Abstract:
A coating is formed by chemical vapour deposition an electrically heated filament which is passed through an end plate into a deposition chamber and leaves the deposition chamber through a similar end plate. The filament slides through an entrance passage into a first electrode chamber, around part of a wheel electrode into the deposition chamber. The passage of the filament around the wheel electrode provides adequate direct electrical contact. The end plate operates in exactly the same manner. As no mercury or a low-melting point eutectic alloy is used, no contaminants associated therewith are produced and the resultant coated filament is free of such contaminants.

Description:
RELATED APPLICATION 
       [0001]    The present application is related to Ser. No. ______ (Attorney Docket No. 827.1.030) for “Coated Filaments And Their Manufacture,” filed on Aug. 29, 2008. Also this application claims foreign priority benefits under 35 U.S.C. 119 of prior United Kingdom Application No. 0815296.9, filed on Aug. 22, 2008, the entire disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to coated filaments, and to an apparatus and a method for their formation. 
       BACKGROUND TO THE INVENTION 
       [0003]    It is well known to deposit a coating on an electrically conductive filament using chemical vapour deposition techniques. Typically the electrically conductive filament is passed continuously through a deposition chamber containing an appropriate gas or gases whilst the filament is heated by the passage of an electrical current, and the gas or gases deposit a coating on the hot filament. This is a process of “chemical vapour deposition” or CVD and essentially requires the provision of gas seals around the electrical contacts to the filament at both ends of the deposition chamber. 
         [0004]    EP 0 396 333 teaches that silicon carbide may be coated on a tungsten filament which passes through electrodes at the ends of a deposition chamber, the entrance electrode is a pool of mercury and the exit electrode is a mercury/indium amalgam. The pool of mercury and the mercury/indium amalgam both serve the dual function of providing a gas seal around and an electrical contact to the tungsten filament. 
         [0005]    U.S. Pat. No. 3,622,369 and U.S. Pat. No. 4,127,659 both describe similar processes for depositing silicon carbide on a filament. 
         [0006]    EP 0 396 332 teaches that an exit electrode for a ceramically coated filament should, instead of using mercury, utilise a liquid metal mixture of mercury/indium or mercury/cadmium amalgam or a gallium/indium mixture. 
         [0007]    EP 0 450 760 teaches that carbon may be coated on a filament which comprises a tungsten core coated with silicon carbide and is passed through mercury electrodes at the ends of a deposition chamber. 
         [0008]    EP 0 598 491 teaches that a layer of titanium carbide can be deposited on a tungsten core as an intermediate layer, an outer layer being of silicon carbide. Again, mercury electrodes are used at the ends of the deposition chamber. 
         [0009]    These CVD techniques for producing coated filaments can be applied to different electrically conductive core materials capable of being heated electrically by the direct application of electrical current, or by induction, and to a range of coatings provided by an appropriate selection of reactive gas or gases. 
       SUMMARY OF THE INVENTION 
       [0010]    We have found that these techniques for producing coated filaments inevitably result in mercury contamination of the coated filament. Such contamination occurs by the physical contact of the filament with liquid mercury forming the entry electrode, and by physical contact of the coating with liquid mercury forming the exit electrode. Further contamination occurs due to the production of mercury vapour by both of the electrodes. Some of this mercury vapour adheres to the filament as it approaches the deposition chamber and some adheres to the coating as the coated filament leaves the exit electrode. Mercury vapour also enters the deposition chamber and mingles with the gas or gases that produce the coating with the result that mercury may be incorporated within the coating. Mercury vapour additionally issues from the vicinity of both mercury pools and constitutes a potential health hazard. Similar problems occur with the use of liquid metal as the electrodes, for instance mercury/indium, mercury/cadmium or gallium/indium, in which case the contaminants would of course be mercury, indium, cadmium, and/or gallium. 
         [0011]    As a result, the coated filament is compromised by contaminants which are on, within or under the coating. To some extent surface contaminants can be cleaned off the surface of the coating, but contaminants within or under the coating cannot readily be removed. 
         [0012]    According to one aspect of the invention a filament coating apparatus comprises a deposition chamber in which a coating is to be applied to the filament, a first electrode structure having an entrance passage permitting the filament to slide into the electrode chamber and an exit passage permitting the filament to slide from the electrode chamber into the deposition chamber, a second electrode structure having an entrance passage permitting the coated filament to slide out of the deposition chamber into the second electrode chamber and an exit passage permitting the coated filament to slide out of the second electrode chamber, the first electrode chamber housing a first roller electrode means providing direct electrical contact with the filament, and the second electrode chamber housing a second roller electrode means providing direct electrical contact with the coated filament. In this manner the filament can be coated without the use of liquid metal as the electrode and will not be contaminated by mercury, indium, cadmium or gallium. 
