Abstract:
An apparatus and method for thermal spraying a metal coating on a substrate is accomplished with a modified pulsejet and optionally an ejector to assist in preventing oxidation. Metal such as Aluminum or Magnesium may be used. A pulsejet is first initiated by applying fuel, air, and a spark. Metal is inserted continuously in a high volume of metal into a combustion chamber of the pulsejet. The combustion is thereafter controlled resonantly at high frequency and the metal is heated to a molten state. The metal is then transported from the combustion chamber into a tail pipe of said pulsejet and is expelled therefrom at high velocity and deposited on a target substrate.

Description:
ORIGIN OF THE INVENTION 
     The invention described herein was made by an employee of the United States Government, and may be manufactured and used by the government for government purposes without the payment of any royalties therein and therefor. 
    
    
     FIELD OF THE INVENTION 
     This invention is in the field of the surface deposition of protective metallic coatings. 
     BACKGROUND OF THE INVENTION 
     There are several known methods of thermal spraying. In these methods, a coating material, such as a metal in the form of powder is fed into a flame. The flame melts the metal powder, so that it can be deposited onto a surface as a coating. An important measurement of quality in most thermal spraying methods is the adhesion of the coating on the surface. A higher velocity thermal spray is generally preferred as the impingement of the coating material onto the deposition surface at higher velocity, typically results in coatings which exhibit better adhesion to the deposition surface. An additional concern common to most methods of thermal spraying is to avoid overheating the coating material which can lead to vaporization or oxidation and reduce the overall quality of the coating produced. In addition, it is also desirable to produce small droplets of material to ensure even coating and maximize surface to volume ratios in order to enhance adhesion and quality of the coating produced. 
     In the field of thermal spraying, there are several methods that attempt to optimize the velocity of the deposition without degrading the quality of the material to be deposited. Most thermal spray methods seek to reduce the residence time in the heating device to minimize the formation of oxides in the coating material. Also, many thermal sprays use a coating material in powder form in order to optimize the surface to volume ratio of the coating material. However, the use of powder may require special delivery and metering equipment and can lead to delivery problems within the thermal spray device. 
     Systems known to exist which may be somewhat functionally similar to the technique of this application utilize pulsed detonation technology (rather than resonant deflagration) to achieve high velocity, molten material. Pulsed detonation systems, while achieving higher temperatures and velocities than the instant invention are far more complex to achieve and control. They require multi-valved actuation and forced fuel and air. As such they are non-mobile and very expensive. Their operational frequencies (pulse rates) are also considerably lower than pulsejet based combustion systems of the instant invention so that high deposition rates are more difficult to achieve. 
     U.S. Pat. No. 2,926,855 discloses an Atomizing and Spraying Apparatus wherein an acoustic jet resonator has a chamber and tube which are both excited at their natural frequency and heated by the pulsating flow of exhaust gases from the internal combustion device to spray a liquid. This reference teaches spraying a liquid material using exhaust fumes. 
     U.S. Pat. No. 6,745,951 B2 to Barykin et al discloses using a detonation spray gun to produce high energy explosions to thermally spray a coating initially supplied as a powder. This reference requires the use of coating material in a powder form and special precautions to detonate gases without causing continuous explosions or a distribution of the powder within the barrel of the device due to the highly explosive nature of the reactant gases. 
     U.S. Pat. No. 4,232,056 teaches a method for using a thermospray gun to melt a metallic coating material and impinge the molten coating particles against a metallic substrate. The thermospray gun utilizes an oxy-fuel gas flame spraying gun or electric arc gun in a continuous process. 
     U.S. Pat. No. 6,579,573 B2 teaches a method for forming a nanostructured coating using ultrasound to form a solution with dispersed nanostructured particles using an ultrasonic horn as a sound source. This reference discloses a high velocity oxy-fuel (HVOF) for depositing a coating. High velocity oxy-fuel processes are continuous and require high outputs of energy to maintain a high velocity stream. 
     None of the references employ a pulsejet having metal wire fed into the combustion chamber to produce high volume, high velocity surface deposition of a protective metallic coating. 
     SUMMARY OF THE INVENTION 
     A method has been devised for high volume, high velocity surface deposition of protective metallic coatings on otherwise vulnerable surfaces. The structure which carries out the method is also disclosed herein. The method is a form of thermal spraying whereby the material to be deposited is heated to the melting point by passing it through a flame. In such systems the molten material is normally transported to the deposition surface by the jet formed from the combustion products. Normally, because steady combustion occurs at relatively low gas velocities, the speed at which the molten particles impinge on the deposition surface is low. This in turn yields relatively low adhesion characteristics for the deposited material. The method described herein utilizes non-steady combustion processes (i.e. high frequency, periodic, confined volume) which generate not only higher velocities, but also use a resonant process requiring no external actuation or control, and no high pressure supply of fuel or air. Optionally, as disclosed hereinbelow combustion chamber pressure may be used to control the deposition process if desired. Velocity increases or decreases as a function of combustion chamber pressure increasing or decreasing and, therefore, velocity may be controlled by varying the fuel-air ratio and/or by increasing the mass of the fuel and the air in a desired proportion within the combustion chamber. 
