AISI 4140 low alloy steel was plasma nitrided to form a nitride layer of thickness 15 μm by conventional DC plasma nitriding method and TiN film of 5 μm was coated by arc ion plating method. The interfacial structure between TiN and iron nitride was characterized by optical microscope, SEM and XRD. A new layer, so called "black layer" was formed from the decomposition of iron nitride during Ti ion bombardment. The thickness of the black layer was increased with increasing ion bombardment time. The black layer was characterized as an α-Fe phase transformed from the iron nitride by XRD. The black layer was observed by SEM and TEM to investigate the detailed microstructure. Two distinct sublayers were observed in the black layer. The upper black layer was of very fine grains and the lower of 10 times larger grains. The fine grains were irregular and around 50 nm in size.
In order to clarify the formation mechanism of the black layer, DSC method was used as a thermal analysis of iron nitride phase transformation. As an iron nitride was consisted of $γ'-Fe_4N$ and $ε-Fe_3N$ phase after plasma nitriding which was one of the main process of duplex coating of low alloy steel, $γ'-Fe_4N$ and $ε-Fe_3N$ powders were separately prepared by the different processing conditions of gas nitriding method in the fluidized bed. From the DSC thermal analysis, the phase transformation path of $γ'-Fe_4N$ was identified as $γ'-Fe_4N → γ-Fe → α-Fe$ and that of $ε-Fe_3N$ was identified as $ε-Fe_3N → ε-Fe_2.5N + γ'-Fe_4N → γ'-Fe_4N → γ-Fe → α-Fe$. From the Kissinger plot of the phase transformation from the iron nitride to α-Fe, an activation energy was 64.8∼65.5(kcal/mole) which was well coincident with the activation energy of the self-diffusion of Fe in the γ-Fe phase. From the comparison of the activation energy, the rate controlling step of the phase transformation of the iron nitride was the self-diffusion process of Fe in γ-Fe phase. From the mechanism of the phase transformation of the iron nitride, it is essential to reduce its surface temperature during Ti ion bombardment below the temperature of 630℃ for the suppression of the black layer formation. It was confirmed that the formation of black layer was suppressed by reducing bias voltage below 600 V in the Ti ion bombardment process of duplex coating.
The adhesion strength of the duplex coating measured by scratch test was dropped rapidly even if the black layer was thin. The rapid reduction of adhesion strength in the duplex coating was related to the interfacial microstructure in which the delamination of TiN film was occurred. A lot of pores were observed along the interface by TEM. It is suggested that these pores play an important role for the decrease of the adhesion strength of TiN film in the duplex coating. The adhesion strength was improved to 60N by the suppression of the black layer and pores.