         [0013]    Preferably, sealing means is provided to inhibit the escape of gas from the deposition chamber into either electrode chamber. 
         [0014]    Each electrode chamber may be provided with a gas inlet to supply gas at a pressure greater than an operational pressure within the deposition chamber. Alternatively, each electrode chamber may be provided with a gas outlet to reduce its internal pressure to below atmospheric pressure. Each sealing means may comprise a gas outlet to remove gas escaping into its electrode chamber from the deposition chamber. 
         [0015]    The first roller electrode means may comprise an electrode wheel positioned relative to its entrance passage and its exit passage to ensure adequate direct electrical contact with the filament. Similarly, the second roller electrode means may comprise an electrode wheel positioned relative to its entrance passage and its exit passage to ensure adequate direct electrical contact with the coated filament. Preferably the entrance passage to the first electrode and the exit passage from the second electrode are both horizontal. 
         [0016]    Alternatively, at least one of the roller electrode means may comprise at least two opposed wheels positioned to press against opposite sides of the filament or the coated filament to ensure adequate direct electrical contact, and at least one of the opposed wheels is an electrode. In this event the roller electrode means preferably comprises three wheels positioned such that two of them press against one side of the filament or the coated filament and the third wheel is pressed against the opposite side of the filament or coated filament between the first and second wheels. Preferably one of the wheels of the roller electrode means is positioned to guide the filament or coated filament into alignment with the appropriate exit passage. 
         [0017]    Alternatively, at least one of the roller electrode means may comprise a roller electrode mounted for rotation about an axis that is oblique to a line between the associated entrance passage and the associated exit passage whereby the filament or coated filament can be wound at least once around the roller whilst passing from the entrance passage to the exit passage to ensure adequate direct electrical contact. The roller electrode may have a spiral surface for engaging the filament. 
         [0018]    The first roller electrode and the second roller electrode may form an electrical circuit for heating the filament to cause chemical vapour deposition of the coating from a gas or gases within the deposition chamber. Alternatively, the first roller electrode and the second roller electrode may form an electrostatic circuit to produce an electrostatic charge to cause physical vapour deposition of the coating from material within the deposition chamber. 
         [0019]    According to another aspect of the invention, a method of manufacturing a coated filament may include passing an electrically-conductive filament over a first roller electrode into a deposition chamber, withdrawing the coated filament from the deposition chamber over a second roller electrode, using the first roller electrode to establish direct electrical contact with the filament, using the second roller electrode to establish direct electrical contact with the coated filament to provide an electrical heating circuit through the filament, and passing at least one thermally-reactive gas into the deposition chamber to form the coating by chemical vapour deposition (CVD). Preferably leakage of the thermally-reactive gas past the roller electrodes is prevented. 
         [0020]    Alternatively, a method of manufacturing a coated filament may include passing an electrically-conductive filament over a first roller electrode into a deposition chamber, withdrawing the coated filament from the deposition chamber over a second roller electrode, and using the first roller electrode to establish an electrostatic circuit to cause physical vapour deposition (PVD) of the coating from material within the deposition chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention is now described, by way of example only, with reference to the accompanying drawings in which: 
           [0022]      FIG. 1  is a diagrammatic side elevation of a known filament coating apparatus using chemical vapour deposition CVD, 
           [0023]      FIG. 2  is an enlarged vertical cross-section, taken on the line  2 - 2  in  FIG. 1 , illustrating a known combined entrance electrode and entrance sealing means; 
           [0024]      FIG. 3  is a diagram showing, in vertical section, one form of filament coating apparatus as taught by the present invention; 
           [0025]      FIG. 4  is an enlarged vertical cross-section similar to  FIG. 2  but illustrating the provision of an alternative roller electrode means as taught by the present invention, and 
           [0026]      FIG. 5  is similar to  FIG. 4  but illustrates the provision another form of roller electrode. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    With reference to  FIGS. 1 and 2 , a known construction of filament coating apparatus is indicated generally by arrow  10  and consists of a long vertical tube  11  closed by end plates  12  and  13 . The tube  11  is about 4 metres long and is made of borosilicate glass. The upper end plate  12  acts as an entrance electrode  14  and as a housing for an entrance sealing means  15  as shown in  FIG. 2 . The lower end plate  13  acts as an exit electrode  16  and as a housing for an exit sealing means  17  which is of identical construction to the entrance sealing means  15 . 
         [0028]    A suitable electrically-conducting filament  18 , for instance a tungsten wire or a carbon fibre, is fed from a supply spool  19 , through an entrance passage  20  in the entrance electrode  14  into the longitudinal tube  11 , and progresses through an exit passage  21  in the exit electrode  16  to a storage spool  22 . The supply spool  19  and the storage spool  22  form parts of an otherwise unshown spooling mechanism which continually moves the filament  18  at an appropriate speed through the tube  11 . 
