In this research, effects of changes in Aluminizing conditions on microstructure of Pt - aluminide coating applied oil a Ni - base superalloy GTD -111, has been studied. A thin layer (i.e.68,#mm ) of Pt was electroplated onto the surface of the .samples, and then they were aluminized by pack cementation technique under various conditions of time, temperature, rate of heating and pack powder compositions In addition, by application of a thin Ni layer on the substrate before applying Pt, the source of nickel available for diffusing into Pt layer during Aluminizing process enhanced and the need for an extra heat treatment cycle before Aluminizing process was practically omitted. Addition of a nickel layer, also prevented scaling of Pt layer during its electroplating and Aluminizing processes that helps enhancement of cohesiveness of the coated layers.
The general microstructure of the coating consisted of four layers, which are PtA12layer internal diffusion layer; external diffusion layer and interdiffusion zone. The structure of each layer has been studied by optical and scanning electron microscopes as well as XRD technique.
The results show the presence of the original Pt - electroplated layer had no effect on the thicknesses of the coating layers, but higher Aluminizing time and temperature had increased the thicknesses of interdiffusion and internally diffused layers. In addition, at high temperature, Aluminizing with a lower heating rate caused an increase in the thickness of internal diffusion layer. Aluminizing with a lower heating rate at high temperature (more than 900°C) had increased the thickness of interdiffusion laver Attempt has been made to justify , the changes occurred in microstructures and thick nesses of various coating layers as they exposed to different Aluminizing conditions.