Stock material or miscellaneous articles – All metal or with adjacent metals – Having aperture or cut
Reexamination Certificate
1999-10-05
2001-01-09
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Having aperture or cut
C428S652000
Reexamination Certificate
active
06171711
ABSTRACT:
TECHNICAL FIELD
This invention pertains to methods for preparing a protectively coated, apertured article to be recoated with a nonoriginal layer of protective coating. In particular, the invention is a method for preconditioning a transpiration cooled gas turbine engine component, such as a turbine blade or vane, so that the flow capacity of the transpiration cooling passages is not degraded by subsequent recoating of the component.
BACKGROUND OF THE INVENTION
The blades used in the turbine section of a gas turbine engine are susceptible to oxidation, corrosion and thermally induced damage from hot combustion gases flowing through the engine's gaspath. Turbine blades are therefore made from a temperature tolerant, corrosion resistant substrate material to which various protective coatings are applied. In addition, turbine blades usually include numerous transpiration cooling passages, each of which extends from an internal cavity to a gaspath exposed surface of the blade. Each passage has a throat in the vicinity of the cavity and a mouth that extends outwardly from the throat to the exposed surface. During engine operation a cooling medium is supplied to the internal cavity, and at least a portion of the medium flows through the cooling passages to transpiration cool the blade. As the medium discharges from the passages, it disperses over the blade surface to form a cooling film that further shields the blade from thermal distress.
Because turbine blades are expensive, a variety of refurbishment techniques have been developed to restore deteriorated or damaged blades to serviceable condition. The specific details of the various refurbishment techniques depend on the nature and extent of blade damage and deterioration. However certain procedures are almost invariably carried out during refurbishment. For example, it is customary to relieve any residual blade stresses by heating the blades to an elevated temperature for a predetermined period of time. In addition, existing protective coatings are usually removed from the blades, and nonoriginal replacement coatings are applied prior to returning the blades to service.
When a nonoriginal coating is applied to a blade having transpiration cooling passages, excess coating can accumulate in the mouth of each passage. This phenomenon is known as “coatdown” and restricts the flow capacity of the affected passages. Unless coatdown is prevented, or its flow restricting effect is reversed, the effectiveness of the transpiration cooling and film cooling will be diminished, and the blade's useful life will be reduced.
One way to prevent coatdown is to coat the blade by vapor deposition, a coating process that causes little or no coatdown. However the equipment for applying coatings by vapor deposition is expensive, and therefore it is economically unattractive to use the vapor deposition equipment to apply coatings that can be applied by more cost effective means.
One way to reverse the effects of coatdown is to erode the excess coating by propelling a high velocity, precisely focused stream of abrasive particles into the mouth of each affected passage. However the erosive treatment can be inaccurate and nonrepeatable. Therefore the effectiveness of the treatment must be assessed by verifying that a gauge pin, representative of the minimum acceptable passage dimension, is insertable into each passage. Although the erosive process is effective in restoring the flow capacity of a passage, it is also tedious and time consuming since a typical turbine blade has nearly two hundred passages, each of which must be treated and gauged individually. Moreover, the gauge pins are necessarily fragile due to the small diameters of the passages (typically on the order of 0.3 millimeters or about 0.012 inches) and occasionally break, leaving a pin fragment lodged in the passage. Extraordinary measures, such as electro-discharge machining, must often be employed to clear the fragment from the passage.
Thus, it is seen that existing methods for avoiding or reversing the effects of coatdown are unsatisfactory. Accordingly, a time efficient, cost effective and trouble free method of accommodating the coatdown phenomenon is sought.
SUMMARY OF THE INVENTION
It is a primary object of the invention to reverse the effects of coatdown and to do so in a way that is time efficient and cost effective.
According to the invention, an auxiliary coating is diffused into an existing coating on a transpiration cooled article, and into the substrate material exposed at the periphery of the transpiration cooling passages to form a diffusion zone. The mouth of each passage is enlarged when the article is subsequently subjected to a stripping agent capable of attacking the diffusion zone. The enlargement of the passage mouths compensates for the coatdown expected to occur when a nonoriginal layer of coating is deposited on the article.
Ideally the chosen stripping agent is one that is effective against not only the diffusion zone, but also against the auxiliary coating and the existing coating. Therefore, the compensatory enlargement of the passage mouths occurs concurrently with the stripping of the existing coating. The delay and expense associated with applying the auxiliary coating are negligible, especially in comparison to the time and cost required to abrasively erode excess coating from a multitude of passages. The diffusion of the auxiliary coating is similarly benign in terms of its cost and processing time since the diffusion can be carried out while the article is being stress relieved.
According to one aspect of the invention, the auxiliary coating is formed from a commercially available, adherable coating tape that is easily applied to the article.
In one detailed embodiment, the substrate is a nickel base alloy, the existing coating is a member of the class of MCrAlY coatings, and the auxiliary coating comprises aluminum and is diffusible into both the substrate and into the existing coating.
The method of the invention is advantageous in that the required processing steps are carried out largely in parallel with the stripping and stress relieving procedures customarily accomplished during refurbishment of a transpiration cooled article. Accordingly, the disclosed method contributes only negligibly to the expense and time required for article refurbishment, but dispenses with the significantly expensive and time consuming process of abrasively eroding excess coating from the mouths of the cooling passages. Moreover, the disclosed method is repeatable and predictable so that the effectiveness of the method can be verified with a simple aggregate flow capacity test of the article, rather than with a tedious, trouble prone pin check of each individual passage. Finally, it has been observed that the edges of the passage outlets are smoother and somewhat wider than is typical of either abrasively eroded passages or newly installed passages. As a result, the refurbished article may benefit from improved dispersion of the cooling film.
The foregoing features and advantages and the method of the invention will become more apparent in light of the following description of the best mode for carrying out the invention and the accompanying drawings.
REFERENCES:
patent: 3927952 (1975-12-01), Kirby
patent: 4121894 (1978-10-01), Cretella et al.
patent: 4141127 (1979-02-01), Cretella et al.
patent: 4402992 (1983-09-01), Liebert
patent: 4768700 (1988-09-01), Chen
patent: 4953777 (1990-09-01), Griffith et al.
patent: 5685971 (1997-11-01), Schroder et al.
patent: 5702288 (1997-12-01), Liebke et al.
patent: 5728227 (1998-03-01), Reverman
patent: 5813118 (1975-12-01), Roedl et al.
Draghi Peter J.
Shurman Brian J.
Simard Dale A.
Wrabel Peter
Baran Kenneth C.
United Technologies Corporation
Zimmerman John J.
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