Abstract:
A process for repairing a workpiece includes the steps of: providing a workpiece having at least one interior surface requiring restoration; providing a source of repair material; and depositing the repair material onto the at least one interior surface using a technique which is a near non-line of sight technique.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/426,680, filed Dec. 23, 2010, entitled CATHODIC ARC VAPOR DEPOSITION COATINGS FOR DIMENSIONAL RESTORATION OF SURFACES. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to a process for dimensionally restoring surfaces on a workpiece using a near non-line of sight technique such as cathodic arc vapor deposition technique. 
         [0003]    Cathodic arc deposition was developed to apply simple coatings, such as titanium nitride, as a hard coat for workpieces such as drill bits. Cathodic arc deposition has been used to apply wear resistant and corrosion resistant coating systems on workpieces. 
         [0004]    In use, some workpieces, such as turbine engine components, suffer loss of desired dimensions. It thus becomes necessary to repair them by performing dimensional restoration. Conventional thermal sprays require moderate to near-normal angle of approach to the substrate to achieve adherence, limiting applications where only shallow angle of approach is available. Such repairs have been difficult to perform on certain portions of the workpieces with limited line-of-sight (LLOS) such as hook attachments, seal grooves, and bolt holes. Current methods for LLOS dimensional restoration are restricted to plating processes or recoil directed thermal sprays. Plating materials with high temperature capability such as those operating at temperatures greater than 1000° F., are not commercially available. Choice of plating materials for titanium is much more restricted. Welding is frequently not an option because of mechanical property debits and distortion. 
         [0005]    Near non-line of sight (NLOS) dimensional restoration approaches for repair are currently dominated by plating processes; however, electroless nickel, electrolytic nickel and chrome (Cr-III or Cr-VI) plates are limited on parts with service temperatures over 800° F. for chrome and electroless nickel and to 1000° F. for electrolytic nickel 
         [0006]    Certain bolts used on turbine engines are used to align and tie the high pressure compressor cases together. These are press-installed in to tight tolerance, heavily loaded holes in each stage case. These holes can wear beyond limits with no known repair other than flange replacement. Local weld repair of the holes induces too much distortion on the other holes in the flange or leaves unacceptable levels of residual stress and fatigue property debit from the weld process. 
         [0007]    Repair of worn and damaged cases with seal slot lands requires removal of the rear flange to access the repair area with plasma spray. The flange is subsequently restored by electron beam welding on a new flange. Such repairs have had limited success because the post-weld heat treatment frequently results in irreparable distortion. Vane hooks are similarly weld repaired when worn but may not have sufficient strength after extensive weld repair. 
         [0008]    Other applications push the acceptable temperature limits of nickel plate, but only at reduced allowable stresses on the plate. These include diffuser case stator hooks and turbine disk shoulder snap diameters. Also note that use of chrome plate is increasingly hampered by EH&amp;S restrictions world-wide and its viability as a repair technique is more and more becoming a method of last resort. 
       SUMMARY 
       [0009]    Cathodic arc vapor deposition provides an alternative approach for performing dimensional restoration of a part, such as a hole, in a case used on a turbine engine. In particular, cathodic arc vapor deposition enables one to coat down the sides of the holes or to both faces of a hook. Further, coating down the inner diameter of tubes is also possible, particularly where the restoration of material near the end of the tube where the largest wear takes place is of concern. 
         [0010]    In accordance with the present disclosure, there is provided a process for repairing a workpiece which broadly comprises the steps of: providing a workpiece having at least one interior surface requiring restoration; providing a source of repair material; and depositing the repair material onto the at least one interior surface using a technique which is a near non-line of sight technique. 
         [0011]    Other details of the cathodic arc vapor deposition coatings for dimensional restoration of surfaces are set forth in the following detailed description and the accompanying drawings wherein like reference numerals depict like elements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic representation of a cathodic arc vapor deposition apparatus being used to coat a hole in a workpiece; 
           [0013]      FIG. 2  is a schematic representation showing a mask placed over a portion of the workpiece; 
           [0014]      FIGS. 3(   a )- 3 ( i ) are photomicrographs showing build-up of a coating along a hole area in a diffuser case. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Cathodic arc vapor deposition is a method of applying coatings that do not require full line of sight. Cathodic arc vapor deposition can coat areas not accessible via thermal spray coatings and can apply coatings that result in little or no base material debit. Cathodic arc vapor deposition can coat parts to near-net shapes without requiring subsequent machining. Cathodic arc can be applied without introducing structural lifing debits as part of the surface preparation or coating processes. 
