Formation of aluminide coatings on nickel by a fluidised bed CVD process

The present work concerns the formation and modelling of aluminide diffusion coatings onto nickel and nickel alloys by means of a fluidised bed chemical vapour deposition (FBCVD) process. In this process, the treated object is suspended within the FB reactor, which contains the treating agent. The latter is a mixture of powders of the donor of the coating-forming element (FexAly), and of a filler material (Al2O3). As an activator of the process, a halide compound is used (NH4Cl). The bed is fluidised by inert gas (Ar). The precursor vapours of the element to be deposited are formed in situ, by the reaction of the donor with the activator. The produced coatings are characterised by optical microscopy, SEM and EPMA. The modelling of the nickel–aluminide coating growth on nickel, has been based on the diffusion equations of aluminium in pure nickel, considering the intermetallic compounds predicted by the Ni–Al phase diagram. The model predicts the aluminium composition profiles as functions of the time and the concentration of Al in the donor, for the aluminisation conditions under consideration. The model is in satisfactory agreement with experimental results.