Boriding of Fe and Fe–C, Fe–Cr, and Fe–Ni alloys; Boride-layer growth kinetics

Specimens of pure Fe and of Fe-0.8 mass % C, Fe-0.5 mass % Cr, Fe-4.0 mass % Cr, Fe-4.0 mass% Ni, and Fe-10.0 mass% Ni alloys were borided in boriding powder. A boron-compound layer developed consisting of a surface-adjacent “FeB” sublayer on top of an “Fe₂B” sublayer. Layer-growth kinetics were analyzed by measuring the extent of penetration of the “FeB” and “Fe₂B” sublayers as a function of boriding time and temperature in the range 1025–1275 K. Layer growth is dominated by B diffusion through “FeB/Fe₂B”. This diffusion process is of strongly anisotropic nature. Consequently, ragged interfaces occur between the substrate and the boride layers. The depths of the tips of the most deeply penetrated “FeB” and “Fe₂B” needles have been taken as measures for diffusion in the easy [001] diffusion directions. Assuming unidirectional B diffusion and parabolic growth, a simple model of layer growth has been given. It accounts for the specific volume difference between “FeB” and “Fe₂B”. In contrast with earlier work, the model provides values for the kinetic parameters for growth of each of the phases in the boron-compound layer.