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 the deposition chamber. The entrance passage is formed from misaligned portions which press the filament into direct electrical contact with their walls. A tube communicates with a chamber between the ends of the passage and acts as a sealing means to prevent gas escaping from the deposition chamber through the entrance passage. 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 Serial No. (Attorney Docket No. 827.1.031) 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. 0815283.7, 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, an entrance electrode defines an entrance passage permitting the filament to slide into the deposition chamber whilst retaining direct electrical contact with the entrance passage, an exit electrode defines an exit passage permitting the coated filament to slide out of the deposition chamber whilst retaining direct electrical contact with the exit passage. 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]    A sealing means may be provided to prevent gas escaping from the deposition chamber through either the entrance passage or the exit passage. 
         [0014]    The sealing means for the entrance passage preferably comprises a gas inlet to supply gas to a working clearance between the filament and the entrance passage at a pressure greater than an operational pressure within the deposition chamber. Alternatively, the sealing means for the entrance passage may comprise a gas exhaust to remove gas escaping from the deposition chamber through a working clearance between the filament and the entrance passage. 
         [0015]    The sealing means for the exit passage preferably comprises a gas inlet to supply gas to a working clearance between the coated filament and the exit passage at a pressure greater than an operational pressure within the deposition chamber. Alternatively, the sealing means for the exit passage may comprise a gas exhaust to remove gas escaping from the deposition chamber through a working clearance between the coated filament and the exit passage. 
         [0016]    The entrance passage is preferably formed as two intersecting misaligned passages having a cross-section that will allow the filament to slide through them whilst being pressed by their misalignment into direct electrical contact with the entrance passage. The exit passage is preferably formed as two intersecting misaligned passages having a cross-section that will allow the coated filament to slide through them whilst being pressed by their misalignment into direct electrical contact with the exit passage. 
         [0017]    Preferably, the misaligned passages forming the entrance passage are positioned to guide the filament to the centre line of the deposition chamber, and the misaligned passages forming the exit passage are positioned to receive the coated filament from the centre line of the deposition chamber. 
         [0018]    At least one of the passages may be defined by a capillary tube in electrical contact with the appropriate electrode. One or more of the capillary tubes may be detachable from its electrode. 
         [0019]    The entrance and exit electrodes preferably 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, at least one of the electrodes may form an electrostatic circuit to produce an electrostatic charge to cause physical vapour deposition of the coating from material within the deposition chamber. 
         [0020]    According to another aspect of the invention a method of manufacturing a coated filament may include passing an electrically-conductive filament into a deposition chamber through an entrance passage in an entrance electrode, withdrawing the coated filament from the deposition chamber through an exit passage in an exit electrode, pressing the filament into direct electrical contact with the entrance passage and pressing the coated filament into direct electrical contact with the exit passage to establish 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). Alternatively, a method of manufacturing a coated filament may include passing an electrically-conductive filament into a deposition chamber through an entrance passage in an entrance electrode, withdrawing the coated filament from the deposition chamber through an exit passage in an exit electrode, pressing the filament against the wall of the entrance passage and the coated filament against the wall of the exit passage 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 an enlarged vertical cross-section similar to  FIG. 2  but illustrating an improved entrance electrode and entrance sealing means in accordance with the present invention, and 
           [0025]      FIG. 4  illustrates a modification of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    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 may be made, for instance, from borosilicate glass, quartz glass, or of metal provided that it is insulated from the end plates  12 ,  13 , or of ceramics. 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 . 
         [0027]    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 . 
         [0028]    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 100-200 μm and is defined by a watchmaker&#39;s ruby or sapphire  24 . 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. 
         [0029]    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 . 
         [0030]    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. 
         [0031]    The use of mercury has several disadvantages due to its toxicity. Operators of such known filament coating apparatus inevitably come into physical contact with mercury vapour, and/or liquid mercury droplets, with the result that bodily levels of this toxin increase. 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. 
         [0032]    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. 
