Abstract:
A method for texturing a surface to form anchoring structures for a coating. The method includes: traversing an energy beam ( 10 ) along a path ( 30 ) on a solid substrate surface ( 12 ) to cause a melt pool ( 16 ) to move along the path; controlling power and motion parameters of the energy beam effective to establish a wave front ( 18 ) in the melt pool; and terminating the energy beam at an end ( 34 ) of the path when the wave front contains sufficient energy to create a protrusion ( 22 ) of material above the surface at the end of the path as the melt pool solidifies.

Description:
FIELD OF THE INVENTION 
       [0001]    Aspects of the present invention relate to thermal barrier coating systems for components exposed to high temperatures, such as encountered in the environment of a combustion turbine engine. More particularly, aspects of the present invention are directed to techniques that control laser irradiation to form directionally-aligned, three-dimensional structures that are effective to improve adherence of a layer applied to the textured surface. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is known that the efficiency of a combustion turbine engine improves as the firing temperature of the combustion gas is increased. As the firing temperatures increase, the high temperature durability of components of the turbine must increase correspondingly. Although nickel and cobalt based superalloy materials may be used for components in the hot gas flow path, such as combustor transition pieces and turbine rotating and stationary blades, even these superalloy materials are not capable of surviving long term operation at temperatures that sometimes can exceed 1,600 degrees C. or more. 
         [0003]    In many applications, a metal substrate is coated with a ceramic insulating material, such as a thermal barrier coating (TBC), to reduce the service temperature of the underlying metal and to reduce the magnitude of temperature transients to which the metal is exposed. TBCs have played a substantial role in realizing improvements in turbine efficiency. However, one basic physical reality that cannot be overlooked is that the thermal barrier coating will only protect the substrate so long as the coating remains substantially intact on the surface of a given component through the life of that component. 
         [0004]    High stresses that may develop due to high velocity ballistic impacts by foreign objects and/or differential thermal expansion can lead to damage and even total removal of the TBC (spallation) from the component. It is known to control a roughness parameter of a surface in order to improve the adhesion of an overlying thermal barrier coating. U.S. Pat. No. 5,419,971 describes a laser ablation process where removal of material by direct vaporization (e.g., without melting of material) is purportedly used to form three-dimensional structures at the surface being irradiated. U.S. Pat. No. 8,536,483 describes ablation of coatings with high power pulsed laser beams directed by scanning optics, and mentions that some configurations may remove coating to achieve a desired surface roughness. These methods are generally limited to removing material to create the desired texturing, (e.g., do not generally form structures extending outside the surface), and thus processes that can provide improved structural formations conducive to enhanced adhesion are needed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The invention is explained in the following description in view of the drawings that show: 
           [0006]      FIG. 1  is a sectional view of a surface of a substrate being irradiated with an energy beam that is controlled to form directionally aligned, three-dimensional protrusions in the surface. 
           [0007]      FIG. 2  is a top view of the surface of the substrate of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]    In accordance with one or more embodiments of the present invention, structural arrangements and/or techniques conducive to formation of three-dimensional anchoring structures on a surface exposed to controlled energy beam are described herein. In the following detailed description, various specific details are set forth in order to provide a thorough understanding of such embodiments. However, those skilled in the art will understand that embodiments of the present invention may be practiced without these specific details, that the present invention is not limited to the depicted embodiments, and that the present invention may be practiced in a variety of alternative embodiments. In other instances, methods, procedures, and components, which would be well-understood by one skilled in the art have not been described in detail to avoid unnecessary and burdensome explanation. 