         [0029]    With reference to  FIG. 2 , the entrance electrode  14  is made of metal or glass and defines the upper and lower ends of the entrance passage  20  which are separated by a small reservoir  23  containing a pool of about 0.5 cm 3  of liquid mercury retained by surface tension in the much narrower lower end of the entrance passage  20 , this has a diameter of typically 10-300 μm and is defined by a watchmaker&#39;s ruby  24 . Instead of ruby, sapphire, ceramic or glass may be used but would need to be replaced more frequently. The pool of liquid mercury has a dual function in forming a sealing means that allows the filament  18  to slide through the entrance passage  20  whilst providing indirect electrical contact between the end plate  12  and the filament  18 . Instead of using mercury, other low melting point eutectics have been used. 
         [0030]    A potential difference of typically 4 KV is applied across the electrodes  14 ,  16  to their respective mercury contacts with the filament  18  thereby causing a current to flow through the filament  18  and its coating to create a desired temperature rise, typically to between 800° C. and 1500° C. Reactive gases are passed into the tube  11  through an inlet  25 , and exit through an outlet  26 . These gases react at, or near, the hot surface of the filament  18  and deposit a coating of which the thickness increases as the filament passes through the tube  11 . The coating thickness of the coated filament where it enters the exit passage  21  is typically 5-10 times the diameter of the filament. For this reason, the diameter of the exit passage  21  is correspondingly larger than that of the entrance passage  20 . Apart from having a larger exit passage  21 , the configuration and operation of the exit electrode  16  is identical to that already described with reference to the entrance electrode  14 . 
         [0031]    The coated filament has a variety of uses dependant on the composition of the coating, for instance the fabrication of high performance metal-matrix composites. 
         [0032]    The use of mercury has several disadvantages due to its toxicity. Operators of such known filament coating apparatus could come into physical contact with mercury vapour, and/or liquid mercury droplets, should they fail to follow appropriate health and safety guidelines. Some of the mercury is transferred to the surface of the filament and to the coated filament by the liquid mercury in the reservoirs  23 , and any mercury leaking into the tube  11  may become incorporated in the filament coating and/or be entrained in the waste gas exiting through the gas outlet  26  thereby necessitating precautions in its disposal. Traces of mercury on or in the coated filament are a potential hazard to users of the coated filament and could also adversely affect the physical properties of the coating, its adherence to the filament, and particularly its adherence to the metal in a metal-matrix composite. 
         [0033]    Attempts have been made to replace the mercury with a variety of low-melting point eutectic alloys, but these all incur the release of associated toxins and suffer from equivalent disadvantages. 
         [0034]      FIGS. 3 ,  4  and  5  use the same reference numerals as  FIGS. 1 and 2  to denote equivalent elements which have the same function as already described. However the embodiments of  FIGS. 3 ,  4  and  5  overcome the above-mentioned disadvantages by modifying the end plates  12 ,  13  to define electrode chambers  32  and  33  containing respective roller electrodes in the form of electrode wheels  34 ,  35  to achieve direct electrical contact with the filament  18  and the coated filament  36 , without the use of mercury or any other low-melting point alloy. 
         [0035]    With specific reference to  FIG. 3 , a potential difference +V−0V is applied across the electrode wheels  34 ,  35  to cause the heating current to flow along the filament  18  from its point of contact with the electrode wheel  34  to the point of contact between the electrode wheel  35  and the coated filament  36 . The filament  18  enters the entrance passage  20  to the electrode chamber  32  via a sealing means  37  and leaves the electrode chamber  32  via the sealing means  15 , whilst the coated filament  36  enters the electrode chamber  33  via the sealing means  17  and leaves the electrode chamber  33  via the exit passage  21  and a sealing means  38 . The electrode chambers  32 ,  33  are preferably operated at a sub-atmospheric pressure applied through outlets  39 ,  40  with the sealing means  15 ,  17  acting as gas to vacuum seals and the sealing means  37  and  38  acting as air to vacuum seals. In this manner, the sealing means  15 ,  17  inhibit escape of the reactant gases from the tube  11  into the electrode chambers  32 ,  33  and the sealing means  37 ,  38  inhibit the entry of atmospheric air into the electrode chambers  32 ,  33 . Any reactant gas and/or atmospheric air entering either of the electrode chambers  32 ,  33  will be sucked out through the respective outlet  39 ,  40  and can be fed through a suitable cleaner/neutraliser. Instead of operating the electrode chambers  32 ,  33  at a sub-atmospheric pressure, an innocuous gas for instance argon, or nitrogen, or some other gas appropriate to the coating process) may be supplied at a pressure slightly greater than that in the tube  11  via respective inlets  41  and  42 . With either embodiment, the reactive gases within the tube  11  are isolated from the surrounding atmosphere. 
