Coating processes – Solid particles or fibers applied
Utility Patent
1998-12-21
2001-01-02
Parker, Fred J. (Department: 1762)
Coating processes
Solid particles or fibers applied
C427S190000, C427S191000, C427S421100, C118S308000, C118S309000, C239S079000, C239S080000
Utility Patent
active
06168828
ABSTRACT:
TECHNICAL FIELD
This invention relates to the field of gas detonation coating apparatus for industrial use for applying protective coatings to workpieces.
BACKGROUND ART
Many industrial applications exist where materials are exposed to severe environmental conditions of heat, wear and corrosion. Spray coating processes utilizing powder coating materials offer high quality protection in some of these applications. A common method of spray coating is the detonation gun process. This process uses kinetic energy from the detonation of combustible mixtures of gases to deposit powdered coating materials on workpieces.
Typical coating materials used in conjunction with detonation guns in the spray coating process include powder forms of metals, metal-ceramic, ceramic, erosion resistant, thermal protection, electrically insulating, electrically conductive, and other coating materials. In addition powder forms of other materials can be utilized in conjunction with the detonation gun process for parts cleaning, hole drilling, making powders, and other conceivable applications.
A typical detonation gun functions in the following manner. A certain amount of a combustible gas mixture, oxygen and acetylene for example, is fed into a tubular combustion chamber have a closed end and an open end where it is subsequently ignited by a spark plug. The ignition of the gas brings about detonation and the formation of a shock wave. The shock wave travels down the combustions chamber to the open end which is attached to a tubular barrel. A suitable coating powder is typically injected into the barrel in front of the propagating shock wave and is subsequently carried out the open end of the barrel and deposited onto a substrate positioned in front of the barrel. The impact of the powder onto the substrate produces a high density coating with good adhesive characteristics. The process is repeated in a rapid fashion until the workpiece is coated to satisfaction. Between successive ignitions an inert gas, such as nitrogen, may be fed into the combustion chamber after the ignition to halt combustion and prevent backfire into the fuel and oxygen supply and to purge the barrel of combustion products.
The mechanics of detonation are key to the operation of the detonation gun. Detonation produces shock waves that travel at supersonic velocities, as high as 4000 m/s, and elevated temperatures, as high as 3137° C. Detonation in the detonation gun is controlled by the type of fuel used, such as propane, acetylene, butane, etc., the fuel and oxygen mixture ratio, the initial pressure of the gases in the combustion chamber, and the geometry of the combustion chamber. After ignition of the fuel and oxygen mixture deflagration produces an initial detonation wave front that increases the temperature and pressure within the combustion chamber which in turn propagates ignition of the combustible mixture throughout the combustion chamber. Given the correct combination of parameters, the detonation continues to propagate until all available fuel and oxygen is consumed. The detonation front moves toward the open end of the combustion chamber and into the barrel. It is of particular importance that the combustion chamber be of sufficient length, for the specific detonable mixture in use, to complete the transition from deflagration to detonation before entering the barrel or the detonation wave front may not be sustained within the barrel. It is also important in the operation of a detonation gun to produce as strong a shock wave as possible and direct it to the barrel as efficiently as possible so that a large amount of the kinetic energy of the detonation wave goes directly to carrying the powder out of the barrel and onto the substrate.
At a fixed moment in time the detonation wave front is made up of a system of individual stationary detonation cells. The behavior of detonation at the cell level is an important attribute in the control and operation of a typical detonation gun. The detonation cell is a multidimensional structure which includes both the detonation wave front and transverse detonation waves moving perpendicular to the detonation front. The frontal surface of a detonation cell consists of convex shaped mach wave. Behind the mach wave is a reaction zone where the chemical reactions take place that lead to detonation. At the edge of the cell transverse shock waves form a substantially right angles to the frontal surface of the detonation cell. The transverse waves have acoustic tails that extend from the aft edges oft he transverse waves and define the aft edge of the detonation cell. The transverse waves move from cell to cell and reflect off of each other and off of any limiting structure such as the combustion chamber wall. Once detonation has been initiated the reaction continues in a fairly stable fashion. However, the detonation wave front structure can be negatively by collisions with reflecting transverse waves and reflecting refracted waves from the detonation front while moving through the combustion chamber. These collisions diminish the intensity of the detonation cells and therefor lessen the amount of kinetic energy available to be transferred to the coating powder. This reduction in energy transferred to the coating powders translates into a reduction of the coatings produced in terms of density and adherence with the substrate. The residuum of detonation wave front moves from the combustion chamber into the barrel and out onto the workpiece.
The size of the detonation cell is another important attribute in the control and operation of a detonation gun. Cell size is a function of the molecular nature of the fuel, the initial pressure within the combustion chamber, and the fuel/oxygen ratio. The particular cell size for certain conditions can be determined experimentally. The width of a cell, Sc, is measured along the wave front between successive transverse waves. The length of a cell, Lc, is the perpendicular distance from a line tangent to the wave front measured to the intersection point of the acoustic tails from adjacent transverse waves. The typical ratio of cell width to cell length is Sc=0.6 Lc for the detonable gases under consideration. The physical parameters of a particular typical detonation gun, such as the geometry and operating pressures, are determined by the cell size of a particular fuel and oxygen mixture.
The operating pressure within the combustion chamber is influenced by the behavior of the detonation cells. Prior to ignition the pressure within the combustion chamber is controlled by the fuel and oxygen supply pressures and the geometry of the combustion chamber. After ignition of the mixture the pressure within the combustion chamber increases and reaches a maximum when detonation occurs. As the detonation wave travels down the barrel and reaches the open end of the barrel a peak rarefaction pressure is measured within the combustion chamber. A positive pressure peak is then subsequently measured within the combustion chamber due to the presence of reflected waves from the detonation wave front.
In a typical detonation gun the coating powder, such as Amperit, is fed either directly into the barrel directly or into the combustion chamber and then carried into the barrel by inert gases ahead of the detonation wave. For example, a certain powder feeder utilizes a continuous supply of air or inert gas to carry the powder fed from a continuous source through a valve arrangement and finally into the gun. The operation of the valve is coordinated with the firing of the spark plug so that the powder and carrying gases are in position along the barrel to be properly effected by the detonation wave. Typically the valves are opened by mechanical means such as a cam and tappets or a solenoid. The disadvantage of these mechanisms is that they often limit the frequency at which the gun can fire because the valve must be opened far enough and long enough to permit the passage of the proper amount of powder through the valve. These mechanisms also pose reliability problems i
Barykin Georgy Yur'evich
Chernyshov Alexandr Vladimirovich
Aerostar Coating S.L.
McDermott & Will & Emery
Parker Fred J.
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