Method for coating a blade underplatform

Abstract

In a method (804) for coating a metallic blade (20) substrate (22), the blade substrate has: an airfoil section (40); a root section (42); and a platform section (44) between the root section and the airfoil section. The method comprises: applying (844, 846) a maskant to a portion of an underside (52) of the platform bearing an MCrAlY coating and a portion of the root lacking an MCrAlY coating; and heating (848), the heating causing aluminum depletion from the MCrAlY into the maskant.

Description

BLADE UNDERPLATFORM COATINGCROSS-REFERENCE TO RELATED APPLICATION

[0001] Benefit is claimed of U.S. Patent Application No. 63 / 735,502, filed December 18, 2024, and entitled “Blade Underplatform Coating”, and U.S. Patent Application No. 63 / 689,588, filed August 30, 2024, and entitled “Blade Underplatform Coating”, the disclosures of which are incorporated by reference herein in their entireties as if set forth at length.BACKGROUND

[0002] The disclosure relates to gas turbine engines. More particularly, the disclosure relates to coatings for metallic substrates.

[0003] Gas turbine engines (used in propulsion and power applications and broadly inclusive of turbojets, turboprops, turbofans, turboshafts, industrial gas turbines, and the like) include metallic components (e.g., with nickel-based superalloy substrates) having ceramic barrier and other coatings (e.g., environmental barrier coatings (EBC), thermal barrier coatings (TBC), abradable coatings, and the like) and corrosion coatings. The ceramic coatings are typically along gas-path-facing surfaces. Corrosion coatings are typically on other surfaces such as internal cooling passageway surfaces, blade and vane platform underside surfaces, vane shroud outer diameter (OD) surfaces, and the like.

[0004] Turbine blades require protection on the airfoil gaspath surface, the tip and under platform regions for differing failure modes. They also require a thermal barrier coating (TBC) to be deposited on the airfoil surface to give it thermal protection with exceptional spall life. Currently, multiple steps are required to coat each of these areas with different coatings for adequate protection. An example may require a platinum plating step (overall exterior), an aluminizing step (overall interior and exterior), heat treatment (forms the aluminide from the prior steps), application of overlay coatings (under platform such as higher MCrAlY by cathodic arc or low pressure plasma spray (LPPS)), heat treatment, then chromizing (e.g., slurry chromizing under platform particularly non-line-of-sight (NLOS) regions missed by the high-Cr MCrAlY) and heat treatment step, finally a TBC application (ceramic principally to gaspath surfaces such as electron beam physical vapor deposition (EB-PVD)). In MCrAlY M stands for Ni, Co, Fe or combination of two or three of the three. However typical commercially MCrAlY used as bondcoats have little, if any, Fe (such as NiCrAlY, NiCoCrAlY and CoNiCrAlY with not more than 5 weight percent Fe, if any).Minor intentional elements typically include Hf, Si, and Zr. Often, Hf, Si, and Zr are not more than 1.0 weight percent but Si may be up to 5 weight percent and Hf to 2 weight percent in some cases. And there may be commercial or inevitable impurities.

[0005] United States Patent 6,435,830, Allen et al., August 20, 2002, “Article having corrosion resistant coating”, (the ‘830 patent), the disclosure of which is incorporated by reference in its entirety herein as if set forth at length, discloses a high-Cr MCrAlY along the under-platform surfaces of a blade.SUMMARY

[0006] One aspect of the disclosure involves a method for coating a metallic blade substrate, the blade substrate comprising: an airfoil section; a root section; and a platform section between the root section and the airfoil section. The method comprises: applying a maskant to a portion of an underside of the platform bearing an MCrAlY coating and a portion of the root lacking an MCrAlY coating; and heating, the heating causing aluminum depletion from the MCrAlY into the maskant.

[0007] A further embodiment of any of the foregoing embodiments may additionally and / or alternatively include: applying the MCrAlY coating to the platform underside and the root; and before the applying of the maskant, machining the applied MCrAlY from said portion of the root.

[0008] A further embodiment of any of the foregoing embodiments may additionally and / or alternatively include applying an MCrAlY coating to an airfoil of the casting.

