Abstract:
The present invention provides a pulsed carbon plasma apparatus to produce a diamond-like carbon coating over an extended object, the coating having a high degree of thickness uniformity achieved by scanning the plasma flow over the surface of the object. The pulsed carbon plasma apparatus of the invention comprises a carbon plasma flow scanning device having at least one pair of deflecting coils, where the deflecting coils have, in the scanning plane, a different number of turns on opposite sides. The object may be made of metal, ceramic, glass or plastic. The coatings may be used to improve life and operating performance of tools and machine parts, and as decorative coatings.

Description:
RELATED APPLICATION  
       [0001]    This application claims priority to PCT Patent Application No. PCT/RU01/00041, filed February 1, 2001, the disclosure of which is incorporated by reference in its entirety for all purposes. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates to an apparatus for applying coatings in vacuum and in particular to applying super-hard wear-resistant carbon coatings in vacuum and more specifically to a pulsed carbon plasma apparatus.  
         BACKGROUND  
         [0003]    The invention is applicable for producing coatings in vacuum to an object, for example diamond-like carbon coatings, to extend life of cutting, shaping and measuring tools, friction units and parts of machines, as well as to improve biological compatibility of implants in medicine, and to extend life of video and audio read/write heads in electronics.  
           [0004]    A vacuum coating apparatus for coating materials with a diamond-like coating using a pulsed plasma beam comprising electrodes coupled to a voltage source, a storage and a deflecting coil is known from U.S. Pat. No. 5,078,848. The apparatus described in U.S. Pat. No. 5,078,848, however, has an insufficiently reliable arc striking device and does not permit depositing coatings on extended objects. Furthermore, the coating produced is highly nonuniform in thickness.  
           [0005]    The closest prior art is a pulsed carbon plasma apparatus comprising a consumable graphite cathode and an anode, both accommodated in a housing of a vacuum chamber and having a common geometrical axis, the consumable graphite cathode and the anode being electrically coupled to a capacitive storage shunted to a dc charger, and an arc striking device disposed in the vacuum chamber and connected to an initiation unit (see e.g. A. I. Maslov et al.,  Journal of Experimental Instruments and Methods,  No. 3, 1985, pages 146-149). This apparatus has a restricted region of depositing a coating, and the coating produced is highly nonuniform. Focusing the plasma flow by the use of a focusing solenoid improves efficiency in the center, but still further increases the thickness nonuniformity of the coating. This method for controlling the energy characteristics suffers a number of deficiencies. With variation in the voltage of the capacitor, the charge of the capacitor is modified, resulting in variation in the cathode erosion region and, consequently, in the coating deposition area, leading to inhomogeneities. Furthermore, with a reduction in the capacitor voltage below a certain value, the reliability of initiating a discharge pulse is reduced. An increase in the capacitor voltage above a predetermined threshold leads to uncontrolled electrical breakdowns between electrodes, resulting in contamination of the carbon plasma and deterioration of the properties of the diamond-like condensate that is formed on the object.  
         SUMMARY OF THE INVENTION  
         [0006]    The object of the present invention is to provide a pulsed ion plasma apparatus, in particular a pulsed carbon plasma apparatus, which makes it possible to deposit a coating over an extended object, and to improve the thickness uniformity of the coated layer by scanning the plasma flow over the surface of the object. The pulsed carbon plasma apparatus of the invention comprises a carbon plasma flow scanning device having at least one pair of deflecting coils, where the deflecting coils have, in the scanning plane, a different number of turns on opposite sides.  
           [0007]    Another feature of the invention is the geometrical axes of the deflecting coils being aligned with the geometrical axis of the cathode and the anode.  
           [0008]    Another feature of the invention is each of the deflecting coils comprising, in a scanning plane passing through the geometrical axis, a different number of turns on opposite sides so that a number of turns on one side of the deflecting coil is n, and a number of turns on the other side is (n+1), where n is an integer of 1, 2 or 3.  
           [0009]    Another feature of the invention is the deflecting coils of each pair being turned relative to each other through 180° around the geometrical axes so that in the scanning plane the number of turns on one side of one of the coils is different from the number of turns on the same side of the other coil.  
           [0010]    Another feature of the invention is the number of pairs of coils being selected in accordance with a shape of the object to be coated, and defining a number of scanning planes, the coils being arranged along the geometrical axis, but turned around the axis through a predetermined angle.  
           [0011]    Another feature of the invention is the consumable graphite cathode having a shape selected from a cylinder, prism or a ring.  
