Formation of chromium nitride coatings on carbon steels by pack cementation process

Abstract

Chromium nitride coatings are usually applied to metal surface by physical vapour deposition processes, which not only have line-of-sight restrictions but also require capital intensive facilities. This study is thus undertaken in an attempt to form chromium nitride coatings on the surface of carbon steels by pack cementation process, which offers an efficient method to form chromium nitride coatings on metal surfaces of complex geometries. Thermal chemical calculations were undertaken to identify suitable activators for the process. Experiments were then carried out using pack powder mixture of a composition 30Cr2N-2NH4Cl-68Al2O3 (wt.%). Two grades of carbon steels with compositions differing mainly in carbon content were used as substrates. The process was carried out in the temperature range of 1000 °C–1100 °C. The microstructures of the coatings were characterised using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The phases in the coatings were analysed by X-ray diffraction (XRD). In all cases, the coatings produced had an outer Cr2N layer, but the inner microstructures depended on carbon content in steel substrate. The effects of processing temperature and time on the microstructures of the coatings were also discussed.