost, technologically mature, and widely available commercially. The coaxial injection method overcomes several undesirable effects associated with prior art high pressure injection methods.

Description:
TECHNICAL FIELD  
       [0001]     The present invention is directed toward a design for a gas collimator, and more particularly, toward a gas collimator for a kinetic spray nozzle and a low pressure injection method.  
       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, nitrogen 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 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 main gas temperature was increased. Increasing the main gas temperature decreases its density and thus increases its velocity. The velocity varies approximately as the square root of the main gas 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 and of the substrate. Once an initial layer of particles has been formed on a substrate subsequent particles not only eliminate the voids between previous particles bound to the substrate by compaction, but also engage in particle to particle bonds. The bonding process is not due to melting of the particles in the main gas stream because the temperature of the particles is always below their melting temperature.  
         [0004]     The above kinetic spray methods all relied on high pressure particle powder feeders. These powder feeders are very expensive and can cause erosion of the throat of the kinetic spray nozzle. In addition, high pressure systems are prone to clogging at the throat of the nozzle, which limits the main gas temperatures that can be used.  
         [0005]     A recent improvement was disclosed in U.S. application Ser. No. 10/117,385, filed Apr. 5, 2002. In this improvement the particle powder is introduced through the side of the nozzle in the diverging section, which allows a low pressure powder feeder to be used. Low pressure powder feeders are very common, inexpensive and reliable. This method suffers from erosion of the nozzle sidewall opposite the point of powder introduction, especially when hard materials are sprayed. In some cases, the edges of the spray path produced by this method are saw-toothed and not clean well defined edges such as are obtained using the prior art high pressure method described above. The reason for this appears to be asymmetric assimilation of the particles into the gas stream. Both the high pressure and the low pressure prior art systems suffer from turbulence in the entraining main gas associated with high velocity flow, especially when the main gas goes through a right angle as it is introduced into the converging section of the nozzle. Turbulence significantly reduces the deposition efficiency of the kinetic spray system. Thus, the kinetic spray process requires higher main gas temperatures to obtain efficient deposition of particles.  
       SUMMARY OF THE INVENTION  
       [0006]     In one embodiment, the present invention is a gas collimator for a kinetic spray nozzle comprising a collimator having a central hole surrounded by a plurality of gas flow holes and a length of from 10 to 30 millimeters with the gas flow holes having a hydraulic diameter of from 0.5 to 5.0 millimeters.  
         [0007]     In another embodiment, the present invention is a kinetic spray nozzle comprising a supersonic nozzle having a gas collimator located between a premix chamber and a mixing chamber; the mixing chamber located adjacent to a converging section of the nozzle; a throat located between the converging section and a diverging section of the nozzle; the collimator having a central hole surrounded by a plurality of gas flow holes and a length of from 10 to 30 millimeters; and the gas flow holes having a hydraulic diameter of from 0.5 to 5.0 millimeters.  
         [0008]     In another embodiment, the present invention is a method of applying a material via a kinetic spray process comprising the steps of providing a particle powder; providing a converging diverging supersonic nozzle having a gas collimator having a central hole surrounded by a plurality of gas flow holes and a length of from 10 to 30 millimeters; the gas flow holes having a hydraulic diameter of from 0.5 to 5.0 millimeters; directing a flow of a gas through the collimator and the nozzle, the gas having a temperature insufficient to cause melting of the particles in the nozzle; and entraining the particles in the flow of the gas and accelerating the particles to a velocity sufficient to cause the particles to adhere to a substrate positioned opposite the nozzle. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which like parts throughout the views have the same reference number:  
         [0010]      FIG. 1  is a general schematic layout illustrating a kinetic spray system for performing the method of the present invention;  
         [0011]      FIG. 2  is an enlarged cross-sectional view of a prior art kinetic spray nozzle used with a high pressure powder feeder in a kinetic spray system;  
         [0012]      FIG. 3  is an enlarged cross-sectional view of a prior art kinetic spray nozzle used with a low pressure powder feeder in a kinetic spray system;  
         [0013]      FIG. 4  is an enlarged cross-sectional view of a kinetic spray nozzle of the present invention used with a high pressure powder feeder in the kinetic spray system;  
         [0014]      FIG. 5  is an enlarged cross-sectional view of a kinetic spray nozzle of the present invention used with a low pressure powder feeder in the kinetic spray system;  
         [0015]      FIG. 6  is a graph showing the pressure at the end of an injector in a kinetic spray nozzle of the present invention used with a low pressure powder feeder in the system versus the main gas temperature;  
