Patent Publication Number: US-8535783-B2

Title: Ceramic coating systems and methods

Description:
BACKGROUND 
     The disclosure relates to ceramic coatings. More particularly, the disclosure relates to substrate preparation for ceramic coatings. 
     Components that are exposed to high temperatures, such as a component within a gas turbine engine, typically include protective coatings. For example, components such as turbine blades, turbine vanes, blade outer air seals (BOAS), and compressor components (e.g., floatwall panels) typically include one or more coating layers that function to protect the component from erosion, oxidation, corrosion or the like to thereby enhance component durability and maintain efficient operation of the engine. 
     As an example, some conventional turbine blade outer air seals include an abradable ceramic coating that contacts tips of the turbine blades such that the blades abrade the coating upon operation of the engine. The abrasion between the outer air seal and the blade tips provide a minimum clearance between these components such that gas flow around the tips of the blades is reduced to thereby maintain engine efficiency. Over time, internal stresses can develop in the protective coating to make the coating vulnerable to erosion and spalling. The outer air seal may then need to be replaced or refurbished after a period of use. 
     Similarly, the turbine blades may have an abrasive tip coating which properties are chosen to abrade the BOAS abradable coatings. 
     SUMMARY 
     One aspect of the disclosure involves an article having a metallic substrate. The substrate has a first surface region and a plurality of blind recesses along the first surface region. The substrate has perimeter lips at the openings of the plurality of recesses and extending partially over the respective associated recesses. A ceramic coating is along the first surface region. 
     In various implementations, the article may be a gas turbine engine component (e.g., a blade outer airseal or a combustor floatwall panel). A substrate may be one of a casting and an outer layer of a multi-layer metal laminate. The coating may comprise a stabilized zirconia (e.g., gadolinia-stabilized zirconia). An MCrAlY bondcoat may be between the coating and the substrate. The recesses may be arranged in a regular pattern. The recesses may have a transverse dimension at the lip of 85-98% of a transverse dimension below the lip. 
     The article may be manufactured by a method comprising indenting the first surface region to form the indentations. The indenting raises portions of the first surface region at perimeters of the indentations. The raised portions are deformed partially into the indentations so as to form the lips. The coating is applied to the substrate. 
     In various implementations, the deforming may comprise a pressing. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial sectional view of a substrate and indenter during indentation. 
         FIG. 2  is a partial sectional view of a substrate and die during a post-indentation coining. 
         FIG. 3  is a partial view of a surface region of the coined substrate. 
         FIG. 4  is a partial sectional view of the substrate after a first stage of coating. 
         FIG. 5  is a partial sectional view of the substrate after a second stage of coating. 
         FIG. 6  is a partial sectional view of the substrate after smoothing. 
         FIG. 7  is a partially schematicized central longitudinal sectional view of a turbine engine. 
         FIG. 8  is a view of a blade outer airseal. 
         FIG. 9  is a view of a combustor floatwall panel. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     An exemplary indentation process starts with a metal substrate. An exemplary substrate is a casting or machining (e.g., of a nickel- or cobalt-based superalloy for a gas turbine engine component such as an airseal or a combustor component). Alternative substrates may be roll or other sheet material for use in such components. The substrate  20  ( FIG. 1 ) is placed in an indenting press and the indenters  22  are pressed into a first surface region  23  of the substrate creating indentations  24  (which form blind recesses in the substrate). The indenting causes material flow outward from the indentations into areas therebetween so as to raise the surface  26  above the initial level  26 ′. The distribution of any raising of the surface  26  will depend upon the distribution of the indenters. If the indenters are sufficiently far away, then at least portions of the surface  26  between the indenters will not be raised. This material flow may create especially elevated zones  28  comprising raised lips immediately around the indentations. The indenters may then be extracted. 
     After the indenting, the substrate may be transferred to a different press. In the exemplary implementation, one or more second dies  30  ( FIG. 3 ) in one or more stages deform (coin) the raised lips  28  over/into/across the indentations. This may leave the lips (now shown as  28 ′) deformed/pressed/coined flush or subflush to the remainder of the adjacent surface or may still leave the lips  28 ′ merely less proud of the adjacent area. 
