Abstract:
Components of a dental brace including arch wires and brackets are coated with a hard, wear resistant material that has a low coefficient of friction. The aesthetically pleasing coating encapsulates the substrate material preventing the release of toxins from the substrate material that would otherwise occur due to wear, galling or corrosion. The coating includes a first layer of a metal which is preferably Titanium, Zirconium or Hafnium, a second layer that is preferably a Nitride of the metal used in the first layer and a third layer that is preferably a Nitride of the metal used in the first layer and has approximately two metal atoms for every Nitrogen atom. The coating is preferably applied using a physical vapor deposition source such as a cathodic arc source with a controlled gas atmosphere.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention pertains generally to coated dental braces and methods for coating dental braces. More particularly, the present invention pertains to dental braces that are coated with a hard, wear resistant coating. The present invention is particularly, but not exclusively, useful for brackets and arch wires for dental braces having a metal Nitride coating.  
         BACKGROUND OF THE INVENTION  
         [0002]    It is often necessary to use dental braces to correct the irregular positioning of teeth. Components of a dental brace typically include a plurality of brackets and an arch wire. In use, a bracket is bonded to each tooth on one or both jaws of the patient. The arch wire is then positioned in a slot of the bracket and tightened to selectively move the patient&#39;s teeth into their correct positions. The interaction between the arch wire and the bracket is sometimes referred to as “sliding mechanics.” Typically, the arch wire is tightened periodically (e.g. once every three to four weeks) to slowly move the teeth into their correct positions.  
           [0003]    Tightening of the arch wire causes the arch wire to move relative to each bracket. This sliding contact between the arch wire and bracket can cause wear and galling at the contact surfaces of both the arch wire and bracket. The effects of wear and galling, in turn, can present a resistance to sliding that can interfere with the tightening process and lead to a nonuniform distribution of stresses in the arch wire that can adversely affect the correction procedure. Heretofore, brackets have been generally made of stainless steel. More recently, to avoid the Nickel and Chromium in stainless steel, brackets have been made of a Titanium alloy that is, however, relatively soft and susceptible to wear and galling at the contact surfaces. Accordingly, it would be desirable for the contact surfaces between the brackets and arch wire to have a low coefficient of friction that is maintained over the course of the correction procedure.  
           [0004]    In addition to interfering with the sliding contact between the brackets and arch wire, wear and galling can result in the removal of material from the arch wire or brackets that can result in the unnecessary exposure of the patient to a hazardous material. For example, arch wires are typically made of either a stainless steel alloy containing Chromium and Nickel or a Titanium-Nickel alloy. These materials, especially Chromium and Nickel, are generally considered to be toxins, and may be carcinogenic, to which patient exposure should be avoided. Along these same lines, corrosion can result in the release of potentially toxic materials if the materials used to prepare the braces and arch wires are not corrosion resistant. Corrosion can also affect the aesthetics of the braces which is an important factor in the design of dental braces.  
           [0005]    In light of the above, it is an object of the present invention to provide components for dental braces that are coated with a material that is hard, wear resistant, corrosion resistant, and has a low coefficient of friction. It is another object of the present invention to provide methods for coating components for dental braces with a hard, wear resistant, low-friction coating. It is yet another object of the present invention to provide a coating for dental brace components that is aesthetically pleasing and that encapsulates the component substrate material to prevent patient exposure to toxins and allergens in the substrate material.  
         SUMMARY OF THE PREFERRED EMBODIMENTS  
         [0006]    The present invention is directed to coated dental braces and methods for coating dental braces. Components of a dental brace including arch wires and brackets are coated with a coating material that is hard, wear resistant and has a relatively low coefficient of friction. By encapsulating the substrate material (i.e. the underlying material that the uncoated arch wires and brackets are made of) the coating prevents the release of substrate material into the patient&#39;s mouth that would otherwise occur in the absence of the coating due to wear, galling or corrosion.  
           [0007]    For the present invention, the coating includes a first layer of a metal which is preferably Titanium, Zirconium or Hafnium. The first layer overlays and contacts a portion or all of the dental brace component and preferably overlays portions of the component that will be visible when the dental brace is worn. Importantly, the first layer overlays the portion of each component that contacts and interacts (i.e. wear surfaces) with other components. In one implementation of the present invention, the entire arch wire is coated and all portions of each bracket are coated except the surface of the bracket that is to be bonded to a tooth (hereinafter referred to as the bonding face).  