[0009] A further embodiment of any of the foregoing embodiments may additionally and / or alternatively include after the heating, applying a ceramic coating to the airfoil.

[0010] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the applying the MCrAlY coating to the platform underside and the root and the applying the MCrAlY coating to the airfoil are from the same source.

[0011] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the applying of the maskant comprises applying a powder maskant.

[0012] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the maskant comprises AI2O3, nickel, chromium, and additional aluminum.

[0013] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the maskant comprises by weight percent 30 to 60 (or 30.0 to 60.0) said AI2O3, 20 to 60 (or 20.0 to 60.0) said nickel, 0.5 to 5.0 said chromium, and 0.5 to 8.0 saidadditional aluminum, optionally wherein a ratio of said chromium to said additional aluminum is at least 1:2.

[0014] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the maskant consists essentially of Ni, Cr, and Al in one or more powders and AI2O3 powder. For example, this may include separate powders of all three of Ni, Cr, and Al. Or it may include NiAlCr. Or it may include one or more of NiAl and Nh Al powder, mixed with Cr powder, among other options including combinations of the foregoing.

[0015] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the maskant comprises, by weight percent: at least 20 (or at least 20.0) AI2O3, 40 to 50 (or 40.0 to 50.0) Ni; 1.0 to 10.0 Cr; 0.5 to 8.0 additional aluminum; and up to 5.0 other.

[0016] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the maskant substantially lacks a halide activator.

[0017] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the heating does not form nitride particles.

[0018] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the aluminum depletion is by an overall increment of 2.0 to 7.0 weight percent; and a chromium depletion, if any, of the MCrAlY is by an overall increment of not more than 5.0 weight percent.

[0019] A further aspect of the disclosure involves a blade having a metallic blade substrate comprising: an airfoil section; a root section; and a platform section between the root section and the airfoil section and having an outer diameter surface and an underside. The airfoil section has a coating comprising: a bondcoat; and a ceramic coating atop the bondcoat. The underside has a coating having, relative to the bondcoat, a difference in weight percent of: -4 to 2 Cr; -3 to 3 Co; and -7 to -2.5 Al. Or said difference may be -4.0 to 2.0 Cr; - 3.0 to 3.0 Co; and -7.0 to -2.5 Al.

[0020] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the underside coating has, relative to the bondcoat, a difference in weight percent of: -3.0 to 1.0 Cr; -2.0 to 2.0 Co; and -5.5 to -3.0 Al.

[0021] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the underside coating has, relative to the bondcoat, a difference in weight percent of: 0.5 to 3.0 Ta greater; 0.5 to 3.0 W greater; and 0.10 to 1.0 Mo greater.

[0022] A further aspect of the disclosure involves a method for coating a metallic substrate, the method comprising: applying a first MCrAlY coating; machining the substrate after application of the first MCrAlY coating to remove said first MCrAlY coating from a first region of the substrate; applying an Ni, Cr, Al, and AI2O3 maskant to the first region and a second region where the first MCrAlY remains; and heating. The heating causes: aluminum depletion from the first MCrAlY in the second region without nitride formation. The Ni, Cr, and Al may be in one or more powders as discussed above and the AI2O3 may be in powder. Optionally, the powders may be in a sintered form.

[0023] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, said heating also heats a third region from where the first MCrAlY coating has not been removed and to which region the maskant was not applied.

[0024] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, said heating causes less, if any, aluminum depletion from the first MCrAlY in the third region than from the first MCrAlY in the first region.

[0025] In a further embodiment of any of the foregoing embodiments, additionally and / or alternatively, the metallic substrate is a metallic blade substrate comprising: an airfoil section; a root section; and a platform section between the root section and the airfoil section and having an outer diameter surface and an underside. The first region is along sides of the root and the second region is along the underside.

[0026] 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

[0027] FIG. 1 is a photograph of three test coupons after a Type 2 hot corrosion test using a baseline MCrAlY after diffusion in contact with an Ni / Al / Cr / AhO3 maskant.

[0028] FIG. 2 is a photograph of three test coupons having just the baseline MCrAlY after a Type 2 hot corrosion test.

[0029] FIG. 3 is an electron micrograph of a baseline MCrAlY after chromization.

