Characterization and Diffusion Kinetics of Silicon on AISI D2 Steel

This paper presents the simultaneous influence of two different activators (NH4Cl and NaF) on the kinetic evolution during siliconising of steel. The AISI D2 tool steel surface was coated by pack siliconising through the growing of silicon on the surface. Pack siliconising was accomplished by employing a mixture of powders of Si 12 wt% + NH4Cl 0.5 wt% + NaF 0.5 wt% + Al2O3 at 923, 1073 and 1223 K for 1 to 5 h, respectively. The thermodynamics of the different chemical reactions were calculated for the forecast of the coating growth mechanisms of pack siliconising. The microstructure and the precipitation evolution of the silicides were analysed by Energy Dispersive X-ray Spectroscopy line scan (EDS-line scan), X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). The ranges of the silicide layers thickness resulted between 19 and 337 mu m depending on the employed processing parameters. The maximum hardness related to the siliconising parameters for fabricating the different thickness of the intermetallics on the surface of the AISI D2 tool steel and the ranges of the microhardness of the substrate after siliconising is between 750 and 800 HV. The growth of the FeSi, Fe2Si and FeSi2 on the sub-layer is a function of the treatment temperature and time, their growth behavior was used for revealing the kinetics during the process. The results showed that the diffusion coefficient (k) was enhanced by increasing the treatment temperature. Activation energy (Q) resulted 138 KJ/mol. The crystal growth rate resistance (k) are from 1.141 x 10(-8) to 1.078 x 10(-6) m(2)/s.