     Each exemplary indenter  22  is cylindrical (e.g., an outer surface  40  along a lower/distal portion  42  is cylindrical (e.g., a right circular cylinder, although other cross-sections and varying cross-sections are possible)). Each indenter extends upward/outward from a distal/lower face  44 . An exemplary indenter diameter D 1  along the cylindrical portion is essentially identical to the diameter D 2  of the indentation it leaves. The exemplary indentation base  50  is essentially flat, meeting the adjacent lower portion  52  of the indentation sidewall  54  at a right angle. An exemplary pre-coining indentation depth or height (to the apexes of the lips) is H 1  ( FIG. 1 ). Exemplary D 1  and D 2  are about 60 mil (1.5 mm), more broadly, 1.0-2.5 mm or 0.5-4.0 mm. An exemplary post-coining indentation depth or height is H 2 . An exemplary diameter at the coined lip  28 ′ is D 3 . Exemplary D 3  is less than 98% of D 2 , more narrowly, 85-98% or 88-95%. An exemplary protrusion ΔR, where ΔR=(D 2 −D 3 )/2 (for a circular indentation) of the lip is 1-7.5% of D 2 , more narrowly 2-5% or 15-115 micrometers, more narrowly, 15-65 micrometers. For example, when D 2 =1.5 mm, an exemplary protrusion is 0.75 mm. Alternatively, an exemplary ΔR might be 2-15% of a local radius (e.g., 2-15% of 0.5 D 2 ). Exemplary H 1  is 20 mil (0.5 mm), more broadly, 0.2-1.0 mm or 0.1-2.0 mm. An exemplary coining depth H 1 -H 2  (ΔH) is 15-115 micrometers, more narrowly, 15-65 micrometers. Alternatively, exemplary ΔH is 5-20% of H 1 . An exemplary web thickness T 1  ( FIG. 1 ) between adjacent indentations is 20 mil (0.5 mm), more broadly, 0.1-4.0 mm or 20-200% of D 1 , more narrowly, 30-100%. 
     The indentations may be arranged in one or more regular arrays. For example, depending upon the nature of the particular article (e.g., the BOAS) local curvature may require slight deviations from an exact regular pattern/array and larger surface features may interrupt arrays or separate distinct arrays. An exemplary regular pattern/array of the indentations is a two-dimensional (2D) hexagonal array ( FIG. 2 ). In such an array, an exemplary on-center spacing S is 130-250% of D 2 . 
     Alternative indentation planforms or cross-sections include polygonal (e.g., triangular, square, hexagonal) indentations and annular indentations. Their respective transverse dimensions would correspond to the diameters above. The ΔR of an annular indentation would correspond to the diameter of the circular indentation. 
     With roll-formed sheet metal as the substrate, the pressing and coining may be performed as continuous processes (e.g., via rollers). The resulting sheet material may then be laminated to other layers and further formed into the shape of the ultimate component (e.g., for an exemplary floatwall, various features may be machined, mounting features may be secured to the laminate, and the laminate may be deformed to the frustoconical segment shape). 
     Coating may be via a multi-stage process appropriate to the particular end use. This may involve applying a mere thermal barrier coating (e.g., on the combustor panel). On a BOAS segment it may involve an abrasive coating (for abrading blade tips) or abradable coating (to be abraded by blade tips). 
     An exemplary coating process is a multi-stage process. The exemplary process includes depositing a bondcoat and then depositing one or more additional coating layers (e.g., ceramic). An exemplary bondcoat is an MCrAlY (where M is at least one of nickel, cobalt, and iron) deposited via high velocity oxy-fuel (HVOF) deposition. An exemplary ceramic abradable coating comprises one or more stabilized zirconia layers (e.g., a GSZ and/or a yttria stabilized zirconia (YSZ)) via air plasma spray (APS). 
     During the spraying process, the protrusion of the lips above the lower portion of the indentation sidewall tends to shield the sidewall and the peripheral portion of the base. The result ( FIG. 4 ) is a reduction in the amount of coating available to bridge the junction of the sidewall and the base (the corner of the cross-section).  FIG. 4  shows the bondcoat  60  as having a thickness T 2  along the raised, flattened surface regions between the indentations. Approaching the indentation, the coating tapers around the lip leaving the underside  64  of the lip and the indentation sidewall  54  therebelow largely uncoated. Similarly, in a central region of the indentation base  50 , the thickness is shown as T 3  which may be similar to (e.g., slightly less than) T 2 . Near the periphery of the base  50 , the coating tapers off in thickness. Thus, in distinction to a bridging situation, the coating may taper so as to thin toward the periphery to the base rather than thicken toward the periphery of the base. 