[0030] FIG. 4 is an electron micrograph of the baseline MCrAlY with an Ni / Al / Cr / AhO3 masked diffusion.

[0031] FIG. 5 is an electron photomicrograph of a region of the coated substrate.

[0032] FIGs. 5A-E, respectively, show nickel, aluminum, chromium, silicon, and hafnium contents via SEM / EDX mapping of the FIG. 5 region.

[0033] FIG. 6 is a view of a gas turbine engine blade.

[0034] FIG. 7 is a schematic cutaway view of the blade.

[0035] FIG. 8 is a flowchart of a blade manufacture process.

[0036] Like reference numbers and designations in the various drawings indicate like elements.DETAILED DESCRIPTION

[0037] With reference to an example article as a Ni-based superalloy blade, a reactive mask is applied to the bare substrate root and the MCrAlY-coated platform underside and the combination is heated. The reactive mask comprises a mixture of Ni, Al, Cr, and AI2O3. As discussed further below, it is believed that when brought to temperature in a relatively inert environment, the mask getters Al from the MCrAlY. It may further form an alpha chromium phase in the coating. The aluminum depletion may improve corrosion / oxidation resistance.

[0038] Mask format may include dry packed powder. This may include introducing the powder to a robust container (e.g., metallic such as nickel alloy or nickel superalloy or ceramic) surrounding the blade root. Pressing of the blade or of a fixture surrounding the blade may provide firm contact between the powder and the root and the platform underside. Alternatively, it may comprise a form-fitting assembly such as of sintered pieces formed from such powder. Such material is available from APV Engineered Coatings, Inc. of Akron Ohio as M-l, M-7, and M-17 products. These have been typically introduced as powder in a masking box. Although sintered root masking pieces have also been used. The M-l maskant broadly includes: (1) alumina; and (2) in one or more phases nickel, aluminum, and chromium. Those phases are believed to essentially be one or more NiAlCr intermetallic phases. Various sources describe similar net elemental compositions as including an intermetallic of Ni, Cr, and Al mixed and optionally presintered.

[0039] Several Ni-enriched compositional variants exist. The M-7 maskant is a powder blend of the M-l maskant and Ni powder. This may add an elemental nickel phase. The M- 17 maskant is often produced as a mixture of M-l and M-7. Nominal M-l composition by weight percent is 28.5 Ni, 6.7A1, 2.9 Cr, balance AI2O3. Several Ni-enriched compositional variants by weight percent up to 55.0 Ni, down to 5.0 Al, down to 1.0 Cr, balance AI2O3 plus impurities may be utilized.

[0040] Thus, one group of example compositions may be 25.0 to 55.0 Ni, 5.0 to 10.0 Al, 1.0 to 5.0 Cr, balance AI2O3, plus impurities. A relatively Ni enriched group may have 40.00- 50.00 Ni, 5.00-10.00 Al, 1.00-5.00 Cr, balance AI2O3.

[0041] The Ni, Al, and Cr may be a blend of powders or a presintered powder (whose particles themselves are sintered Ni, Al, and Cr particles). Or the form may be an intermetallic of Ni and Al mixed and optionally presintered with Cr.

[0042] More broadly example maskant comprises by weight percent 30 to 60 or 30.0 to 70.0 or 30.0 to 60.0 AI2O3, 20 to 60 or 20.0 to 60.0 nickel, 0.5 to 10.0 chromium, and 2.0 to 20or 2.0 to 20.0 or 2.0 to 10.0 additional aluminum (beyond that in the alumina). Optionally aratio of said nickel to said additional aluminum is 3:1 to 20:1. Alternatively, example nickel- enriched maskant comprises by weight percent at least 20 AI2O3 or at least 20.0, 40 to 50 nickel or 40.0 to 60.0 or 40.0 to 50.0, 1 to 5 chromium or 1.0 to 5.0, and 5 to 10 additional aluminum or 5.0 to 10.0 and up to 5.0 total other (subject to variations below).

[0043] Other variations, beyond the example 5.0 total other may include any further metal in the element at up to the content in the substrate alloy or, more narrowly, up to half the content in the substrate alloy.

[0044] Other variations may substitute further oxides for at least a portion of the alumina. Preferably, these may be relatively inert such as zirconia in not providing their metal for diffusion into the MCrAlY.

