ABSTRACT
A production-scale vapour aluminising process has been developed which facilitates the protective aluminide coating of the surface of internal cooling channels/passages in nickel-base superalloy turbine blades and vanes, without recourse to reduced pressure or pressure pulsing. Initial studies confirmed that aluminide coatings, thus formed, were present both at the entrance to and exit from these channels. Explicit restraints, however, precluded the necessary detailed determination of the characteristics of the vapour aluminide coating and hence the elucidation of any variation in these characteristics along the length of the cooling channels in an actual blade/vane. To circumvent this problem, a previously devised experimental reactor set-up has been utilised to simulate the vapour aluminising of the internal channels in a blade/vane. Small, rectangular Inconel 738 samples were suspended in three strategic locations in the reactor coating chamber, representing the entrance to, the mid-point of and the exit from the cooling channels. After the vapour aluminising process, the coating formed on each of these samples was subjected to the required detailed characterisation employing the following techniques: X-ray diffraction, profilometry, scanning electron microscopy, optical microscopy, microhardness testing and Auger electron spectroscopy. The results thus obtained are presented and discussed and any variation in the characteristics of the vapour aluminide coating with increasing sample distance from the source powder mixture identified.
