Abstract:
A method for applying a coating by a cathodic is provided. The method includes the steps of: a) providing a cathodic arc coater that includes a power source and utilizes a disk-shaped cathode, the cathode having an evaporative surface extending between a first end surface and a second end surface, wherein the evaporative surface has an area; b) determining a maximum acceptable power density for the evaporative surface; and c) applying a magnitude of electrical current from the power source to the cathode, wherein the electrical current magnitude divided by the area is equal to or less than the maximum acceptable power density for the evaporative surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to apparatus for vapor deposition of coatings in general, and to cathodic arc vapor deposition apparatus in particular. 
     2. Background Information 
     Vapor deposition as a means for applying a coating to a substrate is a known art that includes processes such as chemical vapor deposition, physical vapor deposition, and cathodic arc vapor deposition. Chemical vapor deposition involves introducing reactive gaseous elements into a deposition chamber containing one or more substrates to be coated. Physical vapor deposition involves providing a source material and a substrate to be coated in an evacuated deposition chamber. The source material is converted into vapor by an energy input, such as heating by resistive, inductive, or electron beam means. 
     Cathodic arc vapor deposition involves a source material and a substrate to be coated placed in an evacuated deposition chamber. The chamber contains only a relatively small amount of gas. The negative lead of a direct current (DC) power supply is attached to the source material (hereinafter referred to as the “cathode”) and the positive lead is attached to an anodic member. An arc-initiating trigger, at or near the same electrical potential as the anode, contacts the cathode and subsequently moves away from the cathode. When the trigger is still in close proximity to the cathode, the difference in electrical potential between the trigger and the cathode causes an arc of electricity to extend therebetween. As the trigger moves further away, the arc jumps between the cathode and the anodic chamber. The exact point, or points, where an arc touches the surface of the cathode is referred to as an arc spot. Absent a steering mechanism, an arc spot will move randomly about the surface of the cathode. 
     The energy deposited by the arc at an arc spot is intense; on the order of 10 5  to 10 7  amperes per square centimeter with a duration of a few to several microseconds. The intensity of the energy raises the local temperature of the arc spot to approximately equal that of the boiling point of the cathode material (at the evacuated chamber pressure). As a result, cathode material at the arc spot vaporizes into a plasma containing atoms, molecules, ions, electrons, and particles. Positively charged ions liberated from the cathode are attracted toward any object within the deposition chamber having a negative electrical potential relative to the positively charged ion. Some deposition processes maintain the substrate to be coated at the same electrical potential as the anode. Other processes use a biasing source to lower the potential of the substrate and thereby make the substrate relatively more attractive to the positively charged ions. In either case, the substrate becomes coated with the vaporized material liberated from the cathode. 
     Cathodic arc deposition has historically been used to apply relatively thin coatings (2-5 mils; 1 mil=25.4 microns=25.4×10 −6  m) to substrates. The deposition rate of cathodic arc coaters is typically relatively slow (e.g., 0.3 mil/hr), but has not been a substantial concern in view of the thin coatings. Applying a relatively thick coating (10-200 mils) has heretofore not been practical because of the substantial time required to apply the coating at the deposition rate of currently available cathodic arc coaters (e.g., at 0.3 mil/hr, a 150 mil thick coating would take at least 500 hrs) 
     It may be possible to slightly increase the deposition rate of an existing cathodic arc coater by only increasing the current applied. The increase in deposition rate is limited however, by the apparatus currently available. Significantly increasing the current in currently available cathodic arc coaters is likely to cause the portion of the cathode proximate the arc to undesirably melt or increase the chance that undesirable macroscopic particles will be liberated. In addition, most currently available cathodic arc coaters could not accommodate an appreciable increase in current without incurring significant damage. 
     In short, what is needed is an apparatus and method for cathodic arc vapor deposition of material on a substrate that is capable of operating at a high deposition rate. 