     Hence, the disclosed system is potentially simpler than conventional thermal spraying systems. Furthermore, the high heat transfer rates developed allow the deposition material to be introduced, not as an expensive powder with high surface area to volume, but in convenient rod-form, which is also easier and simpler to feed into the system. 
     Thermal spray coating is not a new technology. It has been around for quite some time and is well developed. There are different techniques utilized which depend on the objective function of the coating, the environment to which the coated piece will be subjected, and the coating material used. In any application, quality is ultimately measured by how well the coating material adheres to the sprayed surface. Adhesion is markedly improved when the coating material is applied at high velocity. There are also the issues of heating temperature and residence time within the combustion chamber. The goal is to achieve a liquid form of the material to be deposited; however, care must be used because excessive heating can lead to vaporization of the deposition material, or worse, chemical reactions such as oxidation. Furthermore, the droplets of deposited material must be small to ensure uniform coating and to maximize surface area to volume ratios in order to enhance adhesion. Because of all the requirements, flame spraying systems are complex, costly, and generally require the part to be brought to the coating machine rather than the other way around. As described briefly above, the instant invention utilizes a low cost combustion system to heat the material. The particular combustion technique naturally generates periodic high velocity flows which greatly enhance adhesion and heat transfer. 
     Furthermore, the residence times in the combustion device are low and will therefore result in contact with the deposition surface before significant reaction has occurred. Typically, a pulsejet operates at frequencies in excess of 100 Hz. For example, a pulse jet may operate at 220 Hz with the dimensions in this application. Furthermore, the combustion device is mechanically simple, portable, and lightweight and therefore is a mobile, high volume flame spray unit. The combustion device is self-aspirating, requiring no external air or fuel supply energy. The only external power required would be that which controls and actuates the feeding of the coating material into the device. Alternatively, a controller may be used to control the air-fuel ratio and volume. The invention disclosed is inexpensive, mobile, and may produce an exceptionally high material deposition rate, at very high impingement velocity, thus resulting in a quality thermal coating. 
     In testing, a small access port on the side of the combustion chamber section was utilized. Aluminum material to be deposited was inserted through this port as a 1/16″ thick aluminum rod. As such, the rod was fixed such that it protruded approximately 1.25 inches into the 2.5 inch diameter combustion chamber. The notion here was to operate the pulsejet only long enough to melt and deposit this amount of material on the sample. This pulsejet produces approximately 4.25 lbf of thrust when operating. This pulsejet operates at 220 Hz. The thrust production results from a periodic high speed jet which is emitted (due to periodic rapid deflagration) from the tailpipe, downstream of the combustion chamber. The pulsejet was operated for approximately three seconds on a methanol nitromethane mixture to produce a deposition sample. A simple “fingernail” test indicated good adhesion with no preparation performed on the sample surface before the coating. Post-test examination of the aluminum rod indicated that at least half of the 1.25 inch length inserted into the pulsejet combustion chamber was melted. 
     A method for thermally spraying a metal coating is disclosed and claimed using a modified pulsejet. First a pulsejet is initiated using fuel, air and a spark plug. Next, a solid metal is continuously fed into the combustion chamber of a pulsejet. The heat of combustion is coupled with a high pressure wave produced from combustion to melt a high volume of metal material. A fine molten spray is produced through the interaction with combustion-driven, gasdynamic waves. The waves quickly carry the high volume of metal material at high velocity toward the end of the tail pipe of the pulsejet with low residence time within the pulsejet. A vacuum is formed at the front of the combustion chamber as a high pressure wave or waves travel toward the end of the tail pipe. A substrate is placed in proximity to the end of the tail pipe and the metal material entrained in the products of combustion impinge the surface of a substrate at high temperature and high velocity. Fuel and air are drawn through a valve in the head of the pulsejet into the combustion chamber wherein the vacuum is formed following the combustion of the fuel and the air of the previous cycle. 
     A pulsejet cycle can be thought of generally as comprising the following sequence: fuel and air are drawn into the combustion chamber through a valve arrangement in the head of the pulsejet; combustion of the fuel and air occurs when the valves in the head of the pulsejet are closed isolating the fuel and the air in the hot combustion chamber of the pulsejet; expulsion of the products of combustion from the combustion chamber through the tail pipe of the pulsejet; and, formation of a vacuum in the combustion chamber of the pulsejet and opening the valves of the head of the pulsejet. The instant invention takes advantage of the pulsejet and continuously feeds solid metal wire into the combustion chamber wherein it is melted into droplets and is conveyed out of the pulsejet in high volume and at high frequency and velocity with the metal kept at high temperatures and short residence times within the combustion chamber. Additionally, the metal may be fed into the pulsejet in the tail pipe section thereof. The metal may also be fed radially or axially into the pulsejet at several different locations. The valve system in this invention is simple and self-actuating after the initial ignition using a spark plug. The pulsejet is lightweight and highly mobile and simple to operate at high frequency. 