         [0016]    While cathodic arc vapor deposition has been used as a means to install wear resistant and corrosion resistant coatings onto a surface of smaller parts in OEM, cathodic arc vapor deposition has not been used in repair techniques for dimensional restoration of an object. The LLOS advantages of cathodic arc coating have not yet been utilized. 
         [0017]    Cathodic arc vapor deposition is a process which uses electrical biasing and inert gas pressure to enhance coverage to include areas of limited accessibility or that lack line-of-sight. Coating composition is flexible and can be made from any material. Coatings are fully dense and metallurgically bonded with bond strength capability in excess of 9 ksi. Coating compositions have an operating temperature capability equal to or better than base allow substrate. 
         [0018]    As shown in  FIG. 1 , cathodic arc vapor deposition can be used to restore the dimensions of a workpiece, such as the walls  11  of a hole  10  in a case  12 . The hole  10  may have a diameter less than 0.25 inches. The case  12  is placed within a chamber  14  and spaced from a source  16  of the coating material by a distance “d”. The distance “d” can be varied as desired. The source  16  is a cathode having an evaporative surface. 
         [0019]    Prior to being placed in the chamber  14 , the walls  11  and other areas of the workpiece to be coated may be subjected to a treatment to remove any deleterious material, such as engine run contaminants and oxides, and provide a clean surface for the coating. 
         [0020]    The coating material forming the source  16  may comprise a supply of any desired coating material. The coating material may comprise a restoration alloy having desirable strength properties, desirable wear properties, desirable lubrication properties, desirable electronic properties, desirable corrosion resistance properties, or a metal which closely matches the metal forming the case  12 . By “closely match”, it is meant that the restoration alloy is formed of a composition similar to or the same as the parent material from which the case  12  is formed. Examples of such materials include nickel-based alloys, nickel-based superalloys, titanium, titanium-based alloys, cobalt alloys, cobalt superalloys, stainless steels, corrosion resistant steels, i.e. Greek Ascaloy steel, and high temperature steels, i.e. Thermospan steel. 
         [0021]    The source/cathode  16  is connected to a negative lead of a direct current power supply (not shown) and the positive lead of the power supply is attached to an anodic member. An inert gas such as Argon is introduced into the chamber  14 . 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 cathode material vaporizes into a plasma containing atoms, molecules, ions, electrons, and particles. Positively charged ions liberated from the cathode are attracted toward the negatively charged workpiece  12  and the walls  11  of the hole  10  and are deposited thereon. 
         [0022]    The amperage (source) and bias (case) needed to obtain the desired coating  18  down the hole (a limited line of sight), are dependent on “d” and the aspect ratio of the hole being coated. 
         [0023]    If desired, as shown in  FIG. 2 , masking  20  may be added to keep the coating  18  off certain areas of the workpiece or case  12 . With masking, no bridging is observed using cathodic arc vapor deposition. The use of masking also enables the restorative build-up to take place only at the intended location, rather than over the entire workpiece. 
         [0024]    In one test of a cathodic arc vapor deposition repair technique, a hole area in a diffuser case was coated down the sides of a hole.  FIGS. 3(   a )- 3 ( i ) illustrate the coating which was deposited on the sides of the hole using a cathodic arc vapor deposit.  FIGS. 3(   a ) and  3 ( c ) show the sides of the hole closest to the source.  FIGS. 3(   d ) and  3 ( i ) show the sides of the hole farthest from the source. 
         [0025]    Using cathodic arc vapor deposition to effect dimensional repairs has a number of advantages. First, it is a near non-line of sight coating technique. Second, restored materials deposited using this method have a high bond strength. Third, it allows the use of coating materials, such as Stellite-31 base material. Such materials are desirable because they are thermally stable well beyond the highest allowable service temperatures for nickel or chrome plates. Fourth, cathodic arc vapor deposition results in no grit blast fatigue debit like thermal spray coatings and no hydrogen pick-up fatigue debit as per most electrolytic applied plating. Fifth, cathodic arc deposition may allow for as-applied surfaces to be released without subsequent machining. 
         [0026]    Use of cathodic arc deposition for dimensional restoration can extend to holes and to hooks, but can also be considered for seal ring grooves such as on turbine seal plates or on bearing housings, i.e. a trough-like shape where out on the sides of the groove is the critical dimension. There is also a potential use as an alternate to Chrome plate. 
         [0027]    The cathodic arc repair technique is advantageous in that it can be used to coat holes having a depth greater than 0.060 inches. Further, it can be used to repair slots in workpieces that have a width less than 0.25 inches and/or a depth greater than 0.060 inches. 
         [0028]    Although the process described herein has been shown and described with respect to 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 instant disclosure.