Plasma nitridation of aluminized high purity iron

Surface treatment of high-purity iron by powder liquid coating aluminization and plasma nitridation is investigated with respect to the mechanism of hardening and nitrogen diffusion. Grain boundaries in the aluminized layer are found to be preferentially nitrided in the early stage of nitridation, accompanied by the formation of γ′-Fe4N platelets in the substrate. Aluminization and nitridation increase the hardness from HV110–120 for the original α-Fe(Al) to HV1200–1500 for the modified specimen, which consists of an α-Fe(Al) matrix with γ′-Fe4N and rocksalt AlN. Transmission electron microscopy observation reveals the AlN to be present in the form of platelets of 2–3 nm in thickness with an orientational relationship of (0 0 1)α//(0 0 1)AlN and [1 1 0]α//[1 0 0]AlN (Baker–Nutting relationship). The kinetics of plasma nitridation is formulated, and good agreement with the experimental results is obtained when first- or second-order reactions are assumed for the formation of AlN under constant N flux at the surface.