Structure and mechanical properties of carbon–nickel and CNx–nickel nanocomposite films

DC sputtered carbon–nickel and carbon-nitride–nickel thin films were investigated by high resolution electron microscopy, X-ray microanalysis and nanoindentation to clarify the influence of Ni addition on the structure, formation and mechanical properties. The films were deposited in argon or nitrogen plasma at temperatures from 25 to 800 °C onto NaCl and SiO2 substrates. The microstructures can be described as nanocomposites, built from Ni/Ni3C nanocrystals in a carbon/CNx matrix. The mechanical properties of the films were found to be strongly dependent on the growth temperature along with the changes of the structure. The highest hardness of 14 GPa was measured for the film grown at 200 °C, while low values (down to 2–6 GPa) were obtained for films deposited at high temperatures. The decrease of the hardness is thought to be primarily the consequence of morphological changes of the crystalline phase. In films grown at room temperature and at 200 °C, an amorphous or nearly amorphous matrix was observed, while at higher temperatures up to 800 °C, the matrix showed relatively high ordering in the form of curved graphite/CNx layer stacks. At all temperatures above 200 °C, the Ni/Ni3C particles were encapsulated in three- to five-layer thick graphitic shells following exactly the surface of the Ni particles. TEM measurements confirm that nickel enhances the formation of ordered C/CNx structures as graphitic shells and fullerene-like domains in sputter-deposited films. Nanoindentation experiments show that the CNx–Ni films have slightly higher hardness compared to the C–Ni films.