     The invention consists of a process for thermally spray coating metal with pulsed resonant combustion. The apparatus used in this process is a modified pulsejet. The modified pulsejet includes, generally, a head, a combustion chamber, and a tail pipe. The head includes a fuel line, an air line, an eductor, and one or more valves. The combustion chamber is located next to the head and has a sparkplug for initiating combustion. The spark plug may run for several cycles as the pulsejet heats up and begins firing on its own. An access port in the head allows metal wire to be fed therein continuously in solid form. The combustion chamber is formed by the head on one end and a tail pipe on the other end. The tail pipe has a smaller diameter than the combustion chamber. 
     According to the process, fuel is aspirated from the fuel line into the pulsejet and air is ported through the air line. The spark plug ignites the fuel in the combustion chamber. The combustion provides heat to melt the metal coating material and a pulse wave propels the metal coating material at high velocity down through the tail pipe where it exits the pulsejet and is deposited on a surface. The process is resonant and it relights itself in the next several cycles without requiring additional use of the spark plug. 
     The method for thermal spraying of coatings using resonant pulsed combustion includes, more specifically, the following steps: initiating the pulsejet; inserting continuously a high volume of metal into a combustion chamber of a pulsejet; combusting resonantly a fuel air mixture in the combustion chamber; heating the metal into a molten metal; producing a fine molten spray through interaction with combustion-driven, gasdynamic waves; moving the molten metal from the combustion chamber into a tail pipe of the pulsejet; transporting the molten metal downstream within the tail pipe of the pulsejet at a high velocity; expelling the molten metal from the tail pipe of the pulsejet in a thermal spray at a high velocity and high frequency oscillation through a thrust augmentation rig; entraining a volume of gas around the molten metal; and depositing the molten metal as a thermal spray onto a sample at the end of the tail pipe. Use of the augmentation rig is optional and could be used for entrainment of inert gas to minimize oxidation. 
     The pulsejet produces thrust when operating. The thrust production results from a periodic high speed jet which is emitted (due to periodic deflagration) from the tailpipe, downstream of the combustion chamber. In the invention, the material to be deposited is melted in the combustion chamber, then carried downstream and ejected from the tailpipe at high speed wherein it impinges and solidifies on the substrate surface. 
     The device is self-aspirating and self-actuating at a high frequency (˜220 Hz) and low residence time of melted material within the pulsejet to minimize the opportunity for oxidation. In another example, an ejector or a thrust augmentation rig can be located at the end of the pulsejet to entrain an inert gas to reduce oxidation of the coating material. 
     The device uses a process that is non-steady, periodic, high frequency, high volume, self-aspirating, and self-actuating. The combustion used in this process is non-steady and takes place in a confined volume of the combustion chamber. The process is periodic with a spark plug igniting fuel that is fed into the combustion chamber in the first step. The combustion produces heat and a pulse that include one or more waves. The heat melts the solid coating material and the pulse wave moves the melted coating material. 
     The pulse wave carries the molten metal material from the combustion chamber down the tail pipe and ejects the molten metal material from the pulsejet with high velocity as it impinges on the surface of a sample. When the pulse wave moves the melted coating material from the combustion chamber down the tail pipe, a vacuum, or low pressure is formed in the combustion chamber next to the head. This low pressure allows the valve to open and receive fuel from the head. The fuel is then ignited in the combustion chamber and the next cycle of combustion takes place. The metal material is melted and the next pulse wave is formed to carry this material down the tail pipe and impinge the coating material into the surface outside of the pulsejet at high velocity. The high velocity ensures that the coating material impinges into and onto the substrate with greater adhesion. The high frequency (˜220 Hz) ensures a low residence time which reduces the time for oxidation or other degradation of the coating material to take place due to the exposure to high heat before it reaches the deposition surface. The process repeats at high frequency. 
     A high volume of coating material can be moved with each combustion step and the process occurs at high frequency, so that a high amount of coating material can be deposited over time. The coating material can be fed into the combustion in a solid rod form. Introduction of the coating material in a solid form is preferred due to cost and material handling convenience. The solid coating material can be fed in continuously as a wire to thermally spray a high volume of coating material in a faster amount of time. As an example a 1/16″ aluminum wire was used as previously stated, but other sizes, shapes, forms, and compositions of coating material could be used. For example, wire made from magnesium could also be used. The coating material preferably has a high thermal conductivity and melts in the range of 1100-1500° C. Coating material composition, feed rate, and diameters can be modified to control the deposition rate and various qualities of the coating. Coating material can be introduced in a variety of access port locations into the combustion chamber. Wire is fed continuously with a continuous feeding mechanism at controllable rates. Feed locations of the coating material can include other sites such as coaxially in the combustion chamber, transversely into the combustion chamber, and transversely or coaxially in the tail pipe. 