     DISCLOSURE OF THE INVENTION 
     According to the present invention, a method for applying a coating by a cathodic arc is provided. The method includes the steps of: a) providing a cathodic arc coater that includes a power source and utilizes a disk-shaped cathode, the cathode having an evaporative surface extending between a first end surface and a second end surface, wherein the evaporative surface has an area; b) determining a maximum acceptable power density for the evaporative surface; and c) applying a magnitude of electrical current from the power source to the cathode, wherein the electrical current magnitude divided by the area is equal to or less than the maximum acceptable power density for the evaporative surface. 
     An advantage of the present invention method is that a cathodic arc deposition process is enabled that will permit thick coatings (e.g., 10-200 mils) to be applied to a substrate within a commercially practical period of time. The present invention method and apparatus enables deposition rates up to and beyond 4.0 mils per hour, thereby more than doubling the deposition rate possible with currently available cathodic arc coaters of which we are aware. 
     Another advantage of the present apparatus and method is that the integrity of the cathode is maintained during the erosion process. The present invention permits the cathode to be eroded using a high magnitude current without causing undesirable melting of the cathode, or appreciable undesirable macroscopic particle formation. 
     These and other objects, features and advantages of the present invention will become apparent in light of the detailed description of the best mode embodiment thereof, as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic view of the present invention cathodic arc vapor deposition apparatus. 
         FIG. 2  is a diagrammatic view of the contactor in contact with a cathode. A magnetic field generator is disposed inside of the contactor. 
         FIG. 3  is a diagrammatic view of the contactor, magnetic field generator, and cathode, illustrating magnetic field configuration. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , an apparatus for cathodic arc vapor deposition on a substrate, hereinafter referred to as a “cathodic arc coater”  10 , is provided having a vessel  12 , means  14  for maintaining a vacuum in the vessel  12 , a cathode  16 , a contactor  18 , and means  20  for sustaining an arc of electrical energy between the cathode  16  and an anode. A coolant supply  22  maintains the coater  10  within acceptable temperatures by cycling coolant through cooling passages within the vessel  12  and contactor  18 . In the preferred embodiment, the means  14  for maintaining a vacuum in the vessel  12  includes a mechanical rough vacuum pump and a high volume diffusion-type vacuum pump piped to the interior of the vessel  12 . Other vacuum means may be used alternatively. A cathodic arc coater  10  as described in this paragraph is disclosed in U.S. Pat. No. 6,036,828, which is hereby incorporated by reference. 
     Referring to  FIGS. 1-3 , the cathode  16  is a substantially cylindrical disk having an axially extending evaporative surface  24  extending between a pair of end surfaces  26 , 28 . The coating to be deposited dictates the material composition of the cathode  16 . The end surfaces  26 , 28  are substantially parallel with one another. The axial length  30  of the cathode  16  is equal to or greater than the anticipated final width of the erosion pattern  32  (shown in phantom) along the evaporative surface  24  of the cathode  16 . Keeping the erosion pattern  32  between the end surfaces minimizes the possibility that the arc will leave the evaporative surface  24  of the cathode  16 . 
     The cathode  16  has a maximum acceptable evaporative surface  24  heat transfer flux that occurs when subjected to a particular power density value. The term “heat transfer flux value” is defined as the average heat transfer value exiting a unit of evaporative surface  24  area of the cathode  16 . The term “power density” is defined as the magnitude of electrical power introduced into the cathode  16  (i.e., “cathode amperage”) divided by the area of the evaporative surface  24 . A portion of the cathode amperage/electrical energy introduced into the cathode  16  exits the cathode  16  via an electrical arc extending between the cathode  16  and an anode, but a significant portion of the energy exits the cathode  16  in the form of thermal energy. The thermal energy is a function of the electrical resistance provided by the cathode material (e.g., a cathode that is more electrically conductive will produce less thermal energy than a cathode that is less electrically conductive). According to the present invention, the thermal flux (thermal energy/unit area) exiting the evaporative surface  24  must be below a particular value. That value will depend principally on the cathode material, is associated with a particular power density value, and is dictated by the maximum amount of heat transfer that can occur for that cathode material while maintaining macroscopic particle creation to a tolerable level. The tolerable level will depend upon the application, but in all cases a tolerable level is that which can occur and still produce a coating operable for the application contemplated. An example is provided below. 