         [0016]      FIG. 7  is a graph comparing the deposition efficiency of the nozzles shown in  FIGS. 2, 3 , and  5 ;  
         [0017]      FIG. 8A  is an end view of a prior art gas collimator;  
         [0018]      FIG. 8B  is an end view of a gas collimator designed according to the present invention;  
         [0019]      FIG. 9A  is a graph comparing the loading of a substrate by a nozzle having a prior art gas collimator versus a nozzle having a gas collimator designed according to the present invention; and  
         [0020]      FIG. 9B  is a graph comparing the deposition efficiency of a nozzle having a prior art gas collimator versus a nozzle having a gas collimator designed 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 to be coated. The work holder  18  is preferably designed to move a substrate relative to a nozzle  34  of the system  10 , thereby controlling where the powder material is deposited on the substrate. In other embodiments the work holder  18  is capable of feeding a substrate past the nozzle  34  at traverse rates of up to 50 inches per second. 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 pounds per square inch) 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 pressure of the main gas generally is set at from 150 to 500 pounds per square inch (psi), more preferably from 300 to 400 psi. The powder feeder  30  is either a high pressure powder feeder or a low pressure powder feeder depending on the design of the nozzle  34  as described below. When the powder feeder  30  is a high pressure feeder  30  preferably the pressure is set at a pressure of from 25 to 100 psi, and more preferably from 25 to 50 psi above the pressure of the main gas. When the powder feeder  30  is a low pressure feeder the pressure is preferably from 60 to 125 psi, more preferably from 60 to 100 psi, even more preferably from 60 to 90 psi, and most preferably from 70 to 80 psi. The powder feeder  30  mixes particles of a spray powder with the high or low pressure air and supplies the mixture to a supplemental inlet line  48  of the nozzle  34 . Preferably the particles are fed at a rate of from 20 to 1200 grams per minute, more preferably from 60 to 600 grams per minute to the nozzle  34 . A computer control  35  operates to control the powder feeder  30 , the pressure of air supplied to the powder feeder  30 , the pressure of air supplied to the air heater  32  and the temperature of the heated main gas exiting the air heater  32 .  
         [0023]     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. The particles preferably have an average nominal diameter of from 60 to 110 microns, more preferably from 63 to 106 microns, and most preferably from 63 to 90 microns. The substrate materials 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.  
         [0024]     Depending on the particles or combination of particles chosen the main gas temperature may range from 600 to 1200 degrees Fahrenheit. The main gas has a temperature that is always insufficient to cause melting within the nozzle  34  of any particles being sprayed. For the present invention it is preferred that the main gas temperature range from 600 to 1200 degrees Fahrenheit depending on the material that is sprayed. What is necessary is that the temperature and exposure time of the particles 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 inlet temperature is above 1000° F.  
         [0025]      FIG. 2  is a cross-sectional view of a prior art nozzle  34  and its connections to the air heater  32  and a high pressure powder feeder  30 . This nozzle  34  has been used in a high pressure system. 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 gas collimator  40  and into a chamber  42 . This prior art gas collimator  40  is a disc approximately 1 millimeter in thickness, see  FIG. 8A  for an end view. The collimator  40  includes a central injector hole  108  for receiving a powder injector tube  50 . A series of gas flow holes  110  surround the injector hole  108 . 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 .  
         [0026]     The mixture of high pressure air and coating powder is fed through the supplemental inlet line  48  to the powder injector tube  50  comprising a straight pipe having a predetermined inner 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 .  
         [0027]     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 5.5 to 1.5 millimeters, with from 4.5 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.  
         [0028]     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  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 the particles are never heated to their melting point, 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 are always at a temperature below the main gas temperature. The particles exiting the nozzle  54  are directed toward a surface of a substrate to be coated.  
         [0029]     It is preferred that the exit end  60  of the nozzle  54  have a standoff distance from the surface to be coated of from 10 to 80 millimeters and most preferably from 10 to 20 millimeters. 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. Upon impact the kinetic sprayed particles stick to the substrate surface if their critical velocity has been exceeded. 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, because the kinetic energy of the particles must be converted to thermal and strain energies via plastic deformation upon impact. This critical velocity is dependent on the material composition of the particle and the type of substrate material. In general, harder materials must achieve a higher velocity before they adhere to a given substrate. The nature of the bonds between kinetically sprayed particles and the substrate is discussed in the article in Surface and Coatings Technology 154, pp. 237-252, 2002, discussed above.  
