Vapour aluminide coating of internal cooling channels, in turbine blades and vanes

A production-scale vapour-aluminizing process has been developed that facilitates the protective aluminide coating of the surface of internal cooling channels in superalloy turbine blades and vanes, without recourse to either reduced pressure or pressure pulsing. The plant employed to apply this process industrially is briefly described. Vapour aluminide coatings thus formed on a nickel-base superalloy have been characterized in their as-coated condition. The techniques employed for this purpose comprised X-ray diffraction, Auger electron spectroscopy, optical microscopy-Nomarski differential interference contrast and bright field illumination, and microhardness testing. It has thus been determined that such aluminide coatings consist largely of β-NiAl and essentially have a four-zone microstructure that is discussed in some detail. This microstructure diverges markedly from those observed for commensurate coatings formed by other vapour-aluminizing processes. Two illustrative examples of the industrial application of this vapour-aluminizing process, to coat the linear internal cooling channels in a turbine blade and the serpentine channel in a vane, are presented. In both cases, it has been established that the internal surfaces have been coated with an aluminide layer that exhibits a good uniformity of thickness throughout. The coated internal cooling channel surfaces have additionally been shown to be clean and free from any extraneous material.