Abstract:
A method of removing a coating ( 14 ) from a substrate ( 12 ) by applying both vibratory mechanical energy ( 16, 20 ) and an energy beam ( 32 ) to the coating. Localized combination of thermally and mechanically induced stressed in the coating result in the formation of cracks ( 34 ) in the coating.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of materials technology, and more particularly to the removal of coating materials from an underlying substrate. 
       BACKGROUND OF THE INVENTION 
       [0002]    Coatings are used in many applications to provide improved protection of an underlying substrate material from damage caused by environmental exposure. For example, paints are used to prevent rusting of metal or rotting of wood, and ceramic thermal barrier coatings are used to protect gas turbine engine components from the harsh combustion environment existing inside the engine. However, coatings also degrade due to environmental exposure, and they must sometimes be removed and refreshed, often accompanied by a local repair of the underlying substrate material which may have degraded as a result of a degradation of the coating. 
         [0003]    It is known to remove coatings in a variety of ways. Abrasive procedures such as grit blasting are used to remove coatings by mechanical action. Chemicals are used to dissolve coatings. Heat is used to remove paint by burning, and intense localized heat applied by a laser energy beam is used to dislodge ceramic thermal barrier coatings by causing localized vaporization and a resulting shock wave. Coatings are designed to adhere tightly to the underlying substrate, so as the performance characteristics of coatings improve, they become ever more difficult to remove with known techniques. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The invention is explained in the following description in view of the drawings that show: 
           [0005]      FIG. 1  is a schematic illustration of a component having a coated surface exhibiting a standing wave induced by vibratory mechanical stimulation of the component, and wherein coating material in a region of a trough of the wave is being heated by a laser beam. 
           [0006]      FIG. 2  is the component of  FIG. 1  after the vibratory mechanical stimulation has been controlled to move the standing wave such that the region of heated coating material now resides at a peak of the standing wave, thereby causing a fracturing the coating material in that region. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0007]    The present inventors have found that known techniques for the removal of ceramic thermal barrier coatings are becoming increasingly undesirable. Chemical methods require the handling and disposal of highly toxic compositions, and mechanical and thermal processes are often inadequate to remove the latest generations of highly adherent coatings. Laser processes can be effective, but they must be carefully controlled to achieve coating removal while avoiding substrate damage. Accordingly, the inventors have developed an improved coating removal process which synergistically combines mechanical energy with thermal energy to remove even highly adherent coatings at processing temperatures that may be lower than experienced during prior art laser removal processes. 
         [0008]      FIG. 1  illustrates a step in one embodiment of the present invention. A component  10  includes a substrate material  12  covered by a coating material  14 . Of particular interest to the inventors is a gas turbine engine component formed of a superalloy substrate material coated with a thermal barrier coating including a metallic bond coat and a ceramic top coat, although one skilled in the art will recognize that the invention is not limited to such components and may be useful for the removal of a large variety of coatings from a variety of different substrate materials. 
         [0009]    An electro-mechanical vibration transducer  16  is in contact with the component  10  and is used to impart vibratory mechanical energy into the component  10 . The transducer  16  may be any known type of device which converts electrical signals into mechanical energy, such as a magnetic transducer or a piezoelectric transducer. The transducer  16  may be operated through a controller  18  to selectively control the magnitude and frequency of vibrations induced into the component  10 , and in particular, to induce a wave  20  in at least the coating  14  and an underlying surface portion of the component  10 .  FIG. 1  exaggerates the illustration of the wave  20  to schematically show two peaks  22  and one trough  23  along the component surface  26 . One skilled in the art will appreciate that peaks  22  and troughs  23  may not be visible to the naked eye in an actual embodiment, although they will generally be detectable by an instrument  28 , for example an optical instrument such as a camera or laser rangefinder, or a strain gage, etc. The instrument  28  may also be connected to the controller  18  to provide feedback for controlling the transducer  16  to produce a desired form and magnitude of wave  20  in the component  10 . 
