Patent Description:
Gas turbine engines (used in propulsion and power applications and broadly inclusive of turbojets, turboprops, turbofans, turboshafts, industrial gas turbines, and the like) include abrasive tip coatings on the blades of their compressor and turbine sections. The abrasive tip coating may interface with an abradable coating on the inner diameter (ID) surface of a blade outer air seal.

An exemplary compressor blade has a titanium alloy or nickel-based superalloy substrate. The abrasive coating includes a base/bond layer formed as an electrolytic nickel strike (via Wood's nickel solution) atop the substrate. Exemplary thickness is <NUM> micrometers to <NUM> micrometers. After the strike, abrasive grit is tacked to the strike layer utilizing a sulfamate nickel plating chemistry. Exemplary abrasive grit material may include cubic boron nitride, silicon carbide, aluminum oxide, titanium boride, or other superabrasives. Exemplary grit size is between ASTM standard mesh <NUM> and <NUM>. Exemplary tack layer matrix thickness is <NUM> micrometers to <NUM> micrometers.

After the tack, the remainder of the nickel matrix is applied to complete the coating utilizing a sulfamate nickel plating chemistry. Exemplary matrix thickness (including tack layer) is <NUM> micrometers to <NUM> micrometers. Exemplary abrasive grit are no more than <NUM>% encapsulated.

The completed coating is baked in an air, argon, or vacuum atmosphere (e.g., at <NUM>°F to <NUM>°F (<NUM> to <NUM>)) to stress relieve the plated deposited and to abate hydrogen embrittlement.

In examining production blades, we have observed property alterations associated with cobalt (e.g., in the vicinity of <NUM> to <NUM> weight percent) in the nickel strike base layers.

In engineering applications, Wood's nickel plating solutions are typically only used to deposit a "strike" layer on top of auto-passivizing substrates such as stainless steels or nickel superalloys. The low pH of the Wood's nickel solution, coupled with the vigorous off-gassing of the process, effectively removes the naturally occurring oxide scales on the substrate and allows for a thin layer of pure nickel to be deposited. This thin layer of nickel is well adhered to the substrate and forms a suitable base to which subsequent electrochemically deposited layers can strongly adhere. Because of its ability to strongly adhere to both passivizing substrates and subsequent coating layers, nickel strike plating layers are sometimes referred to as a bond coat. The deposit itself is typically characterized by being a very thin, pure nickel layer (e.g., <NUM> to <NUM> micrometers) directly on top of the substrate and typically has high internal tensile stresses. Additionally, Wood's nickel deposits may contain nano-pore stringers which align with the columnar-grain microstructure. An exemplary Wood's nickel plating bath has <NUM>/L nickel chloride and <NUM>/L hydrochloric acid.

Sulfamate nickel plating is the primary process used in engineering applications due to its inherently low internal stress (typically <NUM> to <NUM> MPa), high deposition rates, and high cathode efficiency. The deposit is characterized by being a pure nickel deposit with fine grain, columnar structure. Sulfamate nickel may be plated over <NUM> micrometers in thickness. An exemplary sulfamate nickel plating bath has <NUM> to <NUM>/L nickel sulfamate, <NUM> to <NUM>/L nickel chloride, and <NUM> to <NUM>/L boric acid.

Outside the field of plated blade tip coatings, Watts nickel is the most commonly used type of nickel electroplating, due to its application for decorative purposes and for occasional engineering purposes. The Watts nickel bath can be modified in several different ways depending on the desired properties (e.g., bright nickel plate), but is typically used without additives for engineering applications. The Watts nickel plating deposit is characterized by being a pure nickel deposit with fine grain, columnar structure and relatively high internal tensile stress (<NUM> to <NUM> MPa). Watts nickel may be plated over <NUM> micrometers in thickness. An exemplary Watts nickel plating bath has <NUM> to <NUM>/L, nickel sulfate, <NUM> to <NUM>/L nickel chloride, and <NUM> to <NUM>/L boric acid. Document <CIT> discloses a blade having an airfoil having a tip, the blade comprising:a metallic substrate; and a coating system atop the substrate at the tip and comprising:a first strike layer of nickel, an abrasive layer, comprising: a matrix and an abrasive at least partially embedded in the matrix; and a second layer of nickel between the first layer and the matrix.