         [0009]    The inventors of the present invention propose innovative utilization of an energy beam to form protrusions on a surface of a substrate. These protrusions act as three-dimensional anchoring structures that enhance adherence of a layer that is subsequently applied to the surface of the substrate. In addition to providing an anchor for a subsequently applied layer, the three dimensional anchoring structures may offer increased thermal conduction (akin to fins in radiators), improved lubricity etc. In one non-limiting embodiment, as shown in  FIG. 1 , an energy beam  10  moving in a direction of travel may be applied to a surface  12  of a solid substrate  14  to form a melt pool  16  on the surface  12  of the solid substrate  14 . For example, as shown in the right-side of  FIG. 1 , the energy beam  10  may be arranged to melt a relatively shallow layer on the surface  12  of the solid substrate  14 . Power and motion parameters of the energy beam  10  are controlled in a manner that may cause the melt pool  16  to move. Energy from the energy beam  10  and plasma created at the substrate surface may contribute to the motion, also referred to herein as a scooping effect, of the melt pool  16 . The plasma force may be effective to cause directional expulsion of the material. Various energy beam types for use with the method described herein include laser beams such as, for example, ytterbium fiber, diode, neodymium YAG, carbon dioxide and, most especially, such lasers operated in a pulsed mode. Further, the energy beam may be an alternate source like an electron or plasma beam. 
         [0010]    In an exemplary embodiment the energy beam  10  may cause a wave front  18  to form in the melt pool  16 . However, a visible wave front  18  in the melt pool  16  is not necessary in order to form the desired protrusion, much as a wave in the middle of the ocean may be undetected, yet contain adequate energy to create a large wave when it strikes a shoreline. When a wave front  18  is formed, it may be formed in front of the energy beam  10 , behind, and/or adjacent to the energy beam  10 . If there exists a wave front  18  behind the energy beam  10  and in front of the energy beam  10 , the two wave fronts  18  may unite when the energy beam is terminated to form a single wave front  18 . Whether or not an actual wave front  18  per se is formed, the energy and motion of the energy beam  10  are effective to form a liquid protrusion  20  that extends above the surface  12 , and the energy and motion are controlled to ensure that the liquid protrusion  20  solidifies while it is above the surface  12  to form a solidified protrusion  22 . This is visible in the left side of  FIG. 1 , which shows a three-dimensional anchoring structure  24  that was formed earlier in time (due to the direction of travel of the energy beam  10  from left to right). The energy beam may be terminated at any time once the melt pool has enough energy to form the liquid protrusion  20 . In the exemplary embodiment shown in  FIG. 1 , where a wave front  18  is formed, an interaction of the wave front  18  and adjacent solid/unmelted (i.e. not melted by the energy beam  10 ) substrate  26  can be utilized to cause the wave front  18  to curl and extend over the adjacent solid substrate  26 . The cantilevered wave front  18  solidifies in this position, thereby forming a cantilevered protrusion, referred to herein as a hook  28  of the three-dimensional anchoring structure  24 . This exemplary embodiment is not limiting, however, and the protrusion need not overhang adjacent solid substrate  26 . 
         [0011]    In an exemplary embodiment, the energy beam  10  may be a pulsed laser beam and the motion may be accomplished using laser scanning optics (e.g. galvanometer driven mirrors) and commensurate optics control software and controller(s). Alternately, the surface  12  may be moved relative to the energy beam  10 . The surface to be textured may be a substrate, such as a superalloy used in a gas turbine engine component. Typical superalloys for use in the preferred embodiment of surface modification include, but are not limited to, CM 247, Rene 80, Rene 142, Rene N5, Inconel-718, X750, 617, 738. 792, and 939, PWA 1483 and 1484, C263, ECY 768, CMSX-4, Hast-X and X45. In such case, the protrusions will be formed in the superalloy substrate and may act to improve adherence of a bond coat applied to the superalloy substrate. 
         [0012]    Alternately, or in addition, the surface to be textured may be a bond coat (e.g. an MCrAIY material) that has been applied to a superalloy substrate. In this case, the protrusions will be formed in the bond coat and may act to improve adherence of a thermal barrier coating (TBC) applied to the bond coat. However, the preceding examples are not meant to be limiting, and the process may be applied to a variety of surfaces. The component may be a new component or a stripped and repaired component, such as a turbine blade or vane. Alternately, the substrate can be a repaired component where significant bond coat is left on the component to be refurbished. In this instance the bond coat may be textured in anticipation of the application of the TBC. 
