Patent Description:
Gas turbine engines, such as those that power modern commercial and military aircraft, generally include a compressor section to pressurize an airflow, a combustor section to burn a hydrocarbon fuel in the presence of the pressurized air, and a turbine section to extract energy from the resultant combustion gases.

Gas turbine engines have rows of circumferentially spaced airfoils mounted on respective rotor disks for rotation about an engine axis. Advanced configurations feature shroud-less hollow airfoils manufactured of lightweight materials. The airfoils are designed to high tolerances and typically include a multi-layer coating to accommodate significant operational requirements.

Airfoil coatings erode over time and may be removed and reapplied as part of a repair strategy. The multi-layer coatings can include a structural adhesive. A bond coat primer can also be used to facilitate attachment or enhance the properties of the structural adhesive to bond the underlying substrate to a subsequent layer. Some bond coat primers are 'toughened' by addition of an elastomer to the epoxy resin. These toughened bond coat primers can be resistant to removal by chemical or mechanical means. Although grinding or grit blasting may remove the bond coat primer, frequently the underlying substrate is also partially removed using these techniques, which can result in the component no longer being able to meet the high tolerance geometric requirements of a gas turbine engine. As such, the 'toughened' bond coat primers have conventionally been removed laboriously through hand abrasion techniques over several hours.

<CIT> discloses a method to strip and recoat erosion coatings that are applied to fan blades and structural guide vanes.

<CIT> discloses a composition and method for stripping polyvinyl butyral primed coating.

In accordance with a first aspect of the disclosure, a method of repairing a defectively manufactured multi-layer component of a gas turbine engine, having a substrate material, a coating, a structural adhesive, and a primer layer is provided in accordance with claim <NUM>.

In accordance with a second aspect of the disclosure, a method of repairing a damaged multi-layer component having a substrate material, an anodized layer, a primer layer, a structural adhesive, and a coating is provided in accordance with claim <NUM>.

Many components within a gas turbine engine require various multi-layer coatings in order to withstand the cyclic high pressures and temperatures or other stresses present in the gas turbine engine under load. As these coatings wear or are damaged, the coatings can be repaired by removal and reapplication of the coatings, extending the useful life of the component. However, some toughened epoxy primers can be resistant to removal by chemical or mechanical means. Herein is disclosed a method for repairing coatings, which include a toughened epoxy layer, using a hybrid technique to first swell the toughened epoxy primer using a suitable composition followed by mechanical abrasion to remove the toughened epoxy layer. The hybrid technique significantly reduces the time needed by a user to mechanically remove the toughened epoxy layer. For example, it can take a typical user several hours to remove the toughened epoxy layer using a hand abrasion technique. In contrast, when the toughened epoxy layer is swelled first, then the user can remove the toughened epoxy layer through hand abrasion techniques in less than an hour.

<FIG> is a cross-sectional view of a multi-layer component. <FIG> shows component <NUM> including substrate material <NUM>, anodized layer <NUM>, primer layer <NUM>, structural adhesive <NUM>, and coating <NUM>. Component <NUM> can be, for example, a compressor blade, turbofan blade, a turboprop propeller blade, tilt rotor prop, vane strut, or other component with a toughened epoxy layer.

Substrate material <NUM> of component <NUM> can be formed of any metal or alloy. For example, substrate material <NUM> can be selected from the group consisting of aluminum, aluminum alloys, titanium, titanium alloys, stainless steel, Hastelloy™ or other metal alloys, Inconel™ or other nickel alloys and alloys of nickel, chromium, and iron, or combinations thereof. Substrate material <NUM> can be solid throughout or a hollow structure. For example, a hollow structure can include an interior formed of a different material such as a composite material. The hollow structure can also include one or more cavities to reduce the overall weight of component <NUM> and/or to provide cooling holes or channels to component <NUM>.

Anodized layer <NUM> is formed on the surface of substrate material <NUM>. Anodized layer <NUM> is a protection layer of the base metal typically used to reduce oxidation of underlying substrate material <NUM> during component <NUM>'s useful working life. For example, when substrate material <NUM> is formed of aluminum, anodized layer <NUM> is an aluminum oxide layer and formed through an acid anodization process. The aluminum oxide layer can be from, for example, <NUM> inches (<NUM>) to <NUM> inches (<NUM>) thick and can have a microscopic roughness. The anodization process changes the crystal structure of the aluminum at the surface of the aluminum and reduces the amount of corrosion and further oxidation of underlying substrate material <NUM> compared to substrate material <NUM> formed of aluminum without anodized layer <NUM>. Substrate material <NUM> formed of metals or alloys other than aluminum can have anodized layer <NUM> formed of metals or alloys other than aluminum oxide.