According to an aspect of the invention, there is provided a blade as claimed in claim <NUM>.

Optionally, the second layer has higher fracture toughness than the at least one of the first layer and the matrix.

Optionally, the second layer is more ductile than the at least one of the first layer and the matrix.

Optionally, the second layer is tougher than the matrix.

Optionally, the second layer is tougher than the first layer.

Optionally, the first layer is <NUM> micrometers to <NUM> micrometers.

Optionally, the second layer is <NUM> micrometers to <NUM> micrometers.

Optionally, the second layer comprises, by weight, at least <NUM>% nickel and <NUM>% to <NUM>% cobalt.

Optionally, the second layer comprises, by weight, no more than <NUM>% elements other than said nickel and cobalt.

Optionally, the abrasive is selected from the group consisting of: cubic boron nitride; silicon carbide; aluminum oxide; titanium boride; and combinations thereof. The matrix is at least <NUM>% nickel by weight.

According to another aspect of the present invention, there is provided a method for manufacturing the blade as described in any of the above embodiments, and claimed in claim <NUM>.

According to another aspect of the present invention, which is not claimed independently, a method for manufacturing the blade described in any of the above embodiments comprises: applying the first layer by Wood's nickel strike; applying the second layer by sulfamate nickel plating; applying the abrasive with a tack portion of the matrix; and applying a further portion of the matrix without further abrasive.

Optionally, the applying the abrasive with the tack portion of the matrix is by sulfamate nickel plating and the applying the further portion of the matrix without further abrasive is by sulfamate nickel plating.

Optionally, the second layer is tougher than the tack portion.

According to another aspect of the present invention, which is not claimed independently, a blade has an airfoil having a tip. The blade also has a metallic substrate and a coating system atop the substrate at the tip and having a nickel-based first layer, an abrasive layer having a matrix and an abrasive at least partially embedded in the matrix, and a second layer between the first layer and the matrix. The second layer has by weight at least <NUM>% nickel and <NUM>% to <NUM>% cobalt.

Optionally, the first layer has, by weight, either less than <NUM> weight percent cobalt or at least <NUM> weight percent more cobalt than the second layer.

According to another aspect of the present invention, which is not claimed independently, a method for applying an abrasive system to a blade tip comprises: applying a strike layer; applying an interlayer after the applying of the strike layer; and applying an abrasive layer having a matrix and an abrasive at least partially embedded in the matrix. The interlayer is tougher or more ductile than at least one of the strike layer and the abrasive layer matrix.

Optionally: the blade has a forged or cast metallic substrate; the strike layer is applied directly to the substrate; the interlayer is applied directly to the strike layer.

According to another aspect of the present invention, which is not claimed independently, a method for applying an abrasive system to a blade tip comprises: applying a strike layer; applying an interlayer after the applying of the strike layer; and applying an abrasive layer having a matrix and an abrasive at least partially embedded in the matrix. The applying of the strike layer is a Wood's nickel strike. The applying of the interlayer is selected from the group consisting of: a nickel plating via a different process than the applying of the strike layer; a nickel alloy plating comprising by weight nickel and <NUM>% to <NUM>% cobalt; electroless plating of nickel/phosphorous-graphene composite; electrodeposition of copper or copper alloy; and electrodeposition of an aluminum-manganese alloy.

Optionally, the applying of the interlayer is selected from the group consisting of: Watts nickel plating; a nickel alloy plating consisting of by weight nickel, <NUM>% to <NUM>% cobalt, and no more than <NUM>% by weight elements other than said nickel and cobalt; electroless plating of nickel/phosphorous-graphene composite having <NUM> to <NUM> volume percent phosphorous and <NUM> to <NUM> volume percent graphene; electrodeposition of at least <NUM> weight percent pure copper; and electrodeposition of Al-<NUM>.

Optionally: the blade has a forged or cast metallic substrate; the strike layer is applied directly to the substrate; and the interlayer is applied directly to the strike layer.

According to another aspect of the present invention, which is not claimed independently, a blade has an airfoil having a tip. The blade comprises a nickel alloy substrate and a coating system atop the substrate at the tip. The coating system has: a first layer of at least <NUM>% weight nickel with less than <NUM>% cobalt; a second layer comprising by weight nickel and <NUM>% to <NUM>% cobalt; and an abrasive layer, comprising a matrix and an abrasive at least partially embedded in the matrix.

