Simulation of the growth kinetics of boride layers formed on Fe during gas boriding in H2-BCl3 atmosphere

The modeling of the boriding kinetics is considered as a necessary tool to select the suitable process parameters for obtaining boride layer of an adequate thickness. Therefore, the simulation of the growth kinetics of boride layers has gained much attention for last years. The majority of the published works described the kinetics of the pack-boriding or paste-boriding. In this study, the model of growth kinetics of two-phase boride layer (FeB+Fe2B) on pure Fe was proposed for gas boriding. Displacements of the two interfaces (FeB/Fe2B and Fe2B/substrate) resulted from a difference of the arrival flux of interstitial boron atoms to one phase and the departure flux of the boron atoms from this phase to the second phase. The mass balance equations were formulated. The measurements of thickness of both zones (FeB and Fe2B), for different temperature of boriding, were used for calculations. Based on the experimental data, the parabolic growth constants AFeB and BFe2BBFe2B versus the temperature of boriding were determined. The linear relationships were accepted. As a consequence, the activation energies (QFeB and QFe2BQFe2B) were calculated. The calculated values were comparable to other data derived from gas boriding. The presented model can predict the thicknesses of the FeB and Fe2B zones (XFeB and YFe2BYFe2B, respectively) formed on pure Fe during gas boriding. Additionally, the diffusion annealing after boriding was analyzed. This process was carried out in order to obtain a single-phase boride layer (Fe2B). The relationship between the reduction in FeB zone (dXFeB) and the growth in Fe2B phase (dYFe2BdYFe2B) was determined. The time tXFeB=0, needed for the total elimination of FeB phase in the boride layer was calculated and compared to the experimental data.

JBFeB, JBFe2B, JBFe—diffusion fluxes, CB,Fe/Fe2BCB,Fe/Fe2B, CB,Fe2B/FeCB,Fe2B/Fe, CB,FeB/Fe2BCB,FeB/Fe2B and CB,s/FeB—boron contents at the interfaces Fe/Fe2B/FeB and at the surface, dxFeB, dYFe2BdYFe2B—infinitesimal growth (or reduction) of borides xFeB, YFe2BYFe2B—thickness of FeB and Fe2B borides, AFeB, BFe2BBFe2B—parabolical growth constants, QFeB,QFe2BQFe2B—activation energy of borides, t—boriding time, T—absolute temperature, R—gas constant tXFeB=0—time needed for the total elimination of FeB phase, gas boriding—diffusion process in H2-BCI3 atmosphere providing the two-phase boride layers, diffusion annealing—the second stage of process in which the single phase boride layer is obtained. Figure optionsDownload as PowerPoint slideHighlights
► The model of growth kinetics of two-phase boride layer on pure Fe was proposed for gas boriding.
► The mass balance equations were formulated.
► The parabolic growth constants and the activation energies were determined.
► The diffusion annealing, carried out in order to obtain a single-phase boride layer, was analyzed.
► The time for the total elimination of FeB phase was calculated and compared to the experimental data.