[0045] Other variations may substantially reduce Al relative to the commercial masking products. This may enhance Al diffusion from the MCrAlY. Likewise increasing the additional aluminum above about 10 weight percent will start to compromise the Al diffusion from the MCrAlY and may result in aluminum-enrichment rather than depletion.

[0046] We believe that, due to various activity differences between the Ml maskant pack and the coating, Al migrates from the coating into the maskant and depletes Al from the coating. As a result, Cr enriches in the coating. Enriched Cr promotes the in-service formation of Cr2Oa, which is more stable than AI2O3 in the temperature regimes that Type 1 and Type 2 hot corrosion are the primary damage mechanism to typical MCrAlY coating. Therefore, the enriched Cr enhances performance of the coating, as demonstrated in testing. As is discussed further below, this may provide oxidation resistance similar to that provided by during pack or vapor chromizing (despite having a lower Cr content) while reducing detrimental nitride and oxide formation (that reduce ductility) associated with use of halide activator during pack or vapor chromization. Type 1 is high temperature frontal attack from sulfidation. Type 2 is low temperature pitting corrosion attack from sulfidation.

[0047] Bondcoat source material may be a conventional MCrAlY. Specific examples are high-chromium MCrAlY. An example high-Cr MCrAlY may have by weight Al of 5.0% to 15.0% (more narrowly 6.0% to 12.0%) and Cr of 15.0% to 50.0% (more narrowly 25.0% to 40.0%) and / or an Al:Cr weight ratio of 1:3 (more broadly 1:1.5 to 1:10 or 1:2 to 1:6 or 1:3 to 1:5). Example applied thickness is 50 micrometers to 100 micrometers, more broadly 25 micrometers to 200 micrometers.

[0048] Example maskants comprise or consist essentially of a mixture of AI2O3 and Ni, Cr, Al (in one or more phases) powder(s). As discussed above, there may be departures fromthe commercial compositions tailored to further enhance the aluminizing and avoidance of nitride formation.

[0049] FIG. 2 is a photograph of three test coupons (1 inch (2.54 cm diameter) having a baseline MCrAlY after a Type 2 hot corrosion test. This is characteristic of some prior art processes. FIG. 1 is a photograph of three test coupons after a Type 2 hot corrosion test using the baseline MCrAlY after diffusion in contact with an Ml maskant. The coupons of both tests were a single crystal nickel-based superalloy having a nominal by weight percent composition of 5 Cr, 2 Mo, 6 W, 3 Re, 8.7 Ta, 5.6 Al, 10 Co, 0.1 Hf, balance nickel (or 5.0 Cr, 2.0 Mo, 6.0 W, 3.0 Re, 8.7 Ta, 5.6 Al, 10.0 Co, 0.1 Hf, balance nickel). In both sets of specimens, the MCrAlY was applied and diffused to the composition noted above. The FIG. 1 coupon was then masked with Ml maskant via a dry press process and heat treated at 1080°C for six hours. Both sets of specimens were then exposed for 100 hours at approximately 1350°F (732°C) to a CMAS-simulant salt. The MCrAlY was applied by cathodic arc deposition to a thickness of 50 micrometers to 100 micrometers. The baseline MCrAlY alone showed substantially greater visible bulk surface attack / pitting. The test was continued until a total of 500 hours whereupon attack rate was measured via microscope upon sectioning. A relative attack rate was measured as a depth of attack of 4.9 mil (125 micrometers) for an NiAl-coated coupon, 2 mils (50 micrometers) for the baseline MCrAlY, and 0.1 mil (2.5 micrometers) for the Ml masked / diffused MCrAlY.

