Phase depth distribution characteristics of the plasma nitrided layer on AISI 304 stainless steel

The phase depth distribution of a plasma nitrided layer on AISI 304 stainless steel at 400–420 °C was investigated by glancing-angle X-ray diffraction through step-wise electrio-polishing removal. The nitrogen depth profile was measured by energy-dispersive X-ray. The results confirm the dominant phase in the nitrogen-rich layer as an expanded austenite. The amount of the lattice expansion relative to the substrate austenite decreases from the layer surface inwards, due to the decrease in nitrogen content. The amount of lattice expansion from planes of (2 0 0) is always larger than that from (1 1 1) at a specific depth. However, the amount of distortion does not decrease monotonically with the decrease in nitrogen content with depth. It is related to the different arrangement of the nitrogen atoms in the fcc lattice at various nitrogen contents. Observations in situ on the surface morphology were performed by scanning electron microscopy. When the strain resulting from supersaturating nitrogen is sufficient to initiate the slip system activity, the slip bands are observed on the surface of the nitrided layer.