Patent Application
20160059346
This invention relates generally to the field of materials technology, and more particularly to the removal of coating materials from an underlying substrate.
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.
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.
The invention is explained in the following description in view of the drawings that show:
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.
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.
As illustrated in
Subsequent to the step illustrated in
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.
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.
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.
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'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'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.
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.
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.
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.