[0050] FIG. 3 shows a section of the baseline MCrAlY after a chromization. This is characteristic of some other prior art processes. FIG. 4 shows a comparable section of the same MCrAlY with the M-l masking treatment in the absence of chromizing or aluminizing (e.g., vapor phase or pack). The chromized baseline of FIG. 3 shows substantial nitride formation as the layer of black spots 320. In contrast, there are not such nitrides in the masked variant of FIG. 4. FIG. 4 shows substrate 300 and a diffusion zone with an inner precipitate sublayer 302 and an outer sublayer 304 with the masked / diffused MCrAlY 306 thereatop. FIG. 4 shows added boxes of three sampled regions of the masked variant and Table I has compositional data for samples in such regions. This reflects a slight drop in chromium content near the surface as well as a drop in aluminum. Table II shows estimated compositions pre-mask and final for a hypothetical example along with estimated ranges of the final composition and the delta between the final and the pre-mask compositions.Table IFIG. 4 Region Content (Weight Percent)Table II U / P Coating Content (Weight Percent)

[0051] In other variations, for a final value of a given element an alternative range may be formed by pairing the lower value of the narrower range with the upper value of the broader or the upper value of the narrower range with the lower value of the broader.

[0052] In other variations, for a delta value of a given element an alternative range may be formed by pairing the lower value of the narrower range with the upper value of the broader or the upper value of the narrower range with the lower value of the broader.

[0053] FIG. 5 is an electron micrograph map of the region of the coated substrate. FIGs.5A-E, respectively, show nickel, aluminum, chromium, silicon, and hafnium contents. In FIG. 5B it is seen that the coating has very little bulk aluminum with there being some small isolated high concentration areas. Chromium in FIG. 5C has largely coagulated in varioussublayers as alpha chromium but with noticeable partial thin sublayer on the surface. This thin layer is believed to increase resistance to Type 2 hot corrosion. It is also seen from these views that Al and Cr account for essentially all the FIG. 5 black spots evidencing the lack of nitride.

[0054] FIG. 6 shows an example article 20 as a blade. The blade comprises a metallic substrate 22 (FIG. 7). An example substrate comprises a unitary metallic casting (e.g., a single-crystal (SX) casting of a nickel-based superalloy) and defines the overall gross features of the blade. The substrate and blade thus include an airfoil 40 and an attachment feature 42 (e.g., a firtree root). The blade and substrate may further include a platform 44 between the airfoil and the firtree root.

[0055] The firtree root 42 extends from an inboard end (inner diameter (ID) face) 50 forming an inboard end of the blade to an outboard end at an underside 52 of the platform (underplatform (U / P) surface). A central radial plane is shown as 502. The airfoil 40 extends from an inboard end at a fillet 46 joining an outer surface (gaspath-facing surface) 54 of the platform to a tip 60. The airfoil extends from a leading edge 62 to a trailing edge 64 and has a pressure side surface 66 and a suction side surface 68.

[0056] The tip 60 has a primary radially-outward facing surface 70. The surface 70 may at least partially surrounds a tip squealer pocket (not shown) extending radially inward from the tip surface 70. In some embodiments, an abrasive coating may be applied along the surface 70 and the TBC system may be applied along the pressure and suction side surfaces of the airfoil, the fillet, and the gaspath-facing surface of the platform.

[0057] The root has a front (fore) face 56, an aft face 57, a first side 58, and a second side 59.

[0058] FIG. 7 shows the cooling passageway system 100 as including multiple trunks 102A, 102B, 102C extending from respective inlets 104A, 104B, 104C along the inner diameter face 50 of the root. Depending upon blade configuration, the trunks may branch in multiple spanwise cavities optionally with turns such that a cavity with tipward flow is termed an up pass and a cavity leg with rootward flow is termed a down pass. Various of the cavity legs may discharge to the tip / tip pocket. Additionally, there may be a tip flag leg 130 passing in a rearward to the trailing edge from one of the more forward trunks. The example trailing edge slot 140 is fed by the most rearward trunk.

[0059] In a specific example of a manufacturing process 800 (FIG. 8), in a casting process 802, the substrate precursor is cast 820 (e.g., investment cast with a sacrificial ceramic feedcore or optionally a ceramic feedcore with refractory metal core (RMC)components assembled therewith such as to cast a discharge slot or the like). The as-cast casting (e.g., single crystal (SX) or directionally solidified (DS)) may be deshelled 822 (e.g., mechanical breaking) and decored 824 (e.g., alkaline and / or acid leaching and / or a thermo- oxidative decoring).

[0060] One or more machining steps 826 (e.g., cutting, milling, abrasive machining, and the like) may de-gate and yield a final substrate surface (e.g., at least along significant portions of the airfoil lateral surface, and the platform OD and ID surfaces).