     The fuel for combustion in this example is a mixture of methanol and nitromethane. Other fuels such as gasoline may be used. Fuel consumed in a periodic rapid deflagration process produces a high speed jet from the tail pipe at the end of the pulsejet. A pulsejet produces a vortex in the exhaust region outside the tailpipe with each pulse. The exhaust consists of flame spray droplets of coating material, exhaust fumes (combustion products) and air. Air is drawn radially into the tailpipe from the ambient environment surrounding the pulsejet following the expulsion of the exhaust therefrom as the pressure within the combustion chamber is below the ambient pressure. 
     This pulsejet produces approximately 4.25 lbf of thrust which results from a periodic high speed jet emitted from the tailpipe downstream of the combustion chamber. The quality of the thermally deposited coating is influenced by the operating temperature of the pulsejet and the velocity of the exhaust gases. Both the operating temperature and the velocity of the exhaust gases can be adjusted by controlling the thrust. The combustion chamber pressure can be monitored and is directly related to thrust. The diameter in this example is 2.5 inches at its maximum with the tail pipe diameter of 1.25 inches. Characteristics of the modified pulsejet include simple ignition, smooth self-actuation, and self-aspiration which enables a mobile operation. In one example, a device capable of producing a significant thrust of nearly 4.25 lbf weighs approximately 1 pound. The thrust and hence velocity can also be adjusted by changing the fuel flow or the size of the pulsejet including the diameter. 
     The thermal spray coating exits the tail pipe at a high velocity. The axial velocity at the tail pipe has different component velocities. The velocities at the tail pipe can be changed based on the pressure in the combustion chamber. The pressure in the combustion chamber can be changed by altering the feed rate of the fuel and air into the head of the pulsejet. Further, the final qualities of the metal coating deposited on the surface can be adjusted based on the velocities at the end of the tail pipe. 
     Further, unsteady ejectors typically can augment thrust by entraining a lot of fluid, and mixing very rapidly. Additionally, ejectors in this application can be used to entrain fluid to prevent the effluent from the primary jet from reacting with ambient air. It is also possible to optimize the amount of mixing to maintain high velocity and high temperature of the molten deposition material. Entrainment and mixing are controlled by the ejector diameter and length. An ejector may be optimized specifically to maintain high velocity and high temperature of the effluent. The ejector design may be considered to have different dimensions from an ejector design which augments thrust. The ejector may be used to localize the injection of inert gas. One illustration of how this may be done is shown in  FIG. 2C . 
     An effluent comprising a molten metal material is ejected from the tailpipe at high velocity. A flow of inert gas is released from a pressurized ring to combine with the effluent at the entrance to the ejector. The flow of inert gas surrounds the effluent from the primary jet and prevents it from reacting with ambient air. A secondary rig effluent comprising effluent from the primary jet and an inert flow of gas exits the ejector for deposition on the substrate. 
     The entrainment and mixing of the effluent from the primary jet with the flow of inert gas are controlled by the ejector diameter and length. The ejector helps to prevent the effluent from reacting with ambient air. The ejector is used to keep mixing to a minimum and maintain a high jet velocity and high temperature of the coating material. An ejector may be used which is optimized differently from a thrust augmentation rig. The ejector can be used to localize the introduction of inert gas around the effluent. This design would be portable and avoid having to place the entire apparatus in a giant tank filled with inert gas. 
     The combustion chamber of the pulsejet includes a pressure tap which can be connected to a pressure transducer controller for measuring the pressure in the combustion chamber. The average pressure can be used to monitor the thrust of the pulsejet to better adjust for the deposition rate and quality of the desired coating. 
     The high frequency pulsing produces gas dynamic waves which are believed to break the coating into fine particles, producing a more even coating. The gas dynamic waves are formed as part of the combustion which produces heat, pressure, and sound. Selection of the metal material for the coating, dimensions of the composition of the combustion chamber, length, and diameter of the pulsejet, and type of fuel, can be used to adjust the properties of the gas dynamic waves in order to have the optimal effect of the final coating. 
     The pulsejet is made of materials able to withstand the combustion and the melting temperature range of the metal material to be coated. In this example, the valve body is Aluminum, the combustion chamber and tail pipe are made from Inconel, and the valve covering is made from blue spring steel. The combustion device used in the pulsejet is self-actuating and self-aspirating as a result no external air or fuel supply energy is required after starting the device. Initially air is simplified to the pulsejet an ignition source is provided. The pulsejet includes a simple single valve actuation mechanism which reduces cost, weight, and increases the ease of operation. As a result, a high frequency, high volume thermal spray coating operation can be achieved using a lightweight device that is portable making the thermal spray operation mobile. The thermal spray coating device is portable to accommodate the coating of parts more conveniently than having to bring parts to a stationary, immobile thermal spray coating device. 
     The method is a form of thermal spraying wherein the material to be deposited is heated to the melting point by passing it through a flame. The method of this invention utilizes intense heat necessary to melt a metallic coating material and high velocity pulses to impinge the metallic coating on a deposition surface. By utilizing heat and velocity together, the problem of a high pressure wave extinguishing the flame does not exist. Additionally, because the heat of the flame and pressure of the wave are coordinated, less energy is required to maintain and fuel a flame continuously. 
