Understanding the deformation and cracking behavior of Cr-based coatings deposited by hybrid direct current and high power pulse magnetron sputtering: From nitrides to oxynitrides

Highlights: • Elastic-Plastic deformability of thin hard Cr-based nitrides and oxynitrides. • Knowledge on cracking behavior using nanoindentation force-displacement curves. • Understanding the cracking behavior under static and dynamic loading conditions. • More crack formations and column separations in oxynitrides compared to nitrides. • Lower cohesion between microstructural columns in oxynitrides compared to nitrides. -- Abstract: The increasing demand for corrosion and wear reduction in modern tribological systems places high requirements on hard coatings deposited by physical vapor deposition (PVD) technology in numerous technical applications e.g., plastic processing, die casting or machining. One possible approach to withstanding the excessive wear is coating of tools with PVD hard coatings of the system Cr-Al-O-N. Each of the constituent elements can provide the coating with particular characteristics in point of its performance under different loading conditions. Therefore, the ongoing improvement in modern surface engineering requires comprehensive insights into the elastic-plastic deformation and cracking behavior of such coatings. For this purpose, several modifications and extensions of established experiments are required to overcome the limitations involved in characterization of thin hard coatings e.g., as a result of the μm ranges. In the presented work, three coatings CrN, (Cr,Al)N and (Cr,Al)ON deposited on quenched and tempered AISI 420 steel substrate were investigated. All coating systems were deposited through a hybrid technology, consisting of direct current and high power pulse magnetron sputtering (dcMS/HPPMS). The deformation and cracking behavior of the coatings and coating/substrate compounds were studied through application of static loadings by nanoindentation and Rockwell tests as well as dynamic loading conditions using nanoscratch tests. Complementary quantitative investigations were performed by means of depth profiling using confocal laser scanning microscopy (CLSM). Qualitative analyses of Rockwell imprints and nanoscratch tracks were conducted through scanning electron microscopy (SEM). Based on the results, precise analyses of nanoindentation force-displacement curves can improve the understanding about the possible crack formation in the coatings. Compared to (the binary) CrN, the ternary coating system (Cr,Al)N exhibits more promising characteristics in point of elastic-plastic deformation behavior and crack resistance. The crack resistance decreases through incorporation of oxygen into the ternary nitride and deposition of quaternary oxynitrides (Cr,Al)ON. Such character of oxynitrides can be overseen taking into consideration only the static loadings and therefore, applying of the both static and dynamic loading conditions are required to gain precise knowledge on the deformation and cracking behavior of thin hard coatings.