[0061] Such machining (if any) may yield near net or other surfaces along the root and tip (which will be subject to subsequent machining). Allowance for subsequent machining on these or other areas of the blade or analogous areas of an alternative component allows for coatings to temporarily be applied (when such coatings ultimately remain on other areas) but then be machined away. As is seen below, this may help yield an efficient process for providing the desired diverse combination of coatings across different regions of the ultimate article.

[0062] A coating process 804 uses an MCrAlY deposition 830 / 834 (e.g., cathodic arc) and a masked diffusion 848 of the root and U / P surface.

[0063] The MCrAlY such as a high-Cr MCrAlY of the type normally used for localized under-platform (U / P) protection (e.g., the ‘830 patent), is applied to substantially the entire exterior of the casting from the root (precursor) lobes / serrations (e.g., pre-grinding) upwards to the tip (optionally overspraying the tip depending on the tip coating variation). The key areas for coating in this step are: the platform (outer diameter (OD) gaspath surface and inner diameter (ID) under-platform (U / P) surface and edges); and the airfoil lateral surface (pressure side and suction side). In this example, the root and in some implementations, tip are less relevant and are left unmasked and subject to overspray or the like.

[0064] The example process has separate stages: 830 for coating the root and underplatform (U / P) surface; and 834 for coating the platform gaspath surface and airfoil. This process may cover all areas in 10 micrometers to 250 micrometers, more narrowly 12.0 micrometers to 100 micrometers, more narrowly 25 micrometers to 100 micrometers, more narrowly 25 micrometers to 50 micrometers of MCrAlY coating. As noted above, root coverage is optional and simply avoids the difficulty of locally masking the root while coating the U / P surface.

[0065] An example application process is cathodic arc or plasma spray or PVD (e.g.,EB-PVD). The cathodic arc or PVD may be in a vacuum chamber. An example process first applies the MCrAlY to the airfoil and platform gaspath surfaces by fixturing 828 the castingsuch as in a fixture that holds the root and may mask the root and U / P surface but not react therewith. After the MCrAlY application 830, the casting may be defixtured / refixtured 832 such as in a fixture holding the airfoil to expose the root and U / P surface. Then the MCrAlY is applied 834 to the U / P surface and root. In this fixture or an additional fixture, there may be a diffusion heat treat 836 to improve bonding of the MCrAlY. Example diffusion is at about 1080°C for about four hours (more broadly, 1000°C to 1200°C for one hour to ten hours). Following the diffusion 836, the airfoil and platform gaspath surface may be peened 838 (e.g., shot peened) to densify the MCrAlY coating thereon. This peening may omit the U / P surface and root.

[0066] After the peening, the root may be machined 840 to remove the MCrAlY and essentially finish the root shape. An example root machining 840 is an abrasive grinding such as abrasive wheel grinding.

[0067] Additionally, the example process forms 842 at least some outlet holes. Example outlet hole formation may include one or more of electro discharge machining (EDM) or laser drilling (e.g., laser percussion / ablation).

[0068] As discussed above, the Ni / AI / Cr / AEOs mask may then be applied. For the example dry powder process, this is a two-step process with initial application 844 followed by packing / compaction 846. For example, as discussed above, the blade may be placed with its root in a container and powder poured in place for initial introduction followed by packing such as with a hydraulic press acting on the blade or on an end effector driving the powder into contact with the blade. As noted above, an alternative may involve clamping sections of a sintered mask over the root and U / P surface.

[0069] A second diffusion heat treat 848 may follow and may have parameters similar to those of the earlier diffusion 836. As discussed above, the effect of this process is to cause reaction between the maskant and the MCrAlY on the U / P surface while having essentially no reaction between the maskant and the root material which is the machined superalloy surface. Similarly, during this process there is essentially no effect on the MCrAlY on the blade and platform gaspath surface.

[0070] After the diffusion 848, the mask is removed 850. In the example process, after such demasking 850, there may be a blast / hone process 852. This may be a cosmetic blast or vapor hone so as to remove any surface contamination.

