Nanoindentation response analysis of Cu-rich carbon-copper composite films deposited by PVD technique

The micromechanical properties of sputter deposited amorphous carbon-copper (a-C/Cu) composite films with different carbon content (6.8-19.8 at.%) were investigated. Atomic force and electron microscopy studies confirmed that the a-C/Cu films have a two-phase microstructure of amorphous sp2/sp3-bonded carbon as a composite matrix with embedded copper nanocrystallites encapsulated in a graphene shell. Load-displacement curves obtained in nanoindentation experiments followed Meyer's law with the exponent varying slightly within the interval 2.0-2.2 depending on the penetration depth and carbon content. The reverse indentation size effect was observed which was most likely the result of compacting and densification of the films due to grain boundary sliding. The apparent hardness and elastic modulus versus displacement exhibited a monotonic increase within the whole indentation range. To determine the true hardness and the true elastic modulus a simple empirical technique based on the strain gradient plasticity approach was proposed. The elastic-plastic strain gradients were calculated from the nanoindentation loading-unloading curves and a micromechanical model of the a-C/Cu film samples was developed based on the strain gradients' fine structure (coating's fingerprint). The a-C/Cu films exhibited reasonable true hardness (1.4-2.8 GPa) combined with good plasticity which are significant prerequisites for solid lubricant applications.