Plasma foil aluminizing of steel at a temperature below aluminum melting point

In this research, steel AISI $4340$ was aluminized by plasma foil, where the specimen was masked by an aluminum $(\rm Al$–$1.6$ wt $\%~\rm Si)$ foil with a thickness of $50~\mu$m and was placed in cathode position of glow discharge plasma reactor. By gradual increasing the pressure and voltage of plasma, cathode current rate increased that resulted in heating up of the specimen. The process was designed for sample treatment at $600^{\circ}$C (below the melting point of aluminum), where for comparison, another experiment was also carried out at $750^{\circ}$C. Aluminizing was conducted for about $5$ min at process temperature. The coatings were characterized by field emission scanning electron microscopy equipped with energy dispersive spectroscopy and X‑ray diffraction. The coating consisted of a $90~\mu$m-thick double layer of $\rm Al$-rich $\rm FeAl_3$ and $\rm Fe_2\rm Al_5$ phases beneath $\rm Al$ during treatment at $600^{\circ}$C. It is concluded that the effect of plasma was not only heating the sample but also providing the $\rm Al$–steel diffusion couple and enhancing the interdiffusion of iron and aluminum elements in $\rm Al$-rich iron aluminides. Plasma foil aluminizing at $750^{\circ}$C resulted in the formation of a multi-layer coating consisting of iron aluminide phases $(\rm Fe_2\rm Al_5$ and $\rm FeAl)$ with a thickness of $\sim140~\mu$m. A comparison of the coating thickness obtained in this study with those reported in hot dip aluminizing showed that ions' bombardments on the sample surface during plasma process increased the coating thickness by activating the surface and enhancing the interdiffusion of iron and aluminum in aluminides.