ESTIMATION OF THE EFFECTIVE BORON-DIFFUSION COEFFICIENT IN THE Fe2B LAYERS GROWN ON GRAY CAST IRON

The present work evaluates the effective diffusion coefficient of boron in the Fe2B layers grown on a substrate from gray cast iron. The boride layers were generated using powder-pack bonding in the temperature range of 1173-1273 K for (2, 4, 6 and 8) h of treatment. First, the diffusion coefficient of boron in Fe2B (free of chemical stresses) was obtained through solving the mass-balance equation at the (Fe2B/substrate) interface using the experimental values of parabolic-growth constants taken from the literature. Second, a simple equation was derived to evaluate the effective diffusion coefficient of boron in Fe2B by considering the effect of chemical stresses. The estimated values of boron activation energies were 154.8 kJ/mol and 164.8 kJ/mol with and without the presence of chemical stresses. Furthermore, the calculated values of effective diffusion coefficients were found to be sensitive to the change in the bonding temperature and to the increase in the upper boron concentration in Fe2B.