Abstract:
A compliant, impact-absorbing layer ( 27 ) on a thermal barrier coating (TBC) ( 26 ) on a substrate ( 24 ). The impact-absorbing layer ( 27 ) has an internal structure of planar grains ( 28 ) oriented parallel to the substrate so the impact-absorbing layer preferentially fractures horizontally and it blocks vertical cracking. A ceramic armor layer ( 30 ) on the impact-absorbing layer has a higher density, and is fractured ( 32 ) into fracture plates ( 33, 34 ) of a designed size. This provides a thermal barrier with particle impact-resistance that may be applied to gas turbine components where needed.

Description:
FIELD OF THE INVENTION 
     The invention relates to particle impact resistant thermal barrier coatings, particularly on internal turbine components. 
     BACKGROUND OF THE INVENTION 
     Some components of gas turbine engines, such as vanes and blades, operate at temperatures up to about 1500° C. Ceramic thermal barrier coatings (TBCs) are used to insulate such components from heat, reduce surface oxidation, and reduce wear and damage caused by ingestion of foreign objects from the external air intake or from debris within the engine. Impacts from foreign objects and debris can spall the TBC, reducing its life. Hard particles commonly ranging from about 5 to 100 microns in diameter erode surfaces bounding the working gas flow path. The present coating and method reduces and controls such damage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  is a conceptual sectional view of a multi-layer thermal barrier coating on a component substrate per aspects of the invention. 
         FIG. 2  is a top view of the top layer of  FIG. 1 . 
         FIG. 3  illustrates a method according to aspects of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a sectional view of a component substrate  22  having a surface  23  with a bond coat  24  and a thermal barrier coating (TBC)  26 . The substrate may be made of a high-temperature structural material such as a nickel-based superalloy or a ceramic matrix composite. The bond coat  24  may be any type suitable for the materials of the substrate and the TBC as known in the art. For example, the bond coat  24  may be an MCrAlY alloy, where M is selected from the group of Ni, Co, Fe and their mixtures, and Y can include yttrium Y, as well as La and Hf. The bond coat may be applied for example by sputtering, electron beam vapor deposition, or low pressure plasma spraying, to provide a dense, relatively uniform layer such as about 0.02 mm to 0.25 mm thick. 
     The TBC  26  may comprise yttria-stabilized zirconia (YSZ) or a gadolinium zirconate (GZO) such as Gd 2  Zr 2 O 7  and/or other TBC materials known in the art. The TBC layer  26  may cover the exterior surface  23  of a turbine component in the working gas flow. Two additional protective layers  27  and  30  may cover some or all of the TBC  26  for particle impact protection. 
     Impact-absorbing layer  27  is a relatively soft anisotropic layer that absorbs the energy of particle impacts and stops vertical crack propagation. Layer  27  may be applied by a thermal spray process, such as plasma spray, that produces overlapping pancake-like lamellae  28  called “splats” with respective diameters oriented parallel to the substrate surface  23 , forming a porous, compliant, planar-grained layer. The overlapping splats  28  block vertical crack propagation. “Vertical” means normal to the substrate surface  23 . Layer  27  may have less than 75% of theoretical density, due to voids  29 . A desired density can be achieved by setting thermal spray parameters such as feedstock, plasma gas composition and flow rate, energy input, torch offset distance, and substrate cooling, as known in the art. 
     Armor layer  30  is a relatively hard layer designed to crack along vertical fractures  32  into a geometry of fracture plates  34  ( FIG. 2 ) with perimeters  33 . These plates limit impact damage horizontally to a diameter or zone, because any impact-induced horizontal cracks will stop at a vertical crack  32 ,  33 . The plates  34  may have an average diameter larger than the average diameter of splats  28  in the impact-absorbing layer  27  to spread the load of the impact and allow a larger volume of the underlying layer  27  to be used to absorb the impact energy. The plates  34  form an impact-absorbing armor in conjunction with the impact-absorbing layer  27 . The fracture plates  34  may be made small enough to recoil from the particle impacts to absorb energy, yet large enough to spread the energy over a larger area than either the impact particle size or the absorbing layer grain size. For example, the fracture plates  34  may range in size from 0.25 to 2.0 mm and especially from 0.5 to 1.5 mm. A desired size range can be achieved for a given thickness of the armor layer by setting thermal spray parameters as known in the art. Alternately, a honeycomb pattern of score lines may be laser-engraved on the armor layer to promote vertical cracks in a geometry of fracture plates of a predetermined size. The armor layer may have greater than 90% of theoretical density, and especially greater than 95%. 
     Each protective layer  27 ,  30  has a specialized role. These two layers work synergistically to limit damage both horizontally and vertically, and to absorb impact energy, thus protecting the TBC  26 . To reduce cost and weight, the protective layers  27 ,  30  may be limited to areas where damaging particle impacts occur, such as the leading edges of blades, vanes, and other parts. 
     All layers  24 ,  26 ,  27 , and  30  may be applied by a thermal spray process such as plasma spray or high velocity oxygen fuel spray. The protective layers  27  and  30  may use the same materials as layer  26 , but with different spray parameters. Alternately, different materials may be used for different layers. The thickness of layer  30  may be engineered in conjunction with its hardness such that process shrinkage of layer  30  produces fracture plates  34  of the desired sizes. 
       FIG. 3  illustrates a method  40  per aspects of the invention, including the steps of:  42 —Form a thermal barrier coating (TBC) on a surface;  44 —Form an impact-absorbing layer on the TBC including planar grains oriented parallel to the surface;  46 —Form an armor layer on the impact-absorbing layer with fracture plates of a design size range. 
     The impact-absorbing layer  27  may have 10-35% greater porosity than the armor layer  30 , and especially 15-35% more porosity. For example, the TBC  26  may be formed of 7-9 mol % YSZ with 9-15% porosity, the impact-absorbing layer  27  may be formed of 7-9 mol % YSZ with 25-35% porosity, and the armor layer  30  may be formed of 7-9 mol % YSZ with 2-10% porosity. 
     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.