     The method described herein utilizes non-steady high frequency combustion processes which take place in a confined volume. This type of combustion process provides higher temperatures and heat transfer rates which are capable of spraying a higher volume of metallic coatings with a much higher impingement velocity of the thermal spray on the deposition surface. The design of this device is also greatly simplified as a resonant process is self-actuating requiring no external actuation and no high pressure supply of fuel or air. Further, the high heat transfer rates allow the deposition material to be introduced in a solid rod form. As a result, greater efficiency of this thermal spraying method enables a simplified delivery system and lightweight device to be used for thermal spraying. 
     It is an object of this invention is to provide a method of thermally spraying metallic coatings with good adhesion to a deposition surface. 
     It is an object of the invention is to use a high volume, high velocity, thermal spray to achieve high quality coatings with strong adhesion to the deposition surface. 
     It is an object of this invention to provide a method of thermally spraying metallic coatings at high volumetric rates. 
     It is an object of this invention is to provide a method of thermally spraying metallic coatings with low residence time within the device and thus decreased oxidation. 
     It is an object of this invention is to provide a method of thermally spraying metallic coatings inexpensively using a light weight pulsejet. 
     It is an object of this invention is to provide a method of thermally spraying metallic coatings by adjusting the velocity of the pulsejet exhaust to effect the quality of the final metallic coating deposited. 
     It is an additional object of this invention to provide a method to thermally spray metallic coatings surrounded by inert gas. 
     It is an object of the invention to control the rate at which the metal wire is inserted into the combustion chamber. 
     These and other objects of the invention will be best understood when reference is made to the drawings and the description herein below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of some of the process steps of the invention. 
         FIG. 2  is a side view of the pulsejet apparatus used for thermally spraying metallic coatings using resonant pulsed combustion. 
         FIG. 2A  is a side perspective of the pulsejet apparatus used for thermally spraying metallic coatings using resonant pulsed combustion. 
         FIG. 2B  is a side view of the pulsejet apparatus used for thermally spraying metallic coatings with a thrust augmentation rig for providing an inert gas blanket. 
         FIG. 2C  is an enlarged side view  200 C of inert flow entrained into an ejector with the main effluent flow from the primary jet towards the target. 
         FIG. 3  is an enlarged view of the head and combustion chamber components of the apparatus. 
         FIG. 3A  is a cross-sectional view taken along the lines  3 A- 3 A of  FIG. 3  of the combustion chamber of the pulsejet apparatus for thermally spraying metallic coatings using resonant pulsed combustion. 
         FIG. 3B  is a cross-sectional view taken along the lines  3 B- 3 B of the combustion chamber of the pulsejet apparatus for thermally spraying metallic coating using resonant pulsed combustion. 
         FIG. 3C  is an end view of the valve seat taken along the line  3 C- 3 C in  FIG. 3 . 
         FIG. 3D  is a cross-sectional view of the head taken along line  3 D- 3 D of  FIG. 3C . 
         FIG. 3E  is a cross-sectional view of an axial feed of wire from the head through a hollow bolt and into the combustion chamber. 
         FIG. 4  is a graph of Combustion Chamber Pressure Fluctuations (pressure reading−ambient pressure) (psi) vs. time (sec). 
         FIG. 5  is a graph of Near Exit Plan Velocity Profile (PIV) of velocity (ft/sec) vs. time (msec.) 
         FIG. 6  is a schematic illustration of the pulsejet and the velocity profile exiting the pulsejet. 
         FIG. 6A  is an enlarged view of the pulsejet and the velocity contour plot. 
     
    
    
     The drawings will be best understood when reference is made to the description and claims which follow herein below. 
     DESCRIPTION OF THE INVENTION 
       FIG. 1  is a diagram  100  of some of the process steps of the invention. A method for thermally spraying a metallic coating, includes the steps of initiating a pulse jet  101 ; inserting, continuously, a high volume of metal into a combustion chamber of the pulse jet  102 ; combusting and controlling, resonantly, at high frequency a fuel-air mixture in the combustion chamber  103 ; heating the metal to a molten state  104 ; transporting the molten metal from the combustion chamber into a tail pipe of the pulse jet  105 ; producing a fine molten spray through interaction with combustion-driven, gasdynamic waves  106 ; transporting the molten metal within the tail pipe of the pulse jet at a high velocity  107 ; expelling the molten metal from the tail pipe of the pulse jet in a thermal spray at a high velocity  108 ; and, depositing the molten metal in a thermal spray onto a sample at the end of the tail pipe  109 . 