           [0008]    The coating further includes a second layer that overlays and contacts the first layer. The second layer is preferably a Nitride of the metal used in the first layer. For example, for a coating having Titanium as the first layer, the second layer is preferably Titanium Nitride (TiN). Similarly, for a coating having Zirconium as the first layer, the second layer is preferably Zirconium Nitride (ZrN) and for a coating having Hafnium as the first layer, the second layer is preferably Hafnium Nitride (HfN). Note; the abbreviations (e.g. TiN, ZrN and HfN) are used herein as a shorthand rather than an exact chemical label, and do not suggest that the stoichiometry of the indicated compound must be exactly as stated in the abbreviation.  
           [0009]    The coating further includes a third layer that overlays and contacts the second layer. For the present invention, the third layer is the top layer, constituting the material that is exposed and visible when the braces are worn. The third layer is preferably a Nitride of the metal used in the first layer and has approximately two metal atoms for every Nitrogen atom. For example, for a coating having Titanium as the first layer and Titanium Nitride (TiN) as the second layer, the third layer is preferably a so-called di-Titanium Nitride (Ti x N) wherein the Nitrogen level has been reduced to obtain a bright, lustrous, silver look. Similarly, for a coating having Zirconium as the first layer and Zirconium Nitride (ZrN) as the second layer, the third layer is preferably a so-called di-Zirconium Nitride (Zr x N). Likewise, for a coating having Hafnium as the first layer and Hafnium Nitride (HfN) as the second layer, the third layer is preferably a so-called di-Hafnium Nitride (Hf x N).  
           [0010]    In accordance with the present invention, the coating is preferably applied using a physical vapor deposition source such as a cathodic arc source with a controlled gas atmosphere. Other operable techniques such as magnetron sputtering may also be used. During coating deposition, the brackets and arch wires are held in fixtures and the fixtures are rotated in a planetary movement about a central axis. In greater detail, the brackets are held in a fixture that includes a plate that is formed with a plurality of relatively shallow slots. Each slot extends around the plate in a closed loop. A plurality of brackets are placed somewhat loosely in each slot with the bonding face of each bracket oriented face-down in the slot. The remainder of the bracket protrudes from the slot, thus exposing bracket surfaces other than the bonding surface to the vapor in the chamber. Accordingly, these exposed surfaces are coated while the bonding face remains uncoated. Because each slot is formed in a closed loop, brackets are prevented from ‘walking’ off the plate in spite of the fact that the brackets are subject to rotational movement and minor vibrations. The slotted plate separates the brackets and prevents the brackets from movement (i.e. tipping) into non-desirable orientations during coating.  
           [0011]    The arch wires are held in a fixture that includes a pair of wire screens with each screen creating a plurality of apertures. The screens are aligned parallel to each other and spaced apart to allow the arch wires to hang from screen to screen. More specifically, a first end of each arch wire is inserted into a respective aperture of the first screen and the second end of each arch wire in inserted into a respective aperture of the second screen. This cooperation of structure allows a plurality of arch wires to be uniformly spaced from each other in the deposition chamber. Further, the screens function as an ionization diffuser as the arch wires are being coated.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:  
         [0013]    [0013]FIG. 1 is a perspective view of the mouth of a patient wearing dental braces;  
         [0014]    [0014]FIG. 2 is an enlarged, perspective view of a coated bracket for dental braces;  
         [0015]    [0015]FIG. 3 is an enlarged cross-sectional view of a portion of the coated bracket shown in FIG. 2 as seen along line  3 - 3  in FIG. 2 showing the coating layers;  
         [0016]    [0016]FIG. 4 is a perspective view of a fixture for supporting the brackets during the coating process;  
         [0017]    [0017]FIG. 5 is a cross-sectional view as seen along line  5 - 5  in FIG. 4 showing a bracket positioned in a slot of the fixture;  
         [0018]    [0018]FIG. 6 is a perspective view of a fixture for supporting arch wires during the coating process;  
         [0019]    [0019]FIG. 7 is a schematic plan view and control diagram of a deposition apparatus for use in the invention;  
         [0020]    [0020]FIG. 8 is a schematic perspective view of a detail of the deposition apparatus of FIG. 6; and  
         [0021]    [0021]FIG. 9 is a schematic cross-sectional view of a preferred cathodic arc source, taken along lines  9 - 9  of FIG. 8. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]    Referring to FIG. 1, dental braces are shown positioned on the teeth of a patient&#39;s upper jaw and generally designated  10 . As shown in FIG. 1, the dental braces  10  include a plurality of brackets  12  and an arch wire  14 . A bracket  12  is bonded to each tooth  16  on the patient&#39;s upper jaw and a ligating module (e.g. a rubber band) is used to attach the arch wire  14  to a respective bracket  12 . The ends of the arch wire  14  can then be attached to buccal tubes (not shown) that are mounted on the patient&#39;s molars. The buccal tubes allow the arch wire  14  to be tightened by the orthodontist to selectively move the patient&#39;s teeth into their correct positions.  