           [0012]    Another feature of the invention is the anode having a shape selected from a hollow cylinder or hollow prism, the side walls of the cylinder or prism being formed by rods, the longitudinal axis of which is parallel with the longitudinal axis of the cylinder or prism.  
           [0013]    The present invention is suitable for producing ultra-hard carbon coatings with a high degree of thickness uniformity over an extended object. The object may be made of metal, ceramic, glass or plastic. The coatings may be used to improve life and operating performance of tools and machine parts, and as decorative coatings. The present invention may be modified to use other cathode materials, for example metals and oxides, to produce suitable coatings of such materials. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The main features of the invention will become apparent upon examination of the accompanying drawings wherein:  
         [0015]    [0015]FIG. 1 shows a schematic diagram of a pulsed carbon plasma apparatus in accordance with the invention.  
         [0016]    [0016]FIG. 2 shows a cross section taken at line II-II of FIG. 1.  
         [0017]    [0017]FIG. 3 shows a cross section taken at line III-III of FIG. 1.  
         [0018]    [0018]FIG. 4 shows a schematic diagram of a pulsed carbon plasma apparatus comprising two pairs of deflecting coils, in accordance with the invention.  
         [0019]    FIGS.  5 ( a, b, c, d ) shows different arrangements of coils used to coat objects of different shapes, in accordance with the invention.  
         [0020]    FIGS.  6 ( a, b, c ) shows different embodiments of the anode and the cathode, in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    The pulsed carbon plasma apparatus of the invention comprises a carbon plasma flow scanning device having at least one pair of deflecting coils, which, in a scanning plane, have a different number of turns on opposite sides; a consumable graphite cathode and an anode, both accommodated in a vacuum chamber and having a common geometrical axis, the consumable graphite cathode and the anode being electrically coupled to a capacitive storage shunted to a dc charger; and an arc striking device disposed in the vacuum chamber and connected to an initiation unit. The deflecting coils are arranged in the vacuum chamber so that the geometrical axes of the deflecting coils are aligned with the geometrical axis of the cathode and the anode, and wound in one direction, one output of each coil in the pair of deflecting coils being connected to the anode, and another output being connected, via a controlled switch, to a positive output of the capacitive storage. Each of the deflecting coils comprise, in a scanning plane passing through the geometrical axis, a different number of turns on opposite sides so that a number of turns on one side of the deflecting coil is n, and a number of turns on the other side is (n+1), where n is an integer of 1, 2 or 3. The deflecting coils of each pair are turned relative to each other through 180° around the geometrical axes so that in the scanning plane the number of turns on one side of one of the coils is different from the number of turns on the same side of the other coil.  
         [0022]    The outputs of the deflecting coils connected to the anode are preferably disposed in a plane perpendicular to the scanning plane and passing through the geometrical axis, on opposite sides of coils of one pair, and the outputs connected, via a controlled switch, to a positive output of the capacitive storage, are also disposed in a plane perpendicular to the scanning plane and passing through the geometrical axis, on opposite sides of coils of one pair.  
         [0023]    The number of pairs of coils is advantageously selected in accordance with a shape of the object to be coated, and defines a number of scanning planes, the coils being arranged along the geometrical axis, but turned around the axis through a predetermined angle.  
         [0024]    The apparatus preferably comprises a controlled inductance to provide electrical coupling between the consumable graphite cathode and a negative output of the capacitive storage.  
         [0025]    The consumable graphite cathode preferably has a shape selected from a cylinder, prism or ring.  
         [0026]    The anode preferably has a shape selected from a hollow cylinder, hollow prism, the side walls of the cylinder or prism being formed by rods, the longitudinal axis of which is parallel with the longitudinal axis of the cylinder or prism.  
         [0027]    The controlled switch preferably comprises a control unit to determine an operation algorithm of the deflecting coils.  
         [0028]    The arc striking device is preferably either a pulsed plasmatron or a pulsed laser.  
         [0029]    Referring to FIG. 1, the pulsed carbon plasma apparatus of the invention comprises a consumable graphite cathode  2  and an anode  3 , both accommodated in a vacuum chamber  1  and having a common geometrical axis a-a, and electrically coupled to a capacitive storage  4  shunted to a dc charger  5 . An arc striking device  6  is disposed in the vacuum chamber  1  and connected to an initiation unit  7 .  
