Growth Mechanism and Kinetics of Siliconizing of AISI D2 Tool Steel

The present paper goes into details of the kinetics of silicide layer growth on AISI D2 during surface siliconizing. Pack cementation was employed in order to siliconize the steel surface. Siliconizing was conducted by using powder mixtures Si 12 wt.% + NH4Cl 0.5 wt.% + Al2O3 at 923, 1073 and 1223 K for 2 to 6 h, respectively. Thermodynamic calculations showed that growth mechanisms of the coating comprise many different chemical reactions. The microstructure and silicides precipitation evolution were analyzed through scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction analysis (XRD). The silicides layers thickness falls in the range 32-154 mu m. The coatings hardness varied between 750 and 800 HV, being dependent on the process parameters. The kinetics measurement revealed the growth of the FeSi, Fe2Si and FeSi2 sub-layers as a function of treatment time and temperature. The results illustrated that the diffusion coefficient (k) increased with the treatment temperature. Activation energy (Q) was calculated as 788.83 kJ.mol(-1). The crystal growth rate resistance (K) ranged from 5.2 x 10(-9) to 3.1 x 10(-8) cm(2).s(-1).