On the plastic deformation of chromium-based nitride hard coatings deposited by hybrid dcMS/HPPMS: A fundamental study using nanoscratch test

Abstract

Tribological behavior and elastic-plastic deformation of hard coatings deposited by physical vapor deposition (PVD) under normal and lateral loads can be investigated using innovative nanoscratch tests. However, a comprehensive study of thin PVD coatings requires a precise insight into mechanisms of the observed plastic deformation. Nanoscratch analyses on nanostructured PVD hard coatings and investigations of the mechanisms of resulted plastic deformation are the subjects of current research. In the presented work, two coatings CrN and (Cr,Al)N deposited on quenched and tempered AISI 420 steel substrate were investigated. The coatings were synthesized using a hybrid technology, consisting of direct current and high power pulse magnetron sputtering dcMS/HPPMS. Nanoscratch tests were performed with constant normal loads applying Berkovich and conical indenters. Despite significant hardness, these coatings deformed plastically under applied forces. Plastic deformation of the coatings was investigated by means of confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were applied to explore the mechanism of plastic deformation of the investigated coatings. Based on the results of CLSM and SEM, (Cr,Al)N exhibited a higher resistance against plastic deformation compared to CrN. The TEM investigations furthermore revealed that neither micro-crack formation nor dislocation motion inside the individual grains led to the observed plastic deformation. The dominating mechanism of the plastic deformation of investigated coatings is explained by sliding of grain boundaries (GBS) as the main reason.