Abstract:
A bond coat is applied to a substrate of a gas turbine engine component the component. A barrier coat is applied atop the bond coat. The applying of the bond coat includes: applying a first layer having an as-applied first roughness; and applying a second layer atop the first layer, the second layer having an as-applied second roughness, greater than the first roughness. In the resulting coating system, the first and second layers may have different properties (e.g., greater porosity, pore size, and/or splat size for the second layer).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to gas turbine engines. More particularly, the invention relates to thermal barrier coatings for gas turbine engines. 
         [0002]    Gas turbine engine gaspath components are exposed to extreme heat and thermal gradients during various phases of engine operation. Thermal-mechanical stresses and resulting fatigue contribute to component failure. Significant efforts are made to cool such components and provide thermal barrier coatings to improve durability. 
         [0003]    Exemplary thermal barrier coating systems include two-layer thermal barrier coating systems. An exemplary system includes a NiCoCrAlY bond coat (e.g., low pressure plasma sprayed (LPPS)) and a yttria-stabilized zirconia (YSZ) thermal barrier coat (TBC) (e.g., air plasma sprayed (APS)). While barrier coat layer is being deposited or during an initial heating cycle, a thermally grown oxide (TGO) layer (e.g., alumina) forms atop the bond coat layer. As time-at-temperature and the number of cycles increase, this TGO interface layer grows in thickness. U.S. Pat. Nos. 4,405,659 and 6,060,177 disclose exemplary systems. 
         [0004]    Exemplary TBCs are applied to thicknesses of 5-40 mils and can provide in excess of 300° F. temperature reduction to the base metal. This temperature reduction translates into improved part durability, higher turbine operating temperatures, and improved turbine efficiency. 
         [0005]    Nevertheless, there remains need for improvement in component durability. 
       SUMMARY OF THE INVENTION 
       [0006]    One aspect of the invention involves a method for coating a gas turbine engine component. A bond coat is applied to a substrate of the component. A barrier coat is applied atop the bond coat. The applying of the bond coat includes: applying a first layer having an as-applied first roughness; and applying a second layer atop the first layer, the second layer having an as-applied second roughness, greater than the first roughness. 
         [0007]    In various implementations, the method may be implemented in the remanufacturing of a baseline component or the reengineering of a configuration thereof. 
         [0008]    Another aspect of the invention involves a gas turbine engine component comprising a metallic substrate. A coating is on the substrate. The coating includes a bond coat and a barrier coat atop the bond coat. The bond coat comprises: a base layer; and a second layer atop the base layer and having a greater characteristic pore size and/or splat size than the base layer. 
         [0009]    In various implementations, a TGO may be between the bond coat and barrier coat. The second layer may have a greater characteristic porosity than the base layer. 
         [0010]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a sectional view of coated substrate. 
           [0012]      FIG. 2  is a flowchart of a process for coating the substrate of  FIG. 1 . 
       
    
    
       [0013]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0014]      FIG. 1  shows a coating system  20  atop a superalloy substrate  22 . The system may include a bond coat  24  atop the substrate  22  and a TBC  26  atop the bond coat  24 . The exemplary bond coat  24  includes a base layer  28  and an intermediate layer  30 . As is discussed below, the properties of the base layer  28  may be chosen for adhesion to, and protection of, the substrate  22  while the properties of the intermediate layer may be chosen for adhesion to the TBC  26 . Exemplary substrates are of nickel- or cobalt-based superalloys used for hot gaspath components such as: turbine section blades; turbine section vanes; turbine section blade outer air seals; combustor shell pieces; combustor heat shield pieces; combustor fuel nozzles; and combustor fuel nozzle guides. 
         [0015]    An exemplary coating process  100  includes preparing  102  the substrate (e.g., by cleaning and surface treating). A precursor of the bond coat base layer  24  is applied  104 . An exemplary application  104  is of an MCrAlY, more particularly a NiCoCrAlY material. Advantageous high temperature protective properties for the base layer  24  may be associated with properties that are disadvantageous for adhesion to the TBC  26 . For example, advantageously high density and low porosity for protection may be associated with a surface roughness that is lower than desired for TBC adhesion. An exemplary as-applied roughness of the base layer  28  is less than 300 microinches R a  (e.g., 200+/−40 microinch R a  or less). An exemplary application is via a spray from a powder source of less than 45 microns particle size to achieve said roughness. An exemplary application is via a high-velocity oxy-fuel (HVOF) process. An exemplary application is to a thickness of 0.003-0.010 inch. LPPS, VPS, EBPVD, cold spray, and any other appropriate process may be used to provide a dense, low oxide, base layer  28  that provides good oxidation and corrosion resistance. 
