Improvement in nano-hardness and corrosion resistance of low carbon steel by plasma nitriding with negative DC bias voltage

In this work, we study the effect of plasma nitriding on nano-hardness and corrosion resistance of low carbon steel samples. The plasma was generated through a radio-frequency inductively coupled plasma source. The substrate temperature increased (by the self-induced heating mechanism) with the treatment time for increasing negative bias voltages. X-rays diffraction analysis revealed the formation of nitride phases (epsilon-Fe2-3N and gamma-Fe4N) in the compound layer of the treated samples. A phase transition occurred from 3.5 kV to 4.0 kV and was accompanied by an increase in the volume fraction of the gamma-Fe4N phase and a decrease in that of the epsilon-Fe2-3N phase. Auger electron spectroscopy revealed a deep diffusion of the implanted nitrogen beyond 320 nm. The nano-hardness increased by similar to 400% for the nitrogen-implanted samples compared to the untreated state, the nitride phases are believed to participate to the hardening. Potentiodynamic polarization measurements revealed that the plasma nitriding has improved the corrosion resistance behavior of the material. When compared to the untreated state, the sample processed at 4.0 kV exhibits a shift of +500 mV and a reduction to 3% in its corrosion current. These results were obtained for relatively low bias voltages and short treatment time (2 h).