         [0036]    The electrode wheels  34 ,  35  are positioned relative to their respective inlet and outlet passages  20 ,  21  and have a diameter selected so that the filament  18  or the coated filament  36  is in adequate direct electrical contact whilst not being damaged by the radius of curvature. In this manner, the filament  18  can enter horizontally and the coated filament  36  can exit horizontally, thereby minimising the height of the filament coating apparatus  10 . 
         [0037]    With specific reference to  FIG. 4 , the end plate  12  is modified so that the filament  18  enters the electrode chamber  32  through a first plate  42  defining the entrance passage  20 , and leaves the electrode chamber  32  through a second plate  43  defining an exit passage  44 . The passages  20 ,  44  have internal diameters that will provides a close sliding clearance for the filament  18  of which the diameter is typically 14 μm. To ensure good wear resistance, the plates  42 ,  43  are made out of ruby or sapphire, for instance jeweller&#39;s rubies or sapphires may be used, but could be made of ceramic or glass if more frequent replacement is acceptable. The roller electrode means  14  comprises three wheels  45 ,  46  and  47  positioned such that the wheels  45 ,  46  press against one side of the filament  11  whilst the third wheel  47  is opposed to the wheels  45 ,  46  and is pressed against the opposite side of the filament  18  by a compression spring  48  to provide even filament tension. As illustrated, the wheels  45 ,  46  and  47  are mounted for rotation from respective brackets carried by the end plate  14  to receive the potential +V. Provided the tension in the filament  18  is adequate to ensure adequate direct electrical contact, one of the wheels  46  or  47  may be omitted. It will be noted from the drawing that the filament  18  is deflected as it passes the wheels  45 ,  46  and  47  thereby ensuring excellent electrical contact with the filament  18 . Good electrical contact is essential to avoid arcing. The degree of overlap between the wheels  45 ,  46  and  47  may be fixed for a filament  18  or coated filament  36  of particular diameter but may be variable to accommodate different diameters. 
         [0038]    The electrode chamber  32  can be provided with an outlet  39  and an inlet  41  which can be operated as described with reference to  FIG. 3 . 
         [0039]    The exit electrode  16  will be constructed in the same manner as the entrance electrode  14  as just described with reference to  FIG. 4 , the only point of difference being that the exit passage  21  in the lower end plate is essentially of greater diameter to permit the much larger diameter of the coated filament to slide through it. 
         [0040]    With reference to  FIG. 5 , the roller electrode means  14  comprises a roller electrode  49  mounted for rotation about an axis X-X that, as shown, is oblique to a line between the entrance passage  20  and the exit passage  44 . The filament is wound once around the roller electrode  49  to ensure adequate electrical contact. The roller electrode  49  is shaped to define a spiral surface  50  which engages the filament  18  and retains it in position whilst the electrode  49  rotates to ensure adequate direct electrical contact. Although the roller electrode is shown journaled from the plates  42  and  43 , it could be mounted for rotation from other structure electrically connected to the entrance electrode  14 . 
         [0041]    The exit electrode  16  will again be constructed in the same manner as the entrance electrode  14  as just described with reference to  FIG. 5 , the only point of difference being that the exit passage  21  in the lower end plate  12  is essentially of greater diameter to permit the much larger diameter of the coated filament to slide through it. 
         [0042]    If desired, the longitudinal tube  11  could be sufficiently large to process several filaments  18  using either single end plates  12 ,  13  serving respectively as entrance and exit electrodes  14 ,  16 , or could carry a separate pair of electrodes for each filament. 
         [0043]    The various roller electrodes  34 ,  35 ,  45 ,  46 ,  47  or  49  may have flat or grooved contact surfaces, and such grooves may be v-shaped or radiused. Further rollers or wheels may be provided to operate in one or more planes to align or otherwise control the path of the filament or of the coated filament. 
         [0044]    The various roller electrodes  34 ,  35 ,  45 ,  46 ,  47  or  49  may be formed from metal or from an alternative conducting material. They may be designed to wear preferentially to the filament  18  or the coated filament  36 , or be hard enough to withstand such wear. 
         [0045]    To this point the description has related to apparatus for, and methods of, chemically depositing a coating on a filament  18 . The apparatus and method can also be applied to the physical deposition of a coating on a filament, for instance by sputtering, electrostatic painting or vacuum deposition. In such cases the roller electrode means  14  and  16  can form part of an electrostatic circuit to produce an appropriate electrostatic charge on the filament. 
         [0046]    Although various embodiments of the invention have been shown and described herein, they are not meant to be limiting. Those of skill in the art may recognize certain modifications to these embodiments, which modifications are meant to be covered by the spirit and scope of the appended claims.