Abstract:
A uniform coating of radar-absorbing material (RAM) is produced on small or intricate parts by suspending the part in a vessel, slowing filling the vessel with RAM slurry without turbulence from the bottom up, subsequently draining the slurry slowly without turbulence to leave a coating of RAM on the part, and repeating the process until a coating of sufficient thickness is obtained.

Description:
This application is a divisional of Ser. No. 08/889,785 filed Jul. 8, 1997, now U.S. Pat. No. 6,001,425. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a coating process for intricate parts, and more specifically to a process for coating parts with a ceramic radar-absorbing material (RAM) by flooding a vessel containing the part with a RAM slurry. 
     BACKGROUND OF THE INVENTION 
     In many military applications, there is a strong need to make aircraft, vehicles and other objects, including their component parts, as invisible to radar as possible. A number of techniques for accomplishing this purpose are well known. One such technique is to coat metallic parts with a ceramic radar-absorbing material. 
     Conventionally, the ceramic RAM is suspended in particulate form in a wet slurry which is sprayed onto the substrate of the part to be processed. Although this process is easy to use and is performable with readily available equipment, and has proven generally suitable for its intended purpose, it possesses inherent deficiencies which detract from its overall effectiveness and desirability. Specifically, the spray process has several disadvantages: for one, coatings of small, complex parts or parts with small internal diameters are difficult to obtain consistently; secondly, the spray process does not lend itself well to automation because variables such as coating thickness are difficult to control; and thirdly, a sprayed coating sometimes has difficulty adhering to the part with the result that electromagnetic performance is degraded. 
     Furthermore, because the slurry is a mixture of heavy and light particulates, it is important to maintain the slurry in a homogenous consistency. This can be done by maintaining the slurry in an agitated and/or flowing state, which keeps the heavier particles in suspension. 
     Other prior art methods include the following: 
     Nishio et al. U.S. Pat. No. 5,091,222 describes a method of ceramic coating in which the workpiece is dipped into a ceramic solution; 
     Van &#39;T Veen et al. U.S. Pat. No. 5,089,299 shows apparatus for applying a micropore coating to a ceramic substrate, in which the workpiece is moved with respect to the ceramic suspension. This is undesirable because movement of the part can disrupt the uncured coating. 
     Reed et al. U.S. Pat. No. 4,208,454 shows a coating process in which an alumina slurry is forced though a workpiece by a vacuum. 
     In view of the shortcomings of the prior art, it is desirable to provide a process which will uniformly coat parts regardless of their size or complexity, and will not be subject to the inherent inconsistencies arising from variations in spray patterns and from non-homogenity of the slurry. In this regard, although the prior art has recognized to a limited extent the nature of this problem, the proposed solutions have, to date, been ineffective in providing a satisfactory remedy. 
     SUMMARY OF THE INVENTION 
     The present invention specifically addresses and alleviates the above mentioned deficiencies associated with the prior art. More particularly, the present invention comprises positioning the part in a vessel, and flooding the vessel with a uniformly rising level of RAM slurry. When the part has become completely submerged, the RAM slurry is drained from the vessel, and the coated part is cured or dried. The process may be repeated as often as desired to obtain a thicker coating. 
     In the preferred embodiments of the invention, even flooding and recycling of the RAM slurry is obtained by introducing the slurry, and also removing it, from the bottom of the vessel. The slurry is preferably stored in a variable-volume container which is preferably subjected to pressure to force the slurry into the vessel, and to a vacuum or positive-displacement device to draw the slurry out of the vessel. This method not only allows the slurry to be readily reused from one part to the next but it also allows it to be stored in a sealed, contamination-free container. 
     These, as well as other advantages of the present invention will be more apparent from the following description and drawings. It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation of a first embodiment illustrating a basic aspect of the invention; 
     FIG. 2 is a side elevation of an inclined turntable illustrating a step in a method of coating parts in accordance with the invention; 
     FIG. 3 is a schematic view of an embodiment illustrating certain principles of the invention; 
     FIGS. 4 a  through  4   c  are schematic perspective views of an apparatus carrying out the three basic sequential steps of the inventive method; 
     FIGS. 5 a  and  5   b  are elevations of another embodiment carrying out the teachings of the invention; 
     FIG. 6 is an elevation illustrating a modification of the embodiment of FIG. 5; 
     FIG. 7 is an elevation of a sample panel showing a preferred embodiment of protection for the back of the sample panel; 
     FIG. 8 is a schematic view of an arrangement for maintaining circulation of the slurry during the use of the invention; 
     FIG. 9 is a schematic view of a dual pressure embodiment of the invention; and 
     FIGS. 10 a  and  10   b  schematically illustrate a plasma bag embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiments of the invention, and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the functions and the sequence of steps for constructing and operating the invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and sequences may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention. 
     FIG. 1 shows the invention in its most basic form. A panel  10  to be coated with ceramic RAM is suspended in a vessel  12  of an appropriate inert material such as Plexiglas. A partition  14  separates the panel  10  from an inlet  16  through which a RAM slurry  18  is introduced into the vessel  12 . The slurry  18  flows around the bottom end of partition  14  and gradually rises in the vessel  12  until it covers the panel  10 . After a short dwell time, during which the panel  10  is fully immersed in the slurry  18 , the drain valve  20  is opened. The slurry  18  then flows slowly out of the vessel  12  and leaves on the panel  10  a thin coating  19  (FIG. 2) of RAM. 
