Method of maintaining a non-obstructed interior opening in kinetic spray nozzles

#The present invention is directed to a method for maintaining a non-obstructed interior opening in a kinetic spray system nozzle. The invention further permits one to increase the air flow temperature in the system thereby increasing deposition efficiency.

#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.

#A new technique for producing coatings on a wide variety of substrate surfaces by kinetic spray, or cold gas dynamic spray, was recently reported in an article by T. H. Van Steenkiste et al., entitled “Kinetic Spray Coatings,” published in Surface and Coatings Technology, vol. 111, pages 62-71, Jan. 10, 1999. The article discusses producing continuous layer coatings having low porosity, high adhesion, low oxide content and low thermal stress. The article describes coatings being produced by entraining metal powders in an accelerated air stream, through a converging-diverging de Laval type nozzle and projecting them against a target substrate. The particles are accelerated in the high velocity air stream by the drag effect. The air used can be any of a variety of gases including air or helium. It was found that the particles that formed the coating did not melt or thermally soften prior to impingement onto the substrate. It is theorized that the particles adhere to the substrate when their kinetic energy is converted to a sufficient level of thermal and mechanical deformation. Thus, it is believed that the particle velocity must be high enough to exceed the yield stress of the particle to permit it to adhere when it strikes the substrate. It was found that the deposition efficiency of a given particle mixture was increased as the inlet air temperature was increased. Increasing the inlet air temperature decreases its density and increases its velocity. The velocity varies approximately as the square root of the inlet air temperature. The actual mechanism of bonding of the particles to the substrate surface is not fully unknown 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 not only on the material of the particle but also on the size of the particle. 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 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 air stream because the temperature of the air stream is always below the melting temperature of the particles and the temperature of the particles is always below that of the air stream.

#In a first series of experiments the effect of utilizing second particle population of copper in combination with a first particle population of tin was tested. In Table 1, below, are presented the results of testing addition of a copper particle population to a tin particle population. All of the samples were run through a de Laval-type nozzle having a throat of 3 millimeters and a rectangular shaped opening of approximately 4.7 millimeters by 12 millimeters. The main gas temperature was set at 400° F. 1 TABLE 1 Percent Copper by Volume Run Time, Minutes Observations 0.0 4 Nozzle throat completely plugged. 6.0 20 Small build-up of material in the nozzle. 12.0 20 Nozzle completely clean. 25.0 20 Nozzle completely clean.

#As can be seen from the data above, inclusion of a small portion of copper along with the tin enables the tin to be run for a much longer period of time. In the absence of copper, tin completely plugged the nozzle within 4 minutes, whereas in the presence of copper after a run of 20 minutes the nozzle was still perfectly clean. Tin has a melting point of 232° C., while copper has a melting point of 1083° C., thus the cooper can scour the nozzle and keep it clean whereas the tin will stick to it.

#In this example, the addition of a copper particle population to a tin particle population was tested utilizing a de Laval nozzle having a throat diameter of 2 millimeters and a rectangular shaped opening of approximately 2.8 millimeters by 27.4 millimeters. The combination of copper with tin was tested at a series of copper levels and main gas operating temperatures. 2 TABLE 2 Main Gas Temperature, Percent Copper Run Time, Degrees F. by Volume Minutes Observations 400 0.0 0.5 Nozzle completely plugged. 400 6.0 20 A small amount of build-up observed inside the nozzle. 400 12.0 20 Nozzle extremely clean. 400 25.0 20 Nozzle extremely clean. 200 25.0 20 Nozzle extremely clean. 300 25.0 20 Nozzle extremely clean. 500 25.0 20 Nozzle extremely clean.