         [0030]    The apparatus further comprises a device  8  for scanning carbon plasma flow over a surface of an object  9 , the scanning device  8  having at least one pair of deflecting coils  10 ,  11  arranged in the vacuum chamber  1 . The geometrical axes of the coils  10 ,  11  are aligned with the geometrical axis a-a of the cathode  2  and anode  3 . The deflecting coils  10 ,  11  are wound in one direction. Output  12  of the deflecting coil  10  and output  13  of the coil  11  are connected to the anode  3 . Output  14  of the deflecting coil  10  and output  15  of the deflecting coil  11  are connected, via a controlled switch  16 , to a positive output  17  of the capacitive storage  4 .  
         [0031]    [0031]FIGS. 2 and 3 schematically show turns of the deflecting coils  11  and  10 , respectively, lying in planes perpendicular to the geometrical axis a-a and a scanning plane. Referring to FIG. 1, in the described embodiment, the scanning plane coincides with the plane of the drawing. In the scanning plane passing through the geometrical axis a-a, each of deflecting coils  10  and  11  has a different number of turns on opposite sides.  
         [0032]    The number of turns  18  (FIG. 2) on one side of the deflecting coil  11  relative to line b-b is n, where n equals one in the described embodiment. The number of turns  19  and  20  on the other side of the deflecting coil is (n+1), where n is an integer of 1, 2 or 3, and equals two in the described embodiment. In possible embodiments, the smallest number of turns may be two and three, and the largest number of turns may be three and four.  
         [0033]    The number of turns  21  (FIG. 3) on one side of the deflecting coil  10  relative to line c-c is n, where n equals one in the described embodiment. A number of turns  22  and  23  on the other side of the deflecting coil is (n+1), where n is an integer of 1, 2 or 3 and equals two in the described embodiment. In possible embodiments, the smallest number of turns may be two and three, and the greatest number of turns may be three and four.  
         [0034]    Therefore, in the scanning plane (FIG. 1) passing through the geometrical axis a-a, each of the deflecting coils  10  and  11  has a different number of turns on opposite sides, i.e. the coil  10  has a turn  21  on one side, and turns  22  and  23  on the other side. The coil  11  has a turn  18  on one side, and turns  19  and  20  on the other side.  
         [0035]    The deflecting coils  10  and  11  (FIGS. 2, 3) are turned relative to each other through 180° around the geometrical axis a-a so that in the scanning plane the number of turns on one side of one of the coils is different from the number of turns on the same side of the other coil. In FIGS. 2 and 3, the scanning plane passes through the axis d-d perpendicular to the plane of the drawing.  
         [0036]    Outputs  12  and  13  (FIGS. 2 and 3) of the deflecting coils  10  and  11 , respectively, connected to the anode  3  are disposed in a plane perpendicular to the scanning plane and passing through the geometrical axis, on opposite sides of the coils  10  and  11  of one pair, and outputs  14  and  15  connected, via a controlled switch  16 , to a positive output  17  of the capacitive storage  4 , are also disposed in a plane perpendicular to the scanning plane and passing through the geometrical axis, on opposite sides of the coils  10  and  11 .  
         [0037]    The number of pairs of coils is selected according to the shape of the object to be coated and defines the number of scanning planes. FIG. 4 shows an embodiment where the apparatus comprises two pairs of deflecting coils  10 ,  11 , and  24 ,  25 . The coils  10 ,  11 ,  24 ,  25  are arranged along the geometrical axis a-a and turned around the axis through a predetermined angle (not shown). FIGS.  5 ( a, b, c, d ) shows an arrangement of windings in the pairs of coils and outputs of the coils. FIGS. 5 c  and  5   d  show an arrangement of windings  26 ,  27 ,  28  and  29 ,  30 ,  31  of the coils  24  and  25 , respectively. Output  32  of the coil  24  and output  34  of the coil  25  are connected to the anode  3 , and output  33  of the coil  24  and output  34  of the coil  25  are connected, via the controlled switch, to the positive output  17  of the capacitive storage  4 .  
         [0038]    The pulsed plasma apparatus further comprises a controlled inductance  36  (FIG. 1) to provide electrical coupling between the consumable graphite cathode  2  and the negative output  37  of the capacitive storage  4 .  
         [0039]    The consumable graphite cathode  2  (FIGS. 6 a, b, c ) has a shape of a cylinder, or prism, or ring.  
         [0040]    The anode  3  then has a shape of a hollow cylinder (FIG. 6 a ) or a hollow prism (FIG. 6 b ), respectively. The side walls of the cylinder or prism may be formed by rods  20   38 , the longitudinal axis of which is parallel with the longitudinal axis of the cylinder or prism.  
         [0041]    The controlled switch  16  comprises a control unit (not shown) to define an operation algorithm of the deflecting coils.  