         [0033]    With reference to  FIG. 3 , the present invention overcomes these disadvantages by modifying the end plate  12  to utilise an entrance sealing means that prevents the escape of reactive gas from the tube  11  past the end plate  12  without the use of mercury or any other low-melting point eutectic alloy, and to modify the end plate  12  to serve as the entrance electrode  14  again without the use of mercury or any other low-melting point alloy. These modifications to the upper end plate  12  are also incorporated in the lower end plate  13 . 
         [0034]    The end plate  12  is electrically conducting and is formed preferably from a metal, such as stainless steel, or from graphite. The entrance passage  20  has a small chamber  27  communicating with a tube  28  which is connected to an unshown waste gas disposal system having a gas pressure slightly less than the pressure of the reactive gases in the tube  11 . In this manner the chamber  27  and the tube  28  act as an entrance sealing means by preventing any of the reactive gases from escaping to atmosphere through the working clearance between the filament  18  and the upper portion of the entrance passage  20 . Alternatively, the tube  28  may supply an innocuous gas (for instance argon, or nitrogen, or some other gas appropriate to the coating process) under a pressure slightly greater than that in the tube  11 , whereby the tube  28  and chamber  27  again act as an entrance sealing means. With either embodiment, the respective entrance sealing means isolates the reactive gases within the tube  11  from the surrounding atmosphere. 
         [0035]    It will be noted from  FIG. 3  that the entrance passage  20  is formed as two misaligned portions  201  and  202  of which the axes intersect in the region of the chamber  27 . Each portion  201 ,  202  has an internal diameter that will provides a close sliding clearance for the filament  18  of which the diameter is typically between 20 μm and 200 μm. The upper portion  201  is displaced laterally (typically 1 mm or less) from a centre line  29  of the tube  11  whilst the lower portion  202  converges towards the centre line  29 . 
         [0036]    The filament  18  is deflected as it passes from the upper portion  201  into the misaligned lower portion  202  thereby ensuring excellent electrical contact between the filament  18  and the entrance passage  20 . Good electrical contact is essential to avoid arcing. 
         [0037]    In operation, the filament  18  can abrade its contact points in the upper and lower portions  201  and  202  of the entrance passage  20 . Such abrasion will be minimal because typical spooling tensions are likely to be only of the order of 10 g or less. Over an extended period, such wear could necessitate replacement of the entire entrance electrode  14 . However, this can be avoided by the modification shown in  FIG. 4  in which the upper portion  201  and lower portion  202  of the entrance passage  20  are defined by separate straight capillary tubes  30  and  31  inserted in the entrance electrode  14 . These capillary tubes  30  and  31  may conveniently be formed from standard capillary tube stock used in the manufacture of disposable hypodermic needles and could either be cast as inserts into the upper end plate  12 , or fitted into bores formed to receive them. In the latter event, the capillary tubes  30  and  31  could be replaceable but would preferably be a press fit, or locked with a grub screw, in order to prevent arcing. The use of capillary tubes  30  and  31  formed from capillary tube stock has the added advantage that they are made of a much harder material than the entrance end plate  12  and will experience much less abrasive wear. 
         [0038]    Instead of using two straight capillary tubes  30 ,  31  to form the misaligned portions  201  and  202 , the entrance passage  20  could be defined by a single curved capillary tube cast as an insert into the end plate  12  with the aperture for mounting the tube  28  and the chamber  27  being subsequently machined in the end plate  12  to intersect such curved capillary tube to the extent necessary to provide communication between its bore and the tube  28 . The use of a single curved capillary tube maximises electrical contact with the filament  18  and spreads potential wear over a larger contact area. 
         [0039]    As already indicated, the construction and operation of the lower end plate  13  is generally identical to that just described for the upper end plate  12 . The only point of difference is 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. 
         [0040]    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. 
         [0041]    If desired, at least one of the reactive gases may be passed into the tube  11  through at least one of the end plates  12  or  13  and this gas may, for instance, be supplied through the tube  28 . In this connection it should be noted that some of the reactive gases that may be used are innocuous within the environment inside the end plates  12 ,  13  whilst their reacted residues within the tube  11  must be kept away from the electrodes  14  and  16 . 
         [0042]    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 electrodes  14  and  16  can form part of an electrostatic circuit to produce an appropriate electrostatic charge on the filament. 
         [0043]    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.