       FIG. 2  is a side view  200  of the pulsejet apparatus used for thermally spraying metallic coatings with resonant pulsed combustion.  FIG. 2  illustrates the pulsejet  207  comprising head  210 , combustion chamber  220 , and tail pipe  230 . The head  210  has a stationary air line  201  descending from the upper left and is also connected to a fuel line  202 . Head  210  is adjacent to the combustion chamber  220  and resides generally leftwardly of the combustion chamber. Valve seat  319  is located between the combustion chamber  220  and head  210  as illustrated in  FIG. 3 . Combustion chamber  220  includes spark plug  222  located on the top thereof and extending radially inwardly as viewed in  FIG. 3 . A tail pipe  230  is integral with the combustion chamber  220  and extends rightward therefrom as viewed in  FIGS. 2 ,  2 A,  2 B and  3 . Combustion chamber  220  and the tail pipe  230  are generally cylindrically shaped and are made of Inconel. Alternatively, the chamber  220  and the tail pipe  230  can be of other materials including a ceramic material. Head  210  has internal geometry shaped in the form of a venturi  314  for the eduction of fuel as viewed in  FIG. 3D . The outer diameter of the combustion chamber  220  gradually slopes down to a reduced diameter of the tail pipe  230 . The tail pipe  230  diameter is approximately one half the diameter of the combustion chamber  220 . Pulsejet  207  is spaced apart a distance  208  from substrate surface  205 . 
     The pulsejet rests on two supports: a first support  203  and a second support  204 . Spaced apart  208  from the end of the tail pipe  230  is a substrate surface  205 . The substrate surface  205  has a thermally sprayed metal coating  231  deposited thereon in a generally circular shape located generally in-line with the tail pipe  230  as illustrated in  FIG. 2A . 
       FIG. 2A  is a side perspective view  200 A of the pulsejet apparatus  207  used for thermally spraying metal coatings using resonant pulsed combustion.  FIG. 2A  illustrates the head  210  at one end of the pulsejet  207 . The pulsejet  207  is generally in the shape of a tube with a wider diameter at one end (head portion) and a generally decreasing diameter towards the opposite end (tail pipe portion). The pulsejet  207  rests on a first support  203  and a second support  204 . 
     The head includes an eductor  212 . The eductor  212  has an inlet  211  open to atmosphere and a fuel line  202 . A starting air line  201  is also located in the head  210  and initially supplies air for educting fuel into the combustion chamber  220  much like a carburetor. Adjacent to head  210  is a combustion chamber  220  and between head  210  and combustion chamber  220  is a valve seat  319  as viewed in  FIG. 3A . The valve seat  319  is also shown from a rear view of the head  210  in  FIG. 3C . The venturi  314  leading from the head  210  to the valve seat  319  is shown in the cross-sectional view in  FIG. 3D . Referring to  FIG. 3D , valve passageways  313  through the head  210  are illustrated as is the valve seat  319  on the face of head  210 . The combustion chamber  220  has an access port  221  located on one side with a metal wire  206  inserted into the access port  221  by an automatic feeding mechanism  216 . Fitting  221 A secures and seals the metal wire  206  to the combustion chamber  220 . 
     Alternative access ports  221 B and  221 C are illustrated in  FIG. 2A  for the admission of wire. Mounted in the top of the combustion chamber  220  is a spark plug  222  which is used to initially begin combustion within the pulsejet. Tail pipe  230  is formed integrally with the combustion chamber  220  and extends rightward with viewing  FIG. 2A . The combustion chamber  220  is connected to the head  210  on one side and connected to the tail pipe  230  at the other end. At one end the combustion chamber  220  has a larger diameter approximately equivalent to the diameter of the head  210  at its widest point. At the other end, the diameter of the combustion chamber  220  is reduced to match the diameter of the tail pipe  230 . The diameter of the combustion chamber  220  at one end is approximately twice the diameter of the tail pipe  230 . The outer diameter of the pulsejet  207  is gently sloped from its widest value near the combustion chamber  220  to the tail pipe  230  wherein the diameter is reduced. 
     Still referring to  FIG. 2A , at the end of the pulsejet, separated by a distance  208  from the pulsejet is the substrate  205 . Substrate  205  is illustrated as having a thermally sprayed metal coating  231  thereon as represented by reference numeral  231 . The deposited thermally sprayed metal coating  231  is generally cylindrically shaped with a pattern slightly larger in diameter than the tail pipe  230  of the pulsejet  207 . 
       FIG. 2B  is a side view  200 B of the pulsejet apparatus  207  for thermally spraying metal coatings using resonant pulsed combustion with the ejector  233  spaced apart from the tail pipe  230 . Reference numeral  234  signifies the entrance to the ejector  233  wherein entrained inert gas may be used to prohibit oxidation of the thermally sprayed metal coating. Entrainment of inert gas may be routed through the entranceway  234  of the rig or entrainment may occur without the use of the ejector  233  at all. For instance, it is possible for the tail pipe  230  to be surrounded by inert gas with the inlet of the pulsejet (i.e., the head) open to atmosphere as an oxygen source. The combustion chamber of the pulsejet includes a pressure tap  323  located on the sides of the combustion chamber  220  as illustrated in  FIG. 3 . Still referring to  FIG. 2B , reference numeral  238  is the distance between the tail pipe  230  and the entrance of the ejector  234 . Reference numeral  208 B is the distance between the tail pipe  230  and the substrate  205  and reference numeral  242  is the distance between the ejector and the substrate  205 . 
