Studies and modelling of high temperature diffusion processes in selected high performance structural coating systems

The central theme of the work undertaken in this thesis involved modelling of diffusion – numerical and microstructural – occurring during high temperature exposure of selected materials and coatings. The materials and coatings and their high temperature treatments included carburization of steel (nonsteady-state diffusion of iron carburized at 950o C after 7.1 hours), a two component Cu-Ni diffusion couple subjected to diffusion anneal at 1054oC for 300 hours, a three component Pt-Ni-Al solid alloy subjected to oxidation and diffusion anneal, Multicomponent Ni -aluminide and Pt-aluminide coatings on MAR M002 subjected to 150 hours of diffusion treatment at temperature 1273K, a Jr and Jr/Pt Low-activity aluminide / MAR M002 system at 1100oC after 100 hours, aluminise coating on low alloy steels at 650oC, Jnnovatial coatings- Ti45Al8Nb coated with Al2Au subjected to air oxidation at 750oC for 1000 hours, and Ti45Al8Nb coated with TiAlCrY subjected to air oxidation at 750oC for 500 hours. Such coatings are being increasingly used to protect materials against high temperature (600-1000oC) degradation by oxidation. The demands for using such coatings have arisen because of the need to increase the efficiency by increasing the operation temperature in many areas of technological applications such as power operation, aero engines, and energy conversion systems and in processing industries. However the one of the major obstacles to use these coatings to prevent high temperature oxidation of materials is the degradation of the coatings due to the coating/substrate interdiffusion. Jnterdiffusion of critical elements from the coating to the substrate will deplete the protective scale forming elements in the coating