Metal concentration dependent mechanical properties of electrodeposited nickel incorporated diamond like carbon (Ni-DLC) thin films studied by nanoindentation

Abstract

Abstract Effect of nickel incorporation in diamond-like carbon (DLC) matrices has been investigated on the mechanical properties of Ni-DLC thin films by nanoindentation. DLC and Ni-DLC thin films of different nickel content were electrodeposited on ITO coated glass substrates at room temperature and low voltage. Raman spectrum analysis confirms the formation of the sp2 and sp3 phases of carbon in DLC and in some of Ni-DLC thin films. Microstructural studies by SEM reveal a variation in microstructures from finely granular to cluster like features with change in molar concentration of nickel in the electrolyte. GIXRD profiles demonstrate the presence of graphite, diamond and crystalline nickel phases in the thin films. EPMA study indicates the increase in nickel content in carbon matrix of prepared thin films with increase in nickel ions in the electrolyte. Depth controlled mode of nanoindentations was performed on the films to investigate their mechanical properties like hardness, elastic modulus, stiffness and elasto-plastic properties of the thin films. Nickel concentration dependent bilayer nature of heterogeneous growth structure has been revealed through force-distance curves. A hard and strong adhesive layer of DLC is formed immediately above the ITO surface, over which a much softer less adhesive layer is produced. With the increase in nickel content, bottom layer grows in thickness by the expense of the top layer. Hence at very low nickel content, the effect of top soft layer dominates the mechanical properties of the films, while at high nickel content gradually the effect of the soft layer vanishes. With increasing nickel content hardness of the films also shows an increasing trend contrary to the usual behavior of metal incorporated DLC thin films which may be due to multiple counter-interacting processes occurring simultaneously within the carbon matrix inducing changes in phase and hence in sp2/sp3 hybridization ratio.