Abstract:
A stand-off for maintaining the separation between an electrode and an object during a flow-through electroplating process is disclosed. The stand-off comprises a substantially streamlined shape that mitigates the effects of shadowing during deposition.

Description:
UNITED STATES GOVERNMENT INTEREST 
     The inventions described herein may be manufactured, used and licensed by or for the U.S. Government for U.S. Government purposes. 
    
    
     FEDERAL RESEARCH STATEMENT 
     The inventions described herein may be made, used, or licensed by or for the United States Government for government purposes without payment of any royalties thereon or therefore. 
     FIELD OF THE INVENTION 
     This invention relates generally to the field(s) of electro-chemical deposition and, more particularly to electroplating technology. 
     BACKGROUND OF THE INVENTION 
     Electro-chemical deposition processes are used to deposit materials on exposed surfaces of objects. Electro-plating is one well-known electro-chemical deposition process. To plate an object using an electro-plating process, the object and an electrode are placed in a bath of electroplating solution. The electrode is placed in proximity to the object to be plated and a voltage is applied between the electrode and the object. In the presence of the electric field, current flows through the electroplating solution and a chemical reaction occurs, the result of which is the deposition of the plating material on the object. Electro-plating is a commonly-used process for applying a layer of metal to an object. 
     To increase their lifetime, the interior surface of gun-barrels, such as those used in artillery pieces and tanks, are often coated with chromium using a “flow-through” electro-plating process. In order to coat the interior surface of a gun barrel, a copper electrode (of the appropriate diameter with respect to the center bore of the gun barrel) is inserted into the barrel during plating. Electroplating solution is flowed through the region between the electrode and the barrel while a voltage is applied between the electrode and the barrel. In the presence of the applied voltage, a current flows through the electroplating solution and chromium deposits on the interior surface of the gun barrel. 
     Conventional hard-chromium is electroplated using electric current of approximately 6,000 amperes. Low-contraction (LC) chromium, however, is highly desirable in many applications, including for coatings of gun barrel interiors. Unfortunately, the plating of LC chromium requires the use of a much higher current—as high as 48,000 amperes. The electric field associated with the electroplating of LC chromium induces a substantial mechanical force between the electrode and the gun barrel. As a result of this force, the electrode can bends to one side and electrically short to the gun barrel. Even if the electrode does not short to the gun barrel, however, the bending effect results in an uneven deposition of chromium on the gun barrel. 
     In an effort to eliminate bending of the electrode, stand-offs are inserted into the electrode. These stand-offs are installed both radially and axially along the length of the electrode, and provide a mechanical “stop” that helps maintain the separation between the electrode and the interior surface of the gun barrel. While these stand-offs do reduce the bending of the electrode, they interfere with the flow of electroplating solution through the length of the gun barrel. Due to flow effects, such as stagnation and eddying, the stand-offs cause a “shadowing” effect that reduces the plating thickness near the locations of the stand-offs. 
     Methods and apparatus which mitigate the problems associated with bending of the electrode while reducing the shadowing effect, is therefore desirable. 
     SUMMARY OF THE INVENTION 
     An advance is made in the art according to the principles of the present invention directed to a stand-off and electrode for use in electroplating systems—particularly electroplating systems that utilize high electric current and/or high electric fields during the coating process. The present invention is particularly well-suited for use in flow-through low-contraction chromium electroplating systems. In some embodiments, each of a plurality of stand-offs has a first end that is threaded. This threaded portion mates to any of a plurality of holes located in the electrode. Each of the plurality of stand-offs also has a second end that tapers to a small point wherein it may contact the inside surface of the object to be coated. In addition, each stand-off includes a body portion, between the first and second end, that is substantially streamlined for the direction of electroplating solution flow during the electroplating process. As a result, the present invention provides in an improvement in coverage uniformity as compared to electroplating systems known in the prior-art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  depicts a method for plating the inner wall of an object in accordance with an illustrative embodiment of the present invention; 
         FIG. 2A  depicts a cross-sectional side view of an electroplating system in accordance with the illustrative embodiment of the present invention; 
         FIG. 2B  depicts an end view of an electroplating system in accordance with the illustrative embodiment of the present invention; 
         FIG. 3A  depicts a side view of details of a stand-off in accordance with the illustrative embodiment of the present invention; 
         FIG. 3B  depicts a top view of details of a stand-off in accordance with the illustrative embodiment of the present invention; and 
         FIG. 3C  depicts a front view of details of a stand-off in accordance with the illustrative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a method for plating the inner wall of an object in accordance with an illustrative embodiment of the present invention. Method  100  is particularly suitable for electro-chemical deposition of low-contraction chromium on the inside of a cylindrical object, such as a gun barrel. Method  100  is described below with reference to  FIGS. 2A-2B  and  FIGS. 3A-3C . 
       FIGS. 2A and 2B  depict a cross-sectional side view and end view (respectively) of an electroplating system in accordance with the illustrative embodiment of the present invention. Electroplating system  200  is suitable for plating the inside surface of objects, such as the inner wall of a gun barrel. Electroplating system  200  comprises object  202 , electrode  206 , stand-offs  208 , and terminals  210  and  212 . 
     Object  202  is a gun barrel that comprises an electrically conductive, metallic cylinder having inner wall  204  and a center bore. The center bore and wall thickness of object  202  are suitable for launching a projectile, such as an artillery shell. In some embodiments, inner wall  204  is an electrically-conductive layer that is bonded to a cylinder comprising a material that is not electrically conductive. In some embodiments, object  202  is an object other than a gun barrel and comprises a shape that is different than a cylinder. It will be clear to those skilled in the art how to make and use object  202 . 
     Method  100  begins with operation  101 , wherein stand-offs  208  are attached to electrode  206 . 