         [0016]    After the application  104 , the precursor may be diffused  106 . An exemplary diffusion is via heating (e.g., to at least 1900° F. for a duration of at least 4 hours) in vacuum or nonreactive (e.g., argon) atmosphere. The exemplary diffusion  106  is effective to create a metallurgical bond between the base layer and the substrate. The diffusion may also reduce the diffusion path length for protective oxide-forming species. Alternatively diffusion steps may occur after applying the intermediate layer and/or the TBC, if at all. 
         [0017]    After the application  104  and the optional diffusion  106 , the intermediate layer  30  may be applied  108 . The exemplary intermediate layer may be of essentially the same material as the base layer precursor and may be applied via similar techniques. However, it is preferable that the intermediate layer be applied to yield advantageous adhesion of the TBC. The intermediate layer  30  may have a surface roughness that may be greater than that of the base layer  28 . An exemplary as-applied roughness is 300-800 microinch R a , more narrowly, 500+/−100 microinch R a . This may be 150-300% (or more) of the as-applied roughness of the base layer  28 . Such roughness may be achieved by using a coarser source powder (e.g., at least 150+% of the characteristic particle size of the base layer source powder) and/or varying application parameters. An exemplary powder size is 45-70 microns. Other properties may differ from the base layer (e.g., as discussed below). 
         [0018]    In ascending order of typical roughness, alternative methods for the application  108  include: EBPVD, cold spray, HVOF and LPPS, APS, wire arc and wire flame. Other options include slurry methods where a slurry is made with an optional binder and powder, then applied to the base layer by spraying, dipping, brushing, etc. Then the binder baked off and the metallics sintered for adhesion. The slurry has large particles that produce the roughness. The slurry may include fine particles and/or elements or alloys that melt below the sintering temperature, to promote sintering and adhesion of the intermediate layer to the base layer by sintering and/or brazing. 
         [0019]    Exemplary thickness of the intermediate layer  30  is less than (e.g., 10-50% of) the thickness of the base layer  28 . For example, the absolute and relative thicknesses may be chosen to make the oxidation and corrosion resistant base layer as thick as possible to maximize the effect of those properties. The rough intermediate layer need only be thick enough to provide desired improvements in TBC bonding. An exemplary intermediate layer thickness is at least 0.001 inch, more narrowly 0.002-0.004 inch would be required. 
         [0020]    After the application  108 , the TBC  26  may be applied  110 . The exemplary application  110  is of a yttrium-stabilized zirconium oxide (e.g., 6-8% yttrium by weight, nominal 7YSZ). An environmental barrier coat (“overcoat”—not shown, if any) may then be applied  112 . An exemplary overcoat is one that is not wet by, nor reacts with calcium-magnesium-alumino-silicates (CMAS) or ingested dust or sand. 
         [0021]    Generally, for good oxidation and corrosion resistance, the base layer  28  would have some to all of the following attributes relative to the intermediate layer  28 : lower roughness; greater density; smaller pores; lower porosity (volume fraction), smaller oxide particles; and less oxide content (mass fraction); smaller splats, and smaller oxide stringers. Various of these properties may be observed by metallography (e.g., with use of etchant). To the extent measurable, the vestigal surface roughness may differ in the same way as the as-applied surface roughness. 
         [0022]    The splat structure results from the impact of spray droplets. The droplets flatten and solidify, leaving traces of the individual splat structure within the coating as further splats build up. An exemplary characteristic splat size of the intermediate layer  30  may be at least twice that of the base layer  28 . The characteristic may be a median, mean, or modal value, with or without weighting based upon splat size. This may be measured as a cross-sectional area in a cross-sectioning perpendicular to the coating surface. 
         [0023]    In development, splats may readily be observed by adding a tagging component to the sprays. The tagging component may then highlight the splat interfaces. However, once process parameters are finalized, the tagging component may be eliminated. In the absence of a tagging component, a dye may be infiltrated into the coating after coating application. An exemplary dye is a rhodium-B fluorescent dye. 
         [0024]    In some reengineering or remanufacturing situations, the foregoing teachings may be applied to reduce total bond coat thickness while improving or maintaining TBC adhesion and/or oxidation resistance. Other combinations of such benefits may also be achieved. In the reengineering from a baseline bond coat, the baseline could have properties in the paragraph above falling in between those of the base layer  28  and intermediate layer  30 . Performance (e.g., spall resistance) may be measured by observation or direct testing. An exemplary observation comprises thermal cycling with differential heating and cooling (heating one portion of the coating while cooling another portion of the part). Spallation may be observed after a sufficient number of cycles. 
         [0025]    One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, and applied as a reengineering of an existing component, details of the existing component may influence or dictate details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.