         [0023]    A better appreciation of a bracket  12  can be obtained with reference to FIG. 2. As shown, the bracket  12  includes a base  18  having a bonding face  20  that is bonded to the tooth  16 . In some cases, a mesh (not shown) is interposed between the tooth  16  and the bonding face  20  to facilitate removal of the bracket  12  at the conclusion of the corrective procedure. As further shown in FIG. 2, the bracket  12  includes contact surfaces  22  which will be in sliding contact with the arch wire  14  during use (See FIG. 1). Also shown, four projections  24   a - d  extend from the base  18  allowing a ligating module to be wrapped around the projections  24   a - d  to secure the arch wire  14  to the bracket  12 .  
         [0024]    Referring now to FIG. 3, a coating  26  is shown applied to a substrate  27  that can be either the bracket  12  or the arch wire  14 . In the case of the bracket  12 , the substrate  27  is typically made of a corrosion resistant material such as a Titanium alloy or stainless steel. In the case of the arch wire  14 , the substrate is typically made of stainless steel or a shape memory alloy such as a Nickel-Titanium alloy. In either case, as shown in FIG. 3, the coating  26  includes a first layer  28  of a metal which is preferably Titanium, Zirconium or Hafnium. The use of a metal for the first layer  28  ensures that the coating  26  adheres strongly to the substrate  27 . The coating  26  preferably overlays and contacts portions of the bracket  12  that will be in sliding contact with the arch wire  14  (i.e. contact surfaces  22  shown in FIG. 2) and all other surfaces that will be visible when the dental brace  10  is worn. Accordingly, the coating  26  preferably overlays and contacts the entire arch wire  14 .  
         [0025]    Continuing with FIG. 3, it can be seen that the coating  26  further includes a second layer  30  that overlays and contacts the first layer  28 . The second layer  30  is preferably a Nitride of the metal used in the first layer  28  having approximately one metal atom for each Nitrogen atom (i.e. a monometal Nitride). For example, for a coating  26  having Titanium as the first layer  28 , the second layer  30  is preferably Titanium Nitride (TiN). Similarly, for a coating having Zirconium as the first layer  28 , the second layer  30  is preferably Zirconium Nitride (ZrN) and for a coating having Hafnium as the first layer  28 , the second layer  30  is preferably Hafnium Nitride (HfN). Note: as indicated above, the abbreviations (e.g. TiN, ZrN and HfN) are used herein as a shorthand rather than an exact chemical label, and do not suggest that the stoichiometry of the indicated compound must be exactly as stated in the abbreviation.  
         [0026]    With continued reference to FIG. 3, it can be seen that the coating  26  further includes a third layer  32  that overlays and contacts the second layer  30 . As shown, the third layer  32  is the top layer, constituting the material that is exposed and visible when the dental braces  10  are worn. The third layer  32  is preferably a Nitride of the metal used in the first layer  28  and has approximately two metal atoms for every Nitrogen atom (i.e. a di-metal Nitride). For example, for a coating  26  having Titanium as the first layer  28  and Titanium Nitride (TiN) as the second layer  30 , the third layer  32  is preferably di-Titanium Nitride (Ti x N). Similarly, for a coating  26  having Zirconium as the first layer  28  and Zirconium Nitride (ZrN) as the second layer  30 , the third layer  32  is preferably di-Zirconium Nitride (Zr x N). Likewise, for a coating  26  having Hafnium as the first layer  28  and Hafnium Nitride (HfN) as the second layer  30 , the third layer  32  is preferably di-Hafnium Nitride (Hf x N).  
