Study and modelling of the characteristics of aluminum coating on pure nickel and nickel superalloy using HTLA process

Nickel based superalloys are amongst the most common choices of materials for gas turbine blades due to their exceptional mechanical properties at higher temperatures. However, for most applications they require the use of coatings to protect them against the chemically aggressive gases in contact with them. This thesis studies the behavior of aluminum coatings formed by an out-of-the-pack CVD process on both pure nickel samples and CMSX4 samples by means of SEM analyses of the samples’ surface and of the profile of the aluminized layers. Samples were oxidized for 100 h at 1100°C and again analyzed using the SEM. It was found that the aluminizing process executed on pure nickel with different values for parameters such as temperature and time affect not only the thickness of the aluminized layers, but also the grain sizes of the surface, as well as their boundaries’ shapes, and that this affected the form of the alumina layer formed during oxidation. The analysis of the chemical composition of the aluminized layer is far more complex in CMSX4 and is discussed in detail in the thesis. A model based on the work of Heckel and implemented in Matlab at Polimi for the simulation of the development of the aluminized layer in pure nickel was reviewed and improved in order to allow an increased degree of flexibility in the choice of simulating parameters. An extensive sensitivity analysis was performed with the model, and the simulation results were compared to the chemical profiles measured using the SEM, showing good accuracy.