Examination of cobalt contamination in the nickel strike on production blades has led to the conclusion that cobalt alloyant added to an intermediate layer may improve fatigue performance by reducing crack propagation from the abrasive layer into the blade substrate. As is discussed below, the interlayer may be tougher than the matrix, than the strike, or both.

<FIG> shows a blade <NUM>. The blade may be used in a compressor or turbine section of a gas turbine engine (e.g., a compressor blade shown). The blade has an airfoil <NUM> extending from an inboard (inner diameter (ID)) end <NUM> to an outboard (outer diameter (OD)) end <NUM>. In the exemplary blade, the outboard end forms a free tip which may include features such as a squealer pocket, cooling outlets, and the like. Alternative blades may include shrouded tips. The airfoil includes respective pressure <NUM> and suction <NUM> sides extending between a leading edge <NUM> and a trailing edge <NUM>. Yet alternative blades may be on integrally bladed rotors (IBR).

The exemplary inboard end <NUM> merges at a fillet <NUM> with the outboard end of an attachment feature <NUM>. The exemplary attachment feature is dovetail or firtree root having an inboard end <NUM>, an outboard end <NUM> at the fillet <NUM>, a forward end <NUM>, an aft end <NUM>, and lateral faces (sides) <NUM>, <NUM>. The lateral sides have the parallel convolution form providing the dovetail or firtree to be received in a complementary disk slot.

In alternative blades (not shown), at the inboard end <NUM> of the airfoil, the blade includes a platform having an inboard face (underside), an outboard face (gaspath face), a leading/forward end, a trailing/aft end, and lateral/circumferential ends. The attachment feature <NUM> depends from the underside of the platform.

An optional internal cooling passageway system (not shown) includes one or more outlets along the root inboard end with passageways extending to one or more inlets along the airfoil (typically including trailing edge outlets, leading edge outlets, surface outlets along the pressure and/or suction side, and tip outlets).

The exemplary blade is formed having a substrate <NUM> having a surface <NUM>. In an exemplary embodiment, the substrate is a nickel-based superalloy (e.g., a cast single-crystal alloy for a separate blade or a powder metallurgical (PM) alloy or a cast and forged alloy for a separate blade or IBR). <FIG> shows an abrasive coating system <NUM> atop the surface at the tip. Additional coatings (not shown - e.g., thermal barrier coating systems and/or environmental barrier coating systems) may be atop the surface <NUM> elsewhere on the substrate. The abrasive coating system <NUM> is formed as a modification of a baseline (e.g., prior art discussed above) abrasive coating system by adding an interlayer discussed below. The coating system <NUM> comprises; a strike layer <NUM> (e.g., electrolytic Ni) atop the substrate; the added interlayer <NUM> atop the strike layer; and an abrasive coating <NUM> atop the interlayer. The abrasive coating comprises a particulate abrasive <NUM> embedded in a matrix <NUM> (e.g., electrolytic Ni). The matrix has an exposed outer surface <NUM>. The coating system may be implemented as a modification of a baseline coating system otherwise similar, but lacking the interlayer <NUM>. As such, the interlayer may add thickness between substrate and abrasive coating <NUM>.

The interlayer <NUM> has greater fracture toughness (e.g., plane strain fracture toughness (KIc)) than one to all of the strike layer and matrix layer(s) discussed below. KIc may be measured by ASTM E399-<NUM> ("Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness KIc of Metallic Materials", ASTM International, West Conshohocken, Pennsylvania). The fracture toughness provides crack arresting capability for cracks starting at the abrasive-matrix interface and propagating down into the substrate.

Additionally, the fracture toughness of the interlayer <NUM> prevents cracks originating underneath the coating (in the substrate, substrate-coating interface, or strike layer) from propagating into the coating, which could result in coating failure, spallation, or loss of abrasive grits. For example, the inclusion of the abrasive grits in the metal matrix results in high stress concentrations zones at the grit-matrix interface. Additionally, the typically sharp-cornered geometry of abrasive particles creates concentrated stress regions. The elevated stress may result in the initiation of a crack. The crack can propagate down through the coating and into the substrate material, causing failure of the entire blade. By being tougher than the matrix <NUM>, the interlayer <NUM> prevents or delays the propagation of these cracks into the substrate material. By being tougher than the strike <NUM>, the interlayer <NUM> prevents cracks originating in the strike layer, at the coating-substrate interface, or in the substrate itself from propagating into the coating, which could result in coating failure, spallation, or loss of abrasive grits.