         [0027]    Referring now to FIG. 4, a fixture  34  is shown for holding the brackets  12  during coating. As shown, the fixture  34  is formed as a circularly shaped plate that is formed with a plurality of relatively shallow slots  36   a - c  with each slot  36   a - c  extending on the plate in a closed loop. As best seen in FIG. 5, brackets  12  are placed somewhat loosely in each slot  36  with the bonding face  20  of each bracket  12  oriented face-down in the slot  36 . As shown, portions of the bracket  12  (except the bonding face  20  and the edges  38  of the base  18 ) protrude from the slot  36  leaving these portions exposed to receive coating  26 . With the slot  36  formed as a closed loop, brackets  12  are prevented from ‘walking’ off the plate in spite of the fact that the brackets  12  are subject to rotational movement (see description below) and minor vibrations during coating. Also, the slots  36  separate the brackets  12  and prevent the brackets  12  from movement (i.e. tipping) into non-desirable orientations during coating.  
         [0028]    Referring now to FIG. 6, a fixture  40  is shown for holding a plurality of arch wires during coating, for which two exemplary arch wires  14   a  and  14   b  have been shown for clarity. As shown, the fixture  40  includes a pair of wire screens  42 ,  44  that are mounted on a base  46 . Wire screen  42  is formed with a plurality of apertures, for which exemplary apertures  48   a  and  48   b  have been labeled. Similarly, wire screen  44  is formed with a plurality of apertures, for which exemplary apertures  49   a  and  49   b  have been labeled. As further shown, the screens  42 ,  44  extend from the base  46  and are aligned substantially parallel to each other. The screens  42 ,  44  are spaced apart to allow the arch wires  14   a ,  14   b  to drape from screen  42  to screen  44 . In one implementation, as shown, one end of arch wire  14   a  is inserted into aperture  48   a  of screen  42  and the second end is inserted into aperture  49   a  of screen  44 . Similarly, one end of arch wire  14   b  is inserted into aperture  48   b  and the second end is inserted into aperture  49   b . This cooperation of structure allows a plurality of arch wires  14  to be uniformly spaced from each other during coating. Although the exemplary screens  42 ,  44  shown are sized to hold about sixteen arch wires  14 , it is to be appreciated that larger screens having more apertures could be used to hold arch wires  14  during coating.  
         [0029]    [0029]FIGS. 7 and 8 depict a preferred deposition apparatus  50  for coating the brackets  12  and arch wires  14  (not shown), although other operable deposition apparatus may be used. The deposition apparatus  50  includes a chamber  52  having a body  54  and a door  56  that may be opened for access to the interior of the chamber  52  and which is hermetically sealed to the body  54  when the chamber  52  is in operation. The interior of the chamber  52  is controllably evacuated by a vacuum pump  58  pumping through a gate valve  60 . The vacuum pump  58  includes a mechanical pump and a diffusion pump operating together in the usual manner. The interior of the chamber  52  may be controllably backfilled to a partial pressure of a selected gas from a gas source  62  through a backfill valve  64 . The gas source  62  typically includes several separately operable gas sources. The gas source  62  usually includes a source  62   a  of an inert gas such as argon and a source  62   b  of Nitrogen gas, each providing gas selectively and independently through a respective selector valve  65   a  or  65   b . Other types of gas can also be provided as desired.  
         [0030]    The pressure within the chamber  52  is monitored by a vacuum gage  66 , whose output signal is provided to a pressure controller  68 . The pressure controller  68  controls the settings of the gate valve  60  and the backfill valve  64  (and, optionally, the selector valves  65 ), achieving a balance of pumping and backfill gas flow that produces a desired pressure in the chamber  52  and thence pressure reading in the vacuum gage  66 . Thus, the gaseous backfilled atmosphere within the chamber  52  is preferably a flowing or dynamic atmosphere.  
         [0031]    At least two, and preferably four as shown, linear deposition sources  70  are mounted within the interior of the chamber  52  in a circumferentially spaced-apart manner. In FIG. 7, the four deposition sources are identified as distinct sources  70   a ,  70   b ,  70   c , and  70   d , as they will be addressed individually in the subsequent discussion. The four deposition sources  70  are generally rectangular bodies having a greatest rectilinear dimension elongated parallel to a source axis  72 . This type of deposition source is distinct from either a stationary point source or a point source that moves along the length of the substrate  27  during deposition procedures.  