     The cathode evaporative surface  24  is sized to create a power density that in turn produces an average heat transfer flux through the evaporative surface  24  that is equal to or less than the maximum acceptable heat transfer flux value for a given cathode amperage. Heat transfer at the point of the arc is greater than the maximum acceptable heat transfer flux value. 
     The substrates  82  to be coated are mounted on a platter  34  that preferably rolls into and out of the vessel  12 . The platter  34  includes means for rotating the substrates  82  (not shown). 
     The contactor  18  includes a head  36  attached to a shaft  38 . The head  36  is positioned inside the vessel  12  and the shaft  38  extends from the head  36  to outside the vessel  12 . An insulative disk  40  (see  FIG. 1 ) electrically insulates the contactor  18  from the vessel  12 . The contactor  18  preferably further includes a cooling tube  42  coaxially positioned within the shaft  38 , a coolant inlet port  44  (see  FIG. 1 ) connected to the cooling tube  42 , and a coolant exit port  46  connected to the passage  48  formed between the coaxial coolant tube  42  and shaft  38 . The coaxial arrangement between the cooling tube  42  and the shaft  38  allows coolant from the coolant supply  22  to enter the cooling tube  42  and return via the passage  48  between the shaft  38  and the cooling tube  42 , or vice versa. 
     The contactor  18  head includes a cup  50 , a shaft flange  52 , and a magnetic field generator  54 . The shaft flange  52  is fixed to the shaft  38  and the cup  50  is removably attached to the shaft flange  52 . The cup  50 , shaft flange  52 , and shaft  38  are fabricated from an electrically conductive material such as a copper alloy. 
     The magnetic field generator  54  includes a ferromagnetic centerpiece  56 , and a plurality of magnets  58 . The centerpiece  56  includes at least one side surface  60  extending between two end surfaces  62 . The magnets  38  are preferably permanent magnets, although alternative magnetic field sources such as electromagnetics may be used. The magnets  38  are attached to the centerpiece  56 . In all embodiments, the number of magnets  38  can be varied to accommodate the process at hand. 
     Referring to  FIGS. 1 and 2 , apparatus  64  is included for rotating the magnetic field generator  54 . The rotation apparatus  64  includes a rod  66  extending through the coolant tube  42  and into the head  36  where it connects with the ferromagnetic centerpiece  56 . The opposite end of the rod  66  is connected to a variable speed drive motor  68  via a drive belt  70 . 
     In some embodiments, the cathodic arc coater  10  includes an actuator  72  for selectively actuating the contactor  18  into electrical contact with the cathode  16 . The actuator  72  includes a pair of two-way actuating cylinders  74  (e.g., hydraulic or pneumatic) acting between the vessel  12  and a shaft flange  76  attached to the contactor shaft  38 . Mechanical apparatus may be used in place of the actuating cylinders  74 . A commercially available controller (not shown) can be used to control the position and force of the cylinders (or mechanical apparatus). 
     The cathodic arc coater  10  includes a biasing source  78  for electrically biasing the substrates  82 . Negatively biasing the substrates  82  relative to the anode makes the substrates  82  electrically attractive to positive ions liberated from the cathode  16 . A contact electrically connects the biasing source  78  to the platter  34 . The substrates  82 , which are electrically connected to the platter  22 , are consequently electrically connected to the biasing source  78 . Other means for electrically connecting the substrates  82  to the biasing source  78  may be used alternatively. 
     Deflector shields  80  are used throughout the coater  10  to confine the vaporized cathode materials in the area of the substrates  82 . The deflector shields  80  attached to the vessel  12 , platter, and contactor  18  also minimize undesirable material build-up on those surfaces. In the preferred embodiment, the deflector shields  80  attached to the vessel  12  are electrically connected to the vessel  12  and are made of an electrically conductive material resistant to corrosion, such as stainless steel. 
