Effect of bias voltage on microstructure and properties of Ti-doped graphite-like carbon films synthesized by magnetron sputtering

Ti-doped graphite-like carbon (GLC) films with different microstructures and compositions were fabricated using magnetron sputtering technique. The influence of bias voltages on microstructure, hardness, internal stress, adhesion strength and tribological properties of the as-deposited GLC films were systemically investigated. The results showed that with increasing bias voltage, the graphite-like structure component (sp² bond) in the GLC films increased, and the films gradually became much smoother and denser. The nanohardness and compressive internal stress increased significantly with the increase of bias voltage up to −300 V and were constant after −400 V. GLC films deposited with bias voltages in the range of -300--400 V exhibited optimum adhesion strength with the substrates. Both the friction coefficients and the wear rates of GLC films in ambient air and water decreased with increasing voltages in the lower bias range (0--300 V), however, they were constant for higher bias values (beyond −300 V) . In addition, the wear rate of GLC films under water-lubricated condition was significantly higher for voltages below −300 V but lower at high voltage than that under dry friction condition. The excellent tribological performance of Ti-doped GLC films prepared at higher bias voltages of −300--400 V are attributed to their high hardness, tribo-induced lubricating top-layers and planar (2D) graphite-like structure.