         [0030]      FIG. 3  is a cross sectional view of a prior art nozzle  34  for use with a low pressure powder feeder. The de Laval nozzle  54  is very similar to the high pressure one shown in  FIG. 2  with the exception of the location of the supplemental inlet line  48  and the powder injector tube  50 . In this prior art system the powder is injected after the throat  58 , hence a low pressure feeder  30  can be used. The collimator  40  is the same as shown in  FIG. 2 .  
         [0031]      FIGS. 4 and 5  show a nozzle  54  and a gas collimator  40 ′ designed in accordance with the present invention.  FIG. 4  shows a cross-sectional view of a high pressure nozzle  54  designed according to the present invention, while  FIG. 5  is of a low pressure nozzle  54  designed according to the present invention. An end view of the collimator  40 ′ is shown in  FIG. 8B . The collimator  40 ′ is much longer than the prior art collimator  40 . Preferably the collimator  40 ′ has a length of from 10 to 30 millimeters, and more preferably from 25 to 30 millimeters. The collimator  40 ′ is preferably formed from a ceramic material so that it can withstand the temperature and pressures of the main gas. The collimator  40 ′ can, however, also be made from any metal or alloy capable of withstanding the main gas temperatures and pressures. The collimator  40 ′ has a central hole  114  for receiving the injector tube  50  and this central hole  114  is surrounded by a plurality of gas flow holes  116 . In  FIG. 8B  the holes  116  are shown as hexagonal honeycomb shaped holes, however, other shapes such as circular shapes and other shapes will work as well. It is preferable that the hydraulic diameter for an individual hole  116  be from 0.5 to 5.0 millimeters. It is also preferable that the ratio of the hydraulic diameter of the holes  116  to a length of the collimator  40 ′ be from 1:5.0 to 1:50.0. Finally, it is preferable that the ratio of the total open space in a cross-sectional area of the collimator  40 ′ to the cross-sectional open area of the mixing chamber  42  be from 0.5:1.0 to 0.9:1.0.  
         [0032]     The only differences between the nozzle  54  in  FIG. 5  versus  FIG. 4  are the length of the injector tube  50  and the diameter of the throat  58 . In the low pressure nozzle  54  of  FIG. 5  the injector tube  50  is longer and it extends into the diverging section of the nozzle  54 . Because the injector tube  50  extends through the throat  58  the throat  58  must be wider. The throat  58  is widened such that a gap exists between the outside of the injector tube and the inside diameter of the throat  58 . This gap provides a cross-sectional air flow area that is equivalent to that of  FIG. 4  and so that it provides from 15 to 50 cubic feet per minute (cfm) of air flow, more preferably from 25 to 35 cfm.  
         [0033]     The distance from the end of the throat  58  to the end of the injector tube  50  in the low pressure nozzle shown in  FIG. 5  effects the deposition efficiency of the particles. Computer modeling indicates that it is preferable that the end of the injector tube  50  be located within the first ⅓ of the diverging section of the nozzle  54  to get maximal acceleration of the particles. Preferably the injector extends from 2 to 50 millimeters, and more preferably from 5 to 30 millimeters beyond the throat  58  into the diverging section of the nozzle  54 . In an actual test two injector  50  lengths were compared. The first extended 12 millimeters beyond the throat  58  and the second extended 38 millimeters beyond the throat  58 . For both nozzles  54  the particles were aluminum powder, feed rate was 1 gram per second, traverse speed was 2 inches per second, and the main gas temperature was 900° F. The substrate was aluminum. The nozzle  54  with the shorter injector tube  50  had a deposition of 325 grams per square meter and the longer injector tube  50  had a deposition of only 295 grams per square meter. Thus the shorter tube  50  was more efficient. In addition, it was found that the present invention eliminated the sawtooth edges found in use of the prior art low pressure nozzle. The edges of passes using the collimator  40 ′ of the present invention were clean and sharp like those found using high pressure kinetic spray systems. The present invention also eliminates the nozzle  54  sidewall erosion found in the prior art low pressure nozzle  54 . Using the low pressure nozzle  54  of the present invention also permits the main gas pressure to be increased independently of the powder feeder  30  pressure. This permits an increase in the total mass flow rate which in turn increases deposition efficiency.  