Primer layer <NUM> is applied on top of anodized layer <NUM>. Alternatively, primer layer <NUM> can be applied directly onto substrate material <NUM>, which typically is applied after a mechanical abrasion of substrate material <NUM> to remove any surface contaminants. The mechanical abrasion can be, for example, grit blasting or hand abrasion. Primer layer <NUM> can increase the bond strength between anodized layer <NUM> and any subsequent layers. Primer layer <NUM> can have a thickness from, for example, <NUM> inches (<NUM>) to <NUM> inches (<NUM>). Primer layer <NUM> is a toughened epoxy primer. For example, in one embodiment primer layer <NUM> is formed of product number EC-3924B, commercially available from <NUM>™. Although not wanting to be limited by theory, it is believed that toughened epoxies have an elastomer component that makes them more resistant to chemical and mechanical removal means compared to epoxies without an elastomer component. Epoxies without an elastomer component can be brittle and easier to remove compared to toughened epoxies. It is also believed that the toughened epoxy may infiltrate into groves present on the surface of an anodized layer, making the removal of the toughened epoxy difficult without also removing substrate material.

Primer layer <NUM> can include a second primer layer to impart various characteristics. For example, the second primer layer can include active corrosion resistant moieties to protect underlying substrate material <NUM> from corrosion damage. One embodiment may include hexavalent chromium as the active corrosion resistant moiety. One embodiment may include a non-(hexavalent chromium) moiety as the active corrosion resistant moiety.

Structural adhesive <NUM> is formed of a composition selected from the group consisting of polyurethanes, polyimides, epoxy based materials, or combinations thereof that bond together substrate material <NUM> with coating <NUM>. Structural adhesive layer <NUM> can have a thickness from <NUM> inches (<NUM>) to <NUM> inches (<NUM>). Structural adhesive <NUM> together with primer layer <NUM> provides a strong bonding network between anodized layer <NUM> and coating <NUM>.

Coating <NUM> helps to further protect underlying substrate material <NUM> from corrosion and impact damage. For example, coating <NUM> can protect substrate material <NUM> from corrosive elements such as rain or from impact damage such as from birds, ice, or other objects. In one embodiment, coating <NUM> is a metallic sheath, which covers a leading edge of an airfoil. The metallic sheath can be formed of, for example, titanium, stainless steel, aluminum alloys, or nickel alloys. Coating <NUM> can also be, for example, an airfoil cover, wear pads, or seals. Coating <NUM> can be a thickness from <NUM> inches (<NUM>) to <NUM> inches (<NUM>).

The layers on substrate material <NUM> of component <NUM> work as a system to decrease corrosion and impact damage to substrate material <NUM> during the component's useful working life. Nonetheless, the layers can erode over time due to the high pressures and temperatures present within a gas turbine engine under load. The layers can also be damaged by an object striking the surface of component <NUM> under load. For example, during engine operation a bird, ice, or other object can enter the gas turbine engine and cause impact damage to component <NUM>. Alternatively, one or more layers of component <NUM> can be defectively manufactured and, as such, be removed and reapplied. For example, the primer layer may be applied too thinly or thickly or have voids, porosity, or bubbles, leading to an improper cure. In one embodiment, only the defectively manufactured primer layer is removed and subsequently reapplied. Alternatively, only the structural adhesive and the primer layer are removed and subsequently reapplied.

For further example, the structural adhesive may not reach a required temperature, pressure, or remain at the required temperature and pressure for the specified duration, leading to an improper cure. Additionally, the structural adhesive man create a bondline during cure or have porosity, resulting in inadequate bond strength in the final component. These manufacturing defects can be repaired before the component is put into service.

<FIG> are cross-sectional views of component <NUM> undergoing repair process <NUM>. <FIG> shows component <NUM> in need of repair at damaged site D and includes substrate material <NUM>, damaged anodized layer <NUM>, damaged primer layer <NUM>, damaged structural adhesive <NUM>, and damaged coating <NUM>. Although anodized layer <NUM>, primer layer <NUM>, structural adhesive <NUM>, and coating <NUM> are shown in <FIG> as all being damaged at site D, in other embodiments only one layer is damaged or defective. Component <NUM> can be, for example, a compressor blade, turbofan blade, a turboprop propeller blade, tilt rotor prop, vane strut, or other component with a toughened epoxy layer.

<FIG> shows component <NUM> with damaged site D including substrate material <NUM>, damaged anodized layer <NUM>, damaged primer layer <NUM>, and damaged structural adhesive <NUM>. Damaged coating <NUM> has been removed. In one embodiment, coating <NUM> is a metallic sheath on the leading edge of an airfoil. Usually, the metallic sheath is destroyed as the sheath is mechanically pried off of component <NUM>.

Alternatively, a polymer coating can be removed through mechanical means, such as mechanical or hand abrasion, grit blasting, grinding, or sanding or through chemical means, such as application of a chemical or paint stripper or other suitable stripping agent, which removes the polymer coating, but does not etch or otherwise damage underlying substrate material <NUM>.

<FIG> shows component <NUM> with damaged site D including substrate material <NUM> and damaged anodized layer <NUM>, and damaged primer layer <NUM>. Damaged structural adhesive <NUM> has been removed. Structural adhesive <NUM> can be removed through mechanical means such as mechanical or hand abrasion, grit blasting, grinding, or sanding or through chemical means such as application of a chemical or paint stripper or other suitable stripping agent, which removes the structural adhesive, but does not etch or otherwise damage underlying substrate material <NUM>. Alternatively, component <NUM> does not include structural adhesive <NUM> when a polymer such as polyurethane is used as coating <NUM>. Notably, primer layer <NUM> is resistant to removal by conventional mechanical or chemical means.