         [0042]    The arc striking device  6  may be a pulsed plasmatron (FIG. 1), or a pulsed laser (not shown).  
         [0043]    The pulsed carbon plasma apparatus operates in the following manner. Upon evacuating the vacuum chamber  1  (FIG. 1), the capacitive storage  4  is charged from the dc charger  5 . An operation algorithm of the deflecting coils  10  and  11  is defined by the control unit of the switch  16 . To initiate a pulsed discharge, a voltage pulse is provided from the initiation unit  7  to the arc striking device  6 , for example a pulsed plasmatron generating a plasma beam in the direction of the consumable graphite cathode  2 .  
         [0044]    When the striking plasma hits the surface of the consumable graphite cathode  2 , a plurality of cathode spots appear on the surface, the cathode spots being the source of a highly-ionized carbon plasma. A pulsed vacuum arc discharge occurs between the cathode  2  and the anode  3  at the expense of the energy stored in the capacitive storage  4 . The greatest portion of electrons (approximately 80-90% of the total discharge current) passes to the anode  3 . The remaining electrons compensate for the charge of carbon ions moving toward the object  9 , thereby generating a quasi-neutral carbon plasma beam. The capacitive storage  4  discharges across the following circuit: the consumable graphite cathode  2 , the anode  3 , the deflecting coils  10  or  11 , simultaneously or separately, as specified by the control unit of the switch  16 .  
         [0045]    The magnetic field of the deflecting coils  10  and  11  magnetize the electron component of the carbon plasma so that the plasma ceases to be isotropic. Under such conditions, electrons may freely move only along lines of force of the magnetic field. An electrical potential appears within the plasma volume, which provides deflection of the ion component of the carbon plasma from the region of a stronger magnetic field generated by the asymmetric deflecting coils  10  and  11  to the region of a weaker magnetic field. There are three options of enabling the deflecting coils  10  and  11 .  
         [0046]    Option 1:  
         [0047]    The switch  16  enables the deflecting coil  10 . In this case, the plasma flow  39  is directed upwards relative to the axis a-a of symmetry and a carbon coating is deposited on the upper part of the surface of the object  9 .  
         [0048]    Option 2:  
         [0049]    The switch  16  enables the deflecting coil  11 . In this case, the plasma flow  40  is directed downwards with respect to the axis a-a of symmetry and a carbon coating is deposited on the lower part of the surface of the object  9 .  
         [0050]    Option 3:  
         [0051]    The switch  16  enables both deflecting coils  10  and  11 . The plasma flow  41  is directed along the axis a-a of symmetry and a carbon coating is predominantly deposited on the surface of the object  9 , which intersects the axis of symmetry of the carbon pulsed plasma apparatus.  
         [0052]    By specifying an operation algorithm of the deflecting coils  10  and  11  through the control unit of the switch  16 , the region of depositing a diamond-like carbon coating on the object  9  may be extended, and the thickness uniformity of the coating produced is improved.  
         [0053]    By specifying a number n of turns in the deflecting coils within the range of n=1, 2 or 3, the angle of deflection of the plasma carbon flow  39 ,  40  and  41  from the axis a-a of symmetry of the pulse carbon plasma apparatus is modified. The greatest deflection angle corresponds to n=1, as in this case the magnitude of the magnetic field generated by one turn is approximately half as large as that of the magnetic field generated by two turns.  
         [0054]    With the number of turns above  3 , the plasma carbon flow is refocused. This manifests itself as a dramatic reduction in the plasma flow diameter, resulting in increased nonuniformity of the coating thickness. Further, a “magnetic plug” effect occurs which closes the carbon plasma within the space between the consumable graphite cathode  2  and the anode  3 , thereby reducing efficiency of the pulsed carbon plasma apparatus.  
         [0055]    The energy characteristics of the carbon plasma govern the properties of the diamond-like carbon coating produced on the object. With deficiency in energy, formation of the carbon condensate with predominantly diamond-type coupling is unfeasible. With excess energy, irradiation defects accumulate in the formed carbon coating and impair properties of the diamond-like condensate. Taking into account that carbon exhibits a great number of allotropic modifications, the possibility of modifying energy characteristics of the carbon flow within a wide range opens opportunities to produce carbon coatings with predetermined characteristics.  
         [0056]    By varying the inductance value by changing the number of turns, the discharge pulse duration, carbon plasma energy characteristics, the erosion factor of the consumable graphite cathode and the angle of deflection of the carbon plasma flow may be controlled.