Reactive magnetron sputtering deposition and characterization of niobium carbide films

• The structure for the NbCx films depends strongly on the carbon content.
• The hardness of NbCx films is strongly dependent on the structure.
• The tribological behavior depends strongly on the amount of amorphous carbon.
• The NbC film is a potential material for protective coating.

Niobium carbide thin films have been deposited on Si(100) substrates by direct current reactive magnetron sputtering using CH4 as a carbon source. With increasing FCH4FCH4 from 4 to 22 sccm, the carbon content for the film increases from 32.7 to 68.7 at.% gradually, accompanying with a phase transition from hexagonal-Nb2C to cubic-NbC, and at the highest carbon content, the film exhibits a typical nanocomposite structure consisting of NbC nanocrystallites embedded in an amorphous hydrocarbon (a-C:H) matrix. The morphology, mechanical properties and tribological behavior for the films exhibit a significant dependency on the amount of the amorphous carbon in the nanocomposite structure. The film surface becomes smooth with increasing the carbon content, corresponding to a transition from columnar crystallites to free of columnar features. In addition, the increase in carbon content for the films leads to an increase in the compressive stress for the well-crystallized film, but the excessive amorphous phase partially relaxes the stress. The NbCx film with 53.9 at.% carbon content shows the maximum hardness 25.0 GPa. Both friction coefficient and wear resistance are improved by increasing the content of the surplus amorphous carbon.