       FIG. 2C  is an enlarged side view  200 C of the flow of inert gas  246  entrained into an ejector  233  with the effluent flow  247  from the primary jet to produce a secondary rig effluent  248  towards the target  205 . The ejector  233  has a width  244  and a length  243  which can be modified to change the characteristics of the secondary rig effluent  248 . When the effluent  247  is ejected from the tailpipe  230 , a pressurized ring  245  releases a flow of inert gas  246  to surround the effluent  247  as the effluent  247  approaches the entrance  234  of the ejector  233 . This flow of inert gas  246  prevents the effluent  247  from reacting with the ambient air. The effluent  247  carries a high temperature molten material at high velocity for use in depositing on a surface as a coating. The flow of effluent  247  and inert gas  246  enter the ejector  233  to form a secondary rig effluent  248  which will be expelled from the ejector  233  to coat the substrate surface  205 . 
       FIG. 3  is an enlarged view  300  of the head and combustion chamber components of the apparatus. The head  210  includes valve seat  319  and valve retainer  318  which prevents the over extension of valve seat  319 . A valve retainer  318  is located next to the valve seat  319  and prevents the valve cover  317  from being extended too far when opened. A valve retainer bolt  315  is inserted through the valve retainer  318 , and valve seat  319  and into the head  210 . The combustion chamber  220  has a spark plug  222  inserted into the top side of the combustion chamber  220  as shown in  FIG. 3 . A spark plug gasket  328  is located on the outside of the combustion chamber  220  with a spark plug nut  329  located on the inner side of the combustion chamber  220  to hold the spark plug  222  in place. 
       FIG. 3A  is a cross-sectional view  300 A taken along the lines of  3 A- 3 A of  FIG. 3  and illustrates the valve seat  319  in the head  210  of the apparatus for thermal spray of coatings using resonant pulsed combustion in juxtaposition with fitting  221 ,  221 A and feeding mechanism  216  for feeding metal wire  206  into the combustion chamber. Valve seat  319  has a valve cover  317  with individual flappers which correspond to valve passageways  313  equally spaced apart from each other and equally spaced radially from the center point of the head  210 . The valve has a threaded receptacle  309 . 
       FIG. 3B  is a cross-sectional view  300 B taken along the lines  3 B- 3 B of  FIG. 3  and illustrates the combustion chamber  220  of the pulsejet apparatus for thermally spraying a metal coating using resonant pulsed combustion illustrating a pressure tap  323  which may be used with a controller  350  for controlling the air-fuel mixture of the pulsejet and hence the combustion within the combustion chamber  220 . The combustion chamber pressure is related to the velocity of the discharge of the combustion products and the molten metal which are expelled out of the pulsejet  207 . Referring to  FIGS. 3 and 3B , controller  350  is illustrated as interfacing a line to the controller  324  with the pressure tap  323  of the combustion chamber and the fuel flow inlet  212  with dotted lines. Necessarily included within the dotted lines are fittings and valves necessary to accomplish the stated objectives. 
       FIG. 3C  is an end view  300 C of valve seat  319  of the head  210  illustrating passageways  313  therethrough and a threaded receptacle  309 .  FIG. 3D  illustrates a cross-sectional view of the head  210  taken along the lines  3 D- 3 D of  FIG. 3C  illustrating a venturi  314  formed within passageway  313 . 
       FIG. 3D  is a cross-sectional view  300 D of the head taken along line  3 D- 3 D of  FIG. 3C  illustrating the venturi  314  and the length of the valve passageway  313  in the head  210 . The valve seat  319  is located at one end of the head  210 . 
       FIG. 3E  is a cross-sectional view  300 E of an axial feed of wire from the head through a hollow bolt  360  and into the combustion chamber  220  of a pulsejet  207 . The head  210  has an air inlet  201 , a fuel line  202 , and an aerodynamic strut  360  with wire  206  fed therethrough. The wire  206  follows a path along the path of a guides  361  through the center of a hollow bolt  362  and into the combustion chamber  220 . An air line  201  is used to start the flow of fuel from the fuel line  202  to the head  210  and along the valve passageway  313  where it passes the valve cover  317  and enters the combustion chamber  220 . The fuel is ignited initially with a spark from the spark plug  222 . The spark plug  222  is inserted though the wall of the combustion chamber and is held in place with a spark plug gasket  328  and a spark plug nut  329 . 
       FIG. 4  is a graph  400  of Combustion Chamber Pressure Fluctuations p-p ambient  (psi) vs. time (sec) illustrating pressure fluctuations in the combustion chamber  220  as a function of time. Pressure was measured with a transducer connected to the pressure tap in the side of the combustion chamber demonstrating the resonant periodic cycle of the pulse within the combustion chamber operating at approximately 220 Hz. The rapid cycling within the combustion chamber demonstrates the low residence time of each pulsed thermal spray of metal. Pressure, as previously stated, is a parameter that can be monitored to control the thermal spraying process and the discharge velocity of the pulsejet. Time averaged pressure of the curve presented in  FIG. 4  may be useful in controlling the thermal spraying of the metal coating. A specific instant in time t 1  is identified on the graph with reference numeral  401 . See  FIG. 6  wherein the profile of the discharge velocity at time t 1  is illustrated. 
