Interface-dominated deformation mechanisms in Cr/CrN/Cr-DLC multilayer triggered by nanoindentation

Abstract

The diamond-like carbon (DLC) film traditionally possesses low toughness owing to high intrinsic compressive stress. The incorporation of other elements into the DLC film provides an effective approach to improving toughness, but it is still limited and produces some detrimental effects. In this study, a new strategy of designing the Cr/CrN multilayers to serve as a functional transition layer and doping Cr into DLC film is attempted to simultaneously improve the strength and ductility of DLC film. Herein, a profound understanding of the deformation mechanisms of Cr/CrN/Cr-DLC multilayers with different bilayer periods (BPs) during the indentation process is investigated. The maximum strength and toughness values are achieved by the Cr/CrN/Cr-DLC multilayer with BP of 10 nm (P10). The cooperation of multiple strengthening mechanisms involving the diminishment of dislocation motion within individual layers and the formation of coherent twin boundaries (CTBs) and Lomer-Cottrell locks (L-C locks) contributed significantly to the high hardness of P10. The predominant toughening mechanism of P10 is the formation of a substantial number of high-density stacking faults (SFs) in CrN sublayers, which is attributed to that the pre-existed CTBs with numerous kinks in CrN can reduce the stacking fault energies (SFEs) for P10. Few deformation twins facilitated the deformation of P10. In contrast, a versatile deformation mechanism appears in Cr/CrN/Cr-DLC with BP of 100 nm (P100) involving dislocation motion, SFs, deformation twins, and, of particular note, amorphization. However, the released energy related to deformation in P100 is limited, leading to cracks in the Cr-DLC top layer. This multilayer structure can provide additional strengthening and toughening mechanisms to endow the DLC film with profound mechanical properties in harsh service conditions.