Effects of substrate bias voltage on the microstructure, mechanical properties and tribological behavior of reactive sputtered niobium carbide films

Niobium carbide films have been deposited on Si(100) substrates using direct current reactive magnetron sputtering in discharging a mixture of CH4 and Ar gas. The effects of substrate bias voltage (Vb) and methane flow rate (FCH4) on the phase structure, composition, morphology, mechanical properties and tribological behavior for NbC films have been explored. For the film grown at FCH4 = 6 sccm, a phase transition from a mixture of hexagonal-Nb2C and cubic-NbC phases to cubic-NbC phase occurs with increasing the absolute value of Vb and no CC bonding appears. In contrast, for the film deposited at FCH4 = 16 sccm, only the cubic-NbC phase is observed with increasing the absolute value of Vb and the CC bonding appears. If FCH4 is fixed at either 6 or 16 sccm, as the absolute value of Vb is increased, the growing film surface becomes smoother, and the compressive stress increases. This can be attributed to the increase in the carbon ion bombarding energy, which leads to promoting the diffusion of adsorbed atoms and more carbon speciesʹ occupying the interstitial positions. It is found that the hardness (H) increases first, and then decreases after reaching a maximum value with increasing the absolute value of Vb. The friction coefficient for the film obtained at FCH4 = 16 sccm is lower than that at FCH4 = 6 sccm, which may be ascribed to the presence of either graphite or amorphous carbon in the film grown at FCH4 = 16 sccm. Furthermore, a high stress results in a poor wear resistance.