         [0013]    In an exemplary embodiment where the surface to be textured is a bond coat disposed on a superalloy substrate, approximately 125-300 microns (0.005 inches-0.012 inches) of bond coat may be applied to the superalloy substrate. The energy beam  10  is controlled such that the energy beam  10  is pulsed along a path  30  across the surface  12  using the laser scanning optics, initiating at a beginning  32  of the path  30  and terminating at an end  34  of the path  30 . In an exemplary embodiment, energy beam parameters include a speed of the energy beam  10  that may be 0.02 meters/second, a power output that may be 1 kW, a frequency that may be 0.01 kHz, and a duration that may be a 50,000 microsecond pulse. Using these parameters, the energy beam  10  may form a divot  36  of the three-dimensional anchoring structure  24  having a divot depth  38  from a divot bottom  40  to the surface  12  of about 30 microns. The path  30  may be approximately one millimeter long. The three-dimensional anchoring structure  24  may have a structure depth  42  approximately 60 microns from the divot bottom  40  to a top  44  of the hook  28 . The result is a process that can quickly and efficiently produce a pattern of the three-dimensional anchoring structures  24  through rapid scanning of the pulsed energy beam. One skilled in the art will appreciate that the energy beam  10  may be controlled (e.g., power and focal point etc.) to achieve desired divot characteristics and desired dimensions of the three-dimensional anchoring structures  24 . 
         [0014]    In an alternate embodiment, instead of maintaining a constant power, the power of the energy beam  10  may be varied to maximize the formation of the liquid protrusion  20 . For example, the power may be spiked immediately before its termination to enhance a propulsive effect of the energy beam  10 . Likewise, other parameters may be varied as desired to achieve the desired three-dimensional anchoring structures  24 . 
         [0015]    Optionally, mechanical assistance may also be used to mechanically drive the formation of the liquid protrusion  20  and the associated solid protrusion  22 . For example, an assist gas  46  may be used, such as, for example, laser fiber cooling air that is properly oriented to push the melt pool  16 . Alternately, other forms of mechanical assistance can be used, such as a discrete source of assist gas  46 , or ultrasonic energy etc. Alternatively, a further application of an energy beam may be used to apply a mechanical push to the protrusion  20  by creating a shock wave in the melt pool  16  via rapid vaporization of material. 
         [0016]    Also optionally, a flux  48  may be prepositioned on the surface  12  where the energy beam  10  is to traverse the surface  12 . The flux will assist coupling of the laser beam optical energy. The flux  48  will be melted by the energy beam  10  and incorporated as a molten slag  50  over the melt pool  16 , where such slag  50  acts to protect the melt pool  16  from atmospheric contaminants. After treatment, the solidified slag  50  resulting from the flux melting may be removed by any of the well-known techniques, such as mechanical brushing, grit blasting etc. 
         [0017]    The flux  48  may also be formulated to control a viscosity of the melt pool  16 . Reducing the viscosity results in a faster fluid flow velocity, and this promotes formation of the three-dimensional anchoring structures  24 . In contrast, increasing the viscosity results in slower fluid flow velocity, and this has the opposite effect. Small additions of silicon are effective to reduce viscosity and promote good metal motion. Therefore, an amount of silicon in the flux  48  may be adjusted to influence the formation of the three-dimensional anchoring structures  24 . Embodiments of flux  48  may include at least 0.25 wt. % silicon, or at least 0.50 wt. % silicon, or 0.50-0.75 wt. % silicon. 
