In situ SEM and AFM observations of surface morphology of plasma nitrided layer of austenitic stainless steel

Abstract

Surface morphology changes of the layer on austenitic stainless steel plasma nitrided for different times at low temperature (400–420°C) were studied by in situ SEM observation. The results show that nitriding does not modify the annealed twin, grain size and shape. However, when for a process time of 240 min, the strain in the nitrided layer (γN) resulting from supersaturating nitrogen can be great enough to initiate slip system activity so that a high slip band density appears on the layer surface. Obvious surface convex occurs in annealed twin band and matrix zones, which indicates that the greater slip amount has been produced in these zones. Local convex surface zones or differences in magnitude of the amount of slip in various zones can be attributed to the grain orientation relative to the direction of the compressive stress in the layer. The presence of slip bands and the occurrence of convex surface zones indicate that the planar compressive stress in the nitrided layer reaches the planar yield strength, so that a surface yield phenomenon occurs. Atomic probe microscopy was employed to study the surface topography. The mean microroughness value R _a was 19·851 nm over a 15 × 15 μm² area of AFM image that included slip strips. The spacing between the slip strips was of the order 100 nm, and the slip step of order 10 nm.