         [0032]    A substrate support  74  is positioned in the chamber  52 . The substrate support  74  produces a compound rotational movement of a fixture  34  (or fixture  40  if arch wires  14  are being coated) mounted thereon. The preferred substrate support  74  includes a rotational carriage  76  that rotates about an axis  78 , driven by a rotational drive motor  80  below the rotational carriage  76 . Mounted on the rotational carriage  76  are at least one and preferably six, as shown, planetary carriages  82 . The planetary carriages  82  are rotationally driven about a rotational axis  84  by a planetary drive motor  86  below the planetary carriages  82 . The speeds of the rotational drive motor  80  and the planetary drive motor  86  are controlled by a rotation controller  88 . The rotation controller  88  preferably rotates the rotational carriage  76  at a rate of about 1 revolution per minute (rpm).  
         [0033]    Continuing with FIGS. 7 and 8, for deposition processing of brackets  12 , a plurality of fixtures  34  as described above can be stacked and mounted on the planetary carriage  82 , as shown. For commercial operations, six to ten fixtures  34  having approximately 200-500 brackets  12  per fixture  34  are typically mounted on each planetary carriage  82  in the manner described, as illustrated for one of the planetary carriages  82  in FIG. 7. Alternatively, for deposition processing of arch wires  14  (not shown), one or more fixtures  40  as described above can be mounted on the planetary carriage  82 . For commercial operations, a fixture  40  having approximately 100-350 arch wires  14  is typically mounted on each planetary carriage  82 .  
         [0034]    The temperature in the chamber  52  during deposition is controlled using a heater  92  that extends parallel to the deposition sources  70  on one side of the interior of the chamber  52 . The heater  92  is preferably a radiant heater operating with electrical resistance elements. The temperature of the heating array is monitored by a temperature sensor  94  such as an infrared sensor that views the interior of the chamber  52 . The temperature measured by the sensor  94  is provided to a temperature control circuit  96  that provides the power output to the heater  92 . Acting in this feedback manner, the temperature controller  96  allows the temperature of the heating array to be set. In the preferred processing, the heating array is heated to a temperature of from about 1000° F. to about 1700° F.  
         [0035]    [0035]FIG. 9 illustrates a cathodic arc source  100  used in the preferred form of the deposition source  70 . The cathodic arc source  100  includes a channel-shaped body  102  and a deposition target  104 . The deposition target  104  is in the form of a plate that is hermetically sealed to the body  102  using an 0-ring  106 , forming a water-tight and gas-tight hollow interior  108 . The interior  108  is cooled with cooling water flowing through a water inlet  110  and a water outlet  112 . Two spirally shaped (only sections of the spirals are seen in FIG. 9) permanent magnets  114  extend parallel to the source axis  72 . Positioned above the deposition target  104  exterior to the body  102  is a striker electrode  118 . A voltage V ARC  is applied between the striker electrode  118  and the deposition target  104  by an arc source power supply  120 . V ARC  is preferably from about 10 to about 50 volts.  
         [0036]    The metallic material that initially forms the deposition target  104  is deposited onto the substrate, in this case an arch wire  14 , together with, if desired, gas atoms producing gaseous species from the atmosphere of the chamber  52 . In the preferred embodiment, the deposition target  104  is made of Zirconium (Zr) or Titanium (Ti) or Hafnium (Hf).  
         [0037]    To accomplish the deposition, an arc is struck between the striker electrode  118  and the deposition target  104 , locally heating the deposition target  104  and causing Zirconium, Hafnium or Titanium atoms and/or ions to be ejected from the deposition target  104 . (The deposition target  104  is therefore gradually thinned as the deposition proceeds.) The striking point of the arc on the deposition target  104  moves in a racetrack course along the length of the deposition target  104 . A negative bias voltage V BIAS  is applied between the deposition target  104  and substrate  27  (i.e. the bracket  12  or arch wire  14 ) by a bias power supply  122 , so that any positively charged ions are accelerated toward the substrate  27 .  