         [0018]    The formation of the anchoring structures  24  may also be promoted by relatively deeper penetration of the melt pool  16 . Such penetration can be affected by flow that can be driven toward or away from the heat source (e.g. the energy beam  10 ) with more or less downward flow or penetration in the melt pool. Sulfur promotes a positive temperature coefficient of surface tension. As a result of the Marangoni effect, this increases penetration and promotes the formation of the three-dimensional anchoring structures  24 . Aluminum has the opposite effect. Therefore, the sulfur and/or aluminum content of the flux  48  may be regulated to influence the formation of the three-dimensional anchoring structures  24 . Embodiments of flux  48  may include at least 0.010 wt. % sulfur, or at least 0.020 wt. % sulfur, or 0.010-0.030 wt. % sulfur. 
         [0019]    The shape of the bottom of the melt pool  16  and the speed of travel of the melt pool across the surface  12  will also affect the formation of the anchoring structures  24 , much as the speed of an ocean wave and the shape of a beach affect the shape of waves upon a shoreline. Accordingly, the energy beam  10  may be controlled in a manner effective to impart a desired shape/size to the anchoring structures  24 . 
         [0020]    In lieu of using a flux, one may control environmental conditions using a suitable enclosure while performing the foregoing process. For example, depending on the needs of a given application, one may choose to perform the energy beam process under vacuum conditions in lieu of atmospheric pressure, or one may choose to introduce an inert gas in lieu of air. 
         [0021]    As can be seen in  FIG. 2 , the three-dimensional anchoring structures  24  are elongated in the direction of travel. That is, the three-dimensional anchoring structure  24  is oval-shaped with a narrow axis  60  transverse to the direction of travel and of approximate dimension of a diameter of the energy beam  10 . A long axis  62  is oriented parallel to the direction of travel. A length  64  of the three-dimensional anchoring structure  24  is characterized by the pulse duration and travel speed of the energy beam  10  plus an overhang length  66  of the hook  28 . The hook  28  serves to mechanically interlock with a subsequently applied layer, thereby improving adherence of the applied layer. 
         [0022]    The direction of travel of the energy beam  10  may be one-way along a straight traversal, which would form three-dimensional anchoring structures  24  and hooks  28  that are all aligned with the direction of travel. This traversal may be repeated such that several parallel rows of three-dimensional anchoring structures  24  are formed, all with aligned hooks  28 . Alternately, additional energy beam traversals may be parallel but with varying directions of travel, or the traversals may be patterned in any arrangement and have a plurality of directions of travel. This would result in a pattern having hooks  28  that point in a plurality of directions, and this would increase bond strength in multiple directions. The various described foregoing processes may be iteratively performed throughout the surface  12  to form a large number of three-dimensional anchoring structures  24  thereon. Moreover, three-dimensional anchoring structures  24  may be selectively distributed throughout surface  12 . For example, surface regions expected to encounter a relatively large level of stress may be engineered to include a larger number of three-dimensional anchoring structures  24  per unit area compared with surface regions expected to encounter a relatively lower level of stress. 
         [0023]    The energy beam  10  may be angled such that it points into the direction of travel, and this may enhance the scooping effect. Alternately, the direction of travel may be curvilinear, for example an arc. This may form a three-dimensional anchoring structure  24  where the solidified protrusion  22  forms a sweeping overhang or the like. 
         [0024]    In the preceding detailed description, various specific details are set forth in order to provide a thorough understanding of the invention and its various embodiments. However, those skilled in the art will understand that embodiments of the present invention may be practiced without these specific details, that the present invention is not limited to the depicted embodiments, and that the present invention may be practiced in a variety of alternative embodiments. In other instances, methods, procedures, and components that would be well-understood by one skilled in the art have not been described in detail to avoid unnecessary and burdensome explanation. 
         [0025]    Furthermore, various operations have been described as multiple discrete steps performed in a manner that is helpful for understanding embodiments of the present invention. However, the order of description should not be construed to infer that these operations must be performed in the order they are presented, nor that they are even order-dependent unless otherwise so described. Moreover, repeated usage of the phrase “in one embodiment” does not necessarily refer to the same embodiment, although it may. Lastly, the terms “comprising”, “including”, “having”, and the like, as used in the present application, are intended to be synonymous unless otherwise indicated. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.