Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, such alloys alone are often susceptible to damage by oxidation and hot corrosion attack and may not retain adequate mechanical properties. For this reason, these components are often protected by an environmental and/or thermal-insulating coating, the latter of which is termed a thermal barrier coating (TBC) system. Ceramic materials, and particularly yttria-stabilized zirconia (YSZ), are widely used as a thermal barrier coating (TBC), or topcoat, of TBC systems used on gas turbine engine components. These particular materials are widely employed because they can be readily deposited by plasma spray, flame spray and vapor deposition techniques. A commonly used type of TBC is a coating based on zirconia stabilized with yttria, for example about 93 wt. % zirconia stabilized with about 7 wt. % yttria. This general type of TBC has been reported in such United States patents as U.S. Pat. No. 4,055,705, U.S. Pat. No. 4,328,285, and U.S. Pat. No. 5,236,745, which are incorporated herein by reference.
During routine operation of the engine and aircraft, the coated surfaces, particularly those in or near the flowpath (intake, compression, combustion and exhaust) of the gas turbine engine, are subjected to heat, pressure and other forces can cause the coating to suffer localized damage such as spallation due to thermal fatigue and stress, defects, impact damage and other mechanical damage. For these reasons, the coated surfaces must be routinely inspected and meticulously repaired to avoid further damage to the coated surfaces and the underlying substrate. Maintenance personnel must inspect all visible surfaces, often requiring the use of flashlights, mirrors, and other inspection tools to access remote areas such as the small cooling holes and exhaust ports in a gas turbine engine. In the case of aircraft turbine engines and large power generation turbines, removing the turbine from service for repairs results in significant costs in terms of labor and downtime. For these reasons, removing components having TBCs that have suffered only localized damage such as spallation is not economically desirable. As a result, components identified as having only localized coating damage are often analyzed to determine whether the damage has occurred in a high stress area, and a judgment is then made as to the risk of damage to the turbine due to the reduced thermal protection of the component that could lead to catastrophic failure of the component.
Once a localized damaged coating area is located by inspection, and the decision is made to effect a field repair in situ, maintenance personnel must clean and prime the damaged area. Optionally, the field personnel may also apply a repair coating composition to the damaged surface, such as the compositions described in commonly-owned U.S. Pat. No. 6,413,578, for example. Current tools for cleaning, priming and optionally applying a repair coating are inadequate. Known tools are often too large, too small, or otherwise insufficient to perform the cleaning, priming and repairs. For example, maintenance personnel often use spray bottles and other containers for dispensing cleaning solutions such as solvents and detergents, beakers containing primers and coating repair compositions, and multiple brushes for cleaning and applying cleaning solutions, primers and repair compositions. In addition to the cumbersome use of so many different items, this situation creates a safety hazard to the personnel as well as a mechanical hazard to subsequent operation of the engine. While maintenance protocols require an accounting of each item used in the engine area, the use of a large number of tools makes the accounting process difficult, and more prone to errors that can have catastrophic results.
For all these reasons, there exists a continuing need for maintenance tools that can be effectively and efficiently used in limited access areas of engine and flowpath areas to clean, prime, and optionally repair damaged coated surfaces. There is additionally a need for maintenance tools and cleaning methods that can be used to perform one or more of the tasks of cleaning, priming, and repairing of coated surfaces in situ.
Accordingly, it would be desirable if a cleaning, priming and repair method were available that could be performed on localized damaged areas of TBC on turbine hardware in field and in situ, without necessitating that the component be removed from the turbine, so that downtime and scrappage are minimized.