     The means  20  for sustaining an arc of electrical energy between the cathode  16  and an anode includes a direct current (D.C.) power supply. In the preferred embodiment, the positive lead of the power supply is connected to the vessel  12 , thereby making the vessel  12  act as an anode. The negative lead of the power supply is electrically connected to the contactor shaft  38 . Alternative embodiments may use an anode (not shown) disposed inside the vessel  12 . An arc initiator  81 , maintained at or near the electrical potential of the vessel  12 , is used to initiate an arc. 
     Referring to  FIG. 1 , in the operation of the present invention cathodic arc coater  10 , a plurality of substrates  82  and a cathode  16  are attached to the platter  34  and loaded into the vessel  12 . The substrates  82  have been previously degreased and substantially cleaned, although each will likely have some molecular contaminant and oxidation remaining on its exterior surface. The actuating cylinders  74  subsequently actuate the contactor  18  into electrical contact with the cathode  16  and the vessel  12  is closed. 
     The mechanical rough vacuum pump is operated to evacuate the vessel  12  to a predetermined pressure. Once that pressure is reached, the high volume diffusion vacuum pump further evacuates the vessel  12  to near vacuum conditions. The substrates  82  are then cleaned of any remaining contaminants and/or oxidation by a method such as “sputter cleaning”. Sputter cleaning is a process well known in the art and will not be described in detail here. Other cleaning methods may be used alternatively. After the substrates  82  are cleaned, the contaminants are purged typically using an inert gas. 
     Prior to initiating an arc, several steps are performed. The substrates  82  are electrically biased via the biasing source  78 , making them electrically attractive to positive ions emitted from the cathode  16 . The substrates  82  are rotated at a predetermined rotational speed. The power supply is adjusted to produce a cathode amperage that establishes an arc having a predetermined current, but no arc is initiated. The vacuum pumps are operated to establish and maintain a predetermined vacuum pressure of gas within the vessel  12 . Coolant is cycled through the cooling passages within the vessel  12  and contactor  18 . Specific process parameters will depend upon factors such as the substrate material, the material to be coated, and the desired characteristics of the coating, etc. 
     Once the aforementioned steps have been completed, the arc initiator  81  is brought into and out of contact with the evaporative surface  24  of the cathode  16 , causing an arc to jump between the arc initiator  81  and the evaporative surface  24 . The arc initiator  81  is subsequently moved a distance away from the cathode  16 , preferably radially outside of the substrates  82 . Once the arc initiator  81  is no longer proximate the cathode  16 , the arc jumps between the cathode  16  and the deflector shields  80  electrically connected to the vessel  12  (or directly between the cathode  16  and the vessel  12 ). 
     The magnetic field generator  54  positioned in the contactor  18  drives the arc spot along the evaporative surface  24  of the cathode  16 . To be more specific, each side magnet produces a magnetic field that permeates the cathode  16  and runs substantially parallel to the cathode evaporative surface  24 . The direction of the magnetic field vector  57  depends upon the orientation of the magnet poles, and all the magnets  58  are oriented in like manner. A vector  59  representing the electric arc, in contrast, extends away from the evaporative surface  24  in a substantially perpendicular direction. Together, the magnetic field and the electric current of the arc create a force (the Hall effect) on the arc that causes the arc to travel a distance around the circumference of the cathode  16 . The dwell time of the arc at any particular arc spot is inversely related to the Hall effect force; i.e., an increase in the Hall effect force, causes a decrease in the dwell time. A person of skill in the art will recognize that decreasing the dwell time reduces the occurrence of macroscopic particles that can adversely affect the uniformity and surface finish of the deposited coating. 