         [0034]     In  FIG. 6 a  graph is shown illustrating the pressures at the end of a low pressure nozzle  54  designed in accordance with the present invention and having an injector tube  50  that extends 25 millimeters beyond the throat  58  at various main gas temperatures. The main gas pressure was kept constant at 300 psi. While the measured pressures in  FIG. 6  somewhat underestimate the true pressure at the end of the injector  50 , the results demonstrate the existence of the low pressure region. This is why the injection method permits the use of low pressure powder feeders  30 .  
         [0035]      FIG. 7  shows the results of a series of comparative studies using the nozzles  54  shown in  FIGS. 2, 3 , and  5 . The Y-axis is the particle loading per square meter on the substrate and the X-axis is the powder feed rate. For all nozzles  54  the main gas temperature was 800° F., the particles were an alloy of Al—Zn—Si (80-12-8) sprayed onto aluminum, the particle size was 53 to 106 microns, the traverse speed was 2 inches per second, and the main gas pressure was 300 psi. Reference line  100  was generated using a prior art high pressure nozzle  54  as shown in  FIG. 2  using an injection pressure of 350 psi. Reference line  102  was generated using a low pressure nozzle  54  as shown in  FIG. 5  designed according to the present invention. Reference line  104  was generated using a prior art low pressure nozzle  54  designed as shown in  FIG. 3 . The results show the new collimator  40 ′ in a low pressure nozzle  54  increases the amount of deposited particles on the substrate significantly at all feed rates versus the prior art low pressure nozzle  54  and collimator  40 . The new low pressure nozzle  54  is still not as efficient as the prior art high pressure nozzle  54 .  
         [0036]     The collimator  40 ′ designed in accordance with the present invention also increased the efficiency of high pressure nozzles  54 . In a comparison a nozzle  54  designed as shown in  FIG. 2  was compared to a high pressure one designed according to the present invention as shown in  FIG. 4 . The results are shown in  FIGS. 9A and 9B . In all of the tests the powder was an alloy of Al—Zn—Si (80-12-8) sprayed onto aluminum, the feed rates were kept constant at 0.5 grams per second, particle size 53 to 106 microns, the main gas pressure was 300 psi, the powder feeder  30  pressure was 350 psi., and the results are the average of 12 runs.  
         [0037]     In  FIG. 9A  the loading per square meter of substrate is shown. Reference bar  118  represents the results from a high pressure powder feed nozzle  54  designed according to the present invention with a main gas temperature of 700° F. and a traverse speed of 4 inches per second. Reference bar  120  represents the results from the same nozzle  54  as reference bar  118  except the traverse speed was increased to 5 inches per second. Reference bar  122  represents the results from a prior art nozzle  54  designed in accordance with  FIG. 2  with a prior art collimator  40 , a main gas temperature of 800° F. and a traverse speed of 3 inches per second. The results demonstrate the benefits of the collimator  40 ′ designed according to the present invention. The collimator  40 ′ of the present invention permits for much higher depositions at higher traverse speeds and lower main gas temperatures. The ability to use a lower main gas temperature also results in less clogging of the throat  58 .  
         [0038]     In  FIG. 9B  the deposition efficiency is shown. Reference bar  124  represents the results from a high pressure nozzle  54  designed according to the present invention with a main gas temperature of 700° F. and a traverse speed of 4 inches per second. Reference bar  126  represents the results from the same nozzle  54  as reference bar  124  except the traverse speed was increased to 5 inches per second. Reference bar  128  represents the results from a prior art nozzle  54  designed in accordance with  FIG. 2  with a prior art collimator  40 , a main gas temperature of 800° F. and a traverse speed of 4 inches per second. The results demonstrate the benefits of the collimator  40 ′ designed according to the present invention. The collimator  40 ′ of the present invention permits for much higher deposition efficiencies at the same and at higher traverse speeds all with lower main gas temperatures. The deposition efficiency was over twice as high with the collimator  40 ′ at the same traverse speed and a lower main gas temperature, compare reference bars  124  and  128 . Even when the traverse speed was increased to 5 inches per second, a 25% increase, the deposition efficiency was still twice as great with the prior art collimator  40 , compare reference bars  126  and  128 .  
         [0039]     In the present invention it is preferred that the nozzle  34  be at an angle of from 0 to 45 degrees relative to a line drawn normal to the plane of the surface being coated, more preferably at an angle of from 15 to 25 degrees relative to the normal line. Preferably the work holder  18  moves the structure past being nozzle  34  at a traverse speed of from 0.25 to 6.0 inches per second and more preferably at a traverse speed of from 0.25 to 3.0 inches per second.  
         [0040]     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.