Fracture toughness of thin films can be very difficult to measure. Thus most literature studies do not report toughness, or they use non-standard methods for measurement. Elongation to failure (e.g., ductility) is perhaps the best property to use for estimating fracture toughness in lieu of fracture toughness measurements. Ductility of electrodeposited or autocatalytically deposited metal coatings may be measured by using the ASTM B489 bend test method (<NPL>).

A first group of examples of the interlayer <NUM> is nickel-cobalt alloys (e.g., <NUM>% to <NUM>% cobalt, by weight, more particularly <NUM>% to <NUM>%, or about <NUM>% ). Exemplary alloys are balance nickel plus impurities (e.g., other elements at levels of <NUM> weight percent or less total, more broadly <NUM> or <NUM> weight percent total or less and <NUM> or <NUM> weight percent individually or less). Aluminum, copper, lead, zinc, iron, and chromium are common impurities in plating. Potential intentional alloying elements are manganese, iron, copper, tin, zinc, boron, phosphorus and/or tungsten. These may be included in the aforementioned <NUM> or <NUM> weight percent total and <NUM> or <NUM> weight percent individually. Practical difficulties in consistency of intentional ternary plating, quaternary plating, or beyond tends to limit the likelihood of significant amounts of intentional alloyants, particularly beyond ternary systems.

If intentional alloyants are included, manganese in amounts up to <NUM> weight percent may be included to mitigate the embrittling effects of sulfur and to increase the strength of the deposit. Iron in amounts up to <NUM> weight percent may be included to reduce cost associated with nickel metal or to modify the ductility and hardness of the interlayer. Copper in amounts up to <NUM> weight percent may be added to modify the interlayer corrosion resistance, magnetic, and thermophysical properties of the interlayer. Tin in amounts up to <NUM> weight percent may be added to modify the interlayer corrosion resistance and inhibit the penetration of oxidation. Zinc in amounts up to <NUM> weight percent may be added to modify the interlayer corrosion resistance properties. Boron in amounts up to <NUM> weight percent may be added to modify the interlayer hardness and wear resistance. Phosphorous in amounts up to <NUM> weight percent may be added to modify the interlay corrosion resistance, hardness, and wear resistance. Tungsten in amounts up to <NUM> weight percent may be added to modify the interlayer hardness and wear resistance.

An exemplary process starts with a forged powder metallurgical (PM) or cast substrate (Ni-based superalloy or Ti-alloy (e.g., Ti6Al4V or other TiAl alloy) post-machining. Depending on implementation, a thermal barrier coating system and/or environmental barrier coating system may be on relevant areas of the substrate surface.

A preparation step may include the technician(s) preparing the substrate tip such as by solvent cleaning and/or roughening (e.g., using a grit blasting process utilizing alumina, garnet, or other media at pressures up to <NUM> KPa (e.g., <NUM> KPa to <NUM> KPa).

The preparing may include a post-roughening cleaning of the substrate tip (e.g., using an alkaline or solvent based solution such that light oils, greases and fingerprints are removed). In one example, the airfoil is cleaned in a light duty alkaline solution at no greater than <NUM> for less than <NUM> minutes (e.g., <NUM> to <NUM> for <NUM> minutes to <NUM> minutes).

The preparing may further include a subsequent etching (e.g., using electroless or electrolytic processes in an acid based solution). The acid solutions may consist of mixtures of hydrochloric acid and water; sulfuric acid, hydrofluoric acid, and water; or nitric acid, hydrofluoric acid, and water. The solutions may be used in conjunction with an anodic current (e.g., <NUM> to <NUM> A/dm<NUM> (Amperes per square decimeter)). Solution may be at room temperature or heated to up to <NUM>. In one embodiment, the airfoil is etched in a muriatic acid solution at room temperature and an anodic current of <NUM> A/dm<NUM> for <NUM> to <NUM> minutes.

