Microstructure and Mechanical and Oxidation Properties of Multilayer Aluminide Coatings Formed Over P91 Steel

摘要

Iron aluminide-based oxidation-resistant and permeation barrier coatings were formed on P91 steel using pack cementation and post-coating annealing treatment. Fe2Al5 layers of variable thicknesses were formed at the coating temperatures in the range of 600-700 degrees C. The as-coated alloy annealed at 625-750 degrees C for different duration revealed the formation of multilayered coating structure that consisted of FeAl2, FeAl and Fe(Al) phases of varying thicknesses. Knoop hardness of Fe2Al5 phases was found to be 700 HK. High-resolution nanoindentation mappings were carried out for heat-treated specimen to establish structure-property interlink at the micron length scale. Hardness and modulus maps were generated using large numbers of arrays of nanoindentations at intervals of 2 mu m. Scratch tests performed at varying loads revealed superior scratch resistance of FeAl as compared to Fe2Al5 phase. Oxidation studies under pure O-2 environment of as-coated alloy in the temperature range of 700-1000 degrees C for 8 h revealed the parabolic growth of alumina scale.