       FIG. 5  shows the coated substrate after application of the ceramic material  70 . In the exemplary implementation, the as-applied ceramic material  70  more than fills the indentations. The indentations are, however, associated with relatively recessed regions  72  in the coating surface  74  which may be interspersed with relatively elevated regions  76 . A subsequent machining process may flatten the coating by removing the elevated areas ( FIG. 6 ). This may involve removing material from both the elevated and recessed regions to smooth/level the coating (e.g., close to accommodating overall curvature of the substrate such as the original pre-indentation shape of a cast or machined substrate). An exemplary peak bondcoat thickness T 2  is 5-8 mil (0.13-0.20 mm), more broadly, 0.05-0.50 mm. An exemplary final thickness T 4  of the ceramic material away from the indentations is 5-40 mil (0.13-1.0 mm), more broadly, 0.05-2.0 mm.  FIG. 5  further shows faults  78  associated with the indentation and extending outward through the coating. The faults have the tendency to provide some accommodation of differential thermal expansion and interrupt crack propagation. 
     In general, the segmentation of the coating provided by the indentations helps the coating accommodate differential thermal expansion (e.g., of the coating and substrate) to avoid spalling. The lips, by reducing bridging across the indentations help. With substantial bridging, the accommodation of differential thermal expansion is partially compromised. 
       FIG. 7  shows a turbine engine  100  (e.g., a turbofan) having a fan  102 , one or more compressor sections  104 , a combustor  106  and one or more turbine sections  108 , and a case  110 . The exemplary two-spool engine has high speed/pressure compressor and turbine sections on the high speed spool and low speed/pressure compressor and turbine sections on the low speed spool.  FIG. 7  also shows a blade  112  in the first blade stage of the low-pressure turbine. The blade stages rotate about the engine centerline or central longitudinal axis  114 . Tips of the blade stage move in close facing proximity to a circumferential array  116  of BOAS segments. 
       FIG. 8  shows a blade outer air seal (BOAS) segment  120 . Relative to an installed condition, a downstream/aftward direction  500 , radial (outward) direction  502 , and circumferential direction  504  are shown. The BOAS has a main body portion  122  having a leading/upstream/forward end  124  and a trailing/downstream/aft end  126 . The body has first and second circumferential ends or matefaces  128  and  130 . The body has an ID face  132  (along which the indentations may be formed) and an OD face  134 . To mount the BOAS to environmental structure (e.g., a main portion of the case), the exemplary BOAS has a plurality of mounting hooks. The exemplary BOAS has a single central forward mounting hook  142  having a forwardly-projecting distal portion recessed aft of the forward end  124 . The exemplary BOAS has a pair of first and second aft hooks  144  and  146  having rearwardly-projecting distal portions protruding aft beyond the aft end  126 . 
     The assembled ID faces of the circumferential array of BOAS segments thus locally bound an outboard extreme of the core flowpath through the engine. The BOAS  122  may have features for interlocking the array. Exemplary features include finger and shiplap joints. The exemplary BOAS  122  has a pair of fore and aft fingers  150  and  152  projecting from the first circumferential end  128  and which, when assembled, are positioned radially outboard of the second circumferential end  130  of the adjacent BOAS. 
     The exemplary combustor is an annular combustor having inboard and outboard walls each having an outer shell and an inner heat shield. Each exemplary wall heat shield is made of a longitudinal and circumferential array of panels as may be the shells. In exemplary combustors there are two to six longitudinal rings of six to twenty heat shield panels (floatwall panels). Each panel ( FIG. 9 ) has a generally inner (facing the combustor interior) surface  240  and a generally outer surface  242 . Mounting studs  244  or other features may extend from the outer surface  242  to secure the panel to the adjacent shell. The panel extends between a leading edge  246  and a trailing edge  248  and between first and second lateral (circumferential) edges  250  and  252 . The panel may have one or more arrays of process air cooling holes  254  between the inner and outer surfaces. The indented surface may be the inner surface  240 . The panel is shown having a circumferential span θ and a cone-wise length L. At a center  260  of the panel, a surface normal is labeled  510 , a cone-wise direction  512  normal thereto, a circumferential direction  516  and a radial direction  514 . 
     One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, the nature of the particular article (e.g., BOAS or floatwall panel) may influence details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.