<FIG> shows component <NUM> with damaged site D including substrate material <NUM> and damaged anodized layer <NUM>. Damaged primer layer <NUM> has been removed. Primer layer <NUM> is a toughened epoxy layer. For example, grit blasting using a plastic media easily removes structural adhesive <NUM>, but leaves primer layer <NUM> intact. Although not wanting to be limited by theory, it is believed that primer layer <NUM> includes an elastomeric component making primer layer <NUM> flexible and able to absorb a greater amount of abrasive energy compared to a more brittle primer layer that does not include an elastomeric component.

A swelling solvent is applied to the surface of primer layer <NUM>, which swells as at least some of the swelling solvent is absorbed by primer layer <NUM>. Primer layer <NUM>, while in a swelled state, is much more susceptible to abrasive techniques. For example, the time required for primer layer <NUM> removal by hand abrasion is reduced by ten-fold when primer layer <NUM> is in a swelled state compared to when primer layer <NUM> is in a non-swelled state. The swelling solvent can be any solvent capable of swelling primer layer <NUM>. For example, a neutral peroxide paint remover available from PPG Aerospace, product code Eldorado PR-<NUM>, can be used to swell primer layer <NUM>. The swelling solvent can evaporate, returning primer layer <NUM> to a non-swelled state. The swelling solvent can be re-applied, which results in the re-swelling of primer layer <NUM>.

<FIG> shows component <NUM> with substrate material <NUM>. Damaged anodized layer <NUM> has been removed. Anodized layer <NUM> can be removed by chemical etching. For example, chromic-phosphoric acid etching (CPAE) can be used to remove anodized layer <NUM> without etching substrate material <NUM>.

<FIG> shows component <NUM> with substrate material <NUM> and new anodized layer <NUM>'. New anodized layer <NUM>' is formed by acid anodization. For example, the acid can be selected from the group consisting of chromic acid, phosphoric acid, boric sulfuric acid, or sulfuric acid. The removal and application of an anodized layer is specific for the embodiment when substrate material <NUM> is aluminum. When substrate material <NUM> is another metal or alloy, a different corrosion prevention layer can be removed and reapplied. Alternatively, the removal and reapplication step of anodized layer <NUM> can be omitted and primer layer <NUM> removed and reapplied directly onto anodized layer <NUM> or directly onto substrate material <NUM>.

<FIG> shows component <NUM> with substrate material <NUM>, new anodized layer <NUM>', and new primer layer <NUM>'. New primer layer <NUM>' is applied to the surface of new anodized layer <NUM>'. <FIG> shows component <NUM> with substrate material <NUM>, new anodized layer <NUM>', new primer layer <NUM>', and new structural adhesive <NUM>'. New structural adhesive <NUM>' is applied to the surface of new primer layer <NUM>'. <FIG> shows repaired component <NUM> with substrate material <NUM>, new anodized layer <NUM>', new primer layer <NUM>', new structural adhesive <NUM>', and new coating <NUM>'. New coating <NUM>' is applied to the surface of new structural adhesive <NUM>'. In one embodiment, new coating <NUM>' is a metallic sheath covering the leading edge of an airfoil. New coating <NUM>' can also be, for example, an airfoil cover, wear pads, or seals.

Components within a gas turbine engine wear under load, are damaged during operation, or can be defectively manufactured. Repair process <NUM> can extend the useful life of a worn, damaged, or defective component within a gas turbine engine. Repair process <NUM> reduces the time a user spends removing a toughened epoxy layer adhered to a metal component without significant loss of the underlying substrate metal. While some other treatments may effectively remove a toughened epoxy layer, they can also remove a significant amount of underlying substrate metal. The removal of significant amounts of underlying substrate metal can weaken the overall structural integrity of the component such that it can no longer withstand the cyclic high pressures and temperatures present under load in a gas turbine engine or the component may no longer meet the stringent geometric tolerances of the component in a gas turbine engine.

Claim 1:
A method of repairing a defectively manufactured multi-layer component (<NUM>) of a gas turbine engine that includes a substrate material (<NUM>), a coating (<NUM>), a structural adhesive (<NUM>), and a primer layer (<NUM>), the method comprising:
removing the coating (<NUM>) from the multi-layer component (<NUM>), wherein the coating (<NUM>) is formed of a metal or alloy;
removing the structural adhesive (<NUM>) from the multi-layer component (<NUM>) and exposing the primer layer (<NUM>);
applying, to the primer layer (<NUM>), a solvent that causes the primer layer (<NUM>) to swell to form a swelled primer layer (<NUM>);
abrading away the swelled primer layer (<NUM>) from the multi-layer component (<NUM>); and
rebuilding the multi-layer component (<NUM>) by forming a new structural adhesive (<NUM>') on a new primer layer (<NUM>') on the substrate material (<NUM>) and a new coating (<NUM>') on the new structural adhesive (<NUM>').