     Electrode  206  is a copper rod having a diameter appropriate for the center bore of object  202 . Electrode  206  comprises a plurality of threaded holes for receiving a plurality of stand-offs  208 . The threaded holes are arrayed on the surface of electrode  206  in a pattern that has a radial and a longitudinal component. The pattern of the threaded holes is suitable for providing adequate support between electrode  206  and object  202  when a high electric field is induced between them. In some embodiments, electrode  206  acts as an anode during the process of electroplating. Although the illustrative embodiment comprises a plurality of stand-offs wherein the stand-offs are arranged at 90 degree increments around the circumference of electrode  206 , it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein a plurality of stand-offs are arranged in any manner suitable to maintain the relative position between electrode  206  and inner surface  204  in the presence of an applied electric field associated with the process of electro-chemical deposition. 
     Stand-offs  208  are mechanically rigid supports that provide support between electrode  206  and object  202  during the process of electroplating. Stand-offs  208  are shaped to be substantially streamlined in the direction of the flow of solution through the region between electrode  206  and object  202 . In other words, the shape of stand-off  208  is designed to provide minimal perturbation to the flow of electroplating solution through the gap between electrode  206  and object  202 . Stand-offs  208  comprise a material that is: (1) electrically insulating so that the stand-offs are capable of withstanding electric fields associated with electroplating without exhibiting electrical breakdown; and (2) substantially chemically inert with respect to the solution used for electroplating object  202 . Suitable materials for stand-off  208  include, without limitation, ceramics, high-density plastics, and glass. In some embodiments, stand-offs  208  comprise a material that degrades slightly during the process of electroplating. Stand-off  208  is described in more detail below and with respect to  FIGS. 3A-3C . The specific positions of stand-offs  208  along electrode  206  is a matter of design choice. The positions are selected, however, to ensure that the physical relationship between electrode  206  and object  202  remains substantially fixed during the process of electroplating. 
     In some embodiments, stand-offs  208  comprise alternative connective elements to screw threads, such as swage fittings, pressure fittings, etc. It will be clear to those of ordinary skill in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein stand-offs  208  are attached to electrode  206  using connective elements other than screw threads. 
     At operation  102 , electrode  206 , with attached stand-offs  208 , is inserted into the center bore of object  202 . 
     At operation  103 , electroplating solution is flowed through the cavity between electrode  206  and inner surface  204 . The direction of the flow of electroplating solution is a matter of design choice, and in some embodiments of the present invention the flow is opposite that shown in  FIG. 2A . 
     Terminals  210  and  212  are electrical terminals suitable for introducing the electrical voltages and currents associated with the process of electroplating. Terminal  210  is electrically connected to electrode  206 , and terminal  212  is electrically connected to electrically conductive inner wall  104  of object  202 . In some embodiments, terminal  212  is electrically connected to inner wall  104  through the thickness of the sidewall of object  202 . In some embodiments, terminals  210  and  212  are suitable for carrying electric currents as high as 50,000 amperes. 
     At operation  104 , a voltage differential is applied across terminals  210  and  212 . As a result, a flow of electric current is established through a path that includes terminal  210 , electrode  206 , the electroplating solution, inner surface  204 , and terminal  212 . In some embodiments, the magnitude of the established electric current is as high as 50,000 amperes. 
       FIGS. 3A ,  3 B, and  3 C depict a side view, top view, and front view (respectively) of details of a stand-off in accordance with the illustrative embodiment of the present invention. Stand-off  208  comprises body  302 , cone  304 , and threaded portion  308 . 
     Body  302  is a structural element that has an elliptical cross-section having length L 1  along its major axis, and length L 2  along its minor axis. In some embodiments, length L 1  is within the range of approximately 0.5 inches to approximately 2 inches. In some embodiments, L 1  is approximately 1 inch. In some embodiments, length L 2  is within the range of approximately 0.25 inches to approximately 1 inch. In some embodiments, L 2  is approximately 0.5 inches. The value of lengths L 1  and L 2  is a matter of design, and is influenced by the magnitude of the voltage applied to terminals  210  and  212 , the desired separation between electrode  206  and inner surface  204 , and the flow rate of electroplating solution through the region between electrode  206  and inner surface  204 . 
     Cone  304  is a tapered structural element whose cross-section transitions in size from that of the cross-sectional shape of body  302  to point  306 . In some embodiments, the height of cone  304  is within the range of approximately 0.5 inches to approximately 2 inches. In some embodiments, the height of cone  304  is approximately 1 inch. The shape of cone  304  is a matter of design; however, cone  304  should provide: (1) sufficient mechanical stability in the presence of the electric field associated with the process of electroplating; and (2) be able to withstand the force associated with the flow of electroplating solution. 
     The shape of body  302  and cone  304  is chosen to provide a substantially streamlined shape for the flow of electroplating solution. As a result of the shape of body  302  and cone  304 , electroplating solution does not become substantially depleted behind stand-off  208  (relative to the direction of the flow) due to stagnation or eddying of the fluid. 
     Threaded portion  308  comprises a thread that is suitable for mating to the threaded holes located on electrode  206 . 
     It is to be understood that the above-described embodiments are merely illustrative of the instant invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. For example, in this Disclosure, numerous specific details are provided in order to provide a thorough description and understanding of the illustrative embodiments of the instant invention. Those skilled in the art will recognize, however, that the invention can be practiced without one or more of those details, or with other methods, materials, components, etc. 
     Furthermore, in some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the illustrative embodiments. It is understood that the various embodiments shown in the Figures are illustrative, and are not necessarily drawn to scale. Reference throughout the disclosure to “one embodiment” or “an embodiment” or “some embodiments” means that a particular feature, structure, material, or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the instant invention, but not necessarily all embodiments. Consequently, the appearances of the phrase “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout the Disclosure are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.