     The slurry is preferably a ceramic slurry containing a combination of very dense and light metallic particles, as is well known in the art. Preferably, it is introduced into the vessel  12  at a rate which causes the level of slurry  18  in the vessel to rise about 0.5-1.0 cm per minute, producing a homogeneous and even coating. After a dwell time of about 1 min., the slurry  18  is drained at the same rate. 
     A single application of slurry will deposit only a thin RAM coating. The thickness of the coating varies between about 0.13 and 0.25 mm depending upon the viscosity of the slurry, which typically ranges from 100 to 10,000 centripoise. Consequently, it is desirable to repeat the process several times until the desired thickness has been built up. The panel  10  may then be placed, if desired, on a rotating table  21  (FIG. 2) which may advantageously be rotated at a about 3 rpm at an inclination of about 17° to evenly distribute the coating by cold flow. When a sufficient thickness of coating has been built up, the panel  10  may then be heated to cure the ceramic. 
     Because the single use of the slurry exemplified by the embodiment of FIG. 1 is wasteful, it is preferable to reuse it by a system illustrated basically in FIG.  3 . In that figure, the slurry  18  is stored in an appropriate reciprocatable apparatus  22  which is connected through a conduit  24  to the bottom of the vessel  12  in which the panel  10  is suspended. Pushing the plunger of the device  22  injects the slurry  18  into the vessel  12  at a fully controllable rate, while withdrawing the plunger causes the slurry to be returned into the device  22  at an also fully controllable rate. 
     FIGS. 4 a  through  4   c  illustrate, in a schematic fashion, a more practical version of this concept. In FIG. 4 a , a slurry tank  26  is pressurized to force the slurry  18  into the vessel  12  (FIG. 4 b  ). After the workpiece  28  has been coated, a vacuum is applied to the tank  26  and the slurry is returned to the tank  26  (FIG. 4 c  ). 
     A practical application of this principle to the embodiment of FIG. 1 is shown in FIGS. 5 a  and  5   b . In FIG. 5 a  the valve  30  is opened to a supply  32  of inert gas, forcing the slurry  18  in tank  26  into the vessel  12 . In FIG. 5 b , the valve  30  is switched to the vacuum supply  36 , and the slurry  18  is sucked out of the vessel  12 . 
     It is important for the uniformity of the coating on panel  10  that the slurry  18  rise uniformly in vessel  12  without causing any flow patterns on panel  10 . To this end, it may be preferable to terminate the T fitting  34  in downwardly pointing outlets, so that any flow turbulence will be confined to the bottom of the vessel  12  (FIG.  6 ). 
     Some parts, such as electronic circuitry, may have to be protected from the slurry  18  during the coating of the substrate exemplified by panel  10 . This is typically done by a plastic coating to which the ceramic RAM does not adhere. However, as shown in FIG. 7, the plastic coating  40  is preferably confined to an area no closer than about 1 cm from the edge of panel  10 , as there is a danger that solvents in the plastic coating  40  on the back side of panel  10  may migrate around the edge of panel  10  during the cure, and interfere with the adhesion of the RAM coating to the front side of panel  10 . 
     To avoid a settling of the slurry  18 , it may be advantageous to use a system such as that shown in FIG.  8 . In that figure, a compressed inert gas  42  such as nitrogen may be used to provide the pressure to force slurry from the tank  26  into the vessel  12 . The slurry  18  in the tank  26  is continuously circulated by a pump such as the roller pump  44  depicted in FIG.  8 . To withdraw the slurry  18  from the vessel  12 , the pressurizing nitrogen gas may simply be vented at  46 , or a vacuum may be applied to the line  46 . 
     FIG. 9 illustrates a further refinement of the invention. In accordance with that modification, the vessel  12  is closed, and separate pressure sources  48 ,  50  are applied to the vessel  12  and the tank  26 , respectively. This approach has several advantages: for one, it allows the introduction into vessel  12  of inert gases such as nitrogen or argon to prevent skinning (i.e. the formation of a dried film or skin on the surface of the coating) and to promote drying of the coating; and for another, it allows emptying of the vessel  12  by positive pressure from source  48  rather than by a vacuum from source  50 . This reduces loss of volatiles in the slurry  18  while maintaining the slurry  18  free from contamination. 
     As shown in FIG. 9, the vessel  12  can be filled by making the pressure at  48  smaller than that at  50  (solid lines on gauges  49 ,  51 ), and emptied by making the pressure at  48  greater than that at  50  (dotted lines on gauges  49 ,  51 ). 
     In the foregoing embodiments, the natural agitation caused by the flow of the slurry has been used to maintain its particulates in suspension. Another method of agitating the slurry  18  is shown in FIG. 10, in which a plasma bag  52  is enclosed in the tank  26 . As the pressure in tank  26  is increased, the bag  52  is deformed from all sides, thus maintaining the slurry  18  agitated during the filling and emptying of vessel  12  without allowing the pressure medium to aerate it. 
     It is understood that the exemplary ceramic RAM film coating process as described herein and shown in the drawings represents only presently preferred embodiments of the invention. Indeed, various modifications and additions may be made to such embodiments without departing from the spirit and scope of the invention. Thus, other modifications and additions may be obvious to those skilled in the art and may be implemented to adapt the present invention for use in a variety of different applications.