[0071] One or more layers of ceramic coating may then be applied 854 (e.g., via physical vapor deposition or thermal spray) to the airfoil and platform gaspath surface. There may be further steps such as a subsequent tip coating 856 and further heat treat 858.

[0072] The effect of the masked diffusion 848 is to aluminum deplete the MCrAlY and slightly chromium enrich. As is discussed further below, this chromium enrichment may be less than that which would occur in pack chromization wherein additional halide or nitride activator (chromium halide slurry, particularly CrCh and / or CrFa slurry (e.g., with an organic solvent / carrier such as glycol ether)) is mixed in with the Ni / Cr / AI / Al Oa powder(s). Such activator is associated with undesirable nitride formation as noted above.

[0073] This change to the U / P MCrAlY may yield a difference in such ultimate MCrAlY composition versus that of the MCrAlY bondcoat under the airfoil and platform gaspath surface ceramic coating even though the two MCrAlYs originated as the same source material and had the same as-applied composition. For example, relative to the final bondcoat MCrAlY composition, the U / P MCrAlY may have one or more delta values such as in Table II and described thereafter.

[0074] In an example of further variations, the diffusion 848 may be integrated with an aluminization such as of the interior passageways and the airfoil and platform gaspath surfaces. In such situations, particular attention may be paid to mask engagement to prevent halide vapor from encountering the U / P surface.

[0075] The example boundary between the U / P MCrAlY coating and the gaspath MCrAlY plus ceramic may fall along a lateral perimeter of the platform and the transition need not be abrupt. Thus, the respective identified properties may exist only over portions of the U / P surface, root, and gaspath surface.

[0076] The coating and layer thickness, composition, and properties may be measured at a single location or across an area. Such area may be a total or continuous area (e.g., truly continuous or continuous ignoring cooling hole outlets). With an area, that may be at all locations or an average (mean, median, or mode). A representative area is at least 1.0 square centimeters of the U / P surface or gaspath surface.

[0077] Or, a representative area may be a fraction or percentage of a relevant area. For example, for a ceramic / MCrAlY coating on a blade, the relevant area for MCrAlY includes pressure and suction side surfaces of the airfoil and the gaspath-facing facing outer diameter surface of the platform; whereas the U / P MCrAlY may be on a fraction of the U / P surface. A representative such fraction or percentage is at least 10% or at least 50% or at least 90%.

[0078] The measurement may also be over a depthwise region of less than the entirety of a layer to discount boundary effects and contamination. An example depth wise limitation (optionally paired with an area limitation as discussed above) may be a continuous region of at least 25% of a layer thickness or at least 50%. For typical MCrAlY thicknesses aboveapplied either to the U / P MCrAlY or the bondcoat MCrAlY, example thickness may be a additionally or alternatively be characterized as a continuous region of at least 20 micrometers or at least 25 micrometers or at least 50 micrometers.

[0079] Measurement may be via a sufficiently statistically significant sampling such as using SEM / EDX.

[0080] The use of “first”, “second”, and the like in the following claims is for differentiation within the claim only and does not necessarily indicate relative or absolute importance or temporal order. Similarly, the identification in a claim of one element as “first” (or the like) does not preclude such “first” element from identifying an element that is referred to as “second” (or the like) in another claim or in the description.

[0081] Where a measure is given in English units followed by a parenthetical containing SI or other units, the parenthetical’s units are a conversion and should not imply a degree of precision not found in the English units.

[0082] One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, when applied to an existing baseline article or coating process or group of materials, details of such baseline may influence details of particular implementations. Accordingly, other embodiments are within the scope of the following claims.

Claims

CLAIMSWhat is claimed is:

1. A method (804) for coating a metallic blade (20) substrate (22), the blade substrate comprising: an airfoil section (40); a root section (42); and a platform section (44) between the root section and the airfoil section, the method comprising: applying (844, 846) a maskant to a portion of an underside (52) of the platform bearing anMCrAlY coating and a portion of the root lacking an MCrAlY coating; and heating (848), the heating causing aluminum depletion from the MCrAlY into the maskant.

2. The method of claim 1 further comprising: applying (834) the MCrAlY coating to the platform underside and the root; and before the applying of the maskant, machining (840) the applied MCrAlY from said portion of the root.

