Effect of bias voltage on the microstructure and properties of Nb-DLC films prepared by a hybrid sputtering system

Highlights: • Nb-DLC films are fabricated using a hybrid deposition system combing HiPIMS and PDC magnetron sputtering system. • The Nb content and the sp³‒C bonding fraction in films can be tailored by modifying the bias voltage. • The microstructure, wettability and mechanical properties evolution of Nb-DLC films were systematically studied. -- Abstract: Diamond-like carbon (DLC) films with Nb doping were fabricated by a hybrid sputtering system comprising a high power impulse magnetron sputtering unit and a pulse direct current magnetron sputtering source. The effect of bias voltage on the morphology and chemical bonding characteristic of the Nb-DLC films were investigated. Results showed that the bias voltage played a significant role on the microstructure of the films. The surface morphology revealed a granular microstructure and the average size of these granules decreased with increasing bias voltage. Accordingly, the cross-sectional morphology evolved from an apparent columnar structure to a compact and dense nanocomposite structure with NbC nanocrystallites embedding in the amorphous carbon matrix at a high bias voltage. The integrated area ratio of the D and G peaks (I_D/I_G) of the Raman spectra decreased with increasing bias voltage, indicating the sp³‒C bonding fraction was improved in the films. The mechanical properties including hardness and elastic modulus, internal stress and elastic recovery, as well as the wettability property of films with different bias voltages were also investigated. With increasing bias voltage, the high sp³‒C fraction, modulated stress and formed hard carbide phase were all conducive to the enhancement of the hardness. Furthermore, all the films exhibited hydrophobic behavior and this tendency weakened at relatively high bias voltage conditions.