Patent Grant
7475831
The present invention is directed toward a design for a supersonic nozzle, and more particularly, toward a nozzle for a kinetic spray system.
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.
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 and increases its pressure. 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.
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.
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. One problem encountered with both low pressure and high pressure nozzles is the inability of certain types of particles to achieve critical velocity even at higher main gas temperatures and pressures. Thus, it would be advantageous to design a supersonic nozzle allowing particles to achieve higher velocity with the same main gas temperature and pressure.
In one embodiment, the present invention is a supersonic kinetic spray nozzle comprising: a converging region and a diverging region separated by a throat; at least a portion of the diverging region adjacent the throat having a cross-sectional expansion rate of at least 1.0 millimeters squared per millimeter. This expansion rate varies linearly with the variation in the cross-sectional area of the throat. The rate of at least 1.0 millimeters squared per millimeter is favored for a throat cross-sectional area of 9.08 millimeters squared.
In another embodiment, the present invention is a kinetic spray system comprising: a supersonic nozzle having a converging region and a diverging region separated by a throat; at least a portion of the diverging region adjacent the throat having a cross-sectional expansion rate of at least 1.0 millimeters squared per millimeter; at least one powder injector connected to the nozzle with one of a low pressure or a high pressure powder feeder connected to said injector; and a high pressure source of a heated main gas connected to the nozzle.
In another embodiment, the present invention is a method of applying a material via a kinetic spray process 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 at least a portion of said diverging region adjacent said throat having a cross-sectional expansion rate of at least 1.0 millimeters squared per millimeter; 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 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.
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:
Referring first to
The spray system 10 further includes an gas compressor 24 capable of supplying gas pressure up to 3.4 MPa (500 pounds per square inch) to a high pressure gas ballast tank 26. The gas ballast tank 26 is connected through a line 28 to both a powder feeder 30 and a separate gas heater 32. The gas heater 32 supplies high pressure heated gas, 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 above the main gas pressure, 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 10 to 200 psi total, more preferably from 10 to 100 psi total, even more preferably from 10 to 90 psi total, and most preferably from 10 to 60 psi. total. The powder feeder 30 mixes particles of a spray powder with the high or low pressure gas 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 gas supplied to the powder feeder 30, the pressure of gas supplied to the gas heater 32 and the temperature of the heated main gas exiting the gas heater 32.
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 250 microns, more preferably from 60 to 200 microns, and most preferably from 60 to 150 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.
Depending on the particles or combination of particles chosen the main gas temperature may range from 600 to 1300 degrees Fahrenheit (° F.). 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 1300° F. 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.
The mixture of high pressure gas 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 gas collimator 40 into the mixing chamber 42.
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. In this prior art nozzle 54, the expansion rate of the interior cross-sectional area of the diverging region ranges from 0.1 mm2/mm to 0.50 mm2/mm.
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 1300 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.
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.
Knowing the gas flow field of a nozzle 54 the particle acceleration within the nozzle 54 can be calculated. As discussed above, the particles within the gas field are accelerated by the drag force of the gas field. The drag force (D) acting on the particles is expressed by the following equation: D=1/2CD g (Vg−Vp)2 Ap wherein Vg and Vp are the gas and particle velocity respectively; g is the main gas density; Ap is the projected area of the particle; and CD is the drag coefficient of the particle, which is a function of the Reynolds number and the Mach number. It can be seen from the equation that the overall particle acceleration potential for the nozzle flow will be proportional to gVg2. It has been found that in most nozzles 54 the particle acceleration potential is highest in the diverging region of the nozzle 54 from just downstream of the throat 58 through approximately the first ⅓ of the diverging region of the nozzle 54. In this portion of the diverging region, the particle acceleration potential increases very rapidly with relatively large values for the gas density. As the gas expands further downstream, the gas density decreases very rapidly as it approaches the exit end 60 of the nozzle 54. Knowing the cross-sectional flow areas of a nozzle 54 it is possible using one-dimensional isentropic flow analysis to calculate the effect of the expansion profile of the diverging region of the nozzle 54 on the particle acceleration. It has been found by the present inventors that rapidly expanding the cross-sectional flow area in the first ⅓ of the diverging region of the nozzle 54 leads to a dramatic increase in the particle velocity achievable using the same main gas temperature. The effect of the rapid expansion of the diverging region immediately following the throat 58 is to cause a rapid decrease in the gas pressure and a corresponding rapid increase in the gas velocity. The rapid increase in the gas velocity is important in achieving rapid acceleration of the particles.
The nozzles 54′ and 54″ shown in
Utilizing nozzles 54′ 54″ designed according to the present invention it has been found that the deposition efficiency of particles can be increased utilizing the same main gas temperature and pressure relative to the prior art nozzles 54. This has important benefits in manufacturing because it allows one to utilize a lower main gas temperature while still getting efficient coating of a substrate. In practice it has been found that an expansion rate of at least 1 mm per millimeter right at the downstream side of the throat 58 provides a significant benefit to the coating performance of a modified kinetic spray nozzle 54. Preferably, the expansion rate is at least 2 mm per millimeter, more preferably 5 mm per millimeter and most preferably 10 mm2 per millimeter. It is especially beneficial if this rapid expansion rate and the transition to a standard expansion rate occurs in the first third of the diverging region adjacent the throat 58.
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.
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