     The individual magnetic fields of the magnets  58  disposed along the side surface(s) of the ferromagnetic centerpiece  56 , in close circumferential proximity to one another, collectively force the arc to circle the cathode evaporative surface  24  along an arc path. The number of magnets  58 , the relative spacing of magnetic fields emanating from the magnets  58 , and the intensity of those magnetic fields can be adjusted to satisfy the application at hand. In some applications, however, it is advantageous to further include a magnet  84  (see  FIG. 3 ) disposed proximate the center of the ferromagnetic centerpiece  56 . The magnetic field of the centrally located magnet appears to favorably influence the geometry of the magnetic fields emanating from the magnets  58  disposed along the side surfaces  60  of the ferromagnetic centerpiece. 
     The energy delivered by the arc causes the material at the arc spot to vaporize, thereby liberating atoms, molecules, ions, electrons, and particles from the cathode  16 . The biased substrates  82  attract the ions, causing them to accelerate toward the substrates  82 . The ions strike the exterior surface of the substrates  82 , attach, and collectively form a coating of the cathode material. 
     The rate at which material is liberated from the cathode  16  and deposited onto the substrate(s) within the vessel  12  (i.e., the deposition rate) is predominantly a function of the magnitude of the cathode amperage. The maximum deposition rate for a given cathode material is dictated by the maximum acceptable heat transfer flux value for the evaporative surface  24  of the cathode  16 , which is a function of the arc current magnitude. 
     The maximum acceptable heat transfer flux for a given disk-shaped cathode  16  comprising a particular material can be determined by empirical methods, including but not limited to, inspection of the applied coating to ascertain density, grain size, etc. Once the maximum acceptable heat transfer flux, and therefore power density, is known for the given cathode material, the deposition rate can be increased by increasing both the cathode amperage and the surface area of the cathode  16  in a ratio that maintains the heat transfer flux at or below the determined maximum acceptable heat transfer flux value. 
     As an example, a cathode  16  comprising a titanium alloy (e.g., Ti-8Al-1Mo-1V) is provided having a four-inch diameter and a two-inch axial height. Cathode amperage is applied to the cathode  16  and increased until the frequency and/or magnitude of macro particles and cathode melting exceeds a predetermined tolerable level. Analysis of the coatings applied at different cathode amperages provides the information necessary to establish the predetermined tolerable level. In our experience, 450 amperes of electrical power applied to the above-described four-inch diameter titanium alloy cathode  16  produces a power density of approximately 16 amperes per square inch of evaporative surface  24 , which in turn produces a maximum acceptable heat transfer flux out of the evaporative surface  24  of the cathode  16 . The deposition rate at a cathode amperage of 450 amperes is in the range of approximately 1.5 mils to 2.0 mils per hour. Increasing the magnitude of the electrical power applied to the same cathode  16  geometry yields a higher deposition rate, but the applied coating is less desirable. 
     Increasing the cathode evaporative surface area by, for example, increasing the diameter to six inches while maintaining the axial height at two-inches, decreases the power density and heat transfer flux out of the evaporative surface  24 . As a result, the current applied to the cathode  16  can be increased. A current of approximately 600 amperes applied to the six-inch diameter cathode  16  comprising the aforesaid titanium alloy, creates the same power density and heat transfer flux as 450 amperes does for the four-inch diameter cathode  16 . At a cathode amperage of 600 amperes, however, the deposition rate increased to within the range of approximately 3.5 mils to 4.0 mils per hour; i.e., at least twice the deposition rate possible with the four-inch cathode  16 . 
     Referring to  FIG. 1 , when a coating of sufficient thickness has been deposited on the substrates  82 , the power supply is turned off and the arc extinguished. The vessel  12  is purged with inert gas and brought to ambient pressure. The contactor  18  is actuated out of contact with the cathode  16  and the platter is removed from the vessel  12 . The substrates  82  are subsequently removed from the platter and new substrates  82  attached. The loaded platter is then inserted back into the vessel  12  in the manner described earlier and the process repeated. 
     Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and the scope of the invention.