Role of Pt content in the microstructural development and oxidation performance of Pt-aluminide coatings produced using a high-activity aluminizing process

The present study highlights the effect of Pt content on the microstructure of Pt-aluminide coatings produced using a single-step high-activity aluminizing process. The amount of Pt in the coating was varied by changing the thickness of the initial electroplated Pt layer between 1 and 15 mu m. The aluminium uptake from the pack was found to be almost the same for all the coatings produced using a Pt layer of thickness 2.5 pm and above, with a somewhat lower uptake for the coating corresponding to a 1 mu m thick Pt layer. The coating microstructure, which consisted of an outer two-phase (PtAl2 in a matrix of NiAl) layer, an intermediate NiAl layer and an interdiffusion layer, was also found to be independent of the Pt layer thickness when it was in the range 2.5-10 mu m. In the case of the 1 mu m Pt layer, however, the whole of the Pt remained in solid solution in the NiAl phase. For a Pt layer thickness exceeding 10 mu m, on the other hand, a continuous surface layer of PtAl2, phase was observed. The above mentioned influence of the thickness of the Pt plated layer on the microstructure of the Pt-aluminide coatings observed in the present investigation could be explained in terms of the Pt concentration in the diffusion layer resulting from the interdiffusion between the Pt layer and the superalloy substrate during the pre-aluminizing diffusion treatment. Cyclic oxidation tests on these Pt-aluminide coatings reveal that the presence of Pt in aluminide coatings, in general, enhances oxidation resistance. However, in order to fully realize the beneficial effects of Pt on oxidation behaviour, a certain minimum Pt content in the coating was found to be necessary. (C) 1998 Elsevier Science S.A. All rights reserved.