Microstructure and mechanical property of diamond-like carbon films with ductile copper incorporation

In this paper, a ductile and non-carbide former Cu was incorporated into diamond-like carbon (DLC) films to modify the microstructure and property of the films using a hybrid ion beam system comprising an ion beam source and a magnetron sputtering unit. The composition, microstructure, residual stress and mechanical property of the DLC films with Cu doping were characterized carefully using X-ray photoelectron spectroscopy, transmission electron microscopy and Raman spectroscopy, stress-tester, and nanoindentation as a function of Cu concentration. The results reveal that the doped Cu atoms had low solubility in the as-deposited DLC films. The maximum solubility was found to lie around 1.93 at.%. When the Cu concentration was lower than this solubility, the doped Cu atoms dissolved in the carbon matrix, and the film exhibited the typical amorphous structure of DLC and showed a low residual stress and high elastic recovery due to the dissolved Cu atoms which could play a role of the interstitial atoms for stress relaxation through the distortion of the atomic bond length and angle. As the doped concentration exceeded the solubility, Cu nanocrystalline was formed in the carbon matrix, which could significantly improve the elastic resilience of the film through strain release via sliding of the nanocrystalline in the amorphous carbon matrix. It is worth noting that when the doped Cu concentration approached the solubility limit, amorphous nano-clusters were formed in the carbon matrix due to the segregation of Cu, resulting in the decrease of the number of the interstitial atoms, and thus caused the increase in the residual stress and the decline in the elastic recovery.