BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a gas turbine engine blade.
FIG. 2 illustrates a portion of a gas turbine engine blade having an anti-fretting wear coating applied in accordance with exemplary embodiments of the invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
With reference to FIG. 1, a gas turbine engine blade 30 is illustrated. The gas turbine engine blade 30 has an airfoil 36 including a pressure side 38, against which a flow of gas impinges during service operation, and an oppositely disposed suction side 40. The gas turbine blade 30 further includes a downwardly extending shank42, and an integral attachment in the form of a dovetail 44, which attaches the gas turbine blade 30 to a gas turbine disk (not shown) of the gas turbine engine. A platform 46 extends transversely outwardly at a location between the airfoil 36 and the shank 42 and dovetail 44.
The blade 30 may be any gas turbine engine blade including a compressor blade or a turbine blade, and more particularly may be either a low pressure turbine blade or a high pressure turbine blade. During operation, the dovetail 44, and particularly the pressure side 48 of the dovetail 44 is subjected to contact with the gas turbine disk by vibration and rubbing resulting in wear to the dovetail 44. This wear may be increased when the blade 30 and disk are of different base alloy compositions, such as a titanium-base alloy blade and a nickel-base alloy disk.
Referring now to FIG. 2, a portion of the blade 30 serves as a substrate 15 to which the anti-fretting wear coating is applied in accordance with exemplary embodiments of the invention. Typically, the wear coating is applied to the dovetail 44, and more typically to the pressure face 48 of the dovetail 44, which has at least one surface that mates with a corresponding surface of the gas turbine disk, and both of which are subjected to a significant amount of rubbing during engine operation.
The substrate 15 may be constructed of any operable material. Examples include nickel-base alloys such as nickel-base superalloys strengthened by the precipitation of gamma-prime or a related phase, iron-base alloys, cobalt-base alloys, and titanium-base alloys.
A substrate 15 of particular current interest is titanium aluminide (TiAl), including gamma titanium aluminides and alpha-2 titanium aluminides. One particularly suitable titanium aluminide for use as the substrate 15 has a composition of about 32 to about 33.5 weight percent (wt %) aluminum, about 4.5 to about 5.1 wt % niobium, about 2.4 to about 2.7 wt % chromium, about 0.04 to 0.12 wt % oxygen, up to about 0.020 wt % nitrogen, up to about 0.015 wt % carbon, up to about 0.10 wt % iron, up to about 0.001 wt % hydrogen, up to about 0.050 wt % impurities, and the balance titanium.
Prior to coating, the surface of the substrate 15 may be prepared by dry or wet blasting to a surface roughness of about 80 to about 150 microinches Ra, as well as masking any areas that do not need coated. An anti-fretting wear coating 20 is applied overlying the substrate 15. The anti-fretting wear coating 20 comprises a high temperature bond coat 22 and, optionally, a layer of dry-film lubricant 24. The high temperature bond coat 22 is applied by air plasma spraying techniques using either a powder or wire feed. By “high-temperature bond coat” is meant a bond coat comprising any material that has a composition stable above about 343° C. (650° F.), such as a nickel-chromium alloy. It has been discovered that methods according to exemplary embodiments of the present invention result in high temperature bond coats that may be stable from about 343° C. (650° F.) up to about 704° C. (1300° F.).
One suitable high temperature bond coat 22 is a nickel-chromium alloy having a composition of about 58 to about 62 weight percent (wt %) nickel, about 14 to about 18 wt % percent chromium, about 1.3 to about 1.7 wt % silicon, and a total of about 0.23 maximum wt % of impurities, which is commercially available as METCOLOY® 33 from Sulzer Metco of Winterthur, Switzerland. The high temperature bond coat is typically applied to a thickness of about 0.0254 mm (0.001 inches) to about 0.305 mm (0.012 inches).
Optionally, the anti-fretting wear coating also comprises a high temperature dry film lubricant 24 applied overlying the high temperature bond coat 20. The dry film lubricant 24 typically comprises graphite and may further comprise either one or both of silicates (for example, LOB1800 available from Everlube Products of Peachtree City, Ga.) or aluminum phosphates (for example, EVERLUBE® 853, also available from Everlube Products) and may be applied to a thickness of about 0.013 mm (0.0005 inches) to about 0.102 mm (0.004 inches). The application of the dry film lubricant 24 may be by spraying, brushing, dipping or any other suitable methods, but typically is applied by spraying followed by a heat treatment cycle to cure it.
The combination of the APS application of the high temperature bond coat 22 and dry film lubricant 24 results in an anti-fretting wear coating that reduces friction, and thus wear, between the coated gas turbine engine blade and the disk. Embodiments of the present invention may reduce the coefficient of friction (both sliding and break) between the mated components to less than about 0.6 and more preferably to less than about 0.4. Thus, the application of the high temperature bond coat 22 by air-plasma spraying protects the mating surfaces of the gas turbine engine blades to which it is applied, such as the dovetail pressure face 48 of a low pressure turbine blade, while in service.
The method of applying the high temperature bond coat 22 by APS has the further advantage of permitting the blades to be inspected and/or repaired at each service interval. At a service interval, each blade can be separated from its disk and the APS-applied high temperature bond coat removed by grit blasting, chemical stripping, or water jet stripping by way of example only. Once removed, the underlying substrate may be inspected for cracks or other possible sources of failure in need of repair. Such inspection and repair is not currently feasible when HVOF application techniques are used, since the HVOF coatings cannot readily be removed without possible damage to the underlying substrate.
Following inspection and any needed repairs, the anti-fretting wear coating can then be re-applied to the dovetails 44 so that the repaired blades 30 may be returned to service, thereby permitting continued use of turbine blades that otherwise may have been discarded.
Wear and friction results are shown with respect to the following example of the invention, which has been reduced to practice. These results demonstrated that methods of applying a high temperature bond coat by APS techniques resulted in wear and friction that were generally at least as good or better as those typically found in bond coats applied by HVOF techniques.
Shoes of titanium aluminide were coated with a METCOLOY® 33 bond coat by APS to a thickness of about 0.064 mm (0.0025 inches) to about 0.114 mm (0.0045 inches). Several samples were coated with a layer of dry-film lubricant over the bond coat to a thickness of about 0.013 mm (0.0005 inches) to about 0.051 mm (0.002 inches). Both LOB1800 and EVERLUBE® 853 dry film lubricants were used in separate tests and the results combined and averaged. Sliding wear tests were conducted on the samples according to GE Aviation Specification E50TF76 with parameters modified to match the performance requirements for the specific application at temperatures of 427° C. (800° F.) and 538° C. (1000° F.) and applied pressures between 34.5×103 kPa (5,000 psi) and 137.9×103 kPa (20,000 psi). The results were compared with sliding tests on bare titanium alumnide, as well as coated and uncoated samples in which the bond coat was applied by HVOF. Averaged results are shown in Table 1 below.
−4.0 × 10−3
TiAl + M33
−1.3 × 10−4
TiAl + M33 (APS)
−5.5 × 10−5
TiAl + M33 + DFL
−1.6 × 10−3
TiAl + M33 + DFL
−8.4 × 10−4
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.