         [0038]    V BIAS  is preferably from about −30 to about −600 volts. The value selected for V BIAS  determines the energy of ionic impact against the surface of the substrates, a phenomenon termed ion peening. In a typical case, V BIAS  is initially selected to be a relatively large negative voltage to achieve good adherence of the metallic first layer  28  (see FIG. 3) to the bracket  12  or arch wire  14 . V BIAS  is subsequently reduced (made less negative) when overlying hard layers are deposited, to achieve a uniform, fine microstructure in the layers. The values of V BIAS  are desirably maintained as low as possible, consistent with obtaining an adherent coating  26 . V BIAS  is more positive than −600 volts, and most preferably more positive than −400 volts. If V BIAS  is too negative, corona effects and backsputtering may occur at some regions of the bracket  12  or arch wire  14 . Thus, while higher V BIAS  voltages may be used in some instances, generally it is preferred that V BIAS  be more positive than −600 volts. The cathodic arc source  100  is preferred, but other types of sources, such as sputtering sources, may also be used.  
         [0039]    The cooperative selection of the material of the deposition target  104  and the gases introduced into the deposition chamber  52  from the gas source  62  allows a variety of coatings  26  to be deposited onto the bracket  12  or arch wire  14 , within the constraints discussed previously. In the case of brackets  12 , the thickness of the coating  26  is preferably from about 1 to about 10 micrometers. On the other hand, in the case of an arch wire  14 , the thickness of the coating  26  is preferably from about 0.25-5 micrometers. If the coating thickness is less than about 1 micrometer, the physical properties of the coating  26  are insufficient to produce the desired results. If the coating thickness is more than about 10 micrometers, the coating  26  has a high internal stress that leads to a tendency for the coating  26  to crack and spall away from the bracket  12  or arch wire  14  during deposition or during service.  
         [0040]    These general principles are applied in preparing the coatings  26  of interest, as described previously in relation to FIG. 3. The coating  26  of FIG. 3 includes a metallic first layer  28 , such as metallic Zirconium, Hafnium or Titanium, that contacts and overlays surface of the bracket  12  or arch wire  14 . The metallic first layer  28  aids in adhering the overlying layer(s) to the surface of the substrate. The metallic first layer  28  is preferably quite thin, on the order of from about 100 Angstroms to about 1000 Angstroms thick. The metallic first layer  28  is deposited by backfilling the deposition chamber  52  with a small partial pressure of about 5 microns of an inert gas, such as flowing argon (flowing at a rate of about 200-450 standard cubic centimeters per minute (sccm) in the apparatus used by the inventors), and then depositing metal, such as Zirconium, Hafnium or Titanium, from the deposition target  104  with V BIAS  about −400 volts. Because the argon does not chemically react with the metal, a metallic first layer  28  is deposited.  
         [0041]    As shown in FIG. 3, a second layer  30 , which is a metal Nitride having approximately one metal atom per atom of Nitrogen, overlies the metallic first layer  28 . The second layer  30  is deposited by backfilling the deposition chamber  52  with a small partial pressure of about 5 microns of flowing Nitrogen (flowing at a rate of about 150-500 seen in the inventors&#39; apparatus), and then depositing metal, such as Zirconium, Hafnium or Titanium, from the deposition target  104  with V BIAS  about −50 volts. The metal combines with the Nitrogen to produce the metal Nitride in the second layer  30 . The second layer  30  is preferably of a thickness of approximately 0.25 to 5 micrometers.  
         [0042]    Also shown in FIG. 3, a third layer  32 , which is a metal Nitride having approximately two metal atoms per atom of Nitrogen, overlies the second layer  30 . The third layer  32  is deposited by backfilling the deposition chamber  52  with a small partial pressure of about 5 microns of flowing Nitrogen (flowing at a rate of about 150-500 seen in the inventors&#39; apparatus), and then depositing metal, such as Zirconium, Hafnium or Titanium, from the deposition target  104  with V BIAS  about −50 volts. The metal combines with the Nitrogen to produce the metal Nitride in the third layer  32 . The third layer  32  is preferably of a thickness of approximately 0.25-5 micrometers with the total thickness of the coating  26  being from about 1 to about 10 micrometers.  
         [0043]    While the particular dental braces and methods for coating as herein shown and disclosed in detail are fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that they are merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.