Microstructure of the composite oxynitrided chromium layers produced on steel by a duplex method

The paper describes the microstructure of the composite layers produced on chromium pre-coated AISI 1045 steel by oxynitriding under glow discharge conditions (duplex method). The layers were examined by the cross-section method using transmission electron microscopy. Their microstructure was composed of the 4 zones: a near-surface CrN–Cr2O3 mixture zone about 1 μm thick, a Cr2N zone about 4 μm thick, a chromium zone–the thickest–about 50 μm, and the thinnest (Cr,Fe)7C3 zone about 0.5 μm thick. Cr2O3 also occurred in the form of nano-crystalline aggregates, about 1 μm in size, situated in the upper part of the Cr2N zone. All the zones had a fine-grained structure, except the finest nano-crystalline zone situated at the surface. It was found that, during oxynitriding, the cracks present in the chromium coating are filled up by chromium nitrides and oxides. This may explain the observed improvement of the corrosion resistance.