3. The method of claim 1 further comprising: applying an MCrAlY (830) coating to an airfoil of the casting.

4. The method of claim 3 further comprising: after the heating, applying a ceramic (854) coating to the airfoil.

5. The method of claim 1 wherein: the applying the MCrAlY coating to the platform underside and the root and the applying the MCrAlY coating to the airfoil are from the same source.

6. The method of claim 1 wherein: the applying of the maskant comprises applying a powder maskant.

7. The method of claim 1 wherein: the maskant comprises AI2O3, nickel, chromium, and additional aluminum.

8. The method of claim 1 wherein: the maskant comprises by weight percent 30 to 60 said AI2O3, 20 to 60 said nickel, 0.5 to 5.0 said chromium, and 0.5 to 8.0 said additional aluminum, optionally wherein a ratio of said chromium to said additional aluminum is at least 1:2.

9. The method of claim 1 wherein: the maskant comprises by weight percent 30.0 to 70.0 said AI2O3, 20.0 to 60.0 said nickel, 0.5 to 5.0 said chromium, and 0.5 to 8.0 said additional aluminum, optionally wherein a ratio of said chromium to said additional aluminum is at least 1:2 optionally with no more than 5.0 weight percent total other including impurities.

10. The method of claim 1 wherein: the maskant consists essentially of Ni, Cr, and Al in one or more powders and AI2O3 powder.

11. The method of claim 1 wherein the maskant comprises, by weight percent: at least 20 AI2O3,40 to 50 Ni;1.0 to 10.0 Cr;0.5 to 8.0 additional aluminum; and up to 5.0 other.

12. The method of claim 1 wherein: the maskant substantially lacks a halide activator.

13. The method of claim 1 wherein: the heating does not form nitride particles.

14. The method of claim 1 wherein: the aluminum depletion is by an overall increment of 2.0 to 7.0 weight percent; and a chromium depletion, if any, of the MCrAlY is by an overall increment of not more than 5.0 weight percent.

15. The method of any of claim 1 to claim 14 wherein:the substrate is a nickel-based single crystal substrate.

16. A blade (20) comprising: a metallic blade substrate (22) comprising: an airfoil section (40); a root section (42); and a platform section (44) between the root section and the airfoil section and having an outer diameter surface and an underside, wherein: the airfoil section has a coating comprising: a bondcoat; and a ceramic coating atop the bondcoat; the underside has a coating having, relative to the bondcoat, a difference in weight percent of:-4 to 2 Cr;-3 to 3 Co; and-7 to -2.5 Al.

17. The blade of claim 16 wherein: the underside coating has, relative to the bondcoat, a difference in weight percent of:-3.0 to 1.0 Cr;-2.0 to 2.0 Co; and-5.5 to -3.0 Al.

18. The blade of claim 17 wherein: the underside coating has, relative to the bondcoat, a difference in weight percent of:0.5 to 3.0 Ta greater;0.5 to 3.0 W greater; and0.10 to 1.0 Mo greater.

19. The blade of any of claim 16 to claim 18 wherein: the substrate is a nickel-based single crystal substrate.

20. A method (804) for coating a metallic substrate (22), the method comprising:applying (830) a first MCrAlY coating; machining (840) the substrate after application of the first MCrAlY coating to remove said first MCrAlY coating from a first region of the substrate; applying (844) a Ni, Cr, Al, and AI2O3 maskant to the first region and a second region where the first MCrAlY remains; and heating (848), the heating causing: aluminum depletion from the first MCrAlY in the second region without nitride formation.

21. The method of claim 20 wherein: said heating also heats a third region from where the first MCrAlY coating has not been removed and to which region the maskant was not applied.

22. The method of claim 21 wherein: said heating causes less, if any, aluminum depletion from the first MCrAlY in the third region than from the first MCrAlY in the first region.

23. The method (804) of claim 20 wherein: the metallic substrate is a metallic blade (20) substrate (22) comprising: an airfoil section (40); a root section (42); and a platform section (44) between the root section and the airfoil section and having an outer diameter surface and an underside; the first region is along sides of the root; and the second region is along the underside.

24. The method of any of claim 20 to claim 23 wherein: the substrate is a nickel-based single crystal substrate.

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