       FIG. 5  is a graph  500  of Near Exit Plan Velocity Profile (PIV) velocity (ft/sec) vs. time (msec.). This graph shows high velocity of the thermally sprayed metal of approximately 1700 ft/s released from the pulsejet in the exit plane near the end of tail pipe  230 . In addition to illustrating high velocity discharge of the pulsejet apparatus, this graph also illustrates the dynamic characteristics of the thermal spray wherein the velocity is approximately negative 300 ft/sec around 2.6 to 3.2 seconds after combustion is initiated. This graph shows that in addition to achieving the high velocity to impinge the thermal spray on a sample, the thermal spray has unique bi-directional flow properties which make it possible, it is believed, to further breakdown the particles of molten metal into very small particles which enhances the coating ability. 
       FIG. 6  is a side view  600  of the pulsejet with velocity contour plot of the exhaust plume at time t 1  designated by reference numeral  401  from  FIG. 4 . This plot shows the profile of different velocities in the exhaust plume at the end of the tail pipe outside of the pulsejet as the plume emanates therefrom. Units expressed in  FIG. 6  are in inches with velocities ranging from about 200-1100 ft/sec.  FIG. 6A  is an enlarged view of a portion of  FIG. 6  illustrating the velocity profile with better resolution. Reference numeral  602  illustrates a contour line of 200 ft/s and reference numeral  603  illustrates a contour line of 1100 ft/s. 
       FIG. 6A  is an enlarged view of the tail pipe  230  and the enlarged contour plot  601  of exhaust velocities shown at t 1 . Velocity contours are shown with a high velocity contour  603  located near the center of the velocity contour plot at a velocity of approximately 1100 ft/s. Lower velocity contours are located further from the tail pipe  230  at  602  showing a velocity of 200 ft/s. 
     LIST OF REFERENCE NUMERALS 
     
         
           100  Selected process steps 
           101  Process step of initiating a pulsejet 
           102  Process step of inserting, continuously, a high volume of metal into a combustion chamber of the pulsejet 
           103  Process step of combusting and controlling, resonantly, at high frequency a fuel-air mixture in the combustion chamber 
           104  Process step of heating the metal to a molten state 
           105  Process step of transporting the molten metal from the combustion chamber into a tail pipe of the pulse jet 
           106  Process step of transporting the molten metal within the tail pipe of the pulse jet at a high velocity 
           107  Process step of expelling the molten metal from the tail pipe of the pulse jet in a thermal spray at a high velocity 
           108  Process step of depositing the molten metal as a thermal spray onto a surface at the end of the tail pipe 
           200  Side view of pulsejet 
           200 A Perspective view of pulsejet 
           200 B Side view of pulsejet with thrust augmentation rig 
           201  Air line 
           202  Fuel line 
           203  First support 
           204  Second support 
           205  Substrate surface 
           206  Metal wire 
           207  Pulsejet 
           208  Distance from pulsejet to substrate surface 
           208 B Distance from tail pipe to substrate 
           210  Head 
           211  Inlet 
           212  Eductor 
           216  Feeding mechanism 
           220  Combustion chamber 
           221  Access port 
           221 A Fitting for access port 
           221 B Additional location for access port 
           221 C Additional location for access port 
           222  Spark plug 
           230  Tail pipe 
           231  Deposited coating 
           233  Ejector 
           234  Entrance to ejector 
           238  Distance from pulsejet to ejector 
           242  Distance from pulsejet to substrate surface 
           243  Length of ejector 
           244  Width of ejector 
           245  Pressurized ring 
           246  Flow of inert gas 
           247  Primary jet effluent 
           248  Secondary rig effluent 
           300  Assembly view of pulsejet 
           300 A Cross-Sectional view of combustion chamber along line  3 A- 3 A 
           300 B Cross-Sectional view of combustion chamber along line  3 B- 3 B 
           300 C End view of head along  3 C- 3 C 
           300 D Cross-Sectional view of head along  3 D- 3 D 
           309  Threaded Receptacle 
           313  Valve passageway 
           314  Venturi 
           315  Valve retainer bolt 
           317  Valve cover 
           318  Valve retainer 
           319  Valve Seat 
           323  Pressure tap 
           324  Line to controller 
           328  Spark plug gasket 
           329  Spark plug nut 
           350  Controller 
           360  Aerodynamic strut 
           361  Guide 
           362  Hollow bolt 
           400  Graph of combustion chamber pressure fluctuations 
           401  Specific instant in time t 1  relating to  FIG. 6   
           500  Graph of near exit plane velocity profile 
           600  Side view of pulsejet with contour plot exhaust velocities shown at t 1    
           600 A Enlarged view of pulsejet with contour plot 
           601  Enlarged contour plot of exhaust velocities at t 1    
           602  Contour, 200 ft/s 
           603  Contour, 1100 ft/s