An exemplary step for depositing the electrolytic strike layer <NUM> is directly onto the substrate from a Wood's nickel solution. The strike layer may also be called the "bond layer" as its purpose is to form a strong bond to both the substrate material and the subsequent coating layers. In an exemplary embodiment the strike layer is deposited from the Wood's nickel solution at <NUM> to <NUM>° C (e.g., typical room//factory temperature) and at a cathodic current density up to <NUM> A/dm<NUM> (e.g., about <NUM> A/dm<NUM>, more broadly <NUM> A/dm<NUM> to <NUM> A/dm<NUM>) for <NUM> to <NUM> minutes. Exemplary deposition of the strike layer <NUM> is to a thickness TS of <NUM> micrometer to <NUM> micrometers, more particularly, <NUM> micrometers to <NUM> micrometers or about <NUM> micrometers. The exemplary as-deposited strike layer consists essentially of pure nickel (no alloying elements). Impurity levels may be at levels of <NUM> weight percent or less total, more broadly <NUM> or <NUM> weight percent total or less and <NUM> or <NUM> weight percent each element individually or less. Aluminum, copper, lead, zinc, iron, and chromium are common impurities in plating. Also, intentional alloyants are possible. For example, high-Co Ni-Co systems are known (e.g., the <NUM> to <NUM> weight percent Co noted above).

An exemplary step for depositing the high fracture toughness interlayer <NUM> is on top of the strike layer (e.g., directly atop) such as via an electrolytic or autocatalytic chemical process. An exemplary process uses standard off-the-shelf nickel sulfamate plating solution with the addition of cobalt metal and/or cobalt sulfamate liquid such that the concentration of cobalt in the plating bath is <NUM> to <NUM> parts per million, more specifically, <NUM> to <NUM> parts per million, or about <NUM> parts per million. In the exemplary process, the nickel-cobalt plating solution is heated to <NUM>° F (<NUM>° C), and a cathodic current density of <NUM> amps per square foot (<NUM> amps per square decimeter) is applied for <NUM> to <NUM> minutes, or about <NUM> minutes. Exemplary deposition of the interlayer <NUM> is to a thickness TI of <NUM> micrometers to <NUM> micrometers, more particularly, <NUM> micrometers to <NUM> micrometers, or <NUM> to <NUM> micrometers, or about <NUM> microns. Exemplary interlayer <NUM> fracture toughness is at least <NUM> MPa/(m<NUM>/<NUM>), more particularly, <NUM> MPa/(m<NUM>/<NUM>) to <NUM> MPa/(m<NUM>/<NUM>) or <NUM> MPa/(m<NUM>/<NUM>) to <NUM> MPa/(m<NUM>/<NUM>), or about <NUM> MPa/(m<NUM>/<NUM>). This may be contrasted with exemplary fracture toughness <NUM> MPa/(m<NUM>/<NUM>) for the matrix. Thus, exemplary differences may be or at least <NUM> MPa/(m<NUM>/<NUM>) or <NUM> MPa/(m<NUM>/<NUM>) to <NUM> MPa/(m<NUM>/<NUM>). Strike layer toughness is expensive to quantitatively measure due to low thickness.

Ductility of the interlayer <NUM> may be <NUM>% to <NUM>%. Ductility of the matrix <NUM> may be <NUM>% to <NUM>%. It is possible that the interlayer is slightly less ductile than the matrix but higher strength so as to provide higher fracture toughness and resistance to fatigue cracking. As with toughness, ductility is also difficult to measure for the strike.

