Influence of elevated temperature and reduced pressure on the degradation of iron nitride compound layer formed by plasma nitriding in AISI D2 tool steels

Abstract The stability of an iron nitride compound layer prepared by plasma nitriding of AISI D2 tool steel is investigated using in situ high‐temperature X‐ray diffraction at rising temperature up to 470°C and for an isothermal treatment at 450°C. Both kinds of experiment were carried out in vacuum. For all treatments, the iron nitride compound layer, which consists mainly of ε‐Fe3N, decomposes and forms an α‐Fe ‘black’ layer. The stability of the iron nitride compound layer is discussed in terms of the phase composition, the relative change of the lattice parameter, the nitrogen content and the decomposition kinetic. The decomposition takes place via a phase transition in two steps: (i) the out‐diffusion of nitrogen from ε‐Fe3N above an annealing temperature of 300°C and (ii) the transformation of ε‐Fe3Ny into α‐Fe when the nitrogen content (y) declines below 0.66. The second step is accompanied by the formation of Fe3C precipitates at about 450°C

Microstructure and adhesion characteristics of duplex coatings with different plasma‐nitrided layers and a Cr‐Al‐Ti‐B‐N physical vapor deposition coating

Abstract A duplex treatment consisting of plasma nitriding (PN) and physical vapor deposition (PVD) significantly improves the thermal, tribological, and corrosion resistance of forming tools, and especially if they are intended for applications subject to high mechanical loads. This study investigates the influence of nitriding on the properties of a conventionally heat‐treated AISI D2 tool steel, coated with a Cr‐Al‐Ti‐B‐N layer, while the effect of the presence or absence of a compound layer is discussed. PN is performed at 510–520°C using different N2‐H2 gas mixtures. The Cr‐Al‐Ti‐B‐N layers are deposited at 480°C using a combination of cathodic arc evaporation and magnetron sputtering. The samples are characterized using electron probe microanalysis with wavelength‐dispersive X‐ray spectroscopy, optical and scanning electron microscopy, glancing‐angle X‐ray diffraction, surface hardness measurements, profilometry, Rockwell indentation, and scratch tests. These techniques reveal relationships between the depth gradient of the chemical composition and microstructure of the nitrided interlayer, the adhesion of the PVD coating, and the hardness of the tool steel. Although the PVD process induces a structural transformation in the compound layer, this transition does not have a negative influence on the adhesion of the PVD coating