         [0010]    As illustrated in  FIG. 1 , a standing wave  20  may be induced into the coating  14 , and a heat source, for example laser  30 , may be used to heat that portion of the coating  14  in the region  24  of the trough  23  by projecting a beam of energy  32  onto the surface  26 . Other sources of heat may be used, such as other forms of beam energy or a heated gas jet, for example. Both the mechanical wave action and the heating process function to impart stress into the coating  14 . Heating tends to expand the coating  14  and to create differential thermal expansion stresses. The wave action generates both tensile and compressive stresses in different regions of the coating  14 . 
         [0011]    Subsequent to the step illustrated in  FIG. 1 , the transducer  16  is controlled to move the standing wave  20  such that a peak  22  is positioned within the region  24  that was heated, as illustrated in  FIG. 2 . This movement tends to further expand the coating  14  in region  24  and to generate cracks  34  in the coating  14 , thereby facilitating its release and removal from the substrate  12 . Some additional mechanical cleaning may be required to completely remove the fractured region  24  of the coating  14 , such as light wire brushing. 
         [0012]    Advantageously, the selective simultaneous application of vibratory mechanical energy and heat energy will create complex, complementary stress patterns in the coating  14 , resulting in the overstressing and mechanical fracture of the coating  14 .  FIGS. 1 and 2  illustrate one embodiment where the coating  14  is subjected to relatively moving stress patterns which result in at least local transient stress conditions within the region  24  where the strength limit of the coating is exceeded, resulting in the formation of cracks  34 . An alternative embodiment may include the heating of a peak region of a standing wave in a coating followed by movement of the wave such that a trough of the wave moves to the heated region of the coating. This alternative embodiment generates a different transient stress pattern in the coating than does the embodiment of  FIGS. 1 and 2 , but advantageously would be performed in a manner that also results in a local stress condition within the region  24  where the strength limit of the coating is exceeded, resulting in the formation of a crack  34 . 
         [0013]    In another embodiment, a transducer  16  may be controlled to move a wave  20  across the surface  26  of a coating  14 , and simultaneous scanning of an energy beam  32  onto the surface  26  in a manner responsive to the movement of the wave  20 , such as maintaining the beam  32  in a trough or on a peak or at any other selected location relative to the wave  20 . The position of the wave  20  may be detected by any known technique, such as with a camera  28 , and input to controller  18  for use in controlling the source  30  of the beam energy. 
         [0014]    In another embodiment, a static pattern of heating may be generated on a surface  26  of a coating  14  to produce a temperature gradient pattern of relatively hot and cold regions which create differential thermal stress patterns in the coating  14 . Then, a pattern of mechanical waves  20  may be swept across the surface  26  to interact with the heating pattern to fracture the coating  14  at locations where additive stresses exceed the fracture limits of the coating material. 
         [0015]    Parameters of the laser beam  32  may be selected in response to the material of the coating  14  such that a sufficient portion of the beam&#39;s energy is absorbed by the coating  14  to raise a temperature of the coating  14  to above a temperature of the substrate  12 , or at least to expand the substrate relative to the coating, to exert tensile stress on the coating. The resulting temperature differential contributes to the stress pattern generated in the coating  14 . Alternatively, parameters of the laser beam  32  may be selected such that the coating  14  is largely transparent to the beam  32  so that a sufficient portion of the beam&#39;s energy is transmitted to the substrate  12  to raise a temperature of the substrate  12  to above a temperature of the coating  14 . Again, the temperature difference between the substrate  12  and coating  14  will contribute to the generated stress pattern. 
         [0016]    In an embodiment where the substrate  12  is heated to a temperature above a temperature of the coating  14 , tensile force is generated in the coating  14 . Vibratory mechanical energy may then be applied to the component  10 , such as at a resonant frequency of the component  10 , to excite the coating mechanically to a magnitude sufficient to cause fracture of the coating  14  as a result of complementary tensile stresses in the coating  14 . 
         [0017]    Methods of repairing coated components  10  may include the removal of at least a portion of the coating  14  using one of the processes described herein, repair of the underlying substrate  12  as necessary, and the re-application of coating material  14  of the same or different composition. Such methods benefit by the avoidance of the use of caustic chemicals or grit, and they have a reduced chance of damaging the component  10  as a result of the application of beam energy  32  when compared to prior art processes. 
         [0018]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.