In an exemplary tack step, abrasive grits are "tacked" directly on top of the interlayer <NUM> utilizing a sulfamate nickel plating chemistry. The abrasive grit material may be cubic boron nitride, silicon carbide, aluminum oxide, titanium boride, or other superabrasive. Abrasive grit may be between ASTM standard mesh <NUM> and <NUM> in size. In the exemplary embodiment the abrasive grit are cubic boron nitride with an ASTM standard mesh size between <NUM> and <NUM>. The matrix 118A of the tack layer has sufficient thickness such that abrasive grit are secured to the surface. Exemplary tack matrix 118A thickness TT is <NUM> micrometers to <NUM> micrometers, more broadly, <NUM> micrometers to <NUM> micrometers. Exemplary sulfamate nickel plating solution is heated to between <NUM> to <NUM>. Exemplary cathodic current density is <NUM> A/dm<NUM> to <NUM> A/dm<NUM>. Exemplary duration is for <NUM> minutes to <NUM> minutes. The exemplary layer consists of the matrix and abrasive. Exemplary as applied matrix 118A consists essentially of pure nickel (no alloying elements). Impurity levels may be at levels of <NUM> weight percent or less total, more broadly <NUM> or <NUM> weight percent total or less and <NUM> or <NUM> weight percent each element individually or less. Aluminum, copper, lead, zinc, iron, and chromium are common impurities in plating.

An exemplary step for applying a remainder 118B of the matrix (overlayer) is also via a sulfamate nickel plating chemistry. The chemistry and parameters may be generally similar to those used with the tack. Exemplary thickness TO is greater than that of the matrix layer 118A (e.g., <NUM> micrometers to <NUM> micrometers, more broadly, <NUM> micrometers to <NUM> micrometers or <NUM>% to <NUM>%). Exemplary sulfamate nickel plating solution is heated to between <NUM> to <NUM>. Exemplary cathodic current density is less than the current density used for the matrix layer 118B because the presence of the abrasive particles reduces the conductive surface area of the airfoil tip. In this way, the effective current density is relatively constant due the smaller surface area. Exemplary cathodic current density is <NUM> A/dm<NUM> to <NUM> A/dm<NUM> based on the original airfoil tip surface area. Exemplary duration is for <NUM> minutes to <NUM> minutes. Exemplary as applied matrix 118B consists essentially of pure nickel (no alloying elements). Impurity levels may be at levels of <NUM> weight percent or less total, more broadly <NUM> or <NUM> weight percent total or less and <NUM> or <NUM> weight percent each element individually or less. Aluminum, copper, lead, zinc, iron, and chromium are common impurities in plating. The exemplary application of the matrix 118B leaves the abrasive grit no more than <NUM>% encapsulated. Exemplary overall matrix thickness TM is <NUM> micrometers to <NUM> micrometers, more specifically <NUM> micrometers to <NUM> micrometers, or about <NUM> micrometers. The matrix is more likely to have alloyants than is the strike layer. Exemplary matrix alloyants are manganese, iron, copper, tin, zinc, boron, phosphorus and/or tungsten. These may be included in the aforementioned impurity levels of <NUM> or <NUM> weight percent total and <NUM> or <NUM> weight percent individually. Practical difficulties in consistency of intentional ternary plating, quaternary plating, or beyond tends to limit the likelihood of significant amounts of intentional alloyants, particularly beyond ternary systems.

If intentional alloyants are included in the 118B layer of the matrix (overlayer), manganese in amounts up to <NUM> weight percent may be included to mitigate the embrittling effects of sulfur and to increase the strength of the deposit. Iron in amounts up to <NUM> weight percent may be included to reduce cost associated with nickel metal or to modify the ductility and hardness of the overlayer. Copper in amounts up to <NUM> weight percent may be added to modify the corrosion resistance, magnetic, and thermophysical properties of the overlayer. Tin in amounts up to <NUM> weight percent may be added to modify the overlayer corrosion resistance and inhibit the penetration of oxidation. Zinc in amounts up to <NUM> weight percent may be added to modify the overlayer corrosion resistance properties. Boron in amounts up to <NUM> weight percent may be added to modify the overlayer hardness and wear resistance. Phosphorous in amounts up to <NUM> weight percent may be added to modify the overlayer corrosion resistance, hardness, and wear resistance. Tungsten in amounts up to <NUM> weight percent may be added to modify the overlayer hardness and wear resistance. In some uses, electroplated wear coatings contain up to <NUM> wt% tungsten. However that much tungsten would embrittle the deposit, and hence is not a suitable level for application to the interlayer <NUM>.

An optional stress relief step may involve heating. Exemplary heating is in air or under vacuum. Exemplary heating is at <NUM>°F to <NUM>°F (<NUM> to <NUM>) for <NUM> minutes to <NUM> minutes to stress relieve the plated deposited and to abate hydrogen embrittlement.

An alternative interlayer <NUM> composition, not according to the invention, is un-alloyed nickel deposited out of the Watts nickel plating solution which produces deposits with considerably high elongation to failure (analogous to fracture toughness) compared to the preceding (e.g., strike <NUM>) and subsequent (e.g., matrix) layers. An exemplary standard off-the-shelf Watt's plating solution is used (e.g., <NUM>/L nickel sulfate, <NUM>/L nickel chloride). Exemplary nominal plating conditions: pH <NUM>, temperature <NUM> F (<NUM> C), current density <NUM> amp/sq-ft (<NUM> amps per square decimeter). Thickness TI may be the same as initially noted above.

Potential advantages relative to the Ni-Co alternatives are the simplicity of operating the Watt's nickel plating solution, as there no need to balance the precise concentrations of dissolved nickel and cobalt metal in solution, as well as the lower cost of the Watt's nickel plating solution compared to nickel sulfamate and cobalt sulfamate solutions.

Further alternative interlayer <NUM> materials, not according to the invention, are electroless nickel/phosphorous-graphene composites. An exemplary composite is nickel-based with <NUM> volume % of phosphorus (more broadly <NUM> to <NUM> volume %) and <NUM> volume % of graphene nano-particles (more broadly <NUM> to <NUM> volume %). This may be applied by autocatalytic deposition methods from a nickel and phosphorus containing solution with <NUM> milligrams per liter of suspending graphene nano-particles. Potential advantages relative to the alternatives are the amorphous microstructure of electroless nickel-phosphorus-graphene composites which can provide enhanced corrosion protection to the substrate. Additionally, the electroless deposition method creates a deposit that has high uniformity, ensuring a consistent interlayer thickness regardless of the position of the airfoil in the plating tank. Thickness TI may be the same as initially noted above.

Further alternative interlayer <NUM> materials, not according to the invention, are electrodeposited aluminum-manganese alloys. An exemplary alloy is Al-<NUM>. 8Mn (<NUM> atomic percent Mn - see<NPL>). A broader compositional range is <NUM> to <NUM> atomic percent Mn, balance Al plus impurities. Exemplary application is by electrodeposition from a chloroaluminate ionic liquid solution containing <NUM>-ethyl-<NUM>-methylimidazolium chloride and anhydrous manganese chloride. Potential advantages relative to the alternatives are the significantly lower density of the Al-Mn alloy compared to the nickel-based interlayer options. This is relevant for applications where weight savings are critical. Impurity levels may be at levels of <NUM> atomic percent or less total, more broadly <NUM> or <NUM> atomic percent total or less and <NUM> or <NUM> atomic percent each element individually or less.

Further alternative interlayer <NUM> materials, according to the invention, are electrodeposited copper. For example, the interlayer <NUM> may be deposited from an acid or alkaline copper plating solution. The deposition may leave the layer <NUM> as essentially pure copper (e.g., at least <NUM>% or at least <NUM>% by weight). Thickness TI may be the same as initially noted above. Alternatively, the electrodeposited copper may be intentionally alloyed with up to <NUM> weight percent zinc or up to <NUM> weight percent tin (e.g., reflecting use of known bronze or brass plating techniques).

The identification of particular layers does not preclude interface diffusion, particularly in-service. Accordingly, interface zones of a layer of a given composition may depart from the nominal composition specification or range while leaving the core within the nominal specification or range.

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.

Claim 1:
A blade (<NUM>) having an airfoil (<NUM>) having a tip (<NUM>), the blade (<NUM>) comprising:
a metallic substrate (<NUM>); and
a coating system atop the substrate (<NUM>) at the tip (<NUM>) and comprising:
a first layer (<NUM>) of at least <NUM>% weight nickel;
an abrasive layer (<NUM>), comprising:
a matrix (<NUM>); and
an abrasive (<NUM>) at least partially embedded in the matrix (<NUM>); and
a second layer (<NUM>) between the first layer (<NUM>) and the matrix (<NUM>) and with higher plane strain fracture toughness or an increased elongation to failure than at least one of the first layer (<NUM>) and the matrix (<NUM>),
wherein the second layer (<NUM>) comprises, by weight, at least